Bureau of Mines Report of Investigations/l978 

Methane Emissions From Gassy Coals 

in Storage Silos 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Report of Investigations 8269 

Methane Emissions From Gassy Coals 

in Storage Silos 

By J. E. Motta, J. C. I,aScola, and Fred N. Kissell 

UNII'EID STATES DEPARTMENT OF THE INTERIOR 

Cecil I>. Andrus, Secretary 

BUREAU OF MINES 
hla.tta, Joscpli I.: 

Methane emissions from gassy coals in storage silos ' by 

J. E. Matta, J. C. LaScola, and Fred N. Kissell. !Washington] : U.S. Dept. of the Interior, Bureau of Mines, 1978. 

1.i 1'. : diagra~ns : 2- cnl. (I<cj>ort uf invcstig;ltions - I3urr,lu 01 

5lincs : 8269) 

13ibliopr:lphy: p. 11. 

I. (:o;ll Storagr. 2. 13ituniinotrs cn;ll - Stor.1~~. Z, Jli*tll:i~~c~ 

4. \line g:lsesA I. I-;~Scnl;l, John ('., joint ;ltrtlior, 11. Kisscll, 

Frcd N., joint author. 111. I;nited Stntrs. t3urc.1~1 c>f \lines. I\'. . , 1 itle. \'. Series: [Iniced Stntrs. 13ureau ol \lines. I<cport of 

in\.estig.xtions - 13urcau of \lines : 82(10. 

TN23.U7 no. 8269 622.06173 

I:.S. 1)cyt. of the Int. Librilry 
CONTENTS 

Page 

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

Introduction ............................................................. 1 

Silo ventilation ......................................................... 3 

Silo usage ............................................................... 3 

Conveyor belt grab samples ............................................... 4 

Methane concentration above the stored pile .............................. 8 

Methane concentration within the stored pile ............................. 10 

Conclusions .............................................................. 11 

References ............................................................... 12 

Appendix . .-Theoretical calculation of the gas emission from a coal silo .. 13 

ILLUSTRATIONS 

1 . Gas emitted from conveyor belt grab samples .......................... 4 

2 . Gas emitted from grab sample at Federal No . 2 mine (log-log plot) .... 6 

3 . Calculated gas emission from a coal silo using the Federal No . 2 

grab sample and assuming a constant8-hour filling rate ............ 7 

4 . Methane concentration gradient in coal silo .......................... 8 

5 . Average methane concentration in silo versus gas emitted within 

24 hours from the corresponding grab sample ........................ 9 

TABLE S 

1 . General information concerning silos investigated .................... 2 

2 . Gas emission from grab samples ....................................... 5 
METHANE EMISSIONS FROM GASSY COALS IN STORAGE SILOS 

by 

Jm E, Matta, Jm C. LaScola, and Fred N. Ki ssel I 

ABSTRACT 

The methane gas emitted from coal samples collected from the conveyor 

belts dumping into silos was measured by the Bureau of Mines. Approximately 

50 pct of the total gas desorbed into a sealed can within 1 week was released 

during the first 24 hours. No simple correlation between the gassiness of the 

coal stored and the methane concentration in the silo open space above the 

coal was found. This was probably because the gassier coals were stored in 

open-top silos, which were better ventilated. Although the methane concentration in the open space above the coal pile was less than 1 pct for all 34 

silos investigated, a methane measurement in the coal pile showed that high 

methane concentrations can exist. The methane released in the pile appears to 

accumulate and not liberate freely into the open space above the coal pile. 

Probably this gas is released during reclaiming operations, but further 

research is required to substantiate this claim. 

INTRODUCTION 

Coal can be stored in a number of ways It can be stored under 

water, in open pits, bins, bunkers, stockpiles, and silos. The current trend 

is to store coal in stockpiles or silos at the mine site, so that a predetermined quantity can be stored and easily loaded into railroad cars. In the 

past, coal was generally loaded into railroad cars the day it was mined; however, the use of the unit train, which affords reduced transportation rates 

for trainloads or partial trainloads of coal, has made temporary stockpiling 

at the mine a necessity (7). In addition to convenience in handling the coal, 

the use of silos lessens the possibility of physical and chemical changes in 

the coal, thereby preserving the utilization value of the fuel. 

l~esearch physicist. 

2~athematician. 

3 Physical research scientist. 

All authors are with Pittsburgh Mining and Safety Research Center, Bureau of 

Mines, Pittsburgh, Pa. 

4~nderlined numbers in parentheses refer to items in the list of references 

preceding the appendix. 
Subbituminous coal can be stored without much degradation in a closed bin with 

minimum air circulation, a slow rate of temperature change of the bin interior, and 

a low temperature (5). However, when gaseous bituminous coal is stored in a confined space such as a silo, methane accumulation can become an explosion hazard. 

The English were aware of this problem as early as 1875 when the Royal Commission 

inquired into the spontaneous combustion of coal and gas explosions in ships (6) . Current Federal regulations (8) requires the methane content in the air of any struc 

ture, enclosure, or other facility to be less than 1.0 vol-pct. As deeper and gassier coals are mined, problems associated with methane accumulations in silos will 

increase. 

The purpose of the present investigation was to study the possibility of a hazard resulting from methane accumulations in silos. Methane concentrations were measured in the open space above the coal and in the coal pile. Coal samples were 

collected from the conveyor belt that dumps the coal into the silo. The gas emitted 

from the samples was measured to determine the relationship between the gassiness of 

the coal stored and the methane concentration in the silo. This information is relevant because improper storage inay result in high methane concentrations. Now, with 

even deeper and more gassy coalbeds being mined and with the growing practice of 

short- and long-term storage of fine coal, the explosion hazard due to methane emission is worthy of investigation. Most of the silos investigated in this study store 

large quantities of coal extracted from mines that emit large volumes of methane 

(table 1) and, therefore, methane accumulations in these silos are possible if sufficient ventilation is not maintained. 

TABLE 1. - General information concerning silos investigated 

Mine 

Loveridge No. 22.. . Federal No. 2.. 

Valley Camp No. 3.. 

Allison ................. Do.. ................. 

Nelms No. I........ 

Nelms No. 2........ 

Do .................... 

Beatrice.. ......... 

Virginia-Pocahontas 

No. 1. 

Virginia-Pocahontas 

No. 2. 

Virginia-Pocahontas 

No. 3. 

Virginia-Pocahontas 

No. 4. 

Oak Grove .......... 

Number 

of 

silos 

1 

2 

1 

1 

1 

1 

1 

6 

1 

1 

1 

1 

1 

1 

Seam 

Pittsburgh.. ... ........ .do.. ...... ..... do ........ do ........ .do.. ...... 

Lower Freeport. 

..... do ........ do ........ 

Pocahontas 

No. 3. 

..... do ........ 

.... .do.. ...... 

.... .do.. ...... 

.... .do. ....... 

Mary Lee.. ..... 

Coal 

stored 

Clean.. 

Raw.. ... Clean.. 

Raw ...... Clean.. 

Raw .... Raw. ... Crushed 

Raw. .... 

Raw. ... 

Raw. .... 

Raw.. ... 

Raw. .... 

Raw. ... 

Daily 

mine 

production, 

tons 

9,200 

4,000 

4,200 

4,500 

4,500 

4,500 

- 

- 

3,300 

3,000 

1,100 

2,400 

850 

800 

Type 

of top 

Closed 

.do.. 

..do.. 

do.. 

. .do.. 

..do.. 

..do.. 

Open.. 

.do.. 

..do.. 

.do. . 

.do. . 

.do. . 

Closed 

with 

fan. 

Mine 

methane 

emission, 

MM cu ft/ 

day 

11.6 

8.1 

2.0 

.2 

.2 

2.1 

.4 

.4 

5.6 

3.9 

3.4 

3.3 

1.9 

1.3 

Capacity 

of each 

silo, 

tons 

10,000 

2,500 

10,000 

6,800 

17,400 

5,000 

2,500 

2,000 

2,000 

2,000 

2,000 

5,000 

5,000 

6,000 
SILO VENTILATION 

Silos can be divided into two types according to their type of ventilation--open top and closed top. Open-top silos allow large air movements above 

the coal pile, thus reducing the hazard of a methane explosion. However, opentop silos provide no protection against precipitation, and wetting of some 

coals increases the chance of spontaneous combustion. Furthermore, the movement of large volumes of air through a silo can readily entrain dust into the 

atmosphere. 

Although closed-top silos provide the protection not afforded by open-top 

silos, methane may accumulate above the coal pile during storage of gassy 

coals. Ventilation is provided by various types of openings at the top of the 

silo. Naturally ventilated silos depend upon wind to move air through the 

openings, although some silos employ exhaust fans to insure sufficient air 

movement. 

Typically, ventilation of closed-top silos is through rectangular holes 

approximately 1 by 2 feet, spaced along the outer edge of the silo immediately 

below the concrete roof. This scheme is subject to slight variations. For 

example, in the silo at Nelms No. 1 mine, the side vent holes are expanded 

into a single 18-inch-high slit, starting at the point where the conveyor belt 

crosses the side of the silo, to a point halfway around the circumference. 

Some silos have round, covered ducts, 4 to 10 inches in diameter, that protrude through the roof to provide additional openings for ventilation. Openings on the roof such as access holes, inspection holes, and open space around 

the transfer point and conveyor belt also enhance air circulation. 

When air movement due to the prevailing wind is insufficient or methane 

emission is high, exhaust fans are mounted on top of the silo. In a design 

used at the OakGrovemine, eight I-beams support the concrete roof. To insure 

adequate ventilation, 8- by 16-inch vent holes appear on one side of the silo 

only, one for each of the nine spaces between the I-beams. On the other side 

of the silo, a metal duct passes up through the roof from each space and manifolds into a 30-inch exhaust fan powered by a 13.4-bhp motor. Air sweeps 

through the silo and flows parallel along the I-beams at a total rate of 

16,600 cu ftlmin. 

A compromise between an open- and closed-top arrangement is used at the 

Nelms No. 2 mine. Here, a cluster of six open-top silos is covered by a 

slanted metal roof mounted high above the top edge. This allows free air 

movement while providing fair protection against precipitation. 

SILO USAGE 

Coal freshly mined from a working face may be temporarily stored underground in small batches and then brought to the surface. In a typical processing operation where silos are used, coal is conveyed by belt and dropped 

into the top of the raw coal silo during each mining shift. Coal is unloaded 

from the raw-coal silo through feeders in the silo base, onto a conveyor belt, 

and into the preparation plant at a continuous rate determined by the capacity 
of the plant. Since the plant capacity is usually large enough to handle a 

full day's mine production in one plant shift, coal does not normally remain 

in the raw coal silo more than 24 hours. After processing, the coal is conveyed by belt to the clean-coal silo where it will be loaded onto a unit train. 

Unit trains are loaded as often as daily or as seldom as weekly. 

CONVEYOR BELT GRAB SAMPLES 

To ascertain the gas emission potential of gassy coals stored in a silo, 

random coal samples of a few pounds each were collected in sealed canisters. 

All of the coal samples tested originated from coalbeds with a high gas content, 

as indicated by the Bureau of Mines direct method for determining gas content 

(3) and presented in table 2, and also the daily mine methane emissions 

(table 1). The samples were collected from the conveyor belt emptying into 

the silo. Gas emission from the grab samples was measured periodically by 

bleeding the gas into an inverted water-filled graduated cylinder. Chromatograph analyses indicated that the gas released from the coal was essentially 

methane. Figure 1 shows some typical gas emission curves. Table 2 lists all 

the samples gathered plus the total gas emitted within various time intervals. 

As indicated from the figure and table, the gas emission rate decreases significantly with time. In fact, for the grab samples collected, approximately 50 

pct of the total gas released within 1 week was emitted during the first 24 

hours . 

I I I 

Va. Pocahontas, No.2 

Va. Pocahontas, No. I 

Va. Pocahontas, No.4 

0 

-, 

100 200 300 400 

TIME, hr 

FIGURE 1. - Gas emitted from conveyor belt grab samples. 
TABLE 2. - Gas emission from grab samples 

2~ashes indicate that measurements were terminated since gas was no longer being 

emitted. 

3~mission curves for these samples are plotted in figure 1. 

Silo 

Gas Total gas emitted, cu ft/ton after-- 

content,' 

cu ft/ton 

PITTSBURGH 

Sample 

No. 

24 hr 48 hr 1 week 3 weeks2 

- 

- 

38.4 

16.7 

28.8 

26.6 

24.6 

31.4 

- 

- 

- 

2 weeks2 

11.2 

10.2 

11.5 

5.8 

9.0 

9.1 

7.5 

12.3 

2.0 

2.8 

1.0 

1 

2 

3l 

2 

1 

2 

3 

4 

1 

2 

3 

Loveridge, 

West Virginia. 

Federal No. 2, 

West Virginia. 

Valley Camp No. 3. . 

Allison.. ........... 

LOWER FREEPORT 

- 

- 

46.4 

19.8 

35.4 !} 

- 

- 

- 

190 

190 

190 

19 0 

60-130 

60-130 

60-130 

60-130 

60-130 

60-130 

60-130 

} clean. 

} Raw. 

Clean. 

) Raw. 

Clean. 

13.8 

12.9 

15.7 

7.5 

11.6 

11.3 

9.7 

14.9 

2.0 

2.9 

1.7 

NelmsNo. 1 ......... 

Nelms No. 2.. ....... 

23.4 

22.8 

28.2 

12.2 

23.6 

22.0 

20.3 

26.7 

2.8 

4.0 

4.5 

60-130 

60-130 

60-130 

60-130 

60-130 

60-130 

60-130 

60-130 

60-130 

60-130 

60-130 

60-130 

4.2 

4.5 

4.5 

4.2 

5.2 

5.9 

4.8 

4.6 

5.2 

4.8 

4.1 

4.8 

POCAHONTAS 

7.5 

8.7 

7.9 

8.2 

; - 

- 

5.7 

6.8 

6.2 

6.1 

10.6 

11.3 

8.7 

9.1 

9.7 

9.6 

8.4 

9.7 

Beatrice. ........... 

Virginia-Pocahontas 

No. 1. 

Virginia-Pocahontas 

No. 2. 

Virginia-Pocahontas 

No. 3. 

Virginia-Pocahontas 

No. 4. 

1 

2 

3 

4 

1 

2 

3 

4 

5 

6 

7 

38 

;I - 

- 

3.3 

3.6 

4.2 

3.8 

3.6 

3.1 

3.3 

3.3 

3.7 

3.4 

2.8 

3.3 

350-600 

350-600 

350-600 

350-600 

350-600 

350-600 

350-600 

Raw. 

. 

MARY LEE 

Oak Grove. .......... 

1 

2 

1 

1 

2 

1 

1 

178.1 

67.6 

106.3 

33.6 

18.4 

31.8 

85.8 

63.9 

55.0 

71.1 

30.7 

14.2 

20.5 

l~irect -method results. 

500 

5 00 

500 

500 

197.9 

95.3150.5 

93.0 

145.8 

47.0 

26.0 

53.0 

31 

2 

3 

4 

230.9 

179.6 

109.2 

163.6 

43.7 

32.8 

68.2 

121.8 

120.0 

101.7 

45.9 

85.8 

85.9 

67.4 

31.1 

242.6 

189.8 

115.0 

170.4 

44.2 

35.5 

75.5 

166.6 

161.5 

145.9 

64.0 

0 Raw. 

186.2 

179.2 

165.5 

72.1 

198.9 

191.3 

178.2 

77.4 

} Raw. 
According to Miiller (4), - the total gas emitted, Q, can be expressed as 

fo llows : 

where a is a constant, F is the coal tonnage, t is the time, and the exponent 

x ranges from 0.3 for coal size under 8 mm to 0.7 for grain size between 25 to 

120 mm. Figure 2 is a log-log plot of the gas emitted per ton during the 

first 48 hours for the Federal No. 2 sample shown in figure 1. Here, the initial time, t = 0, is the time when the coal is taken from the silo belt. The 

slope of the dashed line, 0.54, corresponds to the exponent x for this coal. 

The best fit to the other five emission curves shown in figure 1 resulted in 

exponents that ranged from 0.4 to 0.8. Although these values are consistent 

with those obtained by Miiller, it should be noted that Miiller chose the time 

of cutting the coal as t = 0. The choice of initial time does affect the 

exponent. For example, ignoring the first 2 hours of gas emission from the 

Federal No. 2 grab sample, which would be equivalent to an additional 2-hour 

delay in transporting the coal to the silo belt altered the exponent to 0.68. 

I 2 4 6 8 10 20 30 405060 

TIME, hr 

FIGURE 2. - Gas emitted from grab sample at Federal No. 2 mine (log-log lot). 
The grab-sample emission curve in figure 2 can be used to estimate gas 

emission from the Federal No. 2 silo. Using this emission rate for the coal 

and assuming that an empty silo is filled at a constant rate over an 8-hour 

shift, the methane emission rate from the stored coal is shown in figure 3. 

Also superimposed is the quantity of coal stored versus time. The derivation 

of this emission curve is presented in the appendix. The emission rate 

increases with time to the 0.54 power until the silo is filled, then it begins 

to decrease. The highest emission rate occurs when the silo is just filled; 

that is, when the coal is gassiest and the quantity stored is maximum. 

Although the maximum gas emission rate is only about 40 cfm, this could result 

in a large methane concentration in a short time if proper ventilation is not 

provided. 

0 120 240 360 480 600 720 840 960 

TIME, min 

FIGURE 3. - Calculated gas emission from a coal silo using the Federal No. 2 grab sample and 

assuming a constant 8-hour filling rate. 
METHANE CONCENTRATION ABOVE THE STORED PILE 

Methane gas concentrations in the unfilled upper part of the silo were 

measured with a handheld portable gas detector. One end of a 100-foot section 

of 318-ID flexible plastic tubing marked at 5-foot intervals was attached to 

the gas detector; the other end was lowered into the silo for sampling. Occasionally, samples were taken in vacuum bottles, which were later analyzed 

using a gas chromatograph. Methane concentrations measured using the gas 

detector and chromatograph agreed within '0.1 pct. 

Figure 4 shows typical curves for the methane concentration versus the 

distance from the top of the silo. The curves are based on concentration 

measurements at 10-foot intervals from the top near the center axis of the 

silo down to the coal surface. If the end of the tube actually touched the 

coal surface, concentrations of a few percent were often measured; therefore, 

to avoid measurement inconsistencies, the tubing end was always lifted a few 

inches from the coal surface. As seen from figure 4, there are no significant 

concentration changes throughout the unfilled portion of the silo. However, 

all of the open-top and most of the closed-top silos investigated do display a 

slight decrease in methane toward the top of the silo owing to better 

circulation. 

0 10 20 30 40 50 60 70 80 

DISTANCE FROM TOP OF SILO, ft 

I 

FIGURE 4. - Methane concentration gradient in coal silo. 

KEY ---Open-top silo 

Closed-top silo 

- -- -1 I Indicates surface of - / 

/ 

/ coal pile 

I I 

- 

I 

- 

I 

I - 

- -------- -I 

I 

I I 
Figure 5 is a plot of the average methane concentration in the silo above 

the coal pile versus the gassiness of the coal as determined from the random 

grab samples for each silo. Since the coal is usually stored for approximately 24 hours, this time period was used to determine the gassiness of the 

coal from the emission curves. The average methane concentration above the 

coal was estimated from the methane concentration midway between the top of 

the silo and the coal surface. For the silos shown in figure 4, the deviation 

from the average5 and midway concentration is less than 10 pct. 

Prior to this investigation, it was thought that there might be a correlation between the coal gassiness and the average methane concentration above 

the coal surface. Higher methane concentrations might be observed for gassier 

coals. However, figure 5 shows no obvious correlation, probably because the 

gassier coals were stored in open-top silos where better air circulation 

reduces the methane concentration. 

An example of this improved air circulation is the Beatrice silo at 

Keen Mountain, Va. (fig. 5). In 1970, this Beatrice silo, which was 

I 1 1 I I I I 1 - - yison (raw) 

- - Valley camp No.3 T/ 

Va. Pocahontas, No.2 

TJ - 1 I-c-l I0 I - 1 - - T~llison(clean) 

1 

-C+ Nelms No. I 

0 - Open-top silos 

Closed-top silos w /fan - T/O~~ Grove - 

LFT ,Va. Pocahontas. Nol 1 - I-o< 

(raw) 1 - 

t 1 I r a o 

TOTAL GAS EMITTED FROM AVERAGE GRAB 

SAMPLE WITHIN 24 HOURS, ft3/ton 

FIGURE 5. - Average methane concentration in silo versus gas emitted within 24 hours from 

the correspond ing grab sample. 

5The average methane concentration was determined from the methane concentrations taken from the top of the silo down to the coal surface at 10-foot 

intervals. 
originally constructed as a closed-top silo, exploded. Subsequently, it was 

reconstructed as an open-top silo, and current tests indicated essentially no 

methane above the coal. 

ME THANE CONCENTRATION WITHIN THE STORED PILE 

To investigate the methane concentration in the coal pile, a section of 

polyethylene tubing was extended halfway down into an empty closed-top silo, 

where it was buried by coal as the silo was filled over an 8-hour period. 

About 2 hours after the silo was filled, a methane concentration of about 14 

pct was measured in the coal piie. This is similar to what one would estimate 

from a methane calculation assuming no ventilation. 

Assuming no ventilation into the coal pile, the percent of methane concentration, C, which is initially zero, is governed by the following relation: 

where Q,, V, and t are the gas emission rate, free space in the coal pile, and 

time, respectively. For the silo studied, V is approximately 50,000~ cu ft, 

and the average methane emission during the first 10 hours was about 20 

cu ft/min. The average methane emission was determined from the random grab 

samples taken from this silo and using equation A-13, given in the appendix. 

Equation 2 predicts an approximate methane concentration of 21 pct after 

10 hours. 

The fair agreement between the calculated and measured concentration in 

the coal pile indicates that there was little ventilation throughout the pile. 

The methane concentration above the coal pile was less than 0.5 pct. It 

appears that although the methane concentration above the coal is low, large 

concentrations in the coal pile may exist. 

As noted earlier, when investigating the air space above the coal pile, 

if the sampling-tube end actually touched the coal pile, methane measurements 

of a few percent were obtained. Also, at some of the silos visited, the 

workers noted that similar methane concentrations are detected below the silo 

in the vicinity of the feeders when coal is being unloaded from the silo. 

Similar problems have been reported during stockpile operations (7), further 

indicating that there can be high methane concentrations within tKe coal pile. 

Large methane concentrations in the coal pile might generate an explosive 

atmosphere below the silo during reclaiming operations if proper ventilation 

is not maintained. At the Valley Camp No. 3 mine in West Virginia, two 10,000- 

cu ft/min exhaust fans force air into the vicinity of the feeders, reducing 

the methane concentration below 1 pct during reclaiming operations. The fans 

operate simultaneously with the conveyor belt that unloads coal out of the 

silo. 

^The free space in the coal pile was determined by calculating the empty volume of the silo minus the volume of coal stored using 1.3 g/cu cm as the 

density. 
Although this investigation was designed to study the possibility of 

hazardous methane concentrations in and above the coal pile, it appears now 

that a followup study of the silo reclaiming area is also warranted. 

CONC LUS IONS 

1. Even for stored gassy coals, open-top silos appear to provide sufficient ventilation above the coal to keep the methane concentration far below 

1 pct. If degradation of the coal is a concern, a slanted metal roof of the 

sort used at the Nelms No. 2 mine can provide protection while maintaining 

good ventilation. 

2. High methane concentrations can exist in the coal pile. It is possible that this gas is released during reclaiming operations, resulting in 

high concentrations below the silo if this area is not ventilated properly. 

However, to substantiate this, further research is required. 
Barkley, J. *R. The Storage of Coal. BuMines IC 7235, 1943, 14 pp. 

Fliigge, G. Methane Gas Emissions During Mining and Hauling. Gluckauf, 

v. 25, May 1972, 7 pp. 

Kissell, F. N., C. M. McCulloch, and C. H. Elder. The Direct Method of 

Determining Contents of Coalbeds for Ventilation Design. BuMines 

RI 7767, 1973, 17 pp. 

Miiller, H. Restgasinhalt und Ausgasung der Fzrderkohle (Residual and 

Emitted Gas During Coal Haulage). Ph.D. Dissertation, Tech. Univ. 

Berlin, Berlin, Germany, 1966, 162 pp. 

Parry, V. F., and J. B. Goodman. Storage of Subbituminous Coal in Bins. 

BuMines RI 3587, 1941, 10 pp. 

Rowan, T. Spontaneous Combustion and Explosions Occurring in Coal 

Cargoes: Their Treatment and Prevention. E. and F. N. Spon, London, 

1882, 97 pp. 

Stahl, R. W., and C. J. Dalzell. Recommended Safety Precautions for 

Active Coal Stock Piling and Reclaiming Operations. BuMines IC 8256, 

1965, 7 pp. 

U.S. Code of Federal Regulations. Title 30--Mineral Resources; Chapter I-- 

Mining Enforcement and Safety Administration; Subchapter 0--Coal Mine 

Health and Safety; Part 77--Mandatory Safety Standards, Surface Coal 

Mines and Surface Work Areas of Underground Coal Mines. Federal 

Register, v. 39, No. 126, June 28, 1974, pp. 24006-24008. 

3Title enclosed in parentheses is translation from the language in which the 

item was originally published. 
APPENDIX. - -THEORE TICAL CALCULATION OF THE GAS 

EMISSION FROM A COAL SILO 

To determine the total gas emitted from a silo, one should take into 

account the filling time of the silo. This can be considered by dividing the 

silo into discrete vertical intervals, n, of equal size and summing the gas 

released from the coal in each interval. The total amount of coal stored, F, 

can be expressed as the sum of the coal stored in each interval; namely, 

where fi is the amount of coal stored in the ith interval. Assuming a constant silo filling rate, the total time to fill the silo, TF, is 

where At is the time to fill an interval; that is, 

where b is the filling rate. 

Now, if Qi (t) equals the total gas emitted from the ith interval at time, 

t, then the total gas emitted, Qr (t), for t>TF is 

Q~ (t) = Ql (t) + Q2 (t - At) + Q3 (t - 2At) + . . . Qn (t - [n-ll~t). (A-4) 

According to Miiller (4), - Q,(t) can be expressed as 

Qi (t) = af, tX, 

where a and x are constants depending upon the coal stored, 

QT (t) = af, [tX + (t-~t)~ + . . . (t - [n-l]~t)~ 1. (A-6) 

The gas emission rate from the silo, E(t), for t>TF can then be written as 

Using equations A-6, A-3, and A-2 and taking the limit 

E(t) = ab [tX - (t-TF)'] , for t r TF. 
Similarly for t<TF by summing the gas emitted for only the filled intervals, 

the total gas emission rate can be expressed as 

E (t) = abtX , for t s TF . (A-11) 

The total gas emitted can be determined from the emission rates by integration; 

that is, 

ab Q~ = - tx+l , for t I TF; x+l (A-12) 

and QT = ab [tX+l - (t-T,)X+l] , for t 2 TF. 

Fliigge (2) developed an equation similar to A-12 for the specialized case 

x = 0.5 using a double integration technique. 

-U.S. GOVERNMENT PRINTING OFFICE: 1978-703-103129