COAL CHARACTERIZATION FOR CARBON DIOXIDE
SEQUESTRATION PURPOSES
G. Di FEDERICOI, I. CAMPLONEI, S. BRANDINII, J.
BARKERII
Centre for CO2 Technology, University College London
I
Department of Chemical Engineering, UCL, Torrington Place,
WC1E 7JE, London, UK
II
Department of Earth Sciences, UCL, Gower Street,
WC1E 6BT, London, UK
1. Introduction
The greenhouse gas effect helps to regulate the temperature of our planet. It is the
result of heat adsorption by a number of gases in the atmosphere called
greenhouse gases because they effectively ‘trap’ heat in the lower atmosphere.
Water vapour is the most abundant greenhouse gas, followed by carbon dioxide
and methane. Without a natural greenhouse gas effect, the temperature of the
Earth would be about –18°C instead of its present 14°C [1]. So, the concern is not
with the fact that there is a greenhouse gas effect, but whether human activities
are leading to an enhancement of this effect. Reduction in CO2 emissions is the
most quoted solution for global warming in the short time. At the third
Conference of Parties of the Framework Convention on Climate Change (FCCC)
in Kyoto (1997), developed countries agreed to stabilise emissions of greenhouse
gases through a 5.2% reduction from 1990 levels by 2008-12. In order to stabilise
atmospheric carbon dioxide concentrations it will be necessary to reach more
than 60% reduction by 2050 [2].
TABLE 1: CO2 emissions in 1996 and reductions agreed in Kyoto [3]
Nation
USA
Russian Federation
Germany
UK
Canada
Poland
Total emission (1996)
million tonnes/year
5043
1500
924
571.4
507
373
Reduction required
%
7
0
21
12.5
6
6
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2. CO2 Sequestration
Carbon dioxide sequestration is one of the options that are being considered for
the reduction of greenhouse gas emissions. It would enable the world to
continue to use fossil fuels and at the same time reduce emissions from their
combustion. It involves two main stages that are the gas capture and its storage
in suitable geological structures. In the capture operation CO2 is separated from
flue gases produced in combustion processes using adsorption, absorption,
cryogenic or membrane systems. The storage is realised by injecting the carbon
dioxide in the coal seams, depleted gas reservoirs, depleted oil reservoirs, etc
[4].
The present work is focused on the characterisation of coal for CO2
sequestration and simultaneous production of methane. The anthropogenic
methane contained in coal can be regarded as a significant source of energy.
This methane is usually referred to as coal-bed methane (CBM) in the literature.
CBM is retained in coal in three ways: first, as free gas within the pore space or
fractures in coal; second, as adsorbed molecules on the organic surface of coal;
and third, dissolved in groundwater within the coal. The depletion techniques
used to recover CBM allow the recovery of 50% of the total amount contained
in the reservoir. Carbon dioxide is more strongly adsorbed in coal than methane
therefore the injection of carbon dioxide will enhance the production of
methane with recoveries of up to 100% [5]. This process is often referred to as
enhanced-CBM (ECBM).
2.1. GAZ TRANSPORT IN COAL
The gas transport through the coal reservoir is governed by the permeabilitycontrolled flow in the macropore system and by the diffusion-controlled flow in
the micropore system. The diffusion-controlled flow obeys Fick’s law. Clarkson
and Bustin [6] estimated that the inverse of the CO2 effective diffusion time
constant, D` D l 2 , for Cretaceous Gates Formation is between 6∙10–3 s–1 and
3∙10–2 s–1 in the macropores and between 2∙10–3 s–1 and 4∙10–3 s–1 in the
micropores.
The permeability-controlled flow is generally considered to be laminar, so it
obeys D’Arcy’s law [7].
m
k
P

(1)
with  the gas density, k the permeability, P the pressure. Permeability is a
measure of coal ability to transmit fluids therefore knowing the coal
permeability the injection pressure can be estimated. Typical values for the
permeability are between 0.1 and 250 md for US coals [8].
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The permeability of a coal seam varies with effective external stress, which can
be defined as the difference between the external stresses applied on a rock and
the internal pressure within the pore space of the rock. Increasing the external
stress the permeability decreases because the system shrinks and the pressure
drops in the matrix increase. Coal matrix shrinkage and the resulting change in
cleat or fracture system porosity can have a profound effect on reservoir
permeability and thus on production (or injection) performance. In fact, gas
desorption causes a decrease in internal pressure, so it is expected that this would
cause an increase in the effective stress and hence a decrease in permeability.
Another important effect that occurs in the sequestration of CO2 in coal is the
swelling of the structure.
Coal swells upon exposure to carbon dioxide and the extent of expansion
increases with increasing carbon dioxide pressure. The application of the external
stress limits the extent of coal swelling. In this project the swelling is evaluated
measuring the radial and the axial variation for the sample size under various
stress conditions. It will be useful to develop a physical model to predict the
swelling and the shrinkage in a coal reservoir and the associated changes in
permeability in order to predict the maximum amount of CO2 that can be
sequestered.
Most available laboratory data, show that CO2 adsorption causes more strain and
swelling than CH4 and N2 because it is adsorbed in larger concentration by coal
and also suggest that CO2 causes more swelling on a unit of concentration basis
[9].
2.2. COAL ADSORPTION
The sequestration process is controlled by the absorptive capacity of coal in
terms of the maximum amount adsorbed and its rate of sorption. Data available in
the literature is sparse but provides insight into the applicability of simple
isotherm models. Experimental data for Fruitland Coal A [10] show favourable
shaped isotherms with carbon dioxide more strongly adsorbed than methane and
nitrogen (Figure 1). This behaviour is the driving force for the ECBM process in
which CO2 replaces CH4.
Figure 1 shows that the experimental data can be correlated using the
Langmuir (eq. 2) and the truncated virial (eq. 3) isotherms.
bP
1  bP
(2)
bp
 exp 1  A1q 
q
(3)
q  qs
where q is the quantity sorbed, P the pressure. qs, b, A1 are the models’
parameters determined by regression of the single component data.
3
Amount adsorbed (mol/g-coal)
0.0012
Virial
Langmuir
0.001
carbon dioxide
0.0008
methane
0.0006
0.0004
nitrogen
0.0002
0
0
20
40
60
80
100
120
140
Pressure (bar)
Figure 1. Single component sorption isotherms for Fruitland Coal A at
115ºF.
Amount adsorbed (mol/g-coal)
0.0012
0.001
0.0008
0.0006
methane
carbon dioxide
0.0004
0.0002
0
0
0.1
0.2
0.3
0.4
0.5
0.6
EVP-scaled
Figure 2: Single component sorption isotherms for Fruitland Coal A at 115ºF
with scaled pressures.
The Langmuir parameters obtained from the Fruitland Coal A
experimental data show a molecule dependent saturation capacity which is
thermodynamically inconsistent [11]. For nitrogen qs = 7.416·10–4 mol/g-coal,
methane qs = 1.014·10–3 mol/g-coal and carbon dioxide qs = 1.327·10–3 mol/gcoal, therefore the Langmuir model should be considered only as a useful fitting
function. It is interesting to note [10] that by plotting the data on a reduced
4
pressure basis, p/p*, where p* is the extrapolated vapour pressure (EVP) using a
simple two constant Clapeyron form, CO2 and CH4 isotherms are in reasonable
agreement as shown in figure 2.
Using the Ideal Adsorbed Solution theory it is possible to obtain a
reasonable agreement with the experimental binary adsorption data as shown in
Table 2.
TABLE 2. CH4-N2 equilibrium data [10].
Pressure
(bar)
25.77
Gas
Total sorption
Composition Experimental
ymethane
(mol/g)
0.6865
4.21E-04
Total sorption Total sorption
IAS-Langmuir
IAS-Virial
(mol/g)
(mol/g)
4.56E-04
4.52E-04
55.44
0.7085
6.20E-04
6.34E-04
6.23E-04
83.87
0.7203
7.07E-04
7.16E-04
7.23E-04
27.48
0.0832
2.55E-04
2.43E-04
2.52E-04
57.26
0.0906
3.84E-04
3.72E-04
3.76E-04
82.56
0.0954
4.46E-04
4.38E-04
4.49E-04
25.57
0.1478
2.63E-04
2.62E-04
2.75E-04
58.62
0.1633
4.32E-04
4.21E-04
4.27E-04
83.94
0.1713
5.22E-04
4.90E-04
5.03E-04
3. Design of test cell
With the aim of characterizing coal for sequestration purposes, a test cell has
been designed and assembled. What makes this a challenging problem is that
coal will undergo swelling as a consequence of CO2 adsorption, thus modifying
the solid properties, in particular permeation [12]. The coal can swell by 1-4%
in volume and it is function of the mechanical stress state [13,14] and this
swelling can have a large effect on the coal permeability. Therefore it is
necessary to reproduce, as close as possible, the stress conditions of the coal
sample in the coal reservoir. For typical UK coal the pressure along the bedding
plane is lower than the overburden pressure (i.e. along the vertical direction) by
a factor of 0.8 to 1 [15].
A schematic diagram of the test cell is shown in Figure 3. The different stresses
are obtained by flowing water at different pressures in the two compartments,
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thus obtaining the required pressure ratios. The water is also used to maintain
the cell at isothermal conditions, which may vary from 25 to 60 ºC. Two HPLC
pumps are used to maintain the water pressure constant and a schematic
diagram of the system is shown in Figure 4.
Figure 3. Test cell.
Figure 4. Experimental set-up.
To investigate the carbon dioxide adsorption process for a given coal sample,
the gas is injected from 1 (Fig.3) and the outlet stream 2 is analysed. The gas
injected can be pure carbon dioxide or its mixtures with nitrogen to reproduce
streams coming from a power plant. The gas pressure can be varied from
atmospheric to 120 atm. In order to quantify the extent of the swelling a
measurement of the piston 5 displacement is used to evaluate the variation in
the longitudinal length during the experiment. It is also possible to measure the
diameter changes of the sample 6 using a strain gauge belt. The use of multiple
strain gauge belts to follow the propagation of the swelling along the axis is
being considered. When combined with unconstrained adsorption isotherm
measurements this test cell should provide the necessary experimental
information to characterize coal at the coal matrix scale. The results will be
included in a large-scale reservoir simulator currently being developed at
UCL’s Earth Sciences Department.
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4. Conclusions
Experiments will be performed in order to evaluate the kinetics of
adsorption/desorption, the effect of the stress state and swelling on permeability
and how it affects the coal structure. They will be carried out with nitrogen,
carbon dioxide and methane as pure components or mixtures. From the analysis
of the results it will be possible to understand and predict the swelling
phenomenon and lead to a physical model for the sequestration process that will
consider adsorption, permeability and swelling.
5. Notation
CBM
Coal Bed Methane
ECBM
Enhanced Coal Bed Methane
EVP
Extrapolated Vapour Pressure
FCCC
Framework Convention on Climate Change
SCF
Standard Cubic Feet
A area of cross-section of flow
D` effective diffusivity
D diffusivity
l diffusion path length
m mass flow rate
2
k permeability (1 md = 9.8692 10–4
)
 fluid viscosity
P pressure
pout outlet pressure
p pressure drops along the coal sample
pav the average pressure between the injection pressure and outlet pressure
 gas density
Acknowledgements
Financial support from the Leverhulme Trust (Philip Leverhulme Prize), Royal
Society (Royal Society Wolfson Research Merit Award) and EPSRC is
gratefully acknowledged.
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6. References
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2. Royal Commission on Environmental Pollution Report 22, June 2000.
Energy – The Change in Climate
3. European Monitoring and Evaluating Programme web site:
http://www.emep.no
4. National Energy Technology Laboratory website: www.netl.doe.gov
5. Pashin J. C., Groshong Jr. R.H., Carrol R.E., Enhanced coalbed methane
recovery through sequestration of carbon dioxide: potential for a marketbased environmental solution in the Black Warrior Basin of Alabama, in
www.netl.doe.gov.
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