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Solid waste is lifted into a feed system consisting of two retractable, isolation gates. The system is programmed to assure one of the two
gates remains closed at all times. After the first door closes,
nitrogen is used to pressurize the feeding chamber to minimize the
amount of air that can enter into the reactor with the feedstock. After
the materials enter the feed chamber, a hydraulic-powered ram feeder
pushes the waste feedstock into the plasma reactor. The feeding system
is designed to accommodate 30-gallon (113 liters) medical waste bags
and 250mm x 250mm x 250mm (10” x 10” x 10”) boxes. The section of the
feeder closest to the plasma reactor is refractory lined to ensure the
feeding chamber remains within a prescribed temperature limit, also
ensuring that any plastic bags containing medical waste do not
thermally degrade in the feeding chamber. A load cell monitors the
quantity of feedstock being introduced into the feeding sub-system.
The waste then enters the plasma reactor, made of mild steel and lined
with refractory and insulation, where the high temperature created by
the plasma torch dissociates the molecules that make up the waste into
their elemental constituents. The plasma reactor allows for a residence
time of 2.0 seconds based on a design basis gas flow of approximately
2.1 Nm3 (75 SCF)/minute.
The syngas is then processed further in a secondary reaction chamber
also made of mild steel. Depending on the operating mode and the waste
being processed, in this subsystem, the syngas can be further
conditioned to be used in one of several energy recovery options. If a
syngas utilization system is not available, the gas is transformed to
produce nitrogen, oxygen, carbon dioxide and water vapor. The residence
time in this subsystem is approximately 3.0 seconds, depending on the
waste being processed.
The resulting gas, at a temperature of approximately 1,100°C (2,000°F)
is then fed through a gas cleaning and conditioning system, where the
gases are rapidly cooled to ensure that there is no potential for the
generation or re-association of any undesired complex molecules or
formation of new compounds, such as dioxins or furans. The gas is then
cleaned to remove any entrained particulate matter and/or acid gases.
The gas cleaning and conditioning system consists of a venturi/packed
bed scrubber. The packed bed scrubber also serves to remove excess
moisture from the gas in conjunction with a cooling tower. A caustic
solution is added to the recirculating water in the venturi scrubber to
scrub the acid gases. Cooled water is recycled throughout the system
and there is a minimal amount of discharged water and dissolved salts
from the entire system.
Any inorganic constituents in the waste are melted (vitrified) by the
non-transferred arc torch in the graphite-lined plasma reactor bottom
into an environmentally safe, leach resistant, vitrified matrix. The
removal of the vitrified matrix presents no hazards of any kind to
personnel, requires no special tools and does not disrupt the operating
process. The vitrified matrix can be used in a variety of applications
including roadbed/fill construction, blast media and concrete aggregate.
Typical outputs associated from the PTDR-100 system are:
- 5 kilograms (11 pounds) per hour of the glass matrix
- 750 Nm3 (25,000 SCF) per hour of clean gas
- 4m3 per day or approximately 45 gallons per hour of water discharge, of which less than 2% is salt
The 100 kW plasma generation system utilized within the PTDR-100 system is comprised of two graphite electrode torches that function in two distinct operating modes: transferred and non-transferred. The transferred-arc and non-transferred-arc torches run at different times in the operating sequence. The transferred-arc torch is used during the initial start-up (to bring the plasma reactor to operating temperatures) and for the melting of the inorganic constituents in the waste feedstock; the non-transferred torch operates at all other times, particularly during feeding operations. In reality both operating modes reflect a transferred arc operation. In the "transferred-arc" operating mode, the arc is transferred between a bottom mounted graphite electrode plate and the transferred-arc torch. During non-transferred arc operations, the arc is transferred between the two torches.
The transferred-arc torch, mounted at the top of the plasma vessel, moves up and down within the plasma reactor, while the non-transferred-arc torch, mounted laterally and angled horizontally in the reactor, moves in and out along a generally radial direction. Due to this movement, the torches are housed within a sealing and insulating assembly. This assembly insulates the torch body and ensures that its structural elements are maintained within a prescribed temperature range. This avoids the need for additional cooling, which would remove excess thermal energy from the torch and thereby reduce the electrical-to-thermal efficiency.
The entire plasma generating system has an electrical-to-thermal efficiency greater than 80% and requires no pressurized external supply of carrier gas. The system supplies its own gas flow - approximately 5 liters (1.25 gallons) per minute of air. This small flow of air enhances the thermal energy distribution within the reactor.
The torch system is powered by an advanced Insulated Gate Bipolar Transistor (IGBT) power supply that provides the following advantages over other plasma torch power supplies:
- Requires an input current approximately 30% less than silicon controlled rectifier (SCR) systems
- Power factors around 0.97
- Low harmonic distortion (approximately 10 times less than SCR systems
- High arc stability compared to SCR systems
- Control panel size is approximately 66% smaller than a comparable SCR system
The PTDR-100 system can handle a wide variety of waste feedstocks including:
- Biomedical wastes, including infectious, pathological, chemo chemo
- Universal and/or industrial waste streams such as batteries and electronic waste, solvents and sludges
- Contaminated soils
- Incinerator fly ash
- Pharmaceutical waste
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