international journal of hydrogen energy 40 (2015) 1639e1650
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Solar hydrogen production via thermochemical iron oxideeiron sulfate water splitting cycle
* Rahul R. Bhosale , Anand Kumar, Leo J.P. van den Broeke, Shahd Gharbia, Dareen Dardor, Mehak Jilani, Jamila Folady, Mashail Shaif Al-Fakih, Mahsa Ali Tarsad
Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar article info abstract
Article history: This paper reports the thermodynamic analysis of solar H2 production via two-step ther- Received 18 September 2014 mochemical iron oxideeiron sulfate (IOeIS) water splitting cycle. The first step belongs to
Received in revised form the exothermic oxidation of FeO via SO2 and H2O producing FeSO4 and H2 and second step
19 November 2014 corresponds to the endothermic reduction of FeSO4 into FeO, SO2, and O2. The products,
Accepted 25 November 2014 FeO and SO2 can be recycled to step 1 and hence, reutilized for the production of H2 via Available online 20 December 2014 water splitting reaction. Thermodynamic equilibrium compositions and variations in enthalpy, entropy and Gibbs free energy of the thermal reduction and water splitting re- Keywords: actions were computed as a function of reaction temperatures. Furthermore, the effect of _ Solar fuel molar flow rate of inert Ar (nAr) on thermal reduction temperature (TR) and equilibrium
Iron oxideeiron sulfate water compositions during the thermal reduction of FeSO4 was also examined. Second law h splitting cycle thermodynamic analysis was performed to determine the cycle efficiency ( cycle) and solar h Thermodynamics to fuel energy conversion efficiency ( solar to fuel) attainable with and without heat recu- _ e e Hydrogen production peration for varying nAr (0 30 mol/s) and TR (1280 1510 K). Results obtained indicate h ¼ h ¼ h ¼ cycle 39.56% and solar to fuel 47.74% (without heat recuperation) and cycle 51.77% and h ¼ ¼ solar to fuel 62.43% (by applying 50% heat recuperation) at TR 1510 K. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
favored thermodynamically as it requires process heat at Introduction temperatures above 2723 K for obtaining a significant degree of dissociation of water [1]. Furthermore, to avoid the forma- H2 is considered as one of the potential energy carriers due to tion of an explosive mixture comprised of H2 and O2, a gas its capacity of producing 143 MJ/kg of energy (higher in separator unit (for the separation of H2 and O2 at high comparison with the oil, gas, and coal individually) and its temperatures) needs to be equipped near to the water disso- environmentally friendly nature (burning of H2 produces ciation reactor which further enhances the overall cost of the water with no other polluting gases). Water splitting reaction H2 production process. is considered as one of the most promising ways for the pro- As the direct dissociation of water is not practical due to duction of H2. However, direct water thermolysis is not the high operating temperature and gas separation problem,
* Corresponding author. Tel.: þ974 4403 4168; fax: þ974 4403 4131. E-mail addresses: [email protected], [email protected] (R.R. Bhosale). http://dx.doi.org/10.1016/j.ijhydene.2014.11.118 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. 1640 international journal of hydrogen energy 40 (2015) 1639e1650
attempts are currently underway to achieve H2 production via investigated metal catalysts were loaded either on non- water-splitting reaction at lower operating temperatures and sulfating substrates or substrates forming very stable sul- via bypassing the formation of H2 and O2 explosive mixture. fates. At high reaction temperatures, most of the transition Several thermochemical cycles such as iron oxide cycle [2e6], metal catalysts (such as nickel, manganese based) undergo zinc/zinc oxide cycle [7e10], tin/tin oxide cycle [8,11e13], formation of stable sulfates and resulted as inefficient in e e e mixed ferrite cycle [14 21], ceria cycle [22 24], sulfur iodine terms of their catalytic activity towards the reduction of SO3 e cycle [25 27], and hybrid sulfur cycle [28,29] were investigated into SO2 and O2 [31]. In contrast, the noble metal catalysts (e.g., e towards H2 production via water-splitting reaction. Among platinum based) supported on BaSO4 TiO2, ZrO2, or SiO2 were these, the sulfureiodine cycle (reaction set I) and its variation observed to be active towards the endothermic dissociation of the hybrid sulfur cycle (reaction set II) are more appealing as SO3. Although, the noble metal catalysts are attractive for the required operating temperatures are lower as compared to such reactions, they are less preferable due to the limited other thermochemical cycles. availability and high cost. Reaction set I: sulfureiodine cycle Utilization of metal oxides as the catalytic materials (instead of noble metal catalysts) and converting the sul- H SO /SO þ H O ð673 KÞ (1) 2 4 3 2 fureiodine and hybrid sulfur cycle into a ‘metal oxideemetal sulfate’ cycle operated using concentrated solar energy is one SO34SO2 þ 1=2O2 ð1123 1273 KÞ (2) of the alternative to achieve H2 production at moderate tem- peratures. Solar ‘metal oxideemetal sulfate’ thermochemical þ þ / þ ð Þ SO2 2H2O I2 H2SO4 2HI 393 K (3) cycle utilizes solar energy, metal oxide (MO), SO2 and H2O for
the production of H2 and O2. It is a two-step process in which / þ ð Þ 2HI H2 I2 573 723 K (4) the first non-solar step belongs to the exothermic oxidation of Reaction set II: hybrid sulfur cycle MO by SO2 and H2O producing metal sulfate (MSO4) and H2 (reaction 8). The endothermic step two corresponds to the / þ ð Þ H2SO4 SO3 H2O 673 K (5) solar thermal reduction of MSO4 into MO, SO2, and O2. The MO
and SO2 produced in step 2 are recycled back to step 1 and
SO34SO2 þ 1=2O2 ð1123 1273 KÞ (6) hence can be used in multiple cycles.
MO þ SO2ðgÞþH2OðgÞ/MSO4 þ H2ðgÞðNon solarÞ (8) SO2 þ 2H2O/H2SO4 ð353 393 KÞ (7)
For both sulfureiodine cycle and hybrid sulfur cycle, the / þ ð Þþ ð Þð Þ MSO4 MO SO2 g 1=2O2 g Solar (9) most energy consuming step is the dissociation of SO3 into
SO2 and O2. The reduction of SO3 into SO2 and O2 is possible According to the previous investigations [2e24], among the only under catalytic conditions and takes place at high tem- several MO systems examined for the solar thermochemical perature due to its endothermic nature. Several metal based water splitting application, iron oxide based redox materials catalytic systems have been investigated previously towards are considered as one of the most favorable materials. In this the decomposition of the gaseous SO3 [30]. As sulfation viewpoint, this paper proposes the utilization of the ‘iron e ’ e poisoning is a major concern related to such reactions, the oxide iron sulfate (IO IS) cycle for the production of solar H2
Fig. 1 e Typical two-step solar thermochemical IOeIS water splitting cycle. international journal of hydrogen energy 40 (2015) 1639e1650 1641
via thermochemical water splitting reaction. Typical two-step the IOeIS cycle were performed with the help of commercial solar thermochemical IOeIS water splitting cycle is presented thermodynamic HSC Chemistry software and databases [32] in Fig. 1. In this study, the thermodynamic feasibility of this by assuming a) solar reactor operated in continuous mode cycle was investigated and obtained results are presented in with inlet concentration of FeSO4 equal to 1 mol/s and b) by detail. At first, the thermodynamic equilibrium compositions considering FeO, SO2(g), H2O(g), FeSO4,H2(g), and O2(g) as the during the solar thermal reduction of FeSO4 under inert at- reactive species. mosphere and oxidation of FeO via water splitting reaction were determined. The variation of the reaction enthalpy, en- H2 production via water splitting reaction tropy and Gibbs free energy for the thermal reduction and water splitting steps with respect to the operating tempera- Fig. 2a and b represent variations in the equilibrium compo- tures were studied. Furthermore, the maximum theoretical sition for step 1 with respect to water splitting temperature solar energy conversion efficiency of the IOeIS cycle is (TS) (in absence of Ar). According to the results presented, the determined by performing the second law thermodynamic production of H2 via oxidation of FeO by SO2 and H2Ois analysis over different solar reactor temperatures and with/ possible below 483 K. It is further observed that at lower without considering the heat recuperation. temperatures, FeSO4 and H2 are the major species, whereas
the molar concentration of FeO, H2O and SO2 increases with
the increase in TS. D D D Chemical thermodynamic equilibrium Variations in HWS, SWS, and GWS as a function of TS for the production of H2 via oxidation of FeO by SO2 and H2Ois Simplified chemistry of the solar thermochemical IOeIS water presented in Fig. 3a and b. Following are the mathematical correlations derived from the obtained HSC simulation results splitting cycle involving thermal reduction of FeSO4 and water D D D splitting via FeO oxidation is shown below: for the estimation of HWS, SWS, and GWS at different TS. À Á À Á D ¼ 6 2 þð : Þ ð : Þ FeO þ SO2ðgÞþH2OðgÞ/FeSO4 þ H2ðgÞ (10) HWS 9 10 Ts 0 0129 Ts 123 08 300 900 K (12) / þ ð Þþ = ð Þ FeSO4 FeO SO2 g 1 2O2 g (11) À Á À Á DS ¼ 9 10 6 T2 þð0:0499ÞT 257:35 300 900 K The equilibrium thermodynamic calculations related to WS s s (13) À Á À Á D ¼ 5 2 þð : Þ : GWS 2 10 Ts 0 2532 Ts 119 36 300 900 K (14)
Fig. 2 e a) and b) Variations in equilibrium composition as Fig. 3 e Variations in a) enthalpy and entropy, and b) Gibbs a function of TS during H2 production via oxidation of FeO free energy as a function of TS during H2 production via ¼ _ ¼ ¼ _ ¼ by SO2 and H2O(P 1 atm, nAr 0 mol/s). oxidation of FeO by SO2 and H2O(P 1 atm, nAr 0 mol/s). 1642 international journal of hydrogen energy 40 (2015) 1639e1650
À Á À Á According to the results presented in Fig. 3a, DHws is varied D ¼ 6 2 ð : Þ þ : STR 3 10 TR 0 029 TR 297 6 300 900 K (16) by 11.35% with the increase in the TS from 300 to 900 K. Pre- À Á À Á sented results further indicate that more amount of 5 2 DGTR ¼ 1 10 T ð0:296ÞTR þ 361:29 300 900 K (17) exothermic heat will be released if the water splitting reaction R is carried out at 300 K ( 117.94 kJ/mol) instead of 900 K According to Fig. 4a, energy required for performing the D ( 104.56 kJ/mol). Likewise, DSws is also observed to be altered endothermic thermal reduction of FeSO4 i.e. HTR is decreased D by 10.79% (from 242.39 J/K mol to 216.23 J/K mol) due to the by 6.50% as the TR increased from 300 to 1500 K. Similarly, STR increase in the TS from 300 K to 900 K. As the water splitting is also reduced by 9.047% with the increase in TR (from 300 to reaction has a negative enthalpy and entropy change, this 1500 K). It is further realized that the thermal reduction of reaction seems to be more feasible at lower temperatures. The FeSO4 is more viable at high temperature as the reaction D data related to change in the Gws with respect to TS presented enthalpy and entropy has positive variation. This realization in Fig. 3b indicate a decrease of 149.40% in DGws with the is further confirmed from the data reported in Figs. 4b and 5. D decrease in the TS from 900 to 300 K. Furthermore, the results According to Fig. 4b, GTR for the thermal reduction of FeSO4 reported confirm that the production of H2 via oxidation of becomes negative above 1290 K (in absence of Ar) which
FeO by SO2 and H2O is feasible below 483 K. confirms that this reaction is possible at higher temperatures as compared to water splitting reaction. Furthermore, the
Step 2: thermal reduction of FeSO4 variations in equilibrium composition related to the thermal reduction of FeSO4 also confirm the above mentioned
Similar to the H2 production step, equilibrium thermody- observation. namic analysis of the thermal reduction of FeSO4 (in absence According to the results reported in previous investigations of Ar) was also simulated with the help of HSC Chemistry [2e24], the thermal reduction of MO can be achieved at lower software and databases. Variations in DHTR, DSTR, DGTR, and temperatures if inert carrier gas is used during the thermal equilibrium compositions as a function of TR are presented in reduction step. Based on this observation, we examined the Figs. 4(a and b) and 5, respectively. The mathematical ex- effect of Ar as an inert carrier gas during the thermal reduc- pressions (15)e(17) represents the change in the DHTR, DSTR, tion of FeSO4 in a solar reactor. The results reported in Fig. 6a _ DGTR with respect to TR. and b shows the influence of molar flow rate of Ar (nAr)on À Á À Á equilibrium composition of gaseous and solid species during DH ¼ 7 10 6 T2 ð0:0079ÞT þð363:93Þ 300 900 K TR R R thermal reduction of FeSO4. It is observed that the slope of the
(15) decrease in the molar concentration of FeSO4 and increase in
the molar concentration of FeO, SO2 and O2 is shifted towards _ ' lower TR as the nAr increases. As per the Le-Chatelier s prin- ciple, due to the inclusion of the inert gas during the solar
thermal reduction of FeSO4, the thermodynamic equilibrium
shifts towards the lower TR as the entropy of the product gaseous species decreases. _ Effect of nAr on TR required for complete conversion of
FeSO4 into FeO, SO2 and O2 is shown in Fig. 7. The results re- ported indicate that in absence of Ar, the complete conversion _ is possible at TR ¼ 1510 K. As the nAr increases upto 5 mol/s, the
required TR for complete conversion decreases to 1410 K. A
further decrease in the TR down to 1280 K is observed with the
Fig. 4 e Variations in a) enthalpy and entropy, and b) Gibbs Fig. 5 e Variations in equilibrium composition as a free energy as a function of TR during reduction of FeSO4 function of TR during reduction of FeSO4 into FeO, SO2 and ¼ _ ¼ ¼ _ ¼ into FeO, SO2 and O2 (P 1 atm, nAr 0 mol/s). O2 (P 1 atm, nAr 0 mol/s). international journal of hydrogen energy 40 (2015) 1639e1650 1643