Patent Application
20050164139
The invention concerns a new process for to reduce emission of pollutants of the type generated by incomplete combustion, e.g., particulates, unburned hydrocarbons and carbon monoxide, while avoiding increasing the production of NO2.
Diesel engines have a number of important advantages over engines of the Otto type. Among them are fuel economy, ease of repair and long life. From the standpoint of emissions, however, they present problems more severe than their spark-ignition counterparts. Emission problems relate to particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others. NO is of concern because it is believed to undergo a process known as photo-chemical smog formation, through a series of reactions in the presence of sunlight and hydrocarbons, and is significant contributor to acid rain. NO2 on the other hand has a high potential as an oxidant and is a strong lung irritant. Particulates (PM) are also connected to respiratory problems. As engine operation modifications are made to reduce particulates and unburned hydrocarbons on diesel engines, the NO2 emissions tend to increase.
After treatment devices, such as diesel particulate filters (DPFs) and diesel oxidation catalysts (DOCs), have been proposed to reduce the emission of particulates and gaseous hydrocarbons and carbon monoxide from diesel engines. These devices are greatly stressed in older engines and are in need of efficiency improvements in newer engines. In all cases, they are expensive due in significant part to the cost of precious metals used required to be effective. It would be desirable to reduce the cost of DPF devices and/or improve their efficiency.
NO2, being a strong oxidant, has been recognized by the art for playing a useful role in burning diesel particulates. Cooper, et al., U.S. Pat. No. 4,902,487, implements this reaction through the use of a heavily catalyzed DOC upstream of an uncatalyzed DPF. The heavily catalyzed DOC converts NO present in the exhaust to NO2, which oxidizes carbon particulates to help regenerate the filter. As a first element in Example 2 of that patent, a conventional ceramic monolith supported catalyst was employed containing approximately 80 gm/ft3 Pt. Typical loadings of platinum are reportedly 30 to 90 gm/ft3 of DOC volume. More recently a manufacturer of such devices has introduced a system which utilizes a heavily catalyzed DPF to help with low temperature regeneration. Total precious metal loadings are now reportedly 90 to 120 gm/ft3. A result of this approach is large quantities of excess NO2 escaping the system. NO2 is a strong lung irritant and concentrations have been limited in exhaust gas by MSHA and are proposed to be capped at 20% of exhaust nitrogen oxides by CARB. However, in this type of system, the art finds it necessary to utilize high platinum loadings to achieve satisfactory regeneration despite the high cost of the platinum and the associated problems of NO2 emission.
Another commercial effort has been made to improve regeneration of the soot filter of the Cooper type, and generate high NO2 emissions and aid DPF regeneration through the use of cerium or iron fuel additives. See U.S. Pat. No. 6,767,526 to Blanchard, et al., which employs a DOC with a DPF or a DPF alone with fixed platinum loadings of unspecified concentrations sufficient to oxidize NO to NO2. It does not address the high cost of platinum related to the Cooper system or the adverse effect of NO2 emissions.
Another commercially tested system uses a DOC upstream of a new wire mesh filter but needs the heavily catalyzed DOC which forms high NO2 in the exhaust to regenerate the uncatalyzed wire mesh filter. See, for example, EP 1 350 933.
In U.S. Pat. No. 6,023,928, Peter-Hoblyn and Valentine describe a platinum FBC with a DOC or DPF and or Pt/Ce with a catalyzed or uncatalyzed DPF but does not describe minimizing platinum loadings or reduction in NO2.
What is needed is a system that provides good PM reduction while minimizing the generation and escape of NO2.
It is an object of the invention to provide a system that provides good PM reduction through a catalytic exhaust treatment while minimizing the escape of NO2.
It is another object of the invention to provide a system that can reduce system costs by lowering the requirements for platinum catalyst while maintaining the apparent benefit of NO2 as an aid to soot oxidation in a DPF.
It is yet another object of the invention to provide an effective diesel particulate reduction system that provides good PM reduction efficiency while minimizing the use of precious metals and reducing the quantity of NO2 emitted from the exhaust.
A yet further object of the invention is to provide an emissions control system that is durable and able to oxidize soot at low exhaust temperatures without a need for frequent cleaning.
It is yet another and specific object of the invention to fill a need in the art for a system for emissions control for diesel engines which is based on the recognition of adverse effects of excessive catalyst loading on a DPF support and thereby enables a new balance to be achieved between the level of PM reduced and cost, durability, secondary emissions and maintenance intervals.
These and other objects are accomplished by the invention, which provides an improved diesel exhaust treatment system.
In a principal aspect, the invention provides a method for reducing particulate emissions from a diesel engine while also controlling emissions of NO2 as a percent of exhaust total nitrogen oxides, comprising: adding a fuel borne catalyst comprising platinum and cerium and/or iron at a total metal concentration of from 2 to 15 ppm in the fuel to a diesel fuel; and passing exhaust produced by the combustion through a diesel particulate filter having substrate with a precious metal catalyst thereon, the catalyst be present on the substrate in an amount of less than 15 grams per cubic foot of substrate.
In another aspect the invention provides a new robust diesel particulate control system involves the use of a lightly catalyzed wire mesh filter used in conjunction with a low dose rate FBC comprising platinum in combination with cerium and/or iron at total catalyst levels of under 15 ppm and preferably 4-8 ppm. This system has demonstrated high levels of particulate reduction of 60-75% especially when used with ULSD (<15 ppm S) without the substantial increase in NO2 emissions accompanying heavily catalyzed devices that rely on the formation of upstream NO2 to oxidize soot collected in a downstream filter.
As noted, the invention provides improved systems for diesel operation and preferably employs an FBC and an emissions after treatment device comprising a lightly catalyzed diesel particulate filter, DPF, e.g., of conventional or wire mesh construction. The term FBC refers to fuel borne catalyst, which is typically a fuel soluble or suspended composition having a metal component that is released to the combustion chamber in active form during the combustion of the fuel in the diesel engine. The terms DPF and FBC will all be explained in greater detail below and are also known to the art as evidenced by the above citations.
The invention employs an emissions after treatment device treatment comprising a catalyst substrate that can be a DPF alone or with a DOC, the catalyst substrate being lightly catalyzed with precious metal, e.g., a platinum group metal. The catalyst loading will be less that the art has seen the need for to convert NO to NO2 for use as a soot oxidant, preferably having a metal loading of less than 15 gm/ft3 platinum group metal loading, desirably less than 10 gm/ft3, and most preferably 3 to 5 gm/ft3. These low catalyst loadings aid in burning soot, without creating so much NO2 that excessive emission of the NO2 becomes an environmental problem. Among the suitable precious metals for catalyzing the DPF are those identified in the Cooper, et al., patent identified above, and particularly comprises platinum group metal.
In one embodiment of the invention, a lightly catalyzed DPF contains less than 15 grams per cubic foot (gm/ft3) platinum group metal loading, desirably less than 10 gm/ft3, and preferably 3 to 5 gm/ft3, used with a platinum and cerium FBC at 0.015-0.5 ppm Pt and 0.5-8 ppm Ce and/or iron. Higher and lower levels of additives may be employed for portions of a treatment or operation cycle. A further discussion of FBC compositions is provided below.
The improved systems of the invention significantly reduce PM, e.g., by 50 to 90% in preferred embodiments, e.g., when used with ultra low sulfur diesel fuel and does not increase NO2 above baseline and has demonstrated the ability to maintain low NO2 emissions, e.g., to below 35%, e.g., preferably to below 25%, of total nitrogen oxide species while also minimizing the use of platinum group metals.
Among the diesel fuels suitable for use in the invention are those which typically comprise a fossil fuel, such as any of the typical petroleum-derived fuels including distillate fuels. The diesel fuel can be of any of those formulations disclosed in the above priority patent applications, which are incorporated by reference herein in their entireties. A fuel can be one or a blend of fuels selected from the group consisting of distillate fuels, including diesel fuel, e.g., No. 2 Diesel fuel, No. 1 Diesel fuel, jet fuel, e.g., Jet A, or the like which is similar in boiling point and viscosity to No. 1 Diesel fuel, ultra low sulfur diesel fuel (ULSD) and biologically-derived fuels, such as those comprising a “mono-alkyl ester-based oxygenated fuel”, i.e., fatty acid esters, preferably methyl esters of fatty acids derived from triglycerides, e.g., soybean oil, Canola oil and/or tallow.
Jet A and Diesel No. 1 are deemed equivalent for applications of the invention, but are covered by different American Society For Testing and Materials (ASTM) specifications. The diesel fuels are covered by ASTM D 975, “Standard Specification for Diesel Fuel Oils”. Jet A has the designation of ASTM D 1655, “Standard Specification for Aviation Turbine Fuels”. The term ultra low sulfur diesel fuel (ULSD) means No. 1 or No. 2 diesel fuels with a sulfur level no higher than 0.0015 percent by weight (15 ppm) and some jurisdictions require a low aromatic hydrocarbon content e.g., less than ten percent by volume.
The process of the invention employs a fuel-soluble, multi-metal catalyst, i.e., an FBC, preferably comprising fuel-soluble platinum and either cerium or iron or both cerium and iron. The cerium and/or iron are typically employed at concentrations of from 0.5 to 20 ppm and the platinum from 0.0005 to 2 ppm, with preferred levels of cerium and/or iron being from 5 to 10 ppm, e.g., 7.5 ppm, and the platinum being employed at a level of from 0.0005 to 0.5 ppm, e.g., less than 0.15 ppm. In some embodiments, the treatment regimen can call for the utilizing higher catalyst concentrations initially or at defined intervals or as needed—but not for the whole treatment as has been necessary in the past. The cerium and/or iron are preferred at levels of cerium and/or iron being from 2 to 10 ppm, e.g., 3-8 ppm, and the platinum being employed at a level of from 0.05 to 0.5 ppm, e.g., from 0.1 to 0.5 ppm, e.g., 0.15 ppm, for typical operations. The tests below run at these levels show surprising results in terms emissions utilizing a lightly catalyzed DPF.
The cerium and/or iron FBC is preferred at concentrations of 1 to 15 ppm cerium and/or iron w/v of fuel, e.g., 4 to 15 ppm. A preferred ratio of cerium and/or iron to platinum for the FBC is from 100:1 to 3:1, e.g., more typically will be from 75:1 to 10:1. A formulation using 0.15 ppm platinum with 7.5 ppm cerium and/or iron is exemplary.
An advantage of low levels of catalyst (about 3 to 15 ppm total), preferably below 12 ppm and more preferably below 8 ppm, is the reduction in ultra fine particles resulting from metal oxide emissions. Data published under the European VERT program show that at high FBC dose rates of 20 ppm, or 100 ppm, cerium the number of ultra fine particles increases dramatically above baseline. However, for a bimetallic used at 0.5/7.5 or 0.25/4 ppm there is no significant increase in the ultra fine particle number. It has been found that at low levels of FBC there is not a separate ultrafine oxide particle peak and metal oxides are contained in the soot over the entire particle size distribution. A further advantage of the low dose rates prescribed by the current invention is a reduction in the contribution of metal ash to overall engine emissions. For an engine meeting 1998 US emission standards, particulate emissions are limited to 100,000 μg/hp-hr (0.1 gram/hp-hr). A cerium FBC used at 30 ppm in fuel represents a metal catalyst input loading to the engine of 6000 μg/hp-hr of metal or roughly 6% of untreated engine emissions. Therefore, low levels of catalyst used in the present invention of less than 8 ppm and preferably 4 ppm as a bimetallic or trimetallic FBC will, for example, contribute only 800-1600 μg/hp-hr of catalyst loading to the engine or 0.8-1.6% of baseline soot emissions. This has the advantage of reduced metal ash emissions and reduces the contribution of the FBC to overall particulate mass emissions or loading of metal ash to downstream emission control devices.
The fuel can contain detergent (e.g., 50-300 ppm), lubricity additive (e.g., 25 to about 500 ppm), other additives, and suitable fuel-soluble catalyst metal compositions, e.g., 0.1-2 ppm fuel soluble platinum group metal composition, e.g., platinum COD or platinum acetylacetonate and/or 2-20 ppm fuel soluble cerium or iron composition, e.g., cerium as a soluble compound or suspension, cerium octoate, ferrocene, iron oleate, iron octoate and the like. The fuel as defined, is combusted without the specific need for other treatment devices although they can be used especially for higher levels of control on diesels.
Among the specific cerium compounds are: cerium III acetylacetonate, cerium III napthenate, and cerium octoate, cerium oleate and other soaps such as stearate, neodecanoate, and other C6 to C24 alkanoic acids, and the like. Many of the cerium compounds are trivalent compounds meeting the formula: Ce (OOCR)3 wherein R=hydrocarbon, preferably C2 to C22, and including aliphatic, alicyclic, aryl and alkylaryl. Preferably, the cerium is supplied as cerium hydroxy oleate propionate complex (40% cerium by weight) or a cerium octoate (12% cerium by weight). Preferred levels are toward the lower end of this range.
Among the specific iron compounds are: ferrocene, ferric and ferrous acetyl-acetonates, iron soaps like octoate and stearate (commercially available as Fe(III) compounds, usually), iron napthenate, iron tallate and other C6 to C24 alcanoic acids, iron penta carbonyl Fe(CO)5 and the like.
Any of the platinum group metal compositions, e.g., 1,5-cyclooctadiene platinum diphenyl (platinum COD), described in U.S. Pat. No. 4,891,050 to Bowers, et al., U.S. Pat. No. 5,034,020 to Epperly, et al., and U.S. Pat. No. 5,266,083 to Peter-Hoblyn, et al., can be employed as the platinum source. Other suitable platinum group metal catalyst compositions include commercially-available or easily-synthesized platinum group metal acetylacetonates, including substituted (e.g., alkyl, aryl, alkyaryl substituted) and unsubstituted acetylacetonates, platinum group metal dibenzylidene acetonates, and fatty acid soaps of tetramine platinum metal complexes, e.g., tetramine platinum oleate.
The invention can employ a DPF alone or it can be used with other devices including DOCs, particulate reactors, partial filters or NOx adsorbers can also be used and benefit from reduced engine out emissions of the current invention. See the examples below, for the engine out results and the benefits of the FBC with catalyzed DPF devices to reduce NO2 and particulate emissions. While not wishing to be bound by any theory, the unexpectedly good results with after treatment devices as well as for engine out emissions, may be because the platinum is not present in amounts sufficient to produce excessive amounts of NO2 and yet produces some NO2 or other chemical species which is sufficient to foster oxidation of the carbon in the particulates in the presence of low levels of cerium and/or iron. NO2 is a strong lung irritant and can be generated in large quantities by traditional use of heavily catalyzed aftertreatment devices such as DOCs, DPFs or combinations. The net result of the limited NO2 production due to low platinum concentrations and the cerium and/or iron being present in low but sufficient amounts is to produce greater than expected reductions in particulates (as well as other species resulting from incomplete oxidation) and at the same time control the amount of NO2 generated and released. Unlike the prior art, then, the invention has found that high NO2 production rates are not necessary and, indeed, has found a way to provide emissions less irritating to humans.
The preferred CWMF is a stainless steel wire mesh filter with an alumina wash coat that is catalyzed with a light coating of precious metal. In the subject invention this catalytic loading is under 15 gr/cu ft and typically 7-14 gr/cu ft which has the advantages of reduced cost, lower conversion of sulfur to sulfate and reduced NO2 emissions. Used with an FBC the CWMF exhibits good PM reductions of 45-75% as well as low temperature soot oxidation without the need for upstream NO2 generation.
A typical CWMF filtration unit employs multiple rings of wire mesh formed from a mat of wire mesh as described in EP Application EP1 350 933 A1. That EP Application describes the use of a catalyzed wire mesh filter in conjunction with an upstream oxidation catalyst to generate NO2. It also teaches the use of wire diameter, the formation of the mat, compression density and other features to adjust filter performance.
In the current invention, a wire diameter of 0.35 mm is preferred to give a good balance of filtration, durability and backpressure.
Each wire mesh ring has a hollow center core and multiple rings are compressed together between two end plates to form a filter module core. Typically, six filter rings of 274 mm outside diameter with a 90 mm hollow center core are used for engines of 6-9 liters. Additional rings are used for larger engines. The filter module is placed inside a stainless steel can with gas flow directed around the outside circumference of the wire mesh by a distribution cone on the front plate of the filter module. The module is supported in the can such that gases travel length wise along the axis of flow between the filter module and stainless steel can. The module diameter is less than the can such that an air space exists between the module and the can. The exit plate of the filter module fits tightly against the steel can preventing gases from escaping. Dirty exhaust gases are forced to travel through the depth of the filter module and exit out the hollow center core. Two 10 mm safety relief ports in the outer edge of the end plate allow a small portion of gas to escape untreated and serve to prevent catastrophic failure to the engine should the filter block or become plugged with soot. Even with the two ports, overall reduction remains at 45-75% for particulates when used in the current invention.
A preferred embodiment will comprise a six section wire mesh assembly that is 232.5 mm long and 274 mm in diameter. It is compressed between two end plates using eight bolts and nuts spaced equally around the end plate circumference. The filter module has a distribution cone on the front plate to direct gas flow to the outer circumference of the filter module. The module is placed inside a stainless steel can with the eight bolts and spacer bars suspending the module inside of the can to allow gas to flow around the outside circumference and through the depth of the filter rings. The slightly larger diameter end plate prevents gases from escaping without passing through the wire mesh.
A preferred wire diameter is 0.35 mm although 0.2 mm to 0.5 mm can be used. The wire mesh mat is wound around a hollow center core of 90 mm diameter. The wire mesh is coated with an alumina wash coat and catalyzed with 14 gr/cu ft of platinum although levels of 7-10 gr/cu ft or lower can be used effectively with a FBC treated fuel.
The FBC comprises a fuel additive containing platinum in conjunction with cerium, iron or combinations of cerium and iron. In an alternative embodiment a cerium FBC can be used with the CWMF at dose rates of 2-15 ppm although bimetallic and trimetallic compositions incorporating a platinum FBC are preferred.
The following examples are presented to further explain and illustrate the invention and are not to be taken as limiting in any regard. Unless otherwise indicated, all parts and percentages are by weight.
Testing was conducted on a 1990 DTA-466 International 7.6 liter engine over three twenty minute hot transient test cycles. Average emissions for NOx, NO and NO2 and particulates were measured in grams/hp-hr are presented in the table below.
Baseline emissions on commercial No. 2D (>300 ppm Sulfur) and ULSD (<15 ppm Sulfur) showed similar NO2 emissions as a percentage of total NOx species at 17 and 18% of total nitrogen species. Particulates were slightly lower for the ULSD at 0.244 gram/hp-hr.
Installation in the exhaust of a heavily catalyzed diesel oxidation catalyst (HCDOC) with 75 g/cu ft loading of PGM and a lightly catalyzed wire mesh filter (LCWMF) with 14 g/cu ft loading of platinum group metal (PGM) used with a bimetallic platinum/cerium FBC at 0.5/7.5 ppm in ULSD fuel produced reduction in particulates of 59%, but increased NO2 emissions to 58% of total nitrogen oxide species. The cerium additive was cerium hydroxy oleate and the platinum additive was platinum COD.
When the DOC was removed, particulate reduction efficiency decreased slightly to 57% but NO2 was only 25% of total nitrogen oxide species. After a further 25 hours of operation on treated fuel both particulates and NO2 were further unexpectedly reduced.
One unexpected positive result observed in the testing was the reduction in both particulate emissions and percentage NO2 when the FBC was added to either baseline No. 2D or ULSD without the installation of any after treatment devices. For No. 2D, the particulates were reduced by 15% from 0.253 to 0.215 on treated fuel (Pt/Ce at 0.15/7.5 ppm) and NO2 decreased from 17% to 13%. For the ULSD, the particulates decreased from 0.244 to 0.207 with the addition of FBC (Pt/Ce at 0.5/7.5 ppm) to the fuel while NO2 decreased by 15% from 18% to 12%. Thus there are benefits to the use of the FBC alone or with catalyzed after treatment devices to reduce particulates and other emissions. Highly catalyzed DOCs, advocated by the prior art as important aids in particulate reduction due to their generation of NO2, are shown here to be no more effective than the right FBC for particulate reduction and can adversely affect NO2 emission. This has not been disclosed in the prior art.
Notes:
DOC = 75 gr/cu ft PGM loading
CWMF = 14 gr/cu ft PGM loading
FBC = 0.5 ppm Pt/7.5 ppm Ce, unless noted
A preferred system will contain a wire mesh DPF of the type of EP 1350933, having a loading of 14 gm/ft3 of platinum metal. This device with its construction and loading is of a type not heretofore known. Data from a 1990 (Certified 1991 Emissions) Cummins 8.3 liter engine were generated over replicate hot start test cycles on an engine dymamometer. Baseline particulate emissions for No. 2D and ULSD were similar at 0.202 gr/hp-hr and 0.201 gr/hp-hr. Overall NO2 emissions as a percentage of total nitrogen oxides were also similar for the baseline fuels at 15% and 14%.
Use of FBC treated No. 2D fuel at 0.15/7.5 with a six section wire mesh filter catalyzed with 14 gr/cu ft precious metal reduced PM by 71% to 0.059 gr/hp-hr with NO2 emissions at 20%.
Testing of the same CWMF with FBC treated ULSD using 0.5 Pt/7.5 Ce increased PM reduction to 77% with NO2 at 33% of total NOx emissions. Thus high PM reductions were achieved without extensive emissions of NO2 as typical of more heavily catalyzed devices using 25-90 gr/cu ft of precious metal loading.
This example presents engine dynamometer data generated on a 1995 DT 466 Navistar engine over a single cold and triplicate hot test cycles.
Baseline on No. 2D fuel gave PM emissions of 0.106 gr/hp-hr with NO2 at 23% of overall NOx. Use of ULSD with 0.15/7.5 Pt/Ce FBC decreased PM emissions by 31% with NO2 reduced to 19% of total NOx emissions.
Installation of a CWMF catalyzed at 14 gr/cu ft and operated with FBC treated ULSD decreased PM further to 0.035 gr/hp-hr representing at 67% overall reduction. Emissions of NO2 were at 35% of overall NOx. Good reductions in HC, CO and NOx were also obtained for the ULSD/FBC/CWMF combination.
While the invention relates to the use of a catalyzed wire mesh filter with a low level FBC, it will be recognized that the benefits of low precious metal loading on the filter substrate as well as high PM reductions and low NO2 generation will extend to other filter types. In testing of a lightly catalyzed Cordierite ceramic filter with 3 gr/cu ft of platinum used with a platinum/cerium FBC at 0.5 ppm/7.5 ppm in ULSD, particulates were reduced from 0.082 gr/hp-hr to 0.007 gr/hp-hr. That represents over 90% reduction in particulates. Emissions of NO2 were at 16% of total nitrogen oxide emissions with the FBC/DPF versus 11-13% on baseline fuel.
The above description is intended to enable the person skilled in the art to practice the invention. It is not intended to detail all of the possible modifications and variations which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such modifications and variations be included within the scope of the invention which is seen in the above description and otherwise defined by the following claims. The claims are meant to cover the indicated elements and steps in any arrangement or sequence which is effective to meet the objectives intended for the invention, unless the context specifically indicates the contrary.
This is a continuation in part of and claims priority to pending U.S. patent application Ser. No. 10/306,954 filed 29 Nov. 2002, which claims priority to U.S. Provisional Patent Application No. 60/354,435 filed 04 Feb. 2002 (hereinafter, both referred to as priority applications).
| Number | Date | Country | |
|---|---|---|---|
| 60354435 | Feb 2002 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10306954 | Nov 2002 | US |
| Child | 11038288 | Jan 2005 | US |
