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1-25
-------
Although MHFs were used for incineration of sludge long before the advent of the fluidized-bed
incinerators, they are falling from favor. Because of the increasing severity of the requirements im-
posed for pollution prevention, the off-gases product can no longer be released to the atmosphere
and substances that cause noxious odors must be decomposed at high temperatures before their
discharge. As a result, with the MHF, the off-gases must either be recycled to the hot region of the
incinerator or they must be led to separate burners operating at temperatures higher than 800 °C
(1,500°F). It is precisely this requirement that has been primarily responsible for the demise of the
multiple-hearth incinerator.
Few multiple-hearth incinerators have been constructed recently, at least in West Germany,
because the fuel costs involved in removing the odors from the flue gases often exceed the cost of
incineration. Economic factors favor using the fluidized-bed incinerator, which has a high enough
temperature to degrade the odors before emission to the atmosphere. The capacity of a multiple-
hearth incinerator, expressed in terms of the dry solids in the sludge, is only about one-third of that
of a fluidized-bed incinerator of comparable size.
Fluidized-bed incinerators in Germany are similar to U.S. designs. Controlled amounts of
dewatered sludge are fed into the fluidized bed of sand, which is heated to 750°-850°C (1,400° -
Table 1-9. Total production of nonstabilized wastewater sludge.
(In millions of cubic yards)
Country
Austria
Belgium
Denmark
Finland
France
Germany
Greece
Ireland
Italy
Netherlands
Norway
Portugal
Sweden
Switzerland
Spain
Turkey
United Kingdom
United States
1.8
3.4
3.1
58.9
98.1
—
0.5
31.4
5.2
1.8
—
6.3
3.8
—
—
48.4
160.9
Am.ount
Generated
(Year)
(1979)
(1977)
(1977)
(1980)
(1980)
—
(1980)
(1979)
(1979)
(1980)
—
(1978)
(1979)
—
—
(1980)
(1980)
Percentage
Incinerated
1.7
9.0
—
15.0
8.0
—
—
5.0
0.6
—
—
—
9.5
—
—
4.0
25.0
Cubic yards x 0.7646 = cubic meters.
1-26
-------
1,600°F). The constituent water evaporates, the combustible substances burn in the bed or
freeboard, and the combustion residues are swept out of the incinerator by the flue gases. The dust
content of the flue gases may be as high as 200 g/m3 (87 gr/cu.ft) and is reduced below the legally
permissible level of 100 mg/m3 (0.04 gr/cu.ft.) almost exclusively by electrostatic precipitators. In
some cases, wet scrubbers have been installed to eliminate the gaseous pollutants (e.g., sulfur diox-
ide, hydrogen fluoride, and hydrogen chloride). The operating costs for fluidized-bed incinerators
was about 300-500 DM ($114-$ 190) per ton of dry solids in 1983, including dewatering.
A combination of the multiple-hearth and fluidized-bed designs has been developed for the pur-
pose of direct heat-transfer drying of sludge and removal of noxious odors from flue gases. This
type of incinerator is equipped with a predrying and distribution zone that may have up to six
hearths; the fluidized-bed chamber is located underneath. Forty to sixty percent of the water contain-
ed in the waste material is evaporated in the predrying zone; the vapors flow through the upper sec-
tion of the combustion zone where all volatile components should burn out. Due to the highly effec-
tive drying operation, the needed cross section of the fluidized-bed zone becomes correspondingly
smaller.
To ensure the maximum burnout of the flue gases, an after-burning chamber may be linked to
this type of incinerator. In this type of plant, the combustion chamber may be kept even smaller
with corresponding savings in capital costs.
Rotary kilns are comparatively expensive to install, and the capacity of units of similar size is
much less than that of the MHF or FBF.
WASTEWATER SLUDGE INCINERATION IN JAPAN
Sludge incinerators were first installed in Japan in 1962. Table I-10 shows the number and type
for 1977, 1981, and 1983 and illustrates that the multiple-hearth type is predominant both in number
and in total incineration capacity. In particular, all incinerators with a capacity of 150 dry tons/ day
(t/d) or more are MHFs. The largest size is rated at 300 wet tons per day, based on loading rates
that are more conservative than those typical in the United States.
The percentage of total incinerating capacity accounted for by MHFs has dropped. In 1977 it
was 89 percent, but in 1981 it was down to 84 percent. The share represented by FBFs has in-
creased, and new types have recently been installed. These include wet oxidation, pyrolysis process,
and melting furnaces.
The first installation of a fluidized-bed incinerator was in the early 1970s. Most of these are in
smaller treatment plants that require intermittent operation due to low sludge production, and
capacities are up to 100 tons per day, wet cake basis. Most of the plants that plan to construct new
incinerators are smaller-size facilities, so the ratio of fluidized-bed units will grow larger in the
future.
Rotary kilns and moving-bed type incinerators are seldom considered the most suitable design
and are built only infrequently.
1-27
-------
Table 1-10. The number and types of sludge incinerators in Japan.
Multiple-hearth
Fluidized-bed
Inclined grate,
moving bed, and
others
Process devel-
opment units
(PDU)
Pyrolysis
Melting
furnace
Wet oxida-
tion
1977
No. of No. of Capacity
Units Plants (tons/d)
74 61 5,831
9 8 305
6 4 159
1981
No. of No. of Capacity
Units Plants (tons/d)
81 58 7,859
21 6 164
15 13 469
1983
No. of
Units
80
13
10
5
2
3
1-28
-------
CHAPTER II. IMPROVING SLUDGE INCINERATION METHODS
INCINERATOR DESIGN PRACTICES
This chapter utilizes case studies to discuss how several operational problems were overcome at
existing incineration facilities. The key to engineering these changes is the understanding of basic
design considerations, since these remedial actions began with what amounted to redesigns. It is
therefore necessary to present information required in the design of a typical incineration facility.
Although the discussion that follows presents the design procedures for a multiple-hearth incineration
process, many of the same concepts apply to other sludge combination processes as well.
Specifying the Feed
The first step in the design of a municipal sludge incineration system is defining the feed that
the system must handle. The usual parameters are:
1. Feed rate - stated in kilograms (pounds) of wet-basis cake per hour
2. Properties of the feed -
a. Percent solids (preferable to expressing as percent moisture)
b. Percent combustibles in the solids (volatiles plus fixed carbon)
c. Gross, or higher, heating value of the combustibles (HHV)
d. Ultimate analysis of combustibles
e. Presence of chemicals (e.g., lime) that react endothermically
Not often stated, but highly desirable, are softening and fusion points of the ash as determined
by ASTM Method D-1857-68 if a representative sample of the sludge or ash can be obtained.
In many instances, precise information on the feed is not known when specifications are
prepared. Instead, ranges of expected values are given to ensure that the furnace meets the needs of
the wastewater treatment plant (WWTP). All too often the designer increases this uncertainty when
communicating with the furnace manufacturer by specifying very wide and unrealistic ranges of
values for the feed parameters. The numerical permutations and combinations that result from this
practice prompt equipment designers to supply a single piece of equipment and expect it to operate
over an unrealistic range of conditions. It is perhaps analogous to specifying a car for use in carry-
ing a large family, pulling a camper on vacation and, at the same time, city driving, and getting 15
km/1 (35 mpg) fuel economy. The responsibility for making a "best guesstimate" on the sludge feed
and keeping this estimate within values that can be satisfied by a single-size piece of equipment is
clearly the designer's.
An effective alternative to specifying ranges is to specify various possible modes of plant opera-
tion and then develop, for each of these modes, the two major parameters mentioned previously,
feed rate and properties of the feed. One must then decide on:
• Minimum and maximum furnace exhaust temperature and
• Minimum percent oxygen in the exhaust gas or, in other words, the amount of excess air.
IM
-------
Prior to finalization of the specifications, heat and material balances should be prepared for each
case. A summary table should indicate, as a minimum, the following items:
1. Sludge combustion air requirement - mass flow rate and volume rate, usually in kg/hr (Ib/hr)
and 1/s (cfm)
a. Shaft cooling air recycle
b. Ambient air
2. Auxiliary fuel requirement - kJ/hr (Btu/hr) or in fuel volume terms
3. Auxiliary fuel combustion air requirement - units same as (1) above
4. Furnace exhaust flue gas volume - actual 1/s (cfm)
After preparation of the summary table that indicates minimum and maximum values for each
parameter, this table should be examined to determine if the adjustments required of the individual
equipment items are within the useful operating range of the equipment.
A single factor applied to the quantity of sludge to be processed should be the sole basis for
establishing the sizing of individual components. This will result in a harmonious design of all com-
ponents of the system.
Understanding the Combustion Process
Problems in the incineration of sludge solids cannot be solved without a thorough and complete
understanding of the combustion process. Although "hit or miss" approaches will occasionally yield
the desired results, they cannot be relied on to keep a MHF—or any furnace—operating properly.
The combustion control logic of a MHF is not understood by most engineers, and many furnaces in
place today have been improperly designed; in addition, information contained in some operating
manuals provided by manufacturers is inaccurate.
One approach to design is to consider that the MHF operates as a number of individual furnaces
connected in series. The mechanical design (i.e., size and number of hearths; size, number, and
location of burners; and size, number, and location of combustion air nozzles) and the combustion
control logic should reflect this consideration.
The heat and material system balances that have traditionally been used as a basis of design for
the MHF treat the MHF as a "black box." This is not to say that the First Law of Thermodynamics
is invalid. The answers obtained by this "black box" approach certainly represent overall fuel and
air requirements but do not give any clue to understanding the combustion processes occurring on
the individual hearths. Without this understanding, it is impossible to determine, for example, the
proper location of the auxiliary fuel burners. This usual approach gives the total heat required in the
furnace, but installing one single large burner somewhere in the furnace would usually not represent
an intelligent design. Additionally, the "black box" approach gives no clue as to the control loops
necessary for control of the furnace.
"THERMAL JUMP" REVISITED
In the early years of municipal sludge incineration, the theory of the "thermal jump" derived
from the work of Rudolphs and Baumgartner was used to justify a moderate exhaust temperature of
nominal 430 °C (800 °F) in the gases leaving the furnace. Their paper stated that "distillation of
II-2
-------
volatile matters from sludge containing 25 percent solids did not occur until 80 to 90 percent of the
moisture had been driven off, regardless of the temperature." Stated another way, starting with a 25
percent solids cake, volatilization should not occur until the total solids content of the sludge is be-
tween 63 and 77 percent.
More recent data indicate that self-sustaining combustion takes place when the mass reaches a
total solids concentration as low as 48 percent. For combustion to occur, volatiles must be driven
off, and, therefore, these data appear contradictory. This contradiction can be explained by a more
detailed examination of the phenomena taking place within the MHF.
As the sludge proceeds on its downward, serpentine path through the MHF, moisture is con-
tinuously evaporated. The sludge on the hearth develops furrows caused by the action of the rabble
teeth. When sufficient moisture has been evaporated, the very volatile parts of the sludge on the
upper ridge of the furrow begin to undergo destructive distillation before they are turned over by the
next pass of the rabble arm. The exact point at which this occurs is difficult to ascertain and is ob-
viously affected by many variables. The previously mentioned value of 48 percent total solids is not
an unreasonable estimate, however, if it is recognized that this is an average value for all sludge
within the hearth area, and the sludge on the upper ridges is substantially drier.
Ideally, to ensure complete combustion, the hearth where these volatiles begin to distill off
should have active combustion with visible flame and the hearth temperature should be 760 °C
(1,400°F) minimum. In addition, the volatiles should be exposed to this temperature for a defined
length of time. Unfortunately, this is not always possible to achieve in actual practice. Certain
organic materials called condensables may escape. The condensables and odors are the result of in-
complete combustion of volatile organic compounds. These compounds are products of pyrolysis and
materials distilled off from sludge during the drying process before the sludge reaches the active
combustion hearth. The condensables are the material normally caught in the liquid impingement
train of the EPA Method V paniculate test, the fraction often referred to as the "back-half catch."
A number of MHFs operating with exhaust temperatures in the 400 °C (800 °F) range have had
odor problems and have failed particulate emission tests because of the high contribution of the
back-half catch. These problems have been corrected by:
1. Substantially increasing the temperature of the combustion (hottest) hearth, which in turn in-
creases the temperature of the hearth immediately above the combustion hearth. This is
where volatilization is most likely to occur, and sufficient temperature is provided in the
gases to combust the distilled organics.
2. Providing an afterburner (either at the zero or top hearth or as an external unit), which
operates at sufficient temperature (nominal 760 °C [1,400°F]) to ensure complete combustion.
Of the two methods described above, the afterburner approach gives the greater degree of con-
fidence, especially where there are low boiling organics present. Both methods imply the use of aux-
iliary fuel to reach a higher temperature in the gases. When a dewatered sludge with an adequately
high total solids content is available, a 760 °C (1,400°F) exhaust temperature can be achieved
without the use of auxiliary fuel.
Furnace Operation
Incineration of sludge solids in an MHF occurs in four distinct zones (see Figure 1-3):
1. Moisture evaporation;
II-3
-------
2. Distillation and combustion of volatiles;
3. Combustion of fixed carbon; and
4. Ash cooling.
The boundary between zones may occur part-way across a given hearth. Assuming that the feed
rate and thermodynamic properties of the sludge are a given (i.e., not subject to control), there are
three primary variables that can be manipulated in a conventional-style MHF with all sludge combus-
tion air going to the bottom hearth(s):
1. Flow rate of sludge combustion air;
2. Auxiliary fuel firing rate; and
3. Rotational speed of the rabble arms (rpm).
Thus, the following parameters are the controlled variables of the combustion process:
1. MHF exhaust temperature (temperature of the uppermost hearth prior to the effect of a zero-
hearth afterburner or external afterburner);
2. Excess air in the MHF exhaust gas; and
3. Temperature of the combustion hearth.
Temperatures are usually measured by thermocouples, connected to appropriate instrumentation.
There is no instrument called an excess air meter. What is actually measured is volume percent ox-
ygen in the exhaust gas, either hot or cooled in a sampler. Measurement of hot flue gas containing
much water vapor from sludge moisture and combustion products is termed "wet" basis. If, on the
other hand, measurement is made on gases that have been cooled and scrubbed and have only
residual moisture at 100 percent relative humidity, it is termed "dry" basis. Future discussions refer
only to percent oxygen and, unless otherwise stated, this is on a "dry" basis. A handy reference
formula for converting percent oxygen (dry basis) to excess air is:
Percent Excess air = [O2/(21 - O2)] x 100
O2 = Percent oxygen (dry)
The temperature of the combustion hearth is henceforth referred to as the temperature of the "hottest
hearth."
A Look Inside a Multiple-Hearth Furnace
To fully appreciate why the conventional "black box" heat and material balances are inadequate
for the design and evaluation of the MHF combustion process, it is necessary to examine the com-
bustion process as it actually takes place. The results of a series of heat and material balances are
shown graphically in Figures II-1 and II-2. Since these graphs are for comparative purposes only, it
is advantageous to make certain simplifying assumptions.
• Assume shell heat loss is zero. Normally it amounts to only 2-3 percent.
II-4
-------
• Assume all shaft and rabble arm cooling air is recycled back to the MHF as sludge combus-
tion air. As a result there will be no net heat loss from the heated shaft cooling air being
discharged to the atmosphere.
• Assume zero percent combustibles in the ash. Normally this value would be 1-4 percent.
For these calculations, two differently conditioned and dewatered sludges were chosen. One
sludge was chemically conditioned (CCS) and the other was thermally conditioned (TCS). The ther-
modynamic properties of each follow.
CCS TCS
Percent total solids 25 40
Percent combustibles 65 60
Combustible heating value 23,000 26,000
(U/kg combustibles)
Combustible ultimate analysis
Carbon 50.64 53.73
Hydrogen 7.22 7.71
Oxygen 37.14 33.06
Nitrogen 5.00 5.50
Sulfur 0.00 0.00
TOTAL 100.00 percent 100.00 percent
In the graphs, the theoretical temperature of the products of combustion is plotted against per-
cent total solids at 9.0 percent oxygen (75 percent excess air). The conventional "black box" heat
and material balance would indicate, for the 25 percent total solids (TS), an exhaust temperature of
approximately 450 °C (850 °F). However, if it is assumed that combustion begins when the TS
reaches 50 percent, then the temperature on the hottest hearth (combustion hearth), where the 50
percent TS sludge is burning at 9.0 percent oxygen, should be approximately 980 °C (1,800°F).
Thus, what many have observed is shown graphically: the temperature of the hottest hearth is
significantly higher than the exhaust temperature. A temperature of 980°C (1,800°F) would probably
cause clinkers, and therefore the furnace operation would have to be modified.
Figures II-1 and II-2 also show the effect of operating at increased percent oxygen (excess air)
in the flue gas. It is commonly stated in the literature that MHFs are operated at excess air rates in
excess of 100 percent to assure oxygen for combustion. This large quantity of excess air is not re-
quired for complete combustion but is required to maintain the temperature of the hottest hearth at a
level that will avoid clinker ing or thermal stress to the furnace. Even though the desired result has
been achieved in existing MHFs, the simplified approach used heretofore has prevented an adequate
combustion control logic from being developed.
A value of 6.0 percent oxygen (40 percent excess air), under the proper combustion conditions
of time, temperature, and turbulence, is sufficient for complete combustion. In Figure II-1 the
temperature is plotted for values of 6.0 percent oxygen (40 percent excess air) and 0 percent oxygen
(0 percent excess air). The problems of excessive temperatures on the combustion hearth are obvious
in these situations relative to those with greater excess air. It is for this reason that where MHFs
have been operated at nominally 6.0 percent oxygen (40 percent excess air) a starved-air combustion
mode has been used.
II-5
-------
3000
2800
2600
2400
2200
2000
u. 1800
O
| 1600
a
| 1400
a>
1200
1000
800
600
400
200
65% Combustibles
10,000 BTU/lb Combustibles
I
I
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
25 30 35 '40 45 50
Percent Total Solids
55
60
Figure 11-1. Heat and material balances using 23,000 kJ/kg (10,000 Btu/lb) combustibles
(°C = (°F - 32) 5/9).
Figure II-2 shows data for TCS. At 10.7 percent oxygen (103.3 percent excess air), the furnace
exhaust temperature is 760 °C (1,400°F), adequate to assure complete combustion and deodorization
without the use of auxiliary fuel. Because of the ballasting effect of the high excess air, the
temperature of the hottest hearth, at 50 percent TS, is approximately 870 °C (1,600°F), or only
110°C (200 °F) hotter than the exhaust temperature. Therefore, in a MHF with adequate air handling
capacity, a TCS can be easier to incinerate.
Hearth-by-Hearth Balances
To perform a hearth-by-hearth heat and material balance, it is necessary to have an extensive
data base to develop the "rate and heat transfer equations" that determine the success of any
mathematical model of this type. Some information on these has been published, but most MHF
manufacturers and knowledgeable consultants consider this information proprietary. Parameters that
should be included in a hearth-by-hearth furnace simulation model are:
II-6
-------
Q.
E
0)
1800
1600
1400
1200
1000
800
600
400
200
40
40% Total Solids
60% Combustibles
11,000 BTU/lb Combustibles
1
I
50 60
Percent Total Solids
1800
1600
1400
1200
1000
800
600
400
200
Figure II-2, Heat and material balances using 26,000 kJ/kg (11,000 Btu/lb) combustibles
(°C - (°F - 32) 5/9).
II-7
-------
1. Moisture evaporation rate (kg/m2/hr or Ib/ft2/hr) as a function of hearth temperature and gas
flow rate;
2. Heat loss to rabble arms as a function of temperature and the number of rabble arms on a
hearth;
3. Shell heat loss as a function of surface area and hearth temperature;
4. Air leakage through flap gate feeders and hearth doors;
5. Percent total solids at onset of volatile distillation and combustion; and
6. Separate combustion rates (kJ/m2/hr or Btu/ft2/hr) and heat release rates (Btu/fT3/hr) for both
volatiles and fixed carbon.
With a comprehensive model (incorporating a hearth-by-hearth heat and material balance), a sen-
sitivity analysis can be made for the wide variety of sludges that the furnace is likely to encounter in
a typical wastewater treatment plant, thereby making it possible to optimize the design and cost-
effectiveness of the MHF.
Moisture Evaporation Rate
With the exception of the fixed carbon combustion and ash cooling hearths, moisture evaporation
rate dominates the processes on all other hearths within the MHF. Moisture evaporation consists of
three steps:
1. Sensible heating phase — Before any evaporation can take place, heat must be added to the
sludge until the vapor pressure of the free water in the sludge exceeds the vapor pressure of
the water vapor in the flue gases. For typical sludge incineration in a MHF, the sludge will
reach approximately 71 °C (160°F) before rapid evaporation begins.
2. Constant rate phase — Once the above phase is reached, evaporation usually takes place at
a nearly constant rate over a certain range of moisture.
3. Critical moisture point — When the sludge has reached the critical moisture point, the dry-
ing rate occurring in the constant rate phase begins to fall. At this point (nominally 48-50
percent TS) the percent total solids in the upper ridges is higher than the average, and
volatilization of the combustibles begins to occur.
An optimum rabble arm speed is where the width of the level portion in the valley of the fur-
rows is approximately 3 cm (1 in). When rabble speed is too fast, this width will increase. When it
is too slow, it will fill in with sludge. Both of these have the effect of reducing the projected area
exposed to the hot gases and radiation from the roof.
When an attempt is made to "move the fire," or change the location of the hottest hearth by
slowing down the speed of the rabble arms, it can only be done at the expense of increasing the in-
ventory of sludge on a hearth, which can lead to a "runaway" (uncontrolled burning) furnace should
this large inventory of sludge begin to burn. Additionally, volatilization is likely to occur on a hearth
that is not up to proper combustion temperature (760°C [1,400°F] minimum), and unburned fuel
gases, including tars and oils, will be discharged from the furnace (observed as smoke).
II-8
-------
Attempts to control furnace operation by varying the speed of the rabble arms have largely prov-
ed unsatisfactory. Since a definite cause-and-effect relationship between rabble arm speed
(manipulated variable) and any other controlled variable has never been satisfactorily established,
rabble arm speed has been eliminated from the list of parameters considered in the hearth-by-hearth
heat and material balance.
Combustion Control Logic
The importance of correct combustion control logic for a MHF has been stressed. A MHF con-
trol circuit developed from incorrect hypotheses cannot succeed. MHF operators should not be
blamed for improper operation of their furnaces when they are not given the proper instruction
and/or control system.
In seeking economical MHF operation, there are a number of variables that should be
controlled:
• Temperature of flue gas leaving the furnace,
• Excess air (measured as percent oxygen),
• Temperature in the combustion hearth gases, and
• Location of the combustion hearth.
In a typical MHF fed at a constant rate, there are a number of conditions that can be
manipulated to help keep the controlled variables at their set points:
• Mass flow rate of sludge combustion air (recycled shaft cooling air plus outside air),
• Rotational speed (rpm) of the central shaft,
• Firing rate (temperature set point) of auxiliary fuel burners, and
• Hearth location of burners currently firing.
In a typical MHF, with almost all of the sludge combustion air introduced in the bottom hearth,
the combustion hearth is always the hottest hearth (HH). All hearths located above and below it are
at lower temperatures.
It is desirable that the temperature of the HH always be maintained at set point temperature.
Temperature control is achieved by varying the flow rate of sludge combustion air (SCA). In a
stable burning mode, the flow rate of the SCA is decreased in order to increase HH temperature.
Conversely, the flow rate of the SCA is increased in order to decrease HH temperature.
The maximum set point temperature for the HH is determined by either (1) the temperature, as
measured at the wall, at which the sludge begins to form clinkers, typically 870° - 980 °C (1,600° -
1,800°F) or (2) temperature limitation of the furnace, typically 1,000°C (1,900°F), which is based
on the grade of firebrick used and the alloy in the rabble teeth. A further discussion of slagging and
clinker formation is presented in Chapter III.
To maintain the burner flame safety circuit in a "purged" safety condition, a burner in the up-
permost hearth that receives auxiliary fuel is always lit. Unless it is needed to provide additional
heat to the furnace, this burner will remain on low fire.
II-9
-------
For safety reasons, all burners should be turned ON by the MHF operator and not by the con-
trol system. When the combustion control logic circuit determines that a burner should be turned
ON, a light on the control panel will signal the operator. The control panel lights, however, will
also tell the operator WHICH burner should be started. The control circuit will automatically turn
burners OFF.
When the combustion control logic circuit determines that more auxiliary fuel is needed in the
furnace, it will signal the operator as to WHICH burner to light. Once this is done, the combustion
control logic circuit varies the fuel firing rate until the desired results are achieved. The control cir-
cuit increases the fuel firing rate by AUTOMATICALLY increasing the burner set point temperature
on the "selected" fired hearth. Conversely, when less auxiliary fuel is required, the control circuit
AUTOMATICALLY decreases the burner set point temperature on the fired hearth, which in turn
decreases the amount of auxiliary fuel being fired into the furnace. This type of control loop is com-
monly called cascade control.
The maximum set point temperature for any fired hearth should be at least 50 °C (100°F) less
than the set point temperature of the HH. This avoids control circuit problems by assuring that the
control logic will not confuse a fired hearth with the HH.
DEWATERING SLUDGE
Many processes are available for removing water from sludge (dewatering) and thus preparing it
for combustion. Each of these processes can be designed in various ways, make use of different
commercial equipment, and be operated in alternative modes. Their objective is to ultimately pro-
duce a high-solids cake and thereby minimize auxiliary fuel usage in the subsequent combustion
process.
Thickening, conditioning, and dewatering are the common processes for removing water from
sludges. Other processes such as drying and dehydration are less common. Some processes (like
composting and combustion) result in water removal, but that is not ordinarily their primary
purpose.
Dewatering of sludge to produce a feed to the incinerator is a critical step for the process of
combustion. Both centrifugal and filter-type dewatering equipment have been greatly improved in re-
cent years. It no longer holds true that incineration is an unreasonable consumer of fuel. Many in-
cinerators today operate in an "autogenous" mode, using no fuel for moisture evaporation at all.
The burning quality or heat content of a sludge as fed to an incinerator may principally be im-
proved in three ways:
1. Remove water from the sludge more effectively by using the best available type of dewater-
ing equipment with the most appropriate conditioning process.
2. Before feeding sludge to the incinerator, dry the sludge partially or completely, in addition to
dewatering it, by using the heat from combustion that would otherwise be wasted.
3. Add a combustible material to either the dewatered or undewatered sludge as an augmenta-
tion of its heat value in relation to its moisture.
Dewatering Equipment
The most common approach is to design new facilities with the latest and most cost-effective
type of dewatering machine. In centrifuges, this is currently the variable-speed backdrive type that
11-10
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has reached the market in the past decade. In filtering equipment, the continuous-belt filter press has
taken a major share of the market once held by the vacuum drum filter. Another filtering method is
the recessed plate filter press, which was adapted from the chemical industry and is commonly pro-
vided in an automated version.
A strong endorsement of the belt press comes from the city of Hartford, CT. Similarly,
dramatic improvements in dewatering with the belt press were reported by the wastewater treatment
plants in Rochester, NY, and Duluth, MN. In these cases, the substitution was for rotary vacuum
drum filters. Some plants that have attempted to use continuous-belt filter presses in lieu of cen-
trifuges, however, have reported dissatisfaction with performance; examples of this are: Philadelphia
Southwest, Denver Metro, and Central Contra Costa in California. The principal problem with the
belt press arises when the sludge feed varies in quality and this causes a change in the required
polymer dose. The operator must be alert to such changes and modify the dose to meet the new con-
dition. Failure to do this results in the sludge being squeezed out at the sides and causes a severe
housekeeping problem. A centrifuge will give more solids in the centrate and a wetter cake in the
same situation but not cause a housekeeping problem.
Carver-Greenfield Dewatering (Drying) Process
The Carver-Greenfield (C-G) process was developed specifically for application to "liquid-
solid" slurries and has been successfully applied to various industrial slurries as well as industrial
and municipal wastewater treatment plant sludges. In the C-G process, water is essentially extracted
from sludge using a multiple-effect evaporator or vapor recompression dryer that results in a con-
siderable economy of steam compared to single-effect heat dryers.
A schematic diagram of the process as designed for the Hyperion Energy Recovery System
(HERS) in Los Angeles, CA, is presented in Figure II-3. Dewatered sludge is first mixed with an
oil, such as light-high-boiling solvent, which serves as a carrying or fluidizing medium. The carry-
ing oil assures that fluidity is maintained in all phases of the evaporation cycle and that formation of
scale or corrosion of the heat exchangers is minimized. Sludge-oil slurry is then pumped to a
multiple-effect evaporator where water is vaporized. The remaining solids-oil mixture is then cen-
trifuged and hydro-extracted (i.e., steam stripped) to separate the carrying oil from the solids.Carry-
ing oil is recycled for reuse in the evaporative cycle while the solids are removed for other pur-
poses, including subsequent combustion or reuse in agriculture. Oil and grease content (i.e., freon or
hexane extractables) of the Hyperion sludge varies from about 8 to 15 percent of the dry sludge
solid weight. These nonpolar components dissolve in the carrying oil. A sidestream of the carrying
oil is continuously withdrawn from the C-G evaporator and distilled to separate light fluidizing oil
from higher-molecular-weight sludge oils. Fluidizing oil is returned to the C-G process, and the
sludge oil is stored for subsequent combustion in the fluidized-bed gasifier. Startup of the Los
Angeles HERS System is scheduled for late 1985, with full operation in early 1986.
Continuous Belt Filter Press
Hartford, CT, Case History. Hartford, CT, began pilot testing belt filter presses in the spring
of 1978. Test results showed that significantly drier sludge cake was produced at a higher production
rate with a belt filter press (BFP) than could be accomplished with the existing vacuum filters. The
plant staff then conducted side-by-side performance tests of the best performing BFPs to select the
first BFP for procurement and installation. The first BFP was installed in 1979 and its startup and
shakedown were carefully monitored. Despite numerous mechanical problems and excessive
downtime (25 percent), the BFP quickly performed so cost effectively that approval for acquiring a
second press was granted only 4 months after installation of the first one. The payback period for
the first press was only 6 weeks. In selecting the second BFP, performance tests were again con-
11-11
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ducted to evaluate overall performance, mechanical design, and maintenance features of competitive
presses and to incorporate the most desirable requirements into the bid specifications. The second
BFP was supplied by a different manufacturer than the first one and was installed in December
1979, just 8 months after the first one.
Since Hartford was one of the first plants to try the BFP, operational difficulties were expected.
Initial BFP operation included problems with the bearings, spray water pump, filter screen cleaning,
filter screen tracking, and filter screen seam closures. With assistance from the manufacturer, the
first press was retrofitted and upgraded for more reliable operation. The second through fourth BFPs
were from a different supplier and had fewer mechanical problems.
As more experience was gained, improvements were made in several key operating conditions.
Filter screen seam closure wearing was reduced by using scraper blades of a higher molecular
weight plastic, and an increase of from 500 hours to an average of 1,500 hours of filter screen
operating life resulted. Proper polymer conditioning of the sludge was a problem on all the BFPs. A
two-component liquid polymer mix was developed in experiments by a polymer supplier to reduce
dosage requirements to the same level as was required for the vacuum filters. Changes in the sludge
conditioning tank to improve polymer/sludge mixing also helped reduce dosage requirements and in-
creased operational flexibility for adjusting to the sludge's variable characteristics. Maintaining a
constant BFP feed by mixing the raw primary and waste-activated sludges from three plants requires
close operator control. Sludge blend variations of only 5-10 percent can cause a press screen plug,
resulting in sludge squeezing out at the ends of the rollers. This results in a reduction in cake solids,
lost production, and a messy cleanup job. In spite of these operating problems associated with reduc-
ing a new operating technology to routine production line practice, the operational improvements and
cost savings achieved with the BFPs at Hartford were dramatic.
Energy savings realized from the BFP conversion were significant. From the time of the plant's
startup in 1972, the activated-sludge mixed-liquor suspended solids (MLSS) concentration had
averaged 4,000-5,000 mg/1, requiring approximately 32.8 m3/s (100 million cubic feet per day) of
dissolved air. With the BFPs, the increase in dewatered sludge production has enabled the MLSS
level to be lowered to a more desirable 2,000 mg/1 range. The resulting decrease in the dissolved
oxygen demand reduced the daily air usage to approximately 18 m3/s (55 million cubic feet per
day). This reduction, in turn, reduced the electrical energy requirements of a 2,238 kW (3,000 hp)
air compressor by 20 percent, which amounted to a $200,000 per year savings in electricity costs.
Also, each vacuum filter had a 53.3 kW (71.5 hp) requirement as compared to 16.4 kW (22 hp) for
each belt press. This reduction in electrical use resulted in an estimated savings of $25,000 per year.
In addition, the elimination of the vacuum pumps resulted in a maintenance savings of $6,000 per
year. In total, these savings amounted to $231,000 per year.
The average specific fuel consumption or gallons of oil per dry ton (dt) sludge solids and the
moisture-to-volatile (M/V) ratio for the Hartford incinerator operations for the years 1978-81 are
shown in Table II-1.
The savings resulting from the belt filter presses are reflected in the sharp reduction in the
sludge cake M/V ratio, particularly in 1980 when the major fuel reduction was achieved. The net
reduction of an average of almost 0.34 I/kg (82 gal/dt) of oil would translate into savings of over
3.21 x 103m3 (848,000 gal) of oil at the 1982 dry ton production level of 9.41 x 106 kilograms
(10,351 tons). Coupled with the dramatic reduction in fuel consumption, there was also a 57 percent
gain in the volatile solids incineration rate per operating equipment hour, which is the key produc-
tion performance parameter. Furthermore, the average incinerator hours of operation per day for two
incinerators also dropped from 46.5 in 1978 to 35.7 in 1981, a 23 percent decrease. This meant that
only two of the three plant incinerators had to be used routinely.
11-13
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Table 11-1. Moisture-to-volatile ratio for Hartford incinerator operations.
Year Percent change
Variable 1978 1979 1980 1981 (1978-81)
Percent solids
Sludge cake M/V
Fuel consumption
13.8
8.6
125.2
14.5
8.1
116.1
18.5
5.8
60.5
19.5
5.4
43.5
+ 4
- 37
- 65
(gal/dt)1
Incineration rate 0.7 0.7 1.0 1.1 +57
(volatile tons/
incinerator/hr)2
\2
'gal/dt = 0.004 I/kg
2t/hr = 907.2 kg/min
These substantial results were accomplished after a considerable amount of time and effort was
invested by the Hartford plant management, staff, and operating personnel. The experience of Hart-
ford with the belt filter presses serves as a classic example of the opportunities that exist in many
plants throughout the country to achieve cost-effective performance by the adoption and modification
of new operating technologies.
Improvement of Centrifugal Dewatering by Steam Injection
Kansas City, KS, Case History. Municipal Wastewater Treatment Plant No. 1 is located
southwest of downtown Kansas City and situated on the Kansas River. The design flow is 0.3 m3/s
(7 mgd) with potential to expand to a maximum of 0.92 m3/s (21 mgd). Plant No. 20 is a complete
mix-activated, sludge-type secondary treatment plant with aerobic sludge digestion and sludge in-
cineration. The flow diagram for the wastewater treatment process is shown in Figure II-4.
Typically, secondary sludge alone will mechanically dewater to cake solids of 10-14 percent.
Following extensive testing and development work at Northwest Bergen County WWTP in
Waldwick, NJ, a centrifugal dewatering system was provided at Kansas City, employing steam
heating of the secondary sludge and an eddy current back-drive for the centrifuge. Steam was used
to heat the secondary sludge to 73.9°C (165°F) just ahead of the centrifuge. The eddy current back-
drive for the centrifuge automatically adjusts the differential speed between the bowl and the scroll
to maximize solids' residence time in the centrifuge and to keep the centrifuge operating at full load
regardless of percent sludge solids in the incoming feed. It is not known if this sludge heating with
steam can be universally applied to all secondary or activated sludges. It certainly worked well at
Kansas City Plant No. 20, as shown in Table II-2. The fire tube boiler is designed for an operating
pressure of 700 kPa (100 psig). It is normally operated at 400 kPa (60 psig), as this is all that is re-
quired for steam injection into the sludge feed line to the centrifuge. Also, operation much below
400 kPa (60 psig) (saturated steam 153°C [307 °F]) is not recommended because of potential corro-
sion problems at lower steam pressures and temperatures. The steam requirement at Kansas City
Plant No. 20 for the conditions specified in Table II-2 and Figure II-4 is 1,114 kg/hr (2,455 Ib/hr)
of 400 kPa (60 psig) saturated steam. The fire tube boiler is rated at 1,800 kg/hr (4,000 Ib/hr), so
some hot gas is bypassed around the boiler to balance the steam supply with steam demand.
11-14
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Steam
Steam
Thickened
Secondary Sludge
Waste
Heat
Boiler
Shutoff
Valves
MB
Water
Venturi *T^
Scrubber k
Thickened
Primary Sludge
Fluidizing
Air Blower
Dorr-Oliver
Stamford, CT
To Turbo
Generator
Kaw Point
Plant No. 1
Only
Stack
Water
Tray
Cooler
Effluent
Figure II-4. Municipal Wastewater Treatment Plant No. 1, Kansas City, KS.
DRYING SLUDGE BEFORE COMBUSTION
Drying the sludge before incineration has not been adopted in the United States, except where
cocombustion of sludge and solid wastes is the objective. Stamford, CT, has done this successfully
in a rotary dryer for several years (see Chapter III). Attempts at Harrisburg, PA, to use a "porcu-
pine" dryer have been unsuccessful, although this unit has worked satisfactorily in Europe. At the
Flint River WWTP in Clayton County, GA, the Heil dryer is used and the sludge is pelletized.
Two examples of systems for combustion sludge that is burned by itself are located at plants in
Minneapolis-St. Paul, MN, and Norwalk, CT. At the Metro WWTP serving the Twin Cities, two
rotary dryers were provided in the facilities started up in 1983 to provide a dry sludge option in case
a potential offsite use was developed or if very wet feed was being delivered from the sludge
dewatering system. As of this time, these dryers have not been used. Because thermally-conditioned
sludge is developed very effectively, the main operating problem has been too "hot" a feed to the
furnaces, instead of excess moisture. At the Norwalk plant, an add-on dryer system of the fluidized-
bed type was started up in 1983 and is reported to be operating successfully. This system links the
previously installed Fluosolids® combustor with the new dryer by lifting hot sand to the dryer,
where the feed sludge becomes mixed with the sand. The now-dried sludge solids mixed with sand
are dropped back to the main combustion chamber and burned. A further linkage of the two vessels
is that the fluidizing air for the dryer is heated in the second stage of the air preheater that extracts
energy from the hot combustion gases. The first stage provides a hot air stream to the windbox of
the combustor. The benefit of such integration is savings in fuel. The water is taken away from the
11-15
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Table 11-2. Improving dewatering by stream injection-
Municipal Wastewater Treatment Plant No. 20, Kansas City, KS.
Secondary Sludge Heating
Sludge feed
Secondary sludge - Percent solids in centrifuge cake
Primary sludge - Percent solids in centrifuge cake
Ratio of primary solids/secondary solids
Yes
23
27
1.7 to 1
No
12
27
1.7 to 1
Percent solids in composite centrifuge cake
(feed to combustor)
Fluosolids® combustor
Capacity - Pounds feed solids per hour
Auxiliary fuel - Btu x 106 per ton solids
Power - kWh per ton solids
Operating costs (47.4 tons/solids per week)
Operator hours/week - shifts/week
Operator labor cost ($25,000/year/person)
dollars/ton solids
Auxiliary fuel cost ($5/million Btu) -
dollars/ton solids
Power cost (50 per kWh) - dollars/ton solids
Total labor, fuel, and power
Savings per ton of solids
Annual savings (2,275 tons/year solids)
25
1415
4.18
290
72-9
19.78
$20.90
$14.50
$55.18
$33.00
$75,000
18.5
1079
8.56
380
96-12
$26.37
$42.81
$19.00
$88.18
1 kg = 2.20462 Ib 1 kJ = 0.948 Btu 1 Mg = 1.1023 ton
sludge solids at a much lower "cost" in calories per gram (Btu's per pound) of water than it would
be in the combustor. The moist off-gas leaves the dryer at a much lower temperature, perhaps
100° - 130 °C (220° - 260 °F), than it would if the drying was being done in the main combustor.
This moist, odorous gas is then wet-scrubbed to remove its moisture burden before being routed
back through the first stage of the preheater to the main combustor windbox and into the combustor,
where any remaining odors are destroyed.
11-16
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Another major benefit of the Norwalk installation, as there would be with any predrying pro-
cess, is that the capacity of the combustion device is expanded greatly. Much larger amounts of
sludge solids can be burned per hour because the limitation caused by necessary drying in part of the
combustion unit is no longer present. At Norwalk the burning capacity was doubled. Thus, a plant
with a sludge disposal load in excess of its burning capacity might find that adding a predrying step
expands its capability and reduces fuel consumption per unit of solids handled at a much lower cost
than if it elected to add another combustion system.
Drying ahead of combustion, of course, is not new. It has been practiced at Allegheny County
Sanitary District's plant (Alcosan) in Pittsburgh, PA, for many years, as well as in other locations
with Raymond-type flash dryer equipment that is linked to a combustor.
Energy-Efficient Dehydration Prior to Combustion
Heat drying to remove moisture prior to combustion, as discussed in the previous sections, can
be accomplished in a number of different ways. It can be thermodynamically advantageous provided
that the moisture is removed with less energy than that required to accomplish the same thing in a
furnace. Two heat drying processes are examined here: indirect contact steam dryers and the C-G
multiple-effect evaporation process. Other drying systems are available but generally require greater
energy input than the two examined here.
An energy balance for indirect steam drying shows that the thermal requirement for drying is
about 2,910 U/kg (1,250 Btu/lb) of water removed, which equates to 99,000 MJ/day (94 MBtu/day)
if it is assumed that the sludge is dried to 80 percent solids. Energy recovery from the furnace ex-
haust is about 77.2 percent of input, or about 104,430 MJ/day (99 MBtu/day). Recovered energy
essentially balances the requirement of the steam dryers. Net energy cannot be recovered unless the
cake solids concentration is significantly increased above the 20 percent assumed.
A four-effect C-G process will thermodynamically operate at about 930 kJ/kg (400 Btu/lb) of
water removed and produce a product with about 1 percent moisture. An energy balance for C-G
drying and combustion shows that 24,000 MJ/day (32 MBtu/day) is required for the C-G process
and about 111,000 MJ/day (105 MBtu/day) is recovered from the combustion system; thus, signifi-
cant net energy production can be accomplished even with 20 percent sludge solids.
Energy-efficient dehydration of wet sludge cake can produce a material capable of autogenous
combustion without the need for supplemental fuels. Sufficient steam can, in some cases, be
generated from thermal processing for net electrical power production as well as operation of the
drying process. Importation of fossil or alternative fuels to the treatment plant site is not required,
and the technology involved appears to have a low odor potential compared to other alternatives. For
these reasons, energy-efficient drying of digested, dewatered sludge using the C-G process was
selected by the city of Los Angeles.
Other Drying Methods
In addition to the use of hot gases from the incinerator in external direct or indirect drying
devices, other interesting developments in promoting more efficient drying within the incinerator
have taken place in Germany and Japan.
In Germany, several "hybrid" furnaces have been built that combine the fluidized-bed and the
multiple-hearth configurations in one shell. The MHF portion is above, and the combustion region is
below. Partially dried cake drops into the fluidized bed and is burned with minimal fuel demand.
Also, the diameter of the fluidized bed is lowered by this design because it is not evaporation-
11-17
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limited. Two such Lurgi furnaces have been in service in the Frankfurt treatment plant for several
years.
In Japan, studies have been carried out in experimental systems where gas is recirculated
through the drying zone to increase velocity over the sludge and increase the rate of evaporation.
This was found necessary in units operated in the starved-air combustion (SAC) mode because of the
low excess air supply, which in turn was intended to minimize the oxidation of chromium to the
hexavalent state, the most toxic form. One or two of the hearths are actually operated in reducing
conditions for this purpose, which in turn mandates the lesser air supply. This technology is ex-
pected to become increasingly popular in Japan and those parts of the United States with sludges
high in trivalent chrome.
FUEL NEEDS
Stable operation of a sludge solids combustor requires an equalization between heat in and heat
out; this is known as a heat balance. Heat in can be in the form of sensible heat energy and in-
troduced in the air supply, which is at an elevated temperature, or in the combustible material. Heat
out can be as sensible heat energy contained in combustion products, latent heat of water vaporiza-
tion, radiation and surface losses, and heat that is lost in excess air and equipment cooling. The
balance point is dependent on the amount of excess air that is allowed to pass through the furnace
and the recovery of energy from the gases leaving the furnace. A system requiring no heat other
than what is provided by the sludge cake is termed autogenous.
A furnace being fed sludge cake that does not have sufficient fuel value to balance the heat leav-
ing the system requires auxiliary heat to stay in balance. This is supplied by burners using commer-
cial fuel such as natural gas or fuel oil, which is fired into the furnace to inject heat.
Sludge cake that has an excess of heat content, as compared with the heat needed for drying and
elevating the combustion gas stream to target flue gas temperature, is termed superautogenous. If
superautogenous burning is conducted at high temperatures, equipment damage and ash fusion can
occur. Control of the temperature can be achieved by varying the air supply or by modifying the
cake moisture.
Fuel is not used in a sludge incinerator to burn sludge—it is burned to evaporate water. Any
sludge cake will burn by itself once it gets dry enough. Fossil fuel is only used to remove most of
the water, but sludge solids do not have to be bone dry to burn; self-sustaining combustion will
occur on a sludge lump once it gets to about 50 percent moisture or less as was discussed in an
earlier section. Frequently, dewatering sludge to a higher solids content is more economical than
using fuel in the incinerator to evaporate water.
Fuel is also needed for other reasons:
1. Startup heating and holding at standby of a furnace requires fuel that cannot be furnished by
sludge combustibles. Cooldown may require some fuel to manage the rate of temperature
drop.
2. Safety standards may require that at least one burner is firing at all times.
3. If a mandatory exit temperature is required by air quality permit or policy of plant manage-
ment, some fuel may be required to assure compliance with this requirement.
Heat sources that can be used to supplement the sludge combustibles to achieve heat balance at the
desired furnace exit temperature and oxygen levels are found in Table II-3.
11-18
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Table 11-3. Supplemental energy sources.
Source
Comment
Natural gas
Fuel oil
Air
Scum
Coal
Waste oil
Refuse-derived fuel (RDF)
Woodmill waste or wood chips
Paper mill waste
Industrial oily wastes
Repulped paper
Sometimes only available on an interruptible basis;
normally the lowest cost fuel
More expensive than gas
"Hot" air from the MHF shaft cooling system exhaust;
preheated air from the fluidized-bed windbox
Requires a special concentrator for waste removal and
a metering pump for controlled feeding
Requires dustproof construction within the plant for
handling; must use gas or oil for MHF temperature
control
Practical if a source of reasonable, steady volume is
available. Concern must be exercised regarding handl-
ing and contaminants
A potential fuel; has not yet been successfully used
Possible handling problems (bridging in bulk bias);
wood chips have been used successfully in FBFs
May contain a high ash level due to filler
Not recommended because of potential toxic emis-
sions and corrosion to ducts and scrubber system
Water on the fibers may nullify heat gain from
cellulose; requires dewatering and mixing with sludge
Understanding Incineration to Minimize Fuel Needs
Fluidized-Bed Incineration at Duff in Creek, Toronto. On the north shore of Lake Ontario,
east of metropolitan Toronto, the first stage of one of Ontario's largest sewage treatment plants is in
operation. The Duffin Creek Water Pollution Control Plant is being built in four stages. Stage 1 has
a daily treatment capacity of 289,250 m3 (50 mgd). The sludge combustion and energy recovery
system at this plant is a set of parallel trains of fluidized-bed combustors, each rated at 67 X 106
kJ/hr (63 x 10" Btu/hr).
This combustion facility has several interesting and innovative features. The fluidized-bed reac-
tors are designed to operate either as CWB or HWB units. The exhaust gas-to-air heat exchanger is
piped so that it can be bypassed or so that a part or all of the reactor hot gases flow through the
11-19
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heat exchanger. This means that the fluidizing combustion air to the reactor can be controlled from
about 60 °C (140°F) (heat developed by compression in the blower) with no air preheating when
burning drier sludges, up to a maximum air preheat of 621 °C (1,150°F) when burning wetter
sludges. Part of the steam generated in the water-tube waste heat boiler is used in a turbine to drive
the fluidizing air blower. The balance of the steam is used for process steam and for building
heating. In the future, when three more stages are added and plant capacity has increased, turbo
generators will be added to produce electrical energy that will be used in the wastewater treatment
plant. Venturi scrubber effluent with collected ash is thickened and then dewatered. Thickener
overflow is recycled to the Venturi scrubber. Dewatered ash is blended with dry ash from the waste
heat boiler dust hoppers to produce a damp ash material that is trucked to a landfill.
Successful performance testing was carried out on the system in January 1982. Air quality test
data and operating conditions during testing showed that total hydrocarbons in the stack ranged from
2 to 7 ppm and averaged 4.5 ppm; stack particulates were 0.13 gm/kg (0.25 Ib/ton) of feed solids;
combustibles in the ash were less than 1 percent. To verify the adequacy of the installation, higher
solids cake was imported from the nearby Lakeview plant. This thermally conditioned sludge filter
cake was 34-36 percent solids, and the performance was satisfactory; no fuel was required.
In early 1983, the Duffin Creek Plant was operating at reduced capacity. The belt filters were
not producing the projected 30 percent cake solids but rather cake solids ranging from 17 to 27 per-
cent and averaging about 20 to 22 percent. Also, the amount of volatile solids in the sludge was
below the design level. Because of the higher water content of the sludge and the lower volatile
solid content of the sludge solids, it became desirable and necessary to maximally preheat the air to
a temperature of 621 °C (1,150°F) so as to minimize the auxiliary fuel requirement. With increased
air pre-heating, the gas temperature to the waste heat boiler was lower, resulting in reduced steam
production. When burning a 22 percent solids concentration sludge of which 70 percent of the solids
are volatile, the auxiliary fuel requirement goes from 0 to 0.14 liters of No. 2 oil per kg of solids (0
to 34 gal/ton). The combustor capacity drops from a design of 4,350 to 3,570 kg/hr (9,600 to 7,860
Ib/hr). Steam production drops from a design value of 11,750 to 10,054 kg/hr (25,906 to 22,165
Ib/hr). The fluidizing air blower consumes the same amount of steam, 4,470 kg/hr (9,850 Ib/hr) in
both cases, but in terms of percentage of total steam generated, steam usage goes from 38 percent of
the total to 44 percent of the total. Tests were carried out with other belt presses for sludge dewater-
ing. These newer units in tests produced 4 to 5 percent higher solids content than is being obtained
with the existing units. These drier cakes, coupled with increased sludge generation rates, reduced
the usage of auxiliary fuel and thus reduced operating costs, but this was not considered sufficient.
In October 1983, purchasing commitments were made for membrane-type automatic plate-and-frame
filter presses..
UPGRADING EXISTING MHFs TO REDUCE FUEL NEEDS
Of the various types of combustors in common use, two offer the opportunity to make the dry-
ing process more efficient and two do not. Those that do are the MHF and the IEF. Both the MHF
and the IEF configurations provide a long residence time for the solids and permit separation of the
drying and burning zones for stagewise process control, a feature that is not possible in a FBF and
difficult in a rotary kiln furnace.
A major improvement in using the MHF as a better drying device came through a study at the
city of Indianapolis's Belmont Street Treatment Plant in 1981. This showed that substantial savings
in fuel consumption could be achieved with an appropriate process control strategy. The fundamen-
tals of this strategy are to (1) keep the burn zone low in the furnace, about two hearths above the
bottom; (2) utilize shaft cooling exit air to the maximum degree; and (3) minimize the excess air
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amount and off-gas temperature as much as possible while still achieving desirable air quality stan-
dards. The first and second goals provide the maximum amount of area above the burning sludge for
the drying process and use heat generated by the sludge combustibles instead of purchased fuel. The
third goal prevents heat waste by minimizing mass flow and heat content in the off-gas. These main
goals, together with supporting instructions, were also the basis of revisions implemented in sludge
incinerators at Nashville, TN; Buffalo, NY; and Hartford, CT, all of which are subautogenous feed
situations. The following list is a blend of general operating procedures utilized at Indianapolis and
Nashville:
1. Maintain steady feed rate.
2. Maintain top hearth draft at 0.51 to 2.03 mm (0.02-0.08 in) of water negative.
3. Maintain oxygen level at 4-5 percent at full rated loading, somewhat higher at partial load,
and a maximum of 8 percent at 55 percent of rating. These are for readings in-situ on hot
gases containing vaporized and combustion moisture; after scrubbing and cooling, the values
would be 2-3 percent higher because most of the moisture would be removed.
4. Control burn zone on hearth 6 (of 8, Indianapolis) or 7 (of 10, Nashville). This is managed
mainly by burner firing rate control.
5. Minimize or eliminate any use of burners on hearths above the burn zone. If a choice is
made, use lowest burners and only to control the initiation point of combustion.
6. Keep center shaft speed at a low, steady rate—100 seconds per revolution or longer. Do not
attempt to manage burning zone by varying the shaft speed.
7. Use only shaft cooling return air for sludge combustion.
8. Limit maximum hearth temperature to 843° - 899°C (1,550° - 1,650°F) to prevent formation
of clinkers.
9. Allow top hearth temperature to be what it will; do not use higher hearth burners to reach an
arbitrary standard, although occasional use for smoke abatement during upset conditions is
allowable.
As an added benefit, this strategy is reported to provide improved performance of scrubbing
equipment and better compliance with emission standards.This result is probably due to the lowered
air velocities attributable to less excess air and lower off-gas temperatures, which cause less entrain-
ment of fly ash.
Hartford, CT, Case History
The Hartford Water Pollution Control Plant provides primary and secondary wastewater treat-
ment for more than 170 X 103m3 (45 Mgal) of wastewater per day and generates in excess of 180
Mg (200 tons) of sludge cake per day. The sludge handling facility was originally designed in 1968
with four dissolved air flotation thickeners, five vacuum filters, and three multiple-hearth inci-
nerators. In 1978, before the conversion to belt filter presses described previously in this chapter,
the vacuum filters averaged 13.8 percent cake solids. Production required continuous operation of
three of the five vacuum filters, with two of the three incinerators operating around the clock. The
plant operation experienced the typical production and maintenance problems associated with handl-
11-21
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ing an extremely wet sludge cake. In addition, the Hartford plant started to receive sludge from
satellite plants in East Hartford and Rocky Hill. The incinerator operations were plagued with the
operating problems of handling very wet sludge cake and consuming large amounts of fuel. A Hart-
ford incinerator is shown schematically in Figure II-5. It is equipped for either gas or oil operation
and rated at 11.3 wet Mg (12.5 wet tons) per hour. No common operating procedure was used by
the incinerator operators. Each operator had certain specific practices and techniques for maintaining
Sludge Feed
Gas/Oil
Burners (1)
Burner Air
Cooling Air Exhaust
"Pop"
Damper
Return Damper
Cooling Air
8
11
Return
10
T
Ash
Exhaust
(1)4 Burners/Hearth
(2I1 Damper/Hearth
Rabble Arm
Cooling Air
Auxiliary Air
Figure II-5. Hartford incinerator system schematic.
11-22
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temperatures on various hearth levels and for managing incinerator airflow. The operators' preoc-
cupation with just burning the very wet sludge cake resulted in many inefficient operating practices.
Fuel Reduction Program. Reductions in fuel consumption for sludge cake incineration were
accomplished in two ways. The first was to substitute belt filter presses in place of coil-type vacuum
filters, and the second was to modify the MHF operating methods and upgrade their control system.
The second change involved the acceptance and implementation of advice presented by a specialized
consulting firm. Based on a careful study of equipment and operating practices, a new operating
mode was undertaken, and the collection and analysis of data was improved. The result was more
uniform procedure among the various shift operators and adherence to a greatly revised format of
operating variable control. Overall, this reduced fuel consumption by 83 percent, which in dollar
terms amounted to a savings of over $1,300,000 for 1982 compared to operation in 1978 and before
the changes.
To provide an accurate baseline for comparing the fuel reduction achieved by converting to belt
filter presses and improving incinerator operation, a statistical analysis was made of key performance
data, including temperature and oxygen content of the off-gas, fuel consumption, and air usage for
past operations and each of the years during which changes were made. In addition, the correlation
of specific fuel consumption (SFC) measured in liters of oil consumed per dry kilogram (gal/ton)
with the absolute sludge cake moisture to volatile ratio by weight (M/V) was computed to provide a
more comprehensive measure of change for comparison:
M/V
s x v
where
s — fraction solids in cake
v = fraction volatiles in cake solids
The M/V ratio is a site-specific parameter that is useful at a given plant for process control, but
is limited for interplant comparison to those having similar volatile heat values and running at com-
parable excess air ratios and breech temperatures. The average specific fuel consumption for the
Hartford incinerator operations in 1978 was 0.52 liters of oil per dry kilogram (125 gal/ton). The
sludge cake solids averaged 13.8 percent and the volatiles averaged 77.1 percent. The sludge cake
M/V ratio, which is directly related to and principally determines the specific fuel consumption de-
mand, averaged 8.6; this is relatively high. For example, if evaporative effectiveness is 4,700 kJ/kg
(2,000 Btu/lb) of water and sludge volatiles have 23,000 kJ/kg (10,000 Btu/lb) of heat value, both
expressed in gross heating value, cake having an M/V ratio of 5.0 would be autogenous. Any higher
M/V in this plant would require fuel. At 8.6, the cake would require 3.6 times 4,700 kJ/kg (2,000
Btu/lb) or additional 17,000 kJ/kg (7,200 Btu/lb) of volatile solids to reach balance at the exit gas
temperature and oxygen content because of the large amount of moisture in the sludge cake.
Operational Testing and Analysis. An operational analysis was made of the Hartford in-
cinerator operations and included measurements of airflow; analysis of exhaust gas; and assessments
of key instrumentation and controls, existing operator-specific practices, feed rate management,
airflow management, burner use profiles, hearth temperature profiles, and combustion zone location
and control. A demonstrated and proven kinetic incinerator analytical model was also used to deter-
mine the optimum loading rate and plant operating mode that would result in the minimum possible
fuel consumption. Preliminary investigation of operator practices found that no uniform operating
11-23
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procedure was used. There were several operating practices and lack of controls that were con-
tributing to excessive fuel consumption such as:
• Combustion occurring too high in the incinerator;
• High exhaust gas temperatures;
• High draft settings and too much auxiliary air;
• Misuse of heated rabble arm cooling air;
• Improper burner use choices;
• Improper techniques for controlling combustion zone location; and
• Lack of remote operator controls for airflow dampers and burners.
A preliminary analysis indicated that optimum airflow management could result in a potential
fuel reduction of 70 percent when burning sludge cake with an M/V ratio of 5.0 at an incinerator
loading of 6 tons/hr. Two examples of M/V at 5.0 would be 22 percent solids cake at 71 percent
volatile and 25 percent solids at 60 percent volatile. The two main parameters of furnace off-gas
(breech temperature and oxygen in the off-gas) must be kept as low as possible without violating air
emission limits.
The kinetic analysis for these conditions predicted that the potential fuel consumption for the
Hartford operation with such a dry cake was zero. This analytical result agreed with the empirically
based preliminary estimate drawn from airflow management, since an additional 30 percent of fuel
savings could be reasonably expected from improved combustion zone location control, optimum
burner use, improved load rate management, and the synergistic effect of these operating mode
techniques on fuel consumption. Based on these results and those from similar programs in In-
dianapolis, Buffalo, and Nashville, periods of autogenous combustion were expected with the new
operating mode. Autogenous combustion was achieved several times during the operational trial and
demonstration test for as long as 8 hours. During the period of routine use of the new operating
mode, there were many days in which no fuel was used over a 24-hour period. Based on the opera-
tional trial tests and analyses, a new operating mode with specific instructions and operating settings
was developed. The new operating mode was then demonstrated in full-plant operation for a 2-week
performance demonstration test period. On-the-job operator training in the use of the new mode was
also accomplished at the same time, since this is the only way to ensure continued good operation.
After completion of the successful performance test, the operating mode was further refined for
routine operational use.
The new operating mode was characterized by the following general operating guidelines:
• Maximize the use of the heated rabble arm cooling air return;
• Use the least possible draft (i.e., just slightly negative) to minimize air leakage;
• Combust on the third lowest hearth to maximize drying area;
• Replace cold auxiliary air supply with heated cooling air return;
• Minimize excess air by observing and minimizing oxygen in flue gas;
11-24
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• Use lower hearth burners to maximize drying temperature;
• Eliminate airflow to top hearth burners;
• Control combustion zone location with burner use profile;
• Slow center shaft speed to improve sludge drying;
• Discontinue use of hearth No. 5 burners; and
• Use the following operating parameters in breech:
Oxygen 4-5 percent in the raw off-gas, which equates to about 7.5 percent on a
moisture-free basis after scrubbing
Draft 0.05-0.20 cm (0.02-0.08-in) w.c. (negative)
Temperature 370° - 425 °C (700° - 800 °F)
The specific operating instructions that constituted the new operating mode were given to the in-
cinerator operators and included procedures for sludge load management, incinerator operation con-
trol, specific settings for normal operations, combustion zone location control, standby and startup
operations, and techniques to control sludge cake "burnouts." The most effective incinera-
tion/dewatering configuration was for two BFPs to feed each operating incinerator. The optimum in-
cinerator loading rate was found to be 6.6 Mg (6 wet tons) per hour per incinerator, approximately
half the design basis, based on analysis and trial tests of load rates between 5.0 and 7.7 wet Mg (4.5
and 7 wet tons). The 6.6-Mg (6-ton) per hour load rate was the minimum rate needed to keep up
with the overall plant production rate and still minimize fuel consumption, considering that the
average sludge cake M/V ratio was 4.5. (A 4.5 M/V would mean, for example, 22 percent solids
and 79 percent volatile or 25 percent solids and 67 percent volatile.) The improved operating mode
also enabled a further reduction in the M/V ratio because the new mode allowed the dewatering
presses to be slowed down, resulting in drier cake.
Fuel Reduction Results. The new incinerator operating mode was placed into routine operation
immediately following the 2-week performance test conducted in January 1982. Operational data for
1982 were analyzed to measure and compare the fuel reduction achieved. Shown in Figure II-6 is
the computed least squares correlation of the average specific fuel consumption vs the sludge cake
M/V ratio for the baseline period 1978-81 and for 1982. The improved thermal operating efficiency
achieved is reflected in the change of the slope of the relationship. This result was quite similar to
what occurred in Indianapolis, Nashville, and Buffalo when these plants implemented similar
operating techniques. Figure II-7 shows the average specific fuel consumption for the Hartford
operations from 1978 through 1982. The average specific fuel consumption for 1982 was 0.091/dry
kg (21.1 gal/dry ton) as compared to 0.18 (43.5) for 1981, approximately a 51.5 percent reduction.
With this improvement, the total fuel reduction achieved by Hartford between 1978 and 1982
amounted to 0.43I/dry kg (104 gal/dry ton), or 83 percent, which at the 1982 production level
represented a savings of 4,074.568 m3 (1,076,504 gal) of No. 2 fuel oil as compared to 1978.
In addition to reducing direct fuel consumption, the new operating mode provided increased
operating flexibility with the equipment because the incinerators could now be operated efficiently at
load rates 50-60 percent of capacity, which was not possible before without paying a tremendous
penalty in excess fuel consumption. Incinerator operation is also now characterized by cooler max-
11-25
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125
100
c
•- "c
0.^0
3 >•
O —
06
0) tr
3 O
H- «
£|
'o ra
75
25
I
I
45678
Sludge Cake Moisture to Volatile Ratio (Ib/lb)
Figure II-6. Specific fuel consumption vs sludge cake M/V ratio before and
after incinerator operating mode change at the Hartford plant.
imum operating temperatures, more steady state control, less particulate emissions, and reduced
maintenance on internal incinerator parts.
Cost Savings. The nominal cost savings from reducing incinerator fuel consumption on an an-
nual basis were estimated from the change in the specific fuel consumption from 0.52 to 0.091/dry
kg (125 to 21 gal) of oil/dry ton. Based on 1982 production of 9,388.4 dry Mg (10,351 dry tons),
the savings would be over $1,076,000 per year using an estimated price of $0.26 per liter ($1 per
gallon) for No. 2 fuel oil. The total estimated annual operating cost savings from converting to belt
filter presses and the new incinerator operating mode are over $1,300,000 per year.
AUTOGENOUS COMBUSTION
In Japan
In Japan, there are at least two plants utilizing the principle of autogenous combustion with
MHFs. For dewatering, a belt filter press is used at the Yotsuya Treatment Plant in Takaoka City,
while a recessed plate pressure filter is used at the Hojin Treatment Plant in Nagoya City.
Autogenous combustion at these plants was made possible not only by such factors as optimum
dewatering and incineration processes, but also by the improvements that resulted from the challenge
to operators and other personnel to achieve autogenous combustion.
11-26
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125
125
116
100
c
.o
'
§ >
qj 0}
3 CL
LL. _
'5 o
o
0) =
O) »
CD O
0)
75
83%
60.5
50
43.5
25
21.1
1978
1979
1980
1981
1982
Figure II-7. Average specific fuel consumption for the Hartford operations, 1981-82.
Yotsuya Treatment Plant Case History. The six-hearth incinerator at this treatment plant
started operation in July 1979 and has since operated in an autogenous combustion mode. Sup-
plementary fuel is only needed to heat the furnace at the start of combustion. Incinerating capacity is
30 wet metric tons per day. Feed is dewatered on high-pressure BFPs. In 1981, 19,521 liters (5,157
gal) of heavy oil were used for 8,122 Mg (8,934 tons) of dewatered sludge at 30 percent solids con-
tent. This showed that, on the average, only about 8 liters (2 gal) of heavy oil were used to in-
cinerate one metric ton (1.1 tons) of dewatered sludge solids. This incinerator is clearly one of the
most energy-conserving in Japan, since the consumption of supplementary fuel (heavy oil) by in-
cinerators is commonly 170 liters (45 gal) per ton of solid matter for a MHF. The savings in heavy
oil achieved by autogenous combustion represent about 7 percent of the total operation and
maintenance cost for the treatment plant. Currently, the incinerator operates on a weekly cycle (i.e.,
it starts on Monday and stops on Sunday). Switching to long-term continuous operation is technically
possible, and, if this is done, the number of heatup times will be reduced and costs further lowered.
Discussing the operation in greater detail, thickened sludge at a concentration of about 4 percent
solids is dewatered by high-pressure BFPs after being conditioned with polymer at 0.5-0.8 parts by
weight per hundred (pph) and ferric chloride at 5-7 pph dosage rates. The solids content of the
11-27
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dewatered sludge is about 38 percent. Ferric chloride is added to improve the release of the
dewatered sludge from the filter fabric.
Dewatered sludge is fed from the sludge hopper into the top of the incinerator by a screw-type
conveyor. Both are closed structures. Closed screw-type sludge conveyors are reported to be ex-
cellent from the standpoint of odor control and appearance but have previously been considered un-
suitable for incinerators. They tend to form sludge balls that do not completely burn in the furnace.
This plant's designer reconstructed the rabble arm teeth so that the lumpy sludge is completely com-
busted to ash in the MHF. When the ash drops from the bottom hearth, it is moved by conveyor to
the ash hopper. The ash is removed from the plant by adding about 20 percent water by weight and
is disposed to a landfill.
Sludge combustion air is induced into the bottom stage of the furnace by draft. Additional air is
induced into the middle stage of the furnace. The exhaust gas is cooled and thus dehumidified,
scrubbed, and alkali-washed in a cyclone spray-type scrubber that uses secondary effluent and caustic
soda. It is then deodorized by passing it through acid and sodium hypochlorite scrubbers, further
scrubbed by a wet-type electrostatic precipitator, mixed with hot air from the shaft cooling of the
furnace to prevent a steam plume, and discharged from the stack.
Problems and Solutions
In early trials, incineration by autogenous combustion displayed a number of problems compared
with incineration using supplementary fuel. Even though the heat balance between combustible mat-
ter and moisture indicated that autogenous combustion was possible, stabilized autogenous combus-
tion operation could not be achieved in conventional furnaces. Three major problem areas included
unstable combustion, unburned sludge, and use of large quantities of air. The combustion
temperature and location shifted frequently due to the varying amounts of combustibles in the feed.
Since the combustion zone was narrow, due to a lack of heat from the burners, the sludge retention
time on the hearth was short, and the temperature was frequently low; therefore unburnt sludge
resulted. High air use resulted because the combustion temperature was controlled by the amount of
cooling air; as the air ratio increased, heat loss to the exhaust gas increased and thermal efficiency
deteriorated.
Stabilized autogenous combustion operation resulted from the introduction of the procedures
discussed here. The volume of primary air flowing into the bottom stage of the furnace can be
automatically controlled, taking advantage of the draft effect that changes according to load change
in the furnace and the change of sludge character. This widens the combustion zone and prevents the
lowering of the surface temperature and the discharge of unburned matter in conjunction with a
change in the combustion load. The hearth with the greatest temperature in the combustion zone can
be detected with a sensor and the volume of secondary air directly induced into the combustion zone
can be manipulated so that the temperature on this hottest hearth is held between 700° and 900 °C
(1,300° and 1,650°F). In times of low-load operation, the operation can be governed by moving the
combustion hearth up higher in the furnace. Thus, heat radiated from the furnace walls and the
center shaft can be reduced and the lowering of the thermal efficiency prevented. These control
measures prevent unstable combustion, high air ratios, and discharge of unburnt sludge; they also
minimize clinker and slag formations.
Performance Test of MHFs at Yotsuya. An experimental program was undertaken by the
Japanese to investigate the conditions required for autogenous combustion in a MHF. This was done
by changing both the amount and the solids content of dewatered sludge put into a furnace. The
sludge employed for the study had a volatile solids concentration of 70 percent, and its high heat
11-28
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value was 5,800 kcal/kg (10,453 Btu/lb) VS. The incinerator was designed to handle 30 metric
tons/day (33 tons) of dewatered sludge with a water content of 70 percent and a volatile solids con-
tent of 70 percent.
In the study, stabilized autogenous combustion with an input load range of 9.5-40.6 metric
tons/day (10.5-44.7 tons) was possible if the net heating value (NHV) of the input dewatered sludge
was 400 kcal/kg (720 Btu/lb) cake or more. This range was 32-135 percent of the design value;
thus, it was clearly possible to handle a very wide range of loads. In this test program the NHV of
the dewatered sludge was only 350-370 kcal/kg (631-667 Btu/lb) cake. Maintaining the condition of
autogenous combustion for more than 6 hours was possible in some cases but impossible in others.
This seems to show that if the sludge has a low calorific value of under 350-370 kcal/kg (631-667
Btu/lb) cake, autogenous combustion in the furnace is not practical (NHV takes into account the heat
sink effects of sludge moisture, combustion products, and excess air, all raised to furnace exit
temperature).
The properties of exhaust gas at the furnace outlet were found to be as follows:
NOX concentration 100-260 ppm, related to 12 percent oxygen concentration
HCN concentration ND-66 ppm, related to 12 percent oxygen concentration
Dust 0.2-1.4 g/Nm3
Odor concentration 4,100-7,300 in autogenous combustion; 17,000 in combustion using sup-
plementary fuel
It was found that odor concentration was lower in autogenous combustion than in combustion with
supplementary fuel. When the degree of conversion into NOX and SOX was calculated with respect to
N and S quantities in dewatered sludge put into the furnace, on the assumption that 10 ppm of ther-
mal NOX will be created at 800° - 850°C, the conversion from N into NOX was 2.0-4.7 percent, and
the conversion from S into SOX was 84-120 percent. These values generally agreed with NOX and
SOX production in existing furnaces. The unburned portion of incinerated ashes was nil to 0.67 per-
cent; thus the extent of sludge combustion was very satisfactory.
Hojin Treatment Plant Case History. At the time of its planning, the incinerator at the Hojin
Treatment Plant in Nagoya City was believed capable of autogenous combustion. However, when it
began operation in April 1979, it could not be operated in the autogenous mode. As the control
system for incineration was improved, however, oil consumption went from about 10 liters (2.6 gal)
per metric ton of dewatered sludge to less than 3 liters (0.8 gal), including that required for startup
or maintenance of temperature when not burning.
Thickened sludge with a solids concentration of 3-4.5 percent is dewatered by a horizontal
recessed plate pressure filter after dosing with ferric chloride at about 8 pounds per hundred pounds
(pph) dry sludge solids and slaked lime at 25-30 pph. The solids content of the dewatered sludge is
32-45 percent and the filtration yield is 3-6 kg/m2/hr of dewatered sludge solids. Because of its
relatively low water content, the sludge is mixed by a pug mill and fed to the incinerator. The
slablike sludge cake is crushed in the mill to permit easy handling on a conveyor. This improves
both drying and combustion in the furnace. The incinerator has 10 hearths, but the first does not
function as a drying hearth. It is provided for potential future use as an afterburning chamber for ex-
haust gas deodorization. Nine hearths are used to carry out the incineration process, and the fifth
and sixth are the usual combustion hearths. At present, exhaust gas is treated by water scrubbing,
alkali scrubbing, and electrostatic precipitation.
11-29
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Average incinerator operation conditions prior to upgrading were: furnace outlet temperature,
330 °C (625 °F); miscellaneous heat loss, 8.4 percent of the total heat input; sludge feed rate, 4.0
metric tons/hr (4.4 tons) of sludge solids concentration of 20-40 percent; and a calorific value within
the range of 2,500-4,000 kcal/kg (4,505-7,209 Btu/lb) dry sludge. Since dewatered sludge at the Ho-
jin Treatment Plant typically has a calorific value of about 2,600 kcal/kg (4,700 Btu/lb) dry solids,
the limiting dewatered sludge solids content in autogenous combustion was 34 percent, and the
limiting NHV was 450 kcal/wet kg solids (810 Btu/lb).
Upgrading of operations basically included the development of a combustion control strategy and
refurbishing to permit automatic control. The furnace temperature was formerly controlled using the
temperature in the third hearth as a reference. The fifth hearth, which was the combustion hearth,
was made the index. Temperature control in the sixth and seventh hearths also was made possible
because it was assumed that the combustion hearth could actually be the sixth or seventh hearth,
depending on the amount and properties of the dewatered sludge. The selection of the control hearth
is made manually. So that the gas temperature can be measured without being affected by flames,
the length of projection of a thermocouple into the hearth was reduced. Two thermocouples were in-
stalled in the fifth hearth, with a view towards higher measurement accuracy and ready detection of
the failure of either. Air for autogenous combustion was formerly supplied from the burner blower.
This was changed to a method that supplied combustion air in proportion to sludge input rate, by
positioning an air damper on a duct that branched from the central shaft cooling fan. The furnace
temperature can now be controlled by adjusting the volume of air handled by the exhaust gas fan
rather than by supplying tempering air from the burner blower. Instruments were added and their
control capability was improved. As a result, the variation in furnace temperatures has decreased,
and combustion has stabilized.
Autogenous Combustion Control System
Incineration is stabilized by controlling the air volume and furnace pressure. Three temperature
control levels, 800°, 850°, and 900°C (1,470°, 1,560°, 1,650°F), are available as temperature set-
points and are measured in the fifth hearth (or in the sixth or seventh hearths, if this is where com-
bustion occurs). The process control variables are air volume and furnace draft. Control is executed
by a closed temperature feedback loop: temperature control setpoint—process variable control—
temperature variation— process variable control. In the temperature range of 800° - 900 °C (1,470° -
1,650°F), which is the most desirable operating range, combustion air volume is controlled in pro-
portion to sludge input. Draft in the furnace for 800° - 850 °C (1,470° - 1,560°F) is fixed at -30
mmAq (negative pressure of 30 millimeters measured by a water column draft gauge). When the
temperature exceeds 850 °C (1,470°F), the furnace draft is gradually increased and cooling is ef-
fected. If 900 °C (1,650°F) is exceeded several minutes later, the proportional control of combustion
air is lifted and air for combustion plus cooling is supplied. If the temperature at the fifth stage
reaches 1,000°C (1,830°F), cooling air is increased by operating the burner blower. If 1,050°C
(1,920°F) is reached, sludge feed is stopped. In cases of temperatures below 800 °C (1,470°F), a
temperature rise is generated by reducing draft to -10 mmAq, which reduces the excess air and
quenching effect. If the temperature is still below 800 °C (1,470°F) several minutes later, combustion
air is manually reduced after lifting combustion air control in proportion to sludge input.
Properties of Exhaust Gas
The results of exhaust gas analysis at the outlet of the electrostatic precipitator are characterized
by a small emission of NOX and SOX. NOX emission was 0.2-0.4 g/kg of cake feed. SOX emission
was often below the detection limit value. At other sludge treatment plants in the city, where sludge
is dewatered by vacuum filters and incinerated by MHFs with auxiliary fuel, the NOX is typically
11-30
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0.6-0.8 g/kg of cake feed. As in the case of SOX, this seems to be due mainly to the fact that heavy
oil is not the main fuel.
SUPERAUTOGENOUS (HIGH-CALORIFIC) COMBUSTION
Burning sludge cake with a high net positive heat value requires special consideration. In the
past decade, a number of municipal sludge incinerators were built to combust sludge cake that has
more heat value than the water in the cake requires for evaporation and elevation of flue gas
temperature. These cakes are produced from either thermally conditioned sludge or raw primary
sludge, the latter perhaps containing grease and oil or admixed scum. The increasing adoption of
BFPs and recessed plate filter presses that can more effectively dewater sludges has generated con-
cern about burning this type of dewatered sludge cake in furnaces of the multiple-hearth type, which
were designed for cakes that require significant drying before combustion can begin. FBFs do not
raise these concerns, but have other factors that limit their acceptance.
These feeds are termed "hot" in the sludge processing industry. Other terms that apply are high
calorific and superautogenous, or simply autogenous meaning that the cake maintains itself in com-
bustion without the need for auxiliary heat sources. Sometimes burning these sludges produces a
high flue gas temperature at the furnace breech, which causes excess air emissions or higher than
desirable flue gas temperatures at the furnace outlet. Various approaches have been proposed by
combustion specialists in the sludge incineration field to manage the burning process within accep-
table limits, to prevent damage to the furnace, and to prevent episodes of massive combustion on
several hearths at once. All of these problems have in fact plagued some of the early projects.
Perhaps the first instance of serious damage to a furnace was the loss of the top two hearths at
Kalamazoo, MI, in 1974. This was caused by premature burning of the thermally conditioned sludge
in the topmost part of the furnace. The system had been put into service a few years earlier, and the
design was only suited for handling raw primary sludge cake; it was not appropriate for "hot" feed.
One of the first methods of control, based on the presumption that the volatiles needed more room
for combustion, was to feed to the second or even a lower hearth. This was done by providing an
enlarged drophole immediately below the feed entry chute or, in a few cases, by mounting a screw
conveyor in the sidewall of the furnace to inject cake laterally at the desired hearth. The screw con-
veyor approach, as applied in 1972 at Muskogee, OK, to handle thermally conditioned sludge filter
cake, suffered from burn-back difficulty and has seldom been used. Another project where a great
deal of difficulty occurred was in the MHF at Atlantic County, NJ. The sludge, after heat treatment,
had an unusually high heat value, and it was necessary to modify the furnace internally and provide
additional air for quenching.
A well-known furnace manufacturer has modified the design of the air input system to overcome
much of the past difficulty experienced with hot sludge. This design was applied as a retrofit at
Lansing, MI, and in a new furnace started up in late 1983 in Hawaii. Neither of these cases
represents a complete and perfect adoption of the design principles, but both have shown substan-
tially better performance than could be expected from previous furnace systems. A case history of
the retrofit at Lansing follows. It illustrates the performance of an incinerator that combusts high-
calorific sludge cakes.
Lansing, Ml, Case History
The original incinerator design at Lansing, MI, was for burning a nonautogenous sludge cake;
however, it could not satisfactorily operate within allowable particulate air emission standards while
burning the superautogenous sludge that was produced with the adoption of a thermal sludge condi-
tioning process. In 1978, the sludge was thermally conditioned and dewatered by a rotary vacuum
11-31
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drum filter to a 50-60 percent solids concentration. In addition, the heating value of dry cake was
7,200-7,800 kcal/kg (13,000-14,000 Btu/lb) compared to a 4,200 kcal/kg (7,500 Btu/lb) design value
normally experienced and used for sludge furnace design purposes in the United States.
Modifications to the furnace and its control system were completed in 1982 and corrected the
deficiencies. The main feature of this new design, known as Cyclo-Hearth® , is a distributed sludge
combustion air supply that allows air to be routed where needed for temperature management and
prevents overheating when burning a superautogenous cake. This air management system consists of
two elements: (1) individual supplies to each hearth that are readily modulated with accuracy by
automatic or operator intervention and (2) a swirling effect induced by air jets that operate when
burners are shut off to simulate the burner gas dynamics and thus promote the equalization of
temperatures laterally in a hearth. The Cyclo-Hearth MHF consists of a number of discrete furnaces
(or hearths) acting together in series rather than a single massive operation. The basic design of the
Cyclo-Hearth MHF (see Figure II-8) is implemented through the following provisions. Temperature
Auxiliary
Fuel
Burner
(Typ.)
High
Velocity
Mixing Jet
(Typ.)
*s;
Sludge
Combustion
Air
(Typ.)
Figure 11-8. Schematic of the Cyclo-Hearth® multiple-hearth furnace.
11-32
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is controlled on each hearth by modulating either the burner firing rate or the sludge combustion air
rate, but not both simultaneously. For instance, if a hearth is on burner control with the burners fir-
ing at a low rate and the hearth temperature rises above the set point, combustion control logic will
switch the hearth to combustion air control. Control logic will also switch in the opposite direction
as conditions warrant. Exhaust gas oxygen content (excess air rate) is maintained at the set point
value by allowing the oxygen analyzer to override the temperature control signal to the bottom
hearth. This will allow the bottom hearth sludge combustion air valve to modulate and admit more
air as required. High-velocity mixing jets impart turbulence to unfired hearths by directing a stream
of air tangent to a circle, dividing the hearth area approximately in half. Operation of the jets is only
required on unfired hearths and on fired hearths when an autogenous sludge is being incinerated. By
promoting turbulence, complete combustion, and uniform hearth temperature, these mixing jets allow
greater response to furnace conditions and eliminate control circuit instability caused by delayed
combustion. Furnace draft is controlled by varying the position of the adjustable throat in the Ven-
turi scrubber, to allow better particulate emission control with no increase in electrical power
consumption.
Modification work at Lansing, MI, was restricted to the furnace, sludge combustion air fan, air
distribution system, and sludge feed point. The waste heat boiler, scrubbing system, and induced
draft (ID) fan remained unchanged. The modified furnace is shown schematically in Figure II-9.
Three sludge combustion air ports were added to the first through third hearths, thus enabling this
air to be added wherever needed in the furnace. A damper was placed in each 25-cm (10-inch)
sludge combustion air line to the furnace. These dampers can be controlled by the temperature con-
troller on each hearth or manually by the operator. In the automatic control mode, each hearth
temperature would be maintained at its set point by the flow of sludge combustion air to that hearth.
To provide adequate sludge combustion air, a larger ID fan was needed.
Three mixing jets were added to the first through fifth hearths. The purpose of these jets is to
provide gas-phase turbulence in these hearths when no burners are being used. The mixing jet air is
taken from the burner combustion air lines. Mixing jet air is controlled manually with valves located
in each line to the furnace. The operators determine when and how much mixing jet air is to be add-
ed to the furnace based on the temperature profile of the furnace and visual observation of the hearth
conditions. No modifications were needed in the burner combustion air fan to accommodate this
change, since mixing jet and burner combustion air are not needed simultaneously. The sludge feed
point was moved to a lower hearth, allowing more gas residence time in the top hearths to achieve
adequate combustion of the volatiles. The sludge feed can now be varied between hearths three or
four as determined by the operator. The rabble arms were removed from hearths one and two, and
refractory feed chutes were installed to bring the feed cake from the top of the furnace to the new
feed hearths. A single burner was added to hearth one in case it was necessary to have an ignition
source to ensure combustion of volatiles remaining at that point.
Modification Results. Operation of the modified system began in March 1982, on a 5
day/week, 24 hr/day schedule. The system operated at a wet feed rate of 3.6-4.5 Mg/hr (4-5
tons/hr), with roughly a 50 percent total solids content cake, and was able to meet air emission re-
quirements. With the feed split between hearths three and four, there is adequate gas residence time
in hearths one and two to allow complete combustion of the gaseous volatile material. In addition,
the temperature of these "afterburner" hearths can be controlled by the supplemental air injection.
In a similar fashion, the feed hearth temperatures are also controlled by the addition of sludge com-
bustion air. The operators can control temperatures on these feed hearths at 540° - 650°C (1,000° -
1,200°F) even though the sludge is burning at these conditions. Maintenance of the desired
temperature profile in the furnace has reduced clinker formation to a minor operating problem.
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(
Sli
Com
Air
-*
1
*
1
]
1
i
— -* — y •*-
[3)
(3)
(3)
^ (3)
— »•
2?~
jdge
Dustion
Fan
L
D
SI
'l
Ash
ischa
udge Cak
Feed
=
e
rge
Center
Shaft
Air Fan
—*• / — »- To Main Stack
Hearth No.
1
2
3
4
5
6
7
8
(3) — ^- Breeching
3 (1 )
2 -
(3)
3
(3)
(3)
3
(3)
Mixing
Jets
(3)
"^i
(3)
I
Burners
i ^ ^^omi
> i
' Fuel
Burner
Combustion
Air Fan
Figure II-9. Modified Cyclo-Hearth® furnace configuration.
11-34
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The sludge feed distribution between hearths three and four is determined by the sludge cake
solids content. Should the temperature of hearth three increase in spite of large combustion air rates
to that hearth, indicating a hotter sludge, the operator diverts more sludge to hearth four, thus main-
taining the proper temperature profile. Conversely, when little combustion air is directed to hearth
three and that hearth temperature begins to drop, the sludge feed rate to that hearth is increased. The
modified furnace is also capable of handling fluctuations in feed quite successfully. If the addition of
an autogenous sludge is stopped for a brief period of time (e.g., 10-15 minutes), the operator simply
reduces the amount of air entering the upper portion of the system until the feed problem is cor-
rected. If the feed interruption persists, the volatile material in the furnace is depleted and the
temperature profile cannot be maintained by sludge combustion. The operator then actuates the
necessary burners to maintain the system's temperature profile. Meanwhile, the emission control
system is able to maintain the exhaust gas quality.
Control of the sludge combustion air to the furnace can be accomplished by two methods:
automatic controller and manual loading stations and manual valves. The operators currently run the
furnace in the manual mode. The automatic system, with the original instrumentation, tended to
overcompensate, which resulted in an oscillating temperature profile. Proper tuning of the instrumen-
tation eliminated this problem. The operators do not find temperature control difficult and are able to
keep the furnace operating in a stable mode. Another innovative feature of the modified furnace con-
cerns the method of sludge combustion air addition to the upper hearths. Sludge combustion air ports
on hearths one through three have been placed there for that purpose, while the mixing jets on
hearths one through five were installed only to promote good combustion for burning autogenous
sludge. Each 5-cm (2-in) mixing jet line operates at 0.1 kPa (16 oz/in2) header pressure and
discharges a significant amount of air into the hearth when the valve is open. Consequently, the
1.9-cm (0.75-in) mixing jets are used to provide gross temperature control on a hearth, and the
sludge combustion air manual loading stations are used for fire control.
The Lansing furnace was not equipped with sufficient instrumentation to verify the ability of the
mixing jets to equalize the temperature on a given hearth, but a visual examination did not indicate
problems of uneven temperature profiles with the modified system.
Cyclo-Hearth vs Conventional MHF at Lansing. The most significant improvement of the
Cyclo-Hearth over the conventional MHF is the degree of operator control over the furnace
temperature profile. When burning autogenous sludge, the conventional system was unable to process
the material satisfactorily due to inadequate hearth temperature control, which caused clinker forma-
tion in the furnace and resulted in frequent shutdowns for removal and high wear rates on the rabble
teeth. The modifications provide the needed temperature control at all times, thereby minimizing
clinker formation. The modifications also resulted in a system that demonstrates better stability
response to feed changes than the previous system. The modified furnace responds to changes in the
feed conditions to maintain stable operating temperatures. When processing the superautogenous
sludge using the conventional system, operators had considerable difficulty maintaining temperature
stability, since the furnace lacked the necessary hardware to effect this type of control. The improv-
ed stack emissions of the Cyclo-Hearth over the conventional MHF are the most important results of
the modifications. Due to poor temperature control and stability, the conventional system was not
able to meet the limit of 0.2 gm particulate/kg dry gas at 50 percent excess air. The performance
test for the modified furnace was well under this limit, since 0.032 gm/kg dry gas at 50 percent ex-
cess air was achieved.
WASTE HEAT RECOVERY EQUIPMENT
In early 1983, a new installation of four MHFs fitted with waste heat boilers began generating
steam at the Metropolitan (Metro) WWTP serving 57 municipalities in the major core of the Twin
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Cities area in Minnesota. The plant, located on the Mississippi River in Saint Paul, has a nominal
flow rating of 11 m3/s (250 mgd) and produces 81 Mg/day (90 tons) of raw primary sludge cake
conditioned with polymer and 131 Mg/day (145 tons) of thermally conditioned cake. Typical solids
content of the primary sludge after dewatering by twin-roll presses is 32-35 percent. Typical solids
content of the cake from the thermal conditioning and dewatering process, which handles a blended
sludge that is 3 parts secondary to 1 part primary, is also held at 32-35 percent. Process conditions
in the dewatering step that employs automated membrane presses can be set to achieve solids content
of 55 percent or more, but this burns too hot in the furnace and causes clinkering.
For the first 24 days in January 1984, steam production averaged between 14-18 Mg/hr
(30,000-40,000 Ib/hr), and total steam produced when valued at the equivalent cost of natural gas
that would otherwise be burned in onsite steam generators was worth $113,000. This is based on gas
costs of 0.4C/MJ ($4/MBtu). The 1984 goal is to produce steam valued in this way worth $1.5
million. If valued in No. 2 fuel oil terms at 0.26C/1 ($l/gal), and even allowing for a higher excess
air ratio to avoid smoke, this equivalent value would be approximately doubled. The system at the
Metro plant consists of two MHFs and two furnaces that had been in service for 13 and 9 years,
respectively, plus two 1968 remodeled standby furnaces not fitted with waste heat recovery. All are
designed to accommodate autogenous cake plus scum burning. The top hearth receives no feed and
is the "zero-hearth afterburner." All have provision for the addition of pressurized sludge combus-
tion air at hearths zero through four, seven, and eight. The waste heat recovery steam generators are
rated at 9 Mg/hr (20,000 Ib/hr) at 2.8 x 103 kPa (400 psig). Actually, they are delivering at about
half of that capacity because of reductions in furnace loading. All steam needed by this thermal con-
ditioning process is supplied by this recovery system, and much of the plant's winter building heat
load is also provided. In warmer months, steam will continue to be used for the thermal conditioning
process and air-conditioning; the remainder will be used in turbines that drive equipment such as ID
fans and boiler feedwater pumps.
Table II-4 lists the locations that have reported good-to-excellent results in the operation of
waste heat recovery equipment.
Table 11-4. Successful waste heat recovery installations.
Generating Steam over
125 psig
Generating Steam up to
125 psig or Hot Water
Heating Air for
Combustion Supply
Green Bay, Wl
San Mateo, CA
Cedar Rapids, IA
Davenport, IA
Dubuque, IA
Atlantic County, NJ
Honolulu, HI (Sand Island)
Lansing, Ml
St. Paul-Minneapolis, MN (Metro)
New Rochelle, NY
Hopewell, VA
Campbell-Kenton County, KY
Niles, Ml
Buffalo, NY
Erie County, NY (South
Downs)
Kansas City, KS (Plant
No. 20)
Louisville, KY
Ann Arbor, Ml
Duluth, MN
Redwood City, CA
Amherst, NY
Tonawanda, NY
Watertown, NY
kPa = 0.14465 psi
11-36
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CHAPTER III. COCOMBUSTION OF SLUDGE AND
SOLID WASTES
INTRODUCTION
The rationale for considering the joint combustion of sludge and solid wastes is that sludge, even
after dewatering by conventional methods, lacks sufficient heat value to balance the evaporative
burden of its remaining moisture when it is burned in a typical combustor. A further disadvantage is
usually the need to supply adequate heat to the combustion products and excess air so that odor
emissions are minimized. Solid wastes, on the other hand, typically generate more heat than is
necessary to burn them to innocuous products. By computation, a heat balance is reached when the
per capita quantities of solid wastes and sludge solids are combined, provided that the sludge is
dewatered to about 25-30 percent solids. This is an easy task if the sludge is raw primary sludge
alone, but the refined dewatering is much more costly if the sludge has been digested or if a compa-
nion amount of biological sludge is present. A further complication is that more sludge mass is
created when biological treatment is employed, and the overall cake from any dewatering process,
except that preceded by heat conditioning, is generally wetter. Thus, the technical feasibility of
cocombustion without the need for auxiliary fuel has to take into account the treatment plant process
train.
The trend toward cocombustion in those European countries where incineration is a well-
established and widely applied technology is a natural progression. Solid waste incineration is far
less accepted in the United States, and thus joint combustion technology is considered less. In part,
this is because many earlier solid waste incinerators were serious air polluters. In Minneapolis, MN,
for example, two municipal incinerators that were built in the 1930s under a Public Works Ad-
ministration program were commonly ignited each day by burning auto tires found in the collection
trucks. Public pressure from citizens who were bothered by the air pollution helped bring about a
landfill program that began in the 1960s and terminated incineration.
New York City, where a plethora of apartment house incinerators was installed, has historically
fought hard against municipal incinerators for refuse or solid wastes and, as a result, public accep-
tance of sludge incineration in the near future appears unlikely. A major report for the Interstate
Sanitation Commission (NY-NJ-CT) proposed sludge combustion but called it "pyrolysis," a hi-tech
name used incorrectly in this instance to mean starved-air combustion (SAC). Refuse was to be pro-
cessed into refuse-derived fuel (RDF) and blended in, as was done in the mid-1970s pilot work in
California for the Central Contra Costa Sanitary District. Neither the New York nor the California
work has proceeded to full-scale design for both technical and institutional reasons. In Europe, even
20 years ago, incinerators were built to a higher standard of quality, and air emissions were con-
trolled by costly methods. There, the ability to more economically process two waste products at a
single site with a single management staff and shared maintenance and support workers is seen as a
major advantage; cocombustion also allows a single operating permit to be issued by the regulatory
agency. This is much less likely to be realized in the United States, where it is common to have one
completely separate entity responsible for the sludge and another one for the solid wastes, with the
solid wastes frequently controlled by private ventures rather than public agencies.
Partial appeal of cocombustion rests on the assumption that fossil fuel costs for sludge solids in-
cineration are an insurmountable obstacle and that the heat value of the refuse is needed. However,
in recent years, better methods of dewatering sludge have become widely adopted. This was dis-
cussed and illustrated with case histories in the previous chapter. Furthermore, the concerns about
IIM
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cost and availability of fuel have dissipated, and, as was shown in the previous chapter, more effi-
cient incinerators for sludge can be designed and older ones can be upgraded. Thus, the technical
risks and institutional aspects of a joint combustion facility have taken on a greater significance.
Cocombustion of refuse and sludge solids in the United States is currently practiced in only three
locations using widely divergent technologies.
COMBUSTORS SUITABLE FOR CODISPOSAL
Most burning has been tried in mass-burn refuse incinerators of the usual configurations adopted
for refuse that has had only minimal preparation, such as removal of bulky objects and explosion
risks. Incinerators of the water-wall type that are suited for heat recovery are the more modern type,
although sludge could also be burned in a refractory-wall type incinerator. The action of the grate,
application of combustion air, and the means of dispersal of the sludge are critical elements.
The rotary combustor, a combination of a water-wall incinerator and rotary kiln, has been in-
stalled in only one location in the United States (Gallatin, TN) and is currently burning only refuse.
It should be excellent for handling a mixture of coarsely shredded, moderately prepared refuse and
sludge cake in that it has absolute purging of any noncombustibles. Thermal economy and stability
when burning sludge cake along with refuse would have to be proved. This U.S. installation has
been found to be too small to allow adequate burn time for raw refuse.
The modular refuse incinerator is generally much smaller than the typical mass-burn type, and
its practicality for codisposal has not yet been proven. However, a well-dewatered cake could be
proportioned into the solid wastes/refuse feed and be expected to burn satisfactorily in the available
residence time, provided that sludge cake lumps did not "case harden" and leave a core of wet
solids surrounded by crust or ash. Agitation of the bed in a modular unit is minimal, and sludge ball
discharge with the ash is possible.
Flash-dryer incinerator equipment has been used for many years to dry wastewater sludge, but
has been less successful in incineration. Because the process involves suspension burning of fine par-
ticles, such a system is not at all suited to mixtures of refuse and sludge. However, it can be used in
a design as the method of preparing sludge for mixing with refuse or for injection above the burning
refuse in a conventional incinerator. If the flash-drying method is considered, it can be made cost-
effective by using recycled incinerator gases as the heat source. If, for example, flue gases pass
through a high-pressure boiler and come out at 315° - 370 °C (600° - 700 °F), there should be suffi-
cient heat energy for an effective flash-drying operation.
The MHF, commonly applied in sludge combustion, is likely to be used if the wastewater
agency is in control of design or has such units in service already. In this situation, preparation of
the refuse needs to be more elaborate, usually requiring size reduction to 3-5 cm (1-2 in) maximum
dimension to minimize fouling of the rabble teeth, unless metals are effectively taken out by air
classification. The furnace is self-clearing of noncombustibles if the ash system has been designed
appropriately. European experience indicates that best performance is obtained if sludge cake is fed
to the top hearth and the RDF is fed to the normal combustion hearth. Flashback fires must be
guarded against in the RDF injection design.
The FBF was installed and operated as a cocombustion demonstration facility at both Duluth,
MN, and Franklin, OH. As noted previously, any design must deal with the noncombustibles that
especially tend to collect in a FBF rather than purge out automatically as in a traveling grate or
multiple-hearth unit. However, this problem can be materially lessened with proper design. At
Duluth, the problem of noncombustible matter collecting in the bed was aggravated by the design in-
adequacy of the classifier built into the secondary shredder. This caused a greater loading of non-
III-2
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combustibles than was expected. It was one of many materials handling aspects in the solid
wastes/refuse train that delayed the successful operation of codisposal and was not attributable to the
selection of a FBF. Similar problems at Franklin, OH, were solved within the first few years; then
the system ran well until shutdown (for other reasons) in the late 1970s.
Predrying the Sludge Solids
In the design of a cocombustion system, the thermal balance must account for the water present
in the sludge stream. If heat is to be recovered from the hot gases or if odor control considerations
require some preselected flue gas temperature, then the presence of more moisture than can be
tolerated will mandate that a drying step follow the dewatering of the sludge.
A further factor is the chosen means of combustion. Is it by suspension firing or burning on a
grate along with the refuse? Firing in suspension implies that the sludge solids enter the furnace
from the top, as a free-flowing granular or powdered product. This usually means that the residual
moisture has been brought down to the 5-25 percent range, which dictates the use of a direct drying
or multieffect evaporation step.
If grate firing is planned, a conventional dewatering step may be sufficient, as is employed at
Glen Cove, NY. It is necessary to get the sludge solids distributed evenly over the refuse charge. If
water reduction beyond that achievable by conventional dewatering is dictated by the process' ther-
mal balance, drying to 30-50 percent residual moisture may be sufficient. This can be accomplished
by indirect drying equipment, although such systems as applied in chemical, pharmaceutical, and
food industries have not been economically or technically attractive to the wastewater treatment in-
dustry for sludge use. Virtually all the sludge drying that is done in the United States, whether for
cocombustion (Stamford, CT; Flint River, GA) or marketing of a sludge product (Houston, TX;
Milwaukee, WI; Chicago, IL; Largo, FL), is by the direct drying method. A rotary dryer system of
two units was installed in 1982 at the Metropolitan WWTP serving the Saint Paul-Minneapolis area.
However, it is expected to be used only as a contingency mode of sludge disposal or if an
agricultural-use market is identified. At Harrisburg, PA, trials of an indirect drying device, termed a
hollow-flight jacketed dryer, have not given good results.
In short, drying of sludge makes cocombustion easier, but the need for the added capital invest-
ment and the incurring of operating and maintenance (O&M) costs have to be justified by the value
of the heat that is not needed for evaporation of water in the combustor. Energy is also required for
the heating of water vapor to flue gas temperature or, at a minimum, to the exit temperature of a
waste heat recovery device. Thus, the design decisions for a sludge drying process involve the same
reasoning as for applying a waste heat recovery system: the value of the heat and whether it can be
used beneficially.
U.S. COINCINERATION SITES
The following section describes the coincineration practices at several sites in the United States.
Case histories are presented for systems at Stamford, CT; Glen Cove, NY; Duluth, MN; Flint
River, GA; and some trial operation sites in the United States.
Stamford, CT
This coincineration system is the only operational one of its kind in the United States. It was
proposed in 1968, put into operation in December 1974, and has been operational ever since. Sludge
for this process is produced at the city's 0.9 m3/s (20-mgd) conventional activated sludge treatment
III-3
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plant. Using progressive cavity pumps, the mixture of primary and secondary sludges is pumped to
belt filter presses. Approximately 7.6 m (25 ft) upstream of the belt filter presses, poly electrolyte is
added into the sludge piping. This point of addition allows for better mixing of the sludge and
polymer. The conditioned sludge is dewatered to an average cake solids concentration of 26 percent.
The cake is discharged to a pug mill, where it is combined with previously dried sludge to produce
a mixture with a solids concentration of approximately 65 percent. This mixture is then conveyed to
a rotary dryer. A portion of the hot gas that would normally be wasted through the solid waste in-
cinerator stack is tempered with ambient air and introduced into the dryer at about the same location
as the sludge mixture. As the dryer rotates, the sludge is cascaded through the hot gases, and
moisture is evaporated at a rate of 2,300-3,200 kg (5,000-7,000 Ib) of water per hour. The dried
sludge, with a solids content of 90 percent, is discharged through a diverter gate and divided into
two streams, one of which is recycled to the pug mill while the other is conveyed to the incinerator
and burned. The heat value of the sludge averages 20,930 kJ/kg (9,000 Btu/lb) of volatile solids.
The incinerator is a conventional mass-burning refuse incinerator with rocking grates. It has a
capacity of 330 Mg/day (360 tons/day) and uses electrostatic precipitators for pollution control. The
refuse/solid waste as received with no pretreatment has a heat value as fed of 14,000 kJ/kg (6,000
Btu/lb). It enters the incinerator through a charging hopper and is discharged onto the grates at a
rate of approximately 180 kg/min (280 tons/day). At preset intervals, the grates rock, thus moving
the burning material through the furnace. Combustion is controlled using overfire and underfire air
fans. At the end of the furnace bed, the ash drops into a wet sluice and is conveyed to a truck for
landfill.
The dried sludge, at a rate of about 9.5 kg/min (15 dry tons/day), enters the furnace through
ports in the ceiling and burns in suspension within the first 1m (3 ft) of drop. The hot gases for the
drying system are drawn from the incinerator at 980 °C (1,800°F) at a rate of about 6.1 m3/s
(13,000 cfm) and are reduced to 200° - 400°C (400° - SOOT) by adding ambient air. This
temperature is controlled by the dryer exhaust temperature, which is set at 66-79.4°C (150° -
175°F). As the hot gases pass through the dryer, they pick up moisture and dust which must then be
removed in a cyclone dust collector. These gases are then returned to the furnace for deodorization.
Soon after this system went into operation, it became obvious that several modifications had to
be made to enable the system to work effectively. Many of these were quite simple and most were
designed and installed by plant personnel. The coincineration system was installed in the existing in-
cinerator building. Because space was limited, it was necessary to spread the equipment over five
different floor levels. Many conveyors were needed to move the sludge from one stage of the pro-
cess to another, thereby increasing material handling problems.
At various stages in the system, samples were taken to determine material moisture. It was
observed that as material moisture increased, the amperage of the conveyor motors increased. In ad-
dition, if the operator tried to process too much sludge, the amperage would also increase. Using
these facts, an amperage range was established for ideal sludge moisture content and volume. Por-
table ammeters were used for the determination. After operating for several days to prove that these
ranges were correct, permanent ammeters were installed at the main control panel to monitor the
pug mill and all critical conveyors. These allow the operator to control the process from the main
panel and determine whether to increase or decrease the dry recycle and dewatered cake rates on the
basis of the amperage reading. This has reduced operator fatigue and allows for a more stable pro-
cess. The operator is still required visually to inspect the entire system but at much less frequent
intervals.
Clogged conveyors presented another serious material handling problem. This was caused by rag
buildup on the bearings, changes in the material characteristics, and broken drive belts. As the con-
III-4
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veyor began to get clogged or if the drive belts broke, the rotational speed of the shaft would
decrease or completely stop. Again, this was fairly simple to solve. By attaching speed sensors to
the drive shafts of the gear reducers, this decrease of speed could be sensed. Now, as soon as the
speed decreases below a certain point, the electrical interlock system shuts down the material feed to
that conveyor and an alarm rings at the main control panel, alerting the operator and allowing him
to take action before any serious equipment blockages occur. Because of this, downtime has been
greatly reduced, and the operator is no longer faced with the frustration of having to remove com-
pacted sludge from the conveyor.
Another material handling problem was caused by the thixotropic nature of dewatered sludge
cake; that is, the sludge would change in viscosity as stress was applied. A screw conveyor approx-
imately 20 m (65 ft) long was used to convey the dewatered sludge from the presses to the pug mill.
Problems were encountered almost immediately. The physical characteristics of the sludge cake
began to change dramatically as the sludge proceeded through the conveyor. The sludge became very
sticky, making it difficult to convey and causing excessive torque on the drive motor. It was virtual-
ly impossible to run the system. The screw conveyor had to be replaced by a belt conveyor that
would not alter the physical characteristics of the sludge. Short screw conveyors for dewatered cake
do not seem to affect the sludge characteristics, but as the length of the conveyor increases, this pro-
blem becomes more evident.
Dewatered cake dryness and polymer concentration in the cake also appear to have considerable
effects on the ability of the system to function. Instead of the usual type of dried material, which is
light and fluffy somewhat like the material inside a vacuum cleaner, the sludge begins to form balls,
initially about the size of peas. These balls are dry on the outside and moist on the inside. As these
balls circulate through the drying system, they get larger and larger. Surface area is reduced con-
siderably, resulting in a greater recycle ratio by weight. This, at times, also makes the system vir-
tually impossible to operate. High polymer dosages also tend to make the sludge sticky, creating
drag on the conveyors and difficulty in mixing in the pug mill. Experiments have shown that these
changes occur as the cake solids concentrations drop below 22 percent or the polymer dosage in-
creases beyond 10 gm/kg (20 Ib) of dry polymer per kg (ton) of dry sludge. Therefore, when
designing this type of system it is important that dewatering equipment is specified that will obtain
the desired cake solids. Care in selecting and evaluating polymers will ensure dosage below this
level.
Fires were another serious problem. Most of the fires occurred inside the dryer. To control
these, an automatic water spray system was devised that includes a spray bar located across the
diameter of the feed end of the dryer and two sprays located at the discharge end of the dryer. It
was critical that no water from the spray system be allowed to impinge on the periphery of the
dryer, since thermal shock could possibly cause damage. Five stainless steel fogging nozzles are
spaced evenly along the spray bar. Three nozzles are adjusted to spray the length of the dryer and
two are adjusted downward. The sprays are controlled by a thermocouple in the exhaust end of the
dryer. When the temperature exceeds the set point of 150°C (300°F), a solenoid opens, allowing
water to flow through the sprays, which are set to pulse in intervals of 10 seconds on and 5 seconds
off or can be run continuously in a manual mode. The nozzles at the discharge end are also con-
nected to this system. The combination of the sprays has effectively controlled most fires.
Additional minor problems included spalling of metal from the dryer riding rings and hot spots
in the live-bottom storage bin. The problem of spalling was corrected by the installation of graphite
blocks on each of the four trunnion rolls. This small amount of continuous lubrication has prevented
serious wear on the rings. The hot spots in the corners of the live-bottom bin, which were a source
of smoldering sludge, were corrected by welding a sheet of metal inside the bin to round the corners
preventing a buildup of hot recycled sludge.
III-5
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In conclusion, although many operational problems have occurred with this system, most of
them have been solved, and the system now runs effectively and efficiently. When the plant was
built, the coincineration system was the only means of sludge disposal. An alternative method was
added to provide backup for the coincineration system. This alternative method is a postdewatering
lime stabilization system that costs approximately $1 million/yr to operate, mostly due to the cost of
trucking the sludge 65 km (40 miles) to the only available landfill site. The coincineration system is
extremely economical and energy conservative, since neither the incinerator nor the dryer requires
any external fuel source. It has not added any additional ash handling problems, and it is not a
source of air pollution. Additionally, the dried sludge before incineration can be used as a soil con-
ditioner where a market is available, thus creating a source of revenue for the municipality. Further-
more, excess waste heat can be used to generate steam or electricity, which could supply the
municipality-owned buildings, with the excess being sold to the public utility. The experience at
Stamford indicates that coincineration can be a viable means of sludge disposal.
Glen Cove, NY
One of the newest installations in the United States for joint combustion of refuse and sludge
solids is at Glen Cove, NY. This design is different from those at Stamford, CT, Flint River, GA,
and Duluth, MN, in that here the sludge is dewatered by centrifuge and fed in semiliquid form at
15-25 percent solids directly onto the refuse charge. The intent of the design is that it will evenly
distribute the sludge as a layer on top of the charge, with the sludge having a fuel value that is fair-
ly proportional to that of the refuse on a unit area basis. The Glen Cove plant has two mass-burning
furnaces, each designed for a daily feed of 100 Mg (110 tons) of refuse and 14 Mg (15 tons) of cen-
trifuge cake at 20 percent solids, or 3 Mg (3 tons) of dry sludge solids per day. If the refuse is
assumed to have 28 percent moisture, the dry weight ratio is 24 to 1 as compared to a wet weight
ratio for both of 7.3 to 1. If the refuse, as received, is compared to dry sludge solids, the ratio
would be 37 to 1.
Because the Glen Cove WWTP is not receiving flow as high as its design rate, the amount of
sludge to be disposed of is much less than the design basis. Actually, only about 2.7-7.2 Mg/d (3-8
tons/day) of 18-20 percent solids centrifuge cake are being fed to the furnaces, which are burning
refuse at their design rate total of 200 Mg/d (220 tons/day). Thus, the charging ratio is higher than
design, so the sludge is being handled readily. As a result, the operation of this system is not
presently representative of the per capita ratio of about 14 Ib of dry refuse to 1 Ib of dry sludge, ex-
pressed on the "as received refuse to sludge dry solids" basis. Instead, there is so much excess
refuse compared to sludge that the ratio is greater than 50 to 1. This does not represent a fair
demonstration of the adequacy of this design to handle a balanced per capita basis feed.
The Glen Cove furnaces are the refractory-lined, mass-burning type, equipped with "Kascade"
stokers and automatic combustion air controls to maintain uniform combustion temperatures and con-
ditions. Each furnace is equipped with a 9,080 kg/hr (20,000 Ib/hr) convection boiler rated for 41
kg/cm2 (600 psi) at 250°C (480°F). Available steam is converted to electric power in a 2,500 kW
multistage condensing turbine generator set, which powers the complex of incineration facility and
the adjacent wastewater plant. Excess power can be sold to the Long Island Lighting Company. The
design basis sludge/refuse mixture has an average high heat value of 9,500 kJ/kg (4,100 Btu/lb).
This presumes 11 Mg (12 tons) of water and 3 Mg (3 tons) of sludge solids in the sludge stream to
each furnace. Of course, if the sludge is wetter than the 20 percent level, this heat value would be
reduced. At 16.7 percent, for example, there would be an extra 3 Mg (3 tons) of water to be
evaporated, which would reduce the heat recovery potential and thus the yield of electric power. At
15 percent solids, there would be an extra 4.5 Mg (5 tons) of water to evaporate. The incentive,
then, is to maximize the solids in the centrifuge cake consistent with getting even distribution over
III-6
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the refuse charge. Sludge cake above the 20 percent solids level is only achieved when straight
primary sludge is centrifuged.
Air pollution control is for particulates only, using two field electrostatic precipitators, each siz-
ed for 14,160 L/s (30,000 scfm) at 316°C (600°F). The performance guarantee is to achieve 0.11
g/m3 (0.05 grains/scf) at 12 percent carbon dioxide. The long-term performance of this particulate
removal system will be followed with interest by professionals in the wastewater field, where wet
methods of scrubbing are universally applied to sludge combustion gases.
Both the coincinerator system and the wastewater plant are operated for Glen Cove under con-
tract by a private company. The city estimates a savings of $700,000 per year compared to prior
costs for wastewater and refuse management. The project cost was $24 million.
Fluidized-Bed Coincinerator at Duluth, MN
Because of the fuel oil shortages resulting from the oil embargo of 1974, the Western Lake
Superior Sanitary District in Duluth, MN, and the district's consulting engineers decided to utilize
available solid waste as auxiliary fuel to incinerate sludge produced at the wastewater treatment
facilities. The construction of the facilities was completed in 1979, and the startup began in
November 1979. Performance tests were conducted in June 1980.
The Duluth installation consists of two systems, each having a CWB-type conventional fluidized -
bed reactor with a 6-m (20-ft) diameter bed and 10-m (34-ft) diameter freeboard, dual gas cyclones,
waste heat boiler, Venturi scrubber gas cooler, fluidized air blower and induced draft (ID) fans, and
heat exchanger for plume suppression (Figure III-l). The fluidized air blower, ID fan, and some
pumps are driven by steam turbines powered by the steam from waste heat boilers. The waste heat
boilers are rated for 21,792 kg (48,000 Ib) of steam at 19 kg/cm2 (280 psig). The boilers are the
two-drum, water-tube, natural circulation type, designed for 870 °C (1,600°F) gas inlet. The units
are equipped with soot-blowing capabilities. The sludge was to be fed into the fluidized-bed reactor
through a feed chute at the top of the reactor, and the refuse derived fuel (RDF) was to be fed with
a pneumatic solid waste feed system for injection into the freeboard approximately 0.6 m (2 ft)
above the fluidized bed, pointing downward to the bed. The gas-cleaning equipment consists of dual
gas cyclones, a Venturi scrubber, and a collector/cooler with three impingement plates. The system
includes a hydraulic ash handling system consisting of an ash slurry tank, an ash classifier, and a
fine ash thickener. The RDF preparation system includes coarse and fine shredders, air classifier,
magnetic separators, conveying and metering equipment, and a storage silo.
After completing construction and making the system operational, various attempts were made to
incinerate RDF without the sludge. Troubles developed in the RDF feed system, and undesirable
"freeboard burning" was experienced (i.e., combustion occurred too high in the furnace). As a
result of a malfunctioning air .classification system, large metal or wood objects constantly jammed
the rotary air lock at the RDF pneumatic feed system. The secondary shredder was designed to
shred 95 percent of the material to a size of less than 3.8 cm (1.5 in) in effective diameter. The
solid waste processing facilities had no presorting capability. Therefore, glass and other noncom-
bustible objects entered the primary shredder and were broken up and imbedded in the RDF. The
location of the RDF feed nozzle was incorrect. The majority of RDF burned in the freeboard area,
while fuel oil had to be burned to maintain reasonable bed temperatures. The addition of sludge, by
dropping sludge cake into the bed, made the situation worse. While trying to keep the bed hot by
burning fuel oil, the startup operators were also trying to keep the freeboard cool by spraying a fine
spray of water in the reactor. Fortunately, there was an alternate RDF feed nozzle located near the
top of the fluidized bed and directed downward to the bed. The point of entrance into the reactor
III-7
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III-8
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was approximately 0.6 m (2 ft) below the top of the bed when the fluidized-bed height is 2 m (6 ft).
Although use of this optional feed location made considerable improvement, the freeboard
temperatures were not controllable when coincinerating sludge with RDF. Also, the necessary bed
temperature for combustion could not be sustained when coincinerating.
It became apparent that using the single RDF feed port at the lower location did not satisfy the
original design intent of coincinerating RDF and sludge cake in the bed. Multiple feed points for
RDF needed to be developed. The first approach was to modify the sludge feed port and utilize the
moisture content of sludge to control the freeboard temperature in lieu of freeboard sprays. A series
of attempts were made to spread the sludge over the freeboard. Finally, a special nozzle was
developed to feed the sludge in a small enough particle size to allow the water to evaporate and a
large enough size to have solids end up in the bed to complete incineration. The feed device
developed produces sludge particles approximately 0.3 cm (1/8 in) in diameter most of the time.
Steam is utilized to shear the sludge after it is extruded through a converging nozzle. The extruded
flow is not even, and periodic surges of dispersed sludge take place. With the new sludge feed
device and the RDF feed nozzle, experiments were made with the height of the bed. The bed was
allowed to grow as deep as permitted by the capacity of the fluidizing air blower.
Performance tests successfully demonstrated that the system could operate at design rates while
meeting particulate emission requirements. They also showed that the system was capable of in-
cinerating sludge with fuel oil, sludge with RDF, RDF alone, or all fuels at one time and at adjusted
loading rates. Table III-l shows the average quantities of RDF sludge, and fuel incinerated during
tests. The design capacity tests were conducted for an 8-hour duration. Ash collected during the test
was analyzed in detail and results are shown in Table III-2. Results of emission tests and exhaust gas
composition are shown in Table III-3. While coincinerating sludge with RDF, the excess air rate was
between 48 and 51 percent. The excess air rate during incinerating sludge with fuel oil was about 85
percent. While burning RDF without sludge, fluidizing and cooling requirements resulted in high
levels of excess air. The amount of excess air used during the RDF-only mode was 174 to 232 per-
cent. The visible emissions from the stack were limited to 20 percent opacity. During the tests, 98
percent of the time the opacity was within 10 percent, and only during the last 7 minutes of the
sludge/RDF test did the opacity exceed the 20 percent limit.
During the startup and testing period, the ash handling system was very difficult to operate. The
fine ash thickener continually plugged up, and a small wet cyclone eventually had to be added to the
system to relieve the problem. The fine ash refused to settle in the ash classifier, and the cyclone
dip legs had a tendency to plug near the ash quench tank. In addition, the inert fraction of the RDF
was too big to be elutriated, and the bed continued to grow in size. Arrangements were made to
remove some of the bed regularly, which further aggravated the ash system operation.
During the startup and test burning RDF-only mode, one of the gas cyclones plugged with slag.
The material looked like lava and was named "moonrock" by the plant personnel. It was at least
0.9 m (3 ft) high and located at the bottom of the cyclone. The sample of slag was very hard, black
in color, shiny, and amorphous.
It appeared that the slag material contained some amount of alkali metal silicates, which became
sticky viscous glass when heated. Although there were considerable amounts of A12O3, CaO, and
Fe2O3 present to react with low-melting, alkali metal silicates like Na2O»3SiO2, there were not
enough quantities to convert all of the metal silicates to metal oxide-alkali oxide-silica dioxides. The
retention time in the bed was not long enough to complete the reactions, and burning was taking
place in the ducts and cyclones. The residual amount of low-melting, alkali-metal silicates caused the
ash to agglomerate in the cyclones.
III-9
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Table 111-1. Average quantities of refuse-derived fuel, sludge, and fuel incinerated
during the performance tests.
Refused-derived fuel
Sludge
Refused-derived fuel
Sludge
No. 2 fuel oil (gal)
Sludge
Refused-derived fuel
Note: Ib x 0.454 = kg
Btu/lb x 2.3255 =
Sample
Elements2
Si
Al
Fe
Ca
Mg
Na
K
Ti
Wet
Ibs/hr
17,700
27,217
5,285
15,778
338
29,880
13,787
Id /kg
Table III-2.
Bed
Material
4/25/80
P.C.5
6.0
2.5
7.5
1.0
2.5
0.5
0.4
Dry Percent
Ibs/hr Solids
12,938 76.66
5,852 21.50
3,932 74.22
3,281 20.34
6,884 23.00
10,453 76.10
Ash and bed analysis'1.
Bed
Material
4/26/80
P.C.5
3.5
3.5
7.5
0.85
2.25
0.5
0.7
Percent
Volatiles
68.83
56.30
69.47
63.62
58.80
67.20
(Percent)
Cyclone3
Deposit
4/28/80
P.C.5
17.5
5.0
10.0
1.5
4.0
3.5
1.25
Heating Value
Btu/lb
8,652
10,468
8,963
10,735
19,400
9,884
9,464
RDF
Ash4
4/25/80
P.C.5
8.5
5.0
8.5
1.25
5.0
4.0
0.85
1 During RDF incineration without sludge; semi-quantitative spectrographic analysis
2 As Oxides
3 Melting point of material is 1,000°C (> 1,832°F)
4 Residue after ignition @ 590 °C (1,100°F) is 22 percent w/w
5 Principal constituent
III-10
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Table 111-3. Results of emission tests and exhaust gas composition.
FILTERABLE PARTICULATE CONCENTRATIONS AND EMISSION RATES
Concentration Emission Rate
GR/DSCF Ib/ton dry feed
Sludge with refuse derived fuel 0.028 0.468
Sludge with fuel oil 0.024 0.901
Refuse derived fuel 0.009 0.234
Note: 1 grain = 0.065 grams
1 cubic foot = 28.32 liters
1 Ib/ton = 0.5 gm/kg
EXHAUST GAS COMPOSITION* (Percent Volume)
Sludge with refuse
derived fuel
Sludge with fuel
oil
Refuse derived
CO2
11.5
7.9
6.0
02
7.2
10.0
14.2
CO
< 0.1
< 0.1
< 0.1
Balance
81.3
82.1
79.8
Moisture
26.55
12.1
9.9
*At scrubber exhaust, dry basis except percent v/v moisture
At the time the tests were conducted, FeCl3 and lime were being used to condition the sludge
for vacuum filtration. It was known that the addition of both Fe and Ca would help to further con-
vert the remaining alkali metal silicates. CaO with sodium silicate will form Devitrite
(Na2O-3/CaO6SiO2), which melts at 1,030° C (1,886° F); Fe2O3 reacts with sodium silicate to
form Acmite (Na2O'Fe2O3«4/SiO2), which melts at 955°C (1,751°F). With this assumption, it was
decided to continue the coimcineration tests after the slag was removed from the cyclone. The addi-
tion of limestone and clay to the feed to prevent this scaling gave encouraging results. Similar
results were reported using clay as an additive when incinerating sludges high in sodium. This
eliminated the buildup of molten salts on the bed particles and the resultant bed stickiness. During
the sludge/RDF coincineration, no slagging problems were encountered. Immediately after the com-
pletion of the performance tests in July 1980, the solid disposal facility was shut down for modifica-
tions to the RDF preparation and the ash handling systems.
In conclusion, the startup and modification work at Duluth showed that processed refuse and
wastewater sludge cake could be coincinerated and produce recoverable energy while meeting permit
standards for air emissions. In early 1984, the district purchased wood chips and bark waste from
III-11
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forest product plants in the vicinity and began coincinerating this material with sludge cake. New
belt presses have been started up successfully, replacing the vacuum filters in service, and the
resulting cake is higher in the ratio of volatile solids to moisture. Presently, the cost of the added
fuel in the form of wood industry waste is about $10/ton of wet cake at 16-18 percent solids. This is
considered less expensive than the cost of processing refuse in the RDF-making facilities.
Coincineration at Flint River, GA
The incineration of sludge solids and green wood chips at the Flint River WWTP in Clayton
County, GA, differs from the other U.S. projects described in that the sludge is dried and pelletized
before being fed to the combustor. The hot gases from the Pulse Hearth™ furnace are used to dry
the sludge in rotary triple-pass Heil dryers. The furnace system was started up in late 1982; the
dryers were existing equipment that had previously been heated by natural gas. The rated capacity of
the system is 25,000 MJ/hr (24 million Btu/hr) input. Two furnaces are arranged in series and pro-
duce a 980 °C (1,800°F) off-gas temperature that is tempered down to 540 °C (1,000°F) by mixing
with ambient air before entering the dryers. Exit gases are wet-scrubbed with water only, at a
pressure drop of 15 inches water column, and the air emissions limit of the plant is met. No signifi-
cant odor complaints have been reported. The manufacturer of the furnace claims that the multipass
design of the combustion chamber allows controlled burning of the sludge pellets and wood chips
without creating submicron fines or permitting unburned hydrocarbons to escape. Selection of a
cocombustion system like this would be most attractive in an area where forest products are
manufactured and where a steady supply of the wood chips at an attractive cost can be assured. This
is the case at the Duluth installation.
TYPES OF CODISPOSAL SYSTEMS IN THE UNITED STATES AND EUROPE
Although there are a number of different approaches to codisposal by incineration, most of them
can be categorized into four basic types: Direct Drying - Suspension Firing (DD-SF); Indirect Dry-
ing - Suspension Firing (ID-SF); Direct Drying - Grate Firing (DD-GF); and Indirect Drying - Grate
Firing (ID-GF). Each of the four types involves predrying the sludge cake before feeding it to the
combustion chamber. A fifth type, that does not involve sludge cake predrying, is exemplified by the
Glen Cove, NY, and Duluth, MN, projects in the United States but is not readily evident in Euro-
pean practice.
Figure III-2 illustrates the DD-SF type, where sludge coming from a wastewater treatment plant
(WWTP) is fed into a thermal dryer prior to incineration. In some cases this occurs directly,
whereas in other cases preliminary dewatering is accomplished by mechanical means before thermal
drying. Often a conditioning agent, such as a poly electrolyte (polymer), is added to facilitate
mechanical dewatering. Because these agents are of an organic nature, they do not adversely affect
subsequent incineration. Inorganic agents such as lime and ferric and aluminum salts would be
detrimental to heat value but may prevent rapid slag formation and be beneficial in that way.
However, chloride content is very undesirable. In the dryer, the solids concentration of the sludge is
raised typically to the 75-95 percent level. To promote drying by assuring proper consistency, the
feed sludge is often ground and mixed with a recirculated portion that is already dry. Finally, the
sludge powder is injected or blown into the incinerator in a zone above the burning refuse. Most of
the sludge powder burns in suspension, and complete destruction of all putrescible matter is virtually
assured. The energy required for drying the sludge is provided by removing a portion of hot flue
gases that are then directly brought into contact with the wet sludge. The quantity of flue gases must
be sufficient to furnish all the latent heat of vaporization required by the sludge to be dried. This
heat must be dissipated by evaporating water before the sludge gets dry enough to catch fire or
explode.
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III-13
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During thermal drying, some of the sludge solids volatilize in the form of gases and vapors.
These highly odoriferous substances are returned to the hot zone of the incinerator, usually at a
temperature of 774°C (1,425°F) or more, so that they are safely destroyed by thermal means. If the
highest degree of environmental control is required, as in West Germany, an auxiliary firing system
is installed. A temperature control system monitors the final combustion temperature inside the in-
cinerator. In case this temperature drops below the minimum of what is considered necessary for the
destruction of odoriferous substances (i.e., 802°C [1,475°F]), auxiliary gas- or oil-fired burners will
light up to compensate for any energy deficiency. Thereafter, the mixture of flue gases coming from
both refuse and sludge burning is conveyed through a common air pollution control (APC) system
out through a common stack. The resultant ash contains the inert constituents of both the refuse and
the sludge. If a high degree of thermal destruction is achieved, the amount of carbonaceous matter
will be below 3 percent by weight, and the amount of putrescible matter will be below 0.3 percent
by weight.
There are two major options with regard to the DD-SF type. One is to recover additional energy
for steam generation. This is achieved by inserting a waste heat recovery boiler between the in-
cinerator furnace and the APC system. This has been done successfully and on a large scale in
Europe. The other option is to siphon off either all or some of the dry sludge powder and use it as a
soil conditioner for agricultural and construction purposes.
The ID-SF type differs from the DD-SF type in one important respect. There is no direct con-
tact between the hot flue gases from the incinerator and the wet sludge to be dried. This is
accomplished by means of a heat transfer fluid that is used to extract sufficient heat from the in-
cinerator's hot flue gases. This heat is then transferred to the dryer, where it provides the latent heat
of vaporization. The heat recovery equipment is installed on the outlet side of the incinerator. It may
include steam generators, hot water boilers, and heat exchangers. Steam, water, and oil are com-
monly used as the thermal transfer fluids. As before, the gases and vapors that emanate from the
dryer are returned to the incinerator for thermal destruction. The dried sludge is also conveyed to
the incinerator for suspension firing. The main advantage of the ID-SF type is that sludge drying and
sludge incineration are physically separated. This is particularly helpful in retrofit situations, where
space limitations require separate equipment installations.
A third type, DD-GF, is considered by many engineers as the simplest approach to sludge
disposal in a municipal refuse-fired incinerator. Sludge is received from the treatment plant and par-
tially dewatered by the aforementioned means. The resultant sludge cake is simply transferred to the
incinerator and burned together with the refuse on a grate. The precise mechanism by which this
sludge is added differs from one plant to the next. In one case sludge is mixed with refuse in the
pit, while in another case sludge is dropped into the feed chute above the refuse. Attempts have also
been made to simply spray this sludge into the incinerator at various locations. In all cases, the heat
needed for vaporization of moisture must be transferred to the sludge cake, or more precisely, to
each sludge particle, by direct contact. The effectiveness of any particular approach has aroused con-
troversy among a number of investigators. This is mostly because of the unique heat and mass
transfer phenomena which govern sludge drying.
The ID-GF type is similar to the combustor design discussed above, except that, in addition to
mechanical dewatering, the sludge is dried by indirect thermal means before it is fed to the in-
cinerator. The ID-GF type lends itself to retrofit applications. Here, heat energy is extracted from
the incinerator and transferred to the thermal dryer, which is similar to the process described for the
ID-SF type, except that very fine particles are not needed when grate fired and more residual
moisture can be tolerated. Indirect drying takes place in a separate device under conditions that can
be more closely controlled. Many investigators have established the critical importance of the three
T's (temperature, time, and turbulence) in effective sludge processing. Depending on the design
III-14
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features of the dryer, intimate mixing and agitation of individual sludge particles are promoted for
maximum thermal efficiency.
U.S. AND EUROPEAN CODISPOSAL INCINERATOR SITES
A number of experiences in burning sludge solids and refuse have been reported over the years.
Reasons for the discontinuation of coburning or failures at some of the reporting sites vary; in some
cases it was because not enough effort and investment went into the planning and operational stages.
Tables III-4 through III-6 are a comprehensive listing of these trials in the United States, and Tables
III-7 through III-10 are a listing of those in Europe. Data are presented for the four basic types of
codisposal incinerators discussed previously, with the exception that no U.S. incinerators of the ID-
SF type were identified.
Case Histories of European Installations
The descriptions of the European cocombustion facilities in this section expand on the tabulated
data in the previous section (Tables III-7 through III-10) and should assist in their interpretation.
European cities have adopted the principle of cocombustion much more than have U.S. cities. The
motivations are intense in Europe, and political jurisdictional problems have not been an obstacle. As
a result, solutions have been found that are grounded in technical and economic imperatives. Also, a
high quality of construction has been observed by visitors to European installations. The case
histories that follow present experiences of cocombustion installations at Bielefeld, Goppingen, and
Marktoberdorf in the Federal Republic of Germany and Dordrecht in the Netherlands. A com-
parative evaluation is then presented of seven additional European mass-burning facilities.
The Bielefeld refuse incinerator consists of three units with a capacity of 16 tons/hr each at a
calorific value of 11,000 kJ/kg. With respect to incineration itself, it is a rather conventional plant,
consisting of a bunker, the incinerators, boilers, electrostatic precipitators, wet scrubbers, and a
specially designed wastewater plant. The four following factors convinced the Federal Government to
permit the development: waste was pretreated in order to homogenize it and to mix it with sewage
sludge; scrap was separated prior to incineration; hospital waste was incinerated with other wastes;
and the wastewater treatment system included flocculation and heavy metal ion exchange processes.
The coincineration process design was based on the following throughput:
Household waste 195,000 tons/yr
Commercial waste 85,000 tons/yr
Sewage sludge (40 percent dry matter) 30,000 tons/yr
Hospital waste 10,000 tons/yr
Tons X 0.90718 = metric tons
The investment costs (1983) were 146 million Deutsche Marke (DM) ($58.4 million), about 30 per-
cent of which apply to environmental protection devices. The innovation of interest here is the
pretreatment of the waste. Combustion, energy generation, heat exchanger burden, and flue gas
cleaning are better equalized than in the incineration of untreated refuse. During the mechanical
pretreatment of waste, predried and digested sludge is added in such a way that the mixture could be
incinerated on conventional grate systems without difficulties. Scrap is separated before incineration
to gain better scrap and less heavy metal emissions in the raw flue gas. Raw waste and the sludge
(about 10-15 percent sludge by weight as wet cake containing 40 percent dry matter) are fed into a
Losche ball mill, which has an inner diameter of 6.5 m and is filled with 50 tons of 12-cm-diameter
balls. Inside the mill, which has an energy consumption of 1 MW and a capacity of 50 tons/hr, the
waste is ground and mixed with the sludge. This mixture is then discharged via a trommel screen
III-15
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-------
having 260 X 100 mm holes to a conveyor belt. From here the scrap is magnetically separated
before the waste is stored in an intermediate bunker (2,300 m3) and then it is transported to the hop-
pers for incineration. Results so far indicate that steam production varies only by about plus or
minus 2 percent, the heavy metal content of the flue gas is only about 10 percent of known usual
figures, and the incineration of the shredded and mixed waste on the grates is problem free. From
this viewpoint, the additional investment for the mill (4 percent of the total investment or about 5
million DM or $2 million) seems to be justified. One major problem to date has been the unex-
pectedly large amounts of malodorous off-gases from the mill house. This will be rectified by direc-
ting this polluted air through the secondary air supply system of the incinerator.
An interesting approach to the incineration of household waste and sewage sludge is scheduled
to start in the near future. In 1975, two mass-fired household waste incinerators went into operation
with capacities of 12 tons/hr each, with space for a third unit of the same capacity. With an increas-
ingly difficult sewage sludge disposal problem, the Federal Government decided in 1982 to add to
the existing incinerator a mixing and homogenization device to coincinerate the sludge. The plan in-
cludes installation of a rotary kiln with a length of 32.5 m (107 ft) and an inner diameter of 5.0 m
(16.5 ft). Unshredded household waste and sewage sludge (500 tons/day of waste and 100 tons/day
of sludge with a solids content of about 35 percent) will be fed into the kiln, which will rotate at
0.8-4 rpm. The kiln will be indirectly heated with steam from the incinerator. During its residence
time of about 8 hours, the charge will be mixed, partially ground (from heavy components in the
waste), homogenized, and dried. The off-gases from the kiln will be drawn out of the kiln with hot
air and discharged into the incinerator as secondary air, the mixed charge being subsequently fed on-
to the grates of the incinerator. The retrofitted plant will cost an additional 10 million DM ($4
million). The concept of retrofitting existing incinerators (provided they are state of the art with
respect to flue gas cleaning), enabling them to coincinerate mechanically dewatered sewage sludge
together with household waste, makes this solution an interesting one.
The coincineration plant at Marktoberdorf was built in 1973 and went into operation in 1974.
Approximately 2 tons/hr of municipal waste are incinerated in a mass-burning incinerator with a
grate system. In the adjacent multiple-hearth incinerator, approximately 1 ton/hr of undigested
sewage sludge (about 80 percent water content) is incinerated after dewatering in a centrifuge. The
heat of a portion of the flue gases from the grate incinerator at 800 ° - 1,000 °C is used in a
counterflow manner to dry the sludge in the upper hearths of the sludge incinerator. The odorous
flue gases (250° - 350 °C) are recycled into the waste incinerator for complete combustion. The
balance of the flue gases from the waste incinerator enter a scrubbing system to eliminate dust and
noxious gases before they are released into the atmosphere. In Marktoberdorf there is no heat
recovery apart from the support of sludge incineration. The overall investment costs have been about
5.5 million DM ($2.2 million).
At Dordrecht, Holland, a combined system for refuse incineration, sewage treatment, and drying
and incineration of sludge has been in operation for more than 10 years and has worked well. The
combination results in a minimum of residues, as the sludge is converted to ash. Scrubber water is
taken from the sewage treatment plant effluent and recycled back for treatment. Data are provided
on the effectiveness of the flue gas washing in removal of HC1, HF, SO2, and dust. The sludge in-
cineration flowsheet (Figure III-3) is presented in terms of fates of six heavy metals. Compounds of
cadmium, lead, and zinc are volatilized into the flue gases; chrome, copper, and nickel are found in
the ash from the sludge. The Dordrecht wastewater treatment plant is one of the rare examples in
the Netherlands of a combined installation for the treatment of refuse, sewage, and sludge. The
sludge is dewatered, dried, and incinerated; dewatering is achieved by means of a centrifuge; and
drying, and incineration is by a MHF.
111-31
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' Refuse
Coarse Refuse
Lindeman
Shear
Refuse
(Stor
i
Normal Refuse
Sand
Floating
, W
' Objects Tre
Pr 2C
age) Slud9e
r~ t
["" fcxcess
Sludge
Sludge ' 1-2% d.s
Refuse [ Thickener )
Incinerators X /
(3x7t/h) Slag Etc \ /
/Scru
1 ( (Ra
1 V Flo
Clean
Fluegas 70 °C
h Sludge j 4-7% d.s.
Slag Scrap y
and p— -i Sludge
Flue Gas Ash A h L-p Decanter
1 t' 16-20% d s
/ Multiple \
Part of Flue Gas ^/ Hoarth \
Ref. Inc V Incinerator/
/^\ Flue Gas 425 °C ^~y-^
dial
Washwater
After Scrubber
Sedimentation
™^™"^ (Flu A«:hl . _
^__^^,^ \riy «SM| UUpel-|iA/;>ter R
' Waste Water
(City of Dordrecht)
(Holland)
Degritter
aste Water
atrnent Plant
)0.000 Inh
(Pi. Sed.
Aeration
Vfter Sed.)
Effluent
to River
Effluent
to Scrubbers
ack to
Rinse Water waste water 1 reat-
From Drinking ment Plant
Water Plant
Wet Fly Ash
Figure 111-3. Flow sheet of the Dordrecht installation.
111-32
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In the early 1960s, when the domestic refuse incineration plant built in 1938 started to show
signs of wear after 25 years of service, plans were begun to build a new installation. Because of the
absence of a sewage treatment plant in Dordrecht, all sewage was discharged directly into the Rhine
River. This situation was no longer acceptable, and a combined refuse and sludge treatment plant
was planned. The refuse processing was to be achieved by means of incineration, and the combina-
tion refuse-sludge treatment was to be technically and economically viable. Because it was virtually
impossible to find or create a market for digested sludge in the region surrounding Dordrecht, the
sludge also was to be incinerated. Flue gases were to be cleaned to meet government regulations
covering dust and hydrogen chloride (mostly from PVC). Some attention was to be given to the
recovery of heat; but since the first oil crisis had not yet occurred, no emphasis was placed on this
feature. The necessary equipment for each stage of the process was to be sufficiently tested and
available from reliable companies.
An investigation into the combined composting of refuse and undigested sludge showed that this
was technically possible. However, this method presented problems in relation to shredding and the
iron content in the refuse. Also, the removal of organic and inorganic components that could not be
composted was a problem. The largest drawback, however, was that no stable market could be
found for the compost end product. An investigation showed that thermal sludge conditioning was
not attractive because of the increased load on the sewage treatment plant. Of all the oxidation pro-
cesses, neither the pyrolysis method nor the fluidized-bed one was commercially available at the
time; only the multiple-hearth incinerator, as used in the roasting of ores, could seriously be con-
sidered. Using this method, only the inorganic substances of the sludge either remain as ash or are
entrained in flue gases. It was necessary to dewater the sludge to 15-20 percent dry solids, and aux-
iliary heat had to be provided in a MHF for the incineration of sludge. The combined incineration of
domestic refuse and sludge as practiced in Ebingen and Bulach, demanded the removal of any iron
and the shredding of the refuse prior to the mixing of refuse and sludge because of the high rate of
mechanical wear in the mixers and in the furnaces. With an improperly controlled process or too
low a caloric value in the refuse, the temperature in the upper part of the incinerator would be
lowered. As a result, incomplete incineration and pyrolysis would occur and thus cause odor prob-
lems. This has already occurred at the Bulach installation, which had to be closed down for that
reason.
The combined incineration in grate-type incinerators without prior shredding and mixing of
refuse and sludge leads to the formation of balls of sludge that are carbonized on the outside but un-
burned on the inside. These give odor problems at the disposal site. Spraying of sludge on refuse
causes objections on hygienic grounds. When it was shown that the solid waste incineration capacity
had to be enlarged to accommodate the sludge contribution, it was concluded that the building of a
special sludge incinerator next to the three planned refuse incinerators was the best solution. Figure
III-3 shows the flowsheet of the installation. The temperature of the flue gases from the refuse in-
cinerators is 900° - 1,000°C. The sewage treatment plant has a degritter, primary sedimentation,
aeration, and secondary sedimentation basins. The waste-activated sludge from the secondary
sedimentation is mixed with the inflowing sewage. The sludge from the primary sedimentation is
collected in a thickener. The thickened sludge is further dewatered by a centrifuge and subsequently
discharged into the sludge incinerator. The flue gas from one of the three refuse incinerators (capaci-
ty 7-1/2 tons/hr) is led into the sludge incinerator countercurrent to the sludge, as a result of which
drying and incineration of the sludge occurs. The ash from the sludge incinerator is moistened in a
mixer (Eirich-type) to reduce dust and then discharged into containers for disposal. Some develop-
ment work has been done to find possible applications for the slag that is produced. Some ex-
perimental paving stones of good quality have been produced, using up to 1 kg of slag per 3 kg pav-
ing. Slag might also be used as a road foundation material.
111-33
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In conclusion, there are several advantages of the Dordrecht integrated system for refuse and
sewage sludge incineration. Residues are minimal and it is possible to wash the flue gas with
WWTP effluent. Substances such as HC1 and HF are washed out at the same time as dust. The rise
of the treatment plant's water temperature results in improved sedimentation and (de)nitrification.
COMPARATIVE EVALUATION OF SEVEN EUROPEAN COCOMBUSTION SITES
Between June 26 and July 8, 1982, seven Western European mass-burning facilities that coin-
cinerate wastewater sludge with solid waste at various ratios were visited and evaluated for com-
parison purposes. The facilities varied in size from 72 to 860 Mg/d (80-950 tons/day). Energy in the
form of steam or hot water was recovered by three plants for district heating, by three plants for
generation of electricity, by four plants for drying of sludge prior to combustion, by one plant for
thermal conditioning of sludge in an adjacent facility, and by one plant to maintain set temperatures
in standby furnaces not presently in operation. Only one plant did not recover some energy in a
boiler. All energy recovered at this plant was used, as hot flue gas, to dry sludge in a dryer. All
plants having boilers also had furnaces of water-wall construction, as opposed to the refractory-wall
construction of the plant without boilers.
Regarding sludge processing, two plants received sludge in the refuse pit, where it was mixed
with refuse prior to charging. One plant received sludge cake into a pit, then transported it to a
"slinger," a device very similar to those used to ballistically project pulverized coal onto a traveling
grate in a semisuspension furnace. The slinger-type feeder disintegrates sludge (at about 45 percent
solids) into particles of slightly varying sizes and distributes it uniformly over the burning refuse
layer.
One plant dried the sludge, prior to combustion, in a thin-film, steam jacket dryer. Another
plant dried the sludge, prior to combustion, in a rotary kiln, flue gas dryer, with no dried sludge or
off-gas vapor recycling. Two plants dried sludge, prior to combustion, in mill-type, flue gas dryers,
with dried sludge recycling (to prevent formation of sludge balls from too wet a sludge entering the
dryer) and with off-gas vapor recycling (to lower the dryer operating temperature, thereby prevent-
ing explosions and fires).
The sludge to solid waste ratio as received at the facilities varied between 1:6 and 1:22 for
facilities receiving dewatered sludge, with a value of 1:5.5 for Deauville (France), which received
liquid sludge. The combined moisture content of the sludge and refuse handled at the seven facilities
ranged from 25 to 32 percent. On the basis of the heat balance, all the facilities visited operated at a
ratio of average Btu's released to Btu's required to vaporize the moisture, which was in excess of 4.9:1.
Applicability of European Technology to the United States
Recently, many municipalities in the United States have become interested in the feasibility of coin-
cineration of sludge and solid waste. While a limited number of U.S. municipalities have conducted studies
or experimented with coincineration, little long-term operating experience exists. As of 1984, only two
municipalities out of four with coincinerating facilities in the United States—Stamford, CT, and Glen Cove,
NY—are currently coincinerating sludge and solid waste.
In Western Europe, the high cost of oil and shortage of open land have frequently made coincinera-
tion an economically attractive method for disposing of waste products. As a result, a number of municipalities
in Western Europe have practiced coincineration for many years. Based on the 1982 visit to seven facilities
to compare their technological approaches to coincineration and evaluate the applicability of their technologies
to sludge and solid waste disposal in the United States, the following comments can be made:
111-34
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• The heating value of solid waste in the United States is generally higher than in Europe. This
means that, on the basis of the heat available from a given amount of solid waste, the ratio of
the amount of sludge that can be coincinerated with solid waste should be higher in the
United States.
• With regard to air pollution control standards, Germany has the most stringent standards of
the countries visited. Bamberg used wet scrubbers in conjunction with electrostatic
precipitators. Goppingen was in the process of installing dry scrubbers ahead of their elec-
trostatic precipitators. This level of air emission control is comparable to the most stringent
level imposed by some states in the United States.
• In Europe, most of the coincineration facilities are owned and often operated by the
municipality. In the United States, there is a trend towards full-service contracts for solid
waste, mass-burning facilities with energy recovery. In the full-service approach, the full-
service vendor must assume the risk for any problem associated with coincineration. Since the
introduction of sludge to a mass-burning furnace may increase maintenance costs and reduce
potential vendor profits from the sale of energy, most full-service vendors in the United
States look with disfavor upon coincineration. The full-service approach, therefore, may not
be the best procurement method for implementing coincineration in the United States.
• In Europe, the level of staffing observed at the coincineration facilities appeared significantly
higher than what one typically sees at U.S. mass-burning facilities. It appears that the
Western European people are willing to assume higher costs for what they perceive to be a
superior method for waste disposal in terms of environmental considerations.
• All of the plants visited were originally designed for burning refuse alone and were later
retrofitted with equipment to allow incineration of sludge. As a result, sludge storage and
processing units were often scattered all over the building, which in some cases necessitated
lengthy sludge cake conveyance systems. These problems will presumably be eliminated when
new systems are designed as coincineration facilities rather than retrofits.
The Future of Cocombustion
The stabilization of energy costs in the past few years has resulted in an abatement of the rush
to utilize refuse as an energy source, with time to reflect on the negative aspects of its use. For ex-
ample, it was previously assumed that RDF could be prepared on a routine basis. However, actual
experience with explosions, fires, and other hazards has resulted in a more realistic appraisal and
diminished the interest in grinding and classifying refuse to make a fuel for addition to a sludge
furnace.
There could be a renewal of interest, particularly if the cost of landfilling rises to the levels of
the "true" cost to process refuse in technically sophisticated ways. Also, although cheap, close-in
landfill space that can be used with minimum political complications has been used up, there is plen-
ty of land available if the transportation costs can be met and if higher levels of government declare
that such use is the "highest and best use," as they have done in cases of toxic waste disposal. Fur-
ther, it is possible that regional waste disposal districts could be created and given responsibility for
ultimate disposal of both refuse and wastewater solids. Such a regional agency could apply sound
engineering and economic analysis to arrive at the most environmentally sound yet cost-effective
methods.
In such an integrated disposal program, it must be recognized that any method of thermal con-
version would still result in some type of residual that must find an ultimate resting place. In cocom-
111-35
-------
bustion based on preparing RDF, there are inert residuals from the RDF-making system and from
the combustion system, so the adoption of these systems is not a total solution; landfill space of
some magnitude must still be provided.
In virtually every successful case of cocombustion, the prime combustor has been a refuse-type
furnace. Past efforts to utilize furnaces designed for sludge to burn refuse also have been less than
successful. If it is assumed that a sludge furnace must be used, there is reason to believe that the
fluidized-bed combustor would be more appropriate for the addition of chopped refuse than would
the multiple-hearth design; however, only scattered results are available for proof. The wet-milling
process applied to refuse at Franklin, OH, evidently produced technical success for several years,
after the FBF was modified. At Duluth, MN, the FBF design could not be altered sufficiently during
the startup period to permit stable operation with RDF containing substantial inert matter, although
another design was suggested that could overcome the problems experienced there.
As of 1984, there is no implementation of the starved-air combustion of mixed sludge solids and
highly prepared refuse that was demonstrated in pilot tests by manufacturers of MHFs in New Jersey
and California in the mid-1970s. Both of these manufacturers have gone out of business in the past
few years, as have several other major companies that competed in the municipal market in the past
decade.
In summary, without a strongly motivated industry trying to sell engineered products, without
financial incentives such as construction grants or attractive loans, and without a fear of energy
supply shortage, it seems unlikely that there will be any active development of cocombustion in the
United States in the near future.
111-36
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CHAPTER IV. THE MANAGEMENT OF AIR EMISSIONS, ASH,
AND OTHER RESIDUALS
INTRODUCTION
The combustion of waste matter results in the production of two major types of residuals-
gaseous and solid. After scrubbing, the gaseous stream is dispersed into the atmosphere. The solid
stream, which consists of ash material, must be placed in an ultimate disposal site such as a landfill.
This chapter will discuss the present regulatory situation regarding each and the process considera-
tions that govern efforts toward compliance.
Various forms of solid matter or residuals result from wastewater treatment and may be proc-
essed at the plant site before they are transported to ultimate disposal. Although the major considera-
tion in this chapter is ash produced during sludge incineration, it is appropriate to review all types of
material removed in treating wastewater. These can include: interceptor debris, screenings, grit,
primary settling tank scum, primary sludge, biological treatment sludge, chemical treatment sludge,
final settling tank scum, sand filter backwash, and solids from cleanup and maintenance. Each of
these can carry pathogenic organisms along with organic material, create an odor nuisance, and be
combusted. Most commonly only primary and biological sludges and scum are incinerated. In plants
that use inorganic chemical conditioning of sludge before dewatering, the chemicals, such as lime
and ferric chloride, remain with and add to the ash requiring disposal. There are three main topics
in this review of residuals management: (1) handling of the ash and/or other residuals, (2) environ-
mental risks in ultimate disposal, and (3) utilization instead of disposal.
NATURE OF THE CHALLENGE
About 4 million dry tons of sludge are generated in U.S. municipal wastewater plants each year.
Over a million tons of this are incinerated resulting in about 400,000 tons of ash that must finally be
placed in an environmentally compatible manner.
Handling of Ash in the Plant
Ash that is slurried as it leaves the furnace creates occasional problems within the vicinity of the
furnace, such as rapid wear on pumping equipment and piping, plug-ups at bends or restrictions, and
high humidity and corrosion above the slurry tank. However, there are not the dust and abrasion
problems that can occur in dry ash handling systems. Dry ash handling, which applies principally to
MHFs, is best when the ultimate disposal site is remote from the plant and when a time lag may
occur between generation and shipping. Wet ash handling is most likely to be chosen when a lagoon-
ing site is available on or near the plant property. Over a long span of time, it is probable that ash
stored in this way will have to be moved elsewhere by truck, and this later cleanout and disposal
cost should be factored into any economic analysis. Another element of concern in wet ash lagoon-
ing or final deposit is that the leaching process will start right away, and if a valuable aquifer could
be impacted, higher cost lining may be demanded. Wet ash handling is essentially the only method
for a fluidized-bed system because all of the ash is blown out of the combustor and caught in the
scrubber. The result is a fairly low ash concentration compared to the slurry from an MHF wet
system. The FBF ash slurry is usually thickened in a tank and may also be dewatered on a filter
before it is shipped to disposal. Ash can usually be handled by standard earth-moving equipment at
the landfill site or in the lagoon cleanout process.
IV-1
-------
Environmental Risks in Ultimate Disposal of Ash
Municipal sludge ash is normally exempted from regulation as a solid waste. As a result,
municipal sludge ash can usually be landfilled at a sanitary landfill, unless local or state regulations
deem otherwise. Actually, many tons have been placed over the years without any credible evidence
of hazard to the public or environmental effect. The Resource Conservation and Recovery Act of
May 1980 specifies teachability standards for substances and can result in a toxic waste rating for
the ash. For example, if there is a tannery in the area, the hexavalent chromium can result in a test
leachate exceeding the 5 mg/1 limit. This is the exception rather than the rule, but the problem can
arise, and ash must be checked for compliance.
Landfill is normally the least costly method of ultimate disposal. If the cost becomes too high,
other alternatives may be considered attractive. The same applies if a toxic rating is given to an ash.
In the following discussion on the utilization of ash, the methods will be classified in three groups:
(1) what is presently being done, (2) what has been tested, and (3) what can be done.
Utilization of Ash
Ash is a valuable resource in that its phosphorus has been shown capable of increasing crop
yields. Often the ash contains free lime, which is a beneficial soil additive in many locations. Ash
can also be used as a filler in asphalt road mix. Here the ash serves as the fine material that fills the
small voids among the pieces of gravel and increases the strength of the material. This also leaves a
smaller void for water penetration and helps prevent freezeout in the winter. Another use of ash is
in building products, if the product color is not a problem. Although the iron will often color a con-
crete or cinder block a dull red, the ash will act as a filler that allows the formation of sharp corners
in the cinder blocks and concrete blocks.
Ash has been used as a sludge filter aid for many years at Indianapolis' Belmont Sewage Treat-
ment Plant, but the added cost of erosion and other maintenance is high. Although this practice does
not eliminate ash, it saves operating labor and may eliminate filter-room odor problems that lime
causes. A polymer-conditioned biological sludge cake that is "sticky" releases more easily from the
filter fabric if ash is added. The negative aspect is that the adherent moisture adds to the heat
requirement in incineration and reduces furnace capacity for sludge solids. Ash also tends to migrate
throughout the plant's processes, because filtrate and belt washwater is recycled upstream.
Ash can be used within the plant area to promote the growth of grass or other cover; it can be
spread to 15 cm (6 in) or more of depth and will not wash away. Grass planted in this ash cover
will grow and stabilize the area. If ash is used for these purposes, then the problem of no-vegetation
growth can become one of overvegetation and additional maintenance may be required to keep the
grass cut.
Another use for sludge ash that has been tested but is not in widespread use is the chemical
fixation process. This process converts some wastes into nontoxic, nonpolluting material suitable for
landfill. This type of process often uses the lowest cost material that is locally available as part of
the process material. However, the process is not a big user of ash at the present time.
As chemical fertilizer manufacturers are well aware, ash can contribute to the quality of their
product. In a complex fertilizer plant, it is often necessary to add a filler to achieve the right con-
centrations of fertilizer. Sand is often used for this purpose because it is convenient. When ash is
used for this purpose, it must be completely burned out because any organics mixed with a nitrate
mixture can start a fire if the mixture gets too warm.
IV-2
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A process that eliminates the disposal of ash after incineration is the use of sludge as fuel in the
manufacture of Portland cement. Dried sludge has been substituted for peat or low-heat value coal
used in the cement kilns and leaves no ash. One location using this process had no problems, while
another had a smoke problem. However, the second location was a demonstration project with the
equipment set up temporarily for that purpose.
Using sludge to make bricks can also eliminate the ash disposal problem. It is possible to make
bricks that substitute up to 30 percent sludge for the clay. The bricks resulting from this are
described as common bricks and have a better bonding to mortar than do all-clay bricks. Since
sawdust was formerly used in fire-brick products, it may also be possible to use sludge as a part of
fire bricks or "refractory" products. It is also possible to use ash itself in bricks.
In the future, there may be other methods for eliminating sludge ash. In 1973, for example,
approximately 1.28 million tons of slag were used in this country to produce mineral insulating
wool. Sixty percent of this was blast furnace slag. To make mineral insulation, industry uses
water-cooled, cokefired furnaces that melt the slag and produce the fibers. Since a good deal more
insulation is produced now than in 1973, it seems that a lot of sludge ash could be converted into
insulation. Mineral insulation is a black, pressed-block type of insulation. Although it is primarily
used for industrial insulation, it also comes in sizes that can be used for house insulation.
The "flying saucer process" can convert ash into a usable construction material. It has been
done for fly ash and could also be done for sludge ash. In this process, which is used for upgrading
phosphate rock or iron ore to electric or blast furnace quality, the material is formed into balls on a
rotating disc. The disc is set at an angle, and then ash is fed onto it. As the disc rotates (thus, the
"flying saucer" name), small amounts of water are sprayed onto the ash and a few particles of ash
stick together and start rolling. As the disc turns, more particles of ash stick together forming a
larger particle that rolls and sticks to other particles. As the disc continues to turn, the particles
agglomerate sufficient material to form a ball. The diameter can vary from marble- to teacup-size
balls. Once the balls are large enough, they roll over the lip of the disc and go to a sintering fur-
nace. After sintering, they have enough strength to pass through an electric furnace or blast furnace
without fracturing. This strength is sufficient for the material to be used for construction purposes.
Another method that has not yet been tried in the United States is using ash as a growth pro-
moter on field crops. Soviet scientists have substantially increased the growth of radishes, potatoes,
and beets by using this method. In Japan, as noted previously, ash is sold as a fertilizer. Phosphorus
is a well-known promoter of healthy root growth. It is believed by some that the iron that is magnet-
ized slows down the growth of soil bacteria that consume nitrogen, leaving more nutrient for the
plants. Also, some bacteria beneficial to the plants grow better in the magnetic field created by the
iron.
It would be ideal to be able to sell all the ash for other uses. Analysis of the ash might find
something of value that would make it saleable. For example, the city of Palo Alto, CA, found
32 ppm of gold in its ash (a legacy of Silicon Valley electronics manufacturing) and was able to sell
it. At that time 3.2 ppm of gold in natural ore was considered high enough for profitable recovery.
The expression "all that glitters is not gold" can also be applied to ash in reverse. In Japan,
blast furnace slag is blended with aluminum hydroxide sludge, and the resultant mixture is used for
jewelry and artificial marble. Since jewelry and artificial marble can be made by adding silica, soda
ash, and other blast furnace slag materials, why not substitute the ash from sludge? The color, hard-
ness, and other features of the products could be changed by the types of additives used and the
method of processing.
IV-3
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AIR EMISSIONS CONTROL
Current United States Requirements
The air emissions standards applicable to new designs for sludge incinerators in the United
States include:
• Particulate emission—expressed as pounds per ton or in grams per kilogram dry feed (The
new source performance standard is 0.65 g/kg.)
• Opacity—expressed as a percent of total opaqueness (The standard is 20 percent; some states
limit this to several minutes per hour.)
• Beryllium—expressed in grams per 24 hours from entire site (The standard is 10 g and is
rarely a concern.)
• Mercury—expressed in grams per 24 hours from entire site (The standard is 3,200 g, and is
virtually never even approached.)
Commonly, the beryllium and mercury compliance is verified by analysis and mass calculation of
mass in the feed, assuming that it all leaves the system in the stack gases.
Other pollutants that have been defined in local area codes include: hydrocarbons; carbonyls;
partial combustion products such as aldehydes; carbon monoxide; sulfur oxides; nitrogen oxides;
chlorinated organics such as PCBs and pesticides; and odor. In addition, some jurisdictions apply
a minimum temperature such as 650° or 760 °C (1200° or 1400°F) and retention time at that tem-
perature in lieu of specific limits on organic constituents and odors.
Metal Emissions Control — Wet Scrubbing Systems
The general concern in the regulatory community about higher-molecular-weight metallics has
given rise to investigations in Japan and the United States to determine if these species leave the
incinerating system by the stack in sufficient amount to constitute a risk to the population in the
vicinity of the plant, as well as to the workers. As a result, the United States Environmental Protec-
tion Agency (USEPA) has evaluated the findings from tests on nine sludge incinerators in which the
following metals were determined: cadmium, iron, nickel, lead, chromium, silver, copper, manga-
nese, zinc, and arsenic. There are currently no Federal emission standards specifically governing
emissions of any of these metals from sludge incinerators. The Federal standard for ambient air lead
concentration is 1,500 /tg/m3, and work under way at the Research Triangle Park Laboratory of
USEPA has led to proposal of a cadmium standard of 100 /ig/m3. Of concern to the investigators
was the finding that both lead and cadmium seemed to be fugitive, passing through the existing low-
energy scrubbing systems. This presence was presumably in the form of submicron particles created
by vaporization in reducing conditions in the combustion zone and later condensation of the metals.
However, the median ground level concentration increment was computed to be very small. The
findings of this recent investigation, reported in 1981 but not yet published, restated the findings of
an earlier USEPA task force that had concluded in 1972 (EPA-R2-72-040) that "properly operated
incinerators produce acceptable stack emissions of particulate matter, nitrogen oxide, sulfur oxides,
and odor." In this recent work, the concerns about the heavy metals were also allayed, thus permit-
ting the statement: "The amount of heavy metals contributed to the atmosphere from incineration of
municipal wastewater sludge is insignificant when compared to that already in the atmosphere and
does not appear to create a threat to the environment." The conclusions of this working paper
follow.
IV-4
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1. Five of eight sewage sludge incinerators met EPA's new source performance standards of
0.65 g of particulates per kg of dry sludge solids feed. Of the remaining three, only one
significantly exceeded the standard. None of these incinerators had to meet new source per-
formance standards when constructed, although most of them had undergone some upgrad-
ing of their particle removal devices. This is an encouraging result and indicates no extreme
hardship created by the standard.
2. Overall efficiencies of the scrubbers varied widely and no correlations were evident. When
efficiencies for removal of various particle-size fractions were compared, differences in
overall efficiency could be explained. All scrubbers showed good removals for size fractions
larger than 1.0 /im. However, efficiencies ranged from about 50 to 90 percent for the 0.1 to
1.0 /im fraction. No significant improvements in performance were evident from the use of
higher pressure drops or use of a Venturi scrubber upstream from an impingement scrubber.
This is not unexpected because the scrubbing systems for the plants were totally unrelated.
3. Material balances around the incinerators (not including the scrubbers) for cadmium and
lead show results consistent with the other metals. These metals evidently are being col-
lected by the particle collection apparatus as efficiently as other metals.
4. Silver and zinc are not concentrated into the particulates by the incineration process. How-
ever, those particles that are carried overhead are fine and concentrate in the fines fraction.
Consequently, since the scrubbing systems are less efficient for fine particles, they are not
recovered as efficiently as the bulk of the particles, and their concentration in the particle
catch at the scrubber exit is substantially higher than at the inlet.
5. Cadmium and lead, probably by virtue of the chemical transformations that cause them to
be concentrated into the particulate fraction leaving the incinerator, are fine and are concen-
trated in the fines fraction. Consequently, their concentration in the particle catch at the
scrubber exit is substantially higher than at the inlet. Arsenic probably behaves in the same
way as cadmium and lead.
6. The enrichment of Cd, Pb, probably As, Ag, and Zn that occurs across the scrubber is
directly related to the presence of these metals in high proportion in the 0.1-1.0 /zm fraction
leaving the incinerator.
7. Generally over 90 percent of the Cd, Pb, probably As, Ag, and Zn that leave the scrubber
as particulates are found in the 0.1 to 1.0 pm fraction.
8. The median ground level concentrations for lead, calculated from plume models, averaged
25.8 ng/m3, which is only 2 percent of the actual ambient concentrations in the cities where
the incinerators are located and 1.7 percent of EPA's standard of 1500 ng/m3.
9. For cadmium, the median of the calculated ground level concentrations averaged 1.5 ng/m3.
This level approaches actual ground level concentrations but is only 1.0 percent of a sug-
gested standard of 100 ng/m3.
10. The low concentrations of lead and cadmium relative to concentrations of health significance
indicate that sludge incineration creates little or no threat to health from these metals.
11. The comparable material balances of lead and cadmium when compared to other metals
indicate good collection of these metals by the SASS train and its 0.1 /tin filter. If desired,
IV-5
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use of a filter downstream from the scrubbing system would assure virtually complete col-
lection of metals except mercury.
12. Sulfur dioxide and nitrogen oxides levels are very low compared to ambient standards and
to other sources of these pollutants.
Paniculate Emissions Control — Wet Scrubbing Systems
Particulate emissions from sewage sludge incineration can vary widely. The variables in the
sludge that seem to determine the emissions are (1) nature of the sludge (this can vary by hour, day,
or season), (2) type of incinerator, (3) temperature of incinerator operation, (4) moisture content of
the sludge, (5) volatiles content of the sludge, (6) amount of excess air used for incineration,
(7) sludge feed rate to the furnace, and (8) the type of paniculate removal system following the
incinerator. Perhaps the most important factor related to emission rate is operator knowledge of the
incineration process.
The nature of the sludge varies by location. At any location, sludge varies daily, weekly, or
seasonally and may change entirely over a period of a year because industrial sources can change the
character of the waste they discharge to the sewer or they may discontinue operation. A meat pack-
ing plant could be an example of this. This one plant could dump hair into the system. This would
improve the dewatering capability of the waste treatment plant and result in a sludge requiring a
smaller amount of fuel for incineration. Paper plant wastes are another example of an industrial
discharge that can make a sludge easy to dewater and burn. Emissions can be affected by the type of
sludge (i.e., primary, secondary, or tertiary). As more plants produce secondary and tertiary
sludges, the emissions become harder to remove by conventional water scrubbers regardless of the
pressure drop across the scrubber.
The particulate emissions, prior to passing through the scrubber system, are usually highest for a
fluidized-bed incinerator because the ash is removed from the furnace with the flue gas. Particulate
emissions quantities from multiple hearth incinerators are variable but usually less than fluidized-bed
incinerators because the ash is removed as bottom ash. Electric furnaces have the smallest amount of
particulate emissions, because the sludge is not stirred or mixed during incineration.
The higher temperature of operation in a FBF results in higher levels of emissions, including
heavy metals. Operation at 980°C (1800°F) normally produces 200 to 400 percent more emissions
than operation at 760 °C (1400°F). This increase in heavy metals could also be true for a multiple
hearth incinerator.
The moisture content of the sludge and the amount of excess air in the furnace can also have an
effect on emission rates. High gas velocities within the incinerator resulting from large quantities of
excess air, along with steam from high moisture content sludge, are believed to increase the sus-
pended paniculate load that leaves the furnace with the flue gas. Supplemental fuel requirements are
also increased by the need to evaporate moisture and to heat excess air.
The volatile content of the sludge determines the amount of supplemental fuel required for incin-
eration. Although higher volatile content in the sludge is usually desirable, it can be a problem when
combined with a low moisture sludge. This combination can cause increased emissions because the
sludge will explode (burn rapidly) as it is introduced into the top hearth of a multiple hearth
incinerator.
A high ash content (low volatile solids) sludge appears to reduce emissions. One waste treatment
plant that had a high ash content (the ash kept wearing out the sludge processing equipment) had a
IV-6
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low emission rate despite a malfunctioning scrubber system. Another plant that used ash as a filter
aid had a low emission rate despite the use of a modified wetted wall cyclone as the only particulate
removal system.
A constant sludge feed is desirable for MHF feed incineration because changes in feed rate, both
up or down, can cause smoke and/or increase emissions. Neither the U.S. EPA nor any contractor,
to our knowledge, has initiated studies on the relationship between operating conditions and emission
rates.
Historically the efficiency of a particulate removal system has been related to pressure drop
across the particulate removal equipment. Most incinerators now use a Venturi impingement tray
water scrubber that develops 30 inches or greater of water pressure drop across the system.
A demonstration project was performed at Indianapolis, IN for the purpose of determining
operating methods that would achieve more efficient fuel usage. The contractor who did the work
for Indianapolis has also performed similar work at seven other locations, and it is believed (not
documented) that air emissions were reduced at all locations.
The fuel reduction work has been documented for three of the cities and published in the follow-
ing documents:
1. "Plant-Scale Demonstration of Sludge Incinerator Fuel Reduction," (Indianapolis, IN) EPA
600/2-83-083 NTIS-PB 83-259 697.
2. "Sewage Sludge Incinerator Fuel Reduction at Nashville, Tennessee," EPA 600/2-83-105
NTIS-PB 84-113 075.
3. "Sewage Sludge Incineration Fuel Reduction, Hartford, Connecticut," EPA 600-84-146
NTIS-PB 84-243 096.
These multiple hearth fuel reduction studies were accomplished by temporarily instrumenting
each incinerator. Data on operating modes for the individual furnaces were collected by means of a
computer program. The procedures for operating the furnaces were then used to retrain the opera-
tors, who could then use the controls and instrumentation that already existed to maintain the
reduced fuel consumption and, it is assumed, reduced air emissions.
At this time the U.S. EPA does not anticipate any demonstration projects to relate furnace
operating modes to particulate emissions of multiple hearth incinerators. This work should not be
done until a reliable oxygen analyzer, a constant moisture determination system, and a constant
sludge feed system can be added to multiple hearth incinerators. Procedures that can immediately be
utilized to reduce air emissions from multiple hearth furnaces are (1) constant sludge feed rate,
(2) operator training to achieve fuel efficiency, and (3) proper operation and maintenance of par-
ticulate removal equipment. It is predicted that a 25 percent reduction in supplemental fuel can be
achieved at most multiple hearth sewage sludge incinerator facilities by the plant operating and main-
tenance staff through use of procedures in the fuel reduction documents listed above. These proce-
dures should also reduce air emissions from the multiple hearth incinerator because less excess air
will be required for incineration.
Air Emissions Control — Dry Scrubbing Systems
The Hyperion Energy Recovery System (HERS), as described in Chapter V, is a sludge process-
ing approach that includes the dehydration of sludge to produce a powdery, granular, sludge-derived
IV-7
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fuel (SDF), and subsequent energy recovery from the fuel combustion. The Hyperion plant is located
in Los Angeles, CA, in a nonattainment air basin, and compliance with new source review (NSR)
provisions of the Clean Air Act is the most constraining requirement in terms of design. In addition
to NSR rules, Los Angeles adopted a policy that air emissions from the plant should not be in-
creased as a result of the HERS Project. Present plant air emissions are summarized in Table IV-1.
These emissions result primarily from the use of digester gas in reciprocating diesel engines. As part
of the HERS Project, these engines will be retired from service and replaced by a gas-turbine,
combined-cycle system. Projected emissions from the latter facility under year 2000 conditions are
also presented in Table IV-1. The differences between present plant emission sources that will be
retired and the future gas-turbine engines emissions represent offsets available for the SDF combus-
tion system. Based on previous combustion studies, expected air emission factors, the availability of
control systems, and available emission offsets, NOX was judged to be the most constraining of the
air pollutants. NOX, CO, and hydrocarbons will be controlled by the temperature and stoichiometric
conditions maintained in the combustion process. Particulate emissions will be controlled by a multi-
clone in series with a fabric filter baghouse. A three-stage wet scrubbing system will be used for
control of acid gases (i.e., SOX, HC1, HF) and condensible particulates.
Table IV-1. Plant air emission offsets for HERS system (pounds per day}.
Existing reciprocating
NOX
(as NO2)
1,800
sox
(as SO2)
660
TSP
225
NMHC
(as CH4)
650
CO
1,740
engines using digester
gas (to be retired from
service)
Gas turbine combined
cycle system (year 2000
conditions)
Offsets available for
fluidized-bed system
680
1,120
20
640
55
170
25
625
715
1,025
kg/day = 2.205 x Ibs/days
Essentially all fuel ash will be carried from the combustion system with the exhaust flue gas.
While some dropout is expected in the boiler, paniculate loading on the baghouse will remain high.
The multiclone is designed to reduce ash loading on the baghouse and to remove any large particles
that may not be fully combusted. The presence of such "sparklers" is considered unlikely based on
the fuel particle-size distribution and gas residence time in the combustion system. Fly ash collected
in the pilot combustion tests contained less than 1 percent fixed carbon, an indication of essentially
complete combustion. Nevertheless, the multiclone adds an additional measure of safety to assure
that hot particulates do not burn holes in the bag material. The multiclone will also remove sufficient
particulates to allow flue gas recycling from ahead of the baghouse if necessary. The baghouse will
be an electrostatically augmented or conventional, multicompartment, pulse jet type with a maximum
net air/cloth ratio of 3.0. Baghouse technology is considered as Best Available Control Technology
(BACT) by the local regulatory agency. Based on fly ash particle-size distributions measured in the
pilot combustion tests and expected baghouse removal efficiencies for the particle-size fractions, an
outlet loading of solid particulates less than 0.005 gm/m3 (0.002 gr/dscf [grains/dry standard cubic
feet]) should be achievable.
IV-8
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Soluble Gas Removal in the HERS System
Two Venturis and a tray tower in series comprise the three-stage wet scrubbing system. The
system is designed to maximize use of wastewater secondary effluent and centrate from dewatering,
both of which are readily available within the treatment plant. Secondary effluent is a high quality
water that contains about 200 mg/1 alkalinity. Centrate is less desirable from a quality standpoint but
contains about 4,500 mg/1 alkalinity. The Venturis are supplied with sufficient secondary effluent to
neutralize expected mass emissions of acid gas. Centrate will be used as necessary to augment the
alkalinity. The tray tower is used as a polishing scrubber and will use a sodium sulfite/bisulfite
scrubbing liquor with pH maintained by NaOH addition. Since plant effluent will neutralize most of
the acid gases, use of NaOH is not cost-prohibitive. Another advantage is that the liquid sidestream,
composed primarily of soluble sodium bisulfite, can be easily disposed of with the treatment plant
effluent. Use of lime or limestone for scrubbing would result in a solid precipitate that would have
to be land-disposed.
The wet scrubbing system will also cool the flue gas to within 6°C (10 °F) of the water
temperature. An exit flue gas below 38 °C (100 °F) is expected from the tray tower. In the pilot
combustion tests, flue gas cooling and wet scrubbing were shown to be effective in removing con-
densible particulates. The HERS design provides a high liquid/gas ratio and the maximum cooling
possible with available water supplies. Subcooling below the saturation vapor temperature in the
scrubbing system will remove substantial water from the flue gas and aid in steam plume suppres-
sion. Cooled flue gas from the tray tower is reheated about 1.7°C (35 °F) in the induced draft (ID)
fan and then mixed with 120°C (250 °F) exhaust gas from the gas-turbine, combined-cycle system.
The combined gas flow is discharged through a single stack at about 104 °C (220 °F).
Current Regulatory Requirements in Europe
Neither the separate incineration of sewage sludge nor its coincineration with household refuse
comes under special regulations, but is covered by the general legislation that governs all waste in-
cinerators. Apart from this, the legal conditions for incineration of waste vary from country to coun-
try. Whereas in some countries incineration plants are approved on a case-by-case basis, in other
countries there are uniform countrywide regulations. These standards may go as far as to cover re-
quirements for site location and design, operating methods, emission limits, control of residues, con-
tingency plans, personnel training, financial responsibility, recordkeeping, reporting, monitoring, and
inspection.
In the Federal Republic of Germany, the construction and operation of sewage sludge incinera-
tion plants is approved under a plan approval procedure.
The Federal Government has issued general administrative regulations, with particular attention
to the following:
• Emission limits that are designed to prevent harmful effects and that must not be exceeded;
• Emission limits that reflect the latest state of technology and that may be exceeded; and
• The procedure for the establishment of such limits.
The following are details of the emission limits set in the Technical Instructions for Maintaining
Air Purity of 1974, now under revision: a negative pressure must be maintained in the feed bunker,
and bunker foul air must be fed to the furnace; minimum temperature of 800 °C (1470°F) must be
IV-9
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maintained, at minimum O2 content of 6 percent by volume with a minimum residence time of 0.3
seconds. Emission levels follow:
Dust 100 mg/m3
Chlorine compounds as Cl 100 mg/m3
Fluorine compounds as F 5 mg/m3
Carbon monoxide 1,000 mg/m3
(All related to an O2 content of 11 percent by volume)
The optical density of the waste gas plume shall be better than No. 1 on a Ringelmann chart
(assumed equivalent to 20 percent opacity). In addition, the responsible plant approval authority can
set lower limits for the emissions mentioned above or set limits for other emissions such as SO2,
NOX, or heavy metals.
As far as environmental requirements for disposing of incinerator ash residues and fly ash
residues are concerned, both are treated as municipal solid waste and disposed of in sanitary landfills
in accordance with requirements for municipal solid waste that are designed mainly to protect
groundwater and surface water from contamination by leachate. In some cases, special precautions
must be taken against dust pollution while discharging the residues on the landfill site.
If liquid effluents such as those from wet scrubbing systems must be disposed of, they may need
further treatment prior to discharge, depending on local circumstances. Neutralization, sedimentation,
biological treatment or even evaporation may be required. Often these effluents need not be treated
separately but can be pumped back into the sewage works and handled together with the wastewater.
In Dordrecht, Holland, the raw flue gases contain fly ash and harmful gases including HC1, HF,
NOX, and SO2. Electrostatic precipitation would remove only the solid particles, not the harmful gas.
Before flue gases can be passed through electrostatic precipitators, it is necessary to cool them down
to 250 °C (480 °F). This cooling can be achieved by means of the steam production in a waste heat
boiler, dilution with quench air, or evaporation of water in the gas stream. Steam production was
rejected for economical reasons, the argument being that the installation was too small. Also, there
was some fear that the growing proportion of halogenated plastics present in the refuse would cause
excessive corrosion of the boiler tubes and result in excessive down time. It is important to realize
that these evaluations date back to the 1960s, when energy conservation concerns were not prevalent.
Lowering the flue gas temperature by adding air requires that the suction (ID) fan and elec-
trostatic precipitators be three times larger. This in turn means larger investments and electricity
consumption. The temperature of the flue gases can also be lowered by evaporating water injected
into the gas stream. By cooling the flue gases to such a temperature that only part of the water
evaporates, polar gas molecules can be expected to dissolve in the water phase. Thus, not only the
solid particles, but also harmful gas components are removed.
The temperature of the wash water as well as the flue gases must not exceed 70 °C (158°F) after
the washing process. Because of the choice of radial flow scrubbers, electrostatic precipitators were
not required. The required water is obtained at minimal cost by using the effluent of the wastewater
treatment plant. For a refuse furnace with a capacity of 7,500 kg/h (16,540 Ib/hr), each scrubber is
supplied about 90 m3 water/hr (23,760 gal/hr), of which 20 m3/hr (5,280 gal/hr) is evaporated. The
scrubbed and cooled flue gases are saturated with water at 70 °C; this causes a stack plume on
release into the atmosphere. By tangential introduction of the scrubbed gases into the stack, water
droplets are caught by centrifugal force and flow to the drain. Not only fly ash but also HF, HC1,
IV-10
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and to a lesser extent SO2 will go into solution in the remaining wash water. The average results of
gas scrubbing are given below:
HF
HC1
SO2
Fly ash
Raw flue gas
(mg/m3)
15
850
280
4,400
Scrubbed
(mg/m3)
0.6
75
155
110
Reduction
(percent)
96
91
45
98
Emission
(kg/h)
0.1
9.2
17.6
13.2
After mixing with the wash water (70 °C, 158°F), the temperature of the waste water increases about
8°- 12°C (47° - 54 °F). This temperature increase causes a lower viscosity and therefore improved
presedimentation as well as higher bacterial activity. This, in turn, creates an improved treatment
capability, especially when atmospheric temperatures are low. After the scrubbing process, zinc,
lead, copper, cadmium, and chrome have been found in the wash water. These metals (particles)
partly settle in the washwater (scrubber drain) sedimentation basin. The wash water flowing back in-
to the wastewater treatment plant thus contains heavy metals after sedimentation.
Due to the differences in the refuse composition, the concentrations in the waste water vary
widely as shown in the following table:
Typical values
(Mg/1)
Range
(Mg/1)
Zn
Pb
Cu
Cd
Cr
Ni
Ag
Hg
10,000
1,000
300
150
120
50
10
1
5,000-18,000
500-3,000
120-500
40-250
30-100
20-80
3-20
There were fears that these amounts of heavy metals would adversely influence the biological
treatment process, but investigations have demonstrated that this concern was not justified.
Fates of Heavy Metals at Dordrecht
At the Dordrecht STP, investigations have shown that heavy metals such as Cd, Cr, Cu, Ni, Pb,
and Zn are mostly recycled in the installation. The raw flue gases contain a significant amount of
these metals. As indicated previously, most of these are washed out. A proportion of the metals set-
tles in the sedimentation basin, while the remaining fluid is brought back to the incoming
wastewater. In the treatment plant, the heavy metals are settled or adsorbed on the sludge, while a
small proportion is released with the effluent of the installation.
In sludge incineration, the metals show different behavior because of differences in volatility.
While the volatile compounds or elemental metals Cd, Pb, and Zn evaporate in the sludge furnace
into the flue gases, the less volatile compounds of Cr, Cu, and Ni appear in the bottom ash. In
Figure IV-1 a flow diagram is given, showing that 83 percent of the Cd, 60 percent of the Pb, and
33 percent of the Zn in the sludge are evaporated in the MHF, where the sludge is retained for a
IV-11
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Centrifuged
Sludge
Ash
Sludge 0,31
Ash 0,05
Sludge 5,20
Ash 2,14
Sludge 16,0
Ash 10,9
Sludge 0,18
Ash 0,21
Sludge 0,93
Ash 0,97
Sludge 0,06
Ash 0,11
Evaporation
Condensation
In Mg/a
Fluegas to Scrubber (Out)
»
Fluegas from Refuse Oven (In)
Cd.
0,26
Pb.
3,06
Zn.
5,10
Cr.
0,03
Cu.
0,04
Ni.
0,05
Fluegas
Out 0,44
In 0,18
Out 4,85
In 1,79
Out 20,80
In 15,70
Out 0,17
In 0,20
Out 0,43
In 0,47
Out 0,03
In 0,08
Figure IV-1. Heavy metals flow in the multiple-hearth furnace at Dordrecht.
relatively long period of time. Of the heavy metals in the flue gas from the refuse incinerator, ap-
proximately 75 percent of the Ni, 20 percent of the Cr, and 10 percent of the Cu are condensed. In
sum, much more of the metal compound is evaporated than is condensed (about 8 tons of metal a
year). In Figure IV-2 the flows of heavy metals are given for the WWTP. The computations are not
exact, since amounts per year are multiplied by a mean concentration. For example, for the ingoing
and outgoing quantities, the loading to the installation (sedimentation) is five times the load in the in-
fluent wastewater. This means that a major part of the heavy metals recirculates several times before
being discharged in either ash or effluent. Nearly 20 percent of the heavy metals originate from the
influent wastewater, 50 percent from the flue gases of the refuse incinerator, and 30 percent from
recirculation (mainly from volatilization in the MHF and from "clean" wash water and centrate).
Outgoing proportions are 55 percent in the ash, 35 percent to recirculation, and 10 percent in the
WWTP effluent.
Current Regulatory Requirements in Japan
Sulfur oxides, dust, nitrogen oxides, and hydrogen chloride contained in exhaust gas from
sewage sludge incinerators are regulated by the Air Pollution Control Law, and odorous substances
contained in the exhaust gas are regulated by the Offensive Odor Control Law.
IV-12
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5,1 Sewage Inflow
Wash Water
22,6
0,5
Clean
Wash
Water
Centrifugate
0,2
Sludge
22,7
8,5
Ash
14, 4
To Disposal
Site
Evaporation in Multiple Hearth Furnace
2,9 Effluent
Figure IV-2. Heavy metals flow at the Dordrecht STP (in tons/yr).
Air Pollution Control Law. The Air Pollution Control Law was enacted in 1969 but was later
amended several times, each time stepping up regulation stringency or increasing the number of
substances and facilities covered by the regulation. Table IV-2 summarizes the regulation of exhaust
gas from sewage sludge incinerators by the Air Pollution Control Law as of January 1983. The
regulation of dust and nitrogen oxides, as recently revised and stepped up because the national en-
vironmental standards for ambient air quality compliance rates for suspended particulate matter and
nitrogen dioxide were low, is summarized in Table IV-3. Emission standards are comprised of
general emission standards, special emission standards, areawide pollutant load control standards,
fuel use standards (relative to sulfur oxides), and more stringent prefectural standards.
IV-13
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Table IV-2. Emission gas standards from sewage sludge incinerators by
Air Pollution Control Law (Jan. 1983).
A. Sulfur Oxides
a) General Emission Standards and Special Emission Standards
The emission standard for sulfur oxides is called the "K-value regulation."
Q = K x 1CT3 x He2
where Q = permissible emission volume of sulfur oxides (Nm3/hr)
K = constant specified for each area
16 values ranging from 3.0 to 17.5 for the general emission
standard
3 values ranging from 1.17 to 2.34 for the special emission
standard
He = effective stack height (m, actual stack height plus height of
smoke ascent)
b) Areawide Pollutant Load Control Standards
• 24 regions have been designated as areawide pollutant load control areas.
• Specified factories are factories where the fuel consumption rate is more than
100 to 1,000 l/hr on the basis of heavy oil.
c) Fuel use standards concerning sulfur content rate of petroleum fuels are stipulated
for soot and smoke emitting facilities located where seasonal air pollution is caus-
ed to a high degree by small and medium sources, such as for heating, as well as
for the small-scale factories and establishments not subject to the areawide pollu-
tant load control standards in the designated areas.
d) More stringent prefectural standards cannot be issued.
B. Dust
a) General Emission Standards and Special Emission Standards
Type of
Furnaces
Continuous
Furnaces
Others
Exhaust Gas
Volume
(x 103 NrrWhr)
more than 40
less than 40
—
General Emission
Standards
(g/Nm3)
0.15
0.50
0.50
Special Emission
Standards
(g/Nm3)
0.08
0.15
0.25
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Table IV-2. Emission gas standards from sewage sludge
incinerators by Air Pollution Control Law (Jan. 1983).
(Continued}
b) Measured concentration is applied for the time being. In the future, the concentra-
tion calculated by the standard oxygen concentration collection method will be
applied.
c) More stringent prefectural standards can be issued.
C. Nitrogen Oxides* t
Exhaust Gas Time When
Type of Volume
Furnaces (x103Nm3/hr) until 6/1 7/77
Continuous more than 40 300
Furnaces less than 40 300
Others -
Incinerators Were Installed
from 6/1 8/77 after
until 8/9/79 8/10/79
250
300
250
250
250
250
* Unit is cm3/Nm3.
t Use the standard oxygen concentration collection method.
b) Areawide Pollutant Load Control Standards
• 3 regions have been designated as areawide pollutant load control areas.
• Specified factories are factories where the fuel consumption rate is more than
1.0 to 10 kl/hr on the basis of heavy oil.
c) More stringent prefectural standards can be issued.
D. Hydrogen Chloride
a) General Emission Standard
• 700 mg/Nm3.
• Use the standard oxygen concentration collection method.
E. Remarks
a) Target incinerators are incinerators with an incinerating capacity of 200 kg/hr or
more.
b) Special Emission Standards are applied for new and expanded facilities in areas
where facilities are concentrated and where serious pollution can be expected to
be caused.
c) Areawide Pollutant Load Control Standards are applied for the designated regions
where factories and workshops are so clustered together that it is considered dif-
ficult to meet the environmental air quality standards only by facility emission
standards. Total mass emission reduction plans in the designated regions are
prepared by prefectural governors and areawide pollutant load control standards
are applied for specified factories larger than a specified size.
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Table IV-2. Emission gas standards from sewage sludge
incinerators by Air Pollution Control Law (Jan. 1983).
(Continued)
d) Standard Oxygen Concentration Collection Method (Standard oxygen concentra-
tion for sewage sludge incinerators is 1 2 percent.)
C = x Cs
21 - 08
where: C: collected concentration of the substance
Os: oxygen concentration in the emission gas (%)
Cs: measured concentration of the substance in the emission gas
Offensive Odor Control Law. The major legal weapon against offensive odors is provided by
the Offensive Odor Control Law of 1971. According to the law, prefectural governors are obliged to
designate "Regulated Areas" where it is necessary to prevent offensive odors for the protection of
amenities and to limit the emission concentration of specific odorous substances from the factories
and places of business within the areas. While WWTPs and pump stations would normally be
classed with the factories in the law, sewer lines were made exempt. Legal limits on odorous
substances are given in Table IV-4. These substances are the principal sources of offensive odors
and have been listed since methods have been established to measure their concentration in the air.
The limits have been established based on those concentrations that the majority of people in
regulated areas will accept. Thus, prefectural governors are obliged to set concentration limits on
odorous substances within 2.5-3.5 points of the six-grade classification of odor strength (see Tables
IV-5 and IV-6). The Environment Agency has so far advised prefectural governments to limit con-
centrations to 2.5-3.0 points in residential areas and to 3.0-3.5 points in industrial areas. Most
governments have set limits at 2.5 points for residential areas and 3.0 or 3.5 points for industrial
areas.
The Offensive Odor Control Law stipulated that limits are to be established in terms of
(1) concentration in the air at the plant boundary; (2) concentration of gases at the stack outlet; and
(3) concentration in industrial effluent. There is as yet no practical method of calculating concentra-
tion in industrial effluent. Concentration limits for gases at the stack outlet are set to maintain
satisfactory ground-level concentrations at the plant boundary.
While the Offensive Odor Control Law has been effective to some extent, there still is much left
to be desired. One problem concerns methods of measuring odorous substances. Usually an odor is
sensed even when its concentration is very low, so the personal judgment of local inhabitants often
comes into conflict with instrument measurements. Because of this, odor control by organoleptic
testing is being considered instead of instrument analysis of the eight listed odorous substances.
Among these methods, the triangle airbag method (a kind of air dilution method) developed by the
Tokyo metropolitan government is considered quite effective, and some local governments have
already specified it in pollution control regulations.
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Table IV-3. National environmental standards for ambient air quality and compliance rates.
Sulfur
Dioxide
Compliance Rate (%)
Standard
General Air
Pollution
Monitoring
Station
Automobile
Exhaust
Monitoring
Station
Remarks
Daily average of
hourly values shall
not exceed 0.04 ppm,
and hourly values shall
not exceed 0.1 ppm.
98.4
Carbon
Monoxide
Daily average of
hourly values shall
not exceed 10 ppm,
and average of
hourly values in eight
consecutive hours shall
not exceed 20 ppm.
100
99.1
Suspended
Paniculate
Matter1
Daily average of
hourly values shall
not exceed 0.10 mg/m3,
and hourly values shall
not exceed 0.20 mg/m3.
29.2
Nitrogen
Dioxide2
Daily average of 3.8
hourly values shall 24.4
be within the range 71.8
between 0.04 ppm and
0.06 ppm or below.
38.2 0.06 ppm
45.9 0.04 0.06 ppm
15.9 <0.04 ppm
Photochemical
Oxidants3
Hourly values shall
not exceed 0.06 ppm.
1 Suspended paniculate matter shall mean airborne particles of 10 microns or less in diameter.
2 a) In an area where the daily average of hourly values exceeds 0.06 ppm, efforts should be made to achieve the level of
0.06 ppm by 1985.
b) In an area where the daily average of hourly values is within the range between 0.04 ppm and 0.06 ppm, efforts
should be made so that the ambient concentration be maintained around the present level within the range or not
significantly exceeding the present level.
3 Photochemical oxidants are oxidizing substances such as ozone and paroxiacetyl nitrate produced by photochemical reac-
tions (only those capable of isolating iodine from neutral potassium iodide, excluding nitrogen dioxide).
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Table IV-4. Odorous substance limits according
to Offensive Odor Control Law of 1971.
Odorous substances
Ammonia
Methyl Mercaptan
Hydrogen Sulfide
Dimethyl Sulfide
Trimethylamine
Dimethyl Disulfide
Acetoaldehyde
Styrene
Limits: Air-borne
at Plant Boundary
1 to 5
0.002-0.1
0.02-0.2
0.01-0.2
0.005-0.07
0.009-0.1
0.05-0.5
0.03-20
Concentration
(ppm)
(D*
(0.002)
(0.02)
(0.01)
(0.005)
(0.009)
(0.05)
(0.4)
* Example figures in parentheses are for Yokohama City.
Table IV-5. Odorous substances and their odor strengths.
Odor Strength
0d°roUS 1 2 2.5 3 3.5 4 5
Substances
Ammonia 0.1 ppm 0.6 ppm 1 ppm 2 ppm 5 ppm 1x10 ppm 4x10 ppm
Methyl Mercaptan 0.0001 0.0007 0.002 0.004 0.01 0.03 0.2
Hydrogen Sulfide 0.0005 0.006 0.02 0.06 0.2 0.7 8
Dimethyl Sulfide 0.0001 0.002 0.01 0.05 0.2 0.8 2
Dimethyl Disulfide 0.0003 0.003 0.009 0.03 0.1 0.3 3
Trimethylamine 0.0001 0.001 0.005 0.02 0.07 0.02 3
Acetoaldehyde 0.002 0.01 0.05 0.1 0.5 1 1x10
Styrene 0.03 0.2 0.4 0.8 2 4 2x10
IV-18
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Table IV-6. Six grades of odor strength.
Odor Strength Description
0 No odor
1 Barely detectable (detection threshold level)
2 Barely identifiable level of odor (identification threshold level)
3 Easily identifiable
4 Strong odor
5 Very strong odor
Gas Cleaning in Japan. The greater concern for removal of acidic constituents (chemical
scrubbing) in Japanese regulations, as compared with U.S. regulatory practices, is exemplified by
the following process description at the Yotsuya Wastewater Treatment Facility (WTF) in Takoka
City, Japan. The exhaust gas is first cooled, dehumidified, dedusted, and alkali washed in a cylcone
spray-type scrubber, using secondary and effluent and caustic soda. The exhaust gas is then deo-
dorized by passing it through an acid scrubber and a sodium hypochlorite scrubber, driven by a fan
and further dedusted by a wet-type electrostatic precipitator, and mixed with hot air from the shaft
cooling exit to prevent white plume from the stack when discharged.
The greater concern for chemical scrubbing in Japan apparently results from the practice of
keeping the furnace exit temperature as low as possible to save fuel and using chemical methods to
control organics and odor constituents in lieu of afterburning.
CONTROL OF SLAGGING AND CLINKER FORMATION
A significant problem facing operators of combustion systems is prevention of ash fusion that
results in formation of glassy slag or porous agglomerates called "clinkers." Two case histories
presented in this publication provide valuable information on the subject: the Duluth startup (see
Chapter III) and the downflow gasifier research at the University of California at Davis (see
Chapter V). The problem of slagging and clinker formation has particularly been found in sludge
combustors burning cake produced by dewatering processes that incorporate either polymer or ther-
mal conditioning of sludge. This finding is dramatic when such a process is put in place following
an earlier period of burning a dewatered sludge produced with inorganic chemical conditioning, such
as by ferric chloride and lime addition. The much lower softening and fusion points of the ash result
in more rapid formation of clinkers and slag. An example of this occurred at the Metro Plant in the
Minneapolis-St. Paul area and is described in the case history that follows.
Slagging Problems in the Twin Cities
Sludge cake has been burned at the Metro Plant in the Minneapolis-St. Paul area since 1938.
One of the earliest multiple-hearth installations in a wastewater plant is located there. Lime slurry
and either ferric chloride or ferrous sulfate were used for many years and continued to be included
after the addition of four new furnaces in the 1968-72 period when an added load, secondary treat-
ment sludge, had to be handled. The newer furnaces were fitted with open-register burners, which
turned out to be a source of massive slagging difficulties. These conditions were alleviated when the
IV-19
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burners were changed to the sealed-in type, and slag formation was essentially eliminated during the
period 1976-1981.
The four newer furnaces were then shut down for modification to a configuration more suited to
burning the sludge solids then being produced in the new thermal conditioning process just started
up. The ash from the resulting solids was found to have a fusion point of 1,100°-1,150°C
(2,000°-2,100°F), compared to the 1,400°-1,500°C (2,500°-2,700°F) values typical of the former
ash, which included residuals of the ferric chloride and lime along with the natural sludge mineral
content. Another type of cake was also produced in newly installed dewatering units; this was roll-
pressed raw primary that had been conditioned by polymer. This also had a low fusion point typical
of natural sludge mineral matter. The result of trying to burn these two new types of feeds at tem-
peratures as high as 980 °C (1,800°F) (observed by thermocouple at the furnace wall and obviously
higher in the flame zone of the bed and in drop holes) was the gradual closing of the out-hearth
drop holes and the development of draft difficulties in a matter of weeks. Corrective action included
reducing loading and thus reduced the production rate of the furnace. Other steps were (1) reducing
the maximum temperature of the hottest hearth to 870 °C (1,600°F) and (2) increasing the size of the
drop holes in hearth two where the plugging was most severe.
Slagging is increased by the presence in the ash of lighter metal oxides or carbonates, such as
from sodium and potassium. Another constituent that has been implicated is vanadium, which is
often found in fuel oil. Ferric phosphate has also been known to flux or form a eutectic, causing ash
to form slag at lowered temperatures. As has been discussed in the Duluth case history, calcium and
magnesium appear to raise the softening point, and thus the addition of limestone or dolomite has
the dual advantage of lessening slag trouble and absorbing sulfur dioxide. Plants that continue to use
lime for conditioning and stabilizing of the sludge get this added benefit.
The earlier problem experienced at the Metro Plant, and mentioned previously, was the rapid
formation of slag in the tiles of the open register burners that did not have the premix features of the
sealed-in type that replaced them. Analysis of flame dynamics indicated that there was a zone in the
flame envelope where the gas mixture passed through zero excess air. The problem was most
noticeable when oil was being burned, as was typical for much of the winter when an interruptible
gas supply was off. The apparent source of the trouble was the passage of fly ash through the
influence zone of the burner flame, where it was exposed to very high temperatures due to transitory
conditions in the flame envelope where the fuel to air mix was zero excess air. Thus the ash particle
might be heated to the theoretical maximum of over 1,800°C (3,200°F) in a fraction of a second.
As it cooled when blown away, it could remain "sticky" and adhere to burner tiles or refractory
brick surfaces, gathering other similar particles in a short time. This gave rise to two types of costly
maintenance: buildup in the burner tile that could block the flame and cause heat to back up into the
burner box and damage components, and buildup on the roof of the hearth, in the form of stalactites
as in a cave. These effects caused a deflection of the flame that eroded the brick surfaces of the
hearth roof and wall by the play of flame at approximately 1,500°C (2,800°F) against brick rated
for 1,200°C (2,200°F) duty. Even after the change to the sealed-in burners, the deterioration prob-
lem was not completely solved. It was found necessary to set the new burners to a leaner ratio, so
that the theoretical flame-tip temperature of the mix could not exceed about 1,400°C (2,500°F). This
unfortunately reduced the total heat that the burners in critical hearths could inject into the furnace
and thus reduced the ability to evaporate moisture and the tonnage loading the furnace could handle.
The reason for this adverse effect is that the sealed-in burners are air-limited; that is, they have a
maximum passageway for air and this controls the amount of airflow at a given supply pressure. The
leaning-out, then, can only be done by reducing maximum fuel flow. As a result, furnaces that were
thought capable of 11 to 12 tons/hour wet cake feed rate could only handle about 9 maximum, and
the workers typically ran at 7 to 8 tons. However, long online times became possible — up to 11
months without trouble, instead of 3 to 4 months as before. Also, previously it had often been
IV-20
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necessary to shut down an individual burner because of slag buildup, thus reducing the productivity
of the furnace until the burner was serviced.
THE ROLE OF REGULATORY AGENCIES
Federal, state, and local regulatory agencies have a strong voice in the decisionmaking process
relating to the design and construction of municipal sludge burning systems. The following discus-
sions present viewpoints of Federal and state officials. The state aspect presents a candid review of
the factors that must be considered by an applicant for a permit to avoid costly delays in getting the
request through the procedural steps leading to approval.
Sludge Management Task Force Project
EPA embarked on an effort to evaluate the costs, benefits, and environmental effects of the
disposal and use of sewage sludge across all media—air, land, and water—with the goal of develop-
ing a comprehensive policy and guidelines for sludge management. Consideration of the relative
merits and problems facing the use of sludge combustion processes in managing municipal sewage
sludge is an important aspect of this multimedia examination of sludge management practices. This
effort also includes a serious attempt at undertaking a cross-media comparison of the risks associated
with the different sludge management alternatives on a common basis. Stated simply, the central
issue is:
How does one manage a complex and variable waste in an environmentally protective
and cost-effective manner when the disposal/utilization may involve any medium?
In response to this concern and the perceived need to provide better guidance to local managers
of wastewater treatment facilities, EPA management initiated the current project in early 1982. The
effort is being carried out by a staff task force under the sponsorship of a policy committee com-
prised of the Assistant Administrators for Water and for Solid Waste and Emergency Response and
the Associate Administrator for Policy and Resource Management. The principal objective is a multi-
media examination of sludge management with the development of a cohesive agency policy on
sludge management as the central product. Specific outputs will include comprehensive guidelines, a
series of recommendations concerning revisions to existing regulations or development of new ones,
and recommendations for a continuing program in the sludge management area.
Scope and Objectives. The task force is focusing on sludge resulting from sewage treatment
at publicly owned facilities and will evaluate all major disposal and use options: ocean disposal, land
disposal (landfills and land applications), thermal destruction, and distribution and marketing.
Project Approach. The project is structured around a comparative assessment of environmental
hazards, costs, and benefits of various disposal/utilization options using selected contaminants or
sludge properties as indicators of environmental concern. Data and information for this analysis are
being supplied by a number of distinct tasks, most of which are being carried out by contract
consultants.
Risk Assessment Efforts. The careful comparison of risks across sludge management alter-
natives against the costs and benefits associated with each practice and of controls placed on these
practices are key elements of the project. Medium-specific legislation and protection programs at the
Federal, state, and local levels across the country have rarely addressed the intermediate side effects
to determine where efforts to control one resource (e.g., air quality) should be limited so as not to
overburden another resource (e.g., oceans, ground water, or farmland). The problems that must be
faced when making such comparisons are tremendous; for example, local conditions vary widely,
IV-21
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many of the real or potential effects of sludge disposal and utilization practices are poorly under-
stood, and technical judgments and decisions — particularly cost estimating — must rely on count-
less assumptions.
Thermal Conversion. Thermal conversion systems (i.e., incineration, starved-air combustion,
codisposal, thermal conditioning, etc.) represent only one of the major types of sludge control tech-
nologies being addressed by the task force. These systems have frequently been plagued by their
need for large capital investments and highly trained operators, serious equipment and operation
problems, increasing fuel costs, and tightening emission control requirements. Fuel prices, emission
requirements, sludge characteristics, operating procedures, and operator experiences different from
those that were assumed by project designers have often led to serious system problems that were
expensive to solve. However, under the right circumstances thermal conversion processes have been
implemented as cost-effective and environmentally acceptable sludge volume reduction and disposal
options. Generally subject to economies of scale due to their requirements for substantial capital
investments and energy inputs (either to dewater the sludge or in the form of auxiliary fuel), these
systems have not often been cost-effective for plants below 10 mgd. The recent dramatic increases in
fuel costs (three- to fivefold since 1970) have resulted in abandonment of many of the existing ther-
mal conversion systems that were designed and/or built during the days of cheaper energy because
they have become too expensive to operate. Increased air emission constraints have also led to prob-
lems in operating many of the existing systems.
Recent efforts to retrofit existing thermal conversion equipment and modify inefficient operating
methods (e.g., operating at lower temperatures with less excess air) have led to substantial fuel sav-
ings and improvements in equipment performance and compliance with air pollution requirements by
a number of previously inefficient incineration and thermal conditioning systems. More recently
installed incineration and thermal conditioning systems have also been designed, built, and operated
to be much more energy efficient, corrosion resistant, and less polluting. By combining the most re-
cent advances for effective sludge dewatering with energy efficient incineration, energy recovery/
reuse equipment, and efficient emission control equipment and system operation, it should now be
possible to design thermal conversion systems to be nearly self-sustaining (energywise) and environ-
mentally acceptable. However, total system capital and operating costs and recent concerns over the
potential for classification as "hazardous waste" of the ash from certain incineration systems will
likely continue to confront greater use of thermal conversion systems.
Conclusion. The ongoing task force efforts should result in a better understanding of the status
of each of the major sludge management options and allow for better consideration of their benefits
and risks relative to each other. A serious attempt will be made to compare the virtues and problems
facing thermal conversion practices with those of other major sludge management options.
PROJECT DELAYS ENCOUNTERED FOR SLUDGE COMBUSTION FACILITIES
There are a multitude of obstacles that can stall or stop the implementation of a project. Causes
of delay are divided into technical deficiencies (e.g., incomplete submissions and inadequate alter-
native analyses) and obstacles relating to the decisionmaking environment (e.g., political impacts at
the local and regulatory level, regulatory changes and coordination, problems with individuals, and
public opposition).
The implementation of new, nearby, and cost-effective disposal facilities is foremost in the
minds of many treatment plant operators and authorities, but permit applicants for new facilities fre-
quently complain that the permitting process is long and expensive. Of all the sludge management
modes, the implementation of new sludge combustion facilities may be the most complicated of all
the disposal modes to work through the regulatory process.
TV-22
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The speed with which a long-term disposal mode can be implemented can have a significant
impact on the operating costs for the facility. A stalled project can be devastating to a facility in
both the short and long run. There are many reasons why a project may become stalled altogether.
Some of the reasons are technical and others relate to the decisionmaking environment, and the
blame for the stalled project may rest with the applicant, the regulatory agency, the consultant, the
public, or all four. The result, however, is usually the same — project delays are expressed in
higher costs that the public must ultimately bear.
Technical Reasons for Project Delays
There are two general reasons for project delays: (1) incomplete project submissions or
(2) inadequate alternative analyses. In most cases, the responsibility for incomplete submissions rests
with both the regulatory agency and the consultant. Either the consultant has failed to fully educate
himself on the regulatory requirements or the regulatory agency has failed to fully communicate the
submission requirements. The responsibility for the inadequate alternative analyses rests most fre-
quently with the consultant or the applicant. Either the applicant has placed unrealistic restrictions on
the alternative selection or the consultant has failed to objectively evaluate the variables leading to
selection of the disposal alternative.
Incomplete Project Submissions
Incomplete submissions are more common for sludge combustion projects than for any other
type of sludge management project. The reason that incomplete submissions are so common is that
sludge combustion projects result in four separate discharges that involve air, water, and solid waste
disposal regulations. The complexity of each of these programs is so great that the project review
staff of one program is rarely knowledgeable in the requirements of others, so they may proceed
with approvals without recognizing the permit requirements of other programs, to the detriment of
the project itself. Primary attention is focused on stack gases for these projects, and the majority of
proposals are considered fully completed if they have provided for emission control devices that will
enable the combustion facility to meet air quality standards. However, the proposal must also
address controls of the paniculate emissions of the ash as it is stored, handled, and transported
within the working areas of the facility; it must address the ultimate disposal of the ash; and it must
address treatment and disposal of scrubber water. Although a few proposals may escape one, or
perhaps two, of these requirements in the complexity of the regulatory review, it is difficult to
predict which requirements may be overlooked. Generally, gambling on escaping requirements is not
worth the price of the delays that occur when the deficiencies are identified and planning or calcula-
tions must be reworked.
Stack Emissions. Complete submissions for stack gas emissions will vary from project to pro-
ject depending upon many parameters, including the quantity and quality of pollutants to be discharg-
ed, ambient air quality, proximity of populations, weather and topographical conditions, dimensions
and shape of the stack, fuel quantity and quality, type of emission control device, and type of incin-
eration unit. Permit requirements address the construction of the facility and its operation (source
information). In fact, the complexity and variability of the submission requirements is so great that
the New Jersey Air Pollution Control Program has developed a four-page instruction sheet for com-
pletion of that state's permit application, and New York has a 17-page instruction booklet.
Mistrust of Submission Information. Sometimes submissions on sludge management projects
are deficient for reasons other than the complexity of the regulatory process. The deficiencies often
reflect efforts to conceal impacts or multiple emission sources to avoid regulation. These types of
deficiencies result in the greatest project delays because the consultant or applicant may not be will-
IV-23
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ing to cooperate fully in satisfying the regulatory requirements. Under these conditions, submission
information is usually forwarded piecemeal as it is requested by the agency and is accompanied by
protests regarding the need for the additional information. Agency reviewers are usually well aware
of the behavior of the consultant who is trying to avoid regulation. More often than not they respond
with closer scrutiny of all the information on the application because the reviewer is sensitized to the
possibility of concealment or deception in the project documentation.
Participates from Ash Residual: Ash Storage and Conveyance Areas. There is a second
area of air emissions that is frequently overlooked — particulate emissions from the ash residual.
The transport and handling of this material can necessitate applications for additional permits or, at
the very least, modifications in the operations and maintenance manual for the facility. Ash
generated by a combustion facility must be removed to a final disposal site; more often than not, it
is stored onsite until its volume justifies hauling to a landfill. Conveyance to the storage site and the
storage site itself may expose the ash to air transport if designs do not provide for enclosure of these
facilities. Where ash is pumped in a slurry to be dried and stored in lagoons, the conveyance is not
an air emission concern, but particulate transport from the dried, stored ash continues to require
emission controls.
Particulates from Ash Residual: Ash Within Facility-Enclosed Working Areas. Par-
ticulates from ash can become a health problem within facility structures where plant operators are
exposed to this risk on a daily basis. The problem can stem from both facility design and poor
operation and maintenance. Regardless of the chemical analysis of the ash particulates, the mere ex-
posure to the respiratory irritation of particulate-laden air within the working space is a health con-
cern issue for operators and a liability concern for employers. Designs should be carefully reviewed
prior to submission to the agencies to assure that all measures have been taken to reduce operator
exposure to ash particulates. If these measures include venting to the outside, air permits would be
required for each vent.
Disposal of Ash. In addition to the particulate emissions from the ash, the ash itself is a
discharge problem that every sludge combustion proposal must address. Although the volume of
sludge ash residual is less than 10 percent of the volume of the dewatered sludge, ultimate disposal
of this solid waste still requires resolution. More often than not, the solution is landfilling, and a
choice must be made between hauling to a commercial landfill or construction of a facility landfill.
Disposal in a commercial landfill has become increasingly difficult to secure. Many commercial
landfills have become sensitized to enforcement for environmental or health impacts. In New Jersey,
for example, solid wastes are commonly subjected to a waste classification procedure that utilizes the
EP Toxicity Test to determine whether the waste is compatible with the licensing limitations of the
landfill. Beyond this classification check, many commercial landfills in New Jersey are now requir-
ing additional toxicity testing for 129 priority pollutants for their own protection. Public opposition
to landfilling has grown as a result of experiences with the uncontrolled dumping practices of the
past; today, construction of new landfills in New Jersey is at a virtual standstill, and capacity in
existing landfills is dwindling rapidly.
Hauling costs to remote landfills must also be taken into consideration. These factors usually
drive consultants to the consideration of onsite disposal of the ash, either in a lined landfill or a
pond. Where landfills are proposed for the project, most air permitting reviewers and consultants
will recognize the need to apply for a permit; however, disposal in onsite drying ponds may not be
recognized as a landfill. A project may be well under way before the need to apply for a landfill
permit is noted. The project is then determined to be technically deficient for lack of the necessary
landfilling submissions, and delays are incurred while information is developed for the landfill per-
mitting agency.
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Scrubber Water. The fourth combustion facility discharge that is frequently overlooked is the
scrubber water. Since most sludge combustion facilities are an integral component of the WWTP,
thermal process flow diagrams routinely designate scrubber water to "return to the head of the
plant"; in other words, out of sight, out of mind. But this is, of course, wishful thinking; the quality
and quantity of the return stream must be evaluated for its impact on the operation of the treatment
plant.
Whereas air quality and stack emissions may permit the incineration of additional volumes of
sludge, treatment plant performance and discharge and surface water quality standards may not per-
mit additional volumes of scrubber water. Such was the case when expansion of the combustion
facility at the treatment plant in Wayne Township, NJ, was sought. It was hoped that expansion of
the Wayne incinerator would be accomplished to process sludge from other treatment plants in the
state that were unable to locate disposal alternatives as a result of closure of sludge landfills; such
was not to be the case because the treatment plant could not hydraulically accommodate the scrubber
water.
Where treatment plant overload is a concern, direct treatment and discharge should be con-
sidered, but scrubber water is difficult to treat because incineration destroys biologically degradable
organics. Successful treatment must then involve physiochemical processes, which are expensive to
construct and operate.
Combustion projects may be stalled or completely blocked because of the costs of disposing of
scrubber water. If this disposal problem is not identified until design and permitting of the air
quality consideration is near completion, it is extremely expensive to return the project to the early
planning stages and begin development of another disposal alternative. Therefore, early planning for
sludge combustion should address disposal of scrubber water as an integral component of the
feasibility studies and before investments have been spent in detailed planning and design.
Summary of Discharge Considerations. During development of sludge combustion pro-
posals, consultants must be meticulous in their attention to solutions for each of the four discharges
from these facilities (stack emissions, particulates from ash, ash, and scrubber water), and agency
reviewers must be sensitive to the regulatory interests of their sister agencies so that deficiencies can
be identified early in the project planning process. It is a tragedy when a combustion project has
completed planning and design stages only to discover that disposal costs for scrubber water or ash
cause this alternative to exceed the costs of other sludge management alternatives such as land
application.
Regulatory submissions should be short, simple, and to the point. Maps, tables, and graphs
should be used wherever possible, and the text should be confined to summaries and conclusions.
The bigger document is not necessarily the better document, and the most impressive submission is
the one that secures all the necessary approvals in the shortest amount of time.
Inadequate Alternative Analyses
A third technical reason for project delay is the inappropriate project. When a combustion pro-
ject is initiated through the air permitting agency, it is of no concern to that agency whether the
combustion disposal mode is an appropriate management alternative for the particular sludge
problem. The application is ordinarily handled solely on the merits of its ability to meet air emission
requirements. It is of no consequence that pelletizing or land application might be more cost-
effective or environmentally sound solutions to the particular problem. Air permitting agencies are
not in the business of sludge management; their business is air quality protection. Therefore, when
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projects are processed through the air permitting program, it is usually the public that raises the
issue of the appropriateness of combustion reduction for the problem. Usually the public becomes in-
volved when the user costs are disclosed, at which point environmental and public interest groups
and neighboring communities are asked for lower cost solutions to sludge problems. Since the cost
of proper sludge disposal is many times more expensive than the old dumping practice that may have
been used, any cost increase may be considered too great and will trigger demands for evaluation of
other alternatives.
Consultants are well advised to perform their alternative analyses with every expectation that the
selected alternative will be questioned. Unfortunately, user costs are not usually disclosed until the
development of the project is nearing completion, and selection of another alternative means aban-
donment of previous work. The unfortunate consultant who has advanced the project may find
himself facing a difficult cost problem if he has not performed a thorough alternative analysis. Will
expended costs plus the costs to reevaluate a less costly alternative outweigh continuing with the ex-
isting proposal in the face of public opposition? This eleventh-hour dilemma can be avoided if the
evaluation is properly conducted in the initial stages of project development.
Applications for sludge combustion projects that are initiated through sludge management pro-
grams or the 201 Federal Wastewater Grant Program should focus on alternative analyses in initial
planning stages. These analyses become the primary concern of both the applicant and the agency
long before publication of the public notice on user costs. Although development and review of the
alternative analyses may be a lengthy process, if properly performed, these functions identify cost
problems before excessive effort has been spent to develop an alternative that cannot be defended
against public opposition.
Presentation of Alternatives
The variables that should be included in the alternative analysis of a sludge management project
are discussed in the following paragraphs. Because the future of landfilling and ocean disposal alter-
natives is questionable at best, the discussion focuses on the comparative concerns between land ap-
plication and combustion of sludge.
Alternative analysis begins with evaluation of the sludge quality and quantity. Generally, treat-
ment plants that receive large volumes of industrial wastes should be concerned about the suitability
of their sludge for land application. Incineration looks like a viable solution for these sludges at the
first cut. Unfortunately, areas with high levels of industrial wastewater discharge are usually the
same ares that are limited for air emissions due to industrial and other discharges to the air. In addi-
tion, these areas are usually remote from available properties for land application. Transportation
costs for land application proposals become a significant consideration for these projects.
The alternative analysis for such sludge problems resolves itself into an analysis of the costs in-
volved in cleaning the sludge through pretreatment versus reduced application rate over an expanded
land area plus the cost of hauling. These costs are then compared to the costs of pretreatment versus
the costs of higher emission controls and management of the ash, ash particulates, and scrubber
water for a combustion disposal mode for the same sludge problem.
Alternative evaluation for sludge projects in rural nonindustrial areas seldom results in selection
of combustion modes because land costs are lower than those in urban areas, suitable land applica-
tion sites are usually available at short hauling distances, and the smaller size treatment plants usual-
ly do not require the technically skilled operators needed to manage combustion facilities. This does
not mean that all urban projects should automatically select combustion and all rural projects should
select land application. In some situations, the time for implementation may be the determining
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factor. Time is, of course, money in the face of escalating costs, and planning, design, and construc-
tion of combustion projects is a lengthy process even under the best conditions. Land application
may prove much more feasible for some projects to implement when time factors are taken into
consideration.
The experience of the Philadelphia sludge management program is a valuable case in point.
Philadelphia had the resources to secure the skilled operators necessary to manage a sludge combus-
tion facility. Sludge quality and quantity and the proximity of suitable disposal sites all weighed
against selection of a land application alternative. Their Northeast treatment plant in particular had a
serious problem with high concentrations of cadmium in the sludge. This may be one of the greatest
problems land application projects must face, yet Philadelphia has one of the most successful land
application programs in the country today—encompassing composting, reclamation of strip-mine
areas, and widescale marketing to the public and a multistate nursery industry. Why did Philadelphia
choose the land application route instead of incineration? They were faced with a 1981 deadline to
cease ocean disposal of sludge, so that the time required for implementation of each alternative
became the critical issue. The cadmium problem was addressed through institution of stringent
pretreatment requirements and strong enforcement of those requirements. The Philadelphia case is
important because it points up the importance of pretreatment in the alternative analysis for sludge
management.
In New Jersey, sludge analyses have indicated a mercury problem in that state's sludge. Mer-
cury is always a cause for concern in incineration projects because emission control technology is
questionable, at best. To compound the problem, the Passaic Valley Sewerage Commission (PVSC)
facility is located in Newark, where ambient air quality already poses a serious problem for the im-
plementation of any combustion proposal. PVSC is presently playing out its appeal options to con-
tinue ocean disposal. Should they lose their arguments for continued ocean disposal, the mercury
issue will demand that they evaluate the alternative of high technology air emission control costs ver-
sus institution and enforcement of a pretreatment program.
These examples touch briefly on many of the variables that merit consideration in an alternative
analysis for sludge management: sludge quality and quantity, hauling costs, costs for management of
the four combustion facility discharges, cost and availability of suitable land, air quality, water quali-
ty, and pretreatment. Failure to accurately evaluate management alternatives will most assuredly
result in the project being stalled by either the public or the regulatory agency.
Decisionmaking Reasons for Project Delays
Delays that relate to the decisionmaking environment are not as easily remedied as delays for
technical reasons. First of all, they often appear to be beyond the control of the project. The pro-
posal may be textbook perfect, but the project may still encounter interminable and frustrating delays
while costs escalate. The problem may be political, one of personalities, or lack of experience. It
may rest with the applicant, the consultant, the agency, or the public.
Political Environment. The political environment is the prime example of the source of delays
that are beyond the control of the project managers. Although decisions on sludge disposal projects
should ideally be technical and financial, they are often controlled by the political climate surround-
ing the project. Changes in political climate are frequently accompanied by directives to reevaluate
the previous decision on the project; the longer a project takes to complete the planning, design, and
implementation stages, the greater is its exposure to this type of change.
Local Elections. The most obvious political reason for project delay is a change in local con-
trol. Where proposals have become the center of controversy they are almost guaranteed to become
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a major issue. When they become political issues rather than technical issues, local support for the
project may be completely reversed following local elections.
There is a fine line between the consultant who recognizes that he is working for the local
government to implement a solution to the government's sludge disposal problem and the consultant
who works for the local government to implement the project the government believes it wants.
Consultants are hired because they possess the regulatory and technical knowledge that the applicants
lack. Consultants and regulatory reviewers work with sludge management projects on a daily basis,
but the vast majority of applicants may see only one sludge project in a lifetime. The consultant who
turns project decisions over to the applicant without educating the applicant to make the decisions
does himself and his applicant a disservice.
When a consultant is hired to solve a particular sludge problem, he should exercise care not to
permit the proposal to become associated with a particular faction. Before beginning work on the
project, he should assist in the education of all interested parties on the technical, cost, and
regulatory considerations and involve them in resolution of issues.
Public and bipartisan support for a sludge management proposal does not necessarily assure
project implementation. There are two other traps at the local level to which a project can fall prey.
One trap is controversy over authority membership. Sludge management projects are frequently
developed on a regional basis. Regional sludge planning is generally performed on behalf of area
facilities by a municipality or sewerage authority acting as a lead applicant or by a loosely formed
planning group comprised of representatives from facilities within the regional planning area.
At completion of the planning process, decisions about membership in the regional authority
may be raised. Project implementation may be stopped dead in its tracks if the lead agency refuses
to extend its voting membership on the authority to include planning participants, insisting that
customer contracts be drawn. Nonmembership planning participants may then withdraw their project
endorsement, fearing they will have no voting control over the costs imposed upon them for use of
the facility if they are relegated to the role of customer. Without regional involvement in the project,
it is impossible for the lead agency to fund the proposed project. A regional project is far too large
for the needs of the lead agency alone, and, in fact, had the project been developed solely for the
use of the lead agency, an entirely different project might have been selected.
Months or years of planning can go down the drain with these standoffs, and there is no easy
solution to this problem. Both applicants and consultants should be aware that this problem can oc-
cur in any regional project that extends beyond member municipalities. It is, therefore, advisable in
these regional situations to resolve the issue before planning begins.
Controversies over facility ownership are identical in concept to the authority membership
problem. The difference is that they occur during development of codisposal projects where the solid
waste management agency* and the sewerage agency are separate. Each agency may assert its right to
facility ownership while the project flounders and costs escalate. Consultants should approach this
problem as they would the previous situation. The issue should be resolved before any time, money,
or effort has been spent on the codisposal alternative. If it cannot be resolved, the sludge project
should go forward independently.
Funding for Sludge Combustion Facilities
Wastewater facilities programs have been affected by significant reductions in funding for the
201 Grant Program. The timing of this funding loss has been particularly difficult for sludge
projects. It was not until the recent years of the 201 Program that the magnitude of the sludge pro-
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blem was identified. The bulk of New Jersey's sludge management projects did not appear on the
state's priority list until 1979-80.
When faced with a major program change such as this, the applicant and the consultant may
look to the agencies for assistance. For example, there are opportunities within the development of
the 201 Priority List Methodology that will permit sludge management projects to realize the greatest
possible advantage under the present funding circumstances. New Jersey's priority methodology, for
example, has placed most of the state's sludge management projects in the innovative and alternative
category where they are awarded the highest number of project priority points.
If a shift in program emphasis is identified as the reason for a project's delay, the applicant (or
consultant) may be tested. Dealing with changes in program emphasis should begin long before the
program changes occur. Some changes have widespread public support; the change in emphasis to
hazardous control programs was illustrative of this political climate. At such times constructive
remedies should be recommended to state and Federal representatives that will make the program
changes less damaging to projects that are being processed, and contingency implementation plans
should be developed.
Contingency implementation plans are a frequently ignored remedy that deserves some discus-
sion here. Most applicants who are caught in the middle of a change in program emphasis feel as if
the rug has been pulled out from under them. If the change in program emphasis merely means that
the project will be delayed, contingency plans should identify an interim disposal option until the
long-term project can be implemented. However, if the change in the program is more damaging
(e.g., a change in funding eligibility or an increase in emission control requirements, both of which
may raise project costs above what the applicant is able to pay), contingency plans must be more
involved.
Development of contingency plans can be approached in several different ways. Facility sizing
can be adjusted to make the project more economical. This adjustment can be either an increase or a
decrease. An increase in size may produce increased emissions with attendant increased emission
controls, but these increased costs may be economical if the combustion facility is able to draw com-
pensating revenues from customers using the facility. A reduction in initial facility sizing may lower
emission control requirements and costs for the facility. A smaller volume 5- or 10-year phased
facility may then be brought online while financing for future years' volumes is developed.
Alternative funding sources should always be sought. This approach is applicable regardless of
the contingency plan. The applicant should investigate private funding for development of the facili-
ty. Placing the full faith and credit of the municipality behind the private developer will lower
private financing costs and create investment interest in the private sector.
Finally, a new sludge management plan may have to be developed. As previously discussed, a
pretreatment program may become a necessary component of the new alternative analysis. The
reduction in emissions it achieves may make the original project financially feasible again, or it may
make a land application alternative feasible as it did for Philadelphia. In most cases, sludge manage-
ment staff in the agencies will give assistance in the development of contingency plans if requested.
However, it is clear that the change in program emphasis may be a difficult obstacle to project
implementation.
Regulations
Air Permitting Program. Projects are dependent on their ability to secure funding and
necessary permits. Permits are primarily regulated through the Clean Air Act, although the Water
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Pollution Control Act controls discharge of scrubber water. Funding may be regulated through the
state or Federal grant programs or by local contracts laws if it is a private funding source.
The two regulatory programs that have had the greatest impact on sludge combustion facilities
have been the 201 Wastewater Facilities Grant Program and the Air Emissions Permitting Program,
which combine to address both the funding and the permitting requirements for combustion projects.
All of the new proposals for sludge combustion projects under consideration in New Jersey today,
for example, were developed through the Federal 201 Grant Program. Both the 201 and the Air
Emissions Programs have been touched upon in earlier discussions. The programs have two different
purposes that are not necessarily coordinated or complementary.
It is the purpose of the 201 Grant Program to provide funding to construct the most en-
vironmentally sound and cost-effective solution that will correct an identified source of water pollu-
tion caused by unacceptable discharges of domestic sewage and residuals. The Air Emissiqns Permit-
ting Program, however, has the purpose of assuring that air emissions will not cause violations of
the air quality standards or hazardous substance emission standards.
Comparing these two purposes, it can be seen that cost evaluation and alternative evaluation are
not a consideration in the air permitting program. It is also apparent that the scope of the air permit-
ting program is much broader than combustion of sludge, and, in fact, it is estimated that sludge
facilities may represent as little as 1-2 percent of the total number of projects being processed
through the New Jersey Air Permits Review Section. Most air permitting activity is on more routine
projects such as vents, storage tanks, and spray facilities.
In New Jersey, sludge combustion projects are grouped together with fossil-fired utility plants
and major refining projects and classed as "Major Projects." These projects have been determined
to have the potential for causing significant impacts to air quality and warrant a higher degree of
scrutiny than other air emission proposals. Higher scrutiny results in slower processing when com-
pared to the vast majority of air emission permits, so applicants must not expect to secure permits
within the same period of time that they obtained a permit for the pressure release valve on the
gasoline storage tank at their municipal garage. Such will not be the case, and the longer review
period should not be considered a delay.
Changes in Ambient Standards. Very few changes have occurred in the Air Permitting
Regulations. The Clean Air Act established the National Ambient Air Quality Standards for various
pollutants. In addition to these standards, EPA set separate emission standards for substances it con-
sidered to be extremely hazardous to public health and welfare. These substances include arsenic,
asbestos, benzene, beryllium, mercury, and vinyl chloride and are regulated under the National
Emission Standards for Hazardous Pollutants. No ambient standards have been set for these
substances, and their controls are related to state-of-the-art and best available control technologies.
Clearly, these substances do not comprise the sum total of potential emissions that would be a
public health concern; there are numerous other candidate substances for inclusion in the emission
standards awaiting promulgation at the Federal level. Under the states' authority to promulgate
regulations that are stricter than the Federal regulations, many states have moved forward with
development of their own hazardous and toxic control programs, including organic and metal emis-
sion standards. New York has added 215 new substances, and New Jersey has added 11 organics.
Consultants and applicants must keep abreast of potential amendments to state and Federal emis-
sion standards that could impact their projects, and they should be prepared to submit additional
technical information quickly if amendments occur midproject.
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Changes in Modeling Guidance. There are two other areas of the air permitting program that
applicants/consultants should watch for changes in the requirements—modeling and measurement. In
1978 EPA issued guidelines for computer modeling programs that were acceptable for use in the
permitting program. Proposed revisions to those guidelines were made in 1981 and opened for public
comment, but they have never been formally revised and reissued. Instead, EPA has issued an inter-
nal document, "Workshops on Air Quality Modeling," which addresses some changes in modeling
techniques that relate to state-of-the-art refinements.
The consultant must keep informed of changes in the modeling guidelines because deviation
from these guidelines is only acceptable where it can be justified to the satisfaction of the agency.
Usually, modeling changes only require amplification of previously submitted project information,
but even amplification can result in delays that would be unnecessary if the consultant remained in-
formed of the impending program changes through close communication with the agencies.
Changes in Measurement Techniques. Measurement techniques that are accepted by the air
permitting program also undergo changes based on improved instrumentation and research. Ap-
plicants and consultants should make sure their emission measurements are performed in accordance
with currently accepted techniques. For example, ambient paniculate measurement may be revised to
measure only those particulates that are less than 10 microns in diameter.
Stack testing of total particulates can change. Obviously, if such a change in particulate
measurement were to be effected during the life of the project, delays could be incurred while alter-
native measurement methods are implemented. Again, the best protection against this type of
regulatory obstacle is advance knowledge of changes through close communication with the agencies.
Coordination of Air Permitting with 201 Grants
The 201 Wastewater Grant Program has undergone many changes since its creation in 1972.
Regulations have been changed, and the Federal Water Pollution Control Act itself has undergone
revision. Although previously discussed changes in program emphasis have seriously affected sludge
projects in the Grant Program by reductions in nationwide funding allocations, most of the
regulatory and statutory changes have not. The primary causes for project delays in the 201 Program
appeared to be a result of the former three-step structure of the grant program, which is poorly
adapted to the air permitting program.
Step 1 of the 201 Program was the planning phase. A planning grant was issued to an applicant
upon approval of a plan of study for Step 1 and other necessary assurances. The second step of the
201 Program was design, but design grants could not be issued until completion of Step 1 (approval
of the selected alternative) and issuance of a permit to construct the selected plan.
Obviously, the 201 Program does not wish to allocate grant monies for designs of facilities that
can never be permitted. The air permitting program cannot issue a permit without a design, so that
applicant is then in a "Catch 22" situation: he should not design without a permit and he cannot
secure a permit without a design. Since enactment of the 1981 amendments to the Clean Water Act,
the problem has worsened. The owner must fund the design work "up front" and hope to be reim-
bursed in a later construction grant.
There are several ways to resolve this problem. One approach is a program for issuance of con-
ceptual air permits. If it is acceptable to both agencies, the project can then proceed to design. New
Jersey, for example, has resolved this problem in a different way. The New Jersey Air Pollution
Control Program does not give conceptual approvals of design and operation, but it has developed a
process whereby approval can be given without final design completion by using key design and
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operating parameters of the selected combustion process and the selected air pollution control
system.
The New Jersey Air Program has also developed an outline plan of study for sludge incineration
projects that has been incorporated into the 201 Plan of Study for the Step 1 phase of the project.
This air plan of study enables the applicant to develop necessary data for air permitting during the
project planning. The air program then requires the applicant/consultant to submit a list of emission
substances in pounds per hour and tons per year before and after control; at the same time, the ap-
plicant must submit results of air quality modeling if applicable (facilities emitting in excess of 50
tons/yr).
The process was not developed until some projects had been trapped by the permit design re-
quirements , but it is to the credit of these two agencies that they were able to develop a mutually
agreeable procedure that would not unduly burden future applicants. Cooperation among review
agencies is the key to resolution of this problem; the applicant is helpless to resolve this issue alone.
Problems with Project Individuals
There are other delays that originate with individuals who are directly involved in managing and
reviewing the project. These problems are difficult to address because they tread close to personal
criticisms that are not discussed in public; but every project has the potential to encounter these
types of delays.
Inexperienced Project Management. Projects can be delayed if project managers are too in-
experienced to manage them efficiently; both consultants and review agencies can be guilty of this.
The importance of adequate training and supervision cannot be over stressed. If there is a change in
management in the middle of a project, the new project manager may unknowingly require that
previously resolved issues be addressed. Such requests are usually followed by an outcry from the
agency or the applicant depending on the source of the request, and a month or two may pass before
the project resumes its forward motion. But management can also be inexperienced because the in-
dividual is uninitiated. In this case regulatory requirements may be overlooked and meaningful defi-
ciencies may not be identified until the project reaches a point at which substantial revision of the
planning documents and designs becomes necessary.
The inexperienced manager or reviewer may also find it difficult to make decisions. It may be
the first project he has reviewed and he is afraid to make a mistake, so that every form of direction
is checked with the supervisor; or it may be the first project he has ever developed and he is afraid
to select an alternative that might not be the right one. Again, close guidance and training may be
the only solution to this problem. Left alone, both experienced or inexperienced managers or
reviewers may unduly delay making a decision. Experienced consulting and review staff can be a
major asset by cooperating in informed decisionmaking.
The Ostrich Applicant. It is not uncommon for consultants and agencies to find that they are
dealing with an applicant who simply does not want to be involved in the project. After the applicant
has hired the consultant, he may want nothing to do with the project. The applicant may refer letters
and calls from the agency to the consultant. The applicant may be too busy to review the alternative
analysis, visit the proposed site, or attend briefing meetings, or participate in problem resolution.
The consultant and the agencies have no power to implement the project, so it is the applicant who
must ultimately finance the proposal. When it is time for the checks to be written, the project may
be in a precarious position if the applicant has kept himself in the dark during development of the
proposal.
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Keeping the ostrich applicant educated and informed may be one of the hardest problems any
consultant may face. One recommended method is for the consultant to hold a project startup
meeting with the applicant in which he can present examples of sludge projects that have been im-
plemented with their costs. Examples should be representative of similar-sized communities.
If the applicant cancels the meeting, it should be rescheduled. If the applicant suggests that the
consultant proceed without him, the consultant should respectfully decline. No project should begin
unless the consultant has prepared his applicant for the range of costs that could be anticipated. The
meeting discussions should be recapitulated in a letter to the applicant. Similarly, when hard issues
must be addressed during the course of project development, the consultant should insist on getting
the applicant's opinion on important issues. Invariably, if the applicant is allowed to avoid involve-
ment, he will question the consultant on the very same issues when the project is nearing com-
pletion, and it is costly to rework the proposal.
If the consultant continues to have difficulty getting his applicant's full attention, it is sometimes
helpful to request that the regulatory agency schedule the meetings. Most agency staff are very
willing to meet with applicants to discuss the project. In short, it is the responsibility of both the
consultant and the agencies to keep the applicant informed and involved in the development of his
project because it is his project, and it is he who must pay for it.
Personality Conflicts. Also included with obstacles that originate with individuals are those
uncommon delays that are rooted in personality conflicts. On unpleasant and rare occasions, a total
breakdown will occur between or among individuals representing applicant/consultant and the
regulatory agency; or one or more of these individuals may be a cause of public animosity. It is im-
perative that all individuals involved in the project treat each other with respect as professionals.
This begins with the recognition that each has a very important function to fulfill that may not
necessarily conform to the goals and functions of the other individuals but is necessary to the success
of the sludge project.
Public Obstacles
The public can also be a source of project delays. Project managers are often up against a public
that opposes tax increases and refuses to allow any type of facility to be constructed in its
"backyard." This opposition may require additional alternative analyses or environmental impacts,
and the project is delayed for months in an effort to satisfy the public's concerns.
Although project managers (agency and consulting) are the experts in solving sludge problems,
and the public is the learner, the public "pays the tab"; therefore its concerns should never be sum-
marily dismissed. The public should be educated to understand that sludge disposal costs cannot be
expected to remain at the levels of the "good old days" when dumping was uncontrolled and haul-
ing costs were cheap.
Summary
After reviewing the long list of potential obstacles to project implementation, the technical
obstacles may seem small by comparison. This is an extremely important realization for every appli-
cant/consultant because the less exposure a project has to these nontechnical obstacles, the greater
are its chances for implementation. A well-prepared technical submission will speed the project's
earliest implementation.
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CHAPTER V. EMERGING TECHNOLOGY
In this chapter, processes for the thermal conversion of municipal sludge are considered in four
groups:
1. Gasification,
2. Liquefaction,
3. Wet oxidation, and
4. Combustion.
Each group is divided into two subgroups: (1) sludge used as the only feed material and (2)
sludge processed with other carbonaceous wastes.
The processes for the thermal conversion of municipal sludge that have been supported by the
USEPA's Municipal Environmental Research Laboratory (MERL) in Cincinnati, OH, are shown
below.
Type of Process Sludge Alone Sludge Plus Other Wastes
Combustion ICFAR*: low-air, low- Coincineration in FBF
burn zone in MHF (Duluth, MN)
(Indianapolis, IN; Coincineration in MHF
Nashville, TN; (Eagan, MN)
Hartford, CT)
Wet Oxidation Vertical Tube Reactor
(Longmont, CO)
Gasification Simplex-S - lab scale
UC/Davis - pilot scale
Purox - prototype
Andco-Torrax - prototype
Wright-Malta - lab scale
Liquefaction Battelle-Northwest
Worcester Polytechnic
Institute
* Indianapolis Center for Advanced Research
GASIFICATION
The use of coal and wood to produce energy (heat or power) via pyrolysis-gasification processes
has been practiced since the 18th century. The more widely used early gasification processes were
the "blue-water gas" and the "producer gas" processes. These processes produced low to medium
V-l
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energy gas, which was used as fuel for gas engines, steam boilers, ceramic kilns, and metallurgical
furnaces. In Sweden, during World War II, gas generators were used to provide fuel for vehicles,
tractors, and boats. With the availability of cheaper fossil fuels (natural gas and petroleum) after the
end of World War II, commercial use of coal and biomass gasification processes ceased. This was
because communities were well supplied with natural gas or could "crack" propane (or other
hydrocarbon gases) into a suitable utility gas at a lower cost than for gasification processes. Intensive
research into pyrolysis-gasification processes in recent years has provided insights into the process
fundamentals and resulted in development of new biomass gasification processes. The new genera-
tion of gasification technology is directed mainly toward production of medium to high energy gas or
production of synthesis gas. To date, only small-scale, low energy gas biomass gasifiers have been
commercialized worldwide. Some of the new generation technologies such as Purox and Andco-
Torrax have been built in Europe, Japan, and the United States, but none has had successful opera-
tions. The major biomass gasification technologies are listed in Table V-l. A detailed discussion of
each is beyond the scope of this document.
With the exception of the cogasification work, only two processes have been tested on sludge:
Standard Solid Fuel and DUVANT. The results of those tests have not been published, however, so
this section is presented solely on the basis of theoretical considerations and by extrapolation from
work with biomass.
Application to Sludge
One means of gaining insights into the applicability of gasification technologies to sludge is to
consider the differences between municipal sludge and the biomass feed material.
Moisture Content. The moisture content of the biomass feed ranges from dry to as high as 70
percent with a median around 20-25 percent moisture. Approximately 75 percent of the gasifiers are
operated or tested on feed materials with moisture concentrations between 10 and 40 percent. The
moisture content of dewatered municipal sludge usually ranges between 60 and 85 percent. However,
with some types of conditioning and dewatering, moisture levels as low as 50 percent may be
achieved. To achieve moisture levels below 50 percent, some type of drying needs to be employed.
Conventional dryers could easily reduce the moisture to required levels. However, there would be
considerable energy and monetary costs. For instance, it would require approximately 9-16 MJ/kg
(4,000-6,800 Btu/lb) of dry solids to lower the moisture content of sludge from 75 to 20 percent.
Another approach to reducing moisture levels would be a dryer that is integrated into the
technology. For instance,the DUVANT process produces low energy gas that is used to generate
electric power using an engine-driven generator. The heat in the engine exhaust is used to dry the
feed material. This approach may prove advantageous to sludge gasification. However, given the
energy conversion efficiencies of gasifiers and engines and the energy content of sludges, there
would be a limit to the amount of moisture that could be removed.
A third alternative is to develop gasifiers that could operate with high moisture level feed
material. This is not an unreasonable course to pursue. The key considerations would be the impact
of the latent heat of vaporization on operating temperature, the extent to which the moisture could be
used as the agent in the steam gasification, agglomeration problems in the gasifier, and the impact of
higher moisture levels on the product's gas quality. Institute of Gas Technology (IGT) researchers
are working on the gasification of peat with moisture levels as high as 50 percent without predrying.
The Standard Solid Fuels technology has been operated on a 70 percent moisture feed material.
Thus, indications are that such an approach is not unreasonable. The last alternative is to dilute the
sludge moisture levels with drier material (i.e., cogasification). The PUROX researchers have sug-
gested that a municipal solid waste and sludge mixture with sludge moisture as high as 75 percent
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could be successfully gasified, but they noted that dried sludge would provide optimum operation
and the least capital investment because of the reduced water vapor volumes.
Feed Material Size. Gasifiers are operated on uniform feed material; otherwise, a high velocity
burning channel would form in the reactor and reduce its efficiency. Shredders are often employed
to produce a uniform size. In addition, several technologies require feed material in a certain size
range. Municipal sewage sludge is not uniform. Grinders can be employed to produce a small,
uniform, particle-sized material. However, in fixed-bed gasifiers, where bed void fraction is essential
to operation, such small-sized material may cause problems. Sludge may be pelletized or briquetted;
however, that requires external drying to low moisture levels or mixing with a dry material. The key
considerations here are mass and heat transfer resistance and bed movement in the gasifier. Fluid-
bed gasifiers and moving-bed gasifiers may prove to be most adaptable to sludge. Information is re-
quired on the characteristics of these types of gasifiers operated with sludge. Successful test runs
have been made using sawdust and manure; thus it appears that any problems due to size can be
surmounted.
Sludge contains contaminants such as heavy metals, but it appears that these may not be a pro-
blem. The cogasification work suggests that they are retained to a large extent in the ash. Certainly
problems of gasification ash disposal and heavy metals concentrations in exhaust gases are similar to
those for sludge incineration. While sludge ash content is high, it does not overwhelm the other con-
stituents. Certainly any sludge gasification technology will have to accommodate a higher ash pro-
duction rate, but it does not appear to be an insurmountable barrier.
The applicability of gasification technologies can be summarized by grouping existing
technologies as follows:
1. Moving-bed gasifiers - These gasifiers can handle moisture levels up to 70 percent. They
represent the best chance for high moisture sludges.
2. Fluidized-bed gasifiers - These gasifiers will be able to overcome the small-sized limitations.
In addition, these gasifiers have operated with moistures up to 50 percent. With extensive
dewatering and/or employing exhaust gas heat for drying, they may prove successful for
sludge gasification.
3. Fixed-bed gasifiers - These gasifiers have rather strict feed material requirements. Their ap-
plicability will most probably be limited to cogasification of briquets or pellets.
Development Potential
The similarity between municipal sludge and biomass leaves little doubt that it can be gasified.
The big question is how much processing it must undergo. Its highest heating value (HHV) is slight-
ly, but not significantly, less than wood. It does not have any other constituents that will prevent
gasification. In general, sludge must be dewatered to approximately 30 percent solids before it will
combust autogenously. How much must it be dewatered in order for it to be gasified, and what will
be the impact of dewatering on process economics and the process energy balance? Some gasifica-
tion technologies have specific feed material size, shape, and density requirements. Again, these re-
quirements do not represent unsurmountable barriers. Sludge can be ground, pelletized, briquetted,
or otherwise treated to satisfy feed material requirements. The question again is one of impact upon
economics and energy balance. Cogasification technology is an example of a solution to these pro-
blems. It mixes wet sludge with drier municipal solid waste and, in some technologies, pelletizes it
to meet feed material moisture,size, shape, and density requirements.
V-4
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Another factor bearing on the development potential of sludge gasification is its place in the
wastewater treatment plant and a use for its product. Approximately 90 percent of all wastewater
treatment plants are less than 44 l/s(l mgd) in capacity (i.e., they generate less than 1 Mg (2,200 Ib)
of dry solids per day). Many are not located near industrial areas that can use synthesis gas. In
many small plants, sludge wasting is done on an intermittent basis. All these conditions represent
constraints to specific processes but not necessarily to gasification in general. It is apparent that
smaller units that can be operated intermittently may find the most application. In addition, units that
can be coupled with a generator to produce electric power will find greater application, since the
need for fuel gas and/or steam is limited in most wastewater treatment plants and the big energy
users are powered by electricity. The use of gasification technologies that produce medium energy
gas (MEG) may be limited to larger plants, especially plants where 90 percent oxygen would be
available and where the unit costs of cleaning, storing, and selling the gas may be more favorable.
Simplex-S Process. The Simplex process developed at Columbia University converts cellulosic
waste to clean, medium Btu fuel gas through cogasification with coal. The principal innovation of
Simplex is the briquetting step, where coal and cellulosic waste such as municipal solid waste or
forest pulp are pressed into briquets. When these briquets are gasified in a moving-bed gasifier, the
waste fibers act as wicks, absorbing the tars that cause swelling and agglomeration of caking coal.
Because the briquets retain their size and shape throughout the gasifier, the flow of briquets through
the gasifier zones is smooth and stable.
Simplex was originally developed for gasification of eastern bituminous caking coal and refuse-
derived fuel (RDF). Municipalities, however, generate both MSW and sewage sludge, and it is
natural to dispose of MSW and sludge together. Codisposal has been applied to several waste-
disposal technologies such as incineration, pyrolysis, and composting. Codisposal through the
Simplex method, which is called Simplex-S, has several advantages over these conventional
codisposal processes. The destruction of heavy organic wastes in MSW and the safe disposal of
heavy metals contained in sludge are accomplished at a relatively low cost. The nongasifiable com-
ponents of Simplex-S briquets end up embedded in a glassy, nonleachable frit. Thus, they can be
disposed of safely or put to use as road-building aggregates.
GASIFICATION OF DENSIFIED SLUDGE AND WASTEPAPER IN A
DOWNDRAFT PACKED-BED GASIFIER
The codisposal of densified sludge and wastepaper in a cocurrent flow packed-bed gasifier
represents a new application of the thermal gasification process. Advantages of this technology in-
clude lower costs than other incineration or pyrolysis technologies, simple construction and opera-
tion, and the ability to use a variety of fuels including agricultural wastes and other biomass
materials in addition to densified sludge and wastepaper.
Gasification involves the partial combustion of a carbonaceous fuel to generate a combustible gas
(producer gas) containing carbon monoxide, hydrogen, and some hydrocarbon gases and a char rich
in carbon. A process flow diagram for a complete sludge and wastepaper gasification system is
shown in Figure V-l. The key elements of the system are fuel processing, gasification, and gas
utilization.
To gasify sludge and wastepaper in a downdraft gasifier, fuel processing is required. A suitable
gasifier can be made from source-separated wastepaper, sludge, and woodchips by shredding, mix-
ing, and densifying the fuel components. A densification system operated by the Papakube Corpora-
tion of San Diego, CA, was utilized to produce sludge/wastepaper fuels. The Papakube densification
system includes an integral shredder, a metering system that allows moistening of the fuel to the op-
V-5
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Source
Separated _*,,
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Sludge
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_». Electricity
Figure V-1. Flow diagram for a complete sludge/wastepaper gasification system.
timum moisture content, and a modified agricultural feed cuber. The producer gas can be used to
power an internal combustion engine to generate power or it can be used as fuel for a boiler.
Downdraft Gasifier
Developed originally to reduce the quantities of tar in the producer gas, a downdraft gasifier
typically is composed of three subassemblies: (1) a fuel hopper, (2) a firebox, and (3) a char pit.
Fuel flow in downdraft gasifiers is by gravity with air and fuel moving cocurrent through the re-
actor. The University of California at Davis, Department of Civil Engineering pilot-scale, batch-fed
downdraft gasifier is shown in Figure V-2. The fuel hopper is constructed as a double-wall cylinder.
The double wall acts as a condenser to remove the water vapor from the fuel prior to gasification.
The inner cylinder is in the form of a truncated cone to reduce the tendency for fuel bridging. The
firebox is also a double-wall cylinder. The inner cylinder is the actual firebox. Air is supplied by
four tubes to the annular space between the walls that serves as an air plenum to distribute air
evenly to the six tuyeres (air nozzles) that supply air for partial combustion of the fuel. The choke
plate is essentially a large orifice that serves as a constriction in the gasifier firebox and is used to
concentrate both the fuel and gas, creating the very high temperatures necessary to thermally crack
tars. A rotating eccentric grate is located in the char pit immediately below the choke plate. The
grate supports the fuel bed and allows passage of char and gas into the char pit. Producer gas is
drawn off continuously through a pipe manifold on the side of the gasifier. A continuous screw
auger is used to convey char from the char pit to a large char storage container. The design of the
grate is specific to the fuel and operating characteristics of the gasifier.
Operating Parameters
Three important operating parameters directly affect the gasification process: (1) fuel ash con-
tent, (2) air input/gasification rate, and (3) internal gasifier temperatures.
Fuel Ash Content. In addition to lowering the energy content of the fuel, fuel ash, upon
reaching its melting point in the gasifier and then cooling, forms slag. Excessive slag formation in a
downdraft gasifier can block the flow of fuel and char through the gasifier and thus halt the gasifica-
tion process. The tendency for slag formation is a function of the reaction zone temperature, the
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composition of fuel ash, and the percentage of fuel ash. To minimize sludge disposal costs, it would
be ideal to gasify only sludge in the downdraft gasifier. But because there is sufficient ash in the
sludge (between 25 and 40 percent) to cause disruptive slag formations in downdraft gasifiers, slag
formation in the gasification of sludge can be inhibited by lowering the reactor temperature and/or
mixing additives with the sludge to lower the melting point.
Large reductions in the reactor temperature in a downdraft gasifier to inhibit slag formation are
not feasible for two reasons: (1) the energy content and quality (with respect to tar vapor content) of
the producer gas varies directly with the reactor temperature, and higher reactor temperatures pro-
duce a better and cleaner gas; and (2) hot spots are always present around the air inlet nozzles of
downdraft gasifiers, causing slag to form below the tuyeres.
A change in the composition of the ash in the sludge can affect the melting point of the ash.
Based on an elemental analysis of a sludge/wastepaper fuel, it was found that the phase diagram
shown in Figure V-3 can be used to estimate the melting point of sludge/wastepaper fuel. The solid
SiO2
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Figure V-3. Phase diagram for a CaO-Fe203-SiO2 system.
V-8
Fe2O3
-------
triangle in the diagram is an estimate of the sludge/wastepaper ash melting point, about 1,250°C
(2,252°F). If lime (CaO) is added to the mixture, it can be seen that the fuel ash melting point will
be raised. (The addition of lime would move the solid triangle towards the CaO angle, located at the
lower left-hand corner.)
One investigator has proposed the idea of controlling the slagging potential of sludge by adding
source-separated wastepaper (a low ash fuel) to dewatered wastewater sludge. However, in municipal
sludge/wastepaper gasification systems, if all the wastepaper generated in a community were to be
collected and mixed with all the wastewater sludge generated, the resultant fuel ash content would be
about 8.2 percent dry basis (which has been shown to cause severe slagging in downdraft gasifiers).
Therefore, to derive usable gasifier fuel from a mixture of wastewater sludge and source-separated
wastepaper in a community gasification system, wood chips can be added to the mixture. Because
the ash content of woodchips is low (0.1-3.0 percent dry basis), a sludge/wastepaper/wood chip fuel
mixture can be gasified without disruptive slag formations.
Air Input and Gasification Rate. The air input rate is the most easily controlled operating
parameter in the gasification process. In the absence of changes in the fuel composition and gasifier
dimensions, the air input rate directly affects the gasification rate and the temperature in the gasifier
zone and indirectly affects the producer gas composition.
The specific gasification rate, defined as the gas output rate expressed in mass or volume terms
divided by a characteristic gasifier area, is an operational parameter used to compare gasifiers of dif-
ferent sizes. It is advantageous to operate the gasifier at a high specific gasification rate. The most
obvious advantage is that the cost of the gasifier is minimized. Also, a gasifier operated at a high
specific gasification rate will maintain a high reactor temperature, which is necessary to maintain a
producer gas of good quality.
Internal Gasifier Temperatures. The temperature reached in the reaction zone of the gasifier
greatly influences all aspects of the gasifier performance. High internal gasifier temperatures affect
the process in several ways: (1) tars, higher hydrocarbons, and other products of fuel distillation are
thermally cracked to noncondensable hydrocarbons in the hearth zone of the gasifier (see Figure
V-2(b)); (2) the chemical equilibrium for the formation of CO and H2 is favored by high
temperatures; and (3) the tendency for the formation of slag is affected greatly by the internal
gasifier temperature.
Development Status
To investigate the gasification of densified sludge and wastepaper, a pilot-scale gasification
system was designed and constructed. The operating system consists of three component parts: (1)
the batch-fed downdraft gasifier, (2) the data acquisition hardware, and (3) the producer gas burner.
To demonstrate the gasification process and evaluate air pollution emissions, a broad range of
fuels has been tested with the gasifier. Fuels tested include agricultural residue, densified
wastepaper, and densified wastepaper and sludge mixtures containing up to 25 percent sludge by
weight. The sludge fuels were made from mixtures of lagoon-dried primary and secondary sludge
and from recycled newsprint (in full-scale systems, a mixed paper fraction of solid waste could be
used). Mixtures were densified using commercially available equipment.
Preparation of Densified Fuel. It has been possible to develop a densified fuel from source-
separated wastepaper and treatment plant sludge using the Papakube densification system. Bulk den-
sities of the sludge/wastepaper fuels range from 284-595 kg/m3 (17.7 to 37.1 lb/ft3). The highest
V-9
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fuel bulk densities are associated with the largest fraction of sludge, which may indicate that the
sludge acts as a binder for the wastepaper during the densification process. The physical integrity of
the sludge/wastepaper fuel cubes is dependent on the moisture and sludge contents of the mixture. It
was found that sludge/wastepaper cubes of the highest physical integrity are made when the moisture
content of the mixture is about 20 percent (wet basis). The physical integrity of the sludge/waste-
paper cubes also depends directly on the sludge content of the sludge/wastepaper mixture (over the
range of mixtures tested).
Maximum Fuel Ash Content. To date, a fuel with an ash content of 4.85 percent is the
highest tested without significant slag formation. Severe slagging occurred in the gasifier with a fuel
having an ash content of 8.5 percent. Although the addition of lime to the fuel may eliminate some
slagging in the high ash fuels, a more conservative fuel ash content of 5 percent (dry basis) can be
used as a design number until more experience with full-scale sludge/wastepaper gasification systems
is obtained.
Specific Gasification Rate. The highest specific gasification rate obtained with
sludge/wastepaper fuels was 9,500 m3 of producer gas (0°C, 1 atm) per hour per square meter of
choke plate opening area (9,500 m3/hr/m2). This rate is close to the maximum value of 10,000
m3/hr/m2 reported by other investigators for small downdraft gasifiers fueled with wood.
Gasification Process Efficiencies. Temperature, pressure, and process rate data were taken
during each experimental gasifier run. These data and the results of gas, fuel, and char analyses
were used to compute energy balances and process efficiencies. It was found that the lower heating
value of the producer gas generated from the gasification of sludge/wastepaper fuel varied between
5.12 and 5.76 MJ/m3. Comparable values for sludge digester gas and natural gas are about 22 and
36 MJ/m3, respectively.
Air Pollution Emissions. Air pollution emission tests were conducted on combusted producer
gas over a series of four gasifier runs. Particle emissions were determined using modified EPA
Reference Method 5; SO2 emission concentration was determined with a modified EPA Reference
Method 6. Concentrations of NOx and noncondensable hydrocarbons were measured using gas
analyzers. The results of these tests can be summarized as follows: (1) Federal particle emissions
standards for incinerators were met without the use of flue gas cleanup equipment; (2) concentrations
of NOX varied between 60 and 115 ppm; (3) noncondensable hydrocarbon concentrations, based on
hexane, were usually below 1 ppm; and (4) concentrations of SO2 ranged from 0.037 to 0.098 g per
dry cubic meter.
Full-Scale Gasifier Systems
There are no full-scale gasifier systems currently operating with sludge/wastepaper fuels. To ac-
quire a full-scale gasifier system that can be used in a small community, commercially available
gasifier systems originally designed to operate on wood fuels may be purchased and modified, or a
gasification system specially designed for sludge/wastepaper gasification may be designed and
manufactured.
Gasifier Systems for Small Communities
Important criteria for a small community gasification system are low capital costs and simplicity
of operation and maintenance. A gasifier system that is designed to operate only a fraction of the
day can be utilized to sell peak or partial peak power to the local power company. Highly automated
gasifier systems are not needed in small communities because of the availability of relatively cheap,
unskilled labor.
V-10
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If a gasifier/engine/generator system is used to generate electricity from sludge/wastepaper fuels,
it is recommended that: (1) the gasifier/ engine/generator system be located at the local wastewater
treatment plant, (2) a downdraft gasifier be used because of its ability to generate a producer gas
low in tar vapor, and (3) the gasification system be operated in a batch mode rather than in a con-
tinuous mode.
LIQUEFACTION
Liquefaction is the thermochemical treatment of hydrocarbonaceous materials to produce a syn-
thetic oil. During the past decade, rapidly escalating costs and uncertainty of foreign petroleum
resources have stimulated the development of liquefaction technology. Actually, the production of
synthetic fuels from organic matter and other solid fuels is not new. Germany used a liquefaction
technology during World War II. Nevertheless, most of the fundamental information on potentially
competitive technologies in today's economy was developed in the past decade through quite exten-
sive research activities, both in the private sector and under the auspices of the U.S. Department of
Energy (DOE). Table V-2 summarizes the major liquefaction technologies available today.
Only one laboratory study has been done on the liquefaction of sewage sludge; thus, much of
this analysis is based on comparisons with biomass liquefaction. While there is a fairly large amount
of information on coal liquefaction, the differences between coal and sludge are much greater than
the differences among sludge and the other biomasses, especially concerning the most important
areas affecting product quality. Direct and indirect liquefaction are significantly different processes
with respect to assessing applicability and development potential.
Application to Sludge
Direct Liquefaction. Three classes of direct biomass liquefaction technologies have been
studied: pyrolysis, solvent-catalyst using oil, and solvent-catalyst using water. The pyrolysis
technologies require a dry, finely ground feed. The solvent-catalyst technology, which uses oil as the
solvent, also requires a dry, finely ground feed. Sludge would have to be extensively dewatered and
ground to meet the feed material requirements of these technologies. The solvent-catalyst
technologies that use water as the solvent would probably require the sludge to be ground but not
dewatered beyond 20-30 percent solids.
With respect to product oil quality, it has been found that oxygen content and viscosity can be
dramatically lowered by operating at higher temperatures. Studies on the two technologies have been
conducted with operating temperatures between 280° and 360 °C (550° and 680 °F) producing oil
with oxygen contents between 7 and 11 percent. By increasing reactor temperature from 330°C
(630°F) to 425 °C (800°F), product oil oxygen concentration was reduced from an average of 8.2 to
1.7 percent. Thus, product oil quality may be improved by higher reaction temperatures.
There is little doubt that sludge could be used to produce low to medium quality fuel oils using
a direct liquefaction technology. However, there are many questions yet to be answered concerning
economics and energy balance.
Indirect Liquefaction. There are two coal technologies and one biomass technology described
herein as indirect liquefaction technologies. The common element is the use of steam gasification to
produce either synthesis gas or a gas rich in olefins. The biomass liquefaction technology requires a
dry, finely ground feed material. There are a number of gasification technologies that employ steam
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V-13
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gasification and that, with proper adjustments, could produce synthesis gas. The feed moisture re-
quirements of these technologies range from 10 to 45 percent moisture. The steam requirements
range from 0.3 to 1.0 kg/kg dry feed. Thus, for a technology like SFGM, as much as 1.4 kg of
water is used per kg dry feed. This theoretically represents a feed moisture of 60 percent. Again,
there is little doubt that sludge could be used to produce a fuel oil via indirect liquefaction. It would
have to be dewatered somewhere beyond 40 percent solids and ground.
Development Potential
The development potential of sewage sludge as a fuel oil depends on the quality of the fuel oil,
whether or not its production energy balance is positive, economics, and the type of market that
develops for the oil. Work is currently being conducted to determine maximum yield, quantities, and
economics of a continuous direct liquefaction process.
Energy Balance. The primary concern is that the production of the fuel oil should not be a net
energy user. The primary factor affecting the energy balance for sludge liquefaction is the moisture
content of the sludge. There are two direct liquefaction technologies that show promise for avoiding
large energy expenditures for dewatering and drying: synthetic asphalt and indirect biomass liquefac-
tion (IBL). However, for the IBL technologies and the other direct biomass liquefaction technologies
that require a dry feed, a means of drying the sludge needs to be developed that does not have to
pay the energy cost of the latent heat of vaporization of water. For instance, it requires 10-19 MJ/kg
(4,400-8,000 Btu/lb) to dry sludge from 75 to 10 percent moisture using a conventional drying
technology, which is basically the heating value of the sludge. Thus, a negative energy balance is
essentially assured. It appears that the development potential of liquefaction technologies may be
limited from an energy balance standpoint to the two direct liquefaction technologies that do not re-
quire any dewatering. One of these appears to have a positive energy balance.
Economics. The economics of direct biomass liquefaction indicate that this may be the first
economical process where a return on investment for sludge disposal is realized. If a credit equal to
other disposal methods is given the process, it is estimated that producing oil can pay off the capital
expenditure in less than the usual commercial 3-year payback period.
Summary. It appears that the synthetic oil technology has development potential. In addition, it
may prove to be the first process to have a positive return on investment for sludge disposal. The
idea of a return on investment or of the value of sludge as a new resource is new and may be dif-
ficult to accept; however, even if the economics do prove positive, this process may be limited to
plants that now have a large enough sludge capacity, 100 tons a day or more, to find incineration
economical. However, as the process is developed a much smaller plant may be acceptable because
of improvements in the process.
CONVERTING SLUDGE SOLIDS TO FUEL OIL - THE BATTELLE-NORTHWEST PROCESS
The conversion of primary dewatered sewage sludge to fuel oil and an asphalt substitute is
described in the following sections. This conversion utilizes a basic catalyst at 250° - 350 °C (480° -
660°F) under pressure. Energy yields of up to 70 percent of theoretical were obtained as an oil with
a heating value up to 90 percent that of diesel fuel; some samples were fractionally vacuum distilled
and the bottoms used in synthetic asphalt mixes. Preliminary process economics for a 100-ton/day
plant suggest payback in less than 6 years using conservative assumptions. This work was based on
batch processing, but continuous processing of sludge is now being studied, together with a more
detailed assessment of fuel product value and the economics of operation.
V-14
-------
Process Description
The sludge liquefaction process developed by Battelle-Northwest (B-N) is depicted in simplified
form in Figure V-4. This chemical process involves alkaline digestion at high temperature and
pressure and is one of a family of similar processes that has been used to liquefy "biomass."
For the past 10 years, B-N has investigated the fundamental chemistry of biomass liquefaction.
This led to the knowledge that the liquefaction process will work in the absence of expensive gas ad-
ditions, catalysts, and organic solvents. B-N obtained a grant from EPA to study the liquefaction of
sewage sludge, cofunded by the city and county of Honolulu. The liquefaction was successfully per-
formed in batch mode (autoclaves). Preliminary economics of the process appeared favorable. B-N
is now progressing to a continuous mode and will further characterize the product from sludge as
a fuel.
The basic liquefaction process has features in common with wet pressure oxidation and with
pyrolysis—the application of heat and pressure to sludge. It differs considerably in its chemistry. As
a result, it does not yield large amounts of carbon dioxide, as does pressure oxidation, and it does
not produce coke-like char, as does pyrolysis. The oil product will also remain stable at room
temperature for more than 2 years.
The essential simplicity of the process for sludge conversion can be seen in Figure V-4. The
sludge should be dewatered to about 30 percent solid or less, as is already practiced by many
municipalities in the United States and elsewhere. The conversion works at any solids concentration
up to about 90 percent, but at low solids value the process becomes energy consuming because of
the amount of water to be heated. The dewatered sludge is mixed with less than 5 percent by weight
of alkali (sodium carbonate, or possibly lime or cement kiln dust). Some lime may have already
been added as a dewatering aid, minimizing the requirement for additional alkali. The mixture is
passed into the reactor by means of a specialized pumping system capable of handling 30 percent
solids in a water slurry. In the reactor, where the sludge is heated to about 320°C for 1-2 hours, it
is converted to oil, char, gas, and watersoluble organics. Characteristically, the total yield of oil and
char (ash-free) is between 60 and 70 percent of the original weight of organic material in the
feedstock, with between 0 and 10 percent of the organics in the sludge going to carbon dioxide and
water-soluble organics. On a carbon basis, this translates to combined yields of carbon in the oil and
char that approach theoretical.
The B-N conceptual design for a 100-ton/day synthetic asphalt plant, based on work performed
for EPA and using primary Honolulu sludge, considered using a steam-distillation of light oil as a
way of separating light, volatile oil from the products and also reducing the reactor pressure prior to
product recovery. Using this concept, the depressurized product, after removal of the light oil,
would be transferred to a holding tank, where gravity separation of the heavy oil/char fraction would
occur, leaving two layers of oil/char and water. Actually, the product separates out into wastewater
and into an oil/char fraction that still contains about 15 percent water. Depending on the efficiency
of separation under actual operating conditions, the supplemental use of a centrifuge could be
necessary to ensure more complete separation of the oil and water fractions. Prior experience with
the wastewater is that it can be treated by conventional processes in an environmentally acceptable
manner.
The proportions of light oil, heavy oil, char, and ash may vary according to reaction conditions.
In particular, the ash portion of the sludge carries through partly in the char and partly as dissolved
inorganics in the wastewater. Heavy oil, as defined here, is soluble in acetone, while the char is not.
In laboratory experiments, the equivalent fractions are defined as (1) light oil - vacuum distillable
V-15
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below 300 °C and (2) heavy oil - not so distillable, with both (1) and (2) being subtractions of the
acetone extract of the total oil products. The "char" is the acetone-insoluble fraction of the total
product organics. It should also be borne in mind that laboratory experiments were performed in
batch conditions in autoclaves, while the conceptual design is for a continuous process—the only one
that is likely to be economical. Extrapolating from batch to continuous conditions is always uncer-
tain, so the actual proportions of light oil, heavy oil, and char obtainable in a continuous plant may
vary from laboratory-scale batch results.
Other products from the liquefaction process include an off-gas (which is over 98 percent carbon
dioxide with only traces of methane, carbon monoxide, hydrogen sulfide, and other undesirables)
and the ash (which is distributed between the char and wastewater fractions). The char itself is con-
siderably different from pyrolysis char. It is a fine particulate material suspended in the heavy oil
and can be removed by solvent extraction. Alternatively, it can be burned with the oil.
For the work for EPA on synthetic asphalt production, payback options were calculated for each
of these alternatives. Addition of a solvent extraction and solvent recovery step to separate out the
char approximately doubles the payback period, but this is necessary to make synthetic asphalt. On
the other hand, for fuel production, without separation of the char, the economics appear much bet-
ter. It appears that this process could be the first sludge disposal process to yield a return on the
capital investment.
B-N performed a series of batch autoclave reactions using sludge and either sodium carbonate or
lime, under various temperature and pressure conditions. The product was compared with the sludge
used as a starting material and was used to determine heats of combustion and to make a series of
synthetic asphalt samples for testing at the University of Idaho; three of the samples tested out as
satisfactory or superior in some respects to the petroleum asphalt test sample.
CONVERTING SLUDGE TO OIL BY HYDROLIQUEFACTION
This study was undertaken to assess the feasibility of converting municipal wastewater sludge to
liquid and gaseous fuels through reactions with hydrogen at high pressure and temperature. The
work was based on earlier experiments carried out in the Resource Recovery Laboratory at the
Worcester Polytechnic Institute.
These studies involved the hydroliquefaction and hydrogasification over nickel catalysts of
cellulosic substances slurried in paraffin oil at temperatures of 350° - 450 °C (660° - 840 °F) under
hydrogen pressure in the range of 3-8 MPa. Under such conditions, up to 90 percent of cellulosic
substances can be converted to gaseous and liquid fuels. The background, techniques, and ex-
perimental equipment associated with the cellulose and lignite liquefaction studies have been applied
in the present investigation to the hydroliquefaction and gasification of sewage sludge.
Experimental Procedures
For this study, raw and digested sludges were collected from the Deer Island Sewage Treatment
Plant in Massachusetts. These samples were used in experiments either as aqueous suspensions as
received or as suspensions of dry sludge solids slurried in anthracene or paraffin oil.
Experiments were carried out in an autoclave under hydrogen pressure. The principal apparatus
consisted of a magnetically stirred batch autoclave. The maximum safe pressure that could be applied
to this autoclave was 14 MPa. Auxiliary equipment included a hydrogen-feed system, a slurry-feed
device, pressure and temperature recorder-controllers, a wet-test meter for measuring the off-gas,
V-17
-------
and analytical equipment for determining the mass and composition of the liquid and gaseous prod-
ucts. Conditions for the oil and water slurry experiments are listed below.
Initial H2 pressure Operating temperature
Experiment (MPa) (°C)
Oil slurry 8.3 425 (SOOT)
Water slurry 3.5 300 (570°F)
With the total pressure limited to 14 MPa, it was necessary to use lower initial hydrogen pressures
and temperatures for the water slurry experiments because of the vapor pressure generated by the
water. The conversion nearly reached its maximum after about 20 minutes at reaction temperature;
little further conversion was observed after 30 minutes.
Results are evaluated in terms of the total fractional conversion of the toluene-insoluble organic
feed into oils (pentane-soluble substances) and into other substances. These are calculated by the
following relationships:
Conversion to Pentane-Soluble Oil,
(weight of oils in (weight of oils in sewage
v = product slurry) - sludge and carrier oil) 1An
A.~ X 1 (JO
(weight of organic toluene-insolubles in
sewage sludge)
Conversion of Toluene-insolubles,
(weight of organic toluene (weight of organic toluene-
x _ insolubles in sewage sludge) - insolubles in product slurry) .„„
TI ~~ (weight of organic toluene-insolubles in sewage sludge
Thus XQ represents the net oil yield per unit of insoluble organic material in sewage sludge, and XTI
represents the conversion of the insoluble organic material in sewage sludge to all liquid and gaseous
products.
Results and Conclusions
1. Raw sewage sludge and sludge settled in digesters can be largely converted to liquid and
gaseous products by heating the water slurry to about 300 °C (570 °F) under its vapor
pressure. Conversion pf up to 90 percent of the toluene-insoluble organic feed can be achiev-
ed with or without added hydrogen. Neither sodium carbonate, sodium molybdate, nor nickel
carbonate catalyst significantly alters the result.
2. Significant amounts of pentane-soluble oils were not produced from the water-slurried sludge
under any of the conditions studied.
3. If raw or settled digester sludge or the final effluent sludge from the digesters is dried,
ground, and slurried in a carrier oil, up to 90 percent of the toluene-insoluble organic content
can be converted in 20 minutes in the presence of hydrogen at a total initial pressure of 8.3
x 106 Pa and a temperature of about 425 °C (800 °F).
V-18
-------
4. Under these conditions, up to 50 percent of the material so converted may be recovered as
pentane-soluble oils or asphaltenes.
5. Great complexity and high projected investment and operating costs of a commercial plant
are indicated by the results obtained here for dried sludge slurried with oil. These results do
not encourage further development work on hydroliquefaction of sewage sludge.
Note: Conclusion 5 indicates that if the sludge is dried, then the problem of sludge disposal is
solved, and conversion to oil is an unnecessary expense that will not further reduce the disposal
cost of the sludge.
WET OXIDATION - THE VERTICAL TUBE REACTOR
The Vertical Tube Reactor (VTR) is a wet oxidation process developed by the Vertical Tube
Reactor Corporation for the treatment and disposal of municipal sludges and industrial organic
wastes. Although the thermodynamic principles involved in wet oxidation were well established by
Zimpro, the VTR system is a recent and unique engineering application. It uses an extended U-tube
reactor to achieve high reaction pressures and temperatures and long retention times for wet
chemical oxidation. An extended U-tube configuration is achieved by suspending two concentric
tubes from the top of a conventionally cased well (Figure V-5). The waste fluid and air are injected
into the inner tube. As the waste stream and air flow down the well, they undergo natural
pressurization due to increasing hydrostatic head. Thus, water pumps and air compressors must only
be sized to overcome friction and other minor pipe losses. The high pressure experienced in the
reaction zone is generated by the waste columns within the tube.
The location of the VTR reactor in a closed well in the earth results in the surrounding earth
providing some of the thermal insulation for the process. During startup of the system, heat is
pumped into the reactor through the reactor heat exchanger system. As the reactor is operated, the
surrounding earth or rock is heated. Heat losses to the earth or rock decrease with time, until there
is a near steady-state flow of heat to the rock. The design of the reactor is such that the heat of ox-
idation is greater than the amount of heat being lost to the rock plus the heat being washed out in
the reactor effluent. When the heat produced exceeds these losses, the reactor is self-sustaining with
respect to heat, and the heat exchanger system is operated in a manner that removes excess heat
from the system to maintain the desired reactor operating temperatures and allows heat recovery to
be used in other parts of the processing system. The high temperature expected in fullscale VTR
systems will produce a lower recycle load than other wet oxidation systems that run at lower
operating temperatures and hence have higher COD loads in their effluent. The VTR effluent is
completely biodegradable before discharge when the system is in stable operation.
History of Process Development
In the early 1970s, the VTR Corporation began research on the problem of municipal waste
disposal. Investigation suggested that an extended U-tube set in a well was a workable method for
wet oxidation of municipal sludges. It was then determined, theoretically, that the thermodynamics
were compatible with the reactions involved. The equilibrium configuration and hydrodynamics of
bubbles in the VTR system are of great importance in establishing minimum sludge flow rates,
overall system pressure drop, and maximum allowable air injection rates. The company constructed
a bubble flow experiment using a clear acrylic tube to resolve the bubble flow mechanics.
VTR Corporation has developed several computer models of the process; these aid in determin-
ing optimal configurations and conditions such as reactor diameter, COD loadings, flow rates, heat
V-19
-------
Air
Heat Exchanger Line (Out)
Heat Exchanger Line (In)
Air Line
Start of Reaction Zone
350 °F
Downcomer
Upcomer
Heat Exchanger Oil
Bottom of Reactor
Temperature Varies
Reactor Influent (Downcomer)
33°F - 100°F
Reactor Effluent (Upcomer)
5-20 °F Above Influent
Reactor Casing
(Pressure Vessel)
Well Casing
Cement Grout
Surrounding Rock
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Figure V-5. VTR pilot plant.
V-20
-------
loss to environment, compressor requirements, and expected energy production. The laboratory batch
reactor pilot plant and computer modeling efforts have often proceeded concurrently. The VTR
hydrodynamic computer model simulates any steady-state operational configuration of a VTR. Impor-
tant outputs of the model include percent reduction of COD, net heat energy output, air compressor
requirements, and fluid properties such as temperature and pressure profiles in the reactor.
The VTR heat model computes heat loss to the rock as a function of time, an aid in computing
the net heat production of a VTR reactor, and a help in designing the most cost-effective reactor in-
sulation system. The VTR carbonate model computes the solubility of calcium carbonate as a func-
tion of various input parameters.
Initial research and development (R&D) work on the VTR process used a bench-scale laboratory
reactor. The oxygenation and mixing were accomplished by inverting a small tube filled with sludge
and air to simulate the fluid dynamics of a full-size reactor. Pressure in the fixed volume was con-
trolled by heating and cooling with external heaters and coolers. Several models of the laboratory
batch reactor have been built; the current version is an invaluable tool in the VTR evaluation
studies.
600
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350
200
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-Downcomer
-Upcomer-
Reactor Bottom
^Reactor Entrance
10
15
20
25
30
35
40 45
50
55
60
Time In Minutes
Figure V-6. VTR temperature vs time profile.
(°C = (°F - 32) s/9).
V-21
-------
100
200
220
240
260 280 300
Reaction Temperature
320
340
Figure V-7. Average percent reduction in solids, 30-minute reaction time.
(°C = <°F - 32) x 5/9).
VTR Corporation constructed a small pilot plant and developed a significant body of test data by
operating the pilot plant in combination with both laboratory bench-scale testing and computer
studies. A feasibility study in 1979 and 1980 for the city of Montrose, CO, under a Step 1 construc-
tion grant study, provided data on the VTR process with respect to construction and operation costs
and VTR effluent characteristics and treatability.
Current State of Development
The VTR Corporation has constructed a reactor at the Longmont, CO, municipal wastewater
treatment facility within the past few years and will operate it over a 2-year period processing all of
the sludge from a 0.22-7.19 m3/s (5-10 mgd) plant. This will be a prototype facility to demonstrate
the VTR process; appropriate technical and cost data will be monitored.
Operational data for the VTR pilot plant are presented in Figures V-6 through V-8. Figure V-6
shows a profile of the temperature conditions, while Figures V-7 and V-8 illustrate solids' destruc-
tion at 30- and 60-minute reaction times. Table V-3 summarizes the solids' destruction data.
V-22
-------
100
p
E
R
C
E
N
200
220
240
260 280
Reaction Temperature
300
320
340
Figure V-8. Average percent reduction in solids, 60-minute reaction time.
(°C = (°F - 32) 5/9).
Table V-3. Average percent reductions of solid parameters for sludges.
30-Minute Detention
Temperature
240°C (400°F)
260°C (500°F)
316°C (600°F)
343°C (650°F)
TS
24
40
72
74
TVS
44
56
90
94
TSS
63
74
90
90
TVSS
77
87
99
96
60-Minute Detention
TS
25
64
66
66
TVS
40
72
88
89
TSS
98
85
90
93
TVSS
77
97
97
96
Energy Considerations
A properly designed VTR wet oxidation process has the potential to produce more energy than it
consumes. As the organic waste is oxidized, heat is released at a rate of about 14,000 kJ/kg (6,000
V-23
-------
Btu/lb) of oxygen consumed. The heat produced is recovered efficiently due to low heat losses
within the system.
The rate of heat produced by oxidation is solely dependent on the rate of oxygen consumed in
the wet oxidation process. The projected daily oxygen consumption for a 20 cm (8 in) reactor at
Longmont is 4,300 kg (9,500 Ib) of oxygen (assuming a COD reduction of 75 percent of the 5,675
kg [12,500 Ib] COD processed). This corresponds to a heat production rate of 60 x 106 Id (57
million Btu) per day. System energy losses can be broken down into three categories:
1. Heat loss to the surrounding rock,
2. Heat loss from thermal washout, and
3. Electrical energy consumed for air and water pumping.
The heat loss to the rock has been modeled as a function of several parameters. The time
elapsed since reactor startup is the most important parameter in estimating heat loss. The heat loss
tapers off with time, as a result of the surrounding well formation being heated up to a temperature
approaching that of the reactor. The two other parameters that affect heat loss to the rock are the
thermal conductivity of the well casing and the amount of insulation around the reactor.
Heat loss from thermal washout is due to the VTR process steam exiting the reactor at a higher
temperature than when it entered. This loss is kept to a minimum by transferring heat from the
warm up-flowing stream to the cooler down-flowing stream. This recuperative heat transfer insulates
the reactor from the top hole environment, enabling the heat recovery system to control the process
temperatures and to recover heat for possible use for power generation or space heating. The heat
loss from thermal washout is a function of reactor flow rate and temperature differential between ef-
fluent and influent.
Electrical energy consumption is primarily used for compressing air and pumping reactor in-
fluent. The operating conditions used in computing the 20 cm (8 in) reactor's electrical energy
demands assume 18,160 kg (40,000 Ib) of air (4,300 kg [9,500 Ib] of oxygen) injected at 10 kg/cm2
(150 psig) and an influent flow rate of 6 1/s (100 gpm) injected at 10 kg/cm2 (150 psig).
COMBUSTION - THE HYPERION ENERGY RECOVERY SYSTEM
Sludge that has been dried can serve as a useful fuel. Indeed, dried sludge is similar in fuel
value to lower grade coals, peat, refuse, and other biomass materials. Just as refuse can be process-
ed into refuse-derived fuel (RDF), so sludge can be processed into sludge-derived fuel (SDF) by
removal of water prior to combustion. The city of Los Angeles (CLA) is currently implementing a
sludge management program for its Hyperion WWTP that includes the processing of sludge to pro-
duce SDF and subsequent energy recovery from burning the fuel. The Hyperion Energy Recovery
System (HERS) was developed as part of a 5-year facilities planning effort for the Los Angeles/
Orange County metropolitan area (LA/OMA Project). LA/OMA examined all feasible alternatives,
recommended HERS as the best management program for the CLA, and completed the environmen-
tal documentation. A process diagram for HERS is presented in Figure V-9. Primary sludge and
waste-activated sludge will be digested anaerobically, and the digester gas will be used as fuel in a
gas turbine, combined-cycle, power system. Digested sludges will be centrifuge-dewatered and then
heat-dried using the Carver-Greenfield (C-G) multiple-effect evaporation process. Dried sludge will
be used as SDF in a fluidized-bed, gasification/staged, afterburner combustion system with waste
heat recovery. Energy will be recovered in the form of electrical power from gas and steam turbine
V-24
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generators. Steam from the combined cycle system will be used for heating in the C-G and
anaerobic digestion processes.
Intermediate storage facilities are provided at four points in the process train: 1 day of liquid-
digested sludge storage in the digesters, 2.2 days of wet cake storage following mechanical dewater-
ing, 3 days of SDF storage following the C-G process, and 3 days of ash storage following thermal
processing. The intermediate storage facilities will equalize loadings between unit processes, allow
for more steady-state operation, and provide additional backup in the event of temporary process
shutdown.
Both the C-G and fluidized-bed combustion processes are designed on a 3/2 modular basis.
Three process trains are provided, with each designed for 50 percent of the year 2000 sludge pro-
duction. The modular design and the intermediate storage facilities are essential to total process train
system reliability.
Thermal Processing/Energy Recovery
The LA/OMA Project undertook several field and pilot studies to develop design criteria and air
emission factors for sludge combustion. A number of reactor types were tested, including an in-
directly heated pyrolytic system, vertical shaft gasifier, rotary hearth furnace, and MHF. Of par-
ticular importance to development of HERS was the fact that the MHF operated in a stable and
predictable manner during starved-air combustion of SDF at temperatures up to 930 °C (1,700°F)
without evidence of slagging. These studies demonstrated the feasibility of starved-air combustion
(gasification) of SDF using commercially available reactor types.
In 1980 the CLA, in conjunction with the consulting firm of Metcalf and Eddy, undertook sup-
plemental analysis to develop detailed information for design of the HERS facilities. Systems con-
sidered for HERS included the MHF, fluidized-bed combustor (FBC), stoker waterwall boiler,
suspension fired boiler, rotary hearth pyrolyzer, and modular two-stage combustor. The fluidized-bed
system was selected because of its prior successful application in municipal sludge incineration,
adaptability to a wide range of fuel characteristics, potential for in-bed and staged combustion for
control of nitrogen oxide (NOx) formation, ease of feed of SDF and sludge fuel oil, ease of opera-
tion, and competitive economics with other combustion alternatives.
Fuel Characteristics
The C-G process is designed to produce about 221 Mg/d (244 dtpd) of dry sludge fuel and 19
Mgd (21 tpd) of sludge oil for a 240 Mgd (265 dtpd) design total. Characteristics of the fuel that in-
fluence design of the combustion system and air pollution control systems are presented in Table
V-4. The fuel will have a very high adiabatic flame temperature (T) (0 percent excess air) near
1,800°C (3,300°F) because of the low moisture content. Control of the combustion temperature will
be necessary to avoid ash softening and slagging, which begins at about 1,100°C (2,100°F).
Fuel-bound sulfur (FBS) is typical of low to moderate sulfur coals. SOx emission controls will
be required, but flue gas desulfurization methods are well proven. However, the fuel-bound nitrogen
(FBN) at 4.5 percent is significantly higher than that of most solid fuels. Without special combustion
controls and using conventional incineration systems, about 20 percent of the FBN would be ex-
pected to be converted to NOx. This would be equivalent to over 2 tons/day of NOx as NO2. In ad-
dition, flue gas denitrification systems are not well proven in solid fuel combustion trains.
V-26
-------
Table V-4. Sludge-derived fuel characteristics of the HERS process.
Heat value (Btu/lb dry)
Sludge
Oil
Combined
Moisture content
Ash content
Fuel bound nitrogen
Fuel bound sulfur
Ash fusibility
Particle size
5,340 Btu/lb dry (HHV)
18,700
6,410
1 percent of total weight
38 percent of dry weight
4.5 percent of dry weight
1.3 percent of dry weight
2,100° - 2,200°F initial
deformation temperature
2,300 - 2,450°F fluid
temperature
Powder
1 kJ = 0.948 Btu
1 kg = 2.205 Ib
°C =
- 32) 5/9
Air Emission Tradeoff. Air emission factors for the HERS combustion system are presented in
Table V-5. These factors are believed to be achievable in continuous operation. They are based on
results of the combustion test program and the expected efficiency of the air pollution control
system. The close simulation maintained during scaleup and the flexibility in the full-scale system
were considered in developing these factors.
A summary of air emission projections for HERS is presented in Table V-6. Mass emission
rates expected under year 2000 conditions are less than the available offsets for all primary air
pollutants. Even under year 2000 conditions, HERS expects a net decrease in air pollutant emissions
from existing conditions. Reductions are projected to be as much as 329 kg/d (725 Ib/d) for NOx
Table V-5. Air emission constraints on design of HERS fluidized-bed
combustion system (Ibs/day).
NO SO TSP NMHC CO
(as I\l62) (as S62) (as CH4)
Existing reciprocating
engines using digester
gas (to be retired from
service)
Gas turbine combined
cycle system (year
2000 conditions)
Offsets available for
fluidized-bed system
1,800
660
225
650
1,740
680
1,120
20
640
55
170
25
625
715
1,025
* 1 kg = 2.205 Ib.
V-27
-------
Table V-6. Air emission balance for HERS (lbs*/day).
Available offsets
from Table V-5
Estimated emissions
from thermal processing
Direct emission
N0x
(as NO2)
1,120
395
725
S°x
(as S02)
640
145
495
TSP
170
55
115
NMHC
625
20
605
CO
1,025
65
960
reductions
* 1 kg = 2.205 Ib.
and 275 kg/d (605 Ib/d) for NMHC. The latter two pollutants are particularly significant, since they
are precursors of ozone, the most troublesome pollutant in the South Coast Air Basin.
Combustion Test Program
An extensive fluidized-bed test program was conducted by the CLA in conjunction with Dorr-
Oliver, Inc. Objectives of the test program were to develop detailed criteria for design and air emis-
sion factors for subsequent air permit applications. Four fuel types were tested: C-G dried sludge in
powder form; C-G dried sludge that was pelleted; CLA sludge that had been dried in a conventional
indirect heat dryer; and Milorganite, a heat-dried raw sludge. The four fuel types allowed evaluation
of a range of fuel characteristics that spanned the range expected with HERS.
Two combustion modes were examined during the test program: incineration and gasification. In
the incineration mode, all fuel and both stoichiometric and excess air were introduced into the
fluidized bed. In this mode, the reactor was termed a fluidized-bed combustor (FBC). Essentially
complete oxidation of the fuel occurred in the fluidized bed, although some overbed burning was
observed. In the gasification mode, primary air was introduced to the fluidized bed in
substoichiometric amounts. SDF was partially oxidized and gasified to a low kJ (Btu) gas, termed
fuel gas. Fuel gas was then combusted with secondary air by one of three different methods: (1) by
secondary air addition to the freeboard above the fluidized bed; (2) in a second-stage fluidized bed;
and (3) in an afterburner designed for multiple staging of secondary air addition.
Major conclusions and recommendations for the fluidized-bed test program are summarized as
follows:
• No significant differences in combustion characteristics or air emission factors were observed
for the fuel types tested. High combustion efficiencies and fixed carbon burnout were achiev-
ed with all fuel types and without recycle of fly ash at space velocities of about 1 m/s (3 fps).
• NOx emissions were markedly affected by the method of combustion. Gasification in the FBG
with staged afterburner combustion of fuel gas resulted in significantly lower NOx emissions
compared to other combustion modes examined.
V-28
-------
• Principal variables affecting NO are the method of secondary air staging, residence time bet-
ween initial and final air input to the afterburner, exhaust gas O2 content, and afterburner
temperature.
• The lowest NO emissions were achieved with partial secondary air addition to the FBC
freeboard bringing the total oxygen supply to 80 percent of stoichiometric, secondary air addi-
tion to the afterburner bringing the total to 100 percent stoichiometric, and excess air addition
bringing the final oxygen content between 4 and 6.5 percent by dry volume. Residence time
in the afterburner between the secondary and excess air addition should be 2-2.5 seconds.
• SO control by limestone addition to the fluidized bed is enhanced by maintaining oxidative
conditions in the bed. Conversely, NOx control is favored by reducing conditions with staged
combustion of the fuel gas.
• Particle-size distribution of the fly ash was similar regardless of the fuel type.
• Conditions for effective burnout of CO and hydrocarbons in the afterburners are:
— A temperature of 930 °C (1,700°F)
— A residual O2 content above 3 percent
— A gas residence time of 2 seconds from the point of excess air addition
— A high level of gas-phase turbulence.
• Analysis of fuel and ash samples indicated that arsenic, beryllium, cadmium, chromium, cop-
per, lead, nickel, selenium, silver, and zinc should be associated with the ash residue after
flue gas cooling in the boiler. Only mercury was consistently unaccounted for in the ash
residue.
Full-Scale System Design
The pilot combustion program showed that reducing conditions were not compatible with
limestone addition to the fluidized bed for SOx control. Furthermore, the exhaust SOx concentration
necessary to stay within available emission offsets would require a high Ca/S ratio in the bed. On
the other hand, available flue gas denitrification processes, such as selective catalytic reduction, are
in an early stage of development and are very expensive compared to control of the combustion
process itself. Therefore, a decision was made to control NOx using the gasification/staged after-
burner approach.This decision also meant that SO would be controlled by a flue gas desulfurization
system.
Because all fuel types were effectively combusted, it was decided to burn the C-G product in
powder form. Pelletizing offered advantages in fuel conveyance and feeding to the FBC. However,
these advantages were outweighed by the projected costs of pelletizing.
A profile of the HERS combustion system is shown in Figure V-10 and a mass balance in
Figure V-ll. The FBC is designed as a conventional, bubbling bed type with a space velocity of
0.76-0.9 m/s (2.5-3 fps). Temperature in the bed is controlled by regulating the fluidizing air sup-
ply. Since the bed is operated in a substoichiometric zone, heat release is proportional to the quanti-
ty of oxygen added for combustion. Heat balances indicate that the bed temperature should
equilibrate at 954°C (1,750°F) with about 40 percent of TOC (theoretical oxygen for combustion).
Under these conditions a bed area of 10.5 m2 (113 ft2) is required, equivalent to the 3.7 m (12 ft)
diameter shown in Figure V-10.
V-29
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As mentioned previously, the theoretical flame temperature of the SDF will be near 1,800°C
(3,300°F) and must be controlled below this level. Substoichiometric air addition will be used in the
fluidized bed itself. Three primary approaches to temperature control are applicable in the remainder
of the combustion system: (1) dilution with excess air, (2) heat extraction by means of heat transfer
surfaces, and (3) addition of a cool dilutant gas such as flue gas recycled from downstream of the
boiler. Dilution with excess air is not viable for HERS because of the use of staged combustion and
because NOx emissions had been shown to increase dramatically with oxygen content above 8 per-
cent in the final flue gas. Therefore, temperature control options were reduced to the use of heat
transfer surfaces or the addition of recycle flue gas (RFG).
Conceptual designs were prepared for both temperature control alternatives. Use of RFG was
selected because it is well proven, simple to operate and control, can be adjusted "on the run," is
an effective NOx control strategy, has a wide range of turndown capability, and does not require ex-
tremely accurate design calculations. The major disadvantages of using RFG are increased energy
usage because of the need for an RFG fan and increased reactor volume. The advantages were judg-
ed to significantly outweigh the disadvantages.
The first addition of secondary air occurs in the overbed or freeboard zone above the fluidized
bed. Sufficient O2 is added to bring the fuel gas to a nominal 80 percent of TOC in the overbed.
Flue gas from downstream of the waste heat boiler and baghouse is about 200 °C (400 °F) and con-
tains a nominal 5 percent oxygen. RFG from this point is used to control the combustion
temperature to 954 °C (1,750°F). RFG is also used for temperature control in the first stage after-
burner, where secondary air addition brings the fuel gas to 100 percent of TOC.
Because the gas is brought to 100 percent of TOC in the first afterburner, very little fuel value
is likely at the point where excess air is added. A temperature of 930 °C (1,700°F) is required in the
second afterburner for burnout of CO and hydrocarbons. To maintain this temperature, the excess
air will be preheated to about 500 °C (950 °F). Temperature in the first afterburner is regulated by
RFG addition, which is controlled to maintain a temperature of 930 °C (1,700°F) in the second after-
burner. Heat balances indicate that the first afterburner will adjust to about 1,040°C (1,900°F). This
is the maximum temperature maintained in the combustor and is several hundred degrees below the
ash softening temperature. Nevertheless, a vertical downflow reactor was used because of the higher
gas temperature and concern over possible ash softening. Excess air is added to cool the gas back to
930 °C (1,700°F) before the gas is turned in the second afterburner.
The heat recovery steam generator (HRSG) consists of a superheater, evaporator, and
economizer sections. The unit is a straight flow, nonbaffled, three drum, A-type boiler. Feed water
will be received at about 160°C (320 °F), and superheated steam will be produced at 400°C/43
kg/cm2 (750°F/620 psig). Bare tubes are used throughout the HRSG because of the high ash content
of the flue gas. Flue gas temperature from the economizer will be maintained between 190° and
200 °C (380° and 400 °F) to stay safely above the sulfuric acid dewpoint and within temperature
limits imposed by the baghouse's bag material. Boiler feedwater temperature can be reduced if
necessary to modulate exit flue gas temperature and maintain a maximum temperature of 200 °C
(400 °F).
Energy Balance. An energy balance for the HERS system is presented in Figure V-12. Pro-
jected digester gas production in the year 2000 is 2,050 1/s (6.25 million scfd), equivalent to 4.28 x
109 kJ/d (4,060 MBtu/day). About 292 Mg/d (265 dtpd) of SDF will be produced, equivalent to 3.6
x 109 kJ/d (3,400 MBtu/day). The gas turbine generators will produce about 12.6 MW. Each gas
turbine will exhaust to a heat recovery steam generator (HRSG), producing superheated steam at a
nominal 76 kg/cm2 (1100 psig), 430°C (807 °F), and low pressure saturated steam at 2 kg/cm2 (30
psig). Superheated steam will be expanded through a backpressure turbine producing about 2.4 MW.
V-32
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Expanded steam at 11.4 kg/cm2 (165 psig) and low pressure steam will satisfy all process demands
for digester heating and drying in the C-G process. Digester gas serves as the base fuel in the HERS
energy cycle because of its history of reliable production. Most of the HTP electrical requirements
and all process heating demands will be met through the combined-cycle use of digester gas.
Waste heat boilers in the FBG/afterburner combustion trains will produce about 45,000 kg/hr
(100,000 Ib/hr) of superheated steam at 400°C 743 kg/cm2 (750T/620 psig). The steam will be ex-
panded in a condensing turbine with three stages of automatic extraction for deaeration and feed-
water heating. About 10.1 MW will be produced. Secondary effluent from the HTP will be used for
condenser cooling.
With the above fuels, HERS will produce about 25 MW of electrical power and all process
steam requirements. The Hyperion plant is expected to consume a total of about 15 MW for both
wastewater treatment and solids processing, leaving about 10 MW of cogenerated power for export
to the electrical utility grid.
STARVED-AIR COMBUSTION AND PYROLYSIS
United States
The mid-1970s saw a strong interest develop in the possibility of using more fuel-efficient
methods of combusting sludge solids. The Federal Government encouraged furnace manufacturers,
design engineers, and owner agencies to prove out their innovative concepts such as "starved-air"
or substoichiometric burning. This resulted in demonstration projects by Nichols Engineering and
Research in New Jersey and Envirotech-BSP in California.
A first-time, full-scale plant combustion facility was designed then by Alexander Potter
Associates/CDM for Arlington, VA, for starved-air combustion (SAC) of sludge solids. The Arl-
ington system started up in early 1984, and a definitive comparison of normal incineration and
starved-air mode is currently under way. Another project, located at Allegheny County Sanitary
District in Pittsburgh, PA (ALCOSAN), is supposed to use the Zimpro Cyclo Hearth MHF in a
starved-air mode. It was scheduled for startup in late 1984.
SAC is not a discrete chemistry, but rather a part of the spectrum of ratios of sludge fuel value
to oxygen that are possible. The spectrum ranges as follows:
Terminology
Percent Excess Air
Used in USA
"Hot sludge" incineration
Normal incineration
Minimized air (ICFAR mode)
No excess air
Starved-air combustion
Pyrolysis (true)
100 to 200
50 to 150
20 to 60
zero
-20 to -60
-90 to -100
Yes - several
Yes - most common
Yes - a few
No - theoretical only
Yes - prototypes only
No - requires indirect
heating
It must be noted that SAC and pyrolysis require a supplemental oxidation step, or "afterburn,"
to complete the combustion of the fuel factors in the off-gas. These can include carbon monoxide,
hydrogen, lower hydrocarbons, carbonyls such as malodorous aldehydes, and partially oxidized
sulfur and nitrogen compounds. This secondary combustion would be done in a separate chamber
V-34
-------
where excess air conditions exist, and where the gases may become very much hotter, possibly up to
1,100°C (2,000°F), depending on the excess air ratio provided. Thus the total process of SAC or
pyrolysis would produce a gas volume of combustion products comparable to normal incineration,
but the following major advantages accrue to SAC: (1) gas volume flow rate, and thus ash particle
entrainment, is much less in the primary chamber; (2) the secondary oxidation enhances complete
destruction of organics, assuring an odor-free exhaust of zero opacity; and (3) creation of hexavalent
chromium species is minimized, making the ash more acceptable.
SAC may also be called "two-stage combustion" because there are two distinct zones of oxida-
tion. In excess-air incineration, these two blend in the furnace and there is no clear line of demarca-
tion, although a few MHFs have been built with special gas handling systems that permit a separa-
tion of these zones.
MHFs have been built with a feature called an "afterburner/' This may be either the top
("zero") hearth, which does not receive feed and is built to maximize detention time, or a separate
chamber beside the furnace. These furnaces do use excess air in the sludge solids burn zone,
however, and the supplemental holding chamber is only to assure residual burnout and perhaps
smoke control, rather than being an integral part of the thermodynamic design as in SAC.
SAC is not a new art. The processing of wood waste, accomplished in Dutch ovens, is actually
a two-stage process. SAC has been carried out for many years in several applications of multiple-
hearth technology, the most common being the production of barbeque charcoal from biomass such
as agricultural wastes.
Perhaps the easiest way to understand SAC is with the aid of graphs. Figure V-13 depicts a plot
of theoretical temperature of products of combustion versus percent stoichiometric air for a typical
municipal sludge. These temperatures are purely theoretical and no inference is made that any com-
bustion would take place below some minimum temperature—760 °C (1,400°F). The gradually slop-
ing line at the left is intended to represent the evaporation of moisture and terminates at an arbitrary
temperature of 100°C (212°F). In the substoichiometric region (less than 100 percent stoichiometric
air), straight lines have been used to connect known points. In actual practice these will not be
straight, but this assumption is sufficiently accurate for equipment sizing. The temperature at 100
percent stoichiometric and greater can be calculated very accurately. As seen on this graph, in the
substoichiometric region, temperature increases with increasing air, while in the excess air region
(greater than 100 percent stoichiometric), temperature decreases with increasing air. The theoretical
temperature of the products of combustion depicted in this curve never reaches 760 °C (1,400°F) at
any point in the excess air region and therefore does not represent a viable system.
To reach a minimum of 760°C (1,400°F) at the final exit condition of 6.0 percent oxygen (40
percent excess air), auxiliary fuel must be added to the system. A "First Law" analysis will indicate
that the quantity of auxiliary fuel required will be the same whether the fuel is added to the first or
second stage. Figure V-14 depicts two different "combustion paths," one for the fuel added to the
primary chamber and one for the fuel added to the secondary chamber (afterburner). With fuel add-
ed to the primary chamber, the exhaust temperature, and hence the average hearth temperature, is
higher. This will increase the capacity kg/m2/hr (Ib/ft2/hr) of the MHF. Figure V-15 indicates that if
the TS of the sludge is increased to 33 percent, the sludge will be autogenous under the afterburner
exhaust conditions shown (760°C [1,400°F] and 140 percent stoichiometric [40 percent excess air]).
Figure V-16 depicts a typical TCS, which again will be autogenous but additionally allows the use of
a higher temperature or increased excess air in the afterburner. Figure V-17 depicts a "hot" sludge,
which would be a TCS that has undergone some enhanced dewatering technique such as belt press.
Although operation at the 1,300°C (2,400°F) temperature indicated would undoubtedly cause some
slagging problems, the opportunity for high-temperature, autogenous operation is certainly present.
V-35
-------
a
3
CO
I
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1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
0
25% Total Solids
65% Combustibles
10,000 BTU/lb Combustibles
Note: °C = (°F-32) 5/9.
kJ = 0.948 BTU
Kg - 2.205 Ibs
I
I
I
I
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
200
100
20
40 60 80 100 120
Percent Stoichiometric Air
140
160
Figure V-13. Plot of theoretical temperature of products of combustion
percent Stoichiometric air.
vs.
Germany
In addition to the proven waste incineration technologies, there has been increased work on new
thermal waste treatment processes, among them pure pyrolysis and SAC. These processes should
make possible facilities that function cost effectively in areas where there are low rates of
throughput, thereby making possible their installation in smaller service districts. Further, they
should make possible the recovery of energy and/or raw materials and should also be flexible
enough to accept changing combinations of types of wastes. This means that the new facilities must
be able to treat not only household waste, but also other forms of wastes such as wastewater sludge,
and should cause minimal environmental impact.
Pyrolysis means the decomposition of solid or liquid organic materials at high temperatures
(400° - 700 °C) in the absence of air. Other terms for this process are degasification, carbonization,
and dry distillation. Wood, peat, coal, and oil shale have been pyrolized in the past, producing a
carbon-rich coked residue and a hydrocarbon-rich product oil and gas. In principle, organic wastes
or components of waste such as rubber, plastics, paper, textiles, oils, fats, and similar materials are
suitable for pyrolysis. At present two prototypes are under construction in West Germany:
V-36
-------
• Goldshofe near Aalen: Pyrolysis of household waste and wastewater sludge in a rotary kiln,
using the Kiener process, with a throughput of 3 Mg/h (2.7 t/h).
• Gunzburg/Donau: Pyrolysis of household waste and wastewater sludge in a rotary kiln, using
the Babcock-Krauss Maffei process, with a throughput of 2-3 Mg/h (1.9 - 2.7 t/h).
Kiener Process. In the Kiener process, the wastes are transported from a silo into the rotary
kiln after the household wastes are shredded and mixed with wastewater sludge. Inside the kiln, the
wastes are degasified at temperatures of about 450°C. The residues, which consist of inorganic mat-
ter (80-90 percent) and pure carbon (10-20 percent) are discharged via a dry lock and stored on a
landfill site. The pyrolytic gases are then partially combusted in a cracking generator, reaching
temperatures of about 1,100° - 1,200°C (2,010°-2,190°F) and producing a gas consisting mainly of
methane, hydrogen, carbon monoxide, carbon dioxide, nitrogen, and water vapor.
Auxiliary Fuel
in MHF
Auxiliary Fuel in
Afterburner
25% Total Solids
65% Combustibles
10,000 BTU/lb Combustibles
Note °C = (°F-32) 5/9
kJ = 0.948 BTU
Kg - 2.205 Ibs
60 80 TOO 120
Percent Stoichiometric Air
140
160
Figure V-14. Two different combustion paths.
V-37
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33% Total Solids
65% Combustibles
10.000 BTU/lb Combustibles
Note- °C = (°F-32) 5/9
kJ = 0.948 BTU
Kg - 2.205 Ibs
80 100
Percent Stoichiometric Air
120
140
160
Figure V-15. Autogenous sludge.
After further dust separation and gas scrubbing, the final product is burned in a gas engine
coupled with an electricity generator. The exhaust from the gas engine serves to heat the rotary kiln.
The gas scrubbing water, contaminated with predominantly inorganic material, is first cleaned and
then emitted into the main drainage channel.
Babcock-Krauss Maffei Process. The plant in Gunzburg, which is currently shut down, is
capable of handling 22,000 Mg/yr (24,500 t/yr) household waste, 5,400 Mg/yr (6,000 t/yr) commer-
cial waste, and 4,000 Mg/yr (4,500 t/yr) of wastewater sludge containing chromium, thus serving
about 100,000 inhabitants.
The main steps of the process are as follows: the waste is fed into the rotary kiln, where it is
degasified at 400° - 450 °C (750° - 840°F),simultaneously binding the hydrogen chloride and
hydrogen fluoride that evolves by adding lime into the kiln. The residues are quenched and dis-
V-38
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2200
2000
1800
1600
1400
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600
400
200
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40% Total Solids
60% Combustibles
11,000 BTU/lb Combustibles
Note: °C = (°F-32) 5/9.
kJ = 0.948 BTU
Kg - 2.205 Ibs
I
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2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
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20 40 60
80 100 120 140 160 180 200 220 240 260
Percent Stoichiometric Air
Figure V-16. A typical thermally conditioned sludge.
charged wet. The gases are passed through heated cyclones and subsequently incinerated in a com-
bustion chamber with 30-40 percent excess air. Some of the hot flue gases are returned to heat the
kiln, the balance being used for steam production in a waste heat boiler and for generation of
electricity.
An advantage of low-temperature pyrolysis is that for the first time the process permits the ther-
mal disposal of wastes such as waste water sludge and industrial sludges that contain heavy metals.
This advantage may become more important in the future. For example, low-temperature pyrolysis
ensures that wastes containing chromium III are not oxidized to chromium IV, which is highly water
soluble; thus the toxic metals remain in the solid residues and are not released to the environment.
Japan
In the mid-1970s, the presence of hexavalent chromium compounds in ash from furnaces burn-
ing lime-conditioned sludge was pointed out as a serious environmental problem. Trivalent chromium
as a cation (chromic ion), used in the leather tanning industry, among others, was found to be con-
verted to chromate anion in the high temperatures of conventional incineration. The chromate was
V-39
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known to be toxic in aqueous environments, and, in fact, was a commonly used component in
biocides for algae control in cooling towers.
For the prevention of chromate formation and for potential fuel cost savings, these new methods
of combustion were investigated in Japan. Two types of processes have been developed: drying-
pyrolysis and SAC.
The pyrolysis system uses an indirect-type steam dryer, MHF, and waste heat recovery boiler. It
is considered suitable for loadings up to 100 tons per day.
The SAC system is applied in a modified MHF, but the ratio of excess air to combustibles in
the cake is only 10-30 percent. One or two hearths are operated in the reducing condition to prevent
production of hexavalent chromium.
Gas recirculation is used to compensate for the lower velocity that the lesser air volume would
create that would adversely affect the drying process. This is applied only in the upper hearths,
where it does not result in ash entrainment.
The number of such systems has been increasing and they appear to be the preferred future
technology in Japan whenever the chromium content of the sludge causes concern.
COMBINED MH/FB PROCESS
An interesting combination of design features is represented by a configuration developed by
Lurgi in Germany, installed in Frankfurt (the home of Lurgi headquarters), and adopted in Japan in
a city in the Hygo Prefecture. The design consists of a multiple-hearth (MH) system mounted on top
of a fluidized-bed (FB) system.
In Frankfurt, the units of this type are designed to handle 10 tons per hour of filter cake at 30
percent solids. The MH portion has no burners and acts merely as a stirring and transport device to
expose the sludge cake to part of the hot gases from the FB portion. The sludge is heated before
centrifuging, to increase dewatering performance. The odorized gases cooled by the drying process
are recycled into the combustion zone below. This method of operation is advantageous in locations
such as Germany, where the off-gas must leave the combustion zone at 800°C (1,470°F), yet a
minimum of excess air in combustion is desirable for thermal economy.
Advantages cited for this configuration include (1) smaller diameter for the windbox and FB,
since sizing is not controlled by drying rate; (2) low alloy, and thus less costly, rabbling system in
the MH portion; (3) requirement for only a single burner; (4) steadier in operation and thus easier to
control; and (5) the action of MH as a distributor and feed-rate controller to the FB portion.
A Selected Plant, Frankfurt (FRG)
The increasing amount of sludges and other residues originating from the new wastewater treat-
ment plants in and around the city of Frankfurt no longer allowed their composting or landfilling.
The municipality therefore decided to build a new wastewater sludge incinerator of the Lurgi
MH/FB design to handle the wastewater sludge derived from about 2.2 million inhabitants. This in-
cinerator went into operation in 1981. The investment was about 80 million DM ($32 million). The
following waste amounts are treated:
V-41
-------
330 m3/d wastewater sludge
20 m3/d screenings
25 m3/d sand from wastewater pipelines
10 m3/d oil and grease 10 m3/d oil-containing sand
15 m3/d grease sludge
410 m3/d overall sludges
After storage of the sludge, it is pumped through a heat exchanger for preheating by oil that has
been heated to about 240 °C (460 °F) by the incinerator flue gases. After preheating and addition of
fluocculents, the sludge is dewatered in a centrifuge to about 30 percent dry matter. The water goes
back to the wastewater plant, while the sludge is conveyed into the combined MH/FB incinerator
together with screenings. The incinerators have a capacity of 9 Mg/h (10 t/h) each (30 percent dry
matter). Two are in operation with one on standby. After further drying in the five hearths, the
sludge falls into the fluidized bed, where it is incinerated at about 800° - 850°C (1,470° - 1,560°F).
Auxiliary heat for startup or hot standby is provided by the incineration of waste oil. Fans pro-
vide the air for fluidization and oxidation of the waste. The flue gases leave the incinerator with a
temperature of about 850 °C (1,560°F). Their sensible heat is used for drying purposes in the MH,
heating the heat-exchange oil, preheating of the combustion air, and reheating the flue gases after
scrubbing. The remaining 15 percent stays in the flue gases; thus no additional fuel is needed for the
incineration of the sludge.
Flue gas cleaning is by electrostatic precipitators. Any harmful components remaining are
eliminated in a double-stage washing tower/rotary scrubber before the cleaned gases are released into
the atmosphere. This comprehensive flue gas treatment produces satisfactory compliance with the air
quality permit, with minimum adverse impact on the surroundings. The scrubbing water is taken
from the outlet of the wastewater treatment plant while the polluted scrubbing water is sent back to
the wastewater treatment plant.
Investigators have shown that virtually all of the heavy metals are concentrated in the fly ash
where they are firmly bound, being extracted neither by rain nor by leachate in a landfill. Therefore,
the remaining 50 m3/d ash is landfilled together with household waste.
MELTING (SLAGGING) FURNACES IN JAPAN
During the past 4 or 5 years, many new methods of wastewater sludge incineration have been
announced in Japan, including the use of technology from other countries. The need for economy in
energy use, the enactment of much stricter regulations for pollution control, and changes in the pro-
perties of the dewatered cakes caused by the use of polymers have prompted the development of
these new methods. In Japan, the ultimate target of development is the melting (slagging) furnace,
which produces a glassy ash product.
The types of melting furnaces that have come into operation or are now under development
include:
• Electric arc furnace,
• Double cylindrical furnace,
V-42
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• Pulverized fuel furnace,
• Shaft furnace, using coke admix, and
• Tank furnace.
The common requirement of these systems is the need to dry the dewatered cake prior to feeding it
into the furnace to minimize the steam and dry it more efficiently.
The only system in operation now at an actual plant is the electric arc furnace in Kawasaki City.
Others still in the experimental stage are classified as process development units (PDUs). The elec-
tric arc furnace is an adaptation of the type used for melting metal in steel manufacturing and foun-
dry industries. It is not presently considered economical, due to the rapid consumption of electrode
rods and electric power. However, the officials of Kawasaki City selected it because it greatly reduc-
ed ash volume—a major consideration in view of the dense population of the city and the
unavailability of land for ash disposal.
A shaft (cupola) furnace using coke mixed with the sludge cake is now under development by
Osaka Gas Company, Ltd. This is a vertical cylinder lined with refractory brick. Molten slag is
removed at the bottom. Coke is used for its auxiliary fuel value and because it forms a porous bed.
The coke and the fairly dry cake, at a moisture content of about 50 percent, are fed alternately into
the top of the cupola. One problem with this system is that coke must be used to form the bed even
in the case of burning high calorific cake that would burn autogenously; thus energy is wasted.
The double cylindrical furnace applies a film-melting process. It is being developed by two dif-
ferent companies.
The tank furnace is an adaptation of the reverberator used in the glass industry. It can readily be
used for melting ash from conventional incinerators.
The pulverized fuel furnace is similar to the cyclonic burner used in electric power generation
plants. Both horizontal and vertical types are now under development. The process consists of dryer,
crusher, furnace, and gas treatment. The dewatered cake is first dried to a residual moisture of 5-20
percent, then crushed in a hammermill. The particles are fed into the furnace with atomizing air and
burned. Ash forms and melts in the combustion chamber.
The final target of these developments is to burn sludge solids without supplemental fuel, melt
the ash, and make heavy aggregates for construction materials from the slag, a so-called autogenous
melting system. The development activity will continue to verify operating reliability, life of the
refractory brick, and ease of operation. It is expected that in a few more years some of these proc-
esses will come into operation in actual wastewater treatment plants.
NEW METHODS OF WASTEWATER SLUDGE INCINERATION IN JAPAN
New wastewater sludge incinerating methods aimed at reducing the consumption of supplemen-
tary fuel, using fuel other than heavy oil, preventing secondary pollution from exhaust gas and in-
cineration ash, and making effective use of sludge have begun to be practiced. New incinerators, in-
cluding a single-hearth cyclonic furnace (Maebashi City, 50 Mg/day [55 t/day]) and an FBF with
drying hearths (Hygo Prefecture, 45 Mg/day [50 t/day]), are in operation. Of these new wastewater
sludge incinerating methods, autogenous combustion is the ultimate technique aimed at cutting the
consumption of supplementary fuels. Other new technologies have also been adopted on a full-scale
basis, as shown in Table V-7.
V-43
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