Abstract:
Combustion method and apparatus for NO x  reduction are capable of easily achieving NO x  reduction to an exhaust NO x  value of 30 ppm or under. The combustion method is to perform in combination a first NO x  reduction step for suppressing generated NO x  value to 60 ppm or under (at 0% O 2  in exhaust gas, dry basis) by a low NO x  burner, a second NO x  reduction step for recirculating exhaust gas of the low NO x  burner to a burning reaction zone formed by the low NO x  burner, and a third NO x  reduction step for adding water or steam to the burning reaction zone.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a combustion method for NO x  reduction, as well as an apparatus therefor, to be applied to water-tube boilers, reheaters of absorption refrigerators, or the like.  
           [0002]    Generally, as the principle of suppression of NO x  generation, there have been known (1) suppressing the temperature of flame (combustion gas), (2) reduction of residence time of high-temperature combustion gas, and (3) lowering the oxygen partial pressure. Then, various NO x  reduction techniques to which these principles are applied are available. Examples that have been proposed and developed into practical use include the two-stage combustion method, the thick and thin fuel combustion method, the exhaust gas recirculate combustion method, the water addition combustion method, the steam jet combustion method, the flame cooling combustion method with water-tube groups (water-tube cooling combustion method), and the like.  
           [0003]    With respect to small-size once-through boilers, as of today, there has been laid out in Tokyo Metropolis or others a regulation that the exhaust NO x  value of gas-fired boilers should be not more than 60 ppm (at (0% O 2  in the exhaust gas, dry basis; hereinbelow, the unit ppm is expressed at 0% O 2  in the exhaust gas, dry basis, unless otherwise specified), and that the exhaust NO x  value of oil-fired boilers should be not more than 80 ppm for A-type heavy oil and not more than 60 ppm for kerosine. Many manufacturers including the present applicant have cleared these regulation values. However, California in U.S.A. has already laid out a regulation specifying not more than 12 ppm (at 3% O 2  in the exhaust gas, dry basis). The applicant, envisaging that even stricter regulations, e.g. not more than 30 ppm, will be applied in the near future also in Japan, has been performing research and development for further NO x  reduction.  
           [0004]    A prior-art NO x  reduction techniques is proposed in combinations of above-described various suppression principles (see, e.g., Patent Reference 1: Japanese Published Patent Application H07- 103411 , Page 3, FIG. 1). This prior-art technique is a combination of the exhaust gas recirculate technique and the steam jet. However, with this NO x  reduction technique, it is not easy to achieve an exhaust NO x  value of not more than 30 ppm (hereinafter, referred to as “target exhaust NO x  value”).  
           [0005]    That is, the present inventors of this application have found through various experiments and discussions that the following issues exist in order to achieve the target exhaust NO x  value or lower in the prior art.  
           [0006]    First, in the prior art, for reduction of the NO x  value by a functional enhancement of combustion gas temperature suppression with the exhaust gas recirculation, the functional enhancement is to increase the exhaust-gas recirculation quantity. However, implementing this functional enhancement would cause unstable characteristics of the exhaust gas recirculation to be amplified. That is, the exhaust gas recirculation has a characteristic that the exhaust-gas flow rate or temperature changes with changes in combustion quantity or changes in load. An increase in the exhaust-gas recirculation quantity would cause these unstable characteristics to be amplified, making it impossible to achieve a stable NO x  reduction. Also, an increase in the exhaust-gas recirculation quantity would cause the oxygen concentration in the combustion air to lower, resulting in a combustion state of oxygen deficiency, so that the combustion could no longer be continued because of incomplete combustion or discharge of unburned combustibles. Further, a volume increase corresponding to the exhaust-gas recirculation quantity would cause the pressure loss in the air blow passage to increase, thus making it inevitable to increase the cost due to the increase in the blower capacity.  
           [0007]    Also, a functional enhancement of NO x  reduction by steam addition is to increase the quantity of water to be added. This functional enhancement would cause an increase in thermal loss and moreover an increase in the quantity of condensations, posing a problem of corrosion of the constituent equipment due to the condensations.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to provide a combustion method for NO x  reduction, as well as an apparatus therefor, capable of solving these and other issues and easily achieving NO x  reduction with the value of exhaust NO x  under 30 ppm.  
           [0009]    The present invention having been accomplished to solve the above object, in a first aspect of the invention, there is provided a combustion method for NO x  reduction comprising in combination the steps of: a first NO x  reduction step for suppressing generated NO x  value to 60 ppm or under (at 0% O 2  in exhaust gas, dry basis) by a low NO x  burner; a second NO x  reduction step for recirculating exhaust gas of the low NO x  burner to a burning reaction zone formed by the low NO x  burner; and a third NO x  reduction step for adding water or steam to the burning reaction zone.  
           [0010]    In a second aspect of the invention, there is provided a combustion method for NO x  reduction as described in the first aspect, wherein the second NO x  reduction step is performed with a target exhaust NO x  value set to 30 ppm or under (at 0% O 2  in exhaust gas, dry basis) and with an exhaust-gas recirculation quantity set in a stable combustion range of the low NO x  burner, and any NO x  value exceeding the target exhaust NO x  value is reduced by the third NO x  reduction step.  
           [0011]    In a third aspect of the invention, there is provided a combustion method for NO x  reduction as described in the first or second aspect, wherein the third NO x  reduction step is performed by spraying water directly to the burning reaction zone.  
           [0012]    In a fourth aspect of the invention, there is provided a combustion apparatus for NO x  reduction, comprising: a low NO x  burner for suppressing generated NO x  value to 60 ppm or under (at 0%  02  in exhaust gas, dry basis); exhaust gas recirculation means for recirculating exhaust gas of the low NO x  burner to a burning reaction zone formed by the low NO x  burner; and water or steam addition means for adding water or steam to the burning reaction zone.  
           [0013]    Further, in a fifth aspect of the invention, there is provided a combustion apparatus for NO x  reduction, comprising: a low NO x  burner for suppressing generated NO x  value to 60 ppm or under (at 0% O 2  in exhaust gas, dry basis); exhaust gas recirculation means for recirculating exhaust gas of the low NO x  burner to a burning reaction zone formed by the low NO x  burner; and water spraying means for spraying water directly to the burning reaction zone.  
           [0014]    In one embodiment, there is provided a NO x  reduction combustion method as described in any one of the first to third aspects, wherein the NO x  reduction step is performed with an excess air ratio which is determined from a NO x  reduction target value and an excess air ratio versus NO x  characteristic of the NO x  reduction step.  
           [0015]    Before the description of embodiments of the present invention, terms used herein are explained. The combustion gas includes burning-reaction ongoing (under-combustion-process) combustion gas, and combustion gas that has completed burning reaction. Then, the burning-reaction ongoing gas refers to combustion gas that is under burning reaction, and the burning-completed gas refers to combustion gas that has completely burning-reacted. The burning-reaction ongoing gas is indeed a concept of substance, but can also be referred to as flame as a concept of state because it generally includes a visible flame so as to be in a flame state. Therefore, herein, the burning-reaction ongoing gas is referred to also as flame or burning flame from time to time. Further, the burning reaction zone refers to a zone where the burning-reaction ongoing gas is present, and the exhaust gas refers to burning-completed gas that has decreased in temperature under an effect of endothermic action by heat transfer tubes or the like.  
           [0016]    Also, the combustion gas temperature, unless otherwise specified, means the temperature of burning-reaction ongoing gas, equivalent to combustion temperature or combustion flame temperature. Further, the suppression of combustion gas temperature refers to suppressing the maximum value of combustion gas (combustion flame) temperature to a low one. In addition, normally, burning reaction is continuing although in a trace amount even in the burning-completed gas, and so the combustion completion does not mean a 100% completion of burning reaction. The target exhaust NO x  value refers to a target value for the NO x  value exhausted from the NO x  reduction combustion apparatus.  
           [0017]    Next, embodiments of the present invention are described. The present invention is applied to thermal equipment (or combustion equipment) such as small-size once-through boilers or other water-tube boilers, water heaters, reheaters of absorption refrigerators or the like. The thermal equipment has a burner and a group of heat absorbers to be heated by combustion gas derived from the burner.  
           [0018]    An embodiment of the method according to the present invention is a combustion method for NO x  reduction comprising in combination the steps of: a first NO x  reduction step for suppressing generated NO x  value to 60 ppm or under, preferably 50 ppm or under, by a low NO x  burner; a second NO x  reduction step for recirculating exhaust gas of the low NO x  burner to a burning reaction zone formed by the low NO x  burner; and a third NO x  reduction step for adding water or steam to the burning reaction zone. Means for performing the first NO x  reduction step, means for performing the second NO x  reduction step, and means for performing the third NO x  reduction step are referred to as first NO x  reduction means, second NO x  reduction means, and third NO x  reduction means, respectively.  
           [0019]    The first NO x  reduction means is the low NO x  burner. The low NO x  burner may be implemented by a burner that suppresses the generated NO x  value to 60 ppm or under by using any one or combining any ones from among the divided flame combustion method, the self recirculate method, the staged combustion method, the thick and thin fuel combustion method, and other techniques. The low NO x  burner is preferably given by a gas-fired burner, but may also be an oil-fired burner in another embodiment.  
           [0020]    Then, burning reaction is performed in front of the low NO x  burner, by which a burning reaction zone is formed.  
           [0021]    The second NO x  reduction means is what is called exhaust gas recirculation method, in which part of exhaust gas to be discharged into the atmospheric air after having decreased in temperature under an effect of endothermic action by the heat absorbers is mixed into the combustion air by external recirculation via an exhaust-gas recirculation passage, which is an external passage. By a combustion-gas-temperature suppression effect or a decrease in oxygen concentration or the like attributable to this mixed exhaust gas, the NO x  value is reduced.  
           [0022]    The exhaust-gas recirculation quantity by the second NO x  reduction means is set to within the stable combustion range of the low NO x  burner. The stable combustion range refers to a range in which the exhaust CO amount is 100 ppm or under, preferably 50 ppm or under.  
           [0023]    The third NO x  reduction means is water or steam addition to the burning reaction zone. By this water or steam addition, the burning-reaction ongoing gas is cooled so that the combustion gas temperature is suppressed, thus the NO x  value being reduced.  
           [0024]    The water or steam addition is performed, preferably, by spraying water directly toward the burning reaction zone. By doing so, in an embodiment in which gaseous mixture of combustion air and exhaust gas is blown to the low NO x  burner by a blower, it becomes implementable to prevent the blower from corrosion and to fulfill the NO x  reduction while suppressing the increase in the capacity of the blower to a minimum.  
           [0025]    Otherwise, the water or steam addition by the third NO x  reduction means may be done in the exhaust-gas recirculation passage in another embodiment. Furthermore, in an embodiment in which the gaseous mixture of combustion air and exhaust gas is fed to the low NO x  burner by a blower, steam addition may be performed between the low NO x  burner and the blower.  
           [0026]    In the combustion method for NO x  reduction of this embodiment, the target exhaust NO x  value is set to 30 ppm or under, preferably 20 ppm or under. Then, the generated NO x  value by the first NO x  reduction means is set to 60 ppm or under, preferably 50 ppm or under, and subsequently a NO x  reduction is performed by the second NO x  reduction means.  
           [0027]    With this arrangement, given that the generated NO x  value by the first NO x  reduction means is A, the NO x  reduction value by the second NO x  reduction means is B and the target exhaust NO x  value is X, then it is assumed that the third NO x  reduction means fulfills a NO x  value of A-B-X=C. That is, setting the NO x  reduction value by the third NO x  reduction means to C or more makes it possible to achieve the target exhaust NO x  value or under.  
           [0028]    By this method as described above, there can be produced an effect that the target exhaust NO x  value or under can be achieved without incurring the aforementioned issues of the exhaust gas recirculation, and moreover such problems as the corrosion of the equipment can be avoided and further the increase in the blower capacity can be suppressed to a minimum.  
           [0029]    Also, in the foregoing embodiment, preferably, a combustion space where the heat transfer tubes are not present, i.e. the heat transfer tubes have been eliminated, is formed in front of the low NO x  burner, so that the burning reaction is performed in the combustion space, with a burning reaction zone formed there. Desirably, the combustion space has such an area that burning reaction of the fuel jetted out from the low NO x  burner is completed within the zone, but this is not limitative.  
           [0030]    That a combustion space where the heat transfer tubes are not present is formed in front of the low NO x  burner means that the water-tube cooling combustion method is not aggressively performed. As a result of this, it is no longer necessary to take measures for the issues of the water-tube cooling combustion method, i.e., the emission of large amounts of CO or unburned combustibles due to the burning-reaction suppression effect of the water tubes. In particular, the NO x  reduction technique by the water-tube cooling combustion method has an issue that the combustion itself cannot be continued in applications to combustion apparatus using an oil-fired burner, and therefore it is preferable to form in front of the low NO x  burner a combustion space where the heat transfer tubes are not present.  
           [0031]    Further, in the foregoing embodiment, preferably, the water or steam addition is performed by spraying water directly toward the burning reaction zone within the combustion space. By doing so, a stable suppression of the combustion gas temperature is fulfilled. Also, in the embodiment in which the gaseous mixture of combustion air and exhaust gas is blown to the low NO x  burner by a blower, it becomes implementable to prevent the blower from corrosion and moreover to prevent the blower from increasing in load.  
           [0032]    Next, embodiments of the apparatus according to the present invention are described. The present invention includes the following embodiments (1) to (5) of the apparatus corresponding to the foregoing embodiments of the method.  
           [0033]    Embodiment (1): A combustion apparatus for NO, reduction comprising: a low NO x  burner for suppressing generated NO x  value to 60 ppm or under (at 0% O 2  in exhaust gas, dry basis); exhaust gas recirculation means for recirculating exhaust gas of the low NO x  burner to a burning reaction zone formed by the low NO x  burner; and water or steam addition means for adding water or steam to the burning reaction zone.  
           [0034]    Embodiment (2): A combustion apparatus for NO x  reduction as defined in Embodiment (1), wherein with a target exhaust NO x  value of 30 ppm, the target exhaust NO x  value is fulfilled by NO x  reduction effects by the exhaust gas recirculation means and the water or steam addition means.  
           [0035]    Embodiment (3): A combustion apparatus for NO x  reduction comprising: a low NO x  burner for suppressing generated NO x  value to 60 ppm or under (at 0% O 2  in exhaust gas, dry basis); exhaust gas recirculation means for recirculating exhaust gas of the low NO x  burner to a burning reaction zone formed by the low NO x  burner; and water spraying means for spraying water directly to the burning reaction zone.  
           [0036]    Embodiment (4): A combustion apparatus for NO x  reduction, wherein a combustion space where the heat transfer tubes have been eliminated is formed in front of the low NO x  burner.  
           [0037]    Embodiment (5): A combustion apparatus for NO x  reduction comprising: a low NO x  burner for suppressing generated NO x  value to 60 ppm or under (at 0% O 2  in exhaust gas, dry basis), the low NO x  burner being switchable between low combustion and high combustion; exhaust gas recirculation means for recirculating exhaust gas of the low NO x  burner to a burning reaction zone formed by the low NO x  burner in low combustion and high combustion of the low NO x  burner; and water or steam addition means for adding water or steam to the burning reaction zone only in the high combustion of the low NO x  burner. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]    [0038]FIG. 1 is an explanatory view of a longitudinal section of a steam boiler to which an embodiment of the present invention is applied;  
         [0039]    [0039]FIG. 2 is an enlarged sectional explanatory view of a main part of FIG. 1;  
         [0040]    [0040]FIG. 3 is an explanatory view of a bottom face of the main part of FIG. 2;  
         [0041]    [0041]FIG. 4 is a chart showing a NO x  reduction characteristic relative to water spray quantity in the same embodiment;  
         [0042]    [0042]FIG. 5 is a chart showing a NO x  reduction rate characteristic relative to water spray quantity in the same embodiment; and  
         [0043]    [0043]FIG. 6 is a chart showing a wind box pressure characteristic relative to water spray quantity. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]    Hereinbelow, working examples in which the NO x  reduction combustion method and apparatus of the present invention are applied to a once-through steam boiler, which is one type of water-tube boilers, are described in accordance with the accompanying drawings. FIG. 1 is an explanatory view of a longitudinal section of a steam boiler to which an embodiment of the present invention is applied, FIG. 2 is an enlarged sectional view of a main part of FIG. 1, FIG. 3 is an explanatory view of a bottom face of the main part of FIG. 2, FIG. 4 is a chart showing a NO x  reduction characteristic relative to water spray quantity in the same embodiment, FIG. 5 is a chart showing a NO x  reduction rate characteristic relative to water spray quantity in the same embodiment, and FIG. 6 is a chart showing a wind box pressure characteristic relative to water spray quantity.  
         [0045]    Referring to FIG. 1, a steam boiler  1 , which is the NO x  reduction combustion apparatus of this working example, is a boiler having a target exhaust NO x  value of 20 ppm and comprising: a low NO x  burner  2 ; a blower  3  for blowing combustion air to the burner  2 ; an annular-shaped boiler body  4  to the top-face opening of which the low NO x  burner  2  is to be fitted; an exhaust gas recirculation means  5  for mixing, and thereby feeding, part of exhaust gas discharged from the boiler body  4  into the combustion air for the low NO x  burner  2 ; and a water spray means  7  for spraying water to a burning reaction zone  6  formed by the low NO x  burner  2 .  
         [0046]    The low NO x  burner  2  performs the thick and thin fuel combustion method, the self recirculate combustion method and the two-stage combustion method in combination, by which the value of generated NO x  in a state in which neither the exhaust gas recirculation nor the water spray is performed is set to about 50 ppm. This low NO x  burner  2  is composed of a burner body  8 , and a wind box  9  for introducing combustion air to the burner body  8 .  
         [0047]    The burner body  8 , as shown in FIGS. 2 and 3, includes a generally annular-shaped-in-section fuel passage member  11  whose interior is a gas fuel passage  10 , and a cylindrical-shaped air register  12  disposed outside the fuel passage member  11  coaxially. Then, inside the fuel passage member  11  is a primary air passage  13  through which primary air passes, and between the fuel passage member  11  and the air register  12  is a secondary air passage  14 .  
         [0048]    Combustion air for the primary air passage  13  and the secondary air passage  14  is supplied by the blower  3 . In this working example, the proportions of primary air and secondary air are set to 10 to 20% of primary air and 90 to 80% of secondary air.  
         [0049]    Further, a first baffle plate  15  is provided at a position slightly deeper than the lower end of the primary air passage  13  so as to cover the lower-end opening, and a second baffle plate  16  is provided at an upper end of the primary air passage  13  so as to cover the upper-end opening. The first baffle plate  15  has a first opening  17  at a center, and the second baffle plate  16  has a plurality of small-diameter second openings  18 ,  18 , . . . through which primary air passes.  
         [0050]    The secondary air passage  14  also has an annular-shaped third baffle plate  19 . This third baffle plate  19 , as shown in FIG. 3, has six cut-outs  20 ,  20 , . . . . arranged circumferentially at generally equal intervals. By these cut-outs  20 , secondary air is dividedly fed (flow rate: 30 to 50 m/s), by which divided flames are formed.  
         [0051]    Further, in the fuel passage member  11  are provided outer jet holes  21 ,  21 , . . . for jetting gas fuel outward, and inner jet holes  22 ,  22 , . . . positioned at lower end portions and serving for jetting gas fuel inward. These outer jet holes  21  and inner jet holes  22  are provided circumferentially in plural numbers as shown in the figure, and the total opening area of the outer jet holes  21 ,  21 , . . . is set larger than the total opening area of the inner jet holes  22 ,  22 , . . . . The inner jet holes  22  are formed downstream of the first baffle plate  15 .  
         [0052]    Next, the wind box  9  is explained. Referring to FIG. 1, the wind box  9  functions to guide the combustion air blown by the blower  3  to the low NO x  burner  2 , and is composed of an outer cylindrical member  23  closed at its upper and lower ends and a lower-end opened inner cylindrical member  24  placed coaxial therewith.  
         [0053]    Next, the boiler body  4  is explained. Referring to FIG. 1, the boiler body  4  is described in detail in U.S. Pat. NO. 6,269,782(Japanese Published Patent Application 2001-41401), the disclosure of which is hereby incorporated by reference. The boiler body  4  has an upper header  25  and a lower header  26  spaced from each other at a specified distance. Between outer circumferences of these upper header  25  and lower header  26  is disposed an outer wall  27 .  
         [0054]    Between the upper header  25  and the lower header  26 , a plurality of water tubes  28 ,  28 , . . . are arranged in a double annular shape. These water tubes  28 ,  28 , . . . constitute annular-shaped inner first water wall  29  and outer second water wall  30 , with an annular-shaped exhaust gas passage  31  defined between these water walls  29 ,  30 . Then, a first outlet (not shown) for combustion gas that has nearly completed burning reaction is formed at a portion of the first water wall  29 , and a second outlet (not shown) for exhaust gas given by not providing the water tube is formed in the second water wall  30  opposite the first outlet (generally point-symmetrically). Reference numerals  32 ,  33  denote refractory members.  
         [0055]    Then, a space which is surrounded by the upper header  25 , the lower header  26 , the first water wall  29  and the like and in which the water tubes  28  are not present is assigned as a combustion space  34  where an air-fuel mixture of the fuel jetted out from the low NO x  burner  2  and combustion air is burned to form the burning reaction zone  6 . The upper header  25  is fitted with the low NO x  burner  2 , so that the combustion space  6  is formed in front of this burner  2 . The low NO x  burner  2  is inserted from an inward (central portion) of the upper header  25  toward the combustion space  34 , so that the combustion-gas jet direction of the low NO x  burner  2  and the water tubes  28  of the first water wall  29  are generally parallel to each other.  
         [0056]    Further, in the annular-shaped outer wall  27  provided outside the second water wall  30 , an exhaust-gas outlet  35  is provided at a position confronting the second outlet so as to communicate with the exhaust gas passage  31 . A smokestack  36  is connected to this exhaust-gas outlet  35 .  
         [0057]    Next, the exhaust gas recirculation means  5  is explained. This exhaust gas recirculation means  5  makes part of the exhaust gas discharged from the boiler body  4  mixed into the combustion air of the low NO x  burner  2  to suppress the combustion gas temperature and thereby reduce NO x .  
         [0058]    The exhaust gas recirculation means  5  is composed of a first duct  37  branched from the smokestack  36  and connected to an inlet port (not shown) of the blower  3 , the blower  3 , and a second duct  38  that connects a discharge port (not shown) of the blower  3  and the wind box  9  to each other. In the first duct  37  is provided an adjustment damper  39  that can adjust the exhaust gas recirculation rate. Reference numeral  40  denotes a cylindrical-shaped lid member to which the first duct  37  is connected and which is fitted to the inlet port so as to cover it, and fresh air inlets (not shown) composed of a multiplicity of small holes are formed on its peripheral surface. These fresh air inlets may also be formed in a surface of the lid member  40  to which the first duct  37  is connected (a surface indicated by numeral  40  in FIG. 1).  
         [0059]    In this working example, the exhaust-gas recirculation rate by the exhaust gas recirculation means  5  is set to 6%. This value is set by taking into consideration such a range that the blowing performance of the blower  3  is not exceeded and that a stable combustion is ensured (aforementioned stable combustion range).  
         [0060]    Finally, the water spray means  7  is explained. This water spray means  7 , as shown in FIGS. 1 and 2, is implemented by a water spray tube  41  which is disposed at a generally center of the primary air passage  13  so that its forward end confronts the first opening  17  of the first baffle plate  15 . The water spray means  7  is so constructed that water mist is jetted out from a nozzle  42  provided at the forward end of the water spray tube  41  toward the burning reaction zone  6  formed in the combustion space  34  through the first opening  17 .  
         [0061]    The amount of water addition by the water spray means  7  is determined in following manner. As already described, the value of generated NO x  of the low NO x  burner  2  is 50 ppm and the value of NO x  reduction by the exhaust gas recirculation means  5  is 17 to 18 ppm. Since the target exhaust NO x  value of the steam boiler  1  has been set to 20 ppm, the value of NO x  that has to be reduced by the water spray means  7  is 12 to 13 ppm. A spray amount corresponding to this NO x  reduction value is determined from the characteristic view shown in FIG. 4, resulting in 0.4 kg/10 4  kcal. It is noted that the gas fuel in FIG. 4 is a natural gas.  
         [0062]    Now, operation of the working example constituted as described above is explained. When the low NO x  burner  2  and the like are actuated, gas fuel is jetted out from the outer jet holes  21  and the inner jet holes  22 . The gas fuel jetted out from the inner jet holes  22  is mixed with primary air flowing through the primary air passage  13 , by which a small flame as a first burning reaction zone  43  is formed at a position downstream of the first baffle plate  15 . This small flame acts as a pilot burner, enhancing the flame holdability.  
         [0063]    The gas fuel jetted out from the outer jet holes  21  is mixed with secondary air flowing through the secondary air passage  14 , by which a large flame as a second burning reaction zone  44  is formed at a position downstream of the third baffle plate  19 . Since the secondary air is divided by the third baffle plate  19  and fed as such, divided flames are formed. Also, a thick and thin fuel combustion is performed with the small flame in a thick fuel combustion of an about 0.7 air ratio and with the large flame in a thin fuel combustion of an about 1.6 air ratio. Thus, in the low NO x  burner  2  of this working example, its generated NO x  is suppressed to 50 ppm in the state that neither the exhaust gas recirculation nor the water spray is performed, by virtue of the flame division method and thick and thin fuel combustion.  
         [0064]    The low NO x  burner  2  forms the burning reaction zone  6 . The burning reaction zone  6  is composed of the first burning reaction zone  43  where a thick (fuel-rich) fuel-air mixture is burned, and the second burning reaction zone  44  where a thin (air-rich) fuel-air mixture is burned. The first burning reaction zone  43  functions as a flame holding zone as described above.  
         [0065]    Further, by virtue of the arrangement that the exhaust gas recirculation rate by the exhaust gas recirculation means  5  is set to 6%, a NO x  reduction of about 17 to 18 ppm is achieved by combustion-gas temperature suppression of the second burning reaction zone  44  or the like (see FIG. 4).  
         [0066]    Further, water mist jetted out from the water spray tube  41  reaches the second burning reaction zone  44  to suppress the combustion gas temperature of the second burning reaction zone  44 , by which the NO x  value is further lowered by about 12 to 13 ppm so that the exhaust NO x  value becomes not more than the target exhaust NO x  value (see FIG. 4).  
         [0067]    The NO x  reduction effect in this working example is as shown in FIG. 4 as described before, and further, when expressed in conversion to NO x  reduction rate, results in a characteristic as shown in FIG. 5. These figures show that changing the amount of water spray causes the NO x  reduction value to increase in proportion to the amount. Also, FIG. 6 shows that there are almost no pressure fluctuations inside the wind box  9  due to increases or decreases in the amount of water spray. This means that the water spray in this working example does not adversely affect the combustibility.  
         [0068]    Here is explained the flow of combustion gas. Heat is transferred to the first water wall  29  by radiant heat transfer in the combustion space  34 , and combustion gas that has nearly completed burning reaction flows via the first outlet into the exhaust gas passage  31 , where convective heat transfer with the first water wall  29  and the second water wall  30  is performed. Then, the exhaust gas, passing through the second outlet, the exhaust-gas outlet  35  and the smokestack  36 , is discharged into the atmospheric air while part of the exhaust gas is utilized by the exhaust gas recirculation means  5 . The part of the exhaust gas is mixed with the combustion air fed to the low NO x  burner  2  by the blower  3 .  
         [0069]    According to this working example, the following working effects are produced. By virtue of the combination of the NO x  reduction by the low NO x  burner  2 , the NO x  reduction by the exhaust gas recirculation means  5  and the NO x  reduction by the water spray means  7 , it becomes possible to clear the target exhaust NO x  value of 20 ppm over the range of the blowing performance of the blower  3  and without incurring unstable combustion of the low NO x  burner  2 , even without the use of the water-tubes cooling combustion.  
         [0070]    Further, since the water spray by the water spray means  7  is done directly to the burning reaction zone  6 , the target exhaust NO x  value or lower can be achieved without increasing the load of the blower  3 .  
         [0071]    According to the present invention, for example, a NO x  reduction with the exhaust NO x  value under 30 ppm can be easily fulfilled, hence great industrial value.