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Thermodynamic analysis of hydrogen generation via oxidative steam reforming of glycerol
Renewable Energy 36 (2011) 2120e2127
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Thermodynamic analysis of hydrogen generation via oxidative steam reforming of glycerol Guangxing Yang, Hao Yu*, Feng Peng*, Hongjuan Wang, Jian Yang, Donglai Xie School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 June 2010 Accepted 20 January 2011 Available online 21 February 2011
A thermodynamic analysis of the oxidative steam reforming of glycerol (OSRG) for hydrogen production has been carried out with Aspen plus TM. The reaction was investigated at ambient pressure within the carbon-to-oxygen (C/O) ratio of 0.5e3.0, steam-to-carbon (S/C) ratio of 0.5e8.0 and temperature of 400 e850 C. Higher C/O and S/C ratios favor the production of hydrogen from glycerol. The highest hydrogen selectivity is obtained at 600e700 C. To predict the potential technical obstacles in the glycerol reforming process, the OSRG process was compared with oxidative steam reforming of ethanol (OSRE) in terms of hydrogen production, autothermal condition and carbon deposition. The selectivity to hydrogen via OSRG is lower than that via OSRE under identical conditions. To achieve autothermal reforming, higher S/C and C/O ratios are required for reforming of glycerol than for ethanol due to the higher oxygen content in a glycerol molecule. From the viewpoint of thermodynamics, the glycerol reforming is more resistant to the carbon deposition. On the basis of the thermodynamic analysis and the preliminary experimental study, suggestions were proposed to guide the development of the glycerol reforming technique. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Glycerol Ethanol Hydrogen Oxidative steam reforming Thermodynamics Aspen plus TM
1. Introduction In response to the exhausting fossil fuel and the global climate change, the energy solution based upon hydrogen as an energy carrier is highly desired for its high efficiency and low emission, especially as coupled with fuel cells. To ultimately achieve the hydrogen-based energy solution, the source of hydrogen should be renewable and environmental-benign. The sustainable hydrogen generation can be achieved by the conversion of biomass via gasification combined with pyrolysis [1], steam reforming (SR) [2], oxidative steam reforming (OSR) [3], partial oxidation (POX) [4] and autothermal reforming (ATR) [5] from renewable carboxylates, such as bio-ethanol, glycerol, carbohydrates and bio-oil. Among these carboxylates, bio-ethanol can be produced by the fermentation of biomass, and glycerol is a main byproduct in the process of biodiesel refining. There is a glut of glycerol in the market due to the forced regulation on the usage of biodiesel [6]. Hence the hydrogen production from glycerol will be a cost-effective way to utilize glycerol and lower the cost of biodiesel. Hydrogen or syngas can be generated from glycerol by SR [7e9], aqueous-phase reforming [10], pyrolysis [11], supercritical water
* Corresponding authors. Tel./fax: þ86 20 8711 4916. E-mail addresses: [email protected] (H. Yu), [email protected] (F. Peng). 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2011.01.022
reforming [12], and photo-reforming [13]. Some typical catalysts succeeded in the reforming of ethanol have been applied to catalyze the reforming reaction of glycerol, due to their chemical similarity on the aspect of the identical hydroxyl group. Schmidt and co-workers [14] reported that hydrogen was produced by autothermal reforming of glycerol (ATRG) at approximate 1000 C in short contact times of tens of milliseconds on the RheCe catalyst, which is one of the best catalysts for ethanol reforming. Supported nickel catalysts, frequently modified with alkali and alkali earth metals, have been also reported as active catalysts for steam reforming of glycerol (SRG) [15]. Wen et al. [16] reported that supported Pt catalysts were active in the aqueous-phase reforming. Although the achievements in this field are continuously emerging, the mechanism of glycerol decomposition and the suitable process for the reaction are still under debate [17,18]. The thermodynamic analysis will be informative for producing hydrogen from glycerol, which provides a comprehensive understanding on the optimal condition, product distribution and energy efficiency. Adhikari et al. [19] analyzed the production of hydrogen from glycerol and the effects of the process variables by the direct minimization of Gibbs free energy. Wang et al. [20] explored the thermodynamic optimal conditions for SRG. The condition to inhibit carbon deposition was discussed thermodynamically. Their following work was also focused on the thermodynamics analyses of dry reforming [21] and ATRG [22] to produce hydrogen or syngas.
G. Yang et al. / Renewable Energy 36 (2011) 2120e2127
Chen et al. [23] investigated the effect of adding adsorbent of CO2 on the carbon formation, which substantially reduce the lower limit of the water to glycerol ratio. These thermodynamic investigations show that high temperatures, high steam to glycerol ratios and low oxygen to glycerol ratios are feasible for this process. The optimum conditions for the hydrogen production are suggested at temperature of 900e1000 K, steam/glycerol ratio of 9e12, oxygen/glycerol ratio of 0e0.4 and under atmospheric pressure [22]. Some authors have conducted the comparative studies between the thermodynamic and experimental results for SRG. Cui et al. [24] have reported an efficient SRG over La1xCexNiO3 catalyst close to the thermodynamic equilibrium. However, Adhikari et al. [25] reported a SRG far away from the thermodynamic equilibrium. It should also be noted that, although the techniques used in the hydrogen production from glycerol are very similar to the hydrogen production process from ethanol, the glycerol processes reported so far suffered from low H2 yield and serious deactivation compared to the ethanol process. To develop a feasible process for hydrogen production from glycerol, a detailed comparative study between glycerol and ethanol is desired to analyze the technical requirements and potential bottlenecks of the glycerol reforming. In this paper, thermodynamic analysis of the glycerol reforming process is carried out, especially focusing on the OSR, including the ATR process, due to its advantage in the less dependence on the additional energy supply. The results are compared with the corresponding process using ethanol as fuel, in terms of the H2 selectivity, the thermo-neutral condition and the carbon deposition condition. A preliminary experimental investigation was carried out to compare with the thermodynamic results. On the basis of the comparative study, the emphasis of the research and development for the hydrogen production from glycerol is suggested. 2. Computational method and experiments 2.1. Reactions involved in OSRG The total reaction for the steam reforming of glycerol with oxygen addition can be described as Eq. (1):
C3 H8 O3ðlÞ þ xH2 OðlÞ þ yO2ðgÞ /aCO2ðgÞ þ bCOðgÞ þ cH2 OðgÞ þ dH2ðgÞ þ eCH4ðgÞ
ð1Þ
When ethanol is used as fuel, the equation will be in the similar form except for the changes of the stoichiometric numbers. The phase state of the fuel has been selected as either gaseous or liquid state in the existing processes reported. When the fuel is gaseous, an evaporator is needed to vaporize and preheat the fuel. When the fuel is liquid, a nozzle is needed to spray the liquid fuel. Because the glycerol decomposes at its boiling point about 290 C, the state of glycerol has to be selected as liquid for the thermodynamic and experimental investigation in this work. Ethanol was also considered as liquid for a comparative study. Taking glycerol as an example, the major reactions involve:
POX: C3 H8 O3 þ3=2O2 /3CO2 þ4H2 DH0298K ¼ 603kJ=mol
(2)
SRG: C3 H8 O3 þ3H2 O/3CO2 þ7H2 DH0298K ¼ þ128kJ=mol
(3)
When x > 0 and y > 0, the coupling of Eqs. (1)e(2) give the OSR of glycerol (OSRG):
OSRG : C3 H8 O3 þ xH2 O þ yO2 / 3CO2 þ zH2
(4)
The reaction enthalpy depends on the value of y. Higher y results in serious combustion accompanying with the heat release. Otherwise, more water will absorb heat via the endothermic SR reaction.
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When the reaction enthalpy equals to zero, the reaction is denoted as ATR. Water-gas shift reaction (WGSR) plays an important role for hydrogen production and carbon monoxide elimination:
WGSR : CO þ H2 O 4 CO2 þ H2 DH0298K ¼ 40 kJ=mol
(5)
In fact, the system includes many other reactions, such as dehydration, dehydrogenation, pyrolysis, etc., which contribute to the formation of by-products, such as methane, ethylene, acetaldehyde and acetone. 2.2. Computational method for the thermodynamic analysis The thermodynamic analysis for the reforming of glycerol and ethanol was carried out in commercial Aspen plus TM simulator, which allows for a sequential modular simulation for a chemical process including standard unit operations. In this work, the reactor was considered as isothermal and isobaric. The physical properties were calculated from the SoaveeRedlicheKwong method. The literature and experimental studies show that the possible products are H2, CH4, CO, CO2, C2H4, C2H6, H2O, graphite carbon (C) and unreacted oxygen and glycerol (C3H8O3)/ethanol (C2H5OH) and these species are assumed in our simulation. Equilibrium composition of these species was calculated through the Gibbs free energy reactor in Aspen Plus TM. The selectivities to products are defined as follows:
SH2 ; % ¼
FH2 ;out 1 100 Fcarbon;out R
where F is molar flow rate, and R is the moles of H2 produced from one mole glycerol or ethanol in feed normalized by carbon number. R is 4/3 for glycerol and 3/2 for ethanol reforming. This definition does not take the H atom in water into account, which may result in the H2 selectivity higher than 100%.
SCi ; % ¼
n FCi ;out 100 Fcarbon;out
where Ci represents CO, CO2, CH4, C2H4 and C2H6, and n is the number of C atoms in one mole Ci. The conversion of glycerol/ ethanol is defined as:
Xol ; % ¼
Fol;in Fol;out 100 Fol;in
2.3. Experimental setup Ir/La2O3 catalyst was used to catalyze the reforming of glycerol and ethanol. Previous work has demonstrated that the catalyst is highly efficient for the OSR of ethanol (OSRE) [26]. The catalyst was prepared by impregnating appropriate amounts of IrCl3$nH2O (with Ir content of 58 wt%, Shanghai July Chemical Co. Ltd.) on La2O3 (Sinopharm Chemical Reagent Co. Ltd.). The impregnated catalysts were dried at 120 C over night, then calcined at 500 C in air for 2 h. The loading of Ir was 5 wt%. Catalytic tests were performed in a quartz fixed-bed reactor with 9 mm inner diameter. Typically, 60 mg catalysts were sandwiched by quartz wool and packed in the reactor. A thermocouple was inserted into the center of the catalytic bed to monitor the reaction temperature. Before the reaction, the catalyst was reduced with hydrogen of 50 Ncm3/min at 500 C for 40 min. A mixture of glycerol and water with 1:6 M ratio was fed into the reactor through a stainless steel capillary with inner diameter of 0.8 mm by a high pressure syringe pump at flow rate of 0.1 ml/min. Oxygen at
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100
o
C/O=1.2;T=600 C
C/O=1.2; S/C=1
100
H2
80
H2 CO CO2 CH4 C2H4 C2H6
60 40
Selectivity /%
Selectivity /%
80
20
CO CO2 CH4 C2H4 C2H6
60 40 20
0
0 0
1
2
3
4
5
6
7
8
400
500
S/C
600
700
800 o
Reaction temperature / C
Fig. 1. Effect of S/C on selectivities of gas-phase products of the OSRG reaction at C/O ¼ 1.2 and T ¼ 600 C.
room temperature was introduced and mixed with the reactant at the inlet of reactor. The gas hourly space velocity (GHSV) was about 11.5 104 h1. The gaseous product off the reactor was analyzed by a Fuli 9790 Gas Chromatography equipped with a TDX-01 packed column (for the analysis of ethanol in a Thermal Conductor Detector) and an AE electric insulating oil analysis column (for the analysis of CO, CH4, CO2, C2H4, C2H6 in a Flame Ionization Detector). CO and CO2 were converted to CH4 by a methanizer before entering the FID detector. Other products off the reactor were condensed through an ice trap. The glycerol unconverted in liquid was analyzed by a Waters Liquid Chromatography 2695 equipped with a ultimate TM column and a Differential Refraction Detector.
Fig. 3. Effect of reaction temperature on selectivities of gas-phase products of the OSRG reaction at C/O ¼ 1.2 and S/C ¼ 1.
account the increased energy consumption caused by the addition of water, higher S/C ratio is cost-inefficient for the H2 production. On the other hand, the excess steam also quenches the carbon deposition, which will be discussed in detail in Sec. 3.2 later. The selectivity of CH4 decreases with the increasing S/C ratio, indicating that the reforming of CH4 may be enhanced by the excess water. Under the conditions investigated, the formation of ethylene and ethane is neglectable. In the OSRG reaction, the amount of oxygen significantly influences the energetics and product distribution. Fig. 2 shows the 140
a
130 120
3.1. Parametric investigation of OSRG
110
Under all of the conditions studied in this paper, the glycerol conversion approaches to 100%, indicating that the utilization of glycerol is not limited by thermodynamics. Fig. 1 shows the effect of S/C ratio on the selectivity of gaseous product of OSRG reaction at C/O ¼ 1.2 and at 600 C. The addition of steam increases the selectivity of H2. The selectivity of H2 is higher than 100% when the S/C ratio is larger than 2, indicating that H2O will contribute H atoms to generate H2. However, at the S/C ratios higher than 3, the addition of steam to the system has little effect on the H2 formation, since the equilibrium of WGSR limits the yield of H2. Taking into
H2 selectivity /%
3. Results and discussion
90 80
Ethanol Glycerol
70
50
0
2
120
CO2
40 20
H2 selectivity / %
H2 CO CH4 C2H4 C2H6
6
8
10
12
1.0
1.5
2.0
2.5
3.0
C/O Fig. 2. Effect of C/O on selectivities of gas-phase products of the OSRG reaction at S/C ¼ 1 and T ¼ 600 C.
18
20
22
110 100 90 80
60 0.5
16
b
Alcohol/O2=0.7
70
0
14
Glycerol Ethanol
o
60
4
Ethanol(Glycerol)/O2
S/C=1; T=600 C 80
H2O/alcohol=3
60
130
100
Selectivity /%
100
0
1
2
3
4
5
6
7
8
9
10
11
H2O/Ethanol (Glycerol) Fig. 4. Hydrogen and water selectivity of OSRG and OSRE as a function of (a) glycerol (ethanol) to O2 ratio and (b) water to glycerol(ethanol) ratio at 650 C.
G. Yang et al. / Renewable Energy 36 (2011) 2120e2127
product distribution at equilibrium as a function of C/O ratio. The selectivities of H2, CH4 and CO increase with the C/O ratio at a fixed S/C ratio and temperature. The rapid decline of H2 selectivity at low C/O ratio may be caused by the dominant combustion of H2 with O2 to form water. At high C/O ratios, the H2 selectivity will approach the limitation of SRG. From the viewpoint of H2 production, the addition of O2 is not desired. The previous thermodynamic analysis of glycerol reforming indicated that the higher temperatures of 627e727 C favor the reaction [22]. As shown in Fig. 3, the highest H2 selectivity is obtained at temperature of 600 C. A slightly decrease of H2 selectivity occurs at T > 650 C. The methane selectivity decreases with the increase of temperature and approaches to zero at T > 700 C. It can be concluded that the methane steam reforming is accelerated with the increasing temperature and contributes to the formation of H2. It should be noted that the selectivity of CO increases and that of CO2 decreases at T > 650 C. It can be explained by the enhanced reverse WGSR at high temperatures, which may also cause the decrease of H2 in this range of temperature. The above parametric study conducted in Aspen plus TM indicates that the OSRG prefers to the moderate S/C and C/O ratios for a compromise between H2 yield and energy consumption. Due to the limitation of WGSR, the feasible reaction temperature should be selected around 600e700 C. These results are in agreement with
6 5
S/C
4
3.2. Comparative study between OSRG and OSRE The comparison was firstly focused on the H2 yield. For the hydrogen production, higher H2 yield per molecule alcohol is desired under identical conditions. As shown in Fig. 4(a), the H2 selectivity from glycerol reforming is lower than that from ethanol reforming at the same ratio of alcohol to O2. At 650 C and water to alcohol ratio of 3, the limitation value at the low O2 amount, namely the steam reforming, is about 110% in OSGR and 130% in OSRE, corresponding to 47.8 mmol H2 per gram glycerol and 84.8 mmol H2 per gram ethanol. Fig. 4(b) shows the H2 selectivity of OSRG and OSRE as a function of water to alcohol ratio at a fixed alcohol to O2 ratio. To obtain the same hydrogen selectivity, more water has to be consumed in OSRG, which causes higher energy cost to vaporize water. The results suggest that the OSRE process is more efficient than the OSRG from the view of producing hydrogen. It may be due to the higher O/C ratio in a glycerol molecule. It could be explained
b
500 550 600 650 700 750 800
ATRG
the previous reports [19,22,24] on the experimental and theoretical studies on the glycerol reforming, demonstrating that Aspen plus TM can be used as a tool for a robust thermodynamics analysis of the hydrogen production process. In the following section, the OSGR will be compared with the ethanol counterpart to evaluate the process development according to the relatively well-established ethanol reforming process.
6
4
3
3 2
2
1
1 0 0.4
0.8
1.2
1.6
0 0.4
2.0
0.8
1.2
d
ATRG
ATRE 90
80
80
70
50
0.4
500 550 600 650 700 750 0.8
70 500 550 600 650 700 750
60 50
1.2
C/O
2.0
100
90
SH2/%
SH2 /%
100
60
1.6
C/O
C/O
c
500 550 600 650 700 750
ATRE
5
S/C
a
2123
1.6
2.0
0.4
0.8
1.2
1.6
2.0
C/O
Fig. 5. The dependence of S/C on C/O of (a) the glycerol and (b) ethanol reforming under the thermal-neutral condition. (c) and (d) show the corresponding selectivity of hydrogen as a function of C/O in the (c) glycerol and (d) ethanol reforming.
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G. Yang et al. / Renewable Energy 36 (2011) 2120e2127
a
100
800
Glycerol
Carbon free
700 600
Temperature / C
60
o
Selectivity /%
80
40 20
500 400
C/O 0.8 1.0 1.2 Carbon deposition 1.5 Steam reforming
300 200
0 500
550
600
650
700
750
100
o
0.0
Temperature / C
0.6
0.8
1.0
1.2
b
800
Carbon free
Ethanol 700 600
o
500
C/O
400
0.8 1.0 Cabon deposition 1.2 1.5 Steam reforming
300 200 100
0.0
0.2
0.4
0.6
0.8
1.0
1.2
S/C Fig. 7. Carbon deposition and carbon-free regions of (a) the glycerol reforming and (b) ethanol reforming. The lines represent the condition at which 0.1% carbon atoms in the feed are deposited.
S/C and C/O ratios, the coke is formed at lower temperatures in the ethanol reforming than in the glycerol reforming. For instance, the coke will be eliminated above 563 and 332 C for the glycerol reforming at S/C ¼ 1 under the SR condition (C/O ¼ N) and at C/O ¼ 1.5, respectively. However, temperatures up to 657 and 420 C are required in the ethanol reforming. This can be explained 180
Thermodynamic Equilibrium
160 140 120
SH2/%
by the more abundance of O in the glycerol system limits the WGSR and enhances the oxidation of H2, which lower the extent to extract hydrogen from water. The ATR is highly desired to reduce the dependence of the hydrogen production on the external energy and minimize the volume of reactor. Fig. 5(a) and (b) show the relationship between C/O and S/C under the thermal-neutral condition. At a fixed reactor temperature, with increasing C/O ratio, less water is needed to maintain the zero reaction heat, due to the less extent of exothermal oxidation reaction. At the identical temperature and S/C ratio, higher C/O ratio is needed for ATRG than for ATRE because of the higher O/C ratio in a glycerol molecule. Fig. 5(c) and (d) show the selectivity of hydrogen as a function of C/O under the corresponding thermalneutral condition. Under a fixed reactor temperature, a maximum of hydrogen yield will be obtained at C/O ratio of 0.8e1.2, indicating that the ATR should be operated with a specific combination of S/C and C/O. At lower C/O ratios, more H2 and reactant have to be consumed to sustain the evaporation of water. On the other hand, the S/C ratio is lower at higher C/O ratios, thus the H2 selectivity will decrease according to Fig.1, meanwhile the risk of carbon deposition increase. Fig. 5(c) and (d) also indicate that the H2 selectivity strongly depends on the reaction temperature of ATR reaction. For a comparison between ATRG and ATRE, the maximum H2 selectivity under thermal-neutral condition is plotted as a function of reaction temperature, as shown in Fig. 6. The maximum H2 selectivity in the ATR reaction is obtained at about 650 C for both the ATRE and ATRG. More CH4 is produced at low temperatures and reduces the H2 selectivity. The reduction of H2 selectivity at high temperatures may be caused by the lower S/C ratio and the limited WGSR. In the temperature range investigated, the H2 selectivity of the ATRG reaction is always lower than that of ATRE, which emphasize that glycerol will be thermodynamically less efficient for producing hydrogen compared with ethanol. It suggests that a process with the faster kinetics and the more elaborate process development will be highly desired for the utilization of glycerol. Carbon deposition is of importance in the (oxygenated) hydrocarbon reforming to produce hydrogen, because the undesirable carbon formation can deactivate the catalysts dramatically [27,28]. The carbon deposition region was determined for the reforming of glycerol and ethanol, as shown in Fig. 7. Generally, higher temperature, addition of oxygen and higher ratio of S/C will suppress the coke-formation, which agrees well with the previous experimental and theoretical studies [3,24,29e32]. At the identical
0.4
S/C
Temperature / C
Fig. 6. The selectivity pattern when the maximum hydrogen selectivity is obtained under thermal-neutral condition in ATRG and ATRE. Solid lines and symbols: ethanol; dashed lines and open symbols: glycerol. Circle: H2; up triangle: CO; down triangle: CO2; square: H2O; diamond: CH4.
0.2
100 80 60 40 20 1
2
10
100
S/C Fig. 8. Experimental H2 selectivity of SRG in literature and the corresponding thermodynamic limitation. The legends are listed in Table 1. For comparison, some of the H2 selectivity values in literature were recalculated with the definition in Sec. 2.2 according to the provided information.
G. Yang et al. / Renewable Energy 36 (2011) 2120e2127
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Table 1 Legends of the scatters in Fig. 8. Catalyst
Reaction conditions
Source
H2O/Glycerol ¼ 6:1(molar ratio); GHSV ¼ 51,000 h1; 600e900 C
[29]
1 wt% glycerol; WHSV ¼ 2.5 h1; 873 K
[15]
H2O/C3H8O3 ¼ 0e1(weight ratio); 5.4 g/h; 800 C
[35]
H2O/C3H8O3 ¼ 24/1(molar ratio); 0.06 ml/min; 500, 550, 600 and 700 C
[24]
10 wt% of glycerol; GHSV ¼ 7.7 gglycerol/(gcat h); 773, 873 K
[36]
C3H8O3/H2O/He ¼ 2/18/80 (vol%); GHSV ¼ 11,000 ml/(gcat h); 500 C.
[37]
S/C ¼ 3.3; Contact time of glycerin: 13.4 gcat h/mol; 500e600 C
[9]
S/C ¼ 2, T ¼ 650 C
This work
;Al2O3, =Rh/Al2O3,
Ir/Al2O3, Pd/Al2O3, Pt/Al2O3, CeO2/Al2O3, Rh/CeO2/Al2O3, Ni/CeO2/Al2O3, Ru/CeO2/Al2O3, Pd/CeO2/Al2O3, Pt/CeO2/Al2O3 Ni/Al2O3, Ni/La2O3/Al2O3, Ni/MgO/Al2O3, Ni/CeO2/Al2O3, Ni/ZrO2/Al2O3
CNi/Al2O3 LaNiO3, HLa0.3Ce0.7NiO3, ILa0.7Ce0.3NiO3, La0.9Ce0.1NiO3, Pt/Y2O3, Pt/CeZrO2/Y2O3 Ni/La2O3(3 wt%)/Al2O3, =Ni/Al2O3, Ni/La2O3(6 wt%)/Al2O3, Ni/La2O3(15 wt%)/Al2O3, Pt/La2O3(15 wt%)/Al2O3, Pt,Ni/La2O3(3 wt%)/Al2O3 Ir/CeO2,
Co/CeO2,
Ni/CeO2
,Ru/La2O3, BRh/La2O3, 6Ni/La2O3, Ir/La2O3, >Co/La2O3, Pt/La2O3, Pd/La2O3, Fe/La2O3, Ru/Y2O3, Ru/ZrO2, Ru/CeO2, Ru/La2O3,
by the more oxygen atoms in glycerol molecule than in ethanol, which facilitates the formation of gaseous CO or CO2. The above comparison indicates that the glycerol reforming may be more stable thermodynamically, since the coking can be more facilely controlled to inhibit the deactivation caused by carbon deposition. The essential information is provided by the analysis to determine the operation conditions preventing carbon deposition. Nevertheless, it should be noted that the gasification of carbon may be kinetically slow, therefore the pathways resulting in coke formation, such as thermolysis, oligomerization, olefin formation via dehydration and decarbonylation etc., must be carefully tailored by the catalyst design to realize the thermodynamic equilibrium product distribution. 3.3. Discussion The aforementioned thermodynamic comparative results offer a basis for the discussion of the strategy of hydrogen production using the renewable ethanol and glycerol. Both of ethanol and glycerol can be used as the starting materials for the sustainable hydrogen energy, because the considerable H2 yield can be achieved with the reforming process, as widely demonstrated by many researchers [17,18]. However, the high O/C ratio in a glycerol molecule could decrease the hydrogen selectivity according to our analysis. It implies that, to achieve a competitive process with the ethanol-based one, the glycerol reforming has to take the advantage of the glut of glycerol in the market to reduce the cost. The attention should also be focused on the catalyst development to achieve the equilibrium product distribution. In the well-established
ethanol reforming process, some catalysts, including Rh, Ir, Ni and Co etc., have been demonstrated active and stable [33]. In the case of glycerol, higher activity is needed because of the difficulty in breaking CeC bond. Fig. 8 compares the experimental results of SRG in literature over a variety of catalysts with the thermodynamic results calculated by Aspen plus TM. Many catalysts reported give the hydrogen selectivity of the glycerol reforming far away from the equilibrium. Among the catalysts with near-equilibrium products, the noble metal catalysts, such as Rh, Ru and Pt, provide high activity and selectivity. The Ni catalysts possess the high hydrogen selectivity at high S/C ratios. So far, few publications report on the experimental investigation of OSRG [14,28], as listed in Table 2. Rennard et al. [14] reported that the RheCe/ Al2O3eSiO2 catalyst can give a H2 selectivity of 75%, very close to the equilibrium value at 600 C in OSRG. However, the results obtained over Pd [34] and Pt [14] catalysts were far away from the corresponding equilibrium values, indicating there is plenty of room for the improvement of the catalyst. Meanwhile, the information on catalyst stability and deactivation has rarely been involved. The difference between the catalytic performances of the identical catalyst in glycerol and ethanol reforming can be directly compared. Our previous work demonstrated that the Ir/La2O3 catalyst performs well in the OSRE reaction [26]. Fig. 9 compares the selectivity patterns of OSRG and OSRE under the same reaction condition. The OSRG provided lower hydrogen selectivity than the OSRE, agreeing with the aforementioned thermodynamic analysis. The equilibrium selectivity of hydrogen via OSRG and OSRE are 86% and 110%, respectively. In the OSRE, a near-equilibrium H2 selectivity was obtained, however, the hydrogen selectivity of OSRG is obviously lower than the equilibrium one. It is indicative of the
Table 2 Comparison between experimental and equilibrium H2 selectivity in OSRG. Catalyst
H2 selectivity/% Experimental
Equilibrium
RheCe/Al2O3eSiO2 Pt/Al2O3eSiO2 Pd/Cu/Ni/K Ir/La2O3
w70 w20 w75 w60
w75(600 C) w82(700 C) w130 w86
Conditions
Source
C/O ¼ 1; S/C ¼ 1 C/O ¼ 1.4 S/C ¼ 3, C/O ¼ 3.33, T ¼ 700 C S/C ¼ 2, C/O ¼ 1, T ¼ 650 C
[14] [14] [34] This work
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G. Yang et al. / Renewable Energy 36 (2011) 2120e2127
Acknowledgments
a 120
Glycerol
H2 CH4 C2H4
Selectivity /%
100
CO CO2 C2H6
H2 selectivity in equilibrium
80 60
References
40 20 0
0
40
80
120
160
200
Time on stream /min
b 120
Ethanol
H2 selectivity in equilibrium
Selectivity /%
100 H2 CO2
80
CO C2H4
CH4
60 40 20 0
The authors thank the financial support from the National High Technology Research and Development Program of China (863 Program, No. 2009AA05Z102) and the Fundamental Research Funds for the Central Universities of China (No. 2009zm0246).
0
20
40
60
80
100 120 140 160 180
Time on stream /min Fig. 9. Selectivity of gaseous products by (a) OSRG and (b) OSRE. Conditions: C/O ¼ 1, S/C ¼ 2, T ¼ 650 C, feed flow rate ¼ 0.1 ml/min, 60 mg Ir/La2O3 catalyst.
slower kinetics over the Ir/La2O3 catalyst for the glycerol reforming. Thus, the more active and selective catalyst will be highly desired for the glycerol reforming technique in the future.
4. Conclusions The Aspen plus TM software is used as a facile and robust tool for the thermodynamic analysis of the reforming of glycerol and ethanol. The OSRG prefers to the moderate S/C and C/O ratios for a compromise between H2 yield and energy consumption. Due to the limitation of WGSR, the feasible reaction temperature should be selected at 600e700 C. To obtain the maximum of hydrogen yield under the ATRG condition, the C/O ratio should be in the range of 0.8e1.2. The comparative study between the glycerol and ethanol reforming shows that the reforming of glycerol provides lower H2 yield than that of ethanol, due to the more oxygenated composition of glycerol. Thus, it can be suggested that the investigation on the reforming property of the crude glycerol, containing methanol, metal ions and larger biomass molecule, will be highly desired, because the refining of crude glycerol from the bio-diesel production will significantly increase the cost. However, this topic has not been fully convinced of by the community yet [17]. On the other hand, the higher oxygen content of glycerol improves the resistance to the carbon deposition compared to ethanol. On the basis of the thermodynamic analysis combining with the preliminary experimental investigation, it is suggested that the development of new catalyst is highly desired for the glycerol reforming to overcome the kinetic limitation.
[1] Demirbas A. Hydrogen-rich gases from biomass via pyrolysis and air-steam gasification. Energy Sources Part A-Recovery Utilization and Environmental Effects 2009;31(19):1728e36. [2] Yoon SJ, Choi YC, Lee JG. Hydrogen production from biomass tar by catalytic steam reforming. Energy Conversion and Management 2010;51(1):42e7. [3] Cai WJ, Wang FG, Zhan ES, Van Veen AC, Mirodatos C, Shen WJ. Hydrogen production from ethanol over Ir/CeO2 catalysts: a comparative study of steam reforming, partial oxidation and oxidative steam reforming. Journal of Catalysis 2008;257(1):96e107. [4] Marda JR, DiBenedetto J, McKibben S, Evans RJ, Czernik S, French RJ, et al. Noncatalytic partial oxidation of bio-oil to synthesis gas for distributed hydrogen production. International Journal of Hydrogen Energy 2009;34(20):8519e34. [5] Deluga GA, Salge JR, Schmidt LD, Verykios XE. Renewable hydrogen from ethanol by autothermal reforming. Science 2004;303(5660):993e7. [6] Vasudevan PT, Briggs M. Biodiesel production-current state of the art and challenges. Journal of Industrial Microbiology & Biotechnology 2008;35 (5):421e30. [7] Adhikari S, Fernando SD, To SDF, Bricka RM, Steele PH, Haryanto A. Conversion of glycerol to hydrogen via a steam reforming process over nickel catalysts. Energy & Fuels 2008;22(2):1220e6. [8] Buffoni IN, Pompeo F, Santori GF, Nichio NN. Nickel catalysts applied in steam reforming of glycerol for hydrogen production. Catalysis Communications 2009;10(13):1656e60. [9] Hirai T, Ikenaga N, Miyake T, Suzuki T. Production of hydrogen by steam reforming of glycerin on ruthenium catalyst. Energy & Fuels 2005;19 (4):1761e2. [10] Luo NJ, Ouyang K, Cao FH, Xiao TC. Hydrogen generation from liquid reforming of glycerin over NieCo bimetallic catalyst. Biomass & Bioenergy 2010;34(4):489e95. [11] Valliyappan T, Bakhshi NN, Dalai AK. Pyrolysis of glycerol for the production of hydrogen or syn gas. Bioresource Technology 2008;99(10):4476e83. [12] Byrd AJ, Pant KK, Gupta RB. Hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst. Fuel 2008;87(13e14):2956e60. [13] Daskalaki VM, Kondarides DI. Efficient production of hydrogen by photoinduced reforming of glycerol at ambient conditions. Catalysis Today 2009;144(1e2):75e80. [14] Rennard DC, Kruger JS, Schmidt LD. Autothermal catalytic partial oxidation of glycerol to syngas and to non-equilibrium products. ChemSusChem 2009;2 (1):89e98. [15] Iriondo A, Barrio VL, Cambra JF, Arias PL, Guemez MB, Navarro RM, et al. Hydrogen production from glycerol over nickel catalysts supported on Al2O3 modified by Mg, Zr, Ce or La. Topics in Catalysis 2008;49(1e2):46e58. [16] Wen GD, Xu YP, Ma HJ, Xu ZS, Tian ZJ. Production of hydrogen by aqueousphase reforming of glycerol. International Journal of Hydrogen Energy 2008;33(22):6657e66. [17] Adhikari S, Fernando SD, Haryanto A. Hydrogen production from glycerol: an update. Energy Conversion and Management 2009;50(10):2600e4. [18] Vaidya PD, Rodrigues AE. Glycerol reforming for hydrogen production: a review. Chemical Engineering & Technology 2009;32(10):1463e9. [19] Adhikari S, Fernando SD, Haryanto A. Glycerin steam reforming for hydrogen production. Transactions of the ASABE 2007;50(2):591e5. [20] Wang XD, Li SR, Wang H, Liu B, Ma XB. Thermodynamic analysis of glycerin steam reforming. Energy & Fuels 2008;22(6):4285e91. [21] Wang XD, Li MS, Wang MH, Wang H, Li SR, Wang SP, et al. Thermodynamic analysis of glycerol dry reforming for hydrogen and synthesis gas production. Fuel 2009;88(11):2148e53. [22] Wang H, Wang XD, Li MS, Li SR, Wang SP, Ma XB. Thermodynamic analysis of hydrogen production from glycerol autothermal reforming. International Journal of Hydrogen Energy 2009;34(14):5683e90. [23] Chen HS, Zhang TF, Dou BL, Dupont V, Williams P, Ghadiri M, et al. Thermodynamic analyses of adsorption-enhanced steam reforming of glycerol for hydrogen production. International Journal of Hydrogen Energy 2009;34 (17):7208e22. [24] Cui Y, Galvita V, Rihko-Struckmann L, Lorenz H, Sundmacher K. Steam reforming of glycerol: the experimental activity of La1-xCexNiO3 catalyst in comparison to the thermodynamic reaction equilibrium. Applied Catalysis B-Environmental 2009;90(1e2):29e37. [25] Adhikari S, Fernando S, Haryanto A. A comparative thermodynamic and experimental analysis on hydrogen production by steam reforming of glycerin. Energy & Fuels 2007;21(4):2306e10. [26] Chen HQ, Yu H, Peng F, Wang HJ, Yang J, Pan MQ. Efficient and stable oxidative steam reforming of ethanol for hydrogen production: effect of in situ dispersion of Ir over Ir/La2O3. Journal of Catalysis 2010;269(2):281e90.
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Thermodynamic analysis of hydrogen generation via oxidative steam reforming of glycerol Guangxing Yang, Hao Yu*, Feng Peng*, Hongjuan Wang, Jian Yang, Donglai Xie School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 June 2010 Accepted 20 January 2011 Available online 21 February 2011
A thermodynamic analysis of the oxidative steam reforming of glycerol (OSRG) for hydrogen production has been carried out with Aspen plus TM. The reaction was investigated at ambient pressure within the carbon-to-oxygen (C/O) ratio of 0.5e3.0, steam-to-carbon (S/C) ratio of 0.5e8.0 and temperature of 400 e850 C. Higher C/O and S/C ratios favor the production of hydrogen from glycerol. The highest hydrogen selectivity is obtained at 600e700 C. To predict the potential technical obstacles in the glycerol reforming process, the OSRG process was compared with oxidative steam reforming of ethanol (OSRE) in terms of hydrogen production, autothermal condition and carbon deposition. The selectivity to hydrogen via OSRG is lower than that via OSRE under identical conditions. To achieve autothermal reforming, higher S/C and C/O ratios are required for reforming of glycerol than for ethanol due to the higher oxygen content in a glycerol molecule. From the viewpoint of thermodynamics, the glycerol reforming is more resistant to the carbon deposition. On the basis of the thermodynamic analysis and the preliminary experimental study, suggestions were proposed to guide the development of the glycerol reforming technique. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Glycerol Ethanol Hydrogen Oxidative steam reforming Thermodynamics Aspen plus TM
1. Introduction In response to the exhausting fossil fuel and the global climate change, the energy solution based upon hydrogen as an energy carrier is highly desired for its high efficiency and low emission, especially as coupled with fuel cells. To ultimately achieve the hydrogen-based energy solution, the source of hydrogen should be renewable and environmental-benign. The sustainable hydrogen generation can be achieved by the conversion of biomass via gasification combined with pyrolysis [1], steam reforming (SR) [2], oxidative steam reforming (OSR) [3], partial oxidation (POX) [4] and autothermal reforming (ATR) [5] from renewable carboxylates, such as bio-ethanol, glycerol, carbohydrates and bio-oil. Among these carboxylates, bio-ethanol can be produced by the fermentation of biomass, and glycerol is a main byproduct in the process of biodiesel refining. There is a glut of glycerol in the market due to the forced regulation on the usage of biodiesel [6]. Hence the hydrogen production from glycerol will be a cost-effective way to utilize glycerol and lower the cost of biodiesel. Hydrogen or syngas can be generated from glycerol by SR [7e9], aqueous-phase reforming [10], pyrolysis [11], supercritical water
* Corresponding authors. Tel./fax: þ86 20 8711 4916. E-mail addresses: [email protected] (H. Yu), [email protected] (F. Peng). 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2011.01.022
reforming [12], and photo-reforming [13]. Some typical catalysts succeeded in the reforming of ethanol have been applied to catalyze the reforming reaction of glycerol, due to their chemical similarity on the aspect of the identical hydroxyl group. Schmidt and co-workers [14] reported that hydrogen was produced by autothermal reforming of glycerol (ATRG) at approximate 1000 C in short contact times of tens of milliseconds on the RheCe catalyst, which is one of the best catalysts for ethanol reforming. Supported nickel catalysts, frequently modified with alkali and alkali earth metals, have been also reported as active catalysts for steam reforming of glycerol (SRG) [15]. Wen et al. [16] reported that supported Pt catalysts were active in the aqueous-phase reforming. Although the achievements in this field are continuously emerging, the mechanism of glycerol decomposition and the suitable process for the reaction are still under debate [17,18]. The thermodynamic analysis will be informative for producing hydrogen from glycerol, which provides a comprehensive understanding on the optimal condition, product distribution and energy efficiency. Adhikari et al. [19] analyzed the production of hydrogen from glycerol and the effects of the process variables by the direct minimization of Gibbs free energy. Wang et al. [20] explored the thermodynamic optimal conditions for SRG. The condition to inhibit carbon deposition was discussed thermodynamically. Their following work was also focused on the thermodynamics analyses of dry reforming [21] and ATRG [22] to produce hydrogen or syngas.
G. Yang et al. / Renewable Energy 36 (2011) 2120e2127
Chen et al. [23] investigated the effect of adding adsorbent of CO2 on the carbon formation, which substantially reduce the lower limit of the water to glycerol ratio. These thermodynamic investigations show that high temperatures, high steam to glycerol ratios and low oxygen to glycerol ratios are feasible for this process. The optimum conditions for the hydrogen production are suggested at temperature of 900e1000 K, steam/glycerol ratio of 9e12, oxygen/glycerol ratio of 0e0.4 and under atmospheric pressure [22]. Some authors have conducted the comparative studies between the thermodynamic and experimental results for SRG. Cui et al. [24] have reported an efficient SRG over La1xCexNiO3 catalyst close to the thermodynamic equilibrium. However, Adhikari et al. [25] reported a SRG far away from the thermodynamic equilibrium. It should also be noted that, although the techniques used in the hydrogen production from glycerol are very similar to the hydrogen production process from ethanol, the glycerol processes reported so far suffered from low H2 yield and serious deactivation compared to the ethanol process. To develop a feasible process for hydrogen production from glycerol, a detailed comparative study between glycerol and ethanol is desired to analyze the technical requirements and potential bottlenecks of the glycerol reforming. In this paper, thermodynamic analysis of the glycerol reforming process is carried out, especially focusing on the OSR, including the ATR process, due to its advantage in the less dependence on the additional energy supply. The results are compared with the corresponding process using ethanol as fuel, in terms of the H2 selectivity, the thermo-neutral condition and the carbon deposition condition. A preliminary experimental investigation was carried out to compare with the thermodynamic results. On the basis of the comparative study, the emphasis of the research and development for the hydrogen production from glycerol is suggested. 2. Computational method and experiments 2.1. Reactions involved in OSRG The total reaction for the steam reforming of glycerol with oxygen addition can be described as Eq. (1):
C3 H8 O3ðlÞ þ xH2 OðlÞ þ yO2ðgÞ /aCO2ðgÞ þ bCOðgÞ þ cH2 OðgÞ þ dH2ðgÞ þ eCH4ðgÞ
ð1Þ
When ethanol is used as fuel, the equation will be in the similar form except for the changes of the stoichiometric numbers. The phase state of the fuel has been selected as either gaseous or liquid state in the existing processes reported. When the fuel is gaseous, an evaporator is needed to vaporize and preheat the fuel. When the fuel is liquid, a nozzle is needed to spray the liquid fuel. Because the glycerol decomposes at its boiling point about 290 C, the state of glycerol has to be selected as liquid for the thermodynamic and experimental investigation in this work. Ethanol was also considered as liquid for a comparative study. Taking glycerol as an example, the major reactions involve:
POX: C3 H8 O3 þ3=2O2 /3CO2 þ4H2 DH0298K ¼ 603kJ=mol
(2)
SRG: C3 H8 O3 þ3H2 O/3CO2 þ7H2 DH0298K ¼ þ128kJ=mol
(3)
When x > 0 and y > 0, the coupling of Eqs. (1)e(2) give the OSR of glycerol (OSRG):
OSRG : C3 H8 O3 þ xH2 O þ yO2 / 3CO2 þ zH2
(4)
The reaction enthalpy depends on the value of y. Higher y results in serious combustion accompanying with the heat release. Otherwise, more water will absorb heat via the endothermic SR reaction.
2121
When the reaction enthalpy equals to zero, the reaction is denoted as ATR. Water-gas shift reaction (WGSR) plays an important role for hydrogen production and carbon monoxide elimination:
WGSR : CO þ H2 O 4 CO2 þ H2 DH0298K ¼ 40 kJ=mol
(5)
In fact, the system includes many other reactions, such as dehydration, dehydrogenation, pyrolysis, etc., which contribute to the formation of by-products, such as methane, ethylene, acetaldehyde and acetone. 2.2. Computational method for the thermodynamic analysis The thermodynamic analysis for the reforming of glycerol and ethanol was carried out in commercial Aspen plus TM simulator, which allows for a sequential modular simulation for a chemical process including standard unit operations. In this work, the reactor was considered as isothermal and isobaric. The physical properties were calculated from the SoaveeRedlicheKwong method. The literature and experimental studies show that the possible products are H2, CH4, CO, CO2, C2H4, C2H6, H2O, graphite carbon (C) and unreacted oxygen and glycerol (C3H8O3)/ethanol (C2H5OH) and these species are assumed in our simulation. Equilibrium composition of these species was calculated through the Gibbs free energy reactor in Aspen Plus TM. The selectivities to products are defined as follows:
SH2 ; % ¼
FH2 ;out 1 100 Fcarbon;out R
where F is molar flow rate, and R is the moles of H2 produced from one mole glycerol or ethanol in feed normalized by carbon number. R is 4/3 for glycerol and 3/2 for ethanol reforming. This definition does not take the H atom in water into account, which may result in the H2 selectivity higher than 100%.
SCi ; % ¼
n FCi ;out 100 Fcarbon;out
where Ci represents CO, CO2, CH4, C2H4 and C2H6, and n is the number of C atoms in one mole Ci. The conversion of glycerol/ ethanol is defined as:
Xol ; % ¼
Fol;in Fol;out 100 Fol;in
2.3. Experimental setup Ir/La2O3 catalyst was used to catalyze the reforming of glycerol and ethanol. Previous work has demonstrated that the catalyst is highly efficient for the OSR of ethanol (OSRE) [26]. The catalyst was prepared by impregnating appropriate amounts of IrCl3$nH2O (with Ir content of 58 wt%, Shanghai July Chemical Co. Ltd.) on La2O3 (Sinopharm Chemical Reagent Co. Ltd.). The impregnated catalysts were dried at 120 C over night, then calcined at 500 C in air for 2 h. The loading of Ir was 5 wt%. Catalytic tests were performed in a quartz fixed-bed reactor with 9 mm inner diameter. Typically, 60 mg catalysts were sandwiched by quartz wool and packed in the reactor. A thermocouple was inserted into the center of the catalytic bed to monitor the reaction temperature. Before the reaction, the catalyst was reduced with hydrogen of 50 Ncm3/min at 500 C for 40 min. A mixture of glycerol and water with 1:6 M ratio was fed into the reactor through a stainless steel capillary with inner diameter of 0.8 mm by a high pressure syringe pump at flow rate of 0.1 ml/min. Oxygen at
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100
o
C/O=1.2;T=600 C
C/O=1.2; S/C=1
100
H2
80
H2 CO CO2 CH4 C2H4 C2H6
60 40
Selectivity /%
Selectivity /%
80
20
CO CO2 CH4 C2H4 C2H6
60 40 20
0
0 0
1
2
3
4
5
6
7
8
400
500
S/C
600
700
800 o
Reaction temperature / C
Fig. 1. Effect of S/C on selectivities of gas-phase products of the OSRG reaction at C/O ¼ 1.2 and T ¼ 600 C.
room temperature was introduced and mixed with the reactant at the inlet of reactor. The gas hourly space velocity (GHSV) was about 11.5 104 h1. The gaseous product off the reactor was analyzed by a Fuli 9790 Gas Chromatography equipped with a TDX-01 packed column (for the analysis of ethanol in a Thermal Conductor Detector) and an AE electric insulating oil analysis column (for the analysis of CO, CH4, CO2, C2H4, C2H6 in a Flame Ionization Detector). CO and CO2 were converted to CH4 by a methanizer before entering the FID detector. Other products off the reactor were condensed through an ice trap. The glycerol unconverted in liquid was analyzed by a Waters Liquid Chromatography 2695 equipped with a ultimate TM column and a Differential Refraction Detector.
Fig. 3. Effect of reaction temperature on selectivities of gas-phase products of the OSRG reaction at C/O ¼ 1.2 and S/C ¼ 1.
account the increased energy consumption caused by the addition of water, higher S/C ratio is cost-inefficient for the H2 production. On the other hand, the excess steam also quenches the carbon deposition, which will be discussed in detail in Sec. 3.2 later. The selectivity of CH4 decreases with the increasing S/C ratio, indicating that the reforming of CH4 may be enhanced by the excess water. Under the conditions investigated, the formation of ethylene and ethane is neglectable. In the OSRG reaction, the amount of oxygen significantly influences the energetics and product distribution. Fig. 2 shows the 140
a
130 120
3.1. Parametric investigation of OSRG
110
Under all of the conditions studied in this paper, the glycerol conversion approaches to 100%, indicating that the utilization of glycerol is not limited by thermodynamics. Fig. 1 shows the effect of S/C ratio on the selectivity of gaseous product of OSRG reaction at C/O ¼ 1.2 and at 600 C. The addition of steam increases the selectivity of H2. The selectivity of H2 is higher than 100% when the S/C ratio is larger than 2, indicating that H2O will contribute H atoms to generate H2. However, at the S/C ratios higher than 3, the addition of steam to the system has little effect on the H2 formation, since the equilibrium of WGSR limits the yield of H2. Taking into
H2 selectivity /%
3. Results and discussion
90 80
Ethanol Glycerol
70
50
0
2
120
CO2
40 20
H2 selectivity / %
H2 CO CH4 C2H4 C2H6
6
8
10
12
1.0
1.5
2.0
2.5
3.0
C/O Fig. 2. Effect of C/O on selectivities of gas-phase products of the OSRG reaction at S/C ¼ 1 and T ¼ 600 C.
18
20
22
110 100 90 80
60 0.5
16
b
Alcohol/O2=0.7
70
0
14
Glycerol Ethanol
o
60
4
Ethanol(Glycerol)/O2
S/C=1; T=600 C 80
H2O/alcohol=3
60
130
100
Selectivity /%
100
0
1
2
3
4
5
6
7
8
9
10
11
H2O/Ethanol (Glycerol) Fig. 4. Hydrogen and water selectivity of OSRG and OSRE as a function of (a) glycerol (ethanol) to O2 ratio and (b) water to glycerol(ethanol) ratio at 650 C.
G. Yang et al. / Renewable Energy 36 (2011) 2120e2127
product distribution at equilibrium as a function of C/O ratio. The selectivities of H2, CH4 and CO increase with the C/O ratio at a fixed S/C ratio and temperature. The rapid decline of H2 selectivity at low C/O ratio may be caused by the dominant combustion of H2 with O2 to form water. At high C/O ratios, the H2 selectivity will approach the limitation of SRG. From the viewpoint of H2 production, the addition of O2 is not desired. The previous thermodynamic analysis of glycerol reforming indicated that the higher temperatures of 627e727 C favor the reaction [22]. As shown in Fig. 3, the highest H2 selectivity is obtained at temperature of 600 C. A slightly decrease of H2 selectivity occurs at T > 650 C. The methane selectivity decreases with the increase of temperature and approaches to zero at T > 700 C. It can be concluded that the methane steam reforming is accelerated with the increasing temperature and contributes to the formation of H2. It should be noted that the selectivity of CO increases and that of CO2 decreases at T > 650 C. It can be explained by the enhanced reverse WGSR at high temperatures, which may also cause the decrease of H2 in this range of temperature. The above parametric study conducted in Aspen plus TM indicates that the OSRG prefers to the moderate S/C and C/O ratios for a compromise between H2 yield and energy consumption. Due to the limitation of WGSR, the feasible reaction temperature should be selected around 600e700 C. These results are in agreement with
6 5
S/C
4
3.2. Comparative study between OSRG and OSRE The comparison was firstly focused on the H2 yield. For the hydrogen production, higher H2 yield per molecule alcohol is desired under identical conditions. As shown in Fig. 4(a), the H2 selectivity from glycerol reforming is lower than that from ethanol reforming at the same ratio of alcohol to O2. At 650 C and water to alcohol ratio of 3, the limitation value at the low O2 amount, namely the steam reforming, is about 110% in OSGR and 130% in OSRE, corresponding to 47.8 mmol H2 per gram glycerol and 84.8 mmol H2 per gram ethanol. Fig. 4(b) shows the H2 selectivity of OSRG and OSRE as a function of water to alcohol ratio at a fixed alcohol to O2 ratio. To obtain the same hydrogen selectivity, more water has to be consumed in OSRG, which causes higher energy cost to vaporize water. The results suggest that the OSRE process is more efficient than the OSRG from the view of producing hydrogen. It may be due to the higher O/C ratio in a glycerol molecule. It could be explained
b
500 550 600 650 700 750 800
ATRG
the previous reports [19,22,24] on the experimental and theoretical studies on the glycerol reforming, demonstrating that Aspen plus TM can be used as a tool for a robust thermodynamics analysis of the hydrogen production process. In the following section, the OSGR will be compared with the ethanol counterpart to evaluate the process development according to the relatively well-established ethanol reforming process.
6
4
3
3 2
2
1
1 0 0.4
0.8
1.2
1.6
0 0.4
2.0
0.8
1.2
d
ATRG
ATRE 90
80
80
70
50
0.4
500 550 600 650 700 750 0.8
70 500 550 600 650 700 750
60 50
1.2
C/O
2.0
100
90
SH2/%
SH2 /%
100
60
1.6
C/O
C/O
c
500 550 600 650 700 750
ATRE
5
S/C
a
2123
1.6
2.0
0.4
0.8
1.2
1.6
2.0
C/O
Fig. 5. The dependence of S/C on C/O of (a) the glycerol and (b) ethanol reforming under the thermal-neutral condition. (c) and (d) show the corresponding selectivity of hydrogen as a function of C/O in the (c) glycerol and (d) ethanol reforming.
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G. Yang et al. / Renewable Energy 36 (2011) 2120e2127
a
100
800
Glycerol
Carbon free
700 600
Temperature / C
60
o
Selectivity /%
80
40 20
500 400
C/O 0.8 1.0 1.2 Carbon deposition 1.5 Steam reforming
300 200
0 500
550
600
650
700
750
100
o
0.0
Temperature / C
0.6
0.8
1.0
1.2
b
800
Carbon free
Ethanol 700 600
o
500
C/O
400
0.8 1.0 Cabon deposition 1.2 1.5 Steam reforming
300 200 100
0.0
0.2
0.4
0.6
0.8
1.0
1.2
S/C Fig. 7. Carbon deposition and carbon-free regions of (a) the glycerol reforming and (b) ethanol reforming. The lines represent the condition at which 0.1% carbon atoms in the feed are deposited.
S/C and C/O ratios, the coke is formed at lower temperatures in the ethanol reforming than in the glycerol reforming. For instance, the coke will be eliminated above 563 and 332 C for the glycerol reforming at S/C ¼ 1 under the SR condition (C/O ¼ N) and at C/O ¼ 1.5, respectively. However, temperatures up to 657 and 420 C are required in the ethanol reforming. This can be explained 180
Thermodynamic Equilibrium
160 140 120
SH2/%
by the more abundance of O in the glycerol system limits the WGSR and enhances the oxidation of H2, which lower the extent to extract hydrogen from water. The ATR is highly desired to reduce the dependence of the hydrogen production on the external energy and minimize the volume of reactor. Fig. 5(a) and (b) show the relationship between C/O and S/C under the thermal-neutral condition. At a fixed reactor temperature, with increasing C/O ratio, less water is needed to maintain the zero reaction heat, due to the less extent of exothermal oxidation reaction. At the identical temperature and S/C ratio, higher C/O ratio is needed for ATRG than for ATRE because of the higher O/C ratio in a glycerol molecule. Fig. 5(c) and (d) show the selectivity of hydrogen as a function of C/O under the corresponding thermalneutral condition. Under a fixed reactor temperature, a maximum of hydrogen yield will be obtained at C/O ratio of 0.8e1.2, indicating that the ATR should be operated with a specific combination of S/C and C/O. At lower C/O ratios, more H2 and reactant have to be consumed to sustain the evaporation of water. On the other hand, the S/C ratio is lower at higher C/O ratios, thus the H2 selectivity will decrease according to Fig.1, meanwhile the risk of carbon deposition increase. Fig. 5(c) and (d) also indicate that the H2 selectivity strongly depends on the reaction temperature of ATR reaction. For a comparison between ATRG and ATRE, the maximum H2 selectivity under thermal-neutral condition is plotted as a function of reaction temperature, as shown in Fig. 6. The maximum H2 selectivity in the ATR reaction is obtained at about 650 C for both the ATRE and ATRG. More CH4 is produced at low temperatures and reduces the H2 selectivity. The reduction of H2 selectivity at high temperatures may be caused by the lower S/C ratio and the limited WGSR. In the temperature range investigated, the H2 selectivity of the ATRG reaction is always lower than that of ATRE, which emphasize that glycerol will be thermodynamically less efficient for producing hydrogen compared with ethanol. It suggests that a process with the faster kinetics and the more elaborate process development will be highly desired for the utilization of glycerol. Carbon deposition is of importance in the (oxygenated) hydrocarbon reforming to produce hydrogen, because the undesirable carbon formation can deactivate the catalysts dramatically [27,28]. The carbon deposition region was determined for the reforming of glycerol and ethanol, as shown in Fig. 7. Generally, higher temperature, addition of oxygen and higher ratio of S/C will suppress the coke-formation, which agrees well with the previous experimental and theoretical studies [3,24,29e32]. At the identical
0.4
S/C
Temperature / C
Fig. 6. The selectivity pattern when the maximum hydrogen selectivity is obtained under thermal-neutral condition in ATRG and ATRE. Solid lines and symbols: ethanol; dashed lines and open symbols: glycerol. Circle: H2; up triangle: CO; down triangle: CO2; square: H2O; diamond: CH4.
0.2
100 80 60 40 20 1
2
10
100
S/C Fig. 8. Experimental H2 selectivity of SRG in literature and the corresponding thermodynamic limitation. The legends are listed in Table 1. For comparison, some of the H2 selectivity values in literature were recalculated with the definition in Sec. 2.2 according to the provided information.
G. Yang et al. / Renewable Energy 36 (2011) 2120e2127
2125
Table 1 Legends of the scatters in Fig. 8. Catalyst
Reaction conditions
Source
H2O/Glycerol ¼ 6:1(molar ratio); GHSV ¼ 51,000 h1; 600e900 C
[29]
1 wt% glycerol; WHSV ¼ 2.5 h1; 873 K
[15]
H2O/C3H8O3 ¼ 0e1(weight ratio); 5.4 g/h; 800 C
[35]
H2O/C3H8O3 ¼ 24/1(molar ratio); 0.06 ml/min; 500, 550, 600 and 700 C
[24]
10 wt% of glycerol; GHSV ¼ 7.7 gglycerol/(gcat h); 773, 873 K
[36]
C3H8O3/H2O/He ¼ 2/18/80 (vol%); GHSV ¼ 11,000 ml/(gcat h); 500 C.
[37]
S/C ¼ 3.3; Contact time of glycerin: 13.4 gcat h/mol; 500e600 C
[9]
S/C ¼ 2, T ¼ 650 C
This work
;Al2O3, =Rh/Al2O3,
Ir/Al2O3, Pd/Al2O3, Pt/Al2O3, CeO2/Al2O3, Rh/CeO2/Al2O3, Ni/CeO2/Al2O3, Ru/CeO2/Al2O3, Pd/CeO2/Al2O3, Pt/CeO2/Al2O3 Ni/Al2O3, Ni/La2O3/Al2O3, Ni/MgO/Al2O3, Ni/CeO2/Al2O3, Ni/ZrO2/Al2O3
CNi/Al2O3 LaNiO3, HLa0.3Ce0.7NiO3, ILa0.7Ce0.3NiO3, La0.9Ce0.1NiO3, Pt/Y2O3, Pt/CeZrO2/Y2O3 Ni/La2O3(3 wt%)/Al2O3, =Ni/Al2O3, Ni/La2O3(6 wt%)/Al2O3, Ni/La2O3(15 wt%)/Al2O3, Pt/La2O3(15 wt%)/Al2O3, Pt,Ni/La2O3(3 wt%)/Al2O3 Ir/CeO2,
Co/CeO2,
Ni/CeO2
,Ru/La2O3, BRh/La2O3, 6Ni/La2O3, Ir/La2O3, >Co/La2O3, Pt/La2O3, Pd/La2O3, Fe/La2O3, Ru/Y2O3, Ru/ZrO2, Ru/CeO2, Ru/La2O3,
by the more oxygen atoms in glycerol molecule than in ethanol, which facilitates the formation of gaseous CO or CO2. The above comparison indicates that the glycerol reforming may be more stable thermodynamically, since the coking can be more facilely controlled to inhibit the deactivation caused by carbon deposition. The essential information is provided by the analysis to determine the operation conditions preventing carbon deposition. Nevertheless, it should be noted that the gasification of carbon may be kinetically slow, therefore the pathways resulting in coke formation, such as thermolysis, oligomerization, olefin formation via dehydration and decarbonylation etc., must be carefully tailored by the catalyst design to realize the thermodynamic equilibrium product distribution. 3.3. Discussion The aforementioned thermodynamic comparative results offer a basis for the discussion of the strategy of hydrogen production using the renewable ethanol and glycerol. Both of ethanol and glycerol can be used as the starting materials for the sustainable hydrogen energy, because the considerable H2 yield can be achieved with the reforming process, as widely demonstrated by many researchers [17,18]. However, the high O/C ratio in a glycerol molecule could decrease the hydrogen selectivity according to our analysis. It implies that, to achieve a competitive process with the ethanol-based one, the glycerol reforming has to take the advantage of the glut of glycerol in the market to reduce the cost. The attention should also be focused on the catalyst development to achieve the equilibrium product distribution. In the well-established
ethanol reforming process, some catalysts, including Rh, Ir, Ni and Co etc., have been demonstrated active and stable [33]. In the case of glycerol, higher activity is needed because of the difficulty in breaking CeC bond. Fig. 8 compares the experimental results of SRG in literature over a variety of catalysts with the thermodynamic results calculated by Aspen plus TM. Many catalysts reported give the hydrogen selectivity of the glycerol reforming far away from the equilibrium. Among the catalysts with near-equilibrium products, the noble metal catalysts, such as Rh, Ru and Pt, provide high activity and selectivity. The Ni catalysts possess the high hydrogen selectivity at high S/C ratios. So far, few publications report on the experimental investigation of OSRG [14,28], as listed in Table 2. Rennard et al. [14] reported that the RheCe/ Al2O3eSiO2 catalyst can give a H2 selectivity of 75%, very close to the equilibrium value at 600 C in OSRG. However, the results obtained over Pd [34] and Pt [14] catalysts were far away from the corresponding equilibrium values, indicating there is plenty of room for the improvement of the catalyst. Meanwhile, the information on catalyst stability and deactivation has rarely been involved. The difference between the catalytic performances of the identical catalyst in glycerol and ethanol reforming can be directly compared. Our previous work demonstrated that the Ir/La2O3 catalyst performs well in the OSRE reaction [26]. Fig. 9 compares the selectivity patterns of OSRG and OSRE under the same reaction condition. The OSRG provided lower hydrogen selectivity than the OSRE, agreeing with the aforementioned thermodynamic analysis. The equilibrium selectivity of hydrogen via OSRG and OSRE are 86% and 110%, respectively. In the OSRE, a near-equilibrium H2 selectivity was obtained, however, the hydrogen selectivity of OSRG is obviously lower than the equilibrium one. It is indicative of the
Table 2 Comparison between experimental and equilibrium H2 selectivity in OSRG. Catalyst
H2 selectivity/% Experimental
Equilibrium
RheCe/Al2O3eSiO2 Pt/Al2O3eSiO2 Pd/Cu/Ni/K Ir/La2O3
w70 w20 w75 w60
w75(600 C) w82(700 C) w130 w86
Conditions
Source
C/O ¼ 1; S/C ¼ 1 C/O ¼ 1.4 S/C ¼ 3, C/O ¼ 3.33, T ¼ 700 C S/C ¼ 2, C/O ¼ 1, T ¼ 650 C
[14] [14] [34] This work
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G. Yang et al. / Renewable Energy 36 (2011) 2120e2127
Acknowledgments
a 120
Glycerol
H2 CH4 C2H4
Selectivity /%
100
CO CO2 C2H6
H2 selectivity in equilibrium
80 60
References
40 20 0
0
40
80
120
160
200
Time on stream /min
b 120
Ethanol
H2 selectivity in equilibrium
Selectivity /%
100 H2 CO2
80
CO C2H4
CH4
60 40 20 0
The authors thank the financial support from the National High Technology Research and Development Program of China (863 Program, No. 2009AA05Z102) and the Fundamental Research Funds for the Central Universities of China (No. 2009zm0246).
0
20
40
60
80
100 120 140 160 180
Time on stream /min Fig. 9. Selectivity of gaseous products by (a) OSRG and (b) OSRE. Conditions: C/O ¼ 1, S/C ¼ 2, T ¼ 650 C, feed flow rate ¼ 0.1 ml/min, 60 mg Ir/La2O3 catalyst.
slower kinetics over the Ir/La2O3 catalyst for the glycerol reforming. Thus, the more active and selective catalyst will be highly desired for the glycerol reforming technique in the future.
4. Conclusions The Aspen plus TM software is used as a facile and robust tool for the thermodynamic analysis of the reforming of glycerol and ethanol. The OSRG prefers to the moderate S/C and C/O ratios for a compromise between H2 yield and energy consumption. Due to the limitation of WGSR, the feasible reaction temperature should be selected at 600e700 C. To obtain the maximum of hydrogen yield under the ATRG condition, the C/O ratio should be in the range of 0.8e1.2. The comparative study between the glycerol and ethanol reforming shows that the reforming of glycerol provides lower H2 yield than that of ethanol, due to the more oxygenated composition of glycerol. Thus, it can be suggested that the investigation on the reforming property of the crude glycerol, containing methanol, metal ions and larger biomass molecule, will be highly desired, because the refining of crude glycerol from the bio-diesel production will significantly increase the cost. However, this topic has not been fully convinced of by the community yet [17]. On the other hand, the higher oxygen content of glycerol improves the resistance to the carbon deposition compared to ethanol. On the basis of the thermodynamic analysis combining with the preliminary experimental investigation, it is suggested that the development of new catalyst is highly desired for the glycerol reforming to overcome the kinetic limitation.
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