United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-83-105 Feb. 1984
Project Summary
Sewage Sludge Incinerator Fuel
Reduction at Nashville,
Tennessee
Albert J. Verdouw and Eugene W. Waltz
A field demonstration project was
conducted to reduce fuel consumption
in municipal sludge incinerators using a
more fuel efficient incinerator operating
mode, which was developed at Indian-
apolis, Indiana, Belmont Treatment
Plant. The Nashville-Davidson County
Department of Water and Sewerage
Services demonstrated the new operat-
ing mode in Nashville, TN, with the use
of two, conventional, multiple hearth
incinerators.
The more fuel efficient operating
mode was developed from an extensive
program of combustion engineering
measurement, testing, and operational
analysis. Incinerator operators were
given on-the-job training in the new
operating mode during a 30-day dem-
onstration-training period. After 1 year
of routine operations in the new mode,
the use of fuel was reduced more than
40%; this represents fuel cost savings
of approximately $.350,000 per year.
The project exemplified how cost
effective efforts can improve and
optimize existing incinerator operation.
This project also successfully transferred
and applied incinerator operating tech-
nology developed by the City of Indian-
apolis Department of Public Works and
the Indianapolis Center for Advanced
Research under sponsorship of the U.S.
EPA Municipal Environmental Research
Laboratory. The Indianapolis work
included a projection that most plants
could reduce their fuel costs 20% to
50% if fuel efficient operation were
used, and this work confirms that
projection.
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 (see
Project Report ordering information at
back).
Introduction
Because incinerating municipal sewage
sludge often uses large amounts of
auxiliary fuel, increasing energy costs
have made incinerator fuel consumption
a major operating problem. Using more
fuel efficient operating modes can reduce
this consumption, and efforts to optimize
existing operating systems can be cost
effective for municipal operators.
The Department of Water and Sewerage
Services of Nashville-Davidson County
designed a new, more fuel efficient
operating mode by adapting incinerator
operating technology developed in Indi-
anapolis, IN. The Nashville Center
Wastewater Treatment Plant has two 10-
hearth incinerators with a design capacity
of 10 wet tons of sludge cake per
incinerator per hour. With technical
assistance from the Indianapolis Center
for Advanced Research, the Nashville-
Sewerage Department conducted a
series of operational tests, measurements,
and analyses to determine the specific
operating settings and improvements
required to reduce fuel consumption and
achieve steady incinerator operation. A
combustion engineering analyses was
made of the complete incinerator opera-
tion including measurements of specific
air flow rates, fuel flow, and load rate
dependent performance parameters.
Performance tests determined the fuel
consumption performance of the rnciner-
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ators at specific settings for excess air,
load rate, sludge cake moisture and
volatile characteristics, total airflow,
burner use profile, and various airflow
management approaches suited to the
given equipment design and operational
set-up.
A kinetic incinerator analytical model
was also used to predict the lowest fuel
consumption possible with optimum
incinerator operation The model's pre-
dictions of incinerator fuel requirements
provided a process-rate-determined
solution accounting for gas temperatures.
sludge composition, and heat transfer
rates, which vary throughout the inciner-
ator volume.
These test results were then used to
specify a new, more fuel efficient
operating mode for trial testing and
demonstration.
New Operating Mode
In the past, incinerator operation was
characterized by each operator having
his own specific operating practices and
techniques for maintaining temperatures
on various hearth levels and for managing
incinerator airflow. Several common
operating problems were associated with
the operator-specific modes:
• incinerator exhaust temperatures
were too high in the furnace,
• incinerator draft pressure was
greater than necessary,
• rabble arm cooling air return was
underutilized,
• auxiliary air supply was being
overused, and
• burner use pattern were not opti-
mum.
Other problem areas, including the
management of the sludge cake loading
to the incinerators, were also contributing
to high fuel consumption.
The new operating mode included
specific, detailed settings for optimizing
the combustion zone location, airflow
management, fuel flow rates needed for
given sludge cake load ranges, and
moisture and volatile characteristics. The
key features of the operating mode
involved operational settings to maintain:
• a steady sludge cake feed rate,
• the lowest possible incinerator draft
to reduce air leakage,
• the proper oxygen level associated
with a given load rate to maintain
the most efficient excess air level,
• the minimum required fuel flow rate
for a given load,
• control of combustion zone location
with burner use firing profiles and
air flow management, and
• optimum throughput rate with
proper center shaft drive speed.
Results
A full, plant scale, operational demon-
stration test was conducted to determine
how much fuel was saved by using a
more fuel efficient operating mode. The
10-month operational test period was
from November 1980 through August
1981. A 29-month baseline operational
period was used for comparison (January
1, 1978, through May 31, 1980). Plant
operational recoids for both periods were
used to obtain fuel consumption, load
rates, incinerator operating hours, and
sludge cake moisture and volatile data for
comparative analyses. The principal
measure used for comparing fuel con-
sumption was specific fuel consumption
(SFC) defined in cubic feet of gas per dry
ton of sludge cake incinerated. The SFC is
also directly related to the absolute ratio
of the sludge cake moisture to volatile
content (M/V) on a weight basis. An ac-
curate comparison of SFC must account
and correct for the M/V ratio of the
sludge cake being incinerated to avoid a
distorted comparison. For this reason, the
relationship of the daily average SFC with
the sludge cake M/V ratio was computed
and compared for the two periods.
A least squares regression analysis
was made for the SFC versus sludge cake
M/V ratios for both time periods. The
results of these analyses can be compared
in Figure 1. Table 1 shows the respective
averages of the other operational variable
for the two time periods.
As shown in Figure 1, the SFC was
reduced, as well as the basic relationship
between SFC and the sludge cake M/V
ratio, reflected by the decreased rate of
increase of SFC with M/V ratio. At the
low M/V ratio of 6, the computed SFC
difference was 22.6%, and at the high
M/V ratio of 10, the difference was 42%.
For the 10-month period, the average fuel
reduction was 38.9%.
The 38.9% reduction represented a fuel
savings of 5,612 cubic feet of gas per dry
ton of cake incinerated. For the 10-month
period, the total fuel savings was
59,670,154 cubic feet of gas. At this
reduction level, the annual gas savings
was approximately 71,604,180 cubic feet
of gas that, at a gas price of $3.50 per
1000 cubic feet, represents direct
savings of $250,000 per year for incin-
eration fuel.
Operational records were also analyzed
to determine the reduction in standby fuel
resulting from improved sludge load
management to the two incinerators as
part of the new operating procedures. The
plant loading rate was not large enough
to keep both incinerators on line at, or
near, design capacity. The past practice
had been to run both incinerators at
capacity until the sludge inventory level
was reduced to a point that one of the
incinerators had to be put on standby
operation. In the new operating mode,
both incinerators were operated contin-
uously at lower loading rates thus
enabling the standby fuel use to be
drastically reduced. Comparison of
standby fuel use rates during the 10-
month demonstration period with past
operational averages showed a dramatic
75% reduction amounting to 2,300,000
cubic feet of gas per month—an annual
savings of $96,000 per year.
The total combined incineration and
standby fuel savings by the Nashville
operations using the new operating mode
was approximately $350,000 per year.
Conclusions
The principal conclusions from this in-
plant research and demonstration project
were that:
• incinerator auxiliary fuel consump-
tion can be significantly reduced by
using more fuel efficient operating
modes derived from new operating
technology and experience;
• fuel reductions can be easily and
quickly achieved through operational
changes alone in existing facilities
without having to make major or
high cost equipment changes;
• multiple hearth sludge incinerator
technology can be cost effectively
transferred and applied to many
municipal operations because of the
similarity in equipment operational
practices;
• a 40% fuel reduction was achieved
over a 10-month demonstration per-
iod representing an annual cost
savings of approximately $350,000
per year;
• this work serves to substantiate the
analysis made for the Indianapolis
work, which indicated that savings
of 20% to 50% are possible if the
plants are operated in the fuel
efficient mode. ^
The full report was submitted in
fulfillment of Contract No. 68-02-3487 by
Battelle Columbus Laboratories under
the sponsorship of the U.S. Environmental
Protection Agency.
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75
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10 Month Average Reduction
42.3%
22.6%
Figure 1.
6789
Sludge Cake Moisture to Volatile Ratio IM/V) ftb/lb)
Specific fuel consumption comparison.
10
Table 1. Average Daily Values of Key Operating Variables for Baseline anil Demonstration
Variable
Wet Tons
Dry Tons
Percent Solids
Percent Volatiles*
M/V Ratio
SFC (1000 ft /dry ton)
* % Volatiles - thermally
(% volatiles
Baseline Demonstration
Period Period
129 Months) (10 Months)
202
35.7
17.8
61.9
7.7
144
volatile portion of the dry solids
+ % ash - 100% dry sludge)
226
358
161
62.9
8.7
8.8
Percent
Difference
*24
+ 1
- 1.7
* 10
+ 1.0
- 56
Albert J. Verdouw and Eugene W. Waltz are with the Indianapolis Center for
Advanced Research, Indianapolis. IN 46204.
Howard O. Wall is the EPA Project Officer (see below).
The complete report, entitled "Sewage Sludge Incinerator Fuel Reduction at
Nashville. Tennessee," (Order No. PB 84-113 075; Cost: $10.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
ftUS. GOVERNMENT PRINTING OFFICE 1984-759-015/7297
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United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
»GEHC1
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