In a period of high global demand steelworks are focusing on maximum productivity.  To obtain high productivity requires stable furnace operation at high feed rates of reductants and sinter and/or pellets.  These higher feed rates result in increased gas flows leading to the need to maintain or improve permeability in the lower and upper zones of the blast furnace.  At injection rates, greater than 140 kg/tHM, it has been observed that changes were occurring in the operation of the blast furnace. Some of these changes are:

All these changes are interdependent and are influenced by the properties and amount of the injected coal, coke quality and blast conditions, as shown by the Figure 1.

  

The development of the modern injection lances has greatly improved the combustion of coal within the tuyere and raceway.  For coals with a volatile matter greater than 10% there is only small differences between the combustibility of coals and it is likely that these difference can accommodated by adjustment of BF operating conditions.   The impact of unburnt char on BF permeability is not that significant compared to the impact of coke fines and changes to slag viscosity around the raceway. 

The full understanding of the all the physical and chemical mechanisms that influence the devolatilisation, fragmentation and char burnout under the intense conditions within the tuyere and raceway is not yet known.

In the immediate region surrounding the raceway the ash from the injected coal has a major influence on the slag chemistry and therefore the slag viscosity.  This can lead to permeability issues in the lower zone of the BF and result in lower productivity.  The understanding of the relationship between ash/slag chemistry and slag viscosity is growing and will lead to tighter controls on amount and composition of the PCI ash. The co-injection of fine BF slag, fine iron ore or other materials does reduce the effect of PCI ash on slag chemistry.

The influence of ash from injected coal on the physical properties of the dripping slag and the permeability of the region surrounding the raceway are summarized in Figure 1.

[1] Ichida, M., Orimoto, T., Tanaka, T., Sakatani, M., Ueno, H., 2002, Behavior of pulverized coal ash and physical property of dripping slag under high pulverized coal injection operation, , International Blast Furnace Lower Zone Symposium, Wollongong, Australia, 25-27 November 2002.

Good coke quality is recognised by all BF operators as a necessary requirement to achieve good productivity at high injection rates. The cold coke strength is used as the coke parameter to monitor coke quality by many works operating with high PCI rates.  At high injection rates, the BF shaft efficiency can be improved by increasing the reactivity of the coke and this is being actively researched particularly in Japan.