On-line measurement of particle size and shape distributions of pneumatically conveyed particles through multi-wavelength based digital imaging

Abstract

On-line continuous monitoring of pneumatically conveyed particulate material is desirable in many industrial processes such as power generation, steel making, chemical processing and food handling. Co-firing of biomass with coal at existing coal-fired power plants has been widely adopted as one of the main technologies for reducing greenhouse gas emissions. Particle size and shape distributions of pulverised coal and biomass during pneumatic transportation are important parameters in relation to combustion efficiency and pollutant emissions and should therefore be monitored on-line continuously. In this paper, a novel instrumentation system incorporating a colour CCD camera and multi-wavelength laser sources is proposed to achieve on-line continuous measurement of particle size and shape distributions. Unlike the single-laser technique, three solid-state lasers of RGB (Red, Green and Blue) wavelengths are used to illuminate the particle flow field simultaneously whilst the camera captures the images of particles. The advantage of this approach is the simultaneous imaging of three different cross-sections of the pipe without the use of moving or scanning components. Quantitative information of the particles is obtained by decomposing the red, green and blue channels from the primary colour images. The equivalent field of view is estimated to be more than three times greater than that of the single-laser system. The size distribution of particles is determined by processing the particle images through contour-based image processing algorithms whilst the particle shape distribution is obtained by fitting the particles to the closest ellipses and determining their aspect ratios. Experimental results show that the multi-wavelength imaging system is capable of producing statistically more representative and more reliable measurement of particle size and shape distributions than the single-laser technique.