Page 46 - New Trends In Coal Conversion

14 New Trends in Coal Conversion

1.5.1.1.2 Inorganic fraction
Minerals and mineral matter are an integral part of the coal, and they are found in var-
iable percentages. Minerals and mineral matter are very important in coal utilization.
According to Ward (2002), the material classed as “mineral matter” embraces all the
minerals and other inorganic elements occurring in coal, including (1) dissolved salts
and other inorganic substances present in the pore water of the coal, (2) inorganic el-
ements incorporated within the organic compounds of the coal macerals, and (3)
discrete inorganic particles (crystalline or noncrystalline) representing the actual min-
eral components. The first two forms, sometimes referred to as nonmineral inorganics,
are most abundant in the mineral matter of lower rank coals, and with some exceptions
(Ward et al., 2007) they usually disappear from the coal with the increase in coal rank.
Mineral particles may also occur in coal of any rank and are usually the dominant
component of the mineral matter in higher rank coals.
Coals from mines may also contain minerals and fragments of rocks mixed with
coal. This material is an extraneous mineral matter and may be partly removed by
cleaning processes in coal preparation plants.
However, mineral matter closely associated with the macerals (inherent mineral
matter), including both intimately admixed minerals and nonmineral inorganics in
the maceral components, remains a part of the clean coal product and must be taken
into account when assessing the behavior of a coal during its utilization.
As a consequence of this, the chemical composition of the inorganic fraction of
coals is varied. The main minerals that are found in coals are quartz and minerals of
clay, mica, and the chlorite groups. Sulfide, carbonate, and chloride minerals together
with feldspars, barites, and titanium minerals make up the remainder of the suite, along
with lesser concentrations of minerals formed by weathering, such as limonite and sul-
fates of iron and calcium. Of particular interest is the sulfur mineral content. Virtually
all other elements occur sporadically in coal. These “trace” or “minor” elements rarely
constitute more than 1% of the mineral matter, and frequently the proportion is much
lower. More detailed information can be found in Su arez-Ruiz and Ward (2008).

1.5.1.2 Coal rank, determination, and classification of coals
1. Coal rank describes the degree of metamorphism or coalification to which the vegetal debris
have been subjected during its burial history. The rank of a coal is related to the geological
history undergone by the sedimentary basins in which coal seams are found. As mentioned
above many of the physicochemical properties (which influence the technological properties
of a coal) are rank dependent and, therefore, determination of the rank of a coal is essential. In
general, the increase in coal rank from peat and lignite to anthracite coal (denoted as increase
of coal rank) is marked by a progressive decrease in moisture and volatile functional groups
with a consequent increase in the carbon content of the coal. The rank of a coal can be
described using chemical parameters and/or petrological parameters.
As described in Su arez-Ruiz and Ward (2008), whole-coal measurements of coal rank by
means of chemical parameters were used long before specific petrographic indicators, such
as vitrinite reflectance, were introduced as rank parameters. The three conventional analyses
that provide chemical coal rank parameters are proximate analysis, ultimate analysis, and
calorific or heating value. Proximate analysisdthe measurement of moisture, ash, volatile