# A Critical Review of SCWG in the Context of Available Gasification Technologies for Plastic Waste
## 2 Gasification Process  
### Fundamentals of Gasification  
Gasification is a well-known process since the mid-1800s. In this period, a gas was produced from coal to illuminate the streets of important cities such as London. During the second world war gasification played an important role as it was used to produce syngas, which, by the Fischer-Tropsch process, was converted into synthetic gasolines [24]. It was due to lower fossil fuel prices that wood gasification has been discharged. However, coal gasification is well known and is still used around the world. In recent years, gasification has received particular attention as a thermochemical recycling technique. It is possible to gasify a considerable variety of biomass and waste-derived feedstock, such as wood, plastic, municipal solid waste (MSW), sewage sludge, crop residues, agricultural, and industrial waste [24]. Regarding plastic waste, one of the greatest advantages is linked to the possibility of being able to treat heterogeneous and contaminated polymers with limited use of pre-treatment [25; 26; 27]. The gasification of plastic waste has numerous environmental advantages over incineration and other technologies. In the incineration process dangerous substances can be released in the environment such as heavy metals (Hg or Cd), dioxins, furans, HCl, SO\({}_{2}\), NO\({}_{\mathrm{x}}\), HF [28; 29; 30]. For example, polyvinyl chloride and halogenated additives, such as flame retardants, are typically found in mixed plastic waste. This can lead to the risky release of dioxins, other polychlorinated biphenyls and furans [31]. On the contrary, gasification plants, using a sub-stoichiometric amount of oxidant and operating at a lower temperature then combustion produce a smaller quantity of the pollutants mentioned above, i.e., NO\({}_{\mathrm{x}}\)[32]. A comparison between the combustion and gasification products is shown in Table 1[32]. Gasification of waste is also preferred over incineration because it provides a syngas product that can be used for different purposes as opposed to combustion gases [33]. Syngas is defined as a gas mixture of mainly H\({}_{2}\) and CO, produced from gasification of carbonaceous feedstock in oxygen and steam, followed by gas separation to remove CO\({}_{2}\). Instead, the term \"producer gas\" refers to a gas mixture with a low heating value, composed of H\({}_{2}\), CO, CO\({}_{2}\), CH\({}_{4}\), other hydrocarbons with a low molecular weight, and N\({}_{2}\). In this case the gasifying agent used is air [34]. Commonly, the term syngas is used to describe the gaseous product from any type of gasification process, even if this is not perfectly correct. This convention will be herein retained. As described by [PERSON] et al. [35], about 6 EJ of syngas are produced per year. As in the past, the main source of syngas production is coal. This is attributed to the high availability of coal reserves in countries such as China, United States, and Russia. Markets such as China and India are mainly engaged in the production of syngas and coal based derivatives [36]. However, there are already some commercial companies such as Enerkem and Sierra Energy that use waste for syngas production. The end uses of syngas are shown in Figure 2. \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & **Carbon** & **Hydrogen** & **Nitrogen** & **Sulfur** \\ \hline Combustion & CO\({}_{2}\) & H\({}_{2}\)O & NO, NO\({}_{2}\) & SO\({}_{2}\) or SO\({}_{3}\) \\ \hline Gasification & CO & H\({}_{2}\) & HCN, NH\({}_{3}\) or N\({}_{2}\) & H\({}_{2}\)S or COS \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison between the combustion and gasification products [32]. Syngas can be used for the generation of thermal energy by direct combustion of H\({}_{2}\) and CO in heat generation equipment, such as steam boilers, cement kilns, dryers, etc., and for the electricity production.