Kunio Arai

CATALYTIC GASIFICATION OF WOOD BIOMASS IN SUBCRITICAL AND SUPERCRITICAL WATER

ABSTRACT Research in biomass gasification with subcritical and supercritical water is reviewed. Catalytic conversion of biomass in sub- and supercritical water is a low-temperature gasification technique that can be carried out from 473 to 973 K. Research is categorized according to temperature and water density, since reaction mechanisms greatly depend on these variables.

Biomass Conversion in Supercritical Water

Kinetic studies for cellulose decomposition in supercritical water (SCW) were conducted. The cellulose hydrolysis and glucose decomposition rate constant in SCW was measured by using a flow reactor. As the pressure increases, cellulose hydrolysis rate constant increases while glucose decomposition rate constant decreases. Based on this evaluated reaction rate constants, Monte Carlo simulation of cellulose decomposition in SCW was carried out. As a result, 75% of glucose yield can be obtained at 40 MPa, 653 K and τ = 25 msec. As a basic study for cellulose conversion in SCW, decomposition rates for maltose and cellobiose were also evaluated in subcritical and supercritical water at temperatures between 603 and 693 K and pressures from 25 to 40 MPa. In supercritical region, both cellobiose and maltose hydrolysis rate constant increases as the pressure increases. The ratio of hydrolysis rate constant of maltose and cellobiose is 2.3. This value is the same ratio for acidic hydrolysis for maltose and cellobiose at 353 K, atmospheric pressure with 0.1 N HCl.

Local density augmentation from fluorescence lifetime for anthracene N,N-dimethylaniline exciplex in supercritical carbon dioxide

Abstract Local density augmentation around exciplex between anthracene and N , N -dimethylaniline in supercritical carbon dioxide was measured by fluorescence lifetime at 40 °C and at pressures from 9.3 to 19.1 MPa. In the near-critical (9.3–10.5 MPa) region, the exciplex was more stable than that predicted by Kirkwood analysis, which means strong influence of local density augmentation around the exciplex.

Preparation of Metal Oxide Nanowires by Hydrothermal Synthesis in Supercritical Water

Three kinds of single metal oxide (MnO 2 , ZnO, and AlO(OH)) and one complex metal oxide (K 2 O·6TiO 2 ) having nano structure of wire, rod and ribbon were rapidly synthesized by hydrothermal synthesis in supercritical water. Aqueous Mn(NO 3 ) 2 , Zn(NO) 2 , Al(NO 3 ) 3 solutions and mixtures of TiO 2 sols and KOH solutions were used as starting materials, respectively. Syntheses of these nanostructured materials were performed by a flow type apparatus. We investigated the relationship between reaction parameters (temperature, pH and reaction time) and morphologies of the products. Reaction temperatures were 350, 400, and 420 °C. Reaction time is in the range of 1.8 – 116 s. The product was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that particle morphologies strongly depend on pH for MnO 2 and ZnO. MnO 2 nanowires with 10 nm in diameter and ZnO nanorods with 50 nm in diameter were obtained from acidic metal salt solutions. In the case of AlO(OH), temperature and time were key parameters for crystal growth. In the case of K 2 O6TiO 2 , larger fibrous particles with 50 nm in diameter were obtained at higher reaction temperature.