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Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Figure 1.—The CDEM is used to analyze the composition of a coal

and rock dust mixture.

Coal Dust Explosibility Meter

Objective

To enable mine operators and mine inspectors to make quick and

accurate measurements of the explosible nature of coal and rock dust

mixtures.

The Problem

Past research has shown that the accumulation of coal dust in

underground coal mines can be rendered nonexplosible by adding

sufficient quantities of inert rock dust, such as limestone dust. Federal

regulations for underground coal mines require mine operators to dust

mine corridors with an inert rock dust and maintain a total incombustible

content of at least 65% in the entries and at least 80% in the returns,

where the coal dust is expected to be finer in size. Currently, samples

of the deposited coal and rock dust are collected for aboveground

analysis of the inert percentage, which consists of rock dust, ash, and

moisture. The processing time for this analysis can be as long as 2

weeks. In addition, research has shown that measuring the

incombustible percentage is not always sufficient to determine the

explosibility of a sample, especially for finer coal dust. The National

Institute for Occupational Safety and Health (NIOSH), Pittsburgh

Research Center (PRC), has devised a prototype handheld instrument

that can provide a direct assessment of the potential explosibility of a

coal and rock dust mixture.

How It Works

The coal dust explosibility meter (CDEM) is a portable optical

device that determines whether or not a coal and rock dust mixture is

explosible (figure 1). It consists of an optical probe connected to a

small electronics box with a digital display. The principle of

operation of the CDEM is based on the measurement of infrared

radiation reflected from the surface of a homogeneous mixture of two

substances with different optical reflectances, such as light colored rock

dust and dark coal dust (figure 2). Near-infrared radiation is emitted by

a light-emitting diode in the optical module

located behind the window of the optical probe. When the meter is

inserted in the dust mixture, the infrared radiation reflects off the dust's

surface and back to the silicon photodiode sensor, also located in the

optical module. For a given coal volatility, the normalized optical

reflectance of such mixtures is relatively constant at the limit of

explosibility (the amount of rock dust required to inert) and independent

of the coal dust particle size. Samples whose normalized reflectance

measures below the threshold would be identified as explosible; samples

with reflectances greater than the threshold would be nonexplosible.
Figure 2.—Drawing depicting principle for measuring the optical

reflectivity of a coal and rock dust sample.

Test Results

The limit of explosibility for many coals was determined in PRC's

20-liter explosibility chamber using several types of rock dust. As

expected, the percentage of rock dust required to inert the coal dust

increases for finer coal dust size. Coals with mass median particle sizes

in the range of 10 to 70 )m required 90% to 60% rock dust,

respectively, to be inerted. At the finest particle sizes, mixtures are still

explosible even above the federally mandated 80% rock dust

requirement for return airways, confirming the fact that measuring the

rock dust concentration is not sufficient to determine the explosibility

of a sample.

For each mixture, the CDEM optical reflectance was measured at

the limit of flammability and was found to be constant over the range

of particle sizes for a given coal volatility. In practice, the normalized

reflectance to which the meter alarm would be set would depend on

the volatility of the coal seam in which the instrument was being used.

The CDEM thus provides a measurement of the explosibility of a dust

sample over the entire range of coal dust sizes, rather than being

restricted to the two coal sizes (intake and return) in current

regulations.

Continued Efforts

Currently, the CDEM could provide an efficient method to

determine the explosibility of air-dried, homogeneous samples at

aboveground laboratories at the mine site. Research is in progress to

measure and correct for the presence of moisture in the samples by

measuring the electrical resistivity in addition to the reflectivity.

This correction would allow the CDEM to be used on samples

directly from the mine and possibly to provide an in situ explosibility

measurement to eliminate the danger of operating under hazardous

conditions while samples are processed. The CDEM could provide

mine operators and safety inspectors with a valuable means for

determining the explosible nature of coal and rock dust deposits.

Efforts are currently underway to commercialize the CDEM,

along with a related instrument, the reflectance rock dust meter, which

provides a direct measurement of the rock dust percentage in mine

dust samples.

For More Information

To obtain a free copy of a technical paper on the CDEM or answers to technical questions about the device, contact Carrie E. Lucci

or Kenneth L. Cashdollar, National Institute for Occupational Safety and

Health (NIOSH), Pittsburgh Research Center, Cochrans Mill Rd., P. O.

Box 18070, Pittsburgh, PA 15236-0070, phone (412) 892-4308 or (412)

892-6753, fax (412) 892-6595,

e-mail: chl4@cdc.gov or kgc0@cdc.gov

Mention of any company name or product does not constitute

endorsement by the National Institute for Occupational Safety and

Health.

To receive additional information about mining issues or other

occupational safety and health problems, call 1-800-35-NIOSH

(1-800-356-4674), or visit the NIOSH Home Page on the

World Wide Web at http://www.cdc.gov/niosh/homepage.html

As of October 1996, the safety and health research functions of

the former U.S. Bureau of Mines are located in the National

Institute for Occupational Safety and Health (NIOSH).