Furnace Heat Balance

Furnace Heat Balance

To melt steel scrap, it takes a theoretical minimum of 300 kWh/ton. To provide superheat above the melting point of 2768 F requires additional energy and for typical tap temperature requirements, the total theoretical energy required usually lies in the range of 350 to 370 kWh/ton. However, EAF steelmaking is only 55 to 65 % efficient and as a result the total equivalent energy input is usually in the range of 560 to 680 kWh/ton for most modern operations. This energy can be supplied from a variety of sources as shown in the table below. The energy distribution is highly dependent on local material and consumable costs and is unique to the specific meltshop operation. A typical balance for both older and more modern EAFs is given in the following Table:

		UHP FURNACE	Low to Medium Power Furnace
	Electrical Energy	50 - 60 %	75 - 85 %
INPUTS	Burners	5 - 10 %
	Chemical Reactions	30 - 40 %	15 - 25 %
TOTAL INPUT		100%	100%
OUTPUTS	Steel	55 - 60 %	50 - 55 %
	Slag	8 - 10 %	8 - 12 %
	Cooling Water	8 - 10 %	5 - 6 %
	Miscellaneous	1 - 3 %	17 - 30 %
	Offgas	17 - 28 %	7 - 10 %

Of course the above figures are highly dependent on the individual operation and vary considerably from one facility to another. Factors such as raw material composition, power input rates and operating practices (e.g. post-combustion, scrap preheating) can greatly alter the above balance. In operations utilizing a large amount of charge carbon or high carbon feed materials, up to 60 % of the energy contained in the offgas may be calorific due to large quantities of un-combusted carbon monoxide. Recovery of this energy in the EAF could increase energy input by 8 to 10 %. Thus it is important to consider such factors when evaluating the energy balance for a given furnace operation.

The International Iron and Steel Institue (IISI), classifies EAFs based on the power supplied per ton of furnace capacity. For most modern operations, the design would allow for at least 500 kVA per ton of capacity. The IISI report " The Electic Furnace - 1990" indicates that most new installations allow 900 - 1000 kVA per ton of furnace capacity. Most furnaces operate at a maximum power factor of about 0.85. Thus the above transformer ratings would correspond to a maximum power input of about 0.75 to 0.85 MW per ton of furnace capacity.