On the Stability and Characteristics of Biogas/Methane/Air Flames Fired by a Double Swirl Burner

CO2-diluted methane fuel is relevant to biogas combustion applications. Despite its poor heating value and low reactivity, which limit its practical applicability, biogas gains popularity as a renewable fuel. However, implementing it in combustion systems requires either modifying or replacing the existing burners. This study investigates the stability, temperature field, and pollutant emissions of CH4/CO2/air-premixed flames fired by a double-swirl burner. A CH4/air mixture of equivalence ratio, Φout, was used in the outer stream, while a CH4/CO2/air mixture was supplied to the inner stream. The CO2 mole fraction, xCO2 , in the inner fuel blend varied from 0 to 0.4 for various inner stream equivalence ratios, Φin. The stability diagram of these flames was mapped in terms of Φin verses xCO2 for a fixed Φout. Based on the stability map, the inflame temperature field was investigated for six flames. Increasing the %CO2 in the biogas modifies the stability map by increasing the inner stream lean blow-off limits. However, increasing Φout sustains the flame stability, while reducing the CO2 decreases the overall flame blow-off equivalence ratio. Flame size growth with increasing xCO2 requires a longer residence time for efficient combustion. The addition of CO2 physically and chemically affects the thermal flame structure, and hence the pollutant emissions. In this burner, ultra-low NOX emission was reported, while an increase in the CO and unburned hydrocarbons (UHC), with increasing xCO2 was observed. However, the results show that, for a given xCO2 , controlling Φin and Φout could reduce CO and UHC emissions.