Page 34 - New Trends In Coal Conversion

Coal 3

1.2 General concepts about coal

1.2.1 Coal definition
Coal is an organic and combustible sedimentary rock of black or brownish-black color
with a high carbon content and varied physical, chemical, and technological properties
depending on the rank (degree of evolution) reached throughout its geological history.
Whereas the majority of rocks are inorganic, coal consists predominantly of organic
matter, largely derived from a variety of plant remains (higher plants, ferns, fungi,
and algae) and different tissues (leaves, stalks, woody trunks, bark, pollen, spores,
sclerotia, resins, etc.) with an associated mineral matter content corresponding to an
ash yield (ISO 11760, 2005) of less than, or equal to, 50% on a dry mass basis.

1.2.2 Coal formation

The plant debris forming coal was originally deposited in a swampy depositional envi-
ronment and eventually formed a soft, spongy sediment called peat (Fig. 1.1).
Peat in turn is affected by synsedimentary and postsedimentary natural processes.
Among them, the physical and chemical ones induced by sedimentary compaction
and elevated temperatures at increasing burial depths of up to several kilometers
and over periods of time of 100 million years transformed the peat into coal
(Fig. 1.2) through a process called coalification.
The coalification process includes first a biochemical phase (that occurs in the peat
swamp just after organic debris has accumulated and at very shallow depths) followed
by a geochemical phase or coal metamorphism. This second phase involves the largest
and irreversible physical and chemical transformation from the lignite stage to the sub-
bituminous, then bituminous, anthracite, meta-anthracite, and graphite being the final
stage of the organic evolution process (albeit not necessarily pure graphite). Coal meta-
morphism is a function of heat and pressure acting over a period of time (Fig. 1.3). Of

Figure 1.1 (Left) Current peat bog in Ireland. (Right) Cross-section in a current peat bog under
exploitation in Ireland.
Left: Photocredits: M. Misz-Kennan, University of Silesia, Poland. Right Photocredits: T.
Pastor, http://perso.wanadoo.es/teresapastor/turba.htm.