JP2008249273A - Heating furnace - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique for securely preventing corrosion of a heat exchanger by protecting a heat exchanger installed on an exhaust pipe from oxide of alkali metal, oxidized gas, corrosive gas or the like for a heating furnace which preheats air for combustion by utilizing exhaust heat of combustion exhaust gas. <P>SOLUTION: The heating furnace 20 is provided with the exhaust pipe 10 for exhausting the combustion exhaust gas generated by combustion in a furnace 26, and is also provided with: a pipe body 11 for distributing the combustion exhaust gas; and a heat exchange part 12 thermally connected to the combustion exhaust gas flowing the pipe body 11, and exchanging heat between the air for combustion supplied to the furnace 26 and the combustion exhaust gas. A ceramic coating layer 14 for preventing the combustion exhaust gas from directly contacting with the heat exchange part 12 is formed on a surface of the heat exchange part 12 through an intermediate joining layer 13. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a heating furnace provided with an exhaust pipe for discharging combustion exhaust gas generated by combustion in the furnace.

  The heating furnace is a combustion apparatus that burns a burner installed in the furnace and treats an object to be processed by the combustion heat. For example, in a glass melting furnace that is one of the heating furnaces, silicon dioxide (silica sand), sodium carbonate (soda ash), calcium carbonate, etc. are supplied into the furnace as glass raw materials, and the glass raw materials are radiated by the combustion of a gas burner. Is melting. At this time, conventionally, the combustion air supplied to the burner is preheated by using the heat of high-temperature (for example, 800 to 900 ° C.) exhaust gas discharged from the furnace to improve the thermal efficiency of the entire furnace. (For example, refer to Patent Document 1).

  FIG. 5 is a schematic configuration diagram of a conventional heating furnace 50 represented by Patent Document 1. In FIG. The heating furnace 50 includes a burner 51 for burning fuel into the furnace, a blower means 52 for supplying combustion air to the burner 51, a fuel supply means 53 for supplying fuel to the burner 51, and generated in the furnace. And an exhaust pipe 54 for discharging the exhausted combustion exhaust gas. In the heating furnace 50, a combustion air supply pipe 55 extending from the blowing means 52 is disposed so as to pass through the vicinity of the exhaust pipe 54, thereby forming a heat exchanging portion 56. Thus, the combustion air is heated by the heat exchanging unit 56 and then supplied to the burner 51.

  By the way, the combustion exhaust gas discharged from the heating furnace is generally a high-temperature oxidizing atmosphere. This oxidizing atmosphere contains oxidizing gas (for example, NOx, SOx), corrosive gas (for example, hydrogen chloride, hydrogen sulfide), fine particles generated by combustion, and the like. For this reason, materials having high oxidation resistance and corrosion resistance such as stainless steel (for example, SUS310S, SUS316) are usually used for the exhaust pipe of the heating furnace.

  In addition, a coating treatment such as an oxide-based ceramic coating is performed on a portion of the exhaust pipe of the heating furnace where corrosion may occur due to an oxidizing gas or a corrosive gas ( For example, see Patent Document 2).

  Patent Document 2 discloses a technique for protecting the surface (combustion exhaust gas contact surface) of a pipe member constituting a heat exchange part of a waste incinerator with a ceramic film such as aluminum oxide.

Japanese Patent Laying-Open No. 2003-329240 (FIG. 1) JP 10-274401 A (paragraph 0007)

  A heating furnace melts a glass raw material (a mixture of silicon dioxide, sodium carbonate, calcium carbonate, etc.) and produces a glass melting furnace for producing glass products mainly composed of sodium silicate, or a flux of sodium or the like. In the case of a plating furnace or the like for performing flux treatment for stirring and removing impurities by oxidation, alkali metal (Na or the like) is mixed in the combustion exhaust gas. When such flue gas containing alkali metal is cooled by heat exchange with a relatively low-temperature combustion air introduced from the outside in a heat exchange section installed in the exhaust pipe, oxidation of the alkali metal in the flue gas is oxidized. In some cases, the material aggregates and accumulates on the wall surface of the tubular body constituting the heat exchange section. The deposited alkali metal oxide can react with moisture in the combustion exhaust gas and chemically change into a strong alkaline substance. In such a case, even if a highly acid-resistant stainless material is used for the pipe body, the heat exchange part of the exhaust pipe may be corroded from the contact surface of the combustion exhaust gas.

  Moreover, generally only by performing a ceramic coating process on the flue gas contact surface of the heat exchange part of the exhaust pipe, a coating material (for example, oxide-based ceramic) and a material constituting the heat exchange part (for example, stainless steel) Since there is a difference in the coefficient of thermal expansion, if the coating treatment conditions (for example, material, thickness, etc.) are not set properly, the coating layer will peel off from the flue gas contact surface of the heat exchange part, and the exposed stainless steel May be corroded by strong alkaline substances.

  Furthermore, some coating materials contain, as a main component, a metal having a relatively high vapor pressure (ie, a metal having a high diffusion rate) such as aluminum or chromium. Further, a stainless steel ground that is an object of the coating treatment also contains a metal element such as chromium or manganese having a relatively high vapor pressure. When such a coating material is formed on the surface of a stainless steel base material by a known film formation method (eg, CVD method, PVD method, etc.), a metal component having a high vapor pressure in the coating material is formed on the stainless steel base. The metal component having a high vapor pressure in the stainless steel is evaporated. As a result, micropores and cracks are formed in the stainless steel as the base material, and embrittlement progresses, which may lead to a decrease in strength of the exhaust pipe.

  The present invention has been made in view of the above problems, and in a heating furnace that preheats combustion air using exhaust heat of combustion exhaust gas, a heat exchanger installed in an exhaust pipe is replaced with an oxidizing gas, It is an object of the present invention to provide a technology that protects from corrosive gas, alkali metal oxide that can be chemically changed to a strong alkaline substance, and prevents corrosion of the heat exchanger.

  A characteristic configuration of the heating furnace according to the present invention is a heating furnace including an exhaust pipe that discharges combustion exhaust gas generated by combustion in the furnace, the pipe body through which the combustion exhaust gas circulates, and the flow through the pipe body A heat exchange unit that is thermally connected to the combustion exhaust gas and heat exchange between the combustion air supplied to the furnace and the combustion exhaust gas is performed, and the combustion exhaust gas is prevented from coming into direct contact with the heat exchange unit The ceramic coating layer to be formed is formed on the surface of the heat exchange part via the intermediate bonding layer.

  In the heating furnace of this configuration, a coating layer is formed to prevent combustion exhaust gas from coming into direct contact with the heat exchange part. Heat exchange can be performed without direct contact of the alkali metal oxide that can change, and then the combustion exhaust gas can be discharged to the outside. Therefore, corrosion of the heat exchange part installed in the exhaust pipe of the heating furnace can be prevented. In addition, when the thermal expansion coefficient of the material constituting the heat exchange part and the material constituting the coating layer are different, if the two are joined together, internal stress accumulates near the joint interface, and the coating layer is on the surface of the heat exchange part. However, in this configuration, since the coating layer is formed on the surface of the heat exchange part via the intermediate bonding layer, the coating layer is not easily peeled off. This is because the internal stress that would otherwise occur due to the difference in thermal expansion coefficient can be absorbed, dispersed, or reduced by the presence of the intermediate bonding layer. Therefore, by adopting this configuration, it is possible to more reliably prevent corrosion of the heat exchange unit installed in the exhaust pipe of the heating furnace.

  In the heating furnace according to the present invention, the ceramic coating layer may be made of a material selected from the group consisting of aluminum oxide, chromium oxide, titanium oxide, and a mixture thereof.

  In the heating furnace of this configuration, the ceramic coating layer is made of an appropriate material as described above, so the heat exchange unit installed in the exhaust pipe of the heating furnace is a combustion exhaust gas with strong oxidizing and corrosiveness, It is surely protected from alkali metal oxides that can be chemically converted into strongly alkaline substances. Therefore, corrosion of the heat exchange part can be prevented more reliably. In addition, although either of the said materials may be used independently for a ceramic coating layer, it can also be set as the mixture of 2 or more types of the said material.

  In the heating furnace according to the present invention, the intermediate bonding layer may be made of a material selected from the group consisting of platinum, iridium, rhenium, ruthenium, and a mixture thereof.

  In the heating furnace having this configuration, the intermediate bonding layer is made of an appropriate material as described above, and therefore, the difference in thermal expansion coefficient between the material forming the heat exchange part and the material forming the coating layer is determined by intermediate bonding. Depending on the layer, it can be absorbed, dispersed or reduced. For this reason, a coating layer does not peel easily. Therefore, corrosion of the heat exchange part installed in the exhaust pipe of the heating furnace can be prevented more reliably. In addition, although either of the said materials may be used independently for an intermediate | middle joining layer, it can also be set as the mixture of 2 or more types of the said material.

  In the heating furnace according to the present invention, the heat exchanging portion includes a sleeve through which the combustion exhaust gas circulates, and a cylindrical jacket formed around the sleeve and through which the combustion air circulates. You may have.

  In the heating furnace having this configuration, the heat exchanging portion has a sleeve and a cylindrical jacket, and the combustion exhaust gas is circulated through the sleeve. For this reason, the heat exchange between the combustion exhaust gas and the combustion air can be easily performed without substantially disturbing the flow of the combustion exhaust gas. In addition, since the combustion air flows between the sleeve and the cylindrical jacket formed around the sleeve, it is possible to increase the effective area of the portion where heat exchange is performed in the heat exchange section. As a result, good heat exchange efficiency can be achieved. Furthermore, since the sleeve through which the combustion exhaust gas flows has a simple configuration, an intermediate bonding layer and a ceramic coating layer can be easily formed.

  The heating furnace according to the present invention may heat a material containing an alkali metal in the furnace.

  As described above, the heating furnace having this configuration has a feature that it is difficult to be corroded by combustion exhaust gas having strong oxidizing and corrosive properties and alkali metal oxides that can be chemically changed to strongly alkaline substances. For this reason, even if the material containing an alkali metal is heated in the furnace, damage to the heating furnace hardly occurs, and troubles such as repair, maintenance, and cleaning can be saved as compared with the conventional case. As a result, the running cost related to the operation of the heating furnace can be reduced.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the structure described in the following embodiment and drawing, The structure equivalent to these is also included.

  FIG. 1 is a schematic configuration diagram of a heating furnace according to the present invention. In the present embodiment, the heating furnace is used as the glass melting furnace 20. The glass melting furnace 20 is provided with a burner 21, a blowing means 22, a combustion air supply pipe 23, a fuel supply means 24, a fuel supply pipe 25, and an exhaust pipe 10. In the present embodiment, as shown in the figure, four burners 21 are installed toward the furnace 26. The four burners 21 are arranged at predetermined intervals by distributing two burners to left and right opposing positions of the furnace wall 27. The left and right burners 21 are alternately burned at predetermined time intervals to perform alternating combustion. Thereby, the heating nonuniformity of the glass raw material in the furnace 26 is reduced. Examples of the burner 21 used in the glass melting furnace 20 include a direct heating type burner such as a throat mix type gas burner, an indirect heating type burner such as a radiant tube burner, and the like.

  Each burner 21 is connected to a combustion air supply pipe 23 that supplies combustion air from the blower means 22 and a fuel supply pipe 25 that supplies fuel from the fuel supply means 24. The blower 22a as shown in the figure can be used as the air blowing means 22, but it is also possible to supply air from an air cylinder. The fuel supply means 24 may be configured to supply fuel such as gas from a fuel tank (or fuel cylinder) 24a as shown in the figure as needed, but a fuel pipe (gas pipe) buried in the ground or the like. It is good also as a structure which supplies fuel (gas) directly from.

  The combustion states of the four burners 21 can be automatically controlled individually or left and right by control means (not shown). For example, the control means can perform ON-OFF control, PID control, and the like in accordance with a preset value while monitoring the temperature in the furnace 26 with a temperature sensor or the like.

  An exhaust pipe 10 for discharging combustion exhaust gas generated by combustion in the furnace 26 is connected to the furnace wall 27 of the glass melting furnace 20. The configuration of the exhaust pipe 10 will be described in detail with reference to FIGS.

  2, (a) is a longitudinal sectional view of the exhaust pipe 10, and (b) is a transverse sectional view taken along line AA of the exhaust pipe 10 shown in (a). FIG. 3 is an enlarged cross-sectional view of the vicinity of the inner surface (contact surface of the combustion exhaust gas) of the heat exchanger 12 constituting the exhaust pipe 10.

  The exhaust pipe 10 includes a tube body 11 and a heat exchange unit 12. The pipe body 11 is made of a material having high oxidation resistance and corrosion resistance such as stainless steel (SUS310S, SUS316), and is configured so that combustion exhaust gas can flow therethrough. In the present embodiment, the tubular body 11 is divided into two parts, the first tubular body 11a and the second tubular body 11b, but it is of course possible to configure the tubular body 11 as an integral body.

  The heat exchange unit 12 is a heat exchanger that performs heat exchange between combustion air supplied to the glass melting furnace 20 and combustion exhaust gas. The heat exchange unit 12 includes a sleeve 12a in which a stainless material is formed in a cylindrical shape, and a cylindrical jacket 12b formed around the sleeve 12a. At both ends of the sleeve 12a, flange portions 12c for connecting the heat exchange unit 12 main body to the first tube body 11a and the second tube body 11b are provided. In this embodiment, the heat exchange part 12 is inserted | pinched between the 1st pipe body 11a and the 2nd pipe body 11b, and the heat exchange part 12 and the pipe body 11 are connected so that communication is possible. In this way, the heat exchange unit 12 is thermally connected to the combustion exhaust gas flowing inside the tube body 11.

  According to the configuration of the present embodiment, the flange portion 12c of the heat exchanging portion 12 is simply aligned with the first tube body 11a and the second tube body 11b, respectively, and is simply tightened with a bolt or the like. The heat exchange part 12 can be easily attached to the tube body 11. Moreover, what is necessary is just to perform the operation | work reverse to the above, when removing the heat exchange part 12 from the pipe body 11. FIG. Therefore, in the heating furnace 20 of the present embodiment, the heat exchange unit 12 can be easily cleaned and replaced.

  When the tube body 11 is an integral body, the heat exchanging portion 12 may be connected to the upstream end portion or the downstream end portion of the tube body 11, and in this case, only one end portion of the sleeve 12a is flanged. What is necessary is just to provide the part 12c.

  The combustion exhaust gas generated in the furnace 26 flows through the sleeve 12a. On the other hand, between the sleeve 12 a and the cylindrical jacket 12 b, combustion air generated by a blower that is a blowing means 22 is introduced through a combustion air supply pipe 23. At this time, heat exchange between the high-temperature combustion exhaust gas and the low-temperature combustion air is performed through the wall portion of the sleeve 12a.

  In this way, by adopting a configuration in which the combustion exhaust gas circulates inside the sleeve 12a, heat exchange between the combustion exhaust gas and the combustion air can be easily performed without substantially impeding the distribution of the combustion exhaust gas. . In addition, by configuring the combustion air to flow between the sleeve 12a and the cylindrical jacket 12b formed around the sleeve 12a, the effective area of the portion where heat exchange is performed in the heat exchange unit 12 is increased. It is possible to take. As a result, good heat exchange efficiency can be achieved in the heat exchange section 12.

  By the way, when a heating furnace is a glass melting furnace like this embodiment, the combustion exhaust gas contains a lot of alkali metal oxides (sodium oxide). When this combustion exhaust gas is cooled in the heat exchanging section 12, sodium oxide in the combustion exhaust gas aggregates due to a temperature drop due to heat exchange and adheres to the inner wall of the sleeve 12a. The attached sodium oxide reacts with moisture contained in the combustion exhaust gas, and can chemically change to sodium hydroxide, which is a strong alkaline substance. Since sodium hydroxide is corrosive even to a material having high oxidation resistance and corrosion resistance such as stainless steel, there is a possibility of damaging the heat exchanging portion 12.

  Therefore, in the present embodiment, in order to prevent combustion exhaust gas containing sodium from coming into direct contact with the heat exchanging portion 12, as shown in FIG. 3, the stainless steel ground that constitutes the heat exchanging portion 12 (sleeve 12a) is used. A ceramic coating layer 14 is formed on the inner surface via an intermediate bonding layer 13. As the ceramic coating layer 14, an oxide ceramic material having excellent acid resistance and alkali resistance can be used. Examples of the oxide-based ceramic material include aluminum oxide, chromium oxide, titanium oxide, zirconium oxide, barium oxide, etc. In particular, aluminum oxide, chromium oxide, and titanium oxide are performance, workability, availability, and price. Etc. are preferable. Two or more kinds of these oxide ceramic materials may be mixed. In this case, the mixture can be used in the form of a eutectic or mixed crystal.

  For example, the thickness of the ceramic coating layer 14 is preferably set to 0.05 to 5% with respect to the thickness of the stainless steel (base material) constituting the sleeve 12a. For example, when the thickness of the stainless steel material is 10 mm, the thickness range of the ceramic coating layer 14 is 5 to 500 μm. The most preferable thickness of the ceramic coating layer 14 is about 4% of the thickness of the stainless steel (400 μm in the case of a 10 mm stainless steel). If the thickness of the ceramic coating layer 14 is smaller than 0.05% of the thickness of the stainless steel material, the ceramic coating layer 14 cannot sufficiently provide corrosion resistance to the exhaust pipe 10, and if the thickness is larger than 5%, the ceramic coating layer The layer 14 is easily peeled off under the influence of the thermal expansion of the stainless material.

  As described above, by forming the ceramic coating layer 14 under optimum conditions, heat exchange is performed without direct contact between the heat exchange section 12 and combustion oxides or alkali metal oxides having strong oxidizing and corrosive properties. Thereafter, the combustion exhaust gas can be discharged to the outside. As a result, corrosion of the heat exchange unit 12 installed in the exhaust pipe 10 of the heating furnace 20 can be prevented.

  By the way, between the material which comprises the heat exchange part 12, and the material which comprises the ceramic coating layer 14, both thermal expansion coefficient may differ. In such a case, if the ceramic coating layer 14 is bonded to the surface of the heat exchange unit 12 as it is, internal stress accumulates near the bonding interface, and the ceramic coating layer 14 may be peeled off from the surface of the heat exchange unit 12. On the other hand, in the present invention, the ceramic coating layer 14 is formed on the surface of the heat exchanging portion 12 via the intermediate bonding layer 13. For this reason, the ceramic coating layer 14 does not peel easily. This is because the internal stress that would otherwise occur due to the difference in thermal expansion coefficient can be absorbed, dispersed, or reduced by the presence of the intermediate bonding layer 13. As such an intermediate bonding layer 13, a noble metal material is preferably used. In particular, platinum, iridium, rhenium, and ruthenium are preferable because of their excellent performance as a bonding material. An alloy in which two or more of these noble metal materials are mixed can be used as the intermediate bonding layer 13.

  The thickness of the intermediate bonding layer 13 is preferably set to 5 to 20% with respect to the thickness of the ceramic coating layer 14. For example, when the ceramic coating layer 14 is formed on a stainless steel material having a thickness of 10 mm with the most preferable thickness of 400 μm, the thickness of the intermediate bonding layer 13 is 20 to 80 μm. If the thickness of the intermediate bonding layer 13 is less than 5% of the thickness of the ceramic coating layer 14, the effect as the intermediate bonding layer cannot be expected, and if it is greater than 20%, the vapor between the ceramic coating layer 14 and the intermediate bonding layer 13 is extremely high. There is a possibility that an oxide of a noble metal having a high pressure is generated and the ceramic coating layer 14 is peeled off from the intermediate bonding layer 13.

  The types of materials constituting the ceramic coating layer 14 and the intermediate bonding layer 13 can be optimally combined depending on conditions such as components of combustion exhaust gas, temperature, and processing amount. In addition, as a method for forming the ceramic coating layer 14 and the intermediate bonding layer 13, an optimum method can be selected according to the characteristics of the material, for example, a thermal spraying method, a CVD method, a PVD method, a sputtering method, or the like. Can do.

  As described above, when the intermediate bonding layer 13 is formed on the sleeve 12a of the heat exchange unit 12 and the ceramic coating layer 14 is formed on the intermediate bonding layer 13 by an appropriate material, condition, and method, the ceramic coating layer 14 is formed. Does not peel easily. As a result, corrosion of the heat exchange unit 12 installed in the exhaust pipe 10 can be more reliably prevented. Further, since the sleeve through which the combustion exhaust gas flows has a relatively simple configuration, the intermediate bonding layer 13 and the ceramic coating layer 14 can be easily formed.

  As described above, in the heating furnace 20, even when an alkali metal-containing material is heated in the furnace, it is corroded by combustion exhaust gas having strong oxidizing and corrosive properties or oxide of alkali metal that can be chemically changed to a strong alkaline substance. By adopting the exhaust pipe 10 having the feature that it is difficult to be performed, it is possible to save time and labor for repairing, maintaining, and cleaning the heating furnace 20 as compared with the related art. Thereby, the running cost regarding the operation of the heating furnace 20 can be reduced.

[Another embodiment]
Next, another embodiment of the present invention will be described. 4, (a) is a longitudinal sectional view of the exhaust pipe 30 used in the heating furnace according to another embodiment, and (b) is a transverse sectional view taken along the line BB of the exhaust pipe 30 shown in (a). . The exhaust pipe 30 includes a pipe body 31 and a heat exchanging section 32 as in the above embodiment. However, the present embodiment is different in that the tube body 31 is a single body, and the heat exchanging portion 32 is configured as a through tube 32 a that penetrates the side portion of the tube body 31. However, even with such a configuration, the heat exchanging unit 32 is thermally connected to the combustion exhaust gas flowing inside the through pipe 32a. The through pipe 32 a has a ceramic coating layer 14 formed on the outer surface of the through pipe 32 a via the intermediate bonding layer 13 at a portion exposed inside the pipe body 31. The formation conditions for the intermediate bonding layer 13 and the ceramic coating layer 14 are the same as in the above embodiment.

  Also in the present embodiment, heat exchange can be performed without direct contact between the heat exchanging part 32 and the exhaust gas or alkali metal oxide having strong oxidizing and corrosive properties, and then the combustion exhaust gas can be discharged to the outside. . For this reason, corrosion of the heat exchange part 32 installed in the exhaust pipe 30 can be effectively prevented. Moreover, since the exhaust pipe 30 of this another embodiment has a simple configuration, the manufacturing cost can be reduced.

  The anticorrosion technique in the heating furnace of the present invention can be used not only in the glass melting furnace described in the above embodiment and other embodiments but also in various furnaces such as an incinerator, a combustion furnace, and a heat treatment furnace. The anticorrosion technique of the present invention can also be applied to combustion apparatuses other than the heating furnace. For example, in an automobile exhaust heat utilization device that reuses the heat of exhaust gas, a ceramic coating layer can be formed on a tube member constituting a heat exchanger under an appropriate condition via an intermediate joining layer in the same manner as in the present invention. it can. Thereby, corrosion of the heat exchanger due to contact with high-temperature exhaust gas can be prevented.

Schematic configuration diagram of the heating furnace of the present invention (A) longitudinal sectional view and (b) transverse sectional view of the exhaust pipe Enlarged cross-sectional view near the surface of the heat exchanger composing the exhaust pipe (contact surface of the combustion exhaust gas) (A) longitudinal sectional view and (b) transverse sectional view of an exhaust pipe used in a heating furnace according to another embodiment Schematic configuration diagram of a conventional heating furnace

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Exhaust pipe 11 Tubing body 12 Heat exchange part 12a Sleeve 12b Cylindrical jacket 13 Intermediate joining layer 14 Ceramic coating layer 20 Glass melting furnace (heating furnace)

Claims (5)

A heating furnace provided with an exhaust pipe for discharging combustion exhaust gas generated by combustion in the furnace,
A tubular body through which the combustion exhaust gas flows;
A heat exchanging unit that is thermally connected to the combustion exhaust gas flowing through the pipe, and that performs heat exchange between the combustion air supplied into the furnace and the combustion exhaust gas;
With
A heating furnace in which a ceramic coating layer for preventing the combustion exhaust gas from coming into direct contact with the heat exchange part is formed on the surface of the heat exchange part via an intermediate bonding layer.   The heating furnace according to claim 1, wherein the ceramic coating layer is made of a material selected from the group consisting of aluminum oxide, chromium oxide, titanium oxide, and a mixture thereof.   The heating furnace according to claim 1 or 2, wherein the intermediate bonding layer is made of a material selected from the group consisting of platinum, iridium, rhenium, ruthenium, and a mixture thereof.   2. The heat exchange section includes a sleeve through which the combustion exhaust gas flows and a cylindrical jacket formed around the sleeve and through which the combustion air flows. The heating furnace as described in any one of -3.   The heating furnace as described in any one of Claims 1-4 which heats the material containing an alkali metal in a furnace.

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