A Numerical Investigation of Oxygen-Enriched Biogas Counter-Flow Diffusion Flames

Biogas is an attractive biofuel which contributes to renewable energy recovery and pollutant emission reduction. Biogas technology has won universal attention and is continuously in development over the world. Combustion is currently one of the major applications of biogas. However, it encounters several problems related to flame stability and insufficient heating value. In the present paper, oxygen-enhanced combustion is proposed as a solution to overcome these difficulties. A numerical investigation of biogas laminar non-premixed counter-flow oxygen enhanced flames is achieved. The fuel is biogas produced in El-Karma wastewater treatment plant situated in Algeria. Flame structure and NOX pollutant emissions are examined over a wide range of scalar dissipations (from equilibrium to extinction) and O-2 contents (from 21% to 100% by volume), including all oxygen enhanced combustion regimes. The laminar flamelet model is considered with the detailed GriMech 3.0 chemical mechanism. Results show that flame temperature increases with oxygen enrichment and stability limits are improved. Radiation effects dissipated beyond an intermediate value of scalar dissipation for all O-2 levels in the oxidizer. This value increases with O-2 enrichment. NOx emissions are promoted for low and intermediate oxygen levels but drop for high oxygen-level enrichment. Practical systems using biogas as a fuel should operate at O-2 enrichment levels upper than 70% under scalar dissipations slightly greater than 10 s(-1) in order to minimize NOX emissions with acceptable levels of CO and CO2.