Biogas combustion with various oxidizers in a nanosecond DBD microplasma burner

This study concerns the effect of non-thermal plasma discharges on simulated biogas (mixture of CH₄ with 80–20 vol% CO₂) combustion at atmospheric pressure in synthetic air or synthetic air enriched by oxygen. The plasma-assisted, premixed combustion was performed in a porous-plate burner with dielectric barrier discharge microplasmas driven by nanosecond high-voltage pulses at 3 kHz and 10 kHz repetition rates in the burner holes. The characteristics of the plasma-assisted flames and the role of reactive oxygen species in the plasma-assisted combustion process supplying various oxidizers were determined using a spatial flame chemiluminescence scanning technique acquiring OH*, C₂* and CH* emission intensities. From the obtained results, the pathways of combustion enhancement by the plasma were established. During plasma-assisted combustion, the biogas flame stability has improved. The highest plasma impact on the flame stability was observed for the biogas mixture (CH₄-60%/CO₂-40%). The flame lift-off for a stoichiometric mixture was reduced by 54% with the discharge at 10 kHz repetition rate, but a decrease of fuel–air ratio φ resulted in reduced effect of plasma and the lift-off was reduced only by 38–10% with 10 kHz discharge and by 22–7% with 3 kHz discharge. The experiments with oxygen-enriched synthetic air showed that the oxygen addition increases the flammability limit of biogas mixtures (CH₄ with 50–20 vol% in CO₂), and allowed to burn mixtures which were not able to combust under normal conditions. However, the plasma-assisted combustion with oxygen enrichment showed a lower effect on the combustion enhancement than without. During experiments of plasma-assisted combustion, the plasma impact on NO_X emissions was also determined, showing that NO_X concentrations increased with increasing plasma power.