Coupling transport effects of H2 and CO2 on the thermochemical characteristics of biogas-hydrogen opposed diffusion flame

Abstract

With the numerical simulation and the detailed chemical mechanism, the coupling and individual transport effects of H2 and CO2 on the flame temperature, heat release rate (HRR), species distributions and critical radical pools of biogas-hydrogen opposed diffusion flame were analyzed quantitatively. The results show that the H2 transport predominates the changes of major species concentrations near the fuel side through the differential diffusion while it influences species concentrations in the reaction zone mainly through the improved chemical effects. The coupling transport effects of H2 and CO2 enhance the flame temperature with the increased(decreased) HRR in the lower-temperature(high-temperature) zones. The H2 transport dominates the higher HRR at lower temperatures by accelerating exothermic reactions of H + O2 + H2O = HO2 + H2O, H + OH + M = H2O + M and OH + HO2 = O2 + H2O, but predominates the decreased HRR at high temperatures through suppressing H + CH3(+M) = CH4(+M). Furthermore, OH + H2 = H2O + H contributes to the decreased HRR at high temperatures and enhanced HRR at lower temperatures simultaneously with the H2 transport. Besides, it is noted that the CO2 transport can only affect the heat release slightly at high temperatures. It is interesting that, with either H2 or CO2 transport, the peak flame temperature can be increased with the reduced heat release around the peak temperature location, which is resulted from the heat supply from the lower-temperature zone and the suppressed endothermicity, especially the decelerated CO2 dissociation via OH + CO = H + CO2, in the high-temperature zone. The ROP analysis confirms that the CO2 transport suppresses the CH4-related and H2-related chemical kinetics simultaneously, while the H2 transport improves the H2-related chemical kinetics but suppress the CH4-related chemical kinetics considerably. The H, O and OH radical pools can be improved by either H2 or CO2 transport, but their improvements are predominated by the H2 transport. The CO2 transport imposes the moderate influences on the critical radical pools by affecting H2O and CO2 dissociations via OH + H2 = H2O + H and OH + CO = CO2 + H at high temperatures. Overall, the CO2 transport could be neglected when investigating the thermochemistry of the biogas-hydrogen opposed diffusion flame.