On the flame structure and emission characteristics of biogas combustion using the double flame co-firing concept

Biogas, a sustainable energy source, faces combustion challenges due to impurities and compositional variability. This study investigates a novel double-flame concept for premixed co-firing of biogas and methane, aiming to achieve low emissions and enhanced flame stabilization. A comprehensive experimental campaign was conducted using a double-swirl burner configured for dual fuel streams: a fuel-rich biogas/air mixture in the inner stream and a lean methane/air mixture in the outer stream. The effects of inner swirl intensity, CO2 dilution in biogas, and the equivalence ratios of both fuel streams were systematically examined. Diagnostic techniques included Particle Image Velocimetry (PIV) for flow field characterization, fine-wire thermocouples for temperature mapping, and exhaust gas analysis for pollutant measurement. Results show that increasing the inner swirl number (Sin = 0.41, 0.72, and 1.24) enhances mixing, strengthens recirculation, and promotes radial spreading of the high-temperature region. However, excessively high swirl intensities reduce the Damköhler number, adversely affecting flame stability. The imposed stratification between the fuel-rich inner and lean outer streams, combined with enhanced mixing, significantly reduces CO emissions—reaching as low as 4 ppm at Sin = 1.24, while NO levels remain low due to thermal dilution and fuel staging. At high swirl intensities, CO2 dilution in the inner stream has a minor impact on the thermal and flame structures. In contrast, variations in both inner and outer stream equivalence ratios affect flame temperature and flow unsteadiness, influencing thermal NO formation. These findings highlight the critical role of swirl–fuel interactions in governing flame behavior and emissions in staged biogas combustion, offering practical insights for the development of low-emission, fuel-flexible burners.