Particulate pollution is main kind of atmospheric pollution.The fine particles are seriously harmful to human health and environment.Acoustic agglomeration is considered as a promising pretreatment technology for fine particle agglomeration.The mechanisms of acoustic agglomeration are very complex and the agglomeration efficiency is affected by many factors.The most important and controversial factor is frequency.Comparative studies between high-frequency and low-frequency sound source to agglomerate coal- fired fly ash were carried out to investigate the influence of frequency on agglomeration efficiency.Acoustic agglomeration theoretical analysis,experimental particle size distributions （PSDs） and orthogonal design were examined.The results showed that the 20 kHz high-frequency sound source was not suitable to agglomerate coal-fired fly ash.Only within the size ranging from 0.2 to 0.25 μm the particles agglomerated to adhere together,and the agglomerated particles were smaller than 2.5 μm.The application of low-frequency （1000–1800 Hz） sound source was proved as an advisable pretreatment with the highest agglomeration efficiency of 75.3%,and all the number concentrations within the measuring range decreased.Orthogonal design L16 （4） 3 was introduced to determine the optimum frequency and optimize acoustic agglomeration condition.According to the results of orthogonal analysis,frequency was the dominant factor of coal-fired fly ash acoustic agglomeration and the optimum frequency was 1400 Hz.
The formation, micromorphology characteristics and heavy metals enrichment of fine particles generated during coal combustion were studied.The particles were sampled by using impacting classifier, the micromorphology was examined with SEM and the heavy metals content was measured with atomic absorption spectrophotometer.The results indicated that the morphology and heavy metals enrichment of the particles were related to the particle size.Coarse particles commonly showed irregular shapes,while particles were closer to spherical shape with decreasing size.Submicron particles were always spherical and usually attached to big ones.The heavy metals content of the particles increased with decreasing particle size.
