. Introduction
With the global warming in the centre of international concern and discussions, a profound review in
energy policy is being conducted. In this arena, ideas for alternatives to ever-decreasing reserves of fossil
fuels as well as measures to decelerate or reduce the CO2 emissions are urgently required. The efficient
management of biomass, for instance to produce biofuels, is one of the most interesting aspects under
investigation in order to achieve an environmentally-clean and CO2-neutral solution.
A possible method to convert biomass into a biofuel is hydrothermal carbonization, also known as HTC.
The first experiments involving the HTC process were already performed during the first half of the
twentieth century and were aimed at understanding the mechanism of natural coalification [1, 2] although
it was not until recently that this mechanism was proposed for the production of biofuels [3]. The HTC
process is relatively simple, requiring mainly a closed vessel that contains the wet biomass and that is
heated to temperatures between 170 and 250°C over a period ranging from a few hours to a day [4]. The
hydrothermal carbonization process includes several reaction mechanisms, such as hydrolysis,
dehydration, decarboxylation, polymerization and aromatization, although the detailed reactions have
been only well characterized for a few types of biomass, such as cellulose [5]. The process takes place
effectively only in water and is exothermic. The products of the HTC are a solid phase or “HTC-coal”
and a liquid phase, referred as process water. A small amount of gas is also produced.
In the field of biofuel research, most attention has been paid to the liquid and gaseous products, while the
solid phase or char did not receive much consideration, partially explained by the fact that its energy