Flammability and burning rates of low quality biogas at atmospheric condition

Abstract

This study focuses on the flammability and burning rate of simulated and actual biogas-air mixture for equivalence ratio of 0.5 to 1.0. The content of simulated biogas was set at 50% CH4 and 50% CO2 to emulate the typical composition of low quality biogas. Studies on low quality biogas combustion with this particular CH4 and CO2 content are still scarce in literature. It was found that the flammability limits fall within the same range for both simulated and actual biogas from 0.6 to 0.9 which is substantially narrower compared to pure methane. Incidence of flame buoyancy were observed for both simulated and actual biogas at equivalence ratio of 0.6 and 0.7. The maximum burning rates were almost identical for simulated and actual biogas at 21.18 cm/s and 24.74 cm/s respectively. It was also found that the peak of burning rate occurs at the leaner equivalence ratio of 0.8 for both simulated and actual biogas. These observations imply the significance of CO2 on biogas combustion. Compared to pure methane, the reduction in burning rate and the shift in the peak burning rate to the leaner mixture suggest the physical and chemical effects of CO2 on biogas combustion. Physically, the presence of CO2 could adversely affect the temperature driven CH4 preferential diffusion into the reaction zone since CO2 could absorb some of the heat released during combustion. When present at 50% or higher, CO2 could significantly reduce OH radical formation thereby narrowing the flammability limits and reducing the burning rates.