Addressing the challenge of ammonia slip and nitrous oxide emissions from zero-carbon fuelled engines through catalytic aftertreatment solutions

Abstract

Addressing climate change demands, energy security and resilience has necessitated replacing conventional fossil-based fuels with zero and carbon-neutral fuels/energy carriers. The most immediate solution is the partial and progressive substitution of conventional fuels in transportation. The effects of partially substituting gasoline with ammonia/hydrogen (NH₃/H₂) mixtures in a spark ignition (SI) engine are investigated in this paper. The utilization of NH₃/H₂ mixtures is a promising avenue of research since they can be produced from on-board NH₃ reforming, utilising heat energy that is recovered from hot exhaust gases. Experimental results indicate that adding NH₃/H₂ enabled stable engine operation at lean conditions (λ = 1.4), resulting in reduced carbon-based emissions due to the non-carbon nature of NH₃/H₂. Utilising an integrated approach that combined a hemispherical flame geometry model with a thermodynamic model, has revealed that the introduction of NH₃/H₂ significantly enhanced the combustion speed during the initial phase and further improved combustion efficiency. However, nitrogen-based emissions such as NO and NO₂ increased. This work also assessed the performance of a conventional three-way catalyst (TWC) and a double-function ammonia slip catalyst (ASC) in mitigating emissions. The TWC effectively controlled carbon-based emissions and NO under stoichiometric conditions but exhibited reduced efficiency under lean conditions, especially with NH₃ present. The ASC demonstrated high NH₃ conversion efficiency even at low temperatures, making it suitable for engine start-up and warm-up phases. Under steady-state conditions with artificially increased NH₃/NO_X ratios, a significant reduction in NOx emission was achieved with the ASC. However, high NH₃/NO_X ratios increased nitrous oxide (N₂O) formation and NH₃ slip.