Combustion Problems in the Steel Industry

Page 3

produce soaking pit t.emperatures end to end and top to bottom in the area of the charge which are within 10 to 20F as illustrated in Figure 7 typical temperature results are il­lustrated in Figure 8 This high degree of temperature uniformity within the pit and charge are achieved through the use of variable heat release burners such that the flame pattern of the burner and its heat release pattern can be altered to satisfy the temperature conditions existing in the pit at any point in time and such that the heat release pattern within the soaking pit can be adjusted to maintain temperature differentials within the pit of 10 to 20F throughout the heating cycle. With the advent of continuous casting it was considered by many that the days of the soaking pit were numbered. Such would not necessarily appear to be the case. Certain grades of steel such as rim steel cannot be produced by the continuous casting method and are being demanded by steel users for difficult draw operations. These grades of steel can only be produced from the ingot form and, therefore, require reheating in soaking pits for subseqent rolling, therefore, a number of soaking pits will be required in the steel industry to satisfy the more severe metallurgical requirements demanded by steel users. In many cases the use of continuous casting permits the shutting down of old slabbing and blooming mills where ingots were rolled into slab or bloom form thus eliminating the high cost of conversion of the stee~ shape into a saleable product. Some mills, in an effort to reduce the high cost of double heating and rolling while at the same time obtaining some of the advantages of rolling from the ingot form, are successfully rolling direct from the ingot to the finished product thus very substantially reducing their cost of production and improving their competitive position. ~ben close temperature tolerance within the ingots of a soaking pit ch8rge is realized, damage and premature wear on the mill in sub­stantially reduced, internal stress cracking and edge cracking of the product are virtually eliminated and section tolerances within the finished rol2.ed product are brought "7ithin fully acceptable world standards. REHEAT FURNACES The largest users of fuel within the steel plant are the boiler houses, blast furnaces and steel reheating furnaces. Blast furnaces ape boiler houses, in large measure, utilize internally generated fuels. Reheating furnaces, in some cases, utilize internally generated coke oven gas, however, much of thi~ equipment ie operated on purchased fuels such as natural gas and oil. The largest user of purchased fuel is the re­heating operations within a steel mill. It is, therefore, into this area where our efforts should be concentrated to effect meaningful improvements. 15 Rehea t furnaces fall into two main groups -­top fired furnaces and top and bottom fired fur­naces. Figures 9, 10 and 11 illustrate typical top fired furnaces of single, multi-zone and walking beam types. The single zone top fired furnace frequently creates a problem since it has only one heat source and the flame must pass over the already hot steel to bring the steel further back in the furnace to temperature as illustrated in Figure 9. The multi-zone top fired furnace as illustrated in Figure 10 overcomes this problem since fuel input to the zones can be regulated to meet specific heat demand in the various zones of the furnace. These furnaces still have the limitation that stock sections above 5 to 6 inches cannot be properly heated without excessive temperature differential within the piece. Great care must be taken to avoid overheating of the steel sur­face with resultant adverse metallurgical results. This condition is often brought on by attempts to obtain higher than practical tonnage output. The top fired walking beam furnace, Figure 8, permitted spacing out of the bars to be heated in such furnaces to attain more rapid heating of the bars and substantially reduced retention time in the furnace. Generally these furnaces helped in attaining closer temperature uniformity in the steel being heated and reduced slab loss. Top and bottom fired reheat furnaces of 2, 3, 4, and 5 zones as illustrated in Figures 12, 13, 14 and 15 respectively permit the heating of thicker steel sections than is possible on top fired furnaces, however, have the disadvantage that the steel slabs or blooms must be supported on water cooled skid structures within the fur­nace enclosures. High insulation coverage of these skid systems is essentiaJ to avoid exces­sive fuel demand. Net heat loss to the water cooled members can be as high as 70,000 to 100,000 BTU per square foot of water cooled member. TIlese furnaces can have from 750 to 6000 square feet of water cooled surface within the furnace and in continuous strip mills from three to five furnaces must be operated at any one time to satisfy production demand. Insula­tion coverage of 85 to 90 percent on these water cooled members is imperative in today's market to achieve reasonable fuel rates. The high production mills frequently use the fiove zone pusher furnace illustrated in Figure 15 or the walking beam furnace illustrated in Figure 16 and 17. These furnaces can have up to 16 zones of control and can be zoned both longitudinally and transversely to attain specific heat profiles in the steel. The in­clusion of a number of zones permits the instal­lation of process equipment and control concepts to minimize fuel usage per ton processed while at the same time improving overall heating quality, thus improved shape and gage in the finished product. The knowledge is available in the United States to meet or surpass fuel rates being achieved on such furnaces anywhere in the world.