Control systems on municipal incinerators Federal and state particulate matter emissions standards dictate the use of electrostatic precipitators, fabric filters, or scrubbers Norman J. Weinstein and Richard F. Tor0 Recon Systems, Inc. Princeton, N.J. 08540 Even a modern well-designed and operated municipal incinerator cannot meet federal and state regulations for particulate matter emissions without an air pollution control system. Federal emission standards in effect require a control efficiency of at least 93% on a weight basis, but design bases are normally higher because of uncertainties and opacity requirements. To achieve the necessary efficiency, all particles larger than 1-3 must be removed, effectively eliminating the simple air pollution control systems traditionally used on incinerators. Electrostatic precipitators, fabric filters, and certain types of scrubbers appear to be the only commercially available devices that have the capability to meet current emission standards for incinerators. Newer forms of these devices, including charged droplet scrubbers and high-velocity wet precipitators, may have advantages over more conventionaldevices, but these have not yet been commercially demonstrated for municipal incinerator applications.
extraordinary attention to all design and construction details is still required to ensure continuing high-efficiency performance. Mechanical and electrical designs are as important to adequate electrostatic precipitator performance as the basic size parameters. With careful design and operation, efficiency requirements for “dry catch” particles (filterable at 121 “C) can be met. Corrosion resulting from acidic gases can be a problem in precipitator operation. The gas temperature must be maintained high enough to avoid acid condensation on cold surfaces. Hot air purging and preheat burners to minimize acid-gas contact with cold surfaces during shutdown and startup, and sufficient insulation of metal surfaces exposed to outdoor conditions are especially important in corrosion prevention. Hopper heaters are useful in avoiding corrosion and bridging problems resulting from moisture deposition in flyash.
Electrostatic precipitators
Simple devices such as baffled-water-spray chambers traditionally used to protect duct, stack, and fan materials, are inadequate as scrubbers to meet modern particulate emission control requirements, but they can still be useful for flue gas cooling. Much more sophisticated devices are needed to efficiently remove particles in the important 1-5 1.1 size range. Various techniques are used in scrubbing, but all rely on “wetting” the particle with water to increase its size and permit
Not until 1969 were electrostatic precipitators applied to municipal incinerators in the U.S., although they had previously been used in Europe and Japan. Almost all new thermal processing facilities built since 1969, however, have used electrostatic precipitators for particulate emission control (Table 1). Although the design efficiencies for incinerators shown in the table are less than normally required for coal-fired steam boilers,
TABLE 1. Some
ESP
Scrubbers
installations a t thermal processing facilities in the U.S. and Canada Plate Gas Gas Resiarea, teomp, velocity, dence A C F M / F FPS time, s ft2
Capacity, TPD
Furnace typea
Gas flow, AC FM
4x300 1 x 220
ww
536 600
5.5 6.0
3.3 3.3
6.2
55
Special R
112 000 160000
6.6
1 X 360
R
225 000
600
3.6
5.0
4.5
57 225
Dade City, Fla. Chicago, N W
1 x 300 4 X 400
R
ww
286 000 110 000
570 450
3.9 2.9
4.0 4.6
5.7 5.5
Washington, D.C.
6 x 250
R
130 800
550
4.1
3.9
4.9
Plant
Montreal Stamford Stamford
____
- .
-
-
-~
__-
Input, KVA
Pressure Effidrop in. ciency, H,O gage wt %
__
~
0.5 0.5 0.5
95.0
40
0.4 0.2
95.6 96.9
77
0.4
95.0
48
95.0 95.0
R = refractory-lined. W W = waterwall. N o t e : Except for capicity, data refer t o design parameters for one precipitator; several may exist
Volume 10, Number 6, June 1976
545
easier removal from the gas stream. The efficiency of a particular type of scrubber on a given particle size can be related to the energy used to force the gas through the collector and to generate the water sprays. This energy usually is supplied by fans supplying pressure to the gas stream (gas motivated) in venturi or orifice scrubbers; or by pumps supplying pressure to the water stream (liquid motivated) in jet ejector or impact scrubbers. The energy required in either case representsa very significant incinerator operating cost. Since scrubber water requirements are high, recirculation is usually practiced, both to minimize makeup water and the amount of wastewater to be treated. The ratio of recycle to makeup water is determined by the quantity of particles to be removed: by the tolerance of the scrubber design to the concentration of both soluble and insoluble materials in the water, which tend to buildup with increased recycling; and by the amount of water evaporated or otherwise lost. Incinerator stack gases contain gases that dissolve during scrubbing and cause the water to become acidic. As a result, even stainless-steel scrubbers have been known to corrode. Therefore, pH control by the addition of alkali must be practiced. This has two other important effects: First, undesirable acidic gases such as hydrogen chloride, hydrogen fluoride, and sulfur oxides are removed to some degree; and second, some carbon dioxide (C02) is removed, which increases alkali consumption. The removal of CO2 may also have an important regulatory effect. Since emission standards are based on a 12 YO C02 content, the lower CO2 content exiting from a scrubber may require an even lower actual particulate emissions rate. The regulations are not clear on this matter. Several venturi scrubbers have been applied to incinerators (for operating data see Figure 1). It would appear that a pressure drop of at least 22-32 mm Hg (12-17 in. H20) is required to achieve the Federal standard. A ”clear stack” (0.07 g/scm or 0.03 gr/scf) may require more than 37 mm Hg (20 in. H20), although the scrubber water vapor plume tends to reduce this requirement by masking the opacity. The visible water vapor plume is exempt from opacity regulations. Wastewater from the scrubbers can be used to quench the furnace residue prior to treatment or disposal, thereby reducing both water and treatment costs. Fabric filters Baghouses are widely used in industrial applications, but only a few have been built for refuse incineration in the U S . and Europe. In this device, the particle-bearinggas stream is passed through a fabric-filter medium of woven or felted cloth that traps the particles and allows the gas to pass through the pores of the fabric. Although the pores are as large as 100 p , sub-micron particles are captured because particles collect on the cloth to form a fragile porous layer that effectively decreases the pore size. For various economic and practical reasons, fabric filters are usually constructed in tubular form (bags)and several bags are housed together in a steel vessel, the baghouse. The design of fabric filter baghouses depends on several parameters: choice of fabric (based on gas temperature, humidity, and particle characteristics) size-length, diameter, and number of bags (based on an empirically obtained air flow-to-cloth area ratio, and mechanical considerations) method of cleaning (based on particle characteristics and vendor preferences) method of precooling the gases to the operating temperature. To operate continuously, the filter must be intermittently cleaned by manual or mechanical means or pneumatic shaking. The dislodged particles fall to a hopper where they are removed by screw or other types of conveyors. For dry catch particles, there is no apparent reason why fabric filters will not easily meet any existing particulate matter standard. The lack of significant use in incinerators may be due to 546
Environmental Science & Technology
the filters’ dramatic sensitivity to high and low temperature; large space requirements: difficult maintenance; and significant operating costs. A pilot baghouse was operated with some success around 1959, and a recent commercial installation on a municipal incinerator has apparently been operating reasonably successfully (Table 2). Selection of control systems To choose among electrostatic precipitator, scrubber, and fabric filter systems for particulate removal from incinerator gases, several factors need to be considered. The first factor is initial cost, including those for the cooling systems, fans, stack, waste disposal, and other items dependent on the method of particulate matter control. Second, Operating costs, including power, water, maintenance, labor, and waste disposal costs. A third consideration is reliability, which must take into account the best possible estimates for downtime, the effect of downtime on other operations, sensitivity to upsets and ranges of operating conditions, possible degradation of performance with age, and problems induced in associated equipment. Finally, environmental and other considerations, including the ability to meet and exceed emissions standards: removal of non-particulatematter pollutants; the effect on the air quality of surrounding areas: the possibility of undesirable plumes; and the availability of facilities for waste disposal need to be assessed. The initial cost for particulate matter control systems tends to be comparable when the complete system is considered, including startup heaters, insulation, gas coolers, hoppers, and conveyors for precipitators; alloy metal construction, alkali addition, water supply, wastewater disposal, water vapor plume control for scrubbers: and startup heaters, gas coolers, hoppers, conveyors, and pulse air supply for baghouses. However, definitive capital cost estimates are advised for system selection, Energy requirements probably represent the single most important difference among systems. Because of low-pressure drop through an electrostatic precipitator, total energy requirement is low, even though power is required for the corona discharge and the rappers and heaters. Fabric-filter-pressure drops are higher, requiring more energy, but the energy requirement for scrubbers is by far the greatest of the three systems (Table 3). Since all of these systems are highly automated, operating labor requirements are essentially comparable and low when the systems are operating properly. Maintenance material and labor requirements for particulate matter control systems may be very significant when design and preventive maintenance are
FIGURE 1
Performance of venturi scrubbers on incinerators
TABLE 2. Operating and design parameters for a baghouse on municipal incinerator Air flow, m3/min (CFM) Air temperature, "C Fabric Air/cloth ratio, m7/min/mz (C F M/f t ) Bag size, diameter, m (in.) length, m (ft) Number o f bags (approx.) Method of cleaning Design pressure drop, m m Hg (in. H,O)
5090 (180 000) 260 (500) glass fiber 0.61 (2/1)
( O F )
0.14 (5.5) 4.27 (14) 4350 reverse air 3.7-5.6 (2-3)
TABLE 3. Comparison of energy requirements for control systems0 System
Gas motivated scrubber
Gaspressure drop mm Hg (in. H,O)
Additional reading Electrostatic
28.0 (15.0)
Fabric f i l t e r
precipitator
9.3 (5.0)
1.9 (1.0)
kW per 1000 m'/min (hp/lOOO CFM) Fan power Pump power Electrostatic power Total power
103.2 (3.92) 34.5 (1.31) 2.1 (0.08) -
_
_
-
_
_
~~
Dry flyash disposal, as usually practiced with electrostatic precipitators and baghouses, is considered advantageous: but, unless the flyash is carefully handled, a considerable amount of fugitive emissions can occur. The removal of solids in a slurry from scrubbers is less objectionable with incinerators than in other applications because this system can be integrated with the residue system for common water recycle and residue disposal facilities. Scrubbers (or wet precipitators) have at least one major advantage over dry methods of particulate matter removal: The ability to simultaneously remove a significant portion of gaseous emissions. However, present regulations do not require this control: efficient, low-energy, second-stage, gas scrubbers can be added to dry systems at a later date, if extra space and static pressure allowance in the fans are provided. The major disadvantage of scrubbers, in addition to the high-energy requirement, is the formation of visible moisture plumes and the possibility of icing and condensation problems in the surrounding area.
-
~
105.3 (4.00) 34.5 (1.31)
6.8 (0.26)
_
-
15.8 (0.6) ____
22.6 (0.86)
cipitators, baghouses) or f o r tracing water. lines (scrubbers) i s n o t included.
inadequate. Thus, differences among systems may be less important than the care that is tendered to adequate design and operation. Aciddew-point corrosion of metal surfaces can be a problem in all systems, but is more likely to occur when gases are cooled with spray water rather than with steam boilers. Cooling with spray water is sometimes done ahead of either precipitators of baghouses, and is always an integral part of a scrubber operation. Increased pressure drop and plugging can be serious problems with scrubbers or baghouses, but seldom occur with precipitators. Problems of hopper operation, which can occur with baghouses and precipitators, are obviously not a part of scrubber operations. Moderate excursions of temperature may have relatively minor effects on precipitator and scrubber operations, but can have drastic effects on filter bags so that frequent changes involving significant labor and downtime may result. The performance of all particulate matter control systems can deteriorate. Dust buildup on either discharge or collection electrodes will cause diminished precipitator performance. Discharge electrodes in precipitators are subject to deterioration and breakage, sometimes shorting out a section of the precipitator and reducing its effectiveness. Hopper bridging can cause problems in both precipitators and baghouses.Tears in filter bags can have drastic effects on baghouse performance. Deterioration in scrubbers can be caused by failure of spray nozzles, mist eliminators, and poor pump performance.
Niessen, W. R., et al., Systems Study of Air Pollution From Municipal Incineration. Volume I. Arthur D. Little, Inc. Cambridge, Mass. US. Department of Health, Education and Welfare. National Air Pollution Control Administration Contract No. CPA-22-69-23. NTlS Report PB 192 378. Springfield, Va., March 1970. Stabenow, G., Performance of the New Chicago Northwest Incinerator. Proceedings, 1972 National Incinerator Conference. New York. June 4-7, 1972. American Society of Mechanical Engineers. pp 178-194. Ensor, D. S., and Pilat, M. J., Calculation of Smoke Plume Opacity From Particulate Air Pollutant Properties.J. Air Pollut. Control Assoc. 2 1 (8), 496-501 (1971). Fife,J. W., Techniques for Air PollutionControl in Municipal Incineration. Am. Inst. Chem. Eng. Symp. Ser. 70 (137), 465-473 (1974). Hesketh, H. E., Fine Particle Collection Efficiency Relatedto Pressure Drop, Scrubbant and Particle Properties, and Contact Mechanism. J. Air Pollut. Control Assoc. 24 (lo), 939-942 (1974). This article was excerpted from Thermal Processing of Municipal Solid Waste for Resource and Energy Recovery by N. J. Weinstein and R. F. Toro, Ann Arbor Science Publishers, Inc.. P. 0. Box 1425, Ann Arbor, Mich. 48106. 1976. $20. The work was performed pursuant to Contract No. 68-03-0293 with the U.S. Environmental Protection Agency.
Norman J. Weinstein is president of Recon Systems, Inc. He has been a consultant to industry and government for 10 years on environmental problems associated with chemical, petroleum, gasification, and combustion processes. Prior to consulting, Dr. Weinstein spent more than 70 years with fsso Research and Engineering Co. and played a key role in the development of the FlOR fluidized solids process for direct reduction of iron ore.
Richard F. Tor0 is vice president of Recon Systems, Inc., an environmental engineering consulting and testing firm. A chemical engineer by training, Mr. Tor0 is an industrial and governmental consultant on technical, economic and legal aspects of pollution control problems. Coordinated by LRE
Volume IO, Number 6, June 1976 5 4 7
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