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

Jul 14, 2006 - The phenomenological rate value was evaluated on the basis of the size change. In the subcritical water, O2 promoted the size decrease ...
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Ind. Eng. Chem. Res. 2006, 45, 5885-5890

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Visualized Kinetic Aspects of a Wood Block in Sub- and Supercritical Water Oxidation Daisuke Shoji, Nobuko Kuramochi, Kazuko Yui, Hiroshi Uchida, Kiyoshi Itatani, and Seiichiro Koda* Department of Chemistry, Faculty of Science and Technology, Sophia UniVersity, Kioi-cho 7-1, Chiyoda-ku, Tokyo 102-8554, Japan

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 flowtype 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. Introduction As we have previously discussed,1 supercritical water (SCW) technology, in particular supercritical water oxidation (SCWO) technology, is now going to be practically applied for decomposing organic materials and for energy recovery from lowquality fuels. There have been published a large number of studies of biomass, including wood, cellulosic materials, and lignins in sub- and supercritical water in the absence of O2, though less frequently in the presence of O2. The products and their production rates have been investigated particularly for some isolated components of biomass materials. Relevant processes of sub- and supercritical water treatment of biomass for energy and material recovery through gasification (with and without catalysts) and liquefaction have been also extensively studied.2-5 The merits and demerits of the treatment have been discussed, being sometimes compared with traditional steam gasification and combustion. However, a practical SCW or SCWO process for treating biomass including wood is not yet established. Despite the extensive studies mentioned above, the whole reaction progress of a solid substance such as a wood block itself is scarcely documented. Understanding the kinetics of solid substances as a whole, to which mass transport processes and heterogeneous reactions as well as homogeneous reactions may contribute in a complicated manner, is strongly needed for developing the novel SCW and SCWO technology. In the design of slurry, fluidized-bed, and/or packed-bed reaction processes, understanding the kinetics and mechanism of individual particles all through the reaction progress is very important, which knowledge is valuable when devising an optimum combination * To whom correspondence should be addressed. Tel.: + 81-3-32383377. Fax: + 81-3-3238-3361 (at Chemistry Office). E-mail: [email protected].

of relevant processes and performing a reactor design. Despite several kinetic studies on the solid particulate,6 such information is not yet documented. In the previous work,1 we studied the decomposition of a wood block, by directly observing the size change in the absence of O2 by means of shadowgraph imaging. A reaction cell possessing sapphire windows, similar to the one employed in the SCWO study of carbon particles7,8 was employed. From the size change analysis, the reaction rate was evaluated under various reaction conditions. The rate constant increased with the reaction temperature and approximately obeyed the Arrhenius rate law in the subcritical region. It decreased suddenly near the critical point and again increased with increasing temperature at the higher temperatures. This peculiar change of the rate in the vicinity of the critical point was reasonably understood by taking into account the remarkable decrease of the ionic product of water in the neighborhood of critical point, and thus, the reaction was promoted by certain proton-catalyzed reactions in the subcritical region. In the present study, the kinetic behavior of a “hinoki” (a coniferous tree, Chamaecyparis obtusa) wood block in the presence of O2 was studied in sub- and supercritical water by using the same reaction system as before.1 The effects of the O2 concentration, flow rate, and temperature were examined. The reaction was accelerated in the presence of O2, and a certain mass-transfer process played an important role in the rate determination. The products in total organic carbon (TOC) and also the solid residue were analyzed in a preliminary manner. Experimental Section The experimental apparatus is the same as previously reported,1 except that we employed two syringe pumps in the present study instead of one. Very briefly, the reaction cell is a Hasteloy block through which is placed a vertical cylindrical hole (8 mm in diameter) as the space for reaction. To the right

10.1021/ie0604775 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/14/2006

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angles to the hole, two sapphire windows are attached confronting to each other, through which shadowgraph observation of the cell inside is conducted. A cylindrical-shaped wood block (typical diameter ) 3 mm and height ) 4 mm) of hinoki was connected with a Pt wire to one end of a slender Hasteloy rod, to the other end of which was attached a magnet. The typical elemental analysis for the employed hinoki was C/H/O ) 47.4:6.4:46.2 mass % after a 3 h drying in a 100 °C oven. A syringe pump (ISCO 100DM) supplied water through a preheater to the reaction cell. After a sufficient period for stabilizing the temperature, pressure, and flow conditions, the water flow was exchanged to an aqueous H2O2 flow from a second syringe pump (ISCO 100DM). The wood block was then transferred using the outer magnet to the central position of the cell to start the reaction. The preheater was kept at 673 K for yielding O2 via decomposing H2O2 stoichiometrically.9 The temperature range studied was 573698 K, and the pressure was 25 MPa. The concentration of O2 was up to 3.0 mass %. The flow rate will be described in the present paper in terms of the volume flow rate evaluated at normal temperature and pressure at the inlet of the syringe pump. The effluents were sampled every several minutes, for which the amount of TOC was measured, and also preliminary analysis of the components was conducted by means of high-performance liquid chromatography (HPLC) analysis. TOC was analyzed using a TOC analyzer (Shimadzu TOC-VCPH), and the components were analyzed using an HPLC system (Shimadzu LC10ADVP) with the column (Shinwa Chemical Ind. Ltd., ULTRON PS-80H 300 mm).

Figure 1. Time evolution of the shape of hinoki wood. Reaction conditions: 573 K, 25 MPa, and flow rate 0.83 × 10-2 cm3 s-1 with O2 concentration of 2.0 mass %.

Results and Discussion General Behavior. When a hinoki wood block was transferred into the water containing O2 at a high temperature in the subcritical region (573-623 K), the block immediately started to shrink. Similarly to the case of the absence of O2 reported previously,1 emission of some colored materials into the surrounding water was observed, although the color was much thinner. In the presence of O2, the size smoothly decreased to null, being different from the case of O2 absence where a residue remained even after a very long reaction time in most experiments. We also studied the behavior of “buna” (a broad-leaved tree: a Japanese beech, Fagus crenata) in a preliminary manner, and found that its qualitative kinetic behavior in the presence of O2 does not differ so significantly from that of hinoki, although a larger difference was found between a coniferous tree and a broad-leaved tree in the absence of O2, as described in the previous work.1 The present study focused on hinoki can be regarded as a typical example, although comparison among a variety of wood is required and left for a future study. The shadowgraph images for a typical experimental run are shown in Figure 1. The shape decreases gradually. The diameter D along the horizontal direction decreases, and the length decreases more slowly. The asymmetric shape change is principally caused by the inhomogeneous character of the wood block itself but not by the flow field asymmetry. The shrinking rate is larger for the right-angle direction to the lead pipe of the wood block. The diameter along the time is plotted as log(D/D0) in Figure 2 under the three temperatures of 573, 598, and 623 K, where D0 is the initial diameter. Except for the very initial stage, the plots of log(D/D0) approximately make a single straight line, whose slope gives a first-order rate constant k. Though the initial stage within ca. 10 s is considered to suffer from the insufficient

Figure 2. Logarithmic change of D/D0 against time. Reaction conditions: 25 MPa, flow rate of 0.83 × 10-2 cm3 s-1 with O2 concentration of 1.0 mass % at 573 (b), 598 (0), and 623 K (O).

temperature rise as previously estimated,1 the temperature of the wood block is expected to be almost the same as the surrounding fluid temperature in the following stages. The firstorder behavior might be fortuitous, considering that the rate process is affected by a variety of factors, some of which are related to the internal structure of the wood block, and some others which are related to the flow field and so forth. The effects of the individual factors are desirable to be deconvoluted, and the present study will help such deconvolution by analyzing the effects of experimental parameters on the phenomenological first-order rate constant. Reaction Rate and Products in Subcritical Water Oxidation. The rate constant obtained from the time behavior was studied as a function of O2 concentration, flow rate, and temperature. Figure 3 shows the rate constant dependence on the O2 concentration. The reaction rate increases with increasing O2 concentration and gradually saturates above the concentration of 1.5 mass %. Oxygen accelerates the reaction that is responsible for the decrease of the wood block size. The saturation may be caused by the solubility limit of O2 in subcritical water. The data by Japas and Franck10 show that the O2 solubility in water at 573 K and 25 MPa is ca. 1.9 mass %. In the presence of O2, the reaction rates increased almost linearly with increasing flow rate in the smaller flow rate region,

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Figure 3. Effect of O2 concentration on rate constant k. Reaction conditions: 573 K, 25 MPa, and flow rate of 0.83 × 10-2 cm3 s-1 with O2 concentration of 0-3.0 mass %.

Figure 4. Arrhenius temperature dependence of the rate constant k. Reaction conditions: 25 MPa and flow rate of 0.83 × 10-2 cm3 s-1 with O2 concentration of 1.0 (-b-) and 2.0 mass % (- -O- -), and 25 MPa and flow rate of 2.5 × 10-2 cm3 s-1 with O2 concentration of 1.0 mass % (-9-). The case with no O2 is also shown by a dashed line (- - -), taken from the previous report.1

smaller than 2.5 × 10-2 cm3 s-1, and then gradually saturated at larger flow rates. Contrary to this trend, the flow rate in the absence of O2 did not affect the reaction appreciably, as already described in our previous paper.1 The above difference found in the presence and absence of O2 suggests that the supply of O2, which causes the rate acceleration, is limited to some extent by a certain mass transfer process of O2 from the bulk flow to the wood block. The temperature dependence of the rate constants is plotted against the reciprocal temperature in Figure 4 for several concentrations of O2 and flow rates. The rate constants are larger in the presence of O2 than in its absence. However, a smaller temperature dependence is found in the presence of O2, which implies that the reaction accelerated by O2 has apparently a smaller activation energy. The activation energies depend on the O2 concentration (their rough values are 37 kJ mol-1 for O2 of 1.0 mass % and 85 kJ mol-1 for O2 concentration of 2.0 mass % in Figure 4) and on the flow rate (37 kJ mol-1 for 0.83 × 10-2 cm3 s-1 and 70 kJ mol-1 for 2.5 × 10-2 cm3 s-1). It is important to notice that the activation energies are always smaller in the presence of O2 than in the hydrolysis-driven shape decrease in the absence of O2 (ca. 120 kJ mol-1),1 although the activation energies should not be taken vigorously because of the large data scattering and also because various factors affect the temperature dependence in a complicated manner. It is also noted that the peculiar temperature dependence of the rate found in the absence of O21 is not observed in the presence of O2, which implies that the reaction in the presence of O2 is not

Figure 5. Effect of O2 concentration on total TOC. Reaction conditions: 573 K, 25 MPa, and flow rates of 0.83 × 10-2 (b) and 2.5 × 10-2 cm3 s-1 (0) with O2 concentration of 0-3.0 mass %.

Figure 6. Effect of temperature on total TOC. Reaction conditions: 25 MPa and flow rates of 0.83 × 10-2 (b) and 2.5 × 10-2 cm3 s-1 (0) with O2 concentration of 1.0 mass %.

affected strongly by the ionic product of water, contrary to the decomposition reaction in the absence of O2. The effluents were sampled every several minutes, for which the amount of TOC was measured and also preliminary analysis of the components was conducted by means of HPLC analysis. The amounts of TOC as well as the products identified by HPLC will be shown as the ratio of TOC as well as the products against the carbon involved in the wood block in the elemental carbon base. The total TOC values, where “total” means the integration over the whole reaction time, i.e., the time necessary for the complete consumption of the wood block, are shown as functions of the O2 concentration and temperature in Figures 5 and 6, respectively. The total TOC amount is very large in the absence of O2 and becomes smaller in the presence of O2 because of the oxidation progress of organic materials during the decomposition of the wood tissue and also after they are emitted from the wood block to the water. The residence time for the emitted materials to remain in the high-temperature region has been estimated to be ca. 1.6 s at 573 K at the flow rate of 2.5 × 10-2 cm3 s-1, while it is 1.3 s at 623 K. On the other hand, the smaller amounts of total TOC at a slower flow rate in both Figures 5 and 6 are reasonable, considering the longer residence time available for the subsequent oxidation progress at the slower flow rate. The amounts of TOC at individual sampling periods are plotted against the sampling time under different O2 concentrations in Figure 7. The amounts become smaller and also the profile shifts to earlier stages with the increase of the O2 concentration, being consistent with the acceleration of the size decrease in the presence of O2. Preliminary analyses of the TOC components were conducted. The amounts of several components analyzed at the maximum

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Figure 7. TOC time evolution. Reaction conditions: 573 K, 25 MPa, and flow rate of 2.5 × 10-2 cm3 s-1 with O2 concentration of 0 (b), 0.5 (0), and 1.0 mass % (O).

Figure 9. Time evolution of the shape of hinoki wood. Reaction conditions: 673 K, 25 MPa, and flow rate 0.83 × 10-2 cm3 s-1 with O2 concentration of 1.0 mass %.

Figure 8. Composition of TOC at the peak position. Reaction conditions: 573 K, 25 MPa, and flow rate of 2.5 × 10-2 cm3 s-1 with O2 concentration of 0, 0.5, and 1.0 mass %: (a) formic acid, (b) acetic acid, (c) 5-hydroxymethylfurfural, (d) 2-furaldehyde, and (e) others.

peak positions of TOC are given in Figure 8. It is shown that 5-hydroxymethylfurfural (5-HMF) and 2-furaldehyde (2-FA) are produced at relatively large yields in the absence of O2, and then the main components change to organic acids in the presence of O2. There are a lot of unassigned compounds that will be investigated in future. The hinoki wood samples usually contain about 40-50% cellulose, 5-20% hemicellulose, and 20-30% lignin. According to the previous hot compressed water treatment of plant biomass,11,12 most of the hemicellulose and some of the cellulose and lignin were solubilized. Furfurals including 5-HMF are likely derived from the decomposition of cellulose and/or hemicellulose through saccharide intermediates such as glucose.13,14 Cellulose and hemicellulose are also reported to decompose partially to organic acid, involving formic acid and acetic acid.15 Decomposition of glucose that is found as an important intermediate compound from cellulose proceeds very rapidly.16,17 For example, the reaction time of 0.02-2.0 s was reported to be sufficient at a temperature of 573-673 K and a pressure of 25-40 MPa.17 On the other hand, in the presence of O2 or oxidant H2O2, acetic acid with a smaller amount of formic acid is found as the main product from cellulose after 300 s reaction at 673 K and 27.6 MPa, although most of the organic materials are converted to gaseous products.18 The authors obtained acetic acid at the highest yield of 10.5 mass % on a carbon basis. Jin et al.19 also reported that the yield of acetic acid could be increased by the addition of O2 following the hydrolysis of cellulosic materials. In the present experiment at 573 K and 25 MPa, the recovered amount of total TOC was 45% on a carbon basis that contained

Figure 10. Logarithmic change of D/D0 against time. Reaction conditions: 673 K, 25 MPa, and flow rate of 0.83 × 10-2 cm3 s-1 with O2 concentration of 0 (b), 0.5 (O), 1.0 (0), and 2.0 mass % (9).

about 13% of acetic acid and 21% of formic acid on a carbon basis, as shown in Figure 8. The total acids yield may become higher by controlling the reaction time and the O2 concentration. Reaction Behavior in Supercritical Water Oxidation. At the temperature of 400 °C above the critical temperature, the phenomenological behavior of the wood block somewhat changed. As exemplified in Figure 9, the initial stage of a very rapid size decrease (t ) 0-30 s) is followed by the intermediate stage of the almost-constant size ( t ) 90-600 s) and then by a final stage of a gradual decrease of the size (t ) 600-1800 s). The temperature increase by 25 °C seems not to affect the behavior appreciably (not shown). The time evolutions of log(D/D0) under various O2 concentrations are shown in Figure 10. The size of the intermediate solid residue and also the amount of total TOC are plotted against the O2 concentration in Figure 11. The diameter of the intermediate solid residue takes a maximum at 0.5 mass % of O2. This may be explained considering that the intermediate production of the solid residue is accelerated by O2 but also its consumption rate becomes larger with increasing the O2 concentration. The yields of the intermediate residue and their elemental compositions are tabulated in Table 1. Their heating values are also tabulated, being calculated by Dulong’s formula,20 shown below,

heating value (MJ kg-1) ) 0.3383C + 1.442 (H - 1/8O) Carbonization of woody biomass has been rarely studied in sub-

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rate of the intermediate solid residue is in fair agreement with that of the previous activated carbon. This means that a 1:1 stoichiometric ratio of carbon/O2 is obeyed in the residue oxidation, similarly to the activated carbon oxidation. Thus, the very rapid decrease of the size in the initiation stage of the present wood experiments should have required much less O2 compared to the oxidation of the intermediate solid residue. Conclusions

Figure 11. Effect of O2 concentration on relative diameter of the intermediate solid residue (Dr/D0) and total TOC. Reaction conditions: 673 K, 25 MPa, and flow rate of 0.83 × 10-2 cm3 s-1 with O2 concentration of 0-3.0 mass %. Table 1. Elemental Analysis and Heating Value of the Recovered Intermediate Solid Residue samplea

yield (mass %)

a b c d

20.5 17.1 8.8

elemental analysis (mass %) C H O 47.4 66.7 70.5 64.2

6.4 4.9 4.7 3.6

46.2 28.4 24.8 32.2

heating value (MJ kg-1) 16.9 24.5 26.2 21.1

The size decrease of a wood block is accelerated by O2, particularly in subcritical water, because of the acceleration of reaction relevant to the size decrease, although the supplied amount of O2 to the vicinity of the wood block is much smaller than that required for a complete oxidation in the initial stage. The amount of TOC decreases in the presence of O2, and the organic materials are progressively oxidized. Furaldehydes and organic acids are found as important intermediate products during the reaction progress. A wood block also makes an intermediate solid residue in SCWO, which is subsequently oxidized at the similar reaction rate as the activated carbon particles. Acknowledgment

a

Sample legend is defined as follows: (a) initial wood; (b) residue from the reaction at 573 K and 25 MPa for 40 s in a flow of 0.83 × 10-2 cm3 s-1 rate with 0.1 mass % O2; (c) residue from the reaction at 673 K and 25 MPa for 20 s in a flow of 0.83 × 10-2 cm3 s-1 rate with 0.1 mass % O2; and (d) residue from the reaction at 673 K and 25 MPa for 40 s in a flow of 0.83 × 10-2 cm3 s-1 rate with 1.0 mass % O2.

and supercritical water. Inoue et al.20 studied the carbonization process in hot compressed water at 473-623 K and reported that the dehydrogenation and deoxygenation in wood were observed during the treatment. In their experiments, the carbon content in the charcoal increased with the temperature, whereas the charcoal yield decreased with the temperature. The composition of the typical solid residue obtained in the present experiments from the reaction at 673 K, O2 ) 0.1 mass %, is comparable with the charcoal obtained by Inoue et al.20 in hot compressed water at 623 K for 0 h, which contained ca. 72% carbon. The smaller yield and larger oxygen content in the sample “d” depicted in Table 1 are ascribed to the oxidation progress under the higher O2 concentration and the longer reaction time. The control of O2 concentration is important to obtain a solid residue usable as a solid fuel. The size decrease of the intermediate solid residue in the final stage can be compared with the results reported previously by the present authors,21 where the first-order reaction of the radius decrease of a spherical activated carbon was investigated in its O2-mass transfer limitation region (>673 K).21 The radius decreasing rate of dR/dt was 1 × 10-6 m s-1 at 673 K and 25 MPa with O2 of 3.6 mass % under the flow rate of 1.67 × 10-2 cm3 s-1. The initial radius R0 was 2 mm. On the other hand, Figure 10 shows that d(D/2)/dt ) dR/dt is ca. 0.5 × 10-6 m s-1 at the diameter D of 1-0.5 mm (thus, R0 is assumed to be 0.5 mm) at 673 K and 25 MPa with O2 of 1.0 mass % under the flow rate of 0.83 × 10-2 cm3 s-1. Admitting that the rate is proportional to the O2 concentration, and also inversely proportional to the initial radius, the present value for the intermediate residue from the wood block is converted to the value of dR/dt ) 0.45 ×10-6 m s-1 () 0.5 × 10-6 × (3.6/1) × (0.5/2)) at 25 MPa with O2 of 3.6 mass %. Taking into account the geometry difference and also the higher flow rate in the previous activated carbon experiments,21 the present oxidation

The preset study was supported in part by a grant provided by NEDO (via JCII) based on the project “Research & Development of Environmentally Friendly Technology Using SCF” of the Industrial Science Technology Frontier Program (METI), which is greatly appreciated. Literature Cited (1) Shoji, D.; Sugimoto, K.; Uchida, H.; Itatani, K.; Fujie, M.; Koda, S. Visualized kinetic aspects of decomposition of a wood block in sub- and supercritical water. Ind. Eng. Chem. Res. 2005, 44, 2975. (2) Sealock, L. J.; Elliott, D. C.; Baker, E. G.; Butner, R. S. Chemical processing in high-pressure aqueous environments. 1. Historical perspective and continuing developments. Ind. Eng. Chem. Res. 1993, 32, 1535. (3) Matsumura, Y.; Sasaki, M.; Okuda, K.; Takami, S.; Ohara, S.; Umetsu, M.; Adschiri, T. Supercritical water treatment of biomass for energy and material recovery. Combust. Sci. Technol. 2006, 178, 509. (4) Osada, M.; Sato, T.; Watanabe, M.; Shirai, M.; Arai, K. Catalytic gasification of wood biomass in subcritical and supercritical water. Combust. Sci. Technol. 2006, 178, 537. (5) Antal, M. J., Jr.; Allen, S. G.; Schlman, D.; Xu, X.; Divilio, R. J. Biomass gasification in supercritical water. Ind. Eng., Chem. Res. 2000, 39, 4040. (6) Pisharody, S. A.; Fisher, J. W.; Abraham, M. A. Supercritical water oxidation of solid particulates. Ind. Eng. Chem. Res. 1996, 35, 4471. (7) Sugiyama, M.; Kataoka, M.; Ohmura, H.; Fujiwara, H.; Koda, S. Oxidation of carbon particles in supercritical water: Rate and mechanism. Ind. Eng. Chem. Res. 2004, 43, 690. (8) Sugiyama, M.; Tagawa, S.; Ohmura, H.; Koda, S. Supercritical water oxidation of a carbon particle by Schlieren photography. AIChE J. 2004, 50, 2082. (9) Croiset, E.; Rice, S. F. Hydrogen peroxide decomposition in supercritical water. AIChE J. 1997, 43, 2343. (10) Japas, M. L.; Franck, E. U. High-pressure phase equilibria and PVTdata of the water-oxygen system including water-air to 673 K and 250 MPa. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 1268. (11) Mok, W. S.-L.; Antal, M. J., Jr. Uncatalyzed solvolysis of whole biomass hemicellulose by hot compressed liquid water. Ind. Eng. Chem. Res. 1992, 31, 1157. (12) Ando, H.; Sasaki, T.; Kokusho, T.; Shibata, M.; Uemura, Y.; Hatate, Y. Decomposition behavior of plant biomass in hot-compressed water. Ind. Eng. Chem. Res. 2000, 39, 3688.

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(13) Sasaki, M.; Kabyemala, B.; Malaluan, R.; Hirose, S.; Kakeda, N.; Adschiri, T.; Arai, K. Cellulose hydrolysis in subcritical and supercritical water. J. Supercrit. Fluids 1998, 13, 261. (14) Quitain, A. T.; Sato, M.; Daimon, H.; Fujie, K. Qualitative investigation on hydrothermal treatment of hinoki (Chamaecyparis Obtusa) bark for production of useful chemicals. J. Agric. Food Chem. 2003, 51, 7926. (15) Yoshida, K.; Kusaki, J.; Ehara, K.; Saka, S. Characterization of low molecular weight organic acids from beech wood treated in supercritical water. Appl. Biochem. Biotechnol. 2005, 121-124, 795. (16) Holgate, H. R.; Meyer, J. C.; Tester, J. W. Glucose hydrolysis and oxidation in supercritical water. AIChE J. 1995, 41, 637. (17) Kabyemela, B. M.; Adschriri, T.; Malaluan, R.; Arai, K. Glucose and fructose decomposition in subcritical and supercritical water: Detailed reaction pathway, mechanisms and kinetics. Ind. Eng. Chem. Res. 1999, 38, 2888.

(18) Calvo, L.; Vallejo, D. Formation of organic acids during the hydrolysis and oxidation of several wastes in sub- and supercritical water. Ind. Eng. Chem. Res. 2002, 41, 6503. (19) Jin, F.; Zheng, J.; Enomoto, H.; Moriya, T.; Sato, N.; Higashijima, H. Hydrothermal process for increasing acetic acid yield from lignocellulolisic wastes. Chem. Lett. 2002, 32, 504. (20) Inoue, S.; Nakaoka, T.; Minowa, T. Hot compressed water treatment for production of charcoal from wood. J. Chem. Eng. Jpn. 2002, 35, 1020. (21) Koda, S.; Maeda, K.; Sugimoto, K.; Sugiyama, M. Oxidation reactions of solid carbon in supercritical water. Combust. Sci. Technol. 2006, 178, 487.

ReceiVed for reView April 18, 2006 ReVised manuscript receiVed June 9, 2006 Accepted June 16, 2006 IE0604775