Patent Publication Number: US-2018031230-A1

Title: Burner, combustion apparatus, and combustion method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is entitled to the benefit of priority of Japanese Patent Application No. 2016-150122, filed on Jul. 29, 2016, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     i) Field of the Invention 
     The present invention relates to the combustion technology such as a burner and the like to combust a fuel gas. 
     ii) Description of the Related Art 
     There is a burner through which plural thick and thin fuel air-fuel mixtures having air ratios different from each other flow for gas combustion. The air ratio of a thin fuel air-fuel mixture is higher than 1, and the thin fuel air-fuel mixture is an air-rich gas that includes an amount of air more than the amount of air necessary for the complete combustion of the fuel gas. The air ratio of a thick fuel air-fuel mixture is lower than 1, and the thick fuel air-fuel mixture includes an amount of air less than the amount of air necessary for the complete combustion of the fuel gas. As to the combustion of the thin fuel air-fuel mixture, nitrogen oxides (NOx) in the combustion exhaust can be reduced while the stability of the combustion is low. In contrast, as to the combustion of the thick fuel air-fuel mixture, the combustion is highly stable. It is known that, based on the properties of these two, the flame of the thick fuel air-fuel mixture holds the flame of the thin fuel air-fuel mixture to reduce NOx and to enhance the stability of the combustion. 
     It is known, as to the above gas combustion, that sleeve flames are formed using thick fuel burner ports disposed on both sides of a thin fuel burner port of the burner and the main flame on the side of the thin fuel burner port is held by the sleeve flames (for example, JP 2010-261615A). 
     BRIEF SUMMARY OF THE INVENTION 
     As to a burner combusting a fuel gas, the flame of the thick fuel air-fuel mixture having the low air ratio holds the flame of the thin fuel air-fuel mixture having the high air ratio to facilitate reduction of NOx and carbon monoxide (CO) in the combustion exhaust and stabilization of the flame. In this case, the thick fuel air-fuel mixture whose a ratio is high is advantageous to maintain a flame holding performance, while the air-fuel mixture whose the air ratio is low increases the fuel gas unable to be completely combusted due to a shortage of air, and it is therefore difficult to realize reduction of NOx and reduction of CO. A ratio of a thin fuel flame having a high air ratio is advantageously increased to reduce NOx and CO. 
     When, simply, the ratio of the thin fuel air-fuel mixture is increased and the ratio of the thick fuel air-fuel mixture is reduced, any generation of NOx and CO in the combustion exhaust can be suppressed while the flame holding property is degraded for a lifting that refers to blowing the flame off to occur. A problem therefore arises that the flame becomes unstable. 
     When the flow velocity of the air-fuel mixture flowing through the burner port on the flame holding side is reduced by reducing the gas amount on the thin fuel air-fuel mixture side, a flashback that refers to combusting the air-fuel mixture in the burner port may occur due to degradation of the balance between the flow velocity of the air-fuel mixture and the combustion velocity of the flame. 
     In view of the above problems, an object of the present invention is to improve the reduction performance of CO and NOx and to facilitate stabilization of the combustion. 
     According to an aspect of a burner of the present invention, the burner includes plural burner ports that produce flames using a first air-fuel mixture, and plural auxiliary burner ports that are disposed in a circumference of the burner ports and that produce an auxiliary flame using a second air-fuel mixture whose an air ratio is different from an air ratio of the first air-fuel mixture. The air ratios of the first air-fuel mixture and the second air-fuel mixture are each higher than 1. 
     Other objects, features, and advantages of the present invention will become more apparent by reading the embodiments herein with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a diagram of an example of the configuration of a burner according to an embodiment. 
         FIG. 2  is a diagram of combustion fields for a main air-fuel mixture f 1  and an auxiliary air-fuel mixture f 2 . 
         FIG. 3  is a chart of combustion conditions for the burner according to the embodiment. The combustion conditions for the burner are compared to those for a conventional burner. 
         FIG. 4  is a diagram of an example of the configuration of a combustion apparatus. 
         FIG. 5  is a diagram of an example of a burner unit. 
         FIG. 6  is an exploded perspective diagram of the burner unit. 
         FIG. 7  is an enlarged diagram of a portion of the burner unit. 
         FIG. 8  is a cross-sectional diagram taken by cutting along a line VIII-VIII of  FIG. 7 . 
         FIG. 9  is a cross-sectional diagram taken along a line IX-IX of  FIG. 7 . 
         FIG. 10  is a diagram of an example of the state of a main flame F 1  and an auxiliary flame F 2  in the burner unit portion depicted in  FIG. 8 . 
         FIG. 11  is a diagram of an example of the state of the main flame F 1  and the auxiliary flame F 2  in the burner unit portion depicted in  FIG. 9 . 
         FIG. 12  is a diagram of an example of the state of the main flame F 1  and the auxiliary flame F 2  in the cross-section taken along a line XII-XII of  FIG. 7 . 
         FIG. 13  is a diagram of an example of the state of the main flames F 1  and the auxiliary flame F 2  in the cross-section taken along a line XIII-XIII of  FIG. 7 . 
         FIG. 14  is a graph of a result acquired by actually measuring a combustion exhaust gas (for NOx) of the combustion apparatus  20  that includes the burner units  30  and that was loaded on a water heater. 
         FIG. 15  is a graph of a result acquired by actually measuring a combustion exhaust gas (for CO) of the combustion apparatus  20  that includes the burner units  30  and that was loaded on the water heater. 
         FIG. 16  is a graph of an example of an experiment on the burner unit. 
         FIG. 17  is a graph of an example of an experiment on the burner unit. 
         FIG. 18  is a graph showing a relation of a combustion load, an electric current value of a proportional valve, and a rotation rate of a motor. 
         FIG. 19  is a diagram of an example of a hardware relating to an air ratio adjustment of the combustion apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment 
       FIG. 1  depicts an example of the configuration of a burner according to an embodiment. The burner  2  is an example of a burner of this disclosure, and the burner of the invention of this application is not limited to this configuration. 
     The burner  2  is an example of a what-is-called press burner that is formed from pressed plate members, for example, heat-resistant metal plates, such as stainless steel plates. The burner  2  includes plural burner ports for allowing air-fuel mixtures having different air ratios to flow therethrough, and combusts plural flames having different natures. The burner  2  includes, for example, an air-fuel mixture exhaust part  4  exhausting a main air-fuel mixture f 1  that produces main flames, and auxiliary burner ports  6 - 1  and  6 - 2  causing an auxiliary air-fuel mixture f 2  that produces an auxiliary flame by combustion to flow. The main air-fuel mixture f 1  is an example of a first air-fuel mixture of this disclosure and produces the main flames by combustion. The auxiliary air-fuel mixture f 2  is an example of a second air-fuel mixture of this disclosure and produces the auxiliary flame by combustion. The air ratio X 1  of the main air-fuel mixture f 1  has a value higher than 1, and the main air-fuel mixture f 1  is in a what-is-called air-rich state. The air ratio X 2  of the auxiliary air-fuel mixture f 2  is lower than the air ratio X 1  of the main air-fuel mixture f 1  and has a value higher than 1, and the auxiliary air-fuel mixture f 2  is in the what-is-called air-rich state. 
     The air-fuel mixture exhaust part  4  includes a ribbon  8  formed along the outer shape of the burner  2  that has, for example, a rectangular shape. The ribbon  8  is an example of a rectifying unit that rectifies a flow of the main air-fuel mixture f 1 . The ribbon  8  has main burner ports  10  that each exhaust and combust the main air-fuel mixture, and squeezed parts  12 . The main burner ports  10  and the squeezed parts  12  are alternately formed. The squeezed parts  12  are an example of parts that block any air-fuel mixture from being exhausted, and partition the area of the main burner ports  10  on the ribbon  8  so as to determine the number of the main burner ports  10 . For the air-fuel mixture exhaust part  4 , the number of the squeezed parts  12  and the size of each of the squeezed parts  12  determine the area of the opening of the main burner ports  10  through which the main air-fuel mixture flows. 
     The auxiliary burner ports  6 - 1  and  6 - 2  open, for example, on both sides of the ribbon  8  along the longitudinal direction of the burner  2 . The auxiliary burner ports  6 - 1  are disposed to be matched with positions of openings of the main burner ports  10 , and each exhaust the auxiliary air-fuel mixture f 2  on the sides of both edges of the burner  2 . The auxiliary burner ports  6 - 2  are disposed to be matched with positions at which the squeezed parts  12  are formed, and exhaust the auxiliary air-fuel mixture f 2  on the inner side of the burner  2  corresponding to the width of the squeezed parts  12 . The distance between the auxiliary burner ports  6 - 2  facing each other over such a squeezed part  12  is shorter than the distance between the auxiliary burner ports  6 - 1  facing each other over such a main burner port  10 . For the burner  2 , the flowing-out amount of the main air-fuel mixture f 1  is larger compared to that of the auxiliary air-fuel mixture f 2  and the flowing-out velocity of the main air-fuel mixture f 1  is set to be faster compared to that of the auxiliary air-fuel mixture f 2 . 
     The burner  2  has blocking parts  14  each blocking exhausting of any air-fuel mixture. The blocking parts  14  are each formed between the air-fuel mixture exhaust part  4  in which the ribbon  8  is disposed, and the auxiliary burner ports  6 - 1  and  6 - 2 . The blocking parts  14  are each an insulating region insulating the main burner ports  10  from the auxiliary burner ports  6 - 1  and  6 - 2 , and form partitioned regions. 
     &lt;Combustion of Thin Fuel Air-Fuel Mixture f 1  and Thick Fuel Air-Fuel Mixture f 2 &gt; 
       FIG. 2  depicts combustion fields for the main air-fuel mixture f 1  and the auxiliary air-fuel mixture f 2 . When the states of the main air-fuel mixture f 1  and the auxiliary air-fuel mixture f 2  are shifted into their combustion states by ignition, their combustion fields are formed. On the burner  2 , the auxiliary air-fuel mixture f 2  exhausted from the auxiliary burner ports  6 - 1  and  6 - 2  surrounds the main air-fuel mixture f 1  exhausted from the main burner ports  10  in circumference thereof. On the burner  2 , centering main flames F 1  produced by the combustion of the main air-fuel mixture f 1 , an auxiliary flame F 2  is produced at a low position on the side of the circumferential edge of the main flames F 1  and on the side of the burner  2 . An independent flame as one of the main flames F 1  is produced at each of the main burner ports  10  based on the flow velocity and the combustion of the main air-fuel mixture f 1 . The main flame F 1  with a horizontal cross-section having an oval shape is formed in this example while the horizontal cross-section may have a circular shape. 
     The pressure of the main air-fuel mixture f 1  is lower than that of the auxiliary air-fuel mixture f 2  in the blocking part  14 . When this pressure relation is set, the auxiliary flame F 2  runs into the blocking part  14  to produce the auxiliary flame F 2  without independence for each of the auxiliary burner ports  6 - 1  and  6 - 2 . The auxiliary flame F 2  forms a chain-like annular flame surrounding the main flames F 1  whose horizontal cross-section has an oval shape. Thereby, each of the main flames F 1  is independently formed for each main burner port  10  while the auxiliary flame F 2  is present in each interval portion between the main flames F 1 . Each of the main flames F 1  is therefore held in its overall circumference by the auxiliary flame F 2  adjacent thereto. 
     The combustion height of each of the main flames F 1  and the auxiliary flame F 2  is determined based on, for example, the flow velocity of each of the main air-fuel mixture f 1  and the auxiliary air-fuel mixture f 2 , the combustion velocity corresponding to the fuel gas ratio, and the composition of the fuel gas. For example, the combustion height of the auxiliary flame F 2  is set to be lower than that of the main flames F 1 , so that the auxiliary flame F 2  holds the main flames F 1  on the side of the foots of the main flames F 1 , that is, at the portion close to the main burner ports  10 . The opening areas of the main burner ports  10  and the auxiliary burner ports  6 - 1  and  6 - 2  are set such that the gas flow velocity of the auxiliary air-fuel mixture f 2  is lower than the gas flow velocity of the main air-fuel mixture f 1 . 
     &lt;Thick Fuel and Thin Fuel Ratio Balance, and Burner Port Shape in Burner Unit  30 &gt; 
     A conventional thick fuel burner port is shaped to hold the thin fuel flame only in the horizontal direction while the auxiliary burner ports  6 - 2  are each disposed between the main burner ports  10 , the auxiliary flame F 2  produces pseudo circumferential flames during combustion, and the burner  2  can therefore hold the main flames F 1 . 
     Compared to the conventional flame holding in which flames are contacted in parallel faces, in the flame holding conducted by the circular auxiliary flame F 2  formed on the burner  2 , the contact region for the flames, that is, the area is increased and efficient flame holding can thereby be acquired. This form of the flame holding is pseudo overall-circumferential flame holding and forms an ideal flame holding form that is a form for the circular auxiliary flame F 2  to surround the circular main flame F 1 . 
     &lt;Setting of Combustion Conditions for Burner  2 &gt; 
       FIG. 3  depicts the combustion conditions for the burner according to the embodiment, being compared to those for the conventional burner. The set conditions depicted in  FIG. 3 , and the setting approach and the calculation method for the conditions are each an example and the invention of this application is not limited by this configuration. 
     &lt;Balance of Gas Amount Ratio of Main Air-Fuel Mixture and Auxiliary Air-Fuel Mixture (Auxiliary Air-Fuel Mixture/Main Air-Fuel Mixture Ratio)&gt; 
     For the burner  2  according to the embodiment, as depicted in  FIG. 3 , a ratio of the gas amount of the auxiliary air-fuel mixture f 2  to the gas amount of the main air-fuel mixture f 1  (hereinafter, referred to as a “gas amount ratio”) are set to be, for example, 20:80 or ratios close thereto (for example, 16:84 to 24:76) (“New Burner” in  FIG. 3 ). For a conventional ordinary burner, the gas amount ratio is set to be about 30:70 (“Current” in  FIG. 3 ) for the flame holding capacity of the auxiliary flame and prevention of lifting up of the main flame and the like. This gas amount is the fuel gas amount to be supplied. 
     The gas amount ratio for an auxiliary thin fuel combustion is determined corresponding to the performance and a purpose of the burner  2 . For example, when enhancement is desired for the suppression of the noise value or the prevention of oscillating combustion, a ratio of the gas amount of the auxiliary air-fuel mixture f 2  is increased to execute a setting for increasing the load on the side of an auxiliary combustion and, as described later, the ratio of the auxiliary flame F 2  is increased that is a stable flame having its air ratio set to be low. 
     When reduction is desired for the harmful exhaust components such as CO and NOx in the exhaust gas, a setting for increasing a ratio of the gas amount of the main air-fuel mixture f 1  is necessary. The air ratio of a main combustion that is the combustion of the main air-fuel mixture f 1  is set to be higher as compared to the air ratio of the auxiliary flame that is the combustion of the auxiliary air-fuel mixture f 2 , the main combustion is combusted in the air-rich state, and generation of any one of these harmful components can therefore be suppressed. 
     As to the burner  2 , due to the configurations of the main burner ports  10  and the auxiliary burner ports  6 - 1  and  6 - 2 , the auxiliary flame F 2  is present in the interval portion between the main flames F 1 , and the main flames F 1  are held on their overall circumferences by the auxiliary flame F 2  adjacent to the main flames F 1  to thereby enhance the flame holding performance. Any lifting of the main flame, any oscillating combustion, and the like can thereby be prevented even when the gas amount ratio of the auxiliary air-fuel mixture f 2  to the main air-fuel mixture f 1  is set to be 20:80 or the ratios close thereto for the ultra low NOx regulation. 
     &lt;Setting of Combustion Air Amount (Air Ratio, and Air/Fuel Ratio (AFR))&gt; 
     As to the burner  2  according to the embodiment, for example, a combustion air amount of the air-fuel mixture flowing through each of the auxiliary burner ports  6 - 1  and  6 - 2  may be set based on the set gas flow amount (the set air-fuel mixture amount) set in advance as a criterion. This set air-fuel mixture amount set in advance is set to be, for example, a value with which no flashback of the flame occurs in the auxiliary burner ports  6 - 1  and  6 - 2 , based on the combustion velocity corresponding to the components included in the fuel gas, the amount of the air-fuel mixture flowing through the main burner ports  10 , and the like. For example, as described, the amount of the air-fuel mixture flowing on the side of the auxiliary burner ports of the conventional burner having the gas amount ratio set to be 30:70 may be used as this set air-fuel mixture amount. 
     In this embodiment, the case will be described where the amounts of the air-fuel mixtures set for the conventional burner are used as the amount of the air-fuel mixture flowing through each of the auxiliary burner ports  6 - 1  and  6 - 2 . In this case, An area of the auxiliary burner ports is equal to that of the conventional burner. 
     The amount of the air-fuel mixture flowing through the conventional auxiliary burner port is represented by “Q A1 ” and the amount of the air-fuel mixture flowing through the auxiliary burner ports  6 - 1  and  6 - 2  is represented by “Q A2 ”. The conventional air-fuel mixture amount Q A1  can be acquired as follows. In this calculation, the case is assumed where methane (CH 4 ) is the main component. 
       The air-fuel mixture amount  Q   A1   =A *(1+( Q   air *0.8))  (1)
 
     In the equation, “A” represents the supplied fuel gas amount and Q air  represents the theoretical air amount to the fuel gas. The case is assumed for Eq. (1) where the air ratios on the main burner ports and the auxiliary burner ports of the conventional burner are set to be 1.6 and 0.8 respectively. With the conventional burner, the air ratio of the air-fuel mixture flowing through the auxiliary burner ports is 1 or smaller. The air-fuel mixture flowing through the auxiliary burner ports is therefore gas-rich, and maintains an oxygen-lacking state. 
     The burner  2  according to the embodiment is set such that an air-fuel mixture whose an amount is equal to the air-fuel mixture amount Q A1  flows through the auxiliary burner ports  6 - 1  and  6 - 2 . 
       The air-fuel mixture amount  Q   A2 =(⅔) A *(1+( Q   air   *X   1 ))= Q   A1   (2)
 
     When the ratio of the fuel gas amount in the air-fuel mixture is assumed to be small and the fuel gas amount is ignored, the following is acquired. 
       (⅔) A *( Q   air   *X   1 )= A *( Q   air *0.8)
 
         X   1 =1.25  (3)
 
     In Eq. (2), X 1  is the air ratio on the side of the auxiliary burner ports  6 - 1  and  6 - 2  of the burner  2 . 
     For the burner  2  according to the embodiment, the gas amount ratio is changed from 30:70 that is the conventional ratio to 20:80. The ratio of the amount of the fuel gas supplied to the side of the auxiliary burner ports  6 - 1  and  6 - 2  is set to be 20 to be reduced compared to that of the conventional burner. Eq. (2) acquires the air ratio X 1  for maintaining the air-fuel mixture amount in the case where the amount A of the gas flowing for the auxiliary flame is set to be ⅔ of the its original amount. 
     In the case where the amount ratio of the gas flowing on the side of the auxiliary burner ports  6 - 1  and  6 - 2  is reduced, when the air ratio equal to the conventional one is maintained, the flow velocity of the air-fuel mixture is reduced and flashback of the flame may occur. Because of this, in the burner  2 , the air-fuel mixture is maintained by increasing the air amount corresponding to the reduced amount of the gas flowing on the side of the auxiliary burner ports  6 - 1  and  6 - 2 . The gas gets the air-rich state in which the air ratio is higher than 1 due to the increase of the air amount in the air-fuel mixture as above. In the burner  2 , the flow velocity of the air-fuel mixture can be maintained with the air ratio equal to the conventional air ratio by reducing the burner port area of the auxiliary burner ports  6 - 1  and  6 - 2  corresponding to the reduced amount of the gas while the flame holding function may be degraded. When the burner port area of the auxiliary burner ports  6 - 1  and  6 - 2  is set to be larger than that of the conventional burner, more air is caused to flow matching with the increased amount of the area while, because the surface area of the burner  2  is limited, the burner port area of the main burner ports  10  is reduced and the flow velocity of the main air-fuel mixture f 1  is increased. The burner port areas need to be set in a range where the flame holding performance is not degraded. 
     For the burner  2 , a pilot flame holding state is established for the main flames F 1  by the auxiliary burner ports  6 - 1  and  6 - 2  even when the air ratio is higher than 1 on the side of the auxiliary flame that hold the main flames, and the flame holding function is thereby secured. 
     In this case, the main air-fuel mixture amount Q B2  may be set related to the main air-fuel mixture amount Q B1  of the conventional burner, or the main air-fuel mixture amount Q B2  may be set regardless of the main air-fuel mixture amount Q B1 . The auxiliary air-fuel mixture flow velocity V A2  and the main air-fuel mixture flow velocity V B2  may be set from, for example, the opening area of the disposed main burner ports  10  or the disposed auxiliary burner ports  6 - 1  and  6 - 2 , and the supply flow amounts therefor. 
     &lt;Theoretical Air Amount of Fuel Gas&gt; 
     The theoretical air amount for the combustion of methane (CH 4 ) that is a main component of a natural gas and is included in the fuel gas is acquired from a reaction formula below. 
       CH 4 +2O 2 +2(79/21)N 2 →CO 2 +2H 2 O+2(79/21)N 2   (4)
 
     In Eq. (4), the second member and the third member on the left-hand side represent the theoretical air amount (Q air ) to combust 1 [mol] of methane. In this calculation process, it is assumed that the air contains 79 [%] nitrogen (N 2 ) and 21 [%] oxygen (O 2 ). The air necessary for completely combusting 1 [mol] of methane thereby includes 2 [mol] of oxygen and 2*(79/21)=2*3.76 [mol] of nitrogen. The amount of the air including this configuration is acquired from these values using the following equation. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           Q 
                           air 
                         
                         = 
                           
                          
                         
                           
                             2 
                              
                             
                                 
                             
                              
                             
                               O 
                               2 
                             
                           
                           + 
                           
                             2 
                              
                             
                               ( 
                               
                                 79 
                                 / 
                                 21 
                               
                               ) 
                             
                              
                             
                               N 
                               2 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             2 
                              
                             
                               ( 
                               
                                 1 
                                 + 
                                 3.76 
                               
                               ) 
                             
                           
                           = 
                           
                             9.52 
                              
                             
                                 
                             
                             [ 
                             mol 
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     When Q air  of Eq. (2) is substituted by the air amount of Eq. (5), 
     X 1 ≈1.2525 
     is acquired and it can be seen that this value is substantially equal to that of Eq. (3). 
     The theoretical air amount for propane (C 3 H 8 ) included in the fuel gas will be acquired. 
       C 3 H 8 +5O 2 +5(79/21)N 2 →3CO 2 +4H 2 O+5(79/21)N 2   (6)
 
     In Eq. (6), the second member and the third member on the left-hand side represent the theoretical air amount (Q air ) to combust 1 [mol] of propane. The air necessary for completely combusting 1 [mol] of propane includes 5 [mol] of oxygen and 5*(79/21)=5*3.76 [mol] of nitrogen. The amount of the air including this configuration is acquired from these values using the following equation. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           Q 
                           air 
                         
                         = 
                           
                          
                         
                           
                             5 
                              
                             
                                 
                             
                              
                             
                               O 
                               2 
                             
                           
                           + 
                           
                             5 
                              
                             
                               ( 
                               
                                 79 
                                 / 
                                 21 
                               
                               ) 
                             
                              
                             
                               N 
                               2 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             5 
                              
                             
                               ( 
                               
                                 1 
                                 + 
                                 3.76 
                               
                               ) 
                             
                           
                           = 
                           
                             23.8 
                              
                             
                                 
                             
                             [ 
                             mol 
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     When Q air  of Eq. (2) is substituted by the air amount of Eq. (7), 
     X 1 ≈1.221 
     is acquired and it can be seen that this value is substantially equal to that of Eq. (3). 
     When the fuel gas (Type 13A) including about 85 [%] methane and about 15 [%] propane is combusted, the air amount is acquired using Eq. (8) below from the calculation results of Eq. (5) and Eq. (7), and the ratios of the constituent elements included in the air for the combustion. To completely combust 1 [mol] of the 13A gas, the air amount is as follows. 
       0.85*9.52+0.15*23.8=11.7 [mol]  (8)
 
     When Q air  of Eq. (2) is substituted by the air amount of Eq. (8), 
     X 1 ≈1.2427 
     is acquired and it can be seen that this value is substantially equal to that of Eq. (3). 
     For the burner, when the air ratio of the air-fuel mixture is increased, the combustion air amount is increased in proportion to the air ratio. As a result, the flowing-out velocity of the air-fuel mixture is also increased. The degree of stability of the flame is determined based on the balance with the combustion velocity. 
     Generally, the combustion field is formed at a position that is more distant from the burner port plane on which the flame has a stable state, as the combustion air amount is increased, that is, the air ratio is increased. Because the flame temperature is lowered due to the increase of the air amount, the state of the flame transitions toward its more unstable state. When the air amount is further increased, the lifting of the flame and flameout finally occurs. 
     Because the burner  2  is a press burner, the air ratio on the side of the main flames F 1  is high. The amounts of generated CO and NOx are then increased when the air ratio is reduced to effect stabilization of the main flames F 1 . The air ratio on the side of the main flames F 1  can be set to be high using the flame holding by the auxiliary flame F 2 . 
     Because the flame holding function of the auxiliary flame F 2  of the burner  2  is high, the main flames F 1  are stabilized and generation of CO is suppressed even in the combustion region having a high air ratio. 
     The air ratio X 1  on the side of the auxiliary burner ports  6 - 1  and  6 - 2  is, for example, higher than 1 and equal to or lower than 1.6 to establish the air-rich state. When the air ratio X 1  is 1 and the gas amount ratio is changed from the conventional ratio of 30:70 to Z1:(100−Z1), Eq. (2) is as follows. 
       The air-fuel mixture amount  Q   A2 =( Z 1/30) A *(1+( Q   air *1))= Q   A1    (9)
 
     When the ratio of the fuel gas amount in the air-fuel mixture is assumed to be small and the fuel gas amount is ignored, the following is acquired. 
         A *( Q   air *0.8)=( Z 1/30) A *( Q   air *1) 
         Z 1=24  (10)
 
     When the air ratio X 1  is 1.6 and the gas amount ratio is changed from the conventional ratio of 30:70 to Z2:(100−Z2), Eq. (2) is as follows. 
       The air-fuel mixture amount  Q   A2 =( Z 2/30) A *(1+( Q   air *1.6)= Q   A1    (11)
 
     When the ratio of the fuel gas amount in the air-fuel mixture is assumed to be small and the fuel gas amount is ignored, the following is acquired. 
         A *( Q   air *0=( Z 2/30) A *( Q   air *1.6) 
         Z 2=15  (12)
 
     The gas amount ratio is consequently set to be, for example, 15:85 to 24:76. 
     Eq. (10) and Eq. (12) show that the air ratio of the auxiliary air-fuel mixture f 2  is increased when the ratio of the gas in the auxiliary air-fuel mixture f 2  is reduced. On the other hand, the air ratio of the main air-fuel mixture f 1  is reduced. Preferably, the air ratio of the main air-fuel mixture f 1  is equal to or higher than the air ratio of the auxiliary air-fuel mixture f 2 . 
     The gas amount ratio of Z3:(100−Z3) will be acquired with which the air ratio of the main air-fuel mixture f 1  and the air ratio of the auxiliary air-fuel mixture f 2  are both X 3 . When the air ratio of the auxiliary air-fuel mixture f 2  is X 3  and the gas amount ratio is changed from the conventional ratio of 30:70 to Z3:(100−Z3), Eq. (2) is as follows. 
       The air-fuel mixture amount  Q   A2 =( Z 3/30) A *(1+( Q   air   *X   3 ))= Q   A1    (13)
 
     When the ratio of the fuel gas amount in the air-fuel mixture is assumed to be small and the fuel gas amount is ignored, the following is acquired. 
         A *( Q   air *0.8)=( Z 3/30) A *( Q   air   *X   3 )  (14)
 
     Similarly, at the air ratio X 3 , the main air-fuel mixture f 1  has the air ratio of 1.6 for the new gas amount ratio of 20:80, and the following is therefore acquired. 
         B *( Q   air *1.6)=((100− Z 3)/80) B *( Q   air   *X   3 )  (15)
 
     “B” in the equation is the fuel gas amount to be supplied. From Eq. (14) and Eq. (15), Z3 is 15.79. Preferably, the gas amount ratio is set to be 16:84 to 24:76. 
     &lt;Combustion Velocity&gt; 
     The combustion velocity of each of hydrocarbons represented by methane is closely related to the air ratio. The combustion velocity becomes maximal in the vicinity of the air ratio of 1 and is reduced therefrom before and after this air ratio. This is because a formation position of the combustion field becomes distant from the burner port plane on which the combustion is stable, and the flame therefore becomes unstable. Because the combustion velocity is reduced as the air ratio is increased, stable flame holding by a thick fuel air-fuel mixture is indispensable for the thin fuel flame F 1 . 
     For example, a value close to a set value may arbitrary be set as the air-fuel mixture amount of the burner  2 . In this case, for the burner  2 , the auxiliary air-fuel mixture flow amount X 1-1  and the main air-fuel mixture flow amount X 2-1  may arbitrary be set at ratios within the predetermined range based on, for example, the set flow amount. 
     &lt;Combustion Apparatus&gt; 
       FIG. 4  depicts an example of the configuration of a combustion apparatus. The combustion apparatus  20  is an example of a combustion apparatus of the present invention. 
     The combustion apparatus  20  is used as a heat source apparatus for a water heater or a heating water heater that uses a fuel gas or the like as fuel. The combustion apparatus  20  has a combustion chamber  24  disposed in an apparatus housing  22 . The combustion chamber  24  is surrounded by a side wall part  26  of the apparatus housing  22 . A burner  28  that combusts a fuel gas is installed in the combustion chamber  24 . The burner  28  is an example of the burner of this disclosure and includes plural burner units  30  combined with each other, and forms a uniform burner port plane as an example. 
     A supporting part  32  protruding toward the side of the circumference of the combustion chamber  24  is disposed in the upper portion of the side wall part  26 . A heat exchanger not depicted is disposed on the upper face of the supporting part  32 , and the combustion exhaust of the burner  28  is caused to flow through the heat exchanger. The heat of the combustion exhaust acquired from the combustion of the fuel gas is heat-exchanged by the heat exchanger. 
     Plural first fuel supply ports  34 - 1  and second fuel supply ports  34 - 2  are formed in the side wall part  26  of the apparatus housing  22 . The fuel supply ports  34 - 1  are openings to supply the fuel gas to the side of the main burner ports of the burner unit  30 . The fuel supply ports  34 - 2  are openings to supply the fuel gas to the side of the auxiliary burner ports of the burner unit  30 . 
     On the outer side of the fuel supply ports  34 - 1  and  34 - 2 , a fuel supply unit  36  is disposed that is common to the fuel supply ports  34 - 1  and  34 - 2 . The fuel supply unit  36  includes plural first fuel injection nozzles  38 - 1  and second fuel injection nozzles  38 - 2 . The fuel injection nozzles  38 - 1  are disposed on the side of the fuel supply ports  34 - 1  and the fuel injection nozzles  38 - 2  are disposed on the side of the fuel supply ports  34 - 2 . The fuel gas is thereby supplied to the inside of the burner unit  30 . In this example, the fuel supply ports  34 - 1  each have, for example, an oval shape and the fuel supply ports  34 - 2  each have, for example, a circular shape. The fuel injection nozzles  38 - 1  and the fuel supply ports  34 - 1  are used to supply the main air-fuel mixture f 1  to the main burner ports  10 , and the fuel injection nozzles  38 - 2  and the fuel supply ports  34 - 2  are used to supply the auxiliary air-fuel mixture f 2  to the auxiliary burner ports  6 - 1  and  6 - 2 . The opening area of a fuel supply port  34 - 1  is larger than that of a fuel supply port  34 - 2  and these opening areas cause the introduction amount of the air to differ for the supply of the fuel gas to differ the air ratio of the main air-fuel mixture produced on the side of the fuel supply port  34 - 1  from the air ratio of the auxiliary air-fuel mixture produced on the side of the fuel supply port  34 - 2 . 
     Adjustment of the gas amount ratio of the auxiliary air-fuel mixture f 2  to the main air-fuel mixture f 1  is conducted by, for example, adjustment of the ratio of the opening area of the fuel injection nozzle  38 - 2  to the opening area of the fuel injection nozzle  38 - 1 . To set this gas amount ratio to be, for example, 20:80, the ratio of the opening area of the fuel injection nozzle  38 - 2  to the opening area of the fuel injection nozzle  38 - 1  is adjusted to be, for example, 20:80. The opening areas of the fuel supply ports  34 - 1  and  34 - 2  are adjusted to acquire the aimed air ratio using, for example, the opening areas of the fuel injection nozzles  38 - 1  and  38 - 2  as criteria. The opening areas of the fuel supply ports  34 - 1  and  34 - 2  may be adjusted by adjusting the diameters of the openings in view of a difference in the air ratio of the auxiliary air-fuel mixture f 2  to the main air-fuel mixture f 1  and a difference in the degree of flowability between the air-fuel mixtures, the opening areas may be adjusted by blocking a portion of the openings with a blocking member such as a blocking plate, or the ratio of the opening areas may be different from the ratio of the opening areas of the fuel injection nozzles  38 - 1  and  38 - 2 . 
     The side wall part  26  on the side of the fuel supply ports  34 - 1  and  34 - 2  dents toward the inside of the apparatus housing  22 , and the apparatus housing  22  has a fuel supply chamber  39  formed in the apparatus housing  22 . The fuel supply chamber  39  is an example of a means of accumulating therein the fuel gas and supplying the fuel gas to the fuel supply ports  34 - 1  and  34 - 2 , and constitutes the fuel supply unit  36 . 
     The apparatus housing  22  is closed on the side of its bottom face by a bottom face plate  40 . The bottom face plate  40  has an air inlet  42  formed therein. An air supply fan  44  is disposed on the side of the lower face of the bottom face plate  40  and the air supply fan  44  is connected to the air inlet  42 . The air supply fan  44  includes a motor  46 , and the rotation of the motor  46  supplies the combustion air from the air supply fan  44  to the air inlet  42 . The combustion air is introduced into the burner unit  30  in response to the injection of the fuel gas and is used for the combustion of the fuel gas. 
     &lt;Burner Unit  30 &gt; 
       FIG. 5  depicts an example of the burner unit. The burner unit  30  is an example of the burner of the present invention. 
     The burner unit  30  is a what-is-called press burner that is formed by plate members acquired by pressed heat resistant metal plates such as stainless steel plates. Each burner unit  30  includes independent mixing units  48 - 1  and  48 - 2 . In the mixing unit  48 - 1 , the fuel gas supplied from the fuel injection nozzle  38 - 1  and the combustion air are mixed with each other to produce the main air-fuel mixture. In the mixing unit  48 - 2 , the fuel gas supplied from the fuel injection nozzle  38 - 2  and the combustion air are mixed with each other to produce the auxiliary air-fuel mixture. The burner unit  30  includes a main body part  50 , a rectifying part  52 , and a burner port part  54  in this order from the lower side toward the side of the burner ports, and these parts are integrally formed by the plate members. 
     In the main body part  50 , air-fuel mixture entrance ports  56 - 1  and  56 - 2  are disposed and formed at two vertical levels. The air-fuel mixture entrance port  56 - 1  is an opening that has a flat hexagonal shape or that is a long hole and is connected to the fuel supply port  34 - 1  for the main air-fuel mixture f 1  to be introduced. The air-fuel mixture entrance port  56 - 2  is an opening that has a circular shape and is connected to the fuel supply port  34 - 2  for the auxiliary air-fuel mixture f 2  to be introduced. 
     The rectifying part  52  rectifies the flow of the main air-fuel mixture f 1  and the auxiliary air-fuel mixture f 2  that are introduced into the main body part  50  to introduce the air-fuel mixtures f 1  and f 2  into the burner port part  54 . The rectifying part  52  has the ribbon  8  disposed in the air-fuel mixture exhaust part  4 . The ribbon  8  is disposed in the air-fuel mixture exhaust part  4  of the burner unit  30 , and is detachable. 
     For example, as depicted in  FIG. 6 , the burner unit  30  is formed by joining inner wall plates  60  and outer wall plates  62  on the right and the left sides of the ribbon  8 . The inner wall plates  60  and the outer wall plates  62  are formed out of common metal plates. The ribbon  8  is sandwiched between the inner wall plates  60  facing each other, the inner wall plates  60  are joined with each other, and a mixing unit  48 - 1  and the air-fuel mixture exhaust part  4  that exhausts the produced main air-fuel mixture are thereby formed in the joining portion. As to the joined outer wall plates  62 , the mixing unit  48 - 2  and the auxiliary burner ports  6 - 1  and  6 - 2  are formed between the outer wall plates  62  and the inner wall plates  60 . 
     The burner port part  54  is formed on the upper face of the burner unit  30 , includes the plural main burner ports  10  formed by the ribbon  8  at constant intervals, and includes the plural first and the second auxiliary burner ports  6 - 1  and  6 - 2  regularly at constant intervals on the side of the main body part  50 . In the air-fuel mixture exhaust part  4  of this example, 12 main burner ports  10  are formed and arranged in a row by at least one ribbon  8 . In the combustion apparatus  20 , due to the plural burner units  30  disposed in parallel to each other, the main burner ports  10  are arranged in plural rows and plural columns to form the burner port part  54  forming a uniform face portion. The ribbon  8  may be divided into plural pieces and be disposed in the air-fuel mixture exhaust part  4 . 
     In the burner unit  30  except the air-fuel mixture entrance ports  56 - 1  and  56 - 2 , and the burner port part  54 , an edge portion  58  is formed by adhering plate members to each other. This edge portion  58  reinforces the burner unit  30 . 
       FIG. 7  depicts an enlarged view of a portion of the burner unit. 
     The ribbon  8  is formed by, for example, press working from metal plates such as stainless steel and includes 6 metal plates in this embodiment. The ribbon  8  has the main burner ports  10  and the squeezed parts  12  alternately disposed therein. Each of the main burner ports  10  has 5 long burner ports  64  formed therein. The long burner ports  64  are lined in a direction perpendicular to the arrangement direction of the main burner ports  10  and are formed from, for example, 6 metal plates whose bending angles differ from each other. The shape of each of the long burner ports  64  is symmetric in the right-and-left direction with respect to a center line taken in the longitudinal direction of the ribbon  8 . The flow of the main air-fuel mixture f 1  is rectified due to the formation of the plural long burner ports  64  to form parallel flows, and the main air-fuel mixture f 1  flows out from the main burner ports  10 . 
     The width in the longitudinal direction of each auxiliary burner port  6 - 1  is smaller than the width of each long burner port  64  of the main burner port  10 , and the opening area of each auxiliary burner port  6 - 1  is smaller than the opening area of one long burner port  64 . The flowing-out velocity of the auxiliary air-fuel mixture f 2  flowing out from the auxiliary burner ports  6 - 1  can thereby be set to be faster than the combustion velocity of the auxiliary air-fuel mixture f 2 . 
     The auxiliary burner ports  6 - 1  and  6 - 2  are formed by joining the inner wall plates  60  and the corresponding outer wall plates  62  with each other. The inner wall plates  60  and the outer wall plates  62  are common thereto. For example, the inner wall plates  60  are bent in a trapezoidal shape to protrude bending parts toward the side of the squeezed parts  12 , and the outer wall plates  62  are similarly bent into the auxiliary burner ports  6 - 2  to protrude bending parts  66 . Thereby, each auxiliary burner port  6 - 2  has a substantially trapezoidal opening shape and each bending part  66  of the outer wall plates  62  makes an opening area of one of the auxiliary burner ports  6 - 2  small. The auxiliary burner port  6 - 2  formed and disposed as above has the opening area larger than that of the auxiliary burner port  6 - 1  and has the flowing-out amount of the auxiliary air-fuel mixture f 2  larger than that of an auxiliary burner port  6 - 1 . Furthermore, each auxiliary burner port  6 - 2  projects into the blocking part  14  to be close to the squeezed part  12 . Thereby, coupling of the auxiliary flame F 2  ( FIG. 2 ) is facilitated by the auxiliary air-fuel mixture f 2  flowing out from a pair of the auxiliary burner ports  6 - 2 . As to the ratio of the areas of the auxiliary burner ports  6 - 1  and  6 - 2 , the former may be greater than the latter or the latter may be greater than the former. 
       FIG. 8  depicts a cross-section taken by cutting along a line VIII-VIII of  FIG. 7 . A pair of the blocking parts  14  are formed in the burner port part  54  of the burner unit  30 , sandwiching the main burner ports  10  formed by the ribbon  8 , and the auxiliary burner ports  6 - 1  are formed on the outer side of the blocking parts  14 . The opening edge portions of the inner wall plates  60  of the auxiliary burner ports  6 - 1  are disposed in the same plane as that of the main burner ports  10 . In contrast, the outer wall plates  62  are set to be higher than, for example, the inner wall plates  60  by a height of h 1 . The burner port part  54  is thereby surrounded by the opening edge portions of the high outer wall plates  62 . 
     The blocking parts  14  are formed by causing protrusions  68  protruding from the middle portion of the inner wall plates  60  toward the side of the ribbon  8  to abut the ribbon  8 . 
     The auxiliary air-fuel mixture f 2  is guided from the side of the main body part  50  to each auxiliary burner port  6 - 1  through auxiliary air-fuel mixture supply paths  70 . 
       FIG. 9  depicts a cross-section taken along a line IX-IX of  FIG. 7 . In the burner unit  30 , a pair of the blocking parts  14  are formed, sandwiching the squeezed parts  12  of the ribbon  8  and a pair of the auxiliary burner ports  6 - 2  are formed on the outer side of the blocking parts  14 . 
     In the middle portion of the ribbon  8 , protrusions  69  are formed by bending metal plates outward. The protrusions  69  abut the inner wall plates  60 . The halfway portions of the inner wall plates  60  project toward the side of the squeezed parts  12  of the ribbon  8 . The interval between the auxiliary burner ports  6 - 2  facing each other is thereby reduced. The opening area of each of the auxiliary burner ports  6 - 2  is reduce by the bending parts  72  of the outer wall plates  62 . The auxiliary burner ports  6 - 2  are also surrounded by the outer wall plates  62  that are higher by a height h 1 . 
     &lt;Combustion of Main Air-Fuel Mixture f 1  and Auxiliary Air-Fuel Mixture f 2 &gt; 
     When the main air-fuel mixture f 1  and the auxiliary air-fuel mixture f 2  transition into their combustion state due to ignition, combustion fields are formed. With the main air-fuel mixture f 1 , the independent main flames F 1  are produced at each main burner ports  10  based on the flow velocity and the combustion of the main air-fuel mixture f 1 . Compared to the auxiliary air-fuel mixture f 2 , the main air-fuel mixture f 1  has a larger flowing-out amount and has a higher flowing-out velocity. The main air-fuel mixture f 1  flowing out from the main burner ports  10  is surrounded by the auxiliary air-fuel mixture f 2  flowing out from the plural auxiliary burner ports  6 - 1  and  6 - 2 . The shape of the main flame F 1  is, for example, an oval shape, a circular shape, or the like in its horizontal cross-section. 
       FIG. 10  depicts an example of the state of the main flame F 1  and the auxiliary flame F 2  in the burner unit portion depicted in  FIG. 8 . The pair of parts of auxiliary flame F 2  are formed, sandwiching the main flame F 1 . In this case, the auxiliary air-fuel mixture f 2  has a pressure that is higher than that of the main air-fuel mixture f 1  in the blocking parts  14  between the auxiliary air-fuel mixture f 2  and the main flames F 1 . The auxiliary air-fuel mixture f 2  thereby runs into the blocking parts  14 . In the auxiliary flame F 2 , a flow velocity of the air-fuel mixture is lower than that in the main flames F 1  and the burner port area is smaller than that therein, and the auxiliary flame F 2  therefore is smaller than the main flames F 1 . The auxiliary flame F 2  thereby combusts in the vicinity of the burner port part  54  in contrast with the main flames F 1  whose flame length is large and that combust at a position away from the burner port part  54 . The auxiliary flame F 2  combusts in the vicinity of the foots of the main flames F 1  to surround the main flames F 1 . Even with the flames in the what-is-called air-rich state in which the air ratio of the air-fuel mixture is higher than 1, the pilot flame holding state is established and the auxiliary flame F 2  holds the main flames F 1  to block any being blown-off of the flames, by forming the above combustion state. 
       FIG. 11  depicts an example of the state of the main flame F 1  and the auxiliary flame F 2  in the burner unit portion depicted in  FIG. 9 . The auxiliary flame F 2  is formed in the interval portion between the main flames F 1 . The flame length of the auxiliary flame F 2  stretches on the portion of each auxiliary burner port  6 - 2  and the auxiliary flame F 2  becomes higher thereon. The pressure in the blocking parts  14  adjacent to auxiliary flame F 2  is lower than that of the auxiliary air-fuel mixture f 2  and, as above, the auxiliary flame F 2  produced by the auxiliary air-fuel mixture f 2  runs into the blocking parts  14  and the occluding squeezed part  12 . The outer wall plates  62  surround the auxiliary flame F 2  formed at each auxiliary burner port  6 - 2  for unifying the auxiliary flame F 2  to be facilitated. The circumferences of the main flames F 1  are thereby surrounded by the auxiliary flame F 2  without any interval in a circled state for the main flames F 1  to be held. 
       FIG. 12  depicts an example of the state of the main flame F 1  and the auxiliary flame F 2  taken in a XII-XII portion of  FIG. 7 . For one main flame F 1 , auxiliary flame F 2  is formed by the plural auxiliary burner ports  6 - 1  and  6 - 2 . A pressure in the blocking part  14  between the auxiliary burner ports  6 - 1  and  6 - 2  is lower than that of the auxiliary air-fuel mixture f 2 , and the auxiliary flame F 2  run into the side of the blocking part  14 . The auxiliary flame F 2  running thereinto combusts in the vicinity of the squeezed parts  12  and in the vicinity of the long burner ports  64 . Because the flame length of the auxiliary flame F 2  stretches in the portions of the auxiliary burner ports  6 - 1  and  6 - 2 , a wavy flame shape with unevenness is produced. 
       FIG. 13  depicts an example of the state of the main flames F 1  and the auxiliary flame F 2  in the cross-section taken along a line XIII-XIII of  FIG. 7 . The main flames F 1  are each independently formed. While the auxiliary flame F 2  is present in the interval portion between the main flames F 1 , and each main flame F 1  is therefore held on its overall circumference by the auxiliary flame F 2  adjacent to each main flame F 1 . 
     &lt;Effects and Characteristic Features of the Embodiment&gt; 
     (1) Combustion Function 
     The auxiliary burner ports  6 - 1  and  6 - 2  produce the auxiliary flame F 2  and thereby hold the main flames F 1 . The auxiliary flame F 2  is a stable flame and is used within a range where CO and NOx amounts are permitted under a predetermined standard. The main burner ports  10  produce the main flames F 1  to be the main heat source. The main flames F 1  are unstable flames and the flame holding by the auxiliary combustion at the auxiliary burner ports  6 - 1  and  6 - 2  is indispensable therefor. 
     (2) Used Air Ratio 
     The used air ratio of the auxiliary air-fuel mixture f 2  flowing into the side of the auxiliary burner ports  6 - 1  and  6 - 2  is set to be a value higher than 1 for the auxiliary air-fuel mixture f 2  to set an air-excessive state. The region of the used air ratio of the main air-fuel mixture f 1  flowing into the side of the main burner ports  10  is set to be 1.6 or about 1.6 for the main air-fuel mixture f 1  to be set an air-excessive state. The air ratios of the main air-fuel mixture f 1  and the auxiliary air-fuel mixture f 2  differ from each other and are set so that the air ratio of the main air-fuel mixture f 1  is higher than the air ratio of the auxiliary air-fuel mixture f 2 . 
     (3) Degree of Stability of Flame 
     As to the combustion at each of the auxiliary burner ports  6 - 1  and  6 - 2 , the air ratio is higher than 1 while stable flames are acquired by adjusting the flow velocity of the air-fuel mixture. As to the combustion flames at the main burner ports  10 , the air is excessive, the belching velocity is higher than the combustion velocity, and the flame temperature is low. This flame therefore tends to suffer the lifting. 
     (4) Flame Form 
     As to the auxiliary combustion at each of the auxiliary burner ports  6 - 1  and  6 - 2 , the belching velocity is close to the combustion velocity, the auxiliary flame F 2  has a short flame length and is small. As to the main combustion at the main burner ports  10 , the belching velocity is high and the combustion in a high air ratio is established (with a low combustion velocity). Due to this, the main combustion has a long flame length to produce a large flame. 
     (5) Generation of CO 
     The generation of CO can be reduced in the main combustion at the main burner ports  10 , and the generation of CO can be suppressed by setting the auxiliary air-fuel mixture f 2  flowing to the auxiliary burner ports  6 - 1  and  6 - 2  to be air-excessive. 
     (6) Generation of NOx 
     The generation of NOx can be reduced by setting both of the main air-fuel mixture f 1  and the auxiliary air-fuel mixture f 2  to be air-excessive. 
     (7) Lifting and Flashback 
     As to the auxiliary air-fuel mixture f 2  flowing to the auxiliary burner ports  6 - 1  and  6 - 2 , for the reduced gas amount, the burner port areas of the auxiliary burner ports  6 - 1  and  6 - 2  are each set to be equal to or larger than the burner port area of the auxiliary burner port of the conventional burner and more air is caused to flow. No flashback of the flame thereby tends to occur. The auxiliary flame F 2  holds the main flames F 1  on their overall circumference, and the flame holding function is thereby enhanced and no lifting of the main flames F 1  tends to occur. In a thick and thin fuel horizontal arrangement of a conventional burner in which a thin fuel flame positioned on the side of a thick fuel flame is held by the thick fuel flame, the thin fuel flame does not leave the vicinity of the burner port and a stable flame is formed. However, a thin fuel flame away from the side of the thick fuel flame is held by only the thin fuel flames and the length of the thin fuel flame is large. The lifting therefore tends to occur and excessive CO tends to be generated. When the air ratio is high or when the thick fuel/the thin fuel ratio is extremely low, these tendencies are conspicuous. Due to this, the usable combustion region (the air ratio and the combustion load) is limited for this combustion. In contrast, the pseudo overall-circumferential flame holding is established on the burner unit  30  according to the embodiment and the above inconvenience is therefore not present. 
     (8) From the Above, According to the Burner Unit  30  of this Embodiment, the Following Effects can be Acquired. 
     a) The flame holding function of the auxiliary flame for the main flame can be enhanced, stabilization of the combustion of the main flame can be facilitated, and reduction of CO and NOx can be facilitated by the combustion of the main flames and the auxiliary flame. 
     b) The range of the usable air ratio is extended and the air ratio can be reduced due to the reduction of CO and NOx, and the stabilization of the combustion. The air supply capacity of the air supply fan can therefore be suppressed. 
     c) The controllability of the combustion can be enhanced, and downsizing of the burner and an increase of the output thereof can be facilitated. 
     &lt;Thick Fuel/Thin Fuel Ratio Balance (Taking Air Ratio into Consideration)&gt; 
     Items to be controlled of each of the auxiliary burner ports  6 - 1  and  6 - 2  include the burner port shape, the burner port area, the gas amount ratio, and the like. The air ratios of the auxiliary flame F 2  and the main flame F 1  need to be taken into consideration to determine the gas amount ratio thereof. For example, when the air ratio on the auxiliary burner ports  6 - 1  and  6 - 2  is set to be higher than 1, the combustion of the auxiliary flame F 2  is close to that of the main flames F 1 . This auxiliary flame F 2  reduces exhaust of CO and NOx (NOx can be reduced with the air ratio equal to or higher than 1.2) but causes an increase of the belching velocity of the auxiliary air-fuel mixture f 2  constituting the auxiliary flame F 2  and reduction of the flame temperature, and the tendency for the lifting may be enhanced. Taking into consideration this point of view, the burner unit  30  according to the embodiment realizes production of the auxiliary flame F 2  with concurrently suppressed exhausts of CO and NOx by establishing the gas amount ratio balance and reducing the flow amount of the air-fuel mixture flowing through the auxiliary burner port. 
     As to the auxiliary burner ports  6 - 1  and  6 - 2 , the air ratio of the auxiliary flame F 2  is set to be a value close to that of the main flames F 1  because the overall-circumference flame holding is conducted for the main flames F 1  using the auxiliary flame F 2 . The flame holding function is therefore enhanced even when the air ratio of the auxiliary flame F 2  is increased and the auxiliary flame F 2  tends to have the lifting. As a result, CO and NOx mainly generated in the auxiliary flame F 2  can be reduced. When the air ratio of the auxiliary flame F 2  is set to be around the theoretical value of A=1, the thermal NOx becomes conspicuous and the production velocity of the thermal NOx is therefore reduced. In this case, the flame temperature is set to be, for example, lower than 1,800° C. and the air ratio is set to be equal to or higher than 1.2. 
     &lt;Thick Fuel/Thin Fuel Ratio Balance&gt; 
     The gas amount ratio of the air-fuel mixture is determined corresponding to the performance and the purpose of the burner unit  30 . For example, when the suppression of the noise value and the prevention of the oscillating combustion are enhanced, a setting is made for the gas amount ratio to be increased, that is, an increase of the load on the side of the auxiliary combustion, and the rate of the auxiliary flame F 2  to be the stable flame is thereby increased. When reduction of the harmful exhaust components, such as CO and NOx is desired in the exhaust gas, a setting is necessary to reduce the air ratio of the auxiliary air-fuel mixture f 2 . Compared to the auxiliary combustion to be the combustion of the auxiliary air-fuel mixture f 2 , the thin fuel combustion to be the combustion of the main air-fuel mixture f 1  combusts on the side of an excessive air ratio and any generation of these harmful components is therefore suppressed. 
     &lt;Results of Experiments&gt; 
     (1) Relation Between Air/Fuel Ratio and Combustion Exhaust Gas 
       FIGS. 14 and 15  depict results acquired by actually measuring a combustion exhaust gas (for NOx and CO) when the combustion apparatus  20  including the burner units  30  was loaded into a water heater. “A” represents the result of an experiment for the combustion apparatus  20  that is an example of the combustion apparatus of the present invention, and “B” represents the result of measurement using the conventional burner as Comparative Example. The thick fuel/thin fuel ratio was 20:80 and the input was 58.1 [kW]. 
     As depicted in  FIG. 14 , as to the relation between the air/fuel ratio and NOx, the line of the reference value represents the NOx regulation value of California in the U.S. that is an internationally very strict exhaust standard. With the conventional burner, when the air/fuel ratio is set to be high, the “B” is able to be reached to the standard while an NOx exhaust amount lower than the reference value is able to be realized within a wide range of air/fuel ratio when the burner unit  30  is used. Although  FIG. 14  depicts the relation between the air/fuel ratio and NOx, a relation between the air ratio and NOx has the same tendency as or similar tendency to the relation between the air/fuel ratio and NOx. That is, with the conventional burner, when the air ratio is set to be high, the “B” is able to be reached to the standard while the NOx exhaust amount lower than the reference value is able to be realized within a wide range of the air ratio when the burner unit  30  is used. 
     As depicted in  FIG. 15 , as to the relation between the air/fuel ratio and CO [%], the line of the reference value represents the regulation value of ANSI 221.10.3, the North American water heaters standard, that is an internationally very strict exhaust standard. Similarly to the result for NOx, it can be seen that the CO exhaust amount is equal to or smaller than the reference value within a wide range of air/fuel ratio for a burner unit  12 . Although  FIG. 15  depicts the relation between the air/fuel ratio and CO [%], a relation between the air ratio and CO [%] has the same tendency as or similar tendency to the relation between the air/fuel ratio and CO [%]. That is, it can be seen that the CO exhaust amount is equal to or smaller than the reference value within a wide range of the air ratio for a burner unit  12 . CO exhaust amount does not become equal to or smaller than the reference value with the conventional burner. 
     A value in the vicinity of “C” is used as the best air/fuel ratio for the conventional burner while the air/fuel ratio and the air ratio is reduced as a first means for the described burner unit  30 . When the flowing-out velocity of the main air-fuel mixture f 1  is increased, the generation ratio of CO is also increased while, as is apparent from the graph of  FIG. 15 , CO [%] maintains a certain low value even when the flowing-out velocity of the main air-fuel mixture f 1  is increased, that is, the combustion velocity is increased when the air/fuel ratio and the air ratio are increased. The flowing-out velocity of the main air-fuel mixture f 1  is therefore increased as a second means. Working a combination of the first and the second means or either one thereof can reduce exhaust of CO, NOx, and the like, maintaining or increasing the heat amount per unit area of the burner port part  54 . 
     (2) State of Air Ratio and Combustion State for Air-Fuel Mixture Flow Velocity of Burner Unit  30  to be One Example of Invention of this Application 
     As depicted in  FIG. 16 , the burner unit  30  can flow out the air-fuel mixtures for the air-fuel mixtures to each have a set air ratio in the main burner ports  10  and the auxiliary burner ports  6 - 1  and  6 - 2 . Of these, for the auxiliary burner ports  6 - 1  and  6 - 2 , the air ratios were measured for the auxiliary flame being disposed on the right side and left side of the main burner ports of the burner unit  30  as a center as one set. According to the measurement, for the burner  2 , the air ratio of the main flame is 1.66 in average with a certain amount of error, and the air ratio of the auxiliary flame is 1.22 in average for not only the right one but also the left one with a certain amount of error. 
     For the combustion apparatus  20  including the burner unit  30 , for example, as depicted in  FIG. 17 , the air-fuel mixture flow velocity was measured for each predetermined combustion load (Input). According to the measurement, for the burner unit  30  and the conventional burner, the ranges of the air-fuel mixture flow velocity of the auxiliary flame against the combustion load overlap with each other. The burner unit  30  with a reduced gas amount ratio thereby exhibits the combustion state similar to that of the burner combusting the conventional thick fuel and thin fuel air-fuel mixtures. With the burner unit  30 , no flashback of any flame occurs at the auxiliary burner ports  6 - 1  and  6 - 2  by maintaining the air-fuel mixture flow velocity against the combustion load. 
     OTHER EMBODIMENTS 
     (1) It is described in the above embodiment that the air ratio of the auxiliary air-fuel mixture flowing through the burner  2  and the amounts of the air-fuel mixtures are calculated using the values set in advance or the amounts of the air-fuel mixtures of the conventional burner while the calculation is not limited to this. The air ratio of the auxiliary air-fuel mixture and the amounts of the air-fuel mixtures may be set based on, for example, the calculation of the air-fuel mixture flow velocity V A1  and V A2 . For the burner  2 , the air-fuel mixture amount Q A2  may be set such that, for example, the air-fuel mixture flow velocity V A2  is equal to the flow velocity V A1  of the air-fuel mixture exhausted from the auxiliary burner port of the conventional burner. 
     (2) In addition, for the burner  2 , after the values of the gas amount ratio is set, the air ratio X 2-2  of the auxiliary air-fuel mixture f 2  and the auxiliary air-fuel mixture amount Q A3  may be able to be arbitrarily set within predetermined condition ranges. 
     (3) It is described in the above embodiment that the auxiliary burner port  6 - 1  is formed by one single opening while the auxiliary burner port  6 - 1  is not limited to this and the auxiliary burner port  6 - 1  may be formed by plural openings lining with the main burner port  10 . 
     (4) The auxiliary burner ports  6 - 2  may be arranged so that their protruding tips abut on the side of the squeezed part  12  of the ribbon  8 . The auxiliary flame F 2  sandwiching the squeezed parts  12  can thereby be closely cohered and the flame holding function for the main flames F 1  can be enhanced. 
     (5) In the above embodiment, the hole shape of the main burner ports  10  is set to be the flat hexagonal shape while the hole shape may be set to be an oval shape or a circular shape. 
     (6) In the above embodiment, the hole shape of the auxiliary burner ports  6 - 1  is set to be the flat rectangular shape while the hole shape may be set to be an oval shape or a circular shape. 
     (7) In the above embodiment, the hole shape of the auxiliary burner ports  6 - 2  is set to be the trapezoidal shape while the hole shape may be set to be an oval shape or a circular shape. 
     (8) A third auxiliary burner port may be formed in the blocking part  14  located between the auxiliary burner ports  6 - 2  of the above embodiment to hold the main flame. 
     (9) It is described in the above embodiment that the air ratios of the air-fuel mixtures are set to be specific values for the air-fuel mixtures to combust while the air ratios are not limited to this. For the combustion apparatus  20 , the air ratios may be adjusted corresponding to, for example, the combustion amount of the burner unit  30 . The combustion apparatus  20  controls a rotation rate of the motor  46  of the air supply fan  44  and the supply amounts of the fuel gas, for example, to adjust the air ratios of the air-fuel mixtures. In this case, the main air-fuel mixture f 1  and the auxiliary air-fuel mixture f 2  each have the air ratio higher than 1. 
     The air ratios of the air-fuel mixtures are adjusted based on, for example, an electric current value of a proportional valve for gas against the combustion load indicated by a solid line in  FIG. 18  and the rotation rate of the motor  46  of the air supply fan  44  against the electric current value of the proportional valve indicated by a dotted line therein. For example, when the combustion load is 40 [kW], the electric current value of the proportional valve for gas is set to be 88 [mA] that corresponds to the combustion load of 40 [kW] and the rotation rate of the motor  46  is set to be 4,000 [r/min] that corresponds to the electric current value of the proportional valve of 88 [mA]. The relation among the combustion load, the electric current value of the proportional valve, and the rotation rate of the motor  46  depicted in  FIG. 18  is prepared in advance by collecting the electric current value of the proportional valve for gas corresponding to the combustion load and the rotation rate of the motor  46  of the air supply fan  44  to establish a predetermined air ratio. On the basis of the relation among the combustion load, the electric current value of the proportional valve, and the rotation rate of the motor, for example, a relation table showing the relation among the combustion load, the electric current value of the proportional valve, and the rotation rate of the motor is produced. 
       FIG. 19  depicts an example of hardware of the combustion apparatus  20  for the adjustment of the air ratio. A memory part  74  included in the combustion apparatus  20  stores therein the described relation table. The memory part  74  is an example of the storing part that stores therein data, and includes a storing device, such as a flash memory or an electrically erasable and programmable read only memory (EEPROM). A control part  76  included in the combustion apparatus  20  reads the table stored in the memory part  74 , adjusts the degree of opening of the proportional valve for gas  78  included in the combustion apparatus  20  to adjust the supply amount of the fuel gas, and adjusts the rotation rate of the motor  46  of the air supply fan  44  to adjust the supply amount of air. The control part  76  adjusts and maintains the air ratios of the air-fuel mixtures based on the relation table. The memory part  74 , the control part  76 , the motor  46  of the air supply fan  44 , and the proportional valve for gas  78  are connected to each other by a connecting line  80 . 
     The relation table may be set regardless of, for example, the combustion stages based on the switching of the combustion area, such as entire combustion, half combustion, or the like of the burner, or may be set for each combustion stage. 
     The combustion velocity is reduced when the air ratio being equal to or higher than 1 is increased, while the combustion velocity is increased when the air ratio being higher than 1 approaches 1. Variation of the air ratio and variation of the combustion velocity are therefore related with each other. Any waste of the fuel gas is prevented and, in addition, CO and NOx become hard to be generated by adjusting the air ratio corresponding to the magnitude of the combustion amount. For the combustion apparatus  20 , the air ratios are adjusted for the combustion to tend to be stable. 
     (10) It is described in the above embodiment that the air ratio of the main air-fuel mixture f 1  is higher than the air ratio of the auxiliary air-fuel mixture f 2 , that is, the main air-fuel mixture f 1  is more air-rich state than the auxiliary air-fuel mixture f 2 , while the air ratios are not limited to this. The value of the air ratio of the auxiliary air-fuel mixture f 2  may be higher than that of the main air-fuel mixture f 1 . 
     Aspects of the burner, the combustion apparatus, and the combustion method extracted from the described embodiments are as follows. 
     An aspect of the burner includes a plurality of burner ports that produce flames using a first air-fuel mixture; and a plurality of auxiliary burner ports that are disposed in a circumference of the burner ports, the plurality of the auxiliary burner ports producing an auxiliary flame using a second air-fuel mixture whose an air ratio is different from an air ratio of the first air-fuel mixture. The air ratios of the first air-fuel mixture and the second air-fuel mixture are each higher than 1. 
     In the burner, the auxiliary flame produced by the plurality of the auxiliary burner ports may be unified to surround the flames, and the auxiliary flame may combust at a position lower than a combustion position of the flames to hold the flames 
     In the burner, a flow velocity of the first air-fuel mixture may be higher than a flow velocity of the second air-fuel mixture. 
     In the burner, a gas flow ratio of the first air-fuel mixture supplied to the burner ports to the second air-fuel mixture supplied to the auxiliary burner ports may be set to be at a ratio of 80 to 20 or a ratio in vicinity thereof. 
     In the burner, air of an amount calculated based on a supplied gas amount or the gas flow ratio may be supplied to the auxiliary burner ports for a set air-fuel mixture amount. 
     An aspect of the combustion apparatus includes a plurality of burner units. The plurality of the burner units each include a plurality of burner ports that produce flames using a first air-fuel mixture; and a plurality of auxiliary burner ports that are disposed in a circumference of the burner ports, the plurality of the auxiliary burner ports producing an auxiliary flame using a second air-fuel mixture whose an air ratio is different from an air ratio of the first air-fuel mixture. The air ratios of the first air-fuel mixture and the second air-fuel mixture are each higher than 1. 
     An aspect of the combustion method includes producing flames at a plurality of burner ports using a first air-fuel mixture whose an air ratio is higher than 1; and producing an auxiliary flame at a plurality of auxiliary burner ports disposed in a circumference of the burner ports using a second air-fuel mixture whose an air ratio is higher than 1, the air ratio of the second air-fuel mixture being different from the air ratio of the first air-fuel mixture. 
     Effect of the burner, the combustion apparatus, and the combustion method are listed as follows. 
     (1) The flame holding function can be enhanced and stabilization of the flame can be facilitated by surrounding the circumference of one burner port by other burner ports. 
     (2) The flame can be stabilized by the flame holding and reduction of NOx and reduction of CO in the combustion exhaust can be facilitated, by generating the plural air-fuel mixtures whose air ratios are different from each other and whose air ratios are higher than 1. 
     (3) Occurrence of any flashback can be suppressed by not reducing the flow velocity of the air-fuel mixture against the variation of the supply gas amount. 
     As above, the most preferred embodiment and the like of the present invention have been described while the present invention is not limited by the above description. It is obvious that those skilled in the art can make various deformations and changes thereto based on the gist of the invention described in claims or disclosed herein. Not to mention, those deformations and changes are included in the scope of the present invention. 
     According to the burner, the combustion apparatus, and the combustion method that each are an example of the present invention, the flame holding function by the burner can be enhanced, highly stable combustion can be acquired, and air-excessive air-fuel mixtures can be combusted. Advantages can therefore be acquired such as reduction of the exhaust amounts of nitrogen oxides.