Swelling and sorption experiments on methane, nitrogen and carbon dioxide on dry Selar Cornish coal

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Abstract

Sorption isotherms of CO2, CH4 and N2 are determined at 318 K and 338 K for pressures up to 16 MPa in dry Selar Cornish coal using the manometric method. Both equilibrium sorption and desorption were measured. The desorption isotherms show that there is no hysteresis in N2, CH4 sorption/desorption on coal. The time to achieve equilibrium depends on the gases and is increasing in the following order: He, N2, CH4, and CO2. The results show that the sorption ratio between the maximum in the excess sorption N2:CH4:CO2 = 1:1.5:2.6 at 318 K and 1:1.5:2.0 at 338 K. Obtained ratios are within the range quoted in the literature.

Swelling and shrinkage induced by CO2 injection and extraction from Selar Cornish coal have been measured. The experiments have been conducted on unconfined cubic samples using strain gauges measurements at 321 K for pressures up to 4.1 MPa. It has been found that the mechanical deformation is fully reversible.

The density of CO2 in its sorbed phase, has been extrapolated from the excess sorption isotherm calculated including the swelling. The resulting value is unrealistically high. Possible reasons for this behavior are discussed in the text. Absolute sorption for CO2 has been estimated considering also the change in the coal volume due to swelling. The resulting isotherm calculated with or without the swelling is almost the same.

Research Highlights

►Equilibrium in a pressure step can be established if there is no change in pressure. ►For N2 and CH4 the excess sorption isotherm does not show any hysteresis. ►CO2 excess sorption isotherm does show hysteresis. ►The physical meaning of the extrapolated absolute density of CO2 obtained from the excess sorption isotherm is not clear. ► Excess sorption data may not provide enough information to accurately determine all unknowns and explain molecular-scale phenomena. ►The free swelling accounts for most of the decline in the CO2 excess sorption at high pressures.

Introduction

Concerns about global warming generated interest in reducing the emissions of the main greenhouse gas—carbon dioxide (CO2). Large quantities of CO2 are produced during the combustion of fossil fuels. Methods intended to reduce CO2 emission include its storage in geological formations, e.g., saline aquifers and (depleted) gas reservoirs. One of the options is CO2 injection into underground coal in combination with the production of CH4 originally present in coal seams. Another idea is to inject flue gas, i.e. a mixture of N2 and CO2 (Reeves, 2001, Mazumder et al., 2006b). In these cases N2 acts as a stripping agent. This technology is known as flue gas-Enhanced Coalbed Methane (flue gas-ECBM) recovery.

The effectiveness of enhancing methane production by CO2 and/or N2 depends on the sorption behavior of the main constituents. Therefore, knowledge about sorption behavior of CO2, CH4 and N2 is required. Many experimental dry coal-sorption studies have been published in the last years (e.g., Chaback et al., 1996, Clarkson & Bustin, 1999b, Ottiger et al., 2006, Busch et al., 2007, Saghafi et al., 2007, Siemons & Busch, 2007, Day et al., 2008, Prusty, 2008, Gruszkiewicz et al., 2009, Majewska et al., 2009). The measurements quantify the sorption capacity of CO2, CH4 and N2 in different kinds of dry coal.

The time required for attaining thermodynamical equilibrium for the gas sorbed by the coal is an important factor in any in situ application. Recent literature stresses the importance of the latter as an essential factor in establishing proper excess sorption isotherms for gases (Day et al., 2008, Prusty, 2008, Gruszkiewicz et al., 2009, Majewska et al., 2009). The equilibration time depends on gas, temperature of the system and the grain sizes of coal used in the experiments. A few authors studied the kinetics of the gas sorption on coal (Clarkson & Bustin, 1999b, Siemons et al., 2003, Busch et al., 2004, Solano-Acosta et al., 2004, Goodman et al., 2006, Gruszkiewicz et al., 2009). CH4 and CO2 adsorption occurs much faster in fine grained fractions. Siemons et al. (2003) reported that equilibration time is proportional to the grain size up to 1500 μm; above it is more or less constant. The grain size used in sorption experiments usually ranges between 63 μm and 2000 μm (Busch et al., 2006) with the exception of the work of Majewska et al. (2009) who used coal blocks. Equilibration times reported by these authors vary between 1 h and 440 h. Mazumder et al., 2006a, Mazumder et al., 2006b in his coal measured that the cleat spacing in coal is between 500 μm and 5000 μm. In order to maintain the structural integrity of the coal, in this paper the chosen particle size is between 1.5 and 2 mm, that leads to long equilibration time.

Sorption/desorption of gases on coal induces a relevant effect on its mechanical structure: the swelling/shrinkage of the matrix. Coal swelling induced by gas adsorption is a phenomenon extensively studied either using optical systems (Robertson, 2005) or strain gauges (Levine, 1996). Literature data concerning experimental results is remarkable and results are relatively consistent amongst different laboratories (Reucroft & Patel, 1986, Levine, 1996, Robertson, 2005, Shi & Durucan, 2005, Cui et al., 2007, Day et al., 2007, Mazumder & Wolf, 2007, Durucan et al., 2009, Pini et al., 2009, van Bergen et al., 2009). CO2 sorption on coal induces a bigger swelling effect on the coal matrix then CH4 and N2. It has been shown that at high pressure the coal is almost saturated and no sorption and swelling are observed anymore. At pressures above 20 MPa, as the rate of change in adsorbed gas content becomes small, matrix compression dominates and can decrease the volumetric strain (Pan and Connell, 2007).

In this study, the isotherm curves of N2, CH4, CO2 were determined at 318 K and 338 K for pressures up to 16 MPa in dry Selar Cornish coal using the manometric method (Hemert et al., 2009). The authors do not yet address mixed gas sorption but focus on pure gas (de)sorption. The chosen range is representative for in situ conditions. In Europe usually seams suitable for CO2 storage are at depths over 500 m, with correspondingly high reservoir pressures and temperatures (308–338 K, 6–15 MPa).

Swelling/shrinkage measurements have been conducted on Selar Cornish coal with CO2 using strain gauges on unconfined cubic samples at T = 321 K up to 4.1 MPa.

Excess sorption isotherms provides insufficient information for ECBM applications because it is considering the void volume that can be occupied by the gas as constant disregarding that it is reduced by the volume of the gas in its sorbed phase and by the swelling induced by the gas absorption. Therefore it is preferable to use the absolute sorption isotherm which is the total amount of gas that can be sorbed per unit mass of coal. The absolute sorption is considering also the volume occupied by the CO2 in its sorbed phase. In this paper the absolute sorption has been recalculated considering also the changes in volume of the coal induced by the swelling.

In this study all the experimental results are fitted with the Langmuir model (Sakurovs et al., 2007), which adequately represents gas sorption in coal and provides values of parameters that can be used in many reservoir simulations.

Section snippets

Sorption measurements

All experiments are performed with a semi-anthracite, from the Selar Cornish, South Wales Coalfield (vitrinite reflectance is Rmax = 2.41). Maceral and ash content are reported in Table 1.

For the sorption experiments the stored coal blocks are broken, crushed and then sieved. The fraction between 1.5 and 2.0 mm is used in the study. Sieving was brief in order to avoid dust production. Batches of 50 to 70 cm3 are sealed and stored at about 276 K until used in the experiments. Before placing the

Equilibration time for the sorption experiments

In total eight sorption experiments were performed. All experiments, except the one with CO2 were carried out in duplicate. Two sorption experiments have been conducted with N2 at 318 K and two, always with N2 at 338 K. In the methane experiments, the temperature for the first two pressure steps was maintained at 318 K. For the following steps the temperature was 338 K. An extra point at 318 K was measured at the last pressure step, before desorption. This procedure was motivated by the long

Summary of observations

For the experiments with Selar Cornish coal the following observations are made:

  • Experimental results indicate that the time required for attaining sorption equilibrium for N2 is 10 h at 318 K and 27 h at 338 K; for CH4 it is 10 days at 318 K and 30 h at 338 K; for CO2 it exceeds 72 h at 318 K and 338 K. It has been proved by the experiments that the time required for equilibration varies with the type of gas, temperature. Thus, results are in line with the expectation that during flue gas injection

Conclusions

Equilibrium in each of the pressure step can be established after that there is no noticeable change in pressure. To assess it correctly it is necessary to plot the pressure in a logarithmic time scale. This can be accomplished for N2 and CH4. The equilibrium cannot be accomplished for CO2 due to extremely long experimentation time. For N2 and CH4 the excess sorption isotherm does not show any hysteresis. CO2 excess sorption isotherm do show hysteresis. One of the causes can be ascribed to the

Nomenclature

SymbolUnitPhysical quantity
mexc[mol/kg](Gibbs) Excess sorption
mabs[mol/kg]Absolute sorption of gas
nt[mol/kg]Total amount of gas present in the sample cell per mass of coal
M[kg]Coal mass after evacuation
P[MPa]Pressure
ρfilli[mol/m3]Molar density of gas at the i-th filling step
ρeqi[mol/m3]Molar density of gas at the i-th equilibrium step
ρa[mol/m3]Molar density of gas in the adsorbed phase
ρg[mol/m3]Molar density of gas in the free phase
χ[–]Volume ratio of sample cell to reference cell
Vsc[m3]Volume

Acknowledgements

The experiments have been performed under the GRASP (Green-House Gas Removal Apprenticeship) programme. We are really thankful to the Department of Mining and Environmental Engineering at Imperial College of London and particularly to Sevket Durucan for allowing the internship and Shafiuddin Amer Syed for all the laboratory works concerning the swelling experiments. Experiments of gas sorption on coal have been performed at Dietz Laboratories of TU Delft and we are really thankful to Henk Van

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