manner, contained essentially the same cations. In the
solution from the white material, the relative proportions,
in parts per million, were 8.0 Fe, 5.5 Al, 0.8 Ca, 0.9 Mg,
and 0.2 Na. For the orange material, the corresponding
relative concentrations, in parts per million, were 2.6 Fe,
03 Al, 0.4 Ca, 0.1 Mg, and 0.8 Na. These results show
that iron compounds were present in most of the samples,
and especially in the oxidation products. The presence of
clay in the samples is indicated by the aluminum ion.

Set 3.—The white material on the surface of the coal
seen in figure 3, center (sample 3.2C), was dissolved in
HCI and the solution was filtered. Analysis of the fil-
trate by ICAP atomic emission showed the presence of Ca
(340 ppm), Mg (11 ppm), and Fe (4 ppm). The sample
was probably rock dust, which was used by the mine to
cover heated areas. The powder did not dissolve in water,
and when it reacted with the acid, bubbles evolved,
evidence of the presence of carbonate.

The dark orange material on the large chunk (figure 3,
top, sample 3.14) was similarly treated with dilute HCl
and filtered. The solution contained the following relative
concentrations, in parts per million: 170 Ca, 79 Fe, 7 Mg,
and 5 Na. Based on the color and texture of the sample,
its ease of solubility in HCl, and proportions of cations in
the solution, this sample contained a mixture of rock dust
and iron oxidation products.

15

Set 6.—Core 6.3 from heated zone 3 had green material
on one surface. The material was scraped off and treated
with acid, and the filtrate was analyzed. Iron was the most
abundant ion found.

Coal Analyses

As the evidence for the presence of pyrite oxidation
products and pyrite in many of the samples accumulated,
it became desirable to quantify the amount of pyritic sulfur
in at least a few of the samples. The samples analyzed
included samples 5.1 and 5.2, samples from cores 6.3 and
6.5 (from the vicinity of the third heating), and a sample
from core 6.6, which was obtained from the area of the
fourth heating. Proximate and ultimate analyses of the
samples were also obtained, to determine if the pyritic
matter was associated with samples containing mainly car-
bonaceous matter or ash.

The results of the analyses are presented in table 2.
Two of the samples, sample 5.1 and core 6.6, contained
relatively large amounts of pyritic sulfur. The presence of
pyritic sulfur in the mine was thus substantiated. Its ab-
sence from some of the samples could be a function of its
distribution mode in the mine and sampling location and
sampling for the chemical analysis. At least for these
samples, it appeared that the pyritic matter was present
mainly with the carbonaceous matter.

Table 2.—Coal analyses of as-received samples from heated areas

(Weight percent unless otherwise specified)

Set 5! Set 6
Sample 5.1 Sample 5.2 Core 6.3? Core 6.5? Core 6.6"
3.15 5.53 0.25 0.23 0.90
23.03 12.43 5.09 8.90 16.34
48.85 16.50 5.39 00 29.75
24.97 65.54 89.27 91.27 53.01
3.68 2.33 0.88 0.48 2.57
53.11 19.35 6.88 2.82 33.79
1.03 0.47 0.13 0.10 0.64
797 2.05 0.72 0.06 758
9.24 10.26 2.42 5.27 241
24.97 65.54 89.27 91.27 53.01
2.62 1.00 0.03 NA 0.19
453 0.87 0.62 NA 7.02
0.82 0.09 0.07 NA 0.37
9,534 3.169 1.035 NA 6.017

NA Not analyzed.

‘Collected in expanded seam zone.
2Drilled in heated area 3.

“Drilled In heated area 4,


temperatures for the stated test conditions is the coal rank.
Lower rank coals record lower peak temperatures, espe-
cially for the second exotherm. During the temperature
range of the first exotherm, aliphatic portions of the coal
decompose more easily and gaseous species, such as CO,
carbon dioxide (CO,), water, and other volatiles, are
evolved. Tars are also released to an extent, and some
char starts to form. Decomposition of aromatic groups
and char oxidation are more prevalent during the tem-
perature range encompassing the second exotherm, but the
transformations detailed for the lower temperature range
also continue. Thermograms for samples from the other
sets, which contained mostly coal, were similar to the first
thermogram seen in figure 8 for sample 1.4.

When a coal that has been previously heated to tem-
peratures in the vicinity of 350° C is tested in the DSC, the
first exotherm is either missing or largely reduced in size.
And, if the coal has previously been exposed to high tem-
peratures or has been heated for an extended period of
time in the temperature range where it undergoes some
decomposition and is then tested in the DSC, the second
peak often shifts to a higher temperature. Both these ef-
fects are seen in figure 8 for sample 2.24, a floor sample
collected from the second area to undergo a heating (heat-
ed area 2). The first exotherm is barely discernible, while
the peak temperature of the second exotherm is higher
than 590° C. The test was terminated at 590° C to prevent
melting of the aluminum sample pan and cover. A few
other samples from the other heated areas generated sim-
ilar thermograms. Thus, depending on the sampling lo-
cation, there was clear evidence that the carbonaceous
matter had experienced high temperatures.

Thermal Analysis of Extraneous Matter

As was indicated previously, extraneous material was
seen on many of the samples. In addition, set 1 contained
a sample of pastelike red material (table A-1, sample 1.2).
This consistency was probably a result of the flooding of
the area with water soon after the discovery of the heating.
A careful examination of the sample revealed the presence
of both black and white particles in the paste. When some
of the particles were sectioned, concentric layers of black,
red, and white material could be seen. A sample of the
red material that contained a few black particles was
tested in the DSC, and the resultant thermogram, pre-
sented in figure 9, displays a variety of features. The
presence of coal (probably some of the black particles) is
evident by the exotherm with peak temperature at 540° C.
The first exotherm usually associated with a coal’s thermo-
gram is much reduced in size, and it peaks at about
300° C, thus indicating that the sample has been heated

> EXOTHERMIC

ENDOTHERMIC <—

1 1 1
300 “400 500

TEMPERATURE, °C

L
0 700 200 600

Figure 9.-DSC thermogram of sample 1.2 (red paste) from
set 1.

prior to the test in the DSC. Various endotherms, at 55°,
65°, 85° to 90°, and 123° C, and a sharp exotherm with its
maximum temperature at 377° C, are indicative of various
phase changes being undergone by the constituents of the
red material during the test.

The appearance of the many mine samples suggested
the presence of a wide variety of oxidation products. Con-
sideration was therefore given to those inorganic oxida-
tion products generally associated with the oxidation of
coal and its associated minerals as the source of the di-
verse endotherms and exotherm cited above. These dis-
tinct products include various kinds of iron oxides and
hydrous iron oxides, for example, Fe,O;- XH,0 or FEOOH
(3); iron sulfates and double salt compounds, such as
jarosite [KFe,-($0,),-(OH) (4), and also gypsum
(CaSO,-2H,0). Hydrous calcium sulfates [CaSO,-2H,O
and plaster of Paris (CaSO, - 1/2H,0)] form when calcium
carbonate (CaCO;) associated with the coal matrix reacts
with sulfuric acid (H,SO,), which also forms during the ox-
idation process (5-6). As can be seen from the chemical
formulas, these compounds contain varying amounts of
water of hydration, which can be driven off by heat at rela-
tively low temperatures in about the same temperature
range in which the endotherms are clustered. Further-
more, hydrous or amorphous iron oxides undergo exo-
thermic crystallization when heated, to form either alpha
or gamma hematite (a- or 7-Fe,0;) (7). This phase
change usually occurs at about 360° C, but in the presence
of other material, the temperature can shift in either
direction. The sharp exotherm in figure 9 is probably due
to just such a change taking place in the red paste sample,
partially confirming the assumption that it is a hydrous
iron oxide, as also suggested by its hue.


150 T T
140 F |
°
i 130 |
5
= L 4
a 120 KEY
5 Initial temp, °C
a 110 9s 4
= @ 130
Ww
F 100F 4
250
200
-
oO
a is0 &
4 a
g S
3 100 8
50
)
0 5 10 15 20

TIME, h

Figure 21.—Temperature-time traces for sample 7.5 from set
7 In self-heating tests in adiabatic oven at Initial temperatures of
95° and 130° C (top), and O, and CO concentration profiles In
‘self-heating test at an Initial temperature of 130° C (bottom).

“Gn table 5. As can be seen from the data, none of the

samples that were tested at 100° C, or at lower tem-
peratures, underwent thermal runaway, and only 2 of
10 samples that were tested at 110° C achieved thermal
runaway from this starting temperature. These two sam-
ples were samples 11.2 and 11.4 from the floor channel cut
on June 9, 1989. These samples contained 15.1 and 6.8 wt
pet FeS,, and 21.2 and 21.9 wt pct ash, respectively. Of
the samples analyzed by coal analyses, these two contained
the least amounts of ash and the highest amounts of fixed
carbon and volatile matter. Sample 11.1, from the same
floor channel, with an ash content of 30.7 wt pet, but with
little pyrite, self-heated at 120° C, and did not self-heat at
110° C. Temperature-time traces for these two tests, as
well as CO and O, concentrations for the self-heating test
at an initial temperature of 120° C, shown in figure 22, are
similar to the traces in figure 21.

Sample 9.3 (collected March 28, 1989) self-heated and
reached thermal runaway from a starting temperature of

19

Table 5.—Results of adiabatic oven
tests (sets 7 to 11)
‘Set and Initial ~~ Maximum’
date Sample tempera- temperature
collected ture, *C rise, °C
Set 7:
Feb. 28,1989... 7.4 70 26
72 70 25
73 70 24
74 70 28
75 70 25
75 95 20
78 190 0)
78 70 26
7 70 30
Set 8:
Mar, 20,1989... 8.4 100 25
82 100 24
. 100 16
a4 100 24
85 100 18
Set 9;
Mar, 28,1989... 9.3 100 64
93 120 e)
Set 10:
Apr. 21,1989 .. 10.1 110 40
102 110 a4
10.3 110 15.2
10.4 110 34
105 110 40
106 110 40
Set 11:
June 9, 1989... 11.4 110 70
Wt 120 (3)
112 110 i)
113 110 60
114 110 ¢

‘Sample underwent thermal runaway.

120° C, whereas at 100° C, the maximum temperature rise
was 6.1° C, The sample was not tested at 110°C. This
sample was not analyzed; its fixed carbon, ash, and pyritic
sulfur concentrations are not known. Thus, comparisons
cannot be drawn.

Sample 10.3 (collected April 21, 1989), tested at 110°C,
had the highest maximum temperature rise, 15.2° C, of all
the samples tested at this temperature that did not under-
go thermal runaway, as seen in table 5. Based on results
from many other tests, the probability is high that this
sample would have undergone thermal runaway at 120°C.
Of all the samples in table 4, it had the highest amount of
Pyritic sulfur (10.07 wt pet) or pyrite (18.84 wt pet). Its
ash content was 34.41 wt pct, while fixed carbon and vola-
tile matter concentrations were 44,08 and 20.51 wt pet,
respectively.

With the exception of sample 9.3 (collected March 28,
1989), whose composition was not known, the samples that


17.
18.
19.

21.

AvaeNE

A-1,
A-2,
A-3,
A-4.
ASS.
A-6.

. DSC thermograms of samples 7.2 and 7.7 from set7 ...

. Results of adiabatic oven tests ........... 6s eee
. Results of H,O, reactivity tests with samples from unheated areas .

ILLUSTRATIONS

DSC thermograms of cores 6.1 and 6.2 from set 6 ..
DSC thermograms of cores 6.2 and 6.6 from set 6
DSC thermograms of very small amounts of samples 7.

Temperature-time traces for sample 7.5 from set 7 in self-heating tests in adiabatic oven at initial
temperatures of 95° and 130° C, and O, and CO concentration profiles in self-heating test at an initial
temperature of 130° C

Temperature-time traces for sample 11.1 from set 11 in self-heating tests in adiabatic oven at
initial temperatures of 110° and 120° C, and O, and CO concentration profiles in self-heating test at
an initial temperature of 120°C .

}.. Pyritic sulfur concentrations in mine samples as a function of time to maximum temperature in H,O,

reactivity testo... 6. e cece eee eee eee eee eee e eee eee ee bennett teens

TABLES

. Identification of oxidation products by qualitative wet chemical analysis . .

Coal analyses of as-received samples from heated areas .
Gas analysis results for sealed flask tests ..........
Analyses of as-received samples from unheated areas .

Samples collected after first heating ....
Samples collected after second heating . .
Samples collected after third and fourth h
Samples collected from expanded seam zone in northwest part of mine
Various cores from mine ............
Samples collected from unheated areas .

Page

BOG

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

Btu/Ib British thermal unit per pound meal/(s-in) millicalorie per second per inch
°c degree Celsius mg milligram

com centimeter min minute

°C/min degree Celsius per minute mL milliliter

cm?/min cubic centimeter per minute mm millimeter

ft foot am micrometer

g gram pet percent

h hour ppm part per million

in inch vol pet volume percent

m meter wt pet weight percent

meal/s millicalorie per second



16

Sealed Flask Oxygen Adsorption Tests

Samples 5.1 and 5.2 were used, as received, for the tests
in the sealed flasks. The samples were neither ground nor
sieved prior to the test because the amounts available were
small and their particle size distribution was similar to the
size range prescribed for the test, but somewhat coarser,
They were weighed and sealed in the flasks, and after
10 days, gas samples were taken for analysis by gas
chromatography. Pressures were read in each flask during
the test period.

Results of the gas analyses are shown in table 3, to-
gether with results from similar sealed flask tests con-
ducted with a coal obtained from the same area, from a
seam located above the seam of the mine. Both coals
have similar coal analyses. The sample from the higher
seam was ground and sieved and immediately sealed in the
flasks, Two different fractions were tested.

Samples 5.1 and 5.2, which were coarser than the other
samples shown in the table, adsorbed some O, and emitted
small amounts of CO, and much smaller amounts of CO;
the amount of CH, evolved was minute. In all probability,
these samples adsorbed some O, before the tests, whereas
the coal sample from the upper seam was carefully packed,
as large chunks, and sealed in plastic bags to minimize
exposure to air. Also, samples 5.1 and 5.2 were not clean
coal samples, but contained large amounts of inert mate-
tial (ash), 25 and 66 wt pct, respectively, as compared with
12 wt pct for the sample from the upper seam. All the
coal seams located in this coal basin contain large amounts
of CH,. The CH, from samples 5.1 and 5.2 was probably
expelled prior to the tests. The quantity of O, adsorbed by
the samples is representative of coals with low self-heating
tendencies. The pressure drops and pressure drop rates
measured in the flasks with miniature pressure transducers
were likewise indicative of coals with low self-heating
propensities. The quantities of CO emitted by samples 5.1
and 5.2 were smaller by an order of magnitude than the
quantities emitted by the sample of the coal from the up-
per seam. Since the amount of CO emitted depends on
the extent of coal oxidation, these results suggest that the
samples were exposed to air prior to the tests. Also, it is
not possible to determine from the results the fraction

of the O, that is adsorbed by the pyrite and the fraction
that is adsorbed by the coal. Sample 5.1, which contained
45 pet pyritic sulfur and 53.1 pet C, adsorbed less O, than
sample 5.2, which contained only 0.87 pct pyritic sulfur and
only 19.4 pet C (table 2). However, sample 5.2 contained
more moisture (5.5 pct) than sample 5.1. Tests with vari-
ous pyrites in the sealed flask have shown that the pres-
ence of moisture is paramount for O, adsorption. Without
sufficient moisture, the rate of O, adsorption by pyrite is
nil.

Sulfur dioxide (SO,) is not usually reported as one of
the oxidation products of pyrite at ambient temperature.
Braley (13) suggested its formation during oxidation of dry
pyrite, as follows:

FeS, + 30, + FeSO, + SO, t. ©)
The gaseous atmospheres in the flasks were tested for the
presence of SO, Only a trace amount (1 ppm) was de-
tected in the flask with sample 5.1, while none was found
in the flask with sample 5.2, These samples were not
dried prior to the test; both samples contained moisture.
SO, was not expected to be present, as indeed was the
case.

SAMPLES FROM UNHEATED AREAS
Visual Observations

The samples were scanned visually for the presence
of any unique features, such as pyrite, sulfur, or oxida-
tion products. The samples were also observed in the op-
tical microscope. In general, no outstanding features were
seen,

Coal Analyses

Coal analyses were obtained for many of the samples
from the unheated areas. Results of these analyses are
summarized in table 4. In addition to ultimate and prox-
imate analyses, many of the samples were tested for forms
of sulfur. As can be seen in the table, each suite of sam-
ples contained at least a few samples with relatively large

Table 3.—Gas analysis results for sealed flask tests

(Volume percent unless otherwise specified)

Sample Particle Oo CO, co CH, 0,
size, um adsorbed
S1.. NA 17.4 0.18 0.0083 0.0043 36
52 . NA 15.2 58 0025 0027 58
Upper seam coal! . 1,200 x 600 123 23 0240 16.21 87
600 x 300 132 a 0268 13.22 78

NA Notanalyzed.

1A sample of coal from the upper seam was ground and sieved. The two designated fractions were

then tested, each in a flask.


2

Finally, coal analyses verified the presence of large
amounts of pyritic sulfur in at least some of the samples
from both the heated and unheated areas.

Results from the adiabatic oven tests showed that the
minimum SHT of the samples that underwent thermal
Tunaway was >100° C, regardless of the pyrite content.
Thus, the tests indicate that the carbonaceous matter in
the samples is not highly reactive. Some of the samples
tested, which had high percentages of pyritic matter, but
also contained large amounts of ash, may not have had
sufficient organic carbon for thermal runaway to occur in
the adiabatic oven tests. Thus, at present, the role of
Pyrite in lowering the minimum SHT value of a mine sam-
ple has not been clearly established by these adiabatic
oven tests,

Considering all the accumulated data, along with the
low spontaneous combustion potential of the mined coal
and the subfloor carbonaceous matter, the conclusion
was reached that the expanded seams beneath the mine
floor contain amounts of pyrite in such forms (size and

morphology) that enhance their oxidation potential. Fur-
thermore, the oxidation of the pyrite on exposure to the
high flow of air and moisture, especially when the floor
heaves, brings about increases in the temperature, such
that the oxidation of carbonaceous matter in contact with
the pyrite is accelerated.

Pyrite has been cited for many years as one of the fac-
tors capable of affecting the spontaneous combustion of
coal. This is one case in which it has had a major effect.
Other coal mines containing substantial amounts of pyrite
may be worked in the future.

Procedures for easily identifying the presence of pyritic
matter and for evaluating its approximate concentration, as
well as assessing its effect on the self-heating tendency of
the coal associated with it are needed. In addition to coal
analysis, other procedures, such as careful visual ob-
servations, thermal analysis, and tests with H,O,, can help
in this regard. These tests require the acquisition of sam-
ples ahead of mining so that locations rich in pyrite can be
identified.

REFERENCES

1, Smith, A. C,, and C. P. Lazzara. Spontaneous Combustion Studies
of U.S. Coals. BuMines RI 9079, 1987, 28 pp.

2. Miron, Y., A. C. Smith, and C. P. Lazzara, Sealed Flask Test for
Evaluating the Self-Heating Tendencies of Coals. BuMines RI 9330,
1990, 18 pp.

3, Huggins, F. B, G. P. Huffman, D. A. Kosmack, and D. EB.
Lowenhaupt. Mossbauer Detection of Goethite (a-FeOOH) in Coal
and Its Potential as an Indicator of Coal Oxidation. Int. J. Coal Geol.,
v. 1, No, 1, 1980, pp. 75-81.

4, ‘Huggins, F. B., G. P. Huffman, and M. C. Lin. Observations on
Low-Temperature Oxidation of Minerals in Bituminous Coals. Int. J.
Coal Geol, v. 3, No. 2, 1983, pp. 157-182.

5, Pearson, D. B., and H. Kwong. Mineral Matter as a Measure of
Oxidation of a Coking Coal. Fuel, v. 58, Jan. 1981, pp. 63-66.

6. Fuller, BL, Jr. Structure and Chemistry of Coals: Transport
and Nucleation Processes in Mineral Metathesis. J. Colloid and Inter-
face Sci,, v. 89, Oct. 1982, pp. 309-327,

7. Furuichi, R, M. Hachiya, and T. Ishii, DTA Study on a-Fe,0,
Formation From Fe(IIl)-Hydroxides Containing Foreign Metallic Ions.
Thermochim, Acta, v. 133, 1988, pp. 101-106.

8 Weast, RC. (ed.). Handbook of Chemistry and Physics. Chem.
Rubber (Cleveland, OH), 57th e4., 1976, p. B-121.

9. Chase, M, W., Jr, C. A. Davies, J. R. Downey, Jr., D. J. Frurip,
R.A. McDonald, and A. N. Syverud. FeS, (C. Marcasite), JANAP
‘Thermochemical Tables, Part II. J. Phys, and Chem, Ref. Data, v. 14,
Suppl. 1, 1985, p. 1198.

10. Kullerud, G. Sulfide Studies, Ch. in Researches in Geochemistry,
ed, by P. H. Abelson. Wiley, v. 2, 1967, pp. 286-321,

11. Dunn, J. G.,, G.C. De, and P. G. Fernandez. The Effect of Ex-
perimental Variables on the Multiple Peaking Phenomenon Observed
During the Oxidation of Pyrite. ‘Thermochim, Acta, v. 135, 1988,
pp. 267-272.

12, Mackenzie, R. C. (ed). Differential Thermal Analysis, V. 1, Fun-
damental Aspects, Academic Press, 1970, pp. 204-205.

13, Braley, S.A. Acid Coal Mine Drainage. Trans. Soc. Min. Eng,
AIME, v. 205, Mar. 1956, pp. 314-318,

14, Heard, I, and F, B. Senftle, Chemical Oxidation of Anthracite
With Hydrogen Peroxide Via the Fenton Reaction, Fuel, v, 63, Feb.
1984, pp. 221-226.

15, Ward, C. R. Isolation of Mineral Matter From Australian Bi-
tuminous Coals Using Hydrogen Peroxide. Fuel, v. 53, July 1974,
pp. 220-221.


—> EXOTHERMIC

|

ENDOTHERMIC ~— —=EXOTHERMIC

ENDOTHERMIC ——

1 1 1 L 1
ce) 200 400

TEMPERATURE, °C

Figure 19.—DSC thermograms of very small amounts of sam-
ples 7.1 to 7.7 from set 7.

600

1 1 nl 1 nl

ie) 200 400
TEMPERATURE, °C

600

Figure 20.-DSC thermograms of samples 7.2 and 7.7 from
set 7.

The results presented here indicate that the presence of
sulfidic or pyritic matter may be identified even in small
samples, by the presence of the plateau, at about 460° to
480° C, if the pyrite is well distributed throughout the
sample, fine in size, and found in relatively large amounts.
Larger samples are needed to see the jagged pattern, as
well.

Adiabatic Oven Tests

The minimum SHT of a coal sample is determined in
tests in the adiabatic oven. Previously tested coal samples,
which were obtained from various seams from the same
area, had minimum SHT values of 135° C (1).

The tests in the adiabatic oven were conducted with two
goals in mind. The first goal was to see if the samples had
about the same minimum SHT values as previously found.
The second goal was to assess the effect of pyrite in the
samples on the minimum SHT values. Since it was postu-
lated that the presence of pyrite might reduce the min-
imum SHT values of those samples containing pyrite, tests

were at first conducted at temperatures much lower than
135° C. The first batch of samples (collected February 28,
1989) was tested at an initial starting temperature of 70° C,
and none of the samples underwent thermal runaway. Be-
cause of the small increase in temperature (about 3° C)
of the samples during the tests, the next test, with sam-
ple 7.5 from the same batch, was started at a tempera-
ture of 95°C, but the sample did not undergo thermal
runaway. A fresh sample was then tested at a starting
temperature of 130° C, and it did undergo thermal run-
away. Temperature-time traces for the two tests, at 95°
and 130° C, are presented in figure 21. These traces are
typical for tests in which samples do and do not undergo
thermal runaway. The O, and CO concentration profiles
of the exit gas stream were also recorded for each test.
The O, and CO concentration profiles in the self-heating
test of sample 7.5, at the initial temperature of 130° C, are
also shown in the figure. The O, concentration dropped
slightly, while CO concentration showed a rapid rise at the
start of the test and rose slowly thereafter.

Samples collected on March 20 and 28, 1989, were test-
ed next in the adiabatic oven at a starting temperature of
100° C, while samples from the last two batches, April 21,
and June 9, 1989, respectively, were tested at 110°C. Re-
sults of all the tests in the adiabatic oven are summarized


wood, contained two typical exotherms, while the second
thermogram, for the charred portion, contained only one
exotherm. When a sample of the uncharred wood was
heated in the DSC to 340° C and then immediately re-
moved from the DSC to cool and arrest further reaction,
it was found to be charred. When reheated from ambient
temperature, this sample still registered two exothermic
peaks (third thermogram in figure 16). However, the first
exotherm was more shallow. Ratios of the peak heights
for the unheated and heated samples (first and third ther-
mograms) were 1.7 and 2.5, respectively, indicating the
reduction in size of the first peak of the thermogram for
the heated sample. These results imply that the charred
wood block sustained a temperature of at least 340° C, the
same exposure temperature that was inferred for the coal
sample from the floor of the heated area, as discussed in
the "Thermal Analysis of Coal-Shale” section, for sample
2.2A, and more probably an even higher temperature.

Thermal Analysis of Cores

‘Small samples, which appeared of interest visually, were
collected from surfaces of the various cores or from por-
tions of the cores and were tested in the DSC. Only a few
results will be cited; some of the resultant thermograms
were similar to thermograms found for the samples from
the other sets, while others were devoid of thermal
features.

Core 6.1 was received with samples 5.1 and 5.2 (on
September 29, 1988). It consisted mostly of dark gray
sandy material and some limestone. Thermograms of two
samples scraped from the core are presented in figure 17.
The first thermogram is essentially representative of inert
matter and does not contain any significant endotherms or
exotherms, while the second thermogram indicates the
presence of some carbonaceous matter.

Core 6.2 from heated area 1 was analyzed next. A very
small sample, which was mostly soft coal, yielded the
familiar coal thermogram (third thermogram in figure 17),
while a sample scraped from a shale portion showed mini-
mal thermal activity, as observed in the fourth thermogram
in the figure. There was extraneous yellowish white ma-
terial on one edge of the core. The material was removed,
and a relatively large sample was tested in the DSC. The
endotherm, with a peak at 132° C, as shown in the first
thermogram in figure 18, is evidence of the presence of
oxidation products. The second thermogram in the fig-
ure, of another specimen, from the shaley portion of the
core, contained a small exotherm with its peak at 432° C.
This is possibly due to the crystallization of a small
amount of hydrous ferrous oxide to a-Fe,O, and is like-
wise an indication of oxidation products being present. As
mentioned previously, the temperature at which this phase
change occurs can vary, depending on sample composition.

—> EXOTHERMIC

ENDOTHERMIC ~<—

n
200 ‘300 400 500 ‘600
‘TEMPERATURE, °C

°
8

Figure 17.-DSC thermograms of cores 6.1 and 6.2 from set 6,

6.2, extraneous matter
6.2, shaley portion

—> EXOTHERMIC

ENDOTHERMIC <—

L 1 L n 1
0 700 200 300 400 500 600

TEMPERATURE, °C
Figure 18.—DSC thermograms of cores 6.2 (extraneous mattor
‘and shaley portion) and 6.6 from set 6.

Three cores, 6.3, 6.4, and 6.5, were obtained from heat-
ed area 3, One sample, from core 6.4, showed the regular
coal thermogram, while another sample was essentially
without thermal features. These thermograms are not
shown. Thermograms of a few samples from core 6.5
were also devoid of the exothermic behavior associated
with carbonaceous material, although there was some en-
dothermic activity, characteristic of the oxidation prod-
ucts, with endothermic peaks at 122° and 138°C. These
thermograms are also not presented in this report.


6-9-89

L|_ fp

Cooonoo

IZ

z-28.e0—-O
Cea

a

LEGEND

® Date of collection and
location of samples
from unheated areas

Heated areas

° 500 1,000
[nd ao)
Scale, ft

0D

WL |

 C

Seon ooooRomooos

J Oo

OL ZOO OO OOOO DIODE

Yao

eel 7] IN] 1]
dH} lq) ek
H} A) oo
Oo] lg
4) |B) i
HA
Ol el ie
Cy) |G} fo
H] Ih}
O) i
oO} oS
aL
i
2) One
|

re
5

[

Figure 5.—Schematic mine map showing heated areas and locations of unheated areas from which samples were collected.



INTRODUCTION

Self-heating events in underground coal mines are
usually not detected until relatively high temperatures are
attained and/or other phenomena, such as odor and haze,
are observed, An increase in the concentration levels of
carbon monoxide (CO) as a function of time is often the
first signal that the temperature of the coal is increasing
because of self-heating.

The propensity of coals to self-heat is rank associated.
In general, low-rank coals are more susceptible to self-
heating than high-rank coals. Thus, precautions against
the occurrence of spontaneous combustion are more
commonly practiced in low-rank coal mines. However,
heatings and spontaneous combustion episodes can occur
in mines of higher rank coals.

A deep coal mine that mines a low-volatile bituminous
coal has experienced a series of floor self-heatings of vari-
ous degrees of severity over a period of several years. All
of the heatings have occurred in a section of the mine sep-
arated by a fault line, where an expanded seam exists be-
neath the floor. To date, 14 heatings have occurred, 7 of
which were around the gob of a longwall panel extracted
in this section. There is extensive folding and faulting in
the coal basin in which the coal seam is located. Both
normal and strike-slip faulting have occurred, and hori-
zontal and vertical displacement is substantial. Floor

heave is common throughout the mine entries, especially
in tailgate roads and at four-way intersections, Yield
abutment-yield pillar design is now employed and has
helped to reduce floor heave to a great extent, but has not
eliminated it completely. The ground heave exposes the
material below the floor to a continuous supply of air and
moisture. Because of the depth of the mine and the
amounts of methane (CH,) released, high-ventilation rates
are prevalent throughout many of the mine areas, often re-
sulting in high-pressure differentials across stoppings and
other ventilation-control structures. To identify the causes
of these floor heatings, the U.S. Bureau of Mines insti-
tuted a research study to evaluate samples from both heat-
ed and nearby unheated areas of the mine. The samples
were collected by mine personnel and sent to the Bureau
for analysis by several standard laboratory techniques,
including thermal analysis, coal analysis, qualitative chem-
istry, instrumental analysis, and adiabatic oven experi-
ments, The reactivity of many of the samples with aque-
ous H,O, was also assessed. Finally, two samples were
tested in a sealed flask test to measure their avidity for
oxygen (0,).

The conclusions reached as to the causes of the heat-
ings, based on the cumulative evidence obtained by the
analyses of all the samples, are presented.

MINE SAMPLES EVALUATED

SAMPLES FROM HEATED AREAS
AND CORE SAMPLES

Five sets of samples, with each set and/or subset con-
taining multiple samples, were collected in the heated
areas of the mine at different times. Most of these sam-
ples consisted of mixtures of coal, shale, and/or rock, with
the amounts varying from sample to sample, depending on
the strata and the manner in which they were collected.
Clays, silica, mica, and other similar compounds were in-
termingled in the samples. Set 2, which also contained
wood cribbing, was from an area that sustained the worst

heating. A sixth set, consisting of six cores, drilled at
different times and in different areas in the mine, was also
examined. Descriptions of all the samples in these sets are
presented in the appendix, tables A-1 through A-5, for sets
1to 6. The individual samples are designated by numbers
for ease of identification. A few of the samples are shown
in figures 1 to 4. A schematic map of the mine, with
marked locations of some of the heated areas, shown in
figure 5, indicates the extent of the problem, namely, that
the heated areas were not localized but rather widely
spread,


CAUSE OF FLOOR SELF-HEATINGS IN AN
UNDERGROUND COAL MINE

By Y. Miron,' C. P. Lazzara,? and A. C. Smith?

ABSTRACT

This report presents a U.S. Bureau of Mines study to identify the causes of self-heating events
beneath the floor of a deep underground coal mine.

Mine samples from both heated and unheated areas were examined by various techniques, including
visual, microscopic, thermal, chemical, and instrumental tests. The combined results led to the
conclusion that pyrite (FeS,) oxidation was the prime cause of the heatings. However, adiabatic oven
tests of selected samples did not indicate the effect of the pyrite on the self-heating process, probably
because of the experimental conditions,

A fast, simple procedure to assess the concentration of pyrite in the mine samples was developed,
utilizing the reactivity of the sample with an aqueous solution of hydrogen peroxide (H,0,).

3Chemical engineer.

Supervisory research chemist.

3Research chemist.

Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.


14

Core 6.6, from heated area 4, contained both coal and
shiny, metallic crystals, which were seen in the microscope.
All of these samples gave thermograms similar to other
thermograms of samples containing pyrite, with the jagged
features and the endotherm at about 460° C, as observed
in the third and fourth thermograms in figure 18.

Qualitative Chemical Analyses

Both the appearance of the samples and the thermal
analysis results presented a strong possibility that, in ad-
dition to coal oxidation, or jointly with it, oxidation of
sulfide compounds occurred in the mine. Ambient oxida-
tion, or weathering, of pyrite has been studied in great
detail, and much information is available in the literature.
Major oxidation products include H,SO, and iron sulfates,
including ferrous sulfate (FeSO, - XH,O) and ferric sulfate
[Fe,(SO,),-XH,O]. It is easy to identify the anion (SO,?-)
and cations (Fe** and Fe**) formed, as well as the strongly
acidic conditions that result.

To test samples for the presence of the oxidation prod-
ucts, the samples were mixed with water and filtered. The
pH values of the filtered solutions were determined, and
various indicators were added to aliquots of the filtrates.
Characteristic results, which were obtained for many of the
samples from the different locations, are shown in table 1.

Table 1.—Identification of oxidation products

by qualitative wet chemical analysis
Indicator End result Meaning

Aqueous barium ‘White precipitate of Sulfate ions
chloride (BaCi,) barium sulfate (BaSO,). present.
solution.

‘Ammonium hydrox- —Gelatinous precipitate due Fe ions
ide (NH,OH). ‘to formation of hydrous. present.

iron compounds,

Hydrochloric acid Distinctive yellow color of Fe? ions.
(HC). iton chloride (FeCi,). present.

‘Aqueous ammonium Distinctive deep red color Do.
thiocyanate of iron thiocyanate
(NH,SCN) solution. _{Fe(SCN),).

‘Aqueous potassium Distinctive deep blue Fe?* ions
ferricyanide color of iron ferricyanide, present.
Ik,Fe(Cn), Fes{Fe(CN)¢l,.

‘solution.

When oxidation products were present, the pH values
of the water eliminate extracts were usually 2 or less; that
is, they were highly acidic owing to the presence of H,SO,.
The H,SO, that forms during pyrite oxidation reacts with
carbonates associated with the coal or shale. CaCO, con-
verts to calcium sulfate or its hydrated form, CaSO,-2H,0,
the presence of which is therefore also associated with coal

pyrite oxidation. The ferrous and ferric sulfate products
also form many hydrates, among which are szomolnokite
(FeSO,-H,0), rozenite (FeSO,-4H,0), FeSO,-5H,0,
FeSO,:7H,0, and Fe,(SO,)3-9H,O. Such hydrated salts
lose their water of hydration when heated, usually sequen-
tially, depending on the heating rate. These events were
evident in the many thermograms of the various samples
from the mine,
Reactions representing pyrite oxidation, such as

FeS, + 7/20, + H,0 + Fe?* + 280, + 2H*, (A)
Fe’* + 1/40, + H* + Fe** + 1/2H,0, (B)
and
FeS, + 14Fe?* + 8H,0 + 15Fe?*
+ 280,> + 16H*, ©

only describe the global events. Many more reactions are
involved, and the kinetics and mechanism, although fairly
well understood, are not known in detail. What is known,
in addition to the fact that acid, iron sulfates, and hydrous
iron oxides are the main reaction products, is that the
process is highly exothermic, with the overall rate in-
creasing with increasing temperature. If the heat cannot
be dissipated, spontaneous combustion will occur.

Instrumental Analyses

A few selected samples, after suitable preparation, were
analyzed by X-ray diffraction or by ICAP atomic emission
to obtain additional data on their compositions and for
verification of some of the results of the other tests. Some
characteristic results of interest from sample sets 1, 3, and
6 are given here:

Set 1—The red paste (sample 1.2) from set 1 was
evaluated by X-ray diffraction and found to contain Fe,0,,
FeSO,:5H,O, KFe,:(SO,),:(OH), and quartz (SiO,).
The three iron compounds are frequently found among the
oxidation products of pyrite. The same red paste sample
was treated with HCl, and the filtrate was analyzed by
ICAP atomic emission, The relative amounts, in parts per
million, of the main cations present in the filtrate were 204
Fe, 10 Al, and 5 each Ca and Mg, indicating that iron
compounds were the major constituents. The presence of
some clay was shown by the Al ions.

White and orange extraneous material from the various
surfaces of the samples was collected, dissolved in water,
and filtered. The clear solutions, analyzed in the same


Figure 3.-Various samples of coal, shale, and rock dust from Figure 4.—Various samples of coal, shale, and rock dust from
set 3. (See table A-3 for descriptions of each sample.) sets 3 and 4. (See table A-3 for descriptions of each sample.)


150
Q 140
rd
> 130
F:
fj 120
$ KEY
a 110 Initial temp, °C 4
F BII0
@ 120
100 beep
2 p~—> 1 1 1 300
| 250
-
g 200 g
g 150
‘ 8
é 100
50

n°)
4 6 8 10 2 14
TIME, h

Figure 22.-Temperature-time traces for sample 11.1 from set
11 In self-heating teste in adiabatic oven at Initial temperatures
of 110° and 120° C (top), and 0, and CO concentration profiles
In self-heating test at an Initial temperature of 120° C (bottom).

co) 2

underwent thermal runaway had the higher heating val-
ues, fixed carbon, and volatile matter. More systematic
experiments, however, are needed before the effects of ash
concentration on the minimum SHT of a mine sample can
be quantitatively determined. The standard test conditions
in the adiabatic oven test are not conducive to the oxida-
tion of pyrite. Pyrite oxidizes in humid air, initially at a
slow rate, The ferrous sulfate that is formed oxidizes next,
also at a slow rate, to ferric sulfate. Once ferric ions are
present, they oxidize pyrite at an accelerated rate (see
reactions A to C) and, concurrently, the temperature rises,

Large amounts of moisture are required for the formation
of the sulfates, and most likely, sufficient moisture was not
available in the adiabatic oven tests. Examination of the
samples after the tests did not show any oxidation prod-
ucts. Specific experimental conditions must be devised
that will facilitate pyrite oxidation and the quantitative
identification of its effects on the minimum SHT of a mine
sample,

None of the mine samples tested are considered to be
highly reactive to self-heating compared with the self-
heating of coal; coals that have minimum SHT’s > 100° C
are considered as having a low spontaneous combustion
potential (7), These results, then, give additional credence
to the idea of pyrite itself being the primary or the initial
material to self-heat in the mine. The results also indicate
that adiabatic oven tests with the present experimental
conditions and procedures are not able to assess the effect
of pyrite on the self-heating process of the mine sample
and will need to be changed,

Hydrogen Peroxide Reactivity

H,0, is a strong oxidizer and reacts with both organic
and inorganic materials, It has been used in studies of ox-
idation of coals of various ranks (14), as well as for the
isolation of mineral matter from coal (15). In the pres-
ence of ferrous and ferric ions, H,O, undergoes catalytic
reduction and oxidation simultaneously; the combined re-
actions are identified as the Fenton reaction. When coal
contains pyrite, the pyrite decomposes in the H,O,, and
the iron ions, in turn, catalyze the decomposition of the
peroxide, The rate of the overall reaction depends on the
concentration of the pyritic matter, as well as on other fac-
tors, including particle size of the test sample and the py-
ritic matter in it, H,O, concentration, and starting ambient
temperatures.

The results of the tests with many of the samples from
the unheated areas are given in table 6. Temperature in-
creases were related to the concentrations of pyrite in the
samples, but a better correlation existed between the time
to reach maximum temperature and the concentration of
pyritic sulfur in the sample, as determined in the coal
analyses (shown in figure 23). The data can be expressed
by the following formula of a power fit curve:

T = 1261 x POS, (1)


RI} 9415 REPORT OF INVESTIGATIONS /1992

Cause of Floor Self-Heatings in an
Underground Coal Mine

By Y. Miron, C. P. Lazzara, and A. C. Smith

UNITED STATES DEPARTMENT OF THE INTERIOR

BUREAU OF MINES


EXPERIMENTAL PROCEDURES

VISUAL OBSERVATIONS

All of the samples were visually examined and pertinent
observations were noted. Optical microscopy was used in-
termittently to observe the samples and to aid in the
selection of specific specimens for tests by thermal anal-
ysis, qualitative chemical analysis, or instrumental analysis.

THERMAL ANALYSIS

A Du Pont‘ 990 differential scanning calorimeter (DSC)
was used to evaluate various portions from all the samples.
Sample sizes varied from a few milligrams to about 75 mg.
For the evaluation of bulk samples, larger amounts were
used, whereas for specific samples of interest, carefully
collected specimens were usually small in size. The sam-
ples were contained in covered aluminum pans and heated
at 10° C/min in an initial atmosphere of static air, which
changed to a self-generated atmosphere as the test pro-
gressed. In all tests but one, the sensitivity was set at
1 meal (s-in), In the one exception, it was set at 2 mcal/
(s-in). For additional insight into the identity of the com-
pounds present in the samples, some of the heated sam-
ples were cooled in the pans and then reheated, usually at
the same conditions.

CHEMICAL AND INSTRUMENTAL ANALYSES

Standard qualitative chemical methods were used to
identify the presence of ferrous and ferric cations and
sulfate and carbonate anions, These procedures are de-
scribed in greater detail in the section "Experimental Re-
sults and Discussion." In a few cases, X-ray diffraction
analysis and inductively coupled argon plasma (ICAP)
atomic emission spectroscopy were used to identify select
minerals, compounds, and elements.

COAL ANALYSES

Ultimate and proximate analyses, heating values, and
forms of sulfur (sulfatic, pyritic, and organic) were ob-
tained for many of the samples, mostly from the unheated
areas,

ADIABATIC OVEN TESTS
Tests were conducted in an adiabatic oven to determine

the minimum self-heating temperatures (SHT’s) of many
of the samples from the unheated areas. A detailed

‘Reference to specific products does not imply endorsement by the
‘U.S. Bureau of Mines.

description of the test is available in a Bureau publication
(2S The following is a brief description of the test pro-
cedure. The sample was pulverized and sieved, and the
minus 100-mesh (minus 150 m) fraction was dried in an
oven at 67° C with a flow of dry nitrogen (N,). A 100-g
sample was placed in the adiabatic oven and brought to a
preselected initial temperature under a 200-cm*/min flow
of dry N,, The'sample was then exposed to an equal flow
of humidified air. Usually, a series of tests is made, each
test with a fresh sample, in 5° C increments, until a min-
imum initial temperature that produces a sustained exo-
thermic reaction (thermal runaway) is determined. Not
enough material was available to conduct all the tests
necessary for the determination of the minimum SHT’s of
the samples. Accordingly, for some of the samples, tests
were conducted at larger temperature increments, and the
probable minimum SHT values were qualified to within a
range of about 20° C. Still, for other samples, only an
upper or lower limit (go/no-go) was established. The lack
of sufficient material was also the reason for using the
minus 100-mesh fraction rather than the minus 100- plus
200-mesh fraction prescribed for the tests. A schematic of
the adiabatic oven is shown in figure 7,

HYDROGEN PEROXIDE REACTIVITY

1,0, is a strong oxidizer and reacts exothermally with
both inorganic and organic material. When mixed with
coal that contains pyrite, it reacts with the pyrite and
decomposes at a fast rate.

S{talic numbers in parentheses refer to items in the list of references
preceding the appendix at the end of this report.

Jeosier oases

ees {Top heater
Sample oven with
Thermocouples— K cutaway view
Somple —
Stoinless steel j
ossembly—— 1 {Wire sample basket
‘| (76 by Sem)
a {-Side heater
Dewar flask
a) KC iS-emi0) J |
Insulation ——{S

Figure 7.—Schematic of adiabatic oven.


Toe of draw rock
on coal seam

—_>
: +—Normal mine
a aes ee entry floor
wf II" Coat
C hae
+ EI" Rock
B ao
7" Coal
coal | wy
5 Tou
N 4 Rock
+L
66" :
c oO g a o
coal | \, a 26° Boney coal
3
N
iS:

12" Rock to floor
| of graded entry

Figure 6.—Schematic of floor strata showing sampling zones for samples of set 11.


Tt 7 1 Rak alc

Figure 1.—Various samples of coal, shale, and rock dust from
set 2. (See table A-2 for descriptions of each sample.)

SAMPLES FROM UNHEATED
AREAS IN MINE

Five sets of samples from unheated areas were collected
in floor channels, at different times, as detailed in ta-
ble A-6. The individual samples are also desginated by
numbers, for ease of identification. Sites where samples
were collected are identified in the map in figure 5. Many
of the samples contained large amounts of rock shale and
clay compounds, in addition to coal, depending on the
areas from which they were taken, Zone samples con-
tained a mixture of coal, rock, and shale, Coal-rock inter-
face samples contained less rock and shale than zone sam-
ples, while coal samples contained essentially coal only.
As an example, figure 6 depicts the various zones from
which the channel samples were collected on June 9, 1989.

Figure 2—Sample 2.3 (partially charred wood-crib block) from
set 2,


amounts of pyritic sulfur, as high as 10 wt pct (or 18.71 wt
pet FeS,). For example, samples 7.5 and 7.7 from the first
group of samples (collected February 28, 1989) contained
4.60 and 4.39 wt pet pyritic sulfur (8.6 and 8.2 wt pet FeS,,
respectively), while samples 7.2 and 7.4 from the same
group contained only minor amounts. In general, large
amounts of pyritic sulfur were found in the same zones for
all the sets of samples,

None of the samples contained noticeable amounts
of sulfate (or sulfatic matter); that is, none of the sam-
ples have undergone appreciable oxidation. Many of the
samples contained large amounts of ash; this is likewise re-
flected in the low heating values of many of these samples.

Many of the samples from the unheated areas were also
examined by the Bureau’s Twin Cities Research Center’
The sulfur content of these samples was measured and the
results were in very good agreement with both the coal
analyses and H,O, test results reported here.

Thermal Analysis

The identification and quantification of pyrite was main-
ly done by chemical analysis. However, its presence in

“Letter reports prepared in 1988-89 by G. W. Reimers, Twin Cities
Research Center; available from BuMines, Minneapolis, MN.

7

various samples was also established by thermal analysis in
the DSC. The main purpose of analyzing the samples
from the unheated areas in the DSC was to rapidly iden-
tify the presence and thereby the location of oxidation
products of pyritic matter.

All the samples from the first group of samples (collected
February 28, 1989) were analyzed in the DSC. Very small
samples, of the order of 1 mg in size, were used. The
thermograms for samples 7.1 to 7.7 are presented in fig-
ure 19, None of the thermograms show evidence that oxi-
dation products are present in these samples, as there are
no endotherms, The first four thermograms, samples 7.1
to 7.4, contain the usual two exotherms associated with
coal, while the other three, samples 7.5 to 7.7, have an
additional plateau or shoulder, at about 475° C, Somewhat
larger samples of 7.2 and 7.7 were then tested with the
same results, as depicted in the thermograms in figure 20.
Only the thermogram for sample 7.7 displays an endo-
therm at about 478° C, which is thought to be indicative of
the presence of pyritic matter. These results, suggesting
the presence of pyrite in samples 7.5, 7.6, and 7.7, as
deduced from the DSC thermograms, agree with coal anal-
ysis results for 7.5 and 7.7, Likewise, the low pyritic
content in samples 7.1, 7.2, 7.3, and 7.4, based on DSC
thermograms, again parallel coal analyses for samples 7.2
and 7.4,

Table 4—Analyses of as-recelved samples from unheated areas (sets 7 to 8, 10 to 11)

(Weight percent unless otherwise specified)

Set and Heating Proximate analysis. Ultimate analysis Forms of sulfur
date Sample value, Mois- Volatile Fixed Ash Hydro- Car Nitro- Suk Oxy. Suh Py- Organ
collected Btu/lb ture. matter Cc gen bon gen fur gen fatic ritic Ie
Set 7:
Feb. 28, 1989.. 7.2 8189 0.49 «17.28 38.78 «43.45 292 47.74 0.91 0.39 489 0.03 0.08 0.28
74 10,260 67 = 20.42 47.67 31.24 357 5864 1.13 73 469 02 28 4
75 6,045, 55 15.73 29.01 54.71 257 3449 70 466 287 04 4.60 02
7 6,260 89 15.79 29.69 53.63 255 35.01 78 445 358 04 4.39 02
Set 8:

Mar, 20,1989... 8.1 NA NA NA NA NA NA NA NA NA NA 06 475 0
82 3,901 75 13.12 19.52 66.61 NA NA NA NA NA 18 8.42 00
83 5,274 90 15.19 24,50 59.41 NA NA NA NA NA .09 3.44 © .00
84 NA NA NA NA NA NA NA NA NA NA 03 1.31 00
85 3,536 99 12.04 15.96 71.01 NA NA NA NA NA 05 1.47 04

Set 10:
Apr. 21,1989... 10.1 NA 82 NA NA NA NA NA NA NA NA 0311 20
10.2 NA 99 NA NA NA NA NA NA NA NA 05 7 AT
10.3 9,959 1.00 20.51 44.08 34.41 3.38 «49.94.68 1070 89 27 10.07 36
10.4 NA 1.18 NA NA NA NA NA NA NA NA 20 7.03 22
10.5 NA 1.30 NA NA NA NA NA NA NA NA 01 3.02 33
10.6 5,631 143° 1490 26.78 56.89 256 31.87 88 387 393 16 363 08

Set 11:
June 9, 1989... 11.1 10,173 1.48 = 20.62 «47.17 30.73 NA NA NA NA NA 4 4 49
1.20 (11,413 1,66 22.52) 54.64 21.18 = NA NA NA NA NA 49 8.08 = 1.08
11.3 4,684 «1.28 §=— 14.16 = 22.83 61.70 NA NA NA NA NA 27 397 2
11.4 11,609 1.30 2285 53.93 21,92 NA NA NA NA NA 19 3.64 86

NA Not analyzed.


APPENDIX.—DESCRIPTION OF ALL SAMPLES COLLECTED IN MINE

Table A-1.—Samples collected after first

heating (set 1)
(Heating discovered November 1986)
Sample Description

Mixture of coal, shale, rock,

Red, claylike paste,

Mixture of coal, shale, rock.
Do.

Table A-2—Samples collected after second heating (set 2)

(Heating discovered February 1988)

Location
and sample

Description Depth below

surface, in

ot a NAp
Rock dust with some coal .. NAD
Coal, shales... eee eae NAD
Rock, coal from mine floor . . NAp
. NAp
NAp
NAp
NAp
NAp
NAp
NAD
Shale, coal, rock (hard shale), 0-15
Boney coal, rock (hard shale), 15-30
coal.
27 eeveeee Bright coal (reaction zone), 30-45,
shale.
28... eee Rock (shale), main moisture 45-60
zone.
NAD Not applicable.
15,000 ft from area of 1st heating.
2Se0 figure 1.
Temperatures above 300° C were measured in two locations
in the area.
‘See figure 2.

Table A-3.—Samples collected after third (set 3)

‘and fourth (set 4) heatings
(Heatings discovered August 1988)
Sample Description Location
SET 3!
31
Aue Large coal piece with oxidation One crosscut
Products on surface. Coal- away from loca-
shale powder or dirt, tion of 1st heat-
ing. Grab sam-
ples from trench,
24 In below sur-
face,
Bu... Broken coal pieces and coal Do.
powder,
3.2;
Aue Small pieces of coal showing Do.
oxidation products,
Buse Black coal powder ......... Do.
Cree White powder on top of coal Do.
‘and rock pieces.
33:
Aue Large rock-coal pieces with ox- Do.
idation products on surface.
Dark gray-black powder .....

#8

Shiny coal powder scraped
from back of large piece in A.
34:
Auseee Coal pieces covered with rock Do.
dust.
Bu Rock dust containing some Do.
coal and possibly other com-
pounds.
SET 4
4A eee Coal pieces. One large shale- About 1,800 ft
like chunk covered with white from area of 1st
and purple-red matter. heating.
\See figure 3.
2See0 figure 4.

Table A-4.—Samples collected from expanded seam
zone In northwest part of mine (set 5)

(Heating discovered August 1988)
Sample Description Depth below
footwall, in
Ble... Coal, shale, rock; 1- to 25-mm- 15
diam particles.
52.0... Coal, shale, rock; moist ..... 50



CONTENTS

Mine

Samples from heated areas and

samples evaluated

Samples from unheated areas in mine ...

Experimental procedures ...
Visual observations .
Thermal analysis . .
Chemical and instrumental analyses
Coal analyses ...
Adiabatic oven test
‘Hydrogen peroxide reactivity
Sealed flask oxygen sdsorption tests

Experimental results and discussion
Samples from heated areas ..

Samples from unheated areas

Visual observations .
Thermal analysis of coal-s .
Thermal analysis of extraneous matter ..

Thermal analysis of samples containing sulfidic matter
Thermal analysis of other mine materials . .
Thermal analysis of cores ...
Qualitative chemical analyses
Instrumental analyses ..
Coal analyses ......
Sealed flask oxygen adsorption tests

Visual observations .
Coal analyses .

Thermal analysis
Adiabatic oven tests .
Hydrogen peroxide reactivity

Summary ..
References
Appendix—Ds ed in mine ...
ILLUSTRATIONS
1, Various samples of coal, shale, and rock dust from set 2...
2. Sample 2.3 from set 2 ..
3. Various samples of coal, shale, .
4. Various samples of coal, shale, and rock dust from sets 3 and 4 .. .
5. Schematic mine map showing heated areas and locations of unheated areas from which samples were

BASBRES ears

Collected 1.6... eee eee cece eee e eee e eee eee eeeeeeeneneee
Schematic of floor strata showing sampling zones for samples of set 11 .
Schematic of adiabatic oven ..
DSC thermograms of sample 1.4 from set 1 and sample 2.24 from set 2
DSC thermogram of sample 1.2 from set1..........eeeceeeeeeeeee
DSC thermograms of white and varicolored materials found in samples from set 1.
DSC thermograms of both red and black matter found in sample 1.2 from set 1.
DSC thermograms of samples 5.1 and 5.2 from set 5 showing jagged behavior .
DSC thermogram of sample 2.2B from set 2 showing jagged behavior .....
DSC thermograms of samples 2.6 and 2.7 from set 2 showing jagged behavior .
DSC thermograms of oxidation products found in samples 5.1 and 5.2 from set 5.
DSC thermograms of sample 2.3 from set 2

Page

Oo WoO MOIIIIIIIWN NH

10


10

Some of the white particulates found in the red paste
were separated and then heated in the DSC to 145° C.
Multiple endotherms were recorded, as seen in figure 10.
When the sample was then cooled in the sample pan to
ambient temperature and then reheated, most of the endo-
therms were gone, thus supporting the hypothesis of de-
hydration being the origin of these endotherms.

When the extraneous, varicolored (white, yellow,
orange, or red) material was scraped from the surfaces of
samples in all the sets and tested in the DSC, a similar
variety of endotherms was obtained, as seen in figures 10
and 11, The second and third thermograms in figure 10
are also for samples that consisted mainly of white mate-
tial scraped from surfaces. The second thermogram, with

T T T
e White
=
5
= Varicolored
S
g
{
\ \Varicolored
g
=
5s Varicolored
Fa
2 VP
3
2
a
f L a
0 100 200 300 400 500

TEMPERATURE , °C

Figure 10.-DSC thermograms of white and varicolored
materials found In samples from set 1.

—> EXOTHERMIC

ENDOTHERMIC <—

n 1 1
300 400 500
TEMPERATURE, °C

1
0 00 200 600

ewe ATURE, °C

Figure 11.-DSC thermograms of both red and black matter
found In sample 1.2 from set 1. First thermogram obtained at a
sensitivity of 2 meal/(s-in); the eecond at 1 mcal/(s-In).

the large endotherm peaking at 110° C and the two small-
er endotherms at 150° and 335° C, can be the result of
dehydration of sulfate compounds, such as CaSO,-2H,0,
CaSO,-1/2H,O, melanterite (FeSO,-7H,O), siderotil
(FeSO, - 5H,0), and coquimbite [Fe,(SO,),:9H,O]. The
iron sulfates lose some of their water of hydration at high-
er temperatures, above 150° C, as does CaSO,-1/2H,0.
Tron and calcium sulfates are known to be associated with
coal and other carbonaceous materials that have been
exposed to air and underwent oxidation (7).

The number of endotherms, their peak temperatures
and relative sizes (or heights) differed for the different
samples, as is expected when more than one compound is
present, each in a different amount. Here, it is surmised
that a suite of diverse hydrous salts is present in various
proportions. The peak temperatures of the endotherms
match many of the temperatures at which these com-
pounds lose water. However, matching peak temperatures
do not prove equal identities; they only imply that the
probability exists. Based on many prior results of DSC
analyses of known oxidation products in association with
coal, this probability is quite high.

The two thermograms in figure 11 were obtained for
samples that contained both black and red material; the
black matter is probably either coal or shale and the red
matter is one of the hydrous iron compounds. Pyrite
could also be present in the black material. The sec-
ond thermogram in figure 11 contains three exothermic
peaks, at 400°, 440°, and 537° C, instead of the common
two. This behavior has been observed at times for other
coals, though infrequently, On the other hand, the peak
at 440° C could be an indication of pyrite decomposition.
The first thermogram in the figure, for a smaller sample,
was obtained at a lower sensitivity (2 mcal/(s-in)) and
therefore the sizes of the exothermic peaks are much
smaller. Nonetheless, endotherms indicative of extraneous
matter are clearly visible.

Thermal Analysis of Samples
Containing Sulfidic Matter

Two other distinct features observed in the thermo-
grams of many of the samples are described below:

First Feature—This feature is a jagged or pulsing
behavior superimposed on a portion of the exothermic
tise, in the temperature range of 400° to 525°C. Various
examples can be seen in figure 8 for sample 1.4, in fig-
ure 12 for samples 5.1 and 5.2, in figure 13 for sample
2.2B (rock, shale) obtained from the roof over the heated
area, and in figure 14 for samples 2.6 and 2.7 obtained
from the trench, 15 to 30 in and 30 to 45 in below the sur-
face, respectively. Some of the samples from the other
sets also displayed the same behavior.


2

or iron sulfide being a very small component, although the
reactivity of the individual pyrite particles also plays a
role. These active sites require only minute amounts of O,
and can possibly be oxidized by the CO, present in the
atmosphere,

Second Feature —This feature is small exothermic peaks,
at 400° C and at 440° to 450° C, that appear in a few of
the thermograms, which are not presented in this report.
These peaks are thought to be a result of the decomposi-
tion of sulfidic matter, such as pyrite, in the coal or shale
matrix,

Thus, these two distinct features, manifest in the ther-
mograms, point to the presence of iron sulfides and specif-
ically pyrite in many of the samples collected from the
heated areas in the mine.

Samples 5.1 and 5.2 were specifically collected by the
mine personnel because they suspected the presence of
large amounts of pyrite in these samples. Indeed, the
thermograms of these samples displayed the unique fea-
tures associated with pyrite, as described above. In addi-
tion, a specimen of sample 5.1, tested in the DSC to a
temperature of 125° C, showed a variety of the endotherms
associated with sulfide oxidation products (as explained in
the previous section), as can be seen in the first thermo-
gram in figure 15. The second thermogram in this figure
was recorded for another specimen of the same sample.
The endotherm, with a peak at 127° C, indicates the pres-
ence of CaSO,-2H,0, which loses water of hydration at
this temperature. This thermogram also contains the
jagged ridges.

Portions of the coal in sample 5.2 contained small
patches of yellow matter on their surfaces. The matter
was scraped off the surface and heated in the DSC. The
resultant thermograms for this material, which was thought
to be sulfur, were different than all the other thermograms
recorded during this study. Two of these thermograms
(the third and fourth ones in figure 15) contain a large
endotherm, which starts at about 325°C. Neither the
melting nor the boiling points of sulfur coincide with the
endotherms’ initiation or peak temperatures. It is believed
that these endotherms are due to evaporation of a rela-
tively large amount of H,SO, in the sample tested. Fil-
trates of aqueous suspensions of samples 5.1 and 5.2 were
treated with barium chloride and found to contain large
amounts of sulfate ions, indicating the oxidation of the
pyrite and the formation of the acid, in addition to the
formation of iron sulfates, When tested, the samples were
found to be highly acidic. A similar endotherm at 335° C
was seen in the second thermogram in figure 10; it may
also be indicative of the presence of H,SO,.

Thermal Analysis of Other Mine Materials

Many substances undergo physical and/or chemical
changes when heated. Thus, each substance has its own

specific thermogram, which can serve as a fingerprint for
identification. If such materials are exposed to heat prior
to being tested in the DSC, the resultant, changed thermo-
grams can be used to approximate the temperature of the
earlier exposure. This has been shown previously for coal.
The same phenomenon was found for a piece of wood-crib
block (sample 2.3), one of the samples in set 2 from the
batch collected after the second heating. Both uncharred
and charred portions of the wood-crib block were exam-
ined. The first thermogram in figure 16, for the uncharred

T T T T
2g
Fe
=
FS 5.1
5 ae
g
&
8.1 Sh
g
=
Fe
z= 52
= :
8
2
3 NS
1 1 1 1
se) 100 200 300 400 500

TEMPERATURE, °C

Figure 15.DSC thermograms of oxidation products found In
samples 5.1 and 5.2 from set 5.

T T T
2
=
=
&
=
§
a
t Uncharred
S
=
&
= Charred
3
2
Fr
Uncharred and preheated
1 i L 1
° 100 200 300 400 500

TEMPERATURE, °C

Figure 16.—DSC thermograms of sample 2.3 (wood-crib block)

from set 2; uncharred, charred, and uncharred sample preheated
In DSC to 340° C.


Library of Congress Cataloging in Publication Data:

Miron, Yael.
Cause of floor self-heatings in an underground coal mine / by Y. Miron, CP.
Lazzara, and A.C. Smith.
P. cm, — (Report of investigations; 9415)
Includes bibliographical references (p. 22).
Supt. of Docs. no.: I 28.23:9415.
1. Mine fires. 2. Coal—Combustion. 3. Pyrites—Oxidation. I. Lazzara, Charles

P. Hl. Smith, Alex C. III. Title. IV. Series. Report of investigations (United
States, Bureau of Mines); 9415.

TN23.U43 [TN315] 622 s—dc20 [622’.82]  91-38753 CIP



100 T T ‘|
2¢ 1
> €
=2 10k NX 4
ad 4
wu ]
25
Fe
\ ! A
Ol 1.0 10 100

PYRITIC SULFUR CONCENTRATION, pct

21

Table 6.—Results of H,0, reactivity teste with samples
from unheated areas (sets 7 to 11)

Figure 23.—Pyritic sulfur concentrations in mine samples as
function of time to maximum temperature In HO, reactivity test.

where T = time to maximum temperature, min,

and P = pyritic sulfur concentration, wt pet.
The correlation coefficient was found to be 0.9399. This
formula can be used to estimate the pyritic sulfur con-
centration of samples from other areas of the mine. Con-
sidering the large variation in the composition of the many
samples, the results are quite satisfactory.
The results from the chemical analyses and the H,O,
reactivity tests show that pyritic matter was more preva-
lent in certain areas of the floor strata, such as zones
B and C, of the samples collected during February and
March 1989, Samples from zones C, D, E, and F, ob-
tained April 21, 1989, and samples from zones B, C, and
D obtained June 9, 1989, all contained pyrite to various
degrees,

‘Set Maximum = Time to Pyritic
and tempere- maximum sulfur
date Sample ture, tempera- _concen-

collected Increase, ture, tration,

“ct min wt pot

Set 7:
Feb, 28,1989.. 7.1 t) NA NA
72 ° NA 0.08
73 0 NA NA
74 0 NA 2B
75 6 4.60
76 2 9 NA
7.7 4 6 439

Set 8:
Mar. 20, 1989.. 8.1 86 15 75
a2 67 3 8.42
83 63 5 3.44
~ 84 60 13 1.31
85 61 8 1.47

Set 9:
Mar, 28, 1989.. 9.4 48 60 NA
92 65 4 NA
93 64 3 NA
94 60 7 NA
95 58 15 NA
96 58 15 NA

Set 10:
Apr. 21,1989... 10.1 ° NA a
10.2 0 NA a7
10.3 67 27 10.07
10.4 65 32 7.03
10.5 61 47 3.02
106 61 54 3.63
Set 11:

June 9, 1989... 11.4 51 23.0 43
11.2 63 26 8.08
11.3 60 44 3.97
114 60 52 3.64

NA Not applicable or not analyzed.
Chemical analyses (see table 4),

SUMMARY

The samples that were collected after the first heating,
especially the red, pastelike sample, gave strong indication
that oxidation processes were of concern in the mine. The
extraneous matter on the surfaces of the solid samples
from the first heating and the hue of the red paste were
the first visible evidence of the presence of oxidation
products associated with oxidation of a coal that contains
pyrite. Samples collected after the other heatings pre-
sented similar evidence.

Tests in the DSC revealed the presence of many hy-
drated compounds in the extraneous matter, as well as in
other portions of the samples themselves. In addition, the
unique features of the thermograms pointed to the pres-
ence of unoxidized, reactive pyrite in the samples. Since

oxidation products of pyrite include many hydrated sulfate
salts and hydrous iron oxides, the combined DSC results
Presented quite strong evidence, though indirect, that py-
rite was present in areas of the mine and that in some of
these areas it had undergone oxidation.

Qualitative chemistry tests were conducted next and
showed conclusively the presence of relatively large
amounts of sulfate, ferrous, and ferric ions in the samples,
especially in the extrancous matter, all the resultant prod-
ucts of pyrite oxidation. Additionally, the high acidity
associated with some of the samples was further witness to
the same process. These results were also supported by
the results from instrumental analyses of a few of the sam-
ples that showed iron cations to be the most prevalent.


Report of Investigations 9415

Cause of Floor Self-Heatings in an
Underground Coal Mine

By Y. Miron, C. P. Lazzara, and A. C. Smith

UNITED STATES DEPARTMENT OF THE INTERIOR
Manuel Lujan, Jr., Secretary

BUREAU OF MINES
TS Ary, Director


Table A-5.—Various cores from mine (set 6)

Sample Mine Date Description of various portions Location
designation fecelved

‘Sept. 29, 1988 .. Dark gray sandy fireclay, some limestone
Dec. 1, 1988... Dark gray-black layered shale, compact; dark gray massive, Heated area 1.

sandy shale, churned; broken pieces and thin discs in places

along core; some surfaces partially covered with white-yellow-

orange material.
63..... Core 1A...... dO eee Dark gray-black massive, sandy shale, churned; dark-gray Heated area 3.
layered shale, compact, churned; light gray-belge sandy shal
massive shale with limestone; broken chunks showing mica
flakes; a few surfaces partially covered with yellow oxidation

products.
64.0... Core 2A... oe Top edge consisting of thin layer of coaly material; dark gray Do.
massive, sandy shale; gray massive, layered shale; gray

massive shale with a few coal streaks and not much breakup.
65..... Core 3A...... +-do........, Dark gray massive, sandy shale; dark gray layered shale; gray Do.
layered shale with limestone; mica flakes in some areas; a few
‘surfaces partially covered with yellow-orange-red material
Black shale; gray-black shale with coal streaks; black shale with Heated area 4,
white streaks (either interbedded sandstone or limestone);
Gray shale with limestone nodules; a few surfaces partially
covered with red material.

NA Notavallable.
‘Collected after 4th heating.

Table A-6.—Samples collected from unheated
beneath the floor (sets 7 to 11)

Set and date collected Sample __Zone Location?
7: Feb. 28, 1989 7A ‘Ai Floor channel; spad 6670.
72 22 Do.
73 3 Do,
74 81 Do.
75 ci Do.
76 C2 Do.
7 D1 Do.
8: Mar. 20, 1989 .........5 at At Floor channel; spad 6523.
a2 BI Do.
a3 ct Do.
84 ot Do.
a5 a1 Do.
9: Mar, 28, 1989 ........ . 4 AL Do.
9.2 2 Do.
93 Bt Do.
94 ct Do.
95 D1 Do.
96 et Do.
10: Apr. 21, 1989 40.1 A Floor channel; spad 4908.
10.2 B Do,
410.3 c Do.
210.4 D Do.
40.5 E Do.
310.6 F Do.
11: June 9, 1989 .......... MAA A Floor channel; spad 5759.
311.2 B Do.
4113 c Do.
414 D Do.
1Se¢ figure 5.
ne sample.
3Coal, rock interface sample.
‘Coal sample,

INT.BU.OF MINES,PGH.,PA 29550


For the tests, 5-g samples of the minus 60- plus 100-
mesh fraction were used. A sample was placed in a
Dewar flask (250 mL in volume), 35 mL of an 8.4 pct
aqueous H,O, solution was added, and the flask was stop-
pered loosely, The temperature of the reacting mixture
‘was monitored with a type K thermocouple as a function
of time until a maximum value was attained. itori
continued until the temperature fell a few degrees below
the maximum value.

SEALED FLASK OXYGEN ADSORPTION TESTS

The ability of materials, such as coal, to adsorb O, and
the quantity adsorbed over time is a good measure of their

propensity to self-heat. Two samples were chosen to un-
dergo this test, which is described in detail in a Bureau
publication (2). As-received samples, 50 g each, were
sealed in 500-mL flasks in ambient air. The samples were
neither ground nor sieved prior to the test because their
particle size distribution was similar to the size range
prescribed for the test. Miniature pressure transducers
monitored the pressure in the flasks as a function of time.
A gas sample was taken from each flask after 10 days and
analyzed by gas chromatography.

EXPERIMENTAL RESULTS AND DISCUSSION

SAMPLES FROM HEATED AREAS
Visual Observations

Many of the samples from sets 1 to 5 that were mix-
tures of coal, shale, and rock also showed evidence of ex-
traneous material. This material was usually present on
the surfaces in small quantities, but it was seen on a large
portion of the samples. It varied in color from white to
yellow-orange to red, and its physical appearance varied
between dry and moist, and powdery and crystalline, sug-
gestive of oxidation compounds commonly found on coals
that have been exposed to air and moisture,

The dark red paste sample from set 1 was the only
sample different in appearance, consistency, and texture
from all the other samples. It felt like a clayey paste to
the touch, but resembled iron oxide in hue.

The hard cores from set 6 showed a variety of mate-
rials, as described in table A-5. Only small amounts of
extraneous material were seen here and there on a few of
the surfaces. These observations were helpful in guiding
the course of the study and in understanding the results of
the tests.

Thermal Analysis of Coal-Shale

During tests in the DSC, thermograms are recorded.
These thermograms indicate the release (exotherm) or
absorption (endotherm) of heat by the sample. When heat
is not exchanged, a straight baseline is recorded. The
exotherms and endotherms are usually identified by the
temperatures at which their peaks occur and, at times, by
their onset tempefatures as well. The shape and magni-
tude and the peak temperatures of a thermogram are a
function of sample size and geometry, heating rate, com-
position of the sample, and the presence of impurities in

the sample. The nature of the atmosphere in the DSC
also influences the results. For instance, coal will undergo
exothermic oxidative decomposition in an atmosphere of
air or O,, while in an inert atmosphere, it will pyrolyze
endothermally.

Most of the samples from all the sets were mixtures
of coal, shale, and/or rock. The resultant DSC thermo-
grams depended on the relative proportions of these mate-
rials in a sample. When coal was the main ingredient, a
typical coal thermogram was obtained. The first thermo-
gram in figure 8 was obtained for sample 1.4 from set 1.
This thermogram contains a very shallow endotherm ex-
tending from ambient temperature to near 100° C because
of water evaporation, followed by two exotherms. The
first, smaller and shallow exotherm, with a peak tem-
perature of the plateau at about 350° C is overlapped by
the second and much larger exotherm, with a peak tem-
perature of about 560° C. The main determinant of peak

—> EXOTHERMIC

ENDOTHERMIC <—

1 L L 1 L
0 100 200 300 “400 500 600
TEMPERATURE, °C

Figure 8.—DSC thermograms of sample 1.4 (unheated coal)
from set 1 and sample 2.24 (heated coal) from set 2.


so

T T T TT
2
=
e
o
=
=
cay
2
in]
5.1
2
= 52
&
o
=
S
8
2
&
1 1 1 1
° 100 200 300 400 500

TEMPERATURE, °C
Figure 12-DSC thermograms of samples 5.1 and 5.2 from

—> EXOTHERMIC

ENDOTHERMIC <—

1
200

1 L 1
300 “400 "500

TEMPERATURE, °C

7
0 100 600

‘tet 5 showing Jagged behavior.
T T T ——T
s
3
Fe
é
g
f
=
Fe
FS
3
2
&
L \ ! \ 1
0 100 200 300 400, 500 600

TEMPERATURE, °C

Figure 13.-DSC thermogram of sample 2.28 (rock, shale)
from set 2 showing Jagged behavior.

At times, this jagged behavior was preceded by an
endotherm at about 440° C, as shown in the two thermo-
grams in figure 12, the second thermograms in figures 14
and 15, and the last thermogram in figure 18. The cause
for this endotherm is not known. It might be that some
marcasite (also FeS,) is present with the pyrite and is
undergoing change. Marcasite, an isomorph of pyrite,
undergoes a solid-phase transition to pyrite at 450° C (8).
This transformation is kinetically slow; its rate increases
with increasing temperature (9). In the presence of sulfur,
marcasite can invert to pyrite at a temperature as low as
150° C, but in its absence, transformation does not occur
below 400° C (10). However, it is more probable that

Figure 14.—DSC thermograms of samples 2.6 and 2.7 from set
2 showing Jagged behavior.

any pyrite present is undergoing decomposition at this
temperature. The tests in the DSC are conducted in an
atmosphere of static air. As the tests proceed and the
coal decomposes, a self-generated atmosphere results,
which contains CO, CO, and various hydrocarbons as the
major gaseous products. In such an atmosphere, pyrite
will likely undergo decomposition, releasing some sulfur,
which in turn will react with the hydrocarbons and form
hydrogen sulfide and other sulfur-containing compounds.
Even some carbonates, such as dolomite, in the pres-
ence of coal, can decompose endothermally at this low
temperature.

Thermograms with similar jagged behavior have been
reported for pyrite particles heated in the DSC at a con-
stant heating rate (11). The sizes of the jutting ridges and
the spacings between them vary with particle size and
heating rate, among other factors. This phenomenon is
due to the initiation of oxidation reactions in individual
particles or particle clusters at a temperature dependent
on their surface properties, which in turn depends on the
method of sample preparation (12). Extremely small ac-
tive sites that are in poor thermal contact with the rest of
the particles may reach a high instantaneous temperature
during oxidation. In the present study, the size of these
ridges is modulated by the major constituent of the sam-
ple, namely the coal, shale matrix with the reactive pyrite