US3846979A

US3846979A - Two stage combustion process

Info

Publication number: US3846979A
Application number: US00388906A
Authority: US
Inventors: W Pfefferle
Original assignee: Engelhard Minerals and Chemicals Corp
Current assignee: BASF Catalysts LLC; Engelhard Minerals and Chemicals Corp
Priority date: 1971-12-17
Filing date: 1973-08-16
Publication date: 1974-11-12
Anticipated expiration: 1991-11-12

Abstract

A method is disclosed which encompasses a two stage combustion process wherein a carbonaceous fuel, which when burned with stoichiometric amount of air has an adiabatic flame temperature of at least 3,300* F., admixed with air is partially combusted in a thermal zone. The effluent is quenched and then contacted with a solid oxidation catalyst to combust or oxidize the fuel under essentially adiabatic conditions. The operating temperature of the catalyst is preferably substantially above the instantaneous auto-ignition temperature of the fuel-air admixture but below a temperature that would result in any substantial formation of oxides of nitrogen, for example at a temperature of about 1,500* to 3,200* F. The resulting effluent is characterized by high thermal energy useful for generating power and by low amounts of atmospheric pollutants, especially oxides of nitrogen.

Description

United States Patent 1 Pfefferle [111 3,846,979 [4 1' Nov. 12, 1974 TWO STAGE COMBUSTION PROCESS William C. Pt'efferle, Middletown, NJ.

221 Filed: Aug. l6, 1973 [21] Appl. No.: 388,906

Related US. Application Data [63] Continuation-in-part of Ser. No. 209,169, Dec. 17,

1971, abandoned.

[75] Inventor:

[52} US. Cl 60/39.04, 60/39.17, 60/39.23, 60/39.65, 60/D1G. 11, 431/7, 110/8 A [51] Int. Cl F02c 7/00, F02g 3/00 [58] Field of Search 60/39.17, 39.23, 39.65, 60/39.69, 39.02, 39.03, 39.04, 39.05, 302,

[56] References Cited UNITED STATES PATENTS 3,685,972 8/1972 De Palma et a1. 23/288 F 3.791581 2/1974 Handa 60/39.65

FOREIGN PATENTS OR APPLICATIONS 588,160 12/1959 Canada 60/203 France 610/3982 C 1,467,142 l/1967 1,043,717 ll/l958 Germany 23/288 F 271,899 8/1927 Great Britain 60/39.82 C

Primary Examiner-Carlton R. Croyle Assistant Examiner-Robert E. Garrett [57] ABSTRACT A method is disclosed which encompasses a two stage combustion process wherein a carbonaceous fuel, which when burned with stoichiometric amount of air has an adiabatic flame temperature of at least 3,300 E, admixed with air is partially combusted in a thermal zone. The effluent is quenched and then contacted with a solid oxidation catalyst to combust or oxidize the fuel under essentially adiabatic conditions. The operating temperature of the catalyst is preferably substantially above the instantaneous auto-ignition temperature of the fuel-air admixture but below a temperature that would result in any substantial formation of oxides of nitrogen, for example at a temperature of about 1,500 to 3,200 F. The resulting effluent is characterized by high thermal energy useful for generating power and by low amounts of atmospheric pollutants, especially oxides of nitrogen.

PATENTEDHUV I2 I974 REACTION RATE 3.846.979 sum 2M 2 REGION "O" REGION "(3" MASS TRANsFER cONTROL LIMITED OPERATION REGION B TRANSITION TO MASS TRANSFER CONTROL REGION "A" KINETIC cONTROL TEMPERATURE This application is a continuation-in-part of copending application Ser. No. 209,169 filed on Dec. 17, 1971, now abandoned.

This invention relates to a two stage combustion process. In its more specific aspect, this invention relates to a two stage combustion process in which a carbonaceous fuel admixed with air is partially combusted in a thermal zone, the effluent quenched and then catalytically combusted under essentially adiabatic conditions. The resulting effluent is characterized by high thermal energy and low in nitrogen oxides content.

Adiabatic combustion systems from a practical standpoint have relatively low heat losses, and the heat released from the combustion zone appears in the effluent gases as thermal energy for producing power. In general, conventional adiabatic, thermal combustion systems (e.g., engines and power plants) operate at such high temperatures in the combustion zone as to form nitrogen oxides or NO,, and especially including NO. A thermal combustion system operates by contacting fuel and air in inflammable proportions with an ignition source, e.g., spark, to ignite the mixture which then will continue to burn. Flammable mixtures of most fuels for complete combustion normally burn at relatively high temperatures, i.e., about 3,300 F. and above, which inherently results in the formation of substantial amounts of N ln-the case of gas turbine thermal combustors, the formation of N0 hasbeen re duced by limiting the residence time of the combustion products in the combustion zone. However, due to the large quantities of gases being handled, undesirable amounts of NO, are produced. v

It has long been realized that little or no NO, is formed in a system which burns fuel catalytically at relatively low temperatures. This type of combustion, sometimes referred to as catalytic combustion, has been regarded generally as being impractical as a means of providing power and has achieved success only in limited situations. For example, catalytic combustion has been employed to treat tail gas streams of nitric acid plants where the catalytic reaction is employed to heat spent process air containing about two percent (2 percent) oxygen to temperatures of about l,400 F. The heated gases are used to power a turbine which drives the air compressor for the nitric acid plant, but such a system could not be used as a source of primary power because the amount of catalyst required would be impractically large as explained below in greater detail.

For a catalytic reaction, one can plot temperature against rate of reaction as shown in FIG. 3 in the accompanying drawings. For any given catalyst and set of reaction conditions, as the temperature is initially increased the reaction rate is also increased as shown in the kinetic region A of the rate curve of FIG. 3. This rate of increase is exponential with temperature. As the temperature is raised further, the reaction rate than passes through a transition zone where the limiting parameters determining reaction rate shift from catalytic to mass transfer (region B of the curve in FIG. 3). When the catalytic rate increases to such an extent that the reactants cannot be transferred to the catalytic surface fast enough to keep up with the catalytic reaction rate, the reaction shifts to mass transfer control, and the cayalytic reaction rate levels off regardless of further termperature increases; The reaction is then said to be mass transfer limited (region C of the curve of FIG. 3). In mass transfer controlled catalytic reactions, one cannot distinguish between a more active cayalyst and a less active catalyst because the intrinsic catalyst activity is not determinative of the rate of reaction. Re-

to increase the conversion rate for any given system, it

appears essential either to increase the amount of catalyst surface or to increase the rate of mass transfer of reactants to the surface. The former, for practical combustion systems, would require either a catalyst size of such magnitude as to be unwieldy or a catalyst configuration which results in increased specific pressure drop and which would require unwieldy geometry to hold total pressure drop at a low level. For example, in the case of gas turbine engines, the catalytic reactor might very well be larger than the engine itself. On the other hand, increasing the rate of masstransfer of reactants to the catalytic surfaces would result in increased pressure drop and consequently a substantial loss of energy; sufficient pressure drop may not. even be available to provide the desired rate of reaction. Quite obviously,

these approaches, while theoretically possible, are im-- practical.

As disclosed in my co-pending application Ser. No. I

358,411, filed May 8, 1973, I have discovered that it is possible to achieve essentially adiabatic combustion in the presence of a catalyst at a reaction rate many times greater than the mass transfer limited rate. That is, I have found thatcatalytically-supported, thermal combustion surmounts the mass transfer limitation. If the operating temperature of the catalyst is increased substantially into the mass transfer limited region, the reaction rate again begins to increase exponentially with temperature (region D of the curve of FIG. 3). This is an apparent contradiction of catalytic technology and the laws of mass transfer kinetics. The phenomena may be explained by the fact that the catalyst surface and the gas layer near the catalyst surface are above atemperature at which thermal combustion occurs at a rate higher than the catalytic rate, and the temperature of the catalyst surface is above the instantaneous autoignition temperature (defined hereinbelow) of the fuelair admixture. Such fuel-air admixture will, of course, include partially oxidized fuel such as may be produced by incomplete thermal oxidation. The fuel molecules entering this layer spontaneously burn without transport to the catalyst surface. As combustion progresses, it is believed that the layer becomes deeper. The total gas is ultimately raised to a temperature at which thermal reactions occur in the entire gas stream rather than only near the surface of the catalyst. At this point, the

thermal reactions continue even without further contact of the gas with the catalyst as the gas passes through the combustion zone.

The term instantaneous auto-ignition temperature for a fuel-air admixture as used herein and in the appended claims is defined to mean that the ignition lag of the fuel-air mixture entering the catalyst is negligible relative to the residence time in the combustion zone of the mixture undergoing combustion.

Broadly, the present invention encompasses a two stage combustion process wherein a carbonaceous fuel, which when burned with stoichiomet'ric amount of air has a adiabatic flame temperature of at least 3,300 F., admixed with air is partially combusted in a thermal zone. The effluent from the thermal or homogeneous zone is quenched, preferably with air, to substantially arrest combustion. The quenched effluent is then contacted with a solid oxidation catalyst to combust or oxidize at least about ten percent of the fuel under essentially adiabatic condition. The operating temperature of the catalyst is substantially above the instantaneous auto-ignition temperature of the fuel-air admixture but below a temperature that would result in any formation of oxides of nitrogen, for example, at a temperature of l,500 to about 3,200 F.

In many instances, it is advantageous to introduce a fuel in liquid phase rather than vapor prior to passing it to the combustion zone. However, in the second catalytic combustion stage, the fuel must be substantially vaporized prior to contact with the catalyst. The flame in the initial thermal combustion zone serves to quickly vaporize the fuel when charged to this zone as a liquid or as a fine particulate solid. Further, prior thermal combustion permits the turbine or other powerproducing system to respond quickly to operational changes, e.g., speed or power changes, even though a portion of the combustion is accomplished by catalytic means, without substantial formation of NO,. Generally, a catalyst must have adequate volume to afford the desired catalytic reaction time. As the volume of catalyst increases, however, the system will take a longer time to respond to changes in operating conditions. For instance, if the temperature of a catalytic combustion zone is to be changed and the speed of a turbine thus altered, the operation will not be completely effective until most, if not all, of the catalyst is at the new operating temperature. This heating process may be too slow for satisfactory turbine operation if the turbine were to be employed in a system designed for quick response, as is the case in automotive vehicles. Use of the present invention, employing an initial thermal oxidation zone, produces a hot effluent and reduces such response time. Additionally, by virtue of the heat supplied by the initial thermal zone, cold start-up of the system is readily accomplished. A further advantage of the present invention is that since the effluent gases have undergone partial combustion prior to catalytic combustion, a lesser volume of catalyst may be required for obtaining a given overall extent of combustion than in a system using a catalyst for the initial oxidation.

In accordance with the present invention, carbonaceous fuel in the presence of air is thermally combusted substantially continuously during operation. This initial combustion serves to partially combust or oxidize and to vaporize any carbonaceous fuel. At least enough oxidation is required to provide sufficinet heat to rapidly vaporize the fuel. If complete oxidation occurs in the thermal zone, excessive NO, formation results. It is found particularly advantageous to oxidize from about 35 to 90 percent of the fuel by this initial thermal oxidation to carbon oxides and water. Preferably, the free oxygen content of the total feed to the thermal combustion zone is at least about 80 percent by weight of the stoichiometric amount needed for complete combustion of the fuel to carbon dioxide and water to sustain the combustion but through proper design, much greater quantities of oxygen are effective, e.g., up to about 200 percent by weight of the stoichiometric amount. The combustion in the thermal oxidation zone takes place at somewhat lower temperatures than the theoretical adiabatic flame temperature of the fuel when completely combusted with a stoichiometric amount of air. This difference is due to incomplete combustion of the fuel and in lesser degree to radiation losses from the combustion zone. If desired, heat from the thermal combustion zone may also serve to preheat a secondary air supply for the system. The temperature of the flame supported thermal combustion zone is generally from about l,800 F., and may be as high as 3,700 F. or higher, with effluent gas temperatures preferably being in the range of about l,800 to 2,800 F. to minimize NO, formation. The temperature in the thermal combustor can be controlled by varying the fuel-to-air ratio. A further means of temperature control is to use the thermal combustor in indirect heat exchange with, for example, air charged to the system.

The partially oxidized effluent from the thermal zone is quenched to effectively prevent complete thermal combustion of the effluent, thus minimizing NO, and insuring a relatively high concentration of carbon monoxide which aids in preventing NO formation. Quenching is preferably accomplished by admixing the effluent with cooler air, preferably below about 1,500 F but where desired, an inert gas such as nitrogen may be used. In the preferred embodiment of the invention, sufficient air is admixed with the effluent to insure a stoichiometric excess thereof for catalyticallysupported thermal combustion. The-effluent gases from the thermal combustor will contain substantial amounts of carbon monoxide and hydrocarbons due to the partial oxidation of the fuel. Since high temperatures for a prolonged period promote the formation of nitrogen oxides, the thermal partial oxidation is preferably followed by a sufficiently rapid quench or cooling of the flame to minimize such formation. The amount of nitrogen oxides produced can also be minimized by quenching the initial thermal combustion before the global temperature reaches the levels at which significant formation of NO, occurs.

The temperature of the effluent can also be reduced by other means, for example, by expansion of the effluent, by indirect heat exchange or by direct heat exchange through contact with a ceramic heat sink. While quenching minimizes nitrogen oxide formation, this minimization can also be accomplished by increasing the gas flow which in turn reduces the amount of time the reactants remain at a high temperature level. The velocity of the gases passing from the thermal combustion zone to the catalytic oxidation may be from about 50 to 200 feet per second. Creation of gas turbulence in the quenching zone as well as minimization of the distance the gases travel between the thermal oxidation and catalytic oxidation zones also assists in reducing nitrogen oxide formation.

The partially oxidized effluent gases passing to the catalytic combustion zone of this invention are mixed with a sufficient amount of air to adjust the theoretical adiabatic flame temperature to the desired value. The adiabatic flame temperature of fuel-air admixtures or partially oxidized fuel-air admixtures at any set of conditions (e.g., initial temperature and, to a lesser extent, pressure) is established by the ratio of fuel to air. The proportions of the partially oxidized fuel and air charged to the catalytically-supported thermal combustion zone are preferably such that there is a stoichiometric excess of oxygen based on complete conversion of the effluent to carbon dioxide and water. Generally,

- the total amount of air added to the system, that is, the

air added to the thermal combustion zone and any additional air added to the partially oxidized effluent from this zone which passes to the catalytic zone is at least about 1.5 times the stoichiometric amount required for complete combustion of the fuel to water and carbon dioxide and preferably two times the stoichiometric amount.

Although the invention is described herein with particularity to air asthe non-fuel component, it is well understood that oxygen is the required element to support proper combustion. Where desired, the oxygen content of the non-fuel component can be varied and the term air is used herein to refer to the non-fuel components of the admixtures. The fuel-air admixture fed to the catalytically-supported combustion zone may have as low as percent free oxygen by volume or less, which may occur, for example, upon utilization as a source of oxygen, a waste stream wherein a portion of the oxygen has been reacted. The additional or quench air can be at ambient temperature or, preferably, at an elevated temperature achieved, for example, by indirect heat exchange with the thermal combustion zone. The temperature of the mixture of partially combusted gas and air at the inlet to the catalytic combustor is below the instantaneous auto-ignition temperature of the mixture, preferably below about l,500 F., to insure incomplete combustion before reaching the catalytic zone. If desired, additional fuel may be added to the system between the thermal and catalytic combustion zones so long as at least partial vaporization of such fuel occurs before reaching the catalyst. This addition, if any, is done at a temperature high enough to insure vaporization of the fuel.

The catalytically-supported thermal combustion step, in accordance with the proposed embodiment of the invention, is achieved by contacting at least a portion of the partially oxidized carbonaceous fuel, in vaporous form and intimately admixed with a stoichiometric excess of air with a solid oxidation catalyst having an operating temperature substantially above the instantaneous auto-ignition temperature of such fuelair admixture. The admixture is then combusted under essentially adiabatic conditions, and preferably at least about 10 percent of the fuel is combusted. Combustion is characterized by the use of a partially oxidized fuelair admixture having an adiabatic flame temperature substantially above the instantaneous auto-ignition temperature of the admixture but below a temperature that would result in any substantial formation of oxides of nitrogen. Theoretically, the adiabatic flame temperature would be the same throughout the catalyticallysupported combustion zone, but in practical-applications where there is some heat loss, the adiabatic flame temperature is more accurately determined at the catalyst inlet. The resulting effluent from the catalyticallysupported zone is characterized by high thermal energy useful for generating'power and by low amounts of N0 Sustained catalytically-supported, thermal combustion occurs at a substantially lower temperature than in conventional adiabatic thermal combustion and therefore it is possible to operate without the formation of a significant amount of N0 Combustion is no longer limited by mass transfer as it is in the case of conventional catalytic combustion. At the specified operating temperatures, the reaction rate is substantially increased beyond the mass transfer limitation, e.g., at least about five or 10 times greater than the mass transfer limited rate. Reaction rates of up to about 100 or more times the mass transfer limited rate may be attainable. Such high reaction rates permit high fuel space velocities which normally are not obtainable in conventional catalytic reactions. The catalytic zone can employ, for instance, an amount of partially oxidized fuel equivalent in heating value to about 300 pounds of propane per hour per cubic foot of catalyst, and this amount may be at least several times greater, for instance, an amount of such fuel equivalent in heating value to at least about 1,000 pounds of propane per hour per cubic foot of catalyst.

The carbonaceous fuels utilized in the invention may be gaseous, liquid, or solid at normal temperature and pressure. Suitable hydrocarbon fuels may include, for example, low molecular weight aliphatic hydrocarbons such as methane, ethane, propane, butane, pentane;

gasoline; aromatic hydrocarbons such as benzene, toluene, ethylbenzene, xylene; naphtha; diesel fuel; jet fuel; other middle distillate fuels; hydrotreated heavier fuels; and the like. Amongthe other useful carbonaceous fuels are alcohols such as methanol, ethanol, isopropanol; ethers such as diethylether and aromatic ethers such as ethylphenyl ether; carbon monoxide; and low ash chars.

The carbonaceous fuel, ,which when burned with a stoichiometric amount of air (atmospheric composition) at the combustion inlet temperature usually has an adiabatic flame temperature of at least about 3,300 F and is combusted essentially adiabatically in the catalyst zone. Although the instantaneous auto-ignition temperature of a typical fuel may be below about 2,000 F., stable, adiabatic combustion of the fuel below about 3,300" F. is extremely difficult to achieve in many primary combustion systems. It is for this reason that even with gas turbines limited to operating temperatures of 2,000 E, the primary combustion is typically at temperatures in excess of 4,000 F. As stated above, in a preferred form of the present invention combustion in the catalytic zone is characterized by using a fuel-air admixture, having an adiabatic flame temperature substantially above the instantaneous this invention vis-a-vis a conventional thermal system.

A higher temperature within the defined range is desired, however, because the system will require less cat-- alyst and thermal reactions are faster by an order of magnitude or more, but the adiabatic flame temperature employed can depend on such factors as the de-.

sired composition of the effluent and the overall design of the system. It thus will be observed that a partially combusted fuel which would ordinarily burn at such a high temperature as to form No,,, is successfully combusted within the defined temperature range without significant formation of N Although combustion occurs adiabatically, it should be understood that for practical operations, there may be heat losses to the environment from the combustion zone. A loss in temperature as measured by the effluent temperature may be as much as about 300 F. and preferably is not more than about 150 F. Notwithstanding these minor heat losses, the catalytic zone operation from a practical standpoint is considered adiabatic, and the heat of reaction is released primarily in the effluent gases. Thus there may be about four times, preferably at at least about seven times, more heat released (thermal energy) in these gases than is lost from the catalytic combustion zone.

The catalyst used in the catalytic zone generally operates at a temperature approximating the theoretical adiabatic flame temperature of the partially oxidized fuel-air admixture introduced to the catalytic combustion zone. The entire catalyst may not be at these temperatures, but preferably at least a major portion of the catalyst surface is at such operating temperatures. These temperatures are usually from about l,500 to 3,200 F., preferably about 2,000 to 3,000 F. The temperature of the catalyst zone is controlled by controlling the composition of the fuel-air admixture, i.e., adiabatic flame temperature, as well as the uniformity of the mixture. At the higher end of the temperature range, shorter residence times of the gas in the catalytic combustion zone appear to be desirable in order to lessen the chance of forming N0 The residence time is governed largely by temperature, pressure and space throughput, and generally is measured in milliseconds. The residence time of the gases in the catalytic combustion zone may be below about 0.1 second, preferably below about 0.05 second. The gas space velocity may often be, for example, in the range of about 0.5 to 10 or more million cubic feet of total gas (standard temperature and pressure) per volume of total combustion zone per hour. For a stationary turbine burning diesel fuel, typical residence times could be about 30 milliseconds or less; whereas in an automotive turbine engine burning gasoline, the typical residence time may be about 5 milliseconds or less. The total residence time in the catalytically-supported combustion zone should be sufficinet to provide essentially complete combustion of the fuel, but not so long as to result in the formation of N0 Nitrogen oxides found in the effluent may have been introduced to the system from the air supply or-even from the fuel as an impurity. By reason of the invention, combustion occurs, however, without the substantial formation of NO;. Typically, the combustion effluent will contain less than about parts per million by volume of NO, above the amount fed to the combustion system. It is of further significancethat the effluent typically may contain less than'about 10 parts per million by volume hydrocarbons, and in some cases, less than about parts per million by volume of carbon monoxide. Effluents this low in pollutants are most ac* ceptable and are far below any requirements of the Federal Emission Standards established by the Environmental Protection Agency for 1976 for automobile emissions.

The solid oxidation catalysts useful for the invention may include any of a number of catalysts used for the oxidation of fuels. Typically, the catalyst comprises a carrier and an active component with or without the addition of other activators or promoters. These catalysts may include a wide variety of materials as well as configurations or structures. For example, the catalyst may comprise a packed bed of pellets, saddles, rings, or the like. Preferably, the catalyst comprises a monolithic or unitary structure comprising a ceramic substrate or carrier impregnated with one or more catalyticallyactive components. Monoliths of this type may be shaped ceramic fiber, usually in cylindrical form or thin-walled honeycomb-type structures. The flow channels in the honeycomb structures are usually parallel and may be of any desired cross-sectional shape such as triangular or hexangular. The number of channels per square inch may vary greatly depending upon the particular application, and monolithic honeycombs are commercially available having anywhere from about 50 to 2,000 channels per square inch. The substrate or carrier portion of the honeycomb desirably is porous, but may be essentially non-porous, and catalytically is relatively inert. The substrate may be provided with a porous film or coating, typically of alumina, which is impregnated with one or more catalytically-active components. Structures of this type are particularly desirable because the pressure drop of gases passing through them is relatively low, and generally they are selfsupporting. The catalytically-active component of the catalyst is generally metal either in the elemental state or .in the combined state, for example, oxides. Included are heavy refractory types as well asother Transition metals for example, zirconium, vanadium, chromium, manganese, copper, platinum, palladium, iridium, rhodium, ruthenium, cerium, cobalt, nickel, and iron. The particular catalyst and amount employed may depend primarily upon the design of the combustion system, the type of fuel used and operating temperature. The pressure drop of the gases passing through the catalyst, for example, may be below about 10 psi, preferably below about 3 psi, or less than about 10 percent of the total pressure.

The present invention will be described further in connection with the drawings in which:

FIG. 1 is a schematic diagram of a turbine system employing a thermalcombustor, a catalytic combustor and a turbine in accordance with the present invention;

- combustor and a second turbine; and

FIG. 3 has been previously described.

With reference to the FIG. 1, a power shaft ltltcarries a turbine-type air compressor 12 and gas power turbine 14. The shaft 10 can be connected to, say, any suitable power transmission system to use the power imparted to the shaft by turbine 14. Thus, turbine 14 may be employed to operate an electrical generator, an automotive vehicle through an automatic transmission. Turbine 14 may, for example, be a high compression turbine having a compression ratio of 10:1. Turbines gen erally have compression ratios of at least about 2:1 and typically at least about 5:1 causing combustion to occur at elevated pressures relative to ambient pressure. The compression ratios preferred to are the approximate number of atmospheres under which combustion takes place. Automotive vehicles will generally have a turbine with a compression ratio of about 5:1 indicating that combustion takes place at about 75 psia when ambient air is at about one atmosphere; The structure, operation and control of turbines are known in the art, details in this regard will be omitted from this description since they are unnecessary to explain the present invention.

A hydrocarbon fuel, for instance, liquid JP-4 turbine fuel enters thermal combustor 16 by line 18 after passing through pump 20 and valve 22 which regulates the amount of fuel sent to the turbine system. Air to be supplied to the thermal combustor 16 enters compressor 12 by a line 24, is compressed, and exits by line 26. The compressed air flow is then divided between line 26 and line 28 which mates with line 26. Line 28 directs a portion of the compressed air through regulating valve 30 to the thermal combustor 16.

lgnitor 32 in the thermal combustor l6 ignites the fuel in the presence of the air during start-up. As soon as flame supported thermal oxidation is sustained, ignitor 32 is shut down. Thermocouple 34 is positioned at the exit of the thermal combustor to detect the temperature within thermal combustor 16 at this location. The partially combusted fuel is quenched by another portion of compressed air from line 26 passing through regulating valve 37 through line 38 and entering quenching zone 40. The admixture of air and the partially combusted fuel is fed into catalytic combustor 42. While catalytic combustor 42 is represented as separate from turbine 14 in the present drawing, if desired, the catalytic combustor can also be located adjacent to turbine 14 or the catalytic combustor may also be followed by a thermal oxidation zone located before the turbine 14.

The partially combusted fuel and air mixture after entering catalytic combustor 42 is contacted with catalyst 46.

Thermocouples

48 and 50, located adjacent to the initial and final catalyst surface, respectively, detect the temperature of the entering and exiting gases at these locations. The catalytically combusted gases exit the catalytic combustor 42 by line 52, connected to turbine 14, where the gases are expanded in the usual manner to impart rotating motion to shaft 10. The exhaust gases exit turbine 14 by exhaust line 54 and are, for instance, released to the atmosphere.

When operation has been established, the quenched, partially combusted fuel-air admixture entering the catalytic combustor 42 via quenching zone 40 is at 3 velocity prior to or at inlet to the catalyst in excess of the maximum flame propagation velocity.

FIG. 2 illustrates another form of the inventionyin FIG. 2 like numbers designate the same members as in FIG. 1 except that turbine 14 is positioned between the thermal combustor 16 and the catalytic combustor 46 and is driven by the effluent gases from the thermal combustor. After these gases are allowed to expand, they pass through line 36 and are admitted to the catalytic combustor 42. The effluent gases from catalytic combustor 42 are used to drive a second turbine 56 which imparts motion to shaft 58. In this system, turbine 14 is used as a so-called free turbine. If desired, the exhaust line 54 in either FIG. 1 or FIG. 2 can be used, through proper design, as a source of indirect heat for the air passing through valve 30 and line 28 to be admixed with the fuel to the thermal combustor 16.

The following examples will more fully illustrate the embodiments of this invention, in particular the method of essentially completely combusting partially oxidized carbonaceous fuels by contact with a catalyst in order to obtain an effluent containing very small Both pieces of catalyst have approximately 100 flow channels per square inch of cross-section with the walls of the channels having a thickness of 10 mils. The catalysts have similar compositions and are composed of a zircon mullite honeycomb support which carries a coating of alumina containing palladium, chromia, and ceria.

The catalyst for these examples typically can be made by slurrying 2,400 grams .of activated alumina powder, less than 40 mesh in size, in a mixer with a solution prepared by dissolving 2,526 grams Cr(NO -9 H 0 and 1,382 grams Ce(NO 6 H O in 890 ml. H O. The mixture is dried at C. over a weekend. The dried solids are crushed and screened to less than 40 mesh, and then the powder is calcined for 4 hours at 1,000 C. 3,200 grams of the powder is charged to a 3.4 gallon ball mill along with 3,200 ml. H 0 and 145.4 grams of palladium nitrate. The mill is rolled for 17 hours at 54 RPM. The resulting slip has a density of 1.63 grams per ml., a pH of 4.20 and a viscosity of 12 centipoises. 1,625 grams of the asrecovered slip are diluted with 1,180 ml. of a 1 percent nitric acid solution. The zircon mullite honeycomb is dipped in the diluted slip and held for one minute, and then withdrawn from the slip and blown with air to remove excess slip. The

coated honeycomb is dried for 16 hours at 110 C. and then calcined for 2 hours at 500 C. The honeycomb is cooled, and shows a pickup of l 1.0 weight percent slip or coating.

The upstream or initial catalyst in the housing has a catalytic coating of 13.9 weight percent of the catalyst. This coating is 70 weight percent alumina, 14 weight percent Cr O and 16 weight percent CeO based on these components. The catalyst also contains 0.23 weight percent palladium (calculated) disposed in the coating. The subsequent-in-line catalyst has a similar coating of alumina, ceria and chromia which is 11.0 weight percent of the catalyst. The catalyst also contains 0.18 weight percent palladium (calculated) disposed in the coating.

EXAMPLE 1 Type A jet fuel having 14 percent aromatics and a 344 F. to 504 F ASTM Boiling Point range and com- I pressed air in percent of the stoichiometric ratio necessary to fully combust the fuel are supplied by separate inlets to the thermal combustor. The compression ratio is 4 to 1. The mixture is ignited to produce an eflowed to expand in a turbine to produce rotating motion and exhausted through a test chamber to determine pollutant content.

The inlet air temperature is l,200 F. and the quench air temperature is 600 F. The temperature at the outlet of the flame supported thermal combustion zone is in excess of 3,300 F. The outlet temperature from the catalytic unit is 2,500 F. Results of this two stage combustion produce essentially complete oxidation of the fuel, and it is found that the effluent is low in NO, emissions.

EXAMPLE 2 JP-4 fuel and compressed air in 85 percent of the stoichiometric ratio necessary to completely combust the fuel are supplied by separate inlets to the thermal combustor. The compression ratio is 4 to l. The mixture is ignited to produce an effluent of about 40 percent oxidized fuel. The effluent is mixed in the quenching zone with sufficient excess air to give an adiabatic flame temperature of 2,300 F. The quenched effluent air admixture is then passed to the catalytic combustion unit and fully combusted. The effluent from the catalytic unit is allowed to expand in a turbine to produce rotating motion and exhausted through a test chamber as in Example 1.

The inlet air temperature and the quench air temperature are both 400 F. The temperature at the outlet of the flame supported thermal combustion zone is less than 2,800 F. The outlet temperature from the catalytic unit is 2,200 F. Results of this two stage combustion are similar to Example 1.

EXAMPLE 3 Diesel fuel having an 18 percent aromatic content and a 368-644 F. ASTM boiling range and compressed air in 95 percent of the stoichiometric ratio necessary to completely combust the fuel are supplied by separate inlets to the thermal combustor. The compression ratio is to l. The mixture is ignited to produce an effluent of about 65 percent oxidized fuel. The effluent is mixed in the quenching zone with sufficient excess air to give an adiabatic flame temperature of 2,500 F. The quenched effluent air admixture is then passed to the catalytic combustion unit and fully combusted. The effluent from the catalytic unit is allowed to expand in a turbine to produce rotating motion and exhausted through a test chamber as in Example 1.

The inlet air temperature and the quench air temperature are both 600 F. The temperature at the outlet of the flame supported thermal combustion zone is in excess of 3,300 F. The outlet temperature from the catalytic unit is 2,400 F. Results of this two stage combustion are similar to Example 1.

EXAMPLE 4 Diesel fuel identical to that of Example 3 and compressed air in 110 percent of the stoichiometric ratio necessary to completely combust the fuel aresupplied by separate inlets to the thermal combustor. The compression ratio is 20 to l. The mixture is ignited to produce an effluent of about 90 percent oxidized fuel. The effluent is mixed in the quenching zone with sufficient excess air to providean adiabatic flame temperature of l .500 F. with sufficient additional fuel sprayed into the quenched effluent to raise the adiabatic flame temperature to 2,600 F. The quenched effluent air admixture is then passed to the catalytic combustion unit and fully combusted. The effluent from the catalytic unit is allowed to expand in a turbine to produce rotating motion and exhausted through a test chamber as in Example 1.

The inlet air temperature and the quench air temperature are both 750 F. The temperature at the outlet of the flame supported thermal combustion zone is in excess of 3,300 F. The outlet temperature from the catalytic unit is 2,500 F. Results of this two stage combustion are similar to Example 1.

In addition to employing the method of the present invention for powering gas turbines, the combustion system can be employed, for example, as a heat source in steam boilers wherein the heat of the exhaust gases are employed to generate steam as in a water-tube boiler, air heaters, hot water heaters and process furnaces.

What is claimed is:

1. In a method for oxidizing carbonaceous fuel which when burned with a stoichiometric amount of air has an adiabatic flame temperature of at least about 3,300 F., the improvement comprising admixing the carbonaceous fuel and air, partially combusting the fuel in a thermal combustion zone and producing an effluent containing partially combusted fuel, immediately quenching said effluent, and essentially adiabatically oxidizing at a temperature of about 1,500 to 3,200 F. at least about 10 percent-of the fuel by contacting the quenched effluent containing partially combusted fuel with a solid oxidation catalyst.

2. The method of claim 1 wherein the partially combusted effluent is quenched by admixture with additional air.

3. The method of claim 2 wherein the temperature of said catalytic oxidation catalyst is about 2,000 to 3,000 F.

4. The method of claim 3 wherein the temperature of the gases from the thermal combustion zone is about l,800 to 2,800 F.

5. The method of claim 3 wherein the total amount of the air for thermal combustion and the additional air for quenching is at least about two times the stoichiometric amount needed for complete combustion of the fuel.

6. The method of claim 1 wherein the carbonaceous fuel is combusted to about 35 to percent of complete oxidation in the thermal combustion zone.

7. The method of claim 1 wherein the temperature of the thermal combustion zone is about 1,800 to 3,700 F.

8. The method of claim 1 wherein the temperature of the thermal combustion zone is about l,800 to 2,800 F.

9. The method of claim 1 wherein the quenched effluent has a theoretical adiabatic flame temperature of about 1,500 to 3,200 F.

10. In a method for operating a gas turbine which comprises oxidizing a carbonaceous fuel, which when burned with a stoichiometric amount of air has an adiabatic flame temperature of at least about 3,300 F., and air to produce a motive fluid for rotating the turbine, the improvement comprising admixing the carbonaceous fuel and air, partially combusting the fuel in a thermal combustion zone to about 35 to 90 percent of complete oxidation and producing an effluent containing partially combusted fuel, immediately quenching said effluent with air in excess of the stoichiometric amount sufficient to completely oxidize the partially combusted fuel, essentially adiabatically oxidizing at a temperature of about 2,000 to 3,000 F. in a catalytic oxidation zone at least about 10 percent of the fuel by contacting the quenched effluent containing partially combusted fuel with a solid oxidation catalyst, and passing the effluent from the catalytic oxidation zone through a gas turbine to rotate the turbine.

11. The method of claim 10 wherein the partial combustion effluent is further quenched by contact with said catalyst.

12. The method of claim 10 in which the weight ratio of air to partially combusted fuel in the admixture prior to catalytic combustion is above about 20:1.

13. The method of claim 10 wherein the temperature of the thermal combustion zone is about 1,800 to 3,700 F.

14. The method of claim wherein the temperature of the thermal combustion zone is about 1,800 to 2,800 F.

15. In a method for operating a gas turbine which comprises oxidizing a carbonaceous fuel, which when burned with a stoichiometric amount of air has an adiabatic flame temperature of at least about 3,300 F., and air to provide a motive fluid for rotating the turbine, the improvement comprising admixing the carbonaceous fuel and air, partially combusting the fuel in a thermal combustion zone and producing an effluent containing partially combusted fuel, immediately quenching said effluent, essentially adiabatically oxidizing at a temperature of about 1,500 to 3,200 F. in a catalytic oxidation zone at least about 10 percent of the fuel by contacting the quenched effluent containing partially combusted fuel with a solid oxidation catalyst, and passing the effluent from the catalytic oxidation zone through a gas turbine to rotate the turbine.

16. The method of claim wherein the carbonaceous fuel is combusted to about 35 to 90 percent of complete oxidation in the thermal combustion zone.

17. A method for the catalytically-supported thermal combustion of carbonaceous fuel comprising:

a. thermally oxidizing a carbonaceous fuel-air mixture to produce a partially oxidized vaporous effluent having a stoichiometric excess of air;

. quenching said effluent; and

c. contacting said quenched, partially oxidized effluent with a solid oxidation catalyst to combust said partially oxidized effluent under essentially adiabatic conditions to form a final effluent of high thermal energy; said combustion being characterized by said partially oxidized effluent having an adiabatic flame temperature such that upon contact with said catalyst, the operating temperature of said catalyst is substantially above the instantaneous auto-ignition temperature of said partially oxidized effluent but below a temperature that would result in any substantial formation of oxides of nitrogen.

18. A method as defined in claim 17 wherein said quenching is accomplished by introducing air into said partially oxidized effluent, said air being present in a sufficient amount to insure a stoichiometric excess thereof and being at a temperature sufficiently low to insure incomplete combustion of said carbonaceous fuel.

19. A method as defined in claim 17 further comprising the additional step of introducing fuel into said effluent to increase the adiabatic flame temperature thereof.

20. A method as defined in claim 17 further comprising the step of adding air to said final effluent, said air having a temperature lower than that of said final effluent, thereby cooling said effluent.

21. A method as defined in claim 17 wherein said.

final effluent is employed to produce power.

22. A method as defined in claim 17 wherein said adiabatic flame temperature is about 1,500 to 3,200 F.

23. A method as defined in claim 17 wherein said quenched partially oxidized effluent contains at least about 1.5 times the stoichiometric amount of oxygen based on complete combustion of said carbonaceous fuel.

24. A method as defined in claim 17 wherein the residence time of said quenched, partially oxidized effluent undergoing catalytic combustion is less than about 0.05 second. l

25. A method for operating a gas turbine by the catalyticallyrsupported thermal combustion of carbonaceous fuel comprising:

b. quenching said effluent;

c. contacting said quenched, partially oxidized effluent with a solid oxidation catalyst to combust said partially oxidized effluent under essentially adiabatic conditions to form a final effluent of high thermal energy, said combustion being characterized by said partially oxidized effluent having an adiabatic flame temperature such that upon contact with said catalyst, the operating temperature of said catalyst is substantially above the instantaneous auto-ignition temperature of said partially oxidized effluent but below a temperature that would result in any substantial formation of oxides of nitrogen; and

d. passing said final effluent from said catalytic combustion through a gas turbine to cause rotary motion.

Claims

1. IN A METHOD FOR OXIDIZING CARBONACEOUS FUEL WHICH WHEN BURNED WITH A STOICHIOMETRIC AMOUNT OF AIR HAS AN ADIABATIC FLAME TEMPERATURE OF AT LEAST ABOUT 3,300*F., THE IMPROVEMENT COMPRISING ADMIXING THE CARBONACEOUS FUEL AND AIR, PARTIALLY COMBUSTING THE FUEL IN A THERMAL COMBUSTION ZONE AND PRODUCING AN EFFLUENT CONTAINING PARTIALLY COMBUSTED FULE, IMMEDIATELY QUENCHING SAID EFFLUENT, AND ESSENTIALLY ADIABATICALLY OXIDIZING AT A TEMPERATURE OF ABOUT 1,500* TO 3,200* F. AT LEAST ABOUT 10 PERCENT OF THE FUEL BY CONTACTING THE QUENCHED EFFLUENT CONTAINING PARTIALLY COMBUSTED FUEL WITH A SOLID OXIDATION CATALYST.

3. The method of claim 2 wherein the temperature of said catalytic oxidation catalyst is about 2,000* to 3,000* F.

4. The method of claim 3 wherein the temperature of the gases from the thermal combustion zone is about 1,800* to 2,800* F.

6. The method of claim 1 wherein the carbonaceous fuel is combusted to about 35 to 90 percent of complete oxidation in the thermal combustion zone.

7. The method of claim 1 wherein the temperature of the thermal combustion zone is about 1,800* to 3,700* F.

8. The method of claim 1 wherein the temperature of the thermal combustion zone is about 1,800* to 2,800* F.

9. The method of claim 1 wherein the quenched effluent has a theoretical adiabatic flame temperature of about 1,500* to 3, 200* F.

10. In a method for operating a gas turbine which comprises oxidizing a carbonaceous fuel, which when burned with a stoichiometric amount of air has an adiabatic flame temperature of at least about 3,300* F., and air to produce a motive fluid for rotating the turbine, the improvement comprising admixing the carbonaceous fuel and air, partially combusting the fuel in a thermal combustion zone to about 35 to 90 percent of complete oxidation and producing an effluent containing partially combusted fuel, immediately quenching said effluent with air in excess of the stoichiometric amount sufficient to completely oxidize the partially combusted fuel, essentially adiabatically oxidizing at a temperature of about 2,000* to 3,000* F. in a catalytic oxidation zone at least about 10 percent of the fuel by contacting the quenched effluent containing partially combusted fuel with a solid oxidation catalyst, and passing the effluent from the catalytic oxidation zone through a gas turbine to rotate the turbine.

13. The method of claim 10 wherein the temperature of the thermal combustion zone is about 1,800* to 3,700* F.

14. The method of claim 10 wherein the temperature of the thermal combustion zone is about 1,800* to 2,800* F.

15. In a method for operating a gas turbine which comprises oxidizing a carbonaceous fuel, which when burned with a stoichiometric amount of air has an adiabatic flame temperature of at least about 3,300* F., and air to provide a motive fluid for rotating the turbine, the improvement comprising admixing the carbonaceous fuel and air, partially combusting the fuel in a thermal combustion zone and producing an effluent containing partially combusted fuel, immediately quenching said effluent, essentially adiabatically oxidizing at a temperature of about 1, 500* to 3,200* F. in a catalytic oxidation zone at least about 10 percent of the fuel by contacting the quenched effluent containing partially combusted fuel with a solid oxidation catalyst, and passing the effluent from the catalytic oxidation zone through a gas turbine to rotate the turbine.

16. The method of claim 15 wherein the carbonaceous fuel is combusted to about 35 to 90 percent of complete oxidation in the thermal combustion zone.

17. A method for the catalytically-supported thermal combustion of carbonaceous fuel comprising: a. thermally oxidizing a carbonaceous fuel-air mixture to produce a partially oxidized vaporous effluent having a stoichiometric excess of air; b. quenching said effluent; and c. contacting said quenched, partially oxidized effluent with a solid oxidation catalyst to combust said partially oxidized effluent under essentially adiabatic conditions to form a final effluent of high thermal energy; said combustion being characterized by said partially oxidized effluent having an adiabatic flame temperature such that upon contact with said catalyst, the operating temperature of said catalyst is substantially above the instantaneous auto-ignition temperature of said partially oxidized effluent but below a temperature that would result in any substantial formation of oxides of nitrogen.

21. A method as defined in claim 17 wherein said final effluent is employed to produce power.

22. A method as defined in claim 17 wherein said adiabatic flame temperature is about 1,500* to 3,200* F.

24. A method as defined in claim 17 wherein the residence time of said quenched, partially oxidized effluent undergoing catalytic combustion is less than about 0.05 second.

25. A method for operating a gas turbine by the catalytically-supported thermal combustion of carbonaceous fuel comprising: a. thermally oxidizing a carbonaceous fuel-air mixture to produce a partially oxidized vaporous effluent having a stoichiometric excess of air; b. quenching said effluent; c. contacting said quenched, partially oxidized effluent with a solid oxidation catalyst to combust said partially oxidized effluent under essentially adiabatic conditions to form a final effluent of high thermal energy, said combustion being characterized by said partially oxidized effluent having an adiabatic flame temperature such that upon contact with said catalyst, the operating temperature of said catalyst is substantially above the instantaneous auto-ignition temperature of said partially oxidized effluent but below a temperature that would result in any substantial formation of oxides of nitrogen; and d. passing said final effluent from said catalytic combustion through a gas turbine to cause rotary motion.