Hydrogen production experiment by thermo-chemical and electrolytic hybrid hydrogen production process

Transcription

1 Hydrogen production experiment by thermo-chemical and electrolytic hybrid hydrogen production process Toshio Nakagiri a, Shoichi Kato a, Kazumi Aoto a a Japan Atomic Energy Agency, O-Arai, Higashi-Ibaraki, Ibaraki-ken, , Japan, nakagiri.toshio@jaea.go.jp ABSTRACT: Long time hydrogen production experiment by the thermo-chemical and electrolytic Hybrid Hydrogen process in Lower Temperature range (HHLT) was performed. In the present experiment, 50wt% sulfuric acid was circulated in the experimental apparatus at the flow rate of 0.6ml/min, and SO 3 was operated at 550 deg-c. The cell voltage of SO 3 and SO 2 solution were kept to 0.13V and 0.9V, respectively. Stable oxygen generation from SO 3 was observed for about 60 hours and no severe corrosion of Pt plated YSZ electrolyte was observed in the post experimental examination. Nevertheless, hydrogen generation rate in SO 2 solution decreased in several hours. Technical problems to increase oxygen generation rate in SO 3 and hydrogen generation rate in SO 2 solution were extracted from the experimental result and a new experimental apparatus for 1NL/h hydrogen production has been developed. KEYWORDS : hybrid-sulfur process, thermo-chemical process, electrolysis, sulfuric acid, solid electrolyte 1. INTRODUCTION The thermo-chemical and electrolytic hybrid hydrogen production process (thermo-chemical and electrolytic Hybrid Hydrogen process in Lower Temperature range: HHLT) for sodium cooled FBR is proposed by Japan Nuclear Cycle Development Institute (JNC) [1]. HHLT is based on the sulfuric acid (H 2 SO 4 ) synthesis and decomposition processes (named "Westinghouse process") developed earlier [2, 3], and SO 3 decomposition process is facilitated by electrolysis with ionic oxygen conductive solid electrolyte which is extensively utilized for high-temperature electrolysis of water. The hydrogen production experiments to substantiate the whole process of HHLT were already performed and stable production of hydrogen and oxygen for several hours was confirmed. Furthermore, conceptual design of hydrogen production plant with HHLT using a small sized sodium cooled reactor was performed [4]. In the present study, experiment for about 60 hours was performed to investigate the durability of the experimental apparatus in long time operation and technical problems were extracted to develop 1NL/h hydrogen production experimental apparatus. 2. LONG TIME HYDROGEN PRODUCTION EXPERIMENT 2.1 Principle of HHLT Westinghouse process requires high temperature over 800 deg-c only for SO 3 decomposition reaction, and other reactions can be performed below 500 deg-c. If SO 3 decomposition can be performed in lower temperature than 800 deg-c, hydrogen production using lower temperature heat source such as FBR can be realized, and corrosion problems can be reduced. SO 3 is decomposed only 7.8%thermally decomposes in equilibrium condition at 500 deg-c as shown in Fig.1, and also only about 20% in the case membrane reactor technique is used to increase decomposition fraction of SO 3 at 500 deg-c. Therefore, some other energy is required to obtain higher decomposition fraction of SO 3. Electrolysis by ionic oxygen conductive solid electrolyte is applied to increase decomposition fraction of SO 3 in HHLT. HHLT is composed of the reaction shown below. 2H 2 O + SO 2 -> H 2 SO 4 + H 2 - electricity >100 deg-c (1) H 2 SO 4 -> H 2 O + SO deg-c (2) SO 3 -> SO 2 + 1/2O 2 - electricity 500 deg-c (3) Characteristics of HHLT are summarized in next pape. 1/5

2 (1) Low electrical energy required Required energies and entropy changes of reaction for HHLT are shown in Table 1. In HHLT, total required electrical energy is kj/mol for 1 mol H 2 production and it is about half of direct water splitting (204.9 kj/mol at 500 deg-c [5]) Furthermore, required splitting voltage for reaction (3) (E G SO3 ) is expected to be lower than 0.2V at 500 deg-c as shown in Fig.2. E G is calculated by equation (4). E G = -G/(nF) (4) In equation (4), n is number of electron (= 2 for 1 oxygen atom) and F is Faraday constant. Theoretical voltage of sulfuric acid synthesis reaction (equation (1)) is V, therefore, total theoretical voltage of HHLT is expected to be lower than 0.5V, which is about half of the theoretical voltage of direct water splitting (E G H2O ) about 1V. (2) Simple process flow HHLT does not require decomposition and separation process of HI, HBr and separation of O 2 which are required in Iodine-Sulfur (IS) process or sulfur-bromine process [2, 3], therefore, numbers of reactions are much less than those of IS process or sulfur-bromine process, and simple process flow can be achieved. (3) Decrease in corrosion of structural materials Corrosion of structural materials is expected to be suppressed in HHLT than in IS process, etc., because high corrosive iodine or bromine is not used. (4) Higher safety In the case hydrogen and oxygen can be mixed at the temperature higher than spontaneous ignition temperature of hydrogen (500 deg-c in air [6]), hydrogen explosion can be take place. In HHLT, hydrogen is produced in reaction (1), at the temperature lower than 100 deg-c, therefore, possibility of hydrogen explosion is considered to be quite a low. Fi.g1 Thermal decomposition fraction of SO 3 Fig.2 Splitting voltages of H 2 O and SO Experimental apparatus and experimental conditions The flow sheet of experimental apparatus and experimental conditions are shown in Fig.3 and Table 1, respectively. The sulfuric acid thermal decomposition reaction was performed in "H 2 SO 4 vaporizer" at 400 deg-c. Electrolytic SO 3 decomposition reaction was performed in "SO 3 " at 550 deg-c and cell voltage was controlled to be 0.13V by potentiostat-galvanostat (Hokuto Denko Co. Ltd, Model HA-151. Stainless steel pipes of the SO 3 exposed to high temperature sulfuric acid were all plated by gold. O 2 generated in the SO 3 is purged by N 2 and O 2 concentration in N 2 purge gas was measured by O 2 meter (Toray Engineering Corp., LC-750). A tubular 8mol% yttria stabilized zirconia (YSZ, Nikkato Corp, ZR-8Y) with a dimension of 2 mm in thickness was used as electrolyte in the SO 3 electrolysis cell, and Pt electrodes were manufactured on both (inner and outer) surface of the. In this experiment, Pt electrodes (thickness: about 1m) were manufactured by Pt plating for higher durability and higher cell current. Furthermore, the diameter of outlet piping of the SO 3 was increased from 1/2 inch to 3/4 inch to increase gas flow rate in SO 3. The cell current of SO 3 was increased about one order as compared to previous cell. H 2 was produced in "H 2 SO 3 solution " at room temperature. The cell voltage of H 2 SO 3 was controlled by potentiostat and cell current was also measure by the potentiostat. Liquids 2/5

3 (H 2 SO 4 and H 2 O) were carried by roller pumps and flow rate was 0.6ml/min, and gases (SO 3, H 2 O, SO 2, O 2 ) were carried by N 2 purge gas and flow rate of N 2 purge gas was 100ml/min. Solid polymer electrolyte Nafion 117 was used as membrane between anode and cathode, and anolyte and catholyte of the were 50wt% H 2 SO 4 solution. Table 1 Experimental conditions Reactor Temperature (deg-c) Cell voltage (V) H 2 SO vaporizer SO 3 electrolysis cell SO 2 absorber Approx H 2 SO 3 solution Room Temp Fig.3 Flowsheet of experimental apparatus 2.3 Experimental results Total duration of present experiment was about 56 hours. H 2 and O 2 generation rate calculated from measured cell current are shown in Fig.4. O 2 generation rate was about 5ml/h and almost stable generation of O 2 was continued in SO 3. O 2 amount measured with O 2 meter agreed well with the amount calculated from cell current. H 2 generation rate in H 2 SO 3 decreased in several hours and H 2 generation rate was lower than 5ml/h during the experiment. H 2 SO 3 solution was added in the anolyte and the anode surface was polished by emery paper at about 20 hours after the experiment started, and stirring by N 2 gas bubbling around the anode was started at about 40 hours the experiment started, to increase H 2 SO 3 amount supplied for the anode surface. Cell current of H 2 SO 3 solution increased for a time by the operations mentioned above, but decreased in several hours again. The experiment was terminated by blockage of the glass pipe in SO 2 absorber by white powder product which is considered to be the sulfate of stainless steel constituent and sealing metal (see Fig.5) Generation rate of H 2 and O 2 (ml/h) time (h) SO 2 absorber inlet pipe of SO 2 absorber SO 2 absorber white powder Intel pipe of SO 2 absorber Fig.4 Generation rate of H 2 and O 2 Fig.5 Blockage in inlet pipe of SO 2 absorber 2.4 Discussion Durability of SO 3 for high temperature sulfuric acid The performance of Pt plated YSZ electrolyte for SO 3 electrolysis was stable for about 60 hours, and no degradation was observed in post experiment examination. Nevertheless, corrosion of the outlet piping of SO 3 was observed in post experiment observation as shown in Fig.6. SEM-EDS analysis of white powder remained in SO 2 absorber was performed, and constituents of stainless steel (Fe and Cr) and sealing metal (Cu), and sulfur were detected as shown in Table 2. Therefore, white powder is considered to be sulfate of stainless steel and sealing metal constituent. The cause of the corrosion was estimated to be the peeling of plated gold by condensed sulfuric 3/5

4 acid. To improve the durability of the outlet piping of SO 3, sulfuric acid resistant material, such as high Si cast steel, should be used. holder cross section of SO 3 plated gold was peeled off. elbow part of outlet pipe Table 2 Composition of white powder in SO 2 absorber of SEM-EDS analysis element weight fraction (wt%) S Cr 6.35 Fe Cu 3.61 Fig.6 Cut image of outlet piping of SO 3 electrode surface electrode surface 20m 20m cross section Pt paste electrode cross section Pt plated electrode Fig.7 SEM images of Pt paste and Pt plated electrode Technical problems to develop 1NL/h hydrogen production test apparatus (1) Development of higher performance of SO 3 In the present experiment, Pt plated YSZ electrolyte was used, and its performance was one order higher than YSZ with Pt paste electrode. The thickness of the Pt plated electrode is much thinner than Pt paste electrode as shown in Fig.7, therefore, SO 3 or O 2 molecule diffusion on the Pt plated electrode surface to the reaction area called three phase (gas/electrode/electrolyte) boundary is assumed to be much faster than on Pt paste electrode. (2) Development of higher performance H 2 SO 3 solution In this experiment, the hydrogen production rate in H 2 SO 3 solution was almost lower than 5ml/h, and the cause of low production rate is considered to be the lack of H 2 SO 3 supply to anode surface. Therefore, improvement to increase the amount of H 2 SO 3 solution supplied to the anode surface, for instance, use flow type and gas diffusion type electrode, will be required. 3. DEVELOPMENT OF 1NL/h HYDROGEN PRODUCTION EXPERIMENTAL APPARATUS A new experimental apparatus for 1NL/h hydrogen production has been developed considering the technical problems extracted from the result of previous experiment. A photo image of new apparatus is 4/5

5 shown in Fig.8. SO 3 has seven s whose length are about twice of the tube used in the former cell, therefore total electrode area of the new SO 3 is about fourteen times of the former cell. Outlet pipe of SO 3 is made of quartz to prevent corrosion by condensed sulfuric acid. Furthermore, flow type cell is adopted for SO 2 solution to increase the amount of SO 2 supplied to the anode surface. Performance test is under performing and hydrogen production experiment will be performed in this year. SO 3 SO 2 solution H 2 SO 4 vaporizer SO 3 Experimental apparatus for 1NL/h hydrogen production Fig.8 Photo image of the experimental apparatus for 1NL/h hydrogen production 4. CONCLUSIONS The hydrogen production experiment for about 60 hours was performed to investigate the durability of the experimental apparatus by HHLT, and technical problems were extracted from the experimental result. Furthermore, a new experimental apparatus for 1NL/h hydrogen production has been developed. Conclusions are summarized as follows. (1) The performance of Pt plated YSZ electrolyte for SO 3 electrolysis did not degrade during about 60 hours operation in 550 deg-c gaseous sulfuric acid. Nevertheless, outlet piping of the SO 3 was corroded by condensed sulfuric acid. Sulfuric acid resistant material should be used in the new experimental apparatus. (2) It is considered to be necessary to increase the total area of three phase boundary on Pt plated YSZ electrolyte to obtain higher performance of SO 3. (3) The hydrogen production rate in H 2 SO 3 solution for decreased in several hours, and the amount of H 2 SO 3 solution supplied to the anode surface is considered to be not enough. It is considered to be necessary to be increased the amount of H 2 SO 3 solution for higher cell hydrogen production rate. (4) A new experimental apparatus for 1NL/h hydrogen production has been developed, and hydrogen production experiment will be started to evaluate hydrogen production efficiency of the apparatus. References: 1. Y. Kani, "Investigation on hydrogen production by Fast Breeder Reactor (FBR)", Nuclear Viewpoints, Vol.49 No.1, pp (2003), [in Japanese]. 2. IAEA, "Hydrogen as an energy carrier and its production by nuclear power", IAEA-TECDOC (1999). 3. W. Weirich, K. F. Knoche, F. Behr, et al., "Thermo-chemical processes for water splitting - Status and outlook", Nuclear Engineering and Design 78, pp (1984). 4. T. Nakagiri et al., "Development of a new thermo-chemical and electrolytic hybrid hydrogen production system for sodium cooled FBR", ICONE , Beijing, M. W. Chase, Jr., et al., "JANAF Thermo-chemical tables third edition", American Chemical Society and American Institute of Physics, (1985). 6. National Astronomical Observatory, "Rika nenpyo (Chronological Scientific Tables)", Maruzen Co., Ltd, pp. 491 (1996), [in Japanse]. 5/5