Foundries. A cupola with no scrubber working (left) discharges a plume while a scrubber on-line (right) removes air emissions
W e t scrubbers tackle pollution James H. Onnen American Air Filter Co., Inc. Louisville, K y . 40201
Custom-designed scrubber systems are having a great impact in solving major air pollution problems; overall removal efficiencies reach 99.9+ %
W
et scrubbers have been successfully used in combatting many major types of air pollution. Their use actually ranges from the smallest applications, involving unit-type wet dust collectors handling a few hundred cfm, to extremely sophisticated, custom-designed large systems for utility boiler flue gas 994 Environmental Science & Technology
cleaning. This article will cover the custom-designed system since in recent years, it has had the greatest impact in solving major air pollution problems. A great number of considerations are involved in choosing a particular type of high-efficiency pollution control system, but normally this selection is made
feature bined particulate matterlgaseous removal combinations.
from three basic types: baghouse, electrostatic precipitator, and wet scrubber. Evaluation of scrubber systems is often more difficult than evaluation for fabric or electrostatic units, because the scrubber usually is a small part of the overall system. Auxiliary equipment selection and general scrubber process play an important part in overall system quality or suitability.
Scrubber system
Scrubber types
The most widely used scrubber for high-efficiency removal of smoke or fume-sized particles is the venturi type (Figure 1). In the venturi, whether round, rectangular, or other shape, the dirty gas stream is accelerated to a high velocity in a converging enclosure, and water or other liquid is brought into direct contact with the dirty gas stream. Water droplets are either sheared from the scrubher walls by the high-velocity air, or are forced into the gas stream under pressure at or near the restriction. Millions of water droplets, often as or less, form “targets” small as 75 into which even the extremely fine fume or smoke particles “impact.” The relatively large droplets and impacted dust particles can then he removed from the cleaned gas stream by either impingement or centrifugal force in a mist eliminator. Automatically actuated dampers or other devices can be installed to regulate the amount of gas flow, pressure drop, or other process variables to provide considerable flexibility. The performance of a well-designed venturi scrubber is a function of its pressure drop, which, in turn, is a function of gas density and velocity as well as input water rate. Only 15 years ago, high-efficiency venturi scrubhers were seldom installed since air pollution regulations did not require extremely clean stacks. Today, it is common to use venturi systems, at 60-in. w.g. (water gage) pressure drop or more, on such applications as iron- and steelmaking, to produce emission levels below 0.02 grainslscfdg. This outlet dust load often corresponds to an overall efficiency in excess of 99.8-99.9Z. Other scrubber types, such as the tixed or mobile bed, are gaining wide acceptance on applications requiring both high-efficiency particulate matter removal as well as high-efficiency gas
Venturi. This high-eficiency scrubber cuts air emissions io 0.02 grainslscfdg absorption. The bed-type scrubber cannot normally he expected to remove a high percentage of the extremely fine dust such as metallurgical fume, but can achieve efficiencies well in excess of 99% on coarser dust, such as fly ash from oil- or coal-fired boilers. When vertically upward gas flow is used and scrubbing liquid is introduced above the bed, the resulting countercurrent flow provides higher gaseous removal for any given reactant liquid than can be achieved with a venturi scrubber at comparable pressure drop. This feature makes a bed-type unit attractive for boiler flue gas cleaning and other com-
A well-designed scrubber system can he successfully applied to many dust or fume control problems. System auxiliaries include process hooding for hot gas handling, necessary combustion chambers for burning potentially explosive gases, scrubbers and mist eliminators for particulate and gaseous matter removal, plus removal of entrained scrubbing water, direct or indirect gas coolers for condensing moisture and removing heat, fans or blowers for air movement, and sound attenuation as needed. Water recycling, sludge removal, and water-cleaning auxiliaries of proved design are also incorporated as needed to eliminate a secondary water pollution potential. All these components are seldom required for a particular problem, hut properly designed systems will include the necessary components. As with other types of pollution control techniques, scrubbers are most suitable for some problems and less practical for others. Scrubbers are particularly suitable for hot gas streams containing heavy dust loads (Table 1) for many reasons including: Flexibility. A well-designed scrubber system can operate at peak design or at reduced volume and temperature conditions. On hot gas applications, there is no fear of condensation, since the gas can he handled either very hot (1000”F+) or immediately quenched to its saturation temperature of 200°F or
.
Table I .
Scrubber applications Steelmaking Basic oxygen converters Open hearth furnaces Electric arc furnaces Blast furnaces Hot scarfine Sintering Foundry Cupola Dupfexingelectric arc furnaces Shakeout and sand handling Nonferrous metals Copper, brass, and bronze melting Aluminum melting and refining Smelting I
Chemical and fertilizer Black liquor recovery boilers Lime kilns Fertilizer dryers and coolers Asphalt plant-aggregate dryers Power production-fly ash and s u l f u r dioxide Public utility boilers
Industrial boilers Miscellaneous Material handling a n d transfer Acid mists and vapors Buffing and grinding Explosive or hazardous gas streams Volume 6,Number 12, November 1972 995
less. Spray chambers, requiring careful temperature controls to maintain precise temperatures to cool adequately but not approach either water or acid dew points, are unnecessary. Dependability. Proper process design coupled with sound material and equipment selections can ensure many years of life for nearly any application. Most scrubber systems contain very few moving parts-throat adjustment dampers or valves, pumps and motors, and exhausters are the principal items. By providing redundancy in the form of installed or stored “spares,” unscheduled shutdowns can be kept to a minimum. Economy. Both initial and operating costs are extremely important, as savings in one category can frequently be offset in a very short time by premiums in the other. This factor must be weighed either when comparing two scrubber systems or scrubber systems vs. dry-type collectors. Often, the initial cost of a scrubber system is substantially less than that of a dry system, to more than offset four or five years of premium operating power cost. Ducts and other vessels required for scrubbers are compact, making construction less costly for handling either explosive gases or gas streams under high positive or negative pressure. These types of conditions are typical of blast furnaces, noncombustion iron- and steelmaking and refining processes, and petroleum plant catalytic regenerators. Ease of arrangement. Principal components are relatively small and can be installed in a multitude of configurations, making it possible to fit scrubbers in spaces that would not accommodate the single, larger, rectangular shape of a precipitator or baghouse. Installing a system in an existing facility is simplest with a scrubber. Several applications-coal-fired boilers, steel-refining converters, and iron foundry cupolas-warrant more detail owing to requirements of recently enacted or proposed clean air legislation. Steel converter and foundry cupola pollution has for more than 10 years been successfully solved by gas scrubbing. Use of scrubbers on both industrial and utility boilers has more recently been brought about owing to regulations regarding SOz emissions. This demonstrates the broad range of 996 Environmental Science & Technology
scrubber system scope and illustrates strengths inherent in a well-designed system. Steel-refining converters
More than 30 oxygen converters are controlled with scrubber systems, with the vast majority being top blown basic oxygen furnaces, the other being Kaldotype furnaces, and, most recently, the Q-BOP process. There are many basic similarities as well as basic differences among these three processes. In the basic oxygen furnace, or converter, hot metal from blast furnaces and steel scrap are “charged” into the top of refractory-lined vessels with capacities of 300 tons or more. Oxygen is blown into the vessel mouth and combines with carbon in the iron producing heat to melt the charged scrap. Oxygen is blown for 20-25 min/hr with fluxes and alloying materials added to purify the melt and provide the proper metallurgy. During the oxygen blowing period, 1-2 % of the melt is discharged from the vessel in the form of smoke, fume, and slag, along with CO and COZ. The purpose of the fume control system is to control the fume and dust, as well as capture the potentially explosive CO either for further combustion or to be recovered for use as a fuel in other steel plant processes. Normally, fume and gas are captured by a close-fitting water-cooled hood which is capable of handling the burning gases at 3500°F or greater. Configuration, or hood shape and size, are important as the hood must ensure adequate fume capture without allowing smoke and fume to enter the melt shop and must allow for gas combustion in most cases. Membrane wall construction has been the most widely used. Quenching the gas stream frequently begins in the hood, using water sprays to temper the gas stream. Complete and final quenching is performed in a lowenergy venturi or orifice-shaped quencher just beyond the water-cooled hood. Water is introduced into the quencher, with only a portion being evaporated and the remainder being removed by gravity further downstream. Most coarse dust is removed at the quencher. Final gas cleaning takes place in the venturi scrubber, at a pressure drop of 50-60-in. w.g., to reduce the dust
content to 0.02-0.01 grains/scfdg as required. Entrained water is removed in a baffle-type demister, and for reducing steam plume fan size and fan power requirement, a direct contact cooler is used. The system fan can be either direct coupled or driven through a fluid coupling to allow reduced speed during nonoxygen blowing periods. Most water can be directly recycled to the scrubber, with only a portion bled to a water cleaning-sludge handling system. Classifiers are used to remove coarse, gritty solids; thickeners concentrate the finer, fume-size particles and provide clean effluent water; and vacuum filters or centrifuges further dewater concentrated fines from the thickener. Collected solids containing 25-30 % moisture can then be hauled to disposal, and the cleaned water partially recycled and partially bled from the system. Scrubber systems offer great promise for control of the two newest oxygen converter steelmaking methods-both the Q-BOP and LWS processes. These steelmaking methods have one factor in common: each proposes to offer greater steel production from a given vessel size at a lower cost than possible with top blown converters and produce greater variations in scrap and hot metal ratios. In each case, oxygen is blown through tuyeres or ports located at or near the vessel base along with an additional fluid. Fume rates, gas volumes, and temperatures are equal to or greater than that for conventional BOF’S.Two U.S. Steel Q-BOP shops are being presently installed in Gary, Ind., and Fairfield, Ala, ; both will be exhausted by scrubber fume control systems. Foundry cupola
Fume control needs for foundry cupolas depend on a great number of factors, and application of fume control equipment is further complicated by great variations in operating and design conditions. Cupolas are as small as 2-3 and as large as 60+ tons/hr melt rate capacity. Composition of materials charged is dependent not just on the melt rate, but also on type of iron being melted, and other empirically based reasons. A typical “job shop” cupola might be operated as few as 3-4 hr/day one or two days per week; another larger, production cupola could
Table I / .
Scrubbing costs Industry o r application
Oxygen steelmaking Driers and kilns Cupola Coal-fired utility boiler (High S O , high fly ash) Note:
C o s t cfm
IS
General scubber types and pressure drops, in. w.g.
Venturi 50-70 Venturi 10-20 Venturi 35-55 Venturi 12-25 Contactor 7-12
General costlcfm range eq uipment-erection
$4-5 $1.5-2.5 $2.5-3.5 $4-8 $5-7
$3.5-4.5 $1.5-2 .O $2.0-3.0 $7-14 $46
basetl o n saturated, but n o t subcooled. gas flow a t system fans.
operate essentially around the clock for several weeks without shutdown. State and local allowable emission rates for small jobbing cupolas are in excess of 0.15 grainsjscf in certain localities; for some production cupolas, allowable emission levels are 0.05 grains/scf. Scrubber systems, usually incorporating venturi-type scrubbers, are the ideal solution to most cupola emission problems. Literally scores of scrubber systems have been installed. Advantages of a scrubber system are: ability to meet the most stringent emission standards flexibility in size and equipment configuration to fit in existing foundry facilities lower initial cost-usually not offset by higher annual utility costs ability to handle up/down operation, low acid dew point gas streams, and extreme ranges of thermal conditions possibility of installing equipment to meet present codes, with relatively minor changes in power or water requirement for upgrading to meet future more stringent codes. Making iron in cupolas is much like baking a cake-the same ingredients mixed by different “cooks” do not necessarily give the same results. Therefore, a fume control system cannot be designed by theory alone, although lack of theory will assuredly give poor results. Scrap, steel, or iron is charged into the cupola along with alloying elements, fluxes, and coke. Air is blasted through tuyeres located in the lower section. As this air passes through the “bed” of charged materials, it combines with carbon in the coke to produce heat for melting the charge. While most of the coke is consumed to produce CO, some passes downward with the metal to ensure proper carbon content. The resulting gases pass upward through the charge and meet with air drawn through the charge door for control of fumes and dust. Combustion occurs in the cupola
stack, owing to the presence of flame, CO from the bed, and oxygen in air through the charge door. Dust and fume to be removed include coke breeze, iron oxides, nonferrous metal oxides, and up to 10-20Z condensibles. Elements of the scrubber system include refractory-lined ductwork, quencher or saturator, additional connecting duct, high-energy scrubber, mist eliminator ductwork, fan, and final stack. A combustion chamber is sometimes required in addition to the cupola stack; a heat exchanger can be incorporated for recovering heat from the gas stream to preheat blast air or heat building makeup air; and a gas cooling tower is sometimes practical for removing water vapor from the cleaned gas to eliminate a steam plume. In some instances, a scrubber system can be designed to produce less steam plume in cool weather than a baghouse or precipitator. Complete water- and sludgehandling systems and sound attenuation can also be included. Boiler flue gas cleaning
Federal guidelines currently require that stack gas concentrations not exceed 1.2 lb SOz/million Btu input and 0.1 lb particulate/million Btu input for existing utility boilers. Many state and local governments are requiring even lower emission levels, as well as setting ground level ambient concentration limits which must be met. Scrubbers can effectively be used for both fly ash removal and SO2absorption from fossel fuel-fired boilers to meet nearly any known or anticipated stack emission regulation (ES&T, January 1971, p 18). They can be applied on either “raw” flue gas or installed to upgrade existing cyclones or electrostatic precipitators. Coal-fired utility boilers presently on line might have electrostatic precipitators capable of removing 90-99+% fly ash or particulate matter, but have no SOzemission control capability. In such
cases, a scrubber system can be added downstream from the precipitators to achieve the required SO2 emission standard. Regardless of precipitator efficiency, the scrubber would increase the overall fly ash removal efficiency as well. For instance, a properly designed SO2 scrubber installed downstream from an existing 90 % efficient precipitator could decrease the remaining fly ash emission by an additional 90-95+% to give an overall fly ash efficiency of 99-99.5% for the combination. The same scrubber utilizing the proper “reactant” or scrubbing solution could, at the same time, reduce the SO2 emission by 8090+% depending on the scrubber design, energy consumption, and particular type of reactant and reactant rate. A number of scrubber processes are currently available, varying from laboratory study only to full-scale system operation. The processes are generally divided into two categories-disposable and salable product types. The disposable system, regardless of the actual type of scrubber employed, utilizes reactant materials such as lime or limestone in a recycle slurry to react with SOz forming calcium sulfite or sulfate. The resulting precipitates, along with any collected fly ash, are then dewatered and disposed of, having no present intrinsic value. A salable product process typically uses a regenerative type material in solution to react with SO% in the gas stream. In a secondary process external to the scrubber, the resulting compound is treated with a second chemical to ultimately form sulfuric acid and regenerate the reactant. The regenerated reactant is then fed back to the scrubber recycle loop for reuse, and the resulting sulfuric acid is a salable product. Other processes now in development stages are proposed to produce even elemental sulfur. Scrubber systems for SOz and fly ash removal are generally accepted. The disposal system dominates at this time, due to greater simplicity of operation and minimal liability (the utility is not involved in a secondary chemical planttype operation). While disposal of calcium sulfite or sulfate is a liability for the disposal system owner, some utilities consider it less a liability than involvement in a secondary product operation. Volume 6, Number 12, November 1972 997
In a combined precipitator-wet scrubber system, gas is first drawn through rigid electrode-type precipitators for optimum fly ash removal of 93-97 wt %. Precleaned gas is then blown through a bank of mobile bed-type contactor scrubbers for 80+ % SO1 removal and a total of 99.5+% fly ash removal. Advantages of this system are: optimum initial cost, high overall reliability (with standby capacity and component redundancy), minimum operating power requirements, and lowest possible reactant usage. Early attempts at SOs scrubbing brought out many concept weaknesses which have been corrected through perseverance and sound engineering judgment. The most serious problems resulted from scrubber configuration and inadequate process controls. Lack of a clean-cut, or well-defined, wet-dry junction caused material deposits to accumulate and plug scrubbers. Improper monitoring and control of slurry pH, especially when using the highly reactive lime slurries, resulted in quick formation of scale on pipe and collector surfaces. Wet scrubbing future
Gas cleaning by wet scrubbing should continue to be an important factor in many industries. New devices are being designed, developed, and offered to give better performance than some existing equipment. Also, field and laboratory work done by manufacturers and users continues to res& in overall system improvements. Continuing work with mass transfer, heat exchange, and combinations of waterlgas energy is resulting in improved system operation and increased performance. Sulfur dioxide removal, whether from coal- or oil-fired boilers or future possible coal gassification plants, has been proved practical and reliable by gas scrubbing. Wet scrubbing may be the major method of removal for the next several years. Upgrading petroleum catalytic regenerator fume control systems is practical and economical in terms of cost, space, and reliability. With recently proved SOzremoval performance, the scrubber will be effective in handling particulate matter and sulfur oxides present from existing boilers downstream from the regenerator. Two years of continuous operation should easily be met with scrubber systems. Incineration of municipal and industrial wastes creates pollutants that Circle ND. 2 01 Readen’ Scnioc Card 998 Environmental Science & Technology
can effectively be removed by wet scrubbing. Components must be designed for high temperatures and extremely corrosive gases, but these problems are encountered in varying degrees by scrubbers on many other applications. Growing concern over the need for removing condensibles points to scrubbing. Condensibles cannot reasonably be removed in dry-type collectors which must, in order to avoid moisture condensation, operate at temperatures of 250°F or higher. Product recovery or reuse
Although fume control is frequently
a liability, a wet scrubber can be used to recover or enrich a product, particularly in chemical process industries. One excellent example is in the paper industry where wood is reduced to pulp and ultimately processed into paper. Various types of “liquors” are recycled depending on the particular process, for example, soda, Kraft, or sulfite. Within the soda process in particular, it is practical to concentrate black liquor prior to its being burned as fuel in a steam-generating boiler. The liquor can be concentrated in a venturi scrubber while the scrubber is exhausting and cleaning flue gas from the boiler. Within the same paper mill, scrubber systems can be used to exhaust lime kilns which produce calcium hydroxide for use in regenerating other process liquors.
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James H. Onnen i s presently product manager uf wet scrubber systems, American Air Filter Co., Inc. A mechanical engineer by training, Mr. Onnen has spent ouer 12 years in the design and application of wet scrubber systems.
