SORPTION INVESTIGATIONS OF METHANE ON COAL 

by 

J. H. Perkins and Joseph Cervik 

Bureau of Mines Methane Control Program 

Technical Progress Report - 14 

May 1969 

U. S . DEPARTMENT OF THE INTERIOR 
CONTENTS 

Page 

Abstract ................................................................ Introduction ............................................................ Desorption phenomenon in coal ........................................... Discussion of sorption apparatus ........................................ Conclusions ............................................................. 

ILLUSTRATIONS 

Fig . 

1. Sorption apparatus ................................................ 2 . Des~rption curve for Pittsburgh coal. 180 to 200 mesh ............. 
SORPTION INVESTIGATIONS OF METHANE ON COAL 

by 

J. H. Perkins and Joseph Cervik2 

The Bureau of Mines has conducted desorption work on fine coal particles 

at near atmospheric pressure. An apparatus has been developed that uses a 

capacitance manometer to measure desorption of methane from coal particles up 

to 2 inches in diameter and pressures up to 1,000 psi. This will make it possible to duplicate the desorption process as it occurs in its natural underground environment. 

INTRODUCTION 

A comprehensive methane control research program is being conducted by 

the Bureau of Mines to establish the geological structural controls that govern the migration and retention of methane in coalbeds ; determine the physical 

properties of methane and of the coal and other strata with which methane is 

associated; devise methods by which methane may be removed from, or controlled 

in, coalbeds prior to and during coal extraction; develop mechanical equipment and instrumentation needed to control methane; and conduct field trials 

of the methane control techniques developed. This is one of a series of 

reports on specific aspects of the program. 

High methane concentrations that result during mining are, in part, due 

to the desorption of methane from coal. Methane emission due to desorption 

can create serious safety hazards both inside and outside coal mines. It has 

been observed that a lump of coal, after being mined, will continue to liberate methane for months. This "slow bleeding" or desorption is the result 

of methane gas that is stored in the micropore structure of the coal. This 

method of transport has been shown to be governed by Fick's law of diffusion:' 

lPhysicist, Pittsburgh Mining Research Center, Bureau of Mines, Pittsburgh, Pa. 

2Supervisory geophysicist, Pittsburgh Mining Research Center, Bureau of Mines, 

Pittsburgh, Pa. 

"~ervik, Joseph. An Investigation of the Behavior and Control of Methane Gas. 

Min. Cong. J., v. 53, No. 7, July 1967, pp. 52-57. 
where 

q' = volume flow rate ; 

D = diffusion coefficient; 

A = cross-sectional area; 

c = concentration of gas in solid coal; 

1 = length. 

Consequently, the rate of desorption of gas from coal depends upon equilibrated 

pressure, coal particle size, and diffusivity coefficient. 

Several attempts to measure the rate of desorption have been made. Hofer 

and others4 did work with fine coal samples near atmospheric pressure. They 

concluded that in terms of the fractional approach to equilibrium, the law 

governing the rate of sorption is nearly independent of pressure, and that the 

adsorption and desorption curves were identical. This work also showed that 

the rate of adsorption increases eightfold as the particle size decreases from 

6 to 8 to 270 to 325 mesh per inch. However, there have been measurements of 

gas bed pressures higher than 550 psi, and it is well known that mining 

machines degrade coal in sizes from very fine dust to particles several inches 

in diameter. Therefore, there exists a need to work on coals at higher pressures and larger particle sizes.5 

The Bureau of Mines attempted to work with large sizes of coal, using a 

flame ionization detector. This device burns a sample of methane gas in an 

oxygen-hydrogen flame resulting in an ionized methane molecule, and an electrometer measures the change in charge that results from the ionization of the 

gas. The apparatus is capable of measuring methane in parts per million, but 

its sensitivity to atmospheric pressure changes makes it impossible to calibrate for flows of methane less than 1 cu cm/min. 

An apparatus has been developed that allows samples of coal up to 2 

inches in size to be equilibrated at pressures up to 1,000 psi. Desorption 

curves can be obtained for these samples that simulate any conditions underground. This apparatus makes it possible to correlate the rate and amount of 

methane desorbed from coal, with the equilibrated bed pressure of the coal and 

the type of mining machinery used. 

DESORPTION PHENOMENON IN COAL 

As mining progresses, more coal becomes exposed and, thus, more gas can 

be emitted into the mine by the desorption process. 

*Hofer, L.J.E., James Bayer, and Robert A. Anderson. Rates of Adsorption of 

Methane on Pocahontas and Pittsburgh Seam Coals. BuMines Rept. of Inv. 

6750, 1966, 13 pp. 

"Work cited in footnote 3. 
Fick's law illustrates that when a piece of coal is placed in an atmoshere of methane, adsorption of the methane by the coal will take place. The 

'total amount of the methane adsorbed is a function of the environmental pres- 

'sure, temperature, and time. The time for this adsorption to reach equilibrium is a function of the size and shape of the coal particle. The reverse 

of this process, desorption, takes place when the environmental pressure is 

reduced. The amount of gas desorbed and rate of desorption are governed by 

the same factors that control adsorption. It has been shown that although a 

small pile of finely powdered coal (275 to 325 mesh) and a pile of 114-inch by 

0 coal will adsorb the same quantity of gas at 1 atmosphere pressure when the 

pressure is reduced, the fine coal releases its gas in 1 hour; the 114-inch 

coal releases its gas in 30 days. For coal particles of the order of 112- 

inch, with no cracks or other defects, the desorption process may take from 6 

rnonths to 1 year.G 

Experiments have shown that when a mining machine is operating there is 

an increase in methane emission. Ignoring Darcy flow, two factors contribute 

to this increase: 

1. The miner is degrading coal into particles ranging from very fine 

dust to large pieces. This degradation accelerates the release of gas from 

coal by the desorption process. 

2. Coal about 150 feet in advance of the face is desorbing gas because 

of the existence of a draining radius.? This clearly demonstrates the need for 

correlating desorption rates of methane with gas bed pressures and particle 

size. 

DISCUSSION OF SORPTION APPARATUS 

The apparatus developed by the Bureau of Mines (fig. 1) is capable of 

measuring desorption rates for pressures from 1,000 psi to vacuum, and for 

coal particles up to 2 inches in diameter; coal blocks up to 18 inches in 

diameter also can be studied at pressures up to 600 psi. These measurements 

are made by placing the coal sample in a pressure vessel and pressurizing 

the vessel with methane. When the desorption process has ended and equilibrium has been reached, the pressure surrounding the coal particle is reduced 

to simulate the condition the coal would experience in a mine. The pressure 

in the vessel will begin to increase owing to the desorption of methane from 

the coal. This buildup in pressure is monitored by a capacitance manometer 

capable of sensing pressure changes of 0.01 psi. The capacitance manometer 

controls a valve that allows the excess gas to be bled into a second vessel, 

where the quantity of gas desorbed can be calculated by using pressure-volume 

relationships. 

6Work cited in footnote 3. 

?Work cited in footnote 3. 
FIGURE 1. - Sorption Apparatus. 1. Pressure vessel containing th

changes in pressure vessel No. 1; 3. capacitance br

vessel No. 1 due to desorption; 5. pressure meter fo

pressure transducer ); 6. air valve actuated by the 

pressure vessel No. 1 ; 7. reservoir for collecting the 
FIGURE 2. - Desor~tion Curve for Pittsburgh Coal, 180 to 200 Mesh. 

12 I 

With this apparatus, it is possible to correlate particle size to the 

rate of desorption. It should be helpful in predicting the practicability of 

various methods of degasification such as sinking methane drainage boreholes 

into the coalbed. This apparatus will be able to correlate the amount of gas 

desorbed with a drop in bed pressure. Since a borehole will produce a 

draining radius, a drop in bed pressure in the area of the borehole can be 

expected. Provided there are no clay veins or other obstructions in the 

coalbed, estimates of the volume of desorbed gas, based on the expected pressure drop around the boreholes, can then be made. Assuming similar conditions 

at the active face of a coal miqe, this apparatus will yield information that 

will aid in predicting the amount of gas moving into the mine through the 

face. 

II 

Figure 2 shows a desorption curve obtained from a 852-gram, 180- to 

200-mesh, sample of Pittsburgh-bed coal. This figure shows the desorption 

curve obtained from equilibrating the sample at 270.9 psi, and then reducing 

the pressure to 173 psi. It should be noted that 90 percent of the methane 

was emitted within the first 100 minutes, but that some desorption is still 

taking place at 200 minutes. The gas desorbed at time zero is the gas that 

was given off when the pressure was initially reduced. 

10 - - 

3 

0 " 9- - 

- U 

- 

- 

- 

- 

2 - - 

l - 

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

0 20 40 60 80 100 120 140 160 180 200 

TIME, min 

1 I I 1 1 1 1 I I I I I I I 1 1 I , 

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CONCLUS IONS 

The apparatus described is the latest of the Bureau's efforts to develop 

a system to measure desorption rates of methane from large coal samples at 

pressures up to 1,000 psi. This apparatus will now be used to measure the 

rates of desorption from many different samples of coal and attempt to correlate the previously mentioned parameters. 

INT.-BU.0 F MIN ES,PGH.,PA. 13652