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without calcium oxide sorbent: A comparative study of thermodynamic and experimental work
Fuel Processing Technology 91 (2010) 1812–1818
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Hydrogen production by glycerol steam reforming with/without calcium oxide sorbent: A comparative study of thermodynamic and experimental work Xiaodong Wang a,b, Maoshuai Li a, Shuirong Li a, Hao Wang a, Shengping Wang a, Xinbin Ma a,⁎ a b
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Chemical Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, United Kingdom
a r t i c l e
i n f o
Article history: Received 4 February 2010 Received in revised form 23 July 2010 Accepted 1 August 2010 Keywords: Hydrogen Glycerol Steam reforming CaO sorbent Thermodynamic analysis
a b s t r a c t Thermodynamic analysis and experimental tests of glycerol steam reforming with/without calcium oxide (CaO) as a carbon dioxide (CO2) sorbent have been performed and compared in this work. Methanol, ethanol, acetaldehyde, acetone and ethylene do not exist in equilibrium conditions according to the equilibrium calculations. Without CaO present, thermodynamic predictions show that a maximum hydrogen concentration of 67% can be obtained at 925 K, with a water to glycerol ratio (WGR) of 9. In the experiments, the Ni/ZrO2 catalyst fails to catalyze the reactions to thermodynamic equilibrium under the selected conditions as the highest hydrogen concentration obtained is 64%. With the presence of CaO, thermodynamic analysis implies hydrogen purity exceeding 95% can be achieved below 925 K at WGRs of 6 and 9. However, CaCO3 does not exist at temperatures greater than 1025 K. In the experiments, a hydrogen purity of 95% with only 5% CH4 as impurity can be reached at 850 K with a WGR of 9. The Ni/ZrO2 catalyst is not active enough to convert excess CH4 to hydrogen in glycerol steam reforming as CH4 concentrations are usually higher than the equilibrium values. The addition of CaO to this system greatly enhances the hydrogen production while reducing the CO concentration. © 2010 Elsevier B.V. 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. Hydrogen can be used in internal combustion engines or more favourably in proton-exchange-membrane fuel cells (PEMFC) to generate power so that the exhaust emission is simply water vapour [1,2]. Regardless of the source of hydrogen, a complete switch to hydrogen fuel cell vehicles in the transportation sector would likely lead to a significant improvement in health, air quality and climate. The benefits are derived from a complete elimination of common vehicle exhaust. This does not, however, address the effect of greenhouse gases when hydrogen is derived from fossil fuels [3]. 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) [4]. As raw fossil feeds are diminishing resources, chemical and energy-related industries are examining alternative renewable feedstocks. The conversion of renewable materials to hydrogen, which can be used for fuel cells, assists in the utilization of renewable energy sources, while
⁎ Corresponding author. Tel.: +86 22 27406498; fax: +86 22 87401818. E-mail address: [email protected] (X. Ma). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.08.003
conversion to commodity chemicals facilitates the replacement of petroleum by renewable resources. For a long period, it has been proposed to use renewable resources for the production of transportation fuels, especially hydrogen [5–9]. Glycerol, which is the by-product of biodiesel production by transesterification of vegetable oils (triglycerides) and methanol, is a potentially important hydrogen source [10–13]. The production of 10 kg of biodiesel yields approximately 1 kg of crude glycerol. Glycerol is non-toxic, non-volatile, and has high energy density [14]. Furthermore, this biomass-derived glycerol is considered to be renewable and CO2 neutral. Although the global production of biodiesel is still limited, the market price of glycerol has dropped rapidly. If the production of biodiesel increases as predicted, the supply of glycerol will be in excess of demand [15]. Hence, exploitation of such a renewable material for energy production is gathering strong interest. Several studies have been carried out on glycerol steam reforming for hydrogen production. Various catalysts were investigated for glycerol steam reforming, including noble metal–based [16–20] and nickel-based catalysts [21–25]. Noble metal catalysts are well known for their high catalytic activities, whereas nickel-based catalysts have been more extensively studied in reforming reactions because of their high capacities in cleavage of C–C bonds and high selectivity towards hydrogen production combined with a relatively low cost [26]. Several thermodynamic analyses of glycerol reforming were also performed to complement the properties of this topic [27–33].
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One of the difficulties, however, associated with the utilization of the synthesis gas produced by steam reforming for energy production is the high CO2 and CO contents. For all fuel cell systems especially, large CO2 content of the fuel gas greatly drops the efficiency of the system, whereas CO has very strong poisoning effects on the catalyst of PEMFC [34]. For example, in terms of fuel cell applications, a CO concentration of less than 10 ppm is required for low temperature proton exchange membranes and alkaline fuel cells [35]. Furthermore, the cost of separating H2 from a H2-rich gas with impurities incurs major cost penalties [36,37]. In order to enhance the H2 concentration in the product gas, a CO2 sorbent, such as calcium oxide (CaO), can be used in situ to capture CO2 immediately when it is produced. The removal of the CO2 from the reaction system alters the equilibrium composition of the product gas and promotes the production of a H2-rich gas. Besides the capacity of removing CO2 to a very low concentration, CaO may be derived from a range of abundant and inexpensive precursors, which makes it attractive from an economic perspective. Additionally, heat generated from the exothermic gas-solid absorption reaction is available to drive the endothermic reforming reactions [38,39]. With regards to in situ removal of CO2 in glycerol steam reforming, only Dupont and co-workers have investigated a commercial Ni-based catalyst and a calcined dolomite sorbent in sorption-enhanced steam reforming [40]. Their experimental results showed that the effect of temperature in 673–973 K on the products selectivity is strong, with a H2 selectivity that increases with increasing temperature. It is demonstrated that 773 K was an optimum temperature with the longest CO2 breakthrough time and the highest H2 purity of 97%. Dupont et al. also reported a thermodynamic analysis of adsorption-enhanced steam reforming of glycerol and concluded that the most favourable temperature for steam–glycerol reforming is between 800 and 850 K in the presence of a CO2 adsorbent, which is about 100 K lower than that for reforming without CO2 adsorption [41]. In their study, they investigated the effect of in situ removal of CO2 on equilibrium compositions but other effects from CO2 sorbents, sorption equilibrium and kinetics were not considered. As introduced above, heat generated from the exothermic gas–solid absorption reaction can drive the endothermic reforming reactions, which can alter the reaction equilibrium too [38,39]. Based on the method used in Dupont's study, they have not taken this heat effect into account. Nevertheless, this can be solved by using equilibrium reactor module of commercial software and indicating the CO2 sorbent, e.g. CaO. A search through the open literature has failed to unearth any study using CaO as a CO2 sorbent in hydrogen production by glycerol steam reforming. The objective of this work is to study the impact of CaO as a CO2 sorbent on glycerol steam reforming in terms of both thermodynamic analysis and experimental work. In this article we first study product compositions at equilibrium employing the total Gibbs free energy minimization method. Next, hydrogen purities under experiments are investigated over a Ni/ZrO2 catalyst. The kinetics and/or catalysts will play a very important role in the product compositions; therefore finally we present for the first time the comparison of experimental results and thermodynamic reaction equilibrium. 2. Methodology The method of thermodynamic analysis by minimization of Gibbs free energy was introduced in our previous publications [31–33] and was also introduced in detail by other groups [41–43]. At low pressure and high temperature, the system can be considered as ideal [44,45]. The total Gibbs function for a system is given as follows:
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For reaction equilibria in gas phase, fi = ϕˆ i yi P, fi0 = P 0, and since is set equal to zero for each chemical element in its standard state, ΔG0 = ΔGf0i for each component is assumed. The minimum Gibbs free energy of each gaseous species and that of the total system can be expressed as Eqs. (2) and (3), with the Lagrange's undetermined multiplier method.
G0i
0
ΔGfi + RT ln N
∑ ni
i=1
0 ΔGfi
ϕˆ i yi p + ∑ λk aik = 0 p0 k ϕˆ y p + RT ln i 0i + ∑ λk aik p k
ð2Þ ! =0
ð3Þ
With the constraints of elemental balances: N
∑ ni aik = Ak
i=1
ð4Þ
When a solid is considered in the system, Eq. (5) is used in the calculations. 0
ns ΔGfs = 0
ð5Þ
In almost all the thermodynamic analyses of glycerol steam reforming, the authors only considered primary species such as hydrogen, carbon monoxide, carbon dioxide, methane, water, glycerol and solid carbon [27–33]. According to the literature, however, methanol, ethanol, acetaldehyde, acetone and ethene were also detected in the experiments [22,46,47]. Therefore, all the different species mentioned above have been taken into account for thermodynamic calculations in this work. Calcium oxide and calcium carbonate were also involved when calcium oxide is used as a CO2 sorbent. The thermodynamic equilibrium calculations were accomplished with the use of Outokumpu HSC Chemistry 4.0, a chemical reaction and equilibrium software, using the extensive thermochemical database delivered in the software package [48]. Gibbs module of this software is usually used to directly calculate product compositions at equilibrium employing the total Gibbs free energy minimization method. The initial amount of glycerol is assumed to be 1 mol with/without 3 mol of CaO in the system. Thermodynamic analysis was performed between 700 and 1100 K at atmospheric pressure with a water to glycerol ratio (WGR) of 3, 6 and 9, respectively. 3. Experimental 3.1. Catalyst preparation Hydroxide precursors of ZrO2 were prepared from co-precipitation of ZrOCl2·8H2O (Tianjin Kermel Chemical Reagent Co., Ltd., China) and ammonia solution (Tianjin Jiangtian Chemical Technology Co., Ltd., China). The formed white precipitate was washed thoroughly with deionized water until it was free of chlorine and exchanged with ethanol several times. The precipitate was then filtered, dried at 393 K and calcined at 923 K in a N2 flow. The active phases were impregnated by the wet impregnation method with a given amount of nitrate aqueous metal solution on zirconia supports. Nitrate precursor salts were Ni(NO3)2·6H2O (Tianjin Yingda Sparseness & Noble Reagent Chemical Factory, China). The impregnated catalyst samples were dried overnight at 393 K and then calcined at 923 K. Metal loading of the catalyst was set as 15% in weight for Ni. 3.2. Catalytic tests
t
N
X
N
X
0
G = ∑ ni Gi = ∑ ni μi = ∑ni Gi + i=1
i=1
fˆ RT∑ni ln 0i fi
ð1Þ
Catalytic performance tests were conducted in a quartz fixed-bed reactor, in which 0.2 ml of catalyst with/without 0.4 ml of CaO was
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diluted with inert quartz particles. Prior to the catalytic reaction, all catalysts were reduced in a H2 flow (10% in N2) at 823 K for 1 hour. The carrier gas N2 was kept constant at 50 ml/min by mass flow controller. All experiments were run with WGRs of 6 and 9 under atmospheric pressure with a GHSV of 10,200 h−1. The active tests without the presence of CaO were carried out at 923 K, whereas, in order to investigate the influence of CaO addition, the experiments with CaO were performed at 850 K. The reactor effluent was passed through a condenser, in which the condensable products were separated from the non-condensable products. Non-condensed gases were analyzed on-line by a gas chromatograph (BeiFen, SP2100), equipped with a TCD detector, TDX-01 packed column. In order to calculate the glycerol conversion, condensable products were separated with a capillary column (INNOWAX) and analyzed off-line with a gas chromatograph (Agilent, 4890), equipped with a FID detector. 4. Results and discussion Irrespective of in situ CO2 adsorption, complete conversion of glycerol was obtained for the range of temperature and WGR considered in both thermodynamic and experimental parts of this study (not shown). With regards to the various species which may be generated in the reactions, as a matter of fact, methanol, ethanol, acetaldehyde, acetone and ethene do not exist in equilibrium according to the equilibrium calculations. With respects to carbon formation from a thermodynamic perspective, no carbon was formed with WGRs of 6 and 9 regardless of the temperature in the case of glycerol steam reforming without CaO. Only a small amount of carbon (b 0.2 mol) was produced under 825 K at WGR = 3 [31]. With the presence of CaO, carbon formation was further inhibited as thermodynamically no carbon was formed over the entire range of temperature and WGR considered here. 4.1. Glycerol steam reforming without CaO The reactions of glycerol steam reforming can be expressed as follows. C3 H8 O3 ↔4H2 þ 3CO
ΔH298K = 251:2kJ=mol
ð6Þ
CO þ H2 O↔H2 þ CO2
ΔH298K = −41:2kJ=mol
ð7Þ
CO þ 3H2 ↔CH4 þ H2 O
ΔH298K = −206:1kJ=mol
ð8Þ
CO2 þ 4H2 ↔CH4 þ 2H2 O
ΔH298K = −164:9kJ=mol
ð9Þ
CO2 þ CH4 ↔2H2 þ 2CO
ΔH298K = 247:3kJ=mol
ð10Þ
H2 þ CO↔C þ H2 O
ΔH298K = −131:3kJ=mol
ð11Þ
CH4 ↔2H2 þ C
ΔH298K = 74:9kJ=mol
ð12Þ
2CO↔CO2 þ C
ΔH298K = −172:4kJ=mol
ð13Þ
Fig. 1. Thermodynamic equilibrium compositions on dry basis (excluding steam) for glycerol steam reforming without the presence of CaO over a range of temperatures at atmospheric pressure and different WGRs: (a) 3, (b) 6 and (c) 9.
Fig. 1 depicts dry product composition and moles of hydrogen at different temperatures and WGRs at equilibrium. The temperature affects the equilibrium concentrations of the products significantly. The number of moles of hydrogen increases with an increasing temperature when WGR is 3. For WGRs of 6 and 9, hydrogen production increases as temperature increases, before reaching a maximum at around 925 K, where almost no methane is produced, and then begins to decrease at higher temperatures. The greatest quantity of hydrogen (6 mol) is produced at 925 K with a WGR of 9. Moles of hydrogen increase steadily with increasing WGR. Although high WGR enhances the reforming performance significantly, it is believed that higher WGRs require higher reactor volume because of
higher steam volumetric flow, and consume higher input heat duty because of higher vaporization energy. With regards to the dry product compositions, hydrogen and CO concentrations increase with increasing temperature, whereas the concentrations of CO2 and CH4 decrease with an increase in temperature. For all the WGRs (except 3) considered in this study, the concentrations of hydrogen reach a maximum and tend to be stable above 850 K. However, the highest hydrogen concentration of the product of glycerol steam reforming is only 67% (achieved at 925 K and WGR of 9), which is far from the basic requirement of hydrogen purity for PEMFC. The distribution of carbon compound at thermodynamic equilibrium was also studied. CO2 is the
X. Wang et al. / Fuel Processing Technology 91 (2010) 1812–1818
predominant product (over 55%) that includes carbon at all temperatures with a WGR of 9 as shown in Fig. 2. CH4 production is favourable at low temperatures while high temperature promotes CO generation. The adsorption of CO2 in situ is of great significance as a maximum of 73% of CO2 is achieved. The total conversion of glycerol was obtained from the experiments (not shown). The complete decomposition of glycerol is also consistent with the prediction from thermodynamics. However, high glycerol conversion may or may not be caused by an active catalyst in glycerol steam reforming as thermal decomposition of glycerol may achieve 98% conversion above 533 K under N2 flow [49]. Fig. 3 shows the comparison between experimental results and thermodynamic equilibrium. The product compositions are close to the thermodynamic equilibrium values over the investigated reaction conditions as indicated in Fig. 3. The concentrations of H2 and CO2 with a WGR of 9 are, however, 64% and 20%, respectively, which are lower than the thermodynamic prediction. In addition, 13% of CO and 3% of CH4 are higher than equilibrium. Fernando et al. also reported that experimental results over a Ni/MgO catalyst were far from thermodynamic equilibrium [21]. The difference between the experimental results and thermodynamic analysis is primarily due to the influence of catalyst. With highly selective catalysts, the product composition can be quite similar to the thermodynamic equilibrium. Navarro et al. compared the performances of nickel catalysts supported on Al2O3 modified by Mg, Zr, Ce or La. The results showed that the product distribution for the steam reforming of glycerol was strongly dependent on the catalysts. The Ni/Al2O3 (modified by Zr) sample was the most selective catalyst with a gas composition after steam reforming similar to the values predicted by equilibrium [23]. RihkoStruckmann et al. have shown that with a La0.3Ce0.7NiO3 catalyst, the hydrogen yield was close to the thermodynamic equilibrium yield over the whole investigated temperature range (773–973 K) [25]. As expected, a decrease in WGR can weaken steam reforming of glycerol since the proportion of H2 and CO2 decrease when WGR declines from 9 to 6. Moreover, thermodynamic equilibrium could still not be achieved with this catalyst since the concentrations of CH4 and CO are higher than that at equilibrium. It may be attributed to the following reasons. Firstly, decomposition of glycerol to CH4 is highly favourable during the reforming process [18,50]. Secondly, in the reaction equilibrium of the water gas shift reaction, Eq. (7), the conversion of CO is depressed at high temperatures [21,25]. The last reason is that the Ni/ZrO2 catalyst used in this study may not be active enough to convert CH4 and CO to H2 and CO2 effectively under selected conditions. Disregarding these issues, hydrogen purity from glycerol steam reforming without the presence of CaO is still significantly far from the standard for PEMFC.
Fig. 3. Comparison of product compositions on dry basis (excluding steam) between experimental and thermodynamic analysis for glycerol steam reforming without the presence of CaO at 923 K. Experimental conditions: Ni/ZrO2 catalyst, GHSV 10,200 h−1, carrier gas N2 50 ml/min, WGRs (a) 6 and (b) 9. Error bars denote the sample standard deviation of the mean values of experimental results.
4.2. Glycerol steam reforming in the presence of CaO When glycerol steam reforming is coupled with in situ CO2 capture using CaO, the following reaction takes place. One of the advantages of conducting glycerol steam reforming with the presence of CaO is that the heat generated by Eq. (14) can drive the reforming reactions, which makes an energy-efficient route for high-purity hydrogen production. With the presence of CaO, glycerol conversions were also 100% in both thermodynamic analysis and experiments, which is similar to the results without the use of CO2 adsorption. CaO þ CO2 ↔CaCO3
Fig. 2. Carbon compound distribution at thermodynamic equilibrium without the presence of CaO as a function of temperature at WGR = 9.
1815
ΔH298K = −178:8kJ=mol
ð14Þ
Fig. 4 illustrates dry product composition and moles of hydrogen at different temperatures and WGRs with CaO present. This is consistent with the trends in glycerol steam reforming without CaO present, as temperature has a significant effect on the equilibrium concentrations of the products. Throughout the range of WGRs considered in this study, moles of hydrogen produced per mole of glycerol with CaO present increase slightly with increasing temperatures and then decrease with an increase in temperature. It is apparent that high WGRs are more favourable for hydrogen production. More than 6 mol of hydrogen can be produced with WGRs of 6 and 9 below 1000 K. In terms of the dry product composition, the addition of CaO in the system can increase hydrogen purity dramatically. As shown from Fig. 4, hydrogen purities are over 95% below 925 K for WGRs of 6 and 9, and furthermore, almost no CO2 or CO is produced with the only
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the temperature is above 1025 K, CaCO3 cannot be formed at all. The equilibrium gas compositions are equivalent to those predicted when CaO is not present (as shown in Fig. 1). High WGRs can increase both moles and concentrations of hydrogen as shown in Fig. 4. Comas et al. have reported a thermodynamic analysis of ethanol steam reforming using CaO as a CO2 sorbent [51]. Our results are in noticeably good agreement with theirs in terms of the trend of hydrogen production and the behaviour of CaO in the system. Interested readers may find that our present results above 900 K are different from the existing report by Chen et al. [41]. In their study, the trend of numbers of hydrogen produced does not change above 900 K. This is because their calculation was based on the ideal assumption that a certain amount of CO2 can be adsorbed (or removed) completely from the system (no practical sorbent was indicated). However, we have used CaO as a sorbent and considered the fact that its capacity of CO2 adsorption decreases when temperature is higher than 900 K. Fig. 5 describes carbon compound distribution at thermodynamic equilibrium with the presence of CaO as a function of temperature with a WGR of 9. Almost all the carbon element exists in CaCO3 when temperatures are below 900 K. After the sorbent deactivates completely, however, CO and CO2 are main products besides hydrogen with the latter one predominant. Fig. 6 shows the difference between the experimental results and the results from thermodynamic analysis. For WGR of 6, the hydrogen concentration can reach 87% together with 9% CH4, 3% CO and only 1% CO2, which are quite different from the thermodynamic prediction. In the case of WGR of 9, 95% of hydrogen can be obtained in the experiments with only 5% CH4 as impurity. The concentration of CH4 is higher than the thermodynamic value again indicating the catalyst may not be active enough for the conversion of excess CH4 to hydrogen. Although the experimental results of hydrogen purity are still far from thermodynamic equilibrium, it has been shown that the addition of CaO as a CO2 sorbent can obviously enhance hydrogen purity significantly. No CO2 or CO was detected in our experimental tests which may be owing to the trace amount produced. Finally, it must be noted that when using the CO2-sorbent and due to the fixed amount of sorbent in an experimental run, the CO2 concentration in the product gas rises rapidly once the sorbent reaches a certain capacity [40]. 5. Conclusions Thermodynamic analysis and experimental tests of glycerol steam reforming with/without CaO as a CO2 sorbent were performed and compared in this work. Methanol, ethanol, acetaldehyde, acetone and ethylene do not exist in equilibrium conditions according to the equilibrium calculations. In the absence of CO2 removal, thermodynamic
Fig. 4. Thermodynamic equilibrium compositions on dry basis (excluding steam) for glycerol steam reforming with the presence of CaO over a range of temperatures at atmospheric pressure and different WGRs: (a) 3, (b) 6 and (c) 9.
impurity being CH4. It is noteworthy that incorporating CaO to the steam reforming substantially diminishes the CO concentration. Hydrogen concentration, however, declines rapidly between 900 K and 1025 K. It can be attributed to two main reasons, firstly due to the deactivation of the CaO sorbent. This occurs due to the higher reaction temperatures favouring the endothermic decomposition of CaCO3, according to Eq. (14) [38,39]. The second reason is due to the high reaction temperature suppressing the proceeding of water gas shift reaction, Eq. (7), which can be supported by the increased amount of CO and H2O (not shown) when the temperature is above 900 K. When
Fig. 5. Carbon compound distribution at thermodynamic equilibrium with the presence of CaO as a function of temperature at WGR = 9.
X. Wang et al. / Fuel Processing Technology 91 (2010) 1812–1818
fXi0 Gi Gt G0i ΔGf0i
the standard-state fugacity of species i partial molar Gibbs free energy of species i total Gibbs free energy standard Gibbs free energy of species i standard Gibbs function of formation of species i
ΔGfs0 ns N P P0 R T yi ΔG Ο
standard Gibbs function of formation of solid mole of solid number of species in the reaction system pressure of system standard-state pressure of 101.3 kPa molar gas constant temperature of system mole fraction of each substance in gas products standard Gibbs function of molar reaction
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Greek symbols λk Lagrange multiplier X μi chemical potential of species i ϕˆ i fugacity coefficient of species i
Acknowledgements 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.
References
Fig. 6. Comparison of product compositions on dry basis (excluding steam) between experimental and thermodynamic analysis for glycerol steam reforming with the presence of CaO at 850 K. Experimental conditions: Ni/ZrO2 catalyst, GHSV 10,200 h−1, carrier gas N2 50 ml/min, WGRs (a) 6 and (b) 9. Error bars denote the sample standard deviation of the mean values of experimental results.
predictions show that a maximum hydrogen concentration of 67% can be obtained at 925 K with a water to glycerol ratio of 9. In the experiments, the Ni/ZrO2 catalyst fails to catalyze the reactions to thermodynamic equilibrium under the selected conditions as the highest hydrogen concentration obtained is 64%. With in situ CO2 removal, thermodynamic analysis shows that hydrogen purity exceeding 95% can be achieved between 700 and 925 K with WGRs of both 6 and 9. Additionally, CaCO3 cannot be formed above 1025 K, and then the function of CaO as a CO2 sorbent disappears entirely. Although our catalyst is not active enough to reach the thermodynamic equilibrium again with the presence of CaO, a hydrogen purity of 95% with only 5% CH4 in the dry gas composition can be reached at 850 K with a WGR of 9 in the experimental tests. Experimental results over Ni/ZrO2 catalyst are still far from thermodynamic equilibrium, which may be because the Ni/ZrO2 catalyst is not active enough to convert excess CH4 to hydrogen. This may leave room for development of effective and efficient catalysts for hydrogen production by glycerol steam reforming. The addition of CaO to the system shows promising results by greatly enhancing the hydrogen production while reducing the CO concentration. There is, therefore, room for investigations of hydrogen production from glycerol steam reforming with in situ CO2 removal. Nomenclature number of atoms of the kth element present in each aik molecule of species i Ak total mass of kth element in the feed ˆ fi the fugacity of species i in system
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Fuel Processing Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / f u p r o c
Hydrogen production by glycerol steam reforming with/without calcium oxide sorbent: A comparative study of thermodynamic and experimental work Xiaodong Wang a,b, Maoshuai Li a, Shuirong Li a, Hao Wang a, Shengping Wang a, Xinbin Ma a,⁎ a b
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Chemical Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, United Kingdom
a r t i c l e
i n f o
Article history: Received 4 February 2010 Received in revised form 23 July 2010 Accepted 1 August 2010 Keywords: Hydrogen Glycerol Steam reforming CaO sorbent Thermodynamic analysis
a b s t r a c t Thermodynamic analysis and experimental tests of glycerol steam reforming with/without calcium oxide (CaO) as a carbon dioxide (CO2) sorbent have been performed and compared in this work. Methanol, ethanol, acetaldehyde, acetone and ethylene do not exist in equilibrium conditions according to the equilibrium calculations. Without CaO present, thermodynamic predictions show that a maximum hydrogen concentration of 67% can be obtained at 925 K, with a water to glycerol ratio (WGR) of 9. In the experiments, the Ni/ZrO2 catalyst fails to catalyze the reactions to thermodynamic equilibrium under the selected conditions as the highest hydrogen concentration obtained is 64%. With the presence of CaO, thermodynamic analysis implies hydrogen purity exceeding 95% can be achieved below 925 K at WGRs of 6 and 9. However, CaCO3 does not exist at temperatures greater than 1025 K. In the experiments, a hydrogen purity of 95% with only 5% CH4 as impurity can be reached at 850 K with a WGR of 9. The Ni/ZrO2 catalyst is not active enough to convert excess CH4 to hydrogen in glycerol steam reforming as CH4 concentrations are usually higher than the equilibrium values. The addition of CaO to this system greatly enhances the hydrogen production while reducing the CO concentration. © 2010 Elsevier B.V. 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. Hydrogen can be used in internal combustion engines or more favourably in proton-exchange-membrane fuel cells (PEMFC) to generate power so that the exhaust emission is simply water vapour [1,2]. Regardless of the source of hydrogen, a complete switch to hydrogen fuel cell vehicles in the transportation sector would likely lead to a significant improvement in health, air quality and climate. The benefits are derived from a complete elimination of common vehicle exhaust. This does not, however, address the effect of greenhouse gases when hydrogen is derived from fossil fuels [3]. 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) [4]. As raw fossil feeds are diminishing resources, chemical and energy-related industries are examining alternative renewable feedstocks. The conversion of renewable materials to hydrogen, which can be used for fuel cells, assists in the utilization of renewable energy sources, while
⁎ Corresponding author. Tel.: +86 22 27406498; fax: +86 22 87401818. E-mail address: [email protected] (X. Ma). 0378-3820/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2010.08.003
conversion to commodity chemicals facilitates the replacement of petroleum by renewable resources. For a long period, it has been proposed to use renewable resources for the production of transportation fuels, especially hydrogen [5–9]. Glycerol, which is the by-product of biodiesel production by transesterification of vegetable oils (triglycerides) and methanol, is a potentially important hydrogen source [10–13]. The production of 10 kg of biodiesel yields approximately 1 kg of crude glycerol. Glycerol is non-toxic, non-volatile, and has high energy density [14]. Furthermore, this biomass-derived glycerol is considered to be renewable and CO2 neutral. Although the global production of biodiesel is still limited, the market price of glycerol has dropped rapidly. If the production of biodiesel increases as predicted, the supply of glycerol will be in excess of demand [15]. Hence, exploitation of such a renewable material for energy production is gathering strong interest. Several studies have been carried out on glycerol steam reforming for hydrogen production. Various catalysts were investigated for glycerol steam reforming, including noble metal–based [16–20] and nickel-based catalysts [21–25]. Noble metal catalysts are well known for their high catalytic activities, whereas nickel-based catalysts have been more extensively studied in reforming reactions because of their high capacities in cleavage of C–C bonds and high selectivity towards hydrogen production combined with a relatively low cost [26]. Several thermodynamic analyses of glycerol reforming were also performed to complement the properties of this topic [27–33].
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One of the difficulties, however, associated with the utilization of the synthesis gas produced by steam reforming for energy production is the high CO2 and CO contents. For all fuel cell systems especially, large CO2 content of the fuel gas greatly drops the efficiency of the system, whereas CO has very strong poisoning effects on the catalyst of PEMFC [34]. For example, in terms of fuel cell applications, a CO concentration of less than 10 ppm is required for low temperature proton exchange membranes and alkaline fuel cells [35]. Furthermore, the cost of separating H2 from a H2-rich gas with impurities incurs major cost penalties [36,37]. In order to enhance the H2 concentration in the product gas, a CO2 sorbent, such as calcium oxide (CaO), can be used in situ to capture CO2 immediately when it is produced. The removal of the CO2 from the reaction system alters the equilibrium composition of the product gas and promotes the production of a H2-rich gas. Besides the capacity of removing CO2 to a very low concentration, CaO may be derived from a range of abundant and inexpensive precursors, which makes it attractive from an economic perspective. Additionally, heat generated from the exothermic gas-solid absorption reaction is available to drive the endothermic reforming reactions [38,39]. With regards to in situ removal of CO2 in glycerol steam reforming, only Dupont and co-workers have investigated a commercial Ni-based catalyst and a calcined dolomite sorbent in sorption-enhanced steam reforming [40]. Their experimental results showed that the effect of temperature in 673–973 K on the products selectivity is strong, with a H2 selectivity that increases with increasing temperature. It is demonstrated that 773 K was an optimum temperature with the longest CO2 breakthrough time and the highest H2 purity of 97%. Dupont et al. also reported a thermodynamic analysis of adsorption-enhanced steam reforming of glycerol and concluded that the most favourable temperature for steam–glycerol reforming is between 800 and 850 K in the presence of a CO2 adsorbent, which is about 100 K lower than that for reforming without CO2 adsorption [41]. In their study, they investigated the effect of in situ removal of CO2 on equilibrium compositions but other effects from CO2 sorbents, sorption equilibrium and kinetics were not considered. As introduced above, heat generated from the exothermic gas–solid absorption reaction can drive the endothermic reforming reactions, which can alter the reaction equilibrium too [38,39]. Based on the method used in Dupont's study, they have not taken this heat effect into account. Nevertheless, this can be solved by using equilibrium reactor module of commercial software and indicating the CO2 sorbent, e.g. CaO. A search through the open literature has failed to unearth any study using CaO as a CO2 sorbent in hydrogen production by glycerol steam reforming. The objective of this work is to study the impact of CaO as a CO2 sorbent on glycerol steam reforming in terms of both thermodynamic analysis and experimental work. In this article we first study product compositions at equilibrium employing the total Gibbs free energy minimization method. Next, hydrogen purities under experiments are investigated over a Ni/ZrO2 catalyst. The kinetics and/or catalysts will play a very important role in the product compositions; therefore finally we present for the first time the comparison of experimental results and thermodynamic reaction equilibrium. 2. Methodology The method of thermodynamic analysis by minimization of Gibbs free energy was introduced in our previous publications [31–33] and was also introduced in detail by other groups [41–43]. At low pressure and high temperature, the system can be considered as ideal [44,45]. The total Gibbs function for a system is given as follows:
1813 ˆ
For reaction equilibria in gas phase, fi = ϕˆ i yi P, fi0 = P 0, and since is set equal to zero for each chemical element in its standard state, ΔG0 = ΔGf0i for each component is assumed. The minimum Gibbs free energy of each gaseous species and that of the total system can be expressed as Eqs. (2) and (3), with the Lagrange's undetermined multiplier method.
G0i
0
ΔGfi + RT ln N
∑ ni
i=1
0 ΔGfi
ϕˆ i yi p + ∑ λk aik = 0 p0 k ϕˆ y p + RT ln i 0i + ∑ λk aik p k
ð2Þ ! =0
ð3Þ
With the constraints of elemental balances: N
∑ ni aik = Ak
i=1
ð4Þ
When a solid is considered in the system, Eq. (5) is used in the calculations. 0
ns ΔGfs = 0
ð5Þ
In almost all the thermodynamic analyses of glycerol steam reforming, the authors only considered primary species such as hydrogen, carbon monoxide, carbon dioxide, methane, water, glycerol and solid carbon [27–33]. According to the literature, however, methanol, ethanol, acetaldehyde, acetone and ethene were also detected in the experiments [22,46,47]. Therefore, all the different species mentioned above have been taken into account for thermodynamic calculations in this work. Calcium oxide and calcium carbonate were also involved when calcium oxide is used as a CO2 sorbent. The thermodynamic equilibrium calculations were accomplished with the use of Outokumpu HSC Chemistry 4.0, a chemical reaction and equilibrium software, using the extensive thermochemical database delivered in the software package [48]. Gibbs module of this software is usually used to directly calculate product compositions at equilibrium employing the total Gibbs free energy minimization method. The initial amount of glycerol is assumed to be 1 mol with/without 3 mol of CaO in the system. Thermodynamic analysis was performed between 700 and 1100 K at atmospheric pressure with a water to glycerol ratio (WGR) of 3, 6 and 9, respectively. 3. Experimental 3.1. Catalyst preparation Hydroxide precursors of ZrO2 were prepared from co-precipitation of ZrOCl2·8H2O (Tianjin Kermel Chemical Reagent Co., Ltd., China) and ammonia solution (Tianjin Jiangtian Chemical Technology Co., Ltd., China). The formed white precipitate was washed thoroughly with deionized water until it was free of chlorine and exchanged with ethanol several times. The precipitate was then filtered, dried at 393 K and calcined at 923 K in a N2 flow. The active phases were impregnated by the wet impregnation method with a given amount of nitrate aqueous metal solution on zirconia supports. Nitrate precursor salts were Ni(NO3)2·6H2O (Tianjin Yingda Sparseness & Noble Reagent Chemical Factory, China). The impregnated catalyst samples were dried overnight at 393 K and then calcined at 923 K. Metal loading of the catalyst was set as 15% in weight for Ni. 3.2. Catalytic tests
t
N
X
N
X
0
G = ∑ ni Gi = ∑ ni μi = ∑ni Gi + i=1
i=1
fˆ RT∑ni ln 0i fi
ð1Þ
Catalytic performance tests were conducted in a quartz fixed-bed reactor, in which 0.2 ml of catalyst with/without 0.4 ml of CaO was
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diluted with inert quartz particles. Prior to the catalytic reaction, all catalysts were reduced in a H2 flow (10% in N2) at 823 K for 1 hour. The carrier gas N2 was kept constant at 50 ml/min by mass flow controller. All experiments were run with WGRs of 6 and 9 under atmospheric pressure with a GHSV of 10,200 h−1. The active tests without the presence of CaO were carried out at 923 K, whereas, in order to investigate the influence of CaO addition, the experiments with CaO were performed at 850 K. The reactor effluent was passed through a condenser, in which the condensable products were separated from the non-condensable products. Non-condensed gases were analyzed on-line by a gas chromatograph (BeiFen, SP2100), equipped with a TCD detector, TDX-01 packed column. In order to calculate the glycerol conversion, condensable products were separated with a capillary column (INNOWAX) and analyzed off-line with a gas chromatograph (Agilent, 4890), equipped with a FID detector. 4. Results and discussion Irrespective of in situ CO2 adsorption, complete conversion of glycerol was obtained for the range of temperature and WGR considered in both thermodynamic and experimental parts of this study (not shown). With regards to the various species which may be generated in the reactions, as a matter of fact, methanol, ethanol, acetaldehyde, acetone and ethene do not exist in equilibrium according to the equilibrium calculations. With respects to carbon formation from a thermodynamic perspective, no carbon was formed with WGRs of 6 and 9 regardless of the temperature in the case of glycerol steam reforming without CaO. Only a small amount of carbon (b 0.2 mol) was produced under 825 K at WGR = 3 [31]. With the presence of CaO, carbon formation was further inhibited as thermodynamically no carbon was formed over the entire range of temperature and WGR considered here. 4.1. Glycerol steam reforming without CaO The reactions of glycerol steam reforming can be expressed as follows. C3 H8 O3 ↔4H2 þ 3CO
ΔH298K = 251:2kJ=mol
ð6Þ
CO þ H2 O↔H2 þ CO2
ΔH298K = −41:2kJ=mol
ð7Þ
CO þ 3H2 ↔CH4 þ H2 O
ΔH298K = −206:1kJ=mol
ð8Þ
CO2 þ 4H2 ↔CH4 þ 2H2 O
ΔH298K = −164:9kJ=mol
ð9Þ
CO2 þ CH4 ↔2H2 þ 2CO
ΔH298K = 247:3kJ=mol
ð10Þ
H2 þ CO↔C þ H2 O
ΔH298K = −131:3kJ=mol
ð11Þ
CH4 ↔2H2 þ C
ΔH298K = 74:9kJ=mol
ð12Þ
2CO↔CO2 þ C
ΔH298K = −172:4kJ=mol
ð13Þ
Fig. 1. Thermodynamic equilibrium compositions on dry basis (excluding steam) for glycerol steam reforming without the presence of CaO over a range of temperatures at atmospheric pressure and different WGRs: (a) 3, (b) 6 and (c) 9.
Fig. 1 depicts dry product composition and moles of hydrogen at different temperatures and WGRs at equilibrium. The temperature affects the equilibrium concentrations of the products significantly. The number of moles of hydrogen increases with an increasing temperature when WGR is 3. For WGRs of 6 and 9, hydrogen production increases as temperature increases, before reaching a maximum at around 925 K, where almost no methane is produced, and then begins to decrease at higher temperatures. The greatest quantity of hydrogen (6 mol) is produced at 925 K with a WGR of 9. Moles of hydrogen increase steadily with increasing WGR. Although high WGR enhances the reforming performance significantly, it is believed that higher WGRs require higher reactor volume because of
higher steam volumetric flow, and consume higher input heat duty because of higher vaporization energy. With regards to the dry product compositions, hydrogen and CO concentrations increase with increasing temperature, whereas the concentrations of CO2 and CH4 decrease with an increase in temperature. For all the WGRs (except 3) considered in this study, the concentrations of hydrogen reach a maximum and tend to be stable above 850 K. However, the highest hydrogen concentration of the product of glycerol steam reforming is only 67% (achieved at 925 K and WGR of 9), which is far from the basic requirement of hydrogen purity for PEMFC. The distribution of carbon compound at thermodynamic equilibrium was also studied. CO2 is the
X. Wang et al. / Fuel Processing Technology 91 (2010) 1812–1818
predominant product (over 55%) that includes carbon at all temperatures with a WGR of 9 as shown in Fig. 2. CH4 production is favourable at low temperatures while high temperature promotes CO generation. The adsorption of CO2 in situ is of great significance as a maximum of 73% of CO2 is achieved. The total conversion of glycerol was obtained from the experiments (not shown). The complete decomposition of glycerol is also consistent with the prediction from thermodynamics. However, high glycerol conversion may or may not be caused by an active catalyst in glycerol steam reforming as thermal decomposition of glycerol may achieve 98% conversion above 533 K under N2 flow [49]. Fig. 3 shows the comparison between experimental results and thermodynamic equilibrium. The product compositions are close to the thermodynamic equilibrium values over the investigated reaction conditions as indicated in Fig. 3. The concentrations of H2 and CO2 with a WGR of 9 are, however, 64% and 20%, respectively, which are lower than the thermodynamic prediction. In addition, 13% of CO and 3% of CH4 are higher than equilibrium. Fernando et al. also reported that experimental results over a Ni/MgO catalyst were far from thermodynamic equilibrium [21]. The difference between the experimental results and thermodynamic analysis is primarily due to the influence of catalyst. With highly selective catalysts, the product composition can be quite similar to the thermodynamic equilibrium. Navarro et al. compared the performances of nickel catalysts supported on Al2O3 modified by Mg, Zr, Ce or La. The results showed that the product distribution for the steam reforming of glycerol was strongly dependent on the catalysts. The Ni/Al2O3 (modified by Zr) sample was the most selective catalyst with a gas composition after steam reforming similar to the values predicted by equilibrium [23]. RihkoStruckmann et al. have shown that with a La0.3Ce0.7NiO3 catalyst, the hydrogen yield was close to the thermodynamic equilibrium yield over the whole investigated temperature range (773–973 K) [25]. As expected, a decrease in WGR can weaken steam reforming of glycerol since the proportion of H2 and CO2 decrease when WGR declines from 9 to 6. Moreover, thermodynamic equilibrium could still not be achieved with this catalyst since the concentrations of CH4 and CO are higher than that at equilibrium. It may be attributed to the following reasons. Firstly, decomposition of glycerol to CH4 is highly favourable during the reforming process [18,50]. Secondly, in the reaction equilibrium of the water gas shift reaction, Eq. (7), the conversion of CO is depressed at high temperatures [21,25]. The last reason is that the Ni/ZrO2 catalyst used in this study may not be active enough to convert CH4 and CO to H2 and CO2 effectively under selected conditions. Disregarding these issues, hydrogen purity from glycerol steam reforming without the presence of CaO is still significantly far from the standard for PEMFC.
Fig. 3. Comparison of product compositions on dry basis (excluding steam) between experimental and thermodynamic analysis for glycerol steam reforming without the presence of CaO at 923 K. Experimental conditions: Ni/ZrO2 catalyst, GHSV 10,200 h−1, carrier gas N2 50 ml/min, WGRs (a) 6 and (b) 9. Error bars denote the sample standard deviation of the mean values of experimental results.
4.2. Glycerol steam reforming in the presence of CaO When glycerol steam reforming is coupled with in situ CO2 capture using CaO, the following reaction takes place. One of the advantages of conducting glycerol steam reforming with the presence of CaO is that the heat generated by Eq. (14) can drive the reforming reactions, which makes an energy-efficient route for high-purity hydrogen production. With the presence of CaO, glycerol conversions were also 100% in both thermodynamic analysis and experiments, which is similar to the results without the use of CO2 adsorption. CaO þ CO2 ↔CaCO3
Fig. 2. Carbon compound distribution at thermodynamic equilibrium without the presence of CaO as a function of temperature at WGR = 9.
1815
ΔH298K = −178:8kJ=mol
ð14Þ
Fig. 4 illustrates dry product composition and moles of hydrogen at different temperatures and WGRs with CaO present. This is consistent with the trends in glycerol steam reforming without CaO present, as temperature has a significant effect on the equilibrium concentrations of the products. Throughout the range of WGRs considered in this study, moles of hydrogen produced per mole of glycerol with CaO present increase slightly with increasing temperatures and then decrease with an increase in temperature. It is apparent that high WGRs are more favourable for hydrogen production. More than 6 mol of hydrogen can be produced with WGRs of 6 and 9 below 1000 K. In terms of the dry product composition, the addition of CaO in the system can increase hydrogen purity dramatically. As shown from Fig. 4, hydrogen purities are over 95% below 925 K for WGRs of 6 and 9, and furthermore, almost no CO2 or CO is produced with the only
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the temperature is above 1025 K, CaCO3 cannot be formed at all. The equilibrium gas compositions are equivalent to those predicted when CaO is not present (as shown in Fig. 1). High WGRs can increase both moles and concentrations of hydrogen as shown in Fig. 4. Comas et al. have reported a thermodynamic analysis of ethanol steam reforming using CaO as a CO2 sorbent [51]. Our results are in noticeably good agreement with theirs in terms of the trend of hydrogen production and the behaviour of CaO in the system. Interested readers may find that our present results above 900 K are different from the existing report by Chen et al. [41]. In their study, the trend of numbers of hydrogen produced does not change above 900 K. This is because their calculation was based on the ideal assumption that a certain amount of CO2 can be adsorbed (or removed) completely from the system (no practical sorbent was indicated). However, we have used CaO as a sorbent and considered the fact that its capacity of CO2 adsorption decreases when temperature is higher than 900 K. Fig. 5 describes carbon compound distribution at thermodynamic equilibrium with the presence of CaO as a function of temperature with a WGR of 9. Almost all the carbon element exists in CaCO3 when temperatures are below 900 K. After the sorbent deactivates completely, however, CO and CO2 are main products besides hydrogen with the latter one predominant. Fig. 6 shows the difference between the experimental results and the results from thermodynamic analysis. For WGR of 6, the hydrogen concentration can reach 87% together with 9% CH4, 3% CO and only 1% CO2, which are quite different from the thermodynamic prediction. In the case of WGR of 9, 95% of hydrogen can be obtained in the experiments with only 5% CH4 as impurity. The concentration of CH4 is higher than the thermodynamic value again indicating the catalyst may not be active enough for the conversion of excess CH4 to hydrogen. Although the experimental results of hydrogen purity are still far from thermodynamic equilibrium, it has been shown that the addition of CaO as a CO2 sorbent can obviously enhance hydrogen purity significantly. No CO2 or CO was detected in our experimental tests which may be owing to the trace amount produced. Finally, it must be noted that when using the CO2-sorbent and due to the fixed amount of sorbent in an experimental run, the CO2 concentration in the product gas rises rapidly once the sorbent reaches a certain capacity [40]. 5. Conclusions Thermodynamic analysis and experimental tests of glycerol steam reforming with/without CaO as a CO2 sorbent were performed and compared in this work. Methanol, ethanol, acetaldehyde, acetone and ethylene do not exist in equilibrium conditions according to the equilibrium calculations. In the absence of CO2 removal, thermodynamic
Fig. 4. Thermodynamic equilibrium compositions on dry basis (excluding steam) for glycerol steam reforming with the presence of CaO over a range of temperatures at atmospheric pressure and different WGRs: (a) 3, (b) 6 and (c) 9.
impurity being CH4. It is noteworthy that incorporating CaO to the steam reforming substantially diminishes the CO concentration. Hydrogen concentration, however, declines rapidly between 900 K and 1025 K. It can be attributed to two main reasons, firstly due to the deactivation of the CaO sorbent. This occurs due to the higher reaction temperatures favouring the endothermic decomposition of CaCO3, according to Eq. (14) [38,39]. The second reason is due to the high reaction temperature suppressing the proceeding of water gas shift reaction, Eq. (7), which can be supported by the increased amount of CO and H2O (not shown) when the temperature is above 900 K. When
Fig. 5. Carbon compound distribution at thermodynamic equilibrium with the presence of CaO as a function of temperature at WGR = 9.
X. Wang et al. / Fuel Processing Technology 91 (2010) 1812–1818
fXi0 Gi Gt G0i ΔGf0i
the standard-state fugacity of species i partial molar Gibbs free energy of species i total Gibbs free energy standard Gibbs free energy of species i standard Gibbs function of formation of species i
ΔGfs0 ns N P P0 R T yi ΔG Ο
standard Gibbs function of formation of solid mole of solid number of species in the reaction system pressure of system standard-state pressure of 101.3 kPa molar gas constant temperature of system mole fraction of each substance in gas products standard Gibbs function of molar reaction
1817
Greek symbols λk Lagrange multiplier X μi chemical potential of species i ϕˆ i fugacity coefficient of species i
Acknowledgements 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.
References
Fig. 6. Comparison of product compositions on dry basis (excluding steam) between experimental and thermodynamic analysis for glycerol steam reforming with the presence of CaO at 850 K. Experimental conditions: Ni/ZrO2 catalyst, GHSV 10,200 h−1, carrier gas N2 50 ml/min, WGRs (a) 6 and (b) 9. Error bars denote the sample standard deviation of the mean values of experimental results.
predictions show that a maximum hydrogen concentration of 67% can be obtained at 925 K with a water to glycerol ratio of 9. In the experiments, the Ni/ZrO2 catalyst fails to catalyze the reactions to thermodynamic equilibrium under the selected conditions as the highest hydrogen concentration obtained is 64%. With in situ CO2 removal, thermodynamic analysis shows that hydrogen purity exceeding 95% can be achieved between 700 and 925 K with WGRs of both 6 and 9. Additionally, CaCO3 cannot be formed above 1025 K, and then the function of CaO as a CO2 sorbent disappears entirely. Although our catalyst is not active enough to reach the thermodynamic equilibrium again with the presence of CaO, a hydrogen purity of 95% with only 5% CH4 in the dry gas composition can be reached at 850 K with a WGR of 9 in the experimental tests. Experimental results over Ni/ZrO2 catalyst are still far from thermodynamic equilibrium, which may be because the Ni/ZrO2 catalyst is not active enough to convert excess CH4 to hydrogen. This may leave room for development of effective and efficient catalysts for hydrogen production by glycerol steam reforming. The addition of CaO to the system shows promising results by greatly enhancing the hydrogen production while reducing the CO concentration. There is, therefore, room for investigations of hydrogen production from glycerol steam reforming with in situ CO2 removal. Nomenclature number of atoms of the kth element present in each aik molecule of species i Ak total mass of kth element in the feed ˆ fi the fugacity of species i in system
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