Research Activity on Supercritical Fluids Technology at Kumamoto
University
Biomass Treatment with Sub- and Supercritical Water
Water at sub- and supercritical conditions is considered a promising solvent for
degradation of hazardous waste into harmless compounds. By varying the density (astrong function of pressure and temperature), it is possible to change the other propertiesof water, such as the dielectric constant and ion product. The density of water is an
important property in determining reaction mechanisms at supercritical water conditions.
Supercritical water not only participates in a reaction as a solvent but also as a reactant.
At sub- and supercritical water conditions, there are two major reactions: oxidation andhydrolysis. In supercritical water, organic materials are completely converted into
carbon dioxide, water and nitrogen. Another important reaction is the conversion oforganic materials into useful chemical compounds.
Biomass is a term used to describe all the organic materials derived from living
matter and it represents a renewable and alternative source for the production ofchemicals. Biomass is also potentially one of the most promising alternatives forelectrical power production owing to its ecological advantages. The development ofprocess has been proposed to recover chemical resources from biomass waste and zero
emission processes that can accomplish the destruction of the wastes into harmlesscompounds, such as hydrogen, carbon dioxide and methane. Plant biomass
approximately consists of 40–45 wt%of cellulose, 25–35 wt% of hemicellulose, 15–30
wt% of lignin and up to 10 wt% other compounds.
We have reported decomposition of biomass wastes at sub- and supercritical waterin batch and flow reactor. Lignin, which is the second most abundant polymericaromatic organic substance in wood biomass after cellulose, and contains many
oxygen-based functional groups, has been investigated as an alternative source ofchemical compounds. Guaiacol, as model compound of lignin, has been decomposed toproduce other chemical compounds [33]. Lignin has also been decomposed to recoverphenolic compounds, such as cathecol, phenol and cresol [34].
Tar, a part of biomass waste, has high impact on the environment and on humans
but it also contains many chemical compounds that can be used for industrial
applications. This hazardous waste, however, has been discarded without any treatment
because there is currently no method with which tar can be converted to fuels andchemical intermediates easily in an environmentally friendly way. Recovery of phenol
through decomposition of tar at hydrothermal alkaline condition and liquefaction kineticof tar in sub- and supercritical water has been studied [35-37].
Glucose as a part of cellulose, in particular, can be converted into energy using
diverse methods. For example, in a conventional reaction method, it can be converted
into low-molecular-weight saccharides as decomposition products. However, the acidcatalysts such as sulfuric or hydrochloric acids were used in this process. And, theprocess was to be complicate because removal of the harmful catalyst that remains after
the process is required. Therefore, the development of new environmentally friendly
synthetic methods with high economic efficiency is desired. In our laboratory, thedecomposition of glucose has been carried out under various experimental conditions in
subcritical water using continuous-flow reactors. The decomposition products, such as
fructose, glyceraldehydes, glycolaldehyde, 5-HMF, and 2-furfural were obtained [38].
In addition, study of decomposition of biomass model compounds was conductedat hydrothermal electrolysis in batch and flow reactors as shown in Figure 3 (a) and (b),respectively. In this work, we focused on understanding the degradation behavior ofglucose under hydrothermal conditions and to determine how to obtain gaseouscompounds such as hydrogen and carbon dioxide. Electrode oxidation reaction ofglucose at 250 °C was conducted to investigate fundamental reaction characteristics forthe purpose of utilizing glucose solution [39]. Other compounds, such 1-butanol andglycerol were also used as starting materials to produce hydrogen as alternative energy.

(a)(b)
Figure 3. Hydrothermal electrolysis apparatus. (a) autoclave (batch type); (b) flow type.
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p. 882.
[36] Wahyudiono, M. Sasaki, M. Goto, J. Mater. Cycles Waste Manage., Vol. , 2007, p.
[37] Wahyudiono, M. Sasaki, M. Goto, Polym. Degrad. Stab., Vol. 93, 2008, p. 1194.
[38] Saito, T., Sasaki, M., Kawanabe, H., Yoshino, Y., Goto, M., Chem. Eng. Technol.,Vol. 32, 2009, p. 1.
[39] M. Sasaki, K. Yamamoto, M. Goto, J. Mater. Cycles Waste Manag., Vol. 9, 2007, p.
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