Experimental characteristics of adding biogas to premixed self-excited oscillating methane swirling flames

This work is the first to examine the influence of renewable energy biogas addition on the characteristics of methane self-excited thermoacoustic oscillation and pollutant NOx emission. Meantime, the swirl intensity and methane enrichment degree that affect the biogas combustion are also explored. Results indicate that the methane premixed self-excited oscillation swirling flames are effectively suppressed during the biogas addition and present two dynamic combustion states: high-frequency self-excited oscillation and swirl stable combustion. Furthermore, the overall flame temperature and NOx emissions have been reduced, which reveals that the introduction of biogas has effectively restrained thermoacoustic oscillation and limited NOx emissions. Notably, the combustion and emission characteristics of methane/biogas co-firing under low swirl intensity are superior to those under high swirl intensity. However, enriching biogas with more methane results in better flame dynamics of biogas/methane but comes with the cost of higher NOx generation. Regarding the flame and schlieren morphology, with the increase of biogas content in the fuel, the flame shape gradually transforms from the compact and robust M-typed flame to the loose and trumpet V-typed steady flame. The flame schlieren's visu-alization characterizes the flow field and vortex structures' stretching and deformation in methane-air premixed flame with biogas addition. The distribution appearance of the flame flow field and vortex under lean com-bustion are more complex and diverse than those under rich combustion. The study's conclusions may contribute to optimizing and improving the combustion and emission performance of premixed combustors primarily based on fossil fuel methane.