Low-energy thermo-chemical conversion processes of municipal wet waste

Hydrothermal carbonization (HTC) is a low-energy thermochemical process that converts wet biomass into a carbon-rich solid, commonly called hydrochar, for use in a variety of areas, such as soil amendment, biofuels or to produce carbon-based materials. The purpose of this paper is to increase knowledge for the economic valorization of municipal wet waste, considered as a raw material to obtain high value-added products through an HTC process and an additional chemical activation procedure. In the first part of the work, a 4.5-liter batch reactor was designed, built, and used in the HTC experimental campaign by varying the main process parameters, namely reaction time, amount and type of organic waste (e.g. vegetables, fruits, bread, pasta), water concentration, temperature and pressure. In addition, some experiments were conducted by applying the steam explosion technique at the end of the HTC process. The HTC results showed that in biomass with high water content, increasing residence time decreases the hydrochar yield. Considering a dry heterogeneous waste with high carbon content, the yields at the end of the process are much higher. In the second part of this work, the hydrochar samples were treated with a high-temperature activation process based on the use of KOH, obtaining activated carbon. Particularly, the best results were achieved by using high KOH: hydrochar ratios, resulting in high-quality activated carbons with good porosity and a high surface area of 2890 m²/g. Finally, an energy analysis was carried out to evaluate how to make the whole process cost-effective.