Page 55 - New Trends In Coal Conversion

Coal 23

Weathering of bituminous coals changes their fluid properties. In fact, the FSI and
the Gieseler fluidity drop rapidly as weathering increases. This loss of fluidity destroys
the coking ability of coal. When charging coke ovens, a uniform bulk density is desir-
able, and it is common practice to add oil to allow the coal particles to slide around
each other so as to increase bulk density. When the coal is weathered, the oil is taken
up by the increased fracturing in the coal, and so bulk density control becomes difficult
(Crelling et al., 1979).
Coal oxidation tends also to lower the calorific value of coals, which is detrimental
in the case of coals intended for combustion. During the control of coal quality, petrog-
raphy (optical microscopy) can be used to detect oxidized or weathered coals because
weathering changes the petrographic characteristics of coal particles in several ways.
In reflected light, weathered coal particles, especially vitrinite (Crelling, 2008), may
show one or more of the following features: a lower reflectance, giving the weathered
particles a darkish tint; dark reaction rims; increased fractures that are unrelated to cleat
direction (in severe weathering conditions, the particles may become so highly frac-
tured that they form particle aggregates); high relief and sometimes discoloration
around the particle borders and maceral boundaries; and an etched and pitted appear-
ance. Fig. 1.11 shows some images of weathered bituminous coals.

100 µm 100 µm

100 µm
Figure 1.11 Photomicrographs of coal weathering. The increase in coal alteration is from the
left to the right and bottom images. Images taken in optical microscopy reflected white light and
oil immersion objectives. Long side of the image: w485 mm.
Photocredits: D. Luis from the INCAR-CSIC, Spain.