# A Critical Review of SCWG in the Context of Available Gasification Technologies for Plastic Waste
## 2 Gasification Process
### Gasification Mechanism  
As defined by [PERSON] et al. [42], gasification is a thermochemical process in which a solid or liquid carbon-based feedstock is converted into a syngas, working at high temperatures (600-1000 \({}^{\circ}\)C) and in the presence of a sub-stoichiometric amount of oxidizing agent (also called gasifying agent). Heat can be supplied both directly and indirectly to the gasifier. As previously said, syngas is suitable for use in electricity production or for the manufacture of chemicals, hydrogen, or transportation fuels [43]. The syngas is mainly composed of H\({}_{2}\), CO, CO\({}_{2}\), N\({}_{2}\), and some low molecular weight hydrocarbons (CH\({}_{4}\), C\({}_{2}\)H\({}_{4}\), C\({}_{2}\)H\({}_{6}\), etc.). Very small amounts of H\({}_{2}\)S, NH\({}_{3}\), tar, and char may also be produced [44]. Tar and char are unwanted products [45; 46]. Gasification occurs in several stages, involving numerous chemical reactions but it can be explained by summarizing it in four steps: Drying of feedstock, pyrolysis, oxidation, and reduction reactions [47]. These steps are summarized below:  
* Drying: Feedstock with variable moisture content are dried in a drying process in a range of temperatures between 100-150 \({}^{\circ}\)C. In this step no chemical reactions take place and the heat  
Figure 2: End uses of syngas, totally \(\sim\)6 EJ/y [35]. supplied is spent in the phase change between liquid water and water steam [33]. Unlike biomass or coal, plastics have a very low moisture content. Since it is external humidity, drying is a very rapid process, not being subject to diffusion processes [20]. * Pyrolysis: During this process complex chemical reactions of endothermic nature take place. Volatile substances and a solid or carbonized residue are formed. The proportions of these products are influenced by the process conditions such as the heating rate (\({}^{\circ}\)C/s) and the temperature. Furthermore, products distribution is affected by feedstock composition and size [33]. Pyrolysis is a \"delicate\" step, as the melted plastic particles tend to stick together. This causes the formation of agglomerates and defluidization in fluidized bed reactors [48]. An interesting aspect, is that some polymers such as PP, PE, and PS can be almost totally converted into volatile substances when fast pyrolysis is conducted [19; 49]. This usually occurs in common gasification technologies. * Oxidation: At elevated temperatures and in the presence of sub-stoichiometric oxygen, heterogeneous reactions occur between oxidant and raw material, forming carbon monoxide and water steam as shown in Table 2. Oxidation depends on the chemical composition of the feedstock, the nature of oxidant used, and the operating conditions. This step is mainly exothermic and the thermal energy released provides the heat needed for the process [47]. * Reduction: It is an endothermic step in which high-temperature chemical reactions take place in the absence of oxygen [47]. These reactions are shown in Table 2. Steam promotes two reactions: The steam reforming of char and tar (endothermic) and the water-gas shift reaction (exothermic) [50]. Both reactions lead to the formation of H\({}_{2}\). In particular, the reduction of water in steam gasification is the most effective way of increasing H\({}_{2}\) production [51]. CO\({}_{2}\) reacts with char to produce CO. This reaction is known as the Boudouard reaction and it is endothermic in nature [52]. CO\({}_{2}\) may also be recirculated with O\({}_{2}\) within oxy-fuel combustion/gasification [53]. In the case of plastics, it is important to remember that each polymer follows a different degradation path, which determines the composition of the volatile substances that are formed [20; 54]. As explained by [PERSON] et al. [20], for polyolefin the main thermal degradation mechanism is random scission. Random degradation occurs at any random point along the polymer chain [15; 55] and leads to a wide product distribution [56].