To meet the growing demand for sustainable aviation fuel (SAF), conversion pathways are needed that leverage wet waste carbon and meet jet fuel property specifications. Here, we demonstrate Sustainable Aviation Fuel production from food waste–derived volatile fatty acids (VFA) by targeting normal paraffins for a near-term path to market and branched isoparaffins to increase the renewable content long term.
Combining these distinct paraffin structures was shown to synergistically improve VFA-SAF flash point and viscosity to increase the renewable blend limit to 70%. Life cycle analysis shows the dramatic impact on the carbon footprint if food waste is diverted from landfills to produce VFA-SAF, highlighting the potential to meet jet fuel safety, operability, and environmental goals.
Abstract
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.
Over 21 billion gallons of jet fuel are consumed in the United States annually, with demand expected to double by 2050 (1). The aviation sector accounts for 2.5% of global greenhouse gas emissions, with airlines committing to reduce their carbon footprint by 50% before 2050 (2, 3).
Sustainable aviation fuels (SAF) comprise a significant portion of the aviation sector’s strategy for CO2 reductions given the limited near-term prospects for electrification (3⇓–5).
In addition, the low aromatic content of current SAF routes has been shown to reduce soot formation and aviation-related aerosol emissions by 50 to 70% (2, 6, 7), which can significantly impact the net global warming potential. Soot is the primary nucleator of aviation-induced contrails (8), which have a larger effective radiative forcing (57.4 mW/m2) than aviation-emitted CO2 alone (34.3 mW/m2) (3).
Commercial SAF production in the United States currently relies on the hydrotreating of esters and fatty acids (HEFA) using virgin vegetable oils as well as waste fats, oils, and greases. These feedstocks also serve the renewable diesel market, which in 2018, produced ∼300 million gallons of HEFA diesel compared to ∼2 million gallons of HEFA SAF (1).
Global HEFA capacity is estimated at 1.1 billion gallons per year (BGPY) in 2017 (9). HEFA SAF competes with demand for HEFA diesel, with US fossil diesel consumption estimated at ∼47 BGPY (10). Producing HEFA SAF requires an additional catalytic cracking step to convert predominantly C16 and C18 long chain fatty acids into C8 to C18 hydrocarbons suitable for jet fuel.
This consumes additional hydrogen and lowers the jet and diesel fuel yield, making HEFA SAF more expensive to produce than HEFA diesel (11). California’s Low Carbon Fuel Standard (LCFS) has provided significant economic incentive for producing HEFA from low carbon intensity feedstocks (12), with petroleum companies continuing to retrofit existing refineries (13).
Although this expansion will significantly increase biofuel production, the US availability of fats, oils, and greases is capped at ∼1.7 BGPY of jet fuel equivalent (14, 15). As such, efforts are needed to develop alternative feedstocks and conversion routes for SAF that avoid direct competition with food resources.
Wet waste is an underutilized feedstock in the United States, with an energy content equivalent to 10.5 BGPY of jet fuel equivalent (assumed 130.4 MJ/gallon). Wet waste includes food waste (2.5 BPGY), animal manure (4.4 BGPY), wastewater sludge (1.9 BPGY), and the abovementioned waste fats, oils, and grease (1.7 BGPY) (14, 15).
While waste lipid feedstocks may be best suited for HEFA refining, valorization strategies are needed for the remaining wet waste feedstocks. Diverting food waste from landfills is of particular note for reducing greenhouse gas emissions, as landfilling one dry ton of food waste has been estimated to release as much as 1.8 tons of CO2 equivalents, assuming landfill methane is collected and recovered for electricity generation (16, 17).
Globally, food waste accounts for 6% of greenhouse emissions (18). The high moisture content of wet waste restricts the use of conventional thermochemical conversion approaches (e.g., pyrolysis and gasification) used to produce liquid biofuels from terrestrial biomass, directing technology development efforts toward hydrothermal liquefaction, biological conversion, and hybrid processes (19).
Currently, anaerobic digestion to produce biogas is the leading technology to recover energy from wet waste (20). The high moisture content of wet waste limits its transport and necessitates local processing, with the majority of US anaerobic digestion facilities located near population-dense areas and airports (21).
Biogas purification provides a route to pipeline quality renewable natural gas compatible with existing infrastructure. Life cycle analysis has shown that negative carbon intensity can be achieved when producing renewable natural gas from municipal solid waste (−23 g CO2eq/MJ) and dairy waste (−276 g CO2eq/MJ), providing a significant economic driver under the LCFS (12).
While renewable natural gas targets an enormous US market (∼246 BGPY of jet fuel equivalent) (10), producing liquid hydrocarbon fuels from wet waste offers the potential to address the challenge of decarbonizing the aviation sector.
Anaerobic digestion of wet waste can be arrested prior to methanogenesis to generate both short chain (C2 to C5) and medium chain length (C6 to C8) carboxylic acids as precursors for biofuels and biobased chemicals (14, 22⇓⇓⇓⇓–27), hereon collectively referred to as volatile fatty acids (VFAs).
VFA production by arrested methanogenesis offers the potential to utilize existing biogas infrastructure and a wide variety of wet waste feedstocks (14, 22, 28) with ongoing research and development working to increase VFA titers, rates, and yields by tailoring feedstock composition, microbial consortia, fermentation parameters, and online separation technologies (14, 22, 29⇓–31).
Currently, C2 to C5 carboxylic acids are primarily produced from the oxidation of petroleum derivatives, while C6 and C8 carboxylic acids are primarily derived from coconut and palm oil (29). Propionic acid (C3) and butyric acid (C4) address chemical market volumes on the order of 0.1 to 0.2 BGPY (29), while medium chain length carboxylic acids target smaller specialty markets.
Given the availability of wet waste and potential saturation of biobased chemical markets in the long term, VFAs provide a potential target intermediate for catalytic upgrading into low carbon intensity biofuel (23, 25, 26, 32⇓⇓–35).
VFAs can be catalytically upgraded to SAF through carbon coupling and deoxygenation chemistries.
Depending on their chain length, VFAs can be converted into normal paraffins identical to those found in petroleum or undergo an additional carbon coupling step to generate isoparaffin, cycloparaffin, and aromatic hydrocarbons with molecular structures distinct from fossil jet.
Read the whole story here: Toward net-zero sustainable aviation fuel with wet waste–derived volatile fatty acids, March 19, 2021
