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.

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 NOx 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 NOx which is an equilibrium mixture mostly of NO but also containing minor amounts of NO2. Modern antipollution laws either currently in effect or under consideration mandate the reduction of combustion NOx.

[0005] It has been recognized that a fruitful way of controlling NOx 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 NOx 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 104 per hour to about 2×106 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 NOx 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 NOx 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 NOx 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° 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° C., preferably from about 300° C., more preferably from about 500° C., to about 1800° C., preferably to about 1700° C., more preferably to about 1650° C. Furthermore, localized temperature spikes resulting from the ignition and combustion processes reach temperatures less than 2000° C., preferably less than 1800° C. and more preferably less than 1650° 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 NOx 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°-700° C. to insure catalytic activity without undue sintering, preferably about 400°-600° 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°-700° C., preferably about 400°-600° 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 5×106, preferably to about 3×106, more preferably to about 2×106 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° C. to a temperature of 350° C. as it entered the burner. Combustion of hydrogen commenced and the temperature of the combustion products exiting from the burner was 560° 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° 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° 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° 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° 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° 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° 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° C. using a preheater set at 450° 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° 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° 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° 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° 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 NOx 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 NOx 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 NOx 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 NOx levels increase substantially even when the preheat temperature is relatively low. 1 TABLE A Combustion Mixture Composition Preheat % vol Stoichi- Tempera- Example CH4 H2 Air O2 N2 ometry tures ° C. NO NOx 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.

Claims

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 2×106 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 NOx 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 5×106 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 3×106 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 2×106 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 NOx 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 5×106 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.