Page 44 - New Trends In Coal Conversion

12 New Trends in Coal Conversion

are humic coals. The different bands that form humic coals have been described as lithotypes
and classified into four classes, each one having a different microscopic composition and
different properties: vitrain (brilliant layers), clarain (semibrilliant layers), durain (mat
layers), and fusain (fibrous layers). The microscopic composition of each layer is described
in ICCP (1963), Stach et al. (1982), and Taylor et al. (1998).
2. Sapropelic, nonbanded, or massive coals (ICCP, 1963) are homogeneous (in appearance) and
require special conditions for the accumulation and preservation of the original organic mat-
ter (Stach et al., 1982 and references therein).

1.5.1.1 Coal composition
1.5.1.1.1 Organic fraction
In optical microscopy (in reflected white light and oil immersion), the organic fraction
of any coal is made up of entities called macerals (ICCP, 1963, 1971, 1975), which are
the result of the transformation of the original vegetal debris. According to their
physico-optical characteristics, macerals are classified into the following three groups
(ICCP, 1963, 1971, 1975, 1993, 1998, 2001; Sýkorov a et al., 2005; Pickel et al., 2017):
1. The huminite/vitrinite maceral group (ICCP, 1963, 1971, 1975, 1998; Sýkorov a et al., 2005)
has originated from lignocellulosic tissues, which are first gelified by bacterial action and
later coalified during its geological evolution. Huminite macerals are identified in low-
rank coals (Sýkorov a et al., 2005), and these are regarded as the precursors of vitrinite mac-
erals in higher rank coals. The huminite/vitrinite group is mainly composed of aromatic and
hydroaromatic structures, and it is usually the most abundant maceral group, particularly in
coals from the Northern Hemisphere. The properties of vitrinite therefore affect most indus-
trial processes in which coal is used. Vitrinite also occurs as a matrix surrounding other mac-
erals and minerals and has the tendency to swell and agglomerate. It also fuses during the
carbonization of medium-rank coals. This fusion property also influences both the course
and products of combustion, gasification, and hydrogenation of coal. The technological prop-
erties of low-rank coals in which the huminite macerals are identified are linked to those of
the predominant huminite maceral and are influenced by the degree of humification and gel-
ification of the huminite.
2. The liptinite maceral group (ICCP, 1963, 1971, 1975; Pickel et al., 2017) is derived from or-
ganisms and organs that are relatively poor in oxygen such as algae, spores, pollens, cuticles,
and secretions. This group also includes some degradation products and products of second-
ary generation during the coalification process of coal. Chemically, it is characterized by a
higher content in hydrogen and aliphatic structures. During the coalification of coal most lip-
tinite macerals disappear due to thermal transformation, or they develop similar optical prop-
erties to those of vitrinite (at the bituminous stage). The influence of liptinite on the
technological properties of a coal depends on the amount in which it occurs. Because of
its high hydrogen content, the liptinite group yields large amounts of tars and gases during
the carbonization process. Some macerals of the liptinite group also have a high calorific
value.
3. The inertinite maceral group (ICCP, 1963, 1971, 1975, 2001) is derived from plant material
that was strongly altered and degraded under oxidizing conditions before deposition or by
redox, biochemical, and chemical processes at the peat stage. Chemically, it is mostly aro-
matic with comparatively high amounts of carbon and low amounts of hydrogen and volatile