Energy & Fuels 2006, 20, 2743-2747
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Selective Promotion of Catalytic Reactions during Biomass Gasification to Hydrogen R. Hashaikeh,† I. S. Butler,‡ and J. A. Kozinski*,† Energy & EnVironmental Research Laboratory, McGill UniVersity, 3610 UniVersity Street, Wong Building, Montreal, Quebec, Canada H3A 2B2, and Department of Chemistry, McGill UniVersity, Montreal, Quebec, Canada H3A 2B2 ReceiVed May 22, 2006. ReVised Manuscript ReceiVed September 10, 2006
Hydrothermal catalytic gasification of diluted (0.1 M) biomass (glucose) solution into hydrogen, methane, and carbon dioxide gases in hot, compressed water has previously been achieved. However, efficient gasification was limited to low concentrations, mainly because the glucose-water system was found to react in a homogeneous phase at a relatively moderate temperature (238-250 °C). These reactions yielded precipitates that blocked the catalyst surface. In order for this technology to be practical and economically feasible, high glucose concentrations, higher than 1 M, should be processed. The phase behavior of the concentrated glucose solution was studied in supercritical water using a diamond-anvil cell and a continuous-flow reactor. The homogeneous phase reactions were triggered by the presence of acidic media and heat released during the hydrothermal processes. Dehydration of glucose leading to the formation of 5-hydroxymethylfurfural (5-HMF) is suggested as the major step in the evolution mechanism of the homogeneous phase reactions. 5-HMF tends to polymerize forming precipitates of oligomers with a high degree of polymerization. A novel reactor design has been developed to promote selective biomass gasification to hydrogen while preventing undesired precipitation reactions.
Introduction Photosynthesis stores solar energy in the substance of plants (biomass) as chemical energy. This energy can be released by direct combustion, or it can be converted into several forms of energy products such as ethanol, methanol, and hydrogen. Biomass is a sustainable source of energy, and it does not contribute to greenhouse gas (GHG) emissions to the atmosphere. On a larger scale, current systems that utilize energy from biomass through combustion (14% of the world’s energy usage1) are based on burning the biomass in air to produce heat. This heat is then converted to mechanical energy Via heat engines. Existing biomass combustion energy systems exhibit low efficiency (∼20%).2 This low efficiency is a result of inefficient heat engine systems and the energy loss due to water content in the biomass (up to 60 wt %). In order to increase the thermal content in the biomass, one needs to convert biomass and gasify it into other forms of fuel (e.g., CH4, H2) without the necessity to go through drying. Recently, a hydrothermal gasification process of biomass via hot compressed water has seen intensive efforts in both basic and applied research. This research is driven by the increased global awareness and development toward establishing hydrogen economy. The hydrogen economy intends to replace current fossil fuels-based energy systems with hydrogen-based systems. Hydrogen will be the carrier of energy from other resources such as hydropower, wind, solar, and biomass. Hydrothermal catalytic gasification is seen as a promising way to selectiVely promote biomass gasification to hydrogen. Other than the ability * Corresponding author. Fax: (514)398-4492. Tel.: (514)931-1656. E-mail:
[email protected]. † Energy & Environmental Research Laboratory. ‡ Department of Chemistry. (1) McKendry, P. Bioresour. Technol., 2002, 83, 37-46. (2) Kheshgi, H.; Prince, R.; Marland, G. Annu. ReV. Energy EnViron. 2000, 25, 199-244.
to process wet biomass, without the necessity of involving an energy-extensive drying process, the main advantage of hydrothermal processing is its low gasification temperature, which significantly reduces CO concentration within the hydrogen stream. This feature is particularly useful in fuel cell systems since the anode of currently used hydrogen fuel cells has low tolerance for CO. Yu et al.3 have reported complete gasification of a 0.1 M glucose solution in supercritical water at a temperature of 600 °C and pressure of 345 bar (34.5 MPa). They found that treating concentrated glucose solutions resulted in decreased gasification efficiency. In another study, Xu et al.4 successfully achieved 98% gasification efficiency of a 1 M glucose solution in supercritical water at 600 °C and 345 bar using activated carbon as a catalyst. Yet, the gasification efficiency dropped to 51% at a lower temperature of 500 °C. It was later reported5 that the activated carbon used as a catalyst tends to be gasified at 600 °C and 345 bar operating conditions thus shortening its lifecycle. A breakthrough was achieved in 2002 by using noble metals, such as Pt, as a catalyst in subcritical water. Cortright et al.6 demonstrated that hydrogen could be produced from sugars at temperatures near 227 °C in an aqueous-phase reforming process using a platinum-based catalyst. Several biomass-derived materials that are soluble in water, such as ethylene glycol, have been used for the hydrogen generation process. This work6 highlighted that the treatment of a concentrated glucose solution resulted in homogeneous phase reactions that rendered the gasification process at such low temperatures. The use of a catalyst has improved the hydrothermal gasification process of glucose. Low gasification temperatures (3) Yu, D.; Aihara, M.; Antal, M. Energy Fuels 1993, 7, 574-577. (4) Xu, X.; Matsumura, Y.; Stenberg, J.; Antal, M. Ind. Eng. Chem. Res. 1996, 35, 2522-2530. (5) Matsumura, Y.; Xu, X.; Antal, M. Carbon 1997, 35, 819-824. (6) Cortright, R.; Davda, R.; Dumesic, J. Nature 2002, 418, 964-967.
10.1021/ef060233x CCC: $33.50 © 2006 American Chemical Society Published on Web 10/24/2006
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Figure 1. Schematic of the experimental setup used for biomass (glucose) gasification.
have been achieved. An intensive screening study for catalysts that are active in the hydrothermal treatment of organic compounds was undertaken by Elliott et al.7,8 They reported8 that, at operating conditions of 350 °C and 21 MPa, Ni and Ru catalysts are the most promising for effective gasification of the organic compounds tested. In order for the process of hydrothermal gasification of biomass to have practical meaning, it needs to be conducted with concentrated glucose solutions and at low operating temperatures. The above-mentioned studies indicated a decrease in gasification efficiency when using concentrated glucose solutions. This situation is due to the formation of char and tar, which usually leads to the reactor fouling3 or to deactivation of the catalyst. This paper interprets the results of an investigation of the gasification, reactivity, and phase behavior of glucose in hot compressed water. Several process scenarios have been studied including undesired precipitation reactions. New data are presented on the nature as well as the conditions for these reactions to occur. Data presented in this paper allowed for development of a new reactor design to selectively promote catalytic gasification of biomass to hydrogen. Experimental Section The hydrothermal technique requires creating a high-pressure water environment that operates at moderate temperatures (150400 °C). Within this environment, biomass is treated and converted to different products. Specialized equipment that was used to provide such a controlled environment was (1) a continuous-flow reactor (CFR) and (2) a hydrothermal diamond-anvil cell (DAC). Continuous-Flow Reactor (CFR). Gasification of glucose was performed in the process illustrated in Figure 1. Two different glucose concentrations (0.1 and 1 M) were studied. The solutions were pressurized to 100 bar (10 MPa) using a high-pressure HPLC pump (Waters Associates, model 510; flow rate 0.1-9.9 mL/min) and fed into a preheater. The glucose solution stream was preheated to 150 °C, and the temperature of the glucose solution outlet from the preheater was controlled using an on/off temperature controller coupled to an electrical heating tape. The CFR was made of a 316 SS (400 mm long tube, 9.5 mm o.d., 7.9 mm i.d.). The reactor was loaded with 5 g of Pt/Al2O3 catalyst pellets (0.5% Pt on 1/8 in. alumina pellets, Alfa Aesar). A type-K thermocouple was placed in the center of the catalytic tube reactor, and the temperature was controlled and maintained using a tubular electric furnace. A tap water cooling jacket was used to cool the fluid exiting from the reactor down to room temperature. The system pressure was controlled by adjusting the backpressure regulating valve located downstream the cooler. The feed flow rate was controlled at 3 mL/ min. The reactor design allowed for a multiple sample collection. The gas product composition was analyzed using an online GCTCD. Analyses of the liquid residues were conducted using standard (7) Elliott, D.; Sealock, L.; Baker, E. Ind. Eng. Chem. Res. 1993, 32, 1542-1548. (8) Elliott, D.; Sealock, L.; Baker, E. Ind. Eng. Chem. Res. 1994, 33, 558-565.
Figure 2. Carbon balance (TOCof solution - TOCof identified species in solution) for the liquid residue as compared to that of acetic acid.
HPLC, GC-TCD and GC-FID techniques. Analytical procedures and protocols are described in detail elsewhere.9-13 Diamond-Anvil Cell (DAC). The phase behavior of concentrated glucose solutions was also studied under supercritical water conditions using DAC. The experimental setup and procedures were identical to those described in previously published works.9,11-13 Typically, a 1 M glucose solution was loaded into the DAC chamber and pressurized between the two diamonds in the DAC, while the temperature was steadily raised. After the reaction was completed, solid residues were analyzed using FT-IR microscopy (UMA 500, Bio-Rad, Cambridge, MA).
Results and Discussion Glucose Gasification, Reactivity, and Phase Behavior. The continuous hydrothermal treatment of glucose (0.1 M) generated both gaseous and aqueous products. Three main gaseous products were H2, CO2, and CH4. The concentration of H2 produced at temperature of 290 °C was 44% (v/v). Optimum energy conversion efficiency (30%) was obtained at this temperature. Glucose that was not gasified was mainly converted to alcohols and carboxylic acids. The main component in the aqueous stream was acetic acid. When the carbon balance was determined (Figure 2), it was found that there are some carbohydrates that could not be quantified. GC-MS analysis of the liquid products revealed that a significant portion of the liquid residue (9) Hashaikeh, R.; Fang, Z.; Butler, I.; Kozinski, J. Proc. Combust. Inst. 2005, 30, 2231-2237. (10) Hashaikeh, R. Hydrothermal dissolution of biomass and gasification to hydrogen. Ph.D. Thesis, McGill University, 2005. (11) Fang, Z.; Kozinski, J. A. Proc. Combust. Inst. 2000, 28, 27172725. (12) Sobhy, A.; Fang, Z.; Xu, S.; Kozinski, J. Proc. Combust. Inst. 2002, 29, 2493-2500. (13) Fang, Z.; Xu, S.; Kozinski, J. Proc. Combust. Inst. 2002, 29, 24852492.
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Figure 3. DAC observations of the behavior of 1 M glucose solution in hot compressed water, heating rate 0.1 °C/s: (a) T ) 25 °C, (b) T ) 238 °C, (c) T ) 240 °C, (d) T ) 243 °C, (e) T ) 250 °C, (f) solid precipitate after opening DAC.
The chemistry of hydrothermal glucose reactions is rather complex. Most of the published studies agree that dehydration reactions of glucose occur in aqueous solutions producing various amounts of 5-HMF (C6H6O3) and other furan derivatives:14-16
Figure 4. FT-IR spectra of residues compared with the spectrum of glucose.
consisted of furan species; in fact, mainly 5-hydroxymethylfurfural (5-HMF), which is a product of glucose dehydration. When conducting the same gasification experiments using the continuous-flow process for a high concentration of glucose (1 M), gasification was not achieved. Even though, the gasification process seemed to run smoothly early on in the experiments, after a few minutes, the reactor was clogged. It was found that the glucose had undergone pyrolysis to form black deposits on the surface of the catalyst and in the interior of the tube reactor. Visual observation of the pyrolysis process of a concentrated glucose solution in the DAC is shown in Figure 3. The reaction in the DAC started at 238 °C, as indicated by the color change to yellow (Figure 3b). At 240 °C the solution color changes to brown (Figure 3c). At 243 °C, precipitates began to form (Figure 3d). As the temperature was increased, the precipitation rate increased significantly (Figure 3e). Some typical FT-IR spectra of the residues (Figure 3f) are shown in Figure 4. The IR spectral results indicate that glucose had metamorphed into species with a polymer-like character. The residue exhibited three major IR peaks at 698, 758, and 3084 cm-1 and additional peaks at 704, 758, 1495, 1602, 2914, and 3028 cm-1. There were no peaks in common between the residue and a standard spectrum of glucose. It is probable that decomposition of furan-based products, in particular 5-HMF, may have resulted in the formation of oligomers with a high degree of polymerization. The existence of these highly polymerized carbohydrates is apparent from the solid precipitates (Figure 3).
In our work, GC-MS analyses of the liquid products show that 5-HMF is the main component of the hydrothermal treatment of glucose. Srokol et al.15 have studied the hydrothermal degradation of glucose and other monosaccharides in subcritical water at 340 °C/27.5 MPa. They found that, as a result of an increase in the H+ concentration under subcritical conditions, acid-catalyzed reactions do occur. The reaction mechanism involves acid-catalyzed dehydration-cyclization of glucose leading to the formation of 5-HMF.16 The generation of these undesirable byproducts is considered to be the main obstacle in the hydrothermal gasification of biomass. These side reactions occur at temperatures close to the gasification temperature for glucose, which has been shown from previous work to be 250 °C.9 The schematic shown in Figure 5 illustrates possible pathways for hydrothermal treatment of glucose. It is derived from results obtained in CFR and DAC experiments. There are two competing reaction pathways that glucose tends to take: (i) the desired gasification to H2 on the surface of the catalyst (reaction 1) and (ii) the undesired precipitation within the voids of the closepacked catalyst bed (reaction 2):
C6H12O6 + 6H2O f 12H2 + 6CO2 gasification
(1)
C6H12O6 + 6H2O f precipitation
(2)
The gasification process of glucose requires glucose adsorption on the catalyst. This is the first and most important step. Decomposition of glucose to intermediates (e.g., acetic acid, methanol, propanol, propionic acid, and butyric acid9) is believed (14) Antal, M.; Moke, W. Carbohydr. Res. 1990, 199, 91-109. (15) Srokol, Z.; Bouche, A.; Estrik, A.; Strik, R.; Maschmeyer T.; Peters, J. Carbohydr. Res. 2004, 339, 1717-1726. (16) Gandini, A.; Belgacem, M. Prog. Polym. Sci. 1997, 22, 1203-1379.
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Figure 6. Schematic showing potential areas for gasification reactions to occur inside the tube reactor.
On the other hand, because of the acidic media and the dehydrating temperatures, which exist under hydrothermal conditions, acid-catalyzed reactions cause the formation of furfural and furfural derivatives. These are known for their sensitivity to resinification and polymerization mainly because of high susceptibility of the furan ring to electrophilic substitution.16 The precipitate formation observed during the hydrothermal treatment of aqueous glucose is caused mainly by 5-HMF polymerization and condensation reactions including branching and cross linking to form linear and cyclic oligomers. The trouble is that both processes (reactions 1 and 2) could occur at about the same temperature, and the challenge is to selectively promote the first reaction and suppress the second one. Such a problem could possibly be solved by an innovative reactor design. Cortright et al.6 have reported that the gasification efficiency of glucose is at a maximum when working at the saturation pressure of water at a temperature of 237 °C. It was suggested17 that, at the saturation point of water, water vapor dominates the gas-phase thus reducing partial pressures of H2 and CO2 in the reforming gas bubbles. This condition would favor shifting the equilibrium toward H2 production. A more reasonable explanation, however, may be that when working near the saturation pressure, the extent of water evaporation increases resulting in more and larger bubbles being produced in the aqueous media. The bubbles would occupy the void volume in the packed bed catalytic reactor. The bubbles would push the aqueous reactant toward the catalyst surface, which would
promote the more desired catalytic reactions that are responsible for gasification. Design of a New Reactor Promoting Desired Catalytic Reactions. A novel reactor has been designed that takes into account the problem mentioned above. This reactor provides catalytically active sites at the desired gasification temperature, thereby leading to the desirable surface reactions. It also minimizes the temperature in the void volume, thus preventing undesirable liquid-phase reactions. The new reactor takes advantage of the fluid dynamics of the flowing glucose solution stream creating a temperature profile that will ensure optimum gasification temperatures at the surface of the catalyst, while maintaining low temperatures (lower than the temperature reported for reaction 2) away from the surface of the catalyst. When fluids flow through a tube, they maintain a velocity profile across the tube. The velocity at the surface of the tube is zero. The fluid velocity profile evolves into a fully developed flow at the center of the tube. Therefore, the idea was to create a temperature profile across the tube by taking advantage of the specific velocity profile. Maintaining a constant temperature (a temperature that is equivalent to or higher than the desired gasification temperature) at the external surface of the tube would result in having a constant high temperature at the internal surface of the tube. The temperature of the flowing fluid will be lower than that of the surface. The temperature of the flowing fluid will rise with the length of the tube. Besides providing the desired temperature profile, when using small i.d. tubes, the residence time of the reactants in the void fraction would be reduced. If the interior of the tube reactor (high-temperature region) is coated with the desired catalyst material, the temperature distribution within the reactor volume will be as follows: high temperatures (optimum for gasification) will be present where the catalyst is available; low temperatures (optimum for preventing undesired precipitation reactions) will be present where there is no catalyst (Figure 6). This reactor design offers the capability to promote catalytic reactions and suppress undesired homogeneous phase reactions in the catalytic gasification process of glucose. At the present time, the experimental validation of this reactor is in progress. The authors have successfully developed an efficient coating technique to deposit Ru catalyst at the interior of a small i.d. tube reactor.18 Concentrated glucose solution (1 M) was tested for gasification in the developed Ru-coated reactor. The tube reactor was immersed in a fluidized sand bath (FSB, Omega FSB-3) (Figure 7). The sand bath was used to heat the tube reactor and maintain its surface temperature constant. Because the sand is fluidized, it exhibits a temperature differences no more than ( 1°C. The sand was heated and maintained at different temperatures in the range of 200-380 °C using a temperature controller. Pressure in the system was controlled at 100 bar (10 MPa) using the backpressure regulator. The flowing stream was cooled and depressurized. Liquid
(17) Davda, R.; Dumesic, J. Angew. Chem. Int. Ed. 2003, 42, 40684071.
(18) Hashaikeh, R.; Butler, I.; Kozinski, J. Thin Solid Films 2006, doi: 10.1016/j.tsf.2006.07.038.
Figure 5. Schematic showing reaction pathways (gasification and dehydration) for the hydrothermal treatment of glucose.
to be the main catalytic decomposition step. It leads to subsequent direct dehydrogenation producing hydrogen gas and acetic acid (path A, Figure 5). The produced acetic acid is then dehydrogenated (reaction 3) producing hydrogen gas and leaving behind carbon monoxide adsorbates at the surface of the catalyst (path B, Figure 5):
C2H4O2 f 2COad + 2H2
(3)
Finally, the gasification mechanism involves carbon monoxide gasification (reaction 4) generating hydrogen and carbon dioxide through the water-gas shift reaction (path C, Figure 5):
COad + H2O f CO2 + H2
(4)
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Figure 7. Experimental setup for processing concentrated glucose solution. Table 1. Conversion of 1 M Glucose Solution as a Function of Sand Bath Temperature. temperature (°C)
glucose conversion flow rate 6 mL/min
gasification efficiency
250 280 300
22% 49% 53%
0% 5% 10%
products were then collected for analysis. Liquid samples were analyzed for carbon and glucose concentration. It was observed that there was no blockage inside the tube. The undesired homogeneous phase reactions were avoided. Preliminary experiments show that glucose was gasified, converted, and decomposed to several liquid and gas products. Several products in the aqueous phase were detected (e.g., acetic acid, methanol, ethanol, and 5-HMF). Table 1 shows the conversion of 1 M glucose when treated in this reactor. Glucose conversion was tested at different reaction temperatures and different flow rates. High conversions were obtained as compared to feed concentration and size of the reactor. Temperature had significant effects on glucose conversion to 5-HMF. Gasification was obtained at 300 °C. A 10% carbon gasification efficiency was obtained. GC-TCD analysis of the gas stream showed that 58% CO2 and 42% CH4 were the main components. Ru showed no selectivity for hydrogen production from glucose at the tested conditions. The authors have plans to perform kinetic studies of the gasification process using this reactor. Data
will contribute to better understanding of the capacity of the process. Summary and Conclusions The glucose-water system exhibits homogeneous phase reactions at relatively moderate temperatures (238-250 °C). These reactions restrict the catalytic gasification process of glucose to hydrogen in hot, compressed water. They produce undesired products. Most of these products are precipitates that block and poison the catalyst surface. The undesired reactions are stimulated by the acidic media as well as by the heat released under hydrothermal conditions. Without the presence of catalysts, glucose undergoes dehydration leading chiefly to formation of 5-HMF. Other products, such as acetic and propionic acids, were detected, as well. They contribute to the acidity of the reaction environment. The furan ring in 5-HMF has a high susceptibility to linking and branching within acidic media. This situation leads to the formation of oligomers with a high degree of polymerization, built mainly from the furan rings. As a result, condensation and precipitation reactions occur. In order for the catalytic gasification process to be practical on an industrial scale, the undesired precipitation reactions must be controlled and prevented. This challenge can be solved by an innovative reactor design, which is based on creating a desired temperature profile across the reactor. EF060233X
