Comparison and synergistic effect analysis on supercritical water gasification of waste thermoplastic plastics based on orthogonal experiments

Supercritical water gasification (SCWG) is a clean technology of utilizing waste plastics. In this paper, the SCWG characteristics of waste thermoplastic plastics, including polypropylene (PP), polyethylene (PE), and polyethylene terephthalate (PET), and a mixed plastic, were investigated by using the orthogonal experiments. The effect of reaction temperature, reaction time and mass concentration on gasification characteristics was explored and the optimum conditions was obtained. The results show that the reaction temperature has the most important positive effect on the SCWG of PP, PE and PET. The factors that influence the SCWG of mixed plastic follow the order: mass concentration > reaction temperature > reaction time. SCWG efficiency of the mixed plastic is higher than that of all single plastic due to the synergistic effect of different plastic composition. The optimal conditions of mixed plastic are 800 °C, 25 min and 5 wt% and the maximum GE is 106.8%. The energy and exergy efficiency of SCWG system at 650 °C are 47.2% and 37.3%, respectively. The GWP of 1 kg hydrogen produced by SCWG of PE is 3.57 kgCO2eq. The water recycling can greatly reduce the exergy cost of the SCWG system. This study will promote the industrialization of SCWG technology of plastics.