This study deals with a detailed numerical investigation of the microwave heating process in plastic pyrolysis. The pyrolysis of high-density polyethylene (HDPE) was studied using a single-mode microwave cavity, TE10 mode, at 2.45 GHz with two different absorbents, as carbon and silicon carbide, and the results were compared. The temperature distribution inside the sample was determined by solving the conservation equations coupled with the microwave and chemical kinetic equations. Lambert's law was applied to describe the electromagnetic field in the microwave cavity. The effective heat capacity method was used to account for the latent heat in the melting range of plastic. The heat of the reaction was taken into account using firstorder kinetic equations assuming a single-step reaction. One-dimensional model equations were solved using the finite difference method utilising MATLAB codes. The model developed in this study provides a better understanding of the fundamental mechanisms of the microwave pyrolysis of HDPE based on a combination of electromagnetic field and thermal models. The primary focus was to incorporate and investigate the effect of the phase changes and reaction during microwave pyrolysis. The results show that the temperature profile strongly depends on the physical properties of the material. Silicon carbide provides more uniform heating distribution compared with carbon.

Microwave heating has been used in the chemical industry for many years for diverse applications due to advantages, such as volumetric heating, high power density, and fast and easier temperature control. The motivation of this work is to develop a methodology to use potential benefits of microwave as a heat source in plastic pyrolysis. A goal was to use the available knowledge to design and build a rotating microwave reactor, to attain homogeneous temperature distribution in anticipation of producing pyrolysis products with uniform molecular hydrocarbon distribution. Also, it was sought to investigate the effect of process parameters on products' yield and composition, and examine their suitability as fuel and useful chemicals. A rotating microwave reactor was designed and fabricated using a coaxial transmission structure without the limitations of the commonly-used enclosed glass quartz reactor, which made the design appealing for any future industrial-sized microwave reactor. The optimum pyrolysis operating condition led to the production of a suitable product for a fuel application. However these products cannot be used directly as phase change materials due to their low latent heat and very broad melting range. Fractionation of the products may also be used as phase change materials, capable of storing/releasing heat at suitable application temperature. The microwave heating provided a more uniform heating distribution, although it did not alter product composition in comparison to the conventional pyrolysis of plastic. In the small microwave reactor used in this project, microwave benefit was not evident with regards to its effect in creating uniform temperature distribution. The benefit may be clearer in an industrial-sized reactor in which heat penetration in conventional thermal reactor can be a serious problem. The pyrolysed products (oil/waxes) were quantitatively analysed using GC/FID. The results of microwave pyrolysis showed 73% oil/wax yield, with silicon carbide used as a microwave absorbent.