| Along with the increasingly stringent of thermal power plant pollutant emission standards, some key areas limited SO2 emission levels to 50 mg/Nm3, resulting in the traditional wet flue gas desulfurization (WFGD) process unable to meet SO2 removal efficiency requirements. Although retrofitting single-tower to double-loop desulfurization tower was better than other methods, the double-loop WFGD brought some internal structures like collect bowl. How to choose the optimal structures that had a better gas liquid contact performance and a smaller system resistance loss was still lack of theoretical basis. This paper presented a gas-liquid hydrodynamic model for a double-loop flue-gas desulfurization reactor in an Euler-Lagrange framework, along with its application to pilot-scale and full-scale industrial plants. The SO2 absorption mass-transfer mechanism was primarily based on the dual-film theory, and then, it was implemented by FLUENT 6.3.26 with the user-defined function (UDF).The research of the pilot-scale scrubber indicated that the desulfurization efficiency rised by increasing liquid to gas ratios and reduced with the improving of superficial gas velocity. When L/G was higher than 14, the double-loop WFGD could achieve a desulfurization efficiency of more than 98%. The analysis of the gas-liquid mass transfer showed that the desulfurization process in the tower, to a large extent, is "liquid-film limited". The liquid film side mass-transfer coefficient in the upper loop is larger than that in the lower loop, while the gas film side mass-transfer coefficient is almost the same in both the upper and lower loops. Gas film resistance to liquid film resistance ratio was about 0.35 in the lower-loop, while 0.65 in the upper-loop. Studies on the pilot-scale, with an error of less than 5%, indicating that the numerical method can be used to guide the design and optimization of WFGD.The bowl separator was also been studied through RSM (response surface methodology). Results showed that the h/D and the l/D had significant effect to system resistance loss, but the a not; the a and the h/D had significant effect to uniform coefficient, but the l/D impact was weak; all of the a, h/D and the l/D had significant effect to the average velocity; Secondly, a rather low system resistance loss and a better flow field uniformity could be obtained when a≤15°, h/D≈0.11 and l/D≈0.10; Prediction errors of regression equations of P, C and V were less than 10%, indicating that these equations could better predict the performance of bowl separator.The fluid dynamics simulation of a 1000 MW plant found that the flue gas needed to overcome a large resistance caused by the spray slurry, which reduced the stiffness of flue gas and created a "short circuit" phenomenon. The "short circuit" flue gas resulted in higher sulfur dioxide concentration at the inlet side and a large maelstrom at the bottom of the desulfurization tower, which lowered the update rate of flue gas and reduced the valid desulfurization space. The spray density of double-loop wet FGD in the lower loop was rather low, which improved the stiffness of flue gas and the valid desulfurization space. Compared 3 types of collecting apparatus, the trench-gate device was good for the upper-loop gas-liquid contribution. Extra spray layer and collecting apparatus in double-loop tower increased pressure drop of about 400 Pa, and 3 types of double-loop tower resistance losses were similar. The lower-loop slurry with a pre-cooling the flue gas temperature, is conducive to SO2 removal. Double-loop tower could obtained a more uniformity SO2 distribution by reducing the "short circuit" flue gas and SO2 escaping. Considering the better gas-liquid contacting effect of trench-gate collecting device, it could get a higher desulfurization efficiency of 97.25% when the G/L was 13.4, which higher than bowl separator tower by 5.8% and single tower by 13.9%. |
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