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.