Production Of Aromatic Hydrocarbons By Two-Step Catalytic Co-Pyrolysis Using Biomass And LDPE

Biomass,as the only renewable energy with organic carbon,can partially substitute fossil resources.The pollution of waste plastic has been a global environmental problem.Catalytic co-pyrolysis of biomass and plastic to produce high-quality aromatic-rich oil is a promising technologyHowever,there are some issues in conventional processes,such as weak interaction between the two feedstocks,low yield of target products,and poor variability of operation parameters(such as one-step catalytic co-pyrolysis,OSCCP for short).In this dissertation,a novel two-step catalytic co-pyrolysis(TSCCP)process was proposed to solve the above key scientific and technology problems.Based on pyrolysis characteristic temperatures of the three typical biomass components(cellulose,hemicellulose,lignin)and plastic,the catalytic co-pyrolysis of biomass and plastic was divided into two steps including reactions at low temperature and reactions at high temperature.The TSCCP process could effectively alleviate secondary reactions of pyrolysis products derived from cellulose,hemicellulose and plastic,meanwhile activation of lignin was formed at low temperature,thus realizing the efficient conversion of feedstocks toward target aromatics.Moreover,the yield and selectivity of aromatics were further improved through composite molding of biomass and plastic due to interaction enhancement between them.Walnut shell(WNS)and LDPE were used as model compounds of biomass and plastic.The pyrolysis behavior of feedstocks,the generation characteristic of products,the influence and optimization of process parameters were systematically investigated,and the reaction mechanism was analyzed in depth.The major contents and results are listed as follows(1)Catalytic pyrolysis of biomass components and LDPE was carried out.TG-DTG analyses and Py-GC/MS pyrolysis experiments on biomass components and LDPE reveal that,the system of cellulose,hemicellulose with LDPE and the system of lignin with LDPE had different thermal behaviors and product characteristics.Based on the pyrolysis characteristic of reactants and the products distribution along with temperature,the TSCCP process was established to decompose biomass and plastic at a low temperature(Step 1)firstly and then at a high temperature(Step 2).Pyrolysis products of WNS and LDPE by two-step catalytic pyrolysis(TSCP)and TSCCP were compared and analyzed,thereby the advantages of TSCCP in promoting the production of aromatics and in improving the quality of pyrolytic oil were preliminarily proved.(2)The effects of process parameters on TSCCP were studied by Py-GC/MS using WNS and LDPE.The influences of feedstock ratio,reaction temperature and residence time on products distribution were systematically investigated.The regulation mechanisms of products were analyzed,and the optimal experimental conditions were obtained.Under the optimum conditions,the selectivity of oxygenates in pyrolytic oil was reduced to less than 1%,but the selectivity of aromatics was as high as 82.5%,and the content of high-value BTX components in aromatics was over 60%.Results show that the yield of pyrolytic oil,the yield and selectivity of aromatics were all significantly improved by TSCCP compared with the conventional TSCP and OSCCP.(3)Pretreatment of feedstocks by composite molding was proposed,against the problems in conventional mechanical mixing feeding mode,such as uneven mixing of feedstocks and limitation of heat and mass transfer caused by solid-solid contact model Catalytic co-pyrolysis experiments on feedstocks from composite molding and mechanical mixing were performed respectively using a fixed-bed reactor.Results show that,in comparison,the composite molding pretreatment significantly contributed to the production of pyrolytic-oil and target aromatics,owing to the homogenization of feedstocks,the enhancement of contact,and the improvement of synergistic effect.The process parameters optimization was studied systematically by TSCCP using the prepared composite molding feedstocks further,including experiments on reaction temperature,catalyst dosage and residence time.Results show that,under the optimal conditions using the composite molding feedstocks,compared with the conventional TSCP and OSCCP,TSCCP has increased the yield and selectivity of aromatics significantly,meanwhile has reduced the content of oxygen compounds,thus dramatically improving the quality of pyrolytic oil(4)The reaction mechanism of TSCCP was explored in depth.The generation pathway of aromatics was studied through catalytic pyrolysis of small molecular model compounds.The evolution pathway of feedstocks during TSCCP was investigated macroscopically and microscopically through characterizing of chars produced at different temperatures along the pyrolysis procedure,using TG-DTG,Van Krevelen curves,SEM,Raman,FTIR,etc..Finally,the reaction mechanism for TSCCP was concluded.Results reveal that:a)positive synergistic effect was generated between oxygenates(including furans,alcohols,acids,aldehydes,ketones,and phenols)derived from cellulose,hemicellulose and lignin components in biomass and olefins derived from LDPE;b)since the stepwise pyrolysis process was developed based on pyrolysis characteristic temperatures of reactants,the secondary reactions occurred at high temperature in Step 2 for co-pyrolysis products of cellulose,hemicellulose and LDPE were inhibited;c)lignin activation was occurred at low temperature in Step 1;thus the formation of aromatics was significantly promoted under the combined effects of the above three factors.