| The power structure that the coal power proportion is high is difficult to change in China, and the contradictions of the coal electricity supply are becoming prominent, so low-quality anthracite is inevitablely used for power generation, and the efficient and clean combustion technology of low-quality anthracite receives much concern. Because of the large capacity, high parameters and efficiency, supercritical W-flame boiler can improve thermal efficiency, reduce the pollutant emissions, and is the best way for power generation using low-quality anthracite. Supercritical W-flame boiler realizes the purpose of high efficiency and low pollutant emissions by in。c reasing furnace heat load, extending flame travel and coal density separation. As a result of the low quality and poor characteristics in combustion and environmental protection of anthracite, supercritical W-flame boiler technology must be based on the studies of the excellent aerodynamic field characteristics, efficient combustion and low pollutant emission mechanism to ensure that the boiler unit is safe, efficient and environmental. So theoretical analysis and numerical simulation are developed to study the coal combustion and pollutant emission control mechanism in supercritical W-flame boiler. The results have great practical use and not only provide a basis for the design and operation of supercritical W-flame boiler but also have great significance for the coal combustion and pollutant emission control theory.The study subject of this paper was a600MW supercritical W-flame boiler. Based on the basic condition when boiler load was100%and the air distribution on the front and rear wall was symmetrical, software Fluent was used to numerically study the characteristics of aerodynamic field, the process of coal combustion and the release characteristics of NOx within a large range of parameters when coal concentration changed from0.525kg/kg to0.938kg/kg, the primary air velocity ratio with the basic condition changed from0.81to1.25, the air distribution ratio of the front and rear wall changed from6:5to5:6, the over-fire air(OFA) rate changed from6%to15%and boiler load was different, so that the law of pulverized coal combustion and NOx release in supercritical W-flame boiler could be revealed.The aerodynamic field characteristics of supercritical W-flame boiler were systematically studied in this paper. The results indicate that owing to the particular furnace structure and air distribution characteristics, the double-vortex flow structure can be formed in the furnace as the "W" shaped flow field is formed in the lower furnace, which can increase the residence time of coal particle in the furnace and is beneficial to the burnout of pulverized coal. Because of the nose of the furnace, symmetrical flow field can not be formed even if the air distribution is symmetrical. The impact of the nose of the furnace will be balanced by the air distribution style on the front and rear wall, and the symmetry of flow field will be better if the air velocity on the front wall is larger than that on the rear wall, but the air distribution ratio of the front and rear wall can not be too large, the symmetry of flow field is best when the air distribution ratio on the front and rear wall is1.05. The structure of OFA above the arch can effectively reduce the erosion effect of the flue gas of pulverized coal on the throat area, the front wall and the rear wall, and is good at avoiding slagging. The flow field will not be best if the OFA rate is too large or too small, and there exists an optimal OFA rate.Combustion mechanism of pulverized coal and its influencing mechanism in supercritical W-flame boiler were systematically studied in this paper. The results indicate that reasonable pulverized coal combustion is good for the burnout of pulverized coal in the furnace and can improve unit efficiency. The pulverized coal concentration can outstandingly affect the coal combustion process in the furnace, the excess air factor in the lower furnace will decrease and the overall furnace temperature will firstly decrease and then increase as the pulverized coal concentration increases. If the pulverized coal concentration is too high, the amount of unburned pulverized coal increases, so the temperature of the furnace decreases again. As a result of OFA, the oxygen content of the lower furnace decreases, and the amount of unburned pulverized coal will increase as the OFA rate increases, so the overall temperature of the lower furnace will gradually decrease. As the unburned pulverized coal continues to combust in the upper furnace, the overall temperature of the upper furnace will increase. The amount of pulverized coal into the furnace will decrease as the boiler load decreases, so the heat release amount because of pulverized coal combustion will decrease, and the overall temperature of the furnace will decrease gradually.The law of NOx release in supercritical W-flame boiler was systematically studied in this paper. The results indicate that most NOx is generated in the initial stage of pulverized coal combustion, and reach its maximum value at the distance of2-4m away from the primary air nozzles. As the coal concentration increases, the reducing atmosphere in the furnace will be enhanced, and the NOX generated will be reduced by reducing gas such as CO, so NOX emission amounts at furnace exit decrease gradually. The air distribution style of the front and rear wall has great impact on the symmetry of flow field in the furnace, but has little effect on NOX release. The NOx formation in the furnace can be effectively controlled by selecting a reasonable primary air velocity on the front and rear arch, and NOx concentration in the furnace will firstly decrease and then increase as the primary air velocity increases gradually, so there exists a best primary air velocity. OFA has great effects on the substance contents and the generation of NOX in the furnace, NOx emissions at furnace exit decrease from756mg/m3to502.9mg/m3with the OFA rate increasing from6%to15%. The excess air factor in the lower furnace will increase and the reducing atmosphere will be weakened as boiler load decreases, so NOx emissions at furnace exit will increase instead of decrease, and the average NOX mass concentration at furnace exit increases from623.79mg/m3to716.5mg/m3as the boiler load decreases from100%to75%. The results have great reference significance for the combustion adjustment and NOX reduction in the practical operation of supercritical W-shaped flame boiler. |
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