Kinetic modeling investigation on the coupling effects of H2 and CO2 addition on the laminar flame speed of hydrogen enriched biogas mixture

The numerical simulation was conducted to investigate the coupling effects of H-2 and CO2 on the laminar flame speed and reaction pathways of the biogas-hydrogen mixtures. The same proportions of H-2 and CO2 were considered in the study. The dilution effects, chemical effects and thermal effects of H-2 and CO2 were isolated, while the contributions of different effects were also calculated to identify the dominating factor. The results show that H radical pool plays the important role in the variation of laminar flame speed of biogas-hydrogen mixture. The coupling dilution effects of H-2 and CO2 lead to the considerable decrease of H radical and dominate the decreased laminar flame speed. The H-2 chemical effects improve R84 (OH + H-2 = H2O + H) and R3 (O + H-2 = H + OH) quite effectively, which results in the reduced dropping trends of H radical and laminar flame speed from BG60H40 to BG40H60. The CO2 chemical effects suppress the major H radical production reactions of R84, R3, R99 (OH + CO = H + CO2) and R10 (O + CH3 = H + CH2O) more apparently at the fuel-rich condition, while importance of major consumption reactions (R53: H + CH4 = CH3+H-2, R52: H + CH3(+M) = CH4(+M) and R58: H + CH2O = HCO + H2) are higher at the fuel-rich condition. This dominates the more evident decrease of H radical and laminar flame speed at the fuel-rich condition. Based on the ROP analysis, R3, R46 (H + HO2 = 2OH), R52, R84, R99 and R158 (2CH(3)(+M) = C2H6(+M)) have the major contributions on the chemistry transition caused by the coupling effects of H-2 and CO2. The decreased contribution of R158 and R99 indicate the weakened CH4-related chemistry while the increased contributions of R3, R46, R52 and R84 indicate the improved H-2-related chemistry in the biogas-hydrogen flame. Furthermore, coupling effects of H-2 and CO2 or H-2/CO2-related chemistry can be monitored by the contribution variations of R84 and R99 on the OH radical concentration. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.