Combustion characteristics of hydrogen, ammonia, and their blends: A review

With rising global energy demands and the urgent need to decarbonize the transportation sector, clean fuels have become increasingly critical. Hydrogen, ammonia, and other sustainable fuels with low carbon footprints are therefore being investigated in response to these demands. Hydrogen, with its high gravimetric energy density and zero carbon dioxide emissions during use, is a promising candidate for reducing carbon footprints in road transport and aviation sectors. However, its high flammability, low volumetric energy density, and risks of permeation and leakage complicate its storage and transport, necessitating the use of hydrogen carriers. Ammonia emerges as a prominent carrier because it can burn directly without dehydrogenation. However, ammonia's low flame speed, long ignition delay, and associated emissions limit its practical use. Ammonia is often blended with other reactive fuels to address these combustion limitations. However, improvements depend highly on specific mixture ratios and operating conditions, as temperature, pressure, and equivalence ratio significantly affect combustion performance. This review explores hydrogen, ammonia, and their blends, focusing on their reaction mechanisms, flame speed, quenching distance, ignition energy, flammability limits, and NOx formation. The review evaluates both recent and historical kinetic models for predicting these properties, emphasizing those with minimal limitations. The review also covers typical experimental instruments, such as shock tubes, flow reactors, rapid compression machines (RCMs), and others. It concludes that ammonia-hydrogen blends generally exhibit superior combustion performance compared to burning hydrogen or ammonia alone, provided that appropriate kinetic models are used and the reactions responsible for these improvements are accurately modeled.