A water-cooled wall temperature optimization method based on working medium dryness regulation to control coal supply

Abstract

The present application relates to coal-fired power generation technology field, disclose a kind of water-cooled wall temperature optimization method based on working medium dryness regulation and control coal supply quantity.Supercritical once-through boiler or ultra-supercritical once-through boiler when low load operation, especially in the process of rapid load variation, due to the working medium flow in water-cooled wall drops, and in the process of rapid load variation, water and coal imbalance phenomenon occurs, resulting in insufficient liquid cooling at the pipe wall in the dry steam area and the heat transfer coefficient significantly decreases, the pipe wall temperature will sharply rise.The method of the present application supplements a certain amount of coal, so that the working medium at the dry steam area is quickly converted from high dryness wet saturated steam to superheated steam, reducing the peak temperature of the metal pipe wall during load variation, thereby increasing the load variation rate of the unit.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of coal-fired power generation, in particular to a water-cooled wall temperature optimization method based on working medium dryness control to regulate coal supply, which improves the operation flexibility of coal-fired units under the premise of not exceeding the temperature of the water-cooled wall. BACKGROUND

[0002] Renewable energy represented by wind and solar energy is developing rapidly to implement the carbon peak and carbon neutral policy. Coal-fired power plants must give up part of their power generation share to renewable energy power stations to achieve sustainable energy development and transition to high-quality economic development. However, the instability, intermittency and unpredictability of renewable energy generation pose a huge threat to the safe operation of the power grid. This means that existing coal-fired power plants will have to bear a large amount of peak shaving tasks, including increasing the variable load rate and operating load range of the unit. Therefore, coal-fired power plants will continue to play an irreplaceable role for a long time.

[0003] According to survey data, 80% of forced shutdowns of coal-fired power plants are related to in-furnace piping failures, of which 40% are caused by water-cooled wall pipe bursts. Supercritical coal-fired power plants usually adopt variable pressure operation. During deep load regulation of the unit, the temperature, pressure, enthalpy and other physical parameters of the working medium in the water-cooled wall change dramatically. In particular, during low load operation of the unit, due to uneven flame distribution in the furnace, insufficient fluid dynamics and low working medium pressure, the metal pipe wall may overheat or even burst. Therefore, the safety and stability of the water-cooled wall are the key to the reliable operation of coal-fired power plants during deep load regulation.

[0004] In order to solve the problem of safe and stable operation of the water-cooled wall, a boiler water power calculation method is needed. A flow network system method composed of a nonlinear model is proposed in "A General Water Power Calculation Method for Ultra-Supercritical Boilers" (Application Patent No. CN201710028121.0) for the safety check of water-cooled wall flow and metal wall temperature. A water power calculation method that divides the complex water-cooled wall structure into multiple equivalent water-cooled wall pipes is proposed in "A Boiler Water Power Calculation Method, System, Device and Medium" (Application Patent No. CN202310418097.7) to improve the accuracy of water power calculation. The above only considers the water power characteristics of the water-cooled wall under steady-state conditions of the coal-fired unit. However, the flexibility requirement of the coal-fired unit is becoming higher and higher, and during rapid load variation, the metal pipe wall of the water-cooled wall may overheat. Therefore, it is far from enough to only complete the safety check of water-cooled wall flow and metal wall temperature under steady-state conditions. Therefore, a method is needed to calculate the temperature and pressure of the working medium in the water-cooled wall and the metal inner and outer wall temperature online during rapid load variation of the coal-fired unit, as well as an improved control strategy to reduce the metal wall temperature and thus improve the load variation rate of the unit. SUMMARY

[0005] In order to overcome the problems in the prior art, the present invention aims to provide a method for optimizing the water-cooled wall temperature based on the working fluid dryness to control the coal feed rate, thereby improving the unit's load change rate while ensuring that the water-cooled wall temperature is within the allowable range of the material, thus enhancing the unit's flexibility.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for optimizing the water-cooled wall temperature based on the dryness of the working fluid and the control of coal feed rate is proposed. When a supercritical or ultra-supercritical once-through boiler operates at low load, the water-cooled wall experiences a second type of heat transfer degradation when the working fluid is in the high dryness region (0.8–1.0). This causes a sharp increase in the metal tube wall temperature (i.e., the water-cooled wall temperature), especially during rapid load changes, exceeding the allowable temperature of the metal tube material. When the working fluid dryness exceeds a critical value, an improved fuel supply control strategy is activated. This strategy involves multiplying the deviation between the working fluid enthalpy and the enthalpy of dry saturated steam at the same pressure by a coefficient to obtain the supplementary coal feed rate for the coal-fired unit. This allows the second type of heat transfer degradation region to quickly pass through the dryness region, i.e., the working fluid rapidly transforms from wet saturated steam to superheated steam, thereby reducing the metal tube wall temperature (i.e., the water-cooled wall temperature). Once the working fluid in the dryness region has transformed from wet saturated steam to superheated steam, the improved fuel supply control strategy is discontinued, and the conventional fuel supply control strategy continues to be used. The calculation of the supplementary coal feed rate for the coal-fired unit is as follows:

[0008]

[0009] In the formula: B add This indicates the additional coal supply, expressed in kg / s or h. d-sat The enthalpy of dry saturated steam corresponding to the current working fluid pressure is expressed in kJ / kg; h. real The working fluid's real-time enthalpy is represented by M (kJ / kg); the working fluid's mass flow rate within the water-cooled wall is represented by LHV (kg / s); the lower heating value of the fuel used in the coal-fired unit is represented by R (kJ / kg). Q This indicates the proportion of the heat load on the flue gas side at the highest temperature in the second type of heat transfer deterioration zone within the water-cooled wall, which is the area where the heat is evaporated to dryness.

[0010] When a supercritical or ultra-supercritical once-through boiler is operating at low load, the flow rate of the working fluid in the water-cooled wall decreases. As the dryness of the working fluid increases, the heat transfer coefficient decreases. The metal material used in the water-cooled wall pipes is 15GrMoG. When the dryness of the working fluid is 0.995, the metal wall temperature reaches 528℃. The upper limit of the allowable operating temperature of 15GrMoG is 550℃. In order to increase the safety margin, 0.995 is set as the critical value. When the dryness of the working fluid exceeds 0.995 during the load change process, the improved fuel supply control strategy is activated.

[0011] In order to reasonably control the metal wall temperature of the water cooled wall, the metal wall temperature of the water cooled wall needs to be accurately calculated, and the temperature distribution along the radial direction of the metal pipe wall is obtained from the one-dimensional unsteady heat conduction differential equation, as shown in the following formula:

[0012]

[0013] In the formula, p is the density of the working medium, kg / m 3 ; c p represents the specific heat capacity of the metal pipe wall, J(kg K) -1 ; τ is time, s; r represents the radius of the water cooled wall metal pipe, m; T w is the metal pipe temperature, ℃; λ is the thermal conductivity of the metal pipe, W / (m ℃);

[0014] According to the basic theory of heat transfer, the temperature at the boundary needs to be determined, and the following formula is the boundary equation of the inner wall of the water cooled wall pipe:

[0015]

[0016] In the formula, T in is the inner wall temperature of the metal pipe, ℃; J n is the non-uniformity coefficient of the water cooled wall pipe furnace side inner wall heat flux; β is the ratio of the outer diameter to the inner diameter of the water cooled wall pipe; Q out represents the furnace side heat load, W / m 2 ; T f is the working medium temperature, ℃; α in is the heat transfer coefficient of the working medium in the pipe, W / (m 2 ℃);

[0017] For the outer wall of the water cooled wall pipe, the heat flux density on the boundary is specified, and the boundary equation of the outer wall of the water cooled wall pipe is as follows:

[0018]

[0019] In the formula, T out is the outer wall temperature of the metal pipe, ℃.

[0020] The conventional fuel supply control strategy is: the boiler main control command is fitted with the steady state operation data to obtain the initial steady state fuel supply command; in addition, the load change amount and the load instruction in the variable load process are used as the feedforward of the fuel supply instruction; the deviation between the set value and the actual value of the water cooled wall outlet steam temperature is used as the PID to adjust the water fuel ratio; at the same time, the deviation of the main steam and the reheat steam temperature multiplied by a coefficient is used as the feedforward signal; finally, the actual fuel supply instruction is obtained.

[0021] The improved fuel supply control strategy is suitable for supercritical once-through boiler or ultra-supercritical once-through boiler low load operation condition, that is, the working medium pressure in the water cooling wall is in the range of 10MPa to 17MPa.

[0022] The optimization method is suitable for both the internal thread pipe and the light pipe in the water cooling wall.

[0023] Compared with the prior art, the present application has the following advantages:

[0024] 1) Compared with the water dynamics calculation method which can only calculate the working medium temperature and pressure in the water cooling wall under the steady state condition, the present application realizes online calculation of the working medium temperature and pressure in the water cooling wall in the transient process, and simultaneously considers the metal heat storage of the water cooling wall, so that the inner and outer wall temperatures of the metal pipeline during the unit load change process can be calculated in real time.

[0025] 2) The present application proposes a control strategy based on the working medium dryness to regulate the coal supply, which can reduce the peak temperature of the pipe wall in the water cooling wall dry-out area during the load change process, so as to improve the load change rate of the unit under the premise that the metal pipeline of the water cooling wall does not overheat. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The metal pipe wall temperature of the water cooling wall (in the second type of heat transfer deterioration area) during the load increase process from 30%THA to 50%THA under the improved control strategy and the original control strategy is compared. DETAILED DESCRIPTION

[0027] The technical solutions of the present application will be described clearly and completely in combination with the drawings. The solutions in the embodiments are not used to limit the protection scope of the present application, and any equivalent implementation or change which does not deviate from the present application is included in the protection scope of the present application.

[0028] When the super / ultra-supercritical once-through boiler is running at low load, the heat load of the water cooling wall and the working medium flow decrease, and at the same time, the working medium in the water cooling wall is below the subcritical pressure, at this time, the second type of heat transfer deterioration occurs, which is called dry-out. This may cause the wall temperature to exceed the allowable value of the metal material, especially during the rapid load change process. Therefore, the present application proposes a water cooling wall wall temperature optimization method based on the working medium dryness to regulate the coal supply to reduce the metal wall temperature. First, the working medium temperature, pressure and metal pipeline average temperature and other thermal parameters along the metal pipe height direction of the water cooling wall are calculated based on the mass conservation equation, momentum conservation equation, energy conservation equation, state equation, metal heat storage equation and working medium side heat transfer equation.

[0029] The application is a water-cooled wall temperature optimization method based on working medium dryness regulation of coal supply, when the working medium is in a high dryness 0.8-1.0 area, the water-cooled wall temperature of the metal tube wall, i.e. the water-cooled wall temperature, is sharply increased, and especially in the process of rapid load change, the temperature exceeds the allowable temperature of the metal tube material; when the working medium dryness exceeds the critical value, the improved fuel supply control strategy is started, the improved fuel supply control strategy is that the deviation value of the working medium enthalpy and the dry saturated steam enthalpy under the same pressure is multiplied by a coefficient as the supplementary amount of the coal-fired unit coal supply, so that the dryness area, i.e. the second heat transfer deterioration area, quickly passes through the second heat transfer deterioration process, i.e. the working medium is quickly converted from wet saturated steam to superheated steam, so as to reduce the metal tube wall temperature, i.e. the water-cooled wall temperature; after the working medium in the dryness area is converted from wet saturated steam to superheated steam, the improved fuel supply control strategy is stopped, and the conventional fuel supply control strategy is continued to be used. The supplementary amount of the coal-fired unit coal supply is calculated as follows:

[0030]

[0031] In the formula, B add represents the additional supplementary coal supply, kg / s; h d-sat represents the enthalpy of the dry saturated steam corresponding to the current working medium pressure, kJ / kg; h real represents the real-time enthalpy of the working medium, kJ / kg; M represents the mass flow of the working medium in the water-cooled wall, kg / s; LHV represents the low heat value of the fuel used by the coal-fired unit, kJ / kg; R Q represents the proportion of the flue gas side heat load at the highest temperature in the dryness area, i.e. the second heat transfer deterioration area, in the entire water-cooled wall.

[0032] When the supercritical once-through boiler or the ultra-supercritical once-through boiler is in low load operation, the working medium flow in the water-cooled wall decreases, and with the increase of the working medium dryness, the heat transfer coefficient decreases; the metal material used in the water-cooled wall pipeline is 15GrMoG, when the working medium dryness is 0.995, the metal wall temperature reaches 528℃, the upper limit of the allowable use temperature of 15GrMoG is 550℃, in order to increase the safety margin, 0.995 is set as the critical value, and when the working medium dryness exceeds 0.995 in the load change process, the improved fuel supply control strategy starts to be used.

[0033] In order to reasonably control the metal wall temperature of the water-cooled wall, the metal wall temperature of the water-cooled wall needs to be accurately calculated, and the temperature distribution along the radial direction of the metal tube wall is obtained from the one-dimensional unsteady heat conduction differential equation, as shown in the following formula:

[0034]

[0035] In the formula, ρ is the working medium density, kg / m 3 ; cp The specific heat capacity of the metal pipe wall is expressed in J (kg K). -1 τ is time, in seconds; r represents the radius of the water-cooled wall metal pipe, in meters; T w λ is the temperature of the metal pipe, in °C; λ is the thermal conductivity of the metal pipe, in W / (m °C).

[0036] Based on the fundamental theory of heat transfer, it is necessary to determine the temperature at the boundary. The following equation is the boundary equation for the inner wall of the water-cooled tube:

[0037]

[0038] In the formula, T in It is the temperature of the inner wall of the metal pipe, in °C; J n β is the coefficient of non-uniformity of heat flux on the inner wall of the water-cooled wall tube furnace side; β is the ratio of the outer diameter to the inner diameter of the water-cooled wall tube; Q out This indicates the furnace-side heat load, W / m². 2 ;T f It is the working fluid temperature, in °C; α in It is the heat transfer coefficient of the working fluid inside the pipe, W / (m³). 2 ℃);

[0039] For the outer wall of the water-cooled wall tubes, specifying the heat flux density at the boundary, the boundary equation for the outer wall of the water-cooled wall tubes is as follows:

[0040]

[0041] In the formula, T out It is the temperature of the outer wall of the metal pipe, in °C.

[0042] The conventional fuel supply control strategy is as follows: the boiler main control command is fitted with steady-state operating data to obtain the initial steady-state fuel supply command; in addition, the load change and load command during the load change process are used as feedforwards for the fuel supply command; the deviation between the set value and the actual value of the water-cooled wall outlet steam temperature is used as a PID controller to adjust the fuel-water ratio; at the same time, the deviation between the main steam and reheat steam temperatures is multiplied by a coefficient as a feedforward signal; finally, the actual fuel supply command is obtained.

[0043] The improved fuel supply control strategy is applicable to the low-load operation of ultra / ultra-supercritical once-through boilers, i.e., the working fluid pressure inside the water-cooled wall is in the high-pressure to subcritical range of 10MPa to 17MPa.

[0044] This optimization method is applicable to both internally threaded tubes and smooth tubes in water-cooled walls.

[0045] During the load increase process of a coal-fired power unit from 30% THA to 50% THA, after adopting an improved fuel supply control strategy and the original control strategy respectively, the metal wall temperature changes in the region of second-type heat transfer deterioration within the water-cooled wall are as follows:Figure 1 It is seen that the peak metal wall temperature is significantly reduced and kept within the allowable temperature of the material at the same ramp rate of load change with the improved fuel supply control strategy.

Claims

1. A method for optimizing the wall temperature of a water-cooled wall based on adjusting the coal feed rate according to the dryness of the working fluid, characterized in that: When water-cooled walls are operating at low load in supercritical or ultra-supercritical once-through boilers, a second type of heat transfer degradation phenomenon (evaporization) occurs when the working fluid is in the high dryness region (0.8–1.0). This causes a sharp increase in the metal tube wall temperature (i.e., the water-cooled wall temperature) in this region, exceeding the allowable temperature of the metal tube material during rapid load changes. When the working fluid dryness exceeds a critical value, an improved fuel supply control strategy is activated. This strategy involves multiplying the deviation between the working fluid enthalpy and the enthalpy of dry saturated steam at the same pressure by a coefficient to determine the additional coal feed for the coal-fired unit. This allows the second type of heat transfer degradation to quickly pass through the drying region, meaning the working fluid rapidly transforms from wet saturated steam to superheated steam, thereby reducing the metal tube wall temperature (i.e., the water-cooled wall temperature). Once the working fluid in the drying region has transformed from wet saturated steam to superheated steam, the improved fuel supply control strategy is discontinued, and the conventional fuel supply control strategy is resumed. The calculation of the additional coal feed for the coal-fired unit is as follows: In the formula: B add This indicates the additional coal supply, expressed in kg / s or h. d-sat The enthalpy of dry saturated steam corresponding to the current working fluid pressure is expressed in kJ / kg; h. real The working fluid's real-time enthalpy is represented by M (kJ / kg); the working fluid's mass flow rate within the water-cooled wall is represented by LHV (kg / s); the lower heating value of the fuel used in the coal-fired unit is represented by R (kJ / kg). Q This indicates the proportion of the heat load on the flue gas side at the highest temperature in the second type of heat transfer deterioration zone within the water-cooled wall, which is the area where the heat is evaporated to dryness. The conventional fuel supply control strategy is to fit the boiler main control command with the steady-state operating data to obtain the initial steady-state fuel supply command. In addition, the load change and load command during the load change process are used as feedforwards for the fuel supply command; the deviation between the set value and the actual value of the steam temperature at the water-cooled wall outlet is used as a PID controller to adjust the fuel-water ratio; at the same time, the deviation between the main steam and reheat steam temperatures is multiplied by a coefficient as a feedforward signal; finally, the actual fuel supply command is obtained.

2. The method for optimizing the wall temperature of a water-cooled wall based on the control of coal feed rate according to claim 1, characterized in that: When a supercritical or ultra-supercritical once-through boiler is operating at low load, the flow rate of the working fluid in the water-cooled wall decreases. As the dryness of the working fluid increases, the heat transfer coefficient decreases. The metal material used in the water-cooled wall pipes is 15GrMoG. When the dryness of the working fluid is 0.995, the metal wall temperature reaches 528℃. The upper limit of the allowable operating temperature of 15GrMoG is 550℃. In order to increase the safety margin, 0.995 is set as the critical value. When the dryness of the working fluid exceeds 0.995 during the load change process, the improved fuel supply control strategy is activated.

3. The method for optimizing the wall temperature of a water-cooled wall based on the control of coal feed rate according to claim 1, characterized in that: To effectively control the metal wall temperature of the water-cooled wall, it is necessary to accurately calculate the metal wall temperature. The temperature distribution along the radial direction of the metal tube wall can be obtained using a one-dimensional unsteady-state heat conduction differential equation, as shown in the following equation: In the formula, ρ is the density of the working fluid, kg / m³ 3 c p The specific heat capacity of the metal pipe wall is expressed in J (kg K). -1 τ is time, in seconds; r represents the radius of the water-cooled wall metal pipe, in meters. T w λ is the temperature of the metal pipe, in °C; λ is the thermal conductivity of the metal pipe, in W / (m °C). Based on the fundamental theory of heat transfer, it is necessary to determine the temperature at the boundary. The following equation is the boundary equation for the inner wall of the water-cooled tube: In the formula, T in It is the temperature of the inner wall of the metal pipe, in °C; J n β is the coefficient of non-uniformity of heat flux on the inner wall of the water-cooled wall tube furnace side; β is the ratio of the outer diameter to the inner diameter of the water-cooled wall tube; Q out This indicates the furnace-side heat load, W / m². 2 ;T f It is the working fluid temperature, in °C; α in It is the heat transfer coefficient of the working fluid inside the pipe, W / (m³). 2 ℃); For the outer wall of the water-cooled wall tubes, specifying the heat flux density at the boundary, the boundary equation for the outer wall of the water-cooled wall tubes is as follows: In the formula, T out It is the temperature of the outer wall of the metal pipe, in °C.

4. The method for optimizing the wall temperature of a water-cooled wall based on the control of coal feed rate according to claim 1, characterized in that: The improved fuel supply control strategy is applicable to the low-load operation of supercritical once-through boilers or ultra-supercritical once-through boilers, i.e., when the working fluid pressure inside the water-cooled wall is in the high-pressure to subcritical range of 10MPa to 17MPa.

5. The method for optimizing the wall temperature of a water-cooled wall based on the control of coal feed rate according to claim 1, characterized in that: This optimization method is applicable to both internally threaded tubes and smooth tubes in water-cooled walls.

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