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Fundamental Tests on Carbon Dioxide Sequestration Into FUNDAMENTAL TESTS ON CARBON DIOXIDE SEQUESTRATION INTO COAL SEAMS K. Ohga1), K.Sasaki2),G. Deguchi3) and M.Fujioka3) 1) Graduate School of Hokkaido University 2) Akita University 3) Japan Coal Energy Center ABSTRACT Recently, carbon dioxide injection into coal seams has become of major interest as one method of carbon dioxide sequestration; sequestration into coal seams is more advantageous than other geo-sequestration because of the following points; 1) the principal of this sequestration is due mainly to the action of gas adsorption to the coal, therefore carbon dioxide injected into the coal seam is fixed firmly. 2) When carbon dioxide is injected into coal seams, methane can be recovered as a by-product. This will reduce the cost of sequestration. The fundamental research group carried out various laboratory tests, which were adsorption tests of carbon dioxide and methane, replacement tests of methane by carbon dioxide and adsorption tests of coal samples treated by carbon dioxide under supercritical condition. Taiheiyo Coal and Akabira Coal were used in these tests. From the results, it is clear that there is a big difference in adsorption volume of methane between Taiheiyo Coal and Akabira Coal, but there is no difference in carbon dioxide adsorption volume between them at low pressure. In this paper, we will describe the results of other tests. INTRODUCTION Fig.1 shows the main coal fields and coal mines in Japan. There are seven main coal fields. At the end of last year, the Matsushima coal mine in Kyushu was closed and in the beginning of this year, the Taiheiyo Coal Mine was closed; the name of coal mine was then changed from the Taiheiyo to the Kushiro Coal Mine and it was re-established. The mining area of this mine was moved from a deeper to a shallower area. Blue characters in this figure indicate the name of coal fields and red figures indicate coal resources of the coal fields. Most of the coal fields are in Kyushu and Hokkaido. More than 50% of coal is in Hokkaido and the most gassy coal field, which is the Ishikari coal field, is in the center of Hokkaido. The permeability of coal seam of the southern part of the Ishikari coal field is one of highest levels in Japanese coal fields. It is one of the sites proposed for CO2 injection into a virgin coal seam. In order to understand Greenhouse Gas Control Technologies, J. Gale and Y. Kaya (Eds.) © 2003 Elsevier Science Ltd. All rights reserved the mechanism of replacement of methane absorbed on the internal surface of coal by carbon dioxide, we carried out the following tests: 1) adsorption tests using carbon dioxide, methane and nitrogen, 2) replacement tests of methane by carbon dioxide and gas mixtures. 3) adsorption tests of coal samples treated with carbon dioxide under supercritical condition. The coal samples collected in the Ishikari and Kushiro Coal Fields were used. COAL SAMPLES Coal samples used in the absorption tests by pure gases and replacement tests by mixted gases were collected from the No.8 coal seam at Akabira Coal Mine in the Ishikari coal field. To compare the absorption volume and replacement ratio, coal samples collected from the main coal seam at Taiheiyo coal in the Kushiro coal field were used. The space of cleats of the No.8 coal seam was about 2- 4mm. Therefore, the coal samples were crushed and screened to between 4 and 8 mesh. EQUIPEMENT Fig.2 shows the equipment used for the adsorption tests and the replacement tests. The equipment consisted of a reaction vessel (I) to absorb gas on the coal sample and a buffer tank (G) to measure the injection gas volume to the reactor vessel. Pressure sensors (D1,D2) were installed on the reaction vessel and the buffer tank. The injection gas volume into the reaction vessel and released gas were calculated from the change of pressure in the buffer tank, and absorption volume into the coal sample was calculated from the change of the pressure in the reaction vessel. To reduce the effect of temperature change, the whole equipment was placed in a water bath (I) as shown in Fig.2. The temperature of the water was kept at 20C by the temperature controller (J). About 50g of coal sample was put into the Reaction Vessel (H). Released gas was analyzed by gas-chromatography. ADSORPTION TESTS Adsorption tests using carbon dioxide, methane and nitrogen were carried out using the following procedures: To remove the adsorbed gas on the coal sample, gas in the reaction vessel is sucked by a vacuum pump at 40oC, for 24 hours. After this, gas is injected into the buffer tank and the reaction vessel until the pressure in the reaction vessel reached a specified level; measurement of the adsorption gas volume at certain pressure is then carried out. The adsorption volumes are measured under pressure. Fig.3 shows the results of these tests by each gas. The circle line indicates the carbon dioxide adsorption isotherm. The triangle and square lines indicate the methane adsorption isotherm and nitrogen adsorption isotherm, respectively. From the results, the adsorption volume of carbon dioxide on the Akabira coal is three times as much as the methane one, and the adsorption volume of nitrogen is half of the methane adsorption. Greenhouse Gas Control Technologies, J. Gale and Y. Kaya (Eds.) © 2003 Elsevier Science Ltd. All rights reserved REPLACEMENT TESTS Methane replacement tests by carbon dioxide and mixtures of carbon dioxide and nitrogen was carried out as following procedures; the adsorbed gas on the coal sample is removed by a vacuum. Then, methane is injected into the reaction vessel to adsorb methane on the coal sample at about 1Mpa. After the pressure in the reaction vessel reaches the equilibrium pressure, gas in the reaction vessel is released. After the pressure in the reaction vessel reaches atmospheric pressure, carbon dioxide or mixtures of carbon dioxide and nitrogen are injected into the reaction vessel at 0.2Mpa. When the pressure in the reaction vessel reached the equilibrium pressure, the gas in the reaction vessel is released and its gaseous content analyzed by gas-chromatography. Fig.4 shows the results of the tests. From the results, when carbon dioxide concentration increases, the volume of carbon dioxide adsorption increases, the volume of methane adsorption decreases and nitrogen is unchanged. Fig.5 shows the results of replacement tests using Taiheiyo and Akabira coals. From the results, Akabira coal adsorbs methane more than Taiheiyo coal. From the measurement results, the methane replacement ratio and carbon dioxide sequestration ratio are calculated by the following equations: Methane Replacement Ratio =(A-B)/A x 100% Where, A; amount of methane adsorption on coal sample at the atmospheric pressure. B; amount of methane adsorption on coal sample at the atmospheric pressure after replacement. Carbon Dioxide Sequestration Ratio= D/C x 100% Where, C; amount of injected carbon dioxide D; amount of carbon dioxide adsorption after released gas. The results are shown in Table 1. TABLE 1 RESULTS OF REPLACEMENT TESTS CH4 Replacement Ratio (%) CO2 Sequestration Ratio (%) CO2(100%) 57.5 55.8 CO2(30%) N2(70%) 17.3 67.4 CO2(10%) N2(90%) 7.1 69.2 As a result, it was found that the higher ratio of sequestration to coal can be obtained when the lower concentration of carbon dioxide is injected, while methane replacement ratio decreases with decrease of carbon dioxide concentration. There is a big difference between methane replacement ratios, but there is no difference between carbon dioxide sequestration ratios. ESTIMATION OF ADSORPTION VOLUM UNDER MIXED GASES We tried to estimate the adsorption volume of each gas under a mixture of methane, carbon dioxide and nitrogen by using the Markham-Benton equation which is extended langmur equation for three mixed gases. I t is expressed as follows; K y Z = A A A + + K A y A K B yB K C yC Greenhouse Gas Control Technologies, J. Gale and Y. Kaya (Eds.) © 2003 Elsevier Science Ltd. All rights reserved Where, ZA expresses molar fraction of gas A in adsorption phase. YA expresses molar fraction of gas A in gas phase. KA(0.08), KB(0.0298) and KC (0.0027) were calculated from the results of adsorption tests by pure gases. Table 2 shows the molar fraction of each gas in the each phase calculated by the results of replacement tests and estimation molar fraction calculated by using Markham- Benton Equation. There is no difference between the results of replacement tests and estimation results by Markham-Benton equation. COAL SAMPLES TREATED BY CO2 UNDER SUPERCRITICAL CONDITIONS In the case where carbon dioxide is injected into deeper coal seams, it is considered that the temperature and the pressure during injection is greater than the supercritical condition of carbon dioxide, and after injection is stopped, the injected gas diffuses into the coal seam and the pressure drops under supercritical conditions of carbon dioxide. TABLE 2 CO2 100% CH4 CO2 N2 Molar fraction in Gas Phase 0.69 0.31 Molar Fraction in Adsorption Phase 0.45 0.55 Estimation Molar Fraction by Markham-Benton 0.45 0.45 CO2(30%)-N2(70%) Molar fraction in Gas Phase 0.59 0.05 0.35 Molar Fraction in Adsorption Phase 0.82 0.23 0.0 Estimation Molar Fraction by Markham-Benton 0.79 0.18 0.03 CO2(10%)-N2(90%) Molar fraction in Gas Phase 0.40 0.02 0.58 Molar Fraction in Adsorption Phase 0.86 0.12 0.03 Estimation Molar Fraction by Markham-Benton 0.79 0.11 0.10 Therefore, we conducted adsorption tests and replacement tests using coal samples which were treated by carbon dioxide under supercritical condition.
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