US2895297A - Combustion apparatus for burning ash-forming liquid fuel - Google Patents

Description

y 1, 1959 J. A. GARDINER 2,895,297

COMBUSTION APPARATUS FOR BURNING ASH-FORMING LIQUID FQEL I Filed May 14, 1956 2 Sheets-Sheet 1 7 J. 7 Mg Attorney;

July 21 1959 J 'A GARDNER 2,895,297

COMBUSTION APPARATUS FOR BURNING ASH-FORMING LIQUID FUEL Filed May 14, 1956 2 Sheets-Sheet 2 m e For BY 5 United States Patent COMBUSTION APPARATUS FOR BURNING ASH-FORMING LIQUID FUEL John Alan Gardiner, Crookham, near Alder-shot, England, assignor to Power Jets (Research and Development) Limited, London, England, a British company Application May 14, 1956, Serial No. 584,57 7 Claims priority, application Great Britain May 10, 1956 8 Claims. (Cl. 6039.74)

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 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 very serious 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 1n combustion apparatus using fuel oils of the type ll'ldlcated above so that the gases resulting from 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 copending application, Serial No. 584,629, in the name of R. I. Hodge filed on the same day as the present application, in order to effect cornbustion 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 be no 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 efliciency 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 spaced from and injecting fuel towards said end of the duct as a spray of droplets of various sizes, an air stream flowing down said duct from said end so that substantially all the smaller fuel droplets, below a pre-determined size, are reversed and carried away by the stream while the remainder of the droplets escape from the duct and enter the combustion zone in which they are partially burnt so that the combustion gases contain a proportion of unburnt carbon which absorbs the fuel ash.

In combustion apparatus of the type with which the present invention is concerned, it is necessary to make provision for a stabilized flame zone in which combustion is to be initiated, and it is desirable to ensure that there shall be no deposition of unburnt carbon or of ash in this zone. The use of a conventional flame stabilizing bafile is therefore to be avoided. Thus there may be a constriction at the entry to the combustion zone, combustion air being supplied to the combustion zone through the constriction with swirl about the axis thereof so that re-circu1ation along the axis of the constriction is set up and a stabilized flame zone formed. Combustion air may be supplied to the constriction through an annular inlet, and a liner may be mounted Within the inlet forming with the outer wall thereof an annular passage extending to the constriction and provided with swirl vanes for swirling the air passing therethrough.

The combustion apparatus may comprise a vortex combustion chamber with a central axial outlet in one of its side walls and a generally tangential facing inlet in its peripheral wall to which is connected a pre-combustion chamber, the fuel to be burnt being introduced into and ignited in the pre-combustion chamber 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 combustion gases at the outlet of the vortex 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 cross-sectional view of the peripheral part of the combustion apparatus as shown in Figure 1.

In Fig. 1 a gas turbine plant comprises a compressor 1, a turbine 2 driving the compressor through a shaft 3 and also driving a 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.

The combustion apparatus is of the spiral vortex type operating on the principles described in the British Pattent No. 639,468 and is structurally similar in some respects to the apparatus described in British Patent Nos. 719,379 and 719,380. It comprises an outer casing 11 having a peripheral wall 11a and two generally frustoconical end walls 11b and 110, and a vortex combustion chamber 12, likewise having a peripheral wall 12a, and end walls 12]) 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 supports are shown in the last two 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 duct 6 for conveying hot combustion gases to the turbine 2, and its other end wall 12c is also formed With a central aperture 18 from which a tube 19 extends axially into the interior of the chamber.

Its. peripheral wall 12a is formed with two diametrically opposite tangentially facing generally segmentshaped inlet apertures (one of which is shown at 17) to which are connected generally tangentially extending pre-combustion chambers 31 as shown in Fig. 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 extending axially there-from 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 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 are 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 pre-combustion chambers 31 (see Fig. 2).

In operation, the air supply entering the inlet duct 22 is split by partition 24 into two streams. The smaller stream enters the space between wall 23 and duct 16 and enters the latter through rows of holes 16a. The larger stream enters the space between the casing 11 and combustion chamber 12, thus cooling the latter, and is further divided, some passing into the air casings 32 and some into 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 31a, 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 310. 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 cooling air. The sheath has an external flange 3319 by which it is secured to the peripheral wall 12:: of the vortex combustion chamber. At the upstream end of the pie-combustion 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 frusto-conical wall 37 converges to the upstream end of the throat. This last mentioned wall 37 defines with a further frustoconical 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-combustion chamber.

The inner wall 38 terminates in a plane spaced upstream of the throat 35 and has centrally mounted within it a tube 40, constituting a duct opening-at one end into the air inlet passage in the terminal plane of the wall 38. The tube 40 extends through the closed end 32:: of the extension 32 and has an external flange 40a for attachment thereto. A fuel injector 41 is mounted cen trally within the tube, so that it is spaced from and discharges towards the open end thereof. The injector may be of any known type which emits liquid fuel in the form of a spray of droplets of various sizes. Thus it may be'of the known swirl atomizing type having an internal swirl chamber to which the fuel is supplied through one or more tangential swirl ports and from which the fuel is discharged through an axial discharge nozzle 41a as a generally conical spray of atomized fuel. 'The injector 41 is mounted on the end of a fuel pipe 42 supported within the tube 40 by a spider 43, and liquid fuel, being a residual fuel oil of the type re- 4 ferred to above, is supplied to the atomizer by pump 44 from tank 45.

In operation air enters the pre-combustion chamber at a pressure higher than atmospheric, having been compressed by compressor 1, and so, if the end of the tube 4%) remote from the pre-combustion chamber is connected to a region of lower pressure, some of the air will tend to flown down the tube away from the combustion chamber. This flow opposes the movement of the fuel droplets from the injector in the direction of the combustion chamber. As mentioned above, the fuel spray consists of droplets of various sizes, and the larger ones have s-uflicient momentum to escape from the tube 40 into the air inlet passage and so into the pre-combustion chamber notwithstanding the air flow down the tube 4% in the opposite direction. The smaller droplets however, on account of their smaller mass, are reversed by the air flow before they can reach the end of the tube 44) and are carried away by the air flow therein. The other end of tube 40 is connected by a duct 46 to a cyclone separator 47 of known type in which the fuel is separated from the air. The separated fuel is collected in a trap 48 from which it drains through pipe 49 back to the fuel tank 45. The air is discharged through an outlet duct 50 to the region of lower pressure referred to above. Thus it may be connected to atmosphere, or to the inlet of compressor 1, or to some other low pressure region of the combustion apparatus or of the plant. If necessary a suction pump can be incorporated in the outlet duct 50 to accelerate the air flow down the tube 40. By suitable choice of the pressure differential calusing the flow in the tube and of the tube size and hence of the mass flow and velocity of the air in the tube and of the spacing of the injector 41 from the end of the tube in relation to the range of fuel droplet size, it can be arranged that substantially only those droplets of above a pre-determined size are allowed to enter the pre-combustion chamber.

Some of the fuel reversed by the air flow down tube 40 may collect on the wall thereof and to prevent this fuel running into the pre-combustion chamber, the tube is tapered towards the latter, its axis being horizontal so that the fuel runs back towards duct 46.

In some arrangements it may be possible to have the tube with its axis inclined to the horizontal and sloping downwardly away from the combustion chamber end, or it might be vertical and lead downwardly from the bottom of the combustion chamber.

Mounted adjacent to the outer wall 37 of the air inlet passage is a frusto-conical liner 51 forming therewith an annular passage for part 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 gives rise to re-circulation along theaxis thereof. Fuel from the tube 40 enters the upstream end of the pre-combustion chamber and is ignited by an ignition device such as a torch igniter 53, combustion being initiated 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 swirling air to flow along the wall of the throat of the constriction, thus reducing any tendency for reverse flow along the wall to occur with consequent deposition of unburned fuel thereon. This arrangement also reduces the risk of the flame blowing back through the layer of air close to the throat wall.

Fuel oils of the type used in the present apparatus contain a certain proportion of comparatively volatile constituents and at least partial combustion of these constituents takes place in the rpre-combustion-chambers 33.

The burning fuel droplets are carried into the vortex combustion chamber 11 and the less volatile constituents are burnt as the fuel droplets are carried around the axis of the chamber in a spiral vortex path, progressively approaching the central outlet 15 as they are burnt away. Since the size of the fuel droplets burnt is controlled substantially all 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 the fuel droplets are only partially burnt and a predetermined proportioned of unburnt carbon remains associated with the fuel ash from each fuel droplet 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 serves 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 sufficient to support combustion of fuel.

The pro-combustion chamber may, instead of being connected to the inlet to a vortex combustion chamber, constitute the upstream end of a tubular flame tube of the known straight flow type. The flame tube will be designed so that the fuel droplets are not completely burnt when they reach the outlet so that the exhaust gases contain particles of unburnt carbon. If necessary chilling air may be introduced at the flame tube outlet through apertures in the frame tube wall.

I claim:

1. Combustion apparatus for burning ash-forming liquid fuel comprising means defining a combustion zone and an air entry thereto; means to supply compressed air to said entry; a duct opening at one end into said air entry; a fuel injector mounted within and spaced from the air entry end of said duct, said injector being of the type discharging a spray of fuel droplets of various sizes and being arranged to discharge towards said end; means to establish a pressure differential in a sense to cause part of said compressed air to flow down said duct away from said air entry end, the differential being so related to the size of the duct that the air will flow therethrough with a velocity which is such in relation to the mass, dimensions, and velocity of fuel droplets discharged by the injector and its spacing from the air entry end of the duct that substantially all those droplets of less than a predetermined size are reversed and carried away by the air flow before they can reach said air entry end of the duct; an igniter for igniting the remainder of said droplets in the 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 ash is formed, and having an outlet for combustion gases.

2. Combustion apparatus according to claim 1 further comprising a fuel separator having an inlet connected to the end of said duct remote from the air entry end.

3. Combustion apparatus according to claim 1 comprising means for swirling the air flowing through said air entry about the axis thereof.

4. Combustion apparatus according to claim 3 comprising means defining a constriction at said air entry.

5. Combustion apparatus according to claim 4 comprising inner and outer coaxial walls defining between them an annular air inlet leading to said constriction; a liner mounted within said inlet and defining with the outer wall an open-ended annular passage; and swirl vanes within said annular passage arranged to swirl the air passing therethrough about the axis of the constriction.

6. Combustion apparatus according to claim 5 wherein the liner includes a part extending into and parallel to the throat of the constriction.

7. 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, the vortex combustion chamber defining a spiral vortex path from said inlet inwardly to said axial outlet.

8. Combustion apparatus according to claim 7 further comprising means for introducing further air into the vortex combustion chamber in the region of said outlet.

References Cited in the file of this patent FOREIGN PATENTS 719,380 Great Britain Dec. 1, 1954

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