Combustion and Nitrogen Chemistry in Oxy-Fuel Flames

Sammanfattning: Oxy-fuel combustion is a Carbon Capture and Storage (CCS) technology that recently has shown afast progress towards the industrial scale application. In oxy-fuel combustion, oxygen instead of air isused as oxidizer to burn the fuel. The oxygen is mixed with recycled flue-gas in order to control theoverall combustion and heat transfer conditions. The resulting combustion atmosphere has asignificant impact on the combustion chemistry and its interaction with the nitrogen chemistry. Thiswork investigates the influence of oxy-fuel operation on the formation and oxidation of CO and thenitrogen chemistry with special attention on the reburning conditions. An indication of the effects ofthe CO2-rich atmosphere on the combustion chemistry in oxy-fuel flames is the elevated CO-peakconcentration compared to air fired flames. This work characterizes the reactions with CO2 whichform CO in oxy-fuel flame by modeling. The reduction of nitrogen oxides are investigated bycombining a similar modeling approach with experimental work at the Chalmers 100 kWth oxy-fuelunit.The model results show that CO formation in gaseous fired oxy-fuel flames is dominated by thereaction between CO2 and H-radicals. In oxy-fuel combustion of pulverized lignite, char gasificationby CO2 can be the dominating reason for an increased CO formation. However, the importance ofgasification is dependent on char particle temperature (which is related to char properties such asreactivity and particle size), as well as on the mixing conditions between fuel and oxidizer.The results from the propane fired reburning experiments show that the combustion chemistry of anoxy-fuel flame is disadvantageous for NO reburning compared to an air flame. The reduction of finalNO emissions [mg/MJ] to about 30% of those observed in air firing is caused by the recycle. Theeffect of the recycle ratio is particularly significant for cases with low reduction efficiencies in theflame.