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HIGH-TEMPERATURE PREHEATING OF COMBUSTION AIR

For preliminary heating (hereinafter – "preheating") of combustion air supplied into the burner devices, up to the temperature of 400-600ºC, in particular, under operation of Kalugin shaftless hot stoves (KSS) in the conditions of the People's Republic of China on blast furnace gas with the low heat of combustion (<750 kJ/m³) without high-calorific gas addition and for achievement thus the hot blast temperature of 1250-1300ºC, the experts of KALUGIN JSC developed a special system of two high-temperature combustion air preheaters additionally constructed in the system.
 
These preheaters are like KSS the size of which is much smaller than of the main KSS in hot stove system (see Photo ), so in Russian the name meaning "small KSS" is used for them. In English, the abbreviation CAP "Combustion Air Preheater" is applied.
CAPs are operating by the same principle as KSS, but instead of cold blast air is supplied to them from the central combustion air supply station. One by one CAP is in the mode of heating (gas period), and the second in the mode of blasting (or blast period). Part of the air passing through the checker of the preheated CAP, is heated to the temperature of 1100-1200ºC, then it is mixed in the special mixer with the other (cold) part of the air to get the required temperature and is supplied to the burners of KSS and of the CAP being preheated. Thus, it is possible to obtain high blast temperature (up to 1300ºC) running on blast furnace gas only with low calorific content.
 
 
Photo – General view of KSS system at BF No.2, Qinhuangdao Shouqin Metal Materials Co. Ltd. (Qinhuangdao, China). On the right – two small KSS, combustion air preheaters (CAPs)
 
In China, at the blast furnaces of different volumes there are more than 20 KSS systems, 10 of which are equipped with the systems of combustion gas preheating in CAPs: two blast furnaces at Shouqin Iron & Steel Works in Qinhuangdao (volumes of furnaces are 1780 and 1160 m³), four furnaces at the three iron and steel works in the city of Tangshan (V=1780 and 3200 m³), as well in the new complex of the largest blast furnaces in China of volume 5500 m³ each at Shougang Jingtang Iron & Steel Co. in Caofeidian (one furnace is in operation, the second is under construction).
 
In case if the reconstruction of blast furnace is based on construction of a new KSS system not on the place of old hot stoves, but on a new site, it is possible to reuse two old stoves for preheating the combustion air similar to CAPs.
An important advantage of preheating systems with the use of CAPs is that their service life is, like of main KSS, 30 years without repair, which is not achieved in other types of heat exchangers.