Article

Methane and carbon dioxide adsorption-diffusion experiments on coal: Upscaling and modeling

Institute of Geology and Geochemistry of Petroleum and Coal, Aachen University (RWTH-Aachen), Aachen, Germany
International Journal of Coal Geology (Impact Factor: 3.38). 12/2004; 60(2):151-168. DOI: 10.1016/j.coal.2004.05.002

ABSTRACT

Numerical modelling of the processes of CO2 storage in coal and enhanced coalbed methane (ECBM) production requires information on the kinetics of adsorption and desorption processes. In order to address this issue, the sorption kinetics of CO2 and CH4 were studied on a high volatile bituminous Pennsylvanian (Upper Carboniferous) coal (VRr=0.68%) from the Upper Silesian Basin of Poland in the dry and moisture-equilibrated states. The experiments were conducted on six different grain size fractions, ranging from <0.063 to ∼3 mm at temperatures of 45 and 32 °C, using a volumetric experimental setup. CO2 sorption was consistently faster than CH4 sorption under all experimental conditions. For moist coals, sorption rates of both gases were reduced by a factor of more than 2 with respect to dry coals and the sorption rate was found to be positively correlated with temperature. Generally, adsorption rates decreased with increasing grain size for all experimental conditions.

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    • "In this case, CO 2 can be physically adsorbed on organic matter and/or clay minerals in the same way as methane. It is known from other research studies that CO 2 is preferentially adsorbed on the surface of organic matter in comparison to CH 4 e this is particularly apparent in case of coal (Busch et al., 2004; Ceglarska-Stefa nska and Zare ˛ bska, 2005; Battistutta et al., 2010; Pini et al., 2010; Busch and Gensterblum, 2011). Ceglarska-Stefa nska and Zarebska (2005) conducted sorption tests on coals with moderate levels of metamorphism and the ratio of CO 2 /CH 4 sorption was approximately 3:1 (maximum pressure of experiment was 4.0 MPa). "
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    ABSTRACT: Gas retention mechanisms in shales are to some extent similar to that of coal. The gas is adsorbed in organic matter (mostly organic carbon) and clay minerals whereas transport of gas occurs in fractures. In the study two materials were analyzed- coal from the Upper Silesia Coal Basin and shale sample from Baltic Basin. The coal selected for experiments was a bituminous steam coal with 3.8%wt ash content. The shale sample was characterized by rather low TOC (1.1%) but high clay minerals content. The purpose of the study was to compare the high pressure CO2 sorption characteristics of coal and gas shale and relate it to the particle size of samples subjected to tests. The size of the adsorbate has an impact on the sorption equilibrium time and reaching thermodynamic equilibrium occurs much faster in fine grained fractions. On the contrary, the particle size must reflect natural in-situ conditions and accessibility to the nano- and micropores where sorption occurs. In this study both sorbents were crushed and sieved into three particle size fractions and the CO2 sorption was measured. The measurements were performed at the constant temperature of 55 °C and up to the pressure of 15-16 MPa. To model the sorption behavior a three parameter Langmuir model was fitted to experimental values. Results of the sorption tests show that the particle size of the sorbent has an impact on the obtained sorption isotherm. In case of coal, the difference in the sorption capacity could be related to the ash and inertinite content which are passing to fine particle size (<0.1 mm) whereas in shale it could be related to the area of exposed surface and extended time of sorption equilibrium.
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    • "Competitive adsorption behavior of CO 2 and CH 4 in coal seams and carbon nanopore has been reported [2,3,8–18]. Experiments with gas mixtures (CO 2 /CH 4 ) have shown that selective adsorption does not always favor CO 2 and there is clear evidence for preferential adsorption of CH 4 at low pressures [3] [15] [19]. Majewska et al. [19] observed unusually high affinity of bituminous coal for CH 4 over CO 2 at a pressure of 2.6 MPa and a comparable affinity at 4 MPa. "
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    ABSTRACT: Adsorption isotherms of carbon dioxide (CO2) and methane (CH4) provide crucial information for CO2 sequestration and exploitation of coal seam gas. In this work, we focus on the competitive adsorption behavior of CO2 and CH4 in micropores of an intermediate ranked bituminous coal by performing Monte Carlo (MC) simulations at different injection depths from 300 m up to 3280 m with varying injected gas compositions. An extended poromechanical model enables us to relate our simulation results of adsorption to volumetric strain in the coal. Our simulations show that (i) CO2/CH4 adsorption selectivity, defined as the ratio of the mole fractions of the two species in the adsorbed phase relative to the ratio of the mole fractions in the bulk phase, decreases with increasing injection depth for a fixed injected gas composition, (ii) at a given depth, CO2/CH4 adsorption selectivity decreases as the concentration of CO2 in the injected gas increases, (iii) CO2/CH4 adsorption selectivity appears to be a function of pressure and gas composition at a given temperature. The total adsorption increases with increasing concentration of CO2 in the injected gas at constant gas reservoir pressure. The CO2/CH4 adsorption selectivity decreases with increasing bulk CO2 mole fraction after an initial increase at low pressures, (iv) the volumetric strain has a direct correlation with the injected gas composition and increases with the concentration of CO2 in the injected gas. At 370.2 K the largest volumetric strain of 3.6% is predicted at 30 MPa for pure CO2 adsorption.
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    • "The process is started again thereby continuously increasing the pressure in the sample cell until the desired pressure of 25 MPa is reached. Details of the measuring procedure have been reported previously (Busch et al., 2004; Gasparik et al., 2012; Krooss et al., 2002; Weniger et al., 2010). "
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