Effect of oxygen enrichment on the flame stability and emissions during biogas combustion: An experimental study

In this paper, a biogas composition consisting of methane and carbon dioxide mixture in a pilot-scale model burner has been enriched with oxygen and the combustion characteristics including combustion stability and emission behaviour have been experimentally investigated. CH4 and CO2 gases have been burned as premixed and the blowout and flashback values of the mixture have been determined. Experiments have been performed at constant equivalence ratio (0.7) and the number of swirl (s = 1) at 3 kW thermal power. To examine the combustion stability, the external acoustic forced condition has been applied with two loudspeakers. Experimental results shown that the highest flame stability has been achieved in the 24% oxygen enrichment rate. The flame extinction occurred at 530 Pa amplitude of 21% oxygen enrichment ratio, although it fluctuated at the dynamic pressure amplitude of 775 Pa at the 24% oxygen enrichment ratio continued to burn. As the enrichment rate was increased to 28%, at the dynamic pressure of 1213 Pa and forced under 175 Hz frequency condition the flashback occurred at the flame. While the oxygen rate increased to 28% it has been observed that a decrease in stability compared to the 24% oxygen rate. Experimental results show that the emissions of CO are 2247 ppm at 21%, 372 ppm at 24% and 10 ppm at 28%. At the same oxygen enrichment rates, the values of NOx are 16 ppm, 1 ppm and 58 ppm, respectively. The decreasing has been observed in NOx emissions up to 24% oxygen enrichment rate