Ryan Smith, a United Airlines pilot, took off from Houston earlier in the month to fly over the Gulf of Mexico for a 90-minute flight. He wasn't carrying passengers but he did have a special fuel that powered the Boeing 737. The one engine used standard petroleum-based aviation fuel from Texas, while the other engine used gas made entirely from leftover cooking oils and grease from a Los Angeles factory.
According to United, each engine produced about the same amount of carbon emissions (12.660 pounds) and burned 600 gallons. The sustainable fuel is made of plant-based fuels and plants use carbon dioxide during photosynthesis. This means that it has a 70% smaller carbon footprint.
Lauren Riley, Uniteds managing Director for Global Environmental Affairs and Sustainability, said that we wanted to demonstrate that the aircraft could operate with sustainable fuel in the same way as with blended fuel. It worked. This is a significant step on the path to decarbonization.
Think of sustainable aviation fuel (or SAF) as part of a large plant-fuel-engine carbon recycling system, and not a one-way ticket sending carbon from an underground oil patch directly into the atmosphere. According to industry and federal estimates, SAF can reduce carbon emissions by 50-80 percent over a lifetime depending on the type of fuel and energy used in manufacturing. Houston was the first commercial flight to use 100 percent SAF. Passenger flights are limited to 50/50 blends.
SAF can be made by refining plant or animal feedstocks, cooking oils, and solid waste. Seven types have been certified by the Environmental Protection Agency so far. Other certifications are currently in process. SAF can be used in existing jet engines without modification because it is a drop-in fuel. To refine these feedstocks into fuel using heat and chemical catalysts, many companies use renewable solar and wind energy to power their processes. SAF manufacturers must keep track of energy used during each stage of the process to reduce carbon emissions. They even employ auditors to verify their carbon footprint.
Although the Biden administration encourages airlines to use more of this fuel in their flights, there is not enough. Only two US plants make SAF: The World Energy waste oil facility in Paramount, California and the Gevo facility Silsbee in Texas. These facilities take an alcohol-based compound made of corn called isobutanol, and distill it into aviation fuel.
SAF is more expensive than regular aviation fuel because it's so rare. Only about 1 million of the 4 billion gallons United purchases each year are SAF. Riley says that there is still much to be done. There isn't enough [SAF] for everyone.