Simulation And Experimental Study On The Mode Of Decreasing Oxygen And Nitrogen Reduction In Circulating Fluidized Bed Boilers

With the rapid development of the national economy,the state attaches more importance to the environment.Now the country is vigorously developing new energy sources such as nuclear energy,wind energy and solar energy,in the hope that one day it will be able to replace the traditional energy sources with serious pollutants.In recent years,smog weather has frequently appeared.It is not only limited to the north,but the southern region is also affected by smog weather.Therefore,the country continues to raise emission standards to reduce people's emissions of air pollutants.Although the NOx emissions from power plant CFB boilers are lower than those of conventional coal-fired boilers,they are no longer able to meet the country's increasingly stringent emission standards.Therefore,this paper conducts flue gas recirculation transformation of existing CFB boilers in power plants,and conducts in-depth research through power plant experiments and 3D simulation.This method is used to prove the accuracy of the simulation and the superiority and feasibility of the cycle modification scheme.This provides a theoretical basis for the subsequent boiler plant to reduce NOx emissions.According to the structural size and actual installation position of the CFB boiler of the power plant,the position and size of the boiler inlet and outlet and the feed port are determined,and the components are omitted as oil guns and water-cooled walls.The dense phase zone and the dilute phase zone of the boiler are respectively divided into grids.Because the dense phase zone is the main combustion zone of the boiler furnace,the dense phase zone is encrypted,and then the FLUENT software is used to simulate the boiler combustion and the NOx emission of the boiler.By comparing the simulation results with the data obtained from the monitoring room in the actual operation of the power plant boiler,the temperature field and NOx emission results obtained by the three-dimensional simulation are consistent with the power plant data.The simulation results not only meet the temperature range of 850°C~950°C,but also the temperature field distribution in the furnace also meets the power plant boiler structure,and the simulated NOx emission value is controlled within the existing power plant emission range,that is,the simulation value is lower than 300mg/Nm~3.Therefore,the designed three-dimensional model of the boiler and the equation model used for the three-dimensional simulation can meet the calculation requirements.In order to meet the requirements of the boiler NOx emission,the flue gas recirculation modification test is carried out on the power plant boiler to achieve the effect of reducing oxygen and nitrogen.The exhaust fan is used to extract part of the flue gas from the horizontal flue section to the inlet of the primary air draft fan,so that the clean flue gas uses the primary air chamber to participate in the combustion of the pulverized coal in the furnace.With the flue gas recirculation technology,the flue gas recirculation rate is 20%and the boiler excess air ratio 1.2 remains unchanged.Accordingly,the primary air entering the furnace is correspondingly reduced,so that the introduced clean flue gas can both cool and reduce the oxygen content to form a relatively low-oxygen combustion to reduce NOx formation.Boiler combustion should be a synergistic effect of primary air and secondary air.Therefore,after the flue gas recirculation transformation,the secondary air system is also optimized and improved,so that the entire boiler system reaches an optimal state.CFD simulation of boilers under different loads is carried out,including 330MW,210MW,140 M W and 70MW,of which 330MW is full load operation and 70MW is ultra-low load operation.Compare the CFD simulation results obtained before and after the test under the same load,such as the temperature field,temperature distribution range and NOx emission,and emphasis is placed on comparing the changes of NOx emission value before and after transformation.The NOx values are all before denitration.This method is consistent with the plant report.By comparison,it can be found that the temperature field of the modified boiler still maintains a similar trend under four loads,and both are controlled at850°C~950°C,which meets the needs of the actual operation of the power plant.And the NOx emission value of the boiler under the same load after the transformation is significantly reduced.According to the simulation results,when the boiler load is large,the NOx emission value of the boiler is relatively large,which corresponds to the coal supply amount and air supply volume in the actual operation of the power plant.The simulation results obtained by 3D simulation provide a certain theoretical support for the power plant to reduce NOx emissions,and also lay a certain design basis for the power plant to further reduce pollutant emissions.