Heating – Tumbler-type rotary - drum furnace – Having structure agitating or controlling the motion of the...
Reexamination Certificate
2003-03-26
2004-06-15
Wilson, Gregory A. (Department: 3749)
Heating
Tumbler-type rotary - drum furnace
Having structure agitating or controlling the motion of the...
C432S107000, C432S171000
Reexamination Certificate
active
06749425
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an indirect heating furnace suitable for heat treatment of solid inorganic substances, more particularly, for high-temperature heat treatment in cases where direct contact of the solid inorganic substance with high-temperature combustion gas is unfavorable.
2. Description of the Related Arts
In a solid heating process in which direct contact of a substance to be heated with high-temperature combustion gas is unfavorable, an indirect heating furnace (also called an external heating furnace) is generally used.
However, conventional Indirect heating furnaces use metallic shells for reaction tubes. This limits the conventional indirect heating furnace to heating processes requiring temperatures no higher than 900° C.
To overcome this problem, U.S. Pat. No. 5,846,072 (Patent Literature 1) has described that an indirect heating furnace in which the reaction tube is made of ceramics and a screw conveyor is provided in the reaction tube to convey a solid substance to be heated, by which the Indirect heating furnace can be applied to processes requiring temperatures above 900° C.
FIG. 4
shows an example of a general system configuration in the case where a substance to be heated is heat-treated using such an indirect heating furnace provided with a ceramic reaction tube. In the example shown in
FIG. 4
, a case where lime is burned (thermal decomposition of limestone) is illustrated.
In
FIG. 4
, limestone charged into a supply chamber
120
through a raw material charge port
110
is transferred in a reaction tube
140
by the rotation of a screw conveyor
130
, and conveyed into an outlet chamber
150
. The limestone is subjected to heat treatment during the time when it is passed through the reaction tube
140
toward the outlet chamber
150
. The heat-treated product in the outlet chamber
150
is discharged to the outside of the furnace through a chute
160
.
Outside the tube, high-temperature combustion gas, which is generated by a combustion burner
170
, is introduced into the furnace through a gas introduction port
180
to heat the limestone in the reaction tube
140
indirectly via the wall of the reaction tube
140
, and is discharged from the furnace through an exhaust port
190
. The furnace exhaust gas discharged through the exhaust port
190
is sent to an air preheater
200
to preheat combustion air supplied to the combustion burner
170
, by which the heating value of the furnace exhaust gas is utilized effectively.
In the indirect heating furnace as shown in
FIG. 4
, heating is accomplished in three steps, in that heat is transferred from the high-temperature combustion gas supplied into the furnace to the outside wall of the reaction tube
140
, it is conducted through the tube wall, and then the heat is transferred from the tube wall to limestone, which is a substance to be heated. Because of this, the heat transfer efficiency is poor, and the temperature of furnace exhaust gas is as high as about 1000° C. In order to preheat the combustion air using a conventional metallic air preheater
200
with this high-temperature furnace exhaust gas, it is necessary first to lower the gas temperature to about 800° C. by using dilution air to prevent the air preheater from being burned out.
When furnace exhaust gas of about 800° C. is used, the temperature of the obtained preheated air is about 600° C. at the most. Therefore, the heat recovery efficiency is poor, and a large amount of excess heat is wasted. It might be thought that this excess heat can be used to preheat limestone, which is a substance to be heated. However, since this system is used for a process in which direct contact of the substance to be heated with high-temperature combustion gas is unfavorable, the preheating of the substance should also be accomplished indirectly. Thus, it is difficult to improve the thermal efficiency in this way. Further, the amount of exhaust gas discharged from the system increases due to the dilution air, which results in a further increase in heat loss.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an indirect heating furnace which can improve the thermal efficiency dramatically and can increase the throughput significantly in a solids heating process in which direct contact of the solids with high-temperature combustion gas is unfavorable.
To achieve the above object, the present invention provides the following indirect heating furnaces:
[1] An indirect heating furnace for heating a substance in a reaction tube with a high-temperature combustion gas that does not contact that substance, characterized in that the heating takes place in a stationary ceramic reaction tube, and the combustion device supplying the heating high-temperature combustion gas into the furnace is comprised of at least one pair of regenerative burners.
[2] The indirect heating furnace as described in the above item [1], characterized in that the temperature of the high-temperature combustion gas supplied into the furnace is 1000° C. or higher.
[3] The indirect heating furnace as described in the above item [1] or [2], characterized in that the furnace has a screw conveyor for transporting the substance to be heated through the reaction tube, and the essential portion of the screw conveyor is made of ceramics.
REFERENCES:
patent: 3805406 (1974-04-01), Castonoli
patent: 4084521 (1978-04-01), Herbold et al.
patent: 4376343 (1983-03-01), White et al.
patent: 5393225 (1995-02-01), Freiberger et al.
patent: 5846072 (1998-12-01), Willis
patent: 5899689 (1999-05-01), Fontana
patent: 6042370 (2000-03-01), Weide
Frishauf Holtz Goodman & Chick P.C.
JP Steel Plantech Co.
Wilson Gregory A.
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