ACARP Project Number: C15069
Published: March 07
The objective of this report is to review the
literature on the impact of high coal injection rates on blast
furnace performance and summarise the issues faced by blast furnace
operators, current trends in addressing these issues and the likely
future requirements for PCI coal.
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. The better replacement
ratio of low volatile coals is reflected in the better market price
of these coals compared to the price of high volatile PCI
In the present climate of high steel demand high
blast furnace (BF) productivity is the goal of all blast furnace
operators. To ensure long term productivity there has been a trend
in Japan for injection rates to be reduced. The reason for this
reduction is limited coke quality and therefore limiting the
maximum injection rate that can be achieved without impacting on BF
permeability and therefore on BF productivity.
Milling & Handleability
The main operating costs, other than coal costs,
are related to the milling and distribution of the coal to the
blast furnace. LV coals, compared to HV coals, are generally softer
and thus require less energy to grind, but they require a higher
energy to dry the coal down to a moisture level required to
eliminate the risk of handling problems. Free moisture in the
pulverised coal can lead to handling problems in bins and transport
lines. Clays in the coal may also increase the risk of the handling
problems when there is excess free moisture.
Blast Furnace Operation
The injected coal quality can influence the
quality of the hot metal, stability of the blast furnace and top
BF productivity is strongly influenced by the
permeability of the BF furnace to upward gas and downward liquid
flows. The understanding of the factors that influence the
long-term permeability will add in achieving high injection rates
with high productivity.
The development of 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
are only small differences between the combustibility of coals and
it is likely that these differences can be 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
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. This limits the usefulness
of modelling these processes.
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 the amount and composition of the PCI ash. The
co-injection of fine BF slag, fine iron ore or other materials can
reduce the effect of PCI ash on slag chemistry.
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 European works operating
with high PCI rates, though some BF operators prefer to use the hot
strength of coke.
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.
The main cost benefit of PCI is the replacement
of coking coal. The amount of coke replaced by an injected coal
depends on the useable energy released by the PCI coal in the lower
zone of the BF. This energy is the partial combustion heat released
when coal is gasified to CO and H2 less the sensible heat of the
combustion gases and ash.
The value-in-use benefit of PCI depends on its
replacement ratio and ash chemistry plus delivered cost to the
plant as well as its impact on the energy balance of the
steelworks. The costs/benefits associated with changes to the
energy balance of a steelworks depend on the costs of the
Other factors are considered in the selection of
PCI coals, such as, the dual use of semi-soft coals for PCI and a
blend coal for the coke oven, which is seen by some customers as an
advantage by reducing working capital via lower inventory.
There is an economic injection rate, namely the
injection rate at lowest PCI and coking coal costs, for every BF.
This rate depends on the coke quality, the cost of coking and PCI
coals and cost of any additional fuels for meeting the energy
requirements of the steelworks. Most BF’s have PCI systems
that can achieve injection rates greater than the economic