Study On Fine Particle Pollution From Coal-fired Boiler And Application Of Acoustic Agglomeration To Control Its Emission

The energy consumes structure depending on coal fired power plants for a long timemake serious pollution to the environment, in China. Coal combustion is one of theimportant sources of Inhalable Particulate (PM₁₀) emission to the atmosphere. PM₁₀,especially PM_2.5 has become the most important air pollutant in our country and hasattracted the world attention at present, since it not only effects the world climate change,destroys ecology system and history architectures, but also has great adverse effects onhuman health according to epidemiological studies.Flue gas sample experiments emitted from coal fired boiler were performed throughan isokinetically designed system according to source emission US EPA Method 5.Particulate samples were extracted from flue gas of CFB boilers using cascade impactors.Particle size distribution and the fraction of PM₁₀/PM_2.5 in flue gas are calculated atupstream and downstream of the electrostatic precipitator and capture efficiencies ofPM₁₀/PM_2.5/PM_1.0 for ESP were derived in the thesis.Heavy metals and PAHs distribution in PM₁₀ from coal fired flue gas were measuredin experiments. Characteristics of surface properties of different size particles wereanalyzed with low-temperature nitrogen absorption BET experiments. Several toxic heavymetals including Hg Cd, Pb etal concentration in particulates was analyzed and the resultshows the concentrations of medium volatility Cd and Pb increase with decreasingparticle size and tends to enrich in submicron particles. Hg is a kind of volatile element,and 90% of Hg exists in flue gas and fly ash. PAHs concentration in PM₁₀/PM_2.5 sampledat inlet and outlet of ESP were measured and results show that 5/6-ring PAHs are moreabsorbed in small particles. BET surface area of segregated particles can be derived fromhysteresis loops and PM_2.5 has the greatest surface area compared to other particles. Thusmany pollutants such as toxic heavy metals, organic matter and acid gas are prone to beabsorbed or enriched in PM_2.5 within combustion and post-combustion environment.Acoustic agglomeration is an aerosol preconditioning procedure to improve theperformance of conventional particle removal devices. Application of high intensityacoustic field to aerosol containing micron and submicron sized particles results in muchincreased rates of particle collisions, adhesions and thus agglomeration. Such increasedparticle sizes process improves the fractional efficiency of conventional particle removaldevices. Acoustic agglomeration experiments were done in traveling wave field withredispersed fly ash, and effect of parameters on agglomeration processes is investigatedsuch as SPL, frequency, retains time and dust loading concentration.Acoustic agglomeration simulation is established based on the orthokinetic andhydrodynamic interaction as main mechanisms. The entrainment factor and phase relationship of particle's motion compared to acoustic field is computed and they aredependent upon the particle size and frequency. A general expression for the acousticagglomeration frequency function (AAFF), which measures the agglomeration rate ofparticles in a sound field, has been derived through a statistical approach. The acousticintensity, acoustic frequency, particle size distribution and gas temperature effects on theagglomeration frequency function is analyzed through numerical simulation.Aerosol dynamics equation of flue gas acoustic agglomeration is derived and thenumerical scheme to solve the equations is discussed. The results of the numericalsimulation are compared with the experiments results and have a good agreement. Theeffects of importants parameters on acoustic agglomeration process such as soundpressure level, frequency, mass loading and agglomeration time are calculated anddiscussed and an optimal parameters range were recommended according to thesimulation.