U.S. patent application number 09/757581 was filed with the patent office on 2001-05-31 for process for the selective catalytic reduction of the nitrogen oxides contained in a lean exhaust gas.
Invention is credited to Gieshoff, Jurgen, Kreuzer, Thomas, Lang, Jurgen, Lox, Egbert, Tillaart, Hans Vam Den.
Application Number | 20010002244 09/757581 |
Document ID | / |
Family ID | 7895781 |
Filed Date | 2001-05-31 |
United States Patent
Application |
20010002244 |
Kind Code |
A1 |
Gieshoff, Jurgen ; et
al. |
May 31, 2001 |
Process for the selective catalytic reduction of the nitrogen
oxides contained in a lean exhaust gas
Abstract
The invention relates to a process for the selective catalytic
reduction of the nitrogen oxides contained in a lean exhaust gas
from internal combustion engines by reducing the nitrogen oxides by
means of ammonia on a catalyst. The process is characterized in
that, in addition to the lean exhaust gas, a rich gas stream is
produced that is treated in an electrical gas discharge plasma in
order to form the ammonia required for the reduction.
Inventors: |
Gieshoff, Jurgen;
(Biebergemund, DE) ; Tillaart, Hans Vam Den;
(Brockhuizenvoot, NL) ; Kreuzer, Thomas; (Karben,
DE) ; Lox, Egbert; (Hanau, DE) ; Lang,
Jurgen; (Kirchheim-Teck, DE) |
Correspondence
Address: |
Smith, Gambrell & Russell, LLP
The Beveridge, DeGrandi, Weilacher & Young
Intellectual Property Group
1850 M Street, N.W., Suite 800
Washington
DC
20036
US
|
Family ID: |
7895781 |
Appl. No.: |
09/757581 |
Filed: |
January 11, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09757581 |
Jan 11, 2001 |
|
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09493288 |
Jan 28, 2000 |
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Current U.S.
Class: |
423/235 ;
423/212; 423/213.2; 423/213.5; 423/239.1 |
Current CPC
Class: |
B01D 2255/1028 20130101;
F01N 2610/02 20130101; F01N 2240/25 20130101; B01D 2255/1021
20130101; B01D 53/9418 20130101; B01D 2255/2065 20130101; F01N
3/2066 20130101; B01D 2251/2062 20130101; B01D 2255/1023 20130101;
Y02C 20/10 20130101; B01D 2255/20761 20130101; B01D 2255/1025
20130101; F01N 2240/28 20130101; F01N 13/009 20140601; B01D 2255/50
20130101; Y02T 10/12 20130101; B01D 53/9431 20130101; B01D 53/32
20130101; B01D 2255/20738 20130101; F01N 3/2073 20130101; B01D
2255/2042 20130101; Y10S 423/10 20130101 |
Class at
Publication: |
423/235 ;
423/212; 423/213.2; 423/239.1; 423/213.5 |
International
Class: |
B01J 008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 29, 1999 |
DE |
DE 199 03 533.4 |
Claims
We claim:
1. A process for the selective catalytic reduction of the nitrogen
oxides contained in a lean exhaust gas from an internal combustion
engine with one or more cylinders, comprising reducing the nitrogen
oxides by means of ammonia on a reduction catalyst, by a) producing
a rich gas stream with a normalized air/fuel-ratio of less than 1,
b) forming ammonia in the rich gas stream by reacting components of
said steam with one another, c) combining the lean exhaust gas with
the rich gas stream, and d) reducing the nitrogen oxides contained
in the lean exhaust gas on a reduction catalyst using the ammonia
formed as reducing agent.
2. The process according to claim 1, wherein the rich gas stream is
produced by a sub-stoichiometrically operating burner.
3. The process according to claim 1, wherein the rich gas stream
forms part of the exhaust gas stream from the internal combustion
engine and is obtained by operating one cylinder of the engine with
a sub-stoichiometric air/fuel mixture.
4. The process according to claim 1, wherein the rich gas stream is
formed by injecting hydrocarbons into an air stream.
5. The process according to claim 1, wherein the rich gas stream
for forming the ammonia required for the reduction is treated in an
electrical gas discharge plasma.
6. The process according to claim 5, wherein a dielectrically
hindered discharge at atmospheric pressure with a product (d*p) of
the electrode interspacing and pressure of between 0.05 and 100
mm*bar is used as the gas discharge.
7. The process according to claim 6, further comprising: exciting
the discharge in the flow channels of a honeycomb body that has
been obtained by stacking alternately smooth and corrugated metal
sheets on top of one another, wherein either the smooth or
corrugated sheets or both types of sheets are coated with a
dielectric to achieve electrical insulation from one another and
applying the discharge voltage between all smooth and all
corrugated sheets.
8. The process according to claim 7, wherein a catalyst layer of
finely divided support materials and catalytically active
components is applied to the coating of the metal sheets with the
dielectric.
9. The process according to claim 7, wherein the dielectric is
formed by a catalyst coating of finely divided support materials
and catalytically active components.
10. The process according to claim 8, wherein aluminium oxide,
titanium oxide, zirconium oxide, cerium oxide, silicon dioxide,
magnesium oxide or their mixed oxides and zeolites are used as
finely divided support materials.
11. The process according to claim 10, wherein the finely divided
support materials are stabilized with silicon dioxide and/or rare
earth oxides.
12. The process according to claim 10, wherein the catalyst coating
contains at least one of the catalytically active components
platinum, palladium, rhodium and iridium in a highly dispersed
form.
13. The process according to claim 10, wherein the catalyst coating
additionally contains at least one basic oxide of the alkali or
alkaline earth metals of the periodic system of the elements.
14. The process according to claim 6, wherein the discharge is
generated between two parallel plate electrodes, at least one of
which is coated on the mutually opposite faces with a
dielectric.
15. The process according to claim 6, further comprising generating
the discharge in the annular space between two tubular electrodes
arranged concentrically with respect to one another, at least one
of which is coated with a dielectric on the mutually opposite
jacket surfaces of the tubular electrodes.
16. The process according to claim 14, wherein the discharge space
between the electrodes is filled with ceramic pellets.
17. The process according to claim 16, wherein the ceramic pellets
comprise at least one finely divided support material selected from
the group consisting of aluminum oxide, titanium oxide, zirconium
oxide, cerium oxide, silicon dioxide, magnesium oxide, mixed oxides
thereof and zeolites.
18. The process according to claim 17, wherein the finely divided
support materials are stabilized with silicon dioxide and/or rare
earth oxides.
19. The process according to claim 17, wherein the ceramic pellets
contain at least one of the catalytically active components
platinum, palladium, rhodium and iridium in a highly dispersed
form.
20. The process according to claim 17, wherein the ceramic pellets
additionally contain at least one basic oxide of the alkali or
alkaline earth metals of the periodic system of the elements.
21. The process according to claim 14, wherein a catalyst layer of
finely particulate support materials and catalytically active
components is applied to the dielectric.
22. The process according to claim 14, wherein the dielectric is
formed by a catalyst coating of finely particulate support
materials and catalytically active components.
23. The process according to claim 7, wherein the discharge is
excited with a pulsed voltage having a frequency of between 50 Hz
and 250 kHz.
24. The process according to claim 7, wherein the discharge is
excited with an alternating voltage between 0.2 and 15 kV and a
frequency between 50 Hz and 250 kHz.
25. The process according to claim 22, wherein the alternating
voltage is operated in a pulsed manner with a frequency between
0.01 and 10 Hz.
26. The process according to claim 7, wherein the electrode
surfaces are structured three-dimensional.
27. The process according to claim 1, wherein for the selective
catalytic reduction a reduction catalyst is used that contains the
following components: a) titanium dioxide; b) at least one oxide of
tungsten, silicon, boron, aluminum, phosphorus, zirconium, barium,
yttrium, lanthanum and cerium, and c) at least one oxide of
vanadium, niobium, molybdenum, iron and copper.
28. The process according to claim 1, wherein for the selective
catalytic reduction a zeolite catalyst is used that contains
copper, iron, cerium or mixtures thereof on a mordenite type
zeolite.
Description
INTRODUCTION AND BACKGROUND
[0001] The present invention relates to a process for the selective
catalytic reduction of nitrogen oxides contained in a lean-mix
exhaust gas from combustion engines by reducing the nitrogen oxides
by means of ammonia on a catalyst.
[0002] Combustion engines with a lean exhaust gas concern diesel
engines and lean-running petrol engines, i.e. so-called lean-mix
engines. Compared to stoichiometrically operated conventional
engines, diesel engines and lean-mix engines are characterized by
an up to 20% lower fuel consumption. A substantial problem of these
engines is the purification of their exhaust gases. Although the
oxidizable harmful components of the exhaust gas (hydrocarbons HC,
carbon monoxide CO and minor amounts of hydrogen H.sub.2) can on
account of the high oxygen content in the exhaust gas of up to 15
vol. % easily be converted on a catalyst to carbon dioxide and
water, the nitrogen oxides NO.sub.x however that are also formed in
the combustion of the fuel cannot, on account of the preferably
occurring oxidation reactions, be reduced in sufficient amount to
nitrogen N.sub.2.
[0003] In order to solve this problem the process of selective
catalytic reduction (SCR) already known in the case of stationary
combustion units has been proposed. In this process a reducing
agent is added to the lean exhaust gas, by means of which the
nitrogen oxides can be selectively reduced on a catalyst suitable
for this purpose. Ammonia, which reacts with a high selectivity
with the nitrogen oxides to form nitrogen and water, is preferably
used as reducing agent. The ratio of added ammonia to the nitrogen
oxides that are present is about 1:1. Ammonia can be produced
directly from urea with the aid of a hydrolysis catalyst or by
decomposition of a corresponding salt (e.g. carbamate).
[0004] At the present time much effort is being expended on
attempts to incorporate such systems in lorries and trucks. A
disadvantage of this process is that a further operating material
has to be employed. The high expenditure associated with the SCR
technology has up to now prevented its widespread use, in
particular in passenger cars. As an alternative to ammonia, there
may also be used alcohols, hydrogen or hydrocarbons as reducing
agent. These reducing agents however have considerably worse
selectivities than ammonia for the nitrogen oxide reduction in the
lean exhaust gas. Thus, a nitrogen oxide conversion of up to 30% is
obtained in officially prescribed driving cycles using the
alternative reducing agents, whereas conversions of 70% or more are
possible with ammonia.
[0005] The selective catalytic reduction with ammonia thus provides
very good results, but involves a considerable expenditure on
equipment, which up to now has limited its widespread use in
smaller engines.
[0006] It is therefore an object of the present invention to
provide a process for the selective catalytic reduction with
ammonia that is characterized by a simple production of the ammonia
required for the reduction.
SUMMARY OF THE INVENTION
[0007] The above and other objects can be achieved by a process for
the selective catalytic reduction of the nitrogen oxides contained
in a lean exhaust gas from internal combustion engines with one or
more cylinders, by reducing the nitrogen oxides by means of ammonia
on a reduction catalyst. The process is characterized by the
following process stages:
[0008] a) production of a rich gas stream with a normalized
air/fuel-ratio of less than 1,
[0009] b) formation of ammonia in the rich gas stream by reaction
of its components with one another,
[0010] c) combination of the lean exhaust gas with the rich gas
stream, and
[0011] d) reduction of the nitrogen oxides contained in the lean
exhaust gas on a reduction catalyst using the resultant ammonia as
reducing agent.
[0012] The normalized air/fuel-ratio (.lambda.) describes the
composition of the gas stream, and refers to the air/fuel ratio
standardized to stoichiometric conditions. Stoichiometric
conditions exist at a normalized air/fuel-ratio of 1. With a
normalized air/fuel-ratio greater than 1 the gas contains more
oxygen than is necessary for a complete combustion of the
combustible constituents. Such a gas composition is termed lean. A
rich gas composition exists when the oxygen content is less than is
required for a complete combustion of all combustible constituents
of the gas.
[0013] An essential feature of the process according to the
invention is the production of the ammonia required for the
catalytic reduction from a rich gas stream by the reaction of its
components with one another. Such a gas stream may be produced for
example by a burner that is operated with a sub-stoichiometric
air/fuel mixture (.lambda.<1). The rich gas stream can also be
obtained as part of the exhaust gas from the combustion engine if
one cylinder of the engine is operated with a sub-stoichiometric
air/fuel mixture. It is also possible to form the rich gas stream
by injecting hydrocarbons into an air stream. A rich exhaust gas
contains for example, in addition to non-combusted hydrocarbons,
also carbon monoxide, nitric oxide and water vapour. Ammonia can be
formed from these last three substances according to the following
reaction equation:
5CO+2NO+3H.sub.2O.fwdarw.5CO.sub.2+2NH.sub.3 (1)
[0014] Nitric oxide is thus reduced by means of carbon monoxide to
ammonia. The formation of ammonia is not restricted to a chemical
reaction according to the above overall reaction equation. For
example it is also possible to react hydrogen with
nitrogen-containing gas components or with nitrogen to form
ammonia.
[0015] This reduction may be carried out in various ways. It is
possible to initiate the above reaction merely by thermal
activation, in other words by heating the rich exhaust gas. Of
course, the reverse reaction also increases with increasing
temperature, and for this reason reaction pathways in which the
exhaust gas does not have to be thermally heated are more
favourable. An example of a convenient reaction pathway is to carry
out the reaction on a suitable catalyst. As a rule the catalytic
reaction requires lower temperatures, which means that the
influence of the reverse reaction can be reduced.
[0016] It has now been found that ammonia can also be formed in a
rich gas stream by passing the latter through an electrical gas
discharge plasma. The formation of ammonia in an electrical gas
discharge is thermodynamically favoured since the reaction proceeds
at substantially lower temperatures than the catalytic
reduction.
BRIEF DESCRIPTION OF DRAWINGS
[0017] The invention is illustrated in more detail with the aid of
FIGS. 1 to 9, wherein:
[0018] FIG. 1 is a graph showing the formation of ammonia and
nitric oxide by thermal splitting of cetane (C.sub.8H.sub.18);
[0019] FIG. 2 is a schematic diagram of a dielectric barrier
discharge with parallel, flat electrodes; unilaterally
dielectrically hindered discharge;
[0020] FIG. 3 is a schematic diagram of a dielectric barrier
discharge with parallel, flat electrodes; bilaterally
dielectrically hindered discharge;
[0021] FIG. 4 is a schematic diagram of a dielectric barrier
discharge with parallel, flat electrodes; bilaterally
dielectrically hindered; discharge space filled with pellets;
[0022] FIG. 5 is a schematic representation of an internal
combustion engine with additional, sub-stoichiometric burner for
producing a rich exhaust gas partial stream;
[0023] FIG. 6 is a schematic representation of an internal
combustion engine with separate regulation of the cylinders for
producing a rich exhaust gas partial stream;
[0024] FIG. 7 is a schematic diagram of a coaxial reactor with
palladium pellet catalyst;
[0025] FIG. 8 is a schematic diagram of a reactor with honeycomb
structure;
[0026] FIG. 9 is a graph of a No.sub.x concentration in the emitted
exhaust gas as a function of the electrical power consumption of
the gas discharge and the normalised air/fuel-ratio of the exhaust
gas;
[0027] FIG. 10 is a graph of absorption bands of nitric oxide in
the lean synthesis gas mixture after flowing through the plasma
reactor with the gas discharge disconnected;
[0028] FIG. 11 is a graph of absorption spectrum of the synthesis
gas mixture after flowing through the plasma reactor at the moment
of switching from a lean gas composition to a rich gas composition;
and
[0029] FIG. 12 is a graph of absorption bands of ammonia in the
rich synthesis gas mixture after flowing through the plasma reactor
with the gas discharge on.
DETAILED DESCRIPTION OF THE INVENTION
[0030] FIG. 1 shows the formation of ammonia and nitric oxide by
thermal splitting of cetane (C.sub.8H.sub.18). The illustrated
curves are obtained by thermodynamical calculations for two
different normalized air/fuel-ratios (.lambda.=0.984) and
(.lambda.= 0.992) of the adopted gas mixture. The formation of
ammonia decreases with increasing temperature. At the same time the
formation of nitric oxide increases sharply above 600.degree.
C.
[0031] An electrical gas discharge is preferably used for the
formation of ammonia. Suitable for example are microwave
discharges, also discharges with frequencies above 250 MHz, corona
discharges and dielectrically hindered discharges, also termed
barrier discharges. Combinations of these electrical gas discharges
are also suitable. Barrier discharges are preferably used for the
proposed process.
[0032] A barrier discharge can be produced between two metal
electrodes, at least one of which is coated with a dielectric that
prevents a spark or arc formation between the two metal electrodes.
Instead, a plurality of brief and spatially highly localized
microdischarges are formed, whose discharge duration and energy
content are limited by the dielectric. Suitable dielectrics are
ceramic materials, glass, porcelain or insulating plastics such as
for example Teflon.RTM..
[0033] Barrier discharges may be operated at pressures of between
0.1 and 10 bar. The electrical excitation of the discharge is
effected by applying an alternating voltage to the electrodes.
Depending on the pressure in the discharge space, interspacing of
the electrodes, and frequency and amplitude of the alternating
voltage, thin, spatially and timewise statistically distributed
discharge channels of only a few nanoseconds, duration are formed
when a sparking potential is exceeded.
[0034] FIG. 2 shows the basic construction of a plasma reactor in
which a dielectric barrier discharge can be ignited. (2) and (3)
denote two metal electrodes arranged opposite one another and
connected to an alternating voltage source (5). In order to
suppress the formation of a discharge arc between the two
electrodes, the electrode (2) is coated with a dielectric (4). Such
a discharge is described as being unilaterally dielectrically
hindered.
[0035] By applying an alternating voltage to the two electrodes the
desired discharge occurs at a sufficiently high voltage. The
necessary voltage depends on the free distance "d" between the
dielectric and counterelectrode, on the dielectric that is used as
well as the pressure in the discharge space, on the gas
composition, and on any internal fittings there may be between the
dielectric in the discharge space. The distance "d" is preferably
adjusted to between 0.1 and 10 mm. The necessary voltages may be
from 0,2 to 15 kV. The frequency of the alternating voltage may be
selected between 50 Hz and 250 kHz. These alternating voltages may
also be operated in a pulsed manner at low frequency (10 to 0.01
Hz) in order for example to permit the reaction of adsorbed
species.
[0036] FIG. 3 shows a modification of the arrangement of FIG. 2. In
FIG. 3 both electrodes are coated with a dielectric. The gas
discharge forming in the discharge space is therefore described as
bilaterally dialectically hindered.
[0037] FIG. 4 shows a particularly advantageous arrangement of the
plasma reactor. The discharge space is filled with pellets. The
pellets may consist of catalytically active materials or materials
having special chemical properties or surface properties. Also,
inert ceramic spheres have a positive influence on the formation of
the electrical discharge. The electrical discharge that is formed
in a reactor filled with pellets occurs primarily in the form of
sliding discharges at the surface of the pellets. The concentration
of ions and radicals in the spatial vicinity of the surface is
thereby increased. The formation of ammonia in the gas discharge
can be promoted by using catalytically active pellets.
[0038] The catalytically active pellets preferably consist of at
least one finely particulate support material selected from the
group consisting of aluminum oxide, titanium oxide, zirconium
oxide, cerium oxide, silicon dioxide, magnesium oxide, their mixed
oxides and zeolites. The support materials may be stabilized in a
manner known per se with silicon dioxide and/or rare earth oxides
against thermal stresses. Also, the support materials can be
catalytically activated by depositing at least one of the noble
metals of the platinum group, in particular platinum, palladium,
rhodium and iridium, in a highly dispersed form on their surface.
For this purpose the specific surface of the support materials
should be at least 10 m.sup.2/g (measured according to DIN 66132).
Furthermore, it is advantageous if the ceramic pellets additionally
contain at least one basic oxide of the alkali or alkaline earth
metals of the periodic system of the elements.
[0039] In addition to the pellets or as an alternative to the
latter, the dielectric on the electrode surfaces may be provided
with a catalytically active layer comprising finely particulate
support materials and catalytically active components whose
composition may correspond to the aforedescribed composition for
the ceramic pellets. In specific applications the dielectric may be
formed as a catalytically active layer comprising finely
particulate support materials and catalytically active components
on the electrode surfaces themselves. The precondition for this is
that the insulation action of the layer satisfies the requirements
of a dielectrically hindered discharge.
[0040] The electrodes of the plasma reactor may be formed as flat
plates aligned parallel to one another, or may be formed by two
tubular electrodes arranged concentrically with respect to one
another and whereby the electrical discharge is generated in the
space between the two opposing electrodes. In case of the parallel
plate reactor at least one of the electrodes is coated on the
mutually opposite faces with a dielectric. In case of the
concentrical arrangement of two tubular electrodes at least one of
them is coated with a dielectric on the mutually opposite jacket
surfaces.
[0041] In order to facilitate the formation of discharge filaments
spatial inhomogeneities may be provided by employing
three-dimensional structured electrode surfaces that result in high
localized electrical field strengths and thus to the formation of
the discharge. As is known from the literature, the coupled
electron energy in a plasma discharge depends on the product of the
electrode interspacing "d" and pressure "p" (d*p), with the result
that at constant gas pressure specific radical reactions can be
promoted or alternatively suppressed in the plasma simply by
altering the geometry of the reactor. For the proposed process the
product of the electrode interspacing and pressure should be in the
range between 0.05 and 100 mm*bar.
[0042] The discharge can be excited by various types of alternating
voltages. For a high electron density and as simultaneous a
formation as possible of the discharge throughout the whole
discharge space of the reactor, pulsed excitation voltages having a
frequency of between 50 Hz and 250 kHz are particularly
suitable.
[0043] The reactor may be fabricated from any electrically and
thermally suitable material. Plastics, ceramic materials and
glasses may in particular be mentioned. Hybrid constructions of
various metals are also possible.
[0044] All SCR catalysts known from the prior art may be used as
catalyst for the selective catalytic reaction. By way of example
there may be mentioned the catalysts described in U.S. Pat. No.
4,916,107, U.S. Pat. No. 5,116,586 and U.S. Pat. No. 5,300,472.
U.S. Pat. No. 4,916,107 and U.S. Pat. No. 5,300,472 describe
catalysts based on titanium oxide. These catalysts contain in
particular:
[0045] a) titanium dioxide;
[0046] b) at least one oxide of tungsten, silicon, boron, aluminum,
phosphorus, zirconium, barium, yttrium, lanthanum and cerium,
and
[0047] c) at least one oxide of vanadium, niobium, molybdenum, iron
and copper.
[0048] U.S. Pat. No. 5,116,586 discloses an SCR catalyst based on
zeolites that contains copper, iron, molybdenum, cerium or mixtures
thereof on a mordenite type zeolite. This and the above U.S.
patents describing the SCR catalysts are relied on and incorporated
herein by reference for that purpose.
[0049] Moreover, SCR catalysts are also known that contain as
catalytically active component at least one platinum group metal in
a highly dispersed form on a suitable support material. As support
materials there may be used magnesium oxide, aluminum oxides,
silicon oxide, titanium oxide, zirconium oxide and their
mixtures.
[0050] The aforementioned catalysts may exist in the form of
pellets or may be extruded to form monolithic structures, in
particular honeycomb bodies. So-called coating catalysts may also
be used, in which the catalyst is applied in the form of a layer to
an inert support body.
[0051] The proposed process is intended to reduce the concentration
of nitrogen oxides in oxygen-rich, i.e. lean exhaust gases, by
selective catalytic reduction by means of ammonia. In order to
generate the ammonia a rich, low-oxygen gas stream for example in
the form of an exhaust gas partial stream, is required according to
the invention.
[0052] FIGS. 5 and 6 illustrate by way of example two different
possible ways of carrying out the process on a vehicle.
[0053] FIG. 5 shows an internal combustion engine (10) with four
cylinders (11). The engine is operated with a lean air/fuel
mixture, i.e. the normalized air/fuel-ratio .lambda. is greater
than 1 (.lambda.>1). Since the normalized air/fuel-ratio is not
altered by the combustion in the engine, the exhaust gas from the
machine also has a normalized air/fuel-ratio of more than 1.
[0054] The lean exhaust gas from the four cylinders is collected in
the exhaust gas pipe (3) and first passed over a catalyst for the
selective reduction (13) and then over an oxidation catalyst (14).
The oxidation catalyst (14) is optional. Its purpose is to oxidize
ammonia in the event of a possible excess of ammonia and thus
prevent an emission of ammonia.
[0055] In order to generate the rich exhaust gas partial stream, an
additional burner (15) is provided in FIG. 5 that is operated with
a sub-stoichiometric air/fuel mixture (.lambda. <1). A heating
unit installed in the vehicle may preferably be used for this
purpose. In the fuel economy vehicles considered here, such a
heating unit is often required in order to heat the passenger
compartment. The exhaust gas from the burner is passed through the
electrical gas discharge of a plasma reactor (16) in order to form
ammonia, and is then mixed in front of the catalyst for the
selective catalytic reduction, with the exhaust gas from the
internal combustion engine. Reference number (17) denotes the
electrode of the plasma reactor.
[0056] The heating unit thus fulfils on the one hand the object of
providing additional heating for the passenger compartment, and on
the other hand constitutes a simple controllable component for
generating a rich exhaust gas partial stream. In order to improve
the ammonia yield, a heat exchanger that cools the exhaust gas can
be installed between the burner and electrical gas discharge. The
heating efficiency is thus only reduced on account of the slightly
fuel-rich mode of operation, but not through a removal of thermal
energy. FIG. 6 shows a further possible way of producing a rich
exhaust gas partial stream. Here (10) denotes the internal
combustion engine with four cylinders (11) and (11'). In this case
the engine is a direct injection petrol or diesel engine with
controllable injection for each cylinder. The cylinders (11) are
operated with a lean air/fuel mixture (.lambda. >1), and
cylinder (11') is operated with a rich air/fuel mixture
(.lambda.<1). The plasma reactor (16) for generating ammonia is
situated in the exhaust gas pipe from this cylinder (11').
[0057] The two process stages (generation of a rich gas stream and
the formation of ammonia) may also be combined in one stage. This
may be effected for example by injecting fuel together with a
sub-stoichiometric amount of air into a plasma reactor.
[0058] FIG. 7 shows a coaxial plasma reactor. (20) denotes an outer
pipe and (21) an inner pipe of quartz glass. The internal surface
of the inner pipe and the external surface of the outer pipe are
covered with metal electrodes (23) and (22). A dielectric barrier
discharge can be ignited in the annular interspace between the
inner pipe and outer pipe by applying a voltage source (24) to
these electrodes.
[0059] FIG. 8 shows the cross-section through a reactor of
honeycomb construction, perpendicular to the flow channels (33) for
the exhaust gas. The honeycomb body consists of alternating layers
of smooth (31) and corrugated (30) metal sheets that are coated on
both sides with a dielectric (32). The smooth sheets are connected
via a common electrical lead to one pole of the high voltage source
(34) and the corrugated sheets are connected to the second pole of
the voltage source. An electrical discharge is produced in each
flow channel (33) transverse to the flow of the exhaust gas by
applying a voltage. The coating of the metal sheets with the
dielectric electrically insulates the adjacent sheets from one
another. Either the smooth or corrugated sheets or both types of
sheets may be coated with a dielectric to achieve electrical
insulation from one another. Instead of the dielectric coating it
is also possible to insert a dielectric intermediate layer between
adjacent sheets. This intermediate layer may for example be a
ceramic sheet.
[0060] On account of the small cross-sectional dimensions of the
flow channels, the discharge voltage may be chosen to be
correspondingly low. The length of the honeycomb bodies is not
restricted. In FIG. 8 corrugated and smooth metal sheets are
combined with one another to form a sheet stack. Two metal sheets
having different corrugations may however also be combined with one
another. The type of corrugation as well as the cross-sectional
dimensions of the flow channels may be adapted to particular
application requirements.
[0061] Also, with this reactor a catalyst layer of finely divided
support materials and catalytically active components may be
applied to the dielectric layers that insulate the metal sheets
against the gas discharge and also insulate the sheets from one
another. Alternatively, the dielectric sheets themselves may be
formed as a catalytically active layer if comprising finely divided
support materials and catalytically active components this layer
produces an insulating effect that is sufficient to meet the
requirements of the dielectrically hindered discharge. The
composition of this catalytically active layer may correspond to
the already described composition of the ceramic pellets.
[0062] The proposed process has various advantages compared to the
known processes. It may for example be used in different designs of
engine. When used in lean-burn engines the fuel consumption figures
are only slight worse. The generation of ammonia in situ in the
vehicle, without the need to include additional operating
materials, is of particular advantage. The necessary amount of
ammonia can be adjusted through the electrical and geometrical
parameters of the gas discharge as well as by the provision of the
rich exhaust gas partial stream. This adjustment procedure can be
controlled by the engine electronics.
EXAMPLE
[0063] The coaxial reactor of FIG. 7 was used for the following
investigations. Both the outer and inner pipes consisted of 2 mm
thick quartz glass. The outer pipe had an external diameter of 4 cm
and the inner pipe an internal diameter of 2.6 cm. The length of
the reactor was 20 cm and the length of the electrodes 16 cm. The
gas discharge space between the two quartz pipes was filled with a
palladium pellet catalyst (3.2 g of palladium and 35 g of barium
oxide per 1 litre of .gamma.-aluminum oxide pellets).
[0064] The synthesis gas mixture shown in Table 1 was passed
through the reactor at a volume flow of 4.5 Nl/min at atmospheric
pressure and a temperature of 100.degree. C. The product d*p was 6
mm*bar. A barrier discharge was ignited in the reactor by applying
an alternating voltage having a frequency of 1 kHz and an amplitude
of about 11 kV.
[0065] The measurements were carried out at room temperature.
1TABLE 1 Composition of the synthesis gas mixture Concentration
(Vol. -%) Substance at T = 293 K N.sub.2 77 O.sub.2 13 H.sub.2O 10
NO 500 ppm Total 100
[0066] The synthesis gas mixture of Table 1 is lean, in other words
it has a supra-stoichiometric oxygen content (the normalized
air/fuel-ratio .lambda. is greater than 1). Under these conditions
nitric oxide is adsorbed by the pellet catalyst when the gas
discharge is switched on. This can be explained by the conversion
of NO to NO.sub.2 in the plasma reactor under excess air, and
subsequent further oxidation and accumulation as NO.sub.3.sup.-. If
the exhaust gas composition is switched from a lean composition to
a rich composition, then a desorption of the previously absorbed
nitric oxide takes place. At the same time ammonia is formed in the
gas discharge, which very effectively converts the desorbed nitric
oxide to nitrogen and water.
[0067] In the present example the changeover from lean to rich
exhaust gas was effected by disconnecting the oxygen supply and
connecting carbon monoxide.
[0068] FIGS. 9 to 12 illustrate the aforedescribed procedures. FIG.
9 shows the change in the concentration of nitric oxide in the
synthesis gas behind the plasma reactor as a function of the
electrical power coupled to the gas discharge. When the gas
discharge is switched off (electrical power 0) the NO concentration
of the synthesis gas mixture behind the plasma reactor is 500 ppm
according to Table 1. After switching on the gas discharge there is
an immediate reduction in the NO concentration. This is attributed
to the formation of nitrogen dioxide by the plasma discharge, the
nitrogen dioxide being adsorbed very efficiently by the catalyst
pellets.
[0069] The electrical power coupled to of the plasma discharge was
slowly increased. At a power of about 9 watts the composition of
the synthesis gas mixture was changed from lean to rich by
disconnecting the oxygen and feeding carbon monoxide. FIG. 9 shows
that after the changeover of the exhaust gas composition, there is
first of all a marked desorption of nitric oxide under the now
prevailing reducing exhaust gas conditions. At the same time
ammonia is formed by the plasma discharge, which reduces the
desorbed nitric oxide in the presence of the catalyst.
[0070] FIGS. 10 to 12 illustrate spectroscopic investigations of
the exhaust gas during the lean operating phase, during the
changeover from the lean to rich operating mode, and during rich
operating phase. During the lean phase the exhaust gas exhibits
only the absorption bands of nitric oxide. During the switchover
these absorption bands disappear (FIG. 11), while during the rich
operating phase the exhaust gas clearly shows the absorption bands
of the ammonia that is formed (FIG. 12).
[0071] This test was repeated with the following compositions of
the pellets in the plasma reactor:
Pt/BaO/.gamma.-Al.sub.2O.sub.3
Pt--Pd/BaO/.gamma.-Al.sub.2O.sub.3
[0072] PT/.gamma.-Al.sub.2O.sub.3
V.sub.2O.sub.5/.gamma.-Al.sub.2O.sub.3
.alpha.-Al.sub.2O.sub.3
[0073] In all cases a significant formation of ammonia was found in
the gas discharge under reducing exhaust gas conditions.
[0074] Further variations and modifications of the foregoing will
be apparent to those skilled in the art and are intended to be
encompassed by the claims appended hereto.
[0075] German priority application 199 03 533.4 is relied on and
incorporated herein by reference.
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