Study On Effect Of OFA Arrangement And Air Ratio On Combustion Characteristics Of 350MW Supercritical Boiler

With rapid growth of the domestic economy,the air quality is deteriorating.PM2.5 air pollution,which is frequently beyond index in Beijing and the surrounding areas and the increasingly severe acid rain in Southwest China are the sign of the warning of air pollution.In order to save energy,improve the environment,promote the development of high efficiency clean power plant,Ministry of Environmental Protection(MEP),General Administration of Quality Supervision(AQSIQ)and the Electric Power Groups are gradually promoting the near zero emission of air pollutant of thermal power plants.Therefore,research on NO_x formation in boiler combustion process and NO_x emission reducing methods is of a positive practical significance.This paper takes a 350 MW opposed firing boiler as study object,study the effect of distance between OFA nozzle and the top-level burner and the effect of OFA ratio on furnace temperature field,velocity field,flue gas components and NO_x emission by numerical simulation.By comparing and analyzing the result,the main conclusions are shown as follows.With the fixed furnace outlet excess air coefficient,OFA nozzle size and OFA ratio,increasing the distance between OFA nozzle and top-level burner from 3 meters to 7 meters will cause a significant effect to furnace temperature field.When the distance between OFA nozzle to top-level burner is 3 meters,there is an obvious high temperature reburning zone above the OFA nozzle,and the furnace temperature fluctuates widely.When the OFA nozzle move upward,the furnace temperature fluctuation reduces,fullness of flue gas in the furnace firstly increases and then decreases,the temperature filed and velocity filed are more uniform.Meanwhile,when the OFA nozzle move upward,supplementation of oxygen by over-fire air is delayed,CO concentration at furnace outlet increases significantly,boiler efficiency decreases,and the NO_x concentration also decreases significantly.When the distance between OFA nozzle and top-level burner is increased from 3 meters to 7 meters,the NO_x emission decreases from 336mg/m~3(O2=6%)to 252mg/m~3(O2=6%)Comprehensively considering the boiler efficiency,boiler cost and NO_x emission index,the optimum distance between OFA nozzle and the top-level burner is 5 meters.When the OFA nozzle is fixed to the location 5 meters upward the top-level burner,with the fixed air amount and OFA nozzle size,numerical simulation is conducted to study the effect of OFA ratio on combustion and NO_x emission.As OFA ratio increasing,the flue gas temperature in burner zone decreases,and the gas temperature in reburning zone above the OFA nozzle increases.When OFA ratio is 35%,furnace outlet NO_x emission is 226 mg/m~3(O2=6%),reaching the lowest point.Considering the furnace temperature field,ignition stability in burner zone and stable combustion at low load,the optimum OFA ratio is 25% with the 285 mg/m~3(O2=6%)NO_x emission at furnace outlet.