Analysis of post−explosion residues for estimating 

flame travel during coal dust deflagrations 

Chi-keung Man*†, Marcia L. Harris*, and Eric S. Weiss*

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

National Institute for Occupational Safety and Health (NIOSH), 626 Cochrans Mill Road, Pittsburgh, PA 15236, USA.

TEL : +1-412-386-6628

†Corresponding address : cman@cdc.gov

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.

Keywords : coal dust, explosion, rock dust, forensic.

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 explosions1) 3). 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 courses4). (Note : In

NIOSH Publication5), “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 explosion6), 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

2.1 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 .m)

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 .m), 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.

Table1 Coal particle size distributions (sieving data).

Particle size

〔.m〕

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 elsewhere7) 11). 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, crosscuts1

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 stoppings10), thus

creating a single-entry configuration.

Fig.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.

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 requirements5). 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 floor12). 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.

Table2 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

loading, g/m3

Coal size,

wt%

<200 mesh

Total

incombustible

content wt%

511 64a(210)

& 100.5b(330) 150 80

a67.8,

b81.5

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

2.2 Electronic flame sensors

Electronic optical sensors with 0-5 volt outputs were

used to track the flame arrival times as the explosion

propagated down the 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 entry10),

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 246m (22, 132, 183, 233,

283, 355, 453, 550, 649, and 807ft). These sensors were

located mid-height on the right outby rib, as shown in Fig.

1. Flame was considered to have arrived when the sensor

signal was greater than or equal to 1.0 volt.

2.3 Alcohol coking measurements

After each explosion experiment, a series of dust

samples were collected along the entry from locations

between 33m (108 ft) and 245m (807 ft). The alcohol coking

test was carried out by adding approximately 1g of <20

mesh (<850 .m) 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 5min 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 top13). The samples were classified visually like

previous studies12). 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.

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, l arge=4, medium=

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

2.4 Rock dust solubility

The rock dust solubility test is based on the fact that

calcium hydroxide has a much higher solubility in water14)

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 hydroxide6).

The post-explosion dust samples were initially low

temperature ashed (LTA) to remove all combustible

material. This was carried out by placing about 2-3g of

sample (<20 mesh or 850 .m) into a porcelain crucible and

ashing the sample overnight in an oven at 400ο

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 300mL of distilled water in

a 500mL 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. 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 93m (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 nonpropagating 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 153m (502 ft),

which is well beyond the edge of the 91m (300 ft) dusted

zone [Table 2].

Table3 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 104m (340 ft) from the closed end (face). Explosion result : P=propagating, N=non

propagating.

LLEM Result Distance from face, m〔ft.〕

test no.

33 63 93 123 153 182 213 245

(108) (208) (306) (404) (502) (600) (699) (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 travel distances derived from the alcohol coking
test have also been compared with the values measured

using electronic sensors [Figure 2]. 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 the1

volt criterion, is reported in this paper. Additional flame

sensors would be needed to determine the extent of flame

travel more accurately.

Fig.2	 Comparison of minimum flame travel distances using

electronic sensors, alcohol coking test, and solubility

measurements.

The rock dust solubility results are summarized in

Table 4. 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 nonpropagating 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 Fig. 2. 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.

Table4 Solubility of low temperature ashed post explosion dust samples〔n/d=no dust〕.

LLEM test Solubility, wt%

no.

Distance from 33 63 93 123 153 182 213 245

face, m〔ft.〕 (108) (208) (306) (404) (502) (600) (699) (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

4. Conclusions

OMSHR researchers have conducted a series of largescale 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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