US2895293A - Combustion apparatus for and method of burning ash-forming liquid fuel - Google Patents

Description

July 21, 1959 HQDGE 2,895,293

COMBUSTION APPARATUS FOR AND METHOD OF BURNING v ASH-FORMING LIQUID FUEL Filed May 14, 1956 3 Sheets-Sheet l L' AttorneS S 3 Sheets-Sheet 2 R. I. HODGE ASH-FORMING LIQUID FUEL July 21, 1959 COMBUSTION APPARATUS FOR AND METHOD OF BURNING Filed May 14, 1956 n m? m/ v m g m4 R y Q m 2. WWW Om Z i -w, 0mm vmm 1 E w 0% 1 0M1 m no no 3 m 2m 2 +0 no o m me Z J mm on m o mm nmm I I! I \I I f I!) I ll!- wm E Q 21, 1959 v R HQDGE 2,895,293

COMBUSTION APPARATUS FOR AND METHOD OF BURNING ASH-FORMING LIQUID FUEL Filed May 14, 1956 5 Sheets-Sheet 3 Unite tates atent Ronald Inglis Hodge, Church Crookham, near Aldershot,

England, assignor to Power Jets (Research and Development) Limited, fLondon, England, a British company 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., 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 in combustion apparatus using fuel oils of the type indicated 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.

The present invention stems from the realization that in order to effect 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 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 in which fuel droplets, substantially all of which are within a predetermined range of size, are introduced into the combustion zone of the apparatus and are partially burnt therein so that the combustion gases contain a proportion of unburnt carbon which absorbs the fuel ash.

The invention further provides combustion apparatus as aforesaid comprising a fuel injector discharging a spray of fuel droplets of various sizes, and means for separating fuel droplets of sizes outside said pre-deter mined range from the remainder of the spray before it enters the combustion zone.

According to a feature of the invention, only the larger droplets, being substantially all above a pre-determined size, are introduced into the combustion zone. A fluid stream may be led into contact with the fuel spray so that it causes smaller droplets of less than said pre-determined size to be separated from the remainder of the spray.

ice

The injector may be located in a duct leading at one end to the combustion zone, and open at the other end to receive an air stream flowing to the combustion zone, said stream carrying the remainder of the spray to the combustion zone, and the injector may discharge a generally conical fuel spray and the fluid stream may cross the path of the spray. There may be a collector pipe, the open end of which receives the separated droplets.

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 baffle is therefore to be avoided. Thus there may be a constrictionat 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-circulation along the axis of the constriction is set up and a stabilized combustion zone formed.

Difficulty may arise in that the fuel may contain a considerable proportion of comparatively volatile constituents. The conditions which allow the rapid evaporation and efficient combustion of these constituents in known combustion chambers lead to a loss of control over the burning of the non-volatile constituents, and this lowers the effectiveness of the method of reduction of corrosion indicated above. It is desirable therefore to burn the volatile constituents in a manner which gives rise to the minimum disturbance in the flow through the zone in which these constituents are burnt. Accordingly, the fuel for combustion may be introduced into the upstream end of a vaporizing duct leading to the entry to the combustion zone, in which duct at least part of the volatile constituents of the fuel are vaporized.

The combustion apparatus may comprise a vortex combustion chamber with a central axial outlet in one of its side walls and a generally tangentially facing inlet in its peripheral wall to which is connected a pro-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 pcripheral part of the combustion apparatus shown in Figure 1. i

Figure 3 is a section on the line IIIIII in Figure 2.

Figure 4 shows a detail of the arrangement of Figure 2 to a longer scale.

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 British Patent 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 frusto-conical 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 supports 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 duct 6 for conveying hot combustion gases to the turbine 2, and its other end wall 120 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 segment-shaped 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 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 crossectional 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 split by partition 24 into two streams. The smaller stream enters the space between wall 23 and duct 16 and enters the later 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 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 110 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. 1) 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 33b by which it is secured to the peripheral wall 12a of the vortex combustion chamber. At the upstream end of the pre-combustion chamber 31 there is a constriction of substantially venturi shape having a 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 there is a similar short frusto-conical wall 61 converging to the upstream end of the throat. Extending upstream from this last mentioned wall is a tubular vaporizing duct 62. Ali annular air inlet is formed in the wall of this duct immediately upstream of the constriction in which are mounted swirl vanes 63 (see Fig. 3.). The inlet might be replaced by a number of separate tangentially directed inlets or by a 360 volute, the arrangement in any case being such that the air can be admitted therethrough with swirl about the axis of the duct. At the upstream end of the vaporizing duct 62 there is an annular air inlet defined by two frusto-conical walls 64-, 65, the wall 65 being supported from the -wall 64 by axially and radially extending splitters 66. Mounted within the vaporizing duct are a series of telescoping liners 67.

Within the inner Wall 65 is mounted a tube 71 open at both ends and constituting a duct through which further air can flow from the extension 32 into the vaporizing duct. As shown in Fig. 4 two further tubular members 72, 73 are mounted co-axially within the tube 71, the latter being connected to a curved collector pipe 74 leading to the exterior of the vaporizing duct. A liquid fuel injector 75 is mounted in the entry to member 73 within a faired shroud 76. This injector 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 75a as a generally conical spray of atomized fuel made up of droplets of various sizes. Liquid fuel, being a residual fuel oil of the type referred to above, is supplied to this injector from fuel tank 45 by a fuel pump 44 (as in the embodiment of Fig. 2.), through a pipe 77 nested within the pipe 74-. A fairing 78 is mounted Within the tubular member 72 co-axially with the injector, the adjacent ends of the shroud 7 6 and fairing 78 being shaped to form between them an annular gap conforming substantially to the cone angle of the fuel spray. The gap 79 between the members 72, 73 is also approximately aligned with the conical fuel spray as will be shown below.

In operation air from extension 32 enters the tubular member 72, is diverted outwardly by the fairing 78 and so flows into the annular space between the tubular member 73 and shroud 76. In so doing, the air stream crosses the path of the conical fuel spray. This spray is made up of fuel droplets of various sizes, and the smaller ones are diverted by the air stream and carried thereby into the tubular member 73 and so away through pipe 74. The larger particles on account of their greater inertia are not diverted from their original path to such a great extent and so are able to reach the gap 79. A further air stream from the extension 32 flows between the tube 71 and the tubular member 72 and this stream carries the larger fuel droplets into the vaporizing duct 62. The trajectories of the smaller and larger droplets are indicated by the broken lines X and Y respectively.

The pipe 74 is connected to the inlet of a cyclone separator 47 in which the fuel is separated from the air stream. The separated fuel is collected in trap 48 from which it drains through pipe 49 back to the fuel tank 45. The air is discharged through outlet duct 50 to a region of lower pressure, e.g., to atmosphere, or to the inlet of compressor 1, or to some other low pressure region of the combustion apparatus or of the plant, the pressure differential promoting air flow down pipe 74 to remove the smaller droplets. If required, suction may be applied to outlet duct 50 to accelerate the air flow, but normally this will not be necessary as the air supply to the apparatus will be under pressure.

By suitable choice of the relative locations and dimensions of the parts of the apparatus, the air velocities, and the pressure differential promoting flow down tube 74 in relation to the range of size of the fuel droplets discharged from the injector 75, it can be arranged that substantially only those droplets of above a pre-determined size are allowed to enter the vaporizing duct and the pre-combustion chamber. In the embodiment illustrated, gauzes 80 are provided in the inlet to tubular member 72 to reduce the velocity of the air stream flowing therethrough, while the tube 71 is convergent from its inlet end to accelerate the air stream flowing into the vaporizing duct. The end of the tubular member 72 which forms one edge of the gap 79 is aligned with the undeflected path of the fuel spray indicated by the broken line Z.

It will be noted that air flowing through pipe 74 serves to some extent to protect the fuel in pipe 77 against overheating.

The air entering the vaporizing duct is pre-heated by compression and might in some arrangements also be heated by passage through a heat exchanger, and thus serves to effect vaporization of the more volatile constituents of the fuel oil droplets discharged into the vaporizing duct 62. The duct is long enough having regard to its cross-sectional area, the velocity, mass flow and temperature of the air, and the quantity and composition of the fuel that the major part of the volatile constituents of the fuel are vaporized in the duct before they reach its downstream end. Air admitted progressively through the annular gaps between successive liners 67 serves to prevent fuel droplets coming into contact with the duct *wall with the consequent risk of carbon deposition thereon.

At the downstream end of the vaporizing duct, further air is admitted with a strong swirl about the duct axis through swirl vanes 63. This swirling air passes through the constriction at the upstream end of the pre-combustion chamber 31, its velocity and quantity being such in relation to the size and configuration of the constriction as to give rise to re-circulation along the axis thereof. The fuel is ignited by means of a torch igniter 53 or other ignition device, and stable combustion of the volatile constituents of the fuel takes place in the stabilized flame zone set up by the re-circulation. It is to be noted that there is no conventional flame stabilizing baffle on which carbon deposition might occur.

The burning volatile constituents give rise to a flame in the pro-combustion chamber 31 which serves to ignite the non-volatile constituents of the fuel. These are carried into the vortex combustion chamber 11 and are burnt therein as they 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 pre-determined proportion 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 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 sufficient to support combustion of fuel.

In some cases, the proportion of volatile constituents vaporized in the vaporizing duct 62 will be less than in the example described above; e.g., the proportion vaporized may be or less, depending upon the composition of the fuel oil used. The remainder of the volatile constituents will then be vaporized in the pre-combustion chamber. The vaporizing duct 62 and swirl vanes 63 might be omitted altogether, i.e., the tube '71 and the associated fuel supply and droplet separation arrangement could be used in conjunction with a pre-combustion 6 chamber assembly 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 assembly of vaporizing duct, pre-combustion chamber and vortex combustion chamber described above could also be used in conjunction with the fuel supply and droplet separation arrangement described in said copending application, or with that described in a copending application Serial No. 584,630'in the name of M. V. Herbert also filed on the same day as the present application.

I claim:

1. A combustion method for burning ash-forming liquid fuel that comprises the steps of establishing a combustion zone; discharging said fuel as a spray of droplets of various sizes into a region outside said combustion zone; separating from the spray substantially all those droplets of sizes less than a pre-determined size, which separated droplets would, under the conditions of combustion prevailing in said zone, be substantially completely burned; introducing the. remainder of the droplets into the combustion zone; igniting said remainder of the droplets in said zone; partially burning said ignited droplets in said zone so that a proportion of unburnt carbon associated with fuel ash is formed; and discharging said carbon with the combustion gases from said zone.

2. A combustion method for burning ash-forming liquid fuel that comprises the steps of establishing a combustion zone and a vaporizing zone; discharging said fuel as a spray of droplets of various sizes into a region outside said zones; separating from the spray substantially all those droplets of sizes less than a pre-determined size, which separated droplets would, under the conditions of combustion prevailing in said zone, be substantially completely burned; introducing the remainder of the droplets into the vaporizing zone; heating said remainder of the fuel droplets in the vaporizing zone to vaporize at least part of the volatile constituentsof the fuel droplets; introducing the vaporized fuel and said droplets into the combustion zone; igniting said vaporized fuel and the droplets in the combustion zone; partially burning the non-volatile constituents in the combustion zone so that a proportion of unburnt carbon associated with fuel ash is formed, and discharging said carbon with the combustion gases from said zone.

3. Combustion apparatus for burning 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 ofa type which discharges said fuel as a spray of droplets of various sizes; means for separating from said spray substantially all those droplets of sizes less than a predetermined size, which separated droplets would, under the conditions of combustion prevailing in said zone, be substantially completely burned; means for removing said separated droplets from the duct; means [for supplying air to the other end of the duct to carry the remainder of the droplets into the combustion zone; an igniter for igniting said remainder of the 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 fuel ash is formed, and having an outlet for combustion gases.

4. Combustion apparatus for burning 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 a type which discharges said fuel as a spray of droplets of various sizes; means for directing a fluid stream in a direction different from that of and into contact with said spray at such a velocity in relation to the mass, dimensions and velocity of the droplets as to separate and carry away therefrom substantially all those droplets of sizes less than pre-determined size; means to receive said fluid stream with the droplets carried away thereby; means for supplying air to the other end of said duct to carry the remainder of the droplets into the combustion zone; an igniter for igniting said remainder of the 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 fuel ash is formed, and having an outlet for combustion gases.

5. Combustion apparatus for burning ash-forming liquid fuel comprising means defining a combusion zone; a duct leading at one end to said combustion zone; two coaxial open-ended tubular members axially separated by an annular gap mounted within the duct and defining therewith an annular passage; a fuel injector mounted coaxially within that tubular member nearer the combustion zone end of the duct with a radial clearance therearound, said injector being of the type which discharges fuel as a generally conical spray of droplets of various sizes, said annular gap being substantially aligned with said spray; a collector pipe having an open end connected to said last-mentioned tubular member; means to supply air to the end of the other tubular member remote from the combustion zone to impinge upon the fuel spray at such a velocity in relation to the mass, and dimensions and velocity of the droplets as to carry the fuel droplets of less than a predetermined size into the radial clearance around the injector and so into the collector pipe and to the end of the duct remote from the combustion zone to carry the remainder of the droplets into the combustion zone; an igniter for igniting said remainder of the 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 fuel ash is formed, and having an outlet for combustion gases.

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

7. Combustion apparatus according to claim 6 wherein said means comprises a fuel separator having an inlet connected to said collector pipe.

8. Combustion apparatus according to claim 5 having means defining an air entry to said combustion zone and means for swirling the air flowing through said entry about the axis thereof.

9. Combustion apparatus according to claim 8 comprising means defining a constriction at the entry to the combustion zone.

10. Combustion apparatus according to claim 5 wherein said combustion zone is constituted by a vortex combustionchamber 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 said axial outlet.

11. Combustion apparatus according to claim 10 further comprising means for introducing further air into the vortex combustion chamber in the region of said outlet.

12. Combustion apparatus for burning 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 a type which discharges said fuel as a spray of droplets of various sizes; means for separating from said spray substantially all those droplets of sizes less than a predetermined size, which separated droplets would, under the conditions of combustion prevailing in said zone, be substantially completely burned; means for removing said separated droplets from the duct; means for carrying the remainder of the droplets into the combustion Zone; an igniter for igniting said remainder of the 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 fuel ash is formed, and having an outlet for combustion gases.

13. Combustion apparatus for burning 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 a type which discharges said fuel as a spray of droplets of various sizes; means for directing a fluid stream in a direction dififerent from that of and into contact with said spray at such a velocity in relation to the mass, dimensions and velocity of the droplets as to separate and carry away therefrom substantially all those droplets of sizes less than a predetermined size; means to receive said fluid stream together with the droplets carried away thereby; means for carrying the remainder of the droplets into the combustion zone; an igniter for igniting said remainder of the 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 fuel ash is formed, and having an outlet for combustion gases.

References Cited in the file of this patent UNITED STATES PATENTS 2,374,606 McCollum Apr. 24, 1945 2,680,951 Winter et a1 June 15, 1954 2,828,608 Cowlin et al Apr. 1, 1958 FOREIGN PATENTS 1,095,032 France Mar. 26, 1955 435,679 Great Britain Sept. 25, 1935 719,380 Great Britain Dec. 1, 1954

Images