Comparison Of Methane Concentrations At A Simulated Coal Mine Face During Bolting

Charles D. Taylor Edward D. Thimons Jeanne A. Zimmer

NIOSH NIOSH NIOSH

Pittsburgh, PA 15236 Pittsburgh, PA 15236 Pittsburgh, PA 15236

 U.S.A U.S.A. U.S.A.

ABSTRACT

Strategies for monitoring methane levels during roof bolting in an extended cut entry were evaluated. Testing was conducted

at the Pittsburgh Research Laboratory’s methane test gallery. Operating conditions were varied and methane releases at the

face and drill holes were simulated. Methane readings were taken at the face, and at locations outby the face. Results show

that, when the major source of methane was at the drill holes, the highest methane readings were at locations on or 6.1 m (20

ft) inby the bolting machine. Methods for selecting better outby sampling locations, when the majority of the methane is

released at the face, are examined. During roof bolting it is easier to sample at these outby locations rather than at the face. 

Further testing is needed to determine what relationship exists between methane concentrations at the outby locations and at

the source of the methane release.

KEYWORDS: Roof bolting, Methane monitoring, Ignitions

INTRODUCTION

Almost half of all continuous coal mine sections today have

approvals for making extended cuts. Any cut, with a

continuous mining machine, that extends more than 6.1 m 

beyond permanently installed roof support (usually the last

row of roof bolts) is considered an “extended cut.” 

Research at the Pittsburgh Research Laboratory (PRL)

includes the development and testing of sampling strategies

for monitoring methane levels during extended cutting.

Frictional ignitions can occur during mining and bolting

operations. During mining, a flammable mixture of

methane, primarily released from the face, is ignited when it

comes in contact with hot metal deposited on the rock by the

cutting bits.

A review of MSHA roof bolter face ignition reports, for

the period 1981 through 1994, shows that almost all the

ignitions during bolting originated at the collar of the drill

hole. In most cases, methane was ignited by a hot drill bit

that had been cutting hard or abrasive rock. Usually the

flame started at the hole and traveled only a short distance.

The source of the methane during these bolting incidents is

believed to be roof gas feeders at or near the drill hole. 

Methane is liberated at the face at a lower rate than

during mining, but the gas can accumulate at the face if

insufficient face ventilation is provided. For some of the

bolter ignitions, methane at the face could have contributed

to the spread of the flame from the drill hole.

Drilling usually occurs away from the face. During

bolting of an extended cut entry the face could be 11 m

(36 ft) or more from the roof bolter. The primary way to

assure that methane concentrations are not ignitable is to

monitor methane levels near the drill hole. Measurements

must also be taken during bolting to determine methane

concentrations at the face.

Federal regulations require monitoring of methane

levels at the mining face during mining and roof bolting to

assure that gas concentrations do not exceed one percent. 

Monitoring must be conducted:

“At 20-minute intervals, or more often if required in

the approved ventilation plan at specific locations,

during the operation of equipment in the working

place.” (CFR 75.362 (d) (1) (iii))

The readings are to be taken at least 0.3 m (1 ft) from

the face or roof. In most cases the readings are

accomplished by holding a methanometer approximately 

0.3 m from the face.
Figure 1. Methane test gallery.

Figure 2. Model roof bolting machine.

If the distance to the end of the cut is more than 6.1 m, it

is more difficult to comply with the monitoring requirements

because the readings must be taken from under permanently

supported roof. If the entry is not bolted up to the face,

“..extendable probes or other acceptable means...” (CFR

75.362 (d) (2)) must be used to position the methanometer at

the face.

Several different techniques have been tried to measure

methane at the face during bolting of a deep cut. None of

the techniques currently available for making methane

measurements during bolting are easy to use, or have been

widely accepted. Compliance with the methane standard

would be easier if there were alternative sampling locations

outby the face. Outby sampling locations closer to the

bolting operation could also provide better measurements of

methane when the primary liberation point is the drill hole.

During a series of tests at the PRL, an extended cut roof

bolting operation was simulated in the surface test facility.

Methane concentrations were measured at three locations

0.3 m from the face, and eight locations farther outby the

face. Methane concentrations were measured at the face and

outby locations for different gas release locations and

operating conditions. Comparisons of concentrations at the

sampling locations were made using scatter diagrams and

linear models.

Test Facility and Roof Bolter

Testing was conducted in PRL's Methane Test Gallery. One

side of the "L" shaped building (see figure 1) is designed to

model an underground mining face entry which has

dimensions, 5 m (16.5 ft) wide by 2.1 m (7 ft) high by 37 m

(120 ft) long. The return air from the face exits the entry

behind a brattice and wood wall located on the right side of

the entry.

A full-scale model of a roof bolter (figure 2) was used

during these tests. The width of the model bolter was made

slightly narrower than a normal bolter to conform in

proportion to the 5 m width of the entry. The model had

two drill booms, which were kept in a fully raised position,

and an ATRS system (T-bar).

Methane Sampling Locations

Catalytic heat of combustion methanometers were used to

monitor methane levels at each of 11 sampling locations. 

The locations were divided into three areas designated,

“face,” “sweep” and “machine.” The sampling locations

are described below and show in figure 3.

Face Area (Locations 1, 2 and 3)

 1 0.3 m from roof and manifold and 0.6 m (2 ft)

from right wall

 2 0.3 m from roof and manifold at center of entry

 3 0.3 m from roof and manifold and 0.6 m from left

wall

The face instruments remained at the same location for

all tests. Methanometers are usually positioned at one of

these locations to meet current methane sampling

requirements. Concentrations at the three locations were

averaged to determine the “average face concentration.”
Figure 3. Bolter positions and methane release and sampling locations.
Sweep Area (Locations 4, 5, 6 and 7)

 4 0.3 m from roof, 0.6 m from right wall,

 5 0.3 m from roof, 1.8 m (6 ft) from right wall,

 6 0.3 m from roof, 3 m (10 ft) from right wall, and

 7 0.3 m from roof, 4.3 m (14 ft) from right wall

The sweep locations were always 6.1 m inby the “last

row of bolts.” The distance from the face varied with

machine position. A person standing under the last row of

bolts could use a 6.1 m pole to hold a methanometer at any

of the sampling locations in the sweep area.

Machine Area (Locations 8, 9, 10 and 11)

 8 Right end of ATRS "T-Bar," 0.3 m from roof

 9 Middle of ATRS "T-Bar," 0.3 m from roof

 10 Left end of ATRS "T-Bar," 0.3 m from roof

 11 0.3 m from roof, at the last row of bolts and above

the centerline of the bolter

The sampling locations did not change on the machine

but did move when the bolter changed position.

Methane Release Locations

During continuous mining of coal, most methane in the face

area comes from either the newly exposed face, or pieces of

broken coal as they are crushed by the rotating miner head. 

During roof bolting it is more difficult to determine the

major methane emission sources. An analysis of MSHA

roof bolter reports, mentioned earlier, indicates that most of

the methane comes from roof feeders near the drill holes.

Separate tests were conducted to simulate release of

methane from either the face or roof bolt drill holes. 

Release locations are shown on figure 3. To model uniform

face emissions, methane was released through four 3.7 m

long horizontal pipes positioned at the center of the face. 

Holes, 2 mm (1/16 in) in diameter, were drilled 5 cm (2 in)

apart on top and bottom of each pipe. Emissions from the

drill hole were simulated by placing the end of the gas

supply hose against the roof, adjacent to either the right or

left drill booms. The end of the hose was covered with a

perforated plastic bag. The gas release rate for all tests was 

for all tests was 2.4 l/sec (5 cfm).

Operating Conditions

Machine positions, intake flow and curtain setback distance

were varied to simulate different conditions that could be

encountered during roof bolting. There were 12 different

operating conditions, and each test condition was repeated

once. The results of the replicated tests were averaged. The

variables for each test are given in Table 1 and described in

more detail below. Tests with the 12 operating conditions

were repeated for each of the methane release locations.

Machine Position

Tests were conducted with the roof bolter at four different

positions (figure 3). The machine was positioned so the

bolter controls, adjacent to the drill booms, would be under

the last row of bolts. For these tests it was assumed the last

row of bolts was either 8.5 or 12 m (28 or 40 ft) from the

face. Therefore the operators’ controls were 8.5 m from the

face when at positions 3 and 4, and 12 m from the face

when at positions 1 and 2. Moving from positions 1 and 2

to positions 3 and 4 would have required an underground

bolter to have installed three rows of bolts (4-ft centers).

Intake Air Flow and Curtain Position

Total airflow entering the test gallery was approximately

397 m /min (14,000 cfm). Blowing brattice was used to 3

direct intake airflow toward the face. The brattice was

attached to a wood frame that was constructed 0.6 m from

the left side of the entry. Regulator doors (figure 1) were

opened or closed to provide intake curtain flows of either

113 or 198 m /min (4,000 or 7,000 cfm). Intake flow was 3

measured using an anemometer traverse at the inby end of

the brattice.

Curtain setback distance was 12 m for all tests with the

bolter at positions 1 or 2. At bolter positions 3 and 4 tests

were conducted with 8.5 and 12 m setback distances. The

latter distance represented advancement of the curtain to

the last row of bolts.

Data Collection

Prior to the start of each test the desired operating

conditions (curtain setback, intake flow and machine

position) were set up. Next, the valve to release gas from

one of three locations (face, right drill, left drill) was

opened. After allowing 5 minutes for gas and air to mix in

the gallery, data was collected for the next 5 minutes. For

the entire 10 minute test period, data from each

methanometer was down-loaded every 2 seconds to a

personal computer via an A/D conversion board. A

computer spreadsheet was used to calculate the average

methane concentration for each sampling location.

RESULTS

The graph on figure 4 compares the effects of the methane

release location with methane concentrations measured for

the three sampling areas. Concentrations from all tests for 

a given sampling area were averaged for each release

location. The concentrations were highest at the face when

gas was released from the manifold, and highest at the

sweep and machine locations when gas was released from

the right or left drill locations.
Figure 4. Effects of release location on concentration.

Table 1.–Test operating conditions

Test Condition Distance Curtain

from Face (m) ft

Intake Flow

Quantity

Machine: Right or

left side of entry

Machine Distance From

Face

1 40 7000 Right 40

2 40 7000 Right 28

3 40 7000 Left 40

4 40 7000 Left 28

5 28 7000 Right 28

6 28 7000 Left 28

7 40 4000 Right 40

8 40 4000 Right 28

9 40 4000 Left 40

10 40 4000 Left 28

11 28 4000 Right 28

12 28 4000 Left 28

For the manifold release tests, concentrations measured

at the face were compared to concentrations measured at the

sweep and machine locations. Scatter diagrams (figure 5),

drawn for each of the outby locations, include data for all 12

test conditions. The “best straight line” was drawn through

each of the scatter diagrams using the method of least

1

squares. Estimators of each line’s slope(b )and y intercept

0

(b ) are given in Table 2.

The student t distribution was used to determine, for

each straight line, if there is a statistically significant

difference (95% confidence) between the slope of the line

and zero. If the slope is not significantly different from zero

it can be concluded that there is no linear relationship

between the variables (Table 2). “P values” are also given

in Table 2 for each straight line. A “P -Value” less than

0.05 indicates there is a statistically significant

(95% confidence or greater) relationship between the

concentrations at the face and outby locations.
Figure 5. Comparison of methane concentrations at face

and outby locations.

DISCUSSION AND CONCLUSIONS

Changing the methane release location had a large effect on

concentrations measured in each of the sampling areas. The

effects of release location on concentration can be partly

explained by noting the distance between the source and

sampling location. The closer the source, the higher the

concentration. Airflow patterns also had an effect on

concentration.

With the aid of smoke tubes, airflow diagrams were

drawn for each condition tested. Figure 6 illustrates a

typical airflow diagram from the roof bolter tests. In

general, intake air moved up the left side of the entry, across

the face, and back the right side of the entry to the return. 

Eddy currents formed between the right and left side flows. 

All of the methane released from the manifold passed

through the sweep and machine sampling areas. Only part

of the methane released from either drill release location

reached the face.

Based on the distance from the methane source and

airflow patterns during roof bolting, it would be expected

during roof bolting that:

- Gas concentrations will be higher in the sweep and

machine areas when the major source of the methane

is from the drill hole,

- Gas concentrations will be higher at the face when

the major methane source is from the face.

These expectations were confirmed by the sampling data

(figure 4).

Concentrations at the face were compared to

concentrations measured outby the face, for tests having 

manifold gas releases. The average of the three face

readings was used to represent the single reading that

would be taken at the face to determine compliance with

the current methane standard. Scatter diagrams were

drawn to show the relationship between the average face

concentration and concentrations at the eight outby

locations.

Linear regression techniques were used to draw the

“best straight line” though each set of data. A linear

model was used because it is the simplest model, and

there is no reason to believe any other model would fit

the data better.

The student t distribution was used to determine if

there was a significant relationship between

concentrations at the outby locations and the average

concentration at the face. The “t-test” results showed

that, at the 95% confidence level, there were significant

relationships between concentrations at locations 4 and 8

and the face. For the other outby sampling locations

there were no significant relationships to face

concentration.

The airflow pattern (figure 6) shows that airflow from

the face passes over locations 4 and 8. It would be

expected, therefore, that there would be some

relationship between gas levels at the face and the outby

sampling locations.

A test of significance does not prove that an outby

sampling location should, or should not be used as an

alternative sampling location for face sampling. The

results do indicate that, using straight lines as models,

there is a statistically significant relationship between the

average face concentration and the concentrations

measured only at locations 4 and 8. 

The scatter diagrams show how much the data points

were spread around each of the straight lines. Values for

R , a measure of the clustering of points about a 2

regression line, are given in Table 2. Although the

scatter of data around the lines for locations 4 and 8 is 
Figure 6. Face airflow pattern

Table 2.–Results for “best straight lines”

Avg. Face conc.

Vs

Y 0-Intercept (b ) Slope

1

(b ) 

t-test

95% Confidence

(Significance)

R

2

(Percent)

P-Value

Loc. 4 .04 1.29 yes 48.7 .01

Loc. 5 .12 .52 no 9.2 .34

Loc. 6 .12 .72 no 2.5 .62

Loc. 7 .13 1.17 no 4.2 .52 

Loc. 8 .07 1.11 yes 48.1 .01

Loc. 9 .10 .60 no 9.8 .32

Loc. 10 .14 -.35 no 1.6 .69

Loc. 11 .11 .45 no 10.6 .30

less than for the other sampling locations, the R values 2

are still relatively low. When using data measured at the

outby locations to predict face concentrations, the variation

about the regression lines must be taken into account. Part

of the variation was due to changes in the operating

conditions (curtain setback, curtain flow, and 

machine position). Further studies are needed to determine

the relative importance of these and other factors.

The action level for methane at the mining face is

1.0 percent. Concentrations measured during these tests

were less than 0.2 % methane. Future testing will be

conducted with higher methane flow rates to determine if

the same relationships obtained from the current study are

true for face and outby sampling locations. With data

concentrations near the action level, confidence limits can

be determined for the straight line models.

 The results show that methane concentrations were

higher at the machine and sweep locations when methane

was released at the drill hole. Using sampling locations

closer to the drilling operation (e.g., sweep and machine

locations) could provide better predictions of methane

concentrations at the drill holes. Additional testing is

needed to determine what relationship exists between

concentrations at the sweep and machine locations and

locations adjacent to the drill hole. Methane sampling

locations should not be changed unless it can be

demonstrated that the change provides the same or greater

level of safety for the worker.