Determining Flame Travel Measurements from Experimental Coal Dust
 

Explosions 

* C.K.Man , 

* Corresponding author: cman@cdc.gov 

M.L.Harris and E.S.Weiss 

Fires and Explosions Branch, Office of Mine Safety and Health Research (OMSHR), 

National Institute for Occupational Safety and Health (NIOSH), 

Pittsburgh, PA 15236, USA. 

Disclaimer: The findings and conclusions in this paper 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. 

Abstract 

Fine coal particles are produced in underground coal mines during the mining process. These 

particles can cause an explosion if they are entrained and ignited. However, this type of 

explosion hazard in underground coal mines can be reduced by applying generous quantities 

of incombustible rock dust to the entries, including the roof, rib, and floor areas. 

The National Institute for Occupational Safety and Health (NIOSH) Office of Mine Safety 

and Health Research (OMSHR) has conducted a series of large-scale explosion experiments 

at the Lake Lynn Experimental Mine (LLEM) to investigate the inerting effect of limestone 

rock dust. Numerous experiments were conducted using a pulverized Pittsburgh coal dust 

(PPC) mixed with limestone rock dust to achieve total incombustible contents varying from 

67 to 82%. These dust mixtures were entrained and were subsequently ignited, if insufficient 

incombustible was present, by the ignition of a methane-air zone located at the closed end of 

the entry. 

Electronic flame sensors were positioned at regular intervals along the length of the entry to 

measure the flame travel. Post-explosion dust residue samples were taken along the entry 

after each explosion and analyzed to infer flame travel by measuring the thermal 

decomposition of limestone rock dust. Alcohol coking indices of the post-explosion residues 

were also measured. The alcohol coking test method is a forensic tool currently used by the 

Mine Safety and Health Administration (MSHA) in accident investigations. 

The determination of the extent of flame travel by these different measurement methods are 

discussed and compared. The rock dust solubility test described in this paper could assist 

accident investigators in more accurately establishing the flame limits of a dust explosion. 
 

1. Introduction 

In modern coal mines, improved safety practices such as rock dusting, methane control and 

monitoring, improved ventilation systems, use of permissible equipment, and roof bolting 

have led to a steady decline in the number of fatalities over the years and a dramatic decline 

in the number of explosions. Unfortunately, despite these measures, accidents still occur and 

lives are lost from fires and explosions (Gates et al., 2006; Light et al., 2006; Richmond et al., 

1983). In the United States, underground coal mines are required to add sufficient rock dust 

to maintain an incombustible content of 65 percent in the intake and at least 80 percent in the 

return air courses (Code of Federal Regulations, 2010). (Note: In NIOSH Publication No. 2010

151, “Recommendations for a New Rock Dusting Standard to Prevent Coal Dust Explosions in Intake 

Airways”, NIOSH recommends a new standard of 80% total incombustible content (TIC) be required 

in the intake airways of bituminous coal mines in the absence of methane.) If methane is present in 

either the intakes or returns, the amount of rock dust must be increased by 1.0 and 0.4 

percent, respectively, for each 0.1 percent of methane. 

In the United States, mine accidents are investigated by the federal agency, MSHA (Mine 

Safety and Health Administration). There are few specialized forensic techniques for 

investigating underground mine explosions. The alcohol coking index is one such tool 

normally used for this purpose. Accident investigators analyze dust samples for evidence of 

coking to map the extent of flame travel. Limestone can undergo thermal decomposition 

during an explosion (Man and Teacoach, 2009), and this decomposition pattern (measured by 

its solubility) may track flame travel and may provide useful supplementary information to 

accident investigators. 

In this paper, the flame travel during a series of experimental coal dust explosions is 

measured using the conventional alcohol coking test. Results from this test are compared to a 

newer method that involves measuring the rock dust solubility. 

2. Experimental procedures 

Lake Lynn Experimental Mine explosion tests 

Pittsburgh-seam coal has been mined at the OMSHR Safety Research Coal Mine since the 

early 1900s. The coal is ground and/or pulverized on-site and has been used historically as a 

standard coal dust to which other coal dusts can be compared to determine explosibility. The 

coal, which contains about 8% incombustible material and 37% volatile matter (dry basis), 

was prepared to meet three basic size requirements for this research program: coarse dust 

which contained about 20% <200 mesh (75 μm) particles, medium dust which contained 

about 40% <200 mesh, and pulverized dust which contained about 80% <200 mesh. The coal 

was initially ground to 100% below 20 mesh (850 μm), then pulverized to generate the finersized dust particles (PPC). The coarse and medium sizes were prepared by blending various 

amounts of the ground coal with the pulverized coal. The particle size distributions of the 

­
 

 

 

 

 

 

coals were measured by sieving in the laboratory. The size distributions of the three dusts 

used are summarized in Table 1. 

Table 1. Coal particle size distributions (sieving data) 

Particle size (μm) Coarse Pittsburgh 

coal, wt% 

Medium Pittsburgh 

coal, wt% 

Pulverized 

Pittsburgh coal, wt% 

212-850 57.2 28.8 0.2 

150-212 9.0 7.7 0.8 

106-150 8.2 10.7 5.9 

75-106 5.7 13.6 13.5 

53-75 5.9 14.2 22.8 

38-53 3.7 7.5 17.2 

<38 10.3 17.5 39.6 

Full-scale single-entry explosion experiments were carried out at the OMSHR Lake Lynn 

Experimental Mine (LLEM). Details of the experimental mine and the layout of the entries 

have been described elsewhere (Triebsch and Sapko, 1990; Sapko et al., 2000; Sapko et al., 

1987; Weiss et al., 2008; Weiss et al., 1989). The explosion experiments described in this 

paper were carried out in ‘A’ drift (Figure 1). Normally there are seven open crosscuts 

connecting this entry with entries ‘B’ and ‘C’. However, for these explosion experiments, 

crosscuts 1 to 5 (X-1 to X-5), as sequentially numbered starting from the closed end, were 

blocked with reinforced concrete block structures, while crosscuts 6 and 7 (X-6 and X-7) 

were blocked with mortared solid block stoppings (Weiss et al., 2008), thus creating a singleentry configuration. 

The results given in this paper are part of a much larger research investigation which 

examined the size of coal dust found in modern underground mines and the effect of coal 

particle size on rock dust inerting requirements (Cashdollar et al., 2010). A summary of the 

dust mixtures used in these 2008 explosion experiments is given in Table 2. For each of these 

experiments, the coal and rock dusts were first individually weighed to ensure that the correct 

total incombustible content (TIC) was met; then the dusts were mixed together using a barrel 

tumbler. The coal and limestone rock dust mixtures were deposited in the mine by hand to 

ensure a uniform loading of the dust. To enhance the dispersion, half of the dust mixture was 

loaded onto shelves suspended from the roof and the other half on the floor (Cashdollar et al., 

2007). The explosion experiments were initiated by the spark ignition (12-m long) of 

methane-air mixtures at the face, and the dynamic pressure generated by the methane-air 

ignition entrained the adjacent dust mixture. 
 

xtures 

(

e entry. An o

 mounted into the rib) at ten locations within the entry

n 

trol room

ces from the face

55, 453, 550, 649, and 807

, as

er than or equal to 1.0 

mbers are also the 

ven crosscuts (X1

Coal and rock dust m ents using a 

ft) long ignition zone ifferent 

mixtures of coal and rock dust

with al 

the explosion propagated down t ithin a data 

gathering panel (a steel box flus e entry

), which in tu t room. The 

then relayed to the co rs were 

positioned at the following dista 68, 198

m (22, 132, 183, 233, 283, re located 

height on the right outby ri have arrived 

signal was grea

Diagram of LLEM ed by 

numbers at the bottom). The feet) from 

the face. The positions of the s

tible 

t%

.8,

.5

0

5

0

5

0

4

0

 

in the LLEM explosion experi

indicate two adjacent zones of 

flame arri

gathering panel (a steel box flush mounted into the rib) at ten locations within t

connected into the underground instrume

. The sens

7, 40, 56, 71, 86, 108, 138, 

sensors w

shown in Figure 1. Flame was considered t

indica

distances in meters (and

incombu

content 

6

8

77

81

67

71

74

76

71

 

Table 2. Coal and rock dust mixtures tested in the LLEM explosion experiments using a 

12-m (40-ft) long ignition zone (“a” and “b” indicate two adjacent zones of different 

mixtures of coal and rock dust). 

LLEM 

test no. 

Dust zone, m (ft.) Coal dust 

3 loading, g/m 

Coal 

<200 

size, wt% 

mesh 

Total 

incombustible 

content wt%

511 64a (210)

b & 100.5 (330)

150 80 a

b

67.8,

81.5

512 91.4 (300) 200 80 77.0

513 91.4 (300) 200 80 81.5

514 91.4 (300) 200 20 67.0

516 91.4 (300) 200 20 71.5

517 91.4 (300) 200 40 74.0

518 91.4 (300) 200 40 76.4

520 91.4 (300) 200 40 71.0

n

Figure 1. Diagram of LLEM A-drift showing flame sensor locations (indicated by 

numbers at the bottom). The numbers are also the distances in meters (and feet) from 

the face. The positions of the seven crosscuts (X1-X7) are also marked. 

Electronic flame sensors 

Electronic optical sensors with 0-5 volt outputs were used to track the flame arrival times as 

the explosion propagated down th e entry. An optical flame sensor was installed within a data 

gathering panel (a steel box flush mounted into the rib) at ten locations within the entry 

(Weiss et al., 2008), which in turn was connected into the underground instrument room. The 

data were then relayed to the control room located outside of the mine. The sensors were 

positioned at the following distances from the face: 7, 40, 56, 71, 86, 108, 138, 168, 198, and 

246 m (22, 132, 183, 233, 283, 355, 453, 550, 649, and 807 ft). These sensors were located 

mid-height on the right outby rib, as shown in Figure 1. Flame was considered to have arrived 

when the sensor signal was greater than or equal to 1.0 volt. 
 

Alcohol coking measurements 

After each explosion experiment, a series of dust samples were collected along the entry from 

locations between 33 m (108 ft) and 245 m (807 ft). The alcohol coking test was carried out 

by adding approximately 1 g of <20 mesh (<850 μm) post-explosion dust into a test tube 

containing about 15 mL of denatured ethyl alcohol (ethanol). The sides of the test tube were 

then washed down with another 5-10 mL of alcohol. The test tube was set aside for 5 min to 

allow time for the more dense particles to sink to the bottom. As part of this process, coal and 

incombustible material sink to the bottom and coke floats to the top (Man et al., 2009). The 

samples were classified according to the chart shown in Figure 2 (Cashdollar et al., 2007). 

Using alcohol for the test has two advantages: it is a better wetting agent than water and has a 

lower density, which expedites the sink/float separation process. 

Figure 2. Alcohol coking test classifications (Cashdollar et al., 2007). 

The alcohol coking index is obtained by quantifying the visual alcohol coking observations 

according to the amount of coke residue floating on top. The alcohol coking index classifies 

by way of the following 6 categories, representing amounts: very large=5, large=4, 

medium=3, small=2, trace=1, and none=0. 

Rock dust solubility 

The rock dust solubility test is based on the fact that calcium hydroxide has a much higher 

solubility in water (Lide and Haynes, 2009) than calcium carbonate (limestone rock dust). 

Calcium carbonate undergoes some thermal decomposition in an explosion to form calcium 

oxide, which in turn can be hydrated to form calcium hydroxide (Man and Teacoach, 2009). 

The post-explosion dust samples were initially low temperature ashed (LTA) to remove all 

combustible material. This was carried out by placing about 2-3 g of sample (<20 mesh or 

850 μm) into a porcelain crucible and ashing the sample overnight in an oven at 400°C for 20 

hr. This was followed by hydration of the residual inorganic material in order to convert any 
 

 

 

 

calcium oxide into calcium hydroxide. The hydration process was carried out for about 24 hr 

in a modified laboratory desiccator cabinet containing trays of water in place of desiccant 

material. 

For the solubility experiments, 0.5 g of the hydrated, ashed sample was mixed with 300 mL 

of distilled water in a 500 mL beaker to allow the soluble component of the sample to 

dissolve. The beaker was placed in a water and ice bath for 2 hr. The mixture was then 

filtered using a pre-weighed piece of binder-free glass microfiber filter (2.16-in or 55-mm 

diameter) and a standard Büchner funnel and flask. The sample was left to dry in the 

laboratory for 48 hr then reweighed. The amount of material dissolved was calculated by 

comparing the amount of deposit on the filter and the amount of hydrated, ashed sample 

analyzed. 

3. Results 

Table 3 summarizes the extent of flame travel measurements from the mine explosion 

experiments using the alcohol coking test.

Table 3. Alcohol coking indices of post-explosion dust samples (5=very large, 4=large, 

3=medium, 2=small, 1=trace, 0=none, n/d=no dust). The dust zone extended to 104 m 

(340 ft) from the closed end (face). Explosion result: P=propagating, N=nonpropagating. 

LLEM 

test no. 

Result Distance from face, m (ft.) 

33 

(108) 

63 

(208) 

93 

(306) 

123 

(404) 

153 

(502) 

182 

(600) 

213 

(699) 

245 

(807) 

511 P 5 5 5 5 4 3 2 1 

512 P 5 5 5 4 1 1 1 n/d 

513 N 5 5 4 1 1 n/d n/d n/d 

514 P 5 5 5 2 1 0 0 0 

516 N 5 5 3 1 0 0 n/d n/d 

517 P 5 5 4 2 0 0 n/d n/d 

518 N 5 5 3 2 0 0 n/d n/d 

520 P 5 5 5 5 3 1 0 0 

Flame was assumed to be present where any coke 

was found. All the tests show a large concentration of coke in the dusted zones, but this 

concentration progressively decreases beyond 93 m (306 ft), which is near the end of the 

dusted zone. Some of the propagating explosions (LLEM tests #511, #512, and #520) show 

some coke at 182 m (600 ft), but for one test (LLEM test #514), the coke only extended to 

153 m (502 ft). This distance was the same as that for one of the non-propagating explosion 

tests (LLEM test#513). The short flame travel for propagating explosion LLEM test #514 

may be attributed to the relatively large particle size of the coal used during this test, which 

could have hampered the dust entrainment and travel. Conversely, the finer coal dust used in 

the non-propagating explosion (LLEM test #513) resulted in a trace amount of coke at 153 m 

(502 ft), which is well beyond the edge of the 91 m (300 ft) dusted zone (Table 2). 
 

 

 

 

 

 

 

Flame travel distances derived from the alcohol coking test have also been compared with the 

values measured using electronic sensors (Figure 3). The results show a consistent 

correlation. However, the data also show that all the flame travel distances measured from the 

alcohol coking test are relatively high compared to the electronic sensor values. This suggests 

that the flame may not have traveled as far as where there were only traces of coke. Coke 

particles may have been carried by the air current from an explosion beyond where flame was 

actually present. The data appear to correlate better with an offset of 40 m. This may be 

attributed to the position of the flame sensors, and only the minimum flame travel, using the 1 

volt criterion, is reported in this paper. Additional flame sensors would be needed to 

determine the extent of flame travel more accurately. 

The rock dust solubility results are summarized in Table 4. 

Table 4. Solubility of low temperature ashed post-explosion dust samples (n/d=no dust). 

LLEM test no. Solubility, wt% 

Distance from 

face, m (ft.) 

33 

(108) 

63 

(208) 

93 

(306) 

123 

(404) 

153 

(502) 

182 

(600) 

213 

(699) 

245 

(807) 

511 15.3 15.6 5.6 5.0 3.1 0.4 0.2 0.3 

512 7.0 4.2 1.3 0.0 0.0 0.0 0.0 n/d 

513 3.3 1.7 0.0 0.0 0.0 n/d n/d n/d 

514 10.8 6.0 0.9 0.0 0.0 0.0 0.0 0.0 

516 5.2 2.8 0.6 0.0 0.1 0.0 n/d n/d 

517 6.9 4.5 0.6 0.6 0.4 0.0 n/d n/d 

518 4.1 1.9 0.7 0.6 0.9 0.0 n/d n/d 

520 5.0 8.7 5.8 7.1 3.7 2.3 1.5 1.6 

As shown, there is a significant 

amount of soluble material in all the post-explosion samples near the ignition zone. For each 

explosion, the amount of soluble material decreases with distance away from the initial gas 

(ignition) zone and decreases very quickly for the non-propagating explosions (LLEM tests 

#513, #516, and #518). The data indicate that propagating explosions produced at least 5% 

soluble material along the entry, and this peak value occurs often but not always next to the 

ignition zone. A minimum solubility value of 0.5% was adopted as the criterion for the 

determination of the flame travel based on the repeatability of this analytical test. 

The data from the electronic flame sensors are compared to measurements from the rock dust 

solubility experiments in Figure 3. These two methods used for determining the extent of 

flame travel during the LLEM explosion tests resulted in a good correlation. Unlike the 

comparison with the alcohol coking test, the solubility data show little bias, with half the data 

points above the trend line and half below it. 
 

 

 

 

 

Flame travel from alcohol coking test and solubility, m 300 

250 

200 

150 

100 

50 

0 

0 50 100 150 200 250 300 

2 points 

Alcohol coking test 

2 points 

Solubility test 

Flame travel from electronic sensors, m 

Figure 3. Comparison of minimum flame travel distances using electronic sensors, 

alcohol coking test, and solubility measurements. 

4. Conclusions 

OMSHR researchers have conducted a series of large-scale explosion experiments at the 

Lake Lynn Experimental Mine (LLEM) to investigate the inerting effect of limestone rock 

dust and determine, through various methods, the extent of the explosion flame travel. 

A series of electronic optical sensors were used to detect the flame travel within the LLEM 

and provide a reliable measurement as to the minimum extent of flame travel. It is not 

realistic to install these types of sensors in mine entries to measure flame travel in the 

unlikely event of an explosion. However, conducting various tests on post-explosion dust 

samples collected by accident investigators can provide the necessary evidence to deduce the 

extent of explosion flame travel. Typically, alcohol coking tests are conducted on dust 

samples to make this determination. 

In these experiments, the alcohol coking results produced high amounts of coke in the dusted 

zone and lower amounts away from the ignition zone. Precise flame travel distances were 

difficult to establish by the use of the alcohol coking test due to the limited amounts of postexplosion dust available in some areas, especially for those sample areas furthest from the 

methane-air ignition zone. The data also show that all the flame travel distances measured 
 

from the alcohol coking test were relatively high compared to the electronic sensor 

measurements. This suggests that the flame may not have traveled as far as the distances 

containing only traces of coke particles, which may be subject to dynamic effects such as 

airflow and turbulence. Therefore, the flame sensors can only be used to measure the 

minimum flame travel. Since the sensors used in these tests were separated by long distances, 

it is likely that the actual flame was extinguished in between a set of sensors. In addition, the 

force of the explosion may have carried coke particles beyond the flame path. 

A newer forensic tool involving rock dust solubility measurements of the dust samples may 

aid investigators in more accurately determining the extent of the flame travel during 

explosions. Post-explosion dust samples collected just outby the LLEM methane-air ignition 

zone resulted in the highest rock dust solubility values. Dust samples collected after 

propagating explosions gave higher rock dust solubilities than those samples collected after 

non-propagating explosions. There was a good correlation for the extent of flame travel when 

comparing the electronic sensors and the dust sample solubility measurements. However, it 

should be pointed out that these are only preliminary results and more data would be required 

to develop this experimental technique into a forensic tool for use in explosion investigations. 

Acknowledgements 

The authors would like to thank the following OMSHR Physical Science Technicians for 

their efforts in this study: Gregory Green for providing the particle size data and Donald 

Sellers and Frank Karnack for collecting the LLEM post-explosion dust samples. 

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