Experimental study of acoustic agglomeration and fragmentation on coal-fired ash with different particle size distribution

Abstract

As the major constituent of air pollution, inhalable particles, especially the fine particles are inflicting great harm on human health due to their small particle size and absorption of hazardous components on them. However, the current conventional devices used for removing these particles suffer from low removal efficiency. Acoustic agglomeration is considered as a very effective pre-treatment technique for removing fine particles. Fine particles can agglomerate and grow to form large particles in the sound field, which can be easily removed by conventional particle removing devices. In this paper, the acoustic agglomeration and fragmentation of three different kinds of particles with different size distributions were experimentally studied. The fragmentation of agglomerates was calculated using the agglomerates forces. It was found that the particle size distribution and the volume fraction of fine particles affected the agglomeration efficiency. For unimodal particles having a narrow size range, higher the sound pressure level (SPL), greater was the agglomeration efficiency. However, there existed the optimal SPLs at 115dB and 120dB with 25% and 55% agglomeration efficiency (respectively) for bimodal particles with a wide size range. A desirable agglomeration could only be obtained in a narrow SPL range, while the agglomeration efficiency decreased significantly over the range due to the fragmentation of agglomerates. The fragments of agglomerates started to agglomerate again, though the new agglomerates were smaller than the previous agglomerates obtained at higher SPL. The spraying of water droplets can improve the agglomeration efficiency while avoiding the fragmentation of agglomerates for the bimodal particles with a wide particle size range.