A BIOREFINING process that converts wet waste – including food waste and wastewater sludge – into sustainable aviation fuel (SAF) has been developed by researchers in the US. The SAF could be used in commercial flights within a couple of years if approved.
Wet waste includes food waste, animal manure, wastewater sludge, and waste fats, oils, and grease. Food waste sent to landfill produces methane and is responsible for around 6% of greenhouse gas emissions worldwide. Using it as a feedstock to produce fuel can halt this emissions pathway, but the moisture in the waste restricts the use of processes such as pyrolysis and gasification to produce liquid biofuels. Recovering energy from wet waste is typically done by anaerobic digestion to produce biogas.
Researchers at the National Renewable Energy Laboratory (NREL), the University of Dayton, Yale University, and Oak Ridge National Laboratory have developed a new biorefining process for converting wet waste into SAF.
The anaerobic digestion of wet waste can be stopped before methanogenesis occurs, which then generates short and medium chain carboxylic acids which can be used as precursors for biofuels known as volatile fatty acids (VFAs). The VFAs were produced by an industry partner, Earth Energy Renewables, using pilot-scale anaerobic digestion of food waste via arrested methanogenesis.
The VFAs can be converted via ketonisation and hydrodeoxygenation into paraffins – which are identical to those found in fossil jet fuel – or isoparaffins depending on the chain length. After ketonisation, ketones with chain lengths greater than C8 were processed by hydrodeoxygenation to produce paraffin-rich hydrocarbons. Ketones less than C7 needed an extra step of aldol condensation after ketonisation to produce predominantly isoparaffin hydrocarbons.
As the paraffins are identical to those found in fossil jet fuel, the paraffin-rich hydrocarbons can be directly blended with fossil jet fuel at 10%. Previous studies have shown how wet waste can be transformed into VFAs, but did not show how the fuel could meet ASTM International’s fuel property requirements. The paraffin-rich hydrocarbons blended at 10% would be eligible for ASTM’s “fast track” approval process. This could work up to a 20% blend with fossil fuel, while still meeting the ASTM’s specifications. However, beyond this the flash point becomes a limiting property.
The isoparaffins are more complex and don’t qualify for fast-track approval, and would therefore require further study and testing before they could be approved for use in aviation fuel. However, these would enable greater emissions reductions as a blend of both types of SAF could be combined as 70% SAF, 30% fossil fuel.
“The blend of paraffin and isoparaffin blend at higher levels since their limiting fuel properties offset each other in a synergistic manner,” explained Derek Vardon, Senior Research Engineer at NREL and corresponding author of the study. “The paraffin is capped at 20% due to its flash point. In isolation, the isoparaffin is capped at 30% due to its high viscosity at low temperatures. Interestingly, when mixed, the normal paraffins help reduce the viscosity limitation of isoparaffins in a nonlinear manner in the mixture, allowing for a 70% renewable blend (20% normal, 50% iso) instead of the anticipated arithmetic 50% blend (20% normal, 30% iso).”
A life cycle analysis for the paraffin VFA-SAF revealed that the carbon intensity of the fuel is mainly driven by avoided emissions from food waste which gives a negative carbon intensity compared to fossil jet fuel. The paraffin SAF has a carbon footprint of –55g CO2e/MJ compared to 85g CO2e/MJ for fossil jet fuel. This is a reduction of 165% in emissions relative to the fossil jet fuel.
“At the 70% blend level described above, it actually works out to slightly negative (even better than net zero) lifecycle emissions for a 30% fossil, 70% SAF blend,” said Vardon.
The analysis also noted that changes to the management of food waste – such as changes to regulations requiring reductions in food waste in landfills – would change the carbon intensity of the fuel. However, advancements in renewable energy would further benefit the process by lowering emissions from unit operations.
Currently in the US, most commercial SAF production uses feedstocks such as vegetable oil and waste fats to produce fuel using hydrotreating of esters and fatty acids (HEFA). This can be used to produce renewable diesel as well as SAF, resulting in competing demand. Producing HEFA-SAF is also more expensive than producing HEFA diesel, as an additional catalytic cracking step is required to produce the hydrocarbons suitable for aviation fuel.
Vardon explained that while it is not easy to compare the cost effectiveness of HEFA-SAF to VFA-SAF as HEFA is already a commercially deployed technology, VFAs can be used to produce hydrocarbons for both diesel and SAF for the same price.
The paraffin SAF can be used in the near future for short-term emissions reductions, while more long term the blend of paraffin and isoparaffin SAF will lead to greater reductions.
Southwest Airlines has been collaborating with NREL since 2019 to make SAF commercially viable. Michael AuBuchon, Senior Director of Fuel Supply Chain Management at Southwest Airlines, said: “We’re excited to partner with NREL as we continue our journey to make commercially-viable SAF a reality and a part of Southwest’s future. It is undeniable that SAF’s role in reducing emissions across the industry and at Southwest will be significant. NREL’s research could provide a game-changing opportunity to make SAF cost-effective, leading to its larger-scale deployment.”
“If our refining pathway is scaled up, it could take as little as a year or two for airlines like Southwest to get the fuel regulatory approvals they need to start using wet waste SAF in commercial flights,” said Vardon.
SAF from wet waste could produce 20% of the US aviation fuel demand, which means it will have to complement other technologies in reducing aviation emissions. “Our SAF route is not a silver bullet,” said Vardon, “but as a piece of the puzzle it could make a significant dent in an industry notoriously hard to decarbonise.”
Proceedings of the National Academy of Sciences https://doi.org/gjjbbp
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