The Generation and Suppression of NOx and N2O Emissions in the Oxy-Fuel Combustion Process with Recycled CO2 (an Overview)

Abstract

It is shown that combustion of solid fuels in an oxygen atmosphere with recycled flue gases is an efficient method for capture of CO2. The use of the circulating fluidized bed (CFB) technology in the oxy-fuel combustion process reduces the CO2 capture costs and nitrogen oxide emissions compared with flare combustion. The mechanisms of generation and suppression of nitrogen and nitrous oxides during CFB combustion of solid fuels are considered. The basic reactions that occur during the exit and burning of volatiles and combustion of char resulting in generation of NO, N2O, and N2 are indicated. The effect of the secondary air delivery on nitrogen oxide emissions is studied. It is shown that the degree of converting fuel nitrogen into N2O is reduced when the degree of converting nitrogen into NOx is increased. Comparative analysis that the effect of flue gas recycling and other process factors has on generation of NO and N2O during combustion in air, oxygen, and CO2 has been made. It is shown that, owing to recycling, the fractions of NOx and N2O generated from fuel nitrogen during oxy-fuel combustion are significantly lower than those during combustion in air. Mathematical simulation of various complicated processes related to generation and suppression of hazardous pollutant emissions has recently become a generally accepted method that allows the prediction at relatively low costs of such processes during the combustion of various fuels. However, as applied to new technologies for combustion of various biomass types and cofiring of coal and biomass, these methods have not yet been properly developed. The most preferable mathematical models for calculating nitrogen oxide emissions are provided, and the predicted results are compared with experimental data. The lines of further research, predominantly in the simulation of cofiring of coal and biomass, are outlined. It is shown that comprehensive investigations of generation and suppression of hazardous pollutant emissions during cofiring of coals and biomass of various kinds, in particular, during oxy-fuel combustion with recycled CO2 are of topical interest.