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Hydrogen production by glycerol steam reforming with in situ hydrogen separation: A thermodynamic investigation
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 5 ( 2 0 1 0 ) 1 0 2 5 2 e1 0 2 5 6
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journal homepage: www.elsevier.com/locate/he
Technical Communication
Hydrogen production by glycerol steam reforming with in situ hydrogen separation: A thermodynamic investigation Xiaodong Wang a,b, Na Wang b, Maoshuai Li a, Shuirong Li a, Shengping Wang a, Xinbin Ma a,* a
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China b Chemical Engineering, School of Engineering and Physical Sciences, HerioteWatt University, Edinburgh EH14 4AS, Scotland, United Kingdom
article info
abstract
Article history:
Thermodynamic features of hydrogen production by glycerol steam reforming with in situ
Received 1 April 2010
hydrogen extraction have been studied with the method of Gibbs free energy minimiza-
Received in revised form
tion. The effects of pressure (1e5 atm), temperature (600e1000 K), water to glycerol ratio
23 July 2010
(WGR, 3e12) and fraction of H2 removal ( f, 0e1) on the reforming reactions and carbon
Accepted 24 July 2010
formation were investigated. The results suggest separation of hydrogen in situ can
Available online 23 August 2010
substantially enhance hydrogen production from glycerol steam reforming, as 7 mol (stoichiometric value) of hydrogen can be obtained even at 600 K due to the hydrogen
Keywords:
extraction. It is demonstrated that atmospheric pressure and a WGR of 9 are suitable for
Hydrogen
hydrogen production and the optimum temperature for glycerol steam reforming with in
Glycerol steam reforming
situ hydrogen removal is between 825 and 875 K, 100 K lower than that achieved typically
Hydrogen separation
without hydrogen separation. Furthermore, the detrimental influence of increasing pres-
Thermodynamic analysis
sure in terms of hydrogen production becomes marginal above 800 K with a high fraction of H2 removal (i.e., f ¼ 0.99). High temperature and WGR are favorable to inhibit carbon production. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The simplest of molecules, hydrogen, is in principle, the lightest and the cleanest of all fuels because its combustion results in essentially zero emissions and therefore it is expected to play a major part in the energy supply of the future. From an environmental perspective, renewable sources such as solar power and biomass are required, as conventional methods of producing hydrogen from fossil fuels invariably produce carbon dioxide (green house gas) [1]. As raw fossil
feeds are diminishing resources, chemical and energy-related industries are examining alternative renewable feedstocks. Glycerol, which is the by-product of biodiesel production by transesterification of vegetable oils (triglycerides) and methanol, is a potentially important hydrogen source [2]. It has already been shown, thermodynamically, hydrogen can be generated from glycerol steam reforming [3], dry reforming [4], autothermal reforming [5] and dry autothermal reforming [6]. However, most of these studies focused on different reactants (e.g. glycerol, CO2 and/or H2O) in reforming reactions. Only
* Corresponding author. Tel.: þ86 22 27409248; fax: þ86 22 87401818. E-mail address: [email protected] (X. Ma). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.07.140
10253
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 5 ( 2 0 1 0 ) 1 0 2 5 2 e1 0 2 5 6
DH298K ¼ 127:67 kJ=mol
(1)
Eliminating CO2 immediately after it is formed, glycerol conversion and hydrogen production can be enhanced if the reaction is under equilibrium control. Furthermore by selectively removing hydrogen in situ, this reaction equilibrium can be further shifted in favor of converting glycerol and producing hydrogen. Such a process could be achieved with a membrane reactor (MR) which can both carry out steam reforming reactions and remove pure hydrogen [8]. Of particular relevance to this work, Iulianelli et al. investigated the potential of a PdeAg MR in glycerol steam reforming for hydrogen production at 673 K [9,10]. With in situ separation of hydrogen by an MR, higher conversion of glycerol and hydrogen yield were achieved over both Co/Al2O3 and Ru/ Al2O3 as compared with the results obtained from a traditional reactor (TR) [9,10]. The few studies that focused on glycerol reforming with in situ hydrogen extraction have failed to give a thorough image of the process thermodynamic behavior. Studying glycerol steam reforming with in situ hydrogen removal from a thermodynamic perspective, the capacity of hydrogen production and the effects of operating parameters are analyzed in this communication.
2.
Methodology
The method of thermodynamic analysis by minimization of Gibbs free energy used in this study is the same as that introduced in our previous publications [3e5] and was also introduced in detail by other groups [6,7]. The thermodynamic equilibrium calculations were accomplished with the use of Matlab language. The species considered in this work include hydrogen, carbon monoxide, carbon dioxide, methane, water, glycerol and solid carbon. Thermodynamic data were obtained from previous work by Yaws [11]. The initial amount of glycerol was set to be 1 mol. This analysis has been performed over the following variable ranges: pressure 1e5 atm, temperature 600e1000 K, water to glycerol ratio (WGR) 3e12 and fraction of H2 removal ( f ), 0e1. Hydrogen production in the following sections represents a total amount of hydrogen produced nH2 separated þ nH2 unseparated .
a
7 6
Total moles of hydrogen
C3 H8 O3 þ 3H2 O43CO2 þ 7H2
influence of pressure on hydrogen production decreases significantly however when the fraction of H2 removal increases from 0.2 to 0.99. Notably, for the fractional H2 removal at 0.99, there is almost no impact of pressure (1e5 atm) on hydrogen production when the temperature is above 800 K (not shown). A high fraction of H2 extraction may therefore facilitate the steam reforming of glycerol at a high pressure, whereby there is a positive effect in terms of a higher hydrogen permeation driving force (membrane effect) if hydrogen removal is conducted in an MR [9,10]. Conversely, as glycerol steam reforming (Eq. (1)) is a reaction presenting an increase in the number of moles, high pressure gives a detrimental effect on the equilibrium (thermodynamic effect). As superior hydrogen production was achieved, atmospheric pressure was selected accordingly throughout the following discussion. Irrespective of in situ hydrogen separation, complete conversion of glycerol was obtained for the range of pressure, temperature and WGR considered in this study. The WGR was kept constant at 9 to show the effect of temperature, whereas in order to illustrate the influence of WGR, a temperature of 850 K was used throughout the following sections. As can be seen from Fig. 1(a), hydrogen production can be enhanced by
5
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800
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1000
Temperature / K
b
7 6 5
4
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
3 2
3.
Results and discussion
1
3.1.
Hydrogen production
0
Firstly, it is noteworthy that reaction pressure can affect hydrogen production from glycerol steam reforming thermodynamically. Briefly, hydrogen production is inversely related to pressure, regardless of the fractional H2 removal [3]. The
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
3
1
Total moles of hydrogen
Ding et al. [7] reported adsorption-enhanced steam reforming of glycerol, which combined glycerol steam reforming with in situ removal of CO2. Their results, based on a thermodynamic analysis, showed the maximum number of moles of hydrogen that is produced can be increased from 6 to 7, which can be explained by the Le Chatelier’s principle through the following overall reaction:
3
4
5
6
7
8
9
10
11
12
Water to glycerol ratio (WGR)
Fig. 1 e Hydrogen production: effects of temperature (a, WGR [ 9) and WGR (b, T [ 850 K) at atmospheric pressure, n0(C3H8O3) [ 1 mol.
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 5 ( 2 0 1 0 ) 1 0 2 5 2 e1 0 2 5 6
catalysts suffer from carbon related deactivation which additionally has a negative effect on PdeAg MR [9,10].
3.2.
Carbon monoxide production
Fig. 2 depicts the effects of temperature and WGR on CO production at atmospheric pressure. Contrary to hydrogen production, the production of CO declines as f increases at all temperatures, particularly when temperature is higher than 850 K. For the fraction of H2 removal at 0.99 and 1, CO is eliminated completely from equilibrium compositions over the whole range of temperatures considered. With the exception f ¼ 0.99/1, CO production increases with increasing temperature. This appears unavoidable even in adsorptionenhanced steam reforming of glycerol [7]. It could be explained by the water gas shift reaction [Eq. (2)] which is unfavorable at high temperature coupled with methane dry reforming [Eq. (3)] which is favorable at high temperature based on Fernando’s publication [12]. Eq. (3), however, only indicates one way of CO production. Alternatively, CO may generate from direct decomposition of glycerol [Eq. (4)] which was reported to be highly favorable at high temperatures in the experimental work [13,14].
a
1.25
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
1.00
Moles of carbon monoxide
in situ H2 removal over the whole range of temperature investigated. Removing 99e100% of H2, 7 mol of hydrogen (equal to stoichiometry) per mole glycerol can be achieved from temperatures as low as 600 K, which shows better performance than that achieved in situ CO2 removal reported by Ding et al. [7], where similar hydrogen yield requests a temperature greater than 800 K. Indeed, at this temperature this hydrogen yield of ca. 7 mol (i.e., 6.8 mol) requests a fractional hydrogen removal of only 0.8. With regards to the effect of temperature on hydrogen production, at a given f (except 0.99 and 1), hydrogen production increases with temperature, before reaching a maximum at a peak temperature, and then begins to decrease at higher temperatures. Notably this peak temperature decreases with increasing fraction of H2 removal, which is certainly a good indication to find appropriate reaction conditions for further experimental work. This advantage is not observed from in situ CO2 removal, as the peak temperature for hydrogen production is independent of CO2 removal maintained at ca. 950 K [7]. The preferential temperature in terms of hydrogen production is between 825 and 875 K as shown in Fig. 1(a). Hydrogen production increases with increasing H2 fraction removal as shown in Fig. 1(b), notably at f ¼ 0.99 and 1, the number of moles of hydrogen is close to 7 over the entire range of WGRs considered. Moreover, reasonable moles of hydrogen can be generated at f ¼ 0.8 when WGR is above 6. Moles of hydrogen also increase steadily with increasing WGR. Although high WGR enhances the reforming performance, it is believed that higher WGRs require higher reactor volume due to higher steam volumetric flow. Furthermore, the high steam content would also consume higher input heat duty as a result of higher vaporization energy. The benefit with respect to hydrogen production from an increase in WGR from 9 to 12 is marginal. Therefore, the most favorable WGR should be around 9. Theoretical analysis can provide a set of parameters to maximize the hydrogen production and/or glycerol conversion, through which, further experimental work can be guided. However, it is worth noting that in reality conversion and yield/selectivity that can be achieved in experiments also depend on reaction kinetics, reactor design, operation and other process related parameters. In the case of hydrogen production by glycerol steam reforming, the number of studies that have considered in situ hydrogen removal is limited but we can flag the work of Iulianelli et al. who studied in situ hydrogen extraction utilizing PdeAg MR, working over Co/Al2O3 at 673 K, 1 bar, WGR ¼ 6, sweep factor (SF) ¼ 22.8 and WHSV ¼ 1.01 h1. The maximum glycerol conversion was ca. 50% with a hydrogen yield of 38.7%. Under above conditions, a CO-free hydrogen recovery (equal to f ) lower than 5% was also obtained [9]. Under the same reaction conditions instead using a Ru/Al2O3 catalyst and SF ¼ 11.9, the glycerol conversion and hydrogen yield obtained were 12% and 5.3% respectively ( f ¼ 10%) [10]. The reported experimental results were less than that predicted by thermodynamic analysis (both conversion and yield), suggesting possible effects from reaction kinetics, reactor design and type, operation, permeate equilibrium and kinetics. Carbon formation during the reaction is conceivably one of the main reasons that such a difference is observed between experimental data and thermodynamic prediction, as both Co/Al2O3 and Ru/Al2O3
0.75
0.50
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0.00 600
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950
1000
Temperature / K
b
1.0
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
0.8
Moles of carbon monoxide
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0.6
0.4
0.2
0.0 3
4
5
6
7
8
9
10
11
Water to glycerol ratio (WGR)
Fig. 2 e Carbon monoxide production: effects of temperature (a, WGR [ 9) and WGR (b, T [ 850 K) at atmospheric pressure, n0(C3H8O3) [ 1 mol.
12
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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 5 ( 2 0 1 0 ) 1 0 2 5 2 e1 0 2 5 6
DH298K ¼ 41:2 kJ=mol
(2)
CO2 þ CH4 42H2 þ 2CO DH298K ¼ 247:3 kJ=mol
(3)
C3 H8 O3 44H2 þ 3CO DH298K ¼ 251:2 kJ=mol
(4)
Furthermore, our recent investigations of glycerol steam reforming via response reactions (RERs) have proven, in theory, that Eq. (4) becomes the predominant glycerol decomposition pathway above 874 K (paper in preparation). From Fig. 2(b), one can also see that the production of CO decreases with an increase in either WGR or the fraction of H2 removal.
4.5 4.0
Water to glycerol ratio (WGR)
CO þ H2 O4H2 þ CO2
Carbon free region
3.5 3.0
Carbon region
2.5 2.0
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99
1.5 1.0
3.3.
Methane production
0.5 600
The number of moles of CH4 produced decreases with the increase in either temperature or f. The production of CH4 can be inhibited almost entirely when temperature is higher than 850 K, thermodynamically [see Fig. 3(a)]. As can be seen from Fig. 3(b), CH4 production decreases significantly with an increase in WGR. From a thermodynamic perspective, low temperature favors CH4 production. Zhang et al. also reported that glycerol decomposition to CH4 is highly favorable during
a
1.75
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
1.50
Moles of methane
1.25
1.00
0.50
0.25
0.00 600
650
700
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800
850
900
950
1000
Temperature / K
b
1.00
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
Moles of methane
0.75
0.50
0.25
0.00 3
4
5
6
7
8
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10
11
700
750
800
850
900
950
1000
Temperature / K
Fig. 4 e Coke formation range plotted as a function of temperature and WGR at atmospheric pressure.
the reforming process at 673e723 K [15]. In addition, our study according to the RERs of glycerol steam reforming could confirm that glycerol mainly decomposes to methane and carbon dioxide with a small amount of hydrogen below 714 K (paper in preparation).
3.4.
0.75
650
Carbon formation
Carbon is an undesirable product in the reforming of hydrocarbon and oxygenates for hydrogen production, and the formation of carbon can also deactivate the catalysts. The elimination of carbon formation, therefore, is of great importance, especially when crude glycerol is used as a reactant [13]. Carbon formed and free regions at atmospheric pressure were plotted as a function of temperature and WGR in Fig. 4. In general, a WGR around 4.5 is high enough to inhibit carbon production at all temperatures considered in this work. For a given H2 removal fraction (except f ¼ 0.99), the minimum WGR required to avoid carbon formation decreases slightly with an increase in temperature until around 800 K, after which the decreasing trend becomes more significant. The influence of H2 removal fraction on the minimum WGR is negligible above 875 K. Below 875 K, however, the effect of fractional H2 removal differs depending upon whether above or below f ¼ 0.8. The minimum WGR for those f 0.8 increases with increasing f, whereas f > 0.8 a contrary trend is observed. It seems that unless H2 can be separated in situ to a large extent (i.e., f > 0.8), more carbon would form with increasing f at low WGRs (i.e., WGR < 4). Nevertheless, the easiest way to avoid this is to conduct the experiments with a high WGR as the optimum WGR for hydrogen production is ca. 9. Fluidized bed membrane reactors alternatively could be adopted, as it has been reported that both theoretical and experimental studies show that carbon formation is not an issue [16].
12
Water to glycerol ratio (WGR)
Fig. 3 e Methane production: effects of temperature (a, WGR [ 9) and WGR (b, T [ 850 K) at atmospheric pressure, n0(C3H8O3) [ 1 mol.
4.
Conclusions
Thermodynamic properties of glycerol steam reforming with in situ hydrogen separation have been investigated with the
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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 5 ( 2 0 1 0 ) 1 0 2 5 2 e1 0 2 5 6
method of Gibbs free energy minimization. The effects of pressure, temperature, water to glycerol ratio and fraction of H2 removal on the reforming reactions and carbon formation were examined. The results suggest that extraction of H2 in situ can substantially enhance hydrogen production, as 7 mol (stoichiometric value) of hydrogen can be achieved from a temperature as low as 600 K due to the hydrogen separation. It is found that atmospheric pressure and a WGR of 9 are feasible for hydrogen production and the most favorable temperature for glycerol steam reforming with in situ hydrogen removal is between 825 and 875 K, 100 K lower than that achieved typically without hydrogen separation. Moreover, the negative influence of increasing pressure in hydrogen production becomes marginal above 800 K with a high fraction of H2 removal (i.e., f ¼ 0.99). High temperature and WGR are preferable to inhibit carbon formation. The minimum WGR required to eliminate carbon production can be reduced only with a high level of H2 removal. Glycerol steam reforming with in situ hydrogen separation displays considerable potential for hydrogen production.
Acknowledgments The financial support from the Program for New Century Excellent Talents in University (NCET-04-0242) and the Program of Introducing Talents of Discipline to Universities (B06006) are gratefully acknowledged. The authors thank Dr. Alexander B. Foster (Heriot-Watt University) for providing helpful discussion and language help.
references
[1] Davda RR, Shabaker JW, Huber GW, Cortright RD, Dumesic JA. A review of catalytic issues and process conditions for renewable hydrogen and alkanes by aqueousphase reforming of oxygenated hydrocarbons over supported metal catalysts. Appl Catal B 2005;56:171e86. [2] Cortright RD, Davda RR, Dumesic JA. Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water. Nature 2002;418:964e7. [3] Wang XD, Li SR, Wang H, Liu B, Ma XB. Thermodynamic analysis of glycerin steam reforming. Energy Fuels 2008;22:4285e91.
[4] Wang XD, Li MS, Wang MH, Wang H, Li SR, Wang SP, et al. Thermodynamic analysis of glycerol dry reforming for hydrogen and synthesis gas production. Fuel 2009;88:2148e53. [5] Wang H, Wang XD, Li MS, Li SR, Wang SP, Ma XB. Thermodynamic analysis of hydrogen production from glycerol autothermal reforming. Int J Hydrogen Energy 2009; 34:5683e90. [6] Kale GR, Kulkarni BD. Thermodynamic analysis of dry autothermal reforming of glycerol. Fuel Process Tech 2010; 91:520e30. [7] Chen HS, Zhang TF, Dou BL, Dupont V, Williams P, Ghadiri M, et al. Thermodynamic analyses of adsorption-enhanced steam reforming of glycerol for hydrogen production. Int J Hydrogen Energy 2009;34:7208e22. [8] Gallucci F, Basile A, Tosti S, Iulianelli A, Drioli E. Methanol and ethanol steam reforming in membrane reactors: an experimental study. Int J Hydrogen Energy 2007;32: 1201e10. [9] Iulianelli A, Longo T, Liguori S, Basile A. Production of hydrogen via glycerol steam reforming in a Pd-Ag membrane reactor over Co-Al2O3 catalyst. Asia-Pac J Chem Eng 2010;5:138e45. [10] Iulianelli A, Seelam PK, Liguori S, Longo T, Keiski R, Calabro V, et al. Hydrogen 12 production for PEM fuel cell by gas phase reforming of glycerol as byproduct of bio-diesel. The use of a Pd–Ag membrane reactor at middle reaction temperature. Int J Hydrogen Energy, in press, doi:10.1016/j. ijhydene.2010.02.079. [11] Yaws CL. Chemical properties handbook. New York: McGraw-Hill; 1999. [12] Adhikari S, Fernando S, Gwaltney SR, Filip To SD, Bricka RM, Steele PH, et al. A thermodynamic analysis of hydrogen profuction by steam reforming of glycerol. Int J Hydrogen Energy 2007;32:2875e80. [13] Slinn M, Kendall K, Mallon C, Andrews J. Steam reforming of biodiesel by-product to make renewable hydrogen. Bioresour Technol 2008;99:5851e8. [14] Corma A, Huber GW, Sauvanaud L, O’Connor P. Biomass to chemicals: catalytic conversion of glycerol/water mixtures into acrolein, reaction network. J Catal 2008;257: 163e71. [15] Zhang BC, Tang XL, Li Y, Xu YD, Shen WJ. Hydrogen production from steam reforming of ethanol and glycerol over ceria-supported metal catalysts. Int J Hydrogen Energy 2007;32:2367e73. [16] Ye GY, Xie DL, Qiao WY, Grace JR, Jim Lim C. Modeling of fluidized bed membrane reactors for hydrogen production from steam methane reforming with Aspen Plus. Int J Hydrogen Energy 2009;34:4755e62.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
Hydrogen production by glycerol steam reforming with in situ hydrogen separation: A thermodynamic investigation Xiaodong Wang a,b, Na Wang b, Maoshuai Li a, Shuirong Li a, Shengping Wang a, Xinbin Ma a,* a
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China b Chemical Engineering, School of Engineering and Physical Sciences, HerioteWatt University, Edinburgh EH14 4AS, Scotland, United Kingdom
article info
abstract
Article history:
Thermodynamic features of hydrogen production by glycerol steam reforming with in situ
Received 1 April 2010
hydrogen extraction have been studied with the method of Gibbs free energy minimiza-
Received in revised form
tion. The effects of pressure (1e5 atm), temperature (600e1000 K), water to glycerol ratio
23 July 2010
(WGR, 3e12) and fraction of H2 removal ( f, 0e1) on the reforming reactions and carbon
Accepted 24 July 2010
formation were investigated. The results suggest separation of hydrogen in situ can
Available online 23 August 2010
substantially enhance hydrogen production from glycerol steam reforming, as 7 mol (stoichiometric value) of hydrogen can be obtained even at 600 K due to the hydrogen
Keywords:
extraction. It is demonstrated that atmospheric pressure and a WGR of 9 are suitable for
Hydrogen
hydrogen production and the optimum temperature for glycerol steam reforming with in
Glycerol steam reforming
situ hydrogen removal is between 825 and 875 K, 100 K lower than that achieved typically
Hydrogen separation
without hydrogen separation. Furthermore, the detrimental influence of increasing pres-
Thermodynamic analysis
sure in terms of hydrogen production becomes marginal above 800 K with a high fraction of H2 removal (i.e., f ¼ 0.99). High temperature and WGR are favorable to inhibit carbon production. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The simplest of molecules, hydrogen, is in principle, the lightest and the cleanest of all fuels because its combustion results in essentially zero emissions and therefore it is expected to play a major part in the energy supply of the future. From an environmental perspective, renewable sources such as solar power and biomass are required, as conventional methods of producing hydrogen from fossil fuels invariably produce carbon dioxide (green house gas) [1]. As raw fossil
feeds are diminishing resources, chemical and energy-related industries are examining alternative renewable feedstocks. Glycerol, which is the by-product of biodiesel production by transesterification of vegetable oils (triglycerides) and methanol, is a potentially important hydrogen source [2]. It has already been shown, thermodynamically, hydrogen can be generated from glycerol steam reforming [3], dry reforming [4], autothermal reforming [5] and dry autothermal reforming [6]. However, most of these studies focused on different reactants (e.g. glycerol, CO2 and/or H2O) in reforming reactions. Only
* Corresponding author. Tel.: þ86 22 27409248; fax: þ86 22 87401818. E-mail address: [email protected] (X. Ma). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.07.140
10253
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 5 ( 2 0 1 0 ) 1 0 2 5 2 e1 0 2 5 6
DH298K ¼ 127:67 kJ=mol
(1)
Eliminating CO2 immediately after it is formed, glycerol conversion and hydrogen production can be enhanced if the reaction is under equilibrium control. Furthermore by selectively removing hydrogen in situ, this reaction equilibrium can be further shifted in favor of converting glycerol and producing hydrogen. Such a process could be achieved with a membrane reactor (MR) which can both carry out steam reforming reactions and remove pure hydrogen [8]. Of particular relevance to this work, Iulianelli et al. investigated the potential of a PdeAg MR in glycerol steam reforming for hydrogen production at 673 K [9,10]. With in situ separation of hydrogen by an MR, higher conversion of glycerol and hydrogen yield were achieved over both Co/Al2O3 and Ru/ Al2O3 as compared with the results obtained from a traditional reactor (TR) [9,10]. The few studies that focused on glycerol reforming with in situ hydrogen extraction have failed to give a thorough image of the process thermodynamic behavior. Studying glycerol steam reforming with in situ hydrogen removal from a thermodynamic perspective, the capacity of hydrogen production and the effects of operating parameters are analyzed in this communication.
2.
Methodology
The method of thermodynamic analysis by minimization of Gibbs free energy used in this study is the same as that introduced in our previous publications [3e5] and was also introduced in detail by other groups [6,7]. The thermodynamic equilibrium calculations were accomplished with the use of Matlab language. The species considered in this work include hydrogen, carbon monoxide, carbon dioxide, methane, water, glycerol and solid carbon. Thermodynamic data were obtained from previous work by Yaws [11]. The initial amount of glycerol was set to be 1 mol. This analysis has been performed over the following variable ranges: pressure 1e5 atm, temperature 600e1000 K, water to glycerol ratio (WGR) 3e12 and fraction of H2 removal ( f ), 0e1. Hydrogen production in the following sections represents a total amount of hydrogen produced nH2 separated þ nH2 unseparated .
a
7 6
Total moles of hydrogen
C3 H8 O3 þ 3H2 O43CO2 þ 7H2
influence of pressure on hydrogen production decreases significantly however when the fraction of H2 removal increases from 0.2 to 0.99. Notably, for the fractional H2 removal at 0.99, there is almost no impact of pressure (1e5 atm) on hydrogen production when the temperature is above 800 K (not shown). A high fraction of H2 extraction may therefore facilitate the steam reforming of glycerol at a high pressure, whereby there is a positive effect in terms of a higher hydrogen permeation driving force (membrane effect) if hydrogen removal is conducted in an MR [9,10]. Conversely, as glycerol steam reforming (Eq. (1)) is a reaction presenting an increase in the number of moles, high pressure gives a detrimental effect on the equilibrium (thermodynamic effect). As superior hydrogen production was achieved, atmospheric pressure was selected accordingly throughout the following discussion. Irrespective of in situ hydrogen separation, complete conversion of glycerol was obtained for the range of pressure, temperature and WGR considered in this study. The WGR was kept constant at 9 to show the effect of temperature, whereas in order to illustrate the influence of WGR, a temperature of 850 K was used throughout the following sections. As can be seen from Fig. 1(a), hydrogen production can be enhanced by
5
4
2
0 600
650
700
750
800
850
900
950
1000
Temperature / K
b
7 6 5
4
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
3 2
3.
Results and discussion
1
3.1.
Hydrogen production
0
Firstly, it is noteworthy that reaction pressure can affect hydrogen production from glycerol steam reforming thermodynamically. Briefly, hydrogen production is inversely related to pressure, regardless of the fractional H2 removal [3]. The
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
3
1
Total moles of hydrogen
Ding et al. [7] reported adsorption-enhanced steam reforming of glycerol, which combined glycerol steam reforming with in situ removal of CO2. Their results, based on a thermodynamic analysis, showed the maximum number of moles of hydrogen that is produced can be increased from 6 to 7, which can be explained by the Le Chatelier’s principle through the following overall reaction:
3
4
5
6
7
8
9
10
11
12
Water to glycerol ratio (WGR)
Fig. 1 e Hydrogen production: effects of temperature (a, WGR [ 9) and WGR (b, T [ 850 K) at atmospheric pressure, n0(C3H8O3) [ 1 mol.
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 5 ( 2 0 1 0 ) 1 0 2 5 2 e1 0 2 5 6
catalysts suffer from carbon related deactivation which additionally has a negative effect on PdeAg MR [9,10].
3.2.
Carbon monoxide production
Fig. 2 depicts the effects of temperature and WGR on CO production at atmospheric pressure. Contrary to hydrogen production, the production of CO declines as f increases at all temperatures, particularly when temperature is higher than 850 K. For the fraction of H2 removal at 0.99 and 1, CO is eliminated completely from equilibrium compositions over the whole range of temperatures considered. With the exception f ¼ 0.99/1, CO production increases with increasing temperature. This appears unavoidable even in adsorptionenhanced steam reforming of glycerol [7]. It could be explained by the water gas shift reaction [Eq. (2)] which is unfavorable at high temperature coupled with methane dry reforming [Eq. (3)] which is favorable at high temperature based on Fernando’s publication [12]. Eq. (3), however, only indicates one way of CO production. Alternatively, CO may generate from direct decomposition of glycerol [Eq. (4)] which was reported to be highly favorable at high temperatures in the experimental work [13,14].
a
1.25
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
1.00
Moles of carbon monoxide
in situ H2 removal over the whole range of temperature investigated. Removing 99e100% of H2, 7 mol of hydrogen (equal to stoichiometry) per mole glycerol can be achieved from temperatures as low as 600 K, which shows better performance than that achieved in situ CO2 removal reported by Ding et al. [7], where similar hydrogen yield requests a temperature greater than 800 K. Indeed, at this temperature this hydrogen yield of ca. 7 mol (i.e., 6.8 mol) requests a fractional hydrogen removal of only 0.8. With regards to the effect of temperature on hydrogen production, at a given f (except 0.99 and 1), hydrogen production increases with temperature, before reaching a maximum at a peak temperature, and then begins to decrease at higher temperatures. Notably this peak temperature decreases with increasing fraction of H2 removal, which is certainly a good indication to find appropriate reaction conditions for further experimental work. This advantage is not observed from in situ CO2 removal, as the peak temperature for hydrogen production is independent of CO2 removal maintained at ca. 950 K [7]. The preferential temperature in terms of hydrogen production is between 825 and 875 K as shown in Fig. 1(a). Hydrogen production increases with increasing H2 fraction removal as shown in Fig. 1(b), notably at f ¼ 0.99 and 1, the number of moles of hydrogen is close to 7 over the entire range of WGRs considered. Moreover, reasonable moles of hydrogen can be generated at f ¼ 0.8 when WGR is above 6. Moles of hydrogen also increase steadily with increasing WGR. Although high WGR enhances the reforming performance, it is believed that higher WGRs require higher reactor volume due to higher steam volumetric flow. Furthermore, the high steam content would also consume higher input heat duty as a result of higher vaporization energy. The benefit with respect to hydrogen production from an increase in WGR from 9 to 12 is marginal. Therefore, the most favorable WGR should be around 9. Theoretical analysis can provide a set of parameters to maximize the hydrogen production and/or glycerol conversion, through which, further experimental work can be guided. However, it is worth noting that in reality conversion and yield/selectivity that can be achieved in experiments also depend on reaction kinetics, reactor design, operation and other process related parameters. In the case of hydrogen production by glycerol steam reforming, the number of studies that have considered in situ hydrogen removal is limited but we can flag the work of Iulianelli et al. who studied in situ hydrogen extraction utilizing PdeAg MR, working over Co/Al2O3 at 673 K, 1 bar, WGR ¼ 6, sweep factor (SF) ¼ 22.8 and WHSV ¼ 1.01 h1. The maximum glycerol conversion was ca. 50% with a hydrogen yield of 38.7%. Under above conditions, a CO-free hydrogen recovery (equal to f ) lower than 5% was also obtained [9]. Under the same reaction conditions instead using a Ru/Al2O3 catalyst and SF ¼ 11.9, the glycerol conversion and hydrogen yield obtained were 12% and 5.3% respectively ( f ¼ 10%) [10]. The reported experimental results were less than that predicted by thermodynamic analysis (both conversion and yield), suggesting possible effects from reaction kinetics, reactor design and type, operation, permeate equilibrium and kinetics. Carbon formation during the reaction is conceivably one of the main reasons that such a difference is observed between experimental data and thermodynamic prediction, as both Co/Al2O3 and Ru/Al2O3
0.75
0.50
0.25
0.00 600
650
700
750
800
850
900
950
1000
Temperature / K
b
1.0
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
0.8
Moles of carbon monoxide
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0.6
0.4
0.2
0.0 3
4
5
6
7
8
9
10
11
Water to glycerol ratio (WGR)
Fig. 2 e Carbon monoxide production: effects of temperature (a, WGR [ 9) and WGR (b, T [ 850 K) at atmospheric pressure, n0(C3H8O3) [ 1 mol.
12
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DH298K ¼ 41:2 kJ=mol
(2)
CO2 þ CH4 42H2 þ 2CO DH298K ¼ 247:3 kJ=mol
(3)
C3 H8 O3 44H2 þ 3CO DH298K ¼ 251:2 kJ=mol
(4)
Furthermore, our recent investigations of glycerol steam reforming via response reactions (RERs) have proven, in theory, that Eq. (4) becomes the predominant glycerol decomposition pathway above 874 K (paper in preparation). From Fig. 2(b), one can also see that the production of CO decreases with an increase in either WGR or the fraction of H2 removal.
4.5 4.0
Water to glycerol ratio (WGR)
CO þ H2 O4H2 þ CO2
Carbon free region
3.5 3.0
Carbon region
2.5 2.0
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99
1.5 1.0
3.3.
Methane production
0.5 600
The number of moles of CH4 produced decreases with the increase in either temperature or f. The production of CH4 can be inhibited almost entirely when temperature is higher than 850 K, thermodynamically [see Fig. 3(a)]. As can be seen from Fig. 3(b), CH4 production decreases significantly with an increase in WGR. From a thermodynamic perspective, low temperature favors CH4 production. Zhang et al. also reported that glycerol decomposition to CH4 is highly favorable during
a
1.75
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
1.50
Moles of methane
1.25
1.00
0.50
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0.00 600
650
700
750
800
850
900
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1000
Temperature / K
b
1.00
f=0 f = 0.2 f = 0.4 f = 0.6 f = 0.8 f = 0.99 f=1
Moles of methane
0.75
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0.00 3
4
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8
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10
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700
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Temperature / K
Fig. 4 e Coke formation range plotted as a function of temperature and WGR at atmospheric pressure.
the reforming process at 673e723 K [15]. In addition, our study according to the RERs of glycerol steam reforming could confirm that glycerol mainly decomposes to methane and carbon dioxide with a small amount of hydrogen below 714 K (paper in preparation).
3.4.
0.75
650
Carbon formation
Carbon is an undesirable product in the reforming of hydrocarbon and oxygenates for hydrogen production, and the formation of carbon can also deactivate the catalysts. The elimination of carbon formation, therefore, is of great importance, especially when crude glycerol is used as a reactant [13]. Carbon formed and free regions at atmospheric pressure were plotted as a function of temperature and WGR in Fig. 4. In general, a WGR around 4.5 is high enough to inhibit carbon production at all temperatures considered in this work. For a given H2 removal fraction (except f ¼ 0.99), the minimum WGR required to avoid carbon formation decreases slightly with an increase in temperature until around 800 K, after which the decreasing trend becomes more significant. The influence of H2 removal fraction on the minimum WGR is negligible above 875 K. Below 875 K, however, the effect of fractional H2 removal differs depending upon whether above or below f ¼ 0.8. The minimum WGR for those f 0.8 increases with increasing f, whereas f > 0.8 a contrary trend is observed. It seems that unless H2 can be separated in situ to a large extent (i.e., f > 0.8), more carbon would form with increasing f at low WGRs (i.e., WGR < 4). Nevertheless, the easiest way to avoid this is to conduct the experiments with a high WGR as the optimum WGR for hydrogen production is ca. 9. Fluidized bed membrane reactors alternatively could be adopted, as it has been reported that both theoretical and experimental studies show that carbon formation is not an issue [16].
12
Water to glycerol ratio (WGR)
Fig. 3 e Methane production: effects of temperature (a, WGR [ 9) and WGR (b, T [ 850 K) at atmospheric pressure, n0(C3H8O3) [ 1 mol.
4.
Conclusions
Thermodynamic properties of glycerol steam reforming with in situ hydrogen separation have been investigated with the
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method of Gibbs free energy minimization. The effects of pressure, temperature, water to glycerol ratio and fraction of H2 removal on the reforming reactions and carbon formation were examined. The results suggest that extraction of H2 in situ can substantially enhance hydrogen production, as 7 mol (stoichiometric value) of hydrogen can be achieved from a temperature as low as 600 K due to the hydrogen separation. It is found that atmospheric pressure and a WGR of 9 are feasible for hydrogen production and the most favorable temperature for glycerol steam reforming with in situ hydrogen removal is between 825 and 875 K, 100 K lower than that achieved typically without hydrogen separation. Moreover, the negative influence of increasing pressure in hydrogen production becomes marginal above 800 K with a high fraction of H2 removal (i.e., f ¼ 0.99). High temperature and WGR are preferable to inhibit carbon formation. The minimum WGR required to eliminate carbon production can be reduced only with a high level of H2 removal. Glycerol steam reforming with in situ hydrogen separation displays considerable potential for hydrogen production.
Acknowledgments The financial support from the Program for New Century Excellent Talents in University (NCET-04-0242) and the Program of Introducing Talents of Discipline to Universities (B06006) are gratefully acknowledged. The authors thank Dr. Alexander B. Foster (Heriot-Watt University) for providing helpful discussion and language help.
references
[1] Davda RR, Shabaker JW, Huber GW, Cortright RD, Dumesic JA. A review of catalytic issues and process conditions for renewable hydrogen and alkanes by aqueousphase reforming of oxygenated hydrocarbons over supported metal catalysts. Appl Catal B 2005;56:171e86. [2] Cortright RD, Davda RR, Dumesic JA. Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water. Nature 2002;418:964e7. [3] Wang XD, Li SR, Wang H, Liu B, Ma XB. Thermodynamic analysis of glycerin steam reforming. Energy Fuels 2008;22:4285e91.
[4] Wang XD, Li MS, Wang MH, Wang H, Li SR, Wang SP, et al. Thermodynamic analysis of glycerol dry reforming for hydrogen and synthesis gas production. Fuel 2009;88:2148e53. [5] Wang H, Wang XD, Li MS, Li SR, Wang SP, Ma XB. Thermodynamic analysis of hydrogen production from glycerol autothermal reforming. Int J Hydrogen Energy 2009; 34:5683e90. [6] Kale GR, Kulkarni BD. Thermodynamic analysis of dry autothermal reforming of glycerol. Fuel Process Tech 2010; 91:520e30. [7] Chen HS, Zhang TF, Dou BL, Dupont V, Williams P, Ghadiri M, et al. Thermodynamic analyses of adsorption-enhanced steam reforming of glycerol for hydrogen production. Int J Hydrogen Energy 2009;34:7208e22. [8] Gallucci F, Basile A, Tosti S, Iulianelli A, Drioli E. Methanol and ethanol steam reforming in membrane reactors: an experimental study. Int J Hydrogen Energy 2007;32: 1201e10. [9] Iulianelli A, Longo T, Liguori S, Basile A. Production of hydrogen via glycerol steam reforming in a Pd-Ag membrane reactor over Co-Al2O3 catalyst. Asia-Pac J Chem Eng 2010;5:138e45. [10] Iulianelli A, Seelam PK, Liguori S, Longo T, Keiski R, Calabro V, et al. Hydrogen 12 production for PEM fuel cell by gas phase reforming of glycerol as byproduct of bio-diesel. The use of a Pd–Ag membrane reactor at middle reaction temperature. Int J Hydrogen Energy, in press, doi:10.1016/j. ijhydene.2010.02.079. [11] Yaws CL. Chemical properties handbook. New York: McGraw-Hill; 1999. [12] Adhikari S, Fernando S, Gwaltney SR, Filip To SD, Bricka RM, Steele PH, et al. A thermodynamic analysis of hydrogen profuction by steam reforming of glycerol. Int J Hydrogen Energy 2007;32:2875e80. [13] Slinn M, Kendall K, Mallon C, Andrews J. Steam reforming of biodiesel by-product to make renewable hydrogen. Bioresour Technol 2008;99:5851e8. [14] Corma A, Huber GW, Sauvanaud L, O’Connor P. Biomass to chemicals: catalytic conversion of glycerol/water mixtures into acrolein, reaction network. J Catal 2008;257: 163e71. [15] Zhang BC, Tang XL, Li Y, Xu YD, Shen WJ. Hydrogen production from steam reforming of ethanol and glycerol over ceria-supported metal catalysts. Int J Hydrogen Energy 2007;32:2367e73. [16] Ye GY, Xie DL, Qiao WY, Grace JR, Jim Lim C. Modeling of fluidized bed membrane reactors for hydrogen production from steam methane reforming with Aspen Plus. Int J Hydrogen Energy 2009;34:4755e62.