Study On Catalytic Cracking Of Model Tar Species In Pyrolysis Gas By Typical Pyrolysis Char

Pyrolysis or gasification technology provides a bright prospect for the harmless treatment of biomass wastes,but it has limited the widespread use of this technology due to the harmfulness of by-product tar.Many researchers at home and abroad are devoted to the research of tar removal methods,of which catalytic cracking is considered as the most promising method.However,the choice of catalyst influences whether the catalytic reaction can be carried out efficiently.Char,one of the products of pyrolysis or gasification of biomass wastes,has become a promising catalyst due to its low price,abundant pore structure and alkali and alkaline earth metal(AAEM)species which contain active sites contributing to the catalytical reactions.This paper chooses three kinds of materials with a low price and a wide source:wood chips,pulverized coal,sludge particles.Their physical and chemical properties under different pyrolysis temperature,catalytic performance under different conditions and the modified char catalyst by adding nickel,iron and nickel-iron binary alloy are carried out in-depth study,committed to provide data support for developing char catalyst with low cost and high activity.Physical and chemical properties of char at different pyrolysis temperatures are analyzed by means of industrial analysis,elemental analysis,ash analysis,pore and surface area measurement,Fourier transform infrared spectroscopy,scanning electron microscopy and electron spectroscopy.The pulverized coal coal contains the highest fixed carbon,the sludge particles contains the highest ash content,and the wood chips contains the highest volatile content,which result in differences in the char yields.The chars prepared from wood chips,pulverized coal and sludge particles at different pyrolysis temperatures have relatively continuous and complete pore systems,ranging from very small pores to large pores.The adsorption loops of wood char and coal char are similar in the transitional stage,and their pore structures are mainly bottleneck-shaped and open-type holes,while pore structure of sludge char is mainly closed at one end and impermeable.Coal char and wood char show higher specific surface area of micropores at different pyrolysis temperatures and sludge char has higher mesopores specific surface area,all of which changes with temperature.Through the analysis of the surface structure and elements,the increase of pyrolysis temperature causes the char structure to collapse and deform to a certain extent,and also causes the change of the surface element content.The catalytic performance of char produced by pyrolysis of different raw materials was studied,and it is found that the char itself has very different physical and chemical properties,resulting in a large difference in catalytic performance.The chemical composition of char itself has a greater impact on the catalytic cracking of toluene than the specific surface area and pore structure.The initial specific surface area of sludge char is the lowest among the three char,but shows the best catalytic effect because of its high ash content.And in the ash,high Fe content and some Alkali and alkaline earth metal compounds were detected,which can provide active sites.During the catalytic cracking of toluene,the tar molecules are adsorbed on the char surface,which are catalyzed and cracked by the active site to produce light gas and coke.The coke can cause serious deactivation of the catalyst.And the char surface functional groups may react with the coke to produce carbon oxides or new functional groups.At the same time,the temperature increase inhibits the coke catalyst deactivation.The nickel,iron and nickel-iron binary alloy were added respectively in the char to modify it.Results show that catalytic effect of single nickel is best.Nickel exhibits excellent catalytic performance,because the C-H and C-C bonds of hydrocarbon molecule on the nickel metal surface show higher activation ability,while iron prefers having the co-catalysis effect.In the supported catalyst,the char is as a carrier loaded with metal ions,and tar is adsorbed in it to promote the progress of the catalytic reaction.Through the analysis of the initial hydrogen production concentration and the average hydrogen production concentration of the supported catalyst,it is found that the supported catalyst has a very high catalytic potential but the stability needs to be further strengthened.Although nickel has the best catalytic performance,its cost is very high.Considering the economic and catalytic effects,it is more appropriate to add nickel-iron binary alloys to char to improve the catalytic performance.By the way,the sludge char is an alternative way to achieve the innocuous treatment of sludge,because the sludge contains high metal elements,which enhances its catalytic performance.In summary,through the study of this paper,the physicochemical properties,catalytic mechanisms and changing rules of different chars are obtained,which not only provide the data basis for basic application,but also have certain industrial application value and significance.It is an industrially feasible route to catalytically crack tar using char made by cheap,widely sourced materials.