Note: Descriptions are shown in the official language in which they were submitted.
WO 95/28371 218 7 9 ~ ~ PCTlUS95/03i95
APPARATUS AND METHOD FOR TREATING HAZARDOUS WASTE
BACKGROUND OF T E INVENTrnN
This invention relates to the controlled thermal
destruction of waste material, and, more particularly, to
a method and apparatus for the treatment of hazardous and
non-hazardous materials contained in a waste stream
composed of organic (carbonaceous) and inorganic
materials, such that the processed waste contains no
residues that would require subsequent treatment or
landfill disposal. The process involves high temperature
pyrolysis and controlled gasification of organic
materials, and metals recovery and/or vitrification of
.inorganic materials.
Placing hazardous and medical waste in landfills was
until recently the accepted method of disposal. When the
consequences of landfill disposal were investigated more
closely, public opposition and regulatory pressures
restricted the landfill practice and forced the industry
to instead employ incineration, the only other technology
that was economical and seemed to solve the disposal
problem. Incineration proved useful where landfill space
was unavailable or too expensive, but it also was
subjected to public concern. Eventually, new and more
stringent air emission regulations became effective which
most incinerators could not meet. Furthermore, those
incinerators that have remained in operation leave an ash
residue and contains recognizable and potentially
hazardous unburned materials.
Most existing medical waste incinerators are based
on old technologies and were built during the period of
1960-1990, or before the present stricter regulations
became effective. The air pollution control systems on
' the older designs, as well as the design of the
incinerators, are inadequate to meet the present
' 35 standards.
One of the major problems encountered in using
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WO 95/28371 PCTIUS95/03195
incinerators to combust medical wastes is the
heterogeneity of the waste material. This problem
prevents the incinerators. from maintaining a sufficiently
high constant temperature to completely destroy all of
the organic material in the waste. Pollutants that
plague incinerators include the so-called products of
incomplete combustion. For example, a first bag of such
waste may be filled with containers of fluids, blood
soaked bandages, and sharps (syringes, glass, metal
surgical tools and the like), while a second bag may
contain mostly plastics, paper, packing material, pads,-
surgical gowns, rubbers gloves, and the like. These two
bags, fed independently into an incinerator, would create
totally different combustion conditions. The first bag
would quench and cool the combustion process, while the
second bag would accelerate and raise temperatures.
During the low temperature cycle, products of
incomplete combustion and the reformation of potentially
hazardous organic materials, such as dioxin and furan,
are much mare likely. During the high temperature cycle,
particulate, nitrogen oxide and metal oxide emissions
increase, and particularly hexavalent chromium, a
carcinogen. Shredding waste before feeding it into the
combustion vessel would homogenize and mix the waste, but
it is generally not acceptable because of the potentially
infectious nature of the waste and the inherent problem
of disinfecting a shredder having numerous internal
components and small confined places where infectious
material might collect and escape disinfection.
Moreover, many states have laws prohibiting opening bags
of infectious waste prior to their final processing.
Compounding the problem of temperature control
within incinerators is the batch method of feeding that
is commonly used. In this method, a ram system is
normally used to push a charge of waste into-a combustion
chamber. Because the incinerator relies on the waste
itself for fuel, as the waste combusts, chamber
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WO 95/28371 PCTlUS95103195
temperatures vary as the amount
of combustible waste in
the chamber changes. This problem
is especially
pronounced at start-up and
shut-down. Temperatures also
vary with changing feed rates
and incinerators operate
poorly at reduced feed rates.
It is important to achieve
high temperatures because
the destruction of inorganic
waste components commonly
found in medical and other
waste streams requires them.
Only a few incinerator designs
can even reach the high
temperatures required to melt
stainless steel and
borosilicate glass used in
laboratories, and these
incinerators further require
fossil fuel additions to
supplement the combustion process.
The destruction of organic
waste also requires high
temperatures, but instead of
simply melting at high
temperatures, such waste decomposes
and burns if
sufficient air is.present.
The combustion process can
be
self-sustaining only if enough
heat energy is released
during the process to cause
additional material to
decompose. This can be a problem
in incinerators,
however, and especially when
wet and inorganic materials
are present in the feed. Under
such conditions, it is
not possible to maintain a
high, continuous operating
temperature.
Apparatuses that have used
plasma torches to improve
on the low and varying temperature
problem, have only
achieved a partial solution.
For example, U.S. Patent
No. 5,280,757 to Carter discloses
a ram (or batch) feed
system, which causes significant
variation in gas flow
rates and temperatures, and
includes no precautionary
measures to hold the exit gas
temperature at a safe, high
level at which reformation
of more complex organic
compounds.is minimal. The off-gas
piping of Car er is__
composed of stainless steel
and it leads to a steel
cyclone for particulate collection.
The present inventors have
determined that cooling
gas in a six-foot section of
stainless steel pipe causes
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WO 95!28371 PCT/US95103195
the gas temperature to drop into a sufficiently low range
(i.e., into the approximately 350°- 500°C range) to allow
significant reformation of organic compounds, and
particularly polycyclic aromatic hydrocarbons (PAH's).
Car r disclsose5_the.presence of significant levels of
PAH's in his emissions data. The present inventors have
determined that even when the gas temperature is-
sufficiently high and constant in the processing-chamber '
to effect complete dissociation of organic materials, the
gas must be maintained at a sufficiently high temperature
after it leaves the chamber and until it is rapidly
cooled, to reduce the likelihood of organic compounds
reforming.
In recent years, public and regulatory attention has
focused on the problems associated with the disposal of
medical waste generated by hospitals, clinics, medical
offices and research facilities. Numerous new
technologies have emerged which been offered as solutions
to these problems. Most of these new technologies employ
disinfection and/or sterilization methods to reduce or
eliminate the infectious portion of the waste so--that it
may be placed in a landfill after treatment. All of the
disinfection and sterilization technologies leave large
amounts of residue or waste to deal with after the
process is completed.
Also, many waste treatment processes require a '
strict sorting practice before selected waste items can
be treated. Because most of the non-incineration
technologies treat only the infectious portion of the
hospital's waste, human error inherent in sorting the
waste exposes the hospital to liability claims if
infectious waste is later discovered in the non-
infectious waste destined for landfills.
StIM_M.ARv E THE INV N rrnu
The present invention has been made n view of the
above-explained inadequacies of the known waste treating
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CA 02187910 2005-07-07
apparatuses and methods and has as an object to provide a
waste processing system and a method for processing a
wide variety of waste types and compositions, containing
non-uniform amounts and distributions of moisture, while
complying with all applicable air and water emission
standards and producing no residues that require disposal
in a landfill.
It is another object of the invention to provide a
waste processing system which is capable of treating
unsorted waste.
It is another object of the invention to provide a
waste processing system which is capable of maintaining a
constant high temperature throughout the waste treatment
process so as to destroy all complex organic materials
and process the off-gas so as minimize the reformation of
complex organic compounds, and melt all inorganic waste
material into solid residues which pass EPA TCLP tests
and can be recycled or reused.
Additional objects and advantages of the invention
will be learned from the detailed description of a
preferred embodiment of the invention which follows, and
the accompanying drawings, or may be learned by practice
of the invention.
In one aspect of the present invention, there is
provided an apparatus for treating waste consisting of
inorganic and organic components, comprising:
a waste processing chamber;
means for continually feeding waste into the
processing chamber at a controlled feed rate;
a plasma arc torch for heating the processing
chamber to a sufficient temperature to convert organic
components of the waste to a gas comprising hydrogen,
carbon monoxide and carbon dioxide, and to particulate
including carbon particulate, and to convert inorganic
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components of the waste substantially to molten material;
means for withdrawing gas from the processing
chamber as an off-gas;
thermally insulated conduit means for maintaining
the off-gas at an effective temperature to substantially
prevent the formation of complex organic compounds;
means for removing the molten material from the
processing chamber;
first monitoring means for monitoring the amount of
carbon particulate entrained in the off-gas;
means for injecting an oxidant into the processing
chamber in an amount effective to convert a majority of
the carbon particulate to carbon monoxide;
means responsive to the first monitoring means for
controlling the amount of oxidant injected into the
processing chamber so as to minimize the formation of
carbon particulate; and
means for rapidly cooling the off-gas from the
effective temperature to a temperature of less than about
150 °C and for separating the particulate from the cooled
off-gas to form a product gas.
In another aspect of the present invention, there is
provided a method for treating waste consisting of
organic and inorganic components, comprising the steps
of
providing a waste processing vessel having a
processing chamber;
feeding waste into the processing chamber at a
controlled feed rate;
providing a plasma arc torch;
heating the waste in the processing chamber using
the plasma arc torch to convert organic components of the
waste to a gas comprising hydrogen, carbon monoxide and
carbon dioxide, and to particulate including carbon
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CA 02187910 2005-07-07
particulate, and to convert inorganic components of the
waste substantially to molten material,
passing off-gas from the processing vessel through a
thermally insulated conduit to maintain the off-gas at an
effective temperature to substantially prevent the
formation of complex organic compounds;
removing the molten material from the processing
chamber;
monitoring the amount of carbon particulate
entrained in the off-gas using a first monitor;
injecting an oxidant into the processing chamber in
an amount effective to convert a majority of the carbon
particulate to carbon monoxide;
rapidly cooling the off-gas from the effective
temperature to a temperature of less than about 150°C;
separating the particulate from the cooled off-gas
so as to form a product gas comprised of from about 45%
to about 55% of hydrogen and from about 30% to about 40%
of carbon monoxide; and
monitoring the composition of the product gas using
a second gas monitor.
In a further aspect of the present invention, there
is provided an apparatus for treating toxic and hazardous
medical waste consisting of inorganic and organic
components, comprising:
a waste processing chamber;
a first collection means for receiving a charge of
waste, first sealing means for closing the first
collection means to the surrounding environment so as to
isolate the charge of waste therein, a second collection
means for receiving waste from the first collection
means, second sealing means for closing the second
collection means to the first collection means and the
surrounding environment so as to isolate a charge of
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CA 02187910 2005-07-07
waste received therein;
conveyor means for continually feeding waste to the
processing chamber at a controlled feed rate;
means for applying a vacuum pressure within the
processing chamber;
means for venting the second collection means to the
processing chamber;
a plasma arc torch mounted within the processing
chamber for heating the waste to convert organic
components of the waste to a gas comprising hydrogen,
carbon monoxide and carbon dioxide, and to particulate
including carbon particulate, and to convert inorganic
components of the waste substantially to molten material,
the gas exiting the processing vessel as an off-gas;
thermally insulated conduit means for maintaining
the off-gas at an effective temperature to substantially
prevent the formation of complex organic compounds;
an oxidant supply system for injecting an oxidant
into the processing chamber in an amount effective to
convert a majority of the carbon particulate to carbon
monoxide;
a control system for controlling the amount of
oxidant injected into the processing chamber so as to
minimize the formation of carbon particulate; and
cooling means for rapidly cooling the off-gas from
the effective temperature to a temperature of less than
about 150°C and for separating the particulate from the
cooled off-gas to form a product gas.
In another aspect, the present invention is to
provide a method for treating waste consisting of organic
and inorganic components, comprising the steps of:
feeding waste into a first collection means and
closing the first collection means to the surrounding
environment, discharging the waste from the first
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collection means to a second collection means, and
closing the second collection means to the first
collection means and the surrounding environment;
conveying the waste at a controlled rate into a
processing chamber;
heating the processing chamber using a plasma arc
torch to a temperature effective to convert organic
components of the waste to a gas comprising hydrogen,
carbon monoxide and carbon dioxide, and particulate
including carbon particulate, and to convert inorganic
components of the waste substantially to molten material;
maintaining the processing chamber under vacuum to
withdraw the gas exiting the processing vessel as an off-
gas;
passing the off-gas from the processing chamber
through a thermally insulated conduit to maintain the
off-gas at an effective temperature to substantially
prevent the formation of complex organic compounds;
removing the molten material from the processing
chamber, and cooling the removed molten material to form
a non-leachable solid material;
monitoring the amount of carbon particulate
entrained in the off-gas;
injecting an oxidant into the processing chamber in
an amount effective to convert a majority of the carbon
particulate to carbon monoxide and to minimize the
formation of carbon particulate;
rapidly cooling the off-gas from the effective
temperature to a temperature of less than about 150°C;
and
neutralizing acidic gases in the cooled off-gas to
form a product gas.
To achieve the foregoing objects of the invention,
as embodied and broadly described herein, the apparatus
5d
CA 02187910 2005-07-07
for treating waste in accordance with a preferred
embodiment of the invention comprises a waste processing
chamber. Waste is introduced into the processing chamber
at a controlled feed rate, and is heated therein by a
plasma arc torch to temperatures up to about 1650°C so as
to convert the organic portion of the waste substantially
to a gas comprising hydrogen, carbon monoxide and carbon
dioxide, and entrained carbon particulate, and to convert
the inorganic component of the waste substantially to
molten material. The gas exits the processing chamber as
an off-gas at a temperature of at least about 875°C to
1000°C.
The waste processing system in accordance with the
invention removes the molten material from the processing
chamber and subsequently cools it to form a non-leachable
solid material, and or alternatively a recyclable metal.
The waste processing system in accordance with the
invention monitors the amount of carbon particulate
entrained in the off-gas and the carbon monoxide/carbon
dioxide ratio of the off-gas.
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W095128371 218 7 910 PCTIUS95/03195
When an excessive amount of carbon particulate is
detected in the off-gas, an oxidant is injected into the
processing chamber in an amount effective to convert a
majority of the carbon particulate to carbon monoxide.
S The amount of the oxidant injected into the processing
chamber is under process control so as to simultaneously
minimize the formation of carbon dioxide, while
maximizing the conversion of carbon to gas.
To prevent the formation of complex organic
compounds from the off-gas, the waste processing system
maintains the temperature of the off-gas above at least
about 875°C until it is cooled rapidly to a temperature
below about 150°C. During the cooling of the off-gas, a
portion of the particulate is separated from the cooled
gas. Further downstream, any acidic gases in the cooled
off-gas are neutralized and the remaining portion of the'
particulate is separated out to form a product gas which
comprises substantiallyhydrogen and carbon monoxide.
The separated carbon particulate is introduced into the
processing chamber for further processing. The
composition of the product gas is continuously monitored
so as to determine the effectiveness of the oxidant
injections.
The waste processing system also provides for the
safe feeding of waste materials to the combustion
processing chamber. A method and apparatus are disclosed
by which waste is isolated from the surrounding
environment before being crushed, shredded, mixed and
subsequently conveyed for pyrolysis. The waste is fed
into a first collection means after which the first
collection means is, closed to the surrounding
environment. The waste is subsequently discharged into a
second collection means, after which the second
collection means is closed to the first collection means
and the surrounding environment. Thereafter the waste is
ground and mixed to blend solids and liquids in the
waste. The gas from said second collection means is also
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WO 95/28371 21 ~ 7 910 pCT/US9S~03195
vented into the processing chamber.
A disinfectant system is also provided to spray
disinfectant into the second collection means, with the
disinfectant then being conveyed to the processing
chamber for destruction.
' The invention also utilizes a processing chamber
which includes upper, lower and intermediate sections,
" which are assembled in an airtight construction.
BRIEF DESC TPTTON OF TIDE DRAWINGS
In the accompanying drawings:
Fig. 1 is a schematic diagram of an apparatus for
treating hazardous waste in accordance with a preferred
embodiment of the invention;
Fig. 2 is an illustrational view of a waste feed
system and processing vessel of an apparatus in
accordance with the invention;
Fig. 3 is a partially schematic view of a
particulate monitoring system of the apparatus in
accordance with the invention; and
Fig. 4 is a cross-sectional view of the processing
or combustion chamber of the invention.
DETAILED DESCRIPTION OF THF pREF'F' REn FntnODIMEN
With reference to the drawing figures, the waste
processing system of the invention will be described
hereinafter in detail. The system is particularly
suitable for--treating medical type waste. Referring to
Fig. 2, the system includes a waste feed system 10 for
feeding medical waste "W" into a waste processing or
combustion chamber 20 at a controlled rate. The waste
feed system 10 feeds a stream of shredded and compact
waste intothe processing chamber in a continuous manner.
The medical waste may consist of organic and inorganic
components and be in the form of solid or liquid
material. It may include bags of infectious waste,
including blood-soaked sponges, bandages, containers of
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WO 95/28371 2 T 8 7 910 PGT/US95/03195
sharps such as needles, razors;-scalpels and other
instruments. The system is further capable of processing
non-infectious wastes comprised of bags and boxes of
office waste, cafeteria waste, facilities maintenance
waste such as wooden pallets, oils, grease, discarded
light fixtures, yard waste and pharmaceutical waste.
The feed system 10 for medical type waste
illustrated in Fig. 2 includes a charging hopper=11
positioned directly above a feed hopper 12. An airlock
door 13 functions as a sliding cover forthe charging
hopper. When waste is to be placed in the charging
hopper, the door 13 is moved to the opened position as
shown. After charging is completed, the door 13 is
closed in the direction of arrow "A" to cover the
charging hopper. A second, alternately opening, sliding
airlock door 14 separates the charging hopper 11 from the
feed hopper 12 when in its illustrated. closed position.
To charge the feed hopper, the door 14 is opened in the
direction of arrow "B" while door 13 is closed, so as to
prevent the release of any emissions from the feed hopper
and the processing chamber into the environment. Each
door is provided with appropriate seals which cooperate
with seals in the side walls of the charging hopper 11 to
prevent any emissions from leaking from the feed system.
A cantilevered conveyor screw or auger 16, driven by
a variable speed motor "M", shreds, mixes, compresses and
extrudes the waste charge. A feed or conveyor tube 17,
which includes a water-cooled ,jacket 17', surrounds the
cantilevered screw and minimizes heat transfer to the
feed system. A water-cooled feed tube slide gate 18 is .
provided at the inlet port of the processing chamber to
isolate the feed tube from heat when the feed process is
stopped for any significant length of time. The opening
and closing of the gate may be automatically controlled.
A vent system 15 is provided between the feed hopper
and the processing chamber through which fumes and vapors
from the medical waste that are released in the feeding
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R'O 95/28371 PCTIUS95103195
system are drawn into the processing chamber 20 as a
consequence of a vacuum pressure being maintained therein
by a draft fan 19 disposed downstream (see Fig. 1).
The interior of the feed hopper 12 is relatively
open and free of obstructions and contains minimal
' crevices or cracks in which infectious material can
accumulate. This design allows the feed hopper and the
cantilevered screw 16, the only moving part in the feed
hopper, to be easily disinfected by a disinfectant system
21. The disinfectant system 21 includes a supply
container 22 in which an appropriate disinfectant is
retained. The preferred disinfectant is a 6~ solution of
hydrogen peroxide because the solution does not add any
new elements, and it contains no chlorine which would
have to be later neutralized. The container is connected
by supply line 23 to at least one injector nozzle 24
mounted within the feed hopper 12. The disinfectant is
pressurized by a pump 25. It is important that the
nozzles are arranged to ensure that the entire area
within the.hopper is subjected to the disinfectant spray
so that no toxic or hazardous emissions are encountered
when the hopper is opened. After it is applied, the
disinfectant drains into the processing chamber and is
processed as waste.
During operation, the cantilevered screw 16 forms
the liquid and solid waste together into a dense
cylindrical plug. The waste is introduced into the
processing chamber 20 at a controlled rate so as to
expose a controlled amount of surface area of compacted
material to the pyrolysis process to regulate the
formation of product gases. The feed rate is dependent
on the characteristics of the waste and the temperature
and oxygen conditions within the processing chamber. The
waste is preferably continuously fed into the processing
chamber.
In accordance with the invention, the feed rate is
initially calculated based on an estimated energy
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WO 95128371 218 7 910 PCT/US95103195
requirement to process the specific-waste type being
treated. The preferred feed rate is determined by actual
operation of the system, and is selected to maintain a
preferred average temperature within the processing
vessel. A plasma torch 35 described in greater detail
hereinbelow, inputs energy into the processing chamber,-
and the contained waste absorbs the energy in the
pyrolysis process. An excessive feed rate maintained for
a period of time causes the interior temperature to
decrease. Conversely, an inadequate feed rate causes the
chamber to overheat. Accordingly, the proper feed rate
is selected to achieve the preferred average temperature.
A system in accordance with the invention is capable
of processing approximately 1000 pounds of waste per
hour, using a 500 kW plasma torch. A system including a
torch of about one-half of this power rating and a
proportionally smaller processing vessel processes about
500 pounds per hour. It is estimated that these systems
are of approximately the necessary size to meet the needs
of average sized hospitals. A regional processing center
serving numerous hospitals in a large city may use a 1000
kw torch and process up to approximately 25 tons of waste
per day.
As the waste material is introduced into the
processing chamber, it absorbs energy by convection,
conduction and radiation from the plasma flame, the
heated refractory lining and the heated gases circulating
in the vessel. As the organic portion of the waste
material is heated, it becomes increasingly unstable
until it eventually dissociates into its elemental
components, mainly carbon and hydrogen. Oxygen and the
halogens are also liberated if present in the waste. The
time required to achieve dissociation varies slightly for
different materials, but is typically milliseconds for
most compounds at 1100°C. With reference to Fig. 1, the
liberated hydrogen gas expands rapidly and flows from the
vessel to the off-gas piping 26, carrying with it a
WO 95128371 218 l 910 p~~g9~03195
portion of any fine carbon particulate generated by the
dissociation of the waste.
The amount of carbon produced is dependent on the
amount of organic compounds and available oxygen in the
waste. Unless the carbon is oxidized before it exits the
processing chamber, an undesirable heavy particulate load
is carried downstream into a gas quencher 65 and a gas
scrubber 68 (see Fig. 2).
With reference to Fig. 4, the processing chamber 20
is a plasma arc furnace which includes a refractory lined
vessel which, in the preferred embodiment, is generally
cylindrically shaped and constructed in three sections.
The processing chamber is preferably vertically oriented
as shown. The vessel is designed to withstand
temperatures of up to about 1850C in a reducing
atmosphere. The vessel includes a central cylindrical
section 28, an upper, generally semi-spherical section
29
and a lower, generally semi-spherical section 30. The
sections are assembled at flanged joints 31 and 32 so as
to provide an airtight structure. The conveyor feed tube
17 is mounted through the wall of the central section 29
as is the vent system tube 15.
A conventional plasma arc torch 35 is mounted
through an inclined torch receptacle opening 36 provided
in the central section. An access and viewing port 38 is
also provided in the central section. A gas vent or
outlet 40 is provided in the upper section 29 of the
processing chamber. The gas vent system is designed to
convey gas at a temperature of about 875C - 1000C to
the gas quencher 65. The gas pipe 26 is designed to be
airtight to prevent the hot gas from igniting. It is
also refractory lined to maintain gas temperatures above
about 875C.
The lower section 30 of the processing chamber 20
includes a vitreous slag tap 42 which is controlled by
a
' tap positioning system 43. The slag(s) forms a pool "S"
in the lower section. The tap is of a suitable diameter
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W O 95!28371 PCTIUS9b103195
to allow overflow tapping of-the glassy slag. Metal
residue, if it accumulates, can be tapped through a
bottom tap 46, which allows the chamber to be emptied.
An injector 45 is also mounted in the upper section
29 of the processing chamber. The injector supplies
oxygen, preferably in the form of steam, within the
vessel, so as to regulate the products of pprolysis as - -
will be discussed in greater detail hereinbelow. "
The plasma arc torch 35 emits a plasma flame which
heats the interior of the processing chamber 20 to a
uniform temperature preferably in the range of from about
1400°C - 1850°C. A non-transferred type torah is
preferred for treating medical type waste being high in
organics. In comparison to transferred type torches,
non-transferred type torches offer the advantages of
simpler mechanical control requirements as continual
torch motion is not required, greater bulk gas heating
capability, increased arc stability, especially during
the heat up period, simplified furnace design, and
overall greater system reliability.
The plasma heating system further includes a power
supply, a plasma gas compressor and a cooling system (not
shown).
Because the processing chamber 20 is constructed in
sections, the replacement of those sections which are
subject to greater wear and maintenance problems, such as
the lower section, is facilitated at reduced cost and
system downtime.
As illustrated in Fig. 2, the feed of material into
the processing chamber from the feed tube 17 is such that
the waste is introduced into the molten slag "S" which
forms in the lower section 30, rather than being directly
injected into the plasma flame "F". Introduction of the
waste into the slag improves the capture of metals in the
vitreous material and minimizes their entrainment into
the effluent gas.
The waste processing system is designed to minimize
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WO 95/28371 218 7 910 P~~S95103195
surges of carbon particulate during the pyrolysis
process. The system includes a process monitor-and
controls 50 (see Fig. 1) which includes a programmable
logic controller having an imbedded microprocessor (not
shown), and various controls and monitoring devices.
This subsystem monitors process varfables that are
subsequently used to control other process variables to
achieve the desired end product of the waste treating
process. The system is designed to control the
reformation of the organic compounds from the separated
elemental components. This is achieved by controlling
various process temperatures and pressures and also the
injection of an oxidant into the processing chamber 20 to
gasify any excess carbon so as to maximize the
percentages of hydrogen and carbon monoxide, and minimize
the percentages of carbon dioxide, carbon particulate,
and reformed complex organic compounds, in the pyrolysis
product gas stream.
The thermal decomposition (pyrolysis)~of
hydrocarbons releases solid carbon and hydrogen gas.
Hydrocarbon derivatives also contain varying amounts of
oxygen, nitrogen and halogens such as chlorine. Oxygen
and chlorine are free to react with the abundant carbon
and hydrogen and may theoretically reform a wide array of
complex organic compounds. Such complex compounds cannot
form at the high temperatures reached within the
processing chamber at which only a limited number of
simple compounds are stable. The most common and stable
of these simple compounds are carbon monoxide and
hydrogen chloride. There is normally an insufficient
amount of oxygen liberated from the waste material to
convert all of the solid carbon to carbon monoxide gas.
As a result, fine carbon particulate is entrained and
' carried out of the processing chamber by the hydrogen
dominated gas stream. Moisture in the waste liberates
' additional oxygen. But unless the waste moisture content
is uniformly distributed throughout the waste and exceeds
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at least about 30~ by weight, an additional source of
oxygen is required to maximize this conversion.
The amount of oxygen added must be closely
controlled, however, because if excessive oxygen is added
into the process, combustion occurs and forms undesirable
carbon dioxide. The amount of oxidant injected into the
processing chamber is under process control. More
particularly, it is determined by two monitoring systems;
a first monitor 51 for monitoring the amount of carbon
particulate entrained in the gas stream as it exits the
processing chamber, and a second gas monitor 52 located
proximate to the first monitor 51, or alternatively
located downstream of the first monitor for monitoring
the composition of the product gas.
In the processing chamber 20, organic waste
decomposes rapidly to its stable basic constituents of
mostly hydrogen gas, carbon monoxide, carbon particulate
and possibly hydrogen chloride. The waste processing
system includes means for injecting an oxidant into the
processing chamber in an amount effective to convert a
major portion of the carbon particulate to carbon
monoxide. The injection means is preferably an oxidant
supply system 53 which includes a steam generator 53' and
a steam valve 54 which is opened in a controlled manner
to supply steam to the injector 45 which injects
predetermined amounts of steam into the processing
chamber. The steam injected into the processing chamber
converts the free carbon into primarily carbon monoxide.
The carbon monoxide is a desirable end product because it
has fuel value and can be combined with the hydrogen
product to form a product gas which can be,utilized by an
energy recovery system 70. By limiting excess oxygen,
the formation of metal oxides, nitrogen oxides arid sulfur
oxides are minimized.
The fuel gas product can, for example, be burned in
a steam boiler, used to power fuel cells, reformed into
methanol with catalytic converters, or used to power a
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WO 95/28371 218 7 910 PCT/U595/03t95
turbine to produce electricity.
After the oxidant is injected into the processing
chamber, turbulence is created in the chamber to
thoroughly mix the carbon and steam to maximize
gasification of the carbon. The processing chamber
includes a turbulent region 47 (see Fig. 2) into which
the oxidant is injecte8 and through which the exiting gas
and entrained carbon are forced to pass. The carbon and
oxidant must remain in the turbulent region for an
effective amount of time for the oxidation reaction to
occur.
The residence time is the amount of time that the
gas and entrained particulate and steam remain in the
high temperature region of the processing chamber and the
gas piping. Residence time is a function of the gas flow
rate and the distance it travels. At the highest gas
flow rate, the volume of the turbulent,region and the gas
piping that carries the gas to the quencher are such so
as to ensure a sufficient residence time. The turbulent
region improves the probability of reaction between
carbon and oxygen without having to increase the chamber
volume and residence time.
The proper amount of oxidant injected into the
processing chamber 20 is determined downstream and in the
off-gas piping 26 by the first monitor 51 which measures
the amount of free carbon in the gas stream as it exits
the processing chamber. As illustrated in Fig. 3, the
first monitor 51 preferably includes a tap having small
viewing holes 56 in the refractory lining of the outlet
gas piping 26 from the processing chamber. The viewing
holes are fitted with stainless steel pipes 57, water-
cooled jackets 58 surrounding the pipes, nitrogen purge
ports 59, glass pressure windows 60,-a light source 64
and a detector 62. The detector continuously monitors
the gas leaving the processing chamber to measure carbon
particulate.
The first monitor illustrated in Fig. 3 uses a light
WO 95128371 218 7 910 PCT~S95103195
source 64 of a specified wavelength that travels across
the gas pipe to the detector 62_ The amount of light -
that reaches the detector is dependent on the density of
particulate in the gas. The particulate causes
scattering and dispersion of the light emitted from the
light source. '
The output from the detector 62 goes to a signal
processor 63 connected to the process monitor and control '
50 which includes means for controlling the amount of
steam injected into the processing chamber. The
controlling means preferably comprises a number of logic
devices. The detector identifies surges of carbon
particulate in the gas stream that follow the rapid
decomposition of organic material and sends a
corresponding signal to the signal processor, which
processes the signal and directs it to the logic devices.
The logic devices control the opening of the steam valve
54 to cause oxidant to be immediately injected into the
processing chamber until an acceptable carbon particulate
level has been restored.
The gas formed from the dissociation and partial
oxidation of organic material is heated to at least about
900°C - 1150°C in the processing chamber. The waste
processing system comprises means for maintaining the
temperature of the off-gas above a predetermined
temperature until it reaches a means for cooling the off-
gas to a temperature of less than about 150°C and for
separating a portion of the entrained carbon particulate
from the cooled off-gas. The cooling means is preferably
quencher 65. The off-gas is transported from the outlet
port 40 of the processing chamber to the quencher through
piping 26 which is thermally insulated to maintain the
gas temperature above about 875°C until it reaches the
quencher. The quencher is preferably as close to the
processing chamber as possible, in order to minimize heat
loss and cooling until the gas is rapidly cooled below
about 150°C. High temperature thermocouples monitor the
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WO 95/28371 PCTlUS95/03195
gas temperature prior to the steam injection zone and
downstream proximate to the inlet of the quencher.
Various operating parameters are used to maintain
the gas temperature within the preferred operating range.
The operating gas temperature inside the main chamber is
a function of balancing the torch power input and the
feed rate_ The torch provides the necessary amount of
heat to maintain a minimum bulk chamber temperature which
determines the gas temperature. The waste absorbs heat
energy as it is fed into the chamber. Because the torch
power is primarily fixed by its size and operating
parameters, the waste feed rate is used to prevent the
chamber from overheating, and thereby to regulate the
chamber temperature. Another parameter which influences
temperature is the amount of combustion oxidation which
occurs to form carbon dioxide. Allowing additional
excess steam into the chamber allows a,larger percentage
of carbon to oxidize to carbon dioxide. This reaction is
exothermic, and it releases additional heat which tends
to raise temperature. In accordance with the invention,
this reaction may be promoted to raise temperatures at
the beginning of the waste treatment process; however, it
lowers the fuel quality of the product gas and
accordingly is not a preferred aspect of the process.
In the quencher 65, the gas is rapidly cooled with
water sprays to a temperature of about 150°C or less to
prevent the formation of complex organic compounds.
Recirculating water is sprayed as a very fine mist along
the axis of the gas pipe that the gas moves through. As
the hot gas contacts the spray, the quench water is
quickly heated and evaporative cooling quickly lowers the
gas temperature to prevent the reformation of complex
organic molecules. The action of the sprays also washes
out a portion of the carbon and metal particulate
entrained in the gas. The particulate is removed by a
particulate recycling system 66 and introduced into the
feed system through conduit 67. The quench water is
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R'O 95128371 218 7 ~ 10 PCT/US95103195
recirculated and reused.
After the gas is cooled by the quencher 65, it is
drawn by the draft fan 19 into a means for neutralizing
acidic gases in the cooled pff-gas and for separating
substantially the remaining portion of the carbon
particulate therefrom so as to form a product gas. This
means is preferably a wet gas scrubber 68. The
chlorinated organic materials often found in hospital and
other organic waste decompose and in the hydrogen rich
gas, reform as hydrogen chloride. This compound is
neutralized in the scrubber by reacting it with a basic
neutralizing agent to form salts, as the pyrolysis gas
travels through the scrubber.
The particulate that is washed out of the gas by the
scrubber 68 are filtered out of the scrubber water by the
particulate recycling system 66 and then dewatered and
introduced into the feed system 10 through conduit 67 for
further processing. A water handling system 72 supplies
regulated amounts of makeup water and a neutralizing
agent to the quencher 65 and scrubber 68 by way of a
water supply 73 and a neutralizing agent supply 74,
respectively. The water handling system also provides a
discharge to a sewer 75.
After the cooled gas leaves the gas scrubber 68, it
passes to the second gas monitor 52 which preferably
comprises an on-line gas monitor for monitoring the
composition of the product gas. The gas monitor includes
a thermal conductivity analyzer 76 to measure the
percentage of hydrogen, and infrared analyzers 77 to
measure the percentages of carbon monoxide, carbon
dioxide and methane, as a representative of the total
hydrocarbons, in the scrubbed gas. The infrared
analyzers provide a general measure of the proportions of
carbon and oxygen in the process and this measure is used
for monitoring overall process balance.
For example, the second gas monitor 52 can be used
to determine if there are any air leaks in the system.
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WO 95/28371 2 l 8 7 910 PC'frt3S95103195
Such leaks are indicated by low total percentages of
hydrogen, carbon monoxide, carbon dioxide and methane.
If air, being about 80% nitrogen, is leaking into the
system, the total percentage of the four gases will be
less than approximately 92-94%. The gas percentages also
indicate that the system is operating properly.
As explained above, an objective of controlled
pyrolysis is to convert the liberated carbon solids from
the organic waste into carbon monoxide which has fuel
value. If too much oxygen is present, combustion will
occur and convert carbon into carbon dioxide which has no
fuel value. The waste processing system in accordance
with the invention achieves a balance between the amount
of liberated carbon and the amount of oxygen permitted to
react with it. Because pure carbon is more reactive at
the high operating temperature than is carbon monoxide,
additional oxygen injected into the chamber reacts with
the carbon and forms additional carbon monoxide, and not
with carbon monoxide to form carbon dioxide. Carbon
dioxide is also relatively less stable at the processing
temperatures than carbon monoxide.
The amount of carbon liberated when processing
medical waste varies with the amount and type of organic
materials fed into the processing chamber. Generally,
the amount of oxidant needed to accomplish the desired
gasification of the carbon can be determined by
monitoring the percentages of carbon monoxide, carbon
dioxide and methane in the product gas stream. As the
waste composition in the feed varies, however, temporary,
rapid changes occur in the amount of carbon liberated.
Accordingly, an immediate adjustment in the amount of
oxidant injected into the processing chamber is required
to prevent accompanying surges in the amount of carbon
exiting the processing vessel. This requirement mandates
having a monitoring device, namely the first monitoring
means 51, proximate to the processing chamber to measure
carbon particulate at the high gas temperature and direct
19
W095I28371 Z ~ 8 7 910 PCTIUS95103195
a timely response from the oxidant injection system to
increase the amount of oxygen injected until the amount-
of free carbon has been reduced to an acceptable level.-
The inorganic waste materials such as non-toxic
metals, toxic heavy metals, ceramics and-glasses melt in
the processing chamber at a temperature of about 1650°C.
The molten material formed is composed of-glass-like slag
and, in some instances, a metal -layer, which are
separable.
As explained above, the waste processing system
includes means for removing the molten material from the
processing chamber. Referring to Fig. 1, the removing
means preferably comprises the tap device 42, a sealed
water tank 80 and a solid residue handling system 81.
The molten material passes through the tap device and
into the sealed water tank, where the material is rapidly
quenched and caused to fracture into smaller pieces. The
solid glassy slag is essentially inert because the heavy
metals are bound within it. Consequently, the glassy
slag resists leaching in the solid state.
The orientation of the torch with respect to the
interior chamber is such that the flame of the torch is
directed to the area of the tap device 42. The increased
heat transfer from the torch flame to the tap area (and
the molten glass as its level reaches the tap hole)
promotes a continuous flow of molten glass through the
tap hole as the level of glass builds up from scrap glass
and metals in the Waste feed. If the tap area requires
additional heating to start the flow of glass through the
hole, the torch can be lowered so as to increase heat
transfer directly to the tap area. The~tap hole can be
sealed during standby periods by a tap positioning device
43 which closes the tap hole to minimize heat loss. Over
a period of time, a layer of metals may accumulate below
the glassy slag. Certain metals such as iron do not
react readily with the silicates. The glassy material
absorbs some of.these metals, but the metals may
PCT/U595/03195
WO 95128371 218 7 910
accumulate if a large amount is in the waste stream.
This is not a serious problem, however, and if the metals
eventually fill the basin, they can be drained through
the bottom tap 46 and recycled as scrap metal. The
bottom tap 46 is also sealed by the tap positioning
device 43 and can be mechanically opened for draining.
The solid residue handling system 81 preferably
includes a conveyor or other suitable device 82 for
moving the solid slag to a bin 85 for transport and
disposal.
The scrubbed gas is transported to an energy
recovery system 70. Entrained moisture can be removed by
conventional equipment if necessary. The gas formed from
the thermal decomposition (pyrolysis) of organic
materials is mainly hydrogen and carbon monoxide. This
composition of gas is a usable fuel gas and can be used
to recover the energy that was in the waste materials as
described hereinabove.
The waste processing system produces solid residues
in the form of glass-contained metals which pass TCLP
tests and accordingly can be recycled or reused.
Further, the apparatus and method of the present
invention are not limited to the safe disposal of
hospital, medical and related toxic and hazardous waste
but are also effective for use in the safe disposal of
chemical toxins and the like.
The foregoing description of the preferred
embodiment of the invention has been presented to
illustrate the principles of the invention and not to
limft the invention to the particular embodiment
illustrated. It is intended that the scope of the
invention be defined by all of the embodiments
encompassed within the following claims, and their
equivalents.
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