14

REFERENCES

1, Federal Water Quality Administration, U.S. Department of the
Interior. Feasibility Study on Mining Coal in an Oxygen Free Atmos-
phere, NTIS PB-197446, Aug. 1970, 162 pp.

2. Zabetakis, M. G. Flammability Characteristics of Combustible
Gases and Vapors. BuMines B 627, 1965, 121 pp.

3. Paczkowski, V., G. A. Tracey, and A. Wojtyczka. CMI Inert Gas
Mine Firefighting System (contract 40318080, CSE Corp., Monroeville,
PA). BuMines OFR 212-83, 1982, 56 pp.; NIIS PB 84-142678.

4, Vinson, R. P. The Brattice Window Technique.. BuMines RI
8240, 1977, 12 pp.

5. Zabetakis, M. G., R. W. Stahl, and H. A. Watson. Dete:
the Explosibility of Mine Atmospheres. BuMines IC 7901, 1959, 11 pp.

6. Ulery, J. P.,, and D. M. Hyman. The Modified Direct Method
of Gas Content Determination: Applications and Results. Paper in
Proceedings of the 1991 Coalbed Methane Symposium. Univ. AL—
Tuscaloosa, 1991, pp. 489-499.

7. Urosek, J. (Mine Safety and Health Administration). Personal
communication. Further information available from J. Urosek, MSHA,
4800 Forbes Ave., Pittsburgh, PA 15213.

8. Parr, S. W., and P, Barker, The Occluded Gases in Coal, Univ.
IL Eng, Exp. Station Bull. 32, 1909, 28 pp.

INT.BU.OF MINES,PGH.,PA 29741


determine the true exhaust volume if hot gas flow is meas-
ured, A correction factor of 0.125 was derived empirically
during testing in January 1992. The correction factor
multiplied times the hot exhaust gas flow measurement
yields the available inert gas volume for the combined
engine flow. Table 1 contains the experimental data for
determination of the correction factor. This correction
factor may be used to quickly determine the available gas
volume of the system by measuring the hot exhaust from
the 5-in pipe. Subsequent testing in March confirmed the
value of the correction factor. If a more direct measure-
ment is required, the brattice window technique can be
used to determine the cooled gas volume.

Table 1.-Hot and cooled volumes of inert gas for various
engines and conditions

Hot! Cool

Derived

Condition Veloc- Vol vol-_— correction
ity? ume, ume,?_ factor,

fpm cfm cfm —_cool/hot
740 101 13 0.129
1,680 229 28 122
Diesel plus gas 3,280 447 456 NA
DO. ieee eens NA NA 64 NA

March:

Diesel plus gas 53,100 424 50 118
Do. . 55 130
Average 125

NA Not available.
‘Measured in stub pipe.

“Brattice window measured,

35.in pipe.

‘Calculated based on correction factor of 0.125.

SMeasured with Pitot tube at hot air density of 0.0415 Ib/#?.

The minimum cooled gas volume found during testing
with both engines was 50 cfm (table 1). The maximum
tate of coal removal was determined to be 42 cfm. This
calculates to a 16% excess volume of inert gas for the
worst case conditions, that is, minimum gas volume and
maximum coal removal. For average conditions, 56 cfm of
exhaust was present and coal removal was 35 cfm, or 37%
excess inert gas volume.

DELIVERY OF GAS TO AUGER HEAD

Gas concentrations introduced at the collar of the hole
followed the head of the auger mining machine into the
hole and remained in the hole until after removal of the
auger head. Table 2 shows the oxygen concentrations in
the hole measured by inserting PVC pipe over the augers.
This was limited to a maximum of 230 ft. In all cases, low
oxygen gas introduced with the stub pipe at the collar of
the hole was found in the interior of the hole. Early,
unpublished testing (7) found that the shroud technique

also produced similar results; however, owing to the ef-
fectiveness and simplicity of the stub pipe method, only
that method is reported.

Table 2.—Hole measurement results showing low oxygen

concentrations In hole Interior
Hole depth, Probe depth, ‘Oxygen
Hole ft ft concentration,
vol %
Tt. 308 225 10.28
3 360 230 9.16
4 155 140 12.78

NOTE.—Approaching darkness precluded hole measurement
with the augers in place for hole 2.

PRELIMINARY TEST
Hole Data

The first results of this testing were obtained in January
1992, Hole data results with the inert gas system func-
tioning showed that holes had only minimal concentrations
of methane. The data were analyzed and extrapolated by
calculating the effect of methane, as if it were added to the
measured gas concentrations. This analysis showed that at
any concentration of methane encountered, the atmos-
phere in the hole would be nonexplosive because of the
presence of the inert gas and low levels of oxygen. Ta-
ble 3 shows the drilling depths, probe depths, and correct-
ed infrared gas concentrations. The table also computes
the effect of adding 5% methane to the gas mixture as if
a pocket of methane were encountered. With the added
methane, the oxygen, carbon dioxide, and nitrogen concen-
trations would be diluted. New levels of these gases are
computed. Using the method of Zabetakis (2, 5), the ex-
plosibility of the gas mixture is determined by computing
the volume of excess nitrogen and amount of effective in-
ert. At 5% methane, a gas with an effective inert concen-
tration over 34 vol % is considered nonexplosive.

Explosibility Diagram

Figure 5 shows an explosibility diagram and the prelim-
inary data extrapolated as if various methane concentra-
tions were encountered. The figure shows that the closest
approach to the explosive range at the inert gas concentra-
tions measured occurs between 5% and 6% methane con-
centration. At the closest point, an 11% [(1-34/38)x100]
reduction in effective inert gas concentration is required to
reach the explosive range. This figure further demon-
strates that as higher levels of methane are encountered,
the effective inert concentrations move further from the
explosive range.


CONTENTS

Abstract . .
Introduction .
Acknowledgments
Methods . .
System as tested .
Volume testing
Gas testing...
Analysis of explosibility .
Gas content measurement .
Personal testing ....
Results and discussion .
Volume measurements .
Coal volume removed
Inert gas volume ....
Delivery of gas to auger head .
Preliminary test .
Hole data ...
Explosibility diagram
Methane encounter .
Hole data ......
Explosibility diagram
Hole profile .....
Time to reach inert condi
Collar measurements . .
Gas content .....
Personal monitoring .
System safety .
Observations .
Conclusions .
References . .

ILLUSTRATIONS

Inert gas shroud enclosure of auger hole collar
Inert gas stub pipe used to direct inert exhaust into the hole .
. Schematic of inert gas system as tested .

Overview of highwall mining operation as tested .
Explosibility diagram for January ............
Explosibility diagram for March data with observed gas concentrations plotted for all holes . .

NAWAYNE

immediately after auger removal
Corrected infrared gas concentrations at the collar sample location for hole 3 ..

ad

TABLES

Hot and cooled volumes of inert gas for various engines and conditions
Hole measurement results showing low oxygen concentrations in hole interior
Measured and computed gas concentrations in hole . .
Gas chromatograph results—auger in hole .........
Overnight ventilation of open and partially sealed holes ...........
Coalbed gas content of samples from hole 4, No. 9 Coalbed, Henderson County, KY

AYSYNE

Profiles of methane concentrations in holes 1 and 3 at maximum hole depth with augers in place, and ,

P

g

BRBGBERESSeccammmmiuuiranaannereane


KEY 4
Sample locations
33 ft in hole
(Hole collar
{$9130 ft in hole

CHa, %

CO, ppm

ot
9:30

10:15 11:00
TIME, h:min _

Figure 8.—Corrected infrared gas concentrations at the collar
‘sample location for hole 3. Note data at 10:44 and Initial Inerting
of hole were not collar samples.

45 12:30

desorption alone. A more likely scenario would involve
slow desorption over time into a fracture or fissure in the
coalbed or immediate roof. Free methane gas would build
up in the crevice, the amount dependent on the size of
opening and pressure conditions. When the cutting auger
penetrates the methane-filled fissure, the gas immediately
begins to flow into the auger hole because of the pressure
differential, The large lead augers draw this methane to
the face with incoming oxygen, creating potential explosive
conditions. If the cutting bits strike a hard inclusion or the
tock roof, the resulting hot smear could ignite the explo-
sive mixture.

Table 6.—Coalbed gas content of samples from hole 4,
No. 9 Coalbed, Henderson County, KY

Depth, Date and Analysis, om?/g

Sample ft time N, ©O, GH, GH,
sampled

BES... 80 1/7/92 1-025 -0.02 0.014 0.49 0.005
15:13

BE4.. 110 1/8/92 -.16 -02 000 28 .004
15:55

BES... 150 3/5/92 -11 0-07 008) 12.001
16:05

BE6.. 160 3/5/92 -.07 -.05 004 18.001
16:30

Negative sign denotes gas adsorbed or consumed.

PERSONAL MONITORING

The highest personal exposure to carbon monoxide
measured with the passive dosimeters was less than
20 ppm. This occurred on the auger pin puller. The con-
veyor side helper had only a trace of exposure, and the
machine operator had no detectable exposure. It should
be noted that the auger pin puller’s exposure may have
been due to periods when a brattice seal was being in-
stalled over the entrance of the hole. Spot measurements
with the handheld CSE 270 were not able to detect carbon
monoxide at the pin puller’s normal work station.

Note also from the continuous carbon monoxide data
in figure 8 that the collar concentrations averaged about
550 ppm inside the hole. The combination of dilution and
distance from the collar of the hole accounts for the ob-
served low personal exposure to carbon monoxide. The
initial peak of 1,200 ppm occurred at startup of the
gasoline engine when the catalytic converter was still cool.
The effect of pit inversions was not investigated.


ACKNOWLEDGMENTS

The authors gratefully acknowledge the cooperation of
John Urosek, Gary Wirth, and Mike Snyder of the Mine
Safety and Health Administration, Pittsburgh Technical
Support, for their consultation and loan of instrumentation.

The authors also would like to express appreciation for the
contributions of Michael McCullough, John McCullough,
and John Wiegand, of B&E Auger Mining Co., for their
help in conducting the field measurements.

METHODS

This section includes the design of the inert gas system
and measurement of the exhaust gas flows and concentra-
tions, This was accompanied by an analysis of the explo-
sibility of the gas mixtures found, analysis of the gas
content of the coals, and personal monitoring for carbon
monoxide on the machine operators.

SYSTEM AS TESTED

The primary source of the inert gas was a 305-in? Gen-
eral Motors‘ gasoline combustion engine. This was a used
automotive engine that was removed from a vehicle and
mounted on the roof of the auger drill. A new catalytic
convertor, available commercially as an off-the-shelf item,
was installed immediately following the exhaust manifold
of the engine. This convertor burns excess hydrocarbons
and carbon monoxide for cleaner gas emissions, and in this
case, lower oxygen content. This engine was operated at
3,600 to 3,900 rpm.

Originally, it was thought that the diesel engine alone
might produce the necessary volume and quality of inert
gas. However, the oxygen concentration in diesel engine
exhaust varied with engine load. Typical diesel engine
exhaust oxygen concentrations ranged from 17% at no
load to 8% at full load. Preliminary testing showed that
the auger drill engine load characteristics could not con-
sistently produce low enough oxygen concentrations to
teliably prevent explosions.

Although the gasoline engine alone produced oxygen
concentrations in the 1% to 4% range, the volume was in-
sufficient to replace the volume of coal being mined. To
make up for this lack of volume, a small portion of the
auger drill’s Cummins turbocharged 270 diesel engine’s
exhaust was added to the gasoline engine exhaust.

Exhaust gas from the gasoline engine connected
through one branch of a Y pipe, and the other branch of
the Y pipe connected to a split of exhaust from the diesel

‘Reference to specific products does not imply endorsement by the
USS. Bureau of Mines.

engine. The combined exhaust flows joined a piece of
5-in-diameter flexible pipe that connected to a vertical
descending pipe that brought the exhaust to the level of
the hole. The initial shroud design consisted of an in-
verted U-shaped tube that surrounded the collar of the
hole. Flaps of conveyer belt extended from the U tube to
the face, and pie-shaped segments shrouded the auger en-
try. Slots on the face side of the U tube directed the inert
gas into the hole. The stub pipe method replaced the U
tube shroud with an elbow and a 3-in reducing pipe fitting.
A 7-ft length of 3-in pipe extended the exhaust gas dis-
charge about 5 ft inby the collar of the hole. Figure 3
shows a schematic of the inert gas system as tested.

The diesel exhaust was conducted from the engine muf-
fler (no catalytic convertor was used on the diesel fraction)
to the roof of the auger drill through a flexible 5-in steel
pipe. The flexibility was such that the engine carriage
traveled freely with the operation of the auger drill. The
required volume of diesel exhaust was added to the 5-in
steel pipe at the Y pipe. This volume was controlled by a
critical orifice-type restriction with the diesel at 2,200 rpm
(normal operating condition). The orifice size allowed ap-
proximately equal proportions of diesel and gasoline ex-
haust. The remaining diesel exhaust was vented at the
roof of the auger drill.

The cutting head and lead guide augers were 39 in. in
diameter and totaled a length of 26 ft. Once these initial
augers were drilled into the highwall, the stub pipe was
inserted at the top of the hole. The engines ran for about
4 min to fill the initial hole depth with inert gas. Mining
then proceeded with all subsequent auger sections being 20
to 24 in. in diameter and 6 ft in length. Stops were made
periodically to take gas measurements,

VOLUME TESTING

The amount of gas volume required was determined by
measuring the volume and rate of coal removal by the
augers. Each 6-ft auger section advance removed a calcu-
lated volume of 49,7 ft?. Rate of removal was determined


A METHOD TO ELIMINATE EXPLOSION HAZARDS IN AUGER
HIGHWALL MINING

By Jon C. Volkwein' and James P. Ulery”

ABSTRACT

The U.S. Bureau of Mines investigated a method of using inert gas to prevent the formation of
explosive gas mixtures in auger highwall mining of coal. A combination of gasoline and diesel engine
exhaust gases was introduced into the auger drill hole using a short section of pipe located at the collar.
Gas samples were taken and analyzed on-site with infrared detectors for oxygen, carbon dioxide, meth-
ane, and carbon monoxide. Evacuated bottle samples were also taken and analyzed by gas chromatog-
raphy in the laboratory. These gas results were analyzed for explosibility. Coal samples from various
depths were used to obtain the gas content of the coal using the modified direct method. Personal expo-
sure to carbon monoxide was also monitored. The highest methane level observed was 9.55%. The lev-
els of inert gas (carbon dioxide and nitrogen) were sufficiently high to prevent any ignition of the
methane. Results showed that for all conditions during mining, gas concentrations were nonexplosive.
The gas content of the coal was about 0.30 cm?/g. The maximum personal time-weighted average sam-
ple for carbon monoxide was 20 ppm. This system provides a safe, inexpensive, simple method for pre-
venting explosions during auger mining.

1Physical scientist.
icologist.
Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.


UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cfm cubic foot per minute in?
cm*/g cubic centimeter per gram Ib/f?
°F degree Fahrenheit min
ft foot ppm
fe cubic foot psig
fP/min cubic foot per minute rpm
h hour 8

in inch scfh

cubic inch

pound per cubic foot
minute

part per million

pound per square inch gage
revolution per minute
second

standard cubic foot per hour



SYSTEM SAFETY

Since the safety of the system relies on sufficient quan-
tity and quality of inert gas from the engines, these must
be guaranteed. The system as tested in March contained
two simple and independent methods of monitoring its
performance. These were the functioning of the gasoline
engine and the continuous oxygen monitor. Failure of
either of these two components resulted in independent
warnings to the operator of system malfunction.

Safety is also enhanced by the condensing water vapor
from the exhaust inside the cool hole. Water vapor is also
an inerting gas but was not included in this analysis. Fur-
thermore, the condensing water may also serve to suppress
dust.

OBSERVATIONS

Sampling of the interior of auger holes is not casily
accomplished. Turbulence and air eddies near the collar
make accurate measurements in this area difficult. The
data support the thesis that gas concentrations immediate-
ly inby the collar of the hole follow the mining head into
the hole. As augering proceeds, the hole remains in an
inert condition provided a sufficient volume of inert gas is

B

present at the collar. Two measurements are required to
determine this condition: a yolume measurement of the
exhaust gas flow and the concentration of oxygen.

The volume measurement may be taken with a vane
anemometer or Pitot tube, and the appropriate correction
factor applied. The brattice window technique may also
be used for confirmation of the correction factor. For the
operator, a visual check can be made by observing direc-
tion of movement of dust or smoke during periods of max-
imum penetration. This movement should always be out
of the hole,

Measurement of the oxygen level of exhaust alone is
sufficient to indicate the inert condition of the hole. A
level of 12% oxygen in the exhaust in combination with
6% carbon dioxide provided levels in the hole that re-
mained inert. These measurements could be made with a
handheld oxygen detector, bottle samples, or an in-line
continuous oxygen detector. Because of the long time for
analysis, bottle samples should only be used for compli-
ance monitoring or instrument confirmation. Since com-
bustion engines reduce oxygen levels and produce carbon
dioxide, measurement of oxygen alone from a combustion
source implies the production of carbon dioxide. There-
fore, it is not essential that carbon dioxide be measured on
a routine basis.

CONCLUSIONS

The concept of using inert gas to prevent explosions, as
described in this report, resulted in nonexplosive gas con-
centrations under all circumstances when an ignition was
possible. The method described represents the best ap-
proach to date to prevent explosions when auger-type
highwall machines encounter substantial quantities of
methane,

Adequate volumes and concentrations of inert gas may
be produced by a combination of diesel and gasoline en-
gines, or a gasoline engine alone, if adequate gas volumes
can be generated. The gas may be introduced into the
hole via a stub pipe inserted into the hole above augers
after the lead augers have been inserted. Alternatively, if
augers are not reduced in size to allow placement of a
stub pipe, a shroud may be used, which surrounds the col-
lar of the hole with inert gas. Gas volumes for shroud
systems were not determined.

Application of this technique is simple and easily moni-
tored. Mine personnel have good understanding of the

engines used to produce the inert gas, and the plumbing of
this gas into the hole is not complex. The method, there-
fore, should have high acceptance by the miners. It is
recommended that the auger drill operator be provided
with a real-time oxygen indicator that would monitor the
oxygen levels in the exhaust gas and give the operator a
warning if levels rise above 12%.

It must be pointed out that the system should be moni-
tored closely and is dependent on adequate quantities and
concentrations of inert gas as well as timely placement of
the inert gas pipe into the collar of the hole. Measure-
ment of the volume and concentration of gas at the collar
of the hole is indicative of the conditions at the cutter-
head. To ensure that inert conditions are present at the
cutterhead during mining, a sufficient volume of low oxy-
gen concentration gas is required. Successful implementa-
tion of this simple method will improve safety of highwall
mining by eliminating possible explosion hazards,


Figure 2.—Inert gas stub pipe used to direct inert exhaust into the hole. Note small-diameter pipe used for collar
measurements.


RI 9462 REPORT OF INVESTIGATIONS/1993

A Method To Eliminate Explosion Hazards
in Auger Highwall Mining

By Jon C. Volkwein and James P. Ulery

Bureau of Mines



was connected to the collar sample pipe or the hole-
measuring PVC pipe. Samples then passed through a
water trap and the pump, where they were pressurized to
between 5 and 10 psig. Samples were then pumped into
a mobile laboratory established inside a van truck. Here
an evacuated glass bottle sample port was located immedi-
ately prior to entry of the gas into a drying tube and the
infrared gas analyzers (OFC Infrared Instruments). A
Dwyer flowmeter was used to supply a dual infrared car-
bon monoxide-methane monitor in series with an infrared
carbon dioxide monitor at a rate of 4 scfh. The gas flow
to the oxygen infrared monitor was internally regulated to
1 scfh at 5 to 10 psig. The signals from the gas analyzers
were recorded on a Soletech strip chart recorder and on
Metrosonics data loggers. Data from the loggers were
transferred to a computer disk at intervals during the test-
ing. The strip chart produced a hard copy of the tests as
they proceeded, while the computer data were used for
subsequent data calculations. All data collection was time
synchronized with mining and bottle sampling.

The gas sample travel time to the instrument was be-
tween 1 and 3 min depending on hole depth and sample
type. Hole data were collected for at least 5 min, to
ensure that representative samples were being collected.
The infrared gas analyzers were warmed up each day for
40 min and then calibrated with known concentrations
of gas. Bottle gas samples were also taken at intervals,
especially during stable gas concentration periods that
occurred during hole sampling, These bottle samples were
analyzed by a gas chromatograph at the Bureau’s Pitts-
burgh Research Center laboratory. The chromatograph
results were then compared with the infrared results. Any
discrepancies between the infrared data and the gas
chromatograph data were corrected to reflect the gas chro-

matograph analysis results.
ANALYSIS OF EXPLOSIBILITY

The explosibility of a particular gas mixture was ana-
lyzed by the method of Zabetakis (2, 5). This method cal-
culates the effective inert concentration in a particular
mixture, For this testing, only methane was considered as
the fuel source for explosions, While carbon monoxide

and higher hydrocarbons could also be potential fuel
sources, their concentrations from gas chromatograph
analysis were in the parts per million range and, therefore,
not used in the calculations for explosibility. This method
constructs an explosibility diagram that contains three
regions: explosive, nonexplosive, and explosive when mixed
with air. The last region includes situations in which the
fuel source itself is so rich that it will not ignite; however,
dilution with air will lean the mixture such that it may
become explosive. Where these concentrations become
diluted is therefore important.

GAS CONTENT MEASUREMENT

The modified direct method (MDM) test procedure was
used to determine the gas content of coal samples (6).
Samples were obtained periodically from the auger hole
collar while augering was in progress. An attempt was
made to choose samples sized from about 1 by 1 to
3 by 3 in. This was done to minimize fine material, which
may have lost gas more quickly prior to sampling. Sam-
ples of even larger size, although desirable, would not fit
in the air-tight sample containers. Gas content measure-
ments were taken periodically. One major departure from
the described technique was that the "lost gas" was not
calculated. Lost gas refers to gas desorbed from the
sample prior to sealing the sample in the air-tight con-
tainer. Lost gas was not determined because of the low
gas contents expected and because the original lost gas
calculation was designed for use in vertical coring, Sam-
ples were obtained where the inerting procedure was being
tested—the No. 9 Coalbed, Henderson County, KY.

PERSONAL TESTING

Since the exhaust from a gasoline combustion engine
was used, personal exposure to carbon monoxide was mon-
itored, Miners wore Mine Safety Appliances stain tube-
type passive carbon monoxide dosimeters on their lapels
for a few of the test days. Handheld CSE 270 multigas
meters were also used to measure carbon monoxide at the
work areas of auger placement. In addition, carbon mon-
oxide levels were measured during collar sampling,

RESULTS AND DISCUSSION

The implementation of the inert gas concept was shown
to lower oxygen concentrations sufficiently to prevent
explosions when methane is encountered in highwall auger

mining. Both theoretical calculations and experimental
data demonstrate the technique. Figure 4 shows an over-
view of the auger mining machine during testing.


Combined:
exhaust flow

Catalytic

Excess diesel convertor

exhaust vent

Critical:
orifice

Gasoline ‘\
engine \

Diesel
engine

Collar measurement.
location

Figure 3.—Schematic of inert gas system as tested.

using a time study during initial and final auger penetra-
tion, Inert gas volumes were determined by measuring
both hot and cooled gas velocities. The hot gas velocities
were measured at the end of the vertical 5-in-diameter ex-
haust pipe using a vane anemometer or Pitot tube. A 15-s
time interval was used to prevent damage to the anemom-
eter bearings from the exhaust heat. Pitot tube readings
were corrected for air density using a value of 0.0415 Ib/f®
to reflect the elevated temperature of about 500° F.
Cooled velocities were determined by measuring the actual
volume exiting the auger hole using a modified brattice
window method (4). This method determines low airflows
by obstructing an airway with brattice, cutting a small
window in the brattice, and measuring the velocity through
the window. In this case, the brattice covered the auger
hole while the exhaust gas entered the hole through the
stubbed pipe. Cooled gas exited through the hole in the
brattice, which was also the measuring point.

GAS TESTING
Three types of tests were conducted to measure the

inert condition of the hole: collar measurements during
mining, hole measurements with augers in place and

mining stopped, and postmining hole measurements after
augers were removed from the hole.

© Collar measurements were real-time monitoring of
the gas concentrations 4 ft into the collar of the hole.

¢ Hole measurements were taken by stopping the min-
ing operation and pushing the appropriate length of 3/4-in
PVC pipe into the hole, The sample end of the PVC pipe
was surrounded by a 4ft section of 4-in-diameter PVC
drain pipe with both ends tapered and taped to the outside
of the 3/4-in pipe. This "torpedo" shape bridged the auger
flights and allowed casier insertion of the sample pipe.
Using this method, gas concentrations could be obtained
within 11 to 26 ft of the face up to a maximum insertion
distance of 230 ft.

¢ Postmining measurements were taken after removal
of the auger from the hole. This method used the 3/4-in
PVC pipe inserted to various hole depths and for periods
of up to 24 h after completion of the hole. Maximum
depth of measurement was about 300 ft before the weight,
friction, and flexibility of the pipe prevented further
insertion.

Gas samples for all tests were withdrawn by a vacuum
pump through 100 ft of 1/4-in Tygon tubing. This tubing


INTRODUCTION

The auger highwall mining method is an effective meth-
od to recover coal from a reserve when removal of the
overburden by surface mining equipment becomes uneco-
nomical, In this method of mining, a horizontal auger
enters the coal seam from the surface mine bench under
the highwall and the coal is literally drilled in a series of
parallel holes, Historically, coal mined from the surface is
relatively shallow, and over time, methane associated with
the coal has dissipated through the surface. In most cir-
cumstances, little methane has been found associated with
auger mining. However, mining technology has enabled
surface mining of deeper reserves of coal. Furthermore,
environmental constraints have forced the highwall extrac-
tion method to be used to remove coal under wetlands,
further increasing the chances of encountering methane.
Recently, incidents of methane explosions at a few auger
mining operations have resulted in injuries and increased
testing for methane at the collars of auger holes. The fuel
source of the reported explosions was not necessarily lim-
ited to methane, but may also have involved coal dust.

The U.S. Bureau of Mines is currently conducting re-
search to better define the cause of the recent encounters
with methane and to determine if these encounters are
more common than previously thought. In the fall of 1991,
Bureau personnel detected the occurrence of methane in
excess of 2% in the interior of several highwall auger
holes. The occurrence of methane was surprising, yet well
documented. Several mining operations have been direct-
ed to stop mining because 2% methane was detected at
the collar of the hole. While 2% methane in air is non-
explosive, it represents 40% of the lower explosive limit
and is considered in underground mining as the point at
which corrective action must be taken. No technology was
available to ensure the safe resumption of mining.

The Mine Safety and Health Administration (MSHA)
met with the Bureau to discuss what technology might be
available to enable the safe resumption of mining, The
discussion included the difficulty of ventilating through the
solid shafts of the augers, the probability that steel bits
created the ignition source, and the potential solution of
inerting the holes with low oxygen and high carbon dioxide
concentrations from the machine’s diesel exhaust.

Considering the ventilation aspects of the problem, it
was not clear if ventilation could be reliably established.
If some degree of ventilation to the front of the mining
head is achieved, air may combine with methane to bring
the hole atmosphere from a rich, nonexplosive mixture to
an explosive mixture. Furthermore, ventilation may not
prevent a dust explosion in such a mining configuration.
Lack of access through the shafts of auger-type mining
machines further limits the ability to add water or air to

cool bits to prevent an ignition source from developing.
Either of these approaches would also be expensive.

The process of mining coal in an inert atmosphere has
been considered in the past, but apparently never imple-
mented (1)? Clearly, implementation in underground min-
ing would be more complicated. On a mine bench open
to the atmosphere, however, adding inert gas to the mining
head could provide a quick, feasible method to prevent
explosions at auger highwall mining operations. Also, the
problem of how to move the inert gas to the cutting head
of the machine has to be considered.

Preventing explosions on auger mining machines us-
ing inert gas requires three primary considerations: first,
finding a source of inert gas; second, placing the inert
gas at the cutter head; and third, monitoring the hole
atmosphere,

Any gas source having an effective inert gas concentra-
tion of 34 vol % or greater will prevent methane from ig-
niting (2). Sources of inert gas considered for this applica-
tion included liquid nitrogen, modified shipboard inert gas
generators (for hydrocarbon shipping and transfer), jet tur-
bine engine (3), the auger’s diesel engine, and a gasoline
engine. Operation cost, purchase cost, and availability lim-
ited testing to the diesel and gasoline engines. This work
tested each engine, separately and combined.

Two methods were tested to direct the inert gas to
the cutting head of the auger machine, The first was a
shrouded enclosure (fig. 1). The second was a “stub” pipe
inserted part way into the hole after the lead augers had
penetrated the face (fig. 2). Hither of these methods can
supply inert gas to the cutting head; however, the stub pipe
method appeared more practical. The gas at the cutter-
head comes from the collar or entrance of the hole and
reaches the cutting head by simple displacement of the
extracted coal. The total gas volume of the hole is equal
to the volume of coal extracted.

To ensure effectiveness, both company and enforcement
personnel need to know how to monitor the condition of
the inerted hole. Measurement at depth inside the hole is
possible by remote sampling through rigid tubing, but this
method is impractical for routine monitoring. Continuous
monitoring of the exhaust gas stream is an alternative.

The U.S. Bureau of Mines evaluated an inert gas sys-
tem at an auger mining operation at a surface mine near
Owensboro, KY. Coal was mined from the No. 9 Coalbed
in Henderson County, KY. Tests were conducted in Jan-
uary and March of 1992, This work was conducted as part
of the Bureau’s Mine Disaster Prevention Program,

italic numbers in parentheses refer to items in the list of references
at the end of this report.




METHANE ENCOUNTER
Hole Data

Subsequent testing, delayed by weather and equipment
problems, resumed in early March. This testing encoun-
tered substantial levels of methane, Gas levels in the hole
were inert when methane was encountered, which agreed
with the predictions from the preliminary testing. Table 4
shows the gas chromatograph results for all samples taken
inside the hole with augers in place. All holes were non-
explosive, but several measurements at shallow hole depth
had 14% oxygen but also low methane concentrations.
For the first hole, this oxygen dilution occurred during the
time the hole was being measured from the interior to the
exterior and the inert gas generator was off. For the sec-
ond hole, the higher oxygen reading may have been caused

KEY
Projected data

20 VIII Sy
impossibie|
mixtures,

15 Explosive when mixed with air N

ba

$

a

z

3

3 Nonexplosive
3
eT
0 10 20 30 40 50 60 70 80 90 100

EFFECTIVE INERT, vol %
[% EXCESS No+1.5x(%CO9)]

Figure 5.—Explosibility diagram for January. Gas data pro-
Jected as if various levels of methane were encountered.

by a leak in the gasoline engine exhaust line. In all cases
of high oxygen concentrations, methane levels were low.

Table 3,—Measured and computed gas concentrations in hole, percent

Hole Probe Analysis New gas levels! Excess Effective
depth, ft depth, ft 0, CO, N, 0, CO; N, N, inert
26 15 12.72 5.86 81.42 12.08 5.57 77.35 31.43 39.78
50 3% 12.84 5.63 81.53 12.20 5.35 T7745 31.10 39.12
68 40 13.05, 5.36 81.59 1240 5.09 7751 0.40 38.04
80 70 12.95, 5.51 81.54 12.30 5.23 77.46 71 38.57
92 70 12.82 5.74 81.44 12.18 5.45 7.37 31.09 39.27
104 90 11.63 6.17 82.20 11.05 5.86 78.09 36.11 44.90
116 9 12.05 6.06 81.89 11.45 5.76 77.80 34.30 42.93

New gas levels result from dilution by 5% added CH,.

Table 4—Gas chromatograph resulte—auger In hole

Hole Hole depth, Probe depth, ‘Analysis, vol %
f f 0,00, NCH,“ ExoesN, Effective inert
1 308 25 1482 © 4.30 79.27 0.591 22.95 29.40
308 75 4457 4.45 79.37 600 24.00 30.67
308 125 10.69 4.55 78.68 = 5.00 38.03 44.86
308 175 1065 488 7861 4.99 38.14 45.16
308 225 10.28 371 «7337.81 38.26 43,83
116 70 1448-461 «79.19 © 602 (24.16 31.08
3 170 130 10.54 498 7.79 «5.66 37.73 45.20
360 100 1158 «5.55 80.20 1.59 36.19 4452
360 200 1011 398 7.48 7.38 39.06 45.03
360 230 916 327 77.07 «9.85 42.26 47.16
4a 155 140 1278 402 78.64 3.50 30.07 36.10

1Coltar air dilution, see text, Inert generator off.
Exhaust gas leak.


Figure 7 also showed that methane concentrations near
the collar of the hole remained less than 1.0% after auger
removal. The data further showed that the high methane
levels in the hole interior did not flow from the hole at
that concentration. Rather, the methane was diluted in-
side the hole and any explosive gas mixtures created dur-
ing dilution occurred inside the hole. A final observation
from figure 7 was that with the augers in the hole, no
methane was present at the collar despite high levels in the
hole interior.

The long-term ventilation of the holes was determined
by measuring holes 1 and 2 the following morning. Hole 1
was left open to the atmosphere, and hole 2 was partially
sealed with brattice cloth. Table 5 contains the gas con-
centration differences. Concentrations the following morn-
ing showed methane levels in both holes had diminished
from the previous day, although the covered hole still re-
tained 1.7% methane. Hole 1 was ventilated with almost
normal atmospheric levels of oxygen, indicating that this
hole naturally ventilated. The sealed hole, in reality, was
not totally sealed and was partially ventilated. From these
data it is probably advisable to leave the holes open to
self-ventilate rather than try to seal them to maintain inert
concentrations.

Table 5.—Overnight ventilation of open and partially

‘sealed holes
Hole and Time, Sample CH, 0, CO, CO,
condition h depth, vol% vol% vol% ppm

ft

Topen... 0 308 5.94 11.69 313 538
20 8 38 20.60 10 )
2, sealed .. 0 00 456 14.56 247 204
15 300 17 18.9 60 8

Time To Reach Inert Conditions

The theoretical time to fill the initial hole created by
the head and lead augers (3.25 ft diameter by 26 ft deep)
with inert gas depended on the gas flow rate, gas concen-
tration, and the volume occupied by the augers and cut
coal. The empty hole volume was 216 ft? with about one-
half of this volume occupied by the auger steel and cut
coal. This left 108 ft?, At an inert gas flow rate of 56 cfm,
one complete air change occurred in less than 2 min, In-
jecting the inert gas through a high-velocity jet along the
top surface of the hole allowed for efficient penetration of
the jet toward the auger head, which exceeded the theoret-
ical time.

The concentration of the combined diesel and gasoline
engine exhaust stream at the end of the stub pipe con-
nector was 11.6% oxygen and 6.6% carbon dioxide. This

1

gas was directed into the hole with the stub pipe after the
cutting head and lead augers were inserted. For the sec-
ond hole, the depth was 26 ft. The hole measurement
probe was at a depth of 24 ft. The time to reach inert
conditions at the probe inlet was less than 2 min,

It is important to insert the stub pipe as soon as pos-
sible for two reasons. First, less time is required to reach
inert conditions when the initial volume is small. Second,
methane may be encountered near the highwall face. The
initial portion of the data from hole 3 in figure 8 from 9:30
to 9:42 show that when the hole was drilled 44 ft prior to
inserting the inert gas stub pipe, it took about 12 min to
reach inert conditions at the end of the hole sample pipe
located 33 ft into the hole. Furthermore, at that hole
depth, the initial methane concentration was already 2%.

COLLAR MEASUREMENTS

The collar measurement location for hole 3, figure 8,
showed the fluctuations of the gases found immediately
inby the collar of the hole. This measurement location
was 1 ft outby the end of the inert exhaust gas stub pipe.
This area may be subject to influence of outside airflows;
however, there was no noticeable periodicity that might be
attributed to auger placement or penetration. The large
fluctuation in the data at 10:44 was actually from a period
when the sampling instruments were disconnected from
the collar sample and shows the gas composition at a
probe depth of 130 ft. As stated earlier, the initial portion
of the data in figure 8 showed gas conditions 33 ft into the
hole as the hole was initially inerted.

GAS CONTENT

Methane content of the samples from hole 4 taken
in March averaged 0.30 cm?/g and ranged from 0.12 to
0.49 cm?/g. Ethane and higher hydrocarbons were consist-
ently found, at a concentration of 0.001 to 0.005 cm?/g,
respectively, Besides methane, ethane, and higher hydro-
carbons, all samples emitted carbon dioxide. Carbon di-
oxide emitted was an order of magnitude less than the
methane emitted. A summary of the gas content data is
shown in table 6.

All samples showed a distinct affinity for oxygen and
rapidly consumed all oxygen remaining in the container
after sealing. This phenomenon has been well document-
ed by Parr (8) and others, and in the past may have been
partially responsible for restricting the explosibility of
auger holes.

The emission rate of methane from the samples was
so slow that it would seem to preclude that explosive con-
ditions encountered in auger holes could be the result of


Library of Congress Cataloging in Publication Data:

Volkwein, J. C. (Jon C.)
‘A method to eliminate explosion hazards in auger highwall mining / by Jon C.
Volkwein and James P. Ulery.

Pp. cm. — (Report of investigations; 9462)
Includes bibliographical references (p. 14).
1. Mine explosions~Prevention. 2. Gases, Rare—Testing, 3. Coal augers. I.

Ulery, J.P. (James P.). I. Title. Il, Series: Report of investigations (United
States. Bureau of Mines); 9462.

TTMN23.043 622 s—dc20 [622’.83) 92-42010 CIP

ath


Report of Investigations 9462

A Method To Eliminate Explosion Hazards
in Auger Highwall Mining

By Jon C. Volkwein and James P. Ulery

UNITED STATES DEPARTMENT OF THE INTERIOR
Bruce Babbitt, Secretary

BUREAU OF MINES


Figure 4.—Overview of highwall mining operation as tested.

VOLUME MEASUREMENTS
Coal Volume Removed

In order for inert conditions to be maintained at the
head of the machine, the volume of inert gas produced
must exceed the volume of coal removed. Time studies of
coal removal showed that auger sections 17 through 27
required an average of 102s per cycle. Of that cycle time,
approximately 20 s was required for retraction of the kel-
ley bar, leaving 82 s for coal removal. The fastest cycle
time recorded was 90 s, or 70 s for coal removal. Each
added auger section removed a coal cylinder 3.25 ft in
diameter by 6 ft long, or 49.7 ft? of coal. Average coal
removal rate was calculated to be 35 ft?/min with a maxi-
mum removal rate of 42 ft?/min. At greater hole depth,

auger numbers 55 through 60, the average rate of removal
was calculated to be 27.0 ft/min with a maximum rate of
30.3 ft/min. Smaller diameter augers or slower penetra-
tion speeds will decrease this volume, while larger augers
or faster penetration speeds will increase this volume. The
inert gas generation system should be designed to supply
enough inert gas to fill the maximum volume of coal
removed plus a safety factor.

Inert Gas Volume

The available gas volume may be determined by the
hot exhaust gas flow from the engines or the volume of
gas leaving the hole. Hot engine exhaust gas condens-
es and contracts as it cools inside the auger hole, and
consequently, a large correction factor must be used to


10

As methane was encountered, the oxygen levels de-
clined. Simple dilution of the oxygen with methane ac-
counts for this decline. This new gas mixture with dimin-
ished oxygen followed the cutting head deeper into the
hole. This observation may be inferred from the data in
table 4, hole 3. Note that at a hole depth of 170 ft and a
sample probe depth of 130 ft, there was 10.54% oxygen
and 5.66% methane. After the hole’s completion at 360 ft,
the gas levels 100 ft into the hole had changed to 11.58%
oxygen and 1.59% methane. The original high levels of
methane were now deeper in the hole. This is to be ex-
pected based on the premise that outby gases follow the
cutterhead into the hole to replace the removed coal.

Because of the displacement of the coal with whatever
gas is available, methane gas generated at any point in the
hole will follow the auger head into the hole. The inert
exhaust gas from the collar then serves as a reservoir sup-
plying the required displaced volume minus the methane
volume.

Vinert = Vote ~ VMerHane

Theoretically, at some point, the hole could inert itself if
sufficient quantities of methane were present. It is pos-
sible, but unlikely, that quantities of methane in excess of
the hole volume would be encountered. In that situation,
pure methane gas would reach the collar and the zone of
dilution through the explosive range would take place
outside of the hole.

Explosibility Diagram

Figure 6 plots the available hole data on the explo-
sibility diagram. For all hole conditions monitored, no
explosive mixtures were encountered. If high methane

KEY
QExperimental data
20 y y y T T SS
impossible
ixtures,
15 Explosive when mixed with afr
x
3
¢
=
&
8 ,
o Nonexplosive
a
4 1 est 1 1 1 1 n 1
0 10 20 30 40 50 60 70 80 90 100

EFFECTIVE INERT, vol %
[% EXCESS Not 1.5X(%CO2))

Figure 6.-Explosibility diagram for March data with observed
gas concentrations plotted for all holes.

concentrations are encountered, the fuel-rich inert gas mix-
tures may become explosive if diluted with air (the upper
portions of figure 6). Therefore, it is important to main-
tain the inert gas flow until the auger is removed from the
hole. After a hole is completed, the auger head, which is
the primary ignition source, is removed and the inert gas
and methane in the hole will be ventilated with air.

Hole Profile

To better understand how the hole ventilated, holes 1
and 3 were profiled with the augers in place and immedi-
ately after auger removal. This analysis, shown in fig-
ure 7, indicated that the high methane levels present near
the cutting head were gradually diluted inside the hole fol-
lowing extraction of the auger head. At methane concen-
trations over 5%, with the augers removed, sufficient inert
gas was still present to preclude an explosion. When the
inert gas system was left on during auger removal, the di-
lution of gas in the hole took place with inert gas rather
than air, However, auger removal was rapid, about 25 s
per cycle with 15 s per cycle required for pin pulling and
stacking of the auger. Rapid removal of the auger steel
volume slightly exceeded the inert gas volume. This was
not considered significant based on the observed decline in
methane levels,

10 T T T T T 7
Hole 1 a

—®= Auger inhole
—A~ Auger removed

CHa, mol %

L 1 n L

150 200
DEPTH, ft

250 300-350

Figure 7.—Profiles of methane concentrations In holes 1 and
3 at maximum hole depth with augers In place, and immediately
after auger removal.