ELECTROSTATIC PRECIPITATOR WITH PRIMARY- AND SECONDARY COLLECTOR

Abstract: An electrostatic wet precipitator for removing particulate contaminants from a gaseous stream passing through a collector tube (3) having a discharge electrode assembly (16) coaxially disposed therein and a moving liquid film on the inner surface of the tube which acts to ionize the particles in the gas. The gaseous stream mixed with water is introduced tangentially into the tube and causing the liquid to flow against the inner surface of the tube to produce the liquid film. The ionised particles are caused to migrate toward the downwardly-flowing liquid film formed on the inner surface of the collector tube by the combined action of centrifugal and electrostatic force.

Description: ELECTROSTATIC PRECIPITATOR WITH PRIMARY- AND SECONDARY COLLECTOR Electrostatic precipitators are used in many larger boiler plants to clean the fluegas from particles. There are to main types of electrostatic precipitators: • The traditional dry precipitators have primary collectors around the emission electrodes. The particles are collected in the collectors which are vibrated periodically to clean the particles down into a container. Dry precipitators are expensive and need large space. • Wet precipitators have emission electrodes and receiving electrodes to create a so-called CORONA. There are no real primary collectors, the surface of the receiving electrodes are too small for this purpose. Secondary collectors are placed separately downstreams and consist of some type of walls or filling pieces on which water is ejected. The particles are positively charged between the emission electrodes and the receiving electrodes and are attracted to the water, which is in connection with the earth. The water is filtrated and reused. Part of the water is vaporised and follows with the gas and must be compensated with new water. The weakness of the dry precipitators is their high price and large space need. The weakness of the wet precipitators is that it is difficult to find space for large enough surface for the electrodes, to isolate the electrodes in the wet atmosphere and to keep the filling pieces clean because the particles are collected on their surface. This actual innovation uses both primary and secondary collectors, has dry emmissions electrodes and wet primary collector and wet secondary collector. The design allows large space for the emission electrodes, collects a substantial part of the dust in a primary collector consisting of a water film on the inner surface of a vessel, increases the attracting force to the water by centrifugal force and uses filling pieces in a secondary collector on a better way and avoids that many water droplets can follow the outgoing flue gas. The innovation is presented in Fig. 5 and 6. In Fig. 1 to 4 the theoretical coupling between cleaning efficiency, needed collector surface and necessary power need, are presented for a dry precipitator. These figures give only a rough idea as help to understand the magnitude and the high influence of the cleaning efficiency. In reality there are large variations depending on the fuel used, gas temperature, the water content in the gas etc. To make it easier to understand the calculations are made partly for a 1 MW boiler power and partly for accepted dust contend if multicyclones are used as precleaner. In this case a gas flow of 1 m3/s and an incoming dust content of 250 mg/nm3 in dry gas is used. The function of the precipitator is explained in a case where the out going dust content of 25 mg/nm3 in dry gas is accepted. Theory. The precipitator according this innovation uses a large enough primarily collector to catch a substantial part of the dust. From Fig. 1 it can be seen that if the surface of the primary collector is about 2,5 m2/l MW boiler power about half of the dust content can be cached already in the primary collector. The dust content of the fluegas is consequently down at about 125-mg/nm3 dry gas after the primary collector, from Fig. 3. it can be seen that the cleaning efficiency must be about 90% to reach a dust content of 25-mg/nm3 dry gas. From Fig. 2. it can be seen that the total collector surface must be about 28 m2 if we assume that the wet collectors are as efficient as the dry collectors. This is realistic and on the safe side because the particles in dry precipitators are only attracted to plane collectors whence the flue gas in a wet collector must change the flow direction several times. The particles have a certain weight and want to go straight on why they have easy to meet a wet surface and be collected. The surface of wet collectors varies between 50 and 95 m2 collector surface /m3 collector volume. The space need of the wet collector is consequently much smaller than of the dry collectors. Fig.4 shows that the need of electricity is only about 120-watt/MW-boiler power. The demand of 25-mg/nm3 dry gas is quite modest why the precipitator is small with low electricity consumption. If the demand is set on 5-mg/nm3 dry gas the needed collector surface is 80-m2/MW boiler power and the electricity consumption would increase from 120 watt to 810-watt/MW-boiler power. Design and function The design of the innovation is shown in Fig. 5 and 6. The flue gas is entering a cylindrical vessel 03 through a tangential opening 02. Water is injected into the gas at the inlet why the gas temperature is decreased, the gas is saturated with water and steam and excess water flows together with the flue gas. The gas is rotated between the outer wall of the vessel and an inner wall 13. Water droplets are thrown against the grounded inner wall of the vessel due to the rotation and a water film sweeps the wall. The gas flows downwards through the distance between 03 and 13. A device with emission electrodes is hanging on a support 14. The support is electronically isolated from the vessel with isolator 12 and gets high voltage in the magnitude of 40 - 60 kV from a transformer. The electrodes consist of a ring 15, rods 16 and a ring 1 7. The electrodes have pointed teeth which send a so-called Corona field to the nearest grounded surface, which is 03, and functions as a primary collector. The dust particles are charged and start moving against the water film on the inner wall of 03. The dust particles are additionally pushed against the inner wall due to the centrifugal force created by the rotating gas. After the water film has left the emission electrodes it is guided together with the gas with a conical device into a duct with smaller diameter than the vessel. In this duct the water at the wall of the duct is scaled away with a scaling device and taken out through the outlet 19. Dust particles collected into the water are also taken away at this stage. At the next stage the gas flows on downwards and meets the ring formed bed 09 with filling pieces. The inner- and outer diameters are chosen to give the gas a velocity in the magnitude of 2 - 4 m/s at the inlet and 1 - 2 m/s at the outlet. Because the gas is rotating when it enters the bed its real velocity at the surface is much higher, which is advantageous because the particles cannot then follow the gas as easy as at a low velocity when the particles must change direction when they pas the filling pieces. It is a benefit if the velocity is moderately large at the beginning giving good cleaning without too high-pressure loss. The outlet velocity must be low so that not small droplets with dust can follow the gas. Because the bed is ring formed the outlet velocity is automatically lower than the inlet velocity. As an extra safety a drop separator of known art can be installed as the last part of the outer ring. Injection of water is arranged through the grooves 07. The groove located nearest the Centrum is ejecting water also in the inner channel, the other grooves on the top of the bed. The water flow is low or nothing in the groove at the outlet and larger and larger in the direction to the Centrum. The water 1 1 with collected particles is collected at the bottom of the vessel and taken through the pipe 25 away to be cleaned. A cleaning device 04 is installed inside the emission electrodes and is supplied with water through the pipe 05 if the electrodes after an interval need cleaning. During the cleaning the electricity to the electrodes is cut off. A cleaning device 08 cleans via a pipe 22 the inner surface of the bed if it has collected so much dust that the normal water cleaning cannot keep it clean. The outer diameter of the vessel is the same regardless of the demand of efficiency. If the demand is increased the electricity consumption increases considerably according to the Fig. 3. More electrodes 16 are then placed between the rings 15 and 1 7. Because the rods are placed along a large radius there is enough space to increase the efficiency. A larger transformer is needed in that case. The size of the bed must also be adjusted according to the demands. A solution for the water cleaning is presented in Fig. 6. The water from the secondary collector is pumped with a pump 28 into a tank 27 at about the half of the height of the tank. The sediment is falling down into the bottom of the tank and cleaned water is coming up on the top of the tank. It goes from there through the pipe 06 into the grooves 07. The water level is held constant in the tank 1 1 by taking away possible excess water into a tank 23 where also water fills to compensate the water used to saturate the gas. This water is taken from a condenser or from a water supply. The water from the primary collector is collected in a vessel 23, which also gets water from the tankl 1 so that the water level in the tank is held constant. The water is then pumped with the pump 29 into the tank at about half of the height of the tank between the outer wall of the tank and an inner wall 26 located in the tank. Cleaner water is then led from the top of the inner tank through the pipe 01 into the gas inlet 02. For increasing the collector surface on the ring form bed 9 without increasing the pressure loss it is possible to use two or more different types of filling pieces. Filling pieces with large outer dimension, which usually means large channels, low ratio for surface/volume and low relative pressure loss (= pressure loss at a certain velocity compared with other filling pieces at the same velocity). The gas velocity is highest near the centre why the largest pieces are installed there. Further out into the direction to the periphery the gas velocity is lower why it is possible to there install smaller filling pieces with a higher surface/volume ratio. At the outer periphery it is possible to install still smaller filling pieces with a still higher surface/volume ratio. As an example the filling pieces near the centre can have 3.5" diameter with 44-m2 surface/m3 with the incoming velocity of 4 m/s and the outgoing velocity of 2 m/s. Near the periphery 1" filling pieces are used with 2 m/s incoming velocity and 1 m/s outgoing velocity. These filling pieces have a surface of 225 m2/m3. The advantage to use different filling pieces is that near the centre where the gas is heavily loaded with dust particles large filling pieces give low risk for blockage whence nearer the periphery the dust content of the gas is lower why smaller filling pieces can be used with lower blockage risk. The function of the filter has been described for a case with flue gas from a boiler firing solid fuel. The use of this filter is not only limited to this application, the filter is intended to be used with all kind of dust from different sources.

Claims: Patent claims - 1. A device for cleaning gases from dust characterised by that the gas is blended with water and is tangentially directed into a chamber between an outer wall 03 and a conical inner wall 13 and with a high voltage charged emission electrode 15 arranged outside the gas stream so that Corona effect is created between the electrode and the outer wall which is cleaned by a water film and that the particles in the gas are charged with the high voltage and attracted to the water film. 2. A device according to the claim 1 characterised by that even vertical electrodes 1 6 are used for charging of the dust particles. 3. A device according to the claim 1 characterised by that the gas after leaving the emission electrode is directed into a channel with a smaller diameter than 03 where a scaling device 24 placed inside the outer wall of the smaller channel scales away from the wall the water film and the dust particles in the water. 4. A device according to the claim 1 characterised by that the gas is led from the device through a water-flushed bed 9 of filling pieces where the final cleaning is done. 5. A device according to the claim 1 characterised by that the bed 9 of filling pieces has two or more types of filling pieces with pieces with relatively low pressure drop near the centre and pieces with larger surface/volume and higher pressure drop further out in the periphery.

Inventors: ERMA EERO, ERMA, EERO
Applicants: ERMA EERO, ERMA, EERO
Application year: 1998
Main classification: B03C3
IPC codes: B03C3