Wai-Fah Chen.The Civil Engineering Handbook

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Incinerators 13-57

(13.14)

where A/C (gross) is the gross air-to-cloth ratio in ft/min, G is the total gas ﬂow rate in ACFM (wet

basis), and F is the total fabric ﬁlter area in square feet.

The gross air-to-cloth ratio is selected based on prior experience with similar sources and dusts. For

pulse jet baghouses on incinerators, typical values are less than 4.0 ft/min. Speciﬁc values are based on

a number of site-speciﬁc factors including but not limited to the following:

•Average and maximum particulate mass loadings (should be decreased with increasing particulate

loading)

•Typical particle size distributions

•Allowable maximum static pressure drops

•The need for off-line cleaning

•The need for on-line maintenance and inspection

•Purchased equipment costs

Pulse-Jet Bag Cleaning Equipment

Pulse-jet bags are cleaned with an intermittant pulse of compressed air delivered to the top, center of the

bags. The cleaning apparatus includes a source of compressed air, a compressed air manifold, a set of

diaphragm valves, a set of solenoid valves, and compressed air delivery tubes. Cleaning is done on a row-

by-row basis. On-line cleaning means that the gas stream is continuing to ﬂow through the compartments

as the rows of bags are cleaned one by one. During off-line cleaning, a compartment is isolated, while the

rows are cleaned one by one. Off-line cleaning minimizes dust cake discharge problems because the falling

solids from the outside of the bags are not being opposed by an upward ﬂowing unﬁltered gas stream.

FIGURE 13.11 Side elevation cutaway of pulse-jet unit (Richards and Quarles, 1986).

Material discharge

Diffuser

Differential pressure

switch

Remote mounted controls

Dirty gas inlet

Dust discharge

conveyor

Airlock rotary valve

Air manifold

Blower

tube

Pilot valve enclosure

Diaphragm valve

Air

pulse

timer

Pressure switch

AC gross G F

()

13-58 The Civil Engineering Handbook, Second Edition

As a general rule, the minimum baghouse gas temperature should be 50°F above the acid dewpoint

to take into account the gas temperature spatial variability in the baghouse and the short-term ﬂuctuations

in the average gas temperature. Sufﬁcient thermal insulation should be provided so that the gas temper-

ature drop across the baghouse does not exceed 25 to 40°F (depending on the inlet gas temperature).

Also, air inﬁltration should be minimized by selecting proper hatch gaskets and latches, proper solids

discharge valves, and proper shell welding practices.

Fabrics and Support Cages

The selection of fabric materials must be based on the expected gas stream temperatures and acid gas

concentrations. A summary of the general capabilities of commonly used fabrics is shown in Table 13.8.

The long-term temperature limits presented in Table 13.8 are slightly below the general temperature

limits often stated for the various types of materials. The reduced long-term temperature values increase

the service life of the bags. The short-term maximum temperature limits speciﬁed in Table 13.8 are

slightly higher than general temperature values. However, these values should not be exceeded for more

than 15 min. Severe gas temperature spikes will lead to premature bag failure, even if the long-term

temperatures are maintained in the proper range.

It should be noted that for many units, there is only a narrow optimum gas temperature range. There

can be only 100 to 150°F difference between the long-term upper gas temperature limit and the acid

dewpoint related lower gas temperature limits. Proper process control and conscientious maintenance

are necessary to maintain the narrow gas temperature range throughout the baghouse.

In addition to the temperature and acid sensitivities, the abrasion and ﬂex resistance of the material

should be considered. Materials that are vulnerable to ﬂex and abrasion problems should be used only

on cages that provide the maximum support. Finally, the cages should not have exposed sharp edges that

could cut the fabric.

Hoppers and Solids Discharge Equipment

The proper design of the hopper and solids discharge equipment is important in ensuring long-term

reliable operation. Hopper heaters and thermal insulation are important to prevent the hygroscopic,

acidic ash from cooling. This can result in bridging of solids and in hopper overﬂows. Due to the gas

entry ducts in the upper portions of the hoppers, dust re-entrainment blasting of the pulse-jet bags

occurs as the hopper solids levels increase.

Fabric Filter System Instruments

Table 13.9 summarizes the categories of instruments often used on pulse-jet fabric ﬁlters. The gas tem-

perature monitors are especially important since they provide indications of incinerator upset and

baghouse air inﬁltration. The pulse-jet fabric ﬁlter static pressure drop gauges should be mounted in

TABLE 13.8 Fabric Capabilities

Te mperature Limits Chemical Resistance

Flex/Abrasion

ResistanceMaterial Long-Term Short-Term HCl and H

Acryclic copolymer 225 250 Good — Fair

Modacrylic 275 300 Good — Fair

Polyester 250 275 Fair Fair Good

Polyphenylene sulﬁde 375 400 Good — Good

Nylon arimid 400 425 Poor Poor Good

Fluorocarbon 450 500 Good Good Good

Polyimide 450 500 Good Good Good

Fiberglass 500 550 Fair Poor Fair

Stainless steel 1200 1300 Fair — Good

Sources: Richards, J. and Segall, R. 1985. Inspection Techniques for Evaluation of Air Pollution Control

Equipment, Vol. II. EPA Report 340/1-85-022b; PEI, 1986. PEI Associates, Inc. Operation and Main-

tenance Manual for Fabric Filters. U.S. EPA Report EPA-625/1-86-020.

Incinerators 13-59

accessible locations because they are prone to pluggage of one or both of the lines. Finally, compressed

air gauges are necessary on each separate header to identify units with leak problems.

High-Efﬁciency Particulate Absolute Filters

These high-efﬁciency collectors are conceptually similar to fabric ﬁlters. They are used for the control of

relatively small gas volume incinerators ﬁring low-level radioactive wastes. The ﬁlter elements are com-

posed of thick ﬁberglass matts with radiation-resistant binders. The ﬁlters are constructed in small 2-ft

square panels approximately 1 ft deep. The ﬁlter is placed within each of the panels in order to increase

the ﬁltering area. A preﬁlter is often used to reduce the frequency of replacement of the expensive HEPA

ﬁlters. This usually consists of a set of low-efﬁciency panel ﬁlters. The average approach velocities range

between 300 to 500 m (1000 to 1500 ft/min). Particles are collected by the combined action of impaction

and Brownian diffusion on the surfaces of the ﬁlter mat. Unlike fabric ﬁlters, the accumulated material

(or dust cake) is not the main ﬁltering element in HEPA ﬁlters. The particulate removal efﬁciencies are

rated at least 99.97% efﬁcient for DOP droplets with a mean size of 0.3 microns.

The initial static pressure drop across a set of new HEPA ﬁlter panels is between 1 and 1.5 inches W.G.

The units are replaced whenever the pressure drop reaches a preset maximum limit of approximately

2.5 in. High-pressure drops are not desirable, because this increases the risks of leakage through the seals

around the panels and the risks of particle seepage through the ﬁlter elements.

Gas Atomized (Venturi) Scrubbers

A large number of quite different devices fall into the general category of “gas atomized scrubbers.” These

include but are not limited to adjustable throat Venturi, rod decks, and collision scrubbers. The common

element of all of these devices is the utilization of a high-velocity gas stream to atomize a relatively slow

moving injected liquid stream. One commercial type of adjustable throat Venturi is shown in Fig. 13.12.

This includes a wedge, which moves up and down within the diverging section of the throat in order to

vary the cross-sectional area. The wedge is moved by means of a hydraulic actuator below the elbow of

the Venturi diverging section. The particular scrubber shown in Fig. 13.12 has a cyclonic demister for

collection of the liquor droplets formed in the Venturi throat.

A second type of “Venturi” scrubber is the rod deck. The deck consists of one or more horizontal rows

of rods across the throat of the scrubber. Liquor is introduced by means of downward oriented nozzles

above the rod decks. Some models include the provision for movement of one of the decks in order to

vary the cross-sectional area and the operating static pressure drop.

A schematic of a collision scrubber is shown in Fig. 13.13. The gas stream is split into two equal streams

that are directed against each other. Impaction occurs due to the signiﬁcant differences in relative

velocities of the water droplets and the particles in the colliding streams. This particular unit has a

chevron-like demister for collection of the water droplets.

TA BLE 13.9 Pulse Jet Fabric Filter System Instruments

Portable

Process/Equipment Instrument Port

Vessel Parameter Measured Controlled or Sampling Tap

Stack Opacity — —

Fan Fan motor current — —

Fan vibration — —

Pulse jet Fabric ﬁlter

Inlet gas temperature Emergency bypass Yes

Outlet gas temperature — Yes

Compressed air pressure (each separate

header)

— —

Static pressure drop, overall bag house Cleaning system Yes

Static pressure drop, each compartment — Yes

13-60 The Civil Engineering Handbook, Second Edition

All of the fundamental mechanisms employed in gas-atomized scrubbers are particle size dependent.

The two most commonly used mechanisms are impaction and Brownian diffusion. Impaction is an

effective means of capture for particles larger than 0.5 µm, and Brownian diffusion is the primary capture

FIGURE 13.12 Adjustable throat Venturi scrubber (Brady, 1982).

FIGURE 13.13 Collision scrubber (Schifftner, 1989).

Optical

mesh-type or

chevron mist

separator pad

Cyclonic

separator

Liquid

Electric or

hydraulic

actuator

Wetted

elbow

Adjustable

insert

Throat

Inlet

Liquid feeds

Entrainment

separator

(waveform)

Chevron wash

HCI Removal stage

(absorber)

Entrainment

separator

Diffuser

Calvert

collision

scrubber

Fully

wetted

approach

Gas

inlet

Liquid inlet

Horizontal adjustable

throat

Plan View

Incinerators 13-61

mechanism for the particles in the less than 0.1 µm range. In the 0.1 to 0.5 µm range, both collection

mechanisms can be active, but they are not especially effective.

The rate of Brownian diffusion is inversely proportional to the particle size diameter. As particle size

decreases, Brownian diffusion increases. It also increases as the gas temperature increases due to the

increased kinetic energy of the gas molecules striking the small particles. Due to the combined action of

impaction and Brownian diffusion, penetration of particulate matter in gas atomized scrubbers is low

in the >1 micron range and in the <0.10 micron range. However, there is a peak in the penetration curve

(penetration = 1 -%collection efﬁciency/100%) at approximately 0.2 to 0.5 µm. Gas atomized scrubbers

and other air pollution control devices using impaction and Brownian diffusion are least effective in the

submicron particle size range.

Scrubbing systems that can achieve high particulate removal efﬁciencies in the submicron particle size

range utilize a ﬂux force/condensation mechanism to aid capture. Flux force/condensation conditions

are initiated by removing the sensible heat from the gas stream downstream of the quench, so that a

portion of the gas stream water vapor condenses on the particles to be removed. The two primary physical

mechanisms active in ﬂux force/condensation are diffusiophoresis and heterogeneous condensation

(Calvert et al., 1973; Calvert and Jhaveri, 1974). Diffusiophoresis is the net force due to nonequal

molecular collisions around the surface of a particle. The conditions that favor diffusiophoresis occur

when the particle is near another particle or its surface is undergoing condensation. The mass ﬂux of

water vapor toward the condensation surface creates the nonequal moleculer forces on the second particle.

Diffusiophoresis is important only for submicron particles affected by molecular collisions.

The static pressure drop through a Venturi scrubber can be estimated by means of the pilot-scale tests

or by one of the published theoretical equations. The Calvert et al. pressure drop calculation (Yung et al.,

1977) is presented in the following equation.

(13.15)

where g

is the static pressure drop, in W.C.; V is the gas velocity in ft/sec; and (L/G) is the liquid-to-gas

ratio, in consistent units (i.e., liters per minute of liquid to liters per minute of gas). This equation

indicates that the pressure drop is a strong function of the gas velocity through the throat used to accelerate

and atomize the liquid. The pressure drop is directly proportional to the liquid-to-gas ratio. It does not

take into account the relatively small dry frictional energy losses of the gas stream passing through the

restricted throat.

The static pressure drops for most gas atomized scrubbers on hazardous waste incinerators is in the

range of 25 to 60 inches W.C. However, some units operate at pressure drops as high as 100 inches W.C.

(Anderson, 1984). The liquid-to-gas ratios for most commercial gas atomized scrubbers is in the range

of 4 to 15 gal/1000 ACFM. At liquor rates less than 4 gal/1000, efﬁciency drops rapidly due to an

insufﬁcient number of liquor droplet “targets” in the throat. This can be a problem in relatively arid

areas, where makeup water is limited, necessitating a high purge rate. The scrubber system efﬁciency

decreases at high liquor ﬂow rates due to a change in the droplet size distribution formed in the scrubber.

A liquid-to-gas ratio of 10 gal/1000 ACFM is generally considered optimal.

Due to the complexity of most wet scrubber systems, numerous instruments are necessary to monitor

performance. Table 13.10 summarizes the categories of instruments usually necessary and the types of

units used.

Hydrosonics™ Scrubber

This is a group of ejector-type scrubbers. Most of the units used for high-efﬁciency particulate collection

use a fan as a source of motive power. However, one of the designs using a supersonic steam (or

compressed air nozzle) can be used without a fan. The scrubbers consist of a cyclonic pretreatment

chamber, one or more converging section “nozzles” for ﬂue gas, a ring of liquor spray nozzles around

the ﬂue gas converging sections, a gas-liquor mixing section, a long contact throat, and a mist eliminator.

LG V=

()

0 005

13-62 The Civil Engineering Handbook, Second Edition

The units rely on a combination of particle condensation growth and particle impaction. Accordingly,

the relationships presented earlier between pressure drop and scrubber performance also apply to this

category of scrubbers.

Two of the most common types of HydroSonics™ systems used are shown in Figs. 13.14 and 13.15.

The unit in Fig. 13.14 is a Tandem Nozzle design, and the unit in Fig. 13.15 is the SuperSub™ unit having

the steam or compressed air ejector nozzle. For both types of units, the gas stream from the incinerator

initially enters a cyclonic chamber, where the temperature is reduced to approximately the adiabatic

saturation temperature. This chamber also serves as a cyclonic precleaner for the removal of large particles

emitted from the incinerator.

For the unit shown in Fig. 13.15, a compressed air nozzle operating at supersonic velocities is used

for the initial atomization of scrubber liquor and for the generation of suction. The ﬂue gas and atomized

liquor then pass through a subsonic nozzle. A ring of spray nozzles around the subsonic nozzle injects

an additional cocurrent stream of liquor. The ﬂue gas is then accelerated in a long throat, where particle

growth by condensation and particle capture by impaction occur. Water droplets are collected in a low-

pressure drop cyclonic collector or in a horizontally oriented chevron demister vessel.

TABLE 13.10 Gas-Atomized Scrubber System Instruments

Portable

Process/Equipment Instrument Port

Vessel Parameter Measured Controlled or Sampling Tap

Quench Inlet gas temperature Emergency quench —

Outlet gas temperature Emergency quench Yes

Makeup water ﬂow rate — —

Makeup water pressure Incinerator trip —

Emergency water pressure Incinerator trip —

Recirculation liquor ﬂow — —

Recirculation liquor pH Alkali feed rate Yes

Inlet static pressure Induced draft, fan,

gas recirc. damper

—

Venturi Pressure drop Adjustable throat Yes

Recirculation liquor ﬂow — —

Liquor inlet header pressures — —

Recirculation liquor pH Alkali feed rate Yes

Demister Pressure drop Flush water sprays Yes

Recirculation

pumps

Discharge pressure Emergency quench —

Fan Inlet gas temperature — Yes

Fan motor current — —

Fan vibration Fan trip —

FIGURE 13.14 Tandem nozzle Hydro-Sonics scrubber (Holland and Means, 1988). (Reproduced courtesy of

HydroSonics Corporation.)

Tandem Nozzle Fan Drive

Water injected

Turbulent mixing

particulate wetted

Gas inlet

Subsonic

nozzle

Free-jet mixing

Agglomeration

Incinerators 13-63

The compressed air requirements for the supersonic ejector unit are 0.04 to 0.05 pounds per pound

of ﬂue gas (John Zink Co., 1988a). For a ﬂue gas stream of 50,000 ACFM saturated at 180°F, this is

equivalent to approximately 125 to 160 lb/min or 1600 to 2100 SCFM. Compressor horsepower require-

ments based on the manufacturer’s data are 0.06 to 0.07 Hp/lb of ﬂue gas per minute (John Zink Co.,

1988b). For the 50,000 ACFM example, the horsepower required is approximately 185 to 220. The

compressor must be equipped with a means of removing condensed oil so that this material is not

volatilized and recondensed as non-wet-table particles. If low-pressure steam is available from a waste

heat boiler or other source, it can be used instead of compressed air in the nozzle. The steam requirements

are typically on the order of 0.03 pounds per pound of ﬂue gas (John Zink Co., 1988b).

The fan horsepower requirements are a function of the static pressure drop across entire system. The

common range is 10 to 35 in. of water. The fan horsepower requirements range between 2.1 HP/1000 ACFM

for a 10-in. pressure drop to 7.3 HP/1000 ACFM for a 35-in. pressure drop (John Zink Co., 1988b).

The instrumentation requirements for a HydroSonics™ scrubber are similar as those for a Venturi

scrubber.

Ionizing Wet Scrubbers

The ionizing wet scrubber (IWS) utilizes electrostatic charges for capture of particulate matter. The initial

part of the control device is an ionizer section that functions like an electrostatic precipitator. Instead of

grounded precipitator collection plates within the nonuniform electric ﬁeld, the ionizer scrubber uses a

packed bed scrubber downstream of the electric ﬁeld for particle capture. The operating principles of an

ionizing wet scrubber are similar to those of a conventional electrostatic precipitator with two major

exceptions. Due to the removal of collected particulate matter on wetted packing, resistivity is not a

major factor. Also, the migration velocities of the charged particles are lower because the applied electric

ﬁeld does not extend into the packed bed. However, this is offset by the much shorter migration velocities

to the collection surfaces.

The power source for an IWS consists of a standard transformer-rectiﬁer set (T-R set) identical to the

type used on electrostatic precipitators. A separate T-R set is used for each ionizer/packed bed module

in series. For incinerators, the number of modules in series can range from one to four, depending on

the particle size distribution, the mass emission regulatory limits, and the presence of an upstream

adjustable throat Venturi section. The overall power requirements of the units range between 0.2 and

0.4 kVA per 1000 ACFM (Ceilcote Co., 1975).

The ionizer is a set of small diameter negatively charged wires centered between grounded metallic

plates. Alignment is maintained at 3 in. ± 0.25 in. to ensure maximum operating voltages. The actual

operating voltage normally varies between secondary voltages of 20 and 25 kV and secondary amperages

FIGURE 13.15 SuperSub Hydro-Sonics scrubber (John Zink Co., 1988a). (Reproduced courtesy of HydroSonics

Corporation.)

SuperSub Fan Ejector Drive

Stream or

compresses air

Water

Subsonic

nozzle

Turbulent mixing

particulate wetted

Agglomeration

Water injected

Small ejector nozzle

(supersonic)

Free-jet mixing

13-64 The Civil Engineering Handbook, Second Edition

of 25 to 100 mÅ (Ceilcote Co., undated). The operating voltages are controlled by an automatic voltage

controller that utilizes spark rate as the monitored variable. The spark rate is a function of the particle

size distribution, the particulate matter loading, and the ionizer electrode alignment. The spark rate is

usually maintained between 50 and 100 sparks per minute (Ceilcote Co., 1975).

Until the secondary voltage reaches the onset point, there is insufﬁcient voltage to initiate a sustained

corona on the negatively charged discharge electrodes. Once this voltage is exceeded, the secondary

current rises rapidly as the voltage is increased. Generally, the maximum currents and voltages are a

function of the spark rate set by the operator, with 50 to 100 sparks per minute being the manufacturer’s

recommendation for most facilities. However, the controller also has maximum primary current, sec-

ondary current, and primary voltage limits to protect the T-R set. There are also undervoltage limits with

a short time delay to protect against short circuits due to broken wires or failure of an insulator.

The size of the ionizing wet scrubber is based primarily on the actual gas ﬂow rate. The cross-sectional

area of the ionizer is based on the desired superﬁcial velocity necessary to achieve adequate particle

charging. The packed bed size is based primarily on the acid gas removal requirements, not the charged

particle removal requirements. The turndown capability of the IWS system is good. As long as particle

size distribution and loadings remain relatively constant, the performances for gas removal and particulate

removal should improve as the gas ﬂow rate is decreased.

The ionizer is cleaned using a programmable controller. During washing, the ionzier is shut down for

approximately 3 min to prevent electrical sparking related damage to the small diameter discharge wires

and to the T-R set. Generally, cleaning is done on a 4 to 8 h schedule. However, this depends strongly

on the particulate loading.

Acid gas removal is accomplished within a packed bed immediately downstream of the ionizer. This

is usually a 4 ft irrigated bed of 2-in. diameter Tellerette™ packing. A set of sprays is used to maintain

recirculation liquor ﬂow across the packing. An internal sump is used as part of the recirculation loop.

The pH of the liquor is maintained between 6 and 8 by means of alkaline addition. The liquor recirculation

rates within an IWS stage are approximately 10 gal per 1000 ACFM. This includes the deluge water used

on a routine basis to clean the electrodes. Make-up water requirements are generally in the range of 2 gal

per 1000 ACFM per stage.

Packed Bed and Tray Tower Scrubbers

Hydrogen chloride, hydrogen ﬂuoride, and sulfur dioxide are the main pollutants collected in packed

bed and tray tower scrubbers. These are relatively soluble gases that can be collected with high efﬁciencies

in a variety of units. The most common type of packed bed scrubber is the vertical tower with randomly

stacked packing. The counterﬂow arrangement inherent in the vertical tower design has a performance

advantage in that the driving forces for absorption are maximized by this ﬂow arrangement. The gas

stream encounters progressively cleaner liquor as it approaches the scrubber outlet at the top of the vessel.

Another advantage of this approach is that it requires little plant area. The main disadvantage is the

length of the ductwork from the outlet at the top to the inlet of ground-mounted fans. The tray tower

scrubbers share the advantages and limitations of the vertical packed bed scrubbers.

Another common scrubber style is the horizontal crossﬂow packed bed. The conﬁguration of this unit

is compatible with rod deck and ionizing particulate wet scrubbers. Demisters installed in the exhaust

ends of the horizontal vessels are also slightly more efﬁcient than demisters in vertical towers because

the collected liquor drains without being opposed by the gas stream. The main disadvantage is the possible

absorption performance problems caused by the driving force gradiant across the packed bed. Removal

efﬁciency can be high for gas passing across the top of the bed and somewhat lower for gas passing

through the bottom. This is due to the reduction in pH levels as the liquor ﬂows downward through the

bed.

Gas and vapor collection in air pollution control devices is achieved by absorption or adsorption.

Absorption is the dissolving of a soluble component into droplets or sheets of liquid. Adsorption is the

physical bonding of molecules to the surface of dry particles entrained in the gas stream or contained

Incinerators 13-65

within a bed. Some of the newer air pollution control systems use absorption and adsorption in separate

control devices arranged in series.

Absorption is the transfer of a gas or vapor phase compound into a liquid phase. In the gas of hazardous

waste incinerators, the gas or vapor phase compounds primarily include hydrogen chloride, hydrogen

ﬂuoride, and sulfur dioxide. The liquid streams that receive these contaminants generally consist of

recirculated liquids containing sufﬁcient alkali to maintain the design pH for the system. Numerous

chemical engineering texts and handbooks give the design procedures for absorbers. See, for example,

Perry’s Chemical Engineering Handbook, McGraw Hill, a book that is updated at regular intervals. The

key operating conditions for an absorber are the types of packing material, and the liquid-to-gas ratio,

for the system, and the height and diameter of the absorber. Table 13.11 lists some common types of

packing material.

The pressure drop through the packed tower can be estimated using manufacturer supplied data

relating the pressure drop per foot of packing as a function of the type of packing, how it is placed in

the column (random packed or stacked in a pattern), and the liquid loading rate in terms of gallons per

square foot per minute. Typical static pressure drops per foot of packing are generally in the range of

0.1 to 1.5 in. of water (Schifftner and Hesketh, 1983).

Dry Scrubbing Systems

There are two basic styles of dry scrubbing systems in use for hazardous incinerators: spray atomizer

systems and dry injection systems. The spray atomizer systems generally include two ﬂuid nozzles and

rotary atomizers. Spray atomizer systems utilize an evaporating alkali slurry for absorption and adsorp-

tion of acid gases. The systems generally consist of a large atomizer vessel followed by a fabric ﬁlter or

an electrostatic precipitator. Dry injection systems use a ﬁnely divided alkali solid for adsorption of the

acid gases. The dry injection systems require more alkali reagent due to the lower collection efﬁciency

inherently involved. However, dry injection systems are less expensive and easier to operate. Spray

atomizer and dry injection scrubbing systems use recycle loops to increase the utilization of the alkali

materials.

A number of physical processes combine in the removal of acid gases in dry scrubbing systems. Those

systems using atomized liquid droplets initially have gas-phase controlled absorption into the drops as

they are beginning to evaporate. The factors that inﬂuence mass transfer rates are similar to those for

absorption. These include the diffusivity of the acid gas molecule in air, the gas temperature, the liquor

TABLE 13.11 Packing Material Data

Packing Material

Packing Factor

Size ft

/ft

Raschig rings, ceramic and porcelain 1.0 155

2.0 65

3.0 37

Raschig rings, metal 1.0 137

2.0 57

Te l l e r e t tes 1.0 40

2.0 20

3.0 15

Intalox saddles, ceramic 1.0 98

2.0 40

Intalox saddles, plastic 1.0 30

2.0 20

3.0 15

Glitsch ballast saddles, plastic 1.0 33

2.0 21

3.0 16

13-66 The Civil Engineering Handbook, Second Edition

spray rate, the droplet size distribution, and the ﬂue gas distribution around each of the nozzles or rotary

atomizers used in the reaction vessel.

As droplet evaporation continues, the accumulating reaction products lower the droplet water vapor

pressure and reduce the evaporation rate. Mass transfer of pollutants into the droplets begins to be

controlled by the diffusion of water molecules through the precipitating matrix of reaction products and

the undissolved reagent. Eventually, mass transfer of acid gases and volatile metals to the dried alkali

particles is limited by the vapor pressures and diffusion rates of the pollutants within the drying particles.

The important operating variables during this phase include the gas temperature, the size distribution

of the adsorbent particles, the quantity of adsorbent available, and the residence time. These factors also

limit the mass transfer rates of systems using only dry alkali particles.

Dry scrubber system vessels are designed by the equipment suppliers. The information necessary to

select the most appropriate and economical unit for a speciﬁc incinerator should be based on visits to

operating dry scrubbing systems, on available performance data for existing systems, and on information

supplied by the suppliers. In a typical spray dryer dry scrubber, the incinerator ﬂue gas initially enters a

cyclonic chamber for removal of the large particles. The cyclone outlet gas is then treated in an upﬂow

quench reactor for removal of HCl and other acid gases.

The reagent is usually calcium hydroxide slurry at 5 to 15% by weight atomized with compressed air

(Dhargalkar and Goldbach, 1988). The atomizer vessel outlet gas temperature is carefully controlled to

ensure that it does not approach the saturation temperature so closely that the solids are difﬁcult to

handle. The acid gas neutralization reactions in the quench reactor are shown below. The efﬁciency of

acid gas removal is primarily a function of the stoichiometric ratio of alkali (such as calcium hydroxide)

to the combined quantities of acid gas. Typical operating stoichiometric ratios are in the range of 2.5 to

3.5 moles of alkali per mole of acid gas.

The size of the atomizing vessel is based on the evaporating rates of the slurry droplets at the prevailing

gas stream temperatures. Generally, the residence time is between 6 and 8 sec. Nozzle operating pressures

and spray angles are selected by the manufacturer to achieve the necessary initial droplet size populations

for proper evaporation. An atomizer vessel can have one or more spray nozzles.

Calcium hydroxide is the most common alkali used because it is relatively inexpensive and easy to

handle. Calcium oxide (quick lime) is less expensive. However, a lime slaker is necessary in order to

prepare the atomizer feed slurry. Improper operation of the lime slaker can result in reduced effectiveness

of the absorption step. Other possible alkali materials include soda ash and sodium bicarbonate.

To increase acid gas removal efﬁciency, an alkali dry injection system can be installed downstream of

the atomizer vessel. The manufacturer of this type of system uses a mixture of waste alkali materials

termed “TESISORB” for dry adsorption. It is also claimed that this material improves the dust cake

properties in the downstream fabric ﬁlter used for particulate and adsorbent collection (Dhargalkar, 1988).

Compliance Test for Hazardous Waste Incinerators

This section summarizes how to design a trial burn for an incinerator or BIF. The discussion focuses on

setting the operating procedures for the unit. Guidance documents on the various aspects of trial burn

design are available from the U.S. EPA (speciﬁcally, EPA, 1983, 1986a, 1986b, 1989a, 1989b, 1992, 1999,

2000b, 2001). An especially important reference is “Test Methods for Evaluating Solid Waste,” SW-846

(EPA, updated), which is a multivolume description of the sampling and analytical methods. SW-846 is

continuously expanded and updated to reﬂect the latest EPA procedures and is incorporated into the

RCRA regulations by reference.

Limits on operating conditions for incinerators can be set as an absolute or a rolling average limit.

The absolute limit is based on the mean measured value of the control parameter during the trial burn.

It is easy to determine and to monitor, but as discussed below, it is conservative to the point where it

may not be usable for many combustors. If the absolute limit is unacceptable, then an hourly rolling

average (HRA) can be used as an alternative.