Emerging trends in improving Blast Furnace (BF) performance of ironmaking – More environment-friendly and cost effective proposition:
Blast furnace (BF) technology is the central to the crude steel industry and is continually undergoing refinements to improve productivity and reduce operating costs. Continuous improvements in productivity, coke consumption and fuel use within steel works have been driven by competition in world steel market.
The major raw materials generally used in blast furnace technology are high quality of iron ore (lumps, sintered or pallets), coal (ranging from coke to coking coal) as fuel, limestone and dolomite as fluxing materials.
A. At present, the real challenge in ironmaking industry by blast furnace lies to ensure that each process meets the emission limits prescribed by law and for that purpose each process is to examine for (a) use of energy, (b) use of material, (iii) waste generation for the entire life cycle of the project. These factors are believed to be root cause of environmental degradation. Some the processes which can economize on energy, process waste and material inputs are:
(i) High temperature hot blast technology for Blast Furnaces for lower energy consumption.
(ii) Cast House Slag Granulation Technology for utilizing Blast Furnace slag.
(iii) Ceramic welding technology for increasing life and reducing energy loss in coke oven.
(v) Continuous casting technology for reducing energy consumption and process waste in steel casting.
(vi) Water treatment technologies for economical water management.
B. As mentioned, coal / coke is an important raw material, which constitutes about 42% of the cost of sales and 58% of the raw material cost of an Integrated Steel Plant. The conventional blast furnace route of iron making needs prime coking coal. In order to reduce the cost of fuel (coke) and to use more of coking or non-coking coal replacing most of coke, many of the emergent technologies have been developed and tried. Many of these coals, extractable through lowcost mining, can be injected directly through the tuyeres of blast furnaces, substituting good quality coke in a very cost-effective manner.
C. In one example, during the implementation of improved method of coal injection to blast furnace, this inferior coal replaces almost same weight of blast furnace grade coke, which is produced from almost one and one-half unit of prime coking coal (at more than double the cost), after carbonization for several hours at substantial processing cost. The coke ovens are very capital intensive and pollute the atmosphere. Thus, coal injection not only reduces the operating cost, but also saves capital expenditure substantially, while maintaining greener and cleaner environment. More important and helpful to the blast furnace (BF) operator is that it facilitates operational stability and optimization by providing endothermic heat to control the combustion. There are two modes of coal injection:
(a) The pulverized coal injection (PCI) and
(b) The granular coal injection (GCI).
Most of the newly constructed blast furnaces generally have installed PCI, a very common technology available everywhere. However, the existing blast furnaces with no injection facilities can go for GCI, which is less energy intensive and more environment friendly. The savings using appropriate variety of low cost non-coking coal would be of the order of about US$ 4 to 6 million per annum per blast furnace of 1 Mtpa capacity.
Experts opine that, for coal injection, GCI is emerging as a more cost effective and energy efficient technology.
D. The advantage of GCI over PCI is given here:
(i) Reduction in coal preparation costs due to low energy consumption (GCI: 20kWh/t, PCI: 32kWh/t);
(ii) Easier to handle in pneumatic conveying systems since granular coal is less sticking to the conveying pipe.
(iii) System availability is more;
(iv) Granular coal’s coarseness delays gas evolution and temperature rise associated with coal combustion in the raceway. Therefore, it is favorable compared to PCI because of less likely generation of high temperature and gas flows at the furnace walls, which results in high heat losses, more refractory wear and poor utilization of reducing gases;
(v) Granular injection system is superior while using low volatile coal to avoid line plugging and other related problems. Thus, the use of granular coal may increase the range of coals available for blast furnace injection.
(vi) There is a significant economic advantage to using granular coal over pulverized coal, since not only is less grinding equipment required resulting in capital savings, but operating costs are also reduced as approximately 60% less grinding energy is required for granular coal. Capital including infrastructure costs for GCI are lower than those for PCI. (PCI ~ US$ 43,000 whereas GCI ~ US$ 37,500 per daily ton of injected coal).