A method and apparatus for measuring the concentration of a flow of coal particles

Abstract

The present application relates to the technical field of coal dust concentration measurement, and particularly relates to a coal dust particle flow concentration measurement method and device. The measurement method comprises the following steps: arranging the coal dust particle flow concentration measurement device on a fluid pipeline to be measured; feeding a feed ring antenna on the side of a coaxial cavity with a microwave signal to generate resonance in the coaxial cavity; under the action of an inner wave column and an opening at the lower end of the coaxial cavity, part of the electromagnetic field in the coaxial cavity is scattered into the fluid pipeline of the coal dust particles through the stray cavity; the coal dust fluid produces disturbance to the electromagnetic field and changes the resonance frequency when passing through the electromagnetic field scattered into the fluid pipeline; the disturbed microwave signal is received by another feed ring antenna on the side of the coaxial cavity, and the resonance frequency offset before and after the disturbance is determined through a signal processing circuit; based on a previously fitted relationship between the resonance frequency offset and the coal dust concentration, the coal dust concentration is determined according to the resonance frequency offset, and the measurement accuracy is effectively improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of coal dust concentration measurement, in particular to a coal dust particle flow concentration measurement method and device. BACKGROUND

[0002] At present, there are several methods for measuring the concentration of coal dust particle flow in the primary air powder feeding link of a power station boiler, including:

[0003] (1) Heat balance method

[0004] The heat balance method uses the heat conservation of hot air, coal dust and their mixture to calculate the concentration of coal dust. A certain distance in the pneumatic conveying pipeline is selected as the measurement section. The temperatures of air and coal dust before mixing are measured, and the temperature after mixing is detected. The temperatures of coal dust and air before and after mixing will change accordingly. From the temperature change, there is a heat balance equation (1):

[0005]

[0006] In the formula, C w1 , C w3 , C c2 , C c3 are the specific heat of air and coal dust before and after mixing.

[0007] Problem analysis of the heat balance method: this method can only be used to measure the concentration of coal dust in the hot air powder feeding system of the intermediate storage type pulverizing system. For the gas powder feeding and direct blowing type pulverizing system, there is no energy balance process for measurement and calculation during the mixing of air and powder, so the concentration of primary air coal dust cannot be measured.

[0008] (2) Electrostatic (charge induction) method

[0009] The electrostatic method measures the accumulation of static electricity on the particles due to the collision, friction and separation between the particles and the tube wall and between the particles when the particles move with the airflow in the pipeline. The static electricity contains a large amount of information such as particle speed, concentration and particle size. The static electricity of the coal dust particles is measured by using an electrostatic sensor, and then a mathematical modeling method is used to establish the relationship expression between the output voltage of the electrostatic sensor and the concentration of coal dust, so as to obtain the concentration of coal dust.

[0010] Many researchers represented by the University of Kent in the United Kingdom have conducted a lot of research on the model and characteristics of the electrostatic sensor. After ignoring the factors such as two-phase flow pattern change, coal dust fineness, pipeline shape and charge transfer, the model based on electrostatic induction is obtained:

[0011] E = f(C, v)

[0012] Wherein: E is the intensity of the charge induction signal, C is the concentration of the coal powder, and v is the flow rate of the coal powder.

[0013] In actual working conditions, electrostatic method for measuring the concentration of coal powder has many application limitations, mainly including:

[0014] ① The measured medium is mostly low-conductivity medium such as ash and limestone, while the conductivity of coal powder is relatively high, and it is difficult to stably maintain electrostatic charge; ② Due to the existence of charge saturation phenomenon, it is more suitable for application in extremely dilute phase and low flow rate occasions; ③ The sensor itself cannot overcome the interference caused by charge transfer; ④ The original output signal of the sensor has great randomness, and is related to factors such as primary air speed, coal powder quality, coal powder fineness, coal powder humidity, primary air temperature, primary air pipeline direction and shape, and the shape of the sensor itself, so it is difficult to accurately obtain the concentration information; ⑤ The sensor itself deforms under the scouring of the primary air, and the charge sensed by the sensor will also change under the same working condition, so the calibration coefficient will also change, and it is impossible to frequently test and calibrate in application.

[0015] Therefore, in actual application, the electrostatic method for measuring the concentration of coal powder has low precision, and at most can only give the relative value of the concentration of coal powder in the primary air of each coal mill, and the concentration of coal powder in the primary air of different coal mills has no comparability. For some sites, even the detection cannot be carried out.

[0016] (3) ultrasonic method, optical method, and capacitance method

[0017] Ultrasonic wave is a mechanical wave, and its propagation is very dependent on the medium, so its measurement will be affected by many factors of the wind-powder mixed medium, including coal powder speed, temperature, fineness, flow state, local voidage unevenness, etc. The optical method has relatively high cost, and its fundamental is to detect the light intensity and other parameters through the propagation and reflection of light. Placing it in a coal powder pipeline with an average particle diameter of not more than 100 microns and sometimes containing moisture, it is very easy to stick the probe and cannot guarantee the light signal path. The capacitance method is different from the electrostatic method, and is an active detection technology, which is suitable for non-conductive or low-conductive particle flow measurement. When the capacitance method is measured, it is easy to be affected by the charge accumulated on the coal powder particles, and temperature drift is easy to occur on the hardware, which affects the measurement stability and further affects the measurement precision.

[0018] (4) microwave method

[0019] In a thermal power plant, the concentration of coal powder in the primary air pipeline is mainly detected by using the absorption, scattering and resonance of coal powder to microwave, so as to change the amplitude, phase and resonance frequency of the microwave. The concentration of coal powder in the primary air is obtained by detecting the intensity and frequency shift of the microwave signal. Yingnaz Zheng, a scholar from the University of Birmingham in the United Kingdom, detailedly summarized and analyzed the existing technologies and gas-solid two-phase flow process parameter methods of existing products, and considered that among these methods, the microwave technology is considered to be one of the most promising technologies in field application.

[0020] Now the microwave detection field research and development has had a good foundation, the mainstream detection method has the following several kinds: Free space reflection method: sensor emits low energy microwave signal to the metal pipeline conveying coal powder, the microwave meets the coal powder and produces reflection. This method reflects the density of coal powder by measuring the reflected energy of microwave. Free space attenuation method: the way of installing microwave transmitting antenna and microwave receiving antenna at more than 90 degrees angle with the flow direction of coal powder (similar to ultrasonic method), these antennas are usually truncated cone horn antenna. The microwave and coal powder interact and attenuate, the degree of attenuation depends on the mass concentration of coal powder. Waveguide method: the primary air pipeline is regarded as a waveguide, the change of microwave transmission parameter is measured to detect the change of dielectric properties of gas-solid two-phase flow, so as to obtain the concentration of measured object.

[0021] In summary, among the existing measurement methods for measuring concentration, the most widely used is the direct measurement method of heat balance method, but due to the problem of effective heat calculation, the measurement result is unreliable; among the indirect measurement methods, the ultrasonic, optical and capacitance methods have great applicability problems, which leads to the fact that it is almost difficult to apply in the current industrial field; the electrostatic method is widely used in the field, but from the measurement result, although it is claimed that the measurement accuracy is high (5%), it is also measured accurately through calibration, but due to the instability of the accumulated charge of coal powder, the measurement error often exceeds 10%; the microwave method also has practical application in the field, but from the perspective of measurement electronic circuit, the amplitude measurement accuracy based on attenuation is far lower than the frequency measurement. SUMMARY

[0022] The present application provides a sensor with a different principle and structure than any previous field application sensor. The sensor is applied to the pneumatic conveying of particles in the industrial sector, such as the primary air powder conveying of power station boilers. The main problem solved is the inaccurate measurement of the concentration of solid particles in this link.

[0023] To solve the above problems, the present application adopts the following technical solutions to solve them:

[0024] In a first aspect, the present application provides a coal particle flow concentration measuring device, which comprises:

[0025] a sensor structure,

[0026] a coaxial cavity formed on the sensor structure, the lower end of the coaxial cavity is open and sealed with a dustproof cover, an inner waveguide column is coaxially arranged in the coaxial cavity, and two feeding holes for feeding a feed ring antenna are arranged on the side surface of the coaxial cavity;

[0027] a base for welding on the fluid pipeline to be measured, and the lower end of the coaxial cavity is welded on the base;

[0028] A stray cavity is formed on the sensor structure, one end of the stray cavity is formed as a hinged platform for connecting with the base mounting, the other end can be embedded into the fluid pipe through a through hole opened on the fluid pipe, and the bottom end of the inner waveguide column can be flush with the inner wall of the fluid pipe through the dustproof cover and the stray cavity.

[0029] As a preferred embodiment, the dustproof cover is made of organic glass and is bonded with the lower end of the coaxial cavity.

[0030] As a preferred embodiment, a bolt hole is formed in the center of the inner diameter of the upper end of the coaxial cavity, and the inner waveguide column is installed in the bolt hole through threads.

[0031] As a preferred embodiment, the bottom end of the base is arc-shaped for fitting with the outer wall of the fluid pipe.

[0032] As a preferred embodiment, the end of the stray cavity embedded into the fluid pipe is cut into a circular arc shape flush with the curved surface of the inner wall of the fluid pipe, and the outer diameter of the stray cavity is the same as the aperture of the through hole opened on the fluid pipe.

[0033] As a preferred embodiment, the inner diameter of the inner waveguide column is not greater than a preset value, and the preset value is positively correlated with the inner diameter of the fluid pipe to be measured.

[0034] As a preferred embodiment, the inner diameter of the antenna ring of the feed ring antenna is not greater than a corresponding preset value; the microwave signal is fed in through the feed ring antenna, the microwave is at 550-650 MHz, and the microwave is excited in a sweep frequency mode.

[0035] In a second aspect, the present application provides a coal particle flow concentration measuring method, which utilizes the coal particle flow concentration measuring device to achieve the method, and the measuring method comprises:

[0036] The coal particle flow concentration measuring device is arranged on the fluid pipe to be measured.

[0037] One feed ring antenna on the side of the coaxial cavity is connected to the microwave signal and feeds the microwave signal into the coaxial cavity to generate resonance.

[0038] Under the action of the opening at the lower end of the coaxial cavity and the inner waveguide column, part of the electromagnetic field in the coaxial cavity is scattered into the fluid pipe of the coal particle flow through the stray cavity.

[0039] The coal flow disturbs the electromagnetic field when it passes through the electromagnetic field scattered into the fluid pipe and changes the resonance frequency.

[0040] The disturbed microwave signal is received by another feed ring antenna on the side of the coaxial cavity, and the resonance frequency offset before and after disturbance is determined through a signal processing circuit.

[0041] Based on the pre-fitting obtained resonance frequency offset and the relationship between the coal concentration, the coal concentration is determined according to the resonance frequency offset.

[0042] Compared with the prior art, the present application has the following beneficial effects:

[0043] From the simulation results, when the coal powder in the pipeline is fully developed and uniformly distributed, the sensor of the present application can accurately measure the coal powder concentration, and the measurement frequency resolution is 0.8MHz / 1% on average at 20% to 70%. From the sensor end, the frequency resolution of 0.8MHz is very easy to achieve, and the ordinary hardware frequency resolution can reach 1KHz. The measurement accuracy of the market product is generally 5%. From the simulation, the measurement device of the present application can reach 1% resolution. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 It is a structural schematic diagram of the measurement device in Example 1;

[0045] Figure 2 It is a physical picture of the measurement device in Example 1;

[0046] Figure 3 It is a magnetic field distribution diagram of the resonance generated in the coaxial cavity in Example 1;

[0047] Figure 4 It is an electric field distribution diagram of the resonance generated in the coaxial cavity in Example 1;

[0048] Figure 5 It is an electromagnetic field distribution diagram of the stray into the fluid pipeline after the resonance generated in the coaxial cavity;

[0049] Figure 6 It is a mixed dielectric constant model diagram in Example 1;

[0050] Figure 7 It is a simulation curve obtained by taking the coal powder concentration of 20% to 80% as a variable;

[0051] Figure 8 It is the S11 and S21 curves measured by the flow field and electromagnetic field coupling simulation experiment when the coal powder concentration is 50%. DETAILED DESCRIPTION

[0052] In the description of the present application, it is to be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second", and the like are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined with "first", "second" and the like can explicitly or implicitly include one or more of the features. In the description of the present application, unless otherwise stated, the meaning of "a plurality of" is two or more. In the description of the present application, it should be noted that, unless otherwise specified and limited, the terms "mounting", "connecting", "connecting" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, or it can be the communication between two elements inside. For those skilled in the art, the specific meaning of the above terms in the present application can be understood through specific circumstances.

[0053] The present application will be further described below in conjunction with the drawings. The following examples are only used to more clearly illustrate the technical solutions of the present application, and cannot limit the protection scope of the present application.

[0054] Example 1

[0055] In order to make the purpose, technical scheme and advantages of the present application more clear and explicit, the present application will be further described in detail below in conjunction with the drawings and examples.

[0056] In conjunction with Figures 1-2The embodiment provides a coal particle flow concentration measuring device, the measuring device comprises a sensor structure, a coaxial cavity 2 and a stray cavity are formed in the sensor structure, the coaxial cavity 2 is opened at the lower end and is blocked by a dust cover 5, in a specific embodiment, the coaxial cavity 2 is bonded with the cavity by using an organic glass dust cover 5 at the lower end, and the opening end of the coaxial cavity 2 is blocked. An inner wave guide column 3 is coaxially arranged in the coaxial cavity 2, a bolt hole is formed in the center of the inner diameter of the upper end of the coaxial cavity 2, and the inner wave guide column 3 is installed on the bolt hole through screw threads. Two feeding holes for feeding a feed ring antenna 4 are formed on the side of the coaxial cavity 2 and are oppositely arranged, and the two feeding holes are 180°. The sensor structure further comprises a base 1, and the base 1 is used for being welded on a fluid pipeline 6 to be measured. In the embodiment, the base is a square steel plate, the lower surface of which is designed in an arc shape and is closely attached to the pipeline, so that the base is closely attached to and welded on the pipeline, and the lower end of the coaxial cavity 2 is welded on the base 1.

[0057] One end of the stray cavity (the main body is in a cylindrical shape) is formed with a hinged table for being connected with the base 1, specifically, a circular steel strip is welded on the outer wall of the stray cavity to form the hinged table, a hole is punched in the center of the hinged table, and the hinged table is connected with the base 1 by using a bolt; the other end can be embedded in the fluid pipeline 6 through a through hole formed on the fluid pipeline 6, and the bottom end of the inner wave guide column can be flush with the inner wall of the fluid pipeline through the stray cavity. Specifically, in order to realize non-invasive design, a through hole is formed on the fluid pipeline 6 in the embodiment, the hole diameter of the through hole is the same as the outer diameter of the stray cavity, the part of the stray cavity embedded in the fluid pipeline 6 is cut into an arc shape and is flush with the inner wall curve of the pipeline.

[0058] The working principle of the measuring device is as follows: the coal particle flow concentration measuring device is arranged on the fluid pipeline 6 to be measured; one feed ring antenna 4 on the side of the coaxial cavity 2 is connected with a microwave signal and feeds the microwave signal into the coaxial cavity 2 to generate resonance, in the embodiment, the microwave signal is fed in through the feed ring antenna 4, the microwave is 550-650 MHz, the microwave is excited in a sweep frequency mode, and the electric field lines and the magnetic field lines when resonance are as shown in Figure 3 and Figure 4 The part of the electromagnetic field in the coaxial cavity 2 is scattered into the fluid pipeline 6 of the coal particles through the stray cavity, and the electromagnetic field distribution in the fluid pipeline 6 is as shown in Figure 5 Under the action of the opening end of the coaxial cavity 2 and the inner wave guide column 3, the coal flow disturbs the electromagnetic field scattered into the fluid pipeline 6 and changes the resonance frequency; the disturbed microwave signal is received by the other feed ring antenna 4 on the side of the coaxial cavity 2, the resonance frequency offset before and after disturbance is determined through a signal processing circuit; based on the relationship between the resonance frequency offset and the coal concentration obtained by pre-fitting, the coal concentration is determined according to the resonance frequency offset.

[0059] The inner wave guide column 3 in the embodiment has a diameter; the feed ring antenna 4 has a diameter, an inner diameter of the antenna ring, and an antenna connection mode with the feed plug; all of the above are optimized designs. In actual use, to adapt to different fluid pipes 6, the length of the inner wave guide column is generally not less than 94 mm and not more than 114 mm. The inner diameter of the inner wave guide column 3 is not greater than a preset value, and the preset value is positively correlated with the inner diameter of the fluid pipe 6 to be measured. In combination with a specific example, when the measuring device is applied to a fluid pipe 6 with an inner diameter of 500 mm, the corresponding parameter settings are as follows: the diameter of the inner wave guide column 3 is not greater than 12 mm; the inner diameter of the antenna ring is not greater than 26 mm. Correspondingly, when the measuring device is applied to fluid pipes 6 with other inner diameters, the parameter settings for the fluid pipe 6 with an inner diameter of 500 mm can be taken as a reference and set according to a preset ratio. For example, when applied to a fluid pipe 6 with an inner diameter of 600 mm, all parameters need to be adaptively enlarged by 1.1 times to ensure measurement accuracy.

[0060] In addition, the inner diameter of the antenna ring of the feed ring antenna 4 is not greater than 26 mm; the outer thread of the radio frequency adapter is hinged with the inner thread of the antenna, the microwave signal is fed through the feed ring antenna 4, the microwave is at 550-650 MHz, and the microwave is excited in a sweep frequency working mode.

[0061] In combination Figure 6 , the specific principle for determining the concentration of the pulverized coal according to the resonance frequency offset in the embodiment is as follows: the resonance cavity perturbation method can obtain the relationship between the resonance frequency change and the dielectric constant of the mixed medium in the pipe: due to the change in the concentration of the original medium inside the resonance cavity, the size and structure of the electromagnetic field change, thereby causing the resonance frequency in the resonance cavity.

[0062] Through derivation of the Maxwell equation, the relationship between the dielectric constant ε and the resonance frequency f can be obtained as follows:

[0063]

[0064] Further, let Δε = ε mix - ε0(ε mix be the dielectric constant of the air and coal mixture after perturbation, and ε0 be the dielectric constant of the mixture before perturbation), then the above formula can be simplified as:

[0065]

[0066] In the formula, C is a to-be-determined coefficient, f is the measurement frequency, and f0 is the cavity resonance frequency.

[0067] The concentration of the coal particles is calculated by using the mixed dielectric constant model (formula 3): ε mixFor the mixed dielectric constant, which is related to the dielectric constants of air and measured particles and the ratio of air and coal powder, the dielectric constants of air and coal powder are known, so the mixed dielectric constant model can be used to measure the concentration m of coal powder c The calculation is as follows.

[0068]

[0069] In the formula, ε c and ε a are the dielectric constants of the medium obtained in advance, and are general constants, which can be taken as 2.7 and 1 as typical values.

[0070] Finally, the relationship between the measured resonance frequency and the concentration to be measured is derived as formula (4)

[0071]

[0072] In the formula, f r and f ro are the real-time measured resonance frequency and the cavity resonance frequency measured in advance respectively; the difference between f r and f ro is the resonance frequency offset, and the prototype in the relationship is a*m c 3 +b*m c 2 +c*m c +d, and a / b / c / d in the formula are fitted to actual values, that is, formula (4).

[0073] The relationship between the resonance frequency offset and the concentration of coal powder is obtained by taking the concentration of coal powder as a variable in the range of 20% to 80%, and the simulation curve is as shown in Figure 7 As shown in the figure, the S11 curves from right to left are black 20% to yellow 80%, and the abscissa corresponding to the valley value is the resonance frequency. It can be seen that the resonance frequency monotonically decreases with the increase of the concentration. When the concentration is 50%, the S11 and S21 curves measured by the flow field and electromagnetic field coupling simulation experiment are as shown in Figure 8

[0074] According to the simulation results, the sensor of the embodiment is obviously improved compared with the existing measurement method, which benefits from the principle of the resonant cavity, the high-frequency microwave resonance generated by the resonant cavity, the high energy concentration, the sensitivity to coal powder, and the electromagnetic field guided into the pipeline by the inner conductor column. It is difficult to measure a pipeline with a length of 500 mm with high precision in the prior art. The quasi-non-invasive design of the embodiment does not damage the internal coal powder flow field, and the measurement result is accurate. In addition, the model in the embodiment adopts the medium perturbation method, and the model form is obtained by combining formula (2) and formula (3) after derivation. The model can be obtained by direct empirical formula nonlinear fitting, and the data is verified by simulation and experiment.​

[0075] The above description is only the preferred embodiment of the present application, it should be pointed out that for those skilled in the art, without departing from the technical principles of the present application, a number of improvements and variations can be made, which should be considered as the protection scope of the present application.

Claims

1. A method of measuring the concentration of a flow of coal particles, characterized by, The application relates to a coal powder particle flow concentration measuring device. A sensor structure, A coaxial cavity is formed on the sensor structure, the coaxial cavity is open at the lower end and is blocked by a dustproof cover, an inner wave guide column is coaxially arranged in the coaxial cavity, and two feeding holes for loading a feeding ring antenna are arranged on the side of the coaxial cavity; the inner diameter of the inner wave guide column is not greater than a preset value, the preset value is positively correlated with the inner diameter of a fluid pipeline to be measured; the inner diameter of the antenna ring of the feeding ring antenna is not greater than a corresponding preset value; microwave signals are fed in through the feeding ring antenna, the microwave is in a sweep frequency working mode, and the microwave is excited; A base is used for being welded on the fluid pipeline to be measured, and the lower end of the coaxial cavity is welded on the base; the bottom end of the base is arc-shaped so as to be matched with the outer wall of the fluid pipeline; A stray cavity is formed on the sensor structure, one end of the stray cavity is formed into a hinged table for being connected with the base, the other end can be embedded into the fluid pipeline through a through hole arranged on the fluid pipeline, the bottom end of the inner wave guide column can be flush with the inner wall of the fluid pipeline through the dustproof cover and the stray cavity, and the one end of the stray cavity embedded into the fluid pipeline is cut into a circular arc shape and is flush with the curved surface of the inner wall of the fluid pipeline; the outer diameter of the stray cavity is the same as the diameter of the through hole arranged on the fluid pipeline. The measuring method comprises the following steps: The coal powder particle flow concentration measuring device is arranged on the fluid pipeline to be measured; One feeding ring antenna on the side of the coaxial cavity is connected with the microwave signals and feeds the microwave signals into the coaxial cavity to generate resonance; Under the action of the coaxial cavity open at the lower end and the inner wave guide column, part of the electromagnetic field in the coaxial cavity is scattered into the fluid pipeline of the coal powder particles through the stray cavity; The coal powder fluid is disturbed when passing through the electromagnetic field scattered into the fluid pipeline and changes the resonance frequency; The disturbed microwave signals are received by the other feeding ring antenna on the side of the coaxial cavity, the resonance frequency offset before and after the disturbance is determined through a signal processing circuit; The coal powder concentration is determined according to the resonance frequency offset based on the relationship between the resonance frequency offset and the coal powder concentration obtained through pre-fitting; The relationship between the measured resonance frequency and the concentration to be measured is: wherein, and are the real-time measured resonance frequency and the previously measured cavity resonance frequency, respectively; and the difference between the two is the resonance frequency shift, m c is the coal dust concentration to be measured.

2. The method of claim 1, wherein, The dustproof cover is made of organic glass and is bonded with the lower end of the coaxial cavity.

3. The method of claim 1, wherein, A bolt hole is arranged at the center of the inner diameter of the upper end of the coaxial cavity, and the inner wave guide column is installed on the bolt hole through screw threads.

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