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.

Description

TECHNICAL FIELD[0001]The present invention relates to a boiler applied to a water-tube boiler, a vapor boiler, a hot water boiler, and the like.BACKGROUND ART OF THE INVENTION[0002]Generally known principles of suppressing NOx emissions include the suppression of flame (combustion gas) temperatures and a decrease in retention time of combustion gas at high temperatures. As such, various technologies are available for decreasing the emission of NOx by applying these principles. Various methods have been proposed and put into practical use, for example, two-stage combustion, lean-rich combustion, exhaust gas recirculate combustion, water mixing combustion, steam injection combustion, and flame cooling combustion by a water tube group.[0003]Moreover, NOx sources relatively small in capacity such as water-tube boilers are also beginning to be required for a further decrease in emission of NOx due to an increasing awareness of environmental problems. In this case, the decrease in NOx gen...

AI Technical Summary

Benefits of technology

[0017]The inventors of the present application have conducted research for solving the above problems, finding a point at which the amount of emitted nitrogen oxides and carbon monoxide is decreased to substantially zero in a premixed burner combustion region at a low air ratio as close to 1 as possible (the region Z2 in FIG. 17), for which research has been so far hardly conducted for a boiler equipped with an oxidation catalyst to decrease carbon monoxide as described in Patent Documents 3 and 4. As a result, they have studied causes for which the amount of emitted nitrogen oxides and carbon monoxide can be decreased to substantially zero, thus obtaining a new finding that a concentration ratio of oxygen, nitrogen oxides, and carbon monoxide on the primary side of the oxidation catalyst is given as a predetermined reference concentration ratio, thereby an oxidation catalyst is used to decrease the amount of emitted nitrogen oxides and carbon monoxide as close to zero as possible. At the same time, the concentration ratio is adjusted in the vicinity of the predetermined reference concentration ratio, thereby obtaining a new finding that the amount of emitted hazardous substances (nitrogen oxides and carbon monoxide) can be decreased to substantially zero or a permissible value. The present invention has been completed on the basis of these findings. According to the present invention, it is possible not only to decrease the concentration of emitted hazardous substances to substantially zero but also to attain a remarkable energy savings due to the fact that the above decrease can be obtained at an air ratio as close to 1.0 as possible.

[0051]According to the present invention, the concentration ratio is adjusted to the predetermined concentration ratio by the air-ratio adjusting device, whereby the emission amounts of nitrogen oxides and carbon monoxide can be decreased to close to zero as much as possible by using the oxidation catalyst. Further, the decrease in hazardous substances can be realized stably. Further, the configuration of an apparatus can be simplified without requiring separately means for controlling a concentration ratio to be constant. Further, compared with a premixed burner, combustible air and a fuel can be mixed uniformly to be burned, and gas at the predetermined concentration ratio can be generated stably and relatively easily with less generation amounts of hydrocarbon and CO even in the vicinity of a theoretical air ratio.

Problems solved by technology

Moreover, NOx sources relatively small in capacity such as water-tube boilers are also beginning to be required for a further decrease in emission of NOx due to an increasing awareness of environmental problems.

In this case, the decrease in NOx generation inevitably entails an increased amount of emitted CO, thus making it difficult to attain a simultaneous decrease in NOx and CO.

A cause of the above problem is that a simultaneous decrease in emission of NOx and CO is technically incompatible.

In other words, temperatures of combustion gas are kept higher in an attempt to decrease the amount of emitted CO, thus resulting in an insufficient suppression of NOx generation.

The technologies disclosed in Patent Documents 3 and 4 are able to decrease the amount of emitted NOx to a value lower than 10 ppm, but it is difficult to decrease the amount of emitted NOx to a value below 5 ppm.

This is due to the fact that combustion characteristics inevitably entail NOx generation at 5 ppm or greater.

In contrast, at a combustion region Z2 where the air ratio is 1.1 or lower (hereinafter, referred to as “low air ratio”), nitrogen oxides are generated in an increased amount, thus making it difficult to attain a simultaneous decrease in the amount of emitted NOx and CO.

There is also posed a difficulty in controlling a stable combustion due to a possible occurrence of backfire where the air ratio is 1 or lower.

Further, according to the method described in Patent Document 5, unburned hydrocarbons are emitted in a great amount, thus making it difficult to decrease the concentrations of emitted nitrogen oxides and emitted carbon monoxide to substantially zero by using an oxidation catalyst.

Further, the oxidation catalyst characteristics in which an efficiency in reducing nitrogen oxides in the presence of hydrocarbons decrease cannot be used.

Still further, in a step of reducing nitrogen oxides, a catalyst is used, which is different from that used in a step of oxidizing unburned components, resulting in a complicated treatment.

Therefore, the treatment method of Patent Document 6 is not suitable in treating combustion gas resulting from a boiler, which occurs on combustion by a burner and contains excess oxygen.

The technology in Patent Document 7 is subjected to such restriction that fuel is burned at an excessively high concentration, thus making it difficult to find an application for the combustion apparatus such as a burner-equipped boiler in which oxygen is contained in exhaust gas.

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