Thermodynamic Simulation And Experimental Study Of Biomass Gasification And Syngas Methanation

The energy crisis and environmental pollution have promoted the development of clean and renewable energy.Biomass energy has received wide attention as the only renewable energy source that can be converted into three different forms of traditional energy sources:solid,liquid and gas phases.Biomass conversion products are abundant but the quality is uneven.Compared with biochar and bio-oil,bio-SNG?Substitute Natural Gas?has attracted much attention due to its simple processing technology,high added value of products,mature end-use market,and perfect infrastructure such as transportation pipelines.The conversion efficiency of traditional biochemical conversion to produce biogas is low,while as for thermochemiacl conversion,the processes of gas purification,tar and dust removal are complicated and costly.These shortcomings inbibite the promotion and utilization of bio-SNG.Based on this,this study proposes a novel concept of biomass gasification and syngas catalytic methanation coupled with biomethanation to produce CH₄,and carries out related research through thermodynamic simulation and experiment,aiming to improve biomass conversion efficiency,reduce process cost,and convert all gas components into methane.In this thesis,a new and original process route for preparing bio-natural gas by biomass conversion is proposed.Firstly,biomass synthesis gas is prepared by biomass gasification.After the preliminary treatment,the synthesis gas enters the catalytic methanation reactor.In this process,the supplementary hydrogen is injected into the reactor which comes from water electrolyze,and convert all or part of carbon monoxide and carbon dioxide into methane;After the completion of the catalytic methanation reaction,the remaining gas is introduced into the biomass fermentation tank,and the original carbon dioxide in the fermentation tank and the remaining carbon dioxide in the first stage reaction are completely reacted with hydrogen to convert it into methane.In this thesis,the research on biomass gasification and syngas methanation was carried out from two aspects of simulation and experiment.The Aspen Plus software was used to simulate the vaporization process of biomass steam gasification on dual fludized bed in Mid Sweden University.The biomass steam gasification process was modeled using a yield reactor and a Gibbs reactor.The study found that the yield of biomass syngas increased with the increase of reaction temperature,and the hydrogen content increased first and then decreased with the increase of temperature,reaching a maximum of 42%at 850°C.With the rising ratio of steam and biomass in the reaction system,the reforming reaction of tar and steam causes the yield of biochar and tar to decrease,and the hydrogen content increases with the increasing ratio of steam and biomass.The simulation result is similar with the effect of steam/biomass ratio when the water content of biomass feedstock is increasing.As the residence time of the reaction increases,the biomass gasification product decomposes relatively more thoroughly,and the tar decomposes into a small molecule gas,which leads to an increase in gas production and a decrease in tar production.A simulation study on the preparation of bio-SNG by catalytic methanation and biochemical methanation of biomass synthesis gas was carried out by Aspen Plus software.The simulation uses a Rstoic reactor to analyze the technical feasibility of the catalytic methanation coupled with biomethanation process.This work simplifies the simulation of catalytic methanation and biochemical methanation processes,defining the conversion rates and priorities through relevant experimental study data.During the catalytic methanation process,the methanation of carbon monoxide is basically completely converted;the carbon dioxide is partially converted into bio-SNG,the content is reduced from 12%to 9.4%,and the methane content is increased from 9.2%to 87.4%,the quality of the gas product is greatly improved.In the process of biomethanation,since the metabolism of microorganisms is not affected by reaction kinetics and thermodynamics,the process of biochemical methanation of carbon dioxide has no reaction equilibrium limitation,and the carbon dioxide in the reactants can completely converted into methane,the content of methane is further increased from 87.4%to 97%.Through the biomethanation process,the quality of the product gas is further improved,and the gas component has been substantially completely converted into a single component of methane.Ni-Al₂O₃,Ni-ZSM-5 and Ni-MCM-41 catalysts were prepared by impregnation method.The loading of active metal nickel was 7%wt..These three catalysts were applied to the study of carbon dioxide methanation and mainly focus on the catalytic performance of different catalysts at different reaction temperatures.Under the conditions of reaction temperature of150-500?and reaction space velocity of 6000 ml·g^-1·h^-1,the carbon dioxide conversion rate and methane selectivity and yield of the three catalysts all show the same tendency with the increase of temperature which increased with the reaction temperature and then decrease slightly.The overall catalytic performance of the catalyst Ni-MCM-41 is slightly better than that of the catalyst Ni-ZSM-5.The catalytic performance is optimal at a reaction temperature of 450°C with the 73%carbon dioxide conversion rate and 97%methane selectivity on catalyst Ni-MCM-41.The order of the catalytic performance of the three catalysts for carbon dioxide methanation is Ni-MCM-41>Ni-ZSM-5>Ni-Al₂O₃.According to the result of Nitrogen adsorption and desorption characterization,the catalyst Ni-MCM-41 has the largest specific surface area with mesoporous structure,which can effectively improve the dispersion degree of the active metal of the catalyst and improve the catalytic performance.