MITIGATING COAL DUST EXPLOSIONS IN MODERN 

UNDERGROUND COAL MINES 

Marcia L. Harris1

,

1

The National Institute For Occupational Safety and Health, P.O. Box 18070, 

626 Cochrans Mill Rd., Pittsburgh, PA 15236, Email: mharris@cdc.gov 

Kenneth L. Cashdollar2

,

2

The National Institute For Occupational Safety and Health, P.O. Box 18070, 

626 Cochrans Mill Rd., Pittsburgh, PA 15236, Email: kcashdollar@cdc.gov 

 Chi-Keung Man3 

3

The National Institute For Occupational Safety and Health, P.O. Box 18070, 

626 Cochrans Mill Rd., Pittsburgh, PA 15236, Email: cman@cdc.gov 

and Ed Thimons4 

4

The National Institute For Occupational Safety and Health, P.O. Box 18070, 

626 Cochrans Mill Rd., Pittsburgh, PA 15236, Email: ethimons@cdc.gov 

ABSTRACT 

The N ational Institute for Occupational Safety and Health (NIOSH), as part of its

continuing research program for evaluating coal dust explosion hazards, has 

investigated several areas in which current practices may need to be updated in order 

to adequately protect mines against coal dust propagated explosions. In the United 

States, current rock dusting requirements remained largely unchanged since 1969. US 

Title 30 Code of Federal Regulations Section 75.403 is based on a coal dust particle 

size survey performed in the 1920s and later was supplemented by full-scale testing 

of the rock dust ability to inert a coal dust explosion. NIOSH recently conducted a 

comprehensive survey of US underground coal mines to determine the range of coal 

particle sizes found in dust samples collected from the mine entries. Due to 

advancements in technology and modern coal mining techniques, the current coal

dust particles in intake airways are significantly finer than those found in the mines in 

the 1920s. According to past full-scale dust explosion test results, the current rock 

dusting practices used in today’s mines to inert a coal dust e xplosion may not be 

adequate. Other closely related issues such as rock dust testing methods and sampling 

procedures are discussed. 
1. INTRODUCTION 

It has long been known that fine coal dust particles laying in an entryway can 

propagate an explosion down an entry. A coal dust explosion can generate sufficient 

air pressure to disperse dust from entry surfaces and draw it into the expanding 

combustion zone. Heat transfer to coal dust particles results in the production of 

volatiles and tars from these particles. At high temperatures, the product reacts with 

the oxygen in the air and the heat released from this exothermic reaction is converted 

into work of expansion of the semi-confined air. Conversely, entrained rock dust acts 

as a heat sink, drawing energy out of the system. 

In the United States (U.S.), the current rock dust requirements were established by 

the Federal Coal Mine Health and Safety Act of 1969 which mandated 65% 

incombustible content (IC) be present in non-return airways and 80% IC in return 

airways. These values were derived from coal dust particle size research performed 

in the 1920s (Rice and Greenwald, 1929; Nagy, 1981). Mining methods have 

drastically changed since the early 20th century. As the industry moved towards 

increased mechanization and less conventional blasting, the size of the coal dust 

found in the entries of modern coal mines has changed (Sapko et al, 2007). 

Finer coal dust requires more incombustible material to inert it (Rice et al , 1 922; 

Rice and Greenwald, 1929; Nagy, 1981; Cashdollar and Hertzberg, 1989; Weiss et 

al, 1989). Figure 1 illustrates the effect of coal dust particle size on the explosibility 

of the mixture (Sapko et al, 2007).

100 

90 

Non-Propagation 

80

nt, 

70

onte e C

60

bl

Propagation 50

usti

40

mb

30 BEM, non-propagation 

Total Inco

LLEM, non-propagation 

20 BEM, propagation LLEM, propagation 

10 

%

BEM curve LLEM curve 

0 

0 10 20 30 40 50 60 70 80 90 100 

Minus 200 mesh content of coal dust, % 

Figure 1: Effect of particle size of coal dust and IC on the explosibility (Sapko et 

al., 2007). 

 Large scale dust explosion experiments conducted 

at the NIOSH-PRL Bruceton Experimental Mine (BEM) and the Lake Lynn 

Experimental Mine (LLEM) show that dust mixtures with finer coal particles require 

more inert material to render the finer coal dust non-explosible. 

In t he U.S. incombustible material is typically pulverized limestone rock dust. In 

compliance with Federal Regulations 30 CFR 75.403, the Mine S afety and Health 

Administration (MSHA) performs quarterly surveys in every mine (MSHA, 2008). 

Dust samples are gathered and sent to MSHA’s laboratory in Mt. Hope, West 

Virginia, for determination of moisture and incombustible content. No particle size 

analysis is performed. The typical wait time for results is approximately two weeks 

after sample collection. A recent NIOSH research project has determined the current 

size of coal dusts found in modern US coal mines and developed an instrument to 

screen samples almost instantly to determine when hazardous conditions may be 

present (Sapko et al., 2007). 

2. MSHA SAMPLE COLLECTION AND TESTING 
When MSHA performs their quarterly surveys, the dust is collected in a band sample 

at 152 m (500 ft) intervals from the last survey sample point in a new development. 

The band is a 15 cm (6 in) swath collected from the roof, ribs, and floor (up to 2.54 

cm or 1 in deep). The collected dust is sieved through a 10 mesh screen (1.7 mm) and 

a portion of the sample is bagged and sent for analysis (MSHA, 2008). 

3. COAL DUST PARTICLE SIZE 

As a result of the 1920s research, “mine size” coal dust was defined as coal dust that 

passes through a U.S. Standard No. 20 sieve (850 μm) with 20% passing through a 

200 mesh sieve (75 μm) (Rice and Greenwald, 1929). Float coal dust was defined as 

minus 200 mesh coal dust particles that may be deposited on the roof, ribs, and 

timbers in a mine (Nagy, 1981). NIOSH recently conducted a particle size survey of 

US coal mines which concluded that the old definition of “mine size” dust is no 

longer applicable or representative (Sapko et al., 2007). 

Dust samples were routinely collected by MSHA inspectors and sent to the Mt. Hope 

Laboratory. Once the Mt. Hope laboratory performed their testing of the samples, the 

remaining samples were sent to NIOSH for coal particle sizing analyses. 

At N IOSH, the limestone fraction of the samples was leached using hydrochloric 

acid (Sapko et al, 2007). Dilute hydrochloric acid was added to the dust sample in a 

beaker and heated. The acid reacted with the rock dust producing foam and releasing 

carbon dioxide. More acid was added until the foaming stopped. The resulting slurry 

was cooled a nd the residue was filtered from the slurry. The residue consisting of 

coal was rinsed with isopropanol and dried. 

Using a sonic siever, the remaining samples were classified into different size 

fractions. The sonic siever utilizes a repetitive mechanical pulse and a vertical 

oscillating air c olumn to provide particle separation. The sample amount on each 

sieve was recorded once the sieving process was complete. 

MSHA divides U.S. coal mines into districts (Figure 2) . The intake airway dust 

samples were examined and compared according to these MSHA districts. The 

fractions of minus 200 mesh (75 μm) ranged from 26 % in District 8 to 39% in 

District 11. The minus 70 mesh (212 μm) fractions ranged from 59% in District 8 to 

77% in D istrict 1 1. The median diameter of the samples ranged from 116 μm in 

District 11 t o 169 μm in District 8. The overall averages are 32% minus 200 mesh, 

64% minus 70 mesh, with a median dust particle diameter of 147 μm. These samples 

are significantly finer than those of the 1920s survey (Sapko et al., 2007). 
Figure 2: MSHA districts (Sapko et al., 2007). 

The same information was analyzed for the various coal seams sampled. The Blue 

Creek (District 11, Figure 2) and Hazard #4 (District 6, Figure 2) coal seams have the 

finest sized particles with a median diameter of 98 μm and 104 μm respectively. For 

these seams, the finer particle sizes were found in the Blue Creek coal seam 

containing about 43% dust particles minus 200 mesh ( 75 μm) a nd 83% minus 70 

mesh (212 μm) and in the Hazard #4 seam containing 40% minus 200 mesh and 69% 

minus 70 mesh (Sapko et al., 2007). 

4. INERTING REQUIREMENTS 

Coal dust explosibility is strongly dependent on its particle size. Since the average 

particle size is significantly different than that on which current regulations are 

based, current IC requirements may also be different. Based on early BEM data 

shown in Figure 1, more than 72% incombustible material is required to prevent a 

flame propagating explosion with a dust mixture containing roughly 40% minus 200 

mesh (75 μm) coal particles. It would appear better to maintain 75% incombustible 

material in these entries in order to protect against a propagating explosion. 

5. DUST SCOURING 

As a part of the recent full-scale dust explosion testing at LLEM, dust scouring 

measurements were also collected. The dust scoured is indicative of the depth of dust 

participating in an explosion. Two parallel rails filled with a coal dust mixture were 

positioned in the entry. The dust was leveled between the rails. Using a displacement 

gauge mounted o n a portable aluminum bar, measurements were taken of the dust 

levels before and after the explosions ( Figure 3). The dust scoured during an 

explosion ranged from 0.7 mm (0.03 in) to 2.6 mm (0.1 in) with an average of 1.5 

mm (0.06 in). This is much less than the 2.54 cm (1 in) that is specified in the MSHA 

band sampling procedures. Therefore, the current 2.54 cm (1 in) sampling depth of 

dust does not portray what actually participates in an explosion. If there is a layer of 

float coal dust, the dust sample can be diluted with rock dust by sampling to a full 

depth of 2.54 cm (1 in), thereby giving a false sense of protection. A sample depth of 

0.64 cm (1⁄4 in) appears to better represent the level of explosion protection. 
Figure 3: Measuring the amount of dust scoured during an explosion. 

6. COMPLIANCE AND PROTECTION 

Approximately two weeks pass before results of the quarterly rock dust band survey 

are received by the mine inspectors. The required remediation of potentially 

dangerous conditions is delayed because of this time constraint. NIOSH has 

developed a coal dust explosibility meter (CDEM) to address this situation. 

The CDEM is a hand-held instrument that gives real-time readings of the 

incombustible conditions present (Figure 4).

Figure 4: The hand-held coal dust explosibility meter (CDEM) 

Its operation is based on optical 

reflectivity and takes into account the particle size of the coal dust (Sapko and 

Verakis, 2 006). The instrument output includes a red/yellow/green designation. 

Green would indicate the area is adequately protected while red would indicate an

area deficient in rock dust. A yellow response indicates that the area is marginally 

protected. 

A dust sample would be collected as per the MSHA established band sample 

requirements. A portion of the thoroughly mixed sample would be collected into a 

45 ml sample tube containing drying molecular sieves. A small amount of the sample 

is shaken through a funnel that snaps onto the 45 ml sample tube and into the sample 

cup. The funnel is internally fitted with a 20 mesh screen which prevents the 

molecular sieves and larger dust particles from filling the sample cup. The sample 

cup containing the dust mixture is presented to the CDEM for measurement. 
An earlier field study of the instrument in District 2 mines indicated that some areas 

were not adequately protected yet complied with the regulated 65% IC in the intake 

airways (Figure 5). 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

MSHA LTA (% IC)

Green Yellow Red 

0 10 20 30 40 50 60 70 80 90 100 

CDEM Measurement (% IC) 

Figure 5: Coal dust explosibility meter field test results (Harris et al., 2008) 

Among the District 2 samples, 92 out of 104 had ≥ 65% IC, as 

required by current regulations. The CDEM indicated that 27 of these samples were 

within the red or yellow bands and may have been deficient in rock dust. These areas 

most likely would not receive more rock dust since analyses indicated compliance, 

however these areas represent regions where a risk of explosion propagation is

present and would require more rock dust (Harris et al., 2008). 

7. SUMMARY 

Modern underground coal mine conditions have changed since the early 20th century. 

The coal dust deposited in the mine intake airways is significantly finer. As such, 

more rock dust needs to be applied in order to prevent a propagating explosion in 

intake airways. New instrumentation has been developed to assess in real time the 

level of explosion protection that is within an entry. By accounting for varying 

particle sizes, the CDEM can immediately indicate whether more inert material is 

required to protect an entry from a possible propagating explosion. The amount of 

dust which participates in an explosion is less than a quarter inch deep. When 

assessing the rock dusting adequacy within an entry, only the top quarter inch should 

be evaluated. 

REFERENCES 

Cashdollar, K.L. and H ertzberg, M ., 1989, “Laboratory study of rock dust inerting 

requirements: effects of coal volatility, particle size, and methane addition,” 23rd 

International Conference of Safety in Mines Research Institutes, pp. 965-977. 

Harris, M .L., Sapko, M.J., Cashdollar, K .L., and Verakis, H .C, 2 008, “Field 

evaluation of the coal dust explosibility meter (CDEM),” Mining Engineering. Vol. 

60, No. 10, Oct., pp. 50-54. 

MSHA, 2008, “MSHA handbook series, general coal mine inspection procedures and 

inspection tracking system,” U. S. Department of Labor, Mine Safety and Health 

Administration, Coal Mine Safety and Health, Handbook Number: PH08-V-1, 

January 1, 2008, pp. 60-66. 
Nagy, J., 1 981, “The explosion hazard in mining,” U.S. Department of Labor, Mine 

Safety and Health Administration, Information Report 1119, 69 pp. 

Rice, G.S., and G reenwald, H.P., 1929, “Coal-dust explosibility factors indicated by 

experimental mine investigations 1 911 to 1929,” U .S. Bureau of Mines Technical 

Paper 464, 45 pp. 

Rice, G.S., Jones, L.M., Egy, W.L., and Greenwald, H.P., 1922, “Coal-dust explosion 

tests in the experimental mine 1913-1918, inclusive,” U.S. Bureau of Mines Bulletin 

167, 639 pp. 

Sapko, M.J. , and Verakis, H ., 2006, “ Technical development of the coal dust

explosibility meter,” 2006 SME Annual Meeting, preprint 06-044, Quality Business 

Services, SME, Littleton, CO, 5 pp. 

Sapko, M.J., Cashdollar, K.L., and Green, G.M., 2007, Coal dust particle size survey 

of U.S. mines, Journal of Loss Prevention in the Process Industries. Vol. 20, pp. 616-

620. 

Title 30, U.S. Code of Federal Regulations (30 CFR), Section 75.403, July 1, 2008. 

Weiss, E.S ., Greninger, N.B., and Sapko, M.J., 198 9, “Recent results of dust 

explosion studies at the Lake Lynn experimental mine,” 23 rd International 

Conference of Safety in Mines Research Institutes, pp. 843-854.