RI9679 

REPORT OF INVESTIGATIONS/2010 

Recommendations 

for a New Rock Dusting Standard 

to Prevent Coal Dust Explosions 

in Intake Airways
 

Department of Health and Human Services 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 
 

 

Report of Investigations 9679 

Recommendations for a New Rock Dusting Standard 

to Prevent Coal Dust Explosions in Intake Airways 

Kenneth L. Cashdollar, Michael J. Sapko, Eric S. Weiss, Marcia L. Harris, 

Chi-Keung Man, Samuel P. Harteis, and Gregory M. Green 

DEPARTMENT OF HEALTH AND HUMAN SERVICES Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 

Office of Mine Safety and Health Research 

Pittsburgh, PA 

May 2010 
This document is in the public dom ain and may be freely copied 

or reprinted. 

Disclaimer 

Mention of any company or product does not constitute endorsement by the 

National Institute for Occupational Safety and Health (NIOSH). In addition, citations to 

Web sites external to NIOSH do not constitute NIOSH endorsement of the sponsoring 

organizations or their programs or products. Furthermore, NIOSH is not responsible for 

the content of these Web sites. 

Ordering Information 

To receive documents or other information about occupational safety and health topics, 

contact NIOSH at 

Telephone: 1–800–CDC–INFO (1–800–232–4636) 

TTY: 1–888–232–6348 

e-mail: cdcinfo@cdc.gov 

or visit the NIOSH Web site at www.cdc.gov/niosh. 

For a monthly update on news at NIOSH, subscribe to NIOSH eNews by visiting 

www.cdc.gov/niosh/eNews. 

DHHS (NIOSH) Publication No. 2010–151 

May 2010 

SAFER • HEALTHIER • PEOPLETM 
Contents

Executive Summary ......................................................................................................................1 

Introduction ...................................................................................................................................3 

Comparison of International Rock Dusting Requirements ............................................................5 

Experimental Procedures..............................................................................................................8 

Size Data for Intake Airways ...............................................................................................15 

Size Data for Return Airways ..............................................................................................18 

MSHA Dust Survey Results from Intake and Return Airways.............................................18 

Limestone Rock Dust Inerting.............................................................................................19 

Summary.....................................................................................................................................21 

Recommendation........................................................................................................................22 

Acknowledgments.......................................................................................................................22 

References..................................................................................................................................23 

Appendix A: Analyses of Size of Coal Dust Particles from Mine Intake Airways ........................27 

Appendix B: Analyses of Size of Coal Dust Particles from Mine Return Airways .......................39 

Appendix C: Discussion of the Coal Dust and Rock Dust Properties and Experiments ............43 

Limestone Rock Dust Inerting Discussion...........................................................................44 

Effect of Particle Size on Coal Dust Explosibility ................................................................49 
Figures

Figure 1. Effect of particle size of coal dust on the explosibility of Pittsburgh seam bituminous coal as 

tested within BEM.................................................................................................................................5 

Figure 2. MSHA Coal Mine Safety and Health Districts, identified by number. ............................................8 

Figure 3. Original analyses of coal sieve size and analyses of sieve size of acid-leached mixture 

containing 30% medium-sized Pittsburgh coal and 70% limestone rock dust...................................10 

Figure 4. Original analyses of coal sieve size and analyses of sieve size of acid-leached mixture 

containing 30% medium-sized Pittsburgh coal, 60% limestone rock dust, and 10% kaolin clay.......10 

Figure 5. Plan view of the Lake Lynn Experimental Mine (LLEM)..............................................................12 

Figure 6. Side view of LLEM A-drift and D-drift test zones for determining rock dust inerting 

requirements.......................................................................................................................................13 

Figure 7. Placing coal and rock dust mixture on shelves in the LLEM. ...................................................... 14 

Figure 8. Distributing test dust mixture at the LLEM...................................................................................14 

Figure 9. Coal particle size by MSHA district..............................................................................................16 

Figure 10. Coal particle size by coal seam. ................................................................................................18 

Figure 11. Effect of particle size of coal dust on the explosibility of Pittsburgh seam bituminous coal 

as tested within LLEM. .......................................................................................................................19 

Figure C-1. Effect of particle size of coal dust on the explosibility of Pittsburgh seam bituminous coal as 

tested within LLEM. ............................................................................................................................49 
Tables

Table 1. Summary of rock dusting requirements for various nations ...........................................................6 

Table 2. Average coal sizes from intake airways in mines in 10 MSHA Safety and Health Districts .........16 

Table 3. Average coal particle size from intake airways for various coal seams........................................17 

Table A-1. Analyses of size of coal dust particles from intake airways in six MSHA District 2 mines.......28 

Table A-2. Analyses of size of coal dust particles from intake airways in seven MSHA District 3 mines...29 

Table A-3. Analyses of size of coal dust particles from intake airways in seven MSHA District 4 mines...30 

Table A-4. Analyses of size of coal dust particles from intake airways in six MSHA District 5 mines........31 

Table A-5. Analyses of size of coal dust particles from intake airways in five MSHA District 6 mines.......32 

Table A-6. Analyses of size of coal dust particles from intake airways in five MSHA District 7 mines.......33 

Table A-7. Analyses of size of coal dust particles from intake airways in six MSHA District 8 mines........34 

Table A-8. Analyses of size of coal dust particles from intake airways in seven MSHA District 9 mines...35 

Table A-9. Analyses of size of coal dust particles from intake airways in five MSHA District 10 mines.....36 

Table A-10. Analyses of size of coal dust particles from intake airways in seven MSHA District 11 

mines ..................................................................................................................................................37 

Table B-1. Analyses of size of coal dust particles from return airways in 36 mines...................................40 

Table B-2. Analyses of size of coal dust particles from return airways for seven Pittsburgh seam coal 

mines ..................................................................................................................................................42 

Table C-1. Pittsburgh seam coal dust sizes................................................................................................46 

Table C-2. Limestone rock dust sizes.........................................................................................................46 

Table C-3. Average proximate and ultimate analyses of coal used in the LLEM experiments .................. 47 

Table C-4. LLEM inerting tests for Pittsburgh seam coal dust and limestone rock dust using a 40 ft long 

ignition zone .......................................................................................................................................48 
ACRONYMS AND ABBREVIATIONS USED IN THIS REPORT 

% TIC Percentage of Total Incombustible Content 

ASTM American Society for Testing and Materials 

BEM Bruceton Experimental Mine 

CFR Code of Federal Regulations 

Dmed mass median diameter 

Ds surface mean diameter 

Dw mass or volume mean diameter 

hvb high volatile bituminous 

hvCb high volatile C bituminous 

LLEM Lake Lynn Experimental Mine 

LLL Lake Lynn Laboratory 

LTA low temperature ashing 

lvb low volatile bituminous 

MSHA Mine Safety and Health Administration 

mvb medium volatile bituminous 

NIOSH National Institute for Occupational Safety and Health 

NP non-propagation 

OMSHR Office of Mine Safety and Health Research 

P Propagation 

PC personal computer 

TIC Total Incombustible Content 
UNIT OF MEASURE ABBREVIATIONS

USED IN THIS REPORT 

 

Btu/lb British thermal unit per pound 

cm centimeter 

oC degree Celsius 

ft foot 

g/m3 gram per cubic meter 

hr hour 

kPa kilopascal 

m meter 

μm micrometer or micron 

mt/yr million ton per year 

ms millisecond 

% percent 

psi pound-force per square inch 

sec second 

ft2 square foot 

m2 square meter 
Dedication

This report was initially prepared by Kenneth L. Cashdollar and is dedicated to his memory. 

Ken passed away on March 4, 2009. Ken never wavered from his continuing commitment to 

conduct the highest quality, solution-oriented, scientific research focused on reducing the risk of 

explosion fatalities in the mining and chemical industries. 
 

Recommendations for a New Rock Dusting Standard 

to Prevent Coal Dust Explosions in Intake Airways 

Kenneth L. Cashdollar1, 

1 Principal Physical Scientist (deceased), OMSHR, NIOSH, Pittsburgh, PA. 

Michael J. Sapko2, 

2 Principal Physical Scientist (retired), OMSHR, NIOSH, Pittsburgh, PA. 

Eric S. Weiss3, 

3 Senior Mining Engineer (Team Leader, Lake Lynn Laboratory Research and Mine Rescue 

Team), OMSHR, NIOSH, Pittsburgh, PA.

Marcia L. Harris4, 

4 Research Chemical Engineer, OMSHR, NIOSH, Pittsburgh, PA. 

Chi-Keung Man5, 

5 Senior Service Fellow, OMSHR, NIOSH, Pittsburgh, PA. 

Samuel P. Harteis6, 

6 Lead Mining Engineer, OMSHR, NIOSH, Pittsburgh, PA. 

and Gregory M. Green7 

7 Physical Science Technician, OMSHR, NIOSH, Pittsburgh, PA. 

Executive Summary 

The workings of a bituminous coal mine produce explosive coal dust for which adding rock 

dust can reduce the potential for explosions. Accordingly, guidelines have been established by 

the Mine Safety and Health Administration (MSHA) about the relative proportion of rock dust 

that must be present in a mine’s intake and return airways. Current MSHA regulations require 

that intake airways contain at least 65% incombustible content and return airways contain at least 

80% incombustible content. The higher limit for return airways was set in large part because 

finer coal dust tends to collect in these airways. Based on extensive in-mine coal dust particle 

size surveys and large-scale explosion tests, the National Institute for Occupational Safety and 

Health (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. 

MSHA inspectors routinely monitor rock dust inerting efforts by collecting dust samples and 

measuring the percentage of TIC, which includes measurements of the moisture in the samples, 

the ash in the coal, and the rock dust. These regulations were based on two important findings: a 

survey of coal dust particle size that was performed in the 1920s, and large-scale explosion tests 

conducted in the U.S. Bureau of Mines’ Bruceton Experimental Mine (BEM) using dust particles 

of that survey’s size range to determine the amount of inerting material required to prevent 

explosion propagation. 

Mining technology and practices have changed considerably since the 1920s, when the 

original coal dust particle survey was performed. Also, it has been conclusively shown that as the 

size of coal dust particles decreases, the explosion hazard increases. Given these factors, NIOSH 

and MSHA conducted a joint survey to determine the range of coal particle sizes found in dust 

samples collected from intake and return airways of U.S. coal mines. Results from this survey 

show that the coal dust found in mines today is much finer than in mines of the 1920s. This 

increase in fine dust is presumably due to the increase in mechanization. 

In light of this recent comprehensive dust survey, NIOSH conducted additional large-scale 

explosion tests at the Lake Lynn Experimental Mine (LLEM) to determine the degree of rock 

dusting necessary to abate explosions. The tests used Pittsburgh seam coal dust blended as 38% 

minus 200 mesh and referred to as medium-sized dust. This medium-sized blend was used to 
represent the average of the finest coal particle size collected from the recent dust survey. 

Explosion tests indicate that medium-sized coal dust required 76.4% TIC to prevent explosion 

propagation. Even the coarse coal dust (20% minus 200 mesh or 75 μm), representative of 

samples obtained from mines in the 1920s, required approximately 70% TIC to be rendered inert 

in the larger LLEM, a level higher than the current regulation of 65% TIC. 

Given the results of the extensive in-mine coal dust particle size surveys and large-scale 

explosion tests, NIOSH recommends a new standard of 80% TIC be required in the intake 

airways of bituminous coal mines in the absence of methane. The survey results indicate that in 

some cases there are no substantial differences between the coal dust particle size distributions in 

return and intake air courses in today’s coal mines. The survey results indicate that the current 

requirement of 80% TIC in return airways is still appropriate in the absence of background 

methane. 

2 
 

Introduction 

Despite the worldwide research on coal mine safety, coal mine explosions involving fatalities 

and injuries still occur [Dobroski et al. 1996; McKinney et al. 2002; Light et al. 2007]. 

Experimental studies by the Office of Mine Safety and Health Research8 

8

The Pittsburgh Research Center was part of the U.S. Bureau of Mines until 1996, when it was 

transferred to the National Institute for Occupational Safety and Health (NIOSH) and became known as 

the Pittsburgh Research Laboratory. Since 2009, it is referred to as OMSHR.

(OMSHR) and similar 

agencies in other countries have shown that inert rock dust acts as a heat sink, and mixing a 

sufficient quantity of inert rock dust with coal dust will prevent coal dust explosions [Cybulski 

1975; Michelis et al. 1987, 1996; Reed et al. 1989; Lebecki 1991]. The U.S. mining law 

pertaining to rock dusting for the prevention of coal dust explosions was specified in the Federal 

Coal Mine Health and Safety Act of 1969 and was included in the Federal Mine Safety and 

Health Act of 1977 [U.S. Congress 1969 and 1977]. Current regulations are specified in Title 30, 

Part 75, Section 75.403 of the U.S. Code of Federal Regulations (CFR) [30 CFR9 

9 Code of Federal Regulations. See CFR in references. 

3 

2010]. Current 

regulations state that U.S. bituminous coal mines must maintain an incombustible content of at 

least 65% in the non-return (intake) airways and at least 80% in the return airways. Return 

airways require more inert material because there is greater risk of accumulation of finer coal 

dust. The U.S. regulations also require an additional 1.0% incombustible by weight for each 

0.1% of methane in the ventilating air inside intakes and 0.4% additional incombustible for each 

0.1% of methane in returns. 

The total incombustible content (TIC) includes measurements of the moisture in the samples, 

the ash in the coal, and the rock dust. The 65% TIC required for intake airways was adopted 

based on the results of two studies. First, coal dust samples were collected and measured to 

determine the average size of coal dust particles. Next, full-scale experimental mine tests were 

conducted to determine the amount of rock dust required for coal particles of the size collected in 

the survey to be rendered inert [Nagy 1981]. The term “mine-size dust” was adopted in the mid1920s and refers to coal dust that passes through a U.S. Standard 20-mesh sieve (850 μm) and 

contains 20% minus 200 mesh (75 μm). The justification for adopting this definition is given in 

Bureau of Mines Technical Paper 464 [Rice and Greenwald 1929]. Briefly, Technical Paper 464 

indicates that coal dust samples collected from the mine floors had 5% to 40% of the material 

minus 200 mesh (75 μm) and that the values were weighted. For 80% of mines, the final values 

ranged from 15% to 25% through 200 mesh. Therefore, coal dust having 20% passing through 

200 mesh was considered to be typical and termed “mine-size dust.” The authors of Technical 

Paper 464 acknowledge that dust collected from ribs, roof, and timbers was finer, with 40% to 

75% of the particles finer than 200 mesh, though they do not list the distribution of dust that 

would pass through sieves other than 200 mesh. Also missing from the report are details on the 

total number of mines surveyed and the total number of samples analyzed for coal particle size. 

Many years later, Public Law 552 (82nd Congress, 1952) required 65% incombustible content for 

most mines entries but it did not differentiate between intake and return areas. 

The quantities of rock dust required in the return airways in bituminous coal mines in the 

United States were increased to 80% by enactment of Public Law 91–173, the Federal Coal Mine 

Health and Safety Act of 1969. Section 304(a) mandated that coal dust shall be cleaned up and 

not permitted to accumulate in active workings or on electrical equipment. Paragraph (b) noted 

that when excessive dust is raised, water, water plus a wetting agent, or other no less effective 
agent shall be applied to abate dust, especially in distances less than 40 feet from the face to 

minimize explosion hazards. Paragraph (c) required that all underground areas where the 

incombustible content is too low shall be rock dusted to within 40 feet of the face. All crosscuts 

that are less than 40 feet from a working face shall also be rock dusted. Section 304(d) reads as 

follows: 

Where rock dust is required to be applied, it shall be distributed upon the top, floor, and 

sides of all underground areas of a coal mine and maintained in such quantities that the 

incombustible content of the combined coal dust, rock dust, and other dust shall be not less 

than 65 per centum, but the incombustible content in the return air courses shall be no less 

than 80 per centum. Where methane is present in any ventilating current, the per centum of 

incombustible of such combined dusts shall be increased 1.0 and 0.4 per centum for each 0.1 

per centum of methane, where 65 and 80 per centum respectively, of incombustibles are 

required. 

The aforementioned requirement of 80% TIC in return airways represents an increase over 

previous standards for return airways. The entire standard was based on earlier research with 

“mine-size dust.” The incombustible content needed to prevent propagation given a particular 

coal dust size is also dependent, to a lesser extent, on the volatility content of the coal. The 

decision to require all coal dusts except anthracite to have 65% TIC was made in 1927 by the 

Mine Safety Board. Decision No. 5, relating to rock dusting [Rice 1927], was superseded and 

clarified by Decision No. 32 [Mine Safety Board, 1937]. All Federal mine codes and laws since 

the mid-1920s have contained the same requirement. The requirement to have a 65% 

incombustible content for all coals except anthracite was made to simplify rock dusting practices. 

Coals that have a volatile ratio [volatile ratio = volatile content / (volatile content + fixed 

carbon)] of less than 0.2 provide a greater margin of explosion protection than coals having a 

volatile ratio higher than 0.2 [Nagy 1981]. 

The effect of coal particle size on explosibility is illustrated in Figure 1 as adapted from Rice 

et al. [1922] and Rice and Greenwald [1929]. This figure shows the amount of incombustible 

dust required to prevent propagation of an explosion for Pittsburgh high volatile bituminous coal 

dust with 10% to 80% passing through a 200 mesh (75 μm) sieve. Each of the data points is an 

individual explosion test conducted in the NIOSH-OMSHR Bruceton Experimental Mine 

(BEM). The curve is the boundary between mixtures that can propagate an explosion (below 

line) and mixtures that cannot propagate an explosion (above line). These data were used to 

support the 65% incombustible requirement for intake and return airways based on “mine-size 

dust” of the time. 

4 
Figure 1. Effect of particle size of coal dust on the explosibility of Pittsburgh seam bituminous 

coal as tested within BEM. 

Comparison of International Rock Dusting Requirements 

Rock dust has been used for about 100 years as a precautionary measure to protect against 

dust explosions. It is generally agreed that the effectiveness of rock dust lies in its ability to be 

simultaneously dispersed with coal dust, and, by serving as a heat sink, thus prevent flame 

propagation. Most leading coal-producing nations have similar requirements, some more 

stringent and some less stringent than those enforced in the United States. A partial listing of 

these requirements is given in Table 1. Passive barriers have been deployed in most leading coalproducing nations to provide supplemental protection against coal dust explosions. Conveyor 

belt entries have received emphasis. Barriers are designed to quench an explosion immediately 

on arrival at the location [Cybulski 1975, Liebman et al 1974, and Sapko et al 1989]. 

5 
 

 

 

 

 

 

 

 

 

 

 

 

Table 1. Summary of rock dusting requirements for various nations

Country TIC % Volatile matter % Methane % Comments 

Australia 

Queensland 

85–80 (return) 

85–70 (intake) 

— 

— 

85% TIC ≤ 200 m from the face 

80% TIC > 200 m from the face 

85% TIC ≤ 200 m from the face 

70% TIC > 200 m from the face 

Supplemental protection—barriers 

Australia 

NSW 

85–70 (return) 

80–70 (intake) 

— 

— 

85% TIC ≤ 200 m from the face 

70% TIC > 200 m from the face 

80% TIC ≤ 200 m from the face 

70% TIC > 200 m from the face 

Supplemental protection—barriers 

Canada 

(Nova Scotia) 

75 (intake) 

80 (return) 

— 

— 

<1 

>1 

Czech Republic 80 (intake/return) 

85 (intake/return) 

— 

— 

<1 

>1 

Supplemental protection—barriers 

Slovakia 80 (intake/return) 

85 (intake/return) 

— 

— 

<1 

>1 

Supplemental protection—barriers 

Germany 80 (intake/return) — Supplemental protection—barriers 

Japan 78 (intake/return) 

83 (intake/return) 

35 

35 

<1 

>1 

Specific requirements depend on 

ash, moisture and volatile content, 

the gassiness of the seam, and the 

fineness of the rock dust used. 

Poland 70 (intake/return) >10 

>10 

70% in “non-gassy” roadways 

80% in “gassy” roadways 

Supplemental protection—barriers 

South Africa 80 (intake) 

80 (return) 

— 

— 

80 % TIC ≤ 200 m from the face 

65% TIC > 200 m from the face 

80% TIC for 1000 m from the face 

Supplemental protection—barriers 

United Kingdom 50 (intake/return) 

65 (intake/return) 

72 (intake/return) 

75 (intake/return) 

20 

27 

35 

>35 

Supplemental protection—barriers 

United States 65 (intake) 

80 (return) 

— 

— 

1.0 / 0.1 

0.4 / 0.1 

Add 1% TIC / 0.1% methane 

Add 0.4% TIC / 0.1% methane 

6 
From 1985 through 2001, numerous coal dust explosion tests were conducted in the single 

entry D-drift at LLEM to determine the concentration of rock dust required to prevent explosion 

propagation of samples with varying coal dust particle sizes, volatilities, mine entry size, and 

other related properties. The LLEM drifts (20-ft or 6-m wide by 6.5-ft or 2-m high) are more 

representative of current U.S. underground coal mine geometries compared to the much smaller 

BEM entries (9-ft or 2.7-m wide by 6-ft or 1.8-m high). 

The factors that can influence the amount of admixed rock dust required to make coal dust 

inert include coal and rock dust particle size distribution, coal dust volatile content, and the 

additional presence of methane. Much knowledge has been obtained from experimental mine and 

laboratory dust explosion research during the past 3 decades. Investigators have examined the 

effects of rock dust inerting requirements, the minimum explosible coal dust concentrations, the 

effect of volatile matter on the explosibility of coal dusts, the effect of the size of coal and rock 

dust particles, and the effect of background methane in full-scale experimental mines and in 

laboratory test vessels [Sapko et al. 1987a, b; 1989; 1998; 2000; Cashdollar 1996; Cashdollar 

and Hertzberg 1989; Cashdollar and Chatrathi 1993; Cashdollar et al. 1987; 1988; 1992a, b, c; 

2007]. Further research evaluated the effects of pulverized versus coarse coal particle size [Weiss 

et al. 1989], coal volatility, extinguishment, and pyrolysis mechanisms [Hertzberg et al. 1987; 

1988a, b; Conti et al. 1991; Greninger et al. 1991]. The clear cumulative consensus of these 

studies is that dust particle size emerges as the single most influential factor controlling coal dust 

explosion propagation. Therefore, the primary focus of this research was to examine the effect of 

coal particle size of Pittsburgh coal while holding other factors constant. 

To determine compliance with current regulations, inspectors from MSHA periodically 

collect samples of deposited dust from various areas in a mine. The MSHA laboratory 

determines TIC and compares it with the TIC requirement. This TIC requirement is based on a 

mean coal particle size of 20% minus 200 mesh and assumed to be constant throughout the 

intake entries. The size of the coal dust component is not measured by MSHA laboratories as 

part of the explosibility assessment. 

This report presents the results of extensive in-mine coal dust particle size surveys of dust 

samples collected from intake airways in 61 U.S. coal mines, representing all 10 MSHA 

bituminous Coal Mine Safety and Health Districts (Figure 2). MSHA District 1 covers anthracite 

mines in Pennsylvania, which do not require rock dusting. A preliminary version of this research 

with data from 50 mines was published by Sapko et al. [2007]. Samples from return airways in 

36 mines were also size analyzed. A series of large-scale dust explosion tests was then conducted 

at the LLEM using the average of the finest coal particle size from the MSHA district intake 

survey results to determine the incombustible content necessary to prevent explosion 

propagation. 

7 
Figure 2. MSHA Coal Mine Safety and Health Districts, identified by number. 

Experimental Procedures 

To assess current variations in coal particle size from various underground coal mining 

operations, MSHA coordinated the acquisition of mine dust samples from the 10 bituminous 

Coal Mine Safety and Health Districts. The dust samples were among those routinely collected 

by mine inspectors to assess compliance with 30 CFR 75.403. The detailed sampling protocols 

are summarized in the General Coal Mine Inspection Procedures and Inspection Tracking 

System [MSHA 2008]. The samples were sent to the MSHA laboratory at Mt. Hope, WV, and 

analyzed for total incombustible content (TIC). The TIC includes measurements of the moisture 

in the samples, the ash in the coal, and the rock dust. The incombustible analysis procedure 

[Montgomery 2005] begins by passing the sample through a 20-mesh sieve (850 μm) and then 

oven drying the minus 20-mesh material for 1 hr at 105°C. The weight lost during drying 

constitutes the as-received-moisture in the sample. Next, the dried sample is heated in an oven 

that is ramped up over 1.5 hr and held at 515°C for about 2.5 hr to burn off the combustible coal 

fraction, thereby leaving the ash and incombustible material. This low temperature ashing (LTA) 

burns off the coal but does not decompose the limestone rock dust. The amount of the remaining 

ash material plus the as-received-moisture divided by the initial weight is reported as %TIC. 

Portions of each dust sample that were not needed for TIC measurement were sent to NIOSHOMSHR for the analyses of coal particle sizes. 

At OMSHR, the limestone (or marble) rock dust was leached from the sample using 

hydrochloric acid. In this leaching method used in the laboratory, dilute hydrochloric acid was 

added to the dust sample in a beaker and heated on a hotplate. The acid reacted with the 

limestone or marble rock dust, producing foam while releasing carbon dioxide. Sufficient acid 

was added until all foaming stopped. The hotplate kept the slurry near its boiling point for about 

1 hr. After the slurry cooled, the acid-insoluble residue was filtered from the acid. The solid 

residue was rinsed with water and isopropanol and then transferred to a large evaporating dish. 

The residue was dried at 110°C for 3 hr. Agglomerates were broken with a spatula. The residue 

consisted of coal plus other insoluble mineral matter. 

8 
The dried residue was then classified into the different size fractions using a sonic sieve, 

which provided particle separation by combining two motions—a vertical oscillating column of 

air, and a repetitive mechanical pulse. Occasionally the tops of the sieves were brushed to break 

up any remaining agglomerates. The sieves are 8 cm in diameter and include the following sizes: 

20 mesh (850 μm), 30 mesh (600 μm), 40 mesh (425 μm), 50 mesh (300 μm), 70 mesh (212 

μm), 100 mesh (150 μm), 140 mesh (106 μm), 200 mesh (75 μm), 270 mesh (53 μm), and 400 

mesh (38 μm). After the sieving was completed, the weight of sample on each sieve was 

recorded. 

Because the residue from the leaching process contained other inert mineral matter that did 

not react with the acid, a correction to the size analysis had to be made. First, the residue was 

grouped into three size fractions: minus 200 mesh, 200 - 70 mesh, and plus 70 mesh. At 

OMSHR, these three fractions were heated to 515°C to determine the incombustible or non-coal 

content, using an LTA method similar to that of the MSHA laboratory at Mt. Hope. The analyses 

of sieve size were then corrected for the non-coal content (insoluble mineral matter) in the three 

size groupings. The amount of this insoluble mineral matter in the samples varied greatly, but it 

was generally in the 20% to 50% range. For most of the samples analyzed, the insoluble mineral 

matter was finer than the coal particles. Therefore, after correction for the mineral matter, the 

corrected minus 200-mesh amount would be less than the original minus 200-mesh amount 

determined by sonic sieving alone. There was a wide range of correction values, but a value of 

39% minus 200 mesh from the original sieving data might typically be reduced to ~31% minus 

200 mesh after correcting for the mineral matter. Details of the size analyses, listing both original 

and corrected data, are included in the tables of Appendixes A and B. 

The total size analysis procedure (acid leaching, sieving, and correction for remaining 

incombustible matter) was verified by using prepared mixtures of coal and rock dust. First, the 

particle size distribution of the coal sample was determined by sieving. Next, samples of coal 

and rock dust were mixed together, and the rock dust was leached from the mixture. The residue 

was then sieved and corrected via LTA for any remaining incombustible matter in the size 

fractions. Data for a mixture of 30% medium-sized Pittsburgh seam high volatile coal and 70% 

limestone rock dust are shown in Figure 3. Both the cumulative and differential size distributions 

(by mass) are shown. A gold dashed vertical line shows the 200 mesh (75 μm) size and a dotdashed vertical orange line shows the 70 mesh (212 μm) size. Both the original coal (red data 

curves) and acid-leached residue from the mixture (blue data curves) had their size analyses 

corrected via LTA for any remaining incombustible matter. For this mixture, both the percentage 

through 200 mesh and the median size (50% point on the cumulative distribution curve) were 

almost identical for the original coal and the residue from the acid-leached mixture. Figure 4 

shows similar data for a mixture of 30% medium-sized Pittsburgh seam coal, 60% limestone 

rock dust, and 10% kaolin clay (to simulate possible shale dust in the sample). The original coal 

data are shown by the red curves and the acid-leached residue data from the mixture are shown 

by the blue curves. Figure 4 also shows close agreement for the percentage through 200 mesh 

and almost identical median values from the two cumulative curves. Original and acid-leached 

Blue Creek seam and Pocahontas seam samples were compared, but without any added rock 

dust. In general, the size analyses after leaching were within 1% to 3% of the amount of minus 

200 mesh material (data not shown). Therefore, there is no evidence that the acid-leaching 

procedure compromises the accuracy of the sieve analysis of the coal dust. 

9 
 

 

Figure 3. Original analyses of coal sieve size and analyses of sieve size of acid-leached mixture 

containing 30% medium-sized Pittsburgh coal and 70% limestone rock dust. 

 

 

Figure 4. Original analyses of coal sieve size and analyses of sieve size of acid-leached mixture 

containing 30% medium-sized Pittsburgh coal, 60% limestone rock dust, and 10% kaolin clay. 

10 
The large-scale explosion tests were conducted in the LLEM, which is shown in the plan 

view of Figure 5 [Triebsch and Sapko 1990]. This is a former limestone mine, and five new 

drifts (horizontal passageways in a mine) were developed to simulate the geometries of modern 

U.S. coal mines. The mine has four parallel drifts—A, B, C, and D. D-drift is a 1,640-ft-long 

(500-m) entry that can be separated from E-drift by an explosion-resistant bulkhead door. In 

order to simulate room and pillar workings, drifts A, B, and C can be used. These three drifts are 

approximately 1,600 ft long (490-m), with seven crosscuts at the inby end. Drifts C and D are 

connected by E-drift, a 500-ft-long (152-m) entry that simulates a longwall face. Explosion tests 

can be conducted in the single entry D-drift, the multiple entry area of A-, B-, and C-drifts, or 

various other configurations including the longwall E-drift. The entries are about 20 ft wide (6-

m) by about 6.5 ft high (2-m), with cross-sectional areas of 130–140 ft2 (12–13 m2

). The LLEM 

bulkhead door and some of the other infrastructure were designed to withstand explosion 

overpressures of up to 100 psi (7 bar or 700 kPa). Higher pressures have been recorded at areas 

away from these structures. Previous publications described the LLEM coal dust explosion test 

procedures and the results of LLEM explosion research and post-explosion observations [Weiss 

et al. 1989; Greninger et al. 1991; Cashdollar et al. 1992b, c; Sapko et al. 1998; 2000]. 

Each LLEM drift has 10 data-gathering stations inset in the rib, which houses a strain gauge 

transducer to measure the explosion pressure and an optical sensor to detect flame arrival. The 

wall pressure is perpendicular to the gas flow and is the pressure that is exerted in all directions. 

This quasi-static pressure is called the “static pressure” by Nagy [1981, p. 58] to differentiate it 

from the dynamic pressure, although the “static pressure” does vary with time during the 

explosion. The dynamic or wind pressure is directional. The total explosion pressure is the sum 

of the quasi-static pressure and the wind or dynamic pressure. Other instruments such as 

dynamic pressure sensors, heat flux gauges to measure explosion temperatures, optical probes to 

measure dust dispersion, and video cameras may be installed at various locations in the LLEM. 

During the explosion tests, a PC-based National Instruments data acquisition system collected 

the data from the various instruments at a sampling rate of 1,500 to 5,000 samples per second. 

11 
The LLEM dust explosion tests, described in this paper, were conducted in D-drift and more 

recently in a modified single entry section of A-drift. These drifts were isolated from E-drift by 

means of the explosion-resistant movable bulkhead doors (Figure 5). 

Figure 5. Plan view of the Lake Lynn Experimental Mine (LLEM). 

The tested coal dusts were 

prepared in the NIOSH coal grinding and pulverizing facilities located at the OMSHR facility at 

Bruceton. The coal and rock dust particle size data used in the LLEM explosion studies from the 

mid-1980s through 2008 are presented in Appendix C: Table C-1 and Table C-2, and coal 

analysis is presented in Table C-3. The size distributions of the limestone from the 1980s and 

from 2007 are similar, so comparisons of explosion inerting results from these periods are valid. 

The typical D-drift dust explosion test ignition zone (Figure 6) was located in the first 40-ft (12-

m) as measured from the face (closed end). This 10% methane air zone was ignited by electric 

matches. In the rock dust inerting tests, the coal dust and limestone rock dust mixture was placed 

half on roof shelves made of expanded polystyrene and half on the floor as illustrated in Figure 7 

and Figure 8. These roof shelves were suspended 1.5 ft (0.5 m) from the mine roof on 10-ft (3-

m) increments throughout the dust zone. This dust distribution technique, developed through 

extensive testing at BEM and LLEM, is used to enable reproducibility of experimental 

conditions. The length of the dust zones during these inerting tests in D-drift varied as follows: 

210, 270, 390, 420, 450, and 600 ft long (64, 82, 119, 128, 137, and 183 m). These dust zones 

started just outby the end of the 40-ft-long ignition zone, that is, the 210-ft-long dust zone 

12 
extended from 40 to 250 ft (12 m to 76 m) as measured from the face. Although the majority of 

the dust zones were 210 ft long, the longer dust zones were used for several reasons that differed 

depending on the experiment. The extension of flame travel through and beyond the longer dust 

zones for a particular incombustible content was always compared to a similar 210-ft-long dust 

zone to verify that the flame propagation was not being overdriven by the methane ignition zone 

(which would typically travel ~200 ft or ~61 m from the closed end). Non-propagation is defined 

as no sustained flame propagation of the dust mixture. Propagation is defined as flame 

propagation of the dust mixture. 

The nominal dust loading reported for the LLEM tests assumes that all of the dust was 

dispersed uniformly throughout the cross-section. For the LLEM tests, the test drift was 

thoroughly washed down after each test. Dehumidified air was passed through the entry, and the 

entry was allowed to dry several days before dust was loaded for the next test. 

13 

Figure 6. Side view of LLEMA-drift and D-drift test zones for determining rock dust inerting 

requirements. 
Figure 7. Placing coal and rock dust mixture on shelves in the LLEM. 

Figure 8. Distributing test dust mixture at the LLEM . 

14 
Size Data for Intake Airways 

For this study, a total of 217 samples of mine dust from intake airways of 61 coal mines in 

the 10 MSHA bituminous districts were analyzed for particle size. For each mine, samples were 

usually collected from two or more entries. For most analyses, multiple samples from a mine 

entry were combined to give an average size distribution for that entry. Most of the samples were 

band samples, also known as perimeter samples, but some were floor and rib samples, floor and 

roof samples, or floor-only samples [MSHA, 2008, p. 60]. The detailed size data for each sample 

and each mine are listed in the tables of Appendix A. The mines are identified only as A, B, C, 

etc., so that the individual mines remain anonymous. Columns three and four of the tables in 

Appendix A list the incombustible percentage (from the MSHA Mt. Hope Laboratory) and the 

soluble in acid percentage, as measured at NIOSH-OMSHR. Columns five and six of the tables 

list the original size analyses. Column seven lists the weighted average of the ash or 

incombustible fraction of the acid-leached material. The remaining columns list the corrected 

size analyses. Table 2 lists the summary intake coal dust size data by the MSHA Coal Mine 

Safety and Health District. Column two lists the states within each MSHA District from which 

samples were obtained. There may be additional states within some districts from which there 

were no samples obtained. Columns three and four of the table list the number of mines and total 

number of combined samples per district. Columns five through twelve list the average 

percentage through the various sieves. The column for minus 200 mesh (75 μm) lists both the 

average value and the associated standard deviation. The standard deviations for the other sieve 

values are listed in the tables of Appendix A. The last column lists the average and standard 

deviation for the mass median particle diameter (50% point on the cumulative distribution 

curve), which was interpolated from the corrected sieving data. The cumulative size data for 

MSHA Districts 3, 9, and 11 are shown in Figure 9. MSHA District 11 has the finest dust, with 

37% minus 200 mesh, and the western states (District 9) have the coarsest dust, with 27% minus 

200 mesh. District 3 (northern WV, OH, and MD) has an intermediate size. The averages for all 

MSHA Districts are 31% minus 200 mesh, 61% minus 70 mesh, and a mass median particle 

diameter of ~156 μm. This is finer than particles measured in the 1920s. 

15 
Table 2. Average coal sizes from intake airways in mines in 10 MSHA 

Safety and Health Districts 

District States Mines Samples 

−270 mesh 

or 

< 53 μm, 

% 

−200 mesh 

or 

< 75 μm, 

% 

−140 mesh

or 

< 106 μm, 

% 

−100 mesh 

or 

< 150 μm, 

% 

−70 mesh 

or 

< 212 μm, 

% 

−50 mesh 

or 

< 300 μm, 

% 

−40 mesh

or 

< 425 μm,

% 

−30 mesh 

or 

< 600 μm, 

% Dmed, μm 

2 PA 6 20 23 29 ± 4 37 47 59 72 85 95 165 ± 27 

3 OH, MD, 7 22 26 33 ± 9 41 51 62 74 87 96 149 ± 42 

No. WV 

4 So. WV 7 23 25 30 ± 6 38 48 60 73 87 97 165 ± 39 

5 VA 6 20 25 31 ± 8 40 50 62 74 86 96 157 ± 36 

6 Eastern KY 5 24 25 31 ± 7 39 49 59 72 85 96 160 ± 37 

7 Central KY 5 19 29 34 ±10 43 53 62 74 86 95 140 ± 48 

8 IN, IL 6 18 24 29 ± 5 37 47 57 71 85 96 170 ± 31 

9 CO, NM, UT 7 20 21 27 ± 3 36 46 57 71 85 96 172 ± 26 

10 Western KY 5 28 23 29 ± 4 39 50 61 74 86 96 152 ± 24 

11 AL 7 23 30 37 ± 10 48 60 73 84 92 97 128 ± 46 

10 Districts 217 25 31 40 50 61 74 86 96 156 Average 

16 

Figure 9. Coal particle size by MSHA district. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 3 lists the average coal dust particle sizes for intake airways for various coal seams or 

groups of adjacent coal seams. 

Table 3. Average coal particle size from intake airways for various coal seams 

Coal Seams States Mines Samples 

−270 

mesh 

or 

< 53 

μm, % 

−200 

mesh 

or 

< 75 

μm, % 

−140 

mesh 

or 

< 106 

μm, % 

−100 

mesh 

or 

< 150 

μm, % 

−70 

mesh 

or < 

212 

μm, % 

−50 

mesh 

or 

< 300 

μm, % 

−40 

mesh 

or 

< 425 

μm, % 

Dmed, 

μm 

Eastern Bituminous 

Coal Seams 

Pittsburgh, hvb 

Upper or Lower 

Kittanning, hvb 

PA,OH, 

WV 

PA, WV 

9 

3 

36 

6 

25 

20 

32 ± 7 

27 ± 7 

40 

34 

50 

43 

62 

54 

74 

67 

87 

82 

152 ± 34 

187 ± 42 

Eagle, hvb WV 2 5 20 25 ± 7 33 44 56 70 85 187 ± 44 

Powellton, hvb WV 2 7 24 28 ± 5 36 45 56 69 84 180 ± 36 

Pocahontas #3 & 

#5, lvb WV, VA 3 11 26 32 ± 6 40 50 61 73 86 154 ± 36 

Raven, hvb VA 2 6 27 35 ± 10 45 57 70 80 89 138 ± 44 

Alma, Cedar Grove, 

Darby, Upper Elkhorn #1 

or #3, hvb 

Hazard #4, hvb 

KY 

KY 

5 

2 

25 

8 

27 

34 

32 ± 7 

40 ± 12 

40 

49 

50 

60 

60 

69 

73 

78 

86 

87 

154 ± 38 

105 ± 43 

Pratt coal seam, hvb 

Blue Creek 

coal seam, mvb 

AL 

AL 

2 

5 

6 

17 

25 

31 

31 ± 5 

40 ± 10 

40 

50 

51 

63 

63 

76 

77 

86 

89 

93 

155 ± 34 

119 ± 47 

Mid-Eastern Bituminous 

Coal Seams 

Springfield, Illinois #5, 

or W. Kentucky #9 

Herrin, Illinois #6, 

or W. Kentucky #11 

KY, IL, 

IN 

KY, IL 

5 

4 

20 

14 

24 

21 

30 ± 5 

27 ± 3 

39 

36 

50 

47 

61 

58 

74 

71 

87 

84 

155 ± 29 

167 ± 25 

Western Bituminous 

Coal Seams 

various hvCb seams 

in Colorado 

various hvCb seams 

in Utah 

CO 

UT 

4 

2 

9 

4 

21 

18 

27 ± 3 

25 ± 3 

36 

33 

46 

44 

57 

58 

70 

73 

84 

88 

174 ± 29 

177 ± 15 

17 

The eastern bituminous coal seams are those in the Appalachian 

Mountains from Pennsylvania to Alabama. Only the seams that included samples from two or 

more mines are listed. The coal rank is also listed in the first column, with hvb, mvb, and lvb 

indicating high, medium, and low volatile bituminous coal, respectively [ASTM 2008]. The mideastern seams are those in Illinois, Indiana, and western Kentucky. These seams are known by 

different names in different states, as listed in the table. The western coal seams include various 

high volatile C bituminous (hvCb) coals in Colorado or Utah. The coal samples from the Hazard 

#4 seam in Kentucky and the Blue Creek seam in Alabama are the finest, with 40% of the 

samples less than 200 mesh. However, the Hazard seam data are based on samples from only two 

mines and may not represent the area as well as the Blue Creek seam data. The Pittsburgh seam 

coal in OH, PA, and WV has 32% minus 200 mesh. The cumulative size data for the Blue Creek, 

Pittsburgh, and Herrin coal seams are shown in Figure 10. 
Figure 10. Coal particle size by coal seam. 

Size Data for Return Airways 

For this study, a total of 44 samples of mine dust was taken from return airways of 36 coal 

mines in the 10 MSHA bituminous districts and were analyzed for particle size. Samples were 

collected from one or more entries in each mine. Similar to the intake airways, multiple samples 

from a mine entry were combined to give an average size distribution for that entry. Most of the 

samples were band samples, but some were floor and rib samples, floor and roof samples, or 

floor-only samples. The detailed size data for the return airways are listed in Table B-1 in 

Appendix B. For the returns, there was a much larger variation in the coal dust size. Many 

samples had percentages of minus 200 mesh dust, which were similar to those of the intake 

samples. However, 8 of the 44 samples had 60% to more than 80% minus 200 mesh. The only 

coal seam for which there were sufficient samples to calculate a representative average size was 

the Pittsburgh coal seam. The coal samples had an average of 62% minus 200 mesh (Table B-2 

in Appendix B), finer than the intake coal samples from the Pittsburgh seam. 

MSHA Dust Survey Results from Intake and Return Airways 

MSHA, from January 2005 to February 2008, collected and determined the TIC for 65,536 

intake and 60,663 return airway samples from underground coal mines. Each dust sample 

represents about 500 ft (152 m) of mine entry. The intake airways are currently required to 

contain at least 65% TIC. Approximately 87% contained ≥ 65% TIC, while ~13% contained < 

65% TIC and thus were non-compliant. The fact that ~13% of the samples collected were found 

18 
to be non-compliant illustrates the scope of the problem. Considering that each sample may 

represent up to 500 ft (152 m) of mine entry, these ~13%, or 8,323 samples, represent more than 

788 miles (1,268 km) of underground coal mine entries that were deficient. At the other extreme, 

66% of the intake samples contained more than 80% TIC, and ~54% contained more than 85% 

TIC. This indicates that rock dusting efforts exceed requirements in a majority of samples, 

because the average TIC among all samples was ~82% TIC. 

A similar TIC distribution is observed for return airway samples. Current MSHA regulations 

require 80% TIC for return airways. Analysis of 60,663 samples revealed that ~72% of samples 

contained ≥ 80% TIC while ~28% contained < 80% TIC. The average TIC for return samples 

was 85%, which is ~3% higher than the intake average of ~82%. 

The MSHA dust survey data indicate that many areas have more than sufficient inert 

material. However, there are still a large number of areas where rock dusting efforts are 

insufficient to prevent coal dust explosions. 

Limestone Rock Dust Inerting 

Prior to having recent access to the MSHA band samples collected from underground coal 

mines throughout the United States, there was growing evidence from limited dust surveys that 

the coal dust particle size had been decreasing since the promulgation of the existing rock 

dusting regulations. This decrease occurred as new mining technologies were adopted by the 

industry. Numerous coal dust explosion tests have been conducted in the LLEM to specifically 

quantify the concentration of rock dust required to prevent propagation of a high volatile coal as 

a function of coal dust particle size. Table C-4 shows a composite of these experiments. Details 

of these experiments can be found in Table C-4 in Appendix C along with a discussion 

highlighting the specific experimental results. 

19 

Figure 11. Effect of particle size of coal dust on the explosibility of Pittsburgh seam bituminous 

coal as tested within LLEM. 
Following the coal dust survey, additional large-scale explosion experiments were conducted 

using medium-sized dust (38% minus 200 mesh or 75 microns—Table C-1) to better define the 

boundary between explosion propagation and non-propagation. Medium-sized dust was 

formulated with a blend of 2008 pulverized and 2008 coarse dust (Table C-1) of Pittsburgh seam 

coal to represent the average of the finer dusts collected from the survey. However, 

approximately 12% of the collected intake airway dust samples (26 of the 217 samples) ranged 

in size from 39 to 63% minus 200 mesh. These finer than medium-sized coal dust samples were 

collected from mines in 7 of the 10 MSHA Districts and represented approximately 26% of the 

overall mines sampled (16 of the 61 mines). 

The results of the LLEM large-scale explosion tests including the medium-sized coal dust 

are shown in Figure 11. Given the experimental test conditions, the curve is the boundary 

between mixtures that did propagate an explosion (below line) and mixtures that did not 

propagate an explosion (above line). The coal dust particle size has a substantial impact on the 

propagation potential for coal dust. As the coal dust particle size decreases, increasing amounts 

of rock dust are necessary to render the coal/rock dust mixture inert. The greatest impact is 

evident between the particle size of the coarse (20% minus 200 mesh or 75 μm) coal dust and the 

pulverized (80% minus 200 mesh or 75 μm) coal dust. To ensure non-propagation within the 

LLEM, the coarse coal dust required at least 70% TIC and the pulverized coal dust required 

greater than a 79% TIC and less than a 81.5% TIC. Once the 80% minus 200 mesh benchmark 

had been reached, no additional TIC was required to prevent flame propagation with further 

decrease in coal dust particle size. One can clearly see when comparing Figure 1 with the earlier 

BEM data to Figure 11 of the recent LLEM data that the TIC increases from about 60% to 70% 

TIC at the coarse coal particle size end of the figures, while TIC remains at about 80% at the fine 

coal particle size end of both figures. 

The 80% limit is also consistent with explosion temperature thermodynamic limit models for 

coal and rock dust put forward by Richmond et al. [1975; 1979], Hertzberg et al. [1988], Conti et 

al. [1991], and Sapko et al. [2000]. The models were essentially based on a thermal balance 

between the heat generated during the combustion of coal dust and heat absorbed by the 

incombustible material. 

LLEM inerting studies using a medium-sized coal dust showed that at least 76.4% TIC 

(Table C-4) is required to prevent explosion propagation. If one considers the finest size intake 

air way dust collected during the recent survey (63% minus 200 mesh from Table A-2), data in 

Figure 11 indicates that approximately 80% TIC would be required to prevent explosion 

propagation. 

20 
Summary 

Dust explosibility is strongly dependent on the size distribution of the coal particles in a coal 

and rock dust mixture. Underground coal mining technology has changed since the 1920s; that 

is, coal mining has become highly mechanized, creating coal dust with more small size fractions 

than those of the 1920s. Despite this change in technology, particle size surveys from the early 

1900s are still being used as the basis for current rock dusting regulations. Although total 

incombustible content is an important determinant of explosion propagation, coal dust particle 

size also needs to be considered as an essential part of an explosibility assessment in 

underground coal mines. The present coal size study indicates that the coal dust in intake airways 

of U.S. mines is finer than that measured by Rice and Greenwald [1929] in the 1920s. Moreover, 

particle size distributions can vary with coal seam and rank, as shown in Table 2. Current rock 

dust regulations mandating a 65% TIC dust mixture do not fully protect miners since LLEM tests 

have shown that even a ~68% TIC dust mixture with coarse Pittsburgh seam coal dust (20% 

minus 200 mesh) will propagate dust explosions. LLEM inerting experiments also demonstrated 

that at least 76.4% TIC is required to prevent explosion propagation for medium-sized coal dust 

(38% minus 200 mesh)—that is, an average of the finer dust found in modern intake areas. For 

return airways, the current requirement of at least 80% TIC is still sufficient in the absence of 

methane. 

LLEM experiments for high volatile coals have also shown that the TIC required to prevent 

flame propagation becomes much less dependent on coal particle size as the TIC approaches and 

exceeds 80%. Therefore, experimental results support at least an 80% TIC requirement for both 

intake and return airways in the absence of methane. 

21 
Recommendation 

Large-scale explosion testing in the Bruceton and Lake Lynn Experimental Mines confirm 

intake airways require more incombustible content to render the coal dust inert than the 65% TIC 

specified in current regulations. 

NIOSH recommends an 80% TIC in intake airways based on: 

• Explosion temperature thermodynamic limit models for coal and rock dust mixtures, 

• Extensive in-mine coal dust particle size surveys, and 

• Multiple explosion experiments at the Lake Lynn Laboratory. 

Acknowledgments 

The authors acknowledge the invaluable technical guidance from Harry Verakis, General 

Engineer, of the MSHA Approval and Certification Center, Triadelphia, WV, and Terry 

Montgomery, Supervisor Chemist, of the MSHA analytical laboratory, Mt. Hope, WV, for 

providing the dust samples and the incombustible percentage data. We acknowledge the many 

MSHA inspectors for collecting the dust samples in coal mines throughout the United States. 

The authors thank E. Cathy Hedrick, Information Technology Specialist, of MSHA for providing 

the intake and return sample data over the past 3 years. The authors also acknowledge the 

laboratory assistance of the following NIOSH-OMSHR personnel for their efforts in the acidleaching and size analysis of the dust samples: Kenneth Helfrich, Physical Science Technician, 

and Electronic Technicians Jon Jobko and Andrew Mazzella. 

The authors also thank the following NIOSH-OMSHR personnel whose contributions made 

the 2008 LLEM inerting tests possible: William Slivensky, Frank Karnack, Donald Sellers, and 

James Addis, Physical Science Technicians at LLL, for their participation in the installation of 

sensors, the construction of the methane ignition zones, the installation of the gas sampling lines, 

the dispersal of the dust mixtures for each test, performing experiments, and post-explosion dust 

sampling; Kenneth Jackson, Electronics Technician at LLL, for sensor calibrations, operation of 

the data acquisition systems, and data analyses; and Paul Stefko, Foreman I – Miner Mechanic, 

of the Safety Research Coal Mine (SRCM) at OMSHR; Jack Teatino and Joseph Sabo, SRCM 

Miner Mechanics, for mining and then grinding and/or pulverizing the coal to the proper particle 

size and preparing the coal/rock dust mixtures. 

The authors acknowledge the following mechanical technicians with Ki Corporation (a 

NIOSH contractor) for their thorough cleanup of the debris and the wash down of the entry 

following each large-scale explosion test: Timothy Glad (lead), James Rabon, Bernard Lambie, 

and Brandin Lambie. 

22 
References 

ASTM International [2008]. Standard classification of coals by rank, D388–05. In: Annual Book 

of ASTM Standards, vol. 5.06. West Conshohocken, PA: ASTM International. 

Cashdollar KL [1996]. Coal dust explosibility. J Loss Prev Process Ind 9(1):65–76. 

Cashdollar KL, Chatrathi K [1993]. Minimum explosible dust concentrations measured in 20-L 

and 1-m3 chambers. Combustion Science and Technology, 87(1–6):157–171. 

Cashdollar KL, Hertzberg M [1989]. Laboratory study of rock dust inerting requirements: effects 

of coal volatility, particle size, and methane addition. In: Proceedings of the 23rd International 

Conference of Safety in Mines Research Institutes (Washington, DC, September 11–15, 1989). 

Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, pp. 965–977. 

Cashdollar KL, Hertzberg M, Zlochower IA [1988]. Effect of volatility on dust flammability 

limits for coals, gilsonite, and polyethylene. In: Proceedings of the 22nd International Symposium 

on Combustion. The Combustion Institute, Pittsburgh, PA, 1988, pp. 1757–1765. 

Cashdollar KL, Sapko MJ, Weiss ES, Hertzberg M [1987]. Laboratory and mine dust explosion 

research at the Bureau of Mines. In: Cashdollar KL, Hertzberg M, eds. Industrial dust 

explosions. West Conshohocken, PA: American Society for Testing and Materials (ASTM), 

Special Technical Publication (STP) 958, pp. 107–123. 

Cashdollar KL, Weiss ES, Greninger NB, Chatrathi K [1992a]. Laboratory and large-scale dust 

explosion research. Plant/Operations Progress, 11, pp. 247–255. 

Cashdollar KL, Weiss ES, Greninger NB, Chatrathi K [1992b]. Laboratory and large-scale dust 

explosion research. In: Proceedings of the 26th Annual Loss Prevention Symposium (New 

Orleans, LA). Paper No. 72C. AIChE Spring National Meeting, 22 pp. 

Cashdollar KL, Weiss ES, Greninger NB, Hertzberg M, Sapko MJ [1992c]. Laboratory and 

large-scale explosion research at the U.S. Bureau of Mines. In: Proceedings of The First World 

Seminar on the Explosion Phenomenon and on the Application of Explosion Protection 

Techniques in Practice (Brussels, Belgium). Section 1, 16 pp. 

Cashdollar KL, Weiss ES, Montgomery TG, Going JE [2007]. Post-explosion observations of 

experimental mine and laboratory coal dust explosions. J Loss Prev Process Ind 20(1):607–615. 

CFR, Code of Federal Regulations. Washington, DC: U.S. Government Printing Office, Office 

of the Federal Register. 

Conti RS, Zlochower IA, Sapko MJ [1991]. Rapid sampling of products during coal mine 

explosions. Combustion Science and Technology, 75(4–6):195–209. 

Cybulski WG [1975]. Coal dust explosions and their suppression. Translated from Polish. 

Warsaw, Poland: National Center for Scientific, Technical and Economic Information. NTIS No. 

TT 73–54001. 

23 
Dobroski H Jr., Stephan CR, Conti RS [1996]. Historical summary of coal mine explosions in 

the United States. 1981–1994. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, 

IC 9440. 

Federal Coal Mine Safety Act of 1952 (Public Law 82–552). 

Greninger NB, Cashdollar KL, Weiss ES, Sapko MJ [1991]. Suppression of dust explosions 

involving fuels of intermediate and high volatile content. In: Proceedings of the Fourth 

International Colloquium on Dust Explosions, Polish Academy of Sciences, Porabka-Kozubnik, 

Poland, November 4–9, 1990, pp. 208–228. 

Hertzberg M, Cashdollar KL [1987]. Introduction to dust explosions. In: Cashdollar KL, 

Hertzberg M, eds. Industrial dust explosions. West Conshohocken, PA: American Society for 

Testing and Materials (ASTM), Special Technical Publication (STP) 958, pp. 5–32. 

Hertzberg M, Zlochower IA, Cashdollar KL [1988a]. Volatility model for coal dust flame 

propagation and extinguishment. In: Proceedings of the 21st International Symposium on 

Combustion (University of Munich, Germany). Pittsburgh, PA: The Combustion Institute, pp. 

325–353. 

Hertzberg M, Zlochower IA, Edwards JC [1988b]. Coal pyrolysis mechanisms and temperatures. 

Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, RI 9169. 

Lebecki K [1991]. Gas dynamics of coal dust explosion—theory and experiment. In: 

Proceedings of the 24th International Conference of Safety in Mines Research Institutes 

(Donetsk, USSR, September 23–28), vol. 1, pp. 357–373. 

Liebman I, Richmond JK [1974]. Suppression of coal dust explosions by passive water barriers 

in a single entry mine. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, RI 

7815. 

Light TE, Herndon RC, Guley AR Jr., Cook GL, Odum MA, Bates RM Jr., Schroeder ME, 

Campbell CD, Pruitt ME [2007]. Report of investigation, fatal underground coal mine 

explosions, May 20, 2006. Darby mine No. 1, Kentucky Darby LLC, Holmes Mill, Harlan 

County, Kentucky, ID No. 15–18185. Arlington, VA: U.S. Department of Labor, Mine Safety 

and Health Administration. 

McKinney R, Crocco W, Stricklin KG, Murray KA, Blankenship ST, Davidson RD, Urosek JE, 

Stephan CR, Beiter DA [2002]. Report of investigation, fatal underground coal mine explosions, 

September 23, 2001. No. 5 Mine, Jim Walters Resources, Inc., Brookwood, Tuscaloosa County, 

Alabama, ID No. 01–01322. Arlington, VA: U.S. Department of Labor, Mine Safety and Health 

Administration. 

Michelis J [1996]. Large scale experiments with coal dust explosions in connection with road-Tjunctions. In: The Seventh International Colloquium on Dust Explosions; part of the 

International Symposium on Hazards, Prevention and Mitigation of Industrial Explosions 

(Bergen, Norway, June 23–26, 1996). Christian Michelsen Research, pp. 8.50–8.59. 

24 
Michelis J, Margenburg B, Müller G, Kleine W [1987]. Investigations into the buildup and 

development conditions of coal dust explosions in a 700-m underground gallery. In: Cashdollar 

KL, Hertzberg M, eds. Industrial dust explosions. West Conshohocken, PA: American Society 

for Testing and Materials (ASTM), Special Technical Publication (STP) 958, pp. 124–137. 

Mine Safety Board [1937]. Recommendations of the United States Bureau of Mines on certain 

questions of safety as of Oct. 1, 1936. Pittsburgh, PA: U.S. Department of the Interior, Bureau of 

Mines, IC 6946. 

Montgomery T [2005]. Personal communication on incombustible analysis procedures. 

MSHA [2008] Handbook Series, Handbook Number PH–08–V–1, General Coal Mine Inspection 

Procedures and Inspection Tracking System, January 1, 2008, pp. 45–66. 

http://www.msha.gov/readroom/handbook/handbook.htm

Nagy J [1981]. The explosion hazard in mining. Pittsburgh, PA: U.S. Department of Labor, Mine 

Safety and Health Administration, IR 1119. 

Reed D, Michelis J [1989]. Comparative investigation into explosibility of brown coal and 

bituminous coal dust in surface and underground test installations. In: Proceedings of the 23rd 

International Conference of Safety in Mines Research Institutes (Washington, DC, September 

11–15, 1989). Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, pp. 941–964. 

Rice GS, Greenwald HP [1929]. Coal-dust explosibility factors indicated by experimental mine 

investigations 1911 to 1929. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, 

Technical Paper 464. 

Rice GS, Jones LM, Egy WL, Greenwald HP [1922]. Coal-dust explosion tests in the 

experimental mine 1913 to 1918, inclusive. Pittsburgh, PA: U.S. Department of the Interior, 

Bureau of Mines, Bulletin 167. 

Rice GS [1927]. Effective rock-dusting of coal mines. Pittsburgh, PA: U.S. Department of the 

Interior, Bureau of Mines, IC 6039. 

Richmond JK, Liebman I, Miller LF [1975]. Effect of rock dust on the explosibility of coal dust. 

Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, RI 8077, 34 pp. 

Richmond JK, Liebman I, Bruszak AE, Miller LF [1979]. A physical description of coal mine 

explosions, part II. Seventeenth symposium (international) on combustion. Pittsburgh, PA: The 

Combustion Institute, pp. 1257–1268. 

Sapko MJ, Weiss ES, Watson RW [1987a]. Explosibility of float coal dust distributed over a 

coal-rock dust substratum. In: Proceedings of the 22nd International Conference of Safety in 

Mines Research Institutes (Beijing, China, November 2–6, 1987). Beijing, China: China Coal 

Industry Publishing House, pp. 459–468. 

25 
Sapko MJ, Weiss ES, Watson RW [1987b]. Size scaling of gas explosions: Bruceton 

Experimental Mine versus the Lake Lynn Mine. Pittsburgh, PA: U.S. Department of the Interior, 

Bureau of Mines, RI 9136. NTIS No. PB 88–230248. 

Sapko MJ, Greninger NB, Watson RW [1989]. Review paper: Prevention and Suppression of 

Coal Mine Explosions. In: Proceedings of the 23rd International Conference of Safety in Mines 

Research Institutes (Washington, DC, September 11–15, 1989). Pittsburgh, PA: U.S. Department 

of the Interior, Bureau of Mines, pp. 791–807. 

Sapko MJ, Weiss ES, Cashdollar KL, Zlochower IA [1998]. Experimental mine and laboratory 

dust explosion research at NIOSH. In: Proceedings of the International Symposium on Hazards, 

Prevention, and Mitigation of Industrial Explosions: Eighth International Colloquium on Dust 

Explosions (Schaumburg, IL, September 21–25, 1998). Safety Consulting Engineers, pp. 120– 

142. 

Sapko MJ, Cashdollar KL, Green GM [2007]. Coal dust particle size survey of U.S. mines. J 

Loss Prev Process Ind, 20(1): 616–620. 

Sapko M J, Weiss ES, Cashdollar KL, Zlochower IA [2000]. Experimental mine and laboratory 

dust explosion research at NIOSH. J Loss Prev Process Ind. 13(1): 229–242. 

Triebsch G, Sapko MJ [1990]. Lake Lynn Laboratory: a state-of-the-art mining research 

laboratory. In: Proceedings of the International Symposium on Unique Underground Structures, 

vol. 2, Golden, CO, June 11–15, 1990. Colorado School of Mines, pp. 75–1 to 75–21. 

U.S. Code of Federal Regulations [2010]. Title 30 CFR, Part 75, Section 75.400 and 75.403. 

U.S. Code of Federal Regulations [2010]. Title 30 CFR, Part 75, Section 75.2. 

U.S. Congress [1969]. Federal Coal Mine Health and Safety Act of 1969. Public Law 91–173, 83 

Stat. 742, pp. 32–33. 

U.S. Congress [1977], Coal Mine Safety and Health Act of 1977, Public Law 95–164, Section 

304, p. 49. 

Weiss ES, Greninger NB, Sapko MJ [1989]. Recent results of dust explosion studies at the Lake 

Lynn Experimental Mine. In: Proceedings of the 23rd International Conference of Safety in 

Mines Research Institutes (Washington, DC, September 11–15, 1989). Pittsburgh, PA: U.S. 

Department of the Interior, Bureau of Mines, pp. 843–856. 

26 
AppendixA:

Analyses of Size of Coal Dust Particles 

from Mine Intake Airways 

27 
Table A-1. Analyses of size of coal dust particles from intake airways in six MSHA District 2 mines 

Mine Production,

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 μm, 

% 

−70 mesh 

or < 212 μm, 

% 

Ash, 

% 

Corrected size analysis 

−270 mesh 

or < 53 μm, 

% 

−200 mesh 

or < 75 μm, 

% 

−140 mesh 

or < 106 μm, 

% 

−100 mesh 

or < 150 μm, 

% 

−70 mesh or 

< 212 μm, 

% 

−50 mesh or 

< 300 μm, 

% 

−40 mesh or 

< 425 μm, 

% 

−30 mesh or 

< 600 μm, 

% 

Dmed, 

μm 

A >1 74 55 35 64 40 21 25 33 44 56 69 83 92 178 

 81 73 43 70 42 27 34 44 53 64 76 89 97 136 

B >1 54 52 38 71 22 25 32 43 55 68 80 91 97 130 

 60 42 31 61 27 19 25 35 45 57 70 84 95 173 

 82 69 41 72 40 25 31 41 52 64 76 89 97 143 

 56 24 37 60 37 25 29 36 45 55 67 82 94 180 

 72 61 33 59 30 22 27 34 44 54 67 82 94 186 

 85 40 48 79 73 22 31 40 50 60 77 89 97 151 

C >1 86 58 31 62 23 21 26 35 44 57 72 85 95 176 

 88 79 46 74 46 26 32 43 55 67 77 87 95 130 

 75 48 35 62 47 23 27 35 44 57 70 86 97 177 

 64 30 27 50 49 18 21 27 35 46 60 79 94 237 

D >1 70 51 39 65 36 23 30 36 44 55 68 82 93 184 

 93 85 35 71 39 23 28 38 50 63 73 84 94 150 

 67 42 34 59 38 24 28 34 44 55 70 85 96 182 

 50 24 46 71 30 33 38 44 54 66 78 90 98 130 

E <1 75 66 38 64 21 30 36 43 53 63 73 84 95 135 

F <1 90 57 30 65 70 17 24 33 43 55 69 80 90 186 

 90 73 28 61 58 15 22 31 41 54 70 86 96 191 

 88 69 39 66 58 23 31 38 48 58 71 85 96 159 

average for MSHA District 2 23 29 37 47 59 72 85 95 165 

standard deviation 4 4 5 5 5 5 3 2 27 

Notes: 

The incombustible content is the value measured by the MSHA Mt. Hope laboratory. 

The soluble content is the percentage that is soluble in hydrochloric acid (i.e., the calcium carbonate content of the limestone or marble rock dust), as measured at OMSHR. 

The ash includes the ash in the coal plus the insoluble mineral material, as measured at OMSHR. 

28 
29 

Table A-2. Analyses of size of coal dust particles from intake airways in seven MSHA District 3 mines 

Mine 

Production, 

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 

μm, % 

−70 mesh 

or < 212 

μm, % 

ash, 

% 

Corrected size analysis 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 mesh 

or < 106 

μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh 

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

−30 mesh 

or < 600 

μm, % 

Dmed, 

μm 

A >1 55 27 29 61 21 20 27 35 46 60 75 90 98 165 

B >1 68 47 41 68 37 26 30 37 47 59 74 88 97 164 

 70 44 31 61 46 17 22 30 40 53 70 86 97 199 

C >1 82 57 39 66 56 24 31 38 47 57 71 86 96 169 

 97 96 42 71 35 28 38 48 58 70 75 85 93 113 

 95 95 67 84 25 52 63 71 77 81 86 93 98 50 

 90 81 47 77 44 28 36 45 57 66 78 89 97 123 

 87 73 51 77 55 26 32 39 49 56 69 82 94 160 

 86 72 37 71 47 23 32 41 52 63 78 91 98 141 

 88 77 45 74 47 32 40 46 54 63 77 89 97 125 

D >1 83 81 45 76 20 32 39 49 61 72 82 91 97 108 

 77 68 52 82 23 38 45 55 66 78 88 94 98 89 

 91 74 40 72 55 20 25 35 47 59 73 87 95 164 

 72 55 42 67 30 27 32 39 49 60 72 86 96 156 

 46 11 37 62 33 24 29 36 45 56 69 84 96 175 

 41 10 34 62 32 23 28 36 46 57 71 87 97 171 

E >1 80 59 46 82 44 27 34 46 60 74 86 95 99 117 

 79 63 32 66 33 20 26 35 47 62 75 86 93 161 

F >1 83 75 43 75 40 25 32 43 54 66 78 89 97 134 

 75 67 43 69 50 25 31 41 49 60 70 83 94 155 

G >1 58 39 29 55 23 21 28 34 43 54 65 79 92 189 

 72 63 20 44 19 13 18 24 32 42 56 75 92 259 

average for MSHA District 3 26 33 41 51 62 74 87 96 149 

standard deviation 8 9 10 10 9 7 5 2 42 

Notes: The incombustible content is the value measured by the MSHA Mt. Hope laboratory. 

The soluble content is the percentage that is soluble in hydrochloric acid (i.e., the calcium carbonate content of the limestone or marble rock dust), as measured at OMSHR. 

The ash includes the ash in the coal plus the insoluble mineral material, as measured at OMSHR. 
30 

Table A-3. Analyses of size of coal dust particles from intake airways in seven MSHA District 4 mines 

Mine 

Production, 

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 

μm, % 

−70 mesh 

or < 212 

μm, % 

ash, 

% 

Corrected size analysis 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 mesh 

or < 106 

μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh 

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

−30 mesh 

or < 600 

μm, % 

Dmed, 

μm 

A >1 65 51 42 70 30 26 34 44 54 64 75 86 96 131 

 68 54 36 59 26 22 28 35 43 52 65 80 94 195 

 64 35 45 69 43 29 36 44 54 64 75 87 97 133 

 70 61 33 63 25 22 29 37 48 61 73 87 97 160 

 89 83 52 80 35 34 41 50 62 74 85 95 99 106 

 85 81 43 68 25 29 35 42 52 64 77 90 98 138 

B <1 82 40 40 63 61 24 30 36 45 55 68 82 94 181 

 75 — 37 65 65 23 28 36 45 56 70 85 96 176 

C <1 68 18 30 54 59 21 25 32 41 52 65 82 96 199 

 70 15 38 59 61 25 28 35 43 53 66 81 95 192 

 69 21 32 56 57 18 22 27 36 47 61 79 94 231 

 81 35 34 60 70 20 25 33 42 53 69 85 97 196 

 78 33 38 60 68 26 29 35 44 56 67 83 96 179 

D <1 79 38 38 60 63 24 28 34 44 54 66 82 95 186 

E <1 47 9 47 82 37 29 36 47 61 75 88 95 99 114 

 57 30 25 54 29 16 21 28 37 48 63 79 94 224 

 37 3 32 59 27 20 25 33 43 54 67 81 94 188 

 53 18 24 59 40 12 16 23 33 48 68 86 97 221 

F >1 77 58 45 71 48 31 36 44 55 65 78 91 98 129 

 87 73 44 70 52 29 34 43 53 63 77 91 98 137 

G <1 40 23 45 84 23 35 43 53 67 83 93 97 99 98 

 37 16 43 75 22 29 36 45 57 71 85 95 99 123 

 49 — 33 69 29 24 30 39 51 67 83 93 98 147 

average for MSHA District 4 25 30 38 48 60 73 87 97 165 

standard deviation 6 6 7 9 10 9 6 2 39 

Notes: The incombustible content is the value measured by the MSHA Mt. Hope laboratory. 

The soluble content is the percentage that is soluble in hydrochloric acid (i.e., the calcium carbonate content of the limestone or marble rock dust), as measured at OMSHR. 

The ash includes the ash in the coal plus the insoluble mineral material, as measured at OMSHR. 
31 

Table A-4. Analyses of size of coal dust particles from intake airways in six MSHA District 5 mines 

Mine 

Production, 

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 

μm, % 

−70 mesh 

or < 212 

μm, % 

ash, 

% 

Corrected size analysis 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 mesh 

or < 106 

μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

−30 mesh 

or < 600 

μm, % 

Dmed, 

μm 

A <1 64 43 32 58 34 25 31 39 49 60 72 84 95 156 

 57 28 26 52 44 23 29 36 45 57 68 82 94 173 

 68 60 38 61 32 28 34 41 50 59 71 85 96 150 

B >1 66 57 47 72 30 27 30 39 49 61 73 86 96 154 

 63 63 52 78 20 40 47 55 65 75 83 92 98 87 

 87 85 31 56 39 18 26 33 42 51 64 81 94 203 

 48 43 37 62 26 24 31 39 50 60 72 85 95 151 

 35 24 23 48 26 16 21 27 37 47 61 78 92 227 

 65 56 36 62 32 27 33 40 51 62 75 88 97 145 

C <1 83 73 60 94 29 40 54 70 86 93 96 98 99 68 

 69 54 38 77 28 26 32 42 56 75 88 95 98 132 

 78 61 44 83 32 30 36 48 64 80 90 95 98 110 

D <1 52 29 33 59 31 21 28 35 45 56 69 83 95 176 

 57 28 34 62 34 21 27 34 45 57 70 84 96 175 

E >1 82 76 35 62 27 23 29 36 46 57 68 81 93 173 

 72 58 32 65 21 22 28 36 46 62 76 89 98 164 

 67 52 26 62 25 21 22 36 45 58 70 84 96 172 

F <1 77 58 38 61 43 27 32 38 47 58 69 84 96 166 

 80 67 35 60 43 25 30 37 46 57 69 83 96 170 

 76 63 30 58 33 16 26 34 44 55 67 81 94 183 

average for MSHA District 5 25 31 40 50 62 74 86 96 157 

standard deviation 6 8 9 11 11 9 5 2 36 

Notes: The incombustible content is the value measured by the MSHA Mt. Hope laboratory. 

The soluble content is the percentage that is soluble in hydrochloric acid (i.e., the calcium carbonate content of the limestone or marble rock dust), as measured at OMSHR. 

The ash includes the ash in the coal plus the insoluble mineral material, as measured at OMSHR. 
32 

Table A-5. Analyses of size of coal dust particles from intake airways in five MSHA District 6 mines 

Mine 

Production, 

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 

μm, % 

−70 mesh 

or < 212 

μm, % 

ash, 

% 

Corrected size analysis 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 mesh 

or < 106 

μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh 

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

−30 mesh 

or < 600 

μm, % 

Dmed, 

μm 

A >1 63 31 37 64 41 20 25 33 42 53 66 83 96 195 

 54 26 38 61 35 23 28 35 43 54 66 81 95 188 

 51 14 33 58 45 21 26 33 42 53 66 82 95 193 

B <1 36 17 28 56 18 19 25 31 41 52 65 82 96 200 

 40 22 28 56 20 18 23 29 37 50 65 82 96 214 

 37 20 38 63 17 27 33 40 48 58 69 82 94 164 

 35 21 40 64 17 29 35 42 50 60 71 85 96 150 

 35 19 35 62 18 23 29 36 46 57 70 83 94 173 

 37 20 38 64 19 27 32 39 48 58 72 87 97 160 

 73 60 42 70 27 27 33 41 51 62 75 87 96 145 

C >1 77 50 30 55 56 17 22 28 37 48 64 81 95 220 

 73 20 42 65 65 28 32 39 50 61 73 86 97 150 

 73 24 35 59 64 18 24 30 39 49 64 81 96 215 

D >1 76 25 46 82 67 25 29 38 51 63 77 89 97 145 

 76 29 47 72 69 27 34 45 56 64 73 85 97 124 

 74 21 42 75 67 22 30 38 48 61 78 89 97 161 

 71 17 50 77 65 28 33 41 52 62 74 84 95 142 

 72 12 52 83 67 29 34 45 59 69 79 87 96 120 

E <1 84 81 60 79 30 45 55 63 70 77 83 92 98 64 

 84 75 50 79 34 34 42 51 62 71 81 90 97 102 

 64 47 40 64 26 29 34 41 50 59 70 84 96 151 

 86 77 36 69 42 23 29 37 49 61 75 88 97 155 

 56 41 37 67 21 26 32 39 50 62 77 90 98 150 

 56 41 36 62 23 26 32 39 48 59 71 85 96 162 

average for MSHA District 6 25 31 39 49 59 72 85 96 160 

standard deviation 6 7 8 8 7 6 3 1 37 

Notes: The incombustible content is the value measured by the MSHA Mt. Hope laboratory. 

The soluble content is the percentage that is soluble in hydrochloric acid (i.e., the calcium carbonate content of the limestone or marble rock dust), as measured at OMSHR. 

The ash includes the ash in the coal plus the insoluble mineral material, as measured at OMSHR. 
33 

Table A-6. Analyses of size of coal dust particles from intake airways in five MSHA District 7 mines 

Mine 

Production, 

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 

μm, % 

−70 mesh 

or < 212 

μm, % 

ash, 

% 

Corrected size analysis 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 mesh 

or < 106 

μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh 

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

−30 mesh 

or < 600 

μm, % 

Dmed, 

μm 

A <1 79 65 41 67 51 23 29 37 46 55 68 82 95 175 

 79 65 44 74 50 24 29 39 51 60 74 87 96 147 

 81 62 41 68 52 22 29 38 48 58 72 85 96 164 

 78 60 37 65 46 21 27 35 45 55 69 84 95 179 

B <1 92 80 62 78 63 52 56 63 69 75 80 84 90 46 

 92 83 66 82 63 49 54 62 69 76 82 88 94 59 

 92 78 63 83 62 48 54 61 69 78 82 88 94 60 

C <1 89 62 44 77 65 20 27 37 50 61 74 87 97 149 

 87 66 55 83 59 29 37 47 59 71 84 93 98 117 

 96 87 45 74 74 24 28 37 52 63 72 85 95 143 

 90 78 59 86 61 29 36 43 58 69 81 90 97 124 

 91 78 45 70 59 22 29 42 51 59 71 85 96 144 

D <1 61 24 39 64 49 22 27 35 45 55 69 84 95 179 

 74 28 36 63 60 20 26 33 42 53 67 82 95 195 

 77 38 38 66 58 19 25 32 40 52 66 82 95 200 

E <1 88 69 59 85 59 31 35 47 58 67 78 88 96 117 

 91 74 64 83 62 36 41 49 58 66 76 87 96 110 

 82 70 34 61 39 19 23 29 39 49 65 81 94 215 

 84 64 57 75 53 33 37 44 53 62 74 86 96 136 

average for MSHA District 7 29 34 43 53 62 74 86 95 140 

standard deviation 10 10 10 9 8 6 3 2 48 

Notes: The incombustible content is the value measured by the MSHA Mt. Hope laboratory. 

The soluble content is the percentage that is soluble in hydrochloric acid (i.e., the calcium carbonate content of the limestone or marble rock dust), as measured at OMSHR. 

The ash includes the ash in the coal plus the insoluble mineral material, as measured at OMSHR. 
34 

Table A-7. Analyses of size of coal dust particles from intake airways in six MSHA District 8 mines 

Mine 

Production, 

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 

μm, % 

−70 mesh 

or < 212 

μm, % 

ash, 

% 

Corrected size analysis 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 mesh 

or < 106 

μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh 

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

−30 mesh 

or < 600 

μm, % 

Dmed, 

μm 

A >1 92 81 27 54 64 14 18 25 35 47 59 76 92 234 

 97 93 38 66 52 21 27 37 48 60 70 83 95 160 

B >1 82 49 49 69 51 23 27 34 42 51 64 80 94 208 

 81 49 53 77 57 24 29 39 48 58 70 83 95 161 

C >1 75 45 42 71 46 24 27 35 45 58 73 86 97 171 

 68 30 43 79 56 22 27 38 51 65 78 91 98 145 

D >1 67 32 50 68 46 29 33 39 47 57 68 84 96 167 

 78 57 53 74 46 33 38 45 54 65 77 90 98 130 

 65 21 41 63 51 20 26 33 41 52 68 84 96 198 

 68 33 46 69 45 25 30 39 48 58 70 84 95 162 

E >1 84 19 47 73 77 26 30 37 49 59 73 89 98 156 

 82 23 41 66 75 25 30 36 46 56 71 86 97 175 

 76 22 43 67 66 25 29 36 46 57 69 85 96 172 

 79 23 49 70 68 27 30 37 46 55 68 83 96 178 

F >1 86 63 55 80 63 30 33 43 54 65 78 89 96 132 

 73 43 50 74 43 27 32 41 50 60 72 85 95 149 

 88 63 54 83 57 24 30 44 56 67 81 92 98 127 

 67 25 36 61 44 15 17 24 34 47 62 80 95 230 

average for MSHA District 8 24 29 37 47 57 71 85 96 170 

standard deviation 5 5 5 6 6 6 4 2 31 

Notes: The incombustible content is the value measured by the MSHA Mt. Hope laboratory. 

The soluble content is the percentage that is soluble in hydrochloric acid (i.e., the calcium carbonate content of the limestone or marble rock dust), as measured at OMSHR. 

The ash includes the ash in the coal plus the insoluble mineral material, as measured at OMSHR. 
35 

 ,

Table A-8. Analyses of size of coal dust particles from intake airways in seven MSHA District 9 mines 

Mine 

Production, 

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 

μm, % 

−70 mesh 

or < 212 

μm, % 

ash

% 

Corrected size analysis 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 mesh 

or < 106 

μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh 

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

−30 mesh 

or < 600 

μm, % 

Dmed, 

μm 

A >1 84 74 42 71 48 21 28 36 47 56 69 83 94 170 

 59 27 35 62 40 19 26 34 44 54 67 82 94 187 

 88 77 39 71 49 23 31 40 51 61 75 88 97 147 

 81 65 44 70 44 27 33 42 53 61 74 87 96 135 

B >1 83 70 32 62 26 19 26 34 45 56 70 85 97 176 

C >1 92 85 46 74 45 23 30 42 52 61 74 87 97 139 

 60 53 35 63 25 20 26 34 44 57 71 87 97 178 

 71 53 45 71 38 25 31 41 51 63 75 89 97 146 

 53 25 42 66 33 24 28 36 46 56 70 84 95 173 

 85 81 40 68 39 21 27 39 49 59 72 86 96 153 

D >1 81 87 34 63 36 16 22 30 40 49 62 77 92 220 

 78 72 37 64 22 23 30 37 47 58 70 84 95 166 

 78 72 38 70 21 25 31 41 52 64 78 90 98 141 

 82 77 33 61 23 20 26 33 43 54 66 82 95 190 

E >1 76 68 35 61 15 25 30 37 46 57 68 83 94 172 

 53 53 25 50 12 17 21 28 36 47 60 78 93 232 

F <1 40 31 26 55 9 17 24 31 41 53 68 84 95 196 

 56 49 30 63 11 20 28 36 48 61 75 90 99 159 

 47 34 29 59 10 20 27 34 44 56 70 86 97 179 

G <1 54 55 26 64 12 16 21 30 43 60 79 92 98 174 

average for MSHA District 9 21 27 36 46 57 71 85 96 172 

standard deviation 3 3 4 4 4 5 4 2 26 

Notes: The incombustible content is the value measured by the MSHA Mt. Hope laboratory. 

The soluble content is the percentage that is soluble in hydrochloric acid (i.e., the calcium carbonate content of the limestone or marble rock dust), as measured at OMSHR. 

The ash includes the ash in the coal plus the insoluble mineral material, as measured at OMSHR. 
36 

Table A-9. Analyses of size of coal dust particles from intake airways in five MSHA District 10 mines 

Mine 

Production, 

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 

μm, % 

−70 mesh 

or < 212 

μm, % 

ash, 

% 

Corrected size analysis 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 mesh 

or < 106 

μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh 

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

−30 mesh 

or < 600 

μm, % 

Dmed, 

μm 

A >1	 67 49 42 73 41 25 30 39 50 62 77 90 98 148 

 89 76 44 80 47 26 31 43 57 70 83 94 99 126 

 72 na 42 73 41 23 29 39 50 59 71 84 95 147 

 62 39 43 69 35 26 31 40 49 58 71 85 95 154 

 74 52 41 72 44 23 27 37 48 58 74 88 96 161 

 67 57 37 71 35 23 28 38 50 62 74 86 96 150 

 89 82 52 82 41 29 41 56 66 75 85 94 99 93 

 58 27 42 78 43 25 31 41 54 66 80 91 98 135 

 56 32 50 81 40 14 22 37 52 65 79 90 97 144 

 66 34 45 72 49 23 27 35 45 52 65 80 93 195 

B >1	 86 74 37 71 41 21 27 36 48 60 73 85 94 159 

 85 75 35 67 37 22 27 37 49 61 72 85 96 154 

 84 72 37 71 35 25 30 41 51 62 75 87 96 145 

 89 80 35 71 37 21 26 36 48 61 76 87 96 159 

 87 78 36 69 29 22 27 37 48 60 73 86 96 159 

 82 68 31 67 37 18 24 32 43 55 69 83 94 184 

C >1	 75 52 35 63 38 21 26 33 43 53 66 81 94 196 

 83 67 47 76 42 26 31 41 53 64 75 86 96 137 

 89 77 54 80 45 27 38 52 60 67 77 87 96 100 

 68 34 43 77 44 25 32 43 55 66 78 88 96 129 

 79 47 33 70 56 17 24 32 43 53 68 81 94 195 

 92 78 44 78 54 23 30 40 52 62 75 88 97 140 

D >1	 86 74 36 67 42 22 28 38 49 59 71 83 92 156 

 75 59 37 68 37 21 28 39 48 59 72 85 96 159 

 86 70 34 70 42 19 25 38 49 59 72 87 96 153 

 83 64 39 73 45 21 28 40 51 62 76 88 96 145 

E >1	 75 51 48 71 51 27 31 38 47 57 69 82 94 168 

 71 42 46 69 43 27 30 37 46 56 70 83 94 171 

average for MSHA District 10 23 29 39 50 61 74 86 96 152 

standard deviation 3 4 5 5 5 5 4 2 24 

Notes: The incombustible content is the value measured by the MSHA Mt. Hope laboratory. 

The soluble content is the percentage that is soluble in hydrochloric acid (i.e., the calcium carbonate content of the limestone or marble rock dust), as measured at OMSHR. 

The ash includes the ash in the coal plus the insoluble mineral material, as measured at OMSHR. 
37 

Table A-10. Analyses of size of coal dust particles from intake airways in seven MSHA District 11 mines 

Mine 

Production, 

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 

μm, % 

−70 mesh 

or < 212 

μm, % 

ash, 

% 

Corrected size analysis 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 mesh 

or < 106 

μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh 

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

−30 mesh 

or < 600 

μm, % 

Dmed, 

μm 

A >1 90 78 46 72 53 23 27 35 44 55 69 83 95 185 

 91 79 47 79 57 25 31 41 52 66 80 91 98 141 

B >1 89 82 35 62 31 23 28 35 45 55 69 83 95 180 

 54 30 31 57 32 21 28 35 44 55 67 81 94 184 

C >1 85 77 49 80 39 34 40 50 62 75 89 96 99 106 

 86 80 51 83 33 35 42 52 65 78 90 96 99 99 

D >1 94 92 61 97 25 41 53 69 88 96 98 99 100 70 

 58 41 40 71 28 29 36 45 55 68 84 95 99 128 

E >1 71 63 55 91 21 40 50 63 78 90 96 98 99 76 

 89 78 42 73 25 29 36 45 56 70 84 94 99 126 

 84 77 51 91 25 34 45 60 77 90 95 97 98 86 

 91 84 43 66 38 26 31 38 47 57 68 81 93 168 

 90 83 34 56 32 19 23 30 38 48 60 75 90 224 

F >1 68 63 47 93 10 33 45 60 79 92 98 99 100 85 

 72 64 56 94 20 41 52 68 83 93 97 98 99 71 

 62 55 47 85 16 35 45 59 74 86 93 97 99 85 

 55 43 43 80 20 35 44 48 63 78 91 97 99 112 

 66 64 28 70 7 19 27 36 50 69 90 98 99 149 

 45 31 54 92 18 42 51 63 77 91 98 99 99 72 

G <1 47 28 35 64 22 24 29 37 46 58 75 90 98 169 

 59 44 36 67 25 23 29 38 49 61 77 92 98 156 

 58 38 50 87 29 33 42 55 71 84 94 98 99 94 

 40 16 32 56 31 23 28 35 44 54 67 82 94 185 

average for MSHA District 11 30 37 48 60 73 84 92 97 128 

standard deviation 7 10 12 15 15 12 7 3 46 

Notes: The incombustible content is the value measured by the MSHA Mt. Hope laboratory. 

The soluble content is the percentage that is soluble in hydrochloric acid (i.e., the calcium carbonate content of the limestone or marble rock dust), as measured at OMSHR. 

The ash includes the ash in the coal plus the insoluble mineral material, as measured at OMSHR. 
38 
AppendixB:

Analyses of Size of Coal Dust Particles 

from Mine Return Airways 

39 
40 

Table B-1. Analyses of size of coal dust particles from return airways in 36 mines 

Mine 

Production, 

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 

μm, % 

−70 mesh 

or < 212 

μm, % 

ash, 

% 

Corrected size analysis 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 mesh 

or < 106 

μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh 

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

−30 mesh 

or < 600 

μm, % 

Dmed, 

μm 

1 >1 86 74 83 92 24 74 83 87 90 93 95 97 98 ~30 

 80 74 63 80 34 55 62 66 73 79 86 94 99 44 

 87 76 72 88 41 62 69 74 79 83 88 93 98 42 

2 >1 68 53 40 62 45 33 37 42 48 58 71 87 97 155 

3 >1 63 40 57 72 35 44 47 52 59 66 75 85 95 91 

 75 54 76 85 43 68 71 74 77 82 89 95 99 ~20–25 

4 <1 77 69 28 55 19 22 27 33 42 54 66 81 94 188 

5 >1 82 79 85 93 12 78 84 88 90 93 95 98 99 ~36 

6 >1 80 75 59 76 15 52 58 63 68 75 82 90 97 49 

7 >1 91 75 52 82 65 46 52 58 67 76 87 96 99 67 

8 >1 72 45 63 78 43 52 57 62 68 75 82 91 98 45 

9 >1 85 78 42 72 26 29 36 45 57 69 80 91 97 122 

10 <1 46 14 38 83 32 24 30 40 55 79 95 99 100 135 

11 >1 75 60 33 62 38 22 26 34 44 57 72 88 98 176 

12 <1 37 24 27 54 18 19 24 32 42 53 66 80 93 193 

13 <1 70 58 38 64 36 30 35 42 52 64 75 88 97 140 

14 >1 71 75 42 68 22 30 35 43 53 63 75 87 96 137 

15 >1 83 83 47 73 32 31 37 45 56 67 78 89 97 124 

16 >1 76 54 41 64 49 24 27 34 42 54 68 84 96 190 

17 >1 72 19 42 63 61 26 30 36 44 55 67 82 95 184 

18 <1 50 29 32 56 27 24 30 37 45 55 67 82 95 178 

19 <1 92 78 75 90 62 60 64 69 77 83 90 96 99 30 

20 <1 89 68 36 62 60 20 25 33 43 54 68 85 97 189 

21 <1 81 14 43 75 79 27 33 38 47 57 76 90 98 171 

22 <1 86 62 56 75 65 32 35 43 52 61 72 84 94 141 

23 >1 83 53 53 74 59 27 31 37 46 57 72 88 97 170 

24 >1 64 40 38 63 132 22 29 36 45 56 69 83 95 178 

25 >1 62 22 42 63 40 24 28 34 42 52 66 83 96 199 

 56 22 43 65 37 25 29 35 44 55 69 87 97 182 

26 >1 89 79 82 93 59 64 71 77 81 84 90 95 99 30 

 70 70 47 69 28 33 40 47 56 64 75 87 95 121 

 Continued on Next Page 
41 

Mine 

Production, 

Mt/yr 

Incombustible, 

% 

Soluble, 

% 

Size analysis 

−270 mesh 

or < 53 

μm, % 

−70 mesh 

or < 212 

μm, % 

ash, 

% 

Corrected size analysis 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 mesh 

or < 106 

μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh 

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

−30 mesh 

or < 600 

μm, % 

Dmed, 

μm 

27 >1 79 69 46 73 29 31 37 45 56 67 80 91 98 124 

28 >1 77 74 36 60 19 23 28 35 44 53 66 81 93 189 

29 <1 66 53 30 54 12 21 26 32 40 50 63 80 94 211 

30 <1 61 40 35 57 34 23 28 33 41 51 64 80 94 208 

 62 16 39 66 54 25 30 37 46 57 73 88 97 171 

31 >1 88 81 36 62 39 20 26 35 44 51 64 79 93 201 

32 <1 65 21 44 70 52 25 29 35 44 54 69 83 95 186 

33 <1 96 95 81 89 60 65 71 76 80 83 88 93 98 ~25–30 

34 >1 94 90 58 79 36 40 46 54 62 72 82 91 97 91 

 93 88 50 79 26 33 40 49 61 74 87 95 99 109 

 88 82 47 66 42 27 32 38 45 54 66 80 92 183 

35 >1 39 25 38 66 17 28 33 41 50 62 77 91 98 148 

36 <1 41 26 26 55 16 18 23 30 39 52 69 87 97 203 

average for all MSHA Districts 35 41 47 55 65 76 88 97 132 

standard deviation 17 17 16 14 12 10 6 2 62 

Notes: The incombustible content is the value measured by the MSHA Mt. Hope laboratory. 

The soluble content is the percentage that is soluble in hydrochloric acid (i.e., the calcium carbonate content of the limestone rock dust), as measured at OMSHR. 

The ash includes the ash in the coal plus the insoluble mineral material, as measured at OMSHR. 
42 

Table B-2. Analyses of size of coal dust particles from return airways for seven Pittsburgh seam coal mines 

States Mines Samples 

−270 mesh 

or < 53 

μm, % 

−200 mesh 

or < 75 

μm, % 

−140 

mesh or < 

106 μm, % 

−100 mesh 

or < 150 

μm, % 

−70 mesh 

or < 212 

μm, % 

−50 mesh 

or < 300 

μm, % 

−40 mesh 

or < 425 

μm, % 

Dmed, 

μm 

PA,WV 7 10 56 62 ± 15 67 72 78 85 93 58 ± 39 
Appendix C:

Discussion of the Coal Dust and 

Rock Dust Properties and Experiments 

43 
Limestone Rock Dust Inerting Discussion 

From 1985 through 2001, numerous LLEM coal dust explosion tests were conducted in the 

single entry D-drift, and more recently in A-drift (2008), to determine the concentration of rock 

dust required to prevent explosion propagation as a function of coal dust particle size, volatility, 

and other related issues (Table C-1 through Table C-3). 

During the LLEM tests with the pulverized Pittsburgh seam coal dust (~80% minus 200 

mesh or 75 μm), the total incombustible content (TIC) required to prevent an explosion 

propagation was greater than 79% but less than 81.5%. This determination was based on a series 

of 12 explosion tests (Table C-4) [Cashdollar et al. 1987; 1992a,c; Weiss et al. 1989; Greninger 

et al. 1991; Sapko et al. 1989; 1998; 2000]. In two of these tests (LLEM tests #51 and #401), the 

flame ended well within the dust zone. In the three tests (LLEM tests #70, #255, and #386) 

where the TIC was 79%, the flame travel extended to or slightly beyond the end of the dust zone. 

The other 7 tests resulted in flame travel well beyond the dust zone. Non-propagation is defined 

as no sustained flame propagation of the dust mixture. Propagation is defined as flame 

propagation of the dust mixture. 

During the LLEM tests with the coarse Pittsburgh seam coal dust (~20% minus 200 mesh or 

75 μm) [Sapko et al. 1989; Weiss et al. 1989; Greninger et al. 1991], a 70% TIC dust mixture 

prevented an explosion propagation (LLEM test #191). A TIC of ~68% resulted in a propagating 

explosion (LLEM test #71). 

Prior to having recent access to the MSHA band samples collected from underground coal 

mines throughout the country, there was growing evidence from limited dust surveys that the 

coal dust particle size had been decreasing since the promulgation of the existing rock dusting 

regulations. This decrease occurred as new mining technologies were adopted by the industry 

(e.g., mining methods involving increased mechanization). For this reason, several explosion 

tests involving intermediate–sized coal dust particles were conducted within the LLEM. One test 

(LLEM test #88) involved the use of medium-sized Pittsburgh seam coal dust (~45% minus 200 

mesh or 75 μm). To achieve this coal dust blend, pulverized coal dust was added to the coarse 

dust. For this single test, the medium-sized coal dust was mixed with rock dust to result in a 

~67% TIC for the coal/rock dust mixture. Upon ignition of the methane zone, this mixture 

resulted in a propagating explosion. 

Additional tests were later conducted with a blend of pulverized and fine coal dust to provide 

an average coal dust particle size ranging from 83% to 85% minus 200 mesh or 75μm. This 

pulverized-fine dust mixture, when mixed with rock dust to result in a ~79% TIC dust mixture, 

resulted in a propagation (LLEM test #357 and #387). A non-propagation resulted using an 

81.6% TIC pulverized-fine coal dust mixture (LLEM test #353). The results from these tests 

were similar to the tests with the pulverized coal (80% minus 200 mesh or 75μm). 

One additional test (LLEM test #388) was conducted with a finer Pittsburgh seam coal dust 

(95% minus 200 mesh or 75 μm). A propagation resulted after using a ~79% TIC fine coal dust 

mix. 

Based on the LLEM explosion tests, the coal dust particle size has a substantial impact on the 

propagation potential for a coal dust. As the coal dust particle size decreases, increasing amounts 

of rock dust are necessary to render the coal/rock dust mixture inert. The greatest impact is 

evident between the particle size of the coarse (20% minus 200 mesh or 75 μm) coal dust and the 

pulverized (80% minus 200 mesh or 75 μm) coal dust. To ensure non-propagation within the 

44 
LLEM, the coarse coal dust required at least 70% TIC and the pulverized coal dust required 

greater than 79% and less than 81.5% TIC. 

During the first test (LLEM test #517) with the medium-sized coal dust (38% minus 200 

mesh or 75 μm), a 74% TIC dust mixture resulted in a propagation. Two tests (LLEM tests #518 

and #522) were conducted with a ~76% TIC dust mixture and resulted in a non-propagation. The 

results of these medium-sized coal dust inerting tests are summarized in Table C-4. 

45 
46 

Table C-1. Pittsburgh seam coal dust sizes 

Size Year 

−400 mesh or 

< 38 μm, % 

−200 mesh or 

< 75 μm, % 

−100 mesh or 

< 150 μm, % 

−50 mesh or 

< 300 μm, % 

−30 mesh or 

< 600 μm, % DS, μm DW, μm Dmed, μm 

Pittsburgh seam 

bituminous coal from PA 

Coarse 1980s 9 19 30 43 65 96 440 380 

Coarse 2008 10 20 34 53 82 84 320 270 

Medium 2008 16 38 61 79 100 55 166 104 

Pulverized 1980s 41 80 99 100 100 28 48 45 

Pulverized 2008 30 69 98 100 100 33 60 57 

Pulverized-fine 1999 64 85 97 100 100 21 40 31 

Fine 1999 86 95 98 100 100 17 24 <38 

Table C-2. Limestone rock dust sizes 

Size Year 

−400 mesh or 

< 38 μm, % 

−200 mesh or 

< 75 μm, % 

−100 mesh or 

< 150 μm, % 

−50 mesh or 

< 300 μm, % 

−30 mesh or 

< 600 μm, % DS, μm DW, μm Dmed, μm 

Pulverized 1980s 62 76 95 100 100 14 47 24 

Pulverized 2007 54 72 98 100 100 10 51 26 
Table C-3. Average proximate and ultimate analyses of 

coal used in the LLEM experiments 

Proximate analysis 

Moisture 

Volatile matter 

Fixed carbon 

Ash 

Total 

Ultimate analysis 

Hydrogen 

Carbon 

Nitrogen 

Oxygen 

Sulfur 

Ash 

Total 

Pittsburgh Coal 

As received, % 

1.7 

36.5 

55.6 

6.2 

100.0 

5.4 

77.4 

1.5 

8.1 

1.4 

6.2 

100.0 

47 

Heating value = 13,803 Btu/lb 
48 

Table C-4. LLEM inerting tests for Pittsburgh seam coal dust and limestone rock dust using a 40 ft long ignition zone 

LLEM test 

no. –entry Date 

Coal dust 

Size −200 Mesh, % Zone, ft Conc., g/m3 

Rock dust, 

% 

Total Incombustible, 

% 

Flame 

travel, 

ft Result 

49–D 7/17/85 pulverized ~80 40–250 200 70.0 72.3 750 P 

50−D 7/25/85 pulverized ~80 40–250 200 75.0 77.1 500 P 

51−D 8/1/85 pulverized ~80 40–250 200 80.0 81.5 200 NP 

53−D 9/4/85 pulverized ~80 40–640 200 75.0 77.1 750 P 

69−D 4/24/86 pulverized ~80 40–250 200 73.0 75.2 600 P 

70−D 5/1/86 pulverized ~80 40–250 200 77.0 78.8 300 P 

71−D 5/8/86 coarse ~20 40–250 200 65.0 67.8 390 P 

77−D 8/6/86 coarse ~20 40–250 200 50.0 54.0 500 P 

83−D 10/9/86 pulverized ~80 40–250 200 65.0 67.8 750 P 

87−D 11/20/86 coarse ~20 40–250 200 60.0 63.2 600 P 

88−D 11/25/86 medium ~45 40–250 200 65.0 67.2 750 P 

90−D 1/8/87 pulverized ~80 40–430 200 65.0 67.8 750 P 

190−D 6/21/89 coarse ~20 40–310 200 73.0 75.0 175 NP 

191−D 7/12/89 coarse ~20 40–310 200 67.7 70.0 200 NP 

255−D 1/16/91 pulverized ~80 40–490 200 77.2 79.0 445 P 

352−D 9/30/97 pulv/fine ~83 40–250 200 83.0 84.4 150 NP 

353−D 10/27/97 pulv/fine ~83 40–250 200 80.0 81.6 200 NP 

357−D 12/17/97 pulv/fine ~83 40–250 200 77.0 78.8 300 P 

386−D 9/8/99 pulverized 72 40–310 200 77.0 78.8 300 P 

387−D 9/15/99 pulv/fine 85 40–310 150 77.0 78.8 300 P 

388−D 9/23/99 fine 95 40–310 150 77.0 78.8 300 P 

398−D 3/1/01 pulverized ~80 40–460 200 65.0 67.2 750 P 

401−D 3/28/01 pulverized ~80 40–460 200 80.0 81.6 200 NP 

512−A 1/9/08 pulverized 69 40–340 200 75.0 77.0 355 P 

513−A 1/15/08 pulverized 69 40–340 200 80.0 81.5 230 NP 

514−A 1/23/08 coarse 20 40–340 200 64.0 66.9 355 P 

516−A 2/6/08 coarse 20 40–340 200 69.0 71.5 280 NP 

517−A 2/13/08 medium 38 40–340 200 71.7 74.0 355 P 

518−A 2/27/08 medium 38 40–340 200 74.4 76.4 280 NP 

520−A 3/12/08 medium 38 40–340 200 68.5 71.0 550 P 

522−A 3/26/08 medium 38 40–340 200 74.4 76.4 280 NP 
Effect of Particle Size on Coal Dust Explosibility 

The effect of coal dust particle size on explosibility is illustrated in Figure C-1, which 

contains data collected from large-scale explosions conducted in the LLEM from the 1985 

through 2008. This curve shows the amount of incombustible material required to prevent 

propagation for coal dust containing 20% to 85% particles passing a no. 200 sieve (< 75 μm). 

Given the experimental test conditions, the curve is the boundary between mixtures that did 

propagate an explosion (below line) and mixtures that did not propagate an explosion (above 

line). Experimental results also show that the TIC required to prevent flame propagation 

becomes much less dependent on coal particle size as the TIC approaches 80%. 

49 

Figure C-1. Effect of particle size of coal dust on the explosibility of Pittsburgh seam bituminous 

coal as tested within LLEM. 
Delivering on the Nation’s promise:

safety and health at work for all people

through research and prevention

To receive NIOSH documents or more information about

occupational safety and health topics, contact NIOSH at

1 –800 –CDC–INFO (1–800–232–4636)

TTY: 1–888–232–6348

e-mail: cdcinfo@cdc.gov

or visit the NIOSH Web site at www.cdc.gov/niosh. 

For a monthly update on news at NIOSH, subscribe to

NIOSH eNews by visiting www.cdc.gov/niosh/eNews. 

DHHS (NIOSH) Publication No. 2010-151 

SAFER • HEALTH IER • PEOPLETM