| Coal staged conversion and power generation system, integrated gasification combined cycle (IGCC) power generation system and other coal clean utilization technologies are very important. It is noticed that hot gas clean-up is one of the key parts for these promising technologies. Electrostatic precipitation (ESP) is an option with high collection efficiency and convenient operation, while it always runs below 200℃. Theories in high-temperature ESP (400℃ or above) are unclear and experiences in designing and operation are in shortage, which cannot meet the requirement of coal staged conversion and power generation and IGCC systems. There is a drastic shortage of systematic research until now. The present work studies discharging mechanism and characteristics of dust removal in a wire-cylinder electrostatic precipitator at high temperatures (350～850℃), aiming to provide usful data and theories for hot gas clean-up.Firstly, several hot gas clean-up technologies are reviewed, and the development and current situation of electrostatic precipitation are introduced. The purpose and structure of this thesis is presented.Characteristics of direct current (DC) discharge in a wire-cylinder configuration at high temperatures are experimentally studied. Different types of discharge is reported with the increase of the output voltage, the corresponding V-I characteristics and according discharging photos are analyzed, and a criterion principle is proposed to identify the types of discharges under different conditions. Enhancing inter-electrode gap and mean gas free path will increase the breakdown voltage and stability of corona discharge. At 850℃, for a device with inter-electrode gap of 29mm, there is a wide range of stable corona discharge. In air atmosphere with a stainless stick as cathode, corona, glow and arc discharge successively occurs as the increase of output voltage when the inter-electrode gap is 5mm and temperature ranges from 350℃ to 650℃, while only glow and arc discharge occurs when temperature is greater than 700℃. As to a inter-electrode gap of 29mm, corona and arc discharge occurs at temperature of 350～750℃ as the increase of output voltage, and glow discharge follows corona and occurs before arc discharge at 850℃.Theoretical expressions of currents in electronegativity and non-electronegativity gases are deduced. The current ratio of non-electronegativity to electronegativity increases with the decrease of α-η. When α-η=0, the ratio equals to expa(r-ro), reaches its maximum value. At 350～850℃, some of electrons are not attached to the electronegative gas molecules and move to the anode tube. These electrons form an electron current, which account formost of the discharging current. The ratio of the electron current to the total current increases with the increase of the temperature and the port voltage.An analytical solution is proposed for DC negative corona discharge in a wire-cylinder device, where the inter-electrode gap is divided into ionization layer, attachment layer and drift region. The boundary is obtained according to experimental results. Equations in different zones are presented and solved, and the relative errors between the calculation and experimental results are within 5% at 350～750℃.Characteristics of dust removal are experimentally investigated in a high-temperature wire-cylinder ESP. A special insulator that avoides creepage is designed. A mathematical method is summerized for particle collection in the ESP. At 350～700℃, the collection efficiencies can be greater than 0.996 when the inlet mass concentration of particles ranges from 200 to 3600 mg/Nm3.The relationship between the energy consumption index, the applied voltage and the inlet mass concentration of particles is given byφ=CUp2(Up-Uc)/mn.Under all conditions, the thickness of ash cake on the collected electrode decreases along the gas flow direction. Most of particles are deposited on the front 50% of the effective collection area. The calculated results agree well with experimental results, and the relative error is within 8% when the corona discharge is in the second stage or later.Back corona discharge (BCD) is a challenge for ESP. Development of back corona discharge is visibly studied here. The mechanism of interaction between back corona and normal corona discharge (NCD) is analyzed. BCD begins at the surface of the ash layer and then extends toward the electrode gap space as the output voltage increases, finally connecting to the NCD around the cathode line and forming a consecutive discharge channel bridging the two electrodes. Generally, BCD exists with NCD when the temperature is low (i.e.,～350℃). As the temperature increases (i.e., above 500℃), the formation of BCD is more likely to result in SB. The collection efficiency is not affected for the BCD & NCD stage; in contrast, the collection efficiency decreases greatly for the BCD & SB stage. The electric power consumption will increase for all cases in which BCD occurs. |
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