Research On Filtration And Back-Pulse Cleaning Process Of Ceramic Filter For Removing Fine Particles

Research in the past two years has shown that the source of outdoor PM2.5 in China mainly comes from fossil fuel combustion,biomass combustion,and other industrial emissions.Hence,reducing the total PM2.5 emissions in these industries is important for improving the Chinese residents' life quality.Nowadays,there are several technologies available for fine particles' removal,and the ceramic filtration technology has attracted widespread attention recently due to its high fine particle removal efficiency,the capability of enduring high temperature and high pressure operating conditions,and strong chemical corrosion resistance.Therefore,in the present work,several experimental and simulation research on filtration and back-pulse cleaning process of ceramic filter for removing fine particles were conducted,which provides a powerful research tool for the optimal design of ceramic filters,and theoretical support for upgrading the PM2.5 control technology in China.The major focuses and research findings in this work are shown as follows:(1)A novel design method of the ceramic filter's testing apparatus for fine particle removal that could better suit the verification and validation of the computational fluid dynamics(CFD)simulation results was proposed.This novel design method includes a novel dust feed device design with twin screw rods,a novel oblique L-shaped dust insert-type inlet pipe design,a novel measurement method combining actual and CFD testing methods for measuring the fluid velocity of the ceramic filter,and a novel dust cake thickness measurement method.(2)A novel CFD method for predicting the dust cake distribution and pressure drop variation on the ceramic filter at the macro-scale level in 3D was presented for the first time,and the mathematical model and algorithm implementation of this novel CFD method was discussed in detail.This novel CFD prediction method first arranges the grid on the outer surface of the filter element and then stacks the dust cake from the bottom layer in the queue to the adjacent outer layer to calculate the growth rate and pressure drop of the dust cake.Moreover,the impact surface is updated at every time steps,and a drag and lift correction model are applied inside the dust cake zone.Meanwhile,the detailed modeling theory and applicable macro-modeling empirical equations for modeling the deposition process of the particles and the growth process of the dust cake was given,and the mathematical models for modeling the internal and surface flow of the dual porous medium structure with both porous filter media and dust cake layer were provided.Besides,the overall calculation flow of the entire novel CFD method was expounded.(3)A thorough verification and validation of the novel CFD method were performed.Before the verification and validation started,the grid convergence and model analysis was conducted,and the Realizable k-? turbulence model with a L17 million grid mesh number was proved to fit the numerical accurate requirement.By comparing with the experimental data,it can be found that the maximum deviation of steady and transient simulation results for the pressure drop predicting was under 4%and 6%,respectively.Furthermore,the shape and position of some "empty zone"was well predicted by this novel CFD method,and the deviation of the dust cake thickness prediction did not exceed 13%.The verification and validation results showed a good agreement between CFD simulation and experimental results.Moreover,it was found that the dust cake on the filter element surface was unevenly distributed,and there were areas with less deposition particle mass(also can be called as "empty zone")and areas with higher deposition particle mass(also can be called as"concentrated zone").After carefully studied,it is believed that the distribution of these areas is related to the vortex field around them.(4)The total pressure drop variation and dust cake distribution among different filtration modes of the ceramic filter were compared and analyzed in detail by using this novel CFD method.By comparative study,it was found that the "tangential filtration flow" mode had the lowest dust cake growth rate and the minimum overall pressure drop across the filter,but it also had the most nonuniform dust cake distribution.The "downdraft flow filtration" mode had the most uniform dust cake distribution and the best dust collection results.However,its overall pressure drop was also the highest among these three filtration modes.Meanwhile,the "updraft flow filtration" mode had the most sophisticated vortex-flow distribution inside the ceramic filter,and the dust cake distribution of it also varied acutely.Furthermore,it was found that when the direction of the vortex around the filter element surface is perpendicular to and far from the outer surface of the filter element,particles will be drawn into this vortex flow,so that the particle deposition chance was reduced and some "empty zone"were formed in this area.In the other hand,when the direction of the vortex near the filter element surface is not completely perpendicular to the filter element's axis,particles in this area will be sucked into this vortex and the collision probability between deposition particles and candle surface will be increased,leading to forming a "concentration zone" in this area.(5)The simulation of the back pulse process was conducted and the results showed that increasing the NRP value is beneficial to improve the back pulse blow effect,but too high NRP value will significantly increase the energy loss.Meanwhile,when the ratio between the distance form nozzle to the sheet plate and the diameter of nozzle is below two,it can effectively improve the utilization rate of the back pulse blow power.When the outlet velocity of the back pulse flow is supersonic,the straight nozzle and Laval nozzle have the best blowing performance.Furthermore,the pressure and velocity field changes in the upper part of the filter tube were very fierce,while the pressure and velocity field changes in the lower part of the filter tube was significantly reduced.Moreover,the detachment process of the compact dust cake was different from that of the loose dust cake.The detachment of compact dust cake was mainly in the form of crack expansion.Finally,a novel mechanism of dust cake detachment was put forward:the detachment of the compact structure dust cake mainly depends on the propagation of mechanical wave and sound wave in the porous media;while the detachment of the loose dust cake mainly depended on the drag force between the back pulse flow and particles inside dust cake.