JPH0579363B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a desulfurization plant for removing sulfur dioxide gas SO ₂ from treated gas, and particularly to an improvement of an absorption tower PH control device for controlling the PH of circulating liquid.

[Conventional technology]

The schematic structure of a desulfurization plant, for example a wet lime gypsum process waste smoke desulfurization plant using carbon dioxide as an absorbent, will be described with reference to the system diagram shown in FIG.

In FIG. 5, an absorption tower 1 is supplied with a processing gas 3 containing sulfur dioxide gas through a processing gas introduction duct 2.
is introduced from above. A circulating fluid 5 is contained in a tank 4 provided below the absorption tower 1, and this circulating fluid 5 is circulated within the absorption tower 1 by a circulation pump 6 and a circulation pipe 7. The processing gas 3 comes into contact with the circulating liquid 5 in the circulation tower 1, and sulfur dioxide gas contained in the processing gas 3 is removed. In other words, SO ₂ in processing gas 3 is caused by the reaction shown in the following equation ().
It generates H ₂ SO ₃ and flows down. A part of this H ₂ SO ₃ is oxidized by oxygen O ₂ in the processing gas 3, and becomes H ₂ SO ₄ as shown in the following formula (). Also, the rest
H ₂ SO ₃ is oxidized in the tank 4 by oxygen in the air injected from the air pipe 8 and becomes H ₂ SO ₄ .

SO ₂ +H ₂ O → H ₂ SO ₃ ...() H ₂ SO ₃ +1/2O ₂ →H ₂ SO ₄ ...() Then, the treated gas from which sulfur dioxide gas has been removed after passing through the absorption tower is sent to the exhaust duct. 9 and released into the atmosphere as treated gas.

As described above, when the contact with the treated gas 3 is continued in the absorption tower 1, a large amount of H ₂ SO ₄ generated by the absorption reaction and oxidation reaction shown in () and () above is contained in the circulating liquid 5. This makes it difficult to absorb SO ₂ unless some measure is taken. Therefore, an absorbent, such as calcium carbonate, is supplied to the circulating fluid 5 in the tank 4 through an absorbent supply pipe 12 equipped with a flow rate detector 10 and a flow rate control valve 11.
CaCO ₃ is supplied to neutralize the circulating fluid 5 as shown in the following equation () and regenerate it so that sulfur dioxide gas can be easily absorbed.

H ₂ SO ₄ +CaCO ₃ →CaSO ₄ +H ₂ O + CO ₂ ↑
...() A part of the circulating fluid 5 containing CaSO ₄ generated by the above equation () is transferred to another process (not shown) via the transfer pipe 13.

As suggested from the above explanation, the circulating fluid 5
SO ₂ absorption capacity has a great influence on the performance of desulfurization plants. The pH of the circulating fluid 5 is an indicator of the SO ₂ absorption capacity of the circulating fluid 5. That is, the higher the CaCO ₃ concentration and the higher the PH in the circulating fluid 5, the more the SO ₂ absorption reaction is promoted.

A simple idea would be to supply a large amount of absorbent to keep the pH of the circulating fluid high, but this is not preferable from a cost standpoint.

For these reasons, there is a demand for desulfurization plants to be operated at a pH level that allows desired performance to be maintained. This ensures that the desulfurization rate in the absorption tower 1 and, in turn, the sulfur dioxide gas concentration in the treated gas released into the atmosphere, is stably maintained at a predetermined value, and that it is maintained stable against any change in load (amount of SO ₂ at the inlet of the absorption tower). This also leads to being able to follow up with good responsiveness.

By the way, as mentioned above, it is the increase in the H ₂ SO ₄ concentration in the circulating fluid that lowers the PH of the circulating fluid.
On the other hand, what increases the pH of the circulating fluid is
_CaCO3 concentration. Therefore, the pH of the circulating fluid is determined by the balance between the absorbed SO ₂ amount and the CaCO ₃ concentration.

In the conventional desulfurization plant shown in Figure 5,
The PH control device for the circulating fluid 5 is as follows.

That is, a PH detector 14 is attached to the circulation pipe 7, and an output signal _S14 from this PH detector 14 is input to a PH regulator 15. This PH regulator 15 compares a preset PH set value and the output signal from the PH detector 14, and compares the PI or PID
(P: proportional, I: integral, D: differential) feedback control is performed. On the other hand, processing gas introduction duct 2
An output signal S 18 from a multiplier 18 obtained by multiplying the output signal from the processing gas flow rate detector 16 and the output signal from the SO ₂ concentration detector ₁₇ provided midway is also input to the PH regulator 15 . The PH regulator 15 performs feedforward control of P or PD using the output signal _S18 of the multiplier 18. The PH regulator 15 outputs an absorbent flow rate correction signal _S15 which is the sum of the output signals of the feedback control and the feedback control. This signal is input to the absorbent flow rate regulator 19 together with the output signal of the flow rate detector 10, and the opening degree of the flow rate control valve 11 is adjusted. In this way, the pH of the circulating fluid 5 is controlled to a predetermined value.

[Problems to be solved by the invention]

Desulfurization plants are required to respond to high-speed boiler load changes with good responsiveness. but,
In the conventional PH control device, the circulating fluid 5 is
The PH of the reactor could not follow the PH set value and decreased, and as a result, there was a risk that the SO ₂ concentration in the treated gas (outlet SO ₂ concentration) would deviate from the regulation value.

In addition, in order to prevent the SO ₂ concentration of the treated gas from deviating from the regulation value, it is necessary to adjust the circulating fluid 5.
There was a problem in that the PH value was increased higher than necessary in advance, which increased running costs.

The present inventors investigated the causes of the above phenomenon and found the following facts. That is, the amount of CaCO ₃ remaining in the system is approximately proportional to the total amount of treated SO ₂ (processing gas flow rate x SO ₂ concentration in the processing gas). An example of this relationship is shown in FIG. 6th
In the figure, the horizontal axis is the total amount of processed SO ₂ , and the vertical axis is the residual amount in the system.
The relationship between the two is shown as CaCO ₃ .
From Figure 6, when considering the increase in load, not only the increased desulfurization SO ₂ equivalent, but also the residual CaCO ₃ in the circulating fluid.
It can be seen that it is necessary to feed extra absorbent to increase the quantity. Therefore, if residual CaCO ₃ does not increase in response to an increase in load, the neutralization reaction will be suppressed and the PH will decrease, resulting in
The SO ₂ concentration deviates from the regulation value.

Now, when the load increase rate is small (1 to 2%/min), the rate of change in the amount of residual _CaCO3 is small, so
It is possible to follow the conventional PH control device before the deviation between the circulating fluid PH and the PH set value becomes large.
However, when the load increase rate is large (3 to 5%/min), the rate of change in the amount of residual _CaCO3 is large;
Conventional PH control devices cannot keep up with this. For this reason, conventional PH control devices are slow to respond quickly to high-speed load changes.

Furthermore, FIG. 7 shows the relationship between the total amount of treated SO ₂ and the amount of residual CaCO ₃ per unit total amount of SO ₂ treated (10 Kgmol/H). From FIG. 7, it can be seen that as the total amount of treated SO ₂ increases, the amount of residual CaCO ₃ must be increased.

The present invention has been made to solve the above problems, and provides an absorption tower PH control that can quickly make the PH of the circulating fluid follow the set value even in response to high-speed load changes, and can reduce running costs. The purpose is to provide a device.

[Means for solving problems]

From the above explanation, in order to make the PH of the circulating fluid follow the PH setting value with good responsiveness, it is necessary to increase the amount of residual CaCO ₃ , which is the integral amount of the absorbent supply amount, in accordance with the increase in the total amount of SO ₂ processed due to rapid load changes. It can be seen that it is sufficient to increase as shown in FIG. The present inventors have developed the present invention by considering increasing the absorbent flow rate by an amount proportional to the total amount of SO ₂ treated only when the PH deviation becomes too large with only a conventional PH control device when the load increases. Ivy.

That is, the absorption tower PH control device of the present invention includes: a processing gas flow rate detector that detects the flow rate of processing gas introduced into the absorption tower; an SO ₂ concentration detector that detects the SO ₂ concentration in the processing gas; a first multiplier that inputs and multiplies the output signal of the processing gas flow rate detector and the output signal of the SO ₂ concentration detector; a PH detector that detects the PH of the circulating fluid; and the first multiplier. a PH regulator that inputs the output signal of the PH detector and the output signal of the PH detector and outputs an absorbent flow rate correction signal; a function calculator that outputs a binary signal according to the output signal of the PH detector; a second multiplier that multiplies the output signal of the first multiplier and the output signal of the functional operator;
an adder that inputs the output signal of the PH regulator and the output signal of the second multiplier and outputs the sum; an absorbent flow rate detector that detects the flow rate of absorbent supplied to the absorption tower; The present invention is characterized by comprising an absorbent flow rate regulator that receives the output signal of the absorbent flow rate detector and the output signal of the absorbent flow rate detector to adjust the opening degree of the absorbent control valve.

[Effect]

According to such an absorption tower PH control device, the flow rate of the absorbent can be changed according to the output signal of the functional calculator corresponding to the PH deviation and the total amount of SO ₂ to be processed. Therefore, it is possible to perform gentle control when the PH deviation is small, and it is also resistant to high-speed load changes.
Since a larger correction than that made by conventional devices is performed before the PH deviation becomes large to a certain extent, the PH of the circulating fluid can be quickly made to follow the set value.
As a result, it is not necessary to increase the amount of absorbent supplied more than necessary, so running costs can be reduced.

〔Example〕

Embodiments of the present invention will be described below with reference to FIG. Note that the same equipment and the like as the conventional apparatus shown in FIG. 5 are given the same numbers and their explanations will be omitted. Newly installed equipment in the absorption tower PH control device according to the present invention is a function calculator 21 and a second multiplier 2.
2 and an adder 23.

In FIG. 1, the function calculator 21 is the PH detector 1.
4 is input, the PH deviation, that is, the difference between the preset PH setting value and the PH of the circulating fluid 5, is calculated, and a binary signal is output in accordance with the PH deviation. An example of this relationship is shown in FIG. That is, when the PH of the circulating fluid 5 decreases with respect to the PH set value and becomes larger than the preset threshold (β in Fig. 2), the output signal changes from 0.0 to 1.0, and the PH deviation becomes 0.0. hold until

Now, the case of load increase will be explained. As the load increases, the amount of desulfurization in the absorption tower 1 increases, and when the amount of desulfurization exceeds the amount that can be neutralized by residual CaCO ₃ in the circulating fluid 5, the pH of the circulating fluid 5 decreases. PH
The regulator 15 outputs an absorbent flow rate correction signal according to the pH deviation.
S ₁₅ is increased to try to correct the PH of the circulating fluid 5. When the load increase rate and load change range are relatively small, the PH of the circulating fluid 5 is corrected by the PH regulator 15 while the PH deviation is small, and the SO ₂ in the treated gas is
Concentrations will not exceed regulatory limits. On the other hand, when the load increase rate and load change width are large, the amount of residual CaCO ₃ necessary for neutralization cannot keep up with the load increase by only absorbent flow rate correction by the PH regulator 15, and the PH deviation becomes large. Then, the PH deviation is the function operator 2
The output of the function calculator 21 is 1.0 from the time when the PH deviation exceeds the threshold value β in 1 until the PH deviation becomes 0.
The signal is input to the second multiplier 22 .

In the second multiplier 22, the multiplier (first multiplier)
An output signal S ₂₂ proportional to the output signal of 18 is output and input to an adder 23 . In adder 23, PH
The second absorbent flow rate correction signal _S15 by the regulator 15
The output signal _S22 of the multiplier 22 is added and outputted as a set value signal of the absorbent flow rate regulator 19.

According to such an absorption tower PH control device, the PH is adjusted to such an extent that the SO ₂ concentration in the treated gas deviates from the regulation value.
Before the deviation becomes large, absorbent is supplied in an amount proportional to the total amount of SO ₂ being processed at that time by on-off control separately from the existing PH regulator 15, so the PH
Deviations can be kept small. As a result, SO ₂ in the processed gas can be maintained below the regulation value, and even high-speed load changes can be followed with good responsiveness. Therefore, it is not necessary to increase the amount of absorbent supplied more than necessary, and running costs can be reduced.

The results of actually desulfurizing the process gas using the conventional PH control device and the PH control device of the above embodiment are shown in FIGS. 3 and 4, respectively. Note that in both cases, the load change rate was 5%/min.

In the case of the conventional apparatus shown in FIG. 3, the PH deviation increases temporarily when the load increases rapidly, and as a result, the SO ₂ concentration in the treated gas at the outlet of the absorption tower also increases temporarily. On the other hand, in the case of the device of the above embodiment shown in Fig. 4, even when the load increases at high speed,
It can be seen that the PH deviation is suppressed to a small level, and as a result, the SO ₂ concentration in the treated gas can be maintained so as not to exceed the regulation value. Furthermore, since the correction of the absorbent supply amount when the load increases is greater than when using only the conventional PH regulator, it can be seen that the PH adjustment time is short and the responsiveness is good. Also, when the load decreases, residual
The amount of CaCO ₃ is higher than the required amount, and the PH of circulating fluid 5 is PH.
Since it is maintained higher than the set value, PH control similar to that of a conventional PH control device is performed.

〔Effect of the invention〕

As detailed above, according to the present invention, the pH of the circulating fluid can be quickly made to follow the set value even when the load changes rapidly, eliminating the need to use more absorbent than necessary and reducing running costs. Possible absorption tower
It can provide a PH control device.

[Brief explanation of drawings]

Fig. 1 is a system diagram of the absorption tower PH control device in an embodiment of the present invention, Fig. 2 is an output characteristic diagram of the function calculator of the absorption tower PH control device in the embodiment of the present invention, and Fig. 3 is a diagram of the conventional PH control device. A characteristic diagram showing changes in the circulating fluid PH and outlet SO 2 concentration during a high-speed load increase when using a control device. Figure 4 shows a characteristic diagram showing changes in circulating fluid PH and outlet SO ₂ concentration during a high-speed load increase when using a PH control device according to an embodiment of the present invention. Characteristic diagram showing changes in circulating fluid PH and outlet SO ₂ concentration, Figure 5 is a system diagram of a conventional absorption tower PH control device, and Figure 6 is a treatment diagram.
A characteristic diagram showing the relationship between the total amount of SO ₂ and the amount of CaCO ₃ remaining in the circulating fluid. Figure 7 shows the total amount of treated SO ₂ and the total amount of treated SO ₂
It is a characteristic diagram showing the relationship with the amount of residual CaCO ₃ per 10 [Kgmol/H]. 1... Absorption tower, 2... Processing gas introduction duct, 3
... Processing gas, 4 ... Tank, 5 ... Circulating fluid, 6
... Circulation pump, 7 ... Circulation piping, 8 ... Air piping, 9 ... Exhaust duct, 10 ... Flow rate detector, 1
1...Flow rate adjustment valve, 12...Absorbent supply piping, 1
3...Transfer piping, 14...PH detector, 15...PH
Regulator, 16... Processing gas flow rate detector, 17...
SO ₂ concentration detector, 18... Multiplier (first multiplier), 19... Absorbent flow rate regulator, 21... Function calculator, 22... Second multiplier, 23... Adder.

Claims (1)

[Claims] 1 In a desulfurization plant that introduces a treated gas containing sulfur dioxide gas into an absorption tower and desulfurizes it by contacting it with a circulating liquid containing an absorbent and circulating within the absorption tower, the flow rate of the treated gas introduced into the absorption tower a processing gas flow rate detector that detects the SO ₂ concentration in the processing gas, an SO ₂ concentration detector that detects the SO 2 concentration in the processing gas, an output signal of the processing gas flow rate detector and an output signal of the SO ₂ concentration detector, inputting a first multiplier for multiplication, a PH detector for detecting the PH of the circulating fluid, an output signal of the first multiplier and an output signal of the PH detector;
a PH regulator that outputs an absorbent flow rate correction signal;
a functional calculator that outputs a binary signal according to the output signal of the PH detector; a second multiplier that multiplies the output signal of the first multiplier by the output signal of the functional calculator; and the PH detector. an adder that inputs the output signal of the regulator and the output signal of the second multiplier and outputs the sum; an absorbent flow rate detector that detects the flow rate of absorbent supplied to the absorption tower; An absorption tower PH control device comprising: an absorbent flow rate regulator that receives an output signal and an output signal of the absorbent flow rate detector to adjust the opening degree of an absorbent control valve.