The two-stage effect of hydrothermal treatment on hydrogen-rich syngas production during different sludge gasification stages: The properties of sludge and their interaction with the product composition

Hydrothermal treatment (HT) coupled with gasification has emerged as an energy-saving technology to convert sewage sludge (SS) into hydrogen-rich syngas. In this study, the carbonaceous structure evolution of hydrothermal treated sludge (HT-sludge) with a temperature ranging from 120 °C to 240 °C was examined, and it was seen to significantly affect the gasification performance of sludge at different gasification stages (devolatilization, volatile steam reforming, and volatile catalytic reforming). The results showed that the maximum hydrogen-rich syngas yield of 306.05 mL/g db (dry basis) could be obtained at an optimal HT temperature of 160 °C. The effect of HT on sludge subsequent gasification showed a two-stage reaction including solubilization and hydrolyzation stage (120–160 °C) and carbonization stage (200–240 °C). When hydrothermal sludge was pretreated at the solubilization and hydrolyzation stage (120–160 °C), plenty of polarity and saturation organic matter in HT-sludge was generated by the hydrolysis reaction, which enhanced the yields of CH4, aliphatic and mano-aromatic during the devolatilization stage, and the subsequence in the improvement of H2 yield by volatile steam reforming. However, with the HT temperature increasing to the carbonization stage (200–240 °C), the AlPO4 content in biochar accelerated ploy-aromatics cyclization and H2 production during HT-sludge gasification.