Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2001-03-08
2003-10-28
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C431S012000
Reexamination Certificate
active
06638487
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for controlling a NOx removal system in a boiler intended to reduce NOx.
In recent years, there has been a desire for further NOx reduction also in boilers. One of the measures therefor is a method that with a NOx removal system provided in the boiler, ammonia as a reductant is supplied to discharge gas by the NOx removal system, thereby reducing the NOx. In this NOx removal system, ammonia is generated by heating urea water with ammonia generating means having an electric heater, and the amount of ammonia generation is controlled by adjusting the amount of urea water supplied to the ammonia generating means. The amount of ammonia supplied to discharge gas is controlled according to the combustion level of the boiler. For example, in a boiler in which combustion level is controlled in multiple stages of high combustion, low combustion and standby, the ammonia supply in the NOx removal system is also controlled in multiple stages in correspondence to the combustion levels.
In this connection, since piping of a specified length is provided to connect the ammonia generating means and jet nozzles for supplying ammonia to the discharge gas to each other, the ammonia supply cannot be changed over to a specified supply level unless the ammonia remaining in the piping is completely jetted out. This means that even if a changeover signal for combustion level and a changeover signal for ammonia supply are issued simultaneously, there will be a time lag until a changeover to a proper ammonia supply corresponding to the combustion level is actually effectuated. Therefore, in the case of a changeover of combustion level from high to low combustion level, it may occur that ammonia is supplied successively during the time lag so that part of the ammonia flows outside as it is, unfortunately.
In some cases, the NOx removal system is provided with a NOx removal catalyst for accelerating the reduction reaction of NOx by ammonia. In this NOx removal system, first the reductant is adsorbed by the NOx removal catalyst, and then NOx is decomposed into nitrogen and water promptly by a reduction reaction of NOx by the adsorbed reductant. Consequently, even with low-temperature discharge gas, NOx in the discharge gas can be reduced.
In this connection, in the NOx removal system provided with the NOx removal catalyst, there have been some cases where, at an increase of combustion level, for example, at a changeover of combustion level from low to high combustion level, unreacted part of ammonia flows outside as it is. In order to prevent such outflow of ammonia, it could be conceived to provide a large volume of the NOx removal catalyst, but this would incur upsizing of the NOx removal catalyst, inhibiting an efficient use of the NOx removal catalyst.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a method for controlling a NOx removal system in a boiler capable of preventing outflow of unreacted reductant from the boiler due to a delay from a changeover of combustion level of the boiler to a changeover of reductant supply.
A second object of the invention is to provide a method for controlling a NOx removal system in a boiler capable of preventing outflow of unreacted reductant from the boiler upon a changeover of combustion level of the boiler in the case where the NOx removal system is provided with a NOx removal catalyst.
In order to achieve the above second object, we inventors energetically made studies and experiments and so on with respect to the NOx removal system, having acquired the following finding. That is, the finding is that in order to prevent any leak of the reductant in increasing the combustion level, there is a need of adjusting the amount of reductant adsorption according to changes of the upper limit of reductant adsorption amount by the NOx removal catalyst (hereinafter, referred to as “saturation adsorption amount”). In more detail of this finding, since a reduction reaction of NOx in the NOx removal catalyst is caused by the reductant adsorbed by the NOx removal catalyst, a specified amount of reductant needs to have been adsorbed to achieve enough reduction of NOx. However, the reductant can only be adsorbed to the NOx removal catalyst up to the saturation adsorption amount. Also, the saturation adsorption amount decreases more and more with increasing temperature of the NOx removal catalyst, and increases more and more with decreasing temperature of the NOx removal catalyst. Therefore, when the temperature of discharge gas flowing through the NOx removal catalyst is increased due to an increase in combustion level, the NOx removal catalyst is heated by this discharge gas, causing the saturation adsorption amount to decrease. The result would be that the reductant is not adsorbed to the NOx removal catalyst or that adsorbed reductant is released out of the NOx removal catalyst, allowing unreacted reductant to be flowed out of the boiler. Accordingly, in order to prevent any leak of the reductant in increasing the combustion level, the reductant adsorption amount has to be decreased according to a reduction of the saturation adsorption amount. From this point of view, we inventors created a control method for solving the foregoing second object based on this finding.
The present invention having been achieved with a view to solving the foregoing issues, in a first aspect of the invention, there is provided a method for controlling a NOx removal system in which reductant supply is adjusted according to combustion level in a boiler in which the combustion level is controlled in multiple stages, the method comprising: for a decrease of the combustion level, keeping the reductant supply lower than a level corresponding to a changeover-target combustion level for a period of a set time from a changeover of the combustion level.
In a second aspect of the invention, there is provided a method for controlling a NOx removal system in which reductant supply is adjusted according to combustion level in a boiler in which the combustion level is controlled in multiple stages, the method comprising: for a decrease of the combustion level, changing over the reductant supply earlier than a changeover of the combustion level.
In a third aspect of the invention, there is provided a method for controlling a NOx removal system in which reductant supply is adjusted according to combustion level in a boiler in which the combustion level is controlled in multiple stages, where the reductant is reacted in a NOx removal catalyst, the method comprising: for an increase of the combustion level, keeping the reductant supply lower than a level corresponding to a changeover-target combustion level for a period of a set time from a changeover of the combustion level.
Further, in a fourth aspect of the invention, there is provided a method for controlling a NOx removal system in which reductant supply is adjusted according to combustion level in a boiler in which the combustion level is controlled in multiple stages, where the reductant is reacted in a NOx removal catalyst, the method comprising: for an increase of the combustion level, making and keeping the reductant supply at a lower level for a period of a set time lasting until a changeover of the combustion level.
Next, embodiments of the present invention are described. The invention is suitably embodied in a NOx removal system of a boiler. This boiler is, for example, a multi-tubular boiler, and the boiler body has a constitution in which, as an example, a combustion chamber is formed inside annular heat transfer tube arrays while an annular gas passage is formed outside the annular heat transfer tube arrays, where a flue is connected to the gas passage.
The NOx removal system is so constructed that the reductant is jetted out from jet nozzles provided at an outlet of the gas passage into combustion gas to thereby reduce the NOx in the combustion gas. In the case where the reductant is ammonia generated by heating and decomposing urea water, an ammo
Ishizaki Nobuyuki
Isshiki Yukihiro
Masuda Kouichi
Takubo Noboru
Wakasa Satoshi
Birch & Stewart Kolasch & Birch, LLP
Johnson Edward M.
Miura Co., Ltd.
Silverman Stanley S.
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