Research On Numerical Simulation Of Combustion Optimization For Circulating Fluidized Bed Boiler

China’s energy consumption is dominated by coal, of which approximately84%are used directly for combustion. Approximately87%of SO2and67%of the nitrogen oxides being discharged into the atmosphere each year are from the direct combustion of coal, which leads to extremely serious environmental pollution. The CFB combustion technology has been developing rapidly due to the advantages of high efficiency, low pollution and good comprehensive utilization. The "Emission standard of air pollutants for thermal power plants"(GB13223-2011) being published in2011puts forward higher requirements about the desulfurization capacity and nitrogen oxide emissions for the existing CFB boilers in our country. Therefore, it has a very important significance for energy saving and emission reduction to optimize the CFB boiler combustion and reduce emissions on the basis of ensuring the efficiency of combustion.In this paper, the HG-440/13.7-L.HM28CFB boiler is taken as a research object, the FLUENT software is used for the numerical simulation of combustion optimization. The standard k-s turbulence model is used to simulate the gas turbulent flow in gas-solid two-phase. Two-equation competitive reaction model is used to simulate the release of volatile matter in coal particles. The mixture fraction-probability density function model is used to simulate the gas-phase turbulent combustion. The motion/diffusion-controlled combustion model is used to simulate the combustion of coke. The P-1radiation model is used to simulate the boiler radiation and convection heat transfer. The stochastic particle trajectory model is used to simulate the trajectory of coal particle. The post-processing method is used to simulate the formation of nitrogen oxides. The SIMPLE method is used to deal with the pressure-velocity coupling in pressure based solver.Firstly, the combustion of CFB boiler under100%B-MCR condition was simulated. The distribution of flow field, temperature field and concentration fields in the furnace was obtained, the emission conditions of nitrogen oxides and burn-out rate of fuel were obtained. Actual operating data and simulation results were compared, which verified the rationality of the model. Second, used the variable conditions to study the impact of different coal particle size, primary-secondary air ratio and volume of recyclable materials on the distribution of flow field, temperature field and the component field in furnace, including nitrogen oxide emissions and fuel burn-out rate.Finally, the distribution of the flow field, temperature field and concentration fields, the nitrogen oxide emissions and fuel burn rate under100%B-MCR condition and variable conditions were compared and analyzed. The distribution law of speed, temperature, O2, CO2and NO in furnace, the influence of different parameters on the burn-out rate of fuel under variable working conditions were drawn. Summarizing the simulation results under various operating conditions, the optimal working conditions of boiler combustion were obtained after comparative analysis. The research results had a certain reference and guiding significance for the combustion optimization and renovation of CFB boiler.