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Flue gas humidity with fixed pollution source

2020-04-17 15:13:25Source:昶艾电子 Hits:

In the conventional monitoring of flue gas emission, flue gas humidity is an important parameter, and it is also the most difficult to measure accurately. Humidity measurement itself is affected by other factors (atmospheric pressure, temperature), flue gas humidity measurement also needs to face high temperature, high dust, high moisture, negative pressure, corrosion problems.
In addition, humidity calibration is a difficult problem. The reason is that it is difficult to manufacture high temperature humidity generator, so it affects the measurement of humidity online measuring instrument. In order to verify and calibrate the flue gas humidity instrument, there must be a device capable of generating standard humidity source and humidity standard. A humidity measuring method capable of carrying out absolute measurement of humidity can be used as a humidity reference. Gases with known humidity can also be used as benchmarks for humidity. The standard "Sampling Method of Particulate and Gaseous Pollutants in the Exhaust of Fixed Pollution Source" (GB/T16157-1996) prescribes three kinds of flue gas humidity measuring methods: condensation method, weight method and dry-wet ball method, which are used as the reference method of flue gas humidity measurement and can be used for calibrating flue gas humidity instrument. In addition, the humidity generator generates constant humidity at a certain temperature and pressure, and it can also be used to calibrate the flue gas humidity meter


1.Introduction of flue gas humidity measurement method
Dry-wet ball method

The three humidity reference methods in standard GB/T16157-1996 are not ideal in practical application. The weight method and condensation method are complex, the test condition is high, the test time is long. The dry-wet ball method is simple, but the error is large.
The main problem of measuring flue gas humidity by dry-wet ball method is that the flue gas temperature is high, often higher than 100°C, but the dry ball temperature can not reach the actual flue gas temperature.
The temperature is usually between the ambient temperature and the flue gas temperature, resulting in a fixed measurement error. Zong Ningsheng believe that when measuring the humidity of flue gas by using dry-wet ball method, the following operations should be taken into consideration: When the temperature indicator is stable and no longer rises, the readings can be made (5-10 min). The connecting pipe between the sampling pipe and the dry and wet thermometers is shorter, and the pipe wall is not suitable to be thin, so as to prevent the temperature of the flue gas from excessively falling. In the cold climate, heating sampling tubes should be used.
Chang-Ai improved the dry-wet sphere thermometer and heated flue gas sampling tube to avoid the errors of flue gas reaching dew point and steam condensation in the sampling tube.

Figure 1CI-PC39 Outline

According to the actual flue gas temperature at the measuring point, by changing the heating temperature of the sampling tube, the difference between the flue gas temperature entering the dry-wet ball chamber and the flue gas temperature in the sampling point is avoided. The accuracy and stability of the self-made dry-wet-bulb hygrometer were investigated by using the standard dry-wet-bulb method and the weight method. The results show that the humidity data measured by the humidity meter is reliable and effective, and has a sensitive response to the change of the humidity of the flue gas, and can work continuously and stably. Although the impact jet method is the working principle of the dry and wet ball, it is not a common humidity instrument which is similar to the weather department to measure the relative humidity of air. This humidity instrument is a brand-new or even a brand-new innovative design, to achieve the above measurement results, its representative product figure is shown in Figure 1
Chang-Ai measurement engineering has been successfully applied to high temperature and high humidity measurement process, as well as the process with corrosive and dust-containing gases. In many process, in order to ensure the quality of the product, efficient use of energy or to achieve the goal of emission reduction, it is necessary to monitor and control the process gas humidity. CI-PC39 is a process humidity meter for the most stringent industrial requirements, including corrosion resistance, continuous operation and insensitivity to fouling.

Condensation method

The principle of the condensation method is that a certain volume of exhaust gas is extracted from the flue to pass through the condenser, and the water content in the exhaust gas is calculated according to the condensed water quantity and the water vapor content of the saturated gas discharged from the condenser. The principle of the weight method is that a certain volume of exhaust gas is extracted from the flue so that the moisture in the exhaust gas is absorbed by the moisture absorption tube by the moisture absorption agent through the moisture absorption tube filled with the moisture absorption agent, and the weight of the moisture absorption tube in the exhaust gas is the water component contained in the known volume of exhaust gas. The two methods are similar in principle. The mass concentration of flue gas humidity is directly obtained by weighing the moisture content and dividing the sampling volume, and then the mass concentration is converted into the volume percentage.
The dry-wet ball method is easy to operate and has strong adaptability. It is a common online measurement reference method for flue gas humidity. The condensation method and the weight method have higher accuracy, but the test is complicated, the operation of the personnel is required high, the test time is long, and the method is not suitable for the online measurement of flue gas humidity, and can only be used as the laboratory method and the online measurement method for comparison test.

Inhibition method

When a substance is placed between a pair of electrodes, the capacitance between the electrodes will change when it absorbs water vapor. The moisture content of flue gas can be obtained by measuring the capacitance change of the moisture absorption material, which is called a capacitive humidity sensor. The traditional capacitive humidity instrument has some problems in sensor sensitivity, humidity hysteresis, temperature coefficient and long-term stability.
The invention relates to an online volume-resistant high-temperature flue gas moisture meter of a patent product, which is an improved capacitance method moisture meter, and has good corrosion resistance and high sensitivity. It uses polymer film capacitance humidity sensor
As a moisture sensor, a platinum resistance temperature sensor is used for temperature compensation. The working principle is shown in Figure 1, the water vapor passes through the upper electrode of the high molecular film capacitance humidity sensor, and reaches the high molecular active polymer film, because the sensor size is small and the polymer film is very thin, the sensor can make quick response to the humidity change of the surrounding environment.

Figure 2 The Block Diagram of Working Principle of Water Meter with Resistance and Capacity

The water vapor absorbed in the polymer changes the dielectric characteristics of the sensor and changes the capacitance value of the sensor, converts the output signal of the capacitive humidity sensor into a voltage value, and outputs a temperature voltage signal through the temperature sensor to perform temperature automatic compensation. The moisture meter can measure the water content in the range of 0~20%±2% under the smoke temperature ≤180°C.
In recent years, a lot of research has been done on the search for better moisture sensing media. Among them, organic polymer materials have attracted much attention because of their high sensitivity, fast response and small humidity hysteresis. There are two main types of moisture-sensitive media materials: CAB series (cellulose acetate butyrate) and P series (polyimide).
The organic polymer capacitive humidity sensor was originally made of cellulose acetate and its derivatives. At present, cellulose acetate is mostly used. Japan Sakai have compared the properties of various cellulose derivatives and studied the properties of capacitance, temperature and absorption isotherms. The results show that in order to make non-hygroscopic humidity sensor, the water quantity must be restrained and the interaction between molecules must not occur. It is proposed that cellulose acetate material, especially the element made of porous gold electrode, not only has fast response speed, but also has little hygroscopic hysteresis.
Matsuguchi proposed the synthesis of low polymers with acetylene groups at both ends of polyimide. In the low polymer state, the solution is used to dissolve, and the film is coated on the substrate to form a film, and after heating, the polyimide with a stereo-linking structure is obtained, which is not easy to dissolve in water. Since the material is not separated from water when solidified, and it is difficult to form micro-holes on the hardened film, it is a humidity sensitive material with good water resistance. The results show that the humidity sensitive element of the modified polyimide has fast response speed and almost no hysteresis. The temperature coefficient is small, the solvent resistance (acetone) is also good, and the stability is greatly improved.
Chen Xingzhu proposed a new type of capacitive composite dielectric film humidity sensor. Its dielectric film is composed of two types of PI(CAB) which have different linear output and temperature characteristics. Compared with PI and CAB single materials, the composite material has small hysteresis, small non-linear error and small temperature coefficient, and has obvious improvement in repeatability and long-term stability. It provides a new idea for functional design of dielectric material of capacitive humidity sensor. Its representative product figure is shown in Figure 3.

Figure 3 CI-XS200 dew point sensor profile
The main advantages of the capacitance method are high sensitivity, quick response, easy manufacture, easy realization of miniaturization and integration. At present, the online humidity instrument of flue gas in China has many applications, but the long-term stability is not ideal, and most of the long-term use of the drift is serious, causing failure and damage. Capacitive humidity sensor has less corrosion resistance, which often requires higher cleanliness to the environment. Some products still have some phenomena such as lighting failure and electrostatic failure. In short, this is a method that is constantly improving.

Limit current method

According to the in-depth theoretical study, through a large number of experiments, the use of ion flow sensor can achieve accurate measurement of humidity. By changing the voltage applied on the cathode and anode of the sensor, the humidity can be measured. This finding solves the problem that the ordinary humidity sensor can not adapt to high temperature environment (for example, higher than 100°C).
An operating voltage is applied to the anode and cathode of the zirconia to provide an electric field to drive oxygen ions from the cathode to form an oxygen ion current through the zirconia to the anode. When the oxygen concentration in the measured atmosphere is certain, the current value of the zirconia sensor does not increase with the increase of the applied voltage, reaching a constant value, this constant current value is called the limit current value of the oxygen concentration, we call the first limit current value. According to the working principle, when the measured atmosphere contains water vapor, by increasing the applied voltage, the water vapor is also ionized into oxygen ions, and when the concentration of water vapor in the measured atmosphere is fixed, the zirconia sensor outputs a constant current value, which is called the second limit current value.

Figure 4 The Relation Between Limit Current and Applied Voltage

Figure 5 The Curve Diagram of Output Limit Current of Sensor Under Water Vapor

The reaction at the sensor cathode and anode is as follows:
Cathode side :O2+4e- →2O2- (4)
H2O+2e- →H2+O2- (5)
Anode side:O2- → 1/2O2+2e- (6)
According to the gas diffusion limit Ficks rule of the sensor, the first limit current I1 and the second limit current I2 are respectively represented by the following formula under the condition that the diffusion coefficient of oxygen is equal to the diffusion coefficient of water vapor:
I1={-4FDSP/(RTL)}Ln(1-PO2/P) (7)
I2={-4FDSP/(RTL)}{(1+PH2O/2PO2)} (8)
PO2=0.21(P- PH2O) (9)
In the formula: F is the Faraday constant, D is the diffusion coefficient of the mixed gas molecule, S is the area of the diffusion hole, P is the total pressure of the mixed gas, PO2 partial pressure, PH2O water vapor partial pressure, R is the gas constant, T is the absolute temperature, L is the length of the gas diffusion hole, 0.21 is the oxygen content in the air.

The application range of ion current humidity measurement is:

 Chang Ai Co., Ltd., in cooperation with Dr. Zhang Yi Can (Chinese, the first to introduce the extreme current type zirconia sensor into China) and his team, Yang Bang Chao, director of the Institute of Microelectronics and Solid Materials of Chengdu University of Electronic Science and Technology, pioneered the application of ion current sensors in the measurement of high temperature and humidity. In 2006, the company first introduced the GRL-12 high temperature humidity instrument based on ion flow sensor (Figure 5), and before the 2008 Green Olympics, a large number of applications to coking plant in Shanxi, thermal power plant emissions monitoring, for environmental protection and monitoring has made a domestic enterprise due to the obligation and contribution. In the development of more than ten years, Chang'ai Company has developed a large number of humidity analysis instruments based on ion flow sensors, such as CI-PC18 series humidity transmitter, CI-PC19 soil humidity monitor, CI-PC168 series high temperature humidity analyzer, CI-PC193 food industry humidity detection system, CI-PC196 series high temperature humidity analysis system, which represents the product's figure, see Figure 6.


Figure 6 CI-PC18 High Temperature Hygrometer Profile

Sensor structure:

Figure 6 3D Ion flow humidity sensor structure

These products are widely used in the fields of environmental protection, printing and dyeing, timber, building materials, paper-making industry, chemical industry, fiber and pharmacy, as well as the processing and storage industry of food, tobacco, vegetables and grain.


Dry-wet oxygen method

The oxygen sensor of CEMS system is used to measure the oxygen content before and after dehumidification of flue gas. When calculating the moisture content in flue gas, the humidity of flue gas is calculated according to the following formula:

Xsw=1-X,O2/XO2 (1)

In the formula (1), X and O2 are the volume percentage of oxygen in the wet flue gas, %, and Xo2 are the volume percentage of oxygen in the dry flue gas, %.
The main problem of dry and wet oxygen is that two instruments are needed to measure dry and wet oxygen respectively. The error caused by the difference of sampling points and sampling errors is as follows: The error of the two instruments is superposed. These errors are difficult to overcome for this method.

Infrared absorption

Absorption spectroscopy is an important technique in modern humidity measurement, including infrared and ultraviolet absorption. At present, the measurement technology based on near infrared absorption spectrum has become more mature, its measurement accuracy, sensitivity and measurement range are better than the traditional humidity analysis method.
Infrared absorption method uses the principle that water strongly absorbs infrared light with specific wavelength, the water content is different, the absorption degree of light, and obey Lambert-Beer law. By measuring the transmittance of the gas at the absorption wavelength and the reference wavelength, the ratio of the transmittance of the two wavelengths is a function of the water vapor content in the gas. Wan Jia Rong have found that the most commonly used absorption wavelengths are 1.45 μm and 1.94 μm, and the commonly used reference wavelengths are 1.73 μm and 2.1 μm.
There are two kinds of humidity measurement methods based on near-infrared absorption spectroscopy: laser diode resonant attenuation spectroscopy (CRDS) and tunable laser diode absorption spectroscopy (TLDAS). The CRDS resonator is simple in structure and small in size, which ensures the rapid replacement of gas, so CRDS is very suitable for on line measurement. TDLAS is a relatively mature absorption spectrum measurement technology, which has been used in the field of micro-moisture measurement, and has the advantages of high sensitivity and fast response.
However, infrared absorption method used in flue gas humidity measurement needs to avoid the interference of CO2/SO2/NOX sensitive wavelength, which is difficult, coupled with the high price of the instrument, so it is rarely used in flue gas humidity measurement.

High temperature humidity generator

Because the temperature of flue gas is generally higher, it is about 80°C~120°C, while the ordinary humidity generator generates constant humidity at normal temperature, even if the constant humidity occurs at high temperature, it is difficult to ensure that the temperature can be kept constant in use. It is often difficult to meet the requirements if using the normal temperature humidity generator to calibrate the high temperature flue gas humidity meter, which brings great restrictions to the research and application of high temperature humidity measurement. Especially the capacitive humidity sensor, the humidity sensor is sensitive to the temperature in addition to the environmental humidity, and it is easy to produce temperature drift, so it is necessary to develop a high temperature humidity generator.
The high temperature humidity generator can produce stable humidity at higher temperature. It is a convenient and intuitive calibration device for flue gas humidity instrument. Zhang Wen Dong developed a set of high temperature precision humidity generator by using the principle of double-temperature and double-pressure method. The temperature stability experiments were carried out at 50°C, 100°C and 150°C. The duration was 2 hours. The test results of the temperature stability of the saturator oil tank and the test chamber were within 0.02°C. The maximum theoretical uncertainty of the device is ±1.09RH. The accuracy of the device is verified by the test results of the weight method hygrometer. The device can be used for correcting high temperature humidity sensor and transmitter.

2.Summary
The measurement of flue gas humidity is a recognized problem. The method of dry-wet ball as the reference method stipulated in the national standard is easy to produce errors. The condensation method and the weight method are high precision, but the operation is complex, and they can only be used as laboratory methods. The online measurement method of flue gas humidity used in CEMS system in China is capacitance method and limit current method. Both of these methods belong to electronic humidity sensor. The application prospect is broad, but the anti-pollution and long-term stability must be improved. However, the dry-wet oxygen method has a large error, and the infrared absorption method is expensive, so it is used less. The normal temperature humidity generator is also difficult to meet the calibration requirements of flue gas humidity instrument. The development of high temperature and humidity generator is necessary and also a technical problem.


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