Visualized Kinetic Aspects of a Wood Block in Sub- and Supercritical Water Oxidation

Reaction of a wood block of "hinoki" (a coniferous tree, Chamaecyparis obtusa) was studied in the presence of O2 in sub- and supercritical water by observing the change in the size and shape of the block in a flow-type reaction cell, to understand the kinetic aspects of the whole reaction progress. The employed temperature was 573-698 K, the pressure was 25 MPa, and the O2 concentration was 0-3.0 mass %. The phenomenological rate value was evaluated on the basis of the size change. In the subcritical water, O2 promoted the size decrease rate almost linearly up to its solubility limit. Its temperature dependence was weaker than in the absence of O2. The recovered amount of total organic carbon was smaller in the presence of O2 and became richer in organic acids such as formic acid. The reaction rate increased with increasing the flow rate. These results indicated that a certain mass-transfer process was involved in the promotion of the reaction. On the other hand, in the supercritical region, the wood block left an intermediate solid residue after a very rapid initial size decrease. The residue was then gradually consumed. The amount of the residue was dependent on the O2 concentration. However, the initial size decrease of the wood block in the supercritical water was less affected by the presence of O2 than that in the subcritical water. The reaction in the presence of O2 is not affected strongly by the ionic product of water, contrary to the decomposition reaction in the absence of O2. The understanding of the whole reaction progress of the wood block should help in devising a reasonable technology to utilize the biomass wood.