Numerical Simulation For Optimizing Flow Field Of Wet Flue Gas Desulfurization Tower

As main parts of Wet Flue Gas Desulfurization(WFGD)facillities in coal-fired power plants,counter-current Open Spray Towers(OSTs)are used to absorb sulfur dioxide and control particle contamination,such as gypsum slurry.According to the characteristics of transfer and chemical reaction processes within the reactors,the OSTs can be divided into the spray region,the mist eliminator region and the oxidation region.In response to the ultra-low emission standard and the consequent rigid restriction on air pollution discharge,the gas flow field in the spray region and gas-liquid flow in the mist eliminator region have been simulated by CFD method.Based on analysis of the flow behaviour,structure optimization schemes are proposed to improve the uniformity of gas flow in the spray region and the demisting efficiency in the mist eliminator region.Based on Reynolds averaged N-S equations closed by standard k-? turbulence model,the gas flow field in a full-scale OST of an 500 WM coal-fired power plant has been investigated.The characteristic of gas flow hydrodynamics in the spray region is studied.In order to mitigate heterogeneity of the gas flow caused by local high speed airflow,a kind of spiral internal,installed under the spray banks,has been applied in flow field optimizaition in this thesis.The diversion effect due to helical structure can avoid the sneakage phenomenon and improve homogeneity of gas flow.The results showed that the effective width and turns of the spiral internals have relatively significant effect on the flow field.The optimum range of the ratio of the effective width to the radius of OST is from 0.13 to 0.26,while the value of the turns should be selected as 1.5.However,intersection angle between the surface and horizontal direction is less affected and the optimal range between 60° and 90° has been proposed.In order to investigate the multiphase hydrodynamics in the mist eliminator region,gas-liquid flow in the single channel between two vanes of the mist eliminator has been simulated using Eulerian-Lagrangian approach.Gas is regarded as a viscous Newtonian fluid and the turbulence behaviour has been simulated by SST k-co turbulence model.Slurry droplets are calculated by Discrete Random Walk(DRW)model.The gas flow behaviour and the distributions of eight kinds of droplets has been analysed in the mist eliminator channels with the blade spacing of 20,26 and 38 mm.The flow field analysis consists of the velocity distribution and the range of recirculation zone.The sizes of droplets are 10-30?m.The demisting efficiency and the pressure drop have been studied when the gas inlet velocity is from 3 to 6 m/s.Finally,the applicability of the simulation model is verified by comparing with experimental data.Based on the gas flow characteristic after the turning of the channel,a method has been proposed to strengthen the separation of gas and liquid.Triangular,rectangular and arc internals have been installed on the vanes of the mist eliminator to increase velocity and adjust direction of gas flow in the channel.Structure optimization scheme has been specified by investigating the effect of shape and layout of the internals on the performance of the mist eliminator in the spacing of 26 mm.Furthermore,the effect of the triangular internal effctive height and apex angle on the gas-liquid flow pattern have been studied.The results indicate that an increase of the internal height or a decrease of the apex angle could lead to an enlargement of the seperation efficiencies and the pressure drops under different oprating conditions.Furthermore,the height of internal has a significantly influence on the performance of demister while apex angle plays a relatively irrelevant role.Besides,taking both of the separation efficiency and the pressure drop into consideration,the optimum value of the effective height and the apex angle for the internals has been determined.The values of apex angle ranges from 90° to 120° for the internal in height of 3 mm and 4 mm,whilst the angle which has great effect on the pressure drop should select as 120° with regard to the internal in height of 5 mm.