The current and future needs of blast furnace
operators are to maintain a stable and productive blast furnace,
while reducing costs and minimising the environmental impact of
steel production. Coal injection will continue to be a means for
the steel industry to address these needs. As the
understanding of the impact of quality of the injected coal has
increased, there has been a shift from high volatile thermal coals
to low volatile semi-anthracite.
The main criteria used to measure the performance
of a coal for injection are:
Economic Benefit: The main economic benefit is the
replacement of high cost coking coals.
Milling and Handleability: The main operating costs,
other than coal costs, are related to the milling and distribution
of the coal to the blast furnace.
Blast Furnace Operation: The injected coal quality
can influence the quality of the hot metal, productivity of the
blast furnace and top gas composition.
The better replacement
ratio and better milling performance of low
volatile coals makes them the preferred PCI coals at current
injection rates of up to 170 kg/tHM.
Blast furnace stability impacts on the
productivity of a blast furnace. Many aspects of blast
furnace operation influences stability, one of these is the
permeability in the lower zone. While high combustion
efficiency within the raceway is important, especially at injection
rates greater than 160 kg/tHM, other issues relating to coke fines
and slag viscosity have a greater impact on blast furnace
performance. Current research has demonstrated that the
unburnt char from the raceway is preferentially transported to the
upper zone of the blast furnace. On the other hand, the
larger and heavier coke fines trend to accumulate in the deadman
having an adverse effect on lower zone permeability.
The generation of coke fines is a function of
coke quality and the blast through the tuyeres. Due to increase in
blast momentum with higher volatile coals these coal will produce
significantly more coke fines than low volatile coals under the
same blast furnace operating conditions.
In the current times of high steel demand all
steelworks operators a striving for high productivity. High
productivity requires maximizing the total gas throughput per unit
of time and minimizing the specific gas requirement per ton of hot
metal. Therefore the objectives to achieve high productivity
improvement of the permeability in the upper and the lower zones of
the BF and
reduction of the specific gas consumption while acting on the blast
conditioning and by decreasing the reducing agents
Permeability with a BF is strongly influenced by
the PCI rate and the properties of the injected coal, as shown by
the figure below.