GB2107849A

GB2107849A - Combustion turbine combustor

Info

Publication number: GB2107849A
Application number: GB08224547A
Authority: GB
Inventors: Daniel Eugene Carl; Serafino Mario Decorso
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1981-10-26
Filing date: 1982-08-26
Publication date: 1983-05-05
Also published as: MX156590A; AR228788A1; GB2107849B; CA1195513A; IT8222947A0; IT1152043B; BR8204910A

Abstract

A combustion turbine combustor comprises an enclosure having apertures (56) for enabling the flow of gases into the enclosure, a fuel injector (42), a fuel preparation zone (41) for mixing the fuel with the gas flow, means for preventing deposition of coke on interior surfaces (54) of the combustor, and means (36) for causing combustion of the fuel mixture. Combustion may be by catalyst or by flame. The means for preventing coke deposition comprises a hood structure (46) having a plurality of concentric cylindrical segments in a telescoping arrangement which forms a protective boundary of air. This arrangement enables use of a heavy oil fuel in a premixing, prevaporizing combustor. <IMAGE>

Description

SPECIFICATION Combustion turbine combustor This invention relates to combustion turbines and combustors employed therein and more particularly to an improved fuel preparation zone structure for a premixed, pre-vaporized combustor.

In general terms, the typical prior art combustion turbine comprises three sections: a compressor section, a combustor section, and a turbine section. Air drawn into the compressor section is compressed, increasing its temperature and density. The compressed air from the compressor section flows through the combustor section where the temperature of the air mass is further increased. From the combustor section the hot pressurized gases flow into the turbine section where the energy of the expanding gases is transformed into rotational motion of the turbine rotor.

A typical combustor section comprises a plurality of combustors arranged in an annular array about the circumference of the combustion turbine. In conventional combustortechnology pressurized gases flowing from the compressor section are heated by a flame in the combustor before passing to the turbine section. In the flame technique, fuel is sprayed into the upstream end of the combustor by a nozzle. The flame is maintained immediately downstream of the nozzle by strong aerodynamic recirculation.

The lack of thorough mixing of the injected fuel results in pockets of high fuel concentration and correspondingly high combustion reaction temperatures (approximately 45000R-Rankine temperature) in these pockets. Because the reaction temperature is high, hot gases flowing from the combustion reaction must be diluted downstream by cool (approximately 1 000R) air so as to prevent damage to turbine components positioned downstream. In addition, the conventional flame technique produces emissions with significant levels of undesirable chemical compounds including NOx and CO.

Increasing environmental awareness has resulted in more stringent emissions standards for NOX and CO. The more stringent standards are leading to development of improved combustor technologies. One such improvement is a premixing, prevaporizing combustor. In this type of combustor, fuel is sprayed into a fuel preparation zone, where it is thoroughly mixed to achieve a homogeneous concentration which is everywhere within definite limits of the mean concentration. Additionally, a certain amount of the fuel is vaporized in the fuel preparation zone.

Fuel combustion occurs at a point downstream from the fuel preparation zone. The substantially uniform fuel concentration achieved in the fuel preparation zone results in a uniform reaction temperature which may be limited to approximately 20000F to 30000 F. Due to the uniformity and thoroughness of combustion, the premixing, prevaporizing combustor produces lower levels of NOX and CO than does a conventional combustor using the same amount of fuel.

To date, premixing, prevaporizing combustors have generally been fueled by clean fuel oils, such as No. 2 fuel oil, as opposed to heavy oil fuels such as No. 5 fuel oil. A clean fuel oil is characterized by low ash content, low nitrogen content and a low boiling point, so that the fuel will readily vaporize in a fuel preparation zone operating at normal temperatures. Heavy oil is generally avoided as a fuel due to the problems which typically accompany its use.

Heavy oils are characterized by the short autoignition times of the longer hydrocarbon chains, making flashback a greater problem in heavy oils than in clean oils. Flashback is the propagation of a flame from the point of combustion back into the fuel preparation zone. If permitted to continue uncorrected, the presence of flame in the fuel preparation zone will damage the combustor to the extent that the turbine must be shut down and the combustor repaired or replaced. Because of the higher boiling points, it is often impractical to completely vaporize a heavy oil. Furthermore, fully mixing a heavy oil prior to combustion typically causes significant coke deposition on the walls surrounding the fuel preparation zone. Deposits of coke on the walls of a combustor create flow obstructions and irregular gas flow patterns, inhibiting performance of the combustor.On the other hand, failure to mix a heavy oil fuel prior to combustion produces a non-uniform fuel concentration which can result in extremely high reaction temperatures and damage to combustor components, especially to a catalytic element in a catalytic combustor.

Hence, the known prior art combustors do not appear to meet the need for a premixing, prevaporizing combustor capable of successfully utilizing heavy oil fuels. Heavy oil fuels, not widely used in the past because of the problems described above, are attractive as an alternative source of energy.

It is an object of this invention to provide an improved combustion turbine combustor with a view to overcoming the deficiencies of the prior art.

The invention resides in a combustor for a combustion turbine system, comprising: an enclosure for containing a combustion reaction, said enclosure being generally cylindrical and having apertures therethrough near an upstream end to permit influx of compressor discharge air which flows into said enclosure and exits through an open downstream end of said enclosure into a transition duct which leads to a turbine inlet; means for injecting fuel into the flow of gases within said enclosure; a fuel preparation zone within said enclosure downstream of said injecting means, said preparation zone having therein means for at least partially mixing with compressor discharge air and vaporizing the fuel injected by said injecting means; means for preventing the fuel mixture from contacting said enclosure surrounding said preparation zone, and a combustion zone within said enclosure downstream of said preparation zone, said combustion zone having therein means for initiating and supporting combustion of the fuel, the temperature of gases flowing into the transition duct being thereby increased.

In accordance with a preferred embodiment of the invention, deposition of coke is inhibited by a hood structure having means for producing a boundary layer of air along its interior surface so as to prevent impingement of fuel mixture thereon. Combustion may be by flame or by catalyst, in both cases the hood structure enabling use of a heavy oil fuel where such would otherwise be impractical due to wall coking.

The invention will become readily apparent from the following description of an exemplary embodiment thereof when read in conjunction with the accompanying drawings, in which: Figure 1 schematically shows a catalytic combustor arranged to operate a gas turbine in accordance with a preferred embodiment of the invention; Figure 2 shows an elevational view of a catalytic combustor disposed as shown schematically in Figure 1; and Figure 3 shows a section of the combustor of Figure 2 arranged in accordance with the preferred embodiment of the invention.

More particuiarly, there is shown in Figure 1 a generalized schematic representation of a combustion turbine combustor and combustor control system. A turbine or generally cylindrical catalytic combustor 10 is combined with a plurality of like combustors (not shown) to supply hot motive gas to the inlet of a turbine (not shown) as indicated by reference character 12.

The combustor 10 includes a catalytic unit 14 which supports catalytic combustion (oxidation) of fuel-air mixture flowing through the combustor 10.

The combustor 10 includes a zone 11 into which fuel, such as oil, is injected by nozzle means 1 6 from a fuel valve 17, where fuel-air mixing occurs in preparation for entry into the catalytic unit 14. Typically, the fuel-air mix temperature (for example 8000F) required for catalytic reaction is higher than the temperature (for example 7000F) of the compressor discharge air supplied to the combustors from the enclosed space outside the combustor sheils. The deficiency in air supply temperature in typical cases is highest during startup and lower load operation.

A primary combustion zone 1 8 is accordingly provided upstream from the fuel preparation zone 11 within the combustor 10. Nozzle means 20 are provided for injecting fuel from a primary fuel valve 22 into the primary combustion zone 18 where conventional flame combustion is supported by primary air entering the zone 18 from the space within the turbine casing through openings in the combustor wall. The primary and secondary fuel valves 22 and 17 are controlled by respective fuel controls 23 and 24, both of which are operatively connected to a speed and load controf 25.

As a result, a hot gas flow is supplied to the fuel preparation zone 11 where it can be mixed with the fuel and air mixture to provide a heated fuel mixture at a sufficiently high temperature to enable proper catalytic unit operation. In this arrangement, the fuel injected by the nozzle means 1 6 for combustion in the catalytic unit is a secondary fuel flow. The secondary fuel flow is mixed with secondary air 19 from a compressor 21 and the primary combustion products, which supply the preheating needed to raise the temperature of the mixture to the level needed for entry into the catalytic unit.

It shouid be noted that a combustor structured according to the preferred embodiment of the invention is not limited to the catalytic structure described herein. A non-catalytic combustor (not shown) can be used which comprises a single nozzle means injecting fuel into a fuel preparation zone for fuel-air mixing. Combustion of the fuelair mixture occurs at a flameholder or in an open section in a combustion zone downstream of the fuel preparation zone, producing a hot gas flow which is supplied to the turbine inlet. The description hereinafter is directed expressly to a catalytic combustor but applies equally well to a non-catalytic type combustor.

In Figure 2 there is shown a structurally detailed catalytic combustion system 30 embodying the principles described for the combustor 10 of Figure 1. Thus, the combustion system 30 generates hot combustion products which pass through stator vanes 31 to drive turbine blades (not shown). A plurality of combustion systems 30 are disposed about the rotor axis within a turbine casing 32 to supply the total hot gas flow needed to drive the turbine.

In accordance with the preferred embodiment of the invention, the combustor 30 includes a combustor basket or enclosure 40, a catalytic unit 36 and a transition duct 38 which directs the hot gas to the annular space through which it passes to be directed against the turbine blades. The catalytic combustor 30 further comprises a fuel preparation zone internal to the combustor basket 40 at reference character 34.

A fuel preparation zone of the combustor 30 of Figure 2 is shown in section in Figure 3. The fuel preparation zone 41 comprises secondary nozzle means 42 for injecting fuel into the fuel preparation zone and a hood structure 46 for containing the fuel-air mixture until the mixture flows into a combustion zone 48.

The secondary nozzle means 42 injects fuel along an injection plane preferably with respective surrounding jets of air through sidewall scoops 56 for mixing with the primary gas f!ow.

Proper penetration of secondary air/fuel jets is important from the standpoint of air/fuel mixing. If penetration is excessive, the center of the catalytic unit 36 receives too much fuel; if too little penetration is obtain, the edges of the catalyst receive too much fuel. For optimum mixing, the maximum penetration should be approximately 33% of the tubular combustor diameter. Proper jet penetration provides good atomization of secondary fuel, which is the key to achieving rapid fuel vaporization.

The hood structure 46 comprises a plurality of concentric cylindrical sections arranged with a portion of each section overlapping and affixed by appropriate means such as welding to adjacent sections so as to form a telescoping hood. The structure 46 encloses substantially the full volume of the fuel-preparation zone 41 from the nozzle means 42 downstream to the combustion zone 48. Apertures positioned in an upstream face 52 of each cylindrical section of the hood structure 46 direct a flow of gases along an interior surface 54 of each section, forming a protective boundary of air which prevents heavy oil fuel from contacting the interior surface of the hood structure 46. The hood structure 46, while enabling the heavy oil fuel to be thoroughly mixed with incoming gases, prevents deposition of coke on the inside walls of the structure 46 and the fuel preparation zone 41.

It is envisioned that the hood structure 46 may be arranged with cylindrical sections which are non-telescoping or with any other structure which, consistent with the principles of the invention, provides a boundary layer of fluid between the fuel preparation zone 41 and the surrounding combustor enclosure. Such an alternative hood structure embodiment might comprise, for example, a generally cylindrical enclosure having annularly disposed, axially spaced fluid injection means for providing a layer of fluid along the interior surface of the enclosure.

The fuel-air mixture exiting the hood structure 46 flows into a combustion zone 48, which may comprise a catalytic element, for a catalytic combustor, as shown in Figure 3, or a flameholder for a conventional flame combustor (not shown).

Both types of combustion zones are well known in the prior art.

Claims

1. A combustor for a combustion turbine system, comprising: an enclosure for containing a combustion reaction, said enclosure being generally cylindrical and having apertures therethrough near an upstream end to permit influex of compressor discharge air which flows into said enclosure and exits through an open downstream end of said enclosure into a transition duct which leads to a-turbine inlet; means for injecting fuel into the flow of gases within said enclosure; a fuel preparation zone within said enclosure downstream of said injecting means, said preparation zone having therein means for at least partially mixing with compressor discharge air and vaporizing the fuel injected by said injecting means; means for preventing the fuel mixture from contacting said enclosure surrounding said preparation zone, and a combustion zone within said enclosure downstream of said preparation zone, said combustion zone having therein means for initiating and supporting combustion of the fuel, the temperature of gases flowing into the transition duct being thereby increased.

2. A combustor according to claim 1 wherein the fuel comprises a heavy oil fuel.

3. A combustor according to claim 1 or 2 wherein said means for preventing the fuel mixture from contacting said enclosure comprises an apparatus surrounding said preparation zone, said apparatus having a boundary layer of motive fluid separating the fuel mixture from said combustor.

4. A combustor according to claim 1, 2 or 3 wherein said boundary layer apparatus comprises a hood structure having a plurality of concentric cylindrical segments, each segment partially overlapping and affixed to adjacent segments to form a telescoping structure, so that apertures in an upstream face of each segment direct gases axially along the interior surface of each segment to prevent impingement of fuel mixture thereon thereby protecting the hood structure from coke deposition.

5. A combustor according to claim 1, 2, 3 or 4 wherein said means for supporting combustion comprises a flameholder in said combustion zone downstream of the hood structure.

6. A combustor according to claim 1, 2, 3 or 4 wherein said means for supporting combustion comprises a catalytic combustion element in said combustion zone downstream of the hood structure.