JP2933664B2 - Absorbent PH control unit for wet flue gas desulfurization unit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a control device for a wet flue gas desulfurization device, and more particularly to a PH control device for an absorbent slurry suitable for reducing the amount of absorbent used. .

[Related Art] In recent years, as the demand for power generation increases, the size of boilers using fossil fuels as main fuels has also increased, and the effect of power generation boilers on air pollution has been increasing.

The emission regulations of SOx, which account for a large proportion of the pollutants that increase this air pollution, tend to become more stringent year after year. Under this circumstance, since the Second Petroleum Shock, Japan's power generation industry, which has been using oil as its main fuel, is switching to coal fuel, which is cheaper and has a sufficient supply.

However, as boilers have become larger, the boiler has to be started and stopped on a daily basis (hereinafter simply referred to as DS) in order to reduce power generation costs and perform frequent load fluctuations in response to power generation demand.
S operation) is repeated.

As a characteristic of recent power demand, the difference between the maximum and minimum power loads has increased with the increase in nuclear power generation, and there has been a tendency to shift the boiler for thermal power generation from base load to load adjustment. The boiler performs a variable-pressure operation by changing the pressure according to the load, so-called a variable-pressure operation boiler that operates in a supercritical pressure region for a full load and a subcritical pressure region for a partial load, thereby generating electricity at a partial load. This is because the efficiency can be improved by several percent.

However, since the DDS operation is frequently performed on a daily basis, the amount of exhaust gas fluctuates due to the load fluctuation.
Since the amount of soluble acid gas and the amount of fly ash vary depending on the type of coal, the SOx value cannot be reduced below the target SOx value during partial load such as 1/4, 1/2, 3/4 load.

For example, a wet-type flue gas desulfurization device installed in a thermal power plant or the like includes calcium carbonate (CaCo ₃ ), calcium hydroxide [Ca
(OH ₂ )] or a slurry using calcium oxide (CaO) or the like as an absorbent. The absorbent slurry is used to absorb sulfur oxides (SOx) in exhaust gas from boilers and the like and oxidize the resulting calcium sulfite. Most commonly, calcium sulfate is recovered as gypsum.

FIG. 5 shows a schematic system diagram of a conventional wet flue gas desulfurization apparatus using this limestone or lime.

FIG. 4 is a control system diagram of the absorbent slurry in the wet fume desulfurization apparatus.

In FIG. 5, exhaust gas from a boiler or the like (not shown) is introduced into a cooling tower 2 from a flue 1, and in the cooling tower 1, gas-liquid contact with an absorbent slurry supplied by a cooling tower circulation pump 3 causes the exhaust gas to be exhausted. The dust contained therein is removed, and a part of SOx is absorbed and removed. The mist eliminator 6 may be installed to remove mist in the gas sent to the absorption tower 7.

In the absorption tower 7, SOx in the exhaust gas is absorbed and removed by gas-liquid contact between the absorbent slurry supplied from the absorption tower circulation pump 10 and the exhaust gas, and the entrained mist is removed by the demister 8, and is treated from the flue 9. Exhausted as gas.

In the absorption tower circulation tank 11 of the absorption tower 7, an absorbent slurry 14 necessary for absorbing SOx in exhaust gas is stored.
The pump is supplied from the absorbent slurry supply line 22 through the pump 15, the absorbent slurry flow control valve 16, and the absorbent tower circulation tank 11 to the cooling tower circulation tank 5.
A part of the absorption liquid slurry in the inside is supplied from the communication pipe 12.

A part of the absorbent slurry containing calcium sulfite generated by absorbing SOx in the cooling tower circulation tank is extracted by the cooling tower bleed pump 4 and recovered as gypsum in an oxidation tower (not shown).

In the figure, 17 is an exhaust gas flow rate detector, 18 is an inlet SOx concentration detector, 19 is an absorbent slurry flow rate detector, 20 is an absorbent slurry PH detector, and 21 is a cooling tower slurry PH detector.

FIG. 4 shows a control system diagram of a conventional absorbent slurry in this wet flue gas desulfurization apparatus.

In FIG. 4, 16 is an absorbent slurry flow control valve, 17 is an exhaust gas flow detector, 18 is an inlet SOx concentration detector, 19 is an absorbent slurry flow detector, 20 is an absorption tower slurry PH detector, and 22 is an absorber. The same agent slurry supply line as that of FIG. 5 is shown.

23 exhaust gas flow rate detection signal, the inlet SO ₂ concentration detection signal 24, 25 is a multiplier, 26 is the total SO ₂ amount signal, 27 is a function generator, 2
8 is a PH setting signal, 29 is a slurry PH detection signal, 30 is a subtractor, 3
1 is a PH deviation signal, 32 is a function generator, 33 is an absorbent slurry excess correction signal for PH deviation, 34 is a function generator, 35 is total SO
Absorbent slurry excess advance signal for _two amount signal 26, 36 an adder, 37 is absorbent slurry excess signal, 38 is a multiplier, 39
Is an absorbent slurry flow rate setting signal, 40 is an absorbent slurry flow rate detection signal, 41 is a subtractor, 42 is an absorbent slurry flow rate deviation signal,
43 is a controller, and 44 is an absorbent slurry flow rate control signal.

In such a structure, the cooling tower 2 of the wet flue gas desulfurization device
The amount of exhaust gas flowing into the detecting respectively the exhaust gas flow rate detector 17 and the SO ₂ concentration at the inlet SO ₂ concentration detector 18, total enter the exhaust gas flow rate detection signal 23 and the inlet SO ₂ concentration detection signal 24 to the multiplier 25 The SO ₂ amount signal 26 is calculated.

In the function generator 27, the PH setting signal 2 for the total SO ₂ amount signal 26
8 and a deviation from the actual slurry PH detection signal 29 of the absorption tower slurry PH detector 20 is obtained by a subtractor 30.
Generator for the excess slurry ratio correction signal 33 corresponding to the
Ask by 32.

On the other hand, in the function generator 34, an absorbent excess ratio leading signal 35 is provided in correspondence with the total SO ₂ amount signal 26, and the above-mentioned absorbent slurry correction signal 33 is added thereto by an adder 36. Adder
Since the output signal of 36 is the absorbent slurry excess ratio signal 37,
A multiplier 38 multiplies the total SO ₂ amount signal 26 to obtain an absorbent slurry flow rate setting signal 39. The difference between the absorbent slurry flow rate setting signal 39 and the actual absorbent slurry flow rate detection signal 40 from the absorbent slurry flow rate detector 19 is calculated by the subtractor 41 as the absorbent slurry flow rate deviation signal 42, The slurry flow rate of the absorbent slurry supply line 22 is adjusted by opening and closing the opening degree of the absorbent slurry flow control valve 16 through the controller 43 in accordance with the absorbent slurry flow control signal 44.

However, such slurry flow control does not take into consideration that good control characteristics cannot be obtained in a special operation state such as DDS operation.

That is, the oxidation state of the absorbent, the absorption of SO _2, Yotsute the size relationship between the amount of oxidized sulfite to produce the total amount oxidized state (SO ₂ absorption <oxidation amount) and the partial oxidation states (SO ₂ absorption Amount> oxidation amount).

 Hereinafter, the oxidation state will be described with reference to FIG.

In FIG. 6, a solid line A indicates a fully oxidized state, a dashed line B indicates a partially oxidized state, and a broken line C indicates a quasi-fully oxidized state.

In general, the state is a partially oxidized state B at a high load and a fully oxidized state A at a low load.

However, when the operating condition shifts from a low load to a high load in DDS operation or the like, for example, the oxidation state shifts from the full oxidation state A to the partial oxidation state B. Occur.

In the quasi-total oxidation state C, there is no solid sulfite generated by absorbing SO ₂ in the absorbent,
When the sulfite ion becomes difficult to crystallize, the sulfite ion in the absorbent becomes supersaturated, and the partial pressure of SO ₂ in the absorbent increases, so that the desulfurization performance decreases.

In other words, in the quasi-total oxidation state C, the reason why no sulfite is present is that in the quasi-total oxidation state C, the total oxidation state A
From such as absorption of SO ₂ was increased (load increase, etc.)
Occurs transiently.

In the total amount oxidized state ^{_{A, SO 3 2+ + 1 /}} 2O 2 → SO 4 2- sulfites by oxidation reaction of (CaSO _3), all sulfate (CaSO ₄₎
Exists in the form of

From this state, if the amount of SO ₂ absorbed increases, the above oxidation reaction does not proceed, and sulfite ions (SO ₃ ^2- ) are present in the absorbent in a supersaturated state. ²⁺
+ SO ₃ ^2- → CaSO ₃ ) is relatively slow, so sulfite (CaS
O ₃ ) is a quasi-total oxidation state C in which it does not exist. The reason why sulfite ions are difficult to crystallize is that the crystallization reaction (Ca ²⁺
+ SO ₃ ^2- → CaSO ₃ ) requires sulphite nuclei (solids) in order to proceed, but in the quasi-total oxidation state C, there is no solids, which makes crystallization difficult. In addition, in the total oxidation state A, since sulphite ions hardly exist, crystallization of sulphite ions does not pose a problem.

On the other hand, the reason for the increase in the partial pressure of SO ₂ in the absorbing solution is that if the crystallization reaction does not proceed, the sulfite ions in the absorbing solution will be in a supersaturated state and the concentration will increase. P _SO2 = k [SO ₃ ^2- ] (P _SO2 : partial pressure of SO ₂ in the absorbing solution, k: constant) P _SO2 increases in proportion to the sulfite ion concentration ([SO ₃ ^2- ]) from the above equation. Because you do.

Also, supersaturated sulfite ions precipitate on the surface of the absorbent, slowing down the dissolution reaction of the absorbent, and a large amount of absorbent must be supplied in order to maintain the PH of the absorbent that can maintain the necessary outflow performance. Required.

For example, the excess ratio of the absorbent in the partially oxidized state B under the high load and the fully oxidized state A under the low load is at most 5%, but during the transition from the fully oxidized state A to the partially oxidized state B, that is, the quasi-total amount. In the oxidized state C, the excess ratio of the absorbent is 200%
(The amount of the absorbent in the wet flue gas desulfurization unit is tripled), which is not preferable.

As described above, in the conventional PH control apparatus, no consideration is given to the supply of the absorbent in the quasi-total oxidation state C.

[Problems to be Solved by the Invention] The conventional control device has a drawback that the oxidation state of the absorbent shifts to the quasi-total oxidation state, the desulfurization performance decreases, and the target desulfurization rate cannot be secured. There is a drawback that stable desulfurization performance cannot be ensured in the fully oxidized state.

The present invention is intended to solve such a conventional drawback, and an object of the present invention is to provide an absorbent which can secure a target desulfurization rate even in a special operation state, that is, a quasi-total oxidation state. It is to propose a PH control device.

Means for Solving the Problems In order to achieve the above object, the present invention provides an alkaline agent supply line having an alkaline agent flow detector and an alkaline agent flow control valve in a circulation tank, and an absorbent extraction flow detector. and absorbent withdrawal line having the absorption tower outlet SO ₂ concentration detector, the exhaust gas flow detector, inlet SO ₂ concentration detector, PH detector,
Absorbent slurry flow detector, the outlet SO ₂ concentration detector extracts the flow rate by the detection signal from the detector for calculating a connexion absorbent activity prediction signal provided absorbent activity calculator, absorbent absorbent from the active calculator Based on the comparison result between the alkali agent demand signal and the absorbent demand signal based on the activity prediction signal and the alkali agent flow detection signal from the alkali agent flow detector and the absorbent slurry flow detection signal from the absorbent slurry flow detector At least one of the alkali agent flow control valve and the absorbent slurry flow control valve is opened and closed.

[Operation] Since the activity of the absorbent changes depending on the oxidation state of the sulfite in the absorbent, the absorbent activity calculator operates so that the activity of the absorbent can be predicted from the online detection signal. Thereby, the ratio between the amount of the absorbent slurry and the amount of the alkaline agent can be adjusted. In other words, when the activity of the absorbent decreases, instead of adding a large amount of the absorbent, the PH of the absorbent is adjusted by adding an alkaline agent, so that a special operation such as a quasi-total oxidation state is performed. In the state,
The PH can be maintained at the set value and the alkaline agent is consumed, but the amount of the absorbent used can be greatly reduced.

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

FIG. 1 is an absorbent PH control system diagram according to an embodiment of the present invention,
FIG. 2 is a schematic configuration diagram of FIG. 1, and FIG. 3 is a characteristic curve diagram of distribution coefficient and absorbent activity.

In FIGS. 1 and 2, reference numerals 1 to 44 indicate the same components as those of the prior art.

In FIGS. 1 and 2, 45 is an outlet SO ₂ concentration detector, 46 is an outlet SO ₂ concentration detection signal, 47 is an extraction flow rate detector, 48 is an extraction flow rate detection signal, 49 is an alkali flow rate detector, and 50 is an alkali flow rate detector. Alkali flow rate detection signal, 51 is an absorbent activity calculator, 52 is an absorbent activity prediction signal, 53 is a function generator, 54 is a distribution coefficient signal, 55 is a PH correction signal generator, and 56 and 57 are alkaline agent side PH correction Signal, absorbent side PH correction signal, 58 is a multiplier, 59
Is an excess agent correction signal, 60 is a function generator, 61 is a PH deviation correction signal, 62 is a multiplier, 63 is an alkali agent demand signal,
64 is a subtractor, 65 is an alkaline agent flow deviation signal, 66 is an alkaline agent controller, 67 is an alkaline agent supply line, 68 is an alkaline agent flow control valve, 69 is an absorbent slurry demand signal, and 70 is an alkaline agent flow control signal. , 71 are extraction lines.

Before describing the control system diagram of FIG. 1 in such a structure, the absorbent activity calculator 51 will be described.

In the absorbent activity calculator 51, the exhaust gas flow detection signal 23 from the exhaust gas flow detector 17 and the inlet S from the inlet SO ₂ concentration detector 18
O ₂ density detection signal 24, PH detector PH detection signal 29 from 20, the absorbent slurry flow detector 19 absorbent slurry flow rate detection signal 40 from the outlet SO ₂ concentration detection signal from the outlet SO ₂ concentration detector 45
The following calculation is performed from the extraction flow rate detection signal 48 from the extraction flow rate detector 46 and the extraction flow rate detector 47.

Absorbent concentration balance in absorbent Where: V: annular tank volume (), x: absorbent concentration (mo
l /), t: time (h), y: absorbent slurry concentration (-),
Y _P: absorber Purity (-), M _a: absorbent Molecular weight (kg / mol), G
_Sl : Absorbent slurry flow rate (kg / h), R _SO2 : SO ₂ absorption amount (mol /
h), G _d : Extraction flow rate (kg / h), γ _a : Absorbent specific weight (kg
/), _{G g:} exhaust gas flow rate (/ h), x _H2O: exhaust gas moisture (-), C ⁱ _SO2: inlet SO ₂ concentration (ppm), η: desulfurization rate (-), C ^O _SO2: outlet SO ₂ Indicates the concentration (ppm).

Therefore, the above equations (1) to (3) are converted to the exhaust gas flow rate detection signal.
23, inlet SO ₂ concentration detection signal 24, the slurry PH detection signal 29, absorbent slurry flow rate detection signal 40, the outlet SO ₂ concentration detection signal 46,
By calculating using the extraction flow rate detection signal 48,
A predicted value x of the absorbent concentration is determined.

On the other hand, since the amount of the removed SO ₂ is equal to the consumption of the absorbent, R _SO2 = V · γ _a (4) γ _a = k [H ⁺ ] ^α [x) ^γ / [Ca ²⁺ ] ^β ( 5) Here, γ _a : absorbent consumption (mol / h), k: activity of the absorbent, [H ⁺ ]: hydrogen ion concentration (mol /), [C
a ²⁺ ]: Total calcium ion concentration (mol /
), Α, β, γ: indicate constants.

Solving the equations (4) and (5) using the absorbent concentration x obtained from the equations (1) to (3) gives: Therefore, the activity k of the absorbent can be predicted from the above equation.
[Ca ²⁺ ] can be easily estimated from PH and Cl ⁻ concentrations.

In this way, the activity of the absorbent can be predicted by the absorbent activity calculator 51.

Absorbent activity prediction signal which is the output of the absorbent activity calculator 51
According to 52, a function generator 53 obtains a distribution coefficient signal 54 from the relationship shown in FIG. The distribution coefficient signal 54 is input to a PH correction signal generator 55 to obtain an alkaline agent side PH correction signal 56 and an absorbent slurry side PH correction signal 57.

That is, the alkaline agent-side PH correction signal 56 and the absorbent slurry-side PH correction signal 57 are signals corresponding to the distribution ratio of the absorbent and the alkaline agent with respect to the PH feedback correction, and when the activity of the absorbent decreases, The PH feedback correction controls mainly the alkaline agent.

On the other hand, the PH deviation signal 31 is output by the subtractor 30 of the absorbent slurry control system.
Is calculated in the same manner as in the prior art, but in the embodiment of the present invention, the PH deviation signal 31 is sent to the function generator 60, while the absorbent calculated by the function generator 32 of the absorbent slurry control system is used. The excess slurry ratio correction signal 33 is input by the multiplier 58.

Then, in the multiplier 58, the absorbent slurry excess ratio correction signal 33
Is multiplied by the absorbent-side PH correction signal 57 described above to obtain an absorbent excess ratio correction signal 59, which is input to the adder.

The adder 36 adds the excess absorbent rate signal 35 and the excess absorbent rate correction signal 59 to obtain an overall excess absorbent rate signal 37,
The product is multiplied by the SO ₂ amount signal 26 in the multiplier 38 to obtain an absorbent slurry demand signal 69. The absorbent demand signal 69 and the absorbent slurry detection signal from the absorbent slurry flow detector 19
The controller measures the absorbent slurry flow rate deviation signal 42, which is the deviation from 40.
At 43, the absorbent slurry flow rate control valve 16 of the absorbent slurry supply line 22 is opened and closed by an absorbent slurry flow control signal 44 to control the absorbent slurry flow rate of the absorbent slurry supply line 22.

On the other hand, for the alkaline agent, the PH deviation signal 31 is processed as a PH deviation correction signal 61 by a function generator 60, and the PH deviation correction signal 61 is multiplied by the alkaline agent-side PH correction signal 56 by a multiplier 62 to produce an alkaline agent. A demand signal 63 and an alkali agent flow deviation signal 65, which is a deviation from the alkali agent flow detection signal 50 from the alkali agent flow detector 49, are processed as an alkali agent flow control signal 70 by a controller 66, and the alkali agent supply line The alkali agent flow control valve 68 is opened and closed to control the alkali agent flow rate.

As described above, the PH control device according to the present invention uses the absorbent activity calculator 51 to predict the activity of the absorbent online, and according to this activity, the absorbent slurry flow rate of the absorbent slurry supply line 22 The supply ratio between the absorbent slurry and the alkaline agent is controlled by opening and closing the regulating valve 16 and the alkaline agent flow regulating valve 68 of the alkaline agent supply line 67, and the PH of the absorbent is controlled to a set value. is there. Basically, an alkaline agent is used for correcting the pH deviation only when the activity of the absorbent decreases, and the amount of the PH deviation corrected is cut for the absorbent.

Since the present invention is configured as described above, the PH of the absorbent can be maintained at the set value by the alkaline agent even under special operation conditions such as a quasi-total oxidation state, and the consumption of the absorbent can be reduced. .

[Effects of the Invention] According to the present invention, when the activity of the absorbent is extremely reduced as in the quasi-total oxidation state, the PH of the absorbent can be quickly recovered by supplying the alkaline agent. Therefore, stable desulfurization performance can be maintained.

In addition, when the activity of the absorbent decreases, it is possible to prevent the introduction of a large amount of the absorbent, so that the consumption of the absorbent can be reduced, which is more effective than the consumption of the alkali agent.

[Brief description of the drawings]

FIG. 1 is an absorbent PH control system diagram according to an embodiment of the present invention, FIG. 2 is a schematic configuration diagram of FIG. 1, FIG.
A characteristic curve showing the activity of the absorbent is shown on the horizontal axis, and FIG.
FIG. 5 is a schematic diagram of the PH control system, FIG. 5 is a schematic diagram of FIG. 4, and FIG. 6 is a characteristic curve diagram showing the desulfurization rate on the vertical axis and the PH of the absorbent on the horizontal axis. 7 ...... absorption tower, 11 ...... circulation tank, 16 ...... absorbent slurry flow adjustment valve, 17 ...... exhaust gas flow rate detector, 18 ...... inlet SO ₂
Concentration detector, 19: Absorbent slurry flow rate detector, 20: PH
Detector, 22 ...... absorbent slurry supply line, 23 ...... exhaust gas flow rate detection signal, 24 ...... inlet SO ₂ concentration detection signal, 29 ...... slurry PH detection signal, 39 ...... absorbent slurry flow rate setting signal, 40
…… absorbent slurry flow rate detection signal, 45… outlet SO ₂ concentration detector, 47 …… extraction flow rate detector, 49 …… alkali agent flow rate detector, 51 …… absorbent activity calculator, 52 …… absorbent Activity prediction signal, 63 ... Alkaline agent demand signal, 67 ... Alkaline agent supply line, 68 ... Alkaline agent flow control valve, 69 ...
… Absorbent slurry demand signal, 71 …… withdrawal line.

Claims (1)

(57) [Claims] 1. An absorption tower for absorbing sulfur oxides in exhaust gas with an absorbent slurry, a circulation tank for storing the absorbent slurry, and an absorbent slurry supply line having an absorbent slurry adjusting valve to the circulation tank are provided. , the absorption tower inlet gas flow rate detector, inlet SO ₂ concentration detector, the absorbent slurry flow rate setting signal by the detection signal from the PH detector circulation tank, absorbent slurry supply line from the absorbent slurry flow detector An apparatus for controlling the PH of an absorbent by opening and closing an absorbent slurry flow control valve based on a deviation from an absorbent slurry flow detection signal. An alkaline agent flow detector and an alkaline agent flow control valve are provided in the circulation tank. an alkali agent supply line having an absorption agent withdrawal line having a withdrawal flow rate detector of the absorbent, and an outlet SO ₂ concentration detector to the absorption tower outlet, the exhaust gas flow detector, inlet SO ₂ concentration detection , PH detector, absorbent slurry flow detector, the outlet SO ₂ concentration detector, the withdrawn flow detector absorbent activity calculator for calculating an O connexion absorbent activity prediction signal to the detection signal from the provided, absorbent activity calculation Of the alkaline agent demand signal and the absorbent demand signal based on the absorbent activity prediction signal from the detector, the alkaline agent flow detection signal from the alkaline agent flow detector, and the absorbent slurry flow detection signal from the absorbent slurry flow detector An absorbent PH control device for a wet-type flue gas desulfurization device, wherein at least one of an alkali agent flow control valve and an absorbent slurry flow control valve is opened and closed based on a comparison result.