GB2108651A

GB2108651A - Combustion turbine combustor

Info

Publication number: GB2108651A
Application number: GB08219211A
Authority: GB
Inventors: Daniel Eugene Carl; Serafino Mario Decorso; John Allen Schwab
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1981-11-02
Filing date: 1982-07-02
Publication date: 1983-05-18
Also published as: JPS61135171U; MX156338A; JPS5878015A; BE893710A; GB2108651B; IT1152245B; JPH0335986Y2; BR8203784A; IT8221875A0; CA1190051A

Abstract

A combustion turbine combustor is provided with an improved fuel preparation zone which utilizes the arrangement of a static mixing structure (46) to accelerate the flow velocity of combustible gases into a combustion zone. The increased flow velocity reduced the potential of the combustor for flashback and enables use of a fuel-rich configuration in the 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 pre-mixing, pre-vaporizing 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 combustor technology pressurized gases flowing from the compressor section are heated by a diffusion flame in the combustor before passing to the turbine section.

In the diffusion flame technique, fuel is sprayed into the upstream end of the combustor by means of a nozzle. The flame is maintained immediately downstream of the nozzle by strong aerodynamic recirculation. The lack of thorough mixing of the fuel results in pockets of high fuel concentration and correspondingly high combustion reaction temperatures, on the order of approximately 45000R (Rankine temperature). Because the reaction temperature is high, hot gases flowing from the combustion reaction must be diluted downstream by cool (approximately 7000R) air so as to prevent damage to turbine components positioned downstream. In addition, the flame diffusion technique produces emissions with significant levels of undesirable chemical compounds including NOx and CO.

Increasing environmental awareness has resulted in more stringent emission 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.

One problem with premixing, prevaporizing combustors is the destructive potential for flashback, or sudden propagation of 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. This phenomenon has been classically defined to involve the competing relationship between the flow velocity and the flame velocity of the combustible gases. Because the fame velocity is ordinarily characteristic of the fuel used, the flow velocity is more readily within the control of the designer. For further elaboration of the classical approach to flashback see B. Lewis, G. von Elbe, Combustion, Flames and Explosions of Gases, Academic Press, New York (1961).

Flame stability in a combustor was later semiquantified by S. L. Plee and A. M. Mellor. See "Review Of Flashback Reported In Prevaporizing/Premixing Combustors," 32 Combustion and Flame 193-203 (1978). The formula suggested by Plee and Mellow for stability is as follows: 1/0 = rn/(Pa ADb exp (T/C)) where:: = = mass flow rate of combustible gases in the fuel preparation zone P = absolute pressure of the fuel preparation zone A = cross-sectional area of the fuel preparation zone D = cross-sectional diameter of the fuel preparation zone T = absolute temperature of the fuel preparation zone a, b, t c = constants, greater than zero It was reported that the value of 1/0 for a fuel preparation zone is representative of the stability of the combustion flame. With larger values of 1/0 the fuel preparation zone is less prone to flashback. From the equation it can be determined that flame stability is promoted by a reduction in the cross-sectional dimensions of the fuel preparation zone.

Reduction of the cross-sectional area, however, places severe restrictions on the designer by limiting fuel nozzle selection to smaller nozzles. In general, smaller.nozzles suffer from more limited availability, greater expense, and a greater pressure drop across the nozzle as compared to larger nozzles. Pressure loss within the combustor is to be minimized so that the working efficiency of the combustion turbine is maintained.

The danger of flashback becomes especially acute when fuel concentration in the fuel preparation zone is high, as in the case of a fuelrich fuel preparation. Operating a combustor in a fuel-rich configuration can reduce NOx emissions by effectively depleting the oxygen which would ordinarily be available to combine with nitrogen to form NOx. Without specific safeguards directed at preventing flashback, use of a fuel-rich fuel preparation zone would not be possible in a premixing, prevaporizing combustor.

Thus, the known prior art does not appear to meet the need for preventing flashback in a premixing, prevaporizing combustor without reducing the cross-sectional area of the fuel preparation zone.

It is an object of this invention to provide an improved combustor for a combustion turbine 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 combustion reaction, said enclosure being generally cylindrical and having apertures therethrough toward an upstream end to permit influx of compressor discharge gases which flow into said enclosure and exit 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 located downstream of said fuel injecting means, where the fuel is mixed and vaporized to obtain a homogeneous gaseous fuel mixture; a combustion zone located downstream of said fuel preparation zone, said combustion zone initiating and supporting combustion of the fuel mixture, the temperature of gases flowing into the transition duct being thereby increased, and means within said fuel preparation zone for increasing the flow velocity of the fuel mixture through the combustion zone.

The mixing structure is arranged to provide acceleration of the fuel mixture through the combustion zone to permit use of a high fuel concentration within the fuel preparation zone and yet diminish the danger of flashback. The means of combustion may be by flame or by catalyst; in both cases, the increased flow velocity through the combustion zone by virtue of the arrangement of the mixing structure serves to prevent flashback.

The invention will become readily apparent from the following description of exemplary embodiments 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; Figure 3 shows a fuel preparation zone for a combustor arranged in accordance with the preferred embodiment of the invention; Figure 4 shows a static mixer of Figure 3 in section; Figure 5 shows a side view of an alternative internal structure for the static mixer shown in Figure 3.

More particularly, 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 1 2. The combustor 12 includes a catalytic unit 14 which supports catalytic combustion (oxidation) of fuelair 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 shells. 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 1 8 from the space within the turbine casing through openings in the combustor wall. The primary and secondary fuel valves 22 and 17 are controlled respectively by fuel controls 23 and 24 which are in turn controlled by a speed and load control 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 26 from a compressor 27 and 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 should be noted that a combustor structured according to the preferred embodiment of the invention is not limited to the catalytic structure described above. Other combustors include catalytic combustors having no primary combustion zone for preheating the gas flow and non-catalytic combustors. A non-catalytic combustor (not shown) comprises nozzle means injecting fuel into a fuel preparation zone for fuelair mixing. Combustion of the fuel-air 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 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 enclosure or basket 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 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 comprises one or more nozzle means 42 for injecting fuel into the fuel preparation zone, a preliminary mixing area 44, and a static mixer 46. Initial fuel-air mixing occurs in the area 44 when fuel is sprayed into a flow of compressor discharge air. Complete mixing of the fuel-air mixture to obtain a uniform concentration of the fuel throughout the mixture occurs as the mixture flows through the static mixing structure 46. The static mixing structure 46 is arranged to provide efficient fuel mixing while minimizing loss of pressure across the mixing structure 46.

The structure of the static mixing structure 46 is utilized to provide to the designer a means for controlling the flow velocity of the gaseous mixture, supplementing the control inherent in the choice of the cross-sectional dimension of the combustor. The significance of this control element is demonstrated by the following equation for flow velocity: V = rii/dKA where: V = flow velocity of the combustible gases in the fuel preparation zone m = mass flow rate of combustible gases d = density of the combustible gases in the fuel preparation zone K = void fraction of the fuel preparation zone A = cross-sectional area of the fuel preparation zone The void fraction (K) equals the ratio of unobstructed cross-sectional area to total crosssectional area of the fuel preparation zone.Thus, the void fraction for an unobstructed fuel preparation zone equals 1.0. As can be seen from the above equation, the flow velocity of the gaseous mixture may be increased by decreasing the void fraction of the fuel preparation zone. The static mixing structure 46 provides a convenient means for decreasing the void fraction of the fuel preparation zone and thereby increasing the flow velocity of the gaseous mixture, without decreasing the cross-sectional dimensions of the fuel preparation zone. Use of the static mixing structure 46 in this way permits a high fuel concentration (fuel-rich) fuel preparation zone without constraining the combustor dimensions available to the designer.

Figure 4 shows a cross-section of the static mixing structure 46 disclosed in Figure 3. The internal arrangement of the static mixing structure 46 comprises a plurality of layers of corrugated material, such as metal alloy, arranged to define a plurality of passageways 50. The layers of corrugated material may be arranged in serveral continuous sections (not shown), so that when the sections are disposed end-to-end to form a plurality of continuous passageways through the several sections, the passageways of any two adjacent sections form angles of 900 or more with respect to one another. The thickness of the corrugated material may be chosen according to the above equation to provide the desired flow velocity.

Figure 5 shows an elevation of an alternative arrangement for the static mixing structure 46 of Figure 3. The structure disclosed is essentially a flat bar twisted 3600 to create a spiral defining dual passageways. This structure alone may be utilized or, alternatively, it may be coupled with a second 3600 spiral in the reverse direction to increase the degree of mixedness within the static mixing structure 46. In both cases, the thickness of the metal bar is chosen to provide an appropriate void fraction as set forth in the equation above.

Hence, a premixing, prevaporizing combustor may be structured according to the embodiments of the invention to minimize the chance of flashback and thereby enable the combustor to operate in a fuel-rich configuration. By appropriate design of the static mixing structure, flow velocity of the fuel-air mixture through the combustion zone is increased, decreasing the risk of flashback without altering the cross-sectional dimensions of the fuel preparation zone.

Claims

1. A combustor for a combustion turbine system, comprising an enclosure for containing combustion reaction, said enclosure being generally cylindrical and having apertures therethrough toward an upstream end to permit influx of compressor discharge gases which flow into said enclosure and exit 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 located downstream of said fuel injection means, where the fuel is mixed and vaporized to obtain a homogeneous gaseous fuel mixture; a combustion zone located downstream of said fuel preparation zone, said combustion zone initiating and supporting combustion of the fuel mixture, the temperature of gases flowing into the transition duct being thereby increased, and means within said fuel preparation zone for increasing the flow velocity of the fuel mixture through the combustion zone.

2. A combustor according to claim 1 wherein said means for increasing flow velocity comprises a static mixing structure, said structure having a plurality of angled passageways arranged to induce flow characteristics in the fuel mixture flowing therethrough to mix and vaporize the fuel without substantial loss of pressure, said structure having a cross-sectional area which substantially decreases the unobstructed cross-sectional area of said combustor so as to increase the flow velocity in said combustion zone downstream to a predetermined flow velocity.

3. A combustor according to claim 1 or 2 wherein said combustion zone has a flameholder downstream of the static mixing structure.

4. A combustor according to claim 1 or 2 wherein said combustion zone has a catalytic combustion element downstream of the static mixing structure.

5. A combustor according to claim 2, 3 or 4 wherein the static mixing structure comprises a plurality of horizontal sections, each of said sections comprising a plurality of corrugated vertically stacked layers, the stacked layers having a thickness chosen to produce the desired flow acceleration and adjacent layers being offset to define horizontal passageways between the layers, the sections aligned end-to-end to form the continuous angled passageways through the combination of sections, the passageways of each section oriented at an angle with respect to adjacent sections, so that the fuel and gases entering the static mixing structure flow through the angled passageways and are thereby mixed and accelerated to produce the flow of homogeneous fuel mixture.

6. A combustor according to claims 2, 3 or 4 wherein the static mixing structure comprises a spiral structure formed of a metallic bar, having a thickness chosen to produce the desired flow acceleration, with one end of the bar rotated 3600 relative to the other end to define dual passageways.

7. A combustor according to claim 6 wherein the static mixing structure comprises a plurality of spiral structures arranged end-to-end to define dual continuous passageways, adjacent spiral structures spiralling in opposing directions.