Numerical Simulation On Demisting Efficiency Of Wet Flue Gas Desulfurization And The Optimization Of The Flow Filed

Due to the environmental challenges as well as the requirements of environmental protection, people have paid more and more attention to the control of dust, fly ash, mist and other atmospheric pollutants. Mist eliminators are the main equipment for power plants to remove the mist, ash and particles of flue gas, whether the flue gas can consistent with the release standard of flue gas after removing particles is decided by the performance of mist eliminator.In order to study effective method to improve the demisting efficiency, the baffle demister which is widely used in power plant and the Wet Electrostatic Precipitator (WESP) which has achieved the good effect in the removal of PM2.5,sulfuric acid mist, dust, Hg etc. in recent years were studied in this paper. Numerical simulation was used to study the effects of different parameters on the baffle demister demisting efficiency, and to study the interior flow field of wet electric dust collector.Conventional methods as well as the new methods of the flow field uniformity control were simulated numerically. The main contents include the following several aspects:(1) In this paper, Euler-Lagrange method has been used to build the two-phase flow of gas-liquid numerical model in baffle demister which has been widely used in the Wet flue gas desulfurization (WFGD) system of coal-fired power plant. Meanwhile, experimental study has been conducted to verify the simulation results. By changing the structural parameters and operating parameters of the baffle demister, such as plate spacing, blade-shape and gas-velocity, demisting efficiency and droplets separation efficiency of different sizes showing different rules. The moving behavior and depositing performance of droplets with different sizes of different sizes were obtained. The results show that the removal efficiency of droplets less than 16μm will show irregular fluctuations if flow rate is increasing and almost unchanged for increasing plate spacing, the removal efficiency of droplets over 16μm will increase with the increased flow rate, and decreased significantly with the increased plate spacing. The demisting efficiency of trapezoidal plates are greater than the efficiency of triangle plates with plate spacing of 38 mm and the difference of two plates was not significant in minor plate spacing. Removal of the droplets less than 20μm requires higher airflow uniformity at lower gas flow rate. Increased airflow disturbance is in favor of small droplets collision-coalescence when gas flow rate less than 3 m·s-1, and increased airflow disturbance is not conducive to small droplets’ collision-coalescence when gas flow rate over 3 m·s-1.(2) In industrial applications, the uneven distribution of the flow field in WESP lead to decreased efficiency demisting. This paper takes a certain transformation project in wet electrical precipitator of a power plant as a case. The numerical simulation on the flow filed in the WESP of a power plant is to find out the reason of uneven flow distribution in electric field according to the numerical results. The simulation results have been compared with experimental results to verify the accuracy of simulation. By analyzing the simulation results, the influence of original structure of the flue and the conventional method of adjusting the airflow uniform of flow field is obtained, which includes setting the deflector, the airflow distribution board to reform the entrance flue, air horn structure. Conclusion:In order to make the flow rate average at the entrance, the width of the entrance should be increase, an arc guide plate should be Installed at the turning of the entrance flue, a inclined guide plate is arranged in the connecting box. The arc deflector within the flue can play a good role in the organization of the gas. The taper of the horn could not be too small, otherwise it will increase the system pressure drop, and cause the scour of the internal electric field area directly. In order to solve the problem of the uneven flow distribution on the outlet of the horn caused by the defects in the original structure of horn, several inclined plates were set along the side of the larger taper of the horn; Although conventional flow field adjustment method could improve airflow distribution and reduce the peak value of flow rate to a certain extent, the adjustment process requires repeated testing, and the flow field after adjustment still can not satisfy the high demand for uniformity of flow field of some wet electrostatic precipitator.(3) Proposes a new method for regulating the flow field, the utilization of a trapezoidal honeycomb diversion structure which is intertwined by horizontal clapboard and vertical clapboard put in the trapezoidal horn structure. According to relationship between the flow rate of a region on outlet section and corresponding to the area of the region on inlet section, it could adjust the flow rate of the outlet section of the trapezoidal horn in both directions, respectively. The new method can effectively reduce the flow rate deviation and make flow filed more uniform compared with the conventional method, and make the rate of region area on outlet section which flow rate within 2～3m/s be 78%, the flow rate deviation achieves 0.19. In order to avoid interference of the gas flow regulation in both directions, horizontal and vertical partitions inside the horn should retain a certain height difference, the difference is determined according to the size of the trapezoidal horn. The number of horizontal clapboard and vertical clapboard and the number of trapezoidal honeycomb diversion grill are determined by the requirements of uniformity of the flow field, the more the number, the higher uniformity can be realized.