Study Of Combined Acoustic Agglomeration With Other Means To Remove Coal-fired Fine Particles

The suspended fine particulate matter, especially PM2.5has become a major kind of air pollution in most cities of China, which has aroused ex-tensive concern because of its seriously injury to human health and atmospheric environment. The emission concentration standard of particulate matter has become stricter and stricter. The traditional particle filtering devices have high collect efficiency of total particulate matter, but the collect efficiency of fine particulate matter is much low. The agglomeration is an effective pretreatment technology, which agglomerates the fine particles to larger ones. Then through the traditional particle filtering devices the particulate emission will be reduced.Acoustic agglomeration utilizes the high-intensity sound field to make the fine particles collide with each other and agglomerate to larger ones within a very short time, and the concentration of fine particles decreased significantly. It is considered as a promising pretreatment technology for fine particle agglomeration. However, the investigation on acoustic agglomeration is uncompleted. The experimental study is still in laboratory, the agglomeration efficiency is lower than50%, and the impacts of various operating parameters are deficient of comprehensive understanding and the acoustic agglomeration is inefficient. The theoretical research of acoustic agglomeration mechanism has not been completely established yet and the theoretical calculations were oversimplified. In view of these, an experimental and theoretical study is carried out in this paper.In the experimental study, the acoustic agglomeration experimental facility with good expanded performance is designed and built. Coal-fired fly ash is mixed with clean air to simulated dust emission of coal-fired power plant. The combined acoustic agglomerations with other means are investigated to improve the agglomeration efficiency.Comparative studies between high-frequency and low-frequency sound sources are carried out. The results show that the agglomeration efficiency of20kHz high-frequency sound source is just about10%. Only the particles within the size smaller than0.25μm adhere together to agglomerate and the agglomerated particles are smaller than2.5μm. The application of low-frequency sound source is proved as an advised pretreatment with much better agglomeration efficiency of70%. Orthogonal design is introduced to determine the optimum acoustic agglomeration condition. The results reveal that frequency is the dominant factor and the impact of residence time is much lower. The optimum parameters are with frequency1400Hz, SPL150dB, residence time4s.The combined acoustic agglomeration with the addition of the lime seed particles is studied to remove of the coal-fired fly ash particles as the first mode. The bimodal acoustic agglomeration can improve agglomeration efficiency significantly and the increase of agglomeration efficiency is up to20%. The difference and variance analyses of orthogonal design reveal that frequency is the dominant factor and the optimum conditions are with frequency1400Hz, SPL140dB, residence time4s, mass percentage of lime seed particle30%. The combination of seed particles can enlarge the frequency window and reduce the energy consumption of acoustic agglomeration.The combined acoustic agglomeration with spray is studied and the increase of agglomeration efficiency is up to70%. Two kinds of surfactant solution, SDS and Triton X-100are investigated to improve the wettability of coal-fired fly ash and the acoustic efficiency increases further20%. As SDS is non-toxicity in the environment, more economical and better wettability, it is more appropriate as a wetting solvent.The numerical simulation is investigated based on orthokinetic agglomeration, hydrodynamic interaction, acoustic wake effect and Brown agglomeration. The effect of parameters for kernel functions is investigated. For low.frequency acoustic agglomerate the coal-fired fly ash, the contrast of kernel functions reveals that orthokinetic agglomeration is the main mechanism of acoustic agglomeration of coal-fired fly ash.The simulation results show that the optimum frequency is about1500Hz. Larger SPL, higher concentration of initial aerosol and longer residence time are more favorable to the agglomeration efficiency. Higher temperature increases acoustic agglomeration efficiency slightly, which means acoustic agglomeration still work in high temperature environment to agglomerate the fine particulate matter.