Post-Explosion Observations of Experimental Mine 

and Laboratory Coal Dust Explosions 

Kenneth L. ashd dollarS. Weiss 


Pittsburgh Research Laboratory, 


National Institute for Occupational Safety and Health, Pittsburgh, PA, U.S.A. 


Terry G. Montgomery 

Mine Safety and Health Administration, Mt. Hope, W,U.S.A. 

John El Going 

Fike Corporation, Blue Springs, MO, U.S.A. 

Abstract 

The Pittsburgh Research Laboratory (PRL) of the National Institute for Occupational 

Safety and Health (NIOSH) and the Mine Safety and Health Administration (MSHA) conducted 

joint research on dust explosions by studying post-explosion dust samples. The samples were 

collected after full-scale explosions at the PRI, Lake Lynn Experimental Mine and after 

laboratory explosions in the PRL20-L chamber and the Fike 1-m3 chamber. The dusts studied 

included both high and low volatile bituminous coals. Low temperature ashing for 24 hr at 

515°C was used to measure the incombustible content of the dust before and after the explosions. 

The data showed that the post-explosion incombustible content was always as high as or higher 

than the initial incombustible content. The MSHA alcohol coking test -was used to determine the 

amount of coked dust in the post-explosion samples. The results showed that almost all coal dust 

that was suspended within the explosion flame produced significant amounts of coke. 

Measurements of floor dust concentrations after LLEM explosions were compared to the initial 

dust loadings to determine the transport distance of dust during an explosion. All of these data 

will be useful in future forensic investigations of accidental dust explosions in coal mines or 

elsewhere. 

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. 

I. Introduction 

Much progress has been made in preventing underground coal mine disasters, but 

explosions still occur, sometimes producing multiple fatalities. In an explosion, all underground 

miners are at risk. There were serious underground coal mine explosions in July 2000 in Utah 

(2 fatalities and 8 injuries), in September 2001 in Alabama (13 fatalities and 3 injuries), in , January 2006 in West Virginia (12 fatalities and 1 injury), and in May 2006 in Kentucky 

(5 fatalities). There have been other mine explosions in the U.S .A. in recent years that did not 

result in any injuries, but the mine recovery efforts took several months. These events show that 

the mine explosion problem has not yet been solved. Therefore, the Pittsburgh Research 

Laboratory (PRL) of the National Institute for Occupational Safety and Health (NIOSH) and the 

'contact author: Kenneth L. Cashdollar, NIOSH, Pittsburgh Research Laboratory, P.O. Box 18070, Cochrans Mill 

Road, Pittsburgh, PA 1523 6, phone 412-386-6753, fax 4 12-386-6595, e-mail KCashdollaa@cdc.govv 
Mine Safety and Health Administration (MSHA) have conducted joint research to improve the 

forensic investigation of accidental coal mine explosions. 

Explosions in underground coal mines and surface facilities such as processing plants are 

caused by accumulations of flammable gas and/or combustible dust dispersed in air in the 

presence of an i,pition source. Research on the causes and mechanisms of gas and dust 

explosions is needed as a basis for the development of techniques and strategies for explosion 

prevention, suppression, and mitigation. In the coal mining industry, rock dust (usually 

limestone) is added to the coal dust deposits to act as a heat sink and inhibit flame propagation. 

If a sufficient amount of rock dust is added to the coal dust, the mixture is rendered inert. The 

rock dusting regulations are based on the results of full-scale experimental mine tests as 

summarized by Nagy (1 98 1). 

NIOSH-PRZ, conducts dust explosion research in its Lake Lynn Experimental Mine 

(LLEM) and in laboratory chambers. Post-explosion dust sampling is part of this research effort. 

Analysis of these samples provides MSHA with valuable knowledge of dust samples collected 

after well-documented explosions. This knowledge 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 and 

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, and Wade, 1983; Triebsch and Sapko, 1990). This is a forrner 

limestone mine, and five new drifts (horizontal passageways in a mine) were developed to 

simulate the geometries of modem U.S.A. coal mines. The mine has four parallel drifts - A, B, 

C, and D. D-drift is a 520-m (1710-ft) long single-entry that can be separated fiom E-drift by an 

explosion-proof bulkhead door. In order to simulate room and pillar workings, drifts A, B, and C 

can be used. These three drifts are approximately 520 m (171 0 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. The entries are about 6 m wide (20 ft) by about 2 m (6% ft) high, with cross-sectional 

areas of 12- 13 m2 (1 30- 140 ft2). The entries are designed to withstand explosion pressures of 

7 bar or 700 kPa (100 psi). Previous publications described the LLEM coal dust explosion test 

procedures and results of research other than post-explosion observations (Weiss, Greninger, and 

Sapko, 1989; Cashdollar, Weiss, Greninger, and Chatrathi, 1992; and Sapko, Weiss, Cashdollar, 

and Uochower, 2000). 

Each LLEM drift has ten data-gathering (DG) stations inset in the rib wall at the locations 

shown in Fig. I. Each DG station houses a strain gauge transducer to measure the static 

explosion pressure and an optical sensor to detect the flame arrival. The "static pressure" is the 

pressure that is exerted in all directions, while the dynamic or wind pressure is directional (Nagy, 

198 1, p. 5 8). Other instruments such as dynarnic 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 fiom 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 (%-ft) wide strip along the floor, ribs (walls), and roof. Most of the 

LLEM dust explosion tests described in this paper were conducted in the single entry D-drift, 

which was isolated from E-drift by means of the explosion-proof movable bulkhead door 

N Surface quarry I 

Fig. 1. Plan view of the Lake Lynn Experimental Mine (LLEM) 

pace '~efhanegas zone 

1 Fig. 2. Side view of D-drift in the LLEM, showing gas 

ignition zone and dust test zone (not to scale). 

(Fig. 1). The ignition zone for a typical D-drift dust explosion test (Fig. 2) was a methane-air 

mixture at the face. 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 of the dust was dispersed uniformly throughout the cross section. For the LLEM tests, 

D-drift was thoroughly washed down several days before the test. In addition, the access ramp 
--- --- 

at the open end of the drift was wetted down the day of the test. The purpose was to significantly 

minimize contamination of the post-explosion dust samples. 

Dispersion 

Scale, cm 

' Fig. 3. Fike 1 -m3 explosion test chamber. 

Mention of any company or product does not constitute endorsement by NIOSH. 

Dust explosion research was also conduited in the Fike Corporation 1-rn3 (1000-L or 

35 f?) chamber (Cashdollar and Chatrathi, 1993; Going, Chatrathi, and 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 5 1-mm diameter bolts. 

Two variable reluctance pressure transducers were used to measure the explosion pressure. 

PRL-designed optical dust probes (Liebman, Conti, and Cashdollar, 1977; Cashdollar, Liebman, 

and Conti, 198 1) 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-mrn 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 aiid.raised the 

chamber pressure to about 1bar,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. sobbe2 of Germany. They were activated electrically with au 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 1 0-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 

and Hertzberg, 1982; Cashdollar, 1996). The test procedtxes and instrumentation for the 20-L 

chamber were similar to those of the 1 -m3 chamber. 

3. Post-explosion dust analysis procedures 

The low temperature ashng (LTA) measured the 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 pm). The LTA was conducted 

for both 4 hr and 24 hr at 5 15OC on most of the samples. The 5 15°C temperature is intended to 

bum up all the coal but not to decompose the limestone rock dust (CaC03). In order to test this, 

a 24-hr LTA was run on limestone rock dust and the recovery was greater than 99%. The 4 hr 

procedure is the standard one MSHA uses for routine mine dust samples for compliance with 

rock dusting regulations. The 24 hr procedure was frst used at PRL after the 4 hr procedure did 

not appear to bum up all the coal 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 hr" LTA at the two labs. MSHA used separate samples for the 4 hr and 24 hr LTA. 

PRL heated the samples for 4 hr, weighed them, and then re-heated them for an additional 20 hr. 

The MSHA "4 hr" procedure was a 1.5 hr ramp-up and 2.5 hr at 5 15OC for a total of 4 hr heating. 

The PRL "4 hr" procedure was a 1.3 hr ramp-up and 4 hr at 5 15OC. 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 105OC 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 minus 20 mesh post-explosion residue was placed in a 

one-inch diameter test tube. 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 five minutes. 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 (274) 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 fkom 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 p).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 m-um of 76 kPa (1 1 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 flarne from the gas ignition 

zone out to 1 05 m (346 ft). There were small but 

measurable signals on the flame sensors from 1 19 to 

153 rn (390 to 501 ft). These small signals probably 

corresponded to either localized burning or to postcombustion hot particles that were gradually cooling as 

they were carried out the drift by the expanding gases. 

There were no signals on the flame sensors beyond 

153m. The flarne sensor data showed that the dust 

flarne 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 previpus 

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). 

Large 

Small 

Trace 

Fig. 4. ~ualitative alcohol 

coke test observations. 

Distance, ft 

300 400 - 500 600 700 800 

14 

100 150 

Distance, m 

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

After the explosion test in the LLEM, dust samples were collected from the mine floor at 

various distances fiom the face and then sieved minus 20 mesh (850 pm). The calculated dust 

loadings in g/m2before and after the explosion are shown in Fig. 6. The dust loading before the 

explosion includes the 150 g/~3of coal dust and 279 dm3 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 of2.1 m (6.9 ft), the calculated, 
nominal floor loading was 900 g/m2. Th~s calculation includes both the dust on the floor and on 

the shelves. The zone fiom 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 fiom measured areas at various locations. These data are shown as the solid circle data 

points in Fig. 6. It is obvious fiom 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 13 5.6 m (445 fi) fiom 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. 

Distance, ft 

0 100 . 200 300 400 500 600 700 800 

, 4I 8, I 

---.

I , I 

.-... - .. - .... .___--- t
-_ -.- .- --. --_ 1600 "1.411..1-1 


B I ... . . . .. ......................................... ; -*. post-explosion dust 

I 'I , 1400 --------------..---...---p--

a I i ll 

I original dust loading ....... ..., ............................................ 

3 1 200 . .............. -.;.. .... 7--I - .-..- . .. ......

CD .---4. 

I , I -Ab 6 /

.-f 1000 L- - 1-2 f 

tu . #11....1.1' / 1 \. U 

0 '. /' I 800 -- 8 I. .L, 

V) I t /' 1 \. J 
 I' I

0, 600 -- .a' o . r - 1 

'@.'. 0 ........... -.-.....-. .; ...........-. .-;C ....................................................... _--_--,- '. - ................... B / LL 
 400--, &--., / I : 

'. /' ' :\.e* / '. I - /' 'a3) 

200 -- I ,/ 1 

- I/ I 

0 Jan, , , I I I 

0 50 100 150 200 250 ' Distance, m 

, Fig. 6. Dust loading before and after explosion test #471. 

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 (fiorn 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 fiom 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 that a significant 

amount of the coal in this region was burned during the explosion. The incombustible content 

further fiom the face was also -80%, similar to the original pre-explosion content in this zone. 

Since the explosion flame stopped at -1 10 m (360 fi), 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. 

-- 

Distance, ft 

0 100 200 300' 400 500 600 700 800 

100 ' I , I . , I 

-

-a. 
 --..--. -.-- --.- Bkz&-~!-z -.,-.4-~4-------.--- . - '. - : -*---4+--- -@.*4- ! 

; P 1 

... -- -- -- -- - a -"1IP.ll-l I 

1 .-. . .-............. .- ... ...... ...... B 
 . . 

1 I 

.... t........................... ---a , Ii I 

........... ,. -......... . . - . . - . -" , - ' I , 

I B 

... .... ,.................. ... .- .- . -+- post-explosion dust !- I i 1 

:, I 1 I I original dust mixtures . ....... , ................. ................ ....... .. --.- -.-- .- ........-.... 

I II ......-. .- .......... -.. .. 

I I ..................................... --.... - .. ,.. -.- .... .- .-. .-. ..... -............ 

I I 

100 150 

Distance, m 

Fig. 7. Incombustible content before and after LLEM explosion test #471. 

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 -m3data 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 low volatile 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 pm), 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. 
Table 1. Analyses of the bituminous coal dusts and rock dust. 

I I volatility, % / -200 mesh, % / Ds,prn / Dw, pm / Drned, pm /

I 

fine Pittsburgh 

pulverized Pittsburgh (PPC) 

coarse Pittsburgh 

1 

36 

36 

36 

1 

i 

100 

73 

-10 

/ 

/ 

i 

9 

34 

-170 

/ 

1 

i 

14 

58 

-620 

1 

1 

i 

11 

56 

-690 

pulverized Pocahontas 

limestone rock dust 

17 

-- / 

1 

86 

73 

/ 

1 

17 

16 

/ 

1 

41 

50 

1 

1 

27 

-35 

The summary post-explosion LTA data fiom NIOSH-PRL, and MSHA are shown in 

Figs. 8-1 0. Fig. 8 compares the measured ash or incombustible content fiom 4-hr versus 24-hr 

LTA for post-explosion samples from Fike I-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-hr and 24-hr LTA 

analyses for a single sample. Data points along the dotted line in Fig. 8 represent perfect 

agreement between the 4-hr and 24-hr analysis procedures. For 1-m3tests with pure Pocahontas 

coal (no rock dust), the 4-hr LTA gave an incombustible content of 19-29%. The 24-hr LTA 

gave a result of 8- 12% post-explosion incombustible. It appears that not all of the coal and char 

was cornbusted during the 4-hr LTA for the low volatile Pocahontas coal samples. The longer 

24-hr LTA burned all of the coalichar and gave a more accurate result. There was less of a 

difference in the 4-hr and 24-hr data for the high volatile Pittsburgh coal samples. There is little 

differencein the 4-hr and 24-hr data at high (60-80%) LTA values for either the 1 -m3 laboratory 

chamber samples or the LLEM samples. In conolusion, the data show that the 24-hr LTA is 

better than the 4 hr LTA for post-explosion samples with low incombustible content. 

I 

4 Pgh coal, 1-m3 Ii 

pI Poc coal, 1-rn3 

I 

. .!. . , I 

0 I 4 I 1 

I I 

j 

0 I0 20 30 40 50 60 70 80 

X coal & rock dust, 1-m3 1- 

24 hr Ash, % 

Fig'. 8. Comparison of 4-hr versus 24-hr incombustible contents for post-explosion 

samples from tests in the Fike 1-m3 laboratory chamber and the LLEM. 
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. 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 hr at 

5 1'5OC. (For this comparison and the other figures., the incombustible did not include the MSHA 

moisture content, whch 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 

Mit~hell(1963) based on very limited data from early experimental mine explosions. 

---- 

I 

i .a -

I 

I 

. . . .j. 

-- . ... .- ., . .i:11_-- .- 0 PRL I-rn3 data -

. . i + MSHA I-rn3 data 

i PRL LLEM data i -- ! X MSHA LLEM data - .. j I ! , I 0 I I , , , , 

0 10 20 30 40 50 60 70 80 90 

Original Pre-ExplosionLTA, % 

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

from tests in the LLEM and the Fike 1-m3laboratory chamber. 

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 incombustible content as measured by LTA. This shows that the largest 

change in incombustible content occurs at the lowest incombustible contents. 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 at 

high original incombustible contents, even if half of the coal were burned, the post-explosion 

incombustible content would not increase greatly because the coal is only a small fiaction 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 "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 & 

77% rock dust to D-3 10 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 3 10 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 10 m or 30 ft. In general, there was good reproducibility in the coke analyses from 2003 

and 2004 even though the alcohol coking test is somewhat subjective. 

i I i -_LpL I I ---I 0 PRL I-madata -

I i 

+MSHA I -m3data 7 j

i 

I ; I

I -

+ - PRL,LLEM data -

- .y I I / X MSHA LLEM data -

0 10 20 30 40 50 60 70 80 90 

Original Pre-Explosion LTA, % 

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

tests in the LLEM and the Fike 1-m3 laboratory chamber. 

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 been small amounts of dust. The other significant 

exception is test #388, where the flame traveled to 340 ft, but the coke observations were small, 

trace, or none. The difference in ths LLEM test was that the coal 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 bum. 
Table 2. Alcohol coke analyses of post-explosion dust samples. 

LLEM #386, 8 Sept 1999 

distance 1 2003 1 2004 

152 ft / large 1 large 

300 ft small small 

390 ft trace trace 

501 ft none 1 none 

LLEM #388, 23 Sept 1999 

distance 1 2003 2004 

100 ft none 1 small 

201 ft none I small 

300 fi 1 none 1 trace 

390 ft 1 none I none 

LLEM #389, 4 Nov 1999 

distance / 2003 1 2004 

108R / --- 1' large 

211R / --- / iarge 

329 ft 1 :-- 1 large 

PPC& 65% rock dust to 5-340 ft 

PPC& 77% rock dust to D-310 ft PghFines & 77% rock dust flame to 530 17 

flame to 310 fi foD-310 1'7, flame fo 340 ft 

LLEM #390, 7 Dec 1999 

distance / 2003 1 2004 

108 ft 1 - - 1 large 

21Ift I --- iarge 

329 ft I --- large 

1. ----	 I 

626 ft 1 I small 

PPC& 65% rock dust to 5-460ft 

' 

LLEM #398, IMarch 2001 

distance / 2003 / 2004 

100 ft / large 1 --- 300 ft large [ ---

390 ft large I ---

501 ft 1 large I large 

601 ft . large / medium 

750 ft I small 1 small 

LLEM #400, 20 March 2001 

distance / 2003 1 2004 ----I ---I --- ---- --- ---

300 ft / --- large 

390 ft / - medium

501ft --- 1 small 

PPCfo D-250 ff, no rock dusf 

flame to 830 ff PPC& 65%rock dust fo D-460 ff flame to 640 ff 

flame to 1250ft 

r 

LLEM #401, 28 March 2001 

distance / 2003 1 2004 

100 ft / large I v. large 

201 ft 1 large large 

300 A trace I trace 

390 ft 1 none 1 none 

501 ft none I none 

601ft 1 trace i trace 

750 ft 1 small trace 

LLEM #434, 25 March 2003 

distance 1 2003 1 2004 

134 ft I v, large / ---

234 ft v, large ---

304 ft v. large --

403 ft ~..large1 ---

501 ft v. large / ---

598 ft V. large / ---

757 ft I trace I ---

LLEM #434, 25 March 2003 

distance / 2003 1 2004 

band samples 

234 ft v, large --- 304 ft v. large j -- . 403 ft I v. large 1 ---

501 ft 1 V. large / -- - _ - - - _ 1 - - _ 

---I ---I_-

PPC	& 80% rock dusf to D-460 ff PPC& 65% rock dust to C-310 ft PPC& 65% rock dust fo C-310 ff 

flame to 200 ff flame to 770 ft flame to 770 ft 

LLEM #471, 26 Feb 2004 

distance 1 2003 / 2004 

100 ft 1 --- 1 v. large 

201ft / -- / v. large 

300 ft i --- large 

390~ 1 --- 1 small 

LLEM #473, 11 March 2004 

distance / 2003 1 2004 

16 ft 1 --- / large 

100ft / -- I large 

201 --- ft small 

300 --- ft none 

LLEM #474, 18 March 2004 

distance / 2003 1 2004 

100ft / --- v, large 

201ft ( -- v. large 

300 ft --- I large 

390 fi --- 1 trace 

501 ft I --- I trace 

601 ft 1 - - trace 

390ft 1 --- 1 none 

PPC& 65% rock dust to D-250 ff 

coarse Pghcoal & 

50% rock dusf to 12-250ft 

PPC& 65% rock dust to 205 ff flame to 220 ft flame fo 310 ff 

PPC& 80% rock dust, 205-445 ft explosion failed to propagafe 

flame 50 -360 ft because of weak ignition zone 
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) 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. 

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 PRL20-L chamber in 2005, coal dust of 

various sizes was dispersed into methane gas explosi-ons. These explosions were ignited by an 

electric spark. In almost all the tests, large or very large mounts of coke were observed in dust 

samples collected after the tests. Even 30x20 mesh (600-850 pm) Pittsburgh coal showed large 

amounts of coke. Only the 20x10 mesh (850-1200 pm) Pittsburgh coal showed a small amount 

of coke. This shows that even very large coal particles up to 20 mesh (850 pm) 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. CorncBusions 

The experimental mine and laboratory data show that the post-explosion incombustible 

content is as high as or higherthan the pre-explosion incombustible content for both high and 

low volatile coals. The data fiorn the alcohol coking test show that coke is found whenever coal 

particles are dispersed into a flame, and therefore the presence of coke is a good indication of 

flame travel. Coke, as measured by the alcohol coking test, is found after explosions at rock dust 

contents up to 80%. The results of this joint research will assist MSHA in their future 

investigations of coal mine explosions. 

References 

Cashdollar, K. L. (1996). Coal Dust Explosibility. Journal of Loss Prevention in the Process 

Industries, 9,65-76. 

Cashdollar, K.L., and Chatrathi, K. (1 993). Minimum Explosible Dust Concentrations 

Measured in 20-L and 1-m3 Chambers. Combustion Science and Technology, 87, 157-171. 

Cashdollar K. L., and Going, J.E. (2003). Coal Dust Inerting and Post-Explosion Dust Sampling 

Research in a 1-m3 Laboratory Chamber and an Experimental Mine, proceedings of the 

Fall Technical Meeting of the Eastern States Section of the Combustion Institute, 

University Park, PA, October 26-29,2003, pp. 97-1 00. 

Cashdollar, K. L., and Hertzberg, M. (1982). 20-L Explosibility Test Chamber for Dusts and 

Gases. Review of Scientzfic instruments, 56, 596-602. 

' 
Cashdollar, K. L, Liebrnan, I. and Conti, R. S. (1 98 1). Three Bureau ornines Oph'cal Dust 

Probes, U.S.Bureau of Mines RI8542,26 pp. 

Cashdollar, K. L., Weiss, E. S., Greninger, N. B.,and Chatrathi, K. (1992). Laboratory and 

Large-Scale Dust Explosion Research. Plant/Operations Progress, 1 1,247-255. 

Going, J. E., Chatrathi, K., and Cashdollar, K. L. (2000). Flammability Limit Measurements for 

Dusts in 20-L and 1 -m3Vessels. Journal ofLoss Prevention in the Process Industries, 13, 

209-2 19. 

Liebrnan, I., Conti, R. &d Cashdollar, K. L. (1977). Dust Cloud Concentration Probe. Review 

ofScientz3c Instruments, 48, 13 14- 13 1 6. 

Mattes, R. H., Bacho, A., and Wade, L. W. (1983). Lake Lynn Laboratory: Construction, 

Physical Description, and Capability. U.S. Bureau of Mines, IC 891 1,40 pp. 

Nagy, J. (1 981). The Explosion Hazard in Mining. U.S. Mine Safety and Health Administration, 

IR 1119,69 pp. 

Nagy, J., and Mitchell, D. W. (1 963). Experimental Coal-Dust and Gas Explosions. U.S. 

Bureau of Mines RI6344,27 pp. 

Sapko, M. J., Weiss, E. S., Cashdollar, K. L., and Zlochower, I. A. (2000). ~x~erimenkl Mine 

and Laboratory Dust Explosion Research at NIOSH. Journal of Loss Prevention in the 

Process Industries, 13,229-242. 

Triebsch, G. and Sapko, M. J. (1 990). Lake Lynn Laboratory: A State-of-the-Art Mining 

Research Laboratory. Proceedings of the International Symposium on Unique 

Underground Structures, Denver, CO, June 12-1 5, 1990, chap. 75, pp. 75-1 to 75-21. 

Weiss, E. S., Greninger, N. B., and Sapko, M. J. (1 989). Recent Results of Dust Explosion 

Studies at the Lake Lynn Experimental Mine. Proceedings of the 23rd International 

Conference of Safety in Mines Research Institutes, Washington, DC, September 11-15, 

1989, pp. 843-854.