5.1 Introduction

Climate change is a global challenge to be addressed by governments and industries around the world. It is estimated by the Intergovernmental Panel on Climate Change (IPCC) that by the year 2100 the world will see a temperature change of 2.5–7.8 °C above the average for the years between 1850 and 1900 (Bernstein et al. 2007). The aviation industry contributes approximately 2% of total carbon emissions across all industrial sectors and air transportation growth is increasing rapidly due to globalization of trade and travel. Aviation has no credible near‐term fuel alternative in ethanol or electric power due to the energy density required although aviation fuel can be used in fuel cells to produce electricity locally in dispersed applications. Current jet fuel production processes require huge facilities that are complex to operate while using fossil oil as feedstock. Carbon emission reduction in the aviation industry is essential to combat global warming. The aviation industry has achieved several milestones in emission reduction including improved aircraft fuel efficiency and better air traffic control to promote safe, efficient and sustainable air travel. However, these improvements have led to carbon emission reductions of less than 15% (The global aviation industry 2010). Compared to these non‐fuel approaches, the use of alternative jet fuel can achieve a further 50–80% reduction in carbon emission and is, therefore, considered to be the most efficient way to achieve carbon neutral aviation operation. Thus, addressing the source of fuel for the aviation industry is an important part of the answer to achieving a material reduction in greenhouse gas emissions.

Demands from regulatory agencies for compliance with international carbon emission standards have increased. Domestic interests in biojet fuel powered planes have also been rising. Both of these factors are driving the growth of biojet fuel markets. By 2028, the carbon emission standards for international flights will be regulated, requiring airlines to purchase carbon offsets to compensate for growth in their emissions. This has led to a sense of urgency for airlines and their shareholders, resulting in their encouragement of technologies that are likely to bring new biojet fuel products to market. Although the research, development, and commercialization of biojet fuels are still in the early stages, several activities have been initiated by a number of stakeholders in the past 10 years, including the newly approved by American Society for Testing and Materials (ASTM) biojet standards, biojet fuel development community formation, fuel certification test flights, and off‐take fuel agreements between airline companies and early‐stage biojet fuel suppliers (Radich 2015).

The US jet fuel market represents 20 billion gallons per year. All emerging biojet fuel technologies that meet ASTM standards are included in this number. However, as of this date, no clear winning sustainable jet fuel technology exists, a situation that encourages continued research and development in innovative biojet fuel technologies. For biomass‐based biojet fuels, as in the case of biofuels for ground transportation, one of the most important deciding factors in commercial feasibility is having accessible, low‐cost feedstock. This presents a challenge because, unlike crude oil, biomass contains a high level of oxygen (up to 45%) in its major heteropolymers. Biojet technologies have made progress in dealing with these diverse biomass substrates, but more effort is still needed to improve the biomass conversion efficiencies and to reduce the cost of production.

The aim of this chapter is to provide an overview of current opportunities for the development of biojet fuels and highlight the reasons behind the burgeoning efforts. Information gathered through interviews with parties from the private sector and government agencies indicated a growing interest in purchasing and investment for biojet fuels. This chapter also provides a brief summary of the current biojet fuel technologies, and the status of several specific challenges facing the industry is explained, which help the reader to better understand the overall landscape of biojet fuels.

5.2 Overview of the Jet Fuel Market

The current US market for jet fuel is dominated by large petroleum‐based refineries that produce a total of approximately 23.6 billion gallons of jet fuel per year (Figure 5.1). Companies such as Exxon Mobil (Irving, TX, USA), Chevron Corporation (San Ramon, CA, USA), BP (London, UK), Valero (San Antonio, TX, USA), and Marathon Petroleum Group (Findlay, Ohio, USA) account for around 50% of that production volume. The global consumption is about four times that much (Indexmundi 2017; US Energy Information Administration 2017a). The domestic jet fuel consumption is expected to increase by more than 40% between 2016 and 2040 as projected by the Energy Information Administration (EIA) (US Energy Information Administration 2017a) because the increased demand for air travel will more than offset projected aircraft efficiencies gained.

5.2.2 Biojet Fuel Types and Specifications

Currently there are five main biojet fuel technologies that have been developed and approved by ASTM as meeting the standard specifications ASTM D7566 (ASTM 2014), and so can be blended with Jet A or Jet A‐1 fuel certified to Specification D1655 and 16 certifications in preparation (International Air Transport Association 2009):

1. Fischer–Tropsch Synthetic Paraffinic Kerosene (FT‐SPK)
2. Fischer–Tropsch Synthetic Kerosene with Aromatics (FT‐SKA)
3. Hydrotreated Esters and Fatty Acids (HEFA)
4. Synthesized Iso‐Paraffinic (SIP)
5. Alcohol (isobutanol) to Jet Synthetic Paraffinic Kerosene (ATJ‐SPK)

Five more biojet fuel technologies are under review as listed in the following:

1. High Freeze Point Hydrotreated Esters and Fatty Acids (HFP‐HEFA)
2. Virent BioForm Synthesized Aromatic Kerosene (SAK) Jet Fuel (SAK)
3. LanzaTech ATJ‐SPK (Ethanol to Jet)
4. Applied Research Associates Catalytic Hydrothermolysis Jet (ARA‐CHJ)
5. BioForm^® Synthesized Kerosene (SK) Jet Fuel (Virent SK)

Data is being collected for three additional technologies including ATJ‐SKA developed by Byogy (Byogy Renewables, Inc., San Jose, USA), Swed Biofuels (Swedish Biofuels AB, Stockholm, Sweden), and the IH² demonstration scale by Shell (Royal Dutch Shell, UK) which acquired the technology from the Gas Technology Institute (GTI) (Des Plaines, USA) in 2009.

Eight more technologies, listed below, are in exploratory discussions. Figure 5.2 shows some of the technologies that are currently being studied.

1. Vertimas. One‐step catalytic conversion of ethanol to jet, petrol, diesel fuel and chemicals which was originally invented at Oak Ridge National Laboratory.
2. SBI bioenergy. Continuous catalytic process that converts fat, oil or grease into renewable gasoline, diesel and jet fuel with proprietary Process intensification and Continuous Flow through Processing technologies (PICFTR).
3. Joule. CO₂‐derived fuels ‘Sunflow‐J’ jet fuel from specially engineered photosynthetic bacteria, waste carbon dioxide, sunlight and water.
4. Global bioenergies. Biological production of isobutene and process to jet.
5. Eni. hydrogenated vegetable oil (HVO).
6. Enerkem. Municipal waste gasification and catalytic conversion to ethanol followed conversions to biofuels and chemicals.
7. POET. Agricultural waste derived alcohol to jet fuel.
8. Washington State University. Lignin to Jet Fuel (LJ‐D&HDO) through one‐step proprietary catalytic upgrading of lignin waste to jet fuel.

As shown in Figure 5.2, biojet fuel technologies use a broad range of feedstocks such as lignocellulosic biomass, municipal solid wastes, vegetable oils, animal fats, lignin, flue gas, and even seawater. Current biojet fuel technology pathways have been summarized by referring to industry publications, academic research and technology reports (Fahim et al. 2009; Wang and Tao 2016; Wang et al. 2016b; Yang and Laskar 2016). Information has also been gleaned from biofuel development companies and institutions (Biochemtax 2017; LanzaTech 2017; Fellet 2016; US Naval Research Laboratory 2016; Virent 2017; WSU Maegan Murray 2016). Compared to current oil refineries, most of the biojet fuel technologies require more than four functional process units, which is likely to be cost intensive, requiring capital, energy, labor, and water to commercialize (Bridgwater 1975; Gerrard 2000). Among the biojet fuels, the lignin‐based jet fuel technology is one of the most promising, and potentially most sustainable, due to its one‐step process conversion. By 2022, it is projected that 62 million tons of lignin side‐product will be generated annually by cellulosic biomass biorefineries (Ragauskas et al. 2014). The pathways of lignin based jet fuel include depolymerization and hydrodeoxygenation (LJ‐D&HDO) of lignin to a mixture of long‐chain hydrocarbons (C7–C18 cyclic hydrocarbons) that can be made into jet fuel. (Laskar et al. 2014; Laskar and Yang 2012; Laskar et al. 2013; Wang et al. 2015, Wang et al. 2016a; Wang et al. 2017a,b,c; Yang and Laskar 2016) Scaling this process and putting it into production alongside current biorefinery production facilities would significantly improve biomass conversion efficiency and the economics of biofuels and chemicals production.

In discussions of capital cost savings, the co‐processing of biomass by co‐feeding of biomass feedstock with petroleum streams in conventional oil refinery units has received attention (Al‐Sabawi et al. 2012; Huber and Corma 2007; Yanik et al. 2012). Recently, an oil refinery raised great interest by co‐processing biomass and crude oil distillates for instance. Chevron Corporation (San Ramon, CA, USA), Phillips 66 (Houston, USA) and BP (London, UK) are researching the feasibility of co‐processed fuels up to 5 vol% bio‐oils from HEFA and middle crude oil distillates to produce hydrocarbons that could be used in jet fuel (Joseph Sorena and Clark 2015). The compatibility of biojet production united with existing infrastructure motivated the interests in co‐feeding of the biojet feedstock/intermediates with crude oil‐based distillates. Figure 5.2 shows several potential research opportunities in co‐feeding desired by oil refiners.

In 2009, ASTM International Committee on Petroleum Products and Lubricants issued ASTM Standard Specification D7566, titled as Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbon, to certify drop‐in jet fuel from alternative feedstock. It contains five Annexes that define the specifications of the five biojet fuels blended with conversional jet fuel certified by D1655 ASTM standard. In Table 5.1, the specifications of the five approved biojet fuels are presented. The rest of the specifications of biojet fuel pathways in Figure 5.2, such as syngas fermentation and CH, are still under development. The blending limit of each biojet fuel is different. The FT‐SPK, HEFA, and SIP biojet fuels were approved for higher blending feasibility testing (ASTM Committee 2017).

ASTM D7566‐16b
Jet fuel tests		Jet A/A‐1	FT‐SPK	FT‐SKA	HEFA	SIP	ATJ‐SPK
Approved blend, %		100	50	10	50	50	30
Composition
Acidity, total (mg KOH/g)	Max	0.1	0.015	0.015	0.015	0.015	0.015
Aromatics (vol %)	Max	8–25	0.5	20/21.2	0.5	0.5	0.5
Cycloparaffins, mass %	Max		15	15	15		15
Saturated hydrocarbons, mass %	Min					98
Farnesane, % mass	Min					97
Carbon and hydrogen, mass %	Min		99.5	99.5	99.5	99.5	99.5
Nitrogen, mg kg−1	Max		2	2	2	2	2
Water, mg kg−1	Max		75	75	75	75	75
Sulfur, mg kg−1	Max	0.3	15	15	15	2	15
Metalsa), mg kg−1	Max		0.1 per metal	0.1 per metal	0.1 per metal	0.1 per metal	0.1 per metal
Halogens, mg kg−1	Max		1	1	1	1	1
Hexahydrofarnesol	Max					1.5
Olefins, mgBr2 per 100 g	Max					300
Contaminants
Existent gum, mg per 100 ml	Max	7		4	7
FAME, ppm	Max				5
MSEP	Min			90
Volatility
Distillation temperature, °C
10% recovered, temperature (T10)	Max	205	205	205	205	250	205
Final boiling point, °C	Max	300	300	300	300	255	300
Distillation (D86) T90‐T10, °C	Min		22	22	22	5	21
Flash point, °C	Min	38	38	38	38	100	38
Fluidity
Freezing point, °C	Max	−47 Jet A‐1 −40 Jet A	−40	−40	−40	−60	−40
Density @ 15 °C, kg l−1		775–840	730–770	755–800	730–770	765–780	730–770
Viscosity −20 °C, mm2 s−1	Max	8.0
Combustion
Energy density, MJ kg−1	Min	42.8			44.1	43.5
Thermal stability
Temperature, °C		325	325	328	325	355	325
Filter pressure drop, mmHg	Max	25	25	25	25	25	25
Additives
Antioxidants, mg l−1	Max	24	17–24	17–24	17–24	17–24	17–24

^a Metal includes Al, Ca, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Pd, Pt, Sn, Sr, Ti, V, Zn.

The US civilian jet fuels are mainly categorized into Jet A‐1, Jet A, and Jet B, while the US military sets its own standards, which are JP‐1, JP‐2 to JP‐10, each for specific applications. Jet A‐1 is similar to JP‐8 and Jet B is similar to JP‐4. Biojet fuels imitate Kerosene‐type fuels, which have a carbon number between 8 and 16, for example Jet A, Jet A‐1, JP‐5 and JP‐8. While a lot of interest in biojet fuels has been generated, so far there have been little in the way of commercial products available in the market because research and development in biojet fuels is still in the early stages.

Notably, Jet A/A‐1 and the five approved biojet fuels differ in composition. The energy content per unit weight increases in the following order; aromatic, naphthenic, paraffinic for hydrocarbons with the same carbon number (Hemighaus et al. 2006). As shown in Table 5.1, although biojet fuel possesses more cycloparaffins than paraffins, it remains hard to compare fuel efficiencies between biojet fuel and conventional jet fuel due to variations in hydrocarbon profiling. Whether the composition of biojet fuel, especially the new contaminants that arise in these processes, such as FAMES and Halogens, could cause problems to the propulsion and energy system of an airplane engine needs to be confirmed by engine manufacturers (FAA Aviation Rulemaking Advisory Committee 1998). Biojet fuels with low sulfur content have been achieved, which reduces aviation impact on air pollution (Hileman et al. 2009). Jet A/A‐1 has a minimum requirement of 8% vol of aromatics, so the lack of enough aromatics in biojet poses a theoretical risk of losing proper functions of elastomeric materials in jet engines (Karatzos et al. 2014). However, the performance characteristics of biojet fuels can be adjusted by additives, which is generally required to be compatible with aircraft operations.

Hydrocarbons that can be used in jet fuel can be categorized as: paraffins, olefins, naphthenes, and aromatics (Hemighaus et al. 2006). Hydrocarbons in jet fuel are mixtures of compounds from each of these classes that contain different numbers of carbon atoms. Since biomass and crude oil are both converted to a mixture of hydrocarbon compounds, taking a look at the elemental profiling of various biomass and crude oil compositions is an efficient way to understand the different chemical compositions of jet fuels and conversion efficiency in biomass and crude oil refineries (Table 5.2) (Baedecker et al. 1993; Bilba et al. 2013; Canoira et al. 2008; Valkenburg et al. 2008). For example, the oxygen element in biomass is much more than that of crude oil, which requires more energy input to effectively remove excess oxygen and produce hydrocarbons consisting of only carbon and hydrogen atoms. This is one of the reasons why hydrogen hydrotreating is needed in nearly all biojet fuel conversion pathways, and the cost and availability of these industrial processes are considered a risk in the research and development of biojet fuel (National Academies of Sciences, Engineering, and Medicine 2016). What is more, understanding the different element compositions is extremely useful for estimating the potential abundance of air pollutants that are emitted during fuel combustion.

Raw materials, % dry weight	Crude oil	Coal	Wood waste	Switchgrass	Sugarcane bagasse	MSW	Animal fat /vegetable oil
Carbon	84–87	73.6	48.0	46.7	48.6	33.9–84.8	76.6
Hydrogen	11–14	4.7	5.5	5.9	6.3	1.72–15.16	12.0
Oxygen	0–2	5.1	39.1	39.0	45.1	15.8–43.7	12.4
Nitrogen	0–1	1.3	1.4	0.54	0.48	0.12–2.37	2.2
Sulfur	0–3	1.6	0.1	0.13	0.21	0.06–1.4	0.15
Ash mineral	0.1–0.2	13.7	5.9	7.7	3.7	4.4–44.2	0.1–3.7

5.3 Assessment of Environmental Policy and Economic Factors Affecting the Aviation Industry

Aviation should be environmentally sustainable, cause minimal pollution to air and water, and contribute to high quality human life. More and more stringent environmental regulations have been enforced to fight against global warming, affecting the worldwide efforts to reduce carbon footprints. Renewable biojet fuels are significant in such efforts because of the increasing demand for jet fuels.

5.4 Current Activities Around Biojet in the Aviation Industry

Biojet fuels remain the only true alternative for the commercial aviation industry and the military, both facing ambitious near‐term greenhouse gas reduction targets. Thus, the aviation industry is more interested in the biojet fuels as many offtake agreements have been made even though biojet fuels are not commercially available yet. These agreements of taking millions of gallons of biojet fuels by airlines shows a strong commitment towards a green aviation industry. Many test flights have been executed by biojet shareholders in order to enter the jet fuel market.

5.5 Challenges of Future Biojet Fuel Development

As highlighted above, feedstock is a critically important contributor to the cost of biojet fuel and its availability and price are an indicator of the commercialization feasibility of each of the biojet fuel technologies. It remains a question whether or not the currently available biomass is sufficient to meet the jet fuel demand. The Department of Energy Biomass Billion Ton report used a simulated price of $40–60 per dry ton for some biomass feedstock. The energy crop cost is about $650–890 per acre (Downing et al. 1998). Table 5.7 shows the availabilities and prices of each biomass feedstock that can be converted to biojet fuel through various pathways (Langholtz et al. 2016; Ragauskas et al. 2014). The production yields of each biojet fuel technology varied in different research studies (Wang et al. 2016b). The cost of crude oil is $46.10 per barrel as of June 2017, which is equivalent to $319 per ton, calculated by converting gallons of oil to barrels by dividing the volume by 42 gallons per barrel and the specific gravity of crude oil is assumed to be 915 kg m⁻³. Compared to the $319 per ton crude oil, animal fat and vegetable oil as well as algae‐derived lipids are much more expensive (Table 5.7). As of June 2017, jet fuel is priced at $1.29 per gallon which is equivalent to $375 per ton. Thus, the final product crude‐derived jet fuel is much cheaper than the starting raw materials of vegetable oil, animal fat and algae‐derived lipids, making the economics of converting these materials into biojet fuel impractical.

An estimation of the US biojet fuel production capacity from biomass is shown in Table 5.7; it reveals that 4–66.5 billion gallons of biojet fuel per year can be produced according to the biomass availability illustrated in DOE's Biomass Billion Ton report. It indicated that the US has the capacity to produce enough biojet fuel to meet the annual demand of jet fuel if all biomass were used in biojet fuel conversions. However, the biojet fuel production has to compete with ethanol and biodiesel production for biomass feedstock. The ethanol mandate is about 15 billion gallons per year since 2010 while the biodiesel mandate is 1–2 billion gallons per year 2011 to 2017 (US Environmental Protection Agency 2017). For example, the mandates for biodiesel and ethanol in 2016 were 1.90 and 18.11 billion gallons per year, respectively. If the conversion efficiency (gallons per ton) of biomass to ethanol and biodiesel are assumed to be 85 gallons per ton and 267 gallons per ton, 220 million tons biomass would be used in ethanol and biodiesel production. The biojet fuel production would lose about 12 billion gallons per year if 55 gallons per dry ton conversion efficiency is assumed. Thus, the feedstock production and distribution system needs to be well established to ensure an appropriate logistics of biomass feedstock.

The cost of biojet fuels remains a challenge, especially when the cost of crude oil is as low as $46.10 per barrel and the cost of jet fuel is about $1.29 per gallon as of June 2017 (US Energy Information Administration 2017c). It has been reported that the minimum selling price of alternative jet fuels can range from $2–24 (Staples et al. 2014). Biojet fuels can achieve cost‐competitiveness with crude oil‐based jet fuel if crude oil is at least priced at $120 per barrel (National Academies of Sciences, Engineering, and Medicine 2016). The operational cost of biojet fuels is currently high and numerous technical challenges remain. To achieve cost‐competitiveness, biojet fuels will need more research and development effort. Also, it is costly and time consuming to scale up biojet fuel technologies, which is reported to range from US$20 to 50 million over ten to fifteen years (Philippe Novelli 2013). Industrial partnerships are therefore crucial in the scale‐up and demonstration of the viability of drop‐in bio‐based fuels.

In addition, the certification and approval process of biojet fuels takes a long time. One of the reasons may be attributed to the involvement of several functional departments specializing in the certification of biojet fuel. The qualification process starts with Tier 1 which tests fuels for specification properties. Tier 2 focuses on the fit‐for‐purpose properties. Tiers 3 and 4 involve aircraft manufacturers and airlines, including component/rig testing and engine/APU testing. The FAA and ASTM participate heavily in the process (Rumizen 2016). While the first phase of the jet fuel certification is relatively straightforward, which requires 100 gallons of fuel and costs nearly half a million dollars, the requirements for the second phase can vary widely. So far, testing for these phases have required from 30 000 to 200 000 gallons of fuel and have run up costs surpassing $3 million (Rumizen 2017). In addition, there is some level of uncertainty inherent in the certification process.

Private companies are actively involved in biojet fuel commercialization activities (Mawhood et al. 2016). Biojet fuel development is still in the early stages, requiring a lot of effort to overcome challenges. Support from well‐established industrial sectors such as oil refineries and engine or aircraft manufacturers is important for the overall success in biojet fuels. Besides industrial efforts, many federal organizations and agencies have collaborated together in developing alternative jet fuels, including the Department of Agriculture (USDA), Environmental Protection Agency (EPA), Department of Transportation (DOT), Department of Commerce (DOC), Department of Defense (DOD), Department of Energy (DOE) and the National Science Foundation (NSF) with projects such as the Defence Production Act (DPA) and Farm to Fleet (Aeronautics Science and Technology Subcommittee 2016). However, there is still a lack of government incentives for the development of renewable aviation fuel. The only credit biojet fuels can claim is the Renewable Fuel Standards (RFS) (US Environmental Protection Agency 2005), but there is no mandate within RFS that applies to jet fuels (Team‐CAAFI, Reasearch and Development Team 2014). This is an important consideration as it means that the aviation industry is not obligated to use biojet fuels, albeit biojet fuels can generate renewable fuel credits to meet RFS mandates for ground transportation.

5.6 Perspectives

Sustainable aviation fuel is the only option for the airline industry in the future. The mitigation of greenhouse gas emissions in the aviation industry is crucial to combat global climate change. A broad range of renewable alternative jet fuels, also known as biojet fuels (also referred to as biojet), are in development as drop‐in fuels that possess performance characteristics and chemical compositions essentially identical to conventional kerosene jet fuels. Most of the technologies to produce biojet fuels are still in the early stages of research, development, and certification. Notwithstanding this early stage, biojet fuel technologies are the most promising options for alternative energy sources for the airline industry. They present both short‐ and long‐term solutions for replacing crude oil‐derived jet fuels. For example, techno‐economic analyses have shown that a corn stover ethanol plant with an annual capacity of 57.2 million gallons of ethanol would be able to produce an additional 20 million gallons of lignin‐based jet fuel if the catalytic process is applied to upgrade the waste lignin stream (Shen et al. 2019). Results indicated that coproduction of jet fuel from waste lignin can dramatically improve the overall economic viability of an integrated process for corn stover ethanol production. Lignin‐derived jet fuel would offer unique advantages compared to the above‐mentioned six varieties of biojet fuels: (i) uses low cost raw materials without conflicting with food or other biofuel production, (ii) has higher thermal stability, (iii) has higher energy density, (iv) is produced at a lower cost, and (v) reduces greenhouse emissions (Ruan et al. 2019). However, it is also important to realize that the commercialization of new technology presents difficulties even greater than those that have already been overcome in the past for alternative biojet technology development, as well as tremendous dedication, persistence, and financial strength are required to clear this last remaining hurdle. Furthermore, the aviation industry is facing an increasingly stringent regulatory compliance environment, mainly from international carbon emission regulations associated with quantifying emissions and payment of renewable fuel credits. There are both environmental and economic benefits for aviation and biojet fuel groups to be engaged. In about 10 years, commercial airlines will have mandatory and legally‐binding international rules to comply with. It is thus in the interest of all the stakeholders to invest in biojet fuel development now, since biojet fuel technologies will take a long time to be fully developed and be producing at a commercial scale, and to significantly penetrate the conventional jet fuel markets. Many challenges remain, including the availability and price of feedstock, conversion efficiency technical challenges, the tedious and cost‐intensive fuel certifications, and government regulatory factors. These uncertainties and challenges, combined with decreasing oil reserves and volatile fuel pricing, justify strategic alliances for biojet fuel stakeholders to collaborate and contribute to the advancement of viable alternatives to conventional jet fuel.

Acknowledgments

This work was supported by the National Science Foundation I‐Corps #1655505, Sun Grant‐U.S. Department of Transportation (DOT) Award # T0013G‐A‐Task 8, and the Joint Center for Aerospace Technology Innovation with the Bioproducts, Science & Engineering Laboratory and Department of Biological Systems Engineering at Washington State University.

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