U.S. patent application number 12/239385 was filed with the patent office on 2009-02-26 for exhaust gas purifier.
This patent application is currently assigned to NGK INSULATORS, LTD.. Invention is credited to Kenji DOSAKA, Atsuo KONDO, Masanobu MIKI, Takeshi SAKUMA.
Application Number | 20090049821 12/239385 |
Document ID | / |
Family ID | 38580967 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090049821 |
Kind Code |
A1 |
SAKUMA; Takeshi ; et
al. |
February 26, 2009 |
EXHAUST GAS PURIFIER
Abstract
An exhaust gas purer (100) includes a plasma reactor (10) that
includes a plasma generating electrode (1) formed by hierarchically
stacking a plurality of unit electrodes (2) at specific intervals,
the plasma reactor generating plasma in a space formed between the
unit electrodes (2) by applying a pulse voltage between the unit
electrodes (2) so that exhaust gas can be treated via a reactions
and a power supply that generates the pulse voltage, each of the
unit electrodes (2) including a conductive film (4) and a
dielectric (3) disposed on at least one side of the conductive film
(4), the unit electrodes (2) being stacked so that the dielectric
(3) is positioned on at least one of opposing sides of the unit
electrodes (2), the pulse voltage having a pulse width of 1 .mu.m
to 1 mm and a field intensity of 5 kV/cm or more, and the amount of
energy supplied when applying the pulse voltage being 10 J or less
per microgram of a treatment target substance. The exhaust gas
purifier can efficiently remove particulate matter via a reaction
when introducing a treatment target fluid containing particulate
matter into the exhaust gas purifier.
Inventors: |
SAKUMA; Takeshi;
(Nagoya-city, JP) ; KONDO; Atsuo; (Okazaki-city,
JP) ; DOSAKA; Kenji; (Wako-shi, JP) ; MIKI;
Masanobu; (Wako-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 320850
ALEXANDRIA
VA
22320-4850
US
|
Assignee: |
NGK INSULATORS, LTD.
Nagoya-city
JP
HONDA MOTOR CO., LTD.
Tokyo
JP
|
Family ID: |
38580967 |
Appl. No.: |
12/239385 |
Filed: |
September 26, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2007/056146 |
Mar 26, 2007 |
|
|
|
12239385 |
|
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Current U.S.
Class: |
60/275 |
Current CPC
Class: |
B03C 3/08 20130101; B01D
2259/818 20130101; F01N 2240/28 20130101; B01D 53/32 20130101; B03C
3/47 20130101; B03C 2201/30 20130101; F01N 3/0275 20130101; B03C
3/60 20130101 |
Class at
Publication: |
60/275 |
International
Class: |
F01N 3/00 20060101
F01N003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2006 |
JP |
2006-094339 |
Claims
1-5. (canceled)
6. An exhaust gas purifier comprising: a plasma reactor that
includes a plasma generating electrode formed by hierarchically
stacking a plurality of unit electrodes at specific intervals, the
plasma reactor generating plasma in a space formed between the unit
electrodes by applying a pulse voltage between the unit electrodes
so that exhaust gas introduced into the space can be treated via a
reaction; and a power supply that generates the pulse voltage, each
of the unit electrodes including a conductive film and a dielectric
disposed on at least one side of the unit electrodes; the pulse
voltage having a pulse width of 1 .mu.s to 1 ms and a field
intensity of 5 kV/cm or more; and the amount of energy supplied
when applying the pulse voltage being 10 J or less per microgram of
treatment target substance.
7. The exhaust purifier according to claim 6, wherein the pulse
voltage has a pulse width of 20 to 50 .mu.s and a field intensity
of 10 to 200 kV/cm, and the amount of energy supplied is 0.05 to 10
J or less per microgram of the treatment target substance.
8. The exhaust gas purifier according to claim 6, wherein the
distance between the adjacent unit electrodes is 2 mm or less.
9. The exhaust gas purifier according to claim 6, wherein a pulse
cycle when applying the pulse voltage is 10 .mu.s to 10 ms.
10. The exhaust gas purifier according to claim 6, wherein a rise
time of the pulse voltage is 100 .mu.s or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust gas purifier.
More particularly, the present invention relates to an exhaust gas
purifier which can efficiently remove particulate matter via a
reaction when introducing a treatment target fluid containing
particulate matter into the exhaust gas purifier.
BACKGROUND ART
[0002] It is known that a silent discharge occurs when disposing a
dielectric between two electrodes and applying a high alternating
voltage or a periodic pulse voltage between the electrodes, and
active species, radicals, and ions are produced in the resulting
plasma field to promote a reaction and decomposition of gases,
particulate matter, and the like. It is known that this phenomenon
can be utilized to remove toxic components contained in engine
exhaust gas or incinerator exhaust gas.
[0003] For example, a plasma reactor that allows engine exhaust gas
or incinerator exhaust gas to pass trough a plasma field to treat
NO.sub.x, carbon particles, HC, CO, and the like contained in the
engine exhaust gas or incinerator exhaust gas has been disclosed
(e.g., Patent Document 1).
[0004] When treating components contained in exhaust gas by
utilizing plasma, a large amount of energy is required to treat
particulate matter (PM) such as carbon particles as compared with
other chemical compounds contained in exhaust gas.
Patent Document 1: JP-A-2001-164925
DISCLOSURE OF THE INVENTION
[0005] Moreover, the PM removal rate is comparatively low although
a large amount of energy is required. Specifically, the PM removal
performance increases to only a small extent even if the amount of
energy supplied to the exhaust gas purifier is increased.
[0006] When treating exhaust gas using an exhaust gas purifier that
generates plasma between unit electrodes and treats PM via a
reaction, PM is normally charged by plasma when exhaust gas is
introduced between the unit electrodes. The charged PM moves and
adheres to the surface of the electrode due to the force of an
electric field generated between the unit electrodes. The PM is
thus collected on the surface of the unit electrode. The collected
PM is oxidized to be removed from the exhaust gas by oxidizing
active species (e.g., ozone) produced by the effect of plasma.
[0007] As described above, the PM oxidation treatment process using
the exhaust gas purifier that utilizes plasma includes the
collection step and the oxidation step. Since the PM removal
performance is not improved even if the amount of energy supplied
is increased, the rate-limiting step of the PM removal process is
considered to be the collection step. For example, when increasing
the total area of the unit electrodes of an exhaust gas purifier
having a PM removal rate of about 60% in order to improve the
collection capability, it is estimated from the following Deutsch
Equation that the PM removal rate increases by about 20% by
doubling the total area of the unit electrodes, and increases by
about 30% by increasing the total area of the unit electrodes by a
factor of three.
Deutsch Equation
[0008] Collection efficiency (PM removal
efficiency)=100.times.(1-Exp(-G.times.A/Q))
Q: Gas flow rate A: Effective (collection) electrode area G:
Separation rate (dust separation rate due to electrode)
[0009] Specifically, it is considered that the dust collection
capability is improved by increasing the total area of the unit
electrodes so that the PM removal rate tends to increase. However,
the size of the plasma reactor of the exhaust gas purifier
increases to a large extent when increasing the area of the unit
electrodes by a factor of two or three. Moreover, it is necessary
to use a large power supply.
[0010] The present invention was conceived in view of the
above-described problems. An object of the present invention is to
provide an exhaust gas purifier which can efficiently remove
particulate matter via a reaction when introducing a treatment
target fluid containing particulate matter into the exhaust gas
purifier.
[0011] in order to achieve the above object, the present invention
provides the following exhaust gas purifier;
[1] An exhaust gas purifier comprising a plasma reactor that
includes a plasma generating electrode formed by hierarchically
stacking a plurality of unit electrodes at specific intervals, the
plasma reactor generating plasma in a space formed between the unit
electrodes by applying a pulse voltage between the unit electrodes
so that exhaust gas introduced into the space can be treated via a
reaction and a power supply that generates the pulse voltage, each
of the unit electrodes including a conductive film and a dielectric
disposed on at least one side of the conductive film, the unit
electrodes being stacked so that the dielectric is positioned on at
least one of opposing sides of the unit electrodes, the pulse
voltage having a pulse width of 1 .mu.s to 1 ms and a field
intensity of 5 kV/cm or more, and the amount of energy supplied
when applying the pulse voltage being 10 J or less per microgram of
a treatment target substance. [2] The exhaust gas purifier
according to [1], wherein the pulse voltage has a pulse width of 20
to 50 .mu.s and a field intensity of 10 to 200 kV/cm and the amount
of energy supplied is 0.05 to 10 J or less per microgram of the
treatment target substance. [3] The exhaust gas purifier according
to [1] or [2], wherein the distance between the adjacent unit
electrodes is 2 mm or loss. [4] The exhaust gas purifier according
to any one of [1] to [3], wherein a pulse cycle when applying the
pulse voltage is 10 .mu.s to 10 ms. [5] The exhaust gas purifier
according to any one of [1] to [4], wherein a rise time of the
pulse voltage is 100 .mu.s or less.
[0012] The exhaust gas purifier according to the present invention
exhibits an improved PM collection capability since the pulse cycle
of the pulse voltage applied to the unit electrodes is as long as 1
.mu.s to 1 ms. Since the field density is 5 kV/cm or more, a stable
discharge can be achieved. Moreover, charged PM can be effectively
moved. Since the amount of energy supplied when applying the pulse
voltage is 10 J or less per microgram of the treatment target
substance (PM), PM can be oxidized while supplying a minimum amount
of energy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1A schematically shows a plasma reactor that forms one
embodiment of an exhaust gas purifier according to the present
invention. FIG. 1A is a cross-sectional view of the plasma reactor
along a plane perpendicular to one direction (as flow
direction).
[0014] FIG. 1B is a cross-sectional view of the exhaust gas
purifier shown in FIG. 1A along the line A-A'.
[0015] FIG. 2 is a cross-sectional view schematically showing one
embodiment of an exhaust gas purifier according to the present
invention.
[0016] FIG. 3 shows one pulse of a pulse voltage. FIG. 3 shows a
graph that indicates the relationship between time and voltage.
[0017] FIG. 4 shows one pulse of a pulse voltage. FIG. 4 shows a
graph that indicates the relationship between time and voltage.
[0018] FIG. 5 shows one pulse of a pulse voltage applied by exhaust
gas purifiers of Examples 1 and 2. FIG. 5 shows a graph that
indicates the relationship between time and voltage.
EXPLANATION OF SYMBOLS
[0019] 1: plasma generating electrode, 2: unit electrode, 3:
dielectric, 4: conductive film, 10: plasma reactor, 11: power
supply, 12; conductive line, 21: casing, 22: inlet port, 23: outlet
port 31 and 32: waveform, P: one direction (gas flow direction), Q:
another direction, V: space, W: distance between unit electrodes,
100: exhaust gas purifier
BEST MODE FOR CARRYING OUT THE INVENTION
[0020] Embodiments of the present invention are described in detail
below with reference to the drawings. Note that the present
invention is not limited to the following embodiments. Various
modifications, improvements, and the like may be made with regard
to the design without departing from the scope of the present
invention based on common knowledge of a person skilled in the art.
In the drawings, identical elements are indicated by identical
symbols.
[0021] FIG. 1A schematically shows a plasma reactor 10 that forms
one embodiment of an exhaust gas purifier according to the present
invention. FIG. 1A is a cross-sectional view of the plasma reactor
along a plane perpendicular to one direction (gas flow direction).
FIG. 1B is a cross-sectional view of the exhaust gas purifier shown
in FIG. 1A along the line A-A'. FIG. 2 is a cross-sectional view
schematically showing one embodiment of the exhaust gas purifier
according to the present invention, FIG. 2 shows an exhaust gas
purifier 100 along a plane similar to that of the plasma reactor 10
shown in FIG. 1A.
[0022] As shown in FIGS. 1A and 1B, the plasma reactor 10 that
forms the exhaust gas purifier according to this embodiment
includes a plasma generating electrode 1 formed by hierarchically
stacking a plurality of unit electrodes 2 at specific intervals, to
plasma reactor 10 generating plasma in a space V formed between the
unit electrodes 2 by applying a pulse voltage between the unit
electrodes 2 so that exhaust gas introduced into the space V can be
treated via a reaction. In this embodiment the plasma reactor 10
includes the plasma generating electrode 1, and a casing 21 that
receives the plasma generating electrode 1 in a state in which a
PM-containing gas (treatment target fluid) can be introduced into
the space V that is three-dimensionally formed between the unit
electrodes 2 of the plasma generating electrode 1. The casing 21
includes an inlet port 22 through which the treatment target fluid
flows into the plasma reactor 10, and an outlet port 23 through
which the treatment target fluid (treated fluid gas) that has
passed through the space between the unit electrodes and has been
treated is discharged. The unit electrode 2 includes a conductive
film 4 and a dielectric 3 disposed on at least one side of the
conductive film 4. The unit electrodes 2 are stacked so that the
dielectric 3 is positioned on at least one of the opposing sides of
the unit electrodes 2. As shown in FIG. 2, the exhaust gas purifier
100 according to this embodiment includes the plasma reactor 10 and
a power supply 11 that generates the pulse voltage. The stacked
unit electrodes 2 of the plasma generating electrode 1 are
alternately connected to a positive electrode and a negative
electrode of the power supply 11 through conductive lines 12.
[0023] In the exhaust gas purifier 100 according to this
embodiment, the pulse voltage applied between the unit electrodes 2
has a pulse width of 1 .mu.s (microsecond) to 1 ms (millisecond)
and a field intensity of 5 kV/cm or more, and the amount of energy
supplied when applying the pulse voltage is 10 J or less per
microgram of the treatment target substance.
[0024] The pulse width of the pulse voltage is 1 .mu.s to 1 ms, and
preferably 5 to 50 .mu.s. The term "pulse width" of the pulse
voltage used herein refers to a period (pulse width) of a pulse
voltage shown in FIG. 3 between a time when the pulse voltage is
10% of the peak value during a rise and a time when the pulse
voltage is 10% of the peak value during a fall. FIG. 3 shows one
pulse of the pulse voltage. FIG. 3 shows a graph that indicates the
relationship between time and voltage. The PM collection capability
can be improved by thus increasing the pulse width so that the PM
removal rate can be improved. Since PM charged by plasma moves due
to the force of an electric field generated between the unit
electrodes, PM moves toward the unit electrodes and is collected
due to the force of an electric field only when the pulse voltage
is applied between the unit electrodes so that an electric field is
generated. Therefore, an electric field that applies the force to
PM is generated for a long period of time by increasing the pulse
width of the pulse voltage (i.e., voltage application time per
pulse) so that a period of time sufficient for PM to move to the
unit electrodes can be provided, whereby the PM collection
capability can be improved. This improves the PM removal rate. The
expression "PM is charged" used herein means that electrons of
plasma, a substance ionized by plasma, and the like adhere to PM so
that the PM is charged (ionized).
[0025] If the pulse width is less than 1 .mu.s, the PM collection
capability cannot be improved. If the pulse width is greater than 1
ms, it is necessary to increase the amount of energy supplied for a
PM oxidation reaction.
[0026] The field intensity when applying the pulse voltage is 5
kV/cm or more, preferably 10 to 200 kV/cm, and more preferably 20
to 150 kV/cm. A stable plasma discharge can be achieved by setting
the field intensity at 5 kV/cm or more. Moreover, a strong force
acts on the charged PM due to a high field intensity so that the PM
can be effectively moved. If the field intensity is lower than 5
kV/cm, plasma cannot be stably generated. Moreover, a sufficiently
strong force cannot be applied to the charged PM.
[0027] The amount of energy supplied when applying the pulse
voltage is 10 J or less, preferably 0.05 to 10 J, and particularly
preferably 0.1 to 2 J per microgram of PM (i.e., treatment target
substance). Since the PM removal efficiency is not improved even if
the amount of energy supplied is increased to a large extent, it is
preferable to reduce the amount of energy supplied within such a
range that PM can be oxidized from the viewpoint of a reduction in
energy consumption and energy cost. If the amount of energy
supplied is larger than 10 J per microgram of the treatment target
substance, energy is unnecessarily consumed so that the energy cost
increases. It is preferable that the amount of energy supplied be
0.05 J/microgram or more in order to reliably oxidize PM in a PM
oxidation step of the PM oxidation treatment process. If the
collected PM cannot be sufficiently oxidized, PM may be deposited
on the surface of the unit electrode and may serve as a conductor,
whereby plasma may tot be generated between the unit
electrodes.
[0028] In the exhaust gas purifier according to is embodiment, it
is preferable that the distance W between the adjacent unit
electrodes 2 be 3 nm or less, and more preferably 0.1 to 2.0 mm.
The field intensity can be easily increased by setting the distance
between the unit electrodes 2 at 2 mm or less. Moreover since the
distance from the unit electrode when PM moves due to the force of
an electric field decreases, the PM collection capability can be
improved. If the distance between the unit electrodes is longer
than 2 mm, since PM moves over a long distance due to the force of
an electric fields the PM collection capability may decrease. If
the distance between the unit electrodes is long, it may be
necessary to supply a large amount of power, because the voltage
required to obtain a desired field intensity increases and it is
necessary to maintain the PM oxidization capability. Note that the
distance W between the adjacent unit electrodes 2 refers to the
distance between the opposing surfaces of the adjacent unit
electrodes 2 in other words the distance between the opposing
surfaces of the dielectrics 3 of the it electrodes 2.
[0029] In the exhaust gas purifier according to this embodiment,
the pulse cycle when applying the pulse voltage is set at 10 .mu.s
to 10 ms. The term "pulse cycle" used herein refers to a period
between the rise timing (i.e., 0% of the peak value) of one pulse
and the rise timing (i.e., 0% of the peak value) of the next pulse.
The term "pulse cycle" used herein refers to a period between the
rise timing (i.e. 0% of the peak value) of one pulse and the rise
timing (i.e., 0% of the peak value) of the next pulse. Therefore,
it is preferable that the pulse cycle be greater than the pulse
width so that successive pulses do not overlap.
[0030] In the exhaust gas purifier according to is embodiment, it
is preferable that the rise time of the pulse voltage be 100 .mu.s
or less, and more preferably 5 .mu.s or less. It is preferable that
the rise time of the pulse voltage is 100 .mu.s or less, because an
oxidizing excited species can be efficiently generated in the
plasma generating space so that the PM oxidation performance is
improved. If the rise time of the pulse voltage is longer than 100
.mu.s, the collected PM oxidation performance decreases. Therefore,
the PM collected on the surface of the electrode cannot be treated
and is deposited on the surface of the electrode, whereby a
discharge between the electrodes becomes unstable. As a result, the
PM removal performance may deteriorate to a large extent. The term
"rise time" of the pulse voltage used herein refers to a period of
time of a pulse voltage shown in FIG. 4 to rise from 10% of the
peak value to 90% of the peak value. FIG. 4 shows one pulse of the
pulse voltage. FIG. 4 shows a graph that indicates the relationship
between time and voltage.
[0031] In the plasma generating electrode 1 of the plasma reactor
10 shown in FIGS. 1A and 1B, the space V is formed between the unit
electrodes 2 so that each end in one direction (gas flow direction)
P is open and each end in another direction Q is closed.
[0032] As shown in FIGS. 1A and 1B, the unit electrode 2 of the
plasma generating electrode 1 includes the sheet-shaped dielectric
3 and the conductive film 4 disposed inside the dielectric 3. In
this embodiment, the dielectric 3 is disposed on each side of each
conductive film 4. It suffices that the dielectric 3 be disposed on
at least one side of the conductive film 4. It suffices that the
unit electrodes 2 be stacked so that the dielectric 3 is positioned
on at least one of the opposing sides of the unit electrodes 2.
When the dielectric is disposed on the surface of one of a pair of
conductive films, a non-uniform discharge such as a spark discharge
can be reduced as compared with the case of causing a discharge
using only the conductive films 4, whereby a small discharge can be
caused to occur at a plurality of locations. Since such a small
discharge involves a small amount of current as compared with a
spark discharge or the like, power consumption can be reduced.
Moreover, a current that flows between the unit electrodes 2 is
limited due to the presence of the dielectric so that non-thermal
plasma which does not cause an increase in temperature and consumes
only a small amount of energy can be generated.
[0033] In this embodiment, the thickness of the conductive film 4
of the unit electrode 2 is preferably 0.001 to 0.1 mm, and more
preferably 0.005 to 0.05 mm in order to reduce the size of the
plasma generating electrode 1 and to reduce the resistance of the
treatment target fluid which is caused to pass through the space
between the unit electrodes 2 when treating an exhaust gas or the
like.
[0034] The conductive film 4 used in this embodiment preferably
includes a metal exhibiting excellent conductivity as the main
component. As suitable examples of the main component of the
conductive film 4, at least one metal selected from the group
consisting of tungsten, molybdenum, manganese, chromium, titanium,
zirconium, nickel, iron, silver, copper, platinum, and palladium
can be given. In this embodiment, the term "main component" refers
to a component that accounts for 60 mss % or more of components of
the conductive film.
[0035] It is preferable that the conductive film 4 of the unit
electrode 2 be applied to the dielectric 3. As suitable examples of
the application method, printing, rolling, spraying, electrostatic
painting, dipping, knife coating, and the like can be given.
According to these methods, at conductive film 4 exhibiting
excellent surface smoothness after application can be easily
formed. In this case, it is preferable to use a sheet-shaped
ceramic body as the dielectric 3.
[0036] When applying the conductive film 4 to the dielectric, a
powder of a metal mentioned above as the main component of the
conductive film 4, an organic binder, and a solvent such as
terpineol may be mixed to prepare a conductive paste, and the
conductive paste may be applied to the dielectric 3 using the
above-mentioned method. When using a sheet-shaped ceramic body as
the dielectric 3, an additive may optionally be added to the
conductive paste in order to improve the adhesion between the
conductive film 4 and the sheet-shaped ceramic body and improve
sintering properties.
[0037] In this embodiment, it is preferable that the dielectric 3
that forms the unit electrode 2 be a sheet-shaped ceramic body
formed using a high-dielectric-constant material as the main
component. As the material for the sheet-shaped ceramic body,
aluminum oxide, zirconium oxide, silicon oxide, mullite,
cordierite, a titanium-barium type oxide, a
magnesium-calcium-titanium type oxide, a barium-titanium-zinc type
oxide, silicon nitride, aluminum nitride, or the like may be
suitably used. It is preferable to form the dielectric 3 using a
material appropriately selected from these materials and suitable
for generating plasma appropriate for a PM reaction. The plasma
generating electrode can be utilized at a high temperature using a
material exhibiting excellent thermal impact resistance as the main
component of the dielectric.
[0038] For example, copper metallization technology may be utilized
for a rower-temperature cofired substrate material (lower-temperate
cofired ceramic: LTCC) in which a glass component is added to
aluminum oxide (Al.sub.2O.sub.3). Use of the copper metallization
technology enables formation of an electrode exhibiting a low
resistance and a high discharge efficiency, whereby the size of the
electrode can be reduced. This enables a design which prevents
thermal stress, whereby the strength of the electrode can be
improved. When forming the electrode using a
high-dielectric-constant material such as barium titrate, a
magnesium-calcium-titanium type oxide, or a barium-titanium-zinc
type oxide, the size of the electrode can be reduced due to high
discharge efficiency. Therefore, structural design which can reduce
occurrence of thermal stress due to high thermal expansion can be
achieved.
[0039] The relative dielectric constant of the dielectric 3 may be
appropriately determined corresponding to the plasma to be
generated. The relative dielectric constant of the dielectric 3 is
preferably 2 to 10.
[0040] The thickness of the dielectric 3 is preferably 0.1 to 3 mm,
although the thickness of the dielectric 3 is not particularly
limited. If the thickness of the dielectric 3 is less than 0.1 mm,
a pair of adjacent unit electrodes 2 may not be electrically
insulated. If the thickness of the dielectric 3 exceeds 3 mm, the
size of the resulting exhaust gas purifier may not be reduced.
Moreover, the applied voltage must be increased due to an increase
in electrode-to-electrode distance, whereby efficiency may
decrease.
[0041] When using a sheet-shaped ceramic body as the dielectric 3,
it is preferable to fire a ceramic green sheet for a ceramic
substrate to produce a sheet-shaped ceramic body. The ceramic green
sheet may be obtained by forming a green sheet slurry or paste to a
specific thickness using a known method such as a doctor blade
method, a calender method, a printing method, or a reverse roll
coating method. The resulting ceramic green sheet may be subjected
to cutting, shaving, punching, or communication opening formation,
or may be used as an integral laminate in which a plurality of
green sheets are stacked and bonded by thermocompression bonding or
the like.
[0042] As the green sheet slurry or paste, a mixture prepared by
mixing an appropriate binder, sintering agent, plasticizer,
dispersant, organic solvent, and the like into a specific ceramic
powder may be suitably used. As suitable examples of the ceramic
powder, alumina, mullite, cordierite, zirconia, silica, silicon
nitride, aluminum nitride, ceramic glass, glass, and the like can
be given. A suitable examples of the sintering agent, silicon
oxide, magnesium oxide, calcium oxide; titanium oxide, zirconium
oxide, and the like can be given. The sintering agent is preferably
added in an amount of 3 to 10 parts by mass based on 100 parts by
mass of the ceramic powder. As the plasticizer; dispersant and
organic solvent, those used for a known method may be suitably
used.
[0043] The porosity of the sheet-shaped ceramic body is preferably
0.1 to 35%, and more preferably 0.1 to 10%. This enables plasma to
be efficiently generated between the unit electrodes 2 including
the sheet-shaped ceramic body 3 so that energy consumption can be
reduced.
[0044] The casing 21 of the plasma reactor 10 shown in FIGS. 1A and
1B is not particularly limited insofar as the plasma generating
electrode can be placed in the casing 21 and provided in an
automobile or the like so that exhaust gas can be purified. As
shown in FIGS. 1A and 1B, it is preferable that the casing 21 have
the gas inlet port 22 and the gas outlet port 23 and allow the
plasma generating electrode 1 to be disposed inside the casing 21,
for example. The plasma reactor 10 is preferably formed so that
exhaust gas that has entered the plasma reactor 10 through the
inlet port 22 flows into the space V of the plasma generating
electrode, 1, PM is treated while the exhaust gas passes through
the space V, and the exhaust gas that has flowed out from the space
V flows toward the outside through the outlet port 23 outside. When
disposing the plasma generating electrode 1 in the casing 21, it is
preferable to provide an insulating cushioning material exhibiting
heat resistance between the casing 21 and the plasma generating
electrode 1 in order to prevent breakage.
[0045] The material for the casing 21 used in this embodiment is
not particularly limited. For example, it is preferable to use
ferritic stainless steel having excellent conductivity, being
lightweight and inexpensive, and showing only a small amount of
deformation due to thermal expansion, as the material for the
casing 21.
[0046] As shown in FIG. 2, the exhaust gas purifier 100 according
to this embodiment includes the plasma reactor 10 and the power
supply 11 that generates a pulse voltage. The stacked unit
electrodes 2 of the plasma generating electrode 1 are alternately
connected to the positive electrode and the negative electrode of
the power supply 11 through the conductive lines 12.
[0047] The power supply 11 is not particularly limited insofar as
the above-deseribed specific pulse voltage can be generated. As
examples of a power supply that generates a pulse voltage by
ON/OFF-controlling a high-voltage switch, "HVP-10K" (manufactured
by Haiden Laboratory Co., Ltd.) and the like can be given.
[0048] The exhaust gas purifier 100 according to this embodiment
configured as described above may be installed in an automotive
exhaust system, for example. In this case, exhaust gas is caused to
pass through a plasma generated in the space V formed between the
unit electrodes, whereby PM contained in the exhaust gas can be
efficiently oxidized.
[0049] A method of producing an exhaust gas purifier according to
one embodiment of the present invention is described in detail
below.
[0050] A plasma generating electrode that forms the exhaust gas
purifier is produced. A ceramic green sheet used for the
above-described sheet-shaped ceramic body is formed. For example a
sintering agent, a binder such as a butyral resin or a cellulose
resin, a plasticizer such as DOP or DBP, an organic solvent such as
toluene or butadiene, and the like are added to at least one
material selected from the group consisting of alumina, mullite,
cordierite, mullite, silicon nitride, aluminum nitride, ceramic
glass, and glass. The components are sufficiently mixed using an
alumina pot and alumina cobblestone to prepare a slurry for forming
a green sheet. The slurry for forming a green sheet may be prepared
by mixing these materials by ball milling using a mono ball.
[0051] The resulting slur for forming a green sheet is stirred
under reduced pressure to remove bubbles. The viscosity of the slur
for forming a green sheet is then adjusted to a specific value. The
slur for forming a green sheet thus prepared is formed in the shape
of a tape using a tape forming method such as a doctor blade method
to form a plurality of types of unfired ceramic bodies.
[0052] A conductive paste for forming a conductive film disposed on
one side of the unfired ceramic body is prepared. The conductive
paste may be prepared by adding a binder and a solvent such as
terpineol to a silver powder, and sufficiently kneading the mixture
using a triple roll mill, for example.
[0053] The resulting conductive paste is printed on the surface of
one of the unfired ceramic bodies by screen printing or the like to
form a conductive film with a specific shape to obtain a conductive
film-containing unfired ceramic body. In order to supply
electricity to the conductive film from the outside of the unit
electrode after forming the unit electrode by holding the
conductive film between the ceramic bodies, the conductive paste is
preferably printed so that the conductive film reaches the outer
edge of the unfired ceramic body.
[0054] The conductive film-containing unfired ceramic body and
another unfired ceramic body are stacked so that the printed
conductive film is covered. It is preferable to stack the unfired
ceramic bodies at a temperature of 100.degree. C. while applying a
pressure of 10 MPa. Then, the unfired ceramic bodies that have been
stacked with the conductive film held therebetween are fired to
form a unit electrode including the sheet-shaped ceramic body and
the conductive film.
[0055] The resulting unit electrodes are then stacked. In this
case, in order to provide a specific interval between the unit
electrodes, a ceramic rod in the shape of a quadrangular prism is
formed using a raw material similar to that of the ceramic body,
and is placed between the unit electrodes. The thickness of the
ceramic rod corresponds to the distance between the unit
electrodes. The ceramic rods are placed between the unit electrodes
so that the ceramic rods are disposed in parallel to provide gas
passages when treating exhaust gas or the like. The ceramic rod
need not be in the shape of a quadrangular prism, but may be in the
shape of a cylinder, a prism other than a quadrangular prism, or
another prism shape. A plurality of protrusions may be formed on
one side of the ceramic body, and a space may be formed by
disposing the unit electrodes with the protrusions interposed
therebetween. Projections and depressions may be formed on the
ceramic bodies, and a space may be formed by combining the
projections and the depressions. The plasma generating electrode of
the exhaust gas purifier according to this embodiment can be
obtained by hierarchically stacking the unit electrodes with the
ceramic rods interposed therebetween. The casing 21 of the exhaust
gas purifier according to this embodiment may be obtained by
forming ferrite-type stainless steel or the like into a tube having
the gas inlet port 22 and the gas outlet port 23 (see FIGS. 1A and
1B) on opposite ends, for example. It is preferable to form the
casing 21 so that the casing 21 can be divided into two or more
sections in order to insert the plasma generating electrode into
the casing 21.
[0056] The exhaust gas purifier 100 according to this embodiment
may be obtained by providing the plasma generating electrode in the
casing and connecting a specific power supply to the plasma
generating electrode (see FIG. 2).
EXAMPLES
[0057] The present invention is described in detail below by way of
examples. Note that the present invention is not limited to the
following examples.
Example 1
[0058] A plasma reactor configured as shown in FIG. 1 in which the
number of unit electrodes was increased was produced. The exhaust
gas purifier was prepared by connecting the plasma reactor to a
pulse power supply. Exhaust gas containing PM was treated using the
exhaust gas purifier thus obtained, and the PM removal rate was
measured.
[0059] A plasma generating electrode used in the exhaust gas
purifier of this example was produced as follows. A conductive film
(thickness; 10 .mu.m) was screen-printed on the inner side of an
unfired alumina tape substrate (thickness: 1.0 mm when stacking and
firing two substrates) using a tungsten paste to produce a unit
electrode. The outer dimensions of the unit electrode were 90
mm.times.50 mm. The dimensions of the printed conductive film were
84 mm.times.46 mm. Thirty-eight unit electrodes were produced and
stacked to produce a plasma generating electrode. The distance
between the unit electrodes was 0.5 mm. The outer circumference of
the plasma generating electrode was held using a heat-resistant
mat. The plasma generating electrode was placed in a casing made of
430SS to obtain a plasma reactor. A pulse power supply was
connected with the plasma reactor to obtain an exhaust gas
purifier. As the pulse power supply, a high-voltage pulse power
supply of which the pulse width could be increased was used. The
38-stage unit electrodes were alternately connected to the pulse
power supply and ground.
[0060] An exhaust gas treatment test was performed using the
exhaust gas purifier utilizing exhaust gas (200.degree. C., 1
Nm.sup.3/min) containing 21.6 mg of particulate matter (PM) to
measure the PM removal rate. As the pulse voltage application
conditions, the pulse width was set at 30 .mu.s, the field
intensity was set at 31.7 kV/cm, and the amount of energy supplied
was set at 1.4 J per microgram of the treatment target substance.
The term "PM removal rate (%)" refers to a value obtained by
dividing the PM content (g/m.sup.3) in the gas discharged from the
exhaust gas purifier by the PM content (g/m.sup.3) in the gas
flowing into the exhaust gas purifier, and multiplying the
resulting value by 100. The results are shown in Table 1. FIG. 5
shows the waveform of the applied pulse voltage. FIG. 5 shows one
pulse of the pulse voltage of the exhaust gas purifiers of Examples
1 and 2. FIG. 5 shows a graph that indicates the relationship
between time and voltage. In FIG. 5, the waveform 31 indicates the
waveform of the pulse voltage applied in the exhaust gas purifier
of Example 1.
TABLE-US-00001 TABLE 1 PM removal rate (%) Example 1 75 Example 2
55 Comparative Example 1 47
Example 2
[0061] An exhaust gas purifier was produced in the same manner as
in Example 1. The exhaust gas treatment test was performed to
measure the PM removal rate in the same manner as in Example 1,
except that the pulse width was set at 4.7 .mu.s, the field
intensity was set at 35.2 kV/cm, and the amount of energy supplied
was set at 1.78 J per microgram of the treatment target substance.
The results are shown in Table 1. FIG. 5 shows the waveform of the
applied pulse voltage. In FIG. 5, the waveform 32 indicates the
waveform of the pulse voltage applied in the exhaust gas purifier
of Example 2.
Comparative Example 1
[0062] A plasma reactor was produced in the same manner as in
Example 1. An exhaust gas purifier was produced using a
high-voltage pulse power supply that applied a voltage having a
short pulse width as the pulse power supply. The exhaust gas
treatment test was performed to measure the PM removal rate using
the resulting exhaust gas purifier in the same manner as in Example
1, except that the pulse width was set at 0.9 .mu.s, the field
intensity was set at 30 kYV/cm, and the amount of energy supplied
was set at 2.0 J per microgram of the treatment target substance.
The PM removal rate was measured. The results are shown in Table
1.
[0063] As shown in Table 1, the exhaust gas purifiers of Examples 1
and 2 exhibited an excellent PM removal rate due to a due pulse
width.
INDUSTRIAL APPLICABILITY
[0064] The exhaust gas purifier according to the present invention
can be used to remove particulate matter contained in engine
exhaust gas or incinerator exhaust gas. In particular, the exhaust
gas purifier according to the present invention can be suitably
utilized as an exhaust gas purifier which can efficiently remove
particulate matter with a low energy loss and a low cost of
equipment.
* * * * *