Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering

Abstract

Hydrothermal carbonization can be defined as combined dehydration and decarboxy lation of a fuel to raise its carbon content with the aim of achieving a higher calorific value. It is realized by applying elevated temperatures (180-220 o C) to biomass in a suspension with water under 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. Although hydrothermal carbonization has been known for nearly a century, it has received little attention in current biomass conversion research. This review summarizes knowledge about the chemical nature of this process from a process design point of view. Reaction mechanisms of hydrolysis, dehydration, decarboxylation, aromatization, and condensation polymerization are discussed and evaluated to describe important operational parameters qualitatively. The results are used to derive fundamental process design im provements.

A Review on Pilot-Scale Applications of Hydrothermal Treatment for Upgrading Waste Materials

“…The degradation of carboxyl and carbonyl groups yielded CO 2 (as a major gaseous product during HTT) and CO and this could be observed by HTT above 150 °C [17], [18]. In addition, other sources of CO 2 could come from condensation reactions and the cleavage of intramolecular bonds as well as formic acid by dehydration of cellulose [12]. Other important reactions (polymerization, condensation, and aromatization) occurred as subsequent reactions after hydrolysis of cellulose [12], [14] and other soluble polymers were formed through these reactions [19].…”

“…In addition, other sources of CO 2 could come from condensation reactions and the cleavage of intramolecular bonds as well as formic acid by dehydration of cellulose [12]. Other important reactions (polymerization, condensation, and aromatization) occurred as subsequent reactions after hydrolysis of cellulose [12], [14] and other soluble polymers were formed through these reactions [19]. Aromatization and aldol condensation (intermolecular dehydration) could occur simultaneously [14].…”

“…The temperature condition was very important parameter and strongly affected the mechanism of the process. For example, Funke and Ziegler reported that several seconds of residence time might be adequate for a hydrolysis of glucose follow by dehydration at 270 °C; however, this would take several hours at lower temperature condition, 150 °C [12]. The increase of the temperature drastically reduced time to reach the reaction.…”

“…Longer holding time might increase the yield of biochar during the HTC [4], [12]. However, for the HTL, short residence time is preferred for bio-oil production.…”

A Morphological Study on the Carbon Particles from Wood Fiber Waste via Hydrothermal Carbonization Process

“…The solid carbonaceous material, also known as biochar, is defined as fine grained charcoal, high in organic carbon and largely resistance to decomposition. HTC process can release one third of the combustion energy throughout dehydration [4]. Its' carbon efficiency is close to 1 after an adequate reaction time under proper condition.…”