Experimental and modeling study of the adsorption of CO₂ on coal aimed at ECBM recovery

Stefan Ottiger¹, Ronny Pini¹, Giuseppe Storti², Marco Mazzotti¹

¹ ETH Zurich, Institute of Process Engineering, Sonneggstrasse 5, CH-8092 Zurich, Switzerland

² ETH Zurich, Institute for Chemical and Bioengineering, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland

The world's CO₂ concentration has been rising steadily in the last years due to the increasing demand of fossil fuels of a growing world's population. The connection between anthropogenic carbon dioxide emissions and the world's average temperature increase has been established [1]. The effects of the warmer climate include melting of ice caps, warming the sea surface temperature and an increase in the intensity of precipitations and related phenomena, e.g. hurricanes. Therefore, it is of worldwide interest to develop carbon dioxide capture and storage techniques to reduce the greenhouse gas concentration in the atmosphere.

When coal seams form by compaction of plants, the so-called coalbed gases (mainly methane) are generated and accumulated into the coal structure. Such coalbed methane is normally recovered by means of reservoir-pressure depletion, i.e. by pumping out water and degassing the reservoir. A more attractive process with higher yields is the so-called Enhanced Coal Bed Methane recovery (ECBM), whereby carbon dioxide is pumped into the coal seam. Due to higher adsorptivity of carbon dioxide with respect to methane, the carbon dioxide stays in the coal seam and displaces the adsorbed methane. In the end the coal seam contains mainly carbon dioxide which can be stored for geological times. ECBM is therefore attractive from two perspectives. On the one hand, if one is interested in the recovered methane as energy source, ECBM allows for a net CO₂ sequestration, thanks to the above mentioned high CO₂ adsorptivity. On the other hand, if the goal is that of storing CO₂ that has been captured, e.g. in a power plant, the ECBM operation makes it possible to recover methane, thus making CO₂ storage economically interesting.

Currently there are a few field tests in progress, which are increasing our understanding of the flow and retention mechanisms in the coal seam [2]. However, the factors still limiting the implementation of ECBM recovery are economical, as well as technological and scientific, i.e. limited understanding of fundamental issues related to ECBM. Of particular concern is the change in volume of the coal under adsorption conditions, which, in turn affects the porosity and the permeability of the bed. There is in fact literature evidence that CH₄ and CO₂ do not only adsorb on the coal surface, but they also absorb in the coal matrix causing the coal to swell [3].

As a first step towards a better understanding of ECBM, the goal of this study is on the one hand to experimentally characterize pure and multicomponent competitive adsorption of CO₂ and CH₄ on coal and to measure the volumetric changes of the coal matrix caused by the sorption of these gases. In particular, we have been investigating adsorption of CO₂ on two different coals [4]. For the adsorption measurements a Magnetic Suspension Balance (Rubotherm, Bochum, Germany) has been used. The swelling experiments are instead performed in a view cell, which has been used previously to study the expansion of polymers [5].

On the other hand, the goal of this study is to describe the sorption of CO₂ and CH₄ on the coal by a suitable model, therefore accounting for both adsorption and sorption phenomena, including swelling. To this aim, a model proposed by Milewska-Duda and Duda [6] for coal in the frame of the Flory-Huggins theory of polymer solutions will be tested. Accordingly, the coal is described as a cross-linked chain polymer with pores approximated by holes, where adsorption is taking place in the larger pores and sorption in the smaller ones.

[1] IPCC (2005), IPCC special report on: carbon dioxide capture and storage. Cambrige University Press, New York.

[2] Sams W. N., Bromhal G. et al., Field-project designs for carbon dioxide sequestration and enhanced coalbed methane production, Energy & Fuels 19 (2005) pp. 2287-2297.

[3] St. George J. D. and Barakat M. A., The change in effective stress associated with shrinkage from gas desorption in coal, Int. J. Coal Geol. 45 (2001) pp. 105-113.

[4] Pini R., Ottiger S., Burlini L., Storti G., Mazzotti M., Experimental study of CO₂ adsorption on coal and other adsorbents aimed at ECBM recovery. Paper presented at 4-PBAST, May 22-26, 2006, Tianjin, China.

[5] Rajendran A., Bonavoglia B., Forrer N., Storti G., Mazzotti M., Morbidelli M., Simultaneous Measurement of Swelling and Sorption in a Supercritical CO₂-Poly(methyl methacrylate) System, Ind. Eng. Chem. Res. 44 (2005) pp. 2549-2560.

[6] Milewska-Duda J., Duda J., Mathematical Modeling of the Sorption Process in Porous Elastic Materials, Langmuir 9 (1993) pp.3558-3566.