HOT BLAST OPERATION

Hot blast operation involves preheating the cupola blast air and was originally conceived as a means of reducing coke consumption. It was first adopted for foundry use in the late 1940s, early 1950s and by heating the blast air to about 500ºC, charge coke reductions of about 30% were experienced compared to cold blast operation. However, hot blast could also be employed to increase metal temperatures and carbon pick-up and this permitted the use of more steel scrap in the charge mixture.

The advantages of hot blast operation may be summarised as follows:

 reduced coke consumption

 increased metal temperature

 higher melting rate

 reduced sulphur pick-up

 lower melting losses of silicon

 increased carbon pick-up.

It is not possible to obtain all of these benefits simultaneously.

Hot blast cupolas have not been universally used in all European countries but have been very popular in Germany. In many countries initial environmental legislation was more stringent for hot blast cupolas than for cold blast. This is not the case now and the cost of the recuperator is relatively easy to justify on operating cost savings.

Most hot blast cupolas are operated on long campaigns, many with externally water cooled unlined shells in the melting zone. Several of this type of furnace have been used for the production of low sulphur ductile base iron using basic slags.

Blast heating has been carried out using both independently fired and recuperative hot blast systems. However, the high fuel costs and generally poor performance of the independent units has resulted in recuperative systems incorporating combustion of the cupola off-take gases being the most common arrangement.

In recent years there has been an interest in the use of higher blast air temperature in excess of 700ºC. Such very high blast temperatures will result in further enhancement of recarburisation of the iron, while the lower blast rates for a given melt rate and the higher bed temperatures should allow the use of smaller charge pieces such as borings to be successfully melted. It has been suggested that superheated hot blast systems will allow lower grade, smaller coke to be employed without the tapping temperature decrease, which would be experienced with conventional cupolas.

Oxygen Enrichment

Although the benefits of oxygen were known for a considerable time, it was only in the 1970s when the costs of bulk oxygen, pig iron and coke were such as to make its employment economical that it came into common use.

Compared with conventional operation, the continuous use of oxygen resulted in:

 higher metal temperatures and carbon pick-up and lower silicon losses at the same coke levels – allowing metallic charge costs to be reduced by pig iron replacement and a reduction in silicon additions

 reduced coke consumption for a given temperature

 improved tapping temperature recovery at a start of melting or following shutdown periods.

The oxygen could be introduced by three different processes – blast enrichment, tuyere injection, or well injection – in order of increasing effectiveness. However, blast enrichment was simpler and the majority of cupolas adopting oxygen technology employed this system.

More recently, the use of supersonic oxygen injection into cupolas via the tuyeres is becoming an accepted technique and a number of cupolas in Europe have installed such facilities. In this process the tuyere lance nozzles are specially designed to provide an outlet velocity in the range 2–2.5 Mach. It is claimed that this approach results in better oxygen and air blast penetration with a consequent improvement in coke bed temperature.

The injection lances are self-cooled by gaseous oxygen and are mounted centrally in each tuyere (generally tuyeres are water cooled) at a distance of between 100 and 300 mm from the exit. It is claimed that the following effects result:

 blast volume is reduced

 blast air/oxygen distribution is more uniform and furnace internal pressure is reduced

 heat losses from cupola are reduced due to more even coke bed combustion

 charge preheating is improved because there is less temperature variation over the cross-section of the cupola

 charge coke additions are reduced

 melting rate variations of -50% to +40% of nominal melting rate are possible

· furnace shell losses are lower as higher temperatures are achieved in the centre of the cupola

 metal tapping temperature is increased

 silicon losses are reduced

 the blast temperature on hot blast cupolas is increased by the higher off-take gas temperature.

It is also claimed that the process is the first oxygen method which demonstrates a reduction in coke consumption at a constant melting rate such that coke savings fully compensate for the cost of oxygen. It is further suggested that it should be possible to use lower grade coke with consequent further cost savings.

Long Campaign Cupolas

In recent years there has been considerably increased interest in operating cupolas for extended periods, both on a daily basis and also from a refractory campaign point of view (repair after weeks rather than after melting day).

Obviously the well-established hot blast technology already discussed in an earlier section can fulfil this role successfully, However, largely as a result of work in the USA, a number of cold blast units has been adopted for the same purpose.

The cold blast furnaces include similar features to those found in hot blast plants in that the cupola shells (which may be lined or unlined) are fully water cooled and are fitted with water cooled projecting tuyeres. Tuyere blast velocities are considerably higher than conventional cold blast practice to ensure complete penetration of the coke bed, the generation of very high bed temperatures and a minimisation of heat losses, particularly through an unlined shell.