Removal and mechanism analysis of NOx emissions in carbon-free ammonia combustion systems with a secondary fuel injection

Abstract

Ammonia, as a carbon-free renewable fuel of emerging interest, has the potential to substitute conventional fossil fuels. However, one challenge in utilizing such fuel is the risk of high NOx (nitrogen oxide) emissions. This work clarifies how a secondary fuel injection strategy affects pollutant formation and pinpoints the underlying suppression mechanism. For this, a three-dimensional computational model involving a detailed ammonia/oxygen kinetic mechanism is established and validated with experimental data from the literature. Effects of the primary fuel flow rate, secondary fuel injection ratio R, and the dimensionless axial location of the secondary fuel injection l on the emission behavior are evaluated in detail. Introducing such a strategy leads to an approximately 28% NO (nitric oxide) emissions reduction, mainly due to the chemical effect of NO reacting with NH2 from NH3 decomposition downstream of the secondary injection region. Increasing R is associated with a lower NO emission but a higher NH3 that is negligible or notable. Furthermore, varying l is found to result in a slight change in the NO emission, whereas it greatly affects the NH3 consumption. This work confirms the viability of applying a secondary fuel injection strategy to mitigate NOx emissions without a negligible NH3 penalty.