Carbon Dioxide Adsorption in Brazilian Coals

Carbon dioxide (CO 2 ) is one of the most important greenhouse gases. In the period between 1980 and 1998, CO 2 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 CO 2 in deep unminable coal beds is one of the more promising of several methods of geological sequestration that are currently being investigated. CO 2 can adsorb onto coal, and there are several studies demonstrating that CO 2 dissolves in coals and swells them. At very low pressures ( P < 1 bar), CO 2 dissolution does not seem to be a problem; however, high pressures are necessary for CO 2 sequestration ( P > 50 bar). In this study, we evaluated the kinetics and equilibrium of sorption of CO 2 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 CO 2 adsorption capacity of Brazilian coals are in the range of 0.089−0.186 mmol CO 2 /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 CO 2 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 ( K s ) are in the range of 1.0−2.0 min - 1 .