Leaching behavior of the boron (B) and fluorine (F) present in fly ashes formed in pulverized coal combustion has been studied with a batch-type quartz vessel under the conditions of 25 °C and a solution to solid ratio of 100 ml/g according to the Dutch Standard NEN 7341 method (availability test). When each ash sample is stirred in ultra pure water without any acids, the pH of the resulting aqueous mixture is as high as 11-12 in every case, whereas the leaching of B-containing ions or F- ions depends strongly on the type of the ash, and the corresponding leaching extent after 60 min-stirring is in the range of 20 % to 65 % and 15 % to 45 %, respectively. In the experiments at a constant pH of 1.5-9.0 of a fly ash with relatively-high B (1000 μg/g-dry) and F (130 μg/g-dry) contents, more than 70 % of B in the ash is leached rapidly within 10 min, and the rate is larger at a lower pH condition. The leaching of F- ions also occurs within 10 min, and the extent depends significantly on pH and increases in the order of pH 9.0 (32 %) ≈ pH 5.6 (35 %) < pH 1.5 (72 %) < pH 3.0 (80 %). The concentration of the B or F present in each ash sample after acid leaching at pH 1.5 - 2.0 is less than the corresponding regulation limit for water pollution. Several factors determining the leachability of these elements are investigated.

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To circumvent the problem of reactor plugging in supercritical water gasification (SCWG), we proposed an SCWG process operated at intervals with a continuous-flow system (10 kg/d) configured to feed water and feedstock in alternate steps. Hydrogen fermentation residues of soybean fiber and restaurant food waste (SFHFR and RWHFR, respectively) were employed as feedstock, and fine activated carbon was used as catalyst. Feedstocks of 2.0-10.0% SFHFR were gasified in a tubular reactor at 600 °C, 25 MPa with a residence time of 60-105 s. An 8% concentration SFHSR was gasified with a carbon gasification efficiency of 0.75 without plugging. A 4% RWHFR, a feedstock composed of more diverse substances, was gasified with a carbon gasification efficiency of 0.57.

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Our group studied the effects of temperature on the tarry material production of glucose under both subcritical and supercritical water conditions. The glucose solution (1.5 wt%) was gasified in a tubular reactor at 25 MPa pressure. The reaction temperature ranged from 300 to 450 °C. Gas products, char particles, and liquid products were observed, and the product yields were determined as a function of temperature. Tarry material was only produced under the subcritical water condition (300 and 350 °C) and was drastically changed or suppressed under the supercritical condition. To explain this, we speculated that the difference in water properties from the subcritical to the supercritical region altered the reaction mechanism. We also determined the kinetic parameters of glucose decomposition pathways by assuming the first-order reaction. The rate constant of overall glucose decomposition conformed to the Arrhenius behavior. The individual rate constant of glucose decomposition in subcritical temperature increased as the temperature rose but decreased in the supercritical condition. Different reaction mechanisms took place under two distinctly different regimes.

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