| In recent years, with rapid growth of economy, the consumption of fossil fuels increases a lot. Particulate matter from the combustion has become one of the main pollution sources, Especially PM2.5. These fine particles can do great harm to both the environment and human body. And the emission standard of particulate matter in our country has become stricter. However, the removal efficiency of conventional dust removal devices for PM2.5 is low, which makes the study of aerosol pretreatment technology necessary.Acoustic agglomeration is kind of promising pretreatment technology of particulate matter. High intensity sound wave forces particles with different sizes to vibrate with the medium in different amplitudes. Relative movement between particles makes them collide more easily and agglomerate into bigger particles, sequentially the removal efficiency for fine particles of the follow-up dust removal devices is improved. A lot of studies on acoustic agglomeration have been carried out, but the research on the mechanisms is not perfect, almost all the experimental researches were done in the laboratory, there was no practical application, and it is still controversial on the optimal frequency of sound wave. Aiming at these problems, this paper did mechanism and experimental study related with acoustic agglomeration.First of all, the motion of particles in sound wave field is calculated according to the carrying mechanism the influence of sound pressure level (SPL), frequency and particle size on the movement of particles is studied. Then high speed photography experimental set-up is constructed, the movement of DEHs particles in sound field is photographed in a high frame rate. The vibration velocity of particles obtained by analyzing these photos matches the results of calculation very well.Then, the following experimental studies are carried out aiming at particle agglomeration characters in sound wave filed:Firstly, monodispersed DEHs aerosols with different particle size distribution and fly ash from Banshan power plant are used respectively as particle source to study the acoustic agglomeration characteristics. As for the acoustic agglomeration of monodispersed aerosol,0.2μm,0.5μm and 2μm DEHs particles are used. It is found that the acoustic agglomeration effect for submicron particles is not very good, while it is quite effective for particles with diameter of 2μm. The optimal frequency is 1400-1600Hz. A higher SPL can enhance the effect of acoustic agglomeration. Agglomerations in a travelling sound wave field and a standing sound wave field are compared in the study of polydispersed aerosol. The results show that the effect of acoustic agglomeration in a standing wave field is better than that in a travelling wave field under the same SPL.Secondly, the particle size distribution of fly ash from Banshan power plant is detected by using laser particle analyser. It is found that the particle size distribution becomes larger with the increase of SPL, which confirms that the decrease of particle concentration in sound wave field is the result of agglomeration. Then the sound field is connected with the homemade electrostatic precipitator and bag filter, respectively. ELPI is used to measure the concentration and the particle size distribution at the outlet of the dust removal devices with or without acoustic effect, to study the enhancement of acoustic agglomeration on these devices. The experiments show that at the condition if 1400 Hz,148dB, the mass concentration removal efficiency for PM10 of the electrostatic precipitator is raised from 89.05% to 99.28%, and for bag filter, it is raised from 91.29% to 99.19%.Thirdly, the combination effect of acoustic agglomeration and spray is studied. Fly ash from Jiaxing power plant is used as particle source. Water, two kinds of surfactants including SDBS and TX100 and three kinds of agglomerating agents including XTG, PG and CMC are used as different spray sources. The influences of sound wave parameters, spray volume, the concentrations of surfactants and the species of agglomerating agents on the combination effect are studied. It is found that the optimal frequency becomes 1000Hz for the fly ash from Jiaxing power plant. Particle concentration reduces more with the increase of spray volume.There exists a optimal concentration for each surfactant. And among the three kinds of agglomerating agents, CMC is the best one for the combination effect.Then, a variety of acoustic mechanisms are considered, including Orthokinetic interaction, acoustic wake effects, mutual radiation pressure and Brownian movement. The effects of different mechanisms are compared by calculating their agglomeration kernel. The influences of particle, SPL and frequency on agglomeration kernel are also study. It is found that Orthokinetic interaction is the main mechanism. Based on the calculation of agglomeration kernel, grouping method is used to solve the kinetic equation of aerosol agglomeration. Particle concentrations and size distributions under different sound wave field are obtained. The results match the experiments very well.At last, an experimental system of multi-field cooperation based on acoustic agglomeration is designed, aiming to enhace the particle agglomeration efficiency through the cooperation of different methods. |
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