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Office of Solid Waste Management Programs
DEMONSTRATION OF PYROLYSIS AND
MATERIALS RECOVERY IN SAN DIEGO, CALIFORNIA
By Yvonne M. Garbe
Construction of a 200-ton-per-day solid waste dem-
onstration pyrolysis facility located in El Cajon,
California (San Diego County,) will be completed in
December, 1977 (photo A, aerial view.) The first of its
kind, the plant is designed to convert the organic frac-
tion of municipal solid waste into an oil-like liquid fuel
and to recover glass, ferrous and nonferrous metals
from the remaining waste. After mechanical comple-
tion, there will be three to four months of normal
shakedown before the plant begins full operation and
testing in May, 1977.
The $14-million project began in 1972 with a solid
waste demonstration grant—currently amounting to
$4-million—to San Diego County from the U.S. En-
vironmental Protection Agency (EPA.) The pyrolysis
and materials recovery processes being demon-
strated are designed by Occidental Research corpora-
tion. Occidental is contributing SB-million to the
County of San Diego for the project with an additional
$2-million being funded by the county.
A New Product from Solid Waste
Unlike other major pyrolysis systems, which pro-
duce low or medium BTU fuel gas, Occidental's pro-
cess produces a storable pyrolytic "oil" that most re-
sembles typical No. 6 oil. It is a chemically complex
organic fluid with a heat value equivalent to roughly
three-fourths that of No. 6 oil (114,900 BTU/gallon vs.
149,000 BTU/gallon for the average No. 6 fuel oil.) It
is lower in sulfur than even the best of residual oils.
Ms. Garbe has been working for two years in the
Resource Recovery Division, Of-
fice of Solid Waste Management,
U.S. Environmental Protection
Agency and is Project Agency
Engineer for the San Diego dem-
onstration. She earned an M.S.
^rom tne Civil and Environmental
Engineering Department, Univer-
sity of Cincinnati, in 1972. Stephen A. Lingle is Chief,
Technology and Markets Branch, and edits the "Up-
date" series.
The product is more viscous than typical residual oils,
and its fluidity increases with temperature. Hence, it
must be stored, pumped and atomized at somewhat
higher temperatures. Satisfactory atomization under
test conditions has been achieved with steam at 25
psi and 240°F, or about 20°F higher than the atomiza-
tion temperature for typical electric utility fuel oils.
San Diego Gas and Electric Company has been
contracted by the county to purchase and burn the
fuel in one of their existing oil-fired steam electric
power plants. After construction of additional storage
and fuel handling equipment at the utility, initial test
burns of the fuel are scheduled to begin in the fall of
1977.
New Processing and Handling Techniques
In addition to the pyrolysis process, there are sev-
eral new processing and storage techniques incorpo-
rated in the design of the system that are worthy of
attention. Successful demonstration of these items
would be significant in advancing the state-of-the-art
of resource recovery. These include:
• A new storage process for housing shredded
solid waste (photo B.) Primary shredding will
take place during one daily 8-hour shift. During
this shift, primary shredded waste will be con-
veyed to an enclosed storage area where it will
be automatically distributed uniformly onto a
concrete pad by a 360-degree rotating conveyor.
For 20 minutes of each hour during the dayshift,
the conveyor will be automatically positioned
over a receiving bin to allow the incoming
shredded waste to go directly to subsequent
processing steps. During the other 40 minutes,
the waste will be piled onto the pad for use dur-
ing the other two shifts. The pad will hold up to
400 tons, or roughly two days of solid waste
coming to the plant. A front-end loader will be
used to feed the waste to the primary shredder
during the day shift. During the next two shifts,
the loader will move the material from the pad
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into the bin feeding the rest of the plant.
• Doffing roll bins used to store and meter
shredded material (photo C.) These devices are
commonly used in the lumber industry to facili-
tate handling of wood chips. They have been
adopted in this process to act as a buffer stor-
age and to uniformly meter the flow of shredded
waste to both the air classifier and the pyrolysis
reactor. Waste enters through the top of the bin
and is moved along the floor from the back of
the bin towards the front by bottom drag con-
veyors. At the front, it is separated and metered
out of the bin by a series of rotating pick rolls.
• Zig zag air classifier. A new zig zag air clas-
sifier, designed and tested by Occidental, fea-
tures 10 separate bends in the column. Waste is
fed into the classifier at the seventh level from
the bottom. A unique feature of the classifier is
the recirculation of air in a closed loop.
• A comprehensive glass and nonferrous met-
als recovery subsystem (photo D.) In a trommel,
glass and aluminum-rich fractions will be sepa-
rated from air classifier heavies. A glass product
will then be recovered through froth flotation.
Aluminum will be recovered using electromagne-
tic separation.
Process Description
A simplified process flow diagram is shown in Fig-
ure No. 1. Municipal solid waste will be dropped onto
the tipping floor during the first shift of operation. A
front-end loader will push the material onto a con-
veyor for delivery to a horizontal shaft hammermill
driven by a 1000-horsepower electric motor. The
waste will be shredded to a nominal three-inch parti-
cle size (80 percent passing a three-inch screen open-
ing.) From the shredder, the material is conveyed
under an electromagnet to extract the ferrous metals.
The shredded waste will then drop onto a rotating
conveyor in the storage area; there, it will be either
stored for later use, or dropped into a receiving bin
and conveyed to the air classifier. The air classifier
will separate the heavier, mostly inorganic particles,
from the lighter, organic material. About 75 percent of
the air classifier input is expected to be separated as
A-Aerial view of San Diego County's 200 TPD demonstra-
tion pyrolysis facility at El Cajon, California.
B-EI Cajon's storage facility for shredded solid waste.
light fraction.
The light fraction will be dried to a moisture content
of three percent using hot air produced from burning
either combustible gas produced in the pyrolysis reac-
tor or fuel oil. Fuel oil is used for start-up and/or
emergency situations.
After drying, this fraction will be classified fur-
ther, using a series of mechanical processes. A 14
mesh-screen will be used to remove larger particles
for secondary shredding in an attrition mill. In this mill,
waste fed between two disks is ground into extremely
fine particles having a nominal size of minus 14 mesh.
(That"is,"80 percent of the particles could pass
through a screen having 14 openings to the inch.)
Meanwhile, the particles that fall through the 14
mesh-screen will be fed onto an air table where a
combination of vibrating motion and air flow will sepa-
rate the small, light organic particles from dense metal
and glass particles. The light particles from the air
table will be combined with the secondary shredder
output to form the organic-feed-stock, which will be
stored in a second doffing roll bin until it is fed into the
pyrolysis reactor. About 60 percent of the waste input
to the plant is expected to become reactor feedstock.
The pyroiysis reactor is a vertical pipe through
which the organic feedstock is pneumatically trans-
ported using oxygen-free recycled gas from the
pyrolysis reaction. In the reactor, hot particles of char
provide the heat needed to pyrolyze the organic mate-
rial. The char, which is the solid residue remaining
after the pyrolysis reaction, enters the reactor after
having been heated and is mixed turbulently with the
organic material. The pyrolysis reaction takes place in
less than a second as the char-waste mixture pro-
ceeds through the reactor.
A series of cyclones will be used to remove the char
from the reactor gases. After the char is removed, it
will be cooled quickly in an oil decanter to condense
the pyrolytic oil. The remaining gas stream will go
through a series of cleanup steps and be compressed
for plant use. Part of the gas will be used as the
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C-Doffing roll bins used at El Cajon to store and meter
shredded waste.
oxygen-free transport medium. The rest of the gas will
be burned to preheat combustion air for the char
heater, to preheat the reactor transport gas, to supply
heat to the dryer and to oxidize dirty gas streams that
are produced in various parts of the system. The
liquid fuel product will go directly to the oil storage
tanks.
The remainder of the system consists of the glass
and aluminum recovery processes. In the first step of
this process, the heavy fraction from the air classifier
will be passed through a trommel for separation into
three fractions: smaller than 1/2-inch, which becomes
the feed to the glass recovery system; 1/2-inch to 4
inches, which goes to aluminum recovery; and greater
than 4 inches, which is returned to the primary shred-
der. Dense fines recovered from the air table will be
combined with the glass fraction from the trommel.
After crushing to between 20 and 200 mesh, the
glass will be sent to a series of froth flotation cells. In
froth flotation, air is bubbled through a solution of
water and special chemicals which contain the glass.
The chemicals cause the glass particles to adhere to
the bubbles and float off, while impurities sink. By
recirculating both the float and sink fractions several
times, the system is designed to recover 90 percent of
the glass in the glass recovery subsystem feedstock
at a concentration of better than 99.5 percent. The
total recovery of glass from the incoming solid waste,
however, will be no greater than 75 percent, because
there are losses in various parts of the process before
the feedstock is formed. Also, some of the product
glass is too fine to use in a glass container furnace
and must be discarded.
The aluminum recovery system is based on eddy
current separation by linear induction motors. The
1/2-inch to 4-inch fraction from the trommel screen will
pass on a belt over a pair of linear motors. Electrical
current through the motors generates a traveling wave
magnetic field above the belt. The magnetic field
causes conductive materials in the refuse fraction to
be deflected off the side of the belt to a collection bin.
Since ferrous metals have been separated previously
and nonferrous metal in municipal waste consists
primarily of aluminum, the recovered product is ex-
pected to be 85 to 90 percent aluminum. It contains
about 60 percent of the aluminum in the as-received
waste. However, the demonstration will attempt to de-
termine if an aluminum product with both suitable
yield and purity can be simultaneously achieved.
An abbreviated materials balance for the entire pro-
cess is presented in Table No. 1.
Testing and Evaluations
Following plant start-up in May, 1977, a rigorous
evaluation will begin. A third-party engineering firm,
working under contract to EPA, will conduct a com-
prehensive assessment of the environmental, techni-
cal and economic performance of the pyrolysis sys-
tem. In addition, a series of test burns will be con-
ducted at San Diego Gas and Electric's boiler using
varying loads of pyrolysis oil product. The stack emis-
sions will be monitored by the local Air Pollution Con-
trol District and EPA.
Other measures to control and test for effects on air
quality include continuous monitoring of the ambient
air quality in the vicinity of the plant. Due to uncon-
trollable meteorlogical conditions that prevail during
certain seasons of the year, situations of air stagna-
tion occur in the general area where the plant is lo-
cated. This allows NOX accumulations from all
sources to build up which exceed the ambient air
standards. In the event this happens, all NOX stack
emitting industries will have to shut down temporarily
D-EI Cajon's comprehensive glass and nonferrous metals
recovery subsystem.
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until the situation clears. Three air-monitoring units
have been purchased by the county, and stationed
strategically near the plant-site to monitor for these
conditions. It is unlikely this precaution would have
been necessary had the plant been sited elsewhere in
the county.
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Economic feasibility for constructing and operating
larger scales of this system is a major part of the
evaluation to be conducted during the demonstration
year. Like most small-scale demonstration plants, the
El Cajon facility (200-tons-per-day) is not expected to
be economically competitive. It will serve mainly to
test the application of this pyrolysis and materials re-
covery process in solid waste management, and will
define the expected economics of larger systems.
REFERENCES
1) Levy, S. J. San Diego County demonstrates pyrolysis of
solid waste to recover liquid fuel, metals, and glass. En-
vironmental Protection Publication SW-80d.2. Wash-
ington, U.S. Government Printing Office, 1975, 27 pp.
2) Preston, G. T. Resource recovery and flash pyrolysis of
municipal refuse. In Clean Fuels from Biomass, Sewage,
Urban, Refuse and agricultural Wastes Symposium, Or-
lando, Florida, Jan. 27-30, 1976. Chicago, Institute of
Gas Technology, pp. 89-114. •
Reprinted from Waste Age - December 1976
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