Effect Of Flue Gas Recirculation On Low-nitrogen Combustion Characteristics Of Gas Boilers

With the adjustment of national energy structure,the consumption of natural gas is increasing year by year.Therefore,in the boiler industry,gas-fired boilers are replacing coal-fired boilers that are highly polluting and energy consuming.However,gas-fired boilers also have the problem of high NOx emission in using.Aiming at the problem of high NOx emissions,this article through the gas boiler to establish three-dimensional model,numerical simulation was carried out on the model with CFD software Fluent,the boiler combustion situation(flow field,temperature field,the formation of NOx concentration,etc.)and the excess air coefficient,the relationship between the different boiler load,and the introduction of 0%to 20%after the flue gas recirculation,the impact on the combustion and NOx generation.The main work and conclusions are as follows:(1)when the boiler load is 100%,the influence of excess air coefficient on combustion is studied.By establishing z=0 section,the average temperature and maximum temperature on the section are compared,and it is found that the maximum temperature on the section changes little with the increase of excess air coefficient(from 1 to 1.3),and it is stable at about 2160K.With the increase of excess air coefficient,the average temperature of the cross section and the generated concentration of NOx showed a trend of first increasing and then decreasing.When the excess air coefficient was 1.1 and 1.2,the maximum values were 1360K and 110mg/m3,respectively.(2)when the excess air coefficient is 1.1,the influence of boiler load on combustion and NOx generation is studied.It is found that as the boiler load continues to decrease(from 100%load to 40%load),the maximum speed in the furnace also continues to decrease with the reduction of load,from 116m/s at 100%load to 45m/s at 40%load;The highest temperature in the furnace also showed a trend of decreasing from 2245K at 100%load to 2185K at 40%load.Therefore,the NOx production concentration also decreases along with the decrease,from 110mg/m3 at 100%load to 85mg/m3 at 40%load.(3)When the excess air coefficient is 1.1 and the boiler load is 100%,study the effects of different flue gas recirculation rates(0%to 20%)on combustion.It can be seen from the calculation that with the increase of the flue gas recirculation rate,the oxygen concentration at the air inlet continuously decreases,from the maximum oxygen concentration of 21%to the minimum oxygen concentration of 17.52%.The maximum temperature of the combustion in the furnace decreases continuously with the increase of the flue gas recirculation rate.The maximum temperature when no flue gas recirculation is introduced is 2235K,which corresponds to 1984K when the flue gas recirculation rate is 20%,a decrease of 251K.Among them,the temperature drop is the largest in the 5%-10%flue gas recirculation interval.As the temperature decreases,the NOx production concentration also decreases continuously with the increase of the flue gas recirculation rate,from 110mg/m3 when flue gas recirculation is introduced to 38mg/m3 when the flue gas recirculation rate is 20%,about 65%.