Research on the state-of-the-art of efficient and ultra-clean ammonia combustion: From combustion kinetics to engine applications

In the context of a low-carbon economy, given the current technological landscape, relying only on direct electrification for the future energy system is both technically challenging and insufficient, particularly in the heavy transportation sector. This creates a significant demand for carbon-neutral alternatives fuels. Ammonia (NH₃) represents itself as a potential option for decarbonization, attracting great attention of combustion community. Although ammonia is regarded as a promising zero-carbon fuel, its combustion characteristics still limit its practical application in energy transition. The objective of the current work is highlighting the current state-of-the-art as well as the challenges that still remain. The scope of this work spans from the fundamental characteristics and reaction kinetics of ammonia combustion, to its applications in internal combustion engines (ICEs). Current work is divided into four main parts. The first section proposes the potential of ammonia as a fuel and its current research status. The second part covers the fundamental combustion characteristics and reaction kinetics of ammonia co-combustion with other carbon-neutral fuels; The third part discusses its application in ICEs as a fuel, in which research on optical engines is highlighted, helping provide a deeper understanding of flame formation and propagation in cylinder. In the fourth section, as a highlight, the application of advanced artificial intelligence (AI) algorithms in combustion science has been emphasized. Co-combustion of ammonia and carbon-neutral fuels has the potential to achieve real zero-carbon emissions in the entire life cycle. At the fundamental combustion characteristics level, the addition of combustion enhancer can improve laminar flame speed (LFS) and ignition delay time (IDT), greatly promoting ammonia combustion. At the kinetics level, key reaction pathways, interactions between CN components and mechanisms of NO_x/soot formation have been revealed. At engine applications level, a deeper understanding of engine performances and emission characteristics can be obtained according to the fundamental research above. From micro-level combustion kinetics to macro-level engine applications, it is hoped that the current work contributes to a comprehensive understanding of key technologies of ammonia combustion and its applications in the transportation sector.