JP2635798B2 - Waste heat recovery boiler feed water treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a method for treating water supply of an exhaust heat recovery boiler in a combined cycle power plant, and in particular, relates to a method for measuring the amount of hydrazine injected into feedwater for a plant. The present invention relates to a water supply processing method for an exhaust heat recovery boiler, which is changed in accordance with an operation condition so as to keep a hydrazine concentration at a boiler inlet constant.

(Prior art) In general, in this type of waste heat recovery boiler, limit values for dissolved oxygen and PH in feed water are set, and water treatment is performed so as to satisfy the limits to prevent system materials from being attacked by corrosion. doing. Of these, in order to keep the dissolved enzyme concentration below the limit value, hydrazine as a deoxidizer is injected into the feedwater at the inlet of the low pressure economizer of the waste heat recovery boiler and reacted with dissolved oxygen dissolved in the feedwater. , With the aim of changing to ammonia water.

Hereinafter, this water treatment method will be described in more detail with reference to FIG. 6. Water is supplied from the condenser 1 to the waste heat recovery boiler 3 by the low-pressure water supply pump 2, and first flows through the low-pressure economizer 4. Heated. Hydrazine is this low pressure economizer 4
Into the feed water at the inlet of the reactor, and reacts with the dissolved oxygen as the feed water is heated in the low-pressure economizer 4. At this time, the concentration of hydrazine injected is kept within a range of 20 ppb to 40 ppb. About 10% of the injected hydrazine is contained in the low-pressure steam generated in the evaporator 5 and is carried along with the steam.
The water flows through the high-pressure water saving pump 8 together with the water extracted through the high-pressure water supply pump 7 and flows to the high-pressure evaporator 9. When the feedwater temperature rises to about 260 ° C in the high-pressure economizer 8, the 60
% Is thermally decomposed into ammonia and water. The remaining hydrazine flows through the high-pressure steam drum 10 to the high-pressure superheater 11 without being decomposed, and at an outlet temperature of 520 ° C., 100% is thermally decomposed to ammonia. In the drawings, reference numerals 12 and 13 indicate a low-pressure circulation pump and a high-pressure circulation pump. Reference numeral 15 denotes a gas turbine, 16 denotes a steam turbine, and 17 denotes a generator.

(Problems to be Solved by the Invention) By the way, during the treatment of water supply with hydrazine described above,
Only a small amount of the injected hydrazine returns to the condenser 1 together with the steam of the cycle, and when calculating the injection amount, it is necessary to determine the amount assuming that all the hydrazine is consumed by pyrolysis. General. Since hydrazine reacts equimolarly with dissolved oxygen in the feedwater, a condenser 1 is required to make the hydrazine concentration in the feedwater the same as the dissolved oxygen concentration or to such an extent that unreacted hydrazine remains.
Assuming the value of the dissolved oxygen concentration of the above, hydrazine corresponding to this value may be injected. However, in the combined cycle power plant, the steam temperature and the pressure greatly vary depending on the load, and the hydrazine may return to the condenser 1 without being decomposed. That is, as shown in FIG. 7, when hydrazine reaches 200 ° C. or higher, thermal decomposition starts, and
At 100C, 100% of it is pyrolyzed. However, when the load on the plant is reduced to, for example, 25%, the gas turbine 15
Since the temperature of the exhaust gas decreases and the main steam temperature becomes 300 ° C. or less even at the outlet of the high-pressure superheater 11, a large amount of hydrazine is returned to the condenser 1 without being decomposed. When the ratio of hydrazine returned to the condenser 1 when the plant load fluctuates is examined, it is inversely proportional to the plant load as shown in FIG. 8, and the amount of hydrazine returned to the condenser 1 decreases as the load decreases. Increase. This is because hydrazine is injected in proportion to the feedwater flow rate irrespective of the steam temperature or the exhaust gas temperature or the load. As shown in FIG. The hydrazine concentration exceeds the limit. For this reason, excessive (undecomposed) hydrazine is thermally decomposed at the start of the plant, and a large amount of ammonia is generated. Therefore, even if the pH is not adjusted by injecting ammonia, the ammonia concentration in the steam is limited. In some cases, the cooling pipe made of aluminum brass of the condenser 1 may be eroded.

Accordingly, it is an object of the present invention to provide a method for supplying water to a waste heat recovery boiler, which can keep the concentration of hydrazine within a limit value even when the load on the plant fluctuates.

[Structure of the Invention] (Means for Solving the Problems) The water supply treatment method for an exhaust heat recovery boiler according to the present invention employs hydrazine for maintaining a limit value of a dissolved oxygen concentration in feedwater sent to an exhaust heat recovery boiler. Of waste heat recovery boiler in which water is injected into the feed water while adjusting the concentration using a plunger-type pump, the waste heat corresponding to the load signal of the generator or the exhaust gas temperature of the gas turbine or the main steam temperature The corrected feedwater flow rate is determined by multiplying the hydrazine thermal decomposition rate in the recovery boiler by the actually measured feedwater flow rate, and controlling the stroke and rotation speed of the plunger pump in proportion to the corrected feedwater flow rate signal to reduce the hydrazine concentration. It is characterized in that it is adjusted.

(Operation) According to the present invention, when the load on the plant is low and the temperature of the main steam is reduced with the decrease of the exhaust gas temperature and undecomposed hydrazine returns to the condenser, the feedwater flow rate is corrected to reduce the actual amount of feedwater. The flow and the number of rotations of the plunger type pump are controlled assuming that the water is flowing, so that the hydrazine concentration in the feedwater flowing to the inlet of the low-pressure economizer is kept constant.

(Embodiment) An embodiment of the present invention will be described below with reference to FIG. 1 and FIG. In FIG. 1, the same components as those in FIG. 6 explained by the prior art are denoted by the same reference numerals, and the description is omitted.

In FIG. 1, a low-pressure feed pump 2 is connected to a low-pressure economizer 4.
Is detected by a flow detector 21 and guided to a correction flow generator 22 as a feedwater flow signal. Further, the output of the generator 17 is detected by the load detector 23, and is supplied to the corrected flow generator 22 as a load signal. The correction flow generator 22 performs a correction, which will be described in detail later, based on the load signal.The output of the correction flow generator 22 is a pump rotation speed control function generator 24 and a stroke control signal generator 25 described later provided in the hydrazine injection path. Respectively. The rotation speed signal output from the rotation speed control function generator 24 is an arithmetic unit.
The stroke signal from the stroke control signal generator 25 passes through the arithmetic unit 28 and the amplifier 29.
, And is output to the pump 30.

As the pump 30, a plunger type pump suitable for injecting a small amount of a chemical solution is used. A representative of this type has the features shown in FIG. That is, the pump body
A small amount of chemical solution can be injected by adjusting both the increase and decrease of the rotation speed of the piston 32 and the stroke of the piston 32 in the inside of the piston 31. At this time, the stroke is the amount of eccentricity of the eccentric cam 34 with the plunger 33 attached. This is done by adjusting

 Next, the operation of the above configuration will be described.

When the plant is operated at a low load, the steam temperature generated in the exhaust heat recovery boiler 3 is low, and the steam after passing through the high-pressure superheater 11 contains undecomposed hydrazine. For this reason, the load signal detected by the load detector 23 is input to the correction flow rate generator 22, and the correction flow rate signal for the feed water flow rate signal of the flow rate detector 21 is calculated in order to determine the required amount of hydracidin to be injected there. Generated as the corrected feedwater flow rate.

The correction amount for calculating the correction flow rate signal varies depending on the configuration of the plant, but is determined based on a relation curve as shown in FIG. 3 of the correction value for the load determined in advance. For example, when the load is 25%, the correction value is
It is set to 0.6, and a value smaller than 1.0 is output. The relationship between the load and the correction value in FIG. 3 represents the relationship between the thermal decomposition rate of hydracidin and the load, and can be obtained from FIG. 8 showing the result of examining the undecomposed ratio of hydracidin to the load. 0.6 means that 40% of the injection amount of hydracidin returns to the condenser 1 without decomposition.

Here, assuming that the feed water flow rate Q (ton / h), the correction value t, the concentration of hydrazine injected into the feed water d ₁ (ppb), and the chemical solution concentration D, the injection amount of undecomposed hydrazine h ₁ (ml / min) )
It is expressed by the following equation.

h ₁ = Q · (1-t) × 10 ⁶ × (1/60) × d ₁ × 10 ^-9 × (1 /
D) …… (1) For example, when the feedwater flow rate at 25% load (correction value 0.6) is 80 tons per hour, the concentration of hydrazine injected into the feedwater is 30 ppb, and the chemical solution concentration is 1% (D = 0.01). Then
From the formula (1), the injection amount of undecomposed hydrazine, h ₁ (ml / min), is given by h ₁ = 80 × (1-0.6) × 10 ⁶ × (1/60) × 30 × 10 ^-9 × 10 ²
= 1.6 (ml / min).

On the other hand, the hydrazine concentration at the inlet of the low-pressure economizer 4 is set to d ₂ (pp
The injection amount h ₂ (ml / min) of the hydrazine having the chemical solution concentration D required for b) is h ₂ = Q · t × 10 ⁶ × (1/60) × d ₂ × 10 ^-9 × (1 / D)… (2) Therefore, under the same conditions as described above, the injection amount h ₂ (ml / min) of hydrazine required to set the target hydrazine concentration d ₂ to 30 ppb is ( From the expression 2), h ₂ = 80 × 0.6 × 10 ⁶ × (1/60) × 30 × 10 ^-9 × 10 ² = 2.4 (ml
/ min).

Since the value h ₁ obtained by converting the injection quantity of undecomposed hydrazine to Return to the condenser 1 as previously described is 1.6, hydrazine amount h ₃ are actually remaining in the water supply, the following formula h ₃ = (h ₁ + h ₂ ) × 60 × 10 ⁻⁶ × D × (1 / Q) (3) Therefore, h ₃ = (1.6 + 2.4) × 60 × 10 ⁻⁶ × 10 ^-2 × (1/80) = 30 (ppb).

As apparent from the above description, (2) by determining the injection amount h ₂ of hydrazine according to formula, the concentration of hydrazine in the feed water can keep a constant value.

In the equation (2), Q · t can be used as a corrected feedwater flow rate Q ′, which is obtained by multiplying the feedwater flow rate given from the flow rate detector 21 by a correction value. Then, a rotation speed signal and a stroke control signal are generated based on the corrected water supply flow rate.

Further, when the load rises and all the injected hydrazine undergoes thermal decomposition, the correction value becomes 1.0 as shown in FIG. 3, and the injection amount increases with the water supply amount. The load becomes 100% and the water supply flow rate is 1 per hour
When it becomes 80 tons, required injection amount of the chemical concentration of 1% hydrazine h ₄ (ml / min), the (2) from equation _{h 4 = 180 × 1.0 × 10} 6 × (1/60) × 30 × 10 - ⁹ × 10 ² = 9.0 (m
l / min).

When the injection amount increases, the pump stroke is maintained at the maximum stroke, and the rotation speed (the number of stroke reciprocations) is adjusted to adjust the injection amount of hydrazine.

Thus, the load on the plant is low, and the concentration of undecomposed hydrazine in the steam after passing through the high-pressure superheater 11 is high. When this concentration returns to the condenser 1, the feedwater flow rate is corrected by the correction flow rate generator 22 so that By controlling the stroke and the rotation speed of the injection pump 30 that injects hydrazine assuming that a small amount of water is flowing, the hydrazine concentration at the inlet of the low-pressure economizer 4 can be kept constant.

FIG. 4 and FIG. 5 show another embodiment of the present invention. These embodiments show another method of correcting the feed water flow rate. FIG. 4 shows a case where the temperature of the exhaust gas of the gas turbine 15 is detected by the temperature detector 35 and the corrected flow rate generator 22 is used.
In the same manner as the correction by the load signal, a correction value for the exhaust gas temperature is obtained, and the feedwater flow rate is corrected by multiplying the correction value for the feedwater flow rate signal of the flow rate detector 21. The injection volume is controlled. FIG. 5 shows an example in which the high-pressure main steam temperature is detected by the temperature detector 36 and input to the correction flow generator 22, and a correction value for the high-pressure main steam temperature is obtained to correct the feedwater flow rate. The relationship of the correction value to the exhaust gas temperature and the relationship of the correction value to the high-pressure main steam temperature almost correspond to the relationship of the hydrazine decomposition rate to the temperature shown in FIG.
Can be more easily obtained by setting the correction value to 1.0.

In any case, the hydrazine concentration in the feedwater flowing to the inlet of the low-pressure economizer 4 can be kept constant, and has a useful function next to the above embodiment.

[Effects of the Invention] As described above, the present invention multiplies the actual decomposition feedwater flow rate by the hydrazine thermal decomposition rate in the corresponding exhaust heat recovery boiler according to the load signal of the generator or the exhaust gas temperature or the main steam temperature of the gas turbine. To determine the corrected water supply flow rate,
The hydrazine concentration is adjusted by controlling the stroke and rotation speed of the plunger pump in proportion to this corrected water supply flow rate, so that even when the load of the combined cycle power plant fluctuates, the hydrazine concentration is kept within the limit value. It is possible to suppress the increase in PH due to the thermal decomposition of hydrazine at the time of starting the plant.

Therefore, according to the present invention, there is an excellent effect that the cooling pipe of the condenser can be prevented from being damaged by erosion by ammonia.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an apparatus for carrying out the method of the present invention, FIG. 2 is an explanatory diagram of the operation of a plunger pump used in the present invention, and FIG. 3 is a diagram showing a relationship between a load and a correction value. ,
FIGS. 4 and 5 are each a block diagram showing another apparatus for carrying out a different method of the present invention, FIG. 6 is a system diagram showing an example of a conventional combined cycle power plant, and FIG. FIG. 8 is a diagram showing the decomposition rate of hydrazine, FIG. 8 is a diagram showing the ratio of hydrazine returning to the condenser, and FIG.
The figure is a diagram showing the hydrazine concentration at the low pressure economizer inlet. 1 ... condenser 3 ... waste heat recovery boiler 4 ... low pressure economizer 11 ... high pressure superheater 21 ... flow detector 22 ... corrected flow generator 23 ... load detector 24 ... rotation speed Control function generator 25 …… Stroke control signal generator 30 …… Pump 35, 36 …… Temperature detector

Claims (1)

(57) [Claims] In order to maintain the limit value of the concentration of dissolved oxygen in feed water sent to a heat recovery steam generator, hydrazine is injected into the feed water while adjusting the concentration using a plunger type pump. In the boiler feedwater treatment method, the corrected feedwater flow rate is determined by multiplying the measured hydrazine thermal decomposition rate by the corresponding hydrazine thermal decomposition rate in the waste heat recovery boiler according to the load signal of the generator or the exhaust gas temperature or main steam temperature of the gas turbine. A water supply processing method for an exhaust heat recovery boiler, wherein the hydrazine concentration is adjusted by controlling the stroke and the number of revolutions of the plunger type pump in proportion to the corrected feed water flow signal.