An experimental and kinetic modeling study on NH3/air, NH3/H2/air, NH3/CO/air, and NH3/CH4/air premixed laminar flames at elevated temperature

Ammonia combustion helps to realize the off-site use of renewable energy. To understand the combustion characteristics of NH3, laminar burning velocities (SL) of NH3/air, NH3/H2/air, NH3/CO/air, and NH3/CH4/air with various mole fractions of H2, CO, and CH4 in fuel (α), equivalence ratios (ϕ = 0.7∼1.4) at elevated initial temperature (Ti = 298∼423 K) were measured in a constant-volume combustion vessel. Based on updated chemistry of NH2, NNH, N2H2, and NOx, a detailed kinetic model with 169 species and 1268 elementary reactions was developed and validated against the SL, NOx emissions, NH3/NO interaction, and ignition delay times measured by this work and literature. The experimental results show that SL of NH3/H2/air, NH3/CO/air, and NH3/CH4/air flames have different dependence on α. It is nearly exponential for NH3/H2/air flame, non-linear for NH3/CO/air flame, and nearly linear for NH3/CH4/air flame. NH3/air flame has the strongest temperature dependence, followed by NH3/H2/air, NH3/CH4/air, and NH3/CO/air flames. The updated NH3 chemistry, especially NNH chemistry (NNH = N2 + H, N2H2 + H = NNH + H2, and N2H2 + O = NNH + OH), enables the present model obtain better temperature dependence of NH3/air flame. Kinetic modeling analysis shows that the production and reduction of NOx are preferred by the lean and stoichiometric flames of NH3/H2/air, NH3/CO/air, and NH3/CH4/air because of the increased concentrations of NHi and H/O/OH radicals. In the lean flames of NH3/H2/air, NH3/CO/air, and NH3/CH4/air, H2, CO, and CH4 can promote the conversion of NH2 to NO, and relatively inhibit the NO reduction by NH2, resulting in the increase in NO emissions. In the rich flames of NH3/H2/air and NH3/CO/air, NH2 and NH can produce N2H2 and N2H3 through reactions of NH2 + NH = N2H2 +H and NH2 + NH = N2H3, which dominate the promotion and inhibition of laminar flame propagation, respectively.