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Introduction and application development of flue gas online high temperature and humidity analyzer

2020-04-17 16:21:03Source:昶艾电子 Hits:

Introduction and application development of flue gas online high temperature and humidity analyzer

Foreword

The measurement of flue gas humidity in the exhaust gas of fixed pollution source is mainly to obtain dry-based oxygen content to calculate the actual content of flue gas and gaseous pollution emissions. Because of the high temperature of wet flue gas, the large amount of dust, and the large difference of the flue gas components of the boiler or industrial kiln used in the industrial field such as thermal power plant, petrochemical plant, garbage incineration plant, steel-making plant, etc., in addition, the humidity measurement in the world is a difficult problem, so the on line detection of flue gas humidity is very difficult. In the past few years, the measurement of high temperature and humidity of domestic flue gas was mostly done by hand, that is, according to the requirements of GB/T 16157-1996 "The Method of Determining Particulate Matter and Sampling Gaseous Pollutants in the Exhaust of Fixed Pollution Source", the method of weight method, condensation method and dry-wet ball method were chosen to determine, and the average value was input into the CEMS system. With the development and progress of technology and the national environmental protection more and more attention, at present there are four kinds of on line measurement methods of high temperature and humidity of flue gas in China: 1. Resistance capacitance method 2. Limit current type zirconium oxide principle method 3. Impact injection method (dry and wet ball) 4. Infrared spectrum absorption method.


1.Resistance capacitance method
The resistance-capacitance method used in high temperature and humidity measurement of flue gas in China is the capacitance method. The sensors used in this method mostly use polyimide as humidity sensitive material. The polymer humidity-sensitive capacitance made of this material has good electrical performance, and its dielectric constant and dielectric loss are very small. The dielectric constant of polyimide is 2~3 in the state of complete drying, and the dielectric constant of water molecule is about 80 at 20°C. The revival dielectric constant after adsorption of water molecule is:
εu=εr+aWuεh               (1)
Wu=b(p/p0) εr+ aWuεh       (2)

εu is the relative humidity of u%RH composite dielectric constant, εr is the humidity of 0%RH, the dielectric constant of polyimide film, a, b is the structure constant, εh is the dielectric constant of water adsorbed in polyimide film, Wu is the humidity of u%RH, the mass of water adsorbed by the unit mass of polymer, p/p0 is the relative pressure of water vapor equilibrium. When the macromolecule humidity-sensitive capacitance absorbs the gaseous water molecules in the environment, the dielectric constant of the material is changed, thereby causing the capacitance value to change; the corresponding environmental humidity value is calculated by measuring the capacitance value of the change. At present, the capacitance sensor-polymer humidity-sensitive capacitor used in high temperature and humidity measurement in flue gas adopts a flat capacitor structure, which is mainly composed of a glass substrate substrate, a lower electrode, a polymer humidity-sensitive film, an upper electrode and the like. According to the formula of flat capacitor, the relation between capacitance and relative humidity can be expressed as

Figure 1 Structure Diagram of Polymer Humidity-sensitive Capacitance

The capacitance value of Ch humidity-sensitive capacitance, ε0 is vacuum dielectric constant, S is the area of humidity-sensitive capacitance electrode, D is the distance between humidity-sensitive capacitance electrodes, and also the thickness of humidity-sensitive film. From the formulas (1), (2) and (3), it can be seen that the relationship between the water molecular adsorption capacity of the humidity-sensitive capacitance and the relative pressure of water vapor equilibrium should conform to the Herry adsorption isotherm, that is, the relationship between the capacitance capacity and the relative humidity is linear.
In summary, we can see that the measurement of high temperature humidity reflects the relative humidity in the flue gas by the resistance volume method, according to the definition of relative humidity: The relative humidity can be expressed as the ratio of water pressure to saturated water vapor pressure at a certain temperature and pressure. According to the definition, the relative humidity and temperature are closely related, in practical application, we need the humidity value of the flue gas is the volume ratio, convenient to calculate dry oxygen content in the flue gas. Therefore, in order to calculate the volume ratio of water vapor in the flue gas, it is necessary to measure the ambient temperature of the humidity sensor.
According to the actual use of the flue gas humidity meter of the resistance-capacitance method, the resistance-capacitance method has the characteristics of fast response time, small volume, not easy to be damaged when condensed water. The disadvantage is that the temperature of flue gas cannot exceed 170°C, the higher the temperature, the more easily the data will fluctuate and the volume ratio is less than 6% humidity is not easy to measure. The reason is that the humidity is 0-40% (volume ratio) water gas (more than 30°C), with the temperature (more than 100°C) corresponding saturated water pressure, the larger the relative humidity, the smaller the corresponding capacity change will be. But the range or resolution of the capacitance change is limited. Thirdly, the waste incineration flue gas, metallurgical flue gas and so on often have certain corrosivity, the electrode is easy to fail and the life is very short.

2.Principle method of limiting current zirconia
Limited current type zirconia works by using the principle of zirconia oxygen pump
That is, firstly, the zirconia solid electrolyte is heated to high temperature (more than 350°C), simultaneously, the working voltage is applied to the platinum electrodes on both sides of the zirconia solid electrolyte, the oxygen molecule on the cathode side is catalyzed into oxygen ions and is driven by the applied voltage to 'pump' to the anode. The output current of the sensor does not increase with the increase of the applied voltage when the oxygen concentration in the atmosphere is certain, but reaches a constant value, which is called the limit current value under the oxygen concentration, and is generally called the first limit current platform I1. According to this principle, by placing the limit current sensor in an environment containing water vapor and increasing the applied voltage, a significant limit current value can be measured, commonly known as the second limit current stage I2, although this current value contains oxygen molecules and water molecules that are ionized. The two limiting current values are proportional to the oxygen content in the environment and the oxygen content containing water vapor, respectively. The micro-reaction mechanism of zirconia on cathode and anode is as follows:
cathode side             O2+4e-→2O2-                    (1)
                               H2O+2e-→H2+O2-             (2)
Anode side               O2-→1/2O2+2e-           (3)


Figure 2 Limit current type zirconium oxide sensor structure

According to the gas diffusion hole limit Ficks rule of the sensor, under the assumption that the diffusion coefficients of oxygen and water vapor are equal, the two limit current values can be expressed as follows:

In the formula: F is the Faraday constant 
D is the mixing gas diffusion coefficient
S is the area of the zirconium oxide current-limiting hole (diffusion hole) 
P is the total pressure of the mixture gas
R is the gas constant   
T is the operating temperature of zirconia (K)
L is the length of the gas diffusion hole

The oxygen content in the flue gas can be calculated according to the first limit current, and the humidity in the flue gas can be calculated according to the difference between the second limit current and the first limit current. Therefore, the use of the limit current zirconium oxide principle humidity instrument compared with other principles of humidity instrument has a clear advantage, because its nature is oxygen measurement and in order to measure humidity must measure oxygen, for the user, there is no need to install an oxygen measurement analyzer, a humidity instrument can provide two measurement data at the same time.
From the actual use of the limit current zirconium oxide principle method of humidity meter, the advantages are small volume, high measurement accuracy, flue gas temperature can be used from normal temperature to 500°C, high cost performance. The disadvantage is that the machine can not be started in liquid water or liquid water, and the machine is easy to be poisoned in a flue gas atmosphere containing more silicon dioxide or heavy metals such as arsenic and lead.
In fact, when the catalytic electrode of zirconium oxide is mentioned, the platinum electrode is usually needed for desulfurization because of the high sulfur content of coal in China.However, the front and back ends of the desulfurization tower often need to monitor the flue gas, but if the platinum electrode of zirconia works in the high SO2 content flue gas environment for a long time, the service life will also be greatly affected. Therefore, in practical use, such as desulfurization inlet, natural gas sulfur recovery, waste incineration and so on, the zirconium oxide electrode using platinum electrode is easy to be corroded. In recent years, in order to solve the influence of corrosive atmosphere on the service life of limit current type zirconia, the zirconia sensor adopted by PC18 humidity transmitter of Shanghai Chang Ai Electronic Technology Co., Ltd. has made bold innovation. The catalytic electrode of zirconia has been changed to ceramic electrode, which has solved the application of limit current type zirconia under the conditions of medical garbage incineration, petrochemical industry, VOC etc. at one stroke.

PC18 Humidity transmitter


3.Impact jet method (dry-wet ball)
The basic principle of moisture measurement by dry-wet ball method: A temperature sensor is used to measure the temperature of the flue gas as the temperature of the dry ball, a measuring pool is used to load a certain amount of water, then a temperature sensor is arranged in the measuring pool, and the temperature sensor is required to be arranged under the water surface, and then the flue gas continuously impacts the water surface directly above the temperature sensor of the measuring pool, and the measured temperature is taken as the temperature of the wet ball. According to the heat transfer principle and thermodynamics theory, the following mathematical formula can be derived:

in formula:
relative humidity % 
saturated Water Pressure at Wet Sphere Temperature
saturated water pressure at dry ball temperature
atmospheric pressure
the difference between dry and wet ball temperatures
constant, related to wind speed

According to the above mathematical formula, we can clearly know that the impact injection method to measure the humidity in the flue gas is achieved by indirect measurement of the flue gas temperature. Temperature measurement technology is relatively mature and reliable. Even if the working condition is very bad, the change of temperature sensor is very fast.
According to the practical application situation of natural gas sulfur recovery plant, food processing plant, textile plant, garbage incineration plant, etc., the high temperature humidity instrument of the impact spraying method (dry-wet ball) has long service life (currently has been used for five consecutive years and is still running normally), the measured data is accurate and reliable, has strong adaptability to harsh environment, wide temperature adaptation range and small maintenance. The disadvantage is that the price is high, the volume is large, and the water needs to be added regularly.
Although the impact injection method is the working principle of dry and wet ball, but not the common use of the meteorological department to measure the relative humidity of air humidity instrument, such as Shanghai Chang Ai Electronic Technology Co., Ltd. CI-PC39 is a brand new or even a new innovative design, to achieve the above measurement effect.

CI-PC39 Humidity analyzer


4.infrared absorption method
IR broad-spectrum absorption is based on the principle that the selective absorption of water vapor molecules to specific infrared wavelengths varies with their concentration. However, since 1912 when Fowle first proposed the infrared measurement of humidity, the measurement of humidity is slow due to the wide-spectrum absorption technology limited by traditional infrared absorption technology. With the rapid development of semiconductor laser spectroscopy (DLAS) in the 1990s, the on line high temperature and humidity analyzer for flue gas has been developed. Compared with the traditional infrared absorption absorption spectroscopy, the DLAS technique belongs to the narrow-spectrum absorption, because the spectral width (less than 0.0001nm) of the semiconductor laser source is much smaller than the broadening of the gas absorption line. Each gas molecule has its inherent absorption spectrum, when the emission spectrum of the light source coincides with the absorption spectrum of the gas molecule, the absorption intensity is related to the volume fraction of the gas. Looking at the relevant database, we can find that the absorption of water gas is very strong near the absorption line with the wavelength of 1390nm and there is no obvious interference absorption of other gases. When a semiconductor laser with I0 intensity passes through the gas to be measured, if the light source spectrum covers the absorption spectrum of the gas molecule, the light will decay when passing through the gas. According to the Lambert-Beer law, the relation between the intensity I of the exit light and the intensity I0 of the incident light and the gas volume concentration is:

In the formula (1), I and I0 are the exit and incident light intensity respectively;It is the absorption coefficient of medium with unit concentration and unit length at a certain wavelength. C is the concentration of the gas to be measured, L is the optical path.

In order to obtain higher or higher sensitivity and reduce the 1/f noise of laser, DLAS technique usually needs modulation spectrum detection technique. This technique significantly reduces the influence of laser noise on measurement by high frequency modulation. At the same time, a narrow bandwidth band-pass filter can be obtained by setting a larger time constant for the phase-sensitive detector used in phase-sensitive detection technique (detection of harmonic components), thus effectively compressing the noise bandwidth.
When the flue gas high-temperature humidity analyzer developed by DLAS is used for measuring the flue gas, the invention belongs to the non-contact measurement, which can not be caused by sensor poisoning or be interfered by the background gas. The invention has the advantages of fast response time, high accuracy of measurement data, long calibration period and almost no maintenance. The disadvantage is that prices are high.

5.development trend
As we all know, the core components of online analytical instruments in the sensor, our country due to the late start, the basic industry is weak, although after these years of development, domestic analytical instrument manufacturers in mastering the core technology has made great progress, but compared with foreign countries still very obvious gap. Most domestic analytical instrument manufacturers buy foreign sensors and then design the second instrument to enter the market for homogeneous price competition, and it makes people sad that the low-end market of analytical instruments (the high end market is almost monopolized by foreign countries), in the long run, it will affect or delay the development of domestic analytical instrument industry.
At present, due to the price reasons, the online high temperature humidity instrument mainly adopts the resistance capacity method and the limit current type zirconia in the market, the high temperature humidity instrument based on the principle of impact jet method (dry-wet ball) and infrared spectrum absorption method has a very low market share. Shanghai Chang Ai Electronic Technology Co., Ltd., through long-term and unremitting efforts, has independently developed the above-mentioned online flue gas high-temperature humidity instrument limit current type zirconium oxide sensor and the impact injection method of the core components-measuring pool. Some domestic scientific research institutes such as China Electrical Science 49 can also do the sensor of the high temperature humidity instrument by the resistance capacitance method, but the performance is unstable, far from industrialization, at present the market is basically designed abroad. The semiconductor laser diode, the core component of online flue gas high temperature humidity instrument by infrared spectrum absorption method, can not be done by domestic manufacturers. Only Germany, the United States, the Netherlands and several other countries can do it. The lack or deficiency of core technology restricts the development and progress of on line flue gas high temperature humidity instrument.
In the future, the development of flue gas online high-temperature humidity instrument will be restricted by various factors, the humidity instrument of the above four principles will exist at the same time, it can be predicted that the humidity instrument of the resistance capacity method and the limit current type zirconium oxide principle will further improve the reliability and service life, the impact injection method humidity instrument will further reduce the volume, reduce the cost, increase the market share. Infrared absorption moisture meter should be the most promising, also represents the development direction of online gas analysis instrument, but if domestic manufacturers can not solve the development and production of semiconductor laser diode, then it will restrict the cost of reducing, affect the market promotion, even if foreign suppliers reduce the price, domestic enterprises can only work for foreign enterprises, and then conduct low-price competition in China, and fight the price war. We hope that the domestic industry association of on line analytical instruments or relevant government departments can promote cooperation among universities, scientific research institutes and enterprises, complement each other's advantages, break the monopoly of foreign manufacturers at an early date, and let China's analytical instrument industry move toward the high end.




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