From Adsorption Data 

UNITED STATES DEPARTMENT OF THE INTERIOR 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Cecil D. Andrus, Secretary 

BUREAU OF MINES 
This publication has been cataloged as follows: 

Kim, Ann G 

Estimating methane content of bituminous coalbeds from 

adsorption data / by Ann G. Kim. [Washington] : U.S. Dept. 

of the Interior, Bureau of Mines, 1977. 

22 p. : diagrams ; 26 cm. (Report of investigations - Bureau of 

Mines ; 8245) 

Bibliography : p. 13. 

1. Coal mines and mining. 2. Methane. I. United States. Bureau of Mines. 11. Title. 111. Series: United States. Bureau of 

Mines. Report of investigations - Bureau of Mines ; 8245. 

TN23.U7 no. 8245 622.06173 

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

Page 

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

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

Acknowledgments ........................................................ 2 

Methane-adsorption equation ............................................. 2 

Estimating the methane content of coal ................................... 7 

Discussion ............................................................... 11 

Conclusion ......................................................... 12 

Bibliography ............................................................. 13 

Appendix A.--Estimating methane content from adsorption isotherms ........ 14 

Appendix B.--Estimating the effect of moisture on adsorptive capacity 

of coal ................................................................ 21 

ILLUSTRATIONS 

1 . Value of adsorption constants k, and no versus the ratio of fixed 

carbon to volatile matter ........................................ 3 

2 . Variation of adsorption isotherms with rank at 0' C ................ 5 

3 . Variation in adsorptive capacity with temperature and rank 

at 10 atm ........................................................ 5 

4 . Relationship of pressure to depth .................................. 6 

5 . Estimated methane content with depth and rank ...................... 8 

A-1 . Adsorption isotherm ................................................ 14 

A-2 . Log-log plot of volume adsorbed versus pressure .................... 18 

A-3 . Volume adsorbed versus temperature at constant pressures ........... 18 

A-4 . Relationship of k, to fixed carbon ................................. 19 

A-5 . Relationship of k, to the ratio of fixed carbon to volatile matter . 20 

TABLE S 

1 . Adsorption constants for selected coals ............................ 4 

2 . comparison of estimated and direct determination of methane 

content of coal .................................................. 9 

3 . Methane content of shallow. high-volatile bituminous coals ......... 10 

A.1 . Analyses of coal samples ........................................ 16 

A.2 . Adsorption constants for methane on coal ........................... 17 

B.1 . Critical moisture and reduction in adsorptive capacity ............. 22 
ESTIMATING METHANE CONTENT OF BITUMINOUS COALBEDS 

FROM ADSORPTION DATA 

by 

Ann GI Kim' 

ABSTRACT 

The Bureau of Mines estimated the methane content of a coal, which 

depends primarily upon rank and pressure, from the adsorption equation V = kpn, 

where k and n are constants related to rank. By incorporating corrections for 

moisture, ash, and temperature, and estimating pressure and temperature as a 

functionofdepth, the methane content of coal in place can be estimated from 

the following equation: 

Values calculated with this equation generally are in reasonable agreement 

with direct determinations. By assuming a standard moisture and ash content, 

and using the hydrostatic head to estimate pressure, a graph of rank and depth 

versus gas content was constructed. Although estimated values were consistently high for several high-volatile bituminous coals from an area where the 

pressure is known to be less than hydrostatic, the estimated methane content 

for most coals shows reasonable agreement with values determined by the direct 

method. 

INTRODUCTION 

Methane is always present in coal (3-4)2 - - and constitutes a serious safety 

hazard in coal mining. It occurs admixed with other hydrocarbons, C02, N,, 

02, Ha, and He. It is a normal byproduct of the coal-forming process. 

Although much of the gas formed during coalification migrates away from the 

coal, a significant portion is retained in the coal and adjacent rocks. Some 

free gas is present in cracks and fractures, but most is adsorbed on the 

internal surface of micropores within the coal. The amount of gas that the 

coal contains depends primarily upon pressure, temperature, adsorptive capacity, and moisture content of the coal. Permeability, porosity, degree of 

lchemist, Pittsburgh Mining and Safety Research Center, Bureau of Mines, 

Pittsburgh, Pa. 2 Underlined numbers in parentheses refer to items in the list of references 

preceding the appendixes. 
fracturing of the coal and adjacent rocks, and distance from the outcrop may 

also influence the gas content of a coalbed. 

When coal is mined, 60 to 80 pct of the gas that it contains is emitted 

into the mine atmosphere, where gas accumulation creates an explosion hazard. 

The conventional method of dealing with this hazard is through ventilation, 

which dilutes the emitted methane with air to nonexplosive concentrations and 

carries it to the surface. The deeper, higher rank coalbeds that will be 

mined in the foreseeable future will contain more gas than the beds currently 

being worked, and it is possible that ventilation will not be adequate to control methane emission. It may be necessary to drain methane from the coal in 

advance of mining. If the drained gas is to be sold as a fuel, it will be 

necessary to estimate the amount of gas that can be recovered from the coal. 

The methane content of a given coal can be estimated by measuring the 

amount of gas emitted by a core sample recovered during exploratory drilling 

(2-6). This direct method is simple and is considered accurate to 130 pct. 

However, it does require drilling a hole, and the data thus obtained apply 

only to a particular coal at a particular depth. 

A more general estimate can be made using adsorption-isotherm data. An 

adsorption isotherm describes the variation in gas adsorption as a function of 

pressure at a constant temperature. If isotherms are run at several temperatures, the change in adsorptive capacity with temperature can be determined. 

Isotherms for dry and moist coals are run to determine the effect of moisture 

on the volume of adsorbed gas. The information from various isotherms can be 

combined into a general equation for adsorbed-gas volume as a function of temperature, pressure, and moisture content. By applying the adsorption equation 

to different coals, the effect of rank can be determined, and a graph of the 

gas content of high-rank coal can be constructed as a function of rank and 

depth. 

ACKNOWLEDGMENTS 

The adsorption isotherms upon which this report is based were obtained by 

the Energy Conversion Section, Pittsburgh Energy Research Center of the Energy 

Research and Development Administration (ERDA), under contract to the Pittsburgh Mining and Safety Research Center of the Bureau of Mines. Detailed 

reports of these studies have been published elsewhere (1-2, -- -- 8-9). 

METHANE -ADSORPTION EQUATION 

Most of the gas in coal is adsorbed on the internal surface of micropores. 

The amount of gas that a coal can adsorb varies directly with pressure and 

inversely with temperature. The relationship between the volume of gas 

adsorbed by the coal, and pressure and temperature can be described by the 

equation 
where V = volume of gas adsorbed, in cubic centimeters per gram of moistureand ash-free coal; 

P = pressure, in atmospheres; 

T = temperature, in degrees Centigrade; 

k, = a constant, in cubic centimeters per gram per atmosphere; 

no = a constant; 

and b = a constant, in cubic centimeters per gram per degree Centigrade. 1 

Constants k, , no, and b were determined for 18 coals and are listed in table 1.3 

The temperature constant b averages 0.14 cm3/g0 C for coals studied, from 

25 i I I I I I I I r 1 I I 1 I I I I 0.08 I - 

- - 

- - - 

20 - 

- 

- - 

E - - 

C - - 0 - : 15- - .20 

E" - - 

e - - (7, - - 0 

\ - C rn - E - 0 10- - .26 - 0 - - 

- 

- 

- 

5- - 

- .32 - - 

- - 

- - 

- - 

I I 1 I 1 I I I --- I I I I I I I I I 

- -- - -- - - .38 

0 2 4 6 8 10 12 14 16 18 20 

FIXED CARBON /VOLATILE MATTER 

FIGURE 1. - Value of adsorption constants k, and n, versus the ratio of fixed carbon to volatile matter. 

3~erivation of the equation and calculation of constants are discussed in 

appendix A. 
subbituminous to anthracite. The values of k, and no depend on the rank of 

the coal, and can be expressed in terms of the ratio of fixed carbon (FC) to 

volatile ~llatter (VM) : 

and no = 0.315 - 0.01 Fc/vM 

The relationship between the constants k,, no, and the ratio FC/VM is shown in 

figure 1. 

TABLE 1. - Adsorption constants for selected coals 

Coalbed 

Mammoth .................................... Pocahontas No. 3........................... 

Lower Kittanning ........................... Beckley .................................... Mary Lee ................................... Upper Freeport ............................. Lower Freeport ............................. Pittsburgh ................................. Sewell ..................................... No. 5 Block ................................ Somerset B................................. 

Castlegate ................................. Rosebud .................................... Elkhorn No. 3.............................. 

Hazard No. 4............................... 

Pocahontas No. 4........................... 

Stearns No. Z.............................. 

Hazard No. 7............................... 

ND--Not determined. 

Adsorption curves indicate tk~a t the amount of gas adsorbed by coal 

increases with increasing rank at a given temperature and pressure (figs. 2-3), 

and this has been verified bv data on tt~c? gas content of the Mary Lee group cif: 

coalbeds (7). At a given temperature and pressure, higher rank coals (,onfain 

larger volumes of methane than lower rank coals. The volume of adsorbed 

methane increases with increasing pressure, but decreases with increasing 

temperature. 

Pressure and temperature are functions of depth. At a given depth, the 

pressure P is usually assumed to equal the hydrostatic head given by the 

equation-- 
0 10 20 30 40 50 60 

PRESSURE, atm 

FIGURE 2. - Variation of adsorption isotherms with rank at 

o0 C* 

20- - 

- 

0 

> IO- - 

P= I0 atm 1 

1 I I I I 1 I 

0 10 20 30 40 50 60 

TEMPERATURE, O C 

FIGURE 3. - Variation in adsorptive capacity with temperature and rank at 10 atm. 
where P is expressed in 

atmospheres, and h is the 

depth, in meters. However, 

drill-stem tests have frequently shown that pressure 

within a coalbed is substantially less than hydrostatic (fig. 4). For these 

data (fig. 4), the average 

pressure gradient is 0.063 

atm/m with a standard deviation of '0.027 atm/m. 

Since it is primarily 

dependent on pressure, the 

estimated adsorbed gas volume will be only as accurate 

as the pressure value. If 

the pressure at depth or 

pressure gradient for an 

area is known, this value 

should bc used to estimate 

the adsorbed gas content of 

200 300 400 500 a coal. Jf: hydrostatic head 

DEPTH (h),rneter s i.s used, i.t should be realized that this represents a 

FIGURE 4. - Relationship of pressure to depth. limiting case, and actual 

gas content may be lower 

than the estimated value. 

Temperature at a given depth h is estimated as the geothermal gradient multiplied by h and added to the ground temperature. Although the geothermal 

gradient varies in different areas, a commonly used value is 1.8" C/100 m. 

Ground temperature--that temperature not affected by surface heating and 

cooling--is also variable. In this study, 11" C is used. Temperature at 

depth is estimated by the following relationship: 

At the relatively shallow depths considered in this report, it is unlikely 

that an error in estimating T will substantially affect the estimate of gas 

content. 

To estimate the methane content of coal in place, its moisture content 

must also be considered. Moisture in coal reduces its capacity to adsorb 

methane. According to one study (I), below a critical value, the reduction in 

adsorption capacity of a coal is directly related to moisture content .q Above 

chis critical value, additional moisture causes no further reduction in gas 

adsorption. The ratio of volumes of gas adsorbed on wet and dry coal (V,/vd) 

4~he reduction i.n adsorption capacity related to moisture content is discussed 

in appendix B. 
is given in table 1. For most high-rank coals, the minimum volume of methane 

adsorbed on wet coal is between 55 and 85 pct of the volume adsorbed on dry 

coal. In general, the reduction in gas-adsorption capacity is greater for 

lower rank coals. For coals for which experimental data is not available, 

moisture in coal can reasonably be assumed to reduce the volume of adsorbed 

methane by 25 pct. In a general gas-adsorption equation, 0.75 is used as the 

ratio of adsorbed gas in wet and dry coal (v,/v,). 

In the preceding equations, adsorbed gas was estimated per gram of 

moisture- and ash-free (maf) coal. To estimate the gas content of coal in 

place, the calculated gas content is multiplied by the factor (100 minus the 

percentage of moisture minus the percentage of ash)/100. Incorporating the 

factors discussed above, a general equation for estimating adsorbed gas content of coal in place can be written as follows: 

(100 - % moisture - % ash) V, 3 V,cm /g = 100 - [k pno - b (1.8h/100 + ll)]. (lj 

Vd 

If P is estimated from the hydrostatic head, the equation is 

3 (100 -% moisture -% ash) V, 

V,cm /g = 100 - [k, (0.096h)"o -b(1.8h/100 + ll)], (2) Vd 

where h is the depth, in meters. 

ESTIMATING THE METHANE CONTENT OF COAL 

Depending on the amount of information available, the gas content of a 

coal can be estimated in several ways. If the coal is listed in table 1 and 

if the pressure at depth is known, equation 1 can be used directly. For 

example, during drilling in the Castlegate coalbed in Utah, measured pressure 

was 24 atm at a depth of 310 m. From table 1, the constants for this coal are 

k, = 4.9, % = 0.37, b = 0.12, and V, /v, = 0.42. Moisture and ash content 

totaled 8 pct. Using these values in equation 1, the estimated methane content of the Castlegate coal is 5.3 cm3/g; the direct determination for this 

coal was 4.7 cm31g. 

For coals that are not listed in table 1, adsorption constants can be 

estimated from proximate analysis. For a sanple of the Lower Hartshorne coal 

from Oklahoma, the fixed carbon content was 80 pct maf, and volatile matter, 

20 pct maf; moisture and ash content equaled 13 pct. Calculated values for 

the adsorption constants are k, = 8.7 and n, = 0.28. The average value of the 

temperature constant is 0.14 cm3/g0 C, and V, /v, calculated according to 

equations in appendix B is 0.82. Measured pressure in the coalbed was 17 atn~ 

at a depth of 450 m. Using these data to calculate the methane content, a 

value of 11.8 cm3/g is obtained, compared with 11.1 cm3/g obtained by direct 

determination. 

When pressure is estimated as a function of depth, the accuracy of the 

gas-content estimate depends on how closely the pressure gradient approaches 
tent estimated with equation 2 is 9.5 cm3 /g, but the 

2 o direct determination is 3.9 

cm3lg. The measured pres25 ~lrlllllllll~llllllllllllllllllllllllllllllllllll,,llllllll,llllll,ll the normal value of 0.096 

/ sure gradient in this area 

- 

1 

I - - - 

of the Lower Freeport coal is 

0.046 atm/m, approximately 

1 one-half of the normal value. - The anomalously low pressure 

could account for the low 

gas content. 

If it is assumed that 

the sum of the moisture and 

ash content is 10 pct, an 

2s timated gas -content range 

can be calculated for various ranks of coal and 

depicted graphically 

(fig. 5) . Although the 

range of gas content at a 

given depth becomes larger 

- for higher rank coals, a 

reasonable estimate of the 

gas content of most coals 

can be obtained from the 

0 100 

~~~"""~~~"""~~~'""'~~~""1~~~""'~~0 graph. When the estimated 

DEPTH ( h), m gas content is compared with 

atm/m. For example, for the 

Upper Freeport coal at a 

depth of 275 m, the gas conFIGURE 5; - Estimated methane content with depth and the direct determination 

(table 2), the agreement is rank, reasonably good. Direct 

determination is considered accurate to t30 pct, and most of the values estimated from the adsorption graph are within '30 pct of the direct 

determinations. 
TABLE 2. - Comparison of estimated and direct determination 

of methane content of coal 

Coalbed 

Difference 

between direct 

,determination 

and estimated 

range, cm3/g 

Estimated 

range 

ANTHRAC ITE 

cm3/g 

Depth 

Meters ~t~/ton 

Direct 

determination 

Feet cm3/g 

Tunnel No. 19. .. . . . . .. 

Peach Mountain No. 18. 

~t~/ton 

600 

600 

600 

699 

699 

608 

448 

416 

704 

608 

183 

183 

183 

213 

213 

19 

14 

13 

22 

19 , 

13-18 

13-18 

13-18 

14-19 

14-19 

416-576 

416-576 

416-576 

448-608 

448-608 

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

Hartshorne ............ 

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

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

i-1 

0 

0 

+3 

0 

IQW 

302 

267 

253 

226 

451 

395 

174 

169 

148 

77 

651 

643 

621 

529 

494 

484 

466 

232 

416 

-VOLATILE 

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 

MEDIUM-VOLATILE 

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

13 

14 

15 

14 

16 

18 

12 

13 

11 

5 

17 

14 

17 

11 

12 

16 

15 

9 

15 

BITUMINOUS 

416 

448 

480 

448 

512 

576 

384 

416 

352 

160 

544 

448 

544 

352 

384 

512 

480 

288 

480 

BITUMINOUS 

666 

520 

519 

518 

335 

384-480 

384-448 

384-448 

352-488 

13-16416-512 

13-16416-512 

352-416 

352-416 

320-384 

256-320 

480-544 

488-544 

448-544 

488-544 

13-16416-512 

448-512 

14-16448-512 

352-488 

448-512 

12-15 

12-14 

12-14 

11-14 

11-13 

11-13 

10-12 

8-10 

15-17 

14-17 

14-17 

14-17 

14-16 

11-14 

14-16 

HIGH-VOLATILE BITUMINOUS 

0 

0 

+1 

0 

0 

+2 

0 

0 

0 

-3 

0 

0 

0 

-3 

-1 

0 

0 

-2 

0 

2,185 

1,706 

1,703 

1,700 

1,099 

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

Redstone.. . . . . . . . . . . . . Sewell .......,........ Sewickley.. . . . . . . . . . . . Waynesburg ............ 

16 

12 

14 

13 

14 

259 

235 

206 

130 

95 

225 

207 

205 

122 

512 

384 

448 

416 

448 

850 

771 

676 

427 

312 

747 

679 

675 

402 

224 

192 

160 

96 

64 

128 

288 

160 

96 

13-15 

13-14 

13-14 

12-13 

7 

6 

5 

3 

2 

4 

9 

5 

3 

710-11 

9-11 

9-10 

8- 9 

5- 6 

9-11 

9-10 

9-10 

8- 9 

320-352 

288 -352 

288-320 

256-288 

160-192 

288-352 

288-320 

288-320 

256-288 

-3 

-3 

-4 

- 5 

-3 

-5 

0 

-4 

-5 

416-480 

416-448 

13-14416-448 

416-448 

384-416 

+1 

-1 

0 

0 

+1 
For some coals, gas contents estimated from adsorption data may be substantially higher than direct determinations; usually such coals are low in 

rank and relatively shallow. For example, in table 2, direct determinations 

for the Pittsburgh, Sewickley, Redstone, 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. Using the pressure gradient 0.046 atm/m and the values of the constants 

listed for the Pittsburgh coal in table 1, gas contents were calculated for 

the coals in table 3 with the following equation: 

Values calculated with this equation are in better agreement with the directdetermination values (table 3). 

TABLE 3. - Methane content of shallow, high-volatile 

bituminous coals 

The direct determinations in table 3 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 

Pennsylvania-northern West Virginia area illustrates the point that local 

geology, anomalous pressure-depth relationships, and characteristics of individual coals must be considered in evaluating the accuracy of the estimate of 

a coal's 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 known to be high in moisture, and theoretically their gas -adsorption capacity is minimal. However, 

subbituminous coals and lignites have been known to liberate substantial 

quantities of gas. Rather than base the gas content estimates on insufficient 

data, estimates were limited to the higher rank coals. In view of the number 

of variables and assumptions, the gas content estimated from adsorption data 

Coal 

Pittsburgh.. . . . Waynesburg ..... Pittsburgh.. . . . Sewickley.. . . . . Pittsburgh. . . . . Redstone ....... Pittsburgh.. . . . 

Difference, 

cm3 /g 

-2 

-1 

-1 

0 

0 

+1 

-1 

+2 

Depth Direct determination 

Meters 

95 

122 

130 

205 

206 

225 

235 

259 

~n?/g 

2 

3 

3 

5 

5 

4 

6 

7 

Calculated 

Feet 

312 

400 

427 

673 

676 

738 

771 

850 

Fts/ton 

64 

96 

96 

160 

160 

128 

192 

224 

CI$ /g 

4 

4 

4 

5 

5 

5 

5 

5 

Ft3 /ton 

128 

128 

128 

160 

160 

160 

160 

160 
provides a general standard for determining the gassiness of most bituminous 

coals. 

DISCUSS ION 

The estimated methane content of a coal can be used in mine planning and 

resource evaluation. In mining, the amount and rate of methane released 

depend on factors such as rib extent, age of the mine, production rate, gob 

area, and permeability of the coal. However, a simple measure of the gassiness of a mine is the methane content of the coal. When planning a mine, an 

estimate of the coal's gas content can be used to estimate methane production 

rates and ventilation requirements. The following example is based on data 

for the Lower Kittanning coalbed in Pennsylvania. All other factors being 

equal, a mine in which the methane content of a medium-volatile bituminous 

coal is 6.0 cm3Ig at 50 m (163 ft) could produce 1.5 MMft3/day of methane; 

in a mine at 150 m (490 ft) with a methane content of 9.5 cm3/g, the methane 

production rate would be 2.4 MMft3/day. At 250 m (810 ft) , the methane production rate could be 3.2 tIP4ft3/day from coal with a methane content of 11.2 

cm3lg. Ventilation capacity for the deepest mine would have to be at least 

double that of the shallowest mine. 

The estimated gas content can also be used to evaluate potential gas 

resources in coal. For example, assume that a high-volatile bituminous coal 

is known to underlie a 10- by 20-mile area; the average depth to the coal is 

150 m (approximately 500 ft) and average coal thickness is 15 in. The 290 

million tons of coal in this area is inaccessible to strip mining and too thin 

to be economically deep mined. The methane content of this coal (estimated 

from fig. 5) is 8.1 cm3/g, or 260 ft3/ton. Therefore, this relatively small, 

uneconomic coalbed contains 75 billion ft3 of methane. Draining 50 pct of 

this would provide enough gas to heat 30,000 homes for 10 years. Of 727 billion tons of bituminous coal in the coterminous United States, only 60 pct is 

considered minable by current methods .= To get the most of limited energy 

resources, the 290 billion tons of "uneconomic" coals could be considered as 

gas reservoirs, potentially containing over 70 trillion ft3 of high-Btu gas. 

The relationship between rank, depth, and gas content developed in this 

report can be extended to deeply buried coals. An estimated 378 billion tons 

of coal lie between 1,000 and 2,000 m below the surface of the coterminous 

United states." Using 15 cm3 lg (480 ft3/ton) as a minimum gas content, these 

coals could contain 181 trillion cubic feet of gas. Using the data in this 

report as a basis for speculating on the gas content of thin coalbeds and 

deeply buried coals, these coals could contain over 250 trillion ft3 of highBtu gas, the equivalent of a 10-year supply of natural gas. 

=Estimates of coal resources are from: Averitt, P. Coal Resources of the 

United States, January 1, 1974. Geol. Survey Bull. 1412, 1975, 131 pp. 

'work cited in footnote 5. 
CONCLUSION 

The gas-adsorption capacity of coal depends upon pressure, temperature, 

and rank. Since pressure and temperature are functions of depth, the gas content of most coals can be estimated from rank and depth or calculated from the 

general adsorption equation. In some cases, factors such as high moisture 

content, low pressure gradient, and anomalous temperatures should be considered in evaluating the accuracy of the gas-content estimate, The estimates of 

coalbed gas content derived from adsorption data provide reasonable preliminary figures and can be developed using readily available data. 
BIBLIOGRAPHY 

1. Joubert, J. I., C. T. Grein, and D. Bienstock. Sorption of Methane in 

Moist Coal. Fuel, v. 52, 1973, pp. 181-185. 

2. . Effect of Moisture on the Methane Capacity of American Coals. 

Fuel, v. 53, 1974, pp. 186-191. 

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

4. Kim, A. G., and L. J. Douglas. Gases Desorbed From Five Coals of Low Gas 

Content. BuMines RI 7768, 1973, 9 pp. 

5. Kissell, F. N., C. M. McCulloch, and C. H. Elder. The Direct Method of 

Determining Methane Content of Coalbeds for Ventilation Design. BuMines 

RI 7767, 1973, 17 pp. 

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

Methane Content of Bituminous Coalbeds. BuMines RI 8043, 1975, 21 pp. 

7. McCulloch, C . M. , and W . P . Diamond. Inexpensive Method Helps Predict 

Methane Content of Coalbeds. Coal Age, v. 81, No. 6, June 1976, 

pp. 102-106. 

8. Ruppel, T. C., C. T. Grein, and D. Bienstock. Adsorption of Methane/ 

Ethane Mixtures on Dry Coal at Elevated Pressure. Fuel, v. 51, 1972, 

pp. 297-303. 

9. . Adsorption of Methane on Dry Coal at Elevated Pressure. Fuel, 

v. 53, 1974, pp. 152-162. 
APPENDIX A. --ESTIMATING METHANE CONTENT FROM ADSORPTION ISOTHERMS 

Coal is a porous solid that can adsorb gas on the internal surface of its 

micropores. The amount of gas adsorbed depends upon pressure and temperature. 

The adsorptive capacity of a solid is determined by measuring the amount of 

gas adsorbed at various pressures and constant temperature. This procedure is 

repeated at several temperatures to obtain a series of adsorption isotherms 

(fig. A-1) . Empirically, adsorption isotherms can be described by the equation 

where V = volume adsorbed, in cubic centimeters per gram of moisture- and 

ash-free coal; 

P = pressure, in atmospheres; 

k = a constant, in cubic centimeters per gram per atmosphere; 

and n = a constant. 

The constants k and n vary with temperature. Rather than determine these constants for all temperatures of interest, the effect of temperature can be 

estimated by using the following equation: 

FIGURE A-1. - Adsorption isotherm. 

PRESSURE, atm 
where k and n are determined at a reference temperature (in this case O0 C); 

b is a constant, in cubic centimeters per gram per degree Centigrade; and T is 

the temperature, in degrees Centigrade. 

To determine k, and no, equation 1 is converted to logarithmic form: 

Log V = log k + n log P. (A -3) 

Plotting log V versus log P (with values of V and P taken from the 0" C isotherm) gives a straight line in which log ko is the intercept and no is the 

slope (fig. A-2). The temperature constant b is determined by plotting V 

versus T at constant pressure; b is the slope of the straight line (fig. A-3). 

The constants k,, no, and b were determined for the coals listed in 

table A-1. Experimental procedures used to determine the isotherms for dry 

coal are described in a previous report (1). Values of k,, no, and b, determined from the isotherms for dry coal, are listed in table A-2. When k, and 

no are plotted versus fixed carbon (fig. A-4), the straight lines obtained are 

described by the equations: 

and no = 0.55 - 0.004 FC (r = 0.86). (A-5) 

The regression coefficients for these lines, 0.82 and 0.86 for equations A-4 

and A-5, respectively, show that the experimental data are in fair agreement 

with the linear equations. To minimize the deviation from the straight line 

at high and low values of fixed carbon, values of k, are plotted versus FC/VM 

(fig. A-5) to obtain the following equation: 

which has a regression coefficient of 0.93. The constant no can be related to 

k,, since both are functions of rank, by the following equation: 

Values of k, and n,, calculated using equations A-6 and A-7, are listed in 

table A-2. Calculated values are generally within +20 pct of the experimentally determined value. 

The temperature constant b (table A-2) does not vary with rank. It has 

a mean value of 0.14 cm3 /go C with a standard deviation of LO. 02. 
TABLE A.1 . . Analyses of coal samples 

Coal 

Mammoth .............. Pocahontas No . 3 ..... DO ................. Lower Kittanning ..... DO ................. Beckley .............. Mary Lee ............. Upper Freeport ....... Lower Freeport ....... Upper Freeport ....... Mary Lee ............. Upper Freeport ....... Pittsburgh ........... Sewell ............... No . 5 Block .......... Pittsburgh ........... Mary Lee ............. Pittsburgh ........... DO ................. Somerset B ........... Castlegate ........... Rosebud .............. Elkhorn No . 3 ........ Hazard No . 4 ......... Pocahontas No . 4 ..... DO ................. Stearns No . 2 ........ Hazard No . 7 ......... Pittsburgh ........... Pocahontas No . 3 ..... 

Locat ion 

(county and State) 

Schuylkill. Pa ....... Wyoming. W . Va ....... Buchanan. Va ......... Cambria. Pa .......... ..... do .............. Raleigh. W . Va ....... Jefferson. Ala ....... Indiana. Pa .......... ..... do .............. ..... do .............. Jefferson. Ala ....... Indiana. Pa .......... Washington. Pa ....... Randolph. W . Va ...... Boone. W . Va ......... Greene. Pa ........... Walker. Ala .......... Washington. Pa ....... Marion. W . Va ........ Gunnison. Colo ....... Carbon. Utah ......... Rosebud. Mont ........ Floyd. Ky ............ Kentucky ............. McDowell. W . Va ...... Raleigh. W . Va ....... Kentucky ............. ..... do .............. Allegheny. Pa ........ Buchanan. Va ......... 

NA.. Not available . 'Moisture-free basis . 

Proximate analysis. pc t 

Moisture Fixed 

carbon 

Ash Volatile 

matter 
TABLE A.2 . . Adsorp 

....... . . -1 . k,, 

Mammoth ............... Pocahontas No . 3 ...... 

Coalbed 

Lower Kittanning ...... 

~x~erfmental 

Beckley ............... Mary Lee .............. Upper Freeport ........ Lower Freeport ........ Upper Freeport ........ Mary Lee .............. Upper Freeport ........ Pittsburgh ............ Sewe 11 ................ No . 5 Block ........... Pittsburgh ............ Mary Lee .............. Pittsburgh ............ 

Somerset B ............ Castlegate ............ Rosebud ............... Elkhorn No . 3 ......... Hazard No . 4 .......... Pocahontas No . 4 ...... 

Stearns No . 2 ......... Hazard No . 7 .......... Pittsburgh ............ Pocahontas No . 3 ...... 

NA.= Not available . 

:ion constants for methane on coal 

:m?/g atm 

Calculated 1 from 

no 

Experimental 

equation A-6 

Calculated 

from 

equation A-7 
LOG PRESSURE 

FIGURE A-2. - Log-log lot of volume adsorbed versus pressure. 

FIGURE A-3. - Volume adsorbed versus 

temperoture at constant 

pressures. 

0 10 20 30 40 50 

TEMPERATURE,' C 
FIXED CARBON, pct maf 

FIGURE A-4. - Relationship of k, to fixed carbon. 
FIXED CARBON /VOLATILE MATTER 

FIGURE A-5. - Relationship of k, to the ratio of fixed carbon to volatile matter (FC/VM). 
APPENDIX B. --ESTIMATING THE EFFECT OF IWISTURE 

ON THE ADSORPTIVE CAPACITY OF COAL 

The amount of gas that can be adsorbed by coal is iwersely related to 

its moisture content, up to a critical moisture-content value (1-2). - - Above 

this value, which is apparently related to the oxygen content of the coal, 

additional moisture has no effect upon adsorptive capacity. Using the 

equation 

where C,, C,, and C2 are constants,' and X, is the percentage of oxygen, ma£, 

the critical moisture was calculated for most of the coals listed in 

table A-1 ,2 and compared with "as -receivedf' moisture (table B-1) . Generally, 

the calculated critical moisture is greater than the as -received moisture. 

When ratios of adsorptive capacity in moist and dry coal (v,/v~) are calculated with the equation 

(table B-1), the ratio is approximately the same for both critical and asreceived moisture. With the exception of the subbituminous Rosebud coal, the 

ratio of gas adsorbed in wet and dry coal (v,/v*) has a mean value of 0.73 

with a standard deviation of '0.13. In the absence of other data, the amount 

of gas adsorbed in moist coal can be estimated as 75 pct of that adsorbed on 

dry coal. 

l~he constants C, and C2 display some variation with pressure. However, the 

effect is small, and has been ignored in these calculations. 

2~omplete analyses werenot available for all coal samples listed in table A-1. 
TABLE B-1. - Critical moisture and reduction 

in adsorptive capacity 

Mammoth. ............................. Pocahontas No. 3..................... 

Coalbed 

Lower Kittanning ..................... 

Critical 

moisture, 1 

pc t 

Beckley .............................. Mary Lee... .......................... Upper Freeport ....................... Lower Freeport ....................... Upper Freeport ....................... Mary Lee ............................. Upper Freeport ....................... Pittsburgh ........................... Sewell ............................... NO. 5 Block. ......................... Pittsburgh ........................... Mary Lee. ............................ Pittsburgh ........................... 

Somerset B.. ......................... Castlegate ........................... Pittsburgh ........................... Pocahontas No. 3. .................... Rosebud .............................. 1 I I I 

NA--Not available. 

2~alculated from critical moisture. 

3 Calculated from as -received moisture.