international journal of hydrogen energy 40 (2015) 1639e1650 Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/he 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 ( solarÀtoÀfuel) 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 solarÀtoÀfuel 47.74% (without heat recuperation) and cycle 51.77% and h ¼ ¼ solarÀtoÀfuel 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 À 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 À 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 À 1273 KÞ (6) hence can be used in multiple cycles. MO þ SO2ðgÞþH2OðgÞ/MSO4 þ H2ðgÞðNon À solarÞ (8) SO2 þ 2H2O/H2SO4 ð353 À 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 Â À6 2 þð : Þ ð : Þ À 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 À9 Â 10À6 T2 þð0:0499ÞT À 257:35 300 À 900 K The equilibrium thermodynamic calculations related to WS s s (13) À Á À Á D ¼ À Â À5 2 þð : Þ À : À GWS 2 10 Ts 0 2532 Ts 119 36 300 900 K (14) Fig.