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Thermodynamic analysis of ethanol reforming for hydrogen production
Energy 44 (2012) 911e924
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Thermodynamic analysis of ethanol reforming for hydrogen production Shaohui Sun*, Wei Yan, Peiqin Sun, Junwu Chen Institute of Catalysis and Polymer, School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, Henan, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2012 Received in revised form 27 April 2012 Accepted 30 April 2012 Available online 28 May 2012
This work presents the simulated equilibrium compositions of ethanol steam reforming (SR), partial oxidation (POX) and auto-thermal reforming (ATR) at a large temperature range, steam-to-ethanol and oxygen-to-ethanol molar ratios. The simulation work shows that the moles of hydrogen yield per mole ethanol are of this order: SR > ATR > POX. The results are compared with other simulation works and fitted models, which show that all the simulation results obtained with different methods agree well with each other. And the fitted models are in highly consistency with very small deviations. Moreover, the thermal-neutral point in corresponding to temperature, steam-to-ethanol and oxygen-to-ethanol mole ratios of ethanol ATR is estimated. The result shows that with the increasing of oxygen-toethanol mole ratio, the T-N point moves to higher temperatures; with the increasing of steam-toethanol mole ratio, the T-N point moves to lower temperatures. Furthermore, the energy exchanges of the reforming process and the whole process and the thermal efficiencies are also analyzed in the present work and that the energy demands and generated in the whole process are greater than the reforming process can be obtained. Finally, the optimum reaction conditions are selected. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen Ethanol Steam reforming Partial oxidation Auto-thermal reforming Thermodynamic analysis
1. Introduction Hydrogen has been reported as one of the few long-term sustainable clean energy carriers, only generating water vapor as a by-product during the combustion or oxidation process [1]. A global system where hydrogen is produced from renewable energy sources would be in complete balance with the environment, and therefore sustainable [2]. Yet many problems are encountered in the fields of hydrogen production, storage and distribution, as well as engine developments [3]. Liquid oxygenated hydrocarbons from biomass are promising resources for hydrogen because they have high energy density and can be easily stored [4]. Until now, many oxygenated hydrocarbons, such as bio-oil (acetic acid, ethylene glycol and acetone) [5,6], glycerol [7e10], methanol [11e13], ethylene glycol [14e16] and ethanol [17e22], have been used to produce hydrogen. Among all these resources, ethanol has attracted much attention in recent years [17,23e26]. Ethanol can be easily obtained from biomass through fermentation or chemistry method. Besides, it has higher hydrogen content and can be separated easier than other oxygenated hydrocarbons. Ethanol reforming for hydrogen production is a carbon-neutral process as the CO2 generated in the process is consumed for * Corresponding author. Tel.: þ86 371 67781754; fax: þ86 371 67781755. E-mail addresses: [email protected], [email protected] (S. Sun). 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2012.04.059
biomass growth, which offers a nearly closed carbon loop [27]. As the boil point of ethanol is lower than water, there are three primary techniques can used to produce hydrogen: ethanol steam reforming (SR), partial oxidation (POX) and auto-thermal reforming (ATR). The reforming process produces a gas stream primarily composed by hydrogen, carbon dioxide, carbon monoxide and methane. Ethanol SR is a highly endothermic process which needs external heat supplier, while ethanol POX is exothermic and ATR is thermal-neutral(T-N) or slightly exothermic [28]. Ethanol SR is feasible for temperatures higher than 500 K and its H2/CO ratio is higher than POX and ATR [29]. In a POX process, ethanol is reacted with oxygen at sub-stoichiometric ratios above the flame temperature, and the heat generated is to make the reactants to a higher temperature [30]. Ethanol ATR is the combination of SR and POX. In this process, the heat generated from partial combustion can make up for the SR, which makes the system a T-N condition [31]. Hydrogen productions from ethanol in these three ways are shown as follows. The reaction equation of ethanol SR is,
C2 H5 OH þ 3H2 O42CO2 þ 6H2 ; DH 0 ¼ 347:5 kJ=mol
(1)
The reaction equation of ethanol POX is,
1 C2 H5 OH þ O2 42CO þ 3H2 ; DH0 ¼ 14:1 kJ=mol 2
(2)
912
S. Sun et al. / Energy 44 (2012) 911e924
Nomenclature aji C ^ pi fi fi0 G Gi Hi DG0fi
number of atoms of element j in component i heat capacity for component i fugacity for component i in the mixture fugacity for pure component in the reference state Gibbs free energy for the system partial molar Gibbs free energy for component i partial molar enthalpy for component i standard Gibbs free energy of formation of species i
DHfi0
standard enthalpy of formation of species i
ni yi NC NE P
number of moles for component i molar fraction for gas phase for component i number of components in the system number of elements in the system pressure of system
(3)
The reaction of ethanol ATR is,
C2 H5 OH þ 1:78H2 O þ 0:61O2 42CO2 þ 4:78H2 ;
DH 0 ¼ 0 kJ=mol
reference pressure (101.325 kPa) universal gas constant temperature of system (K) reference temperature (298.15 K) correlation coefficient for the fitting equation
Greek symbol mi chemical potential for component i m0i chemical potential of at reference temperature for pure component i ^ fi fugacity coefficient for component i Subscripts I component in the mixture J element in component i
Or
3 C2 H5 OH þ O2 42CO2 þ 3H2 ; DH0 ¼ 554:0 kJ=mol 2
P0 R T T0 R
ð4Þ
Other possible reactions are shown as follows.
C2 H6 O4CO þ C þ 3H2 ; DH0 ¼ 31:32 kJ=mol
(5)
CO þ H2 O4CO2 þ H2 ; DH0 ¼ 41:2 kJ=mol
(6)
CH4 þ H2 O4CO þ 3H2 ; DH0 ¼ 206:1 kJ=mol
(7)
CH4 þ2H2 O4CO2 þ 4H2 ; DH0 ¼ 164:9 kJ=mol
(8)
CO2 þCH4 42CO þ 2H2 ; DH0 ¼ 247:3 kJ=mol
(9)
CO þ H2 4C þ H2 O; DH 0 ¼ 131:3 kJ=mol
(10)
CH4 4C þ 2H2 ; DH0 ¼ 74:9 kJ=mol
(11)
2CO4CO2 þ C; DH 0 ¼ 172:4 kJ=mol
(12)
C2 H6 O4H2 þ CH3 CHO; DH 0 ¼ 67:8 kJ=mol
(13)
C2 H6 O4H2 O þ C2 H4 ; DH 0 ¼ 44:7 kJ=mol
(14)
2C2 H6 O43H2 þ CO þ CH3 COCH3 ; DH 0 ¼ 93:8 kJ=mol
(15)
So far, many efforts have been done on the experimental and thermodynamic analysis of hydrogen production generated from ethanol reforming, while no comparisons and optimization have
been made with these models yet. As experimental validation of a thermodynamic model is not always possible since the experimental data reported in literature is not necessarily at equilibrium conditions. Nonetheless it is still quite useful to compare experimental results with thermodynamics data to observe the trends and confirm how far apart the system is from equilibrium conditions. In the present study, a thermodynamic analysis of hydrogen production from ethanol in these three different technologies (SR, POX and ATR) is carried out with Gibbs free energy minimization method under temperatures from 700 to 1400 K, steam-to-ethanol (S/E) from 0 to 10 and oxygen-to-ethanol (O/E) molar ratios from 0.0 to 3.0. The above reaction equations show that besides some main products, there are many other species coexisting in the reforming system, such as ethylene, acetone and acetaldehyde, etc. The previous researches [21,29,32,33] and our calculation find that ethylene, acetone and acetaldehyde as any thermodynamically viable species do not show up above 250 C. So under the reaction conditions investigated, eight species H2, CO2, CO, CH4, carbon, water, ethanol and oxygen are determined as the equilibrium compositions. The minimization of Gibbs free energy has been implemented with many different methods, such as Aspen-HYSYS software [34], HSC Chemistry [9], GNU Octave numerical language [35], etc. However, the constancy of the thermodynamic results with different methods has not been investigated yet; the thermodynamic results are expressed dispersedly in figures, from which the searched equilibrium composition data under an arbitrary condition is not so accurate; especially, energy efficiencies of ethanol reforming and the T-N point of ethanol ATR have not been analyzed systematically. In this work, the minimization of Gibbs free energy will be programmed with FORTRAN language emulating Ant algorithm. The estimation is based on the basic thermodynamic data of all the proposal compounds, thermodynamically analyzes the SR, POX and ATR of ethanol for hydrogen production, and they were compared with previous theoretical studies. Comparisons between the fitted models and all the simulations show that not only the estimations with different methods agree well with each other but also the fitted equations are in highly accordance with all the estimated results, which provides a simple way for further researchers to determine the equilibrium compositions of ethanol reforming without repeating simulations and exploratory experiments. Meantime, the thermal efficiency of the three technologies are investigated and compared, which is of the order, the efficiency of SR is approach to that of ATR, and they both are higher than that of POX.
S. Sun et al. / Energy 44 (2012) 911e924
1) non-negativity of moles number:
2. Methodology The total Gibbs free energy of a system can be expressed as follow,
G ¼
NC X
ni ,mi
(16)
ni 0;
where, ni is the number of equilibrium moles for species i and mi is the chemical potential which can be expressed as: ^
fi fi0
! (17)
where, m0i is the chemical potential of component i at reference temperature, fi0 is the fugacity for pure component in the reference ^
state and fi is the fugacity for component i in the mixture which can be obtained by: ^
^
fi ¼ fi ,yi ,P
(18) ^
where, yi is the molar fraction of component i in equilibrium and fi is the fugacity coefficient for component i which is very close to 1 at ^
low pressure and high temperature. Assuming fi ¼ 1 and submitting them back into Eq. (16) and Eq. (18) can be obtained,
G ¼
NC X i¼1
ni , m0i þ RTðln yi þ ln PÞ
(19)
The reference chemical potential m0i of the pure component i can be calculated using the following known thermodynamic relations:
vHi vT
¼ Cpi
(20)
P
v Gi vT RT
¼ P
Hi RT 2
Gi ¼ mi
(21)
(22)
After directly solving of Eqs. (20) and (21) for gases at standard atmosphere pressure, the following equation can be obtained,
m0i ðTÞ ¼
T T T DG0fi þ 1 DHfi0 ai Tln T þ T0 T0 T0 T0 bi 2 c T 2T0 T þ T02 i T 3 3T02 T þ 2T03 2 6 di 4 e 3 4 T 4T0 T þ 3T0 i T 5 5T04 T þ 4T05 12 20 (23)
where, the value of DG0fi , DHfi0 , ai, bi, ci and di (constants for polynomial expression of Cpi) are taken from databases [36,37]. As carbon is solid and has no vapor pressure, the total Gibbs free energy including carbon is expressed as:
G ¼
NC1 X i¼1
ni , m0i þ RT ln yi þ ln P þ nC ,m0C
(24)
where nC represents the equilibrium moles of carbon and m0C the reference chemical potential of carbon. In the Gibbs free energy minimization, some restrictions should be satisfied:
i ¼ 1; ..; NC
(25)
where, NC is the number of components.
i¼1
mi ¼ m0i þ RTln
913
2) Mass balance of each atom must satisfy the restriction: NC X
ni aji ¼ bj ;
j ¼ 1; ..; NE
(26)
i¼1
where, NE is the number of element. FORTRAN code is applied to obtain the minimize Gibbs free energy with ant algorithm and the equilibrium compositions of ethanol SR, POX and ATR processes. The equilibrium compositions obtained are fitted using MATLAB 7.1 with non-linear polynomials.
3. Results and discussion 3.1. Thermodynamic product distribution and comparisons In the present study, the thermal equilibrium compositions are calculated with Gibbs free energy minimization implemented with FORTRAN code language while many other researchers have done the thermodynamic analysis using different methods. In this part, the thermodynamic results are compared with some other simulation works [32,34,35,38e40] and the comparisons show that they agree well with each other.
Table 1 Comparisons of H2 yield for ethanol POX. T (K)
O/E
Present simulation
Wanga [36]
Rabensteina [30]
Deviation%
700 700 700 700 700 700 700 700 900 900 900 900 900 900 900 900 1100 1100 1100 1100 1100 1100 1100 1100 1300 1300 1300 1300 1300 1300 1300 1300
0 0.25 0.50 0.75 1.00 1.50 2.00 2.50 0 0.25 0.50 0.75 1.00 1.50 2.00 2.50 0 0.25 0.50 0.75 1.00 1.50 2.00 2.50 0 0.25 0.50 0.75 1.00 1.50 2.00 2.50
0.4932 0.4688 0.4678 0.4500 0.4400 0.4300 0.4200 0.4150 1.8003 1.7827 1.7822 1.726 1.7061 1.6165 1.3146 0.8004 2.9000 2.7792 2.7418 2.4613 2.1232 1.7987 1.2734 0.6178 2.9866 2.9726 2.9657 2.6247 2.1232 1.7302 1.1041 0.5025
0.5 0.47 0.46 0.45 0.44 0.43 0.42 0.42 1.8 1.78 1.78 1.72 1.7 1.6 1.3 0.8 2.9 2.8 2.7 2.5 2.1 1.8 1.2 0.6 3 3 3 2.6 2.1 1.7 1.1 0.5
0.5
0.6 0.1 0.8 0.0 0.0 0.0 0.0 0.5 0.007 0.07 0.06 0.1 0.1 0.4 0.5 0.02 0.0 0.3 0.7 0.7 0.5 0.03 2.6 1.3 0.2 0.4 0.5 0.4 0.5 0.7 0.1 0.2
a
Data is read from the figures.
0.43 0.42 0.42 1.8
1.6 1.3 0.8 2.9
1.8 1.2 0.6 3
1.7 1.1 0.5
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S. Sun et al. / Energy 44 (2012) 911e924
3.1.1. Ethanol partial oxidation The heat generated from ethanol combustion contributes a high temperature to the system and thus it can easily to be started [37]. The POX process for hydrogen production is simulated in the O/E molar ratio range from 0.0 to 3.0 and temperature range from 700 K to 1400 K at atmospheric pressure. And comparisons are made between the H2 yields obtained by the present simulation and that by Wang [38] and Rabenstein (using Aspen TechÔ) [30], which are showed in Table 1. Table 1 shows the same results are obtained by our methodology with those obtained by other different method, with the relative coverage deviations lower than 3%. With this methodology, H2, CO, CO2 and CH4 yields (moles per mole ethanol) under different conditions are calculated which are showed in Fig. 1(aed). From Fig. 1(a), we can see that more than 96% (2.9/3.0) yield of H2 can be achieved at above 1200 K and O/E ratio lower than 0.5. The H2 yield decreases with the decrease of temperature and the increase of O/E ratio. Fig. 1(b) shows that the maximum CO yield (w1.9 mol) appears at about 1200 K with O/E between 0.5 and 0.75. Under a given temperature, the yield of CO increases with the O/E
1400
b 1400
1300
1300
1200
1200
Temperature,K
2.9
0.30
1100
1000
2.7 2.4 2
900
1.7
1.3
0.5
0.20 0.10
700 0.0
0.60 1.0
1.5
2.0
2.5
3.0
0.5
d
1400
Temperature,K
0.10
0.50 0.90
1000
0.30
1.1
1.3
1.7
1.9
1.0
1.5
2.0
Oxygen-to-ethanol molar ratio
3.0
0.050 1100
0.10 1000
0.20
800
0.30 0.40
0.60 0.70
0.5
2.5
0.030
900
1.5
800
700 0.0
2.0
0.010
1200
0.70
0.050
1.5
1400
1300
1200
1.0
Oxygen-to-ethanol molar ratio
Oxygen-to-ethanol molar ratio
1100
0.90 0.60
0.40
800
1300
Temperature,K
1.9
1000
0.90
900
1.5
1100
1.5
1.2
800
700 0.0
2.1 1.8
900
c
3.1.2. Ethanol steam reforming In this work, the thermodynamic compositions obtained with FORTRAN code are compared with other simulation results with Wang [35] (with Lagrange multiplier method), Kafarov [34] (with Aspen-HYSYS software) and Vasudeva [40] (with SQP). The comparisons of H2 yield per mol ethanol at different S/E molar ratios (0e10) and the temperature range from 700 to 1200 K is shown in Table 2. From Table 2, we can see that hydrogen yields of ethanol SR obtained with different methods are in consistency with each other with the relative average deviation no greater than 3%. The
Temperature,K
a
ratio at first then decreases owing to the Eqs. (2), (3) and (5). In Fig. 1(c), the yield of CO2 reaches 95% (1.9/2) at all temperatures with O/E > 2.5 and decreases with the decreasing of O/E ratio but is little affected by temperature. As far as CH4 yield is concerned in Fig. 1(d), it is relatively low (<0.7 mol) and low temperature favors the CH4 to generate due to the reverse of Eqs. (7) and (8). These trends are in accordance with that obtained by other simulation works. Carbon formation in all the three processes is discussed in part 3.3.4.
2.5
3.0
700 0.0
0.50 0.5
1.0
1.5
2.0
Oxygen-to-ethanol molar ratio
Fig. 1. (aed). Effects of temperature and O/E molar ratio on H2, CO, CO2 and CH4 yields from ethanol POX.
2.5
3.0
S. Sun et al. / Energy 44 (2012) 911e924 Table 2 Comparisons of H2 yield for ethanol SR. T (K)
S/E
Present simulation
Wanga [37]
800 800 800 800 800 800 800 800 800 1000 1000 1000 1000 1000 1000 1000 1000 1000 1100 1100 1100 1100 1100 1100 1100 1100 1100
0 1 2 3 4 6 8 9 10 0 1 2 3 4 6 8 9 10 0 1 2 3 4 6 8 9 10
1.1280 1.4663 1.8042 2.5487 2.7509 3.7679 3.9674 4.2301 4.3521 2.6268 3.2972 3.8976 4.4572 4.6713 5.0955 5.1778 5.3505 5.4338 2.9000 3.6889 4.0240 4.5786 4.6203 4.8855 5.0158 5.1464 5.1905
1.2 1.5 2 2.5 2.8 3.8 4 4.3 4.4 2.6 3.3 3.9 4.5 4.7 5 5.2 5.3 5.4 2.9 3.6 4 4.5 4.6 4.9 5 5.1 5.2
a b
Kafarova [32]
Vasudevab [2]
Deviation %
1.08 1.41 1.7464
2.7 2.5 3.4 0.8 0.8 1.0 0.4 0.8 0.5 0.4 0.04 0.02 0. 8 0.3 0.8 0.2 0.4 0.2 0.0 1.1 0.4 0.8 0.2 0.8 0.1 0.4 0.08
2.5 3.7 4.2 4.4103 2.4765 4.0382 4.4 5 5.3 5.3173 2.8898 4.1470 4.5 4.8 5.1 5.1627
Data is read from the figures. Data is from the original literature.
thermodynamic equilibrium yields of H2, CO, CO2 and CH4 (moles per mole ethanol) are showed in Fig. 2(aed). From Fig. 2(a), we can see more than 5.1 mol of H2 close to 90% of the stoichiometry coefficient 6 can be obtained at about 1000 K with S/E > 8. Low temperature and S/E ratio deceases H2 yield due to the reverse of water-gas shift reaction (WGSR, Eq. (6)), which is not as the same as that of ethanol POX. Fig. 2(b) shows that as many as 1.8 mol of CO yield appears at 1150 K with S/E ¼ 1e2, and decreases sharply with the decreasing of temperature while there’s no so significant influence by S/E ratio. That CO2 yield maximized at about 900 K with S/E > 8, with the similar trend to H2 yield can be seen from Fig. 2(c). From Fig. 2(d), the relatively high yield of CH4 (w1.1 mol) is obtained at very low temperature which is opposite to the three productions above and when temperature increases, the yield of CH4 decreases owing to the reverse of Eq. (8). These trends are in accordance with that obtained by others’ simulation works [12,18,30,31,36].
915
maximum yield of H2 can be obtained when O/E ¼ 0.25 and decreases with the increase of O/E molar ratio. From Fig. 3(a), we can see that the H2 yield is of the same trend as that in ethanol SR yet less H2 is yielded (w4.8 mol) in the regions where 5.1 mol are generated in SR owning to the addition of oxygen. Fig. 3(b) shows the maximum yield of CO appears at very high temperature and low S/E ratio. However, the yield of CO increases with the increase of temperature while decreases with the S/E and O/E which is attributed to the Eq. (9) and reverse of WGSR (the reverse of Eq. (6)). In Fig. 3(c), the yield of CO2 reaches 1.6 mol at 900 K and S/E ¼ 10 and shows the same trend as H2 yield with temperature and S/E ratio but the opposite with O/E ratio. From Fig. 3(d), we can see that the yield of CH4 is the lowest (<0.8 mol) in the three technologies and increases with the decrease of temperature and the S/E molar ratio at first then decreases when S/E ratio increases furthermore.
3.2. Fitting the simulation results In this part, we fit the simulation results into equations by which one can get the simulation results more convenient, easier and faster. To improve the correlations as much as possible of fitting the yield equation with the S/E, O/E molar ratio and temperature, we make many attempts and find that the polynomial with appropriate deformation of S/E, O/E, temperature and the yield of production is the most suitable. However, the physical and chemical mechanism is not so clear.
3.2.1. Ethanol POX As we know only the temperature and O/E ratio affect the equilibrium production distribution. Here we assume that (O/ Eþ8) ¼ X1, ln(T) ¼ X2 and YH2 , YCO , YCO2 and YCH4 refer to the moles of H2, CO, CO2 and CH4. And X1 is ranged from 8 to 11 while X2 from 6 to 8. The yields of H2, CO, CO2 and CH4 from ethanol POX are fitted in the following polynomials.
ln YH2 þ 1 ¼ 1:65 105 þ 74:05X1 1:61X2 1:95X12 þ 583:13X22 0:64X1 X2 þ 5:66 103 X11 7:59 105 X21 1:28 104 X12 þ 1:30 106 X22
(27)
with r ¼ 0.9911.
lnðYCO þ 0:021Þ ¼ 5:43 105 þ 722:03X1 5:35 104 X2 3.1.3. Ethanol auto-thermal reforming The process of ethanol ATR is the combination of ethanol SR and POX which not only attains thermally sustained operation, but also maximizes the hydrogen production [38]. The H2 yield (moles per mole ethanol) of ethanol ATR obtained with the present method are also compared with that from other researchers, like Rabenstein [30] and Gracia (GNU Octave numerical language) [33]. The comparisons are listed in Table 3. Table 3 reveals a close agreement between the results obtained with different methods with the relative coverage deviations no greater than 5%. The H2, CO, CO2 and CH4 yields (moles per mole ethanol) of ethanol ATR obtained in the present work are described in Fig. 3(aed). The thermodynamic equilibrium compositions of ethanol ATR are calculated at a temperature range from 700K to 1200 K with S/E ¼ 1, 4, 7, 10 and O/E ¼ 0.25, 0.50 and 0.75. Under the given conditions, the
19:33X12 þ 1:94: 103 X22 0:85X1 X2 þ 6:17 105 X11 2:54 106 X21 1:43 105 X12 þ 4:36 106 X22
(28)
with r ¼ 0.9879.
ln YCO2 þ 0:9 ¼ 6:25 104 53:77X1 þ 5:94 104 X2 þ 1:27X12 208:84X22 þ 0:072X1 X2 5:68 103 X11 þ 3:00 105 X21 þ 1:43 104 X12 5:31 105 X22 with r ¼ 0.9911.
(29)
916
S. Sun et al. / Energy 44 (2012) 911e924
a
1200
b
1100
1200
1.8 1.7
1100
1.6 1.2
1000
900
Temperature,K
Temperature, K
5.1
3.6
1000
0.70 0.90
900
1.8 2.4
800
0.20
3.0 700
0.050
1.2 0
2
4
6
8
700
10
Steam-to-ethanol molar ratio
1200
0.10
d
1100
Temperature,K
1.2 1000
1.3 3
0.30 0.90
0.50
900
0.10
0
2
4
6
Steam-to-ethanol molar ratio
8
10
1200
1100
0.70
Temperature,K
c
0.50
4.8
4.2 800
1.4
1000
0.010 0.30
0.10
900
0.050
0.50 800
800
0.70 700
0
2
4
6
8
10
700
1.1 0
Steam-to-ethanol molar ratio
2
4
0.90 6
Steam-to-ethanol molar ratio
8
10
Fig. 2. (aed). Effects of temperature and S/E molar ratio on H2, CO, CO2 and CH4 yields.
ln YCH4 þ 5:3 ¼ 3:04 104 þ 2:93 103 X1 3:46X2
lnðYCO þ 0:03Þ ¼ 1:75 106 þ 1:68X1 1:70 105 X2 1:39
105:51X12 þ 0:074X22 þ 0:077X1 X2 þ 1:41
103 X12 þ 6:21 103 X22 0:24X1 X2
105 X11 272:68X21 þ 2:44 105 X12
þ 18:11X11 7:98X21 51:06X12 þ 1:36
þ
651:08X22
107 X22
(30) with r ¼ 0.9952.
with r ¼ 0.9874. 3.2.2. Ethanol SR It is assumed that (S/Eþ3.5) ¼ X1, ln(T) ¼ X2 and YH2 , YCO , YCO2 and YCH4 refer to the yield of H2, CO, CO2 and CH4. And X1 is ranged from 3.5 to 13.5 while X2 from 6 to 8. The equations of the yields of hydrogen, carbon monoxide, carbon dioxide and methane from ethanol SR are fitted in the following polynomials.
ln YCO2 þ 0:45 ¼ 1:05 105 1:38X1 1:47 103 X2 þ 0:023X12 þ 69:55X22 þ 0:087X1 X2 52:90X11 2:64 104 X21 þ 7:76 103 X12 þ 80:05X22 with r ¼ 0.9911.
103 X12 993:22X22 þ 0:013X1 X2 1:01X11
103 X12 þ 50:887X22 0:215X1 X2 4:0244X11 with r ¼ 0.9949.
(33)
ln YCH4 þ 2 ¼ 2:80 105 0:15X1 þ 2:72 104 X2 þ 2:01
ln YH2 ¼ 7742:4969 þ 1:5048X1 1086:32X2 1:33 18406:629X21
(32)
þ 1:28 106 X21 þ 0:071X12 2:19 106 X22
(31) with r ¼ 0.9772.
(34)
S. Sun et al. / Energy 44 (2012) 911e924
YCH4 ¼ 5:40 0:39X1 0:070X2 9:13 103 X3 þ 0:089X12
Table 3 Comparisons of H2 yield for ethanol ATR. T (K)
O/E
S/E
Present simulation
Rabensteina [30]
Graciaa [33]
Deviation%
800 800 800 800 800 800 800 800 800 800 800 800 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200
0.25 0.25 0.25 0.25 0.50 0.50 0.50 0.50 0.75 0.75 0.75 0.75 0.25 0.25 0.25 0.25 0.50 0.50 0.50 0.50 0.75 0.75 0.75 0.75 0.25 0.25 0.25 0.25 0.50 0.50 0.50 0.50 0.75 0.75 0.75 0.75
1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10
1.5407 2.8500 3.6375 4.3196 1.4296 2.8393 3.2013 3.9761 1.3382 2.5699 3.3133 3.6453 3.3525 4.5576 4.7131 4.8478 3.0520 4.0000 4.2385 4.4422 2.9951 3.6080 3.9649 4.0663 3.4690 4.0060 4.3327 4.5606 3.0474 3.7797 3.9517 4.1880 2.8861 3.1935 3.6394 3.8260
1.5 2.8 3.6 4.3 1.4 2.8 3.2 4 1.3 2.5 3.3 3.6 3.3 4.6 4.8 4.9 3 4 4.2 4.4 3.0 3.6 4.0 4.0 3.5 4.0 4.3 4.5 3.0 3.7 3.9 4.2 2.8 3.2 3.6 3.8
1.5 2.9 3.6 4.3 1.4 2.8 3.2 3.9 1.2 2.5 3.3 3.6 3.3 4.5 4.7 4.8 3 4 4.2 4.4 2.9 3.6 3.9 4.0 3.5 4.0 4.3 4.6 3.0 3.7 3.9 4.2 2.8 3.2 3.6 3.8
1.2 1.1 0.4 0.2 0.9 0.6 0.01 0.9 4.1 1.2 0.1 0.5 0.7 0.7 0.8 0.7 0.7 0.0 0.4 0.4 1.4 0.09 0.9 0.7 0.4 0.06 0.3 0.7 0.7 0.9 0.5 0.1 1.3 0.09 0.4 0.3
a
0:018X22 1:56 105 X32 0:065X1 X2 1:42 (35)
with r ¼ 0.9619.
YCO ¼ 3:95 þ 0:99X1 0:14X2 þ 6:34 103 X3 0:037X12 þ 4:24 103 X22 9:32 107 X32 þ 7:46 103 X1 X2 (36)
with r ¼ 0.9599.
YCO2 ¼ 1:65 þ 0:41X1 þ 0:16X2 þ 5:09 103 X3 þ 0:072X12 0:011X22 3:46 106 X32 0:052X1 X2 þ 2:82 104 X1 X3 þ 9:65 105 X2 X3
(38)
The thermodynamic equilibrium products have been analyzed for the three different technologies, ethanol POX, SR and ATR under various conditions. In order to analyze the energy requirements and thermal efficiencies for the three technologies, an approximate energy balance is made as shown in Fig. 5. Here, it is assumed that the feed in and products out are under the standard state (298.15 K, 1atm) and there’s no heat loss in the heating and cooling process. And the energy balance for the reformer and the whole system is discussed in the following part.
YH2 ¼ 16:05 þ 0:60X1 þ 0:59X2 þ 0:035X3 0:070X12
1:32 103 X1 X3 2:71 104 X2 X3
104 X1 X3 þ 6:05 105 X2 X3
3.3. The thermal efficiency analysis
3.2.3. Ethanol ATR As there are three variables in the process of ethanol ATR, S/E, O/ E molar ratio and temperature, the fitted models are more complicated than that of ethanol SR and POX. Here O/E ¼ X1, S/ E ¼ X2, T ¼ X3 and YH2 , YCO , YCO2 and YCH4 refer to the yield of H2, CO, CO2 and CH4. And X1 is ranged from 0.25 to 0.75 while X2 from 1 to 10 and X3 from 700 to 1200.The fitted equations are listed as follows.
103 X1 X3 1:99 104 X2 X3
þ 3:90 104 X22 þ 3:85 106 X32 0:015X1 X2 þ 3:38
with r ¼ 0.9611. For further check of the fitted models, comparisons among the fitted models, the present work and other simulation works are made in Fig. 4(aec) to see how far are the fitted models deviate from the simulation works. From Fig. 4(aec), we find that the simulation results obtained with different methods agree well with the fitted models, with very small deviations. It should be noted that one or two points deviate far with the relative error greater than 10%. Along with the comparisons above, we can conclude that the fitted models can be applied well in ethanol POX, SR and ATR under the conditions investigated. With the fitted models, one can calculate the thermodynamic equilibrium compositions easily without doing repeated simulation work.
Data is read from the figures.
with r ¼ 0.9567.
917
(37)
3.3.1. Reforming energy balance Fig. 5 plots the energy flows across the boundaries of a control volume, which in this case is the reformer itself (solid box). The fuel and water enter the reformer and react (completely) to form hydrogen and carbon dioxide. In the reforming process, P P T ¼ ðnp Df HpT Þ ðnr Df HrT Þ, where the reforming heat DHreforming Df HT refers to the formation enthalpy at reaction temperature (T) and subscribes p and r refer to the products and reactants, respectively. Here we bring the dimensionless volume T DHreforming =HVethanol (HVethanol is the combustion heat of ethanol of 1366.8 kJ/mol) as a measurement to analyze the energy T =HVethanol > 0, the balance for the reformer. When DHreforming T =HVethanol < 0, then reforming is endothermic while DHreforming the process is exothermic and T-N condition reaches when T T DHreforming =HVethanol ¼ 0. The volume DHreforming =HVethanol for the reforming process of ethanol POX, SR and ATR is showed in Fig. 6(aed). Fig. 6(aed) confirms the truth once again that ethanol POX is highly exothermic while SR or ATR is preferred as the process is slightly endothermic or exothermic. Fig. 6(a) shows that for 1 mol ethanol reacted, the energy ethanol POX generates is the greatest in which almost all the ethanol is burned to generate heat when 3 mol of oxygen is added in the system. And with the decrease of O/E molar ratio, the ratio of ethanol combustion decreases followed by less heat generated. From Fig. 6(b), a small amount of energy (about 12e18% of ethanol heating value) is demanded for ethanol SR and when temperature increases, so does the energy demanded. From Fig. 6(c) and (d), it is apparent that the reforming heat of ethanol ATR is very low (less than 10% of ethanol heating value) that could be negligible and seen as Thermal-Neutral.
918
a
S. Sun et al. / Energy 44 (2012) 911e924
b
1200
1200
1.7
4.6 1100
1100
1.1
4.4 4.8
1000
4.0 3.5
900
0.70
1.3
Temperature,K
Temperature,K
0.90
1.5
1000
0.50 900
0.30 800
800
2.5 700
0.10
2.0 2
4
6
8
700
10
2
4
Steam-to-ethanol molar ratio
c
d
1200
0.40
0.80
1.6
900
Temperature,K
Temperature,K
1.2 1.4
0.050
1000
700
8
10
0.030 0.10
900
0.20 0.40
800
10
1200
1.0 0.60
8
1100
1100
1000
6
Steam-to-ethanol molar ratio
0.30
800
0.60
0.70 700
2
4
6
8
10
0.80 0.80 2
0.50
0.60
4
6
Steam-to-ethanol molar ratio
Steam-to-ethanol molar ratio
Fig. 3. (aed). Effects of temperature and S/E molar ratio on H2, CO, CO2 and CH4 yields from ethanol ATR with O/E ¼ 0.25.
3.3.2. Thermal neutral point analysis for ethanol ATR process T for ethanol ATR can be The fitted equation of DHreforming expressed as follows. T DHreforming ¼ 1:2 103 275:6X1 þ 40:0X2 þ 2:3X3 4:6X12
0:5X22 2:9X1 X2 0:2X1 X3
(39)
where R ¼ 0.9885, X1 ¼ O/E, X2 ¼ S/E and X3 ¼ Temperature. The T ¼ 0Þ are calculated from the above T-N points ðDHreforming equation with X1 ¼ 0.2, 0.3, 0.4 and 0.5, which are showed in Fig. 7. It is noted that there are some differences between the T-N curves calculated by Eq. (39) and the simulated works. Comparing the T-N conditions in the process of ethanol SR (O/E ¼ 0) and ATR (O/E ¼ 0.25, 0.50) in Fig. 6(b), (c) and (d), we can see that under the T-N conditions, for a fixed O/E ratio, the equilibrium temperature increases with the decrease of S/E ratio; and for a fixed S/E ratio, the equilibrium temperature increases with the
increase of O/E molar ratio. From Fig. 7, we can also get that under a certain S/E ratio, when O/E ratio increases, then the process under the former T-N conditions is exothermic and the T-N point moves to higher temperatures. Because higher temperatures favor the endothermic reaction (SR) and suppress the exothermic reaction (POX), thus make the condition reach T-N again. Also, when O/E ratio is fixed, with the S/E ratio increases, the T-N point moves to lower temperatures as lower temperatures favor the exothermic reaction and suppress endothermic reaction to keep heat balance. These trends are as the same as Li’s [29]. 3.3.3. Energy balance for the whole system The energy balance for the whole system includes the energy for the heating, reforming and cooling processes in the dash box in Fig. 5.
Qin ¼ Qheating þ Qreforming þ Qcooling
(40)
S. Sun et al. / Energy 44 (2012) 911e924
a
b
3.0
ethanol POX,T=1100K
H2 yield
H2 yield
4.5
1.5
0.0
4.0 3.5
1.0
0.0
ethanol SR,T=1000K
5.0
2.0
0.5
6.0 5.5
2.5
919
0.5
the fitted model the present work Kafarov etc Wang etc Vasudeva etc
3.0
the fitting model the present work Wang etc Rabenstein etc
2.5
1.0
1.5
2.0
2.5
3.0
2.0
0
2
Oxygen-to-ethanol molar ratio
c
4
6
8
10
Steam-to-ethanol molar ratio
5.5 5.0
ethanol ATR O/E=0.25
4.5
S/E=10
4.0
H2 yield
3.5 3.0
S/E=1
2.5 2.0
the fitted model the present simulation Rabenstein Gracia
1.5 1.0 700
800
900
1000
1100
1200
Temperature,K Fig. 4. (aec). Comparison between the simulation results and the fitted models of ethanol POX, SR and ATR under various conditions.
As it is assumed above, the feed in and products out are under the standard state and there’s not any heat loss in every process. From the dash box, the system energy can be expressed by the following equation.
Qin ¼ DHR0 ¼
X
np Dc Hp0
X
nr Dc Hr0
Fig. 5. Scheme of the thermal exchange of ethanol SR, POX and ATR.
(41)
where Qin refers to the system energy, DHR0 reaction enthalpy at 298.15 K and DcH is the combustion heat while subscript r refers to the reactants compounds and p the products composition. For convenient discuss, the system energy is expressed with a dimensionless volume Qin =HVethanol for these three technologies which is showed in Fig. 8(aed). When Qin =HVethanol > 0, the whole system is endothermic and when Qin =HVethanol < 0, the whole system is exothermic. From Fig. 8(aed) and Fig. 6(aed), we can see that the total energy exchange including heating, reforming and cooling in the processes of ethanol POX is almost equal to that of the reforming process. In other words, energy necessary for heating and cooling is almost equal, which means that the cooling energy almost makes up the energy for heating in the process of ethanol POX. However, the total energy demands is a little greater than reforming energy for ethanol SR which indirect that the energy needed for heating to reaction temperature is greater than that generated in cooling process. Also from Fig. 8(ced) and Fig. 6(ced), we can see that
920
a
S. Sun et al. / Energy 44 (2012) 911e924
1400
POX
b
1200
SR 0.18
0.19
1300
1100
0.050 0.0
1100
1000
-0.20 -0.10
900
-0.70
-0.40
0 -0.80 -0.90
Temperature,K
Temperature,K
1200
0.14
1000
0.16 900
-0.30
0.10 -0.50
0.050
0.0
800
-0.60
800
700 0.0
0.5
1.0
1.5
2.0
2.5
700
3.0
0
2
4
1200
ATR,O/E=0.25
d
0.10 1100
1200
Temperature,K
Temperature,K
10
0.0125 1100
0.090
1000
0.070 0.055 900
0.020
2
0.00 1000
-0.0125 -0.0250 900
-0.0500
0.040
0.0
0.060
-0.0750
800
-0.040 700
8
ATR,O/E=0.50
0.080
800
6
Steam-to-ethanol molar ratio
Oxygen-to-ethanol molar ratio
c
0.12
4
-0.020 6
-0.100
-0.125 8
10
700
Steam-to-ethanol molar ratio
2
4
6
8
10
Steam-to-ethanol molar ratio
Fig. 6. (aed). The energy exchange for the reforming process of ethanol POX, SR and ATR (plotted by the simulation results).
under the same conditions, when it is exothermic in the reforming process then it becomes endothermic in the whole system. It is the reason that the overall processes of heating and cooling are endothermic and the energy is greater than that generated by the reforming process. 3.3.4. Thermal efficiency for the whole system In general, the thermal efficiency is a measure of energy output divided by energy input. Note that a low heating value (where water is generating as vapor) is used throughout this paper. There are two commonly used definition of thermal efficiency. The first is defined as the heating value of H2 divided by the heat consumption of fuel to run the process ðh ¼ nH2 HVH2 =HVF þ Qin Þ. The second definition subtracts the heat input from the numerator accounts for the extra cost of using some of the product hydrogen to heat the process ðh0 ¼ nH2 HVH2 Qin =HVF Þ [41]. Here the first definition is used and the low heating value can be expressed as
HVi ¼ Dc Hi
(42)
where DcHi is the combustion heat for species i. Submitting Eqs. (41) and (42) into the definition, the thermal efficiency h can be written as follows.
h¼
nH2 HVH2 nH2 HVH2 þ nCO HVCO þ nCH4 HVCH4 þ nC HVC
(43)
The equation is based on the assumption that there’s no heat loss and the energy exchange efficiencies of the heating and cooling processes equal to 1.0. This assume reflects heat exchange efficiency, which provides an upper limit (which is equal to 1.0) to the efficiency of the process. The thermal efficiencies for the whole system of ethanol POX, SR and ATR can be obtained from Eq. (43) and are showed in Fig. 9(aed).
S. Sun et al. / Energy 44 (2012) 911e924 1200
As it can be seen above, all the thermal efficiencies increase with the increase of temperature. And in the process of ethanol POX, with oxygen added into the system, carbon dioxide producing is easier than carbon monoxide and more water is formed than methane, but the amount of H2 yield is very small, thus the thermal efficiencies are very low. And with the O/E ratio increasing, thermal efficiency increases at first owning to less CO and coke and more CO2, but decreases when O/E ratio increases further for the combustion of more ethanol. In the process of ethanol SR, with the S/E ratio increasing, H2 yield increases and other combustible gases as well as coke yield decreases, which makes the thermal efficiency increase too. In the process of ethanol ATR, the thermal efficiencies increase with the increase of S/E ratio and make little differences with the O/E ratio which we can obtain from the above Figures that with the O/E increases, the H2 yield decreases as well as CO, CH4 and coke.
0.0
1100
Temperature,K
O/E=0.5 1000
0.0 900
O/E=0.4 0.0
O/E=0.3 800
0.0 O/E=0.2 700
2
4
921
6
8
10
Steam-to-ethanol molar ratio Fig. 7. The T-N conditions for the reforming process of ethanol ATR (calculated by the fitted equation).
a
b
1400
1200
1300
1100
Temperature,K
1100
0.0
1000
-0.20 -0.10
900
-0.50
-0.80
-0.95
Temperature,K
0.050
1200
1000
0.25 0.23
0.20
900
-0.30 -0.40
-0.60
0.050
800
0.15
-0.70
800
0.0 700 0.0
0.5
1.0
1.5
2.0
2.5
Oxygen-to-ethanol molar ratio
700
3.0
O/E=0.25
d
1200
0.14
Temperature,K
1100
1000
0.13 900
.050 .
0.11
0.080 800
2
4
6
2
10
O/E=0.50 1200
0.040
1000
0.020
0.030
900
800
0.0
-0.10 -0.13 -0.050
-0.080 700
8
Steam-to-ethanol molar ratio
-0.025
0.0
0 0.050
0.10
1100
Temperature,K
c
0
0.17
4
6
Steam-to-ethanol molar ratio
8
10
700
2
4
6
Steam-to-ethanol molar ratio
Fig. 8. (aed). System energy of ethanol POX, SR and ATR (with O/E ¼ 0.25, 0.50).
8
10
922
a
S. Sun et al. / Energy 44 (2012) 911e924
1400
POX
b
SR,O/E=0.0
1200
0.80
1300 1100
0.40
0.85 0.60
0.30
1100
Temperature,K
Temperature,K
1200
0.70 0.20
1000
0.88
1000
0.60
900
900
800
.20 0.5
1.0
1.5
0.40 0.50
2.0
2.5
700
3.0
0
2
1200
O/E=0.25
d
1200
8
10
0.75
0.80
1100
1100
0.80
0.87
0.87
Temperature,K
0.70
Temperature,K
6
O/E=0.50 0.70
0.89
1000
0.85 900
0.60
800
2
0.88 900
800
4
0.85
1000
0.40
0.65
0.50 700
4
Steam-to-ethanol molar ratio
Oxygen-to-ethanol molar ratio
c
0.70
0.50
0.40
700 0.0
0.30
800
0.30
6
0.60 0.50
8
10
700
2
4
6
8
10
Steam-to-ethanol molar ratio
Steam-to-ethanol molar ratio Fig. 9. (aed). The thermal efficiency h for POX, SR and ATR.
3.4. The optimum reaction conditions The selection of operation conditions of a reformer is depending on various targets. Main target is a high hydrogen yield, simultaneously with low carbon monoxide content. Maximum hydrogen efficiency and low carbon monoxide as well as the coke contents which should be avoided as it can make the catalyst deactivate when coating on the surface of the catalyst are possible for ethanol reforming operation. The coke formation of ethanol POX, SR and ATR is showed in Fig. 10(aec) from which we can see that ethanol POX has the largest coke region and coke is always formed for hydrogen production; in the process of ethanol SR, with excess water (S/E > 3), coke can be avoided; when S/E > 1 and O/E ¼ 0.25, coke is neglected at above 1000 K in ethanol ATR. From Fig. 10(aec), we can also see that in the process of ethanol POX, H2 yield is maximized accompanied by coke and CO formation at high temperatures (1200e1400 K) and low O/E ratios
(0.4e0.8). Many experiments [42e47] are carried out at much lower temperatures with appropriate catalysts for low H2 yield like 0.8 mol per mole ethanol obtained by Noronha [48] at 673 K with O/E ¼ 0.4. With lowest theoretical H2 yield and highest coke formation, ethanol POX is not a good way for H2 production. As for ethanol SR, H2 yield increases with the temperature and S/E ratio. Under the optimum conditions in the shadow box where temperature is above 900 K with S/E > 6, the hydrogen yield maximizes, coke and other combustible gases minimizes with high thermal efficiency and a small amount energy needed and many experiments are carried out in the region [49e52]. Combining with ethanol SR and POX, ethanol ATR is a process that attains thermally sustained and maximizes hydrogen production, which is the most preferred in the three technologies. The maximum H2 yield is obtained above 900 K with S/E > 6 and O/ E ¼ 0.25. There are many experiment studies of ethanol reforming to generate hydrogen in the optimum shadow region with different O/E ratios [53e56].
S. Sun et al. / Energy 44 (2012) 911e924
a
b 1200
1400
1300
CO yield H2 yield
1100
1000
2.9
CO yield 1.8
43
900
800
700 0.0
coke free regions 2 1
CH4 yield 0.70 0.5
1.6
1100
1000
H2 yield 1 2 0.0
900
3 4
coke formed regions
coke formed regions
1.0
5.2
Temperature,K
Temperature/K
1200
923
1 Schmidt 2 Schmal 3 Shen 4 Noronha
1.5
2.0
2.5
coke free regions
800
1.0
3.0
700
0
2
Oxygen-to-ethanol molar ratio
4
1 Abello 2 Freni 3 Kunzru 4 Onsan
CH4 yield 6
8
10
Steam-to-ethanol molar ratio
c 1200 O/E=0.25 1.6
Temperature/K
1100
CO yield
4
1000
1
H2 yield 4.6
2
900
3 800
0.70 700
2
1 Gutierrez 2 Kunzru 3 Song 4 Ferrio
CH4 yield
4
6
8
10
Steam-to-ethanol molar ratio
Fig. 10. (aec). Optimum reaction conditions for ethanol POX, SR and ATR.
4. Conclusions This work presents the simulated equilibrium compositions and the thermal efficiencies of ethanol SR, POX and ATR at wide range of temperature, S/E molar ratio and O/E molar ratio. Besides, the simulation results are fitted with polynomials and compared with other researchers’ simulations. Moreover, the energy exchanges and thermal efficiencies of the three technologies are calculated and analyzed. Finally, the optimum reaction conditions are selected. The main conclusions are as follows. < The thermal equilibrium compositions obtained with the present methodology agree well with that obtained with many other different methods and the deviations can be ignored. And the fitted models interpret the production distributions well with highly accuracy < Hydrogen yield and energy exchanges for the three technologies are compared, and the results show that with 1 mol ethanol reacted, the moles of H2 yield is of this order: SR > ATR > POX. What’s more, the process of ethanol SR is endothermic and POX is exothermic while ATR is thermal neutral. < The thermal efficiencies increase with the increasing of temperature and S/E ratio, yet increase with the O/E ratio first in ethanol POX and then decrease but make little difference in ethanol ATR. < With the target to get high hydrogen yield simultaneously with low CO and coke content and also low energy demands, the optimum reaction conditions can be summarized as
T > 900 K and S/E > 6 for ethanol SR and T > 900 K, S/E > 6 and O/E ¼ 0.25 for ATR, respectively. References [1] Tang H, Kitagawa K. Supercritical water gasification of biomass: thermodynamic analysis with direct Gibbs free energy minimization. Chem Eng J 2005; 106:261e7. [2] Barbir F. Transition to renewable energy systems with hydrogen as an energy carrier. Energy 2009;34:308e12. [3] Seiler JM, Hohwiller C, Imbach J, Luciani JF. Technical and economical evaluation of enhanced biomass to liquid fuel processes. Energy 2010;35:3587e92. [4] Lin SSY, Kim DH, Ha SY. Hydrogen production from ethanol steam reforming over supported cobalt catalysts. Catal Lett 2008;122:295e301. [5] Vagia EC, Lemonidou AA. Thermodynamic analysis of hydrogen production via steam reforming of selected components of aqueous bio-oil fraction. Int J Hydrogen Energy 2007;32:212e23. [6] Vagia EC, Lemonidou AA. Thermodynamic analysis of hydrogen production via autothermal steam reforming of selected components of aqueous bio-oil fraction. Int J Hydrogen Energy 2008;33:2489e500. [7] Wang X, Li M, Li S, Wang H, Wang S, Ma X. Hydrogen production by glycerol steam reforming with/without calcium oxide sorbent: a comparative study of thermodynamic and experimental work. Fuel Process Technol; 2010. [8] Zhang B, Tang X, Li Y, Xu Y, Shen W. Hydrogen production from steam reforming of ethanol and glycerol over ceria-supported metal catalysts. Int J Hydrogen Energy 2007;32:2367e73. [9] Chen H, Zhang T, Dou B, Dupont V, Williams P, Ghadiri M, et al. Thermodynamic analyses of adsorption-enhanced steam reforming of glycerol for hydrogen production. Int J Hydrogen Energy 2009;34:7208e22. [10] Chen H, Ding Y, Cong NT, Dou B, Dupont V, Ghadiri M, et al. A comparative study on hydrogen production from steam-glycerol reforming: thermodynamics and experimental. Renew Energy; 2010. [11] Wang N, Perret N, Foster A. Sustainable hydrogen production for fuel cells by steam reforming of ethylene glycol: a consideration of reaction thermodynamics. Int J Hydrogen Energy; 2011.
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[12] Kikuchi R, Faungnawakij K, Eguchi K. Thermodynamic evaluation of methanol steam reforming for hydrogen production. J Power Sources 2006;161:87e94. [13] Wang SD, Wang S. Thermodynamic equilibrium composition analysis of methanol autothermal reforming for proton exchanger membrane fuel cell based on FLUENT Software. J Power Sources 2008;185:451e8. [14] Dumesic JA, Davda RR, Shabaker JW, Huber GW, Cortright RD. Aqueous-phase reforming of ethylene glycol on silica-supported metal catalysts. Appl Catal BEnviron 2003;43:13e26. [15] Dumesic JA, Huber GW, Shabaker JW, Evans ST. Aqueous-phase reforming of ethylene glycol over supported Pt and Pd bimetallic catalysts. Appl Catal BEnviron 2006;62:226e35. [16] Dumesic JA, Shabaker JW, Huber GW, Davda RR, Cortright RD. Aqueous-phase reforming of ethylene glycol over supported platinum catalysts. Catal Lett 2003;88:1e8. [17] Abello MC, Barroso MN, Galetti AE. Ni catalysts supported over MgAl(2)O(4) modified with Pr for hydrogen production from ethanol steam reforming. Appl Catal A-Gen 2011;394:124e31. [18] Basile A, Gallucci F, Tosti S, Lulianelli A, Drioli E. Methanol and ethanol steam reforming in membrane reactors: an experimental study. Int J Hydrogen Energy 2007;32:1201e10. [19] Fishtik I, Alexander A, Datta R, Geana D. A thermodynamic analysis of hydrogen production by steam reforming of ethanol via response reactions. Int J Hydrogen Energy 2000;25:31e45. [20] Homs N, Pereira EB, de la Piscina PR. Efficient hydrogen production from ethanol and glycerol by vapour-phase reforming processes with new cobaltbased catalysts. Bioresour Technol 2011;102:3419e23. [21] Li MH. Thermodynamic analysis of adsorption enhanced reforming of ethanol. Int J Hydrogen Energy 2009;34:9362e72. [22] Hori C, de Assis A. Thermodynamic assessment of hydrogen production and cobalt oxidation susceptibility under ethanol reforming conditions. Energy; 2011. [23] Araque M, Vargas JC, Zimmermann Y, Roger AC. Study of a CeZrCoRh mixed oxide for hydrogen production by ethanol steam reforming. Int J Hydrogen Energy 2011;36:1491e502. [24] da Silva AL, Muller IL. Hydrogen production by sorption enhanced steam reforming of oxygenated hydrocarbons (ethanol, glycerol, n-butanol and methanol): thermodynamic modelling. Int J Hydrogen Energy 2011;36: 2057e75. [25] Noronha FB, da Silva ALM, Mattos LV, den Breejen JP, Bitter JH, de Jong KP. Oxidative steam reforming of ethanol over carbon nanofiber supported Co catalysts. Catal Today 2011;164:262e7. [26] Wang YQ, Wang WJ, Liu YA. Production of hydrogen by ethanol steam reforming over nickel-metal oxide catalysts prepared via urea-nitrate combustion method. Int J Energy Res 2011;35:501e6. [27] Deluga G, Salge J, Schmidt L, Verykios X. Renewable hydrogen from ethanol by autothermal reforming. Science 2004;303:993. [28] Holladay JD, Hu J, King DL, Wang Y. An overview of hydrogen production technologies. Catal Today 2009;139:244e60. [29] Li YD, Liu S, Zhang K, Fang LN. Thermodynamic analysis of hydrogen production from oxidative steam reforming of ethanol. Energy Fuel 2008;22: 1365e70. [30] Adhikari S, Fernando SD, Haryanto A. Hydrogen production from glycerol: an update. Energy Convers Manage 2009;50:2600e4. [31] Tanksale A, Beltramini JN, Lu GQM. A review of catalytic hydrogen production processes from biomass. Renewable Sustainable Energy Rev 2010;14:166e82. [32] Rabenstein G, Hacker V. Hydrogen for fuel cells from ethanol by steamreforming, partial-oxidation and combined auto-thermal reforming: a thermodynamic analysis. J Power Sources 2008;185:1293e304. [33] Gutierrez A, Karinen R, Airaksinen S, Kaila R, Krause AOI. Autothermal reforming of ethanol on noble metal catalysts. Int J Hydrogen Energy 2011;36: 8967e77.
[34] Kafarov V, Hernandez L. Thermodynamic evaluation of hydrogen production for fuel cells by using bio-ethanol steam reforming: effect of carrier gas addition. J Power Sources 2009;192:195e9. [35] Gracia F, Alvarado FD. Steam reforming of ethanol for hydrogen production: thermodynamic analysis including different carbon deposits representation. Chem Eng J 2010;165:649e57. [36] Ma P. Test properties data handbook of organic compounds. Chemical Industry Press; 2006. [37] Zhang H, Cao F, Liu D, Fang D. Thermodynamic analysis for synthesis of dimethyl ether and methanol from synthesis gas. J East China University Sci Technol 2001;27:4. [38] Wang W, Wang Y. Thermodynamic analysis of hydrogen production via partial oxidation of ethanol. Int J Hydrogen Energy 2008;33:5035e44. [39] Wang W, Wang YQ. Thermodynamic analysis of steam reforming of ethanol for hydrogen generation. Int J Energy Res 2008;32:1432e43. [40] Vasudeva K, Mitra N, Umasankar P, Dhingra S. Steam reforming of ethanol for hydrogen production: thermodynamic analysis. Int J Hydrogen Energy 1996; 21:13e8. [41] Lutz AE, Bradshaw RW, Keller JO, Witmer DE. Thermodynamic analysis of hydrogen production by steam reforming. Int J Hydrogen Energy 2003;28: 159e67. [42] Rotzoll G. High-temperature pyrolysis of ethanol. J Anal Appl Pyrol 1985;9: 43e52. [43] Shen WJ, Cai WJ, Wang FG, Zhan ES, Van Veen AC, Mirodatos C. Hydrogen production from ethanol over Ir/CeO2 catalysts: a comparative study of steam reforming, partial oxidation and oxidative steam reforming. J Catal 2008;257: 96e107. [44] Schmal M, Guarido CEM, Cesar DV, Souza MMVM. Ethanol reforming and partial oxidation with Cu/Nb(2)O(5) catalyst. Catal Today 2009;142: 252e7. [45] Mattos LV, Noronha FB. Partial oxidation of ethanol on supported Pt catalysts. J Power Sources 2005;145:10e5. [46] Liguras DK, Goundani K, Verykios XE. Production of hydrogen for fuel cells by catalytic partial oxidation of ethanol over structured Ni catalysts. J Power Sources 2004;130:30e7. [47] Salge J, Deluga G, Schmidt L. Catalytic partial oxidation of ethanol over noble metal catalysts. J Catal 2005;235:69e78. [48] Mattos L, Noronha F. Hydrogen production for fuel cell applications by ethanol partial oxidation on Pt/CeO2 catalysts: the effect of the reaction conditions and reaction mechanism. J Catal 2005;233:453e63. [49] Abello MC, Galetti AE, Gomez MF, Arrua LA. Ni catalysts supported on modified ZnAl(2)O(4) for ethanol steam reforming. Appl Catal A-Gen 2010; 380:40e7. [50] Bueno JMC, Liberatori JWC, Ribeiro RU, Zanchet D, Noronha FB. Steam reforming of ethanol on supported nickel catalysts. Appl Catal A-Gen 2007; 327:197e204. [51] Calles JA, Vizcaino AJ, Carriero A. Hydrogen production by ethanol steam reforming over Cu-Ni supported catalysts. Int J Hydrogen Energy 2007;32: 1450e61. [52] Onsan ZI, Soyal-Baltacioglu F, Aksoylu AE. Steam reforming of ethanol over PtNi catalysts. Catal Today 2008;138:183e6. [53] Fierro V, Akdim O, Provendier H, Mirodatos C. Ethanol oxidative steam reforming over Ni-based catalysts. J Power Sources 2005;145:659e66. [54] Guil-Lopez R, Navarro RM, Pena MA, Fierro JLG. Hydrogen production by oxidative ethanol reforming on Co, Ni and Cu ex-hydrotalcite catalysts. Int J Hydrogen Energy 2011;36:1512e23. [55] Kunzru D, Peela NR. Oxidative steam reforming of ethanol over Rh based catalysts in a micro-channel reactor. Int J Hydrogen Energy 2011;36:3384e96. [56] Song IK, Youn MH, Sea JG. Hydrogen production by auto-thermal reforming of ethanol over nickel catalyst supported on metal oxide-stabilized zirconia. Int J Hydrogen Energy 2010;35:3490e8.
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Thermodynamic analysis of ethanol reforming for hydrogen production Shaohui Sun*, Wei Yan, Peiqin Sun, Junwu Chen Institute of Catalysis and Polymer, School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, Henan, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2012 Received in revised form 27 April 2012 Accepted 30 April 2012 Available online 28 May 2012
This work presents the simulated equilibrium compositions of ethanol steam reforming (SR), partial oxidation (POX) and auto-thermal reforming (ATR) at a large temperature range, steam-to-ethanol and oxygen-to-ethanol molar ratios. The simulation work shows that the moles of hydrogen yield per mole ethanol are of this order: SR > ATR > POX. The results are compared with other simulation works and fitted models, which show that all the simulation results obtained with different methods agree well with each other. And the fitted models are in highly consistency with very small deviations. Moreover, the thermal-neutral point in corresponding to temperature, steam-to-ethanol and oxygen-to-ethanol mole ratios of ethanol ATR is estimated. The result shows that with the increasing of oxygen-toethanol mole ratio, the T-N point moves to higher temperatures; with the increasing of steam-toethanol mole ratio, the T-N point moves to lower temperatures. Furthermore, the energy exchanges of the reforming process and the whole process and the thermal efficiencies are also analyzed in the present work and that the energy demands and generated in the whole process are greater than the reforming process can be obtained. Finally, the optimum reaction conditions are selected. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen Ethanol Steam reforming Partial oxidation Auto-thermal reforming Thermodynamic analysis
1. Introduction Hydrogen has been reported as one of the few long-term sustainable clean energy carriers, only generating water vapor as a by-product during the combustion or oxidation process [1]. A global system where hydrogen is produced from renewable energy sources would be in complete balance with the environment, and therefore sustainable [2]. Yet many problems are encountered in the fields of hydrogen production, storage and distribution, as well as engine developments [3]. Liquid oxygenated hydrocarbons from biomass are promising resources for hydrogen because they have high energy density and can be easily stored [4]. Until now, many oxygenated hydrocarbons, such as bio-oil (acetic acid, ethylene glycol and acetone) [5,6], glycerol [7e10], methanol [11e13], ethylene glycol [14e16] and ethanol [17e22], have been used to produce hydrogen. Among all these resources, ethanol has attracted much attention in recent years [17,23e26]. Ethanol can be easily obtained from biomass through fermentation or chemistry method. Besides, it has higher hydrogen content and can be separated easier than other oxygenated hydrocarbons. Ethanol reforming for hydrogen production is a carbon-neutral process as the CO2 generated in the process is consumed for * Corresponding author. Tel.: þ86 371 67781754; fax: þ86 371 67781755. E-mail addresses: [email protected], [email protected] (S. Sun). 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2012.04.059
biomass growth, which offers a nearly closed carbon loop [27]. As the boil point of ethanol is lower than water, there are three primary techniques can used to produce hydrogen: ethanol steam reforming (SR), partial oxidation (POX) and auto-thermal reforming (ATR). The reforming process produces a gas stream primarily composed by hydrogen, carbon dioxide, carbon monoxide and methane. Ethanol SR is a highly endothermic process which needs external heat supplier, while ethanol POX is exothermic and ATR is thermal-neutral(T-N) or slightly exothermic [28]. Ethanol SR is feasible for temperatures higher than 500 K and its H2/CO ratio is higher than POX and ATR [29]. In a POX process, ethanol is reacted with oxygen at sub-stoichiometric ratios above the flame temperature, and the heat generated is to make the reactants to a higher temperature [30]. Ethanol ATR is the combination of SR and POX. In this process, the heat generated from partial combustion can make up for the SR, which makes the system a T-N condition [31]. Hydrogen productions from ethanol in these three ways are shown as follows. The reaction equation of ethanol SR is,
C2 H5 OH þ 3H2 O42CO2 þ 6H2 ; DH 0 ¼ 347:5 kJ=mol
(1)
The reaction equation of ethanol POX is,
1 C2 H5 OH þ O2 42CO þ 3H2 ; DH0 ¼ 14:1 kJ=mol 2
(2)
912
S. Sun et al. / Energy 44 (2012) 911e924
Nomenclature aji C ^ pi fi fi0 G Gi Hi DG0fi
number of atoms of element j in component i heat capacity for component i fugacity for component i in the mixture fugacity for pure component in the reference state Gibbs free energy for the system partial molar Gibbs free energy for component i partial molar enthalpy for component i standard Gibbs free energy of formation of species i
DHfi0
standard enthalpy of formation of species i
ni yi NC NE P
number of moles for component i molar fraction for gas phase for component i number of components in the system number of elements in the system pressure of system
(3)
The reaction of ethanol ATR is,
C2 H5 OH þ 1:78H2 O þ 0:61O2 42CO2 þ 4:78H2 ;
DH 0 ¼ 0 kJ=mol
reference pressure (101.325 kPa) universal gas constant temperature of system (K) reference temperature (298.15 K) correlation coefficient for the fitting equation
Greek symbol mi chemical potential for component i m0i chemical potential of at reference temperature for pure component i ^ fi fugacity coefficient for component i Subscripts I component in the mixture J element in component i
Or
3 C2 H5 OH þ O2 42CO2 þ 3H2 ; DH0 ¼ 554:0 kJ=mol 2
P0 R T T0 R
ð4Þ
Other possible reactions are shown as follows.
C2 H6 O4CO þ C þ 3H2 ; DH0 ¼ 31:32 kJ=mol
(5)
CO þ H2 O4CO2 þ H2 ; DH0 ¼ 41:2 kJ=mol
(6)
CH4 þ H2 O4CO þ 3H2 ; DH0 ¼ 206:1 kJ=mol
(7)
CH4 þ2H2 O4CO2 þ 4H2 ; DH0 ¼ 164:9 kJ=mol
(8)
CO2 þCH4 42CO þ 2H2 ; DH0 ¼ 247:3 kJ=mol
(9)
CO þ H2 4C þ H2 O; DH 0 ¼ 131:3 kJ=mol
(10)
CH4 4C þ 2H2 ; DH0 ¼ 74:9 kJ=mol
(11)
2CO4CO2 þ C; DH 0 ¼ 172:4 kJ=mol
(12)
C2 H6 O4H2 þ CH3 CHO; DH 0 ¼ 67:8 kJ=mol
(13)
C2 H6 O4H2 O þ C2 H4 ; DH 0 ¼ 44:7 kJ=mol
(14)
2C2 H6 O43H2 þ CO þ CH3 COCH3 ; DH 0 ¼ 93:8 kJ=mol
(15)
So far, many efforts have been done on the experimental and thermodynamic analysis of hydrogen production generated from ethanol reforming, while no comparisons and optimization have
been made with these models yet. As experimental validation of a thermodynamic model is not always possible since the experimental data reported in literature is not necessarily at equilibrium conditions. Nonetheless it is still quite useful to compare experimental results with thermodynamics data to observe the trends and confirm how far apart the system is from equilibrium conditions. In the present study, a thermodynamic analysis of hydrogen production from ethanol in these three different technologies (SR, POX and ATR) is carried out with Gibbs free energy minimization method under temperatures from 700 to 1400 K, steam-to-ethanol (S/E) from 0 to 10 and oxygen-to-ethanol (O/E) molar ratios from 0.0 to 3.0. The above reaction equations show that besides some main products, there are many other species coexisting in the reforming system, such as ethylene, acetone and acetaldehyde, etc. The previous researches [21,29,32,33] and our calculation find that ethylene, acetone and acetaldehyde as any thermodynamically viable species do not show up above 250 C. So under the reaction conditions investigated, eight species H2, CO2, CO, CH4, carbon, water, ethanol and oxygen are determined as the equilibrium compositions. The minimization of Gibbs free energy has been implemented with many different methods, such as Aspen-HYSYS software [34], HSC Chemistry [9], GNU Octave numerical language [35], etc. However, the constancy of the thermodynamic results with different methods has not been investigated yet; the thermodynamic results are expressed dispersedly in figures, from which the searched equilibrium composition data under an arbitrary condition is not so accurate; especially, energy efficiencies of ethanol reforming and the T-N point of ethanol ATR have not been analyzed systematically. In this work, the minimization of Gibbs free energy will be programmed with FORTRAN language emulating Ant algorithm. The estimation is based on the basic thermodynamic data of all the proposal compounds, thermodynamically analyzes the SR, POX and ATR of ethanol for hydrogen production, and they were compared with previous theoretical studies. Comparisons between the fitted models and all the simulations show that not only the estimations with different methods agree well with each other but also the fitted equations are in highly accordance with all the estimated results, which provides a simple way for further researchers to determine the equilibrium compositions of ethanol reforming without repeating simulations and exploratory experiments. Meantime, the thermal efficiency of the three technologies are investigated and compared, which is of the order, the efficiency of SR is approach to that of ATR, and they both are higher than that of POX.
S. Sun et al. / Energy 44 (2012) 911e924
1) non-negativity of moles number:
2. Methodology The total Gibbs free energy of a system can be expressed as follow,
G ¼
NC X
ni ,mi
(16)
ni 0;
where, ni is the number of equilibrium moles for species i and mi is the chemical potential which can be expressed as: ^
fi fi0
! (17)
where, m0i is the chemical potential of component i at reference temperature, fi0 is the fugacity for pure component in the reference ^
state and fi is the fugacity for component i in the mixture which can be obtained by: ^
^
fi ¼ fi ,yi ,P
(18) ^
where, yi is the molar fraction of component i in equilibrium and fi is the fugacity coefficient for component i which is very close to 1 at ^
low pressure and high temperature. Assuming fi ¼ 1 and submitting them back into Eq. (16) and Eq. (18) can be obtained,
G ¼
NC X i¼1
ni , m0i þ RTðln yi þ ln PÞ
(19)
The reference chemical potential m0i of the pure component i can be calculated using the following known thermodynamic relations:
vHi vT
¼ Cpi
(20)
P
v Gi vT RT
¼ P
Hi RT 2
Gi ¼ mi
(21)
(22)
After directly solving of Eqs. (20) and (21) for gases at standard atmosphere pressure, the following equation can be obtained,
m0i ðTÞ ¼
T T T DG0fi þ 1 DHfi0 ai Tln T þ T0 T0 T0 T0 bi 2 c T 2T0 T þ T02 i T 3 3T02 T þ 2T03 2 6 di 4 e 3 4 T 4T0 T þ 3T0 i T 5 5T04 T þ 4T05 12 20 (23)
where, the value of DG0fi , DHfi0 , ai, bi, ci and di (constants for polynomial expression of Cpi) are taken from databases [36,37]. As carbon is solid and has no vapor pressure, the total Gibbs free energy including carbon is expressed as:
G ¼
NC1 X i¼1
ni , m0i þ RT ln yi þ ln P þ nC ,m0C
(24)
where nC represents the equilibrium moles of carbon and m0C the reference chemical potential of carbon. In the Gibbs free energy minimization, some restrictions should be satisfied:
i ¼ 1; ..; NC
(25)
where, NC is the number of components.
i¼1
mi ¼ m0i þ RTln
913
2) Mass balance of each atom must satisfy the restriction: NC X
ni aji ¼ bj ;
j ¼ 1; ..; NE
(26)
i¼1
where, NE is the number of element. FORTRAN code is applied to obtain the minimize Gibbs free energy with ant algorithm and the equilibrium compositions of ethanol SR, POX and ATR processes. The equilibrium compositions obtained are fitted using MATLAB 7.1 with non-linear polynomials.
3. Results and discussion 3.1. Thermodynamic product distribution and comparisons In the present study, the thermal equilibrium compositions are calculated with Gibbs free energy minimization implemented with FORTRAN code language while many other researchers have done the thermodynamic analysis using different methods. In this part, the thermodynamic results are compared with some other simulation works [32,34,35,38e40] and the comparisons show that they agree well with each other.
Table 1 Comparisons of H2 yield for ethanol POX. T (K)
O/E
Present simulation
Wanga [36]
Rabensteina [30]
Deviation%
700 700 700 700 700 700 700 700 900 900 900 900 900 900 900 900 1100 1100 1100 1100 1100 1100 1100 1100 1300 1300 1300 1300 1300 1300 1300 1300
0 0.25 0.50 0.75 1.00 1.50 2.00 2.50 0 0.25 0.50 0.75 1.00 1.50 2.00 2.50 0 0.25 0.50 0.75 1.00 1.50 2.00 2.50 0 0.25 0.50 0.75 1.00 1.50 2.00 2.50
0.4932 0.4688 0.4678 0.4500 0.4400 0.4300 0.4200 0.4150 1.8003 1.7827 1.7822 1.726 1.7061 1.6165 1.3146 0.8004 2.9000 2.7792 2.7418 2.4613 2.1232 1.7987 1.2734 0.6178 2.9866 2.9726 2.9657 2.6247 2.1232 1.7302 1.1041 0.5025
0.5 0.47 0.46 0.45 0.44 0.43 0.42 0.42 1.8 1.78 1.78 1.72 1.7 1.6 1.3 0.8 2.9 2.8 2.7 2.5 2.1 1.8 1.2 0.6 3 3 3 2.6 2.1 1.7 1.1 0.5
0.5
0.6 0.1 0.8 0.0 0.0 0.0 0.0 0.5 0.007 0.07 0.06 0.1 0.1 0.4 0.5 0.02 0.0 0.3 0.7 0.7 0.5 0.03 2.6 1.3 0.2 0.4 0.5 0.4 0.5 0.7 0.1 0.2
a
Data is read from the figures.
0.43 0.42 0.42 1.8
1.6 1.3 0.8 2.9
1.8 1.2 0.6 3
1.7 1.1 0.5
914
S. Sun et al. / Energy 44 (2012) 911e924
3.1.1. Ethanol partial oxidation The heat generated from ethanol combustion contributes a high temperature to the system and thus it can easily to be started [37]. The POX process for hydrogen production is simulated in the O/E molar ratio range from 0.0 to 3.0 and temperature range from 700 K to 1400 K at atmospheric pressure. And comparisons are made between the H2 yields obtained by the present simulation and that by Wang [38] and Rabenstein (using Aspen TechÔ) [30], which are showed in Table 1. Table 1 shows the same results are obtained by our methodology with those obtained by other different method, with the relative coverage deviations lower than 3%. With this methodology, H2, CO, CO2 and CH4 yields (moles per mole ethanol) under different conditions are calculated which are showed in Fig. 1(aed). From Fig. 1(a), we can see that more than 96% (2.9/3.0) yield of H2 can be achieved at above 1200 K and O/E ratio lower than 0.5. The H2 yield decreases with the decrease of temperature and the increase of O/E ratio. Fig. 1(b) shows that the maximum CO yield (w1.9 mol) appears at about 1200 K with O/E between 0.5 and 0.75. Under a given temperature, the yield of CO increases with the O/E
1400
b 1400
1300
1300
1200
1200
Temperature,K
2.9
0.30
1100
1000
2.7 2.4 2
900
1.7
1.3
0.5
0.20 0.10
700 0.0
0.60 1.0
1.5
2.0
2.5
3.0
0.5
d
1400
Temperature,K
0.10
0.50 0.90
1000
0.30
1.1
1.3
1.7
1.9
1.0
1.5
2.0
Oxygen-to-ethanol molar ratio
3.0
0.050 1100
0.10 1000
0.20
800
0.30 0.40
0.60 0.70
0.5
2.5
0.030
900
1.5
800
700 0.0
2.0
0.010
1200
0.70
0.050
1.5
1400
1300
1200
1.0
Oxygen-to-ethanol molar ratio
Oxygen-to-ethanol molar ratio
1100
0.90 0.60
0.40
800
1300
Temperature,K
1.9
1000
0.90
900
1.5
1100
1.5
1.2
800
700 0.0
2.1 1.8
900
c
3.1.2. Ethanol steam reforming In this work, the thermodynamic compositions obtained with FORTRAN code are compared with other simulation results with Wang [35] (with Lagrange multiplier method), Kafarov [34] (with Aspen-HYSYS software) and Vasudeva [40] (with SQP). The comparisons of H2 yield per mol ethanol at different S/E molar ratios (0e10) and the temperature range from 700 to 1200 K is shown in Table 2. From Table 2, we can see that hydrogen yields of ethanol SR obtained with different methods are in consistency with each other with the relative average deviation no greater than 3%. The
Temperature,K
a
ratio at first then decreases owing to the Eqs. (2), (3) and (5). In Fig. 1(c), the yield of CO2 reaches 95% (1.9/2) at all temperatures with O/E > 2.5 and decreases with the decreasing of O/E ratio but is little affected by temperature. As far as CH4 yield is concerned in Fig. 1(d), it is relatively low (<0.7 mol) and low temperature favors the CH4 to generate due to the reverse of Eqs. (7) and (8). These trends are in accordance with that obtained by other simulation works. Carbon formation in all the three processes is discussed in part 3.3.4.
2.5
3.0
700 0.0
0.50 0.5
1.0
1.5
2.0
Oxygen-to-ethanol molar ratio
Fig. 1. (aed). Effects of temperature and O/E molar ratio on H2, CO, CO2 and CH4 yields from ethanol POX.
2.5
3.0
S. Sun et al. / Energy 44 (2012) 911e924 Table 2 Comparisons of H2 yield for ethanol SR. T (K)
S/E
Present simulation
Wanga [37]
800 800 800 800 800 800 800 800 800 1000 1000 1000 1000 1000 1000 1000 1000 1000 1100 1100 1100 1100 1100 1100 1100 1100 1100
0 1 2 3 4 6 8 9 10 0 1 2 3 4 6 8 9 10 0 1 2 3 4 6 8 9 10
1.1280 1.4663 1.8042 2.5487 2.7509 3.7679 3.9674 4.2301 4.3521 2.6268 3.2972 3.8976 4.4572 4.6713 5.0955 5.1778 5.3505 5.4338 2.9000 3.6889 4.0240 4.5786 4.6203 4.8855 5.0158 5.1464 5.1905
1.2 1.5 2 2.5 2.8 3.8 4 4.3 4.4 2.6 3.3 3.9 4.5 4.7 5 5.2 5.3 5.4 2.9 3.6 4 4.5 4.6 4.9 5 5.1 5.2
a b
Kafarova [32]
Vasudevab [2]
Deviation %
1.08 1.41 1.7464
2.7 2.5 3.4 0.8 0.8 1.0 0.4 0.8 0.5 0.4 0.04 0.02 0. 8 0.3 0.8 0.2 0.4 0.2 0.0 1.1 0.4 0.8 0.2 0.8 0.1 0.4 0.08
2.5 3.7 4.2 4.4103 2.4765 4.0382 4.4 5 5.3 5.3173 2.8898 4.1470 4.5 4.8 5.1 5.1627
Data is read from the figures. Data is from the original literature.
thermodynamic equilibrium yields of H2, CO, CO2 and CH4 (moles per mole ethanol) are showed in Fig. 2(aed). From Fig. 2(a), we can see more than 5.1 mol of H2 close to 90% of the stoichiometry coefficient 6 can be obtained at about 1000 K with S/E > 8. Low temperature and S/E ratio deceases H2 yield due to the reverse of water-gas shift reaction (WGSR, Eq. (6)), which is not as the same as that of ethanol POX. Fig. 2(b) shows that as many as 1.8 mol of CO yield appears at 1150 K with S/E ¼ 1e2, and decreases sharply with the decreasing of temperature while there’s no so significant influence by S/E ratio. That CO2 yield maximized at about 900 K with S/E > 8, with the similar trend to H2 yield can be seen from Fig. 2(c). From Fig. 2(d), the relatively high yield of CH4 (w1.1 mol) is obtained at very low temperature which is opposite to the three productions above and when temperature increases, the yield of CH4 decreases owing to the reverse of Eq. (8). These trends are in accordance with that obtained by others’ simulation works [12,18,30,31,36].
915
maximum yield of H2 can be obtained when O/E ¼ 0.25 and decreases with the increase of O/E molar ratio. From Fig. 3(a), we can see that the H2 yield is of the same trend as that in ethanol SR yet less H2 is yielded (w4.8 mol) in the regions where 5.1 mol are generated in SR owning to the addition of oxygen. Fig. 3(b) shows the maximum yield of CO appears at very high temperature and low S/E ratio. However, the yield of CO increases with the increase of temperature while decreases with the S/E and O/E which is attributed to the Eq. (9) and reverse of WGSR (the reverse of Eq. (6)). In Fig. 3(c), the yield of CO2 reaches 1.6 mol at 900 K and S/E ¼ 10 and shows the same trend as H2 yield with temperature and S/E ratio but the opposite with O/E ratio. From Fig. 3(d), we can see that the yield of CH4 is the lowest (<0.8 mol) in the three technologies and increases with the decrease of temperature and the S/E molar ratio at first then decreases when S/E ratio increases furthermore.
3.2. Fitting the simulation results In this part, we fit the simulation results into equations by which one can get the simulation results more convenient, easier and faster. To improve the correlations as much as possible of fitting the yield equation with the S/E, O/E molar ratio and temperature, we make many attempts and find that the polynomial with appropriate deformation of S/E, O/E, temperature and the yield of production is the most suitable. However, the physical and chemical mechanism is not so clear.
3.2.1. Ethanol POX As we know only the temperature and O/E ratio affect the equilibrium production distribution. Here we assume that (O/ Eþ8) ¼ X1, ln(T) ¼ X2 and YH2 , YCO , YCO2 and YCH4 refer to the moles of H2, CO, CO2 and CH4. And X1 is ranged from 8 to 11 while X2 from 6 to 8. The yields of H2, CO, CO2 and CH4 from ethanol POX are fitted in the following polynomials.
ln YH2 þ 1 ¼ 1:65 105 þ 74:05X1 1:61X2 1:95X12 þ 583:13X22 0:64X1 X2 þ 5:66 103 X11 7:59 105 X21 1:28 104 X12 þ 1:30 106 X22
(27)
with r ¼ 0.9911.
lnðYCO þ 0:021Þ ¼ 5:43 105 þ 722:03X1 5:35 104 X2 3.1.3. Ethanol auto-thermal reforming The process of ethanol ATR is the combination of ethanol SR and POX which not only attains thermally sustained operation, but also maximizes the hydrogen production [38]. The H2 yield (moles per mole ethanol) of ethanol ATR obtained with the present method are also compared with that from other researchers, like Rabenstein [30] and Gracia (GNU Octave numerical language) [33]. The comparisons are listed in Table 3. Table 3 reveals a close agreement between the results obtained with different methods with the relative coverage deviations no greater than 5%. The H2, CO, CO2 and CH4 yields (moles per mole ethanol) of ethanol ATR obtained in the present work are described in Fig. 3(aed). The thermodynamic equilibrium compositions of ethanol ATR are calculated at a temperature range from 700K to 1200 K with S/E ¼ 1, 4, 7, 10 and O/E ¼ 0.25, 0.50 and 0.75. Under the given conditions, the
19:33X12 þ 1:94: 103 X22 0:85X1 X2 þ 6:17 105 X11 2:54 106 X21 1:43 105 X12 þ 4:36 106 X22
(28)
with r ¼ 0.9879.
ln YCO2 þ 0:9 ¼ 6:25 104 53:77X1 þ 5:94 104 X2 þ 1:27X12 208:84X22 þ 0:072X1 X2 5:68 103 X11 þ 3:00 105 X21 þ 1:43 104 X12 5:31 105 X22 with r ¼ 0.9911.
(29)
916
S. Sun et al. / Energy 44 (2012) 911e924
a
1200
b
1100
1200
1.8 1.7
1100
1.6 1.2
1000
900
Temperature,K
Temperature, K
5.1
3.6
1000
0.70 0.90
900
1.8 2.4
800
0.20
3.0 700
0.050
1.2 0
2
4
6
8
700
10
Steam-to-ethanol molar ratio
1200
0.10
d
1100
Temperature,K
1.2 1000
1.3 3
0.30 0.90
0.50
900
0.10
0
2
4
6
Steam-to-ethanol molar ratio
8
10
1200
1100
0.70
Temperature,K
c
0.50
4.8
4.2 800
1.4
1000
0.010 0.30
0.10
900
0.050
0.50 800
800
0.70 700
0
2
4
6
8
10
700
1.1 0
Steam-to-ethanol molar ratio
2
4
0.90 6
Steam-to-ethanol molar ratio
8
10
Fig. 2. (aed). Effects of temperature and S/E molar ratio on H2, CO, CO2 and CH4 yields.
ln YCH4 þ 5:3 ¼ 3:04 104 þ 2:93 103 X1 3:46X2
lnðYCO þ 0:03Þ ¼ 1:75 106 þ 1:68X1 1:70 105 X2 1:39
105:51X12 þ 0:074X22 þ 0:077X1 X2 þ 1:41
103 X12 þ 6:21 103 X22 0:24X1 X2
105 X11 272:68X21 þ 2:44 105 X12
þ 18:11X11 7:98X21 51:06X12 þ 1:36
þ
651:08X22
107 X22
(30) with r ¼ 0.9952.
with r ¼ 0.9874. 3.2.2. Ethanol SR It is assumed that (S/Eþ3.5) ¼ X1, ln(T) ¼ X2 and YH2 , YCO , YCO2 and YCH4 refer to the yield of H2, CO, CO2 and CH4. And X1 is ranged from 3.5 to 13.5 while X2 from 6 to 8. The equations of the yields of hydrogen, carbon monoxide, carbon dioxide and methane from ethanol SR are fitted in the following polynomials.
ln YCO2 þ 0:45 ¼ 1:05 105 1:38X1 1:47 103 X2 þ 0:023X12 þ 69:55X22 þ 0:087X1 X2 52:90X11 2:64 104 X21 þ 7:76 103 X12 þ 80:05X22 with r ¼ 0.9911.
103 X12 993:22X22 þ 0:013X1 X2 1:01X11
103 X12 þ 50:887X22 0:215X1 X2 4:0244X11 with r ¼ 0.9949.
(33)
ln YCH4 þ 2 ¼ 2:80 105 0:15X1 þ 2:72 104 X2 þ 2:01
ln YH2 ¼ 7742:4969 þ 1:5048X1 1086:32X2 1:33 18406:629X21
(32)
þ 1:28 106 X21 þ 0:071X12 2:19 106 X22
(31) with r ¼ 0.9772.
(34)
S. Sun et al. / Energy 44 (2012) 911e924
YCH4 ¼ 5:40 0:39X1 0:070X2 9:13 103 X3 þ 0:089X12
Table 3 Comparisons of H2 yield for ethanol ATR. T (K)
O/E
S/E
Present simulation
Rabensteina [30]
Graciaa [33]
Deviation%
800 800 800 800 800 800 800 800 800 800 800 800 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200
0.25 0.25 0.25 0.25 0.50 0.50 0.50 0.50 0.75 0.75 0.75 0.75 0.25 0.25 0.25 0.25 0.50 0.50 0.50 0.50 0.75 0.75 0.75 0.75 0.25 0.25 0.25 0.25 0.50 0.50 0.50 0.50 0.75 0.75 0.75 0.75
1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10
1.5407 2.8500 3.6375 4.3196 1.4296 2.8393 3.2013 3.9761 1.3382 2.5699 3.3133 3.6453 3.3525 4.5576 4.7131 4.8478 3.0520 4.0000 4.2385 4.4422 2.9951 3.6080 3.9649 4.0663 3.4690 4.0060 4.3327 4.5606 3.0474 3.7797 3.9517 4.1880 2.8861 3.1935 3.6394 3.8260
1.5 2.8 3.6 4.3 1.4 2.8 3.2 4 1.3 2.5 3.3 3.6 3.3 4.6 4.8 4.9 3 4 4.2 4.4 3.0 3.6 4.0 4.0 3.5 4.0 4.3 4.5 3.0 3.7 3.9 4.2 2.8 3.2 3.6 3.8
1.5 2.9 3.6 4.3 1.4 2.8 3.2 3.9 1.2 2.5 3.3 3.6 3.3 4.5 4.7 4.8 3 4 4.2 4.4 2.9 3.6 3.9 4.0 3.5 4.0 4.3 4.6 3.0 3.7 3.9 4.2 2.8 3.2 3.6 3.8
1.2 1.1 0.4 0.2 0.9 0.6 0.01 0.9 4.1 1.2 0.1 0.5 0.7 0.7 0.8 0.7 0.7 0.0 0.4 0.4 1.4 0.09 0.9 0.7 0.4 0.06 0.3 0.7 0.7 0.9 0.5 0.1 1.3 0.09 0.4 0.3
a
0:018X22 1:56 105 X32 0:065X1 X2 1:42 (35)
with r ¼ 0.9619.
YCO ¼ 3:95 þ 0:99X1 0:14X2 þ 6:34 103 X3 0:037X12 þ 4:24 103 X22 9:32 107 X32 þ 7:46 103 X1 X2 (36)
with r ¼ 0.9599.
YCO2 ¼ 1:65 þ 0:41X1 þ 0:16X2 þ 5:09 103 X3 þ 0:072X12 0:011X22 3:46 106 X32 0:052X1 X2 þ 2:82 104 X1 X3 þ 9:65 105 X2 X3
(38)
The thermodynamic equilibrium products have been analyzed for the three different technologies, ethanol POX, SR and ATR under various conditions. In order to analyze the energy requirements and thermal efficiencies for the three technologies, an approximate energy balance is made as shown in Fig. 5. Here, it is assumed that the feed in and products out are under the standard state (298.15 K, 1atm) and there’s no heat loss in the heating and cooling process. And the energy balance for the reformer and the whole system is discussed in the following part.
YH2 ¼ 16:05 þ 0:60X1 þ 0:59X2 þ 0:035X3 0:070X12
1:32 103 X1 X3 2:71 104 X2 X3
104 X1 X3 þ 6:05 105 X2 X3
3.3. The thermal efficiency analysis
3.2.3. Ethanol ATR As there are three variables in the process of ethanol ATR, S/E, O/ E molar ratio and temperature, the fitted models are more complicated than that of ethanol SR and POX. Here O/E ¼ X1, S/ E ¼ X2, T ¼ X3 and YH2 , YCO , YCO2 and YCH4 refer to the yield of H2, CO, CO2 and CH4. And X1 is ranged from 0.25 to 0.75 while X2 from 1 to 10 and X3 from 700 to 1200.The fitted equations are listed as follows.
103 X1 X3 1:99 104 X2 X3
þ 3:90 104 X22 þ 3:85 106 X32 0:015X1 X2 þ 3:38
with r ¼ 0.9611. For further check of the fitted models, comparisons among the fitted models, the present work and other simulation works are made in Fig. 4(aec) to see how far are the fitted models deviate from the simulation works. From Fig. 4(aec), we find that the simulation results obtained with different methods agree well with the fitted models, with very small deviations. It should be noted that one or two points deviate far with the relative error greater than 10%. Along with the comparisons above, we can conclude that the fitted models can be applied well in ethanol POX, SR and ATR under the conditions investigated. With the fitted models, one can calculate the thermodynamic equilibrium compositions easily without doing repeated simulation work.
Data is read from the figures.
with r ¼ 0.9567.
917
(37)
3.3.1. Reforming energy balance Fig. 5 plots the energy flows across the boundaries of a control volume, which in this case is the reformer itself (solid box). The fuel and water enter the reformer and react (completely) to form hydrogen and carbon dioxide. In the reforming process, P P T ¼ ðnp Df HpT Þ ðnr Df HrT Þ, where the reforming heat DHreforming Df HT refers to the formation enthalpy at reaction temperature (T) and subscribes p and r refer to the products and reactants, respectively. Here we bring the dimensionless volume T DHreforming =HVethanol (HVethanol is the combustion heat of ethanol of 1366.8 kJ/mol) as a measurement to analyze the energy T =HVethanol > 0, the balance for the reformer. When DHreforming T =HVethanol < 0, then reforming is endothermic while DHreforming the process is exothermic and T-N condition reaches when T T DHreforming =HVethanol ¼ 0. The volume DHreforming =HVethanol for the reforming process of ethanol POX, SR and ATR is showed in Fig. 6(aed). Fig. 6(aed) confirms the truth once again that ethanol POX is highly exothermic while SR or ATR is preferred as the process is slightly endothermic or exothermic. Fig. 6(a) shows that for 1 mol ethanol reacted, the energy ethanol POX generates is the greatest in which almost all the ethanol is burned to generate heat when 3 mol of oxygen is added in the system. And with the decrease of O/E molar ratio, the ratio of ethanol combustion decreases followed by less heat generated. From Fig. 6(b), a small amount of energy (about 12e18% of ethanol heating value) is demanded for ethanol SR and when temperature increases, so does the energy demanded. From Fig. 6(c) and (d), it is apparent that the reforming heat of ethanol ATR is very low (less than 10% of ethanol heating value) that could be negligible and seen as Thermal-Neutral.
918
a
S. Sun et al. / Energy 44 (2012) 911e924
b
1200
1200
1.7
4.6 1100
1100
1.1
4.4 4.8
1000
4.0 3.5
900
0.70
1.3
Temperature,K
Temperature,K
0.90
1.5
1000
0.50 900
0.30 800
800
2.5 700
0.10
2.0 2
4
6
8
700
10
2
4
Steam-to-ethanol molar ratio
c
d
1200
0.40
0.80
1.6
900
Temperature,K
Temperature,K
1.2 1.4
0.050
1000
700
8
10
0.030 0.10
900
0.20 0.40
800
10
1200
1.0 0.60
8
1100
1100
1000
6
Steam-to-ethanol molar ratio
0.30
800
0.60
0.70 700
2
4
6
8
10
0.80 0.80 2
0.50
0.60
4
6
Steam-to-ethanol molar ratio
Steam-to-ethanol molar ratio
Fig. 3. (aed). Effects of temperature and S/E molar ratio on H2, CO, CO2 and CH4 yields from ethanol ATR with O/E ¼ 0.25.
3.3.2. Thermal neutral point analysis for ethanol ATR process T for ethanol ATR can be The fitted equation of DHreforming expressed as follows. T DHreforming ¼ 1:2 103 275:6X1 þ 40:0X2 þ 2:3X3 4:6X12
0:5X22 2:9X1 X2 0:2X1 X3
(39)
where R ¼ 0.9885, X1 ¼ O/E, X2 ¼ S/E and X3 ¼ Temperature. The T ¼ 0Þ are calculated from the above T-N points ðDHreforming equation with X1 ¼ 0.2, 0.3, 0.4 and 0.5, which are showed in Fig. 7. It is noted that there are some differences between the T-N curves calculated by Eq. (39) and the simulated works. Comparing the T-N conditions in the process of ethanol SR (O/E ¼ 0) and ATR (O/E ¼ 0.25, 0.50) in Fig. 6(b), (c) and (d), we can see that under the T-N conditions, for a fixed O/E ratio, the equilibrium temperature increases with the decrease of S/E ratio; and for a fixed S/E ratio, the equilibrium temperature increases with the
increase of O/E molar ratio. From Fig. 7, we can also get that under a certain S/E ratio, when O/E ratio increases, then the process under the former T-N conditions is exothermic and the T-N point moves to higher temperatures. Because higher temperatures favor the endothermic reaction (SR) and suppress the exothermic reaction (POX), thus make the condition reach T-N again. Also, when O/E ratio is fixed, with the S/E ratio increases, the T-N point moves to lower temperatures as lower temperatures favor the exothermic reaction and suppress endothermic reaction to keep heat balance. These trends are as the same as Li’s [29]. 3.3.3. Energy balance for the whole system The energy balance for the whole system includes the energy for the heating, reforming and cooling processes in the dash box in Fig. 5.
Qin ¼ Qheating þ Qreforming þ Qcooling
(40)
S. Sun et al. / Energy 44 (2012) 911e924
a
b
3.0
ethanol POX,T=1100K
H2 yield
H2 yield
4.5
1.5
0.0
4.0 3.5
1.0
0.0
ethanol SR,T=1000K
5.0
2.0
0.5
6.0 5.5
2.5
919
0.5
the fitted model the present work Kafarov etc Wang etc Vasudeva etc
3.0
the fitting model the present work Wang etc Rabenstein etc
2.5
1.0
1.5
2.0
2.5
3.0
2.0
0
2
Oxygen-to-ethanol molar ratio
c
4
6
8
10
Steam-to-ethanol molar ratio
5.5 5.0
ethanol ATR O/E=0.25
4.5
S/E=10
4.0
H2 yield
3.5 3.0
S/E=1
2.5 2.0
the fitted model the present simulation Rabenstein Gracia
1.5 1.0 700
800
900
1000
1100
1200
Temperature,K Fig. 4. (aec). Comparison between the simulation results and the fitted models of ethanol POX, SR and ATR under various conditions.
As it is assumed above, the feed in and products out are under the standard state and there’s not any heat loss in every process. From the dash box, the system energy can be expressed by the following equation.
Qin ¼ DHR0 ¼
X
np Dc Hp0
X
nr Dc Hr0
Fig. 5. Scheme of the thermal exchange of ethanol SR, POX and ATR.
(41)
where Qin refers to the system energy, DHR0 reaction enthalpy at 298.15 K and DcH is the combustion heat while subscript r refers to the reactants compounds and p the products composition. For convenient discuss, the system energy is expressed with a dimensionless volume Qin =HVethanol for these three technologies which is showed in Fig. 8(aed). When Qin =HVethanol > 0, the whole system is endothermic and when Qin =HVethanol < 0, the whole system is exothermic. From Fig. 8(aed) and Fig. 6(aed), we can see that the total energy exchange including heating, reforming and cooling in the processes of ethanol POX is almost equal to that of the reforming process. In other words, energy necessary for heating and cooling is almost equal, which means that the cooling energy almost makes up the energy for heating in the process of ethanol POX. However, the total energy demands is a little greater than reforming energy for ethanol SR which indirect that the energy needed for heating to reaction temperature is greater than that generated in cooling process. Also from Fig. 8(ced) and Fig. 6(ced), we can see that
920
a
S. Sun et al. / Energy 44 (2012) 911e924
1400
POX
b
1200
SR 0.18
0.19
1300
1100
0.050 0.0
1100
1000
-0.20 -0.10
900
-0.70
-0.40
0 -0.80 -0.90
Temperature,K
Temperature,K
1200
0.14
1000
0.16 900
-0.30
0.10 -0.50
0.050
0.0
800
-0.60
800
700 0.0
0.5
1.0
1.5
2.0
2.5
700
3.0
0
2
4
1200
ATR,O/E=0.25
d
0.10 1100
1200
Temperature,K
Temperature,K
10
0.0125 1100
0.090
1000
0.070 0.055 900
0.020
2
0.00 1000
-0.0125 -0.0250 900
-0.0500
0.040
0.0
0.060
-0.0750
800
-0.040 700
8
ATR,O/E=0.50
0.080
800
6
Steam-to-ethanol molar ratio
Oxygen-to-ethanol molar ratio
c
0.12
4
-0.020 6
-0.100
-0.125 8
10
700
Steam-to-ethanol molar ratio
2
4
6
8
10
Steam-to-ethanol molar ratio
Fig. 6. (aed). The energy exchange for the reforming process of ethanol POX, SR and ATR (plotted by the simulation results).
under the same conditions, when it is exothermic in the reforming process then it becomes endothermic in the whole system. It is the reason that the overall processes of heating and cooling are endothermic and the energy is greater than that generated by the reforming process. 3.3.4. Thermal efficiency for the whole system In general, the thermal efficiency is a measure of energy output divided by energy input. Note that a low heating value (where water is generating as vapor) is used throughout this paper. There are two commonly used definition of thermal efficiency. The first is defined as the heating value of H2 divided by the heat consumption of fuel to run the process ðh ¼ nH2 HVH2 =HVF þ Qin Þ. The second definition subtracts the heat input from the numerator accounts for the extra cost of using some of the product hydrogen to heat the process ðh0 ¼ nH2 HVH2 Qin =HVF Þ [41]. Here the first definition is used and the low heating value can be expressed as
HVi ¼ Dc Hi
(42)
where DcHi is the combustion heat for species i. Submitting Eqs. (41) and (42) into the definition, the thermal efficiency h can be written as follows.
h¼
nH2 HVH2 nH2 HVH2 þ nCO HVCO þ nCH4 HVCH4 þ nC HVC
(43)
The equation is based on the assumption that there’s no heat loss and the energy exchange efficiencies of the heating and cooling processes equal to 1.0. This assume reflects heat exchange efficiency, which provides an upper limit (which is equal to 1.0) to the efficiency of the process. The thermal efficiencies for the whole system of ethanol POX, SR and ATR can be obtained from Eq. (43) and are showed in Fig. 9(aed).
S. Sun et al. / Energy 44 (2012) 911e924 1200
As it can be seen above, all the thermal efficiencies increase with the increase of temperature. And in the process of ethanol POX, with oxygen added into the system, carbon dioxide producing is easier than carbon monoxide and more water is formed than methane, but the amount of H2 yield is very small, thus the thermal efficiencies are very low. And with the O/E ratio increasing, thermal efficiency increases at first owning to less CO and coke and more CO2, but decreases when O/E ratio increases further for the combustion of more ethanol. In the process of ethanol SR, with the S/E ratio increasing, H2 yield increases and other combustible gases as well as coke yield decreases, which makes the thermal efficiency increase too. In the process of ethanol ATR, the thermal efficiencies increase with the increase of S/E ratio and make little differences with the O/E ratio which we can obtain from the above Figures that with the O/E increases, the H2 yield decreases as well as CO, CH4 and coke.
0.0
1100
Temperature,K
O/E=0.5 1000
0.0 900
O/E=0.4 0.0
O/E=0.3 800
0.0 O/E=0.2 700
2
4
921
6
8
10
Steam-to-ethanol molar ratio Fig. 7. The T-N conditions for the reforming process of ethanol ATR (calculated by the fitted equation).
a
b
1400
1200
1300
1100
Temperature,K
1100
0.0
1000
-0.20 -0.10
900
-0.50
-0.80
-0.95
Temperature,K
0.050
1200
1000
0.25 0.23
0.20
900
-0.30 -0.40
-0.60
0.050
800
0.15
-0.70
800
0.0 700 0.0
0.5
1.0
1.5
2.0
2.5
Oxygen-to-ethanol molar ratio
700
3.0
O/E=0.25
d
1200
0.14
Temperature,K
1100
1000
0.13 900
.050 .
0.11
0.080 800
2
4
6
2
10
O/E=0.50 1200
0.040
1000
0.020
0.030
900
800
0.0
-0.10 -0.13 -0.050
-0.080 700
8
Steam-to-ethanol molar ratio
-0.025
0.0
0 0.050
0.10
1100
Temperature,K
c
0
0.17
4
6
Steam-to-ethanol molar ratio
8
10
700
2
4
6
Steam-to-ethanol molar ratio
Fig. 8. (aed). System energy of ethanol POX, SR and ATR (with O/E ¼ 0.25, 0.50).
8
10
922
a
S. Sun et al. / Energy 44 (2012) 911e924
1400
POX
b
SR,O/E=0.0
1200
0.80
1300 1100
0.40
0.85 0.60
0.30
1100
Temperature,K
Temperature,K
1200
0.70 0.20
1000
0.88
1000
0.60
900
900
800
.20 0.5
1.0
1.5
0.40 0.50
2.0
2.5
700
3.0
0
2
1200
O/E=0.25
d
1200
8
10
0.75
0.80
1100
1100
0.80
0.87
0.87
Temperature,K
0.70
Temperature,K
6
O/E=0.50 0.70
0.89
1000
0.85 900
0.60
800
2
0.88 900
800
4
0.85
1000
0.40
0.65
0.50 700
4
Steam-to-ethanol molar ratio
Oxygen-to-ethanol molar ratio
c
0.70
0.50
0.40
700 0.0
0.30
800
0.30
6
0.60 0.50
8
10
700
2
4
6
8
10
Steam-to-ethanol molar ratio
Steam-to-ethanol molar ratio Fig. 9. (aed). The thermal efficiency h for POX, SR and ATR.
3.4. The optimum reaction conditions The selection of operation conditions of a reformer is depending on various targets. Main target is a high hydrogen yield, simultaneously with low carbon monoxide content. Maximum hydrogen efficiency and low carbon monoxide as well as the coke contents which should be avoided as it can make the catalyst deactivate when coating on the surface of the catalyst are possible for ethanol reforming operation. The coke formation of ethanol POX, SR and ATR is showed in Fig. 10(aec) from which we can see that ethanol POX has the largest coke region and coke is always formed for hydrogen production; in the process of ethanol SR, with excess water (S/E > 3), coke can be avoided; when S/E > 1 and O/E ¼ 0.25, coke is neglected at above 1000 K in ethanol ATR. From Fig. 10(aec), we can also see that in the process of ethanol POX, H2 yield is maximized accompanied by coke and CO formation at high temperatures (1200e1400 K) and low O/E ratios
(0.4e0.8). Many experiments [42e47] are carried out at much lower temperatures with appropriate catalysts for low H2 yield like 0.8 mol per mole ethanol obtained by Noronha [48] at 673 K with O/E ¼ 0.4. With lowest theoretical H2 yield and highest coke formation, ethanol POX is not a good way for H2 production. As for ethanol SR, H2 yield increases with the temperature and S/E ratio. Under the optimum conditions in the shadow box where temperature is above 900 K with S/E > 6, the hydrogen yield maximizes, coke and other combustible gases minimizes with high thermal efficiency and a small amount energy needed and many experiments are carried out in the region [49e52]. Combining with ethanol SR and POX, ethanol ATR is a process that attains thermally sustained and maximizes hydrogen production, which is the most preferred in the three technologies. The maximum H2 yield is obtained above 900 K with S/E > 6 and O/ E ¼ 0.25. There are many experiment studies of ethanol reforming to generate hydrogen in the optimum shadow region with different O/E ratios [53e56].
S. Sun et al. / Energy 44 (2012) 911e924
a
b 1200
1400
1300
CO yield H2 yield
1100
1000
2.9
CO yield 1.8
43
900
800
700 0.0
coke free regions 2 1
CH4 yield 0.70 0.5
1.6
1100
1000
H2 yield 1 2 0.0
900
3 4
coke formed regions
coke formed regions
1.0
5.2
Temperature,K
Temperature/K
1200
923
1 Schmidt 2 Schmal 3 Shen 4 Noronha
1.5
2.0
2.5
coke free regions
800
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0
2
Oxygen-to-ethanol molar ratio
4
1 Abello 2 Freni 3 Kunzru 4 Onsan
CH4 yield 6
8
10
Steam-to-ethanol molar ratio
c 1200 O/E=0.25 1.6
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1100
CO yield
4
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1
H2 yield 4.6
2
900
3 800
0.70 700
2
1 Gutierrez 2 Kunzru 3 Song 4 Ferrio
CH4 yield
4
6
8
10
Steam-to-ethanol molar ratio
Fig. 10. (aec). Optimum reaction conditions for ethanol POX, SR and ATR.
4. Conclusions This work presents the simulated equilibrium compositions and the thermal efficiencies of ethanol SR, POX and ATR at wide range of temperature, S/E molar ratio and O/E molar ratio. Besides, the simulation results are fitted with polynomials and compared with other researchers’ simulations. Moreover, the energy exchanges and thermal efficiencies of the three technologies are calculated and analyzed. Finally, the optimum reaction conditions are selected. The main conclusions are as follows. < The thermal equilibrium compositions obtained with the present methodology agree well with that obtained with many other different methods and the deviations can be ignored. And the fitted models interpret the production distributions well with highly accuracy < Hydrogen yield and energy exchanges for the three technologies are compared, and the results show that with 1 mol ethanol reacted, the moles of H2 yield is of this order: SR > ATR > POX. What’s more, the process of ethanol SR is endothermic and POX is exothermic while ATR is thermal neutral. < The thermal efficiencies increase with the increasing of temperature and S/E ratio, yet increase with the O/E ratio first in ethanol POX and then decrease but make little difference in ethanol ATR. < With the target to get high hydrogen yield simultaneously with low CO and coke content and also low energy demands, the optimum reaction conditions can be summarized as
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