Heat Exchanger

 The operational efficiency of blast furnaces is largely determined by the specific coke rate per ton of hot metal. Perfection of existing ways to reduce the coke rate and development of new ones has always been one of the most urgent challenges in iron and steel industry. The most important way to reduce the specific coke rate and to increase the productivity of blast furnace is to increase the hot blast temperature.

       Previously, the use of expensive natural gas as a high calorie addition to increase the temperature of the hot blast stove dome was the main way to increase the hot blast temperature. Currently, shortages of natural gas, coking coals and an ongoing hike in their prices make it necessary to search for alternative ways to reduce their consumption without reduction in the hot blast temperature, and rather to enhance the hot blast temperature in parallel with injection of coke substitutes (pulverized coal et al.) into the blast furnace. Application of exhaust gas waste heat recovery system to preheat combustion air and gas for hot stoves became one of the most important steps in this direction.

      Each calorie introduced with preheated air or gas, saves 2-3 calories in the fuel consumption, so it is preferable to impart the heat of waste gas to air or gas instead of exhausting it into the atmosphere. Introduction of waste heat recovery systems (WHRS) in hot blast stove system allows running completely on cheap and available fuel – blast furnace gas without reducing the hot blast temperature.

      The average temperature of exhaust waste gases in the flue of hot stoves system is about 270ºC. In this regard, temperatures of preheated gas and combustion air in the heat recovery systems can reach 180ºC. The checker supporting grid of Kalugin shaftless hot blast stoves allows having the maximum temperature of waste gas at 500°C and its average temperature about 350°C, which makes it possible to preheat the gas and air up to 250°C without an additional firing chamber.

       The most effective WHRS designs are based on preheaters using thermosiphons (see Photo 1) and on tube heat exchangers (see Photo 2). KALUGIN JSC has acquired a vast experience in designing and supplying the waste heat recovery systems of the above two types.

                  

Photo 1 – Heat recovery system on thermosiphons,        Photo 2 – Heat recovery system on tube exchangers,
KSS system of BF No.1 V=1750 m³, Jinan                         KSS system of BF V=350 m³ at SunFlag Iron & Steel Co.
Iron & Steel Co., Ltd. (Jinan, China)                                     (Bhandara, India)

       Over 20 Kalugin hot stove systems were equipped with waste heat recovery systems and have already been in operation (the first system was incepted in 2002) in the People's Republic of China in blast furnace units of various capacities.

       The following options for preheating of gas and (or) air are introduced in all KSS systems operating in China where only lean blast furnace gas with low combustion heat (<750 kJ/m³) is used for the stove heating in order to achieve the hot blast temperature of 1250-1300ºC with no high-calorific value gas addition:

       - Preheating of BF gas and air up to 160-180ºC in the heat exchangers based on thermosiphons (Photo 1) by means of the waste gas heat recovery;

      - Preheating only combustion air up to 500-600ºC in two small size KSS (preheaters) specially constructed in the system, see the section "High-temperature preheating of combustion air";

      - Preheating the gas up to 160-180ºC in the heat exchanger based on thermosiphons with the heat of waste gases, and preheating combustion air up to 450-600ºC in two small size KSS.

       Waste heat recovery units based on tube exchangers are applied in Kalugin hot blast stove systems in India and Ukraine designed by KALUGIN JSC, (see the next page, item 2).

       In Russia, the first waste heat recovery system of KALUGIN JSC design for Kalugin hot blast stoves was commissioned in 2005 in BF No.4 OAO Severstal of 2700 m³ (Cherepovets, Vologda region) capacity, see Photo 3. Preheaters in this system are made of thermosiphons.

       

Photo 3 – System of waste heat recovery on thermosiphons
at BF No.4 OAO Severstal V=2700 m³ (Cherepovets, Russia)


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