Rotary Hearth Furnace

The reader is urged to review sections 1.2, 4.3.2, and 4.6.1.2 for descriptions of rotary hearth furnaces—not to be confused with rotary drum furnaces described in section 4.2.3. 

Example 6.1 This is a case study of a 45 ft (13.7 m) diameter donut (see glossary) rotary hearth furnace, similar to figure 1.8, that was having problems with low production capacity. The inside cross-section dimensions of the donut-shaped, circular gas and load passageway (a circular tunnel furnace) are 4.5 ft (1.37 m) high x 12 ft (2.66 m) wide. Most of the furnace gas flow is counter to the load movement. 

If three baffles had been used, with a moveable baffle between the charge and discharge vestibules, the sawtooth roof rotary furnace would have delivered at least 

Evolution of firing methods for large rotary furnaces. Round furnaces had limited capacity and poor control of gas flow pattern. The first donut rotaries had burners through the sides of both inner and outer walls, but the inner circle of biuners were difficult to get to and to work on. 

The next method was called the sawtooth roof system, wherein each fired zone had one tooth of the sawtooth roof with burners firing through the vertical wall of the tooth toward the charge door, firing counter to the direction of product movement. This system was less expensive for larger diameter products and furnaces because it required fewer burners and less piping, especially if preheated combustion air was used. 

---

In the EASTMET process iron oxide fines (minus 0.1 mm), pulverized coal, and binder are mixed together and pehetized. The green pehets are heated in a dryer to remove moisture and fed to a rotary hearth furnace, where the pehets are placed on a flat rotating surface (hearth) in an even layer one to two pehets deep. As the hearth rotates the pehets are heated to 1250—1350°C, and the iron oxide is reduced to metallic iron in 6 to 10 minutes. 

The rotary-hearth furnace consists of a heating chamber lined with refractory brick within which is an annular-shaped refractory-lined rotating hearth. Around the periphery of the rotating hearth, sand or circulating hquid seals are employed to prevent air infiltration. It can be made semicontinuous in operation. The hearth speed can be... 

FASTMET A DR process, using pulverized coal and iron ore fines, heated in a rotary hearth furnace. Under development by Midrex Corporation and Kobe Steel from 1991 apilot plant was operated by Kobe Steel in Japan in 1996. 

Conventional thermal treatment methods, such as rotary kiln, rotary hearth furnace, or fluidized-bed furnace, are used for waste pyrolysis. Molten salt process may also be used for waste pyrolysis. 

Rotary drum vacuum filters, 11 345, 355-357. See also Drum filters Rotary dryers, 9 121-122 indirect-heat, 9 129 steam-tube, 9 129-130 Rotary fiber spinning process, 13 387 Rotary furnace technologies, 21 391 Rotary hearth furnaces, 13 177—178 Rotary impeller vane meters,... 

FASTMET A DR process, using pulverized coal and iron-ore fines, heated in a rotary hearth furnace. Under development by MIDREX Corporation and Kobe Steel from 1991 a pilot plant was operated by Kobe Steel in Japan in 1996. The first commercial installation was at Kobe Steel s Kakogawa plant in Japan in 2000. Further development of the process took place under the name ITmk3. A variation, known as FASTMELT, conveys the hot iron powder to an adjacent melter. See also MIDREX. 

Numerous publications (10 to 12) have appeared, principally from the GDR, on the required quality properties of brown coal and their influence on the quality characteristics of formed coke. Since the Rheinische Braunkohlenwerke AG is not engaged in formed coke production at present, raw material quality and coking behaviour are of interest only for the production of fine coke using the rotary hearth furnace (13> 14). 

The fNMETCO HTMR process uses industry standard equipment in a unique and patented process. At the heart of the fNMETCO process is the use of coal or carbon products to reduce oxidized metal wastes to their metallic form in a rotary hearth furnace. The technology has been adapted to produce direct reduced iron from ore concentrates, carbon steel waste products, or a combination of these materials. The... 

Fig. 8.38 Flow diagram and artist s impression of a recydmg plant for the production of secondary raw material (DRl) from iron-bearing oxide fines, employing a rotary hearth furnace (FAS-TMET) for the reduction of pellets with coal as reductant (courtesy Midrex, Charlotte, NC,...

Sealing the sides of a car hearth or of disc or donut hearths of rotary hearth furnaces is usually accomplished with sand-seals or water-trough seals. 

Horizontal straight-line continuous furnaces are more common than rotary hearth furnaces, rotary dram furnaces, vertical shaft furnaces, or fluidized bed furnaces. 

Multihearth furnaces (fig. 1.9) are a variation of the rotary hearth furnace with many levels of round stationary hearths with rotating rabble arms that gradually plow granular or small lump materials radially across the hearths, causing them to eventually drop through ports to the next level. 

Fig. 1.8. Rotary hearth furnace, donut type, sectioned plan view. (Disk type has no hole in the middle.) Short-flame burners fire from its outer periphery. Burners also are sometimes fired from the inner wall outward. Long-flame burners are sometimes fired through a sawtooth roof, but not through the sidewalls because they tend to overheat the opposite wall and ends of load pieces. R, regenerative burner E, enhanced heating high-velocity burner. (See also fig. 6.7.)...

In fuel-fired furnaces, the nature of the fuel may make a difference in the furnace design, but that is not much of a problem with modem industrial furnaces and burners, except if solid fuels are involved. Similar bases for classification are air furnaces, oxygen furnaces, and atmosphere furnaces. Related bases for classification might be the position in the furnace where combustion begins, and the means for directing the products of combustion, e.g., internal fan furnaces, high velocity furnaces, and baffled furnaces. (See sec. 1.2.4. and the rotary hearth furnace discussion on baffles in chap. 6.)... 

Rotary Hearth Furnaces Rotary hearth furnaces have no water-cooled skid pipes, so the soak zone can be less than one-fifth of the total furnace length. Very rapid heating results in a short heating zone, but requires a long soak zone for thick material. Rotary hearth furnaces have problems, such as ... 

Upgrading a Rotary Hearth Furnace. Overcoming Problem 1. The... 

Summary Actions to Improve Heating Capacity of Rotary Hearth Furnaces... 

Replace large burners in the center (doughnut hole) of large rotary hearth furnaces with high-velocity burners for better crosswise gas and temperature distribution. 

For metallurgical reasons, some rotary hearth furnaces are divided into sections by radial baffles. Rotary furnaces designed to heat rounds for seamless tube mills have some very special problems (1) furnace pressure control, (2) air/fuel ratio... 

Furnace Pressure Control. Extraction of load pieces may be as frequent as one to four pieces per minute therefore, door maintenance is difficult, with the result that discharge doors are often left open. These doors may be very large to accommodate a peel bar mechanism, so leaving a door open permits a large quantity of furnace gas to escape and results in loss of heat and furnace pressure. This problem, combined with the two-way combustion gas flow of a rotary hearth furnace, necessitates three baffles. This solution is described in the following paragraph. 

Air/Fuel Ratio Control. Air flows may differ to burners in parallel in the same zone on the inside and outside of a rotary hearth furnace donut because of the long runs of air duct and the large number of tees and elbows. High design air velocity creates very different air flows to burners in a zone. One such furnace was designed for an air flow of 70 ft/sec (21 m/s) with three elbows and four tees to each burner. The fan s discharge pressure was 14"wc (3.5 kPa), but the pressure delivered to one burner... 

Rg. 6.5. Sectional view of a rotary hearth furnace (such as fig. 1.8) with enhanced heating. This also could be a car-hearth batch furnace or in-and-out batch-box furnace. In many cases, the higher velocity burners would be smaller (relative to the main burners above) than they appear in this drawing. In other than rotary hearth furnaces, the high-velocity burners should fire between piers and opposite the main burners—to further enhance circulation. 

---