Physical characterization of coal

PoiderTechnology. Revi&v

Physical

30 (1984)

1 - 15

-

1

Paper

Characterization

of Coal

0. P. MAHAJAN Corporafe

Rckarch

Department,

Amoco

Research

Center. Napemilk,

SUMMARY On both mrcroscopic and macroscopic scales, coal is a complex heterogeneous material The organic fraction of coal is composed of microcrystalline and amorphous materials, and exists as small condensed aromatic layers The packing of layers rmproues with rankCoals haJe an extensive porosity; pores uaTy ir- size from micron to molecuhr dimensrons. Most _of the coal surface crea, however, is enclosed in micropores <2 nm in size. Signqicanf res?rlts on densrties, surface area and pore volume distribution have been discussed_ All of these parame;ers show some -dependence on coal rank. Pore structure of coal has a large mfluence on its behauior dunng mining, preparation, and utilization. The mechanism of water adsorption on coal, and Its deperrdence on rank are drfferent from that of non-polar molecules_ The nature of coal surface rather than its surface area determrnea its moisture sorption capacitzr. Exchangeable catzons present in fhe carboxylate groups of low-rank coals also have a maJor effect on water adsorption. Wateadsorptron-desorptlon isotherms OR cocks show hysteresis_

XNTRODUCTIC)N

On both microscopic and macroscopic scales, coal is a complex heterogeneous material composed of organic and inorganic constituents_ The organic fraction of coal is made up of macerals There are three major groups of macerals: vitr,nite, liptmite, and mertinite. The liptmitic macerals have the lowest and the inertites the highest specific gravity. Liptinites are rich in hydrogen; they convert to liquid and gzeo-a fuels more readily than do other maceral groups. Vltrinites are next to liptinites in convertibtity. Liptinites and vitrirntes are the preferred macerals in coking coals.

IL. 60566

(USA.)

The inorganic matter in coals belongs to two broad classes, namely, discrete mineral matter, and minor and trace elements. The discrete mineral matter is usually present as particles larger than 1 pm in size The major discrete minerals in coals are- clays, quartz, gypsum, calcite, and pyrite. Trace and minor elements are more widely distributed m coals and can cover half of the periodic table. The discussion on the inorganic matter m coals, its characterization, and its role in coal utilization and conversion processes is outside the scope of this paper. A vast body of information is avalable in the literature on the physical properties of coals, their characterization, and their influence qn coal behavior and utilization [I - 5]_ In this paper, only the more important and relevant aspects of physical charactenzation of coal are discussed_ The emphasis is largely on sigmficant results of various measurements rather than on the analytical methodologies. The -analyicai techniques for characterizing coal are discussed adequately in ref. 6.

COAL

STRUCTURE

Any discussion on physical chvacterizatlon of coal must invanably start v&h the classical X-ray studies of Cartz and Hirsch [7], 2nd Hu-sch [S] _ Cartz and Hirsch [7] examined several v&rains of different rank_ X-ray scattenngs at high, medium, and low angles were attnbuted to small condensed aromatic layers, packing of aromatic layers parallel to each other, and dlscontinuities jn the structure, respectively Amorphous material in coal also contributes to scattering at high angles. The Important conclusions of the X-ray study are summarized below. Layer size htribution in the vitrams varies in the 0.5 - 3 nm range. Below 89% carbon, about 14% of the carbon m vitrain occurs as 0 Elsevier Sequoia/Printed

in The Netherlands

2

single layers, and the remainder (apart from the amorphous carbon) in layers stacked paralle1 to each other m groups of 2, 3, etc. The fraction of single layers decreases with increase in rank, and at 94% carbon content it amounts to only 10%. This implies that the packing of layers improves with rank_ The fraction of amorphous material in vitrain decreases Imearly from about 38% for a vitrain of -57% carbon content to about 28% for a vitrain of -83% carbcn content. The fraction of amorphous material decreases sharply thereafter with increasing rank; for a vitrain of 94% carbon content, the fraction of amorphous material IS only 10%. Based on the results of an exhaustive X-ray study, Hirsch [S] distinguished between three types of structures present in a wide range of coals (Fig. 1).

Fig. l_ Schematic

model of coal structure

[S]

Open structure This structure is characteristic of low-rank coals with carbon contents up to about 85%. Coals in this range are highly amorphous and porous; the lamellae are connected by crosslinks, and are more or less randomly oriented in all drrections. Liquid structure This structure is characteristic of bituminous coals with carbon contents m the 85 91% range In this range, the number of crosslinks has decreased considerably, and the lamellae show some orientation with the formation of crystallites consisting of two or more of these lamellae.

Anthracitrc structure This structure is common m higher rank coals with carbon contents above 91%. In this structure, the degree of orientation of lamellae with regard to each other has mcreased greatly, and the cross-lmks have disappeared_ As a result, anthracites have very small pores COAL

POROSITY

Based on the results of X-ray and chemical studies, it is now generally agreed that the main building blocks of coal are aromatic and hydroaromatic units. These units are connected by cross-links such as methylene, oxygen, and sulfur On the penphery of the units are located various heteroatom functionalrties. It is the poor alignment of these units m three dimensions which produces extensive porosity in coal. The pores in coal vary in size from large cracks of micron dimensions to apertures which are even closed to helium at room temperature- It is the empty volume, known as pore volume, within the pores of any matenal that enables it to adsorb large volume of gases or liquids A large pore volume does not always imply a large pore surface area, because the latter is dependent upon pore size distribution _ Pores in coal are distmguished accordmg LO the IUPXC classification [9], namely(i) macropores - pores which have diameters >50 nm, (ii) mesogores - pores which are between 2 - 50 nm in dizmeter, and (iii) micropores - pores which have diameters <2 nm. In recent years, micropores have further been divided into very narrow pores or ultramicropores [lo], that is pores less than 0.8 nm in diameter [3], and supermicropores [ 111, which fill the gap between the ultramicropore and mesopore range. Coals have an aperture-cavity type porosity [l - 3]_ Entrance to the pore system is determined by the aperture size, and extent of adsorption is determined by the cavity size Apertures in coal are of molecular dimensions As a result, coals exhibit molecular sieving effects, and show major differences in capacity for adsorbing molecules differing only slightly m their dimensions_ It has recently been pointed out that “details of the pore structure influence the

3

behavior of coal more directly than does virtually any other property” 143: It is only through the mternal pore structure that coal comes mto contact with the reacting environment_ Therefore, it is not surprising that coal porosity has a large mfluence on its behavior dunng minmg, preparatron and utilization. For example, coal porosity plays a myor role in diffusion of methane from coal seams, coal beneficration, gasification, liquefaction, transport of coal in water slurnes as well as production of metallurgical coke, activated carbon, and carbon molecular sieves from coal [l - 33. Characterrzation

of pore

structure

For the quantitative characterization of coal pore structure, one needs to estimate pore volume, surface area, and pore size distnbution. Conventionally, displacement of gases and liquids by coals is used to measure their densities and pore volumes, whereas physical adsorption of gases is used to measure their surface areas, and pore volume and area distributionti In addition, additional information on the nature of porosity and adsorption charactenstics of coal 1s obtained from a study of diffusion of gases [ 3]_

assumed that the gas is not adsorbed at room temperature. This rs not always true [13 - 163 _ However, Maggs et al. [14] have reported that hehum adsorption on raw coals is not appreciable enough to cause signrficant error in the densities_ The variation of helium density of American coals with their carbon contents is shown m Fig. 2. The density irntraIly decreases with increase m carbon content_ It then passes through a broad mmimum in the 82 - 86% carbon content range, and rises sharply as 90%mcarbon content is approached_ Similar trends have also been observed for Bntlsh [18] and Japanese 1191 coals. However, the minimum in the foreign coals occurs at somewhat higher carbon contents than for the American coals. These results suggest that geological history has a marked effect on coal structure. Using multivariant analysis technique for 66 coals, Neavel et al. [20] have reported that the hehum density pHc 1~ related to the elemental composition, expressed on a drymineral-matter-free (dmmf) basis, by the following relationship,hIe = O-023C

+ 0.02920

+ 0.02259,,, Densities

For coals, three different densities can be considered, namely true density, particle density, and apparent density. These densities are discussed briefly below, detarled mformation on them and the methods for their determination are discussed elsewhere [ 2]_ True

-

-

0 0261H

+

0.765

This relationship accounted for 94% of the variance of the helium density Determinaticn of helium density mvolves an elaborate vacuum system, and a cumbersome and time-consuming procedure. Therefore, attempts have been made in the past to

density

density of a porous solid is the weight of a unit volume of the pore-free solid To determine the true density of a porous solid, we need a non-interacting fluid which completely fills all the pores. In reality, no fluid completely fills the pore volume of coals. Therefore, the term true density should be understood in this light. Helium is the smallest molecule available. Therefore, it has the best chance of penetrating the entire porosity in coals Since some porosity in coals may be closed to helium [12], helium density of a coal w-iIl be lower than its true density. When determming densities of carbonaceous solids by helium displacement, it is True

Fig 2 carbon

Vmation content

of helium 1173.

demlty

of

coals

with

4

determine ‘true’ density of coals by the displacement of liquids of small molecular dimensions such as water_ The water densities of coals with carbon contents up to 85% were found to be generally higher than the helium densities [lS, 211 This was attributed to contraction of water m the pores and/or interaction of water with the polar functional groups on the coal surface (this aspect is discussed further in the section on water adsorption characteristics). The water densities of higher rank coals with carbon contents above 85% were lower than the helium densities [lS] _ It was suggested that these coals are strongly hydrophobic in character, as a result of which water cannot completely displace air present in the pores. Ettinger and Zhupakhina [22] suggested that water penetration into the hydrophobic coal structure can be enhanced by usmg a wetting agent. They argue that the polar group of the wetting agent would be turned into water, and the non-polar group toward the coal surface Therefore, subject to penetration by the wetting agent itself, water molecules can penetrate into the coal structure quickly and more completely_ The authors determined the helium and water densities of 14 coals having volatile matter contents, espressed on a dry-ash-free (daf) basis, in the 1.7 - 44.5% range. With three exceptions, the difference m the densities determined by the two methods was less than 1%. This type of agreement was not observed by Nelson [17], who determined densities of 8 coals, varying in carbon content (daf) from 70 9 to 89.5%, by hehum displacement as well as by the Ettinger and Zhupakhma technique_ With the exception of one sample, the helium and water densities for the remainder of the samples differed significantly from each other Neavel et al. [ 201 have recently compared helium densities of 66 coals with those measured by the displacement of water at 303 K. Every possible care was taken to more or less completely displace the air present in the pore network The mean difference between the water and hehum densities was ?0.003 g/cm’; the helium densities were generally larger than the water densities. Further, the difference between the two sets of densities for the coals showed no dependence on rank.

In sharp contrast to the work of Neavel ef aZ_, Nelson et al. [23] found that the water densities of most of the 18 coals they investigated were significantly higher than the corresponding helium densities_ The excellent agreement between the water and helium densities observed by Neavel et al [20] appears somewhat surprising from at least two considerations_ First, water interacts with oxygen functionalities present on coal surface; this aspect will be discussed later in this paper. Second, water induces swelling in coals [24] It is hkely that swelling induced by water may open up some of the porosity which is otherwise closed to helium at room temperature. Particle

density

Particle density is the weight of a unit volume of the solid mcluding pores and cracks. Particle density of coal is usually determined by the displacement of mercury The density is measured at a mercury mtrusion pressure at which the voids between the particles are filled with mercury [25] _ Mahajan and Walker 1251 have measured particle densities of 40 X 70 mesh fractions of 32 coals of different rank by displacement of mercury. Mercury densities of the coals are plotted against their carbon and volatile matter contents in Fig 3 and Fig_ 4, respectively_ There is a considerable scatter m the data in both the figures. Nevertheless, some general trends are discernible. For example, m Fig. 3, densities of coals with carbon contents (80% fall on a straight line of slope OO256, the correlation coefficient is 0.828 A sharp transition is observed at a carbon content of about 80%. For the higher-rank coals, the densities are represented by a straight line of considerably less slope (O.OOSS), and correlation coefficient (0 636)

Carbon

conrent

I% dmmll

Fig 3. Variation of mercury density of coals with carbon content

[25].

TABLE

1

Densities of coals at 298 K [28] PSOC

Rank

COd num-

ber 177

1 12

5

10

,

15

,

20

Volatile

25

30

marmr

35

40

45

50

55

i%dmmfJ

Fig. 4 Van&ion of mercury volatile matter content [25].

den&y

of coal with

Results in Fig. 4 show that mercury density decreases sharply with increase m volatile matter content up to about 20%; it remains essentially constant ir the 20 - 40% volatile matter content range, and decreases sharply thereafter. Ettinger and Zhupakhina [22] determined particle density of coal by the ‘silanization’ method The method mvolves coating the coal surface with a thin film of an organo-silicon compound. The thm film is impervious to water, and has practically no influence on either the weight or volume of coal particles Particle density of coal IS then determined pycnometrically using a very dilute solution of pyridme 111water. For 14 coals with volatile matter contents (d-af) in the 2 1 - 48.6% range, the difference in particle density by the silarnzation method and by geometric measurements on cubes of the coal samples was less than 1% Nelson [ 261 determined particle densities of 8 coals by the mercury displacement and silanization methods For one sample (a HVC bituminous coal), the difference in the two densities was 1 2%; for the remainder of the seven samples, the difference showed a random van&ion between 1.5 to 7.3%. Apparent

densr ty

Apparent density of a coal depends upon its particle size, pore size distribution, molecular size of the fluid, degree of interaction of the fluid with coal (that is, swelling and surface effects), and time aIlowed for penetration of the fluid Toda [27] measured densities of vitrams of 17 coals in methanol and n-hexane at 298 K The lower- and higher-rank coals showed higher densities with a minimum at about

318 254w 268 223 212 248 242 246

Antha-

cite

LV bit MV bit HVA bit HVB bit HVC bit Sbb-A Sbb-B Lignite

Density

(g/cm3, dmmf)

Heli“m

Methanol

Benzene

Tetiin

1.565

1 598

1.521

1.995

1.326 1 294 1 282 1292 l-323 1.340 1.334 1.364

1 402 1.326 1 348 1.409 1.465 1.497 1.512 1 564

1 340 1 286 1.283 l-355 1.372 1392 l-348 1.379

1.295 1 264 i 262 1 285 1.274 1312 1.305 1350

87% carbon content. Nelson et al. [28] measured apparent densities of 9 coals by displacement of methanol, benzene and tetrahn at 298 K The results are given in Table 1. Helium densities of the coals are also included m Table 1 for the sake of comparison_ The methanol densities are consistently higher, and the tetrahn densities are lower than the helium densities The benzene densities of two coals (PSOC-177 and 254W) are lower than the helium densities; for the remamder of the samples, the benzene densities are higher. Several workers U-Ithe past have also reported that densities of coals in organic solvents frequently exceed the helium densities because of irnbibition [18,29]. The lower tetraiin densities in Table 1 are due to much larger size of a tetrahn molecule compared with that of a hehum atom Even though tetrahn is imbibed m coals 1281, it appears that the contribution of swelling to the tetralin density is more than offset by the inability of tetralm molecules to enter into some of the smaller micropores which are accessible to helium In the case of low-rank coals (e g., lignites), only apparent densities can be measured because the drying conditions necessary to completely remove adsorbed water cause irreversible structural changes [ 301. Open

pore

volume

Total open pore volume of the organic phase of coal is calculated from the hehum and mercury densities calculated on a dmmf basis, while open pore volume accessible to a fluid other than helium is calculated from the

mercury density and apparent density in the given fluid [23 _ By determining the apparent density of a coal m fluids of different but known dimensions, it should be possible to obtain the pore size-pore volume distribution_ Surface area The significant approaches used to determine surface area of coal are discussed below. Adsorption of gases The Brunauer, Emmett and Teller (BET) equation has been used extensively to determine surface areas of coals from Nz adsorption isotherms measured at 77 K. It is now generally agreed that N2 adsorption at 77 K does not measure the total surface area of coals for two important reasons. First; due to the activated diffusion limitations, Nz molecules at 77 K do not possess enough kinetic energy to readily penetrate mto the coal micropores. Second, the micropores undergo some decrease m their size at low temperatures_ There is an additional drawback associated with the use of N2 adsorption at 77 K to measure surface area of coals and other microporous carbons. Lamond and Marsh [ 311 have suggested that when the pores are only a few times greater m size than an N1 molecule, N2 at 77 K fills the pores at very low relative vapor pressures. The filling results in reversible capillary condensation before the apparent monolayer capacity IS reached As a result, the BET N, areas are unrealistically high The problems associated with the use of N2 adsorption at 77 K prompted researchers to measure surface area of coals from adsorption of other gases at the highest temperatures possible. In this context, attempts were made to determine the surface areas from sdsorption of neon at room temperaturc 1291, hydrocarbon gases at or near r5om temperature [32, 331, and Kr and Xe at 195 K and/or 273 K [34 - 361. -4dsorptlon of CO, at 195 K [ 37 - 401,273 and 293 K [4I], and 298 K 1421 has been used extensively in recent years to measure surface areas of coals. The CO2 areas greatly exceed those determined from Nz adsorption at 77 K for two reasons. First, minimum dimension of a CO1 molecule IS smaller than

that of a Nz molecule_ Second, kinetic energy of COz molecules at the adsorption temperatures used far exceeds that of Nz molecules at 77 K. Consequently, rate of diffusion of CO2 mto the coal micropores wrll be significantly higher than that of N,. Walker and Km1 [42] determined the BET surface areas of six coals from adsorption isotherms of N, at 77 K, Kr at 195 K, Xe at 273 K, and COz at 195 and 298 K. The N, and Kr areas in each case were consistently lower than the Xe and COz areas. The authors concluded that adsorption of Xe at 273 K and COz at 195 K should usually measure the total surface area of coals, whereas COZ adsorption at 298 K should invariably measure essentially their total surface area In recent years, the Polanyi-Dubmin (P-D> equation has been used extensively to calculate micropore surface area of coals from CO2 adsorption data obtained below 1 atm in a conventional volumetric adsorption apparatus at 273, 293, and 298 K [l, 21 Walker and Pate1 [ 433 have reported excellent agreement between surface areas of coals calculated by the BET and P-D equations from COz adsorption measured at 298 K m two different pressure ranges. This agreement obviates the need to use a high-pressure adsorption apnaratus for measuring COz surface areas of coals. The P-D micropcre area approximates closely the total surface area (from the BET equation) because of the large percentage of microporosity existing in coals [ 441. Marsh and Siemiemewska [41] have reported that higher-rank coals have higher COz surface areas than the lower-rank coals, a shallow mmimum m the area was observed at about8955 carboncontent.Theauthorshave suggested that smce surface areas of coals of different rank are not far from 200 m’/g, the observed variation in surface area with rank is caused by differences m accessibrhty rather than a difference in true surface area. Thomas and Damberger [39] agree with this view. Mahajan and Walker 1251 also measured CO, surface areas (298 K) of a wide variety of coals. The variation of t&se areas with -bon content of coals IS plotted m Fig 5, the COz surface areas of 18 coals measured by Gan et al. [44] are also mcluded. The spread m the areas is much wider than that reported by Marsh and Siemiemewska [41]. Surface areas of the coals m Fig. 5 are seen to vary between

7

adsorbed m coal micropores. Since pores in coal are slit-shaped, molecular areas in such pores are hkeiy to be two to three times greater than those found on a flat surface [42]. Second, Dubmin [46] has advocated that the adsorption force field m the entree volume of micropores results in volume filling of the pores instead of the layer-by-layer filling which is the basis on which the BET equation was derived. This author agrees with Spencer and Bond [47], who have emphasized that surface area of coals has no physical significance and, therefore, should not be reported_ Instead, in the case of microporous materials, monolayer volume or total volume of sorbate uptake should be reported. Fig 5. Variation of CO1 and N2 areas of coals with carbon content, CO1 areas: 0, from Ref. 25; 0, from Ref_ 44. N2 azea_s: 0, from Ref_ 44; -, from Ref_ 45

110 to 425 m*/g. The areas of coals of srmllar carbon content fall within a broad band rather than on a line; coals with the same carbon content differ m their CO2 areas by as much as 275 m’/g. For the sake of companson, the N2 surface areas of coals determined by Gan et al [44] and Nandi and Walker [45] are also included in Fig. 5. The N2 areas are considerably lower than the correspondmg CO2 areas. In general, coals with N, areas >lO m*/g fall m the 76 84% carbon content range. The areas of most coals on both sides of this range are
Effectofdzfferent variablesonsurfacearea measured bygasadsorptron In a recent review, Mahajan [l] has pointed out that surface area of coal measured by gas adsorption vanes as a function of: heat-treatment conditions used to dry coals prior to measuring their surface area, coal weathermg, coal rank, maceral composition, particle size, mmeral matter present in coals, and swelhng occurring during the adsorption process. Only those variables which have the most relevance to the needs of this paper will be discussed in this section. Drying. Prior to measunng surface area of coals by gas adsorption, it is essential to remove the water present in coals without changing their structure. For this purpose, coals are usually dried at 378 - 383 K. This dry_mg procedure poses problems for lowerrank coals. For example, u-r such coals, the-rmal decomposition of functional groups to yield water and CO, can commence at temperatures below 373 K [24]. As pornted out by Mahalan [30], drying hgnites at 383 K changes their structure, and adversely affects their surface area, ion-exchange capacity, and adsorption characteristrcs. Even m spite of these limitations, outgassing at 383 K is still the most practical approach to dry the coal samples _ ParticZe sLze_ Since adsorption of gases on microporous coals is generally diffusioncontrolled [l, 33, rates of adsorption can be enhanced by decreasmg the particle size. Walker et al. [48] have studied the effect of

8 TABLE Effect Particle

2 of partick 51ze

sze

on surface Surface

area

[48]

area (m’lg) NZ

CO2

43- x 65 mesh 65 x 100 100 v 150 200 x 325

Anthracite 234 331 226 224

42 x 65 mesh 65 Y 100 100 .J 150 209 Y 325

HVA bituminous 58.8
43 Y 65 mesh 65 \ 100 100 ’ 150 200 Y 325 -325

HVC 160 156 i58 139 122

Neopentane

8.0 12 5 22 6 34.0

bltummous 10 3 rl.1 18.1 22.0 i-1

1.0 1.3 3.7


11.9 12 9 ‘20 3 12.1

particle size on surface areas of three coals an anthracite, a HVA bituminous, and a HVC brtuminous - measured by adsorption of CO? (295 K), N, (77 K), and neopentane (273 K). For each adsorption point, a contact time of 30 min was allowed The results in Table 2 show that the COZ surface area of anthracite is essentially independent of particle size, indicating that equihbrium is attamed within 30 mm. However, equilibrmm 1s not attained for N2 and neopentane adsorption, and surface areas calculated from their isotherms mcreasc wrth decreasing particle size The NZ area of each size fraction is larger than the corresponding neopentane area. This is due to the larger size of a neopentane molecule (0.62 nm) compared wrth an N, molecule (0.36 nm). For the HVA brtummous coal, equrlrbrium is not attained for any adsorbate. However, for the HVC bituminous coal, equtibrium uptake of CO2 1s attained for the three largest particle size fractions, but further reduction in particle size decreases the CO2 area. T&e N, and neopentane areas pass through a maximum with decrease in particle srze. The authors attnbute this to the closure of some mrcropores due to heat generated during grinding of the coal in air_ It is significant that each particle size fraction of the HVC coal has a larger neopentane area than the N2 area.

This was attributed to the imbibing of neopentane in the coal. Grinding of coals in air, m addition to causing plasiic flow, as discussed above, can also bring about surface oxidation The oxidation can reduce the surface area due to blockage of some of the micropores by chemisorbed oxygen and/or reduction in their srze to such an extent that reactant molecules cannot enter mto the pore system [ 30 J_ One possible way to overcome the drawbacks of grmdmg coals in air 1s to gnnd them under water_ Lebedev et cl. [49J have reported that water content associated with coals during their particle size reductron adversely affects then surface areas They studred three coals, namely a brown coal, a hard coal, and an anthracite_ The coals were ground m a vibromill The surface areas were determmed from low-temperature Ar-adsorption isotherms_ An increase in water content dunng gnndmg signdicantly reduces surface area of all the three coals (Table 3). For example, upon increasing the water content of the anthracite from 1.9 to 32 3 wt.%, surface area decreases from 312 to 102 m*/g_ Grindmg of the hard coal dned at 105 “C increases the area by an order of magnitude from 6 5 to 65-9 m’/g. However, when this coal is associated with the maximum hygroscopic moisture TABLE

3

Effect of water content associated grmding on their surface area [49] COal

Anthracite

Hard

Brown

coal

coal

with

coals

during

mme of gnnding (min)

water content (wt.%)

Surface area

90

1.9 6.2 32-3

312 298 102

(m’/g)

0 5

nil” IlIla 2.2 4 Ob

65 68 9 57.7 310

0 5

12 0 nil” 12 0 23.lb nila 23_lb

11 87 60 44 92 73

20

aDried at 105 “C. “Mawmum hygroscopx

moisture

content-

2 0 0 6 0 5

9

content of 4 wt %, the ground sample has a surface area of only 31 m’/g. For the brown coal associated- wrth -the maximum hygroscop~c water content (23-l wt_%), an increase in grinding time from 5 to 20 min increases the surface area- by about_65%.In contrast, the correspondmg increase for the oven-dried sample is less than 6%. Anthracrtes have BET N, (77 K) areas
X-ray

scattering

The use of adsorption of gases to determine pore structure of coals has three major drawbacks First, the gas adsorption techniques require a pre-drying step to remove water present m coals. Since low-rank coals undergo severe shnnkage upon drymg, gas adsorption techniques may underestimate their porosity. Second, gas adsorption techniques do not provide information on closed porosity_ Third, chemical interactions are sometimes involved between surface functional groups and probe molecules_ The above drawbacks can be largely overcome by using non-mb-usive physical probes such as small-angle X-my scattering (SAXS)_ Since X-radiation ‘sees’ the entire pores radius, the SAXS technique shows promise for determming pore size distnbution for both open and closed pores. The technique is also potentially capable of measuring pore ~1~4 dlstnbution under temperature and pressure conditions of interest in commercial processes_ It has recently been pointed out that S 4XS can be used to charactcnze mlcro-

TABLE 4 Surface

PSOC coal

dl 318 127 130 135 134 95 105 185 188 197 22 212 181 138 “Not

areas

of coals

[ 511

Rank

No

Surface

area

(m’/g)

Adsorption

Anthracite LV bit. LV lxt. MV bit.

MV brc. MV bit HVA bit. HVB HVB HVB HVC

bit bit. bit. bit. HVC bit. HVC bit. SubbIt. A Subblt C

NZ

CO2

SAXS

ND= (1-0 < l-0 t10 <10 < 1-O < 1.0 16.0 49 a 32.0 11.0 99.0 3.7 49.0 25

ND” 186 271 249 227 208 220 105 ND” ND= NDa 189 254 NDa 254

3 91 2.54 2 51 2 51 2.42 2.39 5.66 46.58 8.72 15 95 6 18 18-4 9.16 24.01 7.25

determmed

porous structure of low-rank coals irrespective of then moisture content [ 5 O] _ Kaliiat et al [ 511 have determmed surface areas of coals of different rank from X-ray scattering data. The surface areas are given m Table 4; the uncertamty in these areas is about )40% The authors contend that SAXS measures surface area of only mesopores and macropores For the sake of comparison, N, (77 K) and COP (298 K) surface areas of the coals are also Included in Table 4. The SAXS areas of all the coals in Table 4 are considerably lower (in some cases by an order of magnnude or more) than the zorrespol?dmg CO2 areas. The authors concluded that coals contain many pores with minimum dimensions <2 nm which are accessible to CO* at room temperature. In contrast, the SAXS areas of 9 coal samples are larger than the N, areas. For the remainder of the coals, this trend is reversed_ It was suggested that in the former case the coals contain closed pores which are maccessible to N, at 77 K, whereas in the latter case the coals contain many pores which are accessible to N, but are not detected by X-rays. The contention of Kalliat et al [ 51 J that SAXS does not measure surface area of mlcropores 1s not supported by other workers. For example, Sprtzer and Ulicky [52] compared SAXS areas of two coals St. Nicholas anthracite and Illinois No. 6 bituminous -

10

with the CO1 areas (298 K). The SAXS and CO1 areas of the anthracite were 275 and 234 m*/g, whereas the corresponding arias of the bitummous coal were 179 and 160 m’/g, respectively_ For both the coals, the agreement between the SAXS and CO1 areas is reasonably good_ Electron

mrcroscopy

Gas adsorption and SAXS techniques do not provide any information about the geometric characteristics of the coal pore systemTransmission electron microscopy (TEM) permits direct observation of the geometric charactenstrcs of pores, and correlation of pore distribution with specific macerals. A 7ElJ micrograph of a vitrinite fragment of a Ke.ltucky bituminous coal (Fig. 6) shows a range of pores with diameters in the 2 to >50 nm range The smailest pores, some of which may even be <2 nm, appear to be related to connecting channels or irregularly shaped pores. Stereo pairs of these vltrnnte fragments indicated a connecting network of pores, suggesting high permeability

Fig 7 TEM micrograph of tubular and n-regularpores m exinite. Arrows show the location of pores [53].

Lm et al. [543 also used the SAXS technique to study the nicroporosity of vitrinite in an Illinois No. 6 coal. They observed a trimodal size distribution with peaks at 3,lO and 22 nm These peaks were attributed to micropores, microminerals, and mesopores, respectively. Significantly, the average mesopore size determined by SAXS was in excellent agreement with that determined by TEM_ Pore

Fig. 6. TEM microgrcph Kentucky coal 153 J_

of vitrinite maceral of

The porosity associated with the exmitic maceral of the Kentucky coal IS shown m Fig. 7. The large, irregularly shaped pores form tubular channels, which extend from the center of the spore exine to the boundary between the spore and the surrounding inertinite. TEM studies on two bitummous coals revealed that the finest porosity was associated with the vitrinite macerals, and ranged in size from 2 to 20 nm in diameter [53] Inertinite was the most porous group with pores in the 5 - 50 nm range. Exinitic macerals were the least porous macerals.

volume

dstributron

For characterizmg pore structure of coal, one needs to know surface area, various densities and pore volume, and pore size and pore volume distributions_ These distributions can be determined in several ways. Some of the experimental approaches used for this purpose have recently been reviewed by Mahajan [ 11, Mahajan and Walker [Z], and Spitzer c551. Gan et al [44] have used the hehum and mercury densities of coals as well as N, adsorption (77 K) and mercury penetration data to estimate pore volume distributions in the followmg pore diameter ranges (a) Total open pore volume VT accessible to he!ium , as estimated from helium and mercury densities (h) Pore volume V, contarned in pores > 30 nm in diameter, as estimated from mercury porosimetry

11

TABLE 5 Gross

open

Semple

pore

distrlbutlons Rank

in coals C (W,

daf)

[44]

VT

VI

v2

(cd/g)

(cm3k)

(cm3k)

PSOC-80

Anthracite

90 8

0.076

0 009

0 010

PSOC-127 PSOC-135 PSOC-4 PSOC-105A Rand PSOC-26 POC-197 PSOC-190 PSOC-141 PSOC-87 PSOC-89

LV bit. MV bit. HVA bit HVB bit HVC bit HVC bit. HVB bit HVC bit. Lignite L:pnite Llgnlte

89.5 88.3 83 8 81 3 i9.9 77.2 76.5 75 5 71.7 71.2 63 3

0.052 0.042 0 033 0 144 0 063 0.158 0 105 0 232 0 114 0.105 0.073

0.014 0.016 0.017 0 036 0.017 0.031 0 022 0 040 0.088 0.062 3.064

0 000 0.000 0.000 0 065 0.027 0.061 0 013 0.122 0.004 0.000 0.000

(c) Pore volume V, contained m pores m the diameter range 1 2 - 30 nm, as estimated from N, adsorption isotherms using the Cranston and Inkley method [ 563 _ (d) Pore volume V3 contained m pores (1.2 nm in diameter, as estimated from V, = VT -

(VI+

VA

Gross open pore distributions in various coals are given in Table 5. The proportion of V, is significant for all the coals Its value is a maximum ior the PSOC-SO anthracite, and a m,nimum for the PSOC-89 lignite. These results thus indicate that all coals show molecular sieve properties. From the results m Table 5, it can be concluded that. (1) porosity in coals with carbon contents <75% is primarily due to macropores, (ii) porosity in coals with carbon contents in the 75 - 84% range is predommantly due to micropores and mesopores, and (iii) porosity in coals varymg m carbon contents from 85 to 91% is due to the presence of micropores The Cranston and Inkley method [56] used by Gan et al. [44] to estimate pore srze distribution is limited to pores >1.2 nm m diameter_ Debelak and Schrodt [40] estimated pore volume distnbution for micropores below 1 2 nm from CO2 adsorption isotherms (195 K) using the Medek method [ 571, and the Cranston and Inkley method, which was modified for use with CO2 Pore volume distributions calculated by the two methods agreed in the micropore range -0 6 2 nm; the maximum m pore volume occurred for micropores with radn of 0.7 - 0.8 nm However, in contrast to the Medek method,

V3

v3

(%I

v2

(90)

VI

w-1

(cm3/r3

0

057 0.038 0.026 0.016 0.043 0.039 0 066 0 070 0 070 0.022 0 043 0.009

75 0 73-O 61.9 48.5 29 9 47.0 41.8 66 7 30.2 19.3 40-9 12.3

13.1 IIll nil il 25.1 32 5 38.6 12.4 52 6 3.5 ml nil

11 9 27 0 38.1 51.5 25 0 20.5 19 6 23 9 17.2 77.2 59 1 87 7

which gave volume distribution in the micropore range only, the Cranston and Inkley method yielded appreciable pore volumes in the mesopore range as well. Spitzer [ 553 determined pore size distnbutions of 20 Czechoslovakian coals, varying m rank from semi-anthracite to ligmte, from mercury porosimetry, N2 adsorption (77 K) and methanol adsorption (293 K), hehum and mercury densities, and small-angle X-ray scattermg. Coal porosity was found to range from 1.3 to 43% Porosity in coals with carbon contents less than 76% was mostly due to mesopores. In contrast, mesopores and micropores accounted for a substantial part of the porosity in coals with carbon contents above 80.6%. Spitzer’s use of methanol to study microporosity in coals has to be viewed with some concern. Heat of immersion in methanol was one of the earliest methods to determine surface area of coals [Z] This method, however, is now of historical importance only, because adsorption of methanol on coals involves specrfic interactions between the hydroxyl group of methanol and oxygen functional groups present on the coal surface [58]. Recently, Nelson et al. [2S] have reported that the extent of swelling in coals induced by methanol increases rectrlineanly with mcrease in the coal oxygen content. Spitzer concluded that high-pressure mercury peneixation is an effective method to obtain pore size distribution over a broad range of pore radii, provided that a correction is made for coal compressibility. Similar

12

conclusions have also been reported by Debelak and Schrodt [40], and Nelson [17]. It is possible that at high mercury intrusion pressures coal particles might brtak down and/or some of the closed pores might open up Mahajan and Walker [25] examined this aspect on three coals. After mercury intrusion up to 30 000 psi, the system was depressurized carefully_ Mercury retained m the pores after depressurization was distilled off under reduced pressure. Following this, helium densities of the coal samples were again measured and c0mpare.l with those of the starting raw coals_ No discernible changes in the helium densities of the coals were observed followmg mercury intrusion up to 30 000 psi. Based on this evidence, the authors concluded that coal particles do not break and closed porosity does not open up during mercury intrusion up to 30 000 psi.

TABI_,E

6

Analyses

and surface

Sample

Proximate

NO

(wt

%

Ash

of coals

(m2k,

Volatile matter

8.4

4.0

Bltuminous coals 912 956 8.38 885

4.6 63 38 5.4

18.8 337 390 37.6

0

0

[59]

Surface

analysis

dry)

Anthracite 1

s

areas

area dryI

Fuced carbon

NZ (77

86.4

nd

224

33

76.6 60 0 57.2 57 0

nil nil ml 11

146 125 104 132

17 24 38 42

K)

CO2 (298

HZ0 K)

32

‘D WATER

ADSORPTION

CHARACTERISTICS

E= = 24 $20

-4s will be discussed shortly, the mechanism of water adsorption on coal, and its dependence on coal rank, are distinctly different from those of other adsorbates discussed so far in this paper. Smce water associated with coals plays an important role in their handling, drymg and transport, it is appropnate to conclude this paper with some dlscussion on the water adsorption characteristics of ccal. Those interested in details of the coal-water mteractions are referred to a recent review by Mahajan [ 30]_ Mahajan and Walker [59] have measured water adsorption isotherms at 293 K on 5 coals - one anthracite and four bituminous coals. Prior to measurmg the isotherms, the coals were dned in vacuum at 383 K. Analyses of the coals are given m Table 6. The water adsorption isotherms are plotted in Fig 8. Bituminous coals adsorb more water at low relative vapor pressures than does anthracite. Furthermore, sorption on bituminous coals is characterized by type II isotherms, while anthracite gives a type III isotherm. Tlne sorption-desorption isotherms on all the coals show hysteresis down to zero relative vapor pressure_ A certam amount of adsorbed water is not desorbed even on prolonged outgassing of tire system to a constant weight at 293 K. After completio,r of the adsorption-desorp-

z

16

E

12

g

a 4 0

or 01

01

01

0

010203

0

010203040506

PIP,

Fig 8 Water adsorptlon isotherms on coals Solid pomts denote desorplion data [59].

at 298

K

tion cycle, sorption capacity of the coals m Fig. 8 was fully restored upon outgassing the samples at 383 K. The water BET surface areas and the BET N, (77 K) and CO* (298 K) areas of the coals are given in Table 6. The water areas are appreciably higher than the N, areas but are significantly lower than the corresponding CO2 areas. These results cannot be explamed by molecular sieve effects alone because minimum dimensron of a water molecule is smaller than that of N, and CO1 molecules_ It appears, therefore, that different mechanisms govern the adsorptions of CO2 (or N,) and l&O. It has been suggested that the nature of a carbon surface rather than its surface area largely determmes its moisture sorption characteristics [24, 60, 611. It is now generally agreed that initial sorption of water on coals involves adsorption at the ‘pnmary’ sites containing oxygen functional groups Because of

13

the strong intermolecular forces in water, water adsorbed on the primary sites acts as ‘secondary’ sites for the adsorption of additional water. This mechanism leads to cluster or island formation; the clusters grow in size with increasing surface coverage. At some stage, the clusters on neighboring adsorption sites merge to cover the surface with a statistlcal monolayer of adsorbed water. Later on, with further increase in relative vapor pressure, pores of mcreasmg dimensions are filled with water. Ultimately, the entire pore volume is filled with water. Inorganic matter present in coals also contributes to then moisture scrption capacity. However, this effect is largely confined to low-rank coals, namely sub-bituminous and hgnitic These coals contain significant amounts of carboxyl groups The hydrogen ions of these groups have usually been exchanged for other cations, such as Na+, K+, Ca’+, etc., due to extended contact of embedded coals with water containing different cat:ons. Schafer [SZ] observed that the exchange of H+ ions of carboxyl groups of low-rank coals with different cations increases their moisture sorption capacity; the magmtude of the increase depends upon the amount and type of the exchangeable cation. Cation exchange also increases the amount of water retained by the coals above 383 K. In general, the cations which adsorb more water retain it more strongly above 383 K Mraw and Sllbemagel [63] measured heat capacities of three bed-moist coals - bltumlnous, sub-bituminous, and lignite - at different temperatures_ From these results, the authors concluded that coals contain two types of water. freezable and non-freezable. It was suggest& that non-freezable water is adsorbed m smaller pores while freezable water is present in larger pores Assuming the amount of non-freezable water to be proportional to the coal surface area, the authors calculated a surface area of 133 m’/g for the bituminous coal, and 460 m*/g for both the sub-bituminous and ligmtic coals The surface area of the bituminous coal agrees fanly well with the CO* surface area (128 m’/g) and the SAXS area (140 m’/g). However, surface areas of the sub-bituminous coal and ligmte are appreciably higher than the ‘total’ CO2 areas expected from the

general behavior m Fig 5. These results focus again on the question raised aarlier in this paper, namely, what is the effect of removal of water horn low-rank coals on their surface area? Clearly, there is a need for more research work in this area. From NMR studies on bed-moist coals, Mraw and Silbemagel [63] concluded that the process of freezing, referred to above, does not necessarily involve discrete phase transitions but rather a continuous decrease in the rate of molecular motion with decrease in temperature_

CONCLUSIONS

Our understanding of the three-dirmensional ordenng in coal structure is Iargely based on the X-ray studies of Cartz and Hirsch [7]_ These studies show that coal is composed of both microcrystallrne and amorphous materials_ The crystalline material increases, and amorphous material decreases, with increase m coal rank. Poor crystallinity of coals, partmularly low-rank coals, makes it dlfflcult to determine accurately the aromatic cluster size, spacing, and ordering The above X-ray work was done way back in 1954. Since then, there have been tremendous improvements m X-ray and compute; technologies It is time that these improvements were exploited to look again at coal structure. Coals have an extensive internal porosity. They have a tnmodal distnbution of pore sizes: macropores, mesopores, and mlcropores. Most of the coal surface area, however, is enclosed in micropores. Porosity m coal -is of the aperturecavity type. The apertures are of molecular dimensions As a result, coals show molecular sieve effects m their adsorptive behavior. True densities of coals (determined by displacement of hehum), partrcle densrtles (determined by displacement of mercury), and surface areas (determined by gas adsorption) show dependence on coal rank. It is now generally agreed that adsorption of CO2 at 298 K measures essentially the total surface area of coals. Surface area of coal determined from adsorption of gases depends upon several variables. These include. coal particle size,

14

adsorbate, outgassing time and temperature prior to adsorption measurements, adsorption time, and cross-sectional area of the adsorbate used_ Therefore, the concept of a fixed, definable internal surface area of coals measured by adsorptron of gases has little physical significance. For coals and other mrcroporous materials, it is more appropriate to use monolayer volume or total volume of sorbate uptake in place of surface area. Recently, there have been srgnificant advances in the use cf small angle X-ray scattering to study coal porosity_ In addition, transmission electron microscopy has provided direct observation of the geometric charactenstics of pores, and correlatron of pore distribution wrth specific macerals. Coal porosity has a large influence on rts behavior during mining, preparation, and utrhzation. The major analytical techniques currently used to characterize pore structure do not provide information on coal porosity under condrtrons existing in commercial processes A better understanding of the role of coal pore structure in utrhzation and conversion processes will come from quantitative characterizatron under dynamic condrtrons of temperature and pressure typically existing in commercial processes Reaiistrcally, we are still far away from achievrng these goals However, several promising techniques are emerging_ These mclude small-angle X-ray and neutron scattering, nuclear magnetic resonance spectroscopy, electron mrcroscopy, heat capacity measurements, and X-ray computerized tomography These techniques hold exciting possibilities to investigate not only the coal pore structure but also the interaction of molecular species wrth pore surfaces The mechanism of water adsorptron on coals is different from that of non-polar molecules_ The nature of the coal surface, as determined by surface carbon-oxygen functional groups, largely determines the water adsorption capacity; surface area plays only a minor role_ Exchangeable cations present m the carboxylate groups of low-rank coals also have a major effect on water adsorption_ Heat capacity measurements suggest that coal contains two types of water. freezable, and nonfreezable_ The former is believed to be adsorbed m larger pores, and the latter m smaller pores.

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