(71f) Experimental and Modeling Study of the Adsorption of Co2 on Coal Aimed at Ecbm Recovery

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.