A two-step process for energy-efficient conversion of food waste via supercritical water gasification: Process design, products analysis, and electricity evaluation

The huge amount of food waste (FW), containing high organic matter content and moisture, is difficult to be well treated. Supercritical water gasification (SCWG) can efficiently convert FW to H₂-rich syngas. However, it requires high energy input due to the high temperature and high pressure. This study provided an innovative “two-steps heating process” for the SCWG of FW, which firstly utilized hydrothermal (HT) pretreatment to shorter time of SCWG. The effects of different HT temperature (200 °C, 250 °C, 300 °C, 30 min) to SCWG temperature (480 °C, 30 min) and the different residence time (20 min HT - 40 min SCWG, 30 min HT - 30 min SCWG, and 40 min HT - 20 min SCWG) on total syngas yield, carbon conversion efficiency (CE), cold gas efficiency (CGE), and hydrogen conversion efficiency (HE) were studied. Moreover, the energy input by means of electricity consumption in each experiment was measured to determine the energy saving rate. The optimal condition (200 °C, 20 min HT - 40 min SCWG), obtaining the gas yield (17.22 mol/kg), CE (20.10%), CGE (22.13%), and HE (41.54%), was higher than the gas yield (16.53 mol/kg), CE (19.98%), CGE (20%), and HE (38.08%) of directly SCWG (60 min, 0 °C–480 °C). Moreover, the TOC of derived liquid and the pyrolysis characteristics of solid residues were analyzed. Additionally, it was also observed the HT pretreatment helped to reduce the electricity consumption. The highest energy saving rate was 15.58%.