JPH0418884B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention provides SO ₂
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 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 is transferred from the bottom of the tower to tank 6.
flows down to. 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 supplied to the absorption tube 3 through a circulation line 9 by a pump 8. In addition, some of the circulating fluid is taken out,
In the post-process, CaSO ₃ is converted to CaSO ₄ 2H ₂ O (gypsum)
oxidized to

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 load of a discharge plant (hereinafter referred to as desulfurization load)).
is detected and input 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. Further, the flow rate of the supply line 7 is detected by a flow rate detector 16 and inputted to a flow rate controller 17 .
The flow rate controller 17 controls the regulating valve 18 based on these signals.

That is, in the conventional control method, the output from the controller 12 has a feedback effect, and the load detector 1
The output from 4 has a feed forward effect, and is intended to supply a neutralizing agent equivalent to the amount of SO ₂ absorbed, and if a neutralizing agent equivalent to the amount of SO ₂ entering the system is supplied, the desulfurization rate will always be the same. It is based on the idea that SO ₂ can be absorbed by 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. Secondly, since the transfer system number K is related to PH, it is necessary to neutralize it so that the H ₂ SO ₃ concentration does not become high and the PH does not become 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 _I −CG _O /CG _I …(3) However, CG _I : SO ₂ concentration in the plant inlet gas CG _O : SO ₂ concentration in the plant outlet gas On the other hand, PH increases the H ₂ SO ₃ concentration. Then it goes down,
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 CaCG concentration in the tank 6 is high, the CaCG is discharged from the system through the line 19 at that concentration. Therefore, if the reaction rate γ at low load is small and a large amount of unreacted CaCO ₃ is retained, the raw material (neutralizing agent)
must be supplied in large quantities. In addition, ₃ minutes of CaCO is not used for neutralization reaction in absorption tower 3 and is transferred to line 19.
If it is discharged from the system, it must be neutralized with sulfuric acid in a subsequent process.

As mentioned above, in these methods, unused CaSO ₃
Not only does this increase, but the amount of sulfuric acid used to neutralize it also 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 way to make the PH setting 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 FIG. 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 (for example,
4.7 to 5.7), the CaCO ₃ concentration must be changed approximately 10 times. 25% load
When changing from 100% to 100% at an average rate of 5%/min, the time during this time 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 formula.

A=(L/S)〓 Where L: Flow rate of the liquid flowing down the absorption tower, that is, the circulating liquid flowing through line 9 (m ³ /
Hr) S: Cross-sectional area α: An experimentally determined coefficient with a number between 0.3 and 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 is because α is smaller than 1, so for example α=0.7
This means that even if the circulating liquid volume L is reduced to 1/3, the gas-liquid contact area A will not become less than 45%.

Since the second 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.

That is, the present invention provides a method for desulfurizing the exhaust gas flowing into the absorption tower of a wet lime gypsum desulfurization plant by bringing it into contact with the absorption liquid circulating through the absorption tower using a plurality of pumps. The optimum PH value and the optimum number of operating pumps for the absorption liquid are set respectively, and the supply flow rate of the absorption liquid and the number of operating pumps are controlled based on these set values, and the amount of change in the exhaust gas load and the desulfurization rate are detected, and the exhaust gas load is It is characterized in that when the amount is constant or increasing and the desulfurization rate is lower than the target desulfurization rate, the number of operating pumps is increased.

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

In the present invention, a function as shown in FIG. 3 is stored in a storage device 21 of a computer shown in FIG. Set the number of units N and the optimum operation PH (PH _S ), and use the respective setting signals as the on/off signal of the pump 8 and the PH of the PH controller 12.
Control by outputting as a set value signal. 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 an example, and the specific relationship between the load amount, the optimal number of operating pumps, and the optimal operating PH will vary depending on the amount of exhaust gas, inlet SO ₂ concentration, various impurities contained in the exhaust gas and supply water, etc. Different for each. Furthermore, although the graph shown in FIG. 3 is created in advance during plant design, there are slight deviations when operation begins, so adjustments are made using test run data. In addition, Fig. 3 shows the guaranteed desulfurization rate η _G of 90%, for example.
Then, the PH set value PH _S and the number of pumps N to obtain 92%, which is several percent higher than this, are shown.

According to this method, the circulation flow rate of the absorption liquid 4 is controlled by changing the number of operating pumps 8, so the range of pH to be varied is narrowed, and load tracking becomes easier. or
Since the PH set value is controlled, the number of pumps 8 may be small.

By the way, FIG. 3 shows the optimum characteristics in a static state with no or gentle load fluctuation. For this reason, there is a risk that the target desulfurization rate may be lowered in a transient state during a sudden load change.

Therefore, in the present invention, the desulfurization rate in the transient state is guaranteed based on the control logic shown in FIG.

This control logic is based on the SO ₂ concentration in the inlet flue gas.
S _I and the SO ₂ concentration S _O in the outlet exhaust gas are input to the desulfurization rate calculating means 22 to calculate the desulfurization rate η. The desulfurization rate η obtained here is entered into the comparison means 23 and the set desulfurization rate η _I
compared to Here, the set desulfurization rate η is a target lower limit value that increases the pump power when the guaranteed desulfurization rate η _G is lower than the guaranteed desulfurization rate η G. For example, if the guaranteed desulfurization rate η _G is 90%, the set desulfurization rate η is 91%. This comparison means 23
If η<η _I , outputs a pump increase request signal, that is, a signal nP ₂ for adding one pump to another comparing means 24.

On the other hand, the load signal is input to the differentiating means 25 to calculate the rate of change in the load. This calculated value is input to determination means 26 consisting of a filter and a comparator. The determining means 26 determines whether or not there is a significant long-cycle load change, and whether the load is increasing (or in a steady state).
Alternatively, it is determined whether the load is falling and is output to the comparison means 24. When the load changes significantly over a long period and the load is increasing (or in a steady state), the pump increase signal _nP2 is considered to be valid. Note that if the load cycle is high frequency (for example, 5
If the fluctuation occurs at a frequency of less than 1 minute, the pump does not need to follow the high frequency fluctuation because it can be followed by the capacity of the plant (that is, the unreacted neutralizing agent accumulated in the tank).

Furthermore, the number n of pumps currently in operation and the storage device 21
Comparing means 2 with the optimal number of operating pumps N obtained in
Pump increase signal nP ₁ corresponding to the load compared with 7
is output to the comparison means 24 or a pump reduction signal.
n Outputs _n . The comparator 24 is an element having logical product logic, and outputs a signal nP in which the pump increases with either signal nP ₁ or nP ₂ when the load change is increasing or steady.

Therefore, according to the present invention, even if the desulfurization load suddenly changes, the desulfurization rate can always be maintained at a predetermined value, and the amount of neutralizing agent and sulfuric acid used can be reduced, thereby saving resources and energy. 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 drawings]

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 a control method for a lime-gypsum desulfurization plant, and FIG. 4 is a block diagram showing the control logic of the same control method. 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... Adjustment meter, 13... adder, 14... load detector, 15... flow rate controller, 16... flow rate detector, 1
7...Flow rate controller, 18...Control valve, 21...Storage device, 22...Desulfurization rate calculation means, 23...Comparison means, 2
4... Comparison means, 25... Differentiation means, 26... Judgment means, 27... Comparison means.

Claims (1)

[Claims] 1. When desulfurizing by bringing the flue gas flowing into the absorption tower of a wet lime gypsum desulfurization plant into contact with the absorption liquid that is circulated through the absorption tower using multiple pumps, the optimum absorption liquid is selected according to the exhaust gas load. The PH value and the optimal number of operating pumps are set respectively, and the supply flow rate of absorption liquid and the number of operating pumps are controlled based on these set values, and the amount of change in the exhaust gas load and the desulfurization rate are detected, and the amount of exhaust gas load is constant or A method for controlling a wet lime-gypsum desulfurization plant, characterized by increasing the number of pumps in operation when the desulfurization rate is lower than a target desulfurization rate while the desulfurization rate is rising.