An experimental study on the carbon conversion efficiency and emission indices of air and steam co-flow diffusion jet flames

•Gas flaring has notable contributions to global warming and local air pollution.•A systematic study was done using air and steam co-flow diffusion jet flames.•Carbon conversion efficiency and emission indices of soot and NOx were measured.•Adding assisting fluids had non-monatomic response on emissions of diffusion flames.•Useful flow conditions existed that optimized soot, NOx, and fuel emissions