The field of the invention relates to exhaust aftertreatment devices for internal combustion engines. In particular, the field of the invention relates to selective catalytic reduction (SCR) systems for internal combustion engine exhaust including those suitable for use in industrial processes and in mobile and stationary diesel, natural gas, and other engine applications.
Exhaust from internal combustions engines typically includes oxidized nitrogen gases such as nitrogen oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O), which collectively are referred to as “NOx.” Because NOx can be an environmental hazard, systems have been developed to remove NOx from exhaust by selective catalytic reduction (SCR).
Selective catalytic reduction (SCR) of nitrogen oxides (NOx) using a reducing agent is known in industrial processes as well as in stationary diesel engine applications. Ammonia is a commonly used reducing agent in SCR systems. NOx reacts with ammonia and is catalytically reduced by a SCR to nitrogen gas (N2) with water as a by-product. This reaction may be represented generally as follows:
NOx+NH3→N2+H2O
This reaction may be catalyzed by catalysts referred to as “SCR catalysts.”
Because ammonia is a hazardous substance, typically ammonia is not directly introduced into SCR systems. Rather, ammonia is generated in situ by introducing a less hazardous, ammonia-generating source into the SCR system. One common ammonia-generating source is aqueous urea. In the SCR system, aqueous urea is injected into the exhaust gas flow stream upstream of the SCR. Water in the droplets from the injection solution evaporate leaving urea, which decomposes in heat (i.e., pyrolyzes) to isocyanic acid and ammonia. In water, isocyanic acid then hydrolyzes to create ammonia and carbon dioxide. These reactions may be represented generally as follows:
CO(NH2)2+heat→HNCO+NH3
HNCO+H2O→NH3+CO2
These reactions may be catalyzed by catalysts referred to as “pyrolysis catalysts” and “hydrolysis catalysts,” respectively. The ammonia thus formed may react and reduce NOx in a SCR reaction.
The use of SCR catalysts for mobile applications is problematic. One obstacle is that mobile applications are relatively small in size. This makes it difficult to inject a urea solution into exhaust upstream of an SCR catalyst and decompose and hydrolyze the urea solution completely to ammonia prior to the exhaust arriving at the SCR catalyst (and achieve a high enough ratio of ammonia to NOx). At low diesel engine exhaust temperatures and high gas flow velocities (e.g., about 20 meters per second), a distance of several meters (i.e., a time of 0.1 to 0.3 seconds) between the injector and the SCR catalyst is required for the aqueous urea solution spray to vaporize, for urea to decompose and hydrolyze into ammonia and carbon dioxide, and for the ammonia to become practical uniformly distributed across the flow front before entering the catalyst. Although various solutions to this problem have been suggested, (see, e.g., U.S. Pat. Nos. 6,928,807; 6,361,754; and 6,203,770; and U.S. published application No. 2006/0115402), these solutions involve generating ammonia outside of the SCR system and introducing the ammonia directly into the exhaust stream. In these systems, if the SCR catalyst is not pre-heated prior to introduction of this ammonia, the ammonia will pass through the SCR catalyst and foul the air. Therefore, there is a need for improved SCR systems that are suitable for mobile applications.
Disclosed are apparatuses, systems, and methods for treating exhaust. The disclosed apparatuses, systems, and methods typically include or utilize a catalytic device for converting an aqueous urea solution to ammonia.
The disclosed apparatuses and systems for treating exhaust may include a housing having an upstream inlet for receiving the exhaust, and a downstream outlet for discharging the exhaust, where the housing defines an axial flow path for the exhaust. The disclosed apparatuses and systems further may include an injector configured to inject a urea solution into the exhaust. The disclosed apparatuses and systems typically include a catalytic device for converting urea to ammonia, which is positioned in the axial flow path downstream of the injector (e.g., a permeable catalytic device). The catalytic device has an upstream face that is positioned at an angle relative to the axial flow path (e.g., at an angle of about 20-70 degrees relative to the axial flow path, preferably at an angle of about 30-50 degrees relative to the axial flow path). The catalytic device typically is configured to receive the urea solution from the injector at the upstream face. The disclosed apparatuses and systems further may include a selective catalytic reactor (SCR) for converting NOx to nitrogen gas and water in the presence of ammonia, where the SCR is positioned downstream of the disclosed catalytic device.
Also disclosed are methods of manufacturing the disclosed apparatuses and systems. The methods may include methods for manufacturing an apparatus for treating exhaust. The methods may include: (a) providing a housing having an upstream inlet for receiving the exhaust, and a downstream outlet for discharging the exhaust, where the housing defines an axial flow path for the exhaust; (b) positioning in the housing an injector for injecting a urea solution into the exhaust; and (c) positioning downstream of the injector a catalytic device for converting urea to ammonia. Typically, the catalytic device has an upstream face that is positioned at an angle relative to the axial flow path (e.g., at an angle of about 20-70 degrees relative to the flow path, or preferably at an angle of about 30-50 degrees relative to the flow path). Typically, the catalytic device is configured to receive the urea solution from the injector at the upstream face. Optionally, the methods may include: (d) positioning downstream of the catalytic device a selective catalytic reactor for converting NOx to nitrogen gas and water in the presence of ammonia.
The disclosed methods may include methods for manufacturing a permeable catalytic device for converting urea to ammonia, where the device includes a pyrolysis catalyst, a hydrolysis catalyst, or both. The method for manufacturing the device may include: (a) manufacturing a first portion of the device, where the first portion includes the pyrolysis catalyst and has an upstream face and a downstream face; (b) manufacturing a second portion of the device, where the second portion includes the hydrolysis catalyst and has an upstream face and a downstream face; and (c) positioning the downstream face of the first portion adjacent to the upstream face of the second portion, thereby manufacturing the device; where the manufactured device is parallelogram-shaped in cross-section or trapezoid-shaped in cross-section. Optionally, the method for manufacturing the device further may include coupling the first portion and the second portion.
Also disclosed are methods of treating exhaust that utilize the disclosed apparatuses and systems. In some embodiments, the methods include passing exhaust through the disclosed catalytic devices, where the devices comprise a pyrolysis catalyst, a hydrolysis catalyst, or both.
The exhaust may passively heat the catalytic device and promotes conversion of aqueous urea to ammonia, where aqueous urea is injected into the exhaust flow and deposited on an upstream face of the catalytic device.
The disclosed apparatuses and systems utilize a catalyst device for converting urea to ammonia. In some embodiments, the required distance between the injector and the catalytic device in a SCR system for achieving complete conversion of urea to ammonia is reduced by utilizing the disclosed catalytic device. The disclosed SCR systems may be suitable for mobile applications.
Disclosed are apparatuses, systems, and methods for treating exhaust. The disclosed apparatuses, systems, and methods typically include or utilize a catalytic device for converting an aqueous urea solution to ammonia. The catalytic device may include a pyrolysis catalyst, a hydrolysis catalyst, or both. The catalytic device typically has an upstream face that is positioned at an angle relative to an axial exhaust flow when the device is positioned in an SCR system. The upstream face typically is positioned to receive an aqueous urea solution that is injected from a nozzle of a urea tank.
The disclosed apparatuses and systems typically include a catalytic device having an upstream face that is positioned at an angle relative to an axial flow path. The disclosed apparatuses and systems may include a catalytic device having a downstream face that is positioned at a right angle relative to an axial flow path or at an angle of 90 degrees (±15 degrees) relative to the axial flow path, for example, if flow distribution uniformity is not a concern and the same length of exhaust gas pathlines is not required. In some embodiments, the disclosed apparatuses and systems may include a catalytic device having a downstream face that is positioned at an angle of about 20-70 degrees relative to the axial flow path, or at an angle of about 30-50 degrees relative to the axial flow path). In further embodiments, the upstream face and the downstream face of the catalytic device are positioned at about the same angle relative to the axial flow path. Because the catalytic device has an upstream face that is positioned at an angle relative to the axial flow path, the catalytic device typically is not square-shaped or rectangular-shaped in cross-section. In some embodiments, the catalytic device is trapezoid-shaped in cross-section or parallelogram-shaped in cross-section.
In the disclosed apparatuses and systems, the injector may be positioned at an angle that is not parallel or perpendicular to the axial flow path. In some embodiments, the injector is positioned at an angle of about 20-70 degrees relative to the axial flow path (preferably at an angle of about 30-50 degrees relative to the axial flow path). In further embodiments, the injector may be positioned at an angle of about 90 degrees (±15 degrees) relative to the upstream face of the catalytic device.
The catalytic device catalyzes the conversion of aqueous urea to ammonia. The catalytic device may include a pyrolysis catalyst, a hydrolysis catalyst, or both (preferably both). The device may be monolithic (i.e., composed of single piece of material), or segmented (i.e., composed of two or more separate pieces of material which optionally are coupled together). In some embodiments, the catalytic device is composed of a pyrolysis catalyst and is coated with a hydrolysis catalyst.
The catalytic device may include an upstream portion and a downstream portion, where the upstream portion includes a pyrolysis catalyst and the downstream portion includes a hydrolysis catalyst. The upstream portion and the downstream portion may be separate portions that are placed adjacently and optionally may be coupled. In some embodiments, the upstream portion and the downstream portion are separate portions that are parallelogram-shaped in cross-section. In further embodiments, the upstream portion and the downstream portion are separate portions that are triangular-shaped in cross-section, trapezoid-shaped in cross-section, square-shaped in cross-section, or rectangle-shaped in cross-section and are placed adjacently to provide a catalytic device that is trapezoid-shaped in cross-section or parallelogram-shaped in cross-section.
The catalytic device may comprise a material, including but not limited to: extruded material; wrapped material (e.g., pleated or corrugated material); and layered material. The catalytic device preferably comprises a material selected from a group consisting of extruded monolith material; composite ceramic material, (e.g., as in U.S. Pat. Nos. 6,582,490; and 6,444,006, which are incorporated by reference herein in their entireties); fibrous material; and metallic material, (e.g., flow-through metal foils and the like). Preferably, the catalytic device is permeable or porous. In the disclosed apparatuses and systems, the catalytic device may be heated by exhaust (e.g., passively to a temperature of at least about 200° C. or about 200-700° C. or about 200-300° C.) or by a heat source (e.g., actively by an electronic heat source) to accelerate evaporation and decomposition and enhance hydrolysis or urea to ammonia.
The catalytic devices may include a pyrolysis catalyst, a hydrolysis catalyst, or both (preferably both) for converting urea to ammonia. The catalytic device may be manufactured from material that functions as a pyrolysis catalyst, a hydrolysis catalyst, or both. In some embodiments, the catalytic device is coated with a material that functions as a pyrolysis catalyst, a hydrolysis catalyst, or both.
The pyrolysis catalyst comprises a material that catalyzes the conversion of urea to ammonia (or to isocyanic acid) in the presence of heat (e.g., at a temperature of at least about 200° C.). Suitable pyrolysis catalysts may include, but are not limited to, materials having a relative high heat capacity such as metals or metal alloys. Suitable metals and metal alloys may include, but are not limited to iron or iron alloys (e.g., stainless steel), aluminum or aluminum alloys, and copper or copper alloys. In some embodiments, a pyrolysis catalyst may be coated with a different pyrolysis catalyst or a hydrolysis catalyst. The hydrolysis catalyst comprises a material that catalyzes the conversion of urea to ammonia in the presence of water (or the conversion of isocyanic acid to ammonia in the presence of water). Suitable hydrolysis catalysts may include, but are not limited to, metals and metal oxides (e.g., transition metals or transition metal oxides such as titanium, palladium, platinum, vanadium, chromium, molybdenum, nickel, or oxides thereof).
The disclosed apparatuses and systems typically include a catalytic device for converting aqueous urea to ammonia. The disclosed apparatuses and systems also may include a SCR catalyst. The SCR catalyst comprises, or is coated or impregnated with, a material that catalyzes the conversion of NOx to nitrogen gas and water. SCR catalysts may include, but are not limited to, activated carbon, charcoal or coke, zeolites, vanadium oxide, tungsten oxide, titanium oxide, iron oxide, copper oxide, manganese oxide, chromium oxide, noble metals such as platinum group metals like platinum, palladium, rhodium, and iridium, and combinations thereof.
The catalysts disclosed herein, including the pyrolysis catalysts, hydrolysis catalysts, and SCR catalysts may comprise a support material or may be mounted on a support material. Support material may include, but is not limited to, ceramic substances, zeolites, homogeneous monolith materials, and metals and metal alloys. The disclosed catalytic devices may be utilized in SCR exhaust aftertreatment systems. SCR systems are disclosed in U.S. Pat. Nos. 6,449,947; 6,601,385; 6,722,123; and 7,328,572, the contents of which are incorporated herein in their entireties.
Referring now to the drawings,
In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed. The different configurations, systems and method steps described herein may be used alone or in combination with other configurations, systems and method steps. It is to be expected that various equivalents, alternatives and modifications are possible.