| Wet flue gas desulfurization technology is the most widely flue gas desulfurization technology applied in coal-fired power plant. Nevertheless, there are many problems in its application, particularly high running cost and the pollution caused by the entrained slurry droplets in the desulfurization system. In consequence, it has been a focus that how to improves the utilization of desulfurization slurry droplets and droplets entrainment. In this context, the gas-liquid two phase flow field in the spray tower is studied using the numerical simulation method, and the flow field characteristic, droplet motion characteristics and gas-liquid contaction inside the tower are analyzed in this paper. The research content is divided into the following five parts:(1) The 600MW unit desulphurization spray tower was taken as the object to obtain the two kinds of flow field in the spray tower that when it spraying and not spraying, by establishing mathematical model for it. The results show that spraying can make the distribution of flow field in the spray tower be more uniform compare with that in empty tower. Furthermore, the characteristics of the distribution of flow field in the spray tower with spraying are showed to be the short circuit flow in the side of the inlet of gas, the two high-speed spirals at the bottom of the tower, the velocity of flue gas at center of the tower is lower than that surround. Besides, with the increase of height, the flow field in the tower tend to be more uniform, but the center of the low speed region still exist.(2) The trajectories, escape rate and residence time of droplets with different diameter was obtained by adding tracer particles into the flow field. The results show that the droplets with diameter above 2mm distribute uniformly in the tower, while the motion of droplets with diameter below lmm is significantly influenced by the flow field and these droplets mainly stay in the low velocity area. The escape rate reaches about 100% for the droplet with diameter below 0.2mm, while it sharply decreases with diameter above 0.2mm. Nevertheless, the trend slows down when the escape rate lower than 20%. With the increase of diameter, there is a peak in the curve of residence time, and the droplet diameter corresponding to the peak value is also the point when the trend begins to slow down in the curve of escape rate. Comparing the simulation flow field with the assumed uniform flow field, it can be seen that the inhomogeneity of the field contributes to the decrease of escape rate for droplets with diameter below 0.7mm and the increase of escape rate for droplets with diameter above 0.8mm.(3) The gas-liquid interfacial area and relative speed change function of height was obtained by dividing the tower into several regions along the height and then gather gas-liquid interfacial information respectively. The results show that the total interfacial area decreases with increase of the height in the absorption region, the interfacial area fraction of different size droplets sharply changes with the increase of height, and it is evenly distributed in the bottom of absorption region. But with the increase of the height, the fraction of little droplets increases and dominates. The gas-liquid relative speed increases with increase of the size of droplets. As the fraction of large droplets in the bottom bigger than that in the higher region, the average gas-liquid relative speed of all the droplets decreases with increase of the height.(4) The flue gas volume decreases in the varying load operation. So we need to turn some spraying layers off to save electricity. Nevertheless, the flue field will change. The gas-liquid flow fields were predicted under the conditions of 75% load and 50% load. The results show that the interfacial area and the gas-liquid relative speed increase with the increase of the spraying layer and escape rate of the droplets is similar. The pressure drop bigger, the interfacial area bigger. Simultaneously, the interfacial intensity is weaker in the downstream of the highest spraying layer. so the height of the absorption region is constrained by the highest spraying layer.(5) The influence of the increase of the spacing between the spraying layer and demister, spray layer spacing and initial spraying velocity on the flow field were predicted numerically. The results show that the homogeneity of flow field before the demister is improved with the increase of the spacing between the spraying layer and demister, and demisting efficiency is improved. At the same time, the escape rate decreases.for example, the escape rate with the tower height of 28m reduced by 34.4% compared with that of 24m. but it has nearly no impact on the gas-liquid interfacial area, relative speed and the pressure drop. With the increase of spray layer spacing, the escape rate decreases and gas-liquid interfacial area increase and the pressure drop increase, but has nearly no impact on the gas-liquid relative speed. With the increase of initial spraying velocity, the escape rate decreases, at the same time, gas-liquid interfacial area decreases and the pressure drop increase. Unexpectedly, it has nearly no impact on the gas-liquid relative speed. |
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