| Circulating fluidized bed(CFB)boiler is one of the most mature clean coal combustion technologies in commercial applications.The two main NOx emission control technologies are low-nitrogen combustion technology and flue gas denitrification technology.Selective non-catalytic reduction(SNCR)technology in flue gas denitrification technology is widely used in CFB boilers of power plants because of its simple equipment and low cost.In actual operation,excessive reducing agent is usually injected into the horizontal flue to improve the denitrification efficiency,but it will also cause ammonia escape,which will not only waste the cost but also cause secondary pollution.In order to solve this problem,this paper takes a 75 t/h medium temperature and sub-high pressure circulating fluidized bed boiler as the research object,and studies the characteristics of low nitrogen combustion and ammonia injection in the furnace.The influence of air volume change and reducing agent injection position on NOx emission is simulated and analyzed.In this paper,the geometric model of the research object is meshed,and the main mathematical model is selected through theoretical analysis.Based on the MP-PIC method,the whole process computational particle hydrodynamics(CPFD)model is established.By comparing different types of drag models,it is considered that the Wenyu-Ergun model is more suitable for CFB boilers.The grid independence is verified by the gas-solid flow characteristics.The simulation results and industrial results are compared.The maximum deviation is less than3%,which is in good agreement,and the accuracy of the established mathematical model is verified.According to the field operation data,the original working conditions were simulated and analyzed,and the gas-solid flow characteristics,combustion characteristics and NOx pollutant generation characteristics were obtained.The simulation results show that the particle flow in the furnace presents a typical’ring core’flow characteristics,the velocity vector in the side wall area is downward,and the velocity vector in the center of the furnace is upward.The overall temperature in the furnace gradually decreases with the increase of furnace height.At the bottom of the furnace,the formation of NO is limited due to the strong reducing atmosphere.With the increase of furnace height,it reaches the peak at the transition zone,and continues to be limited by CO in the dilute phase zone,showing a trend of increasing first and then decreasing along the height direction.By adjusting the air distribution parameters,the low nitrogen combustion technology simulation was carried out to study the influence of air volume change on NOx generation.The research results of the adjustment of the upper and lower secondary air volume show that with the increase of the proportion of the upper secondary air,there is a slight restriction on the generation of NO,and the emission of NO is reduced by 30 mg/m~3.However,excessive air classification increases the proportion of the upper secondary air,which will lead to a decrease in combustion efficiency.The research results of primary air adjustment show that reducing the proportion of primary air has a significant effect on controlling NO emissions,but it is necessary to ensure that the primary air volume is sufficient to maintain the circulating fluidized bed material.The effect of ammonia injection retrofit scheme on SNCR denitration efficiency was simulated.The results show that by analyzing and comparing the six groups of different ammonia nitrogen molar ratios,the optimal ammonia nitrogen molar ratio is determined to be1.4,and the denitration effects of the two schemes are compared under the conditions of 100%B-MCR,85%B-MCR and 70%B-MCR respectively.The simulation results show that the denitration effect of moving the ammonia injection point forward to the furnace is better,the utilization rate of the reducing agent is higher,and the emissions of nitrogen oxides and ammonia escape have reached the national standard. |
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