# A Critical Review of SCWG in the Context of Available Gasification Technologies for Plastic Waste
## 1 Introduction
Recycling is an important road, as it provides a solution to the environmental and ecological problems associated with the use of fossil energy sources, the CO\({}_{2}\) emissions, and the quantities of waste that require disposal [14]. Figure 1: Distribution of the European (EU28+NO/CH) use sectors of plastic material in 2018 [4]. We can distinguish four different types of recycling:  
* Primary Recycling: Primary recycling, also called re-extrusion, consists of recycling a single type of polymer, with properties close to the virgin material and free of contaminations. This process uses processing wastes, which are regenerated as they are or added to virgin polymer. This process is characterized by simplicity and low cost [15; 16]. * Secondary Recycling (Mechanical Recycling/Waste to materials): Secondary recycling consists in the mechanical transformation of plastic waste, with the aim of obtaining raw materials for the creation of new objects [17]. Secondary recycling can only be done on thermoplastic polymers as they can be remelted and reprocessed. This process consists of a physical method, in which the plastic waste is shredded, washed, and then melted to make the new product by extrusion. The disadvantages of this method are related to the heterogeneity of plastic waste and the deterioration of products' properties occurring cycle after cycle, due to the decrease in the molecular weight of the recycled polymers in comparison with originals. To partially obviate this last problem, the material can be blended with virgin polymer or specific additives [18]. * Tertiary recycling (Waste to energy/Waste to chemicals): This recycling consists of obtaining, by chemical or physical methods, monomers, oligomers, or other compounds from plastic waste. [15]. In chemical recycling, polymers are chemically depolymerized through chemical reactions. The resulting monomers can be used for new polymerization reactions, to reproduce the original polymer or other products [17; 18]. Physical methods, refer to processes that consist of thermal degradation of polymers, that can be carried out in the complete absence of oxygen (pyrolysis) [19] or in the presence of sub-stoichiometric oxygen (gasification) [20]. Pyrolysis products are a gas, an oil, and a char, instead syngas (syngas is an abbreviation for synthesis gas) is the main product from gasification. * Quaternary recycling (Energy recovery): In quaternary recycling, the waste material is treated to recover energy through incineration [15]. With this method a considerable energy is obtained from the polymers (the calorific value of different plastic polymers is comparable to oil and petroleum), but it is not considered a good solution at an ecological level because it leads to the formation of toxic substances for humans and the environment, such as dioxins [18]. Furthermore, with incineration all the energy used to form the object itself is lost. Recycling plastic waste by the energy recovery method is only logical when the recycling of waste is not possible due to other constraints [21]. In recent years, numerous authors have dealt with the recycling of plastic waste. For example, [PERSON] et al. in 2017 [22] reported a detailed description of the current routes for recycling of polymers, via both mechanical and chemical recycling. In the section of chemical recycling they described not only gasification but also other techniques such as pyrolysis, chemolysis, and fluid catalytic cracking. [PERSON] et al. in a 2018 review [20] analyzed the state of art of the main waste plastic conversion technologies for syngas production. Moreover, the review assessed the influence of operating conditions such as temperature, excess ratio (ER), composition of the power supply, on the conversion efficiency. In a more recent work, [PERSON] et al. [23] examined the chemistry of subcritical and supercritical water and the possibility of generating syngas through the hydrothermal decomposition of different raw materials, not only polymeric wastes but also biogenic and organic wastes.