Investigation On NO_x Reduction Mechanism By Superfine Micronized-Coal Reburning And Numerical Simulation Study On Reconstruction Projects

China is the world's largest coal producing and consuming country. The coal has the absolute predominance in the primary energy, which determines the main position of the coal-electricity power in electricity production. During coal combustion, many kinds of contamination are emitted out, in which nitrogen oxide (NOx) is one of the important substance leading to increased environment pollution. This work is close combined with the national 863 program " Deduce nitrogen oxide emission by pulverized-coal reburning technology" . Considering the tangential fired furnaces popularly used in our country, we have design a pulverized-coal reburning technology reconstruction project than is suitable for our national conditions. In this study, many kinds of means such as experiment, detailed kinetic modeling, numerical simulation and field tests have been adopted.Reburning fuel used in our project is Datong bituminous coal. The heterogenous mechanism has little effect on nitrogen oxide destruction during reburning and homogeneous mechanism has dominated the reburning efficiency. Detailed kinetic model has been utilized to study the reaction process of volatile or its main hydrocarbon compositions with nitrogen oxide. The effect of carbon monoxide in the gas and nitrogen compositions (Hydrogen cyanide (HCN) and ammonia (NH3)) on reburning efficiency has also been studied. Through the sensitivity analysis, we have obtained the element reactions, which are close interrelated with the nitrogen oxide generation. These essential reactions have been divided into two categories. One is trans-hydrocarbon and the other is trans-nitrogen compound. Furthermore, the mode of tree has been used to describe the internal relation of these reactions and clearly demonstrate the substantial characteristic.Pulverized-coal reburning technology is based on sufficient superfine pulverized-coal. Usually, a certain amount of superfine pulverized-coal is contained in tertiary, but the content is too low (10-l5%) to satisfy the request for reburning. In this work, we havepromoted an innovative scheme to obtain superfine pulverized-coal by extracting air from the bottom of finely pulverized- coal separator. At the same time, relative experiments have been applied on large multi-phase flow table. However, it is found that the radius of the coal extracted from the finely pulverized-coal separator is too large, so the scheme above has been further improved. The cylindraceous impingement separator has been developed, which is placed in the cone at the separator bottom. The coal is re-seperated in the new apparatus. Consequently the extracted coal can satisfy the reburning request and the new scheme successfully solves the problem of superfine coal source and collection in the project.Numerical simulation is based on fine grid. With regard to the section of the tangential fired furnace, we have developed three kinds of typical unstructured grid systems by grid processor. One is triangle grid and the other two are quadrangular grids. One of the quadrangular grid is the map grid whose grid lines are parallel to the furnace wall, and the other quadrangular grid is the paving grid whose grid lines are radialized with the center of four burners. Grid number, grid cell orthogonality and smoothness of these three grid systems have been quantitatively compared. With regard to tangential flow in the tangential fired furnace, we have program to compute the grid cell size, the angel between grid line and the flow direction and false diffusion. It has been proved by the accurate computing results that the paving grid can efficiently restrain the false diffusion of the flow near the burner, and thus ensure the simulation convergence. This grid is suitable for the numerical simulation of tangential fired furnace.Four reconstruction projects for reburning have been promoted in this work. By use of the computational fluid dynamic (CFD) software FLUENT, numerical simulations have been processed on the four projects above. The results show the temperature, the composition and the pollution of nitrogen oxide distributions in the furnace. In order to study the movement and the combustion of the coal particles, we have modified the particle sample function in FLUENT by user defined function (UDF) and obtain concerned particle statistical data, including particle residence time, burnout ratio and char content in fly ash. Finally, according to the combustion economical efficiency and the pollution emission level,the best reconstruction project for reburning technology could be determined.Before reconstruction, we have test on the pollution emission level of the 5# furnace in Zhenhai Power Plant. The experimental data is in good correspondence with the simulation results.