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Incinerators 13-47

devices. Even when the processes include an APCD, the ash or combustion products from the waste may

damage the components and reduce their operating efﬁciency. Clearly, any program of co-ﬁring wastes

into such a combustion device must include a high level of waste selectivity and quality control. This

fact must be considered when evaluating the feasibility of using any one of these processes to burn a waste.

A second factor that must be considered is the temperature/time/oxygen regime to which a given BIF

exposes the waste. Many furnaces have high ﬂame temperatures, but the mean gas temperature in the

combustion chamber may be low. For example, most ﬁre-tube boilers have the ﬂame virtually surrounded

by the tube walls. The ﬂame may be hot enough to achieve organic destruction, but the walls cool the

combustion gases quickly, so that the postﬂame zone may not provide the extended residence time at

elevated temperature needed to destroy those organics that escape destruction in the ﬂame zone.

A third factor that must be considered before burning hazardous waste in a process furnace is the

point at which the waste is introduced. Take, for example, the introduction of wastes at a point other

than the ﬂame zone of a cement kiln. Because cement kilns are countercurrent kilns (where the solids

ﬂow in the opposite direction to the combustion gases), the problem is of special concern in co-

countercurrent kilns such as cement kilns. The waste may be vaporized and swept out of the combustion

chamber before it has sufﬁcient time to be destroyed.

The U.S. hazardous waste regulations (40 CFR §260.10) deﬁne boiler as an enclosed device using

controlled ﬂame combustion and having three major characteristics. First, the combustion chamber and

energy recovery section must be of integral design. That means that a boiler must have heat transfer

surfaces in the combustion chamber rather than as an added-on feature in the duct. A combustion

chamber that has a heat exchanger or economizer mounted on the ﬂue gas outlet and no tubes or other

steam-generating surfaces in its walls is not a boiler. Second, the boiler must recover at least 60% of the

thermal energy of the fuel. Finally, at least 75% of the recovered energy must be utilized to perform some

FIGURE 13.8 Two controlled air incinerator conﬁgurations (EPA, 1980).

Secondary

chamber

Primary

chamber

13-48 The Civil Engineering Handbook, Second Edition

function. The function can be internal (for example, to preheat air or pump liquids) or external (for

example, to make electricity). The following processes are listed by the deﬁnition:

1. Cement kilns

2. Lime kilns

3. Aggregate kilns (e.g., lightweight aggregate or asphalt plant aggregate drying kilns)

4. Phosphate kilns

5. Coke ovens

6. Blast furnaces

7. Smelting, melting, and reﬁning furnaces

8. Titanium dioxide chloride process oxidation reactors

9. Ethane reforming furnaces

10. Pulping liquor recovery furnaces

11. Waste sulfuric acid furnaces and halogen acid furnaces

The “Technical Implementation Document for EPA’s BIF Regulations” (EPA, 1992) as well as the EPA

regulations for BIFS give the legal description of the speciﬁc types of furnaces that are regulated. There

are other furnace types that are not listed in the regulations that have operating temperatures and other

conditions which may be adequate for the destruction of hazardous waste and which appear to meet the

criteria of the regulations. They are not regulated at present, but they may be subject to hazardous waste

combustion regulation if used for the purpose. Examples are as follows:

• Glass melt furnaces

•Carbon black furnaces

•Activated carbon retort kilns

The most common industrial furnaces used for waste disposal are cement kilns, lime kilns, lightweight

aggregate plants, blast furnaces, and spent acid recovery furnaces. The main factor that differentiates a

boiler or industrial furnace from an incinerator is the purpose of its use. An incinerator is built and

operated mainly for the purpose of destroying waste materials. While heat may be recovered from this

operation, that is ancillary to the system’s main function, which is waste destruction. A boiler or industrial

furnace, on the other hand, is operated for the purpose of making a physical product or producing energy.

Waste materials may be utilized in doing this, but that is an ancillary purpose to the operation.

Both the boiler and industrial furnace equipment categories can achieve the high organic compound

destructions of incinerators. Emissions test data on three boilers have shown a largely unimpaired

performance under a wide operating window that extends well beyond normal operating practices for

the units (Wool, Castaldini, and Lips, 1989). However, other data have shown that “coﬁring,” if not

properly done, can result in increased emissions (EPA, 1987). In 1991, the U.S. EPA expanded the

hazardous waste incinerator regulations (40 CFR § 260) to include boilers and industrial furnaces, (FR

Vol. 56, No. 35, P7134, and subsequent amendments). These “BIF Regulations” require incinerators and

BIFs to meet the same performance standards with operating requirements adjusted for the unique

characteristics of the individual types of furnaces.

In order to ﬁre hazardous wastes in existing boilers and industrial furnaces, it may be necessary to

add equipment and modify the system. Examples of such modiﬁcations are as follows:

•Install waste storage, blending, pretreatment, and handling facilities.

•Set up sampling and analytical facilities to characterize the wastes and to assure that the waste

composition is acceptable.

•Add burners, guns, nozzles, or other types of equipment to feed the wastes to the furnace.

•Upgrade combustion controls to handle the wastes.

•Add monitors for waste feed rate, oxygen, CO, etc., which are required for a facility burning

hazardous waste.

•Upgrade or modify the air pollution control equipment to meet the BIF emission requirements.

Incinerators 13-49

Waste storage and handling equipment will generally have to be added to a boiler or industrial furnace

that is being modiﬁed to burn hazardous waste. The facilities may consist of little more than a tank truck,

which is hooked directly to the waste feed system; however, in most cases, permanent tankage will be

preferable. Provisions for blending the wastes to maintain a consistent composition must usually be

made. Waste blending is usually needed for incinerators and is often more important for boilers and

furnaces, because boilers will typically have tighter waste speciﬁcations. In all cases, the facilities must

meet the same requirements for spill prevention containment and control as would any waste handling

facility.

A testing program to verify that the wastes received satisfy the speciﬁcations for combustion in the

boiler or furnace must be included in the waste management program. The legal requirements for such

a “waste analysis plan” are beyond the scope of this type of technical handbook. See the Technical

Implementation Document for the BIF Rules (EPA, 1992) and related guidance for legal requirements.

Operationally, it is important for a boiler or industrial furnace to keep tight control on the wastes fed

to the furnace. Undesirable waste properties or components could result in damage to the system or to

the production of off-speciﬁcation products. As a minimum, the testing program should include prop-

erties such as the following:

• Heating value — Large amounts of low heating value materials could have a deleterious impact

on ﬂame stability and furnace performance and could cause a drop in temperature.

• Viscosity, density, vapor pressure — This information is needed to assure that the wastes can be

atomized satisfactorily by the nozzles and burners. Poor waste atomization was the probable cause

of unsatisfactory performance in a number of tests conducted on boilers and industrial furnaces

burning hazardous wastes, discussed below.

• Halogen, sulfur, and nitrogen content — These values should be within bounds to keep acid gas

emissions within acceptable limits, to minimize corrosion, and to maintain product quality.

• Ash and metals — These values need to be controlled to prevent the formation of fumes and to

keep the release of particulate and, speciﬁcally, toxic metals to the environment within acceptable

limits. Ash constituents with a low fusion temperature can cause an unacceptable rate of deposition

on system components, especially boiler tubes.

Clearly the decision to burn hazardous waste in a boiler or industrial furnace, means modiﬁcations

to the equipment and to the operating procedures must be made, to provide for a means of getting the

wastes into the combustion device. Process changes that will usually be required in order to legally burn

hazardous waste are as follows:

• Addition of waste feed nozzles and guns — It is generally not recommended that wastes be fed

through the same ﬁring guns as the primary fuel, as the wastes will have chemical and physical

properties signiﬁcantly different from those of the fuel.

• Addition of automatic waste feed cutoffs — These are legally required for any combustor burning

hazardous waste.

• Addition or upgrading of combustion control equipment — Combustion of hazardous waste

requires tight control of operating parameters. For example, a wide temperature excursion may

be acceptable for normal operation of a boiler, but it is unacceptable when hazardous waste is

burned. Existing controllers may not be able to maintain the tighter bounds on the control

parameters legally required when burning hazardous wastes.

• Addition of continuous monitors — These are required for combustion gas, CO, CO

, O

, and

HC1, and possibly, particulate, mercury, and others as they are developed. Other required monitors

are for temperatures of the ﬂue gases, indicators of combustion gas ﬂow, and indicators of proper

operation of the waste nozzles and of the air pollution control equipment. The waste ﬂame must

be equipped with a cutoff that stops the hazardous waste ﬂow in case any operating parameter

goes outside its permit limits.

13-50 The Civil Engineering Handbook, Second Edition

The existing indicators for the primary fuel feeds must be wired into the waste feed system so that the

primary fuel and the waste fuel are shut off in case of primary ﬂame failure. The control system should

incorporate a means of maintaining the primary ﬂame if the problem lies with the waste combustion

system only. In this way, residual hazardous waste in the combustion chamber will be destroyed, and

operation of the system will continue.

The ﬁnal general area of consideration that will be discussed here is the impact of the waste feed on

the air pollution control device. Very few boilers and industrial furnaces are presently equipped with acid

gas control devices so the impact will be restricted to particulate. Tests on boilers and cement kilns have

shown that burning wastes containing halogens in cement kilns or (for any type of combustor) containing

metals or salts can increase the amount of ﬁne particulate that is formed. In addition to having very

small diameters, the particulate can have a lower resistivity than that produced when only primary fuel

is used. These changes can impact the performance of the air pollution control device. The impact is

especially of concern for an electrostatic precipitator or ionizing wet scrubber because their performance

is especially susceptible to size and resistivity, although the ﬁner particulate could reduce the collection

efﬁciency of any type of device. The impact on all air pollution control equipment should, therefore, be

considered when evaluating the feasibility of burning hazardous waste in a boiler or process furnace.

13.5 Air Pollution Control and Gas Conditioning Equipment

for Incinerators

Incineration is one of the most difﬁcult applications of air pollution control systems. The gas must be

cooled and multiple pollutants must be controlled with very high efﬁciencies. Hydrogen chloride, hydro-

gen ﬂuoride, and sulfuric acid vapors can be highly corrosive if handled improperly. Particulate matter

generated in the incinerators is primarily in the submicron range. Pollutants having special toxicity

problems, such as metal compounds and dioxin/furan compounds, generally enter the systems at very

low concentrations and must be reduced to concentrations that challenge the limits of analytical tech-

niques. The air pollution control system typically can consist of a heat recovery boiler, a quench, a

particulate removal device, and an acid gas removal device. The devices are often mounted in series. For

example, the off-gas treatment system for a typical high-performance rotary kiln incinerator in hazardous

waste duty can include (1) a boiler to partially cool the combustion gas and recover steam; (2) a dry

scrubber, also called a spray dryer, to remove some acid gas and reduce downstream caustic requirements;

(3) a quench to further cool the gases; (4) a reheater to prevent condensation in the fabric ﬁlter; (4) a

fabric ﬁlter; (5) a packed column for further acid gas removal; (6) a HydroSonics scrubber that includes

a steam ejector to provide the motive force for the ﬂue gases and to act as a backup air pollution control

device in case of failure of an upstream component; and (7) a stack. As can be seen, the air pollution

control system represents a major fraction of the total system cost.

Quench

The ﬁrst step in the air pollution control system is the gas cooling, which is usually accomplished by

means of a water quench sometimes preceded by a boiler. The boiler is a noncontact heat exchanger,

which utilizes the heat from the combustion gases to make steam. The quench is a portion of the duct,

or a separate chamber, in which water is sprayed into the duct to cool it by evaporative cooling. The

quench increases the water vapor content of the gas stream substantially and drops the gas temperature

by adiabatic evaporation. Typically, the gas stream exiting the quench reaches the adiabatic saturation

temperature, and further cooling by evaporation is impossible. Any additional cooling must occur due

to sensible heat transfer to the exiting liquid stream.

The solids content of the quench liquor is important. Submicron particles can be formed by droplet

evaporation if aqueous stream containing dissolved solids is injected into the hot portions of the evap-

orative cooler. Suspended solids in the aqueous stream will also be released to the gas stream on

evaporation of the water; however, this particulate will tend to be larger and, hence, more easily removed

Incinerators 13-51

by the particulate control system. Particulate noncompliance conditions can be created if the potential

particulate formation in the quench is not considered.

Because of the critical nature of the quench system, a backup, emergency quench system is desirable.

The emergency quench consists of a separate set of nozzles and water feed lines from an independent

source. The water supply for the emergency quench should be active even during a complete power

failure. A ﬁre supply or gravity water feed from an elevated supply tank are possible sources. The system’s

response time must be short because catastrophic damage can occur to the system within seconds of

quench failure. The required short response time necessitates automatic activation of the emergency

quench system.

Heat Recovery Systems

Many incinerator designs incorporate some form of heat recovery systems. Municipal waste incinerators

(waste-to-energy systems) recover heat through steam tubes in the combustion chamber walls — these

are termed water-wall incinerators. Such water-wall units are not commonly used for the combustion of

hazardous wastes because the relatively cold walls can reduce the incinerator’s combustion zone temper-

ature and, hence, the level of organic compound destruction achieved. A notable exception to this is the

use of large industrial or utility boilers for waste destruction. In this case, the amount of waste burned

is small compared to the amount of fuel used. In addition, the large size of the furnace minimizes the

effect of the cold walls.

In general, however, incinerators burning hazardous wastes do not utilize water-wall designs. They do,

however, often include a boiler, which is mounted in the exhaust gas immediately after the ﬁnal combustion

chamber. The boiler serves two functions. First, it recovers heat from the ﬂue gas in the form of usable

steam. Second, it cools the ﬂue gas without increasing its volume and mass by injecting steam as a quench.

Heat recovery is an important aspect of the boiler. The combustion gas, with its high temperature and

volume, lends itself to the production of usable steam to drive steam ejectors, to provide steam for

atomization in the nozzles, to drive turbine pumps, or for numerous other applications. The steam

coming directly from a hazardous waste incinerator is generally not at a high enough pressure to drive

turbines for electrical generation. If cogeneration is desirable, then the low-pressure steam is fed to a

second furnace to boost its temperature. In most cases, however, an incinerator does not produce enough

steam to warrant the expense of an electricity cogeneration system.

The second purpose for the heat recovery system, gas cooling, makes it a signiﬁcant component in the

incinerator design. The temperature of the combustion gas leaving the secondary combustion chamber is

typically on the order of 1000°C (2000°F). It must be cooled to below about 70°C (160°F) prior to entering

the air pollution control system. A quench cools the gas by evaporating water, which then increases the

mass of the combustion gas stream. A boiler cools the combustion gas by indirect heat transfer without

increasing its mass. If the boiler can cool the combustion gas from 1100 to 250°C (2000 to 500°F), it will

reduce the amount of water that the downstream quench will produce by approximately 75%. The quench,

the air pollution control system, the fan or ejector, and the stack can thus be proportionately smaller.

A number of factors must be considered prior to including a boiler in an incineration system. First,

a boiler is designed to cool the gas stream, and hence, it must be placed downstream from all combustion

chambers that destroy hazardous constituents. If it is placed in the combustion chamber, it will cool the

gases and change the temperature-residence time regime to which the combustion gases are exposed,

potentially causing a lesser destruction of organics. Second, in some cases, the ash particles present in

the secondary combustion chamber exhaust gas may be at least partially melted. Sodium chloride, a

common component of wastes, melts at ﬂame temperatures. The molten particles can solidify on the

cool walls of the boiler tubes, fouling them and even restricting gas ﬂow. The boiler may thus be

impractical for incinerators burning wastes with high salt content or those with ash that has a fusion

temperature (melting point) below that found in the combustion chamber.

Another factor is the presence of corrosive gases in the gas stream. The boiler must be operated to

assure that it has no spots below the combustion gas stream’s dewpoint. Acid gases (such as SO

HC1,

13-52 The Civil Engineering Handbook, Second Edition

or HF) in the combustion gas will dissolve in the condensed aqueous liquid, corroding the tubes. HCl

is especially corrosive to metals, including most forms of stainless steel. HF is also corrosive to metals,

but it attacks silica-based refractory. SO

is relatively insoluble in water and is relatively noncorrosive;

however, it lowers the dewpoint of water substantially, and its presence requires that the boiler be operated

at a higher temperature to prevent condensation. To minimize the risk of condensation, boilers are

typically sized so that the temperature of the exit gas is above 450°F. Furthermore, because of the

particularly corrosive nature of HF on refractory materials, the presence of more than trace amounts of

ﬂuorides may prevent the use of boilers.

A ﬁnal consideration in the use of heat recovery boilers is chlorodibenzodioxins and chlorodibenzo-

furans formation. These contaminants have been found to form at temperatures in the 200 to 370°C

(400 to 700°F) range, which is the same operating range creating the risk of these hazardous materials

forming in the boiler and on the boiler surfaces.

Acid gas control devices are of two types, wet and dry. The wet devices include packed bed scrubbers,

Venturi and Venturi-like scrubbers, “wet” electrostatic precipitators, and proprietary wet scrubbers. Dry

particulate removal devices include fabric ﬁlters, electrostatic precipitators, and high-efﬁciency particulate

absolute (HEPA) ﬁlters. HEPA ﬁlters are used for particulate control, where a high level of particulate

removal is required, such as in radioactive waste incinerators. They are similar in operation to fabric

ﬁlters, although the nature of the fabric is such that a high level of particulate control is achieved. Wet

devices remove acid gases as well as particulate, and the two functions are often combined in one device.

Dry scrubbing is a technique whereby lime or another lime-based sorbent is injected into the hot zone

of the incinerator. The sorbent removes a fraction of the acid gas in suspension and is collected on the

particulate removal device, usually a fabric ﬁlter. Dry scrubbers are usually used in conjunction with

another form of acid gas removal device.

Electrostatic Precipitators

Electrostatic precipitators (ESPs) can be used for two entirely different types of service. “Wet” precipitators

can be used as the principal particulate control device within a wet scrubbing system including a gas

cooler and an acid gas absorber. While basic operating principles of the wet and conventional ESP are

similar, the two different styles are subject to quite different operating problems. They both use the

principle of electrostatic attraction. The incoming particulate is ionized and then collected on charged

plates. In a dry precipitator, the plates are periodically rapped or shaken, and the accumulated dust is

collected in hoppers at the bottom. In a wet precipitator, a continuous stream of recirculated liquor

drains over the plates and removes the accumulated particulate matter. A wet ESP can be used to control

acid gas as well as particulate. Wet ESPs operate at relatively low gas temperatures, and the precipitators

are limited to one or two electrical ﬁelds in series.

Figure 13.9 is a drawing of a conventional dry electrostatic precipitator. In electrostatic precipitators,

particles are electrically charged during passage through a strong, nonuniform electrical ﬁeld. The ﬁeld

is generated by a transformer-rectiﬁer set that supplies pulsed D.C. power to a set of small diameter

electrodes suspended between grounded collection plates. Corona discharges on these electrodes generate

electron ﬂow, which in turn, leads to the formation of negative ions as the electrons travel on the electrical

ﬁeld lines toward the grounded plates. The negative ions also continue on the ﬁeld lines toward the plates.

Within the corona itself, positive ions form, and these travel back to the high-voltage electrodes.

An ESP’s performance depends on the ability of the particulate matter to receive and maintain a charge,

the velocity of the particulate migration to the collection plates, the ability of the particulate to adhere

to the plates after they are captured, and the ability of the system to minimize re-entrainment of the

particulate during the cleaning or rapping cycle.

The ability of the particulate to maintain a charge and to adhere to the collection plates is a function

of the resistivity of the particulate and of the ﬂue gas. The resistivity is a measure of the ability of electrical

charges on the particles to pass through the dust layer to the grounded collection plates. Dust layer

Incinerators 13-53

resistivity is expressed in units of ohm-centimeters, and the values can range from 1 ¥ 10

to more than

1 ¥ 10

. The effective migration velocity increases nonlinearly as the resistivity decreases.

Dust layer resistivity at any given site in a precipitator is the net effect caused by two different paths

of charge conduction. When there are conductive compounds adsorbed on the surfaces of the particles,

the electrons can pass along the particle surfaces within the dust layer to reach the grounded collection

plates. The most common conductive materials on the surfaces of particles include sulfuric acid and

water vapor. When there are conductive materials in the particles, there can be an electrical path directly

through the bulk material. The most common conductive compounds within the precipitated particles

include carbonaceous materials, sodium compounds, and potassium compounds. The overall particle

size distribution affects the resistivity simply because, in both types of conductivity, the electrical current

must pass through a number of separate particles in the dust layer prior to reaching the grounded surface.

Smaller particles form a dust layer with less voids and greater particle-to-particle contact.

Dust layer resistivity is not a constant value throughout a precipitator at any given time, and the

“average” resistivity can vary substantially over time at a given incinerator. This temporal and spatial

variability is due to the extreme sensitivity of resistivity values to the gas temperature, the presence of

vapors such as sulfuric acid mist, and the dust chemical composition. The rate of electrical charge

conduction through the dust layer on the collection plates determines the electrostatic voltage drop across

the layer, and this can affect the ability of the dust to adhere to the plates and the ability of the plates to

attract additional dust. When conductivity by either path is high, the dust layer voltage drop is low. Some

of the weakly held particulate matter can be re-entrained due to rapping with excessive force. Also,

FIGURE 13.9 Conventional dry electrostatic precipitator (Western Precipitation, undated).

Bus ducts assembly

Electrical

equipment

platform

Collecting

surfaces

High voltage

electrodes with

weight

Collecting

surface rappers

Hopper

Field

Chamber

Manhole

Gas

flow

Casing

High voltage

system upper

support frame

Railing

Insulator

compartment

High voltage

system rapper

Insulator compartment

ventilation system

Transformer/rectifier

Reactor

Rapper

control

panel

Chamber

Field

Inlet

13-54 The Civil Engineering Handbook, Second Edition

localized high gas velocity zones through the precipitator can scour off some of the dust layer. In

applications where the dust shows a high resistivity, the precipitator will have to be larger. It is sometimes

possible to modify resistivity by injecting small amounts of sulfuric acid into the gas stream entering the

ESP as a conditioning agent.

One of the ﬁrst steps in precipitator design is to determine the necessary collection plate area for the

efﬁciency desired. The efﬁciency should increase as the speciﬁc collection area is increased. However, the

cost and mechanical complexity of the precipitator also increase with the speciﬁc collection area. Also,

the vulnerability to malfunctions can increase as the size increases. The optimum size that provides for

high-efﬁciency performance without excessive costs and reliability problems must be determined. Equip-

ment sizing must take into account the numerous nonideal factors, which are difﬁcult to express math-

ematically but, nevertheless, have important effects on performance. This has generally been done by the

determination of “effective” migration velocities that include theoretical parameters plus the effects due

to nonuniform particle size distribution, nonuniform gas ﬂow distribution, nonuniform gas temperature

distribution, and rapping re-entrainment. During the design stage, the anticipated dust layer resistivity

range should be carefully evaluated. For existing units, the resistivity variability can be measured by a

variety of instruments, such as the cyclonic probe and the point-to-plane probes. For new units, the

resistivity range in similar incinerators handling similar wastes should be reviewed.

High-efﬁciency precipitators have more than one electrical ﬁeld. Two or more ﬁelds are normally

provided in the direction of the gas ﬂow. For large incinerators, the gas ﬂow can be split into two or

more chambers, each of which has several ﬁelds in series. The sectionalization of the precipitators

improves precipitator performance and reliability. For this reason, sectionalization should be considered

in the preparation of precipitator speciﬁcations. As in the case of precipitator sizing, there is an optimum

balance between the number of independent ﬁelds and the cost. One of the underlying reasons for

sectionalization is the signiﬁcant particle concentration gradiant and dust layer thickness gradients

between the inlet and outlet of the precipitator. At the inlet of the precipitator, the dust layer accumulates

rapidly, because 60 to 80% of the mass is collected rapidly. This makes this ﬁeld more prone to electrical

sparking due to the nonuniformities in the dust layer electrical ﬁelds. Also, the ﬁne particles initially

charged in the inlet ﬁeld but not collected create a space charge in the interelectrode zone. This space

charge inhibits current ﬂow from the discharge electrodes and collection plates. By sectionalizing the

precipitator, the inherent electrical disturbances in the inlet ﬁeld do not affect the downstream ﬁelds.

Another reason for sectionalization is the differences in electrical sparking, which are normally moderate-

to-frequent in the inlet ﬁelds and low-to-negligible in the outlet ﬁelds. The number of ﬁelds in series

and the number of chambers in parallel are generally determined empirically, based on the performance

of previous commercial units. In the case of hazardous waste incinerators, there are some practical limits

to the number of ﬁelds used, simply because the gas ﬂow rates are relatively small.

Because of the high voltage, sparks will occur in a precipitator. The sparking rate needs to be maintained

at a speciﬁed level, and the ESP includes automatic voltage controllers to quench the electrical sparks.

These are electronic devices that reduce the applied voltage to zero for milliseconds and then increase it

in several steps to a level close to the one at which sparking occurred. When sparking rates increase in

a ﬁeld, the net effect is to lower the peak voltages and to lower the applied time of the electrical power.

Rapping Techniques

For dry-type electrostatic precipitators, there are two major approaches to rapping: (1) external roof-

mounted rappers and (2) internal rotating hammer rappers. The external rappers are connected to groups

of collection plates or an individual high-voltage frame by means of rapper shafts, insulators, and shaft

seals. The advantage of these roof-mounted rappers is that there is access to the rapper during operation,

and the intensities can be adjusted for variations in dust layer resistivity. The disadvantage is that the

large number of rapper shaft components can attenuate rapping energy and become bound to the hot

or cold decks.

The internal rotating hammer rappers have individual rappers for each collection plate. Due to the

greater rapping forces possible, these can be used for moderately high resistivity dusts. The disadvantages

Incinerators 13-55

of these rappers are the inability to adjust the frequency and intensity in various portions of the precip-

itator and the inaccessibility for maintenance. Also, the internal rotating hammer rappers can be vulner-

able to maintenance problems, such as shearing of the hammer bolts, distortion of the hammer anvils,

bowing of the support shafts, and failure of the linkages.

The type of rapping system chosen should be based on the anticipated resistivity range, the frequency

of routine incinerator outages, and the cost of the equipment. For both styles of rappers, the high-voltage

frame rapper shafts must include high-voltage insulators to prevent transmission of high D.C. voltages

to external, accessible equipment. These insulators must be kept clean and dry. Purge air blowers with

electrical resistance heaters are used for this purpose.

Hoppers and Solids Discharge Equipment

For dry-type electrostatic precipitators used in dry scrubbing systems, proper design of the hopper and

solids discharge equipment is especially important. These units handle high mass loadings, and some of

the reaction products are hygroscopic and prone to bridging. Hopper heaters and thermal insulation are

important to avoid the hopper overﬂow conditions that could cause an undervoltage trip of a ﬁeld and

that could possibly cause serious collection plate-to-discharge electrode alignment problems.

Gas Distribution

One of the most important steps in ensuring adequate gas distribution is to allow sufﬁcient space for

gradual inlet and outlet transition sections. Units with very sharp duct turns before and after the transition

are also prone to gas distribution problems.

Proper gas distribution is achieved by the use of one or more perforated gas distribution screens at

the inlet and outlet of the precipitator. These are generally hung from the top and cleaned by means of

externally mounted rappers. Location of the gas distribution screens (and ductwork turning vanes) is

usually based on either 1/16

scale ﬂow models or gas distribution computer models.

Fabric Filters

The fabric ﬁlter consists of a series of ﬁlter bags made of fabric and hung from a frame in a “baghouse.”

The gas to be treated enters the baghouse and passes through the fabric of the bags that ﬁlter out

particulate. The cleaned gas exits the baghouse to subsequent cleaning or discharge. The parameters that

inﬂuence baghouse construction are (1) type of ﬁlter bag material; (2) gas-to-cloth ratio, also called facial

velocity; (3) direction of gas ﬂow through the bags; and (4) type of bag cleaning employed.

Particulate collection on a fabric ﬁlter occurs primarily by the combined action of impaction and

interception within the dust cake supported on the fabric. Without a well-developed dust cake, the

ﬁltration efﬁciencies would be very low. Particulate capture efﬁciency in a typical baghouse is normally

very high. Penetration of particles through a fabric ﬁlter is due not to inefﬁcient impaction/interception,

but rather to dust seepage through the dust cake and fabric, gas ﬂow through gaps or tears in the fabric,

gas ﬂow through gaps between the bags and the tubesheet, and gas ﬂow through gaps in tubesheet welds.

The emissions are minimized by operating the unit at proper air-to-cloth ratios and by ensuring that the

bags have not deteriorated due to chemical attack, high temperature damage, ﬂex/abrasion, or other

mechanical damage. The efﬁciency of fabric ﬁlters is not highly sensitive to the inlet particle size distri-

bution. The particle size distribution of the material penetrating the baghouse is similar to the inlet gas

stream particulate due to the emission mechanisms.

The typical baghouse is arranged in compartments with dampers that permit isolation of each com-

partment from the rest with poppet or butterﬂy dampers. In case of failure of one or more bags in one

compartment, the compartment can be isolated, and the system can keep operating until the bags are

replaced or repaired. For some systems, the individual compartments are closed prior to rapping or

shaking in order to minimize the release of particulate during the cleaning cycle.

The two basic styles of fabric ﬁlters used on incinerators are pulse-jet units and reverse air units.

Reverse air units are identiﬁed by bags that are suspended under tension from tube sheets directly above

the hopper. The particulate-laden gas stream enters the interior of the bags through the tube sheet and

13-56 The Civil Engineering Handbook, Second Edition

collects as a dust on the inside of the bags. The bags are cleaned by passing blowing ﬁltered gas from the

exhaust through the bags in a reverse direction to normal gas ﬂow. The reverse ﬂow dislodges the dust

cake into the hopper. In order to accomodate the reverse ﬂow of air necessary for cleaning, reverse air

baghouses have multiple compartments. For cleaning, each compartment is isolated from the ﬂow gas

stream with dampers.

Pulse-jet baghouses are the most common because these are well suited for the relatively small gas

ﬂow rates in hazardous waste incinerators. In pulse-jet units, the fabric is supported on a cage that is

suspended from a tube sheet near the top of the collector. The particulate-laden gas stream enters through

a duct on the side of the baghouse or in the upper portions of the hoppers. The dust cake accumulates

on the exterior surfaces of the bags. The bags are cleaned by the combined action of a compressed air

pulse and the reverse airﬂow induced by this pulse. A set of solenoid valve controlled diaphragm valves

are used to supply compressed air cleaning ﬂow to each row of bags. Some units have multiple compart-

ments to allow for bag cleaning off-line or to allow for maintenance of a portion of the unit during

incinerator operation.

The following discussion of fabric ﬁlters emphasizes pulse-jet-type units. Two sketches of pulse-jet

baghouses are shown in Figs. 13.10 and 13.11. The ﬁrst of these is an isometric sketch that illustrates the

locations of the clean side access hatches on the top and the ﬁltered gas exit duct. The second sketch is

a side elevation showing the bag support and the bag cleaning apparatus.

The air-to-cloth ratio is the total fabric area divided by the total gas ﬂow rate in ACFM (wet basis).

It is deﬁned in Eq. (13.14). The units should be speciﬁed along with the air-to-cloth ratio dimensions

of length/time, because the value of x in English units is quite different from a value of x in metric units.

FIGURE 13.10 Isometric sketch of a top access type pulse-jet unit (Richards and Quarles, 1986).

Gas inlet

Air manifold

Diaphragm valves

Top access hatches

Gas outlet

Fan

Hoppers