Temperature Effect on Hydrothermal Decomposition of Glucose in Sub- And Supercritical Water

Hydrothermal decomposition of glucose was conducted at 573-733 K. The glucose solution (1.5 wt %) was gasified in a tubular reactor at 25 MPa. Gas products, char particles, and liquid products were observed, and the product yields were determined as a function of temperature. Char was only produced under the subcritical water condition (573 and 623 K) and was drastically suppressed under the supercritical condition. Intermediate compounds were produced and further decomposed to other products. This change can be attributed to the change in water properties from the subcritical to the supercritical region. The kinetic parameters of glucose decomposition pathways were also determined by assuming the first-order reaction. The rate constant of overall glucose decomposition showed Arrhenius behavior, but some reactions deviated from Arrhenius behavior in the supercritical region. Since it was expected that radical reactions would obey the Arrhenius equation and not be affected by water properties, but that ionic reactions would be influenced by the dielectric constant or ion product, and deviate from Arrhenius behavior in supercritical water, the temperature effect was considered a good method for distinguishing between ionic and radical reactions. On the basis of this concept, reactions in the reaction network of the supercritical water gasification of glucose were successfully classified into ionic and radical reactions.