Research on Combustion and Emission Characteristics of Ammonia under Preheating Conditions

Abstract

The present study focuses on determining the potential applicability of ammonia (NH3) as a carbon-free alternative fuel through numerical simulations. The combustion and emissions characteristics of premixed NH3-air flame at preheating conditions were numerically investigated. The burning velocity and all species mole fraction were determined using the Miller and Bowman and the Reductive Konnov mechanism. The results show that both the equivalence ratio and preheating temperature have important effects on the laminar burning velocity and adiabatic flame temperature of NH3. The reaction rate of production and mole fraction of H, O, and OH radicals, together with nitrogen radicals such as NH2, NH, and N in the reaction zone, increases as the preheating temperature is increased from 298 to 573 K. Although NO concentrations in the exhaust gas increased with increasing preheating temperature under fuel-lean conditions, they could be maintained at a reasonably low level at all preheating temperatures, mainly due to the increased reduction reaction of NO with NH2, NH, N radicals. The main N radical formation in the NH3-air combustion originates from NH3 rather than N-2, which indicates that the N-2 dilution effect of NH3-air flame is negligible for NO formation. Furthermore, the equivalence ratio has a dominant effect on NO yields, because of the higher reduction reaction and lower flame temperature at higher equivalence ratio conditions, leading to a lower formation of HNO, which indicates a potential in reducing NO emissions in NH3 combustion. Therefore, the preheating temperature and equivalence ratio should be given sufficient consideration in practical operations to minimize NO formation in combustion of NH3 as a clean fuel.