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

Rogers C. B. Morton, Secretary 

BUREAU OF MINES 

Elburt F. Osborn, Director 
This publication has been cataloged as follows: 

Z abetakis, Michael George, 1924- 

Methane control in United States coal mines-1972, by M. G. 

Zabetakis, Maurice Deul, and M. L. Skow. [Washington] U.S. 

Bureau of Mines [I9731 

22 ' p. illus., tables. (U.S. Bureau of Mines. Information circular 

8600) 

1. Methane. 2. Mine explosions. 3. Mine accidents. 4. Coal 

mines and mining-Accidents. I. U.S. Bureau of Mines. 11. Deul, 

Maurice, jt. auth. 111. Skow, Milford L., jt. auth. IV. Title. (Series) 

TN23 .U71 no. 8600 622.06173 

U.S. Dept. of the Int. Library 

I 
CONT ENT S 

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

1ntroduction............................................................ 1 

Background .........aa.aa~a..~~~a.*aaaaaaaaa.aaaaaaaaaaaaaaaaaaaaaaaaaaaa 1 

Mine explosion trends................................................... 3 

Discussion.............................................................. 5 

Basic factors............................................~..~...... 6 

Methane control techniques...... ...aaa.aaaaaaa..aaaa..a.aoaa*aa.aaa 8 

Recommended procedures aaaaaaaaaaaaaeaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 15 

Conclusions.~....................~..................~~..~~~.....~..~ 16 

Bibliography ............................................................ 17 

Appendix. .aa~aa~aaa~~~~~~~~~~~~~~~aaa~aaaaaaaaaaaaaaaaa20 

I ILLUSTRATIONS 

Underground coal product ion, man-shifts , and major coal mine 

explosions in United States mines between 1850 and 1970 

(5-year averages) ..........aeaa..oaaoaaaaaaaaaaaaaaaaaaaaaaaaaaa 2 

Cumulative major coal mine explosions (1840-1970) ................. 3 

United States coal mine fatality rate: 2, per thousand 300-day 

workers and b, per million man-hours exposure................... 5 

Shut-in pressure versus well depth (h) .~

6 

Pressure versus distance from the face............................ 7 

Methane liberation rates of various mines in the Pittsburgh 

coalbed; V,, for Pittsburgh coal in its normal water-saturated 

state..............~...........~.............................. 7 

Effect of an idle period on the methane emission rate of an 

operating mine in Pennsylvania aaaaaaaaaaaaaaaaaaaaa.aaaaaa 8 

Fluctuations in methane flow before and after borehole 

intersection during working and idle periods.. . . . . . . . . . . . . . . . . . . 11 

Total methane flow rates from the study area...................... 11 

Methane flow rates near abandoned wells........................^.. 12 

Perspective view of multipurpose borehole......................... 13 

Location of bleeder holes and gas we11s........~.................. 14 

Initial gas flow rates from horizontal holes drilled in virgin 

coal................~........~.................................. 15 

Total flow (gas emission) from seven horizontal holes drilled 

in virgin coa1............................................. 15 

Methane liberation rates and adsorbed gas volumes for mines 

in the Pittsburgh coalbed versus depth (h)...,.,.........,...... 16 

Histogram of time intervals between successive major explosion 

disasters in United States coal mines for the period 1931-50,... 22 

TABLES 

1. Major mine explosion rates during selected periods from 1836-1970. 4 

2. Fatalities due to ignitions and explosions in underground workings 

of coal mines.........,.~....................................... 6 

3. Methane control techniques considered for underground use......... 10 
TABLES - -Cont inued 

A-1. Observed and expected occurrence of major coal mine explosions 

in the United States during the period 1836-1970.......,........ 21 

A-2. Time intervals, t (in days), between successive major explosions 

in United States coal mines for the period 1931-50.............. 22 
METHANE CONTROL IN UNITED STATES COAL MINES-1972 

by 

M. G. Zabetakis,' Maurice De~l,~ andM. L. Skow3 

ABSTRACT 

This Bureau of Mines report presents a brief summary of the explosion 1 

trends in United States coal mines for the past 160 years. These trends show 1 

that in spite of the overall improvements in mine safety, the number of fatal- 1 

ities from ignitions of methane has actually increased in recent years. Sug- 1 

gestions are made of procedures that can be utilized to reverse this trend. 

INTRODUCTION 

The Bureau of Mines has been interested in mine safety since its inception in 1910. In this connection, a series of studies has been undertaken in 

recent years in an attempt to remove methane from underground mines in a controlled fashion thereby reducing or eliminating the hazards associated with 

the presence of this gas. This report presents a summary of the work conducted in 1972. 

BACKGROUND 

Methane has created a hazard in United States coal mines almost as long 

as underground mining has existed. While coal was first obtained from the 

outcrops in Virginia near the turn of the 18th century, it was subsequently 

removed from shafts as the seam was followed underground. The first explosion 

from an accumulation of methane reportedly occurred near Richmond about 

1810 (9).4 - Other explosions occurred periodically until mining was discontinued, as open flames were used both for illumination and to test for the 

presence of methane during this period. With the development of the flame 

safety lamp in 1815, methane could be detected without the hazards associated 

with open flames. However, even when this gas was found in an underground 

area, little could be done to clear it readily. During this period, a miner 

could use his jacket to dilute the methane by creating a current of air (2, 26) 

lSupervisory research chemist. 

%upervisory geologist. 

3~taff engineer. 

4Underlined numbers in parentheses refer to items in the bibliography preceding the appendix. 
or he could eliminate the gas by igniting it with an open flame; needless to 

say, the latter procedure was extremely hazardous. Unfortunately, forced ventilation was not utilized extensively at that time even in the older European 

mines. 

As mining rates and depths increased, the number of ignitions, and consequently the number of explosions per year, increased markedly. The first 

recorded major explosion (one in which five or more persons were killed) 

occurred in the Black Heath Mine near Richmond, Va., in 1839. Eleven more 

explosions occurred in the next 36 years with a resultant loss of 280 lives. 

During this same period (1839-75), coal production rose from 2.07 million to 

52.8 million tons per year (4). In the next 35 years there were 180 major 

explosions, and the rate of occurrence rose to 17 per year in 1909 and in 1910. mL ----I--- llle llU111"cL zlf fatalities fro= iiia jor exploslo-iis rose frzlm 14 per year in 1876 

to 903 per year in 1907, while coal production rose to 480 million tons in 

FIGURE 1. - Underground coal production, man-shifts, and major coa I mine explosions in 

United States mines between 1850 and 1970 (5Dyear averages). 
1907. The trend was unmistakable: both coal production and the number of 

explosions were increasing rapidly. In essence, the increase in coal production at greater depths resulted in an increase in the methane emission rate; 

this in turn resulted in an increase in the number of accidental ignitions and 

gas and dust explosions. 

The steps taken by the various States and the Federal Government to prevent mine explosions are discussed in detail by Humphrey (9). These included 

the elimination of a number of ignition sources (open flames, black powder, 

and high explosives), improved ventilation, generalized rock dusting, the use 

of sprays at the face, education of underground workers, and improved inspection procedures. A detailed analysis of the trends observed to date, and the 

inferences to be drawn from these trends in view of the present demand for 

coai are eensi2cred in the following sections. 

MINE EXPLOSION TRENDS 

The major mine explosions frequency-of-occurrence data included in 

Humphrey's compilation (2) are summarized in figure 1, along with data 

obtained from more recent Bureau of Mines reports for the period 1955-70. 

While the general trends noted above are evident (that is, a low initial rate 

before 1875, followed by an exponential rise to about 1910, and then a gradual 

decline), a detailed analysis of these data indicates considerable variability 

in the rate at which major explosions have occurred over the years. However, 

if the total number of major explosions against time (fig. 2) is plotted, a 

figure is obtained that can be used to correlate these data by using a series 

of straight lines. These lines define certain periods during which the rate 

of occurrence is fairly constant (table 1). Obviously, there are factors such 

as methane liberation rate, coal production rate, dustiness, ignition sources, 

and behavior of workmen that affect the frequency with which major explosions 

occur. Presumably, during the four periods noted here (1836-75; 1901-30; 

1931-50; 1951-70), these factors interacted in such a way as to yield a fairly 

1830 1850 1870 1890 1910 1930 1950 1970 

FIGURE 2. - Cumulative major coal mine explosions (1840-1970). 
constant rate over each specified interval. This idea can be evaluated statistically by determining the nature of the distribution of these explosions with 

time, the frequency of occurrence, and scatter of the data (see appendix). 

TABLE 1. - MM 

selected periods from 1836-1970 

Major mine 

explosion rate 

(number /year) 

Period : 

1836-75................... 0.30k0.17 

1901-30................... 9.9321.17 

1931-50................... 3.702 .86 

1951-70................... 1.302 .53 

During the 30-year period from about 1875 to 1905, major mine explosions 

occurred at an annual rate, yl, given by the regression equation: 

where X1 is the coded time (year - 1847.5)/5. This regression equation 

accounts for 91.8 percent of the scatter (r = 0.958) in the data about the 

average value for this period. During the period 1885 to 1920, the number of 

man-shifts worked per year (in millions) is given by the expression: 

where X, is the coded time (year - 1882.5)/5. This equation accounts for 

97 percent of the scatter (r = 0.985). An even better fit is obtained for the 

coal production rate during the period 1850 to 1910 (fig. 1). Here we have: 

where X, = X1. This equation accounts for 99.6 percent of the scatter 

(r = 0.998) in the data about the average value. 

Alternatively 

yl = 0.263 exp (0.2950X1) major explosions per year, (4) 

y, = 49.6 exp (0.1952X2) million man-shifts per year, (5) 

y, = 8.0 exp (0,3416X3) million tons per year. (6) 

The exponential rate of increase in major explosions before 1905 almost equals 

the rate of increase in coal production, but each exceeds the rate of increase 

in underground exposure (man-shifts; fig. 1). Both underground exposure and 

coal production increased until about 1920 before dropping, while the major 

explosions rate dropped around 1910, tended to a constant value until about 

1930 and then dropped abruptly. Subsequent changes in the major explosions 
KEY I 00 Five year average rote I a Annual rate 

FIGURE 3. - United States coal mine fatality rate: 0, per 

thousand 300-day workers and h, per mi l l ion 

man-hours exposure. 

rate of occurrence appear to 

parallel the underground 

exposure (man-shifts), while 

coal product ion has remained 

at a relatively high level 

since the mines have 

mechanized. 

DISCUSSION 

While only the major 

mine explosions data (gas 

and dust) are considered in 

the preceding section, these 

have followed the same 

trends exhibited by other 

mine accident statistics 

based on worker exposure. 

For example, a plot (curve 5, 

fig. 3) of the United States 

coal mine fatality rate per thousand 300-day workers over the period 1890- 

1968 (21) shows an increase in this rate until about 1910 when the Bureau of 

Mines was formed, an abrupt drop to a relatively constant level until about 

1930, then a gradual decrease during the next 15 years (as employee turnover 

decreased and the proportion of experienced personnel increased (L)). Subsequently the annual rates have fluctuated about the 5-year average. Unfortunately, the actual underground exposure times are not known with any degree of 

certainty during this extended period. Reliable data, available only for the 

period 1930-72 ,6 are included in curve b, figure 3. The fairly constant 

5-year average rate during the period 1941-70 (about 1.36 fatalities per million man-hours underground exposure) is noteworthy despite the fact that the 

annual rates ranged during this period from a low of 0.98 in 1969 to a high of 

1.80 in 1968 (standard deviation = 0.20). The annual rate subsequently 

dropped to a low of 0.91 per million man-hours underground exposure in 1971 

and more recently (1972) to a new low of 0.71. By comparison, the fatality 

rates associated with coal mining operations in general were 0.72 and 0.56 per 

million man-hours total exposure in 1971 and 1972, respectively. 

Approximately one in eight of the more than 90,000 fatalities that have 

occurred in the ati ion's coal mines during the period 1906-68 were caused by 

gas and dust explosions. Despite improvements in other aspects of mine safety 

in recent years (figs. 1 and 3), the number of fatalities from ignitions of 

methane and from major explosions has actually increased slightly in recent 

years, but the fraction of the industry total ascribed to these causes has 

increased markedly (table 2). Thus, if present trends were to continue, the 

number of fatalities from ignitions of methane could be expected to increase 

still further when production of coal from underground workings increases. 

Fortunately, this trend can be reversed by the implementation of coal degasification techniques developed here and abroad in recent years. These 

6~ompiled by Office of Assistant Director--Technical Support, Washington, D.C. 
techniques can be best described in terms of the basic factors that govern the 

behavior of methane in coal. 

TABLE 2. - Fatalities due to ignitions and explosions 

in underground workings of coal mines 

Basic Factors 

Year 

1956-60............ 

1961-65............ 

1966-70.. .......... 

Methane is a component part of coal that is formed during the coalification process (23, 27, 33); according to Mott (g), approximately 3,000 cubic 

feet of methane is produced in forming 1 ton of medium rank bituminous coal 

(along with copious quantities of carbon dioxide and water). In most United 

States mines, only a fraction of this methane has been retained in the coal 

since the balance is lost to the atmosphere. 

The amount of methane retained by a particular coal is determined in part 

by the hydrostatic pressure at the depth below surface at which the coal is 

found. Figure 4 gives the shut-in pressures measured at the base of a number 

of vertical wells drilled recently into coal from the surface. In general, 

these pressures are less 

800 I 1 than the hydrostatic head: 

Phyd = 0.435hy (7 

where h is the well depth. 

600 - - Similar results have been 

obtained from measurements 

. 

u' - made underground (8). For 

m 

a example, a plot of pressure 

against horizontal distance 

- from the face shows a rather 

steep pressure gradient near 

the face, but this gradient 

decreases rapidly with depth 

of penetration and finally - the pressure approaches the 

hydrostatic value (fig. 5). 

The volume of gas 

adsorbed by a coal at a 

0 500 1,000 fixed temperature is given 

1,500 2,000 by theLangmuir isotherm: 

WELL DEPTH (h), ft 

FIGURE 4. - Shut-in pressure versus well depth (h). 

Percent 

of 

total 

7.86 

11.8 

15.3 

Fatalities 

Industry 

total 

1,654 

1,178 

991 

Ignitions and 

explosions 

130 

139 

152 
0 40 80 120 1 60 

DISTANCE FROM FACE, ft 

F!GURE 5. - Pressure versus distance from the face, 

DEPTH, ft 

FIGURE 6, - Methane liberation rates of various mines in 

the Pittsburgh coa lbed; Vad for Pittsburgh 

coa I in its normal water-saturated state, 

where V, is the saturation 

(monolayer) volume, 

b is a constant, 

and P is the equilibrium 

pressure in psia. 

For example, Vad for Pittsburgh coal in its normal 

water-saturated state is 

given approximately by the 

express ion (11) : 

where P is in psia. Taking 

P to be the hydrostatic pressure, this can be written as: 

where h is the coalbed depth 

in feet, A plot of V,, 

against h is given in figure 6, along with data on 

gross methane liberat ion 

rates per ton of coal mined 

(MLR)' in a number of commercial mines in the Pittsburgh coalbed (10). With 

few exceptions, more methane 

is emitted than that contained (adsorbed) in the 

mined coal under saturation 

conditions, and deviations 

from Vad appear to increase 

with increase in mine depth. 

Thus while the rate of 

change of adsorbed gas with 

depth (dV/dh) is approximately 0.67 cubic feet per 

ap his is actually a fictitious figure but is now 

commonly used as a measure of gassiness during 

periods of active mining. 
FIGURE 7. - Effect of an idle period on the methane emission rate of an operating mine 

s 

cr 

z 

gu 

mlS 

C" C 

z .o W= .- 

WE 

z 

a 

I 

IW 

= 0 

ton of coal per foot depth near the surface, the rate of change in the methane 

liberation rate (d~~~ldh) is actually 2.5 cubic feet per ton per foot depth 

for the more gassy mines. This gives a ratio MLR:Vad of 3.75 for shallow 

mines; and an even larger ratio for the deeper and more gassy mines. Such 

behavior results from the fact that a mine is a dynamic system; the gas 

emitted comes not only from the extracted coal but in practice as much as 70 

to 80 percent of the methane enters the mine atmosphere from the exposed ribs 

and adjacent strata. A striking example is presented in figure 7 which shows 

the effect of a 7-112-week strike-induced idle period on the methane emission 

in an operating mine (10) where the methane emission rate (MER) of about 

1 million cubic feet per day before the strike dropped only to about 0.7 million cubic feet per day during the strike. Assumizlg that the decrease represents the adsorbed gas normally released during mining, then an MLR:V,, ratio 

of approximately 3.3 results; this means that nearly 70 percent of the gas 

enters the mine workings from the ribs and adjacent strata. Admittedly, some 

of the gas liberated during the strike would have entered the mine as desorbed 

gas from the mined coal so that this ratio (3.3) may be a little high. 

C L 

o 

C 

u 

C 

m W 

2- * Bituminous coal strike * 

I- * 

% = 

I I I I I I I 1 I I I I 1 1 I I I I I I 

Assuming that a fixed proportion p of the total methane enters a mine 

through the exposed ribs and adjacent strata, then the methane liberation rate 

can be predicted by determining the gas content of the coal from a core sample. 

For example, if p is 0.75 then: 

10 15 20 25 30 5 10 15 20 25 31 5 10 15 20 25 30 5 10 15 d--- 5ept 1971 0ct 1971 d NO" 1971 d~ec 19714 

MLR = - vad = 4 vld ft3/tonl 

1-P (11) 

In a recent study of cores obtained from a number of rather gassy mines in the 

Pittsburgh, Pocahontas, Illinois No. 5, and Hartshorne coalbeds, Kissell (14) 

actually found average MLR:Vad ratios above 6. 

Methane Control Techniaues 

The control techniques considered to date for use in coal mines can be 

divided into three categories : 
1. Controlled dilution with air (ventilation). 

2. Blocking or diverting gas flow in the coalbed by means of adequate 

seals. 

3. Removal of pure or diluted methane through boreholes. 

The first and most widely used technique involves the use of ventilation air 

to reduce the concentration of methane to a safe level. It is the only one 

used universally in underground mines and is a requirement of Section 303 of 

the Federal Coal Mine Health and Safety Act of 1969 (2). Title 30, U.S. Code 

of Federal Regulations, Subchapter 0, Part 75, Subpart D--Ventilation (2) 

contains the statutory provisions and mandatory standards related to the ventilation of mines. Bthsr methane control techniques can be considered under 

the provisions of Section 301(b) of the Federal coal Mine Health and Safety 

Act and Section 75.316 of the mandatory safety standards in Title 30, 

U.S. Code of Federal Regulations. While a few mines have employed other control procedures (2-6, 12, 17-18, E-g, 28), most mine operators have been 

rather reluctant to attempt anything that might change their mining plans, 

this despite the fact that many of the new procedures will decrease ventilation air requirements while increasing productivity by eliminating much of the 

methane from the active face areas (33). The general tendency to date has 

been to use additional quantities of ventilation air as methane emission 

increases. Thus, the large mines in the Pittsburgh coalbed circulate between 

4 and 15 tons of air per ton of coal extracted. In most cases this is adequate to keep the methane concentration at the exhaust below 0.5 percent. 

However, an examination of recent Federal mine inspection reports shows that 

some mines exhaust air containing from 0.7 to 1.0 percent methane at the fan 

outlets. Although on the average this meets the statutory requirements, the 

distribution of ventilation air through a mine is such that there are probably 

areas of such a mine where the methane concentration is well over 1 percent. 

This is particularly true in newly developing mines or in mines which are 

driving development headings into virgin coal. Under these circumstances 

ventilation alone is usually not adequate, and development is curtailed or 

stopped. Furthermore, as the methane emission rate depends on both the coal 

production rate and depth (g), it would be expected that even more air will 

be needed as mining rates and depths increase unless some of the methane is 

removed before mining proceeds. 

The techniques considered to date in our attempts to prevent the entry of 

methane into the ventilating airstream are summarized in table 3. Briefly, 

there are four regions of interestgothe working space, the gob, the coal seam, 

and the adjacent strata; the latter regions are considered because they can 

and do influence the atmosphere in the working space. Furthermore, as those 

regions represent the seat of the problem, they have been the focus of all the 

initial methane control procedures considered by the Bureau in recent months. 

In particular, attempts are being made to remove the methane before it enters 

the working space. To date, vertical and horizontal boreholes, water seals, 

isolation techniques, improved bleeders, tracer gases, and various combinations to determine the best method of gas control in any particular circumstance have been used. For example, in recent months, we have found that as 
much as 25 percent of the available methane can be removed from a longwall gob 

in an operating mine in the Pocahontas No. 3 coalbed by the use of a single 

partially slotted vertical pipe (2); approximately 180 million cubic feet of 

gas was removed in the first 15 months. A summary of total methane flow 

before and after borehole intersection on November 9, 1970, is given in 

figure 8. At least three coal mining firms are experimenting with such pipes 

in gob areas. 

TABLE 3. - Methane control techniques considered for underground use 

2. Gob............ 

Region 

1. Working space.. 

3. Coal ........... 

Methane control technique 

Coal........... 

Diluting with air 

Ventilate ; isolate 

and ventilate. 

Ventilate; bleeders 

Vertical surface 

boreholes ; horizontal and sloped 

underground bore - 

holes; multipurpose 

borehole. 

4. Adjacent strata I - I~trata .........I Boreholes. 

Sea ling 

Water infusion; 

grout ; plastic 

seal. 

Gob............ 

In a series of experiments in the Pittsburgh coalbed, it was found that 

the bulk of the gas entering the face area in a very gassy mine can be 

diverted by infusing the face with water (3); a summary of the methane flow at 

two locations before and after infusion of an active face in this mine is 

given in figure 9. In a companion study, the method of moving averages was 

used to show the effects of oil and gas wells on the methane flow into the 

same mine (3). Figure 10 shows the methane flow rate and the rate of change 

of this rate with distance along an entry in the vicinity of two abandoned 

we1 1s. 

Removal of CH, 

Chemical process ; 

physical process ; 

biological process. 

Surface boreholes. 

More recently, in a natural drainage experiment in the Pittsburgh coalbed, 

it was found that long horizontal holes that pass near old wells frequently 

produce much more gas than those that penetrate the same coalbed in areas free 

of wells. A schematic of the arrangement used in these studies is given in 

figure 11 (1). Eight 3-inch-diameter horizontal holes were drilled into the 

coal from the base of a cased vertical borehole. One hole was used to measure 

the gas pressure in the coalbed, and the other holes were used to drain water 

and gas (fig. 12). The latter holes ranged in depth from 500 to 850 feet; initial gas flow rates for the individual bleeder holes are given in figure 13. 

If we assume that holes 7, 8, and 1 are shielded from the wells by the other 

holes (particularly 2 and 6), then the initial gas (primarily methane) flow 

rate (GFR) in the absence of the wells would be given by the equation: 

GFR = (0.18 - cos 28) cfm/ft2, (12) 
TIME, days 

FIGURE 9, - Total methane flow rates from the study area. 
where 0 is the angle (displacement) measured clockwise from the face cleat 

located between holes 2 and 

4. A plot of equation 12 

is included in figure 13 

which strikingly illustrates 

the effects of directional 

permeability on the flow; 

in the absence of wells, the 

maximum flow is from holes 

drilled along the butt 

cleats (B) and perpendicular 

to the face cleats (Fj. However, since boreholes that 

pass near wells may have 

flow rates above the predicted values, it is not 

surprising to find that the 

initial flow rates from 

holes 2 and 4 were approximately twice as high as 

those to be expected from 

equation 12. Furthermore, 

comparing the flows from 

DISTANCE (XI, f t be seen that the flow from 

hole 4 which passes near a 

FIGURE 10. - Methane flow rates near abandoned wells. gas well was about 2.5 times 

holes 4 and 7 (two diametri- I 

higher than that from hole 7 

d 1,000 2,000 3,000 4,000 5,000 cally opposed holes), it can 

w 

which does not pass near a 

I 

well. This supports earlier findings of the Bureau on the effects of oil and - - 

gas wells on emission of methane in coal mines (34). 

After the initial flow rates were measured, the seven bleeder holes were 

connected to a common manifold. The total methane emission from these holes 

fell for approximately 50 days and then began to increase (fig. 14). Interestingly, the emission rate fell only by a factor of 2 while the gas pressure 

in the coalbed fell by a factor of 20. However, during this period, water was 

removed continuously from the coal (the average initial water flow rate was 

6.2 gpm per hole; in 50 days this figure fell *to 0.5 gpm per hole), and the 

gas permeability of the coal increased. As a first approximation, the initial 

gas emission rates (GER) were found to be proportional to the inverse of the 

square root of the elapsed time, t (30) : 

GER = 970y000 cfd; 0 5 t 5 40 days. a + t/10 

This simple equation yields values within about 5 percent of the smoothed 

(curve) data given in figure 14. 
FIGURE 11. - Perspective view of multipurpose borehole. 

In addition, the Bureau is presently evaluating the effects of clay veins 

and faults on methane flow. The results of these studies and of others on the 

drainage of gas from isolated panels and vertical and horizontal holes should 

be availab1.e soon. In a few years the results of a series of recently started 

studies should be available on the flow of gas from gob areas, the design of 

effective bleeders, and the improvement of ventilation at the face, However, 

even preliminary results show that technology is now available to permit a 

more realistic approach to methane control in most of the Nation's coal mines. 

Toward this end, a program was initiated late in 1972 to demonstrate that a 

complete mine can be effectively degasified with a concomitant improvement in 

safety and production. The initial results of this work also should be available within a few years. 
No. 2 hole 

FED

LOCATION OF 

WITH HORIZ

FIGURE 12. - Location 
FIGURE 13. - Initial gas flow rates from horizontal holes gas flow rates when boreholes are drilled during drilled in virgin coal. exploration and by deterF B F B F Recommended Procedures 

mining the gas content of 

the coal (V,,) using the 

- direct method of Bertard (2). 

Briefly, this involves the 

taking of a coal core from a - 

vertical hole and measuring 

- the volume of gas that is 

desorbed at atmospheric pres- 

- sure. A1 terna t ively , V,, 

can be calculated from the 

Langmuir isotherm (equation 8) if the pressure and 

temperature of the gas in 

0 20 40 60 80 loo the coalbed are measured. 

ELAPSED TIME, days The approximate methane libcq 0.4 

v- C 

\ 

E u- u .3- 

2- 

LL 

W 

z 

a -1 

I 

b 

eration rate (MLR) can then 

FIGURE 14. - Total flow (gas emission) from seven horizon- be obtained from equation 11. 

tal holes drilled in virgin coal. 

Where V,, and P are not 

known, an idea of the 

= 0 90 180 270 360 encountered in mining can be 

forecast by first measuring DiSPikCEMEriT, e, beg gas reservoir pressures and 

I I 111 I I I, 

W 1 - I' 5 Y W 

X / 2 I- 4 

- X X XTX\{ - X 

- x7 

0.18- 0. I cos 28 - - 

I I I I I I I - 

expected MLR can be obtained by combining equations such as those given here 

for the Pittsburgh coalbed. For example, from equations 10 and 11 we can 

From the work conducted 

todate, it isstrongly 

recommended that adequate 

methane control procedures 

be incorporated in all 

phases of underground mining, 

beginning with initial exploration and continuing until 

mining is completed. The 

problems that will be 

obtain an approximate measure of the methane liberation rate for the Pittsburgh coalbed. This is plotted in figure 15 as 4 V,, along with the MLR prediction curve derived earlier from the average methane emission rate data 

obtained in a number of mines in this coalbed. While there appears to be a 

fairly good correlation, it must be recognized that these procedures are based 

on averages and that the presence of gas and oil wells in the area of an 

active mine may tend to increase the averages, while barriers to free flow 

such as clay veins, faults, and nearby abandoned workings may decrease them. 

Once V,, or MLR is obtained, some thought must be given to the proper 

choice of control procedures. As a rule-of-thumb, we propose that where the 
projected methane liberation 

rate (MLR) approaches 

400 cfm per ton of coal 

mined, ventilation alone 

will be inadequate and that 

direct methane control procedures should be utilized. 

These include the use of vertical and horizontal holes 

to dewater and degasify an 

200 400 600 800 1000 area before the onset of mining; the use of horizontal 

DEPTH (h), ft holes to infuse an active 

I=lcl lDI= 15. _ Methane iibzrGticE rztes =c(sorbed nnc face with water and thus I IVUI\I- YU4 divert the flow of methane; 

volumes for mines in the Pittsburgh coal- the use of long horizontal 

bed versus depth (h). holes to dewater and degasify a section during mining; 

improvement of bleeder entries; the use of vertical holes and slotted pipes to 

drain a gob area; the outlining of isolated panels to degasify the panels 

prior to mining; hydrofracing a block of coal to increase its permeability; 

and rapid dilution of methane with air as it enters the mine. Detailed procedures can be found in a number of references included in the bibliography. 

CONCLUSIONS 

While significant advances have been made in reducing the hazards associated with the mining of coal, the ignition and explosion hazards associated 

with the presence of methane and other combustibles have actually increased. 

From the data obtained in recent years, it can be expected that additional 

problems will arise as mining rates and depths are increased. Accordingly, 

special emphasis must be given to the use of established methane control procedures in all phases of underground mining and to the refinement of these 

techniques by a larger number of mining companies through application and 

innova t ion. 
BIBLIOGRAPHY 

1. Barry, Theodore, and Associates. Industrial Engineering Study of Hazards 

Associated With Underground Coal Mine Production. Volume I--Analysis 

of Underground Hazards and ~atil Accidents. BuMines Contract 

Rept. S0110601, December 1971, 298 pp. 

2. Bertard, C., B. Bruyet, and J. Gunther. Determination of Desorbable Gas 

Concentration of Coal (Direct Method). Int. J. Rock Mech. Min. Sci., 

v. 7, 1970, pp. 43-65. 

3. Cetinbas , Abdurrahman, R. P. Vinson, Joseph Cervik, and M. G. Zabetakis. 

Methane and Dust Control by Water Infusion. Pittsburgh Coalbed 

( Fairview, '42. ) . RiiM4nnc u~~~~~~~ RI 7640, 1972, 1-7 pp, 

4. Eavenson, Howard N. The Pittsburgh Coalbed, Its Early History and 

Development. Trans. AIME, v. 130, 1938, table 1. 

5. Elder, C. H. Use of Vertical Boreholes for Assisting Ventilation of 

Longwall Gob Areas. BuMines TPR 13, 1969, 6 pp. 

6. Ferguson, Pete. Methane Control by Boreholes. Coal Age, v. 77, No. 1, 

January 1972, pp. 76-77. 

7. Fields, H. H. Private communication, 1973. Available upon request from 

H. H. Fields, Pittsburgh, Pa. 

8. Hadden, J. D., and Albert Sainato. Gas Migration Characteristics of 

Coalbeds. BuMines TPR 12, 1969, 10 pp. 

9. Humphrey, H. B. Historical Summary of Coal-Mine Explosions in the United 

States. BuMines IC 7900, 1959, 275 pp. 

10. Irani, M. C., E. D. Thimons, T. G. Bobick, Maurice Deul, and 

M. G. Zabetakis. Methane Emission From U. S. Coal Mines, A Survey. 

BuMines IC 8558, 1972, 58 pp. 

11. Joubert, James I. Private communication, 1972. Available upon request 

from James I. Joubert, Pittsburgh, Pa. 

12. Kalasky, J. D. Ventilation of Deep Coal Mines. Min. Cong. J., v. 58, 

No. 9, September 1972, pp. 43-48. 

13. Kingery, D. S., and D. D. Dornenburg. Effectiveness of Bleeder Entries 

in Ventilating Pillared Areas of Bituminous-Coal Mines. BuMines 

RI 5360, 1957, 24 pp. 

14. Kissell, F. N. Private communication, 1973. Available upon request from 

F. N. Kissell, Pittsburgh, Pa. 
Krickovic , S. , and C. Findlay. Methane Emission Rate Studies in a 

Central Pennsylvania Mine. BuMines RI 7591, 1971, 9 pp. 

Maguire, B. A., E. S. Pearson, and A. H. A. Wynn. The Time Interval 

Between Industrial Accidents. Biometrika, 1952, pp. 168-180. 

Merritts, W. M., W. N. Poundstone, and B. A. Light. Removing Methane 

(Degasification) From the Pittsburgh Coalbed in Northern West Virginia. 

BuMines RI 5977, 1962, 39 pp. 

Merritts, W. M., C. R. Waine, L. P. Mokwa, and M. J. Ackerman. Removing 

Methane (Degasification) From the Pocahontas No. 4 Coalbed in Southern 

West Virginia. BuMines RI 6326, 1963, 39 pp. 

Moore, T. D., Jr., and M. G. Zabetakis. Effect of a Surface Borehole on 

Longwall Gob Degasification (Pocahontas No. .3 Coalbed). BuMines 

RI 7657, 1972, 9 pp. 

Mott, R. A. The Origin and Composition of Coals. Fuel in Science and 

Practice, v. 22, No. 1, 1943, pp. 20-26. 

Moyer, F. T., and M. B. McNair. Injury and Experience in Coal Mining, 

1968. BuMines IC 8556, 1972, 107 pp. 

Recht, J. L., and Others. Accident Facts. Nat. Safety Council, 1971, 

pp. &Of. 

Rice, G. S. Safety in Coal Mining (A Handbook). BuMines Bull. 277, 

1928, 141 pp. 

Ridenour, D. C. Core Drilling of Air Shafts and Manway Portals. Min. 

Cong. J., v. 43, October 1957, pp. 99-101. 

. Ventilation by Large-Diameter Boreholes. Mechanization, March 

1959, pp. 69-70. 

Schweisheimer, W. Ventilation in Mines Down the Centuries. Can. Min. J., 

v. 73, April 1952, pp. 81-82. 

Selden, R. F. The Occurrence of Gases in Coals. BuMines RI 3233, 1934, 

64 PP. 

Spindler, G. R., and W. N. Poundstone. Experimental Work in the Degasification of the Pittsburgh Coal Seam by Horizontal and Vertical 

Drilling. Trans. AIME, v. 220, 1961, pp. 37-46. 

Starr, C. Social Benefit Versus Technological Risk. Science, v. 165, 

1969, pp. 1232-1238. 

Stewart, I. M. Diffusional Analysis of Seam Gas Emission in Coal Mines. 

Can. Min. and Met. (CIM) Bull., April 1974, pp. 62-70. 
31. U. S. Code of Federal Regulations. Title 30--Mineral Resources ; 

Chapter I--Bureau of Mines, Department of the Interior; Subchapter 0-- 

Coal Mine Health and Safety; Part 75--Mandatory Safety Standards, 

Underground Coal Mines. Federal Register, v. 35, No. 226, Nov. 20, 

1970, pp. 17890-17929. 

32. U.S. Congress. Federal Coal Mine Health and Safety Act of 1969. Public 

Law 91-173, Dec. 30, 1969, 83 Stat. 742. 

33. Venter, J., and P. Stassen. Drainage and Utilization of Firedamp. 

BuMines IC 7670, 1953, 22 pp. 

34. Zabetakis, M. G., T. D. Moore, Jr., A. E. Nagel, and J. E. Carpetta. 

Methane Emission in Coal Mines: Effects of Oil and Gas Wells. BuMines 

RI 7658, 1972, 9 pp. 
APPENDIX 

Assuming that a major mine explosionis a rare and random event unaffected 

by the occurrence of any other similar event, then the probability that k such 

explosions will occur in a given period (for example, 1 year) is given by the 

Poisson distribution function: 

where is the average number of such explosions that occur in a unit interval 

of time. For example, during the period 1836-75, the average number of major 

explosions (x) was found to be 12/40 or 0.3 per year. If u is estimated by 

then the expected frequency f, with which 0, 1, 2 ... explosions should occur 

per year during the 40-year period can be computed. These expected frequencies are listed in table A-1 along with the observed frequency f, . Similar 

data are given for the other periods of interest (1901-30; 1931-50; 1951-70). 

It should be noted that the unit interval represents the time during which 

observations are made in each case. This interval was taken as 1 year during 

the period 1836-75 so that f, and f, are the number of years in which 0, 1, 

or 2 or more explosions occurred; however, when the interval is 1 month, f, 

and f, are the number of months in which 0, 1, 2 ... explosions occurred. 

There appears to be a good correlation between f, and f, in most cases. This 

can be checked by computing x2. For example, for the period 1836-75, we have : 

where the second subscript (i) refers to the ith values in the sum (that is, 

f, and f, refer to the first observed and expected frequencies, respectively, 

and so forth). As the calculated X2 is less than the tabulated value 

(x2 .05 = 3.841), there is no reason to reject the hypothesis that major mine 

explosions occurred in a random fashion during the period 1836-75 with an 

annual probability of occurrence of 0.3. Furthermore, as this is a Poisson 

distribution, the variance (2) also should be approximately 0.3. Using this 

value, approximate 95-percent confidence limits for u were computed by use of 

the expression x +2/x/n; these limits are included in table 1 of the main text 

(for example, 0.3k0.17). Similar data are given for the other periods considered, and the averages and measures of scatter are included in figure 1 of 

the main text. From these it can be seen that the averages differ markedly 

from neighboring values at the 0.05 significance level in each case. 
TABLE A-1. - Observed and expected occurrence of major coal mine explosions 

in the United States during the ~eriod 1836-1970 

Period ( interval) 

1836-75 (1 year) ............. 

1951-70 (1 month)....... ..... 

1901-30 (1 month)... ......... 

0 

1 

2 

3 or more 

0 

1 

2 or more 

f, f, 

29.6 29 

8.9 10 

1.5 1 

X2 = 0.0407~ 

.8275 

Alternatively, we can consider the time intervals between major explosions rather than their rate of occurrence (16). During any period in which 

the explosion rate is constant, the probability density function of time 

interval between major explosions should be given by the expression: 

Pr(k) 

0.7408 

.2222 

.0369 

~1 

0.30 

0 

1 

2 

3 or more 

f(t) = E exp (-Et), (A-3) 

Explosions, k, 

(unit interval) 

0 

1 

2 or more 

where E is the expection of a major explosion in a given time interval (for example,l month). For example, if the 74 major explosions that occurred in United 

States coal mines during the period 1931-50 are considered (9, pp. 40-41, 168), 

a list of 73 time intervals that elapsed between explosions can be generated 

(table A-2). From these time intervals a histogram can be constructed 

(fig. A-1). Then, taking E to be l/f, we can write: 

f(t) = 0.36 exp (-0.36 t), (A-4) 

where t is the time in months (actually, 30-day intervals). This curve is 

included in figure A-1. As expected, the exponential equation follows the 

trend of the histogram fairly well. Again, the X2 distribution can be used 

to evaluate the goodness of fit. This same (x2) distribution can be used to 

establish a confidence interval for E or 1/~. For example (16): 
so that in this case, with 90 percent probability, 1/E lies between 2.31 and 

3.40 (f = 2.771 months). By computing this interval for two successive 

periods, we can determine if a statistically significant change has taken 

place in the probability of having a major explosion in a given time interval. 

TABLE A-2. - Time intervals, t (in days), between successive major explosions 

in United States coal mines during the period 1931-501 

> 0.4 IIIIIIIIIII 

- 

- 

- 

- 

- 

- 

- 

0 

INTERVAL BETWEEN DISASTERS, days 

FIGURE A-1. - Histogram of time interva Is between successive 

ma lor explosion d isasters 

in United States coal 

mines for the period 1931- 

5 0.