Adsorption rate characteristics of methane and CO 2 in coal samples from Raniganj and Jharia coalfields of India Santanu Bhowmik a, ⁎, Pratik Dutta b a Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 b Bengal Engineering and Science University, Shibpur, P.O. Botanic Garden, Howrah - 711103, West Bengal, India abstract article info Article history: Received 9 November 2012 Received in revised form 3 February 2013 Accepted 6 February 2013 Available online 16 February 2013 Keywords: Coalbed methane Adsorption rate characteristics Diffusion Pressure-dependency Pore diffusion model Linear approximation of unipore model A set of six sub-bituminous to high-volatile bituminous coal samples was chosen to analyze the adsorption rate during the isotherm tests. The isotherm tests were conducted at 30 °C on dry, powdered coal samples up to a max- imum experimental pressure of ~ 8 MPa and ~ 6.5 MPa for methane and CO2, respectively. It was observed that the rate of adsorption for CO2 is higher than that for methane for the same experimental pressure–temperature con- dition and, for a particular pressure step, the equilibrium was reached earlier for CO2 compared to methane. It was also observed that at an increased pressure range, the rates of adsorption and the diffusion were generally decreased. The experimental data were fitted to unipore diffusion model but this model failed to predict the experimental adsorption kinetics data for the entire time range. However, the modification of unipore model, as suggested by Mianowski and Marecka (2009), was able to satisfactorily predict the experimental kinetics data at various pressure stages for the entire time range with the introduction of a kinetic parameter, C, accounting for the effect of gas diffusivity. The adsorption kinetics data was “unusable” at the initial time range due to variation in temperature–pressure in the sample cell after gas was injected. It was also observed that this “unusable” data is small at lower pressure range (~15 s), but could be significant (~30–50 s) at higher-pressure range. © 2013 Elsevier B.V. All rights reserved. 1. Introduction It is well known that gas production from a coalbed methane (CBM) reservoir is a combined process of desorption, diffusion and Darcy flow. So, the success of a CBM operation depends on the gas storage in the coal matrix, the diffusion of the gases occurred in inter-molecular and molecule to pore-wall boundaries, and the fluid flow through the coal natural fracture system. Mass transfer of gas through the coal primary po- rosity is dominated by diffusion where fluid flow occurs in random man- ner, from high-concentration zone to low concentration zone, towards the secondary porosity. Pressure driven mass transfer, if any, is assumed to be negligible or incorporated with the diffusion (Gas Research Institute (GRI), 1996). It is also known that diffusion is a complex process of bulk diffusion (inter-molecular), Knudsen diffusion (molecule to pore-wall in- teractions) and surface diffusion (mass transfer from sorbed state to free state) (GRI, 1996; Pan et al., 2010; Smith and Williams, 1984a). Studies reported on the sorption kinetics of methane and CO 2 in coal are very few. Ruppel et al. (1974) stated that release of methane from small coal particles due to desorption is interlinked with diffusion. As methane is physically adsorbed into coal, the time required for desorp- tion is negligible compared to that for diffusion. Nandi and Walker (1975) studied the diffusion of methane from anthracite, medium vola- tile and high volatile A (HVA) bituminous powdered coal samples up to a pressure of 400 psi and observed a positive variation of effective diffusivities with pressure, though the effect is reduced from anthracite to HVA bituminous coal. They also observed that effective diffusivity in- creases with increasing methane concentration and decreasing particle size of coal. Smith and Williams (1984b) conducted pulse tracer analysis for laboratory measurement of the diffusivity parameters for methane in coal taking into consideration the desorption and diffusion of methane for both short and long times. Ciembroniewicz and Marecka (1993) conducted sorption of CO 2 at low temperatures of two Polish coals and identified the kinetic parameters. The factors on which the fractional up- take depends were also identified. Finally, the experimental data were evaluated to observe the combined effect of absorption and adsorption. Some of the earlier studies also discussed about the effect of exper- imental condition and/or coal type. It was seen that effective diffusiv- ities increased with a decrease in particle size (Busch et al., 2004; Cui et al., 2004; Gruszkiewicz et al., 2009; Li et al., 2010; Marecka and Mianowiski, 1998; Nandi and Walker, 1975; Siemons et al., 2003). It is established that an increase in temperature reduces the gas sorption capacity. So, at lower temperatures sorption capacity increases and hence, more time is required for equilibrium and diffusion rate also re- duces (Busch et al., 2004; Charriere et al., 2010). However, Li et al. (2010) found no dependencies with temperature for methane and CO 2 . Diffusion studies conducted at variable experimental pressure displayed a direct relationship — i.e., diffusion parameters increased for an increased pressure (Busch et al., 2004; Charriere et al., 2010; Nandi and Walker, 1975). However, the opposite trend of inverse relationship is also reported (Cui et al., 2004). But for all studies, it was observed that CO 2 diffusion rate is higher than that of methane at the same International Journal of Coal Geology 113 (2013) 50–59 ⁎ Corresponding author. Tel.: +1 403 803 9307. E-mail address: santanu789@gmail.com (S. Bhowmik). 0166-5162/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coal.2013.02.005 Contents lists available at SciVerse ScienceDirect International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo