Numerical Simulation Of NO_X Formation Characteristics In Flue Gas Recirculation Combustion Process Of S-CO₂ Coal-fired Boiler

In recent years,the supercritical carbon dioxide（S-CO₂）Brayton power cycle power generation technology has attracted the attention of many researchers at home and abroad due to its high efficiency and flexibility.Countries around the world have also increased their support for the research of this new coal-fired power generation technology.However,the current research shows that the S-CO₂ boiler needs to adopt the different furnace type and heating surface arrangement than the conventional hydraulic boiler,which results in the higher temperature and NO_Xemission in the furnace than the conventional water boiler.The introduction of flue gas recycling technology can reduce the combustion temperature and NO_X emission level in the furnace at the same time,which is a reasonable and targeted operation measure.However,the research on this aspect is still more reasonable than that of the conventional water boiler Less.Therefore,the combustion process and NO_Xgeneration characteristics of S-CO₂ coal-fired boiler under the condition of flue gas recycling are studied in depth.In this paper,a numerical simulation was done to investigate the combustion process and NO_X generation characteristics of a 1000MW supercritical carbon dioxide coal-fired boiler,employing the Computational Fluid Dynamics（CFD）code ANSYS FLUENT.In the process of numerical simulation,the gas-phase turbulent flow is simulated by Realizable k-εequation model,P1 model for radiative heat transfer,Discrete Phase Model for pulverized coal particle movement,two-competing-rates model for devolatilization,eddy-dissipation model for volatile combustion,kinetic/diffusion-limited model for char combustion,and post-treatment process of numerical simulation for NO_X generation were used.In this paper,the effects of flue gas circulation rate,flue gas circulation distribution ratio and flue gas circulation modes on the combustion characteristics and NO_X generation characteristics in the furnace were investigated in detail.The results show that:（1）Flue gas recirculation can effectively reduce the combustion temperature in the furnace,and increasing the flue gas recirculation rate will reduce the NO_X emission at the furnace outlet.When the flue gas circulation rate reaches27%,the NO_X emission concentration of the boiler decreases obviously.At this time,the NO_X concentration at the furnace outlet is 302 ppm.If the recirculation rate continues to increase,the NO_X reduction is not obvious;（2）With the increase of the bottom circulation flue gas distribution ratio,the NO_X concentration at the outlet increases,and the combustion effect is the best when the flue gas circulation distribution ratio is relatively uniform;（3）At the same flue gas recirculation rate,when the recirculation flue gas and secondary air are mixed,the temperature uniformity in the main combustion zone of the furnace increases and the local high temperature value decreases.With the increase of the ratio of flue gas recirculation,the maximum temperature of gas in the furnace decreases.The maximum temperature difference can reach 87K under different injection methods.The research results of this paper provide theoretical basis and guidance for the structural design and optimization design of 1000MW supercritical carbon dioxide coal-fired boiler.