JPH0445205B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a wet lime gypsum method flue gas desulfurization plant.
This invention relates to a control method for a wet lime-gypsum desulfurization plant that has excellent ability to follow large and rapid load changes in SO ₂ absorption equipment.

Generally, an SO ₂ absorption device is constructed as shown in FIG. 1, and desulfurization is performed in the following manner. When the exhaust gas 1 enters the absorption tower 3 from the duct 2, it comes into contact with the circulating absorption liquid 4. SO ₂ in the exhaust gas generates H ₂ SO ₃ in the liquid through the absorption reaction of equation (1), which flows down.

SO ₂ +H ₂ O→H ₂ SO ₃ ...(1) After this, the exhaust gas passes through the exhaust line 5 and is discharged from the chimney.

On the other hand, the liquid that produced H ₂ SO ₃ flows down into tank 6 from the bottom of the column. A neutralizing agent (calcium carbonate, other alkaline substances such as calcium hydroxide) is supplied to the tank 6 from a supply line 7, and this neutralizing agent neutralizes this liquid to produce CaSO ₃ . The neutralized liquid is passed through a circulation line 9 by a pump 8 to an absorption tower 3.
is supplied to A portion of the circulating fluid is taken out, and CaSO ₃ is oxidized to CaSO ₄ .2H ₂ O (gypsum) in a post-process.

The conventional control method for this SO ₂ absorption device is
It is done as follows. A pH detector 11 detects the pH value of the circulating absorption liquid and inputs it to a controller 12. The controller 12 inputs a signal to the adder 13 so that the pH value of the absorption liquid reaching the top of the column is constant.

On the other hand, the load detector 14 detects the amount of SO ₂ entering the system (for example, the product of the exhaust gas flow rate and the inlet SO ₂ concentration), that is, the load of the desulfurization plant (hereinafter referred to as desulfurization load), and inputs it to the adder 13 . The adder 13 adds the signal from the controller 12 and the signal from the load detector 14, and inputs the result to the flow controller 17 as a set value signal. In addition, the flow rate of the supply line 7 is detected by the flow rate detector 1.
6 and input it to the flow rate controller 17. The flow rate controller 17 operates the control valve 18 based on these signals.
control.

That is, in the conventional control method, the output from the controller 12 has a feedback effect, and the output from the load detector 14
The output from has a feed forward effect,
The idea is to supply a neutralizing agent equivalent to the amount of SO ₂ absorbed, and is based on the idea that if a neutralizing agent equivalent to the amount of SO ₂ entering the system is supplied, SO ₂ can always be absorbed at the same desulfurization rate. This idea holds true when, when the desulfurization load increases, the neutralization reaction rate within the system is faster than or at least equal to the rate of increase.

However, desulfurization performance depends on the H ₂ SO ₃ concentration and PH in the liquid.
The lower the H ₂ SO ₃ concentration and the higher the PH,
High absorption of SO ₂ .

The reaction rate of the absorption reaction of equation (1) is expressed by equation (2).

γ=K・A・(CG−CL)……(2) γ: Absorption reaction rate A: Contact area between gas and liquid CG: SO ₂ concentration in gas CL: H ₂ SO ₃ concentration in liquid K: SO ₂ Absorption general mass transfer system Equation (2) explains the control method to keep the pH constant by supplying a neutralizing agent. First, when SO ₂ in the gas is absorbed, the H ₂ SO ₃ concentration in the liquid increases.
The H ₂ SO ₃ concentration CL increases and the absorption reaction rate γ decreases. Therefore, by neutralizing H ₂ SO ₃ , H ₂ SO ₃ in the liquid
It is necessary to keep the concentration CL low. Second, since the transfer coefficient K is a function of PH, it is necessary to neutralize it so that the H ₂ SO ₃ concentration does not become too high and the PH becomes low.

However, since the neutralization reaction rate is very slow, when the rate of increase in load is high, it is not possible to maintain the PH at a predetermined value even if an equivalent amount of neutralizing agent is supplied to the load. As the desulfurization performance decreases as the pH decreases, it is necessary to increase the neutralization reaction rate.

Note that the desulfurization performance is generally expressed by the desulfurization rate η in equation (3).

η = CG ₁ - CG ₀ / CG ₁ ...(3) where CG ₁ : SO ₂ concentration in the plant inlet gas CG ₀ : SO ₂ concentration in the plant outlet gas On the other hand, PH increases as the H ₂ SO ₃ concentration increases. falling,
If there is a large amount of unreacted CaCO ₃ in the liquid,
When the CaCO ₃ concentration is high, it increases.

Based on the above, the following methods can be considered in order to be able to follow sudden increases in load.

A method of operating at a high PH at all times, that is, at any load level, so that a large amount of unreacted CaCO ₃ is stored in the system, to provide some margin for increases in load. Or how to operate at high PH during low load.

However, these methods are extremely uneconomical. That is, when the CaCO ₃ concentration in the tank 6 is high, the CaCO 3 is discharged from the system through the line 19 at that concentration. For this reason, if a large amount of unreacted CaCO ₃ is retained when the reaction rate γ at low load may be small, a large amount of raw material (neutralizing agent) must be supplied. Furthermore
If ₃ minutes of CaCO is not used for the neutralization reaction in the absorption tower 3 and is discharged to the outside of the system from the line 19, it must be neutralized with sulfuric acid in a subsequent step.

As mentioned above, in these methods, unused
Not only does the amount of CaCO ₃ increase, but also the amount of sulfuric acid used to neutralize it increases. In addition, in this method, since there is a margin in the process at low load, the desulfurization performance becomes unnecessarily high.

For the above reasons, in order to reduce the consumption of neutralizer and sulfuric acid, it is sufficient to operate at low PH when the load is low and at high PH when the load is high.

The control method for this operation requires a PH controller 1
There is a method of making the PH setting value of 2 a function of the load. In this case, calculations are made in advance so that the target desulfurization rate is achieved under any load. The functional relationship of the PH setting value to the load to obtain the target desulfurization rate differs from plant to plant, but is approximately as shown in Figure 2.

Although this control method is very good for minimizing the consumption of neutralizer and sulfuric acid, it is not practical in terms of following rapid load changes.

That is, in order to change the PH over a wide range as shown in Figure 2, it is necessary to change the CaCO ₃ concentration within the system in response to the PH change. Set the PH change range to 1.0 (e.g. 4.7
to 5.7), the CaCO ₃ concentration must be changed approximately 10 times. When the load changes from 25% to 100% at an average rate of 5%/min,
The time between this is 15 minutes.

On the other hand, the amount of CaCO ₃ in tank 6 (liquid volume in tank x CaCO ₃
concentration) is about 10 hours of the CaCO ₃ supply amount at 100% load. In other words, CaCO ₃ in the tank 6 remains for 10 hours relative to the supplied amount.

Therefore, for example, such a large amount in 15 minutes
To increase _CaCO3 by 10, add _CaCO3 during this 15 minutes.
must be supplied in large quantities. For this reason, this method cannot be called a practical method in terms of supply equipment.
Difficult to follow load changes.

Based on this, the present inventors considered adding another operation to reduce the range of PH change.

That is, if we return to equation (2), when the load changes, the gas-liquid contact area A may be changed accordingly. This gas-liquid contact area A is a factor in the absorption reaction rate in the packed bed, is influenced by the flow rate of the flowing liquid, and is expressed by the following equation (4).

A=(L/S)〓 ……(4) However, L: Flow rate of liquid flowing down the absorption tower, that is, circulating liquid flowing through line 9 (m ³ /Hr) S: Cross-sectional area α: Determined experimentally A series of numbers from 0.3 to 1.

Therefore, if the gas-liquid contact area A is changed in accordance with the load, there is no need to change the PH as described above, and it becomes easier to follow the load.

However, in the case of a desulfurization device, there are the following problems in making the gas-liquid contact area A correspond continuously to the load.

First, the circulating fluid is a slurry. Therefore, if the solid content contained in the liquid itself or the solid content generated by the reaction within the column adheres to the walls and packing, it is necessary to wash it away with the circulating liquid itself. Therefore, the amount of circulating fluid L cannot be made extremely small, and the minimum amount is
It was found that the load was approximately 1/3 of that at 100% load. This means that since α is smaller than 1, for example, when α=0.7, even if the circulating liquid amount L is reduced to 1/3, the gas-liquid contact area A will not become less than 45%.

Second, since the circulating fluid is slurry and is subject to severe wear, it is impossible to adjust the circulating flow rate with a valve.

For the above reasons, in order to change the gas-liquid contact area A, it is a good idea to change the circulating fluid flow rate L by changing the number of operating circulating fluid pumps.

By providing a large number of pumps and turning them on and off, the gas-liquid contact area A can correspond to a considerable load continuously. However, the price required for the pump and its related components is not proportional to the capacity of the pump, but there is also a fixed price element for each pump, so it is not economical to increase the number of pumps. On the other hand, if the number of pumps is small, changes in the circulating fluid flow rate L will be very discrete.

The present invention was made based on these findings, and aims to provide a control method that is economical and can improve followability to load changes by controlling the number of operating pumps in combination with PH. That is.

In other words, the present invention aims to determine the optimum PH value of the absorption liquid circulating through the absorption tower and the optimum pump for circulating the absorption liquid using a simulation model in accordance with the load amount of exhaust gas flowing into the absorption tower of a wet lime gypsum desulfurization plant. When setting the number of operating pumps and controlling the supply flow rate of absorption liquid and the number of operating pumps based on these set values, first and second simulation models with the same content are provided, and the first simulation model The model is always online and can measure exhaust gas flow rate, inlet SO ₂ concentration, absorption liquid circulation flow rate, and absorption liquid flow rate.
Calculate the desulfurization rate or outlet SO ₂ concentration from the PH, and compare these calculated values with the desulfurization rate or outlet SO ₂ concentration during operation.
Compare the detected concentration value and correct the absorbent neutralization reaction rate constant and the H ₂ SO ₄ oxidation reaction rate constant set in the first and second simulation models based on the deviation, and the second simulation model operates offline as needed;
It is characterized by setting the optimum PH value and the optimum number of operating pumps according to the load amount under expected operating conditions.

The present invention will be explained below with reference to the drawings.

The present invention stores a function as shown in FIG. 3 in a computer, inputs a load amount into this computer, and sets the optimum number of operating pumps and optimum operating PH according to fluctuations in the load amount. Each setting signal is sent to the pump 8 on/off signal and the PH controller 1.
It is output as the PH setting value signal of 2 and controlled. Optimum here means the lowest pH and lowest number of pumps that achieve the target desulfurization rate. Furthermore, M in the figure indicates the minimum required number of pumps. Note that Figure 3 is just an example.
The specific relationship between the load amount, the optimal number of operating pumps, and the optimal operating pH differs from plant to plant depending on the amount of exhaust gas, inlet SO ₂ concentration, various types of impurities contained in the exhaust gas and supply water, etc.

According to this method, the circulation flow rate of the absorption liquid 4 is controlled by changing the number of pumps 8 in operation, so
The PH range becomes narrower, making it easier to follow the load. Also PH
Since the set values are controlled, the number of pumps 8 may be small.

Although the characteristics shown in FIG. 3 are created in advance using the second simulation model, there is no guarantee that the plant will be able to operate according to the predicted characteristics, and the characteristics change day by day for the reasons described below. That is, trace amounts of metals such as halogen and Mn contained in the exhaust gas (that is, present in the boiler fuel) and similar impurities in the raw material neutralizing agent play a catalytic role in various reactions. In addition, CaCO ₃ , which is a neutralizing agent, is supplied to the absorption tower as a solid (powder) mixed with water in the previous process, but the particle size and hardness (i.e., the difference in particle size and hardness after entering the absorption tower) depend on the raw material lot. ease of dissolution).

Therefore, due to these factors, the characteristics of the process change slightly from day to day. Although the range of change in this characteristic is not large, since the purpose of the present invention is to save energy and resources, it must be highly accurate with respect to the target.

The present invention uses the first and second simulation models to perform corrections using the method shown in FIG. The first simulation model 21 is an online real-time simulation model,
The second simulation model 22 is an offline simulation model used when creating the characteristics shown in FIG. First, in the first simulation model 21, the exhaust gas flow rate G, the inlet SO ₂ concentration S ₁ ,
Input the detection signal of the circulation flow rate L of the absorption liquid and the detected pH value PH _D of the absorption liquid. The first simulation model 21 then calculates the desulfurization rate based on these input signals and inputs it to the comparison means 23.

On the other hand, the SO ₂ concentration S _ID at the exhaust gas inlet and the exhaust gas outlet
The SO ₂ concentration S _OD is detected and output to the desulfurization rate calculator 24 . This calculator 24 calculates the desulfurization rate η _D and outputs it to the comparing means 23 .

This comparison means 23 compares the desulfurization rate calculated by the first simulation model 21 and the actually measured desulfurization rate η _D , and outputs the deviation to the reaction constant correction means 25. The correction means 25 corrects the reaction constant included in the simulation model based on this deviation, and feeds back a correction signal to the first simulation model 21. This correction is constantly being carried out to eliminate deviations. The main items to be corrected are the neutralization reaction rate constant and oxidation reaction rate constant, which have a strong influence on the process. The oxidation reaction here is a reaction in which sulfite ions produced by absorption are oxidized to sulfate ions. This reaction occurs in the liquid shown in equation (2).
It reduces H ₂ SO ₃ concentration CL and has a large impact on desulfurization performance. Note that it is also possible to calculate the outlet SO ₂ concentration of the exhaust gas instead of the desulfurization rate and correct the reaction constant based on the deviation between the calculated value and the measured value.

On the other hand, the corrected output from the reaction constant correcting means 25 is
It is also input to the second simulation model 22, and the reaction constants of this simulation model are automatically corrected. When the reaction constants of the second simulation model are corrected, the characteristics shown in FIG. 3 are recalculated to ensure that high-performance operation always meets the target performance.

Therefore, according to the present invention, the desulfurization rate can always be maintained at a predetermined value with the optimum PH and the number of pumps in operation for any desulfurization load, and the amount of neutralizing agent and sulfuric acid used is reduced, thereby saving resources. , energy saving can be achieved. Further, according to this method, the driving power of the pump almost corresponds to the load, and it has remarkable effects such as energy saving.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of the control method of a conventional wet lime gypsum desulfurization plant, Fig. 2 is a characteristic diagram showing the relationship between load amount and set PH value (PH _S ), and Fig. 3 is a diagram illustrating the control method of a conventional wet lime gypsum desulfurization plant. FIG. 4 is a characteristic diagram showing an example of the relationship between the load amount, the optimum number of pumps, and the set PH value in the control method of a lime-gypsum desulfurization plant. FIG. 4 is a block diagram showing the logic for modifying the characteristics shown in FIG. 3. 1... Exhaust gas, 2... Duct, 3... Absorption tower,
4... Absorption liquid, 5... Discharge line, 6... Tank, 7
... Supply line, 8 ... Pump, 9 ... Circulation line, 11 ... PH detector, 12 ... Controller, 13 ...
...Adder, 14...Load detector, 16...Flow rate detector, 17...Flow rate controller, 18...Control valve, 2
1...First simulation model, 22...
2nd simulation model, 23... comparison means, 24... desulfurization rate calculator, 25... reaction constant correction means.

Claims (1)

[Claims] 1. Using a simulation model, set the optimum pH value of the absorption liquid circulating in the absorption tower and the optimum number of operating pumps for absorption liquid circulation, according to the load amount of exhaust gas flowing into the absorption tower of a wet lime gypsum desulfurization plant. When controlling the absorption liquid supply flow rate and the number of operating pumps based on these set values, first and second simulation models with the same content are provided, and the first simulation model is run online. This model calculates the desulfurization rate or outlet SO 2 concentration from the exhaust gas flow rate, inlet SO ₂ concentration, absorption liquid circulation flow rate, and absorption liquid PH, and calculates the desulfurization rate or outlet SO ₂ concentration during operation with these calculated values _. Compare the detected concentration value and correct the absorbent neutralization reaction rate constant and the H ₂ SO ₄ oxidation reaction rate constant set in the first and second simulation models based on the deviation, The second simulation model operates off-line as necessary to set the optimum pH value and the optimum number of pumps in operation according to the load amount under the expected operating conditions. Control method for desulfurization plant.