A method for measuring the thickness of the slag layer on the heating surface of the furnace based on medium-wave infrared detection technology

Abstract

The present invention provides a method for measuring the thickness of the slag layer on the heating surface of a furnace based on medium-wave infrared detection technology. The method first uses a 3.9μm infrared thermal imager to measure the spectral radiation intensity of the furnace heating surface and the furnace flame, then uses a CMOS camera to obtain the spectral radiation intensity of the flame at 3.9μm, and solves the slag layer temperature on the heating surface based on the Newton iteration method, the BOOW emissivity model, and the Hadley ash thermal conductivity coefficient model. By continuously increasing the iteration order until the relative error of the radiation intensity corresponding to the slag layer temperature is less than the allowable error, it is considered that the iteration has converged. The output temperature at this time is considered to be the actual temperature of the slag layer. Then, based on the temperature difference between the actual temperature of the slag layer and the actual temperature of the heating surface during furnace operation, the slag thickness is calculated according to Fourier's law of heat conduction. The slag layer thickness solution algorithm provided by the present invention can obtain the two-dimensional spatial distribution of the slag thickness of the heating surface tube wall based on the spectral radiation intensity of the slag layer, thereby realizing visual monitoring of the slag on the tube wall.

Description

Technical Field

[0001] The invention relates to a method for measuring a slag layer on a heated surface based on a medium-wave infrared detection technology, and in particular relates to slagging detection in a furnace of a thermal power station, and can realize visual detection of slagging. Background Art

[0002] Due to the wide range of coal grades, boiler operation often deviates from the designed coal grade, leading to widespread slagging on the heating surfaces of various boiler heat exchange components. Slagging on the heating surfaces not only increases heat transfer resistance and reduces unit thermal efficiency, but also causes high-temperature corrosion under the action of high-temperature flue gases, seriously impacting the unit's economic efficiency and safety. Currently, power plants are equipped with sootblowers that periodically purge the heating surfaces with high-pressure steam. However, due to the inability to visually assess the actual slagging status of the heating surfaces, traditional fixed-cycle sootblowing strategies cannot meet the practical needs of slagging prevention and control under wide-load operation. Insufficient sootblowing frequency in some areas exacerbates slagging, while excessive sootblowing in other areas damages the heating surfaces. Current detection methods mostly focus solely on the presence or absence of slagging and lack sufficient spatial and temporal resolution. Therefore, coal-fired boiler units urgently need online slagging detection technology to characterize the distribution of slagging characteristics on the heating surfaces, enabling dynamic optimization of sootblowing frequency and location. Online detection technology for the slag layer on the heating surface of a coal-fired boiler with spatial and temporal resolution can effectively solve the shortcomings of current slagging detection technology. Therefore, it is of great significance to develop a method for measuring the thickness of the slag layer on the heating surface of the furnace based on medium-wave infrared detection technology. Summary of the Invention

[0003] In order to achieve visual detection of slag on the heating surface of the furnace, the present invention provides a method for calculating the slag thickness by detecting the self-radiation characteristics of the slag layer, reconstructing the slag layer temperature, and based on a medium-wave infrared thermal imager, providing a temporally and spatially synchronized distribution of the slag layer thickness on the heating surface.

[0004] The technical solutions of the present invention are as follows:

[0005] A method for measuring the thickness of the slag layer on the heating surface of a furnace based on medium-wave infrared detection technology comprises the following steps:

[0006] Step 1: Use a medium-wave infrared thermal imager to measure the spectral radiation intensity of the slag layer and the flame below 3.9 μm, and use a CMOS camera to measure the spectral radiation intensity of the flame at 3.9 μm, thereby obtaining the spectral radiation intensity of the slag layer below 3.9 μm;

[0007] Step 2: The temperature of the slag layer can be calculated using Planck's theorem. The relationship between radiation intensity and temperature is shown in formula (1):

[0008]

[0009] In formula (1), I is the flame spectral radiation intensity, the unit is W / m 3 / sr,c1=3.742×10 -16 W·m 2 , c2=1.4388×10 -2 m·K, λ is the wavelength in nm, T is the calculated temperature of the slag layer in K, ε 3.9μm is the emissivity;

[0010] The emissivity ε is calculated based on the BOOW emissivity model formula (2) and the Hadley ash thermal conductivity model formula (3):

[0011] ε=0.51lgλ ash +2.44 (2)

[0012]

[0013] Where α = 1.5266(1-p) 8.7381 is a function of the ash consolidation degree, λ ash is the thermal conductivity of the slag layer, p is the porosity, f0 is the constant parameter of the continuous solid, κ=k s / k g k is the thermal conductivity of the solid s and gas thermal conductivity k g The ratio of

[0014] The solid thermal conductivity can be obtained based on the Rezaei thermal conductivity model, as shown in formula (4):

[0015]

[0016] The thermal conductivity of gas can be obtained based on the Touloukian thermal conductivity model, as shown in formula (5):

[0017]

[0018] Step 3: By assuming the initial value of emissivity ε0 and the initial temperature T0 as the initial value of iteration, the actual emissivity and actual temperature of the slag layer are obtained;

[0019] Step 4: Construct the spectral radiation intensity deviation equation;

[0020] Step 5: Use Newton iteration algorithm to iteratively solve ε k+1 、T k+1 , k is the iteration order, and the last iteration result is used as the initial value to solve the slag layer emissivity and slag layer temperature. The iteration is considered to have converged when the iteration result does not change with the iteration order. The output temperature at this time is considered to be the actual temperature of the slag layer.

[0021] Step 6: The actual temperature of the slag layer is obtained from step 5. The temperature difference between the actual temperature of the furnace heating surface during operation is calculated. The radiation heat transfer is equal to the heat conduction in steady state. The slag thickness is calculated according to the Stefan-Boltzmann law (6) and the Fourier heat conduction law (7), as follows:

[0022]

[0023]

[0024] In formulas (6) and (7), σ is the blackbody radiation constant, σ = 5.67 × 10 -8 W / (m 2 ·K 4 ), T soot is the flame temperature, T ash is the actual temperature of the slag layer, T act is the actual temperature of the furnace tube wall, ε soot is the flame emissivity, ε ash is the actual emissivity of the slag layer, λ ash is the thermal conductivity of the slag layer, L is the slag thickness, unit is m, q is the heat flux density, unit is W / m 2 .

[0025] Preferably, in step 4, a spectral radiation intensity deviation equation is established to iterate the actual temperature of the slag layer. The deviation equation is the following formula (8):

[0026]

[0027] In formula (8), I is the measured spectral radiation intensity, in W / m 3 / sr,ε ash is the actual emissivity of the slag layer, T soot is the flame temperature, T ash is the actual temperature of the slag layer, λ is the wavelength, and the unit is nm.

[0028] Preferably, the Newton iteration method is used to iteratively solve include:

[0029] a. Constructing a spectral radiation intensity deviation equation according to formula (8);

[0030]

[0031] b. Construct the partial derivative of the deviation equation with respect to the actual temperature of the slag layer and solve the iterative step size;

[0032]

[0033] c. After obtaining the iterative step size, the actual temperature of the slag layer can be corrected based on the following formula (11):

[0034]

[0035] In the above formula, K is the iteration order, Δ is the correction value;

[0036] d. Determine whether the iterative process has converged based on the trend of the corrected slag layer temperature: If the corrected slag layer temperature does not change with the iteration order, stop the iteration and obtain the iterative convergence value. The slag layer temperature at this time is regarded as the actual temperature of the slag layer. Otherwise, repeat step a and use the corrected actual slag layer temperature as the input value to iterate again;

[0037] e. Output the iterative convergence value.

[0038] The calculation method provided by the present invention can obtain a two-dimensional spatial image of the actual temperature of the slag layer through medium-wave infrared thermal imagery equipment, and at the same time construct a two-dimensional distribution image of the slag thickness on the heated surface. It does not rely on the accuracy of the initial value of the iteration and any prior conditions, has a fast iteration speed, and has a high accuracy of the iteration result. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 An overall flow chart of a method for measuring the thickness of the slag layer on the heating surface of a furnace based on medium-wave infrared detection technology provided in an embodiment of the present invention;

[0040] Figure 2 A two-dimensional distribution image of the furnace water-cooled wall slag thickness calculated by the method of the present invention;

[0041] Figure 3 A two-dimensional distribution image of the slag thickness of the furnace screen superheater calculated by the method of the present invention;

[0042] Figure 4 Iteratively reconstruct the furnace water wall temperature image for the present invention;

[0043] Figure 5 Iteratively reconstruct the furnace screen superheater temperature image for the present invention;

[0044] Figure 6 This is the distribution image of furnace measurement points. DETAILED DESCRIPTION

[0045] The specific implementation method of the present invention will be described below with reference to the accompanying drawings.

[0046] The method described in the present invention is to use a medium-wave infrared thermal imager to measure the 3.9 μm narrow-band spectral radiation intensity of the slag layer and the flame, and use a CMOS camera to measure the flame radiation intensity. The two-dimensional spectral radiation intensity data of the slag layer is used as input data, and the slag layer emissivity ε is iteratively solved based on the Newton iteration algorithm.k+1 , slag layer temperature T k+1 , k is the iteration order, the last iteration result is used as the iteration initial value to solve the actual emissivity and actual temperature of the slag layer, and the iteration order is continuously increased until the iteration result does not change with the change of the iteration order, which is considered to be iteration convergence. The output temperature at this time is regarded as the actual temperature of the slag layer. Then, according to the temperature difference between the actual temperature of the slag layer and the actual temperature of the heating surface during furnace operation, the slag thickness is calculated according to the Stefan-Boltzmann law and Fourier heat conduction law. The overall process can be seen in Figure 1 shown.

[0047] Example

[0048] The method for reconstructing the two-dimensional distribution of slag thickness on the heating surface of the furnace according to the present invention comprises the following specific implementation steps:

[0049] 1. The spectral radiation intensity of the slag layer and flame at 3.9 μm is measured using a medium-wave infrared thermal imager, and the flame radiation intensity is measured using a CMOS camera. The spectral radiation intensity of the slag layer at 3.9 μm can be obtained.

[0050] 2. The spectral radiation intensity of the slag layer can be expressed as:

[0051]

[0052] In the above formula, I is the total spectral radiation intensity in the 3.9 μm narrowband, in W / m 3 / sr;c1=3.742×10 -16 W·m 2 , c2=1.4388×10 -2 m·K; λ is the wavelength, in nm; T soot is the flame temperature, in K; ε 3.9μm is the flame emissivity.

[0053] 3. The flame temperature and spectral radiation intensity under narrow band are obtained by combining the two-color method with the Levenberg-Marquarelt method. The specific formula is:

[0054]

[0055]

[0056] In the above formulas (13) and (14), T is the flame temperature; λ1 = 610 μm, λ2 = 530 μm; λ is the monitoring wavelength of the medium-wave infrared thermal imager, λ = 3.9 μm; I1 and I2 are the spectral radiation intensities of red and green monochromatic light, and I is the spectral radiation intensity of the flame, in W / m 3 / sr.

[0057] In order to obtain the actual temperature of the slag layer that is closest to the measured spectral radiation intensity curve, the measured spectral radiation intensity is combined with formula (12) to construct the spectral radiation intensity deviation equation. The actual emissivity and actual temperature of the slag layer are obtained by solving the equation. The spectral radiation intensity deviation equation is as follows:

[0058]

[0059] I is the spectral radiation intensity, unit is W / m 3 / sr;c1=3.742×10 -16 W·m 2 , c2=1.4388×10 -2 m·K; λ is the wavelength, λ=3.9μm; T soot is the flame temperature, T ash is the actual temperature of the slag layer, in K; ε ash is the actual emissivity of the slag layer; ε 3.9μm is the flame emissivity.

[0060] 4. The Newton iteration method is used to solve the spectral radiation intensity deviation equation. The actual slag layer temperature iteration step and the deviation equation have the following relationship:

[0061]

[0062] In the above formula, Δ is the iteration step size.

[0063] The partial derivative of the deviation equation for the actual temperature of the slag layer is solved by adding a small deviation at the derivative point:

[0064]

[0065] In the above formula, δ(T) is the slight deviation of the deviation equation from the actual temperature of the slag layer.

[0066] After obtaining the correction value of the actual temperature of the slag layer according to formula (17), the actual temperature of the slag layer is corrected based on the following formula and the iteration step size:

[0067]

[0068] In the above formula, k is the iteration order and Δ is the iteration step size.

[0069] The spectral radiation intensity curve is calculated based on the actual emissivity and actual temperature of the slag layer after iteration, and the deviation between the radiation intensity after iteration and the measured radiation intensity is evaluated using the 1-norm of the relative residual:

[0070]

[0071] In the above formula, I measure is the measurement value of the medium-wave infrared imaging device, Ical is the calculated radiation intensity value.

[0072] If the relative residual is greater than the allowable error, the actual emissivity and temperature of the slag layer after iteration are used as inputs for re-iteration. Otherwise, the slag layer temperature after iteration is output as the convergence value. The output value is considered the actual temperature of the slag layer surface. Finally, the temperature difference with the actual temperature of the tube wall during furnace operation is calculated, and the slag thickness is calculated based on Fourier's law of heat conduction.

[0073] by Figure 3 The spectral radiation intensity data of the furnace screen superheater shown is a calculation example. First, the 3.9 μm narrow-band spectral radiation intensity of the screen superheater slag layer and flame is measured by a medium-wave infrared thermal imager. Then, the flame radiation intensity is measured using a CMOS camera. The spectral radiation intensity of the slag layer below 3.9 μm can be obtained as shown in the following formula (20):

[0074]

[0075] In formula (20), I is the total spectral radiation intensity in the 3.9 μm narrowband, I ash The spectral radiation intensity of the slag layer below 3.9 μm, in W / m 3 / sr; c1=3.742×10 -16 W·m 2 , c2=1.4388×10 -2 m·K; λ is the wavelength, λ=3.9μm; T soot is the flame temperature, in K; ε 3.9μm is the flame emissivity.

[0076] Then, the flame temperature and spectral radiation intensity in the 3.9 μm narrow band are calculated using the two-color method combined with the Levenberg-Marquarelt method. The specific formula is:

[0077]

[0078]

[0079] In the above formulas (21) and (22), T soot is the flame temperature; λ1=610μm, λ2=530μm; λ is the monitoring wavelength of the medium-wave infrared thermal imager, λ=3.9μm; I1 and I2 are the spectral radiation intensities of red and green monochromatic light, I soot is the spectral radiation intensity, in W / m 3 / sr;ε 3.9μm is the flame emissivity.

[0080] The actual temperature of the slag layer can be obtained from the spectral radiation intensity using Planck's theorem. The relationship between the spectral radiation intensity and the actual temperature of the slag layer is shown in formula (23):

[0081]

[0082] In formula (23), I ash is the spectral radiation intensity of the slag layer, in W / m 3 / sr; c1=3.742×10 -16 W·m 2 , c2=1.4388×10 -2 m·K; λ is the wavelength, λ=3.9μm; T ash is the actual temperature of the slag layer, in K; ε 3.9μm is the emissivity of the slag layer;

[0083] The solution of the slag layer emissivity ε is based on the BOOW emissivity model formula (2) and the Hadley ash thermal conductivity model formula (3):

[0084] ε=0.51lgλ ash +2.44 (2)

[0085]

[0086] Where α = 1.5266(1-p) 8.7381 is a function of the ash consolidation degree, λ ash is the thermal conductivity of the slag layer, p = 0.3 is the porosity, f0 = 0.8 is the constant parameter of the continuous solid, κ = k s / k g k is the thermal conductivity of the solid s and gas thermal conductivity k g The ratio of

[0087] The solid thermal conductivity can be obtained based on the Rezaei thermal conductivity model, as shown in formula (4):

[0088]

[0089] The thermal conductivity of gas can be obtained based on the Touloukian thermal conductivity model, as shown in formula (5):

[0090]

[0091] By assuming the initial value of emissivity ε0 and the initial temperature T0 as the initial value of iteration, the Newton iteration algorithm is used to iteratively solve ε k+1 、T k+1, k is the iteration order, and the previous iteration result is used as the initial value to solve the actual emissivity and actual temperature of the slag layer. The iteration is considered to have converged when the iteration result does not change with the change of the iteration order. The output temperature at this time is considered to be the actual temperature of the slag layer.

[0092] The actual temperature of the slag layer is obtained, and the temperature difference with the actual temperature of the furnace heating surface during operation is calculated. In steady state, the radiation heat transfer is equal to the heat conduction. According to the Stefan-Boltzmann law (6) and the Fourier heat conduction law (7), the slag thickness is calculated as follows:

[0093]

[0094]

[0095] In formulas (6) and (7), σ is the blackbody radiation constant, σ = 5.67 × 10 -8 W / (m 2 ·K 4 );T soot is the flame temperature; T ash is the actual temperature of the slag layer; T act is the actual temperature of the furnace tube wall; ε soot is the flame emissivity; ε ash is the actual emissivity of the slag layer; ash is the thermal conductivity of the slag layer; L is the slag thickness, in m; q is the heat flux, in W / m 2 .

[0096] Finally, by combining formulas (6) and (7), the slag thickness L of the superheater pipe in the furnace screen can be calculated, and the two-dimensional spatial distribution image of the furnace slag layer can be obtained, as shown in Figure 3 As shown, the slag thickness of the screen superheater is about 14 mm. Figure 2 This is the calculation result of the slag thickness of the furnace water-cooled wall. Figure 2 、 Figure 3 The two-dimensional spatial distribution of slagging reconstruction based on the radiation temperature measurement method proposed in this invention is given. Figure 4 The invention iteratively reconstructs the furnace water wall temperature image, Figure 5 The invention iteratively reconstructs the furnace screen superheater temperature image, Figure 6 This is a distribution map of furnace measurement points, collecting radiation information from the left and right water-cooled walls at a furnace floor height of 12.6m and radiation information from the platen superheater at a floor height of 63m.

[0097] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as falling within the scope of protection of the present invention.

Claims

1. A method for measuring the thickness of the slag layer on the heating surface of a furnace based on medium-wave infrared detection technology, characterized in that: The steps include: Step 1: Use a medium-wave infrared thermal imager to measure the spectral radiation intensity of the slag layer and flame below 3.9 μm, and use a CMOS camera to measure the spectral radiation intensity of the flame, thereby obtaining the spectral radiation intensity of the slag layer below 3.9 μm; Step 2: The temperature of the slag layer can be calculated using Planck's theorem. The relationship between the spectral radiation intensity of the slag layer and the temperature of the slag layer is as shown in formula (1): In formula (1), I ash is the spectral radiation intensity of the slag layer, in W / m 3 / sr,c1=3.742×10 -16 W·m 2 , c2=1.4388×10 -2 m·K, λ is the wavelength in nm, T is the calculated temperature of the slag layer in K, ε 3.9μm is the emissivity; The emissivity ε is calculated based on the BOOW emissivity model formula (2) and the Hadley ash thermal conductivity model formula (3): ε=0.51lgλ ash +2.44 (2) Where α = 1.5266(1-p) 8.7381 is a function of the ash consolidation degree, λ ash is the thermal conductivity of the slag layer, p is the porosity, f0 is the constant parameter of the continuous solid, κ=k s / k g is the solid thermal conductivity k s and gas thermal conductivity k g The ratio of The thermal conductivity of solids can be obtained based on the Rezaei thermal conductivity model, as shown in formula (4): The thermal conductivity of gas can be obtained based on the Touloukian thermal conductivity model, as shown in formula (5): In formulas (4) and (5), T ash is the actual temperature of the slag layer; Step 3: By assuming the initial value of emissivity ε0 and the initial temperature T0 as the initial value of iteration, the actual emissivity and actual temperature of the slag layer are obtained; Step 4: Construct the spectral radiation intensity deviation equation; Step 5: Use Newton iteration algorithm to iterate and solve k is the iteration order. The previous iteration result is used as the initial value to solve the actual emissivity and actual temperature of the slag layer. The iteration is considered to have converged when the iteration result does not change with the iteration order. The output temperature at this time is considered to be the actual temperature of the slag layer. Step 6: The actual temperature of the slag layer is obtained from step 5. The temperature difference between the actual temperature of the furnace heating surface during operation is calculated. The radiation heat transfer is equal to the heat conduction in steady state. The slag thickness is calculated according to the Stefan-Boltzmann law (6) and the Fourier heat conduction law (7), as follows: In formulas (6) and (7), σ is the blackbody radiation constant, σ = 5.67 × 10 -8 W / (m 2 ·K 4 ), λ ash is the thermal conductivity of the slag layer, T soot is the flame temperature, T ash is the actual temperature of the slag layer, T act is the actual temperature of the furnace tube wall, ε soot is the flame emissivity, ε ash is the actual emissivity of the slag layer, L is the slag thickness, unit is m, q is the heat flux density, unit is W / m 2 .

2. The method for measuring the thickness of the slag layer on the heating surface of the furnace based on medium-wave infrared detection technology according to claim 1, characterized in that: In step 4, a spectral radiation intensity deviation equation is established to iterate the actual temperature of the slag layer. The deviation equation is formula (8): In formula (8), I is the measured spectral radiation intensity, in W / m 3 / sr,ε ash is the actual emissivity of the slag layer, T soot is the flame temperature, T ash is the actual temperature of the slag layer, λ is the wavelength, and the unit is nm.

3. The method for measuring the thickness of the slag layer on the heating surface of the furnace based on medium-wave infrared detection technology according to claim 1, wherein the Newton iteration method is used to iteratively solve the include: a. Constructing a spectral radiation intensity deviation equation according to formula (8); b. Construct the partial derivative of the deviation equation with respect to the actual temperature of the slag layer and solve the iterative step size; Where δ(T) is the small deviation of the slag layer temperature; c. After obtaining the iterative step size, the actual temperature of the slag layer can be corrected based on the following formula (11): In the above formula, K is the iteration order, Δ is the correction value; d. Determine whether the iterative process has converged based on the trend of the corrected slag layer temperature: If the corrected slag layer temperature does not change with the iteration order, stop the iteration and obtain the iterative convergence value. The slag layer temperature at this time is regarded as the actual temperature of the slag layer. Otherwise, repeat step a and use the corrected actual slag layer temperature as the input value to iterate again; e. Output the iterative convergence value.

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