The influence of CO2 in biogas flammability limit and laminar burning velocity in spark ignited premix combustion at various pressures

Biogas is an alternative energy source that is sustainable and
renewable containing more than 50% CH4 and its biggest impurity or inhibitor is CO2. Demands for replacing fossil fuels require an improved fundamental understanding of its combustion processes. Flammability
limits and laminar burning velocities are important characteristics in
these processes. Thus, this research focused on the effects of CO2 on biogas flammability limits and laminar burning velocities in spark ignited premixed combustion. Biogas was burned in a spark ignited spherical combustion bomb. Spherically expanding laminar premixed flames, freely propagating from spark ignition in initial, were continuously recorded by a high-speed digital camera. The combustion bomb was filled with biogas-air mixtures at various pressures, CO2 levels and equivalence ratios (φ) at ambient temperature. The results were also compared to those of the previous study into inhibitorless biogas (
methane) at various pressures and equivalence ratios (φ). Either the flammable areas become narrower with increased percentages of carbon dioxide or the pressure become lower. In biogas with 50% CO2 content, there was no biogas flame propagation for any equivalence ratio at reduced pressure (0.5 atm). The results show that the laminar burning velocity at the same equivalence ratio declined in respect with the increased level of CO2. The laminar burning velocities were higher at the same equivalence ratio by reducing the initial pressure.