# A Critical Review of SCWG in the Context of Available Gasification Technologies for Plastic Waste
## 4 Co-Gasification  
Co-gasification involves the concurrent conversion of two or three carbonaceous fuels (one of them of fossil origin, such as coal, fuel-oil, or plastic wastes) into a gas with an useable heating value [104]. This technology looks very promising as it is able to reduce GHG (greenhouse gas) emissions by offsetting some of the fossil carbon with bio-based carbon [105].  
In recent years, the co-gasification of biomass and coal has been much studied [106; 107] because it represents an opportunity to manage large quantities of agricultural and woody fuel waste typical of agricultural or forestry waste. In addition, the cost of adapting an existing coal power plant to co-fire biomass is significantly lower than the cost of building new systems dedicated only to biomass power [105]. The partial replacement of coal with waste and biomass materials can be introducing renewable energy and thus resulting in environmental benefits. On the other hand, coal/lignite can be regarded as being beneficial because the security of supply of biomass and waste materials is uncertain, and the quality of the delivered fuel is subject to only limited control [108].  
The co-gasification of coal with other types of fuels allows reducing CO\({}_{2}\), SO\({}_{2}\), and NO\({}_{x}\) emissions, this happens because during the process the volatiles decompose forming free radicals. The latter react with the organic substances present on the coal, so the conversion rate increases while the emissions of these gases are reduced [105]. Co-gasification of mixtures of biomass and plastic wastes, for instance, has been suggested as a useful strategy to prevent problems normally occurring during the gasification of plastics alone such as feeding difficulties and the formation of contaminants [109]. Furthermore, synergistic effects deriving from the use of mixtures of raw materials are exploited in the co-gasification process [110]. As explained by [PERSON] et al., when synergistic effects occur the experimental values, such as product yield or gas composition, differ from those calculated by linear interpolation based on the results for the pure components in the feedstock [111].  
These effects are also observed in co-pyrolysis processes of plastic and biomass. Biomass devolatilize in a temperature range between 200-400 \({}^{\circ}\)C instead plastics (PS, PE, PP, PET, PVC) in a higher temperature range (300-500 \({}^{\circ}\)C). The different devolatilization of the materials makes possible interactions between the volatiles of the plastic and the char of the biomass. Char acts as a donor of radicals in the initiation of the polymer chain scission (this means that there is a transfer of H from the polymer to the biomass). At the same time, coal is also capable of adsorbing volatiles from polymers [111]. One of the first studies on this topic is that of [PERSON] et al., in 2000, in which they studied the thermal decomposition of polypropylene (PP) in the presence of wood flour, lignin, cellulose, and charcoal [112].  
Figure 4: Graphical scheme of the supercritical water gasification (SCWG) of waste plastics.