Technical Market Support » Metallurgical Coal
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 coals.
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 gas composition.
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 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. 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 alternative fuels.
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 rate.