Precipitators, electrostatic

Figure 11.3 Electrostatic precipitation can be used to remove fine particles. (Reproduced with permission from Stenhouse, Pollution Control, in Teja, Chemical Engineering and the Enuironment, Blackwell Scientific Publications, Oxford, U.K., 1981.)...

Ecole Nationale Superieure du Petrole et des Moteurs Formation Industrie end point (or FBP - final boiling point) electrostatic precipitation ethyl tertiary butyl ether European Union extra-urban driving cycle volume fraction distilled at 70-100-180-210°C Fachausschuss Mineralol-und-Brennstoff-Normung fluid catalytic cracking Food and Drug Administration front end octane number fluorescent indicator adsorption flame ionization detector... 

These gases leave the furnace at about 600 K. pass through electrostatic precipitators to remove dust, and the phosphorus is then condensed out. 

In all the above methods, the sulphur dioxide obtained is impure. Dust is removed by first allowing the gases to expand, when some dust settles, then by passage through electrostatic precipitators and finally by washing with water. Water is removed by concentrated sulphuric acid which is kept in use until its concentration falls to 94%. 

Control technology requirements vary according to the scale of operation and type of emission problem. For instance, electrostatic precipitator design requirements for fly-ash control from 1000-MW coal-fired power boilers differ from those for a chemical process operation. In the discussion that follows, priority is given to control technology for the CPI as opposed to the somewhat special needs of other industries. 

H. J. White, Industrial Electrostatic Precipitation, Addison-Wesley Publishing Co., Reading, Mass., 1963. 

S. Oglesby, Jr., and G. B. Nichols, "A Manual of Electrostatic Precipitator Technology," NHS Report PB196380, Southern Research Institute, Birmingham, Ala., 1970. 

P. Coopemian, "Nondeutschian Phenomena ia Electrostatic Precipitation," Preprint 76-42.2APCA. 69th A.nnualMeeting, Portland, Oregon, June 27-]ulj 1, 1976. 

H. W. Spencer, 111, and G. B. Nichols, "A Study of Rappiug Re-entrainment ia a Large Pilot Electrostatic Precipitator," Preprint 76-42.5, 68th MPCA. MnnualMeeting Portland, Oregon, June 27—July 1, 1976. 

M. R. Schioeth, Pulse-Energi dElectrostatic Precipitators, paper at the Third Conference on Electrostatic Precipitation, Albano-Padova, Italy, October, 1987, F. L. Smidth Co., Valby, Denmark. 

E. Bakke, "The AppHcation of Wet Electrostatic Precipitators for Control of Eiue Particulate Matter," Preprint, Symposium on Control of Tine Particulate Emissions from Industrial Sources, Joint U.S.-USSR Working Group, Stationay Source Air Pollution ControlTechnology, San Francisco, Calif, Jan. 15—18, 1974. 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

The gas, along with entrained ash and char particles, which are subjected to further gasification in the large space above the fluid bed, exit the gasifier at 954—1010°C. The hot gas is passed through a waste-heat boiler to recover the sensible heat, and then through a dry cyclone. SoHd particles are removed in both units. The gas is further cooled and cleaned by wet scmbbing, and if required, an electrostatic precipitator is included in the gas-treatment stream. 

Puriftcatioa of the cracked gas is accompHshed by water scmbbiag, an electrostatic precipitator, and Hquid ammonia absorption. 

Air cleaning systems are often used to remove dust or vapors from plant or process exhaust streams. Dust collecting systems such as filters or electrostatic precipitators that handle heavy loads of dust are usually designed to be self-cleaning, but it is stiU. necessary to enter the air cleaner periodically for inspection or repair. Dust deposits inside the equipment are likely to be stirred up and inhaled by unprotected workers. Baghouses are particularly likely to cause exposure because large amounts of dust may be retained in the cloth and released when the bags are handled. 

Cooled dust-laden gas is dedusted in an electrostatic precipitator and sent to the cleaning unit to remove impurities such as arsenic, fluorine, and chlorine before being sent on to the sulfuric acid production plant. 

The combined flue dust from waste heat boiler and electrostatic precipitator, including dust from the ventilation system, is collected in a bin and recirculated to the mixing and pelletizing step, where it is used as a binding reagent. 

The lignitic coals of the northern United States tend to have low sulfur contents, making them attractive for boilet fuels to meet sulfur-emission standards. However, low sulfur content coals have impaired the performance of electrostatic precipitators. The ash of these coals tends to be high in alkaline earths (Ca, Mg) and alkaUes (Na, K). As a result, the ash can trap sulfur as sulfites and sulfates (see Airpollution control methods). 

The Megalopohs station (Fig. 4d) uses hot flue gas to dry the lignite. A cyclone separator and electrostatic precipitator permit rejection of some of the water vapor to the atmosphere rather than to the boiler. Another drying method uses a vertical shaft, heated by combustion gases, for partial drying prior to grinding. 

The potassium combines with the sulfur to form potassium sulfate, which condenses as a soHd primarily in the electrostatic precipitator (ESP) or baghouse. The recovered potassium sulfate is then deUvered to a seed regeneration unit where the ash and sulfur are removed, and the potassium, in a sulfur-free form such as formate or carbonate, is recycled to the MHD combustor. It is necessary also to remove anions such as Cf and E which reduce the electrical conductivity of the generator gas flow. These are present in the coal ash in very small and therefore relatively harmless concentrations. As the seed is recycled, however, the concentrations, particularly of CF, tend to build up and to become a serious contaminant unless removed. 

The oxidant preheater, positioned in the convective section and designed to preheat the oxygen-enriched air for the MHD combustor to 922 K, is located after the finishing superheat and reheat sections. Seed is removed from the stack gas by electrostatic precipitation before the gas is emitted to the atmosphere. The recovered seed is recycled by use of the formate process. Alkali carbonates ate separated from potassium sulfate before conversion of potassium sulfate to potassium formate. Sodium carbonate and potassium carbonate are further separated to avoid buildup of sodium in the system by recycling of seed. The slag and fly-ash removed from the HRSR system is assumed to contain 15—17% of potassium as K2O, dissolved in ash and not recoverable. 

N2, and traces of PH, CO2, E, and S large furnaces generate off-gas at a rate of about 120—180 m /min. In most installations the off-gas is passed through a series of Cottrell electrostatic precipitators which remove 80—95% of the dust particles. The precipitators ate operated at temperatures above the 180°C dew point of the phosphoms. The collected dust is either handled as a water slurry or treated dry. Einal disposal is to a landfill or the dust is partially recycled back to the process. The phosphoms is typically condensed in closed spray towers that maintain spray water temperatures between 20 and 60°C. The condensed product along with the accompanying spray water is processed in sumps where the water is separated and recycled to the spray condenser, and the phosphoms and impurities ate settled for subsequent purification. 

Although most of the particulate in the off-gas from the furnace can be captured by the electrostatic precipitators before condensing the phosphoms, some carryover into the product is inevitable. This particulate is partly separated into the condenser water. The remainder reports to the phosphoms to yield either dirty product or a stable emulsion called phosphoms mud or sludge. Over many years a variety of approaches have been used to minimize the formation of sludge and to recover phosphoms product from the sludge. 

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