Metal wall temperature prediction method based on flow deviation in water wall tube

Abstract

The invention relates to the technical field of coal-fired power generation, discloses a metal wall temperature prediction method based on flow deviation in a water-cooled wall tube, and aims to solve the problem that supercritical and ultra-supercritical boilers frequently participate in deep peak regulation to cause a second type of heat transfer deterioration phenomenon (namely evaporation to dryness) and cause flow deviation in the water-cooled wall tube. And the flow deviation can influence the position of the evaporation drying point, so that the heat flux density of the point is changed, and finally, the metal wall temperature is changed. Through analysis, the correlation among the flow deviation rate, the evaporation drying point height and the working medium heat exchange coefficient is determined. On the basis, the correlation with the metal wall temperature is established, and the metal wall temperature is predicted by using the flow deviation rate, so that the wall temperature change condition during the operation period of the boiler is monitored in real time, and the safe and stable operation of the boiler is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of coal-fired power generation, and specifically relates to a method for predicting the metal wall temperature based on the flow deviation in the water wall tubes. The wall temperature is calculated through the flow deviation rate of the working medium in the tubes to monitor the metal wall temperature in real time. Background Art

[0002] China is vigorously building a new power system with wind power and photovoltaic power as typical representatives. However, the power generation from renewable energy sources is inherently intermittent and random, which poses a serious challenge to the safe and stable operation of the power grid. As of 2023, the proportion of coal-fired power installed capacity has dropped to 39.9%, but the electricity generated by coal-fired power generation units accounts for nearly 60% of the total electricity generation, and coal-fired units undertake 70% of the peak shaving tasks. This means that coal-fired units not only have to continuously operate at low load, but also need to have a higher load change rate.

[0003] The boiler is one of the extremely key equipment in the coal-fired power generation system. Investigation data shows that 80% of the forced shutdowns in thermal power plants are related to pipeline failures in the furnace, and 40% of these accidents are caused by water wall tube bursts. During the low-load operation of the unit, uneven flow distribution, reduced flow velocity, and decreased operating pressure will occur in the water wall, resulting in the metal pipe wall being extremely prone to overheating and even tube bursts, seriously affecting the safe operation of the water wall. Therefore, ensuring the safe and stable operation of the water wall is the core point for the reliable operation of coal-fired power generation units to achieve deep peak shaving. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above problems and provide a method for predicting the metal wall temperature based on the flow deviation in the water wall tubes, which can predict the metal wall temperature through the flow deviation of the working medium in the tubes to monitor the water wall temperature of the boiler during deep peak shaving in real time.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions:

[0006] A method for predicting the metal wall temperature based on the flow deviation in the water wall tubes determines the quantitative relationship between the heat flux density on the combustion side of the furnace at the dry steam point and the flow deviation rate, specifically as follows: During the comparison of the inner wall temperature at the dry steam point position and with the working medium dryness in the same state, since the enthalpy value of the working medium is constant at this time, formula (1) is used to calculate the heat required to reach the current state; when there is a flow deviation in the metal pipe, the interaction between the heat flux density and the flow deviation rate becomes prominent; according to formula (2), with the heat required by the working medium in the current state, the position of the dry steam point can be accurately inferred; through formula (3), based on the position of the dry steam point, the heat flux density at the current position can be obtained;

[0007] Q DRO =M(h f,DRO-h f,in ) (1)

[0008]

[0009] In the formula, Q DRO represents the heat required to heat the working medium at the inlet of the water wall from the subcooled water state to the dry-out state, in kJ; M is the mass flow rate of the working medium, in kg / s; h f,DRO represents the enthalpy value of the working medium in the dry-out state, in kJ / kg; h f,in is the enthalpy value of the working medium at the inlet of the water wall, in kJ / kg; H DRO represents the height of the dry-out point on the water wall, in m; Q cw is the total heat flux density of the water wall, in kJ; R DRO is the ratio of the heat flux density corresponding to the ratio of the water wall height; H cw is the height of the water wall, in m; Q out,DRO is the heat flux density on the furnace combustion side at the dry-out point, in W / m; R heat,DRO is the ratio of the heat flux density on the furnace combustion side at the dry-out point to the total heat flux density of the water wall; A cw,out is the heat transfer area of the water wall, in m 2 ; ζ is the lateral spacing between the metal pipes inside the water wall, in m;

[0010] Analyze the metal wall temperature distribution of the working medium at the same dryness. The enthalpy values of the working medium at the dry-out point and the inlet of the water wall remain unchanged; the inner wall temperature of the metal pipe at the dry-out point can be expressed by the mass flow rate of the working medium and the ratio of the heat flux density on the furnace combustion side at that point, as shown in formula (4); in view of the fact that the working medium at the dry-out point is in a vapor-liquid two-phase mixed state, its temperature is constantly maintained at the saturation temperature corresponding to the pressure; compared with the case where the flow rate in the metal pipe is the average flow rate, when the flow rate deviation rate is n, the inner wall temperature difference of the metal pipe is calculated by formula (5), so as to obtain the increase in the inner wall temperature of the metal pipe at the dry-out point when the flow rate deviation occurs;

[0011]

[0012] Among them, T in is the inner wall temperature of the metal pipe at the dry-out point, in °C; T f is the temperature of the working medium inside the metal pipe, in °C; β is the ratio of the outer diameter to the inner diameter of the water wall pipe; J n is the heat load shunt coefficient of the water wall tube facing the fire side; A is the cross-sectional area of the metal pipe, in m 2 ; λ f is the thermal conductivity of the working medium, in W / (m 2 °C); d in is the inner diameter of the metal pipe, in m; x is the dryness of the working medium; ρ g and ρ lThey are the densities of saturated steam and saturated water corresponding to the working fluid pressure in the pipe, kg / m 3 ; Pr f is the Prandtl number of the working fluid; v represents the kinematic viscosity of the working fluid, m 2 / s; k1 and k2 are the heat transfer coefficient of the working fluid and the exponential factor of the heat flux density after the position of the evaporation dry point moves respectively; n represents that the flow rate deviation rate of the working fluid in the metal pipe is n; 0 represents that the flow rate in the metal pipe is the average flow rate.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] The wall temperature predicted by the present invention can give an early warning of the abnormal increase in the wall temperature. Before the wall temperature approaches the allowable temperature of the steel, the operating personnel can timely adjust the operating parameters of the unit, avoiding potential safety risks and ensuring the safe and stable operation of the coal-fired power generation unit under the deep peak shaving condition. Compared with the traditional monitoring method, the prediction method proposed by the present invention has significant advantages. In the traditional measurement method, if it is necessary to accurately obtain the real-time wall temperature data at various positions of a large number of metal pipes in the water-cooled wall, it is necessary to face the harsh environments such as high temperature, high corrosion, and strong radiation in the furnace, and a large number of thermocouples are laid on each pipe to directly measure the metal wall temperature through this direct contact method. This method not only has great operation difficulty, but also has high hardware costs. In sharp contrast, the present invention only needs to monitor the three key parameters of the working fluid temperature, working fluid pressure, and working fluid flow rate of the metal pipe, and then can calculate the metal wall temperature. It reduces the difficulty of the measurement work, avoids the complex operation of arranging a large number of thermocouples in the harsh environment, significantly reduces the investment in hardware equipment, and thus effectively reduces the cost. At the same time, the method of the present invention also greatly improves the real-time performance and accuracy of the monitoring, can timely and accurately provide key data support for the coal-fired power generation unit, and lays a solid foundation for its efficient and stable operation, having extremely considerable economic benefits and broad and far-reaching social benefits. Description of the Drawings

[0015] Figure 1 It is the combustion-side heat flux density along the height direction corresponding to the peak wall temperature under the 50% THA condition and different flow deviation conditions.

[0016] Figure 2 It is the combustion-side heat flux density along the height direction corresponding to the peak wall temperature under the 30% THA condition and different flow deviation conditions.

[0017] Figure 3 It is the temperature change rate in the convective heat transfer process caused by different flow deviations under the 50% and 30% THA conditions.

[0018] Figure 4Comparison between the simulated and calculated inner wall temperatures of metal pipes under different flow rate deviations at 50% and 30% THA conditions. Detailed implementation manner

[0019] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. The solutions in the embodiments are not intended to limit the protection scope of the present invention. Any equivalent implementation or change without departing from the present invention is included in the protection scope of the present invention.

[0020] A method for predicting the metal wall temperature based on the flow rate deviation in the water wall tube of the present invention is as follows: Determine the quantitative relationship between the heat flux density on the furnace combustion side at the dry steam point and the flow rate deviation rate, specifically as follows: During the comparison of the inner wall temperatures at the dry steam point position and with the same state of the working fluid dryness, since the enthalpy value of the working fluid is constant at this time, use formula (1) to calculate the heat required to reach the current state; When there is a flow rate deviation in the metal pipe, the interaction between the heat flux density and the flow rate deviation rate becomes prominent; According to formula (2), with the help of the heat required by the working fluid in the current state, the position of the dry steam point can be accurately inferred; Through formula (3), based on the position of the dry steam point, the heat flux density at the current position can be obtained.

[0021] Q DRO = M(h f,DRO - h f,in ) (1)

[0022]

[0023] In the formula, Q DRO represents the heat required to heat the working fluid at the water wall inlet from the subcooled water state to the dry steam state, kJ; M is the mass flow rate of the working fluid, kg / s; h f,DRO represents the enthalpy value of the working fluid in the dry steam state, kJ / kg; h f,in is the enthalpy value of the working fluid at the water wall inlet, kJ / kg; H DRO represents the height of the dry steam point on the water wall, m; Q cw is the total heat flux density of the water wall, kJ; R DRO is the ratio of the heat flux density corresponding to the ratio of the water wall height; H cw is the height of the water wall, m; Q out,DRO is the heat flux density on the furnace combustion side at the dry steam point, W / m; R heat,DRO is the ratio of the heat flux density on the furnace combustion side at the dry steam point to the total heat flux density of the water wall; A cw,out is the heat transfer area of the water wall, m 2 ; ζ is the lateral spacing between the metal pipes in the water wall, m;

[0024] Analyze the metal wall temperature distribution of the working fluid at the same dryness. The enthalpy values of the working fluid at the dryout point and the inlet of the water wall remain unchanged. The inner wall temperature of the metal pipe at the dryout point can be expressed as the ratio of the mass flow rate of the working fluid and the heat flux density on the combustion side of the furnace at that point, as shown in formula (4). Given that the working fluid at the dryout point is in a vapor-liquid two-phase mixed state, its temperature is constantly maintained at the saturation temperature corresponding to the pressure. Compared with the case where the flow rate in the metal pipe is the average flow rate, when the flow rate deviation rate is n, the inner wall temperature difference of the metal pipe is calculated by formula (5), thereby obtaining the increase in the inner wall temperature of the metal pipe at the dryout point when a flow rate deviation occurs.

[0025]

[0026] Among them, T in is the inner wall temperature of the metal pipe at the dryout point, °C; T f is the temperature of the working fluid inside the metal pipe, °C; β is the ratio of the outer diameter to the inner diameter of the water wall pipe; J n is the heat load shunt coefficient of the water wall tube facing the fire side; A is the cross-sectional area of the metal pipe, m 2 ; λ f is the thermal conductivity of the working fluid, W / (m 2 °C); d in is the inner diameter of the metal pipe, m; x is the dryness of the working fluid; ρ g and ρ l are the densities of saturated steam and saturated water corresponding to the pressure of the working fluid inside the pipe, respectively, kg / m 3 ; Pr f is the Prandtl number of the working fluid; v represents the kinematic viscosity of the working fluid, m 2 / s; k1 and k2 are the heat transfer coefficient of the working fluid and the exponential factor of the heat flux density after the dryout point position moves, respectively; n represents that the flow rate deviation rate of the working fluid in the metal pipe is n; 0 represents that the flow rate in the metal pipe is the average flow rate.

[0027] The method of the present invention is mainly applicable to supercritical and ultra-supercritical coal-fired power units under low load conditions, that is, the pressure of the working fluid in the water wall is below the subcritical pressure.

[0028] The method of the present invention calculates the wall temperature change amount in the convective heat transfer process by means of the flow rate deviation, and it is necessary to ensure that the working fluid is in the same thermodynamic state. Specifically, it is necessary to maintain the dryness of the working fluid consistent at all times.

[0029] The method of the present invention is applicable to both the two structural forms of the water wall being an internally threaded tube and a smooth tube.

[0030] When a supercritical and ultra-supercritical boiler is operating at low load and facing rapid load change requirements, the heat flux density on the furnace combustion side decreases significantly. In this case, the second type of heat transfer deterioration phenomenon, i.e., dryout, will occur, and the metal pipes inside the water wall will be extremely prone to abnormally high wall temperatures. At this time, the quality dryness is high, so only the wall temperature in the range of 0.8 to 1.0 of the quality dryness needs to be calculated.

[0031] Supercritical and ultra-supercritical boilers face two key problems due to their frequent participation in deep peak shaving. First, when the operating pressure of the working medium is below the subcritical pressure, the second type of heat transfer deterioration phenomenon, i.e., dryout, will be triggered. Second, this process will cause flow rate deviation in the water wall tubes. The combined effect of the two makes the wall temperature in the dryout area rise sharply. The prerequisite for the normal operation of the pipe wall is that the wall temperature does not exceed the allowable temperature of the steel and meets the strength calculation requirements. As the main heating surface of the boiler, the water wall operates in a harsh environment, and it is difficult to directly measure the metal wall temperature. Different flow rate deviation rates will cause the dryout point to move to different extents in position, and then change the heat flux density on the combustion side at the dryout point. For the 50% THA condition and 30% THA condition (heat consumption rate acceptance condition), under different flow rate deviation conditions, the specific situation of the heat flux density on the combustion side along the height direction corresponding to the peak wall temperature is as Figure 1 and Figure 2 shown. From Figure 1 and Figure 2 it can be seen that assuming the flow rate deviation rate is η l , as the flow rate deviation rate increases, the position of the dryout point will move downwards. At the same time, the heat flux density on the combustion side at the dryout point will also increase accordingly. In view of this, the flow rate deviation rate can be used to predict the metal wall temperature. As Figure 3 shown, when the quality dryness is 0.95, the temperature change rate in the convective heat transfer process caused by different flow rate deviation rates is obtained, so as to monitor the change of the wall temperature during the operation of the boiler in real time and ensure the safe and stable operation of the boiler. By comparing with the simulated values of the operation data, as Figure 4 shown, the maximum relative error of the inner wall temperature of the metal pipe is only 1.3%, which fully meets the industry standards, verifying the rationality and reliability of this theoretical formula in related calculations. The wall temperature predicted by the present invention can give an early warning of the abnormal increase of the wall temperature. Before the wall temperature approaches the allowable temperature of the steel, the operating personnel can adjust the unit operation parameters in time, avoiding potential safety risks and ensuring the safe and stable operation of the unit under the deep peak shaving condition.

[0032] Compared with the traditional monitoring method, the prediction method of the present invention does not need to directly measure the metal wall temperature in a harsh environment, reducing the measurement difficulty and cost, while improving the real-time performance and accuracy of the monitoring, providing a strong guarantee for the efficient and stable operation of coal-fired generating units, and having significant economic and social benefits.

Claims

1. A metal wall temperature prediction method based on flow deviation in water-cooled wall tubes, characterized in that: The quantitative relationship between the heat flux density and the flow deviation rate on the combustion side of the furnace at the evaporation point is determined as follows: in the process of comparing the inner wall temperature at the evaporation point position and when the working fluid dryness is in the same state, since the working fluid enthalpy is constant at this time, the heat required to reach the current state is calculated using formula (1); when there is a flow deviation in the metal pipe, the interaction between the heat flux density and the flow deviation rate becomes prominent; according to formula (2), the position of the evaporation point can be accurately inferred with the help of the heat required by the working fluid in the current state; through formula (3), the heat flux density at the current position is obtained according to the position of the evaporation point; Q DRO =M(h f,DRO -h f,in ) (1) In the formula, Q DRO It represents the heat required to heat the working fluid at the water wall inlet from the supercooled water state to the evaporated state, kJ; M is the mass flow rate of the working fluid, kg / s; h f,DRO Indicates the enthalpy value of the working fluid in the evaporated state, kJ / kg; h f,in is the enthalpy of the working fluid at the water wall inlet, kJ / kg; H DRO Represents the height of the evaporation point on the water-cooled wall, m; Q cw is the total heat flux density of the water-cooled wall, kJ; R DRO is the ratio of the heat flux density to the height of the water wall; H cw is the height of the water-cooled wall, m; Q out,DRO is the heat flux density on the combustion side of the furnace at the evaporation point, W / m; R heat,DRO A is the ratio of the heat flux density on the combustion side of the furnace at the evaporation point to the total heat flux density of the water-cooled wall; cw,out is the heat exchange area of ​​the water-cooled wall, m 2 ;ζ is the lateral spacing between metal pipes in the water-cooled wall, m; The metal wall temperature distribution of the working fluid at the same dryness is analyzed, and the enthalpy of the working fluid at the evaporation point and the water-cooled wall inlet remains unchanged; The inner wall temperature of the metal pipe at the evaporation point can be expressed as the ratio of the mass flow rate of the working fluid and the heat flux density on the combustion side of the furnace at that point, as shown in formula (4); since the working fluid at the evaporation point is in a vapor-liquid two-phase mixed state, its temperature is constantly maintained at the saturation temperature at the corresponding pressure; Compared with the case where the flow rate in the metal pipe is the average flow rate, when the flow deviation rate is n, the temperature difference of the inner wall of the metal pipe is calculated by formula (5), thereby obtaining the increase in the temperature of the inner wall of the metal pipe at the evaporation point when the flow deviation occurs; Among them, T in is the inner wall temperature of the metal pipe at the evaporation point, °C; T f is the working medium temperature in the metal pipe, °C; β is the ratio of the outer diameter to the inner diameter of the water-cooled wall pipe; J n is the heat load diversion coefficient of the water-cooled wall tube to the fire surface; A is the cross-sectional area of ​​the metal pipe, m 2 ; λ f is the thermal conductivity of the working fluid, W / (m 2 ℃); d in is the inner diameter of the metal pipe, m; x is the dryness of the working fluid; ρ g and ρ l They are the densities of saturated steam and saturated water corresponding to the working fluid pressure in the tube, kg / m 3 ; Pr f is the Prandtl number of the working fluid; v represents the kinematic viscosity of the working fluid, m 2 / s; k1 and k2 are the exponential factors of the heat transfer coefficient of the working fluid and the heat flux density after the evaporation point position moves, respectively; n represents the flow deviation rate of the working fluid in the metal pipe; 0 represents the average flow in the metal pipe.

2. The metal wall temperature prediction method based on flow deviation in water-cooled wall tube according to claim 1 is characterized in that: This method is mainly applicable to supercritical and ultra-supercritical coal-fired units under low load conditions, that is, the pressure of the working fluid in the water-cooled wall is below the subcritical pressure.

3. The metal wall temperature prediction method based on flow deviation in water-cooled wall tube according to claim 1 is characterized in that: If flow deviation is to be used to calculate the wall temperature change during convective heat transfer, it is necessary to ensure that the working fluid is in the same thermodynamic state. Specifically, the working fluid dryness must be maintained consistent.

4. The method for predicting metal wall temperature based on flow deviation in water-cooled wall tube according to claim 1, characterized in that: This method is applicable to both water-cooled wall structures of internally threaded tubes and smooth tubes.

5. The method for predicting metal wall temperature based on flow deviation in water-cooled wall tube according to claim 1, characterized in that: When supercritical and ultra-supercritical boilers are in low-load operation and face rapid load changes, the heat flux density on the combustion side of the furnace is significantly reduced. In this case, the second type of heat transfer deterioration phenomenon, namely, evaporation and drying, will occur, and the metal pipes in the water-cooled wall will easily have abnormally high wall temperatures. At this time, the working fluid dryness is high, so it is only necessary to calculate the wall temperature when the working fluid dryness is in the range of 0.8 to 1.0.

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