# Management of Plastic Wastes through Recent Advanced Pyrolysis Processes
## 1 Introduction  
In recent decades, the exponential growth in plastic production has led to a corresponding surge in plastic waste generation, presenting a significant environmental challenge globally. According to the United Nations (UN) [1], the global average plastic production stands at 430 million tons annually. Figure 1 represents the percentage of plastic and polymer waste generated by the region in 2019. Inspecting Figure 1, it can be seen that the United States of America, Europe, and China, respectively, produce more plastic waste as compared to the other regions of the world [2]. More than half of these plastics are produced for single-use purposes and are subsequently disposed of following their initial use. Plastics, specifically single-use plastics, significantly contribute to land and water pollution, degrade soil fertility, and damage aquatic ecosystems [3, 4]. Moreover, as the global population grows and society standards improve, plastic waste generation also increases, causing worldwide concern. There are hundreds of different types of plastics available on the market. However, there are six common plastics used in many industrial and daily applications. These plastics are low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC) (refer to Figure 2) [5]. Unfortunately, only PET and HDPE are routinely recycled at most recycling centers. [6]. Other plastics, namely, PVC, LDPE, PS, and PP are typically not recycled since they are difficult and expensive to process. Consequently, the extensive use of these plastics, especially the hard-to-recycle ones, caused the accumulation of much plastic waste in landfills (See Figure 3) that could take hundreds of years to completely degrade, hence contributing to significant plastic pollution in the environment [7], and causing the degradation of marine life, forests, and wildlife, and the growth of landfills. Figure 1: Percentage of plastic and polymer waste generated by region in 2019. Data retrieved from the Organization for Economic Co-operation and Development (OECD) [2]. Figure 2: Some examples of the most common types of industrial plastics. Additionally, the escalating concerns over fossil fuel depletion have increased the interest in alternative energy sources and sustainable waste management solutions. Waste-to-energy (WTE) technology has emerged as a promising approach to address both environmental and energy challenges [8]. The most common WTE technologies in the industry are incineration, pyrolysis, hydrogenation, gasification, anaerobic digestion, and mechanical and biological treatment [9]. WTE processes rely on heat energy to decompose the plastic waste to produce gas, oil, and char. These products can then generate electrical energy or other useful products. The general steps of WTE include waste pretreatment, thermal decomposition, product formation, and energy conversion. The primary difference between the WTE processes is the oxygen supply in each process. For instance, incineration requires a complete oxygen supply, while gasification requires partial oxidation, and pyrolysis and anaerobic digestion, and torrefraction processes only need a limited or no oxygen supply [10; 11]. Pyrolysis has gained attention over most of the other WTE technologies because of its potential advantages in mitigating environmental pollution and reducing the carbon footprint associated with plastic products. This is achieved by minimizing emissions of carbon monoxide and carbon dioxide, as compared to combustion and gasification processes [12]. Table 1 presents the proximate and ultimate analyses of the six common plastic wastes mentioned above. The proximate analysis determines plastic composition regarding moisture, ash, fixed carbon, and volatile matter. The volatile content is the major factor in determining the pyrolysis oil yield. The higher the volatile and lower the ash amount in plastic, the higher the yield of pyrolysis oil.