Low-temperature oxidation of brown coal. 3. Reaction with molecular oxygen at temperatures close to ambient

Low-temperature coal. 3. Reaction at temperatures

oxidation of brown with molecular oxygen close to ambient

Philip D. Swarm* and David G. Evans? Department of Chemical Engineering, University of Melbourne, Parkville, Victoria, 3052, Australia (Received 23 June 1978)

Yallourn brown coal was oxidized with pure molecular oxygen at temperatures of 35 and 70°C until, after about 45 days, no further gain in mass occurred. At this stage, despite the loss of considerable quantities of carbon and hydrogen from the coal as carbon dioxide, carbon monoxide and water, the coal had gained in mass by 15 g per kg of dry coal. Virtually all the gain was accounted for by increases in the content of carboxyl, carbonyl and phenolic groups. These functional groups, which subsequently break down to yield the stable products already mentioned, are formed apparently by attack on the aliphatic structures of the coal. At 35°C. reactions leading to the formation of functional groups such as phenolic groups, and their breakdown to water, predominated; however, at 70°C, the formation of carboxyl groups and their breakdown to carbon dioxide became more important, the amount of water formed being less than that at 35’C. The general course of these reactions was qualitatively similar to that observed by other workers using bituminous coals, but the extent of the reactions was an order of magnitude greater.

The problem of spontaneous combustion of brown coal is well recognized in the brown-coal mining industry, and methods of preventing fires on conveyor belts, in storage hoppers and in the open-cut coal mines themselves have been developed empirically over many years. Large-scale experiments involving the tracing of temperature rise and the propagation of temperature waves in stockpiles have given some insight into the gross characteristics of the spontaneous combustion process. However virtually no work has been done to determine the mechanism of the lowtemperature oxidation of the coal itself, or even its extent, although many such inve.stigations have been carried out with less-reactive, higher-rank coals (see for example the reviews in references l-3). Unfortunately much of this work is difficult to interpret depending on their interests (or theories) some of these earlier workers measured the gain in the mass of the coal and some the composition of the product gas; most ignored changes in the composition of the coal (perhaps because of the difficulty of measuring it precisely enough to be useful); and nearly all ignored the possibility that the gain of coal mass and the composition of the gas could be affected by the adsorption of water or carbon dioxide formed by the oxidation, on the coal surface. However perhaps the most serious difficulty is that in order to speed up the oxidation process and produce measurable changes in a reasonably short time nearly all of these earlier workers used temperatures considerably higher than those used in the studies of spontaneous combustion mentioned above. Despite these limitations the following view of the low*F’resent address:

Parade,

Victorian Ministry for Conservation, East Melbourne, 3002, Australia

t Presentaddress: Centre for Environmental Melbourne,

Parkville,

Victoria,

3052,

0016-2361/79/040276-05%2.00 0 1979 IPC Business Press

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FUEL, 1979, Vol 58, April

Studies, Australia

240 Victoria University

of

temperature oxidation of coals with molecular oxygen gradually emerged over a period of years2: (1) the first step in oxidation is the formation of solid coal/oxygen complexes, (2) these may break down to yield carbon dioxide and water or form more stable functional groups such as carboxyl, carbonyl and ether groups, (3) at the temperatures used in most work (>lOO”C) these also tend to break down to yield gaseous products, and (4) the net result of this oxidation sequence is the loss of aliphatic structures and the production of carbon monoxide, carbon dioxide and water. The best work available on bituminous coals is that of de Vries et al. 4, who oxidized coals of carbon content 84-89% at various temperatures, including some close to ambient temperature, and who performed their measurements in such a way that clear interpretations could be drawn. As will be seen later their results provide a useful comparison with our own work on coal of 67% carbon content. Recently Marinov’ has compared the oxidation, in mixtures of air and water vapour, of three coals of widely different rank, including a coal similar in rank to the one used by us. The strength of this work is in the attention given to the changes caused to the coals themselves. However it is also difficult to interpret, for several reasons: (1) descriptions of sample preparation are inadequate or missing; (ii) the fraction of water vapour in the air is uncertain, with the result that it is not possible to draw up reliable materials balances; (in) the temperature was steadily raised at a rate of 1°C per minute during most of the experiments; and

Low-temperature oxidation of brown coal (31: P. D. Swann and D. G. Evans

then sealed and the coal dried by evacuation for 24 hours at the temperature which was to be used subsequently for the oxidation (35°C or 7O”C, see later), and at a pressure of 1.3 Pa. Drying by evacuation at different temperatures was expected from the preliminary experiments to yield dried coals of slightly different compositions,and this was indeed found to be the case (see Tables 3 and 4). However the bias introduced by this procedure can be allowed for by considering only the changes caused by oxidation, as has been done in Tables 4, 5 and 6. The alter native procedure of drying both coals at the same temperature, say 35”C, irrespective of the subsequent oxidation temperature, could lead to some of the effect of drying being interpreted as an effect of oxidation. Oxidation

figure 1 Apparatus used to evacuate and oxidize coal samples: A, jacketted glass reactor containing coal suspended from quartz spring balance; 6, oxygen admission via purification train; C, first cold trap; D, iodine pentoxide tube; E, second cold trap; F, absolute manometers; G, Pirani gauges; H, vacuum pump

(iv) changes of mass in the coal samples upon oxidation were .determined on a dry basis, apparently by normal analytical methods for moisture determination involving heating to 100-l lO”C, even for experiments in which oxidation had been carried out at temperatures as low as 60°C. As we have shown elsewhere6, heating to 110°C can cause appreciable breakdown of the oxygen-containing functional groups in the coal. Consequently we are not able to make meaningful parisons between Marinov’s results and our own.

com-

EXPERIMENTAL Coal preparation

A 25 kg cube of virgin Yallourn coal was cut from a recently-worked face of the Yallourn open-cut coal mine, using a chain saw, after first removing a layer of coal about 25 cm thick to avoid the possibility of preoxidation. This cube was then stored in a sealed drum under water drained from the coal face to prevent oxidation and other chemical changes. When required the whole cube was ground under the water to minus 3 mm and stored in slurry form. Samples for tests were taken by stirring this slurry thoroughly and withdrawing one-kg grab samples. Preliminary experiments showed that the coal had to be dried for appreciable oxidation to occur (bed-moist Yallourn coal contains 6%70% by mass of water), and that the oxidation of dried coal was influenced by the drying procedure used. For example heating the coal to temperatures higher than the temperature subsequently used for oxidation was found to reduce the extent of the oxygen uptake. Also it was found that drying by evacuation even at temperatures as low as 35°C removed small amounts of 2,6-di-tert-butyl-4 methyl phenol from the coa17, and that the amount removed was affected by the pressure used. The following standard drying procedure was therefore adopted: Slurry samples were roughly drained and placed in a vacuum vessel while still quite wet. The vessel was

As mentioned earlier most work on bituminous coals has concentrated either on the gaseous oxidation products or on the solid oxidation product, but seldom on both. Earlier work in our laboratories’ showed that extensive physical adsorption of water could occur on brown coals at temperatures close to ambient temperature, which could cause difficulties in interpreting data on the gain of mass of the solid or on the composition of the gas. We therefore adopted the following scheme (Figure I): a known mass of coal was placed in a polypropylene bucket suspended from a calibrated quartz spring in a glass reactor (A) provided with temperature and pressure control and means of addition of oxygen and removal of gas samples for chromatographic analysis. The coal was dried by evacuation, as described earlier, then molecular oxygen was admitted via a purification train (B) at the desired pressure, and the progress of oxidation was monitored by following the gain in mass by measuring the extension of the spring with a cathetometer and the change of gas composition by chromatographic analysis. At the end of a run, all the gas in the reactor, including material physically adsorbed on the coal, was removed by evacuation through the first cold trap (C) which retained carbon dioxide and water for later analysis. Oxygen and carbon monoxide passed from the cold trap through a tube packed with iodine pentoxide (D) which oxidized the carbon monoxide to carbon dioxide, then into a second cold trap (E) which retained the carbon dioxide thus formed. This permitted unequivocal measurements of the mass and composition of both solid and gaseous products to be made. Larger samples of solid for functional-group analysis were evacuated and oxidized in a larger reactor in parallel with the reactor just described. In all experiments the oxygen pressure was maintained at 101.3 kPa. Temperaturesused were 35”C, to simulate ambient temperatures at the open-cut mine during summer months, and 7O”C, a temperature commonly mentioned in the literature’ as being of critical importance in spontaneous combustion. During preliminary experiments the coal did not gain significantly in mass after 45 days, indicating that oxidation had ceased or was being balanced by corresponding desorption of oxidation products. The main oxidation experiments were therefore continued for only 45 days. Five runs were performed at each temperature to establish the precision of the results. Analytical methods The gas was analysed chromatographically. The water produced was present in such great quantities that the gas in the sample bulb above the cold trap became saturated

FUEL, 1979, Vol 58, April

277

Low-temperature Tab/e 7 Distribution

oxidation

of brown coal (3): P. D. Swann and D. G. Evans

of gaseous oxidation

products,

g/kg of original dry coal Gaseous products Temperature

Distribution

(“Cl

co2

co

Hz0

Reactor Reactor

atmosphere atmosphere

35 70

12 49

1 5

1 4

on coal on coal

35 70

1 1

0 0

48 34

49 35

35

13 50

1 5

49 38

63 93

Adsorbed Adsorbed Total Total

JO

Table 2

Mass changes caused by oxidation

Reactants

Products’

Dry coal

02

Dry oxrdized

35 70

1000 1000

77 108

1014 1015

aStandard deviations for the various products were calculated H20, 0.2 g/kg dry coal; dry oxidized coal, 0.9 g/kg dry coal

Elemental

14 58

of coal, g/kg of original dry coal

Temperature of oxidation (“C)

Tab/e 3

Total

analyses before and after oxidation,

from the five individual

coal

co2

co

H2O

13 50

1 5

49 38

results as: CO,,

0.6 g/kg dry coal; CO, 0.3 g/kg dry coal;

g/kg of dried evacuated coal Temperature

of oxidation

1°C)

35

70

Sample

C

H

0

N

C

H

0

N

Before oxidation After oxidation, measured After oxidation, calculated

670 664 657

49 48 43

251 269 270

6 6 6

670 652 644

47 46 42

258 277 289

6 6 6

from

Table 2

when the cold trap was raised to ambient temperature, and some water was always present as liquid. This was determined by titration with Karl Fischer reagent. The solid residue was analysed for carbon, hydrogen and oxygen using standard microanalytical techniques, and for functional groups using methods developed by the State Electricity Commission of Victoria from earlier proposals by Brooks and Sternhell’ and Schafer”. Its infrared absorption bands were also measured using 0.5% by mass of dry coal in potassium bromide discs. Particular care was taken during the grinding of the samples and the preparation of the discs to exclude moisture and to maintain uniformity of coal particle size and dispersion in the potassium bromide.

RESULTS The yields of gaseous oxidation products in the reactor atmosphere and adsorbed on the coal were obtained by averaging the results of the five runs at each temperature, as shown in Table 1. The result at 35°C was dominated by adsorbed material, and at both temperatures nearly all the water formed remained adsorbed. If the evacuationdesorption procedure outlined earlier had not been used the water produced would have been grossly underestimated

278

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1979, Vol 58, April

and the gain in mass of the solid overestimated. The oxygen balances (discussed later, see Table5) would have shown very large discrepancies. The mass changes observed after desorption are shown in Table 2. Again all results are averages from five runs at each temperature. In each case the oxygen consumption was calculated by an overall mass balance. The elemental analyses of the coals before and after oxidation are shown in Table 3. The changes calculated from the data of Table 2 agree only qualitatively with the changes measured directly, but are considered to be more reliable because the methods used to obtain the data of Table 2 measure the changes occurring more sensitively. The amounts of oxygen-containing functional groups in the dried, evacuated coals before and after oxidation are shown in Table 4. The infrared spectra obtained on the coals before and after oxidation are shown in Figure 2. The main changes occurring upon oxidation are reductions in the aliphatic peaks (2850 and 2920 cm-l) and increases in the carbonyl peaks (1700 cm-‘). Changes also occurred in the hydroxyl hydrogen-bonding region (3300 cm-l) and the ether oxygen region (1280 cm-l) but with brown coals these regions are too diffuse to permit ltly definite conclusions to be drawn. Using the baselines recommended by Durie and Sternhell”

Low-temperature

for the aliphatic peaks at 2850 and 2920 cm-l, the changes in aliphatics on oxidation were calculated by Wright’s optical density method’*. At both temperatures the reduction in aliphatics on oxidation was approximately 35%. Because of the difficulty in establishing a baseline for the carbonyl peak it was decided to rely on the chemical analysis for this group. DISCUSSION Oxygen balances Oxygen balances for reactions at the two temperatures were constructed from the data of Tables 2 and 4, as shown in Table 5. These balances indicate that nearly all the oxygen not used in the formation of carbon dioxide, carbon monoxide and water is accounted for in increases in the carboxyl, phenol and carbonyl contents of the solid residue. The infrared spectra indicate that the formation of ether groups may contribute to the remaining small deficit. This

Tab/e 4 Amounts of oxygencontaining functional groups in the coals, g of functional group oxygen per kg of original dry coal Sample

Phenol

Carbonvl

79 83

77 84

24 33

4

7

9

85 100

75 79

25 34

15

4

9

Carboxyl

Before oxidation at 35°C After oxidation at 35°C Increase on oxidation Before oxidation at 7O’C After oxidation at 7O’C Increase on oxidation

lOOr-

-

oxidation

of brown coal (31: P. D. Swann and D. G. Evans

would be consistent with the formation of water by condensation of previously formed hydroxyl groups. .Hydrogen balances Hydrogen balances for oxidation at the two temperatures were also constructed from the data of Tables 2 and 4, as shown in Table 6. The gains of hydrogen in reaction products must have come from the coal itself, presumably from the aliphatic and aromatic portions of the coal molecule rather than the functional groups, which themselves showed gains in hydrogen. The hydrogen distribution by type can be determined by proton n.m.r. analysis of coal solubilized by treatment with phenol and para toluene sulphonic acid catalyst13. Work done in our laboratories using this method shows that the hydrogen in Morwell brown coal, which is structurally similar to the Yallourn brown coal used here, is distributed as 19 g of aromatic hydrogen per kg of dry coal, 15 g of aliphatic hydrogen and 14 g of functional-group hydrogen14. Loss of 35% of the aliphatic hydrogen, as indicated by the infrared spectra, corresponds to 5.3 g/kg of dry coal, which agrees well with the gains in hydrogen in the products as shown in Table 6. Although this result is based on meagre data it does seem to indicate that at the temperatures used here there is little or no attack on aromatic structures. .Effect of temperature Perhaps the most striking result in Table 5 is that the considerably greater consumption of oxygen at 70°C is accounted for solely in terms of increased formation of carboxyl groups and carbon dioxide; the other functional groups remain virtually unaltered, and water formation is actually less than at 35°C. It follows that the oxidation process is not just one simple sequence of reactions depending on the formation and breakdown of intermediate products. At some stage in the process (probably at more than one stage) competing reactions must be possible, with increase of temperature favouring the reactions leading to the formation of carboxyl groups and their breakdown to carbon dioxide over the reactions leading to the formation of water.

a

Tab/e 6

Hydrogen

balances, g/kg original dry coal Gain in hydrogen

Wavenumber

In water

COOH

OH

Total gain

35 70

5.4 4.2

0.1 0.5

0.4 0.2

5.9 4.9

( cm“ J

figure

2 infrared spectra of coal samples prepared and oxidized at 70°C: a, evacuated at 70°C; b, evacuated and oxidized at 70°C

Table 5 Oxygen

In solid

Reaction temperature (“C)

balances, g/kg original dry coal Gain in oxygen

Reaction

In solid

In gas

temperature

f” C)

co2

co

“20

COOH

OH

c=o

35 70

9 36

1 3

44 34

4 15

7 4

9 9

Total

Oxygen consumption (Table 2)

74 to1

77 108

FUEL, 1979, Vol 58, April

279

Low-temperattire

oxidation

Table 7 Comparison

of brown coal (3): P. D. Swann and D. G. Evans

of oxygen balances for reaction of high-rank

coals and brown coal at 70°C.

g/kg of original dry coal

Gain in oxygen

(mass % of carbon)

Worker Ref. Ref. Ref. Ref.

89 87 85 84 67

4 4 4 4

Table 5

Compatison

In solid

In gas

Coal rank co

‘420

1 2 3 4

0 0 0 1

3 3 5 8

36

3

34

co2

COOH 1 0 -1 1 15

with high-rank coals

As mentioned earlier, the work of de Vries et aL4is the best available study of the oxidation of bituminous coals with molecular oxygen at temperatures close to ambient temperature. In their work oxidation was carried on long enough to ensure that pseudo-equilibrium had been reached*, and water and carbon dioxide formed during the oxidation were desorbed before measuring the composition of the evolved gas and the gain in mass of solid residue. As the carbon contents of the coals used ranged from 84 to 89% this work permits a comparison of the nature and extent of oxidation of high-rank coals and brown coal, as used in the present study. Temperatures used by them included 60 and 8O”C, so that results expected at 70” C could be obtained with reasonable accuracy by interpolation. Table 7 compares results at 70°C by de Vries et al. with ours. The most striking feature of this comparison is that the extent of oxidation of the brown coal in nearly every respect is an order of magnitude higher than for the bituminous coals (the one exception is the increase in phenolic hydroxyl content). It is difficult to make really precise measurements of the changes in functional-group contents, and in the case of the bituminous coals, for which changes are small, this may lead to inconsistencies. For example the change in functional-group content follows no consistent pattern with coal rank. Moreover the relatively large gain in phenolic hydroxyl for the first coal is probably in error, as it leads to a total oxygen gain greater than the oxygen consumption. Nevertheless the comparison of mass of oxygen appearing in products with mass of oxygen consumed is good enough to confirm that oxidation causes a gain in the content of oxygen-containing functional groups in the bituminous coals, principally accounted for by carboxyl, phenol and carbonyl groups, as with the brown coal. Measurement of quantities of gas produced is inherently more precise, and a consistent pattern of increased produc*Whereas we oxidized coals for 45 days using an oxygen pressure 101 kPa de Vries ef al. collected their data after 6 days oxidation with an initial oxygen pressure of 550 kPa

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FUEL, 1979, Vol 58, April

of

Total

OH

c=o

11

1 1 2 1 9

3 5 9 4

Oxygen consumption 17 10 14 24 101

10 16 24 38 108

tion with decrease in coal rank emerges. Possible reasons for this increase of coal reactivity to oxygen with decrease in rank will be discussed in a future paper. The general conclusion may be drawn that, despite the quantitative differences, the pattern of oxidation of brown coal is similar to that of bituminous coals. However it appears that the reactions producing phenolic groups are not favoured by decrease of coal rank to the same extent as those producing carboxyl and carbonyl groups. ACKNOWLEDGEMENTS We wish to thank the State Electricity Commission of Victoria for providing coal samples and performing functional group analyses, Mr R. D. MacDonald of the Australian Microanalytical Services of the Commonwealth Scientific and Industrial Research Organization for advice and encouragement to one of us (PDS) in the performing of elemental and Karl Fischer analyses, and the Australian Research Grants Committee for sponsoring part of the work. REFERENCES 1

2 3 4 5 6 7 8 9 10 11 12 13 14

Dryden, I. G. C. in Chemistry of Coal Utilization, Supplementary Volume (Ed. H. H. Lowry), Wiley, New York, 1963, Ch. 6, p 272 Van Krevelen, D. W., Coal, Elsevier, Amsterdam, 1961,Ch. 13 Smirnov, R. N. Tr. Inst. Goryuch. Iskop, Akad. NaukS.S.R. 1963,21, 16 De Vries, H. A. W., Bokhoven, C. and Dormans, H. N. M. Brennst.-Chem. 1969,50,289 Marinov,V. N. Fuel 1977,56, 153,158, 165 Allardice, D. J. and Evans, D. G. Fuel 1971,50, 201 Swann, P. D., Harris, J. A., Siemon, S. R. and Evans, D. G. Fuel 1973,52,154 Allardice, D. J. and Evans, D. G. Fuel 1971,50, 236 Brooks, J. D. and Stemhell, S. Aust. J. Appl. Sci. 1957, 8, 206 Schafer, H. N. S. Fuel 1970,49, 197, 271 Durie, R. A. and Sternhell, S. Aust. J. Chem. 1959, 12, 205 Wright, N. Ind. Bngng Chem. (Anal.) 1941, 13, 1 Imuta, K. and Ouchi, K. Fuel 1973,52,174 Hooper, R. J., PhD Thesis, University of Melbourne, 1978