Exhaust gas processing system and method for rotary hearth type reducing furnace

Abstract

Provided are an exhaust gas processing system and method capable of effectively utilizing the sensible heat of an exhaust gas for preheating air for burner combustion in a rotary hearth type reducing furnace while preventing troubles caused by adhesion of dust such as blockage in an exhaust gas processing facility for a rotary hearth type reducing furnace and corrosive deterioration in the facility without increasing the facility costs excessively. The system comprises a radiant heat exchanger 2 for heat exchange between an exhaust gas exhausted from the rotary hearth type reducing furnace and air. The radiant heat exchanger 2 includes an inner cylinder 22 made of metal surrounding a space through which the exhaust gas flows; an outer cylinder 21 disposed on an outer side in a radial direction of the inner cylinder to define a flow channel between the inner cylinder and the outer cylinder 21 for allowing the air to flow so as to exchange heat with the exhaust gas; and a highly thermal conductive refractory 23 applied to an inner side of the inner cylinder 22 so as to cover an inner surface thereof.

Description

TECHNICAL FIELD[0001]The present invention relates to a system and a method for processing an exhaust gas generated in a rotary hearth type reducing furnace (hereinafter, referred to also as the rotary hearth furnace) by reducing carbon composite metal oxide agglomerates, and more particularly, relates to a structure of a heat exchanger for effective recovery of the sensible heat of the exhaust gas and an exhaust gas processing technique using the structure.BACKGROUND ART[0002]There is limited a conventional reduced iron manufacturing process by a requirement of an expensive natural gas as a reductant and a location of the plant which is normally restricted to a producing region of the natural gas. Under these circumstances, attention has been recently focused on a method of manufacturing reduced iron using coal as the reductant; coal is relatively inexpensive and eases the geographical restriction on the plant location.[0003]For example, Patent Document 1 discloses such a manufactu...

AI Technical Summary

Benefits of technology

[0063]In order to confirm the effects of the invention, experiments using the exhaust gas processing system shown in FIG. 1 were conducted. In Example 1 was used, as the radiant heat exchanger 2 , a exchanger including an inner cylinder 22 formed of a steel plate made of stainless steel with an inner diameter of 1.55 m, a length of 8.2 m, and a heat transfer area of 40 m2 and a refractory 23 formed of a silicon carbide refractory (the SiC content is 60% by mass and the rest are chiefly Al2O3 and SiO2) with the thickness of 50 mm and laid to the inner cylinder 22 by casting so as to cover the inner surface thereof. As the exhaust gas cooling tower 4 is used one in which spraying water and introduction of air at normal temperature into the exhaust gas is performed.

[0064]An exhaust gas at about 1,450° C. was exhausted from the rotary hearth furnace 1 at a flow rate of about 11,600 Nm3 / h, and air at normal temperature was introduced into the exhaust gas at a flow rate of about 1,130 Nm3 / h to adjust the temperature of the exhaust gas to about 1,350° C. The exhaust gas subjected to the temperature adjustment was then introduced into the heat exchanger 2. Inside the heat exchanger 2, air at normal temperature as the coolant was flown at about 13,130 Nm3 / h. The exhaust gas temperature at the inlet of the exhaust gas cooling tower 4 was controlled to be 800° C. or above, and the exhaust gas temperature at the outlet of the exhaust gas cooling tower 4 was controlled to be 180° C. or below. Inside the exhaust gas cooling tower 4, the exhaust gas was rapidly cooled so that the temperature of the exhaust gas dropped from 1,070° C. to 170° C. in about 2 seconds.

[0065]The operation was continued for about 190 days under the conditions specified above, while changes of the overall heat transfer coefficient and the preheating air temperature in the heat exchanger 2 were examined during this period. The results of the examination are set forth in FIG. 3 and FIG. 4. In these drawings, the indication of “60%” means that the SiC content in the silicon carbide refractory was 60% by mass. The marks with an indication of “(I)” represent data during idle driving when the rotary hearth furnace was heated merely to maintain the atmosphere temperature and no agglomerates were charged therein. The marks with an indication of “(O)” represent data during the reduction operation when agglomerates were charged into the rotary hearth furnace to manufacture reduced iron (the same applies to FIG. 5 and FIG. 6 described below) .

[0066]As can be obvious from these drawings, noticeable changes were not observed in both the overall heat transfer coefficient and the preheating air temperature of the heat exchanger, even after the operation over a long period. Furthermore, observation of the interior of the inner cylinder of the heat exchanger 2 after the operation over about 190 days showed that no falling off of the silicon carbide refractory was occurred and a thin adhesion layer, which was formed on the refractory surface, could be easily removed by an operator poking it manually with a metal rod.

[0067]These results show that applying the highly thermal conductive refractory to the inner cylinder of the heat exchanger so as to cover the inner surface (heat transfer surface) thereof makes it possible to effectively recover the sensible heat of the exhaust gas without the blockage of the exhaust gas flow channel and corrosion of the heat transfer surface caused by adhesion onto the heat transfer surface.

[0068]In addition, analysis of the components of the exhaust gas at the outlet of the dust collector 5 formed of a bag filter revealed no synthesis (resynthesis) of dioxins. The synthesis seems to be prevented by contribution of rapid cooling inside the exhaust gas cooling tower 4

Problems solved by technology

There is limited a conventional reduced iron manufacturing process by a requirement of an expensive natural gas as a reductant and a location of the plant which is normally restricted to a producing region of the natural gas.

The growth of the dust deteriorates the heat transfer efficiency of the heat exchanger, blocks the exhaust gas flow channel, or shortens the life of the facility caused by metal corrosion.

This method, however, requires an addition of the waste heat boiler to the exhaust gas processing facility, which involves an increase in the facility costs.

This deteriorates the energy efficiency of the rotary hearth furnace.

In addition, this method requires mechanical dust removing means such as soot blow or the hammering mechanism, which is likely to cause facility troubles.

This method, as with the method described in Patent Document 1 above, also cannot avoid the problems, such as adhesion of dust onto the inner surface of the heat exchanger or corrosive deterioration of the inner surface.

This heat transfer tube is a single tube formed of ceramic alone, the use of which is extremely restricted.

Specifically, this type of cylinder is capable of ensuring a required strength in a case where its diameter is small, while hard to use due to its insufficient strength in a case where a considerably large inner diameter is required, for example, when there is a need to let a large volume of exhaust gas pass through the interior of the tube.

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