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A comparative thermodynamic analysis of isothermal and non-isothermal CeO2-based solar thermochemical cycle with methane-driven reduction
Renewable Energy 143 (2019) 915e921
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
A comparative thermodynamic analysis of isothermal and nonisothermal CeO2-based solar thermochemical cycle with methanedriven reduction Tianzeng Ma a, b, Lei Wang a, b, Chun Chang a, Jasurjon S. Akhatov c, Mingkai Fu a, *, Xin Li a, b, ** a b c
Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, 100190, China University of Chinese Academy of Sciences, Beijing, 100049, China Physical-Technical Institute, SPA “Physics-Sun”, Tashkent, 100084, Uzbekistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 November 2018 Received in revised form 1 May 2019 Accepted 12 May 2019 Available online 16 May 2019
Induced by promising hydrogen production of CeO2-based solar thermochemical cycle and evident temperature decreasing effect of methane reduction, a moderately high-temperature solar thermochemical ceria-methane cycle is investigated thermodynamically. In this paper, isothermal and nonisothermal solar-to-fuel efficiencies (hsolar-to-fuel) under different temperatures and reactant ratios are compared carefully. The calculated results indicate that the condition of CH4:CeO2 ¼ 0.5 is favorable for oxygen release, fuel selectivity and methane conversion. The introduction of methane could increase the maximum yield of H2, and more solar energy could be converted to chemical energy as the increase of nH2O:nCeO2. nH2O:nCeO2 ¼ 0.5, Tred ¼ 1400 K and Toxi ¼ 750 K are suggested for the maximum nonisothermal hsolar-to-fuel of 0.35, which is larger than the maximum isothermal hsolar-to-fuel of 0.24. The result shows that non-isothermal solar thermochemical ceria-methane cycle is more feasible for fuel production. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Solar thermochemical cycle Solar fuels Isothermal operation Non-isothermal operation Energy conversion efficiency Ceria
1. Introduction Solar fuel is of great significance in solving environmental pollution and rapid depletion of fossil resources [1,2]. Nowadays, the production methods of solar fuel rely on photochemical, electrochemical and thermochemical technologies. Among above methods, the full spectrum of sunlight can be utilized in solar thermochemical process [3]. Typically, two-step redox processes for splitting H2O/CO2 contains thermal reduction step and watersplitting step respectively. In the thermal reduction step, oxygen is released from high valence state of metal oxides (MOx), leading to reduced material (MOx-d), which could be seen in Eq. (1).
* Corresponding author. ** Corresponding author. Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, 100190, China. E-mail addresses: [email protected] (M. Fu), [email protected] (X. Li). https://doi.org/10.1016/j.renene.2019.05.047 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
d
MOx / MOxd þ O2 2
(1)
Then, MOx-d is oxidized to MOx by taking oxygen from water, leading to the production of H2.
MOxd þ dH2 O/dH2 þ MOx
(2)
Since the performance of metal oxide is significant for solar thermochemical cycle, a lot of redox pairs have been reported in recent years, such as Fe3O4/FeO [4], ZnO/Zn [5], CeO2/Ce2O3 [6] and so on. Ceria is attractive as the stable cubic fluorite structure [7,8], the rapid reaction kinetics and oxygen diffusion rates [9]. Because the thermal stability and high efficiency, ceria has become the benchmark of new material for the thermochemical cycle [10]. Steinfeld et al. [11] experimentally demonstrated that hsolar-to-fuel could reach 5.25% without heat recovery using ceria. Despite of above advantages, ceria still suffers from a relatively high reduction temperature (e.g. exceeding 1773 K), depending on the inert atmosphere [12]. Element doping is often used to affect the thermodynamic
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Nomenclature
hsolardto-fuel The energy conversion efficiency of solar energy to
Tred Toxi Qtotal
Cp Df H
Temperature of the thermal reduction step, K Temperature of the water-splitting step, K The energy required for the solar thermochemical cycle based on ceria, kJ Qred The energy required for the reduction step, kJ neq Equilibrium amount of substance, mol nini Initial amount of substance, mol QH2O-heating The energy required for heating water, kJ Qsolarreactor Solar energy required for the solar reactor, kJ HHV Higher heating value, kJ/mol cCH4 The conversion ratio of methane
properties of materials [13]. The solid solution of ceria-zirconia was proposed to decrease the temperature [14,15]. However, the oxidation rate would decrease [16]. Another approach to decrease the reduction temperature is using carbon or methane as reducing agents [17,18]. The introduction of methane enables the reactions to perform under isothermal or near isothermal condition, as the reaction temperature of thermal reduction step decreases greatly [19]. The isothermal operation eliminates the need for solid-solid heat recuperation and reduces the time of reheating and cooling. Otsuka et al. [20] first investigated the partial oxidation of methane into synthesis gas using ceria without solar energy in 1993. They demonstrated that the use of ceria enables the direct conversion of methane into synthesis (H2/CO ¼ 2). Jeong et al. [21] investigated how the operating temperature and the operating time influence the carbon deposition for methane reforming by using ceria redox system. The operating temperature of the isothermal cycle was 1073 K. Warren et al. [22] theoretically and experimentally studied the solar methane reforming over ceria. Experiment was accomplished in a packed bed type solar reactor, and the extrapolated hsolar-to-fuel was 9.82%. The result indicated that isothermal operation was thermodynamically favorable. Krenzke et al. [23] studied the process to produce syngas using solar energy coupled the partial oxidation of methane and the ceria redox cycle. The solar-to-fuel thermal efficiency was 27% for cycling at 1000 C. Nair et al. [24] studied the methane induced fuel production via solar thermochemical redox cycles, and the experiment was performed in a packed bed solar reactor. Fosheim et al. [25] demonstrated a high efficiency methane reforming with ceria in a fixedbed reactor. The hsolar-to-fuel was 7% and the thermal efficiency was 25% in the near-isothermal condition. Nowadays, many ceria-methane thermochemical cycles are studied under isothermal condition. It is still uncertain whether isothermal condition or non-isothermal condition is more advantageous for the ceria-methane thermochemical cycle. Previous work is performed under different conditions, but the best experimental condition of the ceria-methane thermochemical cycle has not been determined. Moreover, the effect of methane on hydrogen production has not been studied. In the present work, isothermal and non-isothermal hsolar-to-fuel under different temperatures and reactant ratios are compared carefully. 2. Model description In the whole thermochemical process, solar energy is used to drive the reaction and thus no fossil energy would be consumed. The process flow is shown in Fig. 1 which is consisted of two steps, the thermal reduction step and the water-splitting step. In the thermal reduction step, CeO2 is reduced by CH4, producing CeO2-d,
s hab To
d MOx-d MOx S Rred U
chemical energy stored in fuels Specific heat capacity, kJ/(mol$K) Standard molar enthalpy of formation, kJ/mol Stenfan-Boltzmann constant, W/(m2$K4) Solar reactor absorption efficiency Ambient temperature, K Oxygen nonstoichiometry Reduced metal oxide Oxidized metal oxide Production selectivity The ratio of CH4:CeO2 Energetic upgrade factor
Fig. 1. Process flow of solar-aided thermochemical cycles based on CeO2 coupled to CH4. Concentrated sunlight provides the driving force for the production of syngas.
H2 and CO simultaneously. In the water-splitting step, CeO2-d is recovered to CeO2 again. Thermodynamic analysis has been widely used in solving energy issues. In the present work, the thermodynamic and equilibrium composition calculations are performed with the help of commercial thermodynamic HSC Chemistry software [26]. All calculations are based on the following assumptions: C The reaction is performed at the standard atmosphere pressure. C The solar reactor is assumed to be an insulated blackbody absorber. C Without heat recovery. C No kinetic limits. C Steady state operation. C The products could separate naturally without consuming work. The products in the thermal reduction step mainly include CH4, CO, CO2, C, H2 and CeO2-d. The solid C and CeO2-d are easily separated. After the calculation, we find that CH4 could be completely converted. CO and H2 constitute syngas and do not need separate. The solar reactor is assumed to be an insulated blackbody absorber, and this assumption is also used in literature [26,27]. In addition, we also assume that there are not kinetic limits. In this work, the main target is to study the thermodynamic performance, other than the kinetic performance. In addition, some experiments also showed that the kinetic performance of ceria is very good [11]. As for the thermal reduction step, it is performed in the solar reactor and the solar reactor absorption efficiency (hab) is defined by the following equation:
T. Ma et al. / Renewable Energy 143 (2019) 915e921
hab ¼ 1
s Th 4 T0 4
SCO ¼ (3)
IC
The basic parameters are shown in Table 1. The thermal reduction reaction is endothermic, and the driving force is provided mainly by solar energy. However, a part of methane’s own energy offsets some of the solar energy consumption. The net energy required for the reduction step is represented by the following:
Q red ¼
X 0 oxi ni Df HTi red nCH4 ;ini DHTCH nCeO2 ;ini DHTCeO 4 2
(4)
i
ni and DfHi are the equilibrium molar quantity and standard molar enthalpy of formation for any possible products at the reduction step. Based on Qred and hab, solar energy required for the solar reactor (Qsolarreactor) could be expressed by the following:
Q solarreactor ¼
Q red
hab
¼
Q red 1
(5)
sðT4h T40 Þ IC
Besides Qsolar-reactor, the energy induced by water heating should also be taken into account in the water-splitting step and QH2Oheating could be calculated via the following expression: Tð oxi
Q H2 Oheating ¼ nH2 O
Cp ;H2 O ðTÞdT
(6)
T0
In view of above, the total energy required to run the CeO2based solar thermochemical cycle is estimated as:
Q total ¼ Q solarreactor þQ H2 Oheating
(7)
Energetic upgrade factor, U, which is usually used to evaluate the quality of solar thermochemical system, is denoted by the following:
U¼
nCO HHVCO þ nH2 HHVH2 nCH4 ;ini HHVCH4
(8)
It is clear that energy would be stored as U > 1. The energy conversion extent of our system is expressed by hsolar-to-fuel, which is defined as:
hsolartofuel ¼
ðU 1ÞHHVCH4 nCH4 ;ini Q total
nCO;eq nCH4 ;ini
SCO2 ¼
SC ¼
nCO2 ;eq nCH4 ;ini
nC;eq nCH4 ;ini
cCH4 ¼ 1
nCH4 ;eq nCH4 ;ini
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(11)
(12)
(13)
(14)
3. Results and discussion 3.1. Effects of reactant ratio on thermal reduction and watersplitting steps Due to the non-stoichiometric character of CeO2, the direct product in the thermal reduction step is CeO2-d where d 0.5. The ultimate value of d is 0.5, making CeO2-d being Ce2O3. We assume that the initial amount of CeO2 is 2 mol. As seen in Fig. 2, the reduction temperature would drop 1600 K by replacing CeO2¼CeO2-dþd/2O2 with CeO2þdCH4 ¼ CeO2-dþ2dH2þdCO. The evident cooling effect makes the reaction could perform at lower temperature. In the present work, we study the effects of temperature and reactant ratios for the thermochemical ceria-methane cycle. For the CeO2-based solar thermochemical cycle, the upper limit of hydrogen production during the water-splitting step depends on the oxygen releasing extent in the thermal reduction step. Since d could reflect how many oxygen atoms are lost per mole of CeO2, the relationships between 2-d and Rred/Tred are studied and shown in Fig. 3. When 2-d is 1.5, Ce2O3 could be produced and the largest oxygen releasing extent is achieved. It is clear that the amount of oxygen released is small as Rred ¼ 0.125 and Rred ¼ 0.25, indicating that Rred ¼ 0.125 and 0.25 are not suitable for the thermochemical ceria-methane cycle. For the thermal reduction step, not only CO and H2 are
(9)
In addition, the production selectivity and conversion ratio of methane are defined by the following:
SH2 ¼
nH2 ;eq 2nCH4 ;ini
(10)
Table 1 Basic parameters required for the calculation of solar reactor absorption efficiency. Parameter
Value
I C T0
1000 W/m2 3000 suns 298 K 5.67 10-8 W/(m2$K4)
s
Fig. 2. Free Gibbs energy change (DG) of CeO2¼CeO2-dþd/2O2 and CeO2þdCH4 ¼ CeO2dþ2dH2þdCO reactions with respect to temperature as d is 0.5.
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Fig. 3. Variations of 2-d with respect to Rred and Tred. Rred is the ratio of CH4:CeO2 at the thermal reduction step. Tred is the temperature of thermal reduction step.
produced, but also some by-products are produced. To further determine the most suitable Rred, variations of product selectivities are calculated and presented in Fig. 4. As seen in Fig. 4(a), SH2 shows similar profile as Rred is 0.5, 1, 2 and 4. According to the definition of SH2 ¼ nH2,eq/2nCH4,ini, it means that almost all of the hydrogen atoms in CH4 are converted to hydrogen as the value of SH2 is close to 1. For the selectivity of CO (SCO), there is a clear positive correlation between SCO and Tred, as illustrated in Fig. 4(b). High temperature is favorable for the enhancement of SCO. Only at Rred ¼ 0.5, the value of SCO is close to 1, indicating that the carbon atoms of CH4 is almost fully converted to CO there. In addition, even if Tred is 1300 K, the selectivity could also be as high as 0.9. Therefore, we believe that the condition of Rred ¼ 0.5 is beneficial to the production of CO. Variations of SC and SCO2 are further studied. As seen in Fig. 4(b)(d), SC decreases correspondingly as the increase of SCO. It could be explained by the following reasons. The amount of CH4 introduced is constant, and part of carbon element in CH4 changes to CO whereas the other part of carbon element in CH4 converts to CO2
Fig. 4. Variations of (a) SH2, (b) SCO, (c) SC and (d) SCO2 with respect to Tred and Rred.
and C. However, the effect of CO2 could be ignored because SCO2 is almost zero at the given temperature range, as illustrated in Fig. 4(d). Since solid C always leads to the deactivation of the metal oxide, preventing the production of solid C is usually required [27]. It could be clearly seen that high temperature is conducive to reduce the production of solid C. From the perspective of selectivity of all productions, only Rred ¼ 0.5 contributes to obtaining high SH2, SCO and low SC. The conversion ratio of methane, cCH4, is also studied, which could display the degree of conversion at different Tred and Rred. Fig. 5 shows that cCH4 is not sensitive to Rred at any specific temperature, and more CH4 is expected to be converted. It could be seen that the value of cCH4 is close to 1 as Tred>1340 K and Rred ¼ 0.5. Additionally, even at the temperature of 1100 K, the corresponding conversion ratio could reach 0.93 as Rred ¼ 0.5. In the thermal reduction step, we study the effect of Tred and Rred on the value of d, SCO, SH2, SCO2, SC and cCH4. It is expected that more CH4 could convert to CO and H2, and fewer by-products are produced. With the overall considerations, Rred ¼ 0.5 is chosen for our subsequent calculations. As for the water-splitting step (CeO2-dþdH2O/dH2þCeO2), CeO2 and H2 are the two direct products. The recovery of CeO2 is measured via the calculation of 2-d similarly. 2-d approaching to 2 means CeO2-d could recover completely in the water-splitting step. The main influence factors are the ratio of nH2O:nCeO2 and Toxi. The value of 2-d is positively correlated with the recovery of ceria, and the maximum of 2 indicates that the ceria is recovered completely. However, as seen in Table 2, the maximum 2-d is 1.77 as nH2O:nCeO2 is 0.25, meaning that ceria is not recovery completely. For the water-splitting step, lower temperature and larger nH2O:nCeO2 is favorable. Ceria has a good recovery state between the range of 600 K and 800 K as nH2O:nCeO2 is 0.5, 1 and 2. Even if the watersplitting temperature is 1400 K, the value of 2-d still could reach 1.93 as nH2O:nCeO2 is 2. Since heating water requires additional energy, the best condition of nH2O:nCeO2 is further studied. We compare the maximum yield of H2 that could be produced with and without methane at different thermal reduction temperature. For the reduction reaction without methane (CeO2¼CeO2dþd/2O2), decreasing the partial pressure of oxygen is usually beneficial for the decrease of reaction temperature, so we also calculate the maximum H2 yield under different partial pressures of
Fig. 5. Conversion ratio of methane, cCH4, at the thermal reduction step with respect to Tred and Rred.
T. Ma et al. / Renewable Energy 143 (2019) 915e921 Table 2 The effect of Toxi and nH2O:nCeO2 for the value of 2-d as Tred ¼ 1400 K. Toxi (K)
nH2O:nCeO2¼0.25
nH2O:nCeO2¼0.5
nH2O:nCeO2¼1
nH2O:nCeO2¼2
600 700 800 900 1000 1100 1200 1300 1400 1500 1600
1.77 1.77 1.77 1.77 1.76 1.76 1.75 1.73 1.71 1.69 1.67
2.00 2.00 2.00 1.99 1.97 1.93 1.89 1.85 1.80 1.76 1.73
2.00 2.00 2.00 2.00 2.00 1.99 1.96 1.93 1.88 1.83 1.79
2.00 2.00 2.00 2.00 2.00 2.00 1.99 1.97 1.93 1.89 1.85
oxygen. For the reduction reaction with methane (CeO2þdCH4 ¼ CeO2-dþ2dH2þdCO), we do not consider the impact of partial pressure of oxygen as the reaction temperature is not very high. As seen in Fig. 6, the maximum yield of H2 is close to 1 mol at different temperature for the methane reduction reaction. However, the maximum yield of H2 is 0.5 mol as the oxygen partial pressure is 10-10 bar for the reaction without methane, and lower oxygen partial pressure is always difficult to reach. The yield of H2 is positively correlative with the reaction temperature for the same oxygen partial pressure. It could be concluded that the introduction of methane could increase the yield of H2.
3.2. Performance assessment based on energetic upgrade factors Although analyzing the amounts of products is a simple and practical method, it could not reflect the performance of the whole system. Therefore, it is meaningful to study the energetic upgrade factor (U) and energy conversion efficiency. The energetic upgrade factor is the ratio of the high heat value of the exit to the high heat value of the CH4 feedstock [25]. The initial amount of ceria is 2 mol and Rred ¼ 0.5. Fig. 7 shows the variations of U as the change of Tred and Toxi. As seen in Fig. 7(a), there is no corresponding temperature region of U > 1.20 as nH2O:nCeO2 is 0.25. This phenomenon is caused by the insufficient water. U ¼ 1.20 means that extra 20% energy could be
Fig. 6. The maximum yield of H2 with (dashed line) or without (solid line) CH4 under different Tred. Here PO2 presents the oxygen partial pressure.
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stored. The temperature region corresponding to U > 1.20 is very large when nH2O:nCeO2 is 0.5, 1 or 2, and it means that more energy could be stored. As seen in Fig. 7(b), within the entire thermal reduction region, the value of U is positively correlative with Tred. Higher thermal reduction temperature is beneficial for the increase of U. However, the increasing rate of U becomes slowly as the increase of Tred. For the same Tred, the value of U decreases as the increase of Toxi. It could be explained that larger swing of temperature is favorable for the promotion of U. The region of U > 1 become larger as the increase of nH2O:nCeO2. The value of U even could reach 1.24 under the condition of Tred >1340 K and nH2O:nCeO2 ¼ 0.5. There are no apparent changes of U in Fig. 7(bed), it could be concluded that the dependence between nH2O:nCeO2 and U is gradually reduced and the condition of nH2O:nCeO2 ¼ 0.25 is not suitable for the increase of U. For the isothermal reaction, the value of U is positively correlated with the increase of nH2O:nCeO2 as the temperature is 1300 K. The value of U is between 1.08 and 1.12 at nH2O:nCeO2 ¼ 0.25. The value of U is between 1.12 and 1.16 at nH2O:nCeO2 ¼ 0.5. The value of U is between 1.16 and 1.2 at nH2O:nCeO2 ¼ 1. The value of U is between 1.12 and 1.24 at nH2O:nCeO2 ¼ 2. It is clear that more energy could be stored as the increase of nH2O:nCeO2 for the isothermal cycle. 3.3. Evaluation of solar-to-fuel efficiency Based on the previous analysis, Rred ¼ 0.5 is one of the best choice for the thermal reduction step. Therefore, the calculation of hsolar-fuel is based on the condition of Rred ¼ 0.5. The variations of hsolar-to-fuel at Rred ¼ 0.5 is shown in Fig. 8. The best hsolar-to-fuel in Fig.(a)-(d) are 0.23, 0.35, 0.31 and 0.27 under different conditions of nH2O:nCeO2 ¼ 0.25, nH2O:nCeO2 ¼ 0.5, nH2O:nCeO2 ¼ 1 and nH2O:nCeO2 ¼ 2 respectively, and the hsolar-to-fuel of 0.23 is the minimum as nH2O:nCeO2 is 0.25. It could be explained that the amount of water is not enough for this thermochemical cycle. The analysis of 2-d and U has shown the similar result. Therefore, we could believe that the condition of nH2O:nCeO2 ¼ 0.25 is not suitable for the thermochemical cycle. Fig. 8(b) shows the change of hsolar-to-fuel as nH2O:nCeO2 is 0.5. For the reaction of Ce2O3þ H2O¼H2þ2CeO2, the ratio of nH2O:nCeO2 required for the complete reaction is also 0.5. For the nonisothermal reaction, the maximum hsolar-to-fuel is 0.35 as Tred ¼ 1400 K and Toxi ¼ 750 K. As previous analysis, the value of SH2, SCO, cCH4 and U almost reach the optimal condition as Tred ¼ 1400 K. Meanwhile, the condition of Toxi ¼ 750 K is also favorable for the water-splitting step. As seen in Fig. 8(c), the maximum hsolar-to-fuel is 0.24 as T ¼ 1250 K. As previous analysis, higher temperature could promote the thermal reduction step, but it is not beneficial for the water-splitting step. Experiment also confirms that higher temperature could cause larger heat loss [28]. However, low temperature is not benefit to the recovery of ceria. As seen in Fig. 8(b)e(d), it is clear that the maximum hsolar-to-fuel would decrease as the increase of nH2O:nCeO2. This could be explained by Eq. (7) and Eq. (9). The energy could be additional consume as the amount of water is excessive. It could be found that non-isothermal condition is more suitable for the thermochemical ceria-methane cycle to pursue higher efficiency. As nH2O:nCeO2 is 0.25 or 2, the corresponding solar-to-fuel efficiencies are 0.23 and 0.27 respectively, and those values do not exceed 30%. Therefore, we only calculate the hsolar-to-fuel of the second cycle as nH2O:nCeO2 ¼ 0.5 and nH2O:nCeO2 ¼ 1. As seen in Fig. 9(a), it is clear that the maximum non-isothermal hsolar-to-fuel of the second cycle is 0.36 as nH2O:nCeO2 ¼ 0.5, which is a little larger than the corresponding value of 0.35 in the first cycle.
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Fig. 7. The contour plot of energetic upgrade factor (U) with respect to Tred and Toxi, at Rred ¼ 0.5 and (a) nH2O:nCeO2 ¼ 0.25, (b) nH2O:nCeO2 ¼ 0.5, (c) nH2O:nCeO2 ¼ 1, and (d) nH2O:nCeO2 ¼ 2.
Fig. 8. Variations of hsolar-fuel with respect to Tred and Toxi at Rred ¼ 0.5 and (a) nH2O:nCeO2 ¼ 0.25, (b) nH2O:nCeO2 ¼ 0.5, (c) nH2O:nCeO2 ¼ 1 and (d) nH2O:nCeO2 ¼ 2 for the first cycle, and the red dashed line represents the isotherm.
The maximum isothermal hsolar-to-fuel in the second cycle is 0.22 as nH2O:nCeO2 ¼ 1, and it is slightly smaller than the corresponding value of 0.24 in the first cycle. Comparing the maximum nonisothermal and isothermal hsolar-to-fuel of the first and second cycles, it could be found that there are not obvious differences for the maximum hsolar-to-fuel.
4. Conclusion In sum, thermodynamic calculations are studied based on methane reduction of CeO2/CeO2-d for the production of CO and H2. The hsolar-to-fuel is compared under isothermal and non-isothermal conditions. The calculated results indicate that Rred ¼ 0.5 is favorable for the
T. Ma et al. / Renewable Energy 143 (2019) 915e921
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Fig. 9. Variations of hsolar-to-fuel with respect to Tred and Toxi at Rred ¼ 0.5 and (a) nH2O:nCeO2 ¼ 0.5 (b) nH2O:nCeO2 ¼ 1 in the second cycle, and the red dashed line represents the isothermal reaction.
oxygen release, fuel selectivity and methane conversion. After the calculation of 2-d, we find that Rred ¼ 0.125 and Rred ¼ 0.25 are not benefit for the oxygen release of ceria. The H2 and CO production selectivities are more than 90% simultaneously as Rred ¼ 0.5. The conversion ratios of CH4 at Rred ¼ 0.5 are all more than 93% at our given temperature range of 1100e1600 K. In the water-splitting step, CeO2 could be fully recovered at typical experimental temperature range of 600e800 K, and the maximum yield of H2 could increase to 1 mol after the introduction of methane. The value of U is positively correlative with H2O:nCeO2, and nearly extra 20% energy could be stored as T ¼ 1300 K and nH2O:nCeO2 ¼ 2. As nH2O:nCeO2 ¼ 0.5, Tred ¼ 1400 K and Toxi ¼ 750 K, the maximum non-isothermal hsolar-to-fuel is 0.35, which is larger than maximum isothermal hsolar-to-fuel of 0.24 at nH2O:nCeO2 ¼ 1 and T ¼ 1250 K. For the second cycle, there are not obvious differences for the maximum hsolar-to-fuel. It could be found that the non-isothermal reaction has great potential to reach higher hsolar-to-fuel for the solar thermochemical ceria-methane cycle. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant no. 51806209, 51476163), the National Key Research and Development Program of China (Grant no. 2018YFB1502005), Chinese Academy of Sciences President’s International Fellowship Initiative (Grant no.2019VEB0001) and Institute of Electrical Engineering, Chinese Academy of Sciences (Grant no. Y770111CSC). References [1] V. Dasireddy, B. Likozar, Activation and decomposition of methane over Co, Cu, and Fe-based heterogeneous catalysts for COx-free hydrogen and multiwalled carbon nanotubes production, Energy Technol. 5 (8) (2017) 1344e1355. [2] A. Obradovic, B. Likozar, J. Levec, Steam methane reforming over ni-based pellet-type and Pt/Ni/Al2O3 structured plate-type catalyst: intrinsic kinetics study, Ind. Chem. Res. 52 (38) (2013) 13597e13606. [3] G.P. Smestad, A. Steinfeld, Review: photochemical and thermochemical production of solar fuels from H2O and CO2 using metal oxide catalysts, Ind. Chem. Res. 51 (37) (2012) 11828e11840. [4] T. Nakamura, Hydrogen production from water utilizing solar heat at high temperatures, Sol. Energy 19 (5) (1977) 467e475. [5] P. Haueter, S. Moeller, R. Palumbo, A. Steinfeld, The production of zinc by thermal dissociation of zinc oxide - solar chemical reactor design, Sol. Energy 67 (1e3) (1999) 161e167. [6] S. Abanades, G. Flamant, Thermochemical hydrogen production from a twostep solar-driven water-splitting cycle based on cerium oxides, Sol. Energy 80 (12) (2006) 1611e1623. [7] C.W. Sun, H. Li, L.Q. Chen, Nanostructured ceria-based materials: synthesis, properties, and applications, Energy Environ. Sci. 5 (9) (2012) 8475e8505. [8] M. Mogensen, N.M. Sammes, G.A. Tompsett, Physical, chemical and electrochemical properties of pure and doped ceria, Solid State Ionics 129 (1) (2000)
63e94. [9] C.L. Muhich, B.D. Ehrhart, I. Al-Shankiti, B.J. Ward, C.B. Musgrave, A.W. Weimer, A review and perspective of efficient hydrogen generation via solar thermal water splitting, Wires Energy Environ. 5 (3) (2016) 261e287. [10] M. Kubicek, A.H. Bork, J.L.M. Rupp, Perovskite oxides - a review on a versatile material class for solar-to-fuel conversion processes, J. Mater. Chem. A. 5 (24) (2017) 11983e12000. [11] D. Marxer, P. Furler, M. Takacs, A. Steinfeld, Solar thermochemical splitting of CO2 into separate streams of CO and O2 with high selectivity, stability, conversion, and efficiency, Energy Environ. Sci. 10 (5) (2017) 1142e1149. [12] W.C. Chueh, S.M. Haile, A thermochemical study of ceria: exploiting an old material for new modes of energy conversion and CO2 mitigation, Philos. T. R. Soc. A 368 (1923) (2010) 3269e3294. [13] Q.-L. Meng, Y. Tamaura, Enhanced hydrogen production by doping Pr into Ce0.9Hf0.1O2 for thermochemical two-step water-splitting cycle, J. Phys. Chem. Solids 75 (3) (2014) 328e333. [14] S. Abanades, A. Legal, A. Cordier, G. Peraudeau, G. Flamant, A. Julbe, Investigation of reactive cerium-based oxides for H2 production by thermochemical two-step water-splitting, J. Mater. Sci. 45 (15) (2010) 4163e4173. [15] A. Le Gal, S. Abanades, Catalytic investigation of ceria-zirconia solid solutions for solar hydrogen production, Int. J. Hydrogen Energy 36 (8) (2011) 4739e4748. [16] S. Chuayboon, S. Abanades, S. Rodat, Syngas production via solar-driven chemical looping methane reforming from redox cycling of ceria porous foam in a volumetric solar reactor, Chem. Eng. J. 356 (2019) 756e770. [17] G. Leveque, S. Abanades, Thermodynamic and kinetic study of the carbothermal reduction of SnO2 for solar thermochemical fuel generation, Energy Fuel. 28 (2) (2014) 1396e1405. [18] S. Kraupl, U. Frommherz, C. Wieckert, Solar carbothermic reduction of ZnO in a two-cavity reactor: laboratory experiments for a reactor scale-up, J. Sol. Energy T. Asme 128 (1) (2006) 8e15. [19] P.T. Krenzke, J.H. Davidson, Thermodynamic analysis of syngas production via the solar thermochemical cerium oxide redox cycle with methane-driven reduction, Energy Fuel. 28 (6) (2014) 4088e4095. [20] K. Otsuka, T. Ushiyama, I. Yamanaka, Partial oxidation of methane using the redox of cerium oxide, Chem. Lett. (9) (1993) 1517e1520. [21] H.H. Jeong, J.H. Kwak, G.Y. Han, K.J. Yoon, Stepwise production of syngas and hydrogen through methane reforming and water splitting by using a cerium oxide redox system, Int. J. Hydrogen Energy 36 (23) (2011) 15221e15230. [22] K.J. Warren, J. Reim, K. Randhir, B. Greek, R. Carrillo, D. Hahn, J.R. Scheffe, Theoretical and experimental investigation of solar methane reforming through the nonstoichiometric ceria redox cycle, Energy Technol. 5 (11) (2017) 2138e2149. [23] P.T. Krenzke, J.R. Fosheim, J. Zheng, J.H. Davidson, Synthesis gas production via the solar partial oxidation of methane-ceria redox cycle: conversion, selectivity, and efficiency, Int. J. Hydrogen Energy 41 (30) (2016) 12799e12811. [24] M.M. Nair, S. Abanades, Tailoring hybrid nonstoichiometric ceria redox cycle for combined solar methane reforming and thermochemical conversion of H2O/CO2, Energy Fuel. 30 (7) (2016) 6050e6058. [25] J.R. Fosheim, B.J. Hathaway, J.H. Davidson, High efficiency solar chemicallooping methane reforming with ceria in a fixed-bed reactor, Energy 169 (2019) 597e612. [26] A. Roine, Outokumpu HSC Chemistry for Windows, OutokumpuResearch Oy, Pori, Finland, 2013, Version 7.1. [27] A. Obradovi c, B. Likozar, J. Levec, Catalytic surface development of novel nickel plate catalyst with combined thermally annealed platinum and alumina coatings for steam methane reforming, Int. J. Hydrogen Energy 38 (3) (2013) 1419e1429. [28] P. Furler, A. Steinfeld, Heat transfer and fluid flow analysis of a 4 kW solar thermochemical reactor for ceria redox cycling, Chem. Eng. Sci. 137 (2015) 373e383.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
A comparative thermodynamic analysis of isothermal and nonisothermal CeO2-based solar thermochemical cycle with methanedriven reduction Tianzeng Ma a, b, Lei Wang a, b, Chun Chang a, Jasurjon S. Akhatov c, Mingkai Fu a, *, Xin Li a, b, ** a b c
Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, 100190, China University of Chinese Academy of Sciences, Beijing, 100049, China Physical-Technical Institute, SPA “Physics-Sun”, Tashkent, 100084, Uzbekistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 November 2018 Received in revised form 1 May 2019 Accepted 12 May 2019 Available online 16 May 2019
Induced by promising hydrogen production of CeO2-based solar thermochemical cycle and evident temperature decreasing effect of methane reduction, a moderately high-temperature solar thermochemical ceria-methane cycle is investigated thermodynamically. In this paper, isothermal and nonisothermal solar-to-fuel efficiencies (hsolar-to-fuel) under different temperatures and reactant ratios are compared carefully. The calculated results indicate that the condition of CH4:CeO2 ¼ 0.5 is favorable for oxygen release, fuel selectivity and methane conversion. The introduction of methane could increase the maximum yield of H2, and more solar energy could be converted to chemical energy as the increase of nH2O:nCeO2. nH2O:nCeO2 ¼ 0.5, Tred ¼ 1400 K and Toxi ¼ 750 K are suggested for the maximum nonisothermal hsolar-to-fuel of 0.35, which is larger than the maximum isothermal hsolar-to-fuel of 0.24. The result shows that non-isothermal solar thermochemical ceria-methane cycle is more feasible for fuel production. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Solar thermochemical cycle Solar fuels Isothermal operation Non-isothermal operation Energy conversion efficiency Ceria
1. Introduction Solar fuel is of great significance in solving environmental pollution and rapid depletion of fossil resources [1,2]. Nowadays, the production methods of solar fuel rely on photochemical, electrochemical and thermochemical technologies. Among above methods, the full spectrum of sunlight can be utilized in solar thermochemical process [3]. Typically, two-step redox processes for splitting H2O/CO2 contains thermal reduction step and watersplitting step respectively. In the thermal reduction step, oxygen is released from high valence state of metal oxides (MOx), leading to reduced material (MOx-d), which could be seen in Eq. (1).
* Corresponding author. ** Corresponding author. Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, 100190, China. E-mail addresses: [email protected] (M. Fu), [email protected] (X. Li). https://doi.org/10.1016/j.renene.2019.05.047 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
d
MOx / MOxd þ O2 2
(1)
Then, MOx-d is oxidized to MOx by taking oxygen from water, leading to the production of H2.
MOxd þ dH2 O/dH2 þ MOx
(2)
Since the performance of metal oxide is significant for solar thermochemical cycle, a lot of redox pairs have been reported in recent years, such as Fe3O4/FeO [4], ZnO/Zn [5], CeO2/Ce2O3 [6] and so on. Ceria is attractive as the stable cubic fluorite structure [7,8], the rapid reaction kinetics and oxygen diffusion rates [9]. Because the thermal stability and high efficiency, ceria has become the benchmark of new material for the thermochemical cycle [10]. Steinfeld et al. [11] experimentally demonstrated that hsolar-to-fuel could reach 5.25% without heat recovery using ceria. Despite of above advantages, ceria still suffers from a relatively high reduction temperature (e.g. exceeding 1773 K), depending on the inert atmosphere [12]. Element doping is often used to affect the thermodynamic
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T. Ma et al. / Renewable Energy 143 (2019) 915e921
Nomenclature
hsolardto-fuel The energy conversion efficiency of solar energy to
Tred Toxi Qtotal
Cp Df H
Temperature of the thermal reduction step, K Temperature of the water-splitting step, K The energy required for the solar thermochemical cycle based on ceria, kJ Qred The energy required for the reduction step, kJ neq Equilibrium amount of substance, mol nini Initial amount of substance, mol QH2O-heating The energy required for heating water, kJ Qsolarreactor Solar energy required for the solar reactor, kJ HHV Higher heating value, kJ/mol cCH4 The conversion ratio of methane
properties of materials [13]. The solid solution of ceria-zirconia was proposed to decrease the temperature [14,15]. However, the oxidation rate would decrease [16]. Another approach to decrease the reduction temperature is using carbon or methane as reducing agents [17,18]. The introduction of methane enables the reactions to perform under isothermal or near isothermal condition, as the reaction temperature of thermal reduction step decreases greatly [19]. The isothermal operation eliminates the need for solid-solid heat recuperation and reduces the time of reheating and cooling. Otsuka et al. [20] first investigated the partial oxidation of methane into synthesis gas using ceria without solar energy in 1993. They demonstrated that the use of ceria enables the direct conversion of methane into synthesis (H2/CO ¼ 2). Jeong et al. [21] investigated how the operating temperature and the operating time influence the carbon deposition for methane reforming by using ceria redox system. The operating temperature of the isothermal cycle was 1073 K. Warren et al. [22] theoretically and experimentally studied the solar methane reforming over ceria. Experiment was accomplished in a packed bed type solar reactor, and the extrapolated hsolar-to-fuel was 9.82%. The result indicated that isothermal operation was thermodynamically favorable. Krenzke et al. [23] studied the process to produce syngas using solar energy coupled the partial oxidation of methane and the ceria redox cycle. The solar-to-fuel thermal efficiency was 27% for cycling at 1000 C. Nair et al. [24] studied the methane induced fuel production via solar thermochemical redox cycles, and the experiment was performed in a packed bed solar reactor. Fosheim et al. [25] demonstrated a high efficiency methane reforming with ceria in a fixedbed reactor. The hsolar-to-fuel was 7% and the thermal efficiency was 25% in the near-isothermal condition. Nowadays, many ceria-methane thermochemical cycles are studied under isothermal condition. It is still uncertain whether isothermal condition or non-isothermal condition is more advantageous for the ceria-methane thermochemical cycle. Previous work is performed under different conditions, but the best experimental condition of the ceria-methane thermochemical cycle has not been determined. Moreover, the effect of methane on hydrogen production has not been studied. In the present work, isothermal and non-isothermal hsolar-to-fuel under different temperatures and reactant ratios are compared carefully. 2. Model description In the whole thermochemical process, solar energy is used to drive the reaction and thus no fossil energy would be consumed. The process flow is shown in Fig. 1 which is consisted of two steps, the thermal reduction step and the water-splitting step. In the thermal reduction step, CeO2 is reduced by CH4, producing CeO2-d,
s hab To
d MOx-d MOx S Rred U
chemical energy stored in fuels Specific heat capacity, kJ/(mol$K) Standard molar enthalpy of formation, kJ/mol Stenfan-Boltzmann constant, W/(m2$K4) Solar reactor absorption efficiency Ambient temperature, K Oxygen nonstoichiometry Reduced metal oxide Oxidized metal oxide Production selectivity The ratio of CH4:CeO2 Energetic upgrade factor
Fig. 1. Process flow of solar-aided thermochemical cycles based on CeO2 coupled to CH4. Concentrated sunlight provides the driving force for the production of syngas.
H2 and CO simultaneously. In the water-splitting step, CeO2-d is recovered to CeO2 again. Thermodynamic analysis has been widely used in solving energy issues. In the present work, the thermodynamic and equilibrium composition calculations are performed with the help of commercial thermodynamic HSC Chemistry software [26]. All calculations are based on the following assumptions: C The reaction is performed at the standard atmosphere pressure. C The solar reactor is assumed to be an insulated blackbody absorber. C Without heat recovery. C No kinetic limits. C Steady state operation. C The products could separate naturally without consuming work. The products in the thermal reduction step mainly include CH4, CO, CO2, C, H2 and CeO2-d. The solid C and CeO2-d are easily separated. After the calculation, we find that CH4 could be completely converted. CO and H2 constitute syngas and do not need separate. The solar reactor is assumed to be an insulated blackbody absorber, and this assumption is also used in literature [26,27]. In addition, we also assume that there are not kinetic limits. In this work, the main target is to study the thermodynamic performance, other than the kinetic performance. In addition, some experiments also showed that the kinetic performance of ceria is very good [11]. As for the thermal reduction step, it is performed in the solar reactor and the solar reactor absorption efficiency (hab) is defined by the following equation:
T. Ma et al. / Renewable Energy 143 (2019) 915e921
hab ¼ 1
s Th 4 T0 4
SCO ¼ (3)
IC
The basic parameters are shown in Table 1. The thermal reduction reaction is endothermic, and the driving force is provided mainly by solar energy. However, a part of methane’s own energy offsets some of the solar energy consumption. The net energy required for the reduction step is represented by the following:
Q red ¼
X 0 oxi ni Df HTi red nCH4 ;ini DHTCH nCeO2 ;ini DHTCeO 4 2
(4)
i
ni and DfHi are the equilibrium molar quantity and standard molar enthalpy of formation for any possible products at the reduction step. Based on Qred and hab, solar energy required for the solar reactor (Qsolarreactor) could be expressed by the following:
Q solarreactor ¼
Q red
hab
¼
Q red 1
(5)
sðT4h T40 Þ IC
Besides Qsolar-reactor, the energy induced by water heating should also be taken into account in the water-splitting step and QH2Oheating could be calculated via the following expression: Tð oxi
Q H2 Oheating ¼ nH2 O
Cp ;H2 O ðTÞdT
(6)
T0
In view of above, the total energy required to run the CeO2based solar thermochemical cycle is estimated as:
Q total ¼ Q solarreactor þQ H2 Oheating
(7)
Energetic upgrade factor, U, which is usually used to evaluate the quality of solar thermochemical system, is denoted by the following:
U¼
nCO HHVCO þ nH2 HHVH2 nCH4 ;ini HHVCH4
(8)
It is clear that energy would be stored as U > 1. The energy conversion extent of our system is expressed by hsolar-to-fuel, which is defined as:
hsolartofuel ¼
ðU 1ÞHHVCH4 nCH4 ;ini Q total
nCO;eq nCH4 ;ini
SCO2 ¼
SC ¼
nCO2 ;eq nCH4 ;ini
nC;eq nCH4 ;ini
cCH4 ¼ 1
nCH4 ;eq nCH4 ;ini
917
(11)
(12)
(13)
(14)
3. Results and discussion 3.1. Effects of reactant ratio on thermal reduction and watersplitting steps Due to the non-stoichiometric character of CeO2, the direct product in the thermal reduction step is CeO2-d where d 0.5. The ultimate value of d is 0.5, making CeO2-d being Ce2O3. We assume that the initial amount of CeO2 is 2 mol. As seen in Fig. 2, the reduction temperature would drop 1600 K by replacing CeO2¼CeO2-dþd/2O2 with CeO2þdCH4 ¼ CeO2-dþ2dH2þdCO. The evident cooling effect makes the reaction could perform at lower temperature. In the present work, we study the effects of temperature and reactant ratios for the thermochemical ceria-methane cycle. For the CeO2-based solar thermochemical cycle, the upper limit of hydrogen production during the water-splitting step depends on the oxygen releasing extent in the thermal reduction step. Since d could reflect how many oxygen atoms are lost per mole of CeO2, the relationships between 2-d and Rred/Tred are studied and shown in Fig. 3. When 2-d is 1.5, Ce2O3 could be produced and the largest oxygen releasing extent is achieved. It is clear that the amount of oxygen released is small as Rred ¼ 0.125 and Rred ¼ 0.25, indicating that Rred ¼ 0.125 and 0.25 are not suitable for the thermochemical ceria-methane cycle. For the thermal reduction step, not only CO and H2 are
(9)
In addition, the production selectivity and conversion ratio of methane are defined by the following:
SH2 ¼
nH2 ;eq 2nCH4 ;ini
(10)
Table 1 Basic parameters required for the calculation of solar reactor absorption efficiency. Parameter
Value
I C T0
1000 W/m2 3000 suns 298 K 5.67 10-8 W/(m2$K4)
s
Fig. 2. Free Gibbs energy change (DG) of CeO2¼CeO2-dþd/2O2 and CeO2þdCH4 ¼ CeO2dþ2dH2þdCO reactions with respect to temperature as d is 0.5.
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Fig. 3. Variations of 2-d with respect to Rred and Tred. Rred is the ratio of CH4:CeO2 at the thermal reduction step. Tred is the temperature of thermal reduction step.
produced, but also some by-products are produced. To further determine the most suitable Rred, variations of product selectivities are calculated and presented in Fig. 4. As seen in Fig. 4(a), SH2 shows similar profile as Rred is 0.5, 1, 2 and 4. According to the definition of SH2 ¼ nH2,eq/2nCH4,ini, it means that almost all of the hydrogen atoms in CH4 are converted to hydrogen as the value of SH2 is close to 1. For the selectivity of CO (SCO), there is a clear positive correlation between SCO and Tred, as illustrated in Fig. 4(b). High temperature is favorable for the enhancement of SCO. Only at Rred ¼ 0.5, the value of SCO is close to 1, indicating that the carbon atoms of CH4 is almost fully converted to CO there. In addition, even if Tred is 1300 K, the selectivity could also be as high as 0.9. Therefore, we believe that the condition of Rred ¼ 0.5 is beneficial to the production of CO. Variations of SC and SCO2 are further studied. As seen in Fig. 4(b)(d), SC decreases correspondingly as the increase of SCO. It could be explained by the following reasons. The amount of CH4 introduced is constant, and part of carbon element in CH4 changes to CO whereas the other part of carbon element in CH4 converts to CO2
Fig. 4. Variations of (a) SH2, (b) SCO, (c) SC and (d) SCO2 with respect to Tred and Rred.
and C. However, the effect of CO2 could be ignored because SCO2 is almost zero at the given temperature range, as illustrated in Fig. 4(d). Since solid C always leads to the deactivation of the metal oxide, preventing the production of solid C is usually required [27]. It could be clearly seen that high temperature is conducive to reduce the production of solid C. From the perspective of selectivity of all productions, only Rred ¼ 0.5 contributes to obtaining high SH2, SCO and low SC. The conversion ratio of methane, cCH4, is also studied, which could display the degree of conversion at different Tred and Rred. Fig. 5 shows that cCH4 is not sensitive to Rred at any specific temperature, and more CH4 is expected to be converted. It could be seen that the value of cCH4 is close to 1 as Tred>1340 K and Rred ¼ 0.5. Additionally, even at the temperature of 1100 K, the corresponding conversion ratio could reach 0.93 as Rred ¼ 0.5. In the thermal reduction step, we study the effect of Tred and Rred on the value of d, SCO, SH2, SCO2, SC and cCH4. It is expected that more CH4 could convert to CO and H2, and fewer by-products are produced. With the overall considerations, Rred ¼ 0.5 is chosen for our subsequent calculations. As for the water-splitting step (CeO2-dþdH2O/dH2þCeO2), CeO2 and H2 are the two direct products. The recovery of CeO2 is measured via the calculation of 2-d similarly. 2-d approaching to 2 means CeO2-d could recover completely in the water-splitting step. The main influence factors are the ratio of nH2O:nCeO2 and Toxi. The value of 2-d is positively correlated with the recovery of ceria, and the maximum of 2 indicates that the ceria is recovered completely. However, as seen in Table 2, the maximum 2-d is 1.77 as nH2O:nCeO2 is 0.25, meaning that ceria is not recovery completely. For the water-splitting step, lower temperature and larger nH2O:nCeO2 is favorable. Ceria has a good recovery state between the range of 600 K and 800 K as nH2O:nCeO2 is 0.5, 1 and 2. Even if the watersplitting temperature is 1400 K, the value of 2-d still could reach 1.93 as nH2O:nCeO2 is 2. Since heating water requires additional energy, the best condition of nH2O:nCeO2 is further studied. We compare the maximum yield of H2 that could be produced with and without methane at different thermal reduction temperature. For the reduction reaction without methane (CeO2¼CeO2dþd/2O2), decreasing the partial pressure of oxygen is usually beneficial for the decrease of reaction temperature, so we also calculate the maximum H2 yield under different partial pressures of
Fig. 5. Conversion ratio of methane, cCH4, at the thermal reduction step with respect to Tred and Rred.
T. Ma et al. / Renewable Energy 143 (2019) 915e921 Table 2 The effect of Toxi and nH2O:nCeO2 for the value of 2-d as Tred ¼ 1400 K. Toxi (K)
nH2O:nCeO2¼0.25
nH2O:nCeO2¼0.5
nH2O:nCeO2¼1
nH2O:nCeO2¼2
600 700 800 900 1000 1100 1200 1300 1400 1500 1600
1.77 1.77 1.77 1.77 1.76 1.76 1.75 1.73 1.71 1.69 1.67
2.00 2.00 2.00 1.99 1.97 1.93 1.89 1.85 1.80 1.76 1.73
2.00 2.00 2.00 2.00 2.00 1.99 1.96 1.93 1.88 1.83 1.79
2.00 2.00 2.00 2.00 2.00 2.00 1.99 1.97 1.93 1.89 1.85
oxygen. For the reduction reaction with methane (CeO2þdCH4 ¼ CeO2-dþ2dH2þdCO), we do not consider the impact of partial pressure of oxygen as the reaction temperature is not very high. As seen in Fig. 6, the maximum yield of H2 is close to 1 mol at different temperature for the methane reduction reaction. However, the maximum yield of H2 is 0.5 mol as the oxygen partial pressure is 10-10 bar for the reaction without methane, and lower oxygen partial pressure is always difficult to reach. The yield of H2 is positively correlative with the reaction temperature for the same oxygen partial pressure. It could be concluded that the introduction of methane could increase the yield of H2.
3.2. Performance assessment based on energetic upgrade factors Although analyzing the amounts of products is a simple and practical method, it could not reflect the performance of the whole system. Therefore, it is meaningful to study the energetic upgrade factor (U) and energy conversion efficiency. The energetic upgrade factor is the ratio of the high heat value of the exit to the high heat value of the CH4 feedstock [25]. The initial amount of ceria is 2 mol and Rred ¼ 0.5. Fig. 7 shows the variations of U as the change of Tred and Toxi. As seen in Fig. 7(a), there is no corresponding temperature region of U > 1.20 as nH2O:nCeO2 is 0.25. This phenomenon is caused by the insufficient water. U ¼ 1.20 means that extra 20% energy could be
Fig. 6. The maximum yield of H2 with (dashed line) or without (solid line) CH4 under different Tred. Here PO2 presents the oxygen partial pressure.
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stored. The temperature region corresponding to U > 1.20 is very large when nH2O:nCeO2 is 0.5, 1 or 2, and it means that more energy could be stored. As seen in Fig. 7(b), within the entire thermal reduction region, the value of U is positively correlative with Tred. Higher thermal reduction temperature is beneficial for the increase of U. However, the increasing rate of U becomes slowly as the increase of Tred. For the same Tred, the value of U decreases as the increase of Toxi. It could be explained that larger swing of temperature is favorable for the promotion of U. The region of U > 1 become larger as the increase of nH2O:nCeO2. The value of U even could reach 1.24 under the condition of Tred >1340 K and nH2O:nCeO2 ¼ 0.5. There are no apparent changes of U in Fig. 7(bed), it could be concluded that the dependence between nH2O:nCeO2 and U is gradually reduced and the condition of nH2O:nCeO2 ¼ 0.25 is not suitable for the increase of U. For the isothermal reaction, the value of U is positively correlated with the increase of nH2O:nCeO2 as the temperature is 1300 K. The value of U is between 1.08 and 1.12 at nH2O:nCeO2 ¼ 0.25. The value of U is between 1.12 and 1.16 at nH2O:nCeO2 ¼ 0.5. The value of U is between 1.16 and 1.2 at nH2O:nCeO2 ¼ 1. The value of U is between 1.12 and 1.24 at nH2O:nCeO2 ¼ 2. It is clear that more energy could be stored as the increase of nH2O:nCeO2 for the isothermal cycle. 3.3. Evaluation of solar-to-fuel efficiency Based on the previous analysis, Rred ¼ 0.5 is one of the best choice for the thermal reduction step. Therefore, the calculation of hsolar-fuel is based on the condition of Rred ¼ 0.5. The variations of hsolar-to-fuel at Rred ¼ 0.5 is shown in Fig. 8. The best hsolar-to-fuel in Fig.(a)-(d) are 0.23, 0.35, 0.31 and 0.27 under different conditions of nH2O:nCeO2 ¼ 0.25, nH2O:nCeO2 ¼ 0.5, nH2O:nCeO2 ¼ 1 and nH2O:nCeO2 ¼ 2 respectively, and the hsolar-to-fuel of 0.23 is the minimum as nH2O:nCeO2 is 0.25. It could be explained that the amount of water is not enough for this thermochemical cycle. The analysis of 2-d and U has shown the similar result. Therefore, we could believe that the condition of nH2O:nCeO2 ¼ 0.25 is not suitable for the thermochemical cycle. Fig. 8(b) shows the change of hsolar-to-fuel as nH2O:nCeO2 is 0.5. For the reaction of Ce2O3þ H2O¼H2þ2CeO2, the ratio of nH2O:nCeO2 required for the complete reaction is also 0.5. For the nonisothermal reaction, the maximum hsolar-to-fuel is 0.35 as Tred ¼ 1400 K and Toxi ¼ 750 K. As previous analysis, the value of SH2, SCO, cCH4 and U almost reach the optimal condition as Tred ¼ 1400 K. Meanwhile, the condition of Toxi ¼ 750 K is also favorable for the water-splitting step. As seen in Fig. 8(c), the maximum hsolar-to-fuel is 0.24 as T ¼ 1250 K. As previous analysis, higher temperature could promote the thermal reduction step, but it is not beneficial for the water-splitting step. Experiment also confirms that higher temperature could cause larger heat loss [28]. However, low temperature is not benefit to the recovery of ceria. As seen in Fig. 8(b)e(d), it is clear that the maximum hsolar-to-fuel would decrease as the increase of nH2O:nCeO2. This could be explained by Eq. (7) and Eq. (9). The energy could be additional consume as the amount of water is excessive. It could be found that non-isothermal condition is more suitable for the thermochemical ceria-methane cycle to pursue higher efficiency. As nH2O:nCeO2 is 0.25 or 2, the corresponding solar-to-fuel efficiencies are 0.23 and 0.27 respectively, and those values do not exceed 30%. Therefore, we only calculate the hsolar-to-fuel of the second cycle as nH2O:nCeO2 ¼ 0.5 and nH2O:nCeO2 ¼ 1. As seen in Fig. 9(a), it is clear that the maximum non-isothermal hsolar-to-fuel of the second cycle is 0.36 as nH2O:nCeO2 ¼ 0.5, which is a little larger than the corresponding value of 0.35 in the first cycle.
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Fig. 7. The contour plot of energetic upgrade factor (U) with respect to Tred and Toxi, at Rred ¼ 0.5 and (a) nH2O:nCeO2 ¼ 0.25, (b) nH2O:nCeO2 ¼ 0.5, (c) nH2O:nCeO2 ¼ 1, and (d) nH2O:nCeO2 ¼ 2.
Fig. 8. Variations of hsolar-fuel with respect to Tred and Toxi at Rred ¼ 0.5 and (a) nH2O:nCeO2 ¼ 0.25, (b) nH2O:nCeO2 ¼ 0.5, (c) nH2O:nCeO2 ¼ 1 and (d) nH2O:nCeO2 ¼ 2 for the first cycle, and the red dashed line represents the isotherm.
The maximum isothermal hsolar-to-fuel in the second cycle is 0.22 as nH2O:nCeO2 ¼ 1, and it is slightly smaller than the corresponding value of 0.24 in the first cycle. Comparing the maximum nonisothermal and isothermal hsolar-to-fuel of the first and second cycles, it could be found that there are not obvious differences for the maximum hsolar-to-fuel.
4. Conclusion In sum, thermodynamic calculations are studied based on methane reduction of CeO2/CeO2-d for the production of CO and H2. The hsolar-to-fuel is compared under isothermal and non-isothermal conditions. The calculated results indicate that Rred ¼ 0.5 is favorable for the
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Fig. 9. Variations of hsolar-to-fuel with respect to Tred and Toxi at Rred ¼ 0.5 and (a) nH2O:nCeO2 ¼ 0.5 (b) nH2O:nCeO2 ¼ 1 in the second cycle, and the red dashed line represents the isothermal reaction.
oxygen release, fuel selectivity and methane conversion. After the calculation of 2-d, we find that Rred ¼ 0.125 and Rred ¼ 0.25 are not benefit for the oxygen release of ceria. The H2 and CO production selectivities are more than 90% simultaneously as Rred ¼ 0.5. The conversion ratios of CH4 at Rred ¼ 0.5 are all more than 93% at our given temperature range of 1100e1600 K. In the water-splitting step, CeO2 could be fully recovered at typical experimental temperature range of 600e800 K, and the maximum yield of H2 could increase to 1 mol after the introduction of methane. The value of U is positively correlative with H2O:nCeO2, and nearly extra 20% energy could be stored as T ¼ 1300 K and nH2O:nCeO2 ¼ 2. As nH2O:nCeO2 ¼ 0.5, Tred ¼ 1400 K and Toxi ¼ 750 K, the maximum non-isothermal hsolar-to-fuel is 0.35, which is larger than maximum isothermal hsolar-to-fuel of 0.24 at nH2O:nCeO2 ¼ 1 and T ¼ 1250 K. For the second cycle, there are not obvious differences for the maximum hsolar-to-fuel. It could be found that the non-isothermal reaction has great potential to reach higher hsolar-to-fuel for the solar thermochemical ceria-methane cycle. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grant no. 51806209, 51476163), the National Key Research and Development Program of China (Grant no. 2018YFB1502005), Chinese Academy of Sciences President’s International Fellowship Initiative (Grant no.2019VEB0001) and Institute of Electrical Engineering, Chinese Academy of Sciences (Grant no. Y770111CSC). References [1] V. Dasireddy, B. Likozar, Activation and decomposition of methane over Co, Cu, and Fe-based heterogeneous catalysts for COx-free hydrogen and multiwalled carbon nanotubes production, Energy Technol. 5 (8) (2017) 1344e1355. [2] A. Obradovic, B. Likozar, J. Levec, Steam methane reforming over ni-based pellet-type and Pt/Ni/Al2O3 structured plate-type catalyst: intrinsic kinetics study, Ind. Chem. Res. 52 (38) (2013) 13597e13606. [3] G.P. Smestad, A. Steinfeld, Review: photochemical and thermochemical production of solar fuels from H2O and CO2 using metal oxide catalysts, Ind. Chem. Res. 51 (37) (2012) 11828e11840. [4] T. Nakamura, Hydrogen production from water utilizing solar heat at high temperatures, Sol. Energy 19 (5) (1977) 467e475. [5] P. Haueter, S. Moeller, R. Palumbo, A. Steinfeld, The production of zinc by thermal dissociation of zinc oxide - solar chemical reactor design, Sol. Energy 67 (1e3) (1999) 161e167. [6] S. Abanades, G. Flamant, Thermochemical hydrogen production from a twostep solar-driven water-splitting cycle based on cerium oxides, Sol. Energy 80 (12) (2006) 1611e1623. [7] C.W. Sun, H. Li, L.Q. Chen, Nanostructured ceria-based materials: synthesis, properties, and applications, Energy Environ. Sci. 5 (9) (2012) 8475e8505. [8] M. Mogensen, N.M. Sammes, G.A. Tompsett, Physical, chemical and electrochemical properties of pure and doped ceria, Solid State Ionics 129 (1) (2000)
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