Exhaust system for an ammonia-burning internal combustion engine

Abstract

The present invention relates to a system (32) for ammonia combustion and exhaust gas treatment, the system (32) comprising an internal combustion engine (30) for combusting ammonia, an exhaust system (34) comprising an intake (40) for receiving exhaust gas from the combustion engine (30), an upstream injector (46) for adding hydrogen gas to the exhaust gas, and a downstream catalyst article (50), the downstream catalyst article (50) comprising a three-way catalyst (TWC) composition. The present invention further relates to a method for treating exhaust gas from an ammonia internal combustion engine.

Description

[Technical field]

[0001] The present invention relates to exhaust systems for ammonia-burning internal combustion engines, and in particular to a complete system which overcomes the shortcomings in adapting known catalyst articles from gasoline exhaust treatment applications. [Background technology]

[0002] Hydrogen (H 2 ) has been proposed for use as an alternative fuel for internal combustion engines, but ammonia (NH 3 ) is also currently attracting increasing interest as a potential renewable carbon-free energy carrier, both as a fuel for direct use in combustion systems or fuel cells, and as an efficient hydrogen carrier when it is decomposed (17.8% hydrogen content by mass).

[0003] Anhydrous ammonia (NH 3 Ammonia is stored and handled very similarly to liquefied petroleum gas (LPG). For example, ammonia has a boiling point of about -33.5°C at 1.013 bar (1 atmosphere). Similarly, propane, the main component of LPG, has a boiling point of about -42.1°C. Furthermore, ammonia is carbon-free and renewable, in addition to being reasonably practical to handle, store and transport.

[0004] Also, from an energy density standpoint, ammonia is much better than liquefied hydrogen: on a volumetric basis, the energy density of liquefied hydrogen is about 8,491 kJ / liter, compared to liquefied ammonia, which has an energy density of about 11,308 kJ / liter.

[0005] Its chemical formula is NH 3Ammonia is one part nitrogen and three parts hydrogen. In atomic weight terms, ammonia is 14 parts nitrogen to three parts hydrogen, or roughly 82% nitrogen to 18% hydrogen. However, while ammonia is only 18% hydrogen, the fact that there are three hydrogen atoms bonded to a single nitrogen atom allows ammonia to contain significantly more hydrogen by volume than liquefied hydrogen.

[0006] The stoichiometric combustion with 100% theoretical air for ammonia combustion is: NH 3 +0.75(O 2 +3.76N 2 ) → 1.5H 2 O+3.32N 2

[0007] When pure ammonia is ideally burned, H 2 O and N 2 However, this type of combustion is not possible in practice: any fuel containing ammonia must be burned with excess air (greater than 100% theoretical air), whereas under practical conditions the kinetics of ammonia combustion tend to form nitric oxide.

[0008] US Patent No. 8,166,926 describes an internal combustion engine using ammonia fuel. In particular, it discloses a method that includes producing exhaust gas from the internal combustion engine and heating the ammonia with the exhaust gas to provide ammonia at a selected temperature and pressure. This helps to facilitate the operation of the combustion engine. However, the application is silent about the treatment of gaseous components of the exhaust gas.

[0009] One problem with ammonia is flame propagation. Ammonia has a minimum ignition energy of about 680 mJ. In contrast, hydrogen has a minimum ignition energy of about 0.011-0.017 mJ, and gasoline has a minimum ignition energy of 0.8 mJ. This makes it difficult to achieve high cycle efficiencies due to the slow flame propagation speed and long combustion duration of ammonia / air mixtures. A solution to this problem is to use a fuel that contains both ammonia and hydrogen.

[0010] US Patent Application Publication No. 2008 / 0308056 describes an internal combustion engine fueled with hydrogen and ammonia. In particular, it provides a dual fuel system controlled by an electronic control system to adjust the proportions of hydrogen and ammonia used to fuel the engine as desired. In a preferred mode of operation, a catalytic fuel-rich mixture is used to start the engine, and then the proportion of ammonia is adjusted for preferred operation. This application does not focus on the exhaust gas products produced by the engine, but rather focuses on adjusting engine conditions to target zero emissions.

[0011] "Operating limits for Ammonia Fuel Spark-ignition Engine", Mounaim-Rousselle et al. Energies 2021,14(14),4141 provides an up-to-date review of the potential of using ammonia as a carbon-free fuel. The discussion addresses issues related to NOx formation and explores options for reducing NOx by controlling engine operating conditions and fuel balance.

[0012] The use of ammonia fuel has been associated with significant NOx and N emissions, especially when hydrogen is used as a co-fuel. 2 Not only O but also NH 3 And further H 2 Without the aftertreatment device, the following occurs: Lean operation results in high NOx (higher than gasoline) but low unburned NH3 leads to. Rich operation results in low NOx, but high unburned NH 3 This is unacceptable due to its odor and harmfulness. ●H at the intake 2 Increasing the amount of unburned NH 3 is much lower (due to better combustion efficiency), but there is some H 2 The content is Summary of the Invention

[0013] It is therefore an object of the present invention to provide a system with an ammonia combustion engine having improved exhaust emissions, or at least to address the problems associated with exhaust in the prior art, or to provide a commercially viable alternative to exhaust.

[0014] According to a first aspect, there is provided a system for ammonia combustion and exhaust gas treatment, the system comprising: an internal combustion engine for combusting the ammonia; an exhaust system comprising an intake for receiving exhaust gas from a combustion engine, an upstream injector for adding hydrogen gas to the exhaust gas, and a downstream catalytic article; The downstream catalyst article comprises a three-way catalyst (TWC) composition.

[0015] As used herein, the terms "upstream" and "downstream" refer to the location of a component relative to the direction of exhaust gas flow through the exhaust system in use.

[0016] The present disclosure will now be further described. In the following passages, different aspects / embodiments of the present disclosure are defined in more detail. Each aspect / embodiment so defined may be combined with any other aspect / embodiment or aspect / embodiment, unless expressly indicated otherwise. 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 features disclosed in relation to the product may be combined with features disclosed in relation to the method, and vice versa.

[0017] The present invention relates to a system for ammonia combustion and exhaust gas treatment. The system includes an internal combustion engine for combusting ammonia. Internal combustion engines are well known and suitable configurations for engines that combust ammonia and, optionally, hydrogen are well known in the art.

[0018] The system includes an exhaust system having an inlet for receiving exhaust gases from an internal combustion engine. The exhaust system includes a flow path extending from the inlet to an outlet to the atmosphere. Along the flow path, the hot exhaust gases from the engine pass through one or more exhaust system catalyst articles or filters to treat undesirable components of the exhaust. In the present invention, the undesirable components of interest that must be treated are oxides of nitrogen produced in the combustion process.

[0019] The exhaust system comprises an upstream injector for adding hydrogen gas to the exhaust gas and a downstream catalytic article. Injectors are well known in the art and in fact it is common to include ammonia or hydrocarbon injectors in exhaust systems for diesel or gasoline engines. The present invention relies on the injector to provide the hydrogen gas. This typically consists of a valve connected to the engine control to control the release and an injection head or nozzle to ensure easy delivery of hydrogen and mixing with other exhaust gases.

[0020] The downstream catalyst article comprises a three-way catalyst (TWC) composition. TWC composition is a well-known term in the field of gasoline engine exhaust treatment systems. The term TWC refers to three chemical reactions that occur on the surface of the catalyst. A conventional TWC following a gasoline engine oxidizes exhaust gas pollutants - both hydrocarbons (HC) and carbon monoxide (CO) - and converts nitrogen oxide (NOx) into a harmless component, water (H2O). 2 O), nitrogen (N 2 ), and carbon dioxide (CO 2 ) to

[0021] In the context of an ammonia combustion engine, only nitrogen oxides are present, no HC or CO. This means that the TWC cannot operate optimally. As a result, the injection of hydrogen provides a component that can be oxidized while the NOx is reduced.

[0022] As discussed above, in pure ammonia and ammonia-fuel blends, particularly ammonia-hydrogen mixtures, in combustion engines, NO x It is clear that emissions are a significant problem. The solutions adopted in the prior art to date have involved careful engine management to control the nature of the emissions. However, this is not always possible, especially under conditions of high acceleration where the engine cannot respond. It is therefore anticipated that existing ammonia oxidation catalysts or catalytic reduction exhaust systems could be used to prevent such problems. This would include reducing NO x and N.H. 3 This allows both to be addressed.

[0023] However, the inventors have found that the selection of exhaust system components is not so straightforward. In particular, the inventors have found that the provision of a three-way catalyst (TWC) is particularly desirable, since TWCs generally have good hydrothermal stability. However, the inventors have found that TWCs perform poorly in NOx reduction. Without being bound by theory, it is believed that this is due to the lack of hydrocarbons and CO from the ammonia engine. The inventors have found that this problem can be overcome by directly supplementing the exhaust gas with additional hydrogen. This allows the provision of an optimized hydrothermally stable catalyst with good NOx treatment performance. Furthermore, this is advantageous over HC injection, since it has been found to operate better at lower temperatures. In addition, hydrogen can be generated in situ from the ammonia used as engine fuel.

[0024] Furthermore, clear cost and design benefits apply when adapting known catalyst articles from the gasoline engine exhaust system art and applying them to the new field of ammonia combustion engines.

[0025] Preferably, the internal combustion engine further comprises a controller for maintaining stoichiometric engine conditions. Three-way catalysts are effective when the engine operates within a narrow band of air-fuel ratios around the stoichiometric point, such that the exhaust gas composition oscillates between rich (excess fuel) and lean (excess oxygen). Conversion efficiency drops very quickly when the engine operates outside this band. Under lean engine operation, the exhaust contains excess oxygen and NOx reduction is unfavorable. Under rich conditions, the excess fuel consumes all of the oxygen available in front of the catalyst, leaving only the oxygen stored in the catalyst available for oxidation functions.

[0026] The oxygen sensor can be used to provide feedback to keep the air-fuel ratio (AFR) within a desired window as much as possible, and the feedback causes perturbations around the stoichiometric point, meaning the catalyst will alternately see slightly rich and slightly lean conditions.

[0027] Preferably, the system further comprises an ammonia storage vessel and / or a hydrogen storage vessel. Ammonia can be provided in many different forms, and the type of storage required depends on the form of ammonia provided. Ammonia can desirably be provided as a liquid or gas, and in these cases the most suitable storage vessel is a high pressure cylinder. Alternatively, ammonia can be provided as a solid, such as adsorbed on a material such as a metal halide or covalently bound in a metal complex. In these cases the storage requirements may be different. Suitable ammonia storage systems are well known in the art.

[0028] Hydrogen can be provided in many different forms as well. For example, it can be provided as a compressed gas or it can be adsorbed onto a material. Alternatively, hydrogen can be stored as part of a solid, such as lithium metal halide. Thermal treatment of the solid can be used to liberate hydrogen as needed.

[0029] Alternatively, the system may not include a hydrogen storage vessel and instead rely on in-situ generation of hydrogen from stored ammonia, which may be accomplished by catalytic or electrolytic reactions to provide hydrogen on demand.

[0030] The TWC composition is typically provided as one or more washcoat layers or zones on a porous substrate. Suitable porous substrates for the TWC composition include flow-through substrates or wall-flow filter substrates. More preferably, the substrate is a flow-through substrate. Suitable materials for forming the porous substrate include ceramic-like materials such as cordierite, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica magnesia or zirconium silicate, or porous refractory metals.

[0031] Such materials can withstand the environment, particularly the high temperatures encountered in treating exhaust streams, and can be made sufficiently porous. Such materials and their use in the manufacture of porous monolithic substrates are known in the art. Preferably, the porous substrate prior to coating has a porosity of 30-70%, such as 40-65%, most preferably >55%, e.g. >50%, such as 55-70%. Suitable techniques for determining porosity are known in the art and include mercury porosimetry and x-ray tomography.

[0032] Preferably, the TWC composition is provided on 25-75% of the axial length of the substrate extending from the upstream end of the substrate.

[0033] The provision of the composition on the upstream end of the substrate may be accompanied by a downstream catalyst composition provided to capture any ammonia and hydrogen slip. Suitable catalysts are generally known as ammonia slip catalysts (ASC) and include platinum group metals for oxidizing these species. The ASC is preferably provided on 25-75% of the axial length of the substrate extending from the downstream end of the substrate.

[0034] Thus, in one embodiment, the system preferably further comprises a PGM-containing slip catalyst disposed downstream of the downstream catalyst article for converting the slipped hydrogen to water and / or the slipped ammonia to nitrogen and water. According to another embodiment, the downstream catalyst article preferably further comprises a PGM-containing slip catalyst composition disposed extending from the outlet end of the downstream catalyst article for converting the slipped hydrogen to water and / or the slipped ammonia to nitrogen and water.

[0035] Generally, the TWC composition includes three parts: alumina, an oxygen storage component (OSC) such as a ceria-zirconia mixed oxide, and a platinum group metal (PGM) supported on one or both of the alumina and the OSC.

[0036] TWC compositions are generally provided in a washcoat. Layered TWC catalysts can have different compositions for different layers. Traditionally, TWC catalysts have been applied in layers up to 2.5 g / in 3 and 5g / in 3 The washcoat layer may have a total loading of 1000000000 or more.

[0037] The application may be characterized as an "on-wall" application or an "in-wall" application. The former is characterized by the formation of a coating layer on the surface of the channel. The latter is characterized by the infiltration of the catalytic material into the pores within the porous material. Techniques for "in-wall" or "on-wall" application may depend on the viscosity of the material to be applied, the application technique (e.g., spraying or dipping), and the presence of different solvents. Such application techniques are known in the art. Suitable coating methods are described in WO 2011 / 080525, WO 99 / 047260, WO 2014 / 195685, and WO 2015 / 145122, which are incorporated herein by reference.

[0038] The porous substrate has a first face and a second face defining a longitudinal direction therebetween, and at least a first plurality of channels extending in the longitudinal direction and providing a first plurality of inner surfaces, In use, one of the first and second faces is an inlet face (or inlet end) for exhaust gases and the other is an outlet face (or outlet end) for treated exhaust gases.

[0039] Preferably, the TWC compositions include platinum group metal (PGM) components including one or more of Pt, Pd, and Rh, which exhibit good activity and long life.

[0040] Preferably, the one or more platinum group metals are selected from a combination of two or more of Pt, Pd and Rh. The platinum group metals serve to catalyze the reactions required to purify the exhaust gas. Preferably, the platinum group metals are Pt, Pd and Rh; Pd and Rh; or Pd only; or Rh only. More preferably, the PGM component comprises a mixture of Pd and Rh.

[0041] Preferably, the TWC composition includes a support material. Preferably, the TWC includes alumina. This is an advantageous support material because it has a high surface area and is a refractory metal oxide. In other words, alumina serves as a refractory support material. This provides the catalyst composition with good heat capacity, which is required for the high temperature conditions encountered. Preferably, the alumina is gamma-alumina or theta-alumina, which may provide improved hydrothermal stability.

[0042] Preferably, the TWC composition includes an oxygen storage component (OSC), which allows the TWC to store oxygen from the exhaust gas stream, typically when the air-fuel ratio is lean. If not enough oxygen is available from the exhaust stream, the stored oxygen is released and consumed. Lack of sufficient oxygen occurs either when oxygen is not available from NOx reduction, or when certain maneuvers, such as hard acceleration, enrich the mixture beyond the converter's ability to supply oxygen.

[0043] Suitable OSCs comprise or consist of one or more mixed oxides. Preferably, the OSC comprises a ceria and zirconia mixed oxide, optionally including one or more rare earth elements. Preferably, the rare earth elements include one or more of praseodymium, lanthanum, yttrium, and neodymium.

[0044] Preferably, the weight ratio of OSC to alumina is from 15:85 to 85:15, more preferably from 30:70 to 70:30, and most preferably about 50:50.

[0045] The alumina and OSC are typically mixed homogeneously and applied to a substrate such as a ceramic honeycomb in a washcoat. The PGMs can be impregnated into one or both of the alumina and OSC prior to the preparation of the washcoat or by addition to the washcoat. Impregnating the PGMs prior to the preparation of the washcoat allows the formulator to choose which component the PGM is supported on. For example, rhodium can be supported only on the OSC and platinum can be supported only on the alumina. An alternative arrangement is to have the components and / or PGMs in separate layers stacked on top of each other.

[0046] Most preferably, the TWC comprises Pd and Rh provided on an alumina support along with an oxygen storage composition, with such a mixture making up the most typical configuration of the TWC catalyst article.

[0047] Preferably, the TWC composition includes ruthenium. When ruthenium is used in the catalyst composition, it may be impregnated on a suitable support material, such as a rutile phase titania support. The inclusion of ruthenium in the catalyst composition facilitates the low N in the disclosed system for combusting ammonia. 2 It was found to favor O production.

[0048] Preferably, H 2is dosed in an amount sufficient to optimize TWC performance, thus minimizing NOx production. The amount of hydrogen that needs to be dosed varies during operation of the system. One way to determine the amount needed is to use a NOx sensor. Preferably, the system further comprises a NOx sensor located upstream or downstream of the downstream catalyst article. These sensors allow the system to adjust the addition of hydrogen based on measurements obtained from the NOx sensor.

[0049] According to a further aspect, there is provided a method for treating exhaust gas from an ammonia internal combustion engine, the method comprising: It involves injecting hydrogen gas into the exhaust gas upstream of the TWC catalyst.

[0050] Preferably, the method is for the treatment of exhaust gases produced by and treated in the system disclosed herein. [Brief description of the drawings]

[0051] The invention will now be further described with reference to the following non-limiting figures. [Figure 1] 1 shows a system according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] 1 shows an ammonia combustion and exhaust gas treatment system 32. The system 32 includes an ammonia combustion engine 30 having a manifold 40 for passing exhaust gas from the ammonia combustion engine 30 to an exhaust gas duct 34.

[0053] Exhaust gas duct 34 conveys the exhaust gas to a TWC catalytic article 50 as described herein, and the treated exhaust gas is then released to the atmosphere 36. Before entering the TWC catalytic article 50, the exhaust gas is dosed with hydrogen 46 from a hydrogen dosing spray 48.

[0054] The TWC catalyst article 50 may be a washcoated catalyst article as described herein. 2 or NH 3 In order to also address the downstream ASC component, an integrated downstream ASC composition can be provided.

[0055] As used herein, the term "comprising" is interchangeable with the definitions "consisting essentially of" or "consisting of." The term "comprising" is intended to mean that the specified elements are essential, but other elements may be added and still form a structure within the scope of the claim. The term "consisting essentially of" limits the scope of the claim to the specified materials or steps, as well as those that do not materially affect the basic and novel characteristics of the claimed invention. The term "consisting of" closes the claim to the inclusion of materials other than those recited, except for impurities ordinarily associated therewith.

[0056] The foregoing detailed description has been provided for purposes 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 of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

[0057] For the avoidance of doubt, the entire contents of all documents identified herein are hereby incorporated by reference.

Claims

1. 1. A system for ammonia combustion and exhaust gas treatment, said system comprising: an internal combustion engine for combusting the ammonia; an exhaust system comprising an intake for receiving exhaust gas from the combustion engine, an upstream injector for adding hydrogen gas to the exhaust gas, and a downstream catalytic article; The system wherein the downstream catalyst article comprises a three-way catalyst (TWC) composition.

2. The system of claim 1 , wherein the internal combustion engine further comprises a controller for maintaining stoichiometric engine conditions.

3. The system of claim 1 or 2, further comprising an ammonia storage vessel and / or a hydrogen storage vessel.

4. The system of any one of claims 1 to 3, wherein the TWC composition is provided on a flow-through substrate or a wall-flow filter substrate.

5. The system of claim 4 , wherein the TWC composition is provided on 25-75% of the axial length of the substrate extending from an upstream end of the substrate.

6. The TWC composition comprises: (i) a platinum group metal component comprising one or more of Pt, Pd, and Rh; (ii) a support material, preferably alumina; and (iii) an oxygen storage component (OSC).

7. The system of any one of claims 1 to 6, wherein the TWC composition comprises ruthenium.

8. 8. The system of claim 1, further comprising a NOx sensor disposed upstream or downstream of the downstream catalyst article, the system configured to adjust hydrogen addition based on measurements obtained from the NOx sensor.

9. 9. The system of claim 1, further comprising a PGM-containing slip catalyst disposed downstream of the downstream catalyst article for converting slipped hydrogen to water and / or slipped ammonia to nitrogen and water.

10. 9. The system of any one of claims 1 to 8, wherein the downstream catalyst article further comprises a PGM-containing slip catalyst composition disposed extending from an outlet end of the downstream catalyst article for converting slipped hydrogen to water and / or for converting slipped ammonia to nitrogen and water.

11. 1. A method for treating exhaust gas from an ammonia internal combustion engine, the method comprising: The method includes injecting hydrogen gas into the exhaust gas upstream of a TWC catalyst.

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