JPH0619201B2 - How to stop the exhaust heat recovery boiler - Google Patents

Description

The present invention relates to a method for stopping an exhaust heat recovery boiler equipped with a denitration device.

[Conventional technology]

In response to recent changes in power demand, there is a growing tendency to adopt gas turbines for peak loads. As described above, when the gas turbine is used, the high temperature gas that gives the kinetic energy to the gas turbine is guided to the exhaust heat recovery boiler, and the exhaust heat recovery boiler uses the heat to generate steam. The generated steam is guided to a steam turbine and drives it to generate electricity. As described above, combined cycle power generation is performed by the gas turbine and the steam turbine, and thus energy saving is achieved.

Normally, the above-mentioned exhaust heat recovery boiler is equipped with a denitration device for removing nitrogen oxides (NO _x ) contained in the exhaust gas from the turbine. In this denitration device, the catalyst effectively acts only in a certain temperature range, and the effect of the catalyst is greatly reduced at a temperature outside this temperature range. This will be described with reference to the drawings.

FIG. 5 is a graph showing the denitration effective temperature range of the denitration device. In the figure, the horizontal axis represents time and the vertical axis represents temperature.
The hatched range in the figure is the denitration effective range, with the upper limit temperature being T ₂ and the lower limit temperature being T ₁ . The temperature T ₂ is about 400 ° C. and the temperature T ₁ is about 200 ° C. Therefore, if the exhaust gas temperature is not within the temperature range indicated by the diagonal lines, effective denitration is not performed, and a large amount of NO _x is contained in the flue gas.

[Problems to be Solved by the Invention] On the other hand, in recent boiler operation, a transformer operation method is adopted in order to improve efficiency. Figure 6
Shown in (a). In the figure, load is plotted on the horizontal axis and drum pressure is plotted on the vertical axis. As shown in the figure, in the variable pressure operation method, the drum pressure is set to a certain low value when the load is small, and when the load exceeds a certain load point, the drum pressure is changed almost in proportion to the magnitude of the load. To do. For this reason,
The drum pressure drops when the boiler is stopped. When boiler banking is entered from this state, the drum pressure at the time of restart is further reduced as shown in Fig. 6 (b). In the figure, the horizontal axis represents time and the vertical axis represents drum pressure. As shown in the figure, the drum pressure gradually decreases as time elapses from the start of banking, and becomes extremely low when the drum is restarted. For example, during 8 hours of boiler banking, the drum pressure dropped about 20 atg.
The residual pressure is around 0 atg (saturation temperature 183 ° C).

Because of the above-mentioned denitration effective temperature range and variable pressure operation method,
When the boiler is restarted, a large amount of NO _x is included in the smoke emitted from the boiler. That is, although the exhaust gas is supplied to the boiler at the time of restarting the boiler, a heat transfer tube group such as a superheater and an evaporator is arranged on the upstream side of the denitration device.
Since the residual pressure of the drum at restart is low, the temperature of the heat receiving medium in the heat transfer tube group is low, and the supplied exhaust gas is deprived of a large amount of heat by these heat transfer tube groups. As a result, the temperature of the exhaust gas flowing into the denitration device when the boiler is restarted becomes the temperature T ₃ (about 180 ° C.) as shown by the curve A in FIG. 5, and the temperature at which the catalyst effectively acts (hereinafter referred to as the catalyst suitable temperature That is the following. Then, it takes a considerably long time after the boiler is restarted to reach the catalyst suitable temperature. Therefore, the NO _x contained in the exhaust gas is not effectively removed by the denitration device from the time when the boiler is restarted until the exhaust gas temperature reaches the catalyst optimum temperature, and during this period, the amount of NO _{x in} the flue gas increases. Will be done.

The present invention has been made in view of the above circumstances, and an object thereof is to solve the above-mentioned conventional problems and to discharge NO from the boiler at the time of restarting the boiler without using any special device. _Another object of the present invention is to provide a method for stopping an exhaust heat recovery boiler that can reduce _x .

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a drum that generates steam, an evaporator that heats can water of the drum with exhaust gas, and the generated steam that is supplied to a load. In the exhaust heat recovery boiler, the exhaust heat recovery boiler is provided with a main steam pipe, a bypass pipe connected to the main steam pipe to bypass the load, and a denitration device for removing nitrogen oxides in the exhaust gas. When stopping, the amount of steam passing through the main steam pipe and the bypass pipe is respectively controlled to increase the pressure of the drum and increase the residual pressure of the drum when the boiler is restarted.

[Operation] When the boiler is stopped, the amount of steam passing through the main steam pipe and the bypass pipe is individually controlled to increase the pressure of the drum, thereby increasing the residual pressure of the drum when the boiler is restarted and increasing the exhaust gas upstream of the denitration device. The temperature of the heat receiving medium of the side heat transfer tube group is kept high so that the exhaust gas temperature at the time of restarting the boiler becomes the catalyst suitable temperature.

[Examples] Hereinafter, the present invention will be described based on illustrated examples.

FIGS. 1 (a), (b) and (c) are time charts showing a method of stopping an exhaust heat recovery boiler according to an embodiment of the present invention, and FIG.
FIG. 7 is a system diagram of an ordinary exhaust heat recovery boiler to which the stopping method shown in FIGS. First, the exhaust heat recovery boiler shown in FIG. 2 will be described. In the figure, 1 is a gas turbine, 2 is an exhaust gas duct for introducing exhaust gas G from the gas turbine 1, 3 is a superheater, 4 is a first evaporator, 5 is a denitration device, and 6
Is a second evaporator, and 7 is a economizer. The superheater 3, the first and second evaporators 4 and 6, the denitration device 5, and the economizer 7 are arranged in the exhaust gas flow path constituted by the exhaust gas duct 2. 8 is a drum for generating steam, 9 is a steam turbine driven by the steam generated in the drum 8, 10 is a condenser for condensing the steam to return it to water, and 11 is water for supplying water from the condenser 10 to the drum 8. It is a condensate pump.

The water in the condenser 10 is supplied to the water W by the condensate pump 11, is preheated by the exhaust gas G in the economizer 7, and is supplied into the drum 8. The water in the drum 8 descends through the descending pipe 13, is introduced into the evaporators 4 and 6 via the pipes 1a and 14b, and the pipe 15
It returns to the inside of the drum 8 through a and 15b. In this way
While circulating, the steam generated by heating the exhaust gas G in the evaporators 4 and 6 is introduced into the superheater 3 by the saturated steam pipe 16, where it is superheated by the exhaust gas G and passes through the main steam pipe 17 as superheated steam. It is supplied to the steam turbine 9.
Reference numeral 18 is a turbine bypass pipe that is connected to the main steam pipe and bypasses the steam turbine 9 to directly guide steam to the condenser 10. Further, 19 is a steam turbine control valve for adjusting the flow rate of steam to the steam turbine, 20 is a turbine bypass valve for adjusting the steam bypass amount by the supply amount of steam to the steam turbine 9, and 21 is a damper of the exhaust gas duct 2. is there. During the above-described steam generating operation, the denitration device 5 performs an operation of removing NO _x in the exhaust gas G.

Here, the plant shutdown process of the present embodiment accompanying the boiler shutdown will be described based on the graph shown in FIG. When the plant is stopped, in order to keep the metal temperature at the time of stopping the steam turbine 9 high and to shorten the start-up time at the time of start-up, the steam turbine 9 is preceded with the gas turbine 1 held at a certain load. Stop. This state is shown in Fig. 1 (a),
It is shown in (c). The steam turbine 9 is stopped by adjusting the steam turbine control valve 19 to reduce the steam flow rate as shown in FIG. 1 (a). During this period, the surplus steam amount changes the drum pressure of the drum 8 by the turbine bypass valve 20. It is dumped to the condenser 10 via the turbine bypass pipe 18 while controlling. In Figure 1 (a), the shaded area is the amount of excess steam. After the steam turbine 9 is stopped, the gas turbine 1 enters a stop process as shown in FIG.

In the conventional stopping method, when the gas turbine 1 enters the stopping process, the pressure of the drum 8 decreases from the pressure P _{0 as} shown by the solid line C in FIG. 1 (b) due to the cooling. When the gas turbine 1 is stopped, the pressure drops to P ₁ . Then, in this state, the outlet damper 21 of the boiler is fully closed to enter the boiler banking state. During banking, the boiler is cooled by natural heat radiation or the like, so that the pressure of the drum 8 further decreases from the pressure P _{1 as} shown in FIG. 1 (b). Therefore, when the boiler is restarted, the drum pressure becomes extremely low pressure P ₂ , the temperature of the heat receiving medium in the heat transfer tube group is also low, and the exhaust gas G flowing into the denitration device 5 as described above.
Will also decrease the temperature.

However, in the present embodiment, unlike the above-described conventional shutdown method, in the plant shutdown process, the amount of steam dumped to the condenser 10 via the turbine bypass 18 is
A method of narrowing down by the turbine bypass valve 20 is adopted. As a result, the pressure of the drum 8 becomes 2 in FIG. 1 (b).
Increases as shown in dash-dotted D, is boosted to the pressure P ₃ in the time entering the boiler banking state. As described above, during banking, the boiler is cooled and the pressure of the drum 8 is also reduced as shown by the chain double-dashed line D in FIG. 1 (b). However, the drum pressure increased to the pressure P ₃ at the start of boiler banking maintains the pressure P ₄ at the time of restarting the boiler, even if there is a pressure drop due to cooling during banking, and this pressure is maintained by the boiler restarting method in the conventional shutdown method. The pressure is much higher than the starting pressure P ₂ . As a result, the heat receiving medium temperature in the heat transfer tube group is also maintained at a high temperature, the amount of heat taken from the exhaust gas G at the time of restarting the boiler is reduced, the temperature of the exhaust gas G does not decrease, and as shown by the curve B in FIG. , The catalyst can be maintained at a suitable temperature, compared to the conventional stopping method,
The amount of NO _{x in} the flue gas can be significantly reduced.

To give one example, when the pressure of the drum 8 is increased to 60 atg (P ₃ = 60 atg) when the boiler is stopped,
Residual pressure of drum 8 after banking for 8 hours is about 30 at
g (P ₄ ≈30 atg, saturation temperature about 235 ° C.).
As a result, the temperature of the heat receiving medium in the superheater 3 and the evaporator 4 is maintained at about 235 ° C., and the temperature of the exhaust gas G introduced at the time of restart hardly decreases, and the temperature T ₄ ( It flows into the denitration device 5 at about 230 ° C.

Figure 3 shows NO _x emitted from the boiler after restarting the boiler.
It is a characteristic diagram of total amount, exhaust gas amount, and exhaust gas temperature. In the figure, the horizontal axis represents time, the solid line represents the residual drum pressure when the boiler is restarted at 30 atg, and the dashed line represents the residual drum pressure of 10 atg.
The case of atg is shown. As is clear from the figure, when the residual drum pressure is 10 atg, the amount of NO _x discharged from the boiler greatly exceeds the NO _x regulation value for a while, whereas when the residual drum pressure is 30 atg, the boiler The NO _x regulation value will not be exceeded after startup. Therefore, it can be seen that the stopping method of this embodiment for increasing the residual pressure of the drum is extremely effective for reducing NO _x .

FIG. 4 is a system diagram of another exhaust heat recovery boiler to which the stopping method shown in FIGS. 1 (a) to 1 (c) is applied. In the figure, the same parts as those shown in FIG. It should be noted that illustration of various devices in the exhaust gas duct 2 is omitted. Reference numeral 23 is a pressure detector attached to the drum 8 for detecting the drum pressure, 19 'is a steam turbine control valve controlled by a detection signal of the pressure detector 23, and 20' is controlled by a detection signal of the pressure detector 23. It is a turbine bypass valve.

In this embodiment, as shown by the chain double-dashed line D in FIG. 1 (b), the drum pressure is increased during the plant shutdown process. However, if this pressure increase is rapid, the design pressure of the drum 8 may be exceeded. Occurs. The boiler shown in FIG. 4 prevents such a situation. When the drum pressure exceeds a certain pressure, the pressure detector 23 detects this and sends a signal to the steam turbine control valve 19 'and turbine bypass valve 20'. It is possible to suppress the rise of the drum pressure by outputting and controlling these valves in the opening direction to safely raise the drum pressure.

Thus, in this embodiment, in the process of shutting down the plant, the turbine bypass valve is throttled to increase the drum pressure,
Since the residual pressure at the time of restarting the boiler is increased, it is possible to flow into the denitration device at the time of restarting the boiler by simply operating the method without interposing a special device in the system or changing the system. It is possible to prevent the exhaust gas temperature from decreasing and keep it within the catalyst suitable temperature range, and it is possible to greatly reduce the NO _x discharged from the boiler.

In the above description of the steam embodiment, the exhaust gas source is described as the gas turbine, but the exhaust gas source is not limited to the gas turbine, and the exhaust gas source may be applied to an industrial furnace, industrial treatment maintenance, or the like.

〔The invention's effect〕

As described above, in the present invention, in the process of stopping the plant, the amount of steam passing through the bypass pipe that bypasses the load is controlled to increase the drum pressure and increase the residual drum pressure when the boiler is restarted. The NO _x emitted from the boiler can be reduced when the boiler is restarted without using a special device.

[Brief description of drawings]

FIGS. 1 (a), (b) and (c) are time charts showing a method of stopping an exhaust heat recovery boiler according to an embodiment of the present invention, and FIG.
The system diagram of the exhaust heat recovery boiler that applies the stop method shown in Figures (a), (b), and (c). Figure 3 shows the NO discharged from the boiler.
_x Graph showing total amount, FIG. 4 is a system diagram of another exhaust heat recovery boiler to which the stopping method shown in FIGS. 1 (a), (b), and (c) is applied, and FIG. 5 is a denitration effective temperature range Fig. 6 showing the graph
(a) and (b) are characteristic diagrams of the drum pressure in the variable pressure operation method. 1 ... Gas turbine, 2 ... Exhaust gas duct, 3 ... Superheater, 4,6 ... Evaporator, 5 ... Denitration device, 8 ... Drum, 9 ... Steam turbine, 17 ... Main steam pipe, 18 ……
Turbine bypass pipe, 19, 19 '... Steam turbine control valve, 20, 20' ... Turbine bypass valve, 23 ...
Pressure detector.

 ─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-57-179309 (JP, A) JP-A-57-196004 (JP, A) JP-A-59-153003 (JP, A)

Claims (1)

[Claims] 1. A drum for generating steam, an evaporator for heating canned water of the drum with exhaust gas, a main steam pipe for supplying the generated steam to a load, and the load connected to the main steam pipe for loading the load. In a waste heat recovery boiler equipped with a bypass pipe for bypassing the exhaust gas and a denitration device for removing nitrogen oxides in the exhaust gas, when the exhaust heat recovery boiler is stopped, steam passing through the main steam pipe and the bypass pipe A method for stopping an exhaust heat recovery boiler, which comprises increasing the residual pressure of the drum when the boiler is restarted by controlling the respective amounts to increase the pressure of the drum.