Study On Performance And Mechanism Of Tar Catalytic Cracking Over In-Situ Strontium Doping Modified Lanthanide Perovskite

Thermochemical conversion is an effective way to convert coal/biomass to multi-value chemicals.However,the heavy tar generated by the organic matter in coal/biomass during the pyrolysis process condensed in the pipeline outlet,blocking and corroding the pipeline of the reaction system,which poses a threat to industrial production safety.Therefore,it is necessary to develop an adaptive,economical and efficient catalyst for the conversion of tar to light oil and pyrolytic gas.Perovskite catalysts have strong oxygen storage capacity and thermal stability,but the catalytic activity and carbon resistance of perovskite need to be improved.Alkaline earth metal strontium doping can effectively improve the catalytic properties of perovskite.In this thesis,perovskite was modified by in-situ strontium doping.XRD,XPS,BET,O₂-TPD,H₂-TPR and NH₃-TPD were used to characterize the crystal structure,surface ion valence and concentration,pore structure and size,oxygen migration ability,reducibility and surface acid-base of the catalyst before and after the reaction.The performance and mechanism of catalytic cracking of tar over strontium in-situ doping modified catalyst were studied and explained.The main research contents are as follows:（1）A series of strontium in-situ doped perovskite catalysts La_1-xSr_xNi_0.8Fe_0.2O₃（x=0,0.1,0.2,0.3,0.4,0.5,0.6）were prepared by the sol-gel method.The catalytic cracking performance of coal pyrolysis tar at 900℃ was studied,and the effect of catalyst doping ratios on the catalytic cracking products and gas yield of coal tar was revealed.The application of La_1-xSr_xNi_0.8Fe_0.2O₃ perovskite increased the total gas production and H₂ yield,and reduced the tar content.When x≤0.2,the total gas yield and H₂ yield increased with the increase of doping ratio.When the doping ratio continued to increase,the gas yield decreased with the increase of the doping ratio.Among them,Sr 0.2 catalyst showed the highest total pyrolysis gas yield of 34.6mmol/g_coal,H₂ yield of 27.5 mmol/g_coal,100%tar conversion and lowest carbon deposition of 19.4 mg/(g_catal·h)due to its perfect perovskite structure,highly dispersed nanostructure and maximum water adsorption capacity.The pyrolysis of coal produced pyrolytic gas,coke and tar,and the pyrolysis volatiles underwent a secondary catalytic cracking reaction with the help of perovskite oxygen bridges.The lattice oxygen,surface oxygen and adsorbed water in perovskite formed an oxygen migration cycle,and the oxygen vacancy acted as a transfer vessel and circulation accelerator.（2）The catalytic cracking performance of La_1-xSr_xNi_0.8Fe_0.2O₃（x=0,0.1,0.2,0.3,0.4,0.5,0.6）on high-volatile biomass pyrolysis tar was studied with bamboo sawdust as raw material to further explore the catalytic activity and mechanism of modified perovskite catalysts.The results showed that Sr-doped catalysts had a positive catalytic effect on tar conversion during bamboo sawdust pyrolysis,and their addition improved the quality of syngas and low heating value in pyrolysis gas.Sr 0.2 has the highest tar conversion efficiency and total gas yield of 606.7 m L/g_bio,showing the highest syngas yield of 475.0 m L/g_bio and the highest low heating value of 12.86 MJ/Nm³.The oxygen vacancies and metal active sites on the catalyst surface were involved in the catalytic reaction,which promoted the oxidation reaction of surface reactive oxygen species with tar molecules,accelerated the decomposition of-COOH and oxygen-containing compounds in tar molecules,and thus improved the yield of CO and H₂O.H₂O was cleaved into hydrogen and oxygen radicals in the high-temperature reaction,which complemented the lack of lattice oxygen and increased H₂ production.（3）The interaction between coal and bamboo sawdust during the pyrolysis process was explored,and the catalytic cracking effect of different doping ratios of modified perovskite on the volatiles of coal and bamboo sawdust co-pyrolysis was revealed by adding different doping ratios of modified perovskite during the co-pyrolysis process.The results showed that perovskite catalyst had a synergistic catalytic effect with 900℃ co-pyrolytic volatilization of coal and bamboo sawdust.The interaction between the transition metals in bamboo sawdust and the alkali/alkaline-earth metals in coal promoted the decomposition of coal and bamboo sawdust in the pyrolysis stage.The hydrogen in bamboo sawdust supplemented the coal,and the oxygen enhanced the reaction activity in the pyrolysis environment.The primary catalytic volatilization in the pyrolysis stage had a synergistic effect with perovskite catalysis,which promoted the transformation of tar and the increase of gas yield in co-pyrolysis.Sr 0.3 showed the best synergistic effect with pyrolytic volatiles,showing the highest H₂ yield of 33.9mmol/g_mixture and the lowest tar content of 4%.The addition of perovskite contributes to the generation of hydrogen-rich gas and the esterification of tar during the co-pyrolysis of coal and biomass.The catalytic pyrolysis gas yield and tar conversion rate are affected by the metal site,surface acid-base,reactive oxygen species and pore structure of the catalyst.