Bureau of Mines Report of Investigations/l978 

Experimental Studies on the Origin 

and Accumulation of Coalbed Gas 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Report of Investigations 8317 

Experimental Studies on the Origin 

and Accumulation of Coalbed Gas 

By Ann G. Kim 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Cecil D. Andrus, Secretary 

BUREAU OF MINES 

Roger A. Markle, Director 
Abstract ................................................................ Introduction ............................................................ Gas formation during biogenetic coalification ........................... Hydrocarbon gas formation during diagenetic coalification ............... Composition of coalbed gas .............................................. Variation in gas content of coal ........................................ Smry ................................................................. References .............................................................. 

ILLUSTRATIONS 

Coal cycle ......................................................... Structures of cellulose and lignin ................................. Gas produced from wood ............................................. Variation in fixed carbon and heating value of coal with rank ...... Arrangement of micelles within the coal structure .................. Coal formation as a function of time and temperature ............... Total hydrocarbon emission frm coal versus rank of coal ........... Total hydrocarbon emission from coal versus temperature ............ Methane emission £run coal versus temperature ...................... Percentage of methane in emitted gas versus temperature ............ Relative composition of hydrocarbon gas ............................ Constants ko and n, as a function of fixed carbon-volatile matter 

ratio ............................................................ Adsorptive capacity of coal as a function of rank and depth ........ 

TABLES 

Gas produced from plant material ................................... Gas desorbed from peat ............................................. Composition of gas frm plant material and peat .................... Hydrocarbon distribution: Gas from plant material and peat ........ Composition of gas from plant material. peat. and coal ............. Changes in hydrocarbon and methane emission with increased 

temperature ...................................................... Hydrocarbon composition at various temperatures .................... Average composition of gas from coalbeds ........................... Comparison of estimated and direct determination of methane content 

of coal .......................................................... Methane content of shallow. high-volatile bituminous coals ......... 
EXPERIMENTAL STUDIES ON THE ORIGIN AND ACCUMULATION 

OF COALBED GAS 

by 

Ann G. Kim 1 

ABSTRACT 

The Bureau of Mines itwestigated factors that influence the fonnation of 

methane and other hydrocarbon gases in coal, which is an integral part of the 

coalification process. Coalification begins with a biogenetic stage in which 

plant debris decomposes and peat is formed; methane and CO, are the predominant 

gases produced during this stage. During the diagenetic stage of coalification 

hydrogen and the hydrocarbon gases methane, ethane, ethylene, propane, propylene, butene, iso- and normal butane, and pentane are formed by dissociation of 

the organic material. Methane may also be produced by a secondary reaction of 

hydrogen with unsaturates or by the reduction of CO,. The gas contained in 

coalbeds is a mixture of methane, C, through C, alkanes, CO, , 0, , N, , G, and 

He; its heating value is equivalent to that of natural gas. The gas content 

of coal depends on many variables, but an empirical relationship between 

adsorptive capacity, rank, and depth has been observed. 

INTRODUCTION 

Underground mining of coal has always been hazardous, owing partially to 

the release of explosive methane gas. Methane and other gases, natural 

byproducts of the coalification process, are held within the coalbed under 

pressure. When coal is mined, this gas is released and can form an explosive 

mixture with air. The traditional method of dealing with methane emission 

during mining is ventilation, pumping sufficient air through the mine to 

dilute emitted methane to concentrations below the explosive limit. This is 

a costly procedure, and even in moderately gassy mines it is not uncommon for 

methane emission to exceed the ventilation capacity. It is possible that conventional ventilation methods will be inadequate in handling the high methane 

emission rates expected when mines are sunk in deeper, gassier coalbeds. 

Work by the Bureau of Mines on methane problems has also shm that conventional ventilation methods waste a valuable energy resource. Methane in 

coalbeds has a heating value of approximately 1,000 ~tu/scf. U.S . coal mines 

l~hemist, Pittsburgh Mining and Safety Research Center, Bureau of Mines, 

Pittsburgh, Pa. 
vent more than 200 MMcf of this high-Btu gas to the atmosphere daily (2) .2 

Methane cannot be recwered from the ventilation exhaust, where its concentration is less than 1 pct; but if it is drained from the coalbed in advance of 

mining, it can be used as a substitute natural gas (I.). 

The Bureau of Mines has conducted extensive research on methods of alleviating me thane hazards in mining and on conserving a valuable, readily available 

energy resource. Part of this program has dealt with the origin and accumulation of methane in coal. The present report is a summary of experimental 

studies on the gases formed during the biogenetic phase of coal formation, the 

effect of low temperatures on the evolution of gas from coal, the determination of the composition of coalbed gas, and the effect of depth and rank on 

coal's gas adsorption capacity. 

GAS FORMATION DURING BIOGENETIC CQALIFICATION 

The formation of coal is part of a cyclic process (fig. 1). Atmospheric 

COz is incorporated into plants which decay to form peat. Temperature and 

time convert the buried peat to coal, methane, and other gaseous byproducts. 

Eventually the gases are released to the atmosphere, the carbon in the coal is 

converted to CO, by oxidation due to burning or weathering, thus completing 

the cycle . 

The first important stage in the formation of coal is the biogenetic 

stage in which peat is fl formed. Most of the bitu- ~f mosphre minous coals currently mined 

in the United States origiPlants nated in Pennsylvanian age 

peat swamps between 275 and 

300 million years ago. 

These swamps were broad 

coastal plains at or very 

near sea level. High humid- 

%. ity and heavy rainfall 

encouraged rapid and extensive plant growth, resulting 

in large volumes of plant 

debris. The plants of the 

Pennsylvanian age peat 

swamps were large, pithy 

land plants adapted to semiaquatic or marshy conditions. 

Many plants grew to heights 

in excess of 100 feet and 

had diameters of over 4 feet. 

Coal 

CH4, CO2 'underlined numbers inparentheses refer to items in 

FIGURE 1. - Coal cycle. the list of references at 

the end of this report. 
CH2OH I CH20H 

I The poor drainage and the 

H q-o Y,~-O,~O- slow migration of fresh water 

contributed to an acidic, 

low-oxygen environment which 

AH CH~OH I A AH favored anaerobic microbia 1 

activity. The slow rate of 

CELLULOSE anaerobic decay, the lowered 

pH, and accumulation of 

toxic products inhibited 

decomposition and promoted 

the preservation of organic 

material and the accumulation of a carbonaceous 

sediment. 

Plants are composed of 

several classes of chemical 

m3 compounds , of which cellulose 

and lignin (fig. 2) are the 

most important from the 

---__ standpoint of coal formation. 

Cellulose is a linear polyOCH~ mer readily decomposed by microorganisms to C4 , CHSO I ethanol, and various acids. 

I A secondary population of I microorganisms produces 

LlGNlN methane from the acids and 

alcohol. Lignin is an aroFIGURE 2. - Structures of cellulose and lignin. matic polymer that anchors 

cellulose fibers and 

strengthens plant walls; 

little is known about its microbial decomposition. 

Since the microbial decomposition of cellulose and lignin initiates coal 

formation, gases formed during this process may be related to the gases in 

coal. To study gas formation during the biogenetic stage, sterile samples of 

wood, cellulose, and a lignin derivative were placed in an anaerobic environment at 34' C (1). Each sample was inoculated with soil microorganisms, and 

the gases produced were analyzed by gas chromatography. Methane and CO, were 

the predominant reaction products from all samples. Carbon dioxide was 

detected first; methane was detected several days later. Over a period of 

several months , the composition of the gas averaged 95 pct methane and 5 pct 

CO,. Hydrogen was detected in the gas produced initially from wood samples 

(fig. 3) but was not found after the production of methane began. The composition of the gas indicates that two mechanisms are involved in the production 

of methane from plant material--fermentation and reduction of CG . 
TI ME. days 

FIGURE 3. - Gas produced from wood. 

The total amount of gas produced and the rate of production (table 1) are 

consistent with the structure and biodegradability of the source material. 

Both the amount and the rate of production were greater for cellulose, which 

is easily decomposed. Less gas was produced from lignin, a more complex substance that is not readily degraded. Gas production from wood, which contains 

both cellulose and lignin, was intermediate--less gas was produced from wood 

than from the lignin derivative, but it was produced at a faster rate. 

TABLE 1. - Gas produced from plant material 

Higher-molecular-weight hydrocarbons produced from plant material averaged less than 1 pct of the total gas. The ratio of methane to total hydrocarbons averaged 0.999, which is typical of microbially produced gases. 

Source......................... . 

Total gas .................. ml.. 

Total gas ...... ml/g substrate.. 

Time.......................mo.. 

Rate. ............. lr6 mole/d.. 

COa. ......................p ct.. 

q,.......................p~t.. 

Other hydrocarbons ........ pct.. 

CYp /total hydrocarbons.. ....... 

To compare the composition of the gas produced in this experiment with 

that produced by microbial action in swamps, the composition of the gas in 

Wood 

10 

6 6 

8 

4.4 

5.01 

94.99 

0.0005 

0.999 

Cellulose 

15 0 

100 

3 

9.9 

5 .O1 

94.94 

0.0496 

0.999 

Lignin 

derivative 

15 

10 

20 

2.5 

5.00 

94.98 

0.0216 

0.999 
peat was determined. Samples from contemporary bogs in northeastern Pennsylvania were placed in a closed container, and the gases were desorbed by heating to 50° C, stirring the sample, and stripping the gas with helium. The 

hydrocarbons and CQ were collected on an activated carbon trap at -62O C. 

When the trap was heated, the freed gas was collected wer water and analyzed 

by gas chromatography (2). The average amount of gas desorbed from the peat 

was 0.12 cm3/g; the average composition was 96.75 pct CQ , 3.23 pct (2% , and 

0.02 pct other hydrocarbons (table 2). The average ratio of methane to total 

hydrocarbons was 0.987, a value consistent with microbial generation. 

TABLE 2. - Gas desorbed from peat 

The distribution of higher hydrocarbons in the total gas is similar for 

plant material and peat (table 3). However, on a C02-free basis the concentrat ion of higher-molecular-weight hydrocarbons is greater in the gas from 

peat (table 4). The gas fran plant materials and peat contains saturated and 

unsaturated Ca and Cg compounds. The gas fran peat contains an unsaturated C, 

hydrocarbon and two unidentified compounds which were not detected in the gas 

from plant materials. The straight-chain C, isomer predominates in the hydrocarbons from plant material and peat. Although n-pentane (straight chain) is 

the predominant Cg isomer derived from plant material, iso-pentane (branched) 

is the predominant C, isumer detected in the gas fran peat. All three (& isomers were detected in the gas from cellulose and peat. 

Sample number....................... 

Sample weight, as received... ....g.. 

Sample weight, dry. ..............g.. 

Totalgas, air free ............. ml.. 

Total gas ............ ml/g dry peat.. 

CQ ............................p ct.. 

CI&............................pct.. 

Other hydrocarbons .............p ct.. 

C~/total hydrocarbons.. ............ 

TABLE 3. - Composition of gas from plant material and peat, pct 

Source.. ............................ I Wood 1 Cellulose I Lignin I Peat 

1 

3 79 

5 7 

2.02 

0.035 

97.0 

2.98 

0.02 

0.992 

2 

222 

48 

21.37 

0.445 

99.9 

0.0097 

0.0003 

0.970 

Methane ((2%) ....................... Unidentified compound.. ............. Ethylene (C2Y,). .................... Ethane (GHg) ....................... Propylene and propane (6 H, , GI-& ) . . Unidentified compound. .............. Iso-butane (i-C4Hl,) ................ 1-Butene (1-C4%) ................... n-Butane (n-C4Hl,) .................. Neo-pentane (neo-C, H12 ) ............. Iso-pentane (I-C, HI, ) ............... n-Pentane (n-C,H12 ) ................. Carbon dioxide (CQ ) ................ 

3 

889 

19 1 

1.77 

0.009 

98.8 

1.18 

0.02 

0.987 

ND Not detected. 

94.99 

ND 

0.0003 

.0001 

.0007 

ND 

ND 

ND 

ND 

ND 

ND 

ND 

5 .O1 

4 

800 

7 2 

0.23 

0.003 

91.3 

8.59 

0.01 

0.999 

94.94 

ND 

0.0036 

.0062 

.0095 

ND 

.0003 

ND 

0085 

.0066 

. 0000 1 

.0149 

5.01 

Avera ge 

573 

9 2 

6.35 

0.123 

96.75 

3.23 

0.019 

0.987 

derivative 

94.98 

ND 

0.0008 

.OO 10 

.0019 

ND 

.0033 

ND 

.0076 

ND 

,0003 

.0064 

5.00 

3.23 

0.0088 

.0017 

.0026 

.0002 

0137 

.0056 

.0033 

.0140 

Trace 

.0006 

.00007 

96.72 
TABU 4. - Hydrocarbon distribution: Gas from plant material and peat, pct 

Source..................... 

CH, ........................ Unidentified compound ...... CaH4 ....................... C2H6 ....................... C3H6 ) &%. ................ Unidentified compound ...... i-C,Hl, .................... 1-C, ....................... 

n-C,H1, .................... Neo-CB H,, .................. i-C,Hl, .................... n-C& H, .................... ND Not detected. 

Lignin 

derivative 

99.98 

ND 

0.0008 

.,0010 

,0020 

ND 

.0035 

ND 

.0080 

ND 

.0003 

.0067 

Wood Peat 

98.44 

0.2700 

.0520 

.0800 

.0063 

.4200 

.I800 

.I000 

.4300 

Trace 

.0190 

.0023 

Ce 1 lulose 

A comparison of the air-free composition of gas from plant material, 

peat, and coal (table 5) shows an apparent concentration of CQ in peat. However, the peat was water saturated, and both sorbed gas and dissolved gas were 

analyzed. Only the gas phase was monitored in the plant material experiment. 

Carbon dioxide is approximately 20 times more soluble than methane in water, 

and the diffusion coefficient of methane is greater than that of Ch. Because 

of its lower solubility and greater diffusion coefficient, methane is more 

likely to migrate out of the water-saturated peat deposit. During coalification, CQ, formed during the peat stage could be removed with water or reduced 

to methane. 

TABLE 5. - Composition of gas from plant material, 

peat, and coal, pct 

Source.............. Other 

Coal. ............... 97.8 

HYDROCARBON GAS FORMATION DURING DIAGEWTIC COALIFICATION 

The diagenetic stage of coalification, which follows the biochemical 

stage, begins with inundation of the peat bog and deposition of inorganic sediments. Increased temperature and pressure and accumulation of toxic substances cause microbial activity to cease. Coalification comprises a complex 

series of chemical reactions that produce a sequence of coals: lignite, subbituminous, bituminous, and anthracite. During this process the fixed carbon 

content and heating value increase (fig. 4). Changes in chemical and physical 

properties reflect changes in the chemical structure of the coal l'molecule.ll 

During coalification there is an increase in the aromatic character of the 

coal and a loss of alkyl moieties. There is a transition from simple randomly 

arranged molecules to complex clusters of ring compounds oriented parallel to 

the bedding plane (fig. 5). There are several possible mechanisms by which 
5 I00 I 1 1 I I I 1 I 

- T1 

- Meta A--/ 4- A= anthracite 

X e0 - daf =dry, ash-free 7 

I 1 I 

I , Subbit c 1 Subbit h pf Hvbb 1 n -L iai 1- Y I 

W I -. z....- I 

x peat I Subbit a 

LL 40 I I I I I I I I I 1 

5 6 7 8 9 10 I I 12 13 14 15 16 

THOUSAND MOIST Btu 

FIGURE 4. - Variation in fixed carbon and heating value of coal with rank. 

High-volatile bituminous coal 

b35 pct volatile matter) 

Medium-volatile biiius coal 

(-22 pct volatile matter) 

Adhnci,te 

(-5 pct volat~le matter) 

FIGURE 5. - Arrangement of micelles within the coal structure (12). 

gases can be formed during this process. Methane, other hydrocarbons, and 

hydrogen can be produced by splitting off of alkyl side chains, by fragmentation of straight-chain compounds, or by condensation of straight chains to 

ring compounds. Methane can be formed by the reduction of CO, , the reduction 

of unsaturates, or the secondary decomposition of paraffins to lawer-molecularweight gases . 

Both time and temperature increase the rank of coal. Temperature determines the rate at which coalification proceeds, and the duration of heating 

influences the final rank of the coal (fig. 6). Very high temperature 

(>300° C) , associated with tectonic activity, can produce high-rank coals in 

relatively short periods (2, 12). At low temperatures (<50° C) the rank of 

coal tends to remain low for very long periods. The majority of bituminous 

coals forrned at temperatures below 200° C in periods of 50 to 100 million 

years. 

To determine if variations in temperature af fect the evolution of gas from 

coal, seven samples of coal--subbituminous and high-volatile bituminous--were 

crushed, placed in closed test tubes, and heated at 35O, 125O, and 150' C (5). 
~urasiic (g) I 1 1 I I 1 

300 .- - 

- 

Anthracite 

- 

- 

X 

Carboniferous (l2) 

- 

Subbit uminous and lignite - 

0 100 200 300 400 

DURATION OF H EATING, million years 

FIGURE 6. - Coal formation as a function of time and temperature. 

The resulting gases were analyzed by gas chromatography. The coals were from 

mines with little detectable gas emission and were placed under vacuum at 

ambient temperature for 20 hours prior to the experiment to minimize contributions from adsorbed gases. 

At all temperatures, total hydrocarbon emission per sample was related 

to the rank of the coal. Higher rank coals emitted less total hydrocarbons 

than lower rank coals (fig. 7). Total hydrocarbon emission, averaged over all 

samples, increased with temperature (fig. 8). Average hydrocarbon emission 

was 30 x l(r6 cm3/g at 35O C, 96 x l(r6 cm3/g at 125O C, and 172 x l(r6 cm3/g 

at 150' C. The average increase in total hydrocarbons per degree (table 6) 

over the entire temperature range was 1.2 x cm3/g O C, but the increase 

in hydrocarbon emission for each lo C rise in temperature between 125' and 

150° C was four times the increase between 35O and 125' C. 
KEY 

0 150.C 

x 125 'C 

X 

-Rank- 0 I 

30 40 50 60 70 80 90 

FIXED CARBON, pct (dafl 

FIGURE 7. - Total hydrocarbon emission FIGURE 8. - Total hydrocarbon emission 

from coal versus rank of from coal versus temperacoal. ture. Nos. 1-7 key to samples in table 6. 

TABLE 6. - Changes in hydrocarbon and methane emission 

with increased temperature, cm3/g " C 

S ample 

No. 

I..... 

2..... 

3..... 

4 ..... 

5.. ... 6..... 

7.. ... 

Methane 

Coa 1 

Rock Island........... 

Laramie No. 3......... 

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

Mamoth, Iowa.. ....... Uncorrelated bed--Iowa 

Upper Freeport........ 

Pittsburgh ............ 

Tota 1 hydrocarbons 

Average 

0.3 

.3 

.2 

.1 

.3 

.2 

.3 

35" to 

125" C 

0.2 

.2 

.1 

-.l 

.2 

.01 

.I 

125" to 

150" C 

0.6 

.7 

.5 

.9 

.5 

1 .o 

1.2 

Average 

1.4 

1.3 

1.1 

1.4 

.8 

.6 

1.6 

35" to 

125" C 

1.1 

.7 

.7 

.8 

-6 

.2 

1.0 

125" to 

150" C 

2.4 

3.6 

2 .5 

3.7 

1.7 

2.1 

3.6 
Methane emission also increased with temperature (fig. 9), but the percentage of methane in the emitted gas decreased at higher temperatures (fig. 10). 

At 35O C, the hydrocarbon gas contained more than 95 pct methane; at 125O C, 

methane content averaged 50 pct; and at 150° C , methane was about 30 pct of 

the hydrocarbon gas. At 125O and 150° C methane accounted for approximately 

30 pct of the total increase in hydrocarbon emission (table 6). In other 

words, the composition of the gas is not constant with temperature. At higher 

temperatures the gas contained a larger proportion of higher-molecular-weight 

hydrocarbons. 

The average composition of the hydrocarbon gas (table 7) shows that at 

35O C only methane and ethane were emitted from the coal. At 125' C, the gas 

contained ethylene , propane, propylene , is o-butane , normal butane , 1-butene , and pentane, in addition to methane and ethane. The composition of the gas at 

150° C was similar to that at 125O C, except that 1-butene was not detected; 

this may indicate a secondary reaction in which the unsaturated 1-butene is 

reduced to n-butane. The relative composition (fig. 11) shows that methane, 

iso-butane, and pentane predominate at higher temperatures. The concentration 

of iso-butane was 15 to 20 times that of n-butane, and only one of three 

pentane isomers was detected. The relative distribution of hydrocarbons does 

not correspond to their boiling points, molecular weights, or heats of formation. For the lower-molecular-weight compounds, there is a slight correlation 

to the heat of adsorption, but this does not account for the high concentrations of iso-butane and pentane or for the absence of two pentane isomers. 

TEMPERATURE, O C 

FIGURE 9. - Methane emission from coal versus temperature. Nos. 1-7 key to samples in 

table 6. 
KEY 

200 

25 50 75 100 125 150 

TEMPERATURE,' C 

FIGURE 10. - Percentage of methane in emit- FIGURE 11. - Relative composition of hydroted gas versus temperature. carbon gas. 

TABLE 7. - Hydrocarbon composition at various temperatures, 

l(r6 cm3/g 

From the experiment it can be concluded that "demethanation" of the coal 

precursor is too simple a mechanism to explain the formation of hydrocarbon 

gases during coalification. Higher-molecular-weight gaseous hydrocarbons are 

CH4 ........................... C2H4 .......................... CaH8.. ........................ C3&+C3Q ................... Unidentified compound. ........ i-C4H 

1-C qH,........................ 

n-C,H, ........................ C6H12 ......................... 

Total.......... .......... 

28.6 

.1 

ND 

ND 

ND 

ND 

ND 

ND 

ND 

28.7 

ND Not detected. 
also farmed during coalification, but simple fragmentation of a long-chain molecule 

does not account for the observed composition of hydrocarbon gas emitted from coal. 

COMPOSITION OF COALBED GAS 

Coalbed gat is frequently referred to as methane because methane is the major 

component (90 pct) . In many theories of coalification, methane was believed to be 

the only hydrocarbon gas formed. Ethane in coalbed gas was attributed to contamination from natural gas horizons below the coal (11). CO,, h, O,, H,, and He are 

also present in gas drained from coalbeds. Carbon dioxide can be formed by decarboxylation of the coal precursor or by oxidation during early stages of coalification. 

Nitrogen and oxygen in coalbed gas may be occluded during deposition of the organic 

sediment or may be introduced by percolating ground water. Helium in coalbed gas is 

a product of radioactive decay. 

In a previous study (4) gas chromatographic analyses of free-flowing gas from 

coalbeds demonstrated that coalbed gas contains me thane and higher-molecular-weight 

hydrocarbons (table 8). The alkanes , ethane, propane , butane, and pentane were the 

only higher-molecular-weight hydrocarbons detected in gas samples taken directly 

from coalbeds. The unsaturated hydrocarbons emitted when coal was heated were not 

found in coalbed gas samples obtained at ambient temperature. The amounts of butane 

and pentane detected were insufficient to determine the ratios of isomeric forms. 

Carbon monoxide, sulfur compounds, and nitrogen oxides were not detected in gas from 

coalbeds. 

TABLE 8. - Average composition of gas from coalbeds, pct 

Btu/scf.. . 1 1,030 

ND Not detected. 

Coalbed... 

CH, ....... GI&. ..... GI&,...... 

C4qo..*.. 

C,IE,, ..... cq, ....... 0, ........ & ........ & ........ He........ 

Pocahontas Upper IB seam4 

No. 3l 

94.9 

1 .3 

.004 

.002 

Trace 

.3 

.2 

3.2 

.02 

.04 

Lower 

.O1 

ND ' ND 

ND 

.1 

.1 

3.5 

Trace 

.3 

1,022 

I Anthracite3 

98.7 

.1 

Trace 

ND 

ND 

.1 

ND 

1.0 

ND 

Trace 

1,053 

IVirginia. 

a ~enns~lvania and West Virginia. 

~enns y lvania . 4~olorado. 

Oklahoma. 

'~labama . 

Since coal is formed in a reducing environment, the absence of sulfur dioxide 

and oxides of nitrogen is not unusual. However, hydrogen sulfide has not been 

detected in coalbed gas, even though coal contains sulfide sulfur, organically combined sulfur, and sulfates. Carbon monoxide is frequently detected in coal mine air, 

where it occurs as a product of low-temperature oxidation, but it is not found in 

gas drained from coal that has not been exposed to air. The heating value of gas 

drained from coalbeds is approximately 1,000 ~tu/scf, which is comparable with that 
of natural gas. Due to its high caloric value and absence of undesirable components (CO, sulfur compounds), coalbed gas is an excellent substitute for 

natural gas. 

VARIATION IN GAS CONTENT OF CQAL 

The amount of gas a coalbed contains is highly variable. Measured values 

of gas in coal range from 0.001 to 16.5 cm3/g (I., 8). Even coals of the same 

rank may exhibit a 10,000-fold difference in gas content, an indication that 

present gas content of a coalbed is related not only to gas formation during 

coalification but also to the postdepositional history ofthe coalbed. The gas 

content of a coalbed depends primarily upon rank, pressure, and temperature, 

and to some extent upon the permeability and porosity of the coal, the degree 

of fracturing, the distance to the outcrop, and the permeability of adjacent 

strata. 

Coal is a porous solid with two distinct pore systems. Gas can exist as 

free gas in cracks and fractures (macropores) or be adsorbed on the internal 

surface of the micropores. Most of the gas in coal is adsorbed, and an adsorption isotherm can be used to determine experimentally the adsorptive capacity 

of the coal as a function of pressure and temperature. Adsorption isotherms 

for coal can be described by the equation 

V = k, Pno - bT, 

where V is the volume adsorbed in cm3/g (maf) , k, is a constant determined at 

0' C in cm3/g atm, P is the pressure in atmospheres, n, is a constant determined at 0' C, b is a constant in cm3/g ' C, and T is the temperature in ' C 

(6). The constants k, and no are functions of the rank of the coal and can be 

related to the ratio of fixed carbon to volatile matter (fig. 12). Since both 

- 

- - 

- - - 

- - L I I I I I I I I I I I 1 L I I I I I , .38 

0 2 4 6 8 10 12 14 16 18 20 

FIXED CARBON /VOLATILE MATTER 

FIGURE 12. - Constants ko and no as a function of fixed carbon-volatile matter ratio. 
temperature and pressure are 

functions of depth, the equation can be rewritten to 

give adsorptive capacity 

directly as a function of 

the depth of the coal: 

where h is the depth in 

meters . 

The moisture content of 

a coal must also be considered in estimating its 

adsorptive capacity. Moisture in coal reduces its 

capacity to adsorb methane ; 

the maximum reduction is 

related to the oxygen content of the coal. However, 

the experimentally determined ratio of gas adsorbed 

on wet and dry coal (V, /vd ) 

is 0.75 with a variation of 

approximately 10 pct. For 

practical purposes , the 

amount of gas adsorbed on 

moist coal can be estimated 

as 25 pct less than that 

adsorbed on dry coal. 

FIGURE 13. - Adsorptive capacity of coal as a function The preceding calculaof rank and depth. tions estimated adsorbed gas 

per gram of dry, ash-free 

coal. To estimate the gas content of coal in place, it is necessary to correct for its moisture and mineral matter content. Incorporating corrections 

for moisture and mineral matter (MM) content and for the reduction of gas content in moist coal, the equation for adsorbed gas in place is written: 

If it is assumed that moisture and mineral matter content is approximately 10 pct , the adsorbed gas content of various ranks of coal can be calculated and graphed as a function of depth (fig. 13). A reasonable estimate of 

the gas content can be obtained for most coals from the graph. Comparison of 

estimated values with values obtained by direct determination &O) (table 9) 

shows that most estimated values are within 30 pct of the measured value, 

which is the accuracy of the direct method. 
TABLE 9. - Comparison of estimated and direct determination 

of methane content of coal 

Difference 

between direct 

de terminat ion 

and estimated 

range, cm3/g 

+1 

0 

0 

+3 

0 

Coa lbed 

Estimated 

range 

cm3/g 

Direct 

determination 

0 

0 

+1 

0 

0 

+2 

0 

0 

0 

-3 

0 

0 

0 

-3 

- 1 

0 

0 

- 2 

0 

+1 

-1 

0 

0 

+1 

-3 

-3 

-4 

-5 

-3 

-5 

0 

-4 

-5 

LOW-VOLAT ILE BITUMINOUS 

~t~ /ton 

416-576 

416-576 

416-576 

448-608 

448-608 

cm3 /g 

Depth 

Meters ~t~ /ton 

Tunnel No. lg....... 

Peach Mountain No. 18 

Beckley ............. 

Hartshorne.. . . . . . . . . 

New Castle.. . . . . . . . . Pocahontas No. 3. . . . 

Pratt ............... 

Feet 

ANTHRACITE 

183 

183 

183 

213 

2 13 

302 

267 

253 

226 

451 

395 

174 

169 

148 

77 

651 

643 

621 

529 

494 

484 

466 

232 

416 

13 

14 

15 

14 

16 

18 

12 

13 

11 

5 

17 

14 

17 

11 

12 

16 

15 

9 

15 

13-18 

13-18 

13-18 

14-19 

14-19 

600 

600 

600 

699 

699 

12-15 

12-14 

12-14 

11-14 

13-16 

13-16 

11-13 

11-13 

10-12 

8-10 

15-17 

14-17 

14-17 

14-17 

13-16 

14-16 

14-16 

11-14 

14-16 

991 

876 

830 

742 

1,480 

1,295 

571 

553 

488 

252 

2,137 

2,110 

2,038 

1,736 

1,621 

1,588 

1,529 

761 

1,365 

416 

448 

480 

448 

512 

576 

384 

416 

352 

160 

544 

448 

544 

352 

384 

512 

480 

288 

480 

MEDIUM-VOLATILE BITUMINOUS 

384-480 

384-448 

384-448 

352-488 

416-512 

416-512 

352-416 

352-416 

320-384 

256-320 

480-544 

448-544 

448-544 

448-544 

416-512 

448-512 

448-512 

352-448 

448-512 

13-15 

13-14 

13-14 

13-14 

12-13 

Mary Lee.. . . . . . . . . . . 

19 

14 

13 

22 

19 

416-480 

416-448 

416-448 

416-448 

384-416 

608 

448 

416 

704 

608 

320-352 

288-352 

288-320 

256-288 

160-192 

288-352 

288-320 

288-320 

256-288 

HIGH-VOLATILE 

666 

520 

519 

518 

335 

BITUMINOUS 

850 

771 

676 

427 

312 

747 

679 

675 

402 

2,185 

1,706 

1,703 

1,700 

1,099 

16 

12 

14 

13 

14 

Pittsburgh .......... 

Redstone............ 

Sewell. ............. Sewickley ........... Waynesburg .......... 

512 

384 

448 

416 

448 

10-11 

9-11 

9-10 

8- 9 

5- 6 

9-11 

9-10 

9-10 

8- 9 

259 

235 

2 06 

130 

9 5 

225 

207 

205 

122 

7 

6 

5 

3 

2 

4 

9 

5 

3 

224 

192 

160 

96 

64 

128 

288 

160 

96 
For some coals, gas contents estimated £ran adsorption data may be substantially higher than those determined by the direct method; usually such 

coals are low in rank and relatively shallow. For example, in table 9, direct 

determinations for the Pittsburgh, Sewickley , Reds tone, and Waynesburg coals 

are 30 to 65 pct lower than the range predicted from the graph, but the pressure gradient in the area from which these samples were obtained (Pennsylvania 

and West Virginia) is lower than average. Drill-stem tests have shown that 

pressure in the Pittsburgh coal in this area is approximately one-half the 

hydrostatic head. Gas contents calculated using the pressure gradient 

0.046 atm/m are in better agreement with the direct-determination values 

(table 10) 

TABLE 10. - Methane content of shallow, high-volatile bituminous coals 

The direct determinations in table 10 can be described by a linear equation (V = 0.025h - 0.5; correlation coefficient = 0.90), but whether this 

relationship is generally accurate for shallow, low-rank coals cannot be determined from the limited data available. The relationship between gas content 

and depth for shallow, high-volatile coals in the southwestern Pennsylvanianorthern West Virginia area illustrates the point that local geology, anomalous pressure-depth relationships, and characteristics of individual coals 

affect the gas content. 

Lack of data precluded extending the estimate to lower rank coals. Of 

22 coal samples studied, only 4 were not bituminous. The adsorption isotherm 

for anthracite fitted the pattern observed for other coals, but the results 

for subbituminous coals were ambiguous. Such coals are knuwn to be high in 

moisture, ad theore tically their gas-adsorption capacity is minimal. However, 

subbituminous coals and lignites have been knm to liberate substantial quantities of gas. Because of limited data, gas content estimates are considered 

most accurate for bituminous coals. 

Coal 

Pittsburgh ... Waynesburg. . . Pittsburgh.. . Sewickley . . . . Pittsburgh.. . Reds t one. . . . . Pittsburgh.. . Do......... 

SUMMARY 

Coalif ication is a canplex process, and the attendant formation of gaseous byproducts cannot be explained by a single, simple mechanism. During the 

biogenetic stage of coal formation, methane is the predominant gas produced, 

accompanied by CQ and small amounts of higher hydrocarbons. The gas retained 

in peat is predominantly CC&, with some methane and C, through C, hydrocarbons. 

The concentrations of higher hydrocarbons are approximately equal in the gas 

generated from plant material and the gas desorbed from peat. 

Difference, 

cm3 /g 

-2 

- 1 

- 1 

0 

0 

+1 

- 1 

+2 

Calculated 

cm3 /p; 

4 

4 

4 

5 

5 

5 

5 

5 

Depth 

~t~ /ton ' 

128 

128 

128 

160 

160 

160 

160 

160 

Meters 

95 

122 

130 

205 

206 

225 

235 

259 

Direct determination 

Feet 

312 

400 

427 

673 

676 

738 

771 

850 

cm3 /g 

2 

3 

3 

5 

5 

4 

6 

7 

~t~ /ton 

64 

9 6 

9 6 

160 

160 

128 

192 

2 24 
During the diagenetic stage of coalification, methane and other hydrocarbons are produced. In experiments on the effect of temperature on gas 

evolution from coal, methane, iso-butane , and pentane were the predominant 

hydrocarbons detected. As the temperature rose, the amount of hydrocarbons 

emitted increased, but the percentage of methane decreased. Therefore, the 

mechanism of gas formation during coalification cannot be limited to a simple 

demethanation reaction but may involve several reactions such as the fragmentation of long-chain molecules, the splitting of alkyl side chains, and the 

reduction of unsaturates and COz . 

Free-flawing gas from coalbeds is usually more than 90 pct methane with 

other hydrocarbons, CO, , N2 , % , H, , and He. To date, carbon monoxide, sulfur 

compounds, and oxides of nitrogen have not been detected in gas from coalbeds. 

Gas drained directly from coalbeds has an average heating value of approximately 1,000 Btulscf. The gas content of coalbeds is highly variable, but an 

empirical relationship has been observed between adsorptive capacity, rank, 

and depth. 
REFERENCES 

Deul, Maurice, and A. G. Kim. Safety and Economic Implications of the 

Degasification of Coalbeds. Proc. 1st Symp. on Underground Mining, 

NCA/BCR Coal Conf . and Expo 11, Louisville, Ky . , Oct . 21-23, 1975, 

v. 1, 1975, pp. 1-8. 

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

London, 2d ed. , 1961, p. 573. 

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 

Inf ormation Circulars 8558 and 8659. BuMines IC 8733, 1977, 55 pp. 

Kim, A. G. The Composition of Coalbed Gas. BuMines RI 7762, 1973, 9 pp. 

. Low-Temperature Evolution of Hydrocarbon Gases From Coal. 

BuMines RI 7965 , 1974, 23 pp . . Estimating Methane Content of Bituminous Coalbeds From Adsorption 

Data. BuMines RI 8245, 1977, 22 pp. 

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

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

. Gases Desorbed From Five Coals of Low Gas Content. BuMines 

RI 7768, 1973, 9 pp. 

. Gas Chromatographic Method for Analyzing Gases Associated With 

Coal. BuMines RI 7903, 1974, 9 pp. 

McCulloch, C. M., J. R. Levine, F. N. Kissell, and M. Deul. Measuring 

the Methane Content of Bituminous Coalbeds. BuMines RI 8043, 1975, 

22 PP* 

Seldon, R. F. The Occurrence of Gases in Coa 1s . BuMines RI 3233, 1934, 

64 PP* 

Teichmuller , M. , and R. Teichmuller. Geological Aspects of Coal Metamorphism. Ch. in Coal and Coal-Bearing Strata, ed. by D. Murchison and 

T. S . Westoll. E lsevier Publishing Co. , New York, 1968, pp. 254-260. 

*U.S. GOVERNMENT PRINTING OFFICE: 1978-603-00212