Improved combustion stability of biogas at different CO2 concentrations using inhomogeneous partially premixed stratified flames

Abstract

Biogas is a renewable and clean energy source that plays an important role in the current environment of low- carbon transition. If high-content CO₂ in biogas can be separated, transformed, and utilized, it not only realizes high-value utilization of biogas but also promotes carbon reduction in the biogas field. To improve the combustion stability of biogas, an inhomogeneous, partially premixed stratified (IPPS) combustion model was adopted in this study. The thermal flame structure and stability were investigated for a wide range of mixture inhomogeneities, turbulence levels, CO₂ concentrations, air-to-fuel velocity ratios, and combustion energies in a concentric flow slot burner (CFSB). A fine-wire thermocouple is used to resolve the thermal flame structure. The flame size was reduced by increasing the CO₂ concentration and the flames became lighter blue. The flame temperature also decreased with increase in CO₂ concentration. Flame stability was reduced by increasing the CO₂ concentration. However, at a certain level of mixture inhomogeneity, the concentration of CO₂ in the IPPS mode did not affect the stability. Accordingly, the IPPS mode of combustion should be suitable for the combustion and stabilization of biogas. This should support the design of highly stabilized biogas turbulent flames independent of CO₂ concentration. The data show that the lower stability conditions are partially due to the change in fuel combustion energy, which is characterized by the Wobbe index (WI). In addition, at a certain level of mixture inhomogeneity, the effect of the WI on flame stability becomes dominant.