US2979890A - Residual fuels - Google Patents

Description

April 18, 1961 A. s.Ro :c|-m\1v|A ET Af. 2,979,890

RESIDUAL FUELS Filed Deo. l0, 195'? IN VEN TORS` HLBE R T G. ROCCH/N/ By CHHRLES E. TRHUTHHN RESIDUAL FUELS ,'lhert G. Rocchini, Springdale,` and Charles E. Trautman, Cheswick, Pa., assignors to Gulf Research &

Development Company, Pittsburgh, Pa., a corporation of Delaware Y Filed Dec. 1o, 1951, ser. No.f7o1,s47

s claims. (el. tio- 35.0

attacks such parts as boiler tubes, hangers, turbine blades,

and the like. These eiects are particularlynoticeable in gas turbines. `Large gas turbines show promise of becoming anl important type of industrial prime mover.

` However; economic considerations based on the efficiency of'. the gas turbine dictate the use of a fuelfor this purpose which is cheaper than a distillatediesel fuel; otherwise, other forms of poweri s uch as diesel engines become competitive with gas turbines.` Y j One of the main problems arising in the use. of residual fuel oils in gas turbines is thecorrosiveness inducedfby those residual fuels containing sufficient amounts 'of vanadium to cause'corrosion. Where no vanadium is vanadium, or none, asto present no corrosion problems, such non-corrosive fuelfoils 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 lis often prohibitive, and the residual oil loses its competitive advantage. These .factors appear to militate against the extensive useA of ,residual fuel oils for gas turbines. YAside from corrosion, the formation of d eposits upon the burning'of aresiduallfuel in a vgas turbine may result inunbalance of the turbine blades, .illog-v ging of openings and reduced thermal efficiency of the turbine.v

Substantially identical problems are encountered when u singa solid residual' petroleum fuell containing substant1alaf-mounts of vanadium. These fuels are; petroleum residues obtained by known methods of petroleum relining such as deep Yvacuum reduction rof'asphalticfcrudes bottomspfollowed by distillation to'obtainsolidfresidues, coking of liquid 4distillation bottoms, yand theplike. The solid residues thus obtained arevknownyar'iously as petroleum pitches or cokes andi fined :use as"v fuels. .Since present or the amount of vanadium is small, no appreci- 'able corroslon is encountered.:4 While many residual fuel v oilsas normallyobtained in the refinery contain so little the-vanadiumcontent of the original crude oil tendsQto,

rcfincentrate"in the v'residual fractions, and since'jrthe'processing'4 of. the fresidualj fractions to solid residues results in .furthericoncentration of the vanadium in theslid f residues, the vanadium corrosion problem tends to be in-Y tensiied in using the solid residues asfu'el.

. 2,979,890 Patented Apr. 18, 1961 ICC of a residual 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 attackvbecoming progres'- sively 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 Yordinary temperatures. It has nowbeendiscovered that residual petroleum fuels containing vanadium in an amount s'uicient to yield a corrosive vanadium-containing ash upon combustion can berendered substantially less corrosivenotwithstand 'ing the Vnormally corrosive vanadium content, by incorporating therein toform a uniform blend a small amount, sufficient to retard the corrosiveness of thel ash, of an alkali metal tungstate;v In the fuel compositions of the invention, corrosiony due to the vanadium-containing ash is substantially retarded. Y

In the. accompanying drawing, the single figure shows an apparatus for testingvthe corrosivity of residual fueloil compositions.V Y Y .The type of residual fuel oils to which the'inventionis directed is exemplified by No. 5, No.6 and Bunker C fuel 'oils which contain a ysufficient amount of vanadium to form a corrosive ash upon combustion. These are residual type fuel oils obtained from petroleum by methods knownlto the artiv Forexample, residual fuel oils -late fuel oil Estocks, known as cutter stocks, and the invention also includes a residualY fuel koils so obtained,

provided that such oilscontain lsufficientvanadium normallyfvto exhibit the corrosioncharacteristics described herein;I 'It-shouldbe understoodlthat distillate fuel oils themselves contain either no vanadium or such small amounts as 'to present no problem ofjcorrosion. The total ash from commercial-residual. fuel oils usually rangesfrom about 0,02 to `0.2 percent :by weight-The vanadium pentoxide (V205) content of such'ashes ranges from zero to vtrace amounts up to about 5 percent by weightV for low vanadium stocks, exhibiting no significant vanadiumcorrosion problem, vto as much as percent byweight for some of the 4high Avanadium stocks','.exhibit} ing severe corrosion. Y v I n n. i.

The Ytype 0f vanadium-containing solid residual fuelsfto' which-,the invention is directed'is exemplified by the coke Y i obtained in knownmanner by the'delayedthermal coking or' fluidized coking of topped or reduced crude oils and to obtain solid residues, visbreaking of `liquid distillation byd'thexpltches Obtamed m known manner by' the deep vacuum-reduction ofiasphaltic crudesto 'obtain solidv residues. These materials have ashA contents Vofthe order of 0.18 percent by weight, more or less,;and contain corrosive amounts of vanadium when preparedfrom stocks containing substantialamounts of vanadium.' A: typical pitchvexhibiting corrosive characteristics .uponY combustion .had a softening point of 347-F. and a vanadium content, as vanadium, of 578 parts per million;

They corrosion retarding additive of the invention is,v

`as has been stated, an alkali metal tungstate. The addi- Y@The'vanadium-containing\ash present inthe'hot flue gasiobtainedfrom the burning of a residual fuel containing' substantial amounts of vanadium compoundscauses tema Onh sorrosion 9i thewrbinablades andtother tive is employed in thedryform or as a water solution.-

The alkaliV metal tungstates include the sodium,l potas-I sium, lithium, rubidium and cesium tungstates in any of n the ortl1 ometa`or para-tungstate formsLBecauseof' 3 their availability and relatively low cost, the sodium Iand potassium tungstates are preferred.

When employing in residual fuels the dry alkali metal tungstates, it is desirable to use the finely-divided salts. However,- `the degree to which the salts are subdivided is not critical. One requirement for using a tinelydivided material is based upon the desirability of forming a fairly stable dispersion or suspension of the salts when blended with a residual fuel oil. Furthermore, the more finely-ydivided materials are more eicient 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. Thedry salts are ,therefore employed `in a particle size range of less than 250 microns, prefer.- ably less than `50 microns. However, since the alkali metal tungstatesA are soluble in water, it is unnecessary to employ the finely dvided'dry additives when it is desired to employ water solutions thereof. Thus, the salts can be dissolved in water to form more or less concentrated solutions and the solutions then are emulsied in the fuel.

In the practice of the invention with vanadium-containing residual fuel oils, the additive is uniformly blended with the oil in proportion to the vanadium content thereof. This is accomplished by suspending the nelydivided dry salts in the oil, as has been indicated above,

emesso i or by emulsifying oridispersing a water solution of the V salts 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 suitability of the suspensions or emulsions can beV employed.

In the practice of the invention with the solid residual fuels, incorporation of the 'additive of the invention is accomplished in several ways. The additive can be suspended or emulsiedin the liquid vanadium-containing 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 Willproduce the solid'fnel. For example, in the production of a pitch by the deep vacuum reduction of an asphaltic crude'oil, sodium ortho tungstate or a solution thereof in water isslurried with the oil in' proportionto the vanadium content thereof, and the whole subjected to deep vacuum reduction to obtain a pitch containing the additive uniformly dispersed therein. As stll another alternative; particularly with a pitch which'is withdrawnl inmolten form from the processing vessel, the additive canbe mixed with the molten pitch and the mixture al lowed to solidfy after which it is ground to the-desired size... Y

In the case of either liquid or solid residual fuels, the additive vcan be separately fed into the burner as the dry`,salts, an oil dispersion thereof or a water solution thereof. In any such case, it is preferred' to meter the additive into the fuel line just. prior to` the combustion zone. Inagasfturbine plant where the heat resisting metallic parts'are exposed to hot combustion gases at temperatures of the'order of 1200 F. and above, the additive can be `added separately from the fuel either prior to or during combustion itself, or even subsequent to combustion.V However, it may specifically be added, whether in admixture with or separately from the fuel, theA additive `is introduced into said plant upstream of the heat resisting metal parts to be' protected from corrosion. y

The alkali metal tung'states` are employed In'a small amount withrespect to the vanadium-containing residual fuel, sucient to retard the corrosiveness ofthe ash. Although the exact amount of additive to usen will vary in accordance with the vanadium content ofthe specific re-u sidual fuel employed, Yas will be understodby those 4 skilled inthe art, in general a reduction'in corrosivity is already observed with an amountmf the alkali metal tungstate sufficient to yield about 1 atom weight of the tungsten present in the additive per atom weight of vanadium in the residual fuel. However, corrosivity is minimized when an amount of additive is employed yielding from about 3 to about 6 atom weights of tung sten per atom weight of Vanadium in the residual fuel, and appears to be `at a minimum at an atom weight ratio of tungsten to vanadium of 3: 1.

The following specific examples are further illustrative of the invention.

A, series of residual fuel oil compositions employing varying amounts of additive are made up and tested under conditions of burning residual fuel oils in a gas turbine. Identical tests are run on the residual fuel oil containing no additive. In the testing 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 I D. and 0.018 inch O.D. The tip'of the nozzle is ground square and allowed to project slightly through an orifice 16 of approximately 0.020 inch diameter. The orifice is supplied with 65 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, respec4 tively, welded to` two end plates 24 and 25. Cylinder 22 has a diameter of 2 inches and cylinder 23 has a dif ameter 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 (1) inch yopening 27 covered by a baffle plate 28 mounted in front of it to prevent direct blast of ame on the test specimen 29. Opening 27 in end plate 125 discharges `into a smaller cylinder 30 havf ing a diameter of 11/2 inches and a length of 6 inches. The .specimen 29 is mounted near the downstream end of thecylinder approximately 1% inches from the outlet thereof. Combustion air is introduced by means of air inlet 31into the annulus between cylinders 22 and 23, thereby preheating the combustion air, and then through three pairs of 3/16 inch tangential air inlets 32 in the inner cylinder 2,2. The-first pair of airinlets is spaced 1A. inch from end plate 24; the second pair 3%: inch from the first; and the third` 3 inches from the second. The additionaly 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 bysuitableinsulation 34. The test specimen 29 is a metal disc one inch in diameter by 0.125,V inch thick, (with a hole` in the center by means'of, which the specimen is attached to a tube 3S containing,thermocouples. The specimen and tube assembly are mounted on a suitable stand' 36.

In conductinga test in the above-described apparatus, a weighed metal specimen is ,exposed tothe combustion productsofja residual fuel oil, the specimen being maintainedl at a selected Atest temperaturev of, for example,- l350, 14S0'or 71550 F. by the heat of the combustion products. The test isusually` run for ,a period of hours with the rate of fuel feed being l/2 pounclper hour Aand the'rate of atomizing air feed being 2V pounds per hour. The combustion airl entering through air inlett31 is fed at 25 pounds per hour. At the end of the test run the specimeniis reweighed to determine the weightiof deposits and is tliendescaled with a conventional alka line descialin'g salt in molten condition at 4750,o C. Afterdescaling, the specimen is'dipped in 67N 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.

The compounded and uncompounded residual fuel oils are tested in the apparatus just described 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. ployed as a base fuel has the following inspection:

Gravity, A.P.I. 20.6 Viscosity, Furol, Sec.:

122 F. 25.4 Flash, OC: F. 255 Fire, OC: F. 295 Sulfur, B: percent 1.6 Ash: percent 0.04 Vanadium: p.p.m. of oil 182 Sodium: p.p.m. of oil 2 The alkali metal tungstate employed in this series of runs is sodium para tungstate (Na6W7O24-l6H2O).

The following table shows the make-up and the reduction in corrosion and deposits obtained in the tests.

TABLE I Fuel Ex. 1 Ex. 2 Ex. 3 Ex. 4 (Blank) Amount of Additive:

Percent by Wt. of Fuel.- 0. 15 0. 2 0.3 0. 7 Atom Ratio, W:V 0 1.5:1 2:1 3:1 ,6:1 Corrosion, Wt. Loss of Specimen, Mg./Sq. In 1, 580 128 97 7 28 Reduction in Corrosion Due to Additive, Percent 92 94 99. 5 99 Deposits on Specimen, Mg./

Sq. 1, 130 276 115 119 164 Texture of Deposits Hard Hard Pow- Pow- Pow- Scale Dalat dery dery dery pos It is apparent from the 'foregoing data that sodium tungstate is an eificient corrosion inhibitor and is most effective when employed in an amount yielding an atom Weight ratio, W:V, of 3:1. Similar results are obtained employing the other alkali metal tungstates disclosed herein.

The following example is illustrative of the use of the additive of the invention with solid residual fuels.

Example 5 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 578 parts per million. While the pitch is in molten form, add and uniformly blend therein with stirring 1.25 percent by weight of finely powdered dry potassium ortho tungstate (K2WO4-2H2O). Upon cooling and solidfica- The residual oil emf tion, grind the mixture to about mesh. The result ing fuel has an atom weight ratio, W:V, of 3:1.

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

Resort may be had to such modifications and variations as fall within the spirit of the invention and thescope of the appended claims.

We claim:

1. A fuel composition comprising a uniform blend of a residual petroleum fuel yielding a corrosive vanadiumcontaining ash upon combustion and an amount of an alkali metal tungstae suiiicient to yield from about 3 to yabout 6'atom weights of tungsten per atom weight of vanadium in said fuel. l

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

3. A fuel composition comprising a uniform blend of a residual petroleum fuel oil yielding a corrosive vanadium-containing ash upon combustion and an amount of a sodium tungstate su'icient to yield from about 3 to about 6 atom weights of tungsten per atom weight of vanadium in said fuel oil.

4. The fuel composition of claim 3, wherein the tungstate is sodium paratungstate.

5. In a gas turbine plant in which a fuel oil containing vanadium is burned and which includes heat resistant metallic parts exposed to hot combustion gases and liable to be corroded by the corrosive vanadium-containing ash resulting from the combustion of said oil, the method of reducing said corrosion which comprises introducing into l said plant upstream of saidparts an amount of an alkali metal tungstate sufcient to yield from about 3 to about 6 atom weights of tungsten per atom weight of vanadium in said fuel oil.

References Cited in the le of this patent FOREIGN PATENTS

Claims (1)

1. 5. IN A GAS TURBINE PLANT IN WHICH A FUEL OIL CONTAINING VANADIUM IS BURNED AND WHICH INCLUDES HEAT RESISTANT METALLIC PARTS EXPOSED TO HOT COMBUSTION GASES AND LIABLE TO BE CORRODED BY THE CORROSIVE VANADIUM-CONTAINING ASH RESULTING FROM THE COMBUSTION OF SAID OIL, THE METHOD OF REDUCING SAID CORROSION WHICH COMPRISES INTRODUCING INTO SAID PLANT UPSTREAM OF SAID PARTS AN AMOUNT OF AN ALKALI METAL TUNGSTATE SUFFICIENT TO YIELD FROM ABOUT 3 TO ABOUT 6 ATOM WEIGHTS OF TUNGSTEN PER ATOM WEIGHT OF VANADIUM IN SAID FUEL OIL.

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