Liquid heaters and vaporizers – Heat transmitter
Reexamination Certificate
2003-12-09
2004-11-30
Wilson, Gregory (Department: 3749)
Liquid heaters and vaporizers
Heat transmitter
C122S235110, C110S234000
Reexamination Certificate
active
06823821
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a combustion method for NO
x
reduction, as well as an apparatus therefor, to be applied to water-tube boilers, reheaters of absorption refrigerators, or the like.
Generally, as the principle of suppression of NO
x
generation, there have been known (1) suppressing the temperature of flame (combustion gas), (2) reduction of residence time of high-temperature combustion gas, and (3) lowering the oxygen partial pressure. Then, various NO
x
reduction techniques to which these principles are applied are available. Examples that have been proposed and developed into practical use include the two-stage combustion method, the thick and thin fuel combustion method, the exhaust gas recirculate combustion method, the water addition combustion method, the steam jet combustion method, the flame cooling combustion method with water-tube groups (water-tube cooling combustion method), and the like.
With respect to small-size once-through boilers, as of today, there has been laid out in Tokyo Metropolis or others a regulation that the exhaust NO
x
value of gas-fired boilers should be not more than 60 ppm (at 0% O
2
in the exhaust gas, dry basis; hereinbelow, the unit ppm is expressed at 0% O
2
in the exhaust gas, dry basis, unless otherwise specified), and that the exhaust NO
x
value of oil-fired boilers should be not more than 80 ppm for A-type heavy oil and not more than 60 ppm for kerosine. Many manufacturers including the present applicant have cleared these regulation values. However, California in U.S.A. has already laid out a regulation specifying not more than 12 ppm (at 3% O
2
in the exhaust gas, dry basis). The applicant, envisaging that even stricter regulations, e.g. not more than 30 ppm, will be applied in the near future also in Japan, has been performing research and development for further NO
x
reduction.
A prior-art NO
x
reduction techniques is proposed in combinations of above-described various suppression principles (see, e.g., Patent Reference 1: Japanese Published Patent Application H07-103411, Page 3, FIG. 1). This prior-art technique is a combination of the exhaust gas recirculate technique and the steam jet. However, with this NO
x
reduction technique, it is not easy to achieve an exhaust NO
x
value of not more than 30 ppm (hereinafter, referred to as “target exhaust NO
x
value”).
That is, the present inventors of this application have found through various experiments and discussions that the following issues exist in order to achieve the target exhaust NO
x
value or lower in the prior art.
First, in the prior art, for reduction of the NO
x
value by a functional enhancement of combustion gas temperature suppression with the exhaust gas recirculation, the functional enhancement is to increase the exhaust-gas recirculation quantity. However, implementing this functional enhancement would cause unstable characteristics of the exhaust gas recirculation to be amplified. That is, the exhaust gas recirculation has a characteristic that the exhaust-gas flow rate or temperature changes with changes in combustion quantity or changes in load. An increase in the exhaust-gas recirculation quantity would cause these unstable characteristics to be amplified, making it impossible to achieve a stable NO
x
reduction. Also, an increase in the exhaust-gas recirculation quantity would cause the oxygen concentration in the combustion air to lower, resulting in a combustion state of oxygen deficiency, so that the combustion could no longer be continued because of incomplete combustion or discharge of unburned combustibles. Further, a volume increase corresponding to the exhaust-gas recirculation quantity would cause the pressure loss in the air blow passage to increase, thus making it inevitable to increase the cost due to the increase in the blower capacity.
Also, a functional enhancement of NO
x
reduction by steam addition is to increase the quantity of water to be added. This functional enhancement would cause an increase in thermal loss and moreover an increase in the quantity of condensations, posing a problem of corrosion of the constituent equipment due to the condensations.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a combustion method for NO
x
reduction, as well as an apparatus therefor, capable of solving these and other issues and easily achieving NO
x
reduction with the value of exhaust NO
x
under 30 ppm.
The present invention having been accomplished to solve the above object, in a first aspect of the invention, there is provided a combustion method for NO
x
reduction comprising in combination the steps of: a first NO
x
reduction step for suppressing generated NO
x
value to 60 ppm or under (at 0% O
2
in exhaust gas, dry basis) by a low NO
x
burner; a second NO
x
reduction step for recirculating exhaust gas of the low NO
x
burner to a burning reaction zone formed by the low NO
x
burner; and a third NO
x
reduction step for adding water or steam to the burning reaction zone.
In a second aspect of the invention, there is provided a combustion method for NO
x
reduction as described in the first aspect, wherein the second NO
x
reduction step is performed with a target exhaust NO
x
value set to 30 ppm or under (at 0% O
2
in exhaust gas, dry basis) and with an exhaust-gas recirculation quantity set in a stable combustion range of the low NO
x
burner, and any NO
x
value exceeding the target exhaust NO
x
value is reduced by the third NO
x
reduction step.
In a third aspect of the invention, there is provided a combustion method for NO
x
reduction as described in the first or second aspect, wherein the third NO
x
reduction step is performed by spraying water directly to the burning reaction zone.
In a fourth aspect of the invention, there is provided a combustion apparatus for NO
x
reduction, comprising: a low NO
x
burner for suppressing generated NO
x
value to 60 ppm or under (at 0% O
2
in exhaust gas, dry basis); exhaust gas recirculation means for recirculating exhaust gas of the low NO
x
burner to a burning reaction zone formed by the low NO
x
burner; and water or steam addition means for adding water or steam to the burning reaction zone.
Further, in a fifth aspect of the invention, there is provided a combustion apparatus for NO
x
reduction, comprising: a low NO
x
burner for suppressing generated NO
x
value to 60 ppm or under (at 0% O
2
in exhaust gas, dry basis); exhaust gas recirculation means for recirculating exhaust gas of the low NO
x
burner to a burning reaction zone formed by the low NO
x
burner; and water spraying means for spraying water directly to the burning reaction zone.
In one embodiment, there is provided a NO
x
reduction combustion method as described in any one of the first to third aspects, wherein the NO
x
reduction step is performed with an excess air ratio which is determined from a NO
x
reduction target value and an excess air ratio versus NO
x
characteristic of the NO
x
reduction step.
Before the description of embodiments of the present invention, terms used herein are explained. The combustion gas includes burning-reaction ongoing (under-combustion-process) combustion gas, and combustion gas that has completed burning reaction. Then, the burning-reaction ongoing gas refers to combustion gas that is under burning reaction, and the burning-completed gas refers to combustion gas that has completely burning-reacted. The burning-reaction ongoing gas is indeed a concept of substance, but can also be referred to as flame as a concept of state because it generally includes a visible flame so as to be in a flame state. Therefore, herein, the burning-reaction ongoing gas is referred to also as flame or burning flame from time to time. Further, the burning reaction zone refers to a zone where the burning-reaction ongoing gas is present, and the exhaust gas refers to burning-completed gas that has decreased in temperature under an effect of endothermic action by heat transfer tubes or the like.
Also, the combustion gas tempe
Kayahara Toshihiro
Matunari Kenji
Okubo Kyosuke
Tachino Kazuhiro
Takubo Noboru
Miura Co., Ltd.
Wilson Gregory
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