Study And Control Of Nitrogen Oxide Emission Model Of Circulating Fluidized Bed Boiler

With the increasingly strict emission standards of pollutants,the original discharge of pollutants from circulating fluidized bed boilers has been unable to meet the requirements of the latest standards.In order to comply with the requirements of the policy,circulating fluidized bed boilers need to add pollutant removal equipment outside the furnace to make the concentration of pollutants discharged meet the standards.Usually,wet desulfurization devices are added outside the furnace to remove SO2.A SNCR denitrification device is added at the outlet of the furnace to reduce the NOx emission concentration at the outlet.However,with the continuous grid connection of new energy.thermal power units need to constantly change the load to absorb new energy.In the process of load change,the combustion state in the furnace changes dramatically.and the concentration of pollutants at the furnace outlet fluctuates greatly.For NOx,there is no measuring point at the SNCR entrance,and the measuring point at the outlet is far away from the outlet.The fluctuation of the original generation in the furnace greatly increases the difficulty of the control of SNCR,and the overall automatic input rate is at a low level.Therefore,the establishment of a dynamic NOx emission model of circulating fluidized bed boiler can predict the NOx emission concentration,and its dynamic characteristics can be obtained to help design control strategies,which is of great significance for the optimization of SNCR.The work done in this paper to solve the above problems is as follows:(1)The NOx generation mechanism and SNCR reaction mechanism in the circulating fluidized bed boiler were analyzed,and the model structure of NOx generation and SNCR in the circulating fluidized bed boiler was established by combining mathematical modeling and mechanism analysis with parameters such as coal feed volume,air feed volume,ammonia injection volume and bed temperature as input,and the dynamic emission model of NOx in the circulating fluidized bed boiler was established.Combined with the actual historical operating data of a 300MW subcritical circulating fluidized bed unit,the dynamic simulation of typical load condition and variable load condition is carried out.The results show that the model can better fit the NOx emission trend in the dynamic process,ith high accuracy,no matter in the steady-state condition or the variable load condition,and can realize the real-time prediction of NOx of CFB units under variable load.Then,open-loop step tests of coal supply,air supply,urea flow and multiple variables under different working conditions are carried out.The tests show that the model can better reflect the dynamic characteristics of NOx emission process.The feasibility and effectiveness of the proposed method are verified.The research results can provide reference for the optimization of ultra-low emission control of CFB units.(2)Taking the established NOx dynamic emission model as the controlled object,aiming at the operation characteristics of circulating fluidized bed boilers and the large delay characteristics of SNCR.the SGPC control algorithm is proposed to increase the accuracy of the prediction model by introducing the feedforward quantity of coal,air and bed temperature,and controlling the NOx emission concentration by taking the ammonia injection quantity as the control quantity.Achieve ultra-low emission control of NOx under multiple working conditions.In the variable load condition,compared with the original control on site and the traditional PID,the SGPC control method has good performance,reduces the fluctuation of NOx and reduces the denitrification reducing agent.