Process energetics for the hydrothermal carbonisation of human faecal wastes

Hydrothermal carbonisation (HTC) has the capability to convert wet biomass such as sewage sludge to a lignite-like renewable solid fuel of high calorific value. However, to date assessment of the energy efficiency of the HTC process has not been fully investigated. In this work, mass and energy balances of semi-continuous HTC of faecal waste conducted at 200 °C and at a reaction time of 30 min are presented. This analysis is based on recovering steam from the process as well as energy from the solid fuel (hydrochar) and methane from digestion of the liquid product. The effect of the feedstock solids content and the quantity of feed on the mass and energy balance were investigated. The heat of reaction was measured at 200 °C for 4 h using wet faecal sludge, and the higher heating value was determined for the hydrochar. The results indicated that preheating the feed to 100 °C using heat recovered from the process would significantly reduce the energy input to the reactor by about 59%, and decreased the heat loss from the reactor by between 50% and 60%. For feedstocks containing 15–25% solids (for all feed rates), after the process is in operation, energy recycled from the flashing off of steam and combustion of the hydrochar and would be sufficient for preheating the feed, operating the reactor and drying the wet hydrochar without the need for any external sources of energy. Alternatively, for a feedstock containing 25% solids for all feed rates, energy recycled from the flashing off of steam and combustion of the methane provides sufficient energy to operate the entire process with an excess energy of about 19–21% which could be used for other purposes.