Testing Simulator Aids Sustainable Jet Fuel Development
NREL is developing an accurate way to model and predict the performance of sustainable jet fuels that will result in a low-carbon aviation future.
In aviation, petroleum-based Jet A fuel has been the standard for decades and has not changed appreciably since it became widely used in the 1950s. However, since it is a fossil fuel, Jet A produces vast amounts of carbon dioxide (CO2) emissions when it is burned in a jet engine, and much of this CO2 is emitted at high altitudes, where it becomes a significant part of the accumulation of greenhouse gases.
Sustainable aviation fuel (SAF) exists, but testing for effectiveness and safety is expensive. To aid in the testing process, the National Renewable Energy Laboratory (NREL) has developed a computerized simulation to assess the performance and safety of SAF formulas.
Scientists review simulations to reduce jet fuel emissions. Photo used courtesy of NREL
Reducing Carbon Emissions in Aviation
Replacing CO2-emitting fossil fuels with battery power for ground-based electric vehicles (EVs) is simpler than building electric-powered aircraft. Although short-range electric commuter aircraft are under development and appear promising, long-haul aircraft capable of transcontinental and transoceanic flight pose a significant problem. The energy density of batteries made with present-day lithium-ion technologies is insufficient to power such aircraft. As a result, the most promising solution appears to be sustainable aviation fuel (SAF), an energy-dense, renewable fuel that can drop directly into place, decarbonizing flight by replacing traditional fossil-based fuels.
One reason for the extremely high reliability enjoyed by the aviation industry has been that new jet engines have been designed around Jet A, and it has remained unchanged for a long time. Developing SAF has some big shoes to fill—it must be a drop-in fuel completely compatible with existing jet engines and meet the same safety, performance, and operability criteria as existing petroleum jet fuel.
ASTM International sets global jet fuel standards. At present, ASTM-approved SAFs are produced by hydrotreating plant and animal oils, yellow and brown greases, and waste oil, fat, and grease. Hydroprocessing these esters and fatty acids creates a synthetic paraffinic kerosene called HEFA-SPK. ASTM approved this synthetic jet fuel in July 2011, but it can only be used when blended with conventional jet fuel.
Making Sustainable Aviation Fuel for Jet Engines
The chemistry of SAFs is fairly complex to ensure it meets ASTM standards. Completing the required tests requires producing thousands of gallons of fuel for combustion in laboratory-scale combustors and, eventually, full-scale jet engines. This can be extremely expensive, particularly if a new fuel formulation doesn’t pass the myriad of tests.
Process for jet fuel approval. Image used courtesy of NREL
As companies work to fine-tune their processes, they often work with just milliliters of the proposed fuel. That's why, for the past several years, the National Renewable Energy Laboratory (NREL) has been developing computational modeling tools capable of simulating aircraft engine combustors to examine new SAF formulations.
NREL Developing Jet Fuel Testing Tools
By simulating SAF combustion, NREL’s tools could help companies determine if their fuels will meet requirements before investing millions of dollars to produce the large quantities required for ASTM tests.
The simulations also go beyond the ASTM standards, which typically provide specifications for jet fuels measured at sea level. But the surface tension of a SAF droplet is critical for safety and reliability at altitudes of 40,000 feet, where airliners cruise. At such altitudes, the engine must relight instantly if it blows out. NREL has undertaken a range of testing to ensure that its simulations realistically capture the performance of SAFs at extreme operating conditions.
Validation of the NREL modeling of HEFA-SPK is taking place with the help of jet-engine builder General Electric and the Georgia Institute of Technology. The SAF fuel is run through aircraft engine combustor test cells over a variety of conditions, and the performance is compared to the simulation results. One immediate goal is to loosen blending limits for HEFA-SPK. At present, the limit set by the ASTM is 50 percent, and pushing that limit higher, or even to 100 percent, would result in further reductions in carbon emissions.
Meeting Aviation Requirements
The U.S. government has set a national goal of covering the annual projected aviation fuel requirements using 100 percent SAF by 2050—a demand of 35 billion gallons each year. This will require a range of SAF pathways to convert biomass feedstocks into safe and reliable SAFs.
Aviation is among the safest transportation means. Every airplane system must be as reliable as possible. When a passenger boards a commercial airliner, there must not be a hint of doubt about whether the aircraft will perform properly and deliver its passengers safely to the destination. This aversion to risk in aviation carries through to every part of the system, including jet fuel.
At the same time, reducing carbon emissions is a priority for global sustainability. NREL’s computer simulations are an important tool in the future of low-carbon aviation.
