Ammonia as an Aircraft Fuel: A Critical Assessment From Airport to Wake

Zero-emission aviation initiatives have mainly focused on using hydrogen or drop-in biofuels and sustainable aviation fuels (SAF) to replace fossil-based jet fuels to achieve near-term reductions in carbon emissions with minimal impacts on the global aircraft fleet and supporting infrastructure. Despite significant advances in the production of such fuels, scaling up manufacturing capability to be cost-competitive is an ongoing effort. This paper discusses ammonia as a near-zero-emission carrier of green hydrogen for aviation. Ammonia is proposed as a carrier of hydrogen fuel, a thermal sink for compressor intercooling, and cooling of cooling air, for NOx elimination, and for condensation of water vapor to reduce contrail formation. A two-pronged investigation is presented, where first, a holistic discussion on alternative fuels identifies ammonia as a suitable hydrogen carrier for aviation. Second, the implications and potentials of ammonia are discussed and analyzed at the airframe and engine system level. Stemming from the already established fertilizer industry, a robust supply chain for ammonia exists together with experience in handling large quantities of the fluid despite its higher toxicity compared to hydrogen and other alternative aviation fuels of the future. It is found that ammonia requires significantly less water than SAF in production, on par with hydrogen, at comparable life cycle emission levels. The feasibility of heat exchangers for compressor intercooling and turbine-cooled cooling air, enabled by ammonia’s non-coking properties, is demonstrated, and paves the way toward efficient zero-emission engine cores.