United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-84-198 Jan. 1985
Project Summary
Trace Metal Retention When
Firing Hazardous Waste in a
Fluidized-Bed Incinerator
Robert D. Litt and Ted L. Tewksbury
This report describes a bench-scale
fluidized-bed incinerator that will
capture trace metals on the bed material
when firing hazardous waste. The
design is based on limited tests at an
existing laboratory facility. Operating
conditions, operating procedures, and
equipment design are established for
greater than 90 percent trace metal
capture on the bed material. A surrogate
hazardous waste, paint containing lead
chromate. was used in the tests. Other
trace metals were identified that can be
captured by agglomeration on a silica
bed material. The design provides the
capability of operating in either a single-
or double-stage configuration so that
various bed materials or operating
conditions can be used to capture
different trace metals or to provide
more effective capture. The bench-
scale fluidized-bed incinerator will have
the flexibility to operate with several
fuels, bed materials, and fluxing agents,
over a wide range of operating condi-
tions.
This Project Summary was developed
by EPA's Air and Energy Engineering
Research Laboratory. Research Triangle
Park. NC, 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
Emissions of trace metals from the
combustion of hazardous waste material
is of primary concern to EPA's Office of
Solid Waste. Organic material in such
waste can be disposed of by incineration;
however, many trace metals are emitted
into the atmosphere from most convention-
al incinerators. This study was undertaken
to evaluate the potential for fluidized-bed
incinerators to capture the trace metals.
The major activity of this work assign-
ment was to provide a design package for
a versatile bench-scale fluidized-bed
incinerator. This design was to be based
on general experience in fluidized-bed
combustion, with particular attention to
operating conditions expected for trace
metal capture. The bench-scale testing
was preliminary in nature to establish the
feasibility of the system. A detailed
performance analysis of the fluidized-bed
incinerator was not necessary to design
the bench scale unit. Starting with a
feasible system, EPA plans to conduct
subsequent work using the versatilely
designed bench-scale facility to thorough-
ly test the trace metal capture performance
on specific waste materials.
The project objective was to design a
bench-scale fluidized-bed combustor
(FBC) to capture trace metals in the bed
material during hazardous waste incinera-
tion. Special designs (e.g., cascading
FBCs or a spouted bed) and selected bed
materials were evaluated for capturing
the trace metals in the FBC. Preliminary
tests were run in an existing laboratory
facility, firing a surrogate hazardous
waste to establish system feasibility and
performance. Samples were collected to
determine collection efficiency and to
provide design data for the bench-scale
FBC.
Procedure
Four tasks were identified for conduct-
ing the required project work: waste
characterization, testing, FBC design, and
reporting.
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Battelle's background in fluidized-bed
combustion, waste management, and
environmental control technology provided
a starting point in planning the project.
The waste characterization task evaluated
different types of waste materials (e.g.,
paint, paint sludge, or used lubricating oil)
that could be used as the surrogate for
testing. All of these wastes contain trace
metals of interest to EPA. Candidate bed
materials (e.g., silica, limestone, alumina,
or glass beads) were evaluated for
chemical absorption and physical adsorp-
tion of the trace metals onto the bed
material. The operating conditions of the
FBC were selected to match the trace
metal(s) of interest and the bed material.
It was generally preferred to capture the
trace metal as an oxide, but consideration
was given to cascading beds so that
another chemical species of the trace
metal can be captured.
A limited amount of experimental
testing was done to verify the behavior of
the candidate materials in the FBC. After
evaluating the available existing facilities
at Battelle, the 6-in. Spouted Bed Facility
(after modification to operate as a
conventional FBC) was chosen as most
suitable for this work. Modifications
included installing a perforated plate
distributor, reactivating the liquid feed
system and the flue gas cyclone, and
installing a paniculate filter. Sample
points were provided for the bed material,
cyclone capture, and filter capture so that
a material balance and capture efficiency
could be calculated.
The bench-scale FBC was designed for
use by EPA to test a variety of hazardous
waste streams. A detailed design of the
FBC and its integral components (e.g., the
distributor, instrumentation, and materials
of construction) are provided. Specifica-
tions of auxiliary components (e.g., air
supply, start-up burner, and control and
analytical instrumentation) are also
provided so that existing off-the-shelf
equipment can be purchased by EPA. The
design is of such quality that a mechanical
contractor can provide a fixed price bid for
building the facility.
Discussion and Results
Waste Characterization
Consistent with the project objective of
trying to capture trace/heavy metals on
the bed material in a FBC, a review was
conducted to characterize the hazardous
wastes and trace metals of concern.
Based on toxicity and OSHA threshold
limit values (TLVs), six metals were
selected as priority concerns: mercury,
lead, cadmium, chromium, nickel, and
cobalt. Other metals which were consid-
ered are: antimony, arsenic, barium,
beryllium, selenium, silver, and tellurium.
It is thought that all of these metals,
except mercury, can be captured as a
stable oxide at conventional operating
conditions for a FBC. The relatively high
vapor pressure of elemental mercury
presents a special problem that must be
addressed separately.
The mechanism for capture of these
trace metals, except for mercury, is to
agglomerate the metal oxide with silica
in the fluidized bed. The addition of a
small quantity of sodium carbonate
(Na2CO3> to the fluidized bed of silica
results in a "tacky" surface. This
approach utilizes the chemistry of glass
formulations on the surface of the bed
material. At near the softening tempera-
ture, the particles within the fluidized bed
have a tacky surface and thus can stick
to other particles.
This is the proposed basis for capture of
trace metals in the FBC. First, particles of
the trace metal oxides are trapped by the
sticky surface of the bed particles, and the
trace metal oxides are incorporated into
the glass composition. In favorable
circumstances, the trace metals are
trapped as insoluble glasses, greatly
easing the problem of disposing of the
material.
Nearly all of the trace metals in th
form of their oxides react with th
sodium silicate to form glasses of low
water solubility than that of the sodiu
silicate glass.
Experience with particle agglomeratio
has shown that fine particles tend t
attach to the relatively large bed particle
in a FBC operating at an agglomeratin
temperature. The small particles coat th
large particles with a tacky surface. If th
temperature is too high or the surface to
tacky, a point will be reached where th
bed particles stick to each other an
defluidize the bed. An empirical correlatio
between bed temperature and superfici;
fluidizing velocity establishes a range c
stable fluidization.
Paint containing lead-chromate pigmer
was chosen as the surrogate hazardou
waste to be used in testing this captur
mechanism in the laboratory. Paint is
common material which typically contair
one or more metallic pigments and
solvent that is the fuel to be incinerate!
Experimental Test Facility
Figure 1 shows the FBC and auxiliar
equipment used during the testing phas
of the project. The FBC has insid
dimensions of 6 in. (0.15 m) diameter an
84 in. (2.13 m) overall height, and is line
with 2 in. (0.05 m) of castable refractor
Bed
Introduction
Exhaust
Gas
Burner
Distributor Plate
Tank
Feed Pump
Air
Figure 1. Six-in. fluidized-bed combustor.
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The unit is flanged near the top and
bottom: a number of nozzles in both the
sidewalls and the top are for bed
introduction, feed introduction, and
temperature and pressure indicators.
A natural gas burner in the FBC's bed
area heats the bed material prior to
introduction of the waste feed. The
fluidizing air, after passing through the
distributor plate and bed material, is
exhausted from the unit via a water-
cooled stainless steel, 6-in. diameter line
which discharges the dust-laden gases
into a cyclone dust collector. After the
cyclone, the gases pass through a high-
purity glass fiber filter and into the
building exhaust system.
The surrogate waste material was
pumped to a pneumatic feed tube in the
side of the combustor, 4 in. (0.1 m) above
the distributor plate, using a peristaltic
pump. The single-barrel (1 /4-in. diameter)
water-cooled feed tube allowed dispersion
of the feed by varying the amount of air to
the tube.
Test Results
Seven tests were made in the 6-in. FBC
while feeding surrogate hazardous
waste. Six of the tests used paint as the
source of trace metals, and one test was
with used motor oil. A preliminary test,
without trace metals in the fuel, was used
to check out the equipment and establish
baseline operating conditions before
testing the surrogate hazardous wastes.
The primary variables were temperature,
velocity, and the use of sodium carbonate
(Naz/COs) as a fluxing agent. The
combustion temperatures ranged from
1300 to 1600°F (700 to 850°C), and the
superficial velocity varied from 3.3 to 4.0
ft/sec (1.0 to 1.2 m/sec). During the
preliminary test, a shallow, 6-in. bed
depth was tried: considerable difficulty
was encountered in burning mineral
spirits. Therefore, a 12-in. deep bed was
used to get more complete combustion in
all the tests with surrogate hazardous
waste as the fuel. This amounted to 20 Ib
(9 kg) of minus 20 plus 100 mesh sand.
Approximately 90 percent of the
trace metals were captured on the bed
material in a fluidized-bed incinerator.
These tests established the operating
conditions and verified the capture
mechanism. A significant amount of the
trace metals were captured with the bed
material in one of the tests without the
fluxing agent. Thus the requirement for
the fluxing agent is less critical, depending
on the waste material and the operating
conditions. Adding a fluxing agent to the
system is more certain and controllable.
Further testing would maximize the
capture of trace metals and extend the
operation to more materials.
Design
The bench-scale FBC was designed to
burn hazardous wastes containing trace
metals. The system can burn either solid
or liquid waste materials. A fluxing agent
can be added to the fluidized bed to
improve capture of the trace metals on
the bed material. The FBC temperature
can be controlled by modulating the fuel
feed and fluidizing air rates. The 6-in.
diameter combustor is designed for a
heat release rate of about 32,000 Btu/hr
(34 MJ/hr), corresponding to a superficial
velocity of 3 ft/sec (0.9 m/sec) at 1400 °F
(760°C). The design is based on a 12-in.
(0.3-m) deep bed of silica sand, 20 x 100
mesh, for this fluidization condition.
Considerable flexibility is designed into
the system so that a wide range of
operating conditions are possible. Single-
stage operation is possible, using one
FBC. Two FBCs can be operated in series
to provide two separate operating stages.
This feature provides the ability to
operate with two different bed materials,
or fluxing agents, to capture distinct trace
metals in each bed. The FBC design is the
same for both stages. Figure 2 shows the
system with two stages in series.
The upper section of the combustor is
expanded to facilitate solids disengage-
Exhaust
ment from the flue gas. After exiting the
combustor, the flue gases are quenched
to 300°F (150°C) by spray injection of
cooling water. The gases then pass
through a cyclone and filter before being
exhausted from the system.
When operating two FBCs in series, the
gas condition into the second FBC affects
the distributor plate and bed behavior. For
this reason, a cyclone was specified in the
duct between the two units to minimize
plugging caused by dust from the first
stage FBC. Gas temperature into the
second stage FBC may need to be
controlled separately from bed tempera-
ture, but this depends on the conditions
being tested.
Conclusions and
Recommendations
A FBC can capture on the bed material
over 90 percent of the trace metals from
the incineration of a surrogate hazardous
waste. To maximize this trace metal
capture, operating conditions are set at
less than 1350°F (750°C), 4 ft/sec (1.2
m/sec) superficial velocity, and bee
material containing up to 0.5 percent
sodium carbonate (NaaCOa) as a fluxing
agent. These conditions are based on
limited testing to establish a design basis
for this project. More extensive testing
would optimize the system and to exteno
the operation to a wide range of wastes.
Combustor,
Refractory
Lined Exhaust
Combustor.
Refractory
Lined
td
Pump
Air
Figure 2. Six-in. fluidized-bed combustion facility (two-stages in series).
3
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A bench-scale FBC has been designed
to operate in either a single- or double-
stage mode so that testing can be
conducted. Such a facility, built tooperate
on a variety of hazardous wastes, could
be used to determine the trace metal
capture efficiency and operating conditions
for a variety of wastes. It would also be
possible to evaluate the capabilities of a
two-stage operation. Some wastes may
require higher operating temperatures
than those tested in this work to achieve
complete combustion; testing under a
variety of conditions would more fully
capitalize on this work.
R.D.LittandT.L Tewksburyare withBattelle-ColumbusLaboratories, Columbus,
OH 43201.
Robert E. Hall is the EPA Project Officer (see below).
The complete report, entitled "Trace Metal Retention When Firing Hazardous
Waste in a Fluidized-Bed Incinerator," (Order No. PB 85-138 618; Cost: $8.50,
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:
Air and Energy Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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