CONTROL AND MONITORING 

OF GAS IN BLIND AUGER HEADINGS 

Jon C. Volkwein 

Pittsburgh Research Center, P.O. Box 18070, Pittsburgh, Pennsylvania 

ABSTRACT 

Advances in mining technology have enabled deeper seams of 

coal to be mined from the surface. Highwall mining is frequently 

used to mine areas at the perimeters of a surface mine when removal of the overburden becomes uneconomical or threatens 

environmentally sensitive surface features. At the same time, advances in materials and technology have enabled highwall mining 

of coal that create blind headings over 300 meters in length. 

Deeper surface mining and deeper highwall penetration depths 

have lead to more frequent encounters with methane gas. While 

this coal is remotely mined from the surface, underground ignitions of methane have propagated to the surface and injured miners. 

Methods described in this paper show how this hazard can be 

correctly monitored and controlled for auger type highwall mining 

machines. Research indicates that small quantities of gas liberated during mining migrate to the face, creating higher gas concentrations than would otherwise be expected. Using the same 

principles that cause the gas to migrate to the face, researchers 

have shown that inert gas introduced at the collar of the hole can 

inert the entire hole, thus ensuring safe conditions for mining. 

INTRODUCTION 

The highwall coal mining method is used to recover coal reserves from a surface mine when removal of overburden with surface mining equipment becomes uneconomical. Auger and continuous miner head machines are the two major types of equipment used in U. S. highwall mining. The auger type machines use 

a horizontal auger to literally drill coal from under the highwall in 

a series of parallel holes. These augers vary in size from 50 cm 

to 2.3 m in diameter and penetrate up to 150 m under the 

highwall. Continuous miner type machines use a remote controlled continuous mining machine coupled with a continuous 

haulage system to penetrate to depths over 300 m. 

Historically, coal mined from the surface is relatively shallow 

and over time the methane associated with the coal has dissipated through the surface. In most circumstances, little methane 

has been found associated with highwall 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 recover coal under wetlands, further increasing the chances of encountering methane. In 

fact, mine inspectors have been more frequently reporting the 

finding of small accumulations of methane near the entrance to 

auger holes. 

In the early 1990's an increasing incidence of ignitions at 

highwall mining operations was reported. The source of these ignitions was reportedly methane gas, with some involvement of 

coal dust, that was presumed to have been ignited from picks 

striking rock. A number of injuries and one fatality from ignitions 

resulted in the Mine Safety and Health Administration (MSHA) 

requesting that the Bureau of Mines help develop solutions to the 

ignition problem. While the number of occurrences was relatively 

small, the accident rate for the small highwall industry was high 

and aggravated by the fact that no technology was available to 

either effectively evaluate the extent of the problem let alone to 

ensure safe highwall mining. While an inert method to prevent 

ignitions has been developed (Volkwein 1993) operators still have 

no sensors to assess gas conditions during mining. In fact, a recent ignition in January 1996 demonstrates a need for the development of a real time gas sensor for auger machines. 

Approaches to prevent ignitions included ventilation, blast 

shields, and pumping inert gas to the face through the shaft. The 

ventilation of augers is unreliable because of the difficulty in establishing a leak free low pressure intake and return airpath to the 

face. If adequate ventilation can not be guaranteed and monitored, then partial ventilation may create an unsafe condition by 

combining air with a nonexplosive rich methane mixture to create 

an explosive mixture. Furthermore, ventilation may not prevent a 

dust explosion in such a mining configuration. Lack of access 

through the shafts of the augers further limits the ability to add 

water or air to cool bits to prevent an ignition source from developing. Blast shields are considered unacceptable since any unplanned explosion is potentially unsafe. The study of the highwall 

ignition problem has resulted in a better understanding of how 

methane accumulates in auger excavations and in the development of an effective method that uses inert gas to prevent ignitions. 

MONITORING METHODS 

Assessment of the auger excavation environment is a primary 

requirement to determine if in fact any methane gas hazard exists. 

The highwall mining method represents a unique niche between 

surface mining of coal in the open air and underground mining. 

With this method, coal cutting takes place underground, but mining is controlled from a surface location. There has been no U. S. 

legislation requiring that gas measurements be taken and only 

recently has there been an MSHA policy directive to monitor for 

methane at the collar of the excavation. The reason for this originates from an old belief that coal seams above drainage do not 

contain methane gas and therefore no assessment was required. 

However, the 1974 mining law recognized that some "above 

drainage" seams did contain methane and hence the law considered all underground coal to contain gas. (Code of Federal Regulations 1974) This law, plus the recent incidences of ignitions, has 
Figure 1. Current hand held and probe measurement of methane In auger mining. 

prompted an increased need for more accurate gas assessments 

in highwall mining. 

Current Monitoring Practice 

Currently, methane in highwall mining is assessed using underground methods. Theseunderground methods measure gas 

levels with machine mounted methane monitors. or hand held aas 

detectors. The machine mounted monlor can be used for the &ntinuous miner type highwall machine, but cannot be easily 

adapted to an auger machine since the mechanism fills the entire 

excavation and would require multiple electrical connections 

whose safety protection would be difficult to maintain. Auger miners currently use only hand held devices to check for methane 

gas. Safety considerations prevent miners from entering an auger 

excavation to take gas readings and therefore, hand held readings can only be obtained at the collar of the hole or at best using 

a short probe to reach about 3 meters inby the collar as seen in 

figure 1. 

Hand held methods give misleadingly low estimates of true 

face methane gas levels. Operators reported that little methane 

gas was observed at collar measurement locations when coal was 

being extracted, but when augers were being removed from the 

hole after mining, methane could be detected. During research on 

the use of inertgas for prevention of ignitions, methane gas levels were observed to increase with hole de~th. Data trends in fiaure 2 indicates that during auger penetraiion, no gas would be 

detected at the collar of the hole (depth 0) and that small quantities could be observed after removal of the augers. Of more importance is the fact that higher methane levels deep in the hole 

Hole I .- 

KEY 00 

-I - ~uprr in hole 

Auger removed 

DEPTH, m 

Flgure 2. Profiles of in-hole methane content wlth augers In place and lmmedlately 

alter auger removal. 

cannot be detected by collar measurements. 

Gas Movement Inside Hole 

Fufiher study of the situation shows that gas must replace the 

volume of coal removed from the mining face. In most instances : 

this replacement gas is air from the hole collar. However, small 

quantities of methane gas liberated inside the excavation will also 

replace the coal volume removed. In fact, small volumes of gas 

liberated within the hole will tend to follow and volumetrically inas coal is removed, methane gas liberated inside the hole more 

easily replaces the coal that has been removed than air from the 

crease near the cutter head. Figure 3 illustrates the principle that 

I 

1 

outside of the hole. The net result being an accumulation of methane near the cutting head. This is counter intuitive to most ventilation engineers experience where methane levels continually dilute. This does not imply that methane self concentrates, but that 

it travels to the face replacing air as the diluent resulting in an increased methane percentage at the face. Data from Table 1, 

taken under inert gas conditions, show methane concentrations 

40 m into the hole at a hole depth of 52 m were 5.66%. When the 

hole was completed at a depth of 11 0 m, gas concentrations at 40 . m were 1.59%. The high concentration gas originally at 40 m was 

now detected closer to the cutting head. At a distance of 70 m, 

methane concentration was 9.55%. Conditions precluded measurements closer to the cutting head. 

Table 1. Auger hole methane concentrations Increase toward the face 1 

Hole depth. m w ane. vol% 

52 40 5.66 

110 30 1.59 

110 61 7.38 

110 70 9.55 

This discovery ~ of gas movement 

- within the hole lead to the 1 

successful development of an inert gas technique to prevent ignitions on auger machines and will be discussed later. The motive 
1 Replace nt 

~ir,n 1 

Figure 3. Principle of gas volume replacing the removed coal volume and the accumulation of methane within the hole. 

force of gas movement in the hole comes from the removal of the 

coal from the face and the replacement of that coal volume with 

an equal volume of gas. Furthermore, in the absence of other 

ventilation forces, this replacement phenomena occurs in any dead 

heading, including the continuous miner highwall type systems. 

Continuous Assessment of Gas Levels During Mining 

Because collar measurements of methane levels are clearly 

inadequate, continuous measurement methods of in-hole gas levels are being developed. Research techniques used for in-hole 

measurements during the development of the inert gas system 

are not practical for everyday use or even for periodic MSHA inspections. We therefore investigated a method of using a permissible recording hand-held multi-gas monitoring instrument to 

record methane and oxygen levels during mining.(Volkwein 1994) 

Briefly, this monitor was inserted into the center of the auger and 

supported with an inflatable bladder. Air from the hole was 

pumped to the instrument, data recorded and then retrieved after 

the auger was removed from the hole. Details of this method are 

in the reference. Figure 4 shows the methane gas data for a gassy 

coal (using the inert gas system) and a "non-gassy" coal. Note 

that as hole depth increases, methane gas levels increase. Of 

particular interest is that the coal considered non-gassy did in fact 

contain small amounts of methane that did accumulate towards 

the face. The results of the study of recording data during mining 

convinced researchers that electronic instrumentation could survive the stress of being mounted to a rotating auger being used 

to cut coal. Furthermore, data gathered during the testing provide 

compelling evidence that gas levels in auger excavations should 

be continuously monitored. 

Work in progress is focusing on the use of a wireless technique 

to transmit data from the head of the auger to the machine 

operator's station. In principle, perfection of this data transmission 

technique should be useful for transmitting various types of data 

from the interior of auger holes to the operator's location. The criteria for this system is that it be inexpensive, intrinsically safe, 

battery operated, and be able to transmit gas data. The machine 

operator's unit will then decode the signal and give a real time 

measurement of the methane, or oxygen concentration near the 

head of the mining machine. 

IGNITION PREVENTION 

Study of the dynamics of highwall mining lead to a successful 

DEPTH OF HOLE. m 

I 

- -Cali bration gas B 

- 

Figure 4. Continuous methane gas concentrations during mining of a gassy and 

non-gassy coal. 

- 

- 

use of inert gas as a means of preventing highwall ignitions. The 

use of inert gas to prevent ignitions in coal mining had been considered in the past (Cyrus 1970; Poundstone 1975; Castanoli 

1954) but never implemented due to, among other reasons, the 

inability of placing gas at the face of the cutting machine. Previous ideas on using inert gas to prevent ignitions on a coal mining 

face all relied on utilization of a second conduit through which inert gas could be delivered to the face. This could not be easily 

accomplished with auger type machines since the auger fills the 

entire cross sectional area of the hole. Delivering the gas through 

the center of the auger steel would require major modification to 

each auger section and development of a method to get the gas 

into the interior of the auger. Better knowledge of gas movement 

within the auger hole lead to the current technique of filling the 

collar of the auger hole with inert gas and allowing the displacement of the coal to provide the motive force for transporting inert 

gas to the cutting head. 

This method, shown in figure 5, prevents ignitions of both 

methane and dust by reducing oxygen levels below explosive limits. This is done by replacing oxygen with an inert gas consisting 

primarily of nitrogen and carbon dioxide. Inert gas may be obtained from several sources on highwalls, but the simplest approach is to generate the gas on-site using combustion processes 

to burn oxygen from the air. For the initial testing a combination 

of gasoline and diesel combustion engine exhaust gases were 

used to achieve enough gas to replace the coal volume and contain less than 12% oxygen by volume. Alternatively others have 

used furnace type technologies to produce the correct inert gas 

quantity and quality. 

Briefly, the system as tested consisted of combining exhaust 

-. 

I b