Sabert Oglesby, Jr. Southern Research Institute Birmingham, Ala. 35205
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Electrostatic precipitators tackle air pollutants New operational modes find wide industrial applications for conventiona precipitators
f k d v a n c e s in electrostatic precipitator technology -~ within recent years have been primarily in specific dust control problems as opposed to radical changes in the equipment itself. Demands for reduced stack emissions have resulted in the necessity for better characterization of emissions from various processes, and especially for designs that will more reliably control emissions to meet more stringent standards. These trends are forcing a shift in precipitator-design emphasis toward a more rational engineering approach, with less reliance on design by analogy alone and less use of minimal designs. The ultimate user is faced with meeting the new air pollution codes, and consequently is insisting on control equipment that can be relied on to meet these more severe emission requirements. Problems with each type of dust control equipment differ for each application because of variations in dust and gas properties. Consequently, specific procedures for designing and operating precipitators apply to a particular industry or application. Fly ash emission control
The largest single use of electrostatic precipitators is controlling fly ash emissions from coal-fired electric power boilers. Coals burned in power boilers in the United States vary widely, and the ash produced from these coals varies correspondingly. As it influences electrostatic precipitators, the most significant properties of fly ash are its resistivity, particle size, and cohesiveness. Resistivity of fly ash can adversely influence electrostatic precipitators in two ways. If 766 Environmental Science & Technology
it is high (greater than around 2 x 1O1O a - c m ) , the current and voltage at which the precipitator can operate are reduced. If the resistivity is low, excessive reentrainment of the dust can occur. Resistivity of fly ash is dependent on the properties of coal and ash and on flue gas temperature. High sulfur coal will generally produce ash with resistivity below the critical 2 x 1010 a - c m at commonly used temperatures, 325°F or less. However, with low sulfur coal (less than 1 % ) , ash resistivity usually will be above the critical 2 X 1O1O n-cm for flue gas temperatures below about 450'F and above about 270°F. Unfortunately, most fly ash precipitators now in service operate in the neighborhood of 300°F where, for low sulfur coal, fly ash resistivity is generally too high for good precipitator performance. Until recently, the resistivity problem was not given sufficient attention, and many precipitators have been installed in power plants which burn low sulfur fuel, with the result that performance of many of these precipitators has been considerably below design. The problem has been compounded by recent demands to reduce sulfur oxide emissions. As a result, a number of power plants now use low sulfur coals instead of the formerly burned high sulfur coal that caused no fly ash resistivity problem. One solution to the high resistivity problem is reducing flue gas temperature to increase moisture and sulfur trioxide (SO,$) adsorption onto the fly ash surface. By reducing the temperature to the 250-220°F range, dust resistivity can be brought to an ac-
ceptahle value for good precipitation. This approach has been taken in several power plants burning low sulfur western coals. In addition to providing resistivity control, lower flue gas temperatures can increase boiler efficiency due to added heat recovery (if the aiT heater is of sufficient size). An alternate approach to reducing gas temperature is increasing combustion air flow through the air heater and discarding excess air or using it to reheat the flue gas. Primary disadvantages of the low temperature approach are the necessity for maintaining rather close control over flue gas temperature when fuel composition is varied and some loss of plume buoyancy. Low flue gas temperatures are desirable for boiler efficiency as previously mentioned, but fear of excessive corrosion and fouling of the air heater has led to operating policies to keep flue gas temperatures in most plants above 250-260'F. However, with low sulfur coals, law temperature corrosion and fouling are minimized since the SO, level is low. A second approach to the high resistivity fly ash problem is using conditioning agents, In medium-to-high sulfur coals, some sulfur dioxide IS converted to SO, which, in combina-
tion with the moisture present, serves to reduce fly ash resistivity. Tests have shown that SO, added to flue gas in quantities around 10-15 ppm acts in much the same way as the naturally present SO,. The first full-scale power plant utilizing anhydrous SO, for conditioning was installed at Kincardine Power Station of the South of Scotland Electricity Generating Board, although full-scale SO, conditioning was successfully proved in field tests in the United States as early as 1950. The Kincardine plant has been in continuous operation since 1963. Other plants have been installed in Isogo, Japan; the Rugeley Power Station of the Central Electricity Generating Board in England; the Arapahoe and Cherokee power plants of the Public Service Co. of Colorado in Denver, Colo.; and in South Africa. Control panels and evaporation equipment for the anhydrous SO, conditioning system installed at the Rugeley Power Station are shown on page 768. In this country, gas conditioning for fly ash has thus far been rather limited, even though used in smelters and some other applications. Several field tests have been made with both anhydrous SO, and sulfuric acid, which is chemically equivalent to SO,
Hot precipitator. Installation at the Lee Station of the Duke Power Co. shows the completed precipitator installation on the left and the ductwork being constructed on the right (notice the differences in stack emissions)
Volume 5, Number 9,September 1971 767
under flue-gas conditions. Some of these tests have been unsuccessfulprimarily due to other problems, such as high gas velocity, inadequate plate area, improper mechanical condition of the precipitator, etc. However, tests have shown that a properly engineered conditioning plant will reduce high dust resistivity to acceptable limits, and an otherwise satisfactory precipitator installation can be improved by conditioning if the efficiency is limited by excessive sparking or hack corona due to abnormally high dust resistivity. Sulfuric acid and anhydrous SO, injection systems are planned for several precipitators, and some have been installed on fly ash precipitators in this country to reduce ash resistivity. Several variations of ash conditioning have been used with some success. Ammonia (NH,), successfully used in the United States on powdered catalyst dust in 1942, has been used on a trial hasis for reduction in fly ash resistivity in Australia. Ammonia is also used in this country at a power station burning high sulfur coal, and substantial improvement in performance has been reported, although the exact nature of the improvement has not yet been resolved. Hot units
Within recent years, there has been a decided trend toward installing a precipitator ahead of the air heater in electric power generating plants. Flue gas temperatures before the air heater are normally around 600700”F, but can he as high as 9001000°F. These precipitators are termed “hot precipitators” to distinguish them from those operating at the 300°F region. One advantage of the hot precipitator is that dust resistivity at these temperatures is sufficiently low regardless of the sulfur content of the coal, and current densities are high enough for good precipitator performance. The first bot precipitator was installed at the Ravenswood Power Station of the ConsolidatedBdison Co. of New York in 1967. This precipitator is extremely large, handling some 4,300,000 ft3/ min of gas at a 700°F temperature. The collecting plate area for this precipitator is also very large (1,008,000 ft2). This precipitator’s efficiencies are reported to range from 99.2-99.6%. Since the Ravenswood installation, two plants (90 MW) at the Lee Station of the Duke Power Co. have 768 Environmental Science & Technology
been equipped with hot precipitators (page 767), and tests have shown efficiencies of 98-99%. In addition to circumventing the high resisitivity problem, the hot precipitator can be used to collect fly ash from oil-fired boilers. Oil ash can he a low resistivity material at about 300°F and difficult to collect because of reentrainment. It can also be quite tacky at low temperatures and difficult to remove. The Ravenswood plant operates interchangeably on oil and coal, and the hot precipitator not only provides the necessary flexibility for both fuels but also remoyes oil ash to minimize potential fouling in the air heater. Operating electrostatic precipitators at temperatures above 600’F is not particularly unique, since they frequently operate at or above this temperature in cement kilns, metallurgical furnaces, municipal incinerators, and other types of service. Consequently, there is considerable experience with the precipitator itself at the higher temperatures. However, there is limited experience at high temperatures for fly ash collection, but this experience has been good, with few maintenance problems reported. Principal disadvantages of hot precipitators are size and cost of the units themselves and increased cost of ducting, insulation, and supporting structure. Since the gas volume to he handled is about one and one-half times that of the low temperature precipitator, the plate area must be increased in direct proportion to achieve the same efficiency. This is generally reflected as a proportionate cost increase in the precipitator itself. A further cost increase results from the more extensive ducting system required to transport flue gas from the boiler to the precipitator, back to the air heater, and out to the fans and stack. Some plants minimize this by placing the stack on the plant roof, but if tall stacks are used, this may not be feasible. On an installed, erected cost basis, the hot precipitator may cost as much as two and one-half to three times that of a low temperature precipitator. If precautions are not taken to reduce resistivity, such as low temperature operation or conditioning, hot precipitator cost may in fact be less than that of a normal precipitator on a plant burning low sulfur coal. Typically, to achieve 99% collection eficiency on a hot precipitator, a plate-
area to gas-volume ratio of around 230-250 ft2/1000 acfm would be required. The corresponding specific area for a cold precipitator would he around 130-150 ftz/lOOO acfm-if resistivity were not a problem. If the dust resistivity is high and steps are not taken to reduce it, the required specific plate area of a cold precipitator could he as high as 400-500 ft2/ 1000 acfm. Some plants in Australia have been built with this high specific area. Pulp and paper industry
Controlling effluents from recovery boilers in the pulp and paper industry constitutes another large application of electrostatic precipitators. Recovery boilers are used in the woodpulping industry to reprocess chemicals used in cooking to dissolve the lignins in wood. The recovery process is accomplished by combustion in a boiler, which results in a significant fraction of the recoverable chemicals being entrained with flue gases. Recovering these chemicals constitutes a significant economic factor ($30-40/ton), as well as a means for controlling particulate emissions to the atmosphere. From the standpoint of process economics alone, recovering about 90% of the solids in effluent gas is the justifiable limit. How-
Controls. The Rugeley Power Station employs anhydrous SO,, which, when combined with the moisture present, will reduce fly ash resistivify
taken place in the steelmaking process, such as replacing Bessemer converters with open hearth furnaces and replacing open hearth furnaces with the basic oxygen process and the electric furnace. These changes have resulted in different applications of precipitators in the steelmaking operation. One of the most successful applications of precipitators has been in collecting sinter machine dust. Within re. . m. cent years, mcreasea lime content ferrous sintering operations has caused an increase in dust resistivity. As a result, du1st collection by elec"LA.." L"" Le-.. trostatic precipildrvm U ~ J L W L vcw iu effective. Fumes from electric furnaces also present many problems for all types of dust collection equipment because of small particle size. There have been few precipitators installed on electric furnaces in this country, principally due to the lack of moisture in the gas (which tends to give high dust resistivity). One of the m;ijor dust collection problems is associated with the iron foundry. Cupolas in most iron foun. ~...wer~. dries have been equippeo wiin cap collectors that remove only the very coarse particles. With more severe emission standards adopted, many cupolas have been equipped with wet scrubbers or fabric filters. Electrostatic precipitators have not been historically used for cupola-dust collection because of the potential high dust resistivity and wide effluent variations as the charge is introduced and melted. Although a cupola installation in California has apparently operated successfully for many years, some early precipitator installations in this country were unsuccessful and were replaced by other dust collection methods. Within the past few years, two precipitators have been installed on iron cupolas in this country-one at the Trinity Valley Iron and Steel Co. in Fort Worth, Tex., and the other at the Decatur Castings Co., Decatur, Ind. I n these installations, the cupola gas is cooled by water sprays in a cooling tower before being sent to the precipitator. The cupola installation at the Decatur Castings Co. and the associated cooling tower are shown above. The temperature of gas leaving the cooling tower is about 450-500'F to minimize high dust resistivity. The major question regarding precipitators on iron cupolas is that of 1
..
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Cupolas. In iron foundry operations, cupola gas is cooled by water sprays to minimize high dust resistivity, before sent to the electrostatic precipitator
ever, air pollution requirements dictate efficiencies of around 99%, and most plants being installed today have efficiencies in this neighborhood. Since effluent gas from recovery boilers has around 30% moisture, high resistivity has not bken a problem with recovery boiler precipitators. Due to the fineness and cohesiveness of the dust, however, removal from the electrodes can be a problem. Within recent years, several changes have been made in the recovery process to minimize the odor problems characteristic of the wood-pulping industry. These changes have involved eliminating contact between the black liquor and the flue gases in evaporators following the boiler and substituting indirect heat transfer evaporators. This process change has produced some chanees in the nature of the particula te matter produced, and the effect of this change on precipitator performa nce has not been fully ex!.., marac~cris~~cs .,..~~..A...-L.-~ _ , VI me plored. Physical sulfate particles can vary from small spheroids to platelet or nee
