| High-temperature electrostatic precipitator?ESP?is a very promising technology in hot-gas cleaning,for its high efficiency,low pressure drop and wide adaptability.In this paper,two key fundamental factors?corona discharge characteristics and fly ash resistivity?,which affect particle collection efficiency and the stability of high-temperature ESP were studied.The influence of temperature on gas ionization mechanism was analyzed through experimental and theoretical analysis.The feature of fly ash resistivity under high temperature was revealed,and the combined effects of temperature and specific resistivity on particle collection mechanism by ESP were investigated.Firstly,the corona discharge characteristics were studied at temperatures from 293K to 1073K,the discharge current densities and corona onset/breakdown voltages were obtained at different temperatures,and the corona development process was observed.The discharge current increased rapidly with the increase of temperature.In order to reveal the corona current formation mechanism at high temperature,corona discharge experiments were carried out under high temperature/normal pressure and under low pressure/normal temperature.It was found that,when the temperature was higher than 673K,discharge current in high-temperature experiment was larger than that in low-pressure experiment with the same gas density.For instance,the current density was 3.095 mA/m2 at 973K.while the current density was only 1.619 mA/m2 in pressure experiment of 30.9kPa.An additional temperature effect on corona discharge was proposed apart from the decrease of gas density as temperature increased,and an additional temperature current Alt was proposed.?It increased with increasing temperature because of the enhanced molecule kinetic energy and ionization rate.Through analysis of saturated ion current,it was found that the electron-carried current was important at temperatures above 873 K.Ale/Alt increased from 0 to 0.941 when temperature increased from 773 K to 973 K.However,with the increase of temperature,the stable corona ranges between onset and breakdown voltages decreased gradually.The corona discharges became unstable,and localized breakdowns occurred frequently,because the ionization coefficient and the number of electrons greatly increased.When temperature increased from 293 K to 873 K,the stable corona interval decreased from 14 kV to 6.8 kV at discharge gap of 35 mm.It was found that.high-temperature corona discharge stability could be improved and the operating voltage range could be extended by enlarging discharge gap,or replacing ribbon electrode with wire/spiral electrodes.Secondly,a high-temperature dust specific resistivity test system was developed,and the influence of temperature on particle surface conduction and volume conduction was studied experimentally in the temperature range from 293K to 1073K.More than 50 kinds of fly ash were collected from various industries in China,and the chemical composition,particle size distribution,morphology and specific resistivity of each fly ash were measured and analyzed.Further,a fly ash specific resistivity database was built and the characteristics of fly ash resistivity from different industries were compared and analyzed.The effect of chemical elements on particle conduction was analyzed.It was found that there was a negative correlation between the specific resistivity and the Fe,K,Na atomic number of fly ash,while the resistivity was in positive correlation with the atomic number of Ca.The internal conduction of fly ash particles was mainly caused by the alkali metal cation K+,Na+ and Li+,and Fe3O4 could change the atomic arrangement structure to influence the number of charge carriers or migration speed.Based on the database,a statistical model was established to predict the fly ash specific resistivity in the temperature range of 293K to 1073K.And the accuracy of the model was proved to be quite high,with R2 of 0.92.Finally,a series of spectrogram of fly ash specific resistivity variation with its chemical composition and temperature were developed.Finally,the combined effects of temperature and specific resistivity on particle removal by ESP were investigated.Fly ash from coal-fired power station/cement kiln/glass kiln was chosen as the typical dust with medium/high/low resistivity,and the particle removal efficiencies of the typical fly ash at temperatures from 373K to 823K were investigated experimently.It was found that,particle removal efficiency changed significantly at high temperature,due to the change of corona discharge characteristics and fly ash specific resistivity.At temperature of 453K,the specific resistivity of cement kiln ash was up to 1.75×1013? cm,with removal efficiency of 88.95%,much lower than that of coal-fired power station?97.05%?and glass furnace?96.07%?.When the temperature increased to 793K,the removal efficiency of fly ash from coal-fired power station and cement kiln decreased from 95.9%and 95.0%at 403K to 89.92%and 83.74%,respectively,due to the decrease of breakdown voltage at high temperature.However,the particle removal efficiency of glass furnace was sharply reduced to 65.91%,because its specific resistivity was only 1.12×105?·cm at 793K.Finally,the limit particle emission concentrations of ESP at different gas temperatures were explored.The experimental results showed that the limit particle emission concentration was independent of the inlet particle concentration,but depended on fly ash resistivity,the limit particle emission concentration of fly ash from coal-fired power station,cement kiln and glass kiln were 6.24 mg/m3,8.07 mg/m3 and 14.05 mg/m3,respectively. |
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