Study On Coliquefaction Of Coal With Waste Plastics And Its Hydrogen Transfer Pathway Using Radioisotope Tracer Technique

Due to environmental concern and effective utilization of natural resources, the development and background of coprocessing technology of coal and organic waste materials have been reviewed in this paper. In particular the research status of coliquefaction of coal with waste plastics into clean liquid fuels and useful chemical products is discussed in details. In consideration of current condition in China, the multi-significance of developing coliquefaction technology of coal with waste plastics in China have been put forward, which is reducing cost of coal processing and utilization, and reuse of these "white pollutants" as natural resources to improve the people's living environment.First of all, the copyrolysis of coal with waste plastics in hydrogen or nitrogen atmosphere has been studied by non-isothermal TG technique respectively. The results indicate that synergistic effect in copyrolysis of coal with different plastics in nitrogen atmosphere decreases with the characteristic temperature (Tm) value corresponding to the maximum rate of mass loss of different plastics increasing. The effect of addition of different plastics on increasing (dx/dt)^ value for Xianfeng coal is in the following order: PS >PP >LDPE >HDPE, which is opposite order for Tn, value of different plastics. The copyrolysis of Xianfeng coal with HDPE plastic in hydrogen atmosphere presents similar behavior in thermal mass loss with common coal hydropyrolysis, and the overall copyrolysis process can be reasonably described by a two-step reaction. It can be found that the former step pyrolysis reaction is controlled by diffusion mechanism and the later step reaction is well described by a second-order chemical reaction. Moreover, the presence of Xianfeng coal during copyrolysis in hydrogen atmosphere can reduce the activation energy (E) value of HDPE thermal decomposition reaction significantly, the interactions or influences between coal and HDPE plastic are found to occur at the second reaction stage of copyrolysis in hydrogen atmosphere.Based on the above results of coprocessing TG experiments, the coliquefaction of coal with waste plastics has been investigated systematically. The effects of liquefaction reaction time, reaction temperature, cold hydrogen pressure, reaction atmosphere, catalyst and solvent characters on coliquefaction of coal with waste plastics are examined in details. The coliquefaction results show that the addition of different plastics to Xianfeng coal or Ningtiaota coal all results in significantincrease in both total conversion and oil yield, and lowers H2 consumption. It is also found that the plastic plays an important part as a hydrogen donor in coal coliquefaction with waste plastics. From comparison of catalytic activities of different catalysts in coliquefaction of coal with waste plastics, it has been shown that FAMo catalyst passing through 250 mesh can increase the total conversion of coliquefaction and promote the conversion of coal and LDPE into asphaltene and preasphaltene obviously with lowering dosage. Ultrafine FAMo and sulfur-promoted iron oxide (Fe2O3+S) catalyst are found to be more effective in improving hydrocracking from asphaltene and preasphaltene to oil. X-ray photoelectron spectrometry (XPS) is used to characterize the surface of FAMo catalyst and finds that there is elemental sulfur existed on the surface of FAMo. After the FAMo is used as catalyst in coliquefaction of coal with LDPE, 35.52% of molybdenum on its surface will convert to sulfide form of molybdenum and 23.76% of sulfur converts to sulfide sulfur to create MoS2 as an active site on the surface of FAMo catalyst. In addition, the comparative studies on different iron-based catalysts in coliquefaction of coal with waste plastics clearly demonstrate that the ferrous sulfate (FeSO47H2O) has very high catalytic activity for the coliquefaction reaction of coal with LDPE plastic before and after oxidation treatments. Combined the XPS analysis results on spent iron-based catalysts, the traditional explanation on catalytic mechanism of iron-based catalysts is challenged, so a novel viewpoint on catalysis active site of iron-based catalysts has been put forward that the active site in working state of sulfur-promoted iron oxide (Fe2O3+S) catalyst is surface sulfate species Fe2O3[SO42- ] formed, rather than Fe^S in the coliquefaction of coal with LDPE plastic. The catalytic mechanism of iron-based catalysts in coliquefaction reaction of coal with waste plastics is explained using superacids theory. This new viewpoint has been confirmed by our coliquefaction results of coal with waste plastics using a laboratory autoclave and experimental results of Japanese researcher, Kotanigawa et al.To elucidate the hydrogen transfer pathway in complex hydrogen-donating reaction system of coal coliquefaction with waste plastics, for the first time in the world, radioisotope tracer technique using 3H labeled tetrahydronaphthalene (THN) and 3H labeled polyethylene (PE) is used for investigating the behaviors of solvent and hydrogen-rich plastic in coliquefaction course respectively. The tracer experimental results verify that THN solvent is able to transfer an amount of hydrogen at the initial coliquefaction reaction stage of Xianfeng coal with LDPE plastic, which provides a forceful evidence for answering the uncertain question "hydrogen-donating solvent can transfer hydrogen to coal liquefying products in coal catalytic liquefaction course or not?". Besides, it has been first proved that the hydrogen transfer from free radical groups containing hydrogen of LDPE to the primary coalliquefaction products occurs in coliquefaction reaction of Xianfeng coal with LDPE. The experimental phenomena indicate that the free radical groups containing hydrogen of LDPE transfer to coal liquids without going through the transmittance of hydrogen-donating solvents. These investigation results have clearly elucidated the related hydrogen transfer pathway in coliquefaction reaction of coal with waste plastics, and verified that the plastic does play a role of hydrogen donor in coliquefaction of coal with hydrogen-rich plastics, thereby reducing H2 consumption in coliquefaction process.Based on comprehensive analyses on reaction mechanism of coal coliquefaction with waste plastics, a five parameter kinetic model for coliquefaction reaction of coal with LDPE plastic is proposed first, which has not found to be reported by literatures. This model is employed to determine kinetic rate constants of each step reactions involved in coliquefaction of coal with LDPE, the effect of LDPE feed ratio, catalyst and liquefaction temperature on kinetic parameters are also studied. Moreover this model can predict the experimental values of product yields for coal coliquefaction with LDPE as a function of different reaction temperature and time. The synergistic effect between coal and plastics obtained during coliquefaction can be explained from the kinetic studies on coliquefaction of coal with LDPE plastics: the apparent reaction activation energy value of converting further asphaltene and preasphaltene to oil is reduced largely and its kinetic rate constant is raised because of plastic loading, thus promoting oil formation.In summary, the reaction mechanism, hydrogen transfer pathway and reaction conditions of coliquefaction of coal with waste plastics are comprehensively investigated in this paper. From these research works, while a contribution is made to the fundamental theory of coal coliquefaction with organic wastes, these waste materials can realize to be reused as natural resources. All of these indicate that the works in this paper are of great theoretical and practical meanings.