Effect of pressure and atmosphere during hydrothermal treatment on the properties of sewage sludge-derived solid fuel

Efforts to improve the performance of hydrothermal treatment (HT) in producing high-quality solid fuel from sewage sludge were carried out by elevating reaction pressure and modifying reaction atmosphere. Reaction pressure was varied by injecting N₂ at the initial pressures of 0.1, 1.3, and 3.1 MPa, where the N₂ was substituted with air or CO₂ to vary the reaction atmosphere. The dewatering performance of the treated sludge, heating value of its derived product, and the efficiency of the treatment process were used as examination criteria. Increasing the initial reaction pressure to 1.3 and 3.1 MPa evidently improved dewatering performance as it produced cakes with 2.4 and 4.4% lower moisture content than the cake produced with normal initial reaction pressure. However, the negative effect on higher heating value (HHV) was also prominent as it produced cakes with the lower HHV of 0.7 and 1.7 MJ/kg. The utilization of CO₂ as reaction atmosphere leads to the production of cakes with the higher HHV of 0.3–0.5 MJ/kg than the cakes produced under N₂ at the same pressure. Moreover, the application of the initial reaction pressure of 1.3 MPa was found as the prime condition since the treatment effectively produced cake with a high effective heating value with a relatively comparable efficiency to the conventional HT at normal initial pressure.