WO1999047238A1

WO1999047238A1 - Passive regeneration catalytic filter, system and method for soot removal from combustion sources

Info

Publication number: WO1999047238A1
Application number: PCT/US1999/005971
Authority: WO
Inventors: Bruce A. Bishop; Robert L. Goldsmith
Original assignee: Ceramem Corporation
Priority date: 1998-03-19
Filing date: 1999-03-18
Publication date: 1999-09-23

Abstract

A catalytic filtration device for separating a soot containing exhaust gas from a combustion source into a filtrate and a soot containing filter cake having a monolith of porous material and a microporous catalytic membrane coating selected to regenerate the device by in situ oxidation of the soot contained in the filter cake.

Description

PASSIVE REGENERATION CATALYTIC FILTER, SYSTEM AND METHOD FOR SOOT REMOVAL FROM COMBUSTION SOURCES

Reference to Prior Application

This application incorporates by reference and claims the benefit of the filing date of U.S. Provisional Patent Application Serial No. 60/078,524, filed March 19, 1998.

Field of the Invention

Catalytic filter devices and methods of use are employed for collecting particulates from combustion source exhausts at high efficiency with in si tu oxidation of collected soot and capable of regeneration by bac pulsing to remove residual particulates not removed by combustion.

Background of the Invention

Treatment of engine exhaust gases, especially that from diesel engines, to remove particulate matter is a field that has attracted substantial interest and commercial activity over the past fifteen years. For diesel exhaust aftertreatment, a monolith-based diesel particulate filter (DPF) was first patented by Pitcher, Jr., in U.S. Patent No. 4,329,162, issued May 11, 1982. Problems which have hindered the broad acceptance of this technology include :

Durability: thermal (500°C to 600°C) or catalytically assisted (ca. 400°C) DPF regeneration must be performed periodically to oxidize the particulate matter (soot) captured by the filter. This oxidation can induce high thermal gradients and local hot spots within the filter, if not properly initiated or controlled. As a result of these thermal stresses, mechanical failure can occur, leading to loss of particulate removal efficiency. Backpressure: To minimize the number of regeneration cycles and thermal shocks, the regeneration operation is usually performed when the filter is highly loaded with soot. As a result, the diesel engine operates under high backpressure for extended periods, increasing fuel consumption and reducing efficiency.

The inability to remove noncombustible particulates, e. g. , sulfates; oil ash; and exhaust system corrosion products, from the filter results in a build-up of residual pressure drop. In addition, reactions between the inorganics and the filter materials at regeneration temperatures can degrade the filter or catalyst, so that lifetimes are shortened.

Soot removal efficiency is currently limited to the range of about 65 to 90% because of the large pore size of the DPF material.

DPF systems with thermal regeneration are complex and have a high cost for system hardware. This limits their use to large diesel engines. Consequently, recent research efforts have been directed to DPF regeneration, including development of:

1. filter materials that can withstand thermal regeneration;

2. catalysts that minimize the thermomechanical stresses associated with thermal regeneration; and

3. compressed gas backpulse techniques that regenerate the filter nonoxidatively and with minimal thermal stress.

2 An improvement to the capability of DPF' s to remove particulate matter with high efficiency is disclosed in Goldsmith et al . , U.S. Patent No. 5,114,581, issued May 19, 1992. Goldsmith et al . includes the addition of an inorganic microporous membrane, at least to the inlet passageway walls of a DPF. Such devices have been shown to have removal efficiency of particulates from diesel exhaust in laboratory tests of up to 99% (Khalil, N. and Y.A. Levendis, Y.A., "Development of a New Diesel Particulate Control System with Wall-Flow Filters and Reverse Cleaning Regeneration," SAE paper 920567, Reprinted from Diesel Particulate Control, Trap, and Filtration Systems (SP-896) , published by SAE International) . Such devices have subsequently shown particulate removal efficiencies in the range of 95 to 99% in over-the-road testing in a diesel automobile (Kim, S.H. and Levendis, Y.A., "Design of a Diesel Particulate Trap-Incinerator with Simultaneous Filtration and Compressed Air Regeneration (CAR) , " SAE Paper 930367, reprinted from Diesel Exhaust Aftertreatment (SP- 943), published by SAE International). This membrane-coated filter has the novelty of being effectively regenerated by compressed gas backpulsing.

Previously, for non-membrane-coated DPF' s regenerated by thermal oxidation, a means of reducing the required oxidation temperature was developed by Ho eier et al . , U.S. Patent No. 4,759,918, issued July 26, 1988, which included the deposition of a low temperature oxidation catalyst to the inlet passageway walls of a DPF.

Another prior art patent, Takeuchi et al . , U.S. Patent No. 4,746,537, issued May 24, 1988, discloses the application of a y-alumina-supported precious metal catalyst to facilitate soot oxidation in a DPF. The presence of the catalyst increased soot removal efficiency, but the removal efficiency still remained low, about 90% or less.

3 Goldsmith et al . , in U.S. Patent No. 5,221,484, issued June 22, 1993, further adds a catalyst to the membrane- coated DPF disclosed by Goldsmith et al. in U.S. Patent No. 5,114,581, issued May 19, 1992. The latter patent discloses separate catalyst coatings and membrane coatings. Preferably, the catalyst coating is applied within the pore structure of the DPF, but it can also be applied as a layer either beneath or on top of the membrane coating, on either one side or both sides of the monolith passageway walls. However, the disclosure in U.S. Patent No. 5,221,484 is for an oxidation catalyst for the oxidation of gaseous contaminants in the filtrate as the filtrate passes through the catalyzed DPF. Nothing in U.S. Patent No. 5,221,484 suggests, directly or indirectly, that particulate carbonaceous matter (soot) can be oxidized in such a device. U.S. Patent No. 5,221,484 specifies that the filter cake collected in such a device is not removed by in si tu oxidation, but by removal by withdrawal from the inlet end face of the device, primarily by backflushing. A recent innovation for DPF regeneration is a passive technique, which employs a low-temperature, precious metal soot oxidation catalyst deposited within the passageway walls of the filter. Soot is continually oxidized in situ, and the complexity of previously employed, higher- temperature means is potentially eliminated. This passive regeneration occurs at temperatures of <400°C (Internet Web Page entitled "Engelhard Emission Control Products," at site http: //www.dieselnet.com/engelhard/products .html, March 1998) . However, such catalyzed devices can show a gradual increase in pressure drop over time as inorganic particulate matter is not removed by oxidation and can create a residual pressure drop by plugging the pores of the catalyzed DPF.

Thus, the DPF' s developed to date which lack membranes, whether catalyzed to oxidize soot or not, have limitations

4 relative to high efficiency soot removal efficiency, which can be reduced by the addition of a membrane as disclosed in Goldsmith et al., U.S. Patent No. 5,114,581. However, such membrane-coated DPF' s disclosed to date in the prior art and technical literature require an active regeneration, such as by backpulsing with compressed air. Thus, no art disclosed to date describes a monolith filter for soot removal from combustion gases which combines a membrane to achieve high soot removal efficiency and an oxidation catalyst for in si tu oxidation of collected soot.

Summary of the Invention It is therefore desirable to provide a new catalytic filter, system and method which can be employed for the filtration of particulates from a combustion exhaust gas, such as that from diesel engines, with a catalyst that provides a means of in si tu oxidation of collected soot.

Also, an objective of this invention is to provide a catalytic filter which provides substantially complete removal efficiency of particulates from the exhaust gas. Further, the invention provides a catalytic filter, which can be regenerated periodically by backflushing to remove inorganic particulates, which are not removed by in si tu oxidation or combustion.

Additionally, the invention provides a device which can also use a second catalyst coating for the removal of vapor phase organics and carbon monoxide as they pass through the device.

Furthermore, the invention provides a new catalytic filter that has a large amount of filtration surface area per unit volume of the device.

The invention relates to a backflushable filter, system and method, which filter can be treated to apply an oxidation catalyst overlying the membrane coating in that device, so as to catalyze in si tu the oxidation of soot (carbonaceous particulate material) collected on the surface of the porous membrane coating. The resulting device and method provides intimate contact of the collected soot with the overlying catalyst layer. The invention features a catalytic filtration device for separating a soot-containing exhaust gas from a combustion source into a filtrate and a soot-containing filter cake. The device is comprised of a monolith of porous material containing a plurality of passageways extending longitudinally from an inlet end face to an outlet end face, having a plurality of plugs in the ends of the passageways at the inlet end face and at the outlet end face, to prevent direct passage of the exhaust gas through the passageways from the inlet end face to the outlet end face. A microporous catalytic membrane coating selected to separate the exhaust gas into a filtrate and soot-containing filter cake is applied to at least the wall surfaces of the passageways, open at the inlet end face and has a mean pore size smaller than the mean pore size of the porous material. The filter is regenerable by in si tu oxidation of the soot contained in the exhaust gas and collected in the filter cake on the catalytic membrane coating. The filter can also be regenerated by backpulse regeneration to remove non- combustible particulate matter in the filter cake from the inlet end face.

In one embodiment of the device the monolith material is a porous ceramic. In the device, the catalytic membrane coating can be a single microporous layer of an oxidation catalyst, or it can be a non-catalytic membrane with an overlying layer of an oxidation catalyst. Preferably, the soot retention efficiency of the membrane is greater than 95%.

In another embodiment of the device, a second oxidation catalyst is applied within the pores of the device to

6 oxidize hydrocarbon vapors and carbon monoxide in the filtrate as they pass through the filter.

The invention also includes a method for the removal of soot from exhaust gas from a combustion source by introducing an exhaust gas into a soot filter, which includes a monolith of porous material containing a plurality of passageways extending longitudinally from an inlet end face to an outlet end face, having a plurality of plugs in the ends of the passageways at the inlet end face and at the outlet end face, to prevent direct passage of the exhaust gas through the passageways from the inlet end face to the outlet end face. The monolith has a microporous catalytic membrane coating, the membrane applied to at least the wall surfaces of the passageways, open at the inlet end face and of mean pore size smaller than the mean pore size of the porous material. The soot is collected as a filter cake on the surface of the catalytic membrane coating and removed by in si tu oxidation which is catalyzed by the catalytic membrane coating. Noncombustible particulates in the filter cake can be removed by backpulse regeneration from the inlet end face.

In one embodiment of this method, the monolith material is a porous ceramic. The catalytic membrane coating itself can be comprised of an oxidation catalyst, or alternatively, the catalytic membrane coating can be comprised of a noncatalytic membrane with an overlying oxidation catalyst. The soot retention efficiency of the membrane is greater than 95%.

In yet another embodiment, a second oxidation catalyst can be applied within the pores of the device for oxidization of hydrocarbon vapors and carbon monoxide in the filtrate.

Pulsed heating of the filter can be used to achieve soot "light off" temperature. Such pulsed heating can be

7 achieved by resistive heating of the porous monolith. One monolith material for which resistive heating can be used is silicon carbide. An alternative method of achieving pulsed heating is by introduction of hydrogen into the exhaust gas, and the hydrogen is catalytically oxidized either before introduction into the filter or by the catalytic membrane itself.

The method may further include exhaust gas recirculation of a portion of the filtrate to the inlet of the combustion source to reduce formation of NOx.

In a preferred embodiment, the combustion source is a diesel engine.

The invention will be described for the purpose of illustration only in connection with preferred, illustrative embodiments; however, it is recognized that various changes, modifications, additions and improvements may be made to the filter, system and method by those persons skilled in the art without departing from the spirit and scope of the invention. Brief Description of the Drawings

Fig. 1 is a schematic of a process and system containing a catalytic membrane-coated ceramic filter for the collection of particulates from a combustion gas source; and Fig. 2 is a schematic which shows the structure of a portion of a device formed from a monolith filter, showing in cross section the monolith wall, the membrane coating, and an overlying catalyst coating.

Description of the Embodiments Fig. 1 is a schematic that illustrates how the catalytic filter can be employed for removal of soot from a combustion source, a diesel engine indicated as an example. The exhaust gas flow is into one or more diesel particulate filters. Preferably, the filter is constructed from a

8 monolith structure configured in a dead end flow configuration and containing a membrane coating deposited at least on the passageway walls of the inlet passageways, as disclosed previously in U.S. Patent No. 5,114,581, herein incorporated by reference. As the exhaust gas flows through the filter, the soot is collected on the surface of the catalytic membrane coating on the inlet passageway walls. The filtered, soot-free gas is exhausted. A portion of the ■ gas can be employed in an exhaust gas recirculation loop for suppression of NOx formation, as disclosed by Levendis et al., in U.S. Patent No. 5,426,936, issued June 27, 1995, herein incorporated by reference.

It is necessary to remove the soot as it builds on the surface of the catalytic membrane coating. If the exhaust gas temperature is at or above a "light off" temperature, typically above about 350°C-400°C, then the soot can be oxidized continuously in the flowing exhaust gas which contains residual oxygen. If the exhaust gas is at a lower temperature, but still achieves "light off" temperature fairly frequently, such as every hour, then the soot will be oxidized intermittently without addition of heat. If, however, the exhaust gas temperature does not regularly exceed the "light off" temperature, then auxiliary heating of the catalyzed filter must be employed to achieve passive, in si tu oxidation of the collected soot.

One type of auxiliary heating which can be employed is to use an electrically conductive monolith in the filter and to heat the monolith by passing electric current through the monolith, as disclosed by Silentor NoTox A/S (web page 0 posted at http://www.notox.com/Diesel/Regeneration/El- heating/monoheat.htm) . A second means of auxiliary heating is to introduce hydrogen gas into the exhaust gas. Precious metal catalysts, at very low temperatures, readily oxidize the hydrogen, and the heat of oxidation will quickly increase the temperature of the catalyzed filter to soot oxidation temperature. This means of heating has been disclosed by Appleby in U.S. Patent No. 5,813,222, issued September 29, 1998, for cold start heating of catalytic converters for gasoline fueled engines.

As exhaust gas can contain inorganic particulate matter, such as corrosion products and particulates originating in fuel or lubricating oil additives, such matter can collect on the surface of the catalytic membrane coating. As the catalytic membrane coating has a fine macropore structure, comparable to that of the membrane, as disclosed in Goldsmith et al., U.S. Patent No. 5,114,581, such inorganic particulate matter will collect on the surface of the catalytic membrane layer. As this particulate matter will not penetrate and plug the pore structure, it can be removed from time to time by backpulse regeneration. This will minimize any long-term increase in filter pressure drop due to irreversible collection of inorganic particulates on or within the filter structure. While a preferred catalyzed filter configuration is that of a honeycomb monolith, as disclosed in Goldsmith et al . , U.S. Patent No. 5,114,581, it is to be recognized that any other filter structure which contains a large-pored mechanical support, coated first by a fine-pored membrane layer, and subsequently with a soot oxidation catalyst layer overlying the membrane layer, will function in accordance with the disclosures herein.

Fig. 2 shows a schematic of a cross section of the layered structure consisting of a monolith wall coated by a fine-pored membrane layer, and finally coated by a catalyst layer. The porous membrane layer can be as disclosed by Goldsmith, in U.S. Patent No. 4,983,423, issued January 8, 1991, herein incorporated by reference. The catalyst layer can be a catalyst-impregnated carrier coating, as disclosed by Goldsmith et al . , in U.S. Patent No. 5,221,484, herein

10 incorporated by reference. The active catalyst component can be a precious metal, such as: platinum; palladium; ruthenium; rhodium; and mixtures thereof. The carrier can be a relatively inert oxide, such as alumina or silica, or a transition metal oxide such as: ceria; vanadia; titania; zirconia; or mixtures thereof. Various additives, as disclosed in the prior art, can be present to suppress oxidation of sulfur dioxide as well as catalyst poisoning.

As a further embodiment, the porous membrane layer itself can be formed from a soot oxidation catalyst. In this instance, the membrane forming methods disclosed by Goldsmith, in U.S. Patent No. 4,983,423 can be followed to provide a coherent, largely defect-free membrane coating which is adherent to the monolith wall surface. Such membrane coatings, which can function as oxidation catalysts will preferably, but not necessarily, contain an active precious metal oxidation catalyst.

Both the two layer structure comprised of a separate membrane with an overlying catalyst layer and the one layer structure comprised of a catalytic membrane coating layer are referred to herein as a catalytic membrane layer.

Optionally, it is also possible to apply a catalyst within the pores of the filter. This would be effective for the further oxidation of gaseous hydrocarbons and carbon monoxide, if not adequately oxidized by the soot oxidation catalyst.

In summary, the present invention is suitable for the removal of particulate matter from a combustion exhaust gas, in which the preponderance of particulates are carbonaceous in composition (soot) and subject to conversion to gaseous constituents by oxidation. The source can be an internal combustion engine, such as a diesel engine, or any type of engine which emits particulates.

In one embodiment, the membrane coating is a separate coating of a low temperature oxidation catalyst, such as one

11 containing a precious metal such as: platinum; palladium; ruthenium; rhodium; or mixtures thereof, supported or unsupported, capable of oxidizing particulate matter at a relatively low temperature, e.g., 300°C to 500°C, as the particulate matter is collected. The system is to collect particulates which are removed, passively, by in si tu oxidation while the engine is in operation. The application of the membrane coating provides a particulate removal efficiency in excess of 95%, which cannot be achieved readily by a DPF without such a membrane coating.

Further, the monolith-based filter can be treated by backpulsing with compressed gas from time to time to remove inorganic particulates and other residues not removed by the in si tu oxidation. Finally, the degree of particulate removal will be sufficiently high such that exhaust gas recirculation can be practiced for suppression of NOx formation during combustion, without introduction of a harmful level particulate matter into the engine intake.

The invention disclosed herein has several advantages over previously disclosed filters for removal of soot from a combustion source exhaust, including, but not limited to the following advantages described herein.

The catalytic membrane coating will have a mean pore diameter in the range of 0.1 to 2 mm, preferably in the range of 0.1 to 1 mm. As demonstrated previously, and as disclosed in the SAE papers referenced above, this will give

>95% soot removal.

This coating will also prevent penetration of particulate matter (both organic and inorganic) into the underlying monolith support wall structure and thereby prevent associated increase in pressure drop.

The soot oxidation catalyst can be applied as a very thin layer overlying the gas particulate membrane layer. This permits use of a highly concentrated catalyst in

12 intimate contact with soot, and this can be expected to provide a much more effective soot oxidation than that achieved by a catalyst deposited by solution impregnation within DPF pores. Inorganic particles, which are not oxidized, will collect as a surface layer on the catalyst/membrane. These can be removed by occasional backpulse regeneration at a maintenance depot (i.e., not by an on-board regeneration system) . This will eliminate long-term pressure drop increase due to ash build-up within the catalyst/monolith pores.

The utilization of one catalyst for soot oxidation and a second catalyst for hydrocarbon vapor and carbon monoxide oxidation allows the possibility of selection of two separate catalyst compositions, each optimized for the desired oxidative duty.

The combination of a thin, highly concentrated, soot oxidation layer and a low concentration, vapor oxidation catalyst within the pore structure could provide an overall reduction in the mass of precious metal required, reducing this expensive component in catalyzed DPF's.

Example 1

A membrane-coated filter can be prepared according to the procedure as described in Example 2, disclosed in U.S. Patent No. 5,114,581, and herein incorporated by reference. Overlying the membrane coating, a platinum on alumina catalyst can be deposited by the following procedure. The specific example will provide a 0.5 wt % platinum on alumina coating, but this can be varied to achieve the desired precious metal composition and concentration. Also, the following recipe can be adjusted to change the thickness of the catalytic coating overlying the membrane. Finally, as

13 is known in the art of membrane formation, fugitive pore formers can be employed in the slip casting process to create acroporosity in the catalyst coating to control the flow resistance of this layer. 0.26 g of hydrogen hexachloroplatinate is added to 19 g of deionized water. The resulting solution is added drop- wise to 24 g of dry, hydrated pseudoboehmite (Type 11N7-80, Vista Chemical Company) . This results in practically complete saturation of interparticle and intraparticle porosity within the pseudoboehmite mass. The mixture is dried overnight at 90°C and subsequently ground in a mortar to break down large agglomerates. Next, 5 g of the mixture are added to 95 g of water containing nitric acid (pH of 3.0). The mixture is stirred rapidly for four hours to assist in suspension and dispersion of pseudoboehmite particles. To control the viscosity of the suspension, 8 g of polyethylene glycol (Carbowax 20M, Union Carbide) are added, followed by two additional hours of stirring. A small portion of this suspension is extracted, dried, and heated to 600°C in air to yield a powdery solid. Chemical analysis of this solid by inductively coupled plasma-atomic absorption spectroscopy indicated a composition of 0.5 wt % platinum. The suspension can be used to slip-cast a precursor catalyst layer overlying the membrane-coated surfaces of a monolith filter. After drying at 110°C, the monolith is heated to 500°C, first under air and subsequently in a stream composed of 2% hydrogen-98% nitrogen to activate the slip-cast supported catalyst layer. The catalytic membrane-coated monolith is then plugged to prevent direct passage of a feed stock from the inlet end face to the outlet end face of a final soot collection filter.

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Claims

ClaimsWhat is claimed is:

Claim 1. A catalytic filtration device for separating a soot containing exhaust gas from a combustion source into a filtrate and a soot-containing filter cake, which device comprises: a) a monolith of porous material containing a plurality of passageways extending longitudinally from an inlet end face to an outlet end face, having a plurality of plugs in the ends of the passageways at the inlet end face and at the outlet end face to prevent direct passage of the exhaust gas through the passageways from the inlet end face to the outlet end face; and t>) a microporous catalytic membrane coating selected to separate the exhaust gas into a filtrate and soot-containing filter cake, the coating applied to at least the wall surfaces of the passageways open at the inlet end face and of mean pore size smaller than the mean pore size of the porous material, and to regenerate the device by in si tu oxidation of the soot contained in the filter cake.

Claim 2. The device of claim 1 which includes backpulse regeneration means to remove noncombustible particulate matter in the filter cake from the inlet end face.

Claim 3. The device of claim 1 in which the monolith material is a porous ceramic.

Claim 4. The device of claim 1 in which the catalytic membrane coating is comprised of a single microporous layer of an oxidation catalyst.

15 Claim 5. The device of claim 1 in which the catalytic membrane coating is comprised of a noncatalytic membrane with an overlying layer of an oxidation catalyst.

Claim 6. The device of claim 1 characterized by soot retention efficiency of the catalytic membrane coating of greater than 95%.

Claim 7. The device of claim 1 in which a second oxidation catalyst within the pores of the device oxidizes hydrocarbon vapors and carbon monoxide in the filtrate passing through the filter.

Claim 8. The device of claim 1 wherein the oxidation catalyst is selected to oxidize soot at a temperature of about 300 to 500┬░C.

Claim 9. The device of claim 1 which includes a recirculation means to recirculate a portion of the gaseous filtrate to the inlet of the combustion source to reduce formation of NOx.

Claim 10. The device of claim 1 which includes a combustion source of a diesel engine.

Claim 11. A method for the removal of soot from an exhaust gas from a combustion source, which method comprises: a) introducing the exhaust gas into a soot collection filter which includes a monolith of porous material containing a plurality of passageways extending longitudinally from an inlet end face to an outlet end face, and having a plurality of plugs in the ends of the passageways at the inlet end face and at the outlet end face to prevent direct passage of the exhaust gas through the passageways from the inlet end face to the outlet end face, the monolith having a microporous catalytic membrane coating, and the membrane coating applied to at least the wall surfaces of the passageways open at the inlet end face

16 and of mean pore size smaller than the mean pore size of the porous material; b) collecting the soot as a filter cake on the surface of the catalytic membrane coating; and c) removing the collected soot in the filter cake by in si tu oxidation catalyzed by the catalytic membrane coating.

Claim 12. The method of claim 11 which includes backpulse regeneration to remove noncombustible particulate matter in the filter cake from the inlet end face.

Claim 13. The method of claim 11 in which the monolith material is a porous ceramic material.

Claim 14. The method of claim 11 in which the catalytic membrane coating is comprised of a single microporous layer of an oxidation catalyst.

Claim 15. The method of claim 11 in which the catalytic membrane coating is comprised of a noncatalytic microporous membrane with an overlying microporous oxidation catalyst.

Claim 16. The method of claim 11 in which the soot retention efficiency of the membrane is greater than 95%.

Claim 17. The method of claim 11 in which a second oxidation catalyst is applied within the pores of the device for oxidization of hydrocarbon vapors and carbon monoxide in the filtrate.

Claim 18. The method of claim 11 in which pulsed heating of the filter is used to achieve soot "light off" temperature.

Claim 19. The method of claim 18 which includes pulsed heating by resistive heating of the porous monolith.

Claim 20. The method of claim 19 in which the monolith material is silicon carbide.

Claim 21. The method of claim 19 which includes pulsed heating by introducing hydrogen into the exhaust gas and the hydrogen is catalytically oxidized either before

17 introduction into the filter or by the catalytic membrane itself.

Claim 22. The method of claim 11 which includes recirculating a portion of the gaseous filtrate to the inlet of the combustion source to reduce formation of NOx.

Claim 23. The method of claim 11 in which the combustion source is a diesel engine.

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