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:
- 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.
