Experimental Study On Low-temperature Oxidation Of Biomass Tar Based On Modified Transition Metal Catalysts

Biomass is a reliable option to maintain national energy security and optimize energy structure due to its abundant reserve and zero CO2 emission.Biomass gasification is a crucial technology to converse the organics into high-quality gas.However,there is excessively high tar content in syngas,which limits the industrialization of biomass gasification.Low-temperature catalytic oxidation is beneficial to the removal of biomass tar and the economics of the reaction.Therefore,based on the response surface method（RSM）,modified transition catalysts were optimized and prepared,and the experimental researches on low-temperature catalytic oxidation of biomass tar were conducted,which promotes the development of biomass gasification technology significantly.Firstly,based on the RSM,the preparation conditions of Cu-Mn-Ce catalysts were optimized,taking the calcination time,calcination temperature,the concentration of active metals as the impact factors,and the carbon conversion rate（Xc）of benzene as the response.The results show that the optimum preparation condition is Cu O-Mn O content of 30%and Ce O₂ content of 4.4%at the calcination temperature of 620°C for 4.1 h（30-4.4-620-4.1）.Using this catalyst,the confirmatory experiment indicates that the average carbon conversion rate within half an hour(X_c-0.5h)and four hours(X_c-4h)are99.5%and 97.1%at 300°C,respectively,which is in good agreement with the prediction value.The results of XRD analysis show that Cu O is the primary active metal in the catalysts,which is affected easily by the calcination temperature.And the results of XPS analysis indicate that Ce O₂ will increase the oxygen vacancies and improve the oxygen transferability.Besides,the catalytic oxidization of benzene on the Cu-Mn-Ce composite catalyst complies with the Marse-van Krevelen mechanism（MVK）.And the ratio of Cu O-Mn O to Ce O₂ in the catalyst will cause the change of the control step of the redox reaction.Secondly,based on the optimum catalyst,the effects of the space velocity,the concentration of benzene,the type of molding compound,and the water vapor atmosphere on the low-temperature catalytic oxidation were investigated.The results show that the space velocity within a specific range of9000 h^-1-12000h^-1 has few effects on the performance of the catalyst.The Xc of benzene can reach more than 97%.As the concentration of benzene increased from 5000 ppm to 9000 ppm,the Xc remained above 99%at first,and then slowly decreased to 92%.Isopropanol and cresol could be almost completely removed,with Xc of 98.5%and 96.3%,respectively.However,the Xc of methylnaphthalene reduced to 87.9%.The water vapor atmosphere has a terrible effect on the Xc of benzene.When the water vapor concentration is higher,the Xc of benzene will decrease faster.The characterization results show that water vapor is mainly adsorbed on the catalyst surface and competes with the tar compound model for active sites,but does not affect the surface morphology and crystal phase of the catalyst.Besides,the competition could be relieved by increasing the reaction temperature.Lastly,the dynamic model of benzene catalytic oxidation was studied,and the dynamic parameters were solved.The results show that the dynamic process of the overall reaction of benzene at low-temperature catalytic oxidation complies with the Power-law model,and the activation energy and pre-exponential factor of reaction on the 30-4.4-620-4.1 catalyst are 69.81k J/mol and 3.88E+07s^-1,respectively.The effects of preparation conditions on the dynamic parameters were studied,and the results show that the composition and structure of the surface-active material of the catalysts are dominated by the calcination temperature,which leads to the significant changes in activation energy and reaction rate constant.When the calcination temperature rises from 500℃to 700℃,the activation energy increases by 97.7%as well as the reaction rate constant increases at first and then decreases.Therefore,the optimal temperature is around 600℃.However,the effect of calcination time is slight.Combined with the results of RSM,the optimal mass ratio of Cu O-Mn O to Ce O₂ is determined to be at the range of 6.6-8.