Boiler

Abstract

First, an emission amount of nitrogen oxide can be decreased to zero as much as possible and, an emission amount of carbon monoxide is decreased to a permissible range. Second, energy saving by combustion at a low air ratio close to 1.0 is realized. Third, air ratio control is performed stably in a combustion region at a low air ratio. Provided is a boiler, including: a premixed burner; a water pipe group for, by heat exchanging with a gas produced by the premixed burner, suppressing a temperature of the gas to thereby suppress a concentration of nitrogen oxide to a predetermined value or less; an oxidation catalyst for oxidizing carbon monoxide contained in the gas after the passage through the water pipe group by oxygen and for reducing nitrogen oxide contained therein by carbon monoxide; and an air-ratio adjusting device for adjusting an air ratio of the premixed burner, in which: the premixed burner and the water pipe group have such characteristics that when the air ratio is a set air ratio, a concentration ratio of oxygen, nitrogen oxides, and carbon monoxide in the gas on a primary side of the oxidation catalyst becomes a predetermined concentration ratio; the oxidation catalyst has such characteristics that when the concentration ratio is a predetermined concentration ratio, a concentration of nitrogen oxides on a secondary side of the oxidation catalyst is substantially zero while a concentration of carbon monoxide is decreased to substantially zero or a predetermined value or less; and the predetermined concentration ratio is kept to be constant by being controlled to the set air ratio by the air-ratio adjusting device.

1 . A boiler, comprising:
 a premixed burner for burning a hydrocarbon-containing fuel to generate a gas free of hydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;   a water tube group for absorbing heat from the gas generated by the premixed burner;   an oxidation catalyst for oxidizing carbon monoxide contained in the gas after the passage through the water tube group by oxygen and reducing the nitrogen oxides contained in the gas by carbon monoxide; and   an air-ratio adjusting device for adjusting an air ratio of the premixed burner,   wherein the premixed burner and the water tube group have characteristics in which a concentration ratio of oxygen, nitrogen oxides, and carbon monoxide in gas on a primary side of the oxidation catalyst becomes a predetermined concentration ratio, assuming that the air ratio is a set air ratio,   the oxidation catalyst has characteristics in which a concentration of nitrogen oxides on a secondary side of the oxidation catalyst is decreased to substantially zero and a concentration of carbon monoxide on the secondary side of the oxidation catalyst is decreased to substantially zero or a predetermined value or less, assuming that the concentration ratio is the predetermined concentration ratio, and   the predetermined concentration ratio is kept to be constant by being controlled to the set air ratio by the air-ratio adjusting device.   
   
   
       2 . A boiler, comprising:
 a premixed burner for burning a hydrocarbon-containing fuel to generate a gas free of hydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;   a water tube group for absorbing heat from the gas generated by the premixed burner;   an oxidation catalyst for oxidizing carbon monoxide contained in the gas after the passage through the water tube group by oxygen and reducing the nitrogen oxides contained in the gas by carbon monoxide; and   an air-ratio adjusting device for adjusting an air ratio of the premixed burner,   wherein the premixed burner and the water tube group have characteristics in which a concentration ratio K of oxygen, nitrogen oxides, and carbon monoxide in gas on a primary side of the oxidation catalyst becomes a predetermined reference concentration ratio K 0 , assuming that the air ratio is a reference set air ratio,   the oxidation catalyst has characteristics in which a concentration of nitrogen oxides and a concentration of carbon monoxide on a secondary side of the oxidation catalyst are decreased to substantially zero, assuming that the concentration ratio K is the predetermined reference concentration ratio K 0 , and   the predetermined concentration ratio is kept to be constant by being controlled to the reference set air ratio by the air-ratio adjusting device.   
   
   
       3 . A boiler, comprising:
 a premixed burner for burning a hydrocarbon-containing fuel to generate a gas free of hydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;   a water tube group for absorbing heat from the gas generated by the premixed burner;   an oxidation catalyst for oxidizing carbon monoxide contained in the gas after the passage through the water tube group by oxygen and reducing the nitrogen oxides contained in the gas by carbon monoxide; and   an air-ratio adjusting device for adjusting an air ratio of the premixed burner,   wherein the premixed burner and the water tube group have characteristics in which a concentration ratio K of oxygen, nitrogen oxides, and carbon monoxide in gas on a primary side of the oxidation catalyst becomes a first predetermined concentration ratio K 1 , assuming that the air ratio is a first set air ratio,   the oxidation catalyst has characteristics in which a concentration of nitrogen oxides on a secondary side of the oxidation catalyst is decreased to substantially zero and a concentration of carbon monoxide on the secondary side of the oxidation catalyst is decreased to a predetermined value or less, assuming that the concentration ratio K is the first predetermined concentration ratio K 1 , and   the predetermined concentration ratio is kept to be constant by being controlled to the first set air ratio by the air-ratio adjusting device.   
   
   
       4 . The boiler according to  claim 2  or  3 , wherein:
 a formula for determining the predetermined reference concentration ratio K 0  is given as the following formula (1);   the predetermined reference concentration ratio K 0  satisfies the following formula (2); and   the first predetermined concentration ratio K 1  is made smaller than the reference concentration ratio K 0 :
   ([NO x]+ 2[O 2 ])/[CO]=K  (1) 
   1.0≦K=K 0 ≦2.0  (2) 
   
     where [CO], [NOx], and [O 2 ] represent concentrations of carbon monoxide, nitrogen oxides, and oxygen, respectively, and satisfying a condition of [O 2 ]>0. 
   
   
       5 . (canceled) 
   
   
       6 . A boiler, comprising:
 a premixed burner for burning a hydrocarbon-containing fuel to generate a gas free of hydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;   a water tube group for absorbing heat from the gas generated by the premixed burner;   an oxidation catalyst for oxidizing carbon monoxide contained in the gas after the passage through the water tube group by oxygen and reducing the nitrogen oxides contained in the gas by carbon monoxide;   a sensor for detecting an air ratio of the premixed burner; and   an air-ratio adjusting device for controlling the premixed burner to a set air ratio based on a detected signal of the sensor,   wherein the premixed burner and the water tube group have characteristics of air ratio-NOx/CO in which, when the air ratio is adjusted to the set air ratio by the air-ratio adjusting device, a concentration of nitrogen oxides and a concentration of oxygen on a secondary side of the oxidation catalyst is decreased to substantially zero, and a concentration of carbon monoxide on the secondary side of the oxidation catalyst is decreased to substantially zero or a predetermined value or less.   
   
   
       7 . A boiler, comprising:
 a premixed burner for burning a hydrocarbon-containing fuel to generate a gas free of hydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;   a water tube group for absorbing heat from the gas generated by the premixed burner;   an oxidation catalyst for oxidizing carbon monoxide contained in the gas after the passage through the water tube group by oxygen and reducing the nitrogen oxides contained in the gas by carbon monoxide;   a sensor for detecting an air ratio of the premixed burner; and   an air-ratio adjusting device for controlling the premixed burner to a set air ratio based on a detected signal of the sensor,   wherein the premixed burner and the water tube group are configured so that, when the air-ratio adjusting device is used to adjust the air ratio to the set air ratio, the concentration of carbon monoxide on a primary side of the oxidation catalyst is substantially equal to or greater than a value obtained by adding a concentration of carbon monoxide decreased inside the oxidation catalyst due to the oxidation to a concentration of carbon monoxide decreased inside the catalyst due to the reduction.   
   
   
       8 . A boiler, comprising:
 a premixed burner for burning a hydrocarbon-containing fuel to generate a gas free of hydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;   a water tube group for absorbing heat from the gas generated by the premixed burner;   an oxidation catalyst for oxidizing carbon monoxide contained in the gas after the passage through the water tube group by oxygen and reducing the nitrogen oxides contained in the gas by carbon monoxide;   a sensor for detecting an air ratio of the premixed burner; and   an air-ratio adjusting device for controlling the premixed burner to a set air ratio based on a detected signal of the sensor,   wherein the premixed burner and the water tube group are configured so that, when the air-ratio adjusting device is used to adjust the air ratio to the set air ratio, a concentration ratio of the gas before flowing into the oxidation catalyst satisfies the following formula (3):
   ([NO x]+ 2[O 2 ])/[CO]≦2.0  (3) 
   
     where [CO], [NOx], and [O 2 ] represent concentrations of carbon monoxide, nitrogen oxides, and oxygen, respectively, and satisfying a condition of [O 2 ]>0. 
   
   
       9 . (canceled) 
   
   
       10 . A boiler, comprising:
 a premixed burner for burning a hydrocarbon-containing fuel to generate a gas free of hydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;   a water tube group for absorbing heat from the gas generated by the premixed burner;   an oxidation catalyst for oxidizing carbon monoxide contained in the gas after the passage through the water tube group by oxygen and reducing the nitrogen oxides contained in the gas by carbon monoxide; and   an air-ratio adjusting device for adjusting an air ratio of the premixed burner,   wherein the premixed burner and the water tube group have characteristics of air ratio-NOx/CO on a primary side of the oxidation catalyst regarding the gas containing oxygen, nitrogen oxides, and carbon monoxide on the primary side of the oxidation catalyst obtained by adjusting in a vicinity of an air ratio of 1.0 by the air-ratio adjusting device,   the oxidation catalyst has characteristics of air ratio-NOx/CO on a secondary side of the oxidation catalyst obtained by bringing a gas having the characteristics of air ratio-NOx/CO on the primary side into contact with the oxidation catalyst, and   the air-ratio adjusting device controls an air ratio of the premixed burner at a set air ratio at which a concentration of nitrogen oxides on the secondary side of the oxidation catalyst is decreased to substantially zero in a NOx/CO decreasing region of the characteristics of air ratio-NOx/CO on the secondary side.   
   
   
       11 . A boiler, comprising:
 a premixed burner for burning a hydrocarbon-containing fuel to generate a gas free of hydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;   a water tube group for absorbing heat from the gas generated by the premixed burner;   an oxidation catalyst that is brought into contact with the gas containing oxygen, nitrogen oxides, and carbon monoxide after the passage through the water tube group; and   an air-ratio adjusting device for adjusting a ratio between a combustible air amount of the premixed burner and a fuel amount,   wherein the oxidation catalyst has characteristics of decreasing a concentration of carbon monoxide by: when a concentration ratio of oxygen, nitrogen oxides, and carbon monoxide in a gas on a primary side of the oxidation catalyst at which a concentration of nitrogen oxides and a concentration of carbon monoxide on a secondary side of the oxidation catalyst are decreased to substantially zero is a reference concentration ratio, and the concentration ratio is the reference concentration ratio, concentrations of oxygen, nitrogen oxides, and carbon monoxide on the secondary side of the oxidation catalyst being decreased to substantially zero; and when a concentration of oxygen on the primary side is lower than a concentration of the reference oxygen, concentrations of nitrogen oxides and oxygen on the secondary side of the oxidation catalyst being decreased to substantially zero, and   the air-ratio adjusting device adjusts a concentration of oxygen on the primary side of the oxidation catalyst with respect to the concentration of the reference oxygen by adjusting the air ratio based on concentrations of oxygen and/or carbon monoxide on the secondary side of the oxidation catalyst, and decreases a concentration of nitrogen oxides on the secondary side of the oxidation catalyst and the concentration of carbon monoxide on the secondary side of the oxidation catalyst to substantially zero, thereby decreasing the concentration of carbon monoxide or decreasing the concentration of carbon monoxide to substantially zero.   
   
   
       12 . The boiler according to any one of  claims 1  to  3 ,  6  to  8 ,  10 , and  11 , comprising a nitrogen oxide generation suppressing device for suppressing a temperature of the gas to suppress a concentration of nitrogen oxides to a predetermined value or less. 
   
   
       13 . (canceled) 
   
   
       14 . (canceled)