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Carbon Dioxide Adsorption in Brazilian Coals Jose´ Luciano Soares,† Andre´ L. B. Oberziner,† Humberto J. Jose´,† Alı´rio E. Rodrigues,‡ and Regina F. P. M. Moreira*,† Departamento de Engenharia Quı´mica e Engenharia de Alimentos, UniVersidade Federal de Santa Catarina, Campus UniVersita´ rio-Trindade, CEP 88040-900, Floriano´ polis SC, Brazil, and Laboratory of Separation and Reaction Engineering (LSRE), Faculdade de Engenharia da UniVersidade do Porto (FEUP), Rua Dr. Roberto Frias, s/n, CP 4200-465 Porto, Portugal ReceiVed April 6, 2006. ReVised Manuscript ReceiVed August 10, 2006
Carbon dioxide (CO2) is one of the most important greenhouse gases. In the period between 1980 and 1998, CO2 emissions increased more than 21% and projections suggest that the emissions will continue to increase globally by 2.2% between 2000 and 2020 and 3.3% in the developed countries. The sequestration of CO2 in deep unminable coal beds is one of the more promising of several methods of geological sequestration that are currently being investigated. CO2 can adsorb onto coal, and there are several studies demonstrating that CO2 dissolves in coals and swells them. At very low pressures (P < 1 bar), CO2 dissolution does not seem to be a problem; however, high pressures are necessary for CO2 sequestration (P > 50 bar). In this study, we evaluated the kinetics and equilibrium of sorption of CO2 on Brazilian coals at low pressures. The adsorption equilibrium isotherm at room temperature (30 °C) was measured through the static method. The results showed that the Freundlich model or the Langmuir model is suitable to describe the equilibrium experimental results. The CO2 adsorption capacity of Brazilian coals are in the range of 0.089-0.186 mmol CO2/g, which are typical values for coals with high ash content. The dynamics of adsorption in a fixed-bed column that contains granular coal (particle sizes of 0.8, 2.4, and 4.8 mm) showed that the adsorption rate is fast and a mathematical model was developed to describe the CO2 dynamics of the adsorption in a fixed-bed column. The linear driving force (LDF) was used to describe the rate of adsorption and the mass-transfer constants of the LDF model (Ks) are in the range of 1.0-2.0 min-1.
1. Introduction Approximately 80% of the energy used worldwide comes from fossil fuels, and this share is expected to increase until at least the year 2020. The combustion of fossil fuels leads to emissions of carbon dioxide (CO2) into the atmosphere, which is believed to contribute to undesired global warming. CO2 capture is easier from large stationary fossil-fuelbased power generation than in automotive applications, where emissions come from an enormous quantity of small, mobile units.1 Five technology pathways can be used to achieve the reduction of greenhouse gas (GHG) emissions: (a) separation and capture, (b) geologic sequestration (oil and gas reservoirs, unminable coal seams, and deep saline aquifers), (c) terrestrial sequestration (retention of atmospheric CO2 by coupling improved agricultural and forestry practices), (d) oceanic sequestration (deep ocean storage), and (e) novel sequestration systems (novel approaches to chemical, biological, or other processes to recycle or reuse CO2).2 * To whom correspondence should be addressed. E-mail address:
[email protected]. † Departamento de Engenharia Quı´mica e Engenharia de Alimentos, Universidade Federal de Santa Catarina. ‡ Laboratory of Separation and Reaction Engineering (LSRE), Faculdade de Engenharia da Universidade do Porto (FEUP). (1) Bredesen, R.; Jordal, K.; Bolland, O. High-temperature membranes in power generation with CO2 capture. Chem. Eng. Process. 2004, 43 (9), 1129-1158. (2) Klara, S. M.; Rameshwar, D.; Srivastava, R. D.; McIlvried, H. G. Integrated collaborative technology development program for CO2 sequestration in geologic formationssUnited States Department of Energy R&D. Energy ConVers. Manage. 2003, 44 (17), 2699-2712.
CO2 capture technology and storage could be used in combination with other mitigation measures, such as changes in fuel, increases in energetic efficiency, and the use of renewable energy. CO2 can be captured from a variety of anthropogenic sources, such as power plants and large industrial and gas processing plants, and then either stored in the oceans or in geological reservoirs.3,4 Geological sequestration is the capture of CO2 directly from anthropogenic sources and its subsequent disposal deep in the ground for geologically significant periods of time. Sequestration in geological media is not dependent on climate conditions, and it does not compete with agriculture, forestry, or other industries; in addition, it is the only significant sink-oriented option available to landlocked major CO2 producers.5 The successful storage of CO2 and CH4 in geological formations has been demonstrated over a period of 20 years. The largest application is for enhanced oil recovery in North America, which involves ∼30 million tons of CO2 per year and several major CO2 pipelines.6 Currently, 70 000 tons of CO2 per year are injected in a deep coal seam in the United States, and, a similar project, at a smaller (3) Gale, J. Geological storage of CO2: What do we know, where are the gaps and what more needs to be done? Energy 2004, 29 (9-10), 13291338. (4) Gale, J.; Freund, P. Coal-bed methane enhancement with CO2 sequestration worldwide potential. EnViron. Geosci. 2001, 8 (3), 210-217. (5) Bachu, S. Sequestration of CO2 in geological media in response to climate change: road map for site selection using the transform of the geological space into the CO2 phase space. Energy ConVers. Manage. 2002, 43, 87-102. (6) Simbeck, D. R. CO2 capture and storagesThe essential bridge to the hydrogen economy. Energy 2004, 29 (9-10), 1633-1641.
10.1021/ef060149h CCC: $37.00 © 2007 American Chemical Society Published on Web 11/10/2006
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scale, is being developed in Canada.7 One of the most important factors in determining how much CO2 can be sequestered in a coal seam is the sorption capacity of the coals and their porous structure. A significant portion of their total open pore volume is located in micropores (8-20 Å) and submicropores (500 Å) or mesopores (20-500 Å). CO2 can access the finest porosity and firmly adsorb on the coal surface. CO2 is preferentially adsorbed onto the coal structure (more so than methane), and then the CO2 sequestration also would allow the recovery of methane. Quantifying the porosity in coals is essential to understanding how gases such as methane and CO2 are stored in the coal seam.8 Coal beds are naturally fractured, low-pressure, watersaturated reservoirs, where most of the gas is retained in micropore structure of the coal by physical adsorption.8 Although the basic knowledge of injecting CO2 into coal seams comes from fractured reservoir engineering, coal seams are heterogeneous, in terms of lithotype and morphology. This creates a challenge in understanding the subsurface behavior of the injected gas and the coal. Thus, optimization of the sequestration process can be enhanced if the gas uptake behavior of different individual coal lithotypes is well understood and can be represented in computational models.9 One of the difficulties, with respect to modeling the gas emission and adsorption process, is to obtain representative equilibrium and kinetics data, which can be used with more confidence in different phases of reservoir modeling.9 The major objectives of this study were to measure the adsorption behavior of methane, nitrogen, and CO2 on Brazilian coals at 30°C and low pressure (
