Methods on combustion enhancement for low active ammonia and its research progress

Ammonia (NH3) is a zero-carbon fuel with broad application prospects and is expected to play a crucial role in achieving the “dual carbon” strategic goals. However, burning pure ammonia presents challenges such as ignition difficulty, narrow combustion limits, slow flame propagation speed, and high nitrogen oxide (NOx) emissions. These issues result in less efficient and less stable combustion of ammonia, limiting its application in power devices. To overcome these challenges, blending highly reactive components can enhance the combustion characteristics of ammonia, offering a promising approach for its utilization. This strategy has become an effective technical method for improving ammonia fuel combustion. This article reviews the research progress on combustion enhancement of ammonia with low activity by blending the active components, focusing on ignition delay, laminar flame propagation, and species profiles. The blending of hydrogen and highly reactive hydrocarbon fuels with ammonia both exhibits a nonlinear promoting effect on the ignition delay times of ammonia. The addition of a small amount of active components can significantly shorten the ignition delay time. At different temperatures, the common mechanism for ignition enhancement involves promoting the accumulation of active radical pools (such as H, O, and OH), thereby accelerating chain reactions. Fuels with different molecular weights exhibit varying effects on enhancing laminar flame speeds when blended with ammonia. When blended with small-molecule fuels (such as hydrogen and syngas), the laminar flame speed shows a nonlinear trend characterized by a slow initial increase followed by a rapid rise as the blending ratio increases. In contrast, large-molecule hydrocarbon fuels exhibit the opposite trend. The laminar flame speed of ammonia/methane blended fuel increases linearly with the methane blending ratio. The laminar flame speeds of all blended fuels show a significant linear correlation with the peak mole fraction of (H + OH). The increase in radical concentration after blending with active components is the intrinsic mechanism accelerating the increase in laminar flame speed. Furthermore, blending active components and increasing pressure can reduce the initial oxidation temperature of ammonia and shift the temperature range for NO and N2O formation to lower temperatures. NOx formation is strongly influenced by the type of active component and reaction conditions, exhibiting complex generation and evolution patterns following their addition. Studies indicate that introducing active components is an effective combustion control method for improving the performance of low-reactivity fuels like ammonia. For such fuels, chemical control strategies prove highly effective. These findings provide valuable insights for optimizing blending strategies, developing efficient ammonia combustion technologies, and advancing chemical kinetics models. Blending high active fuels with ammonia enables a low-carbon transition using existing infrastructure; however, it faces challenges in real-time combustion control and NOx reduction under varying conditions. The large-scale application of ammonia will contribute to a sustainable energy system.