Energy & Fuels 2003, 17, 107-112
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Characterization of Water Adsorbed on Bituminous Coals Alan L. McCutcheon,* Wesley A. Barton, and Michael A. Wilson Deans Unit, College of Science, Technology and Environment, University of Western Sydney, Hawkesbury Campus Building H4, Locked Bag 1797, Penrith South DC, NSW 1797, Australia Received May 1, 2002
Isothermal water vapor adsorption and desorption on eight Australian bituminous coals, varying in rank, was measured as a function of vapor pressure. The isotherms were measured within the temperature range of 15 °C to 40 °C and at relative vapor pressures up to ca. 0.9. These data enabled the effects of high- and low-pressure hysteresis to be separated and quantified, and the calculation of the net heat of water adsorption as a function of water coverage. High-pressure hysteresis is attributed to “ink bottle” type pore structure, which diminishes as coal rank increases. Application of a model to isothermal adsorption data has enabled the relative amounts of water binding to primary and secondary adsorption sites on coal to be quantified. Proximity of the primary sites to one another and the distribution of water between primary and secondary adsorption sites provide an explanation for the changes in the net heat of water adsorption as a function of the water coverage on coal.
Introduction From previous studies that have investigated water adsorption and desorption on coals varying widely in rank, it is well accepted1-3 that adsorbed water is attached to the coal surface through oxygen-containing functional groups on the coal surface, and that these functional groups form hydrogen bonds with the adsorbed water molecules. Allardice and Evans1 argued that water molecules attached directly to the coal surface form secondary adsorption sites on which additional water molecules can adsorb, thereby forming clusters of water molecules. Kaji et al.2 further suggested that, with additional adsorption as a result of increased water vapor pressure, these water clusters expand and finally condense to fill the capillary pore structure of the coal particles. For a set of coals ranging widely in rank, Kaji et al. reported a correlation between the water holding capacity of coal and the product of the weight fraction of oxygen contained in the coal and the specific surface area. The water holding capacity of coal was defined as the amount of water held by the coal following its immersion in distilled water and was quantified by determining the amount of water removed after drying to a constant weight following immersion. The net heat of water adsorption is a useful parameter to quantify the extent of interaction of water with the coal surface. Mahajan and Walker3 studied water adsorption on six coal samples varying in rank from subbituminous to anthracite and reported that heats of adsorption at relative pressures greater than 0.03 are almost equal to the heat of condensation for water (44.4 * Author to whom correspondence should be addressed. (1) Allardice, D. J.; Evans, D. G. Fuel 1971, 50, 236-252. (2) Kaji, R.; Muranaka, Y.; Otsuka, K.; Hishinuma, Y. Fuel 1986, 65, 288-291. (3) Mahajan, O. P.; Walker, P. L., Jr. Fuel 1971, 50, 308-317.
kJ/mol). This suggests that interaction between the coal surface and water molecules is weak. Mahajan and Walker argued that hydrogen bonding should produce higher heats of adsorption at low surface coverage. They referred to Dacey et al.,4 who reported results for water adsorption on Saran charcoals with a heat of adsorption of 63 kJ/mol for a surface coverage of only 1%, whereas at about 5% surface coverage the heat of adsorption approached the heat of water condensation. Combined water adsorption and desorption isotherms, for a given coal sample, produce a hysteresis loop where water uptake is greater for the desorption isotherm when compared with uptake for the adsorption isotherm at the same relative pressure. In previous studies1,3 and in this study, adsorption/desorption hysteresis has been observed with a relative water vapor pressure approaching zero. Relative water vapor pressure is the water vapor pressure relative to the saturated pressure at a particular temperature. This study presents experimental data for the isothermal adsorption/desorption of water at temperatures between 15 °C and 40 °C and at relative pressures up to 0.9 on a set of Australian coals in the bituminous rank range. The data enable a comparison of water uptake by the bituminous coals, quantification of the adsorption/desorption hysteresis phenomenon, and calculation of the net heats of adsorption. The net heats of adsorption were calculated for seven of the eight coals used in this study. The adsorption data are analyzed using a model which was originally developed by D’Arcy and Watt5 to study water adsorption on wool. This model distinguishes and quantifies water molecules on pri(4) Dacey, J. R.; Clunie, J. C.; Thomas, D. G. Trans. Faraday Soc. 1958, 54, 250-256. (5) D’Arcy, R. L.; Watt, I. C. Trans. Faraday Soc. 1970, 66, 12361241.
10.1021/ef020101d CCC: $25.00 © 2003 American Chemical Society Published on Web 12/21/2002
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Table 1. Proximate and Ultimate Analyses for the Coals Studied proximate analysis
ultimate analysis (% daf)
coal moisture (%)a ash (%)b VM (%)c C1 C2 C3 C4 C5 C6 C7 C8 a
3.2 3.4 5.1 1.1 0.7 1.3 0.9 1.0
10.3 7.3 9.8 5.7 16.1 13.7 12.5 5.3
35.4 33.2 32.0 37.0 25.4 20.8 16.9 20.3
Adsorbed. b Dry. c Dry ash free.
d
C
H
Sd
Oe
81.1 83.6 84.3 85.2 87.2 88.3 89.1 89.9
5.2 5.4 5.1 5.7 5.0 4.8 4.8 4.9
0.7 0.6 0.6 0.6 0.7 0.6 0.7 0.7
11.1 8.7 8.1 6.3 5.1 4.4 3.3 2.6
Total sulfur. e By difference.
Table 2. Surface Area of Coal Samples Measured by Applying the BET Model to CO2 Adsorption Isotherms
a
coal
coal rank C (% C)a
surface area (m2 g-1)
C1 C2 C3 C4 C5 C6 C7 C8
81.1 83.6 84.3 85.2 87.2 88.3 89.1 89.9
164 187 173 127 128 127 135 131
Dry ash free.
mary or surface sites and on secondary adsorption sites. The results are consistent with the calculated net heats of adsorption as a function of water coverage. Experimental Section For this study, eight Australian bituminous coals were obtained from the Hunter Valley in New South Wales and the Bowen Basin in Queensland. Proximate and ultimate analysis data for these coals are shown in Table 1. Surface area values for the coals displayed in Table 2 were determined by applying the Brunauer, Emmett and Teller (BET) model6 to CO2 isothermal adsorption data measured gravimetrically at 26 °C within the relative fugacity range of 0-0.4 (where fugacity is pressure corrected for nonideal behavior of gas). The BET model quantified the amount of CO2 adsorbed to provide monolayer coverage. In calculating surface areas, the accepted cross-sectional area7 of 0.253 nm2 for an adsorbed molecule of CO2 was multiplied by the number of molecules that produce monolayer coverage. The BET equation was also used to calculate monolayer converage of water XM from water adsorption data. The densities of two size fractions with particles between 2 and 3 mm and less than 45 µm, respectively, were measured for four coals. The density measurements were made by quantifying the volume of helium gas displaced separately by each coal sample (referred to as the helium or true density) using a helium pycnometer. The results are displayed in Table 3. Water adsorption/desorption isotherms were measured gravimetrically, at temperatures in the range of 15 °C to 40 °C, on the eight bituminous coal samples using a Hiden Intelligent Gravimetric Analyzer (IGA). All samples used for the sorption measurements consisted of ca. 200 mg of material crushed to less than 250 µm. Prior to water uptake measurements, samples were outgassed in the IGA to a constant weight at 70 °C and under a vacuum pressure of ca. 10-6 millibars. When the temperature was raised to ca. 110 °C for one sample, no further weight loss was observed. (6) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. (7) Walker, P. L., Jr.; Verma, S. K.; Rivera-Utrilla, J.; Davis, A. Fuel 1988, 67, 1615-1623.
Figure 1. Water uptake at a relative pressure of 0.9 for a range of bituminous coals varying in rank. Table 3. Change in the Density of Coal Particles Due to Crushing the Particles -3 + 2 mm to -45 µm density of coal particles (g/cm3) coal rank a %C particles 2 mm2 particles 45 µm2 density difference 88.9 88.4 87.4 83.0 a
1.360 1.367 1.361 1.323
1.401 1.390 1.368 1.332
0.041 0.023 0.007 0.009
Dry ash free.
The net heat of adsorption (q) for a particular coverage of water on the coal surface was calculated using the following equation:
ln
( ) P P0
x
)
q + const RT
(1)
where P and P0 are the water vapor pressure and the saturated vapor pressure, respectively, R is the gas constant, T is the temperature, and x signifies a constant coverage of water. By plotting ln(P/P0) against 1/T, the net heat was calculated from the slope.
Results and Discussion The results are presented and discussed under four headings, i.e., equilibrium water uptake, adsorption/ desorption hysteresis, heat of adsorption, and adsorption modeling. Isothermal Equilibrium Water Uptake. Previous studies2,3 reported a trend between coal rank and extent of water uptake, particularly at high relative pressures, indicating that water uptake increases with a decrease in rank. This trend was determined using coals varying widely in rank from low rank lignites (e.g., 66.1% C daf) to high rank anthracites (e.g., 90.9% C daf). For this study the coals were limited to the bituminous rank range with carbon content from 81.1% to 89.9% (daf). Results from this study, displayed in Figure 1, show water uptake at a relative pressure of 0.9 as a function of coal rank, which confirms a trend but the data are scattered. Part of this scatter is probably due to variations in the nature and quantity of mineral matter contained in the coal and therefore in its effect on water uptake.8 The water uptake at a relative pressure of 0.9 (8) McCutcheon, A. L.; Barton, W. A. Energy Fuels 1999, 13, 160165.
Water Adsorbed on Bituminous Coals
Figure 2. Water adsorption/desorption isotherms for a higherrank (87.2%C dry ash free) and a lower-rank (84.3%C dry ash free) bituminous coal.
Figure 3. Correction of hysteresis (for Coal C3) for the contribution due to the “ink-bottle” effect.
is about 3 wt % and about 7 wt % (dry coal) for the higher rank and the lower rank groups of coals, respectively. Adsorption/Desorption Hysteresis. For all of the bituminous coals studied, adsorption/desorption hysteresis was observed as a difference in water uptake at each relative pressure during the adsorption and desorption processes. Examples of adsorption/desorption hysteresis are shown in Figure 2 for two bituminous coals used in this study. The higher-rank coal (87.2 wt % C daf) shows a reasonably uniform degree of hysteresis as a function of relative pressure. For the lowerrank coal (84.3 wt % C daf) shown in Figure 2, this swelling hysteresis is also evident at lower relative pressures, but is augmented appreciably at relative pressures above ca. 0.45 by a second type of hysteresis called here “high-pressure hysteresis”. In this study the extent of hysteresis for adsorption/ desorption isotherms (see Figure 2) was quantified for the set of eight bituminous coals by separately integrating the adsorption and desorption isotherms and calculating the difference. This measure of hysteresis correlated linearly with coal oxygen content (wt % O daf). The correlation was substantially improved, particularly for the lower rank coals, when the desorption isotherms for these coals were corrected for highpressure hysteresis, which generates an inflection in the desorption isotherm at a relative pressure of ca. 0.45 (see Figure 3). This correction was achieved by constructing a desorption isotherm for each lower rank coal, which ran almost parallel with the adsorption isotherm
Energy & Fuels, Vol. 17, No. 1, 2003 109
Figure 4. Integrated difference between adsorption/desorption isotherms plotted against coal oxygen content.
within the relative pressure range of ca. 0.4 to 0.8. This provided a desorption isotherm, in relation to the adsorption isotherm, that was similar to those observed for the higher rank coals (see Figure 3). The integrated adsorption/desorption difference obtained after this correction is plotted against coal oxygen content in Figure 4. This plot demonstrates a strong linear correlation (r2 ) 0.956) between the extent of hysteresis, after removing the influence of high-pressure hysteresis, and the oxygen content of the coals. The type of water hysteresis described here as “lowpressure hysteresis” is not new. Baker et al.9 separately compared water and nitrogen adsorption on chromia gels. It was reported that water, on account of its small molecular size, was able to penetrate the micropore structure where it was believed to partially rehydrate the Cr3+ ion causing the pore structure to swell which presumably enhanced water penetration into the pores. It was found for the water desorption cycle, unlike nitrogen desorption, that a substantial hysteresis occurred down to the low relative pressure region of the isotherm. For coals, Ceglarska-Stefanska and Czaplinski10 suggest the phenomenon is due to swelling in the presence of water in the pore structure of coal causes due to the pressure exerted by clusters of water molecules. Because the extent of water clusters is related to the number of hydrophilic sites or oxygen-containing functional groups, which in turn is dependent on the coal rank,7 it is not surprising that we observe here that lower rank coals swell to a greater extent. An explanation is needed for the “high-pressure hysteresis” for lower rank bituminous coals. Some of the pores in the lower rank coals could be not fully open at the neck. These narrow openings fill readily but drain with more difficulty due to capillary blockage. This phenomenon is commonly known as the ink-bottle effect.11 Why are they not present in the higher rank coals? It is possible that narrow apertures are closed as the coalification process proceeds, thereby forming pores that are inaccessible to moisture vapor or gases12 in higher rank bituminous coals. Such pores should be present but not accessible unless opened by crushing. To support this view, the helium density or the true (9) Baker, F. S.; Sing, K. S. W.; Stryker, L. J. Chem. Ind. 1970, 718. (10) Ceglarska-Stefanska, G.; Czaplinski, A. Fuel 1993, 72, 413417. (11) Katz, S. M. J. Phys. Chem. 1949, 53, 1166-1186. (12) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9, 324-330.
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Figure 5. Net heat of water adsorption, for a water uptake of 0.5 millimol/g (dry coal), plotted as a function of coal oxygen content (% daf).
Figure 6. Net heats of adsorption as a function of coverage (expressed as number of monolayers) for bituminous coal samples.
density was measured for a number of coal samples (similar to the samples used for the sorption studies) with particles sized to 2 mm2 diameter and again after the particles were crushed to -45 µm2 diameter. From the helium density results (Table 3), there is a significant increase in the difference between the density measurements for the two particle size ranges as the coal rank increases above a carbon content of ca. 87.5 wt % C (daf), suggesting more closed pores are present in the higher rank coals. Heat of Adsorption. The net heat of adsorption is a useful parameter to determine the extent of interaction between the coal surface and adsorbed water molecules as a function of their coverage. By plotting ln(P/P0) against 1/T, the net heat can be calculated from the slope. As shown in Figure 5, the net heat of water adsorption, for a small uptake of 0.5 millimol/g of dry coal, increases with increasing oxygen content of the coal. The uptake value of 0.5 millimol/g of dry coal is an arbitrary low value that was selected to ensure it represented water bonding to primary sites. The trend in Figure 5 can be interpreted in terms of the concept, proposed by Allardice and Evans1 and others,3 that water uptake by coals initially involves adsorption at “primary” or hydrophilic sites containing oxygencarbon functional groups. Furthermore, because free water has high intermolecular forces, the adsorbed water, and particularly the monolayer water, acts as less energetic “secondary sites” for the adsorption of additional water, thereby generating “clusters” of water molecules on the coal surface. Therefore, for a fixed water uptake per unit weight of coal, a greater proportion of water molecules are adsorbed at “primary” sites on coals with higher oxygen content, thereby producing a higher heat of adsorption. The net heat of adsorption as a function of coverage, expressed in terms of the number of water monolayers (X/Xm), was determined for the eight coals used in this study. The results for a selection of the coals are displayed in Figure 6. The curves obtained for the other coals studied are similar to the central cluster including coal C3. This clustering of curves suggests that the primary sites for many bituminous coals are similar,13
resulting in comparable monolayer (ie. X/Xm ) 1) binding energies per water molecule. Coals C7 and C8 which are of highest rank have significantly different trends in their heats of adsorption which fall more sharply with increasing submonolayer coverage when compared to the other coals. Figure 6 shows that the net heat of adsorption between 1 and 3 after monolayer coverage is similar, but not identical. Since the net heat is the difference between the isosteric heat and the latent heat, the isosteric heats14,15 should be similar and we can discuss this in terms of molecular adsorption. For the net heat of adsorption of molecules after monolayer coverage to be similar to that at monolayer saturation, monolayer saturation there should not be a large difference between the bonding enthalpy of the final first water layer molecule and the second layer that binds. This is best explained in terms of a decrease in significance of the energy of binding to the surface to that of another water molecule. It is signicant that the data in Figure 6 by extrapolation gives the net heat on average on multilayer formation (2.125 kJ/mol) to be similar to that given by Monazam et al.16 on other coals so the phenomenon is probably common. Adsorption Modeling. Explanations for data such as that in Figure 6 in which significantly different trends in heats of adsorption occur between different coals with increasing submonolayer would therefore appear to need a multisite approach which involves the integration of a very large number of different molecular adsorption sites. However, quite a simple model does suffice. Equation 2 describes a model by D’Arcy and Watt 5 and by Barton et al.17 to estimate the number of primary and secondary sites for water adsorption on nonporous carbon. The equation as used by Barton et al.:
(13) Blom, L.; Edelhausen, L.; Van Krevelen, D. W. Ph.D. Thesis, Delft, The Netherlands, 1960; reproduced in van Krevelen, D. W. Coals Typology, Chemistry, Physics and Constitution; Elsevier: Amsterdam, 1961; p 173.
a)
skh SKh + 1 + Kh 1 - kh
(2)
assumes that there is only one type of surface site for (14) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Pres: London, 1982; p 16. (15) Glass, A. S.; Larsen, J. W. Energy Fuels 1994, 8, 284-285. (16) Monazam, E. R.; Shadle, L. J.; Evans, R.; Schroeder, K. Energy Fuels 1998, 12, 1299-1304. (17) Barton, S. S.; Evans, M. J.; MacDonald, A. F. Langmuir 1994, 10, 4250-4252.
Water Adsorbed on Bituminous Coals
primary adsorption and only one type of site for secondary adsorption. In this equation, a ) mass adsorbed by one gram of adsorbent, S ) mass adsorbed on primary sites (or moles), s ) mass adsorbed on secondary sites (or moles), K ) a constant which measures the attraction between a primary site and the adsorbate, k ) a constant which measures the attraction between a secondary site and the adsorbate, and h ) P/P0 ) relative pressure. The first term in the equation is a Langmuir type contribution and quantifies adsorption on primary sites, while the second term is a BET-type contribution and quantifies adsorption on the secondary sites. In applying this model to water adsorption on a coal surface, the adsorption sites were also grouped into primary and secondary types. For the primary site group, it was assumed that these sites had similar attractive forces indicated by the comparable molar heats of adsorption. Glass and Larson15 calculate specific adsorption energies for various structures on coal and show that these are identical to those obtained for p-flourophenol, thus offering strong circumstantial evidence that phenolic OH with similar electron supply capacity as an F-substituent are the primary adsorption sites for various molecules on coal. Hence it is probable that adsorption is at these sites. Because the net heat of adsorption varies with water uptake at less than submonolayer level as noted above, it must be true that different water molecules interact with each other. Even for ether groups the net heat is 23 kJ/mol which is substantially larger that that observed here, so that adsorbed water must reduce its interaction energy by hydrogen bonding with other water or other structures in some way. The primary site therefore is not just a simple 1:1 adsorption site for water, but where water adsorbs and interacts with other water in a distinctive way. For the secondary site group, it was assumed that these sites correspond to water molecules whose hydrogen-bonding with other water molecules is enhanced by their proximity to the surface binding sites resulting in the formation of molecular clusters which are quite different to the primary sites, and are possibly bi- and trilayer sites. The constants, K and k, represent attraction between water molecules and primary and secondary sites, respectively, and were fixed at separate values before quantifying the distribution of adsorbate on the two types of sites by applying the least squares technique to determine optimum values for the variables S and s. To estimate the correct values for K and k, eq 2 was treated as a four-variable equation and a nonlinear curve fitting routine, using the Marquardt-Levenberg algorithm, was employed to fit this equation separately to experimental adsorption data for each coal sample. This procedure provided values for K varying from 11 to 26 and values for k varying from 0.3 to 0.8 for the different coal samples. In the final least-squares analysis to determine S and s, K and k were fixed at their average values of 16 and 0.6, respectively. Equation 2 was then treated as a two-variable equation. The model provided a good fit to all isothermal adsorption data for the coals studied (r2 > 0.98). In Table 4 values for S and s are provided for the coals investigated. A plot (Figure 7) of S as µmol/m2 versus coal oxygen content
Energy & Fuels, Vol. 17, No. 1, 2003 111
Figure 7. Plot of adsorbed moles on primary sites versus coal oxygen content. Table 4. Values of Parameters in the Modified D’Arcy and Watt Model for a Range of Bituminous Coals and the Calculated Primary Site Density Using CO2 Surface Area (BET) Results coal
S (mg/g)
s (mg/g)
[S × 100/(S + s)] (%)
S (µmol/m2)
C4 (85.2%C) C2 (83.6%C) C3 (84.3%C) C1 (81.1%C) C5 (87.2%C) C6 (88.3%C) C7 (89.1%C) C8 (89.9%C)
13.4 21.1 18.9 18.6 6.7 5.7 4.8 3.1
17.2 42.9 39.8 40.0 19.3 19.5 15.9 19.6
44 33 32 32 26 23 23 14
5.86 5.56 6.06 6.32 2.90 2.49 1.98 1.31
shows a reasonably linear plot, with some scatter suggesting that the data also can be extrapolated back to suggest Langmuir behavior for primary adsorption on oxygen sites. This confirms that oxygen content is a good indicator of the primary adsorption sites or carbon oxygen functional groups on the surface. The scatter may exist because of some variation in oxygen carbon structures in different coals, but also because a continuum of sites exists in some spacial areas of some coals. From Table 4 the percentage of water bound to primary sites (100S/(S + s)) reaches its lowest value for coal C8 and coal C7 has the next lowest value along with a low density of primary adsorption sites. Shifts in the distribution of water molecules binding to the two different types of sites are reflected in the net heats of adsorption for the submonolayer region (see Figure 6). For coal C8 and to a lesser extent coal C7, the higher heats of adsorption, as a result of water binding to primary sites, are not sustained to a high fraction of monolayer coverage. This indicates that the slope of the plot of net heat as a function of coverage reflects the distribution of water molecules bound to primary and secondary sites. Huang and Bodily18 discussed changes in the hydrogenbonding patterns for water molecules bound to high and low densities of oxygen functional groups on the coal surface. For high oxygen content coals the oxygen functional groups are in close proximity to one another, enabling the formation of bridged clusters of water molecules. Klier and Zettlemoyer19 discussed water (18) Huang, H.; Bodily, D. M. 7th Int. Coal Sci. Conf., Banff, Canada, 1993; pp 411-414. (19) Klier, K.; Zettlemoyer, A. C. J. Coll. Interface Sci. (No. 2) 1977, 58, 216-229.
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cluster stabilities and, referring to earlier quantum mechanical calculations performed mainly by Del Bene and Pope20 and Kistenmacher et al.,21 described how water molecules forming larger clusters, for example dimers through to cyclic tetramers, become energetically more stable with energies per water molecule changing from -12.8 to -43.9 kJ/mol, respectively. Klier and Zettlemoyer argued that, in the case of water adsorption on silanols where water forms hydrogen bonds with hydroxyl primary sites, higher coverage is energetically favored where clusters are formed as a result of the energy stabilization effect. In this study, for coal C7 and particularly coal C8 for which the net heats of adsorption fall more rapidly with increasing coverage than for other coals (see Figure 6), it is believed that there is much less lateral interaction between water molecules associated with adjacent primary sites. This view is supported by comparing the density of the primary adsorption sites. The results are given in the final column of Table 4 calculated using the D’Arcy and Watt model and normalized by the surface area determined by applying the BET model to CO2 isotherms.22,23 For coal C7 and again particularly for coal C8, the primary site density is significantly less than for the other coals, suggesting that the clusters of water molecules are less likely to form bridges between adjacent primary sites by means of hydrogen-bonding. Hence these coals have significantly different trends in their heats of adsorption (20) Del Bene, J. E.; Pople, J. A. Chem. Phys. Lett. 1969, 4, 426428. (21) Kistenmacher, H.; Popkie, H.; Clementi, E. J. Chem. Phys. 1973, 58, 1689.
McCutcheon et al.
which fall more sharply with increasing submonolayer coverage when compared to the other coals. Conclusion It has been demonstrated that there are two distinct types of water adsorption/desorption hysteresis, namely, the low-pressure hysteresis that is associated with coal swelling and the high-pressure hysteresis which is associated with pores having restricted openings and referred to as “ink-bottle” type pores. Low-pressure hysteresis correlates substantially better with coal oxygen content after the effect of high-pressure hysteresis has been removed. The high-pressure hysteresis supports the view that narrow aperture or “ink-bottle” type pores exist in the lower rank coals. Furthermore, it has been demonstrated from density measurements that the number of closed pores increases with increasing rank, while the high-pressure hysteresis relates to the population of “ink-bottle” type pores that are accessible. Application of a model to isothermal adsorption data has enabled the relative number of water molecules adsorbed on primary and secondary sites to be determined. These results provide an explanation for differences in the rate of change of the net heat of adsorption as a function of water coverage for different coals. EF020101D (22) Gan, H.; Nanadi, S. P.; Walker, P. L., Jr. Fuel 1972, 51, 272277. (23) Mahajan, O. P. Carbon 1991, 29, 735-742.
