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: 

Kim, Ann G 

Gases desorbed from five coals of low gas content, by A. 

G. Kim and L. J. Douglas. [Washington] U.S. Bureau of 

Mines [I9731 

9 p,, illus,, tables, (UdS, Bureau of hlincs, Kcport of investigations 7768) 

Includes bibliography, 

1, Degassing of coal, 2, Gas, 3, Methane, I, U,S, Bureau of 

hlines, 11. Douglas, Louise J,, jt, auth, III, Title, (Series) 

TN23 .U7 no. 7768 622.06173 

UdS, Dept, of the Int. Library 
CONTENTS 

Page 

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

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

Formation and retention of gases in coal ................................. 2 

Experimental method ...................................................... 3 

Results ............................................................... 4 

Conclusion ............................................................... 8 

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

ILLUSTRATION 

1 . Cylinder containing coal sample ...................................... 4 

TABLE S 

1 . Description of coal samples ........................................ 3 

2 . Average composition of gas ........................................... 5 

3 . Air- and C0,- free composition of gas ................................. 7 

4 . Relative composition of hydrocarbon gas .............................. 7 

5 . Lower flammability limit of gas mixtures ............................. 8 

6 . Estimated residual hydrocarbons and hydrogen in coal ................. 8 
GASES DESORBED FROM FIVE COALS OF LOW GAS CONTENT 

by 

A. G. Kim 1 and L. J. Douglas 

ABSTRACT 

Methane and other hydrocarbon gases, byproducts of coalification, are 

emitted from coal at widely varying rates and often present ventilation problems during mining. The air in some coal mines apparently contains no methane. 

To determine if coals from mines which had no history of detectable gas emission contained adsorbed methane, samples of coal at the shaft bottom and at an 

active face were obtained and placed in sealed metal cylinders. The atmosphere around the coal was sampled and analysed by gas chromatography. Methane, 

ethane, ethylene, propane, propylene, and butane were found to have been 

desorbed from these coals. Hydrogen and helium were also detected. An estimate of the amount of residual gas per ton of coal indicated that coal samples 

from the active face generally contained more adsorbed gas than coal samples 

obtained from the shaft bottom, although in both instances the total amount of 

gas was relatively small. 

INTRODUCTION 

Methane and other hydrocarbon gases are normal byproducts of the coalification process. The amount of methane retained in the coalbed is highly variable and dependent upon many factors--physical, chemical, and geological. The 

rate of methane emission during mining is related to the concentration of 

methane in the coalbed. Any methane released into the mine atmosphere must be 

diluted with air to prevent the formation of an explosive gas mixture. 

Although it is known that methane continues to be desorbed from coal for long 

periods of time, in some mine workings there is no evidence of methane emission. Possible explanations for this phenomenon are that either the methane 

has been completely desorbed from the coal prior to mining, or the rate of 

methane emission is so low that the concentration of methane in the mine 

atmosphere (assuming adequate ventilation) is usually negligible. To investigate the first possibility, the experiment described in this report was 

designed as a simple semiquantitative method for demonstrating the presence or 

absence of hydrocarbon gases in coals from mines which had no history of methane product ion. 

chemist. 
FORMATION AND RETENTION OF GASES IN COAL 

Coalification, the process by which coal is formed, can be described as a 

series of biochemical and geochemical reactions which transform plant material 

into a combustible, carbonaceous solid. Methane is the primary gaseous 

byproduct of this process, but other hydrocarbons, CO,, and hydrogen are also 

produced. 

Coalification begins with a biochemical stage during which plant materials are partially decomposed by anaerobic microorganisms, humic substances 

are formed, and peat accumulates. Also produced during this stage are methane, 

CO,, and trace amounts of ethane, ethylene, propane, propylene, butane, and 

pentanes. These gases may be retained in the peat by adsorption. Atmospheric 

nitrogen and oxygen may also be adsorbed on the peat during deposition. Inundation and the deposition of inorganic sediments usually terminates the development of peat and initiates the geochemical phase of coalification. 

During the geochemical stage of coalification, the time, temperature, and 

pressure cause physical and chemical changes in the coal. Rank designations 

such as lignite, bituminous, and anthracite are roughly equivalent to different stages in a sequential transformation. With increased rank, coal becomes 

darker and more lustrous. Physical changes are accompanied by an increase in 

carbon content, aromatic character, and calorific value, and a decrease in 

moisture, volatile matter, oxygen, and hydrogen content. There are several 

theoretically possible mechanisms by which gases may be produced during coalification. Methane and other hydrocarbons can be produced by the removal of 

alkyl side chains from aromatic molecules or by the condensation of straight 

chain molecules into ring structures. The decomposition of resins and waxes 

can also yield a mixture of hydrocarbon gases. Hydrogen, also a primary product of coalification, could react with C2 to C, hydrocarbons to form methane. 

Oxidation of the coal by occluded oxygen results in the formation of C02. 

Although it is not a reaction product, helium, produced by decay of radioactive minerals, is also present in coal (3-4, 5) ., The gas phase in coal, 

therefore, has been found to be a mixture of methane, the C2 to C, hydrocarbons, C02 , oxygen, nitrogen, hydrogen, and he1 ium. However, the postdeposi - tional history of the coalbed affects the amount and composition of the gas 

retained. 

Physically, coal is a highly porous solid with two distinct pore systems. 

The macropore system consists of cracks and fractures; the micropore system is 

similar to a molecular sieve structure with an average pore diameter of 5 to 

20 A. Gases in the coalbed can exist as free gas in the macropore system or 

adsorbed on the surface of the micropores. The diffusion coefficient, activation energy of desorption, adsorbate density, permeability, surface area, 

and pressure control the rate at which gas can flow from the micropores. Flow 

in cracks and fractures depends upon pressure, the degree of fracturing, and 

the permeability of adjacent strata. Emission from the bed into a mine is 

2Underlined numbers in parentheses refer to items in the list of references at 

the end of this report. 
generally diffusion controlled; the rate of emission is given by Fick's law: 

where dq/dt is the volume flow rate, D is the diffusion coefficient, A is the 

area, C is the concentration of gas, and L is the diffusion path length. 

The total volume of gas released per mass of coal (Vt) is 

V, = A 11 - exp(t/h )"I for 1 > n > 0, 

where A is the initial gas content of the coal, t, is a time constant, t is 

the time elapsed since the start of emission, and n is an empirical constant. 

With time, the volume of gas emitted increases; the rate of emission decreases 

as the volume of gas retained in the coal decreases (1-2, 8). 

EXPERIMENTAL METHOD 

Basically the experimental method consisted of placing samples of coal in 

a sealed container, allobing gas in the coal to desorb, sampling the atmosphere in the container, and analysing it by gas chromatography. Coal samples 

were obtained from six mines in five coalbeds (table 1). All mines had no 

history of gas emission. In the majority, samples were taken both at the 

active face to obtain freshly mined coal, and from the shaft where the coal 

had been exposed for relatively long periods of time. The coal was sealed in 

a cylinder of either cast iron or aluminum (fig. 1); in the top of the cylinder were a pressure gage and a valve. Volume of the cylinder was approximately 0.1 cubic foot. Cylinders were weighed before and after coal samples 

were collected, and the difference was taken as the weight of the coal. 

TABLE 1. - Description of coal samples 

In the laboratory, a ~amilton~ lecture bottle septum was attached to the 

valve on the cylinder. The valve was opened and a sample of the gas in the 

Sample 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

3~eference to specific brands is made for identification only and does not 

imply endorsement by the Bureau of Mines. 

Coal seam 

UpperFreeport ........ Lararnie No. 3......... 

..... do .................... ..... do .................... ..... do .................... Uncorrelated bed ...... ..... do .................... No. 1 (Rock Island). .. ..... do .................... Mamoth ............... ..... do .................... 

Location 

PrestonCounty,W. Va.. 

Weld County, Colo ...... do ................ do ................ do ................ Lucas County, Iowa ..... do ................ Mercer County, Ill.. ... do ................ Monroe County, Iowa .... do ................ 

Sample obtained Rank 

Face ........... HvAb 

Shaft .......... SubB 

Face ........... SubB 

Shaft .......... SubB 

Face ........... SubB 

Shaft. ......... HvCb 

Face ........... HvCb 

Shaft.. ........ HvCb 

Face ........... HvCb 

Shaft. ......... HvCb 

Face ........... HvCb 
CO, , H, , He, and the C1 to C5 hydrocar 

 

bons are summarized in table 2. Air, 

trapped in the cylinder when the coal 

samples were obtained, was the major 

component of the gas. With time some of 

the coal was oxidized, resulting in a 

decrease in the percentage of oxygen and 

an increase in the amount of CO,. The 

percentage of hydrogen, helium, and 

hydrocarbons remained relatively constant with time. Helium was desorbed 

from only two samples, one obtained in 

FIGURE 1. - Cylinder containing the shaft (No. 7) and one from the 

coal sample. active face (No. 10). Hydrogen was 

detected in the gas from all face samples and in the gas from two of the five 

shaft samples. Methane was desorbed from all samples, as was ethane. Small 

amounts of ethylene and propane-propylene were detected; only trace amounts of 

butane were found and no pentanes were detected in the desorbed gas. The 

amount of hydrogen and hydrocarbons desorbed was greater from coals obtained 

at the face than from coal samples obtained in the shaft. Since desorption is 

a continuous process, the concentration of gas within the coal would be lower 

in coals that had been exposed for longer periods. 
TABLE 2. - Average composition of gas, percent 

N,.......... 

CO, ......... % .......... He .......... CH4 ......... C&. ....... 

C4qo.. ..... ND ND 

Total. . , 100.0038 ,100.0045 

ND--Not detected. 

Tr--<0.001 percent (trace). 

Isample numbers refer to table 1. 

2~ropane-propylene elute from gas chromatograph as 1 peak. 
Assuming that oxygen and nitrogen are related to air in the cylinder and 

C02 is formed by oxidation of the coal, the hydrogen, helium, and hydrocarbons 

were considered the gases actually desorbed from the coal. The relative composition of the desorbed gas is given in table 3. 

Since hydrogen constituted an unusually large percentage of the gas from 

most of the coals, the possibility that the hydrogen was formed during the 

experiment was considered. One possible mechanism involved the formation of 

sulfuric acid from pyrite in the coal. If the acid reacted with the metal 

cylinder, hydrogen would have been produced. There was no correlation between 

either the sulfate or pyritic sulfur of the coal and the hydrogen content, and 

the cylinders showed no signs of corrosion. The possibility that the hydrogen 

was a byproduct of the oxidation of the coal was also considered. No relationship was found between the concentrations of CO, and Hz. With one exception, the gas samples from coals obtained at the face contained substantially 

more hydrogen than gases from coals obtained in the shaft, indicating that 

despite the anomalously high concentration, hydrogen was desorbed from the 

coal. However, the origin of the hydrogen could not be unequivocally established with this experimental method. 

Previous analyses have shown that the hydrocarbon gas desorbed from coal 

with a high gas content usually consists of over 95 percent methane; the concentrations of C, to C, alkanes generally decrease with increasing molecular 

weight. Ethylene and propylene are the only alkenes detected, and only at low 

concentrations. In the hydrocarbon gas desorbed from coals of low gas content, 

methane was the major component, but in the majority of samples it constituted 

less than 90 percent of the hydrocarbon gas (table 4). In four of the five 

pairs of samples, gas from face coal contained a higher percentage of methane 

than gas from the shaft coals. Ethane was the second most abundant hydrocarbon. The relative amounts of ethylene and propane-propylene varied. Butane 

was detected occasionally; pentane was not found in the desorbed gas. 

The lower flammability limit for methane is 5 percent. When other combustible gases are present with methane, the flammability limit of the mixture 

depends upon the proportion and flammability limit of each gas (3, 2). The 

lower flammability limit for the mixture of hydrocarbon gases desorbed from 

the low-gas-content coals generally falls between 4 and 5 percent. If hydrogen is included in the calculation of the lower flammability limit, it again 

falls between 4 and 5 percent (table 5) . 

The coal samples used in this experiment were obtained from mines which 

did not have detectable gas emissions. Since a mixture of combustible gases 

is being desorbed from the coals, the rate of desorption must be very low, and 

the rate of desorption is related to the volume of gas in the coal. To obtain 

an estimate of the amount of gas in the coal, the volume of gas in the cylinder was calculated. This, multiplied by the proportion of hydrocarbons and 

hydrogen (table 2), divided by the weight of the coal, was considered a minimum value for the residual gas in the coal (table 6). Since there are insufficient data about residual gas in coal, these numbers cannot be related to an 

"average" gas content of coal. 
TABLE 3 . . Air- and CO,. free composition of gas, percent 

Samp lel ........ H, ............. He ............. CH , ............ C2J& ........... C2H ........... c3 G2 .......... c3 %2 .......... 

ND- Not detected . Sample numbers refer to table 1 . ~ro~ane-~ro~~lene elute from gas chromatograph as 1 peak . 

C4q0 .......... 

Total ..... 

TABLE 4 . . Relative composition of hydrocarbon gas. percent 

1 

95.45 

ND 

4.52 

. 02 

. 005 

) . 005 

ND 

100.00 

2 

1.10 

ND 

85.71 

4.40 

4.40 

4.40 

ND 

100.01 

Samp lel .......... CH, .............. C2G ............. C2H ............. c, G2 ............ c,%" ............ C4H ............ 

Total ....... 

3 

98.67 

ND 

1.04 

. 16 

. 05 

. 08 

ND-- Not detected . 'sample numbers refer to table 1 . ~ro~ane-~ro~~lene elute from gas chromatograph as 1 peak . 

1 

99.32 

. 46 

. 11 

) . 11 

ND 

100.00 

ND 

100.00 

2 

86.67 

4.44 

4.44 

4.44 

ND 

99.99 

4 

7.69 

ND 

65.38 

7.69 

11.54 

7.69 

8 

87.40 

8.40 

1.68 

1.68 

. 84 

.100.00 

5 

93.87 

ND 

6.03 

. 05 

. 03 

. 02 

ND 

99.99 

3 

78.00 

12.00 

6.00 

4.00 

ND 

100.00 

9 

97.02 

2.97 

ND 

. 005 

ND 

99.995 A 

ND 

100.00 

4 

70.83 

8.33 

12.50 

8.33 

ND 

99.99 

1

ND 

.100.00 

5 

98.36 

. 82 

. 55 

. 27 

ND 

100.00 

ND 

99.99 

6 

78.48 

12.66 

1.27 

7.59 

ND 

100.00 

7 

98.45 

1.29 

. 13 

. 13 

ND 

100.00 

. 84 

100.00 

ND 

99.995 

10
TABLE 5. - Lower flammability limit of gas mixtures 

Sample numbers refer to table 1. 

Hydrocarbons, percent 

in air 

4.98 

4.47 

4.20 

3.94 

4.92 

4.20 

4.95 

4.64 

4.90 

4.19 

TABLE 6. - Estimated residual hydrocarbons and hydrogen in coal 

Hydrocarbons + Hz, 

percent in air 

4.04 

4.43 

4.03 

3.94 

4.05 

4.20 

4.41 

4.64 

4.86 

4.05 

11 1 1,563 

Isample numbers refer 

samp lel 

Weight of True specific I Residual 1 Residual 

coal, g gravity 

CONCLUSION 

1.38 

1.48 

1.43 

1.49 

1.45 

1.46 

1.43 

1.44 

1.56 

1.54 

1.43 

Although the methods used were fairly simple, this experiment did demonstrate that methane and other hydrocarbons are contained in coals from which 

there is no apparent methane emission. Ethane, ethylene, propane, propylene, 

butane, and hydrogen in addition to methane were detected in the desorbed gas. 

The estimated amount of residual gas is small. However, lack of critical data 

about the gas content of coal prevents making comparisons between the residual 

hydrocarbons, 

ft3/ton 

gas content of low-gas and "normal1' coals. 

hydrogen, 

ft3/ton 

to table 1. 

0.0072 

. 00 14 

.0015 

.0007 

.0114 

.0012 

.0168 

.0015 

.5 24 

.0015 

. 0 145 

0.151 - 

.I13 

.00005 

.I75 - 

.004 

- 

.018 - 

.017 
I REFERENCES I 

1. Airey, E. M. Gas Emission From Broken Coal. An Experimental and Theoretical Investigation- Internat. J. Rock Mechanics and Min. Sci., v. 5, 

1968, pp. 475-494. 

2. Cervik, Joseph. Behavior of Coal-Gas Reservoirs. BuMines TPR 10, 1969, 

10 PP 

3. Cooper, B. S., and S. G. Murchison. Organic Geochemistry of Coal. Ch. in 

Organic Geochemistry, ed. by G. Elinton and M. J. H. Murphy. SpringerVerlag, New York, 1969, pp. 699-726. 

4. Francis, W. Coal: Its Formation and Composition. Edward Arnold, Ltd., 

London, 2d ed., 1961, 806 pp. 

5. Jones, G. W. Inflammability of Mixed Gases. BuMines Tech. Paper 450, 

1929, pp. 17-18. 

6. Kim, A. G., and L. J. Douglas. Hydrocarbon Gases Produced in a Simulated 

Swamp Environment. BuMines RI 7690, 1972, 15 pp. 

. A Gas Chromatographic Method for Analyzing Mixtures of Hydrocarbon 

and Inorganic Gases. J. Chromatographic Sci., in press. 

8. Patching, T. H. The Retention and Release of Gas in Coal. A Review. 

Min. and Met. Bull., v. 63, 1970, pp. 1302-1308. Canel 

9. Zabetakis, M. G. Flammability Characteristic of Combustible Gases and 

Vapors. BuMines Bull. 627, 1965, pp. 114-115. 

INT.-BU.OF MINES,PGH.,P A. 18539