JPH11347368A - Method and apparatus for controlling ammonia injection in denitration apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an absolute injection control apparatus capable of solving a problem such that leaked ammonia increases temporarily at a start time and at a time of a rise in load in a denitration apparatus relatively low in the concn. of NOx in exhaust gas at its inlet and operated in a high NH3 /NOx molar ratio of 1.0 or more in order to obtain a predetermined denitration ratio. SOLUTION: NOx-containing exhaust gas is introduced into a denitration apparatus having a denitration catalyst built therein and the wiping-out of an ammonia injection amt. to the total NOx amt. in the exhaust gas is performed in a molar ratio (NH3 /NOx) of 1.0 or more and, at a time of low load of 30% or less, an ammonia injection amt. is increased or decreased by about 10% centering around a molar ratio (NH3 /NOx) at a short interval.

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and a control apparatus for controlling the injection of ammonia in a denitration apparatus, and more particularly to a method and a control apparatus for controlling the injection of ammonia in a denitration apparatus for removing nitrogen oxides (NOx) in exhaust gas. , N in exhaust gas
The present invention relates to an ammonia injection control method and a control device for a denitration apparatus suitable for reducing the Ox concentration to a target value and reducing leaked ammonia when the load increases.

[0001]

2. Description of the Related Art NOx in exhaust gas emitted from power plants, various factories, automobiles, and the like is a causative substance of photochemical smog. As an effective method of removing NOx, NOx in exhaust gas is denitrated by selective catalytic reduction with ammonia using a catalyst. The law is widely used mainly in thermal power plants. In recent years, the amount of exhaust gas containing NOx has been increasing due to the development of industry, and there is a trend that further reduction of NOx is required in order to comply with environmental standards. In addition, it is desired that CO contained in combustion exhaust gas besides NOx be suppressed to a very low level, and realization of a catalyst and a process for removing them is an important issue.

Although the NOx concentration at the inlet of the denitration reactor is relatively low, the exhaust gas amount is large and the
In a plant where the amount of x is large and the removal treatment is required in order to prevent pollution, when operating at a high molar ratio of 1.0 or more in order to obtain a predetermined denitration rate, the load at startup and when the load changes There is a case where a phenomenon in which the leak ammonia (NH ₃ ) in the downstream region of the denitration device temporarily rises during the rise. Regarding the high-concentration discharge of unreacted NH ₃ , when the sulfuric anhydride SO ₃ is contained in the exhaust gas, the unreacted ammonia reacts with this and reacts with (NH ₄ ) ₂ SO ₄ or (NH ₄ ) HSO
Generate ₄ . These products adhere to heat exchangers such as boiler air heaters (air preheaters) in the downstream and cause an increase in pressure loss due to blockage of exhaust gas passages, which hinders the operation of denitration equipment and boilers. . In addition, there is a problem with high-concentration discharge of exhaust gas not containing SO ₃ from a combustion device using LNG as fuel because NH ₃ itself has an odor. In order to solve these problems, emission of unreacted NH ₃ must be suppressed as much as possible.

FIG. 5 shows a conventional technique relating to control of the amount of ammonia injected into a denitration apparatus. The calculation of the molar ratio (NH ₃ / NOx) is performed based on the measured value by the NOx concentration analyzer 101 at the inlet of the denitration device 1 and the set value given by the NOx concentration setting device 105 at the outlet of the denitration device. The calculation is performed by the calculation unit 106. The difference between the analysis value of the inlet NOx concentration analyzer 101 and the set value of the outlet NOx concentration setter 105 is obtained by a differentiator 105a, and this difference is divided by a divider 105b using the analysis value of the inlet NOx concentration analyzer 101. The denitration rate is obtained by calculation, and this value is input to the function generator 90a to obtain the ammonia injection molar ratio. The difference between the set value of the outlet NOx concentration setting device 105 and the analysis value of the outlet NOx concentration analyzer 104 is obtained by a differentiator 104a, and the molar ratio is corrected by the adder 107 based on the difference to obtain a corrected molar ratio 107a. On the other hand, the analysis value of the inlet NOx concentration analyzer 101 is multiplied by the processing exhaust gas amount by the multiplier 101a.
To obtain the total NOx amount 111, and multiply this by the corrected molar ratio 107a in the multiplier 107b to obtain the ammonia injection amount 112.

At this time, when the load of the boiler, which is the source of the exhaust gas, changes according to the load command 131, it is detected that the load state has changed, and the primary differentiating circuit 108 (only after the load change) which operates only when the load changes. Bias correction of the ammonia injection amount is performed by the adder 107c in advance of the bias correction until the target load is reached. The obtained ammonia injection amount 20 is given as a set value of a control circuit composed of the ammonia flow meter 103 and the ammonia flow control valve 102. Thus, the ammonia injection amount is controlled so that the NOx amount at the outlet of the denitration device is set to a predetermined value. Where 1
03b is a proportional integrator, 103c is an electropneumatic converter.

[0005] When the boiler load rises, the amount of fuel changes in proportion to the load. Therefore, in consideration of safety, a method of introducing an excessive amount of air into the fuel amount as compared with the normal load setting is performed. Due to the excess air due to the air rich, the NOx concentration also temporarily increases. A primary differentiating circuit 1 capable of correcting a certain amount of bias from a load change to a target load in response to a sudden rise in NOx at the denitration device inlet.
08 and an upper / lower limit controller 110 that can arbitrarily determine the fixed amount. Here, 113 is a monitor relay which detects whether the load is changing or not, connects the switch 114 from a to c during the change, and sends an ammonia increase (or decrease) signal to the adder 107c. It is supposed to be sent.

In addition to the prior art shown in FIG. 5, the NH ₃ injection control may be controlled by a constant NH ₃ / NOx molar ratio. In this case, the NH ₃ injection amount is changed to the NH ₃ injection amount (kg / h).
= Dry exhaust gas volume (kg / h) x Inlet NOx concentration (ppm) x
10 ^-6 × molar ratio (-) and controls as, but the boiler load demand signal thereto during boiler load changes and the preceding signal of the NH ₃ injection control, the load change amount per differential value (unit time load demand signal In some cases, the injection amount is controlled by adding an NH ₃ injection amount in proportion to ( ₃ ).

[0007]

SUMMARY OF THE INVENTION Inlet NO of denitration reactor
x In a plant with a relatively low concentration,
NH to obtain_Three/ NOx high molar ratio of 1.0 or more
In denitration equipment operated with
NH proportional to the load request according to the load request signal _ThreeInjection
If the volume has been determined, temporarily increase
NH from denitration equipment_ThreePhenomenon that increases
Can be seen.

[0008] This is because the catalyst adsorbs at low temperature at low load.
NH_ThreeFrom the catalyst due to the temperature rise accompanying the load increase
Desorbed, but because of a high molar ratio of 1.0 or more,
NH _ThreeThe adsorption amount is larger than the NOx concentration,
Considering the increase in NOx_ThreeLarge amount of desorption
This is because they are discharged to the reactor outlet at a stretch.
As described above, the denitration is performed at a high molar ratio of 1.0 or more.
In the reactor, when the load changes,
Problem has occurred.

SUMMARY OF THE INVENTION An object of the present invention is to provide a denitration apparatus capable of solving the above-mentioned problems of the prior art and suppressing a temporary increase in the leak NH ₃ at the time of load increase, particularly at the time of starting, and obtaining a predetermined denitration rate. An object of the present invention is to provide an ammonia injection control method and a control device.

[0010]

The invention claimed in the present application to achieve the above object is as follows. (1) The exhaust gas containing NOx (nitrogen oxide) is introduced into a denitration device having a built-in catalyst for denitration, and the amount of injected ammonia is adjusted in accordance with the total amount of NOx in the exhaust gas.
An ammonia injection control method for a denitration apparatus for selectively reducing and removing Ox, wherein the ammonia injection amount is changed at short intervals during a low load operation.

(2) The NOx-containing exhaust gas is introduced into a denitration device having a built-in denitration catalyst, and the amount of ammonia injected is adjusted in accordance with the total NOx amount in the exhaust gas, so that the NOx in the exhaust gas is reduced.
In the ammonia injection control method of the denitration apparatus for reducing and removing x, the ratio of the ammonia injection amount to the total NOx amount in the exhaust gas is set at a molar ratio (NH ₃ / NOx) of 1.0 or more, and the molar ratio is set at the time of low load operation. About 10
A method for controlling the injection of ammonia in a denitration apparatus, comprising increasing and decreasing the amount of injected ammonia at short intervals.

(3) A denitration device incorporating a denitration catalyst is
Means for introducing exhaust gas having a relatively low Ox content, and means for injecting ammonia into the exhaust gas such that the molar ratio of the amount of injected ammonia to the total NOx amount in the exhaust gas is a high molar ratio of 1.0 or more. Means for detecting whether the operating load of the exhaust gas discharge plant is a low load of 30% or less, and a molar ratio of the ammonia injection amount when the detection load of the detecting means is a low load of 30% or less. An ammonia injection control device for a denitration device, comprising: means for increasing and decreasing at short intervals.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG. In this embodiment, when the load of a plant (for example, a boiler) that generates exhaust gas exceeds 30% of the rated load, the amount of ammonia injected into the denitration apparatus is set to a molar ratio of 1.1, and this molar ratio is set to the total NOx of the treated exhaust gas. The set ammonia flow rate 20 is obtained by multiplying the amount 111 by the multiplier 9, and the set ammonia flow rate and the measurement value of the ammonia flow meter 103 are input to the controller 10. The controller 10 adjusts the ammonia flow rate based on the deviation. The opening of the valve 102 is adjusted. When the load of the plant is 30% or less, a signal having a molar ratio of 1.21 which is 10% higher than the molar ratio signal based on the signal from the signal device 5 is obtained from the signal device 3 and the molar ratio is 1 based on the signal from the signal device 5. .1
The molar ratio signal 0.99, which is reduced by 10%,
Thus, the signals are alternately switched at predetermined intervals (120 seconds in FIG. 1) and input to the multiplier 9.

The operation of the control sequence shown in FIG. 1 will be described in further detail. In FIG. 1, 6, 7, 8
Are AND circuits, and all input signals are "1".
The output signal is output only when the output signal has become. Otherwise, the output signal is "0". Reference numerals 11 and 12 denote time delays. The output becomes "1" after a time "T" since the "1" signal is applied to the input, and the output is kept at 1 as long as the input is "1". In this embodiment, the “T” is set to 120 seconds. 13 is a wipe-out, and the input X is "1"
In this case, the input Z is "0" and the output Y is "1", and the input Z is "1" and the output Y is 0. Numerals 14 and 15 denote knot circuits. When the input X is "1", the output Y = 0, and when the input X is "0", the output Y becomes "1". Reference numeral 16 denotes an OR circuit, and if any of the inputs X, Y, and W is not "0" but "1", the output Z is "1".

When the boiler load exceeds 30%,
Since the output signal of the knot circuit 15 is “1”, the molar ratio signal 1.1 from the signal device 2 passes through the AND circuit 6 and is input to the multiplier 9 via the OR circuit 16.

When the boiler load becomes 30% or less, the signal of the signal device 5 is input to the knot circuit 15, the AND circuit 7, and the time delay wipeout 13, and the knot circuit 15
Becomes 0, and the output of the AND circuit 6 becomes 0. In the AND circuit 7, the molar ratio from the signal 3 is 1.
21 and the input signal to the knot circuit 14 is "0", the output signal thereof is "1".
A signal having a molar ratio of 1.21 is output as an output of the AND circuit 7, and is input to the multiplier 9 via the OR circuit 16 to adjust the amount of ammonia injected into the denitration apparatus. Since the input Z is 0 when the signal from the device 5 is input, a “1” signal is output as the output Y and input to the time delay 11. After a predetermined time (in this embodiment, after 120 seconds), an output signal “1” is output from the time delay 11 and input to the knot circuit 14 and the AND circuit 8. Therefore, the output signal of the knot circuit 14 becomes “0”, and the output of the AND circuit 7 becomes 0.

At the same time, the AND circuit 8 has a molar ratio of 0.99.
Starts to be output and is input to the multiplier 9 via the OR circuit 16 to control the ammonia flow control valve. At this time, the output of the AND circuit 8 is input to the time delay 12,
After a predetermined time (in this embodiment, after 120 seconds), an output signal "1" (= Z) is added from the time delay 12 to the time delay wipeout 13, and the output signal of the 13 becomes zero at this time.

Accordingly, the output signal of the knot circuit 14 becomes "1", and the AND circuit 7 outputs a signal having a molar ratio of 1.21. On the other hand, the output of the AND circuit 8 is "0". Then, the output of the time delay 12 also becomes 0, and the output of the signal “1” starts from the time delay wipeout 13. After a predetermined time (120 seconds in this embodiment), the output signal of the time delay 11 becomes “1”. ". Thus, the output from the AND circuit 7 ends, and the output from the AND circuit 8 starts.

As described above, the ammonia flow control valve alternately repeats the control with the molar ratio of 1.21 and the molar ratio of 0.99 every predetermined time. When the boiler load exceeds 30%, the signal from the signal unit 5 becomes "0",
The output signal of the knot circuit 15 becomes "1", a signal having a molar ratio of 1.1 is output from the AND circuit 6, and the output signals of the AND circuits 7 and 8 become "0".

As described above, in the present invention, excessive NH ₃ adsorption to the catalyst is suppressed by changing the molar ratio at short intervals during low-load operation near the minimum operating load, and the plant is operated at low load. When the temperature of the exhaust gas is low, the temperature of the exhaust gas rises due to an increase in the load of NH ₃ adsorbed on the catalyst, thereby preventing a temporary sudden rise in the amount of discharged NH ₃ due to desorption, reducing the amount of leak NH ₃ at the reactor outlet, and Since the adsorbed ammonia is used for the denitration reaction, a predetermined denitration rate can be obtained, and the performance as a denitration device can be improved.

Hereinafter, in order to confirm the effect of the present invention, FIG.
3 units of denitration catalyst 30 with a unit length of 450 mm and a catalyst length of 0.5 m were installed in a horizontal flow type test apparatus shown in Fig. 3 and NOx at the inlet of denitration reactor at 100% load.
Assuming a load change at the time of startup of a combined cycle plant including a gas turbine, a boiler, and a generator having a relatively low concentration of 30 ppm, a test was performed in an operation at a normal molar ratio of 1.1.

FIG. 3 shows an embodiment of the present invention at a low load.
N with the molar ratio changed from 0.99 to 1.21 at 2 minute intervals
H_ThreeShows the behavior of each value with load change in injection volume control
You. FIG. 4 shows a comparative example in which the molar ratio of the prior art is 1.
NH at 1 constant_ThreeThe behavior in controlling the injection amount is shown. DeNOx rate 8
0% or more (outlet NOx: 6ppm or less), leak NH _Three:
The embodiment of the present invention and the prior art
Compare.

In the conventional control method, as shown in FIG. 4, when the load rises, the leak NH ₃ temporarily increases and exceeds 10 ppm. In addition, the outlet NOx at the time of low load before load increase
Is about 4 ppm on average. However, in the control method of the present invention, as shown in FIG. 3, the outlet NOx at the time of low load before the load rises is reduced to about 3 ppm on average, and the leak NHx is reduced.
The temporary rise of ₃ was suppressed and kept below 10 ppm at all times. Therefore, even when the load changes, the exit NO
x, the leak NH ₃ can be suppressed to the target value or less, and the performance of the denitration device can be improved.

In the embodiment described above, the ammonia injection amount is controlled at a constant molar ratio during normal operation. However, the present invention is not limited to the above embodiment, and is shown in FIG. As in the prior art, based on the denitration rate determined from the NOx concentration at the inlet and outlet of the denitration device,
It is needless to say that the present invention can be applied to a case where the molar ratio of the injected ammonia is calculated and the amount of ammonia injected into the denitration apparatus is controlled based on the calculated molar ratio.

[0025]

According to the present invention, during low load operation, excessive adsorption of ammonia to the catalyst of the denitration device is reduced, and a temporary increase in the amount of leaked ammonia to the downstream of the device when the load is increased is suppressed. The performance of the device can be enhanced. Also,
It is possible to reduce troubles caused by leak ammonia in the heat exchange device provided in the downstream area of the denitration device.

[Brief description of the drawings]

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

FIG. 2 is a view showing a test device for confirming the effect of the present invention.

FIG. 3 is a diagram showing test results according to the present invention using the apparatus of FIG. 2;

FIG. 4 is a diagram showing the results of a comparative test using the apparatus of FIG. 2 according to a conventional technique.

FIG. 5 is a diagram showing an example of a conventional technique.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Denitrification apparatus, 2 ... Signal signal of molar ratio 1.1, 3 ... Signal signal of molar ratio 1.21, 4 ... Signal signal of molar ratio 0.99, 5 ...
Detection signal device with boiler load of 30% or less, 6, 7, 8, ... AND circuit, 9 ... multiplier, 10 ... ammonia flow rate controller,
11, 12: time delay, 13: time delay wipe-out, 14, 15: knot circuit, 20: ammonia flow setting signal, 30: denitration catalyst, 101: ammonia flow control valve, 103: ammonia flow meter.

Claims (3)

[Claims] 1. An exhaust gas containing NOx (nitrogen oxide) is introduced into a denitration device having a built-in catalyst for denitration, and the amount of injected ammonia is adjusted according to the total amount of NOx in the exhaust gas to select NOx in the exhaust gas. An ammonia injection control method for a denitration apparatus, wherein the ammonia injection amount is changed at short intervals during a low-load operation. 2. A denitration apparatus which introduces NOx-containing exhaust gas into a denitration apparatus having a denitration catalyst built therein and adjusts an ammonia injection amount in accordance with a total NOx amount in the exhaust gas to reduce and remove NOx in the exhaust gas. In the ammonia injection control method, the ratio of the ammonia injection amount to the total NOx amount in the exhaust gas is set to a molar ratio (NH ₃ / NOx) of 1.0 or more, and about 10% ammonia injection is performed at a low load operation centering on the molar ratio. An ammonia injection control method for a denitration apparatus, characterized in that the amount is increased or decreased at short intervals. 3. A NOx removal device with a built-in NOx removal catalyst
Means for introducing exhaust gas having a relatively low content concentration, and means for injecting ammonia into the exhaust gas so that the molar ratio of the amount of injected ammonia to the total amount of NOx in the exhaust gas is a high molar ratio of 1.0 or more. A means for detecting whether the operating load of the exhaust gas discharge plant is a low load of 30% or less, and reducing the molar ratio of the ammonia injection amount when the detection load of the detecting means is a low load of 30% or less. An ammonia injection control device for a denitration device, comprising: means for increasing and decreasing at intervals.