Analysis Of Wall Temperature Characteristics Of Supercritical Carbon Dioxide Coal-fired Boiler With Flue Gas Circulation

Supercritical carbon dioxide（S-CO₂）Brayton cycle,as a new type of cycle system,can achieve higher cycle efficiency than Rankine cycle under the same parameters.However,the temperature of the inlet working fluid of the cooling stave of the S-CO₂ coal-fired boiler is relatively high,which easily leads to over-temperature of the cooling stave and affects the safety of the circulation system.Therefore,in this paper,a thermal calculation model of a 1000 MW S-CO₂boiler is constructed and flue gas recirculation is used to reduce the wall temperature of the furnace cooling wall.Secondly,this paper proposes a structural design scheme of the S-CO₂boiler suitable for flue gas recirculation,and explores the effect of flue gas recirculation on the temperature of the cooling wall of the S-CO₂ boiler and the operation safety and economy.By constructing a thermal calculation model of the S-CO₂boiler,the paper compares the differences in the furnace structure and wall temperature characteristics of the S-CO₂boiler and the typical steam boiler.Considering the heat transfer characteristics of supercritical CO₂ working fluid,the thermal calculation model of S-CO₂ boiler was constructed on the basis of the traditional steam boiler thermal calculation method,and a1000 MW S-CO₂ boiler was designed.Compared with the typical 1000 MW supercritical steam boiler,the maximum wall temperature of the S-CO₂ boiler is 690°C,which is much higher than 450°C of the steam boiler,so measures need to be taken to reduce its wall temperature.This paper analyzes the influence of different flue gas recirculation rates on the temperature and operating characteristics of the cooling stave of the S-CO₂ boiler,and proposes a structural design scheme of the S-CO₂ boiler suitable for flue gas recirculation.The calculation results show that when the S-CO₂ boiler directly adopts the flue gas recirculation and the flue gas recirculation rate is 20%,the maximum cooling wall temperature of the S-CO₂ boiler decreases from 690℃to 640℃,but the recirculated flue gas leads to the superheater.The temperature of the working fluid at the outlet decreases,the temperature of the flue gas at the furnace outlet decreases,the temperature of the working fluid at the outlet of the reheater increases,and the temperature of the exhaust gas from the furnace increases.Therefore,it is necessary to optimize the structure of each heating surface.After the optimized design,the temperature of the working fluid at the outlet of the boiler superheater and reheater reaches the design value of 620°C,the exhaust gas temperature is reduced to 123°C,and the thermal efficiency of the boiler is increased from 92.8%before optimization to 94.0%,which is comparable to the one without flue gas recirculation.Efficiency is comparable.This paper explores the effect of flue gas recirculation on the safety and economy of S-CO₂ boiler operation.Compared with the boiler without flue gas recirculation,the system in this paper adds two recirculation fans with a power of 450 k W.When the flue gas recirculation rate is 20%,the electricity consumption rate of the S-CO₂ boiler plant increases by about 0.1%,and the coal consumption for power supply increases by0.21g/（k W·h）,a relative increase of 0.08%.Compared with the Rankine cycle with the same power generation,the coal consumption rate of the S-CO₂cycle is reduced by 10.5g/（k W·h）,a relative increase of 4.11%,which shows that the flue gas recirculation has little effect on the power generation economy of the S-CO₂ boiler.The simulation calculation shows that the recirculation of flue gas reduces the concentration of H₂S and CO in the furnace wall area,the high temperature corrosion rate will decrease,and the safety of boiler operation will be improved.