| Under the background of rapid urbanization in China,fuel gas produced from biomass gasification has been a key constitution and supplement of energy supply due to its advantages such as cleanliness,high efficiency,flexibility et al.However,because of its relatively high toxic CO content and low heating value,the primary biogas usually needs upgrading to meet the criteria through the combination of water gas shift and methanation.In this dissertation,a novel integrated unit was proposed and according to the“active component-promoter-support”characteristic of heterogeneous catalyst,a bifunctional catalyst suitable for biogas methanation coupling with water gas shift was successfully designed and thoroughly investigated applying the interdiscipline knowledge of thermal engineering,chemistry and materials design.Firstly,based on the active component for methanation and water gas shift,Ni-M(M=Mo,Fe,Co,Mn,Cr)bimetallic catalyst was prepared by co-impregnation method.Their catalytic performance were carefully studied and further compared to select the best second metal M.The preliminary results showed that Ni-Mn and Ni-Cr had higher CO conversion,CH4 selectivity and CO2 growth rate for low H2/CO biogas methanation coupling with water gas shift due to their higher Ni dispersion.Further comparison of Ni-Mn and Ni-Cr found that the former catalyst had superior catalytic performance and less deposited carbon than the latter in the wide range of H2/CO and H2O/CO.Thus,Mn was finally selected as the second metal to cooperate with Ni.After being catalyzed by Ni-Mn,the CO content of biogas was reduced to safety value and the lower heating value(LHV)was slightly improved.The Ni and Ni-Mn O2 surface was modelled by Material Studio software.The adsorption of reactant and product molecules were both calculated using density functional theory(DFT)to explain the synergy effect between Ni and Mn metals.It was found that the steric effect of Ni-Mn O2 surface forced the molecules to be adsorbed on the fcc site,which was kept a distance from large Mn O2molecule.The adsorption energy of Ni-Mn O2 surface was lower than Ni(111)surface,demonstrating the synergy effect of Ni-Mn bimetals.Secondly,based on the Ni-Mn bimetallic components,Re promoter was added using co-impregnation or step impregnation methods and the calcination was conducted in a microwave oven or muffle furnace.The impregnation sequence and heating method was investigated.It was found that the Ni-Mn/Re-Al2O3(MV)prepared by step impregnation along with microwave heating presented the best catalytic performance for methanation coupling with water gas shift.The Re additive promoted the reaction from structural and electronic effect two sides.Step impregnation method enhanced the reaction due to denser electron cloud and stronger anti-coking ability from Re Ox-Al2O3composite support.Microwave heating could efficiently increase Ni dispersion.Under the condition of 350°C,H2/CO=0.8 and H2O/CO=1,the CO content of biogas catalyzed by Ni-Mn/Re-Al2O3(MV)was declined to safety value and the lower heating value was increased to 9.71 MJ/Nm3during the 110-h stability test.Thirdly,based on the Ni-Mn-Re structure system,the Mn-containing water processing material,manganese sand was employed as catalyst support in order to reduce the production cost.Ni and Re was loaded by co-impregnation method and the effect of modification reagent including HNO3,NH3?H2O and H2O2 on the catalyst activity was studied.It was found that the Ni-Re/MMS-H2O2catalyst supported on the H2O2 modified manganese sand showed the best catalytic performance for methanation coupling with water gas shift.Its anti-sintering and anti-coking ability was enhanced due to the steric effect and increased oxygen vacancy number on the surface derived from H2O2 modification.Re promoter benefited the reaction from structural and electronic aspects.Under the condition of 400°C,H2/CO=0.8 and H2O/CO=1,the CO content of biogas catalyzed by Ni-Re/MMS-H2O2 was decreased to safety value and the lower heating value was increased to 10.63 MJ/Nm3,which satisfied the National Standard of Type II fuel gas and implied a promising future application.At last,the intrinsic kinetics and shaping of Ni-Re/MMS-H2O2were studied for the scale-up of reaction apparatus and catalyst production.According to Weisz-Prater criteria and Mears criteria,the internal and external diffusion was considered to be eliminated when the catalyst particle size and gas hourly space velocity was 12-14 meshes and 2000 h-1,respectively.Twenty five sets of intrinsic tests were carried out by orthogonal design method to obtain the relationship between reaction rate and influential factors including temperature,pressure and gas composition.The mathematic expression was then fitted using empirical,synergetic and independent models.Among the three models,the empirical model had the best fitting of methanation coupling with water gas shift.The average relative error(ARE)of synergetic model was around20%and the independent model possessed a large ARE of CO2 formation rate.The catalyst powder was shaped in lab scale using an extrusion apparatus as practice.The effect of binder type,water/powder ratio and binder/powder ratio on the compressive strength of shaped catalyst was investigated.The radial and axial compressive strength of shaped catalyst using CMC and PAM binder were larger than PEO.Both water/powder ratio and binder/powder ratio displayed an optimum value when using CMC as binder.The insufficient water and binder could decline the viscosity of catalyst pug,leading to the fracture of shaped catalyst caused from low compactness while the excessive water and binder would make the catalyst pug stagnate on the screw and inflict coherence among the shaped catalyst.Besides,excessive binder may further reduce the compressive strength due to newly formed pores during calcination.The catalytic activity using the three binders was in the following order:PEO?CMC>PAM.However,the shaped catalyst using PEO could be easily pulverized while that using PAM owned smaller BET surface area. |
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