Study On Kinetics Of Typical Plastics Catalytic Pyrolysis And Analysis Of Carbon Deposit Characteristics

In recent years,it is promising to substitute the traditional pyrolysis technology with the catalytic pyrolysis technology to selectively convert waste plastics into available products.However,the existing technologies are still not mature enough for converting waste plastics into gaseous fuels and chemical raw materials with low cost.The key limiting factor is the development of catalysts with excellent performance.In this paper,the effects of Ni-based catalysts based on different supports on the pyrolysis mechanism and products of waste plastics are investigated.Ni-based catalysts are widely available and inexpensive,and have a strong ability to activate C=C and C=H bonds in polymers.In addition,the anti-carbon deposition characteristics of the catalysts are also studied to improve the performance of pyrolysis catalytic.First,the pyrolysis and catalytic pyrolysis charateristics of HDPE and PP were analyzed by a thermogravimetric analyzer coupled with Fourier transform infrared analysis(TG-FTIR).It was found that the pyrolysis of PP was easier than HDPE under three heating rates(5,10,15^oC/min).The pyrolysis products of the two plastics are mainly saturated alkanes and alkenes and the product content is different due to the differences in the polymerizable monomers of the two plastics.Subsequently,the iso-conversional methods of KAS and FWO and model-fitting method of CR were applied to determine the kinetic parameters of the two kinds of plastic pyrolysis.The kinetic functions of HDPE and PP can be described by Spherical symmetric phase boundary reaction model(R3)and Circular Symmetric Phase Boundary Response Model(R1)respectively.After adding the Ni/Al₂O₃and Ni/HZSM-5,the two catalysts can effectively reduce the energy consumption of reaction and promote the generation of gas products.However,a small amount of carbon will be deposited on the catalyst surface after the reaction.GC results showed that Ni/HZSM-5 could increase the content of isobutane in saturated alkanes products for the two plastics.For Ni/Al₂O₃,the content of methane in saturated alkanes of HDPE can be increased,while the gas products distribution of PP does not change significantly.KAS method and FWO method were used to calculate the catalytic pyrolysis activation energy of the two plastics under the two catalysts,and the results showed that the two catalysts could significantly reduce the reaction activation energy,in which the reduction of Ni/HZSM-5 was larger than that of Ni/Al₂O₃.Secondly,by combination with CR method,the reaction mechanism functions of HDPE and PP over the Ni/Al₂O₃were both in accordance with Nucleation Control Model(P1.5),and the first-order chemical reaction model(F1)and Cylindrical symmetrical phase boundary reaction model(R2)were obtained under Ni/HZSM-5,respectively.Secondly,SEM,BET,XRD,TPO and FTIR were used to characterize the deactivated Ni/HZSM-5 with severe carbon deposition,and to explore the carbon deposition behavior on its surface.The results showed that the specific surface area and pore volume of the catalyst decreased by more than 50%after the catalytic pyrolysis of plastics due to the serious coke covering and sintering.At the same time,the crystal phase of the deactivated catalyst changed from bimodal to unimodal near 23.16°,29.91°and 45.24°.The carbon accumulation components are complex,mainly aromatic compounds.The decomposition of carbon can be achieved by high temperature heating in air atmosphere.Compared with the surface area carbon decomposition after catalytic pyrolysis of the two plastics,the carbon accumulating components produced by PP are more complex than those produced by HDPE.