U.S. patent application number 10/293020 was filed with the patent office on 2004-05-13 for internal combustion engine with scr and integrated ammonia production.
Invention is credited to Mulligan, D. Neal.
Application Number | 20040088970 10/293020 |
Document ID | / |
Family ID | 32229573 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040088970 |
Kind Code |
A1 |
Mulligan, D. Neal |
May 13, 2004 |
INTERNAL COMBUSTION ENGINE WITH SCR AND INTEGRATED AMMONIA
PRODUCTION
Abstract
NO.sub.x emissions from an internal combustion engine fueled by
a gaseous hydrocarbon fuel can be reduced by catalytically
producing a hydrogen and carbon monoxide fuel gas stream from the
gaseous hydrocarbon fuel and a portion of the hot exhaust gas from
the internal combustion engine. Furthermore, ammonia is also
produced catalytically by reacting a portion of the hydrogen
produced with ambient nitrogen present in the exhaust gas. The
ammonia produced is used in connection with a selective catalytic
reduction reactor to treat the remaining hot exhaust gas produced
from the internal combustion engine, resulting in a treated exhaust
gas stream having near-zero NO.sub.x emissions.
Inventors: |
Mulligan, D. Neal; (Reno,
NV) |
Correspondence
Address: |
CHRISTIE, PARKER & HALE, LLP
P.O. BOX 7068
PASADENA
CA
91109-7068
US
|
Family ID: |
32229573 |
Appl. No.: |
10/293020 |
Filed: |
November 13, 2002 |
Current U.S.
Class: |
60/286 ; 60/276;
60/295; 60/301 |
Current CPC
Class: |
F02M 21/0227 20130101;
Y10S 123/12 20130101; F01N 2610/04 20130101; Y02T 10/32 20130101;
F01N 2610/03 20130101; F01N 2240/02 20130101; F01N 2610/02
20130101; F01N 2240/25 20130101; F02M 21/0206 20130101; Y02T 10/30
20130101; F01N 3/2073 20130101; F01N 2240/30 20130101; F01N 2610/08
20130101 |
Class at
Publication: |
060/286 ;
060/301; 060/276; 060/295 |
International
Class: |
F01N 003/00; F01N
003/10 |
Claims
What is claimed is:
1. An emission control system for use with a lean burn internal
combustion engine of the type receiving a source of fuel and
producing a hot exhaust gas stream containing water vapor,
nitrogen, NO.sub.x, and oxygen, the emission control system
comprising: a first reactor adapted to receive a first portion of
the hot exhaust gas stream and a fuel gas to produce an
intermediate exhaust stream containing hydrogen and ammonia; a
separator adapted to separate the intermediate exhaust stream to a
hydrogen-rich stream and an ammonia-rich stream; a conduit adapted
to recycle the hydrogen-rich stream to the internal combustion
engine as at least a portion of the source of fuel; and a selective
catalytic reduction reactor adapted to treat a second portion of
the exhaust gas stream with the ammonia-rich stream and produce a
treated exhaust stream substantially free of NO.sub.x.
2. The emission control system of claim 1 wherein the fuel gas is
selected from the group consisting of methane, ethane, propane,
butane, and combinations thereof.
3. The emission control system of claim 1 wherein the fuel gas is
natural gas.
4. The emission control system of claim 1 wherein the first reactor
includes a first reaction section adapted to reform at least a
portion of the fuel gas and water vapor to hydrogen, and a second
reaction section adapted to convert at least a portion of the
nitrogen and hydrogen to ammonia
5. The emission control system of claim 1 wherein the first reactor
further produces carbon monoxide.
6. The emission control system of claim 1 wherein the separator
further comprises a heat exchanger adapted to condense water and
ammonia from the intermediate exhaust stream, thereby producing the
ammonia-rich stream.
7. The emission control system of claim 1 wherein at least about 2%
of the fuel gas is converted in the first reactor.
8. The emission control system of claim 1 further comprising an
oxidation reactor for oxidizing carbon monoxide in the treated
exhaust stream, thereby producing a treated exhaust gas stream
substantially free of carbon monoxide.
9. The emission control system of claim 1 wherein the hot exhaust
gas stream contains at least about 0.5% oxygen.
10. The emission control system of claim 1 wherein the internal
combustion engine includes a plurality of combustion chambers, the
first portion of the hot exhaust gas stream is produced from at
least one combustion chamber and the second portion of the hot
exhaust gas stream is produced from the remaining combustion
chambers.
11. A method for reducing the emissions from a lean burn internal
combustion engine of the type receiving a source of fuel and
producing a hot exhaust gas stream containing water vapor,
nitrogen, NO.sub.x, and oxygen the method comprising the steps of:
a first reaction step for reacting a first portion of the hot
exhaust gas stream with a source of fuel gas in a reactor to
produce an intermediate exhaust stream containing hydrogen and
ammonia; separating the reformed exhaust stream to a hydrogen-rich
stream and an ammonia-rich stream; recycling the hydrogen-rich
stream to the internal combustion engine as at least a portion of
the source of fuel; and a second reaction step for reacting a
second portion of the exhaust gas stream with the ammonia-rich
stream to produce a treated exhaust stream substantially free of
NO.sub.x.
12. The method of claim 11 further comprising the step of selecting
a fuel gas from the group consisting of methane, ethane, propane,
butane, and combinations thereof.
13. The method of claim 11 wherein the fuel gas is natural gas.
14. The method of claim 11 wherein the first reaction step
comprises a reforming reaction step for reforming hydrocarbon fuel
and water vapor to hydrogen, and an ammonia production step for
converting nitrogen to ammonia.
15. The method of claim 11 wherein the first reaction step further
produces carbon monoxide.
16. The method of claim 11 wherein the separating step further
includes a step for condensing water and ammonia from the
intermediate exhaust stream.
17. The method of claim 11 wherein at least about 2% of the fuel
gas is converted by the first reaction step.
18. The method of claim 11 further comprising an oxidation step for
oxidizing carbon monoxide in the treated exhaust stream, thereby
producing a treated exhaust gas stream substantially free of carbon
monoxide.
19. The method of claim 11 wherein the hot exhaust gas stream
contains at least about 0.5% oxygen.
20. The method of claim 11 further comprising the step of selecting
an internal combustion engine with a plurality of combustion
chambers, wherein the first portion of the hot exhaust gas stream
is produced from at least one combustion chamber and the second
portion of the hot exhaust gas stream is produced from the
remaining combustion chambers.
Description
FIELD OF THE INVENTION
[0001] This invention relates to an emission control system for use
with a gaseous fuel powered internal combustion engine. More
particularly, it relates to a system for producing a source of
hydrogen for supplementing the fuel for the engine and for
producing a source of ammonia for use in a selective catalytic
reduction reactor used to treat the exhaust gas stream, thereby
producing an exhaust gas with near-zero NOx emissions.
BACKGROUND OF THE INVENTION
[0002] Hydrogen is known to burn cleaner in internal combustion
engines than more traditional fuels such as gasoline and diesel.
However, hydrogen is difficult to store, and convenient sources of
hydrogen are not readily available. Light hydrocarbons such as
methane, propane and butane, and mixtures of light hydrocarbons
such as natural gas are more readily available than hydrogen and
easier to store than hydrogen. While such fuels tend to burn
cleaner than gasoline or diesel, such gases are not as clean
burning as hydrogen. Current production engines can often use such
alternative fuels without any substantial engine modification, and
when operating under lean-burn conditions, such fuels can result in
low emission levels that are below current legal standards.
[0003] According to U.S. Pat. Nos. 5,660,602; 5,666,923 and
5,787,864 which are incorporated by reference, a clean burning
alternative gaseous fuel is disclosed for use in internal
combustion engines. Such a fuel includes approximately 21 to 50%
hydrogen and the balance natural gas.
[0004] According to U.S. Pat. No. 6,397,790 and pending application
Ser. No. 09/541,541 which are incorporated by reference, apparatus
and methods can be used to produce hydrogen from gaseous fuels for
combustion in internal combustion engines. According to these U.S.
patents and U.S. Pat. No. 6,405,720 which is also incorporated by
reference, emissions can further be reduced by using high levels of
exhaust gas recirculation in internal combustion.
SUMMARY OF THE INVENTION
[0005] According to the present invention, a method and apparatus
for reducing the emissions from a lean burn internal combustion
engine are set forth. The internal combustion engine is generally
of the type that burns a mixture of fuel and air, producing a hot
exhaust gas stream containing steam, nitrogen, NOx, and oxygen. The
method includes a reaction step for reacting a first portion of the
hot exhaust gas stream with a source of fuel gas in a reactor to
produce an intermediate exhaust stream containing hydrogen. At
least a portion of the hydrogen is further reacted with nitrogen to
form ammonia. The resulting intermediate exhaust gas stream which
now contains ammonia and hydrogen is cooled, condensing
ammonia-saturated water. The remaining gaseous components
consisting of hydrogen, ammonia, nitrogen, carbon dioxide, and
unreacted fuel are then recycled to the internal combustion engine
to either supplement the fuel to the internal combustion engine, or
if a sufficient amount of combustibles are present, the recycled
gas is the exclusive source of fuel for the internal combustion
engine.
[0006] The condensed ammonia-saturated water is vaporized through
heat exchange with the intermediate exhaust stream leaving the
reactor. This vapor is then mixed with a second portion of the
exhaust gas stream where it is reacted in a selective catalytic
reduction reactor to produce a treated exhaust stream substantially
free of NOx. Optionally, the treated exhaust gas stream may be
further treated by known oxidation reactions to remove any
remaining carbon monoxide. The source of oxygen for such oxidation
reactions is the hot exhaust gas stream when the engine is operated
under lean burn conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The drawing FIGURE is a schematic representation of the
emission control system of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0008] Referring to the drawing FIGURE, an internal combustion
engine using the emission control system of the present invention
is illustrated schematically. The invention is intended for use
with a lean-burn internal combustion engine 10 of the type that
includes an intake manifold 12 for receiving combustion air 14 and
fuel, and which produces a hot exhaust gas stream 16. Examples of
such internal combustion engines include piston engines, rotary
engines, and turbine engines. The primary constituents of such a
hot exhaust gas stream are water vapor, nitrogen, and carbon
dioxide. Because the internal combustion engine is operated under
lean-bum conditions, the hot exhaust gas stream will also include
an amount of unused oxygen. Additionally, pollutants such as oxides
of nitrogen (NO.sub.x) and carbon monoxide are generally found in
the hot exhaust gas stream. When using the emission control system
of the present invention, the volumetric concentrations of the
various constituents of the hot exhaust gas stream will be between
about 6 and 7% carbon dioxide, between about 72 and 75% nitrogen,
between about 5 and 8% water vapor, between about 0.5 and 10%
oxygen, between about 1 ppm and 10 ppm NO.sub.x, and between about
250 and 800 ppm carbon monoxide. For purposes of this
specification, unless otherwise set forth, all percentages and
concentrations are provided on a volumetric basis.
[0009] According to the embodiment illustrated, the hot exhaust gas
stream exits the internal combustion engine and splits into two
streams. The first hot exhaust gas stream 18 is combined with a
source of hydrocarbon fuel gas 22 and fed to a catalytic reactor
24. Preferred hydrocarbon fuel gases include methane, propane,
butane and combinations of such gases. A particularly preferred
hydrocarbon fuel gas is natural gas which consists primarily of
methane with small amounts of ethane, propane, butanes, higher
order hydrocarbons such as pentanes and hexanes, and relatively
inert gases and impurities such as carbon dioxide. One skilled in
the art will recognize that while the term "hydrocarbon fuel gas"
is used in this specification, the term is meant to broadly refer
to fuels such as propane, butane, and higher order hydrocarbons
which can be stored in a liquid form, but which are readily
vaporized to a gaseous form.
[0010] In the catalytic reactor, a portion of the water vapor from
the hot exhaust gas stream reacts with the hydrocarbons of the
hydrocarbon fuel gas to form hydrogen. This is achieved by
well-known reforming reactions. For methane, the reaction is a
two-step reaction with the first step being the steam reforming
reaction:
CH.sub.4+H.sub.2O=>3H.sub.2+CO
[0011] In the second step, a portion of the carbon monoxide is
further reacted with additional water vapor according to the
water-gas shift reaction:
CO+H.sub.2O=>H.sub.2+CO.sub.2
[0012] In order to support the reactions set forth above, in one
embodiment of the invention, the ratio of hydrocarbon fuel gas to
hot exhaust gas entering the catalytic reactor is maintained
between about 0.02 and 0.35.
[0013] In addition to providing the necessary water vapor for the
above reactions, the high temperature of the hot exhaust gas stream
supplies at least a portion of the heat necessary to promote the
endothermic part of the reforming reaction. Additional heat may be
provided by the partial oxidation of a portion of the hydrocarbons
from the fuel gas. Because the internal combustion engine operates
under lean-burn conditions, the unused oxygen present in the hot
exhaust gas is generally sufficient to promote some partial
oxidation reactions in the catalytic reactor without the need for
additional oxygen. Not only does the partial oxidation reaction
supply additional heat to the reactor, it has the added benefit of
producing additional hydrogen and carbon monoxide. The partial
oxidation reaction for methane can be described by the following
reaction:
CH.sub.4+1/2O.sub.2=>2H.sub.2+CO
[0014] As mentioned above, the reforming reactions are generally
endothermic. Without the introduction of additional heat to the
reactants, the temperature of the reactor can tend to drop. As the
reaction temperature drops, the reaction equilibrium tends to shift
to favor formation of hydrocarbons rather than hydrogen and carbon
monoxide. In one embodiment of the invention, the reactor
temperature is controlled by increasing or decreasing the partial
oxidation reactions. This is done by controlling the amount of air
introduced to the internal combustion engine and more particularly
by controlling the fuel to air ratio for the internal combustion
engine. By increasing the air levels relative to the fuel levels,
the levels of oxygen in the exhaust will tend to increase,
promoting the partial oxidation reactions and raising the reactor
temperature. In yet another embodiment, an amount of additional air
or other source of oxygen can be introduced directly to the
reactor.
[0015] In addition to using partial oxidation to supplement the
heat provided by the first hot exhaust gas stream, the heat
remaining in the balance of the hot exhaust gas stream can be
recovered and used as heat for the reforming reactions. In one
example, such heat can be recovered by passing the balance of the
hot exhaust gas stream through a heat exchanger to heat the
hydrocarbon fuel gas. In another example, the catalytic reactor can
be provided with multiple stages. Between each stage, the
intermediate reactants can be heated in a heat exchanger through
which the balance of the hot exhaust gas stream is passed. In still
another example, the catalytic reactor may include a jacket through
which the balance of the hot exhaust gas stream is passed.
[0016] The catalytic reactor can take various forms. In exemplary
embodiments, a suitable reforming catalyst is provided on Raschig
rings packed within a reactor vessel, or on a ceramic, cordierite,
or metal matrix monolith construction placed within a reactor
vessel. Suitable catalysts include any metal which promotes the
reformation reaction. Preferred catalysts include nickel-based
catalysts. According to one embodiment, about 10% of the
hydrocarbons in the fuel gas are converted by the reactions
described above. In another embodiment, a suitable catalyst,
reactor space velocity, and reactor temperature is chosen to
selectively convert particular hydrocarbons in the hydrocarbon fuel
gas. For example, because methane requires a relatively high
temperature to convert, reactor parameters can be chosen to
selectively convert the non-methane hydrocarbons in the hydrocarbon
fuel gas such that little of the methane is converted. This is
particularly beneficial when the natural gas contains high amounts
of non-methane hydrocarbons.
[0017] The catalytic reactor will generally operate at a fairly
high temperature in the range of about 450.degree. to 800.degree.
C. The optimum temperature will depend to some extent on the space
velocity within the reactor as well as the level of reformation
desired. While higher temperatures generally promote the desired
reforming reactions, high reactor temperatures can result in higher
exhaust gas NO.sub.x emissions. Therefore, a preferred range of
operation for the reactor is about 500.degree. to about 600.degree.
C.
[0018] In addition to catalytically reforming the fuel gas to
produce a syngas containing hydrogen and carbon monoxide, the
catalytic reactor also converts a portion of the nitrogen present
in the hot exhaust gas to ammonia by the following reaction:
N.sub.2+3H.sub.2=>2NH.sub.3
[0019] According to one embodiment, this reaction takes place
simultaneously with the reforming reaction through the use of a
suitable catalyst. In another embodiment, separate catalytic
reactors may be used, the first for producing hydrogen and carbon
monoxide by the reforming reactions and the second for producing
ammonia. In yet another embodiment, the reforming reactions and
ammonia-production reactions are carried out in two different
reaction zones within a single catalytic reactor. In one example of
such an embodiment, a single reactor includes two catalytic
sections arranged in series. Suitable catalysts for the ammonia
production reaction are nickel and/or ruthenium-based, and may be
provided on a suitable carrier such as those disclosed above for
the reforming catalyst.
[0020] According to the above reactions, the reactor effluent 26
contains unreacted fuel gas, hydrogen, carbon monoxide, ammonia,
water vapor, nitrogen, carbon dioxide and possibly other
constituents such as NO.sub.x. The temperature of the reactor
effluent will generally be in the range of 400.degree. to
600.degree. C. The reactor effluent will contain sufficiently high
levels of water vapor that, if recycled directly to the internal
combustion engine, some of the water vapor might condense, leading
to poor engine performance. Therefore, at least a portion of the
water vapor should be removed from the reactor effluent. According
to such an embodiment, the reactor effluent is cooled and the
resulting condensate and gas mixture are separated from one
another. Since ammonia has an affinity for water, an additional
benefit of such a separation step is to separate at least a portion
of the ammonia from the reactor effluent. The result of the
separation step is to produce an ammonia-rich stream of condensed
water 28 and a hydrogen-rich recycle gas stream 32. In order to
remove a sufficient amount of water vapor from the reactor effluent
to permit high levels of recycle, it is generally desired to cool
the reactor effluent to a temperature in the range of about
13.degree. to 50.degree. C.
[0021] In the embodiment shown in the drawing FIGURE, the
separation of the ammonia-rich stream of water from the
hydrogen-rich recycle gas stream is achieved by cooling the reactor
effluent in a heat exchanger (36). In this heat exchanger, the
reactor effluent is cooled using ambient air. The cooled reactor
effluent then passes into a separator 38 where the hydrogen-rich
recycle gas stream exits from the top of the separator and the
ammonia-rich condensed water stream is drawn from the bottom of the
separator. The liquid ammonia-rich stream of water exiting
separator 38 then passes through a second heat exchanger 34 where
it is vaporized by heat transfer with the hot reactor effluent
gases. It should be noted that not only does heat exchanger 34
vaporize the ammonia-water mixture, it also cools the reactor
effluent gases thereby reducing the size required for heat
exchanger 36. While the drawing illustrates the use of a pair of
heat exchangers and a separator, any number of different
arrangements of apparatus may be used for the separation step such
as a combination of one or more heat exchangers with one or more
integral or distinct separator vessels.
[0022] The hydrogen-rich recycle gas stream is then recycled back
to the intake manifold of the internal combustion engine where it
mixes with combustion air. According to the drawing, the
hydrogen-rich recycle gas stream is supplemented by a stream of
fuel gas 48. However, in another embodiment, the hydrogen-rich
recycle gas is the sole source of fuel for the internal combustion
engine. The molar ratio of methane to hydrogen plus carbon monoxide
in the fuel fed to the internal combustion engine generally ranges
between about 1 and 4, and is preferably about 2.4. When the
specific power requirements of the engine are low, a ratio of about
4 may be used. Such ratios can be controlled either by adding
supplemental fuel gas to the intake manifold of the internal
combustion engine, or by controlling reaction conditions in the
reforming reactor.
[0023] Within the internal combustion engine, the air and fuel are
combusted by well-known techniques to produce useful work and the
hot exhaust gas stream. In one embodiment of the invention, the
first hot exhaust gas stream constitutes between about 10 and 50%,
and preferably between about 20 and 30% of the total hot exhaust
gas stream exiting the internal combustion engine. According to
another embodiment, the hydrogen-rich recycle gas stream is
combined with air in a ratio between about 0.1 and 1 and more
preferably between about 0.4 and 0.67.
[0024] Because the hot exhaust gas stream contains high levels of
noncombustible gases such as water vapor, carbon dioxide, and
nitrogen, high levels of charge dilution are achieved. This in turn
results in an increased heat capacity for the gases fed to the
internal combustion engine. This increase in heat capacity reduces
the peak temperature of the combustion process which is very
important in reducing NO.sub.x emissions from the internal
combustion engine.
[0025] However, while some level of charge dilution is desired, it
should also be noted that too much dilution of the combustion
charge can adversely affect engine performance by causing misfire.
This is often the case for internal combustion engines operating on
traditional hydrocarbon fuels. Misfire generally results when the
charge mixture becomes too lean to support complete combustion.
This not only results in a severe drop in engine efficiency, but
also results in high emissions of unburned hydrocarbons in the
engine exhaust. Consequently, while high levels of charge dilution
are desired to produce low NO.sub.x emissions, near-zero NO.sub.x
emissions are difficult to achieve because of misfire. However,
because hydrogen has a very broad range of flammability, far wider
than the flammability ranges of most hydrocarbons, the inclusion of
hydrogen in the fuel charge fed to the internal combustion engine
permits an engine to operate at higher levels of charge dilution
without misfire.
[0026] According to one embodiment of the invention, when operating
the internal combustion engine, it is desired to operate the engine
under a sufficiently lean burn conditions to provide about 0.5 to
11% oxygen in the exhaust gas. The amount of lean burn is
controlled by regulating the amount of fuel introduced to the
reactor, the amount of fuel introduced directly to the intake
manifold, and optionally the amount of air entering the intake
manifold. The appropriate amount of fuel can be determined by an
oxygen sensor placed in the hot exhaust gas stream.
[0027] Alternatively, the amount of lean burn can be regulated by
knowing the mass flow rate of air and fuel to the internal
combustion engine. According to an embodiment of the present
invention, an equivalence ratio ((I)) based on fuel to air ratio
compared to stoichiometric on a mass basis of 0.98 will produce
approximately 0.5% oxygen in the exhaust gas and an equivalence
ratio of 0.5 will produce approximately 11% oxygen in the exhaust
gas.
[0028] It should also be noted that while the drawing FIGURE shows
the hydrogen-rich recycle gas entering the internal combustion
engine through an intake manifold, various other configurations are
possible for introducing the fuel to the engine. For example, the
hydrogen-rich recycle gas may be directly injected into one or more
combustion chambers of the internal combustion engine. Similarly,
while the hot exhaust gas stream may be drawn from a single exhaust
manifold and then split to produce the first and second hot exhaust
gas streams, other configurations are possible. As one example, if
the internal combustion engine is a multiple cylinder engine with a
combustion chamber associated with each cylinder, one or more
combustion chambers can be dedicated to producing the first hot
exhaust gas stream while the remaining combustion chamber or
chambers are dedicated to producing the second hot exhaust gas
stream. In such a configuration, the two sets of combustion
chambers can be independently operated with different air to fuel
ratios, different ignition timing, and even different fuels to
optimize engine performance or minimize emissions. For example, if
the internal combustion engine is a four cylinder spark ignition
engine, the first cylinder can be dedicated to producing the first
hot exhaust gas stream which is fed to the catalytic reactor while
the second, third, and fourth cylinders are dedicated to producing
the second hot exhaust gas stream. In such an embodiment, the
second, third and fourth cylinders can be operated for optimum
engine performance even though such operation may result in higher
NO.sub.x levels than the first cylinder. However, the inclusion of
the SCR reactor in the emission control system will insure that the
treated exhaust stream achieves near-zero NO.sub.x levels.
[0029] After being heated in heat exchanger 34 against the hot
reactor effluent, the ammonia-rich water (steam) stream is combined
with the second hot exhaust gas stream 42 in a selective catalytic
reduction (SCR) reactor. There, the NO.sub.x present in the second
hot exhaust gas stream reacts with the ammonia according to known
reactions to produce nitrogen and water. For nitric oxide and
nitrogen dioxide, the reactions are as follows:
4NH.sub.3+4NO+O.sub.2=>4N.sub.2+6H.sub.2O
4NH.sub.3+2NO.sub.2+O.sub.2=>3N.sub.2+6H.sub.2O
[0030] Other oxides of nitrogen react similarly with ammonia and
oxygen to produce nitrogen and water as is well known in the art.
The treated exhaust 46 produced from the SCR reactor primarily
contains nitrogen, water vapor, carbon dioxide and oxygen, and is
substantially free of NO.sub.x. Some carbon monoxide may be
present, and so, such carbon monoxide can optionally be oxidized to
carbon dioxide by the remaining oxygen present in the treated
exhaust using a catalytic converter (not shown.) For purposes of
this description, the term "near-zero" when used to describe
NO.sub.x emissions means that the given stream contains less than
about 10 ppm NO.sub.x, and when used to describe CO emissions means
that the given stream contains less than about 5 ppm CO.
[0031] While the invention has been described in terms of preferred
embodiments, the claims appended hereto are intended to encompass
other embodiments which fall within the spirit of the
invention.
* * * * *