Date of Award
Doctor of Philosophy (PhD)
Pulverized coal combustion is an important source of electricity production worldwide. However, it is also a primary air pollution emission (particulate matters, mercury, etc.) source. Electrostatic precipitators (ESPs) are widely used in coal-fired power plants to control particulate matters. This dissertation contributes in the study of the pollution formation from coal combustion processes and pollution control based on ESPs in the following three aspects: (1) explore novel applications of ESPs in particulate matters and mercury removal; (2) develop and evaluate an compact electrostatic precipitator for sampling purpose, which can support offline fly ash particle analysis; (3) study the submicrometer particle formation during co-firing of coal with sawdust.
Part.1. Three new topics in flue gas treatment using ESPs were studied: (1) elemental mercury (Hg0) oxidation by a soft X-ray irradiation enhanced ESP; (2) secondary aerosol formation in the ESP during flue gas treatment; (3) submicrometer particle capture by a pressurized ESP.
In the first study, in-situ soft X-rays were coupled to a DC (direct current) corona-based (ESP). The device showed Hg0 oxidation efficiency of 49.0% without soft X-rays and soft X-rays could enhance the oxidation to an efficiency of 65.6%. Hg0 was oxidized by the oxidizing radicals formed due to corona discharge and soft X-ray irradiation. Detailed mechanisms of radical formations and Hg0 oxidation were proposed and discussed based on ozone generation measurements and Hg0 oxidation behavior observations in single gas environments (O2, N2, and CO2). The effects of flue gas compositions and particulate matters on the performance of the studied device were also evaluated.
The second study reported the new aerosol formation (or secondary aerosol formation) in the ESP during flue gas treatment for the first time. The size distributions and chemical compositions of the secondary aerosols were characterized. It is very likely that they were H2O-H2SO4 aerosols with a unimodal size distribution and a peak diameter around 20 nm. They were formed through the following pathway: (1) SO2 in the flue gas was oxidized by the oxidizing radicals, reacted with H2O in the ESP, and eventually formed H2SO4 (vapor); (2) H2O-H2SO4 SAs were firstly formed through ion-induced nucleation; (3) SAs grew through condensation and nucleation. During the particle formation, organics were also absorbed on SAs. The effects of SO2 concentration, water content in the flue gas, ESP voltage, and flow rate through the ESP on the new aerosol formation were also evaluated.
In the third study, the performance of the pressurized ESP, a promising particle control solution for pressurized coal combustion, was studied. The ESP was operated under 1 atm, 2 atms, and 3 atms, respectively. Current-voltage characteristics of the ESP show that ionization was more difficult with higher gas pressure. Under the same ESP voltage, higher gas pressure resulted in less particle capture due to insufficient particle charging. However, under the same ESP current, the ESP with higher gas pressure worked better. A modified Deutsche-Anderson (D-A) equation was developed and the experimental data fitted well with the estimated particle capture efficiencies from the modified D-A equation. The capture of fly ash particles by the pressurized ESP was also studied. The charging status of fly ash particles greatly influenced the capture efficiencies.
Part.2. In order to characterize the fly ash particles, proper particle sampling is needed. In this part, an electrostatic nanoparticle sampler was developed. It is a two-stage ESP. A parametric study was first performed to identify the optimal operating condition of the sampler. Under the optimized conditions, the sampler achieved a collection efficiency of more than 90% for particles ranging from 50 to 500 nm, and 57% to 78% of the particles were collected on the SEM stub in the sampler. The collection efficiencies of oleic acid particles were lower than those of NaCl particles in the size range from 50 to 150 nm, due to lower charging efficiencies of oleic acid particles.
Part.3. Co-firing of coal with biomass for electricity generation has economic and environmental benefits, which makes it a promising alternative of coal combustion. To better understand this process, submicrometer particle formation during co-firing of coal with sawdust was studied in this part. Particle size distributions, elemental compositions, and organic compositions of particles were analyzed in detail. Coal contributed more than sawdust in submicrometer particle formation and the formation of organic part of the particles was closely related with the formation of inorganic part. Higher sulfur content in coal and higher combustion temperature resulted in higher concentrations of submicrometer particles and organics. Higher moisture content resulted in lower overall particle concentrations but higher particle concentrations in the size range between 80 nm to 500 nm and more organics in the particles.
Pratim Biswas, Richard Axelbaum, John Gleaves, Jung-Tsung Shen, Brent Williams, Ye Zhuang