Sapko *, Eric S. Weiss, Kenneth L. Cashdollar, Isaac A. Zlochower 

Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA 15236, USA 

Abstract 

This paper describes dust explosion research conducted in an experimental mine and in a 20-L laboratory chamber at the Pittsburgh 

Research Laboratory (PRL) of the National Institute for Occupational Safety and Health (NIOSH). The primary purpose of this 

research is to improve safety in mining, but the data are also useful to other industries that manufacture, process, or use combustible 

dusts. Explosion characteristics such as the minimum explosible concentration and the rock dust inerting requirements were measured 

for various combustible dusts from the milling industries. These dusts included bituminous coals, gilsonite, oil shales, and sulfide 

ores. The full-scale tests were conducted in the Lake Lynn experimental mine of NIOSH. The mine tests were initiated by a 

methane-air explosion at the face (closed end) that both entrained and ignited the dust. The laboratory-scale tests were conducted 

in the 20-L chamber using ignitors of various energies. One purpose of the laboratory and mine comparison is to detennine the 

conditions under which the laboratory tests best simulate the full-scale tests. The results of this research showed relatively good 

agreement between the laboratory and the large-scale tests in detennining explosion limits. Full-scale experiments in the experimental 

mine were also conducted to evaluate the explosion rf!sistance characteristics of seals that are used to separate non-ventilated, inactive 

workings from active workings of a mine. Results of these explosion tests show significant increases in explosion overpressure due 

to added coal dust and indications of pressure piling. 

1. Introduction 

There has been a notable decline in the frequency and 

severity of mine explosions since the early part of the 

twentieth century. Among the major safety measures 

responsible for this decline are the use of general rock 

dusting, development of permissible explosives and electrical equipment, improved ventilation, and improved 

methods for detecting hazardous conditions. Although 

these advances in mine safety are noteworthy, the problem of mine explosion prevention is not completely 

solved, for serious mine explosions still occur (Sapko, 

Greninger & Watson, 1989; Dobroski, Stephan & Conti, 

1996). For many mines, the almost continual deposition 

of fine-sized float coal dust on the floor, ribs, and roof 

of mine entries and returns, coupled with the intermittent 

application of rock dust, has resulted in stratified layers 

* Corresponding author. TeL: +1-412-386-6619, fax: +1-412-386-

6595. 

E-mail address: zia50@cdc,gov (MJ. Sapko). 

of dust (Sapko, Weiss & Watson, 1987a). Over the years, 

changes in mining technology have produced increased 

amounts of finer float coal dust, which has compounded 

safety matters (Sapko, Weiss & Watson, 1987a). Frictional ignitions of methane are also a serious concern, 

and their potential for disastrous consequences is well 

known. 

Much knowledge has been obtained from the fullscale explosion research conducted by mine safety 

research establishments during the past two decades 

(Sapko et aI., 1989, 1987a; Weiss, Greninger & Sapko, 

1989; Cashdollar, Sapko, Weiss & Hertzberg, 1987; 

Nagy, 1981; Michelis, Margenburg, MOller & Kleine, 

1987; Reeh & Michelis, 1989; Sobala, 1987; Cashdollar, 

Weiss, Greninger & Chatrathi, 1992; Greninger, Cashdollar, Weiss & Sapko, 1990; Sapko, Weiss & Watson, 

1987b; Lebecki, 1991; Michelis, 1996). This research 

has provided important practical data and a better understanding of the fundamental explosion processes. Previous Pittsburgh Research Laboratoryl (PRL) experi1 The Pittsburgh Research Laboratory was part of the U.S. Bureau 

of Mines before its transfer to the National Institute for Occupational 

Safety and Health (NIOSH) in October 1996. 
s zone 

Coal dust zone D drift 

E drift 

C driff B driftl 

A driff 

LEGEND 

=t= Explosion - proof door 

o 100 200 ! ! ! 

Scole, m 

PLAN VIEW OF LAKE LYNN 

Fig. 1. Plan view of the Lake Lynn experimental mine. 

mental mine research has included studies of float coal 

dust hazards (Sapko et al., 1989), minimum explosible 

coal dust concentrations (Weiss et al., 1989; Cashdollar 

et al., 1987, 1992; Greninger et al., 1990), rock dust 

inerting requirements (Weiss et aI., 1989; Cashdollar et 

al., 1987, 1992; Greninger et aI., 1990), the effects of 

pulverized versus coarse coal particle size (Weiss et al., 

1989), and secondary explosion hazards associated with 

oil shale and sulfide ore dusts (Weiss, Cashdollar, 

Sapko & Bazala, 1996). 

The present publication provides data on the effect of 

volatile matter on the explosibility of fuel dusts such as 

coals and gilsonite. Included are data on the minimum 

explosible concentration (MEC) and the amount of limestone rock dust required to inert the fuel dusts. The data 

from full-scale experimental mine and laboratory tests 

are also compared to thermodynamic model calculations. 

Some of these experimental data2 were presented previously (Greninger et aI., 1990). Also included are 

explosion propagation data from full-scale gas and dust 

explosions that were used to evaluate the explosion 

resistance characteristics of seals that are used to isolate 

unused (gob) areas of mines. 

2. Experimental facilities and test procedures 

The full-scale explosion tests were conducted in 

PRL's Lake Lynn experimental mine (Mattes, Bacho & 

Wade, 1983; Triebsch & Sapko, 1990) (LLEM) shown 

in Fig. 1. This is a former limestone mine, and five new 

drifts were developed to simulate the geometries of mod2 Note that some of the data have been revised slightly from those 

listed previously, based on a re-analysis of the data. 

ern US coal mines. The mine has four parallel drifts 

A, B, C, and D. Drifts C and D are connected by Edrift. a 152-rn long entry which simulates a longwall 

face. Most of the dust explosion tests described in this 

paper were conducted in the single entry D-drift, which 

was isolated from E-drift by means of an explosionprDof movable bulkhead door (Fig. 1). D-drift is 520 m 

long with a cross-sectional area ranging from 11.2 to 

13.0 m2 (average 12.8 m2). The average height in D-drift 

is 2.1 m and the average width is 6.0 m. The D-drift 

face (closed end) is formed by the closed bulkhead door. 

D-drift was instrumented (Fig. 2) with pressure transducers located at the face and at 10, 15,31,46,61,91, 

119, 153, 183, and 229 m from the face. Optical flame 

sensors were located at 5, 10, 15, 23, 31, 38,46,53,61, 

76. 91. 105, 119, 136, 153, 168, 183, 198; 213, and 

229 m from the face. 

The ignitor for all of these D-drift dust explosibility 

tests was a 12-m long zone of a 10% methane in air 

mixture at the face (closed end). This 152-m3 methaneair zone was ignited near the center of the face using 

electric matches grouped in a single-point configuration. 

In tests involving pure fuel (gilsonite or coal) dust, all 

the dust was placed on roof shelves to enhance the dispersion. In the rock dust inerting tests, the fuel dust and 

limestone rock dust mixtures were placed half on roof 

shelves and half on the floor (Fig. 2). The nominal dust 

loading reported for the LLEM tests assumes that all of 

the dust was dispersed uniformly throughout the cross 

section. The D-drift dust test zone extended from the end 

of the methane zone (12 m) out to 76, 94, 152, or 195 m 

from the face. The length of the total test zone listed in 

the remainder of this paper will be the total distance 

from the face. including the 12-m long methane ignitor zone. 
-PortalInstrument c 

OP 

stations BOP c TCP OP Of' HF BOP HF TCP HF Tel' 

F F F F F FFFFFFF F PT F PT PT F PT F PT F PT F PT PT PT PT F F PT 

4 4 4 4 4 4 4 • I I + HHIIi I I I 

I I I I I I I , I , I I I , I I , , , I , , It. I I I -I I I , 0 15 30 45 60 75 90'120 180'210 225 

-DISTANCE. m 

LEGEND 

PT PreSSllrl! transducer TCP 3 color. pyrometer C 16-mm movie camera F L-Sc400 flame DP Dust probe Ac Cross sectional Q rea BOP Bi-direction probe HF Heotflux 

Fig. 2. Side view of D-drift in the Lake Lynn experimental mine, showing instrumentation. 

Various explosion resistant seal designs were evaluated through full-scale explosion tests in C-Drift in the 

LLEM. Fig. 3 shows an expanded view of the seal test 

area. The seals were constructed in the crosscuts 

between B- and C-drifts; these crosscuts are approximately 2.0 m high by 5.8 m wide, giving an average 

cross sectional area of 11.6 m2 • Methane was injected 

into the closed end of C-drift. A plastic diaphragm was 

used to contain the 9% CHcair mixture within the first 

14.3 m of the entry (~190-m3 zone). Electric matches, 

located at the face or closed end of the entry, were used 

to ignite the flammable CH4 -air mixture. Barrels filled 

with water were located in the gas zone to act as turbulence generators to increase the flame speed and achieve 

the desired pressure loading. Each data-gathering station 

in C-drift houses a pressure transducer and optical flame 

N 

30 I '--__ ...1...-__ ..... 

o 

Scale, m 

) 

A 

sensor located at distances of 4,26,41, 56, 71, 93, 123, 

153, 182, and 231 m from the face. 

The laboratory data in this paper were collected in the 

PRL 20-L explosibility test chamber (Cashdollar et al., 

1987, 1992; Cashdollar & Hertzberg, 1985) shown in 

Fig. 4. The test procedure includes the .partial evacuation 

of the chamber and the dispersion of the dust by a blast 

of air from the bottom. The ignition source is energized 

after the pressure has returned to about 1 atm or I bar 

absolute and the dust has been uniformly dispersed. The 

ignition sources were strong chemical ignitors with energies of 2500 or 5000 J. The instrumentation for the 20-L 

chamber includes a pressure transducer and optical dust 

probes (Cashdollar, Liebman & Conti, 1981; Conti, 

Cashdollar & Liebman, 1982) for monitoring dust dispersion uniformity. Details of the operating procedures 

Ltl9tmd 

I Test seal 

-.- Data-oatherino station 

2 Crosscut number 

Fig. 3. Diagram of seal test area in LLEM C-Drift. 
Fig. 4. 20-L laboratory explosibility test chamber. 

Table 1 

Proximate analyses and heating values for the "as received" Pittsburgh 

seam bituminous coal and gilsonite 

Pittsburgh coal Gilsonite 

Moisture, % 2 1 

Volatile matter, % 36 84 

Fixed carbon, % 56 15 

Ash, % 6 0 

Heating value, callg 7700 9900 

and dust dispersion uniformity measurements are in 

Cashdollar et al. (1992), Cashdollar and Hertzberg 

(1985) and Cashdollar and Chatrathi (1993). 

The pulverized coal that was used w.as Pittsburgh 

seam high volatile bituminous with a volatility of 36% 

as measured by ASTM standard test method D3175 

(ASTM, 1999). The gilsonite was a natural bitumen or 

asphaltum that is mined in Utah; it has a volatility of 

84%. Additional proximate analysis data and heating 

values are listed in Table 1. Both fuel dusts were pulverized and had 80-91 % by weight minus 200 mesh 

«75 J..lm). Additional size data are listed in Table 2. The 

Table 2 

gilsonite was finer in size, with 36% less than 20 Ilm, 

compared to 15% less than 20 Ilm for the coal. The surface mean diameter is Ds, the mass or volume mean 

diameter is Dw , and the mass median diameter is D med• 

In addition to the two fuel dusts, the size analysis data 

of the limestone rock dust are also listed in Table 2. The 

particle size data are from a combination of sonic sieve 

and Coulter Counter data. 3 

3. Single entry explosion studies 

In the LLEM D-drift tests, the methane explosion at 

the face both entrains and ignites the dust. The flame 

from the methane-air ignition zone travels about 65 m, 

even in the absence of any fuel dust in the test zone. 

The ignitor also produces an overpressure of -1 bar 

(-100 kPa). For a dust explosibility test, if the flame 

does not reach the end of the test zone, the result is a 

non propagation (NP). For the shorter test zones (76 or 

94 m long), the result is considered a marginal propagation (M) if the flame just reaches the end of the test 

zone or slightly beyond. If the flame travels more than 

30 m beyond the short test zones, the result is considered 

a propagation (P). For the longer test zones (150, 152, 

or 195 m), the result is considered a propagation if the 

flame travels more than 10 m past the end of the test 

zone. The propagation criterion for these mine tests is, 

therefore, more stringent for the shorter test zones. 

Measurement of the flame travel distance is, of course, 

limited by the number and locations of the flame sensors. 

For these tests, the flame sensors were located at intervals of about 31 m for test numbers up to 230 and at 

intervals of about 15 m for tests after test number 234. 

The estimated flame travel distance is interpolated 

between flame sensors, based on the strengths of the signals at the sensors. 

These propagation criteria are the same as those used 

in a previous PRL publication (Greninger et aI., 1990). 

They were developed to give a scientific determination 

of the conditions under which an explosion would continue to propagate beyond the influence of the ignitor. It 

is understood, however, that even localized burning that 

3 Mention of any company name or product does not constitute 

endorsement by NIOSH. 

Particle size distributions for gilsonite, Pittsburgh coal, and limestone rock dusts 

Dust <20 Jlm, % 

. Pittsburgh coal 15 

Gilsonite 36 

Limestone rock dust 46 

<75 Jlm, % 

80 

91 

75 

30 

19 

14 

50 

37 

46 

44 

28 

25 
extends the length of the CH4 ignitor flame can be a 

significant hazard. 

One of the questions in both the laboratory and mine 

tests was whether or not a particular ignition source 

could "overdrive" the system. To evaluate the possibility 

of overdriving for the LLEM studies, some tests were 

made with longer (150-, 152-, or 195-m) test zones 

instead of the 76- or 94-m test zones used for the 

majority of the tests. These longer zone tests would 

determine whether the flame continued to propagate 

beyond any possible influence of the ignitor. In the 

20-L laboratory tests, different ignitor strengths were 

used to evaluate the effect of "overdrjving." At some 

ignitor strength, the ignitor energy and flame volume 

would become too large for the 20-L chamber. In this 

situation, the ignitor flame would be sufficient to combust enough dust so that the result appears to be an 

explosion although there is no real propagation beyond 

the ignitor flame. The 20-L laboratory data were also 

compared to data from a much larger I-m3 (lOOO-L) 

chamber (Cashdollar & Chatrathi, 1993) that would not 

be "overdriven" by the same ignitor. 

The LLEM data for the various dust explosibility tests 

are listed in Table 3. In the first column is the LLEM 

test number. In the second column is the total length of 

the test zone as measured from the face (including both 

the l2-m methane zone and the dusted zone). The third 

column lists only the fuel dust part of the nominal loading; it does not include the rock dust part of the coalrock dust mixtures. The limestone rock dust content of 

the dust mixture is listed in column four. The total inert 

or incombustible content of the dust mixture is listed in 

column five; this includes not only the limestone rock 

dust but also the ash and moisture in the fuel dust. The 

ratio of rock dust to fuel dust is listed in the next column. 

The flame travel distance from the face is listed in column seven. This estimated distance is interpolated 

between the last flame sensor to detect the flame and the 

next sensor that does not detect the flame, based on the 

strengths of the signals at the sensors. The flame travel 

distance is rounded to the nearest 5 m. The result of the 

test is listed in the last column, based on the previously 

described propagation criteria. 

3.1. Minimum explosible concentrations 

The minimum nominal dust loading for gilsonite in 

the LLEM was studied using test zones of 94 and 152 m 

in length. Based on the previously listed flame travel 

criteria, the 30-g/m3 loading for the 94-m long test zone 

(LLEM test 200) was a marginal propagation, but the 

pressure was very low. With the longer 152-m test zone 

(LLEM test 236), the 30-g/m3 test was a nonpropagation. 

This showed that the flame from the 152-m3 methaneair ignition zone could overdrive the results somewhat 

for the minimum loading tests if a short test zone was 

used. The test results from the longer test zone may be 

more accurate in the scientific sense, but the results from 

the shorter test zone are still useful in the practical sense 

because they are more conservative. At concentrations 

less than the limit, the fuel dust can still contribute 

energy and extend the ignitor flame length. At 35 g/m3, 

both the tests with the shorter zone (LLEM test 203) and 

with the longer zone (LLEM tests 235 and 243) propagated beyond the dusted zone. Therefore, the best value 

for the minimum nominal concentration (lean limit) for 

flame propagation in the LLEM is about 35 g/m3 for the 

gilsonite dust, based on the data from the longer test 

zone. 

For the pulverized Pittsburgh coal (PPC) tests with 

the 76-m test zone, the 40-g/m3 dust loading produced 

a marginal propagation and the 50-g/m3 loading produced an apparently significant propagation with flame 

travel out to 135 m. These data were reported previously 

in Weiss et aI. (1989). In later tests with a longer 152-

m test zone, a dust loading of 55 g/m3 failed to propagate 

to the end of the test zone in LLEM test 239 and almost 

reached the end of the test zone in test 247. In LLEM 

test 242, the 60-g/m3 dust loading propagated well 

beyond the test zone. Therefore, the best value for the 

minimum nominal concentration for the Pittsburgh coal 

dust is about 60 g/m3, based on the data from the longer 

test zone. Due to the limited number of mine tests, these 

minimum concentration data for the gilsonite and coal 

dusts are rounded to the nearest 5 g/m3. 

Minimum explosible concentrations (lean flammable 

limits) were also measured in the 20-L laboratory 

chamber using strong chemical ignitors of 2500- and 

5000-1 energy (Cashdollar et aI., 1992; Cashdollar & 

Chatrathi, 1993; Cashdollar, Hertzberg & Zlochower, 

1989; Cashdollar, 1996). For the gilsonite, the measured 

minimum explosible concentration was ~35 g/m3 with 

the 2500-1 ignitor and ~30 g/m3 with the 5000-1 ignitor. 

For the Pittsburgh coal, the minimum explosible concentration was ~80 g/m3 with the 2500-1 ignitor and 

~60 g/m3 with the 5000-1 ignitor. The repeatability of 

the laboratory MEC data was about ±1O%. The question 

of "overdriving" in laboratory chambers was also investigated by comparing the MEC values from the 20-L 

chamber with those from a much larger I-m3 (lOOO-L) 

chamber (Cashdollar & Chatrathi, 1993) at the Fike Corporation in Blue Springs, MO. The minimum explosible 

concentration for a dust should be that value that is independent of ignition energy. The I-m3 MEC data 

(Cashdollar & Chatrathi, 1993) for each of the two dusts 

were relatively independent of ignition energy. The 

MEC for gilsonite was 36 g/m3 and the MEC for the coal 

was 80 g/m3 in the I-m3 c.hamber with a 10-kJ ignitor 

(Cashdollar & Chatrathi, 1993). This comparison shows 

that the 5000-1 ignitor in the 20-L chamber overdrives 

the system somewhat for these dusts. 
Table 3 

Sununary of full-scale test results for Pittsburgh coal and gilsonite dusts 

LLEM test No. Test zone, m Fuel dust Limestone rock Total inert, % Rock dust to Harne travel, m Result' 

loading, glm3 dust, % fuel ratio 

Pulverized giIsonite dust 

201 94 25 0.0 0.0 90 NP 

200 94 30 0.0 0.0 110 M 

236 152 30 0.0 0.0 110 NP 

203 94 35 0.0 0.0 ~180 P 

235 152 35 0.0 I 0.0 ~160 P 

243 152 35 0.0 1 0.0 -185 P 

202 94 40 0.0 1 0.0 -190 P 

229 94 100 79.8 80.0 4.0 -190 P 

224 94 100 84.8 85.0 5.6 -190 P 

227 94 150 84.8 85.0 5.6 >230 P 

217 94 100 89.9 90.0 8.9 75 NP 

226 94 150 89.9 90.0 8.9 75 NP 

Pulverized Pittsburgh bituminous coal dust 

55 76 30 0.0 8 0.0 75 NP 

57 76 40 0.0 8 0.0 100 M 

54 76 50 0.0 8 0.0 135 P 

239 152 55 0.0 8 0.0 110 NP 

247 152 55 0.0 8 0.0 145 NP 

242 152 60 0.0 8 0.0 -185 P 

240 152 67 0.0 8 0.0 >230 P 

81 76 150 65.0 67.8 1.9 -185 P 

52 76 200 65.0 67.8 1.9 >185 P 

83 76 200 65.0 67.8 1.9 >230 P 

82 76 300 65.0 67.8 1.9 230 P 

80 76 100 70.0 72.4 2.3 125 P 

79 76 150 70.0 72.4 2.3 200 P 

49 76 200 70.0 72.4 2.3 230 P 

78 76 300 70.0 72.4 2.3 205 P 

69 76 200 73.0 75.2 2.7 205 P 

50 76 200 75.0 77.1 3.0 160 P 

53 195 200 75.0 77.1 3.0 >230 P 

70 76 200 77.0 78.8 3.3 100 M 

255 150 200 77.2 79.0 3.4 -150 M 

51 76 200 80.0 81.5 4.0 75 NP 

• NP denotes nonpropagation, M denotes marginal propagation, and P denotes propagation. 

3.2. Limestone rock dust inerting 

For the limestone rock dust inerting tests in the experimental mine, the results were based on te'Sts at the most 

hazardous fuel dust concentration. In LLEM tests 49 and 

78-80, the coal dust loading was varied from 100 to 

300 g/m3 while the total inert content of each of the coal 

and rock dust mixtures remained constant at 72% (Table 

3). With the 200-glm3 coal dust loading (LLEM test 49), 

the flame traveled about 230 m from the face with a 

corresponding overpressure (beyond the influence of the 

methane ignitor) of approximately 0.8 bar (80 kPa). In 

the 72% total inert tests with 100-, 150-, and 300-g/m3 

coal dust loadings, the flame travel and overpressures 

were lower than for the 200-g/m3 test. Similar data were 

obtained in the LLEM tests 81-83, where the cofj dust 

loading was varied from 150 to 300 g/m3 and the total 

inert content was held constant at 68%. With the 200-

glm3 coal concentration (LLEM test 83), the flame traveled greater than 230 m from the face with an overpressure of about 1 bar (100 kPa). With the 150- and 300-

g/m3 tests at 68% inert, the flame travel and overpressures were less than for the 200-g/m3 test. In each of 

these series of tests, the 200-g/m3 coal concentration provided the strongest explosion. The LLEM rock dust 

inerting tests with the gilsonite dust (at 85% total inert) 

showed that a concentration of 150 g/m3 produced the 

longest flame travel and largest pressures. Therefore, the 

determination of the final amount of limestone rock dust 

necessary to inert the two fuel dusts was based on LLEM 

tests at a dust loading concentration of 200 glm3 for the 

Pittsburgh pulverized soal and 150 g/m3 for the pulverized gilsoII'ite, since these concentrations produced the 

most violent explosions. Assuming ideal uniform dispersion of the dust in the LLEM, maximum intensity would 

be expected to occur just above the stoichiometric concentrations of 210 g/m3 for Pittsburgh coal and 90 g/m3 

for gilsonite. These stoichiometric values assume that 
mainly the volatiles are burning and that the volatility 

of the coal is -50% under the high heating rates in 

flames (see following section on "Explosion Limit 

Models"). 

The rock dust inerting tests with pulverized gilsonite 

and Pittsburgh coal dusts were conducted using test 

zones 76-, 94-, 150-, and 195-m long (Table 3). For the 

Pittsburgh pulverized coal dust loading of 200 glm3, a 

coal and limestone rock dust mixture of 79% total inert 

content (LLEM tests 70 and 255) produced a marginal 

propagation for both the shorter and longer test zones in 

the single entry at the Lake Lynn mine. In LLEM test 

70, the flame extended somewhat beyond the end of the 

test zone but at a significantly lower speed and much 

lower pressure value compared to a typical propagating 

explosion. In LLEM test 255, the flame radiation was 

very weak as the flame approached the end of the dusted 

zone. An 81.5% total inert content mixture (LLEM test 

51) extinguished the flame within the dusted zone and 

resulted in a nonpropagation. Therefore, the rock dust 

content necessary to prevent an explosion of the Pittsburgh coal in the single entry of the LLEM was 78-

80%, and the corresponding total inert content (including 

the moisture and ash in the coal) was about 80-82%. 

The gilsonite dust at loadings of 100 g/m3 (LLEM test 

217) and ISO glm3 (LLEM test 226) did not propagate 

past the test zone when mixed with rock dust to give a 

total inert content of 90%. However, these same gilsonite 

loadings propagated over twice the length of the test 

zone at 85% total inert content or lower. Therefore, the 

rock dust content necessary to inert the gilsonite in the 

LLEM is about 88-90%, and the total inen content is 

also 88-90%. 

The amount of limestone rock dust necessary to inert 

the Pittsburgh coal dust and gilsonite dust was also measured in the 20-L chamber, using 50OO~1 ignitors 

(Cashdollar et ai., 1989; Cashdollar, 1996; Cashdollar & 

Hertzberg, 1989). For these tests, regular limestone rock 

dust was used instead of the fluidized rock dust used for 

some previous laboratory tests (Cashdollar et ai., 1987, 

1992; Greninger et a!., 1990). The tests were conducted 

over a wide range of concentrations to determine the 

worst case. For the Pittsburgh coal, the rock dust content 

necessary to inert the mixture was about 74%, and the 

corresponding total inert content was about 76%. For the 

gilsonite dust, the rock dust content needed to inert the 

mixture was 90%, and the corresponding total inert content was also 90%. The repeatability of the laboratory 

data was about ±2% rock dust. These laboratory data 

show reasonably good agreement with the full-scale 

experimental mine data. 

A summary of the experimental mine and 20-L laboratory limestone rock dust inerting data for various fuel 

dusts is shown in Fig. 5. The vertical axis shows the 

amount of rock dust in the mixture necessary to inert the 

various fuel dusts. The horizontal axis is the moisturef. 90 

.: 

a: 

w 80 ~ 

~ 

to 70 

is 

g '" 60 

'" lL 

0 

~/' 50 

§ 

.. 40 

30 a 

---- .... - ...... ;:;.-;,;..;;;. ..... - ..... ',,"" 

.' "" ... :/ 

r /; 

K_y 

-LLEM 

...... ·BEM 

- - 2.0~ ~ c:hQmber 

10 20 30 40 50 60 70 80 90 100 

VOLATILITY, % 

Fig. 5. Rock dust inerting requirements for dusts of different volatilities, showing comparison among Lake Lynn experimental mine 

(LLEM) data, Bruceton experimental mine (BEM) data, and 20-L laboratory data. 

ash-free volatility of the fuel dusts. The dotted curve is 

a summary of earlier inerting tests in the Bruceton 

experimental mine (BEM) using a strong ignition source 

(Nagy, 1981; Richmond, Liebman & Miller, 1975); it 

corresponds to the amount of rock dust necessary to inert 

various coals. The Lake Lynn experimental mine data 

from this report and Weiss et al. (1989) are shown as 

the solid curve. The 20-L laboratory data (Cashdollar et 

aI., 1989; Cashdollar, 1996; Cashdollar & Hertzberg, 

1989) for the same dusts are represented by the dashed 

curve. The uncertainties in both the experimental mine 

and laboratory data are of the order of ±3% inert content. 

The data show that it takes slightly more rock dust to 

inert the coal dusts in the larger cross section LLEM as 

compared to that required in the older BEM. The laboratory data show reasonably good agreement with the mine 

data; therefore, the 20-L chamber can be used for preliminary testing before full-scale mine testing. The regulations for the mining industry, however, are based on 

the results of the full-scale mine tests. 

In addition to the coal and gilsonite dusts, the PRL 

studied two other types of dusts from the mining industry that are much more difficult to ignite and propagate 

a flame oil shales and sulfide ores (Weiss et al., 1996). 

For the oil shales, explosions did not occur in either laboratory or experimental mine tests for oil assays less than 

-85 LIt (-20 gaUton). For the sulfide ores, explosions 

did not occur with sulfur contents less than about 20-

25 %. Further details on this research are in Weiss et 

al. (1996). 

4. Explosion limit models 

Two models were compared with the full-scale and 

laboratory explosion limit data. The first model is based 
on a thermal balance between the heat generated during 

combustion of fuel dust and heat abstracted by incombustible material. It is based on the model proposed by 

Richmond et aL (1975) and Richmond, Liebman, 

Bruszak & Miller (1979) to describe the limit explosion 

propagation conditions in a large gallery or mine. This 

model is similar to that of Palmer and Tonkin (1968, 

1971) who investigated the explosibility of organic 

industrial dusts and coal dust, when mixed with inorganic inert dusts. Volatilization of the fuel dust is 

assumed to occur rapidly near the leading edge of the 

flame. The volatiles evolved rapidly from the coal in the 

flame front are assumed to be significantly higher than 

those measured by the ASTM D3175 standard test 

(ASTM, 1999). In addition to the combustion of the volatiles, there may be some combustion of the remaining 

"fixed carbon" or char, but this is neglected in the model. 

The fixed carbon char and the added inert dusts are 

assumed to behave as heat sinks in limiting the 

maximum temperature. The limestone rock dust is 

assumed to decompose entirely in the flame. An equilibrium flame temperature, corresponding to the limit flame 

temperatures for hydrocarbon-air mixtures, is assumed. 

Heat losses to the walls in large diameter galleries such 

as the experimental mine should be negligible and are 

not included in the model. 

The energy balance equation is therefore the heat for 

the rock dust decomposition plus the heating of rock dust 

plus the heating of coal char plus the heating of the products equals the available heat: 

Y)fd+[YrCr+(Yc - Xv)Cc]~T+(pCp)~T=HvXv (1) 

where Y, is the concentration of rock dust, Hd is the heat 

of decomposition of the rock dust (426 kcal/kg), Cr is 

the average over temperature for the specific heat of the 

rock dust (0.29 kcalJkg°C), Yc is the concentration of 

fuel dust, Xv is the concentration of volatiles 

(:.:;stoichiometric), Cc is the average specific heat of the 

char (0.25 kcal/kg°C), ~T is the temperature change 

from ambient to flame, p is the average density of flame 

produc,fS, Cp is the average specific heat of the flame 

products, and Hv is the heat of combustion of the volatiles. For this model, pCp is taken as 0.35 kcal/m3

0C. 

Hv is taken as 9000-10,000 kcal/kg for the coal volatiles 

and as 10,000 kcal/kg for the gilsonite volatiles. These 

input data are the same as those used by Richmond et 

al. (1975, 1979) and Richmond, Liebman, Bruszak & 

Miller (1979) except that they used only Hv=IO,OOO 

kcal/kg for the coal rather than a range of values. It is 

understood that some of the input data for this model are 

only approximate. This model (Richmond et aI., 1975) is 

assumed to be independent of dust particle size as long 

as the coal is <34 jlm and the rock dust is <14 jlm. 

Additional details on the model are in Richmond et al. 

(1975, 1979). 

The second model is more sophisticated than the first 

model, but still has some uncertainties in input data. For 

this model, Hv is taken as 9300 kcal/kg for the coal volatiles, calculated based on the measured heats of combustion (moisture and ash free or maf) of Pittsburgh coal 

and of coke derived from the coal. Hv is taken as 10,000 

kcal/kg for the gilsonite volatiles, based on the measured 

heat of combustion (maf) for the gilsonite. This second 

model uses the NASA-Lewis CEA-400 Fortran computer code, which is an updated PC version of the CEC80 code (Gordon & McBride, 1976), which is described 

in detail in Conti, Zlochower and Sapko (1991). An earlier version of the model is discussed in Hertzberg, Zlochower and Cashdollar (1988). Adiabatic equilibrium 

calculations of product gas temperatures, pressures, and 

compositions were made for the various fuel dust and air 

mixtures. The calculations give the maximum expected 

explosion temperatures and pressures for the gas mixtures in the absence of a detonation. This model considers each of the flame products individually. The program computes the equilibrium product composition 

from the listed reactants and their standard energies of 

formation by examining all possible product species 

(consisting of combinations of the reactant atoms) whose 

temperature dependent thermodynamic properties are 

listed in an auxiliary table. The thermodynamic properties accessed by the program are taken predominantly 

from the JANAF thermodynamic data compilation 

(Chase, Davies, Downey, Frurip, McDonald & Syverud, 

1985). The product composition is obtained by minimizing the free energy of the system. Constant pressure calculations were performed at one atmosphere to determine the adiabatic temperatures. Such temperature 

calculations were used to derive limit flame temperatures 

and compositions. 

For both models, the volatiles evolved rapidly from 

the coal in the flame front were assumed to be significantly higher than those measured by the ASTM D3175 

standard test and listed in Table 1. The Pittsburgh coal 

volatility was taken as 50%, based on rapid pyrolysis 

experiments (Cashdollar et al., 1989; Hertzberg, Zlochower & Edwards, 1988), rather than the 36% value 

listed in Table I. The gilsonite volatility was taken as 

90%, somewhat higher than the 84% value listed in the 

table. The limit flame temperature for both models was 

taken as 1300 to 1500 K. 

U sing the two models, limit coal dust and gilsonite 

concentrations were calculated and compared with 

experimental data in Table 4. For the Pittsburgh coal, 

the calculated minimum concentrations ranged from 72 

to 97 glm3 for the Richmond-Liebman (Eq. (1» model 

and from 81 to 102 glm3 for the NASA-Lewis CEA400 model, compared to the 80 g/m3 in both laboratory 

chambers and the 60 g/m3 value from LLEM. The calculations assume that only the volatiles (-50%) bum. If 

some of the fixed carbon bums or if the volatiles are 

higher than 50%, the calculated MEC values would be 
Table 4 

Calculated and experimental minimum explosible concentration 

(MEC) of Pittsburgh coal and gilsonite dusts 

Dust Minimum explosible concentration (MEC), glml 

20-L & LLEM Eq. (1) CEA-400 

lOOO-L model model 

chambers calculation calculation 

Pittsburgh 80±1O 60±5 72-97 81-102 

coal 

Gilsonite 35±5 35±5 39-47 36-46 

lower. There is some evidence that more of the fixed 

carbon may participate in the mine explosion due to the 

thicker flame zone (Conti et aI., 1991). For the gilsonite, 

the calculated minimum concentrations ranged from 39 

to 47 g/m3 for the Richmond-Liebman model and from 

36 to 46 g/m3 for the NASA-Lewis model, which are 

both in fairly good agreement with the experimental 

value of 35 g/m3 in both the laboratory and mine tests. 

For both models, the lower end of the calculated range 

of MEC-values agrees better with the experimental 

values for the coal and the gilsonite. The lower end of 

the calculated range of MEC-values corresponds to the 

lower value (1300 K) of the limit flame temperature 

or .IT=I000°C. 

The models were also used to predict the amounts of 

limestone rock dust necessary to inert the Pittsburgh coal 

(PPC) and gilsonite. The calculations were made at nearstoichiometric (for the volatiles) concentrations of 

220 g/m3 for the PPC and 100 g/m3 for the gilsonite, 

which would be the "worst case" conditions. The Richmond-Liebman model predicts limit rock dust concentrations of 76-82% and the NASA-Lewis CEA-400 

model predicts limit rock dust concentrations of 72-78% 

to inert the Pittsburgh coal, as shown in Table 5. Experimental rock dust concentrations to inert the PPC ranged 

from 78 to 80% for the mine tests and -74% for the 

20-L chamber. Experimental rock dust concentrations to 

inert gilsonite ranged from 88% to 90% for the mine and 

-90% for the 20-L chamber, compared to 86-88% for 

the Richmond-Liebman model and 84-88% for the 

Table 5 

Limestone rock dust required to inert Pittsburgh coal and gilsonitc 

Dust Rock dust to inert. % 

20-L 

chamber 

Pittsburgh 74±2 

coal 

Gilsonitc 90±2 

LLEM 

78-80 

88-90 

Eq. (I) 

model 

CEA-400 

model 

calculation calculation 

76-82 72-78 

86-88 84-88 

NASA-Lewis model. For both models, the ranges of calculated values overlapped the ranges of experimental 

values for the coal and the gilsonite. If the models were 

modified so that not all of the rock dust decomposed, 

the values calculated by the models would be higher. 

The agreement is rather good between model predictions and experiments for both the minimum concentrations (lower flammable limits) and for the rock dust 

iiler.ting concentrations, especially when considering the 

uncertainties in the model assumptions and in the experimental data, including variations in dispersion methods. 

Additional experimental research on coal dust and coalrock dust mixtures, including flame temperature 

measurements and gas sampling, is needed to better 

refine these models. Important issues that remain include 

the estimation of the best value for the volatility of the 

coal under flame conditions, the extent of char (fixed 

carbon) burning, and the extent of limestone rock dust 

decomposition in mine explosions. 

5. Explosion dynamics 

As described by Nagy (1981), a gas and/or dust 

explosion in a mine passageway develops two types of 

destructive pressures static and dynamic. The hot 

combustion products expand and exert a force equally 

in all directions. This is the static pressure, which is the 

pressure measured in a closed volume. In a mine, the 

hot gases also expand and flow through the mine roadways or passageways, pushing air ahead. This flow of 

gas at high speed generates a wind or dynamic pressure, 

which is directional. Both the static and dynamic pressure8 can cause damage during a mine explosion. The 

static pressure rise can destroy stoppings in side entries 

perpendicular to the direction of gas flow. The dynamic 

pressure gives rise to wind forces that can disperse coal 

dust and that can move objects. The speed and duration 

of the moving air in a mine explosion entrains dust from 

the mine surfaces forming a combustible cloud which 

when ignited causes the most damage in the underground 

workings. The dynamic pressure is proportional to the 

square of the air speed as shown by the following equation (Nagy, 1981; Kuchta, 1985): P=O.5pv2 , where p is 

the air density and v is the air speed. 

The maximum static pressure for a coal-air explosion 

in a laboratory chamber is about 7 bar (700 kPa). In an 

open mine entry the maximum static pressure is generally less than that due to the pressure release from the 

gas flow in the mine entry. However, maximum pressures in a mine may exceed 7 bar with pressure piling or 

a detonation. In pressure piling, the fuel-air mixture 

ahead of the flame front is compressed. The flame then 

bums through this compressed mixture with a correspondingly increased maximum explosion pressure 

(increased by the compression ratio). The next volume 
of unburned mixture is then compressed additionally to 

produce an even greater explosion pressure, etc. For 

example, Cybulski (1975) discusses a coal dust 

explosion experiment (#1397) in a dead end entry where 

the peak pressure was at least 4) bar (4.1 MPa), causing 

considerable damage to the Polish Experimental Mine 

Barbara. Dust explosions in open entries in the Bruceton 

experimental mine have developed pressures exceeding 

10 bar (1.0 MPa) (Nagy, 1981). Pressure piling can, in 

some cases, lead to a detonation, where shock waves 

develop at the flame front and the propagation is supersonic relative to the unburned reactants. The maximum 

explosion pressure of a detonation may be double that 

of a deflagration (Kuchta, 1985). 

A schematic (Hertzberg & Cashdollar, 1987) of a dust 

explosion in a mine entry is shown in Fig. 6. The effect 

of turbulence is most pronounced and devastating when 

the flammable volume is partially confined as in a mine 

passageway, with one end open and with ignition at the 

closed end. Hot burned gases generated behind the flame 

front expand toward the open end and push the unburned 

mixture outward. The Reynolds number of the unburned 

mixture flow in the tube or corridor rapidly exceeds the 

critical threshold for the generation of turbulence. The 

flame speed then increases as the flame propagates into 

that turbulent mixture. This acceleration of the flame 

further increases the turbulence, which further accelerates the flame. This process is idealized in Fig. 6B, 

where the combustion wave is characterized 

(Hertzberg & Cashdollar, 1987) as an accelerating piston 

exerting a pressure force on the unburned gas in the tube, 

accelerating it toward the open end. The flow disturbance associated with the piston-like motion is transmitted to the gas ahead of it at the velocity of sound, 

Co, in the medium. Thus, only a finite volume of gas 

ahead of the piston is compressed by its motion. The 

region of gas ahead of the disturbance front cannot sense 

I-------oot-------; 

:::::::::::::::::::-1-1-----

... vf···.······· .... 1 Co r ~:::: :Compressedgas::::: I ~ ..••... volume ......• I • • • • • • • • • • • • • • • • • • • f-Disturbanee 

:::::::::::::::::::t front 

Fig. 6. Propagation of a dust flame from the closed end of a tunnel 

(A) and idealization of that process as an accelerating piston (8). 

the motion of the piston. Only the gas volume in the 

space between the piston and the leading edge of the 

sonic disturbance is compressed in the time t, and that 

volume is AoCot, where Ao is the cross sectional area of 

the mine passageway and t is the time during which the 

piston has moved. If Po is the initial cold gas density, 

during the time t, the mass of gas accelerated by the 

piston is given by: m=paAoCot. That mass of gas, which 

was initially at rest, is accelerated to the piston velocity, 

Vf' From Newton's law, the product of that mass and 

its instantaneous change in velocity, .lv, must equal the 

impulse, which is the product of the pressure force, F, 

and the time over which it is applied (Hertzberg & Cashdollar, 1987; Richmond & Liebman, 1975). Thus: 

m.lv=F.lt=.lPAot 

paAoCot(vf-O)=.lPAot 

which gives: 

.lP=PoCoVf 

(2) 

(3) 

(4) 

where Vf is the flame speed (or piston velocity) and .lP 

is the explosion overpressure at the leading edge of the 

flame. Hence, the pressure generated by the explosion is 

linearly proportional to the flame speed, with the speed 

of sound, Co as the constant of proportionality. The equation relating these parameters is called the acoustic 

approximation (Richmond & Liebman, 1975). The general validity of this equation and its more exact refinements has been demonstrated in many gas and dust 

explosion tests at the Bureau of Mines Bruceton experimental mine (Richmond & Liebman, 1975). 

The acoustic approximation (Eq. (4» shows that the 

higher the flame speed driving an explosion in a tunnel 

or mine entry, the higher is the explosion pressure. The 

faster the flame speed is, the smaller is the compressed 

gas volume ahead of the wave and the more difficult it 

is for the compression force to be relieved by the gas 

motion, resulting in a higher pressure 'generated by the 

propagating flame front. When the combustion wave 

accelerates to reach the velocity of sound in the 

unburned mixture (or exceeds it), there can be no gas 

motion ahead of the combustion wave. Thus there can 

be no release or venting of the pressure. Under these 

conditions, the combustion process can transit from a 

deflagration to a detonation. 

5.1. Explosion tests to evaluate the strength of 

underground mine seals 

This section discusses some of the flame propagation 

characteristics as they relate to full-scale testing of the 

explosion resistance of seals used in underground coal 

mines. Federal regulations require that abandoned areas 

of underground mines must be either ventilated or isolated from active workings through the use of seals cap-
able of withstanding the destructive forces from an 

explosion. In recent years the Lake Lynn experimental 

mine has been used extensively to evaluate the performance of new seal designs for use in US coal mines 

(Greninger, Weiss, Luzik & Stephan, 1991; Weiss, 

Greninger, Stephan & Lipscomb, 1993; Weiss, Cashdollar, Mutton, Kohli & Slivensky, 1999). As described in 

the "Experimental Facilities and Test Procedures" section, the seals are constructed in the crosscuts between 

B- and C-drifts (Fig. 3). The explosion resistance characteristics of the seals are evaluated by using gas and dust 

explosions conducted in the LLEM C-drift. For an 

explosion giving a static pressure of ~ 1.4 bar (~140 kPa) 

against the seals, a 190-m3 zone of 9% CH4-air was 

ignited at the face. For a stronger explosion, 80 kg of 

coal dust was loaded onto shelves that were suspended 

from the mine roof on 3-m increments for 64 m beyond 

the ignition zone (Weiss et aI., 1999). This coal dust 

loading provided a nominal concentration of 100 g/m3 , 

assuming the dust was uniformly dispersed over the 

entry cross section. To produce an even stronger 

explosion, a third test was run with 160 kg of coal dust 

or a nominal concentration of 200 glm3 . The results of 

the seal evaluations are in the references (Greninger et 

aI., 1991; Weiss et aI., 1993, 1999) and will not be discussed here. However, various aspects of the explosions 

used to evaluate the seals will be considered in relation 

to explosion propagation. 

Fig. 7 shows the flame displacement as a function of 

time for LLEM test 348, which was the gas and 100-

g/m3 coal dust explosion (Weiss et aI., 1999). For convenience, the flame travel is divided into four zones. In 

zone 1, the gas zone is ignited at the face (closed end) 

of the mine entry, and the flame expands hemispherically 

at a nearly constant rate until it contacts the walls and 

roof of the mine entry. It then starts to move outward 

in the mine entry. The average flame speed in zone 1 

was about 15 mls. In zone 2 (4-26 m from the face), 

the flame moves linearly down the mine entry at an aver-

\00 

eo 

e 

1&.1 60 

(.) 

z 

fI) 

~ 40 

Q 

20 

o 0,\ 0.2 0.3 0.4 0.5 0.6 0,7 o.e 

TIME, s 

7. Flame displacement versus time for LLEM test 348 (methane 

gas plus 100 glm3 coal) propagating in C-Drift. 

age flame speed of 83 mls. The water barrels (Fig. 3) 

located in the first third of zone 2 serve as obstacles to 

generate turbulence and increase the flame speed. The 

effect of the induced turbulence is evident in zone 3, 

where the average flame speed was 340 mls. In zone 

4 (56-93 m) the flame propagation averages 440 mis, 

primarily due to the rapid consumption of dispersed coal 

dust that was pushed ahead by the expanding combustion 

products. Flame propagation occurred beyond the 93 m 

station but was not included in the flame speed calculation because the flame slowed down due to venting 

into B-drift through failed seals and open crosscuts. 

A summary of the flame propagation data for LLEM 

tests 347, 348, and 349 is listed in Table 6. LLEM test 

347 was the gas ignitor zone only. Test 348 for the gas 

zone and added 100 g/m3 of coal was described in the 

previous paragraph. Test 349 was for the gas zone and 

200 glm3 of added coal. The measured flame speeds 

through zones 1 to 3 were similar for all three tests. The 

closed end ignition of the 14-m-Iong gas zone (~21O-m3 

volume) resulted in flame propagation for just beyond 

56 m. The influence of added coal dust to the combustion 

process is indicated by the much higher flame speeds as 

the explosion propagates through zone 4. The 200-g/m3 

nominal coal dust loading produced an average flame 

speed about 1.7 times faster than that for the l00-g/m3 

loading. The consequence of this increase in flame speed 

is best illustrated by the significant increase in resulting 

explosion pressures. 

Fig. 8 shows the static pressures as a function of distance from the face for the three LLEM explosions. 

Additional details of these tests, including tables of peak 

pressure versus distance and times of peak pressure are 

in Weiss et al. (1999). The flame essentially acts as a 

piston to drive the flow ahead of it. The maximum static 

pressures within the flow, as well as the maximum gas 

speeds, occur near the flame front in agreement with the 

theory of one-dimensional combustion dynamics. LLEM 

test 347 shows the peak pressures at each station as a 

Table 6 

Gas and dust flame speeds from explosions propagating in LLEM 

C-drifta 

Flame Flame speed. mls 

Zone Distance. m LLEM test LLEM test LLEM test 

1 

2 

3 

4 

0-4 

4 - 26 

26 - 56 

56 - 93 

347 348 349 

Gas only 

14 

103 

390 

Gas+loo 

glm' PPC 

15 

83 

340 

440 

Gas+2oo 

g!m3 PPC 

13 

100 

335 

740 

• Gas ignition zone was 14-m-long or -2ID-m3 of 9% CH4 in air. 

Average flame speeds were measured over indicated zones downstream 

from the point of ignition. 
LLEM 347 LLEM 348 LLEM 349 

200 3'0 500 300 

150 Slatic pressure 2'0 400 

200 300 SialiC PfftIU .. 

100 150 

100 200 

50 50 100 0 25.6m 300 0 

0 250 400 25.6m 

25.6m 200 100 150 300 

50 

100 200 

50 100 0 

0 250 40.8m 0 

200 400 40.8m 

C 150 4O.8m 150 

a.. 100 300 

.:.: tOO 50 200 

0 100 50 2'0 56.1m 

W 200 0 

a:: 0 150 56.1m 500 

::> 56Jm 100 

en 100 50 400 

en 0 71.5m 300 350 

W 50 300 200 

a:: 0 250 100 

a.. 200 

71.3m 150 0 

100 100 500 7Um 

U 50 

50 0 400 

~ 350 92..7m 300 

0 300 I- 92.7m 250 200 

en 100 200 100 

150 0 

50 100 92.7m 

50 400 

0 0 122.8m 300 

300 

250 200 

100 200 100 

50 150 0 100 

50 200 IZ2.Bm 

0 0 100 152.7m 200 152.7m 

50 150 0 

100 100 152.7m 

0 50 

182.3m 0 0 182.3m .m 

0 0.5 1.0 1.5 2.0 2.5 0 0.5 1.0 1.5 2..0 2..5 0 0.5 1.0 U5 2.0 2.5 

TIME AFTER IGNITION, s 

Fig. 8. Pressure wave diagrams for three explosions: LLEM test 347 (CH4 gas zone only), LLEM test 348 (gas plus 100 g/m3 coal), LLEM test 

349 (gas plus 200 glm3 coal). 

wave or piston traveling outward from the face. Both the 

100- and 200-g/m3 explosions show strong compression 

waves being driven in both directions by the accelerating 

flame. These strong compression waves originate near 

the 93-m station, in zone 4 where the flame speed is a 

maximum. The compression wave then travels both 

inward and outward from the 93 m station. The compression wave travels faster towards the face than away 

from the face due to the presence of hot gases behind 

the flame front. The pressure history from the gas zone 

(LLEM test 347) produces maximum overpressures of 

about 1.5-1.7 bar (150:-170 kPa) at the 26- to 56-m stations and decays to about 0.6 bar (60 kPa) at the 

182-m station. Upon closer inspection of the 26-m 

station pressure histories for the LLEM test 348 and test 

349 explosions, one can identify that portion (initial peak 

at -1.5 bar) associated with the gas zone combustion 

prior to the later, higher peak due to the reflected compression wave from the dust combustion. The maximum 

pressures for the three tests were -1.7 bar for LLEM 

test 347, -3.9 bar for LLEM test 348, and -6.0 bar for 

LLEM test 349. The increase in maximum pressure from 

the 1 DO-g/m3 coal dust explosion (LLEM test 348) to 

the 200-glm3 coal dust explosion (LLEM test 349) is 

consistent with the increase in flame speed in zone 4 for 

the latter explosion (Table 6). 
6. Conclusions 

The minimum nominal dust loadings for explosion 

propagation in the Lake Lynn experimental mine were 

35 g/m3 for the 84% volatility gilsonite dust and 60 gtm3 

for the 36% volatility Pittsburgh bituminous coal dust. 

These data are based on mine experiments using a test 

zone that was more than twice as long as the flame travel 

from the ignitor; this avoids the problem of "overdriving" that was found for shorter test zones. In the limestone rock dust inerting tests in the LLEM, the Pittsburgh coal required about 78-80% rock dust or 80-82% 

total inert content in the coal-rock mixture in order to 

prevent explosion propagation. The higher volatility gilsonite required about 10% more rock dust than the coal 

to be inerted. Two theoretical models based on a calculated, adiabatic limit flame temperature were shown to 

be consistent with experimental mine and laboratory data 

for both minimum concentrations and the amounts of 

rock dust necessary to inert the coal and gilsonite. These 

results for the coal and gilsonite dusts extend the data 

base for carbonaceous fuels to include a wide range of 

volatilities. Explosion intensities increased significantly 

with increasing coal dust concentrations in both small 

and large scale explosions. 

Basic and applied research on dust explosions continues in NIOSH. The basic research, including laboratory experiments and theoretical studies, focuses on 

improving mine safety through increased knowledge and 

understanding of complicated dust combustion phenomena. Applied research, mostly conducted in the LLEM, 

plays a key role in providing the technical data needed 

to resolve regulatory issues dealing with explosion prevention, detection, suppression, and the performance 

evaluation of new technologies under realistic largescale conditions. 

Acknowledgements 

The authors wish to acknowledge the assistance of G. 

M. Green, K. Jackson, F. Karnack, D. Kohli, D. Sellers, 

and W. Slivensky of the Pittsburgh Research Laboratory 

in conducting the tests and collecting the data. 

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