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Energy & Fuels 2007, 21, 1640-1645
High-Pressure Sorption of Nitrogen, Carbon Dioxide, and their Mixtures on Argonne Premium Coals Andreas Busch,* Yves Gensterblum, and Bernhard M. Krooss Institute of Geology and Geochemistry of Petroleum and Coal, RWTH Aachen UniVersity, Lochnerstrasse 4-20, D-52056 Aachen, Germany ReceiVed September 25, 2006. ReVised Manuscript ReceiVed February 28, 2007
Gas sorption isotherms have been measured for carbon dioxide and nitrogen and their binary mixture (N2/ CO2 ∼80/20) on three different moisture-equilibrated coals from the Argonne Premium Coal Sample Program by the U.S. Department of Energy, varying in rank from 0.25 to 1.68% vitrinite reflectance (VRr). The measurements were conducted at 55 °C and at pressures up to 27 MPa for the pure gases and up to 10 MPa for the gas mixture. The effects of the large differences in equilibrium moisture contents (0.8 to 32.2%) on sorption capacity were estimated on the basis of the aqueous solubility of CO2 and N2 at experimental conditions. Especially for the Beulah-Zap coal with an equilibrium moisture content of ∼32%, the amount of dissolved CO2 contributes significantly to the overall storage capacity, whereas the amounts of N2 dissolved in the moisture water are low and can be neglected. Sorption measurements with nitrogen/carbon dioxide mixtures showed very low capacities for N2. For Illinois coal, these excess sorption values were even slightly negative, probably due to small volumetric effects (changes in condensed phase volume). The evolution of the composition of the free gas phase in contact with the coal sample has been monitored continuously during each pressure step of the sorption tests. This composition changed strongly over time. Apparently, CO2 reaches sorption sites very quickly initially and is subsequently partly replaced by N2 molecules until concentration equilibration is reached.
Introduction Global warming initiated or amplified by vast amounts of anthropogenic CO2 emissions is a major issue human society has to cope with in the long term. Various options for reducing these emissions are being discussed in the media and technical literature. One of these options is the geological storage of CO2 in geological formations such as saline aquifers, depleted oil and gas reservoirs, or unminable coal seams and abandoned coal mines. Numerous national and international research projects are presently addressing these different options and extensive fundamental and applied research is under way. Several ongoing studies are focusing on the possibility of storing CO2 in unminable coal seams or abandoned coal mines by sorption in the microporous structure of coal. Presently, one major issue is to use CO2 to displace adsorbed methane (coalbed methane, CBM) and transfer it to the cleat system, where it can be recovered and used for energy production. The aim of this procedure is to lower the costs for CO2 capture, transport, and injection by the benefits associated with the enhancement of CBM production (CO2 ECBM). However, costs associated with this option can still be very high, depending on the depth of the storage formation, injection pressures, transport costs, injectivity, net CBM production and capture costs. Costs for carbon capture from flue gas are in the order of 23-35 U.S. dollars per ton of CO2.1 These costs depend on the CO2 concentration required for the injection gas and could be reduced if higher N2 contents were tolerable. * Corresponding author. E-mail:
[email protected]. Fax: 49(0)241 8092152. (1) IPCC (Intergovernmental Panel on Climate Change). Special Report on Carbon Dioxide Capture and Storage; Cambridge University Press: Cambridge, U.K., 2004.
In the present study, the high-pressure (up to 27 MPa) sorption of pure N2, CO2, and N2/CO2 mixtures was investigated on three moisture-equilibrated Argonne Premium Coal Samples at a temperature of 55 °C. The objective was to provide fundamental thermodynamic and kinetic information with respect to the injection of flue gas, rather than pure CO2 into unminable coal seams or abandoned coal mines with coals of different rank. The study represents an extension of a Round Robin test, initiated by the U.S. DOE, where the CO2 sorption behavior was assessed under the above-mentioned conditions. In total, six different laboratories from the United States, Australia, and Germany participated in this study.2 Previously, only a few studies3-8 have addressed the issue of high-pressure sorption of N2/CO2 mixtures on natural coals from an experimental and theoretical point of view. (2) Goodman, A.; Busch, A.; Bustin, M.; Chikatamarla, L.; Day, S.; Duffy, G. J.; Fitzgerald, J. E.; Gasem, K. A. M.; Gensterblum, Y.; Hartman, C.; Krooss, B. M.; Pan, Z.; Pratt, T.; Robinson, R. L., Jr.; Romanov, V.; Sakurovs, R.; Schroeder, K.; Sudibandriyo, M.; White, C. M. Inter-laboratory Comparison II: CO2 Isotherms Measured on Moisture-Equilibrated Argonne Premium Coals at 55 °C and 15 MPa. 2007, in press. (3) Chaback, J. J.; Morgan, W. D.; Yee, D. Fluid Phase Equilib. 1996, 17, 289-296. (4) Fitzgerald, J. E.; Pan, Z.; Sudibandriyo, M.; Robinson, R. L., Jr.; Gasem, K. A. M.; Reeves, S. Fuel 2005, 84, 2351-2363. (5) Gasem, K. A. M.; Robinson, R. L., Jr.; Reeves, S. Adsorption of Methane, Nitrogen, Carbon Dioxide, and Their Mixtures on San Juan Basin Coal; DOE Topical Report; U.S. Department of Energy: Washington, D.C., May 2002. (6) Stevenson, M. D.; Pinczewski, W. V.; Somers, M. L.; Bagio, S. E. Adsorption/desorption of multi-component gas mixtures at in-seam conditions. Presented at SPE 23026, Asia Pacific Conference, Perth, Australia, 1991; Society of Petroleum Engineers: Richardson, TX, 1991. (7) Mazumder, S.; van Hemert, P.; Busch, A.; Wolf, K.-H. A. A.; TejeraCuesta, P. Int. J. Coal Geol. 2005, 67, 267-279.
10.1021/ef060475x CCC: $37.00 © 2007 American Chemical Society Published on Web 04/20/2007
High-Pressure Sorption of N and CO2 on Argonne Premium Coals
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Table 1. Argonne Premium Coal Sample Properties9 sample
rank
ash (%)
VM (%)
C (%)
VRr (%)
vitrinite (%)
liptinite (%)
inertinite (%)
Pocahontas #3 Illinois #6 Beulah-Zap
lvb hvb lignite
4.74 14.25 6.59
18.48 36.86 30.45
91.05 77.67 72.94
1.68 0.46 0.25
89 85
5 1
10 10
a
Equilibrium moisture content (after ASTM procedure; see above).
Samples. The Argonne Premium Coal Sample Programme consists of a selection of eight U.S. coals of different rank ranging from 0.25 up to 1.68% VRr. The coals have been characterized comprehensively and have been used as standard and reference samples in numerous studies. In the present study, two Carboniferous (Pennsylvanian) coals (Pocahontas, Illinois) and one Tertiary coal (Beulah-Zap) were used (Table 1). The maceral composition for the Beulah-Zap sample has not been analyzed because of its low maturity. The two Pennsylvanian samples show similar maceral compositions and are dominated by vitrinite (85-91%9). Experimental Section Sample Preparation. The samples were supplied in small sealed glass vials under an inert gas atmosphere. Each vial contained about 5 g of coal. The grain size of the samples was -100 mesh (-0.15 mm). Moisture-equilibration of the coal samples was performed according to a modification of the ASTM D 1412-99 “Standard Test Method for Equilibrium Moisture of Coal at 96 to 97 Percent Relative Humidity” procedure. Deviating from the standard test method, coal samples were equilibriated at 55 instead of 30 °C. In this study, Pocahontas #3 and Illinois #6 samples were equilibrated for 48 h, whereas the Beulah-Zap sample was equilibrated for 72 h. After moisture-equilibration, the coal samples were transferred immediately to the sample cell to prevent surface oxidation. Approximately 1 g of moisture-equilibrated sample was taken for the determination of the moisture content. Experimental Setup and Procedure. The manometric method was used to determine the sorption capacities of the three coals as a function of pressure. Different setups were used for experiments with pure gases and gas mixtures. Sorption Experiments with Pure Gases. The experimental procedure for the single gas sorption experiments has been published elsewhere.10-12 Compared to former studies, the pressure range of the measurements has been extended to ∼25 MPa. For the assessment of CO2 and N2 densities, the corresponding equations of state for CO2 and N2, respectively, were used.13,14 These equations are considered to be the most reliable EOS and are based on the latest and most comprehensive sets of experimental data. Sorption Experiments with N2/CO2 Gas Mixtures. A schematic diagram of the setup for the sorption experiments with gas mixtures is presented in Figure 1. The entire setup is placed in a thermostat oven, ensuring a constant temperature within 0.1 K of the setpoint. Experiments are carried out in an automated procedure. Initially, both the sample cell (volume VSC ≈ 8.00 cm3) and the reference volume (volume VRC ≈ 1.54 cm3) are evacuated to (8) Hall, F. E.; Chunhe, Z.; Gasem, K. A. M.; Robinson, R. L.; Yee, D. Adsorption of pure methane, nitrogen and carbon dioxide and their binary mixtures on wet Fruitland Coal. Presented at SPE 29194, Eastern Regional Conference and Exhibition, Charleston, WV, Nov 8-10, 1994; Society of Petroleum Engineers: Richardson, TX, 1994. (9) Vorres, K. S. Energy Fuels 1990, 4, 420-426. (10) Busch, A.; Gensterblum, Y.; Krooss, B. M. Int. J. Coal Geol. 2003, 55, 205-224. (11) Busch, A.; Gensterblum, Y.; Krooss, B. M.; Littke, R. Int. J. Coal Geol. 2004, 60, 151-168. (12) Siemons, N.; Busch, A. Int. J. Coal Geol. 2007, 69, 229-242. (13) Span, R.; Wagner, W. J. Phys. Chem. Ref. Data 1996, 25, 15091596. (14) Span, R.; Lemmon, E. W.; Jacobsen, R. T.; Wagner, W.; Yokozeki, A. J. Phys. Chem. Ref. Data 2000, 29, 1361-1433.
establish a defined starting condition. The reference volume is located between valves V5 and V6 and includes the volume of the pressure transducer (max. pressure 16 MPa with an error of 0.05% of the full scale value). Both volumes are calibrated by helium expansion as described above. The uncertainty of the calibration is (0.002 cm3 (0.13%) for the reference cell and (0.008 cm3 (0.1%) for the sample cell. The two cells are then separated by closing the shut-off valve. In the next step, a certain amount of gas mixture is admitted to the reference cell from the reservoir. Approximately 15 min are allowed for pressure and temperature equilibration. The switching valve between the reference and the sample cell is opened and the gas mixture is admitted to the sample cell. To monitor the establishment of sorption equilibrium, several pressure measurements are taken at time intervals ranging between 1 and 60 min. Filter disks with 2 µm pore size are used to prevent coal or mineral particles from entering the valves. Homogenization of the gas concentration in the sample cell is achieved by means of an encapsulated circulation pump with magnetic transmission (Rietschle Thomas, type GK-M 04, Figure 1). The pump body has a nominal upper pressure limit of 16 MPa and thus defines the upper limit of the sorption experiments with gas mixtures. Further information on the experimental procedure is given elsewhere.15 To analyze the evolution of the gas composition in the free gas phase, a small amount of gas is transferred to the gas chromatograph (GC) via a microvolume sampling valve (volume: 0.04 µL) and analyzed for N2 and CO2 contents. The entire procedure was repeated until the final pressure and concentration equilibration is reached. Concentration equilibration is usually reached after 4-6 h, whereas the concentration of the free gas phase is analyzed in 30-60 min time intervals. After equilibration, the system pressure is recorded and the cells are separated again. The feed gas composition was measured over the entire experimental pressure range in a separate blind experiment. The CO2 molar fraction (y(CO2)) was 0.22 with a standard deviation of 0.5%. The calibration of the thermocouple detector (TCD) was performed by introducing defined amounts of certified calibration gases (pure N2 and 2% CO2 in helium) onto the GC column with calibrated sample loops of different sizes and recording the output signal. Thus the detector linearity and the stability during the experimental sequence were ensured. For N2/CO2 mixtures, the z-factors (compressibility factors) were calculated by using reference equations of state for natural gases, provided by Ruhr-University Bochum, Germany.16 Mass-Balance Calculation for Surface Gas Excess Sorption Isotherms. The composition (CO2 molar fraction) of the feed gas 0 mixture (yCO ) and of the free gas phase (yCO2) in the measuring 2 cell was determined by gas chromatography (GC). Calibration gases were used to determine the response factors of the thermal conductivity detector (TCD) for N2 and CO2 over a large concentration range. Figure 2 shows the results of the calibration measurements and demonstrates the accuracy of the calibration constants. To control the linearity of the GC/TCD analysis, the composition of the feed gas mixture was measured over the entire experimental (15) Busch, A.; Gensterblum, Y.; Krooss, B. M.; Siemons, N. Int. J. Coal Geol. 2006, 66, 53-68. (16) Kunz, O.; Klimeck, R.; Wagner, W.; Jaeschke, M. The GERG-2004 Wide-Range Reference Equation of State for Natural Gases; GERG Technical Monograph; VDI-Verlag: Du¨sseldorf, Germany, to be published.
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Figure 4. Excess sorption isotherm, amount of CO2 dissolved in moisture water and portion of adsorbed CO2 for moisture-equilibriated Pocahontas coal. Figure 1. Experimental setup for sorption experiments with gas mixtures. V1-V6 Valco valves (VICI AG International).
Figure 5. Excess sorption isotherm, amount of CO2 dissolved in moisture water and portion of adsorbed CO2 for moisture-equilibriated Illinois coal. Figure 2. Amounts of substance (mol) vs detector signal (peak area in mV s) for the calibration of the GC/TCD. Response factors (mol/ (mV s)) for the quantification of N2 and CO2 in gas mixtures during sorption experiments were obtained from the regression lines.
Figure 6. Excess sorption isotherm, amount of CO2 dissolved in moisture water and portion of adsorbed CO2 for moisture-equilibriated Beulah-Zap coal.
Results and Discussion 0 Figure 3. Molar fraction of CO2 (yCO ) in the source gas calculated 2 from TCD signals, as a function of pressure.
pressure range (∼12 MPa). The molar fractions were determined from the chromatographic peak areas by means of the response factors from the calibration tests (see above). It is evident from Figure 3 that the molar fractions calculated from the detector signals show only a small variation over the entire pressure range. The equation of state package GERG-200416 was used for the evaluation of the sorption experiments. This routine yields densities (kg/m3) of gas mixtures as a function of pressure, temperature, and molar fractions of the gas components (CO2 and N2). The mass-balance calculation used for the evaluation of the experimental data is described in the Appendix.
Pure Gases. Carbon Dioxide. Figures 4-6 show the excess sorption isotherms for CO2 measured at 55 °C and pressures up to 25 MPa. Further, the aqueous solubility of CO2 in the equilibrium moisture, calculated according to the relationship of Duan and Sun,17 is given as a function of pressure. Here, we assume that CO2 dissolves in the water volume corresponding to the entire moisture content. A third isotherm shows the excess sorption capacity with the dissolved CO2 amounts subtracted. The isotherm for Pocahontas coal (Figure 4) shows a maximum sorption capacity of ∼1.0 mmol/g and decreases gently above ∼8 MPa to a value of ∼0.7 mmol/g at the final pressure. This decrease has been discussed extensively by Siemons and Busch (2007) and was interpreted as a volumetric (17) Duan, Z.; Sun, R. Chem. Geol. 2003, 193, 257-271.
High-Pressure Sorption of N and CO2 on Argonne Premium Coals
Figure 7. Comparison of CO2 excess and total sorption capacities for Argonne Premium Coals.
Energy & Fuels, Vol. 21, No. 3, 2007 1643
Figure 8. Nitrogen excess sorption isotherms for the three Argonne Premium Coals analyzed.
Table 2. Langmuir Parameters and Calculated Increase in Condensed Phase Volume for Argonne Premium Coals sample Pocahontas #3 Illinois #6 Beulah Zap
VRr (%) nL (mmol/g) PL (MPa) volume increase (%) 1.68 0.46 0.25
1.43 1.31 1.18
1.34 4.79 6.00
1.91 1.97 0.98
effect (volume of the sorbed phase, possibly coal swelling, etc.) that influences the shape of the excess sorption isotherm. Evidently, CO2 dissolution in the pore water is low because of the very low equilibrium moisture content of 0.78%. The Illinois coal (Figure 5) shows a different behavior due to its higher moisture (and ash) contents as compared to the Pocahontas sample (Table 2). Maximum sorption capacity (∼0.9 mmol/g) is reached only at ∼12 MPa, followed by relatively abrupt cutoff and a continuous decrease at higher pressures. Because of the higher equilibrium moisture content (∼8%) of this coal, the proportion of dissolved CO2 is relatively high, rising to ∼0.11 mmol/g at the final pressure. Subtracting the dissolved CO2 from the excess sorbed CO2, the sorption capacity on pure coal material is reduced by ∼16% at the final pressure. The dissolution of CO2 plays an even more important role for the Beulah-Zap coal sample (Figure 6). The contribution of CO2 solubility in the equilibrium moisture amounts up to 50%, which results in much lower pure coal sorption capacities. The pure coal sorption isotherms after subtraction of the portion of dissolved CO2 for the three coals are shown in a synoptic diagram in Figure 7. Evidently, the Beulah-Zap and Illinois coals show similar sorption behavior, whereas the Pocahontas coal sorption isotherm exhibits a much steeper increase in sorption capacity at low pressures. This is followed by a smooth decrease, comparable to those observed for the other two samples. It is evident from these results that moisture can play an important role and influences excess sorption capacities depending on coal rank. It is well-known that moisture reduces sorption capacity, because water molecules compete with the sorbing gas for sorption sites. However, the solubility of CO2 in water is of the same order of magnitude as CO2 sorption on the organic material and increases with pressure and decreases with temperature.16 Figure 7 also shows calculated total sorption isotherms using the approach proposed by Siemons and Busch.12 The proposed method uses one single volume correction factor to calculate total sorption capacities, which combines the effects of sorbed phase volume, the ability of CO2 to dissolve in the coal polymer structure, and several uncertainties associated with He-pycnometry measurements. Information on the fitting procedure and
Figure 9. N2/CO2 excess sorption isotherms for Argonne Premium Coals analyzed (feed gas composition: yCO2 ) 0.22, yN2 ) 0.88).
an application of this concept to various worldwide coals is given elsewhere.12 By this procedure, Langmuir parameters as well as the volume of the condensed phase (coal + sorbate volume) can be calculated. The percentage of estimated increase in condensed phase volume is provided in Table 2. Nitrogen. Nitrogen excess sorption isotherms for the three coals, measured in the moisture-equilibrated state at 55 °C and pressures up to ∼20 MPa, are shown in Figure 8. The BeulahZap coal clearly has the highest N2 sorption capacities, followed by Illinois and Pocahontas samples. Although Beulah-Zap and Pocahontas coals show an almost linear increase in sorption capacity within the experimental pressure range, the Illinois coal sorption isotherm seems to approach a saturation value corresponding to a Langmuir-type isotherm. Additionally, the aqueous solubility of N2 was calculated.18 Generally, these values are low and, at final experimental pressures, account for 2.24, 1.66, and 0.22% of the excess sorption capacity for Beulah-Zap, Illinois, and Pocahontas coals, respectively. Gas Mixtures. Results of the excess sorption measurements with N2/CO2 mixtures are presented in Figure 9. The molar fraction yCO2 of CO2 in the feed gas in all experiments was 0.22 (see Experimental Section) and measurements were conducted up to maximum pressures of ∼10 MPa. As already evidenced by the single-gas sorption measurements, N2 sorption capacities are much lower than the corresponding CO2 sorption capacities. The highest CO2 excess sorption capacities are encountered for the Illinois coal, followed by the Pocahontas and Beulah-Zap samples. N2 excess sorption (18) O’Sullivan, T. D.; Smith, N. O. J. Phys. Chem. 1970, 74, 14601466.
1644 Energy & Fuels, Vol. 21, No. 3, 2007
Figure 10. Composition of the free gas phase versus time for individual pressure steps during a high-pressure sorption experiment with a N2/CO2 gas mixture on Illinois coal (Argonne Premium Coal Programme).
capacities are very low and even negative excess sorption values of approximately-0.02 mmol/g are evident for the Illinois coal. The Beulah-Zap sample exhibits the highest sorption capacities at low pressures with 0.01-0.02 mmol/g over the entire pressure range. Pocahontas coal shows N2 sorption capacities of almost zero for the first data point; however, the sorption values increase up to 0.06 mmol/g at final experimental pressure. The phenomenon of negative sorption values for Illinois coal is considered to be due to volumetric effects associated with gas sorption. Generally, in high-pressure manometric sorption measurements, volumetric effects affecting the ratio of void volume to sample volume will have an impact on the mass-balance calculations. Because excess sorption is calculated under the assumption of an invariant void volume/condensed volume ratio, negative excess sorption values for N2 may be observed. For each pressure step, the composition of the free gas phase in contact with the coal sample was determined repeatedly and its evolution followed over time, as exemplified for the Illinois coal in Figure 10. Evidently, the molar fraction of CO2 in the free gas (yCO2,i) increases with time and concentration equilibration is achieved only approximately 4-5 h after the start of the pressure step. Pressure equilibration, on the other hand, is usually reached after 30 to 60 min, similar to singlegas measurements. This indicates that after the introduction of new feed gas from the reservoir to the measuring cell, CO2 occupies the sorption sites and/or dissolves much faster than nitrogen (kinetic control). Subsequently, however, nitrogen partially replaces sorbed CO2 molecules and yCO2 increases again until thermodynamic equilibrium is reached (thermodynamic control). This effect could be attributed to different sorption times for CO2 and N2, as indicated in another study for CO2 and CH411. There, it was shown that CO2 reaches sorption equilibrium much faster than CH4. This could hold true in this study as well: initially CO2 reaches sorption sites faster than N2; however, it gets replaced by nitrogen because of the initial 0 increase in N2 partial pressure. The feed gas composition yCO 2 is plotted in Figure 10 as well in order to document that the free gas phase in the measuring cell is depleted in CO2, resulting in a selective sorption of CO2 as already concluded from the mass-balance calculations shown in Figure 9. Conclusions A detailed investigation of the high-pressure sorption of pure CO2 and N2 and their mixtures was performed on three moisture-
Busch et al.
equilibrated Argonne Premium Coals of different rank. The objective of this study was to provide fundamental thermodynamic and kinetic information with respect to the injection of flue gas, rather than pure CO2 into unminable coal seams or abandoned coal mines. Experiments were conducted at 55 °C and pressures up to 25 MPa for single gases and up to 10 MPa for gas mixtures. As expected, sorption capacities of all coal samples were much higher for CO2 than for N2. For all three coals, the influence of moisture content on the excess sorption capacity was investigated (equilibrium moisture contents 0.8-32.2%) and the relative contributions of physical sorption and aqueous solubility of CO2 at equilibrium moisture contents were determined. The excess isotherms of the pure gases were then used to estimate the volume changes of the condensed phases (solid coal + adsorbed phase + water) during CO2 sorption following an approach by Siemons and Busch.12 The quantification of the gas-mixture sorption experiments showed that the composition of the free (non-adsorbed) gas phase changes strongly during the individual pressure steps, indicating a kinetic control of the sorption from mixtures on a time-scale of several hours (particle sizes:
