| Waste tires are high molecular thermosetting polymers,which mainly consist of carbon and hydrogen,as well as a small amount of oxygen,nitrogen,sulfur and other elements.The molecular structures of waste tires are stable and difficult to degrade naturally,for which they have strong heat resistance,anti-biological resistance and mechanical resistance.Waste tires are typical bulk industrial wastes in China.With the rapid development of the automobile industry,the "black pollution" caused by waste tires and other rubber products is showing an increasingly serious trend,which needs to be resolved urgently.Rubber and carbon black are main components of waste tires,which account for about 50 wt.% and 35 wt.% of the tire,respectively,for which waste tires have great potential for resource utilization.Compared with other methods,pyrolysis technology is considered to be one of the best ways to deal with waste tires due to its advantages of high resource utilization,low environmental pollution,and the ability to digest waste tires in large quantities,which conforms to the concept of sustainable development.However,there are still many problems in the pyrolysis technology of waste tires.In terms of basic research,the pyrolysis reaction mechanism of waste tires is still unclear,the secondary reactions during the pyrolysis process are serious,and the composition of pyrolysis products is complex,which cannot be directly used.In terms of pyrolysis devices,currently commonly used industrial-scale devices have disadvantages such as poor heat and mass transfer,low thermal efficiency,and inability to maintain continuous and stable operation for a long time.Therefore,with the goal of realizing efficient resource utilization of waste tires,this paper studied in detail the distribution of waste tire pyrolysis products at different temperature stages,the pyrolysis conversion behavior of pyrolysis volatiles and the secondary reaction phenomenon,and summarized the pyrolysis mechanism of waste tires.A new idea of step pyrolysis was proposed,and a set of step pyrolysis pilot plant suitable for efficient recycling and energy utilization of waste tire has been developed.Research on high-value utilization of pyrolysis oil based on catalytic pyrolysis to prepare BTEX and high-value utilization of pyrolysis char based on catalytic pyrolysis to prepare hydrogen-rich gas were carried out to further improve the economics of pyrolysis technology.The results of this paper provided theoretical guidance and technical support for the efficient treatment and disposal of waste tires and the industrial application.In this study,the transformation behavior and the producing characteristics of pyrolysis volatiles from waste tires were first studied.In this study,the pyrolysis process of waste tires was divided into five stages at intervals of 100 ℃,namely below 200 ℃,200～300 ℃,300～400 ℃,400～500 ℃,and 500～800 ℃.The results showed that the pyrolysis reaction mainly occurred in the third stage(300～400 ℃),and the products were mainly hydrocarbons.The volatile hydrocarbons were mainly composed of three types of alkenes,namely sesquiterpenes,D-limonene and isoprene.In this study,it was found for the first time that sesquiterpenes were the main pyrolysis products of waste tires,accounting for 41.80%.The heteroatom products were all generated below 500 ℃.According to the composition and distribution of the pyrolysis products,the detailed migration and transformation behavior of volatiles during the pyrolysis process were summarized.The sulfur contents in char,oil and gas were 60.98 wt.%,29.52 wt.% and 7.64 wt.%,respectively.Sulfur in pyrolysis char mainly existed in the form of sulfide and thiophene,whereas thiophene and sulfite were the main forms of sulfur found in pyrolysis oil.Due to the existence of secondary reactions in the pyrolysis process,the distribution of in-situ pyrolysis products was quite different from that of ex-situ products.The residence time of the gas products in the high temperature zone and the temperature in the high temperature zone were the most important parameters affecting the secondary reactions.The influence of temperature on the secondary reactions was more profound than that of residence time,but the influence of residence time was more general,more obvious and cannot be ignored in the actual experiment process.With the prolongation of residence time and the increase of temperature,the content of aromatic compounds tended to increase,and the aromatic hydrocarbons were mainly converted from alkanes and alkenes.Under the condition of in-situ pyrolysis without any secondary reactions,the content of limonene in the pyrolysis product could reach up to 46.24%,while the content of BTEX reached the highest(50.55%)when the secondary reaction condition was 700 ℃/60 s.Therefore,the yield of BTEX in pyrolysis products could be significantly increased by simply prolonging the residence time of volatiles in the high-temperature zone or increasing the temperature in the high-temperature zone.Conversely,by minimizing the residence time of volatiles or reducing the temperature in the high-temperature zone could enrich limonene.Based on the above results,combined with the characteristics of waste tire pyrolysis products,and considering the overall process economy,a new idea of step pyrolysis was proposed and the research and development of waste tire pyrolysis pilot plant was carried out.A set of independently designed pyrolysis pilot plant suitable for efficient energy resource utilization of waste tires was developed.The quality of pyrolysis products was effectively improved through the idea of "Step pyrolysis-Two-stage condensation-Independent energy supply".Among the pyrolysis products obtained through the step pyrolysis pilot plant,the light transmittance of the toluene extract of the obtained pyrolysis carbon black reached 100%,the flash point of the pyrolysis oil reached 76.5 ℃,and the content of high value-added aromatics in pyrolysis oil reached more than 60%.All products met the requirements of safe and efficient use.In order to further improve the economics of the pyrolysis process,research on the high-value utilization of pyrolysis oil based on catalytic pyrolysis to prepare BTEX was carried out.In single-material catalytic pyrolysis,the tire derived char was used as precursor of the catalyst.A series of carbon-based catalysts loaded with zinc were prepared by impregnation method,and the effects of zinc content,catalytic temperature and catalyst/raw material ratio on the yield of BTEX were studied.Since tire derived char itself contained a large amount of zinc,it had excellent catalytic activity without additional zinc.By increasing the content of zinc,the selectivity to BTEX could be further improved.The increase of catalytic temperature and catalyst amount was beneficial to the formation of BTEX,but it would also lead to the loss of pyrolysis oil and the formation of undesired polycyclic aromatic hydrocarbons.The results showed that under the optimal experimental condition of catalysis temperature of 600 ℃ and ratio of catalyst to raw material of 20,the BTEX content in the product could reach up to 54.70%,and the yield of BTEX under this condition was 10.13 wt.%,which was 5.95 times higher than that under non-catalytic condition.In the catalytic pyrolysis of mixed materials,waste tires and polyacrylonitrile were used as raw materials,and metal-supported HZSM-5 zeolites were used as catalysts.The purpose of this study was to prepare BTEX enriched pyrolysis oil through enhanced Diels-Alder reaction.The effects of catalytic mode,catalyst type and mass ratio of feedstocks were investigated.The primary pyrolysis products of waste tires and polyacrylonitrile were rich in isoprene and dienophiles,respectively,which indicated that it was feasible to promote the Diels-Alder reaction through the strategy of catalytic co-pyrolysis of waste tires and polyacrylonitrile.The highest BTEX yield of22.01 wt.% was obtained under the conditions of ex-situ catalysis,Zn/HZSM-5 as the catalyst,and the ratio of waste tires to polyacrylonitrile at 1:3.The BTEX yield obtained in this study was much higher than that in previous literatures.Finally,research on the high-value utilization of pyrolytic char based on catalytic pyrolysis to prepare hydrogen-rich gas was carried out.A multifunctional carbonbased material based on tire derived char was developed.Tire derived char can be used as microwave-absorbing medium as well as catalyst at the same time to catalyze the pyrolysis of waste tires to obtain hydrogen-rich gas.The effects of the catalyst type,microwave power and catalyst dosage on the hydrogen production rate were studied.When the microwave power was 1000 W and the ratio of catalyst to raw material was 3:1,the highest hydrogen yield and hydrogen conversion reached 27.81mmol/g and 85.57%,respectively.Too low microwave power cannot achieve complete pyrolysis,while excessive microwave power can only promote the yield of target product to a limited extent.The presence of tire derived char could effectively decompose tar into gas,which further increased the hydrogen yield.The pyrolytic char generated by microwave pyrolysis could be used again as a microwaveabsorbing medium and catalyst,achieving recycling utilization of the products. |
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