# A Critical Review of SCWG in the Context of Available Gasification Technologies for Plastic Waste
## 6 Gasification of Plastic Waste: An Overview
### Supercritical Water Gasification (SCWG)  
The first work related to the gasification of plastic material in supercritical water is that of [PERSON] et al. [140]. In 2004, they studied the production of H\({}_{2}\) from cross-linked polyethylene (XPE), using high pressure superheated steam and supercritical water (500-700 \({}^{\circ}\)C and 30 MPa). The optimal operating conditions identified in the study were: 700 \({}^{\circ}\)C, 30 min, 20 molar ratio of water to carbon in XLPE, and 20% by weight of nickel catalyst. Gas production is improved by increasing the temperature and molar ratio of water to carbon in XLPE. In 2016, [PERSON] and [PERSON] [141] studied SCWG of virgin polyolefin plastics (LDPE, HDPE, PP, PS) in a batch reactor at 450 \({}^{\circ}\)C, with a reaction time of 60 min and in a range of pressures between 10 and 38 MPa. This process involves the use of RuO\({}_{2}\) as a catalyst and it aims to identify a possible strategy to eliminate pollutants from ocean waters. The key reactions in the process are steam reforming (which implies the reduction of RuO\({}_{2}\) to Ru metal) and water gas shift reaction. The resulting CO\({}_{2}\) and H\({}_{2}\) reaction was catalyzed by metallic Ru to produce CH\({}_{4}\). This work shows how SCWG in the presence of a catalyst is an effective technique for solving the hydrocarbon pollution of water. [PERSON] and his research group published the first paper about SCWG in 2018 [142]. In this paper, they studied SCWG of high impact polystyrene (HIPS) at a reaction temperature in the range of 500-800 \({}^{\circ}\)C, a reaction time of 1-60 min, a feed concentration of 2-10 wt%, and a reaction pressure of 22-25 MPa. The increase of some parameters such as temperature and reaction time and the reduction of others such as the concentration of feedstock, increase the gasification efficiency of the plastic. The optimal conditions identified in the study are: A temperature of 800 \({}^{\circ}\)C, a reaction time of 60 min, a reaction pressure of 23 MPa, and a feedstock concentration of 3% by weight. In these conditions, the carbon conversion rate of the plastic reaches 94.48% by weight. In 2018, [PERSON] et al. [143] reported about SCWG of Acrylonitrile Butadiene Styrene (ABS) plastics for fuel gas production. The experiments were firstly conducted from 450 to 700 \({}^{\circ}\)C, at 23 MPa. They showed that an increase of reaction time, temperature, and material ratio (water/ABS) could significantly promote the gasification reaction, and the whole reaction process was obviously divided into three stages: The gasification efficiency rapidly increased firstly, maintaining nearly unchanged then, and restarted to grow. Finally, they investigated the subcritical water hydrolysis for oil products recovery from 375 to 450 \({}^{\circ}\)C at 21 MPa and the results showed that most of the monomers were converted into more stable substances at a long residence time. The optimal reaction condition for monomer recovery was determined to be 400 \({}^{\circ}\)C and 3 min through the experimental results. [PERSON], [PERSON] and [PERSON] [144] in 2019 investigated gasification of virgin PP in supercritical water using a quartz tube reactor. The results showed that the optimal conditions for gasification were 750 \({}^{\circ}\)C and 60 min, and the effect of temperature on gasification was more important than time. Moreover, they tried to use artificial seawater for gasification instead of ultrapure water and they saw that metal salts had a positive impact on the gasification of PP plastics, so this process was an effective way to solve marine microplastics pollution. In order to solve the ecological and environmental problems caused by microplastics, [PERSON] has conducted further researches on this problem, which culminated in two articles published in 2019 and 2020. The first one was in regards to ABS microplastics [145], the second PET microplastics [146]. In the first one, he studied SCWG on ABS microplastics using a quartz tube reactor at 1000 \({}^{\circ}\)C and 35 MPa.