Post-explosion observations of experimental mine and 

laboratory coal dust explosions 

Kenneth L.Cashdollara ,Eric S. Weissa, Terry G. ~ontgomery~, John E.Goingc 

'Pitisburgh Rtaearch &bor~tory. NulionaI I1~1fthatcjort?ecypntia~lSaftry tad Hmllh, Pf~tJburgh.PR 15236, USA 

b~hS@y ad Hdth AhkLrtra~imMi. Hope, WV25880. USA

'Fikt Corporatk Blue Sprhs, MO MOIJ, USA 

The Pituburgh Research Laboratory (PRL)of tht National Institute for Occupational Ssfcty and Health NOSH) ad the Mine 

Safety and Hcalth Adnhhtratiou (MSHA) conductedjoint research on dust explosions by studying post-cxplosion dust smples. The 

samples wcrc collected after full-scalewrplosio~at the PRL Lake Lynn Experimental Mine (LLEM), and after laboratoryexplosionsin 

tL PRL 20.1, chamber and the Fike 1 m3chamber. The dusts studied included both high- and low-volatile bituminous mils. Low 

tcmpcrature &ng for 24h at 515 "Cwas used to measure the incombustibIc content of the dust kforc and alter the explosions. Tbe 

data shod that the post-txplosion inccrmbustible content was always as high a#, or bigber than the initial incombustible wntmt. The 

MSHA alcohol coking test was wed to dotermine the amount of coked dust in the post~xplosionsamples. 'Ihe results showsd that 

ahon all coal dust that wu suspended within the explosion 0amc produced significant amounts of coke. Measuremwts ofAoor dust 

eoncentrationsah LLEM explosionswere compared with the iriitialdust loadings to &&ne the transport distanee of dust during an 

explosion. All these data will be useful in future forensic investigations of accidmtal dust explosions in coal mine9, or dsewherc. 

Ktywordc Dut; Exphion; Mine explosion 

Research .Laboratory (PRL)of the National Institute,for 

,Oocuptional Safety and Health (NIOSH) and the Mine 

Much progress bs teen made in prwenting under- Safety and Health Administration (MSHA) have conground coal mine disasters, but explosions still occur, ducted joint research to improve the forensic investigation 

sometimes producing multiple fatalities. In an explosion, techniques for accidental coal mine explosions. 

all underground miners are at risk. Them were serious Explosions in underground coal mines and surfunderground coal mine explosions in July 2000 in Utah facilitiessuch as processing plants are caused by accumula-

(2 fatalitics md 8 injuries), in September 2001 in Alabama tions of flammable gas andlor combustible dust dispersed 

(13 fatalities rtad 3 inpries), in January 2006 in Wcst in air in the presence of an ignition source. Research on the 

Virginia (12 fatalities and 1 injury), and in May 2006 in causes and mechanisms of gas and dust explosions is 

Kentucky (5 fatalities). There have been other mine needed as a bsis for the development of techniques and 

expiosioan in the USA in recent years that did not result strategies for explosion prevention, suppmsion, and 

in any injuries, but the mine recovery efforts took meral mitigation. In the coal mining industry, roek dust (dy 

monthr. These events show that the mine-explosion limestone) is added to the coal dust deposits to act as a 

problmn has not yet been solved. Therefom, the Pittsburgh hat sink and inhibit flame propagation. If a sufkiient 

amount of rock dust is added to the coal dust, the 

mixture is rendered inert. The rock dusting regulations ate 

b& on the mlts of full-scale experimental mine 
NIOSH-PRL conducts dust explosion research in its

Lake Lynn Experimental Mine (LLEM) and in laboratory

vessels. Post-explosion dust sampling is part of this

research effort. Analysis of these samples provides MSHA

with valuable information concerning the characteristics of

dust samples collected after well-documented explosions.

This information can then be used by MSHA in future

explosion accident investigations. Accurate forensic investigations are important for thorough accident interpretation and follow-up recommendations to prevent future

occurrences. Some preliminary data from post-explosion

dust samples were presented previously (Cashdollar &

Going, 2003).

2. Experimental facilities

The full-scale explosion tests were conducted in the

LLEM, which is shown in the plan view of Fig. 1 (Mattes,

Bacho, & Wade, 1983; Triebsch & Sapko, 1990). This is a

former limestone mine, and five new drifts (horizontal

passageways in a mine) were developed to simulate the

geometries of modern USA coal mines. The mine has four

parallel drifts—A–D. D-drift is a 490 m (1600 ft) long single

entry that can be separated from E-drift by an explosionproof bulkhead door. In order to simulate room and pillar

workings, drifts A–C can be used. These three drifts are

approximately 490 m (1600 ft) long, with seven crosscuts at

the inby end. Drifts C and D are connected by E-drift, a

152 m (500 ft) long entry which simulates a longwall face.

Explosion tests can be conducted in the single entry

D-drift, the multiple entry area of A-, B-, and C-drifts, or

various other configurations including the longwall E-drift.

7 bar or 700 kPa (100 psi). Previous publications described

the LLEM coal dust explosion test procedures and the

results of LLEM explosion research other than postexplosion observations (Cashdollar, Weiss, Greninger, &

Chatrathi, 1992; Sapko, Weiss, Cashdollar, & Zlochower,

2000; Weiss, Greninger, & Sapko, 1989).

Each LLEM drift has 10 data-gathering (DG) stations

inset in the rib wall at the locations shown in Fig. 1. Each

DG station houses a strain gauge transducer to measure

the explosion pressure, and an optical sensor to detect the

flame arrival. The wall pressure is perpendicular to the gas

flow and is the pressure that is exerted in all directions. This

omni-directional pressure is called the ‘‘static pressure’’ by

Nagy (1981, p. 58) to differentiate it from the dynamic

pressure, although the ‘‘static pressure’’ does vary with

time during the explosion. The dynamic or wind pressure is

directional. The total pressure is the sum of the omnidirectional pressure and the wind or dynamic pressure.

Other instruments such as dynamic pressure sensors, heat

flux gauges to measure explosion temperatures, optical

probes to measure dust dispersion, and movie or video

cameras may be installed at various locations in the

LLEM. Post-explosion dust sampling was part of the

research effort. The dust samples were usually collected

from a known area (0.37 m2 or 4.0 ft2

) on the floor at

various distances from the face (closed end of the LLEM

drift). A square wooden form was used to define the area.

If insufficient dust was collected from a single square,

multiple squares were collected. In some cases, band

samples were collected. Band samples include dust from a

15 cm (0.5 ft) wide strip along the floor, ribs (walls), and

roof. In the standard MSHA band sampling technique, the

Fig. 1. Plan view of the Lake Lynn Experimental Mine (LLEM).
floor samples are collected to a depth of 1 in. The dust layer

depth after the LLEM explosions was much less than 1 in,

and therefore all the dust was collected. Most of the LLEM

dust explosion tests described in this paper were conducted

in the single entry D-drift, which was isolated from the

E-drift by means of the explosion-proof movable bulkhead

door (Fig. 1). The ignition zone for a typical D-drift dust

explosion test (Fig. 2) was a methane–air mixture at the

face (closed end). This methane–air zone was ignited by

electric matches. In tests involving pure fuel (coal dust), all

the dust was placed on roof shelves to enhance the

dispersion. In the rock dust inerting tests, the coal dust

and limestone rock dust mixture was placed half on roof

shelves and half on the floor. The nominal dust loading

reported for the LLEM tests assumes that all the dust

was dispersed uniformly throughout the cross section.

For the LLEM tests, D-drift was thoroughly washed

down several days before the test to allow the entry to

dry before the day of the test. In addition, the access

ramp at the open end of the drift was wetted down

on the day of the test. The purpose was to significantly minimize contamination of the post-explosion dust

samples.

Dust explosion research was also conducted in the Fike

Corporation 1 m3 (1000 L or 35 ft3

) chamber (Cashdollar &

Chatrathi, 1993; Going, Chatrathi, & Cashdollar, 2000)

shown in Fig. 3. The chamber is spherical with an internal

diameter of 1.22 m (4.0 ft) and a pressure rating of 21 bar.

The two halves of the sphere are connected by twelve

51 mm diameter bolts. Two variable reluctance pressure

transducers were used to measure the explosion pressure.

PRL-designed optical dust probes (Cashdollar, Liebman,

& Conti, 1981; Liebman, Conti, & Cashdollar, 1977)

monitored the dust dispersion. Data from the instruments

were collected by a high-speed PC-based data acquisition

system. The dust injection system for the 1 m3 chamber

consisted of a 5 L dispersion reservoir, a 19 mm pneumatically activated ball valve, and a rebound nozzle. To create

a dust cloud, a weighed sample of dust was placed in the

dispersion reservoir. The reservoir was pressurized with dry

air to a gauge pressure of 20 bar g. The chamber was

partially evacuated to an absolute pressure of 0.88 bar a.

Activation of the ball valve dispersed the dust and air into

the 1 m3 chamber through the rebound nozzle and raised

the chamber pressure to about 1 bar a. The ignitor was

activated 0.6 s after activation of the ball valve. The

ignition sources used for the tests were 5 and 10 kJ

pyrotechnic ignitors manufactured by Fr. Sobbe1 of

Germany. They were activated electrically with an internal

fuse wire and delivered their energy in about 10–20 ms. The

5 kJ ignitor by itself produced a pressure rise of 0.02 bar in

the 1 m3 chamber. For higher ignition energies, multiple

10 kJ ignitors were used. All of the ignitors were positioned

at the center of the chamber and were pointed down

toward the bottom of the 1 m3 chamber. The chamber was

thoroughly cleaned before each test so that there would be

no contamination from residue from previous tests. After

the explosion test, all of the walls were brushed down to

collect as much of the post-explosion dust sample as

possible. A few laboratory tests were conducted in the

NIOSH-PRL 20 L chamber (Cashdollar, 1996; Cashdollar

& Hertzberg, 1982). The test procedures and instrumentation for the 20 L chamber were similar to those of the 1 m3

chamber.

Fig. 2. Side view of D-drift in the LLEM, showing gas ignition zone and dust test zone (not to scale).

Fig. 3. Fike 1 m3 explosion test chamber.

1

Mention of any company or product does not constitute endorsement

by NIOSH.
incombustible content of the dust samples, including the

ash in the coal plus the limestone rock dust. The LTA

analyses were conducted at both the NIOSH-PRL at

Pittsburgh, PA, and at the MSHA laboratory at Mt. Hope,

WV. First, all the dust samples were sieved minus 20 mesh

(850 mm). The LTA was conducted for both 4 and 24 h at

515 1C on most of the samples. The 515 1C temperature is

intended to burn up all the coal, but not to decompose the

limestone rock dust (CaCO3). In order to test this, a 24 h

LTA was run on limestone rock dust and the recovery was

greater than 99%. The 4 h procedure is the standard one

MSHA uses for routine mine dust samples for compliance

with rock dusting regulations. The 24 h procedure was first

used at PRL after the 4 h procedure did not appear to burn

up all the coal or char in post-explosion samples. The

comparison data are presented in the next section on

‘‘Observations and Data.’’ There was a slight difference in

the procedures for the ‘‘4 h’’ LTA at the two labs. MSHA

used separate samples for the 4 and 24 h LTA. PRL heated

the samples for 4 h, weighed them, and then re-heated them

for an additional 20 h. The MSHA ‘‘4 h’’ procedure was a

1.5 h ramp-up and 2.5 h at 515 1C for a total of 4 h heating.

The PRL ‘‘4 h’’ procedure was a 1.3 h ramp-up and 4 h at

515 1C. These differences in NIOSH-PRL and MSHA

ashing procedures were not discovered until after the

samples had been analyzed. In addition, at PRL, the dust

samples were desiccated before the LTA, so that the

incombustible content would be on a dry basis. At MSHA,

the moisture was separately measured by heating the coal

at 105 1C before the LTA analysis.

The alcohol coking analyses were conducted at the

MSHA laboratory at Mt. Hope, WV. For this analysis,

approximately 1 g of the 20 mesh post-explosion residue

was placed in a test tube 2.5 cm in diameter. Approximately

15 mL of denatured ethyl alcohol was added, and the

sample was stirred to ensure that all the particles were

wetted. Then the sides of the test tube were washed down

with approximately 5 mL of alcohol, and the liquid was

allowed to rest for about 5 min. This ensured that all

particles that were more dense than the alcohol would

settle to the bottom. The sample was then classified based

on the amount of coked material that was observed

floating on the surface of the alcohol. The classifications

are based on the reference chart in Fig. 4. This alcohol

coking test measures the amount of material whose density

is less than that of the alcohol. Coal, which has a density of

1.3 g/cm3

, sinks in the alcohol, which has a density of

0.8 g/cm3

. The coke consists of those particles that float.

4. Observations and data

Figs. 5–7 show data from dust explosion test #471 at the

LLEM in February 2004. The dust was dispersed and

ignited by an 8.2 m (27 ft) long zone of 10% methane in air

at the face of D-drift (Fig. 2). A mixture of 35% coal dust

and 65% limestone rock dust was placed on the floor and

on shelves from 8.2 to 62.5 m (27 to 205 ft) from the face.

The coal dust was Pittsburgh seam high-volatile bituminous coal that had been pulverized to 73% minus 200

mesh (75 mm). A second dust zone of 20% coal dust and

80% rock dust was on the floor and on shelves from 62.5 to

135.6 m (205 to 445 ft) from the face. In both dusted zones,

the coal dust concentration was 150 g/m3

. Fig. 5 shows the

maximum pressure versus distance in D-drift during the

explosion. The pressure reached a maximum of 76 kPa

(11 psi) at 30 m (100 ft) from the face. The pressure

decreased gradually to 49 kPa (7 psi) at a distance of

229 m (750 ft). The flame sensors showed significant flame

out to 105 m (346 ft). There were small but measurable

signals on the flame sensors from 119 to 153 m (390 to

501 ft). These small signals probably corresponded to either

localized burning or to post-combustion hot particles that

were gradually cooling as they were carried out of the drift

by the expanding gases. There were no signals on the flame

sensors beyond 153 m. The flame sensor data showed that

the dust flame propagated rapidly through the coal and

65% rock dust mixture. When it reached the 80% rock

dust zone at 62.5 m, the flame slowed and then died at

about 110 m (360 ft). This was expected since previous

experimental mine tests had shown that an explosion of

this size of bituminous coal dust will not propagate

through a mixture with 80% rock dust (Nagy, 1981; Sapko

et al., 2000).

Fig. 4. Qualitative alcohol coke test observations.
After the explosion test in the LLEM, dust samples were

collected from the mine floor at various distances from

the face and then sieved minus 20 mesh (850 mm).

The calculated dust loadings in g/m2 before and after the

explosion are shown in Fig. 6. The dust loading before the

explosion includes the 150 g/m3 of coal dust and 279 g/m3

of rock dust for a total 429 g/m3 of dust from 8.2 to 62.5 m.

Based on the average height of D-drift of 2.1 m (6.9 ft), the

calculated nominal floor loading was 900 g/m2

. This

calculation includes both the dust on the floor and on the

shelves. The zone from 62.5 to 135.6 m had 20% coal and

80% rock dust for a calculated floor loading of 1580 g/m2

.

These pre-explosion dust loadings are shown by the heavy

dashed line in Fig. 6. The post-explosion dust loadings

were calculated from the amount of dust collected from

measured areas at various locations. These data are shown

as the solid circle data points in Fig. 6. It is obvious from

the figure that, after the dust is dispersed, it is carried some

distance by the dynamic pressure or wind of the explosion.

The pre-explosion dust loading stopped at 135.6 m (445 ft)

from the face. After the explosion, significant amounts of

dust were collected at distances up to 229 m (750 ft).

A more violent explosion may carry the dust even farther.

The incombustible contents (measured by LTA) before

and after the explosion are shown in Fig. 7. The original

incombustible amounts are shown as the heavy dashed line.

It shows the original 67% incombustible (from the 65%

rock dust and the additional 2% from the ash in the coal

fraction of the total dust mix) from 8.2 to 62.5 m (27 to

205 ft). The original 81% incombustible zone extended

from 62.5 to 135.6 m (205 to 445 ft). The post-explosion

samples are shown by the solid circle data points. In the

region that had an original incombustible content of 67%,

the post-explosion incombustible was 80%. This shows

0

20

40

60

80

0 50 100 150 200 250

Distance, m

Pressure, kPa

0

2

4

6

8

10

12

Pressure, psi

Fig. 5. Explosion pressures versus distance during test #471 in D-drift at the LLEM.

0

200

400

600

800

1000

1200

1400

1600

0 50 100 150 200 250

Distance, m

Floor Dust Loading, g/m2

0 100 200 300 400 500 600 700 800

Distance, ft

post-explosion dust

original dust loading

Fig. 6. Dust loading before and after LLEM explosion test #471.
that a significant amount of the coal in this region was

burned during the explosion. The incombustible content

further from the face was also 80%, similar to the

original pre-explosion content in this zone. Since the

explosion flame stopped at 110 m (360 ft), the dust

collected beyond that distance would be mainly dust that

had been dispersed, but had not been in the explosion

flame. Therefore, the post-explosion incombustible content

of this dust should be essentially the same as the original

pre-explosion incombustible content. For the post-explosion LTA data, only samples collected within the dusted

zone or slightly beyond were included in Fig. 7. Since

smaller amounts of dust were collected far beyond the

dusted zone, there would be an increased possibility of

contamination from any small amounts of limestone dust

that would come from the roof and ribs of the limestone

mine during the dust explosion test.

The data shown in Figs. 5–7 are examples of the type of

data collected for the various LLEM dust explosion tests.

Post-explosion dust samples from other LLEM dust

explosions from 1999 through 2004 were also collected

and then analyzed at NIOSH–PRL and MSHA. For the

post-explosion incombustible analyses, only samples collected within the dusted zone or up to 30 m (100 ft) beyond

were included in the data summary.

Post-explosion dust samples were collected from both

the experimental mine and laboratory experiments to

compare the pre- and post-explosion incombustible contents and the coke amounts. The 1 m3 data included tests

with pure coal dusts and with various mixtures containing

30–65% limestone rock dust. The coal dusts included both

the high-volatile Pittsburgh seam bituminous and lowvolatile Pocahontas seam bituminous. The LLEM data

were for Pittsburgh high-volatile coal dust with 65–80%

rock dust in the mixture. Most of the tests were with

pulverized coal with 73% minus 200 mesh (75 mm), but

one test was with an even finer size of coal and one was

with a coarse size of coal. The volatility and size data for

the coals tested are in Table 1. The size data are from a

combination of sonic sieving and Coulter Counter analyses. The percentage minus 200 mesh is listed in column

three. The next two columns list the surface and mass

(volume) weighted mean diameters. The last column lists

the mass median diameter. The size data for the limestone

rock dust are listed in the last line of the table.

The summary post-explosion LTA data from

NIOSH–PRL and MSHA are shown in Figs. 8–10. Fig. 8

compares the measured ash or incombustible content from

4 h versus 24 h LTA for post-explosion samples from Fike

1 m3 chamber tests and a few samples from LLEM tests.

The data include samples from tests with Pittsburgh (Pgh)

coal, Pocahontas (Poc) coal, and mixtures of coal and rock

dust. Each data point compares the results of both 4 and

24 h LTA analyses for a single sample. Data points along

the dotted line in Fig. 8 represent perfect agreement

between the 4 and 24 h analysis procedures. For 1 m3 tests

with pure Pocahontas coal (no rock dust), the 4 h LTA

gave an incombustible content of 19–29%. The 24 h LTA

gave a result of 8–12% post-explosion incombustible. It

appears that not all of the coal and char was combusted

during the 4 h LTA for the low-volatile Pocahontas coal

samples. The longer 24 h LTA burned all of the coal/char

and gave a more accurate result. There was less of a

difference in the 4 and 24 h data for the high-volatile

Pittsburgh coal samples. There is little difference in the 4

and 24 h data at high (60–80%) LTA values for either the

1 m3 laboratory chamber samples or the LLEM samples. In

conclusion, the data show that the 24 h LTA is better than

the 4 h LTA for post-explosion samples with low incombustible content.

A comparison of the pre-explosion and post-explosion

incombustible contents is shown in Fig. 9, which includes

NIOSH-PRL and MSHA data for samples from both

the LLEM and the Fike 1 m3 laboratory chamber.

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250

Distance, m

LTA of Floor Dust, %

post-explosion dust

original dust mixtures

Fig. 7. Incombustible content before and after LLEM explosion test #471.
The incombustible content includes the rock dust plus the

ash in the coal. It was measured by low-temperature ashing

of the post-explosion residue for 24 h at 515 1C. (For this

comparison and the other figures, the incombustible did

not include the MSHA moisture content, which was 1%

for these samples.) The dotted line would represent no

change in the incombustible content from pre-explosion

to post-explosion samples. The conclusion is that the

post-explosion incombustible content was always as high

as or higher than the initial incombustible content. This

conclusion is similar to that previously reported by Nagy

and Mitchell (1963) based on very limited data from early

experimental mine explosions.

The pre-explosion versus post-explosion data comparison is shown in a different way in Fig. 10. In this graph,

the ratio of post-explosion to original incombustible

content is plotted as a function of the original or preexplosion incombustible content as measured by LTA.

This shows that the largest change in incombustible

content occurs at the lowest incombustible content. On

average, the post-explosion ash is about twice the original

ash content for the tests with pure coal dust (no rock dust).

This means that about half of the coal is burned during

these explosions. For tests with high original incombustible

contents, even if half of the coal were burned, the postexplosion incombustible content would not increase greatly

because the coal is only a small fraction of the total dust

content.

All of the alcohol coking analyses were conducted by the

MSHA laboratory at Mt. Hope, WV. Table 2 is a summary

of the coking data for samples from LLEM explosion tests.

All of these analyses were conducted ‘‘blind,’’ with the

MSHA laboratory not knowing the type of LLEM

explosion test or the extent of flame travel. The LLEM

test number and date are listed in the heading for the data

set for each LLEM test. All of the post-explosion samples

were collected from the floor, except for those listed as

Fine Pittsburgh 36 100 9 14 11

Pulverized Pittsburgh (PPC) 36 73 34 58 56

Coarse Pittsburgh 36 10 170 620 690

Pulverized Pocahontas 17 86 17 41 27

Limestone rock dust – 73 16 50 35

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80

24 hr Ash, %

4 hr Ash, %

Pgh coal, 1-m3

Poc coal, 1-m3

coal & rock dust, 1-m3

coal & rock dust, LLEM

Fig. 8. Comparison of 4-h versus 24-h incombustible contents for postexplosion samples from tests in the Fike 1 m3 laboratory chamber and the

LLEM.

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70 80 90

Original Pre-Explosion LTA, %

Post-Explosion LTA, %

PRL 1-m3 data

MSHA 1-m3 data

PRL LLEM data

MSHA LLEM data

Fig. 9. Comparison of pre-explosion versus post-explosion incombustible

contents from tests in the LLEM and the Fike 1 m3 laboratory chamber.

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

0 10 20 30 40 50 60 70 80 90

Original Pre-Explosion LTA, %

Post-Explosion/Original LTA ratio

PRL 1-m3 data

MSHA 1-m3 data

PRL LLEM data

MSHA LLEM data

Fig. 10. Post-explosion to pre-explosion incombustible ratio from tests in

the LLEM and the Fike 1 m3 laboratory chamber.
‘‘band samples’’ for test #434. The amounts of coke

(observed using the alcohol coking test) are listed for

samples at various distances from the LLEM face. For

some of the samples, duplicate analyses were conducted in

2003 and 2004. Below each data set, the dust mixture and

location are listed. For example, in test #386, ‘‘PPC and

77% rock dust to D-310 ft’’ means that a mixture of 23%

Pittsburgh pulverized coal (PPC) and 77% limestone rock

dust was loaded in the LLEM out to a distance of 310 ft in

D-drift. For this test, the flame propagated to a distance of

about 310 ft from the face. The ‘‘Pgh fines’’ listed for test

#388 and the ‘‘coarse Pgh coal’’ listed for test #474 refer to

the two other sizes of Pittsburgh coal dust listed in Table 1.

In general, ‘‘large’’ or ‘‘very large’’ amounts of coke were

observed in samples within the flame zone for the LLEM

tests. Beyond the end of the flame, the coke observations

were only ‘‘small,’’ ‘‘trace,’’ or ‘‘none.’’ At the location

where the flame ended, there was a large variation in

observed coke amounts—from ‘‘small’’ to ‘‘large.’’ This is

not surprising since the explosion flame dies out gradually

and the listed flame travel distance is uncertain by about

710 m or 730 ft. In general, there was good reproducibility in the coke analyses from 2003 and 2004 even though

the alcohol coking test is somewhat subjective.

There were a few exceptions in the coke observations. In

test #390, there was only a small amount of coke in the

sample from 626 ft, even though the flame went to 830 ft. In

test #398, there was only a small amount of coke in the

sample from 750 ft, even though the flame went to 1250 ft.

However, in both of these cases, the samples were from far

beyond the original dusted zone and the floor samples may

have contained only small amounts of dust. The other

LLEM #386, 8 Sept 1999 LLEM #388, 23 Sept 1999 LLEM #389, 4 Nov 1999

152 Large Large 100 None Small 108 – Large

300 Small Small 201 None Small 211 – Large

390 Trace Trace 300 None Trace 329 – Large

501 None None 390 None None

PPC and 77% rock dust to D-310 ft, flame to

310 ft

Pgh Fines and 77% rock dust to D-310 ft, flame to 340 ft PPC and 65% rock dust to B-340 ft, flame

to 530 ft

LLEM #390, 7 Dec 1999 LLEM #398, 1 March 2001 LLEM #400, 20 March 2001

108 – Large 100 Large – – – –

211 – Large 300 Large – – – –

329 – Large 390 Large – 300 – Large

– 501 Large Large 390 – Medium

626 Small 601 Large Medium 501 – Small

750 Small Small

PPC and 65% rock dust to B-460 ft, flame to

830 ft

PPC and 65% rock dust to D-460 ft, flame to 1250 ft PPC to D-250 ft, no rock dust, flame to

640 ft

LLEM #401, 28 March 2001 LLEM #434, 25 March 2003 LLEM #434, 25 March 2003

100 Large V. large 134 V. large – Band samples

201 Large Large 234 V. large – 234 V. large –

300 Trace Trace 304 V. large – 304 V. large –

390 None None 403 V. large – 403 V. large –

501 None None 501 V. large – 501 V. large –

601 Trace Trace 598 V. large – – – –

750 Small Trace 757 Trace – – – –

PPC and 80% rock dust to D-460 ft, flame to

200 ft

PPC and 65% rock dust to C-310 ft, flame to 770 ft PPC and 65% rock dust to C-310 ft, flame

to 770 ft

LLEM #471, 26 Feb 2004 LLEM #473, 11 March 2004 LLEM #474, 18 March 2004

100 – V. large 16 – Large 100 – V. large

201 – V. large 100 – Large 201 – V. large

300 – Large 201 – Small 300 – Large

390 – Small 300 – None 390 – Trace

501 – Trace 390 – None

601 – Trace

PPC and 65% rock dust to 205 ft, PPC and 80%

rock dust, 205–445 ft, flame to 360 ft

PPC and 65% rock dust to D-250 ft, flame to 220 ft,

explosion failed to propagate because of weak ignition

zone

coarse Pgh coal and 50% rock dust to D250 ft, flame to 310 ft
dust was very fine in size. It is possible that the finest sizes

of coal dust do not generate significant amounts of coke

particles when they burn, at least as measured by the

alcohol coking test.

The fact that large and very large amounts of coke were

observed within the flame zones for LLEM tests with initial

rock dust contents up to 65%, 77% and even 80% was

initially somewhat surprising. Nagy and Mitchell (1963,

p. 2) had said that ‘‘coke is not formed where the dust

contains more than 50% incombustible,’’ based on

observations after earlier Bruceton Experimental Mine

tests. However, they did not identify how the coke amount

was determined in their studies. Based on their other

comments about coke and the references to photos in their

report, it is possible that their coke amounts were based on

visual observations only. It is likely that the alcohol coking

test is more sensitive in detecting evidence of coke than

visual observations would be.

Krzystolik and Lebecki (1983) also observed coke after

explosions at the Polish Experimental Mine Barbara. They

used an optical microscope to observe the percentage of

coke particles in the dust samples. All of their tests were for

pure coal dust, without any rock dust. They concluded that

the presence of coked particles could be used to estimate

the flame range in most cases. This is consistent with the

results of the present PRL data.

During LLEM gas explosion tests #484 and #485 in

2005, small trays of pulverized and coarse Pittsburgh coal

dust were placed on the floor near the face in the methane

gas ignition zone. Although these trays were within the gas

flame zone, the dust was not dispersed because the dynamic

or wind pressure was very low near the origin of the

explosion. Samples were collected from these trays after the

explosion and only trace amounts of coke were found. This

shows that the coal dust must be dispersed into the flame

before large amounts of coke are generated.

In a series of laboratory experiments in the PRL 20-L

chamber in 2005, coal dust of various sizes was dispersed

into methane gas explosions. These explosions were ignited

by an electric spark. In almost all the tests, large or very

large amounts of coke were observed in dust samples

collected after the tests. Even 30  20 mesh (600–850 mm)

Pittsburgh coal showed large amounts of coke. Only the

20  10 mesh (850–1200 mm) Pittsburgh coal showed a

small amount of coke. This shows that even very large coal

particles up to 20 mesh (850 mm) become coked when they

are in a flame, even though this size of coal is too large to

propagate an explosion in the absence of methane.

5. Conclusions

The experimental mine and laboratory data show that

the post-explosion incombustible content is as high as or

higher than the pre-explosion incombustible content for

therefore the presence of coke is a good indication of the

extent of flame travel. Coke, as measured by the alcohol

coking test, is found after explosions at rock dust contents

even up to 80%. The results of this joint research will

assist MSHA in their future investigations of coal mine

explosions.

Disclaimer: The findings and conclusions in this report

are those of the authors and do not necessarily represent

the views of the National Institute for Occupational Safety

and Health or the Mine Safety and Health Administration.

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