By 2023-08-01 15:38:01
Flame Detection Principles
All fuels burn to emit a certain amount of ultraviolet (UV) and a large amount of infrared (IR) radiation, which includes a spectrum of IR, visible light, and UV. Therefore, the entire spectrum can be used to detect the presence or absence of flames.
Since fuels differ in their composition, the intensity of flame radiation varies. Generally, in a coal flame, in addition to CO2 and water vapor, there are some hot carbon particles, as well as strong IR, visible light, and some UV radiation. UV is usually easily absorbed by combustion products and dust particles, and its intensity decreases rapidly, so coal combustion flames are best detected by visible light and IR flame detectors. On the other hand, for heating furnaces and ignition burners, in addition to a part of CO2 and water vapor, there are many bright carbon black particles, which can also emit more intense visible light, IR, and UV radiation. Therefore, measurements can be made of the three more sensitive components for ignition detection. When using gaseous fuels as the main fuel, the initial combustion zone of the flame has strong UV radiation. In addition, all flames have pulsations, so a low-pass filter (usually硫化铅) with an infrared solid detector can use the pulsation characteristics of flames to detect the flame. However, for power station boilers with multiple burners, the flashing frequency of flames in the nozzle area is different from that of single-burner industrial boilers, especially in the vicinity of the nozzle, so the flashing frequency range is wider.
Lead sulfide (PbS) sensors are a type of lead sulfide photodiode that is particularly sensitive to IR radiation. When fuels burn, chemical reactions produce flashing IR radiation, which stimulates lead sulfide photodiodes to convert light into electrical signals, and then processed by amplifiers to output 4-20mA or 0-10V analog signals. In the spectrum, the wavelength of IR radiation is greater than 600nm, and the spectral sensitivity of this lead sulfide sensor is 600nm-3000nm, which can effectively collect most of the IR radiation and also cover some red light in the visible spectrum, ensuring that the collected flame signal is genuine.
Phosphorus pentoxide (GaP) sensors are a type of phosphorus pentoxide photodiode that is particularly sensitive to UV radiation. When fuels burn, chemical reactions produce flashing UV radiation, which stimulates phosphorus pentoxide photodiodes to convert light into electrical signals, and then processed by amplifiers to output 4-20mA or 0-10V analog signals. In the spectrum, the wavelength of UV radiation is less than 380nm, and the spectral sensitivity of this lead sulfide sensor is 190nm-550nm, which can effectively collect most of the UV radiation and also cover most of the blue light in the visible spectrum, ensuring that the collected flame signal is genuine.
The relationship between flashing frequency and radiation intensity between coal and oil in the presence of flames and no flames is small at low frequencies (10-20Hz). The maximum difference in radiation intensity between coal with flames and without flames is about 300Hz at the point of maximum difference in flashing frequency. The maximum difference in radiation intensity between oil with flames and without flames should be higher than 100Hz to achieve better detection.
The relationship between flashing frequency and radiation intensity depends on the configuration of the burner, the detection method, the fuel type, the operating conditions of the burner (such as fuel-air ratio, air velocity) and the observation angle. In general:
The flashing frequency of the initial combustion zone in the flame is higher, and then decreases towards the end of the combustion zone.
The detector closer to the initial combustion zone of the flame detects more intense high-frequency components (100-400Hz) than the detector;
The viewing angle of the detector is narrower, and the detected flame signal is more genuine; otherwise, the opposite is true.
It can be inferred that the flashing frequency of the entire furnace monitoring is much lower than that of single-burner monitoring.
Flames in burners can be artificially divided into four sections: the black dragon zone, the initial combustion zone, the combustion zone, and the zone starting from the nozzle. The first stage is the dark coal and air mixture, which we call the black dragon zone, with low radiation intensity and low flashing frequency. The second stage is the initial combustion zone. Due to the heating of high-temperature gas and the return flow of flames, coal particles start to burn, producing bright spots of gas and particles. This stage has the characteristics of high radiation intensity but not very bright flames, with the maximum flashing frequency reaching over 100Hz. The third stage is the combustion zone, also known as the complete combustion zone. Each coal particle is fully burned in the mixture of two-stage gas, producing a large amount of heat. This stage has the highest flame brightness and stability, but the flashing frequency is low compared to the initial combustion zone. The fourth section is the zone starting from the nozzle, where most of the coal burning forms fly ash, and a few larger particles continue to burn, eventually forming high-temperature flue gas, with relatively low flame brightness and flashing frequency compared to the initial combustion zone. It should be noted that the above frequencies refer to flashing frequencies, which are different from some flame detection devices that use pulse frequencies. The former is the characteristic feature of fuel mixtures burning flames, while the latter is only a method of displaying flame strength.