United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-146 Oct. 1984
Project Summary
Sewage Sludge Incinerator
Fuel Reduction, Hartford,
Connecticut
Albert J. Verdouw, Eugene W. Waltz, and Paul F. Gilbert
A field demonstration project was
conducted to reduce fuel consumption
in municipal sludge incinerators by
using a more fuel-efficient operating
mode. The Hartford Metropolitan
District Commission in Hartford,
Connecticut, demonstrated a new
operating mode at its Hartford Water
Pollution Control Plant using three
conventional multiple-hearth sludge
incinerators.
The more fuel-efficient incinerator
operating mode was developed from an
extensive program of combustion
engineering measurement, testing, and
operational analysis. Incinerator
operators were then given on-the-job
training in the new operating mode
during a 14-day demonstration test
period. After 12 months of routine
operations with the new operating
mode, a fuel reduction of 61% was
achieved, representing an annual fuel
cost savings of approximately
$250,000.
The Hartford Plant had just
completed a conversion of its sludge
dewatering equipment from vacuum
filters to continuous-belt filter presses
when this project was initiated.
Average specific fuel consumption had
already been reduced by more than 65%.
Together, the annual fuel cost savings
from dewatering and improved
incinerator operations amounted to
$1.3 million.
Comparisons of fuel reductions
achieved in four major cities by means
of new incinerator operating modes are
also reported.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory. Cincinnati, OH,
to announce key findings of the
research project that is fully
documented in a separate report of the
same title fsee Project Report ordering
information at back).
Introduction
The disposal of municipal sewage
sludge by incineration most often
requires the use of large amounts of
auxiliary fuel. Increasing energy costs in
recent years have made incinerator fuel
consumption a major problem for many
municipal operations. To reduce these
costs, the Hartford Metropolitan District
Commission instituted two major
operational changes at its Hartford Water
Pollution Control Plant to drastically
reduce incinerator fuel consumption. The
first step was a conversion to continuous-
belt filter presses; and the second was the
adoption of a more fuel-efficient
operating mode.
The Hartford Plant was one of the first
in the United States to convert to
continuous-belt filter presses for sludge
cake dewatering.This conversion took
place between 1979 and 1982 with the
installation of four belt filter presses. The
conversion to belt filter presses resulted
in dramatic fuel savings of 65% and
increased solids production.
In late 1981, the Hartford plant
engineering staff and incinerator
operators adopted a new, more fuel-
efficient operating mode with the
technical assistance of the Indianapolis
Center for Advanced Research. The new
incinerator operating mode has been
exclusively used in routine operations
-------
since January 1982. The new operating
mode resulted in an additional reduction
in the specific fuel consumption of 51 %.
The combined result of these two major
operational changes was an 83%
reduction in the incinerator fuel
consumption. These results are reviewed
here as an example of how improved
dewatering equipment and incinerator
operating technology offer new options to
those municipal operations that are
currently bound to sludge disposal by
incineration.
Past Operations
The Hartford Water Pollution Control
Plant performs primary and secondary
wastewater treatment for more than 45
million gallons of wastewater per day and
generates in excess of 200 wet tons of
filtered sludge cake per day. The sludge-
handling facility was originally designed
in 1968 with four dissolved-air flotation
thickeners, five drum-type vacuum
filters, and three multiple-hearth incine-
rators.
In 1978, before conversion to belt filter
presses, the vacuum filters averaged
13.8% 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 handling 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 also
plagued with the common operating
problems of handling the very wet cake
and were consuming large amounts of
fuel. The Hartford incinerators (Figure 1)
are equipped for either gas or oil
operation and have a maximum capacity
of 12.5 wet tons per hour. No common
operating procedure was being used by
the incinerator operators. Each operator
had certain practices and techniques for
maintaining temperatures on various
hearth levels and for managing
incinerator airflow. The operators'
preoccupation with just burning the very
wet sludge cake resulted in many
inefficient operating practices such as
high exhaust gas temperatures,
combustion occurring too high m the
furnace, too much draft and auxiliary
airflow, misuse of heated-rabble-arm
cooling air^nd less than opliingm burner
use patterns. The degree of remote
instrumentation and the controls also
handicapped operator performance.
Cooling Air Exhaust
fieturn Damper
Sludge Fet
Gas/Oil
Burners'
*
&£—'
t& l» .f
kN
rll
P*sJ
ffi
Burner Air
f 1
Cr1
b^v7
-j&— /
7 d"
f^j-^
-*— •/
L rf
b&^
-&—-'
f°\
K.1 A
t
Auxiliary Air
1
^
M<
>-
c
J
/?
c
Coo//
/?e
;
2
3
4
5
6
7
8
9
10
11
ngAir
turn
Ud
k Exhaust
?0
\
Ash
'4 Burners/Hearth
21 Damper/ Hearth
abbleArm
noting A ir
Figure 1. Hartford incinerator system schematic.
In view of these operating problems
and the ever-increasing cost of fuel, the
Hartford Metropolitan District Commis-
sion initiated a major program to find new
methods of plant operations and pro-
cesses to regain control of operating costs.
Conversions to the belt filter press and
development of the new inci nerator oper-
ating mode were two of the major
projects that were undertaken.
To provide an accurate baseline for
comparing the fuel reduction achieved by
converting to belt filter presses and in
improving the incinerator operating
mode, a statistical analysis was made of
key operating performance data for past
operations during each of the years in
which changes were made. In addition,
the correlation of specific fuel consump-
tion (measured in gallons of oil per dry
ton) with the absolute sludge cake
moisture to sludge volatile solids ratio by
weight (M/V) was computed to provide a
more comprehensive measure of change
for comparison.
The average specific fuel consumption
for the incinerator operations in 1978
was 125 gallons of oil per dry ton. The
sludge cake solids averaged 13.8%, and
the volatiles averaged 77.1 %. The sludge
cake M/V ratio, which is directly related
to and principally determines the specific
fuel consumption demand, averaged 8.6,
which is high.
Belt Filter Press Conversion
Testing and Operational
Experience
Hartford began pilot testing belt filter
presses in the spring of 1978. Test results
showed that significantly drier sludge
cake was produced at higher production
rates compared with the performance of
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'the existing vacuum filters. The plant
staff then conducted side-by-side
performance tests of the most qualified
machines to select the first press for
procurement and installation. The first
press was installed in 1979 with
a carefully monitored start-up and shake-
down period. Despite numerous
mechanical problems and excessive
downtime (25%), the belt filter press
quickly proved to be so economically
beneficial that the approval for acquiring
the second press was granted only 4
months after the first one was installed.
The payback period for the first press was
only 6 weeks since the savings were $1
million per year.
When the second press was selected,
performance tests were again conducted
to evaluate overall performance,
mechanical design, and maintenance
features of the presses to incorporate
these requirements in bid specifications.
The second press, which was supplied by
a different manufacturer from the first,
was installed in December 1979, just 8
months after the first one
During the first half of 1980, the Hart-
ford plant operated with two belt presses
and one vacuum filter. The mechanical
reliability sought with the second press
was realized along with continued fuel
savings and inprovements in sludge cake
production. With this continued
operational success, the District
purchased a third press from this same
supplier and installed it in December
1980. During 1981, the plant was able to
operate exclusively with belt presses. A
fourth press was placed into operation in
June 1982 to increase capacity and
operational flexibility by pairing two
presses to each operating incinerator.
Since the Hartford plant was one of the
first to try the belt filter presses, the early
operational experiences were problem-
atic, as expected. The initial press
operation suffered from numerous me-
chanical 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, third, and fourth
presses were from a different supplier.
They have had few mechanical problems
and have been satisfactory in operation.
As more operational experience was
gained, improvements were made in
several key operating conditions. The
filter screen seam closure wearing
problem was reduced by using higher-
molecular-weight plastic scraper blades,
which resulted in an increase in filter
screen operating life from an average of
500 hours to an average of 1500 hours.
Proper polymer conditioning of the sludge
cake on all the presses has been a
problem. A two-component liquid
polymer mix was developed from experi-
ments with the polymer supplier to
reduce the dosage requirements to the
same level as that required for the
vacuum filters. Changes in the sludge
conditioning tank to improve polymer
sludge mixing have also helped reduce
dosage requirements and increased the
flexibility to adjust to varying sludge
characteristics. Maintaining a constant
blend in the mixing of the raw primary
and waste-activated sludges from three
plants requires close .operator control.
Sludge blend variations of only 5% to 10%
can cause the press screens to plug and
the sludge to squeeze out the ends of the
rollers, with a resulting loss in percent
solids and production. In spite of these
operating problems associated with
reducing a new operating technology to
routine production line practice, the
operational improvements and cost
savings achieved with the belt filter
presses in Hartford have been dramatic.
Fuel Reduction Results
Figure 2 illustrates the reduction of
specific fuel consumption for the Hartford
incinerator over the period 1978 through
1981 at the average sludge cake M/V
ratio recorded for each year The same
information appears in Table 1 along with
percent solids and incineration rates.
The savings resulting from the belt
filter presses is 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 nearly 82
gallons of oil per dry ton would translate
into a savings of more than 848,000
gallons of oil at the 1982 dry ton
production level of 10,351 dry tons
Coupled with the drastic reduction in fuel
consumption, there also occurred a 57%
gain in the volatile solids incineration rate
per operating equipment hour, which is
the key production performance
parameter. Furthermore, the average
hours of incinerator operation per day
also dropped from 46.5 in 1978 to 35.7
in 1981 — a 23% decrease.
New Incinerator
Operating Mode
These substantial improvements were
accomplished after considerable time
and effort were invested by the Hartford
plant management, staff, and operating
personnel. The experience at Hartford
with the belt filter presses serves as an
1
^
125 r
100
75
50
25
125
116
65%
60.5
43.5
1978
1979 198O
Year
1981
Figun 2. Reduction of specific fuel consumption as a result of conversion to belt filter presses
3
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Table 1. Key Operating Performance Variables for 1978 through 1981
Variable
Percent
Year Change
1978 1979 1980 1981 78-81
Percent solids
Sludge cake (M/Vj
Fuel consumption (gals/dry ton)
Incineration rate (volatile tons per
incinerator /hr)
13.8
8.6
125.2
0.7
14.5
8.1
116.1
0.7
18.5
5.8
60.5
1.0
19.5
5.4
43.5
1.1
+41
-37
-65
+57
example of the opportunities that exist in
many plants throughout the country to
achieve internal improvements with the
adoption and modification of new
operating technologies. At this point,
most operations would have been
contented with the lower fuel consump-
tion levels that had been achieved. But
the dramatic success with the belt filter
presses encouraged the Hartford Metro-
politan District Commission and staff to
pursue other innovative operating
technologies to improve their operations
further.
One of the technologies adopted was
an improved incinerator operating
technique that had been developed by the
City of Indianapolis Department of Public
Works with the Indianapolis Center for
Advanced Research (ICFAR) under the
sponsorship of the U.S. Environmental
Protection Agency (EPA). A more fuel-
efficient operating mode was developed
for the Hartford incinerator operations
from an extensive program of combustion
engineering measurement, testing, and
operational analysis. The development
and testing of the new operating mode
was a cooperative effort by the Hartford
engineering and operating personnel and
combustion engineers from ICFAR. The
new operating mode was derived from
refined operating technology developed
by ICFAR through a major operational
research project conducted on the
Indianapolis incinerators. On-the-job
instruction and training in the use of the
new operating mode was also performed
to demonstrate the potential fuel
reduction and upgrade operator
performance.
Operational Testing
and Analysis
An operational analysis was made of
the Hartford incinerator operations; it
included airflow measurements, exhaust
gas analysis, assessments of key instru-
mentation and controls, existing oper-
ator-specific practices, load-rate manage-
ment, incineration and dewatering modes.
airflow management, burner use profiles,
combustion zone location and control
techniques, hearth temperature profiles,
etc. A kinetic incinerator analytical model
was also used to determine the optimum
load rate and plant operating mode that
would result in the least possible fuel
consumption.
Preliminary analysis of the operator-
specific operating modes found several
common practices that were contributing
to excessive fuel consumption such as:
(1) combustion occurring too high in the
incinerator, (2) high exhaust gas
temperatures, (3) high draft settings and
too much auxiliary air, (4) misuse of
heated-rabble-arm cooling return air, (5)
improper burner use profiles, and (6)
improper techniques for controlling the
combustion zone location. Also contribu-
ting to the high fuel consumption were
other problem areas associated with the
lack of remote operator controls for
airflow dampers and burners.
Results of the airflow measurement
were used to help correct problems found
in the airflow management approaches
being used and to assess the relative
impacts of various previously proven
techniques on the Hartford incinerator
set-up. A preliminary analysis indicated
that through optimum airflow manage-
ment alone, fuel consumption could be
reduced 70% with a sludge cake M/V
ratio of 5.0 and an incinerator loading
rate of 6 wet tons per hour. Furthermore,
the kinetic rate analysis for these
conditions predicted that the potential
fuel consumption for the Hartford
operation with such a dry cake was zero1
This analytical result agreed with the
empirically based preliminary estimate
drawn from airflow management
techniques, since an additional 30% fuel
reduction could be reasonably expected
from improved combustion zone location
control, optimum burner use profiles,
improved load rate management, and the
synergistic effect of these operating
mode techniques on fuel consumption.
Based on these results and previous
ICFAR experience from similar programs
in Indianapolis, 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 when the new
operating mode was routinely used, there
were many days that no fuel was used for
over a 24-hour period.
Based on the operational 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 per-
formance demonstration. On-the-job
operator training in the use of the new
mode was also accomplished at the same
time. After 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 (refer to Figure 1):
• Make maximum use of the heated-
rabble-arm cooling air return.
• Use the lowest possible draft to
minimize air leakage.
• Maintain combustion on hearth Nos.
7 or 8 to maximize the drying area.
• Replace the cold auxiliary air supply
with heated cooling air return.
• Minimize excess air.
• Use lower hearth burners to
maximize drying temperature.
• Eliminate airflow to top hearth
burners.
• Control combustion location with
burner use profile.
• Slow center shaft speed to improve
sludge drying.
• Discontinue use of hearth No. 5
burners.
The incinerator operators received
specific operating instructions that con-
stituted the new operating mode. These
instructions included procedures for
sludge load management, operational
control of the incinerator, general
operating settings, specific settings for
normal operations, combustion zone
location control, standby and start-up
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operations, and techniques to control
sludge cake burn-outs. The trial tests
indicated that the most effective
incineration and dewatering mode was
two belt filter presses paired to each
operating incinerator. The optimum
incinerator loading rate was 6 wet tons
per hour per incinerator, based on
analysis and trial tests of load rates
between 4.5 and 7 wet tons. The 6-ton-
per-hour load rate was the lowest rate
possible to keep up with the overall plant
loading rate and still minimize fuel
consumption, considering average
sludge cake M/V ratio of 4.5. The
improved operating mode also enabled a
further reduction in the M/V ratio,
because the new mode allowed the
presses to be slowed down, resulting in a
small increase in cake solids.
Fuel Reduction Results
The new incinerator operating mode
was placed into routine operational use
directly following the 2-week
performance test conducted in January
1982. The new operating mode has now
been in routine use by Hartford for more
than a year. The 1982 operational data for
the Hartford plant were analyzed to
measure and compare the fuel reduction
achieved. Figure 3 illustrates the
computed least squares correlation of the
average specific fuel consumption versus
the sludge cake M/V ratio for both the
baseline period (1978 to 1981) and for
1982, when only the new mode was
used.
The improved thermal operating
efficiency achieved with the new opera-
ting mode is reflected in the change of the
slope of the relationship. This result was
quite similar to those that occurred in
Indianapolis, Nashville, and Buffalo when
these operations implemented the ICFAR
operating techniques. Figure 4 illustrates
the average specific fuel consumption for
the Hartford operations from 1978
through 1982. The average specific fuel
consumption for 1982 was 21.1 gallons
per dry ton compared with 43.5 gallons
for 1981 —a 51.1% reduction. With this
improvement, the total fuel reduction
achieved by Hartford between 1978 and
1982 amount to 104 gallons per dry ton,
or 83%. At the 1982 production level, this
figure represented a savings of 1.08
million gallons of No. 2 fuel oil compared
with 1978.
In addition to reducing direct fuel con-
sumption, the new operating mode
provided increased operating flexibility
with the equipment, since the
125
100
.
to
75
50
25
4 5 6 78 9
Sludge Cake Moisture to Volatile Ratio flb/fb)
Figure 3. Specific fuel consumption versus sludge cake M/ V ratio before and after the change
in incinerator operating mode.
.§
t
.JjD
125
100 -
75 -
50 -
25 -
125
-
116
60.5
I
83
43.5
21.1 i
1978 1979 1980 1981 1982
Year
Figure 4. Average specific fuel consumption for 1978 to 1982.
incinerators could now be efficiently
operated at loading rates of 50 to 60
percent of capacity. Such operation was
not possible before without paying a
tremendous penalty in excess fuel
consumption. For example, compared
with past operational averages, the
results of the 2-week demonstration test
-------
recorded a 76% fuel reduction for the
entire test period when the specific fuel
consumption was corrected for load rate.
Incinerator operation is also now
characterized by cooler maximum
operating temperature, more steady-
state control, fewer particulate
emissions, and reduced maintenance on
internal incinerator parts.
In spite of the significant fuel reduction
achieved with the new operating mode
during routine operational use, the
Hartford incinerator operation has still
further fuel reduction potential to be
realized from the new operating mode.
Several equipment-related conditions
are currently limiting further gains. The
most critical of these has to do with the
inadequacy of the original rabble-arm
raking pattern. The latter was originally
set up for a wetter sludge cake (M/V =
8.6) than the current average sludge cake
(M/V = 4.5). The drier cake commands a
far different number of rabble teeth,
depending on the hearth being
considered. During the operational tests,
an uneven-sludge-distribution problem
was encountered several times. A
distorted burning pattern resulted, with
sludge combustion occurring on three
hearths simultaneously and uneven
burning taking place on one side of these
hearths. This condition also creates an
increase in incinerator burn-outs, in
which dry sludge cake gets to a point of
uncontrollable autogenous combustion
and the incinerator steady state thermal
equilibrium is lost in the process. Plans
are under way to adjust the rabbling
pattern for the drier sludge cake. To a
smaller extent, some improvement in the
instrumentation and remote operating
controls would also permit improved
operator performance.
Cost Savings
The nominal cost savings from redu-
cing incinerator fuel consumption on an
annual basis was estimated from the
change in the specific fuel consumption
from 125 to 21 gallons of oil per dry ton.
Based on the 1982 production of 10,351
dry tons, the savings would be more than
$ 1.08 million per year using an estimated
cost of $1.00 per gallon for No. 2 fuel oil.
In addition to the incinerator fuel
savings, other energy savings were
realized from the belt filter press
conversion. Since the plant started up in
1972, the activated sludge mixed liquor
suspended solids (MLSS) level to a more
desirable 2,000-mg/L range. The
resulting decrease in the dissolved
oxygen demand reduced the daily air
usage to about 55 million ft3 per day. This
reduction in turn reduced the electrical
energy requirements of a 3,000-hp air
compressor by 20%, which amounted to a
$200,000-per-year savings on electricity
costs. Also, each vacuum filter had
a 71.5-hp requirement, as opposed to 22
hp for each belt press. This reduction in
electrical use resulted in an estimated
savings of $25,000 per year. Together,
these additional savings totaled more
than $231,000 per year.
The total operating costs saved by
converting to belt filter presses and the
new incinerator operating mode is esti-
mated to be more than $1,3 million per
year.
Conclusions
1. Fuel consumption in multiple-
hearth incinerators can be
significantly reduced by improving
dewatering equipment and by
optimizing existing incinerator
operating modes with more fuel-
efficient operating techniques.
2. A conversion from vacuum filters to
continuous belt filter presses
resulted in a 65% fuel reduction.
3. The use of a new incinerator oper-
ating mode in routine operations
achieved an additional 51% fuel
reduction and a reduction in
equipment maintenance problems.
4. Reduction in fuel use from
dewatering and incinerator opera-
ting improvements resulted in
annual cost savings of approximate-
ly $1.3 million.
For more information, contact:
Paul F. Gilbert, Plant Engineer
Hartford Metropolitan District
Commission
555 Main Street
Hartford, CT 06101
(203) 524-5901
or
Eugene W. Waltz, Associate
Director
Energy Engineering and Re-
search Division
Indianapolis Center for
Advanced Research
2442 City-County Building
Indianapolis, IN 46204
(317)236-4546
The full report was submitted in
fulfillment of Contract No. 68-02-3169 by
Battelle Columbus Laboratories under
the sponsorship of the U.S. Environment-
al Protection Agency.
Albert J. Verdouw and Eugene W. Waltz are with the Indianapolis Center for
Advanced Research. Indianapolis, IN 462O4; and Paul F. Gilbert is with the
Hartford Metropolitan District Commission, Hartford, CT06101.
Howard O. Wall is the EPA Project Officer (see below).
The complete report, entitled "Sewage Sludge Incinerator Fuel Reduction,
Hartford, Connecticut," (Order No. PB 84-243 O96; Cost: $10.00. subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
- U S GOVERNMENT PRINTING OFFICE, 1984 - 559-016/7838
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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