JP2000262862A - Denitration control apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent over-denitration and to avoid the deviation of outlet NOx concn. from a lower limit by controlling denitration on the basis of adsorbed NH3 quantity necessary after the elapse of (n) hr from the present point of time. SOLUTION: In a denitration control apparatus injecting NH3 into exhaust gas to react the same with NOx in the exhaust gas on a catalyst and controlling a flow rate of NH3 so that the outlet NOx concn. of the catalyst enters a predetermined range, a necessary NH3 flow rate correcting device 300 calculating the necessary NH3 flow rate at the present point of time on the basis of the inlet NOx flow rate of the catalyst, an outlet NOx concn. set value and the deviation between both measured values and correcting a necessary NH3 flow rate 111 so that a necessary denitration rate is obtained after the elapse of (n) hr from the present point of time is provided. The necessary NH3 flow rate correcting device 300 adds the reacted NOx flow rate at the present point of time and the reacted NOx flow rate after the elapse of (n) hr and substrates the estimate value of adsorbed NH3 quantity to the catalyst at the present point of time from the necessary adsorbed NH3 quantity to the catalyst after the elapse of (n) hr and calculates a correction value 310 correcting a necessary NH3 flow rate on the basis of the added value and the substracted value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a denitration control apparatus suitable for optimally controlling a system having a large delay in a chemical reaction, such as a denitration apparatus for a coal-fired boiler or the like.

[0002]

2. Description of the Related Art The technical background of the present invention will be described.
The denitration device referred to here is a device that injects NH ₃ as a reducing agent into exhaust gas, and reacts the injected NH ₃ with NOx in the exhaust gas on an installed catalyst to form nitrogen and water.

[0003] NH ₃ + NO + ／ · O ₂ → N ₂ + 3/2 · H ₂ O NOx in exhaust gas is converted into nitrogen and water by the reaction shown in the above formula with NH ₃ to be injected. NH _{3 to be} injected
As for the amount, an amount corresponding to the amount of NOx to be processed is injected. The injected NH ₃ is not only used for the reaction with NOx but also adsorbs in an amount that can be adsorbed on the catalyst surface. Therefore, if it is injected beyond this, it flows out of the apparatus as leak NH ₃ . On the other hand, if the amount of NH ₃ to be injected is insufficient for the amount of NOx to be processed, the NOx cannot be processed completely, and the outlet NOx concentration cannot be reduced to a predetermined value or less.

FIG. 5 shows a system diagram of a conventional control apparatus for a denitration control system having a denitration control apparatus according to the present invention. The outlet NOx concentration 107 is compared with an outlet NOx concentration set value 105 by a subtractor 202a, and a deviation, that is, an outlet NOx concentration deviation 108 is output. The outlet NOx concentration deviation 108 is input to a PI (proportional / integral) controller 203a, and its output is a molar ratio correction amount 109.

On the other hand, the outlet NOx concentration set value 105 is input to a function generator 201b to calculate a required molar ratio 106, and the calculated molar ratio 106 and the previous molar ratio correction amount 109 are added to an adder 205c.
And the corrected molar ratio 110 is calculated. The corrected molar ratio 110 is multiplied by an inlet NOx flow rate 104 and a multiplier 204b to obtain a required NH ₃ flow rate 111.

[0006] NH ₃ flow rate of 112 this necessary flow rate _of NH ₃ 11
1 is subtracted from the subtractor 202b to calculate a deviation, that is, an NH ₃ flow deviation 113, which is input to the PI controller 203c, whereby an NH ₃ flow adjustment amount 114 is obtained. Here, the inlet NOx flow rate 104 is calculated as a result of multiplying the inlet NOx concentration 101 and the flue gas flow rate 103 by the multiplier 204a, and the flue gas flow rate 103 is an output of the air flow rate 102 by the function generator 201a. .

FIG. 4 shows the result of controlling the concentration of outlet NOx according to the prior art. Outlet NOx concentration 501, so not in NH ₃ injection rate 503 commensurate with the adsorbed NH ₃ amount, there are cases where significant departure from the set range, as shown in a point. Also, as shown at point b, the leak NH ₃ 502
Due to the excessive injection of the NH ₃ injection amount 503, the temperature temporarily becomes considerably high. Further, the NH ₃ injection amount 503 itself is temporarily injected in a considerably large amount as shown at point c, resulting in wasteful and uneconomical operation.

In FIG. 4, time 500 (unit (se)
c)), the outlet NOx concentration 501, the set lower limit 504 and the set upper limit 505, the leak NH ₃ amount 502 and its upper limit 506 (the 501, 502, 504, and 505 are in units (ppm)), and the NH ₃ injection amount 503 (Unit (m ³ /
h)) In each case, the possible minimum value to maximum value are 0 to 10
This is displayed by making it non-dimensional to 0%.

[0009]

SUMMARY OF THE INVENTION Conventional PI
In accordance with the feedback control system (proportional-integral) controller 203a, but for the purpose of asymptotic to the set value of the outlet NOx concentration, NH ₃ injection can not according to some adsorbed NH ₃ amount on the catalyst, this Therefore, (1) excessive removal of outlet NOx at the time of load reduction such that the amount of adsorption remains, deviating downward from the set range, (2) overshoot of leaked NH ₃ due to excessive NH ₃ injection, (3) ) Problems such as the occurrence of uneconomics due to excessive use of the total amount of NH ₃ occur.

[0010]

In order to solve the above problems, the present invention mainly employs the following configuration.

In a denitration control device for injecting NH ₃ on the catalyst to react with NOx in the exhaust gas and controlling the flow rate of NH ₃ so that the NOx concentration at the outlet of the catalyst falls within a predetermined range, the NOx at the inlet of the catalyst is controlled. The required NH ₃ flow rate at the present time is obtained based on the flow rate and the deviation between the set value of the outlet NOx concentration and the measured value, and the required NH ₃ flow rate is obtained so that the required denitration rate can be obtained after n hours from the present time. ₃ Denitration control device equipped with NH ₃ flow rate corrector that needs to correct flow rate.

In the denitration control apparatus, the required NH ₃ flow rate corrector is configured to multiply the inlet NOx flow rate by the denitrification rate, and to calculate the reaction NOx flow rate at the present time and the inlet NOx flow rate for n hours. The reaction NOx flow rate at the time point of n hours obtained by multiplying the required denitration rate at the time point is added to the amount of NH ₃ adsorbed on the catalyst at the time point of n hours, and The estimated value of the amount of adsorbed NH ₃ is subtracted, and the sum of the reaction NOx flow rate and the amount obtained by dividing the subtracted amount of adsorbed NH ₃ by half of the time interval n is obtained. The sum and the current NH
A denitration control device that compares the _three flow rates and calculates a correction value for correcting the required NH ₃ flow rate.

In the denitration control device, the inputs to the required NH ₃ flow rate corrector are an inlet NOx flow rate and an inlet Nx flow rate.
A denitration control device comprising: an Ox concentration, an outlet NOx concentration, a required denitration rate at a time point n hours after the present time, a predicted span n for determining a time point to be predicted, and a measured NH ₃ flow rate.

In the denitration control device, the required NH ₃ flow rate corrector is a calculation device for estimating the current amount of adsorbed NH ₃ , a calculation device for calculating the current denitration rate, and a required denitration after n hours. A denitration control device comprising: a function generator for calculating a predicted value of a required amount of adsorbed NH ₃ after n hours from the rate.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A denitration control device according to an embodiment of the present invention will be described below with reference to the drawings. In FIG.
1 shows a denitration control device according to the present embodiment. The difference between the prior art, in the calculation of the required flow rate _of NH ₃ 111, must flow demand i.e. the current measurement-based according to the prior art NH ₃
The amount obtained by adding the required NH ₃ flow rate correction value 310 output from the required NH ₃ flow rate corrector 300 to the flow rate 115 by the adder 205d is set as the required NH ₃ flow rate 111.

Here, 101 is the concentration of NOx at the inlet,
102 is an air flow rate, 103 is a combustion exhaust gas flow rate, 104 is an inlet NOx flow rate, 105 is an outlet NOx concentration set value, 10
6 is the required molar ratio, 107 is the outlet NOx concentration, 108 is the outlet NOx concentration deviation, 109 is the molar ratio correction amount, 110 is the corrected molar ratio, 111 is the required NH ₃ flow rate, 112 is the NH ₃ flow rate, and 113 is the NH ₃ flow rate Deviation, 114 is the NH ₃ flow rate adjustment amount, 115 is the required NH ₃ flow rate based on the current measurement base, 20
1 is a function generator, 202 is a subtractor, 203 is a regulator, 2
04 is a multiplier, 205 is an adder, 300 is a necessary NH ₃ flow rate corrector, 301 is a required denitration rate after n hours, 302
Represents a predicted span n, and 310 represents a required NH ₃ flow rate correction value.

The input to the required NH ₃ flow compensator 300 is
Inlet NOx flow rate 104, inlet NOx concentration 101, outlet NO
x concentration 107, required denitration rate 301 after n time is expected span n302 and NH ₃ flow rate 112, necessary NH ₃
The output from the flow compensator 300 is the required NH ₃ flow compensation value 3
It is 10.

Here, the basic concept of the denitration control device according to the embodiment of the present invention will be described.

First, in order to compensate for the delay in the reaction time, the amount of injected NH ₃ at the present time is set so that the required denitration rate is obtained n minutes after the present time. Its denitrification rate due to the use that the amount determined in relation to the adsorption amount of NH _3, based on the estimated value of the adsorption amount of NH ₃ at the present time, NH ₃ set amount, adsorption NH from the current time after n minutes Decide so that the required amount of ₃ remains.

At present, it is assumed that the amount of adsorbed NH ₃ is present on the catalyst surface in an amount of _{CNH 3} per unit area. In this state, N
When H ₃ is injected by G _NH3 ⁱⁿ per unit time, the injected NH ₃ once adsorbs on the catalyst surface and reacts with NOx there. Extra NH ₃ that did not react with NOx
Leaks to the downstream (this is referred to as G _NH3 ^leak ) or adsorbs on the catalyst surface.

Therefore, considering the process between the present time and the passage of n hours, the total amount of injected NH ₃ G _NH3 ⁱⁿ · n is NO
Total amount R of reaction with x, total amount of leak NH ₃ G _NH3 ^leak · n
And the increase of the adsorbed NH ₃ . That is, assuming that the amount of adsorbed NH ₃ after elapse of n hours is C _NH3 ⁿ , the following equation holds.

The total amount of injected NH ₃ G _NH3 ⁱⁿ · n = NO
Total amount of reaction with x R + Total amount of leak NH ₃ G _NH3 ^leak · n
+ Increase in adsorbed NH ₃ (adsorbed amount C _NH3 ⁿ −
At this time, the adsorption amount C _{N H3} ) Here, since the total amount R of the reaction in the first term on the right side is the amount of the NOx that has flowed into the denitration device, it does not deviate from the product of the inlet NOx amount and the denitration rate η.

On the other hand, since the injected NH ₃ is set at the ratio of the molar ratio SM to the inlet NOx, the difference between the molar ratio SM corresponding to the inlet NOx and the denitration ratio η does not fall into the leak NH ₃ of the second term. . That is, the total amount of injected NH ₃ G _NH3 ⁱⁿ · n = product of inlet NOx and denitration rate η + product of inlet NOx and (molar ratio SM−denitration rate η) + increase of adsorbed NH ₃ (after n hours) Adsorption amount of C _NH3 ⁿ −
Because it is adsorbed amount C _{N H3)} at the present time, eventually, injected increase in total G _NH3 ⁱⁿ · n = the product of inlet NOx molar ratio SM + adsorption of NH ₃ NH ₃ (n time after adsorption the amount C _NH3 ⁿ - is adsorbed amount C _{n H3)} at the present time.

Here, with respect to the left side, N
The H ₃ flow rate is G _NH3 ^{in, n} , and the inlet NO after n minutes with respect to the right side
Let the expected value of x be NOx ⁿ and the molar ratio SM be SM ⁿ . At this time, the first term on the left side and the right side is replaced with the average value of the current value and the value after the elapse of n hours to obtain the following.

(G _NH3 ⁱⁿ + G _NH3 ^{in, n} ) · n / 2 = n / 2
^{· (NOx · SM + NOx n} · SM n) + adsorption amount after increase (n times of the adsorption NH ₃ C _NH3 ⁿ - currently adsorption C
_NH3) to now known, by setting the SM ⁿ molar ratio required to obtain the required NOx removal efficiency eta ⁿ of after n time, injected NH ₃ amount after n time determined by the above equation G _NH3 ^{in , n} , the required denitration rate η ⁿ can be obtained.

[0026] In order to actually calculate, it is necessary adsorption amount C _NH3 adsorption amount C _NH3 ⁿ and the current after n hours. For this purpose, the following experimentally known characteristics of the catalyst are applied. That is, a functional relationship between the molar ratio SM = func1 (adsorption amount C _NH3 , inlet NOx, operating conditions) and the denitration ratio η = func2 (molar ratio SM, operating conditions) is used. Here, func1 and func
c2 is a symbol representing the function of the displayed argument. As a result, the current molar ratio SM and therefore the adsorption amount C _NH3 can be known from the current denitration rate η, and if the required denitration rate η ⁿ is set after n hours, the necessary The molar ratio SM ⁿ and the required adsorption amount C _NH3 ⁿ are estimated.

From the above relationship, the NH ₃ flow rate for obtaining the required denitration rate η ⁿ after n hours has elapsed can be calculated. Since the above equation is divided by n / 2, G _NH3 ^{in, n} = − G _NH3 ⁱⁿ + ｛(NOx ・ SM + NOx ⁿ・ S
M ⁿ ) + increase of adsorbed NH ₃ (adsorbed amount C after elapse of n hours)
_NH3 ⁿ - adsorption C _NH3 at present) is / (n / 2)}.

FIG. 2 shows the required NH ₃ flow rate compensator 30.
0 is shown in detail.

(1) The mole ratio SM410 is calculated as the output of the function generator based on the input of the denitration ratio calculation value 345. Product 35 of inlet NOx flow rate 104 and this molar ratio SM410
1 is calculated by the multiplier. Here, the denitration rate calculation value 345 is calculated by inputting the inlet NOx concentration 101 and the outlet NOx concentration 107 to a denitration rate calculator and outputting the same.

(2) On the other hand, the required denitration rate after elapse of n hours is 3
The required mole ratio SM ⁿ 420 after n hours has elapsed is calculated as the output of the function generator with an input of 01. Inlet NOx flow 1
The product 353 of the value 04 and the molar ratio SM ⁿ 420 is calculated by a multiplier.

(3) The above two amounts are added by an adder to calculate an amount 355. That is, the addition amount 355 is the addition amount 355 = product 351 + product 353 = molar ratio SM410.
The product of the required molar ratio SM ⁿ 420 and the inlet NOx 104 = (NOx · SM + NOx ⁿ · S
M ⁿ ).

[0032] (4) While the inlet NOx concentration 101 and an outlet NOx concentration 107 adsorbed NH ₃ amount estimating unit 370 adsorbed NH ₃ amount estimated value 321 of the present time obtained by inputting a after a lapse of n times adsorbed NH ₃ The amount 361 obtained by subtracting 321 from the required amount estimation value 330 and subtracting 321 from this value is half the amount (n) of the predicted span n302 set to an appropriate value in advance.
The amount divided by the divider in (/ 2)) is 363. That is, quantity 363 = quantity 361 / (n / 2) = (quantity 330−quantity 32
1) / (n / 2) = (required amount of adsorbed NH ₃ after elapse of n hours 330−estimated value of adsorbed NH ₃ at present 321) / (n
/ 2) Here, the predicted value of the required amount of adsorbed NH ₃ after the elapse of n hours 330
Is calculated by inputting the required denitration rate 301 after the elapse of n hours to the function generator and outputting the function generator.

The calculation contents of the adsorption NH ₃ estimator 370 are as follows. First, the molar ratio SM is determined by the following experimentally known function based on the adsorbed NH ₃ and the inlet NOx.

Molar ratio SM = fucn1 (adsorbed NH ₃ , inlet NOx) That is, the adsorbed NH ₃ can be calculated from this function conversely if the inlet NOx and the molar ratio SM are given.

Adsorption NH ₃ = func1 ⁻¹ (molar ratio SM, inlet NOx) On the other hand, the denitration rate η is determined from this molar ratio SM, and the denitration rate η = fucn2 (molar ratio SM). The molar ratio can be calculated as SM = fucn2 ⁻¹ (denitration rate η).

Therefore, the adsorbed NH ₃ eventually becomes adsorbed NH ₃ = func1 ^-1 (molar ratio SM, inlet NOx) =
func1 ^-1 (fucn2 ^-1 (denitration rate η), inlet NO
x) = func3 (denitration rate η, inlet NOx) This is calculated if the denitration rate η and the inlet NOx are known.

The inlet NOx concentration 101 and the outlet NOx concentration 107 are input to the adsorption NH ₃ estimator 370 in FIG. 2, and the denitration rate η is calculated from these two amounts. That is, the adsorption NH ₃ is estimated by the function of the above func3.

(5) The sum of the amount 355 of (3) and the amount 363 of (5) is the amount 470. That is, sum 470
Is the product 3 of the inlet NOx flow rate 104 and the calculated denitration rate 345 or the required denitration rate 301 after n hours.
The difference 361 between 51 and 353 and the amount relating to the amount of adsorbed NH ₃ , that is, the difference 361 between the required amount of adsorbed NH ₃ after elapse of n hours 330 and the estimated value 321 of the amount of adsorbed NH ₃ at the present time is calculated by half 340 of the predicted span n. Is the sum of the amount divided by. That is, the amount 470 = {(NOx · SM + NOx n · SM n) + increase in adsorption NH ₃ (adsorption amount after elapse n times C _NH3 ⁿ - adsorption C _NH3 at the present time) / (n / 2)} ( 6) The sum 470 thus calculated is compared with the current NH ₃ flow rate 112, and a difference is calculated by a subtractor. This difference is used as the required NH ₃ flow rate correction value 302 as the required NH ₃ flow rate correction value 302.
This is an output from the flow rate corrector 300. That is, the required NH ₃ flow rate correction value 302 = G _NH3 ^{in, n} = -G _NH3 ⁱⁿ +
｛(NOx · SM + NOx ⁿ · SM ⁿ ) + increased amount of adsorbed NH ₃ (adsorbed amount C after elapse of n hours _{NH 3} ⁿ _−adsorbed amount C at present time
_NH3 ) / (n / 2).

FIG. 3 shows the result of the denitration control by the control device of the present invention. Compared with the control result by the conventional method, control is performed based on the amount of adsorbed NH ₃ required after elapse of n hours, so that denitration can be prevented, and deviation of the outlet NOx concentration from the lower limit is avoided (point a). The NH ₃ injection amount is also optimized to avoid deviation from the upper limit of the leak NH ₃ amount (b
Point) and the maximum flow rate of the NH ₃ injection amount is also suppressed (c)
Point).

[0040]

According to the present invention, the required NH ₃ after elapse of n hours
Controlling based on the amount of adsorption prevents over-denitration, (1)
The deviation of the outlet NOx concentration from the lower limit is avoided and the NH ₃ injection amount is correspondingly optimized. (2) The deviation of the leak NH ₃ amount from the upper limit is avoided, and (3) the maximum NH ₃ injection amount There is an effect that the flow rate is suppressed.

[Brief description of the drawings]

FIG. 1 is a diagram showing an overall configuration of a denitration control device according to an embodiment of the present invention.

FIG. 2 is a diagram showing details of a necessary NH ₃ flow rate corrector of FIG. 1;

FIG. 3 is a diagram showing a trend of a result of denitration control by the control device according to the embodiment of the present invention.

FIG. 4 is a diagram showing a trend of a denitration control result by a control device according to a conventional technique.

FIG. 5 is a diagram showing the contents of a denitration control device according to the related art.

[Explanation of symbols]

101 Inlet NOx Concentration 102 Air Flow 103 Combustion Flue Gas Flow 104 Inlet NOx Flow 105 Outlet NOx Concentration 106 Required Molar Ratio 107 Outlet NOx Concentration 108 Outlet NOx Concentration Deviation 109 Molar Ratio Correction 110 Corrected Molar Ratio 111 Required NH ₃ Flow 112 NH ₃ flow rate 113 NH ₃ flow rate deviation 114 NH ₃ flow rate adjustment amount 115 Required NH ₃ flow rate at present measurement base 201 Function generator 202 Subtractor 203 Controller 204 Multiplier 205 Adder 300 Required NH ₃ flow rate corrector 301 After n hours Required denitration rate 302 Predicted span n 310 Required NH ₃ flow rate correction value

Claims (7)

[Claims] 1. A method for reducing NOx in exhaust gas using NH ₃ on a catalyst.
The denitration control device controls the flow rate of NH ₃ so that the NOx concentration at the outlet of the catalyst falls within a predetermined range. The deviation between the NOx flow at the inlet of the catalyst, the outlet NOx concentration set value, and the measured value When obtains the necessary NH ₃ flow rate at the present time based on, as denitrification rate required by the obtained n time point from the present time, providing a required NH ₃ flow corrector that corrects the required flow rate _of NH ₃ Characteristic denitration control device. 2. The denitration control device according to claim 1, wherein the required NH ₃ flow rate corrector includes a current reaction NOx flow rate obtained by multiplying the inlet NOx flow rate by a denitration rate, and the inlet NOx flow rate. The reaction NOx flow rate at the time of n hours obtained by multiplying the flow rate by the required denitration rate at the time of n hours is added. From the required amount of NH ₃ adsorbed on the catalyst at the time of n time,
The estimated value of the amount of adsorbed NH ₃ on the catalyst at the present time is subtracted, and the added reaction NOx flow rate is subtracted from the adsorbed NH _{3 amount.}
An amount obtained by dividing the amount by a half of the time interval n is obtained, and a total value is obtained, and the total value is compared with the NH ₃ flow rate at the present time.
A denitration control device for calculating a correction value for correcting the required NH ₃ flow rate. 3. The denitration control device according to claim 2, wherein the input to the required NH ₃ flow rate corrector is an inlet NOx flow rate,
A denitration control device comprising: an inlet NOx concentration, an outlet NOx concentration, a required denitration rate after n hours from the present time, a predicted span n for determining a point to be predicted, and a measured NH ₃ flow rate. 4. The denitration control device according to claim 2, wherein the required NH ₃ flow rate corrector estimates a current amount of adsorbed NH ₃ , a calculation device calculates a current denitration rate, and n hours. A denitration control device, comprising: a function generator for calculating a predicted value of a required amount of adsorbed NH ₃ after elapse of n hours from a required denitration rate after elapse. 5. The denitration control device according to claim 4, wherein inputs of the arithmetic unit for estimating the amount of adsorbed NH ₃ at the present time are an inlet NOx concentration and an outlet NOx concentration, and the outputs thereof are the current values. A denitration control device, which is an estimated value of the amount of adsorbed NH ₃ . 6. The denitration control device according to claim 4, wherein the inputs of the arithmetic unit for calculating the current denitration rate are an inlet NOx concentration and an outlet NOx concentration, and the output thereof is a current denitration rate. A denitration control device, characterized in that: 7. The denitration control device according to claim 4, wherein the input of a function generator for calculating a predicted value of a required amount of adsorbed NH ₃ after elapse of n hours from the required denitration rate after elapse of n hours is provided. , The denitration rate required after n hours have passed,
A denitration control device characterized in that the output is the amount of adsorbed NH ₃ required corresponding to the denitration rate after elapse of n hours.