U.S. patent application number 10/337009 was filed with the patent office on 2004-07-08 for low nox burner.
Invention is credited to Carrera, Martin E., Griffiths, David Charles, Reid, I. A. B., Satek, Larry C., Smith, Geoffrey B..
Application Number | 20040131984 10/337009 |
Document ID | / |
Family ID | 32681141 |
Filed Date | 2004-07-08 |
United States Patent
Application |
20040131984 |
Kind Code |
A1 |
Satek, Larry C. ; et
al. |
July 8, 2004 |
Low NOx burner
Abstract
A process for operating a combustion burner employing a
combustion mixture of a carbonaceous fuel, hydrogen and a molecular
oxygen-containing gas and at least one solid combustion catalyst is
disclosed.
Inventors: |
Satek, Larry C.; (Fremont,
IN) ; Carrera, Martin E.; (Naperville, IL) ;
Reid, I. A. B.; (London, GB) ; Smith, Geoffrey
B.; (Surrey, GB) ; Griffiths, David Charles;
(Esher, GB) |
Correspondence
Address: |
BP America Inc.
Docket Clerk
Law Department, M.C. 2207A
200 East Randolph Drive
Chicago
IL
60601-7125
US
|
Family ID: |
32681141 |
Appl. No.: |
10/337009 |
Filed: |
January 6, 2003 |
Current U.S.
Class: |
431/4 |
Current CPC
Class: |
F23C 13/00 20130101;
F23C 2900/9901 20130101 |
Class at
Publication: |
431/004 |
International
Class: |
F23J 007/00 |
Claims
Having described the invention, what is claimed is:
1. A process for operating a combustion burner comprising:
combusting a carbonaceous fuel and hydrogen in the presence of at
least one solid combustion catalyst by bringing a combustion
mixture of the carbonaceous fuel, hydrogen and a molecular
oxygen-containing gas into contact with at least one aforesaid
solid catalyst at a space velocity in the range of from about 1000
to about 2.times.10.sup.6 volumes of combustion mixture per hour
per volume of catalyst, at a sufficient ratio of molecular
oxygen-to-each carbon atom of the carbonaceous fuel to ensure
complete combustion of the carbonaceous fuel to carbon dioxide and
water and at a sufficient ratio of hydrogen to each carbon atom of
the carbonaceous fuel such that the reaction between hydrogen and
oxygen generates sufficient exothermic heat to ignite the
carbonaceous fuel and with a stoichiometry in the range of from
about 0.45 to about 0.95 such that the carbonaceous fuel is
combusted at a temperature which minimizes the presence of NO.sub.x
in the stream of combustion products produced by the burner.
2. The process of claim 1 wherein the oxygen-containing gas is
air.
3. The process of claim 1 wherein the carbonaceous fuel is methane
or natural gas.
4. The process of claim 1 wherein the space velocity is in the
range of from about 1000 to about 5.times.10.sup.6 volumes of
combustion mixture per hour per volume of catalyst.
5. The process of claim 4 wherein the space velocity is in the
range of from about 5000 to about 3.times.10.sup.6 volumes of
combustion mixture per hour per volume of catalyst.
6. The process of claim 1 wherein the mole ratio of molecular
oxygen-to each carbon atom of the carbonaceous fuel is in the range
of from about 2:1 to about 100:1.
7. The process of claim 6 wherein the mole ratio of molecular
oxygen-to each carbon atom of the carbonaceous fuel is in the range
of from about 2:1 to about 50:1.
8. The process of claim 1 wherein the mole ratio of hydrogen to
each carbon atom of the carbonaceous fuel is in the range of from
about 0.01:1 to about 2:1.
9. The process of claim 8 wherein the mole ratio of hydrogen to
each carbon atom of the carbonaceous fuel is in the range of from
about 0.03:1 to about 1:1.
10. The process of claim 1 wherein the stoichiometry is in the
range of from about 0.60 to about 0.95.
11. The process of claim 1 wherein the at least one combustion
catalyst comprises a noble metal.
12. The process of claim 11 wherein the at least one combustion
catalyst comprises a platinum- or palladium-containing component or
both.
13. The process of claim 1 wherein a single combustion catalyst is
employed.
14. The process of claim 1 wherein two combustion catalysts are
employed.
15. A process for the production of a mono-olefin from a paraffinic
hydrocarbon having at least two carbon atoms in a gas stream
passing through a reactor heated by combustion products from a
combustion burner in which a carbonaceous fuel and hydrogen are
combusted in the presence of at least one solid combustion catalyst
by bringing a combustion mixture of the carbonaceous fuel, hydrogen
and a molecular oxygen-containing gas into contact with at least
one aforesaid solid catalyst at a space velocity in the range of
from about 1000 to about 2.times.10.sup.6 volumes of combustion
mixture per hour volume of catalyst at a sufficient ratio of the
molecular oxygen-to-each carbon atom of the carbonaceous fuel for
complete combustion of the carbonaceous fuel to carbon dioxide and
water and at a sufficient ratio of hydrogen to each carbon atom of
the carbonaceous fuel such that the reaction between hydrogen and
oxygen generates sufficient exothermic heat to ignite the
carbonaceous fuel and with a stoichiometry in the range of from
about 0.45 to about 0.95, such that the carbonaceous fuel is
combusted at a temperature which minimizes the presence of NO.sub.x
in the stream of combustion products produced by the burner.
16. The process of claim 15 wherein the oxygen-containing gas is
air.
17. The process of claim 15 wherein the carbonaceous fuel is
methane or natural gas.
18. The process of claim 15 wherein the space velocity is in the
range of from about 1000 to about 5.times.10.sup.6 volumes of
combustion mixture per hour per volume of catalyst.
19. The process of claim 15 wherein the mole ratio of molecular
oxygen-to-each carbon of the carbonaceous fuel is in the range of
from about 2:1 to about 100:1.
20. The process of claim 23 where in the mole ratio of
hydrogen-to-each carbon of the carbonaceous fuel is in the range of
from about 0.01:1 to about 2:1.
21. The process of claim 15 wherein the stoichiometry is in the
range of from about 0.60 to about 0.95.
22. The process of claim 15 wherein the at least one combustion
catalyst comprises a nobel metal.
23. The process of claim 22 wherein the at least one combustion
catalyst comprises a platinum- or palladium-containing component or
both.
24. The process of claim 15 wherein a single combustion catalyst is
employed.
25. The process of claim 15 wherein two combustion catalysts are
employed.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a process for operating a
burner for the combustion of a gaseous mixture comprising a
carbonaceous fuel with reduced NO.sub.x production and more
particularly concerns the use of such a burner in the manufacture
of olefins by cracking paraffins.
[0003] 2. Discussion of the Prior Art
[0004] Combustion burners are employed to provide heat for numerous
applications, such as gas turbines, furnaces, boilers and the like.
The combustion involves a molecular oxygen-containing gas,
generally air. One of the combustion products is NO.sub.x which is
an equilibrium mixture mostly of NO but also containing minor
amounts of NO.sub.2. Modern antipollution laws either currently in
effect or under consideration mandate the reduction of combustion
NO.sub.x.
[0005] It has been recognized that a fruitful way of controlling
NO.sub.x production is to limit the localized and bulk temperatures
in the combustion zone. There are a number of ways to control the
temperature, such as by dilution with excess air, controlled
oxidation using one or more catalysts, or staged combustion using
variously lean or rich fuel mixtures. Combinations of these methods
are also known.
[0006] One example is a combustion process disclosed in Dalla Betta
et al., U.S. Pat. No. 5,281,128 (Jan. 25, 1994) in which a
hydrocarbon fuel is combusted stepwise using specific catalysts and
catalytic structures and, optionally, a final homogeneous
combustion zone. In a first combustion zone, a combustible mixture
comprising a hydrocarbon fuel and an oxygen-containing gas is
contacted with a combustion catalyst comprising palladium, and
partially combusted gas from the first zone is contacted in a
second combustion zone-with a second combustion catalyst comprising
palladium and optionally one or more Group IB or Group VII metals.
Then partially combusted gas from the second combustion zone is
contacted in a third combustion zone with a third combustion
catalyst comprising platinum. Thereafter, optionally, if it is
desired to raise the temperature of gas exiting from the third
combustion zone, any remaining unoxidized hydrocarbon fuel is
oxidized in a fourth zone in the absence of a combustion
catalyst.
[0007] However, no example exists of the use of controlled
oxidation using one or more catalysts in combination with
modifications to the composition of the combustion mixture to
effect reduced NOx emissions, a smooth start up without thermal
shocks and stable operation of the burner.
OBJECTS OF THE INVENTION
[0008] It is therefore a general object of the present invention to
provide an improved process for operating a burner for the
combustion of a gaseous mixture comprising a carbonaceous fuel,
molecular oxygen-containing gas and hydrogen that affords the
aforesaid benefits.
[0009] More particularly, it is an object of the present invention
to provide an improved aforesaid process which permits, a smooth
start up without thermal shocks and a stable operation of the
burner.
[0010] It is a related object of the present invention to provide
an improved aforesaid process that affords reduced NO.sub.x
emissions through the use of controlled oxidation with at least one
combustion catalyst.
[0011] It is another object of the present invention to provide an
improved aforesaid process through the use of a combustion mixture
having a modified composition.
[0012] It is also an object of the present invention to provide an
improved process for the production of a mono-olefin from a
paraffinic hydrocarbon in a reactor that is heated by the
combustion products of an aforesaid burner.
[0013] Other objects and advantages of the present invention will
become apparent upon reading the following detailed description and
appended claims.
SUMMARY OF THE INVENTION
[0014] These objects are achieved by the method of this invention
for operating a combustion burner comprising: combusting a
carbonaceous fuel and hydrogen in the presence of at least one
solid combustion catalyst by bringing a combustion mixture of the
carbonaceous fuel, hydrogen and a molecular oxygen-containing gas
into contact with at least one aforesaid solid catalyst at a space
velocity in the range of from about 10.sup.4 per hour to about
2.times.10.sup.6 per hour, at a sufficient ratio of molecular
oxygen-to-carbonaceous fuel to ensure complete combustion of the
carbonaceous fuel to carbon dioxide and water and at a sufficient
ratio of hydrogen to molecular oxygen such that their reaction
generates sufficient exothermic heat to ignite the carbonaceous
fuel and with a stoichiometry (as defined below) in the range of
from about 0.45 to about 0.95 such that the carbonaceous fuel is
combusted at a temperature at which the presence of NOx in the
stream of combustion products produced by the burner is
minimized.
[0015] The present invention is also a process for the production
of a mono-olefin from a paraffinic hydrocarbon having at least two
carbon atoms in a gas stream passing through a reactor heated by
combustion products from a burner that is operated by the method
indicated hereinabove.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] For a more complete understanding of this invention,
reference should now be made to the embodiments illustrated in
greater detail in the accompanying drawings and described below by
way of examples of the invention. In the drawings:
[0017] FIG. 1 is a graph of the NO.sub.x concentration in the
combustion products from a burner versus the preheat temperature of
the combustion mixture entering the burner at a constant
stoichiometry of 0.75.
[0018] FIG. 2 is a graph of the NOx concentration in the combustion
products from a burner versus the stoichiometry of the combustion
mixture entering the burner.
[0019] It should be understood, of course, that the invention is
not necessarily limited to the particular embodiment illustrated in
the drawings
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] The combustion mixture employed in the method of the present
invention comprises hydrogen in addition to a carbonaceous fuel and
molecular oxygen-containing gas. The carbonaceous fuel employed may
be gaseous or liquid at normal temperature and pressure. Although
suitable carbonaceous fuels that normally are gaseous hydrocarbons,
for example, methane, ethane, and propane, are highly desirable as
a source of fuel for the process of this invention, most fuels
capable of being vaporized at the temperatures employed in the
method of this invention are suitable. For instance, the fuels may
be liquid or gaseous at room temperature and pressure. Examples
include the low molecular weight hydrocarbons mentioned above as
well as butane, pentane, hexane, heptane, octane, gasoline,
aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and
xylene; naphthas; diesel fuel, kerosene; jet fuels; other middle
distillates; heavy distillate fuels (preferably hydrotreated to
remove nitrogenous and sulfurous compounds); oxygen-containing
fuels such as alcohols including methanol, ethanol, isopropanol,
butanol, or the like; ethers such as diethylether, ethyl phenyl
ether, methyl tertiary butyl ether, etc. Low-BTU gases such as town
gas or syngas may also be used as fuels. Typically the fuel is
methane or natural gas. The molecular oxygen-containing gas can be
oxygen itself or oxygen with a diluent. Typically the molecular
oxygen-containing gas is air.
[0021] The mole ratio of molecular oxygen-to-carbonaceous fuel is
sufficient to ensure complete combustion of the carbonaceous fuel
to carbon dioxide and water. The mole ratio of molecular
oxygen-to-each carbon of the carbonaceous fuel is preferably in the
range of from about 2:1 to about 100:1 and more preferably in the
range of from about 2:1 to about 50:1, and most preferably in the
ranges of from about 2:1 to about 30:1. The mole ratio of hydrogen
to molecular oxygen is sufficient that the reaction therebetween
generates sufficient exothermic heat to initiate combustion of the
carbonaceous fuel. The mole ratio of hydrogen to molecular oxygen
is preferably in the range of from about 0.01:1 to about 2:1 and
more preferably in the range of from about 0.03:1 to about 1:1. The
mole ratio of molecular oxygen-to-the combination of hydrogen and
each carbon of the carbonaceous fuel is preferably in the range of
from about 0.5:1 to about 10:1, and more preferably is in the range
of from about 0.5:1 to about 8:1, and most preferably in the range
of from about 0.5:1 to about 3:1.
[0022] The presence of hydrogen in the combustion mixture is
essential in order to achieve the benefits of the method of this
invention. Sufficient hydrogen must be burned in order to generate
sufficient heat to bring the temperature of the combustion mixture
up to a temperature where the combustion of the carbonaceous fuel,
for example, methane, commences and to a higher temperature where
full combustion of the carbonaceous fuel, for example, methane,
occurs. The combustion of hydrogen over the catalyst serves to
spread the combustion of the carbonaceous fuel out, thereby
providing more space and time for effective radiation of heat away
from the burner to the furnace. Enhanced radiant heat loss leads to
cooler burning and therefore lower NO.sub.x production. The
combustion of hydrogen also forms steam in situ as the combustion
product which has a high heat capacity for heat removal. The end
result of these factors is to lower peak temperatures and reduce
NO.sub.x production. The presence of hydrogen also permits
relatively smooth start-up of the combustion reaction without
extreme thermal shocks and thereafter stable operation of the
burner.
[0023] The term "stoichiometry" as employed herein is the quotient
of (a) the amount of oxygen that is necessary for the complete
combustion of hydrogen and each carbon atom of the carbonaceous
fuel to water and carbon dioxide in the combustion mixture to (b)
the amount of oxygen that is available in the combustion mixture.
For example, each mole of methane (or each carbon of the
carbonaceous fuel) requires two moles of oxygen for complete
combustion to carbon dioxide, and each mole of hydrogen requires
0.5 mole of oxygen for complete combustion of the hydrogen to
water. Therefore, if the combustion mixture contains one mole of
methane, 0.5 mole of hydrogen and 3 moles of oxygen, the
"stoichiometry" is the quotient obtained by dividing the sum of 2
moles plus 0.25 mole of oxygen that is required for complete
combustion divided by 3 moles of oxygen that is available in the
combustion mixture, or in other words 0.75. In the method of the
present invention, the stoichiometry is in the range of from about
0.45, preferably from about 0.60, more preferably from about 0.75,
to about 0.95.
[0024] The at least one catalyst employed in the process of this
invention must be capable of supporting combustion. The combustion
of hydrogen involves its exothermic reaction with oxygen to form
water, which generates sufficient heat to maintain the combustion
reactions even when the at least one combustion catalyst does not
support combustion beyond the normal fuel-rich limit of
flammability. For example, under the conditions employed, the heat
generated by the combustion of a portion of the carbonaceous fuel
and hydrogen is sufficient to support combustion of the remaining
carbonaceous fuel and hydrogen. Thus, at least a portion of the
required thermal energy can be obtained by combustion of a portion
of the combustion mixture.
[0025] In addition, if necessary or desired, the combustion mixture
can be preheated, for example, by condensing high-pressure
saturated steam or by combusting off-gas or other fuel source.
Preheat at a temperature greater than 40.degree. C. but below the
onset of the reaction of the components of the combustion mixture
can be used. Ignition of the combustion reactions can be effected,
for example, by preheating the feed to a temperature sufficient to
effect ignition when the combustion mixture is contacted with the
at least one combustion catalyst. In the alternative, the
combustion mixture can be ignited with an ignition source, such as
a spark or flame. Optionally, a small amount of an additional
component that is easier to ignite than the carbonaceous fuel can
be added to the combustion mixture to facilitate ignition of the
carbonaceous fuel. For example, when methane is the carbonaceous
fuel, the addition of a small amount of ethane to the combustion
mixture facilitates ignition of the methane. Upon ignition, the
combustion reaction-generated heat causes the temperature to
undergo a step change jump to a new level at which continuation and
completion of the combustion reaction can be effected.
[0026] The combustion reactions in the method of this invention are
conducted at a temperature in the range of from about 40.degree.
C., preferably from about 300.degree. C., more preferably from
about 500.degree. C., to about 1800.degree. C., preferably to about
1700.degree. C., more preferably to about 1650.degree. C.
Furthermore, localized temperature spikes resulting from the
ignition and combustion processes reach temperatures less than
2000.degree. C., preferably less than 1800.degree. C. and more
preferably less than 1650.degree. C., at which temperatures the
formation of NOx is minimized and at a level of less than 50,
preferably less than 30, and more preferably less than 20 parts per
million parts by volume. The combustion reactions in the method of
this invention are conducted at a pressure in the range of from
about 1 atmosphere to about 100 atmospheres, preferably to about 50
atmospheres, more preferably to about 30 atmospheres.
[0027] The combustion mixture is generally contacted with the
catalyst in the combustion zone prior to or at the inlet to the
catalyst at a gas velocity in excess of the maximum flame
propagating velocity. This may be accomplished by increasing the
air flow or by proper design of the inlet to a combustion chamber,
for example, by restricting the size of the orifice. This avoids
flashback that causes the formation of NO.sub.x as a result of
noncatalytic combustion and reduction in radioactive loss
mechanisms. Preferably, this velocity is maintained adjacent to the
catalyst inlet. Suitable linear gas velocities are usually above
about three feet per second, but it should be understood that
considerably higher velocities may be required depending upon such
factors as temperature, pressure, and composition. At least a
significant portion of the combustion may occur in the catalytic
zone and may be essentially flameless.
[0028] The at least one combustion catalyst employed in the method
of this invention comprises at least one Group VIII metal
component, and optionally at least one promoter or activator,
supported on a catalyst support. The Group VIII metals comprises
iron, cobalt, nickel and the platinum group metals which are
ruthenium, rhodium, palladium, osmium iridium, and platinum. In a
preferred embodiment, the at least one Group VIII metal is a
platinum group metal; and more preferably the platinum group metal
is at least one of platinum and palladium. The Group VIII metal,
especially platinum, is present at a level of from about 1,
preferably from about 2, more preferably from about 3.5, to about
5, preferably to about 4, more preferably to about 4.5 weight
percent, calculated as the elemental metal and based on the total
weight of the catalyst. If the Group VIII is palladium, the
palladium is present at a level of from about 0.5, to about 3,
preferably to about 2, more preferably to about 1 weight percent,
calculated as the elemental metal and based on the total weight of
the catalyst. When two combustion catalysts are employed in the
method of this invention, in a preferred embodiment one such
catalyst comprises at least a platinum-containing component, and
the other such catalyst comprises at least a palladium-containing
component.
[0029] The catalyst optionally comprises at least one promoter or
activator, which is suitably defined as any element or elemental
ion which is capable of enhancing the performance of the catalyst,
as measured, for example, by an increase in catalyst stability and
lifetime. However, in the present context the terms "promoter" and
"activator" do not include the Group VIII metals. Broadly, the
promoter can be selected from Groups IA, IIA, IIIB, IVB, VB, VIB,
IB, IIIA, IVA, VA, and the lanthanide rare earths and actinide
elements of the Periodic Table. Preferably, the promoter is
selected from Groups IB, VIB, IIIA, IVA, VIA, and the lanthanide
elements. Mixtures of the aforementioned promoters and activators
can also be employed.
[0030] More preferably, the promoter or activator is selected from
copper, tin, antimony, silver, indium, and mixtures thereof. Most
preferably, the promoter is copper, tin, antimony, or a mixture
thereof. If a promoter is employed, then a wide range of atomic
ratios of Group VIIIB metal-to-promoter in the fresh catalyst is
suitable, provided that the catalyst is operable in the process of
this invention. The optimal atomic ratio will vary with the
specific Group VIIIB metal and promoter employed. Generally, the
atomic ratio of Group VIIIB metal to promoter is greater than about
0.10 (1:10), preferably greater than about 0.13 (1:8), and more
preferably greater than about 0.17 (1:6). Generally, the atomic
ratio of the Group VIIIB metal to promoter is less than about 2.0
(1:0.5), preferably less than about 0.33 (1:3), and more preferably
less than about 0.25 (1:4). Although the promoter or activator may
be used in a gram-atom amount equivalent to or greater than the
Group VIIIB metal, the promoter or activator nevertheless functions
to enhance the catalytic effect of the catalyst. Compositions
prepared with a promoter alone, in the absence of the Group VIIIB
metal, are typically (but not always) catalytically inactive in the
process. In contrast, the Group VIIIB metal is catalytically active
in the absence of promoter metal, albeit in some instances with
lesser activity.
[0031] In one embodiment, the Group VIII metal and promoter are
supported on a catalytic support. The loading of the Group VIII
metal on the support is within the above-recited concentration
ranges. Once the Group VIII metal loading is established, the
desired aforesaid atomic ratio of Group VIII metal to promoter
determines the loading of the promoter.
[0032] The catalytic support comprises any material which provides
a surface to carry the Group VIII metal, and optionally, any
promoter(s) and support modifiers, as described hereinafter, and is
thermally and mechanically stable under the conditions employed in
the method of this invention. The support typically exhibits
essentially no activity with respect to the oxidation process and
may consequently be regarded as inert. Alternatively, the support
may exhibit some reactivity with respect to the oxidation
process.
[0033] Preferably, the support is a ceramic, such as a refractory
oxide, nitride, or carbide. Non-limiting examples of suitable
ceramics include alumina, silica, silica-aluminas,
aluminosilicates, for example, cordierite, as well as, magnesia,
magnesium aluminate spinels, magnesium silicates, zirconia,
titania, boria, zirconia toughened alumina (ZTA), lithium aluminum
silicates, silicon carbide, silicon nitride, and oxide-bonded
silicon carbide. Mixtures of the aforementioned refractory oxides,
nitrides, and carbides may also be employed, as well as, washcoats
of the aforementioned materials on a support. Preferred ceramics
include magnesia, alumina, silica, and amorphous and crystalline
combinations of alumina and silica, including mullite. Alpha and
gamma alumina are preferred forms of alumina. Preferred
combinations of alumina and silica comprise from about 65 to about
100 weight percent alumina and from essentially 0 to about 35
weight percent silica. Other refractory oxides, such as boria, can
be present in smaller amounts in the preferred alumina and silica
mixtures. Preferred zirconias include zirconia fully stabilized
with calcia (SSZ) and zirconia partially stabilized with magnesia
(PSZ), available from Vesuvius Hi-Tech Ceramics, Inc.
[0034] The catalytic support may take a variety of shapes including
that of porous or non-porous spheres, granules, pellets,
irregularly shaped solid or porous particles, or any other shape
which is suitable for a variety of catalytic reactors, including
fixed bed, transport bed, and fluidized bed reactors. In a
preferred form, the catalyst is a monolith, which means that it is
a continuous structure. Examples of monoliths include honeycomb
structures, foams, and fibers woven into fabrics or made into
non-woven mats or thin paper-like sheets. Foams are sponge-like
structures. More preferably, the support is a foam or fiber ceramic
monolith. Catalysts prepared with foam or fiber supports tend to
have a higher activity as compared with catalysts prepared on solid
spheres or irregularly shaped particles. Additionally, fibers tend
to possess higher fracture resistance as compared with foams and
honeycombs. Preferred ceramic foams available from Vesuvius Hi-Tech
Ceramics, Inc. comprise alpha alumina, zirconia, and mullite with a
porosity ranging from about 5 to about 100 pores per linear inch
(ppi) (2 to 40 pores per linear centimeter (ppcm)). Foams having
about 45 ppi (18 ppcm) are more preferred. The term "porosity," as
used herein, refers to channel size or dimension. It is important
to note that the foam supports are not substantially microporous
structures. Rather, the foams are macroporous, meaning that they
are low surface area supports with channels ranging in diameter
from about 0.1 millimeter to about 5 millimeters. The foams are
estimated to have a surface area less than about 10 square meters
per gram, and preferably, less than about 2 square meters per gram,
but greater than about 0.001 square meters per gram.
[0035] More preferred ceramic fibers, such as those available as
Nextel(R) brand ceramic fibers, a trademark of 3M Corporation,
typically have a diameter greater than about 1 micron, preferably
greater than about 5 microns The diameter is suitably less than
about 20 microns, preferably, less than about 15 microns. The
length of the fibers is generally greater than about 0.5 inch (1.25
cm), preferably greater than about 1 inch (2.5 cm), and typically
less than about 10 inches (25.0 cm), preferably less than about 5
inches (12.5 cm). The surface area of the fibers is very low, being
generally less than about 1 square meter per gram, preferably, less
than about 0.3 square meters per gram, but greater than about 0.001
square meter per gram. Preferably, the fibers are not woven like
cloth, but instead are randomly intertwined as in a mat or matted
rug. Most preferred are Nextel(R) brand 312 fibers which consist of
alumina (62 weight percent), silica (24 weight percent), and boria
(14 weight percent). Non-limiting examples of other suitable fibers
include Nextel(R) brand 440 fibers which consist of gamma alumina
(70 weight percent), silica (28 weight percent), and boria (2
weight percent) and Nextel(R) brand 610 fibers which consist of
alpha alumina (99 weight percent), silica (0.2-0.3 weight percent)
and iron oxide (0.4-0.7 weight percent).
[0036] During preparation of the catalyst composition, various
compounds and/or complexes as well as elemental dispersions of any
of the Group VIII metals, especially platinum group metal, may be
used to achieve deposition of the metal on the composite. Water
soluble Group VIII metal compounds or complexes may be used.
[0037] The platinum group metal may be precipitated from solution,
for example, as a sulfide by contact with hydrogen sulfide. The
only limitation on the carrier liquids is that the liquids should
not react with the Group VIII metal compound and must be removable
by volatilization or decomposition upon subsequent heating and/or
vacuum, which may be accomplished as part of the preparation or in
the use of the completed catalyst composition. Suitable Group VilI
metal compounds are, for example, chloroplatinic acid, potassium
platinum chloride, ammonium platinum thiocyanate, platinum
tetrammine hydroxide, Group VIII metal chlorides, oxides, sulfides,
and nitrates, platinum tetrammine chloride, palladium tetrammine
chloride, sodium palladium chloride, hexammine rhodium chloride,
and hexammine iridium chloride. If a mixture of platinum and
palladium is desired, the platinum and palladium may be in water
soluble form, for example, as amine hydroxides, or they may be
present as chloroplatinic acid and palladium nitrate when used in
preparing the catalyst of the present invention. The Group VIII
metal may be present in the catalyst composition in elemental or
combined forms, for example, as an oxide or sulfide. During
subsequent treatment such as by calcining or upon use, essentially
all of the Group VIII metal is converted to the elemental form.
[0038] In one manner of preparing structures provided with catalyst
compositions of this invention, an aqueous slurry of the
essentially water insoluble calcined composite of alumina and
stabilizing component is contacted with the support. The solid
content of the slurry forms an adherent deposit on the support, and
the resulting supported composite is dried or calcined for a second
time at a temperature which provides a relatively
catalytically-active product. The second drying or calcination
takes place at a temperature low enough to prevent undue sintering
of the mixture. Suitable calcination temperatures are generally
about 300.degree.-700.degree. C. to insure catalytic activity
without undue sintering, preferably about 400.degree.-600.degree.
C. After this second calcination the coating on the support has a
surface area of at least about 75 square meters per gram. Lower
temperatures can be employed to dry the composite if the second
calcination is not performed.
[0039] After the coated support is dried or calcined, a platinum
group metal component may be added to enhance the catalytic
activity of the composite. The platinum group metal may be added to
the coated support in the manner previously described. Preferably,
this addition is made from an aqueous or other solution to
impregnate or deposit the platinum group metal component on the
coated support.
[0040] After addition of the platinum group metal, the resulting
structure is dried and may be calcined for a third time under
conditions which provide a composition having characteristics that
enhance selected reactions. This final calcination stabilizes the
completed catalyst composition so that during the initial stages of
use, the activity of the catalyst is not materially altered. The
temperature of this final calcination must be low enough to prevent
substantial sintering of the underlying coating which would cause
substantial occlusion of the platinum group metal component. Thus,
the calcination may be conducted at temperatures of about
300.degree.-700.degree. C., preferably about
400.degree.-600.degree. C.
[0041] An alternative method of making the catalyst compositions of
this invention if a relatively inert support is used involves
adding the platinum group metal component to the calcined composite
before the composite is deposited on the support. For example, an
aqueous slurry of the calcined composite can be prepared and the
platinum group metal component added to the slurry and mixed
intimately therewith. The platinum group metal component can be in
the form already described and may be precipitated as previously
described. The final mixture containing the platinum group metal
may then be dried or calcined to provide a catalytically-active
composition in a form suitable for deposition on a support or for
use without such deposition as a finished catalyst in either finely
divided or macrosize forms. Subsequent calcinations or drying may
be conducted as described above. The calcined material generally
has a surface area of at least about 25 square meters per gram,
preferably at least about 75 square meters per gram.
[0042] In another embodiment, the catalyst can be supplied as a
metallic gauze. In this form, the gauze acts as both catalyst and
monolith support. More specifically, the gauze can comprise an
essentially pure Group VIII metal or mixture of Group VIII metals,
preferably, platinum group metals, onto which optionally a promoter
is deposited. Suitable gauzes of this type include pure platinum
gauze and platinum-rhodium alloy gauze, optionally coated with the
promoter. The method used to deposit or coat the promoter onto the
gauze can be any of the methods described hereinafter.
Alternatively, a gauze comprising an alloy of a Group VIII metal
and the promoter can be employed. Suitable examples of this type
include gauzes prepared from platinum-tin, platinum-copper, and
platinum-tin-copper alloys. During preparation, one or more of the
Group VIII alloy metals and/or the same or a different promoter can
be deposited.
[0043] When two or more metal components are employed in the method
of this invention, the two or more metal components can be
intermixed on the same support and thus are present in the same
single catalyst. In another embodiment one catalyst containing one
metal component(s) is employed upstream of another catalyst
containing a different metal component(s) so that the combustion
mixture comes into contact with them in series. In that instance,
the combustion catalyst for use as the upstream catalyst is
selected to favor one combustion reaction, namely, the reaction of
hydrogen with molecular oxygen, and combustion catalyst for use as
the downstream catalyst is selected to favor the other combustion
reaction, namely, the reaction of the carbonaceous fuel with
molecular oxygen. In such case, preferably a first catalyst
comprising a platinum-containing component on a gauze is positioned
upstream of a second catalyst comprising a palladium-containing
component on a gauze. A heat shield precedes the upstream catalyst
and serves to prevent upstream radioactive losses which would
reduce the heat applied to the process.
[0044] Thus, in this embodiment, the metal components of the
upstream and downstream combustion catalysts can be on supports
that are separate from one another such that one support containing
the metal component(s) of the downstream catalyst is located
downstream of the support containing the metal component(s) of the
upstream catalyst. In this case, the upstream catalyst would be in
an upstream monolith or bed, for example, while the downstream
catalyst would be in a separate downstream monolith or bed. In this
case, the two support materials could be in physical contact with
each other or physically separated from each other. In the
alternative, the metal components of the upstream and downstream
catalysts can be on the same support but on upstream and
downstream, respectively, sections of that support. In this case,
both the upstream and downstream catalysts would be in the same
monolith or bed, for example, but at different upstream and
downstream sections thereof.
[0045] In the method of the present invention, the space velocity
is in the range of from about 1000, preferably from about 5000,
more preferably from about 10,000, to about 5.times.10.sup.6,
preferably to about 3.times.10.sup.6, more preferably to about
2.times.10.sup.6 volumes of combustion mixture per hour per volume
of catalyst.
[0046] The present invention will be more clearly understood from
the following specific examples.
EXAMPLE 1
[0047] The following example illustrates the method of this
invention and the criticality of the presence of hydrogen in the
combustion mixture. A combustion mixture containing methane,
hydrogen and air was fed to a burner. The feed rates where 0.59
liter per minute of methane, 0.34 liter per minute of hydrogen and
9.1 liters per minute of air, which in terms of its oxygen and
nitrogen components was 1.91 liters per minute of oxygen and 7.2
liters per minute of nitrogen. The burner contained a catalyst
having the following composition and configuration:
[0048] 99.5% weight percent of alumina foam monolith having 45
pores per inch and with a metals loading of 3 weight percent of
platinum and 1 weight percent of palladium and being 15 millimeters
in diameter and 30 millimeters long. The space velocity was 113,000
volumes of combustion mixture per hour per volume of catalyst.
[0049] The combustion mixture was preheated by a heater at
450.degree. C. to a temperature of 350.degree. C. as it entered the
burner. Combustion of hydrogen commenced and the temperature of the
combustion products exiting from the burner was 560.degree. C. The
hydrogen rate of flow into the burner was increased to 0.4 liter
per minute, and the temperature of the combustion products
increased to 650.degree. C. When the hydrogen rate of flow into the
burner was increased to 0.453 liter per minute, the temperature of
the combustion products rose rapidly to 1000.degree. C. as
combustion of methane began. At this point, the flow rates of
hydrogen and methane were reduced to 0.40 liter per minute and
about 0.50 liter per minute, respectively, but the temperature of
the combustion products continued to rise to 1100.degree. C. The
flow rates were then adjusted to 0.29 liter per minute of hydrogen,
0.59 liter per minute of methane, and 9.22 liters per minute of air
and then to 0.26 liter per minute of hydrogen, 0.52 liter per
minute of methane, and 9.22 liters per minute of air, and the
temperature of the combustion products rose to 1428.degree. C. and
then began to fall sharply, indicating instability of the
combustion in the burner. When the flow rate of hydrogen was
increased to 0.4 liter per minute, the temperature of the
combustion products rose again to 1550.degree. C. This procedure of
raising the hydrogen flow rate and reducing the methane flow rate
was used frequently to control and stabilize the burner.
EXAMPLE 2
[0050] In a further preferred embodiment that permits the
temperature of the combustion products to increase more gradually
over the transition from about 700.degree. C. to about 1100, ethane
was incorporated into the combustion mixture because the combustion
of ethane is easier to initiate than the combustion of methane. In
one example, the combustion mixture entering the burner was
preheated to 215.degree. C. using a preheater set at 450.degree. C.
and the burner configuration whose configuration is described in
Example 1. The combustion mixture was made up of methane at 0.59
liter per minute, hydrogen at 0.29 liter per minute and air at 9.0
liters per minute (that is, oxygen at 1.9 liters per minute and
nitrogen at 7.1 liters per minute) and ethane at 0.05 liters per
minute). The space velocity was 112,000 volumes of combustion
mixture per hour per volume of catalyst. The temperature of the
combustion products exiting the burner was 465.degree. C. When
methane in the combustion mixture was reduced to 0.53 liter per
minute and ethane therein was increased to 0.1 liter per minute,
the temperature of the combustion products increased to 651.degree.
C. At this point, ethane in the combustion mixture was reduced to
0.05 liter per minute; and, as the temperature of the combustion
products fell, hydrogen in the combustion mixture was increased to
0.46 liter per minute. The temperature of the combustion products
rose rapidly to 1000.degree. C., at which point the ethane content
of the combustion mixture was reduced to zero, the methane content
was increased to 0.59 liter per minute, and the hydrogen content
was reduced to 0.29 liter per minute. The temperature of the
combustion products began to fall sharply, indicating instability
of the combustion in the burner. When the hydrogen and ethane
contents of the combustion mixture were increased to 0.46 liter per
minute and 0.05 liter per minute, respectively, the temperature of
the combustion mixture rose again to 1300.degree. C. and combustion
stabilized.
EXAMPLES 3-8
[0051] Examples 3-8 were performed under the following conditions.
The catalyst was 99.5% alumina foam monolith having 45 pores per
inch and with a metal loading of 3 weight percent of platinum and 1
weight percent of palladium and being 15 millimeters in diameter
and 60 millimeters long. The total flow rate was maintained at a
constant 10 liters per minute. Air was supplied as a mixture of
oxygen and nitrogen. The space velocity in each example was 56,600
volumes of combustion mixture per hour per volume of catalyst. The
compositions of the combustion mixtures, stoichiometries, preheat
temperatures, concentrations of NO and NO.sub.x and the temperature
of the combustion product existing the burner used are summarized
in Table A below. From the compositions of the combustion mixture,
the stoichiometries of the combustion mixtures in Example 3-8 are
presented in Table A. Also presented in Table A are the
temperatures of the combustion mixtures and the measured
concentrations of NO and NO.sub.x in the combustion products
exiting from the burner for Examples 3-8. These NOx concentrations
are plotted versus the preheat temperatures at a constant
stoichiometry of 0.75 in FIG. 1 and versus the stoichiometry of the
combustion mixtures in FIG. 2.
[0052] The results illustrated in FIG. 1 indicate that at a
constant stoichiometry of 0.75, the NO.sub.x levels fell as the
preheat temperature was reduced. The results illustrated in FIG. 2
indicate clearly the importance of the stoichiometries or in other
words of the relative amounts of oxygen, methane and hydrogen, and
that as the stoichiometries increase above 0.8, the NO.sub.x levels
increase substantially even when the preheat temperature is
relatively low.
1 TABLE A Combustion Mixture Composition Preheat % vol Stoichi-
Tempera- Example CH.sub.4 H.sub.2 Air O.sub.2 N.sub.2 ometry tures
.degree. C. NO NO.sub.x 3 5.9 2.95 91.1 19.1 72.0 0.7 244 5.5 6.0 4
6.3 3.2 90.5 19.0 71.5 0.75 288 6.0 7.0 5 6.3 3.2 90.5 19.0 71.5
0.75 253 5.5 6.0 6 6.3 3.2 90.5 19.0 71.5 0.75 140 4.4 4.7 7 6.7
3.4 89.9 18.9 71.0 0.8 128 7.3 7.9 8 7.1 3.6 89.3 18.8 70.5 0.85
118 11.4 12.3
[0053] From the above description it is apparent that the objects
of the present invention have been achieved. While only certain
embodiments have been set forth, alternative embodiments and
various modifications and will be apparent form the above
description to those skilled in the art. These and other
alternatives are considered equivalents and are within the spirit
and scope of the present invention.
* * * * *