Characterizing the effect of H2O dilution on the combustion and emission characteristics of NH3/H2 mixtures in laminar flame and HCCI engine

H2 addition to NH3 is a feasible solution to enhance the combustion of NH3, whereas increased NOx emission caused by H2 remain challenging. Emulating exhaust gas recirculating (EGR) technology, this study aims to investigate the effect of water (H2O) dilution on the combustion characteristics and NOx emission of NH3/H2 flames. The laminar burning velocity (LBV) of NH3/H2 mixtures with H2O dilution was measured using freely propagating spherical flames. The effects of H2 fraction and equivalence ratio (φ) on the LBV of NH3/H2 mixtures with H2O dilution was comprehensively investigated at initial temperature of 398K and initial pressure of 1 bar. An optimized mechanism was proposed and validated to perform kinetic analysis and NO emissions calculation. H2O dilution reduced the LBV of NH3/H2 mixtures, with a more pronounced effect observed for mixtures at lean conditions. Thermal effects dominate the impact of H2O dilution on LBV, while both thermal and chemical effects lead to the reduction of LBV. Transport effects caused by H2O dilution promotes LBV albeit it accounts for the smallest proportion and increases with the φ. H2O dilution can significantly reduce NO emission of NH3/H2 mixture at lean conditions. To investigate the effect of H2O dilution on NH3/H2 mixture in engine, simulations of HCCI engine based on zero-dimensional reactor shows that a small amount of H2O dilution (∼10 %) can effectively improve indicated mean effective pressure (IMEP), thermal efficiency, slow down heat release rate and significantly reduce NO emission. Additionally, 10 % H2O dilution can widen the range of stable engine operation and reduce the risk of ringing and misfire.