JPH11221446A - Method and apparatus for denitrating exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus for denitrating exhaust gas having good denitration performance even under the conditions in which severe NOx regulation is required during the actuation of a waste heat recovering boiler, and a method for operating the apparatus. SOLUTION: The time for starting the injection of ammonia during the actuation of a denitration apparatus is set up at a point of time when the temperature of a denitration catalyst layer part reaches the boiling point or the vicinity of the boiling point of the condensed water of exhaust gas. Since the reduction of NOx can be attempted even in a low temperature area without lowering the activity of the denitration catalyst, the discharge of NOx can be controlled during the operation of a plant. Although an exhaust gas temperature at the outlet of the catalyst layer increases behind that at the inlet of the catalyst layer, when the injection of ammonia is started at 140 deg.C of an exhaust gas temperature at the inlet, an exhaust gas temperature at the outlet corresponds to 100 deg.C (the boiling point of the condensed water of exhaust gas), and since the temperature of the entire catalyst layer is at the lowest 100 deg.C when the injection of ammonia is started at this point of time, the discharge of ammonia or NOx into the air can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas denitration apparatus, and more particularly to a waste heat recovery boiler for generating steam using waste heat from a power generation gas turbine using natural gas (LNG) as fuel, and the boiler. Related to exhaust gas denitration equipment in a waste heat recovery boiler used in a combined cycle plant that drives a steam turbine using steam from steam. The present invention relates to an exhaust gas denitration apparatus and a method suitable for reducing as much as possible.

[0002]

2. Description of the Related Art As a method for reducing nitrogen oxides (hereinafter abbreviated as NOx) in exhaust gas from gas turbines and boilers, etc., exhaust gas and ammonia gas are contact-reacted in the presence of a catalyst to convert NOx into nitrogen and water. The so-called selective catalytic cracking method (SCR method) that decomposes is widely used in Japan and overseas.

FIG. 4 shows an example of a conventional ammonia injection control method for a denitration apparatus for a waste heat recovery boiler. Ammonia is injected into an exhaust gas 1 discharged from a gas turbine (not shown) introduced into the waste heat recovery boiler A by an ammonia injection nozzle 4 before passing through a denitration catalyst layer 2.

A method of controlling the amount of injected ammonia is as follows. First, the amount of exhaust gas detected by the exhaust gas amount detector 5 obtained from the amount of air used for combustion of fuel in a gas turbine or the like is determined by the amount of NO
The total NOx amount is obtained by multiplying the detection value by the x amount detector 6,
Further, the required ammonia amount is calculated by multiplying by a preset molar ratio obtained by a preset molar ratio setting device 7,
This value is compared with the value detected by the actually measured ammonia amount detector 8, and the ammonia flow control valve 10 provided in the ammonia supply pipe 3 is controlled to inject an appropriate amount of ammonia so as to match the calculated amount of the required ammonia amount. The method is taken.

[0005] The temperature of the exhaust gas at the inlet is detected by the thermometer 9 on the upstream side of the denitration catalyst layer 2, and a signal for opening and closing the ammonia flow control valve 10 together with the temperature set by the ammonia injection start temperature setting device 12. Used as That is, when the boiler plant is started, when the inlet exhaust gas temperature reaches the ammonia injection start set temperature, the ammonia flow control valve 10 is opened to start ammonia injection.

Conventionally, ammonia injection start temperature setting device 12
Is set to about 200 ° C. in the case of a waste heat recovery boiler utilizing waste heat of a gas turbine using LNG as fuel. The reason is that the temperature (about 90 degrees) at which NO ₂ reacts with ammonia to produce ammonium nitrate (ammonium nitrate)
And at a temperature at which the catalytic activity is exhibited (about 20
0 ° C.) or higher.

[0007]

According to the above-mentioned prior art, in a plant using a waste heat recovery boiler installed in a remote place or an industrial area, the environmental regulations at the start-up time are relatively loose, which has been a problem. There was nothing like that.

However, in recent years, combined cycle plants for peak power, which have been increasing in demand in recent years, are often installed in suburban areas.
Due to the frequent repetition of the start-stop of the plant, strict regulation of NOx concentration is being imposed even at the time of start-up of the plant which has conventionally been excluded from the regulation.

In recent years, there has been a tendency to use a moving average of the amount of NOx emission for one hour as a regulation value of the concentration of NOx emission. According to this method, the amount of NOx emission before the denitration device 2 is started is also counted as a target of regulation, and therefore, there are plants that cannot achieve the regulation value by the conventional operation method. For this reason, it has been required that the denitration apparatus start functioning as soon as possible after the start of the plant using the waste heat recovery boiler.

[0010] It is an object of the present invention to provide an exhaust gas denitration apparatus having good denitration performance even under conditions where strict NOx regulations are imposed when the waste heat recovery boiler is started, and an operation method thereof.

[0011]

The above object of the present invention can be solved by the following constitution. That is, in the exhaust gas denitration method for removing nitrogen oxides in the exhaust gas by injecting ammonia into the exhaust gas and bringing the ammonia into contact with the denitration catalyst layer, the start timing of the ammonia injection after the start of the denitration apparatus is determined by the temperature of the denitration catalyst layer. This is an exhaust gas denitration method that is set at the time when the exhaust gas condensed water reaches or near the boiling point.

In the exhaust gas denitration method of the present invention, it is possible to specify the timing of starting injection of ammonia into the exhaust gas based on the temperature of the outlet of the denitration catalyst layer. Further, when there is a predetermined relationship between the denitration catalyst layer inlet temperature and the outlet temperature, the ammonia exhaust gas based on the denitration catalyst layer inlet temperature obtained not from the denitration catalyst layer outlet temperature but from the denitration catalyst layer outlet temperature. It is also possible to specify the start time of injection into the fuel cell.

In the present invention, the denitration catalyst layer provided in the exhaust gas channel, the ammonia injection section provided in the exhaust gas channel on the upstream side of the denitration catalyst layer, and the denitration catalyst layer are provided at the outlet of the denitration device. The exhaust gas thermometer and the ammonia injection start timing, and the ammonia injection start temperature at which the injection of ammonia from the ammonia injection unit is started when the temperature of the denitration catalyst layer by the exhaust gas thermometer reaches or near the boiling point of the exhaust gas condensed water. An exhaust gas denitration device equipped with a setting device is also included.

In the control of the injection amount of ammonia according to the present invention, for example, an exhaust gas meter, an exhaust gas inlet NOx concentration meter, and an ammonia concentration meter are provided in an exhaust gas flow path upstream of the denitration catalyst layer, and the exhaust gas inlet NOx concentration is controlled. A required ammonia amount molar ratio setting device is provided, the NOx amount in the exhaust gas obtained based on the measurement values of the exhaust gas meter and the exhaust gas NOx concentration meter, and the required ammonia obtained from the NOx amount in the exhaust gas and the molar ratio setting device. A control device for obtaining the amount of ammonia to be injected into the exhaust gas based on the difference between the amount and the actually measured amount of ammonia injected by the ammonia concentration meter can be provided in the ammonia injection section.

[0015]

According to the present invention, NOx can be reduced even in a low temperature range without lowering the activity of the denitration catalyst, so that the amount of NOx emission at the start of the plant can be suppressed. That is, for example, 100 ° C. is set as the set temperature of the ammonia injection start temperature as a temperature at or near the boiling point of the exhaust gas condensed water. At 100 ° C., the temperature is higher than the nitrate generation temperature of about 90 ° C., so there is no need to take measures for the production of nitrate. This is because if the temperature detection position of the catalyst is set at the exit portion of the denitration catalyst layer, the exit portion of the denitration catalyst layer has a slow temperature rise rate.

[0016] As in the prior art, after a plant such as a combined cycle plant is started, a denitration catalyst layer is formed.
For example, in order to reach the ammonia injection temperature at 200 ° C., a normal cold start (starting conditions such as starting after a weekend stop) requires 30 minutes or more. During this time, NOx in the exhaust gas discharged from the gas turbine as the exhaust gas source is discharged from the chimney without being subjected to the denitration process. However, in the present invention, about 10 minutes compared to the prior art,
The ammonia injection start time can be advanced, and the NOx concentration discharged from the chimney can be kept within the regulation value.

[0017]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an example of an ammonia injection control device of a denitration device for a waste heat recovery boiler A according to an embodiment of the present invention. Ammonia is injected from an ammonia injection nozzle 4 into an exhaust gas 1 discharged from a gas turbine (not shown) introduced into the waste heat recovery boiler A via an ammonia supply pipe 3 before passing through a denitration catalyst layer 2. Is done.

The method of controlling the ammonia injection amount is as follows. First, the detection value of both the exhaust gas amount detector 5 and the inlet NOx amount detector 6 obtained from the amount of air used for fuel combustion of a gas turbine or the like is multiplied. The required ammonia amount is calculated by multiplying the total NOx amount by a preset molar ratio determined by a preset molar ratio setting device 7, and this value is compared with the actual ammonia amount measured by the actual ammonia amount detector 8. A method for injecting an appropriate amount of ammonia from the ammonia injection nozzle 4 into the exhaust gas by controlling the ammonia flow control valve 10 provided in the ammonia supply pipe 3 so as to match the calculated amount of the required amount of ammonia with the amount of ammonia. It is. To control the opening and closing of the ammonia flow control valve 10, the temperature detected by the denitration catalyst layer outlet thermometer 13 is controlled by the ammonia injection start temperature setting unit 12.
It is opened when the set temperature is reached.

Here, the difference from the prior art shown in FIG. 4 is that the denitration catalyst layer outlet thermometer for detecting the denitration catalyst layer outlet temperature instead of the inlet exhaust gas temperature by the inlet exhaust gas thermometer 9 shown in FIG. 13 and that the set temperature of the ammonia injection start temperature setting device 12 is set to, for example, 100 ° C. as the temperature at or near the boiling point of the exhaust gas condensed water. At 100 ° C., the temperature is higher than about 90 ° C., which is the temperature at which ammonium nitrate is generated.

As the catalyst temperature detection position, the denitration catalyst layer 2
The reason why the outlet portion is selected is that the temperature rising speed at the outlet portion of the denitration catalyst layer 2 is slow. Further, even in the outlet portion of the denitration catalyst layer 2, in the waste heat recovery boiler A of the type in which the exhaust gas flows in the horizontal direction, the temperature rise of the denitration catalyst layer 2 portion below the duct is slower than that of the upper portion of the duct, and the temperature is lower. Since the temperature is relatively low, it is desirable to measure the temperature of the denitration catalyst layer 2 below the duct.

FIG. 2 is a diagram comparing the effect of operating the ammonia injection control device of the denitration device for the waste heat recovery boiler A shown in FIG. 1 with that of the prior art. The horizontal axis indicates the operating time from the start of the gas turbine, and the vertical axis indicates the exhaust gas temperature, the NOx amount at the inlet of the denitration catalyst layer, the NOx amount at the outlet of the denitration catalyst layer, and the average NOx amount per hour at the outlet of the denitration catalyst layer from the bottom. I have. A in FIG. 2 shows the case where the present invention shown in FIG. 1 is implemented, and B shows the case where the conventional technique shown in FIG. 4 is implemented.

In the control device of the present invention shown in FIG. 1, when the exhaust gas temperature at the inlet of the denitration catalyst layer reaches 140 ° C., the injection of ammonia is started, as shown in the figure of the exhaust gas temperature at the bottom. The temperature of the exhaust gas at the outlet of the denitration catalyst layer rises later than the temperature of the exhaust gas at the entrance of the denitration catalyst layer.
00 ° C., that is, when the boiling point of the condensed water of the exhaust gas is reached. At this point, the temperature of the entire catalyst layer has reached 100 ° C. or higher.

On the other hand, FIG. 2 also shows, for comparison, the conventional case of starting injection of ammonia at 200 ° C. From the start of the combined cycle plant to the time when the inlet gas temperature of the denitration catalyst layer 2 reaches 200 ° C., a normal cold start (starting conditions such as starting after a weekend stop) requires 30 minutes or more. . During this time, NOx in the exhaust gas discharged from the gas turbine as the exhaust gas source is discharged from the chimney without performing the denitration process.

However, in the present invention, the ammonia injection start time can be advanced by about 10 minutes as compared with the prior art. Due to the difference of 10 minutes, NO in the exhaust gas
x concentration regulation can be achieved.

Referring to the graph of the average NOx amount per hour at the outlet of the denitration catalyst layer at the top of FIG. 2, when the conventional ammonia injection start temperature is 200 ° C., the regulation value has not been achieved. When the injection of ammonia is started, the regulation value has been achieved. The reason is that a large amount of NOx is already contained in the exhaust gas before the start of the injection of ammonia into the exhaust gas, and by starting the injection of ammonia at 140 ° C., the emission amount of NOx can be reduced efficiently. is there.

This is shown in the graph of the NOx amount at the outlet of the denitration catalyst layer (outlet of the denitration device 2) at the second stage from the upper stage in FIG. 2. A comparison between the case of injection (A of the present invention) and the case of 200 ° C. injection (B of the prior art) shows that the former case can reduce NOx corresponding to the hatched portion in the figure.

From this, it was found that the lower the temperature of the ammonia injection, the more likely the effect would be. However, in the case of the denitration reaction of exhaust gas in an actual machine, it was found that there was a lower limit relating to the essence of the reaction.

FIG. 3 is a graph showing the denitration rate of the catalyst with respect to the exhaust gas temperature. The present invention uses low-temperature catalysts (Ti-V-based catalysts (those having a large V content), Ti-Pt-based catalysts, etc.) that have been increasingly used in recent years. Are shown side by side. Medium temperature catalyst (Ti / W
System, Ti-Mo system, and Ti-V system (those having a small V content) have extremely low activity at around 200 ° C.

That is, even if ammonia is injected at a temperature of 200 ° C. or less, the denitration reaction does not occur. Therefore, the conventional ammonia injection start temperature is set at around 200 ° C. On the other hand, when attention is paid to the low-temperature catalyst, in the low-temperature range of 100 to 200 ° C., the activity is exhibited although the denitration rate decreases.
However, when the temperature is lower than 100 ° C., almost no activity occurs. This is because the condensed water of the exhaust gas adheres on the catalyst surface,
It is presumed that the injected gaseous ammonia is absorbed by the condensed water and does not contribute to the denitration reaction.

The same phenomenon occurs in an actual machine. That is, LNG is generally used as a fuel for a gas turbine of a combined cycle plant, and the water concentration in the exhaust gas becomes a little less than 10% when it is higher than several%. When the exhaust gas containing water is discharged from the gas turbine and passes through the waste heat recovery boiler A, at the time of starting the plant, a water pipe group or a denitration catalyst layer 2 constituting a boiler heat transfer tube (not shown) cooled to about room temperature. As a result, heat is absorbed, and when the temperature of the exhaust gas becomes equal to or lower than the dew point of the exhaust gas (usually 60 to 70 ° C.), water condensation occurs.

The condensed water stays until the boiling point or higher, and covers the surface of the denitration catalyst. Therefore, even if ammonia is injected at a temperature lower than this temperature, the denitration reaction which is a gas-gas reaction does not occur. . Therefore, the boiling point of the condensed water becomes the minimum temperature of the ammonia injection start temperature, no matter how high the low-temperature active catalyst is used.

Next, the effect on the catalytic activity by lowering the ammonia injection start temperature to 140 ° C. at the exhaust gas temperature at the inlet of the denitration device 2 (corresponding to the boiling point of exhaust gas condensed water at the outlet temperature of the denitration catalyst) will be considered.

When the set temperature for starting injection of ammonia decreases, there is a concern about the production of ammonium nitrate generated by NO ₂ and ammonia. This is because, when ammonium nitrate is deposited on the catalyst, pores having catalytic activity are blocked, and catalytic activity decreases. The nitrate generation temperature calculated from the amount of NO _{2 in} the exhaust gas discharged from the gas turbine and the concentration of ammonia was about 90 ° C., and it was found that no problem occurred even when the temperature at the catalyst outlet side was about 100 ° C. Was.

The control system for injection of ammonia of the present invention shown in FIG. 1 is based on the premise that it can cope with any plant startup pattern. However, in a plant in which the operation mode at the time of starting the plant is always the same, and the temperature rise of the denitration catalyst layer 2 is recognized to be always constant, FIG.
The present invention can be implemented using the ammonia injection control device shown in FIG.

In this case, the temperature difference between the outlet temperature of the denitration catalyst layer and the exhaust gas temperature at the inlet is measured in advance, and a value added by that amount is set as the ammonia injection start set value 12 in FIG. For example, if it is known that the temperature of the exhaust gas at the inlet is 140 ° C. when the outlet temperature of the denitration catalyst layer is 100 ° C., the value of the ammonia injection start temperature set value 12 shown in FIG. In this case, the same effect as that of the present invention can be expected with an existing apparatus of a conventional plant as it is.

[0036]

According to the present invention, an operation in which the NOx emission is reduced as much as possible when starting up a combined cycle plant or the like can be realized. For this reason, severe NOx
It is a powerful means by which regulation can be achieved. In addition, since the existing plant can cope with the operation of the denitration device, it becomes possible to cope with strict regulations at the time of starting without modifying equipment such as a gas turbine.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing an effect of the embodiment of the present invention.

FIG. 3 is a diagram comparing temperature characteristics of a catalyst according to an embodiment of the present invention with a conventional catalyst.

FIG. 4 is a diagram showing a conventional technique.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Exhaust gas 2 Denitration catalyst layer 3 Ammonia supply pipe 4 Ammonia injection nozzle 5 Exhaust gas amount detector 6 Inlet NOx amount detector 7 Molar ratio setting device 8 Actual ammonia amount detector 10 Ammonia flow control valve 12 Ammonia injection start temperature setting device 13 Denitration Catalyst layer outlet thermometer A Waste heat recovery boiler

Claims (5)

[Claims] 1. An exhaust gas denitration method for removing nitrogen oxides from exhaust gas by injecting ammonia into the exhaust gas and bringing the ammonia into contact with the denitration catalyst layer, comprising the steps of: An exhaust gas denitration method characterized in that the temperature is set at the time when the temperature reaches or near the boiling point of exhaust gas condensed water. 2. The exhaust gas denitration method according to claim 1, wherein the timing of starting the injection of ammonia into the exhaust gas is specified based on the outlet temperature of the denitration catalyst layer. 3. When there is a predetermined relation between the inlet temperature and the outlet temperature of the denitration catalyst layer, the timing of starting the injection of ammonia into the exhaust gas based on the inlet temperature of the denitration catalyst layer obtained from the outlet temperature of the denitration catalyst layer. The exhaust gas denitration method according to claim 2, wherein the method is specified. 4. A denitration catalyst layer provided in an exhaust gas channel, an ammonia injection section provided in an exhaust gas channel on the upstream side of the denitration catalyst layer, and an exhaust gas thermometer provided at an outlet of the denitration catalyst layer denitration apparatus. An ammonia injection start temperature setting device for starting injection of ammonia from an ammonia injection section when the temperature of the denitration catalyst layer section measured by the exhaust gas thermometer reaches or near the boiling point of exhaust gas condensed water. Exhaust gas denitration equipment. 5. An exhaust gas meter, an exhaust gas inlet NOx concentration meter, and an ammonia concentration meter are provided in an exhaust gas passage on the upstream side of a denitration catalyst layer, and a molar ratio setting device for a required amount of ammonia with respect to the exhaust gas inlet NOx concentration is provided. NO in exhaust gas obtained based on measurement values of exhaust gas meter and NOx concentration meter at exhaust gas inlet
A control device for determining the amount of ammonia to be injected into the exhaust gas based on the difference between the required amount of ammonia obtained from the x amount and the NOx amount in the exhaust gas and the molar ratio setter and the actually measured ammonia injection amount by the ammonia concentration meter is disposed at the ammonium injection unit. The exhaust gas denitration apparatus according to claim 4, wherein: