Hydrogen production from ammonia-rich combustion for fuel reforming under high temperature and high pressure conditions

Ammonia is a promising carbon-free fuel for internal combustion engines. However, the low reactivity and combustion sluggishness make ammonia difficult to be used in a single-fuel way. A small amount of hydrogen addition has been demonstrated beneficial to improving ammonia's reactivity and engine performance. To overcome the safety and cost issues of onboard hydrogen storage on vehicles, we propose to produce hydrogen by in-cylinder ammonia reforming through ammonia-rich combustion. This paper presents an investigation on the hydrogen production from ammonia-rich combustion on a rapid compression machine (RCM) over conditions with varying pressures (22-36 bar), temperatures (1200-1300 K), and equivalence ratios (1.75-2.25). Both major combustion reactants (NH3) and products (H-2 and N-2) were analyzed using a fast sampling system and gas chromatography (GC). Chemical analysis was also conducted to interpret the experimental results. The results showed that current ammonia mechanisms were inadequate to predict the ignition delay time and the hydrogen production trend under engine-relevant and fuel-rich conditions. The experimental hydrogen production increased with the increasing initial temperature, which could be ascribed to the increase in reaction rate constants of key reactions. For the effect of pressure, the hydrogen production was found to first increase and then decrease as the initial pressure increased, which failed to be predicted by simulations. In the tested equivalence ratio range, a moderate fuel-rich equivalence ratio of two produced the highest hydrogen, in which the combined impact of key radical pool buildup (NH3 and NH2) and the change in reaction ratios of key reactions (NH2 + H -> NH + H-2) was responsible.