Preheating scrap preheater

Ferrous foundries consist of two types steel foundries in which electric furnaces (EAF and induction) are used, and iron foundries in which hot-blast cupolas and/or electric furnaces are used. Electric furnaces use virtually 100% scrap charges. Cupolas are shaft furnaces which use preheated air, coke, fluxes, and metallic charges. Scrap is over 90% of the metallic charge. Cupolas accounted for about 64% of total iron foundry scrap consumption in 1994 and electric furnaces accounted for about 34%. The balance was consumed by other furnaces, such as air furnaces. Iron foundry products have a high carbon content and the scrap charge usually contains a high percentage of cast iron or is used in combination with pig iron. 

When the furnace is tilted toward the charging floor, which is on a platform above ground level, soHd scrap is dumped by an overhead crane into the mouth of the furnace. Scrap can form up to 30% of the charge unless it is preheated, when up to 45% may be used. The crane then moves away from the furnace and another crane carries a transfer ladle of molten pig iron to the furnace and pours the molten pig iron on top of the scrap. 

Seven cement kilns in the United States utilize about 6 million scrap tires per year to replace conventional fuels. Cement kilns appear to be ideal for scrap tires because of their high operating temperatures (2,600 F) and good conditions for complete combustion, which minimize air pollution problems. Also, there is no residue, since the ash is incorporated into the cement product. Of the 240 cement kilns in the United States, about 50 are equipped with precalciner/preheaters, making them most suitable for tire combustion. 

In the U.S. there are approximately 240 active cement kilns (52). Of these, there are 50 precalciner/preheater kilns built since 1971, which would be the kilns most likely to burn tdf. However, about 20 percent of these kilns are at locations, such as the southeast Gulf coast, where they can probably obtain petroleum coke at a lower price than tdf, and hence they would not be likely tdf buyers. The remainder could become tdf users if the economics and the environmental permitting procedures were favorable. If 40 cement plants each used the equivalent of 2 million tires per year, there could conceivably be a national usage of 80 million tires per year, or one third of the annual number of scrap tires generated. 

The reactor consisted of a quartz tube (0j = 10 mm) packed, upstream from the catalyst, with quartz scrap to help with inlet gas preheating. The gas flow was 400 standard litres per hour, giving a gas space velocity in the catalyst of 50 000 h"l. A thennocouple placed 5 mm in front of the monolith inlet side was used for temperature measurement. 

The calculated operational costs include depreciation of the investment cost, but exclude personnel costs. The cost difference between the various techniques is rather small. The calculation takes account of higher cokes consumption and more expensive scrap/raw material costs for the cold blast cupola. In the case of air preheating using a plasma torch, the increased electricity use leads to increased costs. In practice, this increase may be balanced by an increased usage of swarf as raw material. The cokeless cupola shows the lowest cost. In this evaluation, however, local cost effects and uncertainties, such as the effect of hourly rate, have not been taken into account, but need to be considered. The quoted data are all for installations with a significant hourly rate. This disfavours the performance of cold blast cupolas. 

Fig. 3.27. Scrap preheater with high-momentum flames driving through the interstices of iron scrap, to preheat it prior to big ladie melting, and to incinerate paint and oil on the scrap.

Furnace feed consists primarily of sinter of the appropriate composition to give the required slag formulation, and coke preheated to around 700°C. Other feed materials such as briquettes made from zinc oxides, and metallic scrap can also be added to the furnace feed. Feed is charged to the furnace using weighed buckets, which are dumped into a double bell seal hopper arrangement located in the furnace roof. The bell seals are water-cooled steel construction and efficient sealing is very important. 

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