Study On The Pyrolysis Characteristics And Synergistic Conversion Of Plastic Solid Waste

Pyrolysis is an important method for the efficient recovery of plastic monomers,fuels,and chemicals from plastic solid waste.However,different types of plastic solid waste exhibit significant differences in pyrolysis characteristics,and our current understanding of the pyrolysis processes of different plastic waste is still limited.Moreover,in practical recycling processes,plastic waste is often not present in isolation but rather mixed with other waste components.The sorting of mixed solid waste can be challenging,and therefore,it is necessary to consider the co-pyrolysis and conversion processes of plastic waste with other organic waste.However,the co-pyrolysis process is more complex,and the interactions between the different components are not yet well understood.In view of the above issues,this work focused on the structural differences of different plastic polymers and systematically investigates the influence mechanisms of the differences in side-chain structures of polyolefin plastics and the differences in backbone linkages of polyester and polyamide plastics on their pyrolysis characteristics.The paper provided in-depth insights into the pyrolysis mechanisms of different types of plastics.Based on a comprehensive understanding of the pyrolysis characteristics of individual plastics,this paper further explored the specific mechanisms of synergistic effects between components in real mixed plastic waste and enlarged the co-pyrolysis apparatus.This enabled stable and continuous co-pyrolysis of plastic waste and typical biomass components and the coproduction of tri-phase co-pyrolysis products.First,six polyolefin plastics with different side-chain structures were used as raw materials to investigate the influence of side-chain structural differences on the pyrolysis characteristics of polyolefin plastics.It was found that the OH side chain in PVA had the weakest thermal stability,primarily undergoing dehydration and dehydrogenation reactions,while the acetoxy side chain in EVA mainly underwent deacetoxylation and deacetylation reactions simultaneously.The deacetylation reaction led to the transformation of the acetoxy side chain in EVA into OH side chains.During the pyrolysis processes of PVA and EVA,the oxygen-containing side chains with lower thermal stability evolved and released oxygen-rich products earlier or slightly earlier than the hydrocarbon products,while in the pyrolysis processes of HDPE,PP,and PS,the release of all hydrocarbon products occurred almost simultaneously.In addition,the dissociation of the CI atom side chain in PVC mainly occurred via elimination reactions of Cl,leading to the generation of a large number of aromatic compounds.Based on the structural evolution and product distribution results,the major evolution pathways for different side chains were proposed.HDPE,PP,and PS mainly underwent the cleavage of C-C bonds in the main chain,while in PVC,PVA,and EVA,the evolution of side chains took place primarily,thereby exerting different effects on the degradation of the carbon skeleton.Regarding polyester plastics and polyamide plastics that contain C-O and C-N linkages in their backbone structures,the influences of the differences in linkages on the degradation processes of the backbone were investigated using PET and nylon 6 as research subjects.Specifically,the degradation of the PET backbone was primarily caused by the cleavage of the alkyl Cα-O bond and its adjacent alkyl C-C bond,resulting in the formation of acid and ester products.On the other hand,the pyrolysis of nylon 6 initially started with the changes in NH₂groups at the end of the chain.Subsequently,the main reaction involved the rapid cleavage of the amide C-N bond in the chain via concerted reaction.At this stage,the backbone structure of nylon6 underwent degradation and predominantly transformed into caprolactam.The cleavage of C-C bonds in PET and the adjacent Cα-O bonds were found to be the most likely to occur,and both followed a competitive reaction mechanism.In comparison to the cleavage of C-C bonds in the nylon 6 backbone,it was observed that the transformation into caprolactam was more likely to happen via the concerted reaction of the amide C-N bonds.Based on a clear understanding of the pyrolysis characteristics of individual plastics,further study was conducted to investigate the synergistic effects between components in the pyrolysis of actual mixed plastic solid waste.The results showed that the synergistic effects not only promoted the release of volatile compounds during the pyrolysis process but also facilitated the further degradation of long-chain hydrocarbon intermediates into CH₄.DG-DAEM analysis indicated that degradation reactions had a more dominant role in pyrolysis of actual mixed solid waste.Additionally,the synergistic effects between the components of mixed solid waste also enhanced the deoxygenation of oxygen-containing compounds and their conversion into hydrocarbon products,thereby improving the quality of the main liquid pyrolysis products.Based on this,a laboratory-scale continuous feeding rotary kiln pyrolysis reactor was further constructed to investigate the continuous synergistic pyrolysis process of polypropylene（PP）and cellulose（CE）and the characteristics of the three-phase products were evaluated.The strongest synergistic effect was observed at PP:CE ratio of 1:1,which not only led to the deep degradation of some straight-chain olefins and waxy olefins into small molecule products but also potentially promoted the conversion of higher carbon-containing oxygen compounds into small molecular gaseous products.Additionally,under this mixing ratio,a lot of reactive free radical intermediates interacted with each other,resulting in the significant generation of synergistic products such as long-chain alcohols in the liquid products.During the synergistic pyrolysis process,PP not only had the possibility of melting and covering the surface of CE particles to form an encapsulated state,thereby retaining some OH groups in the pyrolysis char but also could have an etching effect on the pyrolysis char,thereby affecting the structural properties such as specific surface area and pore volume of the char.