Gases like CO 2 and CH 4 are able to adsorb on the coal surface, but also to dissolve into its structure causing the coal to swell. In this work, the binary adsorption of CO 2 and CH 4 on a dry coal (Sulcis Coal Province, Italy) and its swelling behavior are investigated. The competitive adsorption measurements are performed at 45°C and up to 190 bar for pure CO 2 , CH 4 and four mixtures of molar feed compositions of 20.0, 40.0, 60.0 and 80.0% CO 2 using a gravimetric-chromatographic technique. The results show that carbon dioxide adsorbs more favorably than methane leading to an enrichment of the fluid phase in CH 4 . Coal swelling is determined using a high-pressure view cell, by exposing a coal disc to CO 2 , CH 4 and He at 45 and 60°C and up to 140 bar. For CO 2 and CH 4 a maximum swelling of about 4 and 2% is found, whereas He shows negligible swelling. The presented adsorption and swelling data are then discussed in terms of fundamental, thermodynamic aspects of adsorption and properties which are crucial for an ECBM operation, i.e. the CO 2 storage capacity and the dynamics of the replacement of CH 4 by CO 2 .

[1] An experimental technique is presented to perform gas injection experiments on coal cores confined by an external hydrostatic pressure, which makes use of the so-called transient step method. The experiments are intended to improve the knowledge on the different mechanisms acting during CO 2 storage in coal seams, in particular, those related to permeability. Helium, nitrogen, and carbon dioxide have been injected at pressure ranging from 10 to 80 bars and at confining pressures varying between 60 and 140 bars. The experiments with helium have been used to study the mechanical compliance of the coal core, whereas those with the adsorbing N 2 and CO 2 to study the effects of adsorption and swelling on the flow dynamics. The obtained experimental transient steps were successfully described using a mathematical model, consisting of mass balances accounting for gas flow and adsorption, and mechanical constitutive equations for the description of porosity and permeability changes during injection. A semiempirical relationship between permeability and operating pressures is validated, and the corresponding parameters have been evaluated. Results showed increase in permeability with decreasing effective pressure on the sample and, when an adsorbing gas was injected, a reduction in permeability caused by swelling, with CO 2 having a stronger effect compared to N 2 .

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Data on the adsorption behavior of CO 2, CH 4, and N 2 on coal are needed to develop enhanced coalbed methane (ECBM) recovery processes, a technology where the recovery of CH 4 is enhanced by injection of a gas stream consisting of either pure CO 2, pure N 2, or a mixture of both. The pure, binary, and ternary adsorption of these gases on a dry coal from the Sulcis Coal Province in Italy has been measured at pressures up to 180 bar and temperatures of 45 and 70 degrees C for the pure gases and of 45 degrees C for the mixtures. The experiments were performed in a system consisting of a magnetic suspension balance using a gravimetric-chromatographic technique. The excess adsorption isotherms are successfully described using a lattice density functional theory model based on the Ono-Kondo equations exploiting information about the structure of the coal, the adsorbed gases, and the interaction between them. The results clearly show preferential adsorption of CO 2 over CH 4 and N 2, which therefore indicate that ECBM may be a viable option for the permanent storage of CO 2.

Enhanced coalbed methane recovery (ECBM) increases the recovery of the methane present in a coal seam by injecting CO 2 at high pressure. It is attractive from two perspectives, the valuable methane recovered and the storage of the greenhouse gas CO 2 for geological times. In the framework of a feasibility study for the Sulcis Coal Province in Italy, the adsorption of pure CO 2 and CH 4 on dry coal has been measured at 45 and 608C, using a magnetic suspension balance with in situ density measurement. The results show that the CO 2 adsorption isotherms on coal are similar to those for other standard adsorbents such as silica gel and activated carbon. From the excess adsorption isotherms, the absolute adsorption is calculated using the assumption of constant volume of the adsorbed phase. As expected, CO 2 get adsorbed more than CH 4 in all cases. The Sulcis coal can uptake CO 2 at the reservoir conditions in an amount of about 10% of its mass.

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A gravimetric apparatus is used to measure the excess adsorption at high pressure. The equipment consists of a Rubotherm magnetic suspension balance, which allows to measure also the density of the fluid. In order to obtain the excess adsorbed amount, the measured weight has to be corrected with a buoyancy term, for which the density of the adsorbing fluid has to be known at each experimental conditions. Therefore the homogeneity of density in the high-pressure cell plays a fundamental role in determining the accuracy of the measured excess adsorbed amounts. This paper is intended to show the impact of the actual approach to thermostating the unit on the density distribution of the adsorbing fluid inside the high-pressure cell. Namely, by changing the inlet position of the heating fluid, large differences in the measured excess adsorption are produced. The closer to the critical point of the fluid, the R. Pini, S. Ottiger . A. Rajendran . M. Mazzotti ( ) ETH Zurich, Institute of Process Engineering,

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