## 2 Thermal Pyrolysis
### Degradation Reactions
Disproportionation and methanization also occur. The main gasification reactions are given in [47; 48]. Gasification is a cycle of primary and secondary reactions between the gasifying agent and fuel (its combustible component). Process conditions, such as reactor temperature distribution and variability during operation (startup and shutdown), affect reaction kinetics. The efficiency of this process and the calorific value of the resulting process gas depend on the type of fuel, the converting agent, its quantity, and the dosing method. It depends on the reactor's technology. The processes for generating the above gas components are fundamental to the production of combustible gas, whose calorific value is determined by CO, H\({}_{2}\), CH\({}_{4}\), and C\({}_{n}\)H\({}_{m}\) (higher hydrocarbons). The fuel gasification process aims to produce high-calorific synthesis gas without complex technology. This gas is used to power gas turbines and make chemicals. This review examines pyrolysis, a thermal waste conversion method. Organic materials are decomposed at 400 to 800 \({}^{\circ}\)C under an inert gas, often nitrogen. The pyrolysis temperature of waste plastics depends on the polymer breakdown temperatures. Waste polymer pyrolysis primarily produces free radicals [48; 49; 50; 51]. The process is similar to burning. Some waste polymers, such as PET [20; 21], and catalytic polymerization use ionic reactions [52]. Figure 2 shows free-radical first steps. Initiation, propagation, and termination are the stages of polymer degradation [53; 54]. Polystyrene shows this technique. There are two ways to begin. First, random chain scission occurs; second, terminal group scission takes place. Both scenarios form a primary and secondary transition state radical. In the next stage, free radicals propagate through hydrogen transfer and unzipping reactions. Figure 1: Schematic overview of the combustion process [47], with permission from Elsevier. Recombination or disproportionation of active radical fragments stops degradation. A graphical representation is shown in Figure 2[54]. Table 3[54; 55; 56] shows that, depending on the PS pyrolysis temperature, a mixture of products with varying quantitative and qualitative contents is formed. The findings of significant research on this topic [57; 58; 59; 60; 61; 62; 63] confirm the mechanisms of PS breakdown by free radical reactions that have been proposed for many years. The polymer degradation mechanism is primarily temperature dependent, and different volatile products are produced at different temperatures. At 300, 350, and 420 \({}^{\circ}\)C, styrene, toluene, and alpha-meta-styrene benzene are produced. Aside from volatiles, the highest diversity of final products is formed at 350 \({}^{\circ}\)C [54]. In the case of the second previously indicated method of polymeric material degradation observed during pyrolysis, degradation proceeds via an ionic mechanism. Degradation is most noticeable in the case of catalytic pyrolysis, although  
Figure 2: Schematic overview of the free-radical reactions (based on [53; 54]). it is also seen in the case of PET pyrolysis. [PERSON]'s research [20,21] showed evidence for the thermal ionic breakdown of PET via beta-CH hydrogen transfer, culminating in the production of an oligomer with a carboxyl group. Subsequent experiments and computational analysis [64] showed that the thermal degradation of PET occurs by cyclization, specifically via an intramolecular (ionic) alcoholizing reaction between 250 and 300 \({}^{\circ}\)C. This reaction (intramolecular exchange) is an ionic process that produces cyclic oligomers and is the primary thermal mechanism occurring in PET below 300 \({}^{\circ}\)C. In contrast, the previously described beta-CH hydrogen transfer reaction happens only as a secondary process (Figure 3).