Thermodynamic Study on Hydrogen Generation from Different

Energy & Fuels 2007, 21, 3505–3512

3505

Thermodynamic Study on Hydrogen Generation from Different Glycerol Reforming Processes Nianjun Luo,† Xun Zhao,† Fahai Cao,‡ Tiancun Xiao,*,‡,§ and Dingye Fang‡ Institute of Chemical Technology, ECUST, Shanghai 200237 China, State-Key Laboratory of Chemical Engineering, ECUST, Shanghai 200237 China, and The Wolfson Catalysis Center, Inorganic Chemistry Laboratory, Oxford UniVersity, U.K. ReceiVed February 2, 2007. ReVised Manuscript ReceiVed July 12, 2007

This work is to study the thermodynamic property of a glycerol aqueous reforming process to generate hydrogen for fuel cells. Three ways of processing glycerol have been considered: autothermal reforming, a combination of water aqueous reforming and oxidation; aqueous hydrogen peroxide reforming; and the water aqueous reforming process. Thermodynamic analysis on three of them has been carried out, and the results show that the side reaction of methanation in each process route leads to a dramatic decrease of hydrogen content in the gas product, which consequently should be limited kinetically. Comparison among them indicates that, in the absence of methanation, the hydrogen content produced from water aqueous reforming of glycerol is the highest, followed by that from autothermal reforming and that from aqueous hydrogen peroxide reforming. It has also been shown that the requirements of external energy input to sustain these reforming reactions appear inversely among them. Therefore, it can be concluded thermodynamically that the external energy can be reduced at the price of losing hydrogen yield due to the exothermic oxidation, which consumes hydrogen to release energy.

1. Introduction The continuously deteriorating environmental quality and the depletion of the fossil fuel reserves have caused people to pay more and more attention to seeking diversity of energy resources. Meanwhile, the increasingly strict emission standards for motor vehicles (i.e., Euro IV emission standard came into force in Europe on October 1, 2005, and Euro V will be adopted in 2008) are being issued step by step in the past and coming years to relieve the environmental problem. Under such a circumstance, a clean energy resource, hydrogen generation, and supply are attracting public attention and becoming hot research topics in recent years1 because hydrogen is emission-free to the environment except for the only by-product of water. However, the fuel cell, the device to make use of hydrogen, is very sensitive to carbon monoxide content in hydrogen feedstocks, even 100 ppm CO in hydrogen can deteriorate proton exchange membrane fuel cell (PEMFC) performance. Therefore in the traditional hydrogen-generating processes (such as steam reforming), multistep processes have been combined to purify the hydrogen product so as to meet the requirements of low levels of carbon monoxide content.2 All these factors motivate researchers to explore novel approaches to produce hydrogen efficiently and simply. On the other hand, being considered as renewable and sustainable resources, biomass-derived polyols seem to be * Corresponding author. Tel: 0044-1865-272660. E-mail address: [email protected]. † Institute of Chemical Technology, ECUST. ‡ State-Key Laboratory of Chemical Engineering, ECUST. § Oxford University. (1) Chui, F.; Elkamel, A.; Fowler, M. An Integrated Decision Support Framework for the Assessment and Analysis of Hydrogen Production Pathways. Energy Fuels 2006, 20, 346–352. (2) Davda, R. R.; Dumesic, J. A. Catalytic Reforming of Oxygenated Hydrocarbons for Hydrogen with Low Levels of Carbon Monoxide. Angew. Chem., Int. Ed. 2003, 42, 4068–4071.

Figure 1. Dependence of water to oxygen mole ratio on the reaction temperature in autothermal reforming for achieving energy neutral (no energy input needed for sustaining the reactions) conditions.

Figure 2. Dependence of oxygen to glycerol mole ratio on the reaction temperature in autothermal reforming for achieving energy neutral (no energy input needed for sustaining the reactions) conditions.

promising feedstock for producing hydrogen .3–6 For example, glycerol, a byproduct of biodiesel process from vegetable oils and animal fats,7 is reported8 to be a candidate for starting material for generating hydrogen via aqueous reforming since it has no wide application in the other aspects.

10.1021/ef070066g CCC: $37.00  2007 American Chemical Society Published on Web 09/11/2007

3506 Energy & Fuels, Vol. 21, No. 6, 2007

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Figure 3. Plot of thermodynamic equilibrium product mole fraction of each component as a function of T and Psystem/PH2O(T) without methanation when water aqueous reforming is employed. Table 1. Reaction Heat of Reactions 1–6 reaction heat (kJ/mol) T (K)

water aqueous reforming

oxidation

aqueous hydrogen peroxide reforming

R-WGS

methanation

formation of gaseous water

298 300 325 350 375 400 425 450 475 500

214.52 214.51 214.24 213.74 212.98 211.97 210.70 209.17 207.38 205.29

-510.92 -510.99 -512.01 -513.25 -514.75 -516.50 -518.50 -520.76 -523.27 -526.07

-306.45 -306.48 -306.96 -307.70 -308.74 -310.07 -311.68 -313.58 -315.76 -318.26

41.17 41.16 41.06 40.94 40.80 40.64 40.46 40.26 40.06 39.84

-206.10 -206.20 -207.35 -208.47 -209.57 -210.63 -211.66 -212.65 -213.61 -214.53

-241.82 -241.84 -242.08 -242.33 -242.58 -242.83 -243.08 -243.33 -243.57 -243.81

Recently, we have carried out a thermodynamic analysis9 of aqueous reforming of polyols for hydrogen generation. In this report, we aim to analyze and investigate the possibilities of different routes of reforming glycerol to generate hydrogen and compare them so as to find a more economical and simple process for hydrogen generation. 2. Reforming Technologies Aqueous reforming is reported10,11 to be superior to steam reforming, which suffers from many limitations (i.e., high (3) Asadullah, M.; Ito, S. I.; Kunimori, K.; Yamada, M.; Tomishige, K. Energy Efficient Production of Hydrogen and Syngas from Biomass: Development of Low-Temperature Catalytic Process for Cellulose Gasification. EnViron. Sci. Technol. 2002, 36, 4476–4482.

temperature and high level of CO content), especially for polyols, since aqueous reforming can occur at low reaction temperature, which can reduce external energy input and favor (4) Czernik, S.; French, R.; Feik, C.; Chornet, E. Hydrogen by Catalytic Steam Reforming of Liquid Byproducts from Biomass Thermoconversion Process. Ind. Eng. Chem. Res. 2002, 42, 4209–4215. (5) Shen, L. H.; Xiao, J.; Niklasson, F.; Johnsson, F. Biomass Mixing in a Fluidized Bed Biomass Gasifier for Hydrogen Production. Chem. Eng. Sci. 2007, 62, 636–643. (6) Wang, D.; Czernik, S.; Montane, D.; Mann, M.; Chornet, E. Biomass to Hydrogen via Fast Pyrolysis and Catalytic Steam Reforming of the Pyrolysis Oil or its Fractions. Ind. Eng. Chem. Res. 1997, 36, 1507–1518. (7) Gerpen, J. V. Biodiesel processing and production. Fuel Proc. Technol. 2005, 86, 1097–1107. (8) Soares, R. R.; Simonetti, D. A.; Dumesic, J. A. Glycerol as a Source for Fuel and Chemicals by Low-Temperature Catalytic Processing. Angew. Chem., Int. Ed. 2006, 45, 3982–3985.

Thermodynamic Study on Hydrogen Generation

Energy & Fuels, Vol. 21, No. 6, 2007 3507

Figure 4. Plot of thermodynamic equilibrium product mole fractions of each component as a function of T and Psystem/PH2O(T) with methanation when water aqueous reforming is employed. Table 2. Reaction Equilibrium Constant of Reactions 1–6 T (K)

water aqueous reforming (atm7)

oxidation (atm5.5)

aqueous hydrogen (atm7)

R-WGS

methanation (atm-2)

formation of gaseous water (atm-0.5)

298 300 325 350 375 400 425 450 475 500

3.6733 × 108 6.5422 × 108 4.8666 × 1011 1.3938 × 1014 1.8509 × 1016 1.3109 × 1018 5.5129 × 1019 1.4972 × 1021 2.8063 × 1022 3.8292 × 1023

5.7441 × 10128 1.4526 × 10128 2.0503 × 10121 2.6742 × 10115 2.0585 × 10110 6.6832 × 10105 7.0801 × 10101 2.0054 × 1098 1.2978 × 1095 1.6924 × 1092

9.9923 × 10100 4.3807 × 10100 3.4191 × 1096 1.0142 × 1093 8.7069 × 1089 1.7644 × 1087 7.2254 × 1084 5.3020 × 1082 6.3420 × 1080 1.1465 × 1079

9.6314 × 10-6 1.0759 × 10-5 3.8233 × 10-5 1.1303 × 10-4 2.8830 × 10-4 6.5215 × 10-4 1.3361 × 10-3 2.5202 × 10-3 4.4340 × 10-3 7.3530 × 10-3

8.6590 × 1024 4.9725 × 1024 8.4614 × 1021 3.4739 × 1019 2.8930 × 1017 4.2895 × 1015 1.0247 × 1014 3.6481 × 1012 1.8200 × 1011 1.2109 × 1010

1.1585 × 1040 6.0440 × 1039 3.4734 × 1036 5.7579 × 1033 2.2285 × 1031 1.7184 × 1029 2.3384 × 1027 5.1094 × 1025 1.6639 × 1024 7.6080 × 1022

water–gas shift (WGS) to decrease the content level of carbon monoxide, etc.; thus in this work, glycerol is considered to be treated via aqueous reforming at the beginning. Moreover, from the view of energy efficiency, the endothermal aqueous reforming reaction of glycerol is to couple with an exothermic oxidation reaction to make up an autothermal process, which has little difference with other reports,12 and its possibility to require no external energy input would also be analyzed. In addition, Xiao’s patent13 shows that hydrogen generation can be realized from reforming methanol with peroxide solution instantly starting at room temperature. Based on this, the feasibility has been investigated on glycerol reforming with aqueous hydrogen peroxide solution instead of just water. From above, three different kinds of glycerol reforming processes are proposed in this work: (a) water aqueous reforming

of glycerol, (b) a combination of water aqueous reforming and oxidation, that is, autothermal reforming, and (c) reforming glycerol with aqueous hydrogen peroxide solution. The primary reactions related to the above processes consist of: C3H5(OH)3 + 3H2O f 3CO2 + 7H2 water aqueous reforming (1) 3 C3H5(OH)3 + O2 f 3CO2 + 4H2 oxidation (2) 2 3 C3H5(OH)3 + H2O2 f 2 11 3CO2 + H2 hydrogen peroxide aqueous reforming (3) 2 Of course, the reverse water–gas shift (R-WGS) reaction may occur to give carbon monoxide as an undesirable byproduct in


Luo et al.

Figure 5. Plot of thermodynamic equilibrium product mole fraction of each component as a function of T and m without methanation when aqueous reforming is coupled with oxidation.

the gas stream, and it is likely to be along with the side reactionhydrogenation (in this work methanation is taken for example): CO2 + H2 T CO + H2O reverse water gas shift reaction (4) CO + 3H2 T CH4 + H2O methanation

(5)

1 H2 + O2 f H2O formation of gaseous water 2

(6)

Obviously, process a may contain three independent reactions 1, 4, and 5; process b can be divided into four independent reactions 1, 2, 4, and 5; process c is made up of four independent reactions 1, 3, 4, and 5. Some other side reactions, such as formation of methane from carbon dioxide and hydrogen, can be deduced from the above reactions. Note that process b can also be regarded as the combination of reactions 1, 4, 5, and 6. Here reaction 6 is the formation of (9) Luo, N.; Cao, F.; Zhao, X.; Xiao, T.; Fang, D. Thermodynamic Analysis of Aqueous-Reforming of Polyols for Hydrogen Generation. Fuel 2007, 86, 1727–1736. (10) Cortright, R. D.; Davda, R. R.; Dumesic, J. A. Hydrogen from Catalytic Reforming of Biomass-Dderived Hydrocarbons in Liquid Water. Nature 2002, 418, 9649–9666. (11) Huber, G. W.; Shabaker, J. W.; Dumesic, J. A. Raney Ni-Sn catalyst for H2 Production from Biomass-Derived Hydrocarbons. Science 2003, 300, 2075–2077. (12) Deluga, G. A.; Salge, J. R.; Schmidt, L. D.; Verykios, X. E. Renewable Hydrogen from Ethanol by Autothermal Reforming. Science 2004, 303, 993–997.

gas–water from hydrogen and oxygen. This assumption will facilitate our thermodynamic analysis and its calculation. 3. Calculation and Results 3.1. Reaction Heat. The reaction heat can be obtained from the following equation: ∆HR ) ∆H0R + ∫TT0 (Cproducts Creactants) dT and the results are presented in Table 1. Table 1 indicates that all the reactions are exothermic except for water aqueous reforming and reverse water–gas shift reaction. As a general trend, among the reactions of oxidation, aqueous hydrogen peroxide reforming, methanation, and formation of gaseous water, the released energy increases with increasing reaction temperature, while water aqueous reforming and reverse water–gas shift require lower energy as reaction temperature increases. Considering that water aqueous reforming is endothermic and oxidation is exothermic, an autothermal reaction, process b, is proposed to couple water aqueous reforming with oxidation. Thus, no external energy is required for conducting reforming reactions to produce hydrogen-rich feed. Figures 1 and 2 depict how much oxygen would be needed when conducting the autothermal reaction for achieving energy neutral (no energy input needed for sustaining the reaction) conditions. From Figure 1, it can be seen that to meet the requirement of autothermal reactions in process b, the moles of water are



Figure 6. Plot of thermodynamic equilibrium product mole fraction of each component as a function of T and m with methanation when water aqueous reforming is coupled with oxidation.

around 5 times of those of oxygen and this ratio slightly increases as reaction temperature increases from 298 to 500 K. For the water aqueous reforming, less than 1/5 oxygen to water ratio is needed since water is much more superfluous compared with glycerol feedstock, which can be regarded to convert into products completely. Therefore, for every mole of glycerol, the lowest requirement of moles of oxygen can be calculated and presented in Figure 2 as a trend that seems to keep a constant value of around 0.44 and slightly decreases with increaseing reaction temperature. 3.2. Reaction Equilibrium Constant. The reaction equilibrium constant can be obtained from 0 -∆G298.15K + lnKP ) 298.15R

∫

T

∆HT

dT RT2 and the calculation results are shown in Table 2. Table 2 lists the reaction equilibrium constants of above reactions at different temperatures. It is assumed that water aqueous reforming, oxidation, hydrogen peroxide aqueous reforming, and formation of gaseous water are irreversible at a constant system pressure due to the high value of their reaction equilibrium constants and the coexistence of liquid water. Table 2 also unveils the fact that low reaction temperature leads to higher equilibrium conversion for oxidation, aqueous 298.15K

(13) Xiao, T. Catalytic Reaction between Methanol and a Peroxide. European Patent EP1711431, 2006, pp 10–18.

hydrogen peroxide reforming, methanation, and formation of gaseous water and the fact that high temperature favors water aqueous reforming and reverse water–gas shift reaction. Because the water aqueous reforming reaction can be regarded to be irreversible, due to high H2O/C ratio, choosing low reaction temperature will have little impact on converting glycerol into CO2 and H2 thermodynamically but may be beneficial to prevent CO2 from reacting with H2 to form CO and H2O due to the very low R-WGS equilibrium constant. 4. Equilibrium Component During the operation of processing glycerol to produce hydrogen-rich feed for fuel cells, the side reaction of methanation is expected to not occur in the system. However, it could not be avoided thoroughly in real operation. Therefore, the following work of calculating equilibrium component is carried out under two assumptions, that is, with methanation and without methanation reactions. The following figures present the trends of how the equilibrium fraction of each component in the gas products is influenced by the operation factors. 4.1. Water Aqueous Reforming. As to water aqueous reforming, the investigated variables are reaction temperature and system pressure higher than the saturated water pressure. Figure 3 illustrates the effects of reaction temperature and the ratio of system pressure, Psystem, to the saturated water


Luo et al.

Figure 7. Plot of thermodynamic equilibrium product mole fractions of each component as a function of T and w without methanation when aqueous hydrogen peroxide reforming is considered.

pressure, PH2O(T), on the equilibrium fraction of carbon monoxide, hydrogen, and carbon dioxide in the gas product (dry basis) when only water aqueous reforming occurs without methanation in the system. Figure 3a indicates that low reaction temperature can give a very low mole fraction of carbon monoxide at a very low constant value when the ratio of Psystem to PH2O(T) ranges from 1.05 to 1.5. The higher temperature and higher pressure ratio will dramatically increase the CO mole fraction. Parts b and c of Figure 3 show the changes of hydrogen and carbon dioxide under the condition that reaction temperature ranges from 300 to 500 K and pressure ratio ranges from 1.05 to 1.5. Obviously, both fractions of hydrogen and carbon dioxide reach high plateaus of around 70% and 30%, respectively, which are in good agreement with their own stoichiometric coefficient shown in reaction 1. Therefore, it can be concluded thermodynamically that under the conditions of low reaction temperature and low pressure ratio, the undesirable product of CO can be lowered to meet the requirement for fuel cells. Figure 4 presents another change of the trends of each fraction when methanation is regarded to occur in the reforming reaction system. Within our study area, the equilibrium fraction of carbon monoxide decreases dramatically to less than 10 ppm. Also, the objective product of hydrogen content is sharply reduced from around 70% to less than 4% at the same time. Consequently, most of the gas products are made up of carbon dioxide and methane, which are shown in parts c and d of Figure 4.

4.2. Autothermal Reforming Process. The combination of water aqueous reforming and oxidation focuses on how oxygen impacts the result when autothermal reactions are employed, and consequently the factors investigated here include reaction temperature and the mole ratio of oxygen to glycerol (m) with the system pressure regulated as 1.1 times the saturated water pressure. Figure 5 illustrates how CO, H2, and CO2 are influenced by these two variables: clearly CO content is mainly decided by reaction temperature, and the oxygen/glycerol ratio seems to be the dominant factor that controls H2 and CO2 content in the gas products. Particularly, it appears that the mole fractions of H2 and CO2 are nearly in line with the variable of m, and higher mole ratio of oxygen to glycerol will lead to lower hydrogen and higher carbon dioxide, undesirably. Figure 6 shows the component equilibrium fraction in the autothermal reactions when methanation is considered with Psystem/PH2O(T) equal to 1.1. Compared with Figure 3, the CO content varies with reaction temperature and the oxygen/glycerol ratio does not appear much different on the whole, but the hydrogen mole fraction decreases slightly. The majority of the gas products still consists of CO2 and CH4. Compared with Figure 5, CO2 content increases from less than 40% without methanation to above 50% with this side reaction, and methane makes up more than 30% of the gas products, but it decreases directly as mole ratio m increases from 0 to 1.



Figure 8. Plot of thermodynamic equilibrium product mole fraction of each component as a function of T and w with methanation when aqueous hydrogen peroxide reforming is employed.

5. Discussion

For different processes, the variables wielding influence on the equilibrium fraction of each component have been studied: In water aqueous reforming, reaction temperature and system pressure are investigated; in autothermal reforming, reaction temperature and the oxygen/glycerol ratio are explored; in aqueous hydrogen peroxide reforming, reaction temperature and weight fraction of hydrogen peroxide in water are considered. There is a common result in these three processes: methanation is undesirable in hydrogen generation via reforming glycerol, and due to its existence, the mole fraction of the objective product of hydrogen would decrease dramatically to unacceptable levels in gas products. Therefore, priority should be given to the limitation of reaction rate of methanation and operation should aim to weaken the formation of the alkane kinetically. Comparison of the above figures shows that the hydrogen content generated from water aqueous reforming is the highest, followed by that created from autothermal reactions and by that produced from aqueous hydrogen peroxide reforming; all of them do not take methanation into consideration. However, for autothermal reactions, the external energy necessary for operation can be lessened even to zero, since the released energy from oxidation can make up the requirements for the reforming reaction. It has also been reported14 that steam autothermal

Three kinds of glycerol reforming processes to generate hydrogen-rich feedstock for fuel cells have been discussed with and without considering the side reaction of methanation.

(14) Chen, Z. X; Elnashaie, S. E. H. Economics of the Clean Fuel Hydrogen in a Novel Autothermal Reforming Process. Ind. Eng. Chem. Res. 2005, 44, 4834–4840.

4.3. Aqueous Hydrogen Peroxide Reforming. In this process, the variables influencing the equilibrium fraction are reaction temperature and the weight fraction of H2O2 to H2O (w), both of which have the potential to take part in the reforming reaction as in eqs 1 and 3. Here the system pressure is also regulated as 1.1 times the saturated water pressure at T (K). Figure 7 describes how the equilibrium mole fraction of each component varies with the variables of reaction temperature and weight fraction of hydrogen peroxide to water. Part a demonstrates that the CO content is mainly affected by reaction temperature rather than the weight fraction of hydrogen peroxide within the study area. Hydrogen and carbon dioxide are primarily influenced by the weight fraction of hydrogen peroxide in water, varying from 64% to 70% and from 30% to 36%, respectively, when weight fraction w ranges from 0 to 100%. As shown in Figures 4 and 6, Figure 8 also indicates that methanation plays an important role in the equilibrium fraction of each component during the operation of aqueous hydrogen peroxide reforming. CO2 and CH4 dominate in and make up more than 98% of the gas products, and the objective product of hydrogen is very little, so undesirably.


reforming (not aqueous autothermal reactions proposed in this work) attracts more attention due to the lower cost of hydrogen production. As to aqueous hydrogen peroxide reforming, its advantage lies in that its energy requirement from outside is reduced in comparison with water aqueous reforming and its operation is simplified: no additional oxygen from another pipeline is needed in comparison with autothermal reactions. 6. Conclusion In summary, three kinds of glycerol reforming processes to generate hydrogen for fuel cells, water aqueous reforming, autothermal reforming, and H2O2/water aqueous reforming, have been studied from the view of thermodynamics. The results show that a high fraction of hydrogen can be generated with a

Luo et al.

reasonable level of CO for PEMFCs when the methanation reaction is fully limited kinetically. Comparison of these three kinds of processes shows that each of these three hydrogen production pathways has its own advantage and deserves further investigation and that the savings of the external energy requirement, for example, energy neutral reforming without external energy input, is at the expense of the hydrogen fraction and consequently results in increase of carbon dioxide in the gas products due to oxidation, which must be restricted through kinetic control. Acknowledgment. This work is financially supported by the Natural Science Foundation of China (No. 20676035). EF070066G

Thermodynamic Study on Hydrogen Generation from Different

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