JPH031563B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to automatic control of boilers, and particularly to a steam temperature control method suitable for thermal power plants that require frequent startup and shutdown and high-speed load following performance.

[Background of the invention]

In the conventional steam temperature control method, the main stream was feedback control, which corrects the fuel flow rate and spray flow rate according to the deviation from the set value in response to fluctuations in the steam temperature, which is the control target. Since there are limits to control due to delays in corrective actions due to constants, it has become difficult to respond to the diversification of operations such as an increase in startups and stops, high-load follow-up operation, and variable voltage operation. Therefore, in recent years, predictive control methods have been introduced that make full use of advanced digital technology and have boiler secondary superheater models that can be stored as part of the control equipment, resulting in significant improvements in controllability in actual plants. Confirmed.

Examples of this type of predictive control method include pages 113 to 115 of the magazine "Micom" (1983 special issue) and Japanese Patent Application Laid-open No. 16719/1983.

An outline of the above-mentioned known conventional control system will be described below using FIGS. 4 and 2.

FIG. 4 shows, as a typical example, a steam system of a once-through boiler having a startup bypass system. During normal load operation, feed water supplied from the feed water pump 2 is preheated in the evaporator 9 of the boiler 1, then further heated in the primary superheater 5 and secondary superheater 6, and vented to the turbine 3 of the generator 4. Obtain the required power generation amount.

On the other hand, when the boiler is started, there is a startup bypass system that allows the steam generated by the boiler to drop into the flash tank 7 and vents the dry steam generated there to the secondary superheater.

The purpose of boiler steam temperature control is to control the temperature of turbine ventilation steam to a specified value by adjusting the spray flow rate of fuel control valve 101 and SH attemperator 8 in any of the above operating conditions.

Here, other symbols in this figure are as follows.

102...Primary SH bypass valve 103...Secondary SH bypass valve 104...SH ventilation valve 105...SH pressure reducing valve 106...SH stop valve 107...Flush tank steam dump valve 108...Secondary superheater Inlet pressure 109... Inlet temperature 110... Outlet pressure 111... Outlet temperature 112...Pressure after turbine first stage (=P _Tb ) 113...Main steam flow rate (=f(P _Tb )) Second The figure shows a conventional steam temperature control method using a secondary superheater prediction model.The figure shows a conventional steam temperature control method using a secondary superheater prediction model. This is a method in which the fuel is adjusted in advance based on the steam temperature prediction value n minutes in advance.

That is, the control system has a model 201 that formulates the heat transfer characteristics of the secondary superheater 6, and the main steam temperature 11
1. Secondary superheater inlet temperature 109, and predicted value of main steam temperature n minutes ahead estimated from other parameters 2
03 and the target value 204 obtained from the target temperature setting circuit 202.
This is added to the program signal 207 created from the boiler input command 206 as the combustion amount command 208, and the deviation from the fuel flow rate 209 is further reviewed to create the operation signal 210 for the fuel control valve 101. be.

[Purpose of the invention]

The purpose of the present invention is to provide a steam temperature control method that can fully meet the needs of future thermal power plants by improving the predictive response delay in the conventional secondary superheater model predictive control method and further improving controllability. It is about providing.

[Summary of the invention]

The present invention aims to improve the predicted response in the conventional secondary superheater model predictive control method, and as a means of correcting the response delay of the boiler evaporation section and the primary superheater, which is the cause of this, the present invention aims to improve the predictive response of the conventional secondary superheater model predictive control method. By having a characteristic model and using the predicted value of the primary superheater outlet temperature n minutes ahead obtained from this model as the input parameter of the secondary superheater model, the dynamic characteristics of the boiler can be estimated and controlled more accurately. It aims to improve the quality of life.

[Embodiments of the invention]

An embodiment of the present invention will be described below.

First, the focus of the present invention will be described using FIG.

In Figure 3a, which is a simplified version of Figure 4,
Figure 3 shows the characteristics of the fuel flow rate 210 and the steam temperature of each part of the boiler (primary superheater inlet temperature 301, primary superheater outlet temperature 109, main steam temperature 111) when the opening degree of the fuel control valve 101 is changed. A change in the main steam temperature 111, which is a controlled variable, appears with a dead time of _t3 after the fuel change. here,
When only the secondary superheater 6 is modeled, a dead time t ₂ approximately equal to the input parameter, the secondary superheater inlet temperature 109, appears in its predicted temperature 203. This delay in control correction corresponding to the dead time t ₂ is fatal, especially in areas where the boiler time constant is long at low loads; conversely, by cutting the dead time in half, controllability can be more than doubled. This improvement has been confirmed through actual machine operation.

Therefore, in the present invention, as a means to improve the response delay of the steam temperature upstream of the secondary superheater, we propose a method in which the primary superheater 5 is modeled and this predicted value is used for predictive calculation of the secondary superheater model. It is something to do.

FIG. 1, FIG. 5, and FIG. 6 show a study on realizing the present invention and its specific circuit.

As with the conventional secondary superheater model method, it is necessary to apply it regardless of the boiler type or operating condition, and as shown in Figure 1, once-through flow, which is the most unique operating condition Let us consider the bypass operation state at boiler startup as an example.

(Note that during normal load operation of a once-through boiler, steam from the boiler evaporator flows through a single path passing through the primary and secondary superheaters, making it easy to embody the present invention. Also, in the case of a drum boiler, Since the drum is a constant steam generation source, its implementation is easy as in the case of once-through boiler normal load operation mentioned above.) In Figure 1, the steam path during start-up bypass operation is as follows:
Steam passes from the primary superheater 5 through the SH pressure reducing valve 105 and from the flash tank 7 passes through the SH vent valve 104.
The steam passing through is mixed with the steam passing through the SH desuperheater 8 and sent to the secondary superheater 6. Also, during the startup process
The SH pressure reducing valve 105 gradually opens from fully closed to fully open.
The mixing ratio with flash tank steam also changes from moment to moment.

Therefore, in order to model the primary superheater 5 and obtain an estimated value of the secondary superheater inlet temperature n minutes ahead based on the predicted temperature, the SH pressure reducing valve 105 and the SH ventilation valve 10 must be
It is necessary to know the ratio of the steam flow rate passing through 4 and 4. Although the specific ratio calculation method will be described later, the method for estimating the steam flow rate ratio will be briefly explained below.

Note that in this figure, the signals marked by single circles can be measured, and the signals marked by double circles are estimated values.

In FIG. 1, the outlet pressure 40 of the primary superheater 5
2. Outlet temperature 403, flash tank pressure 40
6, and the inlet pressure 404 and inlet temperature 405 of the SH desuperheater 8 are measurable signals, and from these, the respective steam enthalpies 410, 41
2,414 is obtained by calculation.

On the other hand, the main steam flow rate 417 and the spray flow rate 407 calculated from the pressure 112 after the first stage of the turbine.
The inlet steam flow rate 415 of the SH attemperator 8 can be determined from .

From each state quantity obtained above, for the SH pressure reducing valve passing flow rate 411 and the SH ventilation valve passing flow rate 413,
The following relationship holds true.

F _MIX・H _MIX ＝F ₂₀₁・H ₂₀₁ +F ₂₀₅・H ₂₀₅ ……(1) F _MIX ＝F ₂₀₁ ＋F ₂₀₅ ……(2) Here, F _MIX ……SH desuperheater inlet steam flow rate (code 415 ) H _MIX ...SH desuperheater inlet steam enthalpy (code 414) F ₂₀₁ ...SH pressure reducing valve (valve 201) passing steam flow rate (code 411) H ₂₀₁ ...SH pressure reducing valve passing steam enthalpy (code 410) F ₂₀₅ ...SH vent valve (valve 205) passing steam flow rate (code 413) H ₂₀₅ ...SH vent valve passing steam enthalpy (code 412) Therefore, by solving equations (1) and (2), the SH pressure reducing valve,
And the steam flow rate passing through the SH vent valve can be expressed by the following formula.

F ₂₀₁ =F _MIX・H _MIX −H ₂₀₅ /H ₂₀₁ −H ₂₀₅ ...(3) F ₂₀₅ =F _MIX・H ₂₀₁ −H _MIX /H ₂₀₁ −H ₂₀₅ ...(4) Furthermore, (3), In equation (4), when H _MIX is equal to H ₂₀₁ , that is, in the start-up bypass operation completion state where the steam enthalpy passing through the SH pressure reducing valve is equal to the steam enthalpy at the SH desuperheater inlet (after mixing), (3) Equations (4) are respectively F ₂₀₁ = F _MIX , F ₂₀₅ = 0 ... (5) (∵H _MIX = H ₂₀₁ ), so the above method for estimating the steam flow rate ratio during startup bypass operation is usually used. It can be seen that it can be applied even during load operation.

FIG. 5 shows a specific modeling range in this embodiment. The model is the primary superheater prediction model 50
1. Consists of a startup bypass flow rate model 502, an SH desuperheater model 503, and a secondary superheater prediction model 201, and each model has a predicted estimated value 41
0,414,416,418,509 are obtained.

FIG. 6 shows a steam temperature control circuit according to this embodiment, which differs from the conventional system explained with reference to FIG. 2 in the following points.

That is, among the predicted calculation parameters of the secondary superheater model 201, the superheater inlet steam temperature signal is replaced with the actual measurement signal, and the primary superheater prediction model 501
Starting bypass flow model 5 from the prediction signal 410 of
02 and the SH desuperheater model 503 in that a predicted estimated signal for n minutes ahead is used.

According to the present invention, the following improvement effects can be expected compared to the conventional method.

FIG. 7 shows the response waveform of the predicted value of the secondary superheater outlet temperature in the case of this embodiment, and shows the response delay of the secondary superheater inlet steam temperature 109 to a change in the fuel flow rate control valve opening command 210. t ₂ is improved to a response delay waveform of t ₁ as shown at 508 by applying the primary superheater prediction model, and the response delay of the secondary superheater model prediction waveform is also improved from t ₃ to t as shown at 509. Since it can be obtained with a waveform shortened to ₂ ,
It becomes possible to significantly improve controllability.

In addition, as mentioned above, this method can be used not only during startup bypass operation of a once-through boiler, but also during normal load operation.
The drum can be widely applied to boiler steam temperature control, and the model size is relatively small, so it can be easily accommodated as a function of a control device.

Below, the configuration of each model used in this method will be explained in detail.

FIG. 8 shows the calculation flow of the startup bypass flow rate model shown by calculation block 502 in FIG.

In this figure, calculation blocks 801 and 802 are
The enthalpy of steam passing through the SH reduced pressure H ₂₀₁ (code 808) and the enthalpy of steam passing through the SH ventilation valve H ₂₀₅ (code 809) are calculated from the temperature and pressure, respectively, and the
From the enthalpy temperature characteristics of steam shown in the figure, it can be expressed by the following equation.

H ₂₀₁ = f (P _ISH , T _ISH ) …(6) H ₂₀₅ = f (P _F ) … (7) Here, P _ISH … Primary superheater outlet pressure T _ISH … Outlet temperature P _F ...Flush tank pressure Note that the steam generated in the flash tank is on the saturation curve shown by reference numeral 901 in FIG. 9, and its enthalpy is uniquely determined by the pressure.

Next, calculation block 803 calculates the SH pressure reducing valve and
From the enthalpy of the steam passing through the SH vent valve, the respective steam flow rates F ₂₀₁ (symbol 810) and F ₂₀₅ (symbol 8
11), and can be obtained from the above-mentioned equations (3) and (4). Here, the enthalpy H _MIX of the steam at the inlet of the SH desuperheater is determined from the steam characteristic table as in equation (6).

Next, in the calculation block 804, from the predicted value 816 of the primary superheater outlet temperature, based on the same idea as equation (6), a predicted estimated value H _201.P (symbol 812) of the steam enthalpy passing through the SH pressure reducing valve n minutes ahead is calculated. is calculated, and a calculation block 805 calculates the predicted amount of heat Q _201.P (reference 813) of the steam passing through the SH pressure reducing valve. In addition, calculation block 806 calculates the amount of heat retained by the steam passing through the SH vent valve.
Q ₂₀₅ (code 814) is calculated.

Calculation block 807 calculates the predicted SH value by dividing the sum of these retained heat values by the steam flow rate F _MIX .
Desuperheater inlet steam enthalpy H _MIX.P (code 815)
is obtained.

As described above, the estimated values obtained in calculation blocks 804 to 807 are expressed by the following equation.

H _201.P = f (P _ISH , T _ISH.P ) ...(8) Here, T _ISH.P ... Predicted value of primary superheater outlet temperature Q _201.P = H _201.P・F ₂₀₁ ... ...(9) Q ₂₀₅ = H ₂₀₅・F ₂₀₅ ...(10) H _MIX.P = Q _201.P +Q ₂₀₅ /F _MIX ...(11) Next, as shown in calculation block 503 in FIG.
The calculation processing of the SH desuperheater model will be explained.

In a desuperheater, the response of the post-spray steam temperature to changes in the spray flow rate is extremely fast compared to the response of the steam temperature when the fuel changes, and this characteristic can be treated as a steady model for the spray flow rate. be. Now, since the temperature T _ECO and pressure P _ECO at the spray water take-off point are measurable signals, the enthalpy H _SP of the spray water is expressed by the following equation.

H _SP =f(P _ECO , T _ECO ) (12) On the other hand, the spray water flow rate F _SP is also a measurable signal, and the amount of heat possessed by the spray water is expressed by the following equation.

Q _SP = H _SP・F _SP ……(13) Therefore, the predicted steam enthalpy value H _2SHi.P after spray water is added can be expressed by the following equation (14).

H _2SHi.P = H _MIX.P・F _MIX +Q _SP /F _2SH ……(14) Here, F _2SH ……Main steam flow rate (=f(P _Tb )) P _Tb ……Pressure after the first stage of the turbine Furthermore, from the secondary superheater inlet pressure P _2SHi (measurable) and equation (14), the predicted value of the secondary superheater inlet temperature is calculated.
T _2SHi.P can be expressed by the following formula.

T _2SHi.P = f(H _2SHi.P , P _2SHi ) (15) Next, the configurations of the primary and secondary superheater prediction models shown by calculation blocks 502 and 201 in FIG. 6 will be explained.

Note that the primary superheater prediction model and the secondary overheating prediction model differ only in the measurement points and amount of data provided from the outside, and the calculation processing inside the models is the same. Therefore, the following explanation describes a modeling method targeting boiler superheaters in general.

As shown in Figure 10, when energy conservation is applied to the process in which heat is transferred from the gas liquid to the internal steam via the superheater metal, changes in the superheater outlet steam enthalpy H _SO and metal temperature θ _M occur. is expressed by the following formula.

V・γ・dH _sp /dt=(H _Si −H _SO )・F _S +A・α _nS・(θ _M −θ _S ) ...(16) M _n・C _n・dθ _M /dt=A・α _gn・(θ _G −θ _M ) −A・α _nS・(θ _M −θ _S ) ……(17) Here, V……superheater flow passage volume γ…… 〃 Fluid specific weight H _SO …… 〃 Outlet fluid enthalpy F _S …… 〃 Liquid output amount θ _S …… 〃 Fluid temperature θ _M …… 〃 Metal temperature θ _G …… 〃 External gas temperature A…… 〃 Heat transfer area α _ns …… 〃 From metal to fluid Heat transfer coefficient H _S1 …… 〃 Inlet fluid enthalpy M _n …… 〃 Metal weight C _n …… 〃 Metal specific heat α _gn …… 〃 Heat transfer coefficient from gas to metal Here, superheater external gas temperature θ _G is as follows It is given by the empirical formula of

θ _G =H _u・F _f +H _a・F _a +H _gr・F _gr −K ₁ /Ff・K ₂ ×4 {(H
u・Ff/C _Pg・F _gBF ×6+273)/100} ⁴ /C _p g・F _gBF …
...(18) Here, H _u ...Fuel calorific value F _f ...Heat flow rate H _a ...Air enthalpy F _a ...Air flow rate H _gr ...Recirculating gas enthalpy F _gr ...Recirculating gas flow rate C _pg ... Gas specific heat F _gBF ... Boiler gas flow rate K ₁ , k ₂ ... Constant On the other hand, the heat transfer process in the superheater can be expressed by the following equation using constant pressure specific heat C _P.

C _P = (∂H/∂θ) _P ...(19) Rearranging equations (16) to (19) above and summarizing the superheater outlet fluid temperature X ₁ and metal temperature X ₂ , we get dX ₁ / dt=A ₁₁・X ₁ +A ₁₂・X ₂ +B ₁₁・U ₁ ...(20) dX ₂ /dt=A ₂₁・X ₁ +A ₂₂・X ₂ +B ₂₂・U ₂ ...(21) Simultaneous equation is obtained.

Here, U ₁ ... Superheater inlet liquid temperature U ₂ ... 〃 External gas temperature A ₁₁ ~ A ₂₂ , B ₁₁ ~ B ₂₂ ... Function of steam flow rate and gas flow rate Furthermore, equation (20), (21) By solving the equation, the following state transition equation regarding the superheater outlet temperature is obtained.

X〓 _(i) = Φ _(i-1)・X^ _(i-1) +H _(i-1)・U _(i-1) ……(22) Equation (22) is the estimation of current point (i) This indicates that the value X〓 _(i) is determined from the maximum estimated value X^ _{(i-1) one sampling time ago (i-1)} and the disturbance amount U _(i-1) . Note that Φ _(i-1) and H _(i-1) here are the transition matrix and driving matrix at time (i-1).

Next, a steam temperature prediction calculation method using the heat transfer model expressed by equation (22) will be explained with reference to FIG.

In this figure, the calculation block 1101 is the above (22)
This is a heat transfer model expressed by the equation, and this estimated value
X〓 _(i) is compared with the boiler observed value Y _(i) in block 1107, and this deviation is subjected to error weight correction in block 1103 to obtain the optimal correction value e _(i) . next,
Correct the estimated value X〓 _i with this correction value to obtain the maximum estimated value
X^ _(i) is calculated and used in the next estimation calculation in block 1101, and in calculation block 1105
The estimation calculation after sampling is repeated N times to calculate the predicted temperature 1106 for n minutes ahead.

On the other hand, block 1104 calculates the amount of modification of the model from the optimal modification value e _(i) , and
By correcting 1 from time to time, the estimated value X〓 _(i) is always kept equal to the actual machine.

Note that the prediction calculation in block 1105 is performed using the following equation.

X^ _(i,i+1) =Φ _(i)・X^ _(i) +H _(i)・U _(i) +e _(i) X^ _(i,i+2) =Φ _(i)・X^ _(i,i+1) +H _(i)・U _(i) +e _(i) X^ _(i,N-1) =Φ _(i)・X^(i,N−2) +H _(i)・U _(i) +e _(i) X^ _(i,N) =Φ _(i)・X^ _(i,N-1) +H _(i)・U _(i) +e _(i) Here, X^ _{(i, N)} ... Estimated value for N samples ahead, calculated at time i (prediction time n = N x sampling time Ts) [Effects of the invention] According to the present invention, steam temperature prediction obtained by the primary superheater prediction model By using this value as input to the secondary superheater prediction model, the response of the main steam temperature prediction value is significantly improved compared to the conventional method, and the controllability of the steam temperature can be improved by several steps. can.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the concept of the present invention when applied to a steam system of a once-through boiler, Fig. 2, Fig. 3,
Figure 4 shows the conventional control circuit and predicted response waveforms, Figures 5, 6, and 7 show the modeling range, control circuit, and predicted response waveforms of the present invention, and Figure 8 shows the startup bypass flow rate. Figure 9 shows the enthalpy temperature characteristics of steam, Figure 10 shows the heat transfer mechanism of the superheater, and Figure 11 shows the calculation flow for predicting steam temperature. 501...Primary superheater prediction model, 502...
Startup bypass model, 503...superheater desuperheater model, 201...secondary superheater prediction model.

Claims (1)

[Claims] 1. Increase or decrease boiler input amounts such as feed water flow rate, fuel flow rate, air flow rate, etc. in accordance with the power generation command, and adjust the fuel flow rate or spray flow rate according to the deviation of the steam temperature from the temperature setting value at this time. In a thermal power plant control device that controls and maintains the boiler outlet steam temperature at the rated value, the control system has a dynamic characteristic model of the boiler superheater, and the dynamic characteristic model in the control system is used to control the boiler outlet steam temperature in the primary superheater after the boiler evaporation section. , applied to a secondary superheater, and using an estimated temperature n minutes ahead derived from the predicted temperature of the former model as the inlet steam temperature which is a calculation parameter of the latter model. Control method. 2. In the method described in claim 1,
As a means of calculating the estimated value of the inlet steam temperature of the secondary superheater model n minutes ahead, we have a startup bypass flow path model for a once-through boiler, and the flow rate ratio of superheated steam and flash steam calculated from this model, and 1
1. A steam temperature control method for a thermal power plant, characterized in that a steam enthalpy estimation calculation is performed using a predicted temperature of a secondary superheater model. 3. In the method described in claim 1,
A thermal power plant characterized in that the data constituting each of the above models is corrected based on the deviation between the estimated value of the boiler steam temperature obtained in the prediction calculation process of the primary superheater model and the secondary superheater model and its actual value. Plant steam temperature control method.