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Characteristics and Performance of a Deep-Ocean Disposal System for Low-Purity CO2 Gas via Gas Lift Effect Takayuki Saito,*,† Sanai Kosugi,‡ Takeo Kajishima,§ and Katsumi Tsuchiya| Department of Mechanical Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Shizuoka 432-8561, Japan, Sumitomo Metal Industries Ltd., 8-2, Honshio-cho, Shinjuku-ku, Tokyo 160-0003, Japan, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan, and The University of Tokushima, 2-1 Minami-josanjima, Tokushima, Tokushima 770-8506, Japan Received October 25, 2000
Ocean sequestration of CO2 is a hopeful option to solve the global warming problems. We proposed the Gas Lift Advanced Dissolution (GLAD) System for efficient sequestration of pure CO2 at deep sea. The GLAD system is an inverse-J pipeline set in the ocean between 200 and 3000 m in depth. We have recently improved it to treat low-purity CO2 gas to reduce the cost for separation and capture of CO2 from exhausted gas. The newly developed system, named the Progressive Gas Lift Advanced Dissolution (P-GLAD) System, is to dissolve low-purity CO2 bubbles into seawater at a depth of 200-300 m and at the same time sequestrate CO2 at the deep sea of 1000-3000 m. Previous ideas of deep-sea sequestration of CO2, including storage of liquid CO2 on the deep-sea floor and direct releasing of liquid CO2 into the deep sea, necessitate the consumption of a huge amount of energy, because the realization of these ideas requires both high-purity capture of CO2 from exhausted gas and liquefaction of the CO2. To realize deep-sea sequestration of CO2 with low energy consumption and low environmental impact, we utilize a gas-lift effect to simultaneously dissolve low-purity CO2 gas into shallow seawater and transport the CO2 solution to a great depth. The present paper describes basic characteristics and performance of the P-GLAD system for low-purity CO2 gases, experimentally and numerically. It is demonstrated that the system has satisfactory ability to both dissolve CO2 gas and pump the CO2 solution. To confirm economic feasibility of the P-GLAD System, we also discuss the overall cost estimation including an additional system for CO2 capturing as well as the construction of P-GLAD. The cost for the P-GLAD is estimated to be half of those for previous ideas.
1. Introduction The global warming mainly due to the increase of atmospheric CO2 concentration is getting serious. We must develop countermeasures that can economically and effectively fix a huge amount of CO2 (23 Gt-C/year) emitted by human activities. Ocean sequestration of CO2 is a hopeful option to mitigate the increase in the atmospheric CO2 concentration, because the absorption capacity for CO2 of the ocean is enormous.1 Thus several ideas have been proposed for the ocean disposal of CO2. They are categorized as illustrated in Figure 1: (A) storage of liquid CO2 on the deep sea-floor;2,3 (B) direct release of liquid CO2 into intermediate-depth water from a moving ship;4,5 (C) direct release of gaseous CO2 into shallow water.6 The total system of ideas (A) and (B) * Corresponding author. Tel & Fax: +81-53-478-1601. E-mail:
[email protected]. † Shizuoka University. ‡ Sumitomo Metal Industries Ltd. § Osaka University. | The University of Tokushima. (1) Hoffert, M. I.; Wey, Y. C.; Callegari A. J.; Broecker, W. S. Climatic Change 1979, 2, 53-68. (2) Ohsumi, T. Energy Convers. Manage. 1993, 34, 1059-1064. (3) Brewer, P. G.; Friederich, G.; Peltzer, E. T.; Orr, F. M. Science 1999, 284, 943-945.
consists of a plant to separate and capture pure CO2 from exhausted gas, a compressor to liquefy the CO2, a plant to store the liquid CO2, and a tanker to transport the liquid CO2 to a dumping site. The total system of the idea (C) consists of a plant to separate and capture pure CO2 from exhausted gas, a compressor and underwater pipeline to pump the pure CO2 gas to a releasing site. Long-term isolation of CO2 from the atmosphere more than several hundred years is expected in the ideas of (A) and (B), because the initial injection point is in intermedisate or deep water. As these two ideas necessitate high-purity capture and separation, and liquefaction of CO2, they consume a huge amount of energy and need high cost. Moreover, they cause a secondary environmental impact through acidification due to the super-saturation of CO2 near gas-hydrate surface. In addition, CO2 gas-hydrate is not so stable as expected in the real ocean.7,8 In the idea of (B), CO2 droplets covered with thin film of gas-hydrate rise in the (4) Ohmura, H.; Mori, Y. H. Environ. Sci. Technol. 1998, 32, 11201127. (5) Liro, C. R.; Adams, E. E.; Herzog, H. J. Energy Convers. Manage. 1992, 33, 667-674. (6) Haugan, P. M.; Drange, H. Nature 1992, 357, 318-320.
10.1021/ef0002501 CCC: $20.00 © 2001 American Chemical Society Published on Web 02/14/2001
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Figure 1. Categorization of previous ideas of CO2 sequestration at sea.
seawater, because the density of the droplets is smaller than that of seawater at the pressure equivalent to the injection depth. Since the idea of (C) needs no liquefaction of CO2, it improves an amount of energy consumption. However, density and/or temperature layers in the middle and low-latitude ocean might prevent sinking of CO2 solution to a great depth.9 In addition, a bubble plume causes upward current of CO2 solution. The direct release of CO2 bubbles is not so reliable for long-term isolation. A method of the ocean sequestration has to isolate a huge amount of CO2 from the atmosphere for a long term, several hundred or thousand years, with low cost, low energy consumption and low environmental impact. To realize them simultaneously, the preprocessing should be low-purity separation of CO2 with no liquefaction. On the basis of our previous work regarding the GLAD system,10-15 we have newly developed a P-GLAD (Progressive Gas Lift Advanced Dissolution) system, as illustrated in Figure 2, for low-purity CO2 gas.17-19 In the present paper, we discuss performance, overall cost (7) Teng, H.; Yamazaki, A.; Chun, M.-K.; Lee, H. Energy 1997, 22, 1111-1117. (8) Hirai, S.; Okazaki, K.; Tabe, Y.; Hijikata, K.; Mori, Y. Energy 1997, 22, 285-293. (9) Nagaosa, R.; Saito, T. AIChE J. 1997, 43, 2393-2404. (10) Kajishima, T.; Saito, T.; Nagaosa, R. Energy Convers. Manage. 1995, 36, 467-470. (11) Kajishima, T.; Saito, T.; Nagaosa, R. Energy 1997, 22, 257262. (12) Saito, T.; Kajishima, T. Japanese Patent 2,655,818, 1997. (13) Saito, T.; Kajishima, T. U.S. Patent 5,662,837, 1997. (14) Saito, T.; Kajishima, T.; Nagaosa, R. Environ. Sci. Technol. 2000, in print. (15) Saito, T.; Kajishima, T.; Tsuchiya, K.; Kosugi, S. Chem. Eng. Sci. 1999, 54, 4945-4951. (16) Saito, T.; Kajishima, T.; Tsuchiya, K. Japanese Patent 2,958,460, 1999.
Figure 2. Concept and principle of the P-GLAD system.
estimate, and environmental receptivity of P-GLAD using our experimental and numerical results. First, we describe experimental results on solubility and pumping performance of the system. Second, acidity of the solution released from the system is discussed. Third, an outline of the numerical method is explained showing reasonable agreement between numerical and experimental results. Finally, we compare the cost estimated for the overall P-GLAD system to that of the previous method including direct release of liquid CO2. 2. Experimental and Numerical Methods 2.1. Experimental Setup. The experimental setup used in the present investigation is illustrated in Figure 3. A riser (1) and downcomer (2) are made of acrylic transparent pipes that are 25 mm in diameter and 7.69 m in height. The riser is connected to two pressure vessels (4) and (5) made of stainless steel pipe that are 106.3 mm in diameter and 8.19 m in height. The downcomer is placed inside the vessel (4). The riser and downcomer are set in the situation equivalent to submergence. Large and small remote-controlled valves are attached at the top of the riser to discharge indissoluble gas. The maximum discharging rate of each valve is 2 × 10-6 and 2 × 10-5 kg/s, respectively. We adjusted the valve with gas injection rate. After completely mixing pure CO2 gas (99.9% purity) and pure air (CO < 1 ppm, CO2 < 1 ppm, and CH4
