Biomass Gasification in Supercritical Water: Influence of the Dry Matter

The results concerning the dependence of the dry matter content on the gas ... e.g., to understand the effect of heating rate; concerning this, anothe...
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Ind. Eng. Chem. Res. 2003, 42, 3711-3717

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Biomass Gasification in Supercritical Water: Influence of the Dry Matter Content and the Formation of Phenols A. Kruse,* T. Henningsen,† A. Sınagˇ ,‡ and J. Pfeiffer§ Institut fu¨ r Technische Chemie CPV, Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe, Germany

Biomass was gasified in a continuous stirred tank reactor (CSTR) at 500 °C and various pressures (30-50 MPa), reaction times (1.5-10.5 min), and dry matter contents (1.8-5.4 wt %) of the biomass. These results are compared with the results of experiments in a batch reactor, which was heated to 500 °C with two different heating rates (1 and 3 K/min) using two different dry matter contents (5 and 7.5 wt %). Key compounds for different reaction pathways, in particular phenols, were analyzed. The results concerning the dependence of the dry matter content on the gas formation, total organic carbon content, and phenols concentration are very different. In the CSTR the increase of the dry matter content leads to an increased gas yield, in particular of CH4, and the phenols yield increases. This was not found in the batch reactor. The possible reasons are discussed. Introduction A considerable proportion of the biomass available for energetic utilization exhibits a high water content of up to 95%. For classical gasification or liquefaction processes, this biomass has to be subjected to preliminary drying. To avoid this high drying expenditure, the biomass may be converted in hot, pressurized water or supercritical water. Depending on the reaction conditions, mainly liquid products or burnable gases are formed. An application of particular interest is the generation of hydrogen from biomass.1-3 Because of the high water excess, hydrogen (H2) and carbon dioxide (CO2) are generated at temperatures of 600 °C or higher instead of the synthesis gas produced during classical gasification processes. In addition, small amounts of methane (CH4) and very small amounts of carbon monoxide (CO) result. In this way, one process step of classical H2 generation would no longer be required, namely, the conversion of CO with water into H2 and CO2. Today H2 production is of interest above all for the use of fuel cells in electricity production. Biomass gasification in supercritical water can be regarded as a high-pressure steam reforming process of biomass, in which part of water is transformed to H2. In fact, up to half of the H2 formed after the reaction of biomass in supercritical water originates from water. Another difference as compared to classical gasification in the gas phase is that intermediate biomass decomposition products are dissolved in supercritical water as a result of its high solvent power for organic compounds. Consequently, coke and tar formation is reduced drastically under appropriate reaction conditions. For the development of this new process, several chemical and process technology problems have to be solved. Judging from the experience gained so far, three * To whom correspondence should be addressed. Fax: + 49 7247 822244. E-mail: [email protected]. † E-mail: [email protected]. ‡ E-mail: [email protected]. § E-mail: [email protected].

major “chemical” questions arise for optimizing a process for the generation of H2 from wet biomass: 1. How do higher dry matter contents of the wet biomass affect gasification and how can the gas yield be improved at increased dry matter contents? Higher dry matter contents likely lead to an incomplete gasification and a reduced gas yield.2,14 2. How does the heating rate of the biomass affect the gasification result? In a technical hydrogen production process, it is aimed at using the residual heat of the aqueous flow leaving the reactor for heating the biomass. This means that the biomass is liquefied in the preheater. The chemical compounds formed mainly depend on the heating rate.1 This means that the heating rate also determines the reactivity of the mixture formed in terms of the desired gas generation and the undesired formation of tar or even char/coke. 3. Which effect does the composition of the biomass have on the gasification result? It is demonstrated by the work performed so far that the hydrogen yield is adversely affected by a high proportion of lignin4 and positively affected by a high content of alkali salts.3 However, the detailed relationships between the gas yield and the composition of the biomass are still unknown to a large extent. To answer this questions, a lot work has to be done. This work is only a piece of a huge puzzle! This paper focuses on the effect of the dry matter content (dry matter content ) 100% - water content) on gasification. In the continuous stirred tank reactor (CSTR), the temperature was kept constant at about 500 °C in all experiments. At three residence times and three pressures, the effect of a variable biomass dilution on gasification was studied. (In the following sections, residence time shall be understood to be the space time, i.e., the ratio between the reactor volume and the volume flow under reaction conditions; assuming an approximately ideal behavior of the stirred vessel, the space time is identical with the mean residence time in the reactor). As mentioned above, an increased dry matter content was found before to lead to a higher tar content and decreased gas yield. Tar here means mainly furfurals and phenols. Other studies5 show that fur-

10.1021/ie0209430 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/11/2003

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Figure 1. Schematic representation of the decomposition of cellulose in supercritical water.

furals and phenols behave differently. The concentration of furfurals decreases faster with reaction time than that of phenols and decreases with temperature. The amount of phenols was found to increase with temperature.5 Therefore, phenols can be regarded as the “last hurdle” to get over for complete gasification. Furfurals are very important key compounds, e.g., to understand the effect of heating rate; concerning this, another “piece of puzzle” will be published soon.5 Working on the problems outlined above requires fundamental knowledge of the chemical decomposition of biomass. The main reaction pathways are known for model substances, e.g., for glucose as a substitute for the main constituent of the biomass, i.e., cellulose.6,7 These main reaction pathways are represented in Figure 1 in a simplified manner. It is expected that a rapid hydrolysis of cellulose to sugar units, e.g., glucose and fructose, takes place first. As consecutive reactions of this hydrolytic depolymerization, decomposition reactions to short-chain aldehydes and organic acids compete with dehydration and ring closure to furfural derivatives or phenols. The short-chain aldehydes and acids react to the gaseous products desired. The furfurals and phenols generated are also decomposed to gases under the conditions of biomass gasification in supercritical water.3 However, this decomposition takes place more slowly than that of glucose. Moreover, undesired tar and char/coke may be produced from these aromatic compounds. Some compounds of the substance classes listed in Figure 1 were determined quantitatively in the experiments described. As key substances, they provide information on which reaction paths are pursued to which extent during biomass decomposition. This paper focuses on phenol compounds as key compounds for a reaction pathway from biomass to aromatic compounds, which can be gasified or remain after reaction as tar. It has to be mentioned that phenols additionally can be formed from lignin;8 this is important for other kinds of biomasses such as wood. Experimental Setup The biomass used in the experiments was a finely chopped mixture of carrots and potatoes (Hipp Co.). From the chemical point of view, this biomass nearly exclusively consists of carbohydrates and possesses the chemical composition of CH1.87O0.98N0.02S0.001. The dry matter content of the initial biomass is 10.8 wt % (89.2 wt % water content). The ash content amounts to 6.2 g/kg, with the relevant content of potassium of 1241 mg/ kg (as K2O) having to be mentioned.

Figure 2. Schematic setup of the CSTR device. The boxer-type screw press (by Sitec) is filled from the storage vessels B1 and B2. At the same time, either the pneumatic valves V5 and V8 or V4 and V7 are open; therefore, one side of the screw press is filled when the other side presses biomass into the reactor. The control unit of the screw press controls these pneumatic valves. The upper volumes of storage vessels are filled with nitrogen to prevent air from coming in. The temperature of the reactor (by Premex) is controlled by a thermocouple inside the CSTR, which is stirred via magnetic coupling. After reaction the product mixture expanded to normal pressure via valve V12 is cooled by cooler K1, and phase separation occurs in vessel B5.

The CSTR system (Figure 2) consists of a boxer-type screw press for pressure generation and biomass supply into an autoclave equipped with a stirrer (190 mL internal volume, up to 700 °C and 100 MPa). The screw press provides a continuous supply of biomass into the reactor. When entering the reactor, the nonpreheated biomass is heated abruptly by backmixing with the reactor content. This helps to prevent the measured results from being falsified by extended heating, and it additionally prevents char formation (see point 2 in the Introduction). At the reactor outlet, a pressure control system with a pressure gauge and a pneumatically controlled valve is installed. After expansion, the product mixture discharged is cooled and separated into gas and liquid phases in a phase separator. Then, gas samples are taken, and the gas volume generated is measured using a wet gas meter. In addition, samples are taken from the liquid phase and analyzed with regard to its composition. In the outlet of the reactor and to a major extent in the sample vials, a precipitate is formed. This laboratory facility was run continuously at a constant temperature of 500 °C, while the pressures (30-50 MPa), residence times (1.5-10.5 min), and dry matter contents (1.8-5.4 wt %) were varied. The batch reactor, a tumbling reactor with an internal volume of 1 L, is described elsewhere.3 Here the diluted biomass (5 or 7.5 wt %) was filled into the reactor, and then the reactor was heated at a 1 or 3 K/min heating rate up to 500 °C. The pressure was around 30 MPa at

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this temperature. The temperature of 500 °C was hold for 1 h, and the reactor was cooled. Thereafter, the amounts of the gaseous, solved, and solid products were determined, and samples of every phase were taken and analyzed. The slow heating is generally a disadvantage of the experimental setup, but this batch reactor is usually used for studies on char/coke formation: here the long heating time is desired to form char/coke! Here only some analytical data collected during this study are mentioned. For details concerning the analysis of the gaseous, liquid, and solid phases, see ref 9. The gas composition was measured by gas chromatography (GC). The concentration of selected phenols was determined by solid-phase microextraction (SPME)GC [headspace extraction (60 °C, 35 min extraction time) using a polyacrylate SPME fiber (85 µm layer) before GC analysis]. The total organic carbon (TOC) content was measured by a TOC analyzer (Rosemount Dohrmann DC-190). The solid precipitate was dissolved in methanol and analyzed by GC-mass spectrometry. The content of every kind of phenol was determined colorimetrically (reaction of phenols with 4-nitroaniline to a yellow complex; cuvette test LCK 344, by LangeHach). The colorimetric measurements were conducted with the photometer Cadras 200 by Lange-Hach.

Figure 3. Carbon content in the gas phase and the aqueous phase relative to the carbon fed into the reactor as biomass, as a function of the reaction time and dry matter content (500 °C, 30 MPa, CSTR): f, carbon content in the aqueous effluent; /, carbon content in the gas and in the aqueous effluent. The deficit to 100% is the carbon content of the deposit of organic compounds, which are not solvable in ambient water but are solvable in methanol or tetrahydrofuran.

Results At the outlet of the CSTR, a gas phase, mainly consisting of H2 and CO2, and an aqueous phase, which includes various organic compounds soluted in water, were detected. In addition, a black precipitate was formed at the outlet and in the sample vials, which consists of organic compounds that are not solvable in ambient water. The amount of gas and aqueous solution could be determined quantitatively but not the amount of precipitate. The precipitate is solvable in methanol and tetrahydrofuran and consists of furfurals and phenols with large alkyl groups. The organic compounds found in the aqueous phase were identified as different types of aldehydes, carboxylic acids, phenols, and furfurals. The compounds chosen as key compounds and quantified are ethanol, methanol, formic acid, acetic acid, formaldehyde, acetic aldehyde, levulinic acid, cyclopenten-1-one, furfural, methylfurfural, m/p-cresol, o-cresol, and phenol. All of these compounds contribute to the TOC content in the aqueous solution. The amount of carbon fed into the reactor as biomass is converted to carbon in gaseous compounds such as CO2 and CH4, in substances solved in the aqueous effluent like, e.g., acids and simple phenols, and in compounds that are not solvable in ambient water, forming a precipitate. Figure 3 shows the percentage of the initial carbon input found in the gas and the aqueous effluent as a function of the dry matter content, and the residence time is given. At the lower dry matter content of 1.8 wt %, the carbon fraction in the liquid phase decreases significantly with the residence time. In the gas phase, it increases accordingly. At the highest dry matter content of 5.4 wt %, the carbon fraction of the gas phase is much higher, while the carbon fraction of the aqueous effluent is lower than that in the case of the low dry matter content. This decrease of the TOC content with the initial TOC content, or dry matter content, corresponds at the lowest reaction time roughly to a reaction order of 2 with respect to the TOC content. At the low dry matter content, the carbon content in

Figure 4. Proportion of the main gaseous products and CO in the product gas as a function of the dry matter content (500 °C, 5 min of reaction time, 30 MPa, CSTR).

the aqueous phase decreases distinctively with the reaction time, but at a high dry matter content, the carbon fraction of the aqueous effluent drops only a little bit with the reaction time and the carbon fraction in the gas does not change within the limits of measurement accuracy (about (10% relative). The carbon cannot be balanced 100% because of the formation of a black precipitate. It is not only the carbon fraction in the gas phase that depends on the dry matter content of the biomass but also the gas composition (Figure 4). In contrast to the experiments at lower temperatures,9 H2 is the main product, followed by CO2 and CH4. All other gaseous components, including CO, occur in very small amounts only. With increasing dry matter content, the CH4 fraction increases, while the H2 fraction drops (Figure 4). The yield of all gases, except CO increases with the dry matter content. In addition, the composition of the gas phase changes: At the dry matter content of 1.8 wt %, the yields are 28 mol of H2, 24 mol of CO2, 4.9 mol of CH4, and 0.32 mol of CO per kg of dry mass. At a dry matter content of 5.4 wt %, the yields are 33 mol of H2, 31 mol of CO2, 11 mol of CH4, and 0.28 mol of CO per kg of dry mass. The drastic relative increase of the methane yield at higher dry matter content leads to an increase of the methane fraction in the gas phase. To get information about the species contributing to the TOC content, different kinds of analysis methods were used (see the Experimental Setup section). Looking at Figure 3, it can be assumed that the yield of each component behaves according to the TOC content, which means that it decreases with the dry matter content and with the reaction time, while the change with the

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Figure 5. Yield of phenols (in grams of phenols per g of dry mass) as a function of the reaction time at 500 °C, dry mass contents of 1.8 and 5.4 wt %, and at 30 and 50 MPa in the CSTR.

Figure 6. Concentration of phenols as a function of the dry matter content at 30, 40, and 50 MPa (500 °C, ca. 8 min of reaction time, CSTR). Table 1. Concentration and Yield Range of Selected Compounds at 1.8 and 5.4 wt % Dry Matter Contenta (at 500 °C, in CSTR, different pressures and reaction times) 1.8 wt % dry matter content

5.4 wt % dry matter content

compound

concn (ppm)

yield (10-4 g/g of dry mass)

concn (ppm)

yield (10-4 g/g of dry mass)

phenol o-cresol m/p-cresol methylfurfural

12-32 7.9-14 12-29 2.4-4.0

6.6-18 4.4-7.9 6.6-16 1.3-2.2

32-67 18-28 26-44 0.88-2.3

5.9-12 3.3-5.2 4.9-8.1 0.16-0.42

a

Yields are given relative to the dry matter content.

reaction time is more distinctive at low dry matter content than at high dry matter content. This is true for a lot of key compounds that we have investigated but not for the sum of the phenols! Therefore, phenols are of special interest. Figure 5 shows the yield of phenols (determined colorimetrically; see the Experimental Section) at 30 and 50 MPa, as well as 1.8 and 5.4 wt % dry matter contents, as a function of the reaction time. Two observations have to be pointed out. At first the yield of phenols is higher at the 5.4 wt % dry matter content than at 1.8 wt %, and at second in nearly the whole experimental range the phenol yield increases or remains constant with the reaction time. Both facts are opposed to the behavior of the TOC content, which means that they are opposed to the behavior of the majority of compounds in the aqueous effluent. Figure 6 shows that the concentration of phenols increases drastically with the dry matter content. This corresponds roughly to a reaction order of 2 with respect to the initial TOC content (an exact calculation of the reaction order is difficult because of the black precipitate precipitating very slowly, which leads to a rather high scattering of the TOC content in the liquid phase) or dry matter content. No significant pressure dependence was found. Table 1 shows the concentration and yield ranges of selected compounds at 1.8 and 5.4 wt % dry matter content. The ranges are rather large because of the

dependence of concentration on the pressure and especially on the reaction time. Despite this, it is obvious that the concentration of phenol, o-cresol, and m/p-cresol increases with increasing dry matter content; the yield remains nearly constant. This means that the increase of phenols shown in Figures 5 and 6 corresponds to the formation of phenols, which include higher or several alkyl groups. The content of methylfurfural decreases with the dry matter content as the yield decreases. This is found for most of the intermediates. Other compounds such as furfural and cyclopenten-1-one possess very large concentration ranges; therefore, it is difficult to see trends, e.g., with respect to the dry matter content. The batch experiments lead to completely different results. At first in the batch reactor char/coke was formed. In other words, a black solid was found, which was not soluble in solvents such as water, methanol, acetone, and THF. It consists mainly of carbon. After the batch experiments, the TOC contents in the aqueous phase at 5 wt % dry matter content were 775 ppm (∼3.8% of the carbon input) and 898 ppm (∼4.5% of the carbon input) at a heating rate of 1 and 3 K/min, respectively. The experiments at 7.5 wt % dry matter content lead to an increased TOC contents of 1024 ppm (∼3.4% of the carbon input) and 1151 ppm (∼3.8% of the carbon input) at a heating rate of 1 and 3 K/min, respectively. Here the TOC content increases slightly with the initial dry matter content, which is in contrast with the results of the CSTR experiments. Also the yields of the different gases are different in the batch experiments compared with the results of the CSTR! Figure 7 shows the gas yields for the two different heating rates and the two different dry matter contents. Here the gas yield is nearly the same for both dry matter contents: a slight decrease because the decreased hydrogen yield is observed. The CH4 and CO2 yields are nearly identical for the four experiments, which means that the carbon contents in the gas phase are nearly identical. This is also in contrast with the experiments in the CSTR. Figure 8 shows the concentration and yield of phenols at the different heating rates and dry matter contents of the diluted biomass. The yield is given relative to the dry matter content. Here, in contrast to the CSTR experiments, the yield of phenols decreases with the dry matter content at both heating rates. The concentration of single phenols, here phenol, o-cresol and m/p-cresol, also increases or stays unchanged with increasing dry matter content at constant heating rate (experimental error). Discussion The experiments in the batch reactor and the CSTR lead to completely different results. First, the results of the CSTR are discussed because here a very short heating time guarantees that all products are formed at 500 °C. The thermodynamic calculations10 were done with glucose instead of biomass and predict complete conversion to gases and equilibrium gas compositions of 0.29 vol % CO, 0.86 vol % CH4, 33.4 vol % CO2, and 65.4 vol % H2 for 1.8 wt % dry matter content and 0.65 vol % CO, 10.0 vol % CH4, 36.3 vol % CO2, and 53.1 vol % H2 for 5.4 wt % dry matter content. So, the high H2 and CO2 contents and the small CO content in the gas phase found experimentally in the CSTR are in agreement with the thermodynamic calculations.3,13 Thermody-

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Figure 7. Yield of gases in the batch reactor at two different heating rates (1 and 3 K/min) up to 500 °C (1 h) and two different diluted biomass feedstocks (5 and 7.5 wt %).

Figure 8. Concentration and yield of phenols in the batch reactor for two different heating rates (1 and 3 K/min) up to 500 °C (1 h) and two different diluted biomass feedstocks (5 and 7.5 wt %). The yield is given as weight percent relative to the dry matter content.

namically, the high water excess leads to a preference of the formation of H2 and CO2 instead of CO. During the reaction, CO formed reacts with water via the water gas shift reaction to H2 and CO2. This reaction is catalyzed by a lot of compounds, in particular by alkali salts.3,11,12 Therefore, a very low CO content like that calculated in equilibrium can only be reached here because of the presence of alkali salts occurring in the biomass.3 The increase of the CH4 fraction and the

decrease of the H2 fraction in the gaseous phase with increasing dry matter content found experimentally are also in agreement with the thermodynamic calculation mentioned above. From the kinetic point of view, it is not clear via which reaction pathway the CH4 is formed. It can be a free-radical mechanism or hydrogenation of CO. The second reaction is only possible in the presence of a suitable catalyst. In every case the CH4 is in competition with H2 formation, depending on the temperature and water content. The formation of 1 mol of H2 and CO2 via water gas shift requires 1 mol of water; therefore, this reaction is thermodynamically, corresponding to the Le Chatelier principle, more preferred at higher water content. The formation of CH4 and CO2 from glucose or biomass needs, from a stoichiometric point of view, no water contribution. Therefore, the CH4 formation should be preferred at lower water content, which is found experimentally (see Figure 2). Concerning the liquid phase, the results of the experiments performed in the CSTR show that the content of phenol compounds increases while the TOC content decreases with increasing dry matter content. This means that the network of reaction steps, very simply shown in Figure 1, is shifted toward the phenol com-

Figure 9. Concentration of phenol, o-cresol, and m/p-cresol in the batch reactor for two different heating rates (1 and 3 K/min) up to 500 °C (1 h) and two different diluted biomass feedstocks (5 and 7.5 wt %).

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pounds. Not very much is known about the formation of aromatic compounds at these conditions. There are a lot of investigations of the degradation of glucose,6,7,13-20 cellulose,4,16,21-25 and cellobiose,26 from which we can learn about the chemistry under near-critical or supercritical water conditions. However, in the case of the formation of phenol compounds, these studies are not helpful because detailed analyses have not been done and/or the reaction conditions have been different. Most of these studies were conducted at lower temperatures (e.g., refs 6, 7, 15, and 16) and/or without inorganic compounds (e.g., refs 6, 13, and 16) occurring in biomass or with inorganic compounds usually not occurring in biomass (ZrO2 as the catalyst19). New investigations of glucose conversion in the presence of K2CO3 show that the gas yield is increased, the furfurals yield is decreased, and the phenols yield is increased by this additive.5 It is likely that the salts included in the biomass also influence the different reaction pathways. The phenols found here were gasified in supercritical water as well (see ref 3), but they can also be found as tar. Phenols are decomposed more slowly than other small, aliphatic compounds formed by degradation of cellulose (see Figure 1) and also slower than furfurals.5 The main differences between the experiments in the CSTR and the batch reactor are as follows: 1. Char/coke is formed in the batch reactor but not in the CSTR. 2. The gas yields in the batch reactor are much lower and show only slight decreases of the H2 yield with the dry matter content. The carbon content in the gas phase is not influenced by the dry matter content. In the CSTR the gas yield and relative carbon content in the gas phase increase with the dry matter content. 3. The TOC decreases with the dry matter content in the CSTR and increases in the batch reactor. 4. The yield of phenols increases drastically with the dry matter content in the CSTR and decreases in the batch reactor. The main differences of these two reactors are that the batch reactor heats very slowly compared with the CSTR and that in the CSTR products and fresh biomass are mixed. What are the consequences of these different characteristics of these two reactors? The fast heating in the CSTR avoids and the low heating in the batch reactor improves char/coke formation (see the Introduction section). One explanation could be that furfurals, which are formed by an ionic reaction pathway below the critical point of water, start to polymerize above the critical point because of the formation of a free radical (see ref 9). At high heating rates, less furfurals are formed and therefore less or no coke can be formed. In the CSTR the gas yield and the phenols yield increase with the dry matter content. The backmixing in the CSTR means that in every stage of the reaction (see Figure 1) a reactive form of hydrogen is present. This might be H2 in statu nascendi or potassium formate. Both might be formed by the water gas shift reaction catalyzed by potassium salts because the water gas shift reaction is assumed to progress via the formation of formate.3,11,12 This reactive form of hydrogen could influence different reactions pathways, e.g., by hydrogenation of double bonds or avoidance of bond formation via free radicals by saturation. This idea is supported by the observation that the addition of potassium salt to glucose also leads to an increased gas

yield and increased phenols yield.5 Up to now the knowledge concerning the chemistry is very poor, and therefore no detailed explanation is available. Conclusion In the CSTR no char/coke and no increased tar formation with increasing dry matter content are observed. Only the phenols yield increases with increases in the dry matter content. The reason may be the very fast heating and the backmixing, which leads to the presence of reactive hydrogen during every step of biomass degradation. Here the phenol formation is the last hurdle for complete conversion. The chemistry of phenol formation has to be studied in more detail in the future. Understanding the single reaction steps of biomass conversion might open the opportunity to optimize biomass gasification from a chemical point of view. Acknowledgment We thank J. Hops for her excellent work in doing the GC and colormetric measurements. Our special thanks go to F. Schu¨bel for building up reliable equipment, which made our measurements possible. We thank D. Hager for the electronic supply. The analysis of ions, acids, alcohols, and aldehydes done by S. Habicht and J. Scherwitzel is gratefully acknowledged. We thank the unknown reviewers for their helpful comments, which made this paper much more “readable”. Literature Cited (1) Antal, M. J., Jr.; Allen, S. G.; Schulman, D.; Xu, X.; Divilio, R. J. Biomass Gasification in Supercritical Water. Ind. Eng. Chem. Res. 2000, 39, 4040. (2) Schmieder, H.; Abeln, J.; Boukis, N.; Dinjus, E.; Kruse, A.; Kluth, M.; Petrich, G.; Sadri, E.; Schacht, M. Hydrothermal Gasification of Biomass and Organic Wastes. J. Supercrit. Fluids 2000, 17, 145. (3) Kruse, A.; Meier, D.; Rimbrecht, P.; Schacht, M. Gasification of Pyrocatechol in Supercritical Water in the Presence of Potassium Hydroxide. Ind. Eng. Chem. Res. 2000, 39, 4842. (4) Yoshida, T.; Matsumura, Y. Gasification of Cellulose, Xylan, and Lignin Mixtures in Supercritical Water. Ind. Eng. Chem. Res. 2001, 40, 5469. (5) Sınagˇ, A.; Kruse, A.; Schwarzkopf, V. Key compounds of the Hydropyrolysis of glucose in supercritical water in the presence of K2CO3. Ind. Eng. Chem. Res. 2003, 42, in press. (6) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K. Glucose and Fructose Decomposition in Subcritical and Supercritical Water: Detailed Reaction Pathway, Mechanisms, and Kinetics. Ind. Eng. Chem. Res. 1999, 38, 2888. (7) Luijkx, G. C. A. Hydrothermal Conversion of Carbohydrates and Related Compounds. Doctoral Thesis, Technical University of Delft, Delft, The Netherlands, 1994. (8) Burrows, R. D.; Elliott, D. C. Analysis of chemical intermediates from low-temperature steam gasification of biomass. Fuel 1984, 63, 4. (9) Kruse, A.; Gawlik, A. Biomass Conversion in Water at 330410 °C and 30-50 MPa. Identification of Key Compounds for Indicating Different Chemical Reaction Pathways. Ind. Eng. Chem. Res. 2003, 42, 267. (10) The calculations were done with the computer program Aspen Plus 11.1 from Aspen Technology, Inc. (1981-2001), using the Gibbs reactor module. The property/base method used for estimation of the equilibrium composition was UNIF-DMD (Unifac modified in Dortmund). The different estimation methods for single properties are combined as given for the UNIF-DMD property method by Aspen Plus. (11) Elliott, D. C.; Sealock, L. J. Aqueous Catalyst Systems for the Water-Gas Shift Reaction. 1. Comparative Catalyst Studies. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 426.

Ind. Eng. Chem. Res., Vol. 42, No. 16, 2003 3717 (12) Elliott, D. C.; Hallen, R. T.; Sealock, L. J. Aqueous Catalyst Systems for the Water-Gas Shift Reaction. 2.Mechanism of Basic Catalysis. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 431. (13) Yu, D.; Aihara, M.; Antal, M. J., Jr. Hydrogen Production by Steam Reforming Glucose in Supercritical Water. Energy Fuels 1993, 7, 574. (14) Kruse, A.; Abeln, J.; Boukis, N.; Dinjus, E.; Kluth, M.; Petrich, G.; Sadri, E.; Schacht, M.; Schmieder, H. Gasification of Biomass and Model Compounds in Hot Compressed Water. Wiss. Ber.sForschungszent. Karlsruhe 1999, 111. (15) Nelson, D. A.; Hallen, R. T.; Theander, O. Formation of Aromatic Compounds from Carbohydrates: Reaction of Xylose, Glucose, and Glucuronic Acid in Acidic Solution at 300 °C. In Pyrolysis Oils from Biomass: Producing, Analyzing, and Upgrading; Soltes, E. J., Milne, T. A., Eds.; ACS Symposium Series 376; American Chemical Society: Washington, DC, p 113. (16) Minowa, T.; Fang, Z.; Ogi, T.; Varhegyi, G. Decomposition of Cellulose and Glucose in Hot-Compressed Water under CatalystFree Conditions. J. Chem. Eng. Jpn. 1998, 31, 131. (17) Lee, I.-G.; Kim, M.-S.; Ihm, S.-K. Gasification of Glucose in Supercritical Water. Ind. Eng. Chem. Res. 2002, 41, 1182. (18) Modell, M. Gasification and liquefaction of forest products in supercritical water. In Fundamentals in Thermochemical Biomass Conversion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier Applied Science: London, U.K., 1985. (19) Watanabe, M.; Inomata, H.; Arai, K. Catalytic Hydrogen Generation from Biomass (Glucose and Cellulose) with ZrO2 in Supercritical Water. Biomass Bioenergy 2002, 22, 405.

(20) Amin, S.; Reid, R. C.; Modell, M. Reforming and Decomposition of Glucose in an Aqueous Phase. Am. Soc. Mech. Eng., [Pap.] 1975, 75-ENAs-21. (21) Zhen, F.; Ogi, T.; Minowa, T. Cellulose Decomposition in Hot-Compressed Water with Alkali or Nickel Catalyst. J. Supercrit. Fluids 1998, 13, 253. (22) Sasaki, M.; Fang, Z.; Adschiri, T.; Arai, K. Dissolution and Hydrolysis of Cellulose in Subcritical and Supercritical Water. Ind. Eng. Chem. Res. 2000, 39, 2883. (23) Decompositon of Cellulose by Continuous Near-Critical Water Reactions. Chin. J. Chem. Eng. 2000, 8, 321. (24) Schwald, W.; Bobleter, O. Hydrothermolysis of Cellulose under Static and Dynamic Conditions at High Temperatures. J. Carbohydr. Chem. 1989, 8, 565. (25) Saka, S.; Ueno, T. Chemical Conversion of Various Cellulose to Glucose and its Derivatives in Supercritical Water. Cellulose 1999, 6, 177. (26) Kabyemela, B. M.; Takigawa, M.; Adschiri, T.; Malaluan, R. M.; Arai, K. Mechanism and Kinetics of Cellobiose Decomposition in Sub- and Supercritical Water. Ind. Eng. Chem. Res. 1998, 37, 357.

Received for review November 26, 2002 Revised manuscript received May 22, 2003 Accepted June 3, 2003 IE0209430