A thermal calculation method for a supercritical carbon dioxide boiler

Abstract

The present invention provides a thermal calculation method for a supercritical carbon dioxide boiler, belonging to the technical field of boiler thermal calculation. The method includes segmenting the furnace of the supercritical carbon dioxide boiler according to whether fuel is input, calculating the sectional thermal power of the furnace based on the principle of energy conservation; calculating the heat exchange amount in the flue and the heat absorption amount of the high-temperature superheater; determining the thermal calculation result of the supercritical carbon dioxide boiler based on the sectional thermal power of the furnace, the heat exchange amount in the flue and the heat absorption amount of the high-temperature superheater. In view of the problem in the prior art that the empirical coefficient is not applicable to the thermal calculation of large-scale supercritical boilers when the entire furnace is regarded as a whole, the furnace of the boiler is segmented according to whether fuel is input, and the thermal power of each section is calculated based on the principle of energy conservation. This method of adopting sectional thermal calculation can obtain more accurate calculation results.

Description

Technical Field

[0001] The present invention belongs to the technical field of boiler thermal calculation, and specifically relates to a thermal calculation method for a supercritical carbon dioxide boiler. Background Art

[0002] In order to ensure the normal operation of a supercritical CO 2 boiler, it is necessary to perform thermal calculations on it to obtain correct operating data of the boiler and provide a basis for the working state of the boiler. By performing thermal calculations on a supercritical CO 2 boiler, firstly, it is to clarify the heat absorption of each heating surface, and determine the heating surface area and the arrangement of each heating surface according to the magnitude of the heat absorption of the heating surface and the physical properties of the outlet working medium; secondly, it is the wall temperature change. By calculating, the maximum temperature of each heating surface can be obtained, and corresponding measures can be proposed according to the calculation results to prevent accidents such as tube explosion caused by overheating.

[0003] Currently, the standard referred to in boiler thermal calculations regards the entire furnace as a whole. The accuracy is relatively high when calculating small furnaces, but as the furnace size continues to increase, the disadvantages of calculations according to this standard gradually emerge. Some empirical coefficients are no longer applicable to the thermal calculations of large supercritical boilers. The results obtained by using the zero-dimensional model have a large error from the actual boiler operation results and are difficult to accurately reflect the operating parameters, thus affecting the safe operation of the boiler. Summary of the Invention

[0004] In view of this, the present invention aims to provide a thermal calculation method for a supercritical carbon dioxide boiler to solve the above-mentioned deficiencies of the existing calculation methods.

[0005] To solve the above technical problems, the technical solution provided by the present invention is as follows:

[0006] A thermal calculation method for a supercritical carbon dioxide boiler includes the following steps:

[0007] Segment the furnace of the supercritical carbon dioxide boiler according to whether fuel is input, and calculate the sectional furnace heat based on the principle of energy conservation;

[0008] Calculate the heat exchange amount in the flue and the heat absorption of the high-temperature superheater;

[0009] Determine the thermal calculation result of the supercritical carbon dioxide boiler based on the sectional furnace heat, the heat exchange amount in the flue, and the heat absorption of the high-temperature superheater.

[0010] Further, the furnace is divided into a section with fuel input and a section without fuel input.

[0011] Further, in the section with fuel input, the specific principle of energy conservation followed is:

[0012] The heat entering the section with fuel input is the sum of the fuel input heat and the heat carried in by hot air. A part of the heat is absorbed by the furnace gas-cooled walls of this section and the adjacent upper and lower sections in the form of radiation. A part of the heat is lost due to various reasons, and the remaining heat is carried out of this section by the flue gas generated by combustion.

[0013] Furthermore, the section with fuel input is further divided into several consecutive sections.

[0014] Furthermore, in the section without fuel input, only the heat dissipation loss is considered for heat loss.

[0015] Furthermore, the high-temperature superheater includes: a front platen superheater and a rear platen superheater;

[0016] The heat absorption of the front platen superheater is calculated based on the radiation heat absorption mode;

[0017] The heat absorption of the rear platen superheater is calculated based on the heat absorption mode of radiation plus convection.

[0018] Furthermore, the heat absorption of the front platen superheater is determined according to the relationship between the heat absorption of the front platen superheater and the boiler load. In the relationship between the heat absorption of the front platen superheater and the boiler load, the value of the heat absorption of the front platen superheater corresponds one-to-one with the value of the boiler load.

[0019] Furthermore, the calculation process of the heat absorption of the rear platen superheater specifically includes:

[0020] Set the flue gas inlet platen temperature as the platen outlet temperature of the front platen superheater;

[0021] Assume the flue gas outlet platen temperature;

[0022] Assume the convective heat absorption of the additional surface of the furnace top and the additional surfaces of the gas-cooled walls on both sides of the platen;

[0023] Calculate the radiative heat absorption of the furnace top and the additional surfaces of the gas-cooled walls on both sides of the platen;

[0024] Calculate and check the convective heat absorption until the assumed flue gas outlet platen temperature and convective heat absorption meet the set requirements;

[0025] Take the sum of the assumed convective heat absorption and radiative heat absorption as the heat absorption of the rear platen superheater.

[0026] Furthermore, the calculation and checking of the convective heat absorption specifically include:

[0027] Obtain the average heat transfer temperature difference based on the enthalpy of the working medium entering the platen and the enthalpy increment;

[0028] Obtain the convective heat transfer coefficient according to the heat transfer coefficient on the working medium side and the convective heat release coefficient on the flue gas side;

[0029] Calculate the convective heat absorption of the rear platen superheater based on the average heat transfer temperature difference and the convective heat transfer coefficient;

[0030] Check the calculated convective heat absorption. If the first set condition is not met, re-assume the temperature of the outlet platen and recalculate the convective heat absorption according to the subsequent steps until the first set condition is met;

[0031] Calculate the furnace roof heat absorption and the heat absorption of the gas-cooled walls on both sides of the platen;

[0032] Check the convective heat absorption of the economizer on the additional surface of the furnace roof. If the second set condition is not met, re-assume the convective heat absorption of the additional surface of the furnace roof and the gas-cooled walls on both sides of the platen and recalculate the furnace roof heat absorption and the heat absorption of the gas-cooled walls on both sides of the platen according to the subsequent steps until the second set condition is met;

[0033] If the second set condition is met, the check is completed.

[0034] Furthermore, in the calculation of the sectional heat of the furnace, the flame emissivity uses the effective flame emissivity corrected by the correction factor to participate in the calculation. The calculation formula of the correction factor is as follows:

[0035]

[0036] In the formula, K is the correction factor, and a hy is the flame emissivity.

[0037] In summary, the present invention provides a thermal calculation method for a supercritical carbon dioxide boiler, including segmenting the furnace of the supercritical carbon dioxide boiler according to whether the fuel is input, calculating the sectional heat of the furnace based on the principle of energy conservation; calculating the heat exchange amount in the flue and the heat absorption of the high-temperature superheater; determining the thermal calculation result of the supercritical carbon dioxide boiler based on the sectional heat of the furnace, the heat exchange amount in the flue and the heat absorption of the high-temperature superheater. Aiming at the problem that in the existing method, when the entire furnace is regarded as a whole, its empirical coefficient is not applicable to the thermal calculation of large supercritical boilers, the boiler furnace is segmented according to whether the fuel is input, and the thermal calculation is carried out for each section based on the principle of energy conservation. This method of using sectional thermal calculation can obtain more accurate calculation results. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0039] Figure 1The flowchart of a thermal calculation method for a supercritical carbon dioxide boiler provided by an embodiment of the present invention;

[0040] Figure 2 The schematic diagram of the furnace section provided by an embodiment of the present invention;

[0041] Figure 3 The heat balance schematic diagram of section two provided by an embodiment of the present invention;

[0042] Figure 4 The thermal calculation flowchart of the rear platen superheater provided by an embodiment of the present invention;

[0043] Figure 5 The relationship diagram of the heat absorption of the front platen superheater varying with the load provided by an embodiment of the present invention. Specific embodiments

[0044] To make the objectives, features, and advantages of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the embodiments described below are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0045] Please refer to Figure 1 , this embodiment provides a thermal calculation method for a supercritical carbon dioxide boiler, including the following steps:

[0046] S1: Segment the furnace of the supercritical carbon dioxide boiler according to whether the fuel is input, and calculate the sectional furnace heat based on the principle of energy conservation.

[0047] It should be noted that the thermal calculation of the boiler furnace is an important part of the boiler thermal calculation. When performing the thermal calculation of the furnace of a supercritical CO 2 boiler, due to its large furnace volume, it is difficult to accurately calculate the flue gas temperature using the method of treating the furnace as a whole. At the same time, the problem of weakening radiation intensity in the large space also needs to be considered. Therefore, more accurate calculation results can be obtained by using sectional thermal calculation in this step. For the furnace after being divided into sections, each section follows the principle of energy conservation for thermal calculation, and finally accurate calculation results are obtained by combination.

[0048] S2: Calculate the heat transfer amount in the flue and the heat absorption amount of the high-temperature superheater.

[0049] It should be noted that the calculation of the heat transfer amount in the flue and the heat absorption amount of the high-temperature superheater is part of the boiler thermal calculation, and the traditional calculation method can be used to calculate the heat transfer amount in the flue and the heat absorption amount of the high-temperature superheater.

[0050] The calculation method of heat transfer in the flue is consistent with the traditional method. Except for the rear screen superheater, which is a convection + radiation heat transfer method, the heat transfer of the other heating surfaces is mainly convection heat transfer. After the flue gas flows through the heating surface, the heat is absorbed by the working fluid. The heat transfer coefficient on the flue gas side is consistent with the calculation method of the traditional boiler, and the main difference is on the working fluid side. The working conditions of the working fluid in the heating surface are far from the critical state. The convection heat transfer coefficient of the fully developed, non-phase change working fluid in the circular tube can be calculated using the Ditus-Belt formula with good accuracy.

[0051] S3: Determine the thermal calculation results of the supercritical carbon dioxide boiler based on the furnace segment thermal, the heat exchange in the flue and the heat absorption of the high-temperature superheater.

[0052] It should be noted that the thermal calculation of the boiler unit starts with the combustion of the fuel and the heat balance calculation, and then the various heating surfaces of the boiler unit (furnace, convection heat exchanger, platen superheater, etc.) are calculated according to the flue gas flow direction. The thermal calculation results of each part are calculated according to this method and then summarized to obtain the thermal calculation results of the boiler.

[0053] The present invention provides a method for thermal calculation of a supercritical carbon dioxide boiler. The method aims to solve the problem in the existing method that the empirical coefficient is not suitable for thermal calculation of a large supercritical boiler when the entire furnace is regarded as a whole. The boiler furnace is divided into sections according to whether fuel is added, and thermal calculation is performed on each section based on the principle of conservation of energy. This method of using segmented thermal calculation can obtain more accurate calculation results.

[0054] In one embodiment of the present invention, the furnace is divided into sections with fuel input and sections without fuel input, that is, the furnace sections are divided based on whether fuel is input.

[0055] It should be noted that since the divided furnace sections all need to be thermally calculated based on the principle of energy conservation, and whether fuel is added will affect the heat entering the section, using whether fuel is added as the basis for section division can improve the accuracy of thermal calculation results. Compared with zero-dimensional furnace thermal calculation, segmented calculation has higher calculation accuracy.

[0056] In a further embodiment of the present invention, in the section where fuel is added, the energy conservation principle followed is specifically:

[0057] The heat entering the section where fuel is input is the sum of the heat input by the fuel and the heat substituted by the hot air. Part of the heat is absorbed by the air-cooled walls of the furnace in this section and the adjacent upper and lower sections in the form of radiation, part of the heat is lost due to various reasons, and the remaining heat is carried out of the section by the flue gas generated by combustion.

[0058] In a further embodiment of the present invention, the section where fuel is input is further divided into several consecutive sections.

[0059] As Figure 2 shown, Figure 2 is a schematic diagram of the sectional division of the furnace. In the figure, the boiler furnace is divided into Section 1, Section 2, Section 3, and Section 4. Among them, the first three sections are the sections where fuel is input, and Section 4 is the section where no fuel is input. The following takes the Figure 2 shown furnace section as an example to introduce the heat calculation methods for Section 2 and Section 3 among them, and the heat calculations for the remaining sections are introduced incidentally therein, as follows:

[0060] 1. Section 2

[0061] Please refer to Figure 3 , Figure 3 which is a schematic diagram of the heat balance of Section 2. In Section 2, the heat balance equation is: heat input by fuel + heat input by hot air + other heat sources = heat radiated to Section 3 + heat radiated to Section 1 + heat carried away by flue gas + furnace absorption + loss.

[0062] 1) Heat input by fuel Q 1

[0063] Q 1 = zβQ net,ar B j

[0064] In the formula: β - burnout degree of fuel in this section; Q net,ar - low calorific value of fuel, kJ / kg; B j - fuel input in this section, kg / h; z - fuel input in this section.

[0065] 2) Heat carried away by flue gas Q 2 , first assume the flue gas temperature at the outlet of this section, and then perform iterative calculations and checks.

[0066]

[0067] In the formula: - flue gas temperature at the outlet of this section, °C; VC” - heat capacity of combustion products at the outlet flue gas temperature, kJ / (kg·°C).

[0068] 3) Furnace absorption Q 3

[0069] Q 3 = σa 2 T” 4 ψ 2 F 2

[0070] where: σ - Boltzmann constant; a 2 - emissivity of the furnace in section two; T” - outlet flue gas temperature, K; ψ 2 - heat effective coefficient of section two; F 2 - area of the gas-cooled furnace wall in section two.

[0071] 4) Heat radiated outwards Q 4

[0072] Q 4 = σa 2 T” 4 (ψ'A 12 + ψ”A 23 )

[0073] where: ψ' - heat effective coefficient of this section to the lower section; A 12 - cross-sectional area of section one and section two, m 2 ; ψ” - heat effective coefficient of this section to the upper section; A 23 - cross-sectional area of section two and section three, m 2 .

[0074] 5) Considering ash loss Q 5 and heat dissipation loss Q 6 , which can be obtained from the boiler design.

[0075] 6) Heat of hot air Q 7 , which is the product of the set hot air temperature and the theoretical air enthalpy value.

[0076] 7) Other heat sources Q 8 , if there is flue gas recirculation, overfire air, etc., it is the product of the temperature and enthalpy value entering the furnace. The outlet flue gas temperature of this section can be calculated from the heat balance.

[0077] It should be noted that the heat balance equation of section one is similar to that of section one, the difference is that section one lacks the radiant heat to the adjacent lower section, that is, only the radiant heat to section two.

[0078] 2. Section three

[0079] In the furnace thermal calculation of section three, the heat input by the fuel includes two parts. One part is the heat released by the unburned fuel in section two during combustion in this section, and the other part is the heat input by the fuel in this section. At this time, the heat input by the fuel Q 1 ’ is:

[0080] Q 1 ' = (1 - z)ΔβQ net,ar + β'Q net,ar

[0081] Where: Δβ—the proportion of unburned fuel in the previous section burned in this section; β'—the burnout degree of this section.

[0082] In section three, in addition to the heat input from the fuel, there is also the heat input from the combustion products of the fuel in the lower section, Q y and the radiant heat Q from the lower section f , and the overall calculation still follows the principle of heat balance.

[0083]

[0084]

[0085] Where: —the flue gas temperature at the outlet of the upper section, °C; VC”—the flue gas heat capacity at the upper flue gas temperature; a 3 —the furnace blackness of section three.

[0086] In the subsequent sections without fuel input, the heat only comes from the flue gas input. At the same time, in these sections, the ash loss can also be ignored, and only the heat dissipation loss is considered.

[0087] In an embodiment of the present invention, the high-temperature superheater includes: a front screen superheater and a rear screen superheater;

[0088] The heat absorption of the front screen superheater is calculated based on the radiation heat absorption mode;

[0089] The heat absorption of the rear screen superheater is calculated based on the heat absorption mode of radiation plus convection.

[0090] Please refer to Figure 4 , Figure 4 is the thermal calculation flow chart of the rear screen superheater. In a further embodiment of the present invention, in combination with an example, the calculation process of the heat absorption of the rear screen superheater is introduced as follows:

[0091] S21: Set the flue gas inlet screen temperature to the screen outlet temperature of the front screen superheater.

[0092] In an example, the flue gas inlet screen temperature = the furnace outlet flue gas temperature, 1197 °C, and the enthalpy value is 14806 kJ / kg.

[0093] S22: Assume the flue gas outlet screen temperature, which is 1138 °C, and the enthalpy value is 13991 kJ / kg.

[0094] S23: Assume the convective heat absorption of the additional surface on the furnace top and the additional surfaces of the gas-cooled walls on both sides of the screen.

[0095] The heat absorption of the furnace top Q ld = 71 kJ / kg; the heat absorption of the gas-cooled walls on both sides of the screen Q qlb = 51 kJ / kg; the sum of the heat absorptions

[0096] Convective heat absorption of the screen:

[0097]

[0098] In the formula: - Heat preservation coefficient, 0.996; Hp’ - Enthalpy of flue gas entering the screen, 14806 kJ / kg; Hp” - Enthalpy of flue gas leaving the screen, 13991 kJ / kg;

[0099] The convective heat absorption is calculated according to the above formula

[0100] Radiative heat absorption of the screen:

[0101] S24: Calculate the radiative heat absorption of the additional surfaces of the gas-cooled walls on both sides of the furnace top and the screen.

[0102] When calculating the radiative heat absorption in this step, first determine the radiative attenuation coefficients of triatomic gases and ash particles, and further obtain the radiative attenuation coefficient of the flue gas. Secondly, calculate the flue gas emissivity, and finally calculate the radiative heat absorption.

[0103] 1) Radiative attenuation coefficient of triatomic gases:

[0104]

[0105] In the formula: Pn - Partial pressure of triatomic gases, 0.026852 MPa; S - Effective thickness of the radiation layer, 1.28 m; T pj - Average temperature of the flue gas, 1440.7 K;

[0106] Ky = 3.457 is calculated according to the above formula.

[0107] 2) Radiative attenuation coefficient of ash particles:

[0108]

[0109] In the formula, d h - Average diameter of ash particles, 13 μm;

[0110] K is calculated according to the above formula h = 79.26.

[0111] 3) Radiative attenuation coefficient of the flue gas flow = radiative attenuation of triatomic gases * volume fraction + radiative attenuation of ash particles * ash fraction.

[0112] K = μ h K h + K y r n

[0113] In the formula: μ h- Fly ash concentration, 0.0375; r n - Volume fraction of triatomic gas, 0.274;

[0114] Calculated according to the above formula, K = 3.92.

[0115] 4) Flue gas blackness in the platen area

[0116] a = 1 - e -KpS

[0117] p - Pressure, 0.098 MPa; S - Effective thickness of the radiation layer, 1.28 m;

[0118] Calculated according to the above formula, a = 0.387.

[0119] 5) Angle coefficient from the platen inlet to the outlet:

[0120]

[0121] c - Platen depth, 3.5 m; s1 - Transverse pitch, 14.5 mm;

[0122] Calculated according to the above formula

[0123] Correction factor, ξ r = 0.5, selected according to the coal type, 0.5 for bituminous coal.

[0124] 6) Platen outlet area,

[0125] 7) Radiation heat of the flue gas in the furnace and between the platens to the heating surface behind the platen: = Radiation absorbed by the platen inlet * (1 - Flue gas blackness) * Angle coefficient from the inlet to the outlet / β + 5.67 * 10^-11 * Flue gas blackness * Platen outlet area * Average flue gas temperature^4 * Correction factor / (Fuel input / 3600), calculated by the formula.

[0126]

[0127] - Radiation absorbed by the platen inlet, 599 kJ / kg; a - Flue gas blackness, 0.387; - Angle coefficient from the platen inlet to the outlet, 0.132; β - Related to the flue gas temperature, 0.515; - Platen outlet area, 316.8 m2; T pj - Average flue gas temperature, 1440 K; ξ r - Correction factor, 0.5.

[0128] 8) Radiation heat absorbed by the platen area = Radiation absorbed by the inlet - Radiation of the furnace and flue gas to the heating surface behind the platen

[0129]

[0130] Calculated according to the above formula

[0131] 9) Calculate the radiation absorbed by the additional area of the screen section and the furnace top area

[0132] Heat absorbed by the additional area of the furnace top:

[0133]

[0134] In the formula, - Heat absorbed by the screen section, 305 kJ / kg; - Additional area of the furnace top, 17 m 2 ; - Additional area of the gas-cooled wall, 28.4 m 2 ; - Area of the screen receiving furnace radiation, 410 m 2 .

[0135] Calculated according to the above formula

[0136] Heat absorbed by the additional area of the gas-cooled wall:

[0137]

[0138] In the formula, - Additional area of the gas-cooled wall, 28 m 2 ;

[0139] Calculated according to the above formula

[0140] 10) Radiation heat absorbed by the screen = Radiation absorbed by the screen section - Radiation absorbed by the furnace top - Radiation absorbed by the gas-cooled wall, that is:

[0141]

[0142] Calculated according to the above formula

[0143] S25: Calculate and check the convective heat absorption until the assumed flue gas outlet temperature from the screen and the convective heat absorption meet the set requirements.

[0144] It should be noted that in a further embodiment of the present invention, calculating and checking the convective heat absorption specifically includes:

[0145] S251: Obtain the average heat transfer temperature difference based on the enthalpy of the working medium entering the screen and the enthalpy increment.

[0146] It should be noted that in this step, the inlet screen temperature is obtained based on the enthalpy of the working medium entering the screen, the outlet screen temperature is obtained from the enthalpy increment, and finally the average heat transfer temperature difference is obtained.

[0147] S252: Obtain the convective heat transfer coefficient based on the heat transfer coefficient on the working medium side and the convective heat release coefficient on the flue gas side.

[0148] It should be noted that in this step, the heat transfer coefficient on the working medium side is obtained based on the flow velocity of the working medium, the convective heat release coefficient on the flue gas side is obtained based on the flow velocity of the flue gas, and finally the convective heat transfer coefficient is obtained.

[0149] S253: Calculate the convective heat absorption of the rear screen superheater based on the average heat transfer temperature difference and the convective heat transfer coefficient.

[0150] S254: Check the calculated convective heat absorption. If the first set condition is not met, re-assume the outlet screen temperature and recalculate the convective heat absorption according to the subsequent steps until the first set condition is met;

[0151] S255: Calculate the heat absorption of the furnace top and the heat absorption of the gas-cooled walls on both sides of the screen;

[0152] S256: Check the convective heat absorption of the additional economizer on the furnace top surface. If the second set condition is not met, re-assume the convective heat absorption of the additional surface of the furnace top and the additional surfaces of the gas-cooled walls on both sides of the screen and recalculate the heat absorption of the furnace top and the heat absorption of the gas-cooled walls on both sides of the screen according to the subsequent steps until the second set condition is met;

[0153] S257: If the second set condition is met, the check is completed.

[0154] In an example, the calculation process of the above steps is specifically as follows:

[0155] 1) The enthalpy of the working medium entering the screen = the enthalpy of the working medium at the outlet of the front screen (without desuperheating gas volume, the enthalpy value remains unchanged), and H' is calculated p = 1142.58 kJ / kg.

[0156] Obtained from NIST, 30.34 MPa, the inlet screen temperature is 574.9 °C.

[0157] The enthalpy of the working medium leaving the screen

[0158]

[0159] D - mass flow rate, 12546770 kg / h.

[0160] H″ is calculated p = 1163 kJ / kg, NIST, 30.34 MPa, the outlet screen temperature is 620 °C.

[0161] Average temperature of the working medium = 597.45 °C, 870.45 K, 30.34 MPa, specific volume = 0.0057721 m3 / kg.

[0162] 2) Average flow velocity of the working medium

[0163]

[0164] In the formula: v pj - Average specific volume of the working medium, w - Flow velocity of the working medium, 26.8 m / s.

[0165] 3) Average flow velocity of the flue gas

[0166]

[0167] V y - Flue gas volume, 6.74; v pj - Average temperature of the flue gas, 1167.7 °C; A pj - Average flow area of the flue gas, 325.18 m 2 .

[0168] Calculated w y = 8.2 m / s.

[0169] 4) Heat transfer coefficient from the tube wall to the working medium

[0170] The temperature of 870.5 K is the average temperature of the working medium inside the screen, and the pressure is 30 MPa.

[0171] The formula is:

[0172]

[0173] λ is the thermal conductivity of CO 2 Thermal conductivity, 0.0676 W / m·K; d in Is the inner diameter of 0.053 m.

[0174]

[0175] In the formula: density ρ = 171.46 kg / m 3 ; The characteristic length is the inner diameter of the tube, 0.053 m; μ is the kinematic viscosity, 4.0253*10^-5 Pa·s;

[0176] The flow velocity is the mass flow rate (total: 25093 t / h, 50% enters the rear screen, take 12546.77 t / h, converted to 3485.2 kg / s) × specific volume (reciprocal of density, take 0.0058323) / flow cross-section of the working medium (area of a single tube = 0.25 × π × 0.0532, 17 tubes per screen, a total of 20 screens, total area = 0.25 × π × 0.0532 × 17 × 20 = 0.75 m2) = 27.1 m / s.

[0177] The calculated Re = 6117894.7

[0178]

[0179] Cp is the specific heat capacity at constant pressure of 1271.6 J / kg·K; μ is the kinematic viscosity of 4.0253×10^-5 Pa·s; λ is the thermal conductivity of CO 2 with a thermal conductivity of 0.0676 W / m·K.

[0180] The calculated Pr = 0.757.

[0181] The calculated heat transfer coefficient h = 7053 W / m 2 ·K.

[0182] A large mass flow rate leads to a significant increase in the Reynolds number. The above formula is applicable to the calculation of the Reynolds number in the furnace. The total flow area is larger than that of the platen superheater, resulting in an excessive flow velocity and an excessive heat transfer coefficient.

[0183] Calculation of the outer wall temperature:

[0184] The fouling factor ε = 0.007;

[0185] The convective heat transfer coefficient on the flue gas side is 45 W / m 2 ·°C, fitted according to the 1998 standard.

[0186] The formula for the wall temperature is:

[0187]

[0188] t pj - The average temperature of the working fluid, 597.45°C; a 2 - The heat transfer coefficient from the tube wall to the working fluid, 7053 W / m 2 ·°C; Q p = 964.27 kJ / kg; - The calculated convective heating area, 1614.4 m 2 ;

[0189] The calculated t hb = 650.5°C.

[0190] 5) Radiation heat transfer coefficient

[0191] a f = a 0 a

[0192] In the formula: a - the flue gas emissivity, 0.3876, a 0 - Obtained by fitting; a f = 167.2 W / m 2 ·°C.

[0193] 6) Heat transfer coefficient of flue gas to the tube wall:

[0194]

[0195] Utilization factor, ξ = 0.85, a d - Convective heat transfer coefficient on the flue gas side, 45 W / m 2 ·°C; d - Outer diameter, 0.065 mm; S 2 - Longitudinal pitch, 0.1 m; x hp - Corner coefficient of the rear platen, 0.72; a f - Radiation heat transfer coefficient, 167.2 W / m 2 ·°C.

[0196] Calculated to get a 1 = 196.37 W / m 2 ·°C.

[0197] 7) Convective heat transfer coefficient

[0198]

[0199] Calculated to get k = 53.34 W / m 2 ·°C.

[0200] 8) The maximum temperature difference is obtained by subtracting the inlet working fluid temperature from the flue gas temperature entering the platen

[0201] Δt D = 622 °C

[0202] 9) The minimum temperature difference is obtained by subtracting the outlet working fluid temperature from the flue gas temperature leaving the platen

[0203] Δt X = 518 °C

[0204] The average temperature difference is obtained, Δt = 570 °C;

[0205] Then, the convective heat transfer amount is calculated from the average temperature difference.

[0206]

[0207] Calculated to get

[0208] 10) The difference between the working fluid temperature of the gas-cooled wall and the flue gas temperature, combined with the heat transfer area and the fuel input amount, can obtain the heat absorption of the gas-cooled walls on both sides of the platen.

[0209]

[0210] Where: Δt - Average heat transfer temperature difference; A cq - Area of the side walls on both sides of the platen, 112 m 2 .

[0211] Calculated

[0212] 11) Calculate the heat absorption at the furnace top

[0213]

[0214] Δt - Average heat transfer temperature difference; A ld - Furnace top area, 67.221 m 2 .

[0215] Calculated

[0216] After the calculation is completed, check the previously assumed heat absorption.

[0217] S26: Take the sum of the assumed convective heat absorption and radiative heat absorption as the heat absorption of the rear platen superheater.

[0218] Total heat absorbed by the platen = Convection + Radiation.

[0219]

[0220] Calculated Q p = 964.27 kJ / kg.

[0221] In an embodiment of the present invention, the heat absorption of the front platen superheater is determined according to the relationship between the heat absorption of the front platen superheater and the boiler load. In the relationship between the heat absorption of the front platen superheater and the boiler load, the value of the heat absorption of the front platen superheater corresponds one-to-one with the value of the boiler load.

[0222] It should be noted that the high-temperature superheater is arranged at the top of the furnace and is divided into a front platen superheater and a rear platen superheater. After the working medium absorbs heat in furnace section two and section three, it enters the superheater to further increase the parameters of the working medium to improve the efficiency. The heat absorbed by the front platen superheater is mainly radiation, accounting for more than 90%, so the convective heat transfer can be ignored; the heat absorption mode of the rear platen superheater is radiation plus convection, that is, it absorbs the radiation heat inside the furnace and also conducts convective heat transfer with the flue gas. During the process of the boiler changing its operating conditions, the coal feeding amount and the mass flow rate will also change accordingly. Since the increase in the coal feeding amount makes the flue gas at the furnace outlet have higher heat and the outlet flue gas temperature rises, even though the decrease in the mass flow rate leads to a decrease in the heat transfer coefficient on the working medium side, the increased temperature difference makes the heat absorption of the high-temperature superheater increase. Please refer to Figure 5 , Figure 5 is the relationship diagram of the heat absorption of the front platen superheater changing with the load. From Figure 5It can be seen that as the boiler load gradually decreases, the heat absorption of the front platen superheater and the rear platen superheater increases. During the process of the boiler load decreasing from 100% to 30%, the heat absorption increments of the front platen are 1.17%, 2.92% and 5.02%; the increments of the heat absorption of the rear platen are 0.2%, 2.4% and 5.4%. The increment of the heat absorption of the front platen superheater is relatively uniform because all the heat absorbed by the front platen is radiation, and the gradual increase of the coal feeding amount makes the radiant heat intensity in the furnace increase uniformly; while the heat absorption of the rear platen superheater is mainly convective and supplemented by radiation. As the mass flow rate decreases, the convective heat transfer decreases and the proportion of the radiant heat transfer increases, showing a gentle change when the load decreases less and a more drastic change when the load decreases more.

[0223] The existing methods do not consider the non-uniformity of the flame temperature in the horizontal direction. As the boiler size continues to increase, the weakening of radiation along the forward direction is more obvious, and this non-uniformity will also increase. At this time, the calculation of the furnace emissivity will be affected. In an embodiment of the present invention, a coefficient is used to correct the deviation of the calculation result caused by the increase of the furnace size, and the proposed calculation formula for the correction coefficient is

[0224]

[0225] In the formula, K is the correction coefficient, and a hy is the flame emissivity.

[0226] Compared with the original calculation formula, the corrected effective flame emissivity can show the non-linear change with the furnace emissivity, which is closer to the actual furnace emissivity and can reduce the calculation error.

[0227] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A supercritical carbon dioxide boiler thermal calculation method, characterized in that: The steps include: The furnace of the supercritical carbon dioxide boiler is divided into sections according to whether the fuel is put into use, and the heat of the furnace sections is calculated based on the principle of energy conservation; Calculate the heat exchange in the flue and the heat absorption of the high-temperature superheater; Determine the thermal calculation result of the supercritical carbon dioxide boiler based on the furnace segment thermal power, the heat exchange in the flue and the heat absorption of the high-temperature superheater; The furnace is divided into a section with fuel input and a section without fuel input; in the section with fuel input, the energy conservation principle followed is specifically: The heat entering the section where fuel is input is the sum of the heat input by the fuel and the heat substituted by the hot air. Part of the heat is absorbed by the air-cooled walls of the furnace in the section and the adjacent upper and lower sections in the form of radiation, and part of the heat is lost due to at least one of the heat loss of ash and the heat dissipation loss of the furnace wall. The remaining heat is carried out of the section by the flue gas generated by combustion.

2. The supercritical carbon dioxide boiler thermal calculation method according to claim 1 is characterized in that: The section where fuel is added is further divided into several consecutive sections.

3. The supercritical carbon dioxide boiler thermal calculation method according to claim 1 is characterized in that: In the section without fuel injection, heat loss only considers heat dissipation loss.

4. The supercritical carbon dioxide boiler thermal calculation method according to claim 1, characterized in that: The high temperature superheater comprises: a front screen superheater and a rear screen superheater; The heat absorption of the front screen superheater is calculated based on the radiation heat absorption mode; The heat absorption amount of the rear panel superheater is calculated based on the heat absorption mode of radiation plus convection.

5. The supercritical carbon dioxide boiler thermal calculation method according to claim 4 is characterized in that: The heat absorption of the front panel superheater is determined according to the relationship between the heat absorption of the front panel superheater and the boiler load. In the relationship between the heat absorption of the front panel superheater and the boiler load, the value of the heat absorption of the front panel superheater corresponds to the value of the boiler load.

6. The supercritical carbon dioxide boiler thermal calculation method according to claim 4, characterized in that: The calculation process of the heat absorption of the rear screen superheater specifically includes: Set the flue gas inlet temperature to the front screen superheater outlet temperature; Assuming the smoke exit screen temperature; Assume that the additional surface of the furnace top and the additional surface of the air-cooled wall on both sides of the screen absorbs heat through convection; Calculate the radiation heat absorption of the additional surface of the gas-cooled wall on both sides of the furnace roof and the screen; The convective heat absorption is calculated and verified until the assumed smoke exit screen temperature and the convective heat absorption meet the set requirements; The sum of the assumed convection heat absorption and the radiation heat absorption is taken as the heat absorption of the rear panel superheater.

7. The supercritical carbon dioxide boiler thermal calculation method according to claim 6, characterized in that: The convective heat absorption is calculated and verified, specifically including: The average heat transfer temperature difference is obtained based on the working fluid entering the screen enthalpy and enthalpy increment; The convective heat transfer coefficient is obtained according to the heat transfer coefficient on the working fluid side and the convective heat release coefficient on the flue gas side; Calculating the convective heat absorption of the rear screen superheater based on the average heat transfer temperature difference and the convective heat transfer coefficient; The calculated convective heat absorption is checked. If the first set condition is not met, the screen outlet temperature is re-assumed and the convective heat absorption is recalculated according to subsequent steps until the first set condition is met. Calculate the heat absorption of the furnace top and the heat absorption of the air-cooled walls on both sides of the screen; Check the convection heat absorption of the economizer on the additional surface of the furnace top. If the second setting condition is not met, re-assume the convection heat absorption of the additional surface of the furnace top and the additional surface of the air-cooled wall on both sides of the screen, and then recalculate the heat absorption of the furnace top and the heat absorption of the air-cooled wall on both sides of the screen according to the subsequent steps until the second setting condition is met; If the second setting condition is met, the verification ends.

8. The supercritical carbon dioxide boiler thermal calculation method according to claim 1, characterized in that: In calculating the furnace section heat, the flame blackness is calculated using the flame effective blackness corrected by the correction coefficient. The calculation formula of the correction coefficient is as follows: Where K is the correction coefficient, a hy is the flame blackness.

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