Blast preheat

Increased blast preheat. The temperature has increased from about 650°C to up to 1 lOO C. The low calorific value of the furnace waste gas prevents temperatures above this level fnrm being obtained. It has been demonstrated that an increase of 100°C in... 

The manner in which carbon is utilised for zinc production is affected by heat balance issues as well as chemical requirements for metal reduction. The heat balance is influenced by the amount of slag produced, the blast preheat and the preheat achieved in coke and sinter. Apart from the quantity of zinc oxide reduced, the chemical factors include the lead and iron contents of sinter and the moisture entering the furnace in blast air and in sinter or additives. Based on typical operating conditions as indicated above and a lead production of close to 50 per cent of the zinc production, the empirical formula used to relate carbon consumption to zinc production is given by Equation 6.1 ... 

The combustible components of the gas are carbon monoxide and hydrogen, but combustion (heat) value varies because of dilution with carbon dioxide and with nitrogen. The gas has a low flame temperature unless the combustion air is strongly preheated. Its use has been limited essentially to steel (qv) mills, where it is produced as a by-product of blast furnaces. A common choice of equipment for the smaller gas producers is the WeUman-Galusha unit because of its long history of successful operation (21). 

Air for the hot blast may also be considered a raw material. The air is preheated in stoves to between 900 and 1300°C. Over 1.5 t of air is required to produce 11 of hot metal (pig iron). SoHd, Hquid, or gaseous fuels, eg, coal, fuel oil, or natural gas, may be added to the hot blast at the tuyeres to replace some of the coke. Oxygen may also be added to the hot blast to increase flame temperature. 

Blast air, preheated to 650°C, is deflvered by centrifugal blowers through a refractory-lined busde main to the furnace. Zinc vapor from the reduced sinter is carried out with the furnace gases to a condenser fitted with mechanical rotors that are partly immersed in a shallow pool of molten lead. The lead flows countercurrenfly to the gas and is vigorously agitated by the rotors to create an intense shower of lead droplets throughout the condenser. 

For gas-fired systems the state-of-the-art is represented by the preheater described in Reference 69. A pebble bed instead of a cored brick matrix is used. The pebbles are made of alumina spheres, 20 mm in diameter. Heat-transfer coefficients 3—4 times greater than for checkerwork matrices are achieved. A prototype device 400 m in volume has been operated for three years at an industrial blast furnace, achieving preheat temperatures of 1670 to 1770 K. 

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. 

Heat recovery from furnace off-gas (LCV gas) normally used to preheat coke and blast air LCV gas has also been burned to heat melting baths and generate steam and power. 

Ruhrgas (1) A coal gasification process developed and used in Germany. Pulverized coal is gasified by a blast of preheated air in a vertical shaft, the temperature reaching approximately 1,500°C. 

Blast-Furnace Stoves Blast-furnace stoves are used to preheat the air that is blown into a blast furnace. A typical blast furnace, producing 1500 Mg (1650 U.S. ton) of pig iron per day, will be blown with 47.2 mVs (100,000 std ft /min) of atmospheric air preheated to temperatures ranging in normal practice from 755 to 922 K (900 to 1200°F). A set offour stoves is usually provided, each consisting of a vertical steel cylinder 7.3 m (24 ft) in diameter, 33 m (108 ft) nigh, topped with a spherical dome. Characteristic plan and elevation sections of a stove are shown in Fig. 24-61. The interior comprises three regions in the cylindrical portion, (1) a side combustion chamber. 

Fig. 10. Blast furnace stove for preheating large quantities of air...

Air required for combustion is furnished by turboblowers and preheated in hot-blast stoves lined with refractory-brick checker-work. Commonly,... 

The ISP evolved to fill a very special niche in nonferrous metallurgy because of its capability of treating Lead-zinc concentrates which may also contain appredable amounts of copper. The concentrate is normally-oxidized in a sintering machine to produce a feed for the blast furnace where the zinc oxide is reduced with coke. Some effort has been underway to develop a hot briquetting operation to produce a suitable feed without sintering. Other efforts to improve the economic competitiveness of the process include air preheat and the use of an oxygen-enriched blast to reduce coke consumption. 

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