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Incinerators 13-37

The air pollution control system on a refuse incinerator is designed to control acid gas and particulate.

Typical acid gas control devices are dry or wet scrubbers. Typical particulate removal devices are fabric

ﬁlter baghouses or electrostatic precipitators. Venturi scrubbers have been used for particulate and acid

gas removal; however, the high pressure drop inherent in the design coupled with the high gas ﬂows of

large refuse incinerators tend to result in higher energy usage than with the fabric ﬁlter or electrostatic

precipitator.

The principal emissions of concern from a refuse incinerator are particulate, mercury, and other toxic

metals, chlorodibenzodioxins and chlorodibenzofurans (dioxins and furans), and acid gases. The partic-

ulate forms by the entrainment of ash and other inert material and by the vaporization of salts or metal

compounds in the hot ﬂame zones followed by their condensation in the cooler zones of the system.

Mercury has a signiﬁcant vapor pressure at even the low temperatures encountered in the incineration

system. It can pass through the system as a vapor. Fortunately, it is usually present in only minute

quantities in refuse and does not require special removal techniques. In most cases, chlorodibenzodioxins

and chlorodibenzofurans can form in the combustion chamber from the combustion of chlorinated

materials. The most common chlorinated material in refuse is polyvinyl chloride (vinyl plastic). There

is increasing evidence (Bruce, 1990) that chlorodibenzodioxins and chlorodibenzofurans can form by

the recombination of other compounds in the air pollution control system. Temperatures in the range

of 230 to 400°C (450 to 750°F) (EPA, 1992) appear to favor the formation of these compounds. At this

point, the information is preliminary, but if veriﬁed, it would tend to encourage the use of fabric ﬁlters

[which operate at temperatures below 230°C (450°F)].

Hazardous Waste Incinerators

There are many types of hazardous waste incinerators in use today. The following are examples of six types:

1. Liquid injection

2. Rotary kiln

3. Fluidized bed

4. Fixed and moving hearth

5. Infrared furnace

6. Plasma arc furnace

The liquid injection, rotary kiln, and hearth systems are traditional designs that have been in use for

many years. Fluidized and circulating bed, and infrared combustors represent newer generation designs

for treatment of a variety of solid wastes, often with claims of higher efﬁciency at lower operating costs

for speciﬁc applications. Infrared and plasma arc furnaces are highly experimental at this time and are

not discussed further herein. Numerous tests performed in support of regulatory development and for

RCRA permitting (EPA, 1986a) have shown that, when properly designed and operated, hazardous waste

incinerators can meet virtually any remission and destruction standards required.

Many incinerator designs are available as modular and transportable systems designed to treat con-

taminated materials and wastes directly at the site of contamination. Portable incinerators refer to those

units with major units (i.e., kiln, SCC, APCE, etc.) mounted on trailers for which assembly on-site consists

largely of bolting the modules together and connecting utilities. The capacity of portable incinerators is

limited by restrictions on the size of over-the-road loads. Transportable incinerators are built in prefab-

ricated pieces that are assembled on-site to form the major units. Assembly of a transportable incinerator

typically requires a foundation and signiﬁcant on-site construction that can require several months. The

operating units can be as large as desired and, hence, have a waste treatment capacity approaching that

of ﬁxed-site systems.

Liquid Injection Incinerators

Liquid injection incinerators are currently the most common types used. As the name implies, these

units are designed to incinerate liquid or pumpable slurry and sludge wastes, usually in a single, refrac-

tory-lined cylindrical combustion chamber positioned in a horizontal or vertical arrangement. Often,

13-38 The Civil Engineering Handbook, Second Edition

process vent gases are incinerated in liquid injection incinerators as well. The secondary combustion

chamber of a multiple chamber incinerator is similar in design to a liquid injection incinerator. Most

liquid injection incinerators consist of only a primary combustion chamber; however, for some cases, it

may prove necessary to stage the combustion in multiple chambers.

Typical liquid injection incinerator combustion chamber mean gas residence time and temperature

ranges are 0.5 to 2+ sec and 700 to 1200°C (1300 to 2200°F). The combustion chambers vary in

dimensions with length-to-diameter ratios in the range of 2 or 3 to 1 and a diameter less than 12 ft.

Liquid injection feed rates are as high as 6000 L/h (1500 gal/h) of organic liquid and 16,000 L/h (4000 gal/h)

of aqueous liquid.

The primary advantages of liquid incinerators are their ability to process a wide range of gases and

pumpable liquid wastes and to operate with a minimum of moving parts. Because the wastes are injected

through atomizing nozzles, the physical properties of the waste are important to the safe and efﬁcient

operation of these units. The primary waste properties that must be considered when evaluating a liquid

injection incinerator (or any combustor burning liquid wastes) are the viscosity and solids content.

Viscosity is important because the liquids must be atomized into ﬁne droplets for adequate vaporization

or pyrolysis. The viscosity should be, typically, less than 10,000 Saybolt seconds (SSU). High solids content

can lead to nozzle erosion, plugging, and caking, which can result in poor atomization of the wastes and

less efﬁcient combustion. Wastes are often blended and pretreated to meet burner and nozzle speciﬁcations.

Liquid injection incinerators can be positioned horizontally or vertically. Horizontally ﬁred incinera-

tors are the simplest design in that the waste feed systems, the air inlets, the combustion gas exhaust,

and the air pollution control systems are at ground level and are readily accessible. Piping and ductwork

runs tend to be shorter than in other designs as well. Such incinerators, however, have relatively large

footprints. In addition, any ash in the waste will collect in the combustion chamber so that, unless the

waste has a very low ash content, the furnaces have to be shut down for manual ash removal on a regular

basis.

Vertical liquid injection incinerators may be downﬁre or upﬁre, depending primarily on the type of

wastes being incinerated. Upﬁre incinerators require relatively little space, but they require that the hot

ﬂue gas be brought down to the APCD in refractory-lined ducts. Like the horizontally ﬁred units, they

also tend to accumulate ash in the combustion chamber. Their principal use is for burning clean solvents

with a low ash content under conditions that may not require the use of air pollution control devices.

Upﬁre incinerators are relatively less common for hazardous waste applications, and when used, they are

limited to small installations, less than about 1000 lb/hr waste feed.

Downﬁre incinerators have the burner(s) at the top of the combustion chamber. Figure 13.3 illustrates

a forced draft, downﬁred liquid injection incinerator. In this particular system, the combustion gases

move downward and impinge on a wet quench located at the bottom of the chamber — called a wet-

bottom. In other variations on the design, in the combustion chamber, the wastes, fuel, and air are

introduced into the combustion chamber tangentially, causing a swirling “vortex” gas ﬂow pattern, shown

in Fig. 13.4. The vortex pattern increases the velocity of the gas and tends to increase the mixing of the

gases with air. The swirling or “vortex” ﬂow is imparted by the shape of the air inlets at the bottom of

the combustor and by the arrangement of the air tuyeres in a cyclonic combustor, which are aimed to

inject the combustion air with a tangential component to its ﬂow.

The combination of vortex ﬂow and wet bottom is useful when wastes contain signiﬁcant quantities

of inorganic materials with melting points that lie in the combustor’s temperature range. The melting

inorganic materials form a slag that can stick to the refractory, building up on it and eventually reducing

the combustor’s cross-sectional area. Some slags can corrode or otherwise damage the refractory. Cyclonic

ﬂow combined with a wet bottom tends to sweep the slag and ash from the walls into the wet-bottom,

thereby reducing buildup.

The organic compound destruction achieved by a liquid injection incinerator is largely determined

by how well the particular design (1) atomizes the waste materials, (2) converts the organic constituents

to a vapor by vaporization or pyrolysis, (3) maintains a stable ﬂame, and (4) provides adequate mixing

Incinerators 13-39

FIGURE 13.3 Typical downﬁred liquid incinerator arrangement.

FIGURE 13.4 Tangentially ﬁred vortex combustor (Bonner, 1981).

Fuel

oil

Organic

waste

Makeup air

Waste gas (air)

Aqueous

waste

Makeup

city water

Note:

= Sample Point

Quench

Tank

Venturi

scrubber

Eductor

Tempering Water

Mesh

Pad

Packed Bed

Scrubber

Combustion

Chamber

Recycle water

Burner

Sampling

Platform

Natural

gas

Stack gas

Caustic

(NaOH)

Caustic

(NaOH)

Neutralization

tank

Water

effluent

Annular space

filled with air

under pressure

for tuyers

Baffle shell

Air tuyeres

Tuyere air shell

and plenum

Cooling air port

cast in refractory

slab

Air tuyeres

Refractory wall

Baffle shell

Tuyere

air shell

Cooling air

(forced draft)

Aux. fuel

burner ring

Burner nozzle

Combustion air

Refractory

cooling air

Combustion air

to tuyeres

Refractory wall

Combustion gas

to APCD and stack

Liquid waste

fuel spray

Gas flow

13-40 The Civil Engineering Handbook, Second Edition

of the combustion gases with oxygen at temperatures sufﬁciently high to destroy the organics. Some of

the factors that need to be considered are as follows:

1. Particle size distribution of the droplets produced by the nozzle

2. Droplet impaction on walls or other surfaces

3. Presence of cool zones in the organics’ destruction zone

4. Presence of stagnant zones, where poor mixing of organics and oxygen (localized pyrolyzing

conditions) may occur

The ﬁrst two factors relate to the precombustion zone and the combustion zone of the incinerator. If the

waste droplets do not completely evaporate prior to exiting the ﬂame zone, then there is a risk that a fraction

of the waste may be evaporated and incompletely destroyed. Improper evaporation can occur because the

droplet size is too large, the heating value of the waste drops, or because the nozzle is spraying a fraction

of the liquid onto a cool surface, preventing its ignition. The atomizing nozzle must be selected to match

the properties of the ﬂuids involved and must be properly mounted so that no spray hits any surfaces. In

addition, it is necessary to continuously test the viscosity and heating value and (for some types of nozzles)

vapor pressure of the waste stream to assure that it remains within the nozzle’s design speciﬁcations.

Rotary Kiln Incinerators

The rotary kiln incinerator is the single most common design for the large-scale combustion of solid

hazardous waste. The typical system consists of a solid waste feed system, a pumpable waste feed system,

an auxiliary fuel combustor, a kiln, a transition section, an ash drop and dump system, a secondary

combustion chamber with auxiliary fuel, pumpable waste feed systems, and an air pollution control

system. Most rotary kiln incinerators are the ﬁrst stage of a two- (or more) chamber incinerator. The

secondary combustion chamber (sometimes called an afterburner) destroys the remainder of the vola-

tilized combustible matter released from the heating of the solid wastes. Exceptions exist — most

industrial furnace kilns that have been tested with hazardous waste appear to perform well without a

secondary combustion chamber, as do a few hazardous waste incinerator kilns. For the most part, rotary

kiln hazardous waste incinerators require the secondary combustion chamber to complete the destruction

of the organic constituents released from the solids. The secondary combustion chamber is essentially

identical to a liquid injection incinerator, which was discussed previously.

A rotary kiln is a cylindrical shell mounted on a slight incline from a horizontal plane. It is usually

refractory-lined, although, if the temperature is low enough, it may simply be a metal shell. Its design is

an adaptation of industrial process kilns used in the manufacture of lime, cement, and aggregate materials.

The rotary kiln is used extensively for the incineration of bulk and containerized solids, sludges, liquids,

and gases. Most rotary kiln incinerators consist of two combustion chambers. The ﬁrst chamber is the

kiln. The second chamber is similar to a liquid injection incinerator. It often burns pumpable hazardous

wastes along with the off-gases from the kiln.

Figure 13.5 is a schematic of a rotary kiln incinerator system that illustrates how the two combustion

chambers treat wastes. The kiln consists of a ﬁring end (shown on the left) containing a solids feed system

and a series of nozzles and lances to feed the wastes and supplementary fuels if needed.

Solid materials, either containerized or bulk, are fed to the kiln by many methods. Four common

methods of feeding are gravity feed, conveyors, augur, and shredder. With gravity feed, a chute is built

into the front of the kiln (front is deﬁned as the point at which the wastes are fed), and the wastes are

simply dumped through the chute and into the kiln. In larger kilns, the chute may be left open during

operation. Combustion air is pulled through the chute by the induced draft system. Alternatively, the

chute may be ﬁtted with a door, a ram feeder, or another device that reduces air inﬁltration but allows

solids to be fed into the kiln.

A somewhat more automated method of feeding the solids to the kiln is to use a conveyor, which

transports the waste to a chute (gravity feeder) or directly into the kiln. In the latter case, the conveyor

must be made of a material capable of withstanding the kiln temperatures. Screw augers are also used

to transport bulk wastes into the kiln; however, their use is limited to wastes that will not jam the auger

Incinerators 13-41

and that can be moved by such a device. They can successfully be used for feeding powdered materials

in many cases.

A blender-feeder is a common method for feeding highly inhomogeneous wastes into an incinerator.

It combines a hammer mill with a solid feed system. These devices can be large and powerful enough to

shred steel drums of waste material. Their advantage is that the blended and shredded wastes burn more

smoothly and with a lower probability of ﬂaring up and causing pufﬁng or explosions in the combustion

chamber.

Pumpable wastes can be fed to a kiln in two ways. Those wastes that can be atomized (usually with

viscosity of under about 740 SSU) are fed through atomizing nozzles. Atomization is highly desirable for

autogenous waste or auxiliary fuel. The considerations for the nozzle are similar to those for a nozzle in

a liquid injection incinerator. Those pumpable wastes that cannot be atomized are typically fed to the

kiln through a lance, a pipe discharging directly into the kiln with a minimum of bends or constrictions.

Steam, air, or nitrogen is sometimes injected with the waste into the lance to facilitate the waste’s ﬂow

and to provide limited atomization.

As the kiln rotates, it mixes the solid wastes with combustion air and moves the wastes toward the

discharge end. The constant rotation also promotes exposure of waste surface to the radiant heat from

the ﬂames and hot refractory to enhance heat transfer efﬁciency and release combustible organics into

the gas. As the solids and liquids burn, they produce combustion gases, which are swept down the kiln

into a secondary combustion chamber or (for a single-chamber system) the air pollution control equipment.

FIGURE 13.5 Rotary kiln incinerator schematic. (Reproduced courtesy of LVW Associates, Inc.)

Exhaust gas

(to dump stack, waste

heat recovery and/or air

pollution control system)

Liquid

waste

and/or

auxiliary

fuel

Liquid

waste

and/or

auxiliary

fuel

Combustion air

Rotary seals

Air

Afterbumer

Rotary kiln

Support

rollers

Kiln

shroud

Solid

waste

Ash discharge

(to ash quench)

13-42 The Civil Engineering Handbook, Second Edition

Operation of a rotary kiln incinerator involves several concerns. The ﬁrst concern is the seals at the

front and rear ends. As can be seen in Fig. 13.5, the rotating kiln must slide past the ﬁxed wall at the

front, where the waste feed and burner nozzles are, and at the rear by the ash drop and combustion gas

exhaust. There is no practical way to seal a rotary kiln to withstand positive pressure at the points where

the rotating equipment meets the stationary components. As a result, a rotary kiln is operated under

negative pressure. The system is designed to draw air at a speciﬁed maximum rate through the seals and

other openings. If the seals become worn or damaged, the air inﬁltration can become excessive, and the

incinerator will have trouble maintaining temperature at an acceptable gas ﬂow rate. A properly operating

incinerator must include routine inspections and a regular maintenance program of the seals.

A second potential problem that is of particular concern with rotary kiln incinerators is that of

“pufﬁng.” Normally, the gases leaving the kiln are “pulled” into the secondary combustion chamber by

the pressure differential between the two. If there is a sudden increase in the gas production rate in the

kiln (due to sudden explosion, combustion, or volatilization of a chunk of waste, for example) or a draft

decrease in the secondary combustion chamber (due, for example, to a problem with the fan), the gas

ﬂow rate may exceed the capacity of the downstream equipment, and an over-pressure could result. Flue

gas from the kiln, potentially containing unburned POHCs and PICs, could thus be released as a “puff”

from the rotating juncture between the kiln and the secondary combustion chamber. Normally, the seals

can contain the gas from a speciﬁed level of overpressure. When the level is exceeded or the seals are

damaged or worn, however, pufﬁng could occur.

This problem is of special concern in incinerators burning munitions or other explosive wastes. In

these cases, pufﬁng could occur when a shell or piece of explosive detonates suddenly. These incinerators

are designed to withstand explosions, but pufﬁng can frequently occur then. The burning of drummed

wastes can also lead to pufﬁng. In this case, if the contents of a drum burn rapidly, the effect could be

similar (although usually not as severe) to an explosion, as discussed above. The resultant overpressure

could produce pufﬁng. When pufﬁng may occur, incinerators are equipped with an “emergency vent

stack,” water column, or other emergency relief vents.

The length-to-diameter ratios of rotary kilns can range from 2 to 10. Outside shell diameters are

generally limited to less than 15 ft to allow shipping of the cylinder sections. Rotational speeds of the

kiln are usually measured as a linear velocity at the shell. Typical values are on the order of 0.2 to 1 in./sec.

Te mperatures for burning vary between 800 to 1600°C (1500 to 3000°F). Bulk gas residence times in the

kiln are generally maintained at 2 sec or higher.

The solids retention time in a rotary kiln is a function of the length-to-diameter ratio of the kiln, the

slope of the kiln, and its rotational velocity. The functional relationship between these variables is given

by the following rough approximation (Bonner, 1981):

(13.13)

where t

solids

is the retention time (in min), L is the length of the kiln (in ft), D is the diameter of the kiln

(in ft), S is the slope of the kiln (in ft/ft), and N is the revolutions per minute (rpm). Typical ranges of

these parameters are L/D: 2–10, S: 0.03–0.09 ft/ft, and rotational speed 1 to 5 ft/min (which can be

converted to rpm by dividing by the kiln circumference measured in ft). The retention time requirements

for burnout of any particular solid waste should be determined experimentally or extrapolated from

operating experience with similar wastes. In a movable grate furnace, the retention time is given by the

ratio of the length of the grate, L, and its speed, S.

Air/solids mixing in the kiln is primarily a function of the kiln’s rotational velocity, assuming a relatively

constant gas ﬂow rate. As rotational velocity is increased, the solids are carried higher along the kiln wall

and showered down through the air/combustion gas mixture. Because solids retention time is also affected

by rotational velocity, there is a tradeoff between retention time and air/solids mixing. Mixing is improved

to a point by increased rotational velocity, but the solids retention time is also reduced. Mixing is also

improved by increasing the excess air rate, but this reduces the operating temperature. Thus, there is a

tradeoff between gas and solids retention time and mixing.

tLDSN

solids

()

019.

Incinerators 13-43

The longer the solid waste is kept in the kiln, the cleaner the bottom ash becomes. The additional cost

of prolonging the solid waste retention time in the kiln is small compared to the total cost of incineration.

The solid waste retention time is typically changed by varying the kiln rotational speed. Slowing the

rotation increases the solids’ residence time.

Rotary kiln incinerators can be operated in two modes, slagging and nonslagging. If the kiln is operated

in the nonslagging mode, the ash temperature is kept below its fusion point. In the slagging mode, the

ash temperature is allowed to rise above its fusion temperature, and the ash forms a liquid (or more

accurately semiliquid) mass in the kiln. The type of kiln refractory (or other wall material) and the type

of ash removal equipment used will be inﬂuenced by whether the kiln is a slagging or nonslagging type.

Many types of molten ash, called slags, are corrosive and will dissolve improperly chosen refractory.

Fluidized Bed Incinerators

The ﬂuidized bed incinerator consists of a bed of sand, limestone, or other mineral type of material and

the combustion residue in a refractory chamber. The bed is ﬂuidized by blowing air, and if additional

gas is needed, recirculated combustion gases into the bottom through a set of tuyeres. The ﬂowing gas

agitates the bed material and turns it into an expanded turbulent mass that has properties similar to a

ﬂuid, hence the name. Bed depth typically varies between 1.5 and 10 ft, and gas velocities are typically

in the range of 2.5 to 8.0 ft/s. It is generally desirable to maintain the depth of the bed as small as possible,

consistent with complete combustion and excess air levels, to minimize the pressure drop and power

consumption. Figure 13.6 is a schematic of a ﬂuidized bed incinerator. Trenholm, Gorman, and Jungclaus

(1984) gives performance data for a ﬂuidized bed incinerator burning industrial wastes.

The combustion chamber of a ﬂuidized bed is lined with brick or castable refractory. It has two

distinguishable zones: a ﬂuidized bed zone composed of inert granular bed material that is ﬂuidized by

directing air upward through the bed and a freeboard area extending from the top of the bed to the exit

of the combustor. Wastes and fuel, if needed, are fed directly into the ﬂuidized bed or into the windbox

beneath the bed, where it ignites. Auxiliary fuel nozzles, which are used for startup or supplemental heat,

are located in the windbox (hot-windbox) or in the freeboard cold windbox. The combustion of the

FIGURE 13.6 Fluidized bed incinerator schematic (Battelle Columbus Laboratories, 1972).

Distributor plate

Windbox

Startup preheat

burner

for hot windbox

Tuyeres

Fuel gun

Startup preheat

burner

for cold windbox

Sight

glass

Exhaust and ash

Freeboard

Sand

feed

Waste

inlet

Fluidizing

air inlet

Fluldized

bed

13-44 The Civil Engineering Handbook, Second Edition

wastes and fuel heats the bed material to temperatures high enough that it, rather than the ﬂame, acts

as the combustion source.

The freeboard serves two major functions. First, its larger cross-sectional area slows the ﬂuidizing gas

velocity and keeps the larger bed particles from escaping. Second, it acts as a secondary combustion

chamber for the off-gases from the bed. Higher heating value liquid wastes or auxiliary fuels can be

burned in the freeboard area in a manner analogous to the secondary combustion chamber of a two-

chamber incinerator.

The ﬂuidized bed combustor is especially appropriate for burning tars and other sticky materials. In

fact, it has been used for many years in the petroleum industry to burn still bottoms and other high-

molecular weight residues. The tars coat the bed particles and increase the particles’ sizes and weights.

The enlarged particles tend to remain in the bed until the waste is burned off. The residence time for

wastes in the ﬂuidized bed can be as much as 12 to 14 sec. The rapid mixing of the bed also provides

good agitation, exposing new surfaces to the hot combustion gases.

The ﬂuidized bed has a high thermal mass that helps even out ﬂuctuations in the combustion of highly

heterogeneous wastes. As discussed for rotary kilns, heterogeneous wastes can burn unevenly. When a

highly ﬂammable clump of waste ignites, it can release a puff of gas that overloads the downstream

combustion gas handling system. This can result in pufﬁng, a potentially dangerous condition. The

thermal mass of the ﬂuidized bed reduces such uneven burning, making this type of incinerator a likely

candidate for such wastes.

While sand is the commonly used bed material for ﬂuidized bed incinerators, other materials that

participate in the chemical reaction can be used as well. For example, lime or limestone is sometimes

used in ﬂuidized bed boilers that burn high-sulfur coal. The bed material absorbs the sulfur oxides formed

in the combustion. A similar method is used to absorb the acid gases formed during incineration.

Fluidized bed combustors operate at relatively low bed temperatures, 425 to 800°C (800 to 1500°F),

and freeboard temperatures up to about 1000°C (1800°F) (EPA, 1971). At startup, the bed is preheated

by a burner located above and impinging down onto it. Because of its high thermal mass and excellent

ability to transfer heat from the bed to the incoming waste, a ﬂuidized bed incinerator is capable of

burning materials using a low heating value. Normal incinerators require that the mean heating value of

the combined wastes and fuels be a minimum of about 3300 to 4400 kcal/kg (6000–8000 Btu/lb). Any one

waste stream could have a lower heating value, but the total of all of them should be above this minimum.

A ﬂuidized bed incinerator requires a minimum gross heating value of 2500 kcal/kg (4500 Btu/lb) and

as little as 550 kcal/kg (1000 Btu/lb) if no water is present.

The rapid motion of the ﬂuidized bed can cause attrition of the bed particles and of the refractory.

This creates particulate that will be carried over into the ﬂue gas. As a result, ﬂuidized bed incinerators

often place a greater load on the APCD than similarly sized conventional incinerators. Attrition requires

that the operators keep tight control of the gas ﬂow. The ﬂow must be great enough to ﬂuidize the bed,

but it should not be much greater than required for this purpose. Note that particulate formed by attrition,

while ﬁne enough to be carried out in the ﬂue gas, is rarely sufﬁciently ﬁne to cause difﬁculty with most

types of air pollution control devices. The large quantity of particulate matter captured can overload the

ﬂyash handling system.

The ﬂuidized bed combustor offers several advantages, including the ability to incinerate a wide variety

of wastes. It operates at relatively low and uniform temperatures, thereby tending to have lower NO

emissions than standard combustors. It also achieves a higher combustion efﬁciency because of the high

mixing and large surface area for reaction. The large mass of the bed makes it tolerant of wide variations

in waste heating values. Finally, proper use of additives such as limestone or lime give this type of

incinerator the potential to neutralize acids in the bed.

The ﬂuidized bed combustor has limitations in its applicability. First, it is mechanically complex.

Second, it cannot typically burn wastes with ash that forms particulate much larger than the bed material.

Large ash particles will fall to the bottom of the bed and will eventually cause deﬂuidization. Deﬂuidi-

zation is a phenomenon whereby the bed settles down and is not blown about by the combustion gas.

Incinerators 13-45

Third, the ash formed must have a fusion temperature (melting point) greater than the bed temperature.

If the ash should melt, it would agglomerate the bed and also cause deﬂuidization.

The composition of the ash that a waste material will produce must thus be carefully controlled to

keep the particle size small and prevent the fusion temperature of the bed material from dropping below

the bed temperature. The fusion temperature of the bed material places an upper limit on the combustion

chamber temperature. For purposes of illustration, the fusion temperature of sand, for example, is 900°C

(1650°F). Another disadvantage is poor combustion efﬁciency under low loads.

Multiple Hearth Incinerators

The multiple hearth incinerator was originally developed for ore roasting and drying of wet materials.

It is typically used today to incinerate sewage sludges and liquid combustible wastes, but it is rarely used

for solid wastes. Its design is most appropriate for wastes containing large amounts of water and requiring

long solids residence times. It is illustrated in Fig. 13.7. The furnace is a refractory-lined vertical steel

shell containing a series of ﬂat hearths. Each hearth has a hole in the middle. A rotating shaft with rabble

arms attached at each hearth runs the length of the cylindrical shell. The incinerator is also equipped

with an air blower, burners, an ash removal system, and a waste feeding system. Liquid wastes and

auxiliary fuel can be injected at points in the furnace to assist the combustion of the solids or simply to

destroy the liquid. Multiple hearth furnaces that burn hazardous waste are usually equipped with a

secondary combustion chamber (afterburner).

Solid waste is fed to the incinerator in a continuous stream, usually from an auger, onto the top hearth,

where it is plowed by the rotating rabble arms. They also slowly move the waste across the hearth and

into a hole leading to the lower hearth, where another rabble arm plows the waste. The waste thus falls

from hearth to hearth until it is discharged from the bottom. The bottom-most hearth is usually the

only one supplied with overﬁre air; the other hearths are fed just underﬁre air. The gases in the multiple

hearth incinerator ﬂow upward, countercurrent to the waste. The hot gases from the lower hearths dry

the waste fed to the upper hearths and eventually ignite the dried solids.

Because a multiple hearth incinerator operates in a countercurrent mode, it has the same difﬁculties

in dealing with volatile hazardous organic as a countercurrent kiln. The initial drying zone typically

FIGURE 13.7 Typical multiple hearth incinerator (EPA, 1980).

Rabble

arm drive

Combuustion

air return

Sludge inlet

Floating damper

Cooling air discharge

Flue gases out

Rabble arm

at each hearth

Drying

zone

Combustion

zone

Cooling

zone

Ash discharge

Cooling air fan

13-46 The Civil Engineering Handbook, Second Edition

operates at moderate temperatures; any volatile materials in the solid will evaporate and leave the

combustor without being exposed to a ﬂame. It is, therefore, necessary to duct the off-gases to a secondary

combustion chamber in order to ensure efﬁcient incineration of the volatilized organics.

The incineration process in a multiple hearth furnace occurs in three stages. In the uppermost sets of

hearths, the incoming wastes are dried at moderate temperatures. In this zone, volatile organics can also

be released into the gas stream. Incineration of combustible matter takes place in the middle sets of

hearths. The ﬁnal set of hearths serves to cool the waste prior to discharge. Discharge of the solids is

usually by means of a second auger.

Multiple hearth incinerators are rarely used for hazardous waste destruction. Their principle applica-

tion is for the combustion of sewage sludge. The wet, homogeneous nature of this waste is well suited

to the multiple hearth design. Little test data are available on their efﬁciency of destruction of organics.

One set of published information on tests of four multiple hearth incinerators conducted by the U.S.

EPA (Bostian et al., 1988; Bostian and Crumpler, 1989), indicates the presence of trace amounts of

common organic POHCs in the emissions, even when they were not detected in the sewage sludge. The

reason for this is not given, but the measured levels were very low. The incinerators were equipped with

“afterburners” or secondary combustion chambers, and inlet/outlet tests were conducted at one of the

four sites. Destruction efﬁciencies of 90 to 99+% were observed.

The metal emissions tests conducted as part of the same program showed that small amounts of a

variety of metals (from the sewage sludge) were released from these incinerators. The size of the metal

particulate was not given, but based on the poor removal efﬁciency of the scrubber at one of the sites (less

than 90% and as low as 50% for beryllium), one can probably assume that the particulate was very ﬁne.

Commercial multiple hearth incinerators come in several sizes ranging from 6 to 25 ft in diameter and

from 12 to 75 ft in height. Upper zone temperatures, depending on the heat content of the wastes and

supplementary fuel ﬁring, range from 350 to 550°C (650 to 1000°F); midzone temperatures range from

800 to 1000°C (1500 to 1800°F); and lower zone temperatures range from 300 to 550°C (600 to 1000°F).

Controlled Air Incinerators

Figure 13.8 is a schematic of such a unit. The design offers advantages of lower fuel requirements and

lower particulate formation than similarly sized ﬁxed hearth incinerators. However, they are technically

more complex and require that the equipment and operation be designed and maintained to prevent

random air inﬁltration. Their use, at present, is generally limited to the burning of hospital and patho-

logical wastes.

Controlled air systems are batch units. Waste is continually fed to them, through an air lock or ram

feeder, and the ash accumulates. Periodically, the system is shut down and cooled, and the ash is removed.

Combustion air to the primary chamber is tightly controlled to maintain an oxygen level close to or even

slightly at substoichiometric conditions. This results in pyrolysis of the waste. The combustion gases

from the primary chamber go into the secondary chamber, where they mix with additional air to complete

the combustion process. Such staged combustion results in very low gas ﬂows in the primary chamber

and minimization of particulate releases from the solids.

Boilers and Industrial Furnaces

Many boilers and high-temperature industrial process furnaces (BIFs) operate at conditions suitable for

the destruction of selected types of hazardous wastes, and their use for the purpose is common. The

practice achieves two things. First, it destroys the waste. Second, the heating value of the waste replaces

fossil fuel with an economic beneﬁt. The practice must, however, be approached with caution.

First, wastes, including hazardous wastes, by their nature, have a highly variable composition. Boilers

and industrial furnaces, on the other hand, are primarily operated to produce energy or a product and

are relatively intolerant of impurities or “off-speciﬁcation” raw materials. Furthermore, the systems may

not be capable of withstanding some of the products of combustion of certain types of wastes. For

example, HC1 from the combustion of chlorinated wastes may damage refractory. Finally, many boilers

or high-temperature industrial processes are not equipped with scrubbers or other air pollution control