Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
CATALYZED PARTICULATE OXIDIZER
FOR REDUCING PARTICULATE EMISSIONS
FROM A DIESEL ENGINE AND METHOD
Technical Field
The invention relates to methods that permit a diesel engine to
operate efficiently with low particulate emissions.
Particulate emissions, e.g., PM 10 and PM 2.5, particularly from diesel
engines, are considered health risks by a growing number of regulatory and
health organizations. A number of technologies exist for controlling NOx and
to control diesel particulates. However, to date there is no technology
available to reduce diesel particulate emissions to less than 0.1 g/bhp-Hr
while also controlling NOx and not creating servicing and reliability problems
or requiring ultra-low sulfur (less than 50 ppm) fuel.
Background Art
Diesel engines provide advantages in fuel economy and are favored
for this reason. However, there is a tradeoff between economy on the one
hand, which favors complete combustion, and emissions of NOx, produced
in large quantities under these conditions. Moreover, there is a tradeoff
between NOx and particuiates and hydrocarbon (HC) emissions. There is no
known technology that is available to take full advantage of diesel
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economy without suffering a penalty in terms of increased particulate
and/or NOx emissions,
When primary measures (actions that affect the combustion process
itself) are taken to reduce NOx in diesel engines, fuel economy is usually
reduced and particulate emissions are increased. On the other hand,
combustion conditions selected to reduce pollution from particulates and
obtain good fuel economy, tend to increase NOX.
Among the current strategies to lower NOx emissions, exhaust gas
recirculation (EGR) seems to be a good candidate, but with it, increases in
particulates - in addition to fuel economy - will be major technical
challenges. Injection timing retard (ITR), like EGR, can also be used to
reduce NOx but results in increased fuel consumption and increases
particulate emissions.
The use of particulate traps for diesel engines has become common
due to an inherent trade-off between NOx and particulates - when actions
are taken to reduce one, the other increases. Conceptually, the use of a
trap could permit NOx to be reduced to a great extent by techniques such
as exhaust gas recirculation, engine timing adjustments, or other known
technologies. However, the capture of particulates in a trap can be a
problem due to loss in engine efficiency when the pressure drop across the
trap becomes too high, Moreover, trap regeneration by burning the
particulates can cause physical damage to fihe trap and requires the use
of catalyst coatings, fuel additives or supplemental heaters to assist
regeneration. Low sulfur fuel is also required for most catalyzed systems.
Pressure drops across pass-through catalytic oxidizers are much lower,
but these devices are less effective at particulate removal. Also, these
devices work best when the particulates are relatively wet with fluid
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hydrocarbons and do not work as effectively with dryer particulates of the
type produced by engines operating with exhaust gas recirculation (EGR)
employed for NOx reduction. Generally, they reduce only the soluble
organic fraction (SOF) fraction and are, therefore, limited to only 10 to 50%
reductions.
A hybrid type of mechanism has be disclosed in PCT publication WO
97/232268, to Van Hardeveld, et al. That device is catalyzed to enable the
particulates trapped to burn, but employs what is termed a turbulent flow
precipitator to knock the particulates out of the exhaust stream for capture,
collection and burning. This would tend to increase pressure drop much as
the particulate filter. However, as with catalyzed traps, the burning cannot
occur at low temperatures. When ignition of the particulates does occur, it
can cause structural damage to the apparatus. The problems can be most
severe where the engine is operated for long periods at low load.
Another problem with all precatalyzed devices, including traps and
pass through oxidizers, is that they tend to lose activity too rapidly in the
presence of sulfur. This will continue to be a problem for diesel engines far
into the future because diesel fuel contains significant sulfur. In U. S.
Patent
No. 5,501,714, Valentine and Peter-Hoblyn disclose that the problem can be
corrected for pass-through catalytic oxidizers, but this does not solve the
basic problems with that technology already noted. And, in PCT publication
WO 97/04045, Peter-Hoblyn, Valentine, Sprague and Epperly disclose that
platinum alone or with cerium, copper or iron fuel additives could
significantly reduce the balance point of a particulate trap. However, low-
load conditions may still not be high enough to control back pressure and
prevent excessive heat during regeneration. Also, Jelles, Makkee, Moulijn,
Acres and Peter-Hoblyn reported at 22"° CIMAC Congress in
Copenhagen,
Tuesday, May 19, 1998, that platinum/cerium fuel additives in combination
with a catalyzed ceramic filter removed high levels of soot at lower
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temperatures than catalyzed filters or additives alone, Even with the
enhanced performance of this system at low temperatures, the filter still
suffers from inherently high back pressure and did not oxidize at
temperatures below about 350°C.
Current technology does not provide an adequate solution to the
problem of diesel particulates, especially for engines operated under
conditions necessary to minimize NOX emissions. Oxidizers are not effective
at removing particulates because they principally reduce the SOF; and
while traps are effective at collecting particulates the are troubled with
inherent regeneration and durability problems and high back pressures.
Disclosure of Invention
It is an object of the invention to provide a method and apparatus,
which provide significant particulate reductions.
It is an object of the invention to provide a method and apparatus,
which provide significant, long-term particulate reductions and do so with
a minimum of maintenance.
It is another object of the invention to provide a method and
apparatus that enable optimizing operation of a diesel for reducing
particulates, e.g" to less than 0.1 g/bhp-Hr, while dealing with NOx reduction
through use of engine changes such as EGR and iTR.
It is another object of the invention to provide a method and
apparatus for enabling simultaneous reduction of particulates and NOx from
a diesel engine.
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It is another object of the invention to provide a method and
apparatus that eliminate the poor removal efficiency normally associated
with pass-through catalytic oxidizers,
It is another object of the invention to provide a method and
apparatus that eliminate the fuel economy penalty normally associated with
a diesel particulate trap.
It is yet another, and more specific object of the invention to provide
a method and apparatus, which provide significant, long-term particulate
reductions, e.g., to less than 0,1 g/bhp-Hr, while simultaneously dealing with
NOX reduction through the use of exhaust gas recirculation and/or ITR.
These and other objects are achieved by the present invention, which
provides an improved method and apparatus for operating a diesel engine
with low particulate emissions,
The method of the invention comprises: equipping a diesel engine
with a catalyzed particulate oxidizer having an inlet, an outlet, an enlarged
central chamber and a plurality of parallel plates within the chamber, the
plates having catalyzed, undulating surfaces provided to create large
number of points of contact for particulates in exhaust; operating the diesel
engine under conditions that create an exhaust containing particulates;
and passing the exhaust through the catalyzed particulate oxidizer.
The catalyzed particulate oxidizer is also claimed.
Preferably, the fuel will contain a fuel-soluble organo-platinum group
metal compound, e,g., comprising a platinum group metal selected from
the group consisting of platinum, palladium, rhodium and mixtures of two or
more of these. In an alternative embodiment, an effective platinum group
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metal compound can be added to the exhaust gases before the trap or
combustion air, In an alternative, cerium, iron, copper, manganese or
combinations of any of these with platinum can be used to reduce engine
out particulate loading, including both the soluble and carbon soot fractions
of the soot prior to the oxidizer. The resulting metal activated soot will
also
promote enhanced oxidation when it contacts the catalyzed surfaces.
In another preferred aspect of the invention, the engine is operated
with exhaust gas recirculation and/or injection timing retard.
Description of the Drawings
The invention will be better understood and its advantages will be
more apparent when the following detailed description is read in light of the
accompanying drawings, wherein:
Figure 1 is a schematic representation of a diesel engine with an
exhaust system including a catalyzed particulate oxidizer in accord with the
invention;
Figure 2 is a schematic representation of a catalyzed particulate
oxidizer in accord with the invention;
Figure 3 is an enlarged, cut-away schematic representation of a
portion of a catalyzed particulate oxidizer in accord with the invention; and
Figure 4 is a schematic representation of a diesel engine operating
with exhaust gas recircufation and an exhaust system including a catalyzed
particulate oxidizer in accord with the invention.
Detailed Description of a Preferred Embodiment
The term "Diesel eragine" is meant to include all compression-ignition
engines, for both mobile (including marine) and stationary power plants and
of the two-stroke per cycle, four-stroke per cycle and rotary types.
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The term "hydrocarbon fuel" is meant to include all of those liquid and
gaseous fuels prepared from "distillate fuels" or "petroleum". The term
"distillate fuel" means all of those products prepared by the distillation of
petroleum or petroleum fractions and residues. The term " petroleum" is
meant in its usual sense to include all of those materials regardless of
source
normally included within the meaning of the term, including hydrocarbon
materials, regardless of viscosity, that are recovered from fossil fuels.
The term "diesel fuel" includes "distillate fuels" including diesel fuels
meeting the ASTM definition for diesel fuels or others even though they are
not wholly comprised of distillates and can comprise alcohols, ethers,
organo-vitro compounds and the like (e.g., methanol, ethanol, diethyl ether,
methyl ethyl ether, nitromethane). Also contemplated, are emulsions and
liquid fuels derived from vegetable or mineral sources such as corn, alfalfa,
shale, and coal. These fuels may also contain other additives known to those
skilled in the art, including dyes, cetane improvers, anti-oxidants such as
2,6-di-tertiary-butyl-4-rnethyiphenol, corrosion inhibitors, rust inhibitors
such as
alkylated succinic acids and anhydrides, bacteriostatic agents, gum
inhibitors, metal deactivators, upper cylinder lubricants, antiicing agents
and
the like.
Reference to Figure 1 shows a diesel engine 10 fed fuel from a tank 11.
The fuel is preferably catalyzed with a platinum group metal compound or
one or more other catalyst compounds, such as cerium, iron or manganese.
These latter material can be used alone or with a platinum group metal
catalyst.
Exhaust from the engine will pass through exhaust pipe 12, carrying
catalytic metals released from the fuel additive catalyst compositions of
cerium, and preferably also platinum, to a catalyzed particulate oxidizer
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(CPO) 14, The CPO can be catalyzed either as installed or by building up a
catalyst deposit by operating the engine with a platinum group metal fuel
additive.
The CPO of the invention is schematically shown in longitudinal cross-
section in Figure 2. The CPO 14 is shown as having an inlet 1 b, an outlet 18
and an enlarged central chamber 20. Within the chamber 20 are a plurality
of essentially parallel plates 22 with catalyzed, undulating surtaces provided
to create large number of points of contact for particulates in exhaust which
enters at 16 and exits at 18. The plates will preferably be made of a ceramic,
a silica-alumina composition such as cordierite, silicon carbide, glass or
metal fibers, porous glass or metal substrates, or the like. or a suitable
metal
such as alloys of the type used in automotive exhaust systems. Among the
suitable catalysts, are those known to be useful for catalyzing traps and
pass-through catalytic oxidizers. Prominent among these are platinum group
metals such as platinum, palladium and rhodium. The oxidizer may or may
not be precoated with an alumina washcoat to provide high surface area
prior to catalyzing. It is an advantage of the invention that the washcoat is
not required.
Reference to Figure 3 schematically shows a section of a CPO
enlarged to illustrate the dynamics of the process. Channels 24 are formed
between individual plates 22. The channels are sufficiently wide to permit
the exhaust gases to pass through with minimal pressure drop. The exact
configuration of the channels will vary depending on many design and
manufacturing variables. The peaks 2b and the valleys 28 formed in the
sheets cause the gases to change direction frequently. The particulates,
even though small, have a mass that causes them to impact the walls of the
channels formed by the plates while the gases easily turn following the
undulations in the plates. The particulates are not collected, but are
oxidized
at least partially by frequent impact with catalyzed surfaces of the plates
22.
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The undulations in the drawings are seen to be of chevron shape, but other
suitable shapes, including sinusoidal, flat-topped chevrons, and the like can
also be employed. In some embodiments, it will be desired to install the
plates in sections along the length of the chamber. For example, the plates
could be assembled into 2 to 5 sections, each filling the cross section of the
chamber, but extending only a portion of its length. In this case, the
sections
would be separated by a space of preferably less than 5 inches, e.g, 0.25 to
3 inches.
As noted above, the fuel will preferably also contain a fuel-soluble
organo-platinum group metal compound, e.g., of platinum, palladium or
rhodium. Among these are platinum group metal compounds selected from
the group consisting of platinum acetylacetonate and compounds having
the general formula XPtR,R2 wherein X is a ligand containing at least one
unsaturated carbon-to-carbon bond with an olefinic, acetylenic or aromatic
pi bond configuration and R, and RZ are, independently, benzyl, phenyl,
nitrobenzyl or alkyl having 1 to 10 carbons, e. g., Biphenyl cyclooctadiene
platinum(ll).
Suitable platinum group metal compounds are disclosed for example
in prior U.S. Patent Nos. 4,892,562 and 4,891,050 to Bowers and Sprague,
5,034,020 to Epperly and Sprague, 5,215,652 to Epperly, Sprague, Kelso and
Bowers, and 5,266,083 to Peter-Hoblyn, Epperly, Keiso and Sprague, WO
90/07561 to Epperly, Sprague, Kelso and Bowers, and U. S. Patent
Application Serial No. 08/597,517, filed January 31, 1996, by Peter-Hoblyn,
Valentine and Sprague, hereby incorporated by reference. Where the
application permits, a blend of these compounds can be used with one or
more other platinum group metal compounds such as soaps, acetyl
acetonates, alcoholates, p-diketonates, and sulfonates, e.g., of the type
which will be described in more detail below.
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The platinum group metal compound suitable for use as a fuel or gas-
borne additive and/or other catalyst additive material, can be added in
any manner effective for its intended purpose, such as by adding it to the
fuel in bulk storage, to the fuel in a tank associated with the engine, or by
continuous or intermittent addition, such as by a suitable metering device,
e.g., 27 from tank 29 in Figure 1, into: the fuel line leading to the engine
or
the fuel return line from the engine, or in the form of a vapor, gas or
aerosol
into the air intake, the exhaust gases before the CPO, exhaust gases after
the CPO but before recirculation to the engine, or a mixing chamber or
equivalent means wherein the exhaust gases are mixed with incoming air.
When employed, platinum group metal catalyst compositions are
preferably employed at concenfirations of less than 1 part by weight of
platinum group metal per million parts by volume fuel (ppm), When used for
the purpose of catalyzing an uncatalyzed CPO (or one that has become
inactive), it is possible to higher doses, e.g., from 1 to 25 (or greater)
ppm, to
effect a rapid deposit of catalyst in the CPO. For the purposes of this
descrip-
tion, all "parts per million" figures are on a weight to volume basis, i.e.,
grams/million cubic centimeters (which can also be expressed as
milligrams/liter), and percentages are given by weight, unless otherwise
indicated, Auxiliary catalysts (named so because they are preferably used
with a platinum group metal composition, but can be used without such)
are employed at levels effective for their intended purpose, preferably at
levels of from 1 to 200 ppm of the fuel utilized, e.g., 5 to 60 ppm.
Among the auxiliary catalytic materials are organometaliic salts of
manganese, magnesium, calcium, iron, copper, cerium, sodium, lithium and
potassium, which can be employed at suitable levels, e.g., from about 1 to
about 100 ppm and preferably 20 to 60 ppm of the catalyst metal in
combination with the platinum group metal catalyst in diesel fuels. Among
these are the alcoholates, sulfonates, beta-diketonates and soaps, e.g.,
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selected from the group consisting of stearates, paimitates, laurates,
tallates,
naptl~anates, other fatty acid soaps, and mixtures of two or more of these,
of copper, calcium, magnesium, manganese, iron, cerium, sodium, lithium
and potassium compounds as are known as fuel soluble and useful fuel
additives.
Among preferred cerium compounds are: cerium III acetylacetonate,
and various cerium soaps such as cerium III napthanate, cerium octoate,
cerium stearate, cerium neodecanoate, and the like. Many 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. The dosage level will be at a level of from
about
1 to 100 ppm cerium per million parts of fuel (mg per liter), and preferably
in
the range of from about 5 to 30 ppm, preferably less than 20 ppm. This level
can be reduced significantly over what is currently employed in the art by
using the cerium in combination with a platinum-catalyzed particulate trap.
Reference can be made to the aforementioned WO 97/04045 for a detailed
listing, incorporated herein by reference, of other representative auxiliary
catalyst compositions.
Reference to Figure 4 shows, schematically, a diesel engine 10
operating with exhaust gas recirculation and an exhaust system including a
catalyzed particulate oxidizer 14 in accord with the invention. During EGR
operation, combustion air from intake 13 (at high or low pressure, heated or
cooled) and exhaust gases from line 32 (separated from the main exhaust
gas stream 34) are mixed and fed to one or more cylinders of engine 10
(e.g., either diesel or lean-burn gasoline). The proportion of exhaust gases
recirculated to the engine for forming a combustion air mixture will be
effective to lower the 'production of NOx by the engine utilizing the
combustion air mixture as compared to combustion air not containing
exhaust gases. Typically, from about 0 to about 30% can be recircuiated.
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The combustion air mixture is typically compressed prior to
introduction into engine cylinders) wherein it is further compressed, causing
heating. The appropriate fuel is injected into the cylinders following
compression. The fuel is then combusted with the combustion air mixture to
produce exhaust gases that are discharged through exhaust stream 34. The
cycle just described is repeated continuously as the engine continues to run
in the EGR mode. EGR lowers the combustion temperature and oxygen to
the combustion chamber and reduces the amount of NOx produced, but
as has been observed, it increases production of particulates and unburned
hydrocarbons - again, the compromise between NOx and complete
combustion.
Downstream of exhaust stream 34 is a CPO unitl4. The CPO is
effective within a temperature window of from about 150 to about 650°
C,
depending on the catalyst. During engine operation giving rise to these
temperatures, the exhaust temperature is maintained at the temperatures
most preferred for the CPO. At these temperatures, NOx conversion by EGR
is practical, and the EGR system is therefore operated. At other times, ITR
can be used alone or in conjunction with EGR to reduce NOx.
Figure 4 also illustrates a control system of a type useful to maintoin the
proper operation of EGR and CPO units. The controller 36 can, if desired,
measure any of a number of parameters to assure optimum NOX reduction
and particulate oxidation. The temperature of the exhaust (sensor means 38)
is one parameter of importance. Engine load is another key parameter
(sensor means 40), and this or like factor can be monitored to determine the
amount of NOx being generated and the need for NOX reduction by EGR or
engine timing changes (r5ot shown).
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The sensing means provided for sensing operating parameters
indicative of conditions effective for NOx reduction, sense the appropriate
operating parameter and generate an operation signal representative
thereof. The controller 36 provides control means for comparing one or
more operation signals to appropriate reference values) and determines if
NOx reduction can be effectively operated. The controller then generates
appropriate control signals representative of the result of the comparison.
Means-are provided to be responsive to the control signals for operating the
EGR unit (and/or engine timing changes), as called for by the controller.
Figure 1 shows, as representative of these latter means, valve 42.
The EGR unit and/or engine timing adjustments can be controlled, in
response to a feed-forward controller in response to a number of measured
parameters, including: engine load as represented by various mechanical
or electronic measures such as fuel flow, tack or pulse width, engine speed,
intake air temperature; barometric pressure; intake air humidity; exhaust gas
temperature and/or other parameters effective for particular engines. In
addition, to the extent that sensors are available, trim or feed back control
can be provided based on residual gas species following the CPO, e.g., the
level of NOx, HC or CO. If desired, feedback control can be employed to
trim the system in response to specific gas species, or any other measurable
engine or exhaust gas property.
The above description is for the purpose of teaching the person of
ordinary skill in the art how to practice the present invention, and it is not
intended to detail all of those obvious modifications and variations of it
which
will become apparent to the skilled worker upon reading this description. It
is intended, however, that all such modifications and variations be included
within the scope of the present invention which is defined by the following
claims.
