Study On Catalvtic Removal Of Gasification Tar By Waste Derived Char Catalyst

The generation of municipal solid waste?MSW?in China is growing year by year,causing the occupying of land and serious damage to the ecological environment by random disposal.Compared with incineration,pyrolysis and gasification have gained more attention due to efficient energy recovery and lower emission of pollutants?e.g.dioxins and heavy metals?.Syngas is the target product of gasification,but the by-product tar will restrict the usage of it.Catalytic conversion is one of the most promising methods for the removal of tar due to the advantages of high efficiency and recovery of tar energy.In recent years,waste derived char catalysts have become hot topics.However,the knowledge of waste derived tar formation and characteristics is inadequate.Meanwhile,the physical and chemical properties and the catalytic activity of char catalyst are unclear.Thus,the goal of the present study is to develop cheap and efficient waste derived char catalyst for the cracking of MSW tar.The properties of tar and the catalytic activity,modification,coking and regeneration of char catalyst are investigated.The results offer some theoretical and technical references for the efficient and clean energy utilization of combustible solid wastes.The properties of MSW tar are researched.Results reveal that the formation and composition of different tars vary significantly.The interaction between different waste materials has strong effects on tar formation and composition.The tar yields of pine wood?PW?,tryptone?TP?,PE and PVC at 600oC are 19.1,21.3,60.4 and 18.1%,respectively.Oxygen compounds?e.g.phenols and furans?,nitrogen compounds?e.g.amines and nitriles?,aliphatic compounds?e.g.diene,alkene and alkane?and polycyclic aromatic hydrocarbons?PAHs?are dominant in PW,TP,PE and PVC tar accordingly.The interaction between PW and PE increases the tar yield by 3.7⁴.2%,while the interaction between PW and PVC decreases the tar yield by 7.2¹4.4%.When biomass is mixed with plastic,the quality of tar is improved due to a higher heating value and lower contents of oxygen,water and acid compounds.The properties of waste derived char are investigated.Results show that biochar is more reactive and potential to be a catalyst or catalyst support than sewage sludge char?SSC?.The contents of elements with catalytic activity?K?Ca?Mg and Fe?in biochar are higher than that in SSC,while the contents of inert elements?Si and Al?in biochar are relatively lower.For biochar,the BET surface area is 209.7 m²/g,and the volume percent of micropore is 83.6%.For SSC,the surface area is 31.3 m²/g,and the volume percent of meso-and macropore is 87.3%.The relative content of graphitic carbon is 54.4%for SSC and 72.5%for biochar.The activation energies under combustion and gasification conditions are 50.1 kJ mol^-1 and 137.9 kJ mol^-1 for SSC,and 36.6 kJ mol^-1 and 119.5 kJ mol^-1 for biochar,respectively,revealing that biochar is more active.The synergistic effect of pine wood and PE decreases the char yield by 13.8²2.4%,while the synergistic effect of pine wood and PVC increases the char yield by 15.5²7.9%.When biomass is mixed with plastic,the char has a smaller H/C atomic ratio,more ordered carbon structure and higher stability.SSC is used to crack tar model compound toluene.Results show that the catalytic activity of SSC is lower than that of nickel catalyst and calcined dolomite,but much higher than that of thermal cracking and bottom ash catalyst.The toluene conversion by SSC is 94.5%at 900oC.The main products of toluene cracking are H₂ and coke,and the latter is the main cause of deactivation.Steam and CO2 promote the toluene conversion through reforming reactions and consuming coke.H₂ promotes the toluene conversion through hydrocracking.HCl is negative to the catalytic activity by forming low-melting-point chlorides?e.g.CaCl₂?,which melt and block the pores.The catalytic activity of char is attributed to porous structure,ash content,oxygen functional groups and defects of carbon structure.The modification methods of char catalyst are investigated.Results show that activation and nickel loading effectively improve the activity and stability of char catalyst.Activation not only increases the surface area from 143 to 797 m²/g,but also upgrades the distribution of pore size?more meso-and macropores?,forming hierarchical pore structure.Activation increases the contents of oxygen functional groups?e.g.C-O?and the defects of carbon structure,which are the active sites during tar cracking.When nickel is loaded on biochar by wet impregnation method,the particle size of nickel is between 7.8⁸.3 nm.The distribution of nickel?Ni?on activated biochar is more uniform,which results in a higher catalytic activity and stability.The results suggest that the pre-activation is necessary if char is used as catalyst or catalyst support.Taken yield and catalytic activity?CA?into consideration,activation?C900 or H40?followed by Ni loading is the optimum preparation method of high CA char catalyst,which increases the CA of the original biochar by 136.7%.The properties of coke and regeneration of catalyst are investigated.Results show that filamentous coke can maintain the activity of catalyst and the regeneration within three times can recover its activity.Filamentous coke?carbon nanotubes?and non filamentous coke are produced during toluene cracking.The former has weaker effect on deactivation.In fact,filamentous coke can improve the catalytic activity at the beginning.High Ni loading,low temperature and low steam concentration are beneficial for the formation of filamentous coke.The yield ratio of filamentous coke to non filamentous coke is highest?1.11?with Ni20 at 700oC without steam.When deactivated catalyst is calcined in air at 600oC for 3 hours,its activity can be recovered within three times.The activity drops dramatically after the fourth regeneration.The surface area of regenerated catalyst decreases from 97.7 m²/g to 86.9 m²/g,while the crystal size of NiO increases from 12.8 nm to 18.0 nm.Finally,the MSW tar with complicated composition is cracked by activated biochar catalyst.Results show that the conversion,products distribution and effects on char catalyst of different MSW tars vary.The tar conversion ratio depends on tar composition,shape selectivity of tar molecular and internal diffusion.The conversion of biomass tar is higher than that of plastic tar,and the highest tar conversion of 98.7%is achieved for PW tar at 900oC.Nitrogen compounds are more stable than oxygen compounds,while compounds with 2 benzene rings,especially naphthalene,are most difficult to be cracked by char catalyst.The coke yield from plastic tar is much more than that from biomass tar,which is due to tar composition and other products during pyrolysis?e.g.H2O and HCl?.Plastic tar may lead to olefin polymerization and aromatic polycondensation,while high oxygen content and low carbon content in biomass tar may lead to less coke formation,and water produced from biomass pyrolysis consumes coke.