The main use of coal within the integrated steelworks is the production of coke. The biggest portion of blast furnace coke produced world-wide is consumed in integrated metallurgical works and within steel mills being linked to each other within a trust of companies. Only a very small portion of blast furnace production is represented by merchant coke production plants.

An increasing amount of coal is used for pulverised coal injection and in the production of sinter.

  

Blending of coals with different plastic properties is extensively carried out prior to cokemaking. The blend must fuse at moderate temperatures and resolidify to form cohesive semi-coke. The development of strong, lump coke is completed by heating to high temperature, typically between 1000°C and 1100°C.

Coke is the critical fuel of integrated iron and steel plants It must possess certain properties in order that blast furnaces operate effectively. It must comprise large well graded lumps with few fines. High resistance to breakage during handling and passage down the blast furnace is important. Irregular shapes give high voidage (hence permeability) to coke in layers. It must also have a high internal porosity (about 50%) and possess low to moderate reactivity to gases (principally CO2) in the blast furnace.

The concept of a coke plant with direct combustion of the raw gas would omit the entire gas treatment plant and produce only coke and power. Aside from these achievements with conventional coke oven batteries, the non-recovery technology has also found acceptance. Coke plants of this type are in operation in the USA, Australia and India. One plant in the USA is a heat- recovery coke plant that produces power from the waste gas. Further improvements in plant efficiency and coke quality are achieved with stamping of the coal charge, this is normally conducted in non-recovery ovens but there are some examples of stamp charging of slot ovens.

Coke production is mainly carried out in conventional slot-ovens. 

In a conventional slot oven coking converts the coal to a higher carbon content solid which contains all feed minerals.  Some of the volatile components (gases, vapours and liquids) are used to heat the coke oven while the remainder are recovered as tars for by- products and coke oven gas for use within the steelworks as a fuel.  Conventional coke ovens (slot type ovens) with raw gas recovery have reached dimensions of more than 8 m height and 90 m 3 useful oven volume, boosting the capacity of a single battery to 1.3 Mt/y and the production of a single coke plant with one operating team to 2.6 Mt/y. At the same time, the emissions from batteries and gas treatment facilities have been reduced to the lowest amounts ever. A further increase in energy efficiency and environmental protection for conventional coke batteries is expected from the combustion of the hot raw gas with subsequent generation of electricity. The energy balance of a conventional coke oven is shown below.  The yield, size distribution and strength of wharf coke can be estimated from the properties of the charged coal blend.

The need to improve environmental controls for existing cokemaking facilities and to find more cost- effective methods of producing high quality metallurgical coke has prompted several new and emerging technologies, for example:

Nicol & Durie review major changes in cokemaking and blast furnace steelmaking.

The Chair of Iron and Steel Metallurgy of the Department of Ferrous Metallurgy (IEHK) at the RWTH Aachen University offers learning units and lessons of the course "Ironmaking" online.

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:

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:

Permeability with a BF is strongly influenced by the PCI rate and the properties of the injected coal, as shown by the figure below.

               

The sintering process agglomerates the fine material into a clinker-like aggregate, with a size range that is acceptable to the blast furnace. In modern sinter plants the iron ore fines, fluxes and revert materials are blended to give the required chemical composition for the sinter feed. Weighed amounts of the blended fines, recirculated sinter fines, crushed coke breeze or anthracite (to assist ignition and the propagation of the flame front during the sintering process) and burned lime (to improve micro-pelletisation) are passed to a mixing and balling drum to optimise the permeability of the sintering mix. Sintering is carried out on a continuous travelling grate ("the strand") which travel over a series of wind boxes incorporating butterfly valves to control the air flow through the strand. This allows the speed at which the flame front is pulled through the sinter mix to be controlled. The surface of the bed is ignited under a gas-fired radiant-hood ignition furnace, producing a flame front, which is drawn downwards through the mix by the suction of the main fan(s) as the bed travels along the strand. Bed depth, strand speed and fan suction are normally controlled to finish sintering (i.e. achieve burnthrough) at the penultimate wind box.

Temperature in the ignition hood ranges from about 1,150°C where ignition begins to about 800 C at the exit of the hood. The temperature of the bed reaches about 1,300°C to 1,480°C. Depending upon characteristics of the ore materials and sintering conditions, average production rates of 22 to 43 tonnes/m2/day of grate area are expected, and rates in excess of 49 tonnes/m2/day have been attained. The major source of energy used in the production of sinter is the carbon content of coke breeze or anthracite and flue dust which supplies about 1.6 MJ/kg of sinter produced.