End of life packaging is nowadays one of the major environmental problems due to its short usage time, the low biodegradability, and the big volume occupied. In this context, gasification is one of the most promising chemical recycling techniques. Some non-recyclable or non-compostable waste gasification plants are already operating such as Enerkem Alberta Biofuels in Canada or the Sierra’s FastOx Pathfinder in California. In this review, we have examined works about plastic gasification from the last fifteen years with a specific focus on polyolefin (PP, PE), plastics mix, and co-gasification of plastic with biomass. For each of these, the best operating conditions were investigated. A very in-depth section was dedicated to supercritical water gasification (SCWG). The most used reactors in gasification processes are fluidized bed reactors together with air or steam as gasifying agents. Tar removal is commonly performed using olivine, dolomite, or nickel based catalysts. SCWG has numerous advantages including the inhibition of tar and coke formation and can be used to remove microplastics from the marine environment. In co-gasification of plastic material with coal or biomass, synergistic effects are observed between the raw materials, which improve the performance of the process, allowing to obtain higher gas yields and a syngas with a high energy content.

Finding a proper way to manage the enormous amount of waste plastic that is globally produced, is one of the main environmental challenges of our times. Among the different types of chemical recycling, Hydrothermal Liquefaction (HTL) appears as a potential method for the treatment of plastic waste mixes, for sustainable production of biocrude or chemicals with high added value. In this work hydrothermal liquefaction reactions were carried out on a polymeric residue, obtained from an industrial plastic waste collection and recycling process. The residue has a heterogeneous composition consisting not only of polymers but also paper and metals. Two batch experiments were performed in a stainless-steel Parr autoclave at 340 °C, investigating a residence times of 5 hours and the use of an alkaline catalyst (NaOH). The oils obtained from the reactions, as well as the aqueous phases, have been analysed by different analytical techniques such as: FT-IR spectroscopy, GC-MS, GC-FID, IC. The operating conditions used in this work, allowed the degradation of cellulose and polymers with reactive sites for hydrolysis such as PET, nylon and PVAc, while polyolefins (PE, PP) were not attacked. The use of a basic catalyst favoured a greater hydrolysis rate.

The hydrothermal liquefaction reactions (HTL) in subcritical conditions of a lignin residue has been studied on a lab scale. The starting material was a lignin rich residue co-produced by an industrial plant situated in Northern Italy producing lignocellulosic bioethanol. The reactions were carried out in batch mode using stainless steel autoclaves. The experiments were under the following operating conditions: two different temperatures (300–350 °C), the presence of basis catalysts (NaOH, and NH4OH) in different concentrations and the presence/absence of capping agent 2,6-bis-(1,1-dimethylethyl)-4-methylphenol (BHT). Lignin residue and reaction products were characterized by analytical and spectroscopic techniques such as CHN-S, TGA, GC–MS, EPR, and 1H-NMR with (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (T.E.M.P.O.). The addition of BHT did not significantly affect the yield of char which is formed by radical way. Spectroscopic analysis indicated that the level of radicals during the reaction was negligible. Therefore, the results obtained experimentally suggest that the reaction takes place via an ionic route while radical species would play a minor role.

In this study, we investigate the hydrothermal liquefaction (HTL) of PET separated from a densified postconsumer plastic mix, with the aim of recovering its monomer. This second raw material is made up of 90% polyolefin, while the remaining 10% is made up of PET, traces of metals, paper, and glass. After preliminary separation by density in water, two batch experiments were performed on the sunken fraction (composed mainly of PET) in a stainless steel autoclave at 345 °C for 30 and 20 min. Both trials resulted in similar yields of the three phases. In particular, the solid yield is around 76% by weight. After a purification step, this phase was analyzed by UV–Vis, 1H-NMR, and FTIR spectroscopy and resulted to be constituted by terephthalic acid (TPA), a product of considerable industrial interest. The study proved that the hydrothermal liquefaction process coupled with density separation in water is effective for obtaining TPA from a densified postconsumer plastic mix, which can be used for new PET synthesis.

Anaerobic digestion (AD) is a well-known biological conversion process to obtain a gaseous biofuel from organic matter: in fact, upgrading biogas to biomethane is a mean to substitute conventional natural gas. It is also known that biochar can improve the biogas production in AD processes. In this work, different biochars have been produced from various feedstocks at different process conditions. Biochars obtained from the carbonization of wheat straw (WS) and poplar (P) were produced in a Thermo Gravimetric Analyser at lab scale, at a temperature of 400 °C and 2 h of retention time at the maximum temperature, with a heating rate of 20 °C min−1. Another biochar from poplar (Pc) was also produced in a pilot plant (CarbOn, RE-CORD) working in oxidative pyrolysis conditions, at a temperature range between 500 and 600 °C. Biochars were oxidized with Oxone® using two different methods (ball-milling and simple aqueous solution mixing) to increase the amount of functional groups on their surface. Oxidized biochars (Ws_Ox and P_Ox) were characterized by FTIR, BET, and CEC, and their impact on biogas production was investigated through a lab scale biochemical methane potential (BMP) test using maize silage as substrate. 0.33 g of biochar was used for each treatment. BMP test shows that all batches containing biochar as additive produced more biogas than control (C). WS_Ox and P_Ox produced respectively a + 7.7% and + 11.3% of biogas than C, obtaining the higher productivities with respect to not oxidized biochars. The addition of P and Pc biochars were similar performances in AD, thus highlighting that no significant differences are due to different biochar production scales and process parameters from the same feedstock. This study highlights how in addition to the various examined parameters (nature of the feedstock, pyrolysis parameters, size of biochar and its concentration in AD), also the presence of specific functional groups on the biochar surface influences the AD performance.