Research On Flue Gas Multi-pollutants Control Enhanced By Pulsed Corona Discharge Electrostatic Agglomeration Combined With Lye Absorption

Particulate matter(PM), SOx, NOx, Hg, PAHs and dioxins emitted from coal-fired power plants are becoming one of the most important sources of air pollution in China. Pulsed corona discharge electrostatic agglomeration is an effective method to remove fine particles, which has good particle charging and agglomeration effects. It has also good oxidation effect of SO2, NO and Hg0 to solvable components; meanwhile, the decomposition of PAHs and dioxins in fly ashes can be also achieved. When combined with conventional lye absorption technology, it is possible to simultaneous remove fine particles, SO2, NOx, Hg0, PAHs and dioxins, which reduces the cost of flue gas cleaning. This thesis focuses on the research of flue gas multi-pollutants control, and detailed experimental and theoretical investigations were carried out on flue gas multi-pollutants control enhanced by pulsed corona discharge electrostatics agglomeration combined with lye absorption.An isokinetical particle sampling system was built and experimental study on the characteristics of particulate matter emitted from a 440t/h circulating fluidized bed(CFB) coal-fired boiler. Cascade impactor was employed to sample the particles at both electrostatic precipitator(ESP) inlet and outlet. The particle size distribution and dust resitivity were measured and the influences of coal composition, boiler load, Ca/S and oxygen concentration on the characteristics of PM emissions during coal combustion were investigated. Also the experimental study on porous structure of variously sized fly ash particles was made under different combustion conditions.The experimental set up of pulsed corona discharge electrostatic agglomeration was built and the influences of pulse peak voltage, pulse frequency, residence time and initial particle concentration on the particle charging, agglomeration and collection were discussed. The results indicate that particles were biopolarly charged during the pulsed corona discharge. Diffusion charging is the dominant charging mechanism for particles with diameter<0.2μm, which are positively charged; while field charging is the dominant charging mechanism for particles with diameter>0.2μm, which are negatively charged. The particle collection efficiency curve under pulsed corona discharge electrostatic agglomeration is of "V" shape, which has the lowest collection efficiency of particles with diameter near 0.2μm.Experimental and theoretical study on decomposition of PAHs and dioxins in fly ashes by pulsed corona discharge was also made. The results indicate high energy electrons produced in positive pulsed corona discharge could collide with particles and change the pore structure, which results in PAHs and dioxins decomposition.Wet flue gas desulfurization(WFGD) is widely used in coal-fired power plants. WFGD utilizes lime/limestone solution as absorbent, which removes SO2 effectively, however, NO and Hg0 in flue gas are insoluble to be removed. Based on the research above, pulsed corona discharge was able to oxidize NO, SO2 and Hg0 in flue gas to solvable components, then flue gas multi-pollutants removal can be achieved with WFGD. So pulsed corona discharge combined with lye absorption was proposed to experimentally investigate the simultaneous removal of NO, SO2 and Hg0 in flue gas; meanwhile the chemical reaction dynamics of multi-pollutants during pulsed corona discharge were discussed. The results show good simultaneous removal efficiencies up to 90%,95%,50% and 50% can be achieved for PM1, SO2, NO and Hg0, respectively.The charging and collection electric field intensity in DC were put forward and particle electric charging calculation under pulsed corona discharge was made by a semi-experiential formula. Program was used to calculate the electrostatic agglomeration coefficient and efficiency of bipolar-charged particles. The results indicate that without the consideration of particle collection in agglomeration area, the electrostatic efficiency increases with descreasing particle size. For ultra-fine particles, the number concentration was decreased by 20～30% through electrostatic agglomeration. Fluent software was employed to simulate particle track in DC electric field. The particles were forced to move toward linearly to collection plates by electric field force and particles with larger diameter move faster. The collection efficiency of different particle size was obtained and compared with our experimental results, which agree well with each other.