CN112666217A - Capacitive dew point monitoring device and method - Google Patents
Capacitive dew point monitoring device and method Download PDFInfo
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- CN112666217A CN112666217A CN202011248339.5A CN202011248339A CN112666217A CN 112666217 A CN112666217 A CN 112666217A CN 202011248339 A CN202011248339 A CN 202011248339A CN 112666217 A CN112666217 A CN 112666217A
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Abstract
The invention discloses a capacitance dew point monitoring device and a method, wherein the device comprises: the smoke filtering module is used for filtering impurities in the detected smoke to obtain the filtered detected smoke; the dew point measuring module is used for measuring the dew point of the filtered measured smoke; wherein the dew point measurement module comprises a capacitive sensor; and the condensation processing module is used for reducing the temperature of a capacitance plate of the capacitance sensor in the dew point measuring module, so that dew is formed on the capacitance plate by the filtered measured smoke. The invention can improve the sensitivity of dew point measurement, reduce the error of dew point measurement and further improve the accuracy of the measurement result of the flue gas dew point.
Description
Technical Field
The invention belongs to the technical field of sensors and measurement, and particularly relates to a capacitive dew point monitoring device and method.
Background
The heat loss of the exhaust gas is one of the maximum heat losses of the boiler, and in order to improve the heat efficiency of the boiler, the thermal power plant often uses a mode of reducing the temperature of the exhaust gas. However, during the exhaust process, since the fuel of the boiler usually contains elements such as sulfur, nitrogen and carbon, the exhaust gas contains SO2、SO3、NO、NO2And CO2And (3) combining the acid gas with the water vapor in the flue gas to form corresponding acid steam under low temperature conditions. If the exhaust gas temperature is too low, the wall temperature of the metal convection heating surface is lower than the acid dew point of the exhaust gas, and acid droplets are condensed on the surface of the metal heating surface. The condensed acid liquid drops not only corrode the heating surface, but also can be further combined with fly ash particles in flue gas and iron rust stripped by corrosion of the metal heating surface to cause ash accumulation hardening, block a flue and cause difficulty in maintaining negative pressure, and even possibly cause combustion deterioration in a furnace or abnormal operation of power station equipment. SO contained in flue gas2、SO3If the acidic gas is directly discharged into the atmosphere, the environmental pollution can be caused, so that in order to reach the flue gas emission standard, the desulfurization is an important process in the flue gas treatment and discharge process, and the accurate acquisition of the acid dew point of the flue gas is the basis for realizing the desulfurization process, and has important significance for the stable operation of a power station boiler, the maintenance of equipment and the flue gas treatment.
In the current flue gas acid dew point measuring method, a conductive dew point meter is one of the most common methods, and the main principle is that two electrodes are inserted into a flue gas pipeline, the electrodes are connected with a power supply and a high-precision ammeter in series, cold air is introduced into the electrodes, the temperature of the electrodes is controlled by controlling the flow rate or temperature of the cold air, when the temperature of the electrodes is reduced to be below the flue gas dew point, acid drops formed by condensation on the electrodes are closed to cause a circuit connected with the electrodes in series, and the current of discharge and the temperature of the electrode points are measured, so that the dew point can be measured. The dew point measuring method utilizes an electrode discharge principle, the theory is feasible, but in actual operation, whether acid liquor drops can be condensed on the electrodes or not enables the conduction between the electrodes to be uncontrollable, the sensitivity requirement of the method on the electrodes is high, and the accuracy of a flue gas dew point measuring result can be influenced. Therefore, aiming at the characteristics of measuring the dew point of the flue gas, a dew point measuring device with higher sensitivity, accuracy and reliability needs to be developed.
Disclosure of Invention
The present invention is directed to a capacitive dew point monitoring device and method, which are used to solve one or more of the above problems. The invention can improve the sensitivity of dew point measurement, reduce the error of dew point measurement and further improve the accuracy of the measurement result of the flue gas dew point.
In order to achieve the purpose, the invention adopts the following technical scheme:
the invention discloses a capacitance dew point monitoring device, which comprises:
the smoke filtering module is used for filtering impurities in the detected smoke to obtain the filtered detected smoke;
the dew point measuring module is used for measuring the dew point of the filtered measured smoke; wherein the dew point measurement module comprises a capacitive sensor;
and the condensation processing module is used for reducing the temperature of a capacitance plate of the capacitance sensor in the dew point measuring module, so that dew is formed on the capacitance plate by the filtered measured smoke.
The invention is further improved in that the dew point measuring module specifically comprises: the device comprises a capacitance sensor, a temperature sensor, an air pressure sensor, a conditioning circuit, a microprocessor, an LCD display circuit and a power supply module; the temperature sensor is attached to the middle of the capacitor plate and used for measuring the temperature of the plate; the air pressure sensor is arranged in the middle of the capacitor plate and used for measuring the air pressure of the measured smoke; the capacitance, temperature and air pressure signals are processed by the conditioning circuit and then sent to the microprocessor; when acid gas in the detected flue gas is condensed and condensed, the microprocessor is used for detecting that the capacitance variation exceeds a preset threshold value through the capacitance sensor, controlling the temperature sensor and the air pressure sensor to process and transmit the acquired temperature and pressure signals to the microprocessor, and finishing data processing and transmitting the processed signals to the LCD display circuit to record the temperature and pressure variation curve of the flue gas in the condensation process.
The invention is further improved in that the electrode plate on the side of the capacitive sensor contacting the flue gas is subjected to gold plating treatment.
A further development of the invention consists in that the condensation treatment module comprises:
the cold air input pipeline is used for inputting cold air to the dew point measurement module to reduce the temperature of the capacitor plate, so that water vapor in the smoke near the capacitor plate is condensed into dew on the capacitor plate;
and the hot air output pipeline is used for leading out the hot air in the dew point measuring module.
A further development of the invention consists in that the flue gas filtration module comprises: a smoke filtering material, an air pump and a tee joint;
the air pump is controlled by the microprocessor; when the smoke is filtered, the microprocessor controls the air pump to rotate forwards, so that the smoke is sucked to the gap between the capacitor plates through the filter material and then returns to the flue through the tee joint; during dust removal, the microprocessor controls the air pump to rotate reversely, and the air pump is used for blowing air in the atmosphere to the smoke filtering material from the tee joint to remove dust, so that blockage is prevented.
In a further development of the invention, the flue gas filter material is made of ceramic fibers.
In a further improvement of the invention, when the microprocessor in the dew point measuring module calculates,
theoretical calculation formula of capacitance value of the capacitance sensor:
wherein C is the capacitance of the capacitor, ε is the dielectric coefficient, εrIs the relative dielectric constant of the medium, epsilon0Is the vacuum dielectric coefficient, A is the electrode area, d is the electrode distance;
when the condensation occurs between the electrode plates of the capacitance sensor, the relative dielectric coefficient of the medium is epsilonr1Capacitance value of capacitor at this time
By plotting the capacitance C1Finding out the capacitance C of the jump point along the time t change curve1_stepAnd the dew point temperature measured by the temperature sensor at the moment.
The invention has the further improvement that the microprocessor in the dew point measurement module also comprises air pressure compensation during calculation, and the air pressure compensation method is based on a Haase and Borgmann flue gas acid dew point estimation formula;
the Hasse & Borgmann estimate is:
in the formula, tadpThe estimated value of the acid dew point of the flue gas;
is SO in flue gas2，SO3Partial pressure of gas, atm;
the partial pressure of water vapor in the flue gas is atm;
the compensation formula is as follows:
Td＝k1Tmea+k2Tcom
Tcom＝27.6lgΔP
in the formula, TdFor compensated dew point value, TmeaFor directly measured dew point values, TcomIs according to Hasse&Theoretical compensation amount of dew point obtained by a Borgmann calculation formula; wherein, the delta P is the difference value between the measured smoke pressure value in the pipeline and the smoke pressure value in the set state; k is a radical of1，k2Is a compensation factor.
The invention discloses a capacitive dew point monitoring method, which is based on the capacitive dew point monitoring device and comprises the following steps:
placing a dew point measuring module in a measured flue gas pipeline;
filtering impurities in the detected flue gas through a flue gas filtering module to obtain the filtered detected flue gas;
introducing the filtered measured smoke into a capacitor plate of a capacitance sensor;
the temperature of a capacitance plate of a capacitance sensor in the dew point measuring module is reduced through the condensation processing module, dew is formed on the capacitance plate by the filtered measured smoke, and the dew point is obtained by measuring the temperature of the capacitance plate during capacitance jump.
Compared with the prior art, the invention has the following beneficial effects:
aiming at the problem of measuring the flue gas dew point of a thermal power plant, the invention develops the capacitance type dew point measuring device with high accuracy and high sensitivity based on the measuring principle of the capacitance sensor, can eliminate the influence of factors such as rusting and deformation of an electrode of a conductive dew point meter, easiness in influence of the environment on the flue gas condensation process and the like on the dew point measuring result, can improve the sensitivity of dew point measurement, and reduces the dew point measuring error. Compared with the common electrode discharge method for measuring the dew point of the smoke, the measuring principle of the device is that when the measured smoke is condensed into dew on the polar plate of the capacitance sensor, the dielectric coefficient of a medium in the capacitance sensor changes, the capacitance value changes, and the dew point can be obtained by measuring the temperature when the capacitance jumps, so that the invention can effectively eliminate the influence of uncertain factors of dew point condensation, corrosion of electrodes and other problems on dew point measurement. The device has the advantages of simple structure, high reliability and lower cost.
In the invention, the capacitance sensor measures the capacitance value in real time, and the polar plate on one side contacting with the flue gas is subjected to gold plating treatment so as to prevent the corrosion of the capacitive sensor by condensed acid dew.
In the invention, the dew point measuring module is positioned in the flue gas to be measured, the condensation processing module sucks gas from the outside of the flue to cool the capacitor polar plate in the dew point measuring module, so that water vapor in the flue gas near the polar plate is condensed into dew on the capacitor polar plate, and meanwhile, tiny particles and NO in the flue gasx、SO2、SO3When gas is dissolved in the dew, the dielectric coefficient of the medium between the polar plates is greatly changed, the capacitance value is greatly changed, the temperature of the polar plates at the moment is measured, and the gas pressure compensation is carried outAnd calculating the dew point of the flue gas under the standard air pressure state.
In the invention, the smoke filtering module aims at eliminating the influence of impurities such as large particles in smoke on a dew point measuring result, and comprises a smoke filtering material, an air pump and a tee joint; preferably, the flue gas filtering material is made of ceramic fibers, and has high dust removal efficiency, high strength, high heat resistance and high corrosion resistance; during filtering, dust-containing gas enters a ceramic filter, high-efficiency separation of dust particles and the gas is realized through the processes of interception and capture, inertial collision capture, diffusion capture and the like, and finally the dust particles are discharged to a gap of a capacitor plate of a dew point measurement module through an air suction pump to carry out dew point measurement; optionally, the smoke filtering module works under the control of the microprocessor, and when the smoke is filtered, the air pump rotates forwards to suck the smoke to the gap between the capacitor plates through the filtering material, and then the smoke returns to the flue through the tee joint; after working for a period of time, the air pump is controlled to rotate reversely, and air in the atmosphere is blown to the filtering material from the tee joint to remove dust, so that the filtering material is prevented from being blocked.
In the invention, the condensation processing module forms a gas path between the outer side of the capacitor plate in the dew point measuring module and the atmosphere outside the flue, and blows the air in the atmosphere to the outer side of the capacitor plate under the control of the microprocessor, so that the temperature of the plate is reduced and the plate returns to the atmosphere.
In the invention, for the capacitor with known plate area and plate distance, the capacitance value and the relative dielectric coefficient of the medium between the plates are in stable linear relation; because the condensation speed of the flue gas is high, when the condensation between the electrode plates of the capacitance sensor occurs, the change of the capacitance value can be approximately jump, and the relative dielectric coefficient of the medium is epsilonr1Capacitance value of capacitor at this time
By plotting the capacitance C1Finding out the capacitance C of the jump point along the time t change curve1_stepAnd the dew point temperature measured by the temperature sensor can be repeatedly measured to accurately measure the smoke dew point. The significance of the air pressure compensation is that the temperature is reduced when the flue gas enters the measuring pipeline, and the pipe is arranged at the momentThe air pressure in the air duct changes to influence the dew point measurement result, so that the air pressure compensation needs to be carried out on the dew point measurement result to reduce the dew point measurement error; the compensation method is based on Haase&Borgmann flue gas acid dew point estimation formula; inside the pipe, when SO3、SO2When the NOx acid gas does not reach a saturated state, namely is not condensed into acid mist, the flue gas dew point is irrelevant to the temperature in the pipeline and is only relevant to the air pressure in the pipeline, and the numerical value of the flue gas dew point is increased along with the increase of the pressure in the flue; according to the Dalton partial pressure law, SO is obtained after the mixed flue gas is desulfurized3When the integral pressure intensity of the flue gas is reduced due to the reduction of the partial pressure of the acid gas and the partial pressure of the water vapor, the measured dew point of the flue gas is lower than the dew point temperature in a set state; therefore, the dew point temperature of the flue gas can be compensated according to the variable quantity of the pressure in the condensation process of the gas in the measuring pipeline, so that the influence of the pressure reduction on the dew point measurement value is reduced.
The capacitance dew point monitoring method of the invention utilizes condensation treatment to cool the polar plate of the capacitance sensor, thereby leading the measured smoke to be condensed on the polar plate to form dew; the capacitance jump when the dew condensation of the smoke to be detected is detected, meanwhile, the temperature sensor is used for collecting corresponding temperature change for measurement (repeated measurement can be carried out, the accuracy is further improved), and the measurement result is substituted into an air pressure compensation calculation formula so as to determine the dew point of the smoke to be detected. The method can improve the sensitivity and accuracy of dew point measurement.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art are briefly introduced below; it is obvious that the drawings in the following description are some embodiments of the invention, and that for a person skilled in the art, other drawings can be derived from them without inventive effort.
FIG. 1 is a schematic block diagram of a capacitive dew point monitoring device in accordance with an embodiment of the present invention;
FIG. 2 is a schematic structural diagram of a capacitive dew point monitoring device according to an embodiment of the present invention;
FIG. 3 is a schematic structural diagram of a dew point measuring module according to an embodiment of the present invention;
in the figure, 1, a capacitive sensor; 2. a temperature sensor; 3. an air pressure sensor.
Detailed Description
In order to make the purpose, technical effect and technical solution of the embodiments of the present invention clearer, the following clearly and completely describes the technical solution of the embodiments of the present invention with reference to the drawings in the embodiments of the present invention; it is to be understood that the described embodiments are only some of the embodiments of the present invention. Other embodiments, which can be derived by one of ordinary skill in the art from the disclosed embodiments without inventive faculty, are intended to be within the scope of the invention.
Referring to fig. 1 to 3, a novel capacitive dew point measuring device according to an embodiment of the present invention includes three modules, i.e., a condensation processing module, a flue gas filtering module, and a dew point measuring module. The dew point measuring module is the core of the invention and is used for measuring the dew point temperature of the measured smoke; the smoke filtering module is used for filtering impurities such as large particles in the detected smoke, and the influence of the impurities on dew point measurement is avoided; the condensation processing module is used for reducing the temperature of a capacitor plate in the dew point measuring module, so that the measured smoke is condensed on the plate to form dew.
Optionally, all three modules are operated under the control of the microprocessor.
The measuring process of the capacitive dew point monitoring device provided by the embodiment of the invention is as follows: the dew point measurement module is arranged in the flue gas to be measured, the condensation processing module is used for pumping gas from the outside of the flue to cool the capacitor plate in the dew point measurement module, so that water vapor in the flue gas near the capacitor plate is condensed into dew on the capacitor plate, and meanwhile, tiny particles and NO in the flue gasx、SO2、SO3And (3) when the gas is dissolved in the dew, the dielectric coefficient of the medium between the polar plates is greatly changed, the capacitance value is greatly changed, the temperature of the polar plates at the moment is measured, and the flue gas dew point in a standard pressure state is finally obtained through pressure compensation.
The smoke filtering module comprises a smoke filtering material, an air pump and a tee joint; the selected flue gas filtering material is ceramic fiber, has higher dust removal efficiency, higher strength and heat resistance, and can resist high-temperature gas corrosion and chemical corrosion. During filtering, the dust-containing gas is introduced into the ceramic filter, the dust particles and the gas are efficiently separated through the processes of interception and capture, inertial collision and capture, diffusion and capture and the like, and finally the dust particles and the gas are discharged to a capacitor plate of the dew point measuring module through the air suction pump to carry out dew point measurement so as to eliminate the influence of impurities on a dew point measuring result and obtain a more accurate dew point value; the smoke filtering module works under the control of the microprocessor, and when the smoke is filtered, the air pump rotates forwards to suck the smoke to the gap of the capacitor plate through the filtering material and then returns to the flue through the tee joint; after working for a period of time, the air pump is controlled to rotate reversely, and air in the atmosphere is blown to the filtering material from the tee joint to remove dust, so that the filtering material is prevented from being blocked.
In the condensation treatment process of the embodiment of the invention, a gas path is formed between the outer side of the capacitor plate in the dew point measurement module and the atmosphere outside the flue, and the condensation treatment module blows air in the atmosphere to the outer side of the capacitor plate under the control of the microprocessor, so that the temperature of the plate is reduced and the plate returns to the atmosphere.
Referring to fig. 3, in the embodiment of the present invention, the dew point measuring module includes a capacitance sensor 1, a temperature sensor 2, an air pressure sensor 3, a conditioning circuit, an LCD display circuit, and a power module; the capacitance sensor 1 measures capacitance in real time, and a polar plate on one side of the capacitance sensor, which is contacted with the flue gas, is subjected to gold plating treatment so as to prevent the capacitance sensor from being corroded by condensed acid dew; the temperature sensor 2 is attached to the middle of the capacitor plate and is tightly attached to the plate, and the temperature of the plate is measured in real time; the air pressure sensor 3 is arranged in the middle of the capacitor plate and is used for measuring the air pressure of the measured smoke in real time; the capacitance, temperature and air pressure signals are processed by the conditioning circuit and sent to the microprocessor; when acid gas in the detected flue gas is condensed and condensed, the microprocessor detects that the capacitance variation exceeds a set threshold value through the capacitance sensor 1, controls the temperature sensor 2 and the pressure sensor 3 to process and transmit acquired temperature and pressure signals to the microprocessor, completes data processing, and transmits the processed signals to the LCD display circuit to record the temperature and pressure variation curve of the flue gas in the condensation process.
The signal conditioning circuit comprises a capacitance-voltage conversion circuit, the conversion from the capacitance value of the capacitance sensor to a voltage signal is realized, the output voltage signal is sent to the microcontroller after pre-amplification and filtering processing, and the microcontroller performs data processing and outputs the data to the LCD display circuit; the capacitance-voltage conversion circuit selects the CAV444 linear capacitance-voltage conversion chip, and has the advantages of simple use and wide measurement range.
The embodiment of the invention is based on a capacitance measuring principle, and the device comprises a condensation treatment module, a smoke filtering module, a dew point measuring module, a signal conditioning circuit, a microprocessor, an LCD display circuit and a power supply module. The dew point measuring process is characterized in that the temperature of a polar plate of the capacitance sensor is reduced by a method of introducing cold air, acid gas contained in smoke can be combined with water vapor to form acid vapor under the low-temperature condition, dew is further condensed on the surface of the polar plate along with the reduction of the temperature of the smoke, the dielectric coefficient of the capacitance sensor is changed, so that the capacitance is changed, the temperature at the moment of change is recorded, and the dew point temperature can be obtained. The invention utilizes the capacitance measurement principle of the capacitance sensor to measure the dew point, can completely reflect the process of flue gas condensation, and improves the sensitivity and reliability of the dew point measurement. Therefore, the invention can eliminate the interference of the flue gas condensation process and realize the function of accurately, reliably and sensitively measuring the flue gas dew point.
The research content of the novel capacitive dew point monitoring device provided by the embodiment of the invention comprises the following steps: the structure design of the smoke filtering module, the condensation processing module and the dew point measuring module is adopted; researching the flow of a gas circuit in the condensation processing module to control the temperature of a polar plate of the capacitance sensor, so that smoke can be condensed on the polar plate to form dew; and determining the optimal parameters by researching the influence of structural parameters such as the polar plate distance, the electrode area and the like of the capacitive sensor in the dew point measurement module on the dew point measurement result.
The capacitance type sensor in the dew point measuring module is a core element for measuring the dew point, and when dew is condensed and formed on a polar plate of the capacitance type sensor, the dielectric coefficient of a medium between the polar plates of the capacitance type sensor jumps, so that the jump of the capacitance value is caused. Capacitive sensingTheoretical calculation formula of capacitance value:
therefore, for a capacitor with known plate area and plate distance, the capacitance value and the dielectric coefficient between the plates are in a stable linear relation.
The air pressure sensor provided by the embodiment of the invention is a silicon piezoresistive air pressure sensor, has the characteristics of mature processing technology, high sensitivity, good stability, good dynamic response, small volume, low power consumption and low cost, and is easy to develop towards the direction of integration and intellectualization. When dew point measurement is carried out, the variable quantity of pressure intensity in the condensation process of gas in the measuring pipeline is substituted into an air pressure compensation formula to compensate the dew point temperature of the flue gas, so that the influence of pressure intensity reduction on a dew point measurement value is reduced.
The simulation analysis process of the embodiment of the invention comprises the following steps: through comparing the capacitance value among the dew point measurement process and dew point value curve fit, obtain comparatively optimized structural parameter for capacitive coupling is less between the electrode, and when the flue gas began the condensation, the dielectric coefficient changed, and current capacitance value changes and has clear limit. On the basis of simulation analysis, a prototype is manufactured, then an intelligent analysis model of a test result is established, experimental test comparison is carried out, and the applicability of parameters is determined. If the requirements are met, the design is finished, otherwise, the simulation analysis is returned, and the parameters are corrected again according to the measured result.
Compared with the common electrode discharge method for measuring the dew point of the smoke, the measuring principle of the invention is that when the measured smoke is condensed into dew on the polar plate of the capacitance sensor, the dielectric coefficient of a medium in the capacitance sensor is changed, the capacitance value is changed, and the dew point can be obtained by measuring the temperature when the capacitance jumps, so that the invention can effectively eliminate the influence of uncertain factors of dew point condensation, corrosion of electrodes and other problems on dew point measurement. The invention has the following beneficial effects: in the application process of the dew point measurement module, the capacitance sensor pole plate is cooled by condensation treatment, so that the measured smoke is condensed on the pole plate to form dew, the capacitance jump when the measured smoke is condensed is detected, meanwhile, the temperature sensor is used for collecting corresponding temperature change for repeated measurement, and the measurement result is substituted into an air pressure compensation calculation formula to determine the dew point of the smoke to be measured. Therefore, the method improves the sensitivity and accuracy of dew point measurement, and has the advantages of simple structure, high reliability and lower cost.
As shown in figure 1, the system block diagram of the capacitance dew point monitoring device is shown, the smoke to be measured is firstly filtered to eliminate the influence of impurities in the smoke on a dew point measuring result, a condensation processing module enables a capacitance sensor polar plate in a dew point measuring module to be cooled, water vapor in the smoke near the polar plate is condensed on the polar plate to form dew, and NO in the smoke at the momentx、SO2、SO3When gas and tiny particles are dissolved in dew to form acid mist, dielectric coefficient of a medium between the polar plates is changed greatly, wherein a capacitance sensor outputs a capacitance value as a voltage value through a capacitance-voltage conversion circuit, an output voltage signal is amplified, filtered and the like and then sent to a microprocessor, meanwhile, a temperature sensor processes the acquired signal through a conditioning circuit and then also sends the processed signal to a single chip microcomputer, and the microprocessor processes the data and sends the processed signal to an LCD display circuit to record the temperature at the corresponding moment. The temperature of the capacitance sensor when the capacitance is just transited is determined by repeatedly measuring and comparing the time-varying curves of the capacitance and the temperature, and the accurate dew point temperature of the flue gas can be obtained by carrying out air pressure compensation on the measured value.
The overall structure of the system is schematically shown in figure 2, dew is quickly condensed on the polar plates of the capacitance sensor, and the dielectric coefficient of the dew generates jump, so that the capacitance between the two polar plates is instantaneously changed. Theoretical calculation formula of capacitance value of the capacitance sensor:
c-capacitance capacity (F)
ε -dielectric constant (F/m)
εrDielectric relative permittivity (F/m)
ε0-vacuum dielectricCoefficient (F/m)
A-electrode area (m)2)
d-electrode distance (m)
For a capacitor with known plate area and plate distance, the capacitance and the dielectric coefficient of the medium between the plates are in a stable linear relationship.
When the smoke to be detected is condensed on the polar plate of the capacitance sensor to form dew, the relative dielectric coefficient of the medium is epsilonr1Capacitance value at this time
Drawing C1The change curve finds the capacitance C of the jump point1_stepAnd repeating the measurement process to the measured temperature, and finally determining the dew point temperature of the smoke.
The invention selects a capacitance-voltage conversion chip CAV444, which is an interface integrated circuit capable of measuring the linear conversion proportional voltage output of various capacitive sensor signals. The differential voltage output circuit has the functions of signal acquisition, signal processing and differential voltage output, and the capacitance signal and the output voltage are in a linear relation. The chip can be used for completing capacitance-voltage signal conversion of the capacitance sensor.
In the invention, because the internal pressure of the pipeline changes during flue gas measurement and influences the dew point measurement result, the air pressure compensation needs to be carried out on the dew point measurement result, and the compensation method is based on the Haase and Borgmann flue gas acid dew point estimation formula. The Hasse & Borgmann calculation is:
wherein
Is SO in flue gas3Iso-acid gas partial pressure, atm;
is the partial pressure of water vapor in the flue gas, atm.
Inside the pipe, when SO3、SO2When acid gases such as NO and the like do not reach a saturated state, namely, are not condensed into acid mist, the value of the dew point of the flue gas is irrelevant to the temperature in the pipeline and only relevant to the air pressure in the pipeline, and the value of the dew point of the flue gas rises along with the rise of the pressure in the flue. According to the Dalton partial pressure law, SO is obtained after the mixed flue gas is desulfurized3When the acid gas partial pressure and the water vapor partial pressure are reduced, the integral pressure of the flue gas is reduced, and the dew point temperature of the flue gas is lower than that of the flue gas in a design state. Therefore, the dew point temperature of the flue gas can be compensated according to the variable quantity of the pressure intensity in the condensation process of the gas in the measuring pipeline.
The compensation formula is now given as follows:
Td＝k1Tmea+k2Tcom (2)
Tcom＝27.6lgΔP (3)
Tdfor compensated dew point value, TmeaFor direct measurement of dew value, TcomIs according to Hasse&And 4, theoretical compensation amount obtained by a Borgmann calculation formula. Wherein DeltaP is the difference value k between the measured pressure value of the flue gas in the pipeline and the pressure value of the gas in the design state1，k2The compensation coefficient is obtained by recording the measured dew point value and the pressure variation through multiple experiments under the experimental working condition, fitting a pressure-dew point value curve and then utilizing neural network iterative learning or empirical setting.
In summary, the invention uses a cylindrical capacitor to detect the capacitance change value and the temperature value when the flue gas is condensed into acid mist by using the capacitance value measurement principle, so as to obtain a more accurate flue gas dew point value, aiming at the influence caused by the uncertainty of the circuit conduction caused by the electrode corrosion aging or the flue gas condensation when the flue gas dew point value is measured by an electrode discharge method. The capacitance sensor adopted in the invention has high measurement sensitivity, can clearly record the influence of the smoke condensation in the smoke pipeline on the capacitance value, and improves the measurement sensitivity of the dew point measurement device. According to the invention, through repeated measurement and comparison, a capacitance value measurement curve and an air pressure change curve are fitted, the error influence of the device is eliminated, and accurate, reliable and sensitive flue gas dew point measurement is realized.
Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can make modifications and equivalents to the embodiments of the present invention without departing from the spirit and scope of the present invention, which is set forth in the claims of the present application.
Claims (9)
1. A capacitive dew point monitoring device, comprising:
the smoke filtering module is used for filtering impurities in the detected smoke to obtain the filtered detected smoke;
the dew point measuring module is used for measuring the dew point of the filtered measured smoke; wherein the dew point measuring module comprises a capacitive sensor (1);
and the condensation processing module is used for reducing the temperature of a capacitance pole plate of the capacitance sensor (1) in the dew point measuring module, so that dew is formed on the capacitance pole plate by the filtered measured smoke.
2. The capacitive dew point monitoring device of claim 1 wherein the dew point measuring module comprises: the device comprises a capacitance sensor (1), a temperature sensor (2), an air pressure sensor (3), a conditioning circuit, a microprocessor, an LCD display circuit and a power supply module; the temperature sensor (2) is attached to the middle of the capacitor plate and used for measuring the temperature of the plate; the air pressure sensor (3) is arranged in the middle of the capacitor plate and used for measuring the air pressure of the measured smoke; the capacitance, temperature and air pressure signals are processed by the conditioning circuit and then sent to the microprocessor; when acid gas in the detected flue gas is condensed and condensed, the microprocessor is used for detecting that the capacitance variation exceeds a preset threshold value through the capacitance sensor (1), controlling the temperature sensor (2) and the air pressure sensor (3) to process and transmit the acquired temperature and pressure signals to the microprocessor, and finishing data processing and transmitting the processed signals to the LCD display circuit to record the temperature and pressure variation curve of the flue gas in the condensation process.
3. A capacitive dew point monitoring device as claimed in claim 1, characterised in that the capacitive sensor (1) is gold plated on the side of the plate which is in contact with the flue gas.
4. The capacitive dew point monitoring device of claim 1 wherein the condensation processing module comprises:
the cold air input pipeline is used for inputting cold air to the dew point measurement module to reduce the temperature of the capacitor plate, so that water vapor in the smoke near the capacitor plate is condensed into dew on the capacitor plate;
and the hot air output pipeline is used for leading out the hot air in the dew point measuring module.
5. The capacitive dew point monitoring device of claim 2 wherein said flue gas filtration module comprises: a smoke filtering material, an air pump and a tee joint;
the air pump is controlled by the microprocessor; when the smoke is filtered, the microprocessor controls the air pump to rotate forwards, so that the smoke is sucked to the gap between the capacitor plates through the filter material and then returns to the flue through the tee joint; during dust removal, the microprocessor controls the air pump to rotate reversely, and the air pump is used for blowing air in the atmosphere to the smoke filtering material from the tee joint to remove dust, so that blockage is prevented.
6. A capacitive dew point monitoring device as claimed in claim 5 wherein the fume filtration material is selected from ceramic fibres.
7. The capacitive dew point monitoring device of claim 2 wherein, when the microprocessor in the dew point measuring module calculates,
theoretical calculation formula of capacitance value of the capacitance sensor:
wherein C is the capacitance of the capacitor, ε is the dielectric coefficient, εrIs the relative dielectric constant of the medium, epsilon0Is the vacuum dielectric coefficient, A is the electrode area, d is the electrode distance;
when the condensation occurs between the electrode plates of the capacitance sensor, the relative dielectric coefficient of the medium is epsilonr1Capacitance value of capacitor at this time
By plotting the capacitance C1Finding out the capacitance C of the jump point along the time t change curve1_stepAnd the dew point temperature measured by the temperature sensor at the moment.
8. The capacitive dew point monitoring device according to claim 7, wherein the microprocessor in the dew point measuring module further comprises an air pressure compensation method during calculation, and the air pressure compensation method is based on a Haase & Borgmann flue gas acid dew point estimation formula;
the Hasse & Borgmann estimate is:
in the formula, tadpThe estimated value of the acid dew point of the flue gas;
is SO in flue gas2，SO3Partial pressure of gas, atm;
the partial pressure of water vapor in the flue gas is atm;
the compensation formula is as follows:
Td＝k1Tmea+k2Tcom
Tcom＝27.6lgΔP
in the formula, TdFor compensated dew point value, TmeaFor directly measured dew point values, TcomIs according to Hasse&Theoretical compensation amount of dew point obtained by a Borgmann calculation formula; wherein, the delta P is the difference value between the measured smoke pressure value in the pipeline and the smoke pressure value in the set state; k is a radical of1，k2Is a compensation factor.
9. A capacitive dew point monitoring method, based on the capacitive dew point monitoring device of claim 1, comprising the steps of:
placing a dew point measuring module in a measured flue gas pipeline;
filtering impurities in the detected flue gas through a flue gas filtering module to obtain the filtered detected flue gas;
introducing the filtered smoke to be detected into a capacitor plate of the capacitance sensor (1);
the temperature of a capacitance plate of a capacitance sensor (1) in the dew point measuring module is reduced through the condensation processing module, dew is formed on the capacitance plate by the filtered measured smoke, and the dew point is obtained by measuring the temperature of the capacitance plate during capacitance jump.
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