Page 45 - New Trends In Coal Conversion

Coal 13

matter. Most inertinite remains inert during the carbonization process improving coke
strength, although the less reflective inertinite macerals may retain their plastic properties.
Depending on the type of inertinite and coal rank, different chars are generated during com-
bustion, some of which are highly reactive.

Examples of the three maceral groups identified by optical microscopy are pre-
sented in Fig. 1.9.
The macerals present in coals are not isolated, and they occur together in different
natural associations that are called microlithotypes, which are also identifiable by op-
tical microscopy (ICCP, 1963, 1971; Taylor et al., 1998). Microlithotypes may be
mono-, bi-, or trimaceralic depending on whether the association contains macerals
of one, two, or three different groups. Microlithotypes can also include mineral matter.
The chemical properties of microlithotypes are very similar to those of the predominat-
ing macerals, whereas the physical properties are related to those of the macerals and
also to the combined effect of the association. The degree of heterogeneity in a micro-
lithotype influences its technological behavior, particularly in coal carbonization, com-
bustion, and gasification processes.
The chemical composition of the organic fraction of a coal is made up of different
percentages of carbon, hydrogen, oxygen, nitrogen, and sulfur. Because in a coal the
organic fraction is the predominant fraction, the amounts of carbon, hydrogen, and ox-
ygen account for between 97% and 99% of the mass of pure coal, depending on the
coal type and rank (Table 1.1). Thus, the carbon content increases progressively
with the increase in coal rank from <65% for lignite to w95% for anthracite.
Hydrogen is a minor element and follows an opposite trend with increase in coal
rank. Oxygen decreases correspondingly from above 30% in lignites to <1% in the
highest rank coals. No distinct trends are apparent for nitrogen (usually between
0.3% and 1.5%) or for organic sulfur (0.3%e2%).

100 µm 100 µm
Figure 1.9 Photomicrographs of the three maceral groups, vitrinite, liptinite, and inertinite in a
bituminous coal (R vitrinite: 1.04%; petrographic composition: 79.4% vol. vitrinite, 7.5% vol.
liptinite and 13.1% vol. inertinite). Images taken in optical microscopy, reflected white light and
oil immersion objectives.
Photocredits: D. Luis from INCAR-CSIC, Spain.