Coupling Research Of Low Nitrogen Oxides Combustion And SNCR Denitrification In A Circulating Fluidized Bed Boiler

In order to meet the ultra-low emission standards,the low-nitrogen combustion reformation in coal-fired circulating fluidized bed boilers are actively underway.Since the SNCR technology is advantageously utilized in cyclone separators,this technology is still the first choice for the flue gas denitrification in the cyclone separator of the circulating fluidized bed boiler.After the low-nitrogen reformation,the amount of NOx generated in the furnace is effectively controlled.However,high-efficiency denitrification reactions in the separators are still required to fulfill the ultra-low emission standards.Since temperature at the outlet of furnace is generally around 900°C,the temperature at the cyclone separator is lower than the optimum temperature for the SNCR technology(950°C).Therefore,in actual operation,excessive ammonia is injected into the separators to increase the efficiency of the SNCR denitrification,leading to the issue of excessive ammonia emission.In order to solve this issue,a 75t/h circulating fluidized bed boiler combined with the low-nitrogen combustion and SNCR technology has been studied in this work.A method that alters the combustion characteristics in the furnace by changing the heat-insulating materials has been proposed.This method can increase the flue gas temperature at the outlet of the furnace,and subsequently promote the SNCR denitrification reaction at the cyclone separator,allowing for the full reactions of reductants with NOx.Consequently,the issue of excessive ammonia emission can be effectively addressed.Based on the Euler-Euler method,a three-dimensional mathematical model describing the gas-solid flow and combustion reaction in a circulating fluidized bed was established to simulate the flow field,temperature field,and component field distribution in the circulating fluidized bed boiler.Meanwhile,the pollutant generation and removal models were introduced to analyze the NOx emission characteristics in the furnace,providing the basis for the subsequent numerical calculation of SNCR denitrification.The simulation results showed that the temperature distribution increased firstly and then decreased along the height direction,reaching the maximum temperature of 936°C at 13.2m.In the dense phase zone at the bottom,due to the volatilization of volatiles,both the combustible gas composition and NOx reached maximum at the upper outlet of secondary air.The diagram of reaction rate showed that the coke reduction rate is one order of magnitude higher than the CO reduction rate.It is concluded that the main NOx consumption pathway in the furnace is the coke reduction reaction.According to the characteristics of the temperature distribution in the furnace,the height above the temperature drop point was used as the target area for the insulation material arrangement.Numerical simulation were used to study the variation of the temperature and the NOx concentration of the flue gas at the outlet of the boiler with different insulation material layout areas.The results showed that the whole temperature in the furnace was increased at the presence of refractory belt,leading to the upward shift of the highest temperature and the highest temperature.When the area of the flammable belt reached 69.6m~2,the temperature increased by 23?C,and the volume concentration of NO in the flue gas at the outlet of the furnace increased by 44.8 ppm.The cyclone separator was modeled and the effects of nozzle position,injection velocity,atomization particle size,and ammonia-nitrogen molar ratio on the denitration efficiency and ammonia leakage were studied to obtain the optimal operating parameters.On this basis,based on the different inlet flue gas conditions,the influence of the thermal insulation material arrangement area on the NOx emission concentration and ammonia slip at the outlet of the cyclone separator was investigated.The results showed that when the area of insulation material was 41.76m~2,the injection rate was 10m/s,the atomization particle size was 0.2mm,the ammonia-nitrogen molar ratio was 1.5,and the denitration efficiency at the cyclone separator No.1 and 2 was 77.67%and 76.49%,respectively the NOx concentration of the outlet flue gas can reach 50mg/Nm~3 or less.Compared with the situation without the semblance of the flammable zone,the ammonia slip at the outlet of No.1 and No.2 separators is reduced by 43.6%and 48.4%,respectively,and the escape concentration is lower than 8mg/Nm~3,which meets the national engineering standards.