Heat Recovery in the Iron & Steel Industries
Background
The Iron & Steel industry employs several high temperature furnaces for sinter, coke, iron, and steel production, which account for about 58% of the industry’s energy consumption. While recovery from clean gaseous streams in the industry is common, heavily contaminated exhaust gases from coke ovens, blast furnaces, basic oxygen furnaces, and electric arc furnaces continue to present a challenge for economic waste heat recovery. Heat recovery techniques from these dirty gaseous streams are available, yet implementation has been limited due to high capital investment costs. (US DOE)
Iron and Steel Mills
Steel mills have a number of high temperature heat recovery opportunities. In integrated mills, waste heat can be recovered from coke ovens, blast furnaces for iron production, and basic oxygen furnaces for steel production. There are also opportunities to recover waste heat from the electric arc furnace in steel “mini-mills” that produce steel largely from recycled scrap. About 46% of steel production in the U.S. now comes from these mini-mills.
This section examines WHR opportunities from coke ovens, blast furnaces, basic oxygen furnaces that are major energy consuming parts of integrated steel production, and at electric arc furnaces that are the major energy consuming process at steel mini-mills.
Coke ovens
Producing coke is an essential part of blast furnace operations. The most commonly used process is the byproduct process. In the byproduct process chemical byproducts (tar, ammonia, and light oils) in the coke oven gas are recovered, while the remaining combustible coke oven gas is cleaned and recycled within the steel plant. Waste heat could be recovered from the hot gas exiting the coke ovens at 1200-1800F; however, these gases are full of tars and contaminants that would make heat recovery difficult. Some steel mills in Japan recover about a third of the energy contained in the hot coke oven gas keeping the exit temperature above the condensation point for the tars – about 840F. Another source of waste heat in coke ovens is the waste gases exiting the flue at 400 °F from combustion of the recycled and cleaned coke oven gas.
Blast Furnace
The blast furnace converts iron ore into pig iron. Older blast furnaces had high exhaust temperatures around 900°F. New furnaces have been designed for more efficient heat transfer; consequently, exhaust gases are in the low temperature range. Blast furnace gas is itself a low Btu fuel that is recovered and used within the mill, often blended with other fuels to increase its heating value.
Basic Oxygen Furnace
The basic oxygen furnace (BOF) uses oxygen to refine pig iron into steel. The heat required for the refining and melting process is provided by the exothermic reaction within the furnace. For U.S. production, the very high temperature off-gases from the BOF equal 27 TBtu/yr of waste
heat. BOF gas has a high concentration of carbon monoxide, and like coke oven gas and blast furnace gas, BOF gases offer opportunities for recovery of chemical energy and sensible heat. Heat recovery is more costly and maintenance intensive due to contaminants in the exhaust stream.
Electric Arc Furnace
About 46% of total U.S. steel production comes from scrap-based “mini-mills” that use an electric arc furnace to melt and refine scrap into new steel. Waste heat typically exits the Electric Arc Furnace (EAF) at about 2,200F. The heat can be captured in a waste heat recovery steam boiler for conversion to power, use in district heating operations, or in other on-site needs. The most common form of heat recovery in EAF operation is scrap Preheating. (EPRI)
Stinter plants
In a sinter plant, sensible heat can be recovered both from the exhaust gases of the sinter machine and the off-air of the sinter cooler. Heat recovery can be in different forms:
- Hot air streams both from sinter machine and the sinter cooler can be used for the generation of steam with the installation of recovery boilers. This steam can be used to generate power or can be used as process steam. For inreased heat recovery efficiency, a high-temperature exhaust section should be separated from a low-temperature exhaust section and heat should be recovered only from high-temperature exhaust section.
- Sinter machine exhaust can be re-circulated to the sinter machine, either after going through a heat recovery boiler or without it.
- Heat recovered from the sinter cooler can be recirculated to the sinter machine or can be used for preheating the combustion air in the ignition hood, for pre-heating of the raw mix to sinter machine. It can also be used to produce hot water for district heating. (IETD)
- A good summary of stinter plants can be found here
Examples
- Steel plant converts waste heat from the stinter into electricity, reducing electricity consumption by 6% [link]
- Kalina cycle heat to power generator installed at a steel plant in Japan in 1999 [link]
- Organic Rankine Cycle unit generates electricity from the heat of the off-gas treatment system of the melting electric furnace. in Germany [link]
Further Reading