In the process of VOCs treatment, the real-time monitoring of pipeline organic waste gas is very important, especially the exhaust gas of the combustion process, because if the exhaust gas concentration accumulates to a certain extent, it is easy to cause explosion accidents. At present, there are three types of online instruments for the safety monitoring of VOCs exhaust gas output pipelines, including catalytic combustion method, infrared absorption method and laser diode principle.
1. Catalytic combustion principle
The principle of catalytic combustion is to use the sensing element to produce catalytic combustion when it comes into contact with flammable gas, and the temperature of the sensing element increases accordingly, so that the resistance value of the element increases, and the concentration of the gas is measured by the change of the detection resistance of the Wheatstone bridge. The main advantages of using the catalytic combustion principle are:
(1) When the composition of the exhaust gas is uncertain, it can still be effectively detected;
(2) The detector has the function of automatic calibration;
(3) Easy to use;
(4) It is easy to realize temperature compensation.
The main disadvantages of the catalytic combustion principle are:
(1) The low-temperature catalytic flameless combustion time will be burned off after a long time, and continuous measurement cannot be realized;
(2) The catalytic combustion reaction process takes a certain amount of time;
(3) The sulfur, hydrogen sulfide and halogen elements contained in the sample gas will cause the catalyst to be poisoned and fail.
2. Infrared absorption principle
The principle of infrared absorption is to use some flammable gases (hydrocarbons) to have a strong absorption capacity for infrared radiation of a specific wavelength, and to obtain the concentration of combustible gas by measuring the reduction of infrared energy.
The advantages of flammable gas detectors using infrared absorption principle are relatively low price, long sensor life, good linearity, fast response speed, stable and reliable long-term operation, and no interference from poisons such as hydrogen sulfide. The disadvantage is that it can only detect hydrocarbons, CO2 and other gases that have obvious absorption capacity for infrared radiation energy, the sample gas is required to be clean, and there can be no dust and water vapor and other components that affect the optical path, and 50% LEL methane is usually used for calibration, and the full-scale calibration needs to communicate with the manufacturer to customize the standard gas.
3. Laser diode absorption principle
The laser diode absorption principle is that the laser emission module emits a laser beam through the measured gas, the detector in the photoelectric sensing module receives it, and the analysis control module collects and analyzes the measured measurement signal to obtain the measured gas concentration. The concentration of the measured gas can be obtained by measuring the laser intensity attenuation, which is proportional to the measured gas content between the detection optical paths.
The principle of laser diode absorption is commonly used in VOCs projects. The advantages of the laser diode absorption principle are that it can be used for on-site analysis or sampling analysis, fast response speed, high degree of intelligence, convenient maintenance, and unclean sample gas will not affect the instrument itself; the disadvantage is that only a certain gas component can be detected in the measurement process, such as O2, CO, CH4, etc., there are minimum requirements for pipe diameter and length during installation, nitrogen is required to be continuously purged, large volume, expensive price, temperature and pressure compensation is required, and it is not suitable for small experimental devices.