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

Simultaneous blending of hydrogen (H2) and ammonia (NH3) to hydrocarbon fuels can tackle the safety issues of H2 and improve burning efficiency of NH3. While this strategy brings challenges for soot prediction due to the promotion effect of H2 and the suppression effect of NH3, and the interactions between H2 and NH3. In this study, the simultaneous addition of NH3 and H2 on soot formation was experimentally and numerically investigated in a co-flow diffusion CH4 flame. The interactions between NH3 and H2, and how they impacted different soot formation processes were comprehensively revealed using a detailed soot sectional model. The decrease of peak SVF in CH4 flame caused by NH3 was 0.013 ppm, about 31.6 % smaller than that in CH4/H2 flame (0.019 ppm), indicating that the inhibitive effect of NH3 on soot formation was promoted by H2. The existence of H2 promoted the suppression effect of NH3 on soot nucleation, condensation and HACA processes in the CH4 flame. Compared with CH4/NH3 flame, the pyrolysis rates of NH3, NH2 and NH in the CH4/NH3/H2 flame were higher since more H and OH radicals were generated via H2 decomposition. This led to a larger consumption rate of H and OH radicals, which decreased the reaction rates of CH4+OH=CH3+H2O and C2H4+OH=C2H3+H2O, and promoted the combination of NO and C2H. Both factors accounted for a stronger suppression effect of NH3 on the formation of A1 in CH4/H2 flame than that in CH4 flame, and thus a stronger inhibitive effect on soot inception and condensation. Compared with the CH4 flame, NH3 resulted in a larger decline of H and OH radicals mole fractions in the CH4/H2 flame, which explained the stronger suppression effect of NH3 on the HACA surface growth process in the CH4/H2 flame.