The relative importance of different aspects of
PCI coal quality has varied, as the technology for injection has
improved and the rate of injection increased. In the late 1970's,
triggered by the oil crisis, interest in PCI was renewed and coal
was considered as an economic replacement fuel for oil. As
combustibility was considered to be of importance, the coals used
for PCI were thermal coals. At that time, thermal coals were
readily available and had a much lower cost than hard coking and
semi-soft coking coals. As understanding of the impact of coal
quality on BF performance increased the demand for lower volatile
coals has increased over the last 5 years.
Today, there are many criteria used to measure
the performance of coal injection:
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Economic Benefit. The main cost benefit is the
replacement of high cost coking coal, though other benefits such as
improved productivity have also been observed. The replacement
ratio is kilograms of coke replaced per kilogram
of coal injected and is reported as the "actual" or the "corrected"
replacement ratio. The "corrected" replacement ratio is calculated
by taking account of other changes in the energy and mass balance
of the blast furnace that influence coke rate, for example, blast
temperature.
-
Milling& Handleability.
The main operating costs, other than coal costs, are related to the
milling and distribution of the coal to the blast furnace. The
Hardgrove Grindability Index (HGI) is a good indicator for the
expected milling behaviour of a coal. The high HGI of a soft coal
allows a mill to be operated at a higher mill throughput with the
same or lower mill power requirement. The size distribution of the
coal can impact on combustibility and coal handleability in bins
and transfer lines. Many handling problems are due to inefficient
drying of the pulverized coal in the milling leading to
condensation in bins and feed lines.
-
Blast Furnace Operation. The injected coal quality
can influence the quality of the hot metal, stability of the blast
furnace and top gas composition. The ash from the injected coal can
act as an inhibitor for the oxidising process, is the main
deliverer of undesirable alkalies and consumes melting energy.
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Figure 1 World coke and PCI rates. The
estimated influence of HV and LV coals on coke rate is determined
based on the replacement ratio of typical coals.
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Figure 1 shows how the coke rate varies with
pulverized coal injection rates. The large scatter in this plot is
due to the data being taken from the monthly average figures from a
range of blast furnaces in various countries injecting a wide range
of coals. The best- fit curve to this data does indicate that there
is a reduction in the incremental coke savings at injection rates
over 200 kg/tHM. The different operating philosophy of the European
(EU), Japanese (JP) and Chinese (CH) is reflected in the data.
Generally, the Europeans aim for lower fuel rates while maintaining
productivity, whereas the Japanese aim for higher productivity with
high fuel rates.
