JPH0448921B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a gas turbine, an exhaust heat recovery boiler that uses gas turbine exhaust heat as a heat source, and a steam turbine that uses steam generated in this boiler as a working medium. The present invention relates to a method of operating a combined cycle plant constructed as described above.

[Background of the invention]

Using the gas turbine exhaust heat recovery boiler as an intermediary,
It is known that a plant that combines a steam turbine and a gas turbine has higher thermal efficiency than a plant that uses only a steam turbine or a gas turbine. It is also known that the total output of a combined plant varies depending on the intake air temperature of the gas turbine. FIG. 2 shows a state in which the base operating output of the combined cycle plant changes depending on the intake air temperature of the gas turbine. The reason why the lower the intake air temperature is, the higher the overall output is.The upper limit of the combustion gas temperature of a gas turbine is suppressed, and it remains almost constant regardless of fluctuations in outside temperature. , a large amount of fuel is input into the gas turbine, and the heat drop to atmospheric temperature increases, so the output of the entire plant increases. On the other hand, the overall efficiency of the entire plant improves as the atmospheric temperature increases. The reason for this will be explained later.

In large-capacity power generation plants, there are restrictions on the authorized output of the plant from the viewpoint of safety, and it is prohibited to operate at an output higher than the output specified under certain specified conditions. Now, in Figure 2, if the base operating output W ₀ at atmospheric temperature T ₀ is determined as the authorized output, when the atmospheric temperature becomes lower than T ₀ , the plant output is controlled to be maintained at W ₀ . Therefore, for example, at atmospheric temperature T ₁ , the base operating output W ₁ is forced to perform partial load operation with the base operating output W 1 reduced to W ₀ . Therefore, the overall efficiency decreases from η ₁ to η ₁ ', and the efficiency of T ₀ also decreases compared to η ₀ .

On the other hand, when the atmospheric temperature is higher than T ₀ , the base operating output is always smaller than the authorized output W ₀ , so
The plant can continue to operate at base.

In other words, the problem with combined cycle power plants is that the overall efficiency of the plant deteriorates as the atmospheric temperature decreases, and furthermore, at atmospheric temperatures lower than the authorized output point,
The plant is forced to operate at partial load all the time, further reducing its overall efficiency.

Therefore, efficiency improvements are possible if the temperature of the gas turbine compressor inlet air can be increased with relatively little energy loss.

Conventionally, a system for heating the inlet air of a gas turbine compressor is known as anti-icing. This system is designed to prevent freezing at the compressor inlet during the winter when the atmospheric temperature is negative, and is shown in Figures 3A and 3B.
The figure shows a typical system configuration.

In FIG. 3A, part of the air 3 blown from the compressor 2 is branched off and returned to the intake system 1 to raise the temperature of the intake air above the freezing point.

Further, in FIG. 3B, the combustion gas 6 generated in the combustor 5 performs work in the turbine 7, and a part of the exhaust gas 8 whose temperature has decreased is returned to the intake system 1.

In either system, when it is detected that the atmospheric temperature has fallen below the freezing point, the regulating valve 14 is opened to mix the heated fluid with the intake air.

The purpose of these systems is to prevent the intake system from freezing due to a drop in atmospheric temperature and to safely operate the gas turbine.They are not intended to improve efficiency or maintain output, and the regulating valve 14 is closed when the temperature exceeds the freezing temperature. .

[Purpose of the invention]

An object of the present invention is to provide a control method for operating a combined cycle plant as close to base load as possible even when the atmospheric temperature is low.

A further object of the present invention is to provide a control method that maximizes the difference between efficiency improvement when the intake air temperature is raised and efficiency reduction due to energy loss in intake air heating required for operation at base load.

[Summary of the invention]

According to the present invention, when the outside temperature is lower than a predetermined temperature, that is, when it is lower than the atmospheric temperature corresponding to the authorized output point of the plant, the intake air of the compressor of the gas turbine is heated by the intake air heating means so that the overall output of the plant is lower than the ambient temperature.
Even in 100% load operation, heating is performed to a target temperature that does not exceed the 100% load output value corresponding to the predetermined temperature.

Before describing embodiments of the present invention, it will be explained with reference to FIG. 4 that as the atmospheric temperature increases, the overall plant efficiency improves. In FIG. 4, the gas turbine unit efficiency 17 decreases as the atmospheric temperature increases. This characteristic is generally known, but the reason for this is that the gas turbine compressor is a positive displacement type, and as the intake temperature increases. This is because the air density becomes smaller and the weight flow rate of intake air decreases.

This decreasing trend in intake weight almost matches the trend in the gas turbine exhaust flow rate 19 in the same figure, and as a result, the output decreases as the amount of gas passing through the gas turbine decreases.
Efficiency also decreases.

As described above, in a simple cycle using only a gas turbine, increasing the intake air temperature of the compressor leads to a decrease in output and efficiency, and efforts have been made to lower the intake air temperature.

However, it has been found that in a combined cycle plant that combines a gas turbine and a steam turbine, the overall plant efficiency 16 improves as the atmospheric temperature increases.

Let me explain the reason. In FIG. 4, when the atmospheric temperature rises, the gas turbine exhaust temperature 18 rises and the exhaust flow rate 19 decreases. This exhaust temperature 1
8. The influence of the exhaust flow rate 19 on the output of the steam turbine is as shown in Figures 5A and 5B. In both cases, as the exhaust temperature and exhaust flow rate increase, the amount of evaporation in the exhaust heat recovery boiler increases and the steam Turbine power increases. Since the exhaust heat recovery boiler uses the exhaust gas of the gas turbine as its heat source, it does not use fuel by itself, so the higher the output, the better the efficiency.

In the characteristics shown in Fig. 4, the exhaust flow rate 19 decreases due to the rise in atmospheric temperature, but the increase in steam turbine output due to the rise in exhaust temperature is greater than the decrease in output due to the decrease in exhaust flow rate. The efficiency exhibits an upward-sloping characteristic as shown in FIG. 4 as steam turbine efficiency 20. In other words, the increase in gas turbine exhaust temperature due to the rise in atmospheric temperature is extremely large, so the efficiency improvement due to the increase in steam turbine output is equal to the efficiency decrease due to the decrease in steam turbine output due to the decrease in exhaust flow rate, and the increase in atmospheric temperature. This is because the overall efficiency is on the increasing side even if the decrease in efficiency of the gas turbine is taken into account.

[Embodiments of the invention]

FIG. 1 shows an embodiment of the present invention.

The basic configuration is similar to a conventional combined power generation facility, in which a gas turbine 7 and a steam turbine 12 are coupled by an exhaust heat recovery boiler 9, and a generator 14 is driven.

A steam extraction line 15 from the steam turbine 12 is provided as a system for heating the intake air 1 entering the gas turbine compressor 2. A regulating valve 30 is provided in this line 15 and is controlled by a controller 31.

This steam may be extracted from either the high-pressure main steam system or the low-pressure steam system, and an appropriate intake air temperature can be obtained by adjusting the amount of extracted air.

The problem with conventional plants is that overall efficiency is controlled by atmospheric temperature, so the lower the temperature, the worse the efficiency.

A method for improving this will be explained with reference to FIG.

First, consider T ₁ o'clock when the air temperature is lower than the atmospheric temperature T ₀ corresponding to the authorized output point. Total output is W ₀
(authorized output), the plant will operate under partial load as it is, and both efficiency and output will change on the broken line.

At T ₁ , the output is limited from W ₁ to W ₀ , so the efficiency drops from η ₁ to η ₁ ′.

Here, when the intake air temperature is increased from T ₁ to T ₀ using the intake air heating system of the present invention, the plant efficiency increases from η ₁ ' to η ₀ on the broken line while the output remains at W ₀ .
will be significantly improved.

Next, consider the range where the temperature is higher than T ₀ .

At this time, since the total output is always smaller than W ₀ ,
The plant can continue to operate at base. That is,
Both output and efficiency move on the solid line.

If the required output is W ₂ and _the temperature is T ₀ , then _{the intake air temperature is set to the required output point W 2 (} intake air temperature _{T 2 ,} efficiency η ₂ ) is more efficient.

That is, with the intake air heating system of the present invention, from T ₀
By increasing the intake air temperature to T ₂ , the overall efficiency increases to η ₀
It can be seen that the value is improved from η to η ₂ .

Next, we will consider the influence of the amount of extracted air on plant performance.

Figure 6 shows the rise in intake air temperature and amount of extracted air, and Figure 7 shows
The relationship between steam turbine output fluctuation and extracted air amount is shown.

The amount of extracted air, temperature rise, and output fluctuation differ depending on whether steam is derived from high-pressure main steam or low-pressure steam.

In the case of main steam extraction, the amount of extracted air required to raise the intake air by 7°C is 0.21% in terms of inlet air ratio.At this time, the output of the steam turbine is reduced by 1.8%, which is a 0.6% reduction in the total plant output ratio. It can be seen that there is no practical problem even if the air is extracted for heating the intake air. Similarly, when extracting air from low-pressure steam, the amount of extracted air required to raise the intake air by 7°C is 0.26%, and the output reduction is 1% for the steam turbine, which is 0.3% in terms of the total plant output ratio.

In this way, the plant output reduction due to air extraction is 1%.
Furthermore, this system is quite feasible considering that up to 3% (inlet air ratio) of extracted air is injected into the combustor using a steam injection system to reduce _NOx . I can understand that.

The above explanation has been made on the premise that the overall efficiency improves uniformly as the atmospheric temperature increases, but in reality there is a limit to efficiency improvement.

Figure 8 shows actual measurements of the overall plant output and efficiency in a region where the compressor inlet temperature is higher than in Figure 2.Each characteristic is shown by the solid line, and the predicted value by calculation (dashed line). decreases from Therefore, raising the inlet temperature above a temperature at which the overall efficiency decreases below the predicted value will have a negative result, since the energy loss for heating will increase but the efficiency will not be improved.

The temperature at which efficiency decreases varies depending on the load of the plant, and the smaller the load, the lower the temperature tends to be.

Therefore, the intake air temperature is the critical temperature for efficiency improvement.
It is preferable to keep it lower than T _l . This limit temperature can be determined in advance by actual measurements for the plant.

In addition, the characteristics of the overall efficiency are also experimentally determined.
It can be given to the controller 31 in FIG. 1 as control data.

Next, control within the controller 31 will be explained.
The controller receives the plant load L _d and the atmospheric temperature T _A and calculates the current plant efficiency η _A . This efficiency calculation is performed using a known method. Next, the plant efficiency ηΔ _T1 when the intake air temperature is increased by a constant temperature ΔT is determined from the characteristic curve determined in advance, and then the decrease in plant efficiency due to the heat loss required to raise the intake air by ΔT is calculated. Calculate _ΔT1 ,
Next, the efficiency improvement amount η _×1 is calculated using the following formula.

η _×1 = ηΔ _T1 − (η _A + η′Δ _T1 ) Next, ηΔ _{T2 and} η′Δ _T2 when the intake air temperature is increased by 2ΔT are found in the same way, and the improvement η _×2 is calculated. η _xo is calculated one after another until the limit temperature T _l is reached, and the maximum temperature is found among the calculated η _×1 , η _×2 . . . η _xo .

This temperature is the intake air temperature at which the plant can be operated most efficiently under the plant load L _d and the atmospheric temperature T _A . Once the intake air temperature is determined, the opening degree of the regulating valve 30 is adjusted to adjust the inlet temperature of the air compressor to the above temperature. In this case, an intake air temperature sensor 32 is provided at the inlet of the compressor 2, and the adjustment valve 30 is adjusted so that the detected value of this sensor 32 becomes the optimum intake air temperature.
All you have to do is control the feedback.

In the system shown in FIG. 1, extracted steam is returned to the intake system, but as shown in FIG. You may also do this. In this embodiment, a controller 31 controls a valve 34 that guides exhaust gas toward a heat exchanger 33 and an exhaust gas flow rate control valve 35 . The exhaust gas from the exhaust heat recovery boiler 9 is normally released into the atmosphere from the chimney, but since it still maintains a temperature of around 100°C, it can be used as a heating source for intake air.

In this case, there is a disadvantage that the heat exchanger 33 becomes larger, but the overall efficiency of the plant decreases to zero due to energy loss for heating the intake air.

〔Effect of the invention〕

As explained above, according to the present invention, by heating the intake air, it is possible to control the overall plant efficiency, which has conventionally been controlled by the atmospheric temperature, so that even though the atmospheric temperature is low, it is possible to control the overall efficiency of the plant as if the air temperature were The same effect can be obtained as when the temperature rises, and the plant can always be operated at the most efficient operating point.

[Brief explanation of the drawing]

Fig. 1 is a system diagram of a combined cycle plant that implements the present invention, Fig. 2 is an explanatory diagram of operation points of a combined cycle plant, Figs. 3A and 3B are a system diagram showing an anti-icing system, and Fig. 4 is a system diagram showing the Turbine exhaust characteristics and efficiency characteristics diagrams, Figures 5A and 5B
The figure is a characteristic diagram showing the effect of gas turbine exhaust on steam turbine output, Figures 6 and 7 are illustrations of the effect of extracted air amount, and Figure 8 is a characteristic diagram of actual measured plant output and efficiency. FIG. 9 is a system diagram of another embodiment of the present invention. 1... Intake system, 2... Gas turbine compressed air,
5... Combustor, 7... Gas turbine, 8... Exhaust gas, 9... Exhaust heat recovery boiler, 12... Steam turbine, 14... Generator, 15... Air extraction system for intake air heating, 16... Plant overall efficiency line, 17...Gas turbine single efficiency line, 18...Gas turbine exhaust temperature, 19...Gas turbine exhaust flow rate.

Claims (1)

[Scope of Claims] 1. A gas turbine, an exhaust heat recovery boiler that uses the exhaust heat of the gas turbine as a heat source, a steam turbine that uses steam generated in the boiler as a working medium, and a gas turbine that uses part of the steam as a working medium. In a combined cycle plant equipped with an intake air heating means for heating intake air, when the outside air temperature is lower than a predetermined temperature, the intake air temperature is adjusted by the intake air heating means when the total output of the plant is 100% load operation. A method for operating a combined cycle plant, characterized in that heating is performed to a target temperature that does not exceed a 100% load output value corresponding to the predetermined temperature. 2. In the method described in claim 1,
The target temperature is determined by the efficiency increase of the exhaust heat recovery boiler and steam turbine when the temperature rises from the actual outside air temperature to the target temperature, the efficiency decrease due to the steam used for intake air heating, and the intake air temperature rising to the target temperature. A method for operating a combined cycle plant, characterized in that the temperature is set to a maximum value after subtracting a decrease in efficiency due to this.