International Journal of Hydrogen Energy, Vol.45, No.16, 10164-10175, 2020
2Lower flammability limit of H-2/CO/air mixtures with N-2 and CO2 dilution at elevated temperatures
The lower flammability limits of H-2/CO/air mixtures with N-2 and CO2 dilution were systematic experimentally studied over a wide range of H2 blending ratios (0-100 vol%) with N-2 (0-67 vol %) and CO2 (0-67 vol%) dilution in the fuels under various elevated initial temperatures (298 K -473 K) and atmospheric pressure. The experimentation was conducted via an 8 L stainless steel cylindrical explosion vessel and using the metal wire fusing as the ignition source. The corresponding cases were also calculated using Kondo's correlation proposed based on a limiting flame temperature concept. To gain an insightful understanding of the effect of chemical kinetics at different H-2 fractions and CO2 dilution ratios, sensitivity analysis and H mole fractions were carried out using Chemkin-Pro. The experimental results showed that the lower flammability limits decreased with the increase of H-2 fractions especially when the H-2 content was low (x( H2) <= 0.25). Attributable to the accelerated oxidation of CO by the greater generation of OH from H-2/O-2 reaction, Le Chatelier's Rule tended to relatively over-estimate the lower flammability limits of H-2/CO mixtures with a small amount of H-2. Because of the larger heat capacity, and the inhibition effect on the oxidation of CO and the generation of H radicals, CO2 presented a stronger dilution effect on lower flammability limit than N-2. Moreover, the lower flammability limits for all measured syngas mixtures displayed great linear temperature dependence. A comparison between the experimental data and calculation results showed that, Kondo's correlation provided the satisfactorily accuracy predictions on the lower flammability limits of diluted syngas mixtures with lower H-2 fractions (x(H2) <= 0.5). However, when the H-2 fractions were high and the mixture was highly CO2 diluted, Kondo's correlation over-estimated the lower flammability limits and the prediction error would reach to 30%. The considerably distinctions were not only attributed to the inadaptable assumption against to the growing and lower behaviour of H-2 flame temperature at lower flammability limit, but also caused by the preferential diffusion of H-2, as well as the variation of the chemical effects under high H-2 content and high CO2 dilution conditions. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.