Steam Reforming of Oxygenate Fuels for Hydrogen Production: A

ARTICLE pubs.acs.org/EF

Steam Reforming of Oxygenate Fuels for Hydrogen Production: A Thermodynamic Study Jichao Li, Hao Yu,* Guangxing Yang, Feng Peng, Donglai Xie, Hongjuan Wang, and Jian Yang School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China ABSTRACT: A thermodynamic analysis of the steam reforming of representative oxygenate fuels, including methanol, ethanol, n-propanol, n-butanol, n-hexanol, ethylene glycol, glycerol, glucose, acetic acid, and acetone, was carried out with the Gibbs freeenergy minimization method. The operational regime, energy efficiency, and reformate composition of reforming various oxygenate fuels were studied. It was revealed that the critical steam/carbon ratio, by which there is free carbon deposition in the reforming product, decreases with an increasing oxygen/carbon ratio in oxygenate fuels. The appropriate operating temperature range for steam reforming of oxygenate fuels is within 600700 °C. Fuels with a higher hydrogen/carbon ratio have wider operational windows. The results would offer a guideline toward a rational selection of raw materials for a renewable reformer proton-exchange membrane fuel cell system based on understanding features of oxygenate fuels in the steam-reforming process.

1. INTRODUCTION Hydrogen is considered as a nonpolluting, inexhaustible, efficient, and cost-attractive energy carrier for the future, as used in H2O2 proton-exchange membrane fuel cells (PEMFCs).1 Hydrogen can be produced through electrochemical, thermochemical, photochemical, or photoelectrochemical processes.2,3 Currently, about 95% of global hydrogen production is from the thermocatalytic and gasification processes of fossil fuels, such as natural gas, heavy oils, naphtha, and coal, among which steam reforming (SR) of natural gas contributes the largest volume.4 The extensive usage of fossil fuels is causing severe air pollution, greenhouse effect, and natural resource depletion. To minimize the non-renewability of hydrogen production, there is a growing interest in seeking new materials for the SR processes. Oxygenate fuels can be produced from fermentation, fast pyrolysis, and hydrolysis of biomass. Hydrogen production from oxygenate fuels derivated from biomass is sustainable, because CO2 emitted in the hydrogen production process can be reabsorbed by the growth of biomass via photosynthesis.5,6 Reforming of oxygenate fuels for hydrogen production could be more active and resistant to coking than reforming of hydrocarbons.7 Oxygenate fuels are usually liquid under ambient conditions and are easy to store and transport. The representative biomass-derived oxygenate fuels include alcohol, acid, ketone, ester, etc.8 Fermentation of carbohydrates is the primary technique for generating liquid fuels (ethanol) from renewable biomass resources.8,9 Zeolite upgrading (HZSM-5) of biomass compounds can produce propanol, butanol, and acetone.10 Butanol and acetone can be produced by fermentation of sugar beet, sugar cane, corn, wheat, and lignocellulosic biomass.11 Glycerol is an important byproduct in the production of biodiesel by transesterification of vegetable oils or animal fats.12 Recently, ethylene glycol and other polyols were obtained by the direct catalytic conversion of cellulose.13 Fast pyrolysis of biomass will generate bio-oil, a mixture of acetic acid, ethylene glycol, acetone, ethyl acetate, m-xylene, and glucose.14,15 The SR of methanol for hydrogen production has been studied extensively.1620 In comparison to SR of methane, the reaction r 2011 American Chemical Society

temperature of methanol reforming is much lower (150300 °C). The reformate of methanol contains typically 6070% H2 and less than 5% CO on a dry basis. Bioethanol is a safe alternative material for the SR process, as demonstrated by many reports.4,2123 SR of 2-propanol and n-butanol has been experimentally investigated by Mizuno et al.24 and Bimbela et al.25 with Rh/CeO2 and NiAl catalysts. Glycerol, a byproduct of biodiesel, is currently regarded as a promising alternative for renewable fuels. The reports on SR of glycerol for hydrogen production over nickel and noble metal catalysts are rapidly increasing in the past few years.2628 As a new technique, aqueous reforming of glycerol at low temperature and high pressure can also be used for hydrogen production, as demonstrated by Luo et al.,29 which eliminates the need to vaporize water and facilitates the watergas shift reaction (WGSR) to lower the CO concentration in the reformate. Marquevich and co-workers have produced hydrogen by reforming glucose, xylose, and sucrose.30 Biooil (as a whole or its selected fractions) can be converted to hydrogen via catalytic SR.15,31 In several recent reviews, hydrogen or synthesis gas production from biomass or bioderived compounds is summarized in detail.1,5,32,33 However, a systematic comparative study on features of various oxygenate fuels in the SR process is rarely reported. Herein, we presented a systematic investigation on SR of representative oxygenate fuels in an equilibrium model integrated with a fuel cell for power generation. Attention was focused on the operational regime, energy efficiency, and reformate composition of SR processes with various oxygenate fuels. The work may offer a guideline toward a rational design of a renewable reforming technique based on the understanding of features of oxygenate fuels in the reforming process.

2. METHODOLOGY 2.1. Chemical Reactions. The oxygenate fuels involved in the current study include typical monohydric alcohols, polyols, glucose, Received: October 22, 2010 Revised: April 28, 2011 Published: April 29, 2011 2643

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Table 1. SR Reactions, Enthalpy Changes, and Standard Heats of Combustion of the Oxygenates Investigated in This Paper fuel

ΔH°298 K (kJ/mol)

reaction

ΔHc°298 K (kJ/mol)

methanol

CH4O þ H2O T CO2 þ 3H2

48.97

726.5

ethanol

C2H6O þ 3H2O T 2CO2 þ 6H2

173.54

1367.5

n-propanol


286.09

2019.8

n-butanol


393.11

2675.8

n-hexanol

C6H14O þ 11H2O T 6CO2 þ 18H2

1160.58

3984.4

ethylene glycol

C2H6O2 þ 2H2O T 2CO2 þ 5H2

88.89

1189.2

glycerol

C3H8O3 þ 3H2O T 3CO2 þ 7H2

127.67

1655.3

glucose acetic acid

C6H12O6 þ 6H2O T 6CO2 þ 12H2 C2H4O2 þ 2H2O T 2CO2 þ 4H2

375.22 134.86

2815.8 874.4

acetone


245.43

1790.0

acetic acid, and acetone. Most of these fuels can be produced by fermentation, fast pyrolysis, and hydrolysis. Therefore, the hydrogen production from them can be viewed as renewable. The SR reactions of the above oxygenates can be generally described by Cx Hy Oz þ ðx zÞH2 O T xCO þ ðx þ y=2 zÞH2

ð1Þ

By coupling with the WGSR xCO þ xH2 O T xCO2 þ xH2

ð2Þ

Complete selectivity of H2 at the expense of CO takes the form of eq 3 Cx Hy Oz þ ð2x zÞH2 O T xCO2 þ ð2x þ y=2 zÞH2

ð3Þ

The enthalpy changes of the overall reactions and the heats of combustion for different oxygenate fuels at 298 K and 1 atm are listed in Table 1. 2.2. Computational Method. The thermodynamic analysis of the SR of oxygenate fuels was carried out in a commercial Aspen plus simulator, which allows for a sequential modular simulation of a chemical process covering standard unit operations. The Gibbs free-energy minimization method was used to carry out the thermodynamic analysis. As the system reaches thermodynamic equilibrium, the total Gibbs free energy of the system should reach a minimum under the operation temperature.34,35 Hence DG ¼0 Dni

ð4Þ

The total free energy of the system can be expressed as G¼

∑ni G°i þ ∑ni RT ln yi þ ∑ni RT ln P

ð5Þ

subject to the constrains of element mass balance N

∑ ni Rik ¼ bk i¼1

k ¼ 1, :::, M

ð6Þ

Besides the restrictions of the elemental mass balance, it is necessary to take into account the non-negativity constraints of ni (ni g 0). The Gibbs free energy of species i is calculated by Gi ¼ Hi TSi Z Hi ¼ Hi 298 þ

T

Cpi dT

ð8Þ

Cpi dT T298 T

ð9Þ

T298

Z Si ¼ Si 298 þ

ð7Þ

T

A series of nonlinear equations can be obtained from the above analysis. The reformate composition (ni) can then be derived from solving these equations.

In Gibbs free-energy minimization method, the species coexisting in the system should be defined. According to relevant literature4,36 and our previous studies3740 carried out on methanol, ethanol, and glycerol, hydrogen, carbon monoxide, carbon dioxide, methane, carbon (graphite and solid), ethylene, acetaldehyde, and ethane were considered in the product. In the case of reforming methanol, only hydrogen, carbon monoxide, carbon dioxide, methane, and carbon (graphite and solid) were considered. Because the Gibbs free energy of formation increases with the number of carbon atoms, compounds with more than three carbon atoms are therefore not likely to exist in the system studied. Equilibrium composition of these species was calculated through the Gibbs free-energy reactor model in Aspen plus. The similar assumption on the Gibbs free energy minimization has been adopted by several researches on reforming ethanol, glycerol, acetic acid, acetone, and ethylene glycol for hydrogen production.34,4143 Recent experimental studies indicated that, from a properly designed process and over selected catalysts, the near-equilibrium products can be achieved.44 2.3. Fuel Processor and Model Libraries. The fuel processing system in this work is shown in Figure 1. Oxygenate fuels, at 25 °C and 1 atm, are fed at a constant flow rate of 100 kmol/h. In the case of partial oxidation steam reforming (POSR), oxygen is fed at 25 °C and 1 atm. Byproducts (mainly containing carbon monoxide and a small quantity of methane) and unrecovered hydrogen from a separator (see the third paragraph of this section) can be simply used by a heat exchanger and a catalytic combustor, where the combustible byproduct and residual hydrogen are converted to CO2 and H2O. The reactants are first preheated by the effluent gas of the separator in a heat exchanger to use the sensible heat. Meanwhile, the effluent stream of the separator is cooled to 400 °C. To use the heat of combustion, an adiabatic catalytic combustor is coupled with the reformer. Air is fed into the combustor at 25 °C and 1 atm and guarantees 50% excess of oxygen. The complete combustion of all fuels is assumed. In a well-designed combustion heat exchanger, heat-transfer efficiency can be up to 6585% using microchannels.45 In this study, it was assumed that 50% of the sensible heat and heat of combustion of the effluent gas was recovered and used to heat the reactants and reformer. In the current study, the reformer was considered as isothermal and isobaric, although the strong endotherm leads to axial and radial temperature profiles in a practical case. A heater can be used to maintain the reactor temperature, which equals that of the outlet. Such a simplification cannot guide the reactor design but allows for a realistic estimate of the extent of reaction.42 The reforming section was modeled by minimizing the total Gibbs free energy of the system, composed on an ideal gas phase and a solid phase (carbon and graphite). At low pressure and high temperature, the gas phase could be considered as ideal. Thus, the fugacity coefficient of species i in the gaseous phase was equal to unity. In all calculations, it was assumed P = 1 atm. The hydrogen-rich gas off the reformer can be further separated and purified by various techniques or their combination, such as WGSR, 2644

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Figure 1. Schematic diagram of the steam reformerPEMFC process investigated in this work. methanation, CO preferential oxidization (PROX), pressure swing adsorption (PSA), and Pd membrane.4749 The overall energy efficiency of the reforming process strongly depends upon the purification technique selected. PSA and Pd membrane techniques are energy-extensive because they are operated at high pressures. The catalytic approach, e.g., the combination of a high- and low-temperature WGSR, a methanator, or CO PROX reactor, could reduce the energy consumption of purification. However, the selectivity of the catalytic process strongly depends upon the catalyst performance. Heat integration is of utmost importance because the consumption of thermal energy is a key issue in the design of the reforming systems.50 In this study, we concentrate on the feature of the SR reactor, especially on the effect of oxygenate fuels. Hence, in an extremely simplified model, it was ideally assumed that the separator (modeled by SEP in Aspen Plus) is operated at the same temperature and pressure of the reformer and extracts 90% of hydrogen from the reformate. In the context of hydrogen economy, the energy consumption of a reformer is considered to be ultimately supplied by a PEMFC, with the hydrogen produced by the reformer as fuel. The PEMFC is modeled as an adiabatic stoichiometric reactor (modeled by RSTOIC in Aspen Plus) operating under 80 °C and 1 atm,51,52 with electrical efficiency of 65%.51,53 The oxygen and hydrogen at the cathode and anode was fed at 25 °C and 1 atm.54 Thus, the PEMFC power is 0.65 0.9 ΔH°298 K (241.8 kJ/mol) FH2 (kmol/s), where FH2 is the molar flow rate of hydrogen in reformer outlet and 241.8 kJ/mol is the combustion heat of H2 with the formation of gasphase water. Because the SR reaction is endothermal, the net power output is the difference of the PEMFC power minus the power for heating the reactant and maintaining the reactor temperature ΔE ¼ 0:65 0:9 241:8 ðkJ=molÞ FH2 ðkmol=sÞ Ereactor ðMWÞ

ð10Þ

ΔE > 0 will be a conservative constraint for operation of the reformer. 2.4. Definition of Efficiency. The energy efficiency of the overall system for SR and POSR processes shown in Figure 1 is estimated as efficiency ð%Þ ¼ 100 ð0:65 0:9 FH2 ðkmol=sÞ 241:8 ðkJ=molÞ Ereactor ðMWÞ þ 0:5 Eexchanger ðMWÞÞ=ðFfuel ðkmol=sÞ ΔH°fuel, 298 K ðkJ=molÞÞ

ð11Þ

where ΔH°fuel,298 K is the enthalpy of the fuel and Eexchanger is the energy recovered by the combustion heat exchanger. When (0.5 Eexchanger Ereactor) > 0, the efficiency is calculated by efficiency ð%Þ ¼ 100 ð0:65 0:9 FH2 ðkmol=sÞ 241:8 ðkJ=molÞÞ=ðFfuel ðkmol=sÞ ΔH°fuel, 298 K ðkJ=molÞÞ

ð12Þ

As an example, Table 2 lists the energy, efficiency, temperatures, and selectivity of reforming ethanol under given conditions. The

temperature of feed of 64 °C indicates that a nozzle has to be used to spray the fuels in liquid phase into the reformer.21,55,56

3. RESULTS AND DISCUSSION 3.1. Operational Regimes of Reforming Oxygenate Fuels. Under a given temperature and pressure, the net power output (ΔE) is a function of the steam/carbon ratio (S/C, the molar ratio of steam and carbon atoms in feed for both SR and POSR processes). Figure 2 shows the dependence of ΔE on the S/C ratio at 700 °C with ethanol as fuel. Because the majority of hydrogen energy has to be used to vaporize water, ΔE will be negative when the S/C ratio is too high. Thus, the S/C ratio under which ΔE equals 0 will be the upper limit for reformer operation under these conditions. The operation is also constrained by carbon deposition (when the number of moles of graphite presented in the reformate under equilibrium is higher than 105) at low S/C ratio, which causes significant catalyst deactivation.57,58 These two S/C ratio values define a feasible operational window of a SR reaction. It is worth noting that a maximum ΔE can be found in the operational regime. Under this S/C ratio, the highest ΔE value can be achieved. It should be pointed out that optimal S/C ratios are usually low. In practice, a higher S/C ratio may be recommended to minimize coke formation because of limitations of the catalyst and kinetics. For instance, the S/C ratio is usually kept above 2.5 upon catalytic conversion at temperatures around 500600 °C over the state-of-the-art catalysts. Thus, it will be highly desired to develop a more efficient catalytic system to narrow the gap between thermodynamics and kinetics. In this paper, we focus on the thermodynamic aspect of the oxygenate fuels in the SRPEMFC system. On the basis of the above considerations, the operational regime of the S/C ratio and S/C value with the highest available power output was calculated between the temperature range of 500900 °C, which covers the common operation conditions in the literature.31,46,59 As shown in Figure 3, the feasible S/C ratios are limited in the regimes between the ΔE = 0 line and the coking line. When the S/C ratio is below the coking line, the carbon deposition on catalysts will deactivate the SR process. When the S/C ratio is above the ΔE = 0 line, the produced hydrogen energy cannot support the reformer operation. In addition, maximum ΔE lines are also plotted, under which the maximum power output will be achieved. It should be noted that, for glucose and acetic acid, ΔE is always negative, indicating that it is infeasible to produce hydrogen with them. The results in Figure 3 are instructive for determining the operating conditions for reforming typical oxygenate fuels, including methanol, ethanol, n-propanol, n-butanol, n-hexanol, ethylene glycol, glycerol, and acetone. 2645


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Table 2. Case Study on Ethanol SR at 700 °C with an Optimum S/C Ratioa S/C

Ereactor (MW)

ΔHc (MW)

ΔHs (MW)

EPEMFC (MW)

Tfeed (°C)

Toff (°C)

SH2 (%)

SCO (%)

SCO2 (%)

SCH4 (%)

efficiency (%)

1.06

13.50

17.70

0.71

15.56

64 (l)

902 (g)

66.01

67.83

23.23

8.84

29.65

Ereactor, the power for the endothermal SR reaction; ΔHc, the combustion heat of byproducts; ΔHs, the sensible heat of byproducts; EPEMFC, the PEMFC power; Tfeed, the reactant temperature in the reformer inlet; and Toff, the temperature of combustor off gas. a

Figure 2. Dependence of ΔE on the S/C ratio at 700 °C with ethanol as fuel.

For all of these fuels, the lower limits of the S/C ratio, namely, the coking limits, decrease with increasing the operation temperature, because a high temperature promotes the oxidation, gasification, and reforming of carbon. For the oxygenate fuels, the coking limits under certain temperatures are mainly influenced by the O/C ratio, defined as the atomic ratio of oxygen/carbon in oxygenate fuels for both SR and POSR processes. As shown in Figure 4, a higher S/C ratio is needed for the oxygenate fuels with a lower O/C ratio in the equilibrium model, indicating that the introduction of the oxygen atom in fuel is beneficial for preventing coking. When the O/C ratio is the same, e.g., for fuels of methanol, ethylene glycol, glycerol, and glucose, the coking S/C ratios increase slightly with the carbon number (molecular weight). The similar trend was found for the optimal S/C ratio (ΔE = maximum). However, it is worth noting in Figure 3 that the ΔE = maximum lines intersect with coking lines at very low/ high temperatures, indicating that the moderate temperature is appropriate for operating the SR reaction against coking. The above discussion indicates that the operational window of the SR reaction has to be obeyed; otherwise, the coking at inappropriate temperatures will make the optimal operation impossible. In this work, the operational window of the S/C ratio is represented by the width of the gap between the ΔE = 0 and coking lines at a certain temperature. For all of the fuels studied, the S/C window dramatically reduces at low temperatures, because of the decrease of H2 production, which make the hydrogen production by SR reactions of n-propanol, n-butanol, n-hexanol, and acetone impractical below 600 °C. The temperature window is wider for the oxygenate with a higher O/C ratio, e.g., methanol, ethylene glycol, and glycerol. We calculated the S/C windows at 650 °C, because the overall efficiency of the process reaches a maximum around 650 °C for most of the oxygenate fuels, as will be presented in the next section. Panels a and b of Figure 5 show the dependences of S/C windows for reforming the oxygenate fuels at 650 °C on the O/C and H/C (defined as the atomic ratio of hydrogen/carbon in oxygenate) ratios. For single alcohols, the S/C window is 1.02.5. Methanol has the widest S/C window among the fuels investigated. As the

O/C ratio decreases, the S/C window of single alcohol becomes narrower, indicating that the short-chain alcohols are preferred from the viewpoint of robust operation. When O/C ratios are identical, e.g., methanol, ethylene glycol, and glycerol, the S/C window will be influenced by the H/C ratio. For example, polyol with more carbon atoms has a narrower operational window. For all of the oxygenates studied, a good correlation between the H/C ratio and S/C window was discovered; namely, the S/C window increases with the H/C ratio. This may be caused by the lower fraction of the hydrogen atom in the oxygenate fuels and more carboncarbon bonds to be broken. According to that, it can be deduced that the polysaccharide compounds, e.g., glucose and cellulose, are harder to be handled for producing hydrogen through the SR reaction. Acetic acid has a zero S/C window as mentioned above, suggesting that the highly oxygenated fuels are not preferred. Above-mentioned results indicate that the highly oxygenated and heavy fuels, e.g., glucose, are inappropriate for hydrogen production by SR, because the reaction is strongly endothermal. In these cases, it is more energy-efficient to couple exothermal oxidation with endothermal reforming, as proposed by Trimm and Onsan.60 As proof, we compared the POSR to the SR process in the cases of glucose and acetic acid. Except that oxygen was introduced in a POSR system, all other conditions and definitions were the same for SR and POSR processes. The overall reactions were preset with a fixed O/C ratio of 1:3 as follows: C6 H12 O6 þ 4H2 O þ O2 T 6CO2 þ 10H2 C2 H4 O2 þ 4=3H2 O þ 1=3O2 T 2CO2 þ 10=3H2

ð13Þ ð14Þ

Figure 6 compares the operating limits of POSR and SR of glucose. POSR can significantly reduce the coke formation under the same S/C ratio.61 The coupling of the exothermal oxidation reaction also allows for the operation under a higher S/C ratio.62 Hence, the operation regime is significantly extended in the POSR process compared to that in the SR process. It demonstrates that the POSR process would be more suitable for the conversion of heavy and highly oxygenated fuels, if the kinetic and catalytic issues were addressed. 3.2. Energy Efficiency of the OxygenateFuelReformer PEMFC System. In this work, we used the efficiency, defined in eqs 11 or 12, to evaluate the capacity of producing electrical energy through the combination of a steam reformer and PEMFCs. At a given temperature, the efficiency is a function of the S/C ratio. Figure 7 shows the efficiencies of the oxygenate fuels within the temperature range from 500 to 900 °C at their respective optimal S/C ratios. Because of their infeasible operation in the SR process, the efficiencies of glucose and acetic acid were obtained with the POSR process defined in eqs 13 and 14. The efficiency increases with the temperature at lower temperatures (500650 °C), because high temperatures benefit the extraction of hydrogen from the mixture of fuel and steam via the SR reaction. In the hightemperature range, a plateau of efficiency is reached for the SR process and the efficiency decreases with an increasing operation 2646


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Figure 3. S/C ratios at coking (black 9), ΔE = maximum (red [), and ΔE = 0 (blue 2) for (a) methanol, (b) ethanol, (c) n-propanol, (d) n-butanol, (e) n-hexanol, (f) ethylene glycol, (g) glycerol, (h) glucose, (i) acetic acid, and (j) acetone. 2647


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Figure 4. Dependences of S/C ratios at coking and ΔE = maximum at 650 °C on the O/C ratios of the oxygenate fuels investigated in this work.

Figure 6. Comparison of the operational regimes between SR and POSR reactions of (a) glucose and (b) acetic acid.

Figure 5. Dependences of the S/C window reforming the oxygenate fuels at 650 °C on the (a) O/C and (b) H/C ratios.

temperature for the POSR process (glucose and acetic acid), because the production of hydrogen is restrained by the exothermic WGSR at elevated temperatures. For all of the fuels, the highest efficiency is achieved at 650700 °C, which should be the optimal operating temperature for the SR process of oxygenate fuels. The capacities of various oxygenate fuels for producing electrical energy are significantly different. The results of the efficiency in the simplified process simulation are helpful for evaluating the SR processes. At higher temperatures, all of the oxygenates investigated can achieve high efficiency above 27%, showing the promise of the reformerPEMFC system in energy generation compared to the internal combustion engines with a typical efficiency less than 20%.63 For single alcohols, the efficiency decreases with the chain length. Thus, methanol can produce energy with the highest efficiency, up to 34%, at its optimal S/C ratio through the reformerPEMFC system. It should be noted that

polyols, i.e., ethylene glycol and glycerol, offer quite high energy efficiencies among the fuels investigated, indicating that the conversion of polyols into hydrogen is energy-efficient. This result shows the promise of use of glycerol, which is flooding the market because of the manufacture of biodiesel, for the hydrogen economy. Although polyols, glucose, and acetic acid share the identical O/C ratio, their efficiencies are very different. Glucose produces hydrogen with low efficiency, especially at the temperatures lower than 600 °C or higher than 800 °C. Taking into account the idealization of the model in this work (perfect separation of H2, adiabatic reactor, no kinetic limitation, etc.), they are impractical for the real SR/POSR process. Furthermore, as a reasonable inference, cellulose and analogue are unlikely efficient materials for hydrogen energy use through the SRH2PEMFC system. For acetic acid, one of the major components of bio-oil, the efficiency is low at the temperatures lower than 600 °C or higher than 800 °C, showing a narrow temperature window for an efficient use in a SRH2PEMFC system. 3.3. Composition of Reformate. In the equilibrium model, the reformate of oxygenate fuels mainly contains H2, CO, and CO2. Under a very low S/C ratio, solid carbon forms. Around the S/C ratio with maximum ΔE, other gaseous products are trace. Because olefin, gasoline, and diesel can be produced by the FischerTropsch (FT) synthesis process, the syngas with a low H2/CO ratio is desired as an alternate for petroleum production. The typical H2/CO ratio for the FT synthesis is within 31. Figure 8 shows the H2/CO ratios in the reformate from the oxygenate fuels studied. For comparison, the reforming condition was selected at their highest ΔE values. A high H2/CO ratio could be achieved by the SR reaction at low temperatures. The concentration of carbon monoxide increases rapidly at higher temperatures (>700 °C), because of the limitation of 2648


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show the following: (1) The critical S/C ratio free of carbon deposition in the SR process decreases with the O/C ratio of the oxygenate fuels. (2) The appropriate operating temperature range of the SR of oxygenate fuels is within 600700 °C. (3) The fuels with a higher H/C ratio have wider S/C windows. Among the fuels studied, methanol offers the widest operational regime and the highest energy efficiency. Polyols, represented by ethylene glycol and glycerol, also possess high energy efficiencies. Narrow operating windows and low energy efficiencies were discovered for glucose and acetic acid. In general, heavy fuels with a high oxygenation degree are inappropriate in the SR process for hydrogen production.

’ AUTHOR INFORMATION Figure 7. Energy efficiencies of the reformerPEMFC systems using (a) methanol, ethanol, n-propanol, n-butanol, n-hexanol, ethylene glycol, glycerol, and acetone and (b) glucose and acetic acid. The efficiencies of glucose and acetic acid were calculated under POSR conditions defined as eqs 12 and 13.

Figure 8. H2/CO molar ratio in the reformate of oxygenate fuels at maximum ΔE.

WGSR. The highest H2/CO ratio is obtained on single alcohols. It is interesting that the H2/CO ratios of single alcohols are almost independent of the chain length, indicating that the composition of reformate at the equilibrium state cannot be altered for single alcohols lighter than n-hexanol. A lower H2/CO ratio can be obtained by increasing the oxygenation degree. For example, acetone and acetic acid have lower H2/CO ratios compared to n-propanol and ethanol, respectively. In Figure 8, acetic acid and glucose, which share the same O/C ratio, have nearly the same H2/CO ratio in reformate at the specific temperature in the SR process. The lighter polyol, with the identical O/C ratio, gives a higher H2/CO ratio. Hence, the oxygenate fuels with a higher oxygen content could be considered as raw materials of syngas production, which provides an alternative way to use the oxygenate fuels.

4. CONCLUSION SR of oxygenate fuels produced via biomass fermentation, fast pyrolysis, and hydrolysis is a feasible renewable route for hydrogen production. The operational regime, energy efficiency, and H2/CO ratio of reforming 10 representative oxygenate fuels, including methanol, ethanol, n-propanol, n-butanol, n-hexanol, ethylene glycol, glycerol, glucose, acetic acid, and acetone, were systematically investigated by an equilibrium model. The results

Corresponding Author

*Telephone/Fax: þ86-20-8711-4916. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National High Technology Research and Development Program of China (863 Program, 2009AA05Z102), the Fundamental Research Funds for the Central Universities of China (2009zm0246), and the Guangdong Provincial Science and Technology Project (2010B050200003). ’ NOMENCLATURE Rik = number of atoms of the kth element present in each molecule of species i bk = total number of atomic masses of the kth element in the system G = total Gibbs energy G°i = Gibbs energy of species i at its standard state H = enthalpy M and N = total number of elements and species, respectively ni = number of moles of species i P = total pressure R = gas constant S = entropy or selectivity T = temperature yi = mole fraction of species i ’ REFERENCES (1) Navarro, R. M.; Pena, M. A.; Fierro, J. L. G. Chem. Rev. 2007, 107 (10), 3952–3991. (2) Haryanto, A.; Fernando, S.; Murali, N.; Adhikari, S. Energy Fuels 2005, 19 (5), 2098–2106. (3) Balat, M. Energy Sources, Part A 2009, 31 (1), 39–50. (4) Ni, M.; Leung, D. Y. C.; Leung, M. K. H. Int. J. Hydrogen Energy 2007, 32 (15), 3238–3247. (5) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Angew. Chem., Int. Ed. 2007, 46 (38), 7164–7183. (6) Orucu, E.; Karakaya, M.; Avcı, A. K.; Onsan, Z. I. J. Chem. Technol. Biotechnol. 2005, 80 (10), 1103–1110. (7) Nahar, G. A.; Madhani, S. S. Int. J. Hydrogen Energy 2010, 35 (1), 98–109. (8) Huber, G. W.; Iborra, S.; Corma, A. Chem. Rev. 2006, 106 (9), 4044–4098. (9) de la Piscina, P. R.; Homs, N. Chem. Soc. Rev. 2008, 37 (11), 2459–2467. (10) Gayubo, A. G.; Aguayo, A. T.; Atutxa, A.; Aguado, R.; Bilbao, J. Ind. Eng. Chem. Res. 2004, 43 (11), 2610–2618. 2649


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