(95a) Particle Measurement in High Temperature Gas Based on Mie Scattering

Abstract: The emission of high temperature flue gas from metallurgical, chemical, building materials and other industries causes serious air pollution and partial waste of available heat, so in order to meet the relevant emission standards, additional purification units (high temperature ceramic filters, cyclone separator, etc.) are installed in the industrial process to capture and collect the dusts in the flue gas. As the purification units are important to control the dust concentration, it is necessary to carry out a real-time measurement of the dust concentration in the flue gas in the upstream and downstream of the purification equipment and calculation of the separation efficiency to evaluate its separation performance.

In this paper, a single particle measurement method and the device suitable for 650 ℃ high temperature condition ware were developed based on the basic principle of Mie scattering. In order to meet the demand of measurement at different complicated positions, the detection device was divided into optical sensing system and data acquisition and processing system, between which optical signal transmission was achieved through optical fiber coupling technology. In addition, the optical sensing system consists of a high-temperature aerosol tube and two same optical lens systems. An optical measuring volume was formed in the center of the aerosol tube by the incident light and scattered light, which kept small enough with sufficiently variant shape and structure in order to realize the sequential scattered light of individual particles.

In order to complete the particles measurement under the high temperature (650℃) condition, the beam images in optical sensing system under different temperature conditions were measured in real time by CMOS surface sensor. It is found that the beam images ware deforms when the light divergence angle increases more than 10°, and the deformation becomes more serious as the temperature increases. Depending on the principle of heat transfer, thermal expansion along the airflow direction caused by heat conduction between the duct and the gas is the main cause of this phenomenon. But by sampling the aerosol tube material with an ultra-small linear expansion coefficient, the optical system can be applied to the temperature up to 700 ℃ condition, and in particular the light divergence angle extends to 30 °. According to the principle of gas dynamics, the optical parameters (refractive index) of the gas at different temperatures changes with the change of its state parameters. According to the results calculated by the formula of Ruegur atmospheric refractive index, it is found that the refractive index of the gas decreases with the increase of temperature, which leads to the increase of the divergence angle of the beam in the aerosol tube, and makes the position of the optical beam offset. Based on the geometrical optics theory and the geometric mathematical relationships, the theoretical dynamic model of the optical measuring volume with the change of the gas temperature was established. By calculating the amount of the beam focus offset and the volume of the optical measurement volume at different temperatures, it is found that the volume of the optical measurement body increases with the temperature increasing. However, when the gas temperature was 650 ℃, the volume increase rate of optical measuring volume was less than 1%(extremely small), so the influence of the temperature on the optical system is negligible. The detection performance of the whole detection system was tested by using 2.5μm standard spherical silica particles. It was indicated that the satisfying measuring results can be obtained and applied to the actual site condition measurement at the temperature in range of 650 ℃.