ACARP Project Number: C13063
Published: July 05
Philip Bennett, Graham O'Brien, Don
Evaluating the potential performance of coal
blends in both the thermal and PCI market requires knowledge of the
size distribution of the organic and mineral matter components of a
blend, especially when there are significant differences in the HGI
of the component coals. The size distribution of the organic matter
impacts on combustibility of thermal and PCI coal blends and
handleability of PCI coal blends.
Petrography techniques were used to examine four
size fractions from the PF of single coals and blends. The
development of curve fitting algorithms to the vitrinite and
inertinite reflectance distributions allowed these maceral groups
from individual coals to be identified in the sized PF samples.
From this data the size distribution of vitrinite and inertinite
was determined in single coals and blends.
For most coals, a good estimate of a blend's size
distribution can be made assuming that the size distribution of the
individual coals, milled under the same conditions, are added
together in the proportions of the blend. The exception is when a
very soft coal (HGI 90) is blended with a very hard coal (HGI 35).
In this case preferential milling (more reporting to the smaller
size fractions) of the softer coal occurred.
All coals studied in this project show some sign
of preferential grinding of the softer maceral group when the coal
was milled individually or in a blend. This preferential grinding
of macerals is due to differing strengths of the macerals which
dictates how the size reduction of the maceral varies with energy
used in the breakage of the particle.
Breakage characteristic curves (change in size
reduction per unit of energy) for vitrinite and inertinite were
determined from the milling data of the coals and blends. For these
curves the mill specific power was proportioned to the maceral
groups based on the petrographic analysis and blend composition.
These curves have similar trends as those found for the breakage of
lithotypes. Generally, these trends are for the particle size to
initially decrease rapidly then approach a constant size with
increasing breakage energy. The results indicate that the breakage
characteristic curves of maceral groups in individual coals do not
change when they are blended with other coals.
It is only when the reduction in breakage energy
proportioned to a maceral group of a coal in the blend moves to the
steeper region of its breakage characteristic curve that the
preferential milling of a coal in a blend is observed in the size
distribution of the blend. This would also explain the
non-linearity of Hardgrove Grindability Index (HGI) determined on
some blends when compared to their component coals as HGI is a
measure of size reduction for a fixed energy.
The results show that the breakage of a coal
particle can have three mechanisms, these are:
Vitrinite and Inertinite Breakage : The breakage of both vitrinite
and inertinite consumes energy in the milling process.
Vitrinite Dominated Breakage : The breakage energy of the coal is
dominated by the breakage of the vitrinite.
Inertinite Dominated Breakage: The breakage energy of the coal is
dominated by the breakage of inertinite.
The results also explain why some coals, those
with an inertinite dominated breakage mechanism, do not follow the
generally observed trend between HGI and the maximum vitrinite
It was shown that relationships between mill
specific power and HGI, Rosin Rammler parameters and vitrinite
reflectance and the breakage characteristic curve of vitrinite and
inertinite allows one to determine the mill performance of a coal
or a blend. Currently it is not possible to estimate the breakage
characteristic curve from petrographic analysis. Further milling
testing, under fixed mill conditions, for wider range of coals will
assist identifying methods to predict the breakage characteristic
curves for the vitrinite and inertinite maceral groups.
The ability to predict the size distributions of
individual maceral groups within a coal or a blend could greatly
assist in the prediction of the combustion performance of blends
and the handleability of blends of PF in blast furnace injection
systems. In advanced low NOx burners NOx formation is very
dependent on how the coal composition varies with the size of the
particles. These burners concentrate the coal, mostly the larger
particles, in the inner core of the primary air stream while the
finer particles stay in the outer region of the primary air stream.
This outer region forms a high temperature flame that envelopes the
inner core driving the rapid volatile release from the coal thus
ensuring minimum NOx formation when the primary air mixes with the
secondary air. By selected blending a plant operator could control
the composition ( nitrogen content and volatiles) of the coal that
reports to the inner core and the outer region of the flame for
greater NOx reduction for these burners.