United States Department of the Interior 

Bureau of Mines 
Cover Photograph: A highwall auger mining machine 

with roof-mounted inert gas generator safely mines coal 

in a gas-laden area Photo by J. C. Volkwein, Pittsburgh 

Research Center, U.S. Bureau of Mines. 
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 
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. 

p. cm. - (Report of investigations; 9462) 

Includes bibliographical references @. 14). 

1. Mine explosions-Prevention. 2. Gases, Rare-Testing. 3. Coal augers. I. 

Ulery, J. P. (James P.). 11. Title. 111. Series: Report of investigations (United 

States. Bureau of Mines); 9462. 

TN23.U43 622 S-4~20 [622'.83] 92-42010 CIP 
Page 

Abstract ........................................................................... Introduction ........................................................................ Acknowledgments .................................................................... Methods ........................................................................... Systemastested ................................................................... Volumetesting .................................................................... Gastesting ....................................................................... Analysis of explosibility .............................................................. Gas content measurement ........................................................... Personaltesting ................................................................... Resultsanddiscussion ................................................................ Volume measurements .............................................................. Coal volume removed ............................................................. Inertgasvolume ................................................................ Delivery of gas to auger head ......................................................... Preliminarytest ................................................................... Holedata ..................................................................... Explosibilitydiagram ............................................................. Methaneencounter ................................................................ Holedata ..................................................................... Explosibilitydiagram ............................................................. Holeprofile .................................................................... Time to reach inert conditions ...................................................... Collar measurements ............................................................... Gascontent ...................................................................... Personalmonitoring ................................................................ Systemsafety ..................................................................... Observations ..................................................................... Conclusions ........................................................................ References ......................................................................... 

ILLUSTRATIONS 

1 . Inert gas shroud enclosure of auger hole collar ........................................... 2 . Inert gas stub pipe used to direct inert exhaust into the hole ................................. 3 . Schematic of inert gas system as tested ................................................. 4 . Overview of highwall mining operation as tested .......................................... 5 . Explosibility diagram for January ..................................................... 6 . Explosibility diagram for March data with observed gas concentrations plotted for all holes .......... 7 . Profiles of methane concentrations in holes 1 and 3 at maximum hole depth with augers in place, and 

immediately after auger removal .................................................... 8 . Corrected infrared gas concentrations at the collar sample location for hole 3 .................... 

TABLES 

1 . Hot and cooled volumes of inert gas for various engines and conditions ......................... 2 . Hole measurement results showing low oxygen concentrations in hole interior .................... 3 . Measured and computed gas concentrations in hole ....................................... 4 . Gas chromatograph results-auger in hole .............................................. 5 . Overnight ventilation of open and partially sealed holes .................................... 6 . Coalbed gas content of samples from hole 4, No . 9 Coalbed, Henderson County, KY ............... 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

c fm cubic foot per minute in3 cubic inch 

cm3/g cubic centimeter per gram Ib/ft3 pound per cubic foot 

OF degree Fahrenheit min minute 

ft foot ppm part per million 

ft3 cubic foot ~sig pound per square inch gage 

ft3/min cubic foot per minute rPm revolution per minute 

h hour s second 

in inch scfh standard cubic foot per hour 
A METHOD TO ELIMINATE EXPLOSION HAZARDS IN AUGER 

HIGHWALL MINING 

By Jon C. ~olkwein' and James P. ~le$ 

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, methane, and carbon monoxide. Evacuated bottle samples were also taken and analyzed by gas chromatography 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 exposure to carbon monoxide was also monitored. The highest methane level observed was 9.55%. The levels 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 cm3/g. The maximum personal time-weighted average sample for carbon monoxide was 20 ppm. This system provides a safe, inexpensive, simple method for preventing explosions during auger mining. 

'physical scientist. 

2~eologist. 

Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 
INTRODUCTION 

The auger highwall mining method is an effective method to recover coal from a reserve when removal of the 

overburden by surface mining equipment becomes uneconomical. 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 circumstances, 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 extraction 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 limited to methane, but may also have involved coal dust. 

The U.S. Bureau of Mines is currently conducting research 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 directed to stop mining because 2% methane was detected at 

the collar of the hole. While 2% methane in air is nonexplosive, 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 

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 implemented Clearly, implementation in underground mining 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 using 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 concentration of 34 vol % or greater will prevent methane from igniting (2). Sources of inert gas considered for this application included liquid nitrogen, modified shipboard inert gas 

generators (for hydrocarbon shipping and transfer), jet turbine engine (3), the auger's diesel engine, and a gasoline 

engine. Operation cost, purchase cost, and availability limited 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). Either of these methods can 

supply inert gas to the cutting head; however, the stub pipe 

method appeared more practical. The gas at the cutterhead 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 system 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 January and March of 1992. This work was conducted as part 

of the Bureau's Mine Disaster Prevention Program. 

prevent a dust explosion in such a mining configuration. 

Lack of access through the shafts of auger-type mining 3~talic numbers in parentheses refer to items in the list of references 

machines further limits the ability to add water or air to at the end of this report. 
4 L. 

- a. *.- - th. 

Figure 2.4nert gas stub pipe used to direct Inert exhaust into the hole. Note smail4iameter pipe used for collar 

measurements. 
ACKNOWLEDGMENTS 

The authors gratefully acknowledge the cooperation of The authors also would like to express appreciation for the 

John Urosek, Gary Wirth, and Mike Snyder of the Mine contributions of Michael McCullough, John McCullough, 

Safety and Health Administration, Pittsburgh Technical and John Wiegand, of B&E Auger Mining Co., for their 

Support, for their consultation and loan of instrumentation. 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 concentrations. This was accompanied by an analysis of the explosibility 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-in3 General Motors4 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 consistently produce low enough oxygen concentrations to 

reliably prevent explosions. 

Although the gasoline engine alone produced oxygen 

concentrations in the 1% to 4% range, the volume was insufficient 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 

4~eference to specific products does not imply endorsement by the 

U.S. 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 inverted 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 entry. 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 discharge 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 muffler (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 approximately equal proportions of diesel and gasoline exhaust. 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 calculated volume of 49.7 ft3. Rate of removal was determined 
were measured at the end of the vertical 5-in-diameter exhaust pipe using a vane anemometer or Pitot tube. A 15-s 

time interval was used to prevent damage to the anemometer bearings from the exhaust heat. Pitot tube readings 

were corrected for air density using a value of 0.0415 Ib/ft3 

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 

Collar measurements were real-time monitoring of 

the gas concentrations 4 ft into the collar of the hole. 

r Hole measurements were taken by stopping the mining operation and pushing the appropriate length of 314-in 

PVC pipe into the hole. The sample end of the PVC pipe 

was surrounded by a 4-ft section of Cin-diameter PVC 

drain pipe with both ends tapered and taped to the outside 

of the 314-in pipe. This "torpedo" shape bridged the auger 

flights and allowed easier 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. 

r Postmining measurements were taken after removal 

of the auger from the hole. This method used the 314-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 114-in Tygon tubing. This tubing 
was connected to the collar sample pipe or the holemeasuring 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 immediately 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 carbon 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 testing. 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 between 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 Pittsburgh 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 chromatograph analysis results. 

ANALYSIS OF EXPLOSlBlLlN 

The explosibility of a particular gas mixture was analyzed by the method of Zabetakis (2,5). This method calculates 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 frne material, which 

may have lost gas more quickly prior to sampling. Samples of even larger size, although desirable, would not fit 

in the air-tight sample containers. Gas content measurements 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 container. 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. Samples 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 monitored. Miners wore Mine Safety Appliances stain tubetype 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 monoxide levels were measured during collar sampling. 

RESULTS AND DISCUSSION 

The implementation of the inert gas concept was shown mining. Both theoretical calculations and experimental 

to lower oxygen concentrations sufficiently to prevent data demonstrate the technique. Figure 4 shows an overexplosions when methane is encountered in hqghwall auger view of the auger mining machine during testing. 
Figure 4.4verview 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 102 s per cycle. Of that cycle time, 

approximately 20 s was required for retraction of the kelley 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 ft3 of coal. Average coal 

removal rate was calculated to be 35 ft3/min with a maximum removal rate of 42 ft3/min. At greater hole depth, 

auger numbers 55 through 60, the average rate of removal 

was calculated to be 27.0 ft3/min with a maximum rate of 

30.3 ft3/min. Smaller diameter augers or slower penetration 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 condenses and contracts as it cools inside the auger hole, and 

consequently, a large correction factor must be used to 
determine the true exhaust volume if hot gas flow is measured. 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 measurement is required, the brattice window technique can be 

used to determine the cooled gas volume. 

Table l.4ot and cooled volumes of inert gas for various 

engines and conditions 

~ot' Cool Derived 

Condition Veloc- Val- vol- correction 

ityI3 ume, umeI2 factor, 

fpm cfrn cfrn cool/hot 

January: 

Gas at 2,600 rpm ... 740 101 13 0.129 

Gas at 3,900 rpm ... 1,680 229 28 .I22 

Diesel plus gas ..... 3,280 447 456 NA 

Do. ............ NA NA 64 NA 

March: 

Diesel plus gas ..... '3,100 424 50 .I18 

Do. ............ 3,100 424 55 .I30 

Average ........................... .I25 

NA Not available. 

'Measured in stub pipe. 

2~rattice window measured. 

35-in pipe. 

4~alculated based on correction factor of 0.125. 

'Measured with Pitot tube at hot air density of 0.0415 lb/f?. 

The minimum cooled gas volume found during testing 

with both engines was 50 cfrn (table 1). The maximum 

rate 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 cfrn 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 effectiveneks and simplicity of the stub pipe method, only 

that method is reported. 

Table 2.4ole measurement results showing low oxygen 

concentrations in hole interior 

Hole depth, Probe depth, Oxygen 

Hole ft ft concentration, 

vol % 

1 ....... 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 functioning 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 atmosphere in the hole would be nonexplosive because of the 

presence of the inert gas and low levels of oxygen. Table 3 shows the drilling depths, probe depths, and corrected 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 concentrations would be diluted. New levels of these gases are 

computed. Using the method of Zabetakis (2,5), the explosibility of the gas mixture is determined by computing 

the volume of excess nitrogen and amount of effective inert. At 5% methane, a gas with an effective inert concentration over 34 vol % is considered nonexplosive. 

Explosibility Diagram 

Figure 5 shows an explosibility diagram and the preliminary data extrapolated as if various methane concentrations were encountered. The figure shows that the closest 

approach to the explosive range at the inert gas concentrations measured occurs between 5% and 6% methane concentration. At the closest point, an 11% [(I-34/38)x100] 

reduction in effective inert gas concentration is required to 

reach the explosive range. This figure further demonstrates that as higher levels of methane are encountered, 

the effective inert concentrations move further from the 

explosive range. 
116 90 12.05 6.06 81.89 11.45 5.76 77.80 34.30 42.93 

'~ew gas levels result from dilution by 5% added CH,. 

Table 4.4as chromatograph results-auger in hole 

Hole Hole depth, Probe depth, Analysis, vol % 

ft ft 0, CO, N, CH, Excess N, Effective inert 

1 ... 308 25 '14.82 4.30 79.27 0.591 22.95 29.40 

308 75 '14.57 4.45 79.37 .600 24.00 30.67 

308 125 10.69 4.55 78.66 5.00 38.03 44.86 

308 175 10.65 4.68 78.61 4.99 38.14 45.16 

308 225 10.28 3.71 77.33 7.61 38.26 43.83 

-- 

'~ollar air dilution, see text, inert generator off. 

2Exhaust gas leak. 
Explosibility Diagram 

Figure 6 plots the available hole data on the explosibility diagram. For all hole conditions monitored, no 

explosive mixtures were encountered. If high methane 

KEY 

o Experimental data 

EFFECTIVE INERT, vol % 

Hole 1 

KEY , - * - Auger in hole / 

9' I - Auger removed 4 

10 I 1 1 I 

Hole 3 9' I 

8 - / / - A 

6 - 

4 - 

0 50 100 150 200 250 300 350 

DEPTH. ~ - ft 

[% EXCESS NZ+ 1.5 X(%CO~)] 

Figure 7.--Profiles of methane concentrations in holes 1 and 

Figure 6.--Explosibliity diagram for March data with observed 3 at maximum hole depth with augers in place, and Immediately 

gas concentrations plotted for all holes. after auger removal. 
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 inside the hole and any explosive gas mixtures created during 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 concentration differences. Concentrations the following morning showed methane levels in both holes had diminished 

from the previous day, although the covered hole still retained 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 

1, open . . . o 308 5.94 11.69 3.13 538 

20 308 .38 20.60 .10 0 

2, sealed . . 0 300 4.56 14.56 2.47 204 

15 300 1.7 18.9 .60 8 

gas was directed into the hole with the stub pipe after the 

cutting head and lead augers were inserted. For the second 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 possible 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 

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 concentration, and the volume occupied by the augers and cut 

coal. The empty hole volume was 216 ft3 with about onehalf of this volume occupied by the auger steel and cut 

coal. This left 108 ft3. At an inert gas flow rate of 56 cfm, 

one complete air change occurred in less than 2 min. Injecting 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 theoretical time. 

The concentration of the combined diesel and gasoline 

engine exhaust stream at the end of the stub pipe connector was 11.6% oxygen and 6.6% carbon dioxide. This 

Methane content of the samples from hole 4 taken 

in March averaged 0.30 cm3/g and ranged from 0.12 to 

0.49 cm3/g. Ethane and higher hydrocarbons were consistently found, at a concentration of 0.001 to 0.005 cm3/g, 

respectively. Besides methane, ethane, and higher hydrocarbons, all samples emitted carbon dioxide. Carbon dioxide 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 documented 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 conditions encountered in auger holes could be the result of 
9:30 10~15 11:OO 11:45 12:30 

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. 

~e~ative 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 conveyor 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 installed 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 observed 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. 
SYSTEM SAFETY present at the collar. Two measurements are required to 

determine this condition: a volume measurement of the 

Since the safety of the system relies on sufficient quan- exhaust gas flow and the concentration of oxvm. 

tity and quality of inert gas from the engines, these must The volume measurement may be taken with a vane 

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. Furthermore, the condensing water may also serve to suppress 

dust. 

OBSERVATIONS 

Sampling of the interior of auger holes is not easily 

accomplished. Turbulence and air eddies near the collar 

make accurate measurements in this area difficult. The 

data support the thesis that gas concentrations immediately 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 

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 direction of movement of dust or smoke during periods of maximum 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 remained 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 compliance monitoring or instrument confirmation. Since combustion engines reduce oxygen levels and produce carbon 

dioxide, measurement of oxygen alone from a combustion 

source implies the production of carbon dioxide. Therefore, 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 concentrations under all circumstances when an ignition was 

possible. The method described represents the best approach 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 engines, 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 collar of the hole with inert gas. Gas volumes for shroud 

systems were not determined. 

Application of this technique is simple and easily monitored. 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, therefore, 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 bi: monitored 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. Measurement of the volume and concentration of gas at the collar 

of the hole is indicative of the conditions at the cutterhead. To ensure that inert conditions are present at the 

cutterhead during mining, a sufficient volume of low oxygen concentration gas is required. Successful implementation of this simple method will improve safety of highwall 

mining by eliminating possible explosion hazards. 
REFERENCES 

1. Federal Water Quality Administration, U.S. Department of the 

Interior. Feasibility Study on Mining Coal in an Oxygen Free Atmosphere. 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. Pankowski, V., G. A. Tracey, and A. Wojtyczka. CMI Inert Gas 

Mine Firefighting System (contract J0318080, CSE Corp., Monroeville, 

PA). BuMines OFR 212-83, 1982,56 pp.; NTIS 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. Determining 

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 

Proceeding of the 1991 Coalbed Methane Symposium. Univ. AG 

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.