Process Conditions for Operation of Ignition Catalyst for Natural Gas Combustion

Burning of carbonaceous fuels is associated with formation of air pollutants, among the most troublesome of which are nitrogen oxides (NO.sub.x). Nitrogen oxides form whenever air supported combustion takes place at open flame temperatures. One approach to eliminating nitrogen oxides involves chemically modifying the oxides after their formation. This approach has drawbacks, including the high cost associated with attempting to eliminate 100% of a once-formed pollutant. A more direct method of eliminating nitrogen oxides is to operate the combustion process at a lower temperature so that no formation of nitrogen oxide occurs. Such low temperature combustion can take place in the presence of catalysts, and it is to such a low temperature combustion process that this invention is directed.

This invention relates to a particularly advantageous process for the catalytically supported combustion of carbonaceous materials including natural gas or methane. In a more specific aspect, this invention relates to a process for catalytically-supported combustion of natural gas using a palladium oxide catalyst without the formation of substantial amounts of nitrogen oxides.

In general, conventional adiabatic, thermal combustion systems (e.g., gas turbine engines) operate at such high temperatures in the combustion zone that undesirable nitrogen oxides, especially NO, are formed. A thermal combustion system operates by contacting fuel and air in flammable proportions with an ignition source, e.g., a spark, to ignite the mixture which then will continue to burn. Flammable mixtures of most fuels burn at relatively high temperatures; i.e., about 3300.degree. F. and above, which inherently results in the formation of substantial amounts of NO.sub.x. In the case of gas turbine combustors, the formation of NO.sub.x can be reduced by limiting the residence time of the combustion products in the combustion zone. However, due to the large quantities of gases being handled, undesirable quantities of NO.sub.x are nonetheless produced.

It has long been realized that little or no NO is formed in a system which burns a fuel catalytically at relatively low temperatures. Typically, such catalytic combustion of natural gas or methane, for example, utilizes a preburner or thermal combustor which employs flame combustion to preheat combustion air to a temperature of 700.degree. C. or higher. Once the catalyst is sufficiently hot to sustain catalysis, the preburner is shut down and all the fuel and air are directed to the catalyst. Preheat is then only due to compressor discharge. Such a Catalytic combustor, if operated at temperatures below about 1300.degree.-1400.degree. C., avoids the nitrogen oxide formation which occurs at the higher temperatures which are characteristic of the flame combustion. A description of such a catalytic combustion process and apparatus is found, for example, in U.S. Pat. 3,928,961. See also U.S. Pat. Nos. 4,065,917 and 4,019,316.

Such catalytic combustion as described above which will function effectively at a high space velocity has, however, heretofore been generally regarded as commercially unattractive. A primary reason for this lack of commercial attractiveness has been the absence of an economically competitive method for catalytic combustion of natural gas.

The present invention is directed to a method for operating a catalytic combustor using a palladium containing catalyst and using a novel set of unexpectedly effective operating parameters which permits high catalytic activity, and results in on-going retention and regeneration of such activity. At atmospheric pressure palladium containing catalysts are known to lose activity when subjected to temperatures in excess of about 800.degree. C., at which temperatures palladium oxide decomposes into palladium metal. The interaction of palladium oxide with reducing agents exacerbates such decomposition into palladium metal. One aspect of the present invention is concerned with compensating for an event of an over-temperature (or a continuing series of such over-temperatures) which causes catalyst deactivation. In the event of such over-temperature, the present invention utilizes procedures for regeneration of the catalyst, in-situ. For example, using a typical palladium on alumina catalyst, when start-up or operation of the catalytic combustor results in exposing the ignition catalyst to a temperature in excess of about 800.degree. C. at atmospheric pressure, resulting in loss of catalyst activity, the over-temperature is, according to the present invention, followed by an atmospheric pressure regenerating temperature soak between about preferably 530.degree.-650.degree. C. and more preferably 560.degree.-650.degree. C., which oxidizes the palladium on alumina to active palladium oxide. Even if the entire catalytic combustor does not reach a catalyst inactivating over-temperature, isolated hot spots within the catalytic combustor may be subjected to an over-temperature, and the heat soak of the present invention will provide a catalyst regenerating benefit. Thus, a regenerating temperature soak according to the present invention unexpectedly regenerates the activity lost due to an over-temperature in all or part of the combustor.

As those skilled in the art will appreciate, the above stated temperature ranges are dependent on the partial pressure of oxygen, and at higher pressures, as for example might be encountered in conjunction with generation o combustion effluent useful for operation of gas turbines, the decomposition temperature at which palladium oxide will decompose into metallic palladium will increase, as will the regeneration temperature at which palladium oxide will reform. References hereinafter to these temperatures are all at atmospheric pressures, it being understood that at enhanced partial pressure of oxygen the decomposition and regenerating temperatures will shift upward, and that the determination of such increased temperatures at higher oxygen partial pressures will be a matter well known to those skilled in the art.

Another aspect of the present invention involves the discovery that the temperatures of palladium oxide decomposition and recombination may be varied depending on the metal oxide support used for the palladium oxide, and the present invention is directed to utilizing this variation to optimize catalytic combustion processes.

Catalytically supported combustion processes have been described in the prior art. See, e.g. Pfefferle, U.S. Pat. No. 3,928,961. The use of natural gas or methane in catalytic combustion has been taught in the art, as has the use of a palladium catalyst to promote such combustion/oxidation. See Cohn, U.S. Pat. No. 3,056,646 wherein the use of palladium catalyst to promote methane oxidation is generically disclosed, as is an operable temperature range, 271.degree. C.-900.degree. C. (see column 2, lines 19-25). Note also that this patent states "the higher the operating temperature, the shorter will be the catalyst life and the more difficult will be subsequent ignition after catalyst cooling". Other patents directed to the use of platinum group metals as catalysts for methane oxidation at temperatures above 900.degree. C. include U.S. Pat. No. 3,928,961; 4,008,037; and 4,065,917. The literature also describes the thermal decomposition of PdO to Pd metal at temperatures of 800.degree. C. in air at atmospheric pressure. See Kirk Othmer Encyclopedia of Chemical Technology, Vol. 18, p. 248 which states that palladium acquires a coating of oxide when heated in air from 350.degree. C. to 790.degree. C. but that above this temperature the oxide decomposes and leaves the bright metal.

The present invention finds particular utility in a process for the start-up of catalytically supported combustion. Prior art references directly related to such start-up are Pfefferle, U.S. Pat. No. 4,019,316 and Pfefferle, U.S. Pat. No. 4,065,917.

This invention relates to a method for operating a palladium oxide containing catalytic combustor useful, e.g., for powering a gas turbine. The invention involves maintaining control of the temperature within the catalytic combustor in such a manner as to insure the presence of active palladium oxide. By maintaining the temperature below about 800.degree. C., decomposition of palladium oxide on alumina into metallic palladium is avoided and high catalytic activity is maintained. However, in the event of an over-temperature, or reduction of palladium oxide as a result of chemical interaction with a reducing agent, such as an excess of fuel, regeneration following inactivation due to loss of PdO can be accomplished by bringing a deactivated palladium on alumina catalyst to a temperature within the regenerating temperature range of about preferably 530.degree. C.-650.degree. C., and more preferably 560.degree. C.-650.degree. C., where reoxidation occurs at a reasonable rate.

Further, according to the present invention, the temperatures of palladium oxide decomposition, and the temperatures of palladium oxide reformation are varied by changing the metal oxide support used for the palladium oxide. The temperature ranges stated above are those which are effective for palladium on alumina. However, the temperature for reformation of palladium oxide is, to an extent, dependent on the metal oxide used to support the palladium, and other suitable metal oxide support materials, such as ceria, titania and tantalum oxide have characteristic temperatures at which palladium oxide thereon will decompose and recombine. These characteristic temperatures, which can be determined by those skilled in the art by means such as, for example, thermal gravimetric analysis, permit the selection of appropriate metal oxide support materials, and thus provide control over palladium oxide decomposition/reformation temperature ranges.

The present invention is described and illustrated with reference to the following drawings, in which

FIG. 1 is a partial schematic breakaway view of a preburner/catalytic combustor system which is operable in accordance with the present invention.

FIG. 2 is a thermogravimetric analysis (TGA) plot in air of temperature versus sample weight, as set out on the right ordinate. Superimposed on this TGA plot, is a plot of temperature versus percent conversion of 1% methane in air (an indice of activity), as shown on the left ordinate.

FIG. 1 depicts a combustor with a precombustion chamber, 20, fed by air, 15, which is exiting from compressor, 25, and fuel nozzle, 13, which is connected to fuel line, 14. The fuel and air together pass through mixer, 17, prior to entering the precombustion chamber, 20. Feeding into the precombustion chamber is a preburner 12, also connected to the air line, 15, and fuel line, 14, which sprays hot combustion gases from injector line, 18. The catalyst is positioned on a supporting monolith, 10, from which the hot combustion gases move downstream to drive turbine, 30.

The procedure used to obtain the data graphed in FIG. 2 was as follows: first a sample of a conventional palladium on aluminum oxide catalyst was prepared according to a standard procedure, viz., gamma alumina was calcined at 950.degree. C. for 2 hours and then screened to particle sizes between of 53 and 150 microns. This gamma alumina was used as a catalyst carrier. The use of gamma alumina as a catalyst carrier in this example was, as those skilled in the art will readily appreciate, simply a matter of choice. Other suitable carriers include, for example, modified alumina (i.e. aluminas which contain surface area stabilizers such as silica, barium oxide, lanthanium oxide and cerium oxide) silica, zeolites, titania, zirconia and ceria as well as mixtures of the foregoing. Ten grams of the described alumina carrier was impregnated with a Pd(NO.sub.3).sub.2.6H.sub.2 O solution by the incipient wetness method to give approximately 4 wt% Pd on the finished catalyst. The Pd was then fixed on the catalyst by a conventional reduction with an aqueous hydrazine solution. The reduced catalyst was dried at 120.degree. C. overnight and calcined at 500.degree. C. for 2 hours to give what will hereafter be designated as "fresh catalyst".

The TGA profile of FIG. 2 was generated by heating this fresh PdO Al.sub.2 O.sub.3 catalyst in air at 20.degree. C./min. The heating portion of the graph depicts a weight loss above about 800.degree. C. where decomposition of PdO to Pd metal occurs. Following decomposition, heating continued to 1100.degree. C. where it was held for 30 minutes.

The temperature program was then reversed allowing the catalyst to cool in air. Unexpectedly, no weight increase due to re-oxidation of the Pd metal was observed until about 650.degree. C. below which a sharp increase was observed which plateaus at about 560.degree. C. to 530.degree. C. Upon continued cooling below 530.degree. C. there was a small but steady weight increase down to room temperature. Repeated heating and cooling cycles of the same sample demonstrates the same temperature dependent weight changes.

Referring to the other data graphed on FIG. 2, the percent conversion plot as read on the left ordinate of FIG. 2 is a measure of catalytic activity.

The procedure used to obtain the graphed data on catalytic activity was as follows: a sample of catalyst (0.06 g ), prepared as described above, was mixed with 2.94 g diluent (alpha alumina) which had been screened to a particle size range of from 50 to 150 microns. This 3 g catalyst charge was supported on a porous quartz frit in a 1" diameter quartz reactor tube. The tube was then positioned vertically in a programmable tube furnace. A thermocouple was positioned axially in the catalyst bed for continuous monitoring and connections to a gas (fuel) stream secured. A fuel mixture of 1% methane in zerograde air metered by a mass flow controller was flowed through the system at a rate of 3 liters per minute. The use of methane as a fuel was, as those skilled in the art will readily appreciate, simply a matter of choice. Other suitable fuels would include, for example, natural gas, ethane, propane, butane, other hydrocarbons, alcohols, other carbonaceous materials, and mixtures thereof. The gas exiting the reactor was analyzed by a Beckman Industrial Model 400A Hydrocarbon Analyzer. The analyzer was zeroed on air and spanned to 100% on the fuel mixture at ambient conditions. The procedure wa initiated by ramping the furnace to a selected maximum temperature. This temperature was held for a limited time and then the furnace was shut off and the reactor permitted to cool. A multi-channel strip chart simultaneously recorded the catalyst bed temperature and the concentration of hydrocarbon in the exit gas stream. This data thus provided a profile of the temperature dependence of methane oxidation/combustion.

The activity of the catalyst, as determined by the percent conversion of the methane fuel, was measured at various increasingly higher temperatures and the results were plotted as the dashed line in FIG. 2. FIG. 2 shows that at progressively higher temperatures the percent conversion of the methane becomes greater, until at approximately 800.degree. C. the conversion becomes essentially 100%. At this temperature, the reaction in effect became a thermal reaction as opposed to a catalytic reaction. The activity data in FIG. 2 also demonstrates that the continuous, rapid increase in percent conversion with an increase in temperature is followed by a rapid decrease in percent conversion with a reduction in temperature. The decrease in percent conversion (or activity) undergoes a reversal below about 700.degree. C. during a cooling cycle, at which point percent conversion (activity) begins to increase until a temperature of about 600.degree. C. is obtained. At that point, the catalyst again demonstrated the same activity as the catalyst had initially demonstrated (during the heating cycle at that temperature. This observation was made for all repeated cycles.

Further PdO on Al.sub.2 O.sub.3 samples were pre-calcined in air for 17 hours to 1100.degree. C. followed by cooling in air to room temperature. TGA profiles of these samples were qualitatively identical to second cycles of fresh samples. Thus, in both cases the PdO decomposes to Pd metal during heat-up, and PdO forms below about 650.degree. C. during cool down.

PdO powder was prepared using the identical procedure as or PdO on Al.sub.2 O.sub.3. Heating of this sample clearly showed only one weight loss process between 810.degree. C. and 840.degree. C. in which the PdO decomposes to Pd metal. The weight loss observed, approximately 13%, agrees with decomposition of PdO to Pd.

Samples of PdO/Al.sub.2 O.sub.3 were calcined to 1100.degree. C. in air and evaluated for activity as a function of temperature as described above. During heat-up, conversion was first noted at about 340.degree. C. and slowly rose to 30% at about 430.degree. C. after which percent conversion rapidly increased with temperature up to 90% at about 650.degree. C. Above this temperature the thermal process became significant. The furnace ramp continued to increase catalyst temperature up to 1000.degree. C., well beyond the temperature of decomposition of PdO to Pd metal. The temperature was then reduced and the sample cooled in CH.sub.4 /air. At about 720.degree. C. the thermal process began to extinguish and the conversion fell far below the conversion observed during heat up, demonstrating that the catalyst had lost activity. The catalyst activity at this point became virtually zero.

As the Pd/Al.sub.2 O.sub.3 continued to cool and the conversion due to the thermal component decreased to about 50% there was a sudden unexpected increase in activity at about 680.degree. C. and a maximum of 70% at 650.degree. C. The conversion curve upon continued cooling efectively overlaps that generated during heat up.

The TGA profile on a sample of the same catalyst, calcined to 1100.degree. C. in air for 17 hours clearly showed decomposition of PdO to Pd metal during heating. Upon cooling the large hysteresis in reoxidation is observed to occur around 650.degree. C. and is complete at 575.degree. C. closely paralleling the activity performance.

A sample of fresh PdO on A1203 catalyst was heated in air to 950.degree. C., well beyond the range where any weight loss occurred. The sample was then cooled to 680.degree. C. and held at that temperature for 30 minutes. No weight gain occurred. The sample was then cooled to 650.degree. C. at which temperature weight gain commenced. This example thus demonstrates that the hysteresis depicted in FIG. 2 is a temperature dependent process, not a rate process.

A sample of fresh PdO on Al.sub.2 O.sub.3 catalyst was heated in air to 950.degree. C., and then cooled to 680.degree. C. and held at that temperature for 30 minutes as in Example 5. The activity of the catalyst as indicated by its ability to catalyze the combustion of 1% methane in air was then measured. The catalyst was then cooled to 650.degree. C. and its activity again measured. The activity at 650 was much greater than at 680.degree. C., again demonstrating that the hysteresis depicted in FIG. 2 is a temperature dependent process, not the result of a rate process.

The dependence of palladium oxide decomposition temperature and reformation temperature on the metal oxide support was established by preparing samples of palladium on alumina, on tantalum oxide, on titania, on ceria and on zirconia and measuring in air decomposition and reformation temperatures using thermogravimetric analysis.

Table I lists the temperature (T.sub.D) for onset of PdO decomposition to Pd, the temperature (T.sub.R) for onset of reformation of PdO and the hysteresis equal to the differences (T.sub.D -T.sub.R), all at atmospheric pressure in air for palladium oxide supported on five different metal oxides. Table I shows that palladium oxide on alumina, tantalum oxide, titania, and ceria supports exhibits little variation in decomposition temperature. However, the choice of metal oxide does result in a pronounced effect on the reformation temperature. The differences between decomposition onset and reformation onset temperatures (T.sub.D -T.sub.R) vary from 210.degree. C. for Al.sub.2 O.sub.3 to 44.degree. C. for the CeO.sub.2 supported palladium. Typically, the smaller this difference (and the higher the reformation temperature), the easier it is to regenerate activity in a gas turbine.

The final metal oxide support listed in Table I is ZrO.sub.2. As seen from Table I, zirconia promotes premature decomposition of PdO to Pd at 682.degree. C. and inhibits reformation to a low temperature of 470.degree. C. This catalyst, therefore, has a large range and a relatively low temperature in which Pd metal is stable in an oxidizing environment. This is not a desirable property for methane oxidation.

The method of preparation for the five samples shown in Table I was as follows:

Catapal SB alumina (R-2139-84) was calcined at 950.degree. C. for 2 hours and then sieved to 75/150 micron particle size; 9.61 g of the alumina was impregnated with an aqueous solution of palladium nitrate using the incipient wetness technique. The palladium was then reduced using aqueous hydrazine. This material was dried at 110.degree. C. overnight and then calcined at 500.degree. C. for 2 hours in air to produce the finished catalyst.

5 g of SKK cerium oxide (CeO2) was impregnated with palladium nitrate as was done for the previous sample, adjusting the total volume of the impregnating solution to the incipient wetness of the support. The sample was then reduced, dried, and calcined at 500.degree. C. for 2 hours in air as was done for the Pd on alumina sample.

A 5 g sample of commercially available zirconia (Magnesium Eleckron SC101 Grade) was impregnated with palladium and handled just as was the Pd/ceria sample.

A sample of commercially available titania was calcined at 950.degree. C. for 2 hours and 8.2 g was then impregnated with palladium and handled just as was the Pd/ceria sample.

A 5 g sample of commercially available tantalum oxide (Ta.sub.2 O.sub.5) (Morton Thiokol) was impregnated with palladium just as was the Pd/ceria sample. The low incipient wetness of this material required a two step impregnation with a drying step in between. The rest of the preparation was the same as for the Pd/ceria.

These examples demonstrate that activity of a palladium oxide containing catalyst, as measured by its ability to promote the oxidation of methane, can be preserved by utilizing temperatures below the palladium oxide decomposition temperature which is the temperature at which catalyst deactivation will occur; and that, if activity is lost through over-temperature, original activity can be restored by using a heat soak at an effective temperature which depends on the metal oxide support being used with the palladium, and which effective temperature is below that at which onset of reformation of PdO occurs.