Hydrothermal carbonization of waste biomass: progress report and prospects

The hydrothermal carbonization (HTC) process allows the treatment of wet biomass without the need for a drying pretreatment, which is necessary for traditional thermochemical processes (combustion, air gasification, pyrolysis). Foreseeing milder operational conditions (180-250°C and 20-40 bar) with respect to the other treatments, the HTC presents a more affordable and easier technical applicability. The products resulting from HTC treatment are primarily a solid phase enriched in carbon, called hydrochar, a liquid phase with dissolved organic compounds and a small quantity of a gas phase mainly composed of carbon dioxide. Hydrochar can be utilized in co-combustion with low-rank fossil coals, which can be a very effective and economically feasible way to exploit biomass for energy generation. HTC hydrochar can also be utilized in high-value applications, such as soil improver and for carbon sequestration, as adsorbent (i.e., activated carbon), as catalyst or as catalyst support, and for the generation of nano-structured materials. The present paper aims at an overall analysis of the literature published on the HTC, focusing both on the mechanisms that govern the process and on the characteristics of reaction products and by-products, and at evaluating the influence of the reaction operating conditionson the hydrochar produced. Moreover, the main advantages and disadvantages of the hydrochar in the above mentioned applications are discussed. Keywords: thermochemical conversion, biochar, bioenergy.