United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-86/044 June 1986
Project Summary
Evaluation of
Sewage Sludg
Using Wood Chips for Fuel
Ned J. Kleinhenz and Gary Smith
An environmental and technical evalua-
tion was conducted for the Western L ike
Superior Sanitary District (WLSSD) waste
treatment plant, which uses wood cr ips
to incinerate sewage sludge in fluidizsd-
bed combustors. The most important e ivi-
ronmental factors for evaluation were the
metal contents of the paniculate stick
emissions and of various influent and ef-
fluent streams for the incinerator and the
incinerator air pollution abatement systi im.
The technical evaluation used data < :ol-
lected over the life of the facility to devc lop
a mathematical model of the incineration
and energy recovery system. The me del
was coded in the form of a computer (ro
gram and can rapidly evaluate a wide \ ar-
iety of possible situations in which si ich
an energy recovery approach might be
taken. Thus the technical evaluatioi is
presented in a widely usable and unc er-
standable form.
This Project Summary was developed
by EPA's Water Engineering 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 order-
ing information at back).
Introduction
Background
The WLSSD serves the 500-square mile
area surrounding Duluth, Minnesota,
which is a major port on Lake Superior.
The District provides wastewater treat-
ment (44 million gal/day) and solid waste
disposal (280 tons/day) for approximately
135,000 area residents.
The WLSSD waste treatment plant was
designed in 1975 to incorporate several
new and innovative technologies for ac-
a Fluidized-Bed
e Incinerator
complishing the co-incineration of solid
wastes and sewage sludge. The incinera-
tion facility (completed in 1979) was
designed to use shredded solid waste as
fuel to augment the combustion of the
sewate sludge in one of two fluidized-bed
combustors, each rated at 100 million
Btu/hr. A major portion of the energy re-
quired to operate the facility was provided
by a waste-heat boiler that produces steam
from energy in the hot combustion gases.
The original study was undertaken in
1978 to compile information on the tech-
nical, environmental, and economic perfor-
mance of this innovative co-disposal sys-
tem. However, because of prolonged tech-
nical difficulties encountered at the facili-
ty, the evaluation was not performed un-
til 1984. Up until that point, constant prob-
lems were experienced with the solid
waste processing system; but the fluid-
ized-bed combustors were operating well
using wood or bark chips as the auxiliary
fuel. Since the study had already been
delayed for 5 yr, a decision was made to
complete as much of the evaluation as
possible with the system in its present
configuration.
Objectives
The original study objectives were to
perform a technical, environmental, and
economic evaluation of the refuse/sludge
combustion process at WLSSD. Because
of the 5-yr delay, these goals were modi-
fied to focus primarily on the environmen-
tal and technical issues. The results of the
evaluation provide technical and environ-
mental information about sludge combus-
tion with wood chips that will allow others
to replicate WLSSD's successes and to
avoid their problems.
-------
Because the energy-recovery, co-com-
bustion system has a very limited number
of effluent streams, it was decided that
the most important environmental factors
were the metal contents of the particulate
stack emissions and the metal contents of
the various influent and effluent streams
to the combustor scrubber system. Be-
cause the scrubber and wastewater treat-
ment plant would be in a closed-loop mode
when operated as designed, it is conceiv-
able that the facility could accumulate
metal concentrations that could be toxic
to the biological processes in the waste-
water treatment operation. Thus the fate
of metals in the process became the most
important consideration in the environ-
mental evaluation of this process.
As a result of the extensive evaluation
performed on the combustion process
over the life of the facility, much of the in-
formation needed to perform a technical
evaluation was already available. Thus it
was decided that this information could
best be used by developing a mathemati-
cal model of the incineration and energy
recovery system and to code this model
in the form of a computer program. This
model could readily be used by WLSSD
personnel and by others in the technical
community. By varying several of the in-
put parameters (e.g., fuel characteristics,
sludge characteristics, and energy de-
mand), it is possible to use this model to
perform rapid technical evaluation of a
wide variety of possible situations in
which such an energy recovery approach
might be taken. Actual operating data
were used as the basis for developing this
model, and the performance of the com-
bustor for various operating scenarios can
be predicted through the use of this model.
This approach presents the technical eval-
uation of the combustion and energy re-
covery system in a widely usable and
understandable form.
Procedures
Figure 1 illustrates the current config-
uration of the WLSSD incineration system.
To collect metals data, sewage sludge was
cofired with wood bark chips in the fluid-
ized-bed reactor under the same set of
normal operating conditions on April 10,
11, and 12, 1984. Representative hourly
samples of each of seven major process
streams were acquired over a 6-hr test
period during the 3 days. Hourly samples
of each process stream were composited
to provide one representative daily sam-
ple of each stream for metal analyses. The
mass flow rate of each process stream
was measured or calculated, and the metal
Figure 1. WLSSD process schematic.
concentrations within each stream were
calculated. In addition, operating logs of
1982, 1983, and 1984 were analyzed to
produce the data needed to design those
equations that describe the mass and
energy flows of the incineration and heat
recovery system.
Results
Average Metal Mass Rates
Table 1 presents the average mass rates
of metals detected within the various pro-
cess streams. Elements entering the sys-
tem with the sludge filter cake (SC), wood
chips (WC), and the fluidizing-bed sand
(SA) are summed in the (I In) column. The
elements are listed in decreasing order of
magnitude of total system input. Exponen-
tial notation has been used to allow quick
scanning for order of magnitude
differences.
Metals leave the incineration system via
the fly ash (FA), the scrubber effluent (AS),
and the stack particulate (PF and 1C). The
net scrubber discharge is the result of sub-
tracting the mass rate of metals entering
the scrubber via the scrubber influent (SI)
from the mass rate of metals in the scrub-
ber overflow (SO) and the scrubber dis-
charge (SD). Thus
AS = (SO + SD) - SI
The total mass rate of metals leaving the
system is then
I Out = FA + AS + PF + 1C.
The results of the metal analyses shown
in Table 1 must be viewed in relation to the
designed operating mode of the plant. The
scrubber and wastewater treatment sys-
tem are designed to operate in a closed
loop, and therefore the WLSSD process is
probably concentrating metals.
Mathematical Model of the
Combustor System
The computer model "HEATBAL" was
designed to simulate the operation of the
fluidized-bed combustors at WLSSD. The
source code of the model is written in
HP-3000 Fortran. The source code is divid-
ed into modules and subroutines with
numerous comment lines and long, de-
scriptive variable names. Efficiency and
elegance have been sacrificed to make the
source code easy to understand and easy
to modify by another programmer.
The object code resulting from the com-
pilation of this source code can be stored
for interactive execution from a video ter-
minal. Successive runs of the program are
possible in the same session without ter-
minating the program and restarting.
Upon initiating a run of this program,
users will first be asked if they want a list
of the available modes of analysis. The
following list will then be displayed if a
user responds "Y":
1. Maximum steam production
2. Minimum auxiliary fuel usage
3. Specified steam production
4. Exhaust temperature (heatwork)
5. An economizer with any mode
6. An air preheater with any mode
-------
Table 1. Summary of Average Metal Mass Rates Ig/hr)
Element
Ca
Al
P
K
Ti
Fe
S
Mg
A/a
Zn
Ba
Mn
Cu
Pb
Sr
Cr
V
Ni
Co
Si
Mo
B
Li
Be
Cd
Sludge
Filter
Cake
2.76504*
3. 13E04
1.43E04
7.20E03
1.23E04
9.23E03
9. 51 £03
6. 13E03
3.02E03
1.26E03
5. 15E02
3.64E02
2.02E02
1.44E02
6.88507
3.91E01
3.84E01
3.39E01
2.99E01
2.34E01
1.20E01
—
—
7.09E-01
-
Wood
Chips
4. 14E04
3.40E02
1.30E03
6. 1SE03
4. 91 £01
5. 78E02
1.S2E03
2.59E03
8. 57 £02
3.26E02
1.89E02
1.44E02
1.11 £01
—
5.71 £01
—
—
—
_
_
—
—
—
—
-
Bed
Sand
1. 15E03
1.07E03
7.24507
2.86E02
1.55E02
1. 15E03
9.16EOO
3.44E02
2. 67 £02
1.81EOO
7.25E02
1.78E01
1.47EOO
5.54E-01
2.67EOO
2.67500
3.44EOO
1.22EOO
8.40E-01
1.36 £00
1.87E-01
3.82EOO
—
—
1.91E-01
I. In
6.42E04
3.27E04
1.56E04
1.36E04
1.25E04
1. 10E04
1. 10E04
9.06E03
4. 14E03
1.59E03
7. 1 1E02
5.26E02
2.15E02
1.45E02
1.29EOO
4.18E01
4.18E01
3.50E01
3. 07 £01
2.48E01
1.22E01
3.82EOO
2. 75EOO
7.09E-01
1.91E-01
Fly
Ash
2.09E04
2. 70E04
3.80E03
1.35E04
1.8 7 £03
9. 91 £03
8.86E02
5.33E03
2.60E03
8.91 £02
3.54E02
3. 15E02
1. 13E02
—
6.75E01
3.90E01
2.01E01
2.21E01
8. 73EOO
3. 79502
1.48EOO
5.76E01
_
—
-
Scrubber
Water
Influent
1.5 6 £04
4.66E01
—
1. 19E03
—
1.79E02
4.63E03
3.32E03
3.80E04
6.53EOO
1.96E01
4.72E01
—
—
2.35E01
—
—
—
—
1. 7 3 £03
—
—
—
—
-
Scrubber
Over-
flow
3.69E03
2.23507
—
2.90502
—
2. 76502
1.21 £03
7.92502
8. 75503
2.22500
5.45500
1.69E01
—
—
5.58EOO
—
—
—
—
4. 27 £02
—
—
—
—
-
Scrubber
Dis-
charge
2.22504
1. 75502
—
7.76503
2.77507
7.59502
8.80E03
3.55503
3.00504
1.8 9 507
4.49507
7.77507
9.63500
—
3.77507
—
_
_
—
2.39503
—
—
—
_
-
Net
Scrubber
Discharge
7.03504
9. 73507
—
2.60502
—
7.96502
5.38E03
7.02503
7.30502
7.46507
3.08E01
4.74507
9.63500
—
7.32507
—
_
_
—
7.09503
—
—
—
_
-
Panicu-
late
Filtrable
_t
—
9.545-07
2.24500
7.725-07
7.695-07
7.59500
—
—
—
3. 795-02
3.475-02
—
—
4.495-03
_
—
7.455-02
—
1.26EOO
—
—
_
_
-
Impin-
ger
Catch
_
2. 79500
—
—
—
—
3.49507
—
7.60507
—
2.86E-02
—
—
—
—
_
_
_
—
4.23507
—
_
_
_
-
2 Out
3. 72504
2.77504
3.80503
7.38504
7.89503
7.07504
6.30503
6.35503
3.44503
9.06502
3.85502
3.62502
7.23502
—
8.07507
3.90507
2. Of £01
2.27507
8. 73500
7.57503
7.48500
5.76507
2. 75500
—
-
"Results reported in computer notation, e.g., 2. 76504 = 2.76 x 1O4 = 27,600.
1Dashes indicate that the concentration of the reported element was below the detection limit in one, two, or all three of the samples analyzed.
The user must then specify the mode of
analysis desired for this run.
The full report was submitted in ful-
fillment of Cooperative Agreement No.
R806144 by SYSTECH Corporation for the
Western Lake Superior Sanitary District
under the sponsorship of the U.S. En-
vironmental Protection Agency.
-------
Ned J. Kleinhenz and Gary Smith are with the SYSTECH Corporation, Xenia, OH
45385.
Howard Wall is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of a Fluidized-Bed Sewage Sludge
Incinerator Using Wood Chips for Fuel." (Order No. PB 86-183 092/AS; Cost:
$11.95, 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:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES P>
EPA
PERMIT No G-35
Official Business
Penalty for Private Use S300
EPA/600/S2-86/044
0169064
60604
------- |