Note: Descriptions are shown in the official language in which they were submitted.
~094118286 2 15 ~ 8 ~ ~ PCT/GB94100205
THERMAL TREATMENT OF CARBONACEOUS MATERIAL
The present invention is concerned with the
thermal treatment of carbonaceous material, such as
carbonaceous waste materials.
It is known to dispose of waste materials by
heating them to effect thermal degradation. This can be
carried out in conventionally heated furnaces or, more
commonly in recent times, by using microwave radiation.
Waste materials which are not themselves susceptible to
microwave radiation, can be treated by contacting them with
microwave-heatable materials such as carbonaceous materials.
These processes are described in my PCT patent applications
nos. WO 88/08871, WO 89/04355 and WO 92/02598 to which
reference should be made for further details.
In these processes, either the heating medium
itself or the product of the pyrolysis or both, comprise
carbon and in the treatment of scrap material on a
continuous basis, the amount of carbon builds up. Normally,
it is either recycled or it is treated to remove materials
such as metals, and it is then disposed of to waste. In
many of the known processes, the gaseous pyrolysis products
are recycled or otherwise treated to maintain the energy
balance and reduce the overall cost of treating the waste.
We have now found that further very substantial
economies can be achieved by modifying the known processes
so that the hot carbon is removed from the microwave zone
directly to a second zone in which heat-exchange is
WO94/18286 PCT/GB94/00205
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effected. In this way, significant advantages can be
obtained.
According to the present invention, there is
provided a method of thermal treatment of carbonaceous
material, which comprises
(a) thermally treating a medium comprising carbonaceous
material by means of microwave radiation in a first zone
having an atmosphere under which flame generation is
substantially prevented;
(b) causing at least part of the treated medium to flow to
a second zone; and
(c) heating heat-exchange means directly or indirectly in
the second zone by means of the medium.
In step (c), the hot medium may be contacted
directly with the heat exchange means in an non-oxidising
atmosphere, or oxygen can be present in which case thermal
energy is released by combustion of the hot medium.
According to a first aspect of the present
invention, the medium consists essentially of carbonaceous
material; the latter may consist essentially of elemental
carbon, or it may be capable of being pyrolysed to elemental
carbon by microwave radiation.
According to a second aspect of the present
invention, the medium contains carbonaceous material as
hereinbefore described admixed with a secondary material not
itself susceptible to microwave heating. The nature of the
secondary material is such that contact of the material with
hot elemental carbon can yield heat from the former. The
contact of the carbon and secondary material may merely
involve heat transfer from the former to the latter;
alternatively the contact may involve a chemical interaction
therebetween. A preferred secondary material includes metal
ores susceptible to reduction by hot carbon, for example
zinc or aluminium ores. In this way, metals can be obtained
from the process.
The carbonaceous material typically comprises
~094/18286 2 1 ~ 3 ~ O ~ PCT/GB94/00205
waste hydrocarbon or carbohydrate material. Examples of
such materials include natural or synthetic rubber compounds
(which may be pyrolysable to elemental carbon by microwave
radiation), agricultural waste material such as citrus
fruit peel, olive waste products and nut shell, non-
putrescent domestic waste, toxic waste, hospital waste and
halogenated hydrocarbons, or other types of organic refuse.
In some embodiments (and particularly where the
material being thermally treated is a rubber compound), it
is preferred that the carbonaceous material should contain
carbon filler. A particularly preferred carbonaceous
material is carbon-filled vulcanised rubber, such as a waste
tyre compound in chopped or finely divided form.
The thermal treatment of the medium typically
involves pyrolysis of the carbonaceous material, whereby
fission of carbon-carbon bonds, and of more polar chemical
bonds, yields elemental carbon which can then be fed or
passed to the second treatment zone. The thermal treatment
involves heat transfer or chemical interaction between the
carbonaceous material and any secondary material not
susceptible to microwave heating, such as the reduction of
metal ores as hereinbefore described.
The microwave radiation is preferably employed at
such a power and for sufficient time as to effect the above
described pyrolysis of the carbonaceous material, and, where
appropriate, also treatment of the secondary material as
described above. Typically the temperature of the medium in
the first zone is from about 800CC upwards.
Typically the microwave radiation is supplied to
the first zone by means of at least one, preferably more
than one, microwave generators. In the case where a
plurality of generators are employed, the generators may be
of similar power output, or, alternatively of graduated
power output (for example, of gradually increasing or
decreasing power output in the downstream direction). It is
preferred that the microwave generators operate at a power
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output in the region of 60kw.
The atmosphere of the first zone is preferably
substantially oxygen-free. Preferably the atmosphere is
reducing, typically comprising a hydrocarbon medium. or an
inert gaseous medium such as nitrogen. It is preferred to
use a reducing atmosphere when environmentally sensitive
compounds, such as halogenated compounds and sulphur are
expected in the gas products (for example when the material
being treated comprises CFC's, PCB's or mercaptans). These
compounds would therefore be converted to acids (HF, HCl,
HBr, H2S) which can be readily scrubbed from the exhaust
gases.
It is further preferred that the atmosphere of the
first zone is self-perpetuating, whereby pyrolysis of the
waste material during microwave radiation yields hydrogen
and low molecular weight gaseous hydrocarbons from the
carbonaceous material, so as to provide a reducing
atmosphere in the first zone.
The gaseous hydrocarbon products produced in
thermal treatment step (a) are preferably recycled, the
recycled hydrocarbon products optionally being mixed
together with other air-excluding gases, such as carbon
monoxide or carbon dioxide. The recycling is advantageous
in lessening the requirement for separate supply of gaseous
hydrocarbons, and is also beneficial in the following
instances:
(i) where a decrease in the partial pressure of
condensible products of thermolysis is required, in order to
preclude condensation (in this case, the recycled
hydrocarbon stream would be cooled, condensate removed and
the stream re-heated);
(ii) where a carrier gas is required to remove products
from the treatment zones; and
(iii) where an increase in partial pressure of hydrogen is
required to strengthen the reducing power of the atmosphere
of the first zone.
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'~094/18286 PCT/GB94/00205
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The heat exchange means are in the second zone, so
that a heat exchange fluid can be heated. Typically the
heat exchange fluid comprises water or air, or any other
fluid which can be heated in a substantially controlled
fashion. Heating water to form steam is preferred.
It is further preferred that exit means are
provided for transfer of thermal energy generated in the
method to a heat exchange apparatus or the like, where its
thermal properties can be utilised to generate heat in a
heating system. It is further preferred that gaseous outlet
means are provided, for transfer of hot gaseous by-products
to heat exchange apparatus according to the invention.
In a preferred embodiment, the second zone
contains an atmosphere which promotes combustion of hot
elemental carbon. Typically the second zone atmosphere
comprises an oxygen-containing medium, the oxygen preferably
being present at a level of at least about 5% by volume.
It is preferred that gaseous communication
between the first and second zones is substantially
precluded. Preferably baffle means or other gas precluding
means, such as air locks or the like, are provided between
the first and second zones for this purpose.
The method is typically carried out in a thermally
resistant housing which is preferably resistant to
temperatures of up to about 1800-2000C. The housing may
typically be of fire brick-lined or refractory-lined,
stainless steel or ceramic material.
In a first embodiment of the invention, it is
preferred that step (b) of the method involves feeding the
thermally treated medium along an inclined transfer surface
arranged to communicate between the first and second zones.
Typically, the medium is allowed to flow along the inclined
surface, under the influence of gravity, from the first zone
to the second zone.
The medium may be fed, typically by means of a
gravity feed hopper, into the first zone, to be collected on
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the transfer surface. The first zone may comprise an inner,
substantially circular chamber, in which case the second
zone preferably comprises a circumferential annular chamber
arranged around the inner chamber. Preferably. the transfer
surface is defined by the floor of the chambers, and
typically provides a substantially conical or frustoconical
surface over which the carbonaceous material can flow.
Typically the medium is fed to an apex region of the conical
transfer surface.
In a second embodiment. the first and second zones
respectively comprise first and second chambers. arranged
such that the first chamber is superjacent the second
chamber. The carbonaceous material can sink through the
first chamber, such that at least part of the treated medium
is passed to the second chamber.
The chambers in the second embodiment may be
arranged remote from one another, communication between the
remote chambers being provided by a conduit arranged to
extend therebetween. The conduit may be provided with an
air lock for precluding gaseous communication between the
chambers.
Alternatively, the superjacent first chamber may
be at least partly enclosed by the second chamber, such that
the treated material can be allowed to sink directly from
the first chamber to the second chamber.
There is further provided by the present invention
apparatus for thermally treating a medium comprising a
carbonaceous material, which apparatus comprises:
(a) a housing comprising first and second zones, which
zones are arranged such that at least part of the
medium can flow directly from the first zone to
the second zone;
(b) at least one microwave radiation source arranged
so as to be capable of directing microwave
radiation into the first zone to thermally treat
the medium;
SUBSrlTUTE SHEET (RUL~ 26)
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2153~08
(c) means for substantially precluding gaseous
communication between the zones; and
(d) heat exchange means provided for the second zone,
such that the means can be directly or indirectly
heated in the second zone by at least part of the
thermally treated medium.
The parts of the apparatus according to the
invention may be substantially as hereinbefore described
with reference to the method according to the invention.
The present invention further comprises a heating
system, which comprises:
(i) apparatus according to the invention;
(ii) thermal storage means; and
(iii) a heat exchange system arranged to convey heat
generated in said apparatus to said thermal storage means.
One highly preferred aspect of the present
invention comprises a method of thermal treatment of waste
material not susceptible to microwave radiation, which
comprises contacting the waste material, in a first zone
under an atmosphere where flame generation is substantially
prevented, with a bed of pulverulent material which
comprises carbon in elemental form, or a material which is
capable of being pyrolysed to elemental carbon by microwave
radiation; and heating the pulverulent material by means of
microwave radiation such that thermal energy is transferred
to the waste material to pyrolyse it to carbon: and wherein
carbon is removed from the first zone to a second zone and
immediately treated to release thermal energy therefrom for
recovery by a heat-exchange medium.
In this method, operated on a continuous basis,
carbon is formed in the first zone and thus, to prevent a
build-up, some carbon is removed continuously or
intermittently. According to the present invention, the
utilisation of this carbon to release thermal energ~ is a
surprising and highly advantageous concept since the
material would otherwise not be utilised to release energy.
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The energy can be released by direct heat exchange or by
permitting combustion in the second zone, to release further
heat.
The invention will now be further illustrated with
reference to the accompanying drawings, like parts being
denoted by like numerals, wherein:
Figure l is a schematic representation of a
portion of a first embodiment of apparatus according to the
present invention;
Figure 2 is a schematic representation of a second
embodiment of apparatus according to the present invention;
and
Figure 3 is a schematic representation of a
modification of the apparatus of Figure 2.
Referring to Figure l, there is shown apparatus
generally designated l which comprises a feeder 2a and a
treatment vessel 2b. Treatment vessel 2b comprises a
reducing chamber 3 and a circumferentially extending
oxidising chamber 4.
Although not fully shown in the drawing, reducing
chamber 3 comprises an inner circular zone of vessel 2b, and
oxidising chamber 4 comprises a circumferential annular
chamber around chamber 3.
A plurality of microwave generators 5 are arranged
relative to treatment vessel 2b so as to be capable of
directing microwaves into the reducing chamber 3. The
microwave generators 5 may all be of substantially similar
power output, or alternatively, of graduated power output
(for example, of gradually increasing or decreasing power in
the direction of travel of material to be pyrolysed through
chamber 3 as shown by arrow A).
Transfer surface 6 is inclined so that elemental
carbon present in chamber 3 can flow to oxidising chamber
4.
Feeder 2a is arranged above chamber 3, so that any
waste material contained in feeder 2a can be allowed to fall
SUBStlME SHEET (RULE 26)
V094/18286 PCT/GB94/00205
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onto transfer surface 6 within chamber 3. Feeder 2a
comprises first and second valve members 7 and 8, for
controlling discharge of waste material from feeder 2a.
Feeder 2a includes an intermediate chamber 9, in which waste
material to be treated can be stored if required.
Intermediate chamber 9 is provided with first and
second vacuum pumps 10 and 11, wherein vacuum pump 10 is in
gaseous communication with the external atmosphere and
vacuum pump 11 is in gaseous communication with a
hydrocarbon gaseous recycling system of the apparatus (the
recycling system not being shown in the drawing).
The material to be treated in apparatus 1
comprises a waste material comprising, or pyrolysable to,
elemental carbon.
The waste material is fed into chamber 3 via
feeder 2a. Although feeder 2a is shown as a gravity fed
mechanism, it is of course appreciated that the waste
material could be fed to chamber 3 in a number of ways, such
as an inclined slope feeding mechanism, a substantially
horizontal feed conveyor or the like.
On arrival in chamber 3, the waste material is
subjected to microwave radiation for a sufficient time and
intensity so as to convert the material to a medium
consisting essentially of elemental carbon. Chamber 3
contains an atmosphere 12 under which flame generation is
substantially prevented, and typically comprises a nitrogen
or hydrocarbon reducing gaseous medium. Atmosphere 12 is
self-perpetuating, in that pyrolysis of the waste material
during the microwave radiation, yields low molecular
weight gaseous hydrocarbons from the waste material.
The temperature of material being pyrolysed will
depend on the material itself and may range, for example,
from about 800C upwards, as will be clear to those skilled
in the art. A common temperature range is 800 to 1200CC
but higher temperatures can certainly be used depending on
the material being treated.
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Although not shown in the diagram, chamber 3 is
provided with a gas inlet for communicating at least the
initial reducing atmosphere to chamber 3. Outlet 13
provides an exit for gaseous hydrocarbon by-products which
are transmitted to condensation or distillation processing
systems (not shown). Baffle 14 is provided to obviate
gaseous communication between chambers 3 and 4.
Following pyrolysis of the waste material to
elemental carbon in chamber 3, the elemental carbon medium
is conveyed to chamber 4. Chamber 4 can have a non-
oxidlsing inert atmosphere so that mere heat exchange is
effected. However, it can instead contain an oxygen-rich
atmosphere 15. Inlet 16 is provided to communicate an
oxygen rich gas to chamber 4, and outlet 17 is provided for
removing hot combustion gases from chamber 4. On contact of
the elemental carbon with atmosphere 15, the former combusts
to yield hot gaseous products which escape chamber 4 via
outlet 17.
Outlet 17 conveys the hot gaseous products to a
heat exchange apparatus or the like, where their thermal
properties can be utilised to generate heat in a heating
system.
Heat exchange apparatus (not shown) are provided
in chamber 4, so that a heat exchange fluid, such as water,
can be directly or indirectly heated in chamber 4 by means
of the hot elemental carbon.
Referring to Figure 2, there is shown apparatus
generally designated 1 which comprises a feeder 2a, a
reducing chamber 3 and a subjacent oxidising chamber 4.
A microwave generating source 5 is arranged to
direct microwaves along conduit 18 into reducing chamber 3.
Conduit 18 is provided with a pressure resistant microwave
window 19.
Feeder 2a is provided with first and second vacuum
pumps 10, 11 as described with reference to Figure 1.
Outlet 13 provides an exit for gaseous hydrocarbon by-
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_
products which are transmitted to condensation or
distillation apparatus.
Conduit 20 extends between the base portion 3a of
chamber 3 and the top portion 4a of chamber 4. Conduit 20
is provided with an air lock 21 for precluding gaseous
communication between chambers 3 and 4.
Chamber 4 is provided with an inlet 16 for supply
of an oxygen-rich gas and outlet 17 is provided for the
removal of hot combustion products from chamber 4. Outlet
17 conveys the hot gaseous products to heat exchange
apparatus.
The gaseous supply and temperature conditions
within chamber 4 are such that a fluidised bed combustor is
provided therein.
In use, the carbonaceous material is fed to
chamber 3 and is pyrolysed to yield elemental carbon. The
treated medium, which is rich in elemental carbon, is
allowed to sink through chamber 3 and conduit 20, so as to
be transferred to the oxygen-rich environment of chamber 4.
Referring to Figure 3, it can be seen that the
arrangement of chambers 3 and 4 is similar to that in Figure
2.
However, Figure 3 shows chamber 3 as comprising a
receptacle 22 having an open end 23. Chamber 4 is arranged
to circumferentially enclose the main body 24 of receptacle
22.
In use, the carbonaceous material is pyrolysed in
receptacle 22, the treated medium sinks through receptacle
22 and is passed through open end 23 to chamber 4.
SU~Srl~E SHEET tRULE 26)
