Radiant tube burner

Page 1 — bibliographic record

Page 2

Drawing sheet — no readable text.

Page 3

Drawing sheet — no readable text.

Page 4

Drawing sheet — no readable text.

Page 5

Drawing sheet — no readable text.

Page 6

Drawing sheet — no readable text.

Page 7

air makes the flame short, and therefor, the secondary

RADANT TUBE BURNER combustion should be occured adjacent to the primary combustion, meaning that the secondary combustion is

This application is a division, of application Ser. No. forced to be occured outside of the furnace or within 195,107, filed May 16, 1988. 5 the bung. Sometimes it causes to burst out the end por

FIELD OF THE INVENTION

tion of the burner or the radiant tube, itself. The above described thermal erosion or a rupture problem caused

The present invention relates to a burner, and more by overheating is also possible.

particularly to a radiant tube burner. Therefore, it is not easy to combine a radiant tube O burner using the previous method with the theory of the

DESCRIPTION OF THE PRIOR ART open flame burner in the two stage fuel supply combus Recently, a preheat technology of combustion air has tion.

been developed for increasing thermal efficiency by SUMMARY OF THE INVENTION using large quantities of exhaust gas.

For example, a radiant tube burner is presented in 15 Primarily, the purpose of this invention is to provide U.S. Pat. No. 4,604,051 where a set of burners equipped a low NOx radiant tube burner. Secondarily, the pur with regenerative beds is installed at both ends of a pose of this invention is to provide a long lasting radiant radiant tube, and these burners are operated alterna tube burner without danger of thermal erosion or rup tively to preheat the combustion air using heat accumu turing a fuel nozzle. Thirdly, the purpose of this inven lated in the non-operated burner side of the regenera 20 tion is to provide an appropriate radiant tube burner for tive bed. In the burner, as shown in FIG. 10, the conical a system of receoving heat from an exhaust combustion type regenerative bed 102 is placed inside of the burner gaS.

shell 104 which surrounds the fuel nozzle 103 protrud To achieve these aims, the radiant tube burner of this ing into the radiant tube 101 at the outside of the fur invention contains a primary combustion chamber nace, whereby through the regenerative bed 102 the 25 which is placed outside of the furnace and of which the combustion air is supplied to the combustion chamber fuel gas injection outlet is located approximately more and the combustion gas is exhausted. The same FIG. 10 inside of furnace than the bung of a radiant tube, a pri shows the furnace wall 105, the forced draft fan 106, the mary fuel nozzle which injects a primary fuel into the reversing valve 107, and the ejector 108. primary combustion chamber, and a secondary fuel On the other hand, achieving a low NOx burner has 30 nozzle surrounded by refractory materials which injects been considered to be an important technical issue to a second fuel into the radiant tube from the area adja prevent air pollution. As in the past, a Two Stage Fuel cent to the injection outlet. All combustion air is led Supply Burner has been considered effective in order to into the primary combustion chamber where combus reduce NOx emission in the combustion system. Patents tion takes place. This leads to the secondary combustion relating to such a techniques include Japanese Pat. Nos. 35 in the radiant tube supplied with the secondary fuel and 1,068,772, and 1,104,160, and U.S. Pat. No. 4,505,666. In the low concentration of oxygen from the primary com this type burner, a portion of fuel and all of the combus bustion gas. Thus, a large amount of the total combus tion air is supplied to a primary combustion chamber, tion air injected into the primary combustion chamber is which causes rapid combustion under a high excess air mixed rapidly with a small amount of the primary fuel. ratio, and also slow and gentle combustion following creating high intensity combustion under a high excess the heat diffusion is taken place by supplying the re air ratio. This forms a long flame which reaches from maining fuel (a secondary fuel) to the furnace from the the end of the primary combustion chamber outlet in outer edge of the primary combustion chamber and side of the furnace in the radiant tube, and where it using the low content of residual oxygen in the combus reaches the secondary fuel source, a low excess air ratio tion gas. This two stage fuel supply combustion burner 45 secondary combustion takes place in the radiant tube. is considered capable of achieving low NOx perfor Therefore, providing low flame temperature in the pri CC mary combustion zone, and a low excess air ratio for The above described regenerative radiant tube combustion in the secondary combustion zone provides burner, however, uses regenerative heat to preheat the an extremely low total NOx emission. As an advantage, combustion air up to about 1,000 C., and therefor has a 50 the fuel nozzle is set out from the exhaust gas passage disadvantage of emitting a large quantity of NOx caused and the secondary fuel nozzle is encompassed with by the extremely high flame temperature. It is disadvan refractory materials to avoid direct encounter with the tageous to have NOx emissions over 700 ppm as this high temperature combustion gas, thereby protecting greatly exceeds the emission limitation allowed (120 thermal erosion or a fuel cracking on the fuel nozzles. ppm) by the Japanese Regulation of the Atomospheric 55 Further, in one embodiment of the present invention, Contamination Prevention Law. In addition, this burner the secondary fuel nozzle is placed approximately on has a structural problem which can result in thermal the axis of the primary combustion chamber, whereby erosion or rupture of the fuel nozzle 103 which is pres the second combustion flame is surrounded by the pri ent in the high temperature combustion gas atmosphere mary combustion gas such that the flame is prevented and continuously heated to the point of possible over from directly contacting with inner wall of the radiant heating even it is not in operation. tube, thus local overheating in the radiant tube is pre On the other hand, if the well known open flame vented and therefore the life of the tube is prolonged. burner of the two stage fuel supply combustion method In addition, in this embodiment, it is able to be formed is applied into a radiant tube burner, commonly, the a long flame in the radiant tube by enlarging the flow combustion is not created by using small amount of fuel 65 passage area of the primary combustion gas without until the air is in condition of largely exceeding a rea changing the diameter of the tube in order to slow the sonable excess air ratio. Because the condition is out of primary combustion gas flow velocity, which causes the combustible range. In addition, a reasonable excess the mix of the primary combustion gas and the second

Page 8

ary combustion gas to moderate. Therefor it is further FIG. 2 is a detailed magnification cross-sectional side contributes to prevention of local over heating in the view of the burner.

tube and has uniform thermal distribution even in case FIG. 3 is a cross-sectional view at III-III in FIG. 2. of applying an installed radiant tube or in case of with FIG. 4 is a cross-sectional view at IV-IV in FIG. 2. out changing the diameter of the tube. 5 FIG. 5 is a cross-sectional view of an alternate em Further, the refractory materials structured in the bodiment of the present invention.

primary combustion chamber, which is inserted into the FIG. 6 is a cross-sectional view of other alternate radiant tube, protects the bung portion of the radiant embodiment of the present invention. tube and also prevents contact with the primary com 10 FIG. 7 is a central cross-sectional view of an alternate embodiment of the present invention illustrating a por bustion gas. The result prevents rupturing of the radiant tion tube at the bung. of a burner.

FIG. 8 illustrates an alternate embodiment of the

Further in a preferably embodiment of this invention, present invention.

the secondary fuel nozzle is protruded from the primary combustion chamber, which generate self-recirculation FIG. 9 is a graphic illustration of temperature distri in the combustion gas, so that NOx emission is more 15 bution along a radiant tube in axis direction. FIG. 10 illustrates an example of a radiant tube reduced. Further, in another embodiment of this inven burner incorporating prior art (U.S. Pat. No. 4,604,051). tion, the secondary fuel nozzle is located approximately on the axis of the primary combustion chamber and it DETALED DESCRIPTION injects a secondary combustion fuel both in the axial 20 Now, referring to the embodiments represented by and in the radial directions of the tube. A secondary the figures, the present invention shall be explained in combustion is generated by the primary combustion gas detail.

and the radially injected fuel; and, furthermore, a third combustion is also generated by the axially injected fuel ofFIG. 1 is a schematic illustration of one embodiment and the secondary combustion gas. This is so called a 25 burner. present invention illustrating a radiant tube the three stage combustion system, and is essentially effec This radiant tube burner contains a pair of burners 3 tive in reducing the NOx emissions and provides for an each of which is connected to one end of a radiant tube uniform temperature distribution along the radiant tube. 1. Each of these burners is alternately connected to the According to experimental results, we found the NOx combustion air supply system 9 and to the combustion emission is maintained lower than 100 ppm in this con 30 gas exhaust system 10 respectively intermediated by struction. regenerative bed 2. Burners are alternatively operated In yet another embodiment of this invention, a recu in fire or in flue mode. The combustion exhaust gas is perator attached to one end of the radiant tube receives exhausted through the regenerative bed 2 where the the exhaust combustion gas from the tube, and supplies associated burner is not being operated. On the other preheated combustion air to the burner which is in 35 hand the combustion air is supplied through the regen stalled at the other end of the tube. erative bed 2 where the associated burner is being oper Further this invention discloses a radiant tube burner ated. Each of the previously mentioned burners 3 con which has a set of burners placed at both end of radiant sists of a primary nozzle 4 which injects a portion of the tube, wherein said each burner has a primary combus fuel called the primary fuel, a combustion air duct 5 tion chamber which is placed outside of the furnace and which injects the full amount of combustion air, a pri of which the combustion gas injection outlet located mary combustion chamber 6 wherein the primary fuel is approximately more inside of the furnace than the bung burned with the full amount of combustion air under a of the radiant tube; a primary fuel nozzle which injects high excess air ratio, and secondary fuel nozzle 7 which the primary fuel into the primary combustion chamber injects the remaining fuel called the secondary fuel and which is located away from the exhaust gas pas 45 directly in to the radiant tube at the outlet of the fired sage; and a secondary fuel nozzle which is surrounded primary combustion chamber or at the combustion gas with refractory materials and which injects the second injection outlet 21.

ary fuel into the radiant tube adjacent to the combustion The combustion air duct 5 is connectd to the regener gas injection outlet; and allows to create primary com ative bed 2, so that both the exhaust of combustion gas bustion by supplying the primary fuel and almost the 50 and the supply of the combustion air is done, through total amount of combustion air into the primary com the regenerative bed 2. In the radiant tube 1, the second bustion chamber and the secondary combustion with ary combustion is created by using the remaining alloca the secondary fuel by combining the secondary fuel tion of fuel which is injected from secondary fuel nozzle with low concentrated oxygen remaining in the gener 7 and the residual oxygen in the primary combustion gas ated primary combustion gas; and each burner is con 55 which is not consumed on the primary combustion. The nected through a regenerative bed selectively either to fuel ratio supplied to the nozzle in the primary combus an air supply system for combustion or to an exhaust tion chamber 6 and to the nozzle in the radiant tube 1 is combustion gas system, whereby the burners operate commonly 5 to 50% for primary fuel and 95 to 50% for alternatively by supplying the combustion air through the secondary fuel. The preferable ratio is 20% for the the regenerative bed or to exhaust the combustion gas primary fuel and 80% for the secondary fuel. through the regenerative bed. The total amount of combustion air is supplied to the In addition to the low NOx emission, this invention primary combustion chamber 6. A small amount of the leads to obtaining an uniform temperature distribution combustion air, 5 to 10%, could be injected directly into along the radiant tube as shown in FIG. 9. the radiant tube 1 where it is ignited by the secondary 65 fuel so long as the two stage fuel supply combustion

BRIEF DESCRIPTION OF THE DRAWING concept can accommodate the excess combustion air. FIG. 1 is a schematic representation of a radiant tube The regenerative bed 2 is alternatively connected to burner in accordance with the present invention. the combustion air supply system 9 and the combustion

Page 9

gas exhaust system 10 through the four direction revers ceramics, nitride silicate type ceramics, or sialon (Si6 ing valve 8. The combustion air is supplied by the Al-OgN6-) type ceramics. In this particular embodi forced draft fan 11 of the combustion air supply system ment, the primary combustion chamber 6 is structured 9 through the fired regenerative bed 2 of operated castable refractory 16. The castable refractory 16 is burner 3, and the combustion gas is drafted by the in covered by heat-proof metal member 23 which has a duced fan 12 through the non-fired regenerative bed 2 flange to attached to the radiant tube 1. In this particu of the non-operated burner 3. The flow of combustion lar embodiment, the primary combustion chamber 6, air and combustion gas is changed by a timer (not made of castable refractory 16 and covered with heat shown in the drawing) in specific internals or by mea proof metal member, has two portions where one por suring the exhaust gas temperature by a thermal detec 10 tion is called the back wall 22 can be separated from the tor (not shown in the drawing). If the measured temper other portion for easy replacement of the secondary ature reaches the specific limit or if the set interval is fuel nozzle 7 and the insulator 17. reached, then the reversing value 8 redirects the flow of The injection outlet 21 of the primary combustion combustion air and this operation is synchronized with chamber 6 is located approximately more inside of the changing the supply of fuel. 15 furnace 14 than the bung portion 25 of the radiant tube The fuel supply source 14 provides fuel through a 1. The “approximately more inside' point should be magnetic solenoid valve 13A which is connected to a understood as a point which is at essentially more inside primary fuel nozzle 4 and another magnetic solenoid of furnace than the bung portion 25, or at the same valve 13B which is connected to a secondary fuel noz surface of the furnace inner wall or even at slightly zle 7. There is a pair of magnetic solenoid valve associ 20 more concaved inside than the furnace inner wall. ated with each of the firing chambers. Just after igni The regenerative bed 2 is to accumulate temporary tion, it is possible to have a secondary unstable combus sensible heat from the combustion exhaust gas and tion. This is easily controlled by delaying the opening of could be made from any materials or by any kind of the magnetic solenoid valve 13B associated with the structural methods as long as the materials/methods fired secondary fuel nozzle 7 than the magnetic solenoid 25 neither reacts with the combustion gas nor provides any valve 13A associated with the fired primary fuel nozzle bad effects to the combustion air. Materials for the 4. regenerative bed 2, in general, need to meet the follow The previously mentioned primary fuel nozzle 4 ing requirements; large area for the heat transfer, small should be located at the back of the primary combustion pressure loss, resistance to high temperature, resistance chamber 6, which is set outside of the furnace 14 so as 30 to thermal shock, and resistance to corrosion. Thus to be away from the combustion gas exhaust passage ceramics, such as alumina, or heat-proof material has and, therefore avoid direct contact to the combustion been used. As an example, sponge-like-foamed and gas gas. This particular embodiment has a primary fuel permeable ceramics, called honeycomb ceramics, hav nozzle 4 embedded into the back wall of the insulating ing many honeycomb shaped holes in the direction of firebrick 22 of the primary combustion chamber 6 and 35 gas flow have been used as the regenerative bed materi having the only outlet of the nozzle is opened to the als.

primary combustion chamber 6. A pilot burner 18 is In this description, a gas permeable, regenerative bed provided near the primary fuel nozzle 4. The numeral means not only that the material has many holes as a 19 indicates a sight hole. characteristics or in itself, but, also the material can hold The secondary fuel nozzle 7 is centered on the central 40 permeability within the structure, even if no specific axis of the primary combustion chamber 6 and injects method is set aside for permeability. Therefore, it is the secondary fuel into the radiant tube at the outlet of possible to make a permeable, solid structure where the the primary combustion chamber 6, i.e., at the combus combustion exhaust gas temperature is as high as 1,000 tion gas injection outlet 21. The secondary fuel nozzle 7, C., with heat-proof metal wire, such as FCH-2 made of in detail description, is located in center of the primary 45 Fe, Cr, and Al alloy netted as a wire gauze and then combustion chamber 6 which made of castable refrac piled up in appropriate thickness. This kind of regenera tory 16, protected by refractory insulator 17. On this tive bed material can be packed in the combustion air preferably embodiment, lightweight and less expensive duct 5 or be a cartridge type which provides for easy glass wool cylinder is applied and held by the secondary replacement.

fuel nozzle 7. The method is not limited by the above 50 The configuration of the radiant tube 1 need not be a description. For example, the insulator 17 could be certain shape such as straight type, U type, T type, W made of formed castable refractory or carbon silicate type, O type, or L type known to be in the Prior Art. It ceramics and inside of it, the secondary fuel nozzle 7 could be a new style.

can be embedded; or, the secondary fuel nozzle 7 can be The radiant tube burner described and structured made of refractory materials such as fused quartz or fine 55 above is operated as follows: ceramics in which case, the extra insulator may not be At first, one of burners 3 is fired with combustion air required. supplied by the forced draft fan 11. At the same time, In the case of this embodiment, the secondary fuel high temperature combustion gas in the radiant tube 1 is nozzle 7 is held by the back wall portion 22 of the pri exhausted through the combustion air duct 5 of the mary combustion chamber 6 at one end and the other non-operated burner 3 by the pull draft operation of the end is held at castable refractory 16 by cross shaped induced fan 12. The heat from the combustion gas ex suspension frame 20. The back wall portion 22 is sepa hausted is recovered while passing through the regener rately fabricated to be removable from the main body ative bed 2. After defined time interval, the burner 16 of the primary combustion chamber 6, wherefore the which has been operated is allowed to rest, on the other insulator 17 is able to replace and maintain. 65 hand, the non-operated burner 3 starts combustion. The The primary combustion chamber 6 has a cylindrical combustion gas is exhausted through the regenerative shape and is usually made of refractory material such as, bed 2 associated with burner 3 which has been in the for example, castable refractory, silicon carbide type operational or fired mode and now is in the non-opera

Page 10

tive mode. On the other hand, the combustion air ab the portion of fuel and the full amount of the combus sorbs the heat which has been provided by the exhaust tion air are combusted in laminar diffusion combustion gas and accumulated to the regenerative bed 2 which under high excess air ratio, and a secondary fuel nozzle has been on the non-operated side from the regenerative 7 which injects remaining fuel (secondary fuel) into the bed 2 and then is supplied to burner in preheated condi 5 radiant tube 1 circumferencely at the outlet of the pri tion at, for example, 700 to 1,000 C. The temperature of mary combustion chamber 6. In the burner, the small the radiant tube 1 and the regenerative bed 2 is in amount of fuel is injected to and enveloped in the layer creased gradually by means of changing combustion of the rotating combustion air which injected from the and exhaust alternatively until the set temperature of combustion air duct 5 into the primary combustion both the radiant tube and the regenerative bed is 10 chamber 6, which brings to create a laminar diffusion achieved. Then the system of combustion stays in con combustion. As the result, a long flame formed by the stant operation. The alternative change of combustion combustion reaches radiant tube 1. The secondary com air and the combustion gas is done at appropriate time bustion occurs in the radiant tube both with the balance intervals, such as every 20 seconds to 5 minute, or when of fuel which injected from the secondary fuel nozzle 7 the temperature of the exhausted combustion gas 15 and the oxygen remaining the primary combustion gas. reaches a set value, such as about 200 C. The primary fuel nozzle 4 should be located at the back FIG. 5 illustrates the other embodiment of the sec of the primary combustion chamber 6, which is set ondary fuel nozzle. Here the secondary fuel nozzle 7 outside of the furnace 14 so as to be away from the surrounded with the insulator 17 made of light weight combustion gas exhaust passage and, therefore avoid refractory materials such as glass wool is protruded 20 direct contact to combustion gas. In this embodiment, from the primary combustion chamber 6 to improve on the primary fuel nozzle 4 is embedded into the back wall low NOx performance. If the length of the protrusion is 22 of the primary combustion chamber 6 made of casta too large, the nozzle 7 tends to bend downward due to ble refractory and only the tip of the nozzle is opened to the heat. On the other hand, if the length is too small, the primary combustion chamber 6. In this case the low NOx performance can not be achieved. Therefore, 25 primary combustion fuel nozzle 4 also acts as a pilot the protrusion of the secondary fuel nozzle as well as burner.

the insulator 17 from the primary combustion chamber The secondary fuel nozzle 7 is embedded into the is about 50 to 300 mm, and preferably about 200 mm in primary combustion chamber which constructed of length. The protrusion type secondary fuel nozzle castable refractory 16 and has an injection outlet open achieves better low NOx performance compared with 30 ing in parallel with the primary combustion gas injec the embodiment shown in FIG. 1, since self-recircular tion outlet 21 at the end of castable refrctory 16 con tion flow of the combustion gas occurs around the sec structed chamber 6. The secondary fuel nozzle 7 is ondary nozzle. located further inside of the furnace 14 than the furnace FIG. 6 illustrates characteristics of the secondary fuel wall 15 and has plural openings, for example four open nozzle in yet another embodiment of the present inven 35 ings, circumferentially to the primary combustion gas tion. In this case, the secondary fuel nozzle 7 is pro injection outlet 21. This configuration provides the best truded slightly out from the primary combustion cham result for low NOx performance and empirical results ber 6 into the radiant tube 1 and has one injection outlets reached under 100 ppm of the NOx emissions. But, in 24 at the end in the axial direction and a few injection other hand, if the diameter of the primary combustion outlets 24 in radial direction. The fuel injected in the 40 chamber 6 becomes smaller, the fuel and the combus radial direction through the radial outlets 24 creates tion air mixture and diffusion becomes better and the secondary combustion with a low oxygen concentra flame tends to become diverged and shorter. Therefore tion (about 17%) of combustion air which remains in it forms a short, blue flame which is apppropriate for the the primary combustion gas, and then the fuel injected open flame burner application, but not for this burner. in the axial direction through the axial outlet 24 creates 45 Therefore, it is preferable to possibly enlarge the diame third combustion with the further low-concentrated ter of the primary combustion chamber 6 until second oxygen remaining combustion air (about 1.1%). Thus ary fuel cracking occurs. For example, it is essentially the three stage fuel supply combustion occurs in this possible to make the diameter of the primary combus embodiment. Furthermore, there is a case where the tion chamber 6 larger along with making the diameter amount of the primary fuel becomes nearly zero or 50 of the radiant tube 1 larger.

sometimes completely zero. Then, the secondary com FIG. 8 illustrates a radiant tube burner with another bustion and the third combustion represents, in fact, a embodiment of the recuperator of the present invention. two stage, fuel supply combustion. The operable fuel In this configuration, a radiant tube 1 is equipped with distribution, in this case, is about 5 to 50% for the pri a burner 3 at one end and a recuperator 30 at another mary fuel and 95 to 50% for the secondary fuel which 55 end which preheats the combustion air using the ex includes the fuel for the third combustion. The prefera. haust combustion gas. The temperature of the combus ble fuel ratio is about 20% for the primary and 80% for tion gas in the radiant tube burner commonly is about the secondary (considering 40% for the secondary and 1,000 C. When the exhaust heat of combustion gas is 60% for the third combustion in the case of the three recovered in the recuperator 30, the exhaust tempera stage assuming that the secondary fuel considered as 60 ture may be decreased to about 500 C. and the combus 100%). tion air may be preheated to between about 350 and FIG. 7 illustrates more another embodiment of this 450 C.

invention. In this embodiment, a burner consists of a The recuperator 30 is constructed, for example, with primary fuel nozzle 4 which injects portion of fuel (pri double wall tubings inserted into the radiant tube 1. mary fuel), a combustion air duct 5 which injects a full 65 When the combustion air is introduced into the inner amount of combustion air to the primary combustion tube 31 it is transferred to the outer tube 32 through the chamber 6 in tangential direction of the cylindrical open top end of the inner tube 31. Heat exchange then chamber, the primary combustion chamber 6 wherein takes place between the combustion air in tube 32, and

Page 11

the combustion gas which flows in the radiant tube 1. 2. A radiant tube burner according to claim 1, around the tube 32. The outer tube 32 of the recuperator wherein the amount ratio of said primary fuel to whole 30 is connected by a connecting tube 33 to the combus fuel is in a range of about 5 to about 50%, and the tion air supply duct 5 of the burner 3 located on the amount ratio of said secondary fuel to whole fuel is in opposite side for supplying the preheated combustion the range of about 95 to about 50%. air. In this embodiment illustrated in the figure, the 3. A radiant tube burner according to claim 1, burner 3 has the secondary fuel nozzle 7 which is placed wherein the amount ratio of said primary fuel to whole on the axis of the primary combustion chamber 6, but it fuel is about 20%, and the amount ratio of said second is not limited by the position of the nozzle and it is ary fuel to whole fuel is about 80%. possible to have secondary fuel nozzles 7 which are 10 4. A radiant tube burner according to claim 1, embedded into the castable refractory 16 form the pri wherein said secondary fuel nozzle is located on the axis mary combustion chamber 6 (see FIG. 7). In this case, of said primary combustion chamber and surrounded the thermal efficiency becomes high since the combus with said refractory material. tion exhaust gas from the other end is used to preheat 5. A radiant tube burner according to claim 4, the combustion air. 15 wherein said secondary nozzle is protruded from said Further more, there is a case that the radiant tube injection outlet of said primary combustion chamber. burner in this invention does not recover heat using the 6. A radiant tube burner according to claim 4, regenerative bed 2 or the recuperator 30 (not shown). In wherein said secondary fuel nozzle injects said second this case the thermal efficiency is reduced, but, NOx ary fuel in both axial direction and radial direction of emissions are lower in comparison to the prior art of 20 said radiant tube.

constructing radiant tube burners. 7. A radiant tube burner according to claim 6, What is claimed is: wherein said secondary fuel nozzle injects about 1. A radiant tube burner comprised of: 5-50% of said secondary fuel to radial direction of said a primary combustion chamber which is placed out radiant tube and about 50-95% of said secondary fuel side of a furnace and of which a combustion gas 25 to axial direction of said radiant tube. injection outlet is located approximately more in 8. A radiant tube burner according to claim 6, side of the furnace than the bung of a radiant tube; wherein said secondary fuel nozzle injects about 40% of a primary fuel nozzle which injects a primary fuel said secondary fuel to radial direction and about 60% of into said primary combustion chamber to create a said secondary fuel to axial direction. primary combustion; and 30 9. A radiant tube burner according to claim 1, includ a secondary fuel nozzle surrounded with refractory ing a plurality of said secondary fuel nozzles embedded materials which injects secondary fuel into said inside of the wall of said primary combustion chamber radiant tube from the area adjacent to said combus and said refractory material forms the wall of said com tion gas injection outlet, wherein the primary con bustion chamber, so that said secondary fuel is injected bustion is created by supplying said primary fuel 35 about the periphery of said primary combustion gas and about a total amount of combustion air for the injection outlet.

burner into said primary combustion chamber to 10. A radiant tube burner according to claim 1, generate primary combustion gas, and the second wherein said burner is attached at one end and a recupe ary combustion is created in said radiant tube by rater at the opposite end of said radiant tube in order to supplying said secoondary fuel to the generated take use of said exhaust combustion gas for supplying primary combustion gas having a low concentra preheated combustion air. t e t tion of oxygen.