Biogas combustion characteristics under varying carbon dioxide dilution and hydrogen enrichment

Abstract

This study investigates the combustion characteristics of four different types of gaseous fuels namely methane (CH4), biogas, simulated biogas with varying carbon dioxide (CO2) content and hydrogen enriched biogas under atmospheric condition. Flammability, laminar burning velocity and flame stability of each fuel are among the combustion characteristics investigated using spherical flame method. Measurement of these parameters is important to explain the effects of CO2 and hydrogen on biogas combustion which are still lacking in literature. CH4 flammability range was found to be within the equivalence ratio of 0.7 to 1.3 with peak laminar burning velocity at approximately 36 cm/s which agrees well with previous findings. For biogas, flammability range narrows to equivalence ratio range of 0.6 to 0.9 with a peak laminar burning velocity of around 24 cm/s. For simulated biogas, as CO2 content increased, the flammability range tended to become narrower with appreciable decrease in laminar burning velocity. Peak laminar burning velocity value steadily decreased to 21%, 34% and 45% as CO2 content was increased from 20% to 40% and 50% respectively. CO2 could slow down the reactions that produce radicals important for CH4 dissociation. It could also modify mass and thermal diffusion pattern as indicated by the corresponding changes in Markstein length. For hydrogen enriched biogas, the flammability limits widened to the leaner side from equivalence ratio of 0.4 to 0.9 for 30% and 40% enrichment. Both flame speed and laminar burning velocity were enhanced with hydrogen enrichment especially at 30% and 40% which led to significant increase in maximum laminar burning velocity to 52 % and 88 % respectively. Flame appeared to become less stable under leaner conditions as supported by the occurrence of buoyancy and mild cellularity at the equivalence ratio of 0.4 and 0.5 under 30% and 40% hydrogen enrichment. Simulation revealed dramatic increase in H radical at 30% hydrogen enrichment onwards. These observations imply the significance of hydrogen on biogas combustion both on laminar burning velocity and flame stability.