Catalytic Co-hydropyrolysis Of Biomass And Waste Plastics To Produce Aromatic Hydrocarbons

Lignocellulosic biomass can be converted into fuels or value-added chemicals through thermochemical conversion,so it can be used as an alternative source of fossil fuel resources.However,due to the limitation of low hydrogen/carbon effective ratio(H/Ceff),the content of the target product(hydrocarbon fuel)in bio-oil is relatively low.Hydrogen-rich plastics can be used as hydrogen sources to supplement the low H/Ceff of biomass,and also be cracked into light olefins,which react with large amounts of furans derived from biomass to convert into aromatic hydrocarbons by Diels-Alder reaction.In this work,a new strategy of catalytic co-hydropyrolysis(co-CHP)under solvent-free conditions is designed to selectively convert biomass and plastics into monocyclic aromatic hydrocarbons.(1)The co-CHP of PW and HDPE to selectively produce aromatic hydrocarbons was studied by optimizing reaction parameters(feedstock composition,reaction temperature,residue time,initial hydrogen pressure and catalyst to reactant ratio).The results show that the highest aromatic yield(18.3 wt.%)and the highest aromatic selectivity(67.8%)were obtained.Aromatic hydrocarbons were mainly composed of mono-cycle aromatic hydrocarbons,of which benzene,toluene,and xylene(BTX)had the highest selectivity(51.6-70.6%),while oxygenated compounds were almost nonexistent in bio-oils.(2)Through the optimized design of catalyst,the polymerization reaction can be suppressed and the efficiency of the co-CHP can be improved.The effects of Pd/trapHZSM-5 prepared by different methods(thermal reduction,chemical reduction and self reduction)on the co-CHP of PW and HDPE were studied,and their catalytic activities were compared.The catalyst was characterized by XRD,TEM,BET,NH3TPD and H2-TPD.The Pd/Trap-HZSM-5 catalysts prepared by different methods show significant differences in the particle size,dispersibility and hydrogen adsorption capacity of Pd nanoparticles.The smaller the particle size of the Pd nanoparticles,the stronger the hydrogen adsorption capacity and the higher the aromatic selectivity;The self reduction Pd/Trap-HZSM-5 catalyst showed the highest activity,and combined moderate acidity and hydrogenation function,achieving a high yield of aromatic hydrocarbon(40.5.wt%),high selectivity of monocyclic aromatics(88.9%)and high yield of bio-oil(45.5 wt.%),of which only 0.7 wt.%was polycyclic aromatic hydrocarbons.Under the optimized catalytic system,the synergistic effect between PW and HZSM-5 had been significantly enhanced,and the relative yield of aromatic hydrocarbons had increased by 44.5%.(3)Under the co-CHP reaction system,cellulose,hemicellulose(represented by xylan)and lignin were selected as biomass model compounds(three components of biomass)to investigate the synergistic mechanism of co-CHP.The experimental results show that the synergistic effect of cellulose and high-density polyethylene was the largest,and the synergistic effect increased the yield of aromatic hydrocarbons by 36.2%;Hemicellulose(xylan)and plastic exhibited obviously different degradation temperature range,thus,the synergistic effect was not obvious;while lignin was easy to polymerize to form coke,and the generated coke covered the catalyst surface,making long-chain hydrocarbons derived from HDPE were not cleaved completely to produce aromatic hydrocarbons,resulting in a negative synergistic reaction,which was the main reason for affecting the overall pyrolysis efficiency.