WO1999010632A1

WO1999010632A1 - Diesel exhaust aftertreatment system and process

Info

Publication number: WO1999010632A1
Application number: PCT/US1998/017583
Authority: WO
Inventors: Robert L. Goldsmith
Original assignee: Ceramem Corporation
Priority date: 1997-08-29
Filing date: 1998-08-25
Publication date: 1999-03-04
Also published as: WO1999010633A1

Abstract

A diesel-engine (12), exhaust-gas (32), aftertreatment system and process (10). The exhaust gas is filtered by a ceramic, oxidation-catalyst, monolith filter (24) to remove particulates and soot, and the filtered exhaust gas oxidized (24) to oxidize the noncondensed hydrocarbons. Periodically, the filter is backpulsed with a compressed fluid (14), like compressed air, to remove accumulated particulates and soot from the filter (24).

Description

DIESEL EXHAUST AFTERTREATMENT SYSTEM AND PROCESS

Background of the Invention Particulate filters for the removal of soot from diesel-exhaust sources are well-known and include ceramic monolith filters with passageways and having microporous membranes, to prevent the entrance of soot into the pores of the passageways of the filter and to permit periodic backflushing of the filter. Such filters are described in U.S. Patent 5,114,581 , issued May 19, 1992, hereby incorporated by reference. Catalytic monolith particulate filters are also well-known and also contain a catalyst impregnated or coated onto the passageways, to catalyze a reaction of the filtrate as it passes through the pores of the filter. Where a gas is the feed stock for the filter, the catalyst may be selected to reduce, by catalytic reaction, the oxides of nitrogen in the gaseous filtrate. Such catalytic filters are described, for example, in U.S. Patent 5,221 ,484, issued June 22, 1993, hereby incorporated by reference.

It is desirable to provide a compact and efficient exhaust-gas aftertreatment system and process. Summary of the Invention

The invention relates to an exhaust gas, particularly a diesel gas, aftertreatment system and process to remove particulates, to oxidize gaseous components and to suppress oxides of nitrogen during the exhaust-gas combustion and during exhaust-gas recirculation (ERG). The invention comprises a compact exhaust aftertreatment system which comprises a source of exhaust gas, such as a diesel engine; an exhaust-gas particulate filter means, preferably one or a plurality of ceramic-membrane-coated monolith filters, to remove soot and other particulates from the exhaust gas and to provide a substantially particulate-free exhaust gas for recycle to the engine; backpulsing means to backpulse periodically with a fluid, particularly compressed air, to remove collected soot and other particulates from said filter means; means, such as a bag filter, to collect the soot and particulate matter removed by the backpulsing of said filter; a source of backpulse fluid, preferably compressed air, for the backpulsing means; an oxidation catalyst means applied to the filter means or downstream of the filter means, or preferably both, to oxidize the noncondensed hydrocarbons_.^ and the carbon monoxide in the particulate-free exhaust gas, to provide a filtered, clean exhaust gas; and means to recycle the oxidized, filtered exhaust gas to said engine source, preferably the inlet of said source, to provide an exhaust gas with suppressed NOx formation and to reduce carbon-monoxide emissions.

The invention includes a process for the aftertreatment of exhaust gas from a diesel engine, which process comprises: providing a diesel- exhaust gas for a diesel engine; filtering soot and particulate material from a diesel-exhaust gas employing a filter, such as a porous, ceramic- membrane, oxidation- catalyst monolith, to provide a substantially particulate-free exhaust gas; backpulsing the filter periodically to remove collected soot and particulate matter from said filter; collecting and discarding the removed soot and particulate material; oxidizing noncondensed hydrocarbons and carbon monoxide in the particulate-free exhaust gas employing an oxidation catalyst downstream of the filter, to provide filtered, clean exhaust gas; and recycling all or part of the filtered, clean exhaust gas to an inlet of the diesel engine, thereby providing a diesel-exhaust gas with suppressed NOx function and reduced carbon monoxide.

The system and process provide an efficient, compact aftertreatment system with diesel-engine emission controls for paticulates

removal with catalytic oxidation of the carbon monoxide and organics and suppression of NOx formation by recycling of the treated exhaust gas. Brief Description of the Drawings Figure 1 is a schematic illustration of the diesel-exhaust aftertreatment system of the invention; and Figure 2 is a graphical representation of test data showing the NOx reduction by the system of Figure 1 as the percentage of NOx reduction in the exhaust versus the percentage of EGR. Description of the Embodiments

Figure 1 illustrates a compact, efficient, diesel-exhaust aftertreatment system 10 for a diesel engine which emits exhaust gas.with soot and particulates and which includes a dirty-gas inlet line 32 into a filter chamber 20. The chamber 20 includes a backpulse system which uses compressed air 14 from a storage tank, with a backpulse line 16 and backpulse valve 18 which extend to a blowpipe with nozzles 22 in the chamber 20. The nozzles include Venturis which discharge into gas filters 24, typically a plurality of ceramic monolith filters 24 with a membrane coating and impregnated with an oxidation catalyst. The chamber 20, optionally and preferably, includes another downstream oxidation catalyst 34 for the filtered exhaust gas, which gas is passed through a cooling heat exchanger 36 and a cooled, filtered, oxidized, clean-gas line 38, with an EGR control valve back to the engine 12 for EGR purposes. The system 10 includes an exhaust-gas bypass line 40 for use during the periodic backpulsing of the filters 24 with compressed air 14 through line 16 and valve 18.

Figure 2 represents in graphical form data obtained in tests with a 65 kW Caterpillar (CAT) diesel generator in the system 10 with ceramic monolith oxidation-catalyst filters 24, as described in U.S. Patent 5,221 ,484 and U.S. Patent 5,114,581 (with a membrane coating for backpulsing collected soot).

The system and process comprise three basic elements in combination: gas particulate filters ("filters") to remove soot and other particulates from diesel-engine exhaust gas, the filters regenerable by backpulsing; and a precious-metal (or other composition) catalyst applied to the filters to oxidize noncondensed hydrocarbon vapors and carbon monoxide in the filtered gas as it passes through the filters; or a separate oxidation catalyst located downstream of the filters serving the same purpose; and exhaust-gas recirculation (EGR) of particle-free exhaust gas to the diesel-engine intake for suppression of NOx during combustion.

The preferred filters are ceramic monolith filters, ceramic- membrane-coated, with the number of filters employed selected to match the gas-flow rate. A normal operating filter-face velocity is 4-8 ft/min. Backpulsing typically uses air from a compressed-air reservoir which passes through a backpulse valve, a backpulse blowpipe with backpulse nozzles, with the backpulse jet expanding into backpulse-assist ventu/is. The backpulse air amount is typically <0.1 % to 1% of the filtered exhaust- gas flow, depending on the diesel-exhaust particulate loading. The regeneration blowdown gas can be directed to a separate cold- temperature, disposable bag filter to collect the blowdown particulates. The bag is removed at the required frequency, and this permits simple disposal of the collected particulates. Alternatively, the particulates can be collected in a separate chamber in which they are intermittently oxidized, as described in SAE (Society of Automotive Engineers) Technical Paper No. 930368 (included in SAE Publication "Diesel Exhaust Aftertreatment", SP-943).

The monolith gas filters are impregnated with a precious-metal (or other composition) oxidation catalyst and operated at a temperature at which substantial organics and carbon-monoxide oxidation occurs as the gas passes through the catalyzed filters. As disclosed in U.S. Patent 5,221 ,484, while the use of such catalyzed filters is preferred, an alternative is to use a separate or additional catalytic converter of the same or different oxidation catalyst downstream of the gas filters. Such catalytic converters can be ceramic monolith-based, as are currently and widely used for pollution control for exhaust aftertreatment for automobiles and diesel trucks.

In operation and downstream of the filters and after cooling, the filtered, clean exhaust gas is returned, in an EGR process, to the diesel- engine intake where it is mixed with fresh inlet air. Such an EGR process is disclosed in U.S. Patent 5,426,936 and described in SAE Technical Paper No. 940460 (included in SAE Publication "Diesel Exhaust Aftertreatment" 1994, SP-1020). EGR with filtered, particle-free gas is highly preferred, in that it prevents introduction of particulate soot and sulfates into the diesel engine.

It has been found that EGR, with exhaust gas filtered through the preferred oxidation-catalyst, ceramic monolith filters, significantly suppresses NOx formation, as shown by the data in Figure 2, with NOx reduction efficiency achieved at different engine loads and EGR rates. EGR can be undesirable in that it increases carbon-monoxide generation. However, in the system, the use of an oxidation catalyst specific for carbon-monoxide oxidation reduces carbon-monoxide emissions to levels

5 present without EGR, as set forth in Table 1. The data shown are for measurements made in tests with the 65 kW diesel generator of Figure 2, with the installation of a catalytic converter downstream of the preferred ceramic monolith filters.

In separate tests, the catalyzed filter for volatile organic compound

10 (VOC) oxidation has been successfully tested for VOC oxidation for various industrial sources. Data from tests at an asphalt plant, at two catalytic-filter-space velocities at 22,000 and 40,000/hr. space velocities, show very high VOC removal, close to the detection limit of the GC/FID instrument used for VOC measurements. Comparative results are

15 expected for oxidation of noncondensed hydrocarbons and carbon monoxide in diesel-engine exhaust gas in the system.

The process combines, in a very compact system, a diesel-engine emissions-control process for particulate removal, catalytic oxidation of carbon monoxide and noncondensed organics, and suppression of NOx

20 formation by EGR.

Table 1. Effectiveness Of Catalytic Converter to Reduce Carbon Monoxide Emissions

Carbon Monoxide Concentrations (ppm) at 75% Engine Load

0% EGR 30% EGR 30% EGR After Catalyst

121 535 1 14

Claims

What is claimed is:

Claim 1. An engine-exhaust aftertreatment system which comprises: a) a source of engine-exhaust gas; b) exhaust-gas particulate-filter means to remove soot and other particulates from the engine-exhaust gas; c) a source of compressed fluid; d) backpulsing means to backpulse periodically with the compressed fluid, to remove soot and other particulates from said filter means; e) means to collect the soot and particulate matter removed by the backpulsing means; and f) an oxidation-catalyst means to oxidize the noncondensed hydrocarbons and carbon monoxide in the filtered exhaust gas to provide filtered, clean exhaust gas.

Claim 2. The system of claim 1 in which the engine is a diesel engine.

Claim 3. The system of claim 1 in which the gas-particulate-filter means is comprised of one or more backflushable, porous, ceramic, monolith filters with a plurality of passageways.

Claim 4. The system of claim 3 wherein the filter means contains a membrane coating on the passageways, to prevent the entrance of particulates into the pores of the filter.

Claim 5. The system of claim 1 wherein the oxidation-catalyst means comprises an oxidation catalyst on the passageways of the filter means.

Claim 6. The system of claim 1 in which the oxidation-catalyst means is a separate oxidation catalyst located downstream of the exhaust- gas particulate-filter means.

Claim 7. The system of claim 1 in which the oxidation catalyst is a precious metal selected from the group consisting of platinum, palladium, ruthenium and rhodium and combinations and alloys thereof.

Claim 8. The system of claim 1 which includes means to recycle a portion of the filtered exhaust gas to the inlet of said engine, to suppress NOx formation and to provide a raw exhaust gas with reduced NOx concentration.

Claim 9. The system of claim 1 wherein the source of compressed fluid comprises compressed air.

Claim 10. The system of claim 1 wherein the filter means comprises a ceramic monolith containing an oxidation catalyst therein.

Claim 11. The system of claim 1 which includes: a) a filter-chamber housing;

(i) an outlet for the discharge of particulate-free, filtered, oxidized-treated exhaust gas;

(ii) a plurality of ceramic-monolith exhaust-gas filters to collect particulates from the exhaust gas;

(iii) a plurality of backpulse Venturis connected to the respective ceramic-monolith filters, to backpulse the filters with compressed air as the backpulse source; (iv) a carbon-monoxide and NOx oxidation catalyst downstream of the backpulse Venturis and ceramic filters; and (v) a blowdown-valve discharge opening to discharge periodically the backpulsed, collected particulates from the ceramic filters; and b) a particulate-collecting disposable-bag filter connected to the blowdown valve to collect discharged, backpulsed particulates.

Claim 12. A process for the treatment of exhaust gas from an engine, which process comprises: a) collecting exhaust from an engine source; b) filtering soot and other particulates from the exhaust employing a particulate filter; c) backpulsing the filter periodically with compressed fluid, to remove soot and other particulates from said filter; d) collecting the soot and particulate matter removed by the backpulsing of the filter; e) oxidizing noncondensed hydrocarbons and carbon monoxide in the filtered exhaust gas to provide filtered, clean exhaust gas; and f) discharging the filtered, clean exhaust gas.

Claim 13. The process of claim 12 in which the engine is a diesel engine.

Claim 14. The process of claim 12 in which the gas particulate. filter is comprised of one or more monolith filters.

Claim 15. The process of claim 12 which includes employing a membrane coating on the particulate filter, to prevent particulates from plugging the filter pores.

Claim 16. The process of claim 12 which includes applying the oxidation-catalyst means to the exhaust-gas particulate filter.

Claim 17. The process of claim 12 in which the oxidation-catalyst means is a separate oxidation catalyst located downstream of the exhaust- gas particulate filter.

Claim 18. The process of claim 12 in which the oxidation catalyst is a precious metal selected from the group consisting of platinum, palladium, ruthenium and rhodium and combinations and alloys thereof.

Claim 19. The process of claim 12 which includes recycling a portion of the filtered exhaust gas to the inlet of said engine, to suppress

NOx formation and to provide a raw exhaust gas with reduced NOx concentration.

Claim 20. A diesel-engine-exhaust aftertreatment system which comprises: a) a source of diesel-engine exhaust gas; b) exhaust-gas particulate-filter means comprised of one or more ceramic-membrane-coated monolith filters, to remove soot and other particulates; c) backpulsing means to backpulse periodically with compressed air, to remove soot and other particulates from said filter means; d) a source of compressed air for use in the backpulsing means; e) means of collecting the soot and particulate matter removed by the backpulsing of said filter; f) an oxidation-catalyst means to oxidize the noncondensed hydrocarbons and carbon monoxide in the filtered exhaust fluid, to provide filtered, clean exhaust gas; and g) a means to recycle a portion of the filtered exhaust gas to the inlet of said engine, to suppress NOx formation and to provide a raw exhaust gas with reduced NOx concentration.

Claim 21. The system of claim 20 wherein the exhaust-gas .. particulate filter contains the oxidation catalyst means.

Claim 22. The system of claim 20 in which the oxidation-catalyst means includes a separate oxidation catalyst located downstream of the exhaust-gas particulate filter.