Analysis Of Denitrification Retrofits Of 130t/h Coal-fired Boiler

Nowadays,coal-fired units still account for the highest proportion of generating units in China.In order to control environmental hazards caused by flue gas discharged from coal-fired units.The State promulgated the Standard for Air Pollutants Emission from Thermal Power Plants in 2011,which set the special emission limits of atmospheric pollutants to control the emission concentration of nitrogen oxides below 100 mg/m~3.At the end of 2015,it was decided to implement the ultra-low emission standard for coal-fired power plants in an all-round way,in which the emission concentration of nitrogen oxides was below 50mg/m~3.Therefore,many coal-fired boilers have carried out flue gas denitrification reforms.Currently,denitrification technology mainly includes low nitrogen combustion technology,SNCR and SCR.There are differences in efficiency and economy among these denitrification technologies.At the same time,it will also have a certain impact on the operation conditions of the original boiler.The first denitrification retrofit of 130t/h coal fired furnace adopted low nitrogen combustion and SNCR/SCR combined denitrification technology.Among them,LNB and OFA are combined in low nitrogen combustion transformation technology.As the low nitrogen combustion mode prolongs the burning-out time of pulverized coal and increases the height of the flame center in the furnace,it makes the lower part of the flame folding angle easy to coke,and causes the increase of exhaust temperature and carbon content of fly ash.Under full load condition,boiler efficiency decreases by0.4%.Through the analysis of thermal operation test,it is concluded that the flue gas temperature at furnace outlet rises continuously with the increase of burnout air opening,and the NOx concentration at SCR inlet shows a downward trend under different boiler loads.It is determined that the incinerator opening should be controlled from 70%to 80%at low load,65%to 70%at intermediate load and 60%to65%at high load.It is found that the pulverized coal concentration in the three-stage air duct with 2#and 3#angles is 8%-10%higher than that of 1#and 4#angles.The denitrification efficiency of low nitrogen combustion is the highest when the pulverized coal is completely stopped,and the denitrification efficiency of low nitrogen combustion under the double pulverized coal operation mode is the lowest.Through the analysis of temperature field of SNCR urea lance in each area of130t/h pulverized coal furnace under different loads,the application of urea lance in each area of SNCR denitrification reaction was optimized.Because of the narrow temperature range of SNCR reaction and only one layer of catalyst in SCR reaction zone,the residence time of SNCR/SCR combined denitrification technology is relatively short,which results in low denitrification efficiency and high ammonia escape in the application of 130t/h pulverized coal furnace.In the operation test of meeting the ultra-low emission standard,compared with meeting the special emission limit,urea consumption increased by 15%-20%and ammonia escape increased by more than 5 ppm.SCR denitrification technology was used in the second denitrification transformation of 130t/h pulverized coal furnace.The key factors affecting the denitrification efficiency were compared and analyzed through experiments.It was found that the denitrification efficiency of SCR reactor was the best when the NH3/NOx molar ratio was 1.1 and the inlet flue gas temperature of catalyst was 410 C.The amount of urea decreased by 5%-10%after the catalyst activity was increased by regeneration.By analyzing the distribution of NOx concentration in flue gas after SCR urea spraying gun under different boiler loads,it is found that the concentration on both sides is higher than that in the middle area.Therefore,the flow rate of SCR urea spraying gun is optimized.In the operation test that meets the ultra-low emission standard,the amount of urea used with three-layer catalyst is about 5%lower than that with two-layer catalyst.Through comparative analysis of the application of SNCR/SCR combined denitrification technology and SCR denitrification technology in 130t/h pulverized coal furnace,when meeting the ultra-low emission standard,the urea consumption of SNCR/SCR combined denitrification technology is 38.6%-41.4%higher than that of SCR denitrification technology.