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
are:
- improvement of the permeability in the upper and the lower zones of the BFand
- reduction of the specific gas consumption while acting on the blast conditioning and by
decreasing the reducing agents consumption.
Permeability with a BF is strongly influenced by the PCI rate and the properties of the injected
coal, as shown by the figure below.
