Combustion and Nitrogen Chemistry in Oxy-Fuel Flames

Licentiate thesis, 2011

Oxy-fuel combustion is a Carbon Capture and Storage (CCS) technology that recently has shown a fast progress towards the industrial scale application. In oxy-fuel combustion, oxygen instead of air is used as oxidizer to burn the fuel. The oxygen is mixed with recycled flue-gas in order to control the overall combustion and heat transfer conditions. The resulting combustion atmosphere has a significant impact on the combustion chemistry and its interaction with the nitrogen chemistry. This work investigates the influence of oxy-fuel operation on the formation and oxidation of CO and the nitrogen chemistry with special attention on the reburning conditions. An indication of the effects of the CO2-rich atmosphere on the combustion chemistry in oxy-fuel flames is the elevated CO-peak concentration compared to air fired flames. This work characterizes the reactions with CO2 which form CO in oxy-fuel flame by modeling. The reduction of nitrogen oxides are investigated by combining a similar modeling approach with experimental work at the Chalmers 100 kWth oxy-fuel unit. The model results show that CO formation in gaseous fired oxy-fuel flames is dominated by the reaction between CO2 and H-radicals. In oxy-fuel combustion of pulverized lignite, char gasification by CO2 can be the dominating reason for an increased CO formation. However, the importance of gasification is dependent on char particle temperature (which is related to char properties such as reactivity 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 an oxy-fuel flame is disadvantageous for NO reburning compared to an air flame. The reduction of final NO emissions [mg/MJ] to about 30% of those observed in air firing is caused by the recycle. The effect of the recycle ratio is particularly significant for cases with low reduction efficiencies in the flame.