Numerical Simulation Of NO_x Generation Characteristics In Combustion Process Of S-CO₂ Swirl Opposed Coal-fired Boiler

Compared with the traditional steam cycle,S-CO₂Brayton cycle has the potential application prospect of further improving the power generation efficiency of coal-fired power boilers because of its compact structure,low metal corrosion and high cycle efficiency.In recent years,more and more countries have carried out research on S-CO₂Brayton cycle power generation,and China has also carried out exploratory research on the key technologies of S-CO₂coal-fired power generation.Due to the large difference between S-CO₂heat transfer coefficient and steam,the increase of furnace inner wall temperature has an impact on pulverized coal combustion process and NO_xgeneration characteristics.In this paper,1000MW S-CO₂coal-fired boiler is taken as the research object,and the swirl opposed arrangement is selected for the burner.The pulverized coal combustion and NO_xgeneration characteristics of S-CO₂coal-fired boiler are analyzed,so as to provide theoretical and technical support for the realization of low NO_xS-CO₂coal-fired boiler.Specific work includes:Firstly,a single pulverized coal swirl burner is simulated,and the gas flow rate,combustion temperature,component concentration,particle movement and pollutant emission characteristics in the furnace under different flue gas recirculation rates are analyzed.The effects of different flue gas recirculation rates on pulverized coal combustion characteristics and NO_xformation in the furnace are obtained.The results show that flue gas recirculation can further reduce the generation of NO_xon the basis of low NO_xemission of staged combustion.When the flue gas recirculation rate is 0%,10%and 20%,the maximum NO_xconcentration in the reflux area is 375ppm,160ppm and 155ppm respectively.Under different air consumption coefficients,the NO_xoutlet concentration decreases with the increase of flue gas recirculation ratio,and it is the smallest when the flue gas recirculation rate is 20%.Secondly,a 1000MW S-CO₂swirl opposed coal-fired boiler model is established to explore the effects of different flue gas recirculation rates on pulverized coal combustion process,heat transfer process and NO_xgeneration characteristics of coal-fired boiler.The results show that the flue gas recirculation technology can reduce the gas temperature level in the furnace,delay the ignition time of pulverized coal and increase the ignition temperature of pulverized coal;The larger the amount of recycled flue gas,the faster the overall furnace temperature decreases.Compared with the non circulating flue gas,when the flue gas recirculation rate is 10%,20%,27%and 35%,The ignition temperature of pulverized coal increased by 136k,184k,204k and 220K respectively,the NO_xat the furnace outlet is reduced by 47.6%,50.5%and 54.1%respectively.At the same time,using the flue gas recirculation technology,the high heat load area on the boiler wall is significantly reduced,the flue gas recirculation volume is gradually increased,and the overall heat load of the boiler is gradually reduced.When the flue gas recirculation rate is less than 10%,local over temperature will occur on the boiler wall.When the flue gas recirculation rate is higher than 10%,the boiler wall temperature is lower than 973k.The combustion process and NO_xgeneration characteristics of 1000MW S-CO₂swirl opposed coal-fired boiler with different pulverized coal particle sizes were simulated.The results show that the larger the particle size of pulverized coal,the higher the ignition temperature and the lower the temperature in the furnace.When the particle size of pulverized coal changes from 31μm increases to 71μm,the NO_xconcentration at the furnace outlet decreases from 357 ppm to 250 ppm,a decrease of about 107 ppm,and the NO_xconcentration can be reduced by about 30%.Finally,the combustion process and NO_xgeneration characteristics of 1000MW S-CO₂swirl opposed coal-fired boiler under different burnout air volume are studied.The results show that with the increase of the proportion of burnout air,the temperature of high-temperature gas in the lower furnace decreases,and the NO_xconcentration at the outlet also decreases gradually.When the proportion of burnout air is 25%,the NO_xconcentration decreases to the lowest.If the proportion of burnout air continues to increase,the NO_xconcentration at the furnace outlet increases instead of decreasing.