Dual-filter exhaust gas treatment system

Abstract

The present invention relates to an exhaust-gas treatment system comprising in order from upstream to downstream, a first particulate filter, means for injecting a nitrogenous reductant, and a selective catalytic reduction (SCR) component, wherein the system further comprises a second particulate filter arranged downstream of the means for injecting a nitrogenous reductant.

Description

[0001] DUAL-FILTER EXHAUST GAS TREATMENT SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an exhaust gas treatment system which comprises two particulate filters. A downstream particulate filter is designed to address particulate emissions resulting from the upstream addition of reductants such as ammonia or urea, while maintaining good exhaust treatment performance.

[0004] BACKGROUND OF THE INVENTION

[0005] Exhaust gas generated in lean-burn and diesel engines is generally oxidative. NOx needs to be reduced selectively with a catalyst and a reductant in a process known as selective catalytic reduction (SCR) that converts NOXinto elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant, typically anhydrous ammonia, aqueous ammonia, or urea, is added to an exhaust gas stream prior to the exhaust gas contacting the SCR catalyst. The reductant is absorbed onto the catalyst and the NOXis reduced as the gases pass through or over the SCR catalyst. In order to maximize the conversion of NOx, it is often necessary to add more than a stoichiometric amount of reductant such as ammonia to the gas stream. However, release of the excess ammonia into the atmosphere would be detrimental to the environment. In addition, ammonia is caustic, especially in its aqueous form. Condensation of ammonia and water in regions of the exhaust line downstream can result in a corrosive mixture that can damage the exhaust system. Therefore, ammonia in exhaust gas should be reduced to an acceptable level.

[0006] Particulate emissions are a well-known problem for diesel engine exhaust gas treatment systems. It is known to address these emissions with the use of a filter. Known types of filters include diesel particulate filters (DPF) and catalyzed soot filters (CSF), among others. These capture soot from the exhaust gas and can be regenerated through an increase in temperature to burn off the accumulated soot. The conventional substrate for a DPF or a CSF is a so-called wall-flow filter substrate.

[0007] Emission legislation is getting increasingly tight. Current suggestions for EU7 / VII particulate number (PN) emissions include PN10 (particulates > 10 nm) limits of 6E11 / km (LDVs) or 6E11 / kWh (HDVs). As emissions standards become stricter, it is increasingly important to ensure that the emissions output of an exhaust system can address concerns with particulate emissions as well as species such as NOXand NH3. Accordingly, it is an object of the present invention to provide an exhaust gas treatment system suitable for meeting expected future regulations on particulate emissions, or at least to tackle problems associated therewith in the prior art or provide a commercially viable alternative thereto.

[0008] SUMMARY OF THE INVENTION

[0009] According to a first aspect there is provided an exhaust gas treatment system comprising in order from upstream to downstream, a first particulate filter, means for injecting a nitrogenous reductant, and a selective catalytic reduction (SCR) component, wherein the system further comprises a second particulate filter arranged downstream of the means for injecting a nitrogenous reductant.

[0010] According to a further aspect there is provided a fuel combustion and exhaust gas treatment system comprising an engine and the exhaust-gas treatment system as described herein.

[0011] According to a further aspect there is provided a vehicle comprising the fuel combustion and exhaust gas treatment system as described herein.

[0012] According to a further aspect there is provided a method for the treatment of an exhaust gas, the method comprising passing an exhaust gas through the exhaust gas treatment system as described herein.

[0013] BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, described below, illustrate exemplary embodiments and are not to be considered limiting of the scope of the invention. The figures are not necessarily to scale, and certain features and certain view of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

[0015] FIG. 1 shows a conventional wall-flow filter substrate.

[0016] FIG. 2 shows a first embodiment of a partial wall-flow filter substrate.

[0017] FIG. 3 shows a second embodiment of a partial wall-flow filter substrate.

[0018] FIG. 4 shows a third embodiment of a partial wall-flow filter substrate.

[0019] FIG. 5 shows a fourth embodiment of a partial wall-flow filter substrate.

[0020] FIG. 6 shows a fifth embodiment of a partial wall-flow filter substrate.

[0021] FIG. 7 shows a partial wall-flow filter substrate coated with a SCR catalyst.

[0022] FIG. 8 shows an exemplary configuration of the exhaust gas treatment system described herein. FIG. 9 shows an exemplary configuration of the exhaust gas treatment system described herein.

[0023] FIG. 10 shows an exhaust gas treatment system for performance test.

[0024] DETAILED DESCRIPTION OF THE INVENTION

[0025] The present disclosure will now be described further. In the following passages different aspects / embodiments of the disclosure are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspects / embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous. It is intended that the features disclosed in relation to the product may be combined with those disclosed in relation to the method and vice versa.

[0026] The present invention relates to an exhaust gas treatment system. That is, the system can be used to treat an exhaust gas derived from a combustion process, such as from an internal combustion engine (whether mobile or stationary), a gas turbine for stationary, marine, or locomotive applications, and coal or oil-fired power plants. The system may also be used to treat exhaust gas from industrial processes such as refining, from refinery heaters and boilers, furnaces, the chemical processing industry, coke ovens, municipal waste plants and incinerators, etc. In a particular embodiment, the system is used for treating exhaust gas from a gas turbine or a lean-burn engine. The treatment is performed to remove undesired components of the exhaust gas, such as NOx and particulate matter. Other species such as CO and un-combusted hydrocarbons (HC) may also be treated by components of an exhaust gas treatment system.

[0027] The exhaust gas treatment system as described herein employs a number of well- known exhaust gas treatment components which are all well known to the skilled person. For simplicity the abbreviations are provided here. These include diesel oxidation catalysts (DOC), selective catalytic reduction (SCR) catalyst component, ammonia slip catalysts (ASC), selective catalytic reduction filters (SCRF), ammonia slip catalyst filters (ASCF), diesel particulate filters (DPF), and catalyzed soot filters (CSF). Except as specified herein, the precise nature and formulation of these parts is not critical to putting the invention into practice and the skilled person would be able to identify and employ these components.

[0028] It has recently been observed that the use of a nitrogenous reductant (especially urea / ammonia) injection into an exhaust gas stream can lead to the production of certain aggregated compounds. The polymerized material then takes the form of additional fine particulate matter that can be released to the atmosphere.

[0029] A discussion of these polymerized particulate emissions may be found in SAE 2017- 01-0915. Here it is explained that depending on the urea doser, decomposition reaction tube (DRT) design and operating conditions, incomplete decomposition of injected urea could lead to solid urea deposit formation in the diesel aftertreatment system. The formed deposits could lead to engine back pressure increase and NOx treatment performance deterioration. The formed urea deposits could be further transformed to chemically more stable substances upon exposure to hot exhaust gas, therefore it is critical to understand this transformation process. The results of the author’s experiments indicate that 1) below urea melting temperature (130°C), the formed urea deposits are still primarily urea; 2) at a temperature range of 130- 190°C, urea transforms to a combination of urea, biuret and cyanuric acid; and 3) above biuret melting temperature (190°C), cyanuric acid and ammelide are formed. At greater than 200°C, urea experiences fast chemical transformation via urea decomposition, biuret formation followed by subsequent biuret decomposition, and turns into cyanuric acid in a short time.

[0030] Thus, the polymerisation of urea and ammonia components in the hot exhaust gas can lead to the formation of polymeric particulate matter. This particulate matter is typically very fine, but it is exactly this sort of fine material that is now the subject of ever more stringent regulation. Furthermore, since the formation of the particulate matter can only occur after the reductant is dosed into the exhaust gas treatment system, it is generally formed both after any conventional particulate filter in the system, for example DPF or CSF, and under a lower temperature environment (i.e. , underfloor configurations) where conventional regeneration of such filters would be difficult.

[0031] Although the exact nature of these particulates will vary depending on the nature of the nitrogenous reductant used and the operating conditions, the particulates will be generally referred to as “reductant-derived particulates” hereafter for simplicity.

[0032] Due to the changing exhaust gas flow conditions and changing urea demand depending on the NOXflow, the amount of injected urea solution is constantly changing. Therefore, spray droplet sizes are varying within one injection pulse and over time. It might therefore be a promising approach to optimise the injection conditions either through the re- introduction of air assisted systems, or the application of multiple nozzles with optimised spray droplet sizes.

[0033] The present inventors have now discovered that it is possible to address these concerns through the provision of an exhaust system as described herein. The exhaust gas treatment system has an upstream end for receiving exhaust gas from an engine and this typically comprises a manifold. The exhaust gas treatment system has a downstream end for emitting the treated exhaust gases to the atmosphere. The components of the exhaust gas treatment system can therefore be ordered according to their position from upstream to downstream, with upstream components contacting the exhaust gases sooner than those downstream.

[0034] According to a first aspect there is provided an exhaust gas treatment system comprising in order from upstream to downstream, a first particulate filter, means for injecting a nitrogenous reductant, and a selective catalytic reduction (SCR) component, wherein the system further comprises a second particulate filter arranged downstream of the means for injecting a nitrogenous reductant.

[0035] The exhaust gas treatment system comprises a first particulate filter. The first particulate filter is upstream of the other listed components and is intended to remove the soot components from the exhaust gases. The first particulate filter may itself be downstream of other components and a preferred system comprises a diesel oxidation catalyst (DOC) upstream of the first particulate filter and preferably as the first component of the system. The first particulate filter can be a diesel particulate filter (DPF) or a catalyzed soot filter (CSF). In use these components can be regenerated to address soot build-up.

[0036] The provision of an upstream first particulate filter prevents the build-up of soot on the second particulate filter. This is desirable because it is difficult to handle a large build-up of material in an end-of-system component due to the relatively lower temperatures. It would be more energy intensive to raise the temperature of the second particulate filter as would be required were it to have soot build-up. In contrast, the build-up of reductant-derived particulates is relatively low, such that regeneration can be performed rarely, if necessary.

[0037] In some embodiments, the first particulate filter is a diesel particulate filter (DPF). Diesel particulate filters can be deep-bed filters and / or surface-type filters. In deep-bed filters, the mean pore size of filter walls is bigger than the mean diameter of collected particles. The particles are deposited on the media through a combination of depth filtration mechanisms, including diffusional deposition (Brownian motion), inertial deposition (impaction) and flow-line interception (Brownian motion or inertia).

[0038] In surface-type filters, the pore diameter of the filter walls is less than the diameter of the particulate matter, so particulate matter is separated by sieving. Separation is done by a build-up of collected diesel particulate matter itself, which build-up is commonly referred to as "filtration cake" and the process as "cake filtration".

[0039] It is understood that diesel particulate filters, such as ceramic wall-flow monoliths, may work through a combination of depth and surface filtration: a filtration cake develops at higher soot loads when the depth filtration capacity is saturated and a particulate layer starts covering the filtration surface.

[0040] The most widely used DPFs are wall-flow filters. A conventional wall-flow filter includes a ceramic honeycomb body having longitudinal, generally parallel cell channels formed by a plurality of intersecting porous walls. The cell channels are typically plugged with a ceramic plugging cement to form a checkered pattern of plugs at the end faces of the honeycomb body. The cell channels of the filter typically have some ends unplugged at an inlet end face of the honeycomb body, referred to herein as “inlet channels.” Likewise, typically, the cell channels also have the remaining ends plugged to form a checkered pattern of plugs at an outlet end face of the honeycomb substrate with some ends unplugged, herein referred to as “outlet channels.”

[0041] The monolithic substrates used for a wall flow filter may contain up to about 400 flow passages (or “cells”) per square inch ((2.54 cm)2) of cross section, although far fewer may be used. For example, it may have from 7 to 400, specifically from 100 to 400, cells per square inch (“cpsi”). The cells can have cross sections that are rectangular, square, circular, oval, triangular, hexagonal, or are of other polygonal shapes.

[0042] A conventional porous wall-flow filter 100 as shown in FIG. 1 includes an inlet end 101 , an outlet end 102, and a plurality of generally parallel cell channels (inlet cell channels 111 and outlet cell channels 112) separated by porous cell walls 120. The inlet channels include plugs 130 at the outlet end 102. The outlet channels include plugs at the inlet end 101. The plugs 130 are generally located at the ends of the cell channels and typically have a depth of about 5 to 20 mm.

[0043] Diesel particulate filters (DPF) without catalyst coating generally have limited capacity for trapping particulate matter before the pressure-drop becomes excessive therefore it is necessary periodically to regenerate the diesel particulate filter. Passive regeneration does not readily take place as combustion of the retained particulate matter in the presence of oxygen requires higher temperatures than those typically provided by diesel engine exhaust.

[0044] In some embodiments, the first particulate filter is a catalyzed soot filter (CSF). A catalyzed soot filters (CSF) works by trapping particulates and continuously oxidizing them at normal diesel operating exhaust temperatures.

[0045] A catalyzed soot filter (CSF) generally comprise a platinum group metal and a refractory metal oxide selected from the group consisting of alumina, silica, silica-alumina, alumino-silicates, alumina-zirconia, alumina-chromia, alumina- rare earth metal oxides, titania, titania-silica, titania-zirconia, and titania-alumina. For example, the platinum group metal may be at concentrations of 2 to 150 g / ft3.

[0046] The reactions on the CSF include oxidation of CO and HC and oxidation of NO to NO2 which enables combustion of the particulate matter.

[0047] Downstream of the first particulate filter is a means for injecting a nitrogenous reductant. The means for injecting a nitrogenous reductant is typically a spray-head arranged in the channel for conducting the exhaust gases. The nitrogenous reductant provided by the means for injecting a nitrogenous reductant can be ammonia, hydrazine or an ammonia precursor selected from the group consisting of urea ((NH2)2CO), ammonium carbonate, ammonium carbamate, ammonium hydrogen carbonate and ammonium formate. Ammonia and urea are most preferred alternatives. Preferably the means for injecting a nitrogenous reductant further comprises a reservoir for the nitrogenous reductant.

[0048] Downstream of the means for injecting a nitrogenous reductant is a selective catalytic reduction (SCR) component, which catalyzes the reduction of NOx emissions by the nitrogenous reductant.

[0049] The SCR component generally comprises a flow-through substrate (e.g., a monolithic substrate) and a SCR catalyst. Monolithic substrates are generally made of ceramics or metal. Ceramic substrates (honeycombs) usually have square cells, while most metallic substrates have sinusoidal channels. Other channel cross sections are possible, including triangular, hexagonal, trapezoidal and round. The number of cells can vary between 10 and over 1000 cells per square inch (cpsi), or between 200 and 600 cpsi.

[0050] The SCR component comprises a SCR catalyst. The SCR catalyst generally comprises an oxide of a base metal, a molecular sieve, a metal-exchanged molecular sieve, or a mixture thereof. The base metal can be selected from the group consisting of cerium (Ce), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), tungsten (W), vanadium (V), and mixtures thereof. SCR catalysts consisting of vanadium supported on a refractory metal oxide such as alumina, silica, zirconia, titania, ceria and combinations thereof are well known and widely used commercially in mobile applications. Typical compositions are described in US4,010,238 and US4,085,193, of which the entire contents are incorporated herein by reference.

[0051] The SCR catalyst may comprise vanadium and antimony. Antimony may be present in an amount such that molar ratio of antimony to vanadium is greater than 0.5, or from 0.6 to 0.9. Antimony may be present as Sb20s. By way of example, the SCR catalyst may comprise vanadium in an amount of 2-6 wt%, or 3-5 wt% on a V2O5 basis, and antimony in an amount of 3-8 wt% on an Sb20s basis, relative to the total weight of the SCR catalyst. The form of vanadium and antimony present in the catalyst is not necessarily V2O5 and Sb20s.

[0052] In addition to vanadium and antimony, the SCR catalyst may additionally comprise cerium. The molar ratio of cerium to vanadium is generally greater than 3: 10. The SCR catalyst may comprise cerium in an amount of 1-10 wt% or 2-5 wt% on a CeC>2 basis. The form of Ce present in the catalyst is not necessarily CeC>2. Employing cerium and vanadium in such proportions reduces vanadium volatilisation without reducing the activity of the SCR catalyst. The reduced volatilisation permits a greater loading of vanadium, which is desirable for improving the efficiency of the SCR catalyst. By way of example, the SCR catalyst may comprise 2-6 wt% V2O5, 2-6 wt% CeC>2, and 3-8 wt% Sb20s relative to the total weight of the SCR catalyst. The SCR catalyst can comprise a molecular sieve or a metal-exchanged molecular sieve. As is used herein "molecular sieve" is understood to mean a metastable material containing tiny pores of a precise and uniform size that may be used as an adsorbent for gases or liquids. The molecular sieve can be a zeolitic molecular sieve, a non-zeolitic molecular sieve, or a mixture thereof.

[0053] A zeolitic molecular sieve is a microporous aluminosilicate having any one of the framework structures listed in the Database of Zeolite Structures published by the International Zeolite Association (IZA). The framework structures include, but are not limited to those of the CHA, BEA, FAU, LTA, MFI, AEI, and MOR types. Non-limiting examples of zeolites having these structures include chabazite, faujasite, zeolite Y, ultrastable zeolite Y, beta zeolite, mordenite, silicalite, zeolite X, and ZSM-5. Aluminosilicate zeolites can have a silica-to- alumina molar ratio (SAR, defined as SiO2 / AhO3) from 5 to 100, from 10 to 80, or from about 10 to 30.

[0054] As used herein, the term “non-zeolitic molecular sieve” refers to corner sharing tetrahedral frameworks where at least a portion of the tetrahedral sites are occupied by an element other than silicon or aluminum. Specific non-limiting examples of non-zeolitic molecular sieves include silicoaluminophosphates such as SAPO-34, SAPO-37 and SAPO 44. The silicoaluminophosphates can have framework structures that contain framework elements that are found in zeolites, such as BEA, CHA, FAU, LTA, MFI, MOR and other types described below.

[0055] The SCR catalyst can comprise a small-pore, a medium-pore ora large-pore molecular sieve, or combinations thereof.

[0056] The SCR catalyst can comprise a small-pore molecular sieve selected from the group consisting of aluminosilicate molecular sieves, metal-substituted aluminosilicate molecular sieves, aluminophosphate (AIPO) molecular sieves, metal-substituted aluminophosphate (MeAIPO) molecular sieves, silico-aluminophosphate (SAPO) molecular sieves, and metal substituted silico-aluminophosphate (MeAPSO) molecular sieves, and mixtures thereof. The SCR catalyst can comprise a small-pore molecular sieve selected from the group of Framework Types consisting of AGO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, LTA, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG, and ZON, and mixtures and / or intergrowths thereof. The small pore molecular sieve may be selected from the group of Framework Types consisting of AEI, AFX, CHA, DDR, ERI, ITE, KFI, LTA, LEV, and SFW.

[0057] The SCR catalyst can comprise a medium-pore molecular sieve selected from the group of Framework Types consisting of AEL, AFO, AHT, BOF, BOZ, CGF, CGS, CHI, DAC, EUO, FER, HEU, IMF, ITH, ITR, JRY, JSR, JST, LAU, LOV, MEL, MFI, MFS, MRE, MTT, MVY, MWW, NAB, NAT, NES, OBW, PAR, PCR, PON, PUN, RRO, RSN, SFF, SFG, STF, STI, STT, STW, -SVR, SZR, TER, TON, TUN, UOS, VSV, WEI, and WEN, and mixtures and / or intergrowths thereof. The medium pore molecular sieve may be selected from the group of Framework Types consisting of FER, MFI, and STT.

[0058] The SCR catalyst can comprise a large-pore molecular sieve selected from the group of Framework Types consisting of AFI, AFR, AFS, AFY, ASV, ATO, ATS, BEA, BEC, BOG, BPH, BSV, CAN, CON, CZP, DFO, EMT, EON, EZT, FAU, GME, GON, IFR, ISV, ITG, IWR, IWS, IWV, IWW, JSR, LTF, LTL, MAZ, MEI, MOR, MOZ, MSE, MTW, NPO, OFF, OKO, OSI, RON, RWY, SAF, SAO, SBE, SBS, SBT, SEW, SFE, SFO, SFS, SFV, SOF, SOS, STO, SSF, SSY, USI, UWY, and VET, and mixtures and / or intergrowths thereof. The large pore molecular sieve can be selected from the group of Framework Types consisting of BEA, MOR and OFF.

[0059] A metal exchanged molecular sieve can have at least one metal from one of the groups VB, VI B, VII B, VII IB, IB, or I IB of the periodic table deposited onto extra-framework sites on the external surface or within the channels, cavities, or cages of the molecular sieves. Metals may be in one of several forms, including, but not limited to, zerovalent metal atoms or clusters, isolated cations, mononuclear or polynuclear oxycations, or as extended metal oxides. The metals can be iron, copper, and mixtures or combinations thereof.

[0060] A metal exchanged molecular sieve can contain in the range of from about 0.10 wt% to about 10 wt% of a group VB, VI B, VI IB, VI I IB, IB, or 11 B metal located on extra framework sites on the external surface or within the channels, cavities, or cages of the molecular sieve. The metal exchanged molecular sieve can be a copper (Cu) supported small pore molecular sieve having from 0.1 to 20.0 wt%, 1 wt% to 6 wt%, or from 1.8 wt% to 4.2 wt% copper relative to the total weight of the metal exchanged molecular sieve.

[0061] The metal exchanged molecular sieve can be an iron (Fe) supported small pore molecular sieve having from 0.1 to 20.0 wt%, 1 wt% to 6 wt%, or from 1.8 wt% to 4.2 wt% iron relative to the total weight of the metal exchanged molecular sieve.

[0062] The SCR catalyst is generally applied to the flow-through substrate by a washcoating procedure to form the SCR component. A washcoat slurry that contains the SCR catalyst described herein may contain components such as fillers, binders, stabilizers, rheology modifiers, and other additives. In certain embodiments, the washcoat comprises pore-forming agents such as graphite, cellulose, starch, polyacrylate, and polyethylene, and the like. These additional components do not necessarily catalyse the desired reaction, but instead improve the SCR catalyst’s effectiveness, for example by increasing its operating temperature range, increasing contact surface area of the catalyst, increasing adherence of the catalyst to a substrate and the like. Techniques for applying the washcoats are well known in the art and include the application of a washcoat to the surfaces to be coated. After coating the layers onto the article, they are typically calcined. Calcining is well known in the art and may be carried out in air at temperatures of about 500°C.

[0063] The system further comprises a second particulate filter arranged downstream of the means for injecting a nitrogenous reductant. The second particulate filter may be upstream of the SCR component and used to capture reductant-derived particulates before the ammonia gas passes to the SCR component. In this case the upstream means for injecting a nitrogenous reductant is the cause of the formation of reductant-derived particulate matter. Alternatively, the second particulate filter may be downstream of the SCR component and used to capture reductant-derived particulates after the ammonia gas passes to the SCR component. In this case, the upstream means for injecting a nitrogenous reductant and the SCR component are the cause of both the potentially slipped ammonia as well as the formation of reductant-derived particulates. Since this reductant-derived particulate matter is produced downstream of the first particulate filter there is no other filter body on which it would be retained.

[0064] When the second particulate filter is arranged downstream of the SCR component, preferably it is provided with an SCR coating (making it into an SCRF) or an ammonia slip catalyst (ASC) coating (making it into an ASCF). This can be desirable since it may mean that the total volume of the system can be kept down. For example, another component can be removed from the system (i.e. , no flow-through ASC) or another component can be made smaller. For example, the SCR component may be smaller as the SCR function is spread over the SCR component and the second particulate filter.

[0065] The second particulate filter generally comprises a wall-flow filter substrates, as described herein. When the second particulate filter is provided with an SCR catalyst as described herein, the system may further comprise an ammonia slip catalyst (ASC) downstream of the SCR component. The ASC helps to minimise ammonia slip from the system.

[0066] In some embodiments, the SCR catalyst of the second particulate filter has a coating length of 100% of the filter substrate length (L). The SCR catalyst may have a coating length of from 50 to 90%, from 55 to 85%, or from 60 to 80% of the substrate length (L).

[0067] An ammonia slip catalyst (ASC) generally comprises a flow-through substrate and an ASC catalyst composition. For example, an ammonia slip catalyst may comprise a top catalyst layer comprising a layer of SCR catalyst as described above layered on top of a bottom catalyst layer comprising the ammonia oxidation catalyst layer comprising, e.g., a supported PGM, supported Pt or Pd, or Pt supported on alumina. The top and bottom layers of the ASC are segregated to prevent immediate oxidation of the NH3 which would lead formation of untreated secondary NOx in the exhaust stream. For this reason, the top layer in an ammonia slip catalyst is generally free from noble metals, such as platinum group metals (PGMs). Moreover, the bottom layer containing the PGM-based oxidation catalyst is generally completely covered by the top layer to prevent untreated secondary NOxfrom entering the exhaust stream.

[0068] When the second particulate filter is arranged upstream of the SCR component, it may be provided with a SCR catalyst coating or a hydrolysis catalyst coating.

[0069] In some embodiments, a second particulate filter comprising a SCR catalyst (thus a SCRF), which is arranged upstream of the SCR component.

[0070] In some embodiments, a second particulate filter comprising a hydrolysis catalyst, which is arranged upstream of the SCR component. A hydrolysis coating is a conventional washcoat and will comprise, for example, metal-exchanged zeolite and zirconia. An example of hydrolysis catalysts can be found in W02009118195A1 , which is incorporated herein by reference. The provision of the hydrolysis catalyst coating helps to ensure that the nitrogenous reductant, such as urea, is converted more fully into ammonia for use on the SCR component. The hydrolysis catalyst coating can also help to reduce the reductant-derived particulate buildup captured on the second particulate filter. The provision of the hydrolysis catalyst coating on a filter avoids spray droplets escaping the hydrolysis catalysts.

[0071] The second particulate filter may have a different filtration property from the first particulate filter. That is, the first and second particulate filters are preferably not based on the same substrate and have been selected to be optimised for their intended use. It is desirable that the first particulate filter is optimised for soot capture and for regeneration (active or passive). On the other hand, it is desirable that the second particulate filter, which does not have to deal with soot and which is difficult to regenerate, has a reduced filtration efficiency and, potentially a reduced thermal mass.

[0072] The second particulate filter may have a lower cell density than the first particulate filter. It is conventional for the cell density of a filter to be given in channels per square inch (cpsi) and this reflects how many channels there are extending from an inlet face to an outlet face of the filter component (albeit alternately blocked in a conventional wall-flow filter). All else being equal, the higher cell density reflects a higher filtration efficiency and higher backpressure.

[0073] The second particulate filter may have a higher porosity than the first particulate filter. The higher porosity of the second particulate filter can lead to a lower resistance to the gas flow and a lower associated back-pressure.

[0074] The second particulate filter may have a lower average back pressure than the first particulate filter. A lower back-pressure is desirable towards the end of the exhaust system, particularly since it is intended to capture only the small amount of reductant-derived particulates as compared to the first particulate filter that is designed to capture a larger amount of soot particles. The back pressure of the first filter substrate may be about 10%, or about 20% higher than that of the second filter substrate.

[0075] The second particulate filter may have a lower filtration efficiency than the first particulate filter. This is a reflection of the ability of the first particulate filter to more effectively address the larger amount of soot particles which it encounters.

[0076] Variation of all of these properties is well within the ability of the skilled person, either by selecting different commercially available filter substrates, or by selecting and adjusting the coating methods applied to change the observed properties.

[0077] In some embodiments, the second particulate filter is a partial catalytic wall-flow filter. A partial wall-flow filter is different from a conventional wall-flow filter where alternate channels are blocked at the inlet or outlet end. Instead, a partial wall-flow filter has an inlet end, an outlet end, and a plurality of porous walls forming channels from the inlet end to the outlet end, wherein some of the channels are plugged at one end and some are unplugged flow-through channels. The “partial” indicates that only a part of the flow is through the walls whereas part of the flow passes through the filter without flowing through a wall.

[0078] The partial wall-flow filter substrate is known. See, e.g., US 20110132194A1. A partial wall-flow filter substrate exhibits a combination of plugged channels and unplugged flow- through channels.

[0079] Suitable partial wall-flow filter substrates are described in US 20110132194A1 , of which the entire contents are incorporated herein by reference. FIG. 2 is a first embodiment of a partial wall-flow filter substrate 200. The partial wall-flow filter 200 comprises a plurality of porous walls 220 forming channels 211 , 212, 213, and 214 wherein some of the channels are plugged channels and the remainder are unplugged flow-through channels (213, 214). In this embodiment, the plugged channels (211 , 212) include some channels that are plugged adjacent to an inlet end 201 of filter substrate 200, that is, at or near the inlet end 201. The other channels 211 are plugged adjacent to an outlet end of the filter substrate 200, that is, at or near the outlet end 202. Plugs 230 may be provided at, for example, an end face of some of the channels, while the remaining channels 213, 214 remain open (unplugged). This differs from the conventional wall-flow filter shown in FIG. 1 where all the cell channels are end- plugged (at inlet end or the outlet end).

[0080] A second embodiment of a partial wall-flow filter substrate 300 is shown and described with reference to FIG. 3. In this embodiment, the filter substrate 300 includes a plurality of porous walls 320 to define and form a plurality of channels. The channels include some which are unplugged (unplugged channels 312) and some which are plugged (plugged channels 311). In this embodiment, the plugs 330 are all included on the outlet end 302 of the filter 300. In this embodiment, approximately 50% of the channels are plugged and the remainder comprise flow-through channels.

[0081] A third embodiment of a partial wall-flow filter substrate 400 is shown and described with reference to FIG. 4. In this embodiment, filter substrate 400 includes a plurality of porous walls 420 to define and form a plurality of channels. The channels include some which are unplugged (unplugged channels 415) and some which are plugged (plugged channels 416). In this embodiment, the plugs 430 are all included on the inlet end 402 of filter 400. In this embodiment, approximately 50% of the channels are plugged and the remainder comprise flow-through channels.

[0082] The channels of the partial wall-flow filter substrate may be of different sizes. For example, the hydraulic diameters of the unplugged, flow-through channels may be different from those of the plugged channels. In some embodiments, the hydraulic diameter of the plugged channels is larger than the hydraulic diameter of the unplugged, flow-through channels. In some embodiments, the hydraulic diameter of the plugged channels is smaller than the hydraulic diameter of the unplugged, flow-through channels.

[0083] A fourth embodiment of a partial wall-flow filter substrate 500 is shown and described with reference to FIG. 5. In this embodiment, the filter substrate 500 includes a plurality of porous walls 520 to define and form a plurality of channels. The channels include some which are unplugged (unplugged channels 512) and some which are plugged (plugged channels 511). The plugs are all included on the inlet end 501 of filter substrate 500. Approximately 50% of the channels are plugged and the remainder comprise flow-through channels. In this embodiment, unplugged flow-through channels (512) are bigger in width than the plugged channels (channel 511).

[0084] A fifth embodiment of a partial wall-flow filter substrate 600 is shown and described with reference to FIG. 6. In this embodiment, the filter substrate 600 includes a plurality of porous walls 620 to define and form a plurality of channels. The channels include some which are unplugged (612) and some which are plugged (611). The plugs are all included on the outlet end 602 of filter substrate 600. Approximately 50% of the channels are plugged and the remainder comprise flow-through channels. In this embodiment, unplugged flow-through channels (612) are smaller in width than the plugged channels (611).

[0085] Generally, the partial wall-flow filter substrate may have a porosity of 40 to 75%. Suitable techniques for determining porosity are known in the art and include mercury porosimetry and X-ray tomography.

[0086] FIG. 7 shows a catalytic partial wall-flow filter 700 that comprises a coated with a SCR catalyst. The catalytic partial wall-flow filter 700 includes a plurality of porous walls 720 to form a plurality of channels. The channels include some which are unplugged (unplugged channels 712) and some which are plugged (plugged channels 711). In this embodiment, the plugs 730 are all included on the outlet end 702 of filter 700. In this embodiment, approximately 50% of the channels are plugged and the remainder comprise flow-through channels. A SCR catalyst 750 is coated from the inlet end of the filter 700.

[0087] Methods of applying a catalyst washcoat slurry to filter substrates including partial filter substrates are known. There are many suitable ways to apply the catalyst washcoat slurry to the substrate. For example, coating of the substrate by the washcoat slurry can be performed by immersing the substrate vertically in the slurry such that the desired coating length is achieved. The substrate can be left in the slurry for sufficient time to allow the desired amount of the slurry to move into the substrate. The substrate is removed from the slurry, and excess slurry is removed from the wall-flow filter substrate first by allowing it to drain from the channels of the substrate, then by blowing on the slurry on the substrate with compressed air (against the direction of slurry penetration), and then pulling a vacuum from the direction of slurry penetration.

[0088] Another method for coating the wall-flow filter substrate include the steps of: (a) depositing a pre-determined amount of a washcoat slurry into a containment means at an upper end of the filter substrate using a shower head, wherein the shower head comprises a plurality of apertures arranged to dispense the washcoat slurry onto an upper end face of the filter substrate; and (b) coating the channels having open ends at the upper end of the filter substrate with the pre-determined amount of washcoat slurry from the containment means by applying a vacuum to a lower end of the filter substrate to draw the liquid along the channels having open ends at the upper end of the filter substrate. See, e.g., US20180229228A1 .

[0089] The coated substrates are typically dried at about 110°C and calcined at a higher temperature, e.g., 300 to 500°C.

[0090] The second particulate filter may have a SCR catalyst washcoat loading of between 0.1 g / ins and 5 g / in3, between 0.1 g / in3and 4.5 g / in3, or between 0.5 g / in3and 4 g / in3. The “washcoat loading” refers to the weight of the washcoat (after calcination) per unit volume of the whole monolith filter. The volume of the whole catalytic filter is calculated based on its cross-sectional area and length; it does not take into account the number of channels per square inch.

[0091] In some embodiments, the second particulate filter is an ASC filter (ASCF), which comprises a wall-flow filter substrate and an ASC catalyst composition described above.

[0092] Fig. 8 shows one exhaust-gas treatment system that comprises in order from upstream to downstream, an optional DOC, a first particulate filter which is a CSF, an injector, a second particulate filter comprising a SCR catalyst (thus a SCRF), a SCR component comprising a flow-through substrate and a SCR catalyst, and an optional ASC catalyst (e.g., ASC catalyst on a flow-through substrate). Fig. 9 shows another exhaust-gas treatment system that comprises in order from upstream to downstream, an optional DOC, a first particulate filter which is a CSF, an injector, a SCR component comprising a flow-through substrate and a SCR catalyst, a second particulate filter which is a SCRF or an ASCF, and an optional ASC catalyst (e.g., ASC catalyst on a flow-through substrate).

[0093] According to a further aspect there is provided a fuel combustion and exhaust treatment system comprising an engine and the exhaust gas treatment system as described herein. Preferably the engine is a diesel or lean-burn engine.

[0094] According to a further aspect there is provided a vehicle comprising the fuel combustion and exhaust treatment system described herein, preferably wherein at least the second particulate filter is located in an underfloor location and / or encounters, in normal use, exhaust gases at a temperature of no more than 450°C. The provision of the second particulate filter in an underfloor location occurs because it is preferably at the end of the exhaust system. This gives rises to challenges where the temperature facilitates the formation of the reductant-derived particulate matter but not its destruction, and hence the second particulate filter is a solution to meeting emissions standards.

[0095] According to a further aspect there is provided a method for the treatment of an exhaust gas, the method comprising passing an exhaust gas through the exhaust gas treatment system as described herein.

[0096] EXAMPLE 1 : FILTER A

[0097] A washcoat slurry was prepared by mixing vanadyl oxalate, antimony triacetate, cerium carbonate, a high surface area titania powder, and an aqueous dispersion of colloidal silica. Specifically, the high surface area titania powder employed was DT-51d obtained from Tronox® and the aqueous dispersion of colloidal silica employed was Ludox® AS-40 from Grace. The aqueous washcoat slurry had a pH of 5-8.

[0098] An asymmetric cylindrical cordierite partial wall-flow filter substrate (300 / 12, length = 3.0 inch, diameter = 9.5 inch, porosity=65%, mean pore diameter=16.2 pm) comprises 50% of channels having a square cross-section with dimension of 0.058 inch x 0.058 inch, and 50% having a square cross-section with dimension of 0.044 inch x 0.044 inch. All the bigger channels of the filter substrate are plugged at the outlet end. None of the smaller channels are plugged.

[0099] The filter substrate was coated by the coating method disclosed in US20180229228A1 from the inlet with the above washcoat slurry to a length of about 60% of the substrate length. The coated substrate was dried at 110°C for approximately 15 minutes, and subsequently calcined at 500°C for approximately 10 minutes. The calcined partial filter (FILTER A) contains 92 g / ft3Vanadium (V), 131 g / ft3Antimony (Sb), 87 g / ft3Cerium (Ce), and has a washcoat loading of 2.0 g / in3.

[0100] EXAMPLE 2: FILTER B

[0101] A washcoat slurry was prepared by mixing vanadyl oxalate, antimony triacetate, cerium carbonate, a high surface area titania powder, and an aqueous dispersion of colloidal silica. Specifically, the high surface area titania powder employed was DT-51d obtained from Tronox® and the aqueous dispersion of colloidal silica employed was Ludox® AS-40 from Grace. The aqueous washcoat slurry had a pH of 5-8.

[0102] An asymmetric cylindrical cordierite partial wall-flow filter substrate (300 / 12, length = 3.0 inch, diameter = 9.5 inch, porosity=65%, mean pore diameter=16.2 pm) comprises 50% of channels having a square cross-section with dimension of 0.058 inch x 0.058 inch, and 50% having a square cross-section with dimension of 0.044 inch x 0.044 inch. All the bigger channels of the filter substrate are plugged at the outlet end. None of the smaller channels are plugged.

[0103] The filter substrate was coated by the coating method disclosed in US20180229228A1 from the inlet with the above washcoat slurry to a length of about 80% of the substrate length. The coated substrate was dried at 110°C for approximately 15 minutes, and subsequently calcined at 500°C for approximately 10 minutes. The calcined partial filter (FILTER B) contains 125 g / ft3Vanadium (V), 177 g / ft3Antimony (Sb), 117 g / ft3Cerium (Ce), and has a washcoat loading of 2.7 g / in3.

[0104] EXAMPLE 2: ENGINE TEST

[0105] Fresh FILTER A and Fresh FILTER B were each fitted into an exhaust system of a diesel engine containing an exhaust gas treatment system as shown in Fig. 10, which includes a DOC, a CSF, an urea injector, a partial filter, a V-based SCR catalyst article containing a flow-through substrate, and optional ASC. The engine was run at steady state and at the following conditions to probe the SCR performance of the catalysts:

[0106] - 13-L Heavy Duty Diesel Engine

[0107] - T = 450 °C, Mass Flow = 900 kg / h

[0108] - Ammonia to NOx Ratio (ANR) = 0.6, 1.1 , 1.35

[0109] - Partial Filter brick volume = 213 in3

[0110] - Space Velocity = 210.000 / h

[0111] The pressure drop across the partial filters are shown in Table 1. The NOx conversions, N2O selectivities, and filtration efficiencies at various Ammonia to NOx Ratios are shown in Tables 2-4. The results show that using a partial SCR filter downstream to a CSF resulted in PN10- reduction down to 0.9 x 1011- 2.2 x 1011# / kWh, in a wide range of ANR = 0.6 - 1.35 and extreme conditions of engine operation (T=450 °C, mass flow = 900 kg / h). This allows for a considerable margin in PN10-emissions against the PN10-limits stated in the latest Euro7 HDD-revision (6.0 x 1011# / kWh for WHTC / WHSC cycles).

[0112] Table 1

[0113] Table 2: ANR = 0.6

[0114] Table 3: ANR = 1.1

[0115] Table 4: ANR = 1.35 The foregoing detailed description has been provided by way of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

Claims

Claims:

1. An exhaust-gas treatment system comprising in order from upstream to downstream, a first particulate filter, means for injecting a nitrogenous reductant, and a selective catalytic reduction (SCR) component, wherein the system further comprises a second particulate filter arranged downstream of the means for injecting a nitrogenous reductant.

2. The exhaust-gas treatment system according to claim 1, wherein the first particulate filter is a diesel particulate filter (DPF) or a catalyzed soot filter (CSF).

3. The exhaust-gas treatment system according to claim 1 or claim 2, wherein the second particulate filter is arranged upstream of the SCR component.

4. The exhaust-gas treatment system according to claim 3, wherein the second particulate filter is provided with a SCR catalyst coating.

5. The exhaust gas treatment system according to claim 1 or claim 2, wherein the second particulate filter is arranged downstream of the SCR component comprising a flow-through substrate and a SCR catalyst.

6. The exhaust gas treatment system according to claim 5, wherein the second particulate filter comprises a SCR catalyst.

7. The exhaust gas treatment system according to claim 6, wherein the SCR catalyst comprises vanadium.

8. The exhaust gas treatment system according to claim 6, wherein the SCR catalyst comprises vanadium and antimony.

9. The exhaust gas treatment system according to any of proceeding claims, wherein the second particulate filter is a partial wall-flow filter having an inlet end, an outlet end, and a plurality of porous walls forming channels from the inlet end to the outlet end, wherein some of the channels are plugged at one end and some are unplugged flow-through channels.

10. The exhaust gas treatment system according to claim 9, wherein the channels which are plugged are plugged only adjacent to the inlet end.

11. The exhaust gas treatment system according to claim 10, wherein about 50% of the channels are plugged at one end.

12. The exhaust gas treatment system according to claim 11, wherein the channels that are plugged are plugged only adjacent to the outlet end.

13. The exhaust gas treatment system according to claim 12, wherein about 50% of the channels are plugged at one end.

14. The exhaust gas treatment system according to any preceding claim, wherein the second particulate filter has a lower cell density than the first particulate filter.

15. The exhaust gas treatment system according to any preceding claim, wherein second particulate filter has a higher porosity than the first particulate filter.

16. The exhaust gas treatment system according to any preceding claim, wherein the second particulate filter has a lower filtration efficiency than the first particulate filter.

17. The exhaust gas treatment system according to any preceding claim, wherein the second particulate filter has a lower channel density than the first particulate filter.

18. A vehicle comprising the fuel combustion and exhaust treatment system according to any of claims 1 to 17.

19. The vehicle according to claim 18, wherein the second particulate filter is located in an underfloor location and / or encounters, in normal use, exhaust gases at a temperature of from 270 to 350°C.

20. A method for the treatment of an exhaust gas, the method comprising passing an exhaust gas through the exhaust-gas treatment system according to any of claims 1 to 17.

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