Patent Application
20100146948
The present disclosure is directed to an engine exhaust treatment system and, more particularly, to an engine exhaust treatment system providing enhanced decomposition of a reductant into ammonia and/or other gaseous byproducts which are useful to enhance the selective catalytic reduction of NOx into molecular nitrogen.
Engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may exhaust a complex mixture of chemical species. The air pollutants may be composed of both solid material, such as, for example, particulate matter, and gaseous material, which may include, for example, oxides of nitrogen, such as NO2 and NO (commonly refereed to collectively as “NOx”).
Due to increased environmental concerns, engine manufacturers have developed devices for treatment of engine exhaust after it leaves the engine (sometimes referred to as “after-treatment”). For example, engine manufacturers have employed exhaust treatment devices that utilize catalysts to convert one or more components of the exhaust to different, more environmentally friendly compounds. Catalyst-based devices have been developed for reducing or removing NOx from the exhaust stream. In some systems, NOx may be reduced by selective catalytic reduction (commonly referred to as “SCR”). In such systems, urea may be added to a catalyst-based device where it is broken down into ammonia (NH3) that is stored in (or on) the catalyst. The ammonia stored in or on the catalyst reacts with NOx in the exhaust to thereby convert the NOx to Nitrogen (N2) and water (H2O) by the following formulas:
4NH3+4NO+O2→4N2+6H2O
4NH3+3NO2→3.5N2+6H2O
NO2+NO+2NH3→2N2+3H2O
The use of urea CO(NH2)2 as a precursor to generate the ammonia used during selective catalytic reduction may be desirable in many applications due to the relative ease of storing urea. Specifically, urea is relatively benign and is, therefore, easy to handle and transport. Urea undergoes a relatively complex thermal decomposition process when injected into an exhaust system. This involves both solid phase and a gas phase decomposition pathways. The solid phase pathway may yield solid byproducts that form within the system. These solid byproducts may eventually interfere with the subsequent selective catalytic reduction. One approach which has been used to improve the conversion efficiency of urea is disclosed in U.S. Pat. No. 6,895,747 to Upadhyay et al., having an issue date of May 24, 2005. This reference advocates using a reductant delivery system wherein a mixture of urea and air is injected into the system and is vaporized by a heating element prior to selective catalytic reduction. A hydrolyzing catalyst may be placed in the path of the vaporized reductant prior to selective catalytic reduction.
In accordance with one aspect, the present disclosure provides an engine exhaust treatment system. The treatment system may include a reductant injector adapted to deliver a reductant composition into an exhaust conduit defining an exhaust flow path and optionally may include a reductant mixer in fluid communication with the reductant injector. A selective catalytic reduction device is disposed downstream from the reductant injector. The selective catalytic reduction device is adapted to reduce an amount of NOx in exhaust gases produced by an engine by reaction of the NOx with ammonia. A contact surface of basic metal oxide and/or amphoteric metal oxide is disposed along the exhaust flow path.
In accordance with another aspect, the present disclosure provides an engine exhaust treatment system. The treatment system may include a reductant injector adapted to deliver a reductant composition into an exhaust conduit defining an exhaust flow path and optionally may include a reductant mixer in fluid communication with the reductant injector. A selective catalytic reduction device is disposed downstream from the reductant injector. The selective catalytic reduction device is adapted to reduce an amount of NOx in exhaust gases produced by an engine by reaction of the NOx with ammonia. A coating layer defining a contact surface is disposed along the exhaust flow path. The coating layer includes a first oxide-forming metal of basic or amphoteric character, which may be combined with at least one alloy element characterized by a Gibbs free energy of oxide formation more positive than the Gibbs free energy of oxide formation of the first oxide-forming metal.
In accordance with another aspect, the present disclosure provides a method of reducing an amount of NOx in exhaust gases produced by an engine by treatment of the exhaust gases in an exhaust treatment system. The treatment system may include a reductant injector adapted to deliver a reductant composition into an exhaust conduit defining an exhaust flow path and optionally may include a reductant mixer in fluid communication with the reductant injector. A selective catalytic reduction device is disposed downstream from the reductant injector. The selective catalytic reduction device is adapted to reduce an amount of NOx in the exhaust gases by reaction of the NOx with ammonia and/or other byproducts of urea decomposition. The method includes providing a contact surface of basic metal oxide and/or amphoteric metal oxide disposed along the exhaust flow path upstream from the selective catalytic reduction device.
Although the machine 10 is shown as a truck, the machine 10 could be any type of machine having an engine 18. Accordingly, the traction devices 16 may be any type of traction devices such as, for example, wheels, as shown in
The engine 18 may be mounted to the fame 12 and may include any kind of engine that produces a flow of exhaust gases. For example, the engine 18 may be an internal combustion engine, such as a gasoline engine, a diesel engine, a natural gas engine or any other exhaust gas producing engine. The engine 18 may be naturally aspirated or, in other embodiments, may utilize forced induction (e.g., turbocharging or supercharging).
The exhaust treatment system 20 may include, among other things, an exhaust conduit 26, an oxidation device 28, a selective catalytic reduction device 30 and a reductant injector 32 adapted to inject urea or other reductants as may be desired into the exhaust conduit 26 upstream from the selective catalytic reduction device 30. The exhaust treatment system 20 may also include a particulate trap 34. The particulate trap 34 may include any type of filtration device configured to remove one or more types of particulate matter, such as soot and/or ash, from the exhaust flow of engine 18. By way of example, the particulate trap 34 may include a filter medium configured to trap particulate matter as the exhaust flows through it. The filter medium may include a mesh-like material, a porous ceramic material (e.g., cordierite), or any other material and/or configuration suitable for trapping particulate matter.
In some embodiments, the exhaust treatment system 20 may include combinations of various devices. For example, the selective catalytic reduction device 30 may include a catalyst which is packaged with, or otherwise associated with a filter medium. In some embodiments, the filter medium may, itself be a catalytic material. Likewise, various combinations of multiple units may be used if desired. Thus, while the engine exhaust treatment system 20 is shown with a single selective catalytic reduction device 30, a single reductant injector 32, and a single particulate trap 34, the engine exhaust treatment system 20 may include more than one of any of those units. Such multiple after-treatment devices may be positioned in series (e.g., along exhaust conduit 26) or in parallel (e.g., in dual exhaust conduits; not shown). In some embodiments, the selective catalytic reduction device 30 may be positioned downstream from the particulate trap 34. In other embodiments, the selective catalytic reduction device 30 may be positioned upstream from the particulate trap 34. While the use of a particulate trap 34 may be desirable in many applications, it is likewise contemplated that the particulate trap 34 may be eliminated in applications where particulate generation is relatively low.
In the illustrated exemplary embodiment, the reductant injector 32 is in fluid communication with a reductant supply source 40 such as a tank of liquid urea solution or other reductant such as diesel fuel or other hydrocarbon material. In the illustrated embodiment, the reductant injector 32 is also in fluid communication with an air supply 42 such as a compressor or the like which delivers a supply of air to facilitate injection of the urea or other reductant. However, it is likewise contemplated that the reductant injector 32 may provide airless injection of the urea or other reductant into the exhaust conduit 26 if desired.
In the practice wherein the injected reductant is urea, the urea is decomposed into ammonia and, possibly, other chemical species, such as isocyanic acid (HNCO) and solid byproducts. Ammonia may be retained within the selective catalytic reduction device 30. The ammonia stored in the selective catalytic reduction device 30 may reduce the amount of NOx in the exhaust gases passing through the selective catalytic reduction device 30. Other agents suitable for reducing NOx may also be injected into the exhaust conduit 26 and/or the selective catalytic reduction device 30 if desired. As noted previously, the use of urea as a reducing agent precursor for formation of ammonia may be desirable due to the relative ease of handling urea. In the exemplary practice, the urea is thermally decomposed into ammonia and HCNO. The HCNO then reacts with water on the appropriate catalyst surface (which could be a hydrolysis catalyst or the SCR catalyst itself) to yield carbon dioxide (CO2) and additional ammonia (NH3). The ammonia, in turn, may act as a reducing agent within the selective catalytic reduction device 30. The use of urea as a reducing agent precursor to provide ammonia may be defined generally by the following overall chemical equation:
CO(NH2)2+H2O→CO2+2 NH3
The use of a hydrolysis catalyst may be desirable to promote the reaction of HNCO with water.
Regardless of whether or not a hydrolysis catalyst is used, the urea undergoes a complex thermal decomposition process. This process involves both a gas phase decomposition pathway (into ammonia and isocyanic acid) and a solid phase decomposition pathway. The solid phase pathway tends to yield residual solid byproducts that may build up within the exhaust system. Over time, these solid byproducts may build up around the injection point for the reductant injector and/or within the selective catalytic reduction device 30. Such a build-up may interfere with NOx removal.
The present disclosure addresses the issue of solid byproduct accumulation by use of one or more metal oxide catalysts defining contact surfaces within the system to promote the gas phase decomposition pathway of urea preferentially over the solid phase decomposition pathway. The metal oxide catalysts consistent with this disclosure convert urea into ammonia and CO2, thereby reducing or eliminating the need for a separated hydrolysis catalyst. In other words, the use of the metal oxide catalysts may be perceived as performing both reactions (i.e. the urea thermal decomposition into ammonia and HNCO as well as the reaction of HNCO with water to produce CO2 and additional ammonia) on the same catalyst. Such an approach is significantly different from previous practices wherein the steps of thermal decomposition and hydrolysis are substantially localized at different locations of the exhaust system. Promoting the gas phase decomposition pathway reduces the level of solid byproducts produced during urea decomposition. Moreover, the metal oxide catalysts may be used in the form of a coating applied to walls of the exhaust conduit 26. The coating may reduce the ability of solid byproducts to adhere to the exhaust conduit 26 and also may catalyze the decomposition of urea into ammonia. The coating can be applied following any suitable technique as may be known to those of skill in the art. The coating can be applied as basic metal oxide or as an amphoteric metal oxide, or as metal coating that is subsequently oxidized into the respective basic metal or amphoteric metal oxide. Blends of basic metals and amphoteric metals and their respective oxides may also be used.
As shown schematically in
As shown in
It is believed that the presence of the oxide catalyst within the mixer 45 and/or along the exhaust conduit 26 may promote the gas phase decomposition of the injected reductant to ammonia, thereby reducing the production of solid byproducts and improving overall efficiency of the NOx conversion. By way of example only, and not limitation, one metal oxide catalyst which may be particularly desirable is zinc oxide (ZnO). However, it is likewise contemplated that other metal oxides including CaO, MgO, SrO, BaO, La2O3 and the like may also be used if desired. In this regard, it is contemplated that virtually any basic or amphoteric oxide may provide some benefit. These include oxides of alkali metals, alkaline earth elements and corresponding transition elements. Such metals and their oxides are generally proton acceptors when reacted with water.
According to one exemplary practice, a layer of zinc oxide or other basic or amphoteric metal oxide may be used alone or in alloyed form with other metals to aid in catalyzing urea into ammonia. By way of example only, a layer of zinc oxide may be developed by applying a layer of zinc metal or an alloy containing a substantial percentage of zinc metal, directly to surfaces in contact with the exhaust stream. The application of the zinc layer may be carried out by any suitable deposition process, including thermal spray, physical vapor deposition, electrodeposition (plating), and hot dip galvanizing. Following application of the metal layer, a surface oxide layer is formed by exposure to oxygen at elevated temperatures. According to one exemplary practice, the applied layer of zinc may be oxidized by heating to a temperature of about 260 degrees Celsius in a circulated air furnace for about 10 hours. Such treatment is believed to develop a surface oxide layer of about 1 micron in thickness. Of course, other temperatures and/or heating times may be used as desired. By way of example, temperatures up to about 400 degrees Celsius may be used during the oxidation of zinc thereby approaching the melting point of 419 degrees Celsius. Of course, other temperatures can be used to oxidize other metals and/or metal alloys.
Any suitable application technique may be used to apply the metal layer. By way of example only, it is contemplated that various thermal spray processes may be used. In general, during a thermal spray process an energy source applies heat to a feedstock material to melt or partially melt the material, and the feed stock material is propelled towards a substrate where it is deposited. In most applications, thermal spray coatings range from about 50 microns to about 1 millimeter, although higher and lower thickness levels are possible. One suitable thermal spray process which may be used is the plasma transferred wire arc process (PTWA) a single, consumable wire plasma process. It is contemplated that twin wire arc, also known as wire arc spray, and atmospheric plasma spray processing may also be suitable as each has equipment that is designed for coating internal diameters, which is especially convenient for exhaust components.
Applying a coating layer of pure zinc which is subsequently oxidized by high-temperature treatment provides a highly adherent coating which is effective at performing the intended catalyzing function of promoting the gas-phase decomposition of urea under normal engine operating conditions where the temperature does not rise substantially over about 400 degrees Celsius. However, in some engines, the particulate trap 34 may be periodically regenerated by heating to burn out entrapped particles. During this regeneration cycle, temperatures downstream from the particulate trap may be substantially increased. Such increased temperature may promote delamination of the oxide coating.
It has been found that the addition of alloying elements to the zinc which increase the melting point and which are characterized by suppressed oxidation relative to the zinc may be useful in preventing delamination while nonetheless promoting the desired zinc oxide formation. By way of example only, it has been found that additions of nickel, iron, cobalt or combinations thereof may be beneficial in increasing the high temperature performance of the applied coating while still allowing the formation of zinc oxide on the surface. That is, the zinc will preferentially oxidize in the presence of these other elements due to thermodynamic considerations. Specifically, at typical oxidizing treatment temperatures, the added elements are characterized by a Gibbs free energy of oxide formation more positive than the Gibbs free energy of oxide formation of the zinc. Since a more negative free energy represents a greater thermodynamic driving force for formation, the zinc oxide is formed preferentially relative to oxides of the added elements. Moreover, since alloys of zinc and the added elements have substantially increased melting points relative to pure zinc, the propensity for localized melting is also diminished.
It is contemplated that alloy constituent levels of about 10 percent or greater by weight with the remainder being zinc may be beneficial in many environments of use. By way of example only, and not limitation, when the coating is applied by a thermal spray process using a wire feedstock, a suitable alloy may be provided from the wire feedstock by techniques such as hot dip coating a nickel, cobalt or iron wire with zinc; plating nickel, cobalt or iron on a zinc wire; co-extruding zinc and nickel, cobalt or iron; either in wire or powder form; swaging zinc and nickel, cobalt or iron to form a cold worked alloy blend; or producing a cored wire using a metal sheath and filling with powder to create a desired composition. In the event that a plasma spray process using a powder feedstock is used, a suitable alloy may be provided by using a blend of discrete zinc particles and additive particles; pre-alloyed particles; or a spray dried and sintered particle composition made up of zinc and alloy additions. Of course, combinations of any of these may be used as desired.
It is to be understood that while zinc may be a desirable base metal for the catalyzing coating, any number of other metals that form an effective oxide coating of basic or amphoteric character may likewise be used either alone or in combination with suitable alloying elements. Such alloying elements may be characterized by a Gibbs free energy of oxide formation more positive than the Gibbs free energy of oxide formation of the oxide coating formed by the base metal.
According to one contemplated practice, the coating 50 may be applied to provide a generally smooth surface. Such a smooth surface may be beneficial in reducing the accumulation of any solid byproducts generated by the urea decomposition. However, it is also contemplated that the applied coating may have an irregular surface to enhance contact area if desired. The coating may be uniform throughout the exhaust treatment system or may be localized if desired. As illustrated schematically in
While the use of a coating of metal oxide catalyst may be desirable in many applications, the metal oxide catalyst may also be present in other forms. By way of example only, and not limitation, the metal oxide catalyst may be present in the form of fins, coated inserts or other structures defining contact surfaces adapted for disposition within the mixer 45 and/or exhaust conduit 26.
Features consistent with the present disclosure may be further understood by reference to the following non-limiting example.
A stainless steel laboratory scale exhaust system incorporating a urea mixer upstream from a selective catalytic reduction device was connected to a diesel engine and the level of NOx conversion was monitored. Conversion levels averaged approximately 88%. A thermal spray coating of zinc oxide ZnO was then applied to the urea mixer and at the elbows of the exhaust conduit. The laboratory scale exhaust system was then reconnected to the diesel engine and the level of NOx conversion was monitored. Conversion levels averaged approximately 90%.
Systems consistent with the present disclosure may be suitable to enhance exhaust emissions control for engines. The disclosed systems may be used for any application of an engine. Such applications may include supplying power for machines, such as, for example, stationary equipment such as power generation sets, or mobile equipment, such as vehicles. The disclosed techniques and features may be used for any kind of vehicle, such as, for example, automobiles, locomotives, marine craft, construction machines (including those for on-road, as well as off-road use), and other heavy equipment.
An exemplary technique for enhanced exhaust treatment using the disclosed techniques may include delivering urea into an exhaust treatment system including one or more surfaces of a metal oxide catalyst and reacting the urea with water in the presence of the metal oxide catalysts to promote gas phase decomposition of the urea to ammonia and delivering the ammonia to a selective catalytic reduction device to reduce an amount of NOx in exhaust gases produced by the engine. The enhanced gas phase decomposition of the urea promotes enhances NOx conversion.
It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated.
Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.
Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.
