Bureau of Mines Report of Investigations/l974 

Low-Temperature Evolution 

of Hydrocarbon Gases From Coal 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Report of Investigations 7965 

Low-Temperature Evolution 

of Hydrocarbon Gases From Coal 

By Ann G. Kim 

Pittsburgh Mining and Safety Research Center, Pittsburgh, Pa. 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Rogers C. B. Morton, Secretary 

BUREAU OF MINES' 

Thomas V.-Falkie, ~ir-ector 
CONTENTS 

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

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

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

Experimental method ..................................................... 7 

Results ................................................................. 9 

Discussion .............................................................. 15 

Sumry ................................................................. 18 

References .............................................................. 20 

ILLUSTRATIONS 

1 . Carbon cycle ........................................................ 3 

2 . Total hydrocarbons. 125" C ......................................... 12 

3 . Concentration of methane versus total hydrocarbons. 125" C .......... 12 

4 . Average normalized distribution of hydrocarbon gases. 125" C ........ 13 

5 . Average normalyzed distribution of total increase in hydrocarbon 

gases. AT = 125" to 150" C ........................................ 14 

6 . Concentration of methane versus totalhydrocarbons. 150" C .......... 15 

7 . Total hydrocarbons. 150" C ....................................... ... 16 

TABLES 

1 . Gases produced during coalification ................................. 5 

2 . Description of coal samples ......................................Do. 7 

3 . Average composition of hydrocarbon gases evolved from coal at 

35" C. ppm ....................................................... 9 

Composition of hydrocarbon gases. unsized fraction. 125" C .......... 10 

Composition of hydrocarbon gases. 60/80-mesh fraction. 125" C ....... 10 

Composition of hydrocarbon gases . minus 200-mesh fraction. 125" C ... 11 

Hydrocarbon composition. unsized fraction. 1.50. C. ppm .............. 13 

Hydrocarbon composition. 60/80=mesh fraction. 150" C. ppm ........... 13 

Hydrocarbon composition. minus 200mesh fraction. 150" C. ppm ....... 14 

Boiling point and heats of formation of hydrocarbon gases ........... 17 

Heats of adsorption (Q, ) of hydrocarbon on carbon black. kcal/mole .. 18 
LOW-TEMPERATURE EVOLUTION OF HYDROCARBON GASES 

FROM COAL' 

by 

Ann Go Kim2 

ABSTRACT 

Hydrocarbon gases, primarily methane but also the C, through C, 

paraffins, the and C, olefins and possibly the Cq and C5 olefins, are 

byproducts of coal formation. Although it is generally believed that coal 

formation occurred at temperatures below 200" C, most experimental work on 

gas formation from coal has been performed at temperatures between 200" and 

l,OOO°C. The objective of this Bureau of Mines experiment was to determine 

if observable changes in the gases from coal could be produced at temperatures below 200" C in relatively short periods of time. The experiment 

involved heating coal samples from six coalbeds at constant temperatures and 

analyzing evolved gases. The composition of the gases at three temperatures 

was compared. At 35" C, over 99 pct of the gas is methane; at 125" and 150°, 

methane constitutes approximately half of the hydrocarbon gas. Isobutane and 

pentane are the predominant higher hydrocarbons at temperatures between 100" 

and 150" C. 

INTRODUCTION 

Coal is an essential source of energy for the present and the future. 

However, to efficiently utilize our coal resources, it will be necessary to 

significantly improve the safety of coal mining. The continual release of 

methane from coal and the potential formation of explosive mixtures in the 

mine atmosphere comprise one of the hazards associated with coal mining. 

In its efforts _to improve methane control, the Bureau of Mines has conducted 

research in the application of advanced mining technology, the development of 

reliable monitoring instrumentation, and the determination of physical, geo - 

logical, and geochemical factors that affect the retention and release of 

gas in coal. 

It has been known, probably since the beginning of mining, that coal 

contains a combustible gas, sometimes in sufficient quantity for the coalbed 

co he data in this publication were submitted as a thesis to the University 

1 o-f Pittsburgh in partia-l fulfil-lment-of the requirements for the degree 

of Master of Science. - - . 

2~hemis t . 
- 

to serve as a source of natural gas (41) - ." It was originally believed that 

the hydrscarbon gas in coal consisted entirely of methane. Approximately 

50 years ago ethane was also detected in coalbed gases, but was attributed 

to contamination from natural gas horizons below the coal (38). With the 

development of gas chromatography, a comple analysis of coal gases became 

possible. The number of analyses performed to date is relatively small, 

but they have established that the gas from coal contains the C2 through C5 

paraffins, the C, and Cg olefins, and possibly the C, and C5 olefins in 

addition to methane, carbon dioxide, oxygen, nitrogen; hydrogen and helium 

also occur in coal gas in varying amounts (c, 19, 24-25, 30). 

Current ventilation procedures can be successfully applied by treating 

the coal gas as methane alone. In future mine developments, it may be necessary to treat the gas as the multicomponent mixture it is. This would 

require more extensive knowledge of the formation and composition of hydrocarbon gases in coal. Although coal formation and the associated formation 

of hydrocarbon gases is believed to have occurred at temperatures below 

200°C over very long periods of time (13, - - 44), most experimental studies of 

gas formation from coal have been performed at temperatures from 200" to 

1 ,000" C (6, 16, 28, 21). Since coal decomposition occurs at temperatures 

above 400" C, it is possible that the gas produced during these investigat ions is subs tantially different from the gas produced during coalif ication. 

The objective of the present experiment was to determine if observable 

changes in the gases from coal could be produced in short periods of time 

at temperatures below 200" C. The experimental method consisted of heating 

coal samples at a constant temperature and analyzing the gases evolved. The 

composition of gases produced at three different temperatures was compared 

for coal samples from six coalbeds. The results from this experiment cannot 

be used to elucidate either the mechanism or kinetics of gas production. 

The method does not permit making a distinction between desorbed and formed 

gases. However, the results of this experiment demonstrate that the gas 

evolved from coal at temperatures below 200" C includes saturated and unsaturated hydrocarbons in addition to methane and that the composition of the 

gas is related to temperature. The low-temperature evolution of a complex 

gas mixture from coal, as shown in this and other studies (43, - 45) supports 

the theory that hydrocarbon gas format ion during coalif ication cannot be 

described by a s imple demethanation mechanism. 

Hydrocarbon gases, primarily methane but also most of the C, through C5 

paraffins and olefins, are byproducts of coal formation. Coalification is 

that portion of the carbon cycle by which a series of biochemical and geochemical reactions transform plant material into a combustible solid (fig. 1). 

The rank designations lignite, brown coal, high-, medium-, and low-volat ile 

bituminous coal, and anthracite are roughly equivalent to different stages 

in a sequential transformation from low-rank to high-rank coals. The 

3~nderlined numbers in parentheses refer to items -in the list of references 

at the end of this report. 
g

trace amounts of ethane, ethylene, propane, propylene , the butanes, and pentanes . FIGURE 1. - Carbon cycle. The gases are retained by 

adsorption on the peat. 

There is evidently no preferential concentration of the higher hydrocarbons 

in the peat deposit (20). - Inundation by water and the deposition of inorganic sediments usually terminate the development of the peat bog and initiate the geochemical phase of coalification. 

The geochemical stages of coalif ication can be described as a series of 

chemical reactions involving an increase in the aromatic character of the 

coal and the loss of hydrogen, oxygen, and some carbon (12, 14, 23). After 

the peat is removed from the biosphere, increasing overburden pressure compresses and desiccates the sediment. It has been estimated that between 10 

and 20 ft of peat are eventually compressed to 1 ft of coal (2). Moisture 

content decreases from between 88 to 97 pct for peat to 20 pct for lignite 

as the concentration of humic acids increases. The humic acids are a group 

of polycondensed aromatic compounds with aliphatic side chains containing 

active methoxyl, carboxyl, and carbonyl groups. During coalification, 

removing or closing the side chains leads to an increase in aromatic content. 

The concentration of humic acids diminishes in more mature brown coals and 

becomes negligible in bituminous coals because the humic acids are oxidized 

to neutral compounds (27). The transition from -lignite to bituminous coals 

is characterized by the removal of aliphatic side chains containing oxygen. 
The transition from high-volatile to medium-volatile bituminous coals 

involves the removal of methyl and rnethylene groups. Aromatic clusters grow 

larger and form larnellae oriented parallel to the bedding plane (42). During 

diagenesis to the low-volatile stage, waxy and resinous material are incorporated into the coal. In low-rank coa 1s , extractable paraffinic hydrocarbons show an odd carbon number preference; the distribution of odd to 

even carbon numbers becomes more nearly equal in higher rank coals, the 

change being a function of the degree of diagenesis (5). The formation of 

anthracite involves ring condensation and the growth of large aromatic clusters with a graphitelike structure. At this stage, the coal is composed of 

very large micelles in a more or less ordered arrangement (44). 

The elemental ratios of carbon to hydrogen and carbon to oxygen change 

with rank. With an increase in rank, the aliphatic H/C ratio decreases while 

the aromatic H/C ratio increases. The proportion of aromatic to aliphatic 

carbon also increases (10, 2). Early in the coalif ication sequence, 

infrared spectra shows a decrease in absorption due to stretching bands, 

particularly the hydroxyl and eith linkages. There is a decrease in absorption due to aliphatic carbon and hydrogen, including the elimination of carbony1 groups, and an increase in aromatic character due to the growth of 

polynuclear condensed ring structures (23). 

Pressure, temperature, and time influence the coalification reactions. 

In the early stages of coal formation, static vertical pressure, due to the 

weight of overlying sediments, causes compaction, decreases in porosity, and 

loss of moisture in low rank coals (11). In higher rank coals, experimental 

data suggests that high pressure mayPither cause depolymerization (E) or an 

increase in the graphite structure due to ring condensation (E) . During 

periods of post-depositional folding, coal is subject to horizontal or shear 

pressures caused by tectonic forces, and increases in rank can be related to 

folding or regional uplift (7, - - 40). 

Temperature is generally considered to be the most important factor in 

the catalysis of chemical changes during coalification. Contact metamorphism, 

increased temperature with depth, and thermal conductivity of adjacent rocks, 

which relate the degree of coa lif ication to temperature, indicate that coalforming reactions probably occurred below 200" C. Even at this relatively 

low temperature, estimated rates of reaction could produce all coals through 

anthracite in geologically reasonable periods of time (13). The length of 

time a coal is exposed to a particular temperature affects the degree of 

rank attained, a long time at a low temperature being equivalent to a short 

time at a higher temperature (42). - 

Based on changes in elemental composition, Mott proposed gaseous 

products for changes in rank (31). By using the decrease in the percentage 

of C, H, and 0 per the net increase in fixed carbon, Mott calculated the 

relative amounts of CH, , CO, , SO, and Y that could possibly be formed - during transitions to hieher rank (table 1). Even with this simple approach 

and without including higher molecular weight hydrocarbons, more than 
one set of reaction products is usually possible for each transition. No 

at tempt was made to correlate the hypothetical reaction products with 

actual coalbed gases. 

TABLE 1. - Gases ~roduced during coalification (31) 

Low-rank lignite.. ..... I Low-rank bituminous.. ... .I C02 

Gaseous reaction products 

(i) 62%O + 9 CH, 

(ii) 64% + BCY, + CO, 

Change 

From-- 1 To-- 

Medium-rank 

bituminous (a). 

Wood ................... 

Low-rank bituminous. ... 

Medium-rank 

bitminous (b) . 

Low-rank lignite ......... 

Medium-rank 

bituminous (a). 

Med ium-ra nk 

bituminous (b). 

(i) CH, + 8C0, 

(ii) 10$0 + 3CH4 + 7c0, 

(i) 6CH4 + 4 CO, 

(ii) 5%O + 3CH, + CO, 

Semianthracite.. ....... I Anthracite.. ............ .I H,O + 36CH, 

Semianthrac ite ........... 

In pyrolysis experiments on low-rank coals at temperatures between 150" 

and 1,000" C, the primary gaseous reaction products are carbon monoxide and 

carbon d ioxide , although methane and higher hydrocarbons have been detected 

(16, - - 35). If no attempt is made to limit the amount of oxygen available 

during the experiment or to remove previously sorbed oxygen, oxidation will 

be the primary reaction (a, The oxidation reactions occur readily at all 

temperatures and could obscure the effect of low temperatures on the formation of hydrocarbon gases. 

(i) 72 CH4 + 19c02 

(ii) 16$0 + 64CH4 + 11C0, 

In the absence of definitive experimental evidence, proposed mechanisms 

of hydrocarbon gas formation during coa lif ication must be considered speculat ive. However, there are several plausible mechanisms by which these 

gases could be produced during coal formation. Methane could be produced 

by removing alkyl side chains from aromatic molecules or by the decomposition 

of large straight -chain molecules. It could also form as straight -chain 

molecules are condensed to ring structures. These mechanisms account for 

observed increases in aromatic character with rank (4, 2). In low-rank 

coals, resins and waxes can be converted to mixtures of hydrocarbons; the 

decomposition of exinite also yields methane (5, 2). It has also been 

suggested that methane could form in a later stage, with ethane, ethylene, 

and hydrogen as the primary decomposition products. Methane would then be 

produced by a secondary reaction of these gases (9). If C3 through C5 

hydrocarbons are also produced from the coal, they could decompose further 

to methane, with the coal acting as a catalytic surface. The reduction of 

carbon dioxide to methane is also possible. Carbon-dioxide is believed to 

form early in the coalificarion process, probably-by oxidation in the 
Presence of occluded oxygen or by removal of carbosyl groups from low rank 

bituminous coals. With increased rank, the CO, /CH, ratio decreases (9). 

Generally, the ratio of hydrocarbon gas to C02 is believed to increase with 

increased rank, and the percentage of higher hydrocarbons decreases (43). 

The pos tdepositional history of the coal deposit strongly affects the 

amount of gas retained in the bed. Physically, coal is described as a solid 

colloid; that is, a solid, highly porous dispersion of discrete particles. 

The porosity of the coal is defined as the volume-percentage occupied by 

voids. Coal has two distinct pore systems : The macropore system, consisting 

of cracks and fractures; and the micropore system, similar to a molecular 

sieve structure. The micropores have an average diameter of 5 to 20 Angstroms and a very large surface area. Gas in coalbeds can exist as free gas 

in the macropore system or adsorbed on the surface of the micropores (2). 

The adsorption of a gas on a solid may be either physical or chemical. 

Physical adsorption depends on van der Waal's forces between the adsorbent 

and the adsorbate, pressure, and temperature. It is exothermic (AH<10 kcal/g) 

and produces adsorbed layers several molecules thick. Adsorption is appreciable only at temperatures below the boiling point of the adsorbate. Chemisorption results from a transfer of electrons between the solid and the 

adsorbed molecule. In effect, a chemical compound is formed at the surface 

of the so lid. Chemisorbed layers are unimo lecular . Chemisorption usually 

involves higher heats of adsorption (bH>20 k~al/~). It is an activated 

process that proceeds at a finite rate, which increases rapidly with 

increased temperature. Adsorption can occur at temperatures above the boiling point of the adsorbate (2, 15). Both physical adsorption and chemisorpt ion involve an equilibrium between adsorption and desorpt ion. 

Generally, the amount of adsorbed gas in coal increases with rank, and 

is influenced by pressure, temperature, degree of fracturing, and the 

permeability of the adjacent strata (34). Movement of gases out of the micropores follows Fick's Law of diffusion; if the diffusion coefficient is concentrat ion-independent and adsorption obeys Henry ' s Law, then 

where q' is the volume flow rate, D is the diffusion coefficient, C is gas 

concentration, A is the area, and L is the path length (8, 32). Movement of 

gases is also controlled by the change in the adso-rption-deGrption equilibrium with changes in pressure. The number of desorbing molecules is related 

to the activation energy of desorption, the adsorbate density, the permeability and surface area of the solid, as well as the change in pressure (36). 

Flow through the fracture system is assumed to follow Darcy's Law: 

where K is the fracture permeability, 4P is the change in pressure, and u is 

the dynamic gas viscosity (34). - 
When coal is removed from a bed, gas emission depends upon the crack 

and pore structure and on the size of the coal particles. The total volume 

of gas given off increases with time, whereas the rate of gas emission 

decreases. For the total volume of gas given off, 

Vt = A[1 - exp(-t/t, )" ] for l>n>O, 

where V, is the total volume of gas released per mass of coal, A is the initial gas content of the coal, to is a time constant, t is the time after the 

start of desorption, and n is an empirical factor. Generally, the volume of 

gas emitted increases rapidly then approaches a constant value, indicating a 

mechanism of pressure release followed by diffusion (1). 

Gas exists in coal either adsorbed on the internal surface of the pores 

or free in the pores and fractures. The amount of gas retained or emitted 

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

the coal, and permeability of the adjacent strata. It is very difficult to 

completely degasify coal. Even "nongas sy" coals contain small amounts of 

adsorbed hydrocarbons (2). 

EXPERDENTAL METHOD 

The experimental method consisted basically of heating coal samples at a 

constant temperature and analyzing the gas produced. Seven samples of 

unweathered coal (table 2) were used. Lower rank coals were selected because 

these coals are usually high in volatile matter. Although volatile matter is 

not directly related either to the amount of gas contained in the coal or to 

the amount of hydrocarbon gas that can be produced from the coal, it should 

be an indicator of the ease with which hydrocarbon gases could be formed. To 

minimize the contribution of desorbed gases, all of the coal samples were 

obtained from "nongassy" mines ; that is, mines that had no detectable gas 

emission. 

TABLE 2. - Description of coal samples 

NOTE. ---Daf = dry ash-free. . . 

Rank 

HvCb 

Subbit. 

Subbit. 

HvCb 

HvCb 

'HVA~ 

HvAb 

H 

daf, 

pct 

5.9 

4.8 

5.0 

5.9 

5.5 

5.2 

5.6 

Coa 1 

sample 

1 

2 

3 

4 

5 

6 

7 

Location 

Mercer Co. , Ill. . . 

Weld Co. , Colo.. . . Weld Co . , Co lo. . . . Monroe Co., Iowa.. 

Lucas Co. , Iowa. . . 

Preston Co.,W. Va. 

Allegheny Co., Pa. 

S eam 

No. 1 (Rock 

Island). 

Laramie No. 3. 

Laramie No. 3. 

Mammoth. . . . . . . Uncorrelated 

bed. 

Upper Freeport 

Pittsburgh.. . . 

Moisture, 

pct 

13.9 

21.0 

2 1.8 

14.1 

17.5 

1.5 

.9 

V.M. 

daf, 

pct 

48.4 

41.3 

40.7 

52.2 

46.0 

35.4 

41.4 

Ash, 

dry, 

pct 

17.5 

4.8 

5.1 

10.8 

10.6 

9.4 

6.6 

C 

daf, 

pct 

74.2 

76.3 

76.4 

83.0 

77.9 

86.2 

84.0 
Each sample was crushed, the largest pieces being about 1/4-inch, then 

placed in a vacuum overnight at 90" C to remove some of the adsorbed gases. 

Ten grams of each crushed coal was placed in a pyrex4 test tube. The mouth 

of the test tube was covered with a double layer of Teflon tape. The 

remaining coal was sieved and samples taken from the 60/80- and minus ZOO-mesh 

fractions. All samples were prepared in triplicate. 

In the presence of oxygen, coal can be oxidized at any temperature; 

the rate of oxidation increases with increased temperature. To minimize the 

extent of oxidation, each tube was flushed with nitrogen at 40 m. /min for 5 

min. The tubes, including a blank, were laid horizontally on racks in an oven 

at 125 22" C. To limit the amount of oxygen entering the tubes and to prevent 

the accumulation of hydrocarbon gases in the oven, nitrogen flow was maintained through the oven at a rate of 20 ml/min. A gas sample was obtained by 

inserting a 9-inch stainless steel needle attached to a disposable plastic 

syringe through the Teflon tape, to the far end of the tube and withdrawing 

1-1/2 ml of gas. The hydrocarbon content of the gas was determined by gas 

chromatography , us ing a Poropak column and the flame ionization detector (z) . One-half milliliter of gas was used to flush a standard 5/8-inch needle; then 

a 1-ml gas sample was injected into the chromatograph. The hydrocarbon composition was calculated by comparing the sample with a standzrd gas sample. The 

gas evolved from each sample was analyzed about once every 2 weeks, and 

results from the triplicate samples in each size range were averaged. After 

45 days, the temperature of the oven was raised to 150" C, and sampling and 

analysis of the gas was continued at this temperzture for 3 weeks. Single 

lo-gram samples of each coal in the 60/80 and minus- and minus 200-mesh fractions were prepared in a similar manner and placed in another oven at 35" C. 

Sampling and analytical procedures were the same as those used at the higher 

temperature. Samples were maintained at the low temperature for 2 weeks. 

Because of its simplicity, this system had several inherent sources of 

error. Since pressure was not controlled, an increase in barometric pressure 

would force air into the sample tube. A change of 25 rnm would equal 0.06 x 

lom3 moles of gas or 2.8 pct . Similarly, if the temperature of the oven 

varied 5" C, the number of moles of gas in the tube would change 0.03 x 10'~ 

or 1.4 pct. Each time a gas sample was removed, 1.5 ml of air was drawn 

into the tube, diluting the gas 2.9 pct. Since the syringe used was not a 

precision gas-tight syringe, sample volume could be inaccurate as much as 

3 pct. The total error was estimated at 510 pct; the accuracy of the 

chromatographic analysis was 0.1 ppm; the quantitative sensitivity of the 

analysis was 0.01 ppm. 

*~e-ference to specific brands-of equipment is made for identification only 

and does not imply endorsement by the Bureau of Mines. 
RESULTS 

Each 1-ml sample of gas derived from the coals at 35" C (table 3) 

contained between 4 and 7 ppm total hydrocarbons of which 99.5 pct was methane 

with ethane and trace amounts of higher hydrocarbons. Ethylene was not 

detected in any of the samples. A combined value is given for propane and 

propylene because the chromatographic system used did not completely separate 

these compounds. 

TABLE 3. - Average composition of hydrocarbon gases evolved 

from coal at 35" C, ppm 

Coal sample ....... 

CH, ................ c2 H4. .............. C2 H, ............... C3H ,............... C3H8. ............,. Unidentified 

4.2 

- 

Trace 

- 

- 

- 

- 

'sample numbers refer to those in table 2. 

2 Total hydrocarbon content of 1-ml sample. 

3.8 

- 

.01 

Trace 

- 

4.3 

The gas produced from the coals at 125" C showed only small variations 

in composition over the 45-day period (3 to 15 ppm total hydrocarbons per 

1-ml sample), with generally good agreement for the triplicate samples. The 

average composition of the hydrocarbon gas from each coal is given in parts 

per million per 1-ml sample in tables 4 through 6. Although total hydrocarbons derived from the various size fraction did not show large differences, 

generally more hydrocarbons were produced from the smaller size sample 

(fig. 2). Also, the percentage of methane in the hydrocarbon gas decreased 

with increasing surface area of the coal sample for the majority of coals. 

For example, in sample 1, at 125" C, the hydrocarbon gas from the unsized 

fraction contains 47 pct methane, the 60/80-mesh fraction contains 29 pct 

methane, and the minus 200-mesh fraction contains 21 pct methane. The ratio 

of methane to total hydrocarbons at 125' C decreased at least 20 pct relative 

to the ratio at 35' C, and the percentage of methane varied inversely with 

total hydrocarbons (fig. 3). 

- 

.05 

- 

Trace 

3.7 

- 

.01 

- 

- 

3.81 

- 

Trace 

- 

Trace 

- 

3.71 

6.9 

- 

.02 

Trace 

- 

- 

- 

Trace 

4.8 

- 

.02 

.02 

I 

3.8 

- 

.01 

Trace 

- 

Trace 

- 

Trace 

- 

6.97 

4.8 

- 

Trace 

Trace 

Trace 

3.81 

- 

4.84 

- 

4.8 
TABLE 4. - Composition of hydrocarbon gases, unsized fraction, 125' C 

1To tal, hydrocarbon content of 1-ml sample. 

Coal sample.. 

CH ,.......... 

C,H ,......... C2H,. . I.. .... C,H, ......... 

c3H8 ......... Unident if ied 

compound. ... i-C,H,, ...... 

...... 1-C4H,. 

..... n-C4H1,. 

C,Hl, ........ ~otgl' .... 

TABLE 5. - Composition of hydrocarbon gases, 60/80-mesh fraction, 125" C 

1 

Ppm 

5.8 

.5 

.7 

) .7 

o 

2.5 

Trace 

.2 

2.0 - -- 

12.4 

Pct 

46.8 

4.0, 

5.6 

5.6 

- 

20.2 

- 

1.6 

16.1 

99-9 

2 

1Total hydrocarbon content of 1-ml sample. 

Ppm 

7.3 

.3 

.5 

1.0 

- 

1.5 

Trace 

.2 

3.8 

14.6 

6 

Pct 

50.0 

2.1 

3.4 

6.8 

- 

10.3 

- 

1.4 

26.0 

100.0 

3 

.. Coal sample.. 

cp,. ........... Ppm 

6.0 

.2 

.5 

.2 

Trace 

.5 

Trace 

Trace 

.6 

8.0 

5 

Ppm 

7.1 

.7 

.3 

.7 

- 

1.4 

Trace 

.04 

7.7 

Pct 

75.0 

2.5 

6.3 

2.5 

- 

6.3 

- 

- 

7.5 

100.

Ppm 

8.2 

4 

Pct 

39.6 

3.9 

1.7 

3.9 

- 

7.8 

- 

.2 

42.9 

17.94100.0 

Cc 

C2H ,........... C,H,. ......... 

c,H,, 

C,H, ........... Unidentified 

compound. ..... 

i-C4H,, 

1-C,H,. ........ n -C,H1, ........ C5EZ1 2.......... 

'~otal' ...... 

PC t 

60.3 

Ppm 

5.7 

Ppm 

7.7 

1.2 

1.2 

1.7 

- 

4.0 

Trace 

-1 

4.8 

20.7 

Pct 

29.5 

1 

Pct 

37.2 

5.8 

5.8 

8.2 

- 

19.3 

- 

.5 

23.2 

100.0 

5 

5.1 

4.4 

5.9 

- 

8.8 

- 

1.5 

14.0 

100.0 

1.3 

1.5 

2.1 

- 

4.0 

- 

.3 

4.4 

119.3 

Ppm 

5.3 

Ppm 

6.2 

.2 

.1 

.2 

- 

.9 

- 

.04 

.3 

Pct 

29.1 

2 

1.2 , 6.6 

.8 4.4 

11.3 1 7.1 

6 

Pct 

78.1 

2.5 

1.3 

2.5 

- 

11.3 

- 

.5 

3.8 

7.94100.0 

Ppm 

7.2 

1.0 

.6 

.5 

- 

.8 

Trace 

Trace 

2.6 

-12.7 

3 

6.7 .7 

7.8 / .6 

- 

4.3 

Trace 

.2 

5.1 

18.2 

Ppm 

5.1 

.02 

.5 

.3 

- 

.9 

Trace - 

.3 

Pct 

56.7 

Ppm 

4,5 

10.9 

- 

20.7 

- 

1.6 

22.8 

100.0 

o 

23.6 

- 

1.1 

28.0 

. 99.9 

Pct 

71.6 

.3 

7.0 

4.2 

- 

12.6 

- 

- 

4.2 

Pct 

54.2 

.8 

- 

1.2 

- 

.2 

1.9 

13-.'6" 

7.12199.9 

6.0 

1.2 

3.6 

- 

7.2 

- 

- 

27.7 

99.9 

7.9 .5 

4.7 

1 .1 3.9 .3 

- 

6.3 

- 

- 

20.5 

100.0 

- 

.6 

- 

Trace 

2.3 

8.3 
TABLE 6. - Composition of hydrocarbon gases, minus 200-mesh fraction, 125" C 
TOTAL HYDROCARBONS, ppm 

FIGURE 3. - Concentration of methane versus total 

hydrocarbons, 125O C. 

samples produced a trace 

amount of 1-butene. 

When the temperature 

of the oven was raised to 

150" C , the hydrocarbon 

content of the gas 

increased by from 4 to 150 

ppm (tables 7-9). Methane, 

isobutane , and pentane generally showed the greatest 

increase. The unidentified 

compound that eleuted after 

the propane-propylene peak 

was found more frequently 

than at 125O C, but 1- 

butene was not detected at 

150" C. The average normalized distribution of the 

total increase in hydrocarbons is shown in figure 5. Methane, isobutane, 

and pentane constitute over 

75 pct of the total - - 

increase in hydrocarbons and 
methane decreases with 

increased total hydrocarbons (fig. 6). Gases from 

coal samples 2 and 3 

(Laramie seam, unsized) 

were not included in the 

calculations of the average 

change because over 90 pct 

of the exceptionally large 

increase in hydrocarbon 

content was pentane. For 

CH4 CpH4 C2H6 C3Hs i-C4HI0 II-C4HI0 C5HI2 all other samples the 

C3H8 average increase was 

approximately equal for 

FIGURE 4. - Average normalized distribution of hydro- methane, isobutane , and 

carbon gases, 125O C. pentane. 

TABLE 7. - Hydrocarbon composition, unsized fraction, 150" C, ppm 

l~otal hydrocarbon content of 1-ml sample. 

TABLE 8. - Hydrocarbon composition, 60/80-mesh fraction, 150" C, ppm 

6 

7.7 

.3 

.9 

.5 

- 

2.1 

-. 

Trace 

2.0 

13.5 

5 

8.1 

1.3 

.4 

.5 

Trace 

1.9 

g 

Trace 

3.3 

15.5 

Coal sam~le. ............. CH4................e.0... 

C2H4.. ................... C2H 6.0............. ...... C3H 6..................... 

C3H B..................... 

Unidentified con-,pound.. .. i-C,Y o.................. 

1-C,H,.. ................. n -C4 % ................ .. C,t$ ..................... 

~otal'. ............... 

'~otal hydrocarbon content of 1-ml sample. 

7 

11.9 

-6 

2.5 

1.9 

- 

4.7 

9 

Trace 

6.9 

28.5 

1 

6.3 

1.3 

- 

11.3 

- 

2.6 

- 

.2 

4,.2 

15.9 - 

6 

9.6 

.7 

.6 

- 

.2 

3.7 

- 

0 

- 

14.8 

Coal sample ................ CH, ........................ C2H4 ....................... C2H 6............;.......... 

C3H6 ....................... C,H e-............... ........ Unidentified compound ...... i-C4Y ,...... .............. 1-C4HB.. .................... n-C4y .. ................... C,Y, ...................... ~otal'.. ................ 

3 

7.7 

2.0 

-7 

2 

10.0 

3.0 

1.1 

3.0 

Trace 

9.4 

- 

.4 

122.0 

148.9 

7 

10.4 

2.1 

2.9 

3.3 

Trace 

9.4 

0 

0 

8.5 

36.6 

3 

7.4 

1.8 

.3 

1.0 

- 

3.3 

- 

.I 

8.2 

22.1 

4 

11.1 

3.8 

1.8 

1 

10.2 

3,4 

1.2 > 2.5 

- 

10.3 

- 

.4 

7.9 

35.9 

2 

8.6 

1.7 ' 

.8 

1.0 

- 

2.6 

- 

- 

7.3 

22.0 

4 1 5 

i 3*2 

- I .2 

6.3 7.4 i 

11.8 

2.7 

1.9 

3.2 

Trace 

9.0 

g 

.6 

7.7 

36.9 

g 

-1 

149.4 

167.9 

10.7 

2.1 

1.0 

1.3 

- 

3.0 

- 

9 

4.2 

22.3 

g 

,2 

6.4 

34.1 
TABLE 9. - Hydrocarbon composition, minus 200-mesh fraction, 150" C, ppm 

'Total hydrocarbon content of I-ml sampie. 

CH4 C2 H4 C2H6 C3H6 Unidentified i-C4HI0 n-C4HI0 C5HI2 

C3H8 comp.ound 

FIGURE 5. - Average normalized distribution of toto I increase in hydrocarbon gases, 

AT = 125' to 1 50° C. 

5 

7.2 

1.3 

.6 

1.0 

- 

5.0 

o 

Trace 

4.5 

19.6 

6 

9.5 

1.9 

1.3 

1.8 

1.6 

1.6 

o 

.7 

1.0 

19.4 

7 

10.3 

5.3 

2.0' 

3.6 

1.9 

6.9 

o 

.3 

7.7 

38.0 

4 

5.5 

2.7 

1.3 

1.9 

Trace 

8.8 

o 

Trace 

7.5 

27.7 

Coal sample.. ............. f 1 2 

8.0 

2.5 

.7 

1.2 

- 

4.7 

- 

.2 

9.4 

CH,....................... 

C2H,. ..................... C2 H6 ...................... C3H 6...................... 

C,K, ...................... Unidentified compound. .... i-C,q, ................... 1-C4H,. ................... n-C4H30................... 

C,H1, ..................... 

3 

7.7 

1.8 

o 

1.3 

Trace 

2.5 

- 

Trace 

7.8 

9.9 

3.7 

3.9 

) 4.6 

- 

8.7 

- 

.9 

7.2 

~otall.. ............... ' 38.9 1 26.7 21.1 
TOTAL HYDROCARBONS, ppm 

FIGURE 6. - concentration of methane versus toto I hydrocarbons, 150' C. 

DISCUSSION 

Although the total amount of gases derived from the coal samples was 

small, (16 to 60 ppm total hydrocarbons per gram of coal), the relative 

composition of the gases was significant. At 35' C, the gas was primarily 

methane. At 125" C, methane composed less than 50 pct of the hydrocarbon 

gas. In order of decreasing concentration, the components of the gas were 

methane, pentane, isobutane, propane-propylene, ethane, ethylene, trace 

amounts of 1-butene, and an unidentified compound, 

When the temperature of the oven was raised from 125' to 150° C, the 

amount of hydrocarbon gases increased (fig. 7). As at the lower temperature, 

difference in total hydrocarbons with particle size were not large, Averaged 

over all samples except 2 and 3 unsized, methane, isobutane, and pentane 

constitute over 75 pct of the total increase in hydrocarbons. Samples 2 and 3 

unsized, which came from different mines in the same seam, showed an exceptionally large increase in the amount of pentane released at 150° C, although the 

concentration of pentane at 125O C did not differ significantly from that of 

other samples. 

The retention time and peak shape of the unidentified compound listed 

between propane-propylene and isobutane indicates that it is a low-molecularweight hydrocarbon, probably not acetylene, possibly propyne (HC=CCH,) or 1, 

2 propadiene ($ C=C=C% ) , which- have -the same molecular weight and similar 

boiling points. It is also possible that-the compound is an alcohol or some 
small differences in gas 

chromatographic retent ion 

time, whether the compound 

is isopentane , neopentane , or n-pentane cannot be 

determined in this experiI 2 3 4 5 6 7 ment. Therefore, it is 

COAL SAMPLES listed only as pentane 

FIGURE 7. - Total hydrocarbons, 150° C. (Cs%a - 

At both temperatures, 

changes in hydrocarbon concentration with time were relatively small and not 

consistent, apparently affected by the concentration of CO, formed by oxidation of the coal. Th-e distribution of the hydrocarbons was not time-related. 

Previous analyses of coal gases have not indicated high concentrations of the 

C, and C, compounds, nor have they shown a preferred isomer. 

The distribution of gases does not correspond to the boiling points or 

heats of formation of the hydrocarbons (table 10). If hydrocarbon gases are 

formed during the experiment by dissociation of a large aliphatic chain into 

shorter hydrocarbon fragments, it may be assumed that the amount of each 

hydrocarbon should increase with decreased heat of formation. Although 

dissociation of coal is believed to occur readily during low-temperature 

-carbonization; it has -been suggested that- even the large tar-molecules - 

produced during carbonization are desorbed out of the micorpore structure 

rather than formed from the coal (46). 
TABLE 10. - Boiling point and heats of formation of 

hydrocarbon gases1 

Hydrocarbon pe oil in^ point, c lAHf , kcal/rnole 

CH4 (methane). ........... C,H4 (ethylene). ......... C,H, (ethane) ............ C3H6 ( propylene) ......... C,H, (propane) ........... 1, 2C3H4 (1,2 propadiene) 

C3H4 (propyne).. ......... i-C,Yo (isobutane). ..... 1-C4H, (1-butene). ....... . n-C,l$. (n-butane) ........ ne~-C~l$~ (neopentane). .. i-C5qa (i-pentane) ...... C,Y, ( pentane). ......... 'Weast , R. C. (ed.). Handbook of Chemistry and Physics, 

CRC Press, Cleveland, Ohio, 54th ed., 1973-74, pp. C75- 

C542, D76. 

Comparing the distribution of gases with their heats of adsorption on 

carbon black (table 11) gives a slight indication that desorption may be a 

significant process during the experiment. In most determinations of 

limiting isosteric heats of adsorption on carbon black, differences for 

ethane-ethylene (0.2 kcal/mole) and propane-propylene (0.03 kcal/mole) are 

small, indicating that these compounds would desorb at approximately the 

same rate. There is a larger difference in the heats of adsorption of the 

C, isomers (0.5 kcal/mole), with isobutane having the lower heat of 

adsorption. Heats of adsorption for the C, isomers vary a total of 1.8 

kcal/mole, in the order neopentanecisopentanea-pentane, indicating that 

neopentane may be more readily desorbed. However, the complete absence of 

two pentane isomers cannot be explained by differences in the heat of 

adsorption. Since the heat of adsorption increases with carbon number, 

desorption does not explain the predominance of C4 and C, hydrocarbons. 

Whether the hydrocarbon gases were formed or desorbed during the 

experiments cannot be determined from the data obtained. If they are 

desorbed from the coal, the gases then must have been formed-in some 

previous stage of coalification. Determining the mechanism (s) of hydrocarbon evolution at low temperatures is beyond the scope of this experiment. 
TABLE 11. - Heats of adsorption (Q,) of hydrocarbon on 

- 

carbon black, kcal /mole1 

Adsorbate di corcia2 Hoory3 Ross4 ~alashnikova' 

CH4.. ........ 3.020 3.13 3.11 

C2H4.. ....... - 4.390 4.40 4.30 

C,H ,......... 4.70 4.600 3.78 4.53 

C,H, ......... - 5.900 - - 

C3H ,......... 5.90,6.10 5.870 6.26 5.93 

i-C,Yo.. .... 6.80, 6.95 6.910 - 6.95 

n-C,y o...... 7.35,7.54 7.450 8.10 7.21 

l-C,H,. ...... - 6.95 

neo-C,y,. ... - - - 6.85 

i-C,%, ...... - - 8.04 

n-C,y2.. .... 8.41, 8.43 - 8.61 8.67 

'For purposes of discussion it is assumed that heats of 

adsorption on carbon black and coal are similar, even 

though coal is known to be a less homogeneous adsorbent. 

It is also assumed that adsorption is a reversible 

process with the heat of desorption equal to minus the 

heat of adsorption. 

di Corcia, A. , and R. Samperi. Hydrogen-treated Graphitized Carbon Black. Limiting Isosteric- Heats and 

Entropy Charges Upon Asdorption of Hydrocarbons. 

J. Phys. Chem., v. 77, 1973, pp. 1301-1306. 

3~oory, S. E. , and J. M. Prausnitz. Adsorption of Hydrocarbons on Graphitized Carbon. Trans. Faraday Soc. , v. 63, 1967, pp. 455-460, 

*Ross, S. , J. K. Saelens , and J. P. Olivier. On Physical 

Adsorption XVIIX. Limiting Isosteric Heats of Adsorption of Gases on Graphitized Carbon by the Chromatographic Method. J. Phys. Chem., v. 66, 1962, pp. 696- 

700, 

5~alashnikova, E. Va., A. V. Keselev, R. S. Petrova,. and 

K. D, Scherbokova. (Gas Chromatographic Investigation 

of the Adsorption Equilibrium or Graphitized Thermal 

Carbon Black, I. Henry Constants and Heats of Adsorption of C1 - C,. Hydrocarbons at Zero Coverage.) Chromatographia, v. 4, 1971, pp, 495-500. 

SUMMARY 

not 

Although it could/be determined if the gases were being desorbed or 

formed, the data obtained from this experiment show that in addition to 

methane, most of the C2 through C5 hydrocarbons are formed during coalification. Of the C, through C5 compounds, isobutane and one of the 

pentane isomers are apparently formed more readily than other hydrocarbons. 

This indicates ?hat the mechanisms by which hydrocarbon gases are produced 

during coalification are more complex than simple demethanation. 
I 

This experiment has shown that it is possible to study gas evolution 

from coal in reasonably short periods of time at temperatures below 200" C; 

however, to obtain accurate results, such experiments should be performed 

in a closed, oxygen-free system, preferably in an inert atmosphere at a 

controlled pressure. More elaborate experiments, more expensive in terms 

of time and equipment, are necessary to obtain additional information about 

the formation of hydrocarbon gases in coal. 
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