Experimental, Theoretical And Numerical Simulation Research On Direct Flow Pulverized Coal Low NOx Combustion And Reburning Technology

The research on efficient and clean combustion of coal is the focus which many energy researchers are concerned about all the time. With China's rapid economic and social development, energy and environmental issues have become more apparent. The coal-based energy structure in China will keep in a fairly long period of time. On the basis of the previous research of the domestic and foreign scholars, this thesis is mainly focused on the direct flow pulverized coal low NOx combustion and reburning technology through the experiment, theory and numerical simulation. Moreover, the conventional pulverized coal reburning retrofit was completed in an engineering practice.Based on the thermal analysis and chemical kinetics analysis, the pyrolysis and combustion process of the pulverized coal was studied with the TGA-FTIR (thermal gravimetric analysis and infrared spectroscopy) integrated technique. The volatile matter emitted during the coal pyrolysis process was identified, and the NO emitted during the coal combustion process under different oxygen concentrations was also compared. It is clear that the influence of the oxygen concentration on NO emission is remarkable. The apparent reaction kinetic model for the combustion of Yanzhou bituminous coal was developed by using the linear regression approach.The experiments in a fixed-bed reactor system were conducted. The effects of a series of factors on the NO reduction reaction were investigated, including the pulverized coal and its pyrolysis products, CO, the coal rank, the oxygen concentration, the temperature, the fineness of the pulverized coal, and the water vapor. The effects of various factors on NO production in pulverized coal combustion and NO reduction through pulverized coal reburning were achieved in a one-dimensional drop-tube furnace. In the pulverized coal reburning through the NO reduction using bituminous coal and anthracite and their pyrolysis products in the condition without oxygen(O₂=0%), it is found that the homogenous reduction dominates the early reaction stage of NO reduction and afterwards the heterogenous reduction dominates when bituminous coal was used. However, the heterogenous reduction dominates in the whole reaction stage when anthracite was used. The fast separation of pyrolysis gas and char is in favor of NO reduction. The comparison of the NO reduction using pulverized coal in the conditions between with oxygen(O₂=4%) and without oxygen was carried out. It is found that the unburned coal contributes more to NO reduction in the coal reburning zone. The experimental research on the influences of water vapor on pulverized coal combustion and reburning was conducted. It is found that both low yield of NO and accelerated pulverized coal combustion can be achieved when a certain amount of water vapor was added under the condition of both high temperature and low oxygen concentration, and that not only the NO reduction through pulverized coal but also the burnout ratio were improved by adding a certain amount of water vapor into the reburning zone, which provide some references for the efficient burnout of pulverized coal and low NOx emission.A pulverized coal ignition temperature multivariate regression model was set up, through which the pulverized coal ignition temperature can be forecasted within 2% error by coal property parameters. A one-dimensional low NOx simple model of direct flow pulverized coal was set up, which includes three zones: pulverized coal ignition preparation zone, ignition combustion zone, the burnout zone mixed with the secondary air. The equations were established and solved by numerical analysis, and then the influences of primary air parameters on low NOx ignition and combustion were discussed. The Local Stoichiometric Ratio of Volatile (LSRV), the Virtual Temperature of Ignition (VTI), and the discriminant of low NOx ignition and combustion namely Gx number were brought out, which provide some references for the low NOx design of both the primary air parameters and the spacing between the primary and secondary air vents.For a 410t/h pulverized coal boiler which will be on reburning retrofit in a power station, firstly numerical simulation on the flow, combustion and contamination generation process under different conditions of operations of mills was conducted. The influences of tertiary-air on NOx production were achieved, and the retrofit scheme of two-layer arrangement tertiary-air was brought out. Secondly numerical simulation on different reburning retrofit schemes with different positions of coal reburning injectors was conducted. An optimized scheme was obtained for engineering practice through taking the NO removal rate and the carbon content in ash into account, and the NO removal rate of 47.66% was achieved. The results from numerical simulation were in conformity with retrofit engineering practice. The influences of the size of reburning zone on NO removal rate and the carbon content in fly ash, and the changes of temperature and gas components (including NO) before and after reburning retrofit were studied. The lower reburning injectors which are disturbed by main combustion zone are beneficial to burnout rate but have a negative trend to NO removal rate. Lastly the further reburning retrofit scheme was studied by numerical simulation by which the effect of low NOx emission was discussed. All of these provide some references for the operation of mills, design of tertiary-air and reburning retrofit engineering practice in the power station boilers.