0I
Jack A. Simon, Chief
A
Ab
rbana, ll 61801
INTERNAL SURFACE AREA,
AND POROSITY OF I
Josephus Thomas, Jr., and Heinz H. Damberger
CONTENTS
Page
Page
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1
Abstract
2
Introduction
2
Historical Background
Surface Area by Gas Adsorption.
5
Carbon Dioxide a s a n Adsorbate on Coals 7
Experimental Investigation.
9
Apparatus for Surface Area Measurements. 9
Experimental Conditions for Surface Area
Studies on Coals
9
Reflectance (% ) Measurements.
11
Selection of Samples
12
16
Results and Discussion
.16
RangeofISAinIllinoisCoals
Influence of Coal Petrographic
Composition on ISA
18
ISA of Illinois Coals i n Relation t o That
18
of Other Coals
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Influence of Porosity and Coal Rank
on ISA
Pore-Size Information from Mercury
Porosimetry.
Porosity Information from Moisture
Content of Coal
Pore -Size Information from
Nitrogen Adsorption
Increa sing t h e ISA by Heating.
Interrelation of Porosity, ISA, and
Reflectance in Illinois Coals
Geological Implications from ISA and
Porosity Evaluations.
Summary and Conclusions
Acknowledgments.
References
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FIGURES
1 - Time of passage of a gas molecule a s a function of the reduced differential heat
of adsorption
5
8
2 - Schematic diagram of apparatus for surface area measurements
3 - Sample locations and stratigraphic distribution of samples.
12-13
4 - Map of Illinois with isoranks of Herrin (No. 6) Coal Member depicted by inherent
moisture content; table of correlations between inherent moisture and ranges for
internal surface area values for both whole-coal and vitmin samples; and curve
depicting t h e average change of t h e inherent moisture content with relative depth
.17
i n the Illinois Basin
5 - Internal surface a r e a s of whole-coal and vitrain samples from Illinois a s they relate
.20
t o coal rank
6 - Correlation between coal rank and internal surface area based on a heat-of-wetting
.21
method a s published by Griffith and Hirst (1944)
7 - Correlation between coal rank and internal surface area based on data from the
xenon adsorption method a s published by Kini (1964)
.2 1
8 - Mercury penetration of two Illinois vitrain samples of different rank.
.22
9 - Correlation between calorific values on moist and dry mineral-matter-free b a s e s and
.23
inherent moisture content in high-volatile bituminous c o a l s
10 - Correlation between natural (inherent) and equilibrium moisture contents of high.24
volatile bituminous and subbituminous coals
.25
11 - Relation between inherent moisture content and volume-percent porosity i n Illinois c o a l s .
.
12 - Range of "apparent" average pore diameters of whole c o a l s and vitrains from the
Illinois Basin
.26
13 - Comparison of mean "apparent" average pore diameter with "true" average pore
diameter of whole-coal samples from Illinois
.3 0
14 - Correlation between "wet" and "dry" reflectance under oil immersion and the internal
.32
surface area of whole-coal samples from Illinois
15 Correlation between "wet" and "dry" reflectance under oil immersion and the inherent
moisture content of whole-coal samples from Illinois
.3 2
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( Continued o n next page )
TABLES
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. . . . . . . . . . . . . . . .19
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1 .Surface Areas. Reflectivities. and Analyses of Various Whole-Coal Samples
2 .Internal Surface Areas of Coal Constituents (Herrin (No 6) Coal Member.
Jefferson Co
Illinois)
3 .Surface Areas. Reflectivities. and Analyses of Vitrain Samples
4 .The Effect of Volatile-Matter Removal on Internal Surface Area
..
The efficiency of c o a l gasification, liquefaction, or
solubilization p r o c e s s e s probably depends, among other factors,
upon t h e internal surface area of t h e c o a l used and t h e a c c e s s i bility of t h a t area t o r e a c t a n t s in t h e s e p r o c e s s e s . In addition,
t h e yields of desired components from t h e s e p r o c e s s e s may depend
upon the porosity and t h e internal surface area of t h e c o a l .
In t h i s report, we have d i s c u s s e d methods of measuring
t h e surface area of c o a l , evaluated t h e internal surface area of
Illinois c o a l s on t h e b a s i s of our s t u d i e s , and shown t h e correlations of internal surface area with such c o a l rank parameters a s
calorific value, inherent moisture content, and vitrinite reflectance.
Illinois whole c o a l s of high-volatile A, B, and C bituminous rank (hvAb, hvBb, and hvCb of ASTM classification) were
evaluated by a g a s adsorption method with carbon dioxide a s t h e
adsorbate a t - 7 8 O C. Values ranging from 46 t o 2 9 2 square meters
per gram (m2/g) were obtained, t h e hvAb c o a l s having t h e lowest
values and t h e hvCb c o a l s t h e highest. Values a s m u c h a s 60 m2/g
higher than t h o s e for whole-coal samples were obtained for vitrain
concentrates of t h e same c o a l s . Internal surface area correlated
well with inherent seam moisture content a n d , t h u s , c o a l rank.
Mercury porosimetry data for a n hvAb c o a l and for a n hvCb
c o a l revealed a significant difference i n t h e pore-size distribution
betweenthe two ranks. The hvCb c o a l c o n t a i n ~ da number of pores
with diameters i n t h e range from 35 t o 2 2 0 A, wheteas the hvAb
c o a l contained relatively few pores greater t h a n 35 A i n diameter,
t h e lower limit of t h e pore-size measurements. This difference i n
pore-size distribution is thought t o be a result of differences i n
t h e former depths of burial of t h e c o a l s .
The pore-size distribution a p p e a r s t o have a significant
influence on reflectance measurements of polished c o a l samples.
Depending upon the sample preparation history, t h e pores i n the
polished surfaces a r e filled with either a i r or water, t h u s leading
t o appreciable differences i n t h e measured reflectance values;
t h a t i s , under oil immersion, "wet" samples reflect l e s s light than
"dry" samples. The difference between reflectance values of "wet"
and "dry" samples is g r e a t e s t with hvCb c o a l s and d e c r e a s e s toward t h e hvAb c o a l s , apparently because of t h e lack of larger pores
i n t h e surfaces of t h e l a t t e r c o a l s .
2
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
Studies were a l s o made of the increase i n internal surface
a r e a s of Illinois coals after the removal of small percentages of
volatile matter a t elevated temperatures under controlled conditions.
It i s apparent from t h e s e studies that Illinois c o a l s , particularly
the hvAb c o a l s , a r e far more porous initially than t h e adsorption
measurements with carbon dioxide would indicate. It i s thought
that t h e s e coals contain many pores t h a t are plugged with relatively
low-boiling organic constituents. The removal of t h e s e constituents
a t moderate temperatures (- 200' t o 300° C) i n a helium stream
opens the pore structure, thus bringing about a large increase i n
t h e internal surface a r e a .
INTRODUCTION
The surface area of a solid i s a n important physical parameter, which
c a n generally be correlated with t h e solid's rate of reactivity. In general, t h e
greater t h e surface area, t h e higher the rate of reactivity i n such processes a s
solubilization, c a t a l y s i s , ion exchange , thermal decomposition, and corrosion;
for c o a l s , combustion and weathering a l s o c a n be included.
It i s likely that reactivity r a t e s of coal gasification, liquefaction, and
solubilization depend, among other factors, upon the internal surface area of the
coal used and the accessibility of that area t o the reactants i n t h e s e processes.
In addition, the nature of t h e components released i n t h e s e processes may be influenced by variations i n porosity and internal surface a r e a . To our knowledge,
such relationships have been studied t o only a limited extent.
Physically, coal may be considered a s essentially a compacted colloidal
substance, or hardened gel. It i s interspersed with various-sized interconnecting
pores and capillaries, which produce a large internal surface. An earlier, preliminary study by Thomas, Benson, and Hieftje (l966), for example, showed that
the internal surface a r e a s of Illinois whole coals range from 46 t o 292 square
meters per gram (m2/g).
A clearer understanding of t h e nature of t h e pore structure of c o a l s has
emerged only within t h e past 15 t o 20 years. It is now generally accepted that
the majority of pores in c o a l s are l e s s than 5 i n diameter. Coals have molecular
sieve properties that permit certain molecules to penetrate the pore structure while
excluding other, larger molecules. Temperature plays a n important role i n the
diffusion of g a s e s within t h i s type of pore structure. The recognition that coal
p o s s e s s e s such a n ultrafine structure has been of considerable help i n the u s e of
c l a s s i c a l g a s adsorption methods for estimations of surface a r e a .
In view of the increased interest i n gasification and liquefaction proce s s e s and i n their future impact on t h e coal industry, i t is the object of t h i s
report t o present a more detailed evaluation of t h e internal surface area of Illinois
c o a l s than was presented i n the earlier study, and t o show t h e correlations of
internal surface area with such coal rank parameters a s calorific value, inherent
moisture content, and vitrinite reflectance.
A
HISTORICAL BACKGROUND
Although i t ha s been recognized for many years that coal ha s a porous
structure, only since t h e middle 1950's has general agreement been reached
ISA,
M O I S T U R E , AND POROSITY O F I L L I N O I S COALS
concerning t h e true nature of t h e pore structure and t h e related internal surface
area .* The controversy t h a t existed t o t h a t time w a s largely t h e result of marked
differences between internal surface a r e a s determined from nitrogen (N2) g a s adsorption data and t h o s e determined from heat-of-immersion (heat-of-wetting) d a t a .
In t h e c l a s s i c a l BET g a s adsorption method (Brunauer, Emmett, and Teller, 1938),
N, i s adsorbed on a solid a t a temperature near t h a t of liquid N2 (-196' C ) . Typic a l l y , t h i s method yields low surface area values for c o a l s , ranging from about
0.5 t o 100 m2/g. In the heat-of-immersion method, a calorimeter i s used t o determine t h e heat evolved (which i s related t o surface a r e a ) when a finely divided or
porous substance i s immersed i n a polar liquid such a s methyl alcohol. This method yields much higher surface area values for c o a l s , ranging from about 20 t o more
than 300 m2/g.
Adherents of t h e BET g a s adsorption method (Lecky, Hall, and Anderson,
1951; Malherbe, 1951; Malherbe and Carman, 1952) insisted t h a t t h e i r values
were correct and that t h e greater sorption of polar molecules with t h e heat-ofwetting method resulted from weak chemical bonding with t h e c o a l . The opposition, equally adamant i n t h e i r views, thought that surface a r e a s determined
from t h e sorption of polar molecules, particularly methyl alcohol, were correct
and t h a t t h e low sorption of inert, nonpolar g a s molecules with t h e BET method
w a s due either t o t h e presence of methane i n the pores (Griffith and Hirst, 1944)
or t o t h e slow rate of diffusion of nonpolar molecules i n t o the pores (Maggs, 1952;
Maggs and Dryden, 1952; Zwietering, Oele, and van Krevelen, 1951)
A report by Maggs (1952) generated much research over t h e next few years
t o account for t h e differences. His study, which we and others have verified,
showed t h a t more N, i s adsorbed on the same c o a l sample a t -78' C than i s adsorbed a t t h e much lower temperature of liquid oxygen (-183' C ) . This behavior
i s contrary t o t h a t which normally occurs i n physical adsorption, t h a t is, a lower
temperature brings about a d e c r e a s e i n t h e vapor pressure of the adsorbate (and a
consequent greater adsorption) i n accordance with t h e well-known ClausiusClapeyron equation, t h e integrated expression being
.
where P, and P1 a r e t h e vapor pressures a t temperatures T, and T, , L is t h e molar
heat of vaporization, and R is the g a s c o n s t a n t . Maggs suggested t h a t closure of
pores by thermal contraction w a s a possible explanation for t h e inverse behavior,
but Zwietering and van Krevelen (1954) did not believe that pore closure accounted
completely for t h e reduced a c c e s s i b i l i t y of t h e internal surface a t t h e lower
temperature.
From mercury intrusion mea surements on c o a l and from measurements of
density by helium displacement, van Krevelen (1954) concluded t h a t c o a l p o s s e s s e s
a fine pore structure into which N2 molecules a t very low temperatures cannot diffuse u n l e s s they a r e provided with a sufficient energy of activation. Anderson
e t a l . (1956) reached similar conclusions. They thought t h a t i n most c o a l s a n
appreciable fraction of t h e pores have openings (diameters) of 4.9 t o 5.6 A a t O o C
and about 4 A a t -196O C They believed t h a t low-temperature N2 adsorption isotherms p r o v i d ~a measure of t h e a r e a s only within pores having diameters greater
t h a n about 5 A.
.
h he
2
external surface area of coal is an insignificant fraction ( < 1 m /g) of the total measured
surface area. The emphasis throughout this report is on the internal surface area ( I S A ) .
4
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
The diffusion of g a s e s i n small pores c a n be described without going into
mathematical d e t a i l . For extensive treatments of t h e subject the reader i s referred t o three good t e x t s : Barrer (1951); Carrnan (1956); and deBoer (1968).
Briefly, when t h e molecular s i z e of t h e diffusing gaseous s p e c i e s closely a p proaches the s i z e of t h e pore opening, or aperture, through which it p a s s e s , there
i s a physical interaction between t h e g a s e o u s molecule and t h e aperture. This
interaction i s t h e sum of dispersive and repulsive forces, The repulsive force i s
predominant when t h e pore opening i s sufficiently small relative t o t h e s i z e of
t h e g a s molecule t h a t t h e g a s molecule requires a certain activation energy t o
p a s s through t h e aperture. Small differences i n t h e s i z e s of diffusing g a s molec u l e s c a n result i n large differences i n t h e activation energy n e c e s s a r y for diffusion through a given aperture. As we shall emphasize l a t e r , i n addition t o s i z e
differences,. t h e temperature of t h e g a s molecules plays a n important role i n t h e
diffusion process
The technology of molecular s i e v e s (zeolites) and t h e commercial use of
t h e s e s i e v e s for s e l e c t i v e g a s adsorption a r e based on t h e s e differences i n t h e
diffusion r a t e s of different g a s molecules
Synthetic zeolites a r e produced with
c l o s e l y controlled pore s i z e s t o permit t h e diffusion and sorption of certain g a s
molecules while not allowing the sorption of others. Studies with molecular s i e v e s
of different pore s i z e s have provided further insight into the nature of t h e pore structure of c o a l s . Breck e t a l . (1956) found t h a t the sorption of N, on a 4A sieve
(pores about 4 i n diameter) i n c r e a s e s with temperature from -195" C t o - 7 8 O C,
a phenomenon similar t o that observed with c o a l s . This effect was not observed
with a 5A s i e v e . With t h e l a t t e r s i e v e the volume of N, adsorbed decreased predictably with increasing temperature between -195" C and 0" C.
Lamond (1962) studied t h e adsorption of N, a t -195" C and carbon dioxide
(CO,) a t -78" C on 4A and 5A molecular s i e v e s . With the 5A sieve he found t h a t
the surface a r e a s determined from N, and CO, adsorption were 770 and 695 m2/g,
respectively. With t h e 4A s i e v e , however, t h e N2 surface area value w a s only
1 m2/g, whereas t h e value from 60, adsorption, 610 m2/g, remained large. It is
significant t h a t both t h e 4A and 5A s i e v e s were found t o have large sorption capaci t i e s for CO, a t -78' C. Walker and Geller (1956) found that c o a l s a l s o p o s s e s s
large sorption c a p a c i t i e s for CO, a t -78" C . They reported that surface area valu e s from CO, adsorption a t -78" C were much greater than those from N, adsorption a t -195" C, and were more nearly equivalent t o surface area values from
heat-of-immersion d a t a . They concluded, a s did Anderson e t a l e (1962), t h a t
CO, a t -78" C r e a c h e s most of t h e internal surface of a coal and thus is superior
t o N, a s a n adsorbate for t h e determination of the internal surface area of t h i s
microporous substance.
From t h e s e s t u d i e s and others (Kini e t a l . , 1956; Nandi, Kini, and Lahiri,
l 9 5 6 ) , i t i s now generally accepted t h a t c o a l s p o s s e s s molecular-sieve properties.
By analogy, i t a p p e a r s that c o a l h a s a large number of pores of s i z e s c l o s e t o
t h o s e i n t h e 4A molecular s i e v e . Nitrogen, normally used a s t h e standard adsorbate i n t h e c l a s s i c a l BET method, f a i l s t o reach most of t h e internal surface of
c o a l s a t -195" C b e c a u s e it i s not sufficiently activated a t t h a t temperature t o
di$fuse, i n a reasonable time, into pores having diameters l e s s than about 4 t o
5 A.
The phenomenon of activated diffusion i n c o a l s h a s been d i s c u s s e d by
Zwietering, Overeem, and van Krevelen (1956), Nandi and Walker (1964, 1966),
Anderson and Hofer (1965) , and Walker, Austin, and Nandi (1966). Although
there i s general agreement a s t o t h e nature of activated diffusion, there is some
.
.
I S A , M O I S T U R E , AND POROSITY O F I L L I N O I S COALS
disagreement a s t o the a c t u a l activation energy values t h a t a r e calculated for
various adsorbate molecules. It i s recognized, however, t h a t GO2 a s a n adsorbate a t -78" C i s more highly activated for diffusion than N, a t -195" C a n d , t h u s ,
is a b l e t o diffuse into smaller-diameter pores than c a n nitrogen.
There i s a n apparent paradox here t h a t needs further explanation. Because
the CO, molecule is larger than t h e N, molecule, one would be inclined t o predict,
strictly from s i z e differences, t h a t CO, would diffuse more slowly rather t h a n
more quickly into t h e smaller pores than N2 would. This prediction would be a c curate if t h e two g a s e s were compared a t t h e same diffusion temperature. However,
a s was pointed out earlier, temperature plays a significant role i n t h e diffusion
process, and it is primarily for t h i s reason t h a t we have consistently included t h e
temperature during our d i s c u s s i o n of t h e diffusion o r adsorption of t h e two g a s e s .
In surface area studies by t h e BET g a s adsorption method, g a s e s normally a r e a d sorbed near their boiling points t o insure monolayer coverage of t h e solid surface
a t increasing relative pressures of t h e adsorbate. Carbon dioxide a s a n adsorbate
cannot be compared with N, a t -195O C s i n c e the former is a solid below about
-78' C; on t h e other hand, surface area measurements normally a r e not conducted
with N2 a t -78" C s i n c e monolayer coverage i s not even c l o s e l y approached a t
t h i s temperature. It i s t h i s large difference between t h e adsorption temperatures
of CO, and N, that accounts for t h e greater diffusion rate of the larger GO2 molec u l e a t i t s n e c e s s a r i l y higher adsorption temperature.
This difference i n diffusion
rates i s depicted i n figure 1 , which
9
is reproduced from a report by Thoma s ,
8
Bohor, and Frost (1970) and i s based
on calculations from d e Boer (1968)
;;7
o n t h e diffusion of g a s i n c a p i l l a r i e s .
-w 65
Figure 1 shows t h e time of p a s s a g e (t)
g4
A
of a g a s molecule through a cylindri3
c a l pore, with t h e dimensions shown,
2
diameter= 5 A
a s a function of Qdirf , t h e differenI
t i a l heat of adsorption. The values
'10
12
14
16
18
20
22
24
26
28
chosen for Qdiff a r e reasonably c l o s e
t o t h e accepted values for adsorption
on c l a y minerals, and they should
Fig. 1 - Plot showing time of pas sage (t) of a
g a s molecule a s a function of the renot differ markedly for adsorption on
duced differential heat of a d sorption
c o a l s . In f a c t , t h e Qdifr value;
(Qdif f ) '
could even be reversed for t h e two
g a s e s depicted i n t h e figure without
significantly changing t h e relative times of p a s s a g e of the two g a s e s . It i s apparent t h a t t h e time of p a s s a g e for N, a t -196" C i s more than l o 5 times that for
GO2 a t -78" C. If t h e two g a s e s were compared a t -78" C , t h e time of p a s s a g e
for N, would be l e s s t h a n t h a t for C 0 2 on t h e b a s i s of t h e relative s i z e s of t h e
ga s molecules
+
.
Surface Area by Ga s Ad sorption
Until t h e l a t e 1930's there w a s no satisfactory method for measuring t h e
surface area of both porous and nonporous adsorbents. Apart from t h e aforementioned heat-of-immersion method t h a t wa s commonly used, a number of other
6
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
methods were (and a r e ) used s u c c e s s f u l l y for nonporous s o l i d s . These include
direct microscopic measurements; adsorption of suitable s o l u t e s , such a s fatty
a c i d s , from solution (Harkins and G a n s , 1931); measurement of rate of fluid flow
through packed beds of t h e adsorbent (Carman, 1937, 1938, 1939); and radioa c t i v e exchange, where t h i s technique i s applicable (Paneth, 1922).
G a s adsorption methods have evolved s i n c e the middle 1 9 3 0 8 s . At present,
t h e Brunauer-Emmett-Teller (BET) method i s generally considered t o be t h e " standard" method d e s p i t e t h e fact t h a t i t , too, suffers from some shortcomings.
The BET equation may be written
where V is t h e volume (at standard temperature and pressure) of g a s adsorbed a t
pressure P, Po i s t h e liquefaction pressure of the adsorbate used a t t h e temperature of experiments, C is a constant related t o t h e energy of adsorption, and %
is t h e volume of g a s required t o form a monolayer on t h e solid surface. The value
for Vm is sought from t h e experimental d a t a . A plot of t h e left-hand side of t h e
equation a g a i n s t P/P, (in t h e range from 0.05 t o a bout 0.3 5) i s linear, with t h e
intercept and slope being 1/vmC and (C - 1)& C, respectively. By substitution,
Vm = l / ( s l o p e + intercept). The relative pressure (P/po) range of 0 . 0 5 t o 0.35
corresponds t o a coverage of a l i t t l e l e s s than a monolayer t o about 1 . 5 l a y e r s ,
and data gathering generally is restricted t o t h i s range (unless the whole isotherm
is desired for pore-size evaluations) since higher P/P, values lead t o condensation of t h e adsorbate.
The surface area of the adsorbent i s calculated from k ,from t h e area
occupied by a single g a s molecule, from AvogadroDsnumber, and from t h e molar
volume of the g a s
A value of 16.2 2 i s generally accepted a s the cross-sectional area (cr)
of a n adsorbed nitrogen molecule. One of t h e uncertainties of t h e g a s adsorption
method i n t h e determination of t h e "absolute" surface area of a finely divided o r
porous solid l i e s i n t h e cr value a s s i g n e d t o t h e adsorbed g a s molecule. The u
value is dependent to some extent upon the nature of t h e adsorbent-that
i s , its
structural characteristics-which influences t h e packing of t h e a d sorbate molec u l e s on the surface of t h e adsorbent
During t h e development of t h e g a s adsorption method, Emmett and Brunauer
(1937) emphasized t h e u s e of N, a s a n adsorbate. This g a s has proved t o be a n
excellent choice for surface area determinations because of i t s inertne s s and a l s o
because of t h e ready availability of liquid nitrogen a s a c o ~ s t a n t - t e m p e r a t u r ebath.
Emmett and Brunauer computed t h e cr value of 16.2 A2 for N2 from t h e liquid
molecular volume. Evidence t h a t t h i s value i s c l o s e t o t h e absolute value i s t h a t
surface a r e a s calculated by using t h i s value a r e i n c l o s e agreement with t h o s e
from a n "absolute" heat-of-immersion method described by Harkins and Jura (1944).
Other cr v a l u e s fcr N2 have been suggested. According t o Livingston
(1949), a value of 15 - 4 A' yields surface area values that a r e i n c l o s e agreement
with t h o s e obtained from electron micrographs (values which i n themselves a r e
que stiona ble) Pi%rce and Ewing (19 6 4) , from their studie s on graphite , stated
that a value of 20
w a s c l o s e t o t h e absolute u value with t h i s particular adsorbent. When t h i s uncertainty i n t h e u value i s coupled withdhe inability of N2
t o penetrate c r a c k s or crevices with dimensions of l e s s than 5 A on t h e surface of
.
-
.
ISA,
M O I S T U R E , AND POROSITY O F I L L I N O I S COALS
7
the adsorbent, it is apparent t h a t t h e "absolute" surface area i s not readily
measurable Brunauer (1943) pointed out t h a t "absolute " surface area values
d e t e m i n e d for certain adsorbents may be i n error by a s much a s 20 percent.
However. there i s no better method for determining surface area than t h e g a s adsorption method, and the real value of t h e method l i e s i n i t s excellent reproducibility. From a n analytical standpoint, t h i s permits one to obtain meaningful
numbers with similar type samples and t o place t h e s e samples i n their proper
order for correlation with t h o s e phenomena dependent upon surface a r e a . We a r e
in agreement with Pierce and Ewing (1964), who stated t h a t "in view of t h e s e uncertainties we feel t h a t a s of today there i s PO better choice of standard for area
measurements than N,, that t h e value 16.2 A, for normal packing i s not absolute
but i s a s good a relative standard a s we now have, and t h a t if vapors other than
N, are to be used for area measurements, they should first be standardized by
comparison with N, on the same type of surface a s that of the sample t o be
mea sured "
.
.
Carbon Dioxide a s a n Ad sorbate on Coals
0
Because CO, a t -78O C diffuses into smaller pores ( l e s s than 5 A i n
diameter) than d o e s N, a t -196O C , CO, h a s been used advantageously by several workers in surface area studies on c o a l s . In addition t o t h e previously
mentioned study by Thomas, Benson, and Hieftje (l966), other studies include
those by Walker and Kini (1965); Anderson, Bayer, and Hofer (1965); Marsh and
Siemieniewska (1965); Marsh and O%air (1966); Chiche, Marsh, and Prggermain
(1967);Walker, Cariaso,and Pate1 (1968 a and b); and Walker and Pate1 (1970). Surface
area values from t h e s e studies on c o a l s do not differ greatly. Although there a r e
differences i n methodology a n d , i n some c a s e s , differences i n t h e equations applied, it is not possible, i n view of the uncertainties involved, to s t a t e that t h e
surface area values obtained by one procedure a r e "more correct" than those by
another.
With C 0 2 , a s with N2, one of the major uncertainties i s the o value a s signed t o the absorbate molecule. This will be d i s c u s s e d in greater d e t a i l later.
Additionally, i t has been argued t h a t the BET equation, or for t h a t matter a n y other
equation, does not adequately represent the phenomena that occur i n t h e diffusion
and adsorption of g a s e s i n pores of molecular dimensions. This argument i s well
taken s i n c e , for example, there i s not room t o accommodate more than one molecular layer on each wall between parallel plates 5 apart. With a cylindrical pore
5 A in diameter, there a r e additional wall forces, which restrict buildup of layers.
Thus, the BET theory, which i s based on multilayer buildup, would not appear applica ble However, Mikhail, Brunauer, and Bodor (1968) have recently come t o
the conclusion from pore-structure a n a l y s e s that the BET theory i s correct for a l l
of the surface t h a t i s a c c e s s i b l e t o t h e adsorbate.
We d o not wish t o get too involved i n t h i s controversy i n t h e present
report. We believe that the surface area values obtained on porous substances
by the BET method, although i n error on a n absolute b a s i s , a r e meaningful on a
relative b a s i s i n the comparison of similar samples.
There is little agreement a t present on t h e value of u for CO, a t temperatures near -78' C As pointed out earlier, there i s disagreement on u values a s signed t o different adsorbates since the exact nature of the molecular packing i n
the adsorbed monolayer is not known. CO, appears t o be among t h e most
puzzling of adsorbates i n t h i s regard. Published u v a l u e s for CO, range from
A
.
.
*
110 VOLTS
L
POWER
RECORDER
*
a
SUPPLY
GAS E X I T
BRIDGE
SOAP
F lL M
e
FLOW METER
ROTAMETER
GAS
SAMPLING
VALVE
HELIUM
SUPPLY
N ITROGEI
SUPPLY
CARBON
DIOXIDE
SUPPLY
SAMPLE
HOLDER
Fig. 2
- Schematic diagram
of apparatus for surface area measurements
.
ISA,
M O I S T U R E , AND POROSITY O F I L L I N O I S COALS
9
AZ,~
17.0
calculated by Emmett and Brunauer (1937) on t h e b a s i s of liquid density,
t o 24.4
reported by Pickering and Eckstrom (1952) . From the literature it appears t h a t the majority of investigators using GO., a s a n adsorbate, not only on
coalsobut on other adsorbents a s well, base their calculations on a u value of
17.0 A'
We find, however, t h a t t h e u s e of 17,O A, i n our calculations gives
surface area values t h a t a r e much too low for "standard" materials in comparison
with surface area values obtained on t h e same materials from N, adsorption
(Thomas, Benson, and Hieftje, 1966). To obtain surface a r e a s equivalent t o t h o s e
from N, adsorption, a IJ value of 22.1 i2 for CO, i s n e c e s s a r y in our calculat i o n s . Similarly, Anderson, Bayer, and Hofer ( l 9 6 5 ) , i n studies of CO, adsorption on silica gel, high-temperature carbon, and porous g l a s s , found t h a t u valu e s of 21.9, 19 - 5 , and 21.1 A', respectively, were necessary t o give surface
area values equivalent t o t h o s e obtained from N, adsorption.
.
EXPERIMENTAL INVESTIGATION
Apparatus for Surface Area Measurements
Data for our study were obtained by a dynamic-sorption method. This
rnezhod i s more commonly called the g a s chromatographic method, a s t h e principles
involved i n the measurements a r e similar t o t h o s e used in g a s chromatography.
Figure 2 is a schematic diagram of our apparatus, which i s basically the same i n
design and u s e a s that first described by Nelsen and Eggertsen (1958) and later
refined by Daeschner and Stross (1962). We have converted t h i s basic apparatus
into more versatile equipment for a number of studies-in addition to surface area
deterrninations-in which g a s e s a r e either adsorbed or evolved (Thomas and Frost,
1971).
The general operational d e t a i l s for the apparatus i n surface area determinations and the treatment of the experimental data a r e clearly outlined by Nelsen
and Eggertsen (1958). There i s l i t t l e need here t o elaborate on t h e s e particulars.
Suffice i t t o s a y t h a t the adsorbate g a s i s adsorbed near i t s boiling point a t different relative pressures established by changing t h e flow rate of t h e adsorbate
ga s in a mixed-ga s stream (CO, -t- He), with helium acting a s both the carrier ga s
and t h e diluent g a s . Adsorption or desorption of the adsorbate by t h e sample
produces a peak on the recorder chart a s t h e thermal conductivity detector responds t o differing proportions of g a s e s in the mixed-ga s stream during the adsorption or desorption process. The g a s volume adsorbed (or desorbed) is determined from the peak area and from a calibration curve relating peak a r e a s t o known
volumes of the adsorbate. The thermal conductivity detector response differs for
the same volume of different g a s e s (just a s i n g a s chromatography) ; therefore,
calibration i s necessary for each g a s that i s t o be used a s a n adsorbate.
Experimental Conditions for Surface Area Studies on Coals
Coal samples were crushed and screened, and the 40- by 120-mesh
(about 425 t o 125 ym) sieve fractions were selected for the internal surface area
measurements. A description of a typical experimental "run, " with a d i s c u s s i o n
of the conditions employed, follows :
Approximately 0.200 g coal is weighed into t h e sample t u b e , which i s then
attached t o t h e apparatus. A small heating mantle i s placed around t h e b a s e of
the sample tube, and the sample i s degassed i n a stream of dry He for one hour
a t a temperature of 105" C ,
10
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
Conditions for d e g a s s i n g c o a l s a r e difficult t o e s t a b l i s h . The complete
removal of water from c o a l s and other porous s u b s t a n c e s such a s molecular s i e v e s ,
c e l l u l o s e , and c l a y minerals i s nearly impossible without utilizing very high temperatures, because of t h e strong physical attraction between t h e la s t adsorbed
water layer (layers) and t h e surface of t h e solid. For obvious r e a s o n s , t h e temperatures used t o d e g a s c o a l s must be lower than those used t o d e g a s inorganic
materials.
Degassing commonly i s conducted under vacuum for long periods. However, from unpublished s t u d i e s t h a t we have made with a Cahn Electrobalance
i n the determination of t h e l o s s of moisture versus time a t a given temperature,
i t a p p e a r s t h a t t h e d e g a s s i n g of porous samples i n He i s more rapid than degassing
i n a vacuum a t t h e same temperature. This superiority apparently i s t h e result of
the small atomic s i z e and high thermal conductivity of He, which permit it t o e n t e r
micropores readily, transfer heat rapidly within a particle, and remove adsorbed
water by displacement without being adsorbed t o a significant extent i t s e l f . If
d e g a s s i n g i s conducted on a small quantity of sample a t 105' C i n a helium stream,
very l i t t l e residual water remains after one hour. We have found that surface area
r e s u l t s a r e not significantly different if degassing is conducted a t the same temperature for a much longer period (up t o 16 hours) either i n He or i n a vacuum.
After t h e d e g a s s i n g operation i s completed, t h e g a s mixture of CO, and He
i n a desired ratio i s permitted t o flow over the sample. A Dewar flask containing
a s l u s h of dry ice-absolute ethanol (approximately -78' C) i s then raised into
position around t h e sample tube and adsorption is conducted for 16 hours. This
adsorption period w a s selected for convenience (adsorption could begin i n t h e
l a t e afternoon of one d a y and desorption be conducted t h e next morning) even
though Thomas, Benson, and Hieftje (1966) found t h a t somewhat l e s s than 16
hours is required t o e s s e n t i a l l y reach adsorption equilibrium with nearly a l l Illinois c o a l s . Adsorption equilibrium is not even c l o s e l y approached if adsorption
i s conducted for only 30 minutes (30% complete i n one i n s t a n c e ) , o r even two
hours. Thus, certain s t u d i e s reported i n t h e literature i n which t h e s e short adsorption periods were used no doubt produced low surface area values for the
c o a l s evaluated. With Illinois c o a l s , t h e increase i n uptake of CO, after 24
hours adsorption over t h a t a f t e r 16 hours w a s insignificant. Higher-rank c o a l s
would not n e c e s s a r i l y reach adsorption equilibrium i n t h e same period of time a s
Illinois c o a l s .
Walker and Kini (1965), recognizing t h a t considerable time i s required t o
approach adsorption equilibrium on c o a l s a t -78' C with CO, 9 reported internal
surface area values from adsorption isotherms obtained with CO, a t 25O C rather
than a t -78' C , believing adsorption a t the higher temperature t o be superior t o
t h a t a t t h e lower temperature. It i s true t h a t a t t h e higher temperature a much
shorter time is required t o reach adsorption equilibrium a n d , additionally, advant a g e may be gained by using multipoint BET plots. However, it i s n e c e s s a r y a t
the higher temperature t o u s e high-pressure apparatus i n obtaining adsorption d a t a .
Insofar a s t h e "correctness" of surface area values i s concerned, because of t h e
uncertainties previously d i s c u s s e d ( c r , i n particular), it i s not likely t h a t the surf a c e area values obtained from t h e higher-temperature method necessarily a r e
c l o s e r t o t h e "absolute" values t h a n t h o s e obtained when adsorption occurs for a
longer period a t -78O C .
We u s e single-point BET plots p a s s i n g through t h e origin, recognizing
t h a t some error is introduced, producing slightly lower values (perhaps by 5%) than
t h o s e t h a t would be produced if t h e intercept from multipoint plots were used i n t h e
BET calculations.
ISA,
M O I S T U R E , AND POROSITY O F I L L I N O I S COALS
11
After t h e adsorption s t e p is completed, t h e g a s i s desorbed from the sample by removing t h e dry ice-absolute ethanol bath and quickly replacing it with a
hot water bath ( ~ 9 5 C
" ) . At t h i s temperature t h e adsorbed CO, i s driven rapidly
from t h e sample, and the d e sorption peak, which shows surprisingly l i t t l e "tailing, " becomes t h e measure of the t o t a l g a s adsorbed, a s determined from calibration curves.
To allow for a very slightly higher temperature resulting from the flow of
the g a s over t h e sample, a saturation vapor pressure (Po) of 1450 mm Hg i s used
i n the calculations rather than the value (1413 mm Hg a t -78O C) extrapolated from
Bridgeman's data (1927). A value of 22 . 1 A2 i s used for t h e area occupied by t h e
CO, molecule on the coal surface under t h e existing experimental conditions.
For comparison, surface area determinations were made on a few c o a l s
with N, a s t h e adsorbate,. For t h e s e determinations, a n adsorption period of 30
minutes was used with a liquid nitrogen bath, although i t w a s found t h a t 2 t o 3
minutes i s sufficient t o e s s e n t i a l l y reach adsorption equilibrium. A saturation
vapor pressure (Po ) of 800 mm Hg w a s used in t h e calculations, with a value of
16.3 i Z
for t h e area occupied by a n adsorbed N, molecule near t h e temperature
of liquid nitrogen.
Reflectance (R,) Measurements
The method for determining microscopically t h e reflectance of polished
coal h a s recently been standardized on the national and international l e v e l s (ASTM,
1974 - Designation D 2798; ICCP, 1971). The differences between the two standards a r e very minor; t h e method used i n our laboratory conforms t o t h e standard
methods. A brief description of our setup and procedure follows. For further det a i l s on t h e method, we refer t o t h e two standards and t o Schapiro and Gray (1960),
K6tter (l96O), Murchison (1964), and d e Vries, Habets, and Bokhoven (1968).
We u s e a Leitz Ortholux microscope that i s s e t up for observation under
incident light. Observation under oil immersion, a s opposed t o observation i n
a i r , i s used, not only because of t h e higher magnification achieved but primarily
because of t h e increased contrast obtained between t h e different constituents of
c o a l . Reflectance under oil immersion (R,) i s about 1/5 t o 1/12 lower than t h a t
under a i r (R,)
The 6 0 x oil immersion objective, together with l o x oculars i n t h e
binocular viewing head and a tube factor of 1 . 2 5 , provides a magnification of
750x for observation. The light i s plane-polarized before i t i s directed t o t h e
reflecting coal surface. The green portion of t h e reflected light (546 nm wavelength) i s filtered out and measured t o avoid dispersion effects. With Illinois
c o a l s , only 1 percent or l e s s of the incident light i s reflected under t h e s e conditions. The effective field of measurement i s 4 pm t o 5 pm i n diameter, which
is achieved by t h e insertion of a limiting aperture into the straight tube below t h e
photomultiplier window. The small area of measurement, the low reflectance,
polarization, and filtering a l l reduce t h e amount of light that reaches t h e pickup
surface of t h e photomultiplier tube. It is n e c e s s a r y t o measure minute differences
of about 0.01 percent i n t h e intensity of t h e reflected light. This requires
t h e u s e of a highly stabilized power supply for t h e light source together with a
very sensitive photomultiplier tube (Photovolt M520 system). The power supply
t o the light source c a n be regulated with a constant-voltage transformer. At regular intervals during a measurement, t h e reflectance of a n y c o a l sample i s compared t o that of g l a s s standards (Bausch & Lomb Optical G o . ) , t h e reflectances
of which a r e known under t h e conditions of measurement. For t h e most part,
Bausch & Lomb optical g l a s s e s No. 751278 and No. 827250, with reflectance
.
12
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
values under oil immersion of 0.53 3
and 0.895 percent, respectively, are
used i n the determination of t h e ref l e c t a n c e ~of Illinois c o a l s .
Vitrinite i s the most common
and a l s o the most homogeneous constituent of c o a l , and i s therefore t h e
most suitable constituent for such
measurements i n t h e comparison of
coals. As i s customary, the mean maximum reflectance of vitrinite i s determined from a t l e a s t 100 individual
measurements. The circular stage is
slowly rotated through 3 6 0 degrees,
and the maximum reflectance i s noted.
The method of preparing coal
samples for reflectance measurements
is described in detail i n ASTM Designation D 2797 (ASTM, 1974). The c o a l
i s ground until 100 percent p a s s e s
through a No. 20 sieve (850 ym). A
representative subsample is embedded in a suitable epoxy i n a standardized pellet form ( 1 or 1 1/2 i n c h e s
diameter), and the surface of t h e pell e t i s ground and polished i n several
s t a g e s . The grains exposed i n t h e
polished surface presumably a r e representative of the whole original
sample.
Selection of Samples
Number
of samples
Vitrain
Whole
coal
1
12
Coal
seams
D a n v i l l e (No. 7 )
H e r r i n (No. 6 )
Briar H i l l
(No. 5A)
HarrisburgSpringfield
(No. 5 )
Summum (No. 4 )
Shawnee town
(No. 2A)
Colchester
(No. 2)
DeKoven
Davis
Rock I s l a n d
(No. 1)
Coal i n lower p a r t
of Abbott Fm.
Reynoldsburg
13
Fig. 3 - See caption on facing page.
Since coal gasification, liquefaction, or solubilization proce s s e s no doubt will involve the whole c o a l , it was deemed desirable t o tabulate
internal surface area values for whole-coal samples from various seams. Most
of the samples a r e from active mines. Only two samples a r e from outcrops; i n
addition, several boreholes were sampled. A large number of the samples i n the
present study a r e from the Herrin (No. 6) Coal Member, which i s the most widely
mined coal seam i n Illinois. Figure 3 shows the a r e a l and stratigraphic distribution of the samples. County and section locations a r e included i n the data of
table 1
Whole-coal samples show considerable variation i n mineral-matter content
and i n petrographic composition. Therefore a number of vitrain samples were handpicked from crushed vitrain-rich c o a l samples. All but one of t h e s e were from the
Herrin (No. 6) Coal seam. Vitrain forms the shiny, black, brittle, vitreous,
conchoidally-fracturing, homogeneous-looking bands and l e n s e s i n coal seams; .
it apparently was derived from the coalification of fragments of wood and bark.
Vitrain samples have a low a s h content of only up t o a few percent and a vitrinite
.
(Text continued on page 1 6 )
ISA,
Fig. 3
MOISTURE, AND POROSITY O F ILLINOIS COALS
- Map
of Illinois with sample locations and stratigraphic distribution of samples.
Number or letter abbreviation a t each location refers t o coal seam from which
sample was taken (see stratigraphic column on facing page).
13
TABLE 1 - SURFACE AREAS, REFLECTIVITIES, AND ANALYSES (WHERE OBTAINED) OF VARIOUS WHOLE-COAL SAMPLES
I--'
cP
I
Proximate a n a l y s e s
Location
Sample
no.
Laboratory
designation
Coal
seam
County
Sec.-T.-R.
Vermilion
3-19W-12W
Bureau
-----Christian
---------
Douglas
36-16N-10W
Franklin
8-5s-2E
Franklin
34-6s-3E
Franklin
11-6s-2E
Franklin
20-7s-3E
Gallatin
15-10s-9E
Jackson
25-7s-2W
Jefferson
12-4s-1E
Madison
9-4N-7W
Marion
31-2N-1E
Montgomery
12-12N-5W
Perry
28-6s-4W
Saline
13-9s-5E
St. Clair
15-1s-7W
Vermilion
36-19N-llW
Williamson
28-8s-3E
Gallatin
19-10s-9E
Gallatin
COz
surface
area,
m2Jg
% Ro
Wet
Dry
% Diff.
% Volatile
matter,
As r e c e i v e d
Dry, a s h - f r e e
% Ash,
As
received
% Moisture,
As r e c e i v e d
Ash-free
Ultimate analyses
Calorific
value, Btu/lb
Dry, a s h - f r e e
b
M o i s t , mmf
C
H
N
0
S
Average
% inherent
moisture
(fromfig.4)
5
Franklin
34-65-33
5
Gallatin
13-9s-8E
5
Gallatin
19-10s-9E
5
Perry
21-65-4W
5
Saline
14-9s-5E
5
Williamson
35-9s-4E
4
Gallatin
19-10s-9E
Shawneetown
Gallatin
19-10s-SE
2
Fulton
2-3N-1E
2
Gallatin
19-10s-9E
2
Kankakee
19-31N-9E
2
La S a l l e
15-30N-2E
2
Peoria
7-6N-6E
.DeKoven
Gallatin
19-10s-9E
Davis
Gallatin
19-10s-9E
1
Mercer
26-14N-1W
Willis
Gallatin
30-10s-9E
Coal i n
lower p a r t
of Abbott
Formation
Pope
32-12s-6E
Reynoldsburg
Johnson
32-115-4E
16
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
content of well over 90 volume percent, i n most c a s e s more than 95 percent.
Such relative homogeneity makes vitrain samples somewhat superior t o wholec o a l samples for comparative studies-for example, studies t o show systematic
changes of properties of c o a l s within the Illinois Ba s i n .
RESULTS AND DISCUSSION
Table 1 summarizes our data on internal surface a r e a (ISA), vitrinite reflectance, and chemical composition of t h e whole-coal samples. The data i n
t a b l e 1 (and similar data for vitrain petrographic fractions i n table 3) need some
clarifying comments. Column 1 l i s t s t h e sample numbers used i n t h i s paper for
convenient reference. Column 2 l i s t s t h e laboratory numbers a s s i g n e d t o t h e
samples by our chemical (C) and c o a l petrographic (A) laboratories. These numbers a r e l i s t e d primarily for future reference. Column 3 identifies the c o a l seam
from which t h e sample w a s taken. The samples a r e listed i n stratigraphic order
( s e e fig. 3). Column 4 gives the location from which t h e sample w a s taken,
first by county, then by the section location within t h e county. Within e a c h
seam, t h e samples a r e l i s t e d alphabetically by county. The CO, surface area
values a r e tabulated i n column 5. Reflectance (Ro) values a r e given i n columns
6 and 7 . The meaning of "wet" and "dry" reflectance values and t h e percent difference shown i n column 8 will be d i s c u s s e d later. The r e s u l t s of the proximate
a n a l y s e s (columns 9 , 10, and 11) a r e l i s t e d on both " a s received" and "ash-free"
b a s e s . Calorific values (column 12) a r e on a moist, mineral-matter-free (unit
coal) b a s i s The ultimate a n a l y s e s (columns 13 through 1 7 ) , where obtained,
a r e shown on a dry, ash-free b a s i s . The moisture values (column 11) and t h e
corresponding calorific v a l u e s , l i s t e d on a moist, mineral-matter-free b a s i s
(column 12), a r e enclosed i n parentheses if a significant portion of t h e original
inherent seam moisture appeared t o have been l o s t before t h e moisture a n a l y s i s
or if t h e moisture content appeared too high, a s with sample 12; l o s s of moisture
occurred i n a l l of t h e vitrain samples of table 3 and i n several of t h e whole-coal
samples of table 1
Calorific v a l u e s on a moist, mineral-matter-free ba s i s a r e normally used
t o c l a s s i f y Illinois c o a l s by rank. Only calorific values t h a t a r e not i n parenthes e s c a n be considered t o indicate rank. The moisture value i n column 18 w a s not
determined from t h e sample itself; i t i s the inherent seam moisture content (ashfree b a s i s ) t h a t we would have expected for t h a t particular sample on the b a s i s of
t h e isoranks on t h e map i n figure 4, which will be d i s c u s s e d i n greaterdetail l a t e r .
.
.
Range of ISA i n Illinois Coals
C o a l s i n t h e Illinois Basin a r e high-volatile A, B y and C bituminous i n
rank (hvAb, hvBb, and hvCb of ASTM classification) The ISA values of wholecoal samples range from 46 t o 292 m2/g (table 1). The lowest values found a r e
for hvAb c o a l s from southern Illinois, i n particular t h o s e from t h e Eagle Valley
area i n Gallatin County. ISA values increase rather regularly towards t h e north,
the highest values having been found for t h e hvCb c o a l s i n t h e extreme northern
and northwestern counties of t h e Illinois c o a l basin. This variation coincides
quite well with the long-recognized d e c r e a s e i n rank of Illinois c o a l s i n progressing from the southern part t o t h e northern part of t h e Illinois c o a l basin
(fig. 4; Damberger, 1971; Cady, 1935).
.
ISA,
MOISTURE,
AND POROSITY O F ILLINOIS COALS
% lnhereni
moisture
hole coal
Fig. 4 - Map of Illinois with isoranks of Herrin (No. 6) Coal Member depicted by inherent
moisture content; table of correlations between inherent moisture and ranges for
internal surface area values for both whole-coal and vitrain samples; and curve
depicting the average change of the inherent moisture content with relative depth
i n the Illinois Basin.
17
18
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
Differences i n ISA between adjoining c o a l seams i n a particular area a r e
small, varying only about a s much a s ISA values for samples taken from t h e same
coal seam i n t h a t a r e a . As shown i n t h e next section, some variation i n ISA i s
expected from different parts of t h e same c o a l seam owing to differences i n petrographic composition.
Influence of Coal Petrographic Composition on ISA
Whereas t h e major differences i n ISA for c o a l s within t h e Illinois Basin
are related t o c o a l rank, smaller differences within a given c o a l rank c a n be a t tributed i n part t o variations i n t h e petrographic composition of t h e samples measured. Table 2 g i v e s t h e ISA and t h e petrographic composition of samples of a
vitrain, a clarain, and a fusain t h a t were handpicked from t h e same whole-coal
sample (sample no. 1 1 i n table 1)
.
TABLE 2
-
INTERNAL SURFACE AREAS OF COAL CONSTITUENTS
(HERRIN (NO. 6) COAL MEMBER,
JEFFERSON CO., ILLINOIS)
Petrographic components, v o l . %
Sample type
Vitrinite
Whole coal
79
Vitrain
Clarain
Fusain*
*NO
analysis.
Exinite
Inertinite
Mineral
matter
C02 s u r f a c e
area, m2Ig
6
7
8
91
4
4
1
72
12
9
7
--
--
90+
-
36
I n e r t i n i t e content i s an estimate.
Vitrain, clarain, and whole c o a l s , which i n Illinois a r e made up predominantly of vitrinite macerals , yield t h e largest ISA v a l u e s . Vitrinites include collinite and telinite Exinites embrace sporinite , cutinite , alginite, and resinite
The inertinites include fusinite, semifusinite, micrinite, and sclerotinite Mine r a l matter, for the most part, i s c l a s t i c material (clay minerals, quartz) and
pyrite
The greater part of t h e ISA of whole-coal samples certainly stems from
their vitrinite content. The whole c o a l s of Illinois normally contain 70 t o 85 volume percent vitrinite and t h e vitrinite ha s a large surface area, particularly i n comparison with t h e surface a r e a s of inertinite (mostly fusinite and semifusinite i n I1linois c o a l s ) and mineral matter. Exinite probably is c l o s e to vitrinite i n ISA.
The vitrain samples a r e characterized i n table 3 Vitrinite-enriched handpicked
vitrain samples consistently yield higher ISA values- a s much a s 6 0 m2/g higherthan corresponding whole-coal samples. These differences a r e shown i n figure
5 , a plot of ISA v a l u e s a s related t o rank i n Illinois c o a l s . A certain proportion
of t h e s c a t t e r i n ISA values for t h e whole-coal samples i n figure 5 c a n be attributed t o variations in t h e petrographic compositions of t h e c o a l s .
.
.
.
.
.
ISA of Illinois C o a l s i n Relation t o That of Other Coals
In published studies relating surface area t o c o a l rank, a minimum i n
surface area i s observed i n t h e region of hvAb c o a l t o medium-volatile bituminous
(mvb) c o a l . Figures 6 and 7 include data from two s u c h s t u d i e s , t h e former from
TABLE 3
-
SURFACE AREAS, REFLECTIVITIES, AND ANALYSES (WHERE OBTAINED) OF VITRAIN SAMPLES
Ultimate analyses
Proximate a n a l y s e s
Location
County
Sec.-T.-R.
Vermilion
2-19N-12N
Franklin
13-6S-2E
Gallatin
8-10s-9E
Gallatin
4-10s-9E
Jefferson
ll-4S-1E
Montgomery
34-12N-5W
Peoria
24-9N-5E
Peoria
25-9N-6E
Saline
23-9S-5E
Stark
13-13N-6E
Stark
13-13N-6E
COe
surface
area,
m2/g
% Volatile
matter,
As r e c e i v e d
Dry, a s h - f r e e
269
43.8
46.0
Calorific
value, ~ t u / l b
% Ash,
As
received
% Moisture,
As r e c e i v e d
Ash-free
0.4
(4.4)
(4.5)
As r e c e i v e d
M o i s t , mmf
(13630)
(13730)
As r e c e i v e d
Dry, a s h - f r e e
C
H
%
N
75.8 6.0 1.3
79.7
5.8
1.3
0
S
14.9
11.6
1.57
1.65
Average
% inherent
moisture
(fromfig.4)
14.5
20
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
% Inherent moisture (whole c o a l )
Fig. 5
- Internal
surface areas of whole-coal and vitmin samples from Illinois a s they relate to
coal rank, expressed by average inherent moisture content and calorific value on the
moist, mineral-matter-free basis. Only samples from the No. 5 , No. 6 , and No. 7
Coals are plotted. No drill-hole samples are included.
Griffith and Hirst (1944), who used t h e heat-of-wetting method, and t h e l a t t e r
from Kini (1964), who used t h e g a s adsorption method. Our averaged data a r e
superimposed on t h e i r data i n e a c h of t h e two figures. In both c a s e s the agreement i s excellent i n t h e regions of comparison.
It is a l s o apparent from figures 6 and 7 t h a t the ISA v a l u e s for Illinois
c o a l s practically cover t h e whole range of values reported for c o a l s of a l l ranks
from different parts of t h e world. To account for t h e considerable d e c r e a s e i n
ISA i n proceeding from t h e hvCb c o a l s t o t h e hvAb c o a l s requires a further understanding of t h e differences i n t o t a l porosity and of t h e variations i n pore s i z e
t h a t e x i s t among t h e s e c o a l s . (These differences a r e treated i n s e c t i o n s which
follow .) In turn, a n understanding of t h e porosity variations, together with ISA
values, helps explain t h e apparent minimum i n ISA t h a t e x i s t s i n t h e region of
t h e hvAb c o a l s .
Influence of Porosity and Coal Rank on ISA
The hvCb c o a l s of northern Illinois contain a greater proportion of larger
pores than t h e hvAb c o a l s of southern Illinois. This difference i s apparent not
only from their relative moisture contents but a l s o from mercury porosimetry d a t a .
It i s not apparent from ISA values a l o n e , because extensive surface area c a n be
present within a n ultrafine pore structure, i n which t h e total pore volume (as
determined from moisture content) i s relatively small.
ISA,
MOISTURE, AND POROSITY O F ILLINOIS COALS
21
Fig. 6 Correlation between coal rank
(expressed in % volatile matter, V. M . , in higher-rank
coals and % moisture content
in lower-rank coals) and internal surface area based on a
heat-of-wetting method, a s
published by Griffith and Hirst
(1944). Our data for Illinois
whole coal and vitrain samples
are superimposed (heavy
dashed lines) and show good
agreement with Griffith and
Hirst's data in the region of
comparison.
-
% Moisture
O Kini, 1 9 6 4
\
\
\
\
\
\
\
\
\
sub + lig
Fig. 7 Correlation between coal rank,
expressed by % carbon content
(dry, ash-free b a s i s ) , and internal surface area based on
data from the xenon adsorption
method, a s published by Kini
(1964) Our data for Illinois
coals are superimposed and
show good agreement with
Kini's data in the region of
comparison.
\a
eon and range of values in
extreme southern I l l i n o i s
.
90
85
80
% Carbon ( d a f l
75
70
22
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
Pore -Size Information from Mercury Poro simetry
The s i z e s and relative amounts of pores i n porous materials c a n be determined by t h e intrusion of mercury into t h e s e pores under increasing pressure. The
method is based on t h e fact t h a t increasing pressure i s n e c e s s a r y t o force t h e
mercury into s u c c e s s i v e l y smaller pores The method i s applicable with commerc i a l insotrumentation for determinations of pore s i z e s from 350 pm t o about 0.003 5
pm (35 A), t h e lower limit of measurement a t present. A p r e s s u r e of about 50,000
psia (a bout 3 500 kg/cm2) i s required t o penetrate pores a t the lower limit. It i s
obvious t h a t only a n open-pore structure c a n be penetrated. It i s a l s o obvious
t h a t rather large pores cannot be penetrated if the entrances t o t h e pores a r e very
small (ink-bottle structure)
As we have already pointed ou:, a large proportion of the pores i n c o a l s
a r e much smaller i n diameter than 35 A. Thus, i t might appear that the data from
mercury intrusion would be quite limited for interpretive purposes. Actually, t h e
technique d o e s yield valuable supplemental information about the larger pores
in c o a l s .
Without going into great d e t a i l here about t h e instrumentation and t e s t
procedures, the principle of the method simply involves t h e determination of the
amount of mercury t h a t c a n be forced into the pores of a material a t known increasing pressures. Mercury will penetrate crevices and pore openings with a
strict dependence upon t h e force applied a s given by t h e equation
.
.
PD = -47 c o s 8
where P i s t h e applied pressure, D i s t h e diameter of t h e pore, y i s t h e surface
tension of mercury, and 8 i s t h e contact angle between t h e mercury and t h e surf a c e . Instrumentation i s designed t o permit t h e pressure on the mercury t o be adjusted t o a known value and t o permit t h e determination of the amount of mercury
forced into t h e pores a t t h a t pressure. Only pores of a given s i z e a r e intruded
and filled a t a given pressure. Pressure-volume curves a r e plotted from t h e experimental d a t a . Two such curves a r e shown i n figure 8 for two different vitrain
samples, one from northern Illinois, t h e other from southern Illinois. The ISA
values for t h e s e two samples, determined with CO, , were 304 m2/g and 47 m2/g,
respectively.
The s t e e p rise of t h e two curves a t the low-pressure end i n d i c a t e s only a
filling of t h e void s p a c e s between t h e sample particles. A gradual increase i n
slope over t h e middle segments of t h e
two curves indicates some compression
Pore diameter (pm)
of t h e samples. It i s a t t h e high-pres100
20 10
2 1.0
0.2 0.10
0.02 0.010 0.0035
sure end of t h e curves t h a t differences
0
\
hvCb vitroin ( 3 0 4 m2/g)
i n pore filling a r e manifested, and i t
i s readily apparent that there i s a greathvAb vitrain ( 4 7 rn2/g)
e r ~ r o p o r t i o nof larger pores (larger than
35 A) present i n t h e northern Illinois
hvCb c o a l than i n the hvAb c o a l from
southern Illinois. The i n c r e a s e i n penetration volume (0.158 cm3/g) of mer-
-
A
"
0
0
cury i s appreciable for t h e h v c b c o a l
for pores between 3 5 and 2 20 A (in t h e
10
100
1,000
10,000 50,000
Pressure (psis)
Fig. 8
- Mercury penetration of two Illinois
vitrain samples of different rank.
pressure region greater than 8000 p s i a ,
or about 550 kg/cm2). There i s only a
slight increase i n penetration volume
ISA,
M O I S T U R E , AND POROSITY O F I L L I N O I S COALS
23
(0.0 22 cm3/g), on the other hand, for t h e hvAb c o a l , amounting t o approximately
one-seventh of the pore volume of the hvCb sample; t h i s increase i s a s s o c i a t e d
only with pores 90 i n diameter and smaller. The pressure-volume data show
that there a r e e s s e n t i a l l y no pores greater t h a n 90 i n diameter present i n t h e
hvAb c o a l and t h a t , i n f a c t , only a small proportion of pores are intruded i n the
s i z e range from 90 t o 35 A, t h a t i s , down t o the lower limit of the measurement.
A
Porosity Information from Moisture Content of Coal
Rank in Illinois c o a l s , a s i n other high-volatile bituminous c o a l s , i s expressed by calorific values on a moist, mineral-matter-free or a moist, a sh-free
b a s i s (ASTM, 1974; United Nations, 1956). It c a n be shown that most of the
variation i n calorific value (moist, mmf) is actually due t o t h e inherent moisture
content of t h e coal (fig. 9): calorific value on a dry b a s i s varies only slightly
and within a broad band between 5 and 20 percent inherent moisture contents,
which are typical for Illinois c o a l s , whereas a nearly linear correlation e x i s t s between calorific value (moist b a s i s ) and inherent moisture content for Illinois c o a l s .
In t h i s paper, we use moisture
content and calorific value (moist,
I
mmf b a s i s ) a s equivalent and equally
hvAb
hvBb
hvCb
suitable rank parameters. Conversion of one into t h e other c a n be made
e a s i l y by using t h e "moist" band of
figure 9 .
Since there i s considerable
confusion about t h e meaning of
"moisture in c o a l , " particularly with
regard t o t h e comparison and significance of analytical d a t a , some
detailed discussion of t h i s subject
i s in order here.
A "moist" sample, according
t o ASTM (1974, p . 55), is a sample
t h a t " s h a l l be taken i n a manner most
likely t o preserve inherent moisture
for purposes of a n a l y s i s . " The ASTM
"Classification of Coals by Rank"
(Designation D388) theoretically is
Fig. 9 - Correlation between calorific values
on moist and dry mineral-matter-free
based on t h e natural, or inherent,
bases and inherent moisture content
moisture of the coal seam before i t i s
i n high-volatile bituminous c o a l s .
sampled or mined. It i s assumed that
coal occurs i n nature in a moisturesaturated condition. It should be readily apparent that high-moisture c o a l s , in
particular, c a n l o s e appreciable amounts of water before sampling, i n transit from
the sampling location t o the laboratory, or during other handling procedures
(crushing, sieving, e t c . ) Samples that may have l o s t moisture before sampling,
or after sampling but before a n a l y s i s , " s h a l l not be used for classification on a
moist b a s i s u n l e s s brought t o a standard condition of moisture equilibrium a t 30° C
in a vacuum desiccator containing a saturated solution of potassium sulfate (97
percent humidity) a s specified i n the Method of Test for Equilibrium Moisture of
Coal a t 96 t o 9 7 Percent Relative Humidity and 30" C" (ASTM Designation D 1412,
ASTM, 1974, p. 250-252). It h a s been shown for t h e c o a l s of high-volatile
1
I
.
I
24
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
bituminous (hvb) rank t h a t we have i n Illinois t h a t equilibrium moisture and inherent seam or bed moisture are e s s e n t i a l l y identical (Ode and Gibson, 1960;
Hiickel, 1 9 6 7 ) . In figure 10, we have combined t h e data from Ode and Gibson,
from Hiickel, and from a few samples of Illinois c o a l s for which equilibrium moisture values were determined by Dr. Marlies Teichmiiller (Geologisches Landesamt
NW, Krefeld, West Germany) and by investigators a t t h e Central Laboratory, Peabody Coal Company, Freeburg, Illinois. The d a t a points a r e about equally s c a t tered on e a c h side of the 1 :1 ratio l i n e , a n indication that t h e two moisture cont e n t s a re virtually interchangeable
Because of t h e uncertainties involved i n t h e determination of t h e inherent
(natural) moisture of c o a l , equilibrium moisture data a r e generally more reliable
t h a n inherent moisture d a t a . However, ASTM d o e s not normally require the
.
10
20
O/o Equilibrium moisture
Fig. 10 - Correlation between natural (inherent) and equilibrium moisture contents of highvolatile bituminous and subbituminous c o a l s . Samples from various c o a l b a s i n s
i n U.S.A. and Europe.
I S A , M O I S T U R E , AND POROSITY O F I L L I N O I S COALS
25
determination of equilibrium moisture if proper precautions a r e taken t o preserve
the inherent seam moisture. Equilibrium moisture i s not routinely determined i n
the Illinois State Geological Survey laboratories
The inherent c o a l seam moisture changes quite regularly within the Illinois Basin, both regionally and with depth. Figure 4 gives t h e main r e s u l t s of a
systematic study of several thousand a n a l y s e s carried out over several y e a r s on
c o a l s from t h e entire Illinois c o a l field. The l i n e s of equal inherent seam moisture for Herrin (No. 6) Coal shown i n the map a r e the result of a n averaging proce s s . Thus, moisture values taken from t h i s map a r e averages from many samples
and a r e probably more representative than any single moisture determination would
be. The moisture-depth curve of figure 4 permits the inference of moisture values for seams both below and above t h e Herrin (No. 6) Coal if the distance between the two seams i s known. We have determined t h e average inherent seam
moisture for e a c h of our samples using both the map and t h e depth curve of figure
4 and have listed t h e s e values i n t a b l e s 1 and 3 a s "average inherent moisture
(from fig. 4) " Single moisture evaluations and corresponding calorific values
(moist b a s i s ) t h a t deviated by more than about 10 t o 15 percent (relative) from
t h i s "map moisture" were put i n parentheses s i n c e it c a n be assumed t h a t t h e s e
samples either had l o s t some of their natural moisture or, i n one ca s e , contained
excessive moisture. In t h e d i s c u s s i o n of the relationship between rank and internal surface a r e a s and pore s i z e s (see figs. 5, 1 2 , and 1 3 ) , map-moisture values from figure 4 have been used t o establish our coal rank parameter.
The " a s received" moisture contents of a l l t h e vitrain samples a r e much
lower than anticipated from the values given by t h e isorank l i n e s i n figure 4 (see
table 3 ) . Several factors contributed t o t h e s e low moisture contents. These
samples were taken during the summer. The original samples were spread out on
a iaboratory bench and crushed with a hammer t o permit the handpicking of bright
vitrain p i e c e s . In addition, a number of the samples were stored for a
considerable period of time before
they were submitted for chemical
a n a l y s i s , The vitrain samples from
southern Illinois (Pope, Saline, and
Gallatin Counties) yielded somewhat
lower equilibrium moisture contents
(table 3) than we would have expected
from t h e map (fig. 4) and from the
plot showing the correlation between
natural seam moisture and equilibrium
moisture (fig. 1 0 ) . We are not sure
about the significance of t h i s difference a t t h i s time.
If a l l pores were filled with
water i n nature, the inherent (natural)
seam moisture would give direct information on t h e pore volume, a s
shown in figure 11 Pores in c o a l
% inherent moisture in coal
usually contain, i n addition t o water,
small amounts of gaseous and liquid
Fig. 11 - Relation between inherent moisture
hydrocarbons and CO, , but a s a first
content and volume-percent porosity
approximation, the natural moisture
in Illinois coals.
.
.
.
26
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
content c a n be assumed t o fill most of t h e pore s p a c e of a coal. Thus, pore volume in Illinois c o a l s ranges from about 5 t o about 25 percent (fig. 11).
Pore volume combined with t h e ISA permits t h e calculation of t h e s i z e of
a n average pore. If we a s s u m e that a l l pores a r e spherical i n shape and have
equal diameters, d (in angstroms), and if N i s the number of pores present i n 1
gram of c o a l having a surface a r e a , A (in m2/g), and a moisture content of P/100
gram (in cm3), then:
(N) (S,)
=
P/100 and (N) (8,)
=A
where S, i s the volume (in i3),
S, is t h e surface area (in i z
) of each one of t h e s e
idealized spherical pores, and P is t h e moisture content of c o a l i n weight percent.
By elimination of N and u s e of t h e proper equations regarding a sphere, the diameter, d (in angstroms), of a n average pore can be determined by t h e following
equation:
Spherical pores a r e not actually representative of t h e tortuous pore system
that e x i s t s i n c o a l s , but the relative change i n d is of interest here for comparative purposes. In figure 12 we have plotted t h e "apparent" average pore diameters
of whole c o a l s and vitrain fractions by using the above equation with t h e experimental d a t a , assuming spherical pores.
The pore diameters of t h e whole c o a l s shown are somewhat large because
t h e s e c o a l s contain mineral matter, which d o e s not contribute much t o t h e total
measured surface a r e a . We d o not have enough data t o calculate internal surface
a r e a s on a mineral-matter-free b a s i s : t h e internal surface a r e a s on a mineralmatter-free b a s i s would be somewhat larger and the "apparent" average diameters
smaller, thus moving the whole-coal curves c l o s e r t o t h e vitrain curves i n figures
5 and 12.
The surprising result of t h i s
manipulation of the porosity and internal surface area data (fig. 12), i n view
of our knowledge of pore-size differe n c e s from mercury intrusion and from
relative moisture contents, i s the small
change of t h e "apparent" average pore
s i z e over most of the range of coal rank
i n Illinois c o a l s . A maximum i s even
indicated i n the hvAb region. Then,
a s rank i n c r e a s e s i n the direction of
the mvb c o a l s , the "apparent" averaTe
pore diameter d e c r e a s e s t o about 20 A
i n the higher-rank c o a l s from the southernmost part of the Illinois Basin. Prob0
5
10
15
20
ably a n even further decrease i n the
% Inherent moisture
average pore diameter would be observed
in mvb c o a l s .
Fig. 12 - Range of "apparent" average pore
With decreasing rank i n the
diameters (calculated from inherent
direction of subbituminous and lignitic
moisture contents and ISA values)
c o a l s , the average pore diameters inof whole coals and vitrains from the
c r e a s e rapidly. As we s hall explain
Illinois Basin.
I
I
I
I
I
I
I
L
I
ISA,
M O I S T U R E , AND POROSITY OF I L L I N O I S COALS
more fully later, we believe that t h e surprising c e s s a t i o n of a decrease i n t h e
"apparent" average pore diameter with increasing rank among the hvb c o a l s i s the
result of a clogging of pores and pore entrances by relatively low-boiling organic
matter that evolves from coal during coalification, particularly within the hvb
c o a l s . Such clogging makes a part of the ISA inaccessible even to CO, ; thus ISA
values from CO, adsorption do not represent t h e complete surface area of a l l t h e
pores that contribute to t h e natural moisture content of coal. The "apparent" average pore diameters i n figure 12 therefore are somewhat greater than they should
be throughout the full moisture range, but are markedly greater for the c o a l s containing a bout 3 t o 11 percent inherent moisture.
Pore-Size Information from Nitrogen Adsorption
As emphasized earlier i n t h i s report, N, a t ;195" C cannot diffuse in a
practical time into pores smaller than about 4 t o 5 A i n diameter. In spite of t h e
lower surface area values obtained with N2 a s the adsorbate, however, valuable
supplemental information about t h e relative s i z e s of pores i n c o a l s c a n be gleaned
from comparing N2 ad sorption data with CO, adsorption data . Machin, Staplin,
and Deadmore (1963) studied a number of Illinois c o a l s and reported N2 surface
area values of 47 t o 92 m2/g for hvCb c o a l s , 7 t o 16 m2/g for hvBb c o a l s , and 1 . 8
t o 4 . 5 m2/g for hvAb c o a l s . In a similar study with N, a s t h e adsorbate, we obtained a value of 99 m2/g for t h e Colchester (No. 2) Coal Member i n Kankakee
County (hvCb), 15 m2/g for t h e Herrin (No. 6) Coal i n Jefferson County (hvBb),
and 0.6 m2/g for t h e coal i n the lower part of the Abbott Formation i n Pope County
(hvAb) These c o a l s had yielded surface area values of 250 m2/g, 198 m2/g, and
59 m2/g, respectively, with CO, a s the adsorbate
An examination of t h e ISA values from N2 adsorption data alone shows
that t h e hvCb c o a l s have much larger internal surface a r e a s than t h e hvAb c o a l s .
The surface area values from N2 adsorption fall off markedly i n proceeding from
t h e HvCb c o a l s through the hvBb c o a l s t o t h e hvAb coals; the latter yield values
two magnitudes smaller than those from the hvCb c o a l s . Clearly, the hvCb c o a l s
from northern and western Illinoisocontain a much larger proportion of pores with
pore entrances larger than 4 t o 5 A than d o the higher-rank c o a l s from southern
Illinois
.
.
Increasing t h e ISA by Heating
Controlled-temperature experiments were carried out using a Ca hn Electrobalance, with which weight l o s s of a sample a t given temperatures can readily be
studied a s a function of time. A known quantity of volatile matter was removed
from a given coal sample, and t h e surface area of t h e sample was then redetermined.
The samples were heated i n a He g a s stream. Water w a s removed a t a temperature
of about 120" C until no further weight l o s s occurred. The temperature then was
increa sed t o remove a small percentage of the volatile matter. The temperature
was kept below 330" C (200" C w a s sufficient for low-percentage removal), which
is considerably below the softening temperature of t h e c o a l s (-380" C ) . Results
are shown i n table 4 for three different whole c o a l s (one hvAb, two hvBb)
The increase i n internal surface area a s a function of the removal of volat i l e matter i s readily apparent with either CO, or N, used a s the adsorbate. The
percentage increase i n surface area is greatest for t h e hvAb coal a t t h e same level
of volatile-matter removal. For example, after t h e removal of only 3 - 87 percent
.
28
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
TABLE 4 - THE EFFECT OF VOLATILE-MATTER REMOVAL ON INTERNAL SURFACE AREA
I
Coal member
1
Briar Hill (No. 5A)
Coal Member,
Gallatin County
(hvAb)
% Volatile matter,
dry basis
Volatile matter
removed (as % of
total volatile
matter, dry basis)
Volatile matter
removed (as %
of total coal
s a l e , dry b a s i s
1
COz surface
area, m2l1
1
N2 surface
area, m2/g
1
38.7
Harrisburg (No. 5)
Coal Member,
Franklin County
(hvBb)
Herrin (No. 6)
Coal Member,
Franklin County
(hvBb)
volatile matter, or 10 percent of the t o t a l volatile matter present, t h e surface a r e a ,
with CO, a s the adsorbate, increased from 50 t o 200 m2/g for t h e hvAb coal, a
fourfold increase i n surface area from t h a t of the original dried sample. At t h e same
level of volatile-matter removal, one of the hvBb c o a l s doubled in surface area
(from 144 t o 284 m2/g) and the other hvBb coal increased by almost 50 percent
(from 205 t o 295 m2/g). Surface area values with N2 a s the adsorbate a l s o increased markedly. It i s significant i n light of the discussion which follows,
however, that after t h e volatile-matter removal the N-, surface a r e a s a r e s t i l l not
a s large a s N2 values for hvCb c o a l s , whereas t h e CO, values a r e equal t o those
for hvCb c o a l s .
The increase in internal surface area i s surprisingly large i n view of the
small quantity of relatively low-boiling organic material removed during heating.
The generation of such a large surface with such little weight l o s s is unknown,
for example, in numerous studies involving the thermal decomposition of inorganic
carbonates, o x a l a t e s , and hydroxides,. Thus, i t would appear t h a t the increased
surface area of the heated coal samples c a n be a ssociated only with the unplugging
of a n already existing pore structure t h a t , i n the original unheated sample, was
inaccessible even t o CO, a t -78O C . As t h e data a l s o indicate, the pores i n t h e
hvAb c o a l s of southern Illinois, being smaller on the average than t h o s e i n hvBb
c o a l s or hvCb c o a l s , a r e more e a s i l y plugged and, t h u s , a greater proportion of
the pore structure i n t h e unheated hvAb coal samples is not penetrated by CO,
We d o not know t h e exact nature of the relatively low-boiling organic
material that i s removed a t temperatures up t o 330° C . However, a s e r i e s of
papers, "The Smaller Molecules Obtainable from Coal and Their Significance "
(Palmer and Vahrman, 1972; Rahman and Vahrman, 1971; Spence and Vahrman,
1970; Vahrman, 1970), lends support t o our interpretation. The authors of t h e s e
papers concluded that much larger quantities of lower-boiling hydrocarbons e x i s t
in c o a l than had previously been suspected. These hydrocarbons a r e adsorbed on
the internal surfaces of the micropores a n d , t h u s , have been overlooked simply
because they a r e difficult t o reach with solvents i n extraction studies. In one of
.
I S A , M O I S T U R E , AND POROSITY O F I L L I N O I S COALS
29
their experiments, Palmer and Vahrman (1972) used superheated steam t o heat
coal and remove t h e s e lower-boiling hydrocarbons. At 300" C they removed about
2 t o 5 percent volatile material, which i s quite comparable t o the weight percent
of material that we removed under helium. Analysis of their extracted material
showed that it c o n s i s t s almost solely of hydrocarbons-aliphatic g a s e s , for the
most part, together with small amounts of t a r oil, the bulk of which w a s soluble
i n light petroleum.
The inability of CO, t o penetrate a l l pores re?ults i n the relatively large
"apparent" average pore diameters of about 30 t o 50 A, a s calculated from pore
volume (from inherent moisture content) and CO, internal surface area data (fig.
1 2 ) . The anticipated decrease i n "apparent" average pore diameters i n proceeding
from hvCb rank c o a l s t o hvAb rank c o a l s i s not realized from t h e s e calculations.
In view of t h e data from the controlled-heating experiments, t h i s lack of significant change i n the "apparent" average pore diameters over most of the c o a l ranks
represented among Illinois c o a l s (fig. 12), i s , with little doubt, indicative of t h e
inaccessibility of pore s p a c e s t o CO, due t o plugging. However, if surface a r e a s
were larger, a s they a r e made t o be by t h e controlled-heating experiments, t h e
calculated "apparent" average pore diameters would decrease (see equation on
page 2 6 ) if t h e pore volume i s assumed t o remain essentially constant. A fourfold
increase i n surface a r e a , which occurs i n a n hvAb c o a l after removal of 10 percent of t h e available volatile matter, in recalculation would result i n a reduction
of the "apparent" average pore diameter t o one-fourth that of t h e initial calculated
value used i n figure 12.
The internal surface area data from the controlled-heating experiments
(using ISA value s after 10% total volatile-matter removal) were used for recalculating the average pore diameters. The limited data from t h e three c o a l s studied
are depicted linearly i n figure 13 a s "true" ("T") average pore diameters for whole
c o a l s . The data show t h e expected decrease i n average pore s i z e s in progressing
t o the hvAb c o a l s . The mean values within the range of "apparent" average pore
diameters calculated for the whole c o a l s of Illinois (shown in fig. 12) a r e replotted
i n figure 13 a s "A" whole c o a l for comparison. The area between curve "A" whole
c o a l and curve "T" whole c o a l i s indicative of the degree of plugging that occurs
i n Illinois c o a l s , with t h e larger area i n the region of t h e hvAb c o a l s indicating
greater plugging. Although the "true " average pore diameters, represented by
curve "T" in figure 13, no doubt a r e s t i l l somewhat large (owing t o the u s e of a
spherical-pore model), the rate of decrease (slope of "T") of the average pore
s i z e s from hvCb c o a l s probably reflects rather closely the a c t u a l rate of change
of the average pore s i z e s .
Thus, i t would appear that t h e well-known minimum i n data plots of surface
area versus coal rank i n the region of t h e hvAb c o a l s (figs. 6 and 7) should not be
nearly a s steep or a s extensive a s studies published thus far would indicate. Carbon dioxide a s a n adsorbate g a s cannot enter the compressed and plugged pore s y s tem, nor c a n liquids, such a s methanol, i n the heat-of-wetting method.
Interrelation of Porosity, ISA, and Reflectance i n Illinois Coals
In recent years the reflectance of polished c o a l h a s become increasingly
accepted a s a rank parameter throughout much of t h e coalification s e r i e s . Generally, t h e maceral vitrinite i s used a s t h e reference material because of i t s relative homogeneity and i t s abundance, both i n normal c o a l and a s finely disseminated coaly particles ("phytoclasts, " Bostick, 19 70; "organoliths, " Alpern, 1970)
i n sedimentary rocks.
30
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
///////////////////////////A
Depiuggfg
begins
Pores become increasingly p l u g g e d by
relafilvely low-boiling, h y d r o c a r b o n s
I
I
I
I
I
5
I
10
I
15
Inherent m o i s t u r e
Fig. 1 3
-
Comparison of mean "apparent" ("A") average pore diameter (fig. 1 2 )
with "true" ("T") average pore diameter of whole-coal samples from
Illinois. The latter was calculated from porosity (fig. 11) and from
increased ISA after removal of relatively low-boiling hydrocarbons
during controlled heating. The difference between the mean "apparent"
average pore diameter and the "true" average pore diameter i s probably
indicative of the degree of plugging of pores that are not accessible t o
CO, i n ISA determinations without prior heat treatment of the coal
samples
.
Reflectance increases over the whole coalification series. In lignite, R,
may be a s low a s 0.3 percent. In high-volatile bituminous c o a l s , R, ranges from
about 0.4 t o about 1.2 percent, which i s the range for Illinois c o a l s . The highestrank c o a l s , semianthracites and anthracites, reflect about 2.0 t o 6 . 0 percent (and
higher in certain c a s e s ) of the incident light from a polished surface under oil
immersion.
Most of the studies on coal reflectance have been done i n connection with
efforts to find simple petrographic methods for the prediction of coking properties
of single coals and of coal blends with R, above 0 . 7 percent (that i s , high-volatile
B and A and medium- and low-volatile bituminous coals). It has been only rela tively recently that t h e reflectances of high-volatile C bituminous, subbituminous,
and lignitic c o a l s have been studied t o any great extent. Teichmuller and Teichmiiller (1968, table 1 , p. 248) and M Teichmiiller (1971) indicate that R, of vitrinite i s a good rank parameter only above about 0.7 percent. It would appear that
R, i s fairly closely related t o the volatile-matter content, even i n high-volatile
bituminous coals, for which t h e volatile matter content i s not a reliable rank parameter. At present there i s some doubt a s t o the reliability of R, a s a rank parameter
in the lower-rank c o a l s .
.
ISA,
M O I S T U R E , AND POROSITY O F I L L I N O I S COALS
31
Harrison (1965) reported t h e effect of moisture content on reflectance
values for Illinois c o a l s . He noted that "moist" samples, t h a t is, those prepared
from moist c o a l s , yielded lower reflectance values than samples prepared from
well-dried c o a l s . The effect was most pronounced among the hvCb c o a l s of
northern and western Illinois. The effect a l s o was noted when measurements were
made on samples soon a f t e r t h e y had been wet polished, Harrison recommended
that coal samples be a i r dried for a t l e a s t 15 hours before a briquette i s made, and
that the briquette, a f t e r i t is wet polished, be placed i n a desiccator for a t l e a s t
15 hours before reflectance a n a l y s i s measurements a r e made. This proposal w a s
included i n the recently adopted ASTM Designation D 2797, "Preparing Coal Samples for Microscopical Analysis by Reflected Light" (ASTM, 1974, p. 551-555),
and it has a l s o been adopted for t h e corresponding international standard method
for c o a l s that might be affected by moisture absorption i n the 1971 Supplement
(ICCP, 19 7 1) t o the second edition of the International Handbook of Coal Petrography.
Harrison and Thomas (1966) compared the differences between the reflect a n c e values of "wet" and "dry" samples with ISA values and found a good correlation, A greater disparity existed between "wet" and "dry" reflectance values for
the high-porosity hvCb c o a l s (higher ISA values) than between those reflectance
values for the hvBb and hvAb c o a l s (lower ISA values). Additional samples have
since been analyzed (table 1 l i s t s both "wet" and "dry" reflectance values and
percentage uifferences), and the data have been plotted a s functions of ISA (fig.
1 4 ) , and of moisture content a s taken from the map i n figure 4 (fig. 1 5 ) - There i s
appreciable scatter i n the data ; nevertheless, the trend established in the earlier
study is reconfirmed. Scattering i s t o be expected because of the difficulty i n
replicating moisture content under nonequilibrium conditions.
As Harrison and Thomas (1966) pointed out, the noted differences in reflectance probably are dependent upon t h e number of macropores into which water c a n
reenter. As we have 5hown in preceding sections, there i s a greater proportion
of larger pores ( > l o 0 A) present i n hvCb c o a l s than in hvAb c o a l s . Thus, strictly
speaking, porosity rather than internal surface area is the parameter that i s responsible for variations i n reflectance between "wet" and "dry" samples. However, since internal surface area a l s o h a s been shown t o correlate with porosity
i n Illinois c o a l s , i t i s a satisfactory substitute for porosity for correlation with
reflectance differences. It should be realized that s u c h a correlation with internal surface area would not be obtained for higher-rank c o a l s of lower porosity.
How d o the differences i n macroporosity account for t h e observed differe n c e s i n reflectance for "wet" and "dry" samples, with t h e latter samples producing t h e higher reflectance v a l u e s ? Although we d o not have unequivocal proof
a t t h i s time, one explanation t h a t we find plausible follows.
First, consider that the polished coal surface ~ o n t a i n sa large number of
openings which, for the most part, a r e l e s s than 200 A i n diameter. Since t h e s e
openings are considerably smaller in diameter t h a n the wavelength of visible light,
light cannot enter the pores (as black boxes) and be scattered from beneath the
surface. Thus, the only scattering and reflection of t h e incident light occur a t
the surface, where the pores, either water-filled or air-filled , a r e considered
part of the continuum of the c o a l surface. To account for the observations then,
it i s necessary t o show only that a n immersion oil-air interface h a s a higher reflectance than a n immersion oil-water interface. These reflectances c a n be calculated from Beer's equation
R=
(n' - n")'
(n" n u ) '
+
+
(n'k)2
(n")'
32
I L L I N O I S STATE G E O L O G I C A L SURVEY CIRCULAR 4 9 3
Fig. 14
-
Correlation between "wet" and "dry" reflectance under oil immersion (R,) and the
internal surface area of whole-coal samples from Illinois.
Fig. 15 - Correlation between "wet" and "dry" reflectance under oil immersion (R,) and the
inherent moisture content of whole-coal samples from Illinois.
I S A , M O I S T U R E , AND POROSITY OF I L L I N O I S COALS
33
where R = reflectance, n' = index of refraction of first medium, n" = index of
refraction of second medium, and k = absorptivity. We define R, a s a "dry" (airfilled) pore and R , a s a "wet" (water-filled) pore. The experimentally observed
relationship i s R1 > R ,
Rewriting Beer's equation,
.
and solving for R1 and R , by using the values
n, = index of refraction of oil of cedar (1.515)
n, = index of refraction of a i r ( - 1.0)
n3 = index of refraction of water (1.333)
and neglecting the absorptivity (which i s small and d o e s not change significantly
for hvb c o a l s ) , then
Thus, it is c l e a r t h a t reflectance from a "dry" (air-filled) pore is nearly
10 times greater t h a n t h a t from a "wet" (water-filled) pore when using oil of cedar
a s a n immersion medium.
Coal i s relatively low i n reflectance. If 15 t o 20 percent of t h e c o a l surface i s void s p a c e (a reasonable figure for the hvCb c o a l s of Illinois i n view of
the moisture contents), it should be apparent t h a t t h e presence of "dry" or "wet"
pores c a n significantly a l t e r t h e t o t a l reflectance from the c o a l surface. On t h e
other hand, i f a solid (other than coal) with relatively high reflectance were under
evaluation, the presence of "wet" pores or "dry" pores would make little difference i n t o t a l reflection, although the density of the solid would certainly be a n
important factor influencing t h e reflectance
.
Geological Implications from ISA and Porosity Evaluations
The relatively high porosity of the Illinois hvCb c o a l s and of some of t h e
hvBb c o a l s makes t h e s e c o a l s unusual i n comparison with similar c o a l s of t h e same
rank from other coal fields. Illinois c o a l s tend t o have appreciably higher freeswelling indices, higher contents of hydrogen and volatile matter, and lower
contents of carbon and oxygen than other c o a l s with comparable inherent or equilibrium moisture contents; i n addition, Illinois c o a l s a r e relatively rich i n organic
sulfur and finely disseminated syngenetic pyrite. These characteristics a r e , of
course, typical of coal seams that were laid down under marine influence.
Petra scheck (1952) mentions a "coking coal" t h a t occurred i n a sequence
of noncoking high-volatile bituminous c o a l s of t h e Ostrava Coal Basin, e a s t e r n
Czechoslovakia. The coking c o a l w a s overlain by marine beds and had a higher
than normal sulfur content. Teichmiiller (1955) and Mackowsky (1947, 1949) disc u s s the unusual properties of t h e Katharina seam of the Ruhr Basin, West Germany.
Not only d o the roof rocks reflect marine conditions by the presence of marine
34
I L L I N O I S STATE G E O L O G I C A L SURVEY C I R C U L A R 4 9 3
f o s s i l s , but the c o a l i t s e l f i s a l s o unusual i n comparison with adjacent seams not
having marine roof sediments. The seam is characterized by a s p e c i a l kind of
clarite t h a t has few spores and other protobitumina, i s exceptionally rich i n finegrained micrinite , and ha s much fine -grained syngenetic pyrite disseminated
through i t . Vitrinite of t h i s seam p o s s e s s e s l e s s c e l l structure and reflects l e s s
light than d o other c o a l macerals. The c o a l is rich i n organic sulfur, h a s higher
than normal H/C and H/O ratios, and t e n d s t o swell more than adjacent c o a l s .
Apparently, i t i s t h i s type of c o a l t h a t we have i n Illinois.
Hilt's rule (1873) t h a t c o a l rank i n any given area i n c r e a s e s with depth is
manifested i n t h e Illinois Basin by t h e d e c r e a s e of inherent moisture-and t h u s
a l s o porosity-with depth, approximately a s shown by the rank versus depth
curve i n figure 4.
Coals now a t or near t h e surface have attained appreciably higher c o a l
rank i n southern Illinois than i n northern and western Illinois (fig. 4 ) . Thus i t
would seem t h a t t h e c o a l s of southern Illinois were once more deeply buried t h a n
t h e c o a l s of northern Illinois, probably by more than 6,000 f e e t , judging from the
rank versus depth curve i n figure 4. Since t h e former depth of burial of northern
Illinois c o a l s w a s possibly 4,000 t o 5,000 feet (Altschaeffl and Harrison, l 9 5 9 ) ,
the total depth of burial of c o a l s i n southern Illinois wa s probably some 10,OO 0
f e e t . Such a pattern of southward-increasing sediment t h i c k n e s s within the
Illinois Basin throughout Paleozoic time i s well documented (Buschbach, 1971;
Bond e t a l . , 1971).
However, t h e difference i n burial depth may have been much l e s s than t h e
full 6,000 f e e t . The somewhat unusual coalification pattern of southern Illinois,
d i s c u s s e d elsewhere by Damberger (1971, l 9 7 4 ) , may have resulted from increased
heat flow above deep-seated igneous intrusions sometime a f t e r c o a l formation, and
burial may therefore not have been t h a t much deeper in southern Illinois than i n t h e
north. Such heating a t relatively shallow depths might help explain t h e marked inc r e a s e of internal surface area after t h e removal of only small percentages of volat i l e matter during controlled heating well below t h e softening temperature of the
c o a l (see page 2 7 ) . The geothermal heating and t h e sweating out of relatively lowboiling organic matter from t h e c o a l occurred a t a time when porosity w a s still
quite high and many pore s p a c e s and connections were open, allowing the migration
and deposition i n pores of t h e s e relatively low-boiling hydrocarbons. Many of the
pores could have become blocked during t h i s s t a g e , or during further compression
of t h e c o a l .
Although t h e influence of s u c h deposited organic materials i n pores on
porosity and internal surface area may be quite pronounced i n Illinois c o a l s , we
believe t h a t t h e minimum i n the parabolic curves of rank versus porosity and internal surface area i n t h e region of high-volatile A bituminous t o medium-volatile
bituminous c o a l s (Kini, 1964; Marsh and Siemieniewska, 1965; Toda e t a l . , 1971)
i s t h e result of both t h e presence of a smaller-diameter pore system and t h e prese n c e of pore s clogged with relatively low-boiling organic components (fig. 13).
With further increa s e i n temperature during coalification, t h i s volatile organic
matter i s removed, along with other volatile matter formed by slow decomposition
of c o a l a t elevated temperatures. With increase i n rank there i s a regeneration
of a n open-pore system, e v e n under overburden pressure, a s g a s e o u s products
a r e released from t h e interior of t h e c o a l . The internal surface a r e a , a s measured
by GO, adsorption, changes with increase i n rank through t h e higher-rank c o a l s
by increasing from a minimal value of 25 t o 40 m2/g for mvb c o a l s t o more than
300 m2/g for anthracites (Thomas, Benson, and Hieftje, 1966). Nitrogen surface
area v a l u e s , however, remain small becayse t h e average pore diameters remain
small, for t h e most part l e s s than 4 t o 5 A.
I S A , M O I S T U R E , AND POROSITY OF I L L I N O I S COALS
35
SUMMARY AND CONCLUSIONS
Internal surface area measurements on c o a l s by a single-point BET method
with CQ a s t h e adsorbate provide useful data for t h e comparison of c o a l s of different,rank. In Illinois coals, t h e internal surface a r e a s correlate well with moisture contents. In t h e s e coals, both of t h e s e parameters a l s o correlate well with
porosities
Internal surface a r e a s of whole c o a l s range from 46 m2/g for hvAb c o a l s
t o 292 m2/g for hvCb c o a l s . Most of t h e large ISA values a r e derived from t h e
vitrinite portion of c o a l s : handpicked vitrains containing well over 90 volume
percent vitrinite produced ISA v a l u e s up t o 60 m2/g greater than the values from
the whole c o a l s from which the vitrains were obtained.
Illinois hvCb c o a l s have greater porosity than hvBb c o a l s , which i n turn
have greater porosity than hvAb c o a l s . Mercury-intrusion data show t h a t in Illinois hvCb c o a l s , there i s a large number of pores having diameters in the range
of 90 t o 220 A, whereas i n hvAb c o a l s there a r e virtually no pores i n t h i s s i z e
range. Most pores i n t h e l a t t e r c o a l s appear t o be smaller than 35 A. These differences may be indicative of a progressive increase i n depth of burial from the
northern t o t h e southern part of the Illinois Basin.
The hvAb c o a l s of southern Illinois actually a r e more porous than ISA
measurements with GO, indicate. This is borne out by the large increase in ISA
t h a t accompanies the l o s s of only small quantities of volatile organic matter during controlled-heating conditions. It would appear t h a t , i n addition t o having a
smaller-diameter pore structure, the hvAb c o a l s have much of their internal pore
structure plugged by relatively low-boiling organic constituents. In plots of ISA
versus various rank parameters, reported by several workers, there i s a minimum
i n ISA in t h e curve near t h e hvAb and mvb c o a l ranks, followed by a n increase i n
ISA a s rank i n c r e a s e s through t h e low-volatile bituminous (lvb) coals, the semia n t h m c i t e s , and t h e anthracites. The increase i n ISA i n t h e higher-rank c o a l s
may be a result of natural removal of relatively low-boiling organic constituents
from pores, which reopened a n already existing internal pore structure i n the
course of coalification a s burial depth and temperature increased
Differences that have been observed i n reflectance measurements on Illinois c o a l s probably c a n a l s o be attributed t o the marked differences i n the porosi t i e s of t h e s e c o a l s . Differences between reflectance values of "wet" and "dry"
prepared briquettes of hvCb c o a l s a r e large ; differences between reflectance
values of "wet" and "dry" briquettes of hvAb c o a l s a r e much somaller. This phenomenon i s likely a s s o c i a t e d with the larger pores (35 t o 220 A) that a r e present
i n hvCb c o a l s (but virtually a b s e n t i n hvAb coals) and into which water can readily
reenter during briquette preparation. We have shown t h a t reflectance under oil
immersion i s lower from "wet" (water-filled) pores that from "dry" (air-fil1ed)lpores.
.
.
ACKNOWLEDGMENTS
We a r e pleased t o acknowledge the a s s i s t a n c e of Mr. Larry Spears, a
former member of t h e Coal Section of t h e Illinois State Geological Survey, for h i s
coal-reflectance calculations and a l s o the a s s i s t a n c e of Mr. Evan Johnson i n work
i n the controlled-heating experiments. Special thanks a r e due t o M i s s Susan
Richard son of t h e Micromeritic s Instrument Corpora tion, Norcros s , Georgia, for
obtaining the mercury porosimetry d a t a . We a l s o thank Dr. Marlies Teichmkller
of the Geologische s Landesamt Nordrhein-We stfalen, Krefeld , West Germany, and
the Pea body Coal Company, Freeburg, Illinois, for supplying us with some inherent
moisture data on Illinois c o a l s
.
I L L I N O I S STATE G E O L O G I C A L SURVEY CIRCULAR 4 9 3
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Illinois State Geological Circular 493
38 p . , 15 f i g s . , 4 t a b l e s , 2500 c o p . , 1976
Urbana , Illinois 6 180 1
t
Printed by Authority of State of Illinois, C h . 127, IRS, Par. 5 8 . 2 5 .
CIRCULAR 493