Biofuels for the Production of Sustainable Aviation Fuels

Advanced biofuels can be developed to play an important role as an energy carrier in the United States' energy portfolio. Principally due to concerns regarding land usage and pressure on the environment, it is unlikely that biofuels can be scaled up in a sustainable manner to meet large-scale energy needs. However, biofuels can be used to assist in decarbonization of hard-to-abate sectors that still require portable and energy-dense fuels or in the production of high-value chemicals (HVCs) that usually rely on fossil fuel feedstocks. In this way, advanced biofuels can provide benefits as one of the many tools available to avoid the worst-case outcomes of climate change.

Biofuels have been increasingly gaining attention in recent years as governments set out policies aimed at achieving their stated climate goals. The main area where this is gaining attention is in the aviation industry, which is often referred to as a 'hard-to-abate' sector. The aviation industry is resistant to standard methods of decarbonization such as electrification or direct hydrogen usage, meaning that combustible liquid fuels will continue to be required for long-distance aviation. Therefore, to achieve decarbonization, the creation and usage of Sustainable Aviation Fuels (SAF) has been proposed, and is being pursued by governments and industry leaders as the future of aviation. Sustainable aviation fuel is a blanket term used to describe any aviation fuel that has a lower carbon footprint than traditional fossil fuels, and can include biofuels of all kinds, along with synthetic fuels known as e-fuels.

In the United States, when we think about biofuels, we almost always think about ethanol derived from corn, known as first generation biofuels. These fuels are derived from food crops and are currently used to meet the renewable fuel standard (RFS) that defines the amount of renewable fuel that must be blended into the gasoline supply for road-going vehicles. Although the creation of the domestic ethanol industry has been viewed as a commercial and strategic success, it is generally agreed that it has not produced benefits for the environment and combating climate change, and in some cases has had negative effects. Learning from the mistakes of the first generation biofuels, advanced biofuels have been developed to address these shortcomings. It is these advanced generation biofuels that are being proposed for use in sustainable aviation fuels.

As will be shown in the following analysis, the hypothetical question "what would be required to develop a large-scale advanced biofuels industry for sustainable aviation fuels?" is asked. Sustainable aviation fuel is used here as an example of a hard-to-abate, but other sectors such as global shipping and the production of some high value chemicals such as ethylene show potential for biofuels to play a role. As will be shown, an enormous effort would need to be undertaken, requiring the collection of hundreds of millions of tons of non-edible biomass every year, while also requiring the conversion of tens of millions of acres of land to produce high-quality and sustainable biomass to feed the industry. However if conversion yields of biomass to ethanol could be increased to closer approach the theoretical maximum, more fuel could be produced from the same amount of biomass and less land would be needed for energy crop production. Although enormous, the levels of production are well within the realm of achievability, being of the same magnitude as the contemporary corn ethanol industry. But due to its scale and the acknowledged environmental concerns associated with biofuels, the question then becomes whether it is worth allocating the necessary resources as a national project, or whether those resources could be better used in other areas to address the climate crisis.

Before analyzing the current state of biofuels and their adoption into the aviation industry, it is helpful to define what is meant by the term 'biofuels'. Biofuels are a form of energy carrier that is derived from biomass. Biomass is defined as any organic matter that is derived from plants or animals, or put another way, any organic material excluding those that are fossilized or located in geological formations. Biomass has historically been the main energy carrier utilized by humans, mainly through the combustion of wood for use in cooking, heating, and lighting. According to the World Bioenergy Association, as recently as 2021, traditional forms of biomass made up over 80% of all biomass energy usage in the world, primarily through the burning of wood, charcoal, and wood pellets.

Biofuels are defined by the Intergovernmental Panel on Climate Change (IPCC) as a fuel, generally in liquid form, produced from biomass. Biofuels include bioethanol from sugarcane, sugar beet, or maize and biodiesel from canola or soybeans. Biofuels have been used since antiquity in places such as oil lamps, which utilized seed oils, or animal-based oils such as tallow, ghee, and whale blubber. More recently in the early 1900s, at times of fossil fuel shortages, some vehicles were powered by a type of fuel called 'wood gas.' The contemporary usage of the term biofuels in the United States generally refers to first generation biofuels which are derived from food crops such as the starch-derived sugars from corn that are used to meet the renewable fuel standard.

The position of biofuels in the United States in 2025 lies in somewhat of an awkward place. On the one hand, biofuels have been a commercial success, often reaching price parity on a per-gallon basis with gasoline while producing less direct CO₂ emissions, albeit with significant government subsidies and incentives. Corn-based ethanols have also assisted in achieving energy independence for the United States and have been a key part of the country's energy security. On the other hand, it has been argued that corn-derived ethanol has been a net negative for the environment, with a large host of issues including land usage, soil degradation, the 'food vs fuel' debate, and the use of fossil fuel-derived fertilizers. Outside of the United States, Brazil has been a leader in the production of biofuels sourced from sugar cane, with aggressive policies to support the industry resulting in similar concerns regarding its sustainability credentials.

Advanced Biofuels

In an effort to address the issues presented by first generation biofuels, advanced biofuels (sometimes referred to as their respective generations second, third, and fourth) have been explored. The primary goal of advanced biofuels is to avoid the dependence on food crops as feedstock, so as to not contribute to increased strain on global food prices and put less pressure on the land. Second generation biofuels are developed to utilize lignocellulosic biomass as feedstock, which is biomass composed primarily of cellulose, hemicellulose, and lignin, which can be sourced from agricultural and forest residues, and energy crops. Third generation biofuels utilize algae and cyanobacteria as their feedstock, which allows them to avoid using arable land or fresh water. Fourth generation biofuels include the same concepts as the previous generation, but with the addition of using genetic engineering to theoretically create fuels that are carbon negative.

Due to the technical challenges of third and fourth generation biofuels that present major hurdles, this analysis will primarily focus on second generation biofuels, which includes biofuels synthesized through biological pathways, thermochemical pathways, and through the use of oil-based feedstocks. Second generation biofuels were born out of the desire to address the shortcomings of first generation biofuels, namely the competition with food crops. Although once touted as the future of biofuels and liquid fuels in general, second generation biofuels encountered a number of early hurdles which resulted in the industry stagnating and almost disappearing throughout the 2010s due to a variety of technical and economic challenges.

Feedstock for second generation biofuels can come from a variety of sources including agricultural waste such as corn stover and sugar cane bagasse, forest residues that are unused from logging operations and clearing for fire prevention, and energy crops such as switchgrass and miscanthus. The processing of lignocellulosic matter is generally more complicated than that of first generation feedstock. First generation ethanol production utilizes enzymes to break down starch into fermentable sugars that are found in high concentrations in crops like corn or directly in the case of sugar cane. Second generation feedstocks also contain large amounts of sugars in the form of cellulose and hemicellulose, but these are difficult to access. Plants and trees also have a natural resistance to enzymatic breakdown, so as to not deteriorate and break down in nature. Therefore, second generation biofuels require extensive pretreatment to make the sugars accessible to the enzymes. Additionally, lignocellulosic matter contains a large amount of the compound lignin, which is a natural polymer that provides structural support to plant cells. Breaking down lignin is considered as one of the major challenges of second generation biofuels.

Second generation biofuel pathways

Second generation biofuels can be produced through three main pathways:

Biochemical pathways: This pathway utilizes microorganisms and biological molecules to break down lignocellulosic matter into fermentable sugars that can then be fermented into alcohols such as ethanol.

Thermochemical pathways: This pathway utilizes temperature, sometimes in the presence of a catalyst and other molecules such as oxygen or water, to break down carbon-based matter into liquid or gaseous fuels. The two main forms in which this is achieved are via pyrolysis and gasification, both of which use heat and a controlled oxygen environment to produce fuels such as bio-oil, carbon monoxide, and hydrogen, which can then be further refined into drop-in liquid fuels.

Oil based feedstocks: This pathway utilizes sustainable fats and oils such as used cooking oil and oil from inedible seeds to produce biodiesel through the process of transesterification. The process is almost identical to the methods used to produce first generation biodiesel that have traditionally used edible vegetable oils such as soybean oil and palm oil.

Environmental Concerns

Although often touted as a 'sustainable', 'green', and 'renewable' energy source, biofuels carry some significant environmental concerns that are often overlooked. Like any industrial process, biofuels produce a host of greenhouse gases through regular means such as energy usage, transportation emissions, and any emissions associated with agriculture.

Indirect Land Use Change (ILUC)

Indirect land use change is a term used to describe the changes in land usage as a result of agricultural expansion. This is one of the major concerns of first generation biofuels, and has been one of the main reasons that they have been viewed as a failure at addressing climate change. To put it simply, land usage on Earth is a zero-sum game, meaning that if you want to grow a crop for a new use, you have to take away land from another crop, which in turn means that the displaced crop will have to move, and so on. This 'pushing out' of other crops can often lead to the need to clear forests or move into areas that were previously uncultivated for agricultural use. The clearing of land for agricultural usage is extremely detrimental to the environment, and the CO₂ emissions associated with it can wipe out any benefits gained from the biofuel. Indirect land usage change is a contentious topic, as it can be hard to calculate and quantify due to its complexity and cascading nature. Depending on the methodology of calculation, when indirect land usage change is included in calculations of lifetime CO₂ emissions, even for advanced biofuels, they can produce a net CO₂ emissions level that is higher than the fossil fuels they are replacing.

Soil Organic Carbon (SOC)

Soil organic carbon is a measure of the amount of carbon that is held within the soil. Under normal circumstances, soil acts as a carbon sink, meaning that it absorbs and traps carbon. This is done through a number of mechanisms, including microbial action along root systems and the deposition of dead plant matter into the soil, where it is further broken down and held in place. Changes to soil organic carbon are related to indirect land usage change, as the breaking up of uncultivated land for agricultural usage can lead to a decrease in soil organic carbon. Soil organic carbon losses become more relevant for second generation biofuels, as one of the more promising proposed feedstock options is agricultural residue that are usually left on the field. Current agricultural practices for tilled corn fields in the United States leave almost all of the corn residue on the fields to maintain carbon levels and protect the soil from erosion. Removing a large proportion of this residue can lead to more carbon being released from the soil.

The goal of biofuels

In order to calculate the feasibility of biofuels, as well as their viability in assisting in combating climate change, it is important to define a clear goal. Regardless of the negative press that biofuels have received recently, organizations like the International Energy Agency (IEA) that perform modeling for net zero emissions pathways state that widespread adoption of biofuels to achieve net zero goals is required. This is primarily due to the difficulty in decarbonizing hard-to-abate sectors that are resistant to standard forms of electrification.

Biofuels today are almost exclusively used for road-going transport, which is one of the sectors primed for rapid electrification. Due to this, further penetration of biofuels into the road transport sector is considered unnecessary for addressing sustainability goals. The industry most primed for biofuel usage is the aviation industry, where biofuels that are 'drop-in' replacements for fossil fuels are being developed, tested, and have already passed regulatory approval. Current regulations in the United States allow the usage of a 50% blend of synthetic kerosene derived from bio-ethanol in aviation fuel.

Therefore, the goal for this analysis is as follows:

"Achieve 50% of the United States' domestic aviation fuel needs utilizing ethanol-based sustainable aviation fuel (SAF) without the use of food crops."

In 2019, the annual jet fuel usage for the United States domestic aviation market was 26,061,000,000 gallons (98,645,513,010 L), meaning that annual production of sustainable aviation fuel would need to be 13,030,500,000 gallons (49,322,756,505 L). 2019 was chosen as the jet fuel consumption was greater than the years that followed, meaning that it is a better indicator of the future consumption levels that are expected to continue growing.

What are sustainable aviation fuels?

Sustainable aviation fuel can be generally defined as an alternative aviation fuel that reduces lifecycle greenhouse gas emissions compared to standard fossil fuels by utilizing renewable or waste-based feedstocks, while also not competing in land and water usage with food crops. The technical specification used to designate sustainable aviation fuels is known as ASTM D7566 (Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons). Sustainable aviation fuel does not necessarily need to be a type of biofuel. For example, although not explicitly covered in ASTM D7566, synthetic fuels (also known as e-jet) that utilize a feedstock of green hydrogen produced from renewable energy and carbon supplied from direct air capture (DAC) are also considered sustainable aviation fuels. Although synthetic fuels are theoretically the 'greenest' form of aviation fuel when the feedstocks are renewable, they are considered to be more of a promising future technology, rather than a short to mid term solution to sustainable air travel.

Due to feedstock consideration, the majority of sustainable aviation fuel pathways are based on organic materials, making them fall under the definition of biofuels. Biofuels can be produced through a number of different pathways and feedstocks, each with their own advantages and disadvantages. Currently, the sustainable aviation fuel that is the most cost-competitive, market-proven, and used on a commercial scale is Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene (HEFA-SPK). This process utilizes fats and oils as the main feedstock, such as used cooking oil, animal fats, and vegetable oils. However, due to the limited supply of used cooking oils (UCOs) and the need for virgin plant oils that directly compete with food crops, the scaling opportunities of this process are considered limited. Additionally, the method of HEFA-SPK production inherently requires large quantities of hydrogen which are currently sourced from fossil fuels, further limiting the scalability of the process.

The two most promising technologies that can make an impact on a short- to mid-term basis are Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK) and Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK) due to their large feedstock availability.

Alcohol-to-Jet Synthetic Paraffinic Kerosene (ATJ-SPK): The alcohol-to-jet process utilizes ethanol (or other alcohols such as isobutanol) as its main feedstock (preferably from a 2nd generation source, but also via fermentation of industrial waste gases). The ethanol undergoes the following processes: dehydration, oligomerization, hydrogenation, isomerization, and distillation, resulting in synthetic kerosene. One of the leaders in sustainable aviation fuel production is a company called LanzaJet, which utilizes an alcohol-to-jet pathway and is the first to produce sustainable aviation fuel in this form on a commercial scale globally.

Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK): The Fischer-Tropsch process utilizes syngas, a combination of carbon monoxide (CO), hydrogen (H₂), and sometimes carbon dioxide (CO₂), as its main feedstock, which then undergoes a catalytic conversion to produce synthetic kerosene. To supply syngas, a process called gasification is used, where a carbon-based feedstock such as biomass or municipal solid waste (MSW) is heated at high temperatures above 700°C in a controlled oxygen environment that does not allow full combustion of the material. Multiple Fischer-Tropsch plants have been built and operated in the United States to varying degrees of success.

The main consideration for choosing between ATJ-SPK and FT-SPK comes down to cost vs. complexity. The production of ethanol from second generation feedstocks is a complicated process requiring biomass cultivation and collection, pretreatment, enzymatic breakdown, and fermentation. Each of these steps can be costly and drawn out, but the complexity of each individual process is considered to be relatively low. In comparison, the Fischer-Tropsch process contains far fewer steps and is agnostic to the types of feedstocks used. However, due to the high temperatures required in the gasification reactors to produce syngas and the precise control of oxygen, waste buildup, and temperature levels that are all required to remain optimal, gasification reactor design and construction becomes significantly more complex. Because of these complications, many gasification plants (not including those that use fossil fuels as feedstock) have struggled to remain operational due to equipment failures, whereas second generation alcohol production has primarily been limited by its cost competitiveness with first generation alcohol production.

For the purpose of this analysis, the focus will be placed on the alcohol-to-jet process derived from second generation feedstocks due to it being the sustainable aviation fuel production pathway with the most potential and industry support as of 2025. This is not a denouncement of the Fischer-Tropsch process or a strong endorsement of alcohol-to-jet, but rather a recognition that the alcohol-to-jet process is showing the most promise in the short term.

Second generation (lignocellulosic) biofuels for alcohol production

The key challenge for the alcohol to jet process is the sourcing of sustainable ethanol, so although the last section focused on sustainable aviation fuels, the main focus of the remainder of this analysis will be on sustainable second generation biofuels, specifically lignocellulosic biofuels. As previously mentioned, the processing of lignocellulosic matter is complicated and requires multiple steps, specialized equipment, and bioproducts like enzymes and yeast. However, the main challenge for lignocellulosic biofuels is the sourcing of sustainable biomass feedstocks. Feedstocks for lignocellulosic biofuels that do not compete with food crops come in three main categories: agricultural waste, forest waste, and energy crops.

Agricultural waste and residues

Agricultural waste and residue is often considered the most desirable feedstock for second generation biofuels due to the fact that it is classified as a residue product and valorizing waste streams is a key part of the sustainability of biofuels. In the United States, the largest source of agricultural waste comes from corn production, known as corn stover, but also includes other commercial crop residues like wheat and soybean straw. Outside of the United States, large sources of potential feedstock can be sourced from sugar cane residues known as bagasse, which is produced on a large scale in Brazil. The main concern for agricultural waste and residues is in relation to indirect land use change and its influence on soil organic carbon. When harvesting grains like corn and wheat, the current practice for tillage farming is to leave all of the residue on the field to maintain soil health and prevent erosion. This limits the amount of agricultural residue that can be taken from the field as feedstock.

Forest and logging residues

The residues from logging operations, timber milling, and forest fire prevention have been considered as a potential feedstock for lignocellulosic biofuels. The United States contains a large amount of intensely cultivated forest land, primarily in the southeastern states, followed by the Pacific Northwest. The use of logging residues remains contentious, as proven by the intense opposition to large bioenergy plants (plants that burn timber for power generation instead of fossil fuels) and wood chip pellets that are exported to the European electricity and heating markets. Logging residues, also known as slash, can include residues such as tree tops, branches, and damaged trees. For large logging operations, slash can be collected and burned as a means of clearing the land for the next timber crop, otherwise some it is left on the field to decompose where it can provide benefits to soil health and the ecosystem. Forest residues are appealing as they are the other main feedstock option that has a large near-term feedstock potential. However, there are very genuine concerns related to obvious habitat destruction associated with logging, as well as the somewhat 'shaky' definition of what is considered a forest residue. In the past, many biomass bioenergy plants and wood chip pellet operations that claim to use forest residues have been exposed for simply using whole trees as feedstock.

General waste and residues

General waste and residues are another potential source of feedstock for 2nd generation biofuels. These wastes include products such as yard waste, clean wood from construction and demolition, paper mill residues, and other industrial waste. The key challenge for general waste and residues is collection and transportation, as well as guaranteeing that the waste is not contaminated with products that can be harmful in later processing steps. That being said, general waste and residues have a reasonable near-term potential as a feedstock.

Energy crops

Energy crops can be defined as any crop grown for the primary purpose of producing energy and co-products. Under this definition, corn produced for ethanol could be considered an energy crop, but this analysis will focus on the more specific definition of "dedicated energy crops" that are grown for the sole purpose of producing energy to meet the definition of a second generation biofuel. The most notable energy crops discussed in the context of biofuels are herbaceous crops like switchgrass and miscanthus, woody crops like willow and poplar, and seed oils from the jatropha plant, which are inedible due to their high levels of toxins. In addition to not being in direct competition with food crops, energy crops can contain a range of other desirable benefits to the land such as: the ability to grow on marginal or degraded land, the ability to improve soil health by increasing soil carbon content, low fertilizer requirements, and the ability to improve soil health by increasing soil carbon content. In some specific cases, the levels of soil organic carbon can be large enough to theoretically result in biofuels that are a net carbon sink. Many energy crops are also perennial, meaning they regrow after being harvested, which allows the soil to remain untilled. The benefits of energy crops is another contentious topic as they still present issues such as large-scale indirect land use change, water usage, and habitat destruction.

Switchgrass

As will be shown later on in the feedstock estimates section, the majority of biomass feedstock under any second generation biofuel pathway will largely consist of energy crops. Of the energy crops that are suitable for usage in the United States, switchgrass has been identified as the "model high potential energy crop" by the Department of Energy and has had over three decades of extensive research into its potential.

What is switchgrass and why is it desirable?

Switchgrass is a perennial grass that is native to the areas east of the Rocky Mountains in the United States. Because it is native, it is well adapted to the soil conditions, climate, hydrology, and pests of the region. Switchgrass is a high productivity crop in terms of above-ground biomass production, with yields generally greater than 5 tons per acre per year, but can go higher than 10 tons per acre per year. Switchgrass can be grown in degraded soil with low fertility however, in order to achieve the high yields that would make cultivation profitable, nitrogen fertilizer is still required, albeit at a lower rate than most 1st generation biofuel crops. The root systems of switchgrass are deep and extensive, which allows them to access nutrients and water that is deep in the soil, providing drought resistance. Compared to other agricultural crops, switchgrass has a lower impact on biodiversity, but if it is grown as a monocrop as would be the case required for biofuels, it can still result in a net negative outcomes compared to land that is left untouched. A key desirable trait of switchgrass is its ability to store carbon in the soil via its large root systems, which assists in reducing lifetime CO₂ emissions.

How is switchgrass grown and harvested?

Switchgrass can be grown from seed, which makes it more desirable than other energy crops such as miscanthus, which has to be grown from a rhizome (similar to ginger). Switchgrass generally requires a year to become established, but this can be extended due to issues such as weeds or pests that may require pesticides. Once established, switchgrass requires significantly less pesticides. Under a regular growing season, switchgrass can be harvested after the first year, after which it can be harvested once per year. Being a perennial crop, once established, switchgrass can remain productive for over 10 years, after which its productivity reduces and it should be removed and replanted. Although switchgrass can be grown in areas with degraded soil, nitrogen fertilizer is still required to achieve optimized yields, but the levels of potassium and phosphorus contained in most soil profiles are adequate. Switchgrass can be harvested using well established and commercially available equipment under similar methods to other grass crops such as alfalfa. After harvest, switchgrass needs to be baled so that it can be transported, stored, or further processed.

What are the issues with switchgrass

The cultivation of switchgrass inherently contains a number of issues that can reduce its sustainability as a biofuel feedstock. Switchgrass is often hailed for its ability to grow on degraded land, which some argue can offset the impacts of indirect land use change. However, many argue that the conversion of degraded land that is currently not productive into switchgrass does constitute a form of land use change. Opponents to this argue that switchgrass can help degraded land by increasing soil organic carbon and protecting the soil from erosion due to its large root system. Further, many correctly point out that if switchgrass cultivation is to be viable as a commodity crop, farmers will be resistant to only using marginal land as it will reduce yield and profitability. Another issue with switchgrass cultivation is its influence on soil organic carbon. Once established, it is true that switchgrass actively stores carbon in the soil, which can remain untouched throughout the 10-year lifecycle of the crop. After this, the switchgrass along with its large root system will need to be removed, which most likely will require tillage that can release much of the carbon that has been stored in the soil. Water usage is another concern with switchgrass cultivation, however, it is expected that irrigation will not be heavily utilized as farmers tend to use the costly process of irrigation for higher value crops. Lastly, habitat destruction is another significant issue for switchgrass, as the creation of a commercial market will encourage the conversion of low-density pasture and conservation lands into switchgrass cultivation.

Logistical Considerations

Switchgrass and other agricultural residues are only harvested once or twice per year, which can produce bottlenecks and logistical challenges in the supply chain that can reduce its viability as a feedstock. Baling is the most common option for collecting and storing switchgrass, as the equipment and methods are well established, but this can be inefficient due to storage losses, damage, and transportation issues. Additionally, the low bulk density of baled grasses introduces its own issues in the processing steps, as they can clog equipment due to lack of flowability. Flowability is the ability of a material to pass through a pipe or transportation system easily without blockage. Some researchers contend that the issues related to flowability and bulk volume are the main challenges for biomass processing. One method for addressing these issues is through pelletization, which can provide a flowable and easily stored form of biomass. Pelletization is the process of compressing a material into a small, dense, and flowable form. Pelletization is already used with some perennial grasses such as alfalfa, which can be transformed into pellet animal feed. Pelletization is also heavily used for the wood chip export market primarily for European heating and energy markets, with global production estimated to be over 60 million tons in 2025. Yet, pelletization itself introduces its own unique challenges to second generation ethanol production, as standard pelletization relies on lignin contained in the biomass to bind the pellets together, which can reduce the effectiveness of enzymatic breakdown in the alcohol production steps.

Feedstock and fuel production estimates

The major concern surrounding any biofuels industry is the sourcing of an environmentally sustainable and steady supply of feedstock biomass. Estimating feedstock availability is complex, as the amount of feedstock required is highly dependent on the efficiency of agricultural systems and overall biomass conversion. For example, if a biorefinery can convert 1 ton of lignocellulosic biomass into 200 L of biofuel, this would require half as much biomass as a biorefinery that can convert 1 ton of biomass to 100 L of biofuel to produce the same amount of ethanol. The same applies to agricultural efficiencies, where half as much land would be required for cultivating switchgrass at 10 tons per acre per year compared to 5 tons per acre per year. Equally important, the amount of biomass feedstock 'available' in the United States depends heavily on the asking price, as higher prices incentivize collection that would otherwise be unprofitable. Overall, the main takeaway from the analysis below is that biomass sourced from agricultural and forest land waste and residues cannot provide enough material to sustain a robust second-generation biofuels industry, thus requiring enormous quantities of energy crops to make up the shortfall. Therefore, evaluating the sustainability and feasibility of an advanced biofuels industry becomes a question of how much land are we willing to dedicate to biofuels.

What feedstock is readily available

Agricultural, forestland and general wastes and residues are the most appealing feedstock for second-generation biofuels as they are readily available (sometimes described as low-hanging fruit) for immediate use, potentially constituting the majority of biomass feedstock for an emerging biofuels industry. These materials are particularly attractive as they generally avoid inducing land-use changes that could negate biofuels' sustainability benefits. However, waste and residue supplies are constrained by the need to leave significant biomass amounts on the ground in fields and forests to maintain soil health and prevent erosion. See table 1 for the breakdown of near-term waste and residue biomass.

The combined near-term waste and residue biomass availability is estimated to be 179.17 million dry tons per year.

Table 1: Near-term waste and residue biomass availability

Near-term Waste and Residues	Quantity (million dry tons/year)
Agriculture	140.00
Forestland	30.17
General Waste	9.00
Total	179.17

Biomass and ethanol conversion yields

A key aspect for any biofuel pathway is the efficiency of converting the biomass into a liquid fuel. In the context of sustainable aviation fuels and more specifically, alcohol-to-Jet fuels, there are two key conversion steps. The first conversion step is from lignocellulosic biomass into alcohol (usually ethanol), and the second being the conversion of alcohol into jet fuel (synthetic kerosene). Higher conversion efficiencies are generally desirable as they would require less biomass and therefore less land to produce the same amount of fuel, but this would require additional breakthroughs in hemicellulose (pentose) conversion which cannot be fermented by naturally occurring yeast strains. See table 2 for the conversion yields for biomass to ethanol and ethanol to jet fuel.

Table 2: Conversion Yields for biomass to ethanol and ethanol to jet fuel

Conversion	conversion yields
Biomass to ethanol	280 L ethanol/ton dry biomass
Ethanol to jet fuel	2.0 L ethanol/ L jet fuel

How much sustainable aviation fuel can be produced from waste and residue biomass

The key question that appears when deciding whether to engage in an advanced biofuels industry that requires organic biomass is a question of land usage, i.e how much land is the country willing to allocate to the production of biofuels whilst weighing this against the environmental concerns such as indirect land use change. As will be shown, the biomass available from wastes and residues is not sufficient to meet the requirements of sustainable aviation fuel production to meet the goal of supplying 50% of the United States domestic aviation fuel needs. Therefore, the most viable option in the short to mid term is to utilize energy crops such as switchgrass to make up the shortfall.

Using the biomass availability and conversion yields, it is possible to calculate the amount of sustainable aviation fuel that can be produced from the available waste and residue biomass. Given the biomass availability and assumed conversion yields, it would be possible to produce approximately 13.3 billion gallons (50.2 billion L) of ethanol a year, which could then be converted into sustainable aviation fuel.

"The 179.17 million dry tons of biomass derived from near-term available waste and residues can produce approximately 6.62 billion gallons (25,083,800,000 L) of sustainable aviation fuel per year, just over half the amount required to meet the stated goal of meeting 50% of the United States domestic aviation fuel needs."

How much switchgrass (energy crops) are needed to make up the shortfall

The amount of switchgrass that is required to meet the shortfall in sustainable aviation fuel production is dependent on the agricultural land efficiency of the switchgrass cultivation. This can depend on a lot of factors such as the local climate, the region it is being cultivated in, and how much fertilizer is used. The factor that will have the most impact on switchgrass yields is whether it is grown on marginal land or productive land. Ideally, only marginal land would be used as it would minimize competition with other existing food crops, but the amount of land required would be larger. It should be noted that in a more rigorous analysis, a range of energy crops would be utilized depending on local climate. For example, miscanthus might be grown in the Midwest where it thrives, and energy cane might be grown in southern states.

To calculate the amount of switchgrass and the land required, two yield values are chosen, 3.65 dry tons per acre for marginal land and 10 dry tons per acre for productive land.

Table 3: Summary of switchgrass cultivation requirements

Land type	Yield (dry tons/acre/year)	Land required (acres)
Marginal	3.65	49,087,671
Productive	10	17,917,000

"Between 17,917,000 and 49,087,671 acres of land dedicated to switchgrass cultivation would be required to produce the 6.41 billion gallons (24,238,956,505 L) of sustainable aviation fuel to make up the shortfall in sustainable aviation fuel production."

To put this land usage into perspective, for marginal land usage this is roughly the size of the state of Nebraska, and for productive land usage roughly the size of West Virginia. Regarding marginal land availability, when defined as abandoned cropland, upper estimates are as high as 68 million hectares (168 million acres) in the United States. As another comparison, the area of productive land that is currently dedicated to the cultivation of corn for the production of ethanol lie around 30 million acres.

Other inputs for sustainable aviation fuel production

When considering biofuels, the main focus of inputs tend to be land and feedstock. However, as with all industrial processes, there is a significant amount of other inputs that are required in the production of sustainable aviation fuel and the ethanol required for its production. On the feedstock cultivation side, the primary inputs are fertilizer and water. For the ethanol production process, inputs can include water, energy, enzymes and yeast. And for the sustainable aviation fuel production, the main inputs are hydrogen and catalysts. Shown below in table 4 is a summary of the approximation of other inputs required for sustainable aviation fuel production (estimated for 49.3 billion L of jet fuel production). Note, the values presented represent a total requirement, but do not take into account that many of the inputs can be recycled and reused in the production process, meaning that the actual requirement may be lower.

Table 4: Other inputs required for sustainable aviation fuel production (estimated for 98.6 billion L of ethanol production and 49.3 billion L of jet fuel production)

Input	Process	Value (per year)	Units
Nitrogen	Switchgrass cultivation	1,625,430	Tons
Water	Ethanol production	591.9	Billion L
Water	Jet fuel synthesis	93.7	Billion L
Enzymes	Ethanol production (hydrolysis)	5,553,650	Tons
Yeast	Ethanol production (Fermentation)	892,167	Tons
Catalysts	Jet fuel synthesis	182,075	Tons
Hydrogen	Jet fuel synthesis (hydrogenation)	500,000 - 1.15 million	Tons

Processing infrastructure

Specialized infrastructure will be required to support the production of ethanol and its conversion into sustainable aviation fuel. Using data from existing 1st generation corn ethanol production infrastructure, it is possible to estimate the size and number of facilities that would be required. With regard to the infrastructure for the conversion of ethanol into sustainable aviation fuel, site data from the currently commercialized alcohol-to-jet facility "Freedom Pines" operated by LanzaJet in the state of Georgia was used; however, it should be noted that this facility is first of its kind and operates at a low capacity. If adequate support is provided to support an advanced biofuel industry in the United States, it is expected that many of the corn ethanol production facilities can be converted to process lignocellulosic biomass. Shown in table 5 is the approximate plant number required at three different plant capacities to meet an annual production capacity of 98.6 billion liters of ethanol, while table 6 shows the current makeup of corn ethanol production in the United States.

Table 5: Estimated number of ethanol biorefineries required (estimated for 98.6 billion L of ethanol production and 49.3 billion L of jet fuel production)

Plant Size	Daily capacity (dry tons biomass / day)	Annual capacity (millions of L)	Number of plants required
Small capacity	2,000	204	483
Medium capacity	4,000	409	241
Large capacity	6,000	613	161

Table 6: Estimated number of active corn ethanol production facilities in the United States

Annual capacity (millions of L)	Active plants
0 - 200	38
201 - 400	92
401 - 600	52
601 - 800	6
801+	6
Total	194

Boosting feedstock while reducing competition with food crops and land use changes

Sustainable biomass sourcing which minimizes competition with food crops and land use changes is the main challenge for a successful advanced biofuels industry. If those two issues are not addressed, the environmental and sustainability benefits gained from developing an advanced biofuels industry could be diminished to the point where it is not worth pursuing. Besides the main solutions previously discussed, use of waste and residues for feedstock and the use of energy crops, there exists a number of other indirect actions that can be taken to further minimize the negative impacts of biofuels. These include:

• Improving agricultural efficiency (higher crop yields per unit of land)
• Reducing food waste
• Conversion of pasture land to cropland for use as food crops or biomass for biofuels
• Reducing meat and dairy consumption per capita

Conclusion

Biofuels have experienced waves of support, high production, and utilization over the past century, but moving into the second quarter of the 21st century, it appears that interest in biofuels may be on the cusp of a new era of renewed interest with a focus on advanced biofuels. The initial push for advanced biofuels in the early 2010s was a resounding failure due to a combination of factors, but these failures can provide lessons for future advanced biofuels industries. The utilization of biofuels offers an alternative to fossil fuels in areas that are resistant to electrification and in industries that require fossil fuels for the synthesis of specific high-value chemicals. It is broadly agreed that second-generation biofuels that utilize wastes, residues, and correctly managed energy crops produce less lifetime greenhouse gas emissions when compared to fossil fuels. However, this claim can be disputed depending on how indirect land use change is accounted for. As shown in the analysis above, the utilization of waste and residue biomass as a feedstock for biofuels has the potential to produce approximately 13.3 billion gallons of ethanol per year which could then be converted into 6.62 billion gallons of sustainable aviation fuel, approximately a quarter of the United States' domestic aviation fuel needs. Because of this, tracts of land between 17.9 and 49.1 million acres depending on the land's productivity would be required to produce the sustainable aviation fuel to meet the goal of supplying 50% of the United States' domestic aviation fuel needs. Land usage could be reduced by increasing the conversion yield of lignocellulosic biomass to ethanol, primarily via increasing hemicellulose (pentose) fermentation yields. Due to this enormous land use requirement and the well-documented environmental damage that high-intensity agriculture incurs, it is unclear whether there is value in pursuing an advanced biofuels industry at a large scale. However, the targeted usage of advanced biofuels such as those that only utilize wastes and residues can still assist in decarbonization on a smaller scale so long as the focus remains on sustainable production.