JPH0729023B2 - Wet exhaust gas desulfurization method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a wet exhaust gas desulfurization method and, in particular, when the conditions of the exhaust gas to the desulfurization device are changed, such as when the load changes, appropriate measures are taken. The present invention relates to a wet exhaust gas desulfurization method capable of

(Prior Art) As a wet exhaust gas desulfurization method, a method of using limestone, lime, etc. as an absorbent, absorbing SOx in the exhaust gas of a boiler, etc., and oxidizing the obtained calcium sulfite to recover gypsum is currently widely used. Has been adopted.

The system of the wet exhaust gas desulfurization equipment is shown in FIG.

Exhaust gas from a boiler or the like is introduced into the dust removing tower 2 by the flue 1 and supplied by the dust removing tower circulation pump 4, and is cooled to the saturation temperature by gas-liquid contact with the circulating liquid sprayed into the tower 2 and the exhaust gas is discharged. After the dust contained therein is removed, it is sent to the absorption tower 7. In order to remove the mist in the gas sent to the absorption tower 7, the mist eliminator 6
May be installed.

In the absorption tower 7, the circulating absorption liquid slurry supplied from the absorption tower circulation pump 10 is sprayed into the absorption tower by the nozzle, and SOx in the exhaust gas is absorbed and removed by gas-liquid contact with the exhaust gas, and then entrained in the demister 8. The mist is removed and discharged from the flue 9.

The absorption tower 7 is newly replenished with a slurry necessary for continuously absorbing SOx in the exhaust gas by the pump 13, while the absorption tower bleed (exhaust) pump 11 absorbs SOx and contains calcium sulfite produced. Part of the slurry is extracted and recovered as gypsum in an oxidation tower (not shown).

The object of the present invention relates to a method of controlling the supply amount of the absorbent slurry to the absorption tower and a method of controlling the circulation flow rate of the absorption tower slurry in the system described above, and an example of the conventional technique thereof is shown below.

FIG. 5 shows an example of conventional technology, in which the flow rate of exhaust gas flowing into the desulfurization unit and the SOx concentration in the exhaust gas are detected by the meters 17 and 18, respectively, and the SOx amount is calculated by the multiplier 21. This is a control method for determining a required absorbent slurry replenishment flow rate by multiplying a constant ratio in the coefficient unit 22. Therefore, with this control method, the excess ratio of the absorbent is constant regardless of the inflow SOx amount. Regarding the slurry circulation flow rate to the absorption tower, a constant flow rate control is performed regardless of the inflow SOx amount.

(Problems to be Solved by the Invention) In the control method as described above, when the desulfurization device is operated by reducing the load from high load operation to low load operation, the inflow SOx
The desulfurization performance increases due to the increase in the pH value of the absorption tower slurry caused by the decrease in the amount of SOx and the decrease in the amount of inflowing SOx. For this reason,
During low load operation, excessive consumption of absorbent and unnecessary consumption of absorption tower slurry circulation pump power, and as a result, a large amount of sulfuric acid is consumed when producing gypsum from the absorbent slurry, the operating cost There was a drawback that the cost was high. To such drawbacks of the prior art,
A control method as shown in FIG. 6 was devised in which a correction circuit was added to keep the pH of the absorbent slurry in the absorption tower constant at each load. This method, in order to maintain the desulfurization rate at low load at the target value, so that the pH of the absorption liquid becomes a preset value, from the value of the measuring instrument 20 that measures the pH of the absorption liquid and the pH setter. The pH set value 38 is input to the controller 23, and the controller 23 controls the supplement amount of the absorbent slurry. However, when operating at a constant pH, when the load is low, that is, when the amount of inflowing SOx decreases, the concentration of the absorbent in the absorption liquid in the absorption tower decreases. The amount of slurry held in the absorption tower is generally about 20 hours for the amount of slurry discharged from the absorption tower (which is almost equal to the amount of absorbent supplied), and There is a lag in the change, which causes a lag in the pH response.

Therefore, with this control method, stable pH at low load
Value, when the load is suddenly increased from the state of operating with the absorption liquid slurry property, the liquid composition (absorbent concentration) changes due to the increase in the load, and the pH is temporarily lowered to decrease the desulfurization rate. Will lead to a decline.

Therefore, there is a drawback that a desulfurizer installed in a boiler or the like, which has a large load change, cannot perform stable operation.

It is an object of the present invention to provide a wet exhaust gas desulfurization method which eliminates the above-mentioned drawbacks of the prior art, has good followability to load fluctuations of a boiler and the like, and can reduce power of an absorbent and an absorption tower slurry circulation pump. .

(Means for Solving Problems) In short, the present invention predicts the pH value of the absorption tower slurry by a simulation model built in the control system with respect to the replenishment amount of the absorbent slurry, and Based on this, the amount of absorbent slurry replenishment is determined, and the absorption tower slurry circulation flow rate (equal to the amount of absorbent slurry sprayed into the absorption tower) is also determined so that the desulfurization rate can always follow the target value. It is a thing. That is, the present invention, the exhaust gas containing a sulfur acid component (SOx) is supplied to the absorption tower, and contacted with the absorbent circulating in the absorption tower, in removing SOx in the exhaust gas, is supplied to the absorption tower In the wet exhaust gas desulfurization method in which the pH value of the absorption liquid circulating in the absorption tower is controlled according to the SOx amount in the exhaust gas,
Predict the future pH value of the circulating absorbent by using the following equations (1) and (2) based on the SOx amount, the pH value of the circulating absorbent, the desulfurization rate in the absorber, and the amount of absorbent replenished to the absorber. Then
It is characterized in that the predicted pH value is used to control the supply amount of the absorbent to the absorption tower.

Here, pH: Absorbent slurry pH k: Modified parameters α, β, a, b: Constant η: Set desulfurization rate X: Absorbent concentration in the circulating slurry in the absorption tower (wt%) Gg: Exhaust gas flow rate (Nm ³ / h) XH ₂ O: Moisture C SOx: Inlet SOx concentration (ppm) [Ca ⁺⁺ ]: Calcium ion concentration (g ion /) Δt: Time step G sl: Absorbent slurry replenishment flow rate (kg / h) Xs: Absorption Absorbent concentration in the absorbent slurry supplied to the tower (wt
%) GH ₂ O: Make-up water amount (kg / h) X ^* : Absorbent slurry concentration in the circulation slurry in the absorption tower before Δt (Example) An example of the wet exhaust gas desulfurization method according to the present invention is shown in FIG. 1 and FIG. Shown in Figure 2. In the figure, 10 is an absorption tower circulation pump,
15 is an absorbent slurry replenishment flow rate control valve, 17 is an exhaust gas flow meter,
18 is the inlet SOx concentration meter, 19 is the absorbent slurry replenishment flow meter, 20
Is an absorption tower slurry pH meter, 21 is a multiplier, 23 is a controller, 24 is an electric / air converter, 25 is a desulfurization rate setting device, 26 is a function generator,
27 is a subtractor, 28 is an adder, 29 is a pH prediction calculator, 30 is a pH prediction current value signal, 31 is a parameter correction signal, and 32 is after t minutes.
pH prediction calculation signal, 33 is a pH target value calculator after t minutes, 34 is an outlet SOx concentration meter, 35 is a divider, 36 is an absorption tower slurry circulation flow meter, and 37 is an absorption tower slurry circulation flow rate calculator. .

The control method of the wet type exhaust gas desulfurization apparatus can be divided into the control of the supply amount of the absorbent slurry and the control of the slurry circulation flow rate. For the former, as shown in Fig. 1, the exhaust gas flow meter 17 and SOx
The value generated by multiplying the measurement value of the densitometer by the multiplier 21a is the function generator 2
6a, where the pH set value is set for the absolute amount of SOx in the exhaust gas, the deviation from the actual measurement value of the pH meter 20 is corrected by the function generator 26c, and this is corrected by the function generator 26b. The set value of the excess rate of the absorbent slurry is set with respect to the absolute amount of SOx in the exhaust gas and added by the adder 28a. Further, from the signal of the exhaust gas flow meter 17, the signal of the SOx concentration meter 18, the signal of the desulfurization rate setting device 25, and the signal of the absorbent slurry replenishment flow meter 19, the pH predictive operation device 29 uses the pH predictive operation signal after t minutes. Find 32, 5
The deviation from the signal from the pH target value calculator 33 after
The signals corrected by a are added by the adder 28a. Since the output signal of the adder 28a is the excess ratio of the absorbent slurry,
In the multiplier 21b, it is multiplied by the absolute amount of SOx to set the absorbent slurry flow rate setting signal, and the deviation from the absorbent slurry replenishment flow meter 19 is corrected by the controller 23c to open / close the absorbent slurry replenishment flow rate adjustment valve 15. By doing so, the supply amount of the absorbent slurry to the absorption tower is adjusted.

In the pH prediction calculator 29, the deviation between the signal of the absorption tower slurry pH meter 20 and the pH prediction current value signal 30 is corrected by the controller 23b, and the parameter correction signal 31 is fed back to the pH prediction calculator 29 to be incorporated therein. Automatically correct the simulation model. The outline of the simulation model is shown below.

Here, pH: absorption slurry pH, k: correction parameter, α,
β, a, b: constant, η: set desulfurization rate, X: concentration of absorbent in circulating slurry in absorption tower (about 1 wt%), Gg: exhaust gas flow rate (Nm ³ /
h), XH ₂ O: Moisture, C SOx: Inlet SOx concentration (ppm), [C
a ⁺⁺ ]: calcium ion concentration (g ion /), Δt:
Time step, G sl: Absorbent slurry replenishment flow rate (kg /
h), Xs: Absorbent concentration in the absorbent slurry to be replenished to the absorption tower (about 20 wt%), GH ₂ O: Make-up water amount (kg / h), X ^* : Absorbent of circulating slurry in the absorption tower before Δt Slurry concentration The pH after t minutes is predicted and calculated from the equations (1) and (2).

Next, the pH target value calculator 33 after t minutes determines the pH set value with respect to the absolute value of the SOx amount. Therefore, the pH target value is obtained by linearly predicting the SOx amount from the online measurement data.

For the control of the absorption tower slurry circulation flow rate, as shown in FIG.
The difference between the measured value of the inlet SOx concentration meter 18 and the measured value of the outlet SOx concentration meter was obtained by the subtractor 27, and this value was divided by the measured value of 18.The output signal of the divider 35 and the desulfurization rate setting device 25 The SOx concentration meter 18 and the desulfurization rate setting device are added to the signal whose deviation from the
25, the absorption tower slurry circulation flow rate demand (request amount) is calculated in the absorbent slurry circulation flow rate calculator 37 from the signals of the absorption tower slurry pH meter 20 and the exhaust gas flow meter 17, and this signal is added by the adder 28b and absorbed. Absorption tower circulation pump 1 after correcting the deviation from the measurement signal of tower slurry circulation flow meter 36 by controller 23e
The desulfurization rate is maintained at the measured value by controlling the number of revolutions or the number of units at 0.

The absorption tower slurry circulation flow rate calculator 37 calculates the flow rate demand as follows.

η = 1-BTU / EXP (-RTU1 / RTU2 / RTU3 / RTU4)
(1) RUT1 = f (pH), RTU2 = f (Gg), RTU3 = f (C SOx), RTU4 = f (L / G) (2) where η: desulfurization rate, BTU: constant, pH: Absorption tower slurry pH, G
g: exhaust gas flow rate, C SOx: inlet exhaust gas SOx concentration, L / G: liquid gas ratio (circulation slurry amount / exhaust gas amount), and BTU is calculated from the following formula.

BTU = -loge (1-ηo) ηo is the standard desulfurization rate. If ηo is 0.9, then BTU
≒ 2.30. The values of RTU1, RTU2, RTU3, and RTU4 are obtained from the relationship diagram shown in FIG.

The absorption tower slurry circulation flow rate demand L is calculated using the online measurement data from the relationships of the equations (1) and (2).

(Effects of the Invention) As described above, in the present invention, the pH prediction control by the simulation model is used for the control of the absorbent slurry replenishment flow rate, and the desulfurization rate control is used for the control of the absorption tower slurry circulation flow rate.
The pH responsiveness can be improved and the pH can be quickly tracked to an appropriate set value.The desulfurization rate control by the circulation flow rate of the absorption tower slurry has a quick response, and the desulfurization rate is almost the set value even when the load changes. Therefore, the utility cost, that is, the absorbent consumption amount and the power cost of the absorption tower slurry circulation pump can be reduced.

According to the present invention, the desulfurization rate can be maintained almost at the target desulfurization rate in all operating conditions including the time of load change.
The load response is improved and stable operation can be secured.

In a normal desulfurizer, the consumption cost of the absorbent and the power cost of the absorption tower circulation pump are almost equal, but when the desulfurization rate constant control is performed by adjusting the slurry circulation flow rate, the daily load pattern is shown in Fig. 7. Since the pump power at 25% load is about half that at 100% load, the reduction rate of circulating pump power is Becomes

On the other hand, the excess control of limestone from 1.05 to 1.
It is possible to round down to 02, so Can be reduced.

Therefore, since the effect of reducing the pump power cost is greater, the pH set value at low load is increased and the pH predictive control is used to quickly achieve this optimum pH value even when the load changes. Can be reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing the replenishment amount control of the absorbent slurry in the wet exhaust gas desulfurization method according to the present invention, and FIG. 2 is a drawing showing the absorption liquid circulation amount control in the absorption tower according to the present invention.
FIG. 4 is a relational diagram of factors that determine the circulation flow rate in the embodiment of FIG. 2, FIG. 4 is a system diagram of a conventional wet desulfurization apparatus, and FIGS. 5 and 6 are absorbent slurry of a conventional technique. FIG. 7 is a control system diagram of the replenishment amount, and is a diagram showing a load change of the desulfurization apparatus when the present invention is implemented. 7 ... Absorption tower, 10 ... Absorption liquid circulation pump, 15 ... Absorbent slurry replenishment flow rate adjustment valve, 17 ... Exhaust gas flow meter, 18 ... Exhaust gas inlet SOx concentration meter, 19 ... Absorber slurry replenishment flow meter , 2
0 …… Absorption tower slurry pH meter, 21a, 21b …… Multiplier, 23a, 23
b, 23c, 23d, 23e ... Controller, 25 ... Desulfurization rate setting device, 26
a, 26b, 26c ... Function generator, 27 ... Subtractor, 28a, 28b
…… Adder, 29 …… pH prediction calculator, 30 …… pH prediction current value signal, 32 …… pH prediction calculation signal after t minutes, 33 …… pH minutes after calculation
pH target value calculator, 34 ... Exhaust gas outlet SOx concentration meter, 35 ...
Divider, 36 ... Absorption tower slurry circulation flow meter, 37 ... Absorption tower slurry circulation flow calculator.

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location B01D 53/34 ZAB

Claims (2)

[Claims] 1. An exhaust gas containing a sulfuric acid component (SOx) is supplied to an absorption tower and brought into contact with an absorption liquid circulating in the absorption tower to be supplied to the absorption tower when SOx in the exhaust gas is removed. In a wet exhaust gas desulfurization method in which the pH value of the absorption liquid circulating in the absorption tower is controlled according to the SOx amount in the exhaust gas, the SOx amount in the exhaust gas, the pH value of the circulating absorption liquid, the desulfurization in the absorption tower Based on the rate and the amount of absorbent replenished to the absorption tower, the future pH value of the circulating absorption liquid is predicted using the following formulas (1) and (2), and the absorption to the absorption tower is performed using this predicted pH value. A wet exhaust gas desulfurization method characterized by controlling the supply amount of a chemical agent. Here, pH: Absorbent slurry pH k: Modified parameters α, β, a, b: Constant η: Set desulfurization rate X: Absorbent concentration in the circulating slurry in the absorption tower (wt%) Gg: Exhaust gas flow rate (Nm ³ / h) XH ₂ O: Moisture C SOx: Inlet SOx concentration (ppm) [Ca ⁺⁺ ]: Calcium ion concentration (g ion /) Δt: Time step G sl: Absorbent slurry replenishment flow rate (kg / h) Xs: Absorption Absorbent concentration in the absorbent slurry supplied to the tower (wt
%) GH ₂ O: Make-up water amount (kg / h) X ^* : Absorbent slurry concentration in the circulating slurry in the absorption tower before Δt 2. The wet exhaust gas desulfurization method according to claim 1, wherein a deviation value between the present value of the desulfurization rate in the absorption tower and the setting value from the desulfurization rate setting device is calculated, and the deviation value and further, A wet exhaust gas desulfurization method characterized in that the flow rate of the circulating absorbent in the future is controlled based on the amount of the exhaust gas at the inlet, the SOx concentration in the exhaust gas, the flow rate of the circulating absorbent and the pH value of the absorbent.