On the effects of NH3 addition to a reacting mixture of H2/CH4 under MILD combustion regime: Numerical modeling with a modified EDC combustion model

Abstract

This paper examines the behavior of reacting NH3/H2/CH4 mixtures in moderate or intense low oxygen dilution (MILD) condition. A series of axisymmetric, turbulent reacting flow simulations are carried out incorporating a modified version of eddy dissipation concept and a few reaction mechanisms. The effects of adding a progressively increasing amount of NH3 to a reacting H2/CH4 mixture in moderate condition are investigated. It is observed that addition of NH3 to MILD combustion leads to markedly different behaviors compared to that in conventional combustion. Most notably, the inherently strong preheating of reactants in MILD combustion causes thermal cracking of NH3 prior to ignition. The resultant production of H2 profoundly affects the reacting flow as such increasing the NH3 mass fraction in the fuel blend decreases the flame lift-off. Further, unlike that in conventional combustion, adding NH3 to MILD combustion increases the process reactivity. In addition, the usual flame thickening typically seen in NH3 flames is not observed here, while in keeping with the thermodynamic predictions, NH3 addition lowers the temperature of combustion products. The results also show that in sharp contrast to that reported for conventional combustion, addition of NH3 in MILD condition does not increase the emission of NO, while the mass fraction of NO2 drops slightly. Overall, it is concluded that MILD combustion could be a promising route to NH3 combustion.