US2924938A - Combustion apparatus for burning ash-forming liquid fuel - Google Patents

Description

Feb. 16, 1960 M. v. HERBERT 2,924,933

COMBUSTION APPARATUS FOR BURNING ASH-FORMING LIQUID FUEL Filed May 14, 1956 3 Sheets-Sheet 1 FIG. I.

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W Maggy/m1 B J'- Feb. 16, 1960 M. v. HERBERT 2,924,938

COMBUSTION APPARATUS FOR BURNING ASH-FORMING LIQUID FUEL I Filed May 14, 1956 s Sheets-Sheet 2 FIG Feb. 16, 1960 v, HERBERT 2,924,938

COMBUSTION APPARATUS FOR BURNING ASH-FORMING LIQUID FUEL Filed May 14, 1956 5 Sheets-Sheet 3 Lg I makmumm U PEG. 3

COMBUSTION APPARATUS FoR BURNING ASH-FORMING LIQUID FUEL Michael Vaughan Herbert, Fleet, England, assignor to Power Jets (Research and Development) Limited, London, England, a company of Great Britain This invention relates to combustion apparatus for burning ash-forming liquid fuel, particularly though not necessarily exclusively fuel oil such as residual fuel oil' which on combustion yields an ash containing substances such as vanadium and sodium compounds which at elevated temperatures, e.g., of 650 C. and above, give rise to accelerated corrosion of metallic parts with which the ash comes into contact. Such accelerated corrosion is a veryserious problem in gas turbines operating on low grade fuel oils. It is an object of this invention to provide a method of reducing such corrosion.

It has been found that by controlling combustion in combustion apparatus using fuel oils. of the type indicated above so that the gases rcsultingfrom the combustion of the fuel include a proportion of unburnt carbon, deposition of ash on and consequent corrosion of the parts upon which the ash would normally be deposited can be reduced or eliminated. It would appear that the dry carbon particles tend to absorb the wet and sticky ash, and so much of the ash will be discharged with a substantially reduced risk of its being deposited on, for example, the turbine blades.

. As pointed out in a co-pending application Serial No. 584,629 in the name of R. l. Hodge filed on the same day as the present application, in order to eifect combustion control in the'manner indicated above, it is necessary to control the size of the liquid fuel droplets burnt in the apparatus, since if droplets of a wide range of sizes are burnt, the smaller ones will be burnt away completely to form ash only, while the larger ones are only partly burnt. Thus in the case of the smaller droplets, there will beno unburnt carbon associated with the ash resulting from the combustion of these droplets and this free ash can give rise to corrosion. On the other hand if combustion is controlled so that even the smallest droplets are not completely burnt, the exhaust solids resulting from the combustion of the larger droplets will contain a much greater proportion of unburnt carbon, and the resultant loss of combustion efiiciency may be unacceptable.

Accordingly the present invention provides combustion apparatus for burning ash-forming liquid fuel comprising a duct leading at one end to the combustion zone of the apparatus and a fuel injector discharging fuel into the duct as a generally conical spray of droplets of various sizes, a fluid stream being discharged from all around the spray inwardly thereinto to concentrate subcombustion chamber, the fuel to be burnt being in- 2,924,938 Patented Feb. 16', 198i trodueed into and ignited in the pre-combustion chamher and continuing to burn while being carried around the axis of the vortex chamber in a spiral vortex path. Air may be admitted to mix with and chill the combustioh gases at the outlet of thevortex chamber.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawings, of which:

Figure 1 is a diagrammatic view of a gas turbine plant incorporating combustion apparatus of the spiral vortex type shown in longitudinal cross-section.

Figure 2 is a transverse sectional view of the peripheral part of the combustion apparatus shown in Figure 1.

Figure 3 shows a detail of the arrangement of Figure 2 to a larger scale.

In Figure l a gas turbine plant comprises a compressor 1, a turbine 2 driving the compressor through a shaft 3 and also drivinga load such as an alternator 4. The compressor is connected to supply compressed air through duct 5. to combustion apparatus of the spiral vortex type and the turbine is connected to receive hot combustion gases through duct 6 from the combustion apparatus whereby'it is driven. a

The combustion apparatus is of the spiral operating on the principles described in British Patent No. 639,468' and is structurally similar in some respects to the. apparatus described in British Patent Nos.

It comprises an outer casing 11 719,379 and 719,380. having a peripheral wall 114: and two generally frustoconical end walls 11b and 110, and a vortex combustion chamber '12, likewise having a peripheral wall 12a, and end walls 12b and 120, mounted in the casing by means of supports 13 which maintain the chamber concentric with the casing and restrain it against bodily. movement but permit differential thermal expansion. Such suppoits are shown in the two last mentioned patents. The combustion chamber has a lining 14 of a refractory material. The end wall 12b of the combustion chamber is formed with a central outlet aperture 15 from which extends an axial outlet duct 16 to which is connected the ductv 6 for conveying hot combustion gases to the turbine 2, and its other end wall 12c is also formed with a central aperture 13 from whicha tube 19 extends axially into the interior of the chamber. Its peripheral wall 12a is formed with two diametrically opposite tangentially facing generally segment-shaped inlet apertures (one of which is shown at 17) to which are connected generally tangentially extending pre-combustiou chambers 31 as shown in Figure 2, the pre-combustion chambers and the vortex combustion chamber constituting the combustion zone of the apparatus.

The outer casing 11 similarly has a central aperture in its end wall 11b and a tubular extension 20 extend ing axially therefrom around the outlet duct 16. The end of the annular space between the extension 20 and duct 16 is closed by an end wall 21, and an air inlet duct 22 connected to the duct 5 from the compressor 1 leads laterally into this space. The space is further divided radially by an axially extending tubular wall 23 while the inlet duct 22 is split into two passages of unequal cross-sectional area by a partition 24, the larger passage communicating with the annular space between the extension 20 and the wall 23 and the smaller with the annular space between the wall 23 and duct 16.

The peripheral walls 11a of the casing is further formed with two diametrically opposite apertures to which are connected generally tangential extensions 32 closed at their ends and constituting air casings enclosing the precombustion chambers 31 (see Fig. 2).

In operation, the air supply entering the inlet duct 22 is splitby partition 24 into two streams.

vortex type The smaller stream enters the space between wall 23 and duct 16 and 4 enters the latter through rows of holes 16a. Thelarger stream enters the space between the casing 11 and combustion chamber 12, thus cooling the latter, and is further divided, some passing into theair casings 32 and so into the pre-combustion chambers 31 as combustion air, and the remainder entering the combustion chamber 12 axially through apertures 18. Butterfly valves 25, 26, separately operable by levers 25a, 26a, are provided in the inlet duct 22 for controlling the air streams on each side of the partition 24. The flow through aperture 18 is controlled by a further valve 27 mounted on a threaded rod 28 carried in the end wall 11c of the casing and operable by hand wheel 29. By adjustment of these valves, the magnitudes of the various air streams can be varied relatively to one another.

As shown in Fig. 2, the pre-combustion chamber 31 comprises a circular-section portion 310, an outlet portion 31b of segmental cross section (conforming to the shape of the inlet 17 to the vortex combustion chamber as shown in Fig. l) and'a transition portion 31c. The chamber 31 is enclosed by a similarly shaped outer sheath 33 having a flared end 33a and annularly spaced therefrom by spacers 34 to define an annular passage for cool-, ing air. The sheath has an external flange 33b by which it is secured to the peripheral wall 12:; of the vortex combustion chamber. At the upstream end of the precombustion chamber, there is a constriction of substantially venturi shape having a cylindrical throat 35. of

smaller diameter than the circular-section portion 31a.-

The throat'is connected to the. latter by a short frusto-. conical wall 36 and a further frustoaconicalwall 37 converges to the upstream end of the throat. This last mentioned wall 37 defines with a further frusto-conical wall 38 co-axially within it an annular convergent ,-air inlet passage leading to the constriction, the wall 38 being supported from the wall 37 by axially and radially extending splitters 39. Air flows through this passage from the interior of the extension 32 into the .pre-cornbustion chamber. V i

Mounted within the inner wall 38 defining the annular air inlet to the pre-combustion chamber 31 is a tube ,or duct 81 open at both ends to permit air to flow from the extension 32 into the pie-combustion chamber. Centrally supported within this tube 81 is a fuel injector 82. Thisinjector is of the known swirl atomizing type having an internal swirl chamber to which fuel is supplied through one or-more tangential swirl ports and from which the fuel is discharged through an axial discharge nozzle 82a as a generally conical spray of atomized fuel made up of droplets of various sizes, the spray being discharged in a direction away from the pre-combustion chamber. The injector is carried on a streamlined hollow strut 83 extending from the wall of the tube 81, and

liquid fuel, being a residual fuel oil of the type referred to above, is supplied to the injector from fuel tank 45 by, a fuel pump 44 through a pipe 84 nested within th strut 83. V

A hollow ring 85 is mounted a short distance upstream ofth'e injector, coaxially therewith and just outside the conical fuel spray, and is supported from the, strut 83 by two diametrically opposite tubes 86. Compressed air is supplied to the interior of the strut 83 from pipe 87 and passes through thetubes 86 to the interior of the ring 85 and is discharged inwardly therefrom into the fuel spray through a large number of air nozzles 85a symmetrically distributed all around the circumference thereof. As indicated in the Figure 3, the nozzlesBSa are arranged sothat the air jets emitted. thereby are directed inwardly andaway from the fuel injector, i. e. downstream with respect to the fuel spray, and are .sub stantialiy normal to the envelope of the conical spray of fuel. It hasbeen' found that in conical fuel spray of the type' described there is a naturallyoccurring tendency. for the smaller fuel droplets emitted by the injector to i be concentrated towards the axis ofthespray. The air jets from the nozzles 85a accentuate thistendency as they are able to deflect the smaller droplets towards the central region of the spray but do not aifect the paths of the larger droplets to such a great extent on account of the greaterinertia of the latter. 7 v I J The smaller droplets which become concentrated towards the axis of the spray are removed through the open end of a collector pipe 88coaxial with and spaced from the discharge end of the injector 82. Thispipe is supported by a spider-89 within the tube 81 and extends through the closed end 32a of the extension 32. By suitable choice of the relative position and size of the injector 82, ring 85 and collector pipe 88, and of the air and fuel pressures, it is possible to arrange for. the open end of the collector pipe to receive substantially all the fuel droplets of below a predetermined size.

'In operation, the air entering the pre-cornbustion cham- I ber is at a pressure higher than atmospheric, having been compressed by compressor 1, and so if the other end of the collector pipe 88 is connected to a region of lower pressure, some of the air will tend to flow down the pipe .and carry away smaller fuel droplets. The pipe 88 is connected to the inlet of a cyclone separator 47 in which the fuel is separated from the air stream, the separated fuel being collected in trap 48 from which it drains through pipe 49 back to the fuel tank 45. The air is dis- :charged through outlet duct 50 to a region of lower pres annular passageforpart of the air, tapering towards the constriction. The liner includes a short cylindrical portion 51a projecting into the throat 35 of the constriction. Swirlers 52 in the passage cause the air to flow with swirl about the axis. of the constriction and the quantity of air and its velocity is so related to the size and configuration of the constriction that this swirling air on entering the pre-combustion .chamber 33 givesrise to re-circulation along the axis thereof. The-larger fuel droplets are carried into the pre-combustion chamber by the air flowing through tube 81 and are ignited by an ignition device such as a torch igniter 53, combustion beinginitiated in the stabilized flame zone. set up in the pre-combustion chamber by. the re-circulation.

The short cylindrical portion 51a of the liner causes the swirlingair to flow along the wall of thethroat of theconstriction, .thus reducing any tendencyfor reverse flow along the wall to occur with consequent deposition of unburned fuel thereon. This arrangement alsoreduces the risk of the flame blowing back through the layer of air close to the throat Wall.

Fueloils of the type used in the present apparatus contain v.a-certain proportion of comparativelyvolatile constituents and at least partial combustion ofthese constituents takes place in the pre-combustion chambers 33. The burning fuel droplets are carried into the vortex combu'stion chamberll and the less volatile constituents are burn't -as the fuel droplets are .carried'around the axis of the chamber iri'a spiral vortex path, progressively approa'ching the centraljoutlet 15 .as they are burnt away. Since the. size of.- the fuel droplets burnt is controlled, substantiallyall being above a pre-determined size, they will all be burnt away to substantially the same extent when they reach the outlet. Thus by appropriate design of the vortex combustion chamber it may be arranged that thefuel droplets are. only partially. burnt and a pre-d etenminedproportion of unburnt carbon remains assoclatedwith the fuel ash from each fueldroplet when it reaches the outlet. To ensure incomplete combustion and so to obtain a certain proportion of unburnt carbon in the combustion gases discharged through the outlet duct 16, the gases are mixed with and chilled by the air entering through the tube 19 and holes 16a in the outlet duct 16. Combustion is thus stopped before it is complete and the resultant free carbon in the exhaust solids serve to reduce or eliminate deposition of ash on the turbine blades and elsewhere in the gas turbine as mentioned above. This chilling air constitutes the dilution air required to reduce the combustion gas temperature to a value which can be supported by the turbine, the quantity of air entering the pre-combustion chambers 33 being sufiicient to support combustion of fuel.

It will be noted that the air in strut 83 serves to protect to some extent the fuel in pipe 84 against overheating.

The pro-combustion chamber is as described in a copending application Serial No. 584,577 in the name of J. A. Gardiner filed on the same day as the present application. The device for introducing the fuel droplets as described may be used in conjunction with a vaporizing duct leading to a pre-combustion chamber as described in the above-mentioned co-pending application in the name of R. I. Hodge.

The pre-combustion chamber may, instead of being connected to the inlet of a vortex combustion chamber, constitute the upstream end of a tubular flame tube of the knownstraight through flow type. The flame tube will be designed so that the fuel droplets are not com- If necessary chilling air may be introduced at the flame tube outlet through apertures in the flame tube wall.

The compressed air for the discharge ring 85 may be supplied by a boost pump drawing in air at high pressure from the outlet of the compressor 1.

I claim:

l. Combustion apparatus for burning an ash-forming liquid fuel comprising means defining a combustion zone; a duct leading at one end to said combustion zone; a fuel injector mounted within said duct, said injector being of the type which discharges said fuel as a generally conical spray of droplets of various sizes; means for discharging a fluid stream from all around said conical spray inwardly thereinto at a velocity so related to the mass, dimensions and velocity of the fuel droplets as to cause substantially all the smaller fuel droplets of below a predetermined size to be deflected towards the central region of the spray; a collector pipe having an open end located in said central region of the spray to receive said smaller droplets; means to supplyair to the end of said duct remote from the combustion zone end to carry the remainder of the droplets into said' combustion zone; an igniter for igniting said remainder of the droplets in said combustion zone; said zone being shaped and dimensioned so that said ignited droplets are only partially burnt therein so that a proportion of unburnt carbon associated with the fuel ashis formed, and having an outlet for combustion gases.

2. Combustion apparatus according to claim 1 comprising a hollow discharge ring around the spray formed with at least one nozzle directed to discharge inwardly into said spray and means to supply a fluid to said ring.

3. Combustion apparatus according to claim 2 wherein the nozzzle is directed substantially at right angles to the envelope of the conical spray.

4. Combustion apparatus according to claim 1 comprising means for causing an air flow down said collector pipe from said open end.

5. Combustion apparatus according to claim 4 wherein said means comprises a fuel separator having an inlet connected to said collector pipe.

6. Combustion apparatus according to claim 1 having means defining an air entry to said combustion zone and means for swirling the air flowing through said entry about the axis thereof.

7. Combustion apparatus according to claim 6 comprising means defining a constriction at the entry to said combustion zone.

8. Combustion apparatus according to claim 1 wherein said combustion zone is constituted by a vortex combustion chamber defined by two side walls, one of which is formed with a central axial outlet, and a peripheral wall formed with a generally tangentially facing inlet, and a generally tangentially extending pre-combustion chamber leading to said inlet, the vortex combustion chamber defining a spiral vortex path from said inlet inwardly to i said axial outlet.

9. Combustion apparatus according to claim 8 further comprising means for introducing further air into the vortex combustion chamber in the region of said outlet.

l0. Combustion apparatus for burning an ash-forming liquid fuel comprising means defining a combustion zone;

a duct leading at one end to said combustion zone; a fuel injector mounted within said duct, said injector being of the type which discharges said fuel as a generally conical spray of droplets of various sizes; means for discharging a fluid stream from all around said conical spray inwardly thereinto at a velocity so related to the mass, dimensions and velocity of the fuel droplets as to cause substantially all the smaller fuel droplets of below a predetermined size to be deflected towards the central region of the spray; a collector pipe having an open end located in said central region of the spray to receive said smaller droplets; means to carry the remainder of the droplets into said combustion zone; an igniter for igniting said remainder of the droplets in said combustion zone; said zone being shaped andvdimensioned so that said ignited droplets are only partially burnt therein so that a proportion of unburnt carbon associated with the fuel ash is formed, and having an outlet for combustion gases.

References Cited in the file of this patent FOREIGN PATENTS 1,085,458 France Feb. 2, 1955 719,380 Great Britain Dec. 1, 1954

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