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Long-Term CO2 Sorption on Upper Freeport Coal Powder and Lumps Vyacheslav Romanov* and Yee Soong United States Department of Energy, National Energy Technology Laboratory, Post Office Box 10940, Pittsburgh, PennsylVania 15236 ReceiVed August 11, 2007. ReVised Manuscript ReceiVed January 11, 2008
Powder and lumps of the Argonne Premium Upper Freeport coal were compared in a 9-month-long CO2 sorption-desorption study. On the basis of the slope analysis, CO2 induced the lumps swelling by 7%. The powder swelled by 8% and then rapidly shrank by 3% on desorption. This was verified by a “single point” injection of helium and the pressure-and-density technique, which was sufficiently sensitive. Low-pressure Langmuir-fit parameters suggest that the microporous textures of the powder and lumps are similar. The samples demonstrated greatly increased sticking coefficients and a slightly larger number of the “sorption ready” sites, after the CO2-induced swelling and structural rearrangement resulted in variable degrees of the matrix shrinkage. No permanent changes in the void volume and dry mass were detected after complete desorption. The observed hysteresis in sorption-desorption behavior of both samples upon the approach to the CO2 density fluctuation ridge is interpreted as a contribution of condensation and coalescence of CO2 clusters in mesopores. The differences between the two samples in the magnitude of the enhanced capillary condensation, with or without dissolution, demonstrate the importance of changes in mesoporous texture caused by grinding of coal lumps into powder.
Typically, the CO2 sorption measurements are conducted on crushed and pulverized coal powder. Such data can be misleading if used for the prediction of the CO2 transport and sequestration in coal seams because it does not take into account the differences in texture and porosity. The importance of such differences should not be overlooked, especially, in view of the confinement conditions of potential sequestration sites. It has been demonstrated that coals change their volume and structure after sorption of gaseous and liquid substances.1,2 Various types of volume changes affect the accuracy of sorption measurements by the two major methods, gravimetric and manometric. To eliminate this error, we need to continuously monitor the freephase (void) volume changes during the test. Previously, we suggested that the integration of simultaneous pressure and density measurements, combined with the use of the binary gas mixture technique, will permit in situ volume measurement for plastic materials.3 Using the equation of state (EOS) of the mixture, one can determine the partial density of each of the two components as long as their molar masses are very different.4 Once the density of the inert gas component is known, the void volume and hence the “inaccessible” volume of the sample can be computed by standard volumetric procedure. In this work, some of these ideas were applied in an attempt to interpret the pure CO2 sorption isotherm data for the Argonne Premium Upper Freeport medium volatile bituminous coal, by * To whom correspondence should be addressed. Telephone: 412-3865476. E-mail:
[email protected]. (1) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structure; Meyers, R., Ed.; Academic Press: New York, 1982; Chapter 6. (2) Gray, I. SPE ReserVoir Eng. 1987, February, 28–34. (3) Romanov, V.; Soong, Y.; Schroeder, K. Chem. Eng. Technol. 2006, 29, 368–374. (4) Schein, E.; Keller, J. U. Presented at the American Institute of Chemical Engineers Annual Meeting, San Francisco, CA, November 2003.
introducing a measured (small) amount of helium after the first desorption step. We used the NETL-built manometric/volumetric apparatus, similar to the one used in the previous studies3,5 but with significantly improved accuracy, to collect the CO2 (99.999% purity, Valley Co., Pittsburgh, PA) sorption isotherm data at 55 °C and the pressures up to 14 MPa. Gases were pressurized by the ISCO syringe pump (Model 500D). The sample and the reference cells volumes were determined by injecting helium (99.997% purity, Valley Co., Pittsburgh, PA) and using either the volume readings of the pump or the standard volumes of cylindrical shape (accurate within 0.05 cc). The estimated accuracy of the void volume determination before and after the sorption experiment is 2% at the CO2pressure of ∼4 MPa, preceded by the first significant deviation from the Langmuir-like overall sorption trend. In the same pressure range, the best-fit density of the adsorbed monolayer on the powder sample began to diverge from the reasonable 1.12–1.13 to over 6 g/cm3, for the Langmuir model of both sorption and desorption. (Two-site Langmuir fit was not successful.) This observation and the fact that the abrupt onset of the coal swelling does not appear to have involved a significant change in the total amount of the sorbed CO2 speaks in favor of either gradual dissolution7 of the Langmuir layer, which makes the separation of the excluded volume terms in eq 1 rather artificial, or plasticization of coal.8 The best-fit partition parameters for the Langmuir-like CO2 sorption, νL (for both sorption and desorption at the pressures below 4 MPa), are summarized in Table 2 and are based on the assumption that the sorbate activity near the surface is proportional to its free-phase density, F νL(F) ) νm
KdF 1 + KdF
(2)
The initial step of the desorption plot for the lumps appears almost identical to the final step of the sorption. Hence, the (7) Reucroft, P. J.; Sethuraman, A. R. Energy Fuels 1987, 1, 72–75. (8) Romanov, V. Energy Fuels 2007, 21, 1646–1654.
Long-Term CO2 Sorption
Figure 2. CO2 ([) sorption-desorption hysteresis at F> Fc for Upper Freeport powder tested at 55 °C. After injection of 3% He at the second step of desorption, the next data were interpreted as 3% coal shrinkage (2) and 17% swelling (4) and, alternatively, as hypothetical pure CO2 excess sorption (]).
slope remains the same. However, the first step of the CO2 desorption off the powder is much steeper than the corresponding sorption step. This could mean that either (1) swelling of the sample, i.e., the increase in the total excluded volume of the sorbed CO2-coal system at these pressures, is about 17%, (2) a decrease in the ambient pressure resulted in an increase, rather than a decrease, of the actual (“absolute”) sorbed amount of CO2, or (3) upon desorption, the total excluded sample volume had transformed from swelling by 8% to shrinking by 3%.9 To test the above hypotheses, we premixed 97% carbon dioxide and 3% helium and injected the measured amount of the mixture into a reference cell, prior to the next desorption step, day 170 (Table 1). When the valve between the powder sample and the reference cell was open, the equilibrium total pressure of the free-phase mixture became nearly the same, within experimental error, as the pure CO2 pressure in the isolated sample cell at the preceding step, day 155. Assuming the third of the above desorption hypotheses (3% shrinkage) to be the most likely scenario, we estimated the densities of He by dividing the hypothetical total void volume into the injected He mass and, using the best-fitEOS parameters of the mixture derived during the premixing procedure, the density of CO2 in the free phase. (For the subsequent desorption steps, especially, after day 193, it was assumed that the helium-related corrections are negligible. The errors were decreasing with each step.) The corresponding value of the excess sorption as well as the value corresponding to the first desorption hypothesis are plotted in Figure 2. It appears that the third hypothesis is more in line with the experimental data than the first one. The sensitivity of this test is demonstrated by the excess sorption values corresponding to the hypothetical pure CO2 injection, at the same pressure as the total pressure (9) Supporting Information available. This information is available free of charge via the Internet at http://pubs.acs.org.
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of the 3% helium mixture, which would be offset by ∼0.2 mmol/g (Figure 2). The observed hysteresis in the sorption-desorption behavior of both samples upon the approach to the CO2 density fluctuation ridge10 can be interpreted as a contribution of another sorption mechanism, the condensation and coalescence of CO2 clusters8–11 in mesopores. Theoretically, an accurate description of this liquid-like phase10,12 is challenging because the cohesive forces are enhanced by the pore adhesion fields.13–15 At lower pressures, the anomalous deviations from the Langmuir-like desorption (Figure 1) can be viewed, in a linear approximation, as the void volume adjustments or the partial sample-volume changes (shrinkage, in accordance with the third desorption hypothesis), V*, -0.7% for the powder and -0.5% for the lumps (Table 2). However, no permanent changes in the void volume were detected by the fast He-volume measurements, after complete CO2 desorption. Also, no apparent changes were observed during a visual inspection of the lumps, except for
