Patent Application
20100300078
Exemplary embodiments of the present invention are related to exhaust treatment systems for internal combustion engines.
Diesel engine exhaust gas is a heterogeneous mixture which contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and NOx, as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions, and substrates on which the catalysts are disposed may be provided in a diesel engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated components. Diesel exhaust emission treatment systems may include one or more of a precious metal containing diesel oxidation catalyst (“DOC”), a diesel particulate trap or filter “(DPF”), and a Selective Catalytic Reduction (“SCR”) catalyst device for the reduction of NOx.
One exhaust treatment technology in use for high particulate matter reduction is the DPF. There are several known filter structures that have displayed effectiveness in removing the particulate matter from diesel exhaust gas such as ceramic honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. Ceramic wall flow filters have experienced significant acceptance in automotive applications. The filter is a physical structure for removing particulates from exhaust gas and, as such, accumulating particulates will have the effect of increasing the exhaust system backpressure experienced by the engine. To address backpressure increases caused by the accumulation of exhaust gas particulates, the DPF is periodically cleaned, or regenerated. Regeneration involves the burning of accumulated particulates in what is typically a high temperature (>600 C) environment that may result in an increase in the levels of NOx components in the exhaust gas stream. The DPF may include an SCR catalyst which, with the assistance of upstream injected ammonia (NH3) in the form of gas, liquid or contained in a urea solution, will convert the NOx to nitrogen (“N2”).
One method of generating the temperatures required in the exhaust system for regeneration of the DPF is to deliver excess HC to an oxidation catalyst disposed upstream of the DPF. In the oxidation catalyst, HC is oxidized, resulting in an exothermic reaction that raises the exhaust gas temperature to levels required for DPF regeneration thereby burning or oxidizing the trapped particulate matter and cleaning the trap.
During the regeneration event, some excess HC may pass through the oxidation catalyst to the DPF. Since HC is a regulated exhaust gas constituent, release to the atmosphere should be avoided in order to meet applicable regulations. Similarly, the quantity of ammonia injected into the exhaust gas stream should be limited to that required for complete NOx conversion. However circumstances may occur in which some ammonia is not consumed by the SCR catalyst activity and passes through the DPF. The release of unconverted ammonia is undesirable.
Accordingly, it is desirable to provide an exhaust system configuration that will reduce the levels of unconverted HC, CO and NH3 in the exhaust gas stream resulting from the operation of the DPF.
In an exemplary embodiment of the present invention, an exhaust gas treatment system for reducing constituents in the exhaust gas of an internal combustion engine is provided. A ceramic monolith is disposed within the exhaust gas treatment system and has exhaust flow passages extending therethrough that are defined by longitudinally extending walls therebetween. A first catalyst composition for catalytic reduction of oxides of nitrogen in the exhaust gas is applied to a first portion of the exhaust flow passages. A second catalyst composition for catalytic oxidation of hydrocarbons, carbon monoxide and ammonia in the exhaust gas is separately applied, serially downstream to a second portion of the exhaust flow passages. The second catalytic composition is configured to reduce hydrocarbon, carbon monoxide and ammonia slip past the exhaust treatment system.
In another exemplary embodiment of the present invention, an exhaust gas treatment system for reducing constituents in the exhaust gas of an internal combustion engine comprises a ceramic wall flow monolith filter disposed within the exhaust gas treatment system having exhaust flow passages extending therethrough that are defined by longitudinally extending walls therebetween. A first subset of exhaust flow passages have open inlets and closed outlets to define inlet passages. As second subset of exhaust flow passages have closed inlets and open outlets to define outlet passages. Exhaust gas enters the ceramic wall flow monolith filter through the inlet passages and migrates through, as is filtered by the longitudinally extending walls to exit the filter through the outlet passages. A first catalyst composition that is operable, with an ammonia reductant, top catalytically reduce oxides of nitrogen in the exhaust gas is applied to a first portion of the ceramic wall flow monolith filter. A second catalyst composition for catalytic oxidation of hydrocarbon, carbon monoxide and ammonia in the exhaust gas is separately applied, serially downstream to a second portion of the ceramic wall flow monolith filter.
In yet another exemplary embodiment of the present invention, an exhaust gas treatment system for reducing constituents in the exhaust gas of and internal combustion engine include an oxidation catalyst and a particulate filter downstream of the oxidation catalyst for removal of particulates from the exhaust gas. A first catalyst composition, operable to catalytically reduce oxides of nitrogen in the exhaust gas is applied to a first portion of the particulate filter and a second catalyst composition for catalytic oxidation of hydrocarbon, carbon monoxide and ammonia in the exhaust gas is applied serially downstream to a second portion of the particulate filter.
The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.
Other objects, features, advantages and details appear, by way of example only, in the following detailed description of embodiments, the detailed description referring to the drawings in which:
Referring to
Downstream of the DOC 16, a reductant may be injected into the exhaust gas 20 in the exhaust conduit 14. NH3 in the form of a gas, a liquid or an aqueous urea solution may be used as the reductant and may be mixed with air in the injector nozzle 18 to aid in the dispersion of the injected spray. The exhaust gas stream containing the added NH3 passes through an SCR device, in this case a DPF having an SCR catalyst applied thereto. The DPF/SCR 22 is configured to filter the exhaust gas to remove carbon and other particulates and to reduce the NOx levels resident in the exhaust gas stream. The DPF/SCR 22 just described is typically referred to as a 2-way device as a result of its ability to treat or remove more than one exhaust component.
The DPF/SCR 22 may be constructed with a ceramic wall flow monolith filter 23,
In an exemplary embodiment of the exhaust gas treatment system 10, an SCR catalyst composition 42 preferably contains a zeolite and one or more base metal components such as iron (Fe), cobalt (Co), copper (Cu) or vanadium (V) which can operate efficiently to convert the NOx constituents in the exhaust gas 20 across the operating range of the DPF/SCR 22. The SCR catalyst composition 42 may be applied to the longitudinally extending walls 26 of the inlet passages 28, the outlet passages 34, or both, of the ceramic wall flow monolith filter 23. Due to the porous nature of the ceramic wall flow monolith filter 23, the SCR catalyst may also be applied within the walls of the substrate to increase the contact time between the exhaust gas 20 and the SCR catalyst composition 42. The DPF/SCR 22 operates as an effective particulate filter and SCR system that is useful for remediation of the NOx in the engine exhaust gas while at the same time removing particulate mater therefrom.
To address exhaust backpressure issues caused by particulate accumulation, the DPF/SCR 22 is periodically cleaned, or regenerated. Regeneration involves oxidation or burning of the accumulated particulate matter 40 in what is typically a high temperature (>600 C) environment. In an exemplary embodiment, excess HC is delivered to the DOC 16 for oxidation therein. The exothermal reaction caused by the oxidation of the HC will raise the temperature of the exhaust gas 20 upstream of the DPF/SCR 22 to required regeneration levels thereby burning or oxidizing the trapped particulate matter and cleaning the trap.
During the operation of the DPF/SCR 22 and, particularly during the regeneration event, some excess HC and CO may pass through the oxidation catalyst to the DPF/SCR 22. Since HC and CO are regulated exhaust gas constituents their release to the atmosphere should be minimized in order to meet applicable regulations. Similarly, the quantity of NH3 injected into the exhaust gas 20 should be limited to that required for complete NOx conversion. However circumstances may occur in which some NH3 is not converted by the SCR catalyst activity and passes through the DPF/SCR 22. The release of unconverted NH3 is undesirable.
The unintentional passage of HC, CO and/or NH3 through the DPF/SCR 22 is referred to as “slip”. To reduce the slip of HC, CO and/or NH3 through the DPF/SCR 22, an additional catalyst 44 containing precious group metals such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalyst or combination thereof is applied serially downstream of the SCR catalyst composition 42,
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents maybe substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the present application.
