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.