Impact of CO₂ dilution on biogas combustion in a double-layer porous media burner: From ignition to stable combustion

Achieving efficient and clean combustion of biogas is crucial for advancing the development of a circular economy. To assess the effects of component fluctuations in actual biogas on its combustion performance within a double-layer porous media burner, this study formulated model biogas compositions by incorporating different volume fractions of CO₂ into CH₄. The impact of varying the CO₂ dilution ratio (α) on the combustion characteristics of the CH₄/air mixture, from ignition to stable combustion were systematically examined, encompassing combustion modes, flame propagation characteristics, stable combustion ranges, temperature distributions and pollutant emission mechanisms. The results indicate that the initial stage of ignition reveals four combustion modes: quenching, submerged combustion, surface combustion and blow-off, determinted by α and U_in. At U_in = 0.2 m/s, the flame propagation mode transitioned from a blue flame to combustion wave propagation as α rised from 0 % to 30 %, accompanied by a significant decrease in flame propagation velocity. The upper limit of stable combustion diminishes with increasing α. An increase in α significantly reduces NO emissions. The reduction in NO emission concentration is mainly attributed to the significant decrease in thermal NO, with reaction R178 being the most significantly affected elementary reaction. This study elucidates the fundamental mechanisms governing the impact of CO₂ dilution on combustion behavior and NO emission characteristics in double-layer porous media biogas burners, thereby providing an essential theoretical foundation for the development of highly efficient, stable and low-emission biogas clean utilization technologies.