Page 47 - New Trends In Coal Conversion

Coal 15

matter, together with the calculation of fixed carbondhas been widely used for over
160 years. However, data from proximate analysis (volatile matter content) are flawed as
rank indicators because at low ranks the contributions of the various maceral groups are
divergent, with the aliphatic liptinite macerals contributing more to the volatile matter than
the more aromatic vitrinite and inertinite macerals. Therefore, volatile matter is used as a
rank parameter only above the high-volatile A/medium-volatile bituminous boundary, for
31% of volatile matter (dry, mineral matterefree basis; DMMF), the point where the chem-
istry of liptinites converges with the chemistry of vitrinite. Moisture content has been used as
a rank indicator in lignites and subbituminous coals. For low-rank coals, moisture is an
important factor because the coal needs to be transported, handled, and stored, and the pres-
ence of moisture in large amounts will impede these operations and lead to greater costs.
Moisture also supplants an equal amount of combustible material decreasing its heating
value.
Ultimate or elemental analysis, which measures the C, H, N, S, and O on the basis of ash and
moisture content, is another test used to obtain rank indicators. In fact, the carbon content for
a broad rank range and the hydrogen content for anthracites are common rank parameters.
However, the carbon content in low-rank coals has the same limitations as volatile matter
or fixed carbon from proximate analysis because the carbon content is a function of the vari-
ation in maceral chemistry in addition to being rank dependent.
The calorific value, expressed as MJ/kg, on a moist, mineral matterefree basis, is used as a
rank parameter for ranks lower than medium-volatile bituminous coal. Because the moist ba-
sis implies equilibrium moisture (see full description in Pajares and Díez, 2014), the calorific
value is really the combination of two rank parameters: the equilibrium moisture and the final
calorific value. As with proximate and ultimate analysis, measurement of the calorific value
of the whole coal means that the properties of all the macerals are averaged out.
The most common of the petrological parameters for evaluating coal rank is vitrinite reflec-
tance. Vitrinite reflectance (expressed as % of reflected light) is measured by optical micro-
scopy and takes into account only one coal component. This parameter has the capacity to
provide an indicator (Su arez-Ruiz and Ward, 2008) that is independent of other factors
(such as the coal type or coal grade), the only requirement being that vitrinite is present in
the coal. The reflectance of the vitrinite increases with the increase in coal rank (Fig. 1.3
and Table 1.1).
Determination of all these parameters (chemical and petrographic) is normalized by the ISO
and ASTM standards. Other rank parameters and their range of applicability have been
compiled in Su arez-Ruiz and Ward (2008).
2. Classification of coals. The purpose of any classification scheme of coals is to provide a tool
for a preliminary evaluation of a coal product and for relating a particular coal to others on the
basis of specific and appropriate criteria. Over the years, several significant countries from the
point of view of coal production and consumption have developed their own classifications
of coals using different parameters of a chemical and technological type. The regions that
have developed different classification systems for coals (most for commercial coals) include
Australia, China, Europe (UNECE), North America, Russia, South Africa, and the United
Kingdom.
The ASTM D388-15 (2015) of North America is a Standard Classification of Coal by Rank
and covers the natural series of coals from lignite to anthracite and it is applicable to coals that
are composed mainly of vitrinite. Coals rich in inertinite and/or liptinite cannot be properly
classified using this system. As indicated in this standard, this is because in inertinite and lip-
tinite maceral groups the properties that determine coal rank (such as calorific value, volatile
matter, and agglomerating character) differ greatly from those shown by vitrinite in the same