Objective
To partially recover the energy contained in the exhaust gas in the EAF Nr.4 to preheat the scrap metal being loaded in the same furnace. The recovered energy heats air which then passes through the scrap avoiding oxidation and fouling by dusting fumes.
Measurements have been carried out to determine preheating effects on operation parameters of the electric furnace and to continuously record process variables in the single stages of the preheating and of suction and dedusting of fumes. The test program included preheat operations.
Exhaust fume temperature had an average value below the foreseen one. This was due to problems concerning the connection of the plant and the existing furnace and the way of operation of the suction of fumes. Therefore the radiation exchanger efficiency was not the optimum one. Furthermore, the efficiency was compromised by fouling and inadequate thermal insulation. The high temperatures of the air which had been fixed in the plant design stage of the work, brought troubles to the basket opening system; therefore the preheating air temperature was limited at 400-500 C and did not reach the design 800 C and also if burner contribution is neglected, energy savings of the electric furnace were lower thanthe ones foreseen in the design stage. With reference to the data taken during the last measurements campaign the saving of energy for melting was only 29,48 kWh per ton of liquid steel against the 45 forecasted by designers; melting time saving is 5 minutes against the foreseen 10; the electrodes consumption is 4,5% lower in comparison with conventional plant, and not 7% lower as it had been foreseen. These results were due to the lower preheating temperature (400 C) in comparison with the design one (800 C), to the lower flow (9000 nm3 per hour against 10 000 nm3 per hour) and to the shorter preheating time (50 min against 60) an economic evaluation was carried out pointing out that to-day saving due to the intervention is about 354 M LIT per year and that is would increase to about 545 M LIT per year with a completely efficient heat exchanger. Work on the plant to improve its performance was being carried out and energy savings have gradually increased. It should also be notedthat the plant operated reliably during the project.
The equipment consists principally of two independent circuits:
- one to extract the hot fumes from the furnace Nr. 4 and convey them to a gas/air heat exchanger which extracts part of the energy and redirects the fumes into the scrubber, and
- one to pump ambient air through the heat exchanger and through the baskets carrying the scrap metal.
A medium power burner is located between the heat exchanger and the loading baskets to regulate the air temperature. Through the heat exchanger the air reaches 600 deg C and, 800 deg C after the burner. It is maintained at this temperature irrespective of variations in heat exchange conditions. A demonstration plant was built at "Indumental S.p.A." in San Giorgio in Nogaro (Udine) in which a furnace with a 20 MVA transformer is operating. The final product is 150,000 t/year of continuous casting carbon steel billets of commercial quality. The construction phase involved the installation of the heat exchanger and fan, the piping structures, the air heating furnace, the instrumentation and electric system.
The energy saving was forecast at about 180,000 TOE/year and the payback time about 3-4 years. Initial results necessitated plant modifications, mainly:
a) replacement of a radiation exchanger by a convection one
b) replacement of the dust extractor
c) installation of a cleaning system by vibration
d) connection of exhaust system behind the basket with the scrubber.
Programme(s)
Topic(s)
Call for proposal
Data not availableFunding Scheme
DEM - Demonstration contractsCoordinator
33042 Udine
Italy