Introduction to various analysis principles of commonly used gas analyzers

The various analysis principles of commonly used gas analyzers introduce process analyzers for measuring gas analyzers. In many production processes, especially in the presence of chemical reactions, automatic control based only on physical parameters such as temperature, pressure, and flow rate is often not sufficient. For example, in the production of synthetic ammonia, controlling only the temperature, pressure, and flow rate of the synthesis tower does not guarantee the highest synthesis rate. At the same time, it is necessary to analyze the chemical composition of the inlet gas and control the optimal ratio of hydrogen and nitrogen in order to obtain higher productivity. For example, in addition to the need to control the ratio of fuel to combustion air in the combustion control of the boiler, it is also necessary to analyze the chemical composition of the flue gas online and change the supply of combustion air accordingly so that the furnace can obtain the highest thermal efficiency. In addition, in factories that emit harmful gases, gas analyzers must also be used to continuously monitor harmful gases in order to prevent hazardous accidents that endanger the health of workers, pollute the environment, or cause explosions. Due to the variety of gases being analyzed and the variety of analytical principles, there are a wide variety of gas analyzers. Commonly used thermal conductivity gas analyzer, electrochemical gas analyzer and infrared absorption analyzer.

1. Thermal Conductivity Gas Analyzer A physical gas analysis instrument. It is based on the principle that different gases have different thermal conductivity, and the content of some of the components can be calculated by measuring the thermal conductivity of the mixed gas. This kind of analytical instrument is simple and reliable, and it is suitable for many types of gases. It is a basic analytical instrument. However, it is difficult to directly measure the thermal conductivity of a gas, so in practice, the change in the thermal conductivity of the gas is often converted into a change in resistance, and then measured using a bridge. The heat-sensitive elements of the thermal conductivity gas analyzer mainly include semiconductor sensing elements and metal resistance wires. The semiconductor sensing element has a small volume, a small thermal inertia, and a large temperature coefficient of resistance, so the sensitivity is high and the time lag is small. A bead-shaped metal oxide was sintered on a platinum coil as a sensitive element, and a material that did not respond to gas was wound around the same platinum coil with the same internal resistance and heat value as a compensating element (FIG. 1). These two components constitute a bridge circuit as two arms, that is, a measurement loop. When the semiconductor metal oxide sensitive element adsorbs the gas to be measured, the conductivity and the thermal conductivity change, and the heat dissipation status of the element also changes. The change in the element temperature causes the resistance of the platinum coil to change, and the bridge has an unbalanced voltage output, from which the concentration of the gas can be detected. Thermal conductivity gas analyzers have a wide range of applications. They can also be used as detectors in chromatographic analyzers to analyze other components, in addition to hydrogen, ammonia, carbon dioxide, sulfur dioxide, and low concentrations of flammable gases.

2. Electrochemical Gas Analyzer A chemical gas analyzer. It measures the gas composition based on changes in the amount of ions or changes in current caused by chemical reactions. In order to improve the selectivity, to prevent contamination of the measuring electrode surface and to maintain the performance of the electrolyte, diaphragm structures are generally used. Commonly used electrochemical analyzers are fixed potential electrolysis and galvanic cells. The working principle of the fixed potential electrolytic analyzer (Figure 2) is to apply a specific potential on the electrode and the gas to be measured generates an electrolytic action on the surface of the electrode. As long as the potential measured on the electrode is measured, the specific electrolysis of the measured gas can be determined. Potential, so that the instrument has the ability to choose to identify the measured gas. The galvanic cell analyzer (Fig. 3) electrolyzes the gas to be measured, which diffuses through the membrane into the electrolyte. The measured electrolytic current is measured to determine the concentration of the gas to be measured. By selecting different electrode materials and electrolytes to change the internal voltage of the electrode surface, the selectivity to gases having different electrolytic potentials is achieved.

3. Infrared Absorption Analyzers Analytical instruments that operate on the selective absorption of infrared light of different wavelengths by different component gases. Measuring this absorption spectrum can discriminate the type of gas; measuring the absorption intensity can determine the concentration of the gas being measured. The infrared analyzer has a wide range of applications. It can not only analyze gas components, but also analyze solution components. It has high sensitivity, rapid response, continuous on-line indication, and an adjustment system. The detection part of the infrared gas analyzer commonly used in industry is composed of two parallel optical systems with the same structure.

One is the measuring room and the other is the reference room. Both chambers open or close the light path at the same time or alternately in a certain cycle through the light cutting plate. After the gas to be measured is introduced into the measurement chamber, light having a specific wavelength of the gas to be measured is absorbed, so that the luminous flux passing through the optical path of the measurement chamber and entering the infrared absorption chamber is reduced. The higher the gas concentration, the lower the flux entering the infrared receiving chamber; while the flux through the reference chamber is constant, and the flux entering the infrared receiving chamber is also constant. Therefore, the higher the measured gas concentration, the greater the difference in luminous flux through the measurement chamber and the reference chamber. This difference in luminous flux is projected to the infrared receiving air chamber with an amplitude of vibration of a certain period. The receiving chamber is divided into two halves by a metal film with a thickness of several micrometers. The gas contained in the chamber is sealed with a relatively large concentration of the measured component gas. In the absorption wavelength range, all the incident infrared rays can be absorbed, so that the fluctuating light flux becomes The temperature cycle changes, and then the temperature change can be converted into a pressure change according to the gaseous equation, and then the capacitance sensor is used to detect the concentration of the measured gas after the amplification process. In addition to using capacitive sensors, quantum infrared sensors can also be used to directly detect infrared rays, and infrared interference filters are used for wavelength selection and tunable lasers as light sources to form a new all-solid infrared gas analyzer. This analyzer uses only a light source, a measurement chamber, and an infrared sensor to measure the gas concentration. In addition, if a filter disc containing a plurality of different wavelengths is used, the concentrations of various gases in the multi-component gas can be measured simultaneously.

Similar to the infrared analyzer principle, there are ultraviolet analyzers, photoelectric colorimetric analyzers, etc., which are also used in industry.