Enthalpy change during hydrothermal carbonization of biomass: a critical review

A remarkable number of scientific papers are available in the literature about the process of hydrothermal carbonization (HTC), also called wet or hydrous pyrolysis, applied to biomass substrates. However, the biggest share of it focuses on the characterization of HTC products obtained from different feedstocks. Only a few works are available on the process thermodynamics, particularly about the determination of the enthalpy change and the related heat release or absorption during HTC, which is a key parameter to understand the nature of the process and to evaluate the heat requirements for the design of industrial scale HTC plants. The present review summarizes the research carried out with the aim of assessing the process enthalpy for HTC. Two main approaches have been identified and described, and the research works are sorted into two categories: enthalpy calculation based on the direct application of the Hess's law and enthalpy evaluation based on differential heat measurement. The hypotheses and results obtained by the different authors are critically analysed and discussed, and directions for further research are proposed.