Bound water in brown coal

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BOUND WATER IN BROWN COAL
Leigh M. Clemow
1,2
, W. Roy J ackson
2
, Richard Sakurovs
3
and David J . Allardice
4
1. CRC for Clean Power from Lignite, Mulgrave, Victoria.
2. Centre for Green Chemistry, Monash University, Clayton, Victoria.
3. CSIRO Division of Energy Technology, North Ryde, NSW.
4. Allardice Consulting, Vermont, Victoria.
ABSTRACT
A well known characteristic of brown coals is their strong affinity to water, which is
demonstrated by their high moisture content as mined, high monolayer water capacity
and high non-freezing water content. It is therefore important to establish how these
quantities relate to the coal structure. For several coals, monolayer water capacity
was measured by equilibration at 15% relative humidity at 30°C and the non-freezing
water content by
1
H nuclear magnetic resonance spectroscopy (nmr) and differential
scanning calorimetry (DSC). These properties have been correlated with structural
characteristics of the same coals (elemental analysis, carboxyl and phenol content
determined by aqueous titration). It was found that both of these properties and their
ratio were primarily dependent on the carboxylic acid concentration of the coal. The
phenol groups were less significant in this regard. Therefore to improve the
characteristics of brown coals, attention should be concentrated on processes which
enhance decarboxylation.
1. INTRODUCTION
As far back as 1913, Porter and Taylor (1913) showed from water sorption isotherms:
1. The impossibility to dry coal completely in an atmosphere other than a completely
dry one,
2. The extremely hygroscopic character of coal, and
3. The large changes in moisture content resulting from slight changes in relative
humidity.
These fundamental characteristics of coal have presented coal scientists and
engineers with many problems, both in drying and other applications, and have
therefore prompted many investigations into the nature of the coal-water interactions.
These investigations are further complicated by the many terms used to describe
different types of water and the lack of uniformity in definitions of each of these terms
(Winegartner 1981). The most comprehensive review of this field is by Allardice
(1991).
In the past, data for water sorption isotherms were obtained by equilibrating coal
samples over constant humidity solutions, (the “classic” desicator method of Gauger
1945 and Readett et al. 1986), or by the vacuum microbalance method (Allardice
1968, Allardice and Evans 1971). These techniques require long equilibration times.
Typical water sorption isotherms for coals of all ranks are sigmoid in shape (Allardice
and Evans 1971b; Gauger 1945; Mahajan and Walker 1971). The generally accepted
interpretation of sigmoid isotherms (Allardice 1991) is:
1. The water removed at close to saturation vapour pressure (>0.96 P/P
0
) is free or
bulk water and is contained in macropores,
2. In the convex part of the curve (0.96 to 0.5 P/P
0
) the water is contained in
capillaries, and the depression in vapour pressure can be explained by a capillary
meniscus,
3. Below P/P
0
=0.5 the loss of water is attributed to water coming from multilayers
(Figure 2) on the walls of pores, and
4. Monolayer sorption occurs in the region below 0.1 P/P
0
The number of water molecules required to produce monolayer coverage of the coal
surface can be determined from isotherms by the BET model (Brunauer et al. 1940).
Clemow (2000) showed that the monolayer water capacity for low-rank coals equates
to the water content at 14 ±2% relative humidity. Thus, in this study the samples
were equilibrated in a desiccator at 15% relative humidity to obtain an estimate of the
monolayer water capacities.
Other approaches used to probe the types of water in coal are
1
H nuclear magnetic
resonance spectroscopy (nmr) and differential scanning calorimetry (DSC).
The
1
H nmr technique can differentiate between mobile and rigid hydrogen present in
the coal. At room temperature and below, the mobility can be attributed to the water
in the coal. When the temperature is dropped below 0°C, a proportion of the mobile
signal remains, which is interpreted to mean that some water is not freezing. This
“non-freezing” water is defined as water that is still liquid at -3°C; it corresponds
approximately to water that has had its thermodynamic properties modified
appreciably by the water’s interaction with the coal surface (Barton and Lynch 1994).
DSC can also be used to determine non-freezing water by measuring the heat of
melting of the water that has frozen, and hence the non-freezing water by difference.
This technique has been used on both sub-bituminous and brown coals (Norinaga et
al. 1998).
It is thought that the coal-water interaction which gives rise to “non-freezing” water and
the unusual sorption phenomena is localized to a considerable extent at the
potentially stronger acidic functional groups which provide binding sites for the water
molecules. The stronger acidic functional groups are predominantly carboxylic acids
and phenols and exist either in the free acid or metal carboxylate/phenolate form.
Many techniques have been used to determine the number and type of these groups,
but there are still significant problems in quantitative determination and in
differentiation between carboxylic and phenolic oxygen in coal. Schafer (Schafer
1970; Schafer 1984; Schafer and Wornat 1990) developed ion exchange methods
where cations were exchanged onto the acid sites and the amount of exchanged
metal determined directly, (see Clemow et al. 2000).
2. EXPERIMENTAL
2.1 Samples
A range of low-rank coals from the Latrobe Valley, varying in lithotype, atomic H/C
ratios, inorganic constituents and as-mined moisture contents were selected.
Samples of hydrothermally dewatered Loy Yang Low Ash coal were investigated.
These were prepared by slurrying the coal with water (dry coal:water ratio of 1:2.5)
and loading the slurry (85g for a high porosity and 40 g for low porosity preparation)
into a glass liner which was inserted into a 500 ml, 316SS autoclave. The autoclave
was sealed and leak tested with hydrogen (~8Mpa) and evacuated. The autoclave
was heated to the desired temperature in 35 min, and held at that temperature for 1 h.
The autoclave was then cooled to room temperature, the gases vented, and the
solid/liquid products scraped and washed out with distilled water.
2.2 Methods
The aqueous determinations of carboxylic acid content total acidity were performed as
discussed elsewhere in these proceedings (Clemow et al. 2000), but with the addition
of 0.1ml methanol during the barium exchange step to overcome the hydrophobicity of
some samples.
Equilibrium moisture contents were determined by equilibrating 3-4 g wet coal over
saturated solutions of CaBr
2
.6H
2
O for 15% relative humidity and KNO
3
for 92%
relative humidity (Kolthoff et al. 1969) for one week at 30°C.
DSC was performed using a Perkin Elmer DSC7 instrument incorporating Thermal
Analysis System software to control all aspects of the system. The calorimeter used
was capable of measurements in the range of -150°C to 40°C. The coal (~20mg) was
placed in an aluminium sample pan and sealed. The pan was then placed in the DSC
head with a reference pan, taken to -100°C and held for 10 min. The sample was
then heated to 20°C whilst measuring the heat flow in the sample. Coals of different
known moisture contents were prepared and run. By comparing the amounts of water
detected at different temperatures with the known moisture contents, the amount of
freezing water was calculated.
Proton nuclear magnetic resonance (nmr) measurements were made at a resonance
frequency of 60 MHz using a modified Bruker SXP spectrometer and a Varian V-7305
magnet. Temperature cycling between -30°C and 10°C was performed with a Bruker
variable temperature accessory. After each step change, the temperature was held
constant for at least 10 min before the
1
H nmr transverse relaxation signals were
measured. For each experiment, approximately 100 mg of sample was used.
3. RESULTS AND DISCUSSION
The bound (non-freezing) water in the coal
was measured by two different techniques,
1
H
nmr and DSC which differ in the property of
the water observed, a molecular mobility for
1
H
nmr and a macroscopic enthalpy for DSC, but
both measure the fusion of freezable water. It
would then be expected that the results would
be the same for both techniques, but the DSC
results for the non-freezing water are ~12%
greater than those for
1
H nmr results (Figure
1). A similar phenomenon was observed by
previous workers (Barton and Lynch 1994)
and was explained by the different preparation
methods employed for the samples analysed by the two techniques. This explanation
can be disregarded, as in the current work both samples were prepared in the same
way. The difference can be explained following Plooster and Gitlin 1971 who showed
that the heat of fusion of water adsorbed as a thin film (<10nm) on silica surfaces was
lower than that of bulk water. This would cause an underestimate of the freezing
water and hence an overestimate of the non-freezing water by DSC.
The amount of strongly bound water (monolayer water capacity) in the coal can be
estimated by equilibrating at 15% relative humidity (see above). Comparison of the
monolayer water capacity so obtained with the non-freezing water (using both DSC
and
1
H nmr) (Figure 2) shows that the non-freezing water content was approximately
ten times the level of the monolayer water capacity over the range of monolayer water
capacity studied. It could be argued that this ratio is only a coincidence and that the
non-freezing water is determined by the volume of narrow pores, rather than the
amount of strongly bound water, because, in principle, a depression of freezing point
occurs in narrow pores by virtue of surface tension alone (Defay et al. 1966).
However, this will only lead to a small lowering of the freezing point, whereas the DSC
measurements show that non-freezing water as measured remains un-frozen down to
–100°C.
y =1.1229x
R
2
=0.972
0
5
10
15
20
25
30
35
40
45
50
0 10 20 30 40 50 60
1Hnmr (meq/g)
D
S
C

(
m
e
q
/
g
)
Figure 1 Relationship of Non-Freezing
Water Measured using
1
H nmr and DSC,
Compared with a Line of y=x
y =10.6x
0
10
20
30
40
50
60
70
80
90
0 2 4 6 8 10
Monolayer Capacity (g/100g coal db)
N
o
n
-
F
r
e
e
a
z
i
n
g

W
a
t
e
r

(
g
/
1
0
0
g

c
o
a
l

d
b
)
0
10
20
30
40
50
11 13 15 17
Total Oxygen Content (meq/g coal db)
N
o
n
-
F
r
e
e
z
i
n
g

W
a
t
e
r

(
m
e
q
/
g

c
o
a
l

d
b
)
Figure 2 Relationship of Non-Freezing Water to
Monolayer Capacity
Figure 3 Relationship of Non-Freezing Water to
Total Oxygen Content
Many workers have suggested that the bound water in coal is attached via hydrogen
bonding through oxygen containing functional groups (see above). When the total
oxygen is plotted against the non-freezing water content (Figure 3), a good correlation
is seen, with the non-freezing water content initially increasing rapidly, but probably
starting to approach a plateau at the highest oxygen concentrations. Not surprisingly
a similar correlation is seen when the total acidity of the coals is plotted against the
non-freezing water contents (Figure 4). The total acidity can be broken down into
phenolic and carboxylic acid contents. When the phenolic content is plotted against
the non-freezing water, there is no correlation (Figure 5).
0
10
20
30
40
50
1 3 5 7
Acidic Oxygen Content (meq/g coal db)
N
o
n
-
F
r
e
e
z
i
n
g

W
a
t
e
r

(
m
e
q
/
g

c
o
a
l

d
b
)
Desiccator (92% P/P0) nmr

0
5
10
15
20
25
30
1 2 3 4
Total Phenolic Content (meq/g coal db)
N
o
n
-
F
r
e
e
z
i
n
g

W
a
t
e
r

(
m
e
q
/
g

c
o
a
l

d
b
)
Desiccator (92% P/P0) nmr
Figure 4 Relationship of Non-Freezing Water to
Acidic Oxygen Content
Figure 5 Relationship of Non-Freezing Water to
Phenolic Content
The relationship between non-freezing water and carboxylic acid content is shown in
Figure 6. The increase in non-freezing water content correlates strongly with the
carboxylic acid content. More quantitatively, when the non-freezing water was linearly
regressed on carboxylic acid and phenolic contents (Daniel 1987), the regression
coefficient for carboxylic acid content was 10.6 ±3.7 (90% confidence limit) and for
phenolic was 0.7 ± 4.7 (90% confidence limit). This indicates that the phenolic
content had no significant relation to the non-freezing water, which was entirely
determined within the limits of error by the carboxylic acid content. (A small effect of
phenolics is not excluded). A least squares fit to the correlation in Figure 6, shows
that each carboxylic acid group has approximately ten water molecules associated
with it that do not freeze. The relationship between the equilibrium moisture content
at 92% relative humidity and carboxylic acid content (Figure 6) was similar but the
moisture content was nearly constant above a carboxylic acid content of 2.5 meq/g
coal db.
y =10.868x - 2.9291
R
2
=0.9295
0
5
10
15
20
25
30
0 1 2 3 4 5
Carboxylic Acid Content (meq/g coal db)
N
o
n
-
F
r
e
e
z
i
n
g

W
a
t
e
r

(
m
e
q
/
g

c
o
a
l

d
b
)
Desiccator (92% P/P0) nmr Linear (nmr)

y =1.1823x - 0.9495
0
2
4
6
8
10
12
0 1 2 3 4 5
Carboxylic Acid Content (meq/g coal db)
M
o
n
o
l
a
y
e
r

C
a
p
a
c
i
t
y

(
m
e
q
/
g

c
o
a
l

d
b
)
Figure 6 Relationship of Non-Freezing Water to
Carboxylic Acid Content
Figure 7 Relationship of Monolayer Water
Capacity to Carboxylic Acid Content
The monolayer water capacity was correlated in a similar way to the non-freezing
water with the various oxygen groups. Figure 7 shows the linear relationship with the
carboxylic acid groups. The monolayer water capacity was also linearly regressed on
carboxylic acid and phenolic contents. The regression coefficient for carboxylic acid
content was 1.2 ±0.7 (90% confidence limit) and for phenolic was 0.1 ±1.6 (90%
confidence limit). This suggests that the phenolic content had no significant relation to
the monolayer water capacity, although small effects of phenolics are not excluded.
There is a ~1:1 relationship of monolayer water capacity and carboxylic acids.
The non-freezing water to carboxylic acid ratio (Figure 8) increases with non-freezing
water content up to a non-freezing water content of ~7 meq/g coal db, where the ratio
plateaus at ten, supporting the previous evidence of approximately ten water
molecules being bound reasonably strongly to the coal for every carboxylic acid group.
The relationship of the monolayer water capacity to carboxylic acid ratio with the
monolayer water capacity (Figure 9) again shows that there is a ~1:1 relationship
between the ratio and monolayer water capacity, as was implied by Figure 7. There is
a tendency for the number of strongly bound (monolayer) water molecules per
carboxylic acid group to increase with monolayer water capacity, i.e. carboxylic acid
group concentration. Two factors could influence this. Firstly, at the highest
carboxylic acid concentration (~5 meq/g coal db) the carboxylic acid groups, if
uniformly distributed, will be only 0.7 nm from each other. The interaction of a water
molecule with neighbouring carboxylic acid groups and with water molecules
surrounding them may increase the energy of adsorption (see Brunauer et al. 1967).
Secondly, in polyelectrolytes such as coal, the apparent pKa value of an acid group
may differ from the intrinsic value expected for the group in isolation. This is because
of the interactions with neighbouring acidic groups more or less fixed in position
relative to it by covalent bonds (Borkovec et al. 1997). An increase in the
concentration of acid groups, other things being equal, will under some circumstances
increase their apparent strength (Martin 1964). If hydrogen bonds become stronger
as acid strength increases, this will also increase the energy of adsorption of water
molecules and hence the number of maximally bound waters per carboxylic acid
group.
0
2
4
6
8
10
12
14
0 10 20 30 40 50
Non-Freezing Water (meq/g coal db)
V
a
l
u
e

(
N
F
W
/
C
O
O
H
)

0
0.5
1
1.5
2
2.5
3
0 5 10 15
Monolayer Capacity (meq/g coal db)
V
a
l
u
e

(
M
L
C
/
C
O
O
H
)
Figure 8 Relationship of Non-Freezing Water to
Carboxylic Acid Ratio with the Non-Freezing
Water
Figure 9 Relationship of Monolayer Water
Capacity to Carboxylic Acid Ratio with the
Monolayer Water Capacity
4. CONCLUSION
The hydrophilic sites in brown coals are the strong acid (carboxylic acid) functional
groups, each of which binds approximately one water molecule. A maximum of ten
water molecules are associated significantly, though less strongly, with each strong
acid group. This does not exclude small contributions from other groups.
5. ACKNOWLEDGMENT
The authors gratefully acknowledge the financial and other support received for this
research from the CRC for Clean Power from Lignite, which is established and
supported under the Australian Government’s Cooperative Research Centres
program.
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