Note: Descriptions are shown in the official language in which they were submitted.
7S~
M~THOD FO~ CATALYTIC PURIFICATION OF COMBUSTION
EXHAUST G~SES
Background of the Ir~vention
The present invention relates to a method for purifying
combustion exhaust gases, in particular to a method for
abating nitrogen oxides ("NO ") therein. The method is
broadly applicable to such purpose and may be used for
treatment of boiler flue yases, incineration flue gase~s,
etc., but is particularly useful for purifying the exhaust
gases of internal combustion engines, most particularly,
internal combustion engines capable of operating somewhat
rich of a stoichiometric air-to-fuel ratio. One type of
such engine is the natural gas-fueled internal combustion
engine of the type often employed to operate oil field pumps
and to pump natural gas through natural gas pipelines.
The prior art-is replete with with schemes for abating
pollutants, including nitrogen oxides, in combustion exhaust
gases. For example, U.S. Patent 4,157,316, assigned to the
assignee of this application, discloses a so-calle~ three-
way conversion catalyst utilized to purify engine exhaust
gases by substantially simultaneously oxidizing carbon
monoxide and unburned hydrocarbons and reducing nitrogen
oxides. IJ.S. Patent 4,202,301, also assigned to the assignee
of this application, discloses an air-fuel ratio control
~5'~
system of the "closed-loop" type ~hich is operated in par-tial
response to a control circuit which, in turn, is responsive
to the output of an oxygen sensor mounted in the exhaust
line of the engine. The carburetor or other air-fuel
proportioning device is regulated in at least partial
response to a signal which corresponds ~o -the presence of
oxygen in the engine exhaust gas, as sensed by an oxygen
sensor disposed in the exhaust gas line. Control of the
air-fuel ratio is considered important for, among other
purposes, enhancing the efficiency of a catalyst utilized to
purify the exhaust gases.
U.S. Patent 3,118,727, assigned to the assignee of the
present application, discloses a method for removing N0x
from the tail gas of nitric acid manufacture plants. The
tail gas is disclosed as having a composition comprising up
to about 0.5% by volume of mixed nitrous and nitric oxides,
about 3-4% by volume of oxygen and the balance nitrogen. A
hydrocarbon fuel such as natural gas or methane is admixed
with the gas to provide a fuel-to-oxygen ratio slightly
greater than that resulting from a stoichiometric mixture,
i.e., a slightly rich mixture. The heated combined gases
are passed over a platinum group metal catalyst essentially
containing rhodium or palladium or both, at an initial
reaction temperature below about 1400F, preferably about
690-780F. This treatment results in reduction of N0x to
z~
nitrogen. In a process as described in U.S. Patent 3,118,727,
reasonably steady state conaitions prevai~ and one may
select the amount of added hydrocarbon to be comingled with
the tail gas to prepare the mixture which is passed into
contact with the catalyst. In the operation of an internal
combustion engine there are, of course, other considerations.
Specifically, these are maintaining a reasonable fuel
efficiency and responding to changes in load, changes in
ambient conditions and fuel composition, etc., while keeping
the engine running smoothly. In addition to these re~uirements,
the need to reduce the level of air pollutants in the exhaust
and to conform with legal requirements for doing so, require
reducing the amount of carbon monoxide, hydrocarbons and ~x
emitted in the exhaust.
It is accordina]y an object of the present invention to
provide a method for catalytically purifying combustion
exhaust gases by controlling the air-to-fuel ratio of the
combustible mixture to maintain an exhaust gas containing
sufficient reducing components to reduce NOX to nitrogen
and simultaneously oxidize carbon monoxide to carbon dioxide
in the presence of a suitable catalyst.
It is another object of the present invention to provide
a method for catalytically purifying the exhaust gases of an
internal combustion engine by running the engine somewhat
rich of stoichiometric at a level which enhances both engine
operating efficiency and MOX abatement by providing sufficient
reducing values in the exhaust gas.
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Other objects and advantages of the invention will
become apparent from the following description.
Summary of the Invention
In accordance with the present invention there is
provided a method for cataly-tically purifying combustion
exhaust gases generated by a fuel burning mechanism having
an adjustable air-fuel proportioning device. The method
comprises the following steps. ~ combustible mixture
comprising a hydrocarbon-containing fuel, for example,
natural gas, and air is introduced into the fuel burning
mechanism and the air-to~fuel ratio of the combustible
mixture is controlled by suitable adjustment of the air-fuel
proportioning device to maintain the ratio at selected
values, as described below, which are rich of stoichiometric.
The fuel is combusted in the uel burning mechanism, which
may be, for example, an internal combustion engine, and
produces an exhaust gas containing nitrogen oxides and
carbon monoxide among the products of combustion. The
selected values of the air to fuel ratio are such as to
maintain in the exhaust gas at least the stoichiometric
amount of reducing components necessary to react with all
the nitrogen oxides present therein. The resultant exhaust
gas is then passed through a catalyst zone containing a
catalyst effective for reducing the nitrosen oxides to
nitrogen at the reaction conditions obtaining in the catalyst
zone, and for oxidizing carbon monoxide.
t ~"
In accordance with one aspect of the invention, the
amount o the reducing component produced by the combustion
is at least sufficient to reduce the ~x; the reducing
component comprises carbon monoxide and hydrogen. In
accordance with another aspect of the invention, the air-to-
fuel ratio is controlled to maintain the value, L, of the
ratio of the air-to-fuel ratio to the stoichiometric air-to-
fuel ratio, from about 0.89 to 1.0, prefera~ly 0.96 to 1.0,
more preferably from about 0.99 to 1Ø
In accordance with another aspect of the invention, the
catalyst comprises a platinum group metal catalyst, preferably
platinum and rhodium and, optionally, ruthenium. The
catalyst ~referably further comprises an alumina bearing
support comprising gamma alumina, on which support the
platinum group metal is distended. The alumina may be
coated on a support formed of a different material.
In yet another aspect of the invention, the method
includes controlling the air-to-fuel ra-tio by utilizing the
level of oxygen sensed in the exhaust gas hy oxygen sensor
means having a limited response range, which is inclusive of
the oxygen level resulting in the exhaust gas from combustion
at the stoichiometric air-to-fuel ratio This includes the
following additional steps. The air-to-fuel ratio is periodically
adjusted in the lean direction to a first value selected to
~ ~L7SZ~
result in a level of oxygen in the exhaust gas which lies
within the response range of the sensor means, thereby
periodically triggering a response range-reference signal
generated by the sensor means. The air-to-fuel ratio is
adjusted in response to the reference signal in the rich
direction sufficiently to attain a second value of the air-
to-fuel ratio which deviates from the first value by a
selected amount, whereby the air-to-fuel ratio is periodic-
ally adjusted to the second value in response to the period-
ic triggerin~ of said reference signal. T~is second valuepreferably is selected to maintain the value, L, of the
ratio of the air-to-fuel ratio to the stoichiometric air-to-
fuel ratio from about 0.89 to 1.0, preferably from about
L=0.99 to 1.0, the latter value being preferred when the
fuel is a natural gas.
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Brief Descri~tion of the Drawings
-
Figure 1 is a schematic diagram illustrating one
embodiment of equipment utilizable to carry out the method
of the present invention,
Figure 2 is a graph plotting the percent of conversion
of noxious components of an exhaust yas on the left hand
veritcal axis against the air-~uel ratio of the combustible
mixture on the horizontal axis, and against the voltage of
the output signal of a zirconia-type oxygen sensor disposed
in the exhaust gas, the latter plotted on the right hand
vertical axis.
Description of the Preferred Embodiments
As above stated, one of the objects.of the present
invention is to reduce the nitrogen oxides as well as other
noxious components in the exhaust ga.s of a combustion process.
The present invention, while particularly applicable to
internal combustion engines which are conventionally run
rich of stoichiometric, such as natural gas fuel fired
engines, is applicable to fuel combustion processes generally.
Referring now to ~igure 1, there is shown an engine 2
which may an internal combustion engine fueled by a gaseous
hydrocarbon containing fuel, such as natural gas. Engine 2
is equipped with a fuel-air proportioning device, for example,
a carburetor or fuel injection device 4, into w~ich a hydrocarbon
containing fuel, for example, methane or natural gas, is
--7--
.
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introduced via line 6 and into which air is introduced via
air line 8. As used herein ~nd in the claims, hydrocarbon-
containing fuel is intended to include ~uels which contain
hydrogen-carbon in their composition, such as alcohols and
derivatives thereof. The exhaust gas from engine 2 passes
through exhaust pipe 10 in which is positioned sensor means
12 which is capable of sensing the level of a component of
the exhaust gas within exhaust pipe 10. Sensor means 12 is
preferably an oxygen sensor capable of sensing the level of
o~y~en within the exhaust gases and may be positioned either
upstream or downstream of the catalyst. For example,
zirconia-type oxygen sensor means are well-known and com-
mercially available for this purpose. In segment lOa of
exhaust pipe 10 there is located a sample line ~ which is
capable of being operated by means not shown to withdraw
from exhaust pipe 10 a sample of the raw, i.e., catalytically
untreated, exhaust gas. Sensor means 12 is connected via
electrical connection 14 to a control unit 16 which may be
of any configuration suitable to control the air-to-fuel
(sometimes hereafter abbreviated "A/F") ratio of fuel-air
proportioning device 4 through control linkage 18, at least
partially in response to the output signal of sensor means
12.
Downstream, as sensed in the direction of exhaust gas
flow through exhaust pipe 10, lOa, there is connected to
exhaust pipe 10, lOa a catalytic reactor 20 having respective
inlet and outlet ends of generally truncated-cone configuration,
z~
the central portion of catalytic reactor 20 being preferably
circular or rectangular (including square) in cross section.
The truncated inlet end, together with suitable distribution
plates and/or baffle means within catalytic reactor 20,
serve to distribute the exhaust gas over the face of a
catalyst disposed within catalytic reactor 20 and the
truncated cone ou-tlet sec-tion of catalytic reactor 20 serves
to channel the catalytically treated gases into outlet
segment lOb of exhaust pipe 10. A sample line B is connected
to outlet segment lOb and is capable of being operated, by
means not shown, to withdraw therefrom samples of the
catalytically treated exhaust gas.
In operation, a fuel gas such as natural gas is fed
through fuel line 6 and air is fed through air line 8 into
fuel-air proportioning device 4 in which the fuel gas and
air are proportioned as required for delivery into the
combustion cylinders of engine 2. Device 4 may be a carburetor
or fuel injection device. The fuel is combusted in the
cylinders, the combustion being incomplete in that some
unburnt portion of the fuel and some unreacted oxygen appear
in the exhaust gas. Rxhaust gases exiting engine 2 via
; exhaust line 10 pass over sensor means 12 and the level of
oxygen (or other component) is sensed by sensor 12 and a
signal is generated which is fed to control unit 16. Control
unit 16, which may be any suitable means of control,
, .. . . ~
_g _ .
~5~
adjusts the ~/F ratio setting of proportioning
device 4 to maintain a setting w'nich is rich of stoichiometric
by a selected amount, as described in more detail below.
Essentially, the setting is maintained rich enouyh to
provide in the exhaust gases sufficient reducing values,
essentially carbon monoxide and hydrogen, but also including
some unburnt hydrocarbons, to reduce the NOX formed during
the combustion. The NO reduction is accomplished in the
presence of a catalyst contained within catalytic reactor
20, through which the exhaust gases are passed. Catalytic
reactor 20 comprises a catalyst zone containing therein a
suitable catalyst which is preferably a platinum group metal
containing catalyst. A suitable platinum group metal containing
catalyst, such as a platinum-rhodium catalyst, will cause
lS the NO contained in the exhaust gas, or at least a substantial
portion thereof, to be reduced to nitrogen by reaction with
the reducing values also contained in the exhaust gas. The
elevated temperature of the exhaust gases emanating from an
internal combustion engine, typically about 750 to 1200F,
~0 is sufficiently high to initiate the catalytic NOX reduction
reaction as the exhaust gases contact the platinum group
metal containing catalyst. The catalytically treated gases
are exhausted through segment lOb of exhau~t pipe 10 either
to atmos~here or to other use. For example, the purified
exhaust gases may be used as an inert gas to pressuri~e
underground oil wells or fox any other use as appropriate.
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~ ssentially, the setting is maintained rich enough to
provide in the exhaust gases sufficient reducing values,
essentially carbon monoxide and hydrogen, but also including
some unburnt hydrocarbons, to reduce the M0x formed during
the combustion. The ~x reduction is accomplished in the
presence of a catalyst contained within catalytic reactor
20, through which the exhaust gases are passed. Catalytic
rea~tor 20 comprises a catalyst zone containing therein a
suitable catalyst which is preferably a pla~inum group metal
containing catalyst. A suitable platinum group metal containing
catalyst, such as a platinum-rhodium catalyst, will cause
the N0 contained in the exhaust gas, or at least a substantial
portion thereof, to be reduced to nitrogen by reac-tion with
the reducing values also contained in the exhaust gas. The
elevated temperature of the exhaust gases emanating from an
internal combustion engine, typically about 750 to 1200F,
is sufficiently high to initiate the catalytic N0x reduction
reaction as the exhaust gases contact the platinum group
metal containing catalyst. The catalytically treated gases
are exhausted through segment lOb of exhaust pipe lO either
to atmosphere or to other use. For example, the purified
exhaust gases may be used as an inert gas to pressurize
underground oil wells or for any other use as appropriate.
~75Z~
Referring now -to Figure 2, there is shown a plot of the
percent conversion of noxious components of the fuel gas
against the air/fuel "specific" ratio as defined below. The
percent conversion is plotted on the left hand vertical axis
and the A/F specific ratio is plotted on the horizontal.
axis. On the right hand vertical axis there is plotted in
millivolts the strength of the output signal of an oxygen
sensor (sensor means 12 of Figurè 1) disposed in the exhaust
gas of the engine.
The percent conversion refers to the percent of the
components present in the untr.eated e~haust gas which is
converted by contact with the catalyst. Hydrocarbons and
carbon monoxide are converted by being oxidized to, re-
spectively, water and carbon dioxide. ~x components are
converted by being reduced to nitrogen.
As will be observed from the graph of Figure 2, the
percent conversion of Wx has a fairly high value, about
87%, at an A/F specific ratio (L) of about 0.87 and increases
as the A/F ratio increases, until a specific ratio of about
1.0 is attained, at which point the conversion precipitously
drops along practically a vertical line to a conversion of
about 40~. The percent conversion continues to rapidly
diminish until at an air/fuel specific ratio of about 1.02,
the graph shows less than about 5% WOx`conversion. The
conversion line for carbon monoxide is seen to be quite low,
less than 10%, at A/F specific ratios of up to about 0.92
after which it rapidly increases with increasing A/F specific
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ratio until it attains practically 100~ conversion at an A/F
specific ratio of about 1.03. The hydrocarbon eonversion
curve shows that maximum eonversion of hydroearbons peaks at
about 40% conversion in the vieinity of 0.97 to 1~01 A/F
specific ratio.
The graph of Figure 2 also shows that the response
range of the oxygen sensor means employed, which is a
zireonia-type oxygen sensor, ranges from about 900 to about
600 millivolts (MV) over an A/F ratio range of about 0.~9 to
1.0 about 1.02.
It will be appreciated from the graph of Figure 2 that
there exists a rather narrow range of A/F ratio at whieh the
eonversion not only of ~x but of a substantial proportion
of the two other mâjor noxious eonstituents is optimized.
For the partieular system and operating eonditions repre-
sented.by the graph of Figure 2, it is seen that this band
of A/F ratio is between ahout 0.99 and 1.0~
Operation outside this relatively narrow band of A/F
ratio will not only eause a marked reduction in conversion
of NOx and the other noxious components, but if the A/F
ratio is too far outside the narrow band on the rieh side,
~75iZ~
ammonia will be formed in significant quantities. If the
exhaust gas is subjected to a second catalytic stage of
oxidizing treatment, the ammonia will be oxidized to Nx A
two catalyst stage treatment for exhaust gases is known in
the art and, in fact, is utilizable in conjunction with-the
present invention. Figure 2 shows that the N0x abatement
treatment of the invention, in the typical embodiment
illustrated, will convert a high proportion of C0 but not
more than about 40% of the hydrocarbons present. The low
conversion results from the fact that methane is difficult
to oxidize, the conversion rate of the non-methane hydrocarbons
being much higher than ~0%. It may therefore be desired to
subject the treated exhaus-t gas to a second, oxidizing stage
of catalytic treatment to oxidize hydrocarbons. For example,
the first stage of catalytic treatment for NOX reduction may
be carried out in accordance with the present invention and
followed by a second, e.g., platinum catalyst stage, to
oxidize hydrocarbons and any remaining carbon monoxide.
Such two stage operations are usually operated with air or
other oxygen containing gas (air) injection between stages
in order to ope~ate the second stage in an oxidizing manner.
Ammonia which is formed in the gases being treated will be
oxidized to N0x in the second stage, thus at least partly
undoing the N0x abatement attained in the first stage.
Proper control of the A/F ratio is not only important
to maintain engine (or other combustor) efficiency and
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smooth running, bu-t i5 also important to enhance the ef-ficiency
of the catalyst utilized in the exhaust gas line to purify
the exhaust gas by reducing N0x and oxidizing carbon monoxide.
Such catalysts, which may be used in single or multiple
stages, are employed to reduce nitrogen oxides to nitrogen
to oxide C0 and/or to oxidize unhurnt hydrocarbons.
In order to reduce the nitrogen oxides (NOX) and carbon
monoxide in the exhaust of a natural gas fired engine, the
engine carburetion system must be adjusted so that the
engine runs slightly on the rich side of stoichiometric.
There must be a controlled exces~ of the natural gas fuel
over stoichiometric in the fuel-air mixture fed to the
engine to provide a-t least enough reducing components in the
exhaust gas to react with and reduce the Nx in the exhaust
gas upon contacting the catalyst. Carbon monoxide, as one
of the reducing components, is oxidized to carbon dioxide.
The exhaust gas emanating from the engine may be passed
through a catalytic reactor in which the N0 is reduced,
probably according to the following reactions:
(1) N02 + H2~ ~lO + H20
(2) 2N0 + 2H2) N2 ~~ 2H20
and/or
(3) N02 + C0 > ~ + C2
(~) 2N0 ~ 2C0-) N2 + 2C02
The reducing component may be carhon monoxide and/or hydrogen,
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~5Z~
as shown by the above equations. Further, unburnt hydrocarbons
may also react with and reduce M0x, thereby serving as a
reducing component.
Reference is made in this specification and in the
claims to a value, denominated "L", which may be referred to
as the air--to-fuel "specific ratio". Use of the specific
ratio, sometimes denominated in the art by the Greek letter
lambda, is a conventional useage in the art because it is
useful in avoiding confusion in making comparisons between
different operations. For exmaple, an A/F ratio of 1~.65
(weight of air to weight of fuel) is the stoichiometric
ratio corresponding to the combustion of a hydrocarbon fuel
with an average formula CHl 88. Fuels with differen~ carbon/
hydrogen ratios will require different A/F ratios to produce
a stoichiometric mixture. An "oxygenated hydrocarbon fuel",
i.e., an alcohol will of course have a quite different air-
to-fuel stoichiometric ratio because the fuel introduces
oxygen as well as hydrogen and carbon ratio to the stoichiometric
A/F ratio. The actual A/F ratio is divided by the stoichiometric
A/F ratio so that in this system L=l is a stoichiometric
mixture, L ~ 1 is a fuel-lean mixture and L <1 is a fuel-rich
mixture. For example, at an actual A/F.ratio of 14.5 for a
CHl 88 hydrocarbon fuel, L=14.5/14.65=0.9898 is a fuel-rich
mixture.
The data of Figure 2 was accumulated by operating an
engine equipped in the manner schematically indicated in
~16-
s
~75Z~
Figure 1, and withdrawing from sample line A samples of
catalytically untreated or raw exhaust gas by withdrawing
from sample line B samples of the catalytically or purified
exhaust gas. Analyses to determine the content of, respect-
ively, NOX hydrocarbons and carbon monoxide in the raw andcatalytically purified exhaust gas were conducted in ~he
course of operation over the A/F ratio range indicated in
the graph.
The tests were conducted on an engine as follows.
The following en~ine is supplied with an A/F ratio
control system in accordance with the present invention:
Engine: Waukesha L7042G Natural Gas Engine
Operating HP: 580 HP at 750 RPM.
Exhaust-Pipe Connection: 8"
Fuel: Sweet natural gas.
Control System: R. Bosch oxygen sensor (zirconia type),
unit operates electric motor to control
natural gas inlet valve. Automatic
control system is the type disclosed
20 ~` in commonly assigned copending Canadian
patent application S.N. 396,127, ~iled
concurrently with this application.
Exhaust Gas Flowrate: 2570 SCFM
Pressure Drop Across Catalyst: 2.5 inches El2O.
Exhaust Gas Temperature: 950F
Catalyst: 38 inch diameter x 3 inches deep.
Catalyst Temperature: 950 to 980F.
The exhaust pipe connection (corresponding to lO,lOa in
Figure 1) leads to a catalytic convertor (20 in Figure 1)
having a honeycomb type monolithic catalyst disposed therein.
The catalyst is substantially disc-shaped, 3~ inches in
S aiameter and 3 inches in depth. It has 300 rectangular
cross-section gas flow passages per square inch of inlet
face area, the passages extending parallel to each other
from the substantially circular inlet face of the monoli~h
to the substantially circular outlet face thereof. The
monolith honeycomb is comprised of cordieri-te and has an
alumina (predominantly gamma alumina) coating on the surface
thereof. There is distended upon the alumina coating
catalytic metal comprising platinum, rhodium and, optionally,
ruthenium in an amount effective to catalyze the N0x
reduction reaction (and C0 oxidation reaction) under the
conditions of temperature, gas flow rate, etc., obtaining.
The ruthenium is believed to he useful in repressing the
formation of ammonia. The catalyst is housed within a
convertor 60 inches long to which the exhaust gas line is
connected. Within the convert~r housing is a distribution
plate to aid in distributing the exhaust gas flow across
substantially the entire inlet face of the catalyst.
The following table shows exhaust gas analyses, as
follows.
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Analysis A - Raw exhaust gas ups-tream of the catalyst.
Analysis B - Treated exhaust gas downstream of the
catalyst. For Analyses A and B, the
~ngine was operated without using the
au-tomatic control (schematically illustrated
- by 16 of Figure 1).
Analysis C - Raw echaust gas upstream of the catalyst.
For Analyses C and D the engine was operated with A/F
ratio maintained at a selected value by an automatic control
system (16 of Figure 1). For Analyses A and C, samples were
taken upstream of the catalyst (sample limit A of Figure 1)
for Analyses B and D downstream of the catalyst (sample line
B of Figure 1).
TABL~
Volume Percent (% vol) or Volume Parts Per Million (ppmv)
Specific A/F
Analysis Ratio (L) NOx C0_ . Hydrocarbons
A 0.9 660 pl~nv 3.0% vol 1750 ppmv
B 0.9 75 ppmv 2.7% vol 1250 ppmv
(88.6% con- 10% con- (28% con-
version) version) version)
C 0.99 2790 ppmv 8706 ppmv 1100 p~nv
D 0.99 183 ppmv 870 p~nv 650 ppmv
(93% con- (90% con- (41~ con-
version) version) version)
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~ s~
As shown by Analyses A and B of the ahove example, with
the A/F specific ratio set, at 0.9 and without regulation of
the automatic control unit, the catalyst provided conversions
of 88.6~, 10% and 28%, respectively, for N0 , C0 and hydrocarbons.
As shown by Analyses C and D, with the A/F specific ratio at
0.99 and controlled by the automatic control system, conversions
of 93%, 90% and 41~ were obtained. Figure 2 plots additional
data points of this test.
Generally, it is desired in order to meet existing and
proposed clean air regulations, to obtain 90~ conversion of
the ~10x originally in the raw exhaust gas and a disclosure
of less than 2000 ppmv C0 in the exhaust. The control
system utilized should permit maintenance of a selected air-
to-fuel ratio rich of stoichiometric. It may be that the
A/F ratio necessary for a given case will be outside the
range of response of the oxygen sensor employed. For example,
the graph of Figure 2 is specific for a particular engine
operated with a specific fuel. The range of A/F which
optimizes ~Ix conversion and hydrocarbon and CO conversion
~0 will be somewhat different for each engine and fuel type.
An automatic A/F control system which permits main-
tenance of a selected air-to-fuel ratio is preferred for
internal combustion engine operation, particularly in
remote, unattended locations.
Note in Analysis C that the unburned hydrocarbons in
the raw exhaust gas are substantially reduced as compared to
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the raw exhaust gas obtained from operation at the lower
air-to-fuel ratio ~Analyses ~). This indicates that (in
addition to obtaining higher conversion efficiency of the
noxious components) the maintained air-to-fuel ratio is more
efficient in terms of fuel economy.
Successful operation of the engine described above has
also heen attained with an otherwise identical catalyst
containing platinum and rhodium as the catalytically active
materials. As will be understood by those skilled in the
art, the catalyst may obviously be of any type suitable for
the purpose and conditions of the specific operation to be
carried out. The catalyst support could be in the form of
beads or other particles, rather than the monolithic honeycomb
structure described. The support can be any suitable support,
preferably alumina, e.g., alumina beads, or some other
material, e.g., cordierite, silica, mullite, zirconia, etc.,
or combinations thereof~ preferably with a high surface area
alumina (gamma alumina) coating thereon. ~etal substrate
honeycomb type monoliths with an appropriate alumina coating
carrying the catalytic material may also be helpful.
The catalysts useful in connection with the present
invention and as described above, may be prepared by any
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.
.
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suitable techniques, for example those disclosed in U. S.
Patents 3,565,830, 3,956,188, 3,993,572, 4,134,860 and
4,157,316, all assign the assignee this application.
While the invention has been described with respect -to
specific preferred embodiments, it will be apparent to one
skilled in the art that numerous variations may be made to
the embodiments withou-t departing from the spirit and scope
of the invention.
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