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

Abstract Industrial flaring is a notable global contributor to carbon dioxide emissions and other key pollutants. Introducing a separate assisting fluid near the base of these flames affects their hydrodynamics, thermodynamics, and chemistry, which in turn affects their efficiency and emissions. In this study, a burner constructed of two concentric tubes allowed for various generic burner geometries, where different fuels (methane or propane) and co-flow assisting fluids (air, steam, or inert gases) flowed through the annular space and the center tube, respectively. The effects of the composition and flow rate of the fuel and assisting fluid, as well as the burner head geometry, were investigated in terms of carbon conversion efficiency (CCE) and emission indices of black carbon and oxides of nitrogen. The CCE was observed to be essentially 100% for unassisted flames and remained at that level when assisting fluid, irrespective of composition, was added. However, at some critical flow rate of assisting fluid, the CCE collapsed, and the main flame blew off. In general, the CCE collapse occurred at considerably more mass of air compared to that of steam. Furthermore, adding the assisting fluid co-flow monotonically reduced the black carbon emission by orders of magnitude. On the other hand, the emission of oxides of nitrogen rose slightly with increasing air co-flow flow rate and dropped when steam or other diluents were added. These results showed that there was a range of assisting fluid flow rates, where the CCE was approximately 100%, while the emission of black carbon and nitrogen oxides were highly suppressed.