Bureau of Mines Report of Investigations/l984 

Simulation of Methane Flows 

in Noncoal Mines 

By Andrew B. Cecala and Edward D. Thimons 

UNITED STATES DEPARTMENT OF THE INTERIOR 
UNITED STATES DEPARTMENT OF THE INTERIOR 

William P. Clark, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
Library of Congress Cataloging in Publication Data : 

i 

& 

Cecala, .Andrew B 

Simulation of methane flows in noncoal mines. , 

(Report of investigations ; 8880) 

Includes bibliographical references. , 

Supt. of Docs. no.: I 28.23:8880. 

1. Mine gases. 2. Methane. 3. Gas flow-Simulation methods. 

I. Thimons, Edward D. 11. Title. 111. Series: Report of investigations (United States. Bureau of Mines) ; 8880. 
CONTENTS 

Page 

Abstract ....................................................................... 1 

Introduction ................................................................... 2 

Verification testing ............................................................ 2 

Mine testing ................................................................... 5 

Conclusions..... ............................................................... 9 

ILLUSTRATIONS 

Difference between large volume release and plug release ................... Partial mine map showing verification testing .............................. Tracer gas concentration curves for verification testing ................... Tracer gas being released from gas cylinder ................................ Partial mine map showing.test 1 ............................................ Tracer gas concentration curves for test 1 ................................. Partial mine map showing test 2 ............................................ Tracer gas concentration curves for test 2 ................................. Partial mine map showing dilution and diffusion ............................ TABLES 

1 . Verification test results using 0.024 ft3 of tracer gas .................... 4 

2 . Results from test 1 ........................................................ 5 

3 . Results from test 2 ........................................................ 7 
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

f t foot h hour 

ft3 cubic foot min minute 

f t/min foot per minute PP~ part per billion 
SIMULATION OF METHANE FLOWS IN NONCOAL MINES 

By Andrew B. Cecala' and Edward D. Thimons 2 

ABSTRACT 

The Bureau of Mines has successfully tested a method to simulate 

methane (CH~) flows from face areas through the returns of noncoal 

mines. This technique' involves releasing a small quantity of tracer 

gas at a working area and sampling for that gas downstream in the return airways. By measuring the dilution of the tracer gas, the dilution of any volume of CH4 released at the face can be calculated. This 

information is useful in selecting locations for CH4 monitors and in 

deciding the necessity for permissible equipment in the returns. 

 inin in^ engineer. 

2~upervisory physical scientist. 

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

Methane gas (CH4) hazards exist in a 

number of noncoal mines in the United 

States. Although the mechanism by which 

CH4 is emitted into a noncoal mine may 

vary according to the type of mine, large 

emissions of CH4 are usually associated 

with face blasting. Most of the Louisiana salt mines have experienced gas outbursts at face areas during blasting. It 

is estimated that some of these outbursts 

may involve as much as several million 

cubic feet of gas, some of which is CH4. 

Oil shale mines may experience significant CH4 releases during blasting. At 

the Horse Draw experimental oil shale 

mine in Colorado, Bureau of Mines studies 

showed an average of 40 f t3 of CH4 being 

released per ton of rock broken during 

blastingm3 For oil shale mines having 

similar CH4 levels, blasting that fractures 1,000 tons of rock per shot can be 

expected to release 40.000 ft3 of CH4. 

Large instantaneous CH4 emissions present a safety hazard at face areas, but 

little is known about how the released 

CH4 mixes with air at the face and how 

fast the CH4 concentration dilutes to 

safe levels as the gas cloud travels from 

the face area through the return airways 

of the mine. Such information would be 

of value in selecting locations for CH4 

monitors in return airways and in deciding the necessity for permissible equipment at given locations in a return. 

This study does not simulate an outbursttype release, and its results could only 

be applied after the outburst pressure 

has equalized with that of the mine 

environment. 

The Bureau of Mines has found that the 

tracer gas sulfur hexafluoride (SF6) can 

be used to simulate the behavior of a CH4 

cloud. Sulfur hexafluoride is a colorless, odorless. inert gas that has been 

used for a number of years for various 

research studiesm4 It is available in 

gas cylinders containing approximately 1.5 ft3 of gas. The cylinders are 

weighed before and after use to determine 

the exact volume released. 

VERIFICATION TESTING 

The Bureau procedure assumes that a 

small volume of tracer gas can represent 

a larger volume of CH4 flowing through 

the returns. Also, it is assumed that 

gas clouds of equal size will have identical dilution, independent of their concentration, if they flow from identical 

locations. The Bureau procedure uses a 

small volume of gas to represent a larger 

volume of gas; for example, 1 ft3 of 

tracer gas to simulate a million ft3, of 

CH4. This can be done by either releasing a 1-ft3 plug of SF6 or by releasing 

that 1 ft3 into 1 million ft3 of air. In 

3~apko, M. J. , J. K. ~ichmond, and 

J. P. McDonnell. Continuous Monitoring 

of Methane in a Deep Oil Shale Mine. 

Pres. at 15th Oil Shale Symposium (CO 

School of Mines, Golden, CO, Apr. 28-30, 

1982), 1983, pp. 62-73; available upon 

request from J. H. Gary, CO School of 

Mines, Energy Technol. Center, Golden, 

co . 

the first case, the 1-ft3 plug of tracer 

gas will have diluted into 1 million ft3 

of air at some point as it flows downstream. In either case, the SF6 concentration (1 x for 1 million ft3 CH4 

release volume is assumed to be equivalent to pure CH4. We can then predict 

the peak CH4 concentrations that will occur at given locations in the return by 

measuring the peak SF6 concentrations at 

those locattons. If a peak SF6 concentration of lo-' occurs, there is a peak 

concentration dilution factor of 10 for 

that location, and the estimated peak CH4 

- 

4~urk, Am, S. M. Edmonds, H. L. Mark, 

and G. G. Collins. Sulfur Hexafluoride 

as a Gas-Tracer. Environ. Sci. Technol., 

V. 2, NO. 1, 1968, pp. 44-48. 

Timko, R. J., and Em D. Thimons. Sulfur Hexafluoride as a Mine Ventilation 

Research Tool-Recent Field Applications. 

BuMines RI 8735, 1982, 15 pp. 
 ele ease CASE I Sampling 'location 

/--- ,, /.,-=-- \ 

6- /--- 

3 3 /---- 

G I million ft 3 

i - b 9 > I million ft 2,) 3 

<- ,, (2 ..--, -5-J' 

Average concentration: I x 10; SF6 

Concentration I ft3 1a Peak concentration : 2 I x 10 SF6 

1x10-~ 

C 

Higher 

4 

1 x 10-6 

I 

Lower 

4 

txt 

Lower A 

4 4 

Release 

CASE 2 

/ > 

Sampling location 

4 X C 

FIGURE 1. - Difference between large volume release and plug release. 
and thus a worst-case situation. In order to assume a safety factor and study 

the worst-case situation, the Bureau of 

Mines uses the plug-release method in its 

studies. 

To verify that SF6 plug release provided a worst case, three tests were run 

at the Bureau's Lake Lynn Laboratory. In 

these tests, 0.024 ft3 of SF6 was released into different air volumes (1.0 

ft3, 14,000 ft3, and 56,000 ft3), and 

the air was sampled 325 ft and 1,500 ft 

downstream from the release point (fig. 

2). A gas chromatograph was used to determine the peak SF6 concentrations at 

the sampling locations (table 1). 

Each test was repeated three times; the 

peak concentrations are the averages of 

these three tests. The results show that 

the peak concentrations that occur downstream of the release point decrease as 

the same amount of SF6 is released into 

larger volumes. Therefore, if we use SF6 

to simulate a 56,000-ft3 cloud of pure 

TABLE 1. - Verification test results using 0.024 ft3 

of tracer gas 

LEGEND 

-- Airflow ...... 

...... ...... 

...... 

............ Release area 

c Fan 

Sampling location 

$ Stopping 

FIGURE 2. - Partial mine map showing verification testing at Lake Lynn Laboratory. 

Volume of air 

into which 

tracer gas 

was released, 

ft3 

1.0.......... 

14,000....... 

56,000....... 

Peak concentration 

dilution factor for 

56,000 ft3 of CH, 

Peak gas concentration 

at sampling location, 

Location A 

0.3 

.8 

1.92 

Location A 

1.42 x 10-6 

5.17 x 10-7 

2.23 x lom7 

Location B 

2.5 

3.1 

6.5 

Location B 

1.71 x 10-7 

1.38 x 10-7 

6.6 x lom8 
CH4 flowing from a face area through the 

returns, the lowest (or worst-case) peak 

concentration dilution factor will be obtained by releasing the SF6 as a point 

source or plug. In all three cases, it 

is assumed that an SF6 concentration of 

is equivalent to 100% CH4. The dilution 

factor is then obtained by dividing 4.29 

x lo-' by the peak SF6 concentration, 

which occurs at the sampling location. 

The closer the sampling location is to 

the release point, the greater the difference between a point source and a 

large-volume release. Figure 3 shows the 

flow of gas past both sampling locations, 

which represents an average of three separate runs. The difference between a release of SF6 into 1.0, 14,000, and 56.000 

ft3 is greater at location A than it is 

at location B. This confirms that the 

point source release is always the worst 

case; however, further downstream, the 

difference between a point source and a 

large volume release becomes less. 

lo4 - I 

- 

- 

- 

- 

- 

- 

Location - AB 

Test I o a 

Test 2 o 

Test 3 A A 

'0 5 10 15 20 25 30 35 

TIME, min 

FIGURE 3, - Tracer gas concentration curves for 

verification testing, 

MINE TESTING 

Numerous tests were run at two largeopening mines using SF6 to simulate CH4 

flows, two of which will be presented in 

this report. In all cases, SF6 was released at a working face or bench and 

monitored at several locations along the 

return. The purpose of these tests was 

to determine at each sample location 

(1) the reduction in the peak gas concentration, and (2) the shape and slope of 

the gas concentration curve from initial 

detection to peak concentration. 

In test 1. 2.27 ft3 of SF^ was released 

directly from a gas cylinder (fig. 4) at 

release point R (fig. 5). Samples were 

taken at three locations in the return 

airway, for approximately 2 112 h after 

release. The results of the test are 

shown in table 2. 

Concentration is plotted as a function 

of time in figure 6. The average velocity from the release point to sampling 

location 2 was greater than location 1 

TABLE 2. - Results from test 1 

Location 

1.. ..... 2 ....... 

3.. ..... 

Peak gas 

concentration 

1.02 x 10'~ 

5.71 lom7 

1.49 10-7 

Time to reach 

peak conc, 

min 

3 

6 

15 

Distance from 

release point, 

f t 

310 

750 

1.100 

Calculated 

velocity, 

f t /min 

103 

125 

73 
FIGURE 4. - Tracer gas being released from 

Release location 25-ft roof 

- Airflow 75-ft roof 

Stopping 

0 - 250 500 

Scale, ft 

gas cylinder. FIGURE 5. - Partial mine map showing test 1. 

because the two booster fans (fig. 5) increased the velocity once the gas flowed out 

of the dead heading. 

If the mine operator wants to know the peak concentration of CH4 that will occur at 

sample location 1 if 50,000 ft3 of pure CH4 is released during a blast, the values 

from table 2 can be used in the following relationships: 

SF6 peak concentration at Z (peak CH4 concentration 

sampling locations - 

- at sampling location) . SF^ released (ft3) CH, released (ft3) 9 

1.02 x 10-6, SF, - concentration Z, CH, concentration 3 

2.27 ft3 SF, 50,000 ft3 CH, 
F 

for 

KEY 

Location / 

Location 2 

Location 3 A 

0 20 40 60 80 100 120 140 

TIME, min 

IGURE 6. - Tracer gas concentration curves 

test 1. 

Therefore, a release of 50,000 ft3 of CHI 

would result in a worst-case maximum CH4 

concentration at sampling location 1 of 

2.2%. At sampling locations 2 and 3, the 

peak CH4 concentrations would be 1.2% and 

0.3%, respectively, for the worst-case 

estimate. 

For test 2, the tracer gas was released 

in conjunction with a bench blast. 

Instead of releasing the SF6 directly 

from a gas cylinder, it was placed i:~ an 

airtight sampling bag, which was placed 

over a borehole filled with explosives 

LEGEND 

25-ft roof 

80-ft roof 

Sampling location 

Release ~oint 

Airflow 0~~

0 

Scale, ft 

FIGURE 7. - Partial mine map showing test 2. 

(release point in figure 7). When the 

shot was fired, the bag ruptured, releasing the tracer gas into the entry. Sampling was performed at three locations in 

the return airway. The total sampling 

time was much longer in this test because 

the ventilating airflow was lower. The 

results are shown in table 3. The change 

in gas concentration as a function of 

time is shown in figure 8. 

No one was permitted in the mine during 

blasting. Although the first sample was 

not taken until 30 min after the blast, 

the peak concentrations were still captured for locations 2 and 3, and it is 

believed that the peak value or very 

close to the peak value was also captured 

for location 1. By using these peak concentration values in equation 1 to calculate gas reductions, it can be seen that 

substantial reductions in CH4 concentrations would occur. 

TABLE 3. - Results from test 2 

Time to reach 

peak conc, 

min 

39 

95 

137 

Location 

l... .... 2.. ..... 3 ....... 

Distance from 

release point, 

ft 

1,050 

2,925 

3,900 

Peak gas 

concentration 

2.3 x lo-8 

1.4 x 10'~ 

-9 x lo-8 

Calculated 

velocity, 

f t /min 

27 

31 

28 
30 I I I I I I I 

KEY 

Location / 

Location 2 

Location 3 A 

- 

- 

I 2 3 4 5 6 7 8 

TIME FROM RELEASE, h 

FIGURE 8. - Tracer gas concentration curves for test 2. 

The reduction in CH4 concentration as 

the gas flows from a release point to a 

sampling location in the return will vary 

for every release point and sampling 

location because the airflow pattern between any two mine points is never the 

same. Figure 9 shows that for test 2, 

gas flowing from the release point to the 

first return sampling location undergoes 

normal diffusion as it flows through multiple airways, and is diluted by fresh 

air at the dilution points (crosscuts) 

labeled 1, 2, and 3. These dilution 

points are a major cause of the substantial reduction in the gas concentrations 

as the gas flows to the first sampling 

locat ion. 
I 

LEGEND 

4$zza Dilution airflow 

Diffusion airflow 

A Release point 

Sampling location 

FIGURE 9. - Partial mine map showing dilution and diffusion. 

CONCLUSIONS 

SF6 tracer gas can be used to simulate information obtained can give guidelines 

large-volume flows of CH4 from face areas for selecting CH4 monitor locations and 

through return airways, The study did in deciding where permissible equipment 

not simulate an outburst-type release, is needed. However, because airflow patand the results would be applicable only terns are different for every mine situaafter the outburst pressure has equalized tion, test results from one location 

with that of the mine environment, The should be applied only to that location, 

W.S. GPO: 1984-705-020/5023