An investigation of hydrogen-ammonia combustion inside internal combustion engines

Abstract

Ammonia can become an attractive alternative fuel source on-board of ships, since it does not produce SOx and carbon related emissions (including CO2, CO and soot). However, pure ammonia combustion is too slow to drive an internal combustion engine (ICE), a promoter fuel such as hydrogen is needed to speed up the combustion process. Since the SOFC can be used as a power generator and ammonia cracker, an interesting proposal is to have a hybrid ICE-SOFC power generation system on-board ships. A vast amount of papers concerning directly fueled ammonia SOFCs have been published throughout the years. This concept has already been extensively modeled and successfully tested on large-scale. On the other hand, not much is known about hydrogen-ammonia combustion inside an ICE. Although the flammability behaviour and chemical kinetics during combustion have been widely investigated. Publications concerning the in-cylinder combustion characteristics are scarce. Since there are already a number of working ammonia fueled SOFC models in the literature, the thesis will be mostly emphasised on the investigation of hydrogen-ammonia fueled internal combustion engines. Conducted experiments on automotive sized engines have shown that hydrogen-ammonia combustion during SI mode is possible, but not optimal because of the high auto-ignition resistance and low flame propagation speed of ammonia. Hence, poor engine efficiencies, low power densities and large amounts of unburned fuel are more likely to occur during spark ignition. Compression ignition on the other hand is largely affected by great amounts of NOx production inside the flame zone. The NOx is primarily produced from fuel-bound nitrogen, which can unlike the nitrogen (N2) in air easily re-bond with free oxygen during the combustion process. HCCI combustion is a promising candidate to overcome these disadvantages (i.e. low power density and high NOx emissions).