Chemiluminescence-based characterization of tail biogas combustion stability under syngas and oxygen-enriched conditions

After the biogas is upgraded to CH4, a waste stream (tail biogas) is obtained, which contains methane in the order of few to tens vol.%. However, the composition of tail biogases can be unstable, therefore, safety should be maintained during the onsite combustion processes. Accordingly, a process control system with various sensors needs to be introduced. This study aims to analyze the experimental limits of flame stability using the chemiluminescence technique during the premixed combustion of low calorific tail biogas (mixture of CH4 with 15-30 vol% concentration in CO2) by adding oxygen or syngas. For the experimental investigation, a flat flame burner mounted in an enclosed combustion chamber was used. The stability of the flame was analyzed via the lift-off height by capturing the flame chemiluminescence from the excited OH* (282.9 nm), CH* (387.1 nm), and C-2* (514.0 nm) radicals. This study reveals that the chemiluminescence spatial intensity of species such as OH*, CH*, and C-2 * is enhanced with an increase in the addition of syngas and oxygen. The peak position of the emission intensities in the flame shifts depending on the composition of gases, showing the existence of unstable flame that can reach its blow-off limit. When syngas is added, an excess of air leads to an increase in flame lifting; however, an O-2 surplus results in more stable flame with minimized lifting. Moreover, it was determined that the lifting height of the flame is higher when syngas is added as compared to O-2 enrichment. Finally, the flue gas emissions, such as CO and NOx, were examined and the most influential formation parameter was determined.