Methane Formation by the Reaction of Coalbed Carbon with Water

doi:10.4236/gm.2011.11003

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Geomaterials, 2011, 1, 21-23

doi:10.4236/gm.2011.11003 Published Online April 2011 (http://www.SciRP.org/journal/gm)

Methane Formation by the Reaction of Coalbed

Carbon with Water

V. E. Vigdergauz

Russian Academy of Sciences, Moscow, Russia

E-mail:[email protected]

Received February 14, 2011; revised March 2, 2011; accepted March 21, 2011

Abstract

There is proposed a mechanism of methane and carbon dioxide formation by the direct reaction of carbon with water

during catastrophic events in the mining of coal deposits. Thermodynamics of the reaction is discussed.

Keywords: Methane Formation, Coal Mining, Water, Carbon Dioxide, Reaction Thermodynamics

1. Introduction

Mechanism of methane formation is a key question in an

understanding of the reasons of its “huge reserves” and

the role in “unpredictable catastrophes” [1]. There are

three main reasons for our interest to the problem [2]:

Association of methane with coal has always pre-

sented a safety problem in mines due to the potential for

explosions. Mixtures of methane and air, with the meth-

ane content between 5 and 14 percent by volume, are

explosive. Explosions of such mixtures have been fre-

quent in coal mines and collieries and have been the

cause of many mine disasters;

Interest in the recovery of methane from coalbeds has

increased rapidly over recent years, because of its poten-

tial as an energy resource and to alleviate pollution and

safety problems;

Methane contributes to the adverse environmental im-

pacts of global warming, tropospheric ozone formation

(smog), and potentially stratospheric ozone destruction.

Its evolution during exploitation of coal deposits is a

source of numerous speculations but not really under-

standable till now. Traditionally three mechanisms for

methane formation are considered [2]: (1) nonbiogenic

formation, (2) formation by anaerobic bacteria during

plant material deposition and the early stages of diagene-

sis, and (3) formation from pyrolysis of coal material

during coal maturation.

All three mechanisms do not explain a fast and unpre-

dictable evolution of huge amounts of the gas in the

processes of an exploration of coal deposits. Controver-

sies are connected with the attempts to consider desorp-

tion of methane from the coal despite of its formation.

It is well known that in the case of catastrophic events

during exploitation of coal deposits there are observed

sudden waste of methane and carbon dioxide. One of the

biggest sudden wastes of gases from the coal occurred in

1921 in France in the Gard district on the mine Nord'Alle

[1]. Of the deposit sudden burst such amount of carbon

dioxide that it flood the mine, poured on the surface and

submerge small town that was situated in the lowlands.

During the catastrophic events the volume of the emit-

ted gases in some cases could reach hundreds cubic me-

ters per tone of the thrown coal. The process that leads to

such catastrophic events is not quite understandable till

now. We know two products of the reaction - methane

and carbon dioxide, and one of the reactants – coalbed

carbon. Coal substance contents some amounts of hydro-

gen but these quantities are too small for the balance. The

main question is the source of the huge amounts of hy-

drogen and a probable answer that it is water.

From the practice of coal mining it is known the cool-

ing of the coal bed before the sudden waste [1] that is

shows the endothermic character of the processes which

are observed before the sudden waste event.

The goal of the presented paper is to check up a ther-

modynamic probability of methane nonbiogenic forma-

tion directly from coalbed carbon and water in the condi-

tions of exploitation of coal deposits.

2. Reaction Termodynamics

According to Gibbs equation Gibbs free energy change

under standard conditions is equal to the total energy

changes for the system minus energy lost in the disor-

dering of the system:

22 Methane Formation by the Reaction of Coalbed Carbon with Water

SGHT (1)

There are two common methods of calculating of Δ G

for chemical reactions - determine Δ Hrxn and Δ Srxn and

use Gibbs equation or use tabulated values of free ener-

gies of formation, ΔGf.

The table summarizes molar thermodynamic charac-

teristics of substances which will be used for an analysis.

Standard molar thermodynamic characteristics of sub-

stances [3].

Substances Δ Hf298,kJ/mol Δ

S298,J/mol.K

Gf298,kJ/mol

C (s), graphite 0 32.1 -9.566

H2O(l) -285.83 69.95 -306.675

O2(g) 0 161.1 -48.008

CH4(g) -74.87 188 -130.894

CO(g) -110.53 197.66 -169.433

CO2(g) -393.52 213.79 -457.229

Proposed model describes a formation of methane

from a coal by the direct interaction of carbon with water.

Direct reduction of hydrogen from its oxide by coal in

the form of graphite carbon could be represented by the

equation:

2C + 2H2O = CH4 + CO2 (2)

rxn



= 44.36 kJ/mol of methane

The above reaction 2 is an endothermic one, but the

products entropy increases and the reaction could be

product-favored. Changes of the reaction 2 entropy Δ Srxn

= 197.69 J/mol.K is positive and this reaction could be an

“entropy driven”.

Necessary energy could be produced by parallel exo-

thermic processes of partial or total oxidation of methane,

such as:

CH4 + 2O2 = CO2 + 2H2O (3)

ΔGrxn = - 843.67 kJ/mol of methane

Summary processes of the generation of coalbed meth-

ane and its explosive oxidation are highly exothermic.

Scheme illustrates energy change during the process of

formation and subsequent oxidation of coal methane:

Energy change during the process of formation and subsequent oxidation of coal methane.

Summary energetic effect of carbon oxidation through

the pass with methane formation is near 400 kJ/mol of

carbon.

Reaction energy is undependable from its path and di-

rect oxidation of carbon by oxygen on the reaction:

C + O2 = CO2 (3)

ΔG rxn = - 400 kJ/mol

gives the same energy and the differences are only in the

reaction path and in the values of activation energies.

3. Field Observations

The proposed path of methane formation on the reaction

of water with coal carbon is indirectly confirmed by the

results of environment chemical analysis. By an exten-

sive study of the hydrologic situation in the Fruitland

Formation it was shown that the continuity of water cov-

erage of the formation is a crucial question for the for-

mation of methane from coal beds [4]. Chemical analysis

of gas and water phases in the areas of Coal-bed Meth-

ΔG

CH4 + CO2

2C + 2H2O

2O2

2CO2 + 2H2O

Reaction path

Methane Formation by the Reaction of Coalbed Carbon with Water 23

ane (CBM) formation also confirms the proposed

mechanism [5]. It was observed, that generally, dissolved

anions in water co-produced with CBM contain mainly

bicarbonate (HCO3

−) and chloride (Cl−). The bicarbonate

quantity decreased because this component limits the

amount of calcium (Ca) and magnesium (Mg) through

the precipitation of carbonate minerals. The composition

is controlled in great part by the association of the waters

with a gas phase containing varying amounts of carbon

dioxide (CO2) and methane. Indirect evidence about re-

ductive conditions during CBM formation gives an ob-

servation that “CBM waters are relatively low in sulfate

(SO4) because the chemical conditions in coal beds favor

the conversion of SO4 to sulfide”.

Shvartsev et al. [6] have recently shown for the

Erunakovo region of the Kuznetsk origin (Russia) that

during the coal methane formation a high mineralization

of water and the higher contents of HCO3

− is mainly

observed due to CO2 formation, which is not the product

of mantle genesis but is the product of coal metamor-

phism.

It is mentioned that coal rank increases with burial

depth and CBM content increases with increasing rank,

so that with greater seam depths, gas contents are ex-

pected to increase [7].

In conditions of high pressure and heating (for events

of methane blast, for example) the reduction of hydrogen

from its oxide could possess as chain-type reactions on

the free radical-mechanism:

C + H2O = CH˙ + OH˙… CH˙ + H2O = CH2˙ + OH˙…

CH2˙ + H2O = CH3˙ + OH˙… CH3˙ + H2O = CH4 + OH˙…

(4)

C + 3OH˙ = CO2 + H3O˙… OH˙ + H2O = H3O˙…

C + H3O˙ = H2O + CH˙; C + H˙ = CH˙… et cetera.

The rate of release of methane depends on the tem-

perature and pressure in the coalbed, coal rank and the

size of coal particles.

By the study of temperature–pressure conditions in

coalbed methane reservoirs of the Black Warrior basin

Pashin and McIntyre [8] have recently shown that carbon

sequestration and enhanced coalbed methane recovery

show great promise in subcritical reservoirs.

Presence of water seems necessary for methane for-

mation and methane sudden waste due to desorption

processes only does not explain the phenomena quantita-

tively. Mechanism of methane formation by a slightly

endothermic reaction of carbon and water simultaneously

with highly exothermic processes of the oxidation in the

coal bed explains unpredictable evolution of the huge

amounts of methane during exploitation of coal deposits.

4. Conclusions

Reaction of methane formation from carbon and water is

a slightly endothermic process. Its energy could be com-

pensated by exothermic reactions of partial or total oxi-

dation of methane in the conditions of exploitation of

coal deposits. The proposed mechanism of methane for-

mation is confirmed by the results of the analysis of the

hydrologic situation in the areas of coal-bed methane

formation.

5. References

[1] I. L. Ettinger, “Huge reserves and unpredictable catas-

trophes,” Moscow: Science, 1988.

[2] V. J. Hucka and D. M. Bodily, “Methane Formation in

Utah Coals,” SME 1993, Internet Available:

http://www.onemine.org/search/summary.cfm/Methane-

Formation-In-Utah-Coals?d=838F8CCB0E722E7EFD18

B682C551854549145D57558153FB4E7FFCF7CA897C

2F5766.

[3] http://en.wikipedia.org/wiki/Methane_(data_page)#Ther

modynamic_properties

[4] U. Fehn, G. Snyder, W. C. Riese, et al., Coal bed meth-

ane formation and water movement: application of the

I-129 and Cl-36 systems,” 2001, Paper 23-0 GSA Annual

Meeting.

[5] U. S. Department of the Interior, U. S. Geological Survey,

USGS Fact Sheet FS-156-00, November 2000

http://pubs.usgs.gov/fs/fs-0156-00/

[6] S. L. Shvartsev, V. T. Khryukin, E. V. Domrocheva, et al,

“Hydrogeology of the Erunakovo region of the Kuznetsk

basin in the context of coal methane formation and min-

ing,” Russian Geology and Geophysics. Vol. 47, no.7,

2006, pp. 878-889.

[7] http://www.ags.gov.ab.ca/energy/cbm/coal_and_cbm_intr

o2.html

[8] J. C. Pashin and M. R. McIntyre, “Temperature–pressure

conditions in coalbed methane reservoirs of the Black

Warrior basin: implications for carbon sequestration and

enhanced coalbed methane recovery,” Int. J. Coal Geol.

Vol. 54, 2003, pp. 167-183.