Coal pulverizer is the main auxiliary equipment for the coal-fired unit pulverizing system in thermal power plants. It is a power machine that grinds the raw coal to the fineness of the suspension combustion of the boiler. During the operation of the coal mill, coal and air are oxidized to form CO gas and carbon. At the same time, heat generated by friction will first cause incomplete combustion of the coal powder, thereby generating a large amount of CO gas. The increase of the CO gas concentration in the limited space inside the coal mill reduces the ignition point of the combustible mixture in the coal mill and increases the risk of the coal mill catching fire or exploding.
By detecting the concentration of CO gas on-line, it is possible to detect the signs before the pulverized coal fires (smoke or smoke). The distribution of CO gas inside the coal mill is uniform, and the temperature distribution is not uniform. The change of the concentration of CO gas can more truly and comprehensively reflect the combustion inside the coal mill than the temperature change. In fact, the increase of CO gas concentration often occurs about 1.5 hours before the visible pyrotechnics, that is, before the local temperature begins to change significantly, CO gas detection of the coal mill is an effective means to prevent the coal mill from catching fire or exploding.
"DLT5203-2005 thermal power plant coal and pulverized coal system explosion-proof design technical specifications" requirements: In the combustion explosion sensitivity and higher volatile bituminous coal and lignite, the use of medium-speed mill or double-double coal mill direct-fired milling In the system, a coal mill CO monitoring system should be set up.
The main methods for CO gas detection include infrared absorption method, electrochemical method, electrical method (thermal conductivity type and semiconductivity type), and chromatography. Currently, on-line detection of CO gas concentration generally uses infrared absorption method and electrochemical method.
Infrared Absorption and Electrochemical Comparison
It is not difficult to see that the infrared absorption method has advantages over the electrochemical method in terms of detection technology and maintenance cost. In addition, the infrared gas analysis technology based on infrared absorption method has the advantages of wide measurement range, high sensitivity, high measurement accuracy, fast response, and good selectivity, but there are also some problems in the infrared measurement process: water vapor, CO 2 interference with CO gas.
CO has an infrared absorption wavelength of around 4.6 μm , CO 2 around 4.3 μm, and water vapor in the wavelength range of 1 to 9 μm with almost continuous absorption bands. CO 2 and water vapor overlap with CO's characteristic absorption wavelength range, and the concentration of CO 2 and water vapor is much greater than that of CO, which has obvious interference to CO measurement. Therefore, the sample gas must be dehydrated or pretreated with a cooling or desiccant prior to measurement, or the gas analysis unit must be used to specifically eliminate moisture. At the same time, the filter wavelength unit is used to select the infrared wavelength, and the narrow band optical filter or gas filter gas is used. The chamber limits the infrared radiation to the narrow band of light absorbed by CO to reduce the influence of other components of the flue gas on the measured value, so as to accurately measure the concentration of CO in the flue gas and ensure the safety of the pulverizer industrial site.
The newly developed gas analyzer (low-range on-line) Gasboard-3000Plus is developed by Keith Automation, and is equipped with a special sample pretreatment device to eliminate moisture from the sample gas; the entire gas analysis unit is equipped with a constant temperature device. Prevent condensation of gaseous water in the gas analysis unit under low temperature conditions, affecting the measurement results. The sensor is also equipped with a moisture compensation adjustment device: a mechanical structure design is adopted on the micro-infrared infrared sensor to change the space ratio of the front and rear expansion air chambers, to increase the response sensitivity of the sensor to the measured gas; and to disturb the water vapor by adjusting the blades and the linear correction. The signal is adjusted so that the gas containing non-condensed water is consistent with the signal of N 2 , so that the expansion chamber before and after the sensor is offset by the influence of water vapor, which ensures the accuracy of the CO concentration measurement result.
For the effects of high CO 2 concentration , the Gasboard-3000Plus gas analysis unit uses a special CO 2 interference reduction device equipped with a filter chamber that absorbs CO 2 wavelengths and eliminates the effect of CO 2 on the characteristic absorption wavelength of CO. At the same time, a narrow-band optical filter with excellent filtering effect is adopted, and only infrared radiation having a CO characteristic absorption wavelength passes through, which can effectively block the influence of CO 2 infrared radiation and ensure the accuracy of the CO concentration measurement result.
CO Microfluidic Sensor with CO 2 Filter Chamber
The distribution of CO gas inside the mill is uniform, and the temperature distribution is not uniform. For the same alarm level, the alarm time of CO gas is one hour earlier than the alarm time of temperature. Therefore, a gas analyzer (low-range on-line) Gasboard-3000 Plus is installed at the outlet of the coal mill to accurately detect the CO concentration. Through reasonable use and scientific maintenance, when the CO gas concentration reaches the limit, it can promptly alarm. The operating personnel take appropriate measures to prevent the coal mill from catching fire or explosion and ensure the safe operation of the generator set.
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