Explosion Prevention in United States Coal Mines 

Jürgen F. Brune, Kenneth L. Cashdollar and R. Karl Zipf 

Abstract: 

This paper outlines the legal standards and methods for protecting underground coal 

mines in the United States from explosions of methane gas and coal dust. It will discuss 

inspection and monitoring of mine atmospheres, dilution of methane through ventilation, 

methane drainage prior to, during and post-mining, as well as explosion prevention 

through rock dusting and sealing of mined-out areas. 

Recent research findings of the National Institute for Occupational Safety and Health 

(NIOSH), Pittsburgh Research Laboratory on the magnitude of pressures obtained during 

methane explosions from modeling and full-scale explosion tests at the NIOSH Lake 

Lynn Experimental Mine will be presented. This work is currently being applied to 

modeling the structural response of explosion-proof mine seals to pressures from gas 

explosions. Also, new research will be presented on the inertization of float coal dust in 

mines through the addition of inert stone dust and on the application of the Coal Dust 

Explosibility Meter (CDEM), a handheld device developed by NIOSH to directly 

measure the explosibility of a coal dust - limestone dust mixture based on optical 

reflectivity. 

1. Introduction 

Most coal deposits release significant amounts of methane gas which is flammable when 

mixed with air in concentrations between 5 and 16%. Coal mine operators need to 

carefully ventilate underground mine openings to dilute and render harmless 

concentrations of methane to prevent ignitions and explosions. Krog et al. (2007) 

document that, between 2000 and 2005, the number of frictional ignitions reported in 

underground coal mines in the United States numbered between 34 and 60 per year. 

Although these ignitions are generally small and localized, they have the potential to lead 

to much larger explosions. 

The number of fatalities and injuries resulting from explosions in U.S. coal mines has 

been considerably reduced since 1980, as Figure 1 shows:
Underground Coal Mine Explosion Fatalities & Injuries, 1980 - present

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Figure 1: Underground Coal Mine Explosion Fatalities & Injuries, 1980 – present 

 Fatalities

 Injuries

Still, the recent tragic explosions at the Sago mine (12 fatalities) and at the Darby mine (5 

fatalities) in 2006 have demonstrated that methane gas explosions continue to present a 

significant risk to the coal industry. 

Another explosion hazard in underground coal mines is fine coal dust which can be 

dispersed and ignited by a methane gas ignition. This adds fuel to the ignition, thereby 

increasing the pressure and heat released by the explosion. 

2. Explosion Protection Philosophy 

To protect underground mines from methane gas and coal dust explosions, various 

philosophies can be employed. Basic options are to prevent the occurrence and 

accumulation of flammable bodies of methane and to eliminate sources of ignition such 

as open flames, frictional and electrical sparks, heat from spontaneously combusting coal 

and heat from mining equipment including, but not limited to hot exhaust manifolds and 

frictional heat (belts, brakes). The following sections discuss these options in detail. 

a. Ventilation 

Preventing accumulation of flammable gases is mainly achieved through adequate, 

positive ventilation of all mine openings and through drainage of methane from the 

coalbed prior to and during mining. 
The mine ventilation system must be designed such that all mine openings are ventilated 

es 

or 

s of methane at specific examination points. 

 a given area of the mine is no longer used, it may be sealed by constructing explosionsistant barriers (seals) in all entries leading to this area. Once an area is sealed, it is no 

nger ventilated and methane is permitted to accumulate inside the sealed area. 

ffective May 22, 2007, the United States implemented new regulations requiring seals 

those 

e mine 

 after the coal has been 

nt 

l 

). 

he moisture and ash in the coal, the added rock dust, 

s shale from floor or roof rock). The added rock 

with sufficient quantities of air to dilute all accumulations of methane in intake aircours

to below 1.0% (U.S. Code of Federal Regulations, 30 CFR §75.323). If higher 

concentrations of methane are detected in intake aircourses, all electrically powered and 

mechanized equipment must be shut off and the ventilation controls adjusted such that 

the methane concentration is reduced to a level below 1%. Kissell (2006) points out that

a considerable margin of safety below the lower flammable limit (5%) needs to be 

provided to protect miners from methane explosions 

All working areas of the mine (those areas where miners work) must be inspected f

proper ventilation within three hours prior to the workers entering these areas. All other

active areas of the mine are inspected at least weekly, recording the quantity of 

ventilation air and any concentration

If

re

lo

E

to have at least a 345 kPa (50 psi) explosion resistance and the atmosphere behind 

types of seals to be monitored. 

b. Methane Drainage 

In addition to diluting methane accumulations by providing fresh air to ventilate th

entries, methane may also be drained from the active coalbed as well as from adjacent 

coalbeds above or below the active seam. Methane drainage generally is done prior to 

mining by drilling boreholes into the coal from the surface or from adjacent mine 

openings. Draining methane from gobs that are heavily fractured

extracted (especially in longwall mining operations) can be an efficient method to preve

methane from coalbeds located above the seam being mined from migrating into the 

active workings. 

c. Preventing Accumulation of Combustible or Explosible Coal Dust 

In order to prevent the formation of an explosible coal cloud, rock dust (usually finely 

ground limestone) is spayed over the coal dust in mine entries to create an inert dust 

mixture. To render such dust sufficiently inert, U.S regulations require that rock dust be 

added to the coal dust such that there is a minimum of 65% incombustible matter in al

intake entries and 80% incombustible in return airways (Nagy 1981, 30 CFR §75.403

The incombustible matter includes t

and any other inert mine dust (such a

dust is required to pass at least 70% through a 200-mesh (75 μm) sieve. A higher 

incombustible content is required in return airways because the coal dust carried by the 

ventilating air is assumed to be finer in size. This “float coal dust” is defined as dust that 

passes a 200-mesh (75 μm) sieve. 
Reduction of float coal dust can be achieved by using water sprays to prevent the 

generation of dust and entrainment into the mine ventilation air when cutting and 

fficient amount is added to the coal dust, it 

 in 

d 

 all 

rvals to ensure that appropriate amounts of rock 

ust are present. The combustible content of rock dust samples is determined by lowmperature (515°C) ashing laboratory analysis. It may take several days or even weeks 

efore the mine operator receives the test results. The recently developed Coal Dust 

xplosibility Meter (CDEM; Sapko, Verakis 2006) permits an instant assessment of the 

tion through an optical reflectivity analysis. The CDEM is 

resently in field testing at several mines in the United States and is also undergoing 

) 

xplosions is the elimination of potential 

urces for ignition. All underground equipment that is used within the face areas where 

tified as “permissible” by MSHA. Typically, electrical equipment 

s and longwall shearers 

onitors that are set to warn the equipment operator if methane 

ct 

ed 

s. 

e 

transporting coal. All mining equipment is typically equipped with water sprays. These 

are typically located next to cutting drums, crushers and conveyor transfer points. 

The rock dust acts as a heat sink, and if a su

prevents the propagation of a coal dust explosion (Nagy 1981). Rock dust is spread

the mine entries through pneumatic spray dusters as well as trickle dusters that are use

in the return entries of longwall and continuous miner sections. 

Mine operators and inspectors examine and record the quality of inertization during

ventilation examinations at regular inte

d

te

b

E

adequacy of rock dust applica

p

permissibility approval at the U.S. Mine Safety and Health Administration (MSHA

Approval and Certification Center. 

d. Elimination of Ignition Sources 

The second tier of protection against mine e

so

coal is cut must be cer

is rendered permissible through explosion-proof enclosures that prevent an ignition of 

methane inside electric motors or switchgear from igniting a flammable methane 

atmosphere outside or through intrinsically safe design where currents inside the 

electrical equipment are so small that they cannot ignite a flammable methane-air 

mixture. In addition, mining equipment such as continuous miner

are fitted with methane m

concentrations above 1% are encountered. At methane concentrations above 1.5%, all 

electrical face equipment must be disconnected at the power source. 

Equipment powered by diesel engines is required to pass rigorous standards with respe

to the surface temperature, exhaust gas temperature and exhaust gas quality to be deem

permissible in coal mines (see 30CFR36). 

Smoking and open flame are generally prohibited inside underground coal mine

However, if it becomes necessary to work with an open flame for cutting or welding 

operations, special precautions must be taken to prevent methane ignitions. Thes

precautions include monitoring the atmosphere by a certified mine examiner at the 

welding site for presence of methane. 
If explosives are used in the mining process, they too need to be permissible and 

approved by MSHA. Permissible explosives typically consist of a sheathed design

prevents detonation if the shot is unconfined and thus can be set off without the risk of 

igniting a flammable methane atmosphere. 

 that 

air 

s a 

s. 

0 

nder certain circumstances, it is possible for a methane-air mixture to detonate. An 

using 

e speed of sound relative to the unburned 

press the 

tion wave is reflected from a 

e up to 2.5 times higher, i.e., 

ed. 

 the presence of coal dust, which can add fuel to the explosion if the coal 

si) 

it is unknown 

hether a flammable gas-air mixture exists behind the seals. 

 release by the coal will eventually exceed the upper 

ealed atmosphere fuel-rich inert. Also, slow oxidation 

3. Mine Explosions 

The explosions in the Sago and Darby mines in 2006 have demonstrated that methanemixtures can create violent explosions even without entraining coal dust. This become

potential hazard especially if large amounts of such flammable gas-air mixtures are 

permitted to accumulate inside confined, sealed areas of mine

a. Explosion Pressures 

Ignition of a confined methane-air mix may develop pressures as high as 800 kPa (12

psi) due to the heat of reaction. 

U

explosion transitions into a detonation when the shock wave that travels ahead of the

flame compresses the unburned gas-air mixture to the point where it auto-ignites, ca

the flame front to travel faster than th

atmosphere ahead of the flame front. Pressures from detonations typically reach the 

Chapman-Jouguet detonation pressure of 1.76 MPa (256 psi). The transition to 

detonation may be enhanced and accelerated in mine entries where the gas flows become 

highly turbulent or where the gas explosion has sufficient run-up length to com

unburned gas mixture ahead of the flame. If such a detona

mine seal, the reflected pressure exerted on the seal may b

4.4 MPa (640 psi). 

Explosions resulting from methane-air ignitions inside active workings usually do not 

create such high pressures, provided that the volume of flammable gas mixture is limit

However, if the mine ventilation system is compromised from an explosion, fire, rock 

burst, or other event that damages ventilation controls, violent explosions are also 

possible inside the active working areas of coal mines. The danger of a violent explosion

is enhanced by

dust is not sufficiently inerted by rock dust. 

b. Requirements for Mine Seals 

Based on their documentation of possible explosion pressures, Zipf et al. (2007) 

recommend that mine seals be structurally designed to withstand a 4.4 MPa (640 p

detonation wave if the sealed area atmosphere is not monitored and if 

w

In most coal mines, the methane

flammable limit and render the s

processes within the coal itself may reduce the atmospheric oxygen, thereby further 

reducing the flammability in the sealed atmosphere. 
Depending on the methane release rate of the coal, this self-inertization is achieved 

within a few days to several weeks. 

Once the sealed atmosphere has self-inertized, the danger of an explosion is greatly 

ing 

ence and deterioration. The structural design for the seal must incorporate 

nchorage along the roof, floor and coal ribs and allow for strains due to convergence of 

ine entries. 

onitoring behind sealed areas can be accomplished by inserting gas sampling tubes 

rough the seals. Automatic on-line gas analyzers can continuously draw atmospheric 

these tubes and provide a comprehensive data stream to the mine 

y the 

ch 

ing flame 

om becoming more violent when coal dust is scoured up, becomes airborne, and is 

added to a methane explosion flame. A newly developed Coal Dust Explosibility Meter 

can assess the adequacy of rock dust application instantly. 

Recent mine disasters at the Sago and Darby mines in 2006 have demonstrated the need 

for better protection of miners against explosions within sealed areas of mines. NIOSH 

has released recommendations for the design of mine seals and for atmospheric 

monitoring and inertization within sealed areas of mines to prevent such disasters in the 

future. 

reduced. However, a rise in the barometric pressure will cause fresh air to leak through

the seal, providing additional oxygen to the sealed area atmosphere and possibly dilut

the gas mixture immediately behind the seal back into the flammable range. 

c. Monitoring and Management of Sealed Atmospheres 

If the atmosphere behind the seals is monitored and managed such that inert gas is 

injected behind the seals if a flammable atmosphere is detected, the required seal strength

may be reduced to 345 kPa (50 psi) with a safety factor of 2 that accounts for 

uncertainties in the seal construction process and subsequent damage to the seal from 

converg

a

m

M

th

sample through 

operator. Computerized alert systems can be connected to the gas analyzer to notif

operator of any hazardous atmospheres inside the sealed areas so that inert gases 

(typically nitrogen or Tomlinson Boiler exhaust gas) can be injected. 

4. Summary and Conclusions 

Underground coal mines in the United States rely on a multi-tiered system of protections 

against explosions of methane-air mixtures. A well-designed and permanently monitored 

mine ventilation system is used to dilute and render harmless any hazardous 

concentrations of methane in the air. Mining equipment is designed and certified su

that it does not present an ignition hazard. Other potential ignition hazards are reduced

by prohibiting smoking underground and by taking special precautions when us

cutting and welding equipment. 

Rock dusting in all active areas of the mines effectively prevents small gas explosions 

fr
5. References 

Krog, RB and Schatzel, SJ: Frictional Ignitions in Underground Bituminous Coal 

Operations 1983-2005. 2007 SME Annual Meeting, Denver, CO, Preprint 07-132. 

Littleton, CO: Society for Mining, Metallurgy and Exploration, Inc., 8p. 

Unites States Code of Federal Regulations, Title 30, Part 75 

Nagy, J: The Explosion Hazard in Mining, US Dept. of Labor IR 1119, 1981, 69p. 

Kissell, FN: Handbook for Methane Control in Mining, US Dept. of Health and Human 

Services, NIOSH IC 9486, 2006, 183p. 

Sapko, MJ and Verakis, H: Technical Development of the Coal Dust Explosibility 

Meter, 2006 SME Annual Meeting and Exhibit, St. Louis, Missouri, Preprint 06-044. 

Littleton, CO: Society for Mining, Metallurgy, and Exploration, Inc., 2006, 5p 

Zipf, RK, Sapko, M and Brune, JF: Explosion Pressure Design Criteria for New Seals in 

U. S. Coal Mines, US Dept. of Health and Human Services, NIOSH IC ####, 2007 91p.