Production and storage of hydrogen from methane by applying the redox of iron oxide

Transcription

1 Proceedings of International Symposium on EcoTopia Science 7, ISETS7 (7) Production and storage of hydrogen from methane by applying the redox of iron oxide Fumio Okada, Masakatsu Morioki, Yoshito Umeda, Masao Uchiya, and Kiyoshi Otsuka. Fundamental Research Department, Toho Gas Co. Ltd., Tokai, Japan. Uchiya Thermostat Co., Ltd., Misato. Japan. Uchiya Technical Center,Uchiya Thermostat Co., Ltd., Misato. Japan Abstract: We have studied the safe storage of hydrogen from natural gas by applying the redox of magnetite. In particular, we focused on the features of magnetite reduced with gas reductants (H, CO, CH ) at temperatures 7 ~ 9 K. Suitability of the reductants was judged on the bases of the amount of stored hydrogen per unit weight of iron, and of the presence of undesirable by-products in hydrogen during the subsequent oxidation of the reduced magnetite with water. Hydrogen could reduce magnetite to iron at or below 7 K. However, the reduction of magnetite with methane required a higher temperature than 7 K. In the case of the reducing gases having carbon in their structures (CO, CO + H, CH ), carbon deposition on iron was always observed during reduction of magnetite. The subsequent oxidation of the reduced sample with water generated CO as an impurity in hydrogen. Thus, it is concluded that we should use pure hydrogen for the reduction of magnetite. Therefore, we investigated an additional process, i.e., catalytic methane decomposition on supported-ni catalysts, producing pure hydrogen in advance. Ni/SiO is one of the most effective catalysts for methane decomposition. Thus, we propose the combination process of catalytic methane decomposition on supported-ni catalysts and the use of this hydrogen for the reduction of magnetite. Keywords: Hydrogen storage, Redox of magnetite, Hydrogen production, Methane decomposition.. INTRODUCTION Hydrogen is a fuel for fuel cells (PEFCs), which are expected to be the most efficient converter of chemical energy into electricity. However, one of the major obstacles for fuel cells to come into wide use in our society is the lack of an efficient and low-cost storage system of hydrogen. In this work, we have focused on the safe storage of hydrogen from natural gas by applying the redox of iron oxide [,]. The principle of the method is based on the following two-step process. In Reaction, magnetite (Fe O ) powder is reduced with a reducing gas like methane, converting into iron. In Reaction, the iron produced in Reaction is oxidized with water vapor, generating pure hydrogen. Reaction : Fe O + Reductant (H, CO, CH ) Fe + H O (Storage of hydrogen) Reaction : Fe + H O Fe O + H (Production of hydrogen) We have examined reductants suitable for the reduction of magnetite in Reaction. On the basis of the results, a new process for the production of hydrogen and its storage by applying the above redox reaction is proposed herein.. Experimental The iron oxide samples used in this work were prepared by the impregnation method using aqueous solutions of Cr(NO ) and Ni(NO ) with Fe O. Every impregnated sample was dried at K for h and calcined in air at 7 K for h, followed by further calcination at 77 K for h. The amounts of added Ni and Cr cations were adjusted to each be mol% of the total metal cations. The apparatus used in Reaction and was a gas flow system with a fixed bed of the iron oxide samples at the center of a stainless steel tubular reactor (inner diameter mm) (Fig. ). The total pressure of the gas mixture passing through the fixed bed of iron oxide samples was always ca. kpa. The temperature of the bed was monitored with a thermocouple attached to the outside wall of the reactor at the position of the iron oxide bed. The temperature of the iron oxide samples was controlled by an electric furnace. A cooler was installed as a drain trap after the reactor. The amount of iron oxide samples (pellets of ca. mm diameter) mounted on the bed was always adjusted to be.g. (In the case of Reaction with hydrogen, the amount was adjusted to be. g or. g. ) In order to stabilize the iron oxide samples, every experiment was executed after reducing the samples with hydrogen at 7 K and subsequently oxidizing the reduced samples with water vapor at 6 K. After this pretreatment, the experiments of Reaction were performed at temperatures of 7-9 K. The total flow rate of the gas mixture was L (STP) min -. The entrance flow rate before the electric furnace was controlled by four mass flow controllers, and the exit flow rate after the drain trap was always monitored. The entrance flow rate was checked by a meter installed after the drain trap. The gas composition was confirmed with a gas chromatograph. The amount of consumed hydrogen was determined from the difference between the flow rates at the entrance and at the exit. Corresponding author: F. Okada, okada-f@tohogas.co.jp 8

2 Proceedings of International Symposium on EcoTopia Science 7, ISETS7 (7) Pressure gauge Meter Sample bed Drain trap Gas chromatograph Furnace Thermocouple Mixer Mass flow controller Preheater Pump Fig.. Apparatus for the reduction and reoxidation of the iron oxide samples The reoxidation of the reduced samples with water vapor was followed continuously after the reduction process. We measured the exit flow rate at the drain trap after the electric furnace. The gas composition was analyzed with a gas chromatograph. The produced amount of hydrogen was determined from the flow rate at the exit of the drain trap. X-ray diffraction (XRD) analysis of the samples was performed by Philips Japan Ltd., / PW- using Cu Kα radiation.. Results and discussion.. Reduction with hydrogen and reoxidation of the iron oxide The features of the iron oxide reduced with hydrogen were studied as reference standards. The reduction of the iron oxide samples with hydrogen Consumption rate of H µmol min - g-feo -,, a) b) c) d) e) f) 6 Reduction time min 9 Water tank was performed at 7-7K. The reproduction of hydrogen from the samples reduced with water vapor was performed at 6 K. Fig. shows the consumption rate of hydrogen during iron oxide reduction at 7-7 K. At 7 K, the consumption rate of hydrogen during the reduction of the iron oxide was very slow. The consumption rate increased with the rise in the reducing temperature. A rapid consumption of hydrogen was observed during the early reduction period. After minutes, the consumption rate decreased. In minutes, the consumption rate became very slow, but the reduction did not stop completely in 9 min. Stored hydrogen in Fe wt Reduction Temperature K) Fig.. Stored hydrogen wt% in the iron vs. reduction temperature. FeOx-Ni-Cr Sample weight:.g (pellet), Flow rate L/min, Reduction time: hr Fig.. The consumption rate of hydrogen during iron oxide reduction. FeOx-Ni-Cr Sample weight:.g (pellet), Flow rate L/min, a):7k, b):67k, c):6k, d):7k, e):k, f):7k Fig. shows the relation between the stored hydrogen (by weight % in iron) and the reduction temperature. The iron oxide had been reduced with hydrogen for hr at different temperatures. The amount of stored hydrogen by 8

3 Proceedings of International Symposium on EcoTopia Science 7, ISETS7 (7) weight percent in iron means the amount of stored hydrogen per unit weight of iron. This value is abbreviated as the stored hydrogen wt%, hereafter. The amount of stored hydrogen in iron increased with the rise of the reduction temperature, approaching the theoretical value (.8 wt%) Stored hydrogen in Fe wt Reduction Time hr Fig.. Stored hydrogen wt% in the iron vs. reduction time. FeOx-Ni-Cr sample weight =.g, reduction temperature at K. hydrogen flow rate = L/min Fig. shows the relation between the stored hydrogen in iron and the reduction time at K. At K, the reactivity of the reduced iron oxide was very low. More than hrs was required to recover the stored hydrogen to the value of wt%... Reduction of iron oxide with carbon monoxide and reoxidation of the reduced sample with water vapor At present, hydrogen is produced through the steam reforming process of methane, the main component of natural gas. The hydrogen produced by this process always contains carbon monoxide that deteriorates the performance of the platinum anode. If iron oxide could be reduced with carbon monoxide and the subsequent oxidation of the reduced sample produced pure hydrogen, such a refining process could probably replace PSA in the steam reforming hydrogen production plant. The reduction of the iron oxide samples with carbon monoxide was performed at 7 7 K. The production of hydrogen from the reduced samples with water vapor was performed at 6 K. Table summarizes the results of reduction with several reductants and reoxidation with water vapor. In the case of the iron oxide reduction with carbon monoxide, the amount of stored hydrogen in the iron increased with the rise in the reduction temperature. However, above 8 K, the carbon deposition occurred on the samples. We noticed it by monitoring the exhaust backpressure and temperature of the reactor. Fig. shows the changes in the carbon monoxide entrance and exit flow rates, the exit flow rate of the produced carbon dioxide, the exhaust backpressure and the temperature of the reactor when carbon deposition occurred. The carbon monoxide entrance flow rate was constant until the reactor was blockblocked by carbon deposition. The carbon monoxide exit flow rate decreased and the flow rate of the produced carbon dioxide increased with the passage of time. Because some reaction had occurred, we checked the pressure and temperature of the reactor. As soon as the carbon monoxide exit flow rate decreased, the temperature of the reactor rose rapidly. This reaction is an exothermic reaction. The exhaust backpressure of the reactor rose with decreasing carbon monoxide exit flow rate. The carbon deposition was confirmed by XRD analysis. The XRD pattern of the sample was measured after the reduction of the iron oxide sample with carbon monoxide and reoxidation of the reduced sample with water vapor. The XRD analysis indicated the formation of graphite, iron carbide and magnetite. From these results, we presume that the deposited carbon was produced in the following reaction, CO C + CO () After the reduction of the iron oxide sample with carbon monoxide, the reduced sample was oxidized with water vapor. The amount of stored hydrogen in iron was very small and there was carbon monoxide in the reproducing gas, as shown in Table. We presume that the following reaction (eq. or ) produces the carbon monoxide. Flow rate L/min Pressure MPa. 6 c) b) a) d) f) Time min Temperature K Fig.. Changes during carbon deposition on FeOx-Ni-Cr sample. a): carbon monoxide entrance flow rate, b): carbon monoxide exit flow rate, c):exit flow rate of produced carbon dioxide, d): temperature of reactor, f): exhaust back pressure of reactor. C + H O CO + H Fe C + H O CO + H + Fe () ()[].. Reduction with methane and reoxidation of iron oxide If iron oxide could be directly reduced using methane, it would be quite attractive because other reductants (H, CO, H +CO) are produced through the steam reforming of methane. The reduction of the iron oxide samples with methane was performed at 7 9 K. The production of hydrogen from the reduced samples with water vapor was performed at 6 K. 8

4 Proceedings of International Symposium on EcoTopia Science 7, ISETS7 (7) Table. Summary of Reaction with different reductants and Reaction. ( Sample: FeOx-Ni(mol%)-Cr(mol%)) Reductants Reduction Composition (%) Produced Gas Composition(%) Stored Temp.(K) H CO CO CH H CO CO CH wt% *) H H H H H CO CO Remarks CO C deposited CH No reduction CH No reduction CH No reduction CH No reduction CH No reduction CH CH CH CH CH C deposited Mixed gas Mixed gas Mixed gas Mixed gas Mixed gas Mixed gas Mixed gas *) stored hydrogen wt% in Fe In table, during iron oxide reduction with methane, the amount of stored hydrogen in iron was very small at 7 7 K. At 7 K, the stored hydrogen appeared and increased as the reduction temperature increased. At or above 9 K, carbon deposition occurred. We could not subsequently oxidize the reduced sample with water vapor, because the exhaust backpressure was very high and the water vapor could not pass through the reactor. At reduction temperatures below 87 K, we could oxidize the reduced sample with water vapor. The amount of stored hydrogen in iron was very small at or below 77 K. At 87 K, the stored hydrogen in iron was.9% and there was a little carbon monoxide in the produced hydrogen... Reduction with mixed gases as reductants and reoxidation of iron oxide... Mixed gases with CO and CO Steam reforming is the main process used to produce hydrogen from natural gas. The output of this process is hydrogen, plus carbon monoxide, carbon dioxide, and methane. The features of the reduction of iron oxide with the mixed gases of hydrogen, carbon monoxide, carbon dioxide, and methane, were studied in order to determine the possibility of directly using of these output gases from the steam reformer. The reduction of iron oxide samples with mixed gases of various composition was performed at K. The production of hydrogen from the reduced samples with water vapor was performed at 6 K. 8

5 Proceedings of International Symposium on EcoTopia Science 7, ISETS7 (7) Fig.6 shows the stored hydrogen in the iron oxide reduced with various mixed gases. When we used the reductants without carbon dioxide, the amount of stored hydrogen in iron increased with the passage of time. But when the samples were reduced with mixed gases that included carbon dioxide, the amount of stored hydrogen in iron was very small and hardly increased with reduction time. We presume that the presence of carbon dioxide in the reductant gas interfered with the reduction of iron oxide. Stored hydrogen wt% in Fe Reduction Time hr Fig.6. Stored hydrogen wt% in the iron vs. reduction time in the case of reduction with various mixed gases. FeOx-Ni-Cr sample weight =.g, reduction temperature = K; mixed gas flow rate = L/min. H ( ), CO( ), mixed gas( ),mixed gas(),mixed gas(),mixed gas() In table, during iron oxide reduction with mixed gases that included carbon monoxide and carbon dioxide (Mixed gases - ), there was some carbon monoxide in the produced gas.... Mixed gas (pseudo methane decomposition gas) without CO or CO. One of the authors of this paper, in a previous investigation, found that catalytic methane decomposition over supported-ni catalysts produced pure hydrogen without CO or CO []. Ni(wt%)/SiO is one of the most effective catalysts for this methane decomposition []. Therefore, we studied the features of the iron oxide reduced with Mixed gas (produced by methane decomposition, composed of hydrogen and methane). The reduction of the iron oxide samples with Mixed gas was performed at 6 7 K. The production of hydrogen from the reduced samples with water vapor was performed at 6 K. Table shows that, when iron oxide was reduced with Mixed gas, carbon monoxide was not observed in the produced gas. At the reduction temperature of 7K, the stored hydrogen in iron was.wt%, close to the value observed when pure hydrogen was used as the reductant. It should be noted that we have to raise the temperature to above 87K in order to reduce iron oxide with methane. But at this temperature, carbon deposition occurs. However, if we perform the reduction of iron oxide at or below 7 K, the output gas from the methane cracker can be directly used without any purification.. Conclusion We studied the features of iron oxide reduced with several reductants (hydrogen, carbon monoxide, and several mixed gases). On the basis of the results described above, we conclude as follows:. Hydrogen is the best reductant to reduce iron oxide.. We can not use carbon monoxide and mixed gases having carbon in their structures (except one case) as the reductant, because carbon deposition occurs and there is carbon monoxide in the produced gases.. We can only use the mixed gas produced by methane decomposition as the reductant, because only then, there are neither deposited carbon on the sample nor carbon monoxide in the produced gas. Catalytic methane decomposition can produce hydrogen without CO or CO. The mixed gas so produced can reduce iron oxide without carbon deposition at or below 7K. Ni(wt%)/SiO is one of the most effective catalysts for methane decomposition. We propose the combination process:. Producing hydrogen by catalytic methane decomposition on supported-ni catalysts without CO or CO ; and. Using this hydrogen without any purification for the reduction of iron oxide. REFERENCES. K. Otsuka, T. Kaburagi, C. Yamada and S. Take-naka, J. Power Sources,, () pp.-.. S. Takenaka, V. T. D. Son, C. Yamada, and K. Otsuka, Chem. Lett.,, () pp.-.. T. Akiyama, A. Miyazaki, H. Nakanishi, M. Hisa, A. Tsutsumi, Int. J. Hydrogen Energy, 9, (), pp 7-7. S. Takenaka, S. Kobayashi, H. Ogihara, and K. Otsuka, J. Catal.,7,(), pp S. Takenaka, Y. Shigeta, E. Tanabe and K. Otsuka, J. Catal,, () pp