Bureau of Mines Report of Investigations/l981 

Assessing the Methane Hazard 
Report of Investigations 8525 

Assessing the Methane Hazard 

of Gassy Coals in Storage Silos 

By John C. LaScola, Joseph E. Matta, 

and Fred N. Kissell 

CONTENTS 

Page 

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

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

Measuring the gassiness of the coal ..................................... 2 

Methane concentrations above the stored pile ............................ 3 

Methane concentrations in the reclaiming area ........................... 5 

Ventilation of the reclaiming area ..................................... 6 

Sumary ................................................................. 8 

References .............................................................. 9 

ILLUSTRATIONS 

1 . Gas emitted from conveyor belt grab samples ......................... 2 

2 . Methane concentration gradient in coal silo ......................... 3 

3 . The Nelms No . 2 cluster of open-top silos is covered by a metal roof 

that allows free movement of air .................................. 4 

4 . Methane concentration as coal was loaded below silo .................. 5 

5 . Tracer gas decay curves ............................................. 7 

TABLE 

1 . Measured methane concentrations in reclaiming area .................. 5 
ABSTRACT 

The Bureau of Mines investigated coal storage silos to determine how 

gassy coal must be for methane accumulations in a silo to become hazardous and 

where such accumulations are likely to occur. Methane concentrations were 

measured in the open space above the stored coal pile, in the pile, and in the 

ASSESSING THE METHANE HAZARD OF GASSY COALS 

IN STORAGE SILOS 

John CY LaScola, Joseph E. Matta, 2 and Fred N. Kissell 3 

1 reclaiming area. No methane layering was found in closed-top silos. Reclaiming areas were as gassy as the top part of the silos. Coal samples were collected from the conveyors entering the silos in order to assess the gassiness 

of the coal. No simple correlation was found between the gassiness of the 

coal stored and the measured methane concentrations. However, at mines where 

the average 24-hour gas emission from the conveyor belt samples was 14 ft3/ton 

or more, fans or open tops were used. It appears that a large fraction of the 

methane released during storage remains in the void space between the coal 

particles. 

INTRODUCTION 

When gassy coal is stored in a confined space, methane accumulations can 

become an explosion hazard. Over a century ago, explosions were known to 

occur on sailing ships carrying coal. More recently, coal silos have been the 

site of methane explosions, and as deeper and gassier coals are mined in the 

future, proper silo ventilation will become even more essential. 

The purpose of the present study was to determine how gassy a coal must 

be for methane accumulations to become hazardous during silo storage and where 

that methane was most likely to accumulate. Current silo ventilation practices, some of which are very effective, were also noted. In all, 20 different silos were visited. Coal samples were collected from the conveyor 

'physical scientist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, 

Pa. 

'~esearch physicist, formerly with the Pittsburgh Research Center, Bureau of 

Mines, Pittsburgh, Pa. (now with the U.S. Department of Defense, Aberdeen 

Proving Ground, Md. ) . 3~esearch supervisor, Dust Control and Ventilation, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, Pa. 
entering the silo and analyzed for their methane content. Methane concentrations were measured in the silo open space above the coal, in the pile, and in 

the reclaiming area at the bottom. Most of the mines from which the coal was 

obtained would be considered very gassy, with total methane emissions exceeding 1 million cubic feet per day (I),~ whereas none of the silos visited had 

gas concentrations that exceeded the 1 pct legal limit. 

MEASURING THE GASSINESS OF THE COAL 

In general, the gassier the coalbed is, the more gas the mine and the 

silo contain. If a close correlation existed, silo ventilation needs could be 

calculated from the coalbed gas content before any coal was mined. This is 

not the case, however, and unless some indication of coal gas content is 

available, no comparison of silos at different locations is possible. Accordingly, at each of the locations visited, several grab samples of coal weighing 

a few pounds each were collected from the conveyor belt entering the silo. 

These were sealed in an airtight can equipped with a valve and short hose. 

Methane bleeding from the coal was periodically vented through the hose into 

an inverted water-filled graduated cylinder and measured. Examples of resulting methane emission curves are shown in figure 1. Such curves may be used 

200 
to estimate an emission rate for an entire silo (2) - provided that the filling 

rate and quantity stored are known or estimated. Since coal is usually stored 

for about 24 hours, the gas emitted from the grab samples in a 24-hour period 

can be used as an approximate gassiness index. According to figure 1, 24-hour 

emissions can vary considerably, ranging from 3.3 ft3/ton at Nelms No. 2 mine, 

Harrison County, Ohio, to 86 ft 3/ton at the Oak Grove mine, Jefferson County, 

Ala. 

METHANE CONCENTRATIONS ABOVE THE STORED PILE 

Silos can be divided into two groups depending upon whether their ventilation is open-top or closed-top. Open-top silos allow large air movements 

above the coal pile, thus reducing the hazard of a methane explosion. Closedtop silos are usually ventilated by openings at the top of the silo. These 

are typically rectangular holes, measuring about 1 by 2 feet and spaced along 

the outer edge of the silo immediately below the concrete roof. Openings on 

the roof such as access holes, inspection holes, and open space around the 

transfer point and conveyor belt enhance air circulation. 

As the possibility of some methane layering in the unfilled upper part of 

closed-top silos has been often mentioned, this was the first area checked. 

For this purpose, one end of a 100-foot section of flexible plastic tubing 

w I I I I 1 I I 

KEY 

DISTANCE FROM TOP OF SILO, feet 

FIGURE 2. - Methane concentration gradient in coal silo. 

METHANE CONCENTRATIONS IN THE RECLAIMING AREA 

Methane concentrations were next measured in the reclaiming area, using a 

methanometer and telescopic sampling probe. The highest values were generally 

equal to, or higher than, those on top, and these were measured in the feeders 

or directly above the coal on the belt while coal was being reclaimed. Figure 4 shows how the methane concentration varied as coal was loaded below a 

silo at the Virginia Pocahontas No. 3 mine, Buchanan County, Va. Table 1 

gives some typical methane concentrations measured. 

TABLE Measured methane concentrations in reclaiming area 

Mine 

Allison (Ohio).... ............. Beatrice (Virginia) ............ 

Federal No. 2 (West Virginia).. 

Nelms No. 1 (Ohio)....... ........... V

llCN3 

Ventilation 

Natural.......... 

5,600-cfm fan.... 

Natural.......... 

do.......... 

20000ff

Average 

at 

feeders, 

percent 

0.45 

.2 1 

.34 

.66 

54 

Number 

of 

readings 

32 

Continuous 

recorders. 

3 8 

15 

13 

Maximum 

value 

in 

silo, 

percent 

1.0 

.3 

.81 

1.0 

78 
corresponds to the 625 tph of coal being reclaimed. Since 32 cfm of methane 

was released, it follows that the void space had a methane concentration of 

12 pct. A similar calculation for another silo gave 6 pct. 

To check this further, a long piece of polyethylene tubing was extended 

from the top halfway down into an empty silo at the Federal No. 2 mine, Marion 

County, W. Va. It was eventually covered with coal as the silo was filled 

over an 8-hour period. Samples drawn from the tubing showed 14 pct methane. 

The calculated concentration, based on the emission from conveyor belt grab 

samples, was 21 pct, assuming that none of the methane migrated out of the 

pile (2). - 

VENTILATION OF THE RECLAIMING AREA 

In general those mines that found it necessary to employ open-top silos 

(where 24-hour conveyor belt grab sample gas averaged about 14 ft3/ton) also 

found it necessary to use a fan to disperse methane in the reclaiming area. 

Fan ventilation ranged from 5,000 to 20,000 cfm (table 1). In a few instances 

a tracer gas was used to measure the amount of fresh air purging the reclaiming areas. The tracer gas technique has the advantage that it can be applied 

with or without a fan (3). At the Federal No. 2 mine, sulfur hexafluoride 

(SF,) gas was used to dGtermine the amount of natural ventilation in the 

reclaiming area of a double silo. No fan was used, but openings totaling 

140 ft2 gave access to the outside. A plot of SF, concentration versus time 

(fig. 5) indicated a fresh air quantity of 11,700 cfm. 

At the Virginia-Pocahontas No. 3 mine, the methane emerging from the 

feeders along with the coal was used as the tracer gas. When coal loading 

ended, the methane concentration decayed (fig. 4). A plot of the decay curve 

(minus the baseline concentration) is also shown in figure 5. The slope gives 

8,800 cfm. The silo had a 14,000-cfm fan and it was evident that not all the 

air passing through the fan was ventilating the feeders. The fan was mounted 

above an open doorway. Although this decreased its effectiveness somewhat, 

there would still be considerable natural ventilation if the fan were to fail. 
TIME, min 
SUMMARY 

1. At mines where the average 24-hour gas emission from the conveyor 

belt grab samples was 14 ft 3/ton or more, ventilation was enhanced by using 

open-top silos or a fan at the top, as well as a fan in the reclaiming area. 

2. No methane layering was found in closed-top silos. If anything, the 

methane concentration at the roof was less than that in the rest of the silo. 

3. Reclaiming areas were as gassy as the top part of the silo. Methane 

is released as coal is reclaimed, and a large fraction of the methane released 

during storage appears to remain in the void space between the coal particles. 
REFERENCES 

1. Irani, M. C., J. H. Jansky, P. W. Jeran, and G. L. Hassett. Methane Emission From U.S. Coal Mines in 1975, A Survey: A Supplement to Information Circulars 8558 and 8659. BuMines IC 8733, 1977, 55 pp. 

2. Matta, J. E., J. C. LaScola, and F. N. Kissell. Methane Emissions From 

Gassy Coals in Storage Silos. BuMines RI 8269, 1978, 14 pp. 

3. Matta, J. Em, Em D. Thimons, and F. N. Kissell. Jet Fan Effectiveness as 

Measured With SF6 Tracer Gas. BuMines RI 8310,-1978, 14 pp.