Part of the purpose of the course is to help you to understand why biofuels are needed and how to make them, at the current state-of-the-art.

Why biofuels? To look at the situation a little more broadly, the question then becomes: why alternative fuels?

As climate change becomes an issue of ever-stronger concern in the world, stronger efforts are being devoted to tackling this issue. The International Energy Agency (IEA) has recently proposed the 2°C scenario (2DS) as a way to handle the climate change issue. The 2DS has become a largely used quote by many policymakers and scientists. The 2DS scenario requires that carbon dioxide (CO₂) emissions in 2060 should be reduced by 70% in comparison to the 2014 level. The transport sector plays an important role to achieve this goal, considering that the transportation sector is responsible for about 23% of total CO₂ emissions. Although electricity has been considered a promising option for reducing CO₂ emissions in transportation (Yabe, Shinoda, Seki, Tanaka, & Akisawa, 2012), transport biofuel is estimated to be the key alternative energy in the transport sector (Ahlgren, Börjesson Hagberg, & Grahn, 2017). The share of biofuels in total transportation-fuel consumption by 2060 is predicted to be 31%, followed by electricity at 27% based on the mobility model results of IEA for the 2DS. Biofuel production must be increased by a factor of 10 to achieve this goal (Oh, Hwang, Kim, Kim, & Lee, 2018). In addition to the need for climate change adaptation, the increasing concerns over energy security is another main driver for the policy-makers belonging to the Organisation for Economic Co-operation and Development (OECD) to promote the production of renewable energy (Ho, Ngo, & Guo, 2014). Last but not least, world energy demand will continue increasing. The world energy demand was 5.5 x 10²⁰ J in 2010. The studies predict an increase of a factor of 1.6 to reach a value of 8.6 x 10²⁰J in 2040. The bioenergy delivery potential of the world's total land area excluding cropland, infrastructure, wilderness, and denser forests is estimated at 190 x 10¹⁸ J yr^-1, 35% of the current global energy demand (Guo, Song, & Buhain, 2015).

Energy Demand, Climate Change, Energy Security: Three main reasons to promote sustainable biofuel production and usage.

Credit: Dr. Hilal Ezgi Toraman © Penn State, is licensed under CC BY-NC-SA 4.0.

In short, there are three main reasons to develop biofuels:

1. to meet the needs of increasing energy demand;
2. to reduce greenhouse gas (GHG) emissions; and
3. to improve energy security by reducing dependence on foreign fuel sources because it can be problematic, depending on US domestic fuel production.

We will explore each of these reasons in more depth in the following sections.

The energy needs of most advanced economies in the Western world are increasing at a modest level. However, in some developing economies, where the economy is booming, energy demands are increasing dramatically, e.g., in India and several African countries. The figure below shows the latest gross domestic product (GDP) growth rate of the countries of the world in 2025. While advanced economies had a 1.7% annual percent change in real GDP growth, emerging and developing Asia had a 4.5% annual percent change in real GDP growth, with India having a 6.2% annual percent change in real GDP growth. Emerging and developing economies accounted for over 80% of global energy demand growth. According to the IEA's Global Energy Review 2025, global electricity consumption increased by approximately 1,100 terawatt-hours (TWh) in 2024, more than double the annual average increase over the past decade. Currently, global energy consumption is growing at around 1-2% per year.  This rate is faster than the average rate over the past decade. If many third-world countries were to dramatically increase their standard of living, there are estimates that worldwide energy consumption would double. But where would that energy come from, particularly since there aren't huge stockpiles of crude oil sitting around? Petroleum cannot supply it all, and neither can natural gas or coal.

World Map of Real GDP Growth

Click here for a text description of the World Map of Real GDP Growth.

The image is a world map visualization titled "IMF DataMapper", which displays Real GDP growth (Annual percent change) for the year 2024 across various countries. It uses a color-coded legend to represent different ranges of GDP growth rates:

• Dark green: Countries with 6% or more GDP growth.
• Medium green: Countries with 3% to 6% GDP growth.
• Light green: Countries with 0% to 3% GDP growth.
• Yellow: Countries with 0% to -3% GDP growth (i.e., slight economic contraction).
• Orange: Countries with less than -3% GDP growth (i.e., significant economic contraction).
• Grey: Countries with no available data.

6% or more (Dark Green)

These countries are projected to have robust growth:

• Several countries in Sub-Saharan Africa

• Parts of Southeast Asia and Central Asia

3% – 6% (Medium Green)

These countries are projected to have moderate to high growth:

• Most of North America and South America
• Large parts of Africa
• Eastern Europe
• Most of Asia, including India

0% – 3% (Light Green)

These countries are expected to see modest economic growth:

• United States
• Canada
• Western Europe
• Japan
• Australia
• Some parts of Latin America
• Some parts of the Middle East

0% – (-3%) (Yellow)

These countries are projected to experience a slight economic contraction:

• Mexico
• Venezuela

Less than -3% (Orange)

These countries are expected to face significant economic decline:

• Haiti

No Data (Grey)

No economic data available for:

• Greenland

Credit: IMF DataMapper, World Economic Outlook

The US is highly dependent on crude oil to produce fuels for transportation. The figure below shows how the transportation sector is almost all oil-based, and the other sources barely make a dent in the hold petroleum has. Up until the last few years, the US has been highly dependent on foreign sources of oil. In 2023, petroleum accounted for approximately 89% of the primary energy consumption in the transportation sector, but it contributed less than 1% to the primary energy consumption in the electric power sector. The chart below illustrates the various types and quantities of primary energy sources used in the United States, the primary energy consumption by the electric power sector and end-use sectors, and the sales of electricity from the electric power sector to these end-use sectors. Growth across different parts of the global energy system varied widely in 2024, shaped by both short-term influences and longer-term structural shifts. Worldwide energy demand rose by 2.2%—well above the 1.3% annual average recorded from 2013 to 2023. Around 0.3 percentage points of this increase can be attributed to the effects of extreme weather. Even so, energy use expanded at a slower pace than the global economy, which grew by 3.2% in 2024, roughly in line with its long-term trend. Electricity demand grew faster than both overall energy use and GDP in 2024, rising by 4.3%. In absolute terms, this was the largest increase ever observed outside of post-recession rebounds. The surge was driven by structural factors, including wider use of electricity-intensive appliances such as air conditioning, shifts toward electricity-heavy manufacturing, growing needs from digitalisation, data centres, and AI, and the continued electrification of end uses. Altogether, the power sector accounted for around 60% of the global increase in energy demand.

On the supply side, renewables contributed the largest share of growth (38%), followed by natural gas (28%), coal (15%), oil (11%), and nuclear (8%). However, energy intensity improved by only 1%, extending the recent trend of slower efficiency gains. Energy-related CO₂ emissions rose by 0.8%—a deceleration compared with the 1.2% increase in 2023. (IEA, 2025).”

The US was the world's largest petroleum consumer (EIA, 2012), but was third in crude oil production. Over half of the material that was imported into the US comes from the Western hemisphere (North, South, and Central America, and the Caribbean), but we also imported 29% from Persian Gulf countries (Bahrain, Iraq, Kuwait, Saudi Arabia, and the United Arab Emirates).

The top 5 sources of net crude oil and petroleum imports included 1) Canada, 28%, 2) Saudi Arabia, 13%, 3) Mexico, 10%, 4) Venezuela, 9%, and 5) Russia, 5%. According to CNN Money, the US was behind Russia and Saudi Arabia in oil production for the first three months of 2016. See the World's Top Oil Producers for additional information. However, this situation recently changed, and the US became the world's largest oil producer in 2018 for the first time since 1973 and held the lead position through 2022. U.S. oil refineries obtain crude oil produced in the United States and other countries. Based on EIA, crude oil is extracted in 32 U.S. states as well as in coastal waters. In 2022, five states together made up roughly 72% of the total crude oil production in the United States. In 2022, 98 countries collectively produced around 80.75 million barrels of crude oil, with five of these nations contributing approximately 52% of the global total. The top five crude oil-producing countries and their respective shares of world crude oil production in 2022 were: The United States 14.7%, Saudi Arabia 13.2%, Russia 12.7%, Canada 5.6%, and Iraq, 5.5%. See the following link for further information see America is now the world's largest oil producer.

So, while oil is fairly available currently, there is extensive potentially explosive turmoil in many petroleum-producing regions of the world, and, in several places, the US's relationship with some oil-producing countries is strained. China and India are now aggressive and voracious players in world petroleum markets because of high economic growth (as pointed out in the previous section). Saudi Arabia's production is likely "maxed out," and domestic oil production peaked in 1970. While the US dependence on imported oil has declined after peaking in 2005, it is clear that if any one of the large producers decides to withhold oil, it could cause a shortage of fuel in the US and would cause the prices to skyrocket from an already high price (depending on the type of crude oil, the price of oil is currently $100-$106/bbl) (see U.S. Energy Information Administration). The figure below is a graphic showing the price level of oil from 1950 to the present. As you can see, there has been significant volatility in the price of oil in the last ~50 years. One of the first spikes came in 1974 when the Organization of the Petroleum Exporting Countries (OPEC) became more organized and withheld selling oil to the US. It was a true crisis at that point, with gasoline shortages causing long lines and fights at gas stations, with people filling up only on certain days depending on their license plates. It had a high spike in 1980, but a significant low in 1986. When the price of oil hit a significant low in 1998, the government took steps to lower the tax burden on oil companies. But when the prices went back up, the law remained in place, and currently, oil companies do not have to pay taxes on produced oil. When the reduced tax burden went into place in the late 90s, it made sense, but oil companies have continued to convince Congress with lobbyists that it should stay that way. What do you think?

As seen in the figure below, there were major fluctuations in gasoline prices in the last few years. As we will discuss in a later lesson, several aspects contribute to the price of gasoline, including but not limited to the recent COVID-19 Pandemic. This is a graphic that shows the price volatility for gasoline from 1990 - 2024 (the most recent data available), and the other figure below shows a breakdown of what goes into the price of gasoline.

In recent years, petroleum became less available and more expensive, and replacement-alternative fuels emerged because the economics were beginning to become more favorable. However, due to lower demand and high petroleum supply, prices drastically dropped, which may affect the development of alternative fuels. There is one factor that will most likely reverse this trend, and that is that energy demands will continue to increase worldwide. For future transportation fuel needs, most likely a liquid fuel will be necessary, and no one source will be able to replace petroleum. In the International Energy Outlook 2021 (IEO2021) Reference case, it is anticipated that, without major policy or technological advancements, global energy consumption will rise by nearly 50% over the next 30 years. While petroleum and other liquid fuels are expected to remain the world's primary energy source by 2050, renewable energy sources like solar and wind are projected to expand to almost the same level. It is important to note that Petroleum and other liquids include biofuels.

There is a scientific consensus that greenhouse gas (GHG) production is increasing, which has led to climate change and several other environmental concerns. Despite efforts to make us believe otherwise, much of the severe weather occurring worldwide is indeed due to climate change. There is a significant amount of evidence to substantiate the existence of climate change and the overall warming of the earth. Climate change is due to the Greenhouse Effect; it is a natural effect, caused by CO₂ and water vapor naturally present in the atmosphere. The focus of debate (scientific and political) has been on whether there is also an anthropogenic greenhouse effect, causing further climate change. Carbon dioxide (CO₂) is not the only greenhouse gas (methane, CH₄ is another potent GHG; this will be discussed further in upcoming sections), but most of the debate focuses on it. It is thought that the dramatic increase in CO₂ in the atmosphere is due to the burning of fossil fuels.

The world is highly dependent on fossil fuels; the US is also highly dependent on fossil fuels. As we saw in the charts in Lesson 1.3, global energy consumption will continue to increase over the next 30 years. Fossil-based sources such as coal, natural gas, and petroleum are expected to be the dominant energy source in 2050.

There is a mountain of evidence indicating that the planet is warming. The figure below shows global average surface temperature levels plotted from 1880-2020. The change has been most dramatic in the last 30 years. Yearly global temperatures from 1880 to 2023 relative to the 20th-century average show that Earth's surface temperature has increased by 0.14 degrees Fahrenheit per decade since 1880. The pace of warming has more than doubled since 1981.

The 2023 Global Climate Report from NOAA's National Centers for Environmental Information reveals that every month in 2023 was among the seven warmest on record for that particular month. Additionally, each month from June to December marked the hottest ever recorded for those months. In July, August, and September, global temperatures exceeded the long-term average by more than 1.0°C (1.8°F)—the first time any month in NOAA's records has surpassed this threshold.

The impacts of climate change on our planet can be observed from pole to pole. NOAA tracks global climate data, and here are some notable changes they've recorded, with more details available on the Global Climate Dashboard.

• Global temperatures have increased by about 1.8°F (1°C) from 1901 to 2020.
• Sea level rise has accelerated from 1.7 mm/year during most of the 20th century to 3.2 mm/year since 1993.
• Glaciers are retreating: the average thickness of 30 well-studied glaciers has reduced by over 60 feet since 1980.
• The summer sea ice coverage in the Arctic has diminished by approximately 40% since 1979.
• Atmospheric carbon dioxide levels have risen by 25% since 1958 and by about 40% since the Industrial Revolution.
• Snow is melting earlier than long-term averages indicate.

In the Arctic and Antarctic regions, the ice pack and glaciers are melting, and at an even faster rate than originally anticipated. Scientists have found that increasing atmospheric temperatures are not the only cause of this; the melting is causing water currents to shift and move warmer water around the poles, so melting is happening underneath the ice pack. The figure below shows that the total area of the Arctic Ocean with at least 15% ice coverage each September from 1979 to 2023 shows that, since 1980, the extent of ice surviving the summer has decreased by 13.1% per decade. The figure related to Sea Ice Concentration demonstrates that the sea ice concentration on September 19, 2023, compared to the 1981-2010 average extent for that date (indicated by the gold line), marked the sixth smallest summer minimum ever recorded. The sea levels have also risen by 8-9 inches since 1880, with the rate of increase accelerating during the satellite era.

Another problem could stem from the increased production of natural gas. Natural gas consists primarily of methane. Sources include petroleum and natural gas production systems, landfills, coal mining, animal manure, and fermentation of natural systems. Methane has 25 times the global warming potential of CO₂. The figure below shows the total GHG percentages from various economic sectors. The transportation sector is the largest contributor of greenhouse gas emissions. Greenhouse gas emissions from transportation mainly result from burning fossil fuels in cars, trucks, ships, trains, and planes. Over 94% of transportation fuel is petroleum-based, including primarily gasoline and diesel, which leads to direct emissions. The transportation sector is the largest source of direct greenhouse gas emissions and ranks second when considering indirect emissions from electricity use across all sectors. Although transportation is an end-use sector for electricity, it currently accounts for a relatively small portion of total electricity consumption. Indirect emissions from electricity make up less than 1% of direct emissions in this sector. The next figure shows the emissions of various GHG emissions from 1990-2021. The EPA points out that overall emissions of CH₄ have been reduced by 11% from 1990-2021. However, an article published in Nature (Yvon-Durocher, March 2014) suggests that there may be an unexpected consequence of warming temperatures; global warming can increase the amount of methane evolved from natural ecosystems. So, it remains to be seen what impacts can happen that have not been included in climate change models.

There are several possible responses to abate CO₂ and CH₄: 1) do nothing; 2) reduce CO₂ and CH₄ prudently; 3) drastically reduce energy use; and 4) move to a carbon-free society. The easiest, but quite possibly the most damaging in the long run, is to do nothing - currently, some nations are pushing to at least increase conservation. The use of hybrids has decreased our use of gasoline, as the increase in Corporate Average Fuel Economy (CAFE) standards has had an impact. However, prudent measures to reduce GHG will most likely not be enough to make a huge impact. Therefore, the use of biofuels could have great potential for reducing the impact of CO₂ and CH₄, if done well. However, some actions in South America have shown that if switching to biofuel growth is not handled well, a greater problem can be created. Some rainforest areas were removed from South America to clear land for producing biofuels, but the rainforests that were removed were burned, putting an excessive amount of CO₂ in the atmosphere. Rainforests have grown over long periods, so there was a lot of carbon stored in them - they were also places where exotic animals, plants, and insects lived, so the burning endangered the wildlife species in the rainforests. One thing to always keep in mind: whenever an action is taken in our atmosphere, there is the possibility of a negative consequence that one cannot foresee.

Summary

This lesson was about how using biofuels can benefit society. We looked at increasing energy demands around the world, how economically dependent we are on foreign sources of fuel, and how we don't have much control over what the prices of our fuels will be. We also explored how the growth in GHG emissions is a vital environmental concern and discussed how, without the use of biofuels, we cannot achieve significant reductions in GHG.

Lesson Objectives Review

By the end of this lesson, you should be able to:

• explain why biofuels are a necessary part of our energy portfolio.

References

"U.S. Energy Information Administration - EIA - Independent Statistics and Analysis. EIA's Energy in Brief: How Dependent Are We on Foreign Oil?

Greenhouse Gas Emissions: Greenhouse Gases Overview. EPA. Environmental Protection Agency.

Yvon-Durocher, G., Allen, A.P., Bastviken, D., Conrad, R., Gudasz, C., St-Pierre, A., Thanh-Duc, N., del Giorgio, P.A., "Methane fluxes show consistent temperature dependence across microbial to ecosystem scales," Nature, 507, 488-491.

The chemical compounds that are important for understanding most of the chemistry in this course are organic - that means that the compounds primarily contain carbon, hydrogen, and oxygen atoms (also sulfur and nitrogen). They can also be called hydrocarbons. The basic structures that we will be discussing in this course are called: 1) alkane (aka aliphatic), 2) branched alkane, 3) cycloalkane, 4) alkenes (double-bonds), 5) aromatic, 6) hydroaromatic, and 7) alcohols. First, I will show the atoms and how they are connected using the element abbreviation and lines as bonds, and then I will show abbreviated structural representations.

1. Alkane - atoms are lined up. For stick representation, each corner represents a CH₂ group, and each end represents a CH₃ group.

Name	Atoms and Bonds	Stick Representation
Heptane (7 C atoms)

2. Branched Alkane - still an alkane, but instead of a straight line, the carbons are branched off of each other.

Name	Atoms and Bonds	Stick Representation
Isobutane (4 C atoms)
Isopentane (5 C atoms)

3. Cycloalkanes - again, still an alkane, but forms a ring compound.

Name	Atoms and Bonds	Stick Representation
Cyclohexane (6 C atoms)

4. Alkenes - alkanes that contain a double bond.

Name	Atoms and Bonds	Stick Representation
Pentene (5 C atoms)

5. Aromatic - hydrocarbon ring compound with single and double bonds, significant differences in properties.

Name	Atoms and Bonds	Stick Representation
Benzene (6 C atoms)

6. Hydroaromatics - hydrocarbon ring compound with an aromatic and an alkane in one molecule.

Name	Atoms and Bonds	Stick Representation
1,2,3,4-tetrahydronaphthalene, aka tetralin (10 C atoms)

7. Alcohols - hydrocarbon with -OH functional group.

Name	Atoms and Bonds	Stick Representation
Butanol (4 C atoms)
Ethanol (2 C atoms)

The following table shows common hydrocarbons and their properties. It is important to know the properties of various hydrocarbons so that we can separate them and make chemical changes to them. This is a very brief overview - we will not yet be going into significant depth as to why the differences in chemicals affect the properties.

Much of the content in this particular section is based on information from Harold H. Schobert, Energy and Society: An Introduction, 2002, Taylor & Francis: New York, Chapters 19-24.

The following is a simplified flow diagram of a refinery. Since it looks relatively complicated, the diagram will be broken into pieces for better understanding.

Distillation

We will start with the first step in all refineries: distillation. Essentially, distillation is a process that heats crude oil and separates it into fractions. It is the most important process of a refinery. Crude oil is heated, vaporized, fed into a column that has plates in it, and the materials are separated based on the boiling point. It indicates that as the liquids are separated, the top-end materials are gases and lighter liquids, but as you go down the column, the products have a higher boiling point, the molecular size gets bigger, the flow of the materials gets thicker (i.e., increasing viscosity), and the sulfur (S) content typically stays with the heavier materials. Notice we are not using the chemical names, but the common mixture of chemicals. Gasoline represents the carbon range of ~ C₅-C₈, naphtha/kerosene (aka jet fuel) C₈-C₁₂, diesel C₁₀-C₁₅, etc. As we discuss the refinery, we will also discuss the important properties of each fuel.

The most important product in the refinery is gasoline. Consumer demand requires that 45-50 barrels per 100 barrels of crude oil processed are gasoline. The issues for consumers are, then: 1) quality suitability of gasoline and 2) quantity suitability. The engine that was developed to use gasoline is known as the Otto engine. It contains a four-stroke piston (and engines typically have 4-8 pistons). The first stroke is the intake stroke - a valve opens, allows a certain amount of gasoline and air, and the piston moves down. The second stroke is the compression stroke - the piston moves up and valves close so that the gasoline and air that came in the piston during the first stroke are compressed. The third stroke happens because the spark plug ignites the gasoline/air mixture, pushing the piston down. The fourth stroke is the exhaust stroke, where the exhaust valve opens and the piston moves back up. There is a good animation in How Stuff Works (Brain, Marshall. 'How Car Engines Work' 05 April 2000. HowStuffWorks.com).

You'll notice the x and the y on strokes 1 and 2. The ratio of x/y is known as the compression ratio (CR). This is a key design feature of an automobile engine. Typically, the higher the CR, the more powerful the engine is and the higher the top speed. The "action" is in the ignition or power stroke. The pressure in the cylinder is determined by 1) pressure at the moment of ignition (determined by CR) and 2) a further increase in pressure at the instant of ignition. At higher pressures with the CR, the more likely the pressure will cause autoignition (or spontaneous ignition), which can cause "knocking" in the engine - the higher the CR, the more likely the engine will knock. This is where fuel quality comes in.

For gasoline engines, the CR can be adjusted to the fuel rating to prevent knocking; this fuel quality is known as the "octane" number. Remember the straight-chain alkanes in the chemistry tutorial? The straight-chain alkanes are prone to knocking. The branched alkanes are not. The octane number is defined as 1) heptane - octane number equal to 0, and 2) 2,2,4-trimethylpentane - octane number equal to 100 (this is also known as "octane"). See the figure below for the chemical structures of heptane and octane for octane numbers. Modern car engines require an 87, 89, or 93-94 octane number. However, when processing crude oil, even high-quality crude oil, we can only produce from a distillation yield of 20% with an octane number of 50. This is why crude oil needs to be processed, to produce gasoline at 50% yield with an octane number of 87-94.

Other ways to improve the octane number:

1. Add aromatics. Aromatics have an octane number (ON) greater than 100. They can be deliberately blended into gasoline to improve ON. However, many aromatic compounds are suspected carcinogens, so there are regulatory limits on the aromatic content in gasoline.
2. Another approach to increasing ON is to add alcohol groups. Methanol and ethanol are typical alcohols that can be added to fuel. ON is ~110. They can be used as blends with racing cars (known as "alky").

But even with these compounds, distillation will not produce enough gasoline with a high enough ON. So other processes are needed.

"Cracking" Processes

Thermal cracking

One way to improve gasoline yield is to break the bigger molecules into smaller molecules - molecules that boil in the gasoline range. One way to do this is with "thermal cracking." Carbon Petroleum Dubbs was one of the inventors of a successful thermal cracking process. The process produces more gasoline, but the ON was still only ~70-73, so the quality was not adequate.

Catalytic cracking

Eugene Houdry developed another process; in the late 1930s, he discovered that thermal cracking performed in the presence of clay minerals would increase the reaction rate (i.e., make it faster) and produce molecules that had a higher ON, ~100. The clay does not become part of the gasoline - it just provides an active surface for cracking and changing the shape of molecules. The clay is known as a "catalyst," which is a substance that changes the course of a chemical reaction without being consumed. This process is called "catalytic cracking". The figure below shows the reactants and products for reducing the hexadecane molecule using both reactions. Catalytic cracking is the second most important process of a refinery, next to distillation. This process enables the production of ~45% gasoline with higher ON.

Below is the refining schematic with the additional processing added.

There are also tradeoffs when refineries make decisions as to the amount of each product they make. The quality of gasoline changes from summer to winter, as well as with gasoline demand. Prices that affect the quality of gasoline include 1) the price of crude oil, 2) the supply/demand of gasoline, 3) local, state, and federal taxes, and 4) the distribution of fuel (i.e., the cost of transporting fuel to various locations). Below is a schematic of how these contribute to the cost of gasoline and diesel.

Additional Processes

Alkylation

The alkylation process takes the small molecules produced during distillation and cracking and adds them to medium-sized molecules. They are typically added in a branched way in order to boost ON. An example of adding methane and ethane to butane is shown below.

Catalytic Reforming

A molecule may be of the correct number of carbon atoms but need a configuration that will either boost ON or make another product. The example below shows how reforming n-octane can produce 3,4-dimethyl hexane.

So, let's add these two new processes to our schematic in order to see how they fit into the refinery, and how this can change the ON of gasoline. The figure below shows the additions, as well as adding in the middle distillate fraction names. Typically, naphtha and kerosene, which can also be sold as these products, are the products that make up jet fuels. So, our next topic will cover how jet engines are different from gasoline engines and use different fuel.

The first aircraft used engines similar to the Otto four-stroke cycle, reciprocating piston engines. The Wright Flyer was an aircraft with this type of engine. During WWII, powerful 16-cylinder, high-compression ratio reciprocating engines were developed. However, the military was interested in developing engines that would make airplanes go faster, higher, and farther - this was to reduce the length of flights and provide better international communication. In order to achieve high-speed flight, a dilemma ensued: 1) the atmosphere thins at high altitudes, offering less air resistance to a plane which could lead to higher speeds, but 2) in "thinner" air, it is more difficult to get combustion air into the conventional piston engine. The modern jet engine was developed as part of a term paper by Frank Whittle while at the British Royal Air Force College, covering the fundamental principles of jet propulsion aircraft.

The jet engine begins with a "burner can," where jet fuel is injected and combusted in high-pressure air. The combustion produces a stream of high-temperature, high-pressure gases, as shown in the first figure below. If more power is required, two to four-burner cans can be included, and the high-temperature, high-pressure combustion gases operate a turbine (more about turbines for electricity generation in the lesson on electricity). The second figure depicts these additions. In the third figure, a containment vessel is put around the burner cans; the gases that exit the turbine pass through a nozzle. The gases exiting the nozzle provide thrust for the airplane. The fourth figure shows the completed engine - the high-pressure air comes from the air compressor, which is operated by the turbine.

There are variations on a simplistic jet engine: 1) the fan jet (turbofan), 2) the prop jet (turboprop), and 3) the turboshaft. The fan jet has a large fan in front of the engine to help provide air to the air compressor. It is a little slower than a turbojet but more fuel-efficient. This is the type favored for civilian transport aircraft. The prop jet uses the mechanical work of the turbine to operate a propeller. These types of engines are typically used for commuter aircraft. The turboshaft is a gas turbine engine that uses all of the output of the turbine to turn the blades, without jet exhaust. Helicopters, tanks, and hovercrafts use these types of engines. So, what is the fuel for jets?

Jet Fuel

Conventional jet fuel is composed primarily of straight-run kerosene (straight-chain carbons and accompanying hydrogen, bigger molecules than gasoline). However, there are some purification steps that are needed to ensure that the fuel behaves in jet engines.

The first step is the removal of sulfur. When sulfur is burned, it forms sulfur oxide compounds, such as sulfur dioxide (SO2) and sulfur trioxide (SO3). Because there are multiple sulfur oxide compounds, they are abbreviated into one chemical formula of SOx. These compounds, when combined with water, form acid rain (more on this in the next lesson on coal for electricity generation). Sulfur compounds are corrosive to fuel systems and have noxious odors. Sulfur is removed by reacting it with hydrogen and a metal catalyst; the processes are known as hydrogen desulfurization processes (HDS) and produce H2S (hydrogen sulfide), which is then reacted to solid sulfur.

Another problem that can occur with jet fuel is if it contains too much aromatic compound content. A small amount is actually necessary to lubricate gaskets and O-rings. However, aromatics are suspected carcinogens, and in combustion, aromatics are precursors to smoke and soot. Too much aromatic content can cause problems such as 1) poor aesthetics, 2) carcinogens, and 3) tracking of military aircraft. The way to remove aromatic compounds is the same as for removing sulfur; the aromatic compound is reacted with hydrogen and a metal catalyst to add hydrogen to the aromatic ring. The resulting compounds are heteroaromatics and cycloalkanes.

Another problem that can occur in the middle distillate fractions can occur if the fuel contains waxes. Waxes are higher molecular weight alkane hydrocarbons that can be dissolved in kerosene. At very cold temperatures at high altitudes, wax can either separate as a solid phase or cause the fuel to freeze and cause plugging in the fuel lines. This can also cause a problem called low-temperature viscosity. Viscosity is a measurement of the flow of a fluid; the thicker the fluid gets (and flow is reduced), the higher the viscosity. While the fuel isn't frozen, it is flowing slower and could cause problems for the engine. Again, the reason for the increase in viscosity is similar to having waxes in the kerosene; high viscosity is caused by bigger molecules within the fuel. The way to improve jet fuel properties is to remove the larger molecules. This is called dewaxing.

The last problem we will discuss has to do with nitrogen. Jet fuels do not typically contain nitrogen, but when combusting fuel using air (which contains primarily nitrogen), nitrogen oxide compounds can form, shown as a formula NOx. Because jet engines burn fuels at high temperatures, thermal NOx is a problem. NOx will contribute to acid rain. If there is any nitrogen in the fuel, it would be removed during the removal of sulfur.

A refinery will make ~10% of its product as jet fuel. The Air Force uses 10% of that fuel, so about 1% of refinery output is for military jet fuel. The figure below shows the additional processes just discussed in our schematic.

Rudolf Diesel first developed Diesel engines in the 19th century. He did so because he wanted to develop an engine that was more efficient than an Otto engine and that could use poorer quality fuel than gasoline. The Diesel engine also operates on a four-stroke cycle, but there are some important differences. Diesel engines have a high compression ratio (CR)- a small Diesel engine has a CR of 13:1, while a high-performance Otto engine has a CR of 10:1. Upon the compression stroke (stroke 2), there is a high increase in temperature and pressure. In the third stroke, fuel is injected and it ignites because of the high temperature and pressure of the compressed air. You can see an animation of this at How Stuff Works (Brain, Marshall. 'How Diesel Engines Work' 01 April 2000. HowStuffWorks.com). Diesel engines use fuel more efficiently; and under comparable conditions, a Diesel engine will always get better fuel efficiency than a gasoline Otto engine. Essentially, Diesel engines operate by knocking. The continuous knocking has two consequences: 1) a Diesel engine must be more sturdily built than a gasoline engine, so it is heavier and has a longer life - 300,000-350,000 miles before major engine service, and 2) fuel standards are "backward" from that of gasoline; we want fuel to knock.

Diesel Fuel

Diesel fuel has a much higher boiling range than gasoline. The molecules are larger than gasoline, and the octane scale cannot be used as a guide. The scale that is used for diesel fuel is called the cetane number. The compound, cetane, or hexadecane, C₁₆H₃₄, is the standard where the cetane number is 100. For the cetane number 0 (the other end of the scale), the chemical compound used is methylnaphthalene, an aromatic compound that doesn't knock. Most diesel fuels will have cetane numbers of 40-55, with the value in Europe on the higher end and the value in the US at the lower end of that range. In a refinery, diesel fuels are processed in the same fashion as jet fuels, using hydrogenation reactions to remove sulfur and nitrogen and reacting aromatics to hydro aromatics and cycloalkanes. Dewaxing also must be done to improve viscosity and low-temperature problems, particularly in colder climates. Therefore, the primary processes that are typical in a petroleum refinery apply to diesel fuel as well as jet fuel. Except in airplanes, diesel engines dominate internal combustion engine applications. They are standard for large trucks; dominate railways in North America and other countries; are common in buses; and are adapted in small cars and trucks, particularly in Europe. 

Similar to gasoline, prices that affect the quality of diesel include 1) the price of crude oil, 2) the supply/demand of diesel, 3) local, state, and federal taxes, and 4) the distribution of fuel (i.e., the cost of transporting fuel to various locations). Above is a schematic of how these contribute to the cost of diesel.

Summary

This lesson was a very brief overview - there are entire classes based on this one lecture. In this lesson, we discussed the different transportation engines for vehicles, the fuels used for these vehicles, and how the fuels are produced from a refinery. Gasoline is the lighter fuel used in typical automobile engines, while diesel fuel is used in Diesel engines. Diesel engines get better fuel mileage than gasoline engines - gasoline is lighter than diesel. Here in the US, the primary fuel produced is gasoline (~45-50%).

Lesson Objectives Review

By the end of this lesson, you should be able to:

• explain the chemistry of gasoline, diesel fuel, jet fuel, and fuel oil.
• describe the basics of how these fuels are made by converting from crude oil.
• discuss the utilization of these fuels in cars, trucks, aircraft, and various engine types.
• evaluate necessary fuel characteristics for various vehicle engines.

History of Burning Wood

Wood has been used as a source of energy for thousands of years (the first known use of fire was determined when archeologists made discoveries of humans living 400,000 years ago), and wood was the obvious source to make fire. In the Americas, in 1637, the people of Boston suffered from the scarcity of wood. It became America’s first energy crisis after less than one century of settlement. During the late 1700s, Benjamin Franklin invented a cast iron stove for indoor use. It held heat in the room after the fire burned out. However, it had a design flaw in that it had no way to pull in air, so fires went out quickly. So David R. Rittenhouse added a chimney and exhaust pipe to improve upon it.

Burning Wood

First, we will look at where energy is stored in materials, starting with the methane molecule. The combustion of methane is exothermic (releases heat as the reaction proceeds), but the reaction must be initiated before it will sustain itself with the continued availability of methane and oxygen. The formula below shows the reaction in a stoichiometric format:

$${\text{CH} }_4+4{\text{O}}_2\to {\text{CO} }_2+{\text{H}}_2\text{O (plus heat!)}$$

The figure below shows the same reactants and products, but with the bonds before reaction and after the reaction, on a molecular/atomic level. The number of atoms in each molecule doesn't change, but how they are arranged and connected does. The only real change is how the atoms are linked - these are the chemical bonds. Since ENERGY comes out of a burning system, then it must mean that more energy is stored in 4 C-H bonds and 2 O-O bonds than in 4 H-O and two C-O bonds. The ENERGY released during chemical combustion comes from ENERGY stored in chemical bonds of fuel & oxygen.

We now know the reaction chemistry of methane combustion, but wood is a much more complex material than methane. Wood contains up to 50% water. Water in the wood will reduce the heating value of the wood, and if the wood is very wet, it will lead to a smoky fire. The main components of wood (we will cover this in more depth in a later lesson) are cellulose (what paper is made from) and lignin (the part of a tree that makes it have a sturdy structure). In order to start a fire, you typically must ignite a material that burns easily to begin heating the wood (this can be newspaper or a “fire starter”). The components begin to decompose from the heat (therefore we are not technically “burning” yet), which produces vapors and char. The vapors are called “volatiles” and the char is composed of carbon and ash. The volatiles are what actually begin to burn, producing a flame. The carbon-rich char produces glowing embers or “coals,” which are needed to keep the fire sustained. Wood does not typically contain sulfur, so no sulfur oxides (or SO_x) are produced.

There can be problems with burning wood. The smoke comes from particulates that did not burn or only partially burned which can pollute the atmosphere, and typically come from resins in the trees. It isn’t an issue when one or two people are burning wood, but when thousands of people burn wood in fireplaces. In State College, Pennsylvania, in the winter, one can see smoke in the air from fireplaces. Wood fires in fireplaces can also deposit soot and creosote in the chimneys, which if not cleaned periodically, can ignite. Burning wood (or really most things) will produce an ash material (minerals in wood and coal that react with air under combustion conditions); the ash must be disposed of. Wood smoke also contains a variety of chemicals that can be carcinogenic.

Now let’s begin discussing different biomass sources, how we measure different properties of different biomasses, and how to determine the atomic composition of biomass.

There are four types of biomass resources that can be utilized:

1. agricultural residues
2. energy crops
3. forestry residues
4. processing wastes

Examples of different sources are listed below:

Agricultural Residues:

• Corn stover
• Wheat straw
• Rice straw
• Soybean stalk

Energy Crops:

• Switchgrass
• Sweet sorghum
• Sugar canes
• Algae
• Cattail
• Duckweed

Forestry Residues:

• Saw dust
• Woody chips

Processing wastes:

• Food processing wastes
• Animal wastes
• Municipal solid wastes

As already mentioned, most biomass is at least partially composed of three components: cellulose, hemicellulose, and lignin. The figures below show a diagram of lignocellulose and the biomass broken down into three parts. There will be significantly more discussion on biomass composition in future lessons. Cellulose is a crystalline polymer of ring molecules (6 carbons) with OH and COOH groups (in the first figure below, cellulose is the straight green lines; in the second figure it is the green molecule). Hemicellulose is similar, but has ring molecules with 5 and 6 carbons, and is amorphous in structures, as depicted in the first figure below by the black squiggly line; The second figure shows how it is around the cellulose and more detail of the molecular structure. Lignin is the material that holds it all together and is the light blue line in the first figure below and it is in red in the second figure.

How To Determine Properties of Biomass

There are four common ways to measure the properties of any carbon product, which will also be used for biomass: 1) proximate analysis, 2) ultimate analysis, 3) heat of combustion, and 4) ash analysis.

Proximate analysis

Proximate analysis is a broad measurement to determine the moisture content (M), volatile matter content (VM), fixed carbon content (FC), and ash content. These are all done on a mass basis, typically, and are done in what is called a proximate analyzer – the analyzer just measures the mass loss at certain temperatures. Moisture is driven off at ~105-110°C (just above the boiling point of water); it represents physically bound water only. Volatile matter is driven off in an inert atmosphere at 950°C, using a slow heating rate. The ash content is determined by taking the remaining material (after VM loss) and burning it at above 700°C in oxygen. The fixed carbon is then determined by the difference: FC = 1 – M – Ash – VM.

The following is an example of proximate analysis of lignin, which is part of wood and/or grasses, primarily:

• Moisture (wt%): 5.34
• Ash (wt%): 14.05
• Volatile Matter (wt%): 60.86
• $FC=100-M\left(\%\right)-A\left(\%\right)-VM\left(\%\right)$
• $FC=100-5.34-14.05-60.86=19.75$

Sometimes the moisture content will be removed from the VM and ash contents, on a dry basis:

• $FC=100-M\left(\%\right)-A\left(\%dry\right)-VM\left(\%dry\right)$
• $FC=100-14.05-60.86=25.09$

Ultimate analysis

The ultimate analysis is more specific in that it analyzes the elemental composition of the organic portion of materials. The compositions of carbon (C), hydrogen (H), nitrogen (N), sulfur (S), and oxygen (O) are determined on a mass percent basis and can be converted to an atomic basis. In some cases, chlorine (Cl) will also be analyzed. There are instruments that are designed to measure only the C H N mass percent and then another to measure the S percent; the instrument combusts the material and measures the products of combustion. The following is an example problem for determining the molecular atomic composition of biomass when being provided with an ultimate analysis. Oxygen is usually determined by difference. Water can skew the hydrogen results and must be accounted for.

Heat of combustion

The heat of combustion can be measured directly using a bomb calorimeter. This instrument is used to measure the calorific value per mass (calorie/gram or Btu/lb). It can also be estimated using different formulas that calculate it based on either ultimate or proximate analysis. A common type of calorimeter is the isoperibol calorimeter, which will contain the heat inside the jacket but will accommodate the change in temperature of the water in the bucket; see the schematic below. A sample is placed in a crucible that is put inside of a reactor with high-pressure oxygen. The sample is connected to a fuse and electrical leads that will ignite the sample, all contained within the reactor (sometimes called a bomb calorimeter). The water temperature in the bucket is measured before and after ignition, and with all the other parts calibrated, the specific heat of the water and the change in temperature are used to determine the heat of combustion.

The heating value is determined in a bomb calorimeter. Heating values are reported on both wet and dry fuel bases. For the high heating value (HHV), the value can be determined by normalizing out the moisture in a liquid form. For the low heating value (LHV), a portion of the heat of combustion is used to evaporate the moisture.

Ash Analysis

The minerals in the material, once combusted, turn to ash. The ash can be analyzed for specific compounds that will contain oxygen, such as CaO, K₂O, Na₂O, MgO, SiO₂, Fe₂O₃, P₂O₅, SO₃, and Cl. The original minerals can also be measured. Once the mineral or ash is isolated, it often must be dissolved in various acids and then analyzed. There is other instrumentation available, but the analysis is quite complicated and not often done.

Bulk density is also determined for biomass as a property. It is typically determined by measuring the weight of material per unit volume. It is usually determined on a dry weight basis (moisture-free) or on an as-received basis with moisture content available. For biomass, the low values (grain straws and shavings) are 150-200 kg/m3 (0.15-0.20 g/cm3), and the high values (solid wood) are 600-900 kg/m3 (0.60-0.90 g/cm3). The heating value and bulk density are used to determine the energy density. The figure below shows a comparison of various biomass sources to fossil fuel sources on an energy density mass basis.

Many of the fuel characteristics we have been discussing need to be known for the proper use of biomass in combustion, gasification, and other reaction chemistry.

Now, we will go into gasification and compare it to combustion. Gasification is a process that produces syngas, a gaseous mixture of CO, CO₂, H₂, and CH₄, from carbonaceous materials at high temperatures (750 – 1100°C). Gasification is a partial oxidation process; the reaction takes place with a limited amount of oxygen. The overall process is endothermic (requires heat to keep the reaction going), so it requires either the simultaneous burning of part of the fuel or the delivery of an external source of heat to drive the process.

Historically, gasification was used in the early 1800s to produce lighting, in London, England (1807) and Baltimore, Maryland (1816). It was manufactured from the gasification of coal. Gasification of coal, combined with Fischer-Tropsch synthesis was one method that was used during WWII to produce liquid fuel for Germany because they did not have access to oil for fuel. It has also been used to convert coal and heavy oil into hydrogen for the production of ammonia and urea-based fertilizers. As a process, it continues to be used in South Africa as a source of liquid fuels (gasification followed by Fischer-Tropsch synthesis).

Gasification typically takes place at temperatures from 750-1100°C. It will break apart biomass (or any carbon material), and usually, an oxidizing agent is added in insufficient quantities. The products are typically gas under these conditions, and the product slate will vary depending on the oxidizing agent. The products are typically hydrogen, carbon monoxide, carbon dioxide, and methane. There may also be some liquid products depending on the conditions used. Gasification and combustion have some similarities; the figure below shows the variation in products between gasification and combustion. The table shows a comparison of the conditions.

Zones of Gasification

There are several zones that the carbon material passes through as it proceeds through the gasifier:

1. drying
2. pyrolysis
3. combustion
4. reduction

The schematic below shows the zones and the products that typically occur during that part of the process. First, we will discuss what happens in each zone. We will also be looking at different gasifier designs to show these zones change depending on the design, and each design has advantages and disadvantages.

The drying process is essential to remove surface water, and the “product” is water. Water can be removed by filtration, evaporation, or a combination of both. Typically, waste heat is used to do the evaporation.

Pyrolysis is typically the next zone. If you look at it as a reaction:

$$\text{Reaction 1: Dry biomass}\to \text{Volatiles + Chars (C) + Ash}$$

$$\text{Reaction 2: Volatiles}\to \left( x \right)\text{Tar + }\left( 1-x \right)\text{Gas}$$

where x is the mass fraction of tars in the volatiles. Volatile gases are released from the dry biomass at temperatures ranging up to about 700oC. These gases are non-condensable vapors such as CH₄, CO, CO_2, and H₂ and condensable vapor of tar at the ambient temperature. The solid residues are char and ash. A typical method to test how well a biomass material will pyrolyze is thermogravimetric analysis; it is similar to the proximate analysis. However, the heating rate and oxidizing agent can be varied, and the instrument can be used to determine the optimum temperature of pyrolysis.

Gasification Process and Chemistry: Combustion and Reduction

A limited amount of oxidizing agent is used during gasification to partially oxidize the pyrolysis products of char (C), tar, and gas to form a gaseous mixture of syngas mainly containing CO, H₂, CH_4, and CO₂. Common gasifying agents are air, O₂, H₂O, and CO₂. If air or oxygen is used as a gasifying agent, partial combustion of biomass can supply heat for the endothermic reactions.

$${\text{Reaction 3: C (char) + O} }_2={\text{CO} }_2$$

$${\text{Reaction 4 C} }_{\text{m}}{\text{H}}_{\text{n}}\text{(tar) + }\left( \text{m + n}/4 \right){\text{O}}_2\to {\text{mCO} }_2+\text{n}/2{\text{H}}_2\text{O}$$

Combustion of gases:

$${\text{Reaction 5: H} }_2+1/2{\text{O}}_2\to {\text{H}}_2\text{O}$$

$${\text{Reaction 6: CH} }_4+2{\text{O}}_2\to {\text{CO} }_2+2{\text{H}}_2\text{O}$$$$\text{Reaction 7: CO + }1/2{\text{O}}_2\to {\text{CO} }_2$$

The equivalence ratio (ER) is the ratio of O₂ required for gasification, to O₂ required for full combustion of biomass. The value of ER is usually 0.2 - 0.4. At too high ER values, excess air causes unnecessary combustion of biomass and dilutes the syngas. At too low ER values, the partial combustion of biomass does not provide enough oxygen and heat for gasification.

There are several reactions that can take place in the reduction zone. There are three possible types of reactions: 1) solid-gas reactions, 2) tar-gas reactions, and 3) gas-gas reactions. Essentially, H₂O and CO₂ are used as gasifying agents to increase the H₂ and CO yields. The double-sided arrow represents that these reactions are reversible depending on the conditions used.

Solid-gas reactions include:

$${\text{Reaction 8: C + CO} }_2\leftrightarrow 2\text{CO (Boudouard Reaction)}$$$${\text{Reaction 9: C + H} }_2\text{O}\leftrightarrow {\text{CO + H} }_2\text{(Carbon-Water Reaction)}$$$$\text{Reaction 10: C + }2{\text{H}}_2\leftrightarrow {\text{CH} }_4\text{(Hydrogenation Reaction)}$$

Tar-gas reactions include:

$${\text{Reaction 11: C} }_{\text{m}}{\text{H}}_{\text{n}}{\text{(tar) + mH} }_2\text{O}\leftrightarrow \left( \text{m+n}/2 \right){\text{H}}_2\text{+mCO (Tar Steam Reforming Reaction)}$$$${\text{Reaction 12: C} }_{\text{m}}{\text{H}}_{\text{n}}{\text{(tar) + mCO} }_2\leftrightarrow {\text{n/2H} }_2+2\text{mCO (Tar Dry Reforming Reaction)}$$

Gas-gas reactions include:

$${\text{Reaction 13: CO + H} }_2\text{O}\leftrightarrow {\text{CO} }_2+{\text{H}}_2\text{(Water-Gas Shift Reaction)}$$$${\text{Reaction 14: CO + 3H} }_2\leftrightarrow {\text{CH} }_4+{\text{H}}_2\text{O (Methanation)}$$

The reactions can be affected by reaction equilibrium and kinetics. For a long reaction time: 1) chemical equilibrium is attained, 2) products are limited to CO, CO₂, H₂, and CH₄, and 3) low temperatures and high pressures favor the formation of CH₄, whereas high temperatures and low pressures favor the formation of H₂ and CO. For a short reaction time: 1) chemical equilibrium is not attained, 2) products contain light hydrocarbons as well as up to 10 wt% heavy hydrocarbons (tar), and 3) steam injection and catalysts can shift the products toward lower molecular weight compounds.

Gasifier Designs

There are several types of gasifier designs:

1. updraft
2. downdraft
3. cross downdraft
4. fluidized bed
5. plasma

The first type of gasifier is the updraft design. The advantages include that it is a simple design and is not sensitive to fuel selection. However, disadvantages include a long start-up time, production of high concentrations of tar, and a general lack of suitability for modern heat and power systems.

The downdraft gasifier is similar, but the air enters in the middle of the unit and gases flow down and out. The oxidation and reduction zones change places. Advantages of this design include low tar production, low power requirements, a quicker response time, and a short start-up time. However, it has a more complex design, fuel can be fouled with slag, and it cannot be scaled up beyond 400 kg/h.

The crossdraft design gasifier is similar to the downdraft, it has a quicker response time and a short start-up time; it is also complex in design, cannot use high mineral-containing fuels, and fuel can be contaminated with slag from ash.

With a fluidized bed design gasifier, the action of this gasifier is similar to how water might boil, except the air (or other gas) flows through the fines (the sample and sand) at temperature, creating a bubbling effect similar to boiling. Because of this action, it has the advantages of greater fuel flexibility, better control, and a quick response to changes. But because of these advantages, these types of gasifiers have a higher capital cost, and a higher power requirement, and must be operated on high particulate loading.

One of the new design gasifiers is a plasma gasifier design. Plasma gasification uses extremely high temperatures in an oxygen-starved environment to decompose waste material into small molecules and atoms so that the compounds formed are very simple and form syngas with H₂, CO, and H₂O. This type of unit functions very differently, as electricity is fed to a torch that has two electrodes – when functioning, the electrodes create an arc. Inert gas is passed through the arc, and, as this occurs, the gas heats to temperatures as high as 3,000 °C (Credit: Westinghouse Plasma Corporation). The advantages of such units include:

1. process versatility
2. superior emission characteristics
3. no secondary treatment of byproducts
4. valuable byproducts
5. enhanced process control
6. volume reduction of material fed
7. small plant size

Units such as these are more expensive and scaling up is still in the research stage. These types of units are most commonly used for municipal waste sludge.

General information on gasification

So what products are made, what advantages are there to using various oxidizing sources, how are the byproducts removed, and how is efficiency improved? Besides syngas, other products are made depending on the design. As stated previously, the syngas is composed of H₂, CO, CO₂, H₂O, and CH₄. Depending on the design, differing amounts of tar and char can also be made. For example, for steam fluidized gasification of wood sawdust at atmospheric pressure and 775°C, 80% of the carbon will be made into syngas, 4% of the carbon will produce tar, and 16% will produce char (Herguido J, Corella J, Gonzalez-Saiz J. Ind Eng Chem Res 1992; 31: 1274-82.)

There are multiple uses for syngas, for making hydrocarbon fuels, for producing particular chemicals, and for burning as a fuel; therefore, syngas has a heating value. The heating value can be calculated by the volumetric fraction and the higher heating values (HHV) of gas components, which is shown in this equation:

Other factors are determined for optimal gasification. Thermal efficiency is the conversion of the chemical energy of solid fuels into chemical energy and sensible heat of gaseous products. For high-temperature/high-pressure gasifiers, the efficiency is high, ~90%. For typical biomass gasifiers, the efficiency is reduced to 70-80% efficiency. Cold gas efficiency is the conversion of the chemical energy of solid fuel to the chemical energy of gaseous products; for typical biomass gasifiers, the efficiency is 50-60%.

There are several processing factors that can affect different aspects of gasification. The following table shows the main advantages and technical challenges of different gasifying agents. Steam and carbon dioxide as oxidizing agents are advantageous in making high-heating value syngas with more hydrogen and carbon monoxide than other gases but also require external heating sources and catalytic tar reformation.

Basic design features can also affect the performance of a gasifier. The table below shows the effect of a fixed bed versus a fluidized bed and the differences in temperature, pressure, and equivalence ratio. Fixed/moving beds are simpler in design and favorable on a small scale economically, but fluidized bed reactors have higher productivity and low byproduct generation. The rest of the table shows how increased temperature can also favor carbon conversion and the HHV of the syngas, while increased pressure helps with producing high-pressure syngas without compression to higher pressures downstream.

Product Cleaning

The main thing that has to be done to clean the syngas is to remove char and tar. The char is typically in particulate form, so the particulates can be removed in a way similar to what was described in the power plant facility. Typically for gasifiers, the method of particulate filtration includes gas cyclones (removal of particulate matter larger than 5 μm). Additional filtration can be done using ceramic candle filters or moving bed granular filters.

Tars are typically heavy liquids. In some cases, the tars are removed by scrubbing the gas stream with a fine mist of water or oil; this method is inexpensive but also inefficient. Tars can also be converted to low molecular weight compounds by “cracking” into CO and H₂ (these are typically the desired gases for syngas). This is done at high temperature (1000°C) or with the use of a catalyst at 600-800°C. Tars can also be “reformed” to CO and H₂, which can be converted into alcohols, alkanes, and other useful products. This is done with steam and is called steam reforming of tar; the reaction conditions are at a temperature of ~250°C and pressure of 30-55 atm. The reaction is shown below and is the same reaction as that shown in reaction 11:

Tar steam reforming reaction:

$${\text{Reaction 11: C} }_{\text{m}}{\text{H}}_{\text{n}}{\text{(tar) + mH} }_2\text{O}\leftrightarrow {\text{(m + n/2)H} }_2+\text{mCO}$$

Steam reforming has advantages. It is generally a safer operation since there isn’t any oxygen in the feed gases, and it produces a higher H₂/CO ratio syngas product than most alternatives. The main disadvantage is a lower thermal efficiency, as heat must be added indirectly because the reaction is endothermic.

Syngas Utilization

As stated earlier, syngas has multiple uses. Syngas can be used to generate heat and power, and can even be used to turn a turbine in some engineering designs. Syngas can also be used as the synthesis gas for Fischer-Tropsch fuel production, synthesis of methanol and dimethyl ether (DME), fermentation for the production of biobased products, and production of hydrogen.

So, how is syngas utilized in heat and power generation? Syngas can be used in pulverized coal combustion systems; it help the coal to ignite and to prevent the plugging of the coal feeding system. Biomass gasification can ease ash-related problems. This is because the gasification temperature is lower than in combustion, and once gasified, can supply clean syngas to the combustor. Adding a gasifier to a combustion system helps in the utilization of a variety of biomass sources with large variations in properties. Once the syngas has been cleaned, it can be fed to gas engines, fuel cells, or gas turbines for power generation.

Syngas may also be used to produce hydrogen. When biomass is gasified, a mixture of H₂, CO, CH₄, and CO₂ is produced. Further reaction to hydrogen can be done using water reforming and water-gas shift reactions:

Water reforming reaction for CH₄ to H₂:

$${\text{Reaction 15: CH} }_4+{\text{H}}_2\text{O}\leftrightarrow {\text{3H} }_2+\text{CO}$$

Water-gas shift reaction for CO to H₂ (as shown earlier):

$${\text{Reaction 13: CO + H} }_2\text{O}\leftrightarrow {\text{CO} }_2+{\text{H}}_2$$

Carbon dioxide may also be removed, as it is typically an undesirable component. One method to keep it from going into the atmosphere is to do chemical adsorption:

$${\text{Reaction 16: CaO + CO} }_2\leftrightarrow {\text{C}}_{\text{a}}{\text{CO} }_3$$

Syngas can also be utilized for the Fischer-Tropsch synthesis of hydrocarbon fuels. Variable chain-length hydrocarbons can be produced via a gas mixture of CO and H₂ using the Fischer-Tropsch method. The reaction for this is:

$${\text{Reaction 17: CO + 2H} }_2\to {{\text{(-CH} }_2\text{-)} }_{\text{n}}+{\text{H}}_2\text{O}$$

In order for the reaction to take place, the ratio should be close to 2:1, so gases generated via gasification may have to be adjusted to fit this ratio. Inert gases also need to be reduced, such as CO_2, and contaminants such as H₂S, as the contaminants may lower catalyst activity.

Methanol and dimethyl ether can also be produced from syngas. The reactions are:

$${\text{Reaction 18: CO + 2H} }_2\to {\text{CH} }_3\text{OH}$$$${\text{Reaction 19: CO} }_2+3{\text{H}}_2\to {\text{CH} }_3{\text{OCH} }_3+{\text{H}}_2\text{O}$$

Dimethyl ether (DME) can be made from methanol:

$${\text{Reaction 20: 2 CH} }_3\text{OH}\to {\text{CH} }_3{\text{OCH} }_3+{\text{H}}_2\text{O}$$

Syngas can also be fermented to produce bio-based products. This will be discussed in detail in a later lesson.

Summary

There are several potential crops that can be utilized for combustion and gasification. These include energy crops, crop residues, forest residues, and process wastes. These can be utilized in both combustion processes and gasification processes. For gasification, we looked at several factors:

1. gasification process and chemistry
2. gasifier design and operation
3. syngas cleaning
4. syngas utilization to make a variety of products

Biobased energy and chemical products discussed in the lesson include:

1. heat and power
2. hydrogen
3. F-T hydrocarbon fuels
4. alcohols
5. biochemical and biopolymers.

Lesson Objectives Review

By the end of this lesson, you should be able to:

• explain how wood was used historically to produce heat and electricity;
• evaluate the difference between combustion and gasification, and explain how design features differ depending on the process;
• evaluate how pyrolysis is different from gasification;
• describe the utilization of products from gasification versus pyrolysis, including how the processing of products differs.

References

Schobert, H.H., Energy and Society: An Introduction, 2002, Taylor & Francis: New York, Ch. 4-6.

Wang, L., Biological Engineering, North Carolina A&T University, BEEMS Module C1, Biomass Gasification, sponsored by USDA Higher Education Challenger Program, 2009-38411-19761, PI on project Li, Yebo.

The figure below shows a graphic of the four methods of thermochemical conversion of biomass, with pyrolysis highlighted. We just went over combustion and gasification, and we’ll cover direct liquefaction later on in the semester.

There are differences in each of the thermal processes. For combustion, the material is in an oxygen-rich atmosphere, at a very high operating temperature, with heat as the targeted output. Gasification takes place in an oxygen-lean atmosphere, with a high operating temperature and gaseous products being the main target (syngas production in most cases). Direct liquefaction (particularly hydrothermal processing) occurs in a non-oxidative atmosphere, where biomass is fed into a unit as an aqueous slurry at lower temperatures, and bio-crude in liquid form is the product.

So, what is pyrolysis? There are several definitions depending on the source, but essentially it is a thermochemical process, conducted at 400-600°C in the absence of oxygen. The process produces gases, bio-oil, and a char, and as noted in Lesson 4, is one of the first steps in gasification or combustion. The composition of the primary products made will depend on the temperature, pressure, and heating rate of the process.

There are advantages, both economical and environmental, to doing pyrolysis. They are:

• utilization of renewable resources through a carbon neutral route – environmental potential;
• utilization of waste materials such as lumber processing waste (barks, sawdust, forest thinnings, etc.), agricultural residues (straws, manure, etc.) – economic potential;
• self-sustaining energy – economic potential;
• conversion of low energy in biomass into high energy density liquid fuels – environmental & economic potentials;
• potential to produce chemicals from bio-based resources – environmental & economic potentials.

Pyrolysis was initially utilized to produce charcoal. In indigenous cultures in South America, the material was ignited and then covered with soil to reduce the oxygen available to the material – it left a high carbon material that could stabilize and enrich the soil to add nutrients ([Discussion of applications of pyrolysis], (n.d.), Retrieved from MagnumGroup.org). It has also been used as a lighter and less volatile source of heat for cooking (i.e., “charcoal” grills) in countries where electricity is not widely available and people use fuel such as this to cook with or heat their homes (Schobert, H.H., Energy, and Society: An Introduction, 2002, Taylor & Francis: New York). Not only is there a solid product, such as charcoal, but liquid products can also be produced depending on the starting material and conditions used. Historically, methanol was produced from the pyrolysis of wood.

This process for pyrolysis can also be called torrefaction. Torrefaction is typically done at relatively low pyrolysis temperatures (200-300°C) in the absence of oxygen. The feed material is heated up slowly, at less than 50°C/min, and is done over a period of hours to days – this way the volatiles are released and carbon maintain a rigid structure. In the first stage, water, which is a component that can inhibit the calorific value of a fuel, is lost. This is followed by a loss of CO, CO₂, H₂, and CH₄, in low quantities. By doing this, approximately 70% of the mass is retained with 90% of the energy content. The solid material is hydrophobic (with little attraction to water) and can be stored for a long period of time.

Classification of pyrolysis methods

There are three types of pyrolysis: 1) conventional/slow pyrolysis, 2) fast pyrolysis, and 3) ultra-fast/flash pyrolysis. The table and figure below summarize how each method differs in temperature, residence time, heating rate, and products made.

As mentioned earlier, slow pyrolysis is typically used to modify the solid material, minimizing the oil produced. Fast pyrolysis and ultra-fast (flash) pyrolysis maximizes the gases and oil produced.

Fast pyrolysis is a rapid thermal decomposition of carbonaceous materials in the absence of oxygen at moderate to high heating rates. It is the most common of the methods, both in research and in practical use. The major product is bio-oil. Pyrolysis is an endothermic process. Along with the information listed in the table, the feedstock must be dry; of smaller particles (< 3 mm); and typically done at atmospheric temperature with rapid quenching of the products. The yields of the products are: liquid condensates – 30-60%; gases (CO, H₂, CH₄, CO₂, and light hydrocarbons) – 15-35%; and char – 10-15%.

Ultra-fast, or flash pyrolysis is an extremely rapid thermal decomposition pyrolysis, with a high heating rate. The main products are gases and bio-oil. Heating rates can vary from 100-10,000° C/s and residence times are short in duration. The yields of the products are: liquid condensate ~10-20%; gases – 60-80%; and char – 10-15%.

Credit: Boyt, R., (November 2003), Wood Pyrolysis. Retrieved from Bioenergylists.org

Bio-oil Product Properties

Crude bio-oils are different from petroleum crude oils. Both can be dark and tarry with an odor, but crude bio-oils are not miscible with petro-oils. Bio-oils have high water content (20-30%); their density is heavier than water (1.10-1.25 g/mL); their heating value is ~5600-7700 Btu/lb (13-18 MJ/kg). Bio-oils have high oxygen content (35-50%), which causes high acidity (pH as low as ~2). Bio-oils are also viscous (20-1000 cp @ 40°C) and have high solid residues (up to 40%). These oils are also oxidatively unstable, so the oils can polymerize, agglomerate, or have oxidative reactions occurring in situ which lead to increased viscosity and volatility. The values in the table below compare the properties of bio-oil to petroleum-based heavy fuel oil.

Credit: Czemik, S. and Bridgewater, A.V., 2004. Overview of Applications of Biomass Fast Pyrolysis, Energy Fuels 18, 590-598.

Process Considerations

Several components are necessary for any pyrolysis unit, outside of the pyrolyzer itself. The units and how they are connected are shown in the figure below.

The goal of the process is to produce bio-oil from the pyrolyzer. The bio-oil that’s generated has potential as a transportation fuel after upgrading and fractionation. Some can be used for making specialty chemicals as well, especially ring-structure compounds that could be used for adhesives. The gases that are produced contain combustible components, so the gases are used to generate heat. A biochar is produced as well. Biochar can be used as a soil amendment that improves the quality of the soil, sequesters carbon, or can even be used as a carbon material as a catalyst support or activated carbon. There will also be a mineral-based material called ash once it’s been processed. Typically, the ash must be contained.

The next units to be considered are separation units. Char is solid, so it is typically separated using a cyclone or baghouse. It can be used as a catalyst for further decomposition into gases because the mineral inherent in the char as well as the carbon can catalyze the gasification reactions. The liquids and gases must also be separated. Usually, the liquids and gases must be cooled in order to separate the condensable liquids from the non-condensable gases. The liquids are then fractionated and will most likely be treated further to improve the stability of the liquids. At times, the liquid portion may plug due to heavier components. The non-condensable gases need to be cleaned of any trace amounts of liquids and can be reused if needed.

The next consideration is the heat sources for the unit. Hot flue gas is used to dry the feed. As the flue gas contains combustible gases, they can be partially combusted to provide heat. Any char that is left over is burned as a major supply of heat. And, biomass can be partially burned as another major source of heat.

Another important process to consider is the means of heat transfer. Much of it is indirect, through metal walls and tube and shell units. Direct heat transfer has to do with char and biomass burning. In the fluidized bed unit, the carrier (most often sand) brings in the heat, as the carrier is heated externally and recycled to provide heat to the pyrolyzer.

Types of Pyrolyzers

So, what types of pyrolyzers are used? The more common types are fluidized-bed pyrolyzers. The figures below show schematics of two different types. The advantages of using fluid beds are uniform temperature and good heat transfer; high bio-oil yield of up to 75%; a medium level of complexity in construction and operation; and ease of scaling up. The disadvantages of fluid beds are the requirement for small particle sizes; a large quantity of inert gases; and high operating costs. The circulating fluid bed pyrolyzer, (CFB), shown below, has similar advantages, although medium-sized particle sizes for feed are used. Disadvantages include a large quantity of heat carriers (i.e., sand); more complex operation, and high operating costs.

Two other types of pyrolyzers are the Rotating Cone and the Auger pyrolyzers. The rotating cone creates a swirling movement of particles through a g-force. This type of pyrolyzer is compact, has a relatively simple construction and operation, and has a low heat carrier/sand requirement. However, it has a limited capacity, requires feed to be fine particles, and is difficult to scale up. Auger pyrolyzers are also compact, simple in construction, and easy to operate; they function at a lower process temperature as well (400 °C). The disadvantages of Auger pyrolyzers include long residence times, lower bio-oil yields, high char yield, and limits in scaling up due to heat transfer limits.

Bio-Oil Upgrading

As noted earlier, bio-oil has issues and must be upgraded, which means essentially processed to remove the problems. These problems include high acid content (which is corrosive), high water content, and high instability both oxidatively and thermally (which can cause unwanted solids formation).

The oils must be treated physically and chemically. Physical treatments include the removal of char via filtration and emulsification of hydrocarbons for stability. Bio-oils are also fractionated, but not before chemical treatments are done. The chemical treatments include esterification (a reaction with alcohol to form esters – this will be covered in detail when discussing biodiesel production); catalytic de-oxygenation/hydrogenation to remove oxygen and double bonds; thermal cracking for more volatile components; physical extraction; and syngas production/gasification.

Catalytic de-oxygenation/hydrogenation takes place. A catalyst is used along with hydrogen gas; specialty catalysts are used, such as sulfides and oxides of nickel, cobalt, and molybdenum. Hydrogenation is commonly used in petroleum refining for the removal of sulfur and nitrogen from crude oil and to hydrogenate the products where double bonds may have formed in processing. Catalytic processes are separate processes and use specific equipment to perform the upgrading. One problem can be that there may be components of bio-oil that may be toxic to catalysts.

Esterification reacts to the corrosive acids in bio-oils with alcohol to form esters. An ester is shown below. A discussion of the esterification reaction will be discussed in the biodiesel lesson.

Bio-oil can also be thermally cracked and/or made into syngas through gasification. Please refer to Lesson 2 for the thermal cracking discussion and Lesson 4 for the gasification discussion. One other process that can be utilized is physical extraction, although extraction takes place due to the affinity of some of the compounds to a particular fluid. One example is the extraction of phenols. Phenols can be extracted using a sodium solution such as sodium hydroxide in water; the phenolic compounds are attracted to the sodium solution, while the less oxygenated compounds will stay in the organic solution. Again, this will be discussed in more detail in later lessons. The figure below shows a schematic of a typical processing unit to upgrade bio-oil.

Biomass Pretreatment

Current methods of generating biofuels are primarily from starch or grain, and starch hydrolysis is fairly straightforward. However, because the starch feedstocks are typically food-based, the goal is to develop technologies to produce ethanol from cellulose; cellulose is obtained from lignocellulosic biomass sources and must be pretreated before breaking down into ethanol. Below is a schematic of the differences in processing for starch (current) and cellulose (emerging). Before we go any further, we will have a short tutorial on the various components of lignocellulosic biomass.

When the word carbohydrate is used, I typically think of the carbohydrates in food. Carbohydrates are the sugars and complex units composed of sugars. This section will describe each.

Sugars are also called saccharides. Monomer units are single units of sugars called monosaccharides. Dimer units are double units of sugars called disaccharides. Polymers contain multiple units of monomers and dimers and are called polysaccharides.

So, what are typical monosaccharides? They are made up of a molecule that is in a ring structure with carbons and oxygen. The first figure below shows the structure of glucose; it is made up of C₆H₁₂O₆. Glucose is distinguished by its structure: five carbons in the ring with one oxygen; CH₂OH attached to a carbon; and OH and H groups attached to the other carbons. This sugar is known as blood sugar and is an immediate source of energy for cellular respiration. The second figure shows galactose next to glucose, and we can see that galactose is almost like glucose, except on the No. 4 carbon the OH and H are an isomer and just slightly different (highlighted in red on the galactose molecule). Galactose is a sugar monomer in milk and yogurt. The third figure shows fructose; while it still has a similar chemical formula as glucose (C₆H₁₂O₅), it is a five-membered ring with carbons and oxygens, but two CH₂OH groups. This is a sugar found in honey and fruits.

Glucose structure with carbons numbered.

Credit: Palaeos.com

Galactose structure next to glucose to highlight the main difference in the structures.

Credit: Palaeos.com

Fructose structure.

Credit: Palaeos.com

We also have disaccharides as sugars in food. Disaccharides are dimers of the monomers we just discussed and are shown below. One of the most common disaccharides is sucrose, which is a common table sugar and is shown in the figure below. It is a dimer of glucose and fructose. Another common sugar dimer is lactose. It is the major sugar in milk and a dimer of galactose and glucose. Maltose is also a sugar dimer but is a product of starch digestion. It is a dimer made up of glucose and glucose. In the next section, we will discuss what starch and cellulose are composed of in order to see why maltose is a product of starch digestion.

Sucrose (glucose + fructose) chemical structure.

Credit: World of Molecules

Lactose (galactose + glucose).

Credit: Optushome.com

Maltose (glucose + glucose) chemical structure.

Credit: Maltose Haworth: from Wikimedia Commons

Carbohydrate structure

All carbohydrate polymers are monomers that connect with what is called a glycosidic bond. For example, sucrose is a dimer of glucose and fructose. In order for the bond to form, there is a loss of H and OH. So, another way to show this is:

C₁₂H₂₂O₁₁ = 2 C₆H₁₂O₆ − H₂O

And as dimers can form, polymers will form and are called polysaccharides. Typical polysaccharides include 1) glycogen, 2) starch, and 3) cellulose. Glycogen is a molecule in which animals store glucose by polymerizing glucose, as shown below.

Glycogen as a) macromolecule and at b) the bond level.

Credit: Glico

Starches are similar to glycogen, with a slightly different structure. Starch is composed of two polymeric molecules, amylose and amylopectin. The structures of both are shown below.

Amylose, the part of starch that is not branched.

Credit: Marshall.edu

Amylopectin, the part of starch that is branched.

Credit: nutritionalbiochem.blogspot.com

About 20% of starch is made up of amylose and is a straight chain that forms into a helical shape with α-1,4 glycosidic bonds and the rest of the starch is amylopectin, which is branched with α-1,4, and α-1,6 glycosidic bonds. The figure below shows the structure of cellulose. Cellulose is a major molecule in the plant world; it is also the single most abundant molecule in the biosphere. It is a polymer of glucose and has connectors of the glucose molecule that are different from starch; the linkages are β-1,4 glycosidic bonds. The polymer of cellulose is such that it can form tight hydrogen bonds with oxygen, so it is more rigid and crystalline than starch molecules. The rigidity makes it difficult to break down.

Cellulose structure. The glycosidic bonds are β-1,4 linkages.

Credit: Z.H. Gao, J.Y. Gu, X‐M. Wang, Z.G. Li, X.D. Bai, (2005) "FTIR and XPS study of the reaction of phenyl isocyanate and cellulose with different moisture contents," Pigment & Resin Technology, Vol. 34 Iss: 5, pp.282 - 289

When the word carbohydrate is used, I typically think of the carbohydrates in food. Carbohydrates are the sugars and complex units composed of sugars. This section will describe each.

Sugars are also called saccharides. Monomer units are single units of sugars called monosaccharides. Dimer units are double units of sugars called disaccharides. Polymers contain multiple units of monomers and dimers and are called polysaccharides.

So, what are typical monosaccharides? They are made up of a molecule that is in a ring structure with carbons and oxygen. The first figure below shows the structure of glucose; it is made up of C₆H₁₂O₆. Glucose is distinguished by its structure: five carbons in the ring with one oxygen; CH₂OH attached to a carbon; and OH and H groups attached to the other carbons. This sugar is known as blood sugar and is an immediate source of energy for cellular respiration. The second figure shows galactose next to glucose, and we can see that galactose is almost like glucose, except on the No. 4 carbon the OH and H are an isomer and just slightly different (highlighted in red on the galactose molecule). Galactose is a sugar monomer in milk and yogurt. The third figure shows fructose; while it still has a similar chemical formula as glucose (C₆H₁₂O₅), it is a five-membered ring with carbons and oxygens, but two CH₂OH groups. This is a sugar found in honey and fruits.

Glucose structure with carbons numbered.

Credit: Palaeos.com

Galactose structure next to glucose highlights the main difference in the structures.

Credit: Palaeos.com

Fructose structure.

Credit: Palaeos.com

We also have disaccharides as sugars in food. Disaccharides are dimers of the monomers we just discussed and are shown below. One of the most common disaccharides is sucrose, which is a common table sugar. It is a dimer of glucose and fructose. Another common sugar dimer is lactose. It is the major sugar in milk and a dimer of galactose and glucose. Maltose is also a sugar dimer, but is a product of starch digestion. It is a dimer made up of glucose and glucose. In the next section, we will discuss what starch and cellulose are composed of in order to see why maltose is a product of starch digestion.

Sucrose (glucose + fructose) chemical structure.

Credit: World of Molecules

Lactose (galactose + glucose).

Credit: Optushome.com

Maltose (glucose + glucose) chemical structure.

Credit: Maltose Haworth: from Wikimedia Commons

Carbohydrate structure

All carbohydrate polymers are monomers that connect with what is called a glycosidic bond. For example, sucrose is a dimer of glucose and fructose. In order for the bond to form, there is a loss of H and OH. So, another way to show this is:

$${\mathrm{C} }_{12 }{\mathrm{H} }_{22 }{\mathrm{O} }_{11 }= 2 {\mathrm{C} }_{6 }{\mathrm{H} }_{12 }{\mathrm{O} }_{6 }- {\mathrm{H} }_{2 }\mathrm{O}$$

There is a wide variety of sources for lignocellulosic biomass, which includes agricultural waste (i.e., corn stover), forest waste from furniture and home construction, municipal solid waste, and energy crops. They all look very different, but all are composed of cellulose, hemicellulose, lignin, and other minor compounds. The figure below shows switchgrass (with parts magnified to emphasize different parts of the plant structure). Once you get down to the microfibril structure, you can see the components of the microfibril, which include lignin on the outside layer, hemicellulose on the next layer, and finally, cellulose. Because of the structure, the lignocellulose is difficult to break down, which is known as recalcitrance. In order to get to the cellulose, the cell wall has to be opened up, the lignin has to be removed or separated from the hemicellulose and cellulose, and then the cellulose, crystalline in nature, has to be broken down. All these steps are resistant to microbial attack, so pretreatment methods are used to break it apart. In other words, biomass recalcitrance requires pretreatment.

Switchgrass, to plant cells, to mesh of microfibrils, to microfibril structure, to crystalline cellulose to a cellulose molecule.

Click Here for a text alternative to the image above

The image is a scientific diagram illustrating the structure and composition of cellulose in plant cells. It begins with a depiction of plant cells, highlighting the cell wall, which is the primary location of cellulose.

A magnified section of the cell wall reveals a layered mesh of microfibrils, which are bundles of cellulose chains. These microfibrils are shown in greater detail, illustrating how they are composed of tightly packed cellulose molecules.

The diagram further breaks down the cellulose molecule into its basic building blocks, showing repeating units of glucose and cellobiose. It also includes a representation of crystalline cellulose, where the cellulose chains are aligned in a highly ordered structure, contributing to the strength and rigidity of the plant cell wall.

Overall, the image provides a clear and detailed view of how cellulose is organized from the molecular level up to its role in the plant cell wall, emphasizing its structural importance in plant biology.

Credit: Rosa Estela Quiroz-Castañeda and Jorge Luis Folch-Mallol (2013). Hydrolysis of Biomass Mediated by Cellulases for the Production of Sugars, Sustainable Degradation of Lignocellulosic Biomass - Techniques, Applications and Commercialization, Dr. Anuj Chandel (Ed.)

Pretreatment is the most costly step; however, the only process step more expensive than pretreatment is no pretreatment. Without pretreatment, yields are low and drive up all other costs, more than the amount saved without pretreatment. Increased yields with pretreatment reduces all other unit costs. Below is a schematic of the role pretreatment plays. Pretreatment, depending on the method, will separate the lignin, the hemicellulose, and the cellulose. The schematic shows how these break apart. Part of the lignin and the hemicellulose are dissolved in liquid during hydrolysis, and part of the lignin and the cellulose are left as a solid residue. There is a partial breakdown of the polymeric molecules, and the cellulose is now more accessible to microbial attack.

The role of pretreatment in breaking apart parts of biomass.

Credit: BEEMS

Pretreatment is costly and affects both upstream and downstream processes. On the upstream side, it can affect how the biomass is collected or harvested, as well as the comminution of the biomass. Downstream of pretreatment, the enzyme production can be affected, which in turn will affect the enzymatic hydrolysis and sugar fermentation. Pretreatment can also affect hydrolyzate conditioning and hydrolyzate fermentation. The products made and the eventual final processing also will be affected by pretreatment. However, it is more costly to not do pretreatment.

There are two different types of pretreatment. Physical effects disrupt the higher-order structure and increase surface area and chemical/enzyme penetration into plant cell walls, including mechanical size reduction and fiber liberation. Chemical effects include solubilization, depolymerization, and breaking of crosslinks between macromolecules. The individual components can “swell," depending on the organic solvent or acid used. Lignin can be “redistributed” into a solution, and lignin and carbohydrates can be depolymerized or modified chemically.

The following pretreatment technologies will be discussed in more depth: 1) size reduction, 2) low pH method, 3) neutral pH method, 4) high pH method, 5) organic solvent separation, 6) ionic liquid separation, and 7) biological treatments.

Summary

Lesson 5 covered biomass pyrolysis and biomass pretreatment. Pyrolysis is a thermal treatment in the absence of oxygen and at lower temperature than gasification. The main products of interest are chars that are often used in combustion (with some of the undesirable components removed), and liquids that need to be processed further to remove oxygen functionality and add hydrogen.

The goal of pretreatment is to overcome biomass recalcitrance and improve conversion efficiency/economics. Mechanical size reduction is generally required. Several pretreatment technologies have been developed based on the use of different chemicals. They do the following:

• Low pH pretreatment: hemicellulose removal, acetyl removal, lignin solubilization and re-precipitation, increased surface area
• High pH pretreatment: lignin removal, acetyl removal, hemicellulose solubilization, increased surface area
• AFEX pretreatment: no compositional changes, lignin alternation, cellulose decrystallization, increased surface area
• Organosolv and IL: fractionation of lignin from cellulose and hemicellulose, increased surface area

Lesson Objectives Review

By the end of this lesson, you should be able to:

• explain how pyrolysis is different from gasification and combustion;
• explain how the products are made and used as fuels and chemicals;
• determine which thermal process is best to use depending on biomass source and product utilization;
• describe the basic chemical structures of biomass, namely lignocellulosic biomass;
• evaluate pretreatment options for lignocellulosic options and explain why they are necessary.
• References

Schobert, H.H., Energy and Society: An Introduction, 2002, Taylor & Francis: New York, Ch. 4-6.

He, Brian, Department of Biological and Agricultural Engineering, University of Idaho, BEEMS Module C2, Biomass Pyrolysis, sponsored by USDA Higher Education Challenger Program, 2009-38411-19761, PI on project Li, Yebo.

Shi, Jian, Hodge, D.B., Pryor, S.W., Li, Yebo, Department of Food, Agricultural, and Biological Engineering, The Ohio State University, BEEMS Module B1, Pretreatment of Lignocellulosic Biomass, sponsored by USDA Higher Education Challenger Program, 2009-38411-19761, PI on project Li, Yebo.

The final project will be due at the end of the semester. Toward the end of the semester, the homework will be less, so you’ll have ample opportunity to work on this. However, I am including the expectations now, so that you can begin to work on them.

Biomass Choice

You will be choosing a particular biomass to focus your report on. For the biomass you choose, you will need to do a literature review on the biomass and how and where it grows. Your requirements for location include 1) where it grows, 2) climate, 3) land area requirement, and 4) product markets near location. However, you are not to make a choice that already exists in the marketplace. This includes 1) sugarcane for ethanol production in Brazil and 2) corn for ethanol production in the Midwest of the USA. You need to put thought into what biomass you are interested in converting to fuels and chemicals, as well as where you want to locate your small facility. Most of all, choose biomass and location based on your particular interests, so as to make it interesting to you.

The literature review should consist of a list of at least ten resources that you have consulted from journals and websites of agencies such as IEA. If available, five of these sources should be from the last five years. Please use APA style for your references.

Location Choice and Method of Production

Once you have determined a biomass, choose a location based on previous information. Discuss your reasons for the choice of biomass, location, and desired products for production. Include a map of the area where you want to grow and market your product. You need to be aware of whether or not the biomass you choose can grow in the climate of the area you choose.

You will be choosing a method with which to convert your biomass into fuels. You are expected to include a schematic of the process units and a description of each process that will be necessary to do the biomass conversion; you should include what each process does and a little about the chemistry of each. Show the major chemical reactions that will take place in the process. The figures below show a process diagram and a chemical reaction, so you have an idea of what I expect.

Schematic of the sulfuric acid pretreatment process. This is a typical process schematic or diagram.

Click here for a text description of the sulfuric acid pretreatment process schematic.

Schematic of the sulfuric acid pretreatment process. Biomass is added to dilute sulfuric acid, called acid impregnation (0.5 – 5.0% H2SO4). It then goes to dilute acid pretreatment around (140-200 °C). This pretreatment can take seconds to hours. It then becomes a slurry, which goes into neutralization conditioning. The solids go to enzymatic hydrolysis, where hexose sugars and solid residue (lignin) are separated. The liquids from the neutralization condition go to the acid post-hydrolysis. The slurry can also go to solid/liquid separation. The solid products of this separation go to neutralization conditions, and the liquids go to acid post-hydrolysis. Acid post-hydrolysis yields pentose sugars.

Credit: BEEMS Module B1

Schematic of the reaction of lignin in subcritical water (1. ether cleavage via hydrolysis, 2. cleavage of aldehyde/ketone and methoxy, rearrangement reaction with each other). This is a typical reaction depiction, although most likely more complex than what I expect you to include.

Credit: M. Bembenic and C. Clifford, Energy Fuels, 27 (11), 6681-6694, 2013

Market

The next section has to do with marketing your product. If you don’t have somewhere to sell your product, it will sit in a warehouse, maybe degrade (spoil is a more common term), and you won’t be making money on it. In the location you have chosen, is there a market for the product? If not, is there a location nearby where you can sell it? Discuss how you might market your products in the areas where you want to use biomass and sell products. How might you make the product you are selling appeal to the public? Due to the deregulation of electricity markets in various states, the prices of electricity will vary. Some companies charge more for renewable-based electricity, so they have to appeal to a particular market of people who are willing to spend more on renewable electricity.

Economics

We are going to assume that your process is going to be economic. However, any economic evidence that you can include that supports your process or indicates it would be a highly economical process will be beneficial to your paper. I would also like you to include any research and development that must occur for the process to become viable and economic (i.e., what is the current research on this process?).

Other Factors

Discuss other factors that could affect the outcome of implementing a bio-refining facility. What laws, such as environmental laws, might be in place? What is the political climate of the community you have chosen? What is the national political climate related to the biomass processing you have chosen? Are there any tax incentives that would encourage your process to be implemented or the product to be sold? An example would be something like this: all airlines in the US are expected to include a certain percentage of renewables in the jet fuel they use. So, would your process make jet fuel, and how would you market it to airlines? Include other factors that could “make or break” the facility.

Format

The report should be 8-12 pages in length. This includes figures and tables. It should be in 12-point font with 1” margins. You can use line spacing from 1-2. It is to be clearly communicated in English, with proper grammar, and as free from typographical errors as possible. You will lose points if your English writing is poor.

The following format should be followed:

• Cover Page – Title, Name, Course Info
• Introduction
• Body of Paper (see sections described above)
• Summary and Conclusions
• APA citation style for citations and references.

Grading Rubric:

• Outline: 10 points
• Rough Draft: 30 points
• Final Draft: 30 points
• Presentations (will be uploaded as videos): 30 points
• TOTAL: 100 points

Rubrics specific to each section of the final project are available in the submission dropboxes.

When submitting, please upload your final project to the Final Project Submission Dropbox. Save it as a PDF according to the following naming convention: userID_FinalProject (i.e., ceb7_FinalProject).

Questions

If you have questions:

If there is anything in the lesson materials that you would like to comment on, or don't quite understand, please post your thoughts and/or questions to our Throughout the Course Questions Comments discussion forum and/or set up an appointment during office hours. The discussion forum is checked regularly (Monday through Friday). While you are there, feel free to post responses to your classmates if you can help.

So, at this point, we’ve talked a bit about what lignocellulosic biomass is composed of, what various carbohydrates are chemically, and how to pretreat various biomass sources. Now, we will discuss the use of enzymes in biomass conversion, particularly in cellulose conversion. I’ll first introduce you to cellulases, and then we'll look at a model of enzymatic hydrolysis of cellulose and enzymes for hemicellulose and lignin.

For cellulases, we’ll discuss what they are, provide a brief history, look at glycosyl hydrolases, and, finally, cellulases.

The processing of cellulose in lignocellulosic biomass requires several steps. We’ve discussed pretreatment, where cellulose, lignin, and hemicellulose are separated. Hemicellulose is broken down into xylose and other sugars, which can then be fermented to ethanol. Lignin is separated out and can be further processed or burned depending on the best economic outcome. The first step of processing is then on the cellulose.

Pretreatment helps to decrystallize cellulose. However, it must be further processed to break it down into glucose, as it is glucose (a sugar) that can be fermented to make ethanol, and the liquid product must be further processed to make concentrated ethanol. So, we are focusing this lesson on the enzymatic hydrolysis of starch and cellulose.

Starches are broken down by enzymes known as amylases; our saliva contains amylase, so this is how starches begin to be broken down in our body. Amylases have also been isolated and used to depolymerize starch for making alcohol, i.e., yeast for bread making and for alcohol manufacturing. Chemically, the amylase breaks the carbon-oxygen linkage on the chains (α-1,4-glucosidic bond and the α-1,6-glucosidic bond), which is known as hydrolysis. Once the glucose is formed, then fermentation can take place to break the glucose down into alcohols and CO₂. The amylases were isolated and the hydrolysis of glucose began to be understood in the 1800s.

However, recall that cellulose linkages are β-1.4-glucosidic bonds. These bonds are much more difficult to break, and due to cellulose crystallinity, breaking cellulose down into glucose is even more difficult. It was only during WWII that enzymatic hydrolysis of cellulose was discovered. Instead of enzymes called amylases, the enzymes that degrade cellulose are called cellulases.

Cellulases are not a single enzyme. There are two main approaches to biological cellulose depolymerization: complexed and non-complexed systems. Each cellulase enzyme is composed of three main parts, and there are multiple synergies between enzymes.

Summary

In Lesson 5.1, we went over the requirements for the final project. In a future lesson, you will be expected to choose your biomass and outline your project.

Lesson 5.2 provided an overview of lignocellulosic biomass structure in greater depth than the previous lesson did. The greater depth is needed in order to understand how the enzymes work. You are expected to understand what lignocellulosic biomass is and how the components can break apart (i.e., what the fragments are chemically).

Lesson 5.3 discussed the basic composition of enzymes, how cellulosic enzymes (cellulases) work, and how hemicellulosic and lignitic enzymes work. The homework provided a background of what you need to know about enzymes.

Lesson Objectives

By the end of this lesson, you should be able to:

• explain the requirements for the final report;
• recall the biochemistry of starch and lignocellulosic biomass, as well as go into greater depth on each of these components;
• discuss the basic biochemistry of enzymes;
• evaluate how the enzymes work, and on certain biomass parts, particular enzymes are used, and products that are made.

Back in Lesson 2, I included a chemistry tutorial on some of the basic constituents of fuels. In this lesson, we will be discussing the production of ethanol (CH₃-CH₂-OH) and butanol (CH₃-CH₂-CH₂-CH₂-OH) from starch and sugar. Ethanol, or ethyl alcohol, is a chemical that is volatile, colorless, and flammable. It can be produced from petroleum via the chemical transformation of ethylene, but it can also be produced by fermentation of glucose, using yeast or other microorganisms; current fuel ethanol plants make ethanol via fermentation.

The basic formula for making ethanol from sugar glucose is as follows:

$${\text{C}}_6{\text{H}}_{12 }{\text{O}}_6\to 2{\text{C}}_2{\text{H}}_5{\text{OH+2CO} }_2$$

For fermentation, yeast is needed (other enzymes are used but yeast is most common), a sugar such as glucose is the carbon source, and anaerobic conditions (without oxygen) must be present. If you have aerobic (with oxygen) conditions, the sugar will be completely converted into CO₂ with little ethanol produced. Other nutrients include water, a nitrogen source, and micronutrients.

Here in the US, the current common method of ethanol fuel production comes from starches, such as corn, wheat, and potatoes. The starch is hydrolyzed into glucose before proceeding with the rest of the process. In Brazil, sucrose, or sugar in sugarcane is the most common feedstock. And in Europe, the most common feed is sugar beets. Cellulose is being used in developing methods, which include wood, grasses, and crop residues. It is considered developing because converting the cellulose into glucose is more challenging than in starches and sugars.

The International Energy Agency (IEA) predicts that ethanol will constitute two-thirds of the global growth in conventional biofuels with biodiesel and hydrotreated vegetable oil accounting for the remaining part (2018-2023). Global ethanol production is estimated to increase by 14% from about 120 bln L in 2017 to approximately 131 bln L by 2027. Brazil will accommodate fifty percent of this increase and will be used to fill in the domestic demand (OECD/FAO (2018), “OECD-FAO Agricultural Outlook”).

World production of ethanol-based by country is shown below. The US produces the most ethanol worldwide (~57%), primarily from corn. Brazil is the next largest producer with 27%, primarily from sugarcane. Other countries, including Australia, Columbia, India, Peru, Cuba, Ethiopia, Vietnam, and Zimbabwe, are also beginning to produce ethanol from sugarcane.

The figure below shows the growth of sugarcane in the world, in tropical or temperate regions. Sugar beet production in Europe is the other source of sugar for ethanol. It is grown in more northern regions than sugarcane, primarily in Europe and a small amount in the US. The next figure shows the growth of sugar beets in the world.

Production of ethanol from corn will be discussed in the next section; this section will focus on sugarcane ethanol production. So, what needs to be done to get the sugar from sugarcane?

• The first step is sugarcane harvesting. Much of the harvesting is done with manual labor, particularly in many tropical regions. Some harvesting is done mechanically. The material is then quickly transported by truck to reduce losses.
• The cane is then cut and milled with water. This produces a juice with 10-15% solids, from which the sucrose is extracted. The juice contains undesired organic compounds that could cause what is called sugar inversion (hydrolysis of sugar into fructose and glucose). This leads to the clarification step to prevent sugar inversion.
• In the clarification step, the juice is heated to 115°C and treated with lime and sulfuric acid, which precipitates unwanted inorganics.
• The next step for ethanol production is the fermentation step, where juice and molasses are mixed so that a 10-20% sucrose solution is obtained. The fermentation is exothermic; therefore, cooling is needed to keep the reaction under fermentation conditions. Yeast is added along with nutrients (nitrogen and trace elements) to keep yeast growing. Fermentation can take place in both batch and continuous reactors, though Brazil primarily uses continuous reactors.

The figure below shows a schematic of one process for ethanol production, along with the option to produce refined sugar as well. Sugarcane contains the following: water (73-76%), soluble solids (10-16%), and dry fiber or bagasse (11-16%). It takes a series of physical and chemical processes that occur in 7 steps to make the two main products, ethanol and sugar.

So, why produce both sugar and ethanol? Both are commodity products, so the price and market of the product may dictate how much of each product to make. This is how Brazilian ethanol plants are configured. To have an economic process, all of the products, even the by-products, are utilized in some fashion.

As noted previously, one of the major by-products is the dry fiber of processing, also known as bagasse. Bagasse is also a by-product of sorghum stalk processing. Most commonly, bagasse is combusted to generate heat and power for processing. The advantage of burning the bagasse is lowering the need for external energy, which in turn also lowers the net carbon footprint and improves the net energy balance of the process. In corn processing, a co-product is made that can be used for animal feed, called distiller grains, but this material could also be burned to provide process heat and energy. The figure below shows a bagasse combustion facility. The main drawback to burning bagasse is its high water content; high water content reduces the energy output and is an issue for most biomass sources when compared to fossil fuels, which have a higher energy density and lower water content.

Bagasse can have other uses. The composition of bagasse is: 1) cellulose, 45-55%, 2) hemicellulose, 20-25%, 3) lignin, 18-24%, 4) minerals, 1-4%, and 5) waxes, < 1%. With the cellulose content, it can be used to produce paper and biodegradable paper products. It is typically carted on small trucks that look like they have “hair” growing out of them.

Another crop that has some similarities to sugarcane is sorghum. Sorghum is a species of grass, with one type that is raised for grain and many other types that are used as fodder plants (animal feed). The plants are cultivated in warmer climates and are native to tropical and subtropical regions. Sorghum bicolor is a world crop that is used for food (as grain and in sorghum syrup or molasses), animal feed, the production of alcoholic beverages, and biofuels. Most varieties of sorghum are drought- and heat-tolerant, even in arid regions, and are used as a food staple for poor and rural communities. The figure below shows a picture of a sorghum field.

The US could use several alternative sugar sources to produce ethanol; it turns out corn is the least expensive and, therefore, the most profitable feed and method to produce ethanol. The table below shows a comparison of various feedstocks that could be used to make ethanol, comparing feedstock costs, production costs, and total costs. When you look at using sugar to make ethanol (from various sources), you can see processing costs are low, but feedstock prices are high. However, in Brazil, sugarcane feed costs are significantly lower than in other countries. Notice the data is from 2006.

1. Excludes capital costs
2. Feedstock costs for US corn wet and dry milling are net feedstock costs; feedstock for US sugarcane and sugar beets are gross feedstock costs
3. Excludes transportation costs
4. Average of published estimates

The following pages will describe the process of ethanol production from corn.

Another alcohol that can be generated from starch or cellulose is butanol, a four-carbon chain alcohol. There are usually two isomers: normal butanol (n-butanol) and iso-butanol. Their structures, along with ethanol, are shown below:

Structures of n-butanol, ethanol, and iso-butonal

Name	Atoms and Bonds	Stick Representation
n-Butanol (4 C atoms)
Ethanol (2 C atoms)
Isobutanol (4 C atoms)

There are some advantages of butanol when compared to ethanol:

1. It has a higher energy content than ethanol.
2. It is less hydrophilic than ethanol (less attracted to water).
3. It is more compatible with oil and its infrastructure.
4. It has a lower vapor pressure and higher flash point than ethanol (evaporates less easily).
5. It is less corrosive.
6. N-butanol works very well with diesel fuel.
7. Both n-butanol and iso-butanol have good fuel properties.

The table below shows a comparison of the energy content of various fuels in Btu/gal. The higher the value, the more miles per gallon one can achieve; the Btu/gal value of butanol is close to the value of gasoline, and is higher than ethanol.

Butanol production is also a fermentation process – we’ll go over the differences in a little bit. There is a history regarding butanol production. It was known as the ABE process, or acetone, butanol, and ethanol process. It was commercialized in 1918 using an enzyme named Clostridium acetobutylicum 824. Acetone was needed to produce Cordite, a smokeless powder used in propellants that contained nitroglycerin, gunpowder, and a petroleum product to hold it together – the acetone was used to gelatinize the material. In the 1930s, the butanol in the product was used to make butyl paints and lacquers. It has also been reported that Japanese fighter planes used butanol as fuel during WWII. The process of ABE fermentation was discontinued in the US during the early 1960s due to unfavorable economic conditions (made less expensively using petroleum). South Africa used the process into the 1980s, but then discontinued. There are reports that China had two commercial biobutanol plants in 2008, and currently, Brazil operates one biobutanol plant. There are three species of enzymes commonly used for butanol fermentation because they are some of the highest producers of butanol: Clostridium acetobutylicum 824, Clostridium beijerinckii P260, and Clostridium beijerinckii BA101. The figures below show micrographs of two of the fermentation enzymes used for butanol production.

As in the conversion of starch to ethanol, the plants must be processed in a similar way, so I won’t repeat the five steps we just covered – we just use different enzymes, and end processing may be different because of the different chemicals produced. Starch must be hydrolyzed in acid before using the enzyme. And, as with using cellulose and hemicellulose as the starting material, it must first be pretreated to separate out the cellulose, then treated again to eventually produce glucose in order to make butanol from fermentation. Remember the glucose-to-ethanol reaction? Starch will produce the following products: 3 parts acetone (3 CH₃-CO-CH₃), 6 parts butanol (6 CH₃-CH₂-CH₂-CH₂OH), and 1 part ethanol (1 CH₃-CH₂-OH).

So, what feed materials are used for butanol production? Similar to what is used for ethanol production, which includes: 1) grains, including wheat straw, barley straw, and corn stover, 2) by-products from paper and sugar production, including waste paper, cotton woods, wood chips, corn fiber, and sugarcane bagasse, and 3) energy crops including switchgrass, reed canarygrass, and alfalfa. The table below shows the costs of various biomass sources.

The price and the availability of feeds determine what might be used to produce various biofuels. The feeds most available in the US are corn stover (2.4 x 108 tons/year) and wheat straw (4.9 x 107 tons/year). Other biomass substrates include corn fiber, barley straw, and corn fiber at ~4-5 x 106 tons/year. Yields of butanol from corn and corn products by fermentation are shown in the table below.

The solventogenic Clostridium species can metabolize both hexose and pentose sugars, which are released by cellulose and hemicellulose in wood and agricultural wastes; this is an advantage over other cultures used to produce biofuels. If all the residues available were converted into acetone-butanol (AB), the result would produce 22.1 x 109 gallons of AB. In 2009, 10.6 x 109 gallons of ethanol was produced, but that was only equivalent to 7.42 x 109 gallons of butanol on an equal energy basis.

There are several issues that are a challenge to producing AB in a traditional batch process: 1) product (butanol) concentration is low 13-20 g/L, 2) incomplete sugar utilization (<60 g/L), and 3) the process streams are large. These issues are due to severe product inhibition. Other issues include: 1) butanol glucose yield low, 22-26%, 2) butanol concentration in fermentation is low, 1.5%, 3) butanol concentration of 1% inhibits microbial cell growth, 4) butanol fermentation is in two phases, and 5) feedstock cost is high.

One of the more important considerations of butanol production is limiting the microbial inhibitory compounds. These compounds include some compounds related to lignin degradation, including syringaldehyde, coumaric acid, ferulic acid, and hydroxymethylfurfural.

As an example of one particular process, wheat straw was processed using a separate hydrolysis, fermentation, and recovery process. The following conditions were used: 1) wheat straw milled to 1-2 mm size particles, 2) dilute sulfuric acid (1% v/v) pretreatment at 160 C for 20 min., 3) mixture cooled to 45 C and hydrolyzed with cellulase, xylanase, and β-glucosidase enzymes for 72 h, followed by centrifugation and removal of sediments, 4) fermentation with C. beijerinckii P260 (fermentation gases CO₂ and H₂ were released to the environment, but could be captured, separated and used in other processes, and 5) butanol removed by distillation. For this particular process, the production of ABE was relatively high, with butanol and acetone being the major products. The reaction was done in a batch reactor and no treatment was used to remove inhibitor chemicals. The table below shows the process with wheat straw, barley straw, corn stover, and switchgrass. Wheat straw did not need to be detoxified, but the others did. Detoxification can be done by adding lime (a weak base) or using a resin column to separate out the components.

So, what can be done to overcome butanol toxicity? What kind of downstream processing needs to be done to separate out the wanted components? The butanol level in the reactor has to be kept to a certain threshold in order to reduce toxicity to the culture and utilize all the sugar reactants.

First of all, these are the typical processing steps that must be utilized in some form for most refining units (the upstream processing includes pretreating the raw material, similar to what we discussed in Lesson 5): 1) sorting, 2) sieving, 3) communition (size reduction by milling), 4) hydrolysis, and 5) sterilization. The next main stage is the bioreaction stage: metabolite biosynthesis and biotransformations. The final aspect of processing is downstream processing, and the methods used depend on the products made. To separate solids, filtration, precipitation, and centrifugation take place. Flocculation can also be done. To separate liquids, several processes can be done: 1) diffusion, 2) evaporation, 3) distillation, and 4) solvent-liquid extraction.

For butanol processing, there have been several processes developed to reduce the level of toxicity. These include: 1) simultaneous saccharification, fermentation, and recovery (SSFR), 2) gas stripping (using N₂ and/or fermentation gases – CO₂ and H₂), 3) cell recycling, 4) pervaporation (combination process of permeation/evaporation using selective membranes), 5) vacuum fermentation, 6) liquid-liquid extraction, and 6) perstraction (combination of solvent extraction and membranes for permeation). The goal is to convert all the sugars to acetone and butanol but remove the products as they are produced to decrease toxicity. We’ll discuss more about liquid-liquid extraction (or solvent extraction) when we get to the lesson on biodiesel.

Summary

This lesson continued from the previous lesson, but went into greater depth with the processing aspects of ethanol production. Starch and cellulose must first be converted into glucose before fermentation into ethanol and CO₂. Starch feedstocks include sugarcane in Brazil, sugarbeets in Europe, and corn in the US. In order to process corn, there are five steps: grinding, cooking and liquefaction, saccharification, fermentation, and distillation. Enzymes are needed in saccharification, and yeast is needed in fermentation. Cellulose to glucose requires some additional steps and enzymes in order to break the structure down, but once it gets to the glucose stage, all the processing is the same. Because the water in the ethanol must be removed for use as a fuel, the last steps include distillation and a molecular sieve.

Butanol can be produced in a similar way, but acetone and ethanol also accompany butanol processing. Different enzymes are used. While the concentration of butanol is low when converting from feed materials, butanol has some advantages over using ethanol; it mixes better with gasoline and has a higher energy content.

Lesson Objectives

By the end of this lesson, you should be able to:

• explain similarities and differences between sugar-based and starch-based ethanol production, as well as butanol production;
• describe the differences between wet and dry milling of corn;
• explain process steps in dry milling ethanol and butanol production;
• identify important co-products from corn ethanol and butanol production;
• evaluate the largest factors that affect the economics of ethanol and butanol production.

References

Pryor, Scott; Li, Yebo; Liao, Wei; Hodge, David; “Sugar-based and Starch-based Ethanol,” BEEMS Module B5, USDA Higher Education Challenger Program, 2009-38411-19761, 2009.

Bothast, R.J., Schilcher, M.A., Biotechnological processes for conversion of corn into ethanol, Appl. Microbiol. Biotechnol., 67, 19-25, 2005.

Recall from Lesson 2 the general schematic of a refinery, shown below. Jet fuel is typically in the middle distillate range, also known as naphtha and kerosene. Diesel fuel is heavier (higher molecular weight and longer long-chain hydrocarbons). These fuels do not require as much processing because they can be obtained primarily from the distillation of oil, but because of sulfur/oxygen/nitrogen functional groups and high molecular weight waxes, these fuels must have these components removed. They are hydrotreated (hydrogen is added, sulfur/oxygen/nitrogen is removed, and aromatics are made into cycloalkanes). Waxes are also separated out.

The primary structure we want for jet fuel and diesel fuel is:

Alkane - atoms are lined up. For stick representation, each corner represents a CH2 group, and each end represents a CH3 group.

Name	Atoms and Bonds	Stick Representation
Heptane (7 C atoms)

Cycloalkanes - again, still an alkane, but form a ring compound.

Name	Atoms and Bonds	Stick Representation
Cyclohexane (6 C atoms)

The table below also shows a list of different chemicals and the properties of each. This table is mainly focused on those chemicals that would be in jet and diesel fuels.

There are differences for each of the thermal processes, as described in Lesson 4. Here we focus on direct liquefaction. Direct liquefaction (particularly hydrothermal processing) occurs in a non-oxidative atmosphere, where the biomass is fed into a unit as an aqueous slurry at lower temperatures, with bio-crude in the liquid form being the product. The primary focus of these particular processes is to produce a liquid product that is a hydrocarbon with an atomic H:C ratio of ~2, and a boiling range of 170-280 °C.

Many of the processes developed are based on coal-to-liquid processing. The main purposes of taking coal and biomass into a liquid are to produce liquids, to remove some of the less desirable components (i.e., sulfur, oxygen, nitrogen, minerals), and to make a higher energy density material that will flow.

One of the primary processes to convert coal into liquids directly is through a combination of thermal decomposition and hydrogenation under pressure. There are several single and two-stage processes that have been developed but have not been made commercial in the US. However, China opened a commercial direct liquefaction plant partially based on US designs in 2008. The first figure below shows the general schematic of the plant. The next figure shows the products they make. Design considerations include 1) temperatures of ~400-450°C, 2) hydrogenation catalysts, 3) hydrocarbon solvents that are similar to fuels, 4) naturally occurring aromatics in coal, 5) sulfur, nitrogen, minerals that must be removed in refining of the liquid. Biomass can be processed in a similar manner, but biomass has significantly more oxygen and fewer aromatic compounds and decomposes differently than coal. Other processes have been developed for biomass, that appear to do a better job of processing cellulose. One process is hydrothermal processing in pressurized water using an acid catalyst such as LaCl₃ at 250°C - we won't go into more detail here, but it is different from the direct liquefaction discussion in the next paragraph.

So, what are the differences with direct liquefaction of biomass? On the surface, it looks pretty much the same as the process of coal liquefaction. It is a thermochemical conversion process of organic material into liquid bio-crude and co-products. Depending on the process, it is usually conducted under moderate temperatures (300-400°C, lower than coal liquefaction) and pressures (10-20 MPa, similar or maybe a little higher with primarily hydrogen in coal to liquids) with added hydrogen or CO as a reducing agent. Unlike coal, biomass is “wet”, or at least wetter than coal, and can be processed as an aqueous slurry. When processed as an aqueous slurry, the process is referred to in the literature as hydrothermal processing and can be subcritical to supercritical for water. The figure below shows the conditions for supercritical water; water behaves more like an acid/base system under these conditions. Thus, it can also be a catalyst. There is also a high solubility of organic material in water under these conditions. This mainly occurs along the liquid/vapor line. The basic reaction mechanisms can be described as:

1. depolymerization of biomass;
2. decomposition of biomass monomers by cleavage, dehydration, decarboxylation, and deamination;
3. recombination of reactive fragments.

Different types of biomasses react differently depending on the biomass source. Carbohydrates, such as cellulose, hemicellulose, and starch can decompose in hydrothermal water. The typical product formed under these conditions is glucose, and glucose can then be fermented to make alcohol or further degrade in water to make glycolaldehyde, glyceraldehyde, and dihydroxyacetone. The products made depend on the conditions: at temperatures, ~180°C, products are sugar monomers, but at higher temperatures, 360-420°C, the aldehyde, and acetone compounds are formed.

Lignin and fatty acids also decompose in hydrothermal water, but the products are very different because the substrate is different. For lignin, the products are similar to the building blocks for lignin, (p-coumaryl, coniferyl, and sinapyl alcohols), although the functional groups vary depending on the hydrothermal conditions. Bembenic and Clifford used hydrothermal water at 365°C and ~13 MPa to form methoxy phenols, using different gases to change the product slate (hydrogen, carbon monoxide, carbon dioxide, and nitrogen). For lipid or triglyceride (fats and oils) reaction in hydrothermal water at 330-340°C and 13.1 MPa, the main products are the free fatty acids (HC – COOH) and glycerol (C₃H₈O₃). The free fatty acids can then be reacted to straight-chain hydrocarbons that can be used for diesel or jet fuel, although the temperature usually needs to be a little higher (400°C) for this to take place. The figure below shows the schematic of a hydrothermal water process to convert algae into liquid fuels, making use of heat from an integrated heat and power system. Flue gas from a power generation facility is used to grow algae. Algae is then harvested and concentrated in water. The algae are then reacted in a hydrothermal unit followed by catalytic hydrogenation to make the straight-chain hydrocarbon liquid fuels.

Many types of catalysts can be used, although it depends on the process stage in which catalysts are used and what feed material is used. In hydrothermal processing, the more common catalysts used are acid and base catalysts. Particle size for biomass needs to be fine, with a size of < 0.5 mm. The introduction of the feed into the reactor is also challenging, as it is fed into a high-pressure reactor. Some advantages of using this process for biomass: 1) it is possible to process feeds with high water content, as much as 90%, 2) it is possible to process many different types of waste materials, including MSW, food processing waste, and animal manure, and 3) the process serves the dual roles of waste treatment and renewable energy production.

Process parameters include solids content, temperature, pressure, residence time, and use of catalysts. Often simultaneous reactions are taking place, which makes the overall understanding of the reactions complicated. The types of reactions taking place include solubilization, depolymerization, decarboxylation, hydrogenation, condensation, and hydrogenolysis.

For one particular process, hydrothermal liquefaction requires the use of catalysts. One typical catalyst used is sodium carbonate combined with water and CO to produce sodium formate:

$${\text{Na} }_2{\text{CO} }_3+{\text{H}}_2\text{O + CO}\to {\text{2HCO} }_2{\text{Na + CO} }_2$$

This dehydrates the hydroxyl groups to carbonyl compounds, then reduces the carbonyl group to an alcohol:

$$\begin{array}{c}{\text{HCO}}_2{\text{Na + C}}_6{\text{H}}_{10 }{\text{O}}_5\to {\text{C}}_6{\text{H}}_{10 }{\text{H}}_4+{\text{NaHCO}}_3\\ {\text{H}}_2+{\text{C}}_6{\text{H}}_{10 }{\text{O}}_5\to {\text{C}}_6{\text{H}}_{10 }{\text{H}}_4+{\text{H}}_2\text{O} \end{array}$$

The formate and hydrogen can be regenerated and recycled. Other catalysts used that behave in a similar manner include K₂CO₃, KOH, NaOH, and other bases. For simultaneous decomposition and hydrogenation, nickel (Ni) catalysts are used.

Similar to pyrolysis, the major product of this process is a liquid biocrude, which is a viscous dark tar or asphalt material. Up to 70% of the carbon is converted into biocrude; lighter products are obtained when different catalysts are used. Co-products include gases (CO₂, CH₄, and light hydrocarbons) as well as water-soluble materials. The liquid biofuel has a similar carbon-to-hydrogen ratio as in the original feedstock and is a complex mixture of aromatics, aromatic oligomers, and other hydrocarbons. In this process, the oxygen is reduced and is 10-20% less than typical pyrolysis oils, with a heating value higher than pyrolysis oils, 35-40 MJ/kg on a dry basis. However, the USDA has developed a pyrolysis process using recycled gases that produces a fairly light hydrocarbon with very little oxygen content. (Mullens et al.) I will discuss this more in the next section. The table below shows a comparison of biocrudes from various processes and feed materials. The quality of the biocrude shown from hydrothermal processing is for a heavy biocrude. Other processes will make a lighter material, but also produce more co-products that must be utilized as well.

BEEMS Module, He, Hu, and Li. Mullens et al., USDA.

Many researchers and scientists think that ground transportation will become increasingly dependent upon batteries, as in hybrids and electric vehicles. Reduction in fuel usage has been realized in the last 10 years, due to hybrid automobiles coming into the auto-market. However, this is not a viable option for air travel, which will remain dependent on liquid fuel. Since more fuel will be available for aircraft if less is used for vehicles, petroleum refineries should be able to keep up with demand. However, if there is a concern about emissions, especially the need to reduce CO₂, liquid jet fuels from biomass will by far be the best option. Jet fuel must also go through a qualification process and become certified for use, depending on the source of the fuel and the type of jet engine. As discussed briefly in Lesson 2, jet fuel should have certain properties. The table below shows some of the ASTM qualifications for jet fuel that currently exist.

The Federal Aviation Administration has been working diligently to get some alternative fuels available in the market. The government has aspirational goals for American airlines to utilize alternative jet fuel, with the hope of 1 billion gallons of alternative jet fuel per year by the year 2018. Airlines will need to meet this requirement either by purchasing alternative jet fuel or finding viable methods to produce alternative jet fuel. The expectations for jet fuel are that it be primarily composed of long-chain alkanes (although shorter carbon chain lengths than diesel fuel) with some cycloalkane and/or aromatic content for the necessary O-ring lubrication and other reasons.

There are several known biomass materials that could be utilized in the production of jet fuel. These include fats/oils, cellulose, woody biomass, and coal.

One of the primary sources is vegetable oil (this also includes algae oil and fats from the production of meats). Vegetable oils contain long-chain hydrocarbons connected by three carbons as esters. The fatty acid portion of the oil is easily converted into fatty acid methyl esters (FAMEs) through transesterification to produce biodiesel. We will discuss biodiesel production for transesterification extensively in another lesson, but I will briefly discuss here why the FAA is interested in making jet fuel from fats and oils. Unfortunately, currently, FAMEs are an issue for bio jet fuel and requirements include a limit of 5 ppm, as the FAMEs can cause corrosion, have a high freeze point, and are not compatible with materials in a jet engine. (Fremont, 2010) At present, biodiesel production is not always economical due to the high cost of oils and the method of production, and the ester must be removed for jet fuel.

Therefore, other methods are being evaluated to produce not only biodiesel but also biojet fuel. These process methods include Hydroprocessed Esters and Fatty Acids (HEFA), Catalytic Hydrothermolysis, and Green Diesel. (Hileman and Stratton, 2014) The figure below shows a schematic of the process for HEFA. Jet fuel made from HEFA has been approved for use in airplanes because it has gone through the approval process. The following white paper, Alternative Fuels Specification and Testing, (Kramer, S., 2013, March 1, Retrieved December 16, 2014) includes a schematic of the approval process on page 4; as you can see, it is a thorough and complicated process, and it takes quite some time to get a particular type of fuel qualified.

There are others who want to explore the use of the fluid catalytic cracking (FCC) unit in a refinery to convert vegetable oils into jet fuel. Al-Sabawi et al. (CanmetENERGY) provided a review of various biomass products that have been processed at a lab scale in the FCC unit. They show that the main effect would be on the catalyst used and the lifetime of the catalyst. (Al-Sabawi, 2012)

Mullens and Boateng of the USDA have developed a process to produce pyrolysis oils of low oxygen content (data on properties of fuel in the previous section). (Mullens, 2013) The review paper by Al-Sabawi et al. also discusses the potential processing of pyrolysis oils in the FCC unit. The main requirement is the pyrolysis oils need to be low in oxygen, but additional information on the composition of the oil could tell us whether the FCC unit or another unit in a refinery would be best for processing.

Cellulosic sources for producing alternative jet fuel can be used. By use of gasification of biomass and Fischer-Tropsch processing, a good biojet fuel can be produced, although some additives need to be included to prevent some potential problems. There are also processes to produce medium chain-length alcohols from cellulose, as methanol and ethanol do not have the energy density necessary to allow planes to fly long distances. One of the fuels that made it through the approval process is Synthetic Paraffinic Kerosene (SPK) made by Fischer-Tropsch synthesis. (Hileman and Stratton, 2014) It may also be possible to use by-products from the production of ethanol from corn (corn stover), sugar cane (bagasse), and paper production (tall oil). Westfall et al. (2008) and Liu et al. (2013) have also outlined other potential sources to produce fuels, with most processes including a catalytic deoxygenation aspect. (2008) The next section of this lesson will discuss Fischer-Tropsch and other chemical processes to make liquid fuels; it is an indirect method, as either natural gas or carbon materials that have been gasified must be used in these processes.

Additionally, there are some processes being developed for the production of alternative jet fuel from biomass-natural gas and biomass-coal. Researchers are working to develop processes at the demonstration scale for eventual commercialization. Virent, along with Battelle in Ohio, have produced ReadiJet Fuel using a pilot scale facility. (Conkle et al., 2012a, 2012b) Their paper includes a diagram of a schematic of their process (p. 3), a catalytic process to deoxygenate oils similar to the HEFA process. Liu et al. point out that jet fuel from natural gas has some advantages, especially the transportation aspect of fuels (2013). Jet fuel made from natural gas is made via steam reforming to CO and H₂, then use of the Fischer-Tropsch method to make long-chain alkanes (see additional explanation toward the end of the lesson). The fuel is very clean (no sulfur and no aromatics) and can be a drop-in replacement for petroleum-derived jet fuel - jet fuel made in this way has been thoroughly tested, and the fuel has been qualified for use in military and commercial jet airliners so long as the alternative fuel composes less than 50% of the fuel mixture. Penn State and the Air Force have been involved with the production of a coal-based jet fuel (Balster et al., 2008) that could possibly be co-processed with some type of bio-oil, such as vegetable oil or low-oxygen pyrolysis oil. The potential for using coal-based jet fuel lies in its high-energy density, superior thermal properties, and few issues with lubricity. The following table shows how the fuel produced by Battelle/Air Force and PSU/Air Force meets some of the ASTM requirements; Battelle’s fuel has been certified, but PSU’s fuel has not completely met certification criteria. Recently, Penn State received DOE funding to expand the solvent extraction unit to a continuous reactor and will use solvent from Battelle’s process to extract the coal – the goal is to incorporate coal into a biomass process in a more environmentally sound way than using other solvents. The figure below shows a schematic of the PSU unit. Elliot et al. (2013) at Pacific Northwest National Laboratory have developed a specific hydrothermal process to convert algal water slurries into organic hydrocarbons at subcritical water conditions (350 °C and 20 MPa pressure). The process also includes catalytic processes to remove oxygen, sulfur, and nitrogen, and the liquids generated are most likely of fuel quality.

*No additives were included in these fuels for these tests. Balster et al., Conkel et al.

In Lesson 4, we discussed gasification in depth. We also briefly discussed using syngas to make liquids. In Lesson 8, we will go into a little more depth. These types of processes are called indirect liquefaction.

The primary objective of gasification is to produce a syngas primarily composed of carbon monoxide (CO) and hydrogen (H₂). After gasification, the product needs to be cleaned to remove any liquids, and there are several reactions that can be done to change the H₂/CO ratio or to make different products. This is where we will start this lesson.

Below are the three different process phases that the gas must go through. The first phase is the gasifier and separation of gas, liquid, and solid products. The biomass is not pure carbon, and all the streams will contain a variety of other compounds that may not be wanted or may be harmful.

Solids are removed by a cyclone or an electrostatic precipitator. The particles are similar to what is seen in combustion – ungasified or partially gasified particles. Some mineral matter/ash can also be in the solids. A separator is used to remove the liquids, mainly tars and water, that must be separated and processed for use. The water fraction can be used to react the organic compounds further, and the tars can be distilled and reacted further, similar to the direct liquefaction processes we described in previous sections. In any case, the water must be treated before disposal.

The gases can also contain unwanted gases. Three gases that need to be removed from the gas phase are ammonia (NH₃), carbon dioxide (CO₂), and hydrogen sulfide (H₂S). They are corrosive and/or toxic and need to be removed. The acid gases (H₂S and CO₂) are called acid gases because they can dissolve in water and produce weak acids that can be corrosive to metals. There are a range of processes that can separate out the acid gases. One typical method to remove these harmful gases is called the Rectisol process. Both H₂S and CO₂ are soluble in methanol, while H₂ and CO are not. In the simplified schematic, you can see there are two parts to the process, an absorber, and a regenerator. The raw gas goes into the absorber and comes into contact with the lean solution of methanol. The purified gas goes out the top, and the solution rich in unwanted absorbed gases goes to the regenerator – the acid gases are then separated out from the methanol so that a lean methanol solution comes out the bottom to be recycled for use in the absorber. The H₂S in the acid gas can be burned or reacted with SO₂ to form solid sulfur, which is used for making chemicals. The CO₂ goes out of the stack, but could also be captured if processes to capture it are put into place.

The H₂/CO ratio may not be ideal for downstream synthesis reactions. This figure shows a process to use the water-gas shift reaction to change the ratio of H₂/CO.

Ideally, the gas stream coming off the gasifier followed by a Rectisol unit could be reasonably pure H₂ and CO. The water-gas shift can change the ratio of H₂/CO. The reaction is shown below:

$${\text{CO + H} }_2\text{O}\leftarrow \to {\text{CO} }_2{\text{ + H} }_2$$

This would be the way to make less CO and more H₂, but the reaction can go in reverse to make more CO and less H₂ as well. From coal gasification, we want to shift it to the right as written. Advantages include:

• that it's an equilibrium process;
• can shift reaction in other direction by taking advantage of LeChatlier’s Principle;
• with the same number of moles on both sides, the equilibrium position is independent of pressure – no requirement of compression or release of pressure of shift reactor;
• can be adapted to any operating pressure.

The major disadvantage of the water-gas shift reaction is it’s a CO₂ factory! There are only a few ways of separating CO₂, such as a monoethylamine (MEA) scrubber. You can separate the CO₂ to a 99% concentration, which would be ideal for CCS. Another thing to consider is we do not need to separate the entire gas stream and shift it; we only need to do enough to get the H₂/CO ratio where we want it to be. Once we get to this point, we can be ready to do some synthesis of liquids.

So, what can be done with synthesis gas? It can be burned and used in a gas turbine to heat exchange the heat to produce steam and operate a second turbine for electricity. The gas can be fed to a solid oxide fuel cell to generate electricity. We can also use synthesis gas to generate fuels, chemicals, and materials. In fact, the dominant application of synthesis gas from coal is the production of synthetic hydrocarbons for transportation fuels – Fischer Tropsch (FT) synthesis. This is what is primarily done in South Africa by the company Sasol and was also one of the methods used by the Germans in WWII to generate liquid fuels; in fact, direct liquefaction was the primary method used to produce liquid fuels in Germany in the 1940s. However, it is not the only gasification to liquids process. As noted in Lesson 4, the FT synthesis reaction can be presented by:

$${\text{CO + nH} }_2\to {\left( {\text{-CH} }_{2- }\right) }_{\text{x}}+{\text{H}}_2\text{O}$$

We are taking carbon atoms and building them up as alkanes, containing up to at least 20 carbon atoms. It is really a polymerization process, and it follows polymerization statistics. The figure below shows a typical polymerization statistical function. You will not obtain one single pure alkane from the FT process, and there will be a distribution of products. As with all chemical reactions, you will have reaction variables to adjust, such as temperature, pressure, residence, and the addition of a catalyst. By skillful selection of variables (T, P, t, and catalyst), we can, in principle, make anything from methane to high molecular weight waxes. The intent is to maximize liquid transportation fuel production.

The primary process for FT is the Synthol Process; the schematic is shown in the figure below. The synthesis gas goes into the reactor at 2.2 MPa of pressure and 315-330°C. The product leaves the reactor where the catalyst is recovered, oils are removed by a hydrocarbon scrubber, and the tail gas is recovered. The gas part is recycled, and the rest of the material is then distilled into gasoline, jet fuel, and diesel fractions. The Synthol reactor is a fluid bed reactor that uses an iron-based catalyst.

The liquids produced make very clean fuels. The product is near zero sulfur and low in aromatic compounds, and it is composed of mainly straight-chain alkanes. When considering the carbon-steam reaction, it is an endothermic reaction (the gasification, needs to add heat). In this case, the reaction is “backward," or going the other direction. Therefore, the FT synthesis reaction is an exothermic reaction. Because the reaction is exothermic, heat is generated, so Synthol reactors have internal cooling tubes with steam that when heated generate high-pressure steam that can be used in other processes.

FT diesel fuel is high-quality diesel fuel – we want to have linear alkanes, low aromatic content, and low sulfur. FT diesel fuel has all three aspects of diesel fuel that we want, and has a cetane number ≥ 70 – it is an ideal diesel fuel (recall that good diesel fuel has a cetane number of 55).

Jet fuel made from FT synthesis makes a decent fuel. It is low in aromatic and sulfur content. It is the first bio-based jet fuel that has been certified for use in aircraft and has been tested in blends with major airlines (Virgin). However, for use in military jets, it must be blended because newer designs use the fuel as a coolant for electronics, as the fuel can have issues in these aircraft. For example, alkanes have the lowest density of various compound classes in jet fuel, so FT jet fuel has borderline volumetric density. Alkanes are also likely to undergo pyrolysis reactions at certain high temperatures, and if the fuel is used as a heat-exchange fluid to reduce the heat load, carbon formation can occur – this is mainly a problem for some of the newer military jet aircraft.

FT gasoline that comes straight off of the reaction is not great gasoline, as it has a low octane number. Recall that branched alkanes and aromatic compounds have higher octane numbers. Since FT compounds tend to be straight-chain alkanes, isomerization is required, and an appropriate catalyst must be used for catalytic reforming.

The primary location for gasification and FT synthesis is in South Africa – the gasoline being sold in South Africa has an octane number of 93. An integrated plant will also produce aromatics, waxes, liquid petroleum gas, alcohols, ketones, and phenols in addition to liquid hydrocarbon fuels. The reasoning behind marketing multiple products is that all the products will go up and down in price; when something goes up in price, you make more of it, and when something goes down in price, you may make less. This is a way for plants to maximize their profits.

Methanol Production

Synthesis gas can also be used to produce methanol, and CH₃OH. The current technology for making methanol is fairly mature. Typically, natural gas is used as the feedstock, which is steam-reformed to make CO and hydrogen:

$${\text{CH} }_4{\text{ + H} }_2\text{O}\to {\text{CO + H} }_2$$

Then methanol is synthesized by the reaction:

$$\text{CO + }2{\text{H}}_2\to {\text{CH} }_3\text{OH}$$

However, another methanol synthesis reaction allows for CO₂ to be in the feed gas:

$${\text{CO} }_2{\text{ + 3H} }_2\to {\text{CH} }_3{\text{OH + H} }_2\text{O}$$

However, because water and methanol are infinitely soluble, an additional step is required downstream to isolate methanol from water. Typical operating conditions in the methanol synthesis reactor are 5-10 MPa pressure, 250-270°C, using a copper/zinc catalyst. The reaction is extremely exothermic, so heat must be removed to keep the reaction under control. Similar to the FT reaction, the reactor has a shell and tube heat exchanger where the coolant is circulated through the shell, and catalyst particles are packed into the tubes where the reactant/product liquids flow. The figure below shows a schematic of the methanol synthesis process.

So, what can methanol be used for? It is periodically used as a replacement for gasoline, particularly for racing fuel, as it has a high-octane number. It has no sulfur in it, will produce almost no NO_X due to the low flame temperature, and can be blended with gasoline.

There are also some disadvantages to using methanol as a fuel. It is infinitely miscible with water, it has health and safety issues, provides only half the volumetric energy density of gasoline, and may have compatibility issues with materials in some vehicles.

Enormous tonnages of methanol are produced and handled annually with excellent safety – but within the chemical process industry. However, if the general public is handling methanol, safety may be an issue because methanol has toxic properties. Methanol is being seriously considered as the fuel of choice in the use of fuel cells. However, there is a process to make methanol directly into gasoline, so concerns about methanol aren’t an issue then.

Methanol-To-Gasoline (MTG)

Methanol can be used to make a gasoline product. The process uses a special zeolite catalyst with a pore size such that molecules up to C10 can get out of the catalyst. Larger molecules cannot be made with this process; therefore, a product is made with no carbon molecules greater than C10, which boils in the gasoline range. In this process, aromatics and branched-chain alkanes are made, which means the MTG process produces very high-octane gasoline. Gasoline is the only product. In the reaction, methanol is converted into dimethyl ether (which can be a good diesel fuel) by the following reaction:

$$2{\text{CH} }_3\text{OH}\to {\text{CH} }_3{\text{OCH} }_3{\text{ + H} }_2\text{O}$$

As the reaction progresses, the dimethyl ether is dehydrated further to the product hydrocarbons. The overall reaction is:

$${\text{CH} }_3\text{OH}\to -{\left( {\text{CH} }_2\right) }_{-n }+{\text{H}}_2\text{O}$$

As with the other reactions we’ve looked at in this section of the lesson, the reaction is highly exothermic, so the reactor and process have to be designed to remove heat from the reaction to keep it under control. The conditions for this reaction are 330-400°C and 2.3 MPa. If one wanted to envision how a plant could incorporate all of these processes together, the following would be one scenario:

1. Add an MTG unit to existing natural gas-fed methanol plants (produce high-octane gasoline).
2. Replace the natural gas units with coal and/or biomass gasification and gas conditioning.
3. Add parallel trains of Synthol reactors (produce high cetane diesel).
4. Add a third section, using a solid oxide fuel cell to generate electricity using synthesis gas as the feed material.

The plant then produces gasoline, diesel, and electricity.

Summary

Lesson 7 covered thermochemical methods of converting biomass into fuels. These are the main methods being considered at this point; however, research continues on additional methods and what is included may not be a complete list. The main advantage of fermentation is that it is a natural process that does not require additional chemicals, but the main disadvantage of fermentation is the processes tend to be slow. All of the thermochemical processes require heat and some other process parameters that may make them more expensive – another lesson will discuss the economics behind all of the processes we’ve discussed for comparison.

We discussed both direct and indirect methods for making fuel from biomass. These methods are not just directed towards making ethanol. Many of these processes are used to make hydrocarbon fuels that limit the amount of oxygen in the product, as too much oxygen typically results in either causing the fuel to form unwanted “gums" or a corrosive environment that will cause problems in the units and storage containers. For jet fuels, oxygenated compounds will keep the fuel from being certified for use, so most of the methods presented make deoxygenated jet fuel. I would suggest continuing to monitor the news to see the progression in certification for additional bio-based fuels.

Lesson Objectives

By the end of this lesson, you should be able to:

• explain the chemical characteristics of heavier fuels such as jet fuel and diesel fuel;
• explain thermochemical processes used to produce higher molecular weight compounds, from direct and indirect methods;
• evaluate how thermochemical processes are different from enzymatic processes.

Fat is a generic term for lipids, a class of compounds in biochemistry. You would know them as greasy, solid materials found in animal tissues and in some plants – oils that are solids at room temperature.

Vegetable oil is the fat extracted from plant sources. We may be able to extract oil from other parts of a plant, but seeds are the main source of vegetable oil. Typically, vegetable oils are used in cooking and for industrial uses. Compared to water, oils and fats have a much higher boiling point. However, there are some plant oils that are not good for human consumption, as the oils from these types of seeds would require additional processing to remove unpleasant flavors or even toxic chemicals. These include rapeseed and cottonseed oil.

Animal fats come from different animals. Tallow is beef fat and lard is pork fat. There is also chicken fat, blubber (from whales), cod liver oil, and ghee (which is a butterfat). Animal fats tend to have more free fatty acids than vegetable oils do.

Chemically, fats and oils are also called “triglycerides.” They are esters of glycerol, with a varying blend of fatty acids. The figure below shows a generic diagram of the structure without using chemical formulas.

A generic diagram of oils and fats.

Credit: BEEMS Module B4

So what is glycerol? It is also known as glycerin/glycerine. Other names for glycerol include 1,2,3-propane-triol, 1,2,3-tri-hydroxy-propane, glyceritol, and glycyl alcohol. It is a colorless, odorless, hygroscopic (i.e., will attract water), and sweet-tasting viscous liquid. The following figure shows the chemical structure in two different forms.

Chemical structure of glycerol.

Credit: BEEMS Module B4

So now we need to define what the fatty acids are. Essentially, fatty acids are long-chain hydrocarbons with a carboxylic acid. The following figure shows the generic chemical structure of a fatty acid with the carboxylic acid on it.

Generic carboxylic acid chemical structure.

Click for a text description of the image above.

This image provides a visual and textual explanation of the carboxylic acid functional group, commonly found in organic molecules such as fatty acids. It features two equivalent representations of the same chemical structure:

1. R–C=O with an –OH group attached to the carbon, which is the expanded structural form.
2. RCOOH, the condensed molecular formula.

In both cases, "R" denotes a long hydrocarbon chain, which can vary in structure. The image notes that this chain may be saturated, meaning it contains only single bonds between carbon atoms, or unsaturated, meaning it includes one or more double bonds.

The COOH group is identified as the acidic functional group—a defining feature of carboxylic acids. The image emphasizes that this group can be written in either the expanded or condensed form, both of which are chemically equivalent.

Credit: BEEMS Module B4

The figure below shows different fatty acid chemical structures. The chemical structures are shown as line chemical structures, where each point on the links is a carbon atom and the correct number of hydrogen atoms is dependent on whether there is a single or double bond. Fatty acids can be saturated (with hydrogen bonds) or unsaturated (with some double bonds between carbon atoms). Because of the metabolism of oilseed crops, naturally formed fatty acids contain even numbers of carbon atoms. In organic chemistry, carbon atoms have four pairs of electrons available to share with another carbon, hydrogen, or oxygen atom. Free fatty acids are not bound to glycerol or other molecules. They can be formed from the breakdown or hydrolysis of a triglyceride.

Other long-chain acids, such as steric, palmitic, oleic, and linoleic acids.

Credit: BEEMS Module B4

The fatty acids shown have slightly different properties. Palmitic acid is found in palm oil. The figure below shows the relationship of each fatty acid to its size and saturation. Palmitic and steric acids are saturated fatty acids, while oleic and linoleic acids are unsaturated with different amounts of double bonds. The figure below shows differing amounts of carbon atoms compared to the number of double bonds in the compound.

A series of fatty acids. The ratio represents the carbon atoms: double bonds in the compound.

Credit: BEEMS Module B4

The figure below shows the part of the triglyceride that is a fatty acid and the part that is glycerol, including chemical structures this time. The chemical structure shown here is a saturated triglyceride.

Chemical structure of triglyceride, pointing out the fatty acid parts and the glycerol part.

Credit: BEEMS Module B4

So, we’ve discussed what fats and oils are. Now, what is biodiesel? What is at least one definition? It is a diesel fuel that was generated from biomass. However, there are different types of biodiesel. The most commonly known type of biodiesel is a fuel comprised of mono-alkyl esters (typically methyl or ethyl esters) of long-chain fatty acids derived from vegetable oils or animal fats – this is according to ASTM D6551. An ASTM is a document that contains the standards for particular types of chemicals, particularly industrial materials. This is a wordy definition that doesn’t really show us what it is chemically.

So when we talk about an alkyl group, it is a univalent radical containing only carbon and hydrogen atoms in a hydrocarbon chain, with a general atomic formula of C_nH_2n+1. Examples include:

Another term we need to know about is an ester. Esters are organic compounds where an alkyl group replaces a hydrogen atom in a carboxylic acid. For example, if the acid is acetic acid and the alkyl group is the methyl group, the resulting ester is called methyl acetate. The reaction of acetic acid with methanol will form methyl acetate and water; the reaction is shown below. An ester formed in this method is a condensation reaction; it is also known as esterification. These esters are also called carboxylate esters.

Reaction of acetic acid with methanol to form methyl acetate and water.

Click for a text description of the image above.

This image illustrates a chemical esterification reaction, both in terms of structural formulas and molecular formulas, showing the formation of an ester from a carboxylic acid and an alcohol.

At the top, the structural formulas depict the reaction between acetic acid (CH₃COOH) and methanol (CH₃OH). The acetic acid molecule features a carboxyl group (–COOH), while methanol contains a hydroxyl group (–OH). These two reactants undergo a condensation reaction, where a molecule of water (H₂O) is eliminated, and a new bond forms between the acid and alcohol.

The product of this reaction is methyl acetate (CH₃COOCH₃), an ester, along with water. The reaction is shown as reversible, indicated by the double arrows (⇌), which is characteristic of esterification reactions under equilibrium conditions.

Below the structural representation, the molecular formulas summarize the same reaction:

${\mathrm{CH} }_{3 }\mathrm{COOH} + {\mathrm{CH} }_{3 }\mathrm{OH} \rightleftharpoons {\mathrm{CH} }_{3 }{\mathrm{COOCH} }_{3 }+ {\mathrm{H} }_{2 }\mathrm{O}$

Each compound is labeled:

• Acetic acid
• Methanol
• Methyl acetate
• Water

Credit: BEEMS Module B4

This is the basic reaction that helps to form biodiesel. The following figure shows the different parts of the chemical structure of the biodiesel, the methyl ester fatty acid, or fatty acid methyl ester (FAME).

The chemical structure of the typical biodiesel, methyl ester fatty acid, or FAME.

Credit: BEEMS Module B4

So, at this point, let’s make sure we know what we have been discussing. Biodiesel is a methyl (or ethyl) ester of a fatty acid. It is made from vegetable oil, but it is not vegetable oil. If we have 100% biodiesel, it is known as B100 – it is a vegetable oil that has been transesterified to make biodiesel. It must meet ASTM biodiesel standards to qualify for warranties sold as biodiesel and qualify for any tax credits. Most often, it is blended with petroleum-based diesel. If it is B2, it has 2% biodiesel and 98% petroleum-based diesel. Other blends include B5 (5% biodiesel), B20 (20% biodiesel), and B100 (100% biodiesel). We’ll discuss why blends are used in the following section. And to be clear: sometimes vegetable oil is used in diesel engines, but it can cause performance problems and deteriorate engines over time. Sometimes, vegetable oil and alcohol are mixed together in emulsions, but that it is still not biodiesel, as it has different properties from biodiesel.

So, if straight vegetable oil (SVO) will run in a diesel engine, why not use it? Vegetable oil is significantly more viscous (gooey is a non-technical term) and has poorer combustion properties. It can cause carbon deposits, poor lubrication within the engine, and engine wear, and it has cold-starting problems. Vegetable oils have natural gums that can cause plugging in filters and fuel injectors. For a diesel engine, the injection timing is thrown off and can cause engine knocking. There are ways to mitigate these issues, which include: 1) blending with petroleum-based diesel (usually < 20%), 2) preheating the oil, 3) making microemulsions with alcohols, 4) “cracking” the vegetable oil, and 5) using the method of converting SVO into biodiesel using transesterification. Other methods are used as well, but for now, we’ll focus on biodiesel from transesterification. The table below shows three properties of No. 2 diesel, biodiesel, and vegetable oil. As you can see, the main change is in the viscosity. No. 2 Diesel and biodiesel have similar viscosities, but vegetable oils have much higher viscosity and can cause major problems in cold weather. This is the main reason for converting the SVO into biodiesel.

So, how do we make biodiesel?

The method being described here is for making FAMEs biodiesel. The reaction is called transesterification, and the process takes place in four steps. The first step is to mix the alcohol for reaction with the catalyst, typically a strong base such as NaOH or KOH. The alcohol/catalyst is then reacted with the fatty acid so that the transesterification reaction takes place. The first figure below shows the preparation of the catalyst with the alcohol, and the second figure shows the transesterification reaction.

The catalyst is prepared by mixing methanol and a strong base such as sodium hydroxide or potassium hydroxide. During the preparation, the NaOH breaks into ions of Na+ and OH-. The OH- abstracts the hydrogen from methanol to form water and leaves the CH₃O- available for reaction. Methanol should be as dry as possible. When the OH- ion reacts with H+ ion, it reacts to form water. Water will increase the possibility of a side reaction with free fatty acids (fatty acids that are not triglycerides) to form soap, an unwanted reaction. Enzymatic processes can also be used (called lipases); alcohol is still needed and only replaces the catalyst. Lipases are slower than chemical catalysts, are high in cost, and produce low yields.

Once the catalyst is prepared, the triglyceride will react with 3 mols of methanol, so excess methanol has to be used in the reaction to ensure a complete reaction. The three attached carbons with hydrogen react with OH- ions and form glycerin, while the CH₃ group reacts with the free fatty acid to form the fatty acid methyl ester.

The figure below is a graphic of the necessary amounts of chemicals needed to make the reaction happen and the overall yield of biodiesel and glycerin. The amount of methanol added is almost double the required amount so the reaction goes to completion. With 100 lbs of fat and 16-20 lbs of alcohol (and 1 lb of catalyst), the reaction will produce 100 lbs of biodiesel and 10 lbs of glycerin. The reaction typically takes place at between 40-65°C. As the reaction temperature goes higher, the rate of reaction will increase, typically 1-2 hours at 60 °C versus 2-4 hours at 40°C. If the reaction is higher than 65°C, a pressure vessel is required because methanol will boil at 65°C. It also helps to increase the methanol-to-oil ratio. Doubling the ratio of 3 mols of alcohol to 6 mols will push the reaction to completion faster and more completely.

The following video shows a time-lapsed reaction of transesterification of vegetable oil into biodiesel. It also incorporates the steps after the reaction to separate out the biodiesel (9:44).

The figure below shows a schematic of the process for making biodiesel. Glycerol is formed and has to be separated from the biodiesel. Both glycerol and biodiesel need to have alcohol removed and recycled in the process. Water is added to both the biodiesel and glycerol to remove unwanted side products, particularly glycerol, that may remain in the biodiesel. The wash water is separated out similar to solvent extraction (it contains some glycerol), and the trace water is evaporated out of the biodiesel. Acid is added to the glycerol in order to provide neutralized glycerol.

As briefly discussed, the initial reactants used in the process should be as dry as possible. Water can react with the triglyceride to make free fatty acids and a diglyceride. It can also dissociate the sodium or potassium from the hydroxide, and the ions Na+ and K+ can react with the free fatty acid to form soap. The figure below shows how water can help to form a free fatty acid, and that free fatty acid can react with the Na+ ion to form soap. The sodium that was being used for a catalyst is now bound with the fatty acid and unusable. It also complicates separation and recovery. All oils may naturally contain free fatty acids. The refined vegetable oil contains less than 1%, while crude vegetable oil has 3%, waste oil has 5%, and animal fat has 20%. Animal fats are a less desirable feedstock.

Some of the processes used in making biodiesel are different from what we’ve discussed. The first of these processes we’ll discuss is solvent extraction.

In the process of making biodiesel through transesterification, we noted that biodiesel and glycerol are the products, with some water formation and unwanted potential soap formation. So, the products are liquid, but they are also immiscible (do not dissolve in each other) and have differences in specific gravity. The specific gravity of the products is shown in the table below.

In batch processing, gravity separation is used, and the products remain in the reactor; the reactor then becomes a settler or decanter. Once the reaction is finished, the product mixture then sits without agitation. After 4-8 hours, the glycerol layer settles at the bottom (because it has higher gravity) and the biodiesel settles at the top. However, if a continuous flow facility is utilized, the products separate too slowly in a settler, so a centrifuge is used. A centrifuge will spin the liquids at a very high speed, which helps to promote density separation. The figure below shows a few different types of industrial centrifuges that can be used for biodiesel separation.

One of the issues that can happen during separation is the forming of a layer containing water and soap, in between the glycerol and biodiesel. That will hinder the separation. Another issue is that glycerol contains 90% of the catalyst and 70% of the excess methanol. In other words, the glycerol fraction is kind of the “trashcan” layer of the process. The biodiesel layer also contains some contaminants, including soap, residual methanol, free glycerol, and residual catalyst. The catalyst in biodiesel is extremely problematic if introduced into fuel systems. One way to improve the separation is through water washing with hot water, as the contaminants are soluble in water, but the biodiesel is not. Water washing will remove contaminants such as soap, residual methanol, free glycerol, and catalysts. The water should be softened (had ions removed) and be hot (both the biodiesel and water should be at 60°C). Thorough mixing with the wash water is needed so that all the contaminants can be removed, but the mixing intensity should also be controlled so that emulsions do not form between the biodiesel and water. Sometimes acid is added in the wash process to separate out the soaps. However, the last portion of washing needs to be acid-free, so a step may need to be added to neutralize the glycerol.

There is more than one way to implement the washing process. For batch processes, two of the methods are: a) top spray and b) air bubbling (see figure below). For the top spray, a fine mist of water is sprayed top-down in a fine mist. The water droplets contact the biodiesel as the water flows down, separating out the impurities. Air bubbling is a method that uses air as a mobile phase. Air bubbles through a layer of water and carries water with it on the way up. As the air bubbles burst on the way up, water droplets are released and drop down on the biodiesel at the bottom, contacting the biodiesel and washing out impurities. It can be a relatively slow process; a combination of the two is also possible.

For continuous-flow processes, different equipment is used, which typically incorporates some sort of counter-current flow process. The lighter biodiesel is introduced at the bottom and the heavier water is introduced at the top, and as they flow the fluids contact each other so that the biodiesel at the top has impurities removed and the water flowing down out the bottom contains the contaminants. The figure below shows two types of counter-current units: a) counter-flow washing system and b) rotating disc extractor. Both units contain materials to increase the interaction between water and biodiesel. For the counter-flow system, packing increases the interaction, while for the rotating disk extractor, disks rotate around as the fluid flows through. These types of equipment are typically used on an industrial scale and need precise mechanical design and process control; these units cost much more than the other type of system.

The most problematic step in biodiesel production, however, is water washing. It requires heated, softened water, some method of wastewater treatment, and water/methanol separation. Methanol recovery from water is somewhat costly using methanol-water rectification. Water can also be removed by vacuum drying. One of the alternative methods for removing water is the use of absorbent materials such as magnesium silicate. One company that provides a process for doing this is Magnesol, which is produced by the Dallas Group. Once the magnesium silicate removes the water, it can be regenerated by heating it up and evaporating the water. Methanol must also be removed from the biodiesel; one method for doing this is flash vaporization of methanol.

So, which type of process should be used? Should it be a batch or continuous flow system? Smaller plants are typically batch (< 1 million gallons/yr). They do not require continuous operation 24 hours per day 7 days a week. The batch system provides better flexibility and the process can be tuned based on particular feedstocks. However, in a commercial, industrial setting, most likely a continuous flow system will be used because of increased production and high-volume separation systems, which will increase the throughput. There are automation and process controls, but this also means higher capital costs and the use of trained personnel. It is feasible to have hybrid systems as well.

The primary byproduct is glycerin (aka glycerine, glycerol). It is a polyhydric alcohol, which is sometimes called a triol. It is a colorless and odorless liquid, which is viscous (thick-flowing) and sweet-tasting. It is non-toxic and water-soluble. Parameters to test quality are purity, color, and odor. Glycerol properties and chemical information are shown in the table below.

There are several different applications that glycerol can be used for, including the manufacture of drugs, oral care, personal care, tobacco, and polymers. Medical and pharmaceutical preparations use glycerol as a means to improve smoothness, lubrication, and moisturize – it is used in cough syrups, expectorants, laxatives, and elixirs. It can also be substituted for alcohol, as a solvent that will create a therapeutic herbal extraction.

Glycerol can be used in many personal care items; it serves as an emollient, moisturizer, solvent, and lubricant – it is used in toothpaste, mouthwashes, skin care products, shaving cream, hair care products, and soaps. Glycerol competes with sorbitol as an additive; glycerol has a better taste and a higher solubility.

Since it can be used in medical and personal care products, glycerol can also be used in foods and beverages. It can be used as a solvent, moisturizer, and sweetener. It can be used as a solvent for flavors (vanilla) and food coloring. It is a softening agent for candy and cakes. It can be used as part of the casings for meats and cheeses. It is also used in the manufacture of shortening and margarine, filler for low-fat food, and thickening agents in liqueurs.

Glycerol is also used to make a variety of polymers, particularly polyether polyols. Polymers include flexible foams and rigid foams, alkyl resins (plastics) and cellophane, surface coatings, and paints, and as a softener and plasticizer.

Unfortunately, there is already enough glycerol produced for the glycerol market. Glycerol consumption in traditional uses is 450 million lb/yr, and traditional capacity is 557 million lb/yr. If we produce glycerol from making biodiesel, it has the potential to produce 1900 million lb/yr. Therefore, we need to find a new market for glycerol, or it will be wasted in some fashion.

There is research being done to find new uses for glycerol. This includes use in additional polymers as an intermediate, conversion to propylene glycol for antifreeze, production of hydrogen via gasification, as a boiler fuel (have to remove alkali), in an anaerobic digester supplement, and for algal fermentation to produce Omega-3 polyunsaturated fatty acids.

To ensure quality biodiesel, there are standards for testing the fuel properly to see that it meets specifications for use. ASTM (an international standards and testing group) has a method to legally define biodiesel for use in diesel engines, labeled ASTM D6751. The table below shows the test methods necessary for all the expected standards for biodiesel.

There are advantages and disadvantages to using biodiesel compared to ultra-low sulfur diesel. It has a higher lubricity, low sulfur content, and low CO and hydrocarbon emissions. This makes it good to blend with diesel from petroleum to be able to achieve the required specifications for ultra-low sulfur diesel because ultra-low sulfur diesel has poor lubricity. But as discussed previously, biodiesel has poor cold weather properties. It really depends on the location; for instance, if using biodiesel in the upper Midwest, there could be problems in the winter.

As with all materials, the production and quality of biodiesel is important. Most importantly, the transesterification reaction should reach completion for the highest production and quality. Due to the nature of the transesterification of triglycerides, a small amount of tri-, di-, and mono-glycerides remain. The figure below shows the changes in these compounds as the glycerides react to form biodiesel. Some terminology to be aware of: 1) bound glycerol is glycerol that has not been completely separated from the glyceride and is the sum of tri-, di-, and mono-glycerides and 2) total glycerol combines the bound glycerol with the free glycerol.

Glycerol content in biodiesel must be as low as possible, as ASTM standards state. The biodiesel will not technically be “biodiesel” unless ASTM standards are met, which means it is below the total glycerol specifications. High glycerol content can cause issues with high viscosity and may contribute to deposit formation and filter plugging. Crude glycerol is often a dark brown color and must be refined and purified before use elsewhere. In biodiesel preparation, brown layers will form, and, possibly, white flakes or sediments, formed from saturated mono-glycerides, will fall to the bottom of the tank the biodiesel is being stored in.

Biodiesel is also a great solvent, better than petroleum-based diesel. It can loosen carbon deposits and varnishes that were deposited by petro-diesel and can cause fuel-filter plugging when switching over to biodiesel. Filters should be changed after the first 1,000 miles with biodiesel.

Another issue is the cold weather properties of biodiesel. These properties include cloud point, pour point, and cold soak filtration. Biodiesel can form cloud points at a much higher temperature than petro-diesel, close to the freezing point. The cloud point is the temperature at which crystals begin to form; it can cause the biodiesel to gel and flow slower than it should. Once the pour point is reached (basically completely frozen), the fuel cannot move. It depends on the normal temperature of the climate as to whether the fuel can be used or blended with petrodiesel. What can complicate it more is the saturated or unsaturated fatty acid content. High saturated fatty acid content can lead to higher fuel stability but higher pour points. High unsaturated acid content can lead to lower pour points but less stability for storing. The figure below shows a pour point comparison of biodiesels made from various oils (including fatty acid content) compared to petrodiesel. Petro-diesel pour points are significantly lower than biodiesels.

Cetane number is also an important property for diesel fuels. Cetane number measures the point that the fuel ignites under compression, and this is what we want for a diesel engine. The higher the cetane number, the greater the ease of ignition. Most petro-diesel fuels have a cetane number of 40-50 and meet the ASTM specification for ASTM D975. In general, most biodiesels have higher cetane numbers, 46-60 (some as high as 100), and meet the specifications for ASTM D6751. Because of the higher cetane numbers of biodiesel, the engine running on biodiesel will have an easier time starting and have low idle noise. The table below shows the heat of combustions for various fuels along with their cetane number.

If full-strength biodiesel is used (i.e., B100), most engine warranties will not be covered. It will also require replacing rubber seals in older engines. Blends include B2, B10, and B20 (2%, 10%, and 20% biodiesel, respectively). Adding biodiesel as a blend with ultra-low sulfur should improve lubricity for ultra-low sulfur diesel fuel, which will improve engine wear. Emissions of hydrocarbons, CO, NO_x, and particulate matter are similar to petrodiesel fuels, although can be reduced in some cases.

Biodiesel is stored very similarly to petrodiesel. It is stored in clean, dark, and dry environments. It can be stored in aluminum, steel, fluorinated polyethylene, fluorinated polypropylene, and Teflon types of containers. It is best to avoid copper, brass, lead, tin, and zinc containers.

In another lesson, we will discuss the economics behind using biodiesel.

Summary

In this lesson, you’ve learned about how to make biodiesel from vegetable oils. You’ve had the opportunity to see how it is made and that it’s fairly simple to make. You’ve been provided with information on properties and how biodiesel is typically used. In a future lesson, you will be provided with information on the economics behind biodiesel and ethanol, as well as determining how much energy it takes to make biodiesel, versus the amount of energy that is produced. The additional information is important to determining the use of, and best practices for making, alternative fuels.

Lesson Objectives

By the end of this lesson, you should be able to:

• explain the chemistry of vegetable oil biodiesel, and several ways to make it;
• explain how to utilize biodiesel in a diesel engine;
• evaluate the best uses of biodiesel.

References

Scott W. Pryor ^1*, B. Brian H ², J.H. Van Gerpen ² 1. Department of Agricultural and Biosystems Engineering, North Dakota State University, 2. Department of Biological and Agricultural Engineering, University of Idaho, BEEMS Module B4, Biodiesel, USDA Higher Education Challenger Program, 2009-38411-19761. Contact: Scott Pryor, Scott.Pryor@ndsu.edu

Algae are generated from sunlight, water, CO₂, and nutrients as well as algal cultures. There are more than 30,000 species of algae. One of the major factors in the use of algae to generate fuels is choosing the best species for oil generation and developing methods for removing the oil and making it into a fuel. Fuels that can be made from algae oil are biodiesel, n-alkane hydrocarbons, ethanol, methane, and hydrogen. Algae can also be used for soil conditioners and agrochemicals such as fertilizers and proteins as well as fine chemicals and bioactive substances such as polysaccharides, antioxidants, omega-3 and-6 fatty acids, proteins, and enzymes.

There are currently several applications for algae including: 1) algin – a thickening agent for food processing (brown algae), 2) carrageenan – foods, puddings, ice cream, toothpaste (red algae), 3) iodine (brown algae), 4) agar – growth media in research (red algae), 5) as food (red and brown algae), 6) plant fertilizers, and 7) diatomaceous earth – used for filtering water, insulating, soundproofing. The table below shows some additional applications detailing the species, end product, origin, and main way to culture the algae.

The role of algae in the aqueous world is that they are the base of the aquatic food chain and are photosynthetic organisms. There is a symbiotic relationship between fungi and algae known as lichens. A lichen is a composite organism that emerges from algae or cyanobacteria living among filaments of a fungus in a mutually beneficial relationship. Their properties are plant-like, but lichens are not plants. Lichens help to cause the pigments of algae, reduce harmful amounts of sunlight, and kill bacteria. They can also serve as shelters, such as kelp forming underwater forests and red algae that form reefs.

Algae can have some negative impacts via eutrophication. Eutrophication is the ecosystem response to the addition of artificial or natural substances, mainly phosphates through detergents, fertilizers, or sewage to an aquatic system. It can also be caused by dense blooms of cyanobacteria or algae. These can cause impacts through 1) clogging of waterways, streams, and filters, 2) a decrease in water taste and quality, and 3) potential toxification. Red tide is one event that can be caused by dinoflagellates.

So why make biofuels from algae? There are several reasons. Algae have high lipid content (up to 70%); they grow rapidly and will produce more lipids per area than other terrestrial plants (10-100 times). To grow algae, non-arable land (this can be thought of as land that is not typically used for farming) can be used along with saline or brackish water. Algae don’t have the same competition with generating food or feed as other oil-producing plants. One of the most amazing features is the use of CO₂ in growing algae; it helps grow algae significantly. It also provides nutrient (N, P) removal in agricultural and municipal wastewater.

The table below shows a comparison of annual oil yield from a variety of plants and algae. Even microalgae with lower lipids content (30%) will generate 50.00 m³/ha, significantly higher than palm oil. (Mata et al., 2010)

Algae are eukaryotic organisms, which are organisms whose cells contain a nucleus and other structures (organelles) enclosed within membranes. They live in moist environments, mostly aquatic, and contain chlorophyll.

Algae are not terrestrial plants, which have 1) true roots, stems, and leaves, 2) vascular (conducting) tissues, such as xylem, and phloem, and 3) lack of non-reproductive cells in the reproductive structures. Algae are not cyanobacteria. Cyanobacteria are prokaryotes, which lack membrane-bound organelles and have a single circular chromosome. The figure below shows the cellular composition of blue-algae and the subsequent one shows a micrograph of the cells. The cell has a wall with a gelatinous coat. Just beneath the cell wall is a plasma membrane. Within the cell, there are layers of phycobilisomes, photosynthetic lamellae, ribosomes, protein granules, and circular DNA known as nucleoids. These are typical components of growing plants - however, the components we are interested in are lipid droplets, which are oils that can be extracted from the algae.

Algae is composed of ~ 50% carbon, 10% nitrogen, and 2% phosphorus. The table below shows the composition of various algae looking at the percentages of protein, carbohydrates, lipids, and nucleic acid.

So what are the characteristics of algae?

1. Eukaryotic organisms:

As mentioned above, algae are eukaryotic organisms. The structure of a eukaryote (a typical plant cell) is shown in the first figure below. The second figure below shows the cell structure of a prokaryote, a bacterium, one of two groups of prokaryotic life. Some do not consider the prokaryotes as true algae because they have a different structure, but most include these in the family of algae. There are labels for the different parts of the organisms, but I will not require you to know this information in detail - it is there so if you have a desire to look up more information, you can. The table shows a comparison of both these types of cells.

2. Live in moist environments

These organisms lack a waxy cuticle (the wax in terrestrial plants prevents water loss). There is a wide variety of growth environments for algae. The typical conditions for algae are moist, tropical regions and they can grow in marine and freshwater. Freshwater algae grow in animals, aquatic plants, farm dams, sewage, lakes, rivers, lagoons, snow, mud/sand, and soil.

3. Contain chlorophyll

Algae are mostly photosynthetic, like plants. They have five kinds of photosynthetic pigments (chlorophyll a, b, c, d, and f) and have many accessory pigments that are blue, red, brown, and gold. Chlorophyll is a green pigment found in almost all plant algae and cyanobacteria. It absorbs light and transfers light energy to ATP (adenosine triphosphate).

So how are algae classified?

Algae belong to the Protista kingdom. The figure below shows a schematic of where Protista fits with other classifications of Plantae, Animalia, Fungi, Eubacteria, and Archaebacteria.

Algae can also be classified based on chlorophyll content. The first type is Chromista. These types of algae contain chlorophylls a and c, and examples of the algae include brown algae (golden-brown algae), kelp, and diatoms. These materials are a division of Phaeophyta. These types have a habitat on rocky coasts in temperate zones or open seas (cold waters). The structure is multicellular and they can grow up to 50 m long.

Red algae are another type and contain chlorophyll a, such as marine algae (seaweed). These organisms are in the division of Rhodophyta, which has over 4000 species. These are some of the oldest eukaryotic organisms on Earth (there are 2 billion-year-old fossils). They are abundant in tropical, warm waters. They act as food and habitat for many marine species. The structure ranges from thin films to complex filamentous membranes. These algae have accessory pigments and the phycobilins (red) mask chlorophyll a. The second figure below shows various red algae. Dinoflagellates are unicellular protists, and these are associated with red tide and bioluminescence.

Green algae contain chlorophylls a and b. They are in the division Chlorophyta. This is the largest and most diverse group of algae. It is found mostly in freshwaters and also on land (rocks, trees, and soil). The structures are single cells (Micrasterias), filamentous algae, colonies (Volvox), and leaf-like shapes (Thalli). Terrestrial plants arose from a green algal ancestor. Both have the same photosynthetic pigments (chlorophyll a and b). Some green algae have a cell wall made of cellulose, similar to terrestrial plants.

There are two primary ways that algae reproduce. Some algae are unicellular and demonstrate the simplest possible life cycles. Note that there is a generative phase and a vegetative phase. During the generative phase, cysts are freed. The cysts open to form gametes and then form the zygote. From there, the vegetative phase occurs so the plant grows and new cysts can form. Most algae have two recognizable phases, sporophyte and gametophyte. The second figure below shows a schematic of the two phases. The main difference is a male and female type is required to form the zygote. I will not be expecting you to know the details in depth, but want you to recognize there are differences.