High-pressure methane and carbon dioxide adsorption on dry and moisture-equilibrated Pennsylvanian coals

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Abstract

In the context of research on the reduction of CO2 emissions and the production of coalbed methane (CBM), high pressure adsorption measurements of CH4 and CO2 have been performed on dry and moisture-equilibrated Pennsylvanian coals of different rank (0.72, 1.19 and 1.56% VRr). Adsorption isotherms of the two gases were measured up to pressures of 20 MPa (200 bar), at 40, 60 and 80 °C using a volumetric method.

Total excess sorption capacities for methane on dry coals ranged between 11 and 14 Std. cm3/g coal. The 40 °C sorption isotherms showed a saturation behavior while the 60 and 80 °C isotherms exhibited a monotonous increase over the entire experimental pressure range (up to 20 MPa).

Methane sorption capacities of moisture-equilibrated coals were lower by ∼20–25% than those for dry coals and ranged between 7 and 11 Std. cm3/g coal. No distinct maturity effect was discernible for methane adsorption on the three samples studied, neither in the dry nor in the moist state.

CO2 adsorption isotherms for dry and moist coals showed substantial differences. For dry coals the highest CO2 excess sorption capacities were observed at 40 °C with maximum values of 70 Std. cm3/g within limited pressure ranges.

Carbon dioxide excess sorption for the moisture-equilibrated coals was usually lower than for the dry samples in the low pressure range. All high-pressure CO2 adsorption isotherms for moist samples were bimodal with distinct minima and even negative excess sorption values in the 8–10 MPa (80–100 bar) range. Beyond this range CO2 adsorption capacity increased with increasing pressure. High-temperature (80 °C) sorption capacities for CO2 were very low (<5 Std. cm3/g) in the low-pressure range but reached much higher levels (25–50 Std. cm3/g) above 12 MPa.

The strong bimodal character of the CO2 excess isotherms on moist coals is interpreted as the result of a swelling effect caused by supercritical CO2 and enhanced by water. Some extent of swelling was also inferred for dry coals.

Absolute sorption isotherms for CO2 were calculated assuming a sorbed-phase density of 1028 kg/m3 and compared with literature data. Like the excess isotherms, the absolute isotherms show a distinct decline in the 8–10 MPa pressure interval. At higher pressures, however, they increase monotonously.

Introduction

Reducing carbon dioxide (CO2) emissions in order to control the overall levels of CO2 in the atmosphere has become an international priority in the wake of the Kyoto Protocol. In that protocol, the Netherlands committed to reducing their CO2 emissions by 6% from 2008 to 2012. Despite all past and ongoing efforts put into the development of sustainable energy supply, the world still depends heavily on fossil fuels and will continue to do so for years to come. For this reason, technology options are required that will allow for the continued use of fossil fuels without substantial emissions of CO2. Subsurface storage of CO2 in geological systems is considered as one promising perspective, which is currently being investigated worldwide. This concept can be defended by the basic principle of closed circles: emitted CO2 originates from fossil fuel, taken from the subsurface, and should therefore be returned to the subsurface. In general, the research window for projects on subsurface CO2 storage is slowly but surely shifting from desk studies to demonstrations. One of the options considered in this context is the storage of CO2 in underground coal seams.

The injection of CO2 into coal while simultaneously producing coalbed methane (CBM) combines the production of a “clean”, hydrogen-rich fossil fuel (methane) with CO2 sequestration (see Hamelinck et al., 2001). The advantages expected from injecting CO2 into coalbeds are numerous: apart from the effect of CO2 sequestration it may accelerate CBM production and improve the recovery of CBM (therefore the term Enhanced CBM production, or ECBM). Another benefit of this technology is that coal seams that cannot be profitably mined could still be exploited for the in situ methane, making available substantial amounts of fossil fuel that could otherwise not be used for energy production. Finally, this new technology could be used for the decentralised generation of energy in remote areas close to coal-bearing basins, minimising the costs of energy transportation. To date, only a few experimental ECBM/CO2 field sites have been realised worldwide. A micro-pilot field test was set-up in Alberta (Canada) by the Alberta Research Council Gunter et al., 1997, Gunter et al., 1998, while the world's first large-scale ECBM pilot using CO2 injection is operated by Burlington Resources in the San Juan Basin in New Mexico, USA Schoeling and McGovern, 2000, Reeves and Schoeling, 2000, Gunter et al., 1998. In Europe, the first field experiment with ECBM/CO2 is scheduled to start in 2003 in the Upper Silesian Coal Basin in Poland. The ongoing tests document the great potential of this process for both CO2 sequestration and ECBM production.

Two main advantages must be emphasised that CO2 storage in coal has above other subsurface sequestration options. Firstly, injected CO2 replaces adsorbed CH4 at the internal coal surface, adsorbing firmly to the coal at a near-liquid density. Since the process of gas adsorption has proven its stability through geological time periods, the chances of future CO2 release from non-mineable coal are considered minimal. Secondly, both laboratory experiments and field tests suggest that for two sequestered CO2 molecules, one CH4 molecule is produced. Laboratory experiments showed that this exchange ratio of 2:1 could be even larger at pressures higher than 9.6 MPa, where the gaseous CO2 changes to supercritical CO2 (Hall et al., 1994). This exchange ratio could imply that, in areas rich in coal, energy plants could be developed that would have an emission of CO2 per energy unit that is much lower than that of conventional energy plants or even approaches zero. This would make the technology of ECBM with gas (CO2) injection a clean source of energy.

The exchange ratio is of specific interest, since the present European perspective for ECBM is on the CO2 sequestration potential rather than on the CH4 production. To study this exchange ratio an experimental programme has been set up for adsorption experiments with CH4 and CO2 on coals. This experimental program is unique because it involves experiments with pressures of up to 20 MPa (200 bar; ∼3000 psi).

The project work comprised the measurement, evaluation and interpretation of methane and CO2 adsorption isotherms on three representative Pennsylvanian coal samples of different rank from the Netherlands. The study aimed at the quantification and prediction of gas storage capacity and expected displacement behavior of methane by CO2 under subsurface conditions. The experiments were carried out on dry and moisture-equilibrated coals in order to assess the effect of natural moisture content on the sorption of the two gases. Adsorption isotherms were determined at 40, 60 and 80 °C.

Section snippets

Samples

The major part of the Dutch territory lies within the Northwestern European Coal Basin. The most important coal-bearing deposits in The Netherlands are the Upper Carboniferous (Pennsylvanian) Westphalian A to Upper Westphalian C sequences, which are present throughout the major part of The Netherlands at various depths with a total thickness of up to 3000 m. The coal content of the Westphalian strata varies between 1.5% and 5%, with thickness of individual layers up to a maximum of 3.5 m.

Sample preparation

The crushed samples were divided and aliquots were ground to pass a sieve size of 0.2 mm. The moisture content of the samples as received was determined according to DIN 51718. Between 0.9 and 1 g of coal was weighed to the nearest 0.0001 g and then dried for 90 min in an insulated air cabinet. The sample was then placed in a desiccator over silica gel to cool down and weighed again. For adsorption measurements on dry coals, the samples were vacuum-dried in the adsorption apparatus for 1.5 h at

High-pressure methane adsorption on dry and moisture-equilibrated coals

The isotherms of the high-pressure methane sorption experiments performed on the dry and moisture-equilibrated coal samples are shown in Fig. 3, Fig. 4, Fig. 5. Usually three experiments at different temperatures were carried out on the same moisture-equilibrated sample. It is evident from these figures that, while the 40 °C isotherms for the dry and moist coals exhibit a saturation behavior at high pressures, and in some instances even a slight decrease with pressure (i.e. a maximum), the 60

Pressure and temperature effects

The methane isotherms display a normal Langmuir-type behavior up to a pressure of about 10 MPa. Some isotherms reach a clear maximum beyond this point, which results in a deviation from the Langmuir equation at higher pressures, since this relation predicts a monotonous asymptotic approach to a limiting value at infinite pressure. The effect of pressure increase on the adsorption capacity of coal for CO2 is evaluated below.

The clear negative effect of temperature on the adsorption capacity up

Summary and conclusions

A comprehensive data-set for methane and carbon dioxide adsorption on dry and moisture-equilibrated Pennsylvanian coals from The Netherlands up to 20 MPa has been obtained during this study. While the results of methane sorption tests on dry coals essentially confirmed earlier experimental work, the extension of the experiments to moist coals and to supercritical carbon dioxide revealed an increasing complexity of the gas sorption behavior. Pilot measurements of CO2 adsorption at low pressures

Acknowledgements

This project has been performed under Programme number 234.1 with NOVEM BV, the Netherlands Agency for Energy and the Environment (see Hamelinck et al., 2001). We thank Prof. W. Wagner of Ruhruniversität Bochum for providing the EOS software for methane and carbon dioxide. Furthermore, we gratefully acknowledge the support by Dirk Prinz and Roland Gaschnitz in experimental matters and design of the evaluation procedures. The final manuscript benefited from thorough reviews by J. Close and C.

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