JP2511400B2 - Steam temperature control method for once-through boiler - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steam temperature control system for a once-through boiler, and more particularly to a steam temperature control system for a once-through boiler with improved controllability of the main steam temperature at plant startup.

[Background of the Invention]

A conventional steam temperature control method for this type of boiler is, for example, “Instrumentation”, 1983, special issue, pages 113 to 115, “Boiler steam temperature predictive control using Kalman filter”.
As described in, in the steam temperature control of the once-through boiler, the correction amount of the proportional integral is added to the deviation between the steam temperature target value after n minutes and the steam temperature predicted value after n minutes with respect to the fuel program value. In addition, the vapor temperature is controlled by controlling the fuel based on the value thus obtained.
According to this method, the change in the heat absorption amount of the boiler due to the operation pattern and the fuel property is detected in advance, and no correction is made to the fuel control based on this, so that the steam temperature control is not sufficient.

[Object of the Invention]

The present invention has been made in view of the above points, and an object thereof is to provide a steam temperature control method for a once-through boiler that suppresses fluctuations in steam temperature due to changes in heat absorption.

[Outline of Invention]

In order to achieve the above object, the present invention controls the boiler supply feed water flow rate during start-up bypass operation to be constant, and releases the difference between the feed water flow rate and the flow rate of steam delivered to the turbine to the flash tank by opening the start-up bypass valve. However, it has a startup bypass system that closes the startup bypass valve during normal operation,
In the steam temperature control method of the once-through boiler, in which the basic fuel flow rate of the boiler is set in accordance with the required load value of the turbine and the basic fuel flow rate is corrected by the deviation between the main steam temperature and the set value, the opening degree of the starting bypass valve By controlling the pressure of the primary superheater inlet fluid to a specified value, and obtaining the deviation between the actual opening degree of the startup bypass valve and the preset value of the startup bypass valve that is preset corresponding to the load demand value, The basic fuel flow rate is modified according to the deviation.

That is, when there is a change in the amount of heat absorbed by the boiler, the change immediately appears in the pressure of the inlet fluid of the primary superheater, and further appears as a change in the opening degree of the startup bypass valve that controls the pressure to a specified value. Therefore, by correcting the basic fuel flow rate according to the fluctuation of the opening degree of the startup bypass valve,
The fuel flow rate can be corrected according to the change of the heat absorption amount of the boiler, and thus the fluctuation of the main steam temperature can be suppressed.

Further, in addition to the correction of the basic fuel flow rate described above, it is preferable to correct the basic fuel flow rate according to an increase in the extraction flow rate of the turbine.

Example of Invention

Embodiments of the present invention will be described below with reference to the drawings. FIG. 4 is a system diagram showing a plant to which the embodiment of the present invention is applied. 1 is a boiler, 2 is a fire path, 3 is a forced draft fan, 4
Is a forced draft fan inlet vane, 5 is a fuel tank, 6 is a heavy oil pump, 7 is a fuel heater, 8 is a fuel flow control valve, 9 is a burner, 10 is a primary superheater, 11 is a reheater, and 12 is a secondary. Superheater, 13
Is a economizer, 14 is a superheater stop valve, 15 is a superheater pressure reducing valve, 16 is a desuperheater, 17 is a secondary superheater bypass valve, 18 is a primary superheater bypass valve, 19 is a superheater spray valve, 20 is a flush tank, 21 is a flush tank steam valve, 22 is a turbine governor, 23 is a high pressure turbine, 24 is a low pressure turbine, 25 is a condenser, 26 is a flush tank drain valve, and 27 is a deaerator steam valve. , 28 is a high pressure heater steam valve, 29 is a bleed stop valve, 30 is a deaerator, 31 is a water supply pump, 32 is a water heater, 50 is a main steam temperature detector, 51 is a main steam pressure detector, 52 is a fuel Flow rate detector,
53 is an air flow rate detector, 54 is a steam reducer outlet steam temperature detector,
90 is a load measurement value, 100 is a control device, and 106 is a load request value.

 The operation of such a plant will be described below.

The water supplied to the boiler by the water supply pump 31 is preheated by the economizer 13 using the combustion gas in the flue and enters the furnace 2. The fluid that has passed through the furnace 2 is turned into superheated steam in the primary superheater 10 and the secondary superheater 12, and is supplied to the high-pressure turbine 23 through the turbine governor 22.

In order to protect the water wall, the feed water flowing through the furnace 2 needs to flow through a certain amount or more (about 25% of the boiler rating, which depends on the boiler model). Through the line of the primary superheater bypass valve 18 and the secondary superheater bypass valve 17, the difference between the constant flow rate and the turbine supply flow rate is calculated by the flash tank until the steam flow rate passing through the turbine reaches this constant amount. Operation required to be missed at 20 is required, and this state is called startup bypass operation. The fluid flowing into the flush tank 20 is
Flash tank steam valve 21, flash tank drain valve
It is recovered by the condenser 25 through 26 or by the deaerator 30 through the deaerator steam valve 27. When the generator load is small and the extraction steam pressure is low, a part of the flash tank steam is recovered by the feed water heater 32 through the high pressure heater steam valve 28.

A case of heavy oil is shown as an example of the fuel system, but the heavy oil pressurized by the heavy oil pump 6 is overheated by the fuel heater 7 and the control device
Based on a command from 100, the fuel flow rate control valve 8 controls the flow rate, and the burner 9 causes spray combustion in the furnace.

On the other hand, the combustion air is introduced into the furnace by the forced draft fan 3. This air volume adjustment is performed in the forced draft fan inlet vane 4 based on a command from the control device 100.

By the way, the characteristics at the time of start-up are as shown in FIG. 5, the fuel is supplied in an amount commensurate with the load demand value 106, and the main steam pressure at the turbine inlet is controlled by the superheater pressure reducing valve 15 to be boosted. The generator load 90 is controlled by the deviation between the load demand value 106 and the actual load, but the load rises almost in proportion to the main steam pressure. Primary and secondary superheaters 1 in proportion to generator load 90
The amount of steam passing through 0 and 12 increases. The area A in Fig. 5 is the start-up bypass area, and the boiler supply water flow rate is constant. Therefore, the flow rate through the start-up bypass valve is the boiler supply water flow rate minus the amount of steam passing through the primary and secondary superheaters. It will be a good thing. The primary superheater bypass valve 18 has a function of controlling the inlet pressure of the primary superheater to a specified value and is gradually closed. When the opening of the superheater pressure reducing valve 15 reaches 90%, the superheater stop valve
14 is fully opened, and the role of the overheating pressure reducing valve ends. The steam temperature in this region has large dead time and delay time constant with respect to changes in the fuel amount (dead time 5 to 8 minutes, delay time constant 12 to 14 minutes), and feedback control has been difficult.

Before the present invention was made, as shown in FIG. 5, a method of controlling by the predicted value of the main steam temperature calculated by the main steam temperature prediction model was proposed, and by this, feed back control became possible and applied. . The control device 100 used in this method is configured as follows. 101
Is a main steam temperature detector, 102 is a main steam temperature prediction model, 10
4, 110 is a subtractor, 105, 111 are proportional-plus-integral calculators, 107 is a function generator, and 108 is an adder. Main steam temperature prediction model 1
The signals input to 02 are main steam pressure value, air flow rate, desuperheater outlet temperature, fuel flow rate, and load. The operation of such a control device is as follows.

In this control method, the function generator 107 determines the basic fuel amount of the fuel commensurate with the load from the load request signal 106, and the basic fuel amount is determined by the difference between the main steam temperature n minutes ahead and the set value n minutes ahead. The amount is modified.

With this method, if the heat absorption amount in the boiler changes or the boiler inlet feedwater enthalpy changes due to changes in the operation pattern, the main steam temperature appears after 20 to 30 minutes, so control using the predicted value of the main steam temperature Even if is executed, a phenomenon occurs in which a sufficient control result cannot be obtained. Further, as the load increases, the extraction pressure of the turbine increases, and the supply steam to the feed water heater 32 is switched from the steam line from the flash tank steam valve 28 to the extraction line at high speed. In this case, even if the load does not increase, the steam flow into the turbine increases and the
The flow rate of steam passing through the next superheaters 10 and 12 increases. In this case, the fuel amount does not correspond to the amount of steam passing through the primary and secondary superheaters, so the heat balance is lost and the main steam temperature changes.

 Therefore, the present invention is configured as follows.

 FIG. 1 is a block diagram showing an embodiment of the present invention.

In FIG. 1, the same members as those in FIG. 5 are designated by the same reference numerals for description. The difference between FIG. 1 and FIG. 5 is that the opening signal from the primary superheater bypass valve opening transmitter 201 and the flow signal from the extraction flow transmitter 206 are newly taken in, and the function generator is also included. 202,204,207 and adder 205,208
And a subtractor 203 is provided. Now, the operation of the steam control device 100 will be described, but before that, the principle adopted by this system will be described.

First, the main steam temperature is inversely proportional to the flow rate of steam passing through the primary and secondary superheaters 10 and 12, and is proportional to the fuel flow rate. Further, when the boiler heat absorption changes due to the change of the operation pattern or the fuel property change, the heat absorption amount of the boiler also changes, but these are related to the fuel amount.
This change appears first in the inlet pressure of the primary superheater, and
This appears as a change in the opening of the primary superheater bypass valve 8 that controls the inlet of the secondary superheater to a specified value. This primary superheater bypass valve opening is appropriate for evaluating an appropriate heat absorption amount, and this opening becomes a constant function depending on the load. Also, the steam flow rate is calculated as a function of the load, but this relationship that turbine bleed is taken is not established and it is necessary to use the fuel amount as the primary and secondary superheater passing amount base. A fuel amount proportional to the extraction flow rate was added to the calculated basic fuel amount to balance the fuel and suppress the fluctuation of the main steam temperature.

 Then, the operation of this method will be described below.

First, the function generator 202 having a function as shown in FIG.
The difference from the actual opening from the primary superheater bypass valve opening transmitter 201 is calculated by the subtractor 203, and the calculation result is input to the function generator 204 having a dead zone. 1 above the dead zone
When the next superheater bypass valve changes, the output of the function generator 204 is input to the adder 205. In addition, the extraction flow transmitter 206
The bleed air flow rate is measured with the function generator 207 for this flow rate value.
The load-based signal is added by the adder 208 to the load request value. The output of the adder 208 is a load request base signal in which the extraction flow rate is corrected.

The signal from the adder 208 is given to the function generator 107, and the function generator 107 is used as the basic fuel amount.
Add to 05. The output of the adder 205 is added to the output of the proportional-plus-integral calculator 105 by the adder 108 to control the fuel flow rate with the fuel flow rate being feed back.

〔The invention's effect〕

As described above, according to the present invention, the change in the operation pattern of the once-through boiler, and the disturbance to the main steam temperature due to the change in the heat absorption amount due to the change in the fuel property are suppressed, and the controllability of the main steam temperature is improved. It has the effect of improving.

[Brief description of drawings]

FIG. 1 is a system diagram showing a steam temperature control system for a once-through boiler according to the present invention, and FIG. 2 is used in an embodiment of the present invention, and shows an optimum program value of a primary superheater bypass valve opening with respect to a load command value. FIG. 3 is a waveform diagram showing fuel correction by a change in opening, FIG. 3 is a system diagram showing a once-through boiler plant having a startup bypass system, and FIG. 4 is a diagram showing startup characteristics of a once-through boiler plant having a startup bypass system. FIG. 5 is a block diagram showing a main steam temperature control system which is the basis of the present invention. 1 ... Boiler, 2 ... Furnace, 3 ... Push blower, 4 ...
Push fan fan, 5 ... Fuel tank, 6 ... Heavy oil pump, 7 ... Fuel heater, 8 ... Fuel flow control valve, 9
…… Burner, 10 …… Primary superheater, 11 …… Reheater, 12 ……
Secondary superheater, 13 ... coal saver, 14 ... superheater stop valve, 15 ...
Superheater pressure reducing valve, 16 …… desuperheater, 17 …… secondary superheater bypass valve, 18 …… primary superheater bypass valve, 19 …… heater spray valve, 20 …… flush tank, 25 …… Condenser, 27 ……
Deaerator steam valve, 28 ... High-pressure heater steam valve, 29 ... Bleak stop valve, 100 ... Control device, 101 ... Main steam temperature detector, 10
2 …… Main steam temperature prediction model, 104,110,203 …… Subtractor,
105,111 …… Proportional-integral calculator, 107,202,204,207,211 ……
Function generator, 108, 205, 208 ... Adder, 201 ... Primary superheater bypass valve opening transmitter, 206 ... Extraction air flow transmitter.

Claims (2)

(57) [Claims] 1. A boiler feed water supply flow rate is controlled to be constant during start-up bypass operation, and a difference between the feed water flow rate and the steam supply flow rate to the turbine is released to a flash tank by opening a start-up bypass valve, and during normal operation. A once-through boiler that has a startup bypass system for closing the startup bypass valve, sets the basic fuel flow rate of the boiler in proportion to the load demand value of the turbine, and corrects the basic fuel flow rate by the deviation between the main steam temperature and the set value. In the steam temperature control method, the opening degree of the starting bypass valve is controlled so that the pressure of the primary superheater inlet fluid is a specified value, and the opening degree of the starting bypass valve and the load demand value are set in advance and set in advance. A steam temperature control method for a once-through boiler, wherein a deviation from a set value of a startup bypass valve is obtained, and the basic fuel flow rate is corrected according to the deviation. 2. The boiler supply feed water flow rate is controlled to be constant during the start-up bypass operation, and the difference between the feed water flow rate and the steam output flow rate to the turbine is released to the flash tank by opening the start-up bypass valve. A once-through boiler that has a startup bypass system for closing the startup bypass valve, sets the basic fuel flow rate of the boiler in proportion to the load demand value of the turbine, and corrects the basic fuel flow rate by the deviation between the main steam temperature and the set value. In the steam temperature control method, the opening degree of the starting bypass valve is controlled so that the pressure of the primary superheater inlet fluid is a specified value, and the opening degree of the starting bypass valve and the load demand value are set in advance and set in advance. Obtaining a deviation from the set value of the startup bypass valve, correcting the basic fuel flow rate according to the deviation, and correcting the basic fuel flow rate according to an increase or decrease in the extraction flow rate of the turbine. Steam temperature control system for once-through boiler, characterized.