Thermal Control

This procedure describes the measure to be taken for majors changes in the thermal balance of the furnace. It is emphasised that the action describe are guidelines, and before any action is taken the recent furnace operation and process changes are to be considered.

Changes to the furnace thermal balance should be made taking the following relationship into account.

Parameter	Extent of change	Effect on coke rate (kg./Thm)	Time to Affect Metal Temp. (hrs)	Effect on Metal Temperature
Blast temperature	+100˚C	-5.5	5	+22.5˚C
Blast moisture	+10g/Nm³	3.5	6	-14˚C
Hot Metal Silicon	+0.1%	+2.0		-8˚C
Hot Metal Temp.	+10˚C	+2.5
Co Utilisation	+1%	-4.5	7	+18˚C
Coke Ash	+1%	+4.0		-16˚C
Slag Volume	+10kg/THM	1.2		-5˚C
Heat Flux	+2Gcal/hr	+1.0	7	-4.0˚C
Coke Rate			24
Coal Rate			12 ?
Blast Volume			5

The data is based on Redcar Furnace.

The heat content of coke is 5233Kcal/kg (90%C dry)

The normal order of priority for parameter change is:

1^st      Blast moisture

2^nd     Coke rate

3^rd      Coal rate

4^th      Blast temperature

Assume the heat content of coal is the same as for coke for small changes. ????

When assessing trends in metal quality the following time constant should be taken into account.

Gas Utilisation

Changes in CO utilisation often reflects previous parameter changes (e.g. reduced/increased coke rate) and metal temperature trends. Thus it is important to consider such changes when considering adjustment to the thermal balance based on CO utilisation. A recalibration of the top gas analyser should be initiated if there is a marked change in Eta CO which is not consistent with the current furnace operation. (is there a back up analyser ???, If there is still doubt the back up analyser used as reference, problems arise with both analysers ) Contact the laboratory and request hourly samples to be analysed.

If a change in gas utilisation is identified as an indicator of future metal quality then following should be considered.

If Eta CO shows a significant downward trend compared to the previous 8 hours (e.g. 1% lower for 4 hours or 2% lower for 2 hours) indicating a cooling trend, then the coke rate should be increased on the basis of 1% Eta CO = 4.6%Kg/THM

If Eta CO shows an upward trend then no compensation is to be applied until metal quality shows a warming trend. Then the compensation will be by blast moisture initially and may later be ‘traded’ for a decrease in fuel rate.

Reduced Blast Volume

Reduced blast volume may result in increased metal temperature. This should be taken into account when considering changes to the thermal balance.

A Racing Furnace

If the true charging rate increases above the norm and is maintained, it may result in a cooling trend. This should be taken into account when considering changes to the thermal balance.

Hot blast temperature and flame temperature

If the hot blast temperature is lower than the aim for more than 4 hours then a fuel rate equivalent change is to be made to restore the thermal input. However if by reducing the blast moisture results in a flame temperature above 2250˚C then the coal rate or coke rate should be increased instead.

Heat Flux

Heat flux may change because of the influences of raw materials quality, burden distribution or liquids removal. Significant and prolonged changes in heat flux should be compensated on the basis of 1kg/THM fuel for each 2Gcal/hr changes in heat flux. Caution should be exercised when taking action in response to heat flux – other parameter changes may affect the actual compensation required.  

Stock Sinter

Extensive use of stock sinter may lead to a warming trend in the hot metal. This is due to the moisture content of stock sinter causing a reduction in the dry weight changed. This should be taken into account when considering changes to the thermal balance.

Burdening Policy

If the burdening policy is altered then the fuel rate is to be maintained. Whilst also taking account any changes to the slag volume.

Taphole changeovers

A dip in metal quality may be observed at taphole changeovers and consequently req1uires caution when assessing quality trends at such times.