Figure showing a ventilation reversal during a mine 

fire simulated in a typical Australian underground coal 

mine layout using longwall extraction method 

Coal dust and methane 

In the USA, the National Institute for 

Occupational Safety and Health (NIOSH) does a 

lot of work and has provided news of 

interesting new developments. Marcia L. Harris 

describes an ‘Explosibility Meter: “In 

underground mines, coal dust explosions are 

prevented by the addition of rock dust 

sufficient to render the coal dust inert. Federal 

[US] regulations require 65% and 80% 

incombustible content in samples taken in 

intake airways and return airways, respectively, 

assuming a nominal coal dust size of 20% 

minus 200 mesh. The Coal Dust Explosibility 

Meter (CDEM), a hand-held instrument 

developed by NIOSH uses optical reflectance to 

measure the explosibility of a rock dust and 

coal dust mixture.” The CDEM is now 

commercially available. 

It provides real-time results during rock dust 

surveys instead of waiting weeks for laboratory 

results1. “With real-time results, the potential for a 

disaster can be mitigated immediately. The CDEM 

displays the percent incombustible content as well 

as a colour indicating the relative explosibility of 

the coal and rock dust mixture. A red read-out 

indicates that more incombustible material is 

required to inert the coal and rock dust sample, 

while a green read-out indicates that the dust 

sample is sufficiently inert. When the mixture is 

marginally explosible, a yellow read-out is 

indicated. The red-yellow-green output depends 

upon the particle size distribution of the rock and 

coal dust mixture 

with the finer size 

fraction being 

more explosible.” 

The 

currently 

approved 

method 

for 

determining the incombustible content present 

within an entry in the USA is to collect a band 

sample and send it to a laboratory for low 

temperature ashing (LTA). The process, from 

obtaining samples to reporting the analytical 

results, typically takes several weeks. Thus, 

inadequate inerting may exist for some time 

before the laboratory results could show that 

additional rock dusting is necessary. 

NIOSH personnel accompanied Mine Safety 

and Health Administration (MSHA) inspectors 

on routine band surveys in five underground 

coal mines in southwest Pennsylvania. While 

underground, they used the CDEM to assess 

the explosibilities of the dust samples. “The 

percent incombustible contents determined by 

the CDEM agreed well with those values 

obtained later by LTA at MSHA and NIOSH 

laboratories. Among these intake entry samples, 

92 out of 104 had ≥65% incombustible content 

as required by current regulations. However, the 

CDEM indicated that 27 of these 92 samples 

were within the red or yellow bands, suggesting 

that about a quarter of the samples may have 

been deficient in incombustible content. Mine 

areas represented by these samples would most 

likely not receive additional rock dust since they 

were compliant with current regulations. Yet 

according to CDEM analyses, these samples 

represented areas of the mine where a risk of 

explosion propagation was present and more 

rock dust was required. These samples likely had 

size distributions finer than the 20% minus 200 

mesh assumed in the federal regulations and, for 

this reason, were indicated as potentially 

explosible2 . 

The use of infrared sensors for monitoring 

methane underground is examined by C.D. 

Taylor, J.E. Chilton and A.L. Martikainen. They 

note that “infrared and catalytic heat of 

combustion sensors are commonly used for 

measuring methane, but only the latter are 

currently used 

on 

mining 
 

machines in underground coal 

mines. A series of tests was 

performed to evaluate the 

feasibility of infrared instruments 

for underground use. A test box 

and a full-scale ventilation test 

gallery designed by NIOSH were 

used to compare the performance 

of one heat of combustion (HC) 

and two infrared (IR-1 and IR-2) 

sensors. Response times were 

measured using the test box. The 

90 % response time for the heat 

of combustion sensor (HC) was 

18.5 s and corresponding 

readings for the infrared 

instruments, IR-1 and IR-2, were 

9.8 and 32.5 s respectively. 

“Further testing in the box showed the large 

difference in response times for IR-1 and IR-2 

was due to the design of the environmental 

caps. Both caps have plastic baffles to lessen 

the amount of dust and water reaching the 

infrared sensor heads. The IR-2 instrument also 

has a filter material inside the cap to provide 

increased protection for the sensor. The filter 

material slowed the diffusion of gas through 

the cap and was responsible for the increased 

response time.” 

Tests were conducted in the ventilation test 

gallery to determine how different response 

times would affect methane measurements 

made on a continuous miner. A model machine 

was located at the gallery face. The instruments 

were placed side-by-side on the top of the 

machine, 2.6 m from the face. Intake and 

machine scrubber flows were varied to provide 

six different test conditions. For each of the ten 

minute tests, the average readings obtained 

with the three instruments were approximately 

the same. However, the 

concentrations varied 

considerably during each test. 

The figure shows real-time 

i

i

nstrument measurements 

obtained during part of one 

test. “In general, the faster the 

nstrument response time, the 

faster the concentrations 

changed and the higher the 

peak values measured. 

“Concentration patterns 

measured one foot from the 

face of the gallery were similar 

to those measured on the 

machine except that the peak 

Real-time methane levels obtained on the mining 

machine 

and average concentrations at 

the face were much higher. It was not possible 

to correlate the changes in concentrations 

occurring at the face and on the machine. 

However, it is likely that the fastest response 

instrument (IR-1) provided the best estimates of 

real-time changes in concentration at the face. 

Long-term underground testing is necessary to 

determine if the faster response instruments 

(IR-1 and HC) provide adequate protection for 

the sensor heads or if the improved protection 

of the IR-2 environmental cap is required when 

using a sampling instrument on a mining 

machine.” 
R. Karl Zipf, Jr is a NIOSH Senior Research 

Mining Engineer, Michael J. Sapko is Principal 

Research Physical Scientist (retired) and Jürgen 

F. Brune is Principal Research Mining Engineer. 

They consider NIOSH Information 

Circular–9500: Explosion Pressure Design 

Criteria for New Seals in U.S. Coal Mines. Seals 

are barriers constructed to isolate abandoned 

mining panels or groups of panels from the 

active workings. 

Historically, mining regulations required seals 

to withstand a 140-kPa explosion pressure. The 

Mine Improvement and New Emergency 

Response Act (MINER Act) required MSHA to 

increase this design standard. MSHA published 

the new design standards Sealing of 

Abandoned Areas; Final Rule on April 18, 

2008.” The NIOSH Information Circular 

published July 2007 provides the scientific and 

engineering justification behind the three-tiered 

explosion pressure design criterion in the new 

design standards. It is available at 

http://www.cdc.gov/niosh/mining/pubs/pdfs/20 

07-144.pdf. 

“NIOSH engineers considered the explosive 

atmospheres that can accumulate within sealed 

areas and used thermodynamic calculations 

and simple gas explosion models to estimate 

worst-case explosion pressures that could 

impact seals. Three design pressure-time curves 

were developed for the dynamic structural 

analysis of new seals under the conditions in 

which those seals may be used: unmonitored 

seals where there is a possibility of methane-air 

detonation or high-pressure non reactive shock 

waves and their reflections behind the seal; 

unmonitored seals with little likelihood of 

detonation or high-pressure non reactive shock 

waves and their reflections; and monitored 

seals where the potentially explosive methaneair volume is limited. The diagram below is a 

simple flowchart that illustrates the key 

decisions in choosing between the monitored 

or unmonitored seal design approaches and the 

three design pressure-time curves. 

“For the first condition, an unmonitored seal 

with an explosion run-up length of more than 

50 m, the possibility of detonation or highpressure non reactive shock waves and their 

reflections exists. The recommended design 

pressure-time curve rises to 4.4 MPa and then 

falls to the 800-kPa constant volume (CV) 

explosion overpressure. For unmonitored seals 

with an explosion run-up length of less than 50 

m, the possibility of detonation or highpressure non reactive shock waves and their 

reflections is less likely. A less severe design 

pressure-time curve that simply rises to the 

800-kPa CV explosion overpressure may be 

employed. For monitored seals, engineers can 

use a 345-kPa design pressure-time curve if 

monitoring can ensure that (1) the maximum 

length of explosive mix behind a seal does not 

exceed 5 m and (2) the volume of explosive mix 

does not exceed 40% of the total sealed 

volume. Use of this 345-kPa design pressuretime curve requires monitoring and active 

management of the sealed area atmosphere. 

“NIOSH engineers used these design 

pressure-time curves in the Wall Analysis Code 

from the US Army Corps of Engineers to 

develop design charts for the minimum 

required seal thickness to withstand each of 

these explosion pressure-time curves. These 

preliminary analyses show that seal designs to 

resist these curves can be achieved using 

common seal construction materials at 

reasonable thickness. Successful 

implementation of the seal design criteria and 

the associated recommendations in this report 

for new seal design and construction should 

significantly reduce the risk of seal failure due 

to explosions in abandoned areas of 

underground coal mines. 

References: 

1. Sapko, M. J., and Verakis, H., 2006. Technical 

Development of the Coal Dust Explosibility Meter. 

SME 2006 Annual Meeting, St. Louis, MO, March 26­

29, 2006, Preprint 06-044. 

Sapko, M.J., Cashdollar, K.L., Green, G.M., 2007. 

Coal Dust Particle Size Survey of U.S. Mines. Journal 

of Loss Prevention in the Process Industries, vol. 20, 

pp. 616-620. 

Flowchart for selecting design pressure-time curve for 

new seals.