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.
A recent review on the impact of PCI on blast furnace performance has been completed by CoalTech for ACARP. This expands earlier work using a mass & energy model to show how properties of the injected coal can impact on blast furnace performance Bennett (2004).
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.
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.