Technology News

No. 478 August 1999

U.S. Department of Health and Human Services

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Apparatus for Measuring the Gas Content

of Coal or Rock Core Samples

Objective

To design an apparatus that advances the state of the art in gas

content testing equipment. The goal of the design is to incorporate

improvements in testing sensitivity/accuracy, durability, and ease

of operation over existing equipment. It is also desirable for the

new design to successfully perform gas content determinations of

multiple chemical species from a rock sample.

Background

When coalbeds and many other rock types containing organic

matter are heated during burial, methane gas is formed as a

byproduct. Methane can cause explosions in underground mines

that can lead to fatalities and/or injuries to mine workers. To

achieve safe underground working conditions, Federal law dictates

that all U.S. underground coal mines dilute methane to 1%

concentrations in air within active areas, about one-fifth of the

lower explosive limit. The coal mining industry requires a test to

measure the amount of methane in coal and associated rocks in

order to design adequate ventilation systems to maintain a safe

environment for underground workers. Accurate gas content data

also play an important role in the economic evaluation of the

mining potential of coal reserves. If methane dilution through mine

ventilation is not sufficient to maintain a safe and compliant

underground environment due to high methane emissions, methane

drainage systems must be included in the mine plan, which increase

development and operational costs.

Noncoal mines may also experience gas hazards due to

emissions of methane, ammonia, hydrogen sulfide, and other gases.

The quantity of gas contained in metal/nonmetal ore is generally

less than that contained in coal. Consequently, metal/nonmetal

mines require a gas content testing capability that has a lower

detection limit and a higher degree of accuracy than is generally

found in conventional gas content testing systems developed for

coal. Noncoal mines frequently have a limited amount of

quantitative gas content data. This lack of information can create

unsafe conditions for underground workers when unplanned

increases in gas emissions occur and create a hazardous work

environment.

Approach

The Portable Modified Direct Method Testing Apparatus

(figure 1) was designed, built, and developed by researchers at the

National Institute for Occupational Safety and Health (NIOSH)'s

Pittsburgh Research Laboratory. Previous gas content

measurement apparatus designs utilize a column of water that is

displaced by gas yielded by a rock sample in an airtight container.

Through the use of modern electronic components powered by a

rechargeable nickel-cadmium battery, the new design has eliminated

all of the experimental problems associated with conventional water

displacement technologies. The new design is easier to use,

rugged, compact, lightweight, reliable, and relatively inexpensive,

yet provides more accurate gas content data.

How It Works

The Portable Modified Direct Method Testing Apparatus uses

the ideal gas law to relate differential gas pressure to gas volumes.

To measure the gas content of a rock unit using the modified direct

method, a sample from the rock unit (either a core sample or a grab

sample) is collected and sealed in a lightweight enclosure. The gas

pressure in the enclosure is periodically measured through a series

of pressure transducers. With each pressure measurement the gas

inside the enclosure is bled off, reducing the pressure to

atmospheric, and a sample of the gas is collected. The gas sample

is analyzed to determine the specific gas mixture. The gas pressure

measurements and the gas species identified can be used to

determine the total volume of gas contained in the rock unit and the

specific volume of each gas identified.
 Figure 1.—A, NIOSH’s Portable Modified Direct Method Testing

Apparatus;B, a standard sample container with a retrieved rock

core for gas content testing.

The Portable Modified Direct Method Testing Apparatus has a

number of significant advantages over conventional water

displacement technologies. The new design embodies improvements in sensitivity and accuracy due to the incorporation of

modern electronic components. The benefit of this innovation is

most significant in relatively low gas content rock samples, but

accuracy is enhanced in any gas content test application. By

eliminating the column of water used in water displacement

systems, problems associated with the differing gas solubilities of

specific chemical compounds are eliminated as a source of

measurement error. Without this improvement, gases that are not

very soluble in water, such as methane, tend to resist going into

solution and can be more easily measured than more highly soluble

gases such as carbon dioxide or nitrogen, which readily dissolve

and may not be detected or measured. During the development of

the modified direct method, it was recognized that interaction

between organically rich core samples and oxygen within the

sample container depleted the quantity of oxygen in the container.

This phenomenon was found to be very common in coal core

samples during methane content testing and resulted in the

underestimation of methane contents, nominally on the order of at

least 5% to 10% when the gas produced from the samples was not

chemically analyzed. Subfreezing air temperatures are not a

problem for the Portable Modified Direct Method Testing

Apparatus, but can result in water line freezing with water

displacement methods. The NIOSH apparatus can be used to

perform gas content determinations on essentially any gas whose

concentration can be analytically resolved by chromatography.

The resolution improvements over conventional methods allow gas

content determinations of noncoal rock cores.

Patent Status

A patent for this apparatus was granted (U.S. patent

No. 5,741,959, "Portable Tester for Determining Gas Content Within

a Core Sample"). NIOSH also won a prestigious R&D 100 Award

for the invention; this award recognizes the 100 most

technologically significant new products each year as judged by

R&D Magazine.

For More Information

For additional information about this invention, contact Steven

J. Schatzel or Fred Garcia, NIOSH Pittsburgh Research Laboratory,

Cochrans Mill Rd., P.O. Box 18070, Pittsburgh, PA 15236-0070,

phone: (412) 386-6521 or (412) 386-6536, fax: (412) 386-6891, e-mail:

zia6@cdc.gov or fbg3@cdc.gov

To receive additional information about

occupational safety and health problems, call 1-

800-35-NIOSH (1-800-356-4674), or visit

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

Mention of any company name or product does not

constitute endorsement by the National Institute

for Occupational Safety and Health.