Study Of Measuement System For Dust Concentration Based On Laser Extinction

The flue gas produced by coal-fired power plant is the one of main causes of atmospheric pollution. Studying and developing the monitoring technique for dust concentration is one of primary tasks to tackle the problem of dust pollution. At present, the existing measurement techniques for dust concentration have their own advantages and disadvantages, and it is difficult for them to meet the requirements of online measurement of dust concentration in harsh industrial processes. In this paper, combined with the CCD and optical fiber imaging technique, an online measurement system for dust concentration is studied and developed based on laser extinction.Firstly, the light scattering theory is introduced briefly in this paper. Then Mie scattering theory, Fraunhofer diffraction and total scattering theory, are presented in detail. Based on the light scattering method and particle size distribution function, the measurement equation of laser extinction is derived by mean particle size and extinction coefficient, and the mass concentration of the particle can be further calculated. This thus simplifies the design of measurement system and improves the measurement accuracy of the extinction method.Secondly, based on the measuring equation of laser extinction, an on-line measurement system for dust concentration is designed. It has five subsystems, including the power subsystem, the optical subsystem, the transmission light acquisition subsystem, the data processing subsystem and the air blowing device. Optical system consists of a laser, a spatial filter, a collimating len and a beam splitter, which is used to collimate laser light and filter stray light. It can improve the quality of incident beam. Transmission light acquisition system is made up of the optical fiber and CCD camera. It is used for the transmission and acquisition of the optical signal. The software of the data processing subsystem, which is based on MFC framework, is designed to achieve the online meausment of the dust concentration with laser extinction method. It can acquire the real-time image of transmitted light and is capable of calculating of the concentration, storing data and images. Air blowing device utilizes clean air to clear optical windows and prevents the pollution of windows. All subsystems are highly integrated, which enhances the anti-jamming performance and stability of the system.Finally, a dust generation device is designed to evaluate the performance of the measurement system. The optimized working exposure time of the CCD camera is selected based on the pixel depth of CCD and the output power of the laser. Reasons for measurement errors are analyzed in detail. The experimental results indicates that the zero drift range of the system is within the range of 10g/m3. The measurement results are also compared with those from the weighting method. The measurement system has better accuaracy in high concentration than that in the low one. The developed system is able to online measure the dust concentration with high sensitivity and good stability.