Study On Pyrolysis Characteristic Of Plastic Wastes In The Rotary Kiln With Solid Heat Carrier

Plastic is the primary material of industrial and agricultural development in human society.Due to easy ageing and damage during the usage,many plastic wastes are produced and accumulated in the environment,causing severe environmental pollution problems.Achieving the green and high-value utilization of plastic waste is an essential part of the national development strategy and has become the critical point of plastic waste pollution prevention and recycling.Pyrolysis technology is a promising method for the treatment and resource utilization of organic solid waste.It can convert plastic waste into high value-added energy products such as pyrolysis oil and gas.It conforms with the current policy of a low-carbon and sustainable development model.However,there are many problems to be solved,such as high energy consumption and complex products in the pyrolysis process,which severely limit the application of pyrolysis technology in the field of plastic wastes recycling.Given the problems above,this thesis puts forward the idea of pyrolysis of plastic waste with solid heat carriers in the rotary kiln.After analyzing the pyrolysis behaviour and product evolution mechanism of plastic wastes,this study focuses on the influence of solid heat carrier loading and catalyst on the pyrolysis characteristics of plastic wastes.It investigates the mechanism of pyrolysis of plastic wastes with solid heat carriers.The main research contents and conclusions are as follows:（1）TG/DSC-MS was used to study the pyrolysis behaviour and kinetics of plastic waste,and a combined Py-GC/MS analysis was used to discuss the mechanism of plastic waste pyrolysis.The results showed that the pyrolysis process of plastic wastes could be divided into melting and volatile release stages,both of which are endothermic processes.The products produced from pyrolysis of polyethylene were mainly linear hydrocarbons,and the increased temperature to 600℃led to increase of<C₁₅ hydrocarbons up to 42.4%.However,the free radicals with the methyl side chain produced from the polypropylene pyrolysis were prone to cyclization reactions.Then the cyclo-aliphatic compounds reached 16.8%when the pyrolysis temperature was 600℃.The C-C bonds in the main chain of the polystyrene molecules were more favourable to be cracked and depolymerized,which led to more pyrolysis oil,and the styrene monomer content can reach 78.9%.The apparent activation energy of the pyrolysis of mixed plastic wastes was lower than that of polyolefins and higher than that of polystyrene.The free radicals formed by various types of plastic wastes performed synergistic reactions,and the aromatic hydrocarbons content in the product can reach 44.2%.In the pyrolysis of plastic wastes,the hydrogen transfer andβ-scission play a decisive role in forming pyrolysis products.（2）The effect of solid heat carrier loadings on the pyrolysis characteristic of plastic wastes was explored.Pyrolysis of plastic wastes was carried out in the self-designed rotary kiln with solid heat carrier.Meanwhile,EDEM simulation was performed to investigate the motion and heat transfer behaviour of solid heat carrier particles in the rotary kiln.Then the mechanism of pyrolysis of plastic wastes with solid heat carrier was revealed.The results showed that the yield of pyrolysis oil produced from plastic wastes was significantly increased via pyrolysis with solid heat carriers.The pyrolysis oil yields of polyethylene,polypropylene,polystyrene,and mixed plastic waste increased from 35.7 wt.%,62.1 wt.%,74.6 wt.%,and 51.9 wt.%to43.9 wt.%,76.3 wt.%,89.2 wt.%,and 58.3 wt.%,respectively,when the filling ratio of solid heat carrier increased from 0%to 15%.The optimum solid heat carrier filling ratio for pyrolysis oil production was 15%.The EDEM simulation results suggested that loading more solid heat carrier in the rotary kiln can increase the thickness of the active layer of the bed,and speed up the translational and rotational speed of the solid heat carrier particles,and then improve the heat transfer efficiency in the rotary kiln.The heating rate of plastic wastes in the rotary kiln can reach more than 200℃/s.A higher heating rate can speed up the volatilization in the pyrolysis process,rapidly increase the pressure difference between the interior and outlet of the kiln,reduce the secondary cracking of volatiles,and improve the yield of pyrolysis oil.（3）In view of the complexity of the pyrolysis oil produced from plastic wastes via single pyrolysis technology,the idea of in-situ catalytic pyrolysis with Ni/ZSM-5 catalyst coupled with solid heat carrier was proposed.The Ni loading ratio and catalyst to plastic wastes ratio could affect the BTX aromatic compounds in the pyrolysis oil,and the hydrogen content in the pyrolysis gas was investigated.When the filling ratio of solid heat carrier in the rotary kiln was15%,the 10Ni/ZSM-5 with the Ni loading ratio was 10 wt.%,and its mixing ratio to plastic wastes was 0.5,the oil yield was 45.1 wt.%and 55.2 wt.%for the pyrolysis of polyethylene and mixed plastic wastes,respectively.Meanwhile,the BTX aromatic in the pyrolysis oil from polyethylene and mixed plastic wastes could reach 71.7%and 57.2%,which were higher than the 23.2%and 17.7%without catalyst.Besides,with the addition of catalyst the hydrogen content in the gas increased to 44.0 vol.%and 35.1 vol.%compared with 6.0 vol.%and 4.9vol.%without catalyst.The ZSM-5 framework remains unchanged after the catalytic pyrolysis reaction.The TG curves of Ni/ZSM-5 under the air atmosphere indicated that the deposited carbon could be burned at 400-550℃.（4）The energy,exergy and self-heating ability of the mixed plastic waste pyrolysis with solid heat carriers under different filling ratios were discussed.The energy and exergy distribution of the in-situ catalytic pyrolysis of plastic wastes were also compared and analyzed.The results showed that the pyrolysis with solid heat carriers could increase the total energy efficiency of the mixed plastic waste pyrolysis to 85.8%,compared to the 80.8%without solid heat carriers.The exergy efficiency of mixed plastic waste pyrolysis with solid heat carrier was94.0-95.8%,and its exergy loss were mainly due to the irreversibility of the heat transfer of solid heat carrier and the irreversibility of the pyrolysis reaction.When the filling ratio of solid heat carrier was 15%,by recovering the energy from burning pyrolysis gas and char in the self-heating pyrolysis,the energy obtained from pyrolysis per 1 kg polyethylene,polypropylene,polystyrene,mixed plastic wastes with solid heat carrier was 41.7 MJ,40.0 MJ,35.7 MJ,and38.2 MJ,respectively.The in-situ catalytic pyrolysis of polyethylene and mixed plastic wastes exhibited better self-pyrolysis ability than the pyrolysis without catalyst.However,due to the presence of carbon deposits on the catalyst,the exergy loss with solid heat carrier was slightly higher than that of the pyrolysis without catalyst,and the total exergy efficiency was 92.7-96.1%and 94.8-95.6%,respectively.In summary,the pyrolysis characteristic of various types of plastic wastes,including polyethylene,polypropylene,polystyrene and mixed plastic wastes in a rotary kiln with solid heat carrier was investigated and the mechanism of pyrolysis plastic wastes with solid heat carrier was revealed.In-situ catalytic pyrolysis of plastic wastes with Ni/ZSM-5 catalyst and solid heat carrier to improve the pyrolysis oil and pyrolysis gas quality was proposed.Besides,the energy,exergy distribution and self-heating ability of pyrolysis of plastic wastes with solid heat carrier and in-situ catalytic pyrolysis of plastic wastes were discussed,which provided the foundation of plastic waste pyrolysis with solid heat carrier in the rotary kiln to achieve low energy consumption,stable operation and high quality production.