JPH0340281B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an improved method of operating an exhaust heat recovery boiler so that steam is not generated inside the economizer.

[Technical background of the invention and its problems]

A gas turbine, a generator, and a steam turbine are arranged on one axis, and an exhaust heat recovery boiler is combined with this to form a combined cycle power generation plant. Among these, the exhaust heat recovery boiler is a heat exchanger that recovers the thermal energy contained in the gas turbine exhaust gas and generates steam. This heat recovery boiler is
By passing the exhaust gas from the gas turbine through the exhaust duct, heat exchange is performed with a superheater, evaporator, and energy saver arranged inside the duct. That is, the water supplied from the water supply line to the economizer is heated to near the saturation temperature, becomes hot water, and is then introduced to the drum provided outside the exhaust duct. A portion of the hot water in this drum is sent back to the water supply line via the recirculation water line.
The other part is led to the evaporator and returns to the drum as a two-phase flow of steam and water. The drum separates the steam into a gas phase and a liquid phase, and the saturated steam is led to a superheater and supplied to a steam turbine as superheated steam.

This waste heat recovery boiler is equipped with a recirculation water line that mixes the hot water at the drum outlet into the feed water in the water supply line. This is provided to prevent water droplets from becoming exposed outside the tube. On the other hand, the difference between the temperature inside the drum and the hot water temperature at the outlet of the economizer is called the subcool temperature difference, and it becomes smaller as the load becomes more partial. That is, as the load is reduced and the amount of heat held by the exhaust gas is reduced, the subcooling temperature difference becomes smaller, and eventually the temperature difference disappears. The fact that this subcooling temperature difference disappears means that the water supply is heated to a saturated state in the economizer and steam is generated. When steam is generated in the economizer pipe in this way, disadvantages such as a sudden increase in the pressure loss in the pipe and a sudden change in the drum level occur.

[Purpose of the invention]

The purpose of the present invention is to prevent steam from being generated inside the economizer and to prevent corrosive substances such as carbides and sulfides from being exposed as water droplets on the outside of the pipe.
An object of the present invention is to provide a method for operating an exhaust heat recovery boiler that allows efficient heat exchange.

[Summary of the invention]

In the present invention, when the subcool temperature difference, which is the difference between the temperature inside the drum and the hot water temperature at the outlet of the economizer, becomes less than the set value, the amount of recirculated water is increased to raise the temperature of the water supply at the inlet of the economizer, and as a result, By increasing the flow rate inside the economizer and lowering the hot water temperature at the exit of the economizer to increase the subcooling temperature difference, steam is not generated inside the economizer, and the subcooling temperature difference and subcooling temperature are also reduced. By controlling the amount of recirculated water in accordance with the calculated value of the set value and the calculated value of the higher value of the calculated value of the economizer inlet water supply temperature and the economizer inlet water supply temperature set value, carbide on the outside of the tube,
This invention relates to a method of operating an exhaust heat recovery boiler that prevents corrosive substances such as sulfides from becoming exposed as water droplets.

[Embodiments of the invention]

The present invention will be described below with reference to an embodiment shown in FIG. For example, exhaust gas from a gas turbine in a combined cycle power plant is led to an exhaust heat recovery boiler 2 via an exhaust gas duct 1, and undergoes heat exchange with a superheater 3, evaporator 4, and energy saver 5 disposed inside. It is released into the atmosphere after it cools down. Water is introduced to the economizer 5 from a water supply line 6, and after heat exchange in the economizer 5, the water is introduced to a drum 7 arranged outside the waste heat recovery boiler 2 in parallel. The saturated water in the drum 7 is led to the evaporator 4 via the circulating water pump 8, and the two-phase flow of water and steam that has undergone heat exchange here is led to the drum 7 again. The saturated steam in the drum 7 is led to the superheater 3, where it becomes superheated steam and is supplied to the steam turbine. On the other hand, a part of the saturated water in the drum 7 is guided to a recirculating water line 9 via a circulating water pump 8 and returned to the water supply line 6 via a flow rate regulating valve 10.

Furthermore, in the present invention, a temperature sensor 11 is provided at the inlet of the economizer 5, and the set value of the water economizer feed water temperature is determined by controlling the opening of the flow rate regulating valve 10. A temperature sensor 12 is also provided at the outlet of the economizer 5 to detect the temperature of the hot water by exchanging heat with the economizer 5, and a temperature sensor 13 is also provided on the drum 7 to measure the temperature inside the drum. Detected. The temperatures detected by these temperature sensors 11, 12, and 13 are input to a flow rate regulating valve opening degree control section 14, and the opening degree of the flow rate regulating valve 10 is controlled according to a predetermined procedure described below. has been done.

Next, a method of operating the exhaust heat recovery boiler device of the present invention will be explained. First, in the exhaust heat recovery boiler device, as shown in Fig. 2, in the rated state, the exhaust gas is supplied to the exhaust heat recovery boiler 2 at a temperature TG1 as shown by the solid line, and after exchanging heat with water,
Released into the atmosphere at temperature TG2. In addition, the water supply is set and controlled at the set temperature TW1 and is supplied to the energy saver 5.
It is heated here and guided to the drum 7 at a temperature TW2. The temperature inside the drum 7 is TW3, and this temperature
It is configured to maintain TW3 and temperature TW2 to some extent, and this temperature difference TW3 - TW2 is called a subcool temperature difference. Temperature of drum 7
Saturated water in TW3 becomes saturated steam in evaporator 4,
Thereafter, it is superheated in the superheater 3 to become superheated steam at a temperature TW4.

On the other hand, in a partial load state, the amount of heat in the exhaust gas decreases and is supplied to the exhaust heat recovery boiler 2 at a temperature TG1', and after heat exchange, is released to the atmosphere at a temperature TG2'.
In addition, the water supply is controlled to the set temperature TW1' and the energy saving device 5
is supplied to the drum 7 at a temperature TW2'. The temperature of this drum 7 is TW3', and then it becomes superheated steam at a temperature TW4' in the superheater 3. In this state, the subcooling temperature difference (TW3′−TW2′) becomes smaller as the load becomes more partial. In this way, as the load on the plant is reduced and the heat content of the exhaust gas is reduced, the subcooling temperature difference becomes smaller. Eliminating the subrule temperature difference means that the feed water is heated to a saturated state in the economizer 5 and steam is generated.

In the exhaust heat recovery boiler operating method of the present invention,
The temperature sensor 11 shown in FIG. 1 detects the feed water temperature TW0, the temperature sensor 12 detects the outlet hot water temperature TW2 of the energy saver 5, and the temperature sensor 13 detects the temperature TW3 inside the drum 7. . If the sub-cooling temperature difference (TW3-TW2), which is the difference between the drum internal temperature TW3 and the economizer outlet hot water temperature TW2, falls below the set value, and the economizer feed water temperature TW0
becomes less than the set value TW1, the amount of water in the recirculation line 9 is increased to raise the energy saver inlet temperature TW0. As a result, the internal flow rate of the economizer 5 increases, the outlet hot water temperature TW2 of the economizer 5 decreases, the subcool temperature difference (TW3 - TW2) increases, and steam is generated inside the economizer 5. You can prevent it from happening.

Generally, when the recirculation line 9 is not used and the feed water flow rate is constant, when the feed water temperature is raised, the amount of heat exchange decreases, the exit temperature of the economizer increases, and the subcool temperature difference decreases. However, with the recirculation line 9, an increase in the economizer inlet temperature will increase the flow rate within the economizer, lowering the economizer outlet temperature and increasing the subcool temperature difference.

The temperature conditions TW0, TW2, and TW3 of each of these temperature sensors 11, 12, and 13 are sent as electrical signals to the opening control section 14 of the flow rate control valve 10. In FIG. 3, the outlet hot water temperature TW2 of the economizer 5 is sent to the subtracter 15 as a signal, and the internal temperature TW3 of the drum 7 is sent to the subtracter 15 as a + signal. The value calculated by this subtracter 15 is the subcool temperature difference (TW3-TW2). This value (TW3
−TW2) is sent to the subtractor 16 as a − signal,
The minimum setting value of the subcooling temperature difference is determined by this subtractor 16.
It is calculated as + signal of TWS. Under rated load conditions, the subcooling temperature difference (TW ₃ − TW ₂ ) is large and the set value
The signal will be - more than TWS, but in the partial load state, the subcooling temperature difference (TW3 - TW2) will become smaller and less than the set value (TWS), and the calculated value will be +
It becomes a signal.

The inlet water supply temperature (TW0) of the economizer 5 is sent to the subtracter 17 as a minus signal, and is calculated as a + signal of the inlet feed water temperature setting value TW1 of the economizer 5. The calculated value of the subtractor 17 and the calculated value of the subtractor 16 are input to the high value priority circuit 18, and the calculated high value is led to the proportional integral calculator 19 to determine the opening degree of the flow rate regulating valve 10, The amount of recirculated water flowing through the recirculation line 9 is controlled. In the rated state, the subcooling temperature difference (TW3−TW2) is equal to the set value TWS
Since the value calculated by the subtractor 16 is sufficiently large, the value calculated by the subtracter 16 becomes a - signal. In addition, the inlet water supply temperature of the energy saver 5
TW0 is almost equal to the set value TW1, and the calculated value of the subtracter 17 is ±0. Therefore, the value calculated by the subtractor 17 is given priority in the high value priority circuit 18, and the recirculated water amount is set to the inlet water supply temperature TW0 of the energy saver 5 to the set value TW1.
control to match.

As the load on the plant is reduced, the subcooling temperature difference (TW3-TW2) gradually decreases and approaches the set value TWS, and the calculated value of the subtractor 16 gradually approaches ±0. As the load is further reduced, the subcooling temperature difference (TW3 - TW2) becomes smaller than the set value (TWS), the calculated value of the subtractor 16 becomes +, and the high value priority circuit 18 gives priority,
opening the flow rate regulating valve 10 to increase the amount of recirculated water;
The subcool temperature difference (TW3−TW2) is the set value
Start controlling so that it does not fall below TWS. At this time, since the amount of recirculated water increases, the inlet water supply temperature TW0 of the economizer 5 rises, the calculated value of the calculator 17 becomes a - signal, and thereafter the calculated value of the subtractor 16 takes priority. In this way, stable water level control of the drum 7 can be performed without generating steam within the economizer 5.

On the other hand, in exhaust heat recovery boiler equipment, as the exhaust gas becomes low in temperature, corrosive substances such as carbides and sulfides in the exhaust gas become exposed as water droplets on the outside of the heat exchanger tubes, and this causes damage to the heat exchanger tubes. There is a problem of so-called low-temperature corrosion where corrosion occurs.

As a countermeasure against this low-temperature corrosion, the temperature of the water supplied into the pipe in the lowest exhaust gas temperature range is increased to prevent the exhaust gas that comes in contact with the extreme regions (all external surfaces) of the pipe from falling below the dew point.

Therefore, in the present invention, as described above, the so-called subcool temperature control signal of the subtractor 16 and the so-called feed water temperature control signal of the subtractor 17 in FIG. The second feature is that the amount of recirculated water is adjusted by selecting a high value and operating the flow rate regulating valve 10. That is, the signal after the high value selection by the high value priority circuit 18 is processed by the load as shown in FIG.
At 50%, subcooling temperature control and feed water temperature control are selectively controlled, and effective steam generation prevention measures and low-temperature corrosion countermeasures are implemented through steady operation of the exhaust heat recovery boiler equipment and partial load operation.

〔Effect of the invention〕

As described above, in the present invention, when the subcooling temperature difference falls below the set value and when the water economizer inlet water temperature falls below the set value, the amount of recirculated water is increased to lower the economizer inlet temperature. As a result, the flow rate inside the economizer increases, the outlet hot water temperature of the economizer decreases, and the subcool temperature difference increases, and as a result, the flow rate inside the economizer increases. Ensure that steam is not generated inside.

Furthermore, in the present invention, by adopting high value selection control of the subcooling temperature control signal and the feed water temperature control signal, steam generation prevention and low-temperature corrosion prevention are effective from steady load operation to partial load operation of the exhaust heat recovery boiler device. be implemented efficiently.

[Brief explanation of the drawing]

Fig. 1 is a piping configuration diagram showing a waste heat recovery boiler device to which the waste heat recovery boiler operating method of the present invention is applied, Fig. 2 is a diagram showing the relationship between heat flow rate and temperature of the waste heat recovery boiler, and Fig. 3 4 is a block diagram for controlling the amount of recirculated water applied to the exhaust heat recovery boiler device of the present invention, and FIG. 4 is a characteristic diagram for explaining high value priority selection control by the high value priority circuit used in the present invention. 1...Exhaust duct, 2...Exhaust heat recovery boiler, 3
...Superheater, 4... Evaporator, 5... Energy saver, 6...
... Water supply line, 7 ... Drum, 8 ... Circulating water pump, 9 ... Recirculating water line, 10 ... Flow rate control valve, 11, 12, 13 ... Temperature sensor, 14 ...
Opening degree control section, 15, 16, 17...Subtractor, 18
...High value priority circuit, 19...Proportional-integral calculator.

Claims (1)

[Claims] 1. In an exhaust heat recovery boiler equipped with a recirculating water line that mixes hot water at the drum outlet into the feed water, the difference between the subcool temperature difference, which is the difference between the temperature inside the drum and the hot water temperature at the outlet of the economizer, and the subcool temperature set value. The calculated value and the calculated values of the economizer inlet water supply temperature and the economizer inlet water supply temperature setting value are input into the high value priority circuit, and recirculation is performed in accordance with the high value calculated value obtained in this high value priority circuit. A method of operating an exhaust heat recovery boiler characterized by controlling the amount of water.