Study On Dynamic Characteristics Of Pollutant Emission From Cfb Boiler Under Variable Load

Ultra-low emission of coal-fired flue gas pollutants is one of the important technical indicators for thermal power generation.Circulating fluidized bed?CFB?coal combustion technology is the lowest cost of pollutant control in clean coal technology and the best adaptability in the same coal situation.It has the characteristics of high efficiency desulfurization and low nitrogen combustion in the furnace.In recent years,a lot of research on desulfurization and NOx suppression in CFB boilers have been done.With the increase of the total installed capacity of CFB boilers and the environmental protection requirements of ultra-low emission of coal-fired flue gas,higher requirements are put forward for the generation of combustion pollutants in the full operating range of CFB boilers.In this paper,the dynamic characteristics of pollutant emission of CFB boiler in variable load process were studied by establishing the mathematical model of desulfurization and denitrification and the overall mathematical model of flow,combustion and heat transfer of CFB boiler on a 350MW supercritical circulating fluidized bed boiler,which provides technical support for pollutant control in combustion of CFB boiler.Firstly,the overall dynamic mathematical model of circulating fluidized bed boiler was established.The model integrated the sub-models of flow,combustion,heat transfer and pollutant generation and removal in the furnace.The flow characteristics,combustion characteristics and heat transfer characteristics in dense and dilute phases of the bed were described correctly.Secondly,the dynamic mathematical model of circulating fluidized bed boiler was established on the basis of the overall mathematical model of circulating fluidized bed boiler.The furnace was divided into a series of chambers along the height.The dynamic balance equations of material,carbon mass,gas mass and energy were established in each chamber,and the dynamic changes of parameters in each chamber were calculated.Finally,the combustion dynamic simulation model of 350MW CFB boiler was established based on the platform of MATLAB/SIMULINK.The variation of parameters along furnace height under full load of boiler was been calculated.The change of pollutant emission under step change of coal feed and secondary air was been simulated and analyzed.The influence of parameters such as calcium sulfur ratio,primary and secondary air ratio and upper and lower secondary air ratio on the original SO₂ and NOx emission concentration in furnace was been analyzed.The simulation results show that:?1?Increasing the molar ratio of calcium to sulfur can quickly reduce SO₂ emissions.After reducing load,the SO₂ emission concentration can be decreased to the concentration level before load reduction when the molar ratio of calcium to sulfur increases from 2 to 2.2.?2?Reducing the primary and secondary air ratio can effectively reduce SO₂ and NOx emissions.After increasing load,the NOx emission concentration decreased to a lower level than that before the load is increased when the primary air volume ratio is reduced from 48%to 42%.The SO₂ emission concentration decreased by 11.34%when the primary air volume ratio is reduced from 48%to 45%.?3?Increasing the upper and lower secondary air ratios can also reduce NOx emissions,but it will lead to an increase in SO₂ emission concentration.Therefore,it is necessary to increase the molar ratio of calcium to sulfur to control SO₂ emissions while increasing the upper and lower secondary air ratios to regulate NOx emissions.The NOx emission concentration can be decreased by reducing the primary air ratio or increasing the ratio of upper and lower secondary air.For each reduction of the NOx emission concentration by 1 mg/Nm³,the primary air ratio should be reduced by 0.29%or the ratio of upper and lower secondary air should be increased by 1.36%.The SO₂ emission concentration can be reduced by increasing the ratio of calcium to sulphur.For each reduction of SO₂ emission concentration by 1 mg/Nm³,the ratio of calcium to sulphur needs to be increased by 0.23%.