# A Critical Review of SCWG in the Context of Available Gasification Technologies for Plastic Waste
## 2 Gasification Process
### Variables Affecting the Gasification Process  
[PERSON] et al. [70] studied biomass gasification and identified some variables that influence the gasification rate. Some of these parameters are of fundamental importance also in the gasification of plastic material such as:  
* Size: The smaller the feedstock size, the better would be the heat transfer. The temperature would be uniform resulting in a reaction taking place throughout the particle. In the case of plastic waste due to the great variety of shapes and sizes, shredding is necessary to create a feed material of less than 5 cm in diameter. Some forms of waste plastics such as thin films may require a simple agglomeration step to produce particles of higher bulk density to facilitate feeding [71]. * Temperature: Temperature influences the yield and composition of the syngas [72; 73] and the production of tar and ash. The increase in gasification temperature gives rise to an increase in gas formation and a subsequent decrease in tar and char yields [74]. The greater amount of gas produced at higher temperatures is explained by the larger release of gases during the initial devolatilization stage, and the secondary reactions undergone by the char and tar (char gasification and tar cracking/reforming) [74]. Many studies have shown that the H\({}_{2}\) content in syngas increases as the gasification temperature increases. This phenomenon is due to the chemical breakdown (thermal cracking) of heavy hydrocarbons which favors molar fraction of the permanent gases such as H\({}_{2}\) and CO at high temperature [75]. [PERSON] et al. [76] examined the effects of reaction temperature on biomass gasification in a laboratory scale atmospheric pressure entrained flow reactor. They observed that by increasing the temperature from 1000 to 1350 \({}^{\circ}\)C, the yield of producer gas (defined as the sum of H\({}_{2}\), CO, CO\({}_{2}\), and hydrocarbons up to C\({}_{3}\) species) increased dramatically by 72%. Moreover, a higher temperature was beneficial to decrease the amount of tar. In general, operating temperatures higher than 800 \({}^{\circ}\)C are recommended to minimize tar formation [74]. However, these high temperatures favor the formation of slag from ash agglomeration. Furthermore, higher temperatures are due to a greater oxidation of the feedstock that causes a decrease of the chemical energy of the syngas. * Heating rate: Heating rate is one of the main parameters that affect the decomposition of waste [77]. In particular, it determines the yield and composition of the derived products of pyrolysis step. Pyrolysis involves a process of cracking polymeric structures to convert the feedstock into charcoal and volatile matter [78]. In general, the char yield is expected to increase at lower heating rates. Hence, the slow pyrolysis processes are always preferred for producing charcoal. Higher pyrolysis temperatures, high heating rates, and long residence times lead to the formation of gas products. * Environment and reactor design: Generally, it is observed that the reactive environment (air/oxygen) results in complete gasification of feedstock while inert environment (nitrogen/argon) aids devolatilization (pyrolysis) yielding more char [70]. The equivalence ratio is commonly used to indicate quantitatively whether a fuel oxidizer mixture is rich, lean, or stoichiometric. Therefore, for fuel rich mixtures, the ER \(>\) 1, for fuel lean, ER \(<\) 1, and for stoichiometric mixture, ER = 1 [79; 80]. Commercial gasifiers employ air at a sub stoichiometric quantity to generate producer gas. Plastic material has some particular characteristics given by the low thermal conductivity, the sticky nature when heated, the high volatile content, and the remarkable tar formation. Therefore, a suitable gasifier design for plastic handling must take these aspects into account [20]. Fluidized beds reactors are widely used in the gasification of plastics waste. Plastic gasification studies are often carried out in bubbling reactors [81; 82; 83; 84; 85; 86; 87].