Coal Formation and the Origin of Coal Fires - Coal and Peat Fires: A Global Perspective - page 7

Burial pressure, heat, and time

Peat Lignite Bituminous

Sub-bituminous

Anthracite

Figure 1.1.5. Increasing rank of coal due to pressure, heat, and time. From Kentucky Geological Survey,
University of Kentucky, http://www.uky.edu/KGS/coal/, with permission.

pyrolytic condensation to form polycyclic aromatic ring structures with the loss of side chain carbons. During
geochemical coalification, there is an increase in carbon content, and a decrease in the concentration of oxygen and
hydrogen. Humic structures become more aromatic, and alkyl chains are split off to form CO 2 and CH 4
(Teichmuller and Teichmuller, 1967).

The increase in coal rank with increasing depth (Hilt ' s Rule) was assumed to be a function of increasing
overburden pressure. Also, folding was assumed to accelerate coalification by tangential pressure. However,
experimental evidence suggests that static pressure actually inhibits the chemical processes of coalification. The
increase of rank with depth is as readily explained by increasing rock temperature. Increases in rank along thrust
planes can be related to the frictional heating during tectonic movements. Magmatic contacts also produce local
increases in rank. The maximum depth of burial and the maximum temperature to which the coal was exposed for
long periods of time determine the rank of the coal. An estimate of the maximum temperature required to produce
different coal ranks is given in Table 1.1.1. Coalification is a function of time only if the temperature is sufficiently
high. Coals of the same rank can be produced either by short intense heating or by heating at lower temperatures for
longer periods of time.

It is relatively well established that age is not a primary determinant of coal rank and that tectonic events increase
the rank of coal beds. Most Pennsylvanian age coals in the Appalachian Basin and in the Interior Basin are
bituminous in rank (Figure 1.1.6). The folded and faulted coals affected by the Alleghenian orogeny are
anthracites. The Cretaceous coals of the Powder River Basin (PRB) in the Western United States are primarily
subbituminous, while the younger Paleogene and Neogene coals affected by the formation of the Alps are
bituminous to anthracite.

Table 1.1.1
Estimated temperature, depth of burial, and time required for metamorphism in normal coal series.

Temperature ( C) Depth of burial (m)

˚

Range * Maximum † Maximum * Maximum ‡ Maximum † Time (10 6 years/transition) *

Coal rank

Peat 10 - 20 15 67

Lignite 20 - 40 45 < 1000 1500 1750 28

Subbituminous 40 - 50 80 2300 2400 4

Bituminous 50 - 100 175 < 2100 6100 > 6000 6

Anthracite 100 to > 150 205 > 2700 7600 1.7

* Francis (1961).
† White (2002).
‡ Smith et al. (1994).