Page 201 - Advances In Productive, Safe, and Responsible Coal Mining

180 Advances in Productive, Safe, and Responsible Coal Mining

components. For instance, low-rank bituminous coal can be characterized by the for-
mula C 10 H 7 O.
As shown in Fig. 10.1, as the coalification process progresses, coal of increasing
density is formed under layers of sand and mud over millions of years. Coal rank is
based on density, which proceeds from low to high as follows: peat, lignite, subbitu-
minous, bituminous, and anthracite. The amount of byproduct gases increases with
coal rank; i.e., it is highest for anthracite.
Most of the gases produced during the coalification process escape to the atmo-
sphere, but a small fraction is retained in the coal. The amount of retained gases
depends on a number of factors, such as burial depth, coal rank, type of immediate
roof and floor rock strata, local geological anomalies, and the tectonic pressures
and temperatures prevalent at that time [4].
Methane is the major component of gases in coal, comprising 80%–90% or more of
the total gas volume. As coal is formed, the decomposing organic material produces
methane gas, as well as CO 2 , hydrogen, oxygen, nitrogen, and lower proportions of
other gases like ethane, propane, butane, and argon. During diagenesis, carbon
increases from 60% to 90%, whereas hydrogen decreases from 5.5% to 3%, meaning
that large volumes of methane are released. Biogenic methane is first to form by anaer-
obic bacteria in the early stage of coalification, followed by thermogenic methane at
temperatures of 120–150°C (248–302°F). Although much of the methane generated
by the coalification process escapes to the atmosphere or migrates into the adjacent
reservoirs or rocks, a significant volume remains trapped within the coal.
Coal mine methane (CMM) is therefore a term given to methane gas emitted due to
coal-mining activities either from the coal seam or from surrounding gassy rock for-
mations. Thus, coal and CMM are syngenetic in origin. In fact, CMM may be emitted
from active and abandoned underground and surface coal mines, and as a result of
postmining activities such as coal processing, storage, and transportation. While this
book is focused on coal, it is not out of place to mention that encountering methane gas
in metal and nonmetal underground mines is also common.

Fig. 10.1 Coalification process during geological time.