Patent Description:
Particulate filters, for example, diesel particulate filters (DPFs), filter particulates from the exhaust stream from engines such as engines burning diesel fuel. Many exhaust treatment systems use a selective catalytic reduction (SCR) component which utilize injection of a reducing agent such as ammonia or urea.

<CIT> discloses an exhaust treatment apparatus comprising the features of the preamble of claim <NUM>.

As used herein, an SCR component may comprise an SCR/ASC (ammonia slip catalyst or ammonia oxidation catalyst) component where the function of ASC is ammonia oxidation, and in some embodiments where the particulate filter is downstream of the most downstream situated of the upstream components, it could be uncatalyzed or coated with SCR functionality, ammonia oxidation functionality, or both.

The invention provides an exhaust treatment apparatus according to claim <NUM> for treating an exhaust stream flowing through an exhaust line housing in a downstream direction from an upstream location of the exhaust line housing to a downstream location, the exhaust treatment apparatus comprising: a first particulate filter; an SCR unit disposed downstream of the first particulate filter; and a second particulate filter disposed downstream of the SCR unit; wherein the first particulate filter, the SCR unit, and the second particulate filter are disposed serially in the exhaust line housing and are configured to allow the exhaust stream to flow serially through the first particulate filter, subsequently through the SCR unit, and subsequently through the second particulate filter. The SCR unit comprises a reducing agent doser configured to inject a reducing agent into the exhaust gas in the exhaust line housing downstream of the first particulate filter. The SCR unit further comprises a selective catalytic reduction catalyst.

The first particulate filter may be comprised of a first honeycomb body comprised of intersecting porous ceramic walls comprising a first bulk average porosity as measured by mercury porosimetry, and the second particulate filter is comprised of a second honeycomb body comprised of intersecting porous ceramic walls comprising a second bulk average porosity as measured by mercury porosimetry, wherein the second honeycomb body comprises a second bulk median pore size which is less than a first bulk median pore size of the first honeycomb body.

The first particulate filter is comprised of a first honeycomb body comprised of intersecting porous ceramic walls comprising a first bulk average porosity of <NUM>-<NUM>% as measured by mercury porosimetry, and the second particulate filter is comprised of a second honeycomb body comprised of intersecting porous ceramic walls comprising a second bulk average porosity of <NUM>-<NUM>% as measured by mercury porosimetry.

The first bulk average porosity micrometers. The second honeycomb body may comprise a second bulk median pore size which is less than a first bulk median pore size of the first honeycomb body.

In embodiments, the first particulate filter is comprised of a first honeycomb body comprised of intersecting porous ceramic walls comprising a first bulk median pore size of <NUM>-<NUM> micrometers as measured by mercury porosimetry, and the second particulate filter is comprised of a second honeycomb body comprised of intersecting porous ceramic walls comprising a second bulk median pore size of <NUM>-<NUM> micrometers as measured by mercury porosimetry, and wherein the second bulk median pore size is less than the first bulk median pore size.

In embodiments, the porous ceramic walls comprise a bulk median pore size of <NUM> to <NUM> as measured by mercury porosimetry. In embodiments, the porous ceramic walls comprise a bulk median pore size of <NUM> to <NUM> as measured by mercury porosimetry.

In embodiments, the first particulate filter is comprised of a first honeycomb body comprised of intersecting porous ceramic walls defining axial channels, wherein the first honeycomb body further comprises plugs selectively disposed in at least some of the axial channels to further define inlet channels and outlet channels and to provide a plurality of gas flow paths through selected porous ceramic walls. In embodiments, at least some of the inlet channels have a cross-sectional channel open area greater than a cross-sectional channel open area of at least some of the outlet channels. In embodiments, the second may be <NUM>-<NUM>%. The first bulk average porosity may be <NUM>-<NUM>%. The second bulk average porosity may be <NUM>-<NUM>%. The second honeycomb body may comprise a second bulk median pore size of <NUM>-<NUM> micrometers. The second honeycomb body may comprise a second bulk median pore size of <NUM>-<NUM>
particulate filter is comprised of a second honeycomb body comprised of intersecting porous ceramic walls defining axial channels, wherein the second honeycomb body further comprises plugs selectively disposed in at least some of the axial channels to further define inlet channels and outlet channels and to provide a plurality of gas flow paths through selected porous ceramic walls. In embodiments, a majority of the inlet channels and a majority of the outlet channels in the second particulate filter have essentially the same cross-sectional channel open area.

In another aspect, not forming part of the scope of the claims, a method of treating an exhaust stream is disclosed herein, the method comprising: flowing the exhaust stream through a first particulate filter, then, after flowing through the first particulate filter, flowing the exhaust stream through an SCR unit, then after being exposed to the Selective catalytic reduction catalyst, flowing the exhaust stream through a second particulate filter.

In examples, in the SCR unit, the exhaust stream is mixed with a reducing agent, and the mixture of the exhaust stream with the reducing agent is flowed through a selective catalytic reduction catalyst. In embodiments, after being exposed to the Selective catalytic reduction catalyst, the exhaust stream is flowed through a second particulate filter. In embodiments, prior to entering the first particulate filter, the exhaust stream is exposed to an oxidation catalyst.

In examples, the oxidation catalyst is a diesel oxidation catalyst. In embodiments, the exhaust stream entering the first particulate filter is comprised of exhaust gas and particles. In embodiments, the exhaust stream entering the first particulate filter comprises soot particles. In embodiments, the exhaust stream entering the second particulate filter comprises SCR unit - generated particles.

In examples, at least some of the particles from the exhaust stream are removed by the first particulate filter. In embodiments, at least some of the soot particles from the exhaust stream are removed by the first particulate filter. In embodiments, greater than <NUM>% of the soot particles entering the first particulate filter are removed by the first
particulate filter. In embodiments, at least some of the soot particles entering the second particulate filter are removed the second particulate filter. In embodiments, the flowing through the second particulate filter removes at least some of the particles from the exhaust stream. In embodiments, the exhaust stream entering the second particulate filter comprises SCR unit -generated particles. In embodiments, the flowing through the second particulate filter removes at least some of the SCR unit - generated particles entering the second particulate filter.

In examples, the flowing through the second particulate filter removes at least some of the SCR unit - generated particles entering the second particulate filter. In embodiments, the exposing of the exhaust stream with the reducing agent to the selective catalytic reduction catalyst adds SCR-generated particles to the exhaust stream. In embodiments, the SCR-generated particles comprise SCR reaction byproduct particles. In embodiments, the second particulate filter is configured to remove at least some of the SCR reaction byproduct particles.

In another aspect, not forming part of the scope of the claims, a method is disclosed herein of treating exhaust stream comprising exhaust gas and particles, the method comprising: flowing the exhaust stream through a first particulate filter configured to remove at least some of the particles from the exhaust stream, then flowing the exhaust stream through an SCR unit wherein a reducing agent is introduced into the exhaust gas stream, then flowing the exhaust stream through a second particulate filter configured to remove at least some of the particles from the exhaust stream.

In another aspect, not forming part of the scope of the claims, a method is disclosed herein of treating an exhaust stream comprising exhaust gas and particles, the method comprising: flowing the exhaust stream through a first particulate filter configured to remove at least some of the particles from the exhaust stream, then flowing the exhaust stream through an SCR unit wherein a reducing agent is introduced into the exhaust gas stream to induce a selective catalytic reaction within the exhaust stream, then flowing the exhaust stream through a second particulate filter configured to remove at least some of the particles from the exhaust stream.

In examples, the selective catalytic reaction adds SCR-generated particles to the exhaust stream, and the second particulate filter is configured to remove at least some of the SCR-generated particles. In embodiments, the particles entering the first particulate filter are primarily soot particles. In embodiments, the selective catalytic reaction adds SCR-generated particles to the exhaust stream, and the second particulate filter is configured to remove at least some of the SCR-generated particles. In embodiments, the SCR-generated particles comprise NH3-based particles.

In examples, the method further comprises regenerating the first particulate filter while flowing the exhaust stream through the second particulate filter. In embodiments, the first particulate filter has an internal temperature of greater than <NUM> during the regenerating.

Before describing several exemplary embodiments of the disclosure, it is to be understood that the disclosure is not limited to the details of construction or process steps set forth in the following description. The disclosure is capable of other embodiments and of being practiced or being carried out in various ways.

For exhaust treatment systems that use a selective catalytic reduction (SCR) component which utilize injection of a reducing agent such as ammonia or urea, under non-ideal conditions there is potential that a reducing agent or byproducts thereof or byproducts of the SCR reaction may produce additional particles ("SCR particles") that are added to the exhaust stream and contribute tailpipe out particulate emissions. An exhaust system may comprise an exhaust line comprising a DOC+DPF+SCR architecture with an injector and mixer system upstream of the SCR which delivers a reducing agent like ammonia or urea or mixtures containing ammonia or urea, like Diesel Exhaust Fluid ("DEF") or "AdBlue™" which is an aqueous solution of <NUM>% urea and <NUM>% DI water.

In one aspect, an exhaust treatment apparatus is disclosed herein for treating an exhaust stream flowing through an exhaust line housing in a downstream direction from an upstream location of the exhaust line housing to a downstream location, the exhaust treatment apparatus comprising: a first particulate filter; an SCR unit disposed downstream of the first particulate filter; and a second particulate filter disposed downstream of the SCR unit; wherein the first particulate filter, the SCR unit, and the second particulate filter are disposed serially in the exhaust line housing and are configured to allow the exhaust stream to flow serially through the first particulate filter, subsequently through the SCR unit, and subsequently through the second particulate filter. The SCR unit may comprise a reducing agent doser configured to inject a reducing agent into the exhaust gas in the exhaust line housing downstream of the first particulate filter, and the SCR unit may further comprise a selective catalytic reduction catalyst.

In some embodiments, the first particulate filter is comprised of a first honeycomb body comprised of intersecting porous ceramic walls comprising a first bulk average porosity as measured by mercury porosimetry, and the second particulate filter is comprised of a second honeycomb body comprised of intersecting porous ceramic walls comprising a second bulk average porosity as measured by mercury porosimetry, wherein the second honeycomb body comprises a second bulk median pore size which is less than a first bulk median pore size of the first honeycomb body.

In some embodiments, the first particulate filter is comprised of a first honeycomb body comprised of intersecting porous ceramic walls comprising a first bulk average porosity of <NUM>-<NUM>% as measured by mercury porosimetry, and the second particulate filter is comprised of a second honeycomb body comprised of intersecting porous ceramic walls comprising a second bulk average porosity of <NUM>-<NUM>% as measured by mercury porosimetry.

In some of these embodiments, the first bulk average porosity is <NUM>-<NUM>%.

In some of these embodiments, the first bulk average porosity is <NUM>-<NUM>%. In some of these embodiments, the second bulk average porosity is <NUM>-<NUM>%.

In some embodiments, the second honeycomb body comprises a second bulk median pore size of <NUM>-<NUM> micrometers; in some of these embodiments the second honeycomb body comprises a second bulk median pore size of <NUM>-<NUM> micrometers.

In some embodiments, the second honeycomb body comprises a second bulk median pore size which is less than a first bulk median pore size of the first honeycomb body.

In some embodiments, the first particulate filter is comprised of a first honeycomb body comprised of intersecting porous ceramic walls comprising a first bulk median pore size of <NUM>-<NUM> micrometers as measured by mercury porosimetry, and the second particulate filter is comprised of a second honeycomb body comprised of intersecting porous ceramic walls comprising a second bulk median pore size of <NUM>-<NUM> micrometers as measured by mercury porosimetry, and wherein the second bulk median pore size is less than the first bulk median pore size; ; in some of these embodiments the second honeycomb body comprises a second bulk median pore size of <NUM>-<NUM> micrometers.

In some embodiments, the first particulate filter is comprised of a honeycomb body comprised of intersecting porous ceramic walls comprising a bulk average porosity of <NUM>-<NUM>% as measured by mercury porosimetry.

In some of these embodiments, the porous ceramic walls comprise a bulk average porosity of <NUM>-<NUM>% as measured by mercury porosimetry.

In some of these embodiments, the porous ceramic walls comprise a bulk median pore size of <NUM> to <NUM> as measured by mercury porosimetry.

In some embodiments, the first particulate filter is comprised of a first honeycomb body comprised of intersecting porous ceramic walls comprising a first bulk average porosity as measured by mercury porosimetry, and the second particulate filter is comprised of a second honeycomb body comprised of intersecting porous ceramic walls comprising a second bulk average porosity as measured by mercury porosimetry, and the second bulk average porosity is greater than the first bulk average porosity.

In some embodiments, the first particulate filter is comprised of a first honeycomb body comprised of intersecting porous ceramic walls defining axial channels, wherein the first honeycomb body further comprises plugs selectively disposed in at least some of the axial channels to further define inlet channels and outlet channels and to provide a plurality of gas flow paths through selected porous ceramic walls.

In some of these embodiments, at least some of the inlet channels have a cross-sectional channel open area greater than a cross-sectional channel open area of at least some of the outlet channels.

In some of these embodiments, the second particulate filter is comprised of a second honeycomb body comprised of intersecting porous ceramic walls defining axial channels, wherein the second honeycomb body further comprises plugs selectively disposed in at least some of the axial channels to further define inlet channels and outlet channels and to provide a plurality of gas flow paths through selected porous ceramic walls; in some of these embodiments, a majority of the inlet channels and a majority of the outlet channels in the second particulate filter have essentially the same cross-sectional channel open area.

In another aspect, a method is disclosed herein of treating an exhaust stream, the method comprising: flowing the exhaust stream through a first particulate filter, then, after flowing through the first particulate filter, flowing the exhaust stream through an SCR unit, then after being exposed to the SCR unit, flowing the exhaust stream through a second particulate filter.

In some embodiments, in the SCR unit, the exhaust stream is mixed with a reducing agent, and the mixture of the exhaust stream with the reducing agent is flowed through a selective catalytic reduction catalyst.

In some embodiments, after being exposed to the Selective catalytic reduction catalyst, flowing the exhaust stream through a second particulate filter.

In some embodiments, prior to entering the first particulate filter, the exhaust stream is exposed to an oxidation catalyst; in some of these embodiments, the oxidation catalyst is a diesel oxidation catalyst.

In some embodiments, the exhaust stream entering the first particulate filter is comprised of exhaust gas and particles.

In some of these embodiments, the exhaust stream entering the first particulate filter comprises soot particles.

In some of these embodiments, the exhaust stream entering the second particulate filter comprises SCR unit -generated particles.

In some of these embodiments, at least some of the particles from the exhaust stream are removed by the first particulate filter.

In some of these embodiments, at least some of the soot particles from the exhaust stream are removed by the first particulate filter; in some of these embodiments, greater than <NUM>% of the soot particles entering the first particulate filter are removed by the first particulate filter; in some of these embodiments, at least some of the soot particles entering the second particulate filter are removed by the second particulate filter.

In some of these embodiments, the flowing through the second particulate filter removes at least some of the particles from the exhaust stream.

In some of these embodiments, the exhaust stream entering the second particulate filter comprises SCR unit -generated particles; in some of these embodiments, the flowing through the second particulate filter removes at least some of the SCR unit - generated particles entering the second particulate filter, in some of these embodiments, the flowing through the second particulate filter removes at least some of the SCR unit - generated particles entering the second particulate filter.

In some embodiments, the exposing of the exhaust stream with the reducing agent to the selective catalytic reduction catalyst adds SCR-generated particles to the exhaust stream; in some of these embodiments, the SCR-generated particles comprise SCR reaction byproduct particles; in some of these embodiments, the second particulate filter is configured to remove at least some of the SCR reaction byproduct particles.

In another aspect, a method is disclosed herein of treating exhaust stream comprising exhaust gas and particles, the method comprising: flowing the exhaust stream through a first particulate filter configured to remove at least some of the particles from the exhaust stream, then flowing the exhaust stream through an SCR unit wherein a reducing agent is introduced into the exhaust gas stream, then flowing the exhaust stream through a second particulate filter configured to remove at least some of the particles from the exhaust stream.

In another aspect, a method is disclosed herein of treating an exhaust stream comprising exhaust gas and particles, the method comprising: flowing the exhaust stream through a first particulate filter configured to remove at least some of the particles from the exhaust stream, then flowing the exhaust stream through an SCR unit wherein a reducing agent is introduced into the exhaust gas stream to induce a selective catalytic reaction within the exhaust stream, then flowing the exhaust stream through a second particulate filter configured to remove at least some of the particles from the exhaust stream.

In some embodiments, the selective catalytic reaction adds SCR-generated particles to the exhaust stream, and the second particulate filter is configured to remove at least some of the SCR-generated particles.

In some embodiments, the particles entering the first particulate filter are primarily soot particles.

In some embodiments, the selective catalytic reaction adds SCR-generated particles to the exhaust stream, and the second particulate filter is configured to remove at least some of the SCR-generated particles; in some of these embodiments, the SCR-generated particles comprise NH3-based particles.

In some embodiments, the method further comprises generating the first particulate filter while flowing the exhaust stream through the second particulate filter.

In some embodiments, the first particulate filter has an internal temperature of greater than <NUM> during the regeneration.

In some embodiments, the method further comprises not regenerating the second particulate filter before the first particulate filter is regenerated.

In some embodiments, the method further comprises not regenerating the second particulate filter before the first particulate filter is replaced, or subjected to ash cleaning.

In some embodiments, the reducing agent injection portion is coupled to an SCR unit; and some of these embodiments the reducing agent doser is disposed upstream of an SCR unit; in other of these embodiments the reducing agent doser is incorporated into the SCR unit.

In some embodiments the reducing agent comprises ammonia, urea, or a combination thereof, or a mixture of ammonia or urea with another fluid, such as deionized (DI) water.

In some environments, the exhaust apparatus further comprises a diesel oxidation catalyst (DOC) unit disposed upstream of the particulate filter in the exhaust line.

In some embodiments, the porous material of the honeycomb bodies is comprised of one or more selected from the group of cordierite, aluminum titanate, magnesium titanate, silica carbide, mullite, alumina, spinel, and combinations thereof.

In some embodiments, the exhaust treatment apparatus further comprises one or more catalytic exhaust components disposed within the exhaust line housing.

In some of these embodiments, one or more of the catalytic exhaust components is selected from the group consisting of a DOC component, an SCR component, and an LNT component.

In some embodiments, the exhaust treatment apparatus further comprises a reducing agent injector coupled to the reducing agent injector junction.

In some embodiments the exhaust treatment apparatus further comprises a reducing agent doser.

In some embodiments, the matrix of intersecting walls of the filter body comprises cells present in a pattern of <NUM> to <NUM> cells per square inch.

In some embodiments, the matrix of intersecting walls of the filter body comprises cells present in a pattern of substantially similarly shaped cells.

In some embodiments, the matrix of intersecting walls of the filter body comprises cells present in a pattern of substantially similarly sized cells.

In some embodiments, the outlet channels of the filter body are larger in area than the inlet channels of the filter body.

In some embodiments, at least one of the particulate filters further comprises catalyst material disposed on, in, or both on and in at least a portion of the intersecting walls of the honeycomb body.

<FIG> schematically illustrates an apparatus or a subassembly or an exhaust system comprising an exhaust line comprising a first particulate filter + SCR unit + second particulate filter architecture, such as a DPF + SCR unit + DPF architecture, wherein the SCR unit comprises an injector disposed upstream of a substrate provided with a Selective Catalytic Reduction catalyst material (SCR), with an optional duct disposed in the exhaust line between the SCR and the second particulate filter.

<FIG> schematically illustrates an apparatus or a subassembly or an exhaust system comprising an exhaust line comprising an oxidation catalyst + a first particulate filter + SCR unit + second particulate filter architecture, such as a DOC + DPF + SCR unit + DPF architecture, wherein the DOC is a Diesel Oxidation Catalyst, the SCR unit comprises an injector disposed upstream of a substrate provided with a Selective Catalytic Reduction catalyst material (SCR), with an optional duct disposed in the exhaust line between the SCR and the second particulate filter.

<FIG> lists various exhaust treatment apparatuses that were tested with a heavy duty diesel engine.

<FIG> schematically presents Euro VI based System Out Particulate Number Measurements performed on a Heavy Duty Diesel Engine for five World Harmonized Transient Cycle (WHTC) tests using a <NUM> solid particle counting system (SPCS) for various apparatus of <FIG>.

<FIG> schematically presents System Out Particulate Number Measurements performed on a Heavy Duty Diesel Engine for five World Harmonized Transient Cycle (WHTC) tests using a <NUM> solid particle counting system (SPCS) for various apparatus of <FIG>.

In some embodiments, the second particulate filter is coated, or supports, an ammonia oxidation catalyst.

In some embodiments, the SCR includes an ammonia oxidation catalyst at the downstream end; in other embodiments, the SCR does not include an ammonia oxidation catalyst at the downstream end.

In various other embodiments disclosed herein, the particulate filters are diesel particulate filters (DPFs) in configurations that include: DOC+SCR+DOC+DPF+SCR+DPF; SCR+DOC+DPF+SCR+DPF; SCR+DOC integrated DPF+SCR+DPF; DOC+SCR+DOC integrated DPF +SCR+DPF; DOC integrated DPF+SCR+DPF.

<FIG> schematically illustrates an embodiment of an exhaust line comprised of a DOC, a mixer with reducing agent injection, an SCR, a duct, a second DOC, a DPF, a second mixer with reducing agent injection, a second SCR, a duct, and a second DPF, connected in fluid communication such that exhaust gas flow can flow into the exhaust line and travel in a downstream direction serially through said exhaust line components.

<FIG> schematically illustrates an embodiment of an exhaust line comprised of a mixer with reducing agent injection, an SCR, a duct, a DOC, a DPF, a second mixer with reducing agent injection, a second SCR, a duct, and a second DPF, connected in fluid communication such that exhaust gas flow can flow into the exhaust line and travel in a downstream direction serially through said exhaust line components.

<FIG> schematically illustrates an embodiment of an exhaust line comprised of a mixer with reducing agent injection, an SCR, a duct, a DOC integrated DPF, a second mixer with reducing agent injection, a second SCR, a duct, and a second DPF, connected in fluid communication such that exhaust gas flow can flow into the exhaust line and travel in a downstream direction serially through said exhaust line components.

<FIG> schematically illustrates an embodiment of an exhaust line comprised of a DOC, a mixer with reducing agent injection, an SCR, a duct, a DOC integrated DPF, a second mixer with reducing agent injection, a second SCR, a duct, and a second DPF, connected in fluid communication such that exhaust gas flow can flow into the exhaust line and travel in a downstream direction serially through said exhaust line components.

<FIG> schematically illustrates an embodiment of an exhaust line comprised of a DOC integrated DPF, a mixer with reducing agent injection, an SCR, a duct, and a second DPF, connected in fluid communication such that exhaust gas flow can flow into the exhaust line and travel in a downstream direction serially through said exhaust line components.

<FIG> schematically illustrates an embodiment of an exhaust line comprised of a DOC, a DPF, a mixer with reducing agent injection, an SCR integrated DPF, a duct, and an SCR, connected in fluid communication such that exhaust gas flow can flow into the exhaust line and travel in a downstream direction serially through said exhaust line components.

<FIG> schematically illustrates an embodiment of an exhaust line comprised of a mixer with reducing agent injection, an SCR, a duct, a DOC, a DPF, a second mixer with reducing agent injection, an SCR integrated DPF, a duct, and a second SCR, connected in fluid communication such that exhaust gas flow can flow into the exhaust line and travel in a downstream direction serially through said exhaust line components.

<FIG> schematically illustrates an embodiment of an exhaust line comprised of a mixer with reducing agent injection, an SCR, a duct, a DOC integrated DPF, a second mixer with reducing agent injection, an SCR integrated DPF, a duct, and a second SCR, connected in fluid communication such that exhaust gas flow can flow into the exhaust line and travel in a downstream direction serially through said exhaust line components.

<FIG> schematically illustrates an embodiment of an exhaust line comprised of a DOC integrated DPF, a mixer with reducing agent injection, an SCR integrated DPF, a duct, and an SCR, connected in fluid communication such that exhaust gas flow can flow into the exhaust line and travel in a downstream direction serially through said exhaust line components.

As seen in <FIG>, the SCR integrated filter is not the most downstream component, but the SCR integrated filter is downstream of the most downstream urea injector/mixer component.

<FIG> schematically illustrates an embodiment of an exhaust line comprised of a DOC, a mixer with reducing agent injection, an SCR, a duct, a second DOC, a DPF, a second mixer with reducing agent injection, a second SCR, a third SCR, an ASC, a duct, and a second DPF, connected in fluid communication such that exhaust gas flow can flow into the exhaust line and travel in a downstream direction serially through said exhaust line components. The first DOC and the first SCR are referred to as close-couple or close-coupled catalysts as they are configurable to be in a close-coupled position with respect to a vehicle engine near the inlet of the exhaust line. In <FIG>, the "second DPF", or the most downstream DPF, is downstream of all of the other upstream components, and can serve as a last trap for particulate matter before the exhaust gas exits the exhaust line.

Claim 1:
An exhaust treatment apparatus for treating an exhaust stream flowing through an exhaust line housing in a downstream direction from an upstream location of the exhaust line housing to a downstream location, the exhaust treatment apparatus comprising:
a first particulate filter;
an SCR unit (SCR) disposed downstream of the first particulate filter; and
a second particulate filter disposed downstream of the SCR unit (SCR);
wherein the first particulate filter, the SCR unit (SCR), and the second particulate filter are disposed serially in the exhaust line housing and are configured to allow the exhaust stream to flow serially through the first particulate filter, subsequently through the SCR unit (SCR), and subsequently through the second particulate filter, characterized in that the SCR unit (SCR) comprises a reducing agent doser configured to inject a reducing agent into the exhaust gas in the exhaust line housing downstream of the first particulate filter, wherein the SCR unit (SCR) further comprises a selective catalytic reduction catalyst, wherein the first particulate filter is comprised of a first honeycomb body comprised of intersecting porous ceramic walls comprising a first bulk average porosity of <NUM>-<NUM>% as measured by mercury porosimetry, and the second particulate filter is comprised of a second honeycomb body comprised of intersecting porous ceramic walls comprising a second bulk average porosity of <NUM>-<NUM>% as measured by mercury porosimetry.