Study On PM2.5 Emission And Control In Coal Combustion Boilers Of Power Plants

Coal is the primary energy source in Chinese electrical power industry.Coal combustion is one of the important sources of Inhalable Particulate(PM₁₀) emission to the atmosphere.PM₁₀, especially PM_2.5 has become the most important air pollutant in our country according to the energy construction of China.It has attracted the world attention at present.Size distribution of particles emitted from coal fired boiler was performed according to US EPA Method 5.Particulate samples were extracted from flue gas of a pilot scale 0.5MW coal combustor test facility and circulating fluidized boiler(CFB) power plants using cascade impactor. Particle size distribution and segregated particulate concentration was measured on both experiments,the fraction of PM₁₀/PM_2.5 in flue gas particulates is calculated at upstream and downstream of the electrostatic precipitator or bag house and size-dependent capture efficiencies for PM_2.5 on ESP were derived in the thesis.Particles emitted from coal combustion are usually enriched toxic heavy metals and organic pollutants such as PAHs,VOCs.Several toxic heavy metals including Cd,Pb,Ni,Cu concentration in particulates was analyzed with atomic absorption spectrometer(AAS) and Hg was analyzed with atomic fluorescence spectrometer.The result shows the concentrations of medium volatility Cd and Pb increase with decreasing particle size and tends to enrich in submicron particles.Limestone powder injection in the pilot scale CFB boiler helps to absorb heavy metals gas and reduces heavy concentration in particles and change Hg distribution in combustion products.Both traditional ESP and combined ESP have ability to catch these pollutants.Acoustic agglomeration is an aerosol preconditioning procedure to improve the performance of conventional particle removal devices.Acoustic agglomeration experiments were done in traveling wave field with redispersed fly ash,and effect of parameters on agglomeration processes is investigated such as SPL,frequency,temperature,reaction time and dust loading concentration. Particle size distributions were measured on line with laser particle sizer MS-2000.PM_2.5 especially nano-particles can get higher charge amount under high voltage pulse discharge compared with DC discharge.The main component in particle causing this result is SiO₂. To improve traditional electrostatic precipitator(ESP) removal efficiency on particles smaller than lμm,a new type of fine particle removal system is set up by combine traditional ESP and pulse charge pretreatment technology.High voltage narrow pulse discharge increases the charge amount of fine particles and makes particles easier to be caught by traditional ESP.The efficiency to removal particles smaller than lμm can be much higher than traditional ESP.The influence of charge voltage and form are analyzed to give theoretically support to improve traditional ESP.NO is one of the most important pollutants during combustion.The combined electrostatic precipitator(ESP) has a character of de-NOx with the increasing efficiency of catching PM_2.5. During pulse corona discharge,the formation of ozone and free radical has important effect on NO oxidation.Electron with high energy can also break N-O directly and the dissociative N can form N2.Accordant with dust catch efficiency,line-cylinder chamber is better than line-plate chamber. With the increase of voltage,the de-NOx efficiency becomes higher.Under a certen voltage,the efficiency of de-NOx is strongly influenced by the initial concentration of NO and NO₂.In the reactor developed by this paper,the oxygenation efficiency can be higher than 90%when the voltage is 50kV and the initial NO volume percentage is less than 10^-4.The oxidation outcome NO₂ can be absorbed by wet gas desulphuration(WGD) technology.In the combined ESP,particles are charged first in the pulse electric field and then go to the DC field.Agglomeration and collection work at the same time in DC field.The total efficiency of combined ESP is influenced by these two processes.According to traditional agglomeration theory and ESP theory,a new combined ESP efficiency model is set up.The proportion of this two processes is analyzed by this model.According to the model,the main influence on collection efficiency is the charge ability of high voltage pulse power.The influcen of agglomeration is below 10%.