GB2092911A - Method for catalytic purification of combustion exhaust gases - Google Patents

Abstract

A method for purifying combustion exhaust gases provides for conducting the combustion operation, e.g., running a natural gas-fueled internal combustion engine, sufficiently rich of the stoichiometric air-to-fuel ratio to provide reducing values (carbon monoxide and hydrogen) in at least the stoichiometric quantity needed to reduce nitrogen oxides: the ratio air: fuel (as used) to air: fuel (stoichiometric) is in the range 0.89 to 1.0. The exhaust gas is contacted with a platinum group metal (platinum and rhodium) catalyst to effectuate reduction of NOx formed in the combustion or otherwise contained in the exhaust gas, to nitrogen and oxidation of carbon monoxide.

Description

SPECIFICATION Method for catalytic purification of combustion exhaust gases 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 gases, incineration flue gases, 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 schemes for abating pollutants, including nitrogen oxides, in combustion exhaust gases. For example, U.S. Patent 4,157,316, discloses a so-called three-way conversion catalyst utilized to purify engine exhaust gases by substantially simultaneously oxidizing carbon monoxide and unburned hydrocarbons and reducing nitrogen oxides. U.S. Patent 4,202,310, discloses an air-fuel ratio control system of the "closed-loop" type which is operated in partial 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 to 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, discloses a method for removing NO, 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 1400 F, (760 C) preferably 690-780"F, (365-41 50C).This treatment results in reduction of NO, to nitrogen. In a process as described in U.S. Patent 3,118,727, reasonably steady state conditions prevail and one may select the amount of added hydrocarbon to be coming led 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 requirements, 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 NO, emitted in the exhaust.

It is accordingly 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 Now 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 NO, abatement by providing sufficient reducing values in the exhaust gas.

Other objects and advantages of the invention will become apparent from the following description.

In accordance with the present invention there is provided a method for catalytically purifying combustion exhaust gases generated by a fuel burning mechanism having an adjustable air-fuel proportioning device.

The method comprises the following steps. A 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 fuel 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 nitrogen oxides to nitrogen at the reaction conditions obtaining in the catalyst zone, and for oxidizing carbon monoxide.

In accordance with one aspect of the invention, the amount of the reducing component produced by the combustion is at least sufficient to reduce the NOX; 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 0.89 to 1.0, preferably 0.96 to 1.0, more preferably from 0.99 to 1.0.

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 preferably 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 ratio by utilizing the level of oxygen sensed in the exhaust gas by 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 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 periodically adjusted to the second value in response to the periodic triggering of said reference signal. This second value preferably is selected to maintain the value, L, of the ratio of the air-to-fuel ratio to the stoichiometric air-to-fuel ratio from 0.89 to 1.0, preferably from L=0.99 to 1.0, the latter value being preferred when the fuel is a natural gas.

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 gas on the left hand veritcal axis against the air-fuel 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.

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 gas 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 Figure 1, there is shown an engine 2 which may be 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 which a hydrocarbon containing fuel, for example, methane or natural gas, is introduced via line 6 and into which air is introduced via air line 8. As used herein and in the claims, hydrocarbon-containing fuel is intended to include fuels which contain hydrogen-carbon in their composirion, 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 oxygen 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 commercially availableforthis purpose. In segment 10a of exhaust pipe 10 there is located a sample line A 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 offuel-airproportioning device4through 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, 1 0a, there is connected to exhaust pipe 10, 1 Oa a catalytic reactor 20 having respective inlet and outlet ends of generally truncated-cone configuration, 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 outlet section of catalytic reactor 20 serves to channel the catalytically treated gases into outlet segment 10b of exhaust pipe 10.A sample line B is connected to outlet segment 1 0b 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. Exhaust 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, adjusts the A/F ratio setting of proportioning device 4 to maintain a setting which is rich of stoichiometric by a selected amount, as described in more detail below.

Essentially, 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 NO, 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 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 750 to 1200 F, (400 to 6500C) is sufficiently high to initiate the catalytic NO, reduction reaction as the exhaust gases contact the platinum group metal containing catalyst. The catalytically treated gases are exhausted through segment 1 0b of exhaust pipe 10 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.

Essentially, 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 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 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 750 to 1200 F, (400 to 6500C) is sufficiently high to initiate the catalytic NO, reduction reaction as the exhaust gases contact the platinum group metal containing catalyst. The catalytically treated gases are exhausted through segment 10b of exhaust pipe 10 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.

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 Figure 1) disposed in the exhaust gas of the engine.

The percent conversion refers to the percent of the components present in the untreated exhaust gas which is converted by contact with the catalyst. Hydrocarbons and carbon monoxide are converted by being oxidized to, respectively, water and carbon dioxide. NO, components are converted by being reduced to nitrogen.

As will be observed from the graph of Figure 2, the percent conversion of NOx 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% NOx 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 ratio until it attains practically 100% conversion at an A/F specific ratio of about 1.03.The hydrocarbon conversion curve shows that maximum conversion of hydrocarbons peaks at about 40% conversion in the vicinity 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 zirconia-type oxygen sensor, ranges from 900 to 600 millivolts (MV) over an A/F ratio range of 0.89 to 1.02.

It will be appreciated from the graph of Figure 2 that there exists a rather narrow range of A/F ratio at which the conversion not only of NO, but of a substantial proportion of the two other major noxious constituents is optimized. For the particular system and operating conditions represented by the graph of Figure 2, it is seen that this band of A/F ratio is between 0.99 and 1.0.

Operation outside this relatively narrow band of A/F ratio will not only cause 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 rich side, 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 NOx. 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 NOx abatement treatment of the invention, in the typical embodiment illustrated, will convert a high proportion of CO 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 40%. It may therefore be desired to subject the treated exhaust gas to a second, oxidizing stage of catalytic treatment to oxidize hydrocarbons. For example, the first stage of catalytic treatment for NO, 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 operate the second stage in an oxidizing manner. Ammonia which is formed in the gases being treated will be oxidized to NO, in the second stage, thus at least partly undoing the NOx abatement attained in the first stage.

Proper control of the AIF ratio is not only important to maintain engine (or other combustor) efficiency and smooth running, but is also important to enhance the efficiency of the catalyst utilized in the exhaust gas line to purify the exhaust gas by reducing NO, 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 CO and/orto oxidize unburnt hydrocarbons.

In order to reduce the nitrogen oxides (NO,) 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 excess of the natural gas fuel over stoichiometric in the fuel-air mixture fed to the engine to provide at least enough reducing components in the exhaust gas to react with and reduce the NO, 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 NO, is reduced, probably according to the following reactions: (1) NO2+H2#NO+H2O (2) 2NO + 2H2 ) N2 + 2H20 and/or (3) NO2+CO#NO+CO2 (4) 2NO+2CO#N2+2CO2 The reducing component may be carbon monoxide and/or hydrogen, as shown by the above equations.

Further, unburnt hydrocarbons may also react with and reduce NOX, 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 example, an A/F ratio of 14.65 (weight of air to weight of fuel) is the stoichiometric ratio corresponding to the combustion of a hydrogen fuel with an average formula CHi.s8. Fuels with different 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=1 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 CH1.88 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 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, respectively, NOx hydrocarbons and carbon monoxide in the raw and catalytically purified exhaust gas were conducted in the course of operation over the A/F ratio range indicated in the graph.

The tests were conducted on an engine as follows.

The following engine 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" (20.3 cm.) 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 in copending British Patent Application No. 8203834 Exhaust Gas Flowrate: 2570 SCFM (72.4 m3 per min.) Pressure Drop Across Catalyst: 2.5 inches H20. (63.5 Kg/m2) Exhaust Gas Temperature: 9500F (510 C) Catalyst: 38 inch diameter x 3 inches deep. (96.5x7.6 cm).

Catalyst Temperature: 950 to 980 F. (510 to 527 C).

The exhaust pipe connection (corresponding to 10,1 0a 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, 38 inches in diameter 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 monolith to the substantially circular outlet face thereof. The monolith honeycomb is comprised of cordierite 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 NO, reduction reaction (and CO oxidation reaction) under the conditions of temperature, gas flow rate, etc., obtaining. The ruthenium is believed to be useful in repressing the formation of ammonia. The catalyst is housed within a converter 60 inches long to which the exhaust gas line is connected. Within the convertor 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.

Analysis A - Raw exhaust gas upstream of the catalyst.

Analysis B - Treated exhaust gas downstream of the catalyst. For Analyses A and B, the engine was operated without using the automatic control (schematically illustrated by 16 of Figure 1).

Analysic C - Raw exhaust 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).

TABLE Volume percent (O/o voll or volume parts per million (ppmv) Specific A/F Analysis Ratio (L) NOx CO Hydrocarbons A 0.9 660 ppmv 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 ppmv D 0.99 183 ppmv 870 ppmv 650 ppmv (93% con- (90% con- (41% con version) version) version) As shown by Analyses A and B of the above 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 NOx, CO 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 NO, originally in the raw exhaust gas and a disclosure of less than 2000 ppmv CO 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 NOx conversion and hydrocarbon and CO conversion will be somewhat different for each engine and fuel type.

An automatic A/F control system which permits maintenance 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 the raw exhaust gas obtained from operation at the lower air-to-fuel ratio (Analyses A). 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 been 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.

Metal 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 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.

Claims (11)

1. A method for catalytically purifying combustion exhaust gases generated by a fuel burning mechanism having an adjustable air-fuel proportioning device, the method comprising the steps of: introducing a combustible mixture comprising a hydrocarbon-containing fuel and air into said fuel burning mechanism; controlling the air-to-fuel ratio of the combustible mixture by suitable adjustment of the air-fuel proportioning device to maintain the value, L, of the ratio of the air-to-fuel ratio to the stoichiometric air-to-fuel ratio, from 0.89 to 1.0;; combusting the fuel in said fuel burning mechanism thereby producing an exhaust gas containing nitrogen oxides and carbon monoxide among the products of combustion, said selected values of the air to fuel ratio being 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; and passing the resultant exhaust gas through a catalyst zone containing a catalyst effective for reducing the nitrogen oxides to nitrogen and oxidizing the carbon monoxide to carbon dioxide at the reaction conditions obtaining in said catalyst zone.

2. The method of claim 1 wherein 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 0.96 to 1.0.

3. The method of claim 1 wherein the fuel is natural gas and 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 0.99 to 1.0.

4. The method of any one of claims 1, 2 or 3 wherein said reducing component comprises carbon monoxide and hydrogen.

5. The method of any one of claims 1,2 or 3 wherein said catalyst comprises a platinum group metal catalyst.

6. The method of claim 5 wherein said catalyst comprises platinum and rhodium and, optionally, ruthenium.

7. The method of claim 6 wherein said catalyst further comprises an alumina bearing support comprising gamma alumina, on which support said platinum and rhodium and, optionally, ruthenium, are distended.

8. The method of claims 1-7 wherein the fuel burning mechanism is an internal combustion engine.

9. The method of claims 1-8 including controlling the air-to-fuel ratio by utilizing the level of oxygen sensed in the exhaust gas by 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, by the additional steps of:: periodically adjusting the air-to-fuel ratio in the lean direction to a first value selected to result in a level of oxygen in the exhaust gas which lies within the response range of said sensor means, thereby periodically triggering a response range-reference signal generated by said sensor means; adjusting the air-to-fuel ratio in response to said 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 periodically adjusted to said second value in response to the periodic triggering of said reference signal.

10. The method of claim 9 wherein the first value is that which results from combustion at about the stoichiometric air-to-fuel ratio and the second value is that which maintains the value, L, of the ratio of the air-to-fuel ratio to the stoichiometric air-to-fuel ratio from 0.89 to 1.0.

11. The method of Claim 1 and substantially as hereinbefore described with reference to the accompanying Drawings.