By: 2022 National PT/MT Fellow Philip Piper
Air travel presents a unique problem to decarbonization due to its reliance on high energy density fossil-fuel-derived kerosene (jet fuel). According to the IEA’s 2022 Aviation Report, aviation accounted for around 2% of global emissions in 2021. Furthermore, the FAA’s 2021 Aviation Climate Action Plan found that 90% of US domestic and international aviation CO2 emissions come from direct emission of CO2 engine exhaust, of which 80% is released in-route (above 10,000 ft). While 2% may not seem like much, aviation is one of a few hard-to-decarbonize sectors due to the irreplaceability of low-cost, fossil-fuel-based kerosene. While several companies are developing electric and hydrogen-powered planes, such as Airbus’s ZEROe, they will cost more while delivering less due to the disadvantages of alternative energy storage options compared to kerosene. In contrast, the use of synthetic kerosenes made from clean hydrogen will likely afford lower passenger costs and higher overall efficiency from renewably-sourced energy generation to the point of energy usage in aircraft.
Alternative energy storage options, such as batteries and hydrogen, are less energy dense than fossil fuels. Kerosene stores 12 kWh/kg fuel, which is 40–50 times more than typical Li-ion battery efficiencies of 0.2–0.3 kWh/kg battery. Additionally, airplane weight reduces with time when using expendable fuel resulting in a higher range for the same energy density. Hydrogen, on the other hand, has an impressive 34 kWh/kg H2, nearly three times as much as kerosene. The downside to hydrogen is that its volumetric energy density, 2.4 kWh/L H2, is four times less than that of kerosene at 9.8 kWh / L fuel. The volumetric energy density of Li-ion batteries is even lower than that of hydrogen, at only 0.5–0.8 kWh / L battery. Kerosene offers a convenient balance of both mass- and volume-based energy density.
Airplanes operate best with a fuel that is dense both by volume and mass. A typical gasoline-powered car has around 3400 L of passenger and cargo volume while its fuel is stored in a 60-liter tank. That fuel weighs 45kg compared to a total vehicle weight that is often over 1000kg. The fuel system makes up less than 5% of the vehicle by both volume and mass. In contrast, a Boeing 787–10 has cargo and fuel volumes of 191,000 and 126,000L respectively; that is almost as much fuel as cargo! By mass, 40% of a fully loaded 787–10 is fuel whereas only 23% can be payload. Battery-powered aircraft fuel-equivalent weight and volume would have to increase by over 10x making electric planes a non-starter when comparing passenger costs and aircraft ranges. If hydrogen could be used as a drop-in replacement for kerosene, then the fuel weight would reduce by 35% at the cost of increased fuel volume from 126,000 L to 515,000 L, which is over half the volume of the 787–10 aircraft.
But hydrogen isn’t a direct drop-in replacement for kerosene. Liquid hydrogen seems to be the most plausible high-density storage option, but it requires temperatures of -250°C for liquefaction, which is cold enough to condense air. At these conditions, hydrogen is highly volatile and constantly boiling as heat leaks into the tanks, all while being a colorless, odorless gas that is highly flammable with air. On the other hand, liquid kerosene can be stored in tanks for years without degradation. Hydrogen burns hotter in the air, making it more difficult to manage NOX emissions which are damaging to the environment. And if the hydrogen is used in a fuel cell, efficiency will likely be only equal to or lower than that of a gas turbine engine. Modern kerosene aircraft and natural gas ground-based gas turbines regularly achieve thermal efficiencies of over 50%.
Producing clean hydrogen is also difficult, as is transporting it from production sites to consumption sites. Electrolysis requires around 50kWh/kg H2 and likely won’t improve much past that in the near term (the theoretical limit is 39 kWh/kg H2). Methane pyrolysis approaches convert methane to hydrogen and solid carbon without CO2 release and could improve that number to as low as 10 kWh/kg H2 (limit of 5 kWh/kg H2). Steam methane reforming with carbon capture, sequestration, and storage (SMR with CCS) can also achieve low energy costs of less than 10 kWh/kg H2. Note that the considerably lower energy requirements for natural gas-based hydrogen production would necessitate reductions in fugitive emissions from gas fields to pipelines to refineries in order to be considered CO2-equivalent free. In all three cases, collocating production near liquefaction plants and airports would be required due to hydrogen’s low density. However, not all airports are located near cheap sources of clean electricity for electrolysis, natural gas for methane pyrolysis, and CO2 sequestration sites for reforming.
Producing a high energy-density fuel, such as synthetic kerosene, would allow for production at refineries that are long distances from airports, as is currently done with conventional jet fuels. Clean hydrogen could be produced wherever it is most convenient, whether that be near the windy Great Plains for electrolysis or the Barnett Shale field for pyrolysis and SMR with CCS. Production of aviation-grade fuels has a long history dating back to Germany’s synthetic fuel program in World War 2. While initially used for nefarious purposes, synthetic fuel production is a well-developed technology that is missing two key ingredients: cheap/clean hydrogen and societal accounting for the CO2 externality. Any approach to decarbonizing aviation will result in higher passenger costs, but a quick look at energy densities and aircraft mass/volume breakdowns shows that synthetic kerosene approaches are most promising.
