Note: Descriptions are shown in the official language in which they were submitted.
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PLANT FOR GASIFICATION OF WASTE
BACKGROUND OF THE INVENTION
I . Field of the Invention
The present invention relates to a waste disposal plant and, in particular, to
a plasma
arc torch waste disposal plant for handling various kinds of waste.
2. Prior Art of the Invention
The daily generation of solid wastes is a fact of life in industrialized
society and their
disposal is becoming an ever-increasing problem. In the search for non-
polluting,
more efficient and less costly disposal, Energy from Waste (EFW) technologies
are
being developed such as gasification by means of a plasma arc torch in an
enclosed,
refractory lined, reactor vessel.
Plasma gasification is a non-incineration thermal process which uses extremely
high
temperatures in an oxygen starved environment to completely decompose input
waste
material into very simple molecules. The extreme heat and lack of oxygen
results in
pyrolysis of the input waste material, as opposed to incineration; pyrolysis
being the
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thermal decomposition of matter in the absence of oxygen. The by-products of
the
process are a combustible gas and an inert slag.
The heat source in a plasma gasification system is a plasma arc torch, a
device which
produces a very high temperature plasma gas. The plasma arc centreline
temperature
can be as high as 50,O00OC, and the resulting plasma gas has a temperature
profile of
between 3,000 and 8,000'C.
A plasma gasification system is designed specifically for the type, size and
quantity
of waste material which must be processed. A refractory lined reactor vessel
is
preheated to a minimum wall temperature of approximately 1100 C before any
processing commences, the actual ambient temperature is determined by the
waste
material being processed. The very high temperature profile of the plasma gas
then
provides an optimum processing zone within the reactor vessel through which
all input
waste material is forced to pass. The reactor vessel operates effectively at
atmospheric
pressure. In this environment, all of the volatile input material is
completely
decomposed, while non-volatile input material, such as glass, metals and dirt,
melt and
chemically combine to form a glassy slag.
Pyrolysis through plasma gasification provides for virtual complete
gasification of all
volatiles in the source material, while non-combustible material is reduced to
an inert
slag. With municipal solid waste as the input waste material, the product gas
and slag
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have very distinct characteristics. The product gas is high in hydrogen and
carbon
monoxide, with traces of methane, acetylene and ethylene; therefore, it can be
combusted very efficiently resulting in carbon dioxide and water vapour being
the
majority of gaseous exhaust to the atmosphere. The slag is a homogeneous,
silicometallic mass, monolithic in texture with leachate toxicity levels
orders of
magnitude lower than those of current landfill regulations.
Plasma gasification systems offer considerable versatility as to throughput
capacity.
Plasma arc torches are available commercially in sizes ranging from 50 kW to
over
60 MW; therefore, plasma gasification systems can be implemented at virtually
any
size capacity. The reactor vessel and plasma arc torch are specifically sized
to the
type and quantity of waste material to be processed. There are many plasma arc
torch manufacturers who could provide equipment for use in such systems.
Individual
torches can be selected to operate in particular waste processing applications
where
their operational capabilities can be best applied.
Applicant's United States Patent No. 5,280,757 issued January 25, 1994
describes
plasma gasification of waste. A gasification
plant is required which is useful for processing of many kinds of waste, such
as
municipal solid waste, boxed waste, liquid waste and granular waste. This
processing
must be efficient and safe, to avoid environmental contamination.
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Several United States patents relating to waste disposal vessels are:
United States Patent No. 4,989,522, issued February 5, 1991;
United States Patent No. 5,095,828, issued March 17, 1992.
SUMMARY OF THE INVENTION
The present invention provides a plasma gasification plant which can process
many
forms of solid and liquid waste such as, for example, municipal solid waste,
boxed-
type waste (i.e. biomedical waste), liquid waste and granular-type waste.
Waste feed
and processing mechanisms are provided to efficiently and safely process such
wastes.
According to a broad aspect of the present invention, there is provided a
plant for
gasification of waste such as municipal solid waste, boxed-type biomedical
waste,
granular contaminated solid, and liquid toxic waste, and the like, comprising:
a
refractory lined reactor vessel; a feed mechanism adapted to feed a
predetermined type
of waste into the reactor vessel with minimum exposure to atmospheric air; and
a
processing platform in the reactor vessel for initially receiving the waste.
DESCRIPTION OF THE INVENTION
It has been found that the manner in which the waste material is fed into the
reactor
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vessel can affect the efficiency of processing. The feed systems also affect
the
possibility of environmental contamination by release of hazardous gas to the
environment.
The present plant includes a plurality of feed mechanisms to accommodate solid
type
5 waste material such as municipal solid waste, boxed-type waste material such
as
hospital biomedical waste, granular-type waste material such as contaminated
soil and
liquid waste such as PCB oils. The feed mechanisms are capable of preventing
problematic amounts of air from entering the reactor vessel along with waste
material.
The feed systems are also capable of preventing the passage therethrough of
gases
from the vessel to the environment.
The plant includes a solid waste feed mechanism. The mechanism is useful for
feeding any type of solid waste into the reactor vessel for processing. The
mechanism
provides an access chute to the interior of the vessel having at least a pair
of gas-tight
barriers. The first gas-tight barrier is provided adjacent to the outboard end
of the
chute, while the second barrier is positioned in the chute, intermediate to
the first
barrier and the reactor vessel. The barriers act to provide a gas lock whereby
atmospheric air and hazardous gases can be trapped and evacuated, if required,
thereby
avoiding the passage of such gases along the chute between the plant exterior
and the
reactor vessel during the feed process. The evacuation of the air or gases in
the gas
lock is carried out by a purging system which acts between the barriers.
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The solid waste feed mechanism is provided with a ram mechanism for forcing
the
waste along the chute and into the vessel. The portion of the chute adjacent
the vessel
is formed to cooperate with the shape and position of the ram to allow the
formation
of a loose plug in the chute by compactable solid waste material. The plug,
when
formed, acts in the same way as the second barrier against passage of heat and
large
quantities of gas. Thus, the formation of a plug formed of compactable waste
allows
further wastes to be fed behind the plug without activation of the second
barrier.
A box feeder is provided on the plant of the present invention. Hospital
biomedical
waste is normally packaged in boxes. Since this waste material can be
infectious, it
is essential to input this waste to the reactor in as-received form. Boxed
type
biomedical waste often includes containers of liquid. If the liquid is not
released from
the containers prior to gasification, the containers will burst inside the
vessel causing
a rapid expansion of gaseous product.
The box feeder comprises a chute having an air lock chamber substantially as
described with reference to the solid waste feed mechanism. The chute is sized
to
accept boxes. Where required, the box feeder further comprises a means for
forcing
the box along the chute and into the reactor vessel, for example a hydraulic
ram, and
a means for piercing the box and its enclosed materials, to break open any
containers
of liquid within the box.
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The box feeder can be a separate chute opening into the vessel or can be
incorporated
into the solid waste feed chute.
To facilitate the processing of granular-type waste such as, for example,
contaminated
soil, a screw feed is provided. The screw feed is comprised of a spiral blade
in a
housing and is provided in association with an air lock chamber. The screw
feed is
positioned to input the materials into the vessel at the processing zone. In
one
embodiment, the screw feed is positioned outside the vessel to feed the
material
through a port positioned such that it drops into the processing zone. In
another
embodiment the screw feed is mounted to be retractably, extendable into the
vessel for
input of waste.
A port in the vessel permits the insertion of a liquid waste feeder. The
liquid waste
feeder is a spray head which injects wastes, for example by spraying or
atomization.
The spray head can be positioned to direct the wastes into the hottest portion
of the
plasma gas stream.
The liquid feed port can also function to inject steam into the vessel. The
injection
of steam enhances the processing of dry carbonaceous type waste.
Most waste materials will process very readily once introduced to the high
temperature
processing zone within the reactor vessel. Normally, processing is efficient
even if
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the input waste material falls into the molten slag pool prior to it being
fully gasified.
However, some waste materials, particularly those which contain a high
concentration
of elemental carbon, should be retained in a high temperature oxidizing
environment
until they are completely gasified. In the plant of the present invention, a
processing
platform is formed within the reactor vessel to receive the input waste
material. The
processing platform is formed such that as the material decomposes, the
gaseous
constituent exits the vessel and the molten solid constituent flows into the
molten slag
pool. Flow of the molten solid constituent away from the processing platform
and
into the slag pool, ensures the remaining unprocessed material is continuously
exposed
to the desired high temperature oxidizing environment.
A continuously operating plasma gasification plant requires the removal of
slag from
the vessel during processing without any adverse impact on the overall
efficiency of
the process. A means for allowing the molten slag to flow from the vessel
during
processing, without opening of the vessel to the ambient environment, is
provided.
The input of waste material into the reactor vessel in discrete quantities
causes
fluctuations in the rate of generation of gaseous product which in turn can
cause
fluctuations in the pressure within the vessel. Maintenance of atmospheric
pressure
is desired to maintain the efficiency of the system. For example, these
fluctuations
can be quite dramatic in the processing of boxed material such as biomedical
waste,
which can contain large concentrations of plastics and cellulosic material.
The product
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gas handling system of the present plant is responsive to such fluctuations in
product
gas flow to maintain atmospheric pressure within the reaction zone. A variable
speed
induction system has been provided which is responsive to fluctuations in the
rate of
generation of product gas.
BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiments of the present invention will now be described in
detail
in conjunction with the annexed drawing figures, in which:
Figure 1A is a side elevational view of a plant according to the present
invention with
a side wall of the reaction vessel cut away to expose the vessel interior;
Figure IB is a plan view of a reaction vessel useful in the present invention;
Figure 2 is a side sectional view of a solid waste feed mechanism useful in
the present
invention;
Figures 3A and 3B are side sectional and plan views, respectively, of a boxed
waste
feed mechanism useful in the present invention;
Figure 4 is a side section view of a granular waste feed mechanism useful in
the
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present invention; and
Figure 5 is a side section view of a liquid waste feed mechanism useful in the
present invention.
5
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to Figures 1A and 1B of the drawings, a side elevational view of
a
gasification plant 10 (Figure 1A only) according to the present invention is
10 shown. Plant 10 (Figure 1A only) has a reaction vessel 11 which has been
cut
away to reveal the interior thereof. Reaction vessel 11 houses a plasma torch
12
(Figure 1A only) for gasification of waste introduced thereto by means of
waste
feed mechanisms. Mechanism 12a (Figure 1A only) supports the plasma torch 12
(Figure 1A only) and permits rotational movements to change the focal point of
the plasma gases for optimization of the processing effect.
The waste feed mechanisms include: a solid-type waste feed mechanism,
indicated generally at 13; a box-type waste feed mechanism, indicated
generally
at 14 (Figure IA only); a granular-type waste feed mechanism, indicated at 15
and
a liquid-type waste feed mechanism, indicated generally at 16 (Figure 1A
only).
Mechanisms 13, 14 (Figure 1A only) and 15 feed the waste onto a processing
platform 17, within vessel 11, such that it is directly in the processing zone
of the
plasma torch. Processing
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platform 17 is formed to have an incline sufficient to cause the molten slag,
resulting from processing, to flow away from platform 17, as indicated by
arrows,
and toward a reservoir 18 where a molten slag pool forms. A weir 19 (Figure IA
only) is provided at a slag exit port 20 which provides for removal of molten
slag
during processing without opening of vessel 11 to the ambient environment. A
gas exit port 20a (Figure IA only) is provided as an exhaust for gases. The
plant
is substantially completely gas-tight when in use with the only access to the
reaction vessel being by the feed mechanisms and the exit ports, which are
sealable.
Plant 10 (Figure 1 A only) is mounted on a platform 21 (Figure 1 A only),
which is
hydraulically tiltable about pivotal connection 22 (Figure IA only) for
emptying
all or a portion of the molten slag, as required, after completion of a
gasification
process.
Referring to Figure 2, solid-type waste feed mechanism 13 is shown in greater
detail.
Mechanism 13 comprises a feed-hopper 25 which opens into a chute 26, which in
turn
opens into reaction vessel 11. Feed-hopper 25 diverges slightly as it opens
into chute
26 to prevent blockage. A gas-tight door 27 is provided at the outboard end of
hopper
25 and a hydraulically-driven, heat resistant, and preferably gas-tight, gate
28 is
provided in chute 26. When door 27 is closed and gate 28 is in its lowered
position,
a heat and gas lock chamber is formed therebetween. Chamber 29 can be purged
through valves 30a, 30b to prevent passage of air or gases between the
atmosphere and
the reaction vessel. Purged gas from the reaction process is returned to the
reaction
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vessel through lines (not shown), while air is vented to the atmosphere or
into a
combustion chamber as part of its excess air supply.
A ram 31 is provided to move waste along chute 26. Ram 31 is driven by
hydraulic
mechanism 32. The shield 33 of ram 31 is formed to prevent waste from falling
into
hydraulic mechanism 32 and is sized to fit within chute 26. Hydraulic
mechanism 32
is enclosed by a gas-tight housing and is actuated by a power source having
controls
such as limit switches. The limit switches control the length of the ram's
stroke, to
thereby control the amount of waste fed to the vessel with each stroke.
In use, waste is input to feed-hopper 25 while ram 31 is in the retracted
position and
gate 28 is in its lowered heat and gas-tight position. Door 27 is then closed
and
atmospheric air is purged from the mechanism with nitrogen gas through valves
30a
and 30b. Gate 28 is then raised to permit the waste to be moved along chute 26
by
action of ram 31 and into vessel 11, as indicted by arrow W. Relief valves
(not
shown) can also be provided to prevent a build up of pressure in the hopper
beyond
safe levels.
When the waste is fully input to vessel 11, gate 28 is again lowered and the
gases are
purged, thereby allowing door 27 to be opened without releasing hazardous
gases to
the environment.
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In an embodiment, ram housing can be formed such that it cooperates with the
cross-sectional shape of chute 26 to allow the formation of a plug of
compacted waste when ram 31 is activated. Once a plug is formed, the plug
will act as a heat and gas-tight barrier, in the same way as gate 28 and allow
purging of gas behind the plug and opening of door 27. Such a system allows
for continuous feeding of waste to the hopper as long as a complete plug
remains in chute 26. To ensure a good heat and gas-tight condition, gate 28
can be lowered on top of the plug.
Referring to Figures 3A and 3B, an embodiment of box-type waste feed
mechanism 14 is shown. Mechanism 14 comprises a box feed chamber 35 which
opens into a feed chute 26. Chute 26 in turn opens into vessel 11 (Figure 3B
only). In the embodiment, as shown, box feed mechanism 14 is associated with
the solid waste feed mechanism and chamber 35 is mounted at a side of chute
26.
Chamber 35 has a gas-tight door 36 through which boxes can be fed to chamber
35. A gas-tight gate 37 (Figure 3B only) separates chamber 35 from chute 26.
Gate 37 (Figure 3B only) is actuated by an air or hydraulic mechanism 38
(Figure
3B only) between an open position (as shown in Figure 3B) and a closed, gas-
tight position. When door 36 is closed and gate 37 (Figure 3B only) is in its
gas-
tight position, a gas lock is formed in chamber 35.
A plunger 40 is provided to move the boxed waste from chamber 35 into chute
26. Ram 31 moves box waste along chute 26 and into vessel 11 (Figure 3B only).
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In the preferred embodiment, as shown, a box piercing apparatus 41 (Figure 3A
only) is mounted in chute 26 to be actuated to pierce a box and its contents
to
break open any containers therein. Apparatus 41 (Figure 3A only) comprises a
plurality of stainless steel piercing rods 42 having sharpened tips 42'
(Figure 3A
only) mounted on a moveable base 43. Base 43 is connected to the shaft 44
(Figure 3A only) of a hydraulic mechanism. Apparatus 41 (Figure 3A only) is
enclosed in a gas-tight housing 46 (Figure 3A only).
In use, gate 37 (Figure 3B only) is closed and boxed waste is input to chamber
35
through door 36. Door 36 is then closed and sealed. To avoid the requirement
for
a purging system, preferably chamber 35 is sized to correspond to the shape
and
size of the boxed waste to be introduced so that substantially all of the
atmospheric air is forced from the chamber by input of a box. Alternately, a
purging system can be installed in chamber 35 and used after sealing of door
36 to
remove atmospheric air. Gate 37 (Figure 3B only) is then opened and plunger 40
is actuated to move the box into chute 26. Plunger 40 is retracted and gate 37
(Figure 3B only) is closed. Hydraulic ram 31 is actuated to move the box into
alignment with piercing apparatus 41 (Figure 3A only). Base 43 of apparatus 41
(Figure 3A only) is lowered such that rods 42 pierce the box and its contents.
Apparatus 41 (Figure 3A only) is thereafter raised and gate 28 is opened to
allow
ram 31 to move the box along chute 26 and into vessel 11 (Figure 3B only).
Referring now to Figure 4, an embodiment of a granular waste feed mechanism
15 is shown. Mechanism 15 comprises a feed hopper 50 which opens into a tube
51 housing a rotatable spiral blade 52. Spiral blade 52 has sufficiently small
diameter, when compared with that of tube 51 to prevent jamming of waste. The
clearance
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between the blade and the tube can be determined by the granule size of the
input
waste.
A housing 53 is sealably mounted about an end 51' of tube 51 . Housing 53
opens
into vessel 11 and has mounted therein a gas-tight, heat resistant gate 54.
Gate 54 is
5 hydraulically driven between an open position and a gas-tight, sealed
position. A gas-
tight door 55, disposed on the outboard end of feed-hopper 50, acts with gate
54 to
form a gas-lock chamber 56 therebetween which can be purged by use of valves
57a,
57b and 57c.
Mechanism 15 is adapted to feed the waste directly to the processing zone of
the
10 reaction vessel by insertion of tube 51 into reaction vessel 11. Tube 51 is
slidably
moveably within housing 53 between a position wherein tube 51 is retracted
from
vessel 11 and gate 54 can be closed and a position, as shown in Figure 4,
wherein a
portion of tube 51 extends within vessel 11. Tube 51 is driven by a hydraulic
mechanism 58.
15 In use, with mechanism 15 fully retracted from vessel 11, gate 54 and door
55 are
sealed and shaft 51 and chamber 56 are purged with nitrogen by use of valves
57a and
57c. Door 55 is opened and granular waste is fed to feed-hopper 50. The waste
drops down by gravity into tube 51 and about blade 52. Door 55 is then closed
and
chamber 56 is purged with nitrogen by valves 57b and 57c. Gate 54 is opened
and
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hydraulic mechanism 58 is actuated to drive tube 51 within housing 53 and past
gate
54 to extend into vessel 11. Spiral blade 52 is then actuated to rotate within
tube 51
to carry the waste along tube 51 and input it to vessel 11.
When desired, rotation of blade 52 is stopped and tube is retracted from
vessel 11 and
past gate 54 by hydraulics 58. Gate 54 is then closed and the process can be
repeated.
Referring to Figure 5, a feed mechanism 16 for liquid waste is shown.
Mechanism
16 comprises a spray nozzle 60 for injecting (i.e., spraying or atomizing)
liquids.
Liquids are fed by a pump 61 to nozzle 60 from reservoir 62 through line 63.
Nozzle 60 is preferably positioned in vessel 11 such that the liquid is fed
directly into
the processing zone of the plasma torch 12.
When liquid waste is not being handled, steam can be fed through nozzle 60 to
assist
in the processing of dry carbonaceous waste.
The mechanisms for feeding solid waste, boxed waste, granular waste and liquid
waste, as described, need not all by present in the same plant, as the
presence of more
than one may not be required for the particular processing of waste being
undertaken.
Alternately, the mechanisms can all be present in the plant at all times, but
only be
used as needed.
