# A Critical Review of SCWG in the Context of Available Gasification Technologies for Plastic Waste
## 6 Gasification of Plastic Waste: An Overview
### Polyolefin
#### 6.1.2 Polyethylene  
Arena et al. [84] in 2008 investigated the gasification of recycled polyethylene pellets in a pilot plant bubbling fluidized bed gasifier. The experiments were conducted in various operating conditions: The bed temperature was kept fixed at 850 \({}^{\circ}\)C while the ER was varied. Both sand and olivine were used as the bed material. Air and steam were used as fluidizing agents. The results confirmed that primary methods for tar removal can eliminate or greatly reduce the need for further cleaning downstream of the syngas. The use of olivine leads to a reduction in tar and significantly improves the quality of the gas produced. [PERSON] et al. in 2013 [67] carried out a steam gasification of HDPE chippings in a continuous bench-scale plant in a conical spouted bed reactor. This type of reactor is considered an interesting technology for the continuous steam gasification of plastic waste due to the high level of the heat transfer rates provided by these beds. The effect of temperature and the vapor/plastic mass ratio on the distribution of products (gas and tar) and on their composition, were examined in the study. To reduce the formation of tar, two in situ catalysts were used (olivine and \(\gamma\)-Al\({}_{2}\)O\({}_{3}\)). The study showed that at 900 \({}^{\circ}\)C and with a steam/plastic mass ratio = 2, a syngas rich in hydrogen was obtained together with a low concentration of CO\({}_{2}\), hydrocarbons and polyaromatic tar. This syngas was particularly suitable for the synthesis of hydrocarbons (Fischer-Tropsch) or DME. [PERSON] and [PERSON] in a work of 2013 [85] studied air gasification of PE (obtained by the recycling chain of domestic waste) in a bubbling fluidized bed gasifier, injecting air in three different modes:  
1. At bed bottom without any splitting of the flow rate;
2. At bed bottom and in the splashing zone, that is the volume just above the primary zone, by
3. splitting into two streams the necessary air flow rate;
4. at bed bottom, in the splashing zone and in the freeboard region, by splitting into three
5. streams the air flow rate. The results indicate that airflow staging may be a valid method to prevent or reduce the formation of condensable compounds. In 2015, [PERSON] et al. [91] studied the steam gasification of virgin HDPE in a conical spouted bed reactor followed by steam reforming in a fixed bed reactor in the presence of a Ni commercial catalyst. Gasification was carried out at a temperature of 900 \({}^{\circ}\)C instead steam reforming was carried out in the range of 600-700 \({}^{\circ}\)C. The reforming leads to a full transformation of C\({}^{2+}\) hydrocarbons and tar. Reforming temperature has an important effect on the product yields ob. The better reforming temperature was 700 \({}^{\circ}\)C because:  
* A high H\({}_{2}\) yield was obtained. * The CH\({}_{4}\) conversion was almost full. * A high H\({}_{2}\)/CO ratio was obtained in the product stream. * A low coke yield was obtained. A recent article is that of [PERSON] et al. [137] that in 2018 proposed \"a high energy milling pre-treatment to boost H\({}_{2}\) generation\" during virgin PE gasification. The polymer was co-milled with calcium and nickel hydroxides and then it was subjected to thermal treatment in a glass tube connected to the argon gas source and inserted in a furnace. The experiments were conducted in the range of temperature between 350-700 \({}^{\circ}\)C. Ca(OH)\({}_{2}\) favored the production of hydrogen. It removes the CO\({}_{2}\) generated by cracking PE by trapping it in a solid form (calcium carbonate). The equilibrium of all reactions is therefore shifted towards the formation of H\({}_{2}\). Moreover, high amounts of the hydroxide prevent excessive CH\({}_{4}\) formation, so the gas product is almost pure H\({}_{2}\) (\(\sim\)95%). A summary view of the operating conditions used in the various works is shown in Table 4. As can be seen, the gasification of the PE is carried out mainly using steam as a gasifying agent and both fluidized and fixed bed reactors.