Observations of post-explosion dust samples from an experimental mine. 

C. K. Man*

, K. L. Cashdollar, I. A. Zlochower, and G. M. Green 

National Institute for Occupational Safety and Health 

626 Cochrans Mill Road 

PO Box 18070 

Pittsburgh PA 15236-3611 

* Corresponding author: cman@cdc.gov 

Proceedings of the 6th U.S. National Combustion Meeting 

Abstract

Coal dust explosions are routinely conducted within the full-scale experimental mine at the National Institute for 

Occupational Safety and Health (NIOSH), Pittsburgh Research Laboratory’s (PRL) Lake Lynn Experimental Mine 

(LLEM) to meet the needs of numerous safety and health research programs. Many of these explosion tests over the 

years involved igniting different coal dust and limestone rock dust mixtures (up to 80% by volume rock dust). Following 

four more recent tests, floor dust samples have been collected and analyzed using the alcohol coking test and scanning 

electron microscopy (SEM) and the results compared to estimate the flame travel. 

 

Introduction 

Serious accidents and injuries from fires and 

explosions have decreased considerably over the last 100 

years in the underground coal mining industry [1, 2]. 

However, they still occur today and often result in many 

fatalities, for example, the Sago mine, WV, 12 fatalities 

from explosion of gas and dust; the Darby mine, KY, 5 

fatalities from explosion of gas and dust; and the 

Aracoma mine, WV, 2 fatalities from fire [3-5]. 

All mining related accidents in the United States are 

investigated by the Mine Safety and Health 

Administration (MSHA). For fire and explosion 

accidents, one area of such investigations usually involves 

implementing a mine dust survey to establish the flame 

travel by the fire or explosion, i.e., where it terminated. 

This can be done by close examination of solid residues 

left by a fire or explosion. 

 

Specific Objectives 

This paper describes how flame travel in a coal dust 

explosion can be assessed by conducting a number of coal 

dust explosions in an experimental mine and analyzing 

the post-explosion residue samples in a laboratory using 

two very different techniques – alcohol coking test and 

scanning electron microscopy. 

Experimental Mine and Test Procedures 

Mine Explosion Tests 

All the explosion experiments were carried out at the 

LLEM [6]. The experimental mine has 4 parallel tunnels 

(A, B, C and D) each approximately 20 ft (6.1 m) wide, 7 

ft (2.1 m) high, and over 1600 ft (488 m) long [7]. The 

first three explosion experiments were conducted in 

tunnel ‘D’ and last one in tunnel ‘A’. 

Pulverized Pittsburgh coal (PPC) was used in the 

explosion experiments. The coal was mined from PRL’s 

Safety Research Coal Mine near Pittsburgh, PA and then 

ground and pulverized at an on-site facility. PPC and 

limestone rock dust are fine powders (80% <75μm or 

-200 mesh). Half of the coal dust and limestone rock dust 

mix was scattered onto the floor of the mine in the 

“dusted zone” and half deposited onto shelves suspended 

1.5 ft (0.5 m) from the roof. 

The dust explosions were initiated with a 10% 

methane-air explosion at the closed end of the tunnel. Gas 

samples were taken for subsequent analysis in a gas 

chromatograph in order to verify the methane 

concentration actually used. The gas zone forming this 

primary explosion varied from 12 ft (3.7 m) for LLEM 

#473 to 40 ft (12.2 m) for LLEM #401 and LLEM #511. 

The gas explosions were ignited using electric matches. A 

list of the explosion experiment parameters are 

summarized in Table 1. After each explosion test, dust 

samples were collected along the middle of the tunnel at 

regular intervals. Only floor samples were taken, starting 

from the closed end of the tunnel. Dust was brushed from 

a known collection area, 2 ft by 2 ft (0.6 m by 0.6 m) 

sample area) from the dry concrete mine floor; this dust 

sample collection process was repeated for several sample 

areas at the same relative location if insufficient quantities 

of dust were collected from just one sample area at that 

location. 

Alcohol coke test 

The alcohol coke test was carried out by adding 

approximately 1 g of -20 mesh (<0.8 mm) post-explosion 

sample into a test tube containing about 15 mL of 

denatured ethyl alcohol [8]. The sides of the test tube 

were then washed down with about 5-10 mL of alcohol. 
The test tube was set aside to allow time for the more 

dense particles to sink to the bottom. After 5 minutes the 

sample was classified according to the chart shown in 

Figure 1. 

Table 1 Summary of LLEM explosion parameters, including rock dust content and dusted zone locations. 

Explosion 

test ID 

Tunnel PPC : limestone 

rock dust mix 

Total incombustible 

content, TIC (%) 

Coal dust*, 

3 g/m

Dusted zone, ft and (m) 

LLEM #401 D 20:80 82 200 40 - 460 

(12.2 – 140.2) 

LLEM #471 D 35:65 and 68 150 27 - 205 

(8.2 – 62.5) 

20:80 82 150 205 - 445 

(62.5 – 135.6) 

LLEM #473 D 35:65 68 200 12 - 250 

(3.7 – 76.2) 

LLEM #511 A 35:65 and 68 150 40 - 250 

(12.2 – 62.5) 

20:80 82 150 250 – 580 

(62.5 – 176.8) 

* Equivalent coal dust per unit volume inside the tunnel in the dusted zone only. 

Figure 1 Alcohol coke test chart [8]. 

To quantify the results, the categories of coke were 

assigned an arbitrary value which this paper terms the 

‘alcohol coke index’ such that, very large = 15, large = 

10, small = 4, trace =1 and none = 0. The ‘medium’ 

category shown in Figure 1 was not used in this study 

because it was difficult to differentiate this from the 

‘large’ class. It should be pointed out that the terms ‘coke’ 

and ‘coking’ used in this paper do not relate to those used 

in the metallurgical world as the post-explosion coal 

residues are the result of rapid coal devolatilization and 

are more akin to char than actual metallurgical coke in 

both appearance and physical properties such as porosity 

and strength. 

Scanning Electron Microscope Observations 

The microscopic analysis of post-explosion particles 

was carried out using a Jeol 6400 scanning electron 

microscope. In order to obtain good images of the organic 

components of interest, the post-explosion samples were 

initially acid leached in 8M hydrochloric acid to remove 

all the limestone rock dust. This also removes some of the 

inorganic minerals from the coal. After this, a small 

amount of each test material was mounted onto a piece of 

adhesive tape which in turn was mounted onto a copper or 

aluminum disc which acted as a sample holder. These 

were then treated in a sputter coater under vacuum, which 

deposits a thin conductive layer of gold onto the sample 

via evaporative deposition to minimize charging effects 

by the electron beam [9]. For each explosion residue, 

digital photographs of at least 6 fields of view were 

recorded at relatively low magnifications (between x100 

and x400) in order to obtain a reasonable number of 

particles per field of view for analysis. 

The SEM micrographs were analyzed by eye to 

quantify the extent of coal decomposition. This was done 

by estimating the percentage of coal particles that had 

decomposed in each of the photographs and averaging the 

totals. This percentage was called the ‘SEM coke 

content’. 

Low temperature ashing 

The incombustible contents (IC) of the post-explosion 

floor dust samples were determined using a low 

temperature ashing furnace. About 1 g of each sample, 

sieved to -20 mesh, was weighed in a small ceramic 

crucible. This was heated over 1.5 hours to 515°C and 

held for 20 hours before cooling naturally. The total 

heating and cooling cycle was about 24 hours. 

2 
Results and Discussion 

The laboratory analysis of the post-explosion dust 

samples showing alcohol coke index, along with the SEM 

coke content are summarized in Figures 2-5. The preexplosion total incombustible contents (TIC) in the dusted 

zones are clearly marked. There was only one dusted zone 

in explosions LLEM #401 and LLEM #473 but two 

adjacent dusted zones in LLEM #471 and LLEM #511, 

the areas of which are outlined by the dashed lines. All 

the dusted zones started immediately beyond the closed 

end gas ignition zone which is located nearest the origin 

on the graphs and have been annotated for clarity. 

For LLEM #401 (Figure 2), the single dusted zone 

explosion resulted in floor dust material which had very 

similar incombustible contents compared to the initial 

TIC of 82% except for one sample taken furthest away 

from the initial explosion. This suggests that only a small 

fraction of the coal has been combusted during the 

explosion. It is also possible that the some of the coal 

particles which have been largely consumed in the 

explosion are simultaneously ejected out along the tunnel. 

There is strong evidence that the coal near to the gas zone 

did burn according to both the SEM coke content and 

alcohol coke index data (Figure 2). The results show a 

very high level of coke from the SEM data and a high 

alcohol coke index in this region. In mine accident 

investigations, the presence of either large or very large 

quantities of coke in the post-explosion dust samples are 

indicative of an explosion flame [4]. It is also worth 

pointing out that although 1g of sample was used in the 

alcohol coke tests, the amount of combustible material 

contained in each of this set of samples was in fact quite 

small (approximately 0.2g or less). 

The SEM coke content falls from 100% at around 200 

ft (61 m) from the closed tunnel to a value of almost zero 

at 400 ft (122 m) and beyond. The alcohol coke index 

follows a similar pattern where it falls rapidly from a peak 

value of 15 at 100 ft (30.5 m) to 0 at 400 ft (122 m) where 

there was only a trace or no coke present in the samples. 

This is well before the end of the dusted zone which 

extended to 460 ft (140 m) and suggests the explosion did 

not travel beyond the dusted zone due to the high IC 

(82%) used in this experiment which effectively inerted 

the explosion. 

Figure 3 shows the laboratory data for samples taken 

from explosion LLEM #471. The two dusted zones 

containing 68 and 82% TIC are clearly identified. The 

single floor dust sample taken from the first dusted zone 

had a relatively high incombustible content as expected 

but the subsequent floor dust samples had ICs very 

similar to the initial TIC of the second dusted zone of 

82% which again suggests only a small fraction of the 

coal has been consumed. There is good agreement 

between the SEM coke content and the alcohol coke 

indices with both showing a steady fall in the amount of 

coke material from 200 ft (61 m) to about 450 ft (137 m) 

before reaching a minimum at 500 ft (152 m) and beyond. 

It appears that the coal dust explosion terminated near the 

end of the second dusted zone. 

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000

Distance, ft

% incombusible, SEM coke content (%)

0

5

10

15

0 40 80 120 160 200 240 280

Distance, m

Alcohol coke index

Initial TIC

Floor dust IC

SEM coke content

Alcohol coke index

gas zone

dusted zone

Figure 2 Analysis of post-explosion materials from 

LLEM #401 showing SEM coke contents and alcohol 

coke indices. 

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000

Distance, ft

% incombusible, SEM coke content (%)

0

5

10

15

0 40 80 120 160 200 240 280

Distance, m

Alcohol coke index

Initial TIC zone 1

Initial TIC zone 2

Floor dust IC

SEM coke content

Alcohol coke index

dusted zones

gas zone

zone 1 zone 2

Figure 3 Analysis of post-explosion materials from 

LLEM #471 showing SEM coke contents and alcohol 

coke indices. 

The floor dust IC for explosion LLEM #473 (Figure 4) 

follows a similar trend to LLEM #401 which also had a 

uniform dusted zone where the IC values almost match 

the original TIC, again indicating a low consumption of 

the coal. However, there is a significant difference in the 

SEM and alcohol coke data compared to the two other 

explosions described above. For LLEM #473, the SEM 

and alcohol coke data both start at lower values near the 

gas zone compared to LLEM #401 and LLEM #471. 

These indices fall rapidly and reach about 0 at 300 ft (91 

m) which is only slightly further than the edge of the 

3 
dusted zone. The reason for the smaller amounts of coke 

near the tunnel face (closed end) is probably due to the 

small gas zone, 12 ft (3.7 m), used in this experiment 

which would have resulted in a smaller initial explosion. 

Again, the explosion appears to have terminated at or near 

the end of the dusted zone. 

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000

Distance, ft

% incombusible, SEM coke content (%)

0

5

10

15

0 40 80 120 160 200 240 280

Distance, m

Alcohol coke index

Initial TIC

Floor dust IC

SEM coke content

Alcohol coke index

dusted zone

gas zone

Figure 4 Analysis of post-explosion materials from 

LLEM #473 showing SEM coke contents and alcohol 

coke indices. 

Figure 5 shows the laboratory results from explosion 

experiment LLEM #511. Like LLEM #471 there were 

two adjacent dusted zones containing 68 and 82% TIC 

respectively but this time the dusted zone was extended 

580 ft (176.8 m). As before, the floor dust IC samples 

almost follow that of the initial TIC values. The results 

also show a very high level of coke from both the SEM 

and alcohol coking tests from the tunnel face (closed end) 

to 400 ft (122 m) which is well into the second dusted 

zone. From this point the SEM coke content falls 

gradually but never reaches zero even at 800 ft (244 m). 

Unfortunately no samples were taken beyond this point 

but it is reasonable to believe from the data that the flame 

could have travelled to this region. Again, the size of the 

methane/air zone played an important role in the coal dust 

explosion both in terms with the amount of coke 

generated and the distance traveled along the tunnel in the 

mine. 

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000

Distance, ft

% incombusible, SEM coke content (%)

0

5

10

15

0 40 80 120 160 200 240 280

Distance, m

Alcohol coke index

Initial TIC zone 1

Initial TIC zone 2

Floor dust IC

SEM coke content

Alcohol coke index

gas zone

dusted zones

zone 1 zone 2

Figure 5 Analysis of post-explosion materials from 

LLEM #511 showing SEM coke contents and alcohol 

coke indices. 

Conclusions 

The alcohol coke test is a quick and simple test and 

has been shown to be a useful technique in an explosion 

research laboratory. Observations of partially burnt coal 

particles under a SEM at low magnification can also give 

similar quasi quantitative data which can forensically 

track the flame travel after a coal dust explosion. In this 

study, there appears to be a very good agreement between 

these two different laboratory methods. 

Although the alcohol coke test is in itself a valuable 

tool in mine accident investigations, it may be 

supplemented with SEM coke analysis, especially in 

situations where the amount of sample is limited (to a few 

milligrams). However, SEM studies are time consuming 

and often not easily accessible. 

Acknowledgments 

The authors would like to thank Eric Weiss, Team 

Leader of Lake Lynn Laboratory, and the rest of the LLL 

team for conducting the explosion experiments; Donald 

Sellers, Physical Science Technician at LLL, for 

collecting the post-explosion dust samples; and to 

William Chisholm, Research Physical Scientist, and 

Diane Schwegler-Berry, Biologist, of NIOSH 

Morgantown for their assistance with the SEM. 

4 
References 

1. Richmond, J. K., Price, G. C., Sapko, M. J., and 

Kawenski, E. M., “Historical Summary of Coal 

Mine Explosions in the United States, 1959-81”, 

U.S. Department of the Interior, Bureau of Mines 

Information Circular 8909, 1983. 

2. Dobroski, H. Jr., Stephan, C. R., Conti, R. S., 

“Historical Summary of Coal Mine Explosions in 

the United States, 1981 to 1994”, U.S. Department 

of the Interior, Bureau of Mines, Information 

Circular 9440, 1996. 

3. MSHA Report of Investigation, “Fatal 

Underground Coal Mine Explosion, Sago Mine, 

West Virginia, January 2, 2006”. 

4. MSHA Report of Investigation, “Fatal 

Underground Coal Mine Explosion (Darby Mine 

No. 1), May 20, 2006”. 

5. MSHA Report of Investigation, “Fatal 

Underground Coal Mine Fire, Aracoma Alma Mine 

#1, West Virginia, January 19, 2006”. 

6. Triebsch,G. F.,and Sapko, M. J., “Lake Lynn 

Laboratory: A State-of-the-Art Mining Research 

Laboratory”, Proceedings of the International 

Symposium on Unique Underground Structures, 

Denver, CO, 1990, pp. 75-1 – 75-21. 

7. Sapko, M. J., Weiss, E. S., Cashdollar, K. L., and 

Zlochower, I. A., “Experimental mine and 

laboratory dust explosion research at NIOSH”, 

Journal of Loss Prevention in the Process 

Industries 13:229-242, 2000. 

8. Cashdollar, K. L., Weiss, E. S., Montgomery, T. 

G., and Going, J. E., “Post-explosion observations 

of experimental mine and laboratory coal dust 

explosions”, Journal of Loss Prevention in the 

Process Industries 20:607-615, 2007. 

9. Ng, D. L., Cashdollar, K. L., Hertzberg, M., 

Lazzara, C. P., “Electron Microscopy Studies of 

Explosion and Fire Residues”, U.S. Bureau of 

Mines IC 8936, 1983. 

Disclaimer 

The findings and conclusions in this report have not been 

formally disseminated by the National Institute for 

Occupational Safety and Health and should not be 

construed to represent any agency determination or 

policy. 

5