US20060272559A1

US20060272559A1 - Modular plasma ARC waste vitrification system

Info

Publication number: US20060272559A1
Application number: US11/144,948
Authority: US
Inventors: Samuel Liu
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-06-03
Filing date: 2005-06-03
Publication date: 2006-12-07

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.

Description

TECHNICAL FIELD

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

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.
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.
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.
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.
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° 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.
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

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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

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;
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;
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;
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;
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
FIG. 6 is a schematic diagram of a prior art plasma arc system for comparison to the present invention.
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

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.
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 12 a, bottom 12 b, and sides 12 c and 12 d. Bottom 12 b may have a generally V-shaped configuration. Reaction vessel 12 further includes at least one port or opening 14 a 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 14 a and 14 b, 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.
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 24 a and 24 b). 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 12 b in vessel 12. Slag in metal/slag layer 42 exits through opening or port 36 a into conduit 36. Conduit 36 may be formed of a conductive material, such as silicon carbide, to facilitate the flow of slag 44.
The temperature of slag 44 is maintained in chamber 30 by heaters 32 a and 32 b 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 32 a and 32 b in chamber 30. Heaters 32 a and 32 b may be constructed of silicon carbide or the like. Alternatively or in addition to heaters 32 a and 32 b, 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.
Reaction vessel 12 also includes a plurality of AC joule heating electrodes 50 a and 50 b positioned across from one another on sides 12 c and 12 d, respectively. In addition, electrodes 50 a-50 b 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 50 a-50 b 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.
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.
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.
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.
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.
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.
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 H₂). 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.
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.
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.
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.
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 18×6×6 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.
The TRC acts as a holding chamber for cooling down off-gases from the crucible from about 900° C. to 200° 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.
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° C. to 1300° 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.
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).
As shown in FIG. 5A, a pair of plasma arc electrodes 170 a, 170 b 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° C. to 12,000° 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.
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 (H₂O) into the plasma field. The water combines chemically with free carbon to form hydrogen gas (H₂) 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.
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.
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.
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.
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.
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.
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.
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.

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:

(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×8-ft×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.