Raceways are vital regions of the blast furnace (BF) even though their total volume usually does not exceåd 1% of the inner furnace volume. They supply the process with heat and reducing agents. The role of the raceway becomes even more significant when injecting auxiliary fuels, wastes and/or non-combustible materials.
Theoretical analysis of the heat exchange phenomena in the raceway has been done considering heat transfer by convection and radiation in order to optimise the flame temperature. The raceway oxidising potential has been investigated with co-injeñtion of different fuels.
Experimental studies on raceway phenomena have been carried out using the following methods and facilities at RWTH Aachen, DNTU Donetsk, CENIM, Madrid, Thyssen Krupp Stahl, Duisburg and ACERALIA Corp. Siderurgica, Gijon:
• cold models for exploration of cavity formation;
• mini scale hot injection rigs for investigation of behaviour of PC and other solid materials in the tuyere and oxidising part of the raceway;
• pilot injection plant that simulates conditions in the raceway to the fullest extent;
• coke gasification chamber for investigation of dimensions and processes in the raceway;
• measurements in an industrial BF using a laser technique to determine the raceway extension.
The interaction between coke and blast, raceway formation, dimensions with and without pulverised coal injection (PCI), effect of different injectants and their mixtures on combustion, reduction processes and other phenomena in the raceway have been investigated using the above mentioned equipment and are discussed in this paper.
Tuyere apparatus designs for super high injection rates, for co-injection of various auxiliary reductants, process oxygen and hot reducing gases generated outside the BF are also presented.
To maintain and improve the competitiveness of the blast furnace process, it is necessary to achieve a further considerable decrease in coke and total energy consumption for primary metal production along with minimisation of environmental impacts.
The potential for coke saving by means of improvement of burden preparation, increase in blast temperature and top pressure are exhausted to a considerable extent. Injection of auxiliary fuels such as natural or coke oven gases, oil, pulverised coal and organic wastes has made a considerable contribution toward coke saving in the last two decades.
Coke consumption of about 280-300 kg/t-HM has been achieved at some BFs when the rate of PCI was 200-230 kg/t-HM1-3. Theoretical, laboratory and pilot investigations as well as the latest industrial experience show that the PC rate can be raised up to at least 250 kg/t-HM and BF operating with coke/coal ratio = 50/50 can be maintained.
On the other hand, the average PC rate in Europe rarely exceeds 130-150 kg/t-HM4 (but Corus IJmuiden BF6 operated with a PC rate of 204 kg/t-HM and coke rate of 316 kg/t-HM5) mainly because of problems with complete combustion within the raceway, gas permeability in the shaft, dirtying of the deadman and as a result irregular furnace operation and decrease in productivity. Increased PC rate up to the record level of 266 kg/t-HM under perfect burden and operational conditions at Fukuyama No.3 BF (NKK, Japan) allowed NKK to achieve the coke rate of about 290 kg/t-HM6. Total consumption of injected fuel, including co-injection of natural gas, PC and/or oil does also not exceed 180-230 kg/t-HM7. It should be emphasised that extra high level of PCI or, generally speaking, of tuyere injection is not the aim but a means for coke rate reduction.
To maintain stablå furnace Operation at a higher PCI rate than that which can presently be achieved for only a Short period of time, further study is needed regarding coke quality, raceway phenomena, the influence of gasification, char formation and other processes on BF Performance under actual operational conditions.
Injection of fossil fuels is nevertheless limited because of drop in the flame temperature and further problems in the deadman region and the cohesive zone. The next step for further coke saving should be injection of hot reducing gases.
The maintenance of sufficient oxidising potential and necessary flame temperature (or more generally, thermal State) in the raceway aro pre-requisites for successful blast furnace Operation using tuyere injection.
Compensating for the changes in the raceway Parameters and generally in the blast furnace Operation with injectants (thermal State, slag regime, gas dynamics) is the way for the effective use of gaseous, liquid and solid auxiliary fuels introduced into the hearth through the tuyere apparatus.
The coal combustion Chamber at CEN1M Madrid, Spain simulates conditions in the raceway to the fullest extent (Fig. 1)". The central dement is a combustion Chamber 1810 mm long and 430 mm maximum diameter with a tuyere of 60 mm diameter. In the Chamber several orifices are arranged in order to measure the temperatures and gas composition. Temperatures of air and gas along the Chamber (3 measurement points) as well as gas composition are continuously recorded.
Fig. 1 Pilot injection plant at CENIM Madrid, Spain. Blast temp = 1000-1100"C, 02 up to 25%, 60mm tuyere, PC flow = 40-55 kg/h.
Numerous research studies have been completed using the facilities described in Section 3.1 to investigate the effect of the amount of injectants and their chemical, physical, petrographical and mechanical properties and lance geometry on the ignition and combustion behaviour of different coals, iron and carbon containing dusts and Sludges, wastes as well as many desulphurising, catalytic and other additives and various mixtures. In this section the most important of already published results are summarised and discussed and recent studies are presented.
For complete conversion, and thus effective utilisation of the injected materials, an optimal mixing of the particles with the hot blast at the lance tip must be achieved. Healing up, degassing, pyrolysis and burning must take place in the period between the entrance of the solid into the hot blast and the end of the oxidising atmosphere of the raceway within approximately 15-30 ms.
Low ash coals promote high efficiency PCI. The content and physical properties of coal ash play an important role in optimising the design of the tuyere apparatus, processes in the raceway and the slag regime. The effect of ash softening and melting characteristics on the above parameters is explained as follows.
The temperature of coal particles and the gaseous phase around (hem incrcascs from the value of blast temperature, as they move through the tuyere cavity, to the value of the flame temperature during coke residue burning in the raceway. If there is a greater distance between the tuyere nose and the PC lance tip, a higher blast temperature or an increasing level of coal grinding, the temperalure of PC particles and gaseous phase at the tuyere nose becomes higher. If the PC ash softening temperature is approximately equal to the temperature within the tuyere cavity, ash melting and sticking on the inner surface of tuyere is possible.
Therefore it is necessary to take into account that although moving the PC lance position away from the tuyere nose improves PC combustion conditions due to a longer residence time of PC particles in the tuyere, there is the danger of of ash sticking. From the viewpoint of tuyere protection, PC delivery through the tuyeres could be more advisable than into the blowpipe, depending on PC ash properties.
The following measures intensify PC combustion in the raceway.
• Enriching the blast with process oxygen. However, the non-linear effect of blast oxygen on the degree of PC combustion should be taken into account: the increase in the PC combustion rate becomes smaller with a rise in oxygen content.
• Preliminary mixing of PC with process oxygen before introduction into the tuyere cavity.
• Use of coal blends (usually coals with high and low content of volatile matter) and fuel mixtures to maintain both high combustion degree and high coke/coal replacement ratio.
• PC injection with iron oxides (fine iron ore, iron-containing waste, etc.), carbonates and other oxygen-rich additives.
• Use of chemical and physical phenomena, e.g. catalytic, polarising and other effects.
• Optimisation of coal grinding, depending on operating conditions and coal properties.