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 oxidize, by catalytic reaction, the volatile organic compounds (VOC) and carbon monoxide in the gaseous filtrate. Sucn 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 comorises 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, 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 filtered exhaust gas to said engine source, preferably the inlet of said source, to provide an exhaust gas with suppressed NOx formation.

The invention includes a process for the aftertreatment of exhaust gas from a diesel engine, which process comprises: providing a diesel exhaust gas from a diesel engine; filtering soot and particulate material from a diesel exhaust gas employing a filter, such as a porous, ceramic- membrane-coated 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 applied to the filter or separately 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.

The system and process provide an efficient, compact aftertreatment system with diesel-engine emission controls for particulates 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 Drawinqs 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, dieset 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 optionally impregnated with an oxidation catalyst. The chamber 20 alternatively can include a 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. Either the entire exhaust gas flow can be cooled or only that portion used for EGR.

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 filters 24, as described in U. S. Patent 5,114,581 (with a membrane coating).

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 venturis.

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 cotd- 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 can be 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 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 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 present without EGR, as set forth in Table 1.

The data shown are for measurements made in tests with the 65 kW diesel generator used for the data in 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 (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 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 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 121 535 114