Enhancement of ammonia combustion with partial fuel cracking strategy: Laminar flame propagation and kinetic modeling investigation of NH3/H2/N2/air mixtures up to 10 atm

The low combustion intensity of ammonia (NH3) raises great research needs of combustion enhancement strategies for its practical applications. Considering the high proportion of hydrogen in cracked gas of NH3, partial fuel cracking is a feasible strategy to enhance NH3 combustion. This work reports an experimental and kinetic modeling study on the laminar flame propagation of partially cracked NH3/air mixtures (NH3/H2/N2/air mixtures) up to 10 atm. Laminar burning velocities (LBVs) of partially cracked NH3/air mixtures are measured at various cracking ratios and equivalence ratios using a high-pressure constant-volume cylindrical combustion vessel. Our recently reported NH3/syngas model is updated to simulate the experimental results, which shows satisfactory performance on predicting the partially cracked NH3/air LBVs in this work, as well as the LBV and speciation data of NH3 and NH3/H2 combustion in literature. modeling analysis is performed to provide insight into effects of equivalence ratio, cracking ratio and pressure on laminar flame propagation of partially cracked NH3/air mixtures. The modified fictitious diluent gas method reported in our recent work is adopted to separate thermal and other effects in enhancement of NH3 combustion. The analysis results indicate that thermal effect only plays a minor role in the enhancement of laminar flame propagation of NH3 in partial fuel cracking strategy, while chemical effect should be significant for the enhanced laminar flame propagation. Due to the presence of H2 in cracked gas, reaction H + O2 (+M) = HO2 (+M) shows enhanced importance and makes the laminar flame propagation of partially cracked NH3/air mixtures more pressure-dependent. Furthermore, NO formation characteristics with increasing cracking ratio is also numerically investigated, which shows that fuel NO is the major NOx formation source. The results reveal a dramatic non-monotonic behavior of NO formation as the cracking ratio increases, which originates from the transition of NH3 chemistry to cracked gas chemistry.