Production Of Olefin-rich Jet Fuel Precursors From Biomass Via Fast Pyrolysis And Catalytic Deoxygenation

For the moment,jet fuel is mainly produced by refining petroleum.Due to the shortage of petroleum resources and climate change,biomass-derived jet fuel has become a research hotspot and attracted much attention.Catalytic fast pyrolysis(CFP)of biomass over HZSM-5is an effective way to convert biomass into jet fuel precursors(olefins and aromatics)in one step.However,the proportion of olefins in hydrocarbon products from CFP is too low to meet the demand for becoming jet fuel precursors.In view of the above problem,the coupling relationship between pyrolysis process and catalysis process in CFP was investigated firstly.And then,ex-situ CFP was studied comprehensively and found to be more feasible for olefins production compared with in-situ CFP.Finally,ex-situ CFP was further optimized from two aspects,increasing olefins proportion in hydrocarbon products by introducing steam and improving the catalytic performance of HZSM-5 by catalyst modification.In order to reveal the coupling relationship between pyrolysis process and catalysis process in CFP,catalytic alkylation of naphthalene with methanol over HZSM-5,HZSM-5@silicalite-1,HSAPO-34,HMordenite,HBeta,HUSY,and HMCM-41 were compared in a fixed-bed to determine the influence of catalyst external acidity and the effective pore size of HZSM-5.Molecular dimension information about CFP products was provided by quantum chemical calculations.The results indicated that the effective pore size of HZSM-5 was between the critical molecular diameters of 2,3-and 1,6-DMN(between 7.520 and 7.961 (?)),and only molecules with a smaller critical diameter was able to diffuse into the channels.Only a few components of bio-oil(less than 20%)had a critical molecular diameter smaller than the effective pore size of HZSM-5.Therefore,there were a large number of heavy components in bio-oil that could not be converted efficiently.On this basis,it was proposed that the appropriate enhancement of the cracking of pyrolysis products was beneficial for the generation of target hydrocarbons.Ex-situ CFP was adopted to replace the conventional in-situ CFP for the enhancement of cracking reactions.Ex-situ and in-situ CFP of biomass over HZSM-5 were compared in a twostage fluidized-bed/fixed-bed combination reactor.The effects of weight hourly space velocity,carrier gas flow rate,pyrolysis temperature,catalysis temperature and nascent char on the product distribution of ex-situ CFP were also investigated.Ex-situ CFP gave similar total carbon yield of olefins and aromatics(around 20%)compared with in-situ CFP,and produced significantly more olefins(increased by 75.9%)as well as less coke and polycyclic aromatic hydrocarbons(PAHs).The yields of olefins and aromatics were mainly affected by deoxygenation reactions and aromatization reactions,while the selectivity of specific hydrocarbon products was mainly affected by cracking reactions.Low catalysis temperature was observed to obviously inhibit the cracking capacity of HZSM-5 and produce less ethylene and benzene.Nascent char was effective to reduce catalyst coke yield.Olefins +aromatics slightly decreased by 2.0% with the presence of a small amount of nascent char,while catalyst coke significantly decreased by 15.9%.Ex-situ CFP was optimized from the perspective of increasing olefins proportion in hydrocarbon products by introducing steam.The effects of steam on both pyrolysis and cataysis processes in ex-situ CFP were studied.The results showed steam took part in the biomass pyrolysis reactions and promoted the formation of light compounds in bio-oil which were the main precursors of hydrocarbons.HZSM-5 catalyst was activated at the initial exposure to steam through the adjustment of physicochemical properties caused by rapid dealumination of the framework,leading to an improvement of its catalytic performance for the production of hydrocarbons.In addition,there was a competitive adsorption between pyrolysis vapors and steam on catalyst acid sites,which reduced catalyst activity in all aspects,especially the aromatization reactions.For activated catalyst,when steam fraction increased from 0% to 40%,olefins to monocyclic aromatic hydrocarbons(MAHs)ratio increased from 0.83 to 1.60 and carbon yield of olefins + MAHs was also increased in varying degrees.Ex-situ CFP was optimized from the perspective of improving the catalytic performance of HZSM-5 by catalyst modification.Core-shell structured HZSM-5@silicalite-1 zeolite catalysts were hydrothermally synthesized through the growth of silicalite-1 along the surface of commercial HZSM-5 to inhibit coke formation on the external surface of HZSM-5.The catalyst characterization results indicated that silicalite-1 shell layer was successfully formed on the HZSM-5 core particles.Silicalite-1 coating was proved to be effective to reduce the external acidity as well as keep the internal acidity and channel patency of core HZSM-5 by catalytic probe reactions.After covering 78.8% of the external surface with silicalite-1,olefins+ MAHs increased by 9.6%,coke and PAHs decreased by 22.1% and 19.2% respectively,the residual activity of catalyst increased by 16.4% after 6 consecutive runs.