| The rapid growth of global industry has aggravated global climate change and accelerated the consumption of fossil energy,making the preparation of liquid fuels from renewable resources one of the inevitable trends in the development of the energy industry.The complex composition of plastic waste and increasing labor costs make it difficult to recycle by conventional methods.Meanwhile,polyolefins in plastic waste can be re-refined by pyrolysis into marketable and value-added hydrocarbons.In this thesis,low density polyethylene（LDPE）,polypropylene（PP）,and polystyrene（PS）were used to simulate waste plastics,which are converted to liquid fuels by catalytic pyrolysis.Biochar-loaded metal（biochar-zero-valent metal）catalysts were prepared for the catalytic pyrolysis of plastics using biomass waste and metal compounds（metal oxides or chlorides）as co-feedstock by a one-step carbothermal reduction method;the optimization of plastic catalytic pyrolysis parameters was explored,and the formation mechanism of gasoline hydrocarbons in plastic catalytic pyrolysis was investigated.The catalysts were characterized by BET,FTIR,NH3-TPD,etc.The BET specific surface area of biochar supported metal catalyst is 304-503 m2/g,and the average pore diameter is 1.686-2.414 nm.The larger specific surface area and pore diameter of biochar supported metal catalyst are conducive to the steam catalytic reaction.The FTIR results revealed that the biochar functional groups mainly consisted of O-H,C-H,C=C,C-O and M-O（M=metal）,which enhanced the relevant active centers and provided favorable conditions for the cracking and aromatization reactions.The results by NH3-TPD revealed that the Ni/L strong acidic sites were the strongest and the acidic sites on the catalyst could catalyze a series of reactions such as dehydrogenation and dehydrocyclization of alkanes and olefins.It was found by Raman spectroscopy that the low graphitization and high carbon defects of Ni/L could provide additional pores and vacancies for metal loading,thus improving the catalytic efficiency.Ni/L catalyzed pyrolysis of LDPE with 76.2%selectivity of C4-C12 hydrocarbons.In contrast,the selectivity of Ru/L-catalyzed pyrolysis of C4-C12 hydrocarbons was85.6%,including 46.2%for BTX.The BTX selectivity in the pyrolysis products was increased by 16.0%compared to the Ni/L catalyst.However,it had the highest selectivity for PAHs（31.7%）,while for the Ni/L catalyst,the selectivity for PAHs was only 2.38%.It was further confirmed that the performance of Ni/L catalyst in hydrogenation process is comparable to Ru/L.The Ni/L catalysts have good catalytic effect,high stability and low production cost,and can replace precious metal catalysts.Mixtures of LDPE,PP and PS in different ratios（1:1:1,1:1:2,1:2:1,1:2:2,2:1:1and 2:2:1）were used to simulate real plastic waste to investigate the effects of its catalytic pyrolysis.The chemo-selectivity of catalytic pyrolysis of C4-C12 hydrocarbons for different LDPE/PP/PS ratios of mixed plastics was 1:1:2（87.9%）>1:2:1（87.4%）>1:2:2（86.9%）>2:1:1（84.6%）>1:1:1（83.6%）>2:2:1（78.2%）.The selectivity of C4-C12 hydrocarbons was 85.6%when LDPE:PP:PS（1:1:2）,of which the selectivity of aromatic hydrocarbons was 74.6%,indicating that Ni/L catalysis promoted the cleavage of long-chain radicals of polystyrene and the aromatization of alkanes and olefins.The mechanism of pyrolysis of both PP and LDPE is triggered by a free radical mechanism.When the homolytic chain breaks at any position on the carbon chain,the resulting hydrocarbon contains any number of carbons.PS pyrolysis consists of four main steps:initiation,transfer,decomposition and termination,involving random chain and end-strand breakage mechanisms.There are synergistic reactions between PE,PP and PS.Under the catalytic pyrolysis of Ni/L,PP and LDPE,as co-hydrogenators,can effectively increase the H/C of the feedstock and promote the chain breaking of long-chain hydrocarbons to form small molecules.These small molecules enter the pores of biochar and generate aromatic hydrocarbons through dihydroxylation,Diels-Alder cyclization and aromatization. |
