Hydrochar pellet produced from hydrothermal carbonization of fecal sludge

Hydrochar, a solid product of hydrothermal carbonization of biomass, has been used in many applications, especially on sustainable bio-energy. In this research, the hydrochar produced from fecal sludge was transferred into pellets using a hydraulic press at various mixing ratios of hydrochar, binders (lignin, starch, and calcium hydroxide), and water. The produced hydrochar pellet characteristics such as energy content, pellet density, and compressive strength were determined. The pelletizing process resulted in increased density and compressive strength of the hydrochar pellets, but the energy content was relatively stable. Adding water resulted in decreased density and compressive strength of the hydrochar pellets. The optimum mixing ratio of hydrochar/starch/water for the pelletizing process was suggested to be 85:15:0 by weight. At this optimum condition, energy contents, densities, and compressive strengths of the hydrochar pellets of 20.1–20.5 MJ/kg, 995–1020 kg/m3, and 8.5–9.2 kN/m2, respectively, could be achieved. At complete combustion, particulate matter sizes of 0.4–2.1 μm in diameters were the main PM component, and the emissions of carbon dioxide and sulfur dioxide were estimated to be 176 g and 2 g per 100 g of the hydrochar pellet, respectively.