Experimental and numerical study of the effects of oxygen-enriched air on the laminar burning characteristics of biomass-derived syngas

Abstract The laminar burning velocities (LBVs) of seven different composition stoichiometric mixtures (CO: 15%–20%, H2: 5%–20%, CH4: 5%–15%, CO2: 15%, N2: 50%) under different oxygen levels (O2: 21%–60%) were experimentally measured by spherical expansion flame method. Numerical calculations were conducted with the PREMIX code using seven detailed chemical reaction mechanisms (FFCM-1, GRI 3.0, USC-Ⅱ, San Diego + NOx, HP-mech, Li-2015, CRECK + NOx) and compared with experimental data. The CRECK mechanism provides an over-prediction of the LBV of CH4-free mixtures. San Diego is more prominent than CRECK with the addition of CH4, followed by GRI 3.0 and HP-mech. Three mechanisms—FFCM-1, Li-2015, USC-Ⅱ—that matched the experimental results well were chosen to perform the LBV calculations for a wider range of initial conditions (Φ: 0.6–1.6, Tu: 300 and 400 K, P0: 1 and 3 atm) for variable composition mixtures. The results show that for all cases, the LBV is considerably increased with increasing oxygen concentration. CH4-rich mixtures are more sensitive to oxygen concentrations and work to increase the LBVs mainly through thermal effects. An oxygen-rich environment makes the flammability limit wider even with high initial pressure. H2-rich mixtures accompanied by higher thermal diffusivity are more susceptible to the initial temperature, while chemical effects do not dominate in oxygen-rich environments. In oxygen-rich environments, diffusional-thermal instability is suppressed while the cellular structure of the flame is mainly affected by fuel composition and hydrodynamic instability. The formation of NO in flames of different composition mixtures in an oxygen-rich environment was analyzed.