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Enhanced hydrogen production from biomass via the sulfur redox cycle under hydrothermal conditions
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 0 6 7 4 e1 0 6 8 2
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Enhanced hydrogen production from biomass via the sulfur redox cycle under hydrothermal conditions Putri Setiani, Javier Vilca´ez 1, Noriaki Watanabe*, Atsushi Kishita, Noriyoshi Tsuchiya Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aoba, Aramaki, Aoba-ku, Sendai 9808579, Japan
article info
abstract
Article history:
A new method of hydrogen production from biomass via a sulfur redox cycle at moderate
Received 14 April 2011
temperatures has been proposed. This method, which can utilize excess sulfur from
Received in revised form
hydrocarbon refining processes and waste or geothermal heat, consists of two half cycles:
31 May 2011
(1) hydrogen production from an aqueous alkaline solution at subcritical conditions of
Accepted 2 June 2011
water, where sulfide, HS and S2, acts as a reducing agent of water, and (2) sulfide
Available online 6 July 2011
regeneration under much milder conditions, with an organic compound derived from biomass acting as a reducing agent of polysulfide, Sn2, and sulfur oxyanion, SxOy2,
Keywords:
formed in the first half cycle. During a 60-min reaction of an aqueous sodium sulfide
Hydrogen production
solution, hydrogen production was observed at 280 C and corresponding saturated vapor
Biomass
pressures. Addition of D-glucose, C6H12O6, to the solution after hydrogen production at
Sulfur redox cycle
300 C resulted in sulfide regeneration at temperatures 60 C in the present 10-min
Hydrothermal condition
reaction. Moreover, hydrogen production from glucose via the sulfur redox cycle was
Glucose
demonstrated, where the hydrogen production and sulfide regeneration were conducted at 300 C and 105 C, respectively. Results indicated that hydrogen production from 1 mol glucose was greater than that by hydrothermal gasification of glucose at much higher temperatures up to 500 C. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen plays a significant role in present and future global energy demands. In the present energy infrastructure, which utilizes hydrocarbon products as a main energy source, hydrogen is required in refining processes such as hydrodesulfurization and hydroprocessing. Hydrodesulfurization typically includes procedures for the capture and removal of the resulting hydrogen sulfide gas, which is subsequently converted into elemental sulfur or sulfuric acid [1]. The production of excess sulfur is a significant problem [2], while hydrogen demand is increasing as crude oils are becoming
heavier [3e5]. On the other hand, regarding the future energy plan, utilization of hydrogen as an energy carrier has significant potential. Hydrogen facilitates the use of fuel cells in vehicles with no practical limit [6]. Additionally, hydrogen can be produced from diverse domestic resources, which is an important reason why hydrogen is such a promising energy carrier [7e9]. At present, many studies have been conducted to find effective methods to produce hydrogen from biomass, which is one of the largest and least expensive renewable resources in the world [10,11]. Hydrogen production from biomass under hydrothermal conditions has been widely reported. However, the method presently available for such
* Corresponding author. Tel./fax: þ81 22 795 7386. E-mail address: [email protected] (N. Watanabe). 1 Present address: Frontier Research Center for Energy and Resources (FRCER), School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 1138656, Japan. 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.012
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Fig. 1 e Hydrogen production from biomass via the sulfur redox cycle.
hydrogen production requires high temperatures and pressures at supercritical conditions of water to maximize hydrogen yield [12e19]. The present study describes hydrogen production from biomass via a sulfur redox cycle at moderate temperatures, which can utilize the excess sulfur and waste or geothermal heat (Fig. 1). In this system, hydrogen can be produced from an aqueous alkaline solution containing sulfide, HS and S2, at subcritical conditions of water (S2 is used hereafter to represent both anions) [20,21]. Sulfide having the lowest oxidation number of sulfur, 2, is a strong reducing agent. Additionally, at temperatures >150 C, water has weaker and less persistent hydrogen bonding [22], making the production of hydrogen from water easier. The reaction may be described by xS2 þ ð2x þ y 2ÞH2 O/Sx O2 y þ 2ðx 1ÞOH þ ðx þ y 1ÞH2 ;
sulfide, and sulfide regeneration using an organic reducing agent, are assumed to occur simultaneously. In contrast, in order to ensure a sulfur redox cycle, the new method must be a two-step process, which is essentially different from the existing method. The present study examined hydrogen production from an aqueous alkaline solution of sodium sulfide at 250e320 C and corresponding saturated vapor pressures, because the reaction has not been fully described previously [20,21]. The feasibility of sulfide regeneration by glucose also was evaluated at 60e80 C for the solution resulting from hydrogen production. Finally, hydrogen production from glucose via the sulfur redox cycle was demonstrated, and the efficiency of this new method was described by comparison with the conventional hydrothermal gasification of glucose.
(1)
where the sulfur oxyanion, SxOy2, may be thiosulfate, S2O32, sulfite, SO32, or sulfate, SO42. For hydrogen production to be sustainable, sulfide must be regenerated from sulfur oxyanion(s) by an organic reducing agent [23], such as glucose which can be derived from biomass [24e27], at much lower temperatures (and pressures) by the heat remaining after production of hydrogen. Thus, this new method can be considered to produce hydrogen from biomass. Glucose is the most common carbohydrate and can be classified as a monosaccharide, an aldohexose, and a reducing sugar. Using glucose at low temperatures is also appropriate because its decomposition (e.g., dehydration reaction) starts at 90 C [28]. It should be noted that a single-step hydrogen production from water using sulfur redox reactions has been reported previously [20], in which both hydrogen production using
Fig. 2 e Relation between hydrogen production and temperature.
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Fig. 3 e Electropherograms from CE analysis for (a) starting solution, and liquid samples at (b) 280 C, (c) 300 C, and (d) 320 C.
2.
Experimental
2.1.
Hydrogen production experiment
A total of 1.2 g of sodium sulfide nonahydrate (Na2S.9H2O, 98e102% assay by Wako Pure Chemical Industries, Ltd.) was dissolved in 120 mL distilled water. The solution containing sulfide was loaded into an autoclave (made of Hastelloy C-22 alloy) with an inner volume of 200 mL. The reactor was equipped with a stirrer, pressure gauge, temperature controller, and gas and liquid sampling valves. The temperature controller, consisting of a heater surrounding the reactor and a thermocouple inserted within the reactor, enabled accurate temperature control. The liquid sampling valve was
connected to a sampling tube located near the bottom of the reactor. Before heating was initiated, nitrogen gas was bubbled through the liquid sampling valve for 15 min to prevent the oxidation of sulfur by air. Heating with stirring was then started to increase the temperature to a predetermined value. Reaction temperatures were 250 C, 280 C, 300 C, and 320 C. After 60 min at the prescribed temperature, the reactor was cooled using a fan, and gas and liquid samples were collected.
2.2.
Sulfide regeneration experiment
The solution resulting after hydrogen production at 300 C was used for sulfide regeneration. A total of 90 mg of D-(þ)glucose powder (C6H12O6, 98e102% assay by Nacalai Tesque,
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Table 1 e Hydrogen production and concentration changes in sulfur-containing anions. Temperature ( C) 280 300 320
H2(mmol)
S2 (mmol/L)
S2O32 (mmol/L)
SO32 (mmol/L)
SO42 (mmol/L)
1.2 6.3 12.1
10.1 16.6 36.5
0.0 0.0 0.0
0.3 0.7 1.5
0.0 0.0 0.2
Inc.) was dissolved in 30 mL of the solution in a 50 mL glass beaker. A silicon rubber cap was placed on the beaker and the solution heated and stirred to increase the temperature up to a prescribed temperature. The magnetic stirrer, which measured the temperature of the liquid directly using a thermocouple, enabled accurate temperature control. Reaction temperatures were 60 C, 70 C, and 80 C. After 10 min at the prescribed temperature, a liquid sample was collected.
2.3.
Sulfur redox cycle experiment
The sulfur redox cycle experiment consisted of three half cycles of the first hydrogen production, sulfide regeneration, and second hydrogen production. The two half cycles of the first and second hydrogen productions were conducted at 300 C for 60 min, using the same experimental method described above. Only the starting solutions differed between the two experiments. The first starting solution was the same aqueous sodium sulfide solution described above, while the second was the solution after sulfide regeneration containing organic compound(s). The half cycle of sulfide regeneration was conducted using a modified method, which is discussed in Results and Discussion.
2.4.
Sample analysis
Capillary electrophoresis (CE G1600 AX, Agilent Technologies, Inc.) was used to analyze sulfur-containing anions (S2, S2O32, SO32, and SO42) in the liquid samples. The column was a fused silica capillary column (Agilent Technologies, Inc.), which had an effective length of 24.5 cm, total length of 33 cm, and an internal diameter of 50 mm. The buffer used was an inorganic anion buffer (Agilent Technologies, Inc.), with the pH adjusted to 11.2. Voltage was 18 kV, and temperature was 35 C. The UV wavelengths were 350/50 nm and 235/10 nm for the signal and the reference, respectively. Detailed information of this method has been described previously [29]. Standard solutions for quantification were prepared using distilled water, sodium sulfide nonahydrate (Na2S$9H2O, 98e102% assay by Wako Pure Chemical Industries, Ltd.), sodium thiosulfate pentahydrate (Na2S2O3$5H2O, 99% assay by Wako Pure Chemical Industries, Ltd.), sodium sulfite (Na2SO3, 97% assay by Wako Pure Chemical Industries, Ltd.), and sodium sulfate (Na2SO4, 99% assay by Wako Pure Chemical Industries, Ltd.). Gas chromatography with a thermal conductivity detector (GC-323/TCD, GL Sciences, Inc.) was used to analyze hydrogen in the gas samples. The column was a stainless steel Porapak Q (80/100 mesh) column (GL Sciences, Inc.), which had a length of 2 m, outer diameter of 3.2 mm, and internal diameter of 2.1 mm. Carrier gas was nitrogen, and temperatures used were 120 C, 60 C, and 100 C for the injection, column, and
detector, respectively. Standard hydrogen gas (99.99% purity, GL Sciences, Inc.) was used for quantification.
3.
Results and discussion
3.1.
Hydrogen production using sulfide
GC analysis detected hydrogen in the gas samples after hydrogen production at temperatures 280 C, where hydrogen production increased with temperature (Fig. 2). At 280 C, 300 C and 320 C, 1.2 mmol, 6.3 mmol, and 12.1 mmol hydrogen, respectively, were produced after 60 min. Since no hydrogen production was observed at 250 C, only the results at higher temperatures are discussed. The CE analysis (Fig. 3) showed that sulfide, S2, peak area decreased with increasing temperature. Initial sulfide concentration was 41.7 mmol/L; concentration changes were 10.1 mmol/L, 16.6 mmol/L, and 36.5 mmol/L at 280 C, 300 C, and 320 C, respectively. CE analysis also showed that only sulfite, SO32, was formed at 280 C and 300 C, whereas sulfate, SO42, also formed at 320 C. Concentration changes in sulfite were 0.3 mmol/L, 0.7 mmol/L, and 1.5 mmol/L at 280 C, 300 C, and 320 C, respectively, whereas the concentration change in sulfate was 0.2 mmol/L at 320 C. The amount and/or oxidation number of sulfur increased with temperature. The concentration changes in sulfide and sulfur oxyanions are summarized in Table 1, along with hydrogen production. As shown in Table 1, the mass balance of sulfur could not be confirmed, because of unknown peaks, and limitations of the analytical method which prevented the detection of polysulfide, Sn2, and polythionate, SxO62. Results from GC and CE analyses indicated hydrogen production was accompanied by sulfide consumption and sulfur oxyanion formation, which coincided well with the
Fig. 4 e Relation between H2/S2L ratio and temperature during hydrogen production.
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study hypothesis (Eq. (1)) based on the previous studies [20,21]. However, not all of the results could be explained by Eq. (1). If hydrogen was produced according to Eq. (1), the molar ratio of hydrogen production to sulfide consumption, H2/S2 (mol/ mol), must be at least 2 if the oxidation state of sulfur changes from S2 just to S2þ:
280 C indicated that hydrogen production could be explained by the following reaction:
2S2 þ 5H2 O/S2 O2 3 þ 2OH þ 4H2 ;
5S2 þ 8H2 O/S2 5 þ 9OH þ 4H2 :
(2)
and must be at most 4 if the oxidation state of sulfur changes from S2 to S6þ: S2 þ 4H2 O/SO2 4 þ 4H2 : 2
(3)
Calculating H2/S ratios using experimental results (Table 1) showed that the ratio at 280 C was less than the lower limit, although the ratios at 300 C and 320 C were between the lower and upper limits (Fig. 4). The small H2/S2 ratio at
nS2 þ 2ðn 1ÞH2 O/S2 n þ ð2n 1ÞOH þ ðn 1ÞH2 ;
(4)
where Sn2 is polysulfide with n ¼ 2e5 [30]. In this reaction, the H2/S2 ratio can be as great as 0.8 if n ¼ 5: (5)
2
Considering that the H2/S ratio is approximately unity at 280 C, the reaction described by Eq. (5) may be predominant at this temperature. However, the sulfite production indicated that the reaction described by Eq. (1) occurred at the same time. This type of simultaneous production of polysulfide and sulfur oxyanion under hydrothermal conditions has been described previously [31]. Thus, two types of hydrogen production occurred simultaneously, with the predominant reaction
Fig. 5 e Electropherograms from CE analysis for (a) starting solution, and the liquid samples at (b) 60 C, (c) 70 C, and (d) 80 C.
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Table 2 e Concentration changes of sulfur-containing anions during sulfide regeneration. Temperature ( C)
S2 (mmol/L)
S2O32 (mmol/L)
SO32 (mmol/L)
SO42 (mmol/L)
3.0 4.7 9.4
0.9 0.6 0.0
0.1 0.0 0.6
0.2 0.0 0.0
60 70 80
shifting from Eq. (4) to Eq. (1) with increasing temperature. In addition, the similar H2/S2 ratios at 300 C and 320 C might indicate that the difference in the reaction at these two temperatures was only the reaction rate. A H2/S2 ratio of approximately 3 at these temperatures indicated that the predominant reaction was accompanied by formation of trithionate, S3O62: 3S2 þ 10H2 O/S3 O2 6 þ 4OH þ 8H2 ;
(6)
where the H2/S2 ratio is 2.7.
3.2.
Sulfide regeneration using glucose
The CE analysis showed that sulfite, SO32, and unknown anions were formed during hydrogen production at 300 C, and remained after sulfide regeneration at every temperature (Fig. 5). Nevertheless, concentration of sulfide, S2, generally increased under all conditions tested (Table 2), demonstrating that sulfide regeneration using glucose as a reducing agent is possible at much lower temperatures than those required for hydrogen production. The change in sulfide concentration increased with temperature, and was 3.0 mmol/L, 4.7 mmol/L, and 9.4 mmol/L at 60 C, 70 C, and 80 C, respectively. Small increments in concentrations of sulfur oxyanions (S2O32, SO32, and SO42) may have been caused by sulfur oxidation by air due to use of a glass beaker. The changes in concentration of all the sulfur oxyanions were significantly smaller than those of sulfide. In addition, since unknown peaks remained after the experiment, the sulfide likely was regenerated mainly from undetected sulfur-containing anion(s) such as polysulfide, Sn2, and polythionate, SxO62. Although the reaction equation for sulfide regeneration could not be determined, regeneration of sulfide from sulfur oxyanions and/or polysulfides was possible when using glucose as a reducing agent. Defining the sulfide regeneration ratio as the increment in concentration (Table 2) over the concentration change in hydrogen production at 300 C (Table 1), the regeneration ratio was approximately 60%, even at 80 C (Fig. 6). However, the regeneration ratio tended to increase nonlinearly with temperature, suggesting a higher regeneration ratio may be possible by increasing reaction rate at temperatures >80 C. Fig. 6 shows an extrapolation of results at higher temperatures (dashed line), indicating complete regeneration at temperatures near 100 C.
that would allow the reaction rate of sulfur reduction to be increased with little or no glucose decomposition. Consequently, the following procedure was employed for sulfide regeneration during the sulfur redox cycle experiment. Glucose powder (0.3 wt.%) was added to the solution within the autoclave produced after the half cycle of the first hydrogen production. Nitrogen gas was bubbled through the solution for 15 min to prevent sulfur oxidation by air. Then, the solution was heated with stirring to increase the temperature up to 105 C. Upon reaching 105 C, cooling was initiated immediately, and a liquid sample was collected. The GC and CE analyses for the gas and liquid samples collected after the half cycle of the first hydrogen production provided a H2/S2 ratio of 2.7 mol/mol, which was similar to previous results shown in Fig. 4. The formation of sulfite, SO32, also was confirmed during hydrogen production (Fig. 7a and b). After this first hydrogen production, the half cycle of sulfide regeneration was conducted with the modified method described above. The CE analysis of the liquid sample after this half cycle provided a sulfide regeneration ratio of 89.7%. As expected, the regeneration ratio improved when the modified method was used. Assuming that all the added glucose was involved in the reaction, 0.9 mol sulfide was regenerated by 1 mol glucose. However, the peak area for sulfite and the unknown peak did not decrease (Fig. 7b and c), which explains the incomplete sulfide regeneration. Sulfide could be regenerated only from undetected sulfur-containing compound(s) such as polysulfide and polythionate. This unidentified sulfurcontaining compound was thought to be trithionate, S3O62, as expected by Eq. (6), because the following equation coincided well with the data that about 1 mol sulfide were regenerated by 1 mol glucose: 2 þC6 H10 O7 þ2C6 H10 O8 þ4Hþ þH2 O; S3 O2 6 þ3C6 H12 O6 /3S
3.3. Hydrogen production from glucose via the sulfur redox cycle Results from sulfide regeneration experiments indicated that the optimum temperature for sulfide regeneration was >80 C. However, decomposition (e.g., dehydration) of glucose can occur at temperatures >90 C [28]. Therefore, it was necessary to determine reaction conditions (e.g., temperature and time)
Fig. 6 e Relation between sulfide regeneration ratio and temperature during sulfide regeneration.
(7)
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Fig. 7 e Electropherograms from CE analysis for (a) starting solution, and liquid samples after (b) first hydrogen production, (c) sulfide regeneration, and (d) second hydrogen production.
where it is assumed that glucose is oxidized to common oxidized glucose derivatives; glucuronic, C6H10O7, and glucaric, C6H10O8, acids [32e34]. The half cycle of the second hydrogen production was conducted using the solution after sulfide regeneration. The GC and CE analyses for gas and liquid samples after this half cycle provided a H2/S2 ratio of 2.5 mol/mol, which was just slightly lower than the value obtained from the first hydrogen production. Additionally, the CE analysis for the liquid sample produced only peaks corresponding to the same anions observed in the first hydrogen
production (Fig. 7b and d). These results indicate that the same mechanism is acting in both the first and second hydrogen production even in the presence of organic compound(s). This finding is significant because the solution containing organic compounds can be used repeatedly for hydrogen production even if the organic compounds cannot be removed completely. The present study demonstrated a method of hydrogen production from biomass via the sulfur redox cycle at moderate temperatures. Results indicated that, in theory,
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2.7 mol hydrogen is produced by consuming 1 mol sulfide at approximately 300 C (Eq. (6)), and 1 mol sulfide is regenerated by consuming 1 mol glucose at approximately 100 C, i.e., with no additional energy (Eq. (7)). Thus, hydrogen production from glucose is 2.7 mol hydrogen per 1 mol glucose, and is represented by the following reaction derived from Eqs. 6 and 7: 3C6 H12 O6 þ 5H2 O/C6 H10 O7 þ 2C6 H10 O8 þ 8H2 :
(8)
In contrast, comparison of hydrogen production from glucose by hydrothermal gasification methods reported previously [35] indicated at most 1.6 mol hydrogen per 1 mol glucose even at much higher temperatures up to 500 C [36]. Thus, hydrogen production from glucose by the proposed method is significantly greater and uses much lower temperatures. Although some details remain unclear, the present study provides a method for enhanced (and sustainable) hydrogen production from biomass via the sulfur redox cycle under hydrothermal conditions.
4.
Conclusions
A new method of hydrogen production from biomass via a sulfur redox cycle has been proposed. This method, which can utilize excess sulfur from hydrocarbon refining processes and waste or geothermal heat, consists of two half cycles: (1) hydrogen production from an aqueous alkaline solution at subcritical conditions of water, where sulfide, HS and S2, acts as a reducing agent of water, and (2) sulfide regeneration under much milder conditions, with an organic compound derived from biomass acting as a reducing agent of polysulfide, Sn2, and sulfur oxyanion, SxOy2, formed in the first half cycle. The present study first examined hydrogen production from the aqueous sodium sulfide solution. During the 60-min reaction, hydrogen production was observed at 280 C and corresponding saturated vapor pressures. The feasibility of sulfide regeneration by D-glucose was then evaluated for the solution resulting from hydrogen production. Addition of D-glucose to the solution after hydrogen production at 300 C resulted in sulfide regeneration at temperatures 60 C in the present 10min reaction. Finally, hydrogen production from glucose via the sulfur redox cycle was demonstrated, where the hydrogen production and sulfide regeneration were conducted at 300 C and 105 C, respectively. Results indicated that hydrogen production from 1 mol glucose was greater than that by hydrothermal gasification of glucose at much higher temperatures up to 500 C. Although some details remain unclear, the present study provides a method for enhanced (and sustainable) hydrogen production from biomass via the sulfur redox cycle under hydrothermal conditions.
Acknowledgments This work was supported in part by Idemitsu Kosan Co., Ltd., and Japan Society for the Promotion of Science (JSPS), through a Grant-in-Aid for Challenging Exploratory Research, No. 23656556. The authors thank Mr. T. Fukunaga, Mr. T. Umeki, and Mr. Y. Kawashima of Idemitsu Kosan Co., Ltd. for their valuable discussions throughout this study.
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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Enhanced hydrogen production from biomass via the sulfur redox cycle under hydrothermal conditions Putri Setiani, Javier Vilca´ez 1, Noriaki Watanabe*, Atsushi Kishita, Noriyoshi Tsuchiya Graduate School of Environmental Studies, Tohoku University, 6-6-20 Aoba, Aramaki, Aoba-ku, Sendai 9808579, Japan
article info
abstract
Article history:
A new method of hydrogen production from biomass via a sulfur redox cycle at moderate
Received 14 April 2011
temperatures has been proposed. This method, which can utilize excess sulfur from
Received in revised form
hydrocarbon refining processes and waste or geothermal heat, consists of two half cycles:
31 May 2011
(1) hydrogen production from an aqueous alkaline solution at subcritical conditions of
Accepted 2 June 2011
water, where sulfide, HS and S2, acts as a reducing agent of water, and (2) sulfide
Available online 6 July 2011
regeneration under much milder conditions, with an organic compound derived from biomass acting as a reducing agent of polysulfide, Sn2, and sulfur oxyanion, SxOy2,
Keywords:
formed in the first half cycle. During a 60-min reaction of an aqueous sodium sulfide
Hydrogen production
solution, hydrogen production was observed at 280 C and corresponding saturated vapor
Biomass
pressures. Addition of D-glucose, C6H12O6, to the solution after hydrogen production at
Sulfur redox cycle
300 C resulted in sulfide regeneration at temperatures 60 C in the present 10-min
Hydrothermal condition
reaction. Moreover, hydrogen production from glucose via the sulfur redox cycle was
Glucose
demonstrated, where the hydrogen production and sulfide regeneration were conducted at 300 C and 105 C, respectively. Results indicated that hydrogen production from 1 mol glucose was greater than that by hydrothermal gasification of glucose at much higher temperatures up to 500 C. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen plays a significant role in present and future global energy demands. In the present energy infrastructure, which utilizes hydrocarbon products as a main energy source, hydrogen is required in refining processes such as hydrodesulfurization and hydroprocessing. Hydrodesulfurization typically includes procedures for the capture and removal of the resulting hydrogen sulfide gas, which is subsequently converted into elemental sulfur or sulfuric acid [1]. The production of excess sulfur is a significant problem [2], while hydrogen demand is increasing as crude oils are becoming
heavier [3e5]. On the other hand, regarding the future energy plan, utilization of hydrogen as an energy carrier has significant potential. Hydrogen facilitates the use of fuel cells in vehicles with no practical limit [6]. Additionally, hydrogen can be produced from diverse domestic resources, which is an important reason why hydrogen is such a promising energy carrier [7e9]. At present, many studies have been conducted to find effective methods to produce hydrogen from biomass, which is one of the largest and least expensive renewable resources in the world [10,11]. Hydrogen production from biomass under hydrothermal conditions has been widely reported. However, the method presently available for such
* Corresponding author. Tel./fax: þ81 22 795 7386. E-mail address: [email protected] (N. Watanabe). 1 Present address: Frontier Research Center for Energy and Resources (FRCER), School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 1138656, Japan. 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.012
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Fig. 1 e Hydrogen production from biomass via the sulfur redox cycle.
hydrogen production requires high temperatures and pressures at supercritical conditions of water to maximize hydrogen yield [12e19]. The present study describes hydrogen production from biomass via a sulfur redox cycle at moderate temperatures, which can utilize the excess sulfur and waste or geothermal heat (Fig. 1). In this system, hydrogen can be produced from an aqueous alkaline solution containing sulfide, HS and S2, at subcritical conditions of water (S2 is used hereafter to represent both anions) [20,21]. Sulfide having the lowest oxidation number of sulfur, 2, is a strong reducing agent. Additionally, at temperatures >150 C, water has weaker and less persistent hydrogen bonding [22], making the production of hydrogen from water easier. The reaction may be described by xS2 þ ð2x þ y 2ÞH2 O/Sx O2 y þ 2ðx 1ÞOH þ ðx þ y 1ÞH2 ;
sulfide, and sulfide regeneration using an organic reducing agent, are assumed to occur simultaneously. In contrast, in order to ensure a sulfur redox cycle, the new method must be a two-step process, which is essentially different from the existing method. The present study examined hydrogen production from an aqueous alkaline solution of sodium sulfide at 250e320 C and corresponding saturated vapor pressures, because the reaction has not been fully described previously [20,21]. The feasibility of sulfide regeneration by glucose also was evaluated at 60e80 C for the solution resulting from hydrogen production. Finally, hydrogen production from glucose via the sulfur redox cycle was demonstrated, and the efficiency of this new method was described by comparison with the conventional hydrothermal gasification of glucose.
(1)
where the sulfur oxyanion, SxOy2, may be thiosulfate, S2O32, sulfite, SO32, or sulfate, SO42. For hydrogen production to be sustainable, sulfide must be regenerated from sulfur oxyanion(s) by an organic reducing agent [23], such as glucose which can be derived from biomass [24e27], at much lower temperatures (and pressures) by the heat remaining after production of hydrogen. Thus, this new method can be considered to produce hydrogen from biomass. Glucose is the most common carbohydrate and can be classified as a monosaccharide, an aldohexose, and a reducing sugar. Using glucose at low temperatures is also appropriate because its decomposition (e.g., dehydration reaction) starts at 90 C [28]. It should be noted that a single-step hydrogen production from water using sulfur redox reactions has been reported previously [20], in which both hydrogen production using
Fig. 2 e Relation between hydrogen production and temperature.
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Fig. 3 e Electropherograms from CE analysis for (a) starting solution, and liquid samples at (b) 280 C, (c) 300 C, and (d) 320 C.
2.
Experimental
2.1.
Hydrogen production experiment
A total of 1.2 g of sodium sulfide nonahydrate (Na2S.9H2O, 98e102% assay by Wako Pure Chemical Industries, Ltd.) was dissolved in 120 mL distilled water. The solution containing sulfide was loaded into an autoclave (made of Hastelloy C-22 alloy) with an inner volume of 200 mL. The reactor was equipped with a stirrer, pressure gauge, temperature controller, and gas and liquid sampling valves. The temperature controller, consisting of a heater surrounding the reactor and a thermocouple inserted within the reactor, enabled accurate temperature control. The liquid sampling valve was
connected to a sampling tube located near the bottom of the reactor. Before heating was initiated, nitrogen gas was bubbled through the liquid sampling valve for 15 min to prevent the oxidation of sulfur by air. Heating with stirring was then started to increase the temperature to a predetermined value. Reaction temperatures were 250 C, 280 C, 300 C, and 320 C. After 60 min at the prescribed temperature, the reactor was cooled using a fan, and gas and liquid samples were collected.
2.2.
Sulfide regeneration experiment
The solution resulting after hydrogen production at 300 C was used for sulfide regeneration. A total of 90 mg of D-(þ)glucose powder (C6H12O6, 98e102% assay by Nacalai Tesque,
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Table 1 e Hydrogen production and concentration changes in sulfur-containing anions. Temperature ( C) 280 300 320
H2(mmol)
S2 (mmol/L)
S2O32 (mmol/L)
SO32 (mmol/L)
SO42 (mmol/L)
1.2 6.3 12.1
10.1 16.6 36.5
0.0 0.0 0.0
0.3 0.7 1.5
0.0 0.0 0.2
Inc.) was dissolved in 30 mL of the solution in a 50 mL glass beaker. A silicon rubber cap was placed on the beaker and the solution heated and stirred to increase the temperature up to a prescribed temperature. The magnetic stirrer, which measured the temperature of the liquid directly using a thermocouple, enabled accurate temperature control. Reaction temperatures were 60 C, 70 C, and 80 C. After 10 min at the prescribed temperature, a liquid sample was collected.
2.3.
Sulfur redox cycle experiment
The sulfur redox cycle experiment consisted of three half cycles of the first hydrogen production, sulfide regeneration, and second hydrogen production. The two half cycles of the first and second hydrogen productions were conducted at 300 C for 60 min, using the same experimental method described above. Only the starting solutions differed between the two experiments. The first starting solution was the same aqueous sodium sulfide solution described above, while the second was the solution after sulfide regeneration containing organic compound(s). The half cycle of sulfide regeneration was conducted using a modified method, which is discussed in Results and Discussion.
2.4.
Sample analysis
Capillary electrophoresis (CE G1600 AX, Agilent Technologies, Inc.) was used to analyze sulfur-containing anions (S2, S2O32, SO32, and SO42) in the liquid samples. The column was a fused silica capillary column (Agilent Technologies, Inc.), which had an effective length of 24.5 cm, total length of 33 cm, and an internal diameter of 50 mm. The buffer used was an inorganic anion buffer (Agilent Technologies, Inc.), with the pH adjusted to 11.2. Voltage was 18 kV, and temperature was 35 C. The UV wavelengths were 350/50 nm and 235/10 nm for the signal and the reference, respectively. Detailed information of this method has been described previously [29]. Standard solutions for quantification were prepared using distilled water, sodium sulfide nonahydrate (Na2S$9H2O, 98e102% assay by Wako Pure Chemical Industries, Ltd.), sodium thiosulfate pentahydrate (Na2S2O3$5H2O, 99% assay by Wako Pure Chemical Industries, Ltd.), sodium sulfite (Na2SO3, 97% assay by Wako Pure Chemical Industries, Ltd.), and sodium sulfate (Na2SO4, 99% assay by Wako Pure Chemical Industries, Ltd.). Gas chromatography with a thermal conductivity detector (GC-323/TCD, GL Sciences, Inc.) was used to analyze hydrogen in the gas samples. The column was a stainless steel Porapak Q (80/100 mesh) column (GL Sciences, Inc.), which had a length of 2 m, outer diameter of 3.2 mm, and internal diameter of 2.1 mm. Carrier gas was nitrogen, and temperatures used were 120 C, 60 C, and 100 C for the injection, column, and
detector, respectively. Standard hydrogen gas (99.99% purity, GL Sciences, Inc.) was used for quantification.
3.
Results and discussion
3.1.
Hydrogen production using sulfide
GC analysis detected hydrogen in the gas samples after hydrogen production at temperatures 280 C, where hydrogen production increased with temperature (Fig. 2). At 280 C, 300 C and 320 C, 1.2 mmol, 6.3 mmol, and 12.1 mmol hydrogen, respectively, were produced after 60 min. Since no hydrogen production was observed at 250 C, only the results at higher temperatures are discussed. The CE analysis (Fig. 3) showed that sulfide, S2, peak area decreased with increasing temperature. Initial sulfide concentration was 41.7 mmol/L; concentration changes were 10.1 mmol/L, 16.6 mmol/L, and 36.5 mmol/L at 280 C, 300 C, and 320 C, respectively. CE analysis also showed that only sulfite, SO32, was formed at 280 C and 300 C, whereas sulfate, SO42, also formed at 320 C. Concentration changes in sulfite were 0.3 mmol/L, 0.7 mmol/L, and 1.5 mmol/L at 280 C, 300 C, and 320 C, respectively, whereas the concentration change in sulfate was 0.2 mmol/L at 320 C. The amount and/or oxidation number of sulfur increased with temperature. The concentration changes in sulfide and sulfur oxyanions are summarized in Table 1, along with hydrogen production. As shown in Table 1, the mass balance of sulfur could not be confirmed, because of unknown peaks, and limitations of the analytical method which prevented the detection of polysulfide, Sn2, and polythionate, SxO62. Results from GC and CE analyses indicated hydrogen production was accompanied by sulfide consumption and sulfur oxyanion formation, which coincided well with the
Fig. 4 e Relation between H2/S2L ratio and temperature during hydrogen production.
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study hypothesis (Eq. (1)) based on the previous studies [20,21]. However, not all of the results could be explained by Eq. (1). If hydrogen was produced according to Eq. (1), the molar ratio of hydrogen production to sulfide consumption, H2/S2 (mol/ mol), must be at least 2 if the oxidation state of sulfur changes from S2 just to S2þ:
280 C indicated that hydrogen production could be explained by the following reaction:
2S2 þ 5H2 O/S2 O2 3 þ 2OH þ 4H2 ;
5S2 þ 8H2 O/S2 5 þ 9OH þ 4H2 :
(2)
and must be at most 4 if the oxidation state of sulfur changes from S2 to S6þ: S2 þ 4H2 O/SO2 4 þ 4H2 : 2
(3)
Calculating H2/S ratios using experimental results (Table 1) showed that the ratio at 280 C was less than the lower limit, although the ratios at 300 C and 320 C were between the lower and upper limits (Fig. 4). The small H2/S2 ratio at
nS2 þ 2ðn 1ÞH2 O/S2 n þ ð2n 1ÞOH þ ðn 1ÞH2 ;
(4)
where Sn2 is polysulfide with n ¼ 2e5 [30]. In this reaction, the H2/S2 ratio can be as great as 0.8 if n ¼ 5: (5)
2
Considering that the H2/S ratio is approximately unity at 280 C, the reaction described by Eq. (5) may be predominant at this temperature. However, the sulfite production indicated that the reaction described by Eq. (1) occurred at the same time. This type of simultaneous production of polysulfide and sulfur oxyanion under hydrothermal conditions has been described previously [31]. Thus, two types of hydrogen production occurred simultaneously, with the predominant reaction
Fig. 5 e Electropherograms from CE analysis for (a) starting solution, and the liquid samples at (b) 60 C, (c) 70 C, and (d) 80 C.
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Table 2 e Concentration changes of sulfur-containing anions during sulfide regeneration. Temperature ( C)
S2 (mmol/L)
S2O32 (mmol/L)
SO32 (mmol/L)
SO42 (mmol/L)
3.0 4.7 9.4
0.9 0.6 0.0
0.1 0.0 0.6
0.2 0.0 0.0
60 70 80
shifting from Eq. (4) to Eq. (1) with increasing temperature. In addition, the similar H2/S2 ratios at 300 C and 320 C might indicate that the difference in the reaction at these two temperatures was only the reaction rate. A H2/S2 ratio of approximately 3 at these temperatures indicated that the predominant reaction was accompanied by formation of trithionate, S3O62: 3S2 þ 10H2 O/S3 O2 6 þ 4OH þ 8H2 ;
(6)
where the H2/S2 ratio is 2.7.
3.2.
Sulfide regeneration using glucose
The CE analysis showed that sulfite, SO32, and unknown anions were formed during hydrogen production at 300 C, and remained after sulfide regeneration at every temperature (Fig. 5). Nevertheless, concentration of sulfide, S2, generally increased under all conditions tested (Table 2), demonstrating that sulfide regeneration using glucose as a reducing agent is possible at much lower temperatures than those required for hydrogen production. The change in sulfide concentration increased with temperature, and was 3.0 mmol/L, 4.7 mmol/L, and 9.4 mmol/L at 60 C, 70 C, and 80 C, respectively. Small increments in concentrations of sulfur oxyanions (S2O32, SO32, and SO42) may have been caused by sulfur oxidation by air due to use of a glass beaker. The changes in concentration of all the sulfur oxyanions were significantly smaller than those of sulfide. In addition, since unknown peaks remained after the experiment, the sulfide likely was regenerated mainly from undetected sulfur-containing anion(s) such as polysulfide, Sn2, and polythionate, SxO62. Although the reaction equation for sulfide regeneration could not be determined, regeneration of sulfide from sulfur oxyanions and/or polysulfides was possible when using glucose as a reducing agent. Defining the sulfide regeneration ratio as the increment in concentration (Table 2) over the concentration change in hydrogen production at 300 C (Table 1), the regeneration ratio was approximately 60%, even at 80 C (Fig. 6). However, the regeneration ratio tended to increase nonlinearly with temperature, suggesting a higher regeneration ratio may be possible by increasing reaction rate at temperatures >80 C. Fig. 6 shows an extrapolation of results at higher temperatures (dashed line), indicating complete regeneration at temperatures near 100 C.
that would allow the reaction rate of sulfur reduction to be increased with little or no glucose decomposition. Consequently, the following procedure was employed for sulfide regeneration during the sulfur redox cycle experiment. Glucose powder (0.3 wt.%) was added to the solution within the autoclave produced after the half cycle of the first hydrogen production. Nitrogen gas was bubbled through the solution for 15 min to prevent sulfur oxidation by air. Then, the solution was heated with stirring to increase the temperature up to 105 C. Upon reaching 105 C, cooling was initiated immediately, and a liquid sample was collected. The GC and CE analyses for the gas and liquid samples collected after the half cycle of the first hydrogen production provided a H2/S2 ratio of 2.7 mol/mol, which was similar to previous results shown in Fig. 4. The formation of sulfite, SO32, also was confirmed during hydrogen production (Fig. 7a and b). After this first hydrogen production, the half cycle of sulfide regeneration was conducted with the modified method described above. The CE analysis of the liquid sample after this half cycle provided a sulfide regeneration ratio of 89.7%. As expected, the regeneration ratio improved when the modified method was used. Assuming that all the added glucose was involved in the reaction, 0.9 mol sulfide was regenerated by 1 mol glucose. However, the peak area for sulfite and the unknown peak did not decrease (Fig. 7b and c), which explains the incomplete sulfide regeneration. Sulfide could be regenerated only from undetected sulfur-containing compound(s) such as polysulfide and polythionate. This unidentified sulfurcontaining compound was thought to be trithionate, S3O62, as expected by Eq. (6), because the following equation coincided well with the data that about 1 mol sulfide were regenerated by 1 mol glucose: 2 þC6 H10 O7 þ2C6 H10 O8 þ4Hþ þH2 O; S3 O2 6 þ3C6 H12 O6 /3S
3.3. Hydrogen production from glucose via the sulfur redox cycle Results from sulfide regeneration experiments indicated that the optimum temperature for sulfide regeneration was >80 C. However, decomposition (e.g., dehydration) of glucose can occur at temperatures >90 C [28]. Therefore, it was necessary to determine reaction conditions (e.g., temperature and time)
Fig. 6 e Relation between sulfide regeneration ratio and temperature during sulfide regeneration.
(7)
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Fig. 7 e Electropherograms from CE analysis for (a) starting solution, and liquid samples after (b) first hydrogen production, (c) sulfide regeneration, and (d) second hydrogen production.
where it is assumed that glucose is oxidized to common oxidized glucose derivatives; glucuronic, C6H10O7, and glucaric, C6H10O8, acids [32e34]. The half cycle of the second hydrogen production was conducted using the solution after sulfide regeneration. The GC and CE analyses for gas and liquid samples after this half cycle provided a H2/S2 ratio of 2.5 mol/mol, which was just slightly lower than the value obtained from the first hydrogen production. Additionally, the CE analysis for the liquid sample produced only peaks corresponding to the same anions observed in the first hydrogen
production (Fig. 7b and d). These results indicate that the same mechanism is acting in both the first and second hydrogen production even in the presence of organic compound(s). This finding is significant because the solution containing organic compounds can be used repeatedly for hydrogen production even if the organic compounds cannot be removed completely. The present study demonstrated a method of hydrogen production from biomass via the sulfur redox cycle at moderate temperatures. Results indicated that, in theory,
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2.7 mol hydrogen is produced by consuming 1 mol sulfide at approximately 300 C (Eq. (6)), and 1 mol sulfide is regenerated by consuming 1 mol glucose at approximately 100 C, i.e., with no additional energy (Eq. (7)). Thus, hydrogen production from glucose is 2.7 mol hydrogen per 1 mol glucose, and is represented by the following reaction derived from Eqs. 6 and 7: 3C6 H12 O6 þ 5H2 O/C6 H10 O7 þ 2C6 H10 O8 þ 8H2 :
(8)
In contrast, comparison of hydrogen production from glucose by hydrothermal gasification methods reported previously [35] indicated at most 1.6 mol hydrogen per 1 mol glucose even at much higher temperatures up to 500 C [36]. Thus, hydrogen production from glucose by the proposed method is significantly greater and uses much lower temperatures. Although some details remain unclear, the present study provides a method for enhanced (and sustainable) hydrogen production from biomass via the sulfur redox cycle under hydrothermal conditions.
4.
Conclusions
A new method of hydrogen production from biomass via a sulfur redox cycle has been proposed. This method, which can utilize excess sulfur from hydrocarbon refining processes and waste or geothermal heat, consists of two half cycles: (1) hydrogen production from an aqueous alkaline solution at subcritical conditions of water, where sulfide, HS and S2, acts as a reducing agent of water, and (2) sulfide regeneration under much milder conditions, with an organic compound derived from biomass acting as a reducing agent of polysulfide, Sn2, and sulfur oxyanion, SxOy2, formed in the first half cycle. The present study first examined hydrogen production from the aqueous sodium sulfide solution. During the 60-min reaction, hydrogen production was observed at 280 C and corresponding saturated vapor pressures. The feasibility of sulfide regeneration by D-glucose was then evaluated for the solution resulting from hydrogen production. Addition of D-glucose to the solution after hydrogen production at 300 C resulted in sulfide regeneration at temperatures 60 C in the present 10min reaction. Finally, hydrogen production from glucose via the sulfur redox cycle was demonstrated, where the hydrogen production and sulfide regeneration were conducted at 300 C and 105 C, respectively. Results indicated that hydrogen production from 1 mol glucose was greater than that by hydrothermal gasification of glucose at much higher temperatures up to 500 C. Although some details remain unclear, the present study provides a method for enhanced (and sustainable) hydrogen production from biomass via the sulfur redox cycle under hydrothermal conditions.
Acknowledgments This work was supported in part by Idemitsu Kosan Co., Ltd., and Japan Society for the Promotion of Science (JSPS), through a Grant-in-Aid for Challenging Exploratory Research, No. 23656556. The authors thank Mr. T. Fukunaga, Mr. T. Umeki, and Mr. Y. Kawashima of Idemitsu Kosan Co., Ltd. for their valuable discussions throughout this study.
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