United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/114 Dec. 1985
&EHV Project Summary
Co-Firing of Solid Wastes and
Coal at Ames: Stoker Boilers
J. L. Hall, A. W. Joensen, D. Van Meter, J. C. Even, W. L. Larsen, S. K. Adams,
P. Gheresus, G. Severns, and R. W. White
-f-
•^
This research program's objectives
are to conduct an in-depth evaluation of
the environmental, economic, and tech-
nical aspects of the resource and energy
recovery system located in Ames, Iowa.
The recovery system includes recovery
of ferrous and aluminum metals, prepa-
ration of the refuse-derived fuel (RDF),
storage for the RDF, and co-firing the
RDF with coal in the City of Ames-
owned power plant to produce electric
power.
The full report includes evaluations of
the refuse processing plant operation,
economics of the total system and
individual subsystems, flow stream
characterization, performance of the
stoker-fired steam generators, and en-
vironmental emissions of the stoker-
fired steam generators. Previous studies
at the Ames plant have been reported in
three U.S. Environmental Protection
Agency reports: EPA/600/2-77/205,
"Evaluation of the Ames Solid Waste
Recovery System, Part I—Summary of
Environmental Emissions: Equipment,
Facilities, and Economic Evaluations;"
EPA/600/7-79/229, "Evaluation of
the Ames Solid Waste Recovery Sys-
tem, Part II—Performance of the
Stoker-Fired Steam Generators;" and
EPA/600/7-79/222, "Part III—Envi-
ronmental Emissions of the Stoker-
Fired Steam Generators."
This Project Summary was developed
by EPA's Hazardous Waste 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 ordering information at
back).
Introduction
The Ames Solid Waste Recovery Sys-
tem is a continuously operating system
that is processing municipal solid waste
(MSW) for use as a supplemental fuel in
the steam generators of the Ames M unic-
ipal Power Plant. The total system
consists of a nominal 136-Mg/day (150-
ton/day) processing plant, a 454-Mg
(500-ton) Atlas storage bin, pneumatic
transport systems, and the existing
municipal power plant. The processing
plant incorporates two stages of shred-
ding, ferrous and nonferrous metal
recovery, and an air density separator.
The three steam generators consist of
one pulverized coal tangentially fired unit
and two stoker-fired return traveling grate
spreader units.
The full report is concerned with the
following objectives:
• Evaluation of the refuse processing
plant performance
• Economic evaluation of the Ames
Resource Recovery System
• Characterization of the material flow
streams within the refuse processing
plant and the refuse-derived fuel pro-
duced
• Evaluation of the performance of the
stoker-fired boilers
• Measurement of environmental emis-
sions from the stoker-fired boilers
• Determination of boiler tube corrosion
in the stoker-fired boilers.
The full report on this project presents
the results and conclusions of the tests
performed through the second year of
evaluations(1977), and contains separate
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sections on each of the above six listed
objectives.
Plant Description
This section addresses the operating
experience of the general plant and the
following subsystems:
• Tipping Floor
• Shredder System
• Air Density Separator System (ADS)
• Rejects
• Ferrous Metal Separation System
• Aluminum Separation System
• Pneumatic Conveying System (PSI)
• Atlas Bin
Figure 1 is a block flow diagram of the
processi ng plant. MSW is delivered to the
tipping floor which is 48 m by 32 m and
has two entrances and exits. One is for
commercial trucks and the other for
private automobiles and pickup trucks.
The commercial trucks are weighed on a
truck scale, and the private vehicles are
simply counted. A front-end loader is
used to push the MSW onto the infeed
conveyor C-1.
Raw refuse first enters the plant pro-
cessing system via infeed conveyor C-1
into the first stage shredder, then via
conveyor C-3 through the second stage
shredder. A magnetic belt separator
removes ferrous metal from the material
flow stream between the first and second
stage shredder. Conveyor C-6 transports
the material from the second stage
shredder into the air density separation
subsystem. Light material is transported
via a pneumatic conveying system to a
storage bin (Atlas bin) prior to transport to
the electric power generating plant. The
heavy material drops out of the ADS onto
conveying belts where it is transported to
a reject bin. Material in the reject bin is
periodically transported by truck to a land-
fill. Additional ferrous metal recovery is
achieved from the ADS heavy material by
a magnetic tail pulley and a magnetic
head pulley. This ferrous metal is added
to the ferrous metal recovered by the
magnetic belt separator.
An aluminum separation system
(Almag) composed of a trommel screen
and an electrical eddy current separator
Air to Fan
t
Cyclone
Separation
ADS
— Rott
— Fligl
try Feeder
'Hating Conv.
tt Conv.
Surge
Bin
2nd
Shredder
C-6
C-5
i
C-3
C-
11
Magnetic
Belt
Magnetic
Tail Pulley
C-12
-«*
C-10
C-2
1st
Shredder
C-1
Tipping
Floor
\C-12
C-7
C-7a
C-15
-JL-.
I Non-Comb. \
I Surge Bin \~
I I
Landfill
Legend:
C - Conveyors
E - Bucket Elevators
ADS - Air Density Separator
PSI - Pneumatic Conveying System
Atlas Bin - Storage Bin for RDF
J Almag System\ I Reject \
"J (Inoperative) | I Hopper I
. , 1 I --J
' .Landfill
| Aluminum \
| Storage \
C-9
Figure 1. Flow diagram of Ames Solid Waste Processing Plant.
2
C-13
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was originally installed at the plant, but
this system is now inoperative. It is
shown in dotted lines in Figure 1.
Operation of the process plant during
1977 represented the second year of
operation. Processing occurred on a 5-
day/week basis.
Economic Evaluation
This section presents an economic
evaluation of the Ames Solid Waste
Recovery System for 1977, its second full
calendar year of operation. The experi-
ence gained in the first year of operation
(1976) resulted in a reduction of overall
operating expenses during 1977. While
some costs were obviously reduced (e.g.,
lower overall costs), others actually in-
creased. Those costs which increased
were electrical energy and replaced parts
primarily as a result of general economic
inflation.
Initial capital expenditures have been
estimated at $6.3 million, while initial
capital investment for the refuse process-
ing plant alone has been estimated at
$4.1 million, and the capital costs of the
storage bin and pneumatic transport
system are estimated at $1.3 million.
The total plant expenses during 1977
were $992,270. This amount includes
depreciation and interest on the capital
expenditures and the refuse processing
operational costs. Total revenue was
$498,626 which was derived from the
sale of RDF, metals, wood chips, and
paper; fees charged for commercial and
private haulers; and reimbursements and
refunds.
The amount of refuse processed in
1977 was 43,891 Mg (48,381 tons).
Based on this amount, the net cost for
processing the refuse was $13.90/Mg
($12.61 /ton). The average usage of elec-
trical energy was 52 kWh/Mg of pro-
cessed refuse.
Flow Stream Characterization
Initial flow stream sampling began July
5, 1977, and continued through July 13,
1978, in order to allow for characteriza-
tion on a monthly and a seasonal basis.
Sampling was conducted at 12 locations.
Eleven locations were inside the process
plant and one on top (inlet) of the Atlas
storage bin.
Samples were taken by using a con-
tainer attached to the end of a rod and
passing it back and forth in a free-fall flow
stream until the container was full.
Samples were taken at 1.5 hr intervals
beginning approximately 15 min after
process plant start-up. Weekly composite
samples for each location were gener-
ated, and from these, appropriate sub-
samples were prepared for the various
analyses.
Performance of the Stoker-Fired
Steam Generators
The conceptual design of the solid
waste recovery system specified burning
RDF in suspension in the pulverized coal-
fired unit 7 (33 MW) at a firing rate of 20%
(by heat input) or about 7.26 Mg/hr.
Initial operation in fall 1975 resulted in a
high dropout of unburned material into
the bottom ash hopper. The power plant
then began burning RDF in the stoker-
fired boilers until a solution could be
developed. The final solution was the
installation of a dump grate at the furnace
bottom. It was concluded that a dump
grate configuration is also necessary in
small- to moderate-sized suspension-fired
steam generators.
The major research emphasis was on
the thermal and environmental evaluation
of the stoker-fired units.
Unit 5 is a Riley RP steam generator
with a Riley overthrow spreader and
traveling grate. Unit 6 is a Union Iron
Works steam generator with a Hoffman
underthrow spreader and continuous
return traveling grate.
The RDF is pneumatically conveyed
from the Atlas storage bin through two
31.5 cm transport lines and is blown into
the furnace approximately 3.4 m above
the grate. Two rows of nozzles in the rear
furnace wall and two rows of nozzles in
the front wall supply overfire air for
turbulent mixing of the furnace gases.
Both units have dry pneumatic vacuum
grate ash hopper and mechanical col-
lector hopper ash removal systems.
Initial program objectives were to test
boilers 5 and 6 at 60%, 80%, and 100% of
rated steam load and at RDF firing rates
(based on heat energy input) of 0%, 20%,
and 50%. Three tests were made at each
condition. The experience gained from
firing unit 5 at 100% load with RDF
demonstrated that this firing condition
was not practical. For unit 6, nine tests
were performed at 80% load. High panic-
ulate emissions occurred during tests at
80% of steam load, with varying RDF flow
rates. A block diagram showing all sample
locations and type of samples collected is
shown in Figure 2.
Environmental Emissions of the
Stoker-Fired Boilers
The preceding section includes descrip-
tions of the Ames stoker-fired steam
generators used for the tests. Both units
have dry pneumatic vacuum bottom and
fly ash removal systems. Mechanical dust
collectors are installed on both units.
For this study, the independently con-
trolled variables were load, based on
steam flow, and RDF quantity, based on
heat energy input to the boiler. Nominal
load levels selected were 60%, 80%, and
100% of rated capacity; RDF quantities
were 0%, 20%, and 50% of heat energy
input.
For each steam flow and each quantity
of RDF, three experimental runs were
accomplished. For unit 5 the statistical
design was 3x3x3 full factorial experi-
ment with 27 runs needed to fill the data
matrix of the experiment. These experi-
mental runs were accomplished during
1976. After observing the operation of
unit 5, a steam load of 80% was chosen as
typical of boiler demand. Also at 80% load
wall slagging was reduced, and excess air
supplied for the coal and RDF combustion
was optimum. Therefore, nine additional
tests selected at 80% load were accom-
plished on unit 6. Testing of two boilers of
similar type but different size allowed the
investigators to observe whether boiler
size had any effect on emissions.
Sampling of effluents was done accord-
ing to EPA-prescribed techniques. Stack
effluents, including particulate samples,
were obtained at numerous prescribed
points in the stack cross section. Three
sampling trains operated simultaneously.
An additional train was located before the
particulate collector. Input fuel and grate
ash were sampled at regular 1-hr inter-
vals throughout the test period and then
mixed to yield a composite sample. Hopper
(fly) ash was sampled at the completion of
each experimental run. Combustion airto
the boiler was monitored by wet and dry
bulb thermometry. Steam flow rate,
temperature, and pressure were also
recorded at regular intervals.
The sampling was conducted on a
regular basis except that of heavy organic
species, which were sampled intermit-
tently.
The composite coal and RDF samples
were analyzed in the Ames laboratory.
Ultimate analyses and heating values
were obtained by standard ASTM meth-
ods. Trace elements were determined by
x-ray fluorescence (XRF).
The size distribution of the particulates
after the dust collector was determined
by an Andersen cascade impactor. Par-
ticulate samples obtained with the EPA
Method 5 sampling train were analyzed
by SRF. The impinger solutions from the
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Flow Rate
Ultimate Analysis
Heating Value
Chemical Analysis
& Trace Elements
Ash Softening Temperature
Filter Paniculate
Trace Elements
Impinger Water Trace
Elements
Emission Rates of
Paniculate
Paniculate Trace
Elements
Impinger Water Trace
Elements
Emission Rates of Paniculate
and Gaseous Species
Paniculate Sizing
Humidity
Barometer
Intake
Temperature
Volume Flow
Density
Ultimate Analysis
Heating Value
Chemical Analysis &
Trace Elements
Ash Softening
Temperature
Flow Rate
Chemical Analysis &
Trace Elements
Softening Temperature
Flow Rate
Chemical Analysis &
Trace Elements
Softening Temperature
Figure 2. Sample locations and items sampled.
Method 5 train were analyzed with an
inductively coupled plasma system.
The gases COz, CO, Oz, and N2 in the
stack were determined by Orstat tech-
niques. EPA Method 7 was used for
evaluation of N0« levels, and the EPA
Method 6 train was used for measure-
ment of SOX and chlorides. Grab samples
of stack gas were obtained for measure-
ment of Ci through C5 hydrocarbons by
gas chromatography. Several modifica-
tions of the EPA Method 5 train were
used to collect samples for analysis of
aldehydes and ketones, chlorides, mer-
cury, and other trace metallic elements.
All inputs to and outputs from each boiler
were evaluated including fuel, combus-
tion air, bottom-ash, steam, fly ash, and
stack gas.
Polynuclear aromatic compounds were
sampled by drawing stack gas through a
column of macro-reticular resin and also
by extraction from particulates collected
in the stack. Gas chromatography and
mass spectroscopy were used for identi-
fication.
The uncontrolled particulate emissions
from unit 5 have no discernible trend
within the data scatter for 1976. However,
the data taken during 1977 indicate a
nearly linear increase in emissions of
about 30% from 0% RDF to 50% RDF at a
boiler load of 80%. This trend is similar to
that indicated for unit 6, but the effect is
much less exaggerated in unit 5. At 100%
load, unit 5 was not stable in operation
when the amount of RDF was increased
to 50% which may account for consider-
able uncertainty in the data at this
operating condition. The scatter in the
1976 data also reflects changes in uncon-
trolled factors in the tests while the
operators were learning how to run the
boilers when burning RDF with coal.
Uncontrolled particulate emissions
from unit 6 increase about 100% in a
nearly linear fashion from 0% RDF to 50%
at boiler loads of both 60% and 80%. The
trends for the data for 1976 are similar to
those of the data for 1977. However, at
80% load, the magnitude of the uncon-
trolled particulate emissions on unit 6
was about one-third higher during 1976
than during 1977. In large part, this may
reflect the learning experience of the
boiler operators in controlling the amount
of additional air routed to the boiler for
burning the fuel. Increased air flow
through the boiler appears to help carry
proportionately more fine particulates
through the boiler passages to the partic-
ulate collectors.
The increase in uncontrolled particu-
lates with increases in RDF is believed to
be due to the additional amount of air
routed into the stoker-fired boilers in
order to properly burn the RDF and to
maintain a proper firebed on the boiler
grates. The air flow through the boiler
appears to carry additional particulate
matter (fly ash) through the boiler pas-
sages in nearly direct proportion to the
amount of the increase in air with RDF.
The effect on unit 6 is exaggerated
because of its physical size in cross
sections is about the same as unit 5.
However, boiler 6 generates 12 MW,
while boiler 5 generates 7.5 MW. Thus,
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the air flow through boiler 6 is significant-
ly larger than that through boiler 5.
Conclusions
RDF in combination with coal was
successfully fired in the stoker boilers
with no insurmountable problems. The
operation of the boiler improved in terms
of stability and consistency of measured
variables from 1976 to 1977. This is
believed to be a "learning effect" on the
part of the boiler operators in properly
firing the RDF and coal mixtures.
The combustible properties of the fly
ash and the bottom (grate) ash became
similar as the RDF approached 50%. The
ash softening point of the ash lowered
and the fouling index became more
detrimental as the RDF was increased in
the fuel input.
Uncontrolled particulate emissions
tended to increase with corresponding
increases in the RDF fraction of fuel
input. This appears to be a result of both
lighter particulates and increases in air
flow through the boiler when burning
RDF. Controlled emissions also appeared
to increase with increases of RDF on unit
6, but the trend was uncertain on unit 5.
Both the oxides of nitrogen (NOX) and
oxides of sulfur (SOX) decreased while
chlorides increased significantly with
increases in RDF. No discernible trends
within the data scatter were noted con-
cerning formaldehyde or hydrocarbon
emissions. Increased emissions of the
trace elements copper, lead, and zinc
correspond to increases in RDF. Further
studies of the trace element emissions
are being performed.
Further studies are still in progress at
the Ames facility. Thus, more specific
conclusions cannot be made at this time.
The final data from Ames, both economic
and technical, will provide valuable design
information for future plants and will aid
operators of existing waste-to-energy
plants.
J. L Hall, A. W. Joensen. D. Van Meter, J. C. Even. W. L Larsen, S. K. Adams. P.
Gheresus. and G. Sever ns are with Iowa State University, Ames, I A 50011; and
R. W. White is with Midwest Research Institute, Kansas City, MO 64110.
Michael Black is the EPA Project Officer (see below).
The complete report, entitled "Co-Firing of Solid Wastes and Coal at A mes: Stoker
Boilers,"(Order No. PB 86-115 151/AS; Cost: $22.95, 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:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
. S. GOVERNMENT PRINTING OFFICE:1986/646-l 16/20735
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United Slates
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-85/114
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