# A Critical Review of SCWG in the Context of Available Gasification Technologies for Plastic Waste
## 7 Conclusions and Final Remarks  
The gasification of plastic material as a technology has achieved the technical maturity to disrupt the plastic recycling sector, with demo processes and plant (Enerkem, Sierra Energy) currently in operation at the industrial scale. However, it is currently underexploited compared to other forms of recycling such as mechanical recycling or pyrolysis. This technology draws its foundations from the well-known gasification of coal or biomass. As observed from the analysis of literature, gasification is a very versatile process, because starting from different types of feedstock (pure plastic, plastic mixtures, plastic with biomass) are possible to obtain a gaseous product rich in hydrogen. Regarding the gasification of polyolefin, virgin polymers are commonly used as a model feedstock in many of the analyzed papers. According to our experience in the sector, this leads to results that are not representative of reality, as real recycled polymers generally show important forms of cross-contamination, for example, with PET or PVC. Cross-contamination can lead to the formation of unwanted products. Therefore, it would be interesting to conduct studies not on virgin polymers but on polymers coming from recycling companies. Gasification reactions are also conducted on plastic mixes commonly found in waste streams: PP, PE, PS, PVC, PMMA, PET. Synergistic effects can occur between different polymers increasing the gas yield. Gasification in supercritical water is a poorly studied technology, which has generated great interest in recent years thanks to its numerous advantages, for example, the inhibition of tar and coke formation and the fact that the gaseous product is very clean (no NOx and SOx are generated in SCWG) and the CO concentration is very low, especially with the catalyst to enhance the water-gas shift reaction. This technology can be used for microplastic removal as described by the recent works of [PERSON], but also for the recycling of particular types of plastics (strongly cross-linked polymers). The high costs and complexity of the process made it difficult to implement gasification plants with supercritical water on a large scale. Therefore, it will be important to understand which operating parameters can be optimized for greater energy savings.  
Finally, the last type of technology considered in this review is co-gasification of plastic material with coal or biomass. The latter technology represents a promising form of recycling for those industries (for example in the food sector) that have processing waste both in the form of biomass and plastic packaging. Co-gasification exploits the synergistic effects between the different components of the raw material, allowing to obtain higher gas yields, with a composition rich in H2. The synergistic effects also lead to an increase in the energy content of the syngas.  
Conceptualization, L.R.; writing--original draft preparation, B.C.; writing--review and editing, L.R., D.C., M.F., A.M.R.; supervision and correction, L.R. All authors have read and agreed to the published version of the manuscript.  
This research was partially funded by Fondazione Cassa di Risparmio di Firenze (project no. 2018-0930 \"PIRACY\")  
The authors declare no conflict of interest.