Plastic waste valorization through pyrolysis and in line sorption enhanced steam reforming with CO2 utilization: Thermodynamic analysis

Plastic waste is a major contributor to pollution, as the generation rate is increasing around the world and it mainly ends up in landfills, in the environment or is incinerated. Thus, the development of efficient valorization routes is essential. In this study, the thermochemical conversion of HDPE, PP, PS and PET to H 2 through pyrolysis and in line sorption enhanced steam reforming (SESR) was evaluated based on thermodynamic equilibrium calculations. The steam to plastic (S/P) ratio (0.5–4) and the reforming temperature (400–800 °C) were evaluated. The optimum temperature range was between 400 and 600 °C for all plastics and the optimum S/P ratio range was between the stoichiometric value (2.57 for HDPE and PP, 2.77 for PS and 0.99 for PET) and 4. Under these conditions, 0.43 kg H2 kg −1 plastic for the HDPE and PP, 0.38 kg H2 kg −1 plastic for the PS and 0.15 kg H2 kg −1 plastic for the PET were obtained. In these calculations, only the pyrolysis and SESR steps were considered because they had the highest impact on the performance of the different plastics. However, the sorbent regeneration is also a critical step in the overall process. Therefore, the process was then further developed for the HDPE (reference plastic) by including a step of the sorbent regeneration with CO 2 utilization. Two strategies were evaluated. One based on feeding H 2 and another one based on feeding pyrolysis volatiles and steam to the regeneration reactor in order to convert the CO 2 released to CO and produce syngas with a suitable H 2 /CO molar ratio. They were studied by setting the pyrolysis and SESR temperatures to 500 °C and 600 °C, respectively, and the S/P ratio to 3. Under these conditions, the production of pure H 2 was 0.17 kg H2 kg −1 HDPE in the first case and 0.26 kg H2 kg −1 HDPE in the second one. When H 2 is fed (61 % of the H 2 produced) into the regeneration reactor (regeneration temperature of 750 °C), it was possible to convert 60 % of the CO 2 released from the sorbent and generate 1.36 kg syngas kg −1 HDPE (H 2 /CO molar ratio of 2), whereas when a stream of pyrolysis volatiles and steam is in the feed (31 % of the stream of pyrolysis volatiles and steam fed into the SESR step, with regeneration temperature being 800 °C) these values were 39 % and 0.88 kg syngas kg −1 HDPE , respectively. In brief, the pyrolysis and in line SESR with CO 2 utilization showed great potential and flexibility for the conversion of plastics to H 2 and syngas.