Cathode Characteristics And Collection Performance Of Rare Earth Tungsten Thermionic Emission Electrostatic Precipitation

Rare earth tungsten thermionic emission high-temperature electrostatic precipitation makes gas ionized by means of electron emission properties of cathode. It could make the particles fully charged at a low voltage and achieve an effective dust removal at high temperature. The paper focused on cathode characteristics and collection performance of rare earth tungsten high temperature thermionic emission electrostatic precipitation. It would provide an experimental and theoretical basis for research and development of the technology.Unary and binary rare earth tungsten cathode were prepared for the application in thermionic emission high-temperature electrostatic precipitation:La-W, Ce-W, Y-W, La-Ce-W, La-Y-W, and Ce-Y-W. Experimental system was constructed to analyze the voltage-ampere characteristics and loss mechanism of the cathode in high temperature electrostatic precipitator. And experiments of variable working condition were performed in different temperature, voltage, pressure and atmosphere. In order to explore dust collection performance of rare earth tungsten thermionic emission high temperature electrostatic precipitation, resistance of fly ashes at temperature of 673-1273 K was tested by DR2-1000 dust resistivity tester firstly. Secondly, it was constructed that one-stage and two-stage cylindrical rare earth tungsten thermionic emission high temperature electrostatic dust removal system. Main factors and variation law of collection efficiency were achieved by experiments of variation conditions. Finally, the process of precipitation, that is gas ionization, discharge of particles and trap of charged particles, was calculated and simulated respectively by COMSOL Multiphysics and ANSYS. Through experimental and theoretical research, the main results were obtained as follows:Effective work function of La-W, Ce-W and Y-W were 2.88,3.09, and 3.01 eV respectively. Rare earth tungsten cathode electrostatic precipitation required much lower voltage to obtain the same current density at temperature above 1173 K than traditional electrostatic precipitation. Volt-ampere characteristics of rare earth tungsten were mainly affected by voltage, temperature, atmosphere and environmental pressures. The emission current density of rare earth tungsten between cathode and anode is proportional to the applied voltage and working temperature. At temperature of 1073 and 1173 K, the relationship of emission current density is:N2>CO2>air atmosphere. The difference of emission characteristics in different atmosphere is smaller with the increase of temperature. At different temperatures, the emission current density shows different trends for the effect of negative pressure, temperature and voltage.At high tempertare, loss mechanism of rare earth tungsten in different atmosphere was researched and came into the conculsions as follows. While working in carbon dioxide or micro oxygenatmosphere, the materials formed compact blue tungsten oxide layer on the surface, and the oxide layer prevented the further reaction. In high temperature oxygen-containing atmosphere, tungsten on the surface of rare earth tungsten cathode was oxidized to tungsten trioxide and volatilized, while the rare earth had evaporation on the surface, which caused the loss of materials together. The working life of rare earth tungsten cathode was affected by temperature and oxygen concentration significantly. At 1073 K and 5% oxygen atmosphere, the theory working time of rare earth tungsten materials was more than 6000 hours.At temperatures of 673-1273 K the resistivity decrease with the increase of temperature. And the decrease trendency agreed with Arrhenius equation well. Particle size distribution affected the resistivity of fly ashes. As the temperature increases, the volume conduction played greater role in electrical characteristics. And volume conduction was mainly affected by the chemical composition. The alkali and iron ion played an important role in volume conduction. Their presence reduced resistivity significantly.Experiments of one-stage cylindrical rare earth tungsten thermionic emission high temperature electrostatic precipitation were studied in variable condition and the result presented that the increase of voltage could significantly increase the efficiency of dust removal. At 1073 and 1173 K, more than 90% of the fly ashes could be collected when operating voltage is greater than 4000 V. Raise of working temperature could also increase the collection efficiency. But collection characteristics changed a little with the increase of temperature when it was higher than 1173 K. Increase of dust concentration at entrance decreased the collection efficiency. When concentration was above 55 g/Nm3, working voltage was 1000 V, the collection efficiency was even less than 10%.Dust collector with anode-cathode distance of 22.6 mm had lower dust removal efficiency than it of 16.2 mm. The efficiency increased with rise of the charged area length. When the voltage was higher than 3500 V, the influence of charged area length on dust removal efficiency was not obvious. Experimental result of two-stage thermionic emission electrostatic precipitation presented that dust collection efficiency increased effectively with the increase of temperature, charge voltage, and collection voltage. Thermal emission electrostatic precipitators have a better effect for particle which diameter is greater than 10 μm. Compared with traditional corona-type electrostatic precipitators, the needed voltage of thermal emission electrostatic precipitators is much lower. And the efficiency could achieve more than 95% with suitable collector structure.Gas ionization among rare earth tungsten cathode and cylindrical anode was simulated and the results were presented as follows. In air, due to the emission characteristics of rare earth tungsten cathode in thermal emission electostatic precipitation, a large number of free electrons escaped from the surface of materials and participated in ionizing multiplication. As a result, the air near cathode was ionized and a region of a higher ion density formed. Outside this region, the distribution of free electrons is relatively uniform. The ion density between cathode and anode generally were more than 1014 per m3. And the region of a relatively high concentration of free electrons expanded with the increase of temperature and voltage. Emission current density between cathode and anode decreases with the increase of anode radius, especially at high temperature. Due to high working temperature and low voltage in thermionic emission high temperature dust removal, the discharge of particles mainly depended on the diffusion charge. The charge of particles increased with the raise of temperature. The charge of particles decreased with the increase of radial length and the decrease speed slowed down gradually.Cumulative efficiency along the dust collector channel of tubular thermionic emission high temperature electrostatic precipitator was calculated. And the results of presented that when working temperature was 1073 K, working voltage was 2000 V, and gas flow rate was 1.5 m/s, particle collection efficiency was less than 50% for the particles of 1 μm, it could reach above 70% for the particles of 10μm When temperature increased from 1173 K to 1273 K, the particles larger than 5 μm had been all trapped at wall ID 4 of precipitator. Collection efficiency of the particles with Rosin-Rammler distribution was calculated. The results presented that collection efficiency increased with the raise of voltage as a whole. Reduce of gas velocity and prolong of filter length would improve the dust collection efficiency. However, collection efficiency was less affected by inlet velocity and length of collector when temperature was 1273 K or even higher, and it maintained a high level.