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Oxidation studies on coking coal related to weathering. 4. Oxygen linkages influencing the dilatometric properties and the effect of cleavage of ether linkages
Oxidation studies on coking coal related to weathering. 4. Oxygen linkages influencing the dilatometric properties and the effect of cleavage of ether linkages Helena M. Wachowska*, Fuels
Research
Ottawa,
Centre,
Ontario,
{Received
destroy
at 100°C
artificially
of
reactions. the
groups.
The
hydroxyl
This suggested links which
result
groups
that
form
as measured
of the had
through step.
carbon-carbon Indications
were
could
under vacuum.
were
bituminous
coal (433)
be reduced
Similar
the swelling
samples
properties. peroxides,
to prevent condensa-
groups and the blocking the
swelling
destroyed
properties.
by ether-type
cross-
The cleavage of these ether linkages in the Balmer resulted
in the lowering
coal indicating obtained
by blocking
the
from
measurements.
that
no significant
point and
amount
of poly-
during the low-temperature-
large
increase
condensation
After
of the melting
after cleavage were essentially
occurred
that
resulting
in dilatation
reactions
after
occurring
at
the cleavage of the ether linkages,
the hydroxyl
* Postdoctorate I:ellow, National Research Council of Gnada; permanent address: Department of Coal Chemistry. University of
July
days to partially
reagents to remove
restoring
largely
The oxidation of coal, under conditions which simulate weathering, is well known to destroy the coking properties of coking coal if carried sufficiently far. The ability of the vitrinite to fuse and to bind the inert macerals together is gradually destroyed as oxidation progresses. The intention of this work was to clarify the types of oxygen or carbon linkages that were principally responsible for the loss of the fluid character of the vitrinite as indicated by the Ruhr dilatometer. Previous workers have investigated the types of oxygen bonds created in coal at low temperature’-3. It is generally thought that during the initial stages of oxidation a carbon-oxygen complex is formed which behaves like a peroxygen, probably of the hydroperoxide type. Above a temperature of about 7O”C, the peroxygen is unstable and decomposes. As the result of this decomposition, it is possible that keto, aldehyde, and hydroxyl groups are formed, which may lead to various forms of polymerization and the formation of cross-links that have a profound effect upon the fluid properties of the coal. The study of the changes in acidity of fresh and oxidized
Vol.53,
with
in
bond formation
formation
of the dilatation
Adam Mickiewicz, Poznan, Poland
destroy
The values of these properties
the dilatation
1974,
Resources,
for four
of these functional
influence
properties
in tetrahydrofuran
heat treatment
FUEL,
and
groups were also blocked
removal
little
at low temperature.
cleavage was due to steam
212
Mines
by the Ruhr dilatometer.
days to completely
the same for the fresh and oxidized
the temperature
Energy,
in air at 85°C
coals were then treated
a large increase in dilatation.
oxidation
of
10, medium-volatile
were oxidized
The hydroxyl
the swelling
coal using potassium
merization
Department
of Balmer
for three
weathered
keto and aldehyde tion
concentrate
Moss 3 coal (535)
the swelling characteristics
oxidized
These
Branch,
1974)
of a vitrinite
and high-volatile were
Mines
Canada
4 March
Samples
Biswanath N. Nandi and Douglas S. Montgomery
groups or removing
water
by
coals suggests that oxygen, which is combined with coal during initial low-temperature oxidation, largely forms nonacidic groups4”. According to Lazarov and Angelova6’7 the thermal softening of bituminous coals on heating at 3”C/ min was associated with the cleavage of ether linkages and the formation of new hydroxyl groups. Pampuch’ emphasized the fact that the removal of cross linkages, including oxygen, between the structural units of coal increased its mobility. It has also been reported that lithium aluminium hydride reactions in tetrahydrofuran regenerated almost completely the coking properties of bituminous coal that had been previously oxidized in air’. It is suggested that an attempt to carbonize oxidized coals results in the formation of oxygen cross-linkages. Where these oxygen cross-linkages are split by effective reducing agents, sufficient motion is imparted to the coal units upon heating to allow their approach to within interatomic distances, resulting in carbon-to-carbon bonding and coke formation. On reviewing the various ways in which oxygen groups in coalsuch as peroxides, keto, aldchydc, and hydroxyl groups might either polymerize or cause the initiation of polythere appeared lo be various merization reactions, possibilities. One was that, within the four-day period of
H. M. Wachowska, B. N. Nandi and D. S. Montgomery: Oxidation studies on coking coal related to wea thering (4)
heating the coal in air at 7O”C, the reactive oxygen groups mentioned above might be formed but with little actual polymerization taking place. However, subsequent heating of such oxidized coal in a Ruhr dilatometer at 3”C/min might allow polymerization reactions to proceed at a rate such that, when the normal softening point of the coal were reached, polymerization would have advanced to such a state that no mobility was possible. If such a working hypothesis were correct, then treating the oxidized coal with various reagents to eliminate or block the active functional groups would permit such coal to regain its original contraction and dilatation. To this end, two coals were oxidized and then treated with a variety of reagents to block or eliminate the peroxide, keto, aldehyde, and hydroxyl groups and determine the effect upon the contraction and dilatation. The elimination of the oxygen reactive groups would prevent certain types of polymerization from occurring on heating and decrease the possibility of autooxidation chain reactions similar to those that occur in fats, rubber, oils, and are known to be initiated by peroxides. A limitation of the amount of the oxygen reactive groups would also reduce the hydrogen bonding forces between the units. These changes may be regarded as equivalent to a reduction in molecular weight. A second possibility was that, even at 70°C, lowtemperature cross-linking reactions occurred in the coal during the four-day oxidation period. This might be due to the formation of ether linkages or carbon-to-carbon bonds. Should the latter be the case, then the treatment of coal after oxidation with lithium aluminium hydride prior to determining the dilatation and contraction would be ineffectual in restoring the dilatation. If the oxygen cross-links on the other hand were ether linkages that can be opened by the treatment of the coal using potassium in tetrahydrofuran (THF) with naphthalene following Sternberg’s procedure”, then the dilatation should be greatly increased and both the original coal and the oxidized coal should have the same value. If, on the other hand, the cross-links were carbonto-carbon bonds then the reduction of coal using Sternberg’s method would only be partially capable of restoring the dilatation. Coals of two different levels of rank, one a high-volatile bituminous coal (Moss 3) and the other a low-volatile bituminous coal (Balmer lo), were selected for this investigation. In this work, it was essential that original coals have a large dilatation, so that this test would have the required sensitivity for the detection of the influence of small amounts of oxidation. To obtain the required level of dilatation the Balmer 10 coal was separated by sink-andfloat methods to yield a fraction floating at S.G. 1.30 and containing 80.6% vitrinite. Only this fraction was used in this investigation. For simplicity, this is referred to as Balmer 10 coal, although it represented only 12-l% of this coal. On the other hand, the Moss 3 coal used in this investigation was the whole coal. The conditions for oxidizing the coals were selected on the basis of the following considerations. Chakravarty’ found that at 85°C the rate of peroxygen formation in coal heated in air was very high and after four days reached a maximum value. This temperature and time were therefore selected as representative of the conditions giving rise to the highest concentration of peroxides. Ignasiak, Clugston and Montgomery found in previous work” that oxidation of Moss 3 coal at 100°C for 72 h caused the disaopearance of the dilatation and the almost complete elimination of tile contraction. These conditions of oxidation were therefore
chosen to represent the state after the coking properties had been completely destroyed.
EXPERIMENTAL Two coals were used for the experimental work: high-volatile Moss 3 coal (68.3% vitrinite) and a vitrinite concentrate from low-volatile Balmer 10 coal. Balmer 10 coal was crushed to minus 20 mesh B.S., separated by flotation in a medium of specific gravity 1.30 (using a mixture of pentachloroethane and hydrocarbon fraction from petroleum) and dried until free from solvent. Precautions were taken to exclude air. The fraction thus obtained contained 80.6% vitrinite. All the samples of coal were ground to pass 200 mesh B.S., dried in a vacuum at 70°C and preserved in an inert atmosphere. Samples of oxidized coals were prepared in two series: ‘(:;
oxidized at 85°C for 4 days; oxidized at 100°C for 3 days.
In both cases, the coal was spread out in a thin layer (2-3 mm thick) on trays in thermostatically controlled ovens where it was exposed to air. The proximate analyses and the main chemical composition of the coals investigated are given in Table I.
Treatment peroxides,
of coal with reagents capable of removing keto, and aldehyde groups Hydroquinone. About 10 g of oxidized coal were stirred for 24 h with 3% hydroquinone solution at room temperature. After washing with distilled water, the coal was dried in a vacuum at 70°C for 24 h. Sodium in methyl alcohol. About 7 g of oxidized coal and 1.50 ml of methyl alcohol were introduced into a 300-ml Erlenmeyer flask. The suspension was stirred continuously at room temperature under nitrogen. A total of 5 g sodium was added in small pieces during 4 h. After filtering, the coal was washed with distilled water and dried in a vacuum at 70°C for 24 h. Sodium sulphite. 4 g of oxidized coal were weighed into a round-bottom 200-ml flask. Subsequently, 100 ml of distilled water and 4 g of sodium sulphite were added. The mixture was heated under a reflux condenser at 80°C for 24 h. After cooling and filtering, the cop1 was washed with distilled water and dried in a vacuum at 70°C for 24 h. Sodium borohydride. Ten g of oxidized coal were suspended in 100 ml of distilled water in a 250-ml Erlenmeyer flask that was equipped with a condenser, thermometer, and magnetic stirrer. Then 0.5-g portions of crystalline sodium borohydride were added and the contents of the flask were stirred for 24 h. The temperature was gradually increased to 7O”C, and the total amount of sodium borohydride added was 10 g. This experiment was carried out in an atmosphere of nitrogen. The suspension was acidified with dilute hydrochloric acid to a pH value of 3. The coal, after filtering, was washed with distilled water to remove chloride and was dried in a vacuum at 70°C for 24 h. Hydrojcylamine hydrochloride. Approximately 5 g of oxidized coal were weighed into a SOO-ml flask, to which 5 g of hydroxylamine hydrochloride and 100 ml of 10% potassium hydroxide were added. The suspension was heated for 24 h at 80°C under reflux, cooled, and diluted
FUEL, 1974, Vol.53, July
213
Oxidation studies on coking coal related to weathering (4): H. M. Wachowska, B. N. Nandi and 0. S. Montgomery
Treatment of coals with potassium in tetrahydrofuran in the presence of naphthalene, according to Stern berg
with 200 ml of water. Dilute hydrochloric acid was added to bring the solution to a pH value of 3. The coal, after filtering and washing with distilled water to remove chloride, was dried in a vacuum at 50°C for 24 h.
Treatment of coal with reagents hydroxyl groups
Tetrahydrofuran (1500 ml) after initial purification by passing it through a column about 500 mm high and 40 mm wide filled with activated alumina was then refluxed over 8 g of potassium metal for 60 h under helium. A 120-ml portion of tetrahydrofuran, 4.4 g of potassium, and O-304 g of naphthalene were introduced under helium into a 250-ml Erlenmeyer flask which was provided with a glass stirring bar and ground-joint connections. The contents of the flask were stirred continuously for 24 h. A l-ml sample was removed and diluted to 50 ml with water. This aliquot of the solution was titrated potentiometrically with O-01 N hydrochloric acid. Subsequently, about 6.50 g of coal were added to the 119 ml of tetrahydrofuran remaining in the Erlenmeyer flask and stirring was continued. Every 24 h a l-ml sample of the slurry was removed and diluted with water in a 50-ml volumetric flask. The tip of the pipette was widened to permit the entry of the coal particles. The aqueous mixture was left to stand for 24 h except for occasional agitation. A lo-ml aliquot of this solution was filtered, to remove coal, which was washed with about 50 ml of distilled water. The faltered solution and washings were then titrated. In all experiments, stirring was continued for 120 h. A blank experiment using the same quantities and conditions as above but omitting the coal was made to determine the amount of naphthalene anion and solvent anion produced in the absence of coal.
capable of blocking
Barium-hydroxide blocking of acid OH groups. Approximately O-3 N solution of barium hydroxide was prepared according to Ihnatowicz”. A 2SO-ml Erlenmeyer flask, provided with a magnetic stirrer and ground-joint connections, was charged with 10 g of coal and 150 ml of barium hydroxide solution. The coal was filtered under a pressure of nitrogen, washed with distilled water and dried in a vacuum at 70°C for 24 h. Methylation of acid OH groups. A 4-g portion of coal, suspended in 100 ml of ether, was cooled in ice-water. To this was added a solution of 5 g of diazomethane in 50 ml ether. After 5 h stirring with cooling ice-water, the mixture was left to react for 48 h at 5°C below zero. Subsequently, the methylated coal was isolated and then dried in a vacuum at 70°C for 24 h. Formation of trimethylsilyl ether. The trimethylsilyl ethers of the reduced coals were prepared by refluxing 5 g of sample with 30 ml hexamethyldisilazane in 60 ml tetrahydrofuran for 5 h. After being washed with tetrahydrofuran, the product was dried in a vacuum at 70°C for 24 h.
Table 7
Proximate analyses and main chemical composition A.
Proximate
of coals used in experimental analysis (wt %I
Balmer
No.1 0
Moss No.3
fresh Moisture Volatile
Fixed carbon
2.0
24.2
Ash
B. Ultimate
fresh
1.1 matter
work
28.45
1.5
6.5
73.1
63.0
analysis (wt 96 dry basis) and swelling properties
(Ruhr
dilatometer) Moss No.3
Balmer No.10 Fresh Ash
1.55
Carbon
88.0
Oxidized
Oxidized
85°C
100°C
1.5 86-75
1-55 86.5
Fresh
Oxidized
Oxidized
85°C
100°C
6.75
6-6
81-3
6.6
80-85
80-7
Hydrogen
4.95
4.95
4.75
5.3
4.85
4.8
Sulphur
o-29
0.31
0.28
O-62
0.62
0.63
Nitrogen
1.43
1.45
1-43
1.44
1.48
1.48
3.8 4.65
5.0 4.7
5.5 5-6
4.6 5.55
5.6 6.0
5.8 6.25
70”
12””
nil**
82”
12**
27”
29**
12””
20”
28””
435**
447””
393”
Oxygen Oxygen
(by diff.) (detd.)
Dilatation
(%)
Contraction, Softening
C (%) Point, 0: (“Cl
Temp.
Max.
Temp.
Max. Cont.,
Plasticity l l
421*
Dil. ( Cl
Index,
Bc (“Cl
P.I. = C/(0,
- 8,)
396**
FUEL,
1974,
Vol.53,
July
18”’ 405””
510’
525’”
nil**
507”
510**
nil**
468*
489**
525””
456”
477**
516””
o-57*
0.53” *
0.15**
o-31
o-39**
l
Coal dried in vacuum oven 24 h at 70% * Coal, after oxidation in air-oven, cooled in inert gas prior to making up compressed coal pellet for dilatomtric
214
nil**
tests
0.16””
H. M. Wachowska,
6. N. Nandi
and D. S. Montgomery:
The coal anion mixture referred to above was separated from the potassium after 120 h. The potassium metal usually agglomerates to form a ball. The actual separation was accomplished by applying helium pressure to the coal anion solution and blowing it over into 500 ml of distilled water. The reaction flask was washed with 100 ml of tetrahydrofuran and the washings were blown over into the distilled water. The coal suspension in the water was neutralized with dilute hydrochloric acid to a pH value of 3, stirred for 5 h and allowed to stand for 24 h for complete hydrolysis. The coal product was recovered by centrifuging in 200-ml closed bottles under nitrogen and was washed with distilled water until the sample was free of chloride ion. The coal was dried at 70°C in a vacuum for 16 h.
Hydroxyl determination in coal following cleavage of ether linkages A 500-mg portion of the reaction product was treated as described below, following the method of Knotnerus13. The sample was placed in a 50-ml flask together with exactly 5 ml of a solution of 2-5 g acetic anhydride made up to 50 ml with pyridine. The flask was fitted with condenser and protected by ascarite against moisture and carbon dioxide. The reaction mixture was then refluxed at 115°C for 24 h. After cooling, the contents of the flask were transferred quantitatively to a beaker with 15 ml benzene and 20 ml water. After the decomposition of the excess acetic anhydride, the acetic acid formed was titrated potentiometrically, in the vigorously stirred two-phase system, with about 0.1 N sodium hydroxide. The end-point was sharp at about pH 9. Exactly the same procedure was followed on a blank sample consisting of 5 ml of a solution of 2.5 g acetic anhydride made up to 50 ml with pyridine.
Dilatation test Balmer 10 coal.
Tab/e 2
Effect
of removing
Oxidation
studies on coking coal related
Table I presents the proximate analysis, the ultimate analysis, and the swelling properties as measured in the Ruhr dilatometer of the coals used in this investigation. The change in the oxygen content caused by the oxidation of the Balmer and Moss 3 coals is also shown, as is the effect of this increase in oxygen content upon the dilatation, contraction, plasticity index, softening point, temperature of maximum dilatation, and temperature of maximum contraction. The reduction in the dilatation andcontraction with increasing oxidation of the coal, as well as the elevation of the softening point and temperature of maximum contraction, show that the physical properties of both these coals have responded to oxidation in the normal manner. After oxidation at 85”C, both the Balmer and the Moss 3 coals have a dilatation of 12%. At this level of oxidation, the dilatation should be a sensitive indicator of small changes in the number and types of oxygen functional groups. The contraction of 29 and 28% of these two coals after this mild oxidation indicates that the coals still soften and contract to fill the interstitial space between the original coal particles that are charged to the Ruhr dilatometer in the form of a small cylindrical briquette. When these two coals were oxidized at 100°C the contraction was only 12 and 18% respectively, suggesting that the softening of these coals was restricted. To test the hypothesis that the elimination of the active oxygen functional groups would permit such oxidized coal to regain its original contraction and dilatation, radical acceptors such as hydroquinone were added to the coal to prevent the occurrence of chain reactions. The reduction of peroxides to the corresponding alcohols was carried out with the following reducing agents: sodium in methyl
peroxide,
keto, aldehyde,
and hydroxy max.
groups Temp.of
max.
Dilatation
Contraction
Softening
dilatation
contraction
Plasticity
(%I
(%)
point (“C)
(“Cl
1°C)
Index
Balmer
10 oxidized
100°C
-
nil
12
447
nil
525
0.15
Hydroquinone
nil
27
435
nil
513
0.34
Sodium
in CH30H
nil
12
450
nil
525
0.16
Sodium
sulphite
nil
12
447
nil
525
0.15
Sodium
borohydride
nil
10
456
nil
525
0.14
nil
32
447
nil
525
0.33
nil
12
456
nil
527
0.16
_
12
29
435
525
Hydroquinone
10
23
438
528
489 495 486 483 494 491
0.53 0.40 0.52 0.43 0.46 0.52
Hydroxylamine Barium
hydrochloride
hydroxide
Balmer
10 oxidized
85°C
Sodium
in CH30H
11
25
438
525
Sodium
sulphite
10
22
432
519
Hydroxylamine Barium
hydrochloride
hydroxide
0
26
438
522
12
28
438
522
12
28
396
510
477
0.34
10
31
408
513
477
0.44
28
408
522
492
0.33
Moss 3 oxidized
_ Barium
hydroxide
Hexamethyldisilazane
(4)
RESULTS AND DISCUSSION
Temp.of Treatment
to weathering
-2
85’C
FUEL,
1974,
Vol.53,
July
215
Oxidation studies on coking coal related to weathering 14): H. M. Wachowska, B. N. Nandi and D. S. Montgomery
alcohol, sodium sulphite, and sodium borohydride. The blocking of the hydroxyl groups of the oxidized coals was achieved by treating them with barium hydroxide and hexamethyldisilazane. To determine whether the oxygen in keto and aldehyde groups can affect the dilatation, the oxidized coals were treated with hydroxylamine hydrochloride. The results of these tests are presented in Table 2 and they show that the dilatation of the coal could not be restored by this type of chemical treatment. Though treatment with hydroquinone and hydroxylamine slightly improved the contraction, the results as a whole must be viewed as negative for neither the hydroquinone nor the hydroxylamine treatment resulted in any increase in the dilatation in the Balmer 10 coal oxidized at 85°C. There was no clear evidence that the removal or blocking of the peroxide, keto, aldehyde and hydroxyl groups significantly improved the plastic properties, so it was concluded that polymerization reactions, caused by oxygen functional groups during heating in the Ruhr dilatometer test, were not the principal cause of the reduction of dilatation in coal oxidized below 100°C. Kruczko14 suggested that during the thermal treatment of slightly oxidized coal, the number of free radicals increased as a result of the decomposition of hydroperoxides. Peroxide radicals were assumed to abstract hydrogen atoms from the polymer, creating radical sites which can then combine to form carbon-carbon cross-links. The decrease of the dilatation of oxidized coals may also be explained by postulating the formation of non-reactive oxygen linkages during the low-temperature oxidation of coals. These linkages, possibly ether-type, are considered to result from the thermal decomposition of peroxides and hydroxyl groups at 85 and 100°C. It is known that diary1 or aryl-alkyl ether bonds in coal can be cleaved by treatment with potassium in tetrahydrofuran (THF) in the presence of a small amount of naphthalene as described by Sternberg”. This provided an opportunity to distinguish between carbon-oxygen crosslinks and carbon-carbon cross-links. According to Sternberg, coal treated with potassium in THF in the presence of a small amount of naphthalene reacts according to the following scheme to produce coal anion: K” + C1uH8 + [CluHs] ~ + K+ Coal + [CloHs] - + CluH8 t [Coal lr
/
I
I
Stirring
I
time
Figure 1 Acidity versus stirring time (I) Anion formation in the absence of coal (2) Anion formation in the presence of coal
216
FUEL,
1974,
Vol.53,
July
I
(h
1
1
I
The naphthalene is required as an electron-transfer agent because neither potassium nor coal is soluble in THF. During this reaction, in addition to coal anion, naphthalene anion and solvent anion are produced. After hydrolysis with water, each anion gives an equivalent number of hydroxyl ions, which may be determined by titration with acid. The number of ionic charges per 100 carbon atoms in coal was calculated by subtracting the number of equivalent hydroxyl ions formed by the naphthalene and solvent anions from the total number of equivalents of hydroxyl ions (Figure 1). Table3 shows that, when coat was treated in this manner, the number of charges per 100 carbon atoms was significantly larger than the total number of oxygen atoms per 100 carbon atoms. The large increase in OH groups resulting from the cleavage of the ether linkages is shown in Table 4. It was assumed that many of the anionic charges not associated with the ether-oxygen linkages are located in the aromatic-ring systems and that after hydrolysis these are protonated, thus increasing the hydrogen-to-carbon ratio. Table3
Number
of charges associated
atoms C after treatment
with
K/TH
Number Number
Balmer
10 fresh
Balmer
10 oxi-
dized IOO’C Moss 3 fresh
of
with
coal per 100
F/naphthalene of
for 120 h Number
of
oxygen
oxygen
charges
atoms (by
atoms
per 100
difference)
(determined)
atoms C
per 100
per 100
(average)
atoms C
atoms C
IO.54
3.2,
3.95
IO.57
4.77
485
8.74
3.92
5.12
Table 4 also shows the elemental composition and the weight % of hydroxyl and carbonyl oxygen groups of the fresh Balmer 10 coal and the same coal after treatment with potassium in tetrahydrofuran. When the oxygen linkages of this coal had been cleaved by this treatment, the hydroxyl groups so produced were blocked by different groups by treating successive samples with diazomethane, barium hydroxide, and hexamethyldisilazane following the procedures described in the experimental part of this paper. This was done to facilitate the interpretation of the dilatation measurements. The hydroxyl groups of the fresh coals were determined by acetylation in pyridine according to Blom”. A modification of this procedure was employed for the estimation of hydroxyl groups in the coals previously treated with potassium in tetrahydrofuran, because of the increased solubility of these reduced coals in pyridine. This procedure was developed for asphaltic bitumens13. Table 4 shows that the Balmer 10 coal after oxidation at 100°C has a higher hydroxyl content following the cleavage reaction than the original coal after treatment with potassium in THF, and that the proportion of the oxygen linkages, added by oxidation, that give rise to OH groups on cleavage is relatively small. The carbonyl content was measured by reaction with hydroxylamine hydrochloride16. No carboxyl groups were found to be present using the ion-exchange method
H. M. Wachowska, 5. N. Nandi and 0. S. Montgomery: Oxidation studies on coking coal related to weathering (4) Table 4
Changes in elementary
furan and products
of blocking
composition
and forms of combined
oxygen
due to treatment
with potassium
in tetrahydro-
coals (dry basis)
0
0
C
H
N
S
(diff.)
(detd.)
Ash
DOH (%I
Cc0 (%I
H/C
88.02
4.96
1.43
0.29
3.77
464
1.5
0.93
0.30
0.66
87.0g
5.56
1.35
0.26
4.50
5.92
1.2
2.69
0.29
0.76
f diazomethane
86.88
5.63
1.22
0.24
4.83
-
1.2
0.33
-
0.77
+ barium
86.26
5.32
I.07
0.23
4.95
-
2.2
0.36
-
0.73
84.2,
5.g3
0.93
0.24
4.28
-
4.4
0.09
-
0.83
86.50
4.77
1.43
0.28
5.4,
5.47
1.5
0.98
0.30
0.65
86.45
5.31
1.38
0.26
5.3*
6.32
l-3
2.98
0.30
0.73
81.30
5.2g
1.44
0.62
4.58
5’56
6.8
0.86
0.23
0.77
80.86
5.44
1.18
0.34
6.68
5.95
5.5
2.92
0.20
0.80
Coal Balmer
10 fresh
+ postassium/TH
F/naphthalene
hydroxide
+ hexamethyldisilazane Balmer
10 oxidized
+ potassium/TH
100°C F/naphthalene
Moss 3 fresh + potassium/TH
F/naphthalene
+ diazomethane
80.45
5.80
1.20
0.32
6.73
-
5.5
0.49
-
0.85
+ barium
80.34
5.31
1.29
0.39
6.83
-
5.8
0.58
-
0.78
79.65
6.24
1.23
0.39
5.40
-
7.1
0.00
-
0.93
hydroxide
+ hexamethyldisilazane
Tab/e 5
Effect
of blocking
hydroxyl
groups - dilatation
test Temp.of
Reagent Balmer
10 fresh
+ potassium/THF/naphthalene + barium
hydroxide
dilatation
contraction
PI as&Y ty
(%I
(%I
point
(“C)
(“Cl
Indepr
468 405 439 436 405 525 420 441 456 405 423 402 429
(“C)
70
27
421
510
260
15
375
525
16
378
495
171
17
402
492
+ hexamethyldisilazane
154
7
363
495
nil
12
447
nil
19
378
510
IOO’C
+ potassium/THF/naphthalene + barium
hydroxide
Moss 3 fresh + potassium/THF/naphthalene t barium
hydroxide
max
Softening
155
10 oxidized
Temp.of
Contraction
+ diazomethane Balmer
max.
Dilatation
254 157
14
375
498
82
20
393
507
222
16
351
495
136
20
363
489
+ diazomethane
158
18
369
501
+ hexamethyldisilazane
130
23
357
480
employing calcium acetate”. The quantity of carbonyl oxygen before and after oxidation at 100°C was essentially the same, hence it was concluded that this linkage did not play a significant part in affecting the dilatation. If the dilatation curves in Figures 2 and 3 and the data in Table 5 are compared for the fresh and oxidized Balmer coal before and after treatment with potassium in THF, it will be noted that the softening point was lowered from 421 to 375°C for the fresh coal as compared with 447 to 378°C for the coal oxidized at 100°C. This indicates that the molecular weights were lowered in both cases to the extent that the softening points had essentially the same value; it was attributed to the severing of the ether-oxygen linkages, removing many of the oxygen functional groups and introducing some additional hydrogen into the aromatic-ring system. There was no evidence of a difference in softening point of the reduced coals that might be ascribed to carbon-carbon bond formation during the oxidation step. The larger decline in the softening point of the oxidized coal after treatment
0.57 0.50 0.26 0.50 0.16 0.15 0.45 o-21 0.31 0.29 0.33 0.54 0.31
with potassium in THF was attributed to the fact that the softening point of this coal had been elevated by oxidation, and that after the oxygen-ether linkages were opened the softening point returned to the same value as that of the fresh coal after treatment with potassium in THF. Comparing the dilatation of the fresh and oxidized Balmer coal after treatment with potassium in tetrahydrofuran, it was evident that the dilatation after this treatment was very large: that the coal had been made much more fluid, was entraining a considerable amount of pyrolysis gas and of the additional steam that was probably being generated from the condensation of the OH groups. Such behaviour would be consistent with the opening of the carbonPoxygen bonds. Again there was no evidence that the oxidation of the Balmer coal produced carbon-carbon cross-links, which would have tended to diminish the dilatation of the oxidized Balmer coal after treatment with potassium in THF. The lowering of the softening point of the Moss 3 coal and the large dilatation produced after the treatment of this coal with potassium in tetrahydrofuran followed the same
FUEL,
1974,
Vol.53,
July
217
Oxidation studies on coking coal related to weathering (4): H. M. Wachowska, 8. N. Nandi and D. S. Montgomery
-
300
250
250
200
200
150
2 -
150
100
.-: 2 ;; 0
100
50
50
0
0
450
L80
Temperature
(‘C)
LZO
Figure 2 Dilatation versus (1) Original 121 Original after treatment (3) Original after treatment with diazomethane (41 Original after treatment with hexamethyldisilazane (51 Original after treatment ,vvith barium hydroxide
temperature,
/
I
510
540
and blocking
with
K/THF/naphthalene
and blocking
with
K/THF/naphthalene
and blocking
pattern as was observed with the Bahner coal. To test the hypothesis regarding additional steam generation during dilatation, Balmer and Moss 3 coals were treated with potassium in THF and the resulting coals were subsequently treated with barium hydroxide, diazomethane, and hexamethyldisilazane to replace the OH groups with other groups of greater thermal stability. Figures 2, 3, and 4 as well as Table 5 show that in each case the blocking of the OH groups reduced the dilatation but had comparatively little effect on the softening temperature. During the investigations of the cleavage of the ether linkages with potassium in THF, it was observed that heating the reaction product at 70°C under vacuum for four days led to markedly lower values of the dilatation than did vacuum drying at 70°C for I6 h. The effect of prolonged heating of Moss 3 coal is shown in Figure 5 and Table 6. The decrease in the value of the dilatation may be explained by assuming that the hydroxyl groups undergo condensation
Tab/e 6
Time
Moss
3 coal after treatment
(h)
16 72 96
218
FUEL,
1974,
I
I
I
I
I
I
390
L20
450
L80
510
510
Temperature
Balmer IO coal
with KiTHFinaphthalene with K/THF/naphthalene
I 360
with potassium
in THF.
f ‘C 1
Dilatation versus temperature, Balmer IO coal figure 3 (1) Oxidized 1 OO°C 72 h (2) Oxidized lC8C 72 h after treatment with KITHFinaphthalene (31 Oxidized IOO’C 72 h after treatment with KiTHFlnaphthalene and blocking with barium hydroxide
to form ether iinkages at 70°C and that the water is removed under the vacuum conditions. This might increase the molecular weight of the coal and reduce the amount of gas available to produce a large dilatation A somewhat similar phenomenon has been observed during the drying of humic acids to form anhydrides”. If it is assumed that the dilatation is governed to a considerable extent by the steam production, then the dilatation associated with coal after blocking the OH groups should correspond to the vaIue obtained as the result of exhaustive heating of the cleaved coal at 70°C under vacuum which again eliminates many of the OH groups. It appears that prolonged vacuum drying at 70°C yields a product which has a lower dilatation than the product which has had OH groups removed by blocking. This difference may be due to the average molecular weight of the condensation products being higher than that of the products produced by blocking the OH groups.
Dried in vacua at 7O”C, for different
periods of time
OOH (wt %, dry basis)
Dilatation
Contraction
Softening
Temp.of max. dilatation
Temp.of max. contraction
(%)
(%I
point
(“Cl
i”C)
Index -
2.92 2.02 _
222 108 90
16 19 14
351 351 351
495 489 480
405 429 429
0.29 0.24 0.17
Vo1.53,
July
(“Cl
Plasticity
H. M. Wachowska, 300
--
B. M. Nandi and 0. S. Montgomery:
Oxidation
studies on coking coal related
to weathering
(41
7-1-7
250
200
2 _
150
.-: ;; 2 0
100
50
0
--II
390
420 Temperature
Figure 4 Dilatation versus (1) Original (2) Original after treatment (3) Original after treatment with diazomethane (4) Original after treatment with hexamethyldisilazane (5) Original after treatment with barium hydroxide
temperature,
480
L50 I’C
510
360
1
480
and blocking
with
K/THF/naphthalene
and blocking
with
K/THF/naphthalene
and blocking
Dilatation versus temperature, Moss 3 coal (1) Original dried 16 h (2) Original after treatment with K/THF/naphthalene in vacuum at 70°C dried 72 h (3) Original after treatment with K/THF/naphthalene in vacuum at 70°C dried 96 h (4) Original after treatment with K/THF/naphthalene in vacuum at 7OcC (5) Original after treatment with K/THF/naphthalene and blocking with barium hydroxide dried 96 h in vacuum at 70°C
ACKNOWLEDGEMENTS The authors would like to express their appreciation to A. T. Lloyd for fractionating the Balmer coal according to specific gravity and to S. E. Nixon as well as the personnel of the coal analytical laboratory for many analytical services. Special thanks are due to Dr H. W. Sternberg of the U.S. Bureau of Mines for many helpful suggestions in carrying out the coal reduction procedure in K + THF.
9 10 11 12
REFERENCES Chakravarty, aakravarty,
450 [‘Cl
Figure 5
Moss 3 coal
with K/THF/naphthalene with K/THF/naphthalene
420 390 Temperature
S. L. J. Mines Metals Fuels 1960, 1 S. L. J. Mines Metals Fuels 1960,lO
Von de Vries, H. A. W., Bokhoven, C. and Dormans, H. N. M. Brennst.-Chem. 1969,10,289 Maher, T. P., Harris, J. M. and Yohe, G. R. Rep. Invest. Illinois State Geol. Surv. No.212, 1959
13 14 15 16 17
Brooks, J. D. and Maher, T. P. Fuel, Land. 1957,36,51 Lazarov, L. and Angelova, G. Fuel, Lond. 1968,47, 333 Lazarov, L. and Angelova, G. Fuel, Lond. 1468, 47, 342 Pampuch, R. Rep. Polish inst. Mining No.103, 1951; Dryden, 1. G. C. and Griffith, M. BCURA Monthly Bull. 1954,18,71 Orchin, M., Golumbic, C., Anderson, J. E. and Starch, H. H. Bull. U.S. Bur. Mines No.505, 1952, p 13 Sternberg, H. W., Delle Donne, C. L., Pantages, P., Moroni, E. C. and Markby, R. E. Fuel, Lond. 1971,50,432 lgnasiak, B. S., Clugston, D. M. and Montgomery, D. S. Fuel, Lond. 1972,51,76 Ihnatowicz, A. Komunikat Gliiwnego Instytutu Gdrnictwa No.125, 1952 Knotnerus, J. J. Inst. Petrol. 1956, 42, 355 Kruczko, A. Khimiya tverd. Topl. 1971, No.5, 5 Blom, L., Edelhausen, L. and van Krevelen, D. W. Fuel, Lond. 1957,36, 135 Von Kroger, C., Darsow, G. and h’uhr, K. Erdijl u. Kohle 1965,9,701 Wood, J. C., Moschopedis, S. E. and Elofson, R. M. Fuel, Land. 1961,40,193
FUEL, 1974, Vol.53, July
219
Research
Ottawa,
Centre,
Ontario,
{Received
destroy
at 100°C
artificially
of
reactions. the
groups.
The
hydroxyl
This suggested links which
result
groups
that
form
as measured
of the had
through step.
carbon-carbon Indications
were
could
under vacuum.
were
bituminous
coal (433)
be reduced
Similar
the swelling
samples
properties. peroxides,
to prevent condensa-
groups and the blocking the
swelling
destroyed
properties.
by ether-type
cross-
The cleavage of these ether linkages in the Balmer resulted
in the lowering
coal indicating obtained
by blocking
the
from
measurements.
that
no significant
point and
amount
of poly-
during the low-temperature-
large
increase
condensation
After
of the melting
after cleavage were essentially
occurred
that
resulting
in dilatation
reactions
after
occurring
at
the cleavage of the ether linkages,
the hydroxyl
* Postdoctorate I:ellow, National Research Council of Gnada; permanent address: Department of Coal Chemistry. University of
July
days to partially
reagents to remove
restoring
largely
The oxidation of coal, under conditions which simulate weathering, is well known to destroy the coking properties of coking coal if carried sufficiently far. The ability of the vitrinite to fuse and to bind the inert macerals together is gradually destroyed as oxidation progresses. The intention of this work was to clarify the types of oxygen or carbon linkages that were principally responsible for the loss of the fluid character of the vitrinite as indicated by the Ruhr dilatometer. Previous workers have investigated the types of oxygen bonds created in coal at low temperature’-3. It is generally thought that during the initial stages of oxidation a carbon-oxygen complex is formed which behaves like a peroxygen, probably of the hydroperoxide type. Above a temperature of about 7O”C, the peroxygen is unstable and decomposes. As the result of this decomposition, it is possible that keto, aldehyde, and hydroxyl groups are formed, which may lead to various forms of polymerization and the formation of cross-links that have a profound effect upon the fluid properties of the coal. The study of the changes in acidity of fresh and oxidized
Vol.53,
with
in
bond formation
formation
of the dilatation
Adam Mickiewicz, Poznan, Poland
destroy
The values of these properties
the dilatation
1974,
Resources,
for four
of these functional
influence
properties
in tetrahydrofuran
heat treatment
FUEL,
and
groups were also blocked
removal
little
at low temperature.
cleavage was due to steam
212
Mines
by the Ruhr dilatometer.
days to completely
the same for the fresh and oxidized
the temperature
Energy,
in air at 85°C
coals were then treated
a large increase in dilatation.
oxidation
of
10, medium-volatile
were oxidized
The hydroxyl
the swelling
coal using potassium
merization
Department
of Balmer
for three
weathered
keto and aldehyde tion
concentrate
Moss 3 coal (535)
the swelling characteristics
oxidized
These
Branch,
1974)
of a vitrinite
and high-volatile were
Mines
Canada
4 March
Samples
Biswanath N. Nandi and Douglas S. Montgomery
groups or removing
water
by
coals suggests that oxygen, which is combined with coal during initial low-temperature oxidation, largely forms nonacidic groups4”. According to Lazarov and Angelova6’7 the thermal softening of bituminous coals on heating at 3”C/ min was associated with the cleavage of ether linkages and the formation of new hydroxyl groups. Pampuch’ emphasized the fact that the removal of cross linkages, including oxygen, between the structural units of coal increased its mobility. It has also been reported that lithium aluminium hydride reactions in tetrahydrofuran regenerated almost completely the coking properties of bituminous coal that had been previously oxidized in air’. It is suggested that an attempt to carbonize oxidized coals results in the formation of oxygen cross-linkages. Where these oxygen cross-linkages are split by effective reducing agents, sufficient motion is imparted to the coal units upon heating to allow their approach to within interatomic distances, resulting in carbon-to-carbon bonding and coke formation. On reviewing the various ways in which oxygen groups in coalsuch as peroxides, keto, aldchydc, and hydroxyl groups might either polymerize or cause the initiation of polythere appeared lo be various merization reactions, possibilities. One was that, within the four-day period of
H. M. Wachowska, B. N. Nandi and D. S. Montgomery: Oxidation studies on coking coal related to wea thering (4)
heating the coal in air at 7O”C, the reactive oxygen groups mentioned above might be formed but with little actual polymerization taking place. However, subsequent heating of such oxidized coal in a Ruhr dilatometer at 3”C/min might allow polymerization reactions to proceed at a rate such that, when the normal softening point of the coal were reached, polymerization would have advanced to such a state that no mobility was possible. If such a working hypothesis were correct, then treating the oxidized coal with various reagents to eliminate or block the active functional groups would permit such coal to regain its original contraction and dilatation. To this end, two coals were oxidized and then treated with a variety of reagents to block or eliminate the peroxide, keto, aldehyde, and hydroxyl groups and determine the effect upon the contraction and dilatation. The elimination of the oxygen reactive groups would prevent certain types of polymerization from occurring on heating and decrease the possibility of autooxidation chain reactions similar to those that occur in fats, rubber, oils, and are known to be initiated by peroxides. A limitation of the amount of the oxygen reactive groups would also reduce the hydrogen bonding forces between the units. These changes may be regarded as equivalent to a reduction in molecular weight. A second possibility was that, even at 70°C, lowtemperature cross-linking reactions occurred in the coal during the four-day oxidation period. This might be due to the formation of ether linkages or carbon-to-carbon bonds. Should the latter be the case, then the treatment of coal after oxidation with lithium aluminium hydride prior to determining the dilatation and contraction would be ineffectual in restoring the dilatation. If the oxygen cross-links on the other hand were ether linkages that can be opened by the treatment of the coal using potassium in tetrahydrofuran (THF) with naphthalene following Sternberg’s procedure”, then the dilatation should be greatly increased and both the original coal and the oxidized coal should have the same value. If, on the other hand, the cross-links were carbonto-carbon bonds then the reduction of coal using Sternberg’s method would only be partially capable of restoring the dilatation. Coals of two different levels of rank, one a high-volatile bituminous coal (Moss 3) and the other a low-volatile bituminous coal (Balmer lo), were selected for this investigation. In this work, it was essential that original coals have a large dilatation, so that this test would have the required sensitivity for the detection of the influence of small amounts of oxidation. To obtain the required level of dilatation the Balmer 10 coal was separated by sink-andfloat methods to yield a fraction floating at S.G. 1.30 and containing 80.6% vitrinite. Only this fraction was used in this investigation. For simplicity, this is referred to as Balmer 10 coal, although it represented only 12-l% of this coal. On the other hand, the Moss 3 coal used in this investigation was the whole coal. The conditions for oxidizing the coals were selected on the basis of the following considerations. Chakravarty’ found that at 85°C the rate of peroxygen formation in coal heated in air was very high and after four days reached a maximum value. This temperature and time were therefore selected as representative of the conditions giving rise to the highest concentration of peroxides. Ignasiak, Clugston and Montgomery found in previous work” that oxidation of Moss 3 coal at 100°C for 72 h caused the disaopearance of the dilatation and the almost complete elimination of tile contraction. These conditions of oxidation were therefore
chosen to represent the state after the coking properties had been completely destroyed.
EXPERIMENTAL Two coals were used for the experimental work: high-volatile Moss 3 coal (68.3% vitrinite) and a vitrinite concentrate from low-volatile Balmer 10 coal. Balmer 10 coal was crushed to minus 20 mesh B.S., separated by flotation in a medium of specific gravity 1.30 (using a mixture of pentachloroethane and hydrocarbon fraction from petroleum) and dried until free from solvent. Precautions were taken to exclude air. The fraction thus obtained contained 80.6% vitrinite. All the samples of coal were ground to pass 200 mesh B.S., dried in a vacuum at 70°C and preserved in an inert atmosphere. Samples of oxidized coals were prepared in two series: ‘(:;
oxidized at 85°C for 4 days; oxidized at 100°C for 3 days.
In both cases, the coal was spread out in a thin layer (2-3 mm thick) on trays in thermostatically controlled ovens where it was exposed to air. The proximate analyses and the main chemical composition of the coals investigated are given in Table I.
Treatment peroxides,
of coal with reagents capable of removing keto, and aldehyde groups Hydroquinone. About 10 g of oxidized coal were stirred for 24 h with 3% hydroquinone solution at room temperature. After washing with distilled water, the coal was dried in a vacuum at 70°C for 24 h. Sodium in methyl alcohol. About 7 g of oxidized coal and 1.50 ml of methyl alcohol were introduced into a 300-ml Erlenmeyer flask. The suspension was stirred continuously at room temperature under nitrogen. A total of 5 g sodium was added in small pieces during 4 h. After filtering, the coal was washed with distilled water and dried in a vacuum at 70°C for 24 h. Sodium sulphite. 4 g of oxidized coal were weighed into a round-bottom 200-ml flask. Subsequently, 100 ml of distilled water and 4 g of sodium sulphite were added. The mixture was heated under a reflux condenser at 80°C for 24 h. After cooling and filtering, the cop1 was washed with distilled water and dried in a vacuum at 70°C for 24 h. Sodium borohydride. Ten g of oxidized coal were suspended in 100 ml of distilled water in a 250-ml Erlenmeyer flask that was equipped with a condenser, thermometer, and magnetic stirrer. Then 0.5-g portions of crystalline sodium borohydride were added and the contents of the flask were stirred for 24 h. The temperature was gradually increased to 7O”C, and the total amount of sodium borohydride added was 10 g. This experiment was carried out in an atmosphere of nitrogen. The suspension was acidified with dilute hydrochloric acid to a pH value of 3. The coal, after filtering, was washed with distilled water to remove chloride and was dried in a vacuum at 70°C for 24 h. Hydrojcylamine hydrochloride. Approximately 5 g of oxidized coal were weighed into a SOO-ml flask, to which 5 g of hydroxylamine hydrochloride and 100 ml of 10% potassium hydroxide were added. The suspension was heated for 24 h at 80°C under reflux, cooled, and diluted
FUEL, 1974, Vol.53, July
213
Oxidation studies on coking coal related to weathering (4): H. M. Wachowska, B. N. Nandi and 0. S. Montgomery
Treatment of coals with potassium in tetrahydrofuran in the presence of naphthalene, according to Stern berg
with 200 ml of water. Dilute hydrochloric acid was added to bring the solution to a pH value of 3. The coal, after filtering and washing with distilled water to remove chloride, was dried in a vacuum at 50°C for 24 h.
Treatment of coal with reagents hydroxyl groups
Tetrahydrofuran (1500 ml) after initial purification by passing it through a column about 500 mm high and 40 mm wide filled with activated alumina was then refluxed over 8 g of potassium metal for 60 h under helium. A 120-ml portion of tetrahydrofuran, 4.4 g of potassium, and O-304 g of naphthalene were introduced under helium into a 250-ml Erlenmeyer flask which was provided with a glass stirring bar and ground-joint connections. The contents of the flask were stirred continuously for 24 h. A l-ml sample was removed and diluted to 50 ml with water. This aliquot of the solution was titrated potentiometrically with O-01 N hydrochloric acid. Subsequently, about 6.50 g of coal were added to the 119 ml of tetrahydrofuran remaining in the Erlenmeyer flask and stirring was continued. Every 24 h a l-ml sample of the slurry was removed and diluted with water in a 50-ml volumetric flask. The tip of the pipette was widened to permit the entry of the coal particles. The aqueous mixture was left to stand for 24 h except for occasional agitation. A lo-ml aliquot of this solution was filtered, to remove coal, which was washed with about 50 ml of distilled water. The faltered solution and washings were then titrated. In all experiments, stirring was continued for 120 h. A blank experiment using the same quantities and conditions as above but omitting the coal was made to determine the amount of naphthalene anion and solvent anion produced in the absence of coal.
capable of blocking
Barium-hydroxide blocking of acid OH groups. Approximately O-3 N solution of barium hydroxide was prepared according to Ihnatowicz”. A 2SO-ml Erlenmeyer flask, provided with a magnetic stirrer and ground-joint connections, was charged with 10 g of coal and 150 ml of barium hydroxide solution. The coal was filtered under a pressure of nitrogen, washed with distilled water and dried in a vacuum at 70°C for 24 h. Methylation of acid OH groups. A 4-g portion of coal, suspended in 100 ml of ether, was cooled in ice-water. To this was added a solution of 5 g of diazomethane in 50 ml ether. After 5 h stirring with cooling ice-water, the mixture was left to react for 48 h at 5°C below zero. Subsequently, the methylated coal was isolated and then dried in a vacuum at 70°C for 24 h. Formation of trimethylsilyl ether. The trimethylsilyl ethers of the reduced coals were prepared by refluxing 5 g of sample with 30 ml hexamethyldisilazane in 60 ml tetrahydrofuran for 5 h. After being washed with tetrahydrofuran, the product was dried in a vacuum at 70°C for 24 h.
Table 7
Proximate analyses and main chemical composition A.
Proximate
of coals used in experimental analysis (wt %I
Balmer
No.1 0
Moss No.3
fresh Moisture Volatile
Fixed carbon
2.0
24.2
Ash
B. Ultimate
fresh
1.1 matter
work
28.45
1.5
6.5
73.1
63.0
analysis (wt 96 dry basis) and swelling properties
(Ruhr
dilatometer) Moss No.3
Balmer No.10 Fresh Ash
1.55
Carbon
88.0
Oxidized
Oxidized
85°C
100°C
1.5 86-75
1-55 86.5
Fresh
Oxidized
Oxidized
85°C
100°C
6.75
6-6
81-3
6.6
80-85
80-7
Hydrogen
4.95
4.95
4.75
5.3
4.85
4.8
Sulphur
o-29
0.31
0.28
O-62
0.62
0.63
Nitrogen
1.43
1.45
1-43
1.44
1.48
1.48
3.8 4.65
5.0 4.7
5.5 5-6
4.6 5.55
5.6 6.0
5.8 6.25
70”
12””
nil**
82”
12**
27”
29**
12””
20”
28””
435**
447””
393”
Oxygen Oxygen
(by diff.) (detd.)
Dilatation
(%)
Contraction, Softening
C (%) Point, 0: (“Cl
Temp.
Max.
Temp.
Max. Cont.,
Plasticity l l
421*
Dil. ( Cl
Index,
Bc (“Cl
P.I. = C/(0,
- 8,)
396**
FUEL,
1974,
Vol.53,
July
18”’ 405””
510’
525’”
nil**
507”
510**
nil**
468*
489**
525””
456”
477**
516””
o-57*
0.53” *
0.15**
o-31
o-39**
l
Coal dried in vacuum oven 24 h at 70% * Coal, after oxidation in air-oven, cooled in inert gas prior to making up compressed coal pellet for dilatomtric
214
nil**
tests
0.16””
H. M. Wachowska,
6. N. Nandi
and D. S. Montgomery:
The coal anion mixture referred to above was separated from the potassium after 120 h. The potassium metal usually agglomerates to form a ball. The actual separation was accomplished by applying helium pressure to the coal anion solution and blowing it over into 500 ml of distilled water. The reaction flask was washed with 100 ml of tetrahydrofuran and the washings were blown over into the distilled water. The coal suspension in the water was neutralized with dilute hydrochloric acid to a pH value of 3, stirred for 5 h and allowed to stand for 24 h for complete hydrolysis. The coal product was recovered by centrifuging in 200-ml closed bottles under nitrogen and was washed with distilled water until the sample was free of chloride ion. The coal was dried at 70°C in a vacuum for 16 h.
Hydroxyl determination in coal following cleavage of ether linkages A 500-mg portion of the reaction product was treated as described below, following the method of Knotnerus13. The sample was placed in a 50-ml flask together with exactly 5 ml of a solution of 2-5 g acetic anhydride made up to 50 ml with pyridine. The flask was fitted with condenser and protected by ascarite against moisture and carbon dioxide. The reaction mixture was then refluxed at 115°C for 24 h. After cooling, the contents of the flask were transferred quantitatively to a beaker with 15 ml benzene and 20 ml water. After the decomposition of the excess acetic anhydride, the acetic acid formed was titrated potentiometrically, in the vigorously stirred two-phase system, with about 0.1 N sodium hydroxide. The end-point was sharp at about pH 9. Exactly the same procedure was followed on a blank sample consisting of 5 ml of a solution of 2.5 g acetic anhydride made up to 50 ml with pyridine.
Dilatation test Balmer 10 coal.
Tab/e 2
Effect
of removing
Oxidation
studies on coking coal related
Table I presents the proximate analysis, the ultimate analysis, and the swelling properties as measured in the Ruhr dilatometer of the coals used in this investigation. The change in the oxygen content caused by the oxidation of the Balmer and Moss 3 coals is also shown, as is the effect of this increase in oxygen content upon the dilatation, contraction, plasticity index, softening point, temperature of maximum dilatation, and temperature of maximum contraction. The reduction in the dilatation andcontraction with increasing oxidation of the coal, as well as the elevation of the softening point and temperature of maximum contraction, show that the physical properties of both these coals have responded to oxidation in the normal manner. After oxidation at 85”C, both the Balmer and the Moss 3 coals have a dilatation of 12%. At this level of oxidation, the dilatation should be a sensitive indicator of small changes in the number and types of oxygen functional groups. The contraction of 29 and 28% of these two coals after this mild oxidation indicates that the coals still soften and contract to fill the interstitial space between the original coal particles that are charged to the Ruhr dilatometer in the form of a small cylindrical briquette. When these two coals were oxidized at 100°C the contraction was only 12 and 18% respectively, suggesting that the softening of these coals was restricted. To test the hypothesis that the elimination of the active oxygen functional groups would permit such oxidized coal to regain its original contraction and dilatation, radical acceptors such as hydroquinone were added to the coal to prevent the occurrence of chain reactions. The reduction of peroxides to the corresponding alcohols was carried out with the following reducing agents: sodium in methyl
peroxide,
keto, aldehyde,
and hydroxy max.
groups Temp.of
max.
Dilatation
Contraction
Softening
dilatation
contraction
Plasticity
(%I
(%)
point (“C)
(“Cl
1°C)
Index
Balmer
10 oxidized
100°C
-
nil
12
447
nil
525
0.15
Hydroquinone
nil
27
435
nil
513
0.34
Sodium
in CH30H
nil
12
450
nil
525
0.16
Sodium
sulphite
nil
12
447
nil
525
0.15
Sodium
borohydride
nil
10
456
nil
525
0.14
nil
32
447
nil
525
0.33
nil
12
456
nil
527
0.16
_
12
29
435
525
Hydroquinone
10
23
438
528
489 495 486 483 494 491
0.53 0.40 0.52 0.43 0.46 0.52
Hydroxylamine Barium
hydrochloride
hydroxide
Balmer
10 oxidized
85°C
Sodium
in CH30H
11
25
438
525
Sodium
sulphite
10
22
432
519
Hydroxylamine Barium
hydrochloride
hydroxide
0
26
438
522
12
28
438
522
12
28
396
510
477
0.34
10
31
408
513
477
0.44
28
408
522
492
0.33
Moss 3 oxidized
_ Barium
hydroxide
Hexamethyldisilazane
(4)
RESULTS AND DISCUSSION
Temp.of Treatment
to weathering
-2
85’C
FUEL,
1974,
Vol.53,
July
215
Oxidation studies on coking coal related to weathering 14): H. M. Wachowska, B. N. Nandi and D. S. Montgomery
alcohol, sodium sulphite, and sodium borohydride. The blocking of the hydroxyl groups of the oxidized coals was achieved by treating them with barium hydroxide and hexamethyldisilazane. To determine whether the oxygen in keto and aldehyde groups can affect the dilatation, the oxidized coals were treated with hydroxylamine hydrochloride. The results of these tests are presented in Table 2 and they show that the dilatation of the coal could not be restored by this type of chemical treatment. Though treatment with hydroquinone and hydroxylamine slightly improved the contraction, the results as a whole must be viewed as negative for neither the hydroquinone nor the hydroxylamine treatment resulted in any increase in the dilatation in the Balmer 10 coal oxidized at 85°C. There was no clear evidence that the removal or blocking of the peroxide, keto, aldehyde and hydroxyl groups significantly improved the plastic properties, so it was concluded that polymerization reactions, caused by oxygen functional groups during heating in the Ruhr dilatometer test, were not the principal cause of the reduction of dilatation in coal oxidized below 100°C. Kruczko14 suggested that during the thermal treatment of slightly oxidized coal, the number of free radicals increased as a result of the decomposition of hydroperoxides. Peroxide radicals were assumed to abstract hydrogen atoms from the polymer, creating radical sites which can then combine to form carbon-carbon cross-links. The decrease of the dilatation of oxidized coals may also be explained by postulating the formation of non-reactive oxygen linkages during the low-temperature oxidation of coals. These linkages, possibly ether-type, are considered to result from the thermal decomposition of peroxides and hydroxyl groups at 85 and 100°C. It is known that diary1 or aryl-alkyl ether bonds in coal can be cleaved by treatment with potassium in tetrahydrofuran (THF) in the presence of a small amount of naphthalene as described by Sternberg”. This provided an opportunity to distinguish between carbon-oxygen crosslinks and carbon-carbon cross-links. According to Sternberg, coal treated with potassium in THF in the presence of a small amount of naphthalene reacts according to the following scheme to produce coal anion: K” + C1uH8 + [CluHs] ~ + K+ Coal + [CloHs] - + CluH8 t [Coal lr
/
I
I
Stirring
I
time
Figure 1 Acidity versus stirring time (I) Anion formation in the absence of coal (2) Anion formation in the presence of coal
216
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I
(h
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1
I
The naphthalene is required as an electron-transfer agent because neither potassium nor coal is soluble in THF. During this reaction, in addition to coal anion, naphthalene anion and solvent anion are produced. After hydrolysis with water, each anion gives an equivalent number of hydroxyl ions, which may be determined by titration with acid. The number of ionic charges per 100 carbon atoms in coal was calculated by subtracting the number of equivalent hydroxyl ions formed by the naphthalene and solvent anions from the total number of equivalents of hydroxyl ions (Figure 1). Table3 shows that, when coat was treated in this manner, the number of charges per 100 carbon atoms was significantly larger than the total number of oxygen atoms per 100 carbon atoms. The large increase in OH groups resulting from the cleavage of the ether linkages is shown in Table 4. It was assumed that many of the anionic charges not associated with the ether-oxygen linkages are located in the aromatic-ring systems and that after hydrolysis these are protonated, thus increasing the hydrogen-to-carbon ratio. Table3
Number
of charges associated
atoms C after treatment
with
K/TH
Number Number
Balmer
10 fresh
Balmer
10 oxi-
dized IOO’C Moss 3 fresh
of
with
coal per 100
F/naphthalene of
for 120 h Number
of
oxygen
oxygen
charges
atoms (by
atoms
per 100
difference)
(determined)
atoms C
per 100
per 100
(average)
atoms C
atoms C
IO.54
3.2,
3.95
IO.57
4.77
485
8.74
3.92
5.12
Table 4 also shows the elemental composition and the weight % of hydroxyl and carbonyl oxygen groups of the fresh Balmer 10 coal and the same coal after treatment with potassium in tetrahydrofuran. When the oxygen linkages of this coal had been cleaved by this treatment, the hydroxyl groups so produced were blocked by different groups by treating successive samples with diazomethane, barium hydroxide, and hexamethyldisilazane following the procedures described in the experimental part of this paper. This was done to facilitate the interpretation of the dilatation measurements. The hydroxyl groups of the fresh coals were determined by acetylation in pyridine according to Blom”. A modification of this procedure was employed for the estimation of hydroxyl groups in the coals previously treated with potassium in tetrahydrofuran, because of the increased solubility of these reduced coals in pyridine. This procedure was developed for asphaltic bitumens13. Table 4 shows that the Balmer 10 coal after oxidation at 100°C has a higher hydroxyl content following the cleavage reaction than the original coal after treatment with potassium in THF, and that the proportion of the oxygen linkages, added by oxidation, that give rise to OH groups on cleavage is relatively small. The carbonyl content was measured by reaction with hydroxylamine hydrochloride16. No carboxyl groups were found to be present using the ion-exchange method
H. M. Wachowska, 5. N. Nandi and 0. S. Montgomery: Oxidation studies on coking coal related to weathering (4) Table 4
Changes in elementary
furan and products
of blocking
composition
and forms of combined
oxygen
due to treatment
with potassium
in tetrahydro-
coals (dry basis)
0
0
C
H
N
S
(diff.)
(detd.)
Ash
DOH (%I
Cc0 (%I
H/C
88.02
4.96
1.43
0.29
3.77
464
1.5
0.93
0.30
0.66
87.0g
5.56
1.35
0.26
4.50
5.92
1.2
2.69
0.29
0.76
f diazomethane
86.88
5.63
1.22
0.24
4.83
-
1.2
0.33
-
0.77
+ barium
86.26
5.32
I.07
0.23
4.95
-
2.2
0.36
-
0.73
84.2,
5.g3
0.93
0.24
4.28
-
4.4
0.09
-
0.83
86.50
4.77
1.43
0.28
5.4,
5.47
1.5
0.98
0.30
0.65
86.45
5.31
1.38
0.26
5.3*
6.32
l-3
2.98
0.30
0.73
81.30
5.2g
1.44
0.62
4.58
5’56
6.8
0.86
0.23
0.77
80.86
5.44
1.18
0.34
6.68
5.95
5.5
2.92
0.20
0.80
Coal Balmer
10 fresh
+ postassium/TH
F/naphthalene
hydroxide
+ hexamethyldisilazane Balmer
10 oxidized
+ potassium/TH
100°C F/naphthalene
Moss 3 fresh + potassium/TH
F/naphthalene
+ diazomethane
80.45
5.80
1.20
0.32
6.73
-
5.5
0.49
-
0.85
+ barium
80.34
5.31
1.29
0.39
6.83
-
5.8
0.58
-
0.78
79.65
6.24
1.23
0.39
5.40
-
7.1
0.00
-
0.93
hydroxide
+ hexamethyldisilazane
Tab/e 5
Effect
of blocking
hydroxyl
groups - dilatation
test Temp.of
Reagent Balmer
10 fresh
+ potassium/THF/naphthalene + barium
hydroxide
dilatation
contraction
PI as&Y ty
(%I
(%I
point
(“C)
(“Cl
Indepr
468 405 439 436 405 525 420 441 456 405 423 402 429
(“C)
70
27
421
510
260
15
375
525
16
378
495
171
17
402
492
+ hexamethyldisilazane
154
7
363
495
nil
12
447
nil
19
378
510
IOO’C
+ potassium/THF/naphthalene + barium
hydroxide
Moss 3 fresh + potassium/THF/naphthalene t barium
hydroxide
max
Softening
155
10 oxidized
Temp.of
Contraction
+ diazomethane Balmer
max.
Dilatation
254 157
14
375
498
82
20
393
507
222
16
351
495
136
20
363
489
+ diazomethane
158
18
369
501
+ hexamethyldisilazane
130
23
357
480
employing calcium acetate”. The quantity of carbonyl oxygen before and after oxidation at 100°C was essentially the same, hence it was concluded that this linkage did not play a significant part in affecting the dilatation. If the dilatation curves in Figures 2 and 3 and the data in Table 5 are compared for the fresh and oxidized Balmer coal before and after treatment with potassium in THF, it will be noted that the softening point was lowered from 421 to 375°C for the fresh coal as compared with 447 to 378°C for the coal oxidized at 100°C. This indicates that the molecular weights were lowered in both cases to the extent that the softening points had essentially the same value; it was attributed to the severing of the ether-oxygen linkages, removing many of the oxygen functional groups and introducing some additional hydrogen into the aromatic-ring system. There was no evidence of a difference in softening point of the reduced coals that might be ascribed to carbon-carbon bond formation during the oxidation step. The larger decline in the softening point of the oxidized coal after treatment
0.57 0.50 0.26 0.50 0.16 0.15 0.45 o-21 0.31 0.29 0.33 0.54 0.31
with potassium in THF was attributed to the fact that the softening point of this coal had been elevated by oxidation, and that after the oxygen-ether linkages were opened the softening point returned to the same value as that of the fresh coal after treatment with potassium in THF. Comparing the dilatation of the fresh and oxidized Balmer coal after treatment with potassium in tetrahydrofuran, it was evident that the dilatation after this treatment was very large: that the coal had been made much more fluid, was entraining a considerable amount of pyrolysis gas and of the additional steam that was probably being generated from the condensation of the OH groups. Such behaviour would be consistent with the opening of the carbonPoxygen bonds. Again there was no evidence that the oxidation of the Balmer coal produced carbon-carbon cross-links, which would have tended to diminish the dilatation of the oxidized Balmer coal after treatment with potassium in THF. The lowering of the softening point of the Moss 3 coal and the large dilatation produced after the treatment of this coal with potassium in tetrahydrofuran followed the same
FUEL,
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Oxidation studies on coking coal related to weathering (4): H. M. Wachowska, 8. N. Nandi and D. S. Montgomery
-
300
250
250
200
200
150
2 -
150
100
.-: 2 ;; 0
100
50
50
0
0
450
L80
Temperature
(‘C)
LZO
Figure 2 Dilatation versus (1) Original 121 Original after treatment (3) Original after treatment with diazomethane (41 Original after treatment with hexamethyldisilazane (51 Original after treatment ,vvith barium hydroxide
temperature,
/
I
510
540
and blocking
with
K/THF/naphthalene
and blocking
with
K/THF/naphthalene
and blocking
pattern as was observed with the Bahner coal. To test the hypothesis regarding additional steam generation during dilatation, Balmer and Moss 3 coals were treated with potassium in THF and the resulting coals were subsequently treated with barium hydroxide, diazomethane, and hexamethyldisilazane to replace the OH groups with other groups of greater thermal stability. Figures 2, 3, and 4 as well as Table 5 show that in each case the blocking of the OH groups reduced the dilatation but had comparatively little effect on the softening temperature. During the investigations of the cleavage of the ether linkages with potassium in THF, it was observed that heating the reaction product at 70°C under vacuum for four days led to markedly lower values of the dilatation than did vacuum drying at 70°C for I6 h. The effect of prolonged heating of Moss 3 coal is shown in Figure 5 and Table 6. The decrease in the value of the dilatation may be explained by assuming that the hydroxyl groups undergo condensation
Tab/e 6
Time
Moss
3 coal after treatment
(h)
16 72 96
218
FUEL,
1974,
I
I
I
I
I
I
390
L20
450
L80
510
510
Temperature
Balmer IO coal
with KiTHFinaphthalene with K/THF/naphthalene
I 360
with potassium
in THF.
f ‘C 1
Dilatation versus temperature, Balmer IO coal figure 3 (1) Oxidized 1 OO°C 72 h (2) Oxidized lC8C 72 h after treatment with KITHFinaphthalene (31 Oxidized IOO’C 72 h after treatment with KiTHFlnaphthalene and blocking with barium hydroxide
to form ether iinkages at 70°C and that the water is removed under the vacuum conditions. This might increase the molecular weight of the coal and reduce the amount of gas available to produce a large dilatation A somewhat similar phenomenon has been observed during the drying of humic acids to form anhydrides”. If it is assumed that the dilatation is governed to a considerable extent by the steam production, then the dilatation associated with coal after blocking the OH groups should correspond to the vaIue obtained as the result of exhaustive heating of the cleaved coal at 70°C under vacuum which again eliminates many of the OH groups. It appears that prolonged vacuum drying at 70°C yields a product which has a lower dilatation than the product which has had OH groups removed by blocking. This difference may be due to the average molecular weight of the condensation products being higher than that of the products produced by blocking the OH groups.
Dried in vacua at 7O”C, for different
periods of time
OOH (wt %, dry basis)
Dilatation
Contraction
Softening
Temp.of max. dilatation
Temp.of max. contraction
(%)
(%I
point
(“Cl
i”C)
Index -
2.92 2.02 _
222 108 90
16 19 14
351 351 351
495 489 480
405 429 429
0.29 0.24 0.17
Vo1.53,
July
(“Cl
Plasticity
H. M. Wachowska, 300
--
B. M. Nandi and 0. S. Montgomery:
Oxidation
studies on coking coal related
to weathering
(41
7-1-7
250
200
2 _
150
.-: ;; 2 0
100
50
0
--II
390
420 Temperature
Figure 4 Dilatation versus (1) Original (2) Original after treatment (3) Original after treatment with diazomethane (4) Original after treatment with hexamethyldisilazane (5) Original after treatment with barium hydroxide
temperature,
480
L50 I’C
510
360
1
480
and blocking
with
K/THF/naphthalene
and blocking
with
K/THF/naphthalene
and blocking
Dilatation versus temperature, Moss 3 coal (1) Original dried 16 h (2) Original after treatment with K/THF/naphthalene in vacuum at 70°C dried 72 h (3) Original after treatment with K/THF/naphthalene in vacuum at 70°C dried 96 h (4) Original after treatment with K/THF/naphthalene in vacuum at 7OcC (5) Original after treatment with K/THF/naphthalene and blocking with barium hydroxide dried 96 h in vacuum at 70°C
ACKNOWLEDGEMENTS The authors would like to express their appreciation to A. T. Lloyd for fractionating the Balmer coal according to specific gravity and to S. E. Nixon as well as the personnel of the coal analytical laboratory for many analytical services. Special thanks are due to Dr H. W. Sternberg of the U.S. Bureau of Mines for many helpful suggestions in carrying out the coal reduction procedure in K + THF.
9 10 11 12
REFERENCES Chakravarty, aakravarty,
450 [‘Cl
Figure 5
Moss 3 coal
with K/THF/naphthalene with K/THF/naphthalene
420 390 Temperature
S. L. J. Mines Metals Fuels 1960, 1 S. L. J. Mines Metals Fuels 1960,lO
Von de Vries, H. A. W., Bokhoven, C. and Dormans, H. N. M. Brennst.-Chem. 1969,10,289 Maher, T. P., Harris, J. M. and Yohe, G. R. Rep. Invest. Illinois State Geol. Surv. No.212, 1959
13 14 15 16 17
Brooks, J. D. and Maher, T. P. Fuel, Land. 1957,36,51 Lazarov, L. and Angelova, G. Fuel, Lond. 1968,47, 333 Lazarov, L. and Angelova, G. Fuel, Lond. 1468, 47, 342 Pampuch, R. Rep. Polish inst. Mining No.103, 1951; Dryden, 1. G. C. and Griffith, M. BCURA Monthly Bull. 1954,18,71 Orchin, M., Golumbic, C., Anderson, J. E. and Starch, H. H. Bull. U.S. Bur. Mines No.505, 1952, p 13 Sternberg, H. W., Delle Donne, C. L., Pantages, P., Moroni, E. C. and Markby, R. E. Fuel, Lond. 1971,50,432 lgnasiak, B. S., Clugston, D. M. and Montgomery, D. S. Fuel, Lond. 1972,51,76 Ihnatowicz, A. Komunikat Gliiwnego Instytutu Gdrnictwa No.125, 1952 Knotnerus, J. J. Inst. Petrol. 1956, 42, 355 Kruczko, A. Khimiya tverd. Topl. 1971, No.5, 5 Blom, L., Edelhausen, L. and van Krevelen, D. W. Fuel, Lond. 1957,36, 135 Von Kroger, C., Darsow, G. and h’uhr, K. Erdijl u. Kohle 1965,9,701 Wood, J. C., Moschopedis, S. E. and Elofson, R. M. Fuel, Land. 1961,40,193
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219