US3078663A

US3078663A - Residual fuels containing alkali metal and calcium, barium or strontium compounds

Info

Publication number: US3078663A
Application number: US795676A
Authority: US
Inventors: Albert G Rocchini; Charles E Trautman
Original assignee: Gulf Research and Development Co
Current assignee: Gulf Research and Development Co
Priority date: 1959-02-26
Filing date: 1959-02-26
Publication date: 1963-02-26
Anticipated expiration: 1980-02-26

Description

3,078,663 M, BARIUM TTOH/VEY Feb- 26, 1963 A. G. RoccHlNl ETAL RESIDUAL FUELS CONTAINING ALKALI METAL AND CALCIU OR STRONTIUM COMPOUNDS Filed Feb. 26, 1959 N SMM m ww m Mm MRT. TS E am Y B nl] I wwnkhwk mw NN n w ww Q w.. M n @v ww. a .Pwwvnhhwnhhm.nhbmwnvk NSR N\ n. im... -ULT .vddlj Lglam, ..W/m. mdvm L-. dvwMJ United States Patent Office 3,078,663 Patented Feb. 26, 1963 3,078,663 RESlDUAL FUELS CONTAINING ALKALI METAL AND CALCIUM, BARIUM R STRONTIUM COM- POUNDS Albert G. Rocchini, Oakmont, 'and Charles E. Trautman,

Cheswick, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed Feb. 26, 1959, Ser. No. 795,676 Claims. (Cl. 60-39.02)

This invention relates to vanadium-containing petroleum fuels. More particularly, it is concerned with rendering non-corrosive those residual fuels which con- -tain such an amount of vanadium as normally to yield a corrosive vanadium-containing ash upon combustion.

It has been observed that when a residual type fuel oil containing substantial amounts of vanadium is burned in furnaces, boilers and gas turbines, the ash resulting from combustion of the fuel oil is highly corrosive to materials of construction at elevated temperatures and attacks such par-ts as boiler tubes, hangers, turbine blades, and the like. These effects are particularly noticeable in gas turbines. Lange gas turbines show promise of becoming an important type of industrial prime mover. However, economic considerations based on the efficiency of the gas turbine dictate the use of a fuel for this purpose which is cheaper than a distillate diesel fuel; otherwise, other forms of power such as diesel engines become competitive with gas turbines.

One of the main problems arising in the use of residual fuel oils in gas turbines is the corrosiveness induced by those residual fuels containing suflicient amounts of Vanadium to cause corrosion. Where no vanadium is present or the amount of vanadium is small, no ap preciable corrosion is encountered. While many residual fuel oils as normally obtained in the relinery contain so little vanadium, or none, as to present no corrosion problems, such non-corrosive fuel oils are not always available at the point where the oil is to be used. In such instance, the cost of transportation of the non-corrosive oil to the point of use is often prohibitive, and the residual oil loses its competitive advantage. These factors appear to militate against the extensive use of residual fuel oils for gas turbines. Aside from corrosion, the formation of deposits upon the burning of a residual fuel in a gas tur-bine may result in imbalance of the turbine blades, clogging of openings and reduced thermal eliiciency of the turbine.

Substantially identical problems are encountered when using a solid residual petroleum fuel containing substantial amounts of vanadium. These fuels are petroleum residues obtained by known methods of petroleum retining such as deep vacuum reduction of asphaltic crudes to obtain solid residues, visbreaking of liquid distillation bottoms followed by distillation to obtain solid residues, coking of liquid distillation bottoms, and the like. The solid residues thus obtained are known variously as petroleum pitches or cokes and iind use as fuels. Since the vanadium content of the original crude oil tends to concentrate in the residual fractions, and since thel processing of the residual fractions to solid residues results in further concentrationof the vanadium in the solid residues, the vanadium corrosion lproblem tends to be intensified in using thesolid residues as fuel.

The vanadium-containing ash present in the hot flue gas obtained from the burning of a residual fuel containing substantial amounts of vanadium compounds causes catastrophic corrosion of the turbine blades and other metal parts in a gas turbine. The corrosive nature of the ash appears -to be due to its vanadium oxide content. Certain inorganic compounds of vanadium, such as vanadium oxide (V205), which are formed on combustion of a residua-l fuel oil containing Vanadium compounds, vigorously attack various metals, their alloys, and other materials at the elevated temperatures encountered in the combustion gases, the rate of attack becoming progressively more severe as the temperature is increased. The vanadium-containing ash forms deposits on the parts affected and corrosively reacts with them. It is a hard, adherent material when cooled to ordinary temperatures.

It has already been proposed to employ in corrosive residual fuels small amounts of certain metal compounds to mitigate the vanadium corrosion. Such compounds are of Varying effectiveness and it has not always been possible to reduce vanadium induced corrosion to a minimum amount.

It has now been discovered that residual petroleum fuels containing vanadium in an amount sufficient to yield ya corrosive vanadium-containing ash upon combustion can be rendered substantially non-corrosive by incorporating Itherein to form a uniform blend (1) a small amount of aA vanadium-free compound selected from the group consisting of calcium, barium and strontium compounds suflicient to retard the corrosiveness of the ash, and (2) a small amount of a vanadium-free alkali metal cornpound sufficient to further reduce the corrosiveness of said ash to a minimum. In the fuel compositions of the invention Ithe coaction of the two additive compounds is such that the corrosion is reduced to negligible amounts.

In the accompanying drawing, the single FIGURE shows an apparatus for testing the corrosivity of residual fuel oil compositions.

The type of residual fuel oils to which the invention is directed is exemplified by No. 5, No. 6 and Bunker C fuel oils which contain a sufficient amount of vanadium to from a corrosive ash upon combustion. These are residual type fuel oils obtained from petroleum by methods known to the art. For example, residual fuel oils are obtained as liquid residua by the conventional distillation of total crudes, by atmospheric and vacuum reduction of total crudes, by the thermal cracking of topped crudes, by Visbreaking heavy petroleum residua, and other con- -ventional treatments of heavy petroleum oils. Residua thus obtained are sometimes diluted with distillate fuel oil stocks, known as cutter stocks, and the invention also includes residual fuel oils so obtained, provided that such oils contain suiiicient vanadium normally to exhibit the corrosion characteristics described herein. lt should be understood that distillate fuel oils themselves contain either no vanadium or such small amounts as to present n o problem of corrosion. The total ash from commercial residual fuel oils usually ranges from about 0.02 to 0.2 percent by weight. The vanadium pentoxide (V205) content of such ashes ranges from zero to trace amounts up to about 5 percent by weight for low vanadium stocks, exhibiting no significant vanadium corrosion problem, to as much as percent by weight for some of the high v'anadium stocks, exhibi-ting severe corrosion.

The type of Vanadium-containing solid residual fuels to which the invention is directed is exemplified by the coke obtained in known manner by the delayed thermal coking or liuidized coking of topped Or reduced crude oils and by the pitches obtained in known manner by the deep vacuum reduction of asphaltic crudes to obtain solid residues. These materials have ash contents of the order of 0.18 percent by weight, more or less, and contain corrosive amounts of vanadium when prepared from stocks containing substantial amounts of vanadium. A typical pitch exhibiting corrosive characteristics upon combustion had a softening point of 347 F. and a vanadium content, as vanadium, of 578 parts per million.

Any calcium, barium or strontium compound, organic or inorganic, which is free'from vanadium is used as an additive of this class. Similarly, any organic or inorganic vanadium-free alkali metal compound is employed. The alkali metals include sodium, potassium, lithium, esium and rubidium; sodium and potassium compounds are preferred. Such inorganic alkali metal, calcium, barium and strontium compounds as the oxides, hydroxides,` acetates, carbonates, silicates, oxalates, sulfates, nitrates, halides and the like are successfully employed. In this connection, the mixture of salts present in sea water, as disclosed in our copending application Serial No. 654,812, tiled April 24, 1957, now U.S. Patent 2,966,029, comprises a suitable alkali metal compound. Calcium, barium and strontium carbonates, oxides and hydroxides are preferred inorganic compounds of these metals. The organic compounds of the additives of the invention include the oilsoluble and oil-dispersible salts of acidic organic compounds such as: (l) the fatty acids, e.g., valerie, caproic, 2-ethylhexanoic, oleic, palmitic, stearic, linoleic, tall Oil, and the like; (2) alkylaryl sulfonic acids, eg., oil-soluble petroleum sulfonic acids and dodecylbenzene sulfonic acid; (3) long chain alkyl sulfuric acids, eg., lauryl sulfuric acid; (4) petroleum naphthenic acids; (5) rosin and hydrogenated rosin; (6) alkyl phenols, eg., iso-octyl phenol, t-butylphenol, and the like; (7) alkylphenol sultdes, c g., his(iso-octyl phenol)monosulde, bis(t-butyl phenol)disullide, and the like; (8) the acids obtained by the oxidation of petroleum waxes and other petroleum fractions; and (9) oil-soluble phenol-formaldehyde resins, eg., Vthe Amberols, such as t-butylphenol-formaldehyde resin, and the like. Since the salts or soaps of such acidic organic compounds as the fatty acids, naphthenic acids and rosins are relatively inexpensive and are easily prepared, these are preferred materials for the organic addtives.

When employing in residual fuels the inorganic additives of the invention, it is desirable to use finely-divided materials. However, the degree of subdivision is not critical. One requirement for using a finely-divided material is based upon the desirability of forming a fairly stable dispersion or suspension of the additives when blended with a residual fuel oil. Furthermore, the more finely-divided materials are more el'hcient in forming uniform blends and rendering non-corrosive the relatively small amounts of vanadium in a residual fuel, whether the fuel be solid or liquid. The inorganic additives are therefore ernployed in a particle size range of less than 250 microns, preferably less than 50 microns. However, where the inorganic additives are water-soluble, for example, in the case of strontium chloride, calcium nitrate, sodium carbonate, and the like, itis not necessary to employ finely-divided materials since, if desired, the additives can be dissolved in water ot form a more or less concentrated solution and the water solution emulsied in the fuel.

The organic additives of the invention are oil-soluble or oil-dispersible and are therefore readily blended with residual fuels to form uniform blends. Since on a weight basis in relation to the fuel the amounts of the additives are small, it may be desirable to prepare concentrated solutions or dispersions of the organic additives in a naphtha, kerosene or gas oil for convenience in compounding.

In the practice of the invention with vanadium-containing residual fuel oils, the mixture of additives is uniformly blended with the oil. This is accomplished by suspending the finely-divided dry additives in the oil, emulsifying or dispersing a concentrated Water solution of the Water-soluble inorganic additives in the oil, or dissolving or dispersing the organic additives in the oil. If desired, suitable surface active agents, such as sorbitan monooleate and monolaurate and the ethylene oxide condensation products thereof, glycerol monooleate, and the like, which promote the stability of the suspensions or emulsions can be employed.

, In the practice of the invention with the solid residual accomplished in several ways.

fuels, incorporation of the additives of the invention is The additives can be suspended, emulsied or dissolved in the liquid vanadiumcontaining residual stocks or crude oil stocks from which the solid residual fuels of the invention are derived, and the mixture can then be subjected to the refining process which will produce the Ysolid fuel. For example, in the production of a pitch by the deep vacuum reduction of an asphaltic crude oil, the additives or a concentrate thereof are slurried with the oil in proportion to the vanadium content thereof, and the whole subjected to deep vacuum `eduction to obtain a pitch containing the additives uniformly dispersed therein. As still another alternative, particularly with a pitch which is withdrawn in molten form from the processing vessel, the additives can be mixed with the molten pitch and the mixture allowed to solidify after which it is ground to the desired size.

In the case of either liquid or solid residual fuels, the additives can be separately fed into the burner as concentrated solutions or dispersions. ln such a case, it is preferred to meter the additives into the fuel line just prior to the combustion zone. In a gas turbine plant where the heat resisting metallic parts are exposed to hot combustion gases at temperatures of the order of l200 F. and above, the additives can be added separately from the fuel either prior to or during combustion itself, or even subsequent to combustion. However, they may specifically be added, whether in admixture with or sep arately from the fuel, the additives are introduced into said plant upstream of the heat resisting metal parts to be protected from corrosion.

The calcium, barium and strontium compounds, on the one hand, and the alkali metal compounds, on the other, are employed in small, corrosion retarding amounts with respect to the fuel, and in such amounts with respect to each other as to minimize the corrosiveness of the ash.

While the calcium, barium and strontium compounds coact with the alkali metal compounds to minimize cor rosion, calcium, barium and strontium are not equally effective. Thus, when an amount of a sodium compound equivalent to about 1 atom weight of sodium per atom weight of the vanadium in the fuel is employed in residual fuel compositions with calcium, barium and strontium compounds, respectively, it is found that With the calcium compounds an amount yielding only about 2 atom weights per atom Weight of vanadium already minimizes corrosion, whereas with the barium and strontium compounds amounts yielding about 3 and 4 atom weights, respectively, of barium and strontium per atom weight of vanadium are required. Therefore, in fuel compositions containing about l atom weight of alkali metal compound per atom weight of vanadium, calcium compounds are employed in an amount yielding about 2 atom weights of calcium per atom weight of vanadium, barium compounds are employed in an amount yielding about 3 atom weights of barium per atom weight of vanadium, and strontium compounds are employed in an amount yielding about 4 atom weights of strontium per atom weight of vanadium.

The following examples are further illustrative of the invention.

1 EXAMPLE I With a residual fuel oil uniformly blend 0.024 percent by weight of strontium carbonate and 0.02 percent by weight of sodium carbonate. The residual fuel oil employed has the following inspection:

Sodium, p.p.m. of oil 1l EXAMPLE Il To the same residual fuel of Example I, add and uniformly blend 0.64 percent by weight of a solution of calcium petroleum naphthenate in naphtha containing 5 percent by weight of calcium, and 0.115 percent by weight ofA a wet paste of sodium petroleum naphthenate containing 8 percent by weight of sodium. The resulting cornposition has an atom weight ratio of calcium to vanadium of 2:1 and an atom weight ratio of sodium to vanadium of 1:1.

EXAMPLE III Melt a solid petroleum pitch obtained from the deep vacuum reduction of an asphaltic crude. This pitch has a softening point of 347 F and a vanadium content of 57 8 parts per million. While the pitch i-s in molten form, add and uniformly blend therein 0.47 percent by weight of strontium oxide (SrO) and 0.08 percent by weight of sodium sulfate. Upon cooling land solidilication, grind the mixture to `about 150 mesh. The resultingfuel has an atom weight ratio of strontium to vanadium of 4:1 and an atom weight ratio of sodium to vanadium of 1:1.

EXAMPLE IV With a residual fuel oil uniformly blend 3.8 percent -by weight of a barium petroleum sulfonate containing 5.1 percent by weight of barium and 0.14 percent by weight of a wet paste of sodium petroleum naphtllenate containing 8 percent by weight of sodium. The residual fuel oil employed in this example has the following inspection:

The resulting composition has an atom4 weight ratio of barium to vanadium of 3:1 and an atom weight ratio of sodium to vanadium of 1:1.

ln order to test the effectiveness of the additives of this invention under conditions of burning residual fuels `in a gas turbine, the apparatus shown in the `drawing is employed. As shown therein, the residual oil under test is introduced through line 10 into a heating coil 11 disposed in a tank of Water 12 maintained at such temperature that the incoming fuel is preheated to a temperature of approximately 212 F. From the heating coil 11 the preheated oil is passed into an atomizing head designated generally as 13. The preheated oil passes through a passageway 14 into a nozzle 15 which consists of a #26 hypodermic needle of approximately 0.008 inch LD. and 0.018 inch O.D. The tip of the nozzle is ground square and allowed to project slightly lthrough an orifice 16 of approximately 0.020 inch diameter. The oriiice is supplied with p.s.i.g. air for atomization of the fuel into the combustion chamber 21. The air is introduced through line 17, preheat coil 18 in tank 12, and -air passageways 19 and 20 in the atomizing head 13. The combustion chamber 21 is made up of two concentric cylinders 22 and 23, respectively, welded to two

end plates

24 and 25. Cylinder 22 has a diameter of 2 inches and cylinder 23 has a diameter of 3 inches; the length of the cylinders between the end plates is 81/2 inches. End plate 24 has a central opening 26 into which the atomizing head is inserted. End plate 25 has a one (l) inch opening 27 covered by a baiile plate 28 mounted in front of it to prevent direct blast of ame on the test specimen 29. Opening 27 in end plate 25 discharges into a smaller cylinder 30 having a diameter `of l/2 inches and a length of ,V6 inches. The specimen 29 is mounted near the downstream end of the cylinder approximately 1% inches from the outlet thereof. Combustion air is introduced yby means 4of air inlet 31 into the annulus between cylinders 22 and 23, thereby preheating the combustion air, and then through three pairs of im inch tangential air inlets 32 in the inner cylinder 22. The first pair of air inlets is spaced 1A inch from end plate 24; the `second pair 1% inch from the first; and the third 3 inches from the second.

The additional heating required to bring the combustion products to test temperature is supplied by an electric heating coil 33 surrounding the outer cylinder 23. The entire combustion assembly is surrounded by suitable insulation 34. The test specimen 29 is a metal disc one nchin `diameter by 0.125 inch thick, with a hole in the center by means of which the specimen is attached to `a tube'35 containing thermooouples. The specimen and tube assembly are mounted on a suitable stand 36.

lnconducting a test in the above-described apparatus,

`a weighed metal specimen is exposed to the combustion products of a residual'fuel oil, the specimen being main- .tained at a selected test temperature of, for example,

1350, 1450", or 1550 F. by the heat of the combustion products. The test is usually run for a period ofl hours with the rate of fuel feed being 1/2 pound per hour and the rate of atomizing air feed being 2 poundsV per hour. The combustion air entering through air inlet 31 is feed at 25 pounds per hour. At the end of the test run the specimen' .is reweighed to determine the weight of deyposits and is then descaled With a conventional alkaline descaling salt in molten condition at 475 C. After descaling, the specimen is dipped in 6 N hydrochloric acid containing a conventional pickling inhibitor, and is then washed, `dried and weighed. The loss in Weight of the specimen after descaling is the corrosion loss.

Tests are conducted in the apparatus just described Table l Atom Wt. Corrosion, Deposits Fuel Ratio, Add- Wt. Loss of tive Metal: V Specimen,

Mg./Sq. In. Mg./Sq. In. Nature Uncempounded Fuel of Ex. I..- 1,430 1,151 Hard Scale. Fuel -l- Sodium Carbonate 1:1 133 234 Granular. Fuel -l- Sodium Naphthenate. 3:1 96 121 Scale. Fuel -1- Sodium Carbonate 5:1 91 205 Powdery. Fuel -lstrontium Carbonate 4:1 155 336 De.

Do 5:1 141 280 D0. Fuel Calcium Naphthenate 3:1 119 230 Do. Uncompounded Fuel of Ex. IV 1, 143 231 Crusty Scale. guel -l-i-Bzstriuui Sulioratc -..-.-.8.21 S V 4 1 4-:1 289 270 De.

uc trontium ar ouate o ium r:

Carbonate (Ex. I). NazV: 1 i 0 100 Powdery' Fuel Calcium Naphthcnate Sodium CazV= 1 4 19,. D FNiu-ltiienm (gxirm' s di N h izmir: i o i o' ue arium u ouate o um ap a:

thenate (Ex. IV). {Na:V=1:1.. i 19 185 Soft powdery using a 25-20 stainless steel as the test specimen. The tests are run for 100 hours at a temperature of 1450 F. under the conditions described above. Tests are made with the fuel oil compositions of Examples I, II and 1V, with fuel oil compositions similar to those of theseexamples but containing only one of the additives in varying proportions, and with the uncompouuded residual fuel oils of Examples l and 1V. The preceding table shows Vthe corrosion and deposits obtained,

It will be seen from the preceding table that, although the alkali metal additives and the calcium, barium and strontium additives, respectively, individually tend to reduce corrosion and deposits, substantial corrosion and deposits are nevertheless obtained. This is apparent even when relatively larger amounts of the individual additives Iare employed, for example, in atom weight ratios of additive metal to vanadium on the order of 4:1 and 5:1.

However, when the additives are employed in the combination of the invention, corrosion is unexpectedly minimized and the deposits are loose and powdery. Thus, it will be noted that the alkali metal additives and the ca1- cium, barium and strontium additives when individually employed in the same total additive content as the combined additives in accordance with the invention do not minimize corrosion. The coaction of the combination of additives could therefore not have been predicted from the action of the individual additives. Similar results to those shown for the specific additives employed in the examples and in the preceding table are obtained when using the other alkali metal compounds and calcium, barium and strontium compounds disclosed.

A typical analysis of the 25-20 stainless steel employed in the testing described is shown in the following table in percent by weight:

Table I1 Cr 25 Ni 20 C 0.08 Mn 2.0 Si 1.5 S 0.03 P 0.04 Fe Balance Resort may be had to such modifications and variations as fall within the spirit of the invention and the scope of the appended claims.

We claim:

1. A fuel composition comprising a uniform blend of a major a-mount of a residual petroleum fuel yielding a corrosive vanadium-containing ash upon combustion, a small amount of a vanadium-free compound selected from the group consisting of calcium, barium and strontium compounds, the amount of said calcium, barium and strontium compounds with respect to the vanadium content of said fuel being such as to yield about 2 atom weights of calcium, about 3 atom Weights of barium and about 4 atom weights of strontium, respectively, per atom weightof vanadium in said fuel, and an amount of a vanadium-free alkali metal compound yielding about l atom weight of alkali metal per atom weight of vanadium in said fuel.

2. The fuel composition of claim 1, wherein the fuel is a solid residual petroleum fuel.

3. A fuel composition comprising a major amount of a residual fuel oil yielding a corrosive vanadium-containing ash upon combustion, an amount of a vanadiumfree calcium compound yielding about 2 atom weights of calcium per atom weight of vanadium in said fuel oil and an amount of a vanadium-free sodium compound yielding about 1 atom weight of sodium per atom Weight of vanadium in said fuel o il.

4. The fuel composition of claim 3, wherein the calcium compound is calcium naphthenate and the sodium compound is sodium naphthenate.

5. A fuel composition comprising a major amount of a residual fuel oil yielding a corrosive vanadium-containing ash upon combustion, an amount of avanadium-ftce barium compound yielding about,3 atom weights oft` barium per atom weight of 'vanadium in said fuel oil and an amount of a vanadium-free sodium compound yielding about l atom weight of sodium per atom weight of vanadium in said fuel oil.

6. The fuel composition of claim 5, wherein the barium compound is barium sulfonate and the sodium compound is sodium naphthenate.

7. A fucl oil composition comprising a major amount of a residual fuel oil yielding a corrosive vanadium-containing ash upon combustion, an amount of a vanadium-free strontium compound yielding about 4 atom weights of strontium per atom weight of vanadium in said fuel oil and an amount of a vanadium-free sodium compound yielding about 1 atom weight of sodium per atom weight of vanadium in said fuel oil.

8. The fuel composition of claim 7, wherein the strontium compound is strontium carbonate and the sodium compound is sodium carbonate.

9. In a gas turbine plant in which a fuel oil containing vanadium is burned and which includes heat resisting mctallic parts exposed to hot combustion gases and liable to be corroded by the corrosive vanadium-containing ash resulting from combustion of said oil, the method of reducing said corrosion which comprises introducing into said plant upstream of said parts a small amount of a vanadium-free mixture of (l) a compound selected from the group consisting of calcium, barium and strontium compounds and (2) an alkali metal compound, the amount of said compound selected from the group consisting of calcium, barium and strontium compounds yielding about 2 atom weights of calcium, about 3 atom weights of barium and about 4 atom weights of strontium, respectively, per atom Weight of vanadium in said fuel oil, and the amount of said alkali metal compound yielding about 1 atom weight of alkali metal per atom weight of vanadium in said fuel oil.

l0. The method of claim 9, wherein the alkali metal compound is a sodium compound.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

9. IN A GAS TURBINE PLANT IN WHICH A FUEL OIL CONTAINING VANADIUM IS BURNED AND WHICH INCLUDES HEAT RESISTING METALLIC PARTS EXPOSED TO HOT COMBUSTION GASES AND LIABLE TO BE CORRODED BY THE CORROSIVE VANADIUM-CONTAINING ASH RESULTING FROM COMBUSTION OF SAID OIL, THE METHOD OF REDUCING SAID CORROSION WHICH COMPRISES INTRODUCING INTO SAID PLANT UPSTREAM OF SAID PARTS A SMALL AMOUNT OF A VANADIUM-FREE MIXTURE OF (1) A COMPOUND SELECTED FROM THE GROUP CONSISTING OF CALCIUM, BARIUM AND STRONTIUM COMPOUNDS AND (2) AN ALKALI METAL COMPOUND, THE AMOUNT OF SAID COMPOUND SELECTED FROM THE GROUP CONSISTING OF CALCIUM, BARIUM AND STRONTIUM COMPOUNDS YIELDING ABOUT 2 ATOM WEIGHTS OF CAKCUYNM ABOUT 3 ATOM WEIGHTS OF BARIUM AND ABOUT 4 ATOM WEIGHT OF STRONTIUM, RESPECTIVELY, PER ATOM WEIGHT OF VANADIUM IN SAID FUEL, AND THE AMOUNT OF SAID ALKALI METAL COMPOUND YIELDING ABOUT 1 ATOM WEIGHT OF ALKALI METAL PER ATOM WEIGHT OF VANADIUM IN SAID FUEL OIL.