Gasification of Model Compounds and Wood in Hot Compressed

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Gasification of Model Compounds and Wood in Hot Compressed Water Sascha R. A. Kersten,* Biljana Potic, Wolter Prins, and Wim P. M. Van Swaaij UniVersity of Twente, Faculty of Science and Technology, PO Box 217, 7500 AE Enschede, The Netherlands

In this paper, an experimental investigation is presented that concerns the gasification of glycerol, glucose, and pinewood in supercritical water. The batch experiments were performed in quartz capillary reactors with an internal diameter of only 1 mm. Because these quartz reactors are catalytically inert, the process could be studied in the absence of the interfering catalytic influence of a metal reactor wall, as used in all previous studies. The reaction space has been mapped by performing over 700 experiments in which the temperature, pressure, reaction time, and concentration of the feedstock were varied. The most important observations were that the pressure turned out to have no effect on the conversion and product yields, and that, noncatalytically, complete conversion to the gas phase is only possible for very diluted feedstock solutions (70 wt % moisture. Examples of wet waste streams include the following: vegetable, fruit and garden waste; waste streams from agricultural, food and beverage industries; manure; sewage sludge; and some household wastes. Seaweed and micro-algae are examples of cultivated wet biomass crops. According to estimates of the Energy research Centre of The Netherlands (ECN),4 5.3-12 million tons (dry matter) of wet streams are annually available in The Netherlands. This corresponds to 70160 PJ of energy (assuming 65% efficiency), which represents 2.3%-5.3% of the current Dutch annual energy consumption. This paper examines the conversion of wet streams into H2 and/or CH4 by means of gasification in hot compressed water. Wet biomass cannot be converted economically by traditional techniques such as pyrolysis, gasification, and combustion, because of the large amount of energy required for the evaporation of water. Partial conversion by anaerobic digestion is possible and already practiced for suitable feedstock materials.5 Gasification in supercritical water (SCWG) is a novel technology for the conversion of wet biomass and waste streams to hydrogen or methane-rich gas.6-13 Regarding the useful products of SCWG, hydrogen, and/or methane, it is interesting that the selectivity can be controlled by the process conditions and catalysis. Biomass-derived hydrogen could be applied in the future as a renewable feedstock for fuel cells. Methane from biomass may be attractive as a renewable substitute of natural gas (SNG). The product gases are available at high pressure, which is required for storage and transportation and in many end applications. The CO2 byproduct is almost pure and suitable for sequestration or fertilization in greenhouses. * To whom correspondence should be addressed. Tel.: +31 53 489 4430. Fax: +31 53 489 4738. E-mail: [email protected].

For proper process development of SCWG, data concerning the influence of the process conditions (temperature (T), pressure (P), reaction time (t), concentration (c)) on the gas yields and byproduct formation (char, liquid intermediates) are needed. Laboratory-scale results of SCWG experiments have been reported by several research groups.6,7,10,11,14-25 However, previously reported data are obtained in small metal (e.g., Inconel, Hastelloy, stainless steel) reactors, which, to a variable extent, exhibit catalytic activity.11,22,26 Because of this ill-defined catalytic activity, the reported data on SCWG are difficult to compare and interpret. In this work, it is shown that Inconel indeed does have catalytic influence on SCWG reactions. Results of noncatalytic SCWG experiments are presented. These results have been obtained by experimentation with inert quartz microreactors27 and are supposed to represent the plain (noncatalytic) performance of the process. Results of model compounds and pinewood are presented. The potential of heterogeneous catalysis for SCWG is discussed on the basis of experiments in which Ru/TiO2 catalyst was added to the quartz reactors. Experimental Section The investigation presented here is based on more than 700 experiments that have been conducted in the batch capillary reactors. In a previous paper from our group,27 this measurement technique is described in detail, including an error analysis. The technique uses quartz capillaries with an inner diameter (ID) of 1 mm, an outer diameter (OD) of 2 mm, and a length of 150 mm as batch reactors. An experiment starts by charging a capillary with a known amount of aqueous solution (biomass or a model compound suspended or dissolved in water). Hereafter, the capillary is sealed and rapidly heated in a hightemperature fluidized bed to the desired reaction temperature. The heating time of the capillary in the fluid bed is 5 s. In the heating trajectory, the pressure increases as a result of the evaporating water. By varying the amount of solution in the capillary, the final pressure is known and can be adjusted quite accurately, because the water vapor pressure predominantly determines the total pressure. The reaction time is defined as the time that the capillary has been immersed in the fluid bed, minus the heating time (5 s). In a standard experiment, a reaction time of 60 s has been applied, because this reaction time turned out to be sufficient to reach the maximum conversion at the

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lowest typical SCWG temperature under noncatalytic conditions (600 °C; see the section, “Effects of Reaction Time”). After a certain fixed time at a specific reaction temperature and pressure, the capillary is quenched to ambient conditions, which ensures that all reactions are stopped. Gas products are released in a metal sampling chamber (50 mL) by smashing the capillary with a hammer mechanism. Finally, the gas phase inside the capillary is analyzed, to determine the absolute amount and the composition. On the basis of the readings of a gas chromatograph, the total number of moles present in the sampling chamber, and the amount of feedstock, the mass balance (yields and conversion) is calculated. The aim of the present investigation is mapping of the operating window of noncatalytic SCWG. The following parameters were varied: temperature (400-800 °C), pressure (50-450 bar), and the concentration of organics in the feedstock (1-20 wt %). Different model compounds were used as feedstock (viz., glucose, glycerol, and cellulose). Besides these model compounds, pinewood also was used. Hereafter, every experimental data point presented in the figures shown is an average value that is based on at least six experimental data points (average standard deviation on the carbon conversion is 6%). The investigation is divided into two areas: noncatalytic gasification and catalytic gasification. For the catalytic investigation, ruthenium on TiO2 (Ru/TiO2) has been applied. Moreover, Inconel 625 powder has been used as an additive for identification and quantification of the catalytic influence of the metal wall of the reactors that have been reported in open literature. The carbon conversion, the product yields, and the cold-gas efficiency are chosen here as the main output parameters of the process. The carbon conversion (carbon efficiency) is defined as degree of conversion of carbon from biomass to permanent gases:

Xc )

∑i Nc,i Nc,feed

× 100

The product yield is defined as

Yi )

Ni Nfeed

In case of wood, the elemental composition has been normalized to C6HyOz (glucose C-basis). The cold-gas efficiency (LHV) is defined as

ηLHV )

∑i YiLHVi LHVfeed

As described previously, the capillary is charged in air, resulting in a certain amount of oxygen (air) being inevitably present in the capillary after the sealing. The amount of hydrogen lost due to oxidation by slipped-in oxygen can be estimated from the experimental results of low-concentration glucose solutions (1 wt %) processed at high temperature (800 °C). Typical product gas yields per mole of glucose of SCWG at 800 °C were as follows: 5.5 for CO2, 0.5 for CH4, 0 for CO, 0 for C2- and C3-components, and 6 for H2 (see Figure 4, which will be discussed later in this work). From a carbon balance under these conditions, it follows that carbon gasification is complete for this experiment:

5.5C (CO2) + 0.5C (CH4) ) 6C (in glucose) If the reaction would proceed via the pathway

C6H12O6 + 5H2O f νΗ2 + 5.5CO2 + 0.5CH4 the value of ν would be 10. However, for these low-concentration solutions, ν has been measured to be ( 6. This would mean that 4 mol of H2 per mol of glucose were oxidized by slippedin O2. In absolute numbers, this comes down to ca. 1 × 10-6 mol of H2. The maximum amount of O2 from air present in the capillaries is 9 × 10-7 mol. If we may assume that the O2 inclusion is the same in absolute value, the effect of slipped-in oxygen can be neglected for all other conditions investigated in this work. Catalytic Effects of Metal Reactors As mentioned earlier, all previously reported results on gasification in hot compressed water were obtained in metal reactors. Stainless steel, Inconel, Hastelloy, and corroded Hastelloy were used as construction materials. Both empty tubular reactors and stirred cells were used in the laboratory work that has been performed thus far. Results obtained in tubular reactors were reported mainly by Antal and coworkers.11,12,24 The diameter of the tubular reactors was in the range of 1.44 mm to 6.22 mm with a corresponding specific possible catalytic wall area of 2778 m2 per unit volume reactor to 643 m2 per unit volume reactor. Reynolds numbers were mostly in the range for laminar flow. Nevertheless, the flow in these reactors presumably approached plug flow. Because of the small reactor diameter, radial mixing by molecular diffusion was fast enough to compensate for mixing effects caused by the laminar flow velocity profile (the Peclet number always exceeded 50). Sinag et al.28 conducted SCWG experiments in a 190-mL metal autoclave that was equipped with a stirrer. In their experiments, a cold feed stream was continuously injected into the hot autoclave. Literature data already indicate that the reactor material has a significant effect on the gasification process. Yu et al.,11 for instance, reported results of gasification of glucose in the same experimental setup under identical conditions, but using other metals for the reactor. They found large differences between experiments that were conducted in Inconel, Hastelloy, and corroded Hastelloy reactors. For a test series using 0.6 M acetic acid, they observed 14% carbon conversion in the Inconel reactor, whereas in the corroded Hastelloy reactor, the carbon conversion was as high as 53%. In the present work, catalytically inert quartz capillaries have been used for the investigation of gasification in hot compressed water. These results are supposed to represent the plain performance of the process. Table 1 shows a comparison between glucose gasification tests in the quartz capillaries and in metal reactors (data from literature) for almost identical conditions. Clearly, the results obtained in the noncatalytic quartz tubes deviate from those obtained in metal reactors (see Table 1) in cases where the feedstock contains only a low amount (1.8 wt %) of glucose. Compared to Inconel reactors, the conversion of carbon from the feed to the gas phase is less in the quartz capillaries. Apparently, nickel in the Inconel reactor wall (there is ca. 60 wt % nickel in Inconel 625) catalyzes the glucose decomposition into gaseous components. Results that have been obtained in the stainless steel reactor are similar to those obtained in quartz capillaries, with respect to carbon conversion;

Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006 4171 Table 1. Comparison between SCWG Results Obtained in Inert Quartz Capillaries and Metal Reactorsa Glucose, Low Concentration (1.8 wt %) reactor temperature, T [°C] pressure, P [bar] Xc gas composition [mol %] H2 CO2 CO CH4 C2+ a

Glucose, High Concentration (10 wt %)

ref 6

ref 12

this work

SS 316 tubular 600 250 61

Inconel tubular 600 345 90

quartz batch 600 300 70

Hastelloy tubular 600 280 67.3

ref 8

quartz batch 600 300 69

this work

31.2 41.7 21.8 4.0 1.3

61.6 29 2 7.2 not available

13.3 20 53 6 7.4

43.1 28.2 9.7 11.6 7.4

11.7 8.9 60.5 12.9 6.0

Data from literature.

Figure 1. Effect of Inconel 625 on the carbon efficiency and product-gas yields. Inconel was added to the capillaries in powder form. A 5 wt % glycerol solution in water was used as feedstock. (T ) 600 °C, P ) (300 bar, t ) 60 s.)

however, the hydrogen production, as a result of the water-gas shift reaction, is significantly higher. Al data obtained in the quartz capillaries were compared with (almost) identical tests that have been reported in the literature. Based on this analysis, it can be concluded that the carbon conversion in the quartz capillaries is lower than that in Inconel and Hastelloy reactors. The difference in carbon conversion is high, up to 20%, for low-concentration feeds (for example, 3 wt %) and reduces to zero for solutions that contain >10 wt % of organics. Metal reactors always show more water-gas shift activity than the quartz capillaries, irrespective of the feedstock concentration. Generally, it can be concluded that, in metal reactors, carbon monoxide formation is low and H2 formation is high, compared to the quartz capillaries (see Table 1). This indicates that smallscale metal reactors have a tendency to promote the water-gas shift reaction. To show the catalytic effect of Inconel, Inconel powder (dp ) 100-200 µm) was added to the inert quartz capillaries. Tests were conducted at 600 °C with a 5 wt % glycerol solution. The results are presented in Figure 1. Although the mixing of the powder in the capillary was not ideal, resulting in some scatter in the results, the general trend of the carbon conversion and gas distribution can be observed. The carbon conversion and the yields of CO, CO2, CH4, and H2 increase due to the addition of Inconel powder to the capillary reactor (see Figure 1). Although it is impossible to relate the

amount of Inconel added to the capillaries to the catalytic activity of a metal reactor, it has been made clear that Inconel has a profound influence on the results of glucose gasification. It is clear from the data presented above that the metal laboratory reactors used in previous investigations exhibited a catalytic effect. Metal walls seem to have catalytic activity, with respect to the carbon conversion (total gas production) and the water-gas shift equilibrium. Because of this catalytic effect of the wall, which may also be influenced by corrosion, published data are difficult to compare and interpret. Besides, the catalytic activity of small laboratory equipment cannot be translated to large-scale reactors, which have a much smaller ratio of wall area to volume. As a consequence, reliable information on the chemistry and kinetics of biomass and waste gasification in hightemperature and high-pressure water is missing. The difficulties regarding the interpretation of laboratory-scale data become already clear when comparing the available laboratory data with the few pilot-plant experiments that have been conducted so far. In pilot-plant runs with the UT/BTG facility,30 less H2 and more CO production was observed, compared to the tests conducted in laboratory reactors made from the same material and under similar conditions (T, P, c, τ). This can be ascribed to the lower ratio of catalytic wall area to volume of the pilot facilities. The tubular reactor in the pilot facility of BTG/UT has a specific wall area of 286 m2 per m3 reactor, being ca. a factor of 7 lower than for the majority of the laboratory reactors (see above). Noncatalytic Gasification in Hot Compressed Water It has been demonstrated that tests in small laboratory reactors are obscured by undefined catalytic effects. In this section, results of gasification experiments in catalytically inert quartz capillaries that are supposed to represent the plain performance of SCWG will be presented. Effects of Temperature and Concentration. Decomposition in hot compressed water starts already at temperatures as low as 200 °C, producing mainly tarry products and char. Figure 2 shows a photograph of a capillary reactor after reaction. The reaction medium of 10 wt % glucose solution was exposed to 400 °C and 250 bar for a reaction time of 60 s. The photograph shows that, at such low temperatures, liquid and solid products are formed. Currently, the exact nature of these reaction products cannot be identified with the applied capillary technique. Figure 3 shows the effect of the reactor temperature and the concentration of organics in the feed on the carbon conversion.

Figure 2. Photograph of the content of a capillary reactor after low-temperature gasification. A 10 wt % glucose solution in water was used as feedstock. (T ) 400 °C, P ) (300 bar, t ) 60 s.)

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Figure 3. (a) Influence of the temperature and the concentration on the carbon efficiency for the glycerol, glucose, and pine wood. (b) Influence of the temperature on the gas production for high concentrated glucose and glycerol. (C2 and C3 components are not shown. P ) (300 bar, t ) 60 s.)

For the model compounds (glucose and glycerol), the conversion reaches a constant level beyond 650 °C. Further increasing the temperature up to 800 °C does not change the carbon conversion. Hence, the carbon conversion reaches a maximum value. This maximum conversion is dependent on the concentration of organic material in the feed: 1 wt % glucose in water can be converted completely, whereas the maximum carbon conversion of 17 wt % glucose is only 83%. This dependency of the conversion on the concentration was also found by Antal and co-workers.11 Pinewood has a much lower maximum conversion (ca. 40%) than the model compounds used. Below 650 °C, the carbon conversion is a strong function of the temperature. For glucose and glycerol the difference between 600 °C and 650 °C is, more or less, equivalent to 20% conversion. This result points toward the need for precise control and measurement of the reactor temperature to obtain well-defined experimental results. In that respect, the temperature gradients and peak temperatures, sometimes reported to be present in the reactors used to study SCWG,12 may have influenced earlier results significantly. This has also been discussed by the Antal et al.12 Glucose and glycerol show identical gasification behavior, with respect to the carbon conversion (see Figure 3a). This is rather surprising, given the fact that they are different chemical species: glucose is a sugar and glycerol is an alcohol. It may indicate that the main reactions of gasification in hot compressed water are thermal cracking reactions, just like in dry gasification. Also, in dry gasification, it has been found that the exact nature of the feedstock hardly influences the process performance.31 However, looking at the H2/CO ratio for high concentrations of glycerol and glucose at temperatures of 550, 600, 650, and 700 °C (see Figure 3b), a significant difference is observed. The production of hydrogen, mainly as a result of the watergas shift reaction, is at least 2 times higher in the case of glycerol. Apart from H2, CO, CO2, and CH4, some C2 (C2H4, C2H6) and C3 (C3H6, C3H8) components are produced (∼10%), with ethane being a major component. The carbon conversion of wood is very low, compared to that of glucose and glycerol, which can be ascribed by the presence of lignin in wood. Lignin itself is difficult to gasify and it has been observed that lignin blocks the conversion of wood’s other constituents: cellulose and hemi-cellulose.32,33 The effect of the reactor temperature on the product distribution is discussed based on the results of glucose solutions processed at 300 bar and a reaction time of 60 s, with a low (1 wt %), medium (10 wt %), and high (17 wt %) concentration

of glucose (see Figure 4). The main gas products of the reaction are carbon monoxide, carbon dioxide, hydrogen, methane, ethane, and propane. Along with the carbon conversion, the yield of CO2, H2, and CH4 increase as the temperature increases. Also, the CO yield initially increases, but above 700 °C, CO rapidly decreases at constant carbon conversion (see Figure 4c), indicating that, above this temperature, the water-gas shift reaction starts to proceed. C3 components are not present in the product gas at temperatures of >700 °C and the yield of C2 components strongly decreases at >700 °C. This could point to enhanced reforming of C2 components at >700 °C. The effect of the concentration on the yields can be summarized as follows. H2 and CO2 increase with decreasing concentration while CO, CH4, and C2 decrease. Figure 5 shows that, over the entire temperature range, the measured yields do not resemble the yields predicted by chemical equilibrium. At 5 wt % organics. A high temperature of 800 °C is needed for enhanced water-gas shift activity to produce hydrogen, but equilibrium still is not reached. (c) For glucose solutions with a concentration of 1-20 wt %, a reaction time of 40 s is sufficient to reach the maximum conversion at 600 °C. The relative conversion rates are hardly dependent on the concentration of the feedstock solution, indicating an overall first-order reaction (d) Under identical conversion conditions, pinewood has a considerably lower maximal conversion than glucose and glycerol (e.g., 45% vs 83% at 700 °C). (e) Pressures in the range of 50-500 bar have no influence on the carbon conversion and the product distribution is not affected in the range of 50-450 bar. (f) Up to 700 °C, the product gas can be characterized as fuel gas that consists of CO, H2 CH4, and C2+ components. Above 700 °C, the product gas becomes less complex: at low concentration, an excess of H2 is produced; a medium concentration results in a mixture of H2 and CH4; and a high concentration produces mainly CH4. (3) The addition of K+ or Na+ cations to the reaction mixtures promotes the water-gas shift reaction, which leads to more H2 and less CO but does not increase the carbon conversion. (4) By adding a 3 wt % ruthenium on TiO2 (Ru/TiO2) catalyst, complete conversion of solutions with 1 to 17 wt % glucose is achieved. These experiments show the potential of heterogeneous catalysis. However, a catalyst for the process is still missing and the development of commercial catalysts should be considered as a necessary and essential step in the process development. Nomenclature LHVfeed ) lower heating value of feed dry and ash free [MJ/m3 or MJ/mol] LHVi ) lower heating value of product i [MJ/m3 or MJ/mol] c ) concentration of organics in the feedstock Ni ) number of moles of component i produced, mol Nfeed ) number of moles of feed, mol Nc,i ) number of carbon moles in component i produced, mol Nc,feed ) number of carbon moles in a feed Nproduct-gas ) number of moles permanent gas Nfeed ) number of carbon moles in feed, mol P ) reaction pressure [bar] t ) reaction time [s] T ) reaction temperature [°C] Xc ) carbon conversion [%] (efficiency)

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Yi ) yield of component i, mol/mol ηLHV ) cold gas efficiency Acknowledgment The Dutch government via NWO, SHELL, The European Commission and NEDO are gratefully acknowledged for their financial support and Pacific Northwest National Laboratory is acknowledged for providing the catalyst. Literature Cited (1) Derde energienota; Ministerie van Economische Zaken (MEZ): Den Haag, The Netherlands, 1995. (2) Duurzame energie in opmars, actieprogramma 1997-2000; Ministerie van Economische Zaken (MEZ): Den Haag, The Netherlands, 1997. (3) White Paper on Environmental Liability. European Commission, 2000. (4) Hemmes, K. Vergassing Van natte biomassa/reststromen in superkritiek water (SG), Voor de productie Van groen gas (SNG), SNG/H2smengsels, basis chemicalie¨n en puur H2; Energy research Centre of The Netherlands (ECN): Petten, The Netherlands, 2004. (5) Sarada, R.; Joseph, R. A Comparative Study of Single and Two Stage Processes for Methane Production from Tomato Processing Waste. Process Biochem. (Oxford, U.K.) 1996, 31, 337. (6) Hao, X. H.; Guo, L. J.; Mao, X.; Zhang, X. M.; Chen, X. J. Hydrogen Production from Glucose Used as a Model Compound of Biomass Gasified in Supercritical Water. Hydrogen Energy 2003, 28, 55. (7) Holgate, H. R.; Meyer, J. C.; Tester, W. J. Glucose Hydrolysis and Oxidation in Supercritical Water. AIChE J. 1995, 41, 637. (8) Lee, I.; Kim, M.-S.; Ihm, S.-K. Gasification of Glucose in Supercritical Water. Ind. Eng. Chem. Res. 2002, 41, 1182. (9) Matsumura, Y. Evaluation of Supercritical Water Gasification and Biomethanation for Wet Biomas Utilization in Japan. Energy ConVers. Manage. 2002, 43, 1301. (10) Modell, M. Gasification and Liquefaction of Forest Products in Supercritical Water. In Fundamentals of Thermochemical Biomass; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier Applied Science Publishers, Ltd.: London, 1985; p 95. (11) Yu, D.; Aihara, M.; Antal, M. J. Hydrogen Production by Steam Reforming Glucose in Supercritical Water. Energy Fuels 1993, 7, 574. (12) Antal, M. J.; Allen, S. G.; Schulman, D.; Xu, X.; Divilio, R. J. Biomass Gasification in Supercritical Water. Ind. Eng. Chem. Res. 2000, 39, 4040. (13) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; Van Swaaij, W. P. M.; Van de Beld, L.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Antal, M. J. Biomass Gasification in Near- and Supercritical Water: Status and Prospects. Biomass Bioenergy, in press. (14) Antal, M. J.; Manarungson, S.; Mok, W. S. Hydrogen Production by Steam Reforming Glucose in Supercritical Water. In AdVances in Thermochemical Biomass ConVersion; Bridgwater, A. V., Ed.; Blackie Academic and Professional: London, 1993; p 1367. (15) 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. (16) Kruse, A.; Abeln, J.; Dinjus, E.; Kluth, M.; Petrich, G.; Schacht, M.; Sadri, H.; Schmieder, H. Gasification of Biomass and Model Compounds in Hot Compressed Water. Presented at the International Meeting of the GVC-Fachausschuss “Hochdruckverfahrenstechnik”, Karlsruhe, Germany, 1999; Paper No. 107. (17) Kruse, A.; Gawlik, A. Biomass Conversion in Water at 330-410 °C and 30-50 MPa. Identification of Key Compounds for Indicating Different Chemical Reaction Pathways. Ind. Eng. Chem. Res. 2003, 42, 267. (18) Kruse, A.; Gawlik, A.; Henningsen, T. Biomass Liquefaction and Gasification in Near- and Supercritical Water: Key Compounds as a Tool to Understand Chemistry. Presented at the 4th International Symposium on High-Pressure Technology and Chemical Engineering, Venice, Italy, 2002. (19) Kruse, A.; Henningsen, T.; Sinag, A.; Pfeiffer, J. Biomass Gasification in Supercritical Water: Influence of the Dry Matter Content and the Formation of Phenols. Ind. Eng. Chem. Res. 2003, 42, 3711. (20) Kruse, A.; Meier, D.; Rimbrecht, P.; Schacht, E. Gasification of Pyrocatechol in Supercritical Water in the Presence of Potassium Hydroxide. Ind. Eng. Chem. Res. 2000, 39, 4842. (21) Schmeider, H.; Abeln, J.; Boukis, N.; Dinjus, E.; Kruse, A.; Kluth, M.; Petrich, G.; Sadri, E.; Schacht, M. Hydrothermal Gasification of Biomass and Organic Wastes. In Proceedings of the 5th Conference on Supercritical Fluids and Their Applications, Garda, Italy, 1999; p 347.

(22) Sinag, A.; Kruse, A.; Rathert, J. Influence of the Heating Rate and the Type of Catalyst on the Formation of Key Intermediates and on the Generation of Gases During Hydropyrolysis of Glucose in Supercritical Water in a Batch Reactor. Ind. Eng. Chem. Res. 2004, 43, 502. (23) Xu, X.; Antal, M. J., Jr. Gasification of Sewage Sludge and Organics in Supercritical Water. Presented at the 1997 AIChE Annual Meeting, 1997. (24) Xu, X.; Matsumura, Y.; Stenberg, J.; Antal, M. J. Carbon-Catalyzed Gasification of Organic Feedstocks in Supercritical Water. Ind. Eng. Chem. Res. 1996, 35, 2522. (25) Potic, B.; Van de Beld, L.; Asink, D.; Prins, W.; Van Swaaij, W. P. M. Gasification of Biomass in Supercritical Water. In Proceedings of the 12th European Conference and Exhibition on Biomass for Energy, Industry and Climate Protection; Palz, W., Spitzer, J., Maniatis, K., Kwant, K., Helm, P., Grassi, A., Eds.; ETA Florence, WIP Munich: Amsterdam, 2002; p 777. (26) Diem, V.; Boukis, N.; Habicht, W.; Hauer, E.; Dinjus, E. Reforming of Methanol in Supercritical WatersCatalysis by the Reactor Material. Presented at The Sixth Italian Conference on Chemical and Process Engineering, Pierucci, S., Pisa, Italy, 2003. (27) Potic, B.; Kersten, S. R. A.; Prins, W.; Van Swaaij, W. P. M. A High-throughput Screening Technique for Conversion in Hot Compressed Water. Ind. Eng. Chem. Res. 2004, 43, 4580. (28) Sinag, 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, 3516. (29) Potic, B.; Kersten, S. R. A.; Prins, W.; Assink, D.; Van de Beld, L.; Van Swaaij, W. P. M. Gasification of Biomass in Supercritical Water: Results of Micro and Pilot Scale Experiments. In Proceedings of the 2nd World Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection; Van Swaaij, W. P. M., Fjalistrom, T., Helm, P., Grassi, A., Eds.; ETA-Florence and WIP-Munich: Rome, Italy, 2004; p 742. (30) Van de Beld, L.; Wagenaar, B. M.; Assink, D.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M.; Penninger, J. M. L. Biomass and Waste Conversion in Supercritical Water for the Production of Renewable Hydrogen. Presented at the 1st European Hydrogen Energy Conference, Grenoble, France, 2003. (31) Van der Drift, A.; Van Doorn, J.; Vermeulen, J. W. Ten Residual Biomass Fuels for Circulating Fluidized-bed Gasification. Biomass Bioenergy 2001, 20, 45. (32) Yoshida, T.; Matsumura, Y. Gasification of Cellulose, Xylan and Lignin Mixtures in Supercritical Water. Ind. Eng. Chem. Res. 2001, 40, 5469. (33) Yoshida, T.; Oshima, Y.; Matsumura, Y. Gasification of Biomass Model compounds and Real Biomass in Supercritical Water. Biomass Bioenergy 2004, 26, 71. (34) Kyle, B. G. Chemical and Process Thermodynamics; Prentice Hall PTR: Englewood Cliffs, NJ, 1999. (35) Soave, G.; Barolo, M.; Bertucco, A. Estimation of High-Pressure Fugacity coefficient of Pure Gaseous Fluids by Modified SRK Equation of State. Fluid Phase Equilib. 1993, 91, 87. (36) Bertucco, A.; Barolo, M.; Soave, G. Estimation of Chemical Equilibria in High-Pressure Gaseous Systems by a Modified RedlichKwong-Soave Equation of State. Ind. Eng. Chem. Res. 1995, 34, 3159. (37) Soave, G. Equilibrium Constants from a Modified Redlich-Kwong Equation of State. Chem. Eng. Sci. 1972, 27, 1197. (38) National Institute of Standards and Technology (NIST), Gaithersburg, MD. (39) Kersten, S. R. A.; Prins, W.; Van Swaaij, W. P. M. Reactor Design Considerations for Biomass Gasification in Hot Compressed Water. In Proceedings of the 2nd World Conference and Technology Exhibition on Biomass for Energy, Industry and Climate Protection; Van Swaaij, W. P. M., Fjallstrom, T., Helm, P., Grassi, A., Eds.; ETA-Florence and WIPMunich: Rome, Italy, 2004; p 1062. (40) Schmieder, H.; Abeln, J.; Boukis, N.; Kruse, A.; Kluth, M.; Petrich, G.; Sadri, E.; Schacht, E. Hydrothermal Gasification of Biomass and Organic Wastes. J. Supercrit. Fluids 2000, 17, 145. (41) Elliott, D. C.; Sealock, L. J.; Baker, E. G. Chemical Processing in High-Pressure Aqueous Environment: 2. Development of Catalyst for Gasification. Ind. Eng. Chem. Res. 1993, 32, 1542. (42) Wang, J. A.; Cuan, A.; Salmones, J.; Nava, N.; Castillo, S.; MoranPineda, M.; Rojas, F. Studies of Sol-Gel TiO2 and Pt/TiO2 Catalysts for NO Reduction by CO in an Oxygen-rich Condition. Appl. Surf. Sci. 2004, 230, 94.

ReceiVed for reView August 18, 2005 ReVised manuscript receiVed January 30, 2006 Accepted February 9, 2006 IE0509490