Article pubs.acs.org/IECR
Bench-Scale Operation of a Concrete Sludge Recycling Plant Atsushi Iizuka,*,† Yuka Sakai,‡ Akihiro Yamasaki,‡ Masato Honma,§ Yasuyuki Hayakawa,§ and Yukio Yanagisawa∥ †
Research Center for Sustainable Science and Engineering, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Sendai, Miyagi 980-8577, Japan ‡ Department of Materials and Life Science, Seikei University, 3-3-1 Kichijoji-kitamachi, Musashino, Tokyo 180-8633, Japan § Nippon Concrete Industries, Co. Ltd., 1-8-27, Konan, Minato-ku, Tokyo 108-0075, Japan ∥ Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan ABSTRACT: A bench-scale plant for waste concrete sludge recycling was designed, constructed, and operated. Real concrete sludge generated from a pile and pole production plant and groundwater were used for the experiments. The process mainly consists of the extraction of calcium ions from the concrete sludge into the aqueous phase and the crystallization of calcium carbonate from the solution with CO2. The CO2 was supplied from boilers installed in the plant, where heavy oil is combusted. High-purity calcium carbonate (>99%) was obtained in the process, with particle sizes distributed in the range of 3−30 μm (volume-based), peaking at about 10 μm. A net reduction in CO2 emissions can be achieved based on the process power consumption and the amount of product. The effects of operating conditions on process performance data such as calcium extraction rate and calcium carbonate crystallization rate were examined, which can lead to scaling-up of the plant. ceramics,7 use of concrete sludge in the preparation of water clarification materials,8,9 and application of concrete sludge as a desulfurization agent10 have also all been investigated. Most of these approaches are based on the fact that concrete sludge is strongly alkaline and rich in calcium. Our research group has been involved in developing a new type of recycling process for concrete sludge. The project started in the fiscal year of 2009 and is supported by the Japanese government through NEDO (New Energy and Industrial Technology Development Organization). We have proposed a new type of recycling process for the concrete sludge generated from the production of concrete piles and poles.11 The process consists of two main steps: extraction of calcium ions and crystallization of calcium carbonate. In the extraction step, concrete sludge is diluted with fresh water, into which the calcium content in the concrete sludge is extracted with stirring. After being separated from the solid residue, the calcium-rich solution is reacted with gaseous CO2 to form calcium carbonate as a precipitate. The CO2 gas can be supplied from the flue gas generated by fossil fuel combustion. The water can be reused after crystallization because the alkalinity in the extracted solution is neutralized with carbonic acid. Thus, the calcium content can be transformed into calcium carbonate, which has a wide range of usages. This process can also be regarded as a CO2 fixation process in the form of carbon mineralization when CO2 from flue gas is used. The purpose of the present project was to construct a benchscale plant for recycling concrete sludge with CO2 and to
1. INTRODUCTION Concrete sludge is a construction waste in the form of a slurry composed of water and solid concrete. The composition of concrete sludge is similar to that of fresh concrete. Concrete sludge is strongly alkaline and rich in calcium, just like fresh concrete. The cement component of concrete sludge, however, has a higher degree of hydration depending on the time after preparation of the fresh concrete and the water content. It is estimated that about 1−2% of the fresh concrete prepared for construction use is discarded as concrete sludge because a certain portion of freshly prepared concrete fails to match the quality required for practical use or because excess amounts of fresh concrete are typically prepared to avoid shortages on construction sites. For Japan alone, it can be estimated that several million metric tons (or tonnes, denoted t) of concrete sludge are generated annually, based on a production rate of 86 million m3 of fresh concrete per year. The concrete sludge can be reused as a landfill material after solid/liquid separation followed by neutralization with an acid such as sulfuric acid. However, this treatment process would increase the cost of concrete sludge recycling, which is estimated to be as much as about US$60/t. It is therefore necessary to develop an effective and low-cost recycling or reuse process for the waste. Some methods for recycling concrete sludge have been proposed, including reuse as a raw material in cement production, road-bed materials, soil neutralizers, neutralizers for waste incinerator gas, and fine aggregate for concrete production. Reuse of concrete sludge (after it has been mixed with construction sludge) as a recycled soil,1 use of concrete sludge as a raw material for high-fluidity concrete,2,3 recycling of sludge water for concrete production,4 use of concrete sludge in alkaline-stabilized biosolids,5 development of a concretesludge-based geopolymer,6 addition of concrete sludge to glass© 2012 American Chemical Society
Received: Revised: Accepted: Published: 6099
August 31, 2011 April 4, 2012 April 6, 2012 April 6, 2012 dx.doi.org/10.1021/ie300620u | Ind. Eng. Chem. Res. 2012, 51, 6099−6104
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Figure 1. Process flow diagram of the bench-scale plant.
plant. It should be noted that the total mass of calcium contained in the concrete sludge generated in the factory was estimated to be about 7000 t annually, and the mass of CO2 in the boiler waste gas was estimated to be about 3000 t annually. Figure 2 shows a photograph of the bench-scale plant. The
collect data applicable for the design of a practical plant based on operating experiments with actual concrete sludge from industry and flue gas from oil-combustion boilers. The present article describes the results of the operation of the bench-scale plant from July 2009 to February 2010 and evaluation of the feasibility of the proposed process.
2. PROCESS OUTLINE Figure 1 illustrates the process flow diagram for the recycling of concrete sludge. The process equipment mainly consists of (1) the calcium extraction reactor and (2) the calcium carbonate crystallization reactor. Concrete sludge generated in industry is introduced into the extraction reactor and diluted with a certain amount of water. The contents in the reactor are stirred, and some of the calcium dissolves from the concrete sludge into the aqueous phase. After a certain time of extraction, the solid residue is separated by gravitational sedimentation, and the supernatant is moved to the calcium carbonate crystallization reactor. Flue gas containing CO2 is introduced into the crystallization reactor, and calcium carbonate froms according to the reaction Ca 2 +(aq) + CO32 −(aq) → CaCO3↓ Figure 2. Photograph of the bench-scale plant.
Because of the low solubility of calcium carbonate in water, calcium carbonate precipitates in the reactor. The calcium carbonate can be separated by gravitational sedimentation, and neutralized water is transferred to the extraction reactor for reuse. The flue gas is supplied from boilers equipped for supplying steam to cure the concrete product. Theoretically, the net inputs to the process are concrete sludge and flue gas (CO2), and the net outputs are calcium carbonate and the solid extraction residue. Based on this concept, we designed and constructed a benchscale plant on the campus of the Kawashima-Daini plant of East Nippon Concrete Company Ltd. in Chikusei City, Ibaraki Prefecture, Japan. Concrete poles and piles are produced at this
volume of the extraction reactor (leftmost in the figure) is about 1.0 m3, and that of the calcium carbonate crystallization reactor (third from the left) is about 1.2 m3. The other vessels are the tanks used as buffers or reservoirs for circulation. Table 1 lists detailed specifications of the bench-scale plant.
3. MATERIALS Real concrete sludge generated in the Kawashima-Daini plant was used in this study. In this plant, concrete poles and piles are produced by a centrifugal molding method. The concrete sludge is generated as excess from the fresh concrete used in 6100
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Table 1. Specifications of the Bench-Scale Plant diameter (mm)
height (m)
volume (m3)
Ca extraction reactor
800
1.0
1.0
CaCO3 crystallization reactor first reservoir second reservoir
700
1.4
1.2
800 800
1.0 1.0
1.0 1.0
vessel
stirrer specifications one propeller (200 mm), 0.75 kW two propellers (200 mm), 0.4 kW
water-level meter, thermometer, pH meter
0.75
water-level meter, thermometer, pH meter, gas blower (5.5 kW) water-level meter water-level meter
4. EXPERIMENTAL METHODS AND RESULTS The main objective of the experimental study was to determine optimal operating conditions for the treatment of concrete sludge. 4.1. Calcium Extraction from Concrete Sludge (Experiments Performed Jul 14−16, 2009). The key index for the process efficiency in the extraction step is the extraction ratio of calcium from the concrete sludge. The extraction ratio is measured in relation to the amount of product in the crystallization process, namely, calcium carbonate. Partial extraction of calcium is also essential to prevent the hardening of concrete through the hydration reaction. The extraction ratio depends on the water/sludge ratio, stirring duration, and stirring rate. In addition, the extraction efficiency could be affected by the quality of water recycled from the crystallization reactor. The effects of these operating parameters on the extraction of calcium ions from the concrete sludge were preliminarily examined in laboratory-scale experimental studies.11 The following conditions are based on the results obtained in those studies. The ratio of concrete sludge to water was fixed at 1:19. Fifty kilograms of concrete sludge and 950 kg of groundwater were introduced into the calcium extraction reactor. This ratio was selected by considering the balance of the calcium extraction amount and the operating time based on the laboratory experimental studies conducted prior to the operation.11 A higher ratio of water could result in a higher amount of calcium in one operation, but the sedimentation speed was reduced. During the extraction operation, aliquots of the reactant were intermittently sampled by being filtered through a membrane filter with a pore diameter of 0.20 μm, and the calcium concentration in the water phase was measured by ICP-AES. Stirring was stopped after a certain time of operation, and the solid component was allowed to settle in the reactor. During sedimentation, the sedimentation rate was measured visually through the observation window. The temperature and pH of the reactant were monitored with a thermometer and a pH meter, respectively. In this series of experiments, fresh water was used for each extraction operation. No temperature control was conducted. The performance in the extraction of calcium from concrete sludge diluted with water could depend mainly on the stirring rate and duration. We conducted extraction operations in the extraction reactor at different stirring rates, for a fixed stirring time at 30 min. Figure 3 shows the influence of stirring rate on the calcium concentration extracted into the aqueous phase from the concrete sludge. The calcium concentration increased quickly up to 5 min and remained almost constant thereafter. The final concentration of calcium after 30 min was below the solubility of calcium carbonate under atmospheric conditions, which is about 700 ppm. No marked effect on the calcium extraction rate was observed for stirring rates, despite the variation of the physical properties of the concrete sludge used
Table 2. Chemical Composition of a Concrete Sludge Sample component
composition (wt %)
component
composition (wt %)
CaO SiO2 Al2O3
17.2 3.2 0.6
Fe2O3 watera
1.9 77.1
other equipment
1.5
0.75 0.75
the molding process. The excess concrete is diluted with water to form concrete sludge, and the concrete sludge is transferred to the waste treatment site at the plant. There, the aggregate components are recovered by sieving. The concrete sludge used in this study was the sludge obtained after sieving. The density of the concrete sludge ranged from 1.3 to 1.7 kg/L, depending on the concrete composition required by the specifications for given products. This means that the sludge treatment plant should be adjustable to the varying properties of the concrete sludge. Table 2 lists the chemical composition of a concrete sludge sample measured by X-ray fluorescence (XRF, Rigaku ZSX
a
pump power (kW)
Water as liquid or hydrated water.
Primus II). The major component of the concrete sludge is calcium, with a weight fraction of 17.2 wt % as CaO. The dilution ratio can be assumed to be a factor of 4 based on the normal calcium oxide content of Portland cement of 64 wt %; moreover, cleaning of the cement production facility typically makes water-diluted sludge. It should be noted that concrete sludge samples hardened completely after several hours without further treatment. Flue gas from heavy-oil-burning boilers was supplied to the crystallization reactor. Two sets of operating conditions were used for boiler combustion: low and high combustion loads. The CO2 concentrations in the flue gas were 6−8% for the low combustion load and 11−13% for the high combustion load. For the low combustion load, both the air and fuel volumes are reduced compared to those for the high combustion load. The air ratios for the two combustion loads were almost same. The CO2 concentration was measured with an infrared CO2 monitor (Shimadzu, NOA-7000). The temperature of the supplied flue gas was almost equal to the surrounding air temperature. Note that high-temperature flue gas could be directly used for CaCO3 crystallization, which would accelerate both CO2 dissolution and the CaCO3 crystallization reaction. Groundwater was used in this study. The calcium concentration in the groundwater was 26 ppm as measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, Shimadzu, ICPS-7500). This concentration is negligible compared with the saturation concentration of calcium ions in water of 700 ppm (=mg/L) at 25 °C under atmospheric conditions (400 ppm CO2). The temperature of the groundwater was approximately 15 °C, and it remained almost constant during plant operation. 6101
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concrete sludge increased. A lower calcium concentration in the extracted solution would result in a lower efficiency in CaCO3 crystallization because the CaCO3 crystallization amount is determined by the difference between the calcium concentration in the extracted solution and the saturated calcium concentration under the conditions of the CaCO3 crystallization reactor. Multiple extraction operations would not be highly economical. The total calcium extraction ratio reached about 30 wt % after five extraction operations. 4.2. Calcium Carbonate Crystallization (Experiments Performed Aug 19−21 and Oct 5−7, 2009). The extracted solution was introduced into the calcium carbonate crystallization reactor, where the flue gas from boilers was bubbled into the solution through 10 aerators (Aience Ltd., Aquablaster AS-250 L). The aerators were located 50 cm above the reactor bottom. The flow rate of flue gas introduced to the reactor from the boiler was 0.5 m3/min. The measured CO2 concentration in the flue gas was 13% for the high combustion load and 8% for the low combustion load. The reactor was exposed to the atmosphere; thus, the inner pressure was atmospheric pressure. No stirring was conducted in the crystallization process. The calcium concentration in the water phase was measured by ICP-AES. The temperature and pH of the reactants during the operation were monitored. After the crystallization operation, all contents in the reactor were removed, and the calcium carbonate crystals were separated by sedimentation. The obtained solid was dried, and its physical properties were characterized as follows: The chemical compositions of product particles were measured by XRF using the bulk fundamental parameter method. The crystal phases of the product were analyzed by X-ray diffraction (XRD; Rigaku, Minifrex). Particle size distributions were measured with a laser-scattering particle size measurement apparatus (Shimadzu, SALD2100). The crystallization process performance could be affected by the CO2 concentration, the duration of bubbling, and the addition of seed crystals of calcium carbonate, and the effects of these variables were examined in view of the above-mentioned physicochemical properties of the produced calcium carbonate. After about 1 min of bubbling of the flue gas, white particles appeared in the crystallization reactor. The pH of the solution was about 10 when the crystallization started. The white particles were identified to be calcium carbonate (calcite) by XRD analysis. The generated particles were found to agglomerate to form snowflakelike crystals in 4 or 5 min after the bubbling started. The crystals were broken into small particles with further bubbling. The snowflakelike crystals disappeared even when the bubbling was stopped after their generation. The sedimentation speed of the aggregated calcium carbonate crystals was about 10 cm/min in the reactor. On the other hand, the smaller particles formed by the breaking of the snowflakelike crystals was about 1 cm/min, reflecting the smaller particle size. Figure 5 shows the time variation of the calcium concentration in the water phase during the bubbling of the flue gas. The calcium concentration decreased with time because of the crystallization of calcium ions in the form of calcium carbonate. We conducted the crystallization operations four times, one time with lower CO2 concentration and three times with higher CO2 concentration. The initial decreasing rate of the calcium concentration increased with increasing CO2 concentration. After 9 min of bubbling, calcium concentrations below 200 ppm were obtained for all cases studied. The solubility of calcium ions in the aqueous system with CO2 was
Figure 3. Influence of stirring rate on the calcium concentration in the extraction operation.
in this series of experiments. This result suggests that the masstransfer rate at the surface of the concrete sludge particles cannot be the rate-determining step for the overall calcium extraction rate. Instead, chemical reactions such as the hydration of cement compounds and subsequent generation of calcium hydroxide must determine the observed extraction rate. When the stirring speed was 60 rpm, some of the solid particles agglomerated at the bottom of the reactor during the extraction operation. On the other hand, the reactant was homogeneously mixed when the stirring speed was higher than 120 rpm. From these observations, the stirring speed for the extraction step was set at 120 rpm for subsequent operations. The time necessary for the separation of the solid residue and the liquid phase after stirring was measured. It took several minutes for the sludge to settle after stirring was stopped. The average sedimentation rate of the extraction residue in the reactor was about 10 cm/min, so that 30 min should be sufficient for most of the solid residue to settle at the bottom of the reactor. The gravitational sedimentation time was thus determined to be 30 min. The aqueous phase was then suctioned from the reactor with a pump through a plastic pipe. The solid residue did not show the self-hardening property. Figure 4 shows the calcium concentration in the aqueous phase after multiple extraction operations. Fresh water was used for each operation. With increasing number of cycles of extraction, the concentration of calcium extracted decreased monotonically, and the ratio of calcium extracted from the
Figure 4. Effect of multiple extraction operations on the calcium concentration in the water phase. 6102
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4.4. Characterization of Produced Calcium Carbonate. The purity of the calcium carbonate product was greater than 99 wt %. Other contaminants were silicon and magnesium. From the XRD analysis, the calcium carbonate was determined to be in the form of calcite. The particle diameters were distributed in the range of 3−30 μm, with a peak at about 10 μm according to the laser scattering method. 4.5. Buildup of Scale in the Plant. The buildup of calcium carbonate scale inside the pipe lines is a common issue when calcium ions are involved in a process. The amount of calcium carbonate scale deposited during plant operation was estimated as follows: After one series of experiments in which extraction was performed 20 times and crystallization was performed 10 times, both the calcium extraction and calcium carbonate crystallization reactors were filled with 5 wt % citric acid solution overnight. Then, the calcium concentrations dissolved in the citric acid solutions were measured. The amount deposited inside the extraction reactor was about 100 g of Ca, and that inside the crystallization reactor was about 200 g of Ca. This amount of calcium was equivalent to about 5 wt % of the total calcium in the produced calcium carbonate. 4.6. Process Evaluation. Reducing emissions of CO2 was also one of the main targets of the present study. The net reduction of CO2 avoided is the difference in the amount of CO2 fixed as calcium carbonate and the emissions of CO2 associated with the power consumption for operating the process. Assuming the specific CO2 emission per unit power generation is 0.324 kg of CO2/kWh,12 the ratio of the CO2 fixed to that emitted is about 1.58. Thus, the present process can be recognized as a CO2 fixation process. This amount, however, does not include the CO2 emissions associated with the plant building.
Figure 5. Time variation of the calcium concentration in the water phase during boiler exhaust gas bubbling (including influence of CO2 concentration).
120 ppm for 8% CO2 and 150 ppm for 13% CO2. The calcium concentration observed in the present study reached close to these values with about 9 min of bubbling. In actual operation, because concrete sludge and flue gas are in the waste stream, so that their properties can vary for several reasons, it will be necessary to adjust the bubbling conditions depending on their properties to achieve the maximum performance. Addition of CaCO3 (reagent grade, Wako Pure Chemicals) as a seed material was also examined; however, no acceleration effect on crystallization was observed (data not shown). 4.3. Repeated Operations (Performed on Nov 17−21, 2009, and Jan 26−27, 2010). A cycle of extraction and crystallization was repeated five times to examine the effects of the reuse of water on the process performance. The gravitational sedimentation time after calcium extraction was fixed at 30 min, and the gas bubbling time for the crystallization was 9 min. The gravitational sedimentation time after calcium carbonate crystallization was changed in the range of 30−45 min according to the sedimentation conditions of the product. This experiment was conducted on two different days, and concrete sludge samples with different properties were used. Figure 6 shows the calcium concentration variations in the water phase during repeated operations. The calcium concentration changed as expected with calcium extractions and CaCO3 crystallizations. Thus, process water can be reused at least five times in the treatment process.
5. SUMMARY OF THE RESULTS From the above results, we confirmed the technical feasibility of the proposed concrete sludge recycling process using CO2 in the bench-scale plant. High-purity calcium carbonate can be produced from waste concrete sludge, and net CO2 fixation can be achieved for the overall process. Water can be reused for a number of operating cycles including the extraction of calcium from concrete sludge and the crystallization of calcium carbonate with flue gas bubbling. The process thus can be recognized as both a recycling process for concrete sludge and a CO2 fixation process. Based on the present study, however, process improvement is necessary for various steps for scaling up for construction of a practical plant. First, the extraction ratio of calcium from the concrete sludge should be improved from about 30 wt % after five extraction operations in the present study. However, the effort to increase the calcium extraction efficiency could be costly, and there is some degree of balance in the calcium extraction. In addition, reuse of the solid residue after calcium extraction should be considered. One potential use would be as a calcium-based adsorbent for the removal of heavy-metal ions or toxic anions from water effluents. In this case, it might be better for some of the calcium to remain after extraction: calcium can be used for adsorption or ion exchange, and selfhardening can be avoided by the partial extraction of calcium. The efficiency of the solid/liquid separation could be improved from the present method based on gravitational sedimentation; other possible options include filter press or belt press separations. For the present plant, the crystallization process required no stirring, which is usually necessary to promote the
Figure 6. Calcium concentration variations in the water phase during repeated operations. 6103
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(6) Yang, Z. X.; Ha, N. R.; Hwang, K. H.; Lee, J. K. A Study of the Performance of a Concrete Sludge-Based Geopolymer. J. Ceram. Process. Res. 2009, 10, S72. (7) Tian, Q. B.; Wang, Y.; Xu, L. N.; Wang, X. H.; Gao, H. Preparation and Properties of Glass-Ceramics Made from Concrete Sludge. Rare Metal Mater. Eng. 2007, 36, 979. (8) Nonaka, K.; Iizuka, A.; Yamasaki, A.; Yanagisawa, Y. Preparation of Hydroxyapaptite (HAP) from Concrete Sludge and Evaluation of Its Capacity to Remove Cadmium, Copper and Fluoride Ions. Kagaku Kougaku Ronbunshu 2010, 36 (5), 539−544. (9) Sasaki, T.; Sakai, Y.; Iizuka, A.; Nakae, T.; Kato, S.; Kojima, T.; Yamasaki, A. Evaluation of the Capacity of Hydroxyapaptite Prepared from Concrete Sludge to Remove Lead from Water. Ind. Eng. Chem. Res. 2011, 50, 9564. (10) Fujiwara, N., Kuroki, H. Desulphurization Methods of Flue Gas from Fluidized-Bed Furnace. Japanese Patent H10-267221, 1998. (11) Iizuka, A.; Yamasaki, A.; Honma, M.; Hayakawa, Y.; Yanagisawa, Y. Aqueous Mineral Carbonation Process via Concrete Sludge. Kagaku Kougaku Ronbunshu 2012, 38, 129. (12) Tokyo Electric Power Co., Inc. Homepage, http://www.tepco. co.jp/eco/report/glb/02-j.html (accessed Dec 22, 2010).
mass-transfer rate. However, it is worth examining the effect of stirring for increasing the crystallization rate; of course, the balance should be considered between the cost of power consumption and the efficiency. We continue the present project to expand the scale of the operation partially supported by the Ministry of Land, Infrastructure, Transport and Tourism of the government of Japan. The objective of the next step is to design and construct a practical-scale or pilot-scale plant in the factory of Kawashima-Daini Plant of Nippon Concrete Industry Inc., based on the knowledge obtained from the bench-scale plant. Results of operations of such a plant will appear in future reports.
6. CONCLUSIONS A bench-scale plant for the treatment of waste concrete sludge was designed and constructed for the recycling of the concrete sludge and the fixation of CO2. The influences of the operating conditions on the calcium extraction performance and calcium carbonate crystallization (i.e., CO2 fixation) were examined based on operating experiments. Up to 30 wt % of the calcium content can be extracted from the concrete sludge in the calcium extraction reactor where the sludge was mixed and stirred with water for 60 min. The solid residue had no selfhardening property after extraction, which made its handling much easier for the further use. High-purity calcium carbonate particles were obtained from the extracted solution and the flue gas from boilers. Recycled water after crystallization can be reused for the extraction of calcium ions from the fresh concrete sludge, which can reduce the water use of the overall process. The total process is recognized as a CO2 fixation process in the form of carbonate.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support provided by NEDO (New Energy and Industrial Technology Development Organization), Kawasaki, Japan. The authors also gratefully acknowledge technical support of Nikko Co. Ltd., Akashi, Japan.
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REFERENCES
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