Research On Pyrolysis-catalytic Reforming Of Brominated Flame Retardant Plastics

Waste electrical and electronic equipment(WEEE),an emerging and fast-growing waste stream,contains not only large amounts of valuable metals and plastics,but also a great variety of toxic materials.It has been a global issue in terms of seeking a sound management of WEEE.Brominated flame retardant plastics(BFR-plastics)fraction of WEEE is one of potential resources and hazardous contaminants.For the sake of environmental protection and resource utilization,it has great economic and environmental benefits to deal with BFR-plastics in an effective and harmless treatment.Up to now,feedstock recycling by means of pyrolysis has been proposed as one of the promising technologies for WEEE plastics treatment with the aim of converting WEEE plastics into fuels and chemical feedstocks.However,the conventional pyrolysis cannot effectively remove the toxic bromine existed in the oil derived from BFR-plastics pyrolysis,which has posed a serious impediment on the recycling treatment of WEEE plastics.In our study,the different pyrolysis characteristics of BFR-plastics by means of catalytic pyrolysis and pyrolysis-catalytic reforming were investigated,respectively.Furthermore,the product distribution,and the migration and transformation behaviors of bromine during the pyrolysis processes were demonstrated to provide new ideas and principle for the feedstock recycling treatment of WEEE plastics.The key findings of dissertation are as follows:Firstly,two typical kinds of BFR-plastics(including Br-HIPS and Br-ABS)were prepared and then analyzed by a thermo-gravimetric(TG).It was found that the two kinds of BFR-plastics had two similar decomposition stages,including the low temperature decomposition(320-340 ℃)and the high temperature decomposition(410-430 ℃),which corresponded to the BFR additives release and the polymer matrix decomposition,respectively.Meanwhile,the pyrolysis kinetic model of Br-ABS was determined by TG analysis.Three iso-conversional methods,including Flynn-Wall-Ozawa,Kissinger-Akahira-Sunose and Friedman method,were used to calculate the activation energy of pyrolysis reaction of Br-ABS.Coats-Redfern method and the kinetic compensation effect were applied to optimize and determine the kinetic parameters.It was found that the pyrolysis kinetic model of Br-ABS consisted of two consecutive reactions,namely,nucleation and nuclei growth model and three-order reaction model,which could excellently simulate and predict the real Br-ABS pyrolysis process.Secondly,three zeolite materials(HY,HO and HZSM-5)and two mesoporous solids(MCM-41 and γ-Al2O3)were applied to investigate the catalytic effects of solid acid catalysts on the pyrolysis of BFR-plastics.The characterization of catalysts was determined by Brunauer-Emmett-Teller(BET),X-ray diffraction(XRD),scanning electron microscope(SEM)and temperature programmed desorption of NH3(NH3-TPD).The results showed that the catalysts had different textural and acidic properties,which exhibited different catalytic cracking performance.Catalytic pyrolysis of Br-HIPS and Br-ABS was conducted in a fixed-bed reactor to investigate the catalytic effects on the pyrolysis of BFR-plastics over various solid acid catalysts.The product distribution and the migration and transformation behaviors of bromine during the pyrolysis process were also determined.The results indicated that BFR-plastics produced the maximum yield of liquid,including the oil and wax products during pyrolysis without catalyst.The addition of zeolite catalysts resulted in a significant decrease of the oil yield due to their intense cracking performance.Meanwhile,the zeolite catalysts exhibited severe sharp selectivity of the products from catalytic pyrolysis of Br-HIPS and Br-ABS,which can be attributed to their specific properties,such as acidities and textural properties.However,mesoporous catalyst of MCM-41 exhibited well catalytic cracking performance in transforming the BFR-plastics sample into liquid oil.In addition,in the presence of catalysts,the bromine concentration in pyrolysis oils was reduced to different extents.However,BFR additives from different BFR-plastics had great influence on the debromination performance of each catalyst.In respect to the feedstock recycling of the main compounds produced in the oils,the results suggested that the HZSM-5 and MCM-41 catalyst showed the highest feedstock recycling efficiency,which might make the best recycling use of the BFR-plastics for chemical feedstocks.Based on the research results of catalytic pyrolysis of Br-HIPS and Br-ABS,the catalytic pyrolysis mechanisms of BFR-plastics and the essential relationship among the PS matrix,the BFR additives and the catalyst have been discussed in the thesis.Finally,Fe and Ni modified ZSM-5 and MCM-41 catalysts were prepared by wet impregnation method and further characterized by various physicochemical methods to investigate the textural and acidic properties of modified catalysts.The pyrolysis-catalytic reforming experiments of Br-HIPS and Br-ABS were performed with different modified catalysts in a two-stage fixed bed reactor,with the aim of investigating the product distribution and debromination performance in relation to each BFR-plastic.The results showed that different modified catalysts exhibited catalytic reforming performance on the intermediate products derived from BFR-plastics pyrolysis.The modified zeolite catalysts with high porosity and strong acidities were not only in favor of the catalytic performance of intermediate products from BFR-plastics,but also enhanced the organobromine compounds further cracking.However,in addition to their prominent performance on converting the BFR-plastics into liquid oil,the modified mesoporous catalysts exhibited more excellent catalytic reforming and debromination performance,compared to parent MCM-41 catalyst.The results indicated that the active materials of Fe2O3 and NiO loading on the catalysts played a key role on the catalytic performance.In terms of the debromination performance,the Fe2O3 loaded catalyst was in favor of the fixation of inorganic bromine,while NiO loaded catalyst exhibited better cracking performance on organobromine compounds.