HORNA EXHAUST

FLUID
BED
FURNACE

SUPPLEMENTAL

SLUDGE FEED

GAS EAH AUIS

HEDGING aD

—O

HEAT EXCHANGER

auence

VENTURI

WET SCHUBBLIG

SCRUBBER
* wate C >

DRAIN

RECYCLE WATER

SOURCE U. S. EPA PROCESS DESIGN MANUAL FOR

SLUDGE TREATMENT & DISPOSAL

EPA 625/1-79-011

MAKEUP WATER C3

FIGURE 5. FLOW SHEET FOR SLUDGE INCINERATION IN A

FLUID BED FURNACE

TO ASH
LAGOON


Equipment Sizing

Incineration systems are often oversized. Their selection
and sizing is frequently based only on projected design loads
that either are never realized or are so much higher than the
sludge volumes provided in the initial years of plant operation
that furnace operation cannot be efficiently reduced to match
production. An oversized incinerator must be operated
intermittently, which leads to very high auxiliary fuel costs
associated with reheating the furnace to operating temperature
after cooling down or maintaining the furnace in a "hot" standby
condition. Oversizing is the result of a compounding of the
following factors:

- Use of peaking factors of 1.5 to 2.0 times average
sludge production to define maximum weekly loading
conditions

- Use of overly conservative criteria (i.e., design for
the highest probable moisture content of the dewatered
cake at the highest solids loading)

- Adoption of excessive safety factors for such
parameters as sludge cake loading on MHF hearths or
FBF fluidizing and freeboard velocities.

Excess MHF capacity increases capital and sludge disposal
costs. The unit costs of MHF operation and maintenance will
increase for the following reasons:

- Total labor costs will remain nearly the same whether
operating at full or partial incinerator capacity
because staffing for both cases will be approximately
the same.

- Total electric power costs for units operating at
partial capacity will be nearly equal to costs at full
capacity due to operation of electrical motors at
inefficient levels.

- Unit fuel costs will be higher due to frequent
startups or a need for standby heating for long
periods. Additionally, extended sludge storage can
reduce dewaterability, increasing sludge cake moisture
and the fuel required for evaporation.

- Total maintenance costs will increase because cycling
of the incinerator for intermittent use decreases
refractory life, requiring more frequent replacement
of brickwork.

34


because it has more frequent temperature fluctuations
than the FBF and more hearths that must be repaired
and replaced. However, with a steady sludge feed and
a minimum of shutdowns and startups, maintenance
requirements in these refractory areas are reduced.

- If both types of furnaces use hydraulic ash handling
systems, the general opinion is that there is no
outstanding difference in maintenance between the
two. Dry ash handling for the MHF requires a
considerable degree of maintenance.

- Because the exhaust gases of the FBF carry both ash
and bed media, there is a significant problem of
erosion in flues from the point at which gas exits the
furnace to the scrubber entrance.

- The exhaust gas systems for the FBF, including the
heat exchanger and expansion joints, often require
serious maintenance and must normally be given more
maintenance attention than the MHF exhaust system.
Corrosive attack is reported more often for the FBF
exhaust system. However, the induced draft fan in the
MHF has high maintenance requirements that do not
exist for the FBF.

Electric Power

Under normal operating conditions, electric power
requirements are not significantly different for the MHF and the
FBF systems. In the MHF, the major electrical equipment is air
blowers for the auxiliary fuel system, the induced draft fan, and
the furnace drive. Fluidizing blowers and burners are the major
electrical equipment in the FBPFs. Despite the considerably
higher pressure required for the FBF fluidizing blower, it must
supply only half the air volume of a comparably sized MHF.

Labor Cost

For either type of furnace, a full-time operator is
required when one furnace is operating. If two furnaces are
operating, operator attention will increase similarly for either
type of furnace, requiring supplemental part-time help. The
required skills for operators of either furnace are considered to
be equal.

The FBF is more advantageous where operating with one or
two shifts per day is preferred to 24-hour operation. The MHF
performs more economically under continuous operation. If
furnace capacity exceeds sludge production or sludge dewatering
is not continuous, labor costs are more favorable for the FBF
system.

30


to sense increasing bed temperature is shorter than in the MHF,
which permits the operator to reduce auxiliary fuel flow more
quickly, restoring the excess air to the safer 25 percent
level. Under autogenous burning conditions, water sprays or a
reduction of the sludge feed rate would be necessary to reduce
bed temperature.

If sludge solids content were to change in the opposite
direction, from 30 to 27 percent solids, combustibles would
decrease by 12 percent. If original operation were at 25 percent
excess air, the new condition would be at 37 percent excess
air. This condition could be maintained until dryer sludge was
available or adjustments to fuel or sludge feed rates could be
made.

Ideally, sludge storage should be provided before
dewatering to level out variations in daily sludge feed
quantities and characteristics. The use of sludge blending tanks
will help minimize variations in moisture, chemical, and grease
content in the liquid sludge feed to dewatering. Multiple
dewatering units will also even out variations in the sludge
dewatering process and provide a constant sludge feed rate to the
incinerator. Minimizing these variations over short periods
(minute-to-minute and hour-to-hour) will help establish steady
and fuel-efficient incineration.

EQUIPMENT PROBLEMS

Major equipment problems in MHFs occur with hearths,
rabble arms, central shafts, induced draft fans, dampers, and
thermocouples. FBF components that experience frequent problems
include refractory domes, air distribution piping, and flues.
Components common to both types of incinerators that experience
problems include ash handling systems, sludge conveyors, off-gas
systems, waste heat boilers, and scrubbers.

Failure of Hearths

MHF hearth failures can result from lack of feed rate
control as well as frequent, rapid temperature cycling; but
hearths primarily fail as a result of, sudden chgnges in
temperature. Frequent cycling between 1,300 F and 1,800 F can be
worse than controlled shutdowns. In normal operation where
sludge feed is steady, properly constructed hearths should not
fail in less than ten years, even with weekend or nightly standby
cooling, as long as temperature changes are controlled to less
than 50 F per hour.

Proper record keeping can be helpful in predicting hearth
failure. During internal inspections of the furnace, the high
point on each hearth should be measured from a common benchmark,
such as the bottom of an inspection door, and recorded. An

39


CENTRAL SHAFT {
COOLING STACK —*]

SLUDGE CAKE,
COOLING AIR SCREENINGS,
EXHAUST DAMPER toy AND GRIT.
RETURN AIR Hay £.
DAMPER a
ay
EXHAUST
< |_+- BURNERS
{+- SUPPLEMENTAL
NY | FUEL
> COMBUSTION AIR
SCUM —— >: FOR BURNER
| SHAFT COOLING
AIR RETURN
| SOLIDS FLOW
AUXILIARY
AIR PORTS
RABBLE ARM —fite_ = om La. DROP HOLES
2 OR 4 PER
HEARTH
GAS FLOW—4
CLINKER
BREAKER
ASH
DISCHARGE

SOURCE: U. S, EPA PROCESS DESIGN MANUAL FOR
SLUDGE TREATMENT & DISPOSAL
EPA 625/1-79-011

FIGURE 1. CROSS SECTION OF A MULTIPLE-HEARTH FURNACE


Air Pollution Control/Scrubbers

Approximately 20 pounds of uncontrolled particulates per
ton of sludge and 0.0038 pounds of particulates per gallon of
fuel oil fired (3) are generated in MHFs. The Federal New Source
Performance Standard of 1.3 pounds of particulates per dry ton of
sludge can be met with modern high energy wet scrubbers, such as
the venturi.

Removal efficiency is a direct function of particle size
and the amount of energy (or pressure drop) designed into the
scrubber system. Typical published values (3) for percent
removal and size distribution data from literature (3) for sludge
incineration of different particle sizes are in Table 4.

TABLE 4. SIZE DISTRIBUTION AND REMOVAL
EFFICIENCIES OF PARTICULATES

Particulate Size Distribution Percent Removal Efficiency
Range (microns) by Weight (percent)
<0.5 1.6 75
0.5-1.0 2.2 96
1.0-5.0 8.2 99.9
>5.0 88.0 100

Problems in meeting air pollution requirements have
resulted from excessive submicron particles with a high
percentage of unburned organics, primarily hydrocarbons. Improved
combustion control of the incinerator usually corrects the
problem of unburned organics.

A constant flow of water over the walls of the venturi
scrubber must be maintained to ensure scrubber performance and to
minimize corrosion in the exhaust gas system. Variation in
scrubber water flow may be caused by a plugged strainer in the
supply line or inadequate flow and pressure control in the supply
system.

Venturi scrubbers are generally constructed of high
hardness stainless steel. However, should abrasion of the
venturi throat become a problem, greater wear-resistant material,
such as silicon carbide, can be used.

The impingement tray separator section of a venturi is
also usually constructed of stainless steel. Corrosion at the
liquid gas interface above the tray sections can be avoided by
making the wall thicker at these points or by constructing the

49


abrasion and wear in the pump and piping due to the ash slurry.
The slurry, consisting of bottom ash and fly ash, is abrasive and
accelerates wear in the piping and pump, especially in pipe
elbows. Abrasion-resistant heavy walled pipe and rubber-lined
pumps should be used for this service.

The ash slurry is generally disposed in a lagoon, which
will require cleanup and disposal after a period of time. This
clean-up and long-term disposal is difficult because of abrasion
and accelerated wear on cleanup equipment due to the ash and
retention of water by the ash slurry.

Sludge Conveyors

Sludge cake conveyor systems, normally belts that deliver
sludge to more than one incinerator, may restrict operation of
the incinerator at full capacity. Common practice is to divide
the feed on a conveyor belt with a plow, which is a metal plate
that diverts part of the sludge to an incinerator or another
conveyor. However, precise division cannot be obtained. The
result is that incinerators may not be equally loaded. A
uniform feed rate to more than one incinerator may be maintained
by dedication of a sludge hopper or bunker with a controlled
discharge system to each incinerator. To obtain a fairly uniform
sludge distribution, the bunkers can be fed by a conveyor system
in which automatically-operated plows divide the sludge feed.
These plows alternately divide all sludge on the conveyor to each
bunker at frequently timed intervals. The bunkers would either
be placed directly below the main feed conveyor or would be fed
by individual feed conveyors.

There are two ways to feed sludge into a MHF. The sludge
cake can be directly dropped into the top of the MHF from a
conveyor over the furnace or from a chute or screw conveyor that
receives sludge from a conveyor on the side of the furnace.
Conveyors located over the top of the incinerator have more
maintenance problems because of the high temperature
environment. Sludge cake should be cleaned off these belts prior
to shutdown because the sludge will become baked on and difficult
to remove. Elastomer and rubber conveyors have reduced life in
this hot environment.

Although conveyors located to the side rather than over
the top of the incinerator have reduced maintenance, the side
conveyors require a chute or screw conveyor to feed the sludge.
To avoid bridging of the sludge at the transfer point, and to
ensure free flow of the sludge, the chute angle should be between
70 degrees and 80 degrees. Water sprays to help keep the sticky
sludge moving at the transfer points are not recommended because
of the increased evaporative load to the furnace.

47


ADMINISTRATIVE PROBLEMS

Problems in the administration of MHF and FBF systems are
in the areas of operations management, staffing, and training.

Management personnel should be aware of the technical
complexities of MHF and FBF systems and should have a knowledge
of system O&M needs, including expected results, methods to
evaluate performance, and a system of personnel accountability.

Many of the problems discussed in this section, although
related to the design of the system and the equipment provided,
can often be eliminated or at least mitigated by proper
operations managment. In many instances, properly designed
systems have functioned poorly due to lack of proper management
procedures. Areas in which operations management can be improved
to enhance incinerator system performance include:

- Control of sludge dewatering operation to maintain a
steady, uniform sludge feed to the incinerator

- Control of energy consumption through efficient sludge
dewatering

- Control of sludge feed and ash disposal through
scheduled, thorough maintenance of these systems.

Because of the complexity of the incineration process,
having an O&M staff with the right qualifications is an absolute
necessity. This staff must have a working knowledge of the
process, the ability to troubleshoot process problems, and be
motivated to learn. Salaries commensurate with these
qualifications are also needed to attract and retain highly
skilled personnel.

Quality training is required for personnel who operate,
maintain, and manage these systems. The training normally
provided by the manufacturer during startup is, in most cases,
insufficient to ensure that long-term training needs are met.
Specialized training is required and should include:

- Basic knowledge of routine system startup, shutdown
and standby procedures

- Preventive maintenance requirements and procedures
- System process control theory and testing procedures
- Process troubleshooting techniques

- System economic considerations

53


Incinerator feed characteristics should include a
uniform sludge mixture with a high volatile content,
low grit content, and be greater than 20 to 25 percent
solids to maintain steady operation of the incinerator
and to conserve auxiliary fuel.

Blending tanks and multiple dewatering units should be
provided to minimize variations in solids content and
fead rate to the incinerator.

The ferric chloride dosage for sludge conditioning
should be optimized or eliminated to prevent corrosion
problems. If necessary, corrosion-resistant metals
should be considered for ID fans, drains, and exhaust
flues.

An energy recovery system to reduce plant-wide energy
consumption should be incorporated into the solids
handling design.

MHF support systems should include:

.» sludge storage system with live bottom bins

+ an ash handling system with a sealed ash discharge
outlet from the MHF

- an ID fan with adequate capacity range

+» central shaft cooling air fan and ductwork

+ a venturi scrubber with an impingement tray section

. instrumentation to measure oxygen in the flue gas,
temperatures on all hearths, and draft in the top
hearth.

FBF support systems should include:

+ sludge feed pumps or screw conveyors with
adjustable speed drives

» an external firebox with a windbox burner and
fluidizing air entry for heating the air

+ an auxiliary fuel system with flame safeguards and
direct feed of fuel into the bed

+ fluidizing air blowers with multiple impellers

+ a venturi scrubber with an impingement tray and
demister section

57


can be pressurized such that sufficient clean air forces leakage
across the dampers from the boiler into the bypass duct.

The solutions to the problems raised by chlorides and
sulfur in sludge begin with recognition and consideration of the
problem. Improved dewatering will help chloride removal. Twenty
percent solids sludge contains 4 pounds of water per pound of
solids; 33 percent solids sludge, contains 2 pounds of water per
pound of solids. This increase in solids reduces the inorganic
chloride content by half since all inorganic chlorides are water
soluble. Using polymer or ferric sulfate instead of ferric
chloride in solids coagulation may be preferable to further
reduce the chloride content of sludge.

Proper handling of exhaust gas for either type of
incinerator will minimize corrosion problems. Transition from
hot dry gas conditions to wet scrubber conditions must be
accomplished without an interfacial zone subject to alternate
wetting and drying conditions. The exhaust gas should be kept
hot enough to keep all metal parts well above the temperature at
which sulfuric acid condenses. The gas should pass into the
throat of the venturi scrubber, where the flow of water covers
metal parts and is sufficient to absorb the acidic gases without
a significant change in water pH.

Corrosion problems for all equipment in contact with
incinerator off-gases can be minimized by process temperature
control, insulation for temperature control, and selection of
proper materials. Materials selection can be based on historical
data or on actual testing of samples in an_= existing
installation. The equipment involved includes heat exchangers,
waste heat boilers, ID fans, stacks, duct work, and drains.

Ash Handling

Ash handling problems have been experienced with both
mechanical and hydraulic ash systems.

Mechanical Type. The mechanical (dry) ash system is only
applicable to MHF systems. The dust from non-tight dry ash
systems and the abrasive ash create severe maintenance
problems. The inability to keep the plant clean causes operator
morale problems that further detract from performance. Common
problems include the following:

- The ash drop hole and chute at the bottom of the MHF
can become plugged with large clinkers or loose
brick. This problem can be reduced by adding a
diversion chute and coarse screen. The fine ash will
pass through the screen, but larger objects will be

45


Ash Handling System

There are two types of ash handling systems, hydraulic
(wet) and mechanical (dry). The MHF can use either the hydraulic
or mechanical type; the FBF can only use a hydraulic ash system
because wet ash is discharged from the FBF scrubber. The
hydraulic ash system, seen in Figure 7, has a steel ash hopper,
pump, discharge pipeline, and water supply. The ash drops into
the hopper, which is filled with water. The wetted ash settles
and the resultant ash slurry is pumped to a lagoon or fill area
for further settling.

The mechanical ash system, shown in Figure 8, consists of
screw conveyors, a bucket elevator, an ash bin, and a rotary ash
conditioner. The dry ash is discharged from the furnace to a
bucket elevator, which lifts the ash to a screw conveyor and into
a storage bin. From the bin, the dry ash is conditioned or
wetted by a conditioning screw or rotary drum mixer with internal
water sprays prior to disposal to reduce dust. The conditioned
ash is normally disposed by truck at a landfill.

Scrubber System

The venturi scrubber with an impingement tray separator is
the most commonly used exhaust gas scrubber in municipal
incinerator facilities. As seen in Figure 9, the exhaust gas
leaves the furnace, passing into a precooler section with water
sprays, and then into a quench section in which water flows over
the metal walls, forming a water seal. After quenching, the gas
passes into a venturi section where its velocity increases. This
increases particle collisions, promoting droplet formation. The
gas and liquid pass into a flooded elbow, after which the clean
gas passes through an impingement tray separator that disengages
the liquid from the gas. Following this, any remaining mist is
separated from the gas in the demister section. The clean gas
then passes through an induced draft (ID) fan and out through an
exhaust stack. Scrubbing water can be recycled and/or supplied
by make-up water. Waste scrubbing water from an MHF is normally
recycled into the main process train in a wastewater treatment
plant. With a FBF, the scrubbing water contains the ash from the
furnace and is normally treated and sent to an ash lagoon.

Other Support Equipment

Other equipment directly related to efficient operation of
the MHF includes a well-sealed ash discharge outlet from the
furnace to prevent the infiltration of ash into the furnace and
lance or poke holes at perimeter drop hole locations in the
hearths to allow access for control of slagging.

Additional support system equipment for a MHF includes the
following:

17


The impacts of oversizing the FBF are similar to those for
the MHF. The most significant impact is the added capital cost
for larger FBF unit(s). Labor costs for operation of a FBF are
not significantly affected by oversizing, and may be reduced if
sludge can be processed during one or two shifts per day. As
with the MHF, unit electric power costs will be higher due to
inefficient operation of motors. Maintenance costs for an
oversized FBF will be increased by cyclic operation and increased
needs for repair or replacement of the refractory.

The first and most important step for the design of a
municipal sludge incineration system is to define the sludge feed
characteristics and to establish the operating parameters
required for the furnace and support systems. Two basic criteria
are required:

- Sludge feed rate: pounds of wet cake per hour
- Properties of the sludge feed:
. Percent solids (preferable to moisture content)

. Percent combustibles in the solids (total
volatiles)

. Gross heating value of combustibles
. Analysis of total, or ultimate, combustibles
. Presence of chemicals that react endothermically.

The softening and fusion points of the ash, determined by
ASTM Method D-1857-68, are also highly desirable. These criteria
can be determined if a valid specimen of the sludge or ash can be
obtained.

In many instances, the above criteria are not known with
precision before specifications are prepared. The designer
depends upon the accumulation of in-house data on sludge feed
characteristics. Ranges of expected values are inspected to
ensure that the furnace will meet the needs of the wastewater
treatment plant.

An effective alternative to specifying wide ranges in
these criteria is to develop plant operating mode scenarios.
This will not only facilitate the development of the above
criteria and heat and material balances, but will also provide a
more realistic picture of operation of the solids handling
train. Once these criteria are developed, minimum and maximum
furnace exhaust temperatures and minimum and maximum percentages
of oxygen (excess air) in the exhaust gas can be determined.

35




- Slagging and clinkering may be more easily handled in
a MHF because of the accessibility provided by furnace
doors. Although this advantage is marginal, slagging
in a FBF may cause defluidization or shutdowns for
long periods to remove the slag.

- FBFs can better accommodate grease and scum, using it
as auxiliary fuel in sludge burning. These materials
can burn in the fluidized bed, which supplies good
contact with combustion air.

- The FBF is more easily controlled than the MHF because
it has a simpler burning process. MHF control can be
simplified by low excess air operation and a longer
sludge drying period.

Emissions

Both types of furnaces can meet present federal emission
standards when fitted with appropriate emission control
devices. In many cases, pollutant emission levels may be
decreased through improved combustion control. In the future,
increasingly stringent requirements on emissions may influence
furnace selection.

Under normal operation, FBFs have exhaust temperatures
between 1,400°F and 1,500°F, which destroy odors and
hydrocarbons, and reduce particulates. Because MHF exhaust
temperatures are below this range, it may be necessary to
incorporate afterburning in some MHF installations to destroy
odors and to burn out hydrocarbons. Present afterburning methods
use considerable amounts of auxiliary fuel, raising operating
costs. Given the present use of the MHF, the FBF has an
advantage where afterburning is necessary. If an exhaust
temperature of 1,600°F is required for emission control, the FBF
can be operated at this level without afterburning.

Operational Flexibility

Based on current use in municipal plants and available
manufacturers' products, the FBF provides more flexibility in
furnace operation than does the MHF. The primary advantage of
the FBF is its ability to be operated for less than 24 hours a
day. The FBF can accommodate shorter periods of sludge feed by
operating fewer hours a day and can be placed on standby
overnight without experiencing appreciable heat loss in the
bed. The MHF cannot be operated intermittently for short periods
without maintaining furnace temperatures at the expense of
auxiliary fuel. The FBF is also more responsive to variations in
feed characteristics and rate than is the MHF. In addition, the
FBF is better able to handle scum, grit, and screenings than is
the MHF. The main disadvantage of the FBF is that it has minimum

28


The water and volatile solids content of the sludge
normally establishes the heat demand in the bed once air flow is
set. Fuel is injected into the sand bed as required to maintain
bed temperature or to heat the fluidizing air. Auxiliary burners
may be located either above or below the sand bed. In some
installations, a water spray in the freeboard area or a heat-
removal system in the bed controls furnace temperature.

Both drying and combustion of sludge occur primarily
within the fluidized sand bed. The minimum temperature needed in
the sand bed prior to injection of sludge is approximately
1,300°F. The temperature of the sand bed is controlled between
1,400°F and 1,500°F. Gas residence time is between 5 seconds and
10 seconds.

The freeboard space above the expanded sand bed is
designed to allow disengagement of entrained sand particles.
Sand or other bed media not disengaged in the freeboard zone
leaves the furnace with the ash and must be replaced period-
ically. Media losses are approximately 5 percent of the design
bed volume for every 300 hours of operation. Replacement media
are introduced to the furnace either above or directly into the
bed.

Combustion of gases and entrained sludge solids will
continue in the freeboard area after their separation from the
bed, and adequate detention time and volume must be provided to
complete this combustion prior to exhaust. Freeboard combustion
is evidenced by an increase in temperature between the bed and
freeboard as measured by thermocouples. Temperature increases
across the freeboard section must be monitored to control furnace
operation. The amount of increase that may be expected is unique
to each facility and must be controlled to keep furnace
temperatures below 1,600°F. Increases in the order of 100°F are
considered normal, and considerably higher increases are not
uncommon, but must be limited. Up to 5 percent of the combustion
in the FBF may occur in the freeboard area. Freeboard combustion
in excess of 5 percent may result in incompletely burned organics
passing through the exhaust system.

Effective destruction of organic substances that might
cause odorous exhaust gases occurs when (1) the combustion within
the expanded sand bed is 90 to 98 percent complete, (2) overall
residence time is 5 to 10 seconds with adequate freeboard volume,
and (3) a temperature range between 1,400°F and 1,600°F is
maintained. In normal FBF operation, because the exhaust gases
are maintained at temperatures of 1,400°F to 1,500°F for the
stated time period, unburned hydrocarbon emissions are minimal
and strict hydrocarbon regulations can be met without using an
afterburner. However, operating conditions must be proper and
steady to ensure a continuous low level of emissions.


02

PRECOOL

WATER SPRAY

EXHAUST GASES |
FROM FURNACE

MAKE-UP oe
WATER
VENTURI
ADJUSTABLE
THROAT —__]

>

DEMISTER

ee
SS Fa

IMPINGEMENT
TRAY
SEPARATOR

root |
ELBOW

MAKE-UP
WATER

WASTE SCRUBBING ID
SCRUBBING WATER FAN
WATER
rc

STACK]

RECYCLE PUMP

FIGURE 9. EXAMPLE OF A VENTURI SCRUBBER



a a

EXHAUST BYPASS
STACK —,|] STACK] CENTRAL SHAFT COOLING STACK
a COOLING AiR EXHAUST DAMPER
inne. AIR DAMPER
HOT AIR RETURN
WV
1D —
FAN wy FEED
lo—
© (0)
A ¢ SLUDGE FEED /
J \ L_ CONVEYOR
SCRUBBER BYPASS t
—- Damper] [HEARTH
/ |
' :
DRAIN N
3
ROTARY ASH
FURNACE __| ~ ‘ CONDITIONER,
h dy {
RABBLE &
ARM ORIVE —
t= _ NX °
x]
[~~ BUCKET
L ELEVATOR
COMBUSTION SHAFT COOLING
AIR BLOWER AIR FAN

SOURCE: U.S. EPA OPERATIONS MANUAL
SLUDGE HANDLING AND CONDITIONING
EPA 430/9-78-002.

FIGURE 6. SCHEMATIC OF MHF INCINERATION SYSTEM

16


air requirements to fluidize the bed media that cannot be
reduced. Therefore, the FBF should be operated near design
sludge loading rates even for short periods.

Fuel Requirements

A comparison of the fuel requirements of the MHF and the
FBF must be based on the characteristics and type of sludge feed
to the furnaces. These characteristics include volatile content
and odor potential. FBFs perform better than MHFs where high
volatile content sludges are incinerated. The typical 50/50 mix
of polymer conditioned raw primary/waste activated sludge is in
this category.

The FBF has better fuel efficiency than the MHF if
afterburning is required. In the FBF, all exhaust gases pass
through a zone between 1,400°F and 1,500°F before exiting from
the furnace. Odor causing components of the exhaust gas usually
have ignition temperatures under 1,420°F and are therefore
destroyed.

In the MHF, furnace exhaust gas temperatures range from
600°F to 900°F when maintaining correct operating temperatures in
the combustion zone without afterburning. For sludge with odor
causing exhaust gas constituents, afterburning may be required in
the MHF. However, MHFs may operate with reduced fuel
requirements with less odorous sludges, such as primary and
trickling filter sludges conditioned with lime and ferric
chloride. With these types of sludges, odor causing constituents
are minimal and afterburning would not normally be required.

Conventional auxiliary fuels for both furnaces are oil and
gas. In some cases, digester gas can replace or supplement these
fuels. The FBF can also operate on nonconventional types of fuel
such as coal or refuse derived fuel.

Maintenance

Maintenance and replacement requirements for major
components of the MHF and FBF systems show no distinct advantages
of one system over the other.

- Both MHF rabble arms and FBF air distribution systems
are durable, and neither requires replacement for many
years with ordinary maintenance. Although rabble arms
are more exposed to abrasion and differential heating
than are FBF orifices, long service is attainable if
they are properly designed and maintained.

- Hearths in the MHF and refractory domes in the FBF are

subject to cyclical temperature swings which can cause
damage. The MHF has a greater potential for damage

29


design of the furnace and burner system. Drying and combustion
May occur on the same hearth in some instances.

Furnace gases exit from the MHF at temperatures ranging
between 600°F and 900°F in normal operation without
afterburning. At some sites, air emissions standards and sludge
characteristics require an afterburner to raise the temperature
of the exhaust gases to 1,400°F or higher to destroy odor-causing
constituents and to burn hydrocarbons. Afterburners may either
be inside the top of the furnace or outside the furnace.

Generally, the heating value of the sludge is insufficient
to sustain autogenous combustion, and additional heat is provided
by adding supplemental fossil fuel to the MHF. Auxiliary fuel
burners for supplemental fuel and combustion air ports are
located at selected hearth levels in the furnace, normally below
the combustion zone hearth in the fixed carbon burning zone. The
position of the combustion zone can be modified or changed
depending upon the sludge feed rate, solids content, auxiliary
heat input, and central shaft speed. Burners may operate either
continuously or intermittently on selected hearths to maintain
temperatures best suited to the sludge feed.

Fluid Bed Furnace

The FBF, seen in Figure 4, is a vertical cylindrically-
shaped, refractory-lined steel shell that contains a sand bed
(media), fluidizing air orifices, and auxiliary burners to
produce and sustain combustion. The FBF is normally available
from 9 to 25 feet in diameter. The sand bed is approximately
2.5 feet thick when quiescent, resting on a brick dome or
refractory-lined grid. The sand bed support area contains
orifices, commonly known as tuyeres, through which air is
injected into the furnace at a pressure between 3 psig and 5 psig
to fluidize the bed. The tuyeres are installed at an angle to
the bed to prevent media from flowing back into the windbox. The
structure of the bed support varies depending upon the operating
temperature of fluidizing air. Dewatered sludge is either pumped
or carried by screw conveyors into the sand bed. Sludge may also
be pumped or conveyed into the top of the furnace, but this
practice is not recommended for municipal sludge. When the sand
bed is active and at operating temperature it expands to
approximately double the at-rest volume. Sludge is quickly mixed
within the fluid bed by the turbulent action of the bed.
Evaporation of the water and combustion of the volatile solids
within the sludge rapidly takes place. Combustion gases and ash
leave the bed and are transported through the freeboard area to
the gas outlet at the top of the furnace. Combustion gases and
entrained ash are normally scrubbed in a venturi scrubber. In
some designs, the exhaust gases pass through a gas-to-air heat
exchanger to preheat the fluidizing air. A flow sheet for a FBF
system is shown in Figure 5.

ll


The main thrust of the work performed at the plants in
Indianapolis, Nashville, and Hartford is the establishment of
operating procedures suited to the sludge feed rate and sludge
volatiles, which utilize the MHF volume for combustion in the
most appropriate zone and for drying in a far more efficient
Manner than has been practiced. To achieve fuel savings, the
operation has to be performed with greatly reduced excess air.
The incinerator upgrading work, resulting from pilot project
findings and subsequent recommendations by the Indianapolis
Center for Advanced Research (ICFAR) (4), is estimated to have
cost $20,000 per incinerator in 1980 to 1981 dollars, which
includes operator training. This cost is normally recovered in
fuel savings within a 3- to 6-month operating period, since the
average savings per furnace is estimated at $180,000 per year.

This approach to incinerator upgrading relies mainly on
well trained, informed operators. The inherent strength of the
approach is transmitting knowledge of the full performance
capabilities of the furnace to the staff.

55


applications of the furnace, including sludge incineration, grew
out of technology developed during the 1940s.

Approximately 350 wastewater treatment plants in the
United States have MHF systems. Of these facilities, 271 were
reported to be operating at least intermittently. Available data
on operating MHF facilities indicate that the majority use vacuum
filters to dewater the sludge prior to incineration. Data on 64
operating facilities indicate that sludge feed characteristics
range from 4 to 50 percent solids and from 61 to 42,000 dry
pounds per hour, with averages of 28.2 percent solids and 3,850
dry pounds per hour (1).

Fluid Bed Furnace

The use of the FBF for wastewater sludge disposal has
increased in recent years. These furnaces are characterized by a
combustion process taking place in a fluidizeg sand bed and
operating in a temperature range between 1,400 F and 1,500°F.
All combustion gases and ash leave the bed and exit at the top of
the pressurized furnace. Heat recovery from furnace off-gases by
means of a gas-to-air heat exchanger is a desirable and common
practice. The characteristic feature of the FBF is a constantly
available heat sink in the sand bed, which aids in attaining
steady combustion.

The first municipal application of a FBF incinerator was
in Lynnwood, Washington in 1965. Today, approximately 60
municipal wastewater treatment plants have FBF facilities.
Improvements in the FBF include the development of an air
preheating unit (a hot windbox) in the mid-1960s, and use of
waste heat boilers for energy recovery in 1968.

Approximately 29 FBF facilities in the United States are
reported to be operating. The data indicate that the majority of
the facilities use vacuum filters to dewater sludge prior to
incineration. Reported sludge feed characteristics range from 21
to 40 percent solids and from 300 to 6,040 dry pounds per hour.
Average sludge feed is 2,270 dry pounds per hour at approximately
30 percent solids (1).

PROCESS PROBLEMS

Although incineration systems are a very effective method
of sludge disposal, these systems have had problems in the areas
of design and operation that have limited successful and cost-
effective operation. Design problems have related primarily to
sizing of the furnaces and to variable sludge feed rates and
characteristics. Equipment problems have included the selection,
design, and layout of furnace components and support systems.
Operations and maintenance (O&M) problems have involved the
handling of slag, linkers, screenings, grit, and scum.


TABLE 1. ADVANTAGES AND DISADVANTAGES OF INCINERATION

Advantages

Reduction of volume and weight of wet sludge cake by
approximately 95 percent, reducing volume for disposal

Destruction or reduction of toxics
Potential for recovery of energy from waste heat

Disadvantages

Generally higher capital and O&M costs than for alternative
disposal methods, especially if energy recovery and fuel
efficient operation are not considered

High maintenance requirements due to high temperature
operations

Highly skilled and experienced operators required

Discharges to atmosphere that may require extensive treatment

Factors that affect selection of both the incineration
process and the type of incinerator are plant size and process
flow train, furnace design differences, emissions levels, fuel
requirements, maintenance, power, labor, and chemical sludge
conditioning.

Plant Size

Practical wastewater treatment plant size for incineration
of sludge is primarily dictated by the feasibility of other
volume reduction and stabilization processes at the site, and the
availability of other, lower-cost disposal methods such as
landfilling, land application, or composting. Generally, smaller
plants (less than 10 million gallons per day, mgd) have options
such as these that are viable and less costly for the volume of
sludge they produce. Other factors influencing this choice
include the location of the plant and the availability of the
land for alternative, more economical disposal methods. Smaller
plants in urban areas may find incineration the best method,
whereas plants of equal size in less industrial, more rural areas
may not. Sludge characteristics, such as volatile solids
contents, also affect the decision to incinerate.


ENGLISH TO METRIC UNITS CONVERSION TABLE

Conversion
English Factor Metric Equivalent
British Thermal Unit, 1.055 Kilojoule, kd
Cubic feet per 0.472 Liters per second,

minute, cfm
Degrees Fahrenheit, F
Feet per second,
ft/sec
Foot, ft

Million gallons per
day, mgd

Pounds per gallon,
1lb/gal

Pound per pound, 1b/1lb
Pounds per
square inch, psi

Pounds per ton, 1b/T

0.555 (F-32)

0.305

0.305

43,800

0.120

1000

6895

0.500

L/s

Degrees °
Centigrade, C

Meters per second,
m/s

Meter, m

Milliliters per
second, mL/s

Kilograms per liter,
Kg/L

Gram per kilogram,
g/kg

Pascals, Pa

Gram per kilogram,
g/kg

2 U,5. Government Printing Office: 1986—151-098/42548

62


~  Underutilization of heated cooling return air
- Unsatisfactory burner use patterns.

A new MHF system at San Mateo, California has incorporated
fuel-efficient design features for that type of furnace. The
feed sludge to the MHF has a dry solids content of 25 percent.
The furnace itself has an oversized combustion hearth located
approximately in the middle of the furnace. The exhaust gases
exit the furnace from this combustion hearth, not from the top of
the furnace, at temperatures between 1,400°F and 1,600°F. The
furnace does not have auxiliary burners mounted on the walls;
rather there is a separate combustion chamber mounted externally
on the furnace. This chamber contains the only auxiliary fuel
burner in the system. All fuel burning occurs in it, providing
more precise control of fuel and air combustion. Flue gases from
the drying zone of the furnace are recirculated into this chamber
and then pass into the fixed carbon zone. Excellent design
features include:

- Use of moisture in the sludge to absorb excess heat in
the combustion zone and oxygen in the dry recirculated
off-gases.

- A variable gas recirculation rate. For example, to
accommodate variations in sludge feed solids content,
a decrease in sludge solids can be offset by a
corresponding increase in gas flow through the drying
zone, ensuring that the sludge reaches the combustion
zone with the solids content required for combustion.

- Control of excess air.

- An external combustion chamber that can burn a variety
of available fuels, including waste fuels, without
flame impingement on the rabble arms or on the central
shaft because no burners are mounted in the furnace.

- A suitable residence time and temperature (between
1,400°F and 1,600°F) for exhaust gases to achieve
deodorizat.on, making afterburning unnecessary.

- Return air temperature is maintained without the use
of fossil fuel.

Fluid Bed Furnace

As with the MHF, the sludge feed rate and its fuel value
are the primary factors affecting fuel consumption in an FBF.
Because the FBF is designed with specific fluidizing air
requirements, the air flowrate is more or less fixed. The sludge
feed rate is matched to the air flowrate and is therefore limited

23


ACKNOWLEDGEMENTS

This report was prepared for the U.S. Environmental
Protection Agency by Metcalf & Eddy, Inc., Wakefield,
Massachusetts, under Contract No. 68-03-3208.

Dr. Charles F. von Dreusche of Chavond-Barry Engineering
Corp. provided special consultant services to this project.

Mr. Francis L. Evans III, EPA Project Officer, was
responsible for overall project direction. Other EPA staff who
contributed to this work include:

Mr. Howard Wall, Technical Project Monitor, Water Engineering
Research Laboratory

Dr. Joseph Farrell, Water Engineering Research Laboratory

Mr. Walter Gilbert, Office of Municipal Pollution Control

Metcalf & Eddy staff participating in this project
include:

Allan F. Goulart, Project Director
Thomas K. Walsh, Project Manager
Francis X. Reardon, Mechanical Engineer
Richard Buell, Mechanical Engineer
Elizabeth M. Gowen, Project Engineer

vi


CONTENTS

fo} d -\\[e) 4° ne ar)

Acknowledgements..

1. INtroduction... eee eee cece cece cece eee e ces eeeeee
Purpose.....
Background..........
Process Problems ...

2. Process and Equipment... ..cccccccccevcecveee
Combustion Process Description......
Support System Equipment Description........
Design IMProveMentS.....cecesceccececcsveces
Process Selection and Application.......

3. Common Problems and Solutions........
Process Design Problems.........
Equipment Problems......cseecesceceee
Operation and Maintenance Problems..
Administrative Problems.........ee08
Technical Investigations of

Multiple-Hearth Furnace Problems..........

4, Summary of Design

and Operational ConsiderationS......s.eceeeeees
Improving System Design.....
Improving Existing Systems..........
Desirable Operating Characteristics.........
Improving Plant Operations and Maintenance..

REFECTENCES. . eee cece e cece eee e cece ene e tense e secre eeeeeseee
English to Metric Units Conversion Table...........eseeeee

59


THERMOCOUPLE

SAND
FEED

Va

THERMOCOUPLE

SLUDGE
INLET

FREEBOARD

4

_ EXHAUST AND ASH

4] PRESSURE TAP

SIGHT

When

BURNER

FLUIDIZING —P|

REFRACTORY

4

AIR INLET

WINDBOX

SOURCE. U. S, EPA PROCESS DESIGN MANUAL FOR
SLUDGE TREATMENT & DISPOSAL

EPA 625/1-79-011

TUYERES

FUEL
GUN

PRESSURE TAP

STARTUP
PREHEAT
BURNER
FOR HOT
WINDBOX

FIGURE 4. CROSS SECTION OF A FLUID BED FURNACE

12


and fluid bed furnaces, thereby providing a general understanding
of the incineration process as well as its proper application in
a sludge handling system. It is not intended as a detailed
design guide or as a replacement for other design guides
available from manufacturers or for technical information

contained in published literature. This report should be
regarded as a summary of technical design and operational
information. Detailed discussions on particular topics are

contained in the original investigation document (1) and in the
references cited within the text.

BACKGROUND

Combustion of sludge provides maximum volume reduction,
destroys or reduces most toxic materials, and offers the
potential for energy recovery. Multiple-hearth furnace (MHF) and
fluid bed furnace (FBF) systems have been the most prominent
types of incinerators used in sludge combustion in the United
States for many years. Multiple-hearth furnace incineration has
been widely used for over 40 years. Use of fluid bed furnace
incineration has increased steadily over the last 15 years. Most
of the sludge incineration facilities currently operating in the
United States are MHFs, which outnumber FBFs about eight to one.

Other types of sludge incinerators include the electric
furnace and single hearth cyclonic furnace. The electric furnace
is relatively new and has been used since 1979 in a limited
number of plants. The cyclonic furnace is limited to industrial
applications in the United States.

Multiple-Hearth Furnace

The MHF is durable and simple to operate if sludge feed
quality and rate are reasonably constant. This furnace can
handle variations in sludge characteristics and loading rates if
such changes are experienced over the long range, such as month-
to-month, but hour-to-hour variations present combustion and
operational difficulties. The MHF is best suited to continuous
operation. Because of the time and fuel required to bring the
hearths and internal equipment from a completely cold conditjon
to operating temperatures between 1,400 F and 1,800 F,
intermittent MHF operation is inadvisable.

The MHF was first used more than 100 years ago by the
mining industry to dry and roast ore concentrates. These early
furnaces were constructed of refractory brick, with hearths, a
central shaft, and a rabble system like today's furnace. Wood
and coal were used as heat sources. By 1910, the furnace was
being constructed with a steel shell, which permitted a larger
diameter and more hearths, and oil and gas fuel systems were
added. The use of stainless steel alloys for high temperature


8T

FURNACE

ash 2 }IS®
| a an ete one
= T
c
=——— To ASH LAGOON ASH INLET
DISCHARGE PIPE r4 / TLOAT VALVES
GRID WASH NOZZLE OVERFLOW WATER FLOAT

LEVEL, EMERGENCY
OVERFLOW PIPE
CENTRIFUGAL ASH PUMP LJ os \
& V-BELT DRIVE NoRMaL water / |_+— RAKE-OUT DOOR
ame LEVEL

Grin”

_— AGITATOR NOZZLE

|

ASH SLURRY

SOURCE BEAUMONT BIRCH COMPANY, NJ
(MODIFIED)

FIGURE 7. EXAMPLE OF HYDRAULIC ASH HANDLING SYSTEM


SEPA

United States Office of Mun ptember 1985
Environmental Protectiot Politi in Conte ol ie 546) re 9-85- 902
agen cy - Washi ngton in OC 20460

Multiple-Hearth and
Fluid Bed Sludge Incinerators:

Design and Operational
Considerations



to the physical layout and orientation of the fan and ductwork,
draft control, vibration, sizing, noise, corrosion, and drainage.

Fan inlet conditions may not permit the ID fan to meet its
rated capacity and pressure. Sharp, short radius bends in the
inlet duct and the resulting change in direction of the inlet gas
stream upstream of the fan impeller can cause an unbalanced,
nonuniform flow condition that reduces fan performance. Adequate
space for fan and duct layout is the solution. Where space is
limited, modification of inlet ductwork and use of straightening
vanes with short radii can avoid many flow problems.

Draft control and scrubber pressure drop are usually
obtained by modulating the flow using an automated damper. In
almost all cases, an inlet damper to the fan is preferable to a
discharge damper. Either parallel or radial leaf dampers may be
used. Parallel leaf dampers resemble louvers or venetian blinds
mounted either vertically or horizontally. The louver segments
of radial dampers are mounted radially to the centerline of the
fan shaft. Butterfly-type dampers mounted too close to the fan
inlet can cause flow unbalance and a higher pressure loss than
radial or parallel leaf dampers.

Induced draft fan vibration can be minimized by a
structural analysis of the fan foundation block and by provision
of vibration isolators on the foundation. Where the ID fan is
located high in a steel frame structure, a careful analysis is
required to avoid harmful vibration. Impeller imbalance caused
by the accumulation of particulates, tars, soot, grease, and
water with subsequent loss of part of the build-up, is common in
wastewater treatment plants. Vibration switches that permit
quick fan shutdown are mandatory in a fan subject to accumulation
of grease and solids to prevent damage from excessive
vibrations.

In many instances, improper sizing of the ID fan has
considerably limited furnace capacity and resulted in severe
noise generation. An ID fan must be sized in accordance with air
flow and pressure requirements, and fan tip speed. Selecting a
fan that is too small and operating it at a speed higher than its
normal operating range will cause high noise levels. If properly
sized and selected, the fan capacity will be adequate for the
furnace and the noise level can be maintained within acceptable
limits.

ID fan corrosion is a result of using improper materials
of construction. Because scrubbers do not absorb all the acid
gases produced in combustion, these gases and the mist droplets
carried through the scrubber can produce some high concentrations
of acid. Chloride corrosion of ID fans following wet scrubbers
can be avoided by selection of special stainless steel alloys for
fan parts. Depending on chloride concentrations, a variety of

41


SECTION 2

PROCESS AND EQUIPMENT

The incineration process reduces sludge volume by
evaporating the water and burning the volatile matter contained
in sludge. The efficiency of this process depends upon the
performance of the preceding dewatering process and the operation
of support systems such as ash handling. The combustion process,
MHF and FBF_ incinerators, support equipment, operating
characteristics, and guidelines for MHF and FBF selection and
application are discussed in this section.

COMBUSTION PROCESS DESCRIPTION

Incineration is a two-step oxidation process involving
drying and then combustion or burning in the presence of
oxygen. Drying and combustion may be accomplished in separate
units or successively in one unit, depending upon temperature
constraints and control parameters. The steps are the same in
both MHF and FBF incinerators. The temperature of the feed
sludge is raised to 212°F to evaporate water from the sludge.
Then the temperature of the water vapor and air are increased.
When the sludge solids content reaches approximately 40 percent,
the temperature of the dried sludge volatiles is increased to the
ignition point, which is less than 1,000°F. Complete combustion
of all organic material occurs at furnace operating temperatures
that are in excess of 1,400°F. The sludge solids are converted
to a relatively inert ash. Moisture, particulates, and inert
gases are released through the furnace exhaust system during the
process.

The primary combustible elements in sludge and in most
supplemental fuels are fixed carbon, hydrogen, and sulfur.
Because free sulfur is rarely present in sludge to any
significant extent and is being limited in fuels, sulfur content
can be neglected in determining the fuel value of a sludge. The
fuel value of sludge is based on its carbon and hydrogen
(volatile) content. In conventional solid fuels, volatile solids
content is determined by heating the fuel in the absence of air,
and the combustible content is determined by ignition at
1,336°F, The difference in weight loss between these two
procedures is the fixed carbon content of the fuel. In sanitary
engineering, the volatile content of a fuel, such as sludge, is
determined by heating the sludge in the presence of air at


Administrative problems in management and training of staff and
in system optimization procedures have also plagued these
facilities.

In some cases, these problems have been so serious that
the process has been abandoned. In general, incineration systems
have been shut down as a result of high energy costs making other
sludge disposal methods more economically desirable. Although
these costs are often attributed to improper design or to
improper operation of the incineration system, in a number of
cases poorly dewatered sludge has caused increased fuel
consumption or fuel costs have exceeded those anticipated when
the furnace was designed.

The potential for operational problems, the ability to
minimize or avoid these problems through proper design features
or operational controls, and a careful analysis of the operation
and maintenance costs associated with an incineration system
should all be considered before the process is selected over
other sludge disposal alternatives. Both the benefits and the
potential problems attributed to these systems, as well as side
benefits such as the potential for waste heat recovery, should be
included in such considerations.


- Process data collection, documentation, and evaluation

- Support system O&M, control, fine tuning, and
troubleshooting.

TECHNICAL INVESTIGATIONS OF MULTIPLE-HEARTH FURNACE PROBLEMS

Although the problems and solutions in this section have
been catagorized into design, operation, and administrative
areas, problems overlap and system improvements must be made in
all three categories. Successful operation of an incinerator
system is dependent upon the integration and implementation of
all necessary and required improvements. Technical
investigations of conventional MHF systems in three major cities
illustrate this approach.

The investigation of conventional MHF operating modes in
Indianapolis, Indiana (4)3 Nashville, Tennessee (5)3 and
Hartford, Connecticut (6); and consequent changes in these modes
are fully documented in EPA reports on the respective municipal
facilities. The operational modes recommended and adopted
resulted in remarkable savings in auxiliary fuel consumption for
the plants in these cities. In the case of one plant, the
operational changes resulted in a large decrease in particulate
loading to the scrubbers and the avoidance of retrofitting with
new, energy-intensive scrubbers at high capital cost.

The changes in operation modes for all of these plants
include:

- Maximum use of central shaft cooling air return to
minimize hot air heat loss

- Reduced draft to minimize air in-leakage

- Slow central shaft speed to increase sludge drying
time concurrent with sludge combustion at a selected
lower hearth

- Elimination of air flow to and operation of top
burners resulting in provision of more hearths for
drying at reduced temperatures

- Burner operation under the combustion zone hearths

- Maintenance of excess air at no greater than
50 percent

- Instrumentation and, in some cases, control changes to

Measure and regulate sludge, air, and fuel flows and
to control fuel and air flow remotely.

54


scrubbers can be appropriately handled by variable venturi
throats and/or variable speed fans.

The MHF can also be modified to handle low sludge feed
rates by allowing the sludge to burn on a higher hearth than
normal and permitting the lower hearths to simply transport
ash. For prolonged low solids throughput, revising the operation
to the higher hearths and discontinuing heating of the lower
hearths could be practiced. The entry of excess air could be
sealed off in the lower part of the furnace. Combustion air and
gas would also be proportionately reduced.

Variable Sludge Composition and Feed Rate

Variations in feed rate, heating value, and solids content
of sludges result in unsteady conditions in MHFs and difficulty
in maintaining regular operation at low excess air in FBFs. The
inability to operate at planned excess air levels seriously
affects fuel economy, capacity, and power consumption.

The combustion air requirements of a furnace are directly
related to the combustible solids feed rate of the feed sludge.
The combustible content of the sludge is directly related to its
dry solids content and to the particular constituents of the
sludge, such as volatile solids, scum, grease, and inert solids
content. For a particular sludge, a 50 percent increase in dry
solids content will equal a 50 percent increase in combustible
material. Under steady sludge feed conditions, this represents a
50 percent increase in combustion air requirements. Similarly, a
decrease in the inert solids content of feed sludge will produce
a proportionate increase in the combustible content of the
sludge. Finally, an increase in sludge feed rate to the
incinerator will increase the rate of feed of combustible
material. If all of these changes occur simultaneously, as they
frequently do in municipal installations, they can produce a
radical change in the heat input to the furnace and in combustion
air requirements, resulting in extreme demands on air and fuel
supply systems. Where those demands are beyond the capacity of
installed equipment, excessive temperatures will result.

An increase in the solids content of the sludge feed can
also place additional demands on the MHF. Increasing the solids
content decreases the quantity of water to be evaporated,
decreasing the hearth drying area required. If the solids
content of feed sludge is increased suddenly, it could be dried
to the point where it could ignite while still on an upper drying
hearth. The preceding lower solids content sludge, still burning
at its normal rate on the lower hearths, would contribute heat to
further raise the temperature on the upper hearths, increasing
the combustion rate on the upper hearths. This burning in two
zones of the furnace is termed double burning and results in a
very large increase in combustion air needs. Once the upper

37


Sz

EXHAUST
TO SCRUBBER

DEWATERED -————~}
BED

SLUDGE

WINDBOX

DRYER
FBF

SAND LIFT
BLOWER

EXHAUST GAS

BED

WINDBOX

COMBUSTOR FBF

HEAT
EXCHANGERS
# EXHAUST TO
SCRUBBER AND
|p| ASH DISPOSAL
FLUIDIZING
AIR BLOWERS

FLUIDIZING AIR

FIGURE 10. SCHEMATIC OF FBF SYSTEM AT NORWALK, CONNECTICUT


materials including 316L stainless steel, Iconel 600 series, and
Hastelloy C276, increasing in corrosion resistance, can be
used. Corrosion can also occur on exhaust stacks and other parts
following induced draft fans. The ID fan drains and the stack
drains must be kept clear to allow the corrosive liquids to drain
away. Drains of corrosion-resistant material have been found to
be necessary in many plants.

The combustible materials built-up in the ID fan can be
ignited under certain circumstances. With a loss of scrubber
water, the bypass dampers in a MHF system should open to protect
the ID fan and scrubber from excessive temperature. If the
bypass does not open, the hot gases passing through the exhaust
system can ignite the fan deposits. A combination of water
sprays in the fan and mechanical cleaning are mandatory to reduce
the rate of material build-up in the fans and to protect against
fan fires.

Emergency Bypass Damper

In a MHF system, one of the most common problems is poorly
fitted bypass dampers. Seating of the damper disc is normally
imperfect in the large refractory-lined stacks. As seat areas
deteriorate and become worn, gases that have not been scrubbed
can leak from the stack. Although an inflow of air to a lower
pressure area below the damper disc should occur, untreated gases
can flow from this area. Alternatively, failure of the bypass
stack damper to close reasonably tightly at high furnace draft
can result in major in-leakage of air adding to the gas flow that
the scrubber and the ID fan must handle. The solutions are
provision of a refractory seat with closer seating tolerances and
routine checking by maintenance personnel.

Damper bearings installed on the bypass stack can over-
heat and seize, rendering the damper inoperable. The bearings
should be offset from the stack with an intervening heat shield.

Thermocouples

Ceramic protective shields for thermocouples are prone to
shattering. They have had satisfactory service life in MHFs when
special refractory shapes are provided above the shields to
protect the thermocouple from falling sludge. However, when the
ceramic type shield is placed unprotected and in the path of
dropping sludge, the thermocouple tends to shatter very easily.

Most ceramic shields for thermocouples in MHFs have been
replaced with Inconel 601 shields, eliminating the shattering
problem. Thermocouples with 304 stainless steel shields are
usually not suitable for service in MHF combustion hearths.

Ceramic type thermocouples are used in FBFs because the
frequency of shatter is much lower. The thermocouples are

42


to a narrow design range. Auxiliary fuel consumption is
dependent upon the fuel value of the sludge and the required bed
temperature.

While bed temperature is a function of the sludge
characteristics and feed rate, the furnace exhaust temperature
can be directly controlled by a temperature control system,
consisting of a series of high pressure water sprays that cool
the exhaust gases.

Improvements in fuel efficiency of FBF systems primarily
depend on revisions to support systems and changes in modes of
operation. This is illustrated by modifications to a FBF system
at a wastewater treatment plant in Norwalk, Connecticut, as seen
in Figure 10. The FBF system, installed in 1973, utilized a cold
windbox without heat recovery. Sludge was dewatered by
centrifuge prior to incineration.

The system was revised by replacing the centrifuge with a
belt filter press for dewatering and installing a FBF to act asa
dryer in series with a FBF acting as a combustor. Sludge from
the belt filter press, at approximately 25 percent solids, is fed
directly to the dryer FBF. The dried sludge and sand from the
dryer FBF flow by gravity down to the combustor FBF where the
sludge is burned. A sand lift blower circulates the hot sand
from the combustor FBF to the dryer FBF, providing a constantly
hot bed for drying. Hot exhaust gas (approximately 1,500°F) is
routed from the combustor FBF through two heat exchangers in
series and then through a_ scrubber. These heat exchangers
preheat fluidizing air to approximately 1,200°F for the combustor
and 230°F for the dryer FBF. The key to fuel saving operation is
the use of a low temperature FBF as a dryer. With less fuel
required for evaporating moisture, more air is available to burn
sludge.

These improvements have resulted in reductions in
operating crew, hours of daily operation, and fuel oil use while
doubling sludge capacity. Annual cost savings have been
estimated at between $300,000 and $400,000 (1984 dollars).

PROCESS SELECTION AND APPLICATION

Selection of the incineration process for municipal sludge
is generally based on the results of a technical and economic
evaluation and a comparison with other sludge handling
alternatives. Because the incineration process can handle nearly
all types of sludge, the primary factors in the selection of
incineration are plant size and economics. An economic
comparison of incineration with other sludge handling
alternatives is beyond the intent of this report and is discussed
in other technical literature (2). The major advantages and
disadvantages of incineration are summarized in Table 1.

24


1.

REFERENCES

U.S. Environmental Protection Agency, "Improving Design and
Operation of Multiple-Hearth and Fluid Bed Sludge
Incinerators," EPA Contract No. 68-03-3208, in preparation.

Environmental Protection Agency, "Process Design Manual for
Sludge Treatment and Disposal," EPA 625/1-79-011, September
1979.

Brown and Caldwell, “Central Contra Costa Sanitary District -
Solid Waste Resource Recovery, Full Scale Test Report,"
March 1977.

U.S. Environmental Protection Agency, Indianapolis Center for
Advanced Research, "Plant Scale Demonstration of Sludge
Incinerator Fuel Reduction," EPA 600/2-83-083, March 1983.

U.S. Environmental Protection Agency, "Sewage Sludge Incin-
erator Fuel Reduction at Nashville, Tennessee," EPA 600/2-
83-105, December 1981.

U.S. Environmental Protection Agency, "Sewage Sludge Incin-

erator Fuel Reduction, Hartford, Connecticut," EPA 600/2-84-
146, March 1982.

61


SECTION 1

INTRODUCTION

Improvements in wastewater treatment technology and in the
design and operation of wastewater treatment plants are resulting
in higher-quality effluents with increased sludge production.
Concurrently, the problem of sludge disposal has become more
difficult because many disposal methods have been found to
present risks to public health and safety. Sludge processing and
disposal methods that have received only cursory attention in the
past are now being reevaluated.

PURPOSE

This design information summary report presents data and
best practices relating to the design and operation of multiple-
hearth and fluid bed furnace incineration systems for combustion
of sludges in municipal wastewater treatment plants in the United
States. It is based on an investigation which evaluated the
causes of operations and maintenance problems experienced with
multiple-hearth and fluid bed furnace systems. The data
contained in the report were obtained from technical literature,
discussions with manufacturers, and telephone inquiries and site
visits to municipal wastewater treatment plants. This document
presents process and equipment descriptions, operational
characteristics, process selection and application information,
common problems and solutions, and design and operational
considerations related to incineration of sludge.

The emphasis of this report is on multiple-hearth and
fluid bed furnace incineration systems, not just on the furnace
itself. In incineration, as in sludge handling in general, the
performance and success of each step in the process flow train
depends upon the previous step. Cost-effective operation and
efficient performance of an incinerator depends upon a properly
dewatered and prepared feed sludge. Ease of operation and low
maintenance needs for ash handling systems depend upon the design
and operation of the incinerator that produces the ash. The
performance of the entire sludge handling system, not just the
incinerator, will determine the success of the incineration
process.

The purpose of this report is to summarize concisely the
available design and operational information on multiple-hearth


In a FBF, sufficient air is provided for combustion by
allowing for 15 percent excess air. To account for imperfect
mixing in the combustion zone and to ensure that adequate oxygen
is available, the FBF is typically designed with 30 to 45 percent
excess air capacity. Less excess air capacity may result in
incomplete combustion.

Air supplied to a FBF may either be at ambient temperature
(cold windbox) or heated (hot windbox). In cold windbox design,
the support beneath the sand bed serves as the air distribution
plate. Because this plate is not subject to the same temperature
conditions as that of the hot windbox, it can be constructed of
metal. This metal plate is air cooled, which prevents excessive
expansion, and designed for temperatures up to 1,000°F or
slightly higher. Provisions for plate expansion at much higher
temperatures require greater design attention and cost.

In hot windbox design, seen in Figure 4, the air is heated
by burners within the windbox or by a heat exchanger that
captures heat from the high-temperature exhaust of the furnace.
The hot windbox unit utilizes a refractory brick dome bed beneath
the sand. This construction is somewhat similar to the
refractory hearths of MHFs. The brick domes are often twice as
thick as MHF hearth cross-sections, however, since they support
the sand bed and the air pressure for fluidization. Large,
specially shaped bricks in the dome are pierced by holes 1 to
3 inches in diameter, through which metal air nozzles are placed,
forming the tuyeres for injecting the fluidizing air.

The fluid bed acts as a thermal sink providing substantial
heat storage capacity. This capacity dampens temperature
fluctuations (thermal cycling) that may result from short term
variations in sludge feed properties and feed rates. To indicate
the heat storage characteristics of a FBF, a sand bed suitable
for combustion at a rate of 6,000,000 Btu per hour would absorb
or release about 1,000 Btu to change the expanded bed area
temperature by 20°F. This heat storage capacity also enables
relatively quick startups if the furnace shutdown period has been
short, e.g., overnight, and protects the refractory dome and
support arches from cracking by dampening out temperature
fluctuations.

SUPPORT SYSTEM EQUIPMENT DESCRIPTION

The performance of either a MHF or a FBF incinerator is
dependent upon the provision of proper support system
equipment. This includes ash handling equipment, scrubbers, and
other equipment directly associated with the furnace. A typical
MHF system, including most of the support equipment, is
illustrated in Figure 6.

15


- Enlarging drop holes in the combustion zone

- Increasing turbulent mixing of gases to provide a
uniformity of the fire from one side of the furnace to
the other

- Adding air below the fire, not above the volatile
burning hearths.

Burners

Carbon build-up around burner tiles is an indication of an
improperly adjusted burner. Other problems caused by maladjusted
burners are slag, clinkers, refractory damage, and furnace shell
damage from buildup of molten material around burners. Typical
reasons for lack of proper adjustment are failure to use the
proper orifice plates in making adjustments and setting the
excess air so low that high flame temperatures cause the ash to
slag. Excess air in MHFs must be high enough to reduce slagging
around the burner.

Screenings, Grit, and Scum Disposal

Screenings, grit, and scum demand special handling if
Processed in an _ incinerator system. The impact on the
incinerator, particularly the MHF, has been adverse, and
relatively few such furnaces have performed satisfactorily.
Screenings can clog feed mechanisms and require constant
attention. Operation costs are expensive due to the high
moisture content of the screenings. Grit is abrasive and may be
high in organic matter and relatively dry. Burning grit causes a
deficiency in combustion air and generation of black smoke.
Whether grease and scum are burned separately or in a high
proportioned scum-sludge mixture, burning them results in
uncontrolled burning, slag, and unburned carbon due to lack of
combustion air. Grease and scum have a high heat value and
require combustion air commensurate with heat release. The air
volume for a high proportioned scum-sludge mixture is not
normally available in the air distribution system of a MHF.

The experience of plants utilizing MHFs adapted to burn
screenings, grit, and scum demonstrate that these furnaces have
been largely inadequate for this service. In addition, there is
considerable risk to furnaces and to emission violations caused
by incomplete combustion.

The FBF is much more adaptable to burning grease, oils,
and other materials of high calorific value. Pumping and
extruding fuels directly into the media bed are the normal
feeding methods in the FBF. Within the last 10 years, 10 FBFs
have been designed and put into service to burn both sludge and
grease.

52


DESIRABLE OPERATING CHARACTERISTICS

The key parameters for efficient performance of MHF and
FBF systems include feed sludge characteristics, furnace
operation, air rates, and, for the MHF system, temperature.
These parameters are important considerations in designing a new
system or in optimizing the performance of an operating facility.

- A uniform and constant sludge feed is needed to
Maintain steady furnace temperatures and air rates and
to minimize energy consumption. Screenings, grit, and
scum should not be fed directly into a MHF to avoid
uncontrolled burning, slagging, and air deficiencies.
Grit and scum should only be burned in a FBF when the
feed is under manual control and with the required
excess air.

- Furnace startups and shutdowns should be minimized to
avoid temperature fluctuations, which result in high
auxiliary fuel consumption. Continuous 24-hour
operation of both types of furnaces is preferable.

- The rotational speed of the rabble arms in a MHF
should be less than 2 rpm to increase sludge drying
time and to maximize the use of the hot exhaust air to
dry the sludge.

- Sludge loading rates to the FBF should be maintained
at design values at all times due to minimum
fluidizing air requirements that must be met in order
to conserve auxiliary fuel and energy consumption by
the fluidizing air blowers.

- The temperature in, the MHF combustion zone should be
kept below 1,650 F to prevent slag and clinker
formation. This is accomplished either by decreasing
the oxygen in the fixed carbon burning zone or by
increasing the air flow into the combustion zone.

IMPROVING PLANT OPERATIONS AND MAINTENANCE

The efficient, safe, and cost-effective operation of an
incineration system is dependent upon having well-qualified,
trained personnel, working within a well-managed system with
adequate budget support. The complexity of the incineration
process and specialized equipment of MHF and FBF systems require
that special attention be given to the management, training, and
staffing for these systems to ensure safe and efficient
operation.

59


SECTION 3

COMMON PROBLEMS AND SOLUTIONS

Problems involving design, equipment, operation, and
administration of MHF and FBF systems, summarized in Table 2, are
common at many municipal wastewater treatment plants. These
problems increase the unit cost of incinerator operation, reduce
its operational efficiency, and/or cause equipment or system
failures. Solutions to these problems exist and have been
successfully implemented. The solutions presented can be used as
guidelines for designing and operating MHF and FBF systems,
keeping in mind that specific solutions may have to be modified
to suit a particular plant.

This information, gathered as part of a research study
conducted for EPA, is drawn from a number of sources, including
discussions with manufacturers and consultants, and telephone
inquires and site visits to municipal wastewater treatment plants
with MHF and FBF facilities.

PROCESS DESIGN PROBLEMS

Process design problems relate to selection and appli-
cation of the type of incinerator, as well as the equipment for
the incinerator and its support systems. These support systems
include sludge dewatering and sludge feed facilities. Problems
that relate more specifically to the design and use of other
equipment components are discussed as equipment problems.

Dewatering

Regardless of the type of incinerator, the extent to which
feed sludge is dewatered has a major impact on incineration
efficiency and costs. The low solids content of sludges
dewatered by vacuum filters or centrifuges is a common problem at
plants completed prior to 1979. Sludge having a high moisture
content reduces the equivalent dry solids throughput capacity of
furnaces and requires larger amounts of auxiliary fuel to
evaporate the water prior to or during combustion.

32


TABLE 2. SUMMARY OF MAJOR PROBLEMS WITH MHF AND FBF SYSTEMS

Process Design Problems

Inefficient sludge dewatering
Oversizing of incinerators
Variable sludge composition and feed rate

Equipment Problems

Failure of hearths

Overheating and failure of rabble arms

Cracks in central shafts

Design, draft control, vibration, noise, and corrosion of
induced draft fans

Failure of poorly fitted bypass dampers

Failure of thermocouples

Failure of refractory domes

Sand leakage into air distribution piping

Corrosion in flues

Wear, dust control, and abrasion in ash handling systems

Wear and maintenance of conveyors

Misalignment of off-gas system

Improper sizing, design, and corrosion of boilers

Improper design and corrosion of scrubbers

Operation and Maintenance Problems

Slag and clinkers
Improperly adjusted burners
Screenings, grit, and scum handling

Administrative Problems

Lack of system understanding by senior management personnel
Poorly qualified O&M staff

Insufficient operator training

Lack of process optimization

Drier sludge cakes are achieved at plants that use more
efficient belt presses or recessed plate filter presses. In some
cases, incinerator fuel consumption has been reduced by more than
50 percent as a result. In multiple incinerator systems, drier
feed sludge has also eliminated the use of one or more
incinerators. Other savings occur in labor, power, and
maintenance costs due to the higher solids content and lower
volume of sludge to be incinerated. The cost for supplementing
or replacing dewatering equipment would be expected to be
recovered from O&M savings in three months to two years.

33


Off-Gas System

Air leakage into the furnace off-gas system may prevent
furnace operation at design conditions and reduce efficiency.
For example, misalignment of the precooler to a venturi scru’.er

may not permit closing of the water seal. The air introduced
into the off-gas system at this point will reduce furnace
capacity. Air in-leakage from dampers in interconnecting duct

work may also be sufficient to reduce furnace capacity. A check
for misaligned equipment should be performed in both new and
older plants, and equipment should be properly aligned to avoid
this problem.

Waste Heat Recovery Boilers

The design loading factors for dust, which are solid
particles in the exhaust gas, are critical in sizing a waste heat
recovery boiler. The values in Table 3 are typically used by
manufacturers in the waste heat recovery boiler industry. As
dust loads increase, gas velocity must decrease to _ reduce
erosion. Reduction of gas velocity increases boiler size and
cost. Tube wall thickness should be increased for erosion
protection where required.

TABLE 3. TYPICAL DUST LOAD FACTORS

Percent Dust by Weight Maximum Gas Velocity
in Flue Gas (ft/sec)
10.2 32
0.32 54
0 66.5

Control dampers must be carefully located to avoid dead
spots where dust can build up when the dampers are modulated.
Modulation changes the velocity distribution profile.

Unburned carbon in the exhaust gas can also be a problem,
coating waste heat boilers with soot and reducing heat transfer
efficiency. The soot is removed by steam cleaning. If steam
availability is a problem, a high pressure air cleaning system
may be used.

An auxiliary heating system may be required to protect the

boiler against condensation and corrosion during long standby
periods.

48


A sludge cake feed conveyor system and feeder that
provides a steady, nonvariable input to the furnace.

- A live bottom bin that regulates the dewatered sludge
feed to the incinerator. This type of bin has a
series of augers or screws at the sludge discharge
point to facilitate sludge discharge and to prevent
sludge from bridging.

- Auxiliary fuel burners that are sealed at the furnace
entry to prevent air infiltration into the furnace.

- Burner system blowers and central shaft cooling air
fan.

- Central shaft cooling air return ductwork to hearths
below the combustion zone.

- An induced draft fan designed with an adequate
capacity range to satisfy the desired excess air
levels in the furnace. The fan sizing should make
allowances for anticipated infiltration and variations
in air requirements.

~ An oxygen analyzer to sample flue gases in the exhaust
gas outlet from the top hearth of the furnace.

- Temperature measurement in all hearths and draft
measurement in the top hearth and at selected points.

- Automatic damper-operated air and gas ducts.

- A heat recovery system consisting of a convective
waste heat boiler ahead of the venturi scrubber.

Additional support system equipment for a FBF includes:

- Progressive cavity pumps, piston pumps, or screw
feeders for feeding sludge beneath the surface of the
bed.

- A multi-staged fluidizing air blower.

- A preheat burner mounted in the furnace to raise the
temperature of the inert bed for ignition of auxiliary
fuel and sludge during furnace startup.

- Fuel injectors for feeding auxiliary fuel directly
into the bed.

al


- Readjust rabble teeth to break up lumps and to retain
lumps longer in the fixed carbon hearths

- Change chemical conditioning, if necessary
- Shred very dry sludge cakes.

Slagging occurs in several forms. Oil and dust can create
slag on burner tiles. Grease burned on a hearth may create local
hot spots or a local area of ash melting, causing slag. Sludge
left on rabble arms can be carried through the burner flame zone
and held at the junction with the central shaft, producing a
roaring volatile flame that turns sludge to slag. Lastly, dust
carried in the gases from the lower hearths can pass through
burner or volatile flames, become molten while in suspension, and
then stick to the first contacted surface. .If the surface is
cool enough, the molten droplets freeze and drop away. If the
surface is sufficiently hot, alternating strata of cinder and
glass deposit on the surface. At still hotter surface
temperatures, the molten droplets become glass-like and penetrate
the refractory until the deposit can only be removed by breaking
away the brick surface. The slag is taffy-like while hot, and if
heated another 100 to 400°F, will become fluid and slowly
dissolve the brick.

In general, if the combustion zone temperatures are below
1,650°F, slag and clinkers are not a problem in a MHF. Problems
occur, however, because bed temperatures and hot spots frequently
exceed this value in localized areas.

Solutions to slagging, as well as clinkering, involve
control of air infiltration and oxygen supply. The slagging
problem is usually better understood and correctable where the
analysis is not only based on oxygen measurement in the top
hearth, but also on excess air leakage into the furnace above the
fixed carbon burning hearth.

In the MHF, one solution to slagging may be to reduce the
fixed carbon burning rate by decreasing the oxygen concentration
over the bed. This solution reduces overheating on the
combustion hearth above. Another solution may be to increase the
air flow into the volatile burning hearth after reducing the air
on the fixed carbon burning hearth. Slagging problems with
thermally conditioned sludges can be minimized by sizing burners
for 100 percent excess air. Slagging caused by ash melting when
passing through the burner flame can be eliminated by reducing
flame temperature through use of higher burner excess air. Other
alternatives to alleviate slagging problems include:

- Providing lance or poke holes at perimeter drop hole
locations

51


Each incinerator facility should have a detailed
Management plan that includes procedures to collect,
review, and analyze process data for trend analyses
and process optimization; communication channels
between operators of solids handling processes; and
procedures to evaluate system and personnel
performance.

Training should be provided for operations,
maintenance, and management personnel with emphasis on
"hands-on" training. This training should be
routinely updated to reinforce process control,
troubleshooting techniques, and preventive maintenance
procedures.

A skilled and qualified staff must be present to
monitor and optimize the incineration process.
Management must support this staff by maintaining
adequate spare parts and supplies and by providing
training and commensurate pay.

60


FOREWORD

The construction of more wastewater treatment facilities
with higher levels of treatment has resulted in the need to treat
and dispose of larger amounts of sewage sludge. In recent years,
sludge incinerators have been frequently considered and often
installed as a final disposal alternative. Many of these were
constructed using financial assistance from the construction
grants program of the U.S. Environmental Protection Agency.

Many municipal sludge incinerators have experienced a
variety of design and operational problems. In addition, the
increased cost of energy during the past decade, as well as an
increasing awareness of possible air pollution problems from
incinerator emissions, has raised serious concerns over the
suitability of incinerators for sludge disposal. EPA's Water
Engineering Research Laboratory in Cincinnati, Ohio has studied
sludge incinerators to identify the nature and extent of design
and operational problems, to identify possible problem solutions,
and to determine the applicability of the technology for use as
part of municipal sludge treatment systems.

This summary document is based on that EPA study and is
intended to provide a basic understanding of sludge incineration,
as well as concise information on design considerations,
operational characteristics, and process and equipment problems
and possible solutions. The document will be useful to design
engineers, governmental agency review personnel, municipal
officials, operators, and others who are considering using sludge
incineration in a sludge treatment train, or who are concerned
with optimizing performance of an existing sludge incinerator.
The information in this summary supplements detailed guidance
available elsewhere, which should be considered when making
design or operating decisions. Improvements in the technology,
the ability to integrate the technology into the total treatment
process, and the compatibility of the process with the plant
environment must be considered along with associated costs in
comparing this technology with other treatment alternatives.

iii


increase in the rate of arch settlement indicates an impending
problem. Refractory behavior is relatively unpredictable,
however, and hearths that started out almost flat or even
slightly negative in slope from perimeter to center have been
known to last a year or more. If the hearths rise on initial
heating, they will generally have a normal service life.

Rabble Arms, Teeth, and Central Shaft

Rabble arms are subject to overheating and accumulating
sludge on top of them. A steady sludge feed and adequate cooling
air flow can prevent overheating of the rabble arms and are vital
to rabble arm life. Sufficient cooling air is dependent upon the
design and size of the furnace and central shaft. The
temperature of the cooling air at the central shaft cooling air
stack should be maintained between 250 F and 350 F. Castable
refractory insulation can also protect rabble arms from
overheating and appears to be effective. By placing the rabble
arms for each hearth at an angle to those on adjacent hearths,
accumulation of sludge on the rabble arms of the lower hearths is
avoided.

Rabble teeth may bend if exposed to short term high
temperatures. The solution is to control temperatures by
maintaining a steady sludge feed. Corrosion of the teeth is
generally caused by chlorides and can be minimized by careful
control of ferric chloride use in liquid and solids treatment.
In addition, use of a two-part tooth avoids replacement of the
tooth holder.

Central shaft problems include insulation anchoring and
cracks. Various means have been used to anchor the castable
refractory insulation to the shaft. None have been entirely
effective due to the difference in coefficients of expansion for
the cast iron shaft and the insulation material. Cracks in the
insulation can be repaired with patches, but proper selection and
application of the patch material are required.

Minor expansion cracks in the cast iron shaft should not
be a major concern. The cast iron can be exposed to the fire as
long as the area receiving the radiation is small and the
surrounding cooling wall area is large.

Induced Draft Fans

Serious problems with improperly selected induced draft
fans may be correctable, but often at considerable cost. The
long term effect of operation with a deficient fan can be
extremely costly in terms of reduced sludge disposal capacity,
limited furnace operating time, and excessive power
consumption. Major problems with ID fans are mainly attributable

40


hearth ignites, temperatures on that hearth and all hearths above
may rise, causing burning on yet a third hearth.

These conditions result in unstable combustion in a MHF
due to high temperatures and a lack of sufficient air for
complete combustion. Without this air, the furnace may produce
smoke or exceed safe temperature limits, depending on furnace
control setting and operating conditions prior to the change.
When combustion occurs on the upper hearths, combustion space is
inadequate, and the lower hearths are rendered useless. These
conditions are extremely serious and result in flue damage and
severe smoking, in addition to limiting MHF capacity to a level
far below the design rating.

On a long term basis, if feed sludge is dryer than
anticipated during furnace design, the upper hearths of existing
MHFs may be bypassed by cutting drop holes in the hearths. This
would improve control of the furnace to eliminate double burning,
reducing fuel requirements. However, the previously noted
increase in combustion air needs would not be changed and
sufficient air would have to be supplied.

Because dewatering and incineration operations within a
treatment facility are often separate, the incinerator operator
may not be aware that short term sludge feed rate or quality
changes have occurred until temperature readouts indicate a
markedly changed furnace condition. This effect may not be
apparent in the MHF until the sludge has passed through several
hearths. MHF response to new burner settings and central shaft
speed changes is very slow, and these changes usually are best
made slowly. Because of this sensitivity to changes in sludge
characteristics, the ability to measure temperature and excess
air in the burning zone of a MHF is the key to furnace control.

The effects of variable sludge feed rates on the operation
of FBF units are different from those in the MHF. Generally, the
FBF requires a forced feed injection system to overcome the
pressure in the furnace. Feed rate surges do not occur because
the screw or the progressive cavity pump used to feed the furnace
operates at a set speed or rate. A sludge with an unusually high
solids content may cause “over pressure" stoppage of the feeder
or plugging due to the increased viscosity of the sludge.

The effects of varying the solids content of feed sludge
on a FBF are similar to those for a MHF. If the solids content
of feed sludge is increased from 27 to 30 percent and the sludge
feed rate and combustible content remain unchanged, the feed rate
of combustibles is increased by 10 percent. If the FBF was
operating at 25 percent excess air originally, this same air
flowrate would be equivalent to 15 percent excess air with the
higher solids content sludge. At this lower excess air level,
the chance for smoking is greater. In the FBF, the time needed

38




diameters ranging from 4.5 feet to 29 feet and can have 4 -to
14 hearths. Two access doors with observation ports are
generally provided at each hearth. The rotating central shaft is
a hollow iron column cast in sections. The shaft is normally
insulated with castable refractory, which is a mixture of heat-
resistant aggregate and cement. Insulation renders the shaft
suitable for temperatures of about 800°F for continuous operation
and 1,100°F fer short term operation. Shaft speed is adjustable
between 1/2 afid 2-revdlutions per minute (rpm). Dewatered sludge
is fed into the furnace at the top hearth and proceeds downward
through the furnace from hearth to hearth, moved by rotating
rabble arms with rabble teeth or plows attached to the central

shaft. The arms are normally 25 percent chrome, 12 percent
nickel alloy castings. The rabble arms constantly move the
sludge in the hearths, aiding drying and burning. Ash is

discharged from the bottom of the furnace, and the exhaust gases
are discharged from the top of the furnace.

Air for combustion and cooling of the shaft and rabble
arms is supplied by a fan. A cold air tube from the fan runs up
the center of the shaft; air lances extend from the tube to the
end of each rabble arm as seen in Figure 2. Ambient air is blown
through the cold air tube and lances. The cold air exits from
the lance tips, flowing back to the annular space in the shaft
through the space between the lances and the rabble arm wall.
This flow of air cools the shaft and rabble arms by convection.
The central shaft cooling air is returned to the bottom hearth of
the furnace to be used as sludge combustion air. If all the air
is not needed for combustion, it is discharged to the atmosphere
through the central shaft cooling stack. Because the heated
central shaft cooling air is not contaminated with combustion
air, it may also be used for direct forced warm air heating of
the furnace area. An MHF can also have a combustion air blower
which supplies auxiliary air to the combustion hearth.

The functions of drying the wet feed, combusting sludge
volatiles, complete burning of fixed carbon, and cooling ash are
performed in distinct zones of the furnace from top to bottom as
seen in Figure 3. The first zone (drying zone) consists of the
upper hearths where heated combustion gases flow upward
countercurrent to the descending sludge, thereby drying and
heating the sludge. The second zone (combustion zone) generally
consists of the central hearths. In this zone, the majority of
volatile organics are burned and some of the fixed carbon in the
sludge begins combustion; temperatures reach between 1,400°F and
1,700°F. In the third zone (fixed carbon burning zone), the
burning of the fixed carbon continues and is completed. Ash is
cooled and discharged from the fourth zone, utilizing returned
central shaft air for cooling. The sequence of these zones is
always the same, but the number of hearths employed in each zone
is dependent on the characteristics of the feed sludge and the


separator in a more corrosion-resistant material, such as silicon
carbide.

Because the control actuator on variable throat venturis
may tend to stick and cause erratic operation, the actuator must
be properly sized and an adequate control system provided.

Excessive water carryover from the exhaust stack results
from an inadequately designed demister section in the scrubber.
Proper baffling and drains within the scrubber avoid this
problem.

OPERATION AND MAINTENANCE PROBLEMS

Improved operation of an incinerator system is heavily
dependent on the ability to deal with common operation and
maintenance problems of slag and clinkers; improperly adjusted
burners; and screenings, grit, and scum disposal.

Slag and Clinkers

Clinkers are free lumps occurring in the burning bed of
MHFs that are carried to the cooling hearths. Slag, which can
occur in a MHF or FBF, is the accumulation of fused material on
walls and dropholes, rabble arms, etc. Failure to recognize the
ash melting temperature under both oxidizing and reducing
conditions and failure to make proper tests has resulted in
severe operational problems and increased maintenance costs due
to slag and clinker formation.

. Siag and clinkers result from high temperatures (above
1,650 F) in the combustion zone. These temperatures are caused
primarily by variable sludge feed and, to a lesser extent, by
poor burner control. Observations indicate that the highest
temperatures occur in the upper portion of the combustion hearth
in a MHF where the sludge volatiles mix with air when they are
passing through drop holes in a MHF. The greatest slagging
potential is observed to occur at the drop holes, particularly in
the outer drop hole area. The intense heat from combustion heats
the refractory at the drop hole; when fine ash particles contact
the drop hole, they adhere and build up. Eventually, the drop
holes can plug entirely. To minimize this problem, lance ports
should be located in this area.

The problem of clinkers deserves more extensive treatment
than can be addressed in this report. Operators with these
problems should seek expert advice to:

- Determine where and how much air enters the furnace,
and quantify and adjust burner excess air as necessary

50


SECTION 4

SUMMARY OF DESIGN AND OPERATIONAL CONSIDERATIONS

The selection, design, and operation of a MHF or FBF
incineration system must take into account the integration of the
system into the overall treatment process and the complexities of
the system. Failure to do this has led to many of the past and
current problems that have plagued MHF and FBF incineration
processes. Under proper conditions, these processes can be an
effective part of the sludge processing train.

This section summarizes the more important design and
operational factors to review when considering MHF or FBF
incineration as part of a new or upgraded treatment facility, or
when optimizing the performance of an existing facility.

IMPROVING SYSTEM DESIGN

When planning a MHF or FBF system, the following process
and support system equipment design factors should be considered:

- The selection of an incineration system should be
carefully considered in recognition of plant size and
anticipated sludge production. As a_ general
guideline, FBF systems are most applicable at plants
of 10 mgd or more, and MHF systems at plants of 20 mgd
or more.

- The design of an incineration process should integrate
the incineration and support systems into one design
by developing operating scenarios, calculating heat
and material balances, and defining operating
ranges. One overall system safety factor should be
applied to the estimated quantities of sludge.

- Incineration and support system equipment selection
and sizing should be carefully matched to the rate and
characteristics of sludge production expected from the
mainstream process. Either units that can be
incrementally modified or multiple units should be
provided, where needed, to avoid inefficient operation
of oversized component parts.

56


SLUDGE

FLOW NORMAL
AIR
TEMPERATURES
600° to
DRYING ZONE 900°F
200° to
COMBUSTION ZONE 1700°F
FIXED CARBON 1400° to
BURNING ZONE 1800°F
ASH COOLING o
ZONE 300°F+

AIR
FLOW

SOURCE. U. S. EPA PROCESS DESIGN MANUAL FOR
SLUDGE TREATMENT & DISPOSAL
EPA 625/1-79-011 (MODIFIED)

FIGURE 3. PROCESS ZONES IN A MULTIPLE-HEARTH FURNANCE

10


This document is condensed from an EPA research report
entitled “Improving Design and Operation of Multiple-Hearth and
Fluid Bed Sludge Incinerators", which will be available in late
1985. That report has been subjected to the United States
Environmental Protection Agency's peer review. This document has
undergone an EPA administrative review and has been found to be
consistent with the EPA research report referenced above. The
information in this document is made available for the use of the
technical community. The information contained herein does not
constitute EPA policy, guidance or directive. Design engineers,
municipal officials, and others are cautioned to exercise care in
applying this general information to particular circumstances of
individual wastewater treatment facilities. EPA assumes no
responsibility for use of this information in a particular
situation. Mention of trade names or commercial products does
not constitute endorsement or recommendation for use.

ii


Maintenance labor for MHF and FBF systems is approximately
equal.

Inorganic Chemical Conditioning

Lime, metal salts, and polymers are frequently used as
chemical aids in wastewater treatment processes and in sludge
conditioning. The burning of sludges containing metal salts from
these processes has caused severe slagging and clinkering. Some
FBFs that have burned municipal sludge containing ferric chloride
have had slagging so severe as to plug the gas outlet from the
furnace. Burning sludge containing polymers has caused
clinkering in some MHFs.

In both types of furnaces, sludges containing lime or
metal salts increase O&M costs, fuel consumption, and
corrosion. Higher operating and maintenance costs result from
the added ash and clinkers produced when using these chemicals.
The additional inert materials from chemicals fed into the
furnace also increase fuel consumption. If calcining
temperatures are reached, the endothermic reaction of lime will
require additional energy. At high temperatures the chlorides
carried in ferric chloride sludges will result in accelerated
corrosion of metal parts.

31


6T

SCREW CONVEYOR

FURNACE

CONDITIONER

ASH DISCHARGE
TO LANDFILL

} “ELEVATOR

t— COUNTERWEIGHT
TAKE-UP

SOURCE: BEAUMONT BIRCH COMPANY, NJ
(MODIFIED)

FIGURE 8. EXAMPLE OF MECHANICAL ASH HANDLING SYSTEM


MULTIPLE-HEARTH AND FLUID BED SLUDGE INCINERATORS:
DESIGN AND OPERATIONAL CONSIDERATIONS

by

Metcalf & Eddy, Inc.
Wakefield, Massachusetts 01880

Project Officer

Francis L. Evans III
Wastewater Research Division
Water Engineering Research Laboratory
Cincinnati, Ohio 45268

OFFICE OF MUNICIPAL POLLUTION CONTROL
OFFICE OF WATER
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460


suspended from the furnace wall in the freeboard and fluidizing
sand bed areas and are less subject to physical impact.

Refractory Dome

In FBFs, failure of the refractory dome beneath the sand
bed has been as severe a problem as hearth failure in MHFs. Just
as some MHFs have had long hearth life and others have had
frequent failures, the FBF refractory dome experiences are also
varied, Maintaining constant temperatures or heating and cooling
slowly are normally the key factors in avoiding problems.

An improved practice for FBFs is to introduce the
fluidizing air into a recessed chamber with a burner that heats
the air to prevent localized hot and cold areas in the brick
arch. This practice is probably more important when oil is used
as the auxiliary fuel than with natural gas because of the
luminosity of oil flames. In all cases, consideration should be
given to uniform temperatures on brick domes or arches during
heating.

During startup of a hot windbox FBF, a preheat burner is
used to heat fluidizing air and bring the sand bed to operating
temperatures. Once operating temperatures are reached, they are
maintained by auxiliary burners in the furnace, and the
fluidizing air is heated using waste heat. At this point, the
preheat burner is turned off. When this is done, the burner
should be shut down slowly, allowing the windbox temperature to
gradually reach equilibrium with that of the fluidizing air. The
windbox temperature should not be allowed to drop more than 50 F
per hour to avoid damage to the refractory dome.

FBF Air Distribution Pipes

Maintenance of FBF air distribution pipes can be
difficult. Keeping the pipes locked in place in brick hearths
and sufficiently tight to prevent sand from leaking into the
windbox during shutdown periods is a problem because the metai
pipes have a coefficient of expansion three times that of the
brick hearth. Proper design of the arch in the hearth, which
results in air pipes of different bends and lengths, permits
controlled expansion of piping without allowing sand to leak.

Exhaust Flue Corrosion

Chlorides and sulfur in dewatered sludge cake are
transformed during the combustion process into corrosive
constituents that damage metal components in the furnace and flue
gas system.

At incinerator temperatures, most chloride salts react
with water vapor to produce hydrochloric acid (HCl). Many

43


In general, incineration is cost-effective in larger
installations where the support facilities discussed below are
readily available and alternative disposal options are limited.
An approximate plant size at which incineration becomes feasible
is about 20 mgd for a MHF system and 10 mgd for a FBF system.
Other factors that dictate practical plant size are:

- The opportunity to incorporate energy saving features
into the overall treatment process by waste heat
recovery, steam generation, use of the incinerator for
odor control of off-gases from other processes, and
the use of digester gas for auxiliary fuel.

- The availability of O&M personnel with the
comparatively sophisticated training required to
operate and maintain incineration units. Maintenance
and instrumentation personnel with the necessary
skills are not normally present at smaller plants, and
service contracts are usually quite costly.

- The ability to finance the considerable spare parts
inventories and preventive maintenance programs needed
to minimize downtime, which may not be feasible in
small plants.

For smaller facilities, the FBF is available in smaller
units than the MHF, and the FBF has the flexibility to be
shutdown for short periods without using significant amounts of
auxiliary fuel to restart or maintain heat. This allows small
plants to operate with one shift per day.

Design Comparison

Differences in design between the MHF and the FBF are
primarily in the areas of ash handling, incinerator construction,
feed solids content, and level of instrumentation.

- The MHF can use a dry ash handling system; its dry ash
is lighter and therfore cheaper to haul and landfill
than is wet ash from a FBF.

- The MHF does not require a regular supply of bed media
{sand) and is not likely to incur as much erosion
damage in the exhaust gas system as is the FBF.

- The FBF system requires gas tight construction. MHFs
are under negative pressure and must be designed to
minimize air infiltration to maintain thermal
efficiency, but they are not necessarily gas tight.

- Both incinerator systems must have properly designed
furnace refractories and flues.

27


Before specifications are finalized, heat and material
balances should be revised and finalized for each operating
scenario and a table prepared indicating the following:

- Sludge combustion air required, as both mass flowrate
and volume rate, in pounds per hour and standard cubic
feet per minute, respectively

- Shaft cooling air recycle (if a MHF is considered), in
pounds per hour

- Ambient air temperature, in °P

- Auxiliary fuel required, in Btu per hour, or in fuel
volume terms, including fuel analysis and
characteristics

- Auxiliary fuel combustion air required, in pounds per
hour and standard cubic feet per minute

- Furnace exhaust flue gas volume, in actual cubic feet
per minute.

After this table of values is completed, a summary table
indicating the minimum and maximum values for each parameter
should be prepared. The summary table should then be examined to
determine whether the desired capacity range of individual
equipment items is within the useful and feasible operating range
of available equipment.

The problem of excess furnace capacity in the initial
years of operation may be addressed by two design approaches.
One approach is to install furnaces that can be incrementally
modified to activate. the use of more hearths and/or combustion
volume as sludge quantities increase. An alternative approach is
to use smaller multiple units to achieve incremental increases in
plant furnace capacity. Although multiple units increase the
capital cost of the plant, they provide considerable flexibility
and increased reliability. If all of the projected furnace
capacity is not initially installed, benefits derived from
advanced technology and onsite system improvements may be
realized when the additional units are installed in the future.
The applicability of either approach is dependent on the
projected growth rate of the sludge load as well as other factors
unique to each individual treatment facility.

In existing MHFs, modifications are possible to minimize
the impact of oversizing. Modifications can be made to the MHF
to reduce the number of sludge processing hearths by cutting
holes in the upper hearths to allow sludge to be fed two or three
hearths lower. The burners on the upper hearths may be sealed
off or used as afterburners if required. Reduced gas flows to

36


- An auxiliary fuel system incorporating flame safeguard
controls for burners and a separate furnace fuel
injector system.

- A furnace freeboard temperature control system,
consisting of a high pressure water pump and sprays.

- A gas-to-air heat exchanger for hot windbox FBFs.

- A heat recovery system consisting of a convective
waste heat boiler ahead of the venturi scrubber.

DESIGN IMPROVEMENTS

Contemporary design and operation of MHF and FBF systems
incorporates procedures that conserve auxiliary fuel. These
diverse fuel-saving procedures range from improved’ sludge
dewatering to modifications of the furnace itself. Because the
focus of this document is on improvements to incinerator systems,
this discussion includes a brief summary of operating variables
that affect fuel consumption and a description of two
installations where fuel-saving designs and procedures have been
undertaken.

Multiple-Hearth Furnace

The primary factors that affect fuel consumption in the
MHF are the sludge feed rate and its fuel value. Although these
factors are often not subject to control, the sludge combustion
air flowrate, the auxiliary fuel and sludge combustion rate, and
the rotational speed of the rabble arms are operating variables
that can be controlled. Manipulation of these variables will
directly affect the following:

- Exhaust temperature (temperature of the uppermost
hearth)

- Excess air in the exhaust gas

- Temperature of the gas in the combustion zone.
In order to conserve auxiliary fuel consumption in the MHF, these
variables should be controlled. If not, the following operating
conditions can result, leading to high fuel consumption:

- High incinerator exhaust temperatures

- High combustion zone location in the incinerator

- Greater draft than necessary

22



1,021°F; this temperature is higher than that used for volatile
solids measurement for solid fuels and includes a portion of the
fixed carbon. The terms volatiles and combustibles will be used
interchangeably in this report in accordance with wastewater
industry practice.

Solids with a high percentage of volatiles, such as grease
and scum, haye:high—fuel values. Grit or chemical precipitates
do not have igh fu@1 values because of the large percentage of
inert material in them, and they require auxiliary fuel to burn.

Incinerator operations require air in excess of
theoretical requirements to achieve complete combustion. The
excess air increases the opportunity for contact between the
oxygen contained in the air and the fuel. To ensure complete
combustion, air volumes of 50 to 150 percent in excess of
theoretical requirements must be provided in the combustion
zone. When the amount of excess air is inadequate, only partial
combustion of carbon occurs, and carbon monoxide, soot, and
odorous hydrocarbons are produced.

The amount of excess air required varies with the type of
incinerator, characteristics of the sludge, and the disposition
of the stack gases. Cost-effective operation requires that
excess air be minimized to reduce energy consumption while still
achieving complete combustion. Energy will be consumed by the
operation of air blowers and by using supplemental fuel to raise
the temperature of the combustion products and excess air from
ambient to that of the combustion zone.

The amount of supplemental fuel required is not only
dependent upon the amount of excess air needed for complete
combustion, but also on the water content of the sludge,
radiation losses; and the heating of gas streams and sludge feed
solids. The heat released by the burning sludge must be
sufficient to raise the temperatures of the air and all
substances in the incoming sludge from ambient levels to those of
the exhaust and ash and to compensate for any radiant heat loss
from the incinerator. If the available heat from sludge burning
is sufficient to maintain combustion without the addition of
supplemental fuel, the process is termed autogenous.

Details of combustion theory and procedures to determine
heat balances and fuel requirements are presented in the EPA
publication entitled "Process Design Manual for Sludge Treatment
and Disposal" (2).

Multiple-Hearth Furnace

A cross-section of a MHF is shown in Figure l. The
furnace consists of a circular steel shell with a series of
horizontal hearths made of fire bricks. MHFs are available in


+ a freeboard temperature control system with high
pressure water pump and sprays.

IMPROVING EXISTING SYSTEMS

Although recent technical investigations have primarily
focused on operational improvements to MHFs, improvements to the
support systems, greater efficiency in sludge dewatering, and the
principle of more efficient sludge drying are applicable to both
types of furnaces. The design factors discussed above and the
following key factors from technical investigations should be
used as guides for determining needed modifications to improve
operation and increase the efficiency of an existing MHF or FBF
incinerator system.

‘= Incorporation of sludge dewatering systems that will
produce dryer sludge with an increased volatile solids
content and a more homogeneous product.

- Development of sludge feed systems that will provide a
more constant, continuous, and controllable feed rate
to furnaces.

- Utilization of sludge conditioning chemicals, such as
polymers, that will not produce the slagging and
corrosion problems associated with the use of ferric
chloride.

- Installation and proper maintenance of sludge and ash
conveyance systems to reduce abrasion and to improve
performance.

- Maintenance of design furnace capacity by repair of
all equipment-related air-leakage into an MHF and
minimization of the draft in the top hearth of the
furnace.

- Minimization of hot air heat loss by maximization of
central shaft cooling air return.

- Improvements in sludge drying by reducing central
shaft speed or by reducing or eliminating active
burners above the combustion zone in a MHF and by
using one FBF for sludge drying in a multiple FBF
installation.

- Control of excess air rates and reduction of heat loss

by using an oxygen analyzer to indicate the type of
combustion in a MHF (high/low excess air).

58


metallic chlorides first vaporize, then react with water vapor in
the gas phase, leaving an extremely fine metallic oxide dust plus
hydrochloric acid vapor. Hydrochloric acid is corrosive in the

hot dry state, as well as upon condensation. At metal
temperatures above 800°F, alloys of the type used in MHF rabble
arms and teeth are subject to dry corrosion by HCl. This

corrosion is not generally a noticeable problem but can
contribute to shortened rabble tooth life.

Sources of chloride include sludge conditioning with
ferric chloride, industrial contributions, and saltwater
intrusion into the sewer system. Where ferric chloride is used
to coagulate sludge solids, the ferric chloride will contribute
chloride to the water. The water retained in the dewatered cake
contains the same chloride concentration. The chloride in the
wet cake will be present in the exhaust gas.

Unlike HCl, $04/S03 is corrosive only upon condensation
with water or absorption in water. The most critical corrosion
conditions occur when the temperature of the steel scrubber shell
and flue drop momentarily below the dew point of the furnace
combustion gas, causing condensation to occur. This absorbs HCl
and 803/803 to create acidic solutions. When the temperature
rises or the partial pressure of water diminishes within the flue
duct or vessel shell, the water evaporates, leaving concentrated
acid on the steel. Repeated occurrences of these conditions will
produce concentrations of hydrochloric, sulphuric, and sulphurous
acids, which attack the metal surface on which they
concentrate. These conditions are considerably lessened in areas
where in-leakage of air occurs, diluting the concentration of
corrosive gases near the steel surface.

FBFs are constructed to operate under positive pressure.
The higher pressure reduces the temperature at which the acids in
the exhaust gases condense, causing corrosion in exhaust ducts
and at expansion joints. Although MHFs are not immune to the
same phenomenon, the situation is mitigated because MHFs are not
pressurized and are subject to air infiltration.

Heat exchangers and waste heat boilers located ahead of
the scrubber frequently have not been designed with consideration
for local cold spots (e.g., first tubes contacted by the entering
cold fluid) where metal temperatures are below the acid vapor dew
points. A general rule for waste heat boilers is to generate
Saturated steam greater than 300 psi to maintain boiler metal
temperatures safely above 400°F and to raise steam generation
pressures before acid bearing gases are allowed into the boiler,
unless other specific anti-corrosion measures are taken.

If the boiler is to be bypassed, recognizing that the
dampers cannot make positive gas-tight closure, the boiler casing

44


either

diverted to the other chute for removal. This
solution helps to protect ash conveying screws and
bucket elevators.

Chains and sprockets must be made of materials with
high hardness to protect against the abrasive ash.
The sprockets should be harder than the chain to
prolong sprocket life.

Bucket elevators should never be loaded greater than
80 percent of capacity and loading should be
considerably less.

Most bucket elevators have two chains, one on each
side of the bucket. One chain is driven, and the
other chain is on idler sprockets and used to maintain
bucket alignment. Both chains should be used to avoid
misalignment.

Helical screws in covered troughs move ash horizon-
tally or on an incline. Hard iron bearings support
the screw and increase the bearing life against the
abrasive ash. Hard iron bearings can be very noisy,
generating a screeching sound. Grease lubrication may
reduce the noise problem, but will reduce bearing life
as ash becomes entrained in the grease. Wooden
bearings can be used to solve the problem.

The covers on screw conveyors are very difficult to
keep dust tight. If all the gaskets, screws and clips
on the cover are used, the dust is reduced. After
maintenance, the covers should be put back with all
fasteners in place.

Ash conditioning screws and rotary drum ash
conditioners require proper maintenance to wet the ash
sufficiently to minimize landfill dust problems, and
generally are only moderately successful.

Conditioning screws must be equipped with adequate
horsepower drives for moving the wet ash.

Control of the water for wetting requires frequent
operator attention due to changing ash characteristics
and feed rates.

The ash conveyors should have a vacuum dust removal
system at the transfer points to minimize fugitive
dust.

Hydraulic Ash Handling. Wet ash systems can be used with

the MHF or FBF. Problems with hydraulic ash systems are

46


|
| =
fa OUT HEARTH Yi
LUTE CAP i imp
a
ae
iy
AZ LR STEEL
IN HEARTH A i SHELL
RABBLE ARM —~ a | j
C Aan
HOT AIR = LJ CJ LJ ate
COMPARTMENT ||
COLD AIR
TUBE TN | ARM HOLDING PIN
insucation— 4 t |
i Fire
+ BRICK
Y
AIR LANCE |
RABBLE ARM TEETH an
—=
SAND SEAL
CENTRAL SHAFT
<_ Tr] S GEAR DRIVE
‘AN —>
al
COOLING
r) AIR <+—_
\
SHAFT COOLING
TS O AIR FAN
AIR HOUSING
H a |
f 5 iz i

SOURCE: U. S. EPA PROCESS DESIGN MANUAL FOR
SLUDGE TREATMENT & DISPOSAL
EPA 625/1-79-011

FIGURE 2. INTERIOR CUTAWAY VIEW OF A MULTIPLE-HEARTH FURNACE