U.S. patent application number 10/178130 was filed with the patent office on 2003-12-25 for method for methane oxidation and, apparatus for use therewith.
Invention is credited to Heeps, Andrew, Lyubovsky, Maxim, Roychoudhury, Subir.
Application Number | 20030235523 10/178130 |
Document ID | / |
Family ID | 29734596 |
Filed Date | 2003-12-25 |
United States Patent
Application |
20030235523 |
Kind Code |
A1 |
Lyubovsky, Maxim ; et
al. |
December 25, 2003 |
Method for methane oxidation and, apparatus for use therewith
Abstract
The invention is a method and apparatus for use therewith for
the combustion of methane. The method employs reformation of
methane and oxygen in fuel-rich proportions into carbon monoxide
and hydrogen and residual methane. The carbon monoxide, hydrogen
and residual methane is then combined with oxidant in fuel lean
proportions to continue oxidation in a porous media that absorbs
some of the heat of oxidation and radiates the heat as infrared
radiation.
Inventors: |
Lyubovsky, Maxim; (North
Haven, CT) ; Roychoudhury, Subir; (Madison, CT)
; Heeps, Andrew; (Farmington, CT) |
Correspondence
Address: |
McCormick, Paulding & Huber
City Place II
185 Asylum Street
Hartford
CT
06103-3402
US
|
Family ID: |
29734596 |
Appl. No.: |
10/178130 |
Filed: |
June 24, 2002 |
Current U.S.
Class: |
422/173 |
Current CPC
Class: |
Y02E 20/344 20130101;
F23C 13/00 20130101; Y02E 20/34 20130101; F23C 2900/03002 20130101;
F23C 2900/06041 20130101; F23C 6/045 20130101 |
Class at
Publication: |
422/173 |
International
Class: |
F01N 003/10 |
Claims
What is claimed is:
1. A method for combustion of a fuel including methane, the method
comprising the steps of: providing a fluid stream that includes
fuel and oxygen in fuel rich proportions to a reformation reactor
having a catalyst therein upon which at least a portion of the fuel
stream contacts; reforming, via catalytic reaction, at least a
portion of the methane in the fuel into carbon monoxide and
hydrogen to form an exhaust stream having various fuel constituents
therein; associating oxygen with the exhaust stream, the oxygen
being provided in quantities that cause the ratio of oxygen and the
fuel constituents in the exhaust stream to be in fuel lean
proportions; oxidizing at least a portion of the fuel constituents
in the exhaust stream within a porous media creating a heat of
reaction; and radiating at least a portion of the heat of reaction
from the porous media.
2. The method of claim 1 having the additional step of preheating
the fluid stream prior to the reforming step.
3. The method of claim 1 wherein the porous media has a catalyst
positioned on the surface thereof.
4. A method for combustion of a fuel including methane, the method
comprising the steps of: providing a fluid stream including fuel
and oxygen in fuel rich proportions; reforming at least a portion
of the methane in the fluid stream into carbon monoxide and
hydrogen to create an exhaust stream from the catalyst having
various fuel constituents therein; associating oxygen with the
exhaust stream, the oxygen having a volume in fuel lean proportions
to the fuel constituents within the exhaust stream; oxidizing at
least a portion of the fuel constituents within a porous media
creating a heat of reaction; and radiating at least a portion of
the heat of reaction from the porous media.
5. The method of claim 4 having the additional step of preheating
the fluid stream prior to the reforming step.
6. The method of claim 4 having an additional step of dispersing
the exhaust stream prior to associating with oxygen.
7. A catalytic burner comprising: a reformation reactor having a
catalyst therein suitable for the converting of at least a portion
of the methane in the fuel stream including methane and oxygen in
fuel rich proportions to carbon monoxide and hydrogen for creating
an exhaust stream; a manifold having a plurality of discharges, the
manifold in fluid communication with the exhaust stream and
defining a plurality discharges; a porous media: means defining a
flow path between at least some of the discharges and the porous
media; and means for introducing oxygen into the flow path such
that the exhaust stream and oxygen are in fuel lean
proportions.
8. The catalytic burner of claim 7 wherein the porous media is a
plurality of stacked short-channel screens.
9. The catalytic burner of claim 7 wherein the porous media has a
catalyst positioned on the surface thereof.
10. The catalytic burner of claim 7 further comprising a heat
exchanger, downstream of the porous media for receiving and passing
the fuel therethrough.
11. The catalytic burner of claim 8 wherein the heat exchanger is a
spiral shape tube.
12. The catalytic burner of claim 7 wherein the manifold includes a
hub having a plurality of spokes extending therefrom.
13. The catalytic burner of claim 7 wherein at least a portion of
the reformation reactor is positioned within the porous media.
14. A catalytic burner comprising: a reformation reactor for
reforming an inlet stream; a manifold having a plurality of
discharges; means defining a first flow path between the
reformation reactor and the manifold; a porous media: means
defining a second flow path between at least some of the discharges
and the porous media; and means for introducing oxidant into the
second flow path.
15. The catalytic burner of claim 14 wherein the porous media is a
plurality of stacked short-channel screens.
16. The catalytic burner of claim 14 wherein the porous media has a
catalyst positioned on the surface thereof.
17. The catalytic burner of claim 14 further comprising a heat
exchanger located downstream of the porous media.
18. The catalytic burner of claim 17 wherein the heat exchanger is
a spiral shape tube.
19. The catalytic burner of claim 14 wherein the manifold a hub
having a plurality of spokes extending therefrom.
20. The catalytic burner of claim 14 wherein at least a portion of
the reformation reactor is positioned within the porous media.
Description
FIELD OF THE INVENTION
[0001] The present invention is generally directed to catalytic
combustion, and more specifically to a method and an apparatus for
use therewith for the reformation of methane into partial oxidation
products and the oxidation of those products at a temperature below
the adiabatic temperature thereof.
BACKGROUND OF THE INVENTION
[0002] Methane is an abundant hydrocarbon that is used as a source
of fuel in numerous applications, such as industrial radiant
heaters, gas turbines, home furnaces and cooking equipment. While
methane can be made available in a relatively pure form, it is more
commonly provided as a constituent of natural gas, of which it is
the primary component.
[0003] Natural gas is typically combusted in an open flame, a
process referred to as diffusion burning, which generates certain
pollutants. One particularly undesirable class of pollutants formed
during diffusion burning is nitrous oxides, i.e. NOx. In diffusion
burning, NOx can be formed by any one of three possible mechanisms:
thermal, prompt, and fuel bound. The production of NOx by the
thermal and the prompt mechanisms, however, far exceeds that
produced from the fuel bound mechanism. Consequently, efforts to
reduce NOx pollution focus on reducing NOx formation by the thermal
and/or the prompt mechanisms.
[0004] NOx produced by the thermal mechanism, i.e. thermal NOx, is
often the dominant mechanism. Thermal NOx is formed when the heat
being released by diffusion burning is sufficient to provide the
necessary energy for the nitrogen in the air to combine with the
oxygen in the air. Generally, at flame temperatures below 1700 K,
the production of thermal NOx is insignificant. However, as flame
temperatures increase, the production of thermal NOx increases
sharply.
[0005] Thermal NOx production can be controlled by regulating
reactant stoichiometry. To burn a fuel it must be mixed with an
oxidant. For example, to burn methane oxygen must be provided. The
ratio of the fuel and oxidant, that is methane and oxygen, is the
reactant's stoichiometry. Reactant stoichiometry is expressed in
terms of a fuel/oxidant equivalence ratio, or where the oxidant is
oxygen as a constituent of air--fuel/air ratio. The fuel/oxidant
equivalence ratio is the ratio of the actual fuel/oxidant ratio to
the stoichiometric fuel/oxidant ratio. For example in the case of
methane (CH.sub.4), the combustion reaction is
CH.sub.4+2O.sub.2.fwdarw.CO.sub.2+2H.sub.2O. Therefore, a
stoichiometric fuel/oxidant ratio is one part CH.sub.4 and two
parts O.sub.2. Thus, if a mixture had this ratio of CH.sub.4 and
O.sub.2, the reactant stoichiometry as expressed by the
fuel/oxidant ratio of the mixture would be 1.0 (an actual mixture
having these proportions would be referred to as
stoichiometric).
[0006] A mixture having an equivalence ratio greater than 1.0 is
fuel rich, i.e., in the case of the above methane reaction more
than one part fuel for each two parts of oxygen, and a mixture
having an equivalence ratio less than 1.0 is fuel lean, i.e. in the
case of the above methane reaction less than one part fuel for each
two parts of oxygen. When combustion is adiabatic, stoichiometric
mixtures burn relatively hotter than non-stoichiometric mixtures
and the further away the mixture is from stoichiometric the
relatively cooler it burns.
[0007] NOx production by the prompt mechanism, i.e. prompt NOx, is
a fuel-rich, gas-phase phenomenon. The reaction is quick and
completes within the diffusion flame. The production of NOx by the
prompt mechanism can only be controlled if the diffusion flame is
eliminated, in whole or in part.
[0008] NOx formation from the combustion of methane could be
greatly reduced if methane could be combusted at temperatures below
1700 degrees K and diffusion flame could be avoided. It is well
known in the art that if methane is catalytically combusted, i.e.
oxidized in the presence of a catalyst, the energy within the
methane can be released without the formation, or limited
formation, of thermal and/or prompt NOx.
[0009] A problem, however, with the catalytic combustion of methane
is that methane is a very stable molecule. Thus, it is more
difficult to oxidize than higher order hydrocarbons, such as
propane. Methane can be catalytically combusted under fuel lean
conditions producing combustion temperatures below 1700 degrees K.
When a palladium-based catalyst is used the reaction may become
unstable due to properties of Pd-PdO transformation of the
catalyst. Hysteresis in the catalyst activity makes controlling the
reaction extremely difficult. Platinum based catalysts on the other
hand can provide more stable operation. However, volatility of Pt
at the desired temperatures under lean conditions is very high.
Thus, platinum catalyst lacks durability.
[0010] Based on the foregoing, it is an object of the present
invention to develop a method and an apparatus for the combustion
of methane that overcomes the problems and drawbacks associated
with the prior art.
SUMMARY OF THE INVENTION
[0011] The present invention is directed in one aspect to a method
for the combustion of methane. In the method, a fluid stream
including fuel having methane and oxygen that is in fuel rich
proportions, i.e. having a fuel/oxidant equivalence ratio greater
than 1.0, is provided. The fluid stream flows into a reformation
reactor having a catalyst therein that promotes the reformation of
methane (CH.sub.4) into carbon monoxide (CO) and hydrogen
(H.sub.2).
[0012] The catalyst reforms at least a portion of the methane in
the fluid stream into carbon monoxide and hydrogen creating an
exhaust stream exiting the reformation reactor having various fuel
constituents therein, such as unreformed methane, CO and H.sub.2.
The exhaust stream is then divided into a plurality of exhaust
streamlets by passing the exhaust stream into a manifold having a
plurality of discrete discharges. As a portion of the exhaust gas
exits through a discharge, an exhaust streamlet is formed.
Sufficient oxygen is then added to the exhaust streamlet such that
the fuel constituents therein and the oxygen are in fuel-lean
proportions. Same amount of oxygen should be added to each
streamlet, such that variations in the equivalence ratios between
the streamlets are small. The exhaust and second fluid are added
together, not mixed. In the present invention, it is desired that
the exhaust and the second fluid enter the porous media as distinct
flow steams. It is understood however, that the two fluids will be
in contact along an interface and that incidental diffusion of one
fluid into another will occur. It is expected that if sufficient
time is provided the diffusion combustion would occur at the
interface before the two streamlets can mix. To avoid gas phase
flame oxidation of the exhaust stream, which is undesirable in this
invention, combined stream formed after adding the second stream to
the exhaust stream should be passed into the porous media before
combustion takes place. Finally, at least a portion of the CO,
H.sub.2 and CH.sub.4 in the exhaust streamlets is oxidized by
passing the combined stream through a porous media that absorbs and
then radiates some of the heat generated by the oxidation.
[0013] A catalytic burner suitable for performing the above method
includes a reformation reactor incorporating a catalyst. A manifold
that receives the exhaust stream from the reformation reactor and
passes the exhaust stream through a plurality of discharges forming
part of the manifold thereby creating a plurality of exhaust
streamlets. The exhaust streamlets then enter a flow path where the
exhaust streamlets are directed into a proximally located porous
media. Means for introducing a second fluid into the flow path are
also provided.
[0014] The reformation reactor is a partial oxidation reactor. In a
partial oxidation reactor, the catalyst and its associated
geometry, e.g. substrate and dispersion thereon, defines an
activity relative to the flow rate, i.e., residence time, of the
methane/oxygen thereover such that when the catalyst and the
methane/oxygen interact partial oxidation products and not complete
oxidation products are predominantly formed. In the case of methane
and oxygen, partial oxidation products are H.sub.2 and CO, while
the compete oxidation products are H.sub.2O and CO.sub.2. An
example of a reformation reactor for methane suitable for this
application is disclosed in U.S. Pat. No. 5,648,582, the disclosure
of which is incorporated herein in its entirety.
[0015] As those skilled in the art will appreciate, the
selectivity, i.e. the ability to produce one product in favor of
another, in the reformation process can be manipulated by
controlling the temperature of the fluid stream. In the case of a
fluid stream including methane and oxygen in fuel rich proportions,
preheating of the fluid stream increases the selectivity in the
reformation of methane in favor of H.sub.2 and CO versus CO.sub.2
and H.sub.2O. Therefore, an enhancement to both the method and the
catalytic burner incorporates heating the fuel stream prior to its
entry into the reformation reactor.
[0016] The exhaust and the second fluid are mixing and reacting
inside the porous media to further oxidize at least part of the
exhaust stream to the complete oxidation products. The porous media
absorbs some of the heat created by the exothermic oxidation
reaction and emits it in the form of IR radiation, assuring that
the temperature remains below the adiabatic flame temperature
defined by the reactant stoichiometry of the fuel constituents and
oxygen. A porous media can be any media through which a gas can
flow, while continuously encountering solid surfaces. In other
words, porous media is comprised of alternating regularly or
randomly empty volumes and filled volumes. Empty volumes should
form a continuous network, such that the porous media remains
permeable to permit the flow of a fluid therethrough. The porous
media should have a pore size, which describes the average size of
the empty volume (if the pores size in not round the smaller
dimension), that is generally uniform, but small deviations are
acceptable. Porous media having a few large empty volumes and
otherwise generally uniform smaller volumes could be problematic.
The precise pore size, porosity (ratio of open volume to total
volume) and material is application dependent.
[0017] The material for the porous media should be chosen to
withstand the temperatures generated in the exothermic oxidation
process and effectively emit heat in the form of infrared radiation
(IR). Pore size and porosity are chosen large enough to minimize
pressure drop induced by the porous media but small enough when
compared to the total volume in which the oxidation reaction
between the exhaust and the second stream takes place.
[0018] As those skilled in combustion will readily appreciate, the
reformation reactor requires that the catalyst therein be at a
certain temperature to perform the reformation. The catalyst can be
brought to this temperature by any one or a combination of well
know procedures, such as heating the fluid stream, or direct
heating of the catalyst.
[0019] Regardless of the method chosen, the exhaust gas will have a
temperature upon exiting the catalyst equal to the operational
temperature chosen for the reformation reactor plus the exothermal
resulting from the exothermic oxidation process taking place
therein. It should be remembered that the proportions of fuel
constituents to oxygen within the exhaust stream are still be quite
rich, i.e., the initial stream had fuel rich proportions and
oxidant was consumed along with fuel creating a progressively
richer fuel stream as it passed through the reformation reactor.
Therefore, although the fuel constituents in the exhaust gas will
be quite hot, oxidation will not occur within the exhaust stream
until additional oxidant is added.
[0020] Where the fuel/oxygen stoichiometry, flow rate and IR
radiation are such that porous media is hot enough, oxidation of
fuel inside the porous media will occur upon contact with an
oxidant. Where the porous media is not hot enough to support
oxidation on contact with an oxidant, the porous media can utilize
a suitable oxidation catalyst to sustain the oxidation reaction. It
is understood that a catalyst can be used even if the fuel
constituents are hot enough to support combustion.
DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a side view of the catalytic burner of the present
invention.
[0022] FIG. 2 is a top view of the manifold of the present
invention taken along line 2-2 of FIG. 1.
[0023] FIG. 3. is a side view of a second embodiment of the
catalytic burner of the present invention.
[0024] FIG. 4 is a top view of the catalytic burner taken along the
line 4-4 of FIG. 3 showing the heat exchanger.
DETAILED DESCRIPTION
[0025] As shown in FIG. 1, the catalytic burner, generally referred
to by the reference number 10, is comprised of a reformation
reactor 12, a manifold 14 and a porous media 16. An inlet stream 18
enters the reformation reactor 12 by means of a flow path 20
creating an exhaust stream 24. The manifold 14 and reformation
reactor 12 are connected by a flow path 26 such that the exhaust
stream 24 enters the manifold 14 to exit through a plurality of
discharges 28 (See FIG. 2). Exhaust streamlets 30 are formed by the
discharges 28. The discharges 28 are positioned proximate the
porous media 16, and connected by a flow path 32, such that upon
exiting the discharges 28 the exhaust streamlets 30 enter an inlet
face 15 of the porous media 16.
[0026] An oxidant 34 flows around the manifold 14 permitting the
oxidant 34 to flow into the flow path 35 connecting the discharges
24 to the porous media 16. As shown in FIG. 2, the discharges 28
are positioned to disperse uniformly the exhaust stream 24 as
exhaust streamlets 30 over a dispersion area 36, defined by a
perimeter 38. The hub and spoke design of the manifold 14 assists
in distributing the exhaust streamlets 30 uniformly under the
porous media 16 across the inlet face 15, but the manifold 14 and
discharges 28 therefrom could be of any design, such as port
injectors positioned in the housing 12, thus the invention should
not be considered limited to the manifold 14 shown.
[0027] The flow path 32 and manifold 14 should cooperate to
uniformly disperse the oxidant 34 and exhaust stream 24 across the
inlet face 15 of the porous media 6. It is a feature of this
invention that the oxidant 34 and exhaust streamlets 30 be
associated, but not mixed in the flow path 32. Associated means
that the exhaust streamlets 30 and oxidant 34 are brought in
contact but are not provided sufficient time to inter-defuse, and
therefore, the exhaust streamlets 30 and oxidant 34 generally enter
the porous media 16 as discrete streams.
[0028] In the method of the present invention, the inlet stream 18
has methane and oxygen in fuel rich proportions. Preferably, the
oxygen is provided as a constituent of air. If desired, the methane
can be provided as a constituent of a blended fuel, such as natural
gas. Preferably, the methane and oxygen are highly mixed. The
operational perimeters of the reformation reactor 12, including the
catalyst therein, are selected such that some, or for all practical
purposes all, of the methane is converted primarily into CO and
H.sub.2 instead of CO.sub.2 and H.sub.2O. This creates an exhaust
stream 24 from the reformation reactor 12 having therein at least
the fuel constituents CO, and H.sub.2.
[0029] The fuel constituents in the exhaust stream 24 define an
adiabatic temperature. The exhaust stream 24 is then divided into
exhaust streamlets 30. The exhaust streamlets 30 are then
associated with additional oxygen, generally as a constituent of
air, in fuel lean proportion (exhaust stream to oxygen). It is
preferred that exhaust and oxygen are mixed in a proportion close
to stoichiometric with small excess oxygen. The exhaust streamlets
30 and additional oxygen then pass into the porous media 16 where
mixing and oxidation, which is exothermic, takes place. The porous
media 16 is constructed of materials that absorb some of the heat
of reaction, such that the oxidation occurring in the porous media
16 is below the adiabatic temperature of the fuel constituents. The
heat of reaction absorbed by the porous media 16 is radiated
therefrom in the form of infrared radiation.
[0030] As discussed above and shown in FIGS. 1 and 2, the catalytic
burner 10 has a plurality of discharges 28 that divide the exhaust
stream 24 into exhaust streamlets 30. In the context of the method,
the exhaust stream 24, which is in fuel rich proportion, has
associated with it a certain amount of energy. The energy density
of the exhaust stream is proportional to the amount of fuel passing
through a certain cross-sectional area per unit of time, i.e. to
the volumetric flow rate of the exhaust stream 24. U.S. Pat. No.
5,648,582 suggests that one essential feature of the reformation
reactor 12 is that the inlet stream 18 enters the reactor at very
high space velocity and the reformation reaction occurs at short
residence time. This provides that flow space velocity and
associated energy density in the exhaust stream 24 will also be
high. If the exhaust stream 24 were to be exposed to additional
oxidant as a single stream, excessive amount of heat, associated
with the oxidation reaction, would be released in a small volume of
the porous media 16. This excessive heat could cause deterioration,
or failure, of the porous media 16. The manifold 14 distributes the
exhaust stream 24 over the larger cross-sectional area, effectively
decreasing the energy density in the stream. The energy density
associated with individual exhaust streamlets 30 and any diffusion
flame that maybe associated therewith is considerably lower and may
be adjusted depending on the application. The discharges 28 can
also act as diffusers to reduce further the power density, i.e.,
power per area, of the exhaust stream 24.
[0031] FIG. 3 is a second embodiment of the catalytic burner which
is similar to the previous embodiment, therefore, like reference
number preceded by the number 1 are used to indicate like elements.
In this embodiment, the catalytic burner 110 is positioned in an
interior area 140 of a housing 142. Also positioned within the
interior area 140 is a heat exchanger 144. The reformation reactor
112 is positioned within the porous media 116 as opposed to under
it. In this embodiment, the inlet stream 118 enters a heat
exchanger 144 positioned within the interior area 140 adjacent the
porous media 116. The porous media 116 has a catalyst 146 deposited
on the surface thereof. The catalyst 146 is selected to support the
continued oxidation of the H.sub.2, CO and CH.sub.4 in the exhaust
streamlets 130. The inlet stream 118 flows through the heat
exchanger 144 prior to entering the reformation reactor 112.
[0032] As explained above, in the method of the present invention
an oxidation reaction occurs in the porous media 116. As such, some
of the heat of reaction 147 leaves the porous media 116 and is
conducted into contact with heat exchanger 144, where some of the
heat of reaction is transferred into the inlet stream 118 flowing
therein. Referring to FIG. 4, the heat exchanger 144 is comprised
of a tube 148 that has been formed into a flat coil about a center
point on an axis designated by the letter A.
[0033] The heat exchanger 144 could be of any other design, which
allows part of heat released in porous media 116 to be transferred
into the inlet stream 118, thus, the invention should not be
considered limited to the heat exchanger 144 shown.
[0034] Continuing with FIG. 3, the manifold 114 is adapted to
receive the exhaust stream 130 from the reformation reactor 112. In
this embodiment, the means for introducing additional oxidant 134
between the discharges 128 and the porous media 116 is by the
introduction of additional oxidant 134 into the housing 142 below
the discharges 128. Depending upon the method of operation, the
flow of additional oxidant 134 may be by natural convection or a
pump, such as a fan. In most cases, the introduction point is not
critical as oxygen as a constituent of air will be the oxidant 134
and the air will naturally flow to the desired location. Therefore,
the means could include passages in the housing, or the additional
oxidant 134 could flow from a point above the porous media 116 into
the housing 142.
[0035] In this embodiment, the reformation reactor 112 is shown
positioned within the porous media 116. This is not a requirement
of the invention, as the reformation reactor 112 could be
positioned anywhere including outside the interior area 40.
[0036] In the method of the present invention, this embodiment is
designed to provide the additional step of preheating of the inlet
gas stream 118 using some of the heat of reaction produced by the
exothermic reaction in the porous media 116. Preheating the inlet
stream 118 offers the advantage of increasing the selectively to CO
and H.sub.2 within the reformation reactor 12. This is but one
method of preheating, therefore the invention should not be
considered so limited. Preheating of the inlet stream 118 by other
means such as electric resistance are considered within the scope
of the invention. Preheating of the inlet stream can assist in
starting the catalytic burner.
[0037] The porous media 16, 116 is a media through which a gas can
flow. In the preferred embodiment, the porous media 16, 116 was
made from a plurality of stacked short-channel screens. The
invention should not be considered so limited however, as other
media could be used such as pellets, foams or gauzes and even a
single screen. Generally, porous media are graded by "pore size."
Another important parameter for this invention, however, is
consistency of pore size. The porous media 16, 116 is designed to
promote interaction of the fuel constituents within the exhaust
stream 24 with the additional oxidant 34, 134, extract heat from
the ongoing oxidation, and radiate infrared radiation. Further, the
porous media 16, 116 continually assures that the exhaust stream
24, 124 and oxidant 34, 134 are divided into small pockets. In
other words, the exhaust stream 24, 124 and oxidant 34, 134 cannot
reform into a large volume. These requirements mean that preferably
the pores within the porous media 16, 116 are generally uniform.
Pore size is chosen such that the pores are large enough to
minimize pressure drop but small enough to assure an acceptable
heat release within a pore.
[0038] Although the present invention has been described in
considerable detail with reference to certain preferred versions
thereof, other versions are possible, particularly versions having
more than two catalysts. Therefore, the spirit and scope of the
invention should not be limited to the description of the preferred
versions contained herein.
* * * * *