JPH11128686A - Apparatus for denitrificating stack gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the consumption of NH3 as a reducing agent and to reduce the concn. of NOx at the outlet of a denitrificating catalyst bed to a regulated value or below by disposing an oxidizing agent injection pipe between the 0 catalyst bed and an ammonia decomposition type catalyst bed in an apparatus for denitrificating stack gas. SOLUTION: A reducing agent is injected from a reducing agent injection pipe 2 into waste gas which flows in an apparatus 1 for denitrificating stack gas and the waste gas is introduced into a denitrificating catalyst bed 3, where NOx is removed. An oxidizing agent is then injected from an oxidizing agent injection pipe 5, a part of unreacted NH3 is oxidized to NOx and the waste gas is introduced into an NH3 decomposition type catalyst bed 4, where the remaining unreacted NH3 and the NOx are brought into denitridificating reaction again and removed. When the NH3 decomposing ability of the NH3 decomposition type catalyst bed 4 deteriorates, the oxidizing agent is additionally injected, a part of the unreacted NH3 is oxidized to NOx and denitrification is carried out. The deterioration of the NH3 decomposing ability is compensated and successive operation of the apparatus 1 is enabled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for denitrifying exhaust gas from a commercial or industrial boiler or the like, and particularly to an unreacted ammonia contained in flue gas at an outlet of the apparatus when highly efficient denitration is required. The present invention relates to a flue gas denitration apparatus for reducing unreacted NH ₃ ).

[0002]

2. Description of the Related Art Nitrogen oxides (NOx) in exhaust gas emitted from power plants, various factories, automobiles, etc. are substances that cause photochemical smog and acid rain. ₃ ) Or flue gas denitration by selective catalytic reduction using ammonia water as a reducing agent is widely used mainly in thermal power plants. The chemical reaction formula is shown below. 4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O Normally, the amount of NH ₃ injected into exhaust gas in a denitration device is represented by a volume ratio (molar ratio) to NOx, and as the denitration ratio (NOx removal ratio) increases, Increase molar ratio.
For example, when the denitration rate is 80%, the molar ratio is about 0.8.

In recent years, due to the tightening of environmental regulations, NO
It is expected that emission regulations for x will be strengthened, and the development of highly efficient flue gas denitration technology is an important issue. Feature of high efficiency denitrification technology, and after injection (high molar ratio infusion / molar ratio ≧ 1) in the exhaust gas to slightly excessive NH _3, and then through the gas to the denitration catalyst layer denitrification, then, in the exhaust gas To perform the decomposition treatment of unreacted NH ₃ contained in the above.

[0004] As shown in FIG. 4, the flue gas from the boiler 6 is introduced into the flue gas denitration device 1, and the flue gas is discharged from the air preheater (A / H) 7 located downstream of the flue gas denitration device 1.
Ammonium salt (ammonium acid sulfate) is generated from the reaction between unreacted NH ₃ and sulfur oxide (SOx) in the exhaust gas, and the ammonium salt adheres to the heat exchanger of the air preheater 7 to cause blockage. Suppression of unreacted NH ₃ is an essential element for a plant system. Therefore, NH ₃
A decomposition type catalyst is installed downstream of the denitration catalyst to decompose and remove unreacted NH ₃ , and clean exhaust gas is collected by an electrostatic precipitator (EP) 8.
Through the chimney 9 to the atmosphere.

That is, as shown in FIG. 5, since the NH ₃ resolution of the NH ₃ decomposition type catalyst is reduced during operation at a low temperature of 300 ° C. or less, such as at the time of starting a boiler, the flue gas denitration apparatus is used as shown in FIG. In addition to the injection of the reducing agent (NH ₃ or NH ₃ water) through the reducing agent injection pipe 2 at the entrance of the denitration catalyst layer 3,
Oxidant injection pipe 5 from the oxidant (O ₃ (ozone)) or hydrogen peroxide (H ₂ O ₂₎ or the like is injected, unreacted NH of NH ₃ decomposing catalyst 4 outlet by the oxidation reaction of the NH ₃ → NO Methods for reducing the number ₃ have been studied (see JP-A-55-97231 and JP-B-58-9692). However, as shown in FIG. 6 (6c) with the injection of the oxidizing agent, the oxidizing reaction by the oxidizing agent increases the NOx concentration at the inlet of the denitration catalyst layer and greatly consumes the reducing agent NH ₃ . NH ₃ near the downstream of the denitration catalyst layer is in an insufficient state,
Instead, the result is that the NOx concentration at the outlet of the denitration catalyst layer greatly exceeds the regulation value.

[0006]

In the conventional flue gas denitration apparatus, the oxidation reaction by the oxidizing agent is performed at N
Increasing the Ox concentration, since it consumes large NH ₃ as a reducing agent, NH ₃ becomes insufficient state near wake of the denitration catalyst layer, rather the NOx concentration in the denitration catalyst layer outlet such greatly exceeding the regulation value There was a problem.

An object of the present invention is to provide a flue gas denitration apparatus capable of reducing the consumption of NH ₃ as a reducing agent and reducing the NOx concentration at the outlet of a denitration catalyst layer to a regulated value or less.

[0008]

In order to achieve the above object, a flue gas denitration apparatus according to the present invention comprises a denitration catalyst layer for reducing and removing nitrogen oxides in exhaust gas, and a decomposition treatment for unreacted ammonia. In a flue gas denitration apparatus provided with an ammonia decomposition type catalyst layer, an oxidant injection pipe is provided between the denitration catalyst layer and the ammonia decomposition type catalyst layer.

The oxidizing agent injection pipe may be configured to inject an oxidizing agent when the ammonia decomposition type catalyst layer has deteriorated.

[0010] In a flue gas denitration apparatus provided with a plurality of denitration catalyst layers for reducing and removing nitrogen oxides in exhaust gas and decomposing unreacted ammonia, oxidization is performed between the respective denitration catalyst layers. An agent injection tube may be provided, and the oxidant injection tube may be configured to inject the oxidant in a low temperature range where the ammonia decomposability of each denitration catalyst layer is reduced.

According to the present invention, the NH ₃ decomposition type catalyst NH ₃
Even in a low temperature range of 300 ° C. or lower where the resolution is deteriorated or the NH ₃ resolution of the denitration catalyst layer is reduced, unreacted NH
_Thus, the NOx concentration at the outlet of the denitration catalyst layer can be prevented from significantly exceeding the regulation value.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG. As shown in FIG. 1, a denitration catalyst layer 3 for reducing and removing nitrogen oxides in exhaust gas discharged from power plants, various improvements and automobiles, etc., and unreacted ammonia in exhaust gas discharged from the denitration catalyst layer 3 Flue gas denitration apparatus 1 provided with an ammonia decomposition type catalyst layer 4 for decomposing water
The oxidizing agent injection pipe 5 is provided between the denitration catalyst layer 3 and the ammonia decomposition type catalyst layer 4. The oxidizing agent injection pipe 5 is for injecting the oxidizing agent during operation at either a high temperature or a low temperature when the ammonia decomposition type catalyst layer 4 is deteriorated due to aging or the like.

That is, for the exhaust gas flowing through the flue gas denitration device 1 located downstream of the boiler, the reducing agent is injected into the exhaust gas from the reducing agent injection pipe 2 and flows to the denitration catalyst layer 3. NOx is removed (denitrified) in the denitration catalyst layer 3. After passing through the denitration catalyst layer 3, an oxidant is injected into the exhaust gas from the oxidant injection pipe 5, and the exhaust gas in which a part of unreacted NH ₃ is oxidized to NOx is passed to the NH ₃ decomposition type catalyst layer 4. You. This N
In the H ₃ decomposition type catalyst layer 4, the remaining unreacted NH ₃ and NOx generated by the oxidizing agent cause a denitration reaction again,
Unreacted NH ₃ and NOx are removed.

In the case of T> 300 ° C. without oxidant injection (1a) shown in FIG. 1, the NH ₃ decomposition type catalyst layer 4 has a high NH ₃ resolution due to the high temperature operation, and even without oxidant injection from the oxidant injection pipe 5. NH ₃ decomposing catalyst layer 4 NOx at the outlet, NH ₃ concentration satisfies the regulation value. However, the T ≦ 300 ° C. oxidant injection without (1b), and NH ₃ Resolution of the NH ₃ decomposing catalyst layer 4 for low temperature is lowered, NH ₃ decomposition without oxidizing agent injected from the oxidant injection pipe 5 NH _{3 at the} outlet of the catalyst layer 4
Only the concentration is above the regulation value. When T ≦ 300 ° C. and the oxidizing agent is injected (1c), although the NH ₃ resolution of the NH ₃ decomposition type catalyst layer 4 is lowered due to the low temperature, NH ₃ is injected because the oxidizing agent is injected from the oxidizing agent injection tube 5. ₃ decomposing catalyst 4 NOx at the outlet, NH ₃ concentration is less than both the regulation value.

If the NH ₃ resolution of the NH ₃ decomposition catalyst 4 is deteriorated, an oxidizing agent is additionally injected even during normal high-temperature operation of 300 ° C. or more, and a part of the unreacted NH ₃
By oxidizing x and denitrifying x, the degradation of the NH ₃ resolution is compensated, and the continuous operation of the flue gas denitration apparatus 1 is enabled. As shown in FIG. 2, when the NH ₃ decomposition type catalyst layer is deteriorated with time, the property of improving the denitration performance with a decrease in NH ₃ resolution is utilized.

Next, another embodiment of the present invention is shown in FIG. This embodiment differs from the embodiment shown in FIG. 1 only in that a denitration catalyst layer 3 is arranged instead of the NH ₃ decomposition catalyst layer. A flue gas denitration apparatus 1 comprising at least two upstream and downstream denitration catalyst layers 3 for reducing and removing nitrogen oxides in exhaust gas and decomposing unreacted ammonia, and comprising: An oxidizing agent injection tube 5 is provided between the denitration catalyst layers 3, and the oxidizing agent injection tube 5 injects the oxidizing agent in a low temperature range of 300 ° C. or lower where the ammonia resolution of each denitration catalyst layer 3 decreases. is there. At T ≦ 300 ° C. without oxidant injection (3b) shown in FIG. 3, the NH ₃ resolution of the denitration catalyst layer 3 is reduced due to the low temperature. At the outlet only the NH ₃ concentration is above the regulation value. In addition, when the oxidizing agent is injected at T ≦ 300 ° C. (3c), although the NH ₃ resolution of the denitration catalyst layer 3 is lowered due to the low temperature, the oxidizing agent is injected from the oxidizing agent injection pipe 5 and thus the denitration catalyst layer 3 At the outlet, both NOx and NH ₃ concentrations are below the regulation value.

This other embodiment is used when operating in a low temperature range of 300 ° C. or lower where the NH ₃ decomposition function of the NH ₃ decomposition type catalyst cannot be used, and is inexpensive instead of the NH ₃ decomposition type catalyst. The present embodiment uses a simple denitration catalyst 3 and has the same effect as the embodiment shown in FIG. Specifically, for example, a temperature sensor is inserted into the exhaust gas inlet, and the oxidizing agent injection valve provided in the oxidizing agent injection pipe 5 is opened by a signal that detects a low temperature of 300 ° C. or less of the exhaust gas temperature by the temperature sensor. Injecting a predetermined amount of oxidizing agent into exhaust gas,
The injection amount is determined in advance according to the properties of the exhaust gas, the oxidizing agent, and the like.

According to the invention, since the suppression of the unreacted NH ₃ it is easily in even device outlet in the low temperature range at a high efficiency denitrification, provided corresponding to the unreacted NH ₃ at a low temperature until now No need for extra NH ₃ decomposition catalyst
The equipment can be made more compact and the restrictions on location can be resolved. In addition, since it is possible to easily suppress unreacted NH ₃ in a low temperature range of 300 ° C. or lower where the NH ₃ resolution of the NH ₃ decomposition catalyst cannot be used, the applicable range of high-efficiency denitration can be expanded.

[0019]

According to the present invention, since an oxidizing agent is injected between a plurality of catalyst layers, it is easy to suppress unreacted ammonia at the outlet of the apparatus even if the ammonia resolution of the catalyst layers is reduced, and high efficiency is achieved. The application range of denitration can be expanded. In addition, the consumption of ammonia is reduced to be economical, and the apparatus is downsized, so that restrictions on location conditions are eliminated.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention and a diagram showing a change in the concentration of NOx and NH _{3 in} exhaust gas.

FIG. 2 is a graph showing a change over time in a denitration rate and an NH ₃ decomposition rate of an NH ₃ decomposition type catalyst.

FIG. 3 is a configuration diagram showing another embodiment of the present invention and a diagram showing a change in concentration of NOx and NH _{3 in} exhaust gas.

FIG. 4 is a diagram illustrating an example of a power generation facility.

FIG. 5 is a diagram showing NH ₃ decomposition rate-temperature characteristics of an NH ₃ decomposition type catalyst.

FIG. 6 is a diagram showing a change in the concentration of NOx and NH _{3 in} exhaust gas according to the related art and FIG.

[Description of Signs] 1 flue gas denitration device 2 reducing agent injection pipe 3 denitration catalyst layer 4 NH ₃ decomposition type catalyst layer 5 oxidant injection pipe

Claims (3)

[Claims] 1. A flue gas denitration apparatus comprising: a denitration catalyst layer for reducing and removing nitrogen oxides in exhaust gas; and an ammonia decomposition type catalyst layer for decomposing unreacted ammonia. An exhaust gas denitration apparatus, wherein an oxidizing agent injection pipe is provided between the exhaust gas denitrification apparatus and the ammonia decomposition type catalyst layer. 2. The flue gas denitration apparatus according to claim 1, wherein the oxidizing agent injection pipe injects the oxidizing agent when the ammonia decomposition type catalyst layer is deteriorated. 3. A flue gas denitration apparatus comprising a plurality of denitration catalyst layers for reducing and removing nitrogen oxides in exhaust gas and decomposing unreacted ammonia, between the respective denitration catalyst layers. A flue gas denitration apparatus, comprising: an oxidizing agent injection pipe, wherein the oxidizing agent injection pipe injects the oxidizing agent in a low temperature region where the ammonia resolution of each denitration catalyst layer is reduced.