Experimental Research On Chemical Chain Technology Assisted Catalytic Removal Of Gasification Tar

Tar is an inevitable by-product of coal pyrolysis and gasification process.The content of tar vapor and oil particles accounts for 3-8%at least,which is harmful to corrode the gas pipeline,increasing gas purification load,causing delayed ignition of gas combustion equipment.There are many researches on tar removal.There are two common treatment methods,like catalytic cracking into small molecule gas in the furnace and physical capture outside the furnace.But tar removal efficiency of these two methods is limited and energy consumption is large,which also cause secondary pollution of atmosphere and water.In this paper,based on catalytic cracking technology,chemical looping route is introduced to the catalytic cracking of tar vapor by preparing oxygen carrier.Through multiple cycles of oxygen carrier,an efficient and clean method of tar vapor removal is realized.The main research contents and conclusions are as follows:Firstly,the characteristics of the composite catalyst for coal tar cracking were investigated,the optimal Ca/Mg ratio was selected,and the kinetics analysis of coal tar catalytic cracking process was carried out.The results show that the Ca/Mg catalyst can obviously promote the tar cracking.When the Ca/Mg ratio is 7:3,the tar cracking conversion rate is highest.The activation energy of different reaction stages is calculated that the Ca/Mg catalyst reduced the 15%and 14%of activation energy respectively during the pyrolysis and secondary cracking processes,so the catalyst promoted the rapid cracking of tar.Secondly,the catalytic cracking of coal tar vapor was studied in a two-stage fixed bed reactor with Ca/Mg composite catalyst.It was found that the tar conversion rate increased with temperature.After 700?,the tar conversion slowed down,the CO yield reached the highest,and the CO₂ concentration was lower.The tar conversion rate increased and then decreased with the particle size decrease,and the 0.1-0.2mm particle size was the best.The gas-phase reaction time is controlled by the thickness of catalyst layer.The increase of reaction time is benefit for the tar conversion rate,but increasing becomes slowly after 0.75s.On the basis of the above optimization parameters,the catalyst activity of tar removal keeps high efficiency in 6h,and then the tar conversion rate and gas production decrease rapidly.In order to obtain the deactivation mechanism of the catalyst,the fresh catalyst and the catalyst after 6 reaction hours was characterized.XRD/SEM analysis showed that fine carbon microcrystals accumulating had been appeared on the catalyst surface.For part of CaO absorbed CO₂ to form CaCO₃ impurities,and the sintering phenomenon was not obvious.The addition of Fe₂O₃ improved the carbon deposition resistance and prolonged the inflection point of cracking rate,among which the inflection point of 3%Fe₂O₃/Ca-Mg catalyst was the latest,extending to 16h.TG-DSC was used to quantitatively analyze the carbon deposition rate,to make sure that the catalyst needs to be renewed when the carbon deposition rate reaches 3%?5%.Finally,Fe6Al4Ca1 composite oxygen carriers was prepared by impregnation method.In the process of chemical looping cracking of coal tar,the dual performance of oxygen carrying/catalyzing was investigated.The effects of molar ratio?O/C?,reaction temperature,S/C molar ratio and cycle times on coal tar removal efficiency were discussed.Fe6Al4Ca1 shows the best performance of cracking tar when the temperature is 900?,O/C molar ratio is 3,S/C molar ratio is 0.5.In order to investigate the multi-cycle performance of oxygen carrier,chemical looping reforming of coal tar and air oxidative regeneration of oxygen carrier were carried out alternately in a fluidized bed.Through XRD,SEM,BET characterization analysis,Fe₂O₃ in the reduced oxygen carrier mainly transforms into Fe₃O₄ and FeO,and the formation of calcium ferrite with catalytic effect and the pore forming effect of Ca maintain the multiple cycle reaction activity of the oxygen carrier.However,the exothermic process of Fe3O4 oxidation regeneration causes partial grain melting and local sintering,destroys the surface sites of the oxygen carrier,and results in the decrease of the activity of the oxygen carrier.At this time,the diffusion rate of lattice oxygen to the surface of oxygen carrier decreases,and carbon deposition is formed on the surface.In this paper,the primary pyrolysis of tar,further decomposition of carbon oxygen complex and deposited carbongasification are three stages of chemical looping reforming reaction of coal tar.In view of this mechanism,the control of chemical looping reaction rate will further improve the catalytic performance and service life of coal tar removal,this will be the next step of the project.