Principles and applications of several oxygen analyzers currently on the market

1 Zirconia oxygen analyzer

Zirconia oxygen analyzers, also known as zirconia oxygen analyzers, zirconia analyzers/zirconia oxygen meters/zirconia oxygen gauges/oxygen sensors, are frequently used to measure the oxygen concentration in flue gas during combustion, and are also applicable to the measurement of oxygen concentration in non-combustion gases.

Its principle is that an electrochemical cell (oxygen concentration difference cell, also known as a zirconia head) with a constant temperature inside the sensor generates a millivolt potential, which directly reflects the oxygen concentration in the flue gas.

The key component of the oxygen sensor is zirconia, which is coated on both the inner and outer sides of the zirconia element with porous platinum electrodes to form an oxygen concentration difference cell. It is located at the top of the sensor. In order to maintain the rated operating temperature of the battery, a heater is set in the sensor. The zirconia temperature is kept constant by the temperature controller in the oxygen analyzer.

The zirconia oxygen analyzer consists of an oxygen sensor (also known as an oxygen probe, oxygen detector), an oxygen analyzer (also known as a transducer, transmitting unit, converter, analyzer), and the connecting cables between them.

The zirconia probe is a sensor that uses the zirconia concentration difference potential to determine the oxygen content. Its core zirconia tube is placed in a miniature electric furnace located at the top of the entire probe. The zirconia tube is formed by sintering zirconia material doped with a certain amount of yttrium oxide or calcium oxide at high temperature to form a stable zirconia ceramic sintered body. Due to the oxygen ion vacancies in its cubic lattice, it is a good oxygen ion conductor at high temperatures. Due to this characteristic, at a certain high temperature, when the oxygen content on both sides of the zirconia tube is different, it is a typical oxygen concentration difference cell. In this cell, air is the reference gas, which is located on the outer and inner electrodes respectively with the flue gas. In the actual oxygen probe, air flows through the outer electrode, and flue gas flows through the inner electrode. When the oxygen content P of the flue gas is less than the oxygen content P0 (20.6% O2) of the air, oxygen molecules in the air capture 4 electrons from the outer electrode to form 2 oxygen ions, and the following electrode reaction occurs: O(P0) + 4e- → 2O-2 Oxygen ions quickly migrate to the flue gas side in the zirconia tube, and the opposite electrode reaction occurs at the inner electrode: 2O-2 → O(P0) + 4e- Due to the oxygen concentration difference, oxygen ions migrate from the air side to the flue gas side, and the resulting potential causes oxygen ions to migrate from the flue gas side back to the air side. When these two migrations reach equilibrium, a potential signal E related to the oxygen concentration difference is generated between the two electrodes, and this potential signal conforms to the "Nernst" equation: E = (RT/4F)Ln(P0/P) (1) where R and F are the gas constant and Faraday constant respectively, T is the absolute temperature (K) of the zirconia tube, P0 is the oxygen content of the air (20.6% O2), and P is the flue gas content. From equation (1), it can be seen that under certain high-temperature conditions (generally 600℃), a certain flue gas oxygen content will have a corresponding potential output. In an ideal state, the potential value corresponds to the oxygen content in the high-temperature region.

In an ideal state, when the concentration of the measured flue gas is the same as that of the reference gas, the output potential E value is 0 mV. However, in practical applications, the actual conditions of the zirconia tube and the on-site conditions are not ideal. Therefore, the actual zirconia tube deviates from this value. In fact, the potential output of the zirconia tube at a certain oxygen content is the sum of the theoretical value and the background potential, which we call the background potential or zero potential output by the zirconia tube under no concentration difference condition. The magnitude of this value is different at different temperatures, and it changes with the extension of the zirconia tube service life.

2 Electrochemical oxygen analyzer:

A considerable number of combustible and toxic and harmful gases have electrochemical activity and can be electrochemically oxidized or reduced. Using these reactions, it is possible to distinguish gas components and detect gas concentrations. Electrochemical gas sensors are divided into many subclasses: (1) Primary battery type gas sensors (also called: galvanic battery type gas sensors, also called fuel cell type gas sensors, also called spontaneous battery type gas sensors), their principle is the same as the dry batteries we use, except that the carbon-manganese electrodes of the battery are replaced by gas electrodes. Taking the oxygen sensor as an example, oxygen is reduced at the cathode, and electrons flow to the anode through the ammeter, where lead metal is oxidized. The magnitude of the current is directly related to the concentration of oxygen. This type of sensor can effectively detect oxygen, sulfur dioxide, chlorine, etc. (2) Constant potential electrolytic cell type gas sensors, this type of sensor is very effective for detecting reducing gases, its principle is different from the primary battery type sensor, its electrochemical reaction occurs under the forced current, which is a true coulometric sensor. This type of sensor has been successfully used in the detection of carbon monoxide, hydrogen sulfide, hydrogen, ammonia, hydrazine, etc., and is currently the mainstream sensor for the detection of toxic and harmful gases. (3) Concentration difference cell type gas sensors, gases with electrochemical activity on both sides of the electrochemical cell will spontaneously form a concentration difference electromotive force, and the magnitude of the electromotive force is related to the concentration of the gas. A successful example of this type of sensor is the automotive oxygen sensor and the solid electrolyte type carbon dioxide sensor. (4) Limiting current type gas sensors, a type of sensor for measuring oxygen concentration uses the principle that the limiting current in the electrochemical cell is related to the carrier concentration to prepare an oxygen concentration sensor, which is used for oxygen detection in automobiles and oxygen concentration detection in molten steel.

3 Paramagnetic oxygen analyzer:

Paramagnetic oxygen analyzer: An instrument for measuring the oxygen content in a gas based on the principle that the volume magnetic susceptibility of oxygen is much higher than that of ordinary gases, and it has a very high paramagnetic property in a magnetic field. Paramagnetic oxygen analyzers are also called magnetic effect oxygen analyzers or magnetic oxygen analyzers, which we usually call magnetic oxygen analyzers. They are generally divided into three types: thermal magnetic convection type, pressure mechanical type, and magnetic pressure type oxygen analyzers. Magnetic properties of matter: Any substance will be magnetized under the action of an external magnetic field, exhibiting certain magnetic characteristics. When a substance is magnetized in an external magnetic field, it will generate an additional magnetic field. If the direction of the additional magnetic field is the same as that of the external magnetic field, the substance will be attracted by the external magnetic field; if the direction of the additional magnetic field is opposite to that of the external magnetic field, it will be repelled by the external magnetic field. Therefore, substances attracted by the external magnetic field are usually called paramagnetic substances, or said to have paramagnetism; while substances repelled by the magnetic field are called diamagnetic substances, or said to have diamagnetism. Gases in a magnetic field will also be magnetized. According to the different attraction and repulsion of gas components to the magnetic field, gases are also divided into paramagnetic and diamagnetic. Paramagnetic gases include: O2, NO, NO2, etc.; diamagnetic gases include: H2, N2, CO2, CH4, etc.

Magnetic oxygen sensors are the core of magnetic oxygen analyzers, but the "sensorization" process has also been realized. This type of sensor can only be used for oxygen detection and has excellent selectivity. Only nitrogen oxides in the atmospheric environment can have a slight impact, but because the content of these interfering gases is often very low, the selectivity of magnetic oxygen analysis technology is almost unique!

Of course, magnetic oxygen analyzers are divided into magnetic mechanical type and thermal magnetic type according to the sensor type. The thermal magnetic type has a slightly lower market price, and customers can choose the model according to their needs. Compared with thermal magnetic analyzers, magnetic mechanical oxygen analyzers have the following characteristics: 1. It is an analyzer that directly measures the paramagnetism of oxygen, and during the measurement, it is not affected by changes in the thermal conductivity and density of the gas being measured. 2. Linear scale in the range of 0-100% O2, high measurement accuracy, measurement error can be as low as ±0.1%O2. 3. High sensitivity, in addition to being used for constant measurement, it can also be used for trace oxygen (O2‰) measurement.

Precautions: 1. Magnetic mechanical oxygen analyzers are based on the direct measurement of magnetic susceptibility. Some strongly magnetic gases such as oxygen and nitrogen will seriously interfere with the measurement, so these interfering components should be removed. In addition, some strongly diamagnetic gases will also cause large measurement errors. For example, xenon. If the sample contains a large amount of such gases, they should also be removed or corrective measures should be taken for the measurement results.

2. The volume magnetic susceptibility of oxygen is a function of pressure and temperature. Changes in sample gas pressure and temperature, as well as changes in ambient temperature, will affect the measurement results. Therefore, the pressure of the sample gas must be stabilized to meet the pressure value when calibrating the instrument. The ambient temperature and the entire inspection components should operate within the designed temperature range. Generally speaking, various models of magnetic mechanical oxygen analyzers are equipped with a temperature control system to maintain the detection components at a constant temperature.

3. Both short-term violent vibrations and slight continuous vibrations will weaken the magnetic field strength of magnetic materials. Therefore, this type of instrument often installs sensitive components such as detectors in anti-vibration devices. Of course, the instrument installation location should also avoid vibration sources and take appropriate anti-vibration measures. In addition, no electrical lines are allowed to pass through these sensitive parts to prevent electromagnetic interference and vibration interference.

4 Laser oxygen analyzer

Laser gas analyzers use TDLAS spectroscopy absorption technology to obtain the concentration of gas by analyzing the selective absorption of laser by gas. The difference between it and traditional infrared spectroscopy absorption technology lies in the fact that the semiconductor laser spectral width is much smaller than the broadening of the gas absorption line (the measurement range is narrower, more targeted, and naturally has strong anti-interference ability). Therefore, TDLAS technology is a high-resolution spectral absorption technology. Compared with infrared gas analyzers, laser analyzers have much stronger anti-interference ability and are not affected by H2O and CO2, making the measurement more accurate. Laser analyzers have their own advantages over traditional extraction analyzers. First, laser analyzers are installed in situ and directly installed on process pipelines, eliminating the need for sampling probes, sampling pipelines, and pretreatment systems. Second, laser analyzers are very easy to maintain, reducing maintenance costs. Finally, the response time of laser analyzers can be as low as 1 second, providing data in time for the safety protection system, and can handle key point measurements. It is worth mentioning that although laser analyzers have many advantages, they cannot completely replace traditional analytical instruments. At some measurement points, such as the outlet of the coal mill and the coal powder bin, due to the very high dust content, the light transmittance of the laser will be greatly affected, resulting in poor measurement results. At this time, only traditional extraction analyzers can be used for measurement.

Now let's briefly summarize the applications and advantages and disadvantages of several types of oxygen analyzers:

1. Electrochemical oxygen analyzer: The lowest cost, the sensor can measure from PPM to percentage level, wide range of applications, inexpensive, suitable for most applications. The sensor generally has a service life of 1-2 years, and can be replaced regularly.

2. Zirconia oxygen analyzer: The price is slightly higher than that of electrochemical oxygen analyzers. It is suitable for measuring flue gas and flue oxygen, and can also be used in other occasions. One instrument can complete measurements from ppm level to percentage level without loss of accuracy, and the measurement speed is very fast. The disadvantage is that it needs to be preheated to about 700 degrees, and the gas being measured cannot contain a high concentration of combustible gases such as hydrogen, otherwise the sensor will be damaged. More suitable for online measurement, larger than electrochemical oxygen analyzers.

3. Magnetic oxygen analyzer: Unlike the previous two types of oxygen analyzers, the sensor has no service life limit and can be used for a long time without artificial damage. It is slightly sensitive to flow rate and requires stable inlet pressure. Typical applications are high-purity oxygen analysis, which can complete 99.99% oxygen content analysis. According to the sensor, it can be further divided into thermal magnetic and magnetic mechanical oxygen analyzers. Thermal magnetic oxygen needs to be preheated for a certain period of time, and the price is cheaper than magnetic mechanical type. Percentage level oxygen content applications are more common.

4. Laser oxygen analyzer: The analyzer is fast, the choice for wealthy users, in-situ selection, eliminating various sampling and filtering. Note that if the pipeline dust is too much, a nitrogen generator needs to be equipped for purging. The instrument is very expensive. If it is well maintained, it is quite easy to use, with data output within 1 second. Domestic manufacturers such as Juguang and Beijing produce them, with general performance. At this price range, it is recommended that customers directly choose imported instruments.

Anhui Tianfen Instrument Co., Ltd. specializes in the research and development and production of gas analyzers, oxygen analyzers, zirconia oxygen analyzers, oxygen content analyzers, oxygen content analyzer sensors, oxygen content analyzer transmitters, and customized flue gas analyzers.

Our zirconia oxygen analyzer products offer personalized customization services to customers. We provide OEM services for various instrument integrators, including customized startup welcome interfaces and startup logo displays. Our zirconia oxygen analyzers are fully featured, including standard 4-20mA output, and optional RS485 communication, HART protocol communication, mobile Bluetooth connection, automatic calibration, and automatic purging. Our zirconia oxygen analyzer is a full-featured instrument. We will meet your needs, just let us know your requirements.

Our zirconia oxygen analyzers offer high accuracy, high stability, high reliability, long-term drift-free performance, and high cost-effectiveness. Customers and instrument integrators are welcome to contact us to discuss cooperation.

TEL:18225808093