Comprehensive review on ammonia combustion technologies: Combustion characteristics, potential of hydrogen/methane additions, and emerging applications

Addressing climate change requires the advancement of carbon-free fuels as a main strategy for reducing global greenhouse gas emissions. Among carbon-free fuels, ammonia is gaining attention for its carbon-free combustion and potential as a hydrogen carrier. However, its practical utilization in combustion systems is hindered by low reactivity, narrow flammability limits, slow flame speed, and elevated NOx emissions. This review presents a comprehensive analysis of ammonia combustion characteristics and recent developments aimed at overcoming these limitations. Enhancement strategies such as blending with hydrogen or methane, oxygen-enriched combustion, partial ammonia cracking, plasma-assisted ignition, and oxidizer modification are critically examined. Blending ammonia with 30–60 % hydrogen or 20–50 % methane significantly improves flame speed and stability, while oxygen enrichment up to 28 % O₂ can triple the laminar burning velocity. Partial ammonia cracking enhances reactivity, and plasma-assisted combustion extends lean blowout limits and reduces NOx emissions. Rich-lean staged combustion has proven effective for NOx control in gas turbine systems, with emissions reduced to below 100 ppm in optimized configurations. The combustion behavior of ammonia, both as pure fuel and in blends, is analyzed concerning flame characteristics and emission profiles across varying conditions. Despite these advancements, challenges remain in achieving stable, efficient, and low-emission combustion under operational limitations. This review provides a comprehensive reference for ongoing research and supports the continued advancement of ammonia as a promising carbon-free fuel within future clean energy systems.