Effects of ammonia and hydrogen on the sooting characteristics of laminar coflow flames of ethylene and methane

•Effects of NH3 and H2 on soot formation are experimentally studied in laminar coflow flames of either ethylene or methane.•Detailed chemical kinetic calculations used to understand effects of NH3 introduction to ethylene counter-flow flames.•NH3 chemically inhibits soot formation in both ethylene and methane flames proportionally to the amount of NH3 introduced, with increasing effects on larger precursors.•H2 chemically enhances soot formation in both ethylene and methane flames.