Energy Recovery from Waste
-------
COVER PHOTOGRAPH: The flame pattern, viewed from the top of a
tangentially-fired boiler, shows how solid waste fuel and coal are fired from the
corners of the boiler. (Photograph courtesy of Combustion Engineering, Inc.)
-------
Energy Recovery from Waste
SOLID WASTE AS SUPPLEMENTARY FUEL
IN POWER PLANT BOILERS
This second interim repdrt (SW-36d.ii) on work performed
under Federal solid waste management demonstration grant No. S-802255
to the City of St. Louis,
was written by ROBERTA. LOWE
U.S. ENVIRONMENTAL PROTECTION AGENCY
1973
-------
Acknowledgments
The contributions of the following are gratefully acknowledged: Horner &
Shifrin, Inc., Consulting Engineers, St. Louis, for the sections on engineering
and economics. James D. Kilgroe, National Environmental Research Center,
U.S. Environmental Protection Agency, for the section on air pollution.
Donaldson, Lufkin & Jenrette Securities Corporation, Investment Bankers,
New York, for the section on financing alternatives. E. J. Ostrowski, National
Steel Corporation, Weirton, West Virginia, for the section on magnetic metals.
The success of this project is the result of the progressive and cooperative
spirit of the participating organizations and their principal representatives: G.
Wayne Sutterfield, City of St. Louis, Missouri; Earl K. Dille, Charles J.
Dougherty, and David L. Klumb, Union Electric Company; Dr. Donald P.
Cairns and H. M. Love, Granite City Steel Company; F. E. Wisely, Horner &
Shifrin, Inc.
An environmental protection publication (SW-36d.ii) in the solid waste
management series / 2d printing
Mention of commercial products does not constitute endorsement by the U.S.
Government
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, D.C. 20402 - Price 40 cents
-------
FOREWORD
GROWING CONCERN for the environment has changed our
thinking about solid waste. Although disguised as a nuisance, solid
waste can be an environmental asset. It contains a wealth of
recyclable materials—paper, cardboard, metals, and glass—and offers
the potential for conserving a seriously diminishing resource—fuel.
In this period of concern about shortages of energy and material
resources, the mere existence of untapped resources commands our
attention. Recycling and reuse of waste materials makes good sense
environmentally and economically. Information is emerging to
show that recovering and reusing our resources is sound practice for
more reasons than appear on the surface. When two production
systems are compared, one using virgin materials, the other secondary
or waste materials, the system using wastes almost always causes
less air and water pollution, generates less solid wastes, and consumes
less energy. This is true if the environmental impacts of all activities
in a system are measured—mining, processing, fabrication, manu-
facturing, and the transportation and disposal steps in between.
The Nation's task, then, is to organize our systems and
institutions so that the economy can begin to receive the benefits
and reflect the savings from using more secondary materials. One
way to help accomplish this is through new technology. But
technological advances are usually expensive, are relatively untried,
and therefore entail some risk. The Resource Recovery Act of 1970
enabled the Federal solid waste management program to assist
States and municipalities by assuming part of the risk of trying new
technologies. The result was a significant expansion of the Federal
resource recovery demonstration program. This report describes one
part of that program: the recovery of energy by burning shredded
residential solid waste as a supplementary fuel in power plant boiler
furnaces.
This process was initially studied in 1968 by the City of St.
Louis and the Union Electric Company, with financial support from
the Federal solid waste management program. The results of the
study were encouraging. In 1970, the demonstration was initiated
when the U.S. Environmental Protection Agency's Office of Solid
Waste Management Programs and Office of Air Programs jointly
111
-------
awarded a grant to the City of St. Louis for two-thirds of the cost
of the project.
Operations began in April 1972 and continued intermittently
until May 1973, when construction of the air classifier began.
Operations then ceased while the air classifier was being installed
and resumed in November 1973.
Much has been learned since our first report on this project was
published in April 1972.* The present report is a second interim
summary and includes discussion of the current technical, market-
ing, and economic aspects of the solid-waste-as-fuel concept.
Thorough testing of air emissions and processing practices were
scheduled to begin in late 1973. A third report on the project will
be published as soon as test results are known.
While there is still much to be learned, it is already apparent
that this demonstration represents a practical step in the right
direction and exemplifies the kinds of creative solutions that
government at all levels, industry, and the public must pursue to
bring our environmental and resource conservation problems under
control.
— ARSEN J. DARNAY
Acting Deputy Assistant Administrator
U.S. Environmental Protection Agency
Office of Solid Waste Management Programs
* Horner & Shifrin, Inc. Energy recovery from waste. Washington, U S
Government Printing Office, 1972. 15 p.
IV
-------
Energy Recovery from Waste
SOLID WASTE AS SUPPLEMENTARY FUEL
IN POWER PLANT BOILERS
CONVERTING MUNICIPAL SOLID WASTE into energy is a solid
waste management option that has recently become attractive, both
environmentally and economically. Although a number of Euro-
pean countries have been generating electricity from municipal solid
waste for years, in the United States recovery of heat from
municipal solid waste has been limited. Until recently, it consisted
of relatively inefficient waste-heat boilers installed in conventional
incinerators. In the past five years, however, more sophisticated
solid waste incinerators have been built, which incorporate boilers
for the recovery of steam.
But these newer facilities, known as waterwall incinerators, have
several important limitations. First, reliable markets for steam are
not always readily available. Secondly, new facilities are relatively
expensive both in capital cost and operating cost. Third, their
relative reliability has not always been acceptable.
By burning the solid waste in a utility power plant, the process
can take advantage of an established system for producing,
distributing, and marketing electricity through use of an existing
boiler, or a new power-producing unit designed for this purpose.
Thus, the energy recovered from solid waste can have an assured
market.
Although coal-fired power plant boilers are not without
operating problems of their own, a comprehensive study by Horner
& Sriifrin, Inc., with the close cooperation of the Union Electric
Company and Combustion Engineering, Inc., concluded that such
problems would not be significantly increased, if increased at all, by
burning prepared solid waste as supplementary fuel.1 The concept
'Horner & Shifrin, Inc. Solid waste as fuel for power plants. U.S.
Environmental Protection Agency, 1973. 146 p. (Distributed by National
Technical Information Service, Springfield, Va., as PB-220-316.)
-------
was considered attractive enough for the City of St. Louis, Missouri,
and the Union Electric Company to undertake an innovative joint
venture. With financial support from the U.S. Environmental
Protection Agency, the project began to operate in April 1972.
The Process
Perhaps the most striking aspect of the process is its simplicity.
Domestic solid waste, collected from residential areas of the City of
St. Louis, is ground up in a large hammermill. The shredded wastes
are air-classified and the light combustible waste fraction is fired
pneumatically into existing boilers in the Union Electric Company
system. Magnetic metals are recovered from the heavier, mostly
noncombustible, fraction. The remaining glass, ceramics, and other
nonmagnetic materials are landfilled. All of this can be achieved by
applying existing technology with equipment that already is
commercially available.
Potential Benefits
This waste disposal system promises to be attractive, both
economically and environmentally. The value of the fuel produced,
together with revenues from the ferrous metals and other materials
that may be recovered from the waste for sales, reduce the cost of
disposal. At the same time, energy and materials are conserved, air
and water pollution are decreased, and land required for waste
disposal is reduced by about 95 percent.
Present Status
The results of this experimental project to date are encouraging.
As evidence of this, the Union Electric Company is considering the
adaptation of additional boilers to handle more solid waste fuel.
At the present time, four aspects of the concept must be
evaluated before the experiment can be proclaimed a complete
success. These are: (1) the quality of air emissions from the boiler
stacks, (2) the performance of the air classifier, (3) the long-term
effect on boiler operations and feed mechanisms, (4) the economics
of the process. Air emission tests will be conducted in the fall of
1973, after the air classifier has been installed. The air classifier is
expected to reduce significantly the two major operating problems
encountered so far: blockages in the feeders that inject the solid
waste fuel into the pressurized pneumatic pipeline system; and
excessive internal wear and tear on those pipelines that feed the
solid waste into the boilers.
In general, although the operating results must at this time be
regarded as preliminary, the project is operating essentially as
-------
predicted. A one-year comprehensive evaluation of the system
operating at full capacity was scheduled to begin in late 1973.
THE PROCESSING SYSTEM AND ITS OPERATION
Type of Waste Processed
The only type of solid waste currently accepted at the St. Louis
processing facility is from residential sources. Certain selected
commercial and industrial wastes may be accepted later. The system
was designed to exclude oversized bulky wastes, such as tires,
appliances, furniture, engine blocks, and land-clearing and demoli-
tion wastes. This limitation is a function of the capacity of the
shredders and the fuel quality objective. Operating personnel have
reported, however, that occasionally tires and even mattresses have
been processed without problems. In other circumstances the
system can be designed to accept certain bulky wastes (see
Economics).
Capacity
The processing system was designed to handle 325 tons per
8-hour shift, with a maximum practical throughput of about 650
tons per day. Because of the one-stage shredding operation (see
Hammermill and Particle Size, below), hammer retipping is required
almost daily. This maintenance requires nearly a full 8-hour shift to
complete. Two-stage shredding may permit less frequent scheduling
of hammer retipping. The hammermill, air classifier, and conveyors
were selected to provide a nominal production rate of 45 tons of
raw solid waste per hour.
Redundancy
Because a community will generate waste whether its resource
recovery system is operating or not, a standby disposal method
must be provided. Extra storage space can accommodate waste
during relatively short periods of downtime. For more extended
periods, the waste can be diverted to a standby processing line,
incinerator, or sanitary landfill. The choice will depend upon the
economics of the alternatives and the availability of an incinerator
or landfill.
The St. Louis plant consists of only one processing line, with
the City's incinerator available as a backup method. The existing
incinerator was selected as the standby method instead of an
auxiliary processing line because of the desire to minimize capital
-------
costs in this experimental project. At the power plant, however, to
assure standby capacity, two boilers have been modified so that one
will be available at all times.
Receiving Area
Raw solid waste is discharged from packer-type collection
trucks onto the floor of the receiving building (Figure 1). Front-end
loaders are used to push the solid waste to a receiving belt
conveyor. This method of handling the waste was selected over the
pit and crane method because it would be more economical and
would enable the operator to remove unwanted materials. This
method also permits greater and more uniform production rates.
From the receiving conveyor, the raw solid waste is transferred to
the hammermill.
Hammermill and Particle Size
Residential solid waste, in its raw state, is remarkably heteroge-
neous. After shredding, however, the solid waste becomes more
homogeneous. Shredded waste is generally easier to separate into
salable components than is raw solid waste. Shredding also reduces
odors and makes the waste easier to handle.
In the St. Louis shredder, 30 large metal hammers swing around
a horizontal shaft, grinding the solid waste against an iron grate
until the material is shredded into particles small enough to drop
through the grate openings. This model was selected on the basis of
three operating parameters: the heterogeneous nature of the waste
stream, the production rate required, and the desired control over
the particle size.
The design called for a nominal particle size of 1l/z inches.
Preliminary data show that over 90 percent by weight of the
incoming waste is reduced to particles not greater than one inch in
any dimension. The optimum particle size has not been determined.
Tests scheduled for the fall of 1973 will attempt to determine the
particle size that will provide the best shredding economics,
materials handling, combustibility, and air emissions.
Shredding in one step to a particle size as small as IVfc inches
causes severe wear on the shredder's hammers, requiring mainte-
nance almost daily with the throughput planned by St. Louis.
Single-stage milling (all shredding in one pass through the
shredder) was selected for the prototype system to minimize capital
costs. For future applications, however, its designers, Horner &
Shifrin, Inc., and other experts recommend a two-stage shredding
operation, with air classification between the two shredding steps.
The first shredding would reduce the waste to a particle size of
-------
about 4 to 6 or 8 inches. After removal of the heavier materials by
the air classifier, the second shredding would reduce particle size of
the light fraction to 1 or 2 inches. Hammer wear should be reduced
considerably.
Air Classifier
From the hammermill, the shredded waste is conveyed to the
air classifier. The air classifier separates the heavier, mostly
noncombustible particles from the lighter ones. The shredded waste
is dropped into a vertical chute. A column of air blowing upward
from the bottom of the chute catches the lighter materials, causing
them to fly to the top. The heavier materials drop to the bottom.
By varying the air velocity and the cross-sectional area of the chute,
the percentage split between heavy and light fractions can be
controlled. The St. Louis air classifier is designed to permit 75 to 80
percent of the shredded waste to be separated into the light group
for use as fuel.
The light fraction is expected to be composed of paper, light
cardboard and plastics, textiles and light food wastes, and other
organics, all of which are combustible, plus a small percentage of
light noncombustibles like aluminum foil. It also will contain small
particles of heavier materials such as pulverized glass that stick to
pieces of organic materials.
The heavy fraction is expected to contain ferrous and nonfer-
rous metals, glass, dirt, and other noncombustibles. Certain heavier
combustible materials, such as grapefruit rinds and heavier pieces of
cardboard, plastics, woodchips, and rubber, will also drop into the
heavy group.
By removing the heavier materials—both combustible and
noncombustible—from the fuel, three benefits should result: an
increase in the heating value of the fuel, an increase in the
transportability of the fuel through the pneumatic pipelines, and an
increase in the suitability of the boiler's bottom ash for reuse. And
the ash content of the waste fuel should decrease. The presence of
the small bits of glass and other materials remaining in the fuel is
not expected to have a significant effect on the suitability of the
waste as a fuel.
The light materials are carried pneumatically from the separa-
tion chute to the cyclone separator, where they are removed from
the air stream and allowed to fall onto the conveyor leading to the
storage bin.
Storage and Transportation
At scheduled intervals, quantities of the solid waste fuel are
removed from the storage bin and loaded onto trailer trucks for the
-------
18-mile trip to the power plant. Two trailer trucks, each with a
capacity of 20 to 25 tons, deliver fuel to the power plant around
the clock five days per week. The trucks are loaded by a ram-type
stationary packer and are unloaded by a ram located within the
trailer of the truck. Both the loading and unloading operations are
controlled by the truck driver.
Ferrous Metal Recovery System
The heavy fraction is processed to recover ferrous (magnetic)
metals. The entire heavy fraction is passed under a magnetic belt.
The nonmagnetic materials are hauled away to be landfilled. They
can be further separated for resale when technology and economics
permit. The ferrous metals are then densified in an Eidal nuggetizer
or densifier (Figure 1). After passing under a magnetic drum for a
final cleanup, the ferrous metals are transported to the Granite City
Steel Company, Granite City, Illinois, in trucks owned and operated
by the City.
The densifier and magnetic drum were added in the summer of
1973 to meet the market specifications of the steel industry (see
Markets). Before this equipment was added to the system, the
magnetic metals removed from the waste stream were not market-
able because of low density and impurities.
About 7 percent of the St. Louis waste stream is ferrous metal.
By removing fuel and ferrous metal, the City of St. Louis has
reduced its landfill volume requirements by 95 percent of the solid
waste processed.
Unloading and Transfer at the Power Plant
The trailer trucks unload the fuel into a receiving bin, which is
unloaded continuously into a pneumatic pipeline transport system
(Figure 2). This part of the operation is owned and operated by the
City.
Surge Bin and Firing System
The City's responsibility ends at the point where the City's
pneumatic pipeline discharges the fuel into the utility's surge bin.
The surge bin serves to smooth and distribute the flow of the fuel
from each batch-type delivery into four continuously fed pipelines
leading to the boilers.
The surge bin uses four drag-chain unloading conveyors to move
the solid waste fuel to four separate feeders that introduce the
supplementary fuel into the pneumatic pipeline system. The
pipelines, each about 700 feet long, blow the fuel to firing ports in
each corner of the boiler furnace.
Prior to air classification, larger and heavier particles have
caused feeder blockages, and the glass caused serious wear to the
pipelines, especially at the elbows. These conditions have required
-------
excessive maintenance and an operator to monitor the system. If
the air classifier performs as expected, the system can be operated
on an unattended basis with routine maintenance only.
BOILER MODIFICATION AND OPERATING EXPERIENCE
Boiler Modification
Two identical boilers (Units 1 and 2) at Union Electric
Company's Meramec Plant near St. Louis have been modified to
burn prepared solid waste. They are 125-megawatt tangentially
suspension-fired boilers that were designed to burn pulverized coal
or gas. There are now four coal-firing, one solid-waste-firing, and
five gas-firing ports in each corner of each boiler.
Other than installing a solid-waste-burning port in each corner
of the furnace, no modifications to the boilers were made. The
refuse-burning ports were installed between the two middle coal
burners. No alterations to the pressure parts of the boilers were
necessary. (Pressure parts are the water/steam pipes that line the
inside of the boiler walls.) The prepared solid waste is burned in
suspension, in the same flame pattern as the pulverized coal or gas.
As is typical of large utility boilers, the furnaces have no grates.
Fuels are burned in suspension at temperatures of 2,400 F to 2,600
F. The retention .time of 1 to 2 seconds is not long enough for the
heavier particles of combustible materials to be consumed, and they
fall to the bottom ash hopper along with the noncombustible
materials. Removal of heavier combustibles and noncombustibles
by air classification is expected to result in more efficient
combustion of the solid waste fuel.
The two boilers are 20 years old and are small compared to
newer units in the Union Electric Company system. They are of
modern reheat design, however, and burn 56.5 tons of Illinois coal
per hour at rated load.
At rated load, the quantity of solid waste for each boiler,
equivalent in heating value to 10 percent of the coal,is about 12.5
tons per hour, or 300 tons per 24-hour day. Solid waste will be
fired 24 hours per day, but only five days per week, since City
residential solid waste collections are scheduled on a five-day-per-
week basis.
Boiler Operations
The boiler operators and shift superintendents report that solid
waste firing has had no discernible effect on the boiler furnace or
convection passes. (Convection passes are hot gas passages contain-
ing heat-transfer surfaces between the boiler furnace and the air
-------
pollution control equipment.) Frequent and sudden interruptions
of the solid waste feed have not required any change in operating
techniques. Existing boiler combustion controls easily accommo-
date the variations in solid waste quantity and quality by varying
the amount of pulverized coal fired to the boiler.
The boiler's efficiency or power-producing capability when
firing solid waste in combination with coal is essentially identical to
the "coal only" performance.
Ash Content. The ash content (residue after burning) of raw
refuse, including metallics, is about 25 to 30 percent. Without
magnetic metals, the ash content is in the 20 to 25 percent range.
Removal of the heavier nonbumable particles should reduce the ash
content further, possibly to the 10 to 15 percent range. The ash
content of Illinois bituminous coal, by comparison, is about 10
percent.
Slagging. There has been no indication to date from the St.
Louis experience that solid waste fuel has any greater tendency to
form slag (deposits of melted material) than does Illinois bitumi-
nous coal. The ash fusion (melting) temperatures of solid waste
apparently are similar to those of Illinois bituminous coal. Utility
personnel have voiced the opinion that the furnace appears to be
cleaner when solid waste is fired in combination with coal than
when coal is fired alone. Although the reason for this is not clear, it
is known that paper forms a nonslagging dry ash and that glass and
metals fall into the bottom ash hopper before the heat can affect
them.
Carryover. There has been no evidence to date of any
unburned materials being carried into the back passages of the
boiler by the gas stream.
Corrosion Potential. As part of the testing program of the St.
Louis project, probes have been inserted in the boiler to determine
whether corrosion potential is any greater when solid waste is fired
in combination with coal than when coal is fired alone. The results
of these investigations are not yet available.
Odors. The shredding process homogenizes the wastes and
tends to disperse the odor-producing materials to a sufficient degree
to make odors far less noticeable than from unshredded waste. It
appears that no further treatment such as deodorizing will be
necessary.
AIR POLLUTION CONSIDERATIONS
Utility boiler air pollutants cause justifiable concern because of
their potential health effects. The most significant pollutants are
-------
sulfur dioxide (SO2), nitrogen oxides (NOX), and particulate
matter. The use of solid waste as a supplementary fuel in Union
Electric's coal-fired utility boiler will probably result in a reduc-
tion in SO2 and NOX air pollution emissions. It is believed that
particulate emissions will be essentially unchanged. In-depth air
pollution tests are planned to verify these expectations.
Sulfur Dioxide
Oxidization (combustion) of the sulfur contained in fuels such
as coal and solid waste result in sulfur dioxide, a pollutant that is
emitted from the boiler as a gas. The low-sulfur coal currently used
in the Meramec boilers has a sulfur content of approximately 1.12
Ib per million Btu of fuel value. By contrast the air-classified solid
waste will probably have a sulfur content of approximately 0.5 Ib
per million Btu of fuel value. Using solid waste to provide 20
percent of the boiler heat input would result in an average sulfur
content of 0.996 Ib per million Btu. This sulfur content represents
the maximum potential sulfur emissions that could be expected;
actual emission levels are normally less because some sulfur remains
in the boiler bottom ash or is collected as fly ash. Even the
maximum potential sulfur emissions are significantly less than
Federal and local standards (1.2 Ib per million Btu and 2.3 Ib per
million Btu, respectively).
Nitrogen Oxides
Nitrogen oxides emitted in the boiler flue gases result from
oxidization of nitrogen present in the air needed for combustion
and, to a lesser extent, in the fuel. The quantity of nitrogen oxides
formed generally increases as the temperature of combustion
increases. Since the combustion temperature of solid waste is lower
than that of coal, the nitrogen oxides resulting from burning solid
waste in combination with coal should be less than when coal is
burned alone. During the air pollution tests, measurements will be
made to determine the amount of nitrogen oxides resulting from
various ratios of solid waste to coal.
Particulate Matter
Particulate matter formed during the combustion process is
carried out of the boiler by hot gases. Before leaving the 250-foot
boiler stack, the gases pass through the electrostatic precipitator
(ESP). Particulate emissions are controlled by using an ESP. In the
ESP, the particles are charged by an electric field and collected on
large electrically charged metal plates called electrodes. Periodically
the accumulated dust, or fly ash, is knocked free from the
-------
AIR CLASSIF
Cyclone sepa
RAW REFUSE DELIVERY
Ferrous metals hauled to steel mill
FERROUS METAL RECOVERY SYS7
Figure 1. The fuel preparation and resource recovery system
electrodes and settles into hoppers in the bottom of the ESP. At
scheduled intervals the collected fly ash is pneumatically removed
from the hoppers and sold by the utility to the cement industry.
Based upon tests on incinerators and utility boilers, it is
expected that the quantity of particles emitted from the boiler
stack will probably remain essentially unchanged by using solid
waste as a supplementary fuel.
New Source Performance Standards
To guard against possible health hazards from air pollution
emission, Federal and local air pollution control agencies have
established regulations to limit utility boiler emissions of SO2,
10
-------
STORAGE AND TRANSPORTATION
Stationary packer
ry power
plant
Nonmagnetic metals, glass, and waste
to further separation or to landfill
ceives raw solid waste and produces fuel and ferrous metal.
NOX, and participate matter. Federal regulations, called New
Source Performance Standards (NSPS) and written by the U.S.
Environmental Protection Agency, apply to new utility boilers and
older boilers modified in certain ways after the NSPS went into
effect. Boilers that were already in existence when the NSPS went
into effect are subject to local standards. These local standards are
generally based upon emission levels needed to meet Federal
ambient air quality standards and are often less strict than the
NSPS. NSPS require the best demonstrated commercial control
technology for new or modified boilers, while allowing local
standards to determine the best mechanism to control emissions
from older boilers to safeguard the public health, without the large
11
-------
UNLOADING OPERATION
Receiving bin
FIR ING SYSTEM
Tangentially-fired boiler
Figure 2. The solid waste fuel is delivered to the power plant and is fired
pneumatically into the boiler.
economic costs associated with installing new pollution control
equipment on all existing air pollution sources.
Some utilities have expressed concern that, by modifying their
existing boilers to burn solid waste, they would become subject to
Federal NSPS, thereby requiring the installation of costly new
pollution control equipment. EPA has stated, however, that the
NSPS were never intended to apply to retrofitting a utility boiler to
burn solid waste as a supplemental fuel. Thus, retrofitted existing
boilers would continue to be subject to State and local emission
standards. Indeed, if the scheduled air pollution tests confirm
preliminary conclusions, using solid waste as a supplementary fuel
may enable some utility boilers to meet SO2 and NOX standards
without having to install expensive air pollution control equipment.
MARKETS
The St. Louis demonstration resource recovery project recovers
two products—fuel and ferrous metals. In addition, the fly ash and
bottom ash, which historically have been sold by the utility, are
expected to continue to have market potential.
No other products are being recovered. There are several reasons
for this. First, recovery of other materials is beyond the original
scope of the project, which was limited to demonstrating that solid
waste could be prepared and fired as a supplementary fuel. (The
12
-------
separation of ferrous metals was originally intended only to
improve the quality of the fuel.) Second, the quantity of
nonferrous metals in St. Louis solid waste has not yet been
determined. Third, the value of the glass fraction is limited by its
particle size. Shredding the solid waste in a horizontal hammermill
crushes the glass into particles too small for color sorting, a
procedure that potentially could increase the market value of the
glass. In other situations, however, depending upon the composition
of the waste and the processing procedure, the recovery of
aluminum, glass, and other materials might be profitable.
Fuel
Heating Value. The heating value of refuse is somewhat
variable, depending mainly upon its moisture content. A generally
accepted average value is 5,000 Btu per pound of the light fraction
of air-classified solid waste as fired (10 million Btu per ton).
Composition. Investigations of the quality of residential solid
waste produced in the St. Louis area, although limited in scope,
disclosed characteristics similar to those found in other parts of the
country. A comparison of the ranges of composition of solid waste
and coal for the St. Louis area was made (Table 1).
Market Value. The value of the solid waste fuel varies with the
value of the fuel that it replaces (coal or oil), with the costs of
modifying the boiler, and with the costs of firing the solid waste
fuel.
Fuel costs vary considerably, from a high of $.75 to $.90 per
million Btu for low-sulfur fuel along the eastern seaboard, to a low
of $.20 to $.25 per million Btu for high-sulfur coal in other regions.
Table 1
COMPOSITION OF RESIDENTIAL SOLID WASTE AND COAL SAMPLES
BY WEIGHT AND BY HEATING VALUE*
Heating value (Ibs per
Percent of sample by weight million Btu)
Solid waste Coal Solid waste Coal
Sulfur
Ash
Chlorine
Moisture
0.1
20.0
0.3
30.0
3—4
10—11
0.03—0.05
6—10
0.2
43.0
0.6
64.0
2.6
9.0
0.03
7.0
Btu per pound Solid waste: 4,675 ' Coal: 11,300 to 11,900
*Solid waste is from 210 samples of St. Louis residential waste, taken
April 1972 through February 1973, as received, with magnetic metals removed.
Coal is from three samples of Union Electric Company coals. No analysis has
been made of air-classified solid waste.
13
-------
Thus, the gross value of solid waste fuel at 10 million Btu per ton
could be as high as $9 per ton in some areas and as low as $2 per
ton in others. From this gross value the utility's incremental costs
associated with firing the solid waste fuel must be deducted (see
Economics). The resulting net value of the fuel can represent a
significant economic benefit to a community. And because fossil
fuel costs are increasing rapidly, the value of the solid waste fuel
will increase accordingly.
Mutual Benefit. The net economic value of the fuel is not
necessarily the price that the utility would be expected to pay for
the fuel. The primary reason for this is that the recovery of fuel
from solid waste potentially creates mutual benefits for both the
community and the utility. Some benefits may be expressed in
dollars; others may not. For example, the community can benefit
from lower disposal costs, less air pollution, longer landfill life, and a
possible alternative to unacceptable land disposal practices. At the
same time, the utility can benefit from lower fuel costs, a reliable
source of low sulfur fuel, and an opportunity to provide a
community service.
In actual practice, then, the value of the solid waste fuel is
established according to how the community and the utility
perceive the possible benefits. Any price associated with the solid
waste fuel must be negotiated.
Potential Markets. It has been demonstrated that solid waste
can be burned as a supplement to coal in power plant boilers. But
the concept appears to have even wider possibilities. Many utility
personnel and boiler manufacturers believe that solid waste can be
burned with oil or gas as well as coal. And electric power plants are
not the only potential users. Markets for solid waste fuel can also be
found in private industrial plants, where boilers burn fossil fuels for
the on-site generation of steam for processing, heating, air con-
ditioning, and power.
Magnetic Metals
The National Steel Corporation evaluated the magnetic metals
recovered from the St. Louis waste stream to determine the
suitability of that metal for use as scrap in steelmaking. The
evaluation report, prepared by E. J. Ostrowski of National Steel
Corporation's Research and Development Department, prescribed
how the magnetic materials should be processed and recommended
the material for use "both in the blast furnace and in the basic
oxygen furnace to establish its use potential within the limits
calculated from results obtained in the evaluation."
"The evaluation showed that a described bulk density of
approximately 75 Ib per cu ft can be obtained at a ring setting of
14
-------
one inch on the Eidal Mill. Magnetic separation of the scrap after
the mill improves its chemical quality by removing some of the
combustibles and some aluminum. The density and cleanliness of
the scrap affects the yield obtained during melting as well as the
water-absorbing potential of the scrap. The higher density material
possesses better yields and retains less water." The average yield at
the one-inch ring setting was 94.6 percent.
"The scrap residuals (contaminants) are primarily tin, alumi-
num, and lead. Carbon levels are high in the melts due to low
oxygen levels of the 100-percent scrap melts. The combustibles
present form carbon which is absorbed by the metal. The copper
level of the scrap is low because it has received no thermal
treatment prior to magnetic separation. Tests conducted in the
electric arc furnace on melts in excess of 800 pounds showed the
residuals to be slightly below the mean value of the levels obtained
on the small induction furnace melts. Use of the mean value to
calculate use limits should provide a margin of safety for melting
steel grade specifications."2
Specifications. The following specifications were recom-
mended by National Steel Corporation: (1) The scrap is to be
processed in an Eidal Mill or equal at a ring setting of 1 inch
followed by magnetic separation; (2) the product is to have a bulk
density of 75 Ibs per cu ft regardless of the ring setting noted above;
(3) the product is to be free flowing, free of greases, oils, paints,
and water; (4) scrap from no other source is to be added to the
stream. This will avoid changes in chemical composition which
could be detrimental to the steelmaking operations.
Contract. The City of St. Louis and the Granite City Steel
Company, a subsidiary of National Steel Corporation, signed an
agreement on May 1, 1973, for the sale of 3,750 gross tons of
prepared ferrous metallic scrap at a price of $20 per gross ton, f.o.b.
steel mill. The ferrous scrap will undergo long-term evaluation in
the steel mill's blast furnace.
Market Value. It is estimated that ferrous metal like that
recovered from the St. Louis solid waste stream will command a
price ranging from $13 to $20 per gross ton, f.o.b. steel mill.
Depending upon transportation costs, the net revenue to a city will
range from zero (when the shipping costs equal the price) to almost
$20 per gross ton (2,240 pounds) of scrap (when the processing
facility is located next to the steel mill). Based upon a $20 per gross
ton selling price and $3 per gross ton transport cost, the revenue
2Ostrowski, E. J. Evaluation of Eidal mill processed solid waste ferrous
scrap from St. Louis, Missouri, solid waste recovery system. Weirton, West
Virginia, National Steel Corporation Research and Development Department,
January 7, 1973. 18 p. (Unpublished report.)
15
-------
derived from the sale of the nuggetized ferrous metal is equivalent
to about $1 per ton of raw solid waste.
Fly Ash and Bottom Ash
Fly ash is fuel ash that is carried out of the boiler and collected
by the electrostatic precipitators. The utility has been selling its fly
ash to a cement manufacturer. Firing solid waste as fuel has not
affected the quality of the fly ash, and the cement manufacturer
continues to purchase it.
The coal bottom ash has been used by the Missouri State
Highway Department on snow-covered roads. After unclassified
solid waste was burned in the boiler, the bottom ash contained large
particles of metal, wood, plastics, and other materials, making the
bottom ash unsuitable for application on roads. The air classifica-
tion is expected to improve the quality of the bottom ash.
ECONOMICS
The processing of solid waste into a fuel promises, on the basis
of one year's start-up experience in St. Louis, to be an economically
attractive solid waste disposal option. Although primarily a volume-
reduction process, this concept can be evaluated as a disposal
system when the cost of residue disposal is included.
Economies of Scale
The use of solid waste as supplementary fuel in power plant.
boilers obviously becomes more economically attractive as larger
quantities are processed and fired. There is a practical upper limit,
however, to the quantity of raw solid waste which reasonably can
be handled at one site. This upper limit is on the order of 1,500 to
2,500 tons per day, depending upon the method of delivery.
Vehicle traffic and unloading time are the primary determining
factors. For example, it would be easier to handle larger quantities
of waste by barge or pneumatic pipeline than by packer truck.
The capacity of available equipment imposes a further con-
straint: a reasonable upper limit to the capacity of a single
processing line is about 125 tons of raw solid waste per hour, or
2,000 tons in a 16-hour operating day. To handle more waste at a
single site, parallel processing lines could be used.
There is also a practical lower limit to the capacity of a single
processing line. The size of the object or bundle of raw waste to be
processed is more important than the required throughput in
16
-------
determining the size of the milling equipment. The minimum throat
dimensions of the feed hopper should be about 4 feet square. When
shredding normal residential solid waste to particle sizes of 6 to 8
inches, the throughput of such a mill would be about 30 tons per
hour.
Under normal circumstances, it is considered advisable to
operate a single processing line no more than 16 hours per day to
allow 8 hours for routine maintenance, such as retipping the
hammers in the hammermill.
Capital and Operating Costs
Capital and operating costs fall into three main categories:
processing, transporting, and firing at the power plant. All such
costs are variable, depending upon the circumstances (Table 2).
Processing Facilities. Processing facilities normally would con-
sist of those required to receive, convey, mill, classify, and prepare
the solid waste for shipment to the utility. The capital cost and in
turn the operating cost of such facilities depend upon the operating
schedule and the required throughput. In some cases, it may be
appropriate to process the solid waste during only one working
shift. The processing requirements in other cases may dictate
two-shift operation.
The capital cost per ton of daily capacity is often used as a
guide to estimate the capital cost of solid waste disposal operations.
Assuming a 16-hour-per-day operation, with processing facilities
including two-stage milling and air classification, a 30-ton-per-hour
facility may be expected to have a capital cost in the range of
$3,500 to $4,500 per ton of daily processing capability. A
125-ton-per-hour facility may be expected to have a capital cost of
$2,000 to $3,000 per ton of daily processing capability.
The cost to the community varies with the method of financing
the project. And the method of financing is directly related to the
ownership and management arrangement that is selected. To
illustrate the effect of the financing method, the capital costs per
ton have been calculated to include the cost of money.
Six typical financing alternatives are discussed in more detail
below. It is important to remember, however, that the effect of the
financing alternative on the cost to the community is so significant
that the financing mechanism must be designed as early in the
project's planning stages as possible.
Operating costs for comparable processing facilities also may
vary widely, with the principal variables being labor, maintenance,
and utilities. Operating labor costs will depend upon labor rates as
well as labor practices. The same variables apply to maintenance.
Power costs depend upon the applicable rate structure. Reasonable
17
-------
ranges for the operating costs of processing facilities are illustrated
in Table 2. These costs include the recovery of magnetic metals.
If markets exist, revenues from the sale of ferrous metals can be as
high as $1 per ton of raw solid waste.
The costs we have projected are intended only as guidelines. In
specific circumstances, actual costs may be higher or lower than the
costs presented here.
Transport Facilities. Even greater variability can be expected
in the cost of transportation of the supplementary fuel from the
processing plant to the power plant. The least transportation cost
will occur when the two facilities are located near each other so
that the material can be conveyed by pneumatic pipeline or
Table 2
PROJECTED COSTS FOR A DRY SUPPLEMENTARY FUEL SYSTEM*
Smaller systems Larger systems
(30 tons per hour) (125 tons per hour)
Processing facilities^
Capital cost, per ton of daily $3,500 to $4,500 $2,000 to $3,000
capacity
Capital cost per ton*
Typical public financing $1.40 to $1.80 $ .80 to $1.20
Typical private financing $2.20 to $2.90 $1.30 to $1.90
Operating costs per ton $4 to $6 $3.50 to $5.50
Transportation facilities, including amortization
Simpler cases $.50 to $1 per ton
Complex cases $5 to $6 per ton
Firing facilities
Capital cost, per ton of daily $3,000 to $3,500 $2,000 to $2,500
capacity
Operating costs, including
amortization
Favorable circumstances $.50 to $1 per ton
Less favorable circumstances $2.50 to $5 per ton
* For discussion of projected revenues for fuel and magnetic metals, see
Markets.
tBasic parameters of the processing facilities: two-stage milling, with air
classification after the first hammermill; two 8-hour shifts per day, 250
operating days per year; land costs are not included; residue disposal cost is not
included.
* Typical public financing reflects a 6-percent cost of capital over a 15-year
life. Typical private financing reflects a 10-percent cost of capital over a
10-year life. A shorter life is used in the private sector to assure the desired
return on investment.
18
-------
conveyor belt. Substantially higher costs will result when transport
by truck, rail, or barge becomes necessary. Total transport costs
obviously will depend upon individual situations, and could be as
low as $0.50 to $1 per ton when the processing and power plants
are adjacent to each other. Where the two plants are far apart, the
costs of transportation could be as high as $5 to $6 per ton. As a
typical example, if truck transport over high-speed highways is
available, transport costs will approximate 7 cents per ton-mile of
one-way distance. All of these figures include amortization of
capital equipment. Each situation requires individual consideration.
Firing Facilities. The factors affecting the costs of firing solid
waste as supplementary fuel include the type of boiler to which the
process is applied, the type of normal fuel, the method of firing,
and the configuration required for the firing system. Other factors
include the means of ash disposal, labor practices, and amortization
practices. It normally would be expected that the capital costs of
the firing systems would be borne by the utility, since such systems
usually would be installed on the utility's property. At least a
portion of the operating costs would be borne by the utility for the
same reason.
The costs of adapting a tangentially-fired boiler normally may
be expected to be minimal because such units often may permit the
insertion of solid-waste-firing ports without modifying pressure
parts. Horizontally-fired boilers usually may be expected to require
such pressure part modification, with correspondingly greater cost.
Cyclone-fired boilers would require different treatment, con-
ceivably by introducing the solid waste along with crushed coal, or
pneumatically along with a portion of the combustion air.
Short-term solid waste storage facilities, along with pneumatic
firing systems, may or may not appropriately be located on the
utility's property. The length and configuration of the pneumatic
pipelines will significantly affect the capital cost.
The principal factors which normally would have the greatest
effect upon operating costs of firing facilities are the type of normal
fuel and the means of ash disposal. If the solid waste fuel were fired
in combination with pulverized coal, the additional operating costs
would probably be moderate, since the labor required for ash
handling and disposal probably would be the same whether the
solid waste fuel were fired or not. However, if the solid waste fuel
were fired in combination with oil or gas, the ash handling and
disposal costs would essentially all be attributable to the ash
resulting from the burning of solid waste.
Under the most favorable circumstances, it is possible that the
solid waste fuel-firing costs would be as low as $0.50 to $1 per ton,
including amortization. This would most likely be the case where
19
-------
the modified boiler is tangentially fired with coal and has bottom
ash and air pollution control devices. Under less favorable circum-
stances, the equivalent costs could be on the order of $2.50 to $5
per ton.
Trends in the Cost of Traditional Waste Disposal
The decreasing availability of land for close-in landfills will
undoubtedly tend to push communities toward more costly
disposal methods such as incineration and remote landfill. Open
dumping, which may appear to be the least expensive practice in
terms of disposal cost, and which is still the most common practice,
will no longer be allowed in many areas as enforcement of land
disposal regulations becomes more vigorous. At the same time,
stricter air emission standards have already increased the cost of
conventional incineration. Overall, recovery of energy and materials
from solid waste will become economically more attractive over
time as the costs of alternative disposal methods rise and as the
Nation puts a price on factors such as protecting the environment
and conserving natural resources.
20
-------
SOME ALTERNATIVES TO THINK ABOUT
Q If I want to consider this system for my community, what
issue should I look at first?
A Markets for fuel. Markets for recovered products are critical
to the success of any resource recovery system.
Q Where are the markets for solid waste fuel?
A Both utilities and private industrial plants are potential
customers for solid waste fuel. The most important prerequisites are
that their boilers have ash-handling capabilities and that they be
located within an economical transport distance.
Q Are any communities looking at implementing a solid-waste-
as-a-fuel system ?
A Yes. At least five cities and utilities are publicly committed
to fuel recovery from solid waste. The Connecticut State Solid
Waste Management Plan has identified energy recovery as its
principal component. At least 25 other utilities and seven private
industries have expressed an interest in using solid waste as a fuel.
Q How much of the fuel is replaced by solid waste?
A Although the system in St. Louis was originally designed to
replace 10 percent of the coal with solid waste fuel on the basis of
heating value, the system operated well at a 15 percent replacement
rate. Utility personnel say that 20 percent is realistic. Further
testing is planned to determine the maximum percentage of the fuel
that can be replaced by solid waste.
Q Union Electric Company has been burning solid waste fuel as
a supplement to coal in tangentially suspension-fired boilers. Are
there any other possibilities?
A It appears that solid waste can be used economically as a fuel
in any boiler that has bottom ash-handling and particulate emission
control facilities. This includes front-fired, opposed-fired, cyclone-
fired, and stoker-fired boilers. It also includes boilers currently
burning gas or oil.
Q This system is applicable only to large cities. True?
A Not necessarily. Depending on local conditions, energy
recovery may be the best alternative in smaller communities as well
as large. The critical conditions are alternative disposal costs, the
availability of a boiler, alternative fuel costs, and public opinion
about resource recovery.
21
-------
Q What about bulky wastes?
A The capability of a fuel-processing plant to accept bulky
wastes is simply a function of design. Shredders and conveyors must
be sized to handle larger materials. Any noncombustible or
oversized material will be separated from the waste fuel by the air
classification process. In general, bulky wastes add little to the
heating value of the fuel.
The luxury of disposing of both bulky wastes and municipal
wastes at the same facility must be weighed against the added cost
to the shredded fuel system.
Q Why should I process solid waste into fuel if sanitary
landfilling is less expensive?
A If sanitary landfilling is less expensive, then you probably
should continue to landfill. However, some communities have
indicated that noneconomic factors are important, too, even at a
premium in cost. This is not so surprising as it first appears. For
example, if the additional disposal cost per ton is $3, the average
person would have to pay only $3 more per year. The environ-
mental benefits may be worth the small extra cost. Moreover, the
recovery of energy at a time of energy shortage is sure to provide a
real community benefit.
Q How much energy can be recovered from solid waste?
A The potential energy available from solid waste is significant.
If energy recovery were practiced in all urbanized areas in the
United States, an estimated 800 trillion Btu's could be utilized
annually by 1975. Solid waste is a growing energy source: by 1990,
an estimated 1.2 quadrillion Btu's will be available from residential
and commercial solid waste in urbanized areas.
In comparison, the potential energy in urbanized areas in the
solid waste generated in 1970 in urbanized areas could have
supplied two-thirds of the Nation's residential and commercial
lighting needs, or about one percent of the Nation's total energy
consumption.
Financing Alternatives
Q Like any capital-intensive high-technology project, an invest-
ment in solid waste processing facilities involves some risk. How can
this risk be defined?
A There are three forms of risk exposure.
1. Risk that the town and its economy will not generate the
predicted waste stream.
2. Risk that a future technological breakthrough will render the
present system obsolete.
22
-------
3. Risk that the proposed plan incorporating technology, financ-
ing, and operating structure cannot meet its predicted perform-
ance.
Q How does one deal with the waste generation risk ?
A There is essentially no waste generation risk if the system
provides a disposal alternative at a competitive dump fee. If a
close-in sanitary landfill site is not available, and if open dumps are
prohibited, then there will be no cost-competitive alternative for
disposal other than the resource recovery system.
Q How does one deal with the risk of obsolescence?
A Milling of solid waste is applicable in many resource recovery
technologies. The risk therefore is limited to the end use of the
organic fraction. This kind of risk is inherent in any long-term
venture.
Q How can the risk of performance, the ultimate financial risk,
be assigned?
A The financial risk can be assigned in a variety of ways,
depending upon the financing arrangement. There are basically six
alternatives:
1. Town bears complete risk. Here the town raises funds through
general obligation bonds and directly or indirectly operates the
facility.
2. Town indirectly bears complete risk. Here the town would raise
funds through revenue bonds with debt service guaranteed by
the town. Some "Authority" would be the financing vehicle
and a public or private concern would be contracted for
operation over a long term.
3. Contractor/operator bears complete risk. Contractor/operator
finances by his own means the construction and operation of
the system on the basis of a long-term contract from the town.
4. Contractor/operator and revenue bondholders bear complete
risk. Here the town would raise special revenue bonds secured
solely by revenues from the operations or first lien on the
financed facility, or both operations and first lien. Revenue
bondholders would have indirect control over operation.
5. Revenue bondholders bear complete risk. As in alternative 4,
the town would raise special revenue bonds secured solely by
revenues from operations or first lien on the financed facility or
both. Revenue bondholders through an agent would have
control over the operation.
6. Bondholders and equity investors and contractor/operator bear
23
-------
complete risk. This would be essentially the same as alternative
5, except that bondholders may want equity investors seeking
tax advantages to bear some of the risk. The return to equity
investors would, for the most part, result from the investment
tax credit and accelerated depreciation provisions of the tax
law.
The cost to the community varies with each financing alterna-
tive. Each community must assess its own opportunities. It bears
repeating that the effect of the financing alternative on the cost to
the community is so significant that the financing mechanism must
be designed as early in the project's planning stages as possible.
fia 890R
24
------- |