Study On The Integrated Washing Technique And Mechanism Of Venturi-Airlift Loop For The Purification Of Coal Gas

The gas production of coal gasification contains large amount of coal ash and saturated vapor.It's likely to block the equipment,cause the negative effects on the process and productions in downstream.The content of coal ash has been the critical factor for clean and safe production.Particularly,the removal of micro-particles attracts increasing attention as one of the hot issues in the field of aerosol separation.In this paper,the purification technology,integrated with venturi-and loop scrubber,is proposed for the removal of coal ash from crude gas.That is,two-step atomized dispesion in venturi scrubber-forced annular flow in loop scrubber-supplementary spray and demist,where bubbling scrubber is replaced by loop scrubber for the first time.Based on the characterizations of ash particles' wettability,the hydraulic-and ash removal performances of major equipments were investigated by the combination of experiments and Computational Fluid Dynamics(CFD).The investigation for the wettability of coal ash particles incloudes: The criterion of Gibbs free energy and wetting heat was presented in order to evaluate whether the particles can be wetted spontaneously in thermodynamics.Then,other characteristics,such as the ? potential,particle-size distribution,surface structure,elemental composition of coal ash,as well as BET surface area of ash particles were measured.Therefore,the similarity of particles was also expressed with the characteristic variable,namely size-and surface fractal dimension.It can be helpful to interpret the wettability of coal ash by these characteristics of the particle size distribution,surface microstructure and composition.In the investigation of venturi scrubber,the hydraulic experiments were accomplished at first,where its structure was determined by the simulated results of CFD.Accordingly,the semitheoretical pressure drop model of venturi scrubber was established for gas-liquid phase in annular mist flow regime.Then,the gas-liguid-solid phase ash removal experiments were explored.According to the two-step atomized dispesion technology,the liquid stream is diffused by the gas stream with high speed in venture scrubber.Thus,the perfect annular mist flow regime is formed to cause a significant increase of droplets' interface which is helpful to capture the particles by inertia impaction and diffusion.Note that considerable micro-droplets,with the equivalent dimension and number as the micro-particles,are generated to destroy the stagnation gas film outside the surface of micro-particles by strong shear force.It greatly improves the the wettability of micro-particles.Afterwards,the hydraulic characteristics,such as gas holdup and annular liquid velocity,were measured in the airlift loop scrubber.Specifically,the circular gas rate ratio was defined and its correlation was founded based on the gas holdup data.Besides,it can be found from the ash removal experiments that the forced gas-liquid annular flow is formed so that he bubbles are likely to be broken and agglomerated while circulating in the bed.Consequently,the ash wrapped in the bubbles would have more opportunities to expose to water,thus leading to a high washing efficiency.In addition,the spraying-demisting system can provid supplementary washing and dehydration.It means the liquid film formed in the demister also can capture the remaining particles.In the combined process,the washing mechanism of each stage is different from each other.Geneally,venturi scrubber plays the critical role for the bulk wetting of coal ash,especially for the micro-particles(<10?m).While annular washing can reinforce the capture of particles by promoting the update of bubbles' interface and prolonging the residence time of bubbles.Spraying and demisting are particularly applicable for blocking the remaining relatively large particles by impacting.The cooperation of multi-stages provides a good performance on ash removal.The removal-efficiency experiments of three phases were performed in this work.It indicates that the total efficiency and grade efficiency are both higher than 99.9%,which are much greater than industrial requirements.Generally,the efficiency increases with the increased throat length of venturi scrubber or liquid rate,while it appear to be non-monotone with the increased gas rate.According to the mechanism analysis,the lower removal efficiency of particles(20~50?m)can be explained as the loss on ignition;while for the particles(<2?m),the surfaces wrapped by gas film lead to a poor wettability.The motion behaviors of droplets and bubbles can reasonably describe the cooperative mechanism for each stage.Thus,the principles of particles removal,especially for microparticles,were analyzed systemically.Therefore,we made a series semitheoretical derivation on total removal efficiency.