Nicholas P. Cheremisinoff. Handbook of Solid Waste Management and Waste Minimization Technologies

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INCINERATOR ASH PROPERTIES

Table 2 provides lierature reported values of physical property characterization

data for sludge ash. Sludge ash is a silty-sandy material. A relatively large

fraction of the particles (up to 90% in some ashes) are less than 0.075 mm (No.

200 sieve) in size. Sludge ash has a relatively low organic and moisture content.

Permeability and bulk specific gravity properties are not unlike those of a natural

inorganic silt. Sludge ash is a nonplastic material.

Sludge ash consists primarily of silica, iron, and calcium. The composition of the

ash can vary significantly and depends in great part on the additives introduced in

the sludge conditioning operation. There are no specific data available relative to

the pozzolanic or cementitious properties of sludge ash, but sludge ash is not

expected to exhibit any measurable pozzolanic or cementitious activity. Table 3

lists the range of major elemental concentrations present in sludge ash.

Trace metal concentrations (e.g., lead, cadmium, zinc, copper) found in sludge

ash are typically higher than concentrations found in natural fillers or aggregate.

This has resulted in some reluctance to use this material; however, recent

investigations (leaching tests) suggest that these trace metal concentrations are not

excessive and do not pose any measurable leaching problem.

Table 2. Typical Physical Properties of Sewage Sludge Ash

Property

Gradation (% passing)

4.76 mm (No. 4 sieve)

2.38 mm (No. 8 sieve)

2.00 mm (No. 10 sieve)

0.85 mm (No. 20 sieve)

0.42 mm (No. 40 sieve)

0.21mm (No. 80 sieve)

0.149 mm (No. 100

sieve)

0.074 mm (No. 200

sieve)

Wegman

(10)

Values

Khanbiluardi

(11)

100

Waste

Commission

(6)

100

Gray

(15)

100

47-93

Property

Gradation

passing)

(0.0902

mm)

0.02

0.005

> 0.001

Loss

Ignition

(%)

Moisture Content

Total Weight)

Absorption

(%)

Specific Gravity

Bulk Specific Gravity

Plasticity Index

Permeability

(ASTM D2434

cm/sec)

Wegman

(10)

10-13

Values

Khanbiluardi

(11)

Waste

Commission

(6)

1.4

(10)

0.28

(11)

1.6

(6)

2.60

(10)

2.61

(6)

2.44

2.96

(15)

2.39

2.99

(2)

1.82

(11)

1.27-

1.48

(2)

Nonplastic

(10)

x 10"

10"

- 4 x

10"

4>(6)

Gray

(15)

2-13

Element

Silicon

(Si)

Calcium

(Ca)

Iron

(Fe)

Aluminum

(Al)

Magnesium

(Mg)

Sodium

Oxide

(SiO

)

(CaO)

(Fe

)

(Al

)

(MgO)

(Na

Concentration

Reported

elemental

concentration*

5.6-25.7

1.4-42.9

1.0-16.4

1.1-8.5

0.6-1.9

0.1-0.8

Reported

elemental

concentration^

0.3

Reported

oxides

(10

16)

27.0

21.0

8.2

14.4

3.2

0.5

Reported

oxides

(15)

14.4-57.7

8.9-36.9

2.6-24.4

4.6-22.1

0.8-2.2

0.1-0.7

Table

Typical Range

Elemental Concentrations

Sewage Sludge

Ash

References at the end of this chapter may be consulted for the sources of data in

Tables 2 and 3 as well as additional information.

MULTIPLE HEARTH INCINERATORS

This incinerator is the most prevalent incinerator technology for the disposal of

sewage sludge in the United States because of its low ash discharge. Sludge cake

enters the furnace at the top. The interior of the furnace is composed of a series

of circular refractory hearths, which are stacked one on top of the other. There

are typically five to nine hearths in a furnace. A vertical shaft, positioned in the

center of the furnace has rabble arms with teeth attached to them in order to

move the sludge through the mechanism. Each arm is above a layer of hearth.

Teeth on each hearth agitate the sludge, exposing new surfaces of the sludge to

the gas flow within the furnace. As sludge falls from one hearth to another, it

again has new surfaces exposed to the hot gas. At the top of the incinerator there

is an exit for flue gas, an end product of sludge incineration. At the bottom of the

furnace there is an exit for the ashes. Figure 8 illustrates the key design features.

Partially dewatered sludge is fed onto the perimeter of the top hearth. The rabble

arms move the sludge through the incinerator by raking the sludge toward the

center shaft where it drops through holes located at the center of the hearth. In

the next hearth the sludge is raked in the opposite direction.

Element

(Na)

Potassium

(K)

Phosphorus

Sulfur (S)

Carbon (C)

Oxide

)

(SO

)

Concentration %

Reported as

elemental

concentration^

0.3-1.6

1.2-4.4

0.3-1.2

0.6-2.2

Reported

elemental

concentration^

0.5

Reported

oxides

(10

16)

0.6

20.2

0.9

Reported

oxides

(15)

0.07-0.7

3.9-15.4

0.01-3.4

Figure 8. Multiple hearth incinerator.

This process is repeated in all of the subsequent hearths. The effect of the rabble

motion is to break up solid material to allow better surface contact with heat and

oxygen. A sludge depth of about 1 in. is maintained in each hearth at the design

sludge flow rate. Scum may also be fed to one or more hearths of the

incinerator. Scum is the material that floats on waste

water.

It is generally

composed of vegetable and mineral oils, grease, hair, waxes, fats, and other

materials that will float. Scum may be removed from many treatment units

COMBUSTION

ZONE

COMBUSTION

AIR RETURN

RABBLE

ARM

EACH

HEART

SLUDGE INLET

FLOATING DAMPER

COOLING AIR DISCHARGE

FLUE GAS OUT

DRYING

ZONE

COOLING

ZONE

ASH

DISCHARGE

COOLING AIR

FAN

RABBLE ARM

DRIVE

including preaeration tanks, skimming tanks, and sedimentation tanks. Quantities

of scum are generally small compared to those of other wastewater solids.

Ambient air is first ducted through the central shaft and its associated rabble

arms.

A portion, or all, of this air is then taken from the top of the shaft and

recirculated into the lowermost hearth as preheated combustion air. Shaft

cooling air which is not circulated back into the furnace is ducted into the stack

downstream of the air pollution control devices. The combustion air flows

upward through the drop holes in the hearths, countercurrent to the flow of the

sludge, before being exhausted from the top hearth. Air enters the bottom to

cool the ash. Provisions are usually made to inject ambient air directly into the

middle hearths as well.

From the standpoint of the overall incineration process, multiple hearth furnaces

can be divided into three zones. The upper hearths comprise the drying zone

where most of the moisture in the sludge is evaporated. The temperature in the

drying zone is typically between 425 and 76O

C (800 and 1400

F). Sludge

combustion occurs in the middle hearths (second zone) as the temperature is

increased to about 925

C (1700

F). The combustion zone can be further

subdivided into the upper-middle hearths where the volatile gases and solids are

burned, and the lower-middle hearths where most of the fixed carbon is

combusted. The third zone, made up of the lowermost hearth(s), is the cooling

zone.

In this zone the ash is cooled as its heat is transferred to the incoming

combustion air.

Multiple hearth furnaces are sometimes operated with afterburners to further

reduce odors and concentrations of unburned hydrocarbons. In afterburning,

furnace exhaust gases are ducted to a chamber where they are mixed with

supplemental fuel and air and completely combusted. Some incinerators have the

flexibility to allow sludge to be fed to a lower hearth, thus allowing the upper

hearth(s) to function essentially as an afterburner.

Under normal operating condition, 50 to 100% excess air must be added to an

MHF in order to ensure complete combustion of the sludge. Besides enhancing

contact between fuel and oxygen in the furnace, these relatively high rates of

excess air are necessary to compensate for normal variations in both the

organic characteristics of the sludge feed and the rate at which it enters the

incinerator. When an inadequate amount of excess air is available, only partial

oxidation of the carbon will occur, with a resultant increase in emissions of

carbon monoxide, soot, and hydrocarbons. Too much excess air, on the other

hand, can cause increased entrainment of particulate and unnecessarily high

auxiliary fuel consumption.

FLUIDIZED-BED INCINERATOR

The basic configuration and features of the fluid bed incinerator have already

been described. This technology has been around since the early 1960s. In this

system, air is introduced at the fluidizing air inlet at pressures of 3.5 to 5 psig.

The air passes through openings in the grid supporting the sand and creates

fluidization of the sand bed. Sludge cake is introduced into the bed. The

fluidizing air flow must be carefully controlled to prevent the sludge from

floating on top of the bed.

Fluidization provides maximum contact of air with sludge surface for optimum

burning. The drying process is practically instantaneous. Moisture flashes into

steam upon entering the hot bed. Some advantages of this system are that the

sand bed acts as a heat sink so that after shutdown there is minimal heat loss.

With this heat containment, the system will allow startup after a weekend

shutdown with need for only 1 or 2 hr of heating. The sand bed should be at least

1200

F when operating.

Fluidized bed technology was first developed by the petroleum industry to be

used for catalyst regeneration. These are referred to as fluidized bed

combustors (FBCs) and they consist of a vertically oriented outer shell

constructed of steel and lined with refractory. Tuyeres (nozzles designed to

deliver blasts of air) are located at the base of the furnace within a refractory-

lined grid. A bed of sand, approximately 0.75 meters (2.5 feet) thick, rests

upon the grid. Two general configurations can be distinguished on the basis of

how the fluidizing air is injected into the furnace. In the "hot windbox" design

the combustion air is first preheated by passing through a heat exchanger

where heat is recovered from the hot flue gases. Alternatively, ambient air can

be injected directly into the furnace from a cold windbox.

Partially dewatered sludge is fed into the lower portion of the furnace. Air

injected through the tuyeres, at pressures of from 20 to 35 kilopascals (3 to 5

pounds per square inch gauge), simultaneously fluidizes the bed of hot sand

and the incoming sludge. Temperatures of 750 to 925

C (1400 to 1700

F) are

maintained in the bed. Residence times are typically 2 to 5 s. As the sludge

burns,

fine ash particles are carried out the top of the furnace. Some sand is

also removed in the air stream; sand makeup requirements are on the order of

5% for every 300 hr of operation. Combustion of the sludge occurs in two

zones.

Within the bed itself (Zone 1), evaporation of the water and pyrolysis of

the organic materials occur nearly simultaneously as the temperature of the

sludge is rapidly raised. In the second zone (freeboard area), the remaining

free carbon and combustible gases are burned. The second zone functions

essentially as an afterburner.

Fluidization achieves nearly ideal mixing between the sludge and the

combustion air and the turbulence facilitates the transfer of heat from the hot

sand to the sludge. The most noticeable impact of the better burning

atmosphere provided by a fluidized bed incinerator is seen in the limited

amount of excess air required for complete combustion of the sludge.

Typically, FBCs can achieve complete combustion with 20 to 50% excess air,

about half the excess air required by multiple hearth furnaces. As a

consequence, FBC incinerators have generally lower fuel requirements

compared to MHF incinerators. Fluidized-bed incinerators most often have

venturi scrubbers or venturi/impingement tray scrubber combinations for

emissions control.

ELECTRIC FURNACE

The electric furnace is basically a conveyor belt system passing through a long

rectangular refractory lined chamber. Heat is provided by electric infrared

heating elements within the furnace. Cooling air prevents local hot spots in the

immediate vicinity of the heaters and is used as secondary combustion air within

the furnace. The conveyer belt is made of continuous woven wire mesh chosen of

steel alloy that will withstand the 1300 to 1500

F temperatures. The sludge on the

belt is immediately leveled to 1 in. The belt speed is designed to provide burnout

of the sludge without agitation. The first electric infrared furnace was installed

in the 1970s, and their use is not common.

Electric infrared incinerators consist of a horizontally oriented, insulated

furnace. A woven wire belt conveyor extends the length of the furnace and

infrared heating elements are located in the roof above the conveyor belt.

Combustion air is preheated by the flue gases and is injected into the discharge

end of the furnace. Electric infrared incinerators consist of a number of

prefabricated modules, which can be linked together to provide the necessary

furnace length. A cross section of an electric furnace is shown in Figure 9. The

dewatered sludge cake is conveyed into one end of the incinerator. An internal

roller mechanism levels the sludge into a continuous layer approximately 1 in.

thick across the width of the belt. The sludge is sequentially dried and then

burned as it moves beneath the infrared heating. The ash is discharged into a

hopper at the opposite end of the furnace. Preheated combustion air enters the

furnace above the ash hopper and undergoes further heating by the exiting ash.

The air flow direction is countercurrent to the sludge flow along the conveyor.

Figure 9. Electric arc furnace.

CYCLONE FURNACE

The cyclonic furnace is a single hearth unit where the hearth moves and the

rabble teeth are stationary. Sludge is moved toward the center of the hearth

where it is discharged as ash. The furnace is a refractory lined cylindrical shell

with a domed top. The air, heated with the immediate introduction of

supplemental fuel, creates a violent swirling pattern which provides good mixing

of air and sludge feed. The air, which later turns into flue gas, swirls up

vertically in cyclonic flow through the discharge flue in the center of the domed

Ash

Discharge

Combustion

Air

Woven Wire

Continuous Belt

Cooling Air

Radiant Infrared Heating Elements

Belt Drive

Sludge In

Gas Out

Air

Lock

Exhaust gases leave the furnace at the end of the feed. The excess air can vary

from 20 to 70%. These systems offer the advantage of lower capital cost for

smaller systems. High electricity costs in some areas make this technology too

costly.

roof.

One advantage of these furnaces is that they are relatively small and can be

placed in operation, at operating temperature, within an hour.

As ash falls into a wet sump, turbulence is created by the entrance of water. This

turbulence is necessary so that the ash doesn't collect and cake up. This water

containing the ash is pumped into a holding pond or lagoon, with a residence

time of at least 6 hours. During this time, 95% of the ash will have settled to the

bottom and the overflow is taken back to the treatment plant. There has to be a

minimum of two lagoons with one being used to hold the ash-water discharge and

the other for drying. When dry, the ash is hauled to a landfill or used for

concrete. Mixing one part of ash to four parts cement will produce a slow-setting

concrete with no loss in strength.

ENVIRONMENTAL IMPACT AND CONTROLS

A serious environmental impact of incineration is on air quality. An incinerator's

smoke discharge or flue gas should be colorless. Flue gas is an emission mainly

made up of nitrogen, carbon dioxide, and oxygen. There are traces of chloride

and sulfides in the gas and if these levels become too high, they could cause the

possibility of corrosion. With respect to the color of the discharge again, if there

is a significant amount of particulate matter in the emission, it will be detected by

color. The stream can range from a black to white appearance and will have a

pale yellow to dark brown trail. The discharge should also have no discernable

odor and there should be no detectable noise due to incinerator operation at the

property line. Unfortunately, colored emissions and odor problems do occur and

treatment plants take the proper actions to correct it.

Air pollution controls are critical factors that add significant costs onto these

technologies. Sewage sludge incinerators potentially emit significant quantities of

pollutants. The major pollutants emitted are: (1) particulate matter, (2) metals,

(3) carbon monoxide (CO), (4) nitrogen oxides (NO

), (5) sulfur dioxide (SO

and (6) unburned hydrocarbons. Partial combustion of sludge can result in

emissions of intermediate products of incomplete combustion (PIC), including

toxic organic compounds. Uncontrolled particulate emission rates vary widely

depending on the type of incinerator, the volatiles and moisture content of the

sludge, and the operating practices employed. Generally, uncontrolled particulate

emissions are highest from fluidized-bed incinerators because suspension burning

results in much of the ash being carried out of the incinerator with the flue gas.

Uncontrolled emissions from multiple hearth and fluidized-bed incinerators are

extremely variable, however. Electric incinerators appear to have the lowest rates

of uncontrolled paniculate release of the three major furnace types, possibly

because the sludge is not disturbed during firing. In general, higher airflow rates

increase the opportunity for paniculate matter to be entrained in the exhaust

gases.

Sludge with low volatile content or high moisture content may compound

this situation by requiring more supplemental fuel to burn. As more fuel is

consumed, the amount of air flowing through the incinerator is also increased.

However, no direct correlation has been established between airflow and

paniculate emissions. Metal emissions are affected by metal content of the

sludge, fuel bed temperature, and the level of paniculate matter control. Since

metals which are volatilized in the combustion zone condense in the exhaust gas

stream, most metals (except mercury) are associated with fine particulates and

are removed as the fine particulates are removed.

Carbon monoxide is formed when available oxygen is insufficient for complete

combustion or when excess air levels are too high, resulting in lower combustion

temperatures. Emissions of nitrogen and sulfur oxides are primarily the result of

oxidation of nitrogen and sulfur in the sludge. Therefore, these emissions can

vary greatly based on local and seasonal sewage characteristics. Emissions of

volatile organic compounds (VOC) also vary greatly with incinerator type and

operation. Incinerators with countercurrent airflow such as multiple hearth

designs provide the greatest opportunity for unburned hydrocarbons to be

emitted. In the MHF, hot air and wet sludge feed are contacted at the top of the

furnace. Any compounds distilled from the solids are immediately vented from

the furnace at temperatures too low to completely destroy them. Particulate

emissions from sewage sludge incinerators have historically been controlled by

wet scrubbers, since the associated sewage treatment plant provides both a

convenient source and a good disposal option for the scrubber water.

The types of existing sewage sludge incinerator controls range from low pressure

drop spray towers and wet cyclones to higher pressure drop venturi scrubbers

and venturi/impingement tray scrubber combinations. Electrostatic precipitators

and baghouses are employed primarily where sludge is co-fired with municipal

solid waste. The most widely used control device applied to a multiple hearth

incinerator is the impingement tray scrubber. Older units use the tray scrubber

alone; combination venturi/impingement tray scrubbers are widely applied to

newer multiple-hearth incinerators and to fluidized bed incinerators.