Ammonia: energy carrier only or well-suited sustainable fuel for large engine applications?

In order to achieve the global greenhouse gas emission reduction goals, a transition from an energy system based on fossil fuels to one based on renewable energy is crucial. The European Union will probably continue to cover a large part of its energy needs with imports from regions with abundant wind and solar energy in the future, making the transportation and storage of renewable energy highly relevant. Liquid hydrogen carriers are particularly suitable for storing large amounts of energy. Ammonia has significant advantages over hydrogen with regard to transport and handling since it can easily be liquified at ambient or sub-zero temperatures and has a higher volumetric energy density than liquefied hydrogen. Both, ammonia and hydrogen combustion in large-bore engines, gained interest in the last couple of years as an enabler for the decarbonization of hard-to-abate energy sectors. It is not clear, however, if direct use of ammonia in the engine or the cracking of ammonia into nitrogen and hydrogen and subsequent use of hydrogen in the engine will be the preferred path.
In this article, an assessment of the performance potential and the limitations for ammonia and hydrogen use in large-bore engines is carried out based on a combination of experimental evaluations on singl- cylinder research engines and 1D performance simulations. For ammonia, different combustion concepts will be considered since the emission profiles and favorable excess air ratios differ significantly between premixed combustion with pilot ignition or spark ignition on one hand and mixing controlled combustion with pilot ignition on the other hand. Hydrogen combustion will be evaluated in a spark ignition engine. The exhaust aftertreatment requirements are compared with regard to conversion efficiencies of selected exhaust components, exhaust gas temperature, and dosing needs of a reductant. Finally, the potential impacts on the engine architecture are compared and future research needs highlighted.