Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02211924 1997-07-30
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METHOD FOR PURIFYING EXFLAUST FROM HOOD MANUFACTURING PROCESSES
The present invention relates generally to the
abatement of contaminant laden exhaust from various wood
manufacturing procESSSes, and more particularly to a method that
prevents destruction of the desired properties of the
manufactured wood ,products during the abatement process.
The manufacture of various wood products, such as
oriented strand board, medium density fiberboard, particle board,
or plywood, typically requires processing under elevated
temperatures to achieve certain desired properties and
characteristics for the wood products. One available method for
generating heat is to burn wood fuels such as bark, sander dust,
hogfuel, shavings, or waste lumber, since these fuels are readily
available as natural by-products of the wood manufacturing
process. The heat generated by burning these wood fuels is often
utilized to dry wood chips, fiber, veneer, or other types of wood
selected as components of the ultimate wood product.
The wood selected for drying is often treated with
resins or binders prior to the drying process. During the drying
process, these resins or binders, along with other contaminants
such as volatile organic compounds (VOC's), methanol, and
formaldehyde are emitted by the drying wood. In addition, the
wood fuel burning operation generates and emits other
contaminants, such as carbon monoxide and many hazardous air
pollutants (HAP's). Federal laws and regulations require that
the concentration of these aforementioned contaminants in the
process emissions must be reduced prior to discharge into the
atmosphere.
~ Regenerai~ive thermal oxidizers have heretofore been
utilized to control the concentration of VOC'S, methanol,
formaldehyde, carbon monoxide, and HAP's which are vented to the
atmosphere. In addition, electronic gravel filter beds or wet
electrostai~ic precipitators may be utilized where removal of
particulates such as a blue haze is also required.
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Generally, thermal oxidizers utilize a supplementary
heat source to increase the temperature of the contaminated
process emissions to a level above the ignition temperature of
the combustible contaminants, so ws to oxidize the combustible
contaminants, such as VOC'S, methanol, formaldehyde,_HAP'S, and
carbon monoxide. Regenerative thermal oxidizers recover heat
remaining in the cleansed exhaust gas to increase the temperature
of emissions entering the oxidizer thereby decreasing the amount
of supplemental energy required to raise the emission to its
ignition temperature.
However, when wood fuels are burned during the drying
process, the regenerative thermal oxidizers utilized under
present technology are not fully satisfactory because of the high
temperatures encountered therein, which typically range from
1400°F 0 760°C) to 1500°F(~815°C). More
specifically, the wood
fuel burning operation also produces by-products which include
potassium and sodium. In addition, both the heat recovery media
and the ceramic fiber insulation of the regenerative thermal
oxidizer contain silica. The potassium and/or sodium from the
process emissions react with the silica found in the regenerative
thermal oxidizer at temperatures exceeding 1100°F-to
1200°F, depending upon the concentration of the potassium
and/or sodium in the process emissions. As noted above,
regenerative thernial oxidizers must operate at temperatures well
above these threshold temperatures, thereby inherently-causing
the potassium and sodium to react with silica. Simply lowering
the operating temperature of the thermal oxidizer does not
provide a-suitable solution because at temperatures below 1400°.F,
a regenerative thermal oxidizer will not only fail to
properly remove existing pollutants, but the regenerative thermal
oxidizer will actually generate even more carbon monoxide and
HAP'S. The reaction of potassium and sodium with silica is
highly undesirable and problematic because the reaction destroys
the desired properties and characteristics of the manufactured
wood products, and thereby diminishes the effectiveness and
quality of the products.
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,~~TMMARY OF THE INVENTION
The aforesaid problems are solved, in accordance with
the present invention, by a method for purifying exhaust from
various wood manufacturing processes, which includes a
regenerative catalytic oxidizer. The regenerative_ catalytic
oxidizer utilizes a catalytic oxidation material to effect
oxidation of VOC'S, methanol, formaldehyde, carbon monoxide,
resins, binders, and HAP's at lower peak temperatures than those
temperatures at which regenerative thermal oxidizers function.
The catalytic oxidation materials include but are not limited to,
metal catalysts such as platinum, palladium, rhodium; iridium,
ruthenium, vanadium, copper, manganese, cerium~and chromium, as
well as metal oxide catalysts such as manganese oxide or chromium
oxide, and also any combination of the metal and/or metal oxide
catalysts.
In- accordance with the present invention, the
regenerative catalytic oxidizer effects oxidation of VOC'S,
methanol, formaldehyde, carbon monoxide, and HAP's at
temperatures typically ranging from 500°F to 1000°F
0 538°C), depending upon the constituents and respective
concentrations of the contaminants in the process emissions. By
effecting oxidation at temperatures well below the reaction
threshold temperature of 1100°F, the regenerative
catalytic oxidizer of the present invention precludes the
25' potassium and/or sodium from reacting with the silica, a reaction
which has heretofore compromised the properties and quality of
manufactured wood products.
The present invention advantageously accommodates.the
continuing use of wood burners, as opposed to requiring
alternative fuels which do not generate potassium or sodium, such
as natural gas. In the manufacture of wood, maintaining the use
of already abundant wood scraps as a fuel is beneficial because
an additional fuel cost is not required. In addition, the
present invention obviates a need for an insulating liner on
ceramic fibers in order to otherwise prevent potassium and/or
sodium from exposure to silica. The present invention also
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minimizes auxiliary heat requirements and electrical
consumption, which have heretofore increased abatement
expenses.
In accordance with an aspect of the invention A
method for purifying air contaminated with emissions
produced by a wood manufacturing process wherein a
regenerative catalytic oxidizer is utilized comprising a
plurality of regenerative beds, and, inlet, purge, and
exhaust valves and ducts, for each of the regenerative
beds, each regenerative bed also comprising a
regenerative heat exchange media containing silica
therein, a plurality of catalyst beds wherein each of the
catalyst beds corresponds to one of the regenerative
beds, and a heat source for combusting contaminants in
the emissions, the method comprising the steps of:
providing a plurality of catalyst beds effective to
remove the pollutants in the contaminated emissions, the
catalyst comprising metals or metal oxides selected from
the group consisting of platinum, palladium, rhodium,
iridium, ruthenium, vanadium, copper, manganese, cerium,
chromium, manganese oxide, chromium oxide, and any
combination thereof;
injecting the contaminated emissions which contain
potassium and/or sodium into an inlet regenerative heat
exchange media of the regenerative catalytic oxidizer;
heating the contaminated emissions to a peak
combustion temperature less than approximately 1000° F.
and greater than 500° F., wherein the temperature is
controlled by a thermally actuated control unit;
inputting the contaminated emissions into the
catalyst bed thereby oxidizing the contaminated emissions
at temperatures below 1000° F. and producing cleansed
air, and feeding the cleansed air thus produced into the
heat exchange media thereby precluding reaction of
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potassium and sodium, with the silica comprising the heat
exchange media;
conducting the cleansed air through a combustion
chamber common to the plurality of regenerative beds;
conducting the cleansed air through an exhaust
regenerative bed and out a respective exhaust duct;
cycling valves for recapturing the cleansed air and
controlling feed, exhaust, and purge modes for each
regenerative bed;
recapturing a portion of the cleansed air;
recycling the portion of the cleansed air as purge
air through one regenerative bed and its respective
catalyst bed; and
exhausting the purge air via the common combustion
chamber to and through a different catalyst bed of the
plurality of catalyst beds, and to and through a
corresponding regenerative bed of the plurality of
regenerative beds, for further purge and heating of the
catalyst bed and the corresponding regenerative heat
exchange media.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a diagrammatic representation of a wood
manufacturing/abatement system, in accordance with the
present invention; and
Fig. 2 is a diagrammatic representation of a
regenerative catalytic oxidizer, in accordance with the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In a preferred embodiment, the invention is in a
method for purifying air contaminated with emissions
produced by a wood manufacturing process wherein the
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emission contaminated air is input into a regenerative
oxidizer having regenerative heat exchange media
containing silica therein, and a heat source for
combusting the combustible contaminants in the emissions,
the method further comprising the steps of: heating the
contaminated emissions to a peak combustion temperature
less than approximately 1000°F, regulated by a thermally
actuated control unit; inputting the contaminated
emissions into an oxidation catalyst means located within
said oxidizer, wherein the oxidation catalyst effects
combustion of the contaminated emissions at temperatures
below 1000°F so as to produce cleansed air and
preclude a reaction of potassium or sodium generated by
the wood manufacturing process with the silica in said
oxidizer, said regenerative catalytic oxidizer comprising
a plurality of regenerative beds adapted to cyclically
perform a feed mode, a heat exchanger exhaust mode,
and a purge mode, a network of ducts comprising inlet,
purge and outlet ducts for each bed, a heat exchange
medium for each bed, oxidation catalyst for each bed for
receiving emission feed from each bed respectively, a
combustion chamber that is common to said oxidation
catalyst and plurality of beds, and cycling valves for
recapturing said cleansed air and controlling said feed,
exhaust and purge modes for each bed including a purge
inlet duct for each bed for bed purge and for purge of
its respective oxidation catalyst, recapturing said
cleansed air, recycling the cleansed air as purge air
through one
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regenerative bed and its respective oxidation catalyst, and
exhausting said purge air via said common combustion chamber to
and through a different bed of said plurality for further purge
' of its oxidation catalyst and respective regenerative bed.
Referring to Fig. 1, a wood manufacturing/abatement
system 10 is shown in accordance with the present invention. A
wood burner 12 hears ambient or pre-heated air by burning wood
fuels, such as bark, sander dust, hogfuel, shavings, or waste
lumber. The air heated by wood burner 12 is then utilized by
l0 dryer 14. The dryer 14 uses the hot air to dry a predetermined
type of wood which. is selected as a component of the product
produced by the system 10. For example, the dryer may be
utilized to dry wood chips, fiber, veneer or any other desired
types of wood.
Abatement of the contaminant-laden emissions produced
by wood burner 12, dryer 14, or elsewhere in the manufacturing
process, is then required. If particulates such as a blue haze
are also present, t:he process emissions may be conducted into a
particulate control. device 16 as perhaps an electronic gravel
filter bed or a wet electrostatic precipitator. In systems which
do not generate a :significant particulate concentration in the
emissions, particulate control device 16 may be omitted.
In accordance with the present invention, the
emissions are conducted from dryer 14, or alternatively,
particulate control device 16, into a regenerative catalytic
oxidizer 100 for oxidation of combustible contaminants such as
VOC~s, methanol, fozznaldehyde, carbon monoxide, resins, binders,
and HAP~S..
Referring now to Fig. 2, a regenerative catalytic
oxidizer 100 (hereinafter "RCO") is shown, in accordance with the
present invention a.s comprising three conventional regenerator
beds 102, 104, and 106, which are each provided with a heat
exchange media 108, 110, and 112, respectively. The RCO 100
further comprises three oxidation catalysts 114, 116, and 118.
The oxidation catalysts may be satisfactorily implemented as, for
example, a metal or a metal oxide deposited on a substrate. The
metals or metal oxides that may be utilized include but are not
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limited to, platinum, palladium, rhodium, iridium, ruthenium,
vanadium, copper, manganese, cerium, chromium, manganese oxide,
chromium oxide, as well as any combination thereof. A common
combustion chamber 120 communicates with each oxidation catalyst
114, 116, and 118. Fuel, for example natural gas, is supplied
to the combustion chamber 120 from a fuel controller and burner
122 or other heat source.
A contaminated emission feed duct 124 admits process
emissions from dryer 14 or, if required, particulate control
device 16, into the RCO 100 through inlet ducts 126, 128, and
130. It is noted that the optional particulate control device
16 is preferably positioned upstream of RCO 100 so as to prevent
rapid plugging or ~~fouling~~ of regenerator beds 102, 104, and
106. Cleansed air is conducted away from RCO 100 by outlet ducts
132, 134, and 136, respectively, which feed a cleansed exhaust
output duct 138, with the cleansed air vented to atmosphere by
an exhaust blower 140. A purge input duct 142 cyclically
recaptures cleansed air discharged by blower 140 in order to
purge beds 102, 104, and 106 as well as catalysts 114, 116, and
118 of contaminants trapped therein. The RCO 110 utilizes a
valve network to control the cyclic flow of contaminated
emissions and cleansed air to and from the RCO 100, respectively.
The three chamber system of the RCO 100 operates in
three cycles. In a first exemplary cycle, emissions pass into
regenerator bed 102 to pick up heat therefrom, then through
catalyst 114 for oxidation. Following oxidation of contaminants
to, for example, carbon dioxide and water, the cleansed air then
passes through regenerative bed 104 and catalyst 116, which are
operating in the regenerative, or heat receptive, mode for
discharge to atmosphere or into purge input duct 142, which in
turn conducts purified air to regenerator bed 106 and catalyst
118 to purge the bed and catalyst of contaminants. Thus, each
regenerator bed cyclically performs three modes of operation: a
feed mode, a heat exchange exhaust mode, and a purge mode. ,
It is noted that Fig. 2 shows a three-chamber RCO 100
having three regenerative beds 102, 104, and 106, and three
oxidation catalyst beds 114, 116, and 118, but the present
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invention is not 7-invited by the number of chambers in the RCO
100. It is further noted that an existing regenerative thermal
oxidizer may be equipped with catalytic oxidation beds in order
' to convert an exiting regenerative thermal oxidizer into RCO
100, in accordance with the present invention.
zn operation, the RCO 100 of the present invention
effects oxidation of the aforesaid combustible contaminants at
temperatures ranging from approximately 500°F 0260°C) to
1000°F
0 538°C) depending upon the constituents and the respective
concentrations of the contaminants in the process emissions.
These reduced peak combustion temperatures provided by the RCO
100 preclude a reaction of potassium and/or sodium produced by
wood burner 12 with silica from the heat recovery media 108, 110,
and 112, or from ceramic fiber insulation in the RCO 100. By
preventing the potassium and/or sodium from reacting with silica,
the present invention enhances the quality of wood products
because the desired properties and characteristics of the wood
products are retained.
TrThile the=_ preferred embodiment of the invention has
been disclased, it should be appreciated that the invention is
susceptible of modification without departing from the scope of
the following claims.
