HTC is a thermochemical conversion process for biomass to yield a solid, coal-like product. It has been used for almost a century in different sciences, mainly to simulate natural coalifi — cation in the laboratory. Due to the need for efficient biomass conversion technologies, HTC has attracted some interest as a possible application for biomass in recent years, and R&D projects have been launched to assess its feasibility and discover additional possibilities for applications. HTC has been in use as a method for simulating natural coalification in coal petrology for nearly a century, and many experimental results have been published. It was introduced to this research field by Bergius as early as 1913 and was discussed controversially from then on. HTC is an exothermic process that lowers both the oxygen and hydrogen content of the feed (described by the molecular O/C and H/C ratio) by mainly dehydration and decarboxylation to raise its carbon content with the aim of achieving a higher calorific value. This is achieved by applying temperatures of 180-200 °C in a suspension of biomass and water at saturated pressure for several hours. With this conversion process, a lignite-like, easy-to-handle fuel with well-defined properties can be created from biomass residues, even with high moisture content. Thus, it may contribute to a wider application of biomass for energetic purposes (Behar and Hatcher, 1995; Funke and Ziegle, 2009; Mukherjee et al., 1996; Payne and Ortoleva, 2001; Ross et al., 1991; Siskin and Katritzky, 1991; Wolfs et al., 1960).

Many chemical reactions that might appear during HTC have been mentioned throughout the literature, but just few have been the focus of detailed investigations, for example, the hydrolysis of cellulose. It has been realized that the process is governed in sum by dehy­dration and decarboxylation, which means that it is exothermal. Simultaneously, functional groups are being eliminated to some extent. But the complex reaction network is not known in detail. So, for the time being, only a separate discussion of general reaction mechanisms that have been identified can provide useful information about possibilities of manipulating the reaction. These mechanisms include hydrolysis, dehydration, decarboxylation, conden­sation polymerization, and aromatization. They do not represent consecutive reaction steps but rather form a parallel network of different reaction paths. It is understood that the detailed nature of these mechanisms, as well as their relative significance during the course of reaction, primarily depends on the type of feed.

Although HTC has been known for nearly a century, it has received little attention in current biomass conversion research. Although it received great attention for biomass lique­faction and gasification, a technical implementation of HTC has only been developed with comparably low effort. This may be due to the fact that coal as an energy carrier is inferior to liquid or gaseous fuels. On the other hand, process requirements of HTC are comparably low while producing a fuel that is easier to handle and store because it is stable and nontoxic. Due to these facts, HTC may provide some advantages when considering small-scale, decentralized applications. Moreover, it might become a viable option for the production of functional carbonaceous materials.

The mildest reaction conditions in terms of temperature and pressure are employed in HTC. Lignocellulosic substrates have been extensively examined (Titirici et al., 2007) as reactants at temperatures from 170 to 250 °C over a period of a few hours to a day (Heilmann et al., 2010). Latest research on HTC focused on the preparation of functional carbonaceous materials and achieved interesting results for a future application to produce even more value-added materials. Low-value and widely available biomass can be converted into interesting carbon nanostructures using environment-friendly steps. These low-cost nanostructured carbon materials can then be designed for applications in crucial fields such as separation, energy conversion, and catalysis. Besides controlling the chemistry of carboni­zation (i. e., C—C linkage), two other important prerequisites for the achievement of useful properties are the control over morphology both at nano — and macroscale and the control over functionality by chemical means in HTC (Titirici and Antonietti, 2010).