Dual filter exhaust gas treatment system

Abstract

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

Description

Technical Field

[0001] This invention relates to an exhaust gas treatment system comprising two particulate filters. The downstream particulate filter is designed to address particulate emissions generated by the upstream addition of reducing agents such as ammonia or urea, while maintaining good exhaust gas treatment performance. Background Technology

[0002] Exhaust gases produced in lean-burn engines and diesel engines are typically oxidized. In a process known as selective catalytic reduction (SCR), NOx is selectively reduced using a catalyst and a reducing agent. This selective catalytic reduction removes NOx... x It is converted into elemental nitrogen (N2) and water. In the SCR process, a gaseous reducing agent (usually anhydrous ammonia, ammonia water, or urea) is added to the exhaust gas stream before the exhaust gas contacts the SCR catalyst. The reducing agent is absorbed onto the catalyst, and NO is converted into nitrogen (N2). x NOx is reduced as it passes through or over an SCR catalyst. To maximize NOx conversion, it is often necessary to add a reducing agent, such as ammonia, in excess of its stoichiometric amount to the gas stream. However, releasing excess ammonia into the atmosphere is harmful to the environment. Furthermore, ammonia is caustic, especially in its aqueous form. Condensation of ammonia and water in downstream areas of the exhaust pipe can result in a corrosive mixture that can damage the exhaust system. Therefore, ammonia in the exhaust gas should be reduced to acceptable levels.

[0003] Particulate matter emissions are a well-known problem in diesel engine exhaust treatment systems. These emissions are known to be treated using filters. Known types of filters include diesel particulate filters (DPFs) and catalytic soot filters (CSFs). These filters capture soot from the exhaust gas and can be regenerated by burning off the accumulated soot at higher temperatures. The conventional substrate used for DPFs or CSFs is a so-called wall-flow filter substrate.

[0004] Emissions regulations are becoming increasingly stringent. Current EU7 / VII particulate number (PN) emission proposals include PN10 (particulate matter >10nm) limits of 6E11 / km (LDV) or 6E11 / kWh (HDV). As emission standards become more stringent, ensuring that exhaust system emissions outputs address particulate matter emissions and other related issues is crucial. x The issues surrounding substances such as NH3 are becoming increasingly important.

[0005] Therefore, the object of the present invention is to provide an exhaust gas treatment system suitable for meeting anticipated future particulate matter emission regulations, or at least to solve related problems in the prior art or to provide a commercially viable alternative. Summary of the Invention

[0006] According to a first aspect, an exhaust gas treatment system is provided, which includes, from upstream to downstream, a first particulate filter, a device for injecting a nitrogen-containing reducing agent, and a selective catalytic reduction (SCR) assembly, wherein the system further includes a second particulate filter disposed downstream of the device for injecting the nitrogen-containing reducing agent.

[0007] According to another aspect, a fuel combustion and exhaust gas treatment system is provided, which includes an engine and an exhaust gas treatment system as described herein.

[0008] According to another aspect, a vehicle is provided that includes a fuel combustion and exhaust gas treatment system as described herein.

[0009] According to another aspect, a method for treating exhaust gas is provided, the method comprising passing the exhaust gas through the exhaust gas treatment system described herein. Attached Figure Description

[0010] The accompanying drawings described below illustrate exemplary embodiments and should not be considered as limiting the scope of the invention. The drawings are not necessarily drawn to scale, and for clarity and brevity, some features and views may be shown to scale or schematically exaggerated.

[0011] Figure 1 A conventional wall-flow filter substrate is shown.

[0012] Figure 2 A first embodiment of a partial wall-flow filter substrate is shown.

[0013] Figure 3 A second embodiment of a partial wall-flow filter substrate is shown.

[0014] Figure 4 A third embodiment of a partial wall-flow filter substrate is shown.

[0015] Figure 5 A fourth embodiment of a partial wall-flow filter substrate is shown.

[0016] Figure 6 A fifth embodiment of a partial wall-flow filter substrate is shown.

[0017] Figure 7 A partial wall-flow filter substrate coated with an SCR catalyst is shown.

[0018] Figure 8 An exemplary construction of the exhaust gas treatment system described herein is shown.

[0019] Figure 9 An exemplary construction of the exhaust gas treatment system described herein is shown.

[0020] Figure 10 An exhaust gas treatment system used for performance testing is shown. Detailed Implementation

[0021] This disclosure will now be described further. In the following paragraphs, different aspects / implementations of this disclosure are defined in more detail. Unless expressly stated to the contrary, each aspect / implementation so defined may be combined with any other aspect / implementation or multiple aspects / implementations. In particular, any feature indicated as preferred or advantageous may be combined with one or more other features indicated as preferred or advantageous. Features disclosed relative to a product are contemplated to be combined with those disclosed relative to a method, and vice versa.

[0022] This invention relates to an exhaust gas treatment system. Specifically, the system can be used to treat exhaust gases originating from combustion processes, such as those from internal combustion engines (whether mobile or stationary), gas turbines for stationary, marine, or locomotive applications, and exhaust gases from coal-fired or oil-fired power plants. The system can also be used to treat gases from industrial processes such as refining, from refinery heaters and boilers, heating furnaces, chemical processing industries, coke ovens, municipal waste treatment plants, and incinerators. In a specific embodiment, the system is used to treat exhaust gases from gas turbines or lean-burn engines. This treatment removes unwanted components from the exhaust gases, such as NOx and particulate matter. Other substances such as CO and unburned hydrocarbons (HC) can also be treated by components of the exhaust gas treatment system.

[0023] The exhaust gas treatment system described herein employs many well-known exhaust gas treatment components, which are well known to those skilled in the art. For simplicity, abbreviations are used herein. These include diesel oxidation catalyst (DOC), selective catalytic reduction (SCR) catalyst components, ammonia leak catalyst (ASC), selective catalytic reduction filter (SCRF), ammonia leak catalyst filter (ASCF), diesel particulate filter (DPF), and catalytic soot filter (CSF). Except as specified herein, the precise properties and formulations of these components are not essential to putting this invention into practice, and those skilled in the art will be able to identify and employ these components.

[0024] It has recently been observed that injecting nitrogen-containing reducing agents (especially urea / ammonia) into exhaust gas streams can lead to the formation of certain aggregated compounds. The aggregated material then takes the form of additional fine particulate matter that can be released into the atmosphere.

[0025] A discussion of these polymerized particulate emissions can be found in SAE 2017-01-0915. It explains that incomplete decomposition of injected urea, depending on the urea metering feeder, decomposition reaction tube (DRT) design, and operating conditions, can lead to the formation of solid urea deposits in diesel aftertreatment systems. These deposits can result in increased engine back pressure and deterioration of NOx removal performance. The urea deposits can further transform into chemically more stable substances upon exposure to hot exhaust gases, making it crucial to understand this transformation process. The authors' experimental results indicate that: 1) below the urea melting temperature (130°C), the formed urea deposits are still primarily urea; 2) in the temperature range of 130°C–190°C, urea transforms into a combination of urea, biuret, and cyanuric acid; and 3) above the biuret melting temperature (190°C), cyanuric acid and cyanuric acid amides are formed. At temperatures above 200°C, urea undergoes rapid chemical transformation via urea decomposition, biuret formation, and subsequent biuret decomposition, eventually converting into cyanuric acid within a short period.

[0026] Therefore, the polymerization of urea and ammonia components in hot exhaust gases can lead to the formation of polymeric particulate matter. This particulate matter is typically very fine, but it is precisely this fine material that is now subject to increasingly stringent regulations. Furthermore, because particulate matter formation can only occur after the reducing agent has been quantitatively added to the exhaust gas treatment system, it typically forms after any conventional particulate filters in the system (such as DPF or CSF) and at lower temperature environments (i.e., under-chassis configurations), where routine regeneration of such filters would be difficult.

[0027] Although the exact properties of these particles will vary depending on the nature of the nitrogen-containing reducing agent used and the operating conditions, for simplicity, the particles will generally be referred to as "reducing agent-derived particles" below.

[0028] Due to changing exhaust gas flow conditions and depending on NO x The urea demand varies with the flow rate, and the amount of urea solution injected constantly changes. Therefore, the spray droplet size varies within an injection pulse and over time. Thus, optimizing injection conditions by reintroducing an air-assisted system or applying multiple nozzles with optimized spray droplet sizes may be a promising approach.

[0029] The inventors have now discovered that these problems can be solved by providing an exhaust system as described herein. The exhaust gas treatment system has an upstream end for receiving exhaust gases from an engine, and this upstream end typically includes a manifold. The exhaust gas treatment system has a downstream end for discharging the treated exhaust gases into the atmosphere. Therefore, the components of the exhaust gas treatment system can be sequenced according to their position from upstream to downstream, wherein upstream components come into contact with the exhaust gases faster than downstream components.

[0030] According to a first aspect, an exhaust gas treatment system is provided, which includes, from upstream to downstream, a first particulate filter, a device for injecting a nitrogen-containing reducing agent, and a selective catalytic reduction (SCR) assembly, wherein the system further includes a second particulate filter disposed downstream of the device for injecting the nitrogen-containing reducing agent.

[0031] The exhaust gas treatment system includes a first particulate filter. The first particulate filter is located upstream of the other listed components and is designed to remove soot components from the exhaust gas. The first particulate filter itself may be downstream of the other components, and a preferred system includes a diesel oxidation catalyst (DOC) upstream of the first particulate filter, which is preferably the first component of the system. The first particulate filter may be a diesel particulate filter (DPF) or a catalytic soot filter (CSF). During use, these components can be regenerated to address soot accumulation.

[0032] The upstream first particulate filter is installed to prevent soot accumulation on the second particulate filter. This is desirable because the large amount of material buildup in the end-of-system components is difficult to handle due to the relatively low temperatures. If soot accumulation occurs in the second particulate filter, its temperature would need to be increased, which would be more energy-intensive. In contrast, the accumulation of reducing agent-derived particles is relatively low, resulting in minimal regeneration if necessary.

[0033] In some implementations, the first particulate filter is a diesel particulate filter (DPF). The diesel particulate filter can be a deep-bed filter and / or a surface filter. In a deep-bed filter, the average pore size of the filter wall is larger than the average diameter of the collected particles. Particles are deposited on the medium through a combination of depth filtration mechanisms, including diffusion deposition (Brownian motion), inertial deposition (impact), and streamline interception (Brownian motion or inertia).

[0034] In surface filters, the pores in the filter walls are smaller than the diameter of the particulate matter, thus separating the particulate matter through sieving. Separation occurs by the accumulation of the collected diesel particulate matter itself, which is often referred to as a "filter cake," and the process is called "cake filtration."

[0035] It should be understood that diesel particulate filters, such as ceramic wall-flow filters, can work through a combination of depth filtration and surface filtration: a filter cake is formed under high soot loads when the depth filtration capacity is saturated and the particulate layer begins to cover the filter surface.

[0036] The most widely used DPF is the wall-flow filter. A typical wall-flow filter comprises a ceramic honeycomb cell with longitudinally spaced, generally parallel pore channels formed by multiple intersecting porous walls. The pore channels are typically plugged with ceramic plug cement to form a checkered pattern of plugging at the end faces of the honeycomb cell. The filter's pore channels typically have some unplugged ends at the inlet end face of the honeycomb cell, referred to herein as "inlet channels." Similarly, the pore channels typically also have plugged remaining ends to form a checkered pattern of plugging at the outlet end face of the honeycomb substrate, some of which are unplugged, referred herein as "outlet channels."

[0037] The monolithic substrate for wall flow filters can contain 2.54 cm per square inch. 2 The cross-section may have up to approximately 400 flow channels (or “holes”), although far fewer channels may be used. For example, the monolithic substrate may have between 7 holes / square inch and 400 holes / square inch (“cpsi”), specifically 100 cpsi to 400 cpsi. The holes may have cross-sections in the following forms: rectangular, square, circular, elliptical, triangular, hexagonal, or other polygonal shapes.

[0038] like Figure 1 The conventional porous wall-flow filter 100 shown includes an inlet end 101, an outlet end 102, and a plurality of generally parallel pore channels (inlet pore channel 111 and outlet pore channel 112) separated by porous pore walls 120. The inlet channel includes a blockage 130 at the outlet end 102. The outlet channel includes a blockage at the inlet end 101. The blockage 130 is typically located at the end of the pore channel and typically has a depth of about 5 mm to 20 mm.

[0039] Diesel particulate filters (DPFs) without a catalyst coating typically have limited ability to capture particulate matter before the pressure drop becomes too large, thus requiring periodic regeneration. Passive regeneration is difficult to achieve because the combustion of particulate matter retained in the presence of oxygen requires a higher temperature than is typically provided by diesel engine exhaust.

[0040] In some implementations, the first particulate filter is a catalytic soot filter (CSF). The catalytic soot filter (CSF) works by capturing particles and continuously oxidizing them at normal diesel operating exhaust temperatures.

[0041] Catalytic soot filters (CSFs) typically contain platinum group metals and refractory metal oxides, selected from the group consisting of alumina, silica, silica-alumina, aluminosilicates, alumina-zirconia, alumina-chromium oxide, alumina-rare earth metal oxides, titanium dioxide, titanium dioxide-silica, titanium dioxide-zirconia, and titanium dioxide-alumina. For example, the concentration of platinum group metals can be 2 g / ft. 3 Up to 150g / ft 3 .

[0042] The reactions on CSF include the oxidation of CO and HC as well as the oxidation of NO to NO2, which enables particulate matter to burn.

[0043] Downstream of the first particulate filter is a device for injecting a nitrogen-containing reducing agent. This device is typically a nozzle arranged in a channel for guiding exhaust gas. The nitrogen-containing reducing agent supplied by the device can be ammonia, hydrazine, or an ammonia precursor selected from the group consisting of urea ((NH₂)₂CO), ammonium carbonate, ammonium carbamate, ammonium bicarbonate, and ammonium formate. Ammonia and urea are the most preferred alternatives. Preferably, the device for injecting the nitrogen-containing reducing agent also includes a reservoir for the nitrogen-containing reducing agent.

[0044] Downstream of the device used to inject the nitrogen-containing reducing agent is a selective catalytic reduction (SCR) unit, which catalyzes the reduction of NOx emissions by the nitrogen-containing reducing agent.

[0045] SCR modules typically consist of a flow-through substrate (e.g., a monolithic substrate) and an SCR catalyst. The monolithic substrate is typically made of ceramic or metal. Ceramic substrates (honeycomb) usually have square pores, while most metal substrates have sinusoidal channels. Other channel cross-sections are also possible, including triangular, hexagonal, trapezoidal, and circular. The number of pores can vary between 10 pores / square inch and over 1000 pores / square inch (cpsi), or between 200 cpsi and 600 cpsi.

[0046] This SCR assembly includes an SCR catalyst. SCR catalysts typically comprise oxides of base metals, molecular sieves, metal-exchanged molecular sieves, or mixtures thereof. Base metals may 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 refractory metal oxides such as alumina, silica, zirconium oxide, titanium dioxide, cerium dioxide, 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, the entire contents of which are incorporated herein by reference.

[0047] SCR catalysts may contain vanadium and antimony. The amount of antimony present may be such that the molar ratio of antimony to vanadium is greater than 0.5, or is between 0.6 and 0.9. Antimony may exist as Sb₂O₅. For example, relative to the total weight of the SCR catalyst, the SCR catalyst may contain vanadium in an amount of 2 wt% to 6 wt% or 3 wt% to 5 wt% based on V₂O₅ and antimony in an amount of 3 wt% to 8 wt% based on Sb₂O₅. The vanadium and antimony present in the catalyst are not necessarily in the form of V₂O₅ and Sb₂O₅.

[0048] In addition to vanadium and antimony, SCR catalysts may also contain cerium. The molar ratio of cerium to vanadium is typically greater than 3:10. SCR catalysts may contain cerium in amounts based on CeO2 ranging from 1 wt% to 10 wt% or 2 wt% to 5 wt%. The form of Ce present in the catalyst is not necessarily CeO2. Using cerium and vanadium in such proportions reduces vanadium volatilization without decreasing the activity of the SCR catalyst. The reduced volatilization allows for a larger vanadium loading, which is desirable for improving the efficiency of the SCR catalyst. For example, an SCR catalyst may contain 2 wt% to 6 wt% V2O5, 2 wt% to 6 wt% CeO2, and 3 wt% to 8 wt% Sb2O5 relative to the total weight of the SCR catalyst.

[0049] SCR catalysts can contain molecular sieves or metal-exchanged molecular sieves. As used herein, "molecular sieve" should be understood to mean a metastable material containing micropores of precise and uniform size, which can be used as an adsorbent for gases or liquids. Molecular sieves can be zeolite molecular sieves, non-zeolite molecular sieves, or mixtures thereof.

[0050] Zeolite molecular sieves are microporous aluminosilicates with any of the framework structures listed in the zeolite structure database published by the International Zeolite Association (IZA). 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 with these structures include chabazite, octahedral zeolite, zeolite Y, ultrastable zeolite Y, β-zeolite, mordenite, silica rock, zeolite X, and ZSM-5. Aluminosilicate zeolites may have a silica to alumina molar ratio (SAR, defined as SiO2 / Al2O3) of 5 to 100, 10 to 80, or about 10 to 30.

[0051] As used herein, the term "non-zeolite molecular sieve" refers to a tetrahedral framework in which at least a portion of the tetrahedral sites are occupied by elements other than silicon or aluminum. Specific, non-limiting examples of non-zeolite molecular sieves include silica-aluminophosphates such as SAPO-34, SAPO-37, and SAPO 44. Silica-aluminophosphates may have a framework structure comprising framework elements present in the zeolite, such as BEA, CHA, FAU, LTA, MFI, MOR, and other types described below.

[0052] SCR catalysts may contain microporous, mesoporous, or macroporous molecular sieves or combinations thereof.

[0053] SCR catalysts may comprise small-pore molecular sieves selected from the group consisting of aluminosilicate molecular sieves, metal-substituted aluminosilicate molecular sieves, aluminophosphate (AlPO) molecular sieves, metal-substituted aluminophosphate (MeAlPO) molecular sieves, silica-aluminophosphate (SAPO) molecular sieves, and metal-substituted silica-aluminophosphate (MeAPSO) molecular sieves, as well as mixtures thereof. SCR catalysts may also comprise small-pore molecular sieves selected from the group consisting of ACO, 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, as well as mixtures thereof and / or commensal framework types. Small-pore molecular sieves can be selected from groups of framework types consisting of AEI, AFX, CHA, DDR, ERI, ITE, KFI, LTA, LEV, and SFW.

[0054] SCR catalysts may comprise mesoporous molecular sieves selected from the group consisting of framework types 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, as well as mixtures and / or commensal organisms thereof. Mesoporous molecular sieves may be selected from the group consisting of framework types of FER, MFI, and STT.

[0055] SCR catalysts may comprise macroporous molecular sieves selected from the group consisting of framework types 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, as well as mixtures and / or commensal organisms thereof. Macroporous molecular sieves may be selected from the group consisting of framework types of BEA, MOR, and OFF.

[0056] Metal-exchange molecular sieves may contain at least one metal from one of groups VB, VIB, VIIB, VIIIB, IB, or IIB of the periodic table, deposited on an external framework site on the outer surface or within the channels, cavities, or cages of the molecular sieve. The metal may be in several forms, including but not limited to zero-valent metal atoms or clusters, isolated cations, mononuclear or polynuclear oxygen-containing cations, or as extended metal oxides. The metal may be iron, copper, or mixtures or combinations thereof.

[0057] Metal-exchanged molecular sieves may contain group VB, VIB, VIIB, VIIIB, IB, or IIB metals in the range of about 0.10 wt% to about 10 wt%, located at sites outside the framework on the outer surface or within channels, cavities, or cages of the molecular sieve. Metal-exchanged molecular sieves may be small-pore molecular sieves supporting copper (Cu) having copper in the form of 0.1 wt% to 20.0 wt%, 1 wt% to 6 wt%, or 1.8 wt% to 4.2 wt% relative to the total weight of the metal-exchanged molecular sieve.

[0058] The metal-exchangeable molecular sieve can be a small-pore molecular sieve carrying iron (Fe) having 0.1 wt% to 20.0 wt%, 1 wt% to 6 wt%, or 1.8 wt% to 4.2 wt% of iron relative to the total weight of the metal-exchangeable molecular sieve.

[0059] SCR catalysts are typically applied to a flow-through substrate via a support coating process to form an SCR module. Support coating slurries containing the SCR catalysts described herein may contain components such as fillers, binders, stabilizers, rheology modifiers, and other additives. In some embodiments, the support coating includes pore-forming agents such as graphite, cellulose, starch, polyacrylates, and polyethylene. These additional components do not necessarily catalyze the desired reaction, but rather improve the effectiveness of the SCR catalyst, for example, by increasing its operating temperature range, increasing the catalyst's contact surface area, or increasing the catalyst's adhesion to the substrate.

[0060] The technique of applying a carrier coating is well known in the art and includes applying the carrier coating to the surface to be coated. After the layers are coated onto the article, they are typically calcined. Calcination is well known in the art and can be carried out in air at a temperature of about 500°C.

[0061] The system also includes a second particulate filter disposed downstream of the device for injecting the nitrogen-containing reducing agent. The second particulate filter may be located upstream of the SCR assembly and is used to capture reducing agent-derived particles before ammonia is delivered to the SCR assembly. In this case, the upstream device for injecting the nitrogen-containing reducing agent is the cause of the formation of reducing agent-derived particulate matter. Alternatively, the second particulate filter may be located downstream of the SCR assembly and is used to capture reducing agent-derived particles after ammonia is delivered to the SCR assembly. In this case, the upstream device for injecting the nitrogen-containing reducing agent and the SCR assembly are both the cause of potential ammonia leakage and the cause of reducing agent-derived particle formation. Since this reducing agent-derived particulate matter is generated downstream of the first particulate filter, no other filter body can retain it.

[0062] When the second particulate filter is positioned downstream of the SCR assembly, it is preferably provided with an SCR coating (making it an SCRF) or an ammonia leak catalyst (ASC) coating (making it an ASCF). This may be desirable because it can mean that the overall system volume can be kept low. For example, another component can be removed from the system (i.e., without flow-through ASC), or another component can be made smaller. For example, when the SCR function is distributed across the SCR assembly and the second particulate filter, the SCR assembly can be smaller.

[0063] The second particulate filter typically comprises a wall-flow filter substrate as described herein. When the second particulate filter is equipped with an SCR catalyst as described herein, the system may also include an ammonia leakage catalyst (ASC) located downstream of the SCR assembly. The ASC helps minimize ammonia leakage from the system.

[0064] 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 50% to 90%, 55% to 85%, or 60% to 80% of the substrate length (L).

[0065] Ammonia leak catalysts (ASCs) typically comprise a flow-through substrate and an ASC catalyst composition. For example, an ammonia leak catalyst may comprise a top catalyst layer and a bottom catalyst layer; the top catalyst layer comprises an SCR catalyst layer as described above and is stacked on top of the bottom catalyst layer; the bottom catalyst layer comprises an ammonia oxidation catalyst layer, which comprises, for example, supported PGM, supported Pt or Pd, or Pt supported on alumina.

[0066] The top and bottom layers of an ammonia leak catalyst (ASC) are isolated to prevent immediate oxidation of NH3, which would lead to the formation of untreated secondary NOx in the exhaust stream. Therefore, the top layer of an ASC typically does not contain precious metals such as platinum group metals (PGMs). Furthermore, the bottom layer of an ASC containing PGM-based oxidation catalysts is usually completely covered by the top layer to prevent untreated secondary NOx. x Enter the exhaust stream.

[0067] When the second particulate filter is arranged upstream of the SCR assembly, it can be equipped with an SCR catalyst coating or a hydrolysis catalyst coating.

[0068] In some implementations, a second particulate filter (and thus SCRF) containing the SCR catalyst is arranged upstream of the SCR assembly.

[0069] In some embodiments, a second particulate filter containing a hydrolysis catalyst is arranged upstream of the SCR assembly. The hydrolysis coating is a conventional carrier coating and will contain, for example, metal-exchanged zeolite and zirconium oxide. An example of a hydrolysis catalyst can be found in WO2009118195A1, which is incorporated herein by reference. Providing a hydrolysis catalyst coating helps ensure that nitrogen-containing reducing agents (such as urea) are more fully converted to ammonia for use in the SCR assembly. The hydrolysis catalyst coating also helps reduce the accumulation of reducing agent-derived particles trapped on the second particulate filter. Providing a hydrolysis catalyst coating on the filter prevents spray droplets from escaping from the hydrolysis catalyst.

[0070] The second particulate filter may have different filtration performance than 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 optimized for their intended use. The first particulate filter is expected to be optimized for soot collection and (active or passive) regeneration. On the other hand, the second particulate filter, which does not need to handle soot and is difficult to regenerate, is expected to have reduced filtration efficiency and potentially reduced thermal mass.

[0071] The second particulate filter may have a lower pore density than the first particulate filter. Typically, the pore density of a filter is given as channels per square inch (cpsi), and this reflects the number of channels extending from the inlet side to the outlet side of the filter assembly (although in conventional wall-flow filters the channels are alternately blocked). All else being equal, a higher pore density reflects higher filtration efficiency and higher back pressure.

[0072] The second particulate filter can have a higher porosity than the first particulate filter. The higher porosity of the second particulate filter can result in lower resistance to airflow and lower associated back pressure.

[0073] The second particulate filter may have a lower average back pressure than the first particulate filter. A lower back pressure is required at the end toward the exhaust system, especially because it is designed to capture only a small amount of reducing agent-derived particles compared to the first particulate filter, which 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.

[0074] The second particulate filter may have a lower filtration efficiency than the first particulate filter. This reflects the first particulate filter's ability to handle a larger volume of soot particles more effectively.

[0075] All these performance variations are entirely within the capabilities of those skilled in the art and can be achieved by selecting different commercially available filter substrates or by choosing and adjusting the coating method to alter the measured performance.

[0076] In some implementations, the second particulate filter is a partially catalytic wall-flow filter. A partially wall-flow filter differs from a conventional wall-flow filter where alternating channels are blocked at the inlet or outlet end. Instead, a partially wall-flow filter has an inlet end, an outlet end, and multiple porous walls forming channels from the inlet end to the outlet end, wherein some channels are blocked at one end, and some are unblocked flow channels. "Partial" means that only a portion of the flow passes through the wall, while a portion flows through the filter without passing through the wall.

[0077] Partial wall-flow filter substrates are known. See, for example, US 20110132194A1. Partial wall-flow filter substrates exhibit a combination of blocked channels and unblocked flow channels.

[0078] Suitable partial wall flow filter substrates are described in US 20110132194A1, the entire contents of which are incorporated herein by reference. Figure 2 This is a first embodiment of a partial wall-flow filter substrate 200. The partial wall-flow filter 200 includes a plurality of porous walls 220 forming channels 211, 212, 213, and 214, wherein some of the channels are blocked channels, and the remaining channels are unblocked flow channels (213, 214). In this embodiment, the blocked channels (211, 212) include some channels that are blocked near (i.e., at or near) the inlet end 201 of the filter substrate 200. Other channels 211 are blocked near (i.e., at or near) the outlet end 202 of the filter substrate 200. Blockage 230 may be provided, for example, at the end face of some of the channels, while the remaining channels 213, 214 remain open (unblocked). This differs from Figure 1 The image shows a conventional wall-flow filter where all the pores are end-blocked (at the inlet or outlet end).

[0079] refer to Figure 3 A second embodiment of a partially wall-flow filter substrate 300 is shown and described. 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 unblocked channels (unblocked channels 312) and some blocked channels (blocked channels 311). In this embodiment, all blockages 330 are included on the outlet end 302 of the filter 300. In this embodiment, approximately 50% of the channels are blocked, and the remaining channels consist of flow channels.

[0080] refer to Figure 4 A third embodiment of a partially wall-flow filter substrate 400 is shown and described. In this embodiment, the filter substrate 400 includes a plurality of porous walls 420 to define and form a plurality of channels. The channels include some unblocked channels (unblocked channels 415) and some blocked channels (blocked channels 416). In this embodiment, the blockage 430 is entirely included on the inlet end 402 of the filter 400. In this embodiment, approximately 50% of the channels are blocked, and the remaining channels comprise flow channels.

[0081] The channels in a partial wall-flow filter substrate can have different dimensions. For example, the hydraulic diameter of an unblocked flow channel can differ from the hydraulic diameter of a blocked channel. In some embodiments, the hydraulic diameter of the blocked channel is larger than that of the unblocked flow channel. In other embodiments, the hydraulic diameter of the blocked channel is smaller than that of the unblocked flow channel.

[0082] refer to Figure 5 A fourth embodiment of a partially wall-flow filter substrate 500 is shown and described. 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 unblocked channels (unblocked channels 512) and some blocked channels (blocked channels 511). All blockages are contained at the inlet end 501 of the filter substrate 500. Approximately 50% of the channels are blocked, and the remaining channels comprise flow channels. In this embodiment, the width of the unblocked flow channels (512) is greater than that of the blocked channels (channels 511).

[0083] refer to Figure 6 A fifth embodiment of a partially wall-flow filter substrate 600 is shown and described. 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 unblocked channels (612) and some blocked channels (611). All blockages are present at the outlet end 602 of the filter substrate 600. Approximately 50% of the channels are blocked, and the remaining channels comprise flow channels. In this embodiment, the width of the unblocked flow channels (612) is smaller than that of the blocked channels (611).

[0084] Typically, the substrate of a partial wall-flow filter can have a porosity of 40% to 75%. Suitable techniques for determining porosity are known in the art and include mercury intrusion porosimetry and X-ray tomography.

[0085] Figure 7 A catalytic partial wall-flow filter 700 is shown, comprising a substrate coated with an SCR catalyst. The catalytic partial wall-flow filter 700 includes multiple porous walls 720 to form multiple channels. The channels include some unblocked channels (unblocked channels 712) and some blocked channels (blocked channels 711). In this embodiment, all blockages 730 are included at the outlet end 702 of the filter 700. In this embodiment, approximately 50% of the channels are blocked, and the remaining channels comprise flow channels. An SCR catalyst 750 is coated from the inlet end of the filter 700.

[0086] Methods for applying catalyst carrier coating slurry to a filter substrate that includes a portion of the filter substrate are known. Many suitable methods exist for applying catalyst carrier coating slurry to a substrate. For example, coating a substrate with carrier coating slurry can be done by vertically immersing the substrate in the slurry to obtain the desired coating length. The substrate can remain in the slurry for a sufficient time to allow the desired amount of 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 expelling it from the channels of the substrate, then by blowing compressed air (against the direction of slurry permeation) onto the slurry on the substrate, followed by vacuuming from the direction of slurry permeation.

[0087] Another method for coating a wall-flow filter substrate includes the steps of: (a) depositing a predetermined amount of carrier coating slurry into a receiving device at the upper end of the filter substrate using a spray head, wherein the spray head includes a plurality of orifices arranged to dispense the carrier coating slurry onto the upper end face of the filter substrate; and (b) coating the channel at the upper end of the filter substrate with the predetermined amount of carrier coating slurry from the receiving device by aspirating liquid along a channel having an open end at the upper end of the filter substrate by applying a vacuum to the lower end of the filter substrate. See, for example, US20180229228A1.

[0088] The coated substrate is typically dried at about 110°C and calcined at higher temperatures (e.g., 300°C to 500°C).

[0089] The second particulate filter can have a capacity of 0.1 g / in 3 With 5g / in 3 Between, at 0.1g / in 3 With 4.5g / in 3 Between or at 0.5g / in 3 With 4g / in 3 The SCR catalyst carrier coating loading. "Carrier coating loading" refers to the weight of the carrier coating (after calcination) per unit volume of the entire bulk filter. The volume of the entire 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.

[0090] In some embodiments, the second particulate filter is an ASC filter (ASCF) comprising a wall-flow filter substrate and the aforementioned ASC catalyst composition.

[0091] Figure 8An exhaust gas treatment system is shown, which, from upstream to downstream, includes an optional DOC, a first particulate filter as a CSF, an injector, a second particulate filter containing an SCR catalyst (and thus an SCRF), an SCR assembly including a flow-through substrate and an SCR catalyst, and an optional ASC catalyst (e.g., an ASC catalyst on a flow-through substrate).

[0092] Figure 9 Another exhaust gas treatment system is shown, which, from upstream to downstream, includes an optional DOC, a first particulate filter as a CSF, an injector, an SCR assembly including a flow-through substrate and an SCR catalyst, a second particulate filter as an SCRF or ASCF, and an optional ASC catalyst (e.g., an ASC catalyst on a flow-through substrate).

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

[0094] According to another aspect, a vehicle is provided that includes the fuel combustion and exhaust gas treatment system described herein, preferably wherein at least a second particulate filter is located under the chassis and / or in exhaust gas exposed to temperatures not exceeding 450°C during normal use. The second particulate filter is located under the chassis because it is preferably situated at the end of the exhaust system. This presents a challenge because here the temperature promotes the formation rather than the destruction of reductant-derived particulate matter; therefore, the second particulate filter is a solution to meet emission standards.

[0095] According to another aspect, a method for treating exhaust gas is provided, the method comprising passing the exhaust gas through the exhaust gas treatment system described herein.

[0096] Example 1: Filter A

[0097] A carrier coating slurry was prepared by mixing an aqueous dispersion of vanadium oxalate, antimony triacetate, cerium carbonate, high surface area titanium dioxide powder, and colloidal silica. Specifically, the high surface area titanium dioxide powder used was obtained from Tronox. ® The DT-51d uses an aqueous dispersion of colloidal silica derived from Grace's Ludox. ® AS-40. Aqueous carrier coating slurry with a pH of 5-8.

[0098] The asymmetric cylindrical cordierite partial wall flow filter substrate (300 / 12, length = 3.0 inches, diameter = 9.5 inches, porosity = 65%, average pore size = 16.2 μm) comprises 50% channels with a square cross-section of 0.058 inches × 0.058 inches and 50% channels with a square cross-section of 0.044 inches × 0.044 inches. All the larger channels of the filter substrate are blocked at the outlet end. No smaller channels are blocked.

[0099] Using the coating method disclosed in US20180229228A1, the aforementioned carrier coating slurry was applied to the filter substrate from the inlet to approximately 60% of the substrate length. The coated substrate was dried at 110°C for approximately 15 minutes and then calcined at 500°C for approximately 10 minutes. The calcined portion of the filter (filter A) contained 92 g / ft. 3 Vanadium (V), 131 g / ft 3 Antimony (Sb), 87 g / ft 3 Cerium (Ce), and has a concentration of 2.0 g / inch. 3 The load of the carrier coating.

[0100] Example 2: Filter B

[0101] A carrier coating slurry was prepared by mixing an aqueous dispersion of vanadium oxalate, antimony triacetate, cerium carbonate, high surface area titanium dioxide powder, and colloidal silica. Specifically, the high surface area titanium dioxide powder used was obtained from Tronox. ® The DT-51d uses an aqueous dispersion of colloidal silica derived from Grace's Ludox. ® AS-40. Aqueous carrier coating slurry with a pH of 5-8.

[0102] The asymmetric cylindrical cordierite partial wall flow filter substrate (300 / 12, length = 3.0 inches, diameter = 9.5 inches, porosity = 65%, average pore size = 16.2 μm) comprises 50% channels with a square cross-section of 0.058 inches × 0.058 inches and 50% channels with a square cross-section of 0.044 inches × 0.044 inches. All the larger channels of the filter substrate are blocked at the outlet end. No smaller channels are blocked.

[0103] Using the coating method disclosed in US20180229228A1, the aforementioned carrier coating slurry was applied to the filter substrate from the inlet to approximately 80% of the substrate length. The coated substrate was dried at 110°C for approximately 15 minutes and then calcined at 500°C for approximately 10 minutes. The calcined portion of the filter (filter B) contained 125 g / ft. 3 Vanadium (V), 177 g / ft 3 Antimony (Sb), 117 g / ft 3 Cerium (Ce), and has a concentration of 2.7 g / inch. 3 The load of the carrier coating.

[0104] Example 2: Engine Testing

[0105] Assemble fresh filter A and fresh filter B into the container containing... Figure 10 The exhaust gas treatment system shown is for a diesel engine and includes DOC, CSF, a urea injector, a partial filter, a V-based SCR catalyst containing a flow-through substrate, and optional ASC. The engine was operated under steady-state conditions and under the following conditions to test the SCR performance of the catalyst:

[0106] -13-L heavy-duty diesel engine

[0107] -T=450℃, mass flow rate=900kg / h

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

[0109] -Partial filter brick volume = 213 inches 3

[0110] -Airspeed = 210.000 / h

[0111] The pressure drop across the partial filter is shown in Table 1. NOx conversion, N2O selectivity, and filtration efficiency at various ammonia to NOx ratios are shown in Tables 2 through 4. The results indicate that, over a wide ANR range of 0.6–1.35 and under extreme engine operating conditions (T = 450 °C, mass flow rate = 900 kg / h), using a partial SCR filter downstream of the CSF results in a reduction of PN10- to 0.9 × 10⁻⁶. 11 -2.2×10 11 # / kWh. This allows for a considerable margin in PN10 emissions relative to the PN10 limit specified in the latest Euro 7 HDD revision (6.0 × 10 for WHTC / WHSC cycles). 11 # / kWh).

[0112] Table 1

[0113]

[0114] Table 2: ANR=0.6

[0115]

[0116] Table 3: ANR=1.1

[0117]

[0118] Table 4: ANR=1.35

[0119]

[0120] The detailed description above has been provided by way of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to those skilled in the art and remain within the scope of the appended claims and their equivalents.

Claims

1. An exhaust gas treatment system, the exhaust gas treatment system comprising, from upstream to downstream, a first particulate filter, a device for injecting a nitrogen-containing reducing agent, and a selective catalytic reduction (SCR) assembly, wherein the system further comprises a second particulate filter disposed downstream of the device for injecting the nitrogen-containing reducing agent.

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

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

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

6. The exhaust gas treatment system of claim 5, wherein the second particulate filter comprises an SCR catalyst.

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

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

9. The exhaust gas treatment system according to any one of the preceding 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 a channel from the inlet end to the outlet end, wherein some of the channels are blocked at one end and some are unblocked flow channels.

10. The exhaust gas treatment system of claim 9, wherein the blocked passage is blocked only near the inlet end.

11. The exhaust gas treatment system of claim 10, wherein approximately 50% of the passage is blocked at one end.

12. The exhaust gas treatment system of claim 11, wherein the blocked passage is blocked only at the outlet end.

13. The exhaust gas treatment system of claim 12, wherein approximately 50% of the passage is blocked at one end.

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

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

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

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

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

19. The vehicle of claim 18, wherein the second particulate filter is located under the chassis and / or encounters exhaust gases at a temperature of 270°C to 350°C during normal use.

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

Images