Effect of simultaneous H2 and NH3 addition on soot formation in co-flow diffusion CH4 flame

Simultaneous blending of hydrogen (H) and ammonia (NH) to hydrocarbon fuels can tackle the safety issues of H and improve burning efficiency of NH. While this strategy brings challenges for soot prediction due to the promotion effect of H and the suppression effect of NH, and the interactions between H and NH. In this study, the simultaneous addition of NH and H on soot formation was experimentally and numerically investigated in a co-flow diffusion CH flame. The interactions between NH and H, and how they impacted different soot formation processes were comprehensively revealed using a detailed soot sectional model. The decrease of peak SVF in CH flame caused by NH was 0.013 ppm, about 31.6 % smaller than that in CH/H flame (0.019 ppm), indicating that the inhibitive effect of NH on soot formation was promoted by H. The existence of H promoted the suppression effect of NH on soot nucleation, condensation and HACA processes in the CH flame. Compared with CH/NH flame, the pyrolysis rates of NH, NH and NH in the CH/NH/H flame were higher since more H and OH radicals were generated via H decomposition. This led to a larger consumption rate of H and OH radicals, which decreased the reaction rates of CH+OH=CH+HO and CH+OH=CH+HO, and promoted the combination of NO and CH. Both factors accounted for a stronger suppression effect of NH on the formation of A1 in CH/H flame than that in CH flame, and thus a stronger inhibitive effect on soot inception and condensation. Compared with the CH flame, NH resulted in a larger decline of H and OH radicals mole fractions in the CH/H flame, which explained the stronger suppression effect of NH on the HACA surface growth process in the CH/H flame.