8: Biodiesel, Syngas and Bioaviation fuels

Biodiesel  

Oils (triacylglycerol) produced from biomass can be converted to diesel fuel. In the following sections, we will discuss the synthesis of gases and liquid fuels from nonpetroleum sources, such as biomass, by producing the synthetic gases H2 and CO (collectively called syngas) and condensing them into liquid fuels via the Fischer-Tropsch reaction. This section will focus on using fats to produce biodiesel. Triacylglycerols for biodiesel production can come from plants, animals, algae, and even food industry waste oils. Because biodiesel fuel is composed of carbon-based molecules (often fatty acid esters) with high melting and boiling points, it is often blended with regular diesel (for example, a 20% blend called B20). Still, it can be used at 100% (B100). Biodiesel enriched with unsaturated fatty acids has a lower melting point and is less prone to cold-weather problems.

As with bioethanol production from lignocellulosic food stocks, biodiesel production has evolved through multiple generations, as shown in Figure \(\PageIndex{1}\) below.

Figure \(\PageIndex{1}\):  Palani Vignesh et al., Oil Gas Sci. Technol. – Rev. IFP Energies nouvelles, 76 (2021) 6.  DOI: https://doi.org/10.2516/ogst/2020088. Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0)

Table \(\PageIndex{1}\) below shows each biodiesel generation's advantages and disadvantages. 

Table \(\PageIndex{1}\): Advantages and disadvantages of various biodiesel generations.  Palani Vignesh et al., ibid 

Triacylglycerol feedstocks for biodiesel are often converted to methyl or ethyl esters through an alcoholysis or transesterification reaction, as shown in Figure \(\PageIndex{2}\) below.

Figure \(\PageIndex{2}\): Methanolysis/transesterification of triacylglycerol to produce methyl-fatty acid ester for biofuels

This reaction is simply a base-catalyzed cleavage of the ester bonds in the triacylglycerol. Alternatively, vegetable oil can be hydrogenated at high pressure and temperature to produce a variant called "renewable diesel".

Figure  \(\PageIndex{3}\) below shows feedstocks and processing for first-generation biodiesel production.  

Feedstock: Processing Method	1st Gen Processing
Waste  cooking oil:  esterification/transesterification Food crops:  extraction/transesterification Organic oils: hydrolysis, distillation Animal fat: hydrolysis, fermentation Bioethanol/butanol:  chemical synthesis

Figure  \(\PageIndex{3}\): Feedstocks and processing for first-generation biodiesel production.  Palani Vignesh et al., ibid 

Figure  \(\PageIndex{4}\) below shows feedstocks and processing for second-generation biodiesel production

Feedstock: Processing Method	2nd Gen Processing
Cellulose:  advanced fermentation Hemicellulose:  hydrolysis Lignin:  gasification Tannins: biological synthesis Vegetable oil/animal fats:  hydrogenation

Figure  \(\PageIndex{4}\) below shows feedstocks and processing for second-generation biodiesel production.  Palani Vignesh et al., ibid 

Finally, Figure  \(\PageIndex{5}\) below shows the processing steps for 3^rd and 4^th generation biodiesel from algae.

Figure \(\PageIndex{5}\): Processing steps for 3^rd and 4^th generation biodiesel from algae.  Palani Vignesh et al., ibid 

Let's consider 3^rd and 4th-generation biodiesel production using algae. First, algae can be used to produce a range of products for biofuels and chemical feedstocks. These are reviewed in Figure \(\PageIndex{6}\) below.

Figure \(\PageIndex{6}\): Macroalgal biofuel refinery;  Godvin Sharmila V et al.,  Bioengineered. 2021 Dec;12(2):9216-9238. doi: 10.1080/21655979.2021.1996019. PMID: 34709971; PMCID: PMC8809944. Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/)

To maximize biodiesel production from algae (3^rd- and 4^th-generation), steps in the anabolic pathways for fatty acid and triacylglycerol synthesis could be genetically modified. Below is an overview of triacylglycerol synthesis in microalgae in Figure \(\PageIndex{7}\).

Figure \(\PageIndex{7}\): Schematic illustration of TAG synthesis in microalgae. NADPH; Nicotinamide adenine dinucleotide phosphate; ATP, Adenosine Triphosphate; DHAP, dihydroxyacetone phosphate; G3P/G3pDH, Glyceraldehyde 3-phosphate / G3P dehydrogenase; GPAT, Glycerol 3-phosphate acyltransferase; PA/LPA/LPAAT/PAP, Phosphatidic acid/Lyso-PA/LPA acyltransferase/PA phosphatase; DAG/DGAT, di-Acylglycerol/ DAG acyltransferase; FAT, Fatty acyl-ACP thioesterase; ACP, Acyl-carrier protein; ER, Endoplasmic reticulum; PC, Phosphatidylcholine; PDAT, Phospholipid: DGAT; ACCase, Acetyl-CoA carboxylase; FAS, Fatty acid synthase; KAS, 3-ketoacyl-ACP synthase; FAD, Flavin adenine dinucleotide.   Sharma PK, et al. Front. Mar. Sci. 5:382, 2018. doi: 10.3389/fmars.2018.00382.  Creative Commons Attribution License (CC BY). 

Each step in the combined pathways is a site for optimization, as shown in Figure \(\PageIndex{8}\) below.

Figure \(\PageIndex{8}\): Schematic illustration of different genetic engineering strategies applied in microalgae for biodiesel application. WT, Wild type cells; TR, Transgenic cells; TF, Transcription factor; TCA, Tricarboxylic acid cycle; NADH, Nicotinamide adenine dinucleotide; FA, Fatty acid; LD, Lipid droplet. Sharma PK, et al., ibid

Life-cycle analyses of third-generation biodiesel production indicate they would lead to a net decrease in CO2 emissions, but most appear incomplete.  

Synthetic Gas (Syngas)

What if biomass could produce "gasoline-like" fuel as an alternative to using fossil fuels to power our vehicles and as feedstock for chemical production? We have already discussed the production of bioethanol from 1st-generation (plant starch), 2nd-generation (lignocellulosic), and 3rd-generation (algal) feedstocks. Bioethanol is routinely added to gasoline at up to 15%. It is also found in E85 (or flex fuel), a gasoline blend containing 50% to 80% ethanol. 

Instead of producing ethanol from glucose fermentation, wood could be partially burned to produce "synthetic gases" (CO and H2), called syngas.  These gases could be further burned in vehicles to power them or converted via chemical processes (e.g., the Fischer-Tropsch reaction) into liquid organic fuels.  

Indeed, when fossil fuels were lacking, wood was used to create syngas to power vehicles. During World War II, up to a million cars in Europe were powered by wood gas. A bus powered by wood gas (syngas) generated by a gasifier on a trailer is shown in Figure \(\PageIndex{9}\) below.

Figure \(\PageIndex{9}\):  Bus power by wood gas c. 1943 in Leeds, England. By Ministry of Information Photo Division Photographer, Smith Norman? - http://media.iwm.org.uk/iwm/mediaLib.../large.jpgThis is photograph D 15675 from the collections of the Imperial War Museums. Public Domain, https://commons.wikimedia.org/w/inde...curid=24364067

The syngas emitted was cleaned to remove tars and soot/ash particles by passing through charcoal before entering the vehicle through a tube. Tar with polycyclic aromatic hydrocarbons and methane could be reduced if wood or coal were first converted to char by pyrolysis (heating to high temperatures in the relative absence of air) before use. 

The gases, derived from the incomplete combustion of wood, contain CO and H₂ in varying proportions depending on the burning temperature and the source (wood or coal). A general and very simplified reaction for the incomplete combustion reaction is:

\begin{equation}
\text { Carbon feedstock }+\text { air } \rightarrow \mathrm{CO}+\mathrm{H}_2+\mathrm{CH}_4+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}+\mathrm{N}_2
\end{equation}

Of course, the reaction is not clean, and many organic side products are produced. 

The relative ratios of CO and H₂ produced can be changed by the addition of water in a second reaction called the water gas shift (WGS) reaction (as water shifts the ratio of CO to H₂):

\begin{equation}
\mathrm{CO}+\mathrm{H}_2 \mathrm{O} \leftrightarrow \mathrm{CO}_2+\mathrm{H}_2 \quad(\Delta \mathrm{H}=-41.2 \mathrm{~kJ} / \mathrm{mol} .)
\end{equation}

If run in reverse (rWGS) and at high temperatures, the water-shift reaction could be used to capture carbon. The H₂ could come from the electrolysis of water

\begin{equation}
2 \mathrm{H}_2 \mathrm{O}(\mathrm{I}) \rightarrow 2 \mathrm{H}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g}) \quad(\Delta \mathrm{H}=286 \mathrm{~kJ} / \mathrm{mol})
\end{equation}

The electrocatalytic reduction of CO₂ and H₂O to produce syngas is shown in \(\PageIndex{10}\).

Figure \(\PageIndex{10}\): Syngas Generation by electrocatalytic reduction of CO₂ and H₂O. (after Kang Cheng et al. Advances in Catalysis, 60 (2017). https://doi.org/10.1016/bs.acat.2017.09.003

The diagram shows the oxidation numbers of each element. The cathode acts as a catalyst for the reaction. Since the reaction requires a power source, this process is greener if the electricity is generated from renewable sources (e.g., wind or solar).

The synthesis of syngas (typically described as a mixture of CO, CO₂, and H₂) is widespread, used in various processes, and used to make many products, including hydrocarbons for fuel and oxygen-containing derivatives such as methanol and ethanol. CO and H₂ produce alkanes and alkenes in the Fischer-Tropsch reaction (described below).

\begin{equation}
\mathrm{CO}+2 \mathrm{H}_2 \rightarrow\left(\mathrm{CH}_2\right)+\mathrm{H}_2 \mathrm{O} \quad \Delta \mathrm{H}=-165 \mathrm{~kJ} / \mathrm{mol}
\end{equation}

where CH₂ is a methylene repeat in longer-chain alkanes.

The CO₂ produced in syngas can be somewhat selectively removed by adsorption onto a CaO catalyst, as shown in Figure \(\PageIndex{12}\) below.

Figure \(\PageIndex{12}\):  Adsorption configurations of CO₂ on the surfaces of CaO-based catalysts at 650 °C: (a) CO₂ adsorption on CaO (100) surface; (b) CO₂ adsorption on 10 wt% % Ni/CaO (100) surface. Green, red, gray, and purple balls represent Ca, O, C, and Ni atoms. Zhao, B. et al. Catalysts 2019, 9, 757. https://doi.org/10.3390/catal9090757.  Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)

CO, CH4, and H2 adsorption energies are so small that they have little effect on CO₂ adsorption.

At present, the easiest way to produce syngas is to react natural gas (methane) with steam at very high temperatures (up to 1000 °C) over a Ni catalyst (a process called steam reforming). This results in a high H₂/CO ratio of about 3. It can be done with liquefied natural gas using Ni-ZrO₂-CeO₂-La₂O₃ catalyst. This process would inherently do little to reduce CO₂ emissions. A gasification method converts coal, lignocellulosic biomass, and waste to syngas, with an H₂/CO ratio of <1 for coal and about 0.6-1 for biomass. The electrocatalytic method described above has an H₂/CO ratio of 0-2, depending on the nature of the cathodic catalyst. Syngas can also be produced by partial oxidation of methane. For the subsequent reactions (Fisher-Tropsch), the optimal H₂/CO is about 2. In the gasification of lignocellulosic biomass, the water-gas shift (WGS) reaction is used to increase the H₂/CO ratio. This requires a lot of water and produces CO₂. The water shift reaction use catalysts such as Co–Mo–Al₂O₃, Fe₂O₃–Cr₂O_3, and Cu–ZnO–Al₂O₃) for coal gasification.

Syngas can be used to make small-molecule energy and chemical feedstocks, such as ethanol (as well as liquid alkanes and alkenes). Given the many types of products, it is often essential to selectively produce and purify them for commercial use. One method for ethanol production is shown in Figure \(\PageIndex{13}\) below.

Figure \(\PageIndex{13}\): Conversion of syngas to ethanol proceeds through a tandem mechanism via methanol and acetic acid intermediates using a variety of sequentially positioned catalysts. Kang, J., He, S., Zhou, W. et al. Single-pass transformation of syngas into ethanol with high selectivity by triple tandem catalysis. Nat Commun 11, 827 (2020). https://doi.org/10.1038/s41467-020-14672-8.  Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/.

H-MOR is a zeolite, a microporous, crystalline structure made of aluminosilicate.

Bioethanol can also be converted to CO and H₂ via steam reforming, as shown in the equation below.

\begin{equation}
\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}+\mathrm{H}_2 \mathrm{O}(+\text { heat }) \rightarrow 2 \mathrm{CO}+4 \mathrm{H}_2
\end{equation}

Even with the ability to capture CO₂ from syngas synthesis, a central concern is whether syngas and syngas-derived fuels are associated with lower net CO₂ emissions. A life cycle analysis would be necessary to determine this.

Fischer-Tropsch Synthesis (FTS) of Fuels

Fischer was head of the Kaiser-Wilhelm Institute for Coal Research in Germany at the start of World War I.  Germany had abundant coal but needed oil for the war, so its efforts were redirected toward that end. Fischer and Tropsch developed the water shift reaction discussed above. They deployed new cobalt catalysts to produce oil, ultimately covering 25% of car fuel and 10% of German military fuel needs in World War II. Large amounts were also made in South Africa during the Apartheid regime.

The Fischer–Tropsch synthesis (FTS) is a polymerization-like reaction that is used to convert gas-to-liquids (GTL), coal-to-liquids (CTL), or biomass-to-liquids (BTL) fuels. It starts with syngas (H₂ and CO) produced by gasifying coal or biomass or by steam reforming/partial oxidation of natural gas, with the H₂/CO ratio determined by the water-shift reaction. If coal or biomass is used, a cleanup of residual products with heteroatoms and metal ions is necessary. The clean syngas is then passed into a reactor containing the catalyst required for FTS of fuels. These reaction systems are summarized in Figure \(\PageIndex{14}\).

Figure \(\PageIndex{14}\): A simplified diagram of the Coal-to-Liquids (CTL), Gas-to-Liquids (GTL), and Biomass-to-Liquids (BTL) processes.  Shafer, W.D.; Gnanamani, M.K.; Graham, U.M.; Yang, J.; Masuku, C.M.; Jacobs, G.; Davis, B.H. Fischer–Tropsch: Product Selectivity–The Fingerprint of Synthetic Fuels. Catalysts 2019, 9, 259. https://doi.org/10.3390/catal9030259.  Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

The FTS reaction is conducted at moderate temperatures and pressure to produce fuels (diesel and jet), lubricants, waxes, and chemical feedstocks. The main representative reactions are given by:

N-alkane production:

\begin{equation}
(2 \mathrm{n}+1) \mathrm{H}_2(\mathrm{~g})+\mathrm{nCO}(\mathrm{g}) \rightarrow \mathrm{C}_{\mathrm{n}} \mathrm{H}_{2 \mathrm{n}+2}+\mathrm{nH}_2 \mathrm{O}
\end{equation}

Alkene production:

\begin{equation}
(2 \mathrm{n}) \mathrm{H}_2(\mathrm{~g})+\mathrm{nCO}(\mathrm{g}) \rightarrow \mathrm{C}_{\mathrm{n}} \mathrm{H}_{2 \mathrm{n}}+\mathrm{nH}_2 \mathrm{O}
\end{equation}

The products can range from 1 carbon (CH4) to over 70, depending on the catalyst, T, P, and the H2/CO ratio. The FTS is a polymerization reaction that begins when the gas adsorbs onto the catalytic surface. It is followed by multiple cycles of free radical initiation, propagation, termination, desorption, and reabsorption.

Since this book focuses on structure/function and reaction mechanisms, we would be remiss not to include at least a simplified mechanism for these complex reactions. Several mechanisms have been proposed. Since iron was/is used in the catalyst, and since iron can form iron carbides, Fisher proposed a carbide mechanism. A second enol mechanism has also been proposed. Both are shown in Figure \(\PageIndex{15}\) below.   

Figure \(\PageIndex{15}\):  Proposed mechanisms for the Fischer-Tropsch reaction. Left: A proposed FTS route based on the carbide mechanism; Right: A proposed FTS route based on the enol mechanism. M is the metal surface.

In the carbide mechanism, CO adsorbs on the metal catalyst and dissociates into C and O atoms, which then cover the surface. These are hydrogenated to form H₂O and CH₂ (methylene), and H₂ adds as the reaction proceeds, as shown.  

In the enol mechanism, CO adsorbs without dissociation into atoms. It reacts on the surface with surface-bound H atoms (that arise from the dissociation of adsorbed H₂) to form hydroxymethylene (M-CHOH). This enol condenses with adjacent hydroxymethylenes. The rate-determining step is the hydrogenation of adsorbed CO.

Another proposed mechanism, CO insertion, is shown in Figure \(\PageIndex{16}\)

Figure \(\PageIndex{16}\):  A proposed FTS route based on the CO insertion mechanism.

In this model, CO inserts into a bond from a hydrogen atom to a metal on the catalyst. The rate-limiting step is the hydrogenation of CO to the CH₂ methylene group. The assumed monomer in this mechanism is simply CO, which inserts into metal-carbon bonds. 

Generally, the FTS reaction catalyst has either cobalt or iron ions. The metal catalyst can also be doped with potassium and copper ions and bind silica and alumina. Iron is abundant, cheap, and more effective at promoting the water-gas shift reaction, so it is best for FTS of fuels from coal and biomass, since the syngas derived from them has a lower H₂/CO ratio.

Chemical Synthesis of Food

Suppose fuels can be synthesized from nonpetroleum sources and agricultural waste. Why not use the reactions described above (such as syngas production, the Fischer-Tropsch process, and electrochemical reduction) to make food, not just fuel?  The agricultural sector accounts for about 25% of total greenhouse gas emissions, so it is ripe (no pun intended) for novel ways to produce foodstuffs (think of them as feedstocks for human consumption). 

It should be obvious from the chemistry that the simplest type of food produced by these reactions is fats, rather than the more synthetic and stereochemically complex proteins and carbohydrates.  Currently, the world relies on palm oil, derived from oil palm trees, as a fat source in crackers, cookies, breads, and other food products. Typical palm oil production has been associated with large-scale deforestation of rainforests and their associated impacts on climate and biodiversity.   Americans consume about 8 kg (17.6 lbs) of palm oil products yearly, including cosmetics, cleaning products, and waxes.  Figure \(\PageIndex{17}\) below shows the scale of palm oil production in the world, which might come as a surprise since we more frequently encounter soybean, sunflower, rapeseed (source of canola oil), and olive oils in our stores. 

Figure \(\PageIndex{17}\): Vegetable Oil Production in the World.  Our World in Data.  https://ourworldindata.org/palm-oil 

The general chemical pathways used to synthesize proteins, fats, and carbohydrates are illustrated in Figure \(\PageIndex{18}\) below.  The fat synthesis pathways relevant to this discussion are represented in the central part of the figure.

Figure \(\PageIndex{18}\): Schematic of potential pathways to synthesize food without agriculture. Davis, S.J., Alexander, K., Moreno-Cruz, J. et al. Food without agriculture. Nat Sustain (2023). https://doi.org/10.1038/s41893-023-01241-2. Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/.  

Proteins, fats, and carbohydrates can be synthesized from various carbon feedstocks through chemical and biological pathways (arrows). The weight and color of the arrows indicate the scale at which the different processes have been demonstrated and the energy required per mass unit output, respectively. Dashed lines indicate where energy requirements remain highly uncertain. Circular labels on each arrow further indicate whether the process is typically continuous (C) or batched (B). NH₃ is ammonia, H₂ is hydrogen gas, MeOH is methanol, and AcO^- is acetate (agriculturally produced carbon feedstocks are omitted).

It is estimated that chemical synthesis of fats could yield <0.8 g CO₂ equiv/kcal, compared with>1.5 g CO₂ equiv/kcal for the current biological synthesis and processing of palm oil in Brazil or Indonesia. This advantage would increase if the CO₂ used in chemical synthesis could be captured from the atmosphere, thereby yielding net-zero emissions.

Figure \(\PageIndex{19}\) shows both land use and energy emissions of CO_{2 equiv}/kcal for agriculturally produced fats (panel a) versus equivalents for chemical-synthesized fats.

Figure \(\PageIndex{19}\): Comparison of emissions per calorie of edible fats.  Davis, S.J et al., ibid.

Panels a and b use shading and contours to show grams of CO₂-equivalent GHG emissions per kilocalorie of edible fat produced by conventional agriculture (Panel a) and chemical synthesis (Panel b). Agricultural emissions are shown as the sum of land-use emissions (y-axis) and energy-related emissions (x-axis), and emissions from synthesis are shown as a function of feedstock emissions intensity (y-axis) and energy emissions intensity. Red circles denote specific estimates based on literature and assumed values. Feedstock emissions include process-related conversion of feedstock to CO2—for example, during the extraction of natural gas, the gasification of coal, and the eventual human respiration of fossil feedstock.

Bioaviation Fuel

Now we can turn our attention to the production and analysis of bioaviation fuel, which is more similar in composition to kerosene and diesel than to ethanol. As we saw for bioethanol production, the feedstocks can be 1^st, 2^nd,and 3^rd generation, as shown in Table \(\PageIndex{3}\) below.

Table \(\PageIndex{3}\): Feedstocks for bio-aviation fuel production. Doliente SS, Narayan A, Tapia JFD, Samsatli NJ, Zhao Y and Samsatli S (2020) Bio-aviation Fuel: A Comprehensive Review and Analysis of the Supply Chain Components. Front. Energy Res. 8:110. doi: 10.3389/fenrg.2020.00110. Creative Commons Attribution License (CC BY).

A typical jet aviation fuel (Jet A-1) contains n- and branched-alkanes (often called paraffins) and some alkenes (often called olefins) with 8-16 carbon atoms, cycloalkanes, and aromatics. A comparison of the components of Jet-A1 with a typical bioaviation fuel, Bio-Jet, is shown in Figure \(\PageIndex{20}\) below.

Figure \(\PageIndex{20}\): Molecular-class compositions of (a) Jet A-1 and (b) bio-jet identified by the relative signal area percentage analysis of GC–MS. After Cheon Hyeon Cho, Hee Sun Han, Chae Hoon Sohn, and Jeong Sik Han.  ACS Omega 2021 6 (40), 26646-26658.  DOI: 10.1021/acsomega.1c04002. 

Bioaviation fuel can be made from feedstocks containing triacylglycerols or from readily available feedstocks listed in Table 2 above, producing syngas for use in the Fischer-Tropsch reaction. We've already discussed those reactions above. Instead, let's focus on whether bioaviation fuel, more accurately termed sustainable aviation fuel (SAF), is good for our climate. This presupposes that battery-powered planes and jets are not scalable to our current environmental needs.

Life Cycle Analysis - Jet Fuel from Grasses

As expected, the US is the largest user of aviation fuel and accounts for 25% of aviation CO₂ emissions, as much as all the greenhouse gases emitted from fuel use in Spain. It is estimated that the US will need approximately 30 billion gallons per year (BGY) of jet fuel in 2040. A bio-jet fuel industry based on cellulose as a feedstock could theoretically produce that amount. If the industry costs are estimated at $123 billion, then the cost of jet fuel would be $4.30/gal. About 60% of the costs would arise from converting biomass to sustainable aviation fuel (SAF), as described above. Those costs include the building of the biorefineries. Their cost would be spread over the plants' lifetimes. The fuel would be derived from syngas from the Fischer-Tropsch reaction, a reasonably mature technology. The costs would be much lower (closer to $1/gal) if infrastructure costs were excluded. These total costs are comparable to the price paid for regular jet fuel (around $2.2/gal in 2021). Consumer costs would not go up 2-fold since fuels are only part of the cost paid by passengers (15-25%)

Consumers' current prices for fossil fuels are well below their actual costs. The present price/gal does not include the external costs of using fossil fuels. These include climate change impacts on infrastructure, agriculture, industry, and other sectors, as well as on human health (primarily through negative health consequences and diseases exacerbated by fossil fuel pollution). We all ultimately pay for these hidden costs resulting from a failed market for fossil fuel pricing. On top of this, the fossil fuel industry has been massively subsidized for decades.

Different prices have been placed on carbon emissions from fossil fuels to resolve this issue in the market. The carbon price is based on the number of tons of CO₂ equivalent emitted  ($US/ton CO₂e). If a reasonable carbon price is applied, bio-jet fuels produced from lignocellulosic feedstocks via the Fischer-Tropsch reaction would be theoretically competitive with conventional jet fuels derived from fossil fuels. The extra added cost for sustainable aviation fuel (SAF) compared to traditional aviation fuel at different prices placed on carbon is shown in Table \(\PageIndex{4}\) below, assuming an average cost of $2.20/gal for traditional aviation fuel.

If the cost of traditional jet fuel rose to $3/gal, as it did in the US in March 2022, SAF and conventional jet fuel would cost the same if a carbon price of $100/t CO2e were applied to both. The actual "social" cost of carbon has been calculated (9/22) to be $175/ton CO₂e.  

Factors other than carbon costs should be included in these analyses. These include the issue of sustainable land use for SAF feedstocks. A recent analysis shows that it would be possible to produce 30 billion gall/yr of cellulosic SAF by planting 23.2 Mha (Million hectares, about the size of the state of Wyoming) of marginal agricultural lands (about one-third of croplands and 2/3 noncrop lands) in the Midwest with miscanthus, with a net cost of $4.1/gal and assuming a carbon price of a $50/ t CO2e.  Miscanthus is a rapidly growing, tall perennial grass with high yields that thrives in moderate climates. The life-cycle analysis included interactions among the atmospheric, land-surface, ecosystem, and economic systems. Miscanthus gigantheus is shown below in Figure \(\PageIndex{21}\).

Figure \(\PageIndex{21}\):Miscanthus gigantheus.  https://commons.wikimedia.org/wiki/F...us_Bestand.JPG.   Creative Commons Attribution-Share Alike 3.0 Unported

Figure \(\PageIndex{22}\) shows some data from the study. Four scenarios are presented, each represented by a bar graph. 

Figure \(\PageIndex{22}\): Land availability and conversion by existing use. Excel data and graph from https://dataverse.harvard.edu/datase...910/DVN/VBFLI2. CC0 1.0 Public Domain.

Four scenarios (left to right) produced 30 MG/yr of SAF using a carbon price of $50/t CO2e.

• M25:  only 25% of the marginally useful land was used
• M100-reg4:  marginal land bases with the lowest hydrological and climatic risks
• M100:  all of the marginally included land was made available
• Unrestricted

Panel (a) shows that in each case, about 23.2 Mha of land was converted to growing miscanthus from the available land. Pane (b) shows stacked bars showing the percentages of each land type available and converted for each scenario. The last scenario shows that demand can be met with the lowest % conversion of marginal land to corn/soybeans and other crops. It would appear that the marginal croplands converted to SAF production would be the same lands diverted to bioethanol production. Nevertheless, it would appear that up to 76% of projected aviation fuel needs could be met by planting marginal cropland and noncrop land for cellulosic SAF production. The study found that using available lands in the Plains was not feasible.

Summary

(Summary written by Claude, Anthropic)

This chapter examines three advanced routes to replacing fossil fuels in transportation: biodiesel from triacylglycerols, liquid fuels from syngas via Fischer-Tropsch synthesis, and sustainable aviation fuel (SAF), connecting chemistry and biochemistry to climate policy and life-cycle analysis.

Biodiesel. Diesel fuel (C12–C20 hydrocarbons, 75% saturated, 25% aromatic) releases more energy per liter (36.9 MJ) than gasoline (33.7 MJ) and ignites by compression rather than spark. Biodiesel is produced by transesterification (methanolysis) of triacylglycerols — a base-catalyzed alcoholysis that replaces the glycerol backbone with methanol to produce fatty acid methyl esters (FAME) and free glycerol. First-generation biodiesel from edible crops (palm oil, soybean, rapeseed) is constrained by competition for food and land. Second-generation biodiesel from non-edible waste oils reduces this conflict. Third-generation biodiesel from microalgae avoids competition for cropland entirely and can use wastewater or seawater for cultivation; fourth-generation approaches use genetically engineered organisms or non-biological CO₂ feedstocks. To maximize triacylglycerol yields in microalgae, genetic engineering targets the fatty acid synthesis pathway at multiple nodes: acetyl-CoA carboxylase (ACCase, the committed step), fatty acid synthase (FAS), fatty acyl-ACP thioesterase (FAT), and the Kennedy pathway enzymes GPAT, LPAAT, PAP, and DGAT that assemble triacylglycerols in the endoplasmic reticulum.

Syngas: production and uses. Syngas — a mixture of CO and H₂ — has powered vehicles since World War II, when up to one million European cars ran on wood gas from vehicle-mounted gasifiers. It is produced by incomplete combustion of wood or coal, steam reforming of natural gas (H₂/CO ~3, using Ni catalyst at up to 1000°C), or gasification of coal/biomass (H₂/CO ~0.6–1). The water-gas shift reaction (CO + H₂O → CO₂ + H₂) adjusts the H₂/CO ratio upward; its reverse (rWGS) at high temperatures can be used for CO₂ capture. Electrocatalytic CO₂ reduction (ECR) — using electrons from water oxidation at an anode to reduce CO₂ at a biocathode — offers a potentially carbon-neutral route to syngas, methanol, formate, ethanol, and methane, provided the electricity is renewable. Standard reduction potentials for these half-reactions range from ~−0.1 to −0.5 V vs. SHE, with multi-electron products requiring progressively more reductive conditions. CO₂ can be selectively removed from syngas by adsorption onto CaO-based catalysts, leaving CO, CH₄, and H₂ largely unaffected due to their much lower adsorption energies. Syngas can be converted to ethanol via a tandem catalytic process with methanol and acetate intermediates, using sequentially positioned catalysts, including zeolites (H-MOR).

Fischer-Tropsch synthesis. The FTS reaction — developed in Germany to produce oil from coal during World War II — converts syngas to liquid alkanes and alkenes via a polymerization-like surface reaction, with chain lengths ranging from C1 to >C70, depending on the catalyst, temperature, pressure, and H₂/CO ratio. The optimal H₂/CO ratio is ~2, achieved from biomass gasification using the water-gas shift reaction. Three mechanistic proposals exist: the carbide mechanism (CO dissociates to surface C and O, then is hydrogenated to CH₂ methylene units); the enol mechanism (CO adsorbs intact, reacts with surface H to form hydroxymethylene, which condenses with adjacent species); and the CO insertion mechanism (CO inserts into metal-carbon bonds, with CH₂ as the rate-limiting product). Iron catalysts are preferred for biomass- and coal-derived syngas (lower H₂/CO, active in the water-gas shift) while cobalt catalysts are preferred for natural gas-derived syngas. Both are doped with K and Cu to improve selectivity and stabilized on silica/alumina supports.

Chemical synthesis of food. Agriculture accounts for ~25% of global GHG emissions, and palm oil — the world's most produced vegetable oil — is associated with large-scale tropical deforestation. Chemical synthesis of fats from CO₂, syngas, methanol, or acetate offers an alternative with a GHG footprint of <0.8 g CO₂ equiv/kcal, compared with>1.5 g CO₂ equiv/kcal for palm oil, and has net-zero potential if atmospheric CO₂ is used as the feedstock. Fats are the most tractable food macronutrient to synthesize chemically because they lack the stereochemical complexity of carbohydrates and proteins; chemical pathways to fatty acids and triacylglycerols from small-molecule feedstocks are well established, in contrast to the enzymatic stereoselectivity required for amino acid or sugar synthesis.

Sustainable aviation fuel. Aviation accounts for significant global CO₂ emissions; the US alone contributes 25% of aviation emissions. Meeting projected US demand of ~30 billion gallons per year (BGY) by 2040 with cellulosic SAF is theoretically feasible: a study modeling miscanthus cultivation on 23.2 Mha of marginal agricultural land in the US Midwest found that up to 76% of projected aviation fuel needs could be met at ~$4.1/gal assuming a carbon price of $50/t CO₂e. Bio-jet fuel composition (C8–C16 n-alkanes, branched alkanes, cycloalkanes, and some aromatics) is matched to conventional Jet A-1 specifications through Fischer-Tropsch synthesis from lignocellulosic syngas. At current unsubsidized market prices, SAF costs ~$1.90/gal more than conventional jet fuel; this gap closes to zero at a carbon price of $175/t CO₂e — precisely the estimated social cost of carbon. The current fossil fuel market fails to account for the external costs of climate change, infrastructure damage, agricultural disruption, and health impacts, effectively subsidizing conventional fuels at the expense of the public. The 2026 conflict disrupting Strait of Hormuz oil flows — which carry ~20% of global oil — has sharply accelerated interest in domestically produced SAF as an energy security imperative as well as a climate