Energy & Fuels 2008, 22, 4049–4061
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Meso- and Micropore Characteristics of Coal Lithotypes: Implications for CO2 Adsorption Maria Mastalerz,* Agnieszka Drobniak, and John Rupp Indiana Geological SurVey, Indiana UniVersity, 611 North Walnut GroVe, Bloomington, Indiana 47405-2208 ReceiVed July 9, 2008. ReVised Manuscript ReceiVed September 9, 2008
Lithotypes (vitrain, clarain, and fusain) of high volatile bituminous Pennsylvanian coals (Ro of 0.56-0.62%) from Indiana (the Illinois Basin) have been studied with regard to meso- and micropore characteristics using low-pressure nitrogen and carbon dioxide adsorption techniques, respectively. High-pressure CO2 adsorption isotherms were obtained from lithotypes of the Lower Block Coal Member (the Brazil Formation) and the Springfield Coal Member (the Petersburg Formation), and after evacuation of CO2, the lithotypes were reanalyzed for meso- and micropore characteristics to investigate changes related to high-pressure CO2 adsorption. Coal lithotypes have differing Brunauer-Emmett-Teller (BET) surface areas and mesopore volumes, with significantly lower values in fusains than in vitrains or clarains. Fusains have very limited pore volume in the pore size width of 4-10 nm, and the volume increases with an increase in pore size, in contrast to vitrain, for which a 4-10 nm range is the dominant pore width. For clarain, both pores of 4-10 nm and pores larger than 20 nm contribute substantially to the mesoporosity. Micropore surface areas are the smallest for fusain (from 72.8 to 98.2 m2/g), largest for vitrain (from 125.0 to 158.4 m2/g), and intermediate for clarain (from 110.5 to 124.4 m2/g). Similar relationships are noted for micropore volumes, and the lower values of these parameters in fusains are related to smaller volumes of all incremental micropore sizes. In the Springfield and the Lower Block Coal Members, among lithotypes studied, fusain has the lowest adsorption capacity. For the Lower Block, vitrain has significantly higher adsorption capacity than fusain and clarain, whereas for the Springfield, vitrain and clarain have comparable but still significantly higher adsorption capacities than fusain. The Lower Block vitrain and fusain have much higher adsorption capacities than those in the Springfield, whereas the clarains of the two coals are comparable. After exposure of coal to CO2 at high pressure, vitrains experienced the largest porosity changes among all lithotypes studied. These changes are dominantly manifested in the mesoporosity (decrease in mesopore volume) range, whereas little to no change occurred in the micropore size range. In other lithotypes (clarains, the dominant lithology in the coals studied, and sporadic fusains), the changes were minimal.
1. Introduction Recent concerns about increasing concentrations of CO2 in the atmosphere have triggered investigations into methods to reduce these emissions of CO2 from point sources (coal-fired power plants and natural-gas-burning sources). Storage of CO2 in deep coal beds is one option being considered. First, CO2 can access the finest pores and adsorb firmly to the coal, with minimal chances of its later release.1 Second, injected CO2 displaces adsorbed CH4, improving and accelerating methane recovery from the coal. Several CBM/CO2 field-scale operations have shown to be promising.2-4 Numerous laboratory studies of CO2 adsorption into coal have been conducted over the past decade,5,6,1,7-9 providing information about rates and volumes of adsorbed CO2 as well as * To whom correspondence should be addressed. E-mail: mmastale@ indiana.edu. (1) Krooss, B. M.; van Bergen, F.; Gensterblum, Y.; Siemons, N.; Pagnier, H. J. M.; David, P. Int. J. Coal Geol. 2002, 51, 69–93. (2) Gunter, W. D.; Gentzis, T.; Rottenfuser, B. A.; Richardson, R. J. H. Energy ConVers. Manage. 1997, 38 (Supplement), 217–222. (3) Schoeling, L.; McGovern, M. Pet. Technol. Dig. 2000, 14. (4) Reeves, S. The COAL-SEQ project: Results of the Allison and Tiffany ECBM field studies. Proceedings of the 2nd Annual DOE/NETL Conference on Carbon Sequestration, Alexandria, VA, 2003. (5) Ceglarska-Stefanska, G.; Czaplicki, A. Fuel 1993, 72, 413–417. (6) Clarkson, C. R.; Bustin, R. M. Fuel 1999, 78, 1333–1344.
selectivity of CO2 over methane. These and other studies have provided a basis for discussion about adsorption mechanisms10,7,11 and the changes in the structure of the coal resulting from the adsorption of CO2.12 Despite the great number of recent papers and reports, important questions about controls on CO2 adsorption in coal and the effects of sorption on the coal matrix remain unanswered and the data are often contradictory. This as yet unsatisfactory understanding of CO2 adsorption into the coal beds is partly the reason why industrial-scale injections face unpredictable CO2 injection rates (injectivity) and, consequently, why they have not progressed as expected. Knowledge of carbon dioxide interactions with coals is important to model injectivity changes over time as well as to predict long-term effects of CO2 storage in coal beds. However, the influence of injected CO2 on the coal structure is not well(7) Mazumder, S.; van Hemert, P.; Bruining, J.; Wolf, K.-H. A. A.; Drabe, K. Fuel 2006, 85, 1904–1912. (8) Cui, X.; Bustin, R. M.; Chikatamarla, L. J. Geophys. Res. 2007, 112, 1–16. (9) Bustin, R. M.; Cui, X.; Chikatamarla, L. AAPP Bull. 2008, 92, 1– 15. (10) Milewska-Duda, J.; Duda, J. T.; Nodenski, A.; Lakatos, J. Langmuir 2000, 16, 5458–5466. (11) Goodman, A.; Campus, L. M.; Schroeder, K. T. Energy Fuels 2005, 19, 471–476. (12) Larsen, J. W. Int. J. Coal Geol. 2004, 57, 63–70.
10.1021/ef800544g CCC: $40.75 2008 American Chemical Society Published on Web 10/21/2008
4050 Energy & Fuels, Vol. 22, No. 6, 2008
Mastalerz et al.
Table 1. Characteristics of the Coal Beds Studied seam
mine
moisture (%)
Danville Springfield Buffaloville Upper Block Lower Block
Discovery Somerville Midway Midway Howesville
10.23 8.74 11.91 12.00 13.29
ash (%, dry)
S (%, dry)
heating value (MJ/kg, daf)
heating value (BTU/lb, daf)
Ro (%)
13.40 10.26 10.01 12.30 9.88
4.96 5.04 3.12 10.12 0.92
33.28 33.63 33.72 33.84 34.30
14294 14443 14477 14530 14728
0.57 0.52 0.57 0.57 0.62
understood. Studies show that, upon initial contact of injected fluids with the coal, there is very fast adsorption of the fluid on the coal surface and that this is followed by slow diffusion into the coal.13 This process of diffusion (and dissolution of fluid in the coal) is commonly explained by the principles of polymer chemistry and the transformation of initially glassy, brittle, crosslinked macromolecules of coal into rubbery, viscous, still crosslinked macromolecules, where CO2 lowers the glass-rubber transition temperature.14,15,12,16 In such an interpretation, CO2 acts as a plasticizer; CO2 is, in fact, an effective plasticizer in many polymers.17 It is not definitely known if CO2 sorption is solely a physical phenomenon or if there is also a chemical reaction of CO2 with functional groups of the organic matter of coal; arguments exist for both. For example, on the basis of high-pressure static and dynamic experiments on coal, Mazumder et al.7 conclude that “chemical reactions involving CO2 cannot be ruled out”, although no direct evidence has been demonstrated in that study. Kolak and Burruss18 suggested that, during the process, some polycyclic aromatic hydrocarbons were liberated and mobilized in the coal bed, suggesting chemical interactions. Another study, however, indicates no interactions between injected CO2 and coal functional groups,11 supporting the physical process of adsorption. Regardless of the physical or chemical nature of CO2 sorption, it has been widely documented that CO2 injection at high pressure results in swelling of the coal matrix19,5 and that the amount of swelling depends upon both coal rank and coal type. Whereas the influence of coal rank on gas adsorption is rather predictable, the influence of coal type is not wellunderstood and not easily quantified. Modeling CO2 injections into coal beds is commonly based on the experience from fractured reservoirs and does not take into account the lithological heterogeneity of a coal bed. However, coal is a heterogeneous material, and coal lithotypes (macroscopically recognizable coal bands) have different characteristics with regard to porosity, cleating, chemical composition, hardness, grindability, and so on. These differences are expected to selectively influence CO2 injectivity and cause selective changes in the coal matrix. Therefore, the understanding of how CO2 sorbs into individual lithotypes should eventually be incorporated into coal CO2-injection models, thus contributing to the optimization of the injection process.20 The two principal objectives of this paper are to (1) analyze variations in meso- and micropore characteristics as well as CO2 adsorption capacities among lithotypes of high-volatile bitumi(13) Hsieh, S. T.; Duda, J. L. Fuel 1987, 66, 170–177. (14) Khan, M. R.; Jenkins, R. G. Thermoplastic properties of coal at elevated pressures: Effects of gas atmospheres. Proceedings of the International Conference on Coal Science, Sydney, Australia, 1985. (15) Larsen, J. W.; Flowers, R. A., II; Hall, P.; Carlson, G. Energy Fuels 1997, 11, 998–1002. (16) Karacan, C. O. Int. J. Coal Geol. 2007, 72, 209–220. (17) Wang, W. C.; Kramer, V.; Sachse, E. J. J. Polym. Sci., Part B: Polym. Phys. 1982, 20, 1371–1384. (18) Kolak, J. J.; Burruss, R. C. Energy Fuels 2006, 20 (2), 566–574. (19) Reucroft, P. J.; Patel, H. Fuel 1986, 65, 816–820. (20) Karacan, C. O.; Mitchell, G. D. Int. J. Coal Geol. 2003, 4, 201– 217.
Table 2. Maceral Composition and Vitrinite Reflectance of the Samples Used in This Study Ro (%) vitrain clarain fusain
0.56
vitrain A vitrain B clarain fusain
0.57
vitrain dull clarain fusain
0.62
vitrain clarain fusain
0.62
vitrain clarain fusain
0.62
a
inertinite (vol %)
MMa (vol %)
Springfield, Somerville 89.2 6.0 83.8 1.2 12.0 0.0
3.4 9.8 85.0
1.4 5.2 3.0
Lower Block, Howesville 94.0 2.0 85.6 6.0 72.2 8.4 11.6 13.2
4.0 7.0 18.2 72.4
0.0 1.4 1.2 2.8
Danville, Discovery 96.0 1.0 75.2 8.0 17.0 0.0
1.0 13.6 79.0
2.0 4.4 4.0
Upper Block, Midway 95.0 1.0 78.8 7.6 6.0 0.0
1.0 12.8 90.0
3.0 0.8 4.0
Buffaloville, Midway 95.0 2.0 81.6 3.6 15.0 1.0
1.0 10.4 76.0
2.0 4.4 8.0
vitrinite (vol %)
liptinite (vol %)
MM ) mineral matter.
nous coals to understand to what extent lithotype composition alone can influence CO2 sorption capacity and (2) investigate meso- and microporosity changes in lithotypes after highpressure saturation with CO2 and discuss the implications for the storage of CO2 in coal beds. 2. Experimental Section Five coal beds, the Danville (Discovery Mine), Springfield (Somerville Mine), Buffaloville (Midway Mine), Upper Block (Midway Mine), and Lower Block (Howesville Mine) Coal Members, were selected for this study. The coals are Pennsylvanian in age, and all come from Indiana’s part of the Illinois Basin. Basic characteristics of these coals in the study area are given in Table 1. For this study, fresh coal was collected from these mines. From each coal, vitrain, fusain, and clarain were hand-picked for further analysis. For the Lower Block Coal, in the same location, two separate vitrain bands (A and B) were sampled and analyzed. The petrographic composition of lithotype samples is presented in Table 2. Vitrinite reflectance listed in Table 2 refers to the samples used in this study and may vary from that in Table 1, where general characteristics (usually on the basis of multiple samples) of the coals in specific mines are given. All lithotype samples were run for low-pressure analysis of surface area and meso- and micropore volumes (Table 3). Selected samples were run for CO2 high-pressure isotherms (Table 4). In addition, samples for which high-pressure isotherms were conducted were subsequently re-analyzed for mesoand micropores to determine if any porosity changes occurred as a result of short-term saturation with CO2 at high pressure (Tables 5-8). The analyses were designed in such a way that the coal was exposed to the air for the shortest period possible to minimize weathering and oxidation-related changes. However, some exposure to the air between individual analyses was not possible to avoid.
Characteristics of Coal Lithotypes
Energy & Fuels, Vol. 22, No. 6, 2008 4051
Table 3. Mesopore (Nitrogen Adsorption) and Micropore (CO2 Adsorption) Characteristics of the Lithotype Samples Studied BET saa (m2/g)
BJH mesopore volume (cm3/g)
mesopore size (nm)
10.0 11.0 6.0
0.012508 0.015154 0.012031
5.6 6.1 8.6
115.4 67.7 43.1 15.9
0.090781 0.072819 0.052220 0.036169
vitrain dull clarain fusain
36.1 36.6 13.6
vitrain clarain fusain vitrain clarain fusain
vitrain clarain fusain vitrain A vitrain B clarain fusain
a
D-R micropore surface area (m2/g)
D-R monolayer capacity (cm3/g)
D-A micropore volume (cm3/g)
micropore size (nm)
Springfield, Somerville 125.0 114.2 72.8
27.4 25.0 15.9
0.058914 0.055160 0.031697
1.3 1.4 1.4
4.2 5.0 5.5 9.8
Lower Block, Howesville 144.8 134.1 121.6 98.2
31.7 29.4 26.6 21.5
0.069744 0.062998 0.058224 0.044177
1.4 1.4 1.4 1.4
0.033031 0.044647 0.016367
4.4 5.6 5.5
Danville, Discovery 154.0 111.7 94.2
33.7 24.5 20.6
0.058097 0.054676 0.037242
nd nd nd
71.9 41.9 4.8
0.066538 0.050936 0.007928
4.5 5.5 7.3
Upper Block, Midway 158.4 124.4 91.6
34.7 27.2 20.1
0.073709 0.059848 0.033542
1.4 1.4 1.4
34.6 35.9 5.6
0.035132 0.044549 0.010396
4.8 5.6 8.0
Buffaloville, Midway 140.3 110.5 79.9
30.7 24.2 17.5
0.066248 0.055588 0.030492
1.4 1.4 nd
sa ) surface area.
Table 4. Langmuir Volume and Pressure of the Lithotype Studied Langmuir volume (scf/ton, as received)
Langmuir volume (cm3/g, as received)
vitrain clarain fusain
1352 1330 765
42 42 24
vitrain A vitrain B clarain fusain
1631 3088 1196 1086
51 97 37 34
Langmuir volume (scf/ton, daf)
Langmuir volume (cm3/g, daf)
Langmuir pressure (psi)
Langmuir pressure (MPa)
Springfield, Somerville 1579 1603 1047
49 50 33
540 520 340
3.72 3.59 2.34
Lower Block, Howesville 2113 3878 1516 1356
66 121 47 42
502 1156 436 398
3.46 7.97 3.00 2.74
Table 5. Comparison of Surface Area, Mesopore Volume, and Average Mesopore Size of the Lithotypes from Samples of the Springfield Coal Member before and after High-Pressure CO2 Isotherm Analysis Springfield, Somerville BET surface area Langmuir surface area BJH adsorption volume average pore width
m2/g m2/g cm3/g nm
vitrain original
vitraina
percent change
vitrainb
fusain original
fusaina
percent change
clarain original
claraina
percent change
10.0 16.9 0.012510 5.60
7.8 13.0 0.010798 6.19
-22 -23 -14 11
8.6 14.2 0.011019 5.75
6.0 9.8 0.012030 8.64
7.3 12.1 0.015130 8.86
22 23 26 3
11.0 18.1 0.015150 6.12
9.2 15.1 0.013544 6.48
-16 -17 -11 6
a Lithotype after being subjected to CO at high-pressure conditions (i.e., after running the high-pressure isotherm). b Lithotype not subjected to CO 2 2 but after storing the sample for the same time as the duration of the high-pressure isotherm.
Table 6. Comparison of Lithotypes Micropore Characteristics from Samples of the Springfield Coal Member before and after High-Pressure CO2 Isotherm Analysis Springfield, Somerville average pore width
nm
vitrain original
vitraina
1.34
1.35
micropore surface area monolayer capacity
m2/g m2/g
125.1 27.4
119.1 26.1
micropore surface area micropore volume
m2/g cm3/g
141.1 0.058900
134.6 0.056176
a
percent change 1
fusain original
fusaina
1.37
1.39
Dubinin-Radushkevich -5 72.8 81.7 -5 15.9 17.9 Dubinin-Astakhov -5 77.5 -5 0.031700
85.8 0.034744
percent change 1
clarain original
claraina
1.35
1.34
12 12
114.2 25.0
114.3 25.0
11 10
131.4 0.055200
127.1 0.052924
percent change -1 0 0 -3 -4
Lithotype after being subjected to CO2 at high-pressure conditions (i.e., after running the high-pressure isotherm).
2.1. Surface Area, Pore Volumes, and Pore Size Distribution. Low-pressure gas adsorption measurements were conducted on a Micromeritics ASAP-2020 porosimeter and surface area analyzer. For this analysis, the lithotype samples were slowly ground in a mortar to 60 mesh (250 µm) and, subsequently, split into representative fractions. Splits weighing 1-2 g were analyzed
separately using nitrogen and carbon dioxide gases to obtain information about the mesopore (2-50 nm) and micropore (
