U.S. patent application number 11/144948 was filed with the patent office on 2006-12-07 for modular plasma arc waste vitrification system.
Invention is credited to Samuel Y. Liu.
Application Number | 20060272559 11/144948 |
Document ID | / |
Family ID | 37492872 |
Filed Date | 2006-12-07 |
United States Patent
Application |
20060272559 |
Kind Code |
A1 |
Liu; Samuel Y. |
December 7, 2006 |
Modular plasma ARC waste vitrification system
Abstract
An improved plasma arc waste processing system has a crucible
formed in a horizontally elongated shape with a low-ceiling-height
plenum space above a melt pool and a waste input feed arranged
sideways into the crucible. The plenum space conducts the off-gases
through a channel into a horizontally adjacent, thermal resonant
chamber. The combined volume of the plenum space and thermal
resonant chamber provides a high ratio of plenum volume to surface
area of melt pool for efficient pyrolization of waste at high
throughput rates. The crucible and other system components can be
arranged in standard sized cargo containers, to be modularly
transported and assembled in populated areas or transported to and
from remote areas, such as in medical crisis zones, military
deployments, and war zones. As a further improvement, a water
injector is used to inject water into the plasma field to remove
excess carbon and form useful gas byrpoducts.
Inventors: |
Liu; Samuel Y.; (Honolulu,
HI) |
Correspondence
Address: |
LEIGHTON K. CHONG;GODBEY GRIFFITHS REISS & CHONG
1001 BISHOP STREET, PAUAHI TOWER SUITE 2300
HONOLULU
HI
96813
US
|
Family ID: |
37492872 |
Appl. No.: |
11/144948 |
Filed: |
June 3, 2005 |
Current U.S.
Class: |
110/250 ;
110/242 |
Current CPC
Class: |
F23G 5/16 20130101; F23L
2900/07008 20130101; F23G 5/10 20130101; F23G 2202/20 20130101;
F23G 5/085 20130101; F23G 5/40 20130101; Y02E 20/12 20130101 |
Class at
Publication: |
110/250 ;
110/242 |
International
Class: |
F23G 5/10 20060101
F23G005/10; F23G 7/00 20060101 F23G007/00 |
Claims
1. An improved plasma arc waste processing system comprising: (a) a
crucible formed as a horizontally flat, enclosed container with a
bottom wall and side walls for containing a melt pool in the
crucible, and a ceiling wall connected to the side walls and
defining a plenum space above the top level of the melt pool for
containing a plasma of pyrolized gases generated from waste
material fed onto the melt pool, (b) a waste input feed
communicating into the crucible through the side walls at one side
position of the crucible with a feed opening positioned above a top
level of the melt pool, and (c) one or more plasma arc electrodes
mounted through the ceiling wall with ends thereof positioned above
the top level of the melt pool for generating a high-temperature
ionization plasma from pyrolized waste material fed onto the melt
pool, (d) wherein the ceiling wall is dimensioned to form said
plenum space with a relatively low ceiling height above the top
level of the melt pool, and said plenum space communicates via a
channel formed through the side walls into a thermal resonant
chamber arranged at another side position of the crucible, (e) said
thermal resonant chamber providing an additional plenum volume
along with said low-ceiling-height plenum space for allowing
complete pyrolization of the waste material into elemental
constituents.
2. An improved plasma arc waste processing system according to
claim 1, wherein the waste input feed is fed from a bulk extruder
through the side walls of the crucible.
3. An improved plasma arc waste processing system according to
claim 1, wherein the joule-heating electrodes are mounted through
the side walls at spaced intervals around the circumference thereof
and project into the molten pool for maintaining melt temperatures
in the pool.
4. An improved plasma arc waste processing system according to
claim 1, wherein off-gases generated in the plenum space are led
through a channel in the side wall into the horizontally adjacent
thermal resonant chamber.
5. An improved plasma arc waste processing system according to
claim 1, further comprising an injector for injecting water or
fluid containing water into the plasma field to remove excess
carbon by chemically combining to form hydrogen gas and carbon
monoxide, thereby reducing carbon incandescence of the plasma arc
electrodes.
6. A transportable plasma arc waste processing system comprising:
(a) a crucible formed as a horizontally flat, enclosed container
with a bottom wall and side walls for containing a melt pool in the
crucible, and a ceiling wall connected to the side walls and
defining a plenum space above the top level of the melt pool for
containing a plasma of pyrolized gases generated from waste
material fed onto the melt pool, (b) a waste input feed
communicating into the crucible through the side walls at one side
position of the crucible with a feed opening positioned above a top
level of the melt pool, (c) wherein the crucible is dimensioned to
form said plenum space with a relatively low ceiling height above
the top level of the melt pool, and said plenum space communicates
via a channel formed through the side walls into a thermal resonant
chamber arranged horizontally adjacent the crucible, (d) wherein
said crucible and thermal resonant chamber are dimensioned to fit
in a container of a standard cargo container size, and (e) wherein
other system components for the system are arranged to fit in
containers of the same standard cargo container size as the
crucible container, so that the crucible and its other system
components can be readily transported in modular fashion in the
containers of standard cargo container size.
7. A transportable plasma arc waste processing system according to
claim 6, wherein the crucible and other system components are
arranged to be contained in standard 20-foot or 40-foot length
cargo containers.
8. A transportable plasma arc waste processing system according to
claim 6, wherein the crucible and thermal resonant chamber are
arranged in horizontally adjacent halves of the container.
9. A transportable plasma arc waste processing system according to
claim 8, wherein the container halves for the crucible and thermal
resonant chamber are formed as separate half compartments that are
assembled by sliding them together or apart on rails.
10. A transportable plasma arc waste processing system according to
claim 6, wherein a gas scrubber unit is arranged in another
container of the standard cargo container size.
11. A transportable plasma arc waste processing system according to
claim 10, wherein the thermal resonant chamber has ducts for oxygen
purging with ambient air and a quench conduit for ducting the
off-gases to the scrubber unit in the first-mentioned
container.
12. A transportable plasma arc waste processing system according to
claim 10, wherein the gas scrubber unit is arranged with a
gas-fired auxiliary power generator in its container.
13. A transportable plasma arc waste processing system according to
claim 6, wherein a control room and an electrical equipment room
are arranged together in another container of the standard cargo
container size.
14. A transportable plasma arc waste processing system according to
claim 6, wherein a bulk extruder is arranged to feed waste material
into the crucible through a port from outside a free end of the
container for the crucible.
15. A transportable plasma arc waste processing system according to
claim 6, wherein a drain from the crucible is arranged to drain
molten waste material from the crucible to a vitrification pen
outside the container for the crucible.
16. A transportable plasma arc waste processing system according to
claim 6, wherein the crucible and thermal resonant chamber are
arranged in horizontally adjacent halves of a standard sized
20-ft.times.8-ft.times.8-ft container.
17. A transportable plasma arc waste processing system according to
claim 16, wherein the crucible plenum space plus the thermal
resonant chamber as additional plenum space take up about 75% and
the crucible takes up about 25% of the inner volume of the
container.
18. A transportable plasma arc waste processing system according to
claim 16, wherein the ratio of total plenum volume to surface area
of the melt pool is in the range of about 20:1.
19. An improved plasma arc waste processing system comprising: (a)
a crucible formed with a bottom wall and side walls for containing
a melt pool in the crucible, and a ceiling wall connected to the
side walls and defining a plenum space above the top level of the
melt pool for containing a plasma of pyrolized gases generated from
waste material fed onto the melt pool, (b) one or more plasma arc
electrodes mounted through the ceiling wall with ends thereof
positioned above the top level of the melt pool for generating a
high-temperature ionization plasma field from pyrolized waste
material fed onto the melt pool, and (c) an injector for injecting
water or fluid containing water into the plasma field to remove
excess carbon by chemically combining to form hydrogen gas and
carbon monoxide, thereby reducing carbon incandescence of the
plasma arc electrodes.
20. An improved plasma arc waste processing system according to
claim 19, wherein the injector injects water in controlled amounts
and at sufficiently high pressures to traverse (before evaporation)
into the plasma field so that it can combine therein with the
carbon.
Description
TECHNICAL FIELD
[0001] The present invention generally relates to plasma arc waste
vitrification systems, and more particularly, to a system designed
to be modular in construction so as to be readily deployed on
location and to be highly efficient in waste processing.
BACKGROUND OF THE INVENTION
[0002] With environmental concerns imposing increasing constraints
on the dumping raw solid waste into landfills or ocean beds,
incinerator facilities have been utilized in recent decades to
reduce the volume of waste that has to go into landfills, and to
convert at least a portion of the waste stream to usable
byproducts, fuels, or recoverable energy. However, a major
byproduct of incinerators is waste ash that has to be transported
and buried in landfills as hazardous waste. Landfills permitted for
hazardous waste are becoming increasingly unavailable and impose an
added cost for disposal. The byproducts of incinerators have also
raised heightened public concern over gaseous emissions, requiring
remediation with costly air pollution control systems, and the
possibility of leaching from waste ash disposed in landfills and
contamination of groundwater.
[0003] As an alternative to incinerator systems, waste
vitrification systems have been developed that use electrical
energy to generate a high-temperature plasma for melting solid
waste into its elemental constituents, and encapsulate the waste
elements in glass material to form a vitrified byproduct that is
largely inert and can be used as ground fill, road asphalt
material, or building construction material. Such waste
vitrification systems have significant advantages over incinerators
in that the volume of gaseous products may be significantly less
than that produced by incineration, and there is substantially less
risk of toxicity or contamination from its emissions and
byproducts. Another significant advantage of such waste
vitrification systems is their capability to neutralize hazardous
medical waste and biowaste due to sterilization and destruction of
all bacterial, microbial, and viral matter through the
high-temperature melt process.
[0004] An example of a plasma arc vitrification system is described
in U.S. Pat. No. 5,811,752 issued Dec. 8, 1998 to Titus et al, and
is commercialized through Integrated Environmental Technologies,
LLC, Carle Place, N.Y. This plasma arc system employs a combination
of DC arc-electrode heating to generate a high-temperature plasma
for melting incoming waste, and AC joule heating to maintain a
molten pool of the waste for processing of its molten elemental
constituents into a flow output that is cooled and glassified into
glass particles. The system can also be configured to recover its
off-gas as a fuel capable of generating electricity using an
auxiliary gas turbine or combustion engine.
[0005] An example of an integrated solid waste processing system
suitable for processing biomedical or hazardous waste is described
in U.S. Pat. No. 6,766,751 issued Jul. 27, 2004, to Samuel Y. Liu,
the same inventor herein. In this integrated process, a selected
and classified type of waste container is used to collect a
specified type of waste, and is transported to a waste processing
site in a vehicle having a compartment maintained under negative
pressure relative to atmospheric pressure to prevent leakage
outside of the compartment. The waste processing site is one
constructed to handle biomedical or hazardous waste, such as a
plasma arc vitrification system. The waste container is marked with
electronic indicia and is tracked as it moves through the waste
processing system, so that information on the disposition of the
waste container of biomedical or hazardous waste can be
electronically accessed and monitored from a remote location.
[0006] However, the prior plasma arc systems have had several major
disadvantages which limit their usefulness. They have generally
employed a large, vertically-shaped crucible having a large volume
for providing plenum space to contain waste material ionized to a
plasma above a melt pool and allow completion of pyrolitic
reactions of the material to elemental constituents and capture in
the glass melt or ducting off as gaseous byproduct through a gas
output port in the ceiling wall. The large plenum space has
required the arc electrodes to be of long length to extend from
their ceiling mounting to the plasma zone above the glass melt,
making them more vulnerable to deterioration, and requiring
frequent maintenance. The prior art systems have had the waste
input port positioned through the ceiling wall of the crucible and
were opened to drop the waste material in by gravity feed, thereby
allowing hot gases to escape and the temperatures in the plasma
zone to fall whenever the waste input port was opened during
repeated feed cycles. Due to the difficulty of maintaining the
plasma zone at the desired high ionization temperatures of 10,000
to 12,000.degree. C. with each feed cycle, a longer time was
required for the waste material to stay in the plasma zone for
complete decomposition. Excess carbon collecting in the plasma
field from organic constituents of waste can cause electrical
"crazing" or "incandescence" which results in a loss of arc
conduction and reduced effectiveness of the plasma arc ionization
of waste.
[0007] The prior plasma arc systems also had design flaws which
limited their useful life and required frequent and extensive
maintenance. The relatively long time it took to process a given
quantity of waste material would result in the melt pool becoming
stratified as between metal constituents and non-metal constituents
(slag), which has led to problems with metal deposits clogging and
damaging the refractory lining of the bottom floor of the crucible,
and requiring frequent repair or maintenance. The crucible design
with a large plenum space above the melt pool and arc electrodes
and waste input and gas output ports mounted through the ceiling
wall made the ceiling half of the crucible very heavy and difficult
to disassemble and reassemble for repair or maintenance. The
crucible's large size, e.g., 10-12 feet in height, and heavy
weight, e.g., 40,000 lbs., made construction of a plasma arc
facility a complex construction task requiring fixed siting and
making it difficult to install, disassemble, and move such
facilities into/from populated areas as well as remote areas which
may need its special type of waste processing capability, such as
in medical crisis zones, military deployments, and war zones.
SUMMARY OF THE INVENTION
[0008] In view of the shortcomings of the prior plasma arc systems,
it is a principal object of the present invention to provide an
improved plasma arc system in which the crucible is designed to
minimize deterioration of its structural components and refractory
lining due to its operation.
[0009] It is another object of the invention to provide an improved
plasma arc system in which the crucible is designed to maintain and
extend the expected service life for the DC-arc electrodes and the
AC joule-heating electrodes.
[0010] It is yet another object of the invention to provide an
improved plasma arc system in which the crucible is designed to
facilitate disassembly and reassembly for convenient repair or
maintenance.
[0011] It is yet another object of the invention to provide an
improved plasma arc system in which the crucible is designed to
reduce or eliminate slag/metal stratification, and thereby maintain
consistent content in the vitrified byproduct, and prevent slag
deposit from clogging the crucible bottom and degrading the
refractory lining.
[0012] It is a further object of the invention to provide an
improved plasma arc system in which the operation of the plasma arc
electrodes is modified to reduce or eliminate electrical "crazing"
or "incandescence", and thereby maintain arc conduction at its full
level for effective processing by plasma arc melting of waste.
[0013] It is still a further object of the invention to provide an
improved plasma arc system in which the crucible is designed so
that waste input is not fed into the crucible by gravity feed from
a ceiling input port, in order to reduce heat loss through the
input port during input feed cycles.
[0014] It is another important object of the invention to provide
an improved plasma arc system with components that can be contained
in small volume structures, so that an entire facility can be
easily installed, disassembled, and moved into/from populated areas
as well as remote areas, such as in medical crisis zones, military
deployments, and war zones.
[0015] To achieve the foregoing objects, the present invention
provides an improved plasma arc waste processing system having a
crucible formed as a horizontally flat, enclosed container with a
bottom wall and side walls for containing a melt pool in the
crucible, and a ceiling wall connected to the side walls and
defining a plenum space above the top level of the melt pool for
containing a plasma of pyrolized gases generated from waste
material fed onto the melt pool, a waste input feed communicating
into the crucible through the side walls at one side position of
the crucible with a feed opening positioned above a top level of
the melt pool, and one or more plasma arc electrodes mounted
through the ceiling wall with ends thereof positioned above the top
level of the melt pool for generating a high-temperature ionization
plasma from pyrolized waste material fed onto the melt pool,
wherein the ceiling wall is dimensioned to form said plenum space
with a relatively low ceiling height above the top level of the
melt pool, and said plenum space communicates via a channel formed
through the side walls into a thermal resonant chamber arranged at
another side position of the crucible, said thermal resonant
chamber providing an additional plenum volume along with the plenum
space for allowing complete pyrolization of gases generated from
the waste material into elemental constituents.
[0016] In a preferred embodiment, a waste output drain is formed
through the bottom wall, and a plurality of joule-heating
electrodes are mounted circumferentially spaced around and mounted
through the side walls for heating the melt pool contained in the
crucible. The waste input feed is fed waste material from an auger
to break up and force the waste material onto the melt pool.
Alternatively, it can be formed with a piston-type extruder member
for forcing a predetermined volume of waste material onto the melt
pool. A liquid injector or feed lance is arranged to inject water
into the plasma zone formed by the plasma arc electrodes to remove
excess carbon by chemically combining to form hydrogen gas and
carbon monoxide as a gas byproduct, thereby reducing or eliminating
carbon incandescence.
[0017] The improved plasma arc system is designed to have its
system components contained in a plurality of modular structures
dimensioned to fit in standard cargo spaces of land, sea, or air
vehicles, so that they can be easily transported and assembled on
location, and/or disassembled and removed in modular fashion. This
would allow an entire facility to be easily moved and installed in
populated areas as well as transported into remote areas, such as
in medical crisis zones, military deployments, and war zones.
[0018] For a more complete understanding of the present invention,
the following description describes the invention in greater detail
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic diagram of a preferred embodiment of a
plasma arc system according to the present invention, showing a
plasma arc crucible and thermal resonant chamber (TRC) in one
modular structure, an off-gas scrubber unit in another modular
structure, and an equipment control room and electrical equipment
room in another modular structure;
[0020] FIG. 2A shows a reverse side view of the preferred
embodiment of the plasma arc system of FIG. 1, and FIG. 2B is a
partially cut-away view in greater detail;
[0021] FIG. 3 shows the plasma arc crucible and thermal resonant
chamber (TRC) arranged in respective container halves slidable
together to form one modular container structure;
[0022] FIG. 4 is a schematic diagram illustrating the plasma arc
crucible in a horizontally flat, ring shape with joule-heating
electrodes arranged through its side walls;
[0023] FIG. 5A illustrates the use of a liquid feed injector or
lance to inject water into the plasma field to reduce or remove
excess carbon, and FIG. 5B illustrates an alternative method of
injecting material containing water into the plasma field; and
[0024] FIG. 6 is a schematic diagram of a prior art plasma arc
system for comparison to the present invention.
[0025] Similar reference numerals in the drawings are used to refer
to similar parts throughout the several views of the drawings.
DETAILED DESCRIPTION OF THE INVENTION
[0026] As a background for description of the invention system,
reference is made to a prior art type of plasma arc vitrification
system as described in U.S. Pat. No. 5,811,752 issued Dec. 8, 1998
to Titus et al, and to an integrated solid waste processing system
preferred for processing biomedical or hazardous waste as described
in U.S. Pat. No. 6,766,751 issued Jul. 27, 2004, to the same
inventor as this U.S. patent application, which are incorporated by
reference herein.
[0027] The general principles of operation of a prior art plasma
arc vitrification system will now be briefly described with
reference to FIG. 6. A prior art plasma arc vitrification system 10
includes a reaction vessel 12 having top 12a, bottom 12b, and sides
12c and 12d. Bottom 12b may have a generally V-shaped
configuration. Reaction vessel 12 further includes at least one
port or opening 14a through its top by which waste material 40 is
introduced into reaction vessel 12. The reaction vessel 12 includes
a plurality of ports or openings 14a and 14b, which may include a
flow control valve or the like to control the flow of waste
material 40 into vessel 12 and to prevent air from entering vessel
12. Reaction vessel 12 also includes gas port or opening 16 and
metal/slag pouring port or opening 20. Gas exiting from port 16
preferably will enter conduit 18 and will be sent to a scrubber,
turbine or the like for further processing. Port 16 may be provided
with a flow control valve or the like so that gas formed in
reaction vessel 12 may be selectively released into conduit 18.
Metal/slag port 20 is designed to have a flow control valve or the
like so that metal and/or slag may be removed and introduced into
metal/slag collector 22 at predetermined rates and periods of time
during the process.
[0028] Metal port 20 may be positioned to protrude through the
bottom of unit 12 and elevated a predetermined distance thereabove.
In this manner, port 20 may function as a submerged counter
electrode to arc plasma electrode 24 (electrodes 24a and 24b). Port
20 may also be provided with inductive heating coils 26 to provide
additional heating when it is desirable to pour metal and/or slag.
Inductive heating coils 26 may also be designed to provide cooling
when it is desirable to cease pouring metal and/or slag. Unit 10
may also include auxiliary heater 30 to assist in glass tapping or
pouring. Due to differences in specific gravity, metal in
metal/slag layer 42 moves toward bottom 12b in vessel 12. Slag in
metal/slag layer 42 exits through opening or port 36a into conduit
36. Conduit 36 may be formed of a conductive material, such as
silicon carbide, to facilitate the flow of slag 44.
[0029] The temperature of slag 44 is maintained in chamber 30 by
heaters 32a and 32b for a time and under conditions sufficient to
provide a fluid glass or slag to pour into slag collector 46. Ohmic
heaters are suitable for use as heaters 32a and 32b in chamber 30.
Heaters 32a and 32b may be constructed of silicon carbide or the
like. Alternatively or in addition to heaters 32a and 32b, the
temperature of slag 44 may be maintained by plasma torch 58. Slag
44 then passes through slag pouring conduit 34 and port 38, thereby
exiting chamber 30 into slag collector 46. When hazardous waste is
being processed, it may be desirable to have collector 46 sealably
connected to port 38 such that air and/or gases do not enter or
exit the system.
[0030] Reaction vessel 12 also includes a plurality of AC joule
heating electrodes 50a and 50b positioned across from one another
on sides 12c and 12d, respectively. In addition, electrodes 50a-50b
are positioned so as to be partially or totally submerged in the
slag 42 mix when the process is in use. The positioning of
electrodes 50a-50b can be varied according to the type and volume
of waste being processed. When the waste feed material has a high
metals content for example, the joule heating electrodes may be
raised or lowered in the unit to adjust or "tune" the effective
resistive path between electrodes. This may be desirable or
necessary if the metal layer is allowed to increase to a point
where the electrical path between the joule heated electrodes is
effectively "shorted" due to contact or near contact with the
highly conductive metal layer. In addition, the number of joule
heating electrodes can be varied depending on the type and amount
of waste material being processed.
[0031] The joule heated melter facilitates production of a high
quality pyrolysis gas using the minimum energy input to the
process. This is accomplished because energy input to the arc does
not need to be greater than that required to pyrolyze and melt the
material in the arc zone. The molten bath below the unmelted feed
material is maintained at the desired temperature using joule
heating as opposed to using only an arc as in an arc plasma
furnace. Air/oxygen and/or a combination of air and/or steam may be
added to eliminate char from the melt surface and adjust the redox
state of the glass. The joule heated melter provides energy (i.e.
hot glass) near the sides of the bath where the gas/steam mixture
is introduced.
[0032] The system is employed utilizing a molten oxide pool. The
composition of the molten oxide pool can be modified to have
electrical, thermal and physical characteristics capable of
processing metals, non-glass forming wastes and low-ash producing
wastes in a manner capable of generating a low to medium BTU gas.
The conductivity of the molten pool is controlled by adding melt
modifier materials so that the joule heated portion of the system
can effectively maintain the temperature of the melt even when
under conditions such as 100% joule heating operation. It is
desirable to maintain the electrical resistivity of the molten pool
in a certain range for effective joule heating of the molten oxide
pool. The constituents of the molten pool are chosen to be optimum
for a given waste stream. Melt modifiers may for example include
dolomite (CaCO.sub.3.cndot.MgCO.sub.3), limestone (e.g. calcium
carbonate, CaCO.sub.3), sand (e.g. glass maker's sand), glass frit,
anhydrous sodium carbonate (soda ash), other glass forming
constituents and/or sand combined with metals.
[0033] The hydrogen-rich gas produced by the system can be cleaned
and then combusted in a gas turbine or internal combustion engine
and subsequently used to produce electricity in a generator. The
waste heat from the gas turbine or internal combustion engine can
be used to produce steam for the water-gas reaction in the melter
unit. The electrical power from the gas turbine or internal
combustion engine generator may be supplied to assist the furnace
power supply.
[0034] However, the prior art plasma arc systems as described above
are found to have major problems in design and effectiveness. The
large, vertically-shaped crucible with its refractory liners and
metal framing can weigh over 40,000 pounds, which would be too
heavy to move and requires a fixed installation site. The large
volume between the arc electrodes at the upper surface of the melt
and the melt pool requires the long length of the electrodes to be
exposed to deterioration and requires frequent repair. The
positioning of the waste feed input in the ceiling wall results in
escaping gas, loss of heat, and difficulty in maintaining
operational plasma temperatures when the input port is opened
during feed cycles. The loss of plasma heat and large plenum volume
requires the waste to be resident for longer periods for processing
in the melt, resulting in uneven heating and stratification of
metal from slag in the melt pool. The metal not drained off tends
to form congealed deposits damaging the bottom floor of the
crucible and deteriorating its refractory lining, which then
requires total disassembly and rebuilding of the crucible floor for
maintenance. The vertical crucible design with waste input and
offgas output ports positioned at the top of the vessel makes it
difficult for disassembly and reassembly for repairs or
maintenance. Excess free carbon from organic constituents of waste
that remain too long in the plasma field can cause electrical
"crazing" or "incandescence" of electrical energy conducted through
the carbon to the molten pool, which can result in a loss of
conduction between the plasma arc electrodes generating the plasma
field and thereby reduce the effectiveness of the plasma arc
melting of waste. The vertical design of the crucible also requires
a large volume housing and frame structure to contain it and to
support the ancillary equipment, feed conduits, and electrical
connections for the crucible.
[0035] Referring to FIGS. 1 and 4, a preferred embodiment for an
improved plasma arc waste vitrification system is shown having a
crucible formed as a horizontally flat, enclosed container with a
bottom wall and side walls for containing a melt pool in the
crucible, and a ceiling wall connected to the side walls and
defining a low-ceiling plenum space above the top level of the melt
pool for containing a plasma of pyrolized gases generated from
waste material fed onto the melt pool. The plenum space of the
crucible communicates via a channel formed through the side walls
into a horizontally-adjacent, thermal resonant chamber (TRC) to one
side of the crucible. The thermal resonant chamber (TRC) provides
additional plenum volume along with the crucible's plenum space for
allowing complete pyrolization of gases generated from the waste
material into elemental constituents without the need to increase
the low ceiling height of the plenum space above the top level of
the melt pool. This horizontally-oriented design allows the system
components to be contained in modular structures dimensioned in
standard cargo container sizes so that they can be easily
transported in the standard cargo spaces of land, sea, and air
vehicles and assembled on location, and/or disassembled and removed
in modular fashion. In this example, the system components are
arranged to fit in standard 20-foot or 40-foot length container
cargo spaces.
[0036] Referring again to FIG. 1, the plasma arc crucible is fed
solid waste material by a bulk extruder coupled to a side port into
the crucible at the free end of the container for the crucible. The
off-gas channeled from the crucible plenum into the TRC containing
elemental free carbon is purged with oxygen from ambient air to
form useful gas byproducts (such as CO and H.sub.2). The carbon can
constitute 25% to 35% of organic solid waste, so its removal
reduces the problem of carbon incandescence in the crucible, as
well as converts it to useful gas byproducts that can be burned as
syngas in the auxiliary power generator. The purging with ambient
air in the TRC and exhaust through a quench pipe (on the right side
of the figure) to a scrubber unit allows the hot off-gases to be
cooled down quickly, thereby eliminating the long residency at
elevated temperatures that can result in the formation of toxic
gases such as dioxins and furans.
[0037] The plasma arc crucible and thermal resonant chamber (TRC)
can be arranged in respective halves of one standard container
space. The gas scrubber unit (with bag filtering can be arranged in
another standard container space stacked on top of the first
container. As a further option, a gas-fired auxiliary power
generator can be coupled with the scrubber unit in the one
container space to use the recovered gas byproducts as a syngas
fuel to fire the auxiliary power generator. A control room and
electrical equipment room (for power transformers and SCR
rectifiers) can be arranged in another standard container space
placed adjacent the first two. Thus, all the necessary system
components for a plasma arc waste processing facility can be stored
and transported in standard sized containers for convenient
transport and readily installed on location.
[0038] FIG. 2A shows a reverse side view of the preferred
embodiment of the plasma arc system shown in FIG. 1. The plasma arc
crucible is depicted schematically, showing its plasma arc
electrodes generating a plasma field above a molten pool contained
in the crucible, which is fed waste material input from the bulk
extruder and outputs molten material through an output drain for
cooling and glassification to a vitrified particle byproduct. The
bulk extruder can be of the piston- and cylinder type for pushing a
solid slug of waste material into the melt pool, or of the
auger-type for screw-feeding waste material into the melt pool.
[0039] FIG. 2B shows in greater detail a partially cut-away view of
the system components contained in the standard container spaces.
The components can be arranged in respective half-container spaces
and then assembled together (with a bolted frame) on location.
[0040] Referring to FIG. 3, the assembling of the half-containers
for the plasma arc crucible and the thermal resonant chamber (TRC)
is shown, with the respective halves slidable together on rails to
form one modular structure. The crucible half-container is shown
lined with a high-temperature refractory lining having a high
density such as 200 lbs/cubic-feet (cft). The TRC half-container is
shown lined with a lower temperature refractory lining having a
density such as 70 lbs/cft. The plasma arc crucible can thus be fit
in a standard cargo space of 8 feet height, 8 feet width, and 20
feet length (total volume 1280 cft). With walls lined with 1-foot
refractory liners, the total inner volume would be
18.times.6.times.6 ft, or a total of about 648 cft. The crucible
has an elongated shape such as an oval or trough shape and fits in
25% of the container space (half the volume of one half side of the
container), while the crucible plenum space plus the TRC as
additional plenum space would take up about 75% of the inner
volume, or a total of about 488 cft. For a 6-ft oval-shaped
crucible, the total plenum volume to surface area of the melt pool
would thus be a ratio in the range of about 20:1, which is much
greater than the plenum/surface area ratio of about 5:1 in the
conventional type of plasma arc system. The high plenum/surface
area ratio allows the pyrolization of waste material to take place
more quickly, thereby making more efficient use of the plasma
field, reducing the residency time of the waste in the melt pool,
and increasing the amount of waste processing throughput for any
given crucible volume.
[0041] The TRC acts as a holding chamber for cooling down off-gases
from the crucible from about 900.degree. C. to 200.degree. C.
before it is sent to the scrubbers. A quench (water-cooled) conduit
may also be used to convey the gases to the scrubbers. The quick
cooling down of the gases as they transit through the TRC and the
quench conduit prevents toxic byproducts such as dioxin and furans
from being formed in the gases. The gases can then be passed
through the scrubbers and filtration units to remove ash and
particulates then exhausted safely into the atmosphere.
Alternatively, hydrogen, carbon monoxide, and/or methane gas can be
recovered using gas separator units and used as combustible fuel in
a gas turbine or combustion engine for generation of auxiliary
power used by the facility.
[0042] As shown in FIG. 4, the plasma arc crucible 100 in shown
formed in a horizontally elongated shape, such as an oval or trough
shape, with a bottom wall 110, annular side walls 120, and a
ceiling wall 130 for containing a molten pool of material in the
crucible. The walls are made of a high-temperature refractory liner
material such as silicon carbide to contain molten pool
temperatures of 1100.degree. C. to 1300.degree. C. In a typical
example fitting in a half of a standard 20-ft container space, the
oval crucible has a 6-foot long-side diametral axis, the bottom
wall has a refractory liner thickness of about 3 inches, the side
walls have a refractory liner thickness of about 8 inches, and the
molten pool has a depth of about 9 inches. The melt pool height is
about 12 inches total (including bottom wall), and the plenum space
height above the level of the melt pool can be about 24-36 inches
(including the ceiling wall). A plurality of joule-heating
electrodes are mounted through the side walls 120 at spaced
intervals around the circumference thereof and project into the
molten pool for maintaining melt temperatures in the pool. The
joule-heating electrodes are driven by AC current. In this example,
3 molybdenum electrodes are powered by 3-phase AC current to heat
the melt pool.
[0043] A waste input feed 140 fed from a bulk extruder is oriented
sideways at an inclined angle at a side position through the side
walls 120 so that its feed opening is positioned in proximity above
a top surface of the molten pool. A melt output drain is formed
through the bottom wall and outputs a molten flow to a cooling and
vitrification pen outside the crucible container. Off-gases are led
off through a channel in the side walls communicating into the
adjacent thermal resonant chamber (TRC).
[0044] As shown in FIG. 5A, a pair of plasma arc electrodes 170a,
170b are mounted through the ceiling wall with respective ends
thereof positioned in proximity above the top surface of the molten
pool to deliver high electrical energy into the top surface of the
melt pool that pyrolizes the molten waste material into elemental
constituents. A high temperature plasma field is generated between
the two electrodes, which can have temperatures as high as
10,000.degree. C. to 12,000.degree. C. The arc electrodes are
energized with DC current which delivers electrical energy through
the ohmic resistance of the plasma field. Due to organic material
contained in typical solid waste material, a substantial amount of
carbon material can be pyrolized as free carbon C which has
conductivity that can cause electrical "crazing" or incandescence
of electrical energy in the plasma field, thereby reducing the
ohmic resistance of the plasma field and the electrical energy
delivered to the melt pool.
[0045] As discovered in the present invention, the electrical
"crazing" or incandescence of carbon in the plasma field can be
reduced or substantially controlled by injecting water (H.sub.2O)
into the plasma field. The water combines chemically with free
carbon to form hydrogen gas (H.sub.2) and carbon monoxide (CO)
which are removable as a usable off-gas byproduct. Ideally, the
water is injected in controlled amounts and at sufficiently high
pressures to traverse (before evaporation) into the plasma field so
that it can combine with the carbon. The metering and pressurizing
of the water shot can be obtained using an impeller pressurizer and
a liquid feed lance aimed toward the plasma field. The carbon
incandescence condition can be detected from outside the crucible
using an optical scope, temperature sensor, or plasma conductivity
sensor.
[0046] In FIG. 5B, a simple method for reducing incandescence of
carbon in the plasma field discharges wet waste material containing
water into the zone of the plasma field where it is pyrolized and
release the water to combine with carbon. This method was tested
and found to produce a noticeable reduction of the carbon
incandescence problem.
[0047] In the present invention, the configuration of the crucible
in a horizontally flat, ring shape provides a smaller melt pool
volume and crucible profile which would allow the crucible to be
transportable in standard size cargo containers. Sufficient
throughput is obtained by operating the crucible at higher flow
rates than the conventional plasma arc systems that use a large,
vertically standing crucible. Pushing the waste input feed sideways
through the side walls allows the melt pool to be "force-fed" for
higher flow rates, eliminates the feed input from taking up space
in the ceiling wall, and substantially reduces the bleed-off of
heat and gases caused in conventional crucibles by breaching the
ceiling of the containment during feed cycles. Positioning the arc
electrodes over the melt pool from a lower ceiling overhead, and
the joule-heating electrodes through the side walls to project into
the melt pool ensures that total electrical energy can be delivered
at high intensity into the pool so that the crucible can be
operated at higher flow rates.
[0048] The high intensity delivery of heat closer into the pool and
operation with shorter residency times and at higher flow rates
also result in the melt being driven as a molten, continually
flowing mixture, thereby reducing the tendency of metal elements to
separate and stratify from the slag elements. In the prior types of
crucibles, metal-slag stratification in the metal resulted in metal
congealing in deposits on the bottom wall and drain, and required
draining off of the slag through a separate port. By reducing the
tendency to stratify, the melt containing metal and slag mixed
together can be drained off at continuous high flow rates for
vitrification in the resultant glass particle byproduct. This also
reduces deterioration of the crucible components and the refractory
lining, thereby lessening the down time for repairs and
maintenance. The small-volume crucible with side-wall-mounting of
the joule-heating electrodes and waste input feed also make it
easier to remove the ceiling wall and access the interior of the
crucible for maintenance.
[0049] The use of a water lance or injector can reduce or eliminate
the problem of carbon incandescence, thereby improving the
efficiency of delivering electrical energy into the plasma field
and more effective processing by plasma arc melting of waste.
Reduction of carbon incandescence also reduces the rate of
degradation to the arc electrodes and extends their useful
life.
[0050] The small-volume crucible allows the core unit to be
contained in a standard-sized 20-ft or 40-ft cargo container spaces
for transport mobility. The ancillary thermal resonant chamber,
scrubber/filtration equipment, and control room and electrical
equipment are likewise divided up modularly and made transportable
in standard-sized cargo spaces. Thus, a complete plasma arc waste
vitrification facility can be contained in several transportable
cargo-sized containers, so that they can be easily transported and
assembled, then disassembled and moved in modular fashion. This
transportability allow this type of system to be moved into and
installed in populated areas without occupying a large volume space
that would raise environmental and aesthetic concerns. It also
allows the plasma arc facility to be easily transported by land,
sea, or air into remote areas.
[0051] Another improvement that can be developed for the plasma arc
vitrification system is to provide for automation in the electronic
control of the AC joule-heating and DC plasma arc electrodes.
Instead of separate controls which do not provide for coordination
between the joule heating and the plasma arc generation, a Smart
Electrode Control system can tie the operation of the DC power
controls with the AC power controls as a coordinated operation
using electronic control heuristics or algorithms for optimal
thermal processing. Sensors are used in or around the crucible to
detect voltages, currents, temperatures, and electrode status as
necessary to drive the Smart Electrode Control algorithms. New
types of optical or magnetic sensors may also be developed to gauge
the characteristics of the ongoing plasma arc generating extreme
temperatures within the crucible may be developed to refine the
sensor data and enhance the knowledge base to improve the
efficiency of the plasma arc vitrification system.
[0052] It is understood that many modifications and variations may
be devised given the above description of the principles of the
invention. It is intended that all such modifications and
variations be considered as within the spirit and scope of this
invention, as defined in the following claims.
* * * * *