Adsorption of Pure CarbonDioxide and Methane on DryCoal from the Sulcis CoalProvince (SW Sardinia, Italy)Stefan Ottiger,a Ronny Pini,a Giuseppe Storti,b Marco Mazzotti,a Roberto Bencini,c

Fedora Quattrocchi,d Giorgio Sardu,e and Giuseppe DeriueaETH Zurich, Institute of Process Engineering, Sonneggstrasse 3, 8092 Zurich, SwitzerlandbETH Zurich, Institute for Chemical and Bioengineering, Wolfgang-Pauli-Str. 10, 8093 Zurich, SwitzerlandcIES, Independent Energy Solutions S.r.l. Rome, ItalydINGV, Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, ItalyeCarbosulcis S.p.A., Miniera Monte Sinni, 09010 Cortoghiana (CA), Italy

Published online 14 November 2006 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10169

Enhanced coalbed methane recovery (ECBM)increases the recovery of the methane present in acoal seam by injecting CO2 at high pressure. It isattractive from two perspectives, the valuable methanerecovered and the storage of the greenhouse gas CO2

for geological times. In the framework of a feasibilitystudy for the Sulcis Coal Province in Italy, the adsorp-tion of pure CO2 and CH4 on dry coal has beenmeasured at 45 and 608C, using a magnetic suspen-sion balance with in situ density measurement. Theresults show that the CO2 adsorption isotherms oncoal are similar to those for other standard adsorb-ents such as silica gel and activated carbon. Fromthe excess adsorption isotherms, the absolute adsorp-tion is calculated using the assumption of constantvolume of the adsorbed phase. As expected, CO2 getadsorbed more than CH4 in all cases. The Sulcis coalcan uptake CO2 at the reservoir conditions in anamount of about 10% of its mass. � 2006 AmericanInstitute of Chemical Engineers Environ Prog, 25: 355–364,2006Keywords: adsorption, supercritical CO2, CH4, coal,

ECBM

INTRODUCTION

The world’s average temperature has beenincreasing in the last years, an effect called globalwarming. In part responsible for that through thegreenhouse effect are the anthropogenic gas emis-sions, among which carbon dioxide plays a majorrole. The concentration of carbon dioxide in theatmosphere has been increasing significantly for thepast 150 years mainly due to combustion processesfor electricity production, transportation, and heatingsystems. Its concentration has increased by morethan 30% since the preindustrial era and has reached368 ppm in the year 2000 [1]. To stabilize the CO2

concentration in the atmosphere, it is therefore cru-cial to reduce its emissions by, among other meas-ures, capturing and storing it for an amount of timelong enough [2].

One of the possible approaches for undergroundCO2 storage is its injection in unmineable coal seams,where large amounts of coalbed methane are presentand can be recovered and exploited. The conven-tional method to recover the coalbed methane (pri-mary recovery) is by reducing the hydrostatic pressureof the seam through dewatering, thus producingthrough production wellbores water, whose amountdeclines in time, and gas. The main disadvantages of� 2006 American Institute of Chemical Engineers

Environmental Progress (Vol.25, No.4) December 2006 355

this method are the large amounts of waste water pro-duced and the low yield, which is only 20–60% of thegas in place that could be produced [3]. Enhancedcoalbed methane recovery (ECBM) is a novel technol-ogy that allows increasing methane recovery by inject-ing carbon dioxide through injection wellbores. Thus,ECBM is accomplished with the use of CO2, hencecombining enhanced methane recovery and associ-ated CO2 storage [4]. Field tests of the ECBM technol-ogy have been and are being conducted in severalsites worldwide, thus improving our understanding ofits potential benefits and practical issues [5–9].

Experimental results about adsorption of carbondioxide and methane on different coals cover a broadrange of conditions from sub- to supercritical, andindicate that CO2 get adsorbed more than CH4 always.In the case of coals similar to the one considered here,the relative capacity of coal for CO2 and CH4 varies bya factor of 2–10 with temperature and pressure [10,11]. Therefore, the injected carbon dioxide can dis-place methane and be stored there for geologicaltimes provided that the coalbed geology guaranteesadequate sealing capacities. This would allow storingmore carbon dioxide than that produced by the com-bustion of the recovered methane. If the recoveredmethane is used as an energy source, ECBM allows fora net CO2 storage, due to the aforementioned higheradsorptivity with respect to methane.

To evaluate the storage potential of a coal seam, itis necessary to characterize properties such as theadsorption equilibria of CO2, CH4, and N2 and theirmixtures on coal, the influence of the injection ofCO2 on matrix swelling and permeability, and thetransport of the gases through the coal seam fromand to the adsorption sites. In this study, we focus onthe adsorption properties of coal, and we measurethe adsorption of pure carbon dioxide and methaneon dry coal from the Sulcis Coal Province (SW Sardi-nia, Italy). Such measurements are carried out in theframe of a feasibility study for CO2–ECBM geologicalstorage in the Sulcis Coal Province currently in pro-gress [12, 13].

Beside the need for measuring CO2 adsorption onthe specific natural coal of interest in this study, it isnecessary to carry out adsorption measurements oncoal to clarify some inconsistencies present in the litera-ture about adsorption of supercritical CO2 on coal. As amatter of fact, most measurements of CO2 adsorptionon coal are in a pressure range in which CO2 is still gase-ous. The adsorption isotherms of carbon dioxide andmethane on high-volatile bituminous coals from Indianawere measured at 628F (�178C) at pressures up to500 psi [11]. Busch et al. [14] have measured the adsorp-tion of methane and carbon dioxide on dry Argonnepremium coals at 228C at pressure up to 51 bar for CO2

and at pressure up to 110 bar for CH4, employing a vol-umetric technique, reporting CO2 and CH4 excessadsorption isotherms show a monotonously increasingbehavior with pressure. To investigate the reproducibil-ity of CO2 excess adsorption isotherms measured in dif-ferent laboratories, an interlaboratory comparison ondry Argonne premium coal samples was carried outand reported [15]. The CO2 excess adsorption iso-

therms were measured on the same coals at 22 and558C at pressures up to 70 bar in four independent lab-oratories, but using different manometric or volumetricsetups. The results indicate a very good agreement forhigh-rank coals, but large variations for mid- and low-rank coals; it was concluded that a strict procedure forobtaining measurements will be needed to obtain re-producible isotherms on dried coals [15].

Measurements of supercritical CO2 adsorption oncoal are scarcer and somehow contradictory. On theone hand, some papers in the literature report excessadsorption isotherms that exhibit irregular behavior.Krooss et al. [16] have measured the adsorption ofmethane and carbon dioxide on dry and moistureequilibrated Pennsylvanian coals at 40, 60 and 808Cat pressures up to 200 bar. Their results exhibit pecu-liar effects, such as a sharp increase in CO2 excessadsorption between 80 and 100 bar for the 408C iso-therms on dry coal. In the case of wet coals, all highpressure CO2 adsorption isotherms are bimodal withdistinct minima, with even unphysical negative valuesof the excess adsorption in the same pressure range.These effects on moist coals were attributed to coal-specific behaviors or to other phenomena like swel-ling of the coal matrix caused by supercritical CO2,without being conclusively explained. An anomalousbump near 90 bar was also observed for the adsorp-tion of carbon dioxide on wet Lower Basin fruitlandcoal at 46.18C, which was attributed to the increaseduncertainty in the CO2 density [17]. Finally, Toribioet al. [18] measured the adsorption of CO2 on dryand moist Japanese samples from the Akabira andTaiheiyo coal mines at 35, 45, and 558C at pressuresup to 150 bar and observed an increased amountadsorbed at conditions beyond the critical point. Onthe other hand, there are also data found in the litera-ture exhibiting smooth CO2 excess adsorption iso-therms on coal. Namely, Fitzgerald et al. [19] havemeasured the adsorption of methane, nitrogen, car-bon dioxide, and their mixtures on wet Tiffany coalat 327.6 K at pressures up to 138 bar without observ-ing any peculiar effect above the critical pressure.

All literature data given here were measured usinga volumetric technique. In our laboratory, we havebeen successfully measuring supercritical adsorptionof different adsorbates on standard adsorbents usinga gravimetric magnetic suspension balance, for exam-ple CO2 and N2O on silica gel and 13X zeolite [20–22]. The observed excess adsorption isotherms allincrease to reach a maximum, and then decrease reg-ularly with increasing density. In this paper, we meas-ure using a gravimetric method the CO2 adsorptionisotherms on Sulcis coal at the supercritical condi-tions that prevail in the coal formation. The shape ofthese isotherms is shown to be similar to those meas-ured also at supercritical conditions on commercialadsorbents.

EXPERIMENTAL SECTION

MaterialsA coal sample from the Monte Sinni coal mine

(Carbosulcis) located in the Sulcis Coal Province was

356 December 2006 Environmental Progress (Vol.25, No.4)

used in this study. The sample was drilled in Decem-ber 2004 at a depth of about 500 m, and preservedin a plastic bottle in air. Results of a thermogravimet-ric analysis (TGA) give a coal composition of 49.4%fixed carbon content, 41.2% volatile matter, 2.1% ash,and 7.3% moisture. These values together with a vitri-nite reflectance coefficient (R0 % 0.7) allow locatingthe coal as high-volatile C bituminous [9]. For theadsorption measurements, the coal sample wasground and sieved to obtain particles with diameterbetween 250 and 355 mm. The coal sample was ho-mogeneous enough and the grinding and sievingwas careful enough to guarantee that the smallamount of coal used for measurements (about 10%of the whole) was representative of the whole sam-ple. Since this was a single sample from a specificlocation in the coal seam, extrapolating the resultspresented in this work to the whole coal reservoirshould be done cautiously. Subsequently, it wasdried in an oven at 1058C under vacuum for 1 dayand a sample of 2.89 g was placed inside the mea-surement cell. The gases used in this study wereobtained from Pangas AG (Luzern, Switzerland),namely CO2 and CH4 at purities of 99.995% and Heat a purity of 99.999%. The critical properties of theadsorbates are as follows: Tc (He) ¼ 5.26 K, Pc (He) ¼2.26 � 105 Pa, �c (He) ¼ 69.3 kg/m3; Tc (CO2) ¼304.1 K, Pc (CO2) ¼ 73.7 � 105 Pa, �c (CO2) ¼ 467.6kg/m3; Tc (CH4) ¼ 190.6 K, Pc (CH4) ¼ 46.0 � 105

Pa, and �c (CH4) ¼ 162.7 kg/m3.

Adsorption MeasurementsAll high-pressure adsorption measurements were

performed in a Rubotherm magnetic suspension bal-ance (Rubotherm, Bochum, Germany), which allowsto measure the density �b of the fluid in situ. Themaximum pressure and temperature are 450 bar and2508C, respectively, and the weight of the sample ismeasured with an accuracy of 0.01 mg. The balanceis kept at a constant temperature using a heatingjacket, and the temperature is measured using a cali-brated thermocouple at an accuracy of 0.18C. The ex-perimental setup has been described extensively inprevious publications [20, 21]. However, the most im-portant equations used for data reconciliation aresummarized here for the sake of completeness.

After the coal sample has been placed in the bas-ket, the magnetic suspension balance is evacuatedand the weight M1

0 under vacuum is measured:

M10 ¼ mmet þmcoal

0 ; (1)

where mmet and m0coal are the weights of the lifted

metal parts and of the coal sample, respectively.Then, the system is filled with helium and the volumeof the metal parts and the coal sample V met þ V0

coal

is calculated from the measured weight M1(�Heb , T)

and the density �Heb

Vmet þ V coal0 ¼ M1

0 �M1ð�bHe; T Þ�bHe

: (2)

It has been recently shown that helium does getadsorbed on several adsorbents, e.g. on alumina [23],silica gel [21], and 13X zeolite [22]. However, it is alsoknown that coal adsorbs much less carbon dioxide ormethane than do these standard adsorbents, such asactivated carbon [24]. Therefore, adsorption of heliumcan be neglected in Eq. 2, because in general the af-finity of helium for adsorbents is much lower thanthat of the species mentioned above and because thehelium measurement is performed at the highest pos-sible temperature and density (i.e. at 1028C and pres-sure 188.1 bar).

After evacuating it again, the cell is filled with thegas to be adsorbed and the weight M1(�

b, T) is meas-ured at the desired conditions, i.e. density �b andtemperature T:

M1ð�b; T Þ ¼ M01 þmads � �b½Vmet þ V coal

0 þ V ads�; (3)

where mads and Vads are the absolute amountadsorbed and the volume of the adsorbed phase,respectively. Because the latter cannot be directlymeasured, the adsorption is commonly representedby the excess adsorption G(�b, T), which does notneed any information about the volume of theadsorbed phase:

�ð�b; T Þ ¼ mads � �bV ads ð4Þ¼ M1ð�b; T Þ �M0

1 þ �b½Vmet þ V coal0 �:

The experimental results are generally reported interms of the molar excess adsorption nex, which isdefined per unit mass of coal as

nex ¼ �ð�b; T ÞMmm

coal0

: (5)

Coal CharacterizationCoal is a heterogeneous material and possesses a

variety of pores of different sizes, ranging frommicro- to macropores. Micropores are usually definedas pores smaller than 2 nm, mesopores are between2 and 50 nm, and macropores are larger than 50 nmin size. Generally, coal can be characterized by adual porosity, because it consists of the microporouscoal matrix and of the macroporous fracture networkcalled the cleat system. There are different techniquesto characterize the coal, such as gas adsorption andmercury porosimetry [25]. However, traditional BETsurface measurements with nitrogen at 77 K signifi-cantly underestimate the coal surface area because atthis temperature nitrogen cannot access all the pores[26]. The commonly accepted way to estimate themicropore volume in coal is low-pressure adsorptionmeasurements of CO2 on coal at 273.15 K. Suchmeasurements on Sulcis coal were performed usingan ASAP 2010 by Micromeritics (Brussels, Belgium) ina relative pressure P/P0 range of 0.002–0.032, where

Environmental Progress (Vol.25, No.4) December 2006 357

P and P0 represent the gas pressure and the satura-tion pressure (P0 ¼ 26.142 mmHg at 273.15 K).

The measured low-pressure isotherm is shown inFigure 1, where the volume adsorbed at STP condi-tions V is plotted against the pressure P. In order toevaluate the experimental data, the theory of Dubi-nin–Astakhov has been applied [27, 28]. The Dubi-nin–Astakhov equation is based on the assumptionsof volume filling of micropores and of Gaussian pore-size distribution. It evaluates the degree of microporefilling �micro, which is the ratio between the adsorbedvolume of CO2, V, and the maximum micropore ad-sorbent capacity, V0, at STP conditions as:

�micro ¼ V

V0¼ exp � RT

�E0lnP0

P

� �n� �; (6)

where R is the universal gas constant, n the character-istic exponent of the system and E0 the characteristicadsorption energy of a standard vapor which is usu-ally benzene. � is a given affinity coefficient whichrelates the adsorbed vapor to the standard vapor andis 0.351 for CO2 [27]. In the case of the Sulcis coal,the quantities V0 ¼ 32.95 cm3 STP/g, E0 ¼ 26.62 kJ/mol, and n ¼ 1.924 were determined from a fit of thelow-pressure adsorption isotherm shown in Figure 1using a linear regression in the Dubinin–Astakhovplots. The characteristic CO2 adsorption energy E,equal to the affinity coefficient � times the standardvapor characteristic energy E0, is 9.35 kJ/mol in thecase of CO2 on Sulcis coal. Furthermore, a value of0.070 cm3/g for the micropore volume vmicro wasobtained from the determined maximum microporeadsorbent capacity V0 at STP conditions in the sameway as explained in Ref. [28]. The pore diameter dis-tribution is determined using the approach proposedby Medek [29] and is shown in Figure 2.

A combination of pycnometry and mercury poros-imetry was used in order to estimate the total pore

volume of the studied coal. As explained in [30], thetotal pore volume vtot can be estimated by the follow-ing relationship:

vtot ¼ 1

�Hg� 1

�He

� �; (7)

where �Hg and �He are the densities of the coal esti-mated by mercury porosimetry and by helium pycn-ometry, respectively. True density of coal was mea-sured by the helium pycnometer AccuPyc 1330 (Mi-cromeritics, Brussels Belgium) and it is defined asweight of a unit volume of the pore-free solid. Onthe other hand, particle (bulk) density is defined asthe weight of a unit volume of the solid includingpores and cracks and it was measured using the mer-cury displacement technique (Pascal 440, ThermoElectron Corporation). For Sulcis coal, a total porevolume of 0.202 cm3/g was found. Subtracting themicropore from the total volume, the meso- and mac-ropore volume of the Sulcis coal is therefore 0.132 cm3/g. Unfortunately, because of the rather high compres-sibility of the Sulcis coal, it was not possible to deter-mine by mercury porosimetry the relative volume ofthe meso- and macropores.

RESULTS AND DISCUSSION

Experimental ResultsThe temperature underground, particularly in a

coal seam, is generally a linear function of the depth.In Southern Sardinia, the geothermal gradient isanomalously high with 18C every 15–20 m [12]. Thecoal sample investigated in this work has been drilledat a depth of 500 m. For an ECBM application in theSulcis Coal Province, optimal conditions, in terms ofnatural gas in place and CO2 capacity, are expectedbetween 800 and 1000 m due to the opposing effectsof increasing pressure and increasing temperature

Figure 1. Low-pressure adsorption isotherm of CO2

on Sulcis coal at 08C for the determination of themicropore volume.

Figure 2. Micropore size distribution for Sulcis coalobtained from the low-pressure CO2 adsorptionexperiments using the Dubinin–Astakhov equation.

358 December 2006 Environmental Progress (Vol.25, No.4)

with depth on capacities [13]. Under the assumptionof an average surface temperature of 208C, theexpected temperatures at these depths thus rangefrom 45 to 708C. Therefore, the temperatures of 45and 608C were chosen as representative of the realsituation in the coal seam of interest and used in theadsorption measurements.

In order to check whether the coal behaves differ-ently in adsorption or desorption mode, after the firstdesorption experiment for CO2 at 458C some meas-urements at increasing pressure were repeated. Nosubstantial difference between adsorption and de-sorption was observed; therefore, all measurementswere then performed in desorption mode. The ex-perimentally measured excess adsorption isothermsfor carbon dioxide on Sulcis coal are reported inTable1. Figure 3 shows the excess adsorption of CO2

on coal from the Sulcis Coal Province plotted againstthe pressure P. The isotherms first increase with pres-sure and then decrease in a nonlinear way. It can beseen that the isotherms at 45 and 608C cross over atabout 90 bar. This is due to the phase behavior ofCO2; in fact, when plotting the excess adsorption as afunction of the reduced density �b/�c, where �b and�c are the bulk and critical densities, the intersectiondisappears, as shown in Figure 4 and the isotherm at608C is below the one at 458C. It is worth noting thatall the isotherms increase to reach a maximum,located at about �b/�c � 0.3, and then decreasealmost linearly with increasing density, as observedearlier for CO2 on other adsorbents [20, 22].

Table 1. Experimental data of the excess adsorptionof CO2 on coal from the Sulcis Coal Province.

T [8C] P [bar] r [g/L] nex [mmol/g]

45 5 10.1 1.1612 22.1 1.5524 46.6 1.9036 73.4 2.0546 98.9 2.1256 129.4 2.1564 156.2 2.1568 175.1 2.1479 234.4 2.0885 283.0 2.0290 330.7 1.9594 379.6 1.8696 420.6 1.7898 453.2 1.72

100 477.4 1.68101 501.5 1.63105 548.1 1.55110 595.0 1.48119 645.7 1.40135 701.2 1.32161 756.7 1.24201 810.1 1.17

60 10 17.5 1.2428 51.5 1.7144 83.7 1.8760 126.6 1.9577 180.3 1.9589 231.2 1.91

108 335.3 1.78114 381.4 1.71118 413.3 1.66124 453.7 1.59128 482.7 1.54138 541.4 1.45150 595.3 1.37166 647.8 1.29191 703.9 1.21229 758.7 1.14

Figure 3. Supercritical isotherms for the excessadsorption of CO2 on coal from the Sulcis CoalProvince at 45 and 608C as a function of pressure P.The symbols represent experimentally measuredvalues and are connected by lines to guide the eye.

Figure 4. Supercritical isotherms for the excessadsorption of CO2 on coal from the Sulcis CoalProvince at 45 and 608C as a function of reduceddensity, �b/�c.

Environmental Progress (Vol.25, No.4) December 2006 359

In our laboratory, the same shape of the isothermlike in the case of Sulcis coal has been recentlyobserved for a Japanese coal from the Akabira coalmine measured in the same balance and comparedwith adsorption measurements performed with a vol-umetric technique [31]. Therefore, it seems thatadsorption of carbon dioxide on coal does not behavequalitatively differently from adsorption on otherstandard adsorbents, such as activated carbon or zeo-lites. In our opinion, this indicates that the effectsreported earlier in the literature and mentioned above,which have been measured using different techniquesthan ours, are possibly an experimental artifact.

The same temperature conditions were investi-gated for methane adsorption on the same coal sam-ple and the results are shown vs. pressure and vs.reduced density in Figures 5 and 6, respectively. Theexperimental data points are also listed in Table2. For

methane, the values of the molar excess adsorptionare significantly lower than those for carbon dioxide.The isotherms show only a slight maximum, becauseCH4 is far above its critical temperature. That is alsowhy there is only a small difference whether theadsorption is plotted against pressure or density.

Absolute Adsorption and Application to ECBMMost adsorption measurement techniques, such as

the magnetic suspension balance, yield only theexcess adsorption, which is defined by Eq. 4. How-ever, for a practical application like ECBM, the molarabsolute adsorption nads per unit mass of coal isneeded, which is given by:

nads ¼ mads

Mmmcoal0

¼ �ð�b; T Þ þ �bV ads

Mmmcoal0

: (8)

Therefore, the volume V ads of the adsorbed phase isneeded in order to evaluate this quantity.

Figure 5. Supercritical isotherms for the excessadsorption of CH4 on coal from the Sulcis CoalProvince at 45 and 608C as a function of pressure P.

Figure 6. Supercritical isotherms for the excessadsorption of CH4 on coal from the Sulcis CoalProvince at 45 and 608C as a function of reduceddensity, �b/�c.

Table 2. Experimental data of the excess adsorptionof CH4 on coal from the Sulcis Coal Province.

T [8C] P [bar] r [g/L] nex [mmol/g]

45 2 1.7 0.544 2.9 0.656 4.2 0.728 5.4 0.79

10 6.5 0.8515 9.7 0.9625 16.2 1.1235 22.7 1.2145 29.4 1.2855 36.2 1.3367 44.2 1.3881 54.8 1.42

100 68.7 1.44121 83.8 1.45141 98.4 1.45161 112.9 1.44183 128.2 1.43200 139.5 1.42

60 2 1.3 0.423 1.9 0.475 3.2 0.567 4.5 0.639 5.7 0.68

11 6.9 0.7317 10.1 0.8327 16.5 0.9538 23.0 1.0448 29.7 1.1059 36.8 1.1571 44.6 1.2087 55.1 1.23

107 68.6 1.25128 82.7 1.26153 99.2 1.27174 112.6 1.26199 128.4 1.25

360 December 2006 Environmental Progress (Vol.25, No.4)

Let us consider CO2 adsorption in the high densityregion, where we can assume that the coal is satu-rated, i.e. mads and Vads are by and large constant. Inthis case, Eq. 4 yields a linear relationship between Gand �b, where the volume of the adsorbed phase V ads

is the slope [32]. Such linearity is indeed observed inthe data shown in Figure 4, where V ads/m0

coal is esti-mated by linear regression to be 0.072 cm3/g at 458Cand 0.066 cm3/g at 608C. In order to estimate the CO2

absolute adsorption, we assume that these two valuesare valid for the whole isotherm, i.e. through thewhole density range. Furthermore, these values com-pare very well to the measured micropore volume of0.070 cm3/g reported in Coal Characterization section;the micropore volume is in fact often used for estimat-ing the volume of the adsorbed phase [32, 33].

In the case of methane, the volume of theadsorbed phase cannot be estimated from the slopeof the isotherm because they have been measuredonly below the critical density and no linear region isavailable for regression. Due to the good agreementof the estimated volumes of the adsorbed phase andthe coal micropore volume in the case of CO2, webelieve that it is a reasonable approximation to calcu-late the absolute adsorption isotherms of methane onSulcis coal using the same adsorbed phase volumesmentioned above.

The obtained absolute adsorption isotherms forcarbon dioxide and methane are plotted in Figure 7.As expected, all the adsorbed concentration valuesfor CO2 are larger than those for CH4. For ECBM pur-poses this is important. Methane is in fact present inthe coal seam in an amount that is equal to or smallerthan its saturation capacity at the formation condi-tions, which is from the data in Figure 7 less than theCO2 amount that can be stored at the same condi-tions. Therefore, if the recovered methane is burnedto produce energy (but producing also CO2), due to

this favorable adsorption ratio, the total CO2 balancestill leads to a net CO2 storage. It is worth noting thatthe assumption of a constant volume of the adsorbedphase yields only approximated values of the abso-lute adsorbed amounts. Beside the experimentaluncertainty, this could explain the presence of anunphysical minimum in the absolute CO2 adsorptionisotherms. Nevertheless, we still believe that the ap-plied method gives a good estimate of the absoluteadsorption, particularly for the evaluation of the feasi-bility of an ECBM operation.

Effect of SwellingCO2 and CH4 do not only adsorb in the coal pores,

but they are also absorbed in the coal matrix causingthe coal to swell; as a matter of fact coal swells muchless upon sorption of methane than of carbon diox-ide [34–37]. Swelling is important for several reasons.On the one hand, the macroporous cleat network ispressed together when the coal swells and the cleatwidth and the porosity are decreased. This results ina reduced permeability of the coal which makes thegas transport slower and reduces therefore the injec-tion capacity of CO2. In order to study this effect,measurements of coal permeability are underwayusing a hydrostatic cell [38]. On the other hand, swel-ling is important also for adsorption measurements. Asimilar behavior as for coal is observed for the sorp-tion of CO2 on polymers. A new purely gravimetricexperimental technique was recently applied tomeasure the swelling and sorption properties of CO2

on poly(methyl methacrylate) [39]. The results fromthis method were compared with a conventionaltechnique which included visualization for swellingand gravimetry for sorption. Both techniques can beapplied to measure the swelling of the coal matrix[38].

So far, we have assumed that CO2 is only adsorbedand that the coal does not swell. In this section, wedemonstrate that the absolute adsorption isotherm ofCO2 shown in Figure 7 is unaffected by this assump-tion, hence that the CO2 storage capacity of the Sulciscoal estimated in this work is insensitive to the extentof coal swelling upon CO2 adsorption. To this aim,the equations presented in Coal Characterization arerewritten to include the effect of swelling. The weightM1 (�b, T) measured by the magnetic suspension bal-ance at the bulk density �b and temperature T is:

M1ð�b; T Þ ¼ M10 þmads þms ð9Þ

��b½Vmet þ ð1þ ÞV coal0 þ V ads�;

where mads and ms are the absolute amounts of CO2

adsorbed in the coal pores and sorbed in the coalmatrix, respectively. The amount of swelling is char-acterized by the swelling ratio , which is defined as:

¼ V coalð�b; T Þ � V coal0

V coal0

: (10)

Figure 7. Supercritical isotherms for the absoluteadsorption of CO2 and CH4 on coal from the SulcisCoal Province at 45 and 608C as a function of thepressure P.

Environmental Progress (Vol.25, No.4) December 2006 361

With respect to Eq. 3, the weight M1 (�b, T) accountsfor the additional mass sorbed ms and for the addi-tional buoyancy effect due to the swollen coal vol-ume, V0

coal. Obviously, Eq. 9 simplifies to Eq. 3 inthe case of no swelling, i.e. where ms ¼ ¼ 0.

Equation 9 can be rewritten as

M1ð�b; T Þ �M10 þ �b½Vmet þ V coal

0 � ð11Þ¼ mads þms � �b½V ads þ V coal

0 �:

where the left hand side corresponds to G(�b, T) ascalculated through Eq. 4, and plotted for CO2 in Fig-ures 3 and 4. The right hand side contains the totaluptake of CO2 on coal, mads þ ms, that includes bothadsorption and absorption, i.e. the quantity neededto assess the CO2 storage capacity. When interpretingthe data in Figure 4, particularly the linear part of thecurves, in the light of the right hand side of Eq. 11,we recognize that the slope of the linear regression isgiven by the sum of the volume of the adsorbedphase and of the swollen part of the coal volume.This interpretation relies on the physically soundassumption that both the amount sorbed ms and theswelling ratio reach an asymptotic value when thegas density increases, as confirmed by both literaturedata [37] and our preliminary measurements [38].

Based on these considerations, the procedure usedto obtain the absolute CO2 adsorption data in Figure7 is indeed independent of the extent of swellingundergone by the coal upon CO2 sorption. Therefore,the absolute CO2 adsorption nads shown in Figure 7actually corresponds to the total molar loading ofCO2 per unit mass of coal, nads þ ns. The same con-clusion cannot be drawn for methane, since thereduced density levels at which CH4 adsorption wasmeasured have not allowed reaching the linearregion of the excess adsorption isotherm (see Figure6). From the data in Figure 7 it can be concluded thatin the temperature range 45–608C the Sulcis coal canuptake carbon dioxide at CO2 pressures above 100bar in an amount which is about 10% of its mass.

NOMENCLATURE

E0 characteristic energy of a standard vapor[kJ/mol]

E characteristic energy of the adsorbingfluid [kJ/mol]

mads absolute adsorption in mass units [g]m0

coal initial weight of coal sample [g]Mm molar mass of adsorbate [g/mol]mmet weight of the lifted metal parts [g]ms amount sorbed in the coal matrix [g]M1 weight at measuring point 1 [g]M1

0 weight at measuring point 1 under vac-uum [g]

n characteristic exponent in the Dubinin-Astakhov equation [-]

nads molar absolute adsorption per unit massof coal [mmol/g]

nex molar excess adsorption per unit mass ofcoal [mmol/g]

ns molar amount sorbed per unit mass ofcoal [mmol/g]

P pressure [bar]P0 saturation pressure [bar]R universal gas constant [J/(mol K)]T temperature [K]� specific pore volume [cm3/g]V adsorbed volume at STP conditions [cm3

STP/g]Vmet volume of lifted metal parts [cm3]V0

coal initial volume of coal sample [cm3]V coal final volume of coal sample [cm3]V0 maximum micropore capacity at STP

conditions [cm3 STP/g]Vads volume of the adsorbed fluid [cm3]

GREEK LETTERS

� affinity coefficient [-]G adsorption excess in mass units [g]�micro degree of micropore filling [-]� density [g/cm3] swelling ratio [-]

SUBSCRIPTS AND SUPERSCRIPTS

b bulkc criticalHe heliumHg mercurymax maximummicro microporetot total

ACKNOWLEDGMENTS

Partial support of the Swiss National Science Foun-dation through grant NF 200020-107657/1 is gratefullyacknowledged. The measurements on the Sulcis coalhave been carried out within a project in cooperationwith Istituto Nazionale di Geofisica e Vulcanologia,Rome, Independent Energy Solutions – Rome, Sota-carbo – Cagliari, and Carbosulcis – Cagliari. We grate-fully acknowledge the possibility to measure the low-pressure CO2 adsorption with Lukas Frunz, Institutefor Chemical and Bioengineering, ETH Zurich, themercury porosimetry measurements by Jorg Heinrich,Martin-Luther University Halle-Wittenberg, Germanyand the helium pycnometry measurements with L.Burlini, Geological Institute, ETH Zurich.

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