Report of Investigations 7762 

The Composition of Coalbed Gas 

By Ann G. Kim 

Pittsburgh Mining and Safety Research Center, Pittsburgh, Pa. 

U N I T E D S T A T E S 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 

The composition of coalbed gas. [washington] U.S. Bureau 

of Mines [ 19731 

9 p. tables. (US. Bureau of Mines. Report of investigations 

7762) 

hcludes bibliography. 

1. Gas. 2. Mine gases. L US. Bureau of Mines. 11. Title. 

Ill. Title: Coalbed gas. (Series) 

TN23.U7 no. 7762 622.06173 

U.S. Dept. of the Int. Library 
CONTENTS 

Page 

Abstract .................. ............................................... Intrductim ................. ............................................. Gas in coal: Theory ..................................................... Procedures ............. ................................................... Gases in coal: Data ...................................................... Smv .................................................................. References ............................................................... 

TABLES 

1 . Coalbeds sampled ..................................................... 2 . Composition of gas from Pocahontas No . 3 coal ........................ 3 . Hydrocarbons from Pocahontas No . 3 coal ...... ........................ 4 . Composition of gas from Pittsburgh coal . ............................ 5 . Composition of gas from Kittanning coalbeds .......................... 6 . Cmposition of gas from Western and Southern coals ................... 7 . Ratio of C)h to C% .................................................. 8 . Heat of combustion of coalbed gas and natural gas .................... 
THE COMPOSITION OF COALBED GAS 

by 

Ann G. Kim 1 

ABSTRACT 

Samples of gas were obtained directly from the coalbed during drilling of 

horizontal and vertical boreholes in six different formations. The samples 

were analyzed by gas chromatography for C1 to C5 hydrocarbons and for 4, %, 

5, He, and Cq. Methane in the gas varied from 63 to 99 percent; carbon 

dioxide from 0.1 to 15 percent. The C\/C4 ratio varied and showed no 

apparent correlation with age, rank, or bed. The majority of samples 

contained ethane, propane, and butane; hydrogen and helium were found in some 

samples. Oxygen and nitrogen were usually present, possibly as a result of 

air contamination. A more extensive sampling program would be necessary to 

establish relationship between the amount and composition of the gas in the 

coalbed and other factors. 

INTRODUCTION 

The existence of gas in coal and its release during mining has been 

common knowledge, probably since the beginning of coal mining. Until about 

the middle of the 20th century coalbed gas was considered a mixture of methane 

(firedamp) and carbon dioxide (blackdamp). Ethane, when detected, was 

attributed to contamination from natural gas horizons below the coal (wea 

More recent analyses of coalbed gas have shown it to be a complex mixture, . containing in addition to methane and C4, higher hmologs of methane and 

other inorganic gases (5 ll). 

. GAS IN COAL: THEORY 

Coalification involves a series of biochemical and geochemical reactions 

that transform plant material into a combustible, carbonaceous solid. The 

rank designations, lignite, bituminous, and anthracite are roughly equivalent 

to different stages in a sequential transformation. Law molecular-weight 

hydrocarbons and carbon dioxide are gaseous byproducts of coalification. Aliphatic hydrocarbons can form by removal of alkyl side chains from aromatic 

1 Chemist. 

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

the end of this report. 
molecules, by decomposition of longer straight-chain molecules, or by condensation of straight-chain molecules into ring structures. Carbon dioxide is 

produced by oxidation of organic material, primarily during early stages of 

coalification. The ratio of methane to carbon dioxide is believed to increase 

during coalification. Hydrogen is also thought to be a product of the coalforming process. Oxygen and nitrogen are occluded during the deposition of 

the organic sediment. They may also be introduced into the coalbed by 

percolating ground water. Most of the oxygen is consumed by the formation of 

C% . Helium is a product of radioactive decay (3-5). - - 

The postdepositional history also affects the gas contained in the coal. 

Coal is a highly porous solid with two distinct pore systems. The macropore 

system consists pf cracks and fractures; the micropores have an average diameter of 5 to 20 A. Gases can exist as free gases in the macropores or be 

adsorbed on the surface of the nicropores. The amount of gas in the bed 

depends on temperature, pressure, degree of fracturing, and permeability of 

coal and adjacent strata (1, 12-13). 

PROCEDURES 

During studies on the use of horizontal and vertical boreholes in methane 

emission control (s I), Bureau personnel collected gas samples directly from 

six coalbeds. The gas samples were obtained by inserting a sealed evacuated 

gas sampling bottle with a capacity of about 250 ml into the borehole. The 

tip of the bottle was broken and the bottle was allowed to fill with the 

coalbed gas. Then the tip was capped with a wax-filled cartridge. Contamination with air was a serious problem. Generally a sample containing more than 

8 percent & and 2 percent 4 was considered to be highly contaminated and not 

included in calculation of gas composition. 

The gas samples were analyzed by gas chromatography in the Bureau gas 

analysis laboratory. Samples are introduced into the sample loop of the gas 

chromatograph through a mercury displacement valve. A 1-ml sample is split 

between two parallel columns--molecular sieve 5A colunm and a porous polymer 

(or silica gel) column. The molecular sieve column leads to thermal conductivity and flame ionization detectors in series which detect B, 4, and 

hydrocarbons, methane through pentane. Carbon dioxide is separated by the 

second column and is detected by thermal conductivity detector. Temperature 

of the columns is 100' C; helium is the carrier gas (9). 

A second 1-ml sample was used to determine hydrogen and helium. The 

gases were separated on a molecular sieve 5A column at room temperature; 

thermal conductivity detector and argon carrier gas were used. The sensitivity of analysis was 0.0001 percent for hydrocarbons and 0.01 percent for C&, 

5, H,, He, and C%. The accuracy of analysis is generally 21 percent of the 

amount present. 

GASES IN COAL: DATA 

Table 1 gives coalbeds and locations where gases were sampled, as well as 

the type of borehole and the number of gas samples used to calculate the 
average composition of the gas. The results of the analyses are summarized in tables 2 

and 4 through 6. The average percentage and standard deviation are given for each 

component of the gas. 

TABLE 1. - Coalbeds sampled 

I 1 I Average 

Site 

number 1 Coalbed I Loca t ion 

I ( Buchanan County, Va. I 1 

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

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

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

Pittsburgh. ...... 

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

...a*.. do........ 

Pocahontas No. 3. 

...... .do. ....... Upper Kittanning. 

(feet) 

Dismal Creek, 

Keen Mountain, 

Buchanan County, Va . Grundy, 

Buchanan County, Va . Van Sant, 

Buchanan County, Va . I Bob town, 

Greene County, Pa. 

Marianna, 

Washington County, Pa. 

Fairview, 

Marion County, W. Va. 

.......... d~.......~ 

Ebensburg, 

Cambria County, Pa. 

~oa 1' 

thick10 

11 

12 

13 

ness 

(inches) 

54 

7 borehole samples 

lMethane Bnission From U.S. Coal Mines (2). 

.....a. do........ 

B Seam, Mesaverde 

Formation. 

Lower Hartshorne. 

Mary Lee. ........ 

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

Carbondale, 

Pitkin County, Colo. 

Heavener, 

LeFlore County, Okla. 

Oak Grove Area, 

Jefferson County, A& 2 Vertical.. 

Vertical.. 

Horizontal 

Vertical.. 

I Horizontal 

TABLE 2. - Composition of gas from Pocahontas No. 3 coal 

2 

3 

3 

3 

Site 

.......... 

......... 1.25 

5. .......... 2.79 

2~11 isomers. 

3~r: Trace, less than 

4ND: Not determined. 

The samples from the Pocahontas No. 3 coalbed were taken from horizontal 

boreholes drilled in four mines within one county. The cmposition of the gas from 

No. 2 No. 3 

Percent I 2 Percent 1 2 

No. 4 

Percent 1 2 
TABLE 4. - Composition of gas from Pittsburgh coal 

Site component 

C& ............. 88.91 

H;.. ........... .I m1 

1 ND: Not determined. 

Gl& ............ C3 % ............ CIH .ma.... 

CSYP ........... co,............. 

%. ............. & .............. -- 

5 1 No. 6 I No. 7 1 No. 

f I Percent l + I Percent I 5 1 Percent 

0.04 

- 

- 

- 

10.97 

0.04 

0.05 

The composition of gas from the Kittanning coalbeds is swmarized in 

table 5. Site No. 9 was a horizontal borehole drilled in the Upper Kittanning 

coalbed; site No. 10 was a vertical borehole through the Upper and Middle 

Kittanning coalbeds in the same area. The composition of the gases from both 

boreholes is similar. Methane was the only hydrocarbon; hydrogen and helium 

were not present. 

combustion of natural gas. The heat of combustion for samples which contained 

a high percentage of CQ would be higher if the C4 were removed prior to use. 

TABLE 7. - Ratio of C$ to CC& 

TABLE 8. - Heat of combustion of coalbed gas and natural gas 

Site 

No. 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

Gas I Heat of combustion, I Heat of combustion 

Coa lbed 

Pocahontas No. 3.. 

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

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

Pittsburgh. ....... ....... .do.. ...... ....... .do.. ...a*. 

...... ..do.. ...... Upper Kittanning.. 

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

B Seam, Mesaverde 

Formation. 

Lower Hartshorne. . Mary Lee.. ........ 

ctt 1% 

4 95 

168 

1,051 

325 

8 

38 

20 

6 

955 

550 

7 

1,654 

96 1 

I ~tu/f t3 

Site No. : 

SUMMARY 

% free, ~tu/ft? 

l.............. 

Z.............. 

3.............. 

4. ............. S.............. 

6....... ....... 7*..* .......... 8. ............. 9.............. 

lO.............. 

ll.............. 

12.............. 

13. ............. Natural gas.. .... Natural gas...... 

The data in tables 2 and 4 through 6 show that the gas in coals sampled 

is a variable mixture of C1 to Cg hydrocarbons and inorganic gases, with 

lcalculated from Heat of Combustion for Organic Compounds, 

Handbook of Chemistry and Physics (17). 

a~els and Combustion Handbook (lo). 

'~ner~y in the American Economy, 1850-1975 (15). 

1,058 

1,053 

689 

1,066 

949 

1,043 

1,001 

900 

1,019 

1,059 

938 

1,058 

la 024 

950 

1,035 

1,061 

1,031 

689 

1,069 

1,043 

1,046 

1,011 

1,015 

1,019 

1,060 

1,066 

1,059 

1,026 

- 

- 
methane as the predominant component. There are significant variations in gas 

composition even for samples obtained at different locations in the same bed. 

Ihe greatest variation was in the relative amounts of methane and carbon 

dioxide, which displayed no apparent correlation with age or rank, location, 

or type of borehole. Ethane was found in the majority of samples; propane and 

butane were not as common. Hydrogen and helium were found consistently only 

in samples from the Pocahontas No. 3 coal. 

The small number of beds sampled and the limited number of samples per 

bed precludes drawing general conclusions about the composition of coalbed gas 

or about the effect of factors such as age, rank, permeability, and degree of 

fracturing. A much more extensive sampling program, with respect to the 

number of beds sampled and number of samples per bed, will be necessary to 

determine the significance of variations in the ratio of methane to C4, the 

prevalence of ethane and other higher hydrocarbons, the prevalence of hydrogen 

and helium. Other factors that should be considered are drilling techniques 

and their effect on the composition of the gas; and variations in the composition of samples from a single borehole with time and with changes in reservoir 

gas pressure. 
8. Ionescu, D., C. Caravan, and V. Ruset . Preliminary Research Regarding 

Presence of Gases in the Jilt-Vest Lignite District. Rev. Minelor, 

v. 20, 1969, pp. 368-373. 

9. Irani, H. 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. 

10. Johnson, A. D., and B. H. Auth (eds.). Fuels and Combustion Handbook. 

McGraw Hill Book Co., Inc ., New York, 1951, p. 2%. 

11. Krautsov, A. I., V. A. Sokolw, and H. M. Elinson. (On the Composition 

and Origin of the Gases of Coal Deposits.) Trudy Moskw. Geo1.- 

Razvedochnogo Inst. im S. Ordzhonikidze, v. 28, 1955, pp. 7-14. 

12. Krevelen, D. W. van. Coal, Typology, Chemistry, Physics, Constitution. 

Elsevier Publishing Co., New York, 1961, 514 pp. 

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

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

3Titles enclosed in parentheses are translations from the language in which 

the item was published. 
14. Selden, R. Fa The Occurrence of Gases in Coal. BuMines RI 3233, 1934, 

64 PP. 

15. Schurr, S. H., and B. C. Netschert. Energy in the American Economy, 

1850-1975. John Hopkins Press, Baltimore, Md., 1960, pa 736, 

table 1-11. 

16. Swift, R. A. Methane Drainage in Great Britain. Coal Age, v. 75, No. 2, 

1970, pp. 94-99. 

17. Weast, R. C. (ed.). Handbook of Chemistry and Physics. Chemical Rubber 

Co., Cleveland, Ohio, 1970, pp. D217-D222.