Preparation Of Light Aromatics By Catalytic Co-pyrolysis Of Biomass/Waste Plastics

The pyrolysis of waste biomass to produce high-value chemicals like light aromatic hydrocarbon（BTEX）,including benzene,toluene,ethylbenzene and xylene,which can realize the resource utilization of waste biomass and reduce the dependence of chemical basic raw materials on traditional fossil resources.However,biomass is poor of hydrogen and rich of oxygen,with low pyrolysis conversion efficiency,accompanied by a large number of highly polluting polycyclic aromatic hydrocarbons（PAHs）and catalyst carbon deposition.In this thesis,the co-pyrolysis of biomass and plastics was used to improve the hydrogen carbon ratio of raw materials.The selectivity of BTEX was regulated by metal modified molecular sieve under CO₂atmosphere,and the formation of PAHs and coke were inhibited.Therefore,the synergistic mechanism of CO₂and metal loading was studied;The series catalytic action of Fe₂O₃rich iron tailings and molecular sieve was discussed,and the method of catalytic pyrolysis to produce light aromatics and iron concentrate was studied.Fe/HZSM-5 molecular sieve catalysts with different loading obtained by impregnation method was added into the pyrolysis experiment of biomass and polypropylene in a fixed bed reactor.The gas,liquid and solid three-phase products were collected,detected and characterized.PAHs were quantitatively analyzed by ultrasonic extraction pretreatment technology combined with mass spectrometry single ion monitoring technology（SIM）.The results show that compared with HZSM-5 catalytic pyrolysis in traditional N₂atmosphere,the introduction of CO₂and metal loading increase the proportion of BTEX in pyrolysis oil from 64.1%to 74.8%,and reduce the proportion of heavy aromatics(C₉₊)from 33.7%to22.8%;In CO₂atmosphere,BTEX first increased and then decreased with the increase of Fe loading,while heavy aromatics showed a monotonic downward trend;The increase of CO₂concentration is conducive to the formation of BTEX.In addition,PAHs mainly exist in pyrolysis oil.Through CO₂synergistic metal supported catalytic pyrolysis,low ring,middle ring and high ring PAHs can be reduced simultaneously,the benzopyrene equivalent toxicity（TEQ）of pyrolysis oil can be reduced by 75%,the aromaticity of catalyst carbon deposition can be reduced,and the formation of"hard"carbon deposition that is difficult to oxidize and regenerate can be inhibited.Hard coke decreased from 80%to less than 50%.It is speculated that CO₂molecules may be activated by metal sites and react with H species released in the aromatization process,so as to accelerate the hydrogen transfer rate and promote the forward progress of aromatization reaction;At the same time,CO₂effectively inhibits the further conversion of BTEX to PAHs through Boudouard reaction of carbon deposition.Using the mixture of biomass and polypropylene catalyzed by iron tailings and molecular sieve,BTEX was quantitatively analyzed by internal standard method.The effects of placement mode of two catalytic materials,the ratio of biomass and plastic and reaction atmosphere on the formation of BTEX were studied.The results show that the closer and more uniform the mixing is,the more conducive to the formation of BTEX（up to 80.00%）,indicating that the reduction of the distance between the metal active site and the acidic site of molecular sieve plays a positive role in the aromatization reaction;Compared with biomass pyrolysis alone,the addition of plastic can increase the yield of BTEX by 5 times;Only under the condition of mixed catalysis of iron tailings and molecular sieve grinding,the introduction of CO₂atmosphere can increase the yield of BTEX,which may be due to the need for adjacent metal sites and acid sites in the process of CO₂activation,promoting hydrogen transfer and strengthening aromatization.In addition,the catalyst before and after the reaction was characterized by X-ray diffraction.It was found that due to the reduction gases such as H₂and CO produced in the pyrolysis process,Fe₂O₃of iron tailings was reduced to Fe₃O₄,which was conducive to the subsequent magnetic separation to produce iron concentrate.