Ind. Eng. Chem. Res. 2009, 48, 1435–1442
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Catalytic Gasification of Glucose over Ni/Activated Charcoal in Supercritical Water In-Gu Lee†,‡ and Son-Ki Ihm*,† Department of Chemical and Biomolecular Engineering, Korea AdVanced Institute of Science and Technology, 335 Gwahangno, Yuseong-gu, Daejeon, 305-701, Korea, and Bioenergy Research Center, Korea Institute of Energy Research, 102 Gajeong-ro, Yuseong-gu, Daejeon, 305-343, Korea
Gasification of glucose was carried out over the 16 wt % Ni/activated charcoal in supercritical water at 28 MPa. Effects of temperature (575-725 °C), feed concentration (0.3-0.9 M), and LHSV (6-24 h-1) were investigated on the product distribution. For comparison, the gasification experiments were performed with activated charcoal and also without catalyst. While the Ni/activated charcoal catalyst showed a good yield of hydrogen, it was deactivated due to coke deposition especially at low temperatures below 650 °C and also due to sintering of nickel particles. 1. Introduction Supercritical water gasification (SCWG) is an attractive method for converting wet biomass1-8 or aqueous organic wastes9-12 completely into combustible gases without drying procedure as a pretreatment of feedstocks. An objective of SCWG operation is to produce hydrogen-rich gas while gasification at subcritical water temperatures (less than 374 °C) results in a product gas rich in methane.13 It is known that the formation of hydrogen predominates over that of methane at high temperature.14 For example, Antal and his co-workers1-3 achieved complete gasification of a variety of organic compounds including whole biomass such as water hyacinth, banana tree stem, sewage sludge, wood sawdust, and sugar cane bagasse in supercritical water under conditions with temperatures above 600 °C, pressures of 28-34.5 MPa, and reactor residence times of less than 1 min. Major products formed in their continuous flow system were hydrogen and carbon dioxide with clean water effluents. Current status of SCWG technology research and development is well summarized by Kruse14 and Matsummura et al.15 Amin et al.16 demonstrated experimentally that the supercritical water can suppress the char formation during the decomposition of glucose. Char is known to be a refractory byproduct formed in a significant amount during atmospheric steam gasification of biomass,17 or hydrothermal treatments of glucose16,18 and celluose18,19 in pressurized liquid-phase water (subcritical water) at temperatures up to 350 °C unless appropriate catalyst is used. The potential applicability of supercritical water to treat biomass or its components is mainly due to the unique thermophysical properties of supercritical water. The major properties of supercritical water, such as density, viscosity, dielectric constant, and hydrogen bonding are quite different from those of steam or liquid water.20,21 Supercritical water behaves like a nonpolar organic solvent under gasification conditions, and many kinds of organic compounds and gases are completely dissolved in supercritical water, resulting in a single phase.22 On the other hand, the solubility of inorganic compounds decreases dramatically in supercritical water.23 Furthermore, water in SCWG processes is not only a solvent * To whom correspondence should be addressed. Tel.: 042-350-3915. Fax: 042-350-3910. E-mail:
[email protected]. † Korea Advanced Institute of Science and Technology. ‡ Korea Institute of Energy Research.
but also a major reactant to give significant influences on gasification chemistry.3 Although supercritical water provides relatively nice environments for the gasification of organic compounds, both the extent of gasification and the composition of gaseous products are found to be very sensitive to reactor wall made of nickel composites.1 Furthermore, the composition of gaseous effluent from SCWG of glucose significantly depends on the reactant concentration1 and temperature.2,24,25 Those findings motivated studies of catalyst to obtain hydrogen-rich gas from the complete gasification of concentrated biomass in supercritical water. Although homogeneous catalysts including salts such as KOH and NaOH dissolved in water catalyzed SCWG to produce hydrogen-rich gas, many of the catalysts are known to cause corrosion of reactor wall made of nickel composite tubing.3,14 That is the reason why researchers try to develop heterogeneous catalysts for the SCWG reactions. Amin et al.16 found commercial nickel reforming catalysts to catalyze water gas shift reaction to increase the content of hydrogen product of glucose gasification in water at the critical temperature and pressure (374 °C, 22.1 MPa) for 1 h reaction time. Minowa and Fang19 reported that a reduced nickel catalyst enhanced not only the extent of gasification of cellulose in hot compressed water (subcritical water conditions) but also the water gas shift reaction. They also reported that methane was formed at the expense of hydrogen and carbon dioxide produced. Xu et al.2 demonstrated that activated carbons and charcoals made of biomass are quite stable in supercritical water and enhance carbon gasification efficiency in the SCWG of high concentrations of glucose at 600 °C and 34.5 MPa. However, the catalytic role of the activated carbons on water gas shift reaction was not clear since the reports on the results obtained for hydrogen yields were not consistent even under the identical reaction conditions. Elliott et al.26-28 investigated a variety of commercial catalysts and support materials in hot liquid water at 20 MPa and 350 °C for converting organic compounds to gaseous products rich in methane. They found nickel, ruthenium, and rhodium to be active metals for methane formation. Zirconia and R-alumina were identified to be stable supports in the subcritical water environment, but other forms of alumina reacted with water to form boehmite with significant loss of surface area and physical strength. Watanabe et al.29 found zirconia effective for increasing the hydrogen yield in the SCWG of both glucose and cellulose in a temperature range of 400-440
10.1021/ie8012456 CCC: $40.75 2009 American Chemical Society Published on Web 12/19/2008
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Figure 1. Schematic diagram of the SCWG system: 1, digital balance; 2, feed container; 3, high pressure pump; 4, tee; 5, annulus thermocouple; 6, heat exchanger; 7, heater; 8, packed-bed reactor; 9, furnace; 10, thermocouple; 11, filter; 12, pressure gauge; 13, back-pressure regulator; 14, gas-liquid separator; 15, gas sample outlet; 16, gas flow meter.
°C, but its catalytic activity was much lower than NaOH. Yosida et al.30 studied effect of the amount of nickel catalyst and composition of model biomass feedstock on the carbon gasification ratio in supercritical water at 400 °C and 25 MPa. Yanik et al.31 demonstrated that the effect of catalyst on SCWG varied with the type of biomass, and they also showed that iron based catalysts can catalyze gasification of biomass to produce hydrogen gas. Consequently, previous experimental studies have indicated that nickel catalysts supported on activated charcoal (AC) can play a catalytic role in the SCWG of concentrated biomass. In this work, a nickel/activated charcoal (Ni/AC) catalyst was formulated by an impregnation method and was tested for SCWG with a temperature range between 575-725 °C to investigate its catalytic ability for hydrogen production. Other operating parameters such as feed concentration and LHSV (feeding rate) were also examined. 2. Experimental Section 2.1. SCWG Experiments. A schematic of the apparatus used in this experiment is shown in Figure 1. The reactor was made of a Hastelloy C-276 tubing with 9.53 mm o.d., 6.22 mm i.d., and 670 mm functional length. Before starting the SCWG reaction, the reactor had been exposed to supercritical water conditions for more than 50 h to avoid any unexpected wall effect on the gasification. The glucose feed was introduced into the reactor by a high pressure pump (Waters model 515). The reactant was quickly heated to the reaction temperature by a coiled heater installed on the outer wall of the reactor entrance. The coiled heater was made by wrapping a long rod heater (4.6 mm diameter and 127 mm heated length) around the reactor’s outer wall. The reactor was maintained at a desired temperature by a furnace and another coiled heater at the outer wall of the reactor exit. The packed bed of heterogeneous catalysts (Ni/ AC and AC) in the entrance of the reactor seems to increase reactant heating rate rapidly enough to reach desired temperatures with a coiled heater and a furnace. Therefore, it was not necessary to employ other heating devices. However reactant heating rate was not satisfactory in the absence of catalysts, and an internal rod heater was employed in the entrance of the reactor to facilitate rapid heat-up of the reactant. More detail can be found in our earlier publication.25 Preliminary experiments also showed that the temperature difference between the reactor outer wall and the inside of the reactor was about 5 °C at the gasification temperature of 650 °C when the catalyst (Ni/ AC or AC) was packed, but it was over 10 °C without packing
of the catalysts. Nevertheless, reactor temperature was taken as the average of the axial temperatures measured with 6 type K thermocouples evenly distributed along the reactor axis. The deviation of the reactor temperatures from the average value was less than (10 °C. The axial temperature profiles were reported also in the earlier publication.25 A cooling water jacket was installed in front of the entrance heater to minimize preheating effect on reactant flow entering the reactor. An identical cooling water jacket was also installed at the exit of the reactor to cool down the product flow to ambient temperature. A back-pressure regulator (Tescom model 26-1762-24) was used to reduce the pressure of product flow from 28 MPa to atmosphere. The reactor effluent was disengaged into gas and liquid products in a gas-liquid separator. The flow rate of gas effluent was measured by a wet test meter (Sinagawa model W-NK-0.5A). Liquid product was collected at the bottom of the separator to measure its flow rate. For the catalytic experiments, sintered alumina or sand was first packed in the cold zone of the downstream of the reactor and then the catalyst was consecutively packed in the heating section (about 20 cm3) of the reactor. It was confirmed through initial glucose experiments over only sintered alumina or sand in the cold section of the reactor that they did not contribute to the gasification results at all. For typical experiments with a feeding rate of 240 g/h at 600 °C and 28 MPa, the Reynolds number was about 400 and the flow in the reactor was laminar. The estimated residence time was 26 s at the conditions in the absence of the catalysts. When the catalysts were packed in the reactor, the residence time might be significantly reduced. Glucose and all the reagents used for the analysis of liquid effluent were purchased from Junsei Chemical Co. The glucose reactant solution was prepared with distilled and deionized water. Sand was obtained from a local area, washed several times to remove contaminants, dried, and sieved to have a size distribution of 0.25-0.83 mm. Sintered alumina (Aldrich no. 34,275-0) was purchased and sieved to 0.25-0.83 mm before it was used as a support. AC (Sigma cat. no. C3014) in 0.42-0.83 mm size was used as a catalyst or a support for nickel catalyst. A 16 wt % Ni/AC catalyst was prepared by an incipient wetness method. Nickel (II) chloride hexahydrate (Kanto Chemical Co. cat. No. 28115-00) was used as a precursor. The catalysts were calcined with a nitrogen flow at 500 °C for 3 h, and reduced at 400 °C for more than 10 h under the flow of hydrogen gas. The calcination and reduction of the nickel were conducted just before every single gasification experiment. Gaseous products were analyzed with a gas chromatograph (Donam Instruments model DS 6200). A thermal conductivity detector was employed for the detection of hydrogen, carbon monoxide, carbon dioxide, and methane, and a flame ionization detector for light hydrocarbons. A 80/100 mesh carbosphere molecular sieve was used as column packings, and a mixture of 8% hydrogen in helium was used as a carrier gas. The glucose concentration was measured by dinitrosalicylic acid (DNS) method.32 The COD concentration was determined by the closed reflux titrimetric method,33 and the acidity was measured using a pH meter (Orion model 290A). 2.2. Characterization of Catalysts. Both fresh and spent catalysts (AC and Ni/AC) were characterized to obtain information on the composition, the pore size distribution and surface area, the metal dispersion, and the surface morphologies. Elemental composition was determined by an elemental analyzer (LECO Co. model CHN-100), and ash content, by a thermogravimeter (LECO Co. model TGA-701). The BET surface area,
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Figure 2. Effect of catalyst on carbon gasification efficiency during the SCWG of 0.6 M glucose at 650 °C, 28 MPa, and 12 h-1 LHSV.
pore volume, and pore size distribution were measured by N2 physical adsorption at a temperature of -196 °C using Micromeritics ASAP 2400. Metal dispersion of Ni/AC catalyst was determined by CO chemisorption at 35 °C using Micromeritics ASAP 2020C with stoichiometry factor of 1. Samples were first purged in He flow at 250 °C for 30 min followed by evacuation at 250 °C for 4 h and at 35 °C for 40 min before the CO chemisorption experiments. Surface of the catalysts were measured by SEM system (Hitachi model S-4700) using a cold emission electron source. 3. Results and Discussion 3.1. Catalytic Performance of SCWG. Figures 2 and 3 show the catalytic performances of the gasification of glucose in supercritical water at 650 °C, 28 MPa, and an LHSV of 12 h-1. In this work, LHSV was defined as the volumetric flow rate of the reactant at an ambient temperature divided by the reactor functional volume. The main components of the gaseous products formed were hydrogen, carbon monoxide, carbon dioxide, methane, and ethane. Ethylene, propane, and propylene were detected only in trace amounts. Figure 2 shows that the carbon gasification efficiency was satisfactory with the 16 wt % Ni/AC with almost complete gasification. Carbon gasification efficiency is defined as percentage of the amount of carbon present in gaseous phase divided by the amount of carbon in the glucose feed. During the gasification time up to 4.5 h, the carbon gasification efficiency without catalysts was about 81% and remained virtually constant while the carbon gasification efficiency with the heterogeneous catalysts decreased slightly with the time on stream. Figure 3a shows that Ni supported on AC can make effective catalyst for hydrogen production from glucose gasification in supercritical water. Hydrogen yield obtained with 16 wt % Ni/AC remained almost constant during the entire gasification time, demonstrating that AC is quite stable in supercritical water. Xu et al.2 also found that activated carbons and activated charcoals are relatively stable under SCWG conditions. The AC itself exerted significantly low catalytic activity for hydrogen production compared with the Ni/AC catalyst. Although the SCWG of glucose is found to proceed through a number of chemical reactions,16,24 hydrolysis of glucose and water gas shift reaction, together with methane steam reforming, are the most important reactions as far as hydrogen production is concerned. C6H12O6 + 6H2O f 6CO2 + 12H2
(1)
CO + H2O f CO2 + H2
(2)
CH4 + H2O f CO2 + H2
(3)
The high yields of hydrogen and carbon dioxide combined with very low yield of carbon monoxide and high yield of methane with the 16 wt % Ni/AC indicate that the reaction pathways (1) and/or (2) were favored while the reforming of methane to hydrogen through eq 3 was suppressed. 3.2. Effect of Temperature. Reaction temperature is one of the most sensitive parameters to convert glucose completely to a hydrogen-rich gas in supercritical water.2,24,25 The SCWG’s of the glucose with the 16 wt % Ni/AC catalyst were conducted over a temperature range from 575 to 725 °C. The reactions were also conducted without catalysts and also only with the AC at the same temperature range for comparison. The data point at each temperature in Figures 4-6 was taken as the average of 2-4 product samples which were obtained from the gasification run continued for about 4 h under the reaction conditions. The catalysts may be deactivated with time on stream and the degree of the deactivation can be varied with temperature. For example, carbon gasification efficiency with the Ni/ AC catalyst changed from 89 to 85% at 600 °C, from 98 to 93% at 650 °C, and 100 to 99% at 700 °C for about 4 h operating time. However the variation in the carbon gasification efficiency was not significant (standard deviation is less than (5%), and the deactivation of the Ni/AC catalyst was not significant during the gasification for 4 h. Therefore, the effect of time on stream for each data point was not included in Figures 4-6. Figure 4 shows that the 16 wt % Ni/AC system had a very similar catalytic activity to the AC for the carbon gasification efficiency over the temperature range examined. Virtually complete carbon gasification was realized at about 675 °C with the 16 wt % Ni/AC and AC catalysts, respectively. Figure 5a shows that hydrogen yield from the SCWG of glucose increased with temperature from 575 to 725 °C. The 16 wt % Ni/AC evidently catalyzed the reactions associated with hydrogen production. The hydrogen yield obtained with the 16 wt % Ni/AC was about 2 times higher than that only with the AC or without catalysts. The AC showed a little catalytic activity for hydrogen production at temperatures below 650 °C, but it did not catalyze hydrogen production pathway at temperatures above 700 °C. Meanwhile, Xu et al.2 observed that the content of hydrogen product from the SCWG of glucose depends on the source of AC. For example, during the SCWG of 1.2 M glucose at 600 °C and 34.5 MPa, they obtained hydrogen content of about 40 mol % with spruce wood charcoal and 28 mol % with coconut shell activated carbon. The AC tested in this work which was made from peat turned out to catalyze hydrogen production pathways to a negligible extent. Both the 16 wt % Ni/AC and the AC provided very low carbon monoxide yields compared with those obtained without catalysts as shown in Figure 5b. The carbon monoxide yield without catalysts increased rapidly with temperature up to 600 °C, had a maximum value at 650 °C, and decreased slowly with increasing temperature. This profile of carbon monoxide yield was quite different from that obtained with the catalysts (both 16 wt % Ni/AC and AC) in which carbon monoxide yield steadily decreased with temperature from 575 to 725 °C. A comparison of this result with other gas yields in Figure 5 suggests that carbon monoxide in the presence of the 16 wt % Ni/AC was consumed through rather different pathways from that with the AC. High hydrogen yield with low carbon monoxide yield in Figure 5a and b indicates that the 16 wt % Ni/AC catalyzed water gas shift reaction to produce hydrogen at all the temperatures examined. The Ni/AC catalyst also appeared to be active for methane formation to some extent as
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Figure 3. Effect of catalyst on gas yield during the SCWG of 0.6 M glucose at 650 °C, 28 MPa, and 12 h-1 LHSV.
Figure 4. Effect of temperature on carbon gasification efficiency obtained from the SCWG of 0.6 M glucose under different catalytic conditions at 28 MPa and 12 h-1 LHSV.
shown in Figure 5d. However, since the hydrogen formation rate was higher than the methane formation rate over the temperature range investigated, the methane does not seem to form at the expense of hydrogen. The methane formation could be activated by Ni metal, AC support, or their combination. Ni metal has been reported in an extensive literature to catalyze both hydrogen and methane formations under supercritical water conditions, and its activity depends on the specific reaction environments.13,15 Meanwhile high methane yield with low carbon monoxide and hydrogen yields demonstrates the AC to catalyze methanation pathway to a great extent through eq 4 especially at high temperatures above 650 °C. CO + 3H2 f CH4 + H2O
(4)
Significantly high yield of carbon dioxide with the AC in Figure 5c suggests that the methanation is not a unique catalyzed pathway but the reactions associated with carbon dioxide production such as water gas shift reaction and decarboxylation of liquid-phase intermediates may also be activated under the AC catalyst. A comparison of the gas yields in Figure 5 led us to conclude that the 16 wt % Ni/AC relatively catalyzes water gas shift reaction over the wide temperature range from 575 to
725 °C while the AC catalyzes methanation pathway particularly at temperatures above 650 °C. The concentration of hydrogen among the gaseous products with the 16 wt % Ni/AC catalyst ranged between 0.27 and 0.50 mol fractions over the temperature range investigated. Meanwhile, hydrogen concentrations were 0.20-0.34 with the AC and 0.18-0.39 without catalysts. The hydrogen contents observed without catalysts at temperatures below 675 °C in this work are consistent with our earlier work on gasification obtained in the similar reaction conditions, whereas the values at temperatures above 675 °C were significantly lower than those obtained from the earlier experiments.25 This comparison indicates that the reaction pathways related to hydrogen production are sensitively influenced by a small change in the reactor conditions at high temperatures. Yu et al.1 reported that the gasification of glucose is very sensitive to the change in the reactor wall made of Hastelloy C-276 which was the same material employed in this work. Tang and Kitagawa34 developed a thermodynamic model based on Peng-Robinson equation of state formulations and direct Gibbs free energy minimization and calculated equilibrium values for gaseous products of the SCWG of glucose. They compared our experimental data with the predicted values for H2, CO, CO2, and CH4. Experimental measurements obtained without catalysts were greatly deviated from the equilibrium. While, the composition of the gaseous products obtained with the 16 wt % Ni/AC catalyst was closer to the theoretical values, especially at temperatures above 650 °C. In all the catalytic conditions, ethane yield increased with temperature up to 675 °C with a maximum value of 0.3 mol/ mol and then decreased. The AC showed the highest ethane yields over almost entire temperature range investigated. The formation of ethylene, propane, and/or propylene was almost negligible in the SCWG of the glucose. The 16 wt % Ni/AC and the AC apparently enhanced the destruction of glucose and COD shown in Figure 6. At 650 °C and in the absence of catalyst, essentially complete decomposition of the glucose was realized while the COD destruction remained 86%. This observation indicates that a significant
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Figure 5. Effect of temperature on gas yield obtained from the SCWG of 0.6 M glucose under different catalytic conditions at 28 MPa and 12 h-1 LHSV.
Figure 6. Effect of temperature on glucose and COD destructions obtained from the SCWG of 0.6 M glucose under different catalytic conditions at 28 MPa and 12 h-1 LHSV.
fraction of the glucose reacted would remain associated with liquid-phase intermediates. Studying glucose hydrolysis in supercritical water at 425-600 °C and 24.6 MPa, Holgate et al.24 identified major liquid-phase intermediates including acetic acid, acetonylacetone, acetaldehyde, propenoic acid, furfural, and its derivatives. With the product identification and mechanistic analysis, they suggested that the gasification of glucose in supercritical water proceeds through three kinetic regimes: glucose undergoes rapid hydrolysis to liquid-phase organic intermediates, followed by the slower formation of small quantities of stable light gases including methane and ethylene, and further by the destruction of these trace light gases which ultimately determines the composition of gaseous products. This seemingly complex gasification of organic compounds in supercritical water at high temperatures is known to be conducted through free radical mechanisms, meanwhile polar or ionic reaction mechanisms are predominant in subcritical water.14 The catalysts (16 wt % Ni/AC and AC) lowered the temperature required to achieve the same COD destruction. Since glucose and its reaction intermediates were the unique materials which affect the COD of liquid effluents and almost no tar and char were detected in the experiments, the COD
Table 1. Effect of Feed Concentration on the Gasification of Glucose in Supercritical Water at 650 °C, 28 MPa, and 24 h-1 LHSV catalyst
AC
feed concentration (M)
0.3
H2 CO CO2 CH4 C2H6 C3H8 CGE (%) COD destruction (%)
1.69 0.86 2.40 0.99 0.23 0.03 83.7 91.0
0.6
16 wt % Ni/AC 0.9
0.3
0.6
0.9
2.82 0.19 3.48 1.17 0.21 0.04 90.5 96.0
2.45 0.29 3.24 1.11 0.18 0.03 86.4 93.0
2.00 0.42 2.57 0.96 0.17 0.02 74.6 93.0
gas yield (mol/mol) 0.88 1.20 1.94 0.67 0.11 0.02 72.9 81.0
0.69 1.26 1.39 0.51 0.09 0.02 60.1 81.0
destruction in Figure 6 represents the degree of destruction of organic compounds to gaseous products. The COD destruction appeared to be higher with the AC than with the 16 wt % Ni/ AC at temperatures below 600 °C but gave virtually the same values as the temperature increased. 3.3. Effect of Feed Concentration and LHSV. Table 1 displays the effect of feed concentration on the yields of gaseous products from the SCWG of glucose over AC and Ni/AC
1440 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 Table 2. Change in Physical Properties of the AC and Ni/AC Catalysts after Used in the SCWG of Glucose
item/sample
carbon content (wt %)
ash (wt %)
BET surface area (m2/g)
total pore volume (cm3/g)
fresh AC fresh Ni/AC spent AC at 650 °C spent AC at 700 °C spent Ni/AC at 650 °C spent Ni/AC at 700 °C
93.0 73.3 95.9 94.9 88.7 80.9
5.8 25.4 3.0 2.8 16.4 16.4
920 788 21 314 28 307
0.64 0.50 0.04 0.26 0.05 0.25
Table 3. Metal Dispersion of the Catalysts Used in the SCWG of the Glucose catalyst fresh Ni/AC spent Ni/AC at 700 °C
metallic surface metal crystallite area (m2/g sample) dispersion (%) size (nm) 32.5 10.4
30.5 9.8
3.3 10.4
catalysts. The reaction temperature and pressure were the same as those employed in Figure 3 but the LHSV increased from 12 to 24 h-1. Hydrogen yield decreased with increasing temperature in case of both AC and Ni/AC, and carbon dioxide and methane showed similar yield trends. Table 1 demonstrates again that the Ni/AC was effective to reduce the content of carbon monoxide among gaseous products not only by catalyzing water-gas shift reaction but also by methanation to some
extent. Carbon gasification efficiency was found to be sensitive to feed concentration in case of both AC and Ni/AC. LHSV values ranging from 6 to 24 h-1 did not give significant influence on the gaseous product distribution obtained from the glucose SCWG over the Ni/AC catalyst. The yields of hydrogen and carbon monoxide had a trend to increase slightly with increasing LHSV from 12 to 24 h-1 while the yields of other gaseous components remained constant over the same LHSV range. 3.4. Characterization of Catalysts. The fresh AC employed in this work had relatively high ash content as displayed in Table 2. The ash can contribute to the catalytic activity during the SCWG reaction. The carbon content of both AC and 16 wt % Ni/AC catalysts increased, and their ash content decreased when the catalysts were used in the SCWG of the glucose. It should be noted that the carbon content formed at 650 °C was rather higher than that at 700 °C. Coke could be formed during the glucose gasification,16 contributing to increase in the carbon content. Table 2 also displays the change in BET surface area and total pore volume of the catalysts after used in the glucose SCWG. The fresh AC used had a smaller BET surface area (∼900 m2/g) than typical activated carbons (>1500 m2/g), and this small surface area led to the relatively large pore diameter of 2.78 nm. The catalysts with the mesosize pores (2 < size < 50 nm) may exert greater catalytic ability than catalysts with
Figure 7. Nitrogen adsorption isotherms of the AC and Ni/AC catalysts before and after use.
Figure 8. SEM images of the fresh and spent catalysts: (a) fresh AC; (b) spent AC at 650 °C; (c) spent AC at 700 °C; (d) fresh 16 wt % Ni/AC; (e) spent 16 wt % Ni/AC at 650 °C; (f) spent 16 wt % Ni/AC at 700 °C.
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microsize pores (size < 2 nm) in the SCWG of glucose because mass transfer limitations can occur when bulky molecules such as glucose pass through small pores of the catalyst. The fresh 16 wt % Ni/AC catalyst had similar BET surface area and total pore volume with the fresh AC. However, the spent AC and the spent 16 wt % Ni/AC show quite different properties. BET surface area significantly decreased after the catalysts were used in the SCWG experiments. Antal et al.3 also observed significant reduction in BET surface area of the coconut activated carbon after it was used as a catalyst in the SCWG of biomass. It is noted that BET surface area of used catalysts (both AC and 16 wt % Ni/AC) in Table 2 was larger when it was used at higher temperatures (700 °C). Higher temperatures also gave larger pore volume. These measurements strongly suggest that coke was formed and deposited in small pores of the catalysts during the SCWG of glucose. The deposition occurred more significantly at lower temperatures (650 °C). Physical adsorption isotherms in Figure 7 show that the fresh AC and the fresh 16 wt % Ni/AC has a “type 4” physical adsorption isotherm on the BET classification, which is encountered with materials having pores in the mesosize range.35 The spent catalysts (both AC and Ni/AC) show similar adsorption isotherms with significant reduction in the pore volume. The reduction in pore volume for AC used at 650 °C was much more significant than that of AC used at 700 °C, obviously due to the low temperature coke deposition. The same results were observed with the Ni/AC. One of the critical parameters which affect the activity of metallic catalysts loaded on the surface of solid-form materials is the state of the metal dispersion. Table 3 displays metal dispersion of the fresh and spent 16 wt % Ni/AC catalyst which was exposed to the gasification conditions for 4-5 h at 700 °C. Sintering due to the crystallite growth of the nickel occurred obviously during the glucose SCWG, which resulted in the reduction of metallic surface area and dispersion of the catalysts. Elliott et al.26-28 also observed crystallite growth of nickel catalysts during the gasification of organic compounds in highpressure aqueous environments at 350 °C. Figure 8 shows SEM images for the surface of the AC and Ni/AC catalysts at a magnification of 3000. The AC showed little change in surface morphology after used in the SCWG gasification experiment, demonstrating again that the AC is a stable material in supercritical water. The nickel particles of the catalyst spent at 650 °C are bigger than those of the fresh one, indicating that crystallization of the nickel particles occurred during the glucose SCWG experiment. 4. Conclusions Catalytic SCWG of 0.6 M glucose was investigated with 16 wt % Ni/AC catalyst over a temperature range of 575 to 725 °C at 28 MPa. The SCWG of glucose was also carried out with AC and without catalysts for comparison. The Ni/AC was found to catalyze hydrogen production pathways such as water gas shift reaction and enhance the carbon gasification efficiency. The Ni/AC catalyst was relatively stable in supercritical water. The AC showed some catalytic activity for methanation reaction at temperatures above 650 °C. As the reaction temperature increased, the measured hydrogen yield became closer to the value predicted by a thermodynamic computer program based on Peng-Robinson equation of state formulations and direct Gibbs free energy minimization. Hydrogen yield and carbon gasification efficiency obtained with the Ni/AC catalyst was found to decrease with increasing feed concentration while they remained almost constant with variation in LHSV.
Characterization of the spent catalysts found that the AC contained a significant amount of ash and had an average pore diameter in the mesosize range. Coke deposition occurred in small pores of the AC especially at temperatures below 700 °C, leading to the loss of surface area and small-sized pores. Crystallite growth was also observed for the Ni/AC used over the temperature range examined. Those changes may lead to the significant reduction in active sites of the catalysts such that hydrogen yield from the SCWG of glucose can be decreased in a long-term operation. Acknowledgment This work was financially supported by Ministry of Environment, Republic of Korea, through KIEST. Literature Cited (1) Yu, D.; Aihara, M.; Antal, M. J., Jr. Hydrogen Production by Steam Reforming Glucose in Supercritical Water. Energy Fuels 1993, 7, 574. (2) Xu, X.; Matsumura, Y.; Stenberg, J.; Antal, M. J., Jr. CarbonCatalyzed Gasification of Organic Feedstocks in Supercritical Water. Ind. Eng. Chem. Res. 1996, 35, 2522. (3) Antal, M. J., Jr.; Allen, S. G.; Schulman, D.; Xu, X. Biomass Gasification in Supercritical Water. Ind. Eng. Chem. Res. 2000, 39, 4040– 4053. (4) Penninger, J. M. L.; Rep, M. Reforming of Aqueous Wood Pyrolysis Condensate in Supercritical Water. Int. J. Hydrogen Energy 2006, 31, 1597– 1606. (5) Lu, Y. J.; Guo, L. J.; Ji, C. M.; Zhang, X. M.; Hao, X. H.; Yan, Q. H. Hydrogen Production by Biomass Gasification in Supercritical Water: A Parametric Study. Int. J. Hydrogen Energy 2006, 31, 822–831. (6) D’Jesu´s, P.; Boukis, N.; Czarnetzki, B. K.; Dinjus, E. Gasification of Corn and Clover Grass in Supercritical Water. Fuel 2006, 85, 1032– 1038. (7) Yanik, J.; Ebale, S.; Kruse, A.; Saglam, M.; Yu¨ksel, M. Biomass Gasification in Supercritical Water: Part 1. Effect of the Nature of Biomass. Fuel 2007, 86, 2410–2415. (8) Furusawa, T.; Sato, T.; Sugito, H.; Miura, Y.; Ishiyama, Y.; Sato, M.; Itoh, N.; Suzuki, N. Hydrogen Production from the Gasification of Lignin with Nickel Catalysts in Supercritical Water. Int. J. Hydrogen Energy 2007, 32, 699–704. (9) Blasi, C. D.; Branca, C.; Galgano, A.; Meier, D.; Brodzinski, I.; Malmros, O. Supercritical Gasification of Wastewater from Updraft Wood Gasifiers. Biomass Bioenergy 2007, 31, 802–811. (10) Yan, B.; Wei, C. H.; Hu, C. S.; Xie, C.; Wu, J. Z. Hydrogen Generation from Polyvinyl Alcohol-Contaminated Wastewater by a Process of Supercritical Water Gasification. J. EnViron. Sci. 2007, 19, 1424–1429. (11) Garcı´a Jarana, M. B.; Sa´nchez-Oneto, J.; Portela, J. R.; Nebot Sanz, E.; Martı´nez de la Ossa, E. J. Supercritical Water Gasification of Industrial Organic Wastes. J. Supercrit. Fluids 2008, 46, 329–334. (12) Sricharoenchaikul, V. Assessment of Black Liquor Gasification in Supercritical Water. Bioresour. Technol. 2008, xx. (13) Elliott, D. C. Catalytic Hydrothermal Gasification of Biomass. Biofuels Bioprod. Biorefin. 2008, 2, 254–265. (14) Kruse, A. Supercritical Water Gasification. Biofuels Bioprod. Bioref. 2008, 2, 415–437. (15) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; Swaaij, W. P. M. V.; Beld, B. V. D.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Antal, M. J., Jr. Biomass Gaisification in Near- and SuperCritical Water: Status and Prospects. Biomass Bioenergy 2005, 29, 269– 292. (16) Amin, S.; Reid, R. C.; Modell, M. Reforming and Decomposition of Glucose in an Aqueous Phase. Proceedings of The Intersociety Conference on EnVironmental Systems, San Francisco, CA, 1975; The American Society of Mechanical Engineers (ASME): New York, 1975;ASME Paper No. 75-ENAs-21, 1. (17) Herguido, J.; Corella, J.; Gonzalez-Saiz, J. Steam gasification of Lignocellulosic residues in a Fluidized Bed at a Small Pilot Scale. Effect of the Type of Feedstock. Ind. Eng. Chem. Res. 1992, 31, 1274.
1442 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 (18) Fang, Z.; Minowa, T.; Fang, C., Jr.; Inomata, H.; Kozinski, J. A. Catalytic Hydrothermal Gasification of Cellulose and Glucose. Int. J. Hydrogen Energy 2008, 33, 981–990. (19) Minowa, T.; Fang, Z. Hydrogen Production from Cellulose in Hot Compressed Water Using Reduced Nickel Catalyst: Production Distribution at Different Reaction Temperatures. J. Chem. Eng. Jpn. 1998, 31, 488– 491. (20) Shaw, R. W.; Brill, T. B.; Clifford, A. A.; Eckert, C. A.; Franck, E. U. Supercritical Water: A Medium for Chemistry. Chem. Eng. News 1991, (December), 26. (21) Haar, L.; Gallagher, J. S.; Kell, G. S. NBS/NRC Steam Tables: Thermodynamic and Transport Properties and Computer Programs for Vapor and Liquid States of Water in SI Units; Hemisphere Publishing Corp.: Washington, D.C., 1984. (22) Josephson, J. Supercritical Fluids. EnViron. Sci. Technol. 1982, 16, 548A. (23) Wofford, W. T.; Dell’Orco, P. C.; Gloyna, E. F. Solubility of Potassium Hydroxide and Potassium Phosphate in Supercritical Water. J. Chem. Eng. Data 1995, 40, 968. (24) Holgate, H. R.; Meyer, J. C.; Tester, J. W. Glucose Hydrolysis and Oxidation in Supercritical Water. AIChE J. 1995, 41, 637. (25) Lee, I. G.; Kim, M. S.; Ihm, S. K. Gasification of Glucose in Supercritical Water. Ind. Eng. Chem. Res. 2002, 41, 1182–1188. (26) Elliott, D. C.; Sealock, L. J., Jr.; Baker, E. G. Chemical Processing in High-Pressure Aqueous Environments. 2. Development of Catalysts for Gasificaiton. Ind. Eng. Chem. Res. 1993, 32, 1542–1548. (27) Elliott, D. C.; Sealock, L. J., Jr.; Baker, E. G. Chemical Processing in High-Pressure Aqueous Environments. 3. Batch Reactor Process Development Experiments for Organics Destruction. Ind. Eng. Chem. Res. 1994, 33, 558–565.
(28) Elliott, D. C.; Phelps, M. R.; Sealock, L. J., Jr.; Baker, E. G. Chemical Processing in High-Pressure Aqueous Environments. 4. Continuous-Flow Reactor Process Development Experiments for Organics Destruction. Ind. Eng. Chem. Res. 1994, 33, 566–574. (29) Watanabe, M.; Inomata, H.; Arai, K. Catalytic Hydrogen Generation from Biomass (Glucose and Cellulose) with ZrO2 in Supercritical Water. Biomass Bioenergy 2002, 22, 405–410. (30) Yosida, T.; Oshima, Y.; Matsumura, Y. Gasification of Biomass Model Compounds and Real Biomass in Supercritical Water. Biomass Bioenergy 2004, 26, 71–78. (31) Yanik, J.; Ebale, S.; Kruse, A.; Saglam, M.; Yu¨ksel, M. Biomass Gasification in Supercritical Water: Part II. Effect of Catalyst. Int. J. Hydrogen Energy 2008, 33, 4520–4526. (32) Ghose, T. K. Measurement of Cellulase Activities. Pure Appl. Chem. 1987, 59, 257. (33) Franson, M. A. H. Standard Methods for the Examination of Water and wastewater, 18th ed.; American Public Health Association: Washington, DC, 1992; Part 5220-C, pp 5-8. (34) Tang, H.; Kitagawa, K. Supercritical Water Gasification of Biomass: Thermodynamic Analysis with direct Gibbs Free Energy Minimization. Chem. Eng. J. 2005, 106, 261–267. (35) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice, 2nd ed.; McGraw-Hill: New York, 1993; p 133.
ReceiVed for reView August 14, 2008 ReVised manuscript receiVed October 21, 2008 Accepted November 5, 2008 IE8012456
