United States
Environmental Protection
Agency
Office of Water &
Waste Management
Washington D.C. 20460
SW-743
February 1979
Solid Waste
fxEPA
The
of Existing Municipal
Sludge Incinerators
for Codisposal
-------
THE CONVERSION OF EXISTING MUNICIPAL
SLUDGE INCINERATORS FOR CODISPOSAL
This report (SW-743) was prepared by
Dick Richards and Harvey W. Gershman
U.S. ENVIRONMENTAL PROTECTION AGENCY
1979
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ACKNOWLEDGEMENTS
The authors gratefully acknowledge the direction provided by David B.
Sussman, who served as EPA's Project Officer during the course of this in-
vestigation. The authors also extend their thanks to the following con-
tributors for the information they provided: Ronald Sieger (Brown and
Caldwell), Terry Allen (Envirotech Corporation - BSP Division), Charles
von Dreusche (Nichols Engineering and Research Corporation) and Clarence
Wall (Dorr-Oliver, Inc.).
This report has been reviewed by the U.S. Environmental Protection Agency
and approved for publication. Its publication does not signify that the
contents necessarily reflect the views and policies of the U.S. Environ-
mental Protection Agency, nor does mention of commercial products consti-
tute endorsement or recommendation for use by the U.S. Government.
An environmental protection publication (SW-743) in the solid waste
management series.
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TABLE OF CONTENTS
1. Introduction 1
2. Refuse-Derived Fuel Systems 4
3. Municipal Sludge Incinerators 9
Multiple Hearth Furnaces 10
Multiple Hearth Pyrolysis 14
Fluidized-Bed Furnace 17
Fluidized-Bed Pyrolysis 21
4. Codisposal of Sewage Sludge and RDF 22
5. Technical Considerations for Codisposal in
Existing Sludge Incinerators 27
Codisposal in MHFs 27
Codisposal in FBFs 33
6. Capacity of Existing Incinerators for RDF 35
Methodology For Determining RDF Potential in Multiple
Hearth Furnaces o 35
Methodology For Determining RDF Potential in Fluidized
Bed Furnaces 38
7. Conclusions 44
TABLES
Table 1. Sludge Quantities 2
Table 2. Annual Fuel Consumption in Currently Operating
and Planned Municipal Sludge Incinerators 3
Table 3. RDF Composition 4
Table 4. Characteristics of RDF and Coal 5
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TABLE OF CONTENTS (cont'd.)
Page
Table 5. RDF Production Facility Costs 7
Table 6. Technical Consideration for RDF Use In Existing
Sludge Incinerators 28
Table 7. Hearth Area Determination 37
FIGURES
Figure 1. A typical RDF process. 6
Figure 2. Capital cost/plant capacity correlation for selected
RDF manufacturing projects (ferrous and aluminum
separation). 8
Figure 3. Cross-section of a multiple hearth furnace. 11
Figure 4. Flowsheet for sludge incineration in multiple hearth
furnace. 13
Figure 5. Flowsheet for pyrolysis of sludge in a multiple hearth
furnace. 15
Figure 6. Cross-section of a fluidized bed furnace. 18
Figure 7. Flowsheet for sludge incineration in a fluid bed 20
furnace.
Figure 8. Central Contra Costs Test Project flow diagram. 24
Figure 9. Flow diagram for FBF-coincineration, Western Lake
Superior Sanitary District, Duluth, MN. 26
Figure 10. RDF capacity of multiple hearth furnaces in the
incineration mode. 36
Figure 11. RDF capacity in multiple hearth furnaces in LTC
(pyrolysis) mode. 39
Figure 12. Water evaporation rate in Dorr-Oliver fluidized
bed furnace. 41
Figure 13. Ratio of wet sludge to wet RDF. 42
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THE CONVERSION OF EXISTING MUNICIPAL SLUDGE INCINERATORS FOR CODISPOSAL
Dick Richards and Harvey W. Gershman
1. INTRODUCTION
As a result of the U.S. Environmental Protection Agency's St. Louis/
Union Electric demonstration, the use of refuse-derived fuel (RDF) type
resource recovery systems is receiving wide consideration. A crucial
factor in the implementation of an energy/resource recovery project is
the existence of locally available energy user(s) for the various forms
of refuse-derived energy products available from municipal solid waste.
One such energy product, refuse-derived fuel, offers users the advantage
of being able to utilize existing combustion equipment and thus possibly
avoid additional investments in associated equipment.
Because of the capabilities of their existing facilities, electric
utilities have been identified as the primary market for RDF products to
date. However, marketing experience, a number of studies, and an elec-
tric utility survey all indicate that these utilities have no present
economic incentive to undertake such projectsJ Therefore, a need exists
to identify other users that could consume large quantities of RDF.
With passage of the Federal Water Pollution Control Act (P.L. 92-500)
and its subsequent amendments, the quantities of sludge generated by mu-
nicipal wastewater treatment plants have increased and will continue to
increase dramatically. A 1976 EPA report2 estimated a current municipal
sludge generation rate of 6.21 M dry metric tons/year and predicted an
8.40 M dry tons/year generation rate by 1986. In addition, the National
Needs Survey, the primary source of quantitative information of a national
scope correlating quantities of sludge with disposal techniques, pro-
vided the information regarding sludge disposal found in Table 1. Al-
though these data indicate that incineration will be the primary method
employed for sludge disposal, EPA's current policy "vigorously encour-
ages... land treatment processes..."3
As a result of P.L. 92-500, municipalities are faced with finding
environmentally sound methods for disposing of these sludges. As stated
above, incineration is the most common method for sludge disposal and
will no doubt continue to be for the next 25 years. Through combustion
processes, the high moisture content sludges are reduced to an inert ash
residue. In almost all sludge incinerators, large quantities of fuel oil
and/or natural gas are consumed in order to complete the reduction of
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Table 1
SLUDGE QUANTITIES
(MILLION DRY METRIC TONS PER YEAR)
Disposal Technique 1976 Projected 1990
Incineration 2.27
Recalcination .81
Landfill 1.21
Landspreading .31
Ocean Dumping .29
Total 4.89 5.39
these sludges (Table 2). The equivalent of more than one million barrels
of oil per year is currently being consumed in these incinerators. As
both fuel prices and sludge generation increase, the cost of sludge in-
cineration to municipalities is expected to rise sharply.
The ability to use RDF as a fuel in municipal sludge incinerators
(Sis) could offer the following benefits:
t significant reduction in fuel consumption by municipal
sewage treatment sludge incinerators,
• provision of a hedge against large increases in future
solid waste disposal and sludge incineration costs, and
• significant reduction in solid waste disposal process steps,
such as hauling and landfilling.
Although the application of RDF in Sis is receiving much attention
and is technically feasible,'* no information exists relating to the ex-
tent to which this disposal technique may apply in the many U.S. munici-
palities with municipal sludge incinerators. In order to assess the ap-
plicability of RDF as a fuel in municipal sludge incinerators, EPA's Of-
fice of Solid Waste contracted with Gordian Associates Inc. (EPA Contract
Number 68-01-4427) to undertake this study. The purpose of this study is
to investigate the potential for using RDF in existing sludge incinerators
only. There are several other techniques for the combined disposal of
municipal solid waste and sewage sludge (codisposal) that merit attention
but which are beyond the scope of this study.^
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Table 2
ANNUAL FUEL CONSUMPTION IN CURRENTLY OPERATING
AND PLANNED MUNICIPAL SLUDGE INCINERATORS*
Gas
on
Electricity
Total
Present Sludge
Incinerators
x 1000
BBLS
Fuel Oil
4696+ 765.4
15358* 365.7
2458§ 1.5
- 1132.6
Planned Sludge
Incinerators
x 1000
BBLS
Fuel Oil
1368+ 223.0
24684* 587.7
2612§ 1.6
812.3
Total
x 1000
BBLS
Fuel Oil
6064+ 988.4
40042* 953.4
5070§ 3.1
- 1944.9
* Source: U.S. EPA, A. Hais, personal communication, 1977.
+ Gas: million cu. ft/yr
t Oil: thousand gal/yr
§ Electricity: thousand KWH/yr
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2. REFUSE-DERIVED FUEL SYSTEMS
Refuse-derived fuel (RDF) is produced in the following manner: in-
coming municipal solid waste (MSW) is first processed through shredding
and air-classification and/or screening to separate the non-combustible
from the combustible portion. The percentage of each material in the
RDF produced from MSW is shown in Table 3. The non-combustible portion
may be used for landfill ing, etc., while the combustible portion may be
burned as is, or re-shredded into a smaller, more homogeneous particle
size (fluff RDF). Once shredded, the RDF may be modified chemically so
that it can be disintegrated into a powder (dust RDF); or the RDF may be
densified into briquettes or pellets (densified RDF, or d-RDF). These
two modifications facilitate storage, transportation, and handling of
the fuel. The characteristics of fluff RDF and dust RDF are compared
to those of coal for heat value, particle size, moisture, ash. and
sulfur content in Table 4.
Table 3
RDF COMPOSITION*
Material % Composition
Paper
Plastic
Wood
Glass
Metal
Organics
Miscellaneous
63.3
6.5
2.4
1.0
0.7
0.6
25.4
* Source: U.S. EPA Resource Recovery Seminar, Chicago, Illinois,
June 1977.
Several RDF processing systems are currently offered by private sys-
tem vendors. Figure 1 depicts a simplified diagram of one such RDF pro-
duction system. In addition, a number of RDF facilities have been de-
signed and constructed by municipalities through the services of architects
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Table 4
CHARACTERISTICS OF RDF AND COAL*
Fluff
Dust
Coal
Heat Value (Btu/Lb.)
Particle Size (In.)
Moisture (%)
Ash (%)
Sulfur (%)
5,000-6,000
3/4-1%
20-30
15-21
0.1-0.4
7,800
<0.015
2.0
9.4
0.1-0.6
11,000-14,000
-
3-12
3-11
0.5-4.3
U.S. EPA Resource Recovery Seminar, Chicago, Illinois,
* Source:
June 1977.
and engineers, such as those in Ames, Iowa and Chicago, Illinois. How-
ever, the production of RDF cannot be considered a proven technology
since the reliability of the processing equipment has not been demon-
strated over extended periods of operation.
Examples of the cost for RDF production facilities are shown in
Table 5. This listing cites capital costs for RDF production, materials
recovery (usually only for ferrous metals), and storage only; costs for
feeding and firing are not included here. The wide variation in cost is
due to differences in project design, date of construction, location,
process redundancy, equipment selection, etc. In order to make more ac-
curate deductions from these figures, more specific cost estimates would
be necessary. A recent analysis by Gordian6 normalized several 1975 RDF
project capital costs versus peak capacity. This information is pre-
sented graphically in Figure 2, which provides a "rule of thumb" measure
for RDF plant capital costs. Since 1975 the construction cost index has
escalated 8.6 percent in 1976, 7.2 percent in 1977, and is forecast to
increase another 7.1 percent in 1978.7
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PRIMARY
SHREDDER
SECONDARY
SHREDDER
HEAVIES TO MATERIALS
RECOVERY AND LANDFILLS
FLUFF
RDF
DENSIFIED
RDF
DUST
RDF
Figure 1. A typical RDF process. Source: U.S. EPA.
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Table 5
RDF PRODUCTION FACILITY COSTS*
Location
Capacity
(tons/day)
Capital Cost and
Type RDF Year of Estimate
Monroe County, NY
Milwaukee, Wis.
New Orleans, La.+
Chicago, Illinois
Ames, Iowa
East Bridgewater, Mass.
Bridgeport, Conn.
Hempstead, NY
Baltimore County, MD
2,000
1,000
650
1,000
400
160
1,800
2,000
1,500
Fluff RDF
Fluff RDF
Fluff RDF
Fluff RDF
Fluff RDF
Dust RDF
Fluff RDF
Wet-Pulped
Fiber
Fluff-RDF
$30 million (1976)
$14 million (1975)
$5.7 million (1975)
$16 million (1975)
$5.6 million (1974)
Not Known
$52 million (1975)
$73 million
$10 million
(1976)1
(1975)
* Source: Gordian Associates Inc. and L. B. McEwen, A Nationwide Survey
of Waste Reduction and Resource Recovery Activities (Washington, D.C.: U.S.
EPA Office of Solid Waste, 1976).
+ RDF is produced here. It is not currently sold, but is suitable for
use in applications such as codisposal.
f Cost includes waterwall combustion system.
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100
e
o
o
o
10
Q.
5 5
100
w
(1975)
L *= LORTON
M = MILWAUKEE
T = TENNESSEE VALLEY AUTHORITY
W = WEGMAN
1,000
10,000
Figure 2. Capital cost/plant capacity correlation for selected
RDF manufacturing projects (ferrous and aluminum separation). Source;
Gordian Associates Incorporated.
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3. MUNICIPAL SLUDGE INCINERATORS*
The combustion of municipal sewage sludge in an incinerator provides
for its maximum volume reduction and leaves an inert ash which is suit-
able for landfilling. Although the heat value of sludge is fairly high
(5,000 to 10,000 Btu/lb dry solids), the water content of most sludges
requires the addition of an auxiliary fuel to maintain combustion in
the furnace. The fuel cost is the major operational cost of incineration.
The reduction of this cost can be achieved either by raising the solids
content of the sludge, thereby increasing the net heat value, or by lower-
ing the amount of inert material in the sludge prior to incineration.
Many processes which reduce the moisture content of sludge also low-
er its heating value, as the proportion of inert material increases at
the expense of the volatile fraction. This is most apparent in anaerobic
digestion where bacteria convert much of the volatile matter to methane.
The methane is given off as a digester gas which is frequently used a
fuel in the treatment plant. Physical-chemical sludges have a high con-
tent of inert material, but tend to dewater better than biological
sludges. When incineration is the final step in sludge processing, the
benefits and penalties of the conditioning steps on the furnace's fuel
consumption must be evaluated. Pretreatment of sludge to decrease the
water content prior to incineration may actually increase overall plant
energy requirements.
Sludge incineration is a process which involves drying the sludge
prior to its actual combustion. Drying and combustion may be performed
in separate units, or both processes can be accomplished in the furnace.
Sludge incineration occurs in four steps:
t the temperature of the sludge is raised to 212 F;
• the water is evaporated from the sludge;
• the water vapor temperature and air temperature are increased;
and
• the temperature of the sludge is raised to the ignition point
of the volatiles.
* The discussion presented here is derived from Brown Caldwell's
Incineration-Pyrolysis of Mastewater Treatment Plant Sludge, presented
at the U.S. EPA Technology Transfer Design Seminar, 1977.
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The heat evolved by sludge incineration can be utilized in many ways:
heating and drying of incoming sludge, production of steam for space heat-
ing, powering mechanical equipment, or generating electricity. Auto-
genous or self-sustained combustion is possible with some sludge feeds,
but is dependent on the net heat value of the sludge. Because of the
relatively high temperature of the combustion gases (approximately 1300
to 1700 F), a large amount of the heat evolved is used to raise the tem-
perature of the incoming combustion air and fuel mixture. For success-
ful incineration, proper mixing of the combustion gases, the fuel mix-
ture, and the volatile solids in the sludge are important.
There are two types of furnaces generally used for sludge incinera-
tion in the United States, the multiple hearth and the fluidized bed.
A third type, the single rotary hearth cyclonic furnace, has several in-
stallations in Great Britain and Europe, but is not as widely used in the
U.S. as the other two furnaces. It too, has a potential for sludge in-
cineration. All three furnaces provide an adequate mixing of sludge with
the combustion gases and a residence time which ensures complete combus-
tion. Since U.S.-installed sludge incinerators are almost always of the
first two types, the discussion in this report has been limited to multi-
ple hearth and fluidized bed installations.
Multiple Hearth Furnaces
The multiple hearth furnace (MHF) is the most widely used sludge in-
cinerator in the United States. It is durable, relatively simple to
operate, and can successfully handle a wide variety of sludges and load-
ing rates. However, smooth operation of an MHF requires that the quality
of the feed, particularly its moisture content, remain relatively
constant.
The MHF is a vertically oriented, cylindrically shaped, refractory
lined, steel shell containing a series of horizontal refractory hearths,
one above the other. MHFs are available with diameters ranging from 54
inches to 25 feet, and contain 4 to 13 hearths. A typical cross-section
of an MHF is given in Figure 3. A central shaft runs the height of the
furnace and supports rabble arms above each hearth. There are either two
or four rabble arms per hearth. Each arm contains several rabble teeth
or plows which rake the sludge spirally across the hearth as the arm ro-
tates above each hearth. The sewage sludge is fed at the periphery of the
top hearth and is raked toward the center where it drops to the hearth
below. On the second hearth, the sludge is raked outward to holes at the
periphery of the bed where the sludge then drops to the next hearth. The
hot gases rise counter-current to the sludge. This flow and the alterna-
tive drop hole locations on each hearth provide good contact between the
hot combustion gases and the sludge feed to ensure complete combustion.
The central shaft, which serves as an exhaust passageway for the
cooling air, is normally cast iron and has an inner tube called the cold
10
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FLUE GASES OUT
DRYING ZONE
COMBUSTION ZONE
COOLING ZONE
ASH DISCHARGE
COOLING AIR DISCHARGE
FLOATING DAMPER
y-SLUDGE INLET
RABBLE ARM
AT EACH HEARTH
COMBUSTION
AIR RETURN
RABBLE ARM
DRIVE
COOLING AIR FAN
Figure 3. Cross-section of a multiple hearth furnace. Source:
U.S. EPA.
11
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air tube. Cooling air is fed to the cold air tube by the cooling air
fan. Each of the rabble arms is connected to the cold air tube by a re-
turn tube which returns the heated air to the annular space between the
cold air tube and the outer shell of the central shaft. The heated cool-
ing air is usually taken from the top of the central shaft and re-
injected into the lowest hearth to aid in combustion by preheating the
combustion air.
The MHF can be divided into four zones for incineration. The first
zone, which consists of the upper hearths, is the drying zone where most
of the water is evaporated; the second zone, generally consisting of the
central hearths, is the combustion zone, where temperatures reach 1400
to 1700 F; the third zone is the fixed carbon burning zone, which re-
duces the volatile matter to carbon dioxide; the fourth zone is the cool-
ing zone, which includes the lowest hearths. In this zone, the ash is
cooled by transferring heat to the incoming combustion air. The sequence
of these zones is always the same, but the number of hearths in each zone
is dependent on the quality of the feed, the design of the furnace, and
the operational conditions.
When the thermal quality of the sludge feed is insufficient to sustain
autogenous combustion, burners supply the additional heat by operating
either continuously or intermittently on all or selected hearths. Gen-
erally, off-gas temperatures of 600 F or lower indicate incomplete com-
bustion and a need for supplemental fuel. Off-gas temperatures from 800
to 1600 F indicate complete combustion. Temperatures are usually main-
tained below 1400 F by eliminating preheating the combustion air and by
adding excess air. Excess air requirements for the MHF are generally 75
to 100 percent of stoichiometric requirements when sludge is the furnace
feed material. A flowsheet for the MHF is given in Figure 4. For areas
where air pollution requirements are similar to those for the San Fran-
cisco Bay Area, the MHF would require an afterburner fired with supple-
mental fuel. This requirement would increase fuel consumption, equip-
ment size, and capital and operating costs.
The successful operation of an MHF in the incineration mode requires
a relatively constant feed. Control of the MHF can be difficult, as an
hour or longer is required for the sludge to reach the combustion zone
from the top hearth. Thus, an increase in the moisture content of the
sludge entering the top hearth may extinguish the flame in the lower
hearths, unless it is detected in time. If the moisture content is de-
tected, additional fuel can be burned to ensure that the sludge is dry
before it reaches the combustion zone.
Other MHF operating problems have included the failure of rabble arms
and teeth, the failure of hearths, and the failure of refractories. The
rabble arms and teeth problems have generally been solved by using dif-
ferent construction materials. Difficulties with the refractory are
usually caused by improper operation and comprise a disadvantage
12
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GAS EXHAUST
SHAFT COOLING
SLUDGE
FEED
AIR RETURN
MULTIPLE
HEARTH
FURNACE
SHAFT
COOLING AIR
SHAFT COOLING AIR NOT RETURNED
PRECOOLER-
AND VENTURI
FURNACE
Figure 4. Flowsheet for sludge incineration in a multiple hearth
furnace. Source: Brown & Caldwell.
13
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which must be considered when evaluating an MHF. An MHF generally re-
quires at least 24 hours to cool off or to be brought back to operating
temperature. During intermittent operation, supplemental fuel is usual-
ly fired to maintain the temperature of the furnace during the hours
when it is not being used. When the furnace is heated or cooled too
quickly, the refractories can be damaged. Multiple hearth furnaces are
generally not operated at temperatures above 2200 F; thus, with high
energy fuels, there may be operational problems.
Multiple Hearth Pyrolysis
Pyrolysis is the destructive distillation of organic materials which
is performed under heat and/or pressure and in the absence of oxygen.
The products of pyrolysis are a combustible gas, tars and oils, and a
solid char. In an MHF the heat required for the pyrolytic destruction
of the organic material in the sludge is provided by the combustion of
part of the sludge with or without the addition of auxiliary fuel. As
a limited quantity of oxygen is required for this reaction, starved-air
combustion is a more accurate term for the process.
Due to rising fuel costs and the heat value of the sludge being fed
to incinerators, pyrolysis has been investigated as a means of reducing
the volume of sludge to be disposed without using large amounts of sup-
plementary fuel. In addition, a pyrolytic gas is produced which has a
heat value of up to 130 Btu/standard dry cubic foot (sdcf) using air
for combustion, and is suitable for combustion in an after burner,
boiler, or furnace. Pyrolysis is more efficient than incineration since
only a small amount of excess air must be heated. This process also
allows for a reduction in the size of the off-gas system; which in turn
reduces capital costs. A typical flow sheet for an MHF operated as a
pyrolytic reactor is shown in Figure 5.
There are essentially three different operating modes for the pyroly-
sis of sludge in an MHF. These vary chiefly in the division of the
energy in the sludge between the off-gas (in the form of both a combus-
tible gas and sensible heat) and the char (as unburned carbon). A
char with a lower proportion of unburned carbon will have a lower bulk
density which could reduce landfill ing costs.8
Low Temperature Char Operating Mode (LTC). The pyrolysis reaction
occurs at 1200 to 1300 F. The reaction is sufficient to induce the
destructive distillation of the organics in the sludge, giving off
almost all of the organic vapors that are able to be released. The
char discharged in this mode is a sterile, black, charcoal-like, free-
flowing mixture of powder and granules with a bulk density of approxi-
mately 16 pounds per cubic foot and containing 75 percent ash. The LTC
residue would retain heavy metals.
14
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GAS EXHAUST
SHAFT COOLING AIR NOT RETURNED
SHAFT COOLING AIR
RETURNED TO AFTERBURNER
SHAFT
VENTURI WATER
CONNECTED F-OWER
ASH
COOLING AIR
Figure 5. Flowsheet for pyrolysis of sludge in a multiple hearth
furnace. Source: Brown & Caldwell.
15
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High Temperature Char Operating Mode (HTC). At a temperature of
1600 F or higher, the char formed during this process contains only ash
and carbon, and has a bulk density of 25 pounds per cubic foot. In the
HTC mode more energy is released in the off-gas stream and less in the
char, as compared to the LTC mode. The char, although it has a lower
heating value, may be useful for its activated carbon content. The
activated carbon may be suitable for addition to the waste activated
sludge system in order to improve the functioning of that system.
Char Burning Operating Mode (CBA*). In this mode the char resulting
from pyrolysis in the middle hearths can be combusted and then converted
to ash in the lower hearths. The advantage of the CBA mode is that the
energy content of the sludge is concentrated in the off-gas in the forms
of combustible gas and sensible heat, leaving a minimum quantity of
ash to be landfilled. This ash has a bulk density of approximately 33
pounds per cubic foot.
The autogenous pyrolysis of sludge was successfully demonstrated at
a full-scale MHF project at the Central Costa Sanitary District waste-
water treatment plant at Concord, California near San Francisco. The
sludge had a solids content of 24 percent, a heating value of 9000 Btu/
Ib dry solids, and a volatile solids content of 75 percent. The py-
rolysis gas had a heating value of 90 Btu/scf/ In other projects the
autogenous pyrolysis of sludge was successful with sludges having a
solids content of more than 25 percent. The nominal solids level for
autogenous pyrolysis is a function of the net heating value of the sludge.
An MHF constructed by Envirotech in Cowlitz County, Washington has
been converted for the pyrolysis of sludge, and recently began operation.
Nichols Engineering and Research has been awarded a contract by Arlington
County, Virginia to construct MHFs for sludge pyrolysis. Both the Eimco
BSP Division of Envirotech and Nichols Engineering are actively involved
in ongoing research and development on pyrolysis systems.
During test work by both MHF manufacturers, pyrolysis was found to
have the following advantages over incineration:
• it is easier to control than incineration;
• pyrolysis modes have a more stable operation with little
response to changes in feed;
• they possess more feed capacity per square foot of hearth area;
* Charcoal Burned Ash
16
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t They have fewer air pollutants and an easier particle size
to scrub;
• their fuel consumption is lower;
• a lower sludge solids content is required for autogenous
operation; and
• their operating costs are slightly lower.
However, there are disadvantages associated with pyrolysis:
• the need for an afterburner may limit its use in existing
installations due to space problems;
• more instrumentation is required;
• care must be taken with the bypass stack exhaust since furnace
exhaust is high in hydrocarbons and may be combustible in the
air. This may result in bypassing only after afterburning
with appropriate emergency controls in some areas;
• the furnace exhaust gases are corrosive; and
• combustibles in the ash may create ultimate disposal problems
in the HTC and LTC modes.
However, in this era of high energy costs, the advantages of pyrolysis
seem to outweigh its disadvantages.
Fluidized Bed Furnace
The fluidized bed furnace (FBF) is a vertically oriented, cylindri-
cally shaped, refractory lined, steel shell which contains the bed and
fluidizing air diffusers. The FBF is normally available in sizes from
nine feet to 25 feet in diameter. However, there is one industrial unit
operating with a diameter of 53 feet. A cross-section of the fluidized
bed furnace is shown in Figure 6. The sand bed is approximately 2.5
feet thick and sits on a refractory lined grid containing tuyeres. To
fluidize the bed, air is injected through the tuyeres into the bed at
a pressure of 3 to 5 psig. The bed's expansion is approximately 80 to
100 percent. The temperature of the bed is controlled at 1400 to 1500 F
by auxiliary fuel guns. The fuel (oil or gas) is injected directly into
and burned in the fluid bed. The preheat burners which are located
either above (for cold windbox design) or below (for hot windbox design)
the bed, are used during start-up to raise the temperature of the bed.
In some installations temperature is controlled by a water spray or heat
removal system above the bed which reduces the furnace temperature when
it is too high. The ash is carried out the top of the furnace and is
17
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SIGHT GLASS
EXHAUST 4 1
SAND FEED
FLUIDIZED
SAND
PRESSURE
TAP *•
PREHEAT BURNER
ACCESS
DOORS
THERMOCOUPLE
SLUDGE INLET
FLUIDIZING
AIR INLET
TUYERE
WINDBOX
Figure 6.
U.S. EPA.
Cross-section of a fluidized bed furnace. Source:
18
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removed by air pollution control devices, usually wet venturi scrubbers.
Sand, which is carried out with the ash, must be replaced. Sand loss
is generally five percent of the bed volume every 300 hours of opera-
tion. Furnace feed can be introduced either above or directly into
the bed, depending on the type of feed. Generally, sewage sludge is
fed directly into the bed.
Excess air requirements for the FBF vary from 20 to 40 percent. This
reduces supplementary fuel requirements and provides a more efficient
thermal balance around the furnace than with the multiple hearth furnace.
The mixing caused by the air flowing through the bed and the injection
of sludge directly into the bed ensures complete contact between the
sludge solids and the combustion gases.
The fluid bed incinerator can be arranged in two general configura-
tions. In the first system, the fluidizing air passes through a heat
exchanger or recuperator prior to its injection into the combustion
chamber. This arrangement is known as a hot windbox. It increases the
thermal efficiency of the system by utilizing the high temperature of
the exhaust gases to heat the incoming combustion air. In the second
system, the fluidizing air is injected directly into the furnace and
is known as a cold windbox. A general flowsheet for the FBF is de-
picted in Figure 7.
The FBF has a lower capital cost than the MHF. Its start-up fuel
requirements are very low, and no fuel is needed for start-up following
an overnight shutdown. The fluid bed reactor has a minimum of mechani-
cal components and is relatively simple to operate. The sand bed acts
as a large heat reservoir which minimizes the amount of fuel required
to reheat the system following shutdown. This makes the FBF very at-
tractive for intermittent operation. Since the exhaust temperatures of
the FBF are in excess of 1400 F, an afterburner with supplemental fuel
to comply with air pollution regulations is not required, as it may be
for an MHF in some areas.
Problems were encountered in early FBFs with the feed system and
temperature control for high energy feeds. Screw and pump feeds jammed
because of overdrying of the sludge when it was fed directly into the
bed. Where spray nozzles were used, thermocouples were burned out oc-
casionally. There were also some problems with preheaters and with
scaling of the sand on the venturi scrubber. Dorr-Oliver maintains that
these problems have been addressed and corrected.
The FBF can easily be operated at temperatures of 2200 F and when
appropriately designed, is suitable for high energy sludges. Thermal
efficiency (.i.e. the quantity of auxiliary fuel required per ton of
sludge) can be maintained when the feed is reduced to 60 percent for
the cold windbox design and 75 percent for the hot windbox design. The
19
-------
FURNACE EXHAUST
GAS EXHAUST
WET SCRUBBER
SCRUBBER
WATER
Figure 7, Flowsheet for sludge incineration in a fluid bed furnace,
Source: Brown & Caldwell.
20
-------
greater turndown capabilities of the former are due to the use of over-
bed air. Since there is a minimum amount of air always required for
bed fluidizing, thermal efficiency is reduced by further turndown.
Fluidized-Bed Pyrolysis
FBFs also have potential as pyrolytic reactors. However, the design
of such systems differs from fluidized-bed incineration in that the
fluidizing gases must be preheated to provide heat for the pyrolytic
process.
A pilot scale FBF at West Virginia University has been operated
using both sludge and MSW as the feed. The fluidizing gases were pre-
heated in a combustion chamber (hot bottom) located directly beneath
the fluidized bed chamber and separated from it by a high temperature
grid plate with small holes (0.096 inch diameter) to permit passage of
the heated air into the FBF. MSW was pyrolyzed in this system at
1500 F to yield a pyrolysis gas with a heating value of 421 Btu/scf.
The sludge, which has been digested and consequently had a low fuel
value (3900 Btu/dry Ib), produced a gas with a heating value of
360 Btu/scf.9
A demonstration plant in Japan has successfully pyrolyzed MSW,
plastic wastes, and sludges from a pulp and paper plant.^ The process,
which has been adapted from its original olefin cracking application in
the petrochemical industry, employs two FBFs. One acts as the pyrolytic
reactor and the second preheats the fluidizing air by combustion of the
char produced by the pyrolytic reactor. The gas generated from the
MSW feed ranged from 405 to 438 Btu/scf in four test runs.
The leading FBF manufacturers have not promoted the pyrolysis of
sludge since they believe that the energy content of the sludge can be
efficiently recovered through incineration as sensible heat, due to the
greater heat release capacity of the FBF in the incineration mode. If
producing steam is the object, then pyrolysis has no advantage over in-
cineration. Capital costs for a pyrolytic FBF system would be larger
than those for combustion due to the requirement for a furnace to pre-
heat the fluidizing gases. For this reason, the conversion of FBFs for
pyrolysis would be impractical in most cases. If, however, a combustible
gas is desired, an FBF system is capable of producing a high quality gas
from a variety of feed materials.
21
-------
4. CODISPOSAL OF SEWAGE SLUDGE AND RDF
Municipal wastewater sludge has a high water content and a low net
heating value when compared to other potential fuels. Unit processes
which dewater sludge typically have a high energy demand. By combining
sludge with other materials such as RDF, a combined furnace feed can
be formed which has both a low water content and a heating value high
enough to eliminate the need for supplemental furnace fuel. This tech-
nique is termed codisposal.
Coincineration is incineration using a combination of sewage sludge
and a combustible material, other than natural gas or fuel oil, in a
single furnace. Copyrolysis is pyrolysis using a combination of sewage
sludge and combustible materials, other than natural gas or fuel oil,
in a single reactor.
A variety of materials can be combined with sewage sludge to form
a furnace feed with a higher heat value than a pure sludge feed. Some
of these materials are: municipal solid waste, coal, wood wastes, tex-
tile wastes, bagasse, and farm wastes such as corn stalks, rice husks,
etc. Virtually any material which can be combusted can be combined
with the sludge. The coincineration of coal and sludge, and municipal
waste and sludge, have been investigated in recent test programs.
The coincineration of sludge with processed MSW in multiple hearth
furnaces has been practiced in Great Britain, Germany, and Switzerland.
At Reigate (England) and Alloa (Scotland), MSW is shredded by a hammer-
mill, and the ferrous materials are removed by magnetic separation be-
fore being fed into the top hearth of a Lurgi MHF along with thickened
sludge.'' Considerable difficulties with the conveyor carrying the
shredded MSW to the MHF were encountered during initial operation of
the Reigate plant. Also, cracks were experienced in the rabble shaft
castings due to local overheating, probably caused by occasional high
heat release during firing of refuse with a high energy content. As
of the last report, these problems have been corrected.!2
In Nieder-Uzwill, Switzerland, a Nichols MHF has been converted for
coincineration. After oversized materials have been separated, the MSW
is shredded by a hammermill and ferrous metals are removed by a magnetic
drum. The remaining material is conveyed onto a vibrating screen with
30 mm (1.2 inch) holes. The oversized material is conveyed to the top
hearth of the MHF and introduced with digested sludge (six percent
solids). A separate furnace is used to incinerate large items (mat-
tresses, tires, etc.) and the combustion gases from this furnace are
then fed to the MHF to provide additional heat. The plant operates 24
22
-------
hours per day, and incinerates 86 to 88 tons/day of refuse and 22 to 27
wet tons/dry of digested sludge, which represents an application rate of
about seven wet Ib/ft2/hr.13
In the United States, codisposal was successfully demonstrated in a
16-foot diameter, six-hearth, MHF operated by the Central Contra Costa
Sanitary District at Concord, California. Mixed municipal refuse was
shredded, classified, and screened prior to addition to the MHF, where
the RDF comprised the light fraction from the air classifier. The
sludge had a solids content of 16 percent, a volatile solids content of
75 percent, and a heating value of 9,000 Btu/lb dry solids; whereas the
RDF had a solids content of 75 percent, very few inerts, and a heating
value of 7,500 Btu/lb of dry solids. The furnace feed rate was varied
from pure sludge to pure RDF.
During the test, the furnace was operated in both a pyrolytic mode
and an incineration mode. Approximately 70 to 100 percent excess air
was introduced in the incineration mode to ensure complete combustion.
The pyrolysis mode was run under oxygen-starved conditions at a con-
stant furnace temperature. The volatile gases were combusted in the
afterburner with minimum excess air in order to maximize the temperature.
Afterburner temperatures well over 2000 F were common. The system was
operated for up to 8.3 hours per day during a two-month demonstration
program. Both pyrolysis and incineration modes were tested on feed
material that ranged from pure sludge to pure MSW. Maximum capacity
was obtained in the pyrolysis mode with a feed consisting of 2.5 parts
of MSW to one part of sludge. The feed rate of this material was re-
ported to be 8,716 wet pounds per hour^ which represents a loading
rate of 10.3 wet pounds per square foot per hour. During copyrolysis,
a combustible gas was produced with a heating value of 130 Btu/sdcf.
This combustible gas could be fired in a waste heat boiler for steam
production, used as the fuel for a lime recalcination furnace, or used
for space heating.
A flow diagram of the system is depicted in Figure 8. During the
test, the RDF could be fed to hearths number three or one. Sludge was
always fed to hearth number one. Temperatures were maintained during
pyrolysis by controlling the amount of air fed to the furnace. The
off-gases from the furnace were allowed to burn in an afterburner
with the introduction of combustion air. Afterburner temperatures were
approximately 2200 F, although the pyrolysis gas could be combusted to
produce a temperature as high as 2500 F with no supplemental fuel
addition.
Autogenous combustion could be maintained with an RDF-to-sludge
ratio of one to two using a sludge solids content of 16 percent. Auto-
genous combustion in a pyrolysis mode could be maintained with sludge
alone, but only when the solids content was 24 percent or more.
Energy recovery and general system operation and control were signifi-
cantly improved when the RDF was fed to hearth number three, rather
than to the top hearth.
23
-------
TO
ATMOSPHERE
-AUXILIARY
AFTERBURNER
FAN
LIVE BOTTOM
TRUCK
10 FAN
BLENDER
AIR
L
^
r— BYPASS
DAMPER
O
[
<
{
PLANT IFFLUENT
FROM LAGOON
SLUOGE
FROM
PLANT
-€?
NATURAL GAS
COMBUSTION
AIR FAN
ASH PUMPS
Figure 8. Central Contra Costa Test Project flow diagram. Source: Brown & Caldwell.
-------
MSW and sludge have been coincinerated in an FBF in Franklin, Ohio.
The furnace preparation utilizes a Black Clawson wet pulper to remove
ferrous metals and heavy solids from the pulped refuse. Sludge from a
2.5 MGD secondary treatment plant is added to a major portion of the
pulper effluent, and the combined stream is dewatered to 15 to 20 per-
cent solids by a primary pulp screw press. The furnace feed is further
dewatered to 40 to 45 percent with a disc or V press before being in-
jected into the FBF at the bottom of the bed. There is a build-up in
bed volume with the coincineration scheme, and a small amount of bed
material must be periodically removed from the furnace. The system
has a capacity of ten wet tons/hr, and the feed preparation steps now
reduce the non-combustible fraction to three to six percent.
Another coincineration project utilizing an FBF is under construc-
tion in Duluth, Minnesota for the Western Lake Superior Sanitary Dis-
trict. Unlike the Franklin plant, the MSW will be shredded rather than
pulped, to produce a dry RDF fuel. A flow diagram for this project is
shown in Figure 9. An FBF in Thunder Bay, Ontario is being used to
incinerate hogged bark and wood debris with pulp mill sludge. Tests
have been run using unsorted solid wastes in place of the wood waste.
In their review of codisposal techniques J5 Roy F. Weston, Inc.
stated that large materials will accumulate at the bottom of the fluid
bed, causing ultimate defluidizing of the bed, thus requiring that the
RDF be uniform in size, generally in the one to three inch range. This
has been strongly disputed by Dorr-Oliver, who claims that RDF for use
in an FBF can be as large as six inches in diameter.
25
-------
SEWAGE SLUDGE
MAKE-UP
SON COMBUSTIBLES
1 TO LA.SDFILL
Figure 9. Flow diagram for FBF-coincineration, Western Lake Superior
Sanitary District, Duluth, MM. Source: Consoer, Townsend & Associates.
26
-------
5. TECHNICAL CONSIDERATIONS FOR CODISPOSAL
IN EXISTING SLUDGE INCINERATORS
In order to assess the application of RDF in existing sludge incin-
erators, lengthy discussions were held with key staff members of four
major sludge incinerator manufacturers. As a rerult of those discus-
sions and upon review of the available literature on this subject,
various technical considerations have been identified. These considera-
tions are summarized in Table 6.
Codisposal in MHFs
Mode of Operation. Coincineration in an MHF would be most applicable
in locations where excess incinerator capacity is available and the
solids content of the sludge is low. The addition of RDF would raise the
proportion of combustible solids in the furnace feed, thereby eliminating
the need for auxiliary fuel except during start-up. If heat recovery is
not being considered and furnace capacity is sufficient, it may not be
worthwhile to modify the MHF for pyrolysis.
In most locations, the solids content of the RDF/sludge feed is like-
ly to be high. The heat released during coincineration of such a feed
material would be a problem in many MHFs, since the volume of air re-
quired to cool the furnace would result in the velocity of this gas,
moving upwards through the drop-holes in each hearth, to be high. If
this gas velocity exceeds approximately three feet per second, then RDF
would be prevented from dropping downward through the drop-holes. This
problem would be particularly severe in tall, narrow MHFs, where the
area of the drop-holes at each hearth is small, and could severely limit
the capacity of the furnace. According to Nichols, this problem could
be overcome by split drafting, where a portion of the hot combustion
gases is removed from the middle hearths, where the RDF would be burned,
and conveyed to the off-gas system.
However, for most MHF installations, the manufacturers recommend
that the pyrolysis mode be adopted for codisposal for the following
reasons:
t furnace capacity is increased;
• heat recovery, particularly in a waste heat boiler, is enhanced;
27
-------
Table 6
TECHNICAL CONSIDERATION FOR RDF USE IN
EXISTING SLUDGE INCINERATORS*
Consideration
MHF-Pyrolysis
FBF
RDF Process
• Shred, air classify,
screening
• 65% RDF output
• d-RDF may be preferable
• Trommel, coarse
shred, air
classify
RDF Specification
• <_ 2-3" particle size t.
t Low inerts •
t Low BTU/lb. & moisture •
content variation prefered
Coarse
95% 6"
Low inorganic
content preferable
BTU & moisture
variation not
critical
Feeding
Screw conveyor feed at 1 or
2 points or pneumatic feed
to table feeder
• Ram feeding or
pneumatic feeding
• Feed into sand bed
Site
• Individual analysis required
t Waste heat boiler location
should be as close as possible
Individual analysis
required
Waste heat boiler
location should be
as close as possible
Fuel Useage
• Significant supplementary
fuel still required for
warm up periods
• Due to heat sink,
nature of design,
less supplementary
fuel required
* Source: Gordian Associates Inc.
28
-------
Table 6 (cont'd.)
TECHNICAL CONSIDERATION FOR RDF USE IN
EXISTING SLUDGE INCINERATORS
Consideration
MHF-Pyrolysis
FBF
Incinerator
Modifications
Add feed system
Replace induced draft fan
Seal air linkages
Add hot cyclone venturi
scrubber
Water jacketed ash screw
After burner required
Add waste heat
boilers especially
to older installa-
tions
Add bed-removal
classification sys-
tem if RDF quality
is low
Incinerator
Modifications
• Add and raise rabble arms
• Additional controls &
instrumentation
• Add waste heat boiler
• Anti-corrosion lining
for off-gas system
• (Add split-draft ducting)
Modify/add scrubber
capacity
Make "hot shell"
design in order to
inhibit corrosive
gas condensation
ofHCVs andHFTs
Add feed system
Cost to Convert
t $1-2 M per conversion,
depending upon unit size, for
boiler, air pollution
modifications, etc.
• $.15-.5 M per con-
conversion, depending
upon size, for boiler,
air pollution mods,
etc.
Ash Handling
• Ash output increased has
larger heat sink, requires
water jacketed ash removal
system
With high inorganic
RDF, bed material will
require removal and
classification
Ash at scrubber and
waste heat boiler
will increase
29
-------
Table 6 (cont'd.)
TECHNICAL CONSIDERATION FOR RDF USE IN
EXISTING SLUDGE INCINERATORS*
Consideration
MHF-Pyrolysis
FBF
Air Pollution
• Increase in scrubber capacity
required
Increase in
particulate emissions
and shift in particulate
particle size
Operational
• Around-the-clock operation will
result in lower maintenance
costs and longer equipment life
Intermittent
operation less
harmful due to
heat sink nature
of sand bed material
Waste Heat Recovery • 75 percent of input BTUs
Potential for CBA mode, less for LTC
and HTC modes
50 percent of input
BTUs
* Source: Gordian Associates Inc.
30
-------
• pyrolysis can become autogenous with sludge with a lower
solids content; and
• pyrolysis is easier to control than incineration in an MHF.
Codisposal System Design. The first generation of MHF-codisposal
projects in the United States is likely to be modelled after the demon-
stration plant at the Central Contra Costa Sanitary District treatment
plant. Although MHF manufacturers are confident that this represents
a viable technology as it stands, they recognize that there are a num-
ber of possible refinements which might improve the MHFs' performance
in coincineration and particularly in copyrolysis.
RDF Process. For codisposal in an MHF, the RDF process should in-
clude shredding, air classification, and screening of the MSW. One
process modification currently receiving much attention is the place-
ment of the trommel (rotary screening) unit operation prior to shred-
ding. This process change provides two significant benefits:
• glass and other abrasive inorganics are removed in large
pieces prior to shredding, thus reducing shredder wear,
maintenance, and power consumption; and
t air classification performance improves relative to its
ability to separate large versus very small inorganic
particles, thus improving the quality of the light RDF
fraction.
The resulting RDF should have the following characteristics:
• <_ two to three inch particle size;
• as much wire, string, rags, etc., as possible should be
removed or reduced in length, as these items tend to jam
the rabbling mechanism;
• the water and heat content should be constant to facilitate
control of the furnace, particularly for coincineration; and
• to prevent equipment wear, the abrasive content should be low,
particularly if a screw-feeding machine is used.
Due to volume considerations, densified RDF (d-RDF) may be preferable
so that the necessary Btu input can be fed to the furnace. However, this
would increase capital and operating costs for RDF production.
Furnace Feed System. RDF can be fed to the MHF by a screw conveyor
at one or two locations, or by pneumatic feed to a table feeder. A
screw feeder is required for pyrolysis, since the additional air added
31
-------
by a pneumatic feeder would upset the balance in the pyrolysis condi-
tion. The screw feeder creates an air seal between the atmosphere and
the combustion chamber. Again, control of the incineration and, to a
lesser extent, the pyrolysis processes is highly dependent on maintain-
ing a uniform feed rate.
Waste Heat Recovery. Seventy-five percent of the Btus in the fur-
nace feed can be recovered by means of a waste heat boiler in the CBA-
pyrolysis mode. This figure will be lower for the LTC and HTC pyroly-
sis modes, as a substantial amount of the heat value of the furnace
feed is retained by the char and may not be recovered.
In order to maximize heat recovery, the location of the waste heat
boiler should be as close to the furnace as possible. Finding space
sufficiently close to the furnace to justify heat recovery may be dif-
ficult in existing sludge incinerators. Thus, an individual analysis
of each site is required in order to determine the feasibility of heat
recovery.
Incinerator Modification. The following items must be added to, or
modified on, an existing MHF before it can be used for coincineration
or copyrolysis:
• An RDF feed system should be added.
t Rabble arms should be raised and their number increased in
order to handle the bulky RDF/sludge feed.
• A hot cyclone venturi scrubber is required to handle the
increased quantity of particulates in the off-gas. An
incidental benefit of the LTC-pyrolysis mode may be the
elimination of the need for a scrubber. Due to the large
diameter of the particulate matter, a cyclone located in
front of the afterburner may be sufficiently effective to
meet air quality emission standards.
• A split-draft duct may be added in order to remove some of
the hot combustion gases and convey them to the off-gas sys-
tem. Dampers would be required to control the relative split
of hot gases (as well as the furnace pressure in the pyrolysis
modes).
The following additional modifications are required to retrofit an
MHF for copyrolysis (but not coincineration):
• The MHF should be sealed to prevent air leakages into the
furnace, since additional air cannot be added without up-
setting the balance of the pyrolysis condition.
32
-------
• Reduction of the induced draft fan speed, or alternatively,
closing of the damper is required in order to provide less
air to the system.
• An afterburner is required for the combustion of the pyrolysis
gas. Most states require that the off-gas temperature exceed
1400 F in order to prevent odors.
• A waste heat boiler, located as close to the MHF as possible,
must be added if heat is to be recovered efficiently. This
utilizes the sensible heat in the off-gases in addition to
the heat released during the combustion of these gases.
• A water-jacketed ash removal system is required to remove
the greater quantity of ash/char produced in codisposal.
t Additional controls and instrumentation are required par-
ticularly for the regulation of excess air, since the py-
rolysis process is controlled by limiting the admission of
air into the furnace.
• A corrosion-resistant lining should be applied to the in-
terior walls of the off-gas system, since pyrolysis gas
can be quite corrosive. Sufficient protection may already
exist if the MHF was designed for sludge containing ferric
chloride.
Retrofit Costs. Envirotech estimated that the cost to convert an
MHF to copyrolysis would be $1 to $2 million depending on the size
and characteristics of the unit in question.
Codisposal in FBFs
Mode of Operation. Incineration would be the preferred mode of
operation for codisposal in an FBF. A relatively high quality pyroly-
sis gas (360 to 440 Btu/scf) can be produced by pyrolysis in a fluidized
bed system. However, in most cases, the additional capital and operat-
ing costs would far outweigh any benefits of producing this gas.
Codisposal System Design. The fluidized bed manufacturers believe
that a dry RDF system, like that designed for the Western Lake Superior
Sanitary District in Duluth, Minnesota, would be adequate for codisposal
in FBFs. Wet pulping of the MSW would produce a better feed material,
but the advantages of this system would not justify the additional
capital and operating costs that would be incurred.
RDF Process. The RDF process for codisposal in an FBF would ideally
include trommel ling or rotary screening to remove glass and other
33
-------
abrasives; shredding, and air classification. The RDF can be coarse
(up to six inches in diameter) resulting in lower capital and operating
costs for RDF production.
Furnace Feed System. RDF can be fed by either ram-feeding or by
means of a pneumatic system.
Haste Heat Recovery. Several of the more recent FBFs already have
waste heat boilers installed, with the steam being used to heat the
fluidizing air. Where a waste heat boiler must be added, it may be
difficult to locate it near the furnace. As with the MHF, individual
analysis of each site is required in order to determine the feasibility
of waste heat recovery.
Approximately 50 percent of the Btus in the furnace feed can be
recovered as steam from a waste heat boiler in an FBF system. This is
less than for an MHF (CBA pyrolysis mode) since a greater volume of
excess air is used which reduces the efficiency of the boiler.
Incinerator Modifications. Before an FBF can be used for coincinera-
tion, the following modifications must be made:
- Add a feed system.
- Add a waste heat boiler. Some of the more recent installations
already have this piece of equipment.
- Add a bed-removal classification system for the removal of in-
ert material that builds up in the bed. This would be particu-
larly necessary where the quality of the RDF is low (i.e. high
inorganic fraction).
- Design a "hot shell" to inhibit corrosive condensation of
hydrogen chloride and hydrogen flouride gases. These gases
result from the combustion of plastics in MSW.
- Modify the off-gas system to capture increased particulate
emissions. Discussions with Dorr-Oliver indicate that it
will be necessary only to reset the adjustable throat on the
scrubber in more recent installations.
Retrofit Costs. Dorr-Oliver estimated that the costs for retro-
fitting an FBF would range between $150,000 and $500,000, depending
on the size of the unit and whether addition of a waste heat boiler is
required. This cost does not include the necessary RDF handling equip-
ment (e.g. storage bins) and other site modifications. Retrofit costs
for an FBF are lower than for an MHF, since no afterburner is needed
and only minor modifications of the scrubber are required.
34
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6. CAPACITY OF EXISTING INCINERATORS FOR RDF
In this section, methodologies are presented to estimate the amount
of RDF that can be combined with a given quantity of sewage sludge for
codisposal in an MHF or FBF. For an accurate determination of the
capacity of a particular unit, other factors such as heat content of
the feed and the design of the furnace must be taken into consideration.
Methodology For Determining RDF Potential
in Multiple Hearth Furnaces
The methodology for determining the capacity for RDF comsumption in
a MHF was developed by Gordian in collaboration with Nichols Engineering
and Research Corporation. Two modes of operation, incineration and low
temperature char (LTC) pyrolysis, are considered here. These methodolo-
gies are based on there being a limit to the amount of heat that can be
released in a MHF. Nichols cautions that capacity could also be limited
by the volume of RDF that can be handled by the furnace, particularly
where the RDF is light and bulky. Therefore, it may be necessary for
the RDF to be densified or pelletized in order to achieve the capaci-
ties estimated by the methodology presented here. Pyrolysis results
in a greater furnace capacity than incineration since only part of the
energy in the feed material is released in the furnace, the remainder
being retained in the char (LTC, HTC modes) or in the combustible py-
rolysis gas.
Prior to calculating potential RDF consumption in an MHF, it is neces-
sary to determine the following:
- hearth area of the furnace (square feet),
- application rate of the sludge (wet pounds per square foot
per hour), and
- solids content of the sludge (percent by weight).
For the incineration mode, the RDF capacity is depicted in Figure 10.
The application of the sludge is located on the y-axis, and by reading
across to the appropriate curve for the solids content of the sludge,
the RDF capacity (wet pounds per square foot per hour) is found on the
x-axis. The total capacity of the furnace is obtained by multiplying
this number by the hearth area of the furnace, which can be obtained
from Table 7.
35
-------
9 ,30%
CAPACITY FOR RDF (wet lbs./ft.2/hr.)
Figure 10. RDF capacity of multiple hearth furnaces in the incineration
mode. Source: Gordian Associates Incorporated.
36
-------
Table 7
HEARTH AREA DETERMINATION
Fu'n»«
ID
13"
20"
30"
39"
54"
51V
T
S'/i"
10VS'
12'
1454'
16V
18'
20'
23%' .
26'
28'
OO fa w»n irtciness oi:
6"
' 18'
•@2%'
28"
4"
4-3"
5-6"
6'6"
8'0"
9'6"
1V6"
13'0"
15'6"
17'6"
19'0"
2TO"
24'6"
27-0"
29'0"
9"
44"
•7"
4'9"
6'
7'
8'6"
10'
12'
13'6"
16'
18'
19'6"
21'6"
25
27'6"
29'6"
13/i"
5'6"
6'9"
7'9"
9'3"
10'9"
12-3"
14'3"
16'9"
18'9"
20'3"
22'3"
25'9"
28'3"
30'3"
Mirumufn
ColuTm
neijhi
r
1'3"
r/4"
T/,"
4'
4'
51
61//
6%"
6M"
r
T
8'
8'
8'
8'
8*
SQUARE FEET OF EFFECTIVE HEARTH AREA AND "NORMAL" SHELL HEIGHT
Furnace jutnj chin
1
1
2
4
7
15
24
32
47
80
97
143
131
215
269
-382
463
610
2
4
2'0"
8
2'2"
14
3'6"
31
4'2"
47
4'8"
65
6'3"
94
6'3"
148
8'4"
185
6'9"
286
8'0"
363
8'4"
431
8'4"
538
9-6"
764
1T4"
926
12*7"
1155
13'
3
6
12
19
42
63
96
138
227
287
422
534
634
790
1145
1389
169O
4
8
4'0"
16
4'4"
28
6'
63
7'4"
94
8'3"
130
IQ'10"
188
ID'S"
295
14'0"
390
ITS"
573
13'2"
727
14'3"
863
14'4"
1077
16T1
1528
18'9"
1852
20-9-
2235
.22'
5
10
20
32
74
110
161
235
374
487
716
908
1078
1346
1909
2315
2770
6
12
6'0"
24
6'
37
8'6"
85
10'6"
125
1V10"
193
15'5"
276
1ST1
442
19'8"
575
16'7"
845
18'7"
1068
20'2"
1268
20'2"
1530
22'9"
2292
26'2"
2778
28'10"
3315
31'
7
28
42
98
145
225
323
521
672
988
1249
1483
1849
2674
3241
3850
8
32
7'8"
43
1VO"
112
13'8"
166
15'5"
256
20'0"
364
19'5"
5S9
2-5 -4"
760
2T5"
1117
24'1"
1410
26'0"
1660
26'1"
2034
29-6"
3056
33'7"
3704
36'1 V
4395
4O'
9
288
411
668
857
1260
1591
1875
2350
3438
4167
4930
10
319
24'7"
452
23-10"
736
3TO"
944
26'4"
1400
29'6"
1752
3m"
2060
31 '11"
2600
36-2"
3818
4T
4630
45'
5475
49V
n
1041
1540
1933
2275
2860
4200
5093
6010
12
1123
3T2"
1675
35'0"
2090
37'9"
2464
37'10"
3120
eri\"
4584
48'5"
5556
53'1"
6555
58'
* Source: Envirotech.
+ Hearth area of multiple hearth furnaces.
sludge incinerators is generally 13-1/2 inches.
The wall thickness for
37
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The capacity obtained by this methodology may not be attainable in
a unit in which the hearth is distributed over a large number of hearths
(i.e. a tall and narrow MHF) since the cooling air velocity would be
high due to the small drop-hole area at each hearth. Flue gas velocity
could limit capacity in codisposal since it could prevent the RDF from
falling through .the drop holes. This constraint can be alleviated by
adopting a split-draft furnace design. This design could increase the
heat release capacity of the furnace by as much as two and one-half
times.
RDF consumption in the LTC (pyrolysis) mode can be computed by
using Figure 11. Knowing the application rate of the sludge in wet
pounds per hour (y-axis), the RDF capacity can be read off on the x-
axis. No allowance need be made for the solids content of the sludge
since the amount of combustion occurring in the furnace can be con-
trolled to maintain a maximum level of heat release sufficient for
the evaporation of all the moisture in the sludge.
The LTC mode has the greatest capacity for RDF since a greater
proportion of the energy in the feed material is retained in the char
than in the HTC or CBA modes. The capacity of the CBA mode is only
slightly higher than that of coincineration (with a 30 percent solids
sludge), since, by burning the char in the lower hearths, as much heat
is released as in incineration except for the energy retained in the
pyrolysis gas. The capacity of this mode could also be increased by
adopting a split-draft furnace design. The capacity of the HTC mode
will be intermediate between the CBA and LTC modes-
Methodology For Determining RDF Potential in
Fluidized Bed Furnaces
The methodology for calculating the capacity for RDF consumption in
an FBF was developed by Dorr-Oliver, Inc. This capacity is limited by
either the maximum quantity of heat (Btu/hr) that can be released in
the FBF during combustion (heat release capacity) or by the maximum
quantity of water that can be evaporated. The factors that determine
the capacity of an FBF depend on the heat value and the water content
of the combined sludge/RDF feed to the FBF.
The calculations presented below assume an average sludge with a
70 percent volatile solids content and a heat value of 9,500 Btu/lb
of volatile solids (6,650 Btu/lb dry solids). The RDF was considered
to be 75 percent solids, with a heat value of 5,000 Btu per wet pound.
In addition, it was assumed that any retrofit would include the addi-
tion of heat recovery (e.g., a hot windbox or recuperator) where the
combustion air is preheated. This would increase the capacity of the
unit to consume RDF. The procedure for the calculation of RDF capacity
is as follows:
38
-------
4 6 e 10
CAPACITY FOR RFO (wet lbs./ft.2/hr.)
Figure 11. RDF capacity in multiple hearth furnaces in
LTC (pyrolysis) mode. Source: Gordian Associates Incorporated.
39
-------
(1) The freeboard area (F) of the FBF is calculated using the
diameter of the furnace:
F = Trr2
r = radius = % diameter
(2) The evaporation rate (E) for the FBF can be determined from
a graph supplied by Dorr-Oliver (reproduced as Figure 12) for
the solids content of the sludge to be incinerated.
(3) The water evaporation capacity (W) is then calculated:
W = E x F
(4) The capacity of the unit to incinerate sludge (C) can then
be calculated:
C = W
w
w = percent water in sludge
(5) Using the amount of sludge (S), in wet Ibs/hr that would be
fed to the FBF per hour, the percent capacity that would be
required to incinerate this sludge - the incineration capacity
utilization (U) - is calculated: ,
U = C x 100
(6) Knowing the sludge incineration capacity utilization (U) and
the percentage of dry solids (S) in the sludge, the ratio of
wet RDF to wet sludge (P) can be determined from a series of
plots provided by Dorr-Oliver (reproduced as Figure 13).
(7) The capacity of RDF is then calculated:
R = P x S.
Since the graph provided by Dorr-Oliver did not consider situations
where the incineration capacity utilization (U) is below 25 percent,
in these cases, the calculation of RDF capacity can be based on the heat
release capacity (Btu/hr). (When the solids content of the feed material
is high, the FBF capacity is determined by its heat release capacity
rather than by its capacity to evaporate water.) The heat release capac-
ity of an FBF with heat recovery was given as 188,797 Btu/hr/ft2 by Dorr-
Oliver. The RDF capacity is then calculated as follows:
40
-------
41
40.8
»
cr
to
> 40.6
S-
40.4
-M
re
. 40.2
t/i
^v 40
UJ
I—
c5 39.8
o
£ 39.6
LU
39.4
39.2
39
15 20 25 30
% DRY SOLIDS IN SEWAGE SLUDGE
35
Figure 12. Water evaporation rate in Dorr-Oliver fluidized
bed furnace. Source: Dorr-Oliver.
41
-------
2.4
2.3
2.
2.1
2.0
1.9
1.8
1.7
1.6
O 1.2
o
I I.I
Q_
-^
O
fe '.o
^~
uj
3
-------
(1) The Btu content of the sludge (Hs) is calculated using a
value of 6,650 Btu/lb of dry sludge:
Hs = S1 x 6,650
S = quantity of sludge
(dry Ib/hr)
(2) The remaining heat release capacity (Hr) can be found by
subtraction:
Hr = 188,797 x F - Hs
(3) Using a value of 5,000 Btu/wet Ib of RDF, the RDF capacity
(R) is then calculated by:
R = Hr (wet Ibs/hr)
5,000.
43
-------
7. CONCLUSIONS
In order to determine the feasibility of a codisposal system at
a particular location, a complete review of the system is necessary.
Important considerations are:
- sludge quality and quantity,
- capital costs for the RDF plant,
- capital costs for retrofit of the sludge incinerator,
- method and cost of housing equipment,
- equipment redundancy requirements,
- operating and maintenance costs,
- uses/markets for recovered energy,
- supplemental fuel cost (for start-up and stand-by fuel),
- sludge pretreatment options,
- ash/char disposal options,
- type of operations: continuous or intermittent, incineration
or pyrolysis,
- emergency methods of sludge handling, and
- institutional constraints.
As this report has documented, the codisposal of sludge and solid
waste in existing multiple hearth and fluidized bed furnaces should be
investigated as an alternative energy technology approach to the dis-
posal of these wastes. Under the right circumstances, coincineration
and copyrolysis are viable approaches to handling municipal solid
wastes and sewage sludges. Not only are both solid waste streams dis-
posed of in an environmentally acceptable manner, but benefits can be
accrued by utilizing the waste heat or combustible gases for energy
conservation. As the cost of fuel increases, the importance of energy
conservation will be magnified, and codisposal will become an increas-
ingly attractive approach to a community's sludge and solid waste
problem.
44
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REFERENCES
1. Lowe, R.A. Use of solid waste as a fuel by investor-owned
electric utility companies. Minutes of EPA/Edison
Electric Institute Meeting, Washington, D.C., Mar. 5-6,
1975. Environmental Protection Publication SW-6p.
[Washington], U.S. Environmental Protection Agency,
July 1975. 31 p.
2. Gordian Associates Inc. Overcoming institutional barriers
to solid waste utilization as an energy source; final
report. Prepared for the Federal Energy Administration.
[Washington], Federal Energy Administration, Jan. 28,
1977. 225 p. (Unpublished report.)
3. Electric Power Research Institute and Edison Electric Institute.
Utilization of Refuse Derived Fuels by Electric Utilities,
Seminar/Workshop, San Diego, Calif., Nov. 16-18, 1977.
[115 p.]
4. [R. Bastian.] Municipal sludge management: EPA construction
grants program; an overview of the sludge management
situation. Evironmental Protection Publication 430/9-
76/009; Municipal Construction Division EPA/MCD-30.
Washington, U.S. Environmental Protection Agency, Office
of Water Program Operations, Municipal Construction Divi-
sion, April 1976. 68 p. (Distributed by National Techni-
cal Information Service, Springfield, Va., as PB-266
695/6BE.)
5. Personal communication. A. Hais, U.S. Environmental Protection
Agency, Water and Hazardous Materials Programs, to H.W.
Gershman, Gordian Associates, Sept. 1, 1977.
6. Niessen, W., et al. (Roy F. Weston, Inc.) A review of tech-
niques for incineration of sewage sludge with solid wastes;
final report. Environmental Protection Technology Series
600/2-76/288. Cincinnati, U.S. Environmental Protection
Agency, Office of Research and Development, Municipal En-
vironmental Research Laboratory, Dec. 1976. 238. p.
(Distributed by the National Technical Information Service,
Springfield, Va., as PB-266 355/7BE.)
7. Sussman, D. Co-disposal for solid wastes and sewage sludge.
Waste Age, 8 (7):44, 46, 49, July 1977.
-------
8. Holloway, J.R. Refuse derived fuel. In resource recovery tech-
nology, an Implementation Seminar, Chicago, 111., June 28-
29, 1977. [Washington], U.S. Environmental Protection
Agency, Office of Solid Waste, Resource Recovery Division.
[p. 98.]
9. Holloway, J.R. Refuse derived fuel. [p. 97.]
10. Levy, S. J., and H. G. Rigo. Resource recovery plant implementa-
tion: guides for municipal officials; technologies.
Environmental Protection Publication SW-157.2 [Washington],
U.S. Environmental Protection Agency, 1976. p. 31.
11. Gordian Associates Inc. Overcoming institutional barriers to
solid waste utilization as an energy source, p. 67.
12. Sewerage costs rise at faster pace. Engineering News Record,
Dec. 22, 1977, p. 102.
13. Gordian Associates Inc. Overcoming institutional barriers to
solid waste utilization as an energy source, p. 67.
14. Sieger, R. B., and P. M. Maroney (Brown and Caldwell, Inc.). In-
cineration-pyrolysis of wastewater treatment plant sludges.
Report no. 757-008/7007. Prepared for the U.S. Environmen-
tal Protection Agency Technology Transfer Design Seminar
for Sludge Treatment and Disposal, 1977- [Washington],
U.S. Environmental Protection Agency. 33 p.
15. [J.R. Harrison et al.] Process design manual for sludge treat-
ment and disposal. Environmental Protection Publication
625/1-74/006. [Washington], U.S. Environmental Protection
Agency, Office of Technology Transfer, Oct. 1974. p. 8:12.
(Notebook.)
16. Sieger, R. B., and P. M. Maroney. Incineration-pyrolysis of
wastewater treatment plant sludges, p. 11.
17. von Dreusche, C., and J. S. Negra. Pyrolyzer design alternatives
and economic factors for pyrolyzing sewage sludge in mul-
tiple hearth furnaces. Belle Mead, N.J., Nichols Engineer-
ing & Research Corp., [undated]. [25 p.] (Unpublished
report.)
18. Sieger, R. B., and P. M. Maroney. Incineration-pyrolysis of
wastewater treatment plant sludges, p. 22.
19. [H.R. Harrison et al.] Process design manual for sludge treat-
ment and disposal, p. 8:18.
US GOVERNMENT PRINTING OfFICE 1979-625-610/1367
46
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