Heyne, J., Rauch, B., Le Clercq, P., & Colket, M. (2021). Sustainable aviation fuel prescreening tools and procedures. Fuel, 290, 120004.
This paper outlines the benefits and procedures for prescreening Sustainable Aviation Fuel (SAF) candidates before entering the official ASTM D4054 evaluation process. Specific properties are identified, that if not met, may result in extensive and costly efforts to correct if not recognized until later in the fuel development process. Hence, an approach with specific techniques that use low fuel volumes is suggested that enable (1) early estimates of critical properties and subsequently (2) direct measurement of these properties to guide fuel processing development prior to formally entering the ASTM evaluation process. The process is demonstrated with two exemplary candidate fuels.
Tanzil, A. H., Brandt, K., Wolcott, M., Zhang, X., & Garcia-Perez, M. (2021). Strategic assessment of sustainable aviation fuel production technologies: Yield improvement and cost reduction opportunities. Biomass and Bioenergy, 145, 105942.
The aviation industry has been studying strategies to produce sustainable aviation fuels (SAF) for over ten years. Our objective is to conduct detailed techno-economic analyses (TEA) of six SAF production technologies and to develop a simplified cost estimation method. Triglyceride based Hydroprocessed esters and fatty acids (HEFA) was compared against five lignocellulose-based technologies using standardized criteria. TEA was conducted to determine minimum fuel selling price (MFSP). The base case annual product capacity was fixed at 60 million liters of total fuel, for which SAF MFSPs ranged from 0.88 to 3.86 $ L−1 of fuel. Triglyceride-based HEFA had the best economic performance. Although triglycerides are more expensive than lignocellulose, HEFA is still a very competitive technology due to its high fuel yield (86–91% of feedstock) and low MFSP. Lignocellulosic-based technologies have lower fuel yields (9–23% of feedstock) due to high oxygen content of initial feedstock, resulting in higher fuel cost per unit of fuel. In order to reach a market fuel price, SAF yields from lignocellulosic materials needs to achieve an estimated value of 60%. Such yields are only possible if carbon efficiencies are close to 100%. Therefore, efforts are needed to avoid the removal of oxygen as CO2. Based on these considerations, a new scheme is proposed for SAF production that could result in yields near those needed to achieve cost targets. This proposed integrated biomass/natural gas hybridized concepts to produce inexpensive SAFs should be thoroughly investigated in future work.
Huq, N. A., Hafenstine, G. R., Huo, X., Nguyen, H., Tifft, S. M., Conklin, D. R., … & Vardon, D. R. (2021). Toward net-zero sustainable aviation fuel with wet waste–derived volatile fatty acids. Proceedings of the National Academy of Sciences, 118(13).
With the increasing demand for net-zero sustainable aviation fuels (SAF), new conversion technologies are needed to process waste feedstocks and meet carbon reduction and cost targets. Wet waste is a low-cost, prevalent feedstock with the energy potential to displace over 20% of US jet fuel consumption; however, its complexity and high moisture typically relegates its use to methane production from anaerobic digestion. To overcome this, methanogenesis can be arrested during fermentation to instead produce C2 to C8 volatile fatty acids (VFA) for catalytic upgrading to SAF. Here, we evaluate the catalytic conversion of food waste–derived VFAs to produce n-paraffin SAF for near-term use as a 10 vol% blend for ASTM “Fast Track” qualification and produce a highly branched, isoparaffin VFA-SAF to increase the renewable blend limit. VFA ketonization models assessed the carbon chain length distributions suitable for each VFA-SAF conversion pathway, and food waste–derived VFA ketonization was demonstrated for >100 h of time on stream at approximately theoretical yield. Fuel property blending models and experimental testing determined normal paraffin VFA-SAF meets 10 vol% fuel specifications for “Fast Track.” Synergistic blending with isoparaffin VFA-SAF increased the blend limit to 70 vol% by addressing flashpoint and viscosity constraints, with sooting 34% lower than fossil jet. Techno-economic analysis evaluated the major catalytic process cost-drivers, determining the minimum fuel selling price as a function of VFA production costs. Life cycle analysis determined that if food waste is diverted from landfills to avoid methane emissions, VFA-SAF could enable up to 165% reduction in greenhouse gas emissions relative to fossil jet.
Yang, Z., Kosir, S., Stachler, R., Shafer, L., Anderson, C., & Heyne, J. S. (2021). A GC× GC Tier α combustor operability prescreening method for sustainable aviation fuel candidates. Fuel, 292, 120345.
Fuels derived from sustainable feedstocks have significant potential to reduce greenhouse gas emissions associated with aviation. These alternative aviation fuels need to meet comprehensive safety and compatibility requirements established by an ASTM committee to achieve approval. To date, only seven annexes that define the property and compositional requirements for alternative fuels have been approved due, in part, to the significant cost and time required for evaluation and approval. Here, a Tier α sustainable aviation prescreening tool has been developed to predict the acceptability of novel SAF candidates, with the minimal volume required. Two-dimensional gas chromatography was used to determine the composition and size distribution of 20 alternative fuels, and Monte Carlo sampling was subsequently performed with a hydrocarbon database to predict eight properties that are important for combustor operability. These predicted properties were compared to experimentally measured properties, and uncertainty quantification was performed to validate the Tier α prescreening tool. The result of this validation suggests that four out of eight property confidence intervals are valid. Untuned predictions for properties with lower property variance across a given hydrocarbon type and carbon number, such as density and flash point, perform better than properties with significant property variance, such as freeze point and viscosity. The Tier α evaluation is outside of the ASTM evaluation process; however, this tool can provide feedback on downstream approval issues and facilitate fuel producer chemical process modeling thereby de-risking technology development. Thus, Tier α provides an early, rapid, and cost-effective evaluation for SAF candidates that requires minimal material volumes.
Petersen, A. M., Chireshe, F., Okoro, O., Gorgens, J., & Van Dyk, J. (2021). Evaluating refinery configurations for deriving sustainable aviation fuel from ethanol or syncrude. Fuel Processing Technology, 219, 106879.
This study presents novel comprehensive comparisons of alternative refinery configurations for sustainable aviation fuels (SAF) from bio-ethanol or Fischer-Tropsch bio-syncrude through techno-economic evaluations. Simulations were conducted in Aspen Plus, followed by evaluating the minimum aviation-fuel selling price (MINAFSP) at specific feedstock price, and the maximum tolerable prices of ethanol or syncrude at incentivised SAF prices. FT-syncrude refining scenarios considered (i) a basic refinery, (ii) incorporating synthetic aromatics, (iii) incorporating synthetic alkanes, and (iv) combined technology. Scenarios for refining bio-ethanol to higher alkanes considered (i) the base Heveling Process, (ii) Hybrid Process, (iii) the patented PNNL process, and (iv) the pre-upgrading of ethanol to butanol. Thus, the basic refining of syncrude had the lowest refining efficiency of 76%, a capital cost (CAPEX) of 117 million US$, and a MINAFSP of 0.52 US$/l, while the synthetic aromatics was most efficient at 91%, and costing 0.41 US$/l. For bio-ethanol refining, the Hybrid Process was most efficient at 80% and resulted in a MINAFSP of 0.89 US$/l. When considering an aviation fuel ecotax of 0.33 US$/l for the most viable scenarios, the syncrude was tolerated at 145 US$/bbl, while ethanol was tolerated at 371 US$/m3. Thus, simpler configurations generally had lower efficiency with lower economic potential.
Tanzil, A. H., Zhang, X., Wolcott, M., Brandt, K., Stöckle, C., Murthy, G., & Garcia-Perez, M. (2021). Evaluation of dry corn ethanol bio-refinery concepts for the production of sustainable aviation fuel. Biomass and Bioenergy, 146, 105937.
A typical Dry Grind Corn Ethanol Mill (DGCEM) with a capacity of 230 ML of ethanol per year is used as the baseline for the evaluation of biorefinery concepts for sustainable aviation fuels (SFAs). The main goal is to identify SAF cost reduction opportunities as well as environmental benefits by integrating with existing DGCEM infrastructure. Five SAF production technologies are studied: Virent’s BioForming (VB), Alcohol to Jet (ATJ), Direct Sugar to Hydrocarbon (DSHC), Fast Pyrolysis (FP) and Gasification & Fischer-Tropsch (GFT). We built SAF unit cases with capital cost equal to the studied DGCEM ($115 M). Larger SAF units are unlikely to synergize well with existing DGCEMs. Twelve co-location and repurposing scenarios are evaluated where SAF technologies utilize intermediate products, auxiliary facilities, or unit operations from DGCEM. For each of the scenarios, the minimum fuel selling price (MFSP) and greenhouse gas (GHG) emissions are estimated. Our aim is to identify which SAF technologies can be most efficiently integrated with a corn ethanol mill. Eleven scenarios result in lowered MFSPs in the range of 3–67% reduction, from their corresponding greenfield design cases. The highest reduction is observed when ATJ is produced in a repurposed facility. In the case of GHG we were able to identify one scenario with lower GHG emissions compared with greenfield units. SAF in thirteen scenarios have GHG emission ranging from 13 to 93% of fossil fuel. One of the repurposed scenarios of ATJ is the concept with the best overall performance parameter.
Santos, K., & Delina, L. (2021). Soaring sustainably: Promoting the uptake of sustainable aviation fuels during and post-pandemic. Energy Research & Social Science, 77, 102074.
The aviation industry is a major producer of greenhouse gas emissions, and the current reduction methods of carbon offsets and increasing aircraft efficiency will not be enough to significantly lower emissions by 2050. Considering the prior growth of the aviation industry and to meet the Paris Agreement’s 2-degree Celsius target, aircraft emissions must be reduced rather than simply offset. This Perspective discusses the role of sustainable aviation fuel (SAF) as a more transformative energy resource to decarbonize airlines. The opportunities and challenges in the development of SAF are reviewed to analyze how governments and airlines can move forward. Although airlines struggle with decreased demand during the COVID-19 pandemic, the airline sector can fuel both sustainable flights and economic recovery if governments can harness this unprecedented time to allocate stimulus funds to support SAF. Promoting SAF uptake during this challenging time requires robust, multi-stakeholder partnerships between governments, airlines, airports, fuel producers, and investors. This Perspective overviews several potential measures to promote SAF including policy and regulations, financing contracts, research and development, business-to-business incentives, and business-to-consumer incentives.
Capaz, R. S., Guida, E., Seabra, J. E., Osseweijer, P., & Posada, J. A. (2021). Mitigating carbon emissions through sustainable aviation fuels: costs and potential. Biofuels, Bioproducts and Biorefining, 15(2), 502-524.
In general, the certified pathways for the production of sustainable aviation fuels (SAFs) are still far from being competitive with fossil kerosene, although they have the potential to reduce greenhouse gas (GHG) emissions. However, the mitigation costs related to SAFs and how they compete with the carbon credits market remain unclear. The present study addressed these issues, evaluating SAF pathways based on hydrotreatment (HEFA process) of soybean oil, palm oil, used cooking oil (UCO) and beef tallow; dehydration and oligomerization of ethanol (ATJ technology) obtained from sugarcane, lignocellulosic residues, and steel off-gases; and the thermochemical conversion of lignocellulosic residues using the Fischer–Tropsch (FT) process and hydrothermal liquefaction (HTL). Residue-based pathways had lower mitigation costs. Used cooking oil / HEFA had the lowest value (185 USD tCO2e−1), followed by the thermochemical conversion of forestry residues (234–263 USD tCO2e−1). Of the 1G pathways, SAF production from 1G sugarcane ethanol (SC-1G/ATJ) performed better (495 USD tCO2e−1) than oil-based ones. In comparison with the carbon market, the mitigation costs of SAFs are much higher than the current prices or even future ones. However, several concerns about the credibility of the emission units and their effective mitigation effects indicate that SAFs could play an important role in aviation sector goals. Considering the potential of supplying SAF and mitigating emissions, SC-1G/ATJ was suggested as a preferred alternative in the short term. Of the residue-based pathways, tallow / HEFA and FT of forestry residues are suggested as strategic alternatives.
Kosir, S., Heyne, J., & Graham, J. (2020). A machine learning framework for drop-in volume swell characteristics of sustainable aviation fuel. Fuel, 274, 117832.
A machine learning framework has been developed to predict volume swell for 10 non-metallic materials submerged in neat compounds. The non-metallic materials included nitrile rubber, extracted nitrile rubber, fluorosilicone, low temp fluorocarbon, lightweight polysulfide, polythioether, epoxy (0.2 mm), epoxy (0.04 mm), nylon, and Kapton. Volume swell, a material compatibility concern, serves as a significant impediment for the minimization of the greenhouse gas emissions of aviation. Sustainable aviation fuels, the only near and mid-term solution to mitigating greenhouse gas emissions, are limited to low blend limits with conventional fuel due to material compatibility issues (i.e. O-ring swell). A neural network was trained to predict volume swell for non-metallic materials submerged in neat compounds. Subsequent blend optimization incorporated nitrile rubber volume swell predictions for iso- and cycloalkanes to create a high-performance jet fuel within ‘drop-in’ limits.
The results of this study are volume swell predictions for 3 of the 10 materials -nitrile rubber, extracted nitrile rubber, and polythioether- with holdout errors of 12.4% or better relative to mean volume swell values. Optimization considering nitrile rubber volume swell achieved median specific energy [MJ/kg] and energy density [MJ/L] increases of 1.9% and 5.1% relative to conventional jet fuel and an average volume swell of 6.2% v/v which is within the range of conventional fuels. Optimized solutions were heavily biased toward monocycloalkanes, indicating that they are a suitable replacement for aromatics. This study concludes that cycloalkanes can replace aromatics in jet fuel considering volume swell and other operability requirements while significantly reducing soot and particulate matter emissions.
Zhang, L., Butler, T. L., & Yang*, B. (2020). Recent Trends, Opportunities and Challenges of Sustainable Aviation Fuel. Green Energy to Sustainability: Strategies for Global Industries, 85-110.
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, no clear winning sustainable jet fuel technology exists, a situation that encourages continued research and development in innovative biojet fuel technologies. This chapter provides an overview of current opportunities for the development of biojet fuels and highlights the reasons behind the burgeoning efforts. It presents a 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. Aviation should be environmentally sustainable, cause minimal pollution to air and water, and contribute to high quality human life. 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.
Chao, H., Agusdinata, D. B., DeLaurentis, D., & Stechel, E. B. (2019). Carbon offsetting and reduction scheme with sustainable aviation fuel options: Fleet-level carbon emissions impacts for US airlines. Transportation Research Part D: Transport and Environment, 75, 42-56.
To reduce aviation carbon emissions, the International Civil Aviation Organization initiated the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), which will take effect in 2021. In response, airlines have taken measures through various means, including the use of sustainable fuels. This article investigates the potential effects of a CORSIA-type policy when implemented in the United States. The study uses a combined model of airlines operations and multi-feedstock sustainable aviation fuels (SAFs) to represent decisions of several actors, such as farmers, bio-refineries, airlines, and policymakers. The research employed a life-cycle assessment and Monte-Carlo simulation to evaluate two policy scenarios on the amount of SAF consumption and the resulting emissions. Implementing a CORSIA-type policy could stimulate the demand and production of SAFs, while also reducing air travel growth by increasing airfare. As a result of this combined effect and improved aircraft technology, there is a 3.5% chance that the U.S. airlines industry can reduce greenhouse gas (GHG) emissions by 37.5–50% by the year 2050, compared to the 2005 emission levels. Despite a projected increase in air travel in 2050 by a factor of 2.75 (the median value), the emissions in 2050 are expected to rise to only 120% (the median value) of the 2005 level. The price of petroleum-based aviation fuels followed by the growth rate of the carbon price are the two most important factors to determine whether the CORSIA-type policy would achieve the emission reduction target.
Chiaramonti, D. (2019). Sustainable aviation fuels: the challenge of decarbonization. Energy Procedia, 158, 1202-1207.
Aviation is steadily growing worldwide as well as in the European Union (EU). Overall, EU transports increased their GreenHouse Gas (GHG) Emissions since 1990, while the other energy sectors succeeded in achieving a constant reduction over the same period. In this context, air transport is the most critical area to decarbonize, given the limited number of options that can be implemented, such as optimization of flight routes, increase of jet engine energy efficiency, and few others. Switching to renewable or low carbon fuels is thus the main opportunity for aviation. Large scale deployment of Sustainable Aviation Fuels (SAF) is however a real challenge, as it requires large investments in new production facilities, strong reduction in production costs (over the entire value chain, i.e. including feedstock production, collection and delivery), and considerable investments in ASTM certification. The present work shortly reviews the perspectives of aviation fuel in terms of demand and GHG emission trends, possible routes to jet fuel production, and the status of ASTM certified routes to jet fuel as of today.
Trejo-Pech, C. O., Larson, J. A., English, B. C., & Yu, T. E. (2019). Cost and Profitability Analysis of a Prospective Pennycress to Sustainable Aviation Fuel Supply Chain in Southern USA. Energies, 12(16), 3055.
This study evaluates biorefinery bio-oil feedstock costs at the plant gate for a prospective field pennycress (Thlaspi arvense L.) to sustainable aviation fuel (SAF) supply chain. The biorefinery would supply SAF to the Nashville, Tennessee international airport. Supply chain activities include pennycress production, transporting oilseed to a crushing facility, processing of oilseed into bio-oil, and transporting bio-oil to the biorefinery. The analysis shows profit potential for economic agents in the prospective supply chain. Estimated breakeven cost (profit = 0) of growing, harvesting, and transporting oilseed to a crushing facility is 17.7 ¢ kg−1. A crushing facility can pay up to 23.8 ¢ kg−1 for pennycress oilseed during the first year of production and provide investors 12.5% annual rate of return. Therefore, a profit margin of up to 6.1 ¢ kg−1 is available for the crushing facility to induce prospective pennycress producers to supply oilseed for SAF production. However, the estimated profit margin was sensitive mainly to uncertain oilseed yields, changes in field production costs, and pennycress meal and bio-oil prices. A spatial biorefineries sitting model, the Biofuels Facility Location Analysis Modeling Endeavor, estimated that the least-cost supply chain configuration is to establish three crushing facilities located in Union City, Huntington, and Clarksville, TN, to supply bio-oil to the biorefinery, with the biorefinery sited in an industrial park about 24.14 km from the Nashville international airport aviation fuel storage. Estimated total costs of bio-oil at the biorefinery plant gate are between 83 and 109 ¢ kg−1 if crushing facility oilseed procurement costs are between 17.7 and 23.8 ¢ kg−1 for oilseed.