On the Ground in Colorado, NREL Is Simulating Sustainable Aviation Fuel Combustion during Flight

February 01, 2024

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by Erik F. Ringle (National Renewable Energy Laboratory) Laboratory Supports Industry in Fine-Tuning Sustainable Aviation Fuel Chemistry Ahead of Seeking Approval for Widespread Airplane Use -- ... But changes are coming for jet fuel, and the aviation industry is actively inviting them. Public and private investments are growing to accelerate the production and use of sustainable aviation fuel (SAF), an energy-dense, renewable fuel seen as essential for decarbonizing flight.

Like changes to gasoline and diesel chemistry in the past, adopting SAF means proving the fuel is as safe and reliable as the old stuff—as well as being fully compatible with existing jet engines. For an industry that has built its fleet around a fuel unchanged for 70 years, that could mean a steep learning curve.

"This idea of designing a new jet fuel is like a completely new concept to the aviation industry," said Robert McCormick, a senior research fellow at the U.S. Department of Energy's (DOE's) National Renewable Energy Laboratory (NREL). "Essentially, they've designed the engines around this one fuel: Jet A, which has been the standard petroleum jet fuel for years."

Fortunately, the aviation industry does not have to find a replacement for Jet A alone. In a project supported by the DOE, a cross-disciplinary research team is gathering meticulous fuel chemistry data to equip the industry with an ultra-detailed SAF combustion simulation. Powered by supercomputers, the "virtual jet engine" can predict how SAF performs during flight and provide insights on how to tune it to maximize its safety and performance. The simulations will be validated with data captured in combustion test cells at General Electric and Georgia Institute of Technology.

If successful, the SAF research platform, built using a suite of custom modeling tools called Pele in collaboration with NREL's Computational Science Center, may help industry avoid costly surprises when seeking approval for new SAFs from ASTM International. More fundamentally, it might reveal frontiers never seen before in jet fuel chemistry—insights that could help planes fly further, run cleaner, and perform better than ever before.

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Jet Engine Tests "Drink a Lot of Fuel"— Simulations Drink None

To understand the value of NREL's jet fuel combustion program, one must recognize the risk companies face as they perfect the complex chemistry of turning biomass, wastes, and other renewable resources into a liquid fuel. To get approval for a new SAF formula from ASTM International—which sets global jet fuel standards—companies must prove that it meets a set of safety, performance, and operability criteria to prove that it is functionally identical to petroleum jet fuel.

ASTM standards ensure jet fuel—no matter how it is made—can be "dropped-in" to existing airplanes, whether fueled in Denver, Dublin, or Dubai. But proving it meets those standards requires thousands of gallons of SAF for the many qualification tests, including combustion in a lab-scale jet engine combustor (and even a full-scale jet engine).

Making that volume of fuel is a steep proposition for a nascent industry, McCormick explained. Many companies are only now working with milliliters of liquids in the lab as they fine-tune complex chemical processes.

"It just adds another risk on top of everything else—that you're going to scale up and your fuel is not going to meet the property requirements," McCormick said.

It is for that reason that NREL computational science and fuels and combustion researchers decided to team up. For a few years, researchers Marc Day and Shashank Yellapantula had been developing modeling tools capable of predictive simulations of aircraft engine combustors. These tools, collectively named Pele, build on more than 50 years of DOE investments in applied mathematics research to provide a unique, highly accurate, and efficient tool to model complex reacting flows. Recent improvements, supported under DOE's Exascale Computing Project, enable the Pele codes to run efficiently on DOE supercomputers, which are the world's largest and fastest. With a goal of allaying the risk of SAF qualification, Day and Yellapantula used this framework to build a combustion simulation based on a NASA jet engine combustor—an open-source design.

"Engine tests drink a lot of fuel, and they are expensive," Yellapantula said. "Simulations in the short term can provide information that could directly help researchers tune SAF refining processes to get better fuels."

If NREL could simulate SAF combustion tests, Day and Yellapantula theorized, companies could determine if new fuels meet requirements before investing millions of dollars to produce large volumes for ASTM engine tests. The simulations could flag helpful fuel chemistry changes that companies could realize in small-scale lab experiments before building expensive pilot plants to produce these new fuels.

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How Detailed Fuel Property Data Will Help Build Out NREL's Simulation

Standing in her lab just blocks away from Interstate 70 in Golden, Colorado, NREL Senior Scientist Gina Fioroni—a colleague of McCormick—rests a hand on a shiny machine on a lab bench. It looks brand new—like a top-shelf kitchen appliance just out of the box. A large dial on the front shows pressure readings up to 8000 psi.

"This is a new frontier in SAF property measurements," Fioroni says.

It is an exceedingly rare piece of machinery—with only one other in the world with a similar configuration. It has an equally particular purpose: measuring the surface tension of liquids at extremely high pressures across a wide range of temperatures.

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Together, the equipment allows SAF to be profiled with unprecedented detail, under expected conditions in jet engines, and by only using a few milliliters of fuel.

Team To Simulate Already Commercialized SAF Before Moving to Newer Fuels

Already, Fioroni, McCormick, and colleagues are busy analyzing a kind of SAF already approved by ASTM: hydroprocessed esters and fatty acids synthetic paraffinic kerosene, or "HEFA-SPK." HEFA-SPK is SAF made by hydrotreating plant and animal oils, yellow and brown greases, and waste fat, oil, and greases. ASTM approved it for commercial use in July 2011, although only when blended with conventional jet fuel.

Using the data the fuel properties team provides, Yellapantula will pull the information into NREL's simulations to start exploring its behavioral quirks—from how it disperses in the engine to the kinetics and pollutants it releases as it burns. His team can then adjust the fuel parameters in the computer to understand the limits of its performance.

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As is critical for any model, though, the team will need to validate the results of their HEFA-SPK simulations in the real world. To do this, partners at General Electric and Georgia Institute of Technology will combust HEFA-SPK in a single aircraft engine combustor test cell—observing its impact on combustor performance over a range of conditions. They will then compare these data to the simulation results. That external work is funded by the U.S. Federal Aviation Administration and NASA as part of a larger effort to characterize SAF impact on the ignition and stability of flames within aviation gas turbine combustors.

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Another important value may be to uncover fuel chemistry adjustments ASTM might use to loosen its blending limits for HEFA-SPK. The organization currently sets a blend limit of 50% with Jet A. Pushing the limit higher—ultimately to 100% SAF—would support deeper reductions in carbon emissions.

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HEFA-SPK—currently used in select markets in California—could be advantageous for increasing SAF adoption quickly around the country. But its supply is likely to be limited by finite supplies of vegetable oils and waste fats, oils, and greases—the primary resources or "feedstock" it is made from.

To grow the market by billions of gallons and overcome barriers to using pure SAF in airplanes, the industry must utilize the full breadth of U.S. renewable feedstocks—from forestry slash leftover from logging operations to agricultural waste, municipal solid waste, and perennial energy crops. Each feedstock has its own complex chemical profile that can impact the properties of the final fuel, creating a highly complicated technical, chemical, and financial challenge, Fioroni said.

"If we want to produce billions of gallons of SAF, we will need to start thinking creatively," she said. "There is only so much biomass, so we need other pathways to develop sustainable jet fuels and to understand how their chemistries work in the engine—especially if we blend beyond what's in the market now."

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"We could provide invaluable insights back to the fuel producers to help them optimize their processes," Yellapantula added. "That way, when they put new fuels forward for certification, the process goes much faster and they can avoid costly surprises."

If achieving ASTM SAF standards feels like digging for a needle in a haystack—or scouring a heap of biomass characteristics for the perfect fuel chemistry—NREL's SAF research makes it like a precise, iterative exercise. As the aviation industry looks to embrace a new jet fuel for the first time in decades, that could be just what is needed to flatten the learning curve ahead. READ MORE; includes VIDEO

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National Laboratory Jetfuel (Sustainable Aviation Fuel (SAF)) Department of Energy (DOE) Colorado Aviation Fuel/Sustainable Aviation Fuel (SAF) Investing ASTM D7566 (Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons) ASTM ASTM standards defossilization jet A modeling computer simulation drop-in biofuels/hydrocarbons HEFA (Hydro-processed esters and fatty acids) Hydrocarbon-Hydroprocesed Esters and Fatty Acids (HC-HEFA-SPK) Federal Aviation Administration (FAA) National Aeronautics and Space Administration (NASA) 100% SAF (Sustainable Aviation Fuel) FOG (Fats/Oils/Grease) vegetable oils animal fat grease forest residue/waste Agricultural waste/residue MSW (Municipal Solid Waste) energy crops biomass BioRefineries/Renewable Fuel Production government resources Sustainability

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