Combustor and operating method for gas- or liquid-fuelled turbine

Abstract: In a combustor for a gas turbine, combustor (1) utilises at least 50% of the air supplied thereto by a compressor to mix with the fuel to form a lean mixture, the remainder of the air is utilised for impingement cooling and the spent impingement cooling air is injected as radial jets into a post-primary combustion zone (17) through perforations (6) in a wall (2).

Description: This invention relates to a combustor for a gas- or liquid-fuelled turbine and a method of operating such a turbine. A gas- or liquid-fuelled turbine plant typically includes an air compressor, a combustor and a turbine. The compressor supplies air under pressure to the combustor and a proportion of this air is mixed with fuel in a mixing zone, the mixture being burnt in a primary combustion zone to produce combustion gases to drive the turbine; a further proportion of the air supplied by the compressor is usually utilised to cool the hot surfaces of the combustor. Such a combustor is disclosed in FR-A-2531748. The proportion of air mixed with the fuel determines the temperature range over which the combustion occurs and will affect the quantity of pollutants, specifically NOx and CO, produced by that combustion. Thus a fuel-rich mixture (i.e. with a comparatively low proportion of air) will burn at comparatively higher temperatures and lead to increased production of NOx and CO. The higher temperatures are detrimental to component life and therefore a large amount of coolant air is required to reduce the temperature downstream of the primary combustion zone. Mixing more air with the fuel produces a lean mix which burns at a lower temperature and with the production of less pollutants although less coolant air is then available to achieve the cooling necessary for reasonable component life. Hence a lean mix burn carries with it the implication that the limited amount of cooling air which is in consequence available must be utilised in an effective manner. According to a first aspect the invention provides a combustor (1) for a gas- or liquid-fuelled turbine having a compressor to supply air to the combustor for combustion and cooling, the combustor comprising a mixing zone (15) in which fuel is mixed with a first proportion of the air supplied to the combustor (1), a primary combustion zone (16) downstream of the mixing zone (15), and a post-primary combustion zone (17) further comprising the features of claim 1. It is preferred that the air directed into post-primary combustion zone (17) flows radially thereinto relative to the longitudinal axis (100) of the combustor (1). The apertures (6)may be formed with respective tapered lips (36). In a preferred arrangement the arrangement is such that air entering the post-primary combustion zone (17) is at a temperature of at least 700°C, and depending on the circumstances the temperature is preferably at least 800° C. It is preferred that the arrangement is such that the jets (22) of spent-impingement cooling air entering the post-primary combustion zone mix with the combustion gases therein to produce a substantially uniform radial temperature distribution in said post-primary combustion zone. According to a further aspect the invention provides a method of operating a gas- or liquid-fuelled turbine wherein compressed air is supplied to a combustor (1) for combustion and cooling, a first proportion of the air supplied to the combustor (1) is mixed with fuel in a mixing zone (15) of the combustor (1), a second proportion of the air supplied to the combustor acts to cool a primary combustion zone (16) of the combustor by impingement cooling, all the spent impingement cooling air thereafter being directed into a post-primary combustion zone (17) of the combustor (1) downstream of the primary combustion zone (16), the method being characterised by the fact that the spent impingement cooling air enters the post-primary combustion zone (17), as jets directed transverse to the flow of combustion gases, the first proportion constituting at least 50% of the air supplied to the combustor (1). The invention will be described by way of example with reference to the accompanying drawing the single figure of which shows an axial section of a combustor of a gas turbine plant. In general the combustor is of a size and configuration determined by the overall design and power requirements of the turbine. There will generally be a plurality of combustors distributed around the turbine axis. As shown and in particular described, the combustor 1 is of generally circular cylindrical or 'can' configuration with the longitudinal axis of the cylinder designated 100. The combustor is one of perhaps four or more mounted in enclosures opening into the turbine casing and distributed uniformly around it. The compressor is driven by a compressor turbine which is exposed to the interior of the combustors and is driven by the combustion gases. The compressor turbine is shaft coupled to the compressor stages which supply compressed air to the exterior of the combustor for combustion and cooling. More particularly each combustor 1 comprises concentric inner and outer cylindrical walls 2, 3. The walls 2, 3 are spaced apart to form an annular space or passage 30 therebetween. The wall 2 is generally imperforate apart from a plurality of holes or perforations 6 which as shown form an annular array, each hole being formed with a tapered lip 36 to assist in the formation of cooling air jets as will be described subsequently, and also to stiffen wall 2 of the combustor. The outer wall 3 has a large number of perforations 7, 27 distributed over its surface e.g. in a series of annular arrays or in a helical arrangement. These perforations provide cooling of the inner wall 2 by permitting fine jets of compressed air from the surrounding region to impinge upon the inner wall 2. As shown, perforations 7 are positioned upstream of dilution apertures 6 (as will be explained) and perforations 27 are positioned downstream of aperture 6. Adjacent the left hand (i.e. upstream) ends of the walls 2, 3 and affixed thereto by a conical duct 8 is a fuel injector assembly 11 with an associated air swirler 12 having a multiplicity of ducts 10 which give the entrained air both radial and circumferential velocity components, the flow of air being broadly as indicated by arrows 13. The region 15 is a mixing zone wherein the air entering through the ducts 10 mixes with fuel injected axially by the fuel injector arrangement. The fuel jets themselves are not illustrated specifically but are commonly mounted in a ring on the back plate. Immediately downstream of the mixing zone is a pre-primary combustion zone 25. Boundaries between the zones are not clear cut and are indicated by wavy lines. As mentioned previously, the combustor is completely enclosed in a compressed air enclosure so that air enters the combustor through any available aperture, having a combustion or cooling function according to the aperture. In a typical prior art impingement cooled combustor approximately 20% of total air supplied to the combustor might be entrained through the swirler and the remainder utilised for cooling. However, in the present arrangement a substantially higher proportion of the available air is used for forming the fuel-air mixture so that a very lean fuel-air mixture is formed in zone 15. It is envisaged that at least 50% of the air provided to the combustor is utilised for mixing directly with fuel from the fuel injector 11; a figure of 57% has been found to give highly beneficial results in certain circumstances. With the fuel/air mixture comprising such a high proportion of the available air supply combustion takes place at a lower temperature than in a conventional combustor and this acts to reduce pollution i.e. leads to reduction in the quantities of CO and NOx produced. Obviously with such high proportions of air being used for the initial combustion mixture, a lower proportion of air is available for cooling of the combustor. However, since combustion is taking place at a lower temperature this is partly self-balancing, and, moreover, the combustor involves a particularly effective cooling arrangement to make use of the cooling air available as is described below; in addition the cooling air is utilised to 'burn out' CO in the combustion gases as will be explained. The interior of the combustor 1 downstream from the pre-primary combustion zone 15 comprises in sequence a primary combustion zone 16 extending from the zone 15 to a post-primary combustion zone 17. Beyond the zone 17 is a transition zone 18 in which negligible combustion takes place, leading to the combustor outlet 19, which itself communicates with the inlet to the turbine driven by the combustion gases produced in the combustor 1. As indicated above it is arranged that at least 50% of the air supplied by the compressor is directly mixed with the fuel in the mixing zones 15 of the various combustors. The remainder of that air flows around the combustor 1. This air has a particular flow arrangement as will now be described. In flowing around and along the outer wall 3 the air passes through the perforations 7, 27 as indicated by arrows 20, and impinges on the inner wall 2. This air thereby effects impingement cooling of the combustor 1, more specifically of the inner wall 2 where it surrounds the primary combustion zone 16 and the post-primary combustion zone 17. The air having entered the annular space 30 between walls 2, 3, flows along as indicated by arrows 21, 31 until it reaches the larger holes 6 in the inner wall 2. As arrows 22 show, the air, i.e.. the spent impingement cooling, air enters zone 17 with considerable force and at high velocity in a series of jets in substantially radial directions relative to the axis 100 i.e. transverse to the flow of combustion gases flowing from zone 16, and in zone 17 this air mixes with these combustion gases. The intermixing of this air with the combustion products flowing to zone 17 from zone 16 in these circumstances tends to produce substantially uniform radial temperature distribution and also ensures a sufficient residence time in zone 17 and to a lesser extent, in transition zone 18 to allow reduction, i.e. burning out of the CO pollutant produced in the combustion process. It is necessary to ensure that the temperature of the spent impingement coolant where it discharges into zone 17 is sufficient to ensure that quenching (i.e. excessive cooling) of the combustion product does not occur otherwise the CO will not be further burnt out. It has been found that this temperature should not be less than 700°C and ideally should be at least 800°C. To ensure that the spent impingement cooling air enters the zone 17 with sufficient force/velocity and at the appropriate temperature requires careful design of the walls, 2, 3 and perforations 6, 7, 27. Thus to achieve the desired results i.e. combustion controlled to produce low quantities of pollutants, effective cooling of the combustor, and uniform radial temperature distribution of the combustion products downstream of the primary combustion zone the number, size and positions of the perforations 7 in the outer wall 3 and the entry holes 6 in the inner wall 2 are chosen to suit the particular environment in which the combustor is to operate and to ensure necessary volume and velocity of air entering through perforations 6. The exclusively impingement cooling here described should be contrasted with the more normal cooling arrangement where spent coolant is ejected substantially axially along the interior of the wall 2 of the combustion zone. The walls 23 defining the transition zone 18 may incorporate a further cooling arrangement if required. The wall is shown as a single wall for convenience but could be double walled or some other arrangement. Film or impingement cooling could then be employed.

Claims: A combustor (1) for a gas- or liquid fuelled turbine having a compressor to supply air to the combustor (1) for combustion and cooling, the combustor (1) comprising a mixing zone (15) in which fuel is mixed with a first proportion of the air supplied to the combustor (1), a primary combustion zone (16) downstream of the mixing zone (15), a post-primary zone (17) downstream of the primary combustion zone (16), and a wall (2) of the combustor which wall is cooled by impingement cooling and contains both the primary zone (16) and the post-primary combustion zone (17), the combustor (1) being characterised in that the wall (2) within the post-primary combustion zone (17) is provided with a plurality of apertures (6) such that spent impingement cooling air constitutes a plurality of cooling jets (22) injected solely into the post-primary combustion zone (17) transverse to the flow of combustion gases, said first proportion of air constituting at least 50% of the air supplied to the combustor (1). A combustor as claimed in Claim 1, wherein the air directed into the post-primary zone (17) flows radially thereinto relative to the longitudinal axis (100) of the combustor (1). A combustor as claimed in Claim 1 or Claim 2, wherein the apertures (6) are formed with respective tapered lips (36). A combustor as claimed in any one of Claims 1-3, wherein the arrangement is such that air entering the post-primary combustion zone (17) is at a temperature of at least 700°C. A combustor as claimed in Claim 4, wherein the temperature is at least 800 °C. A combustor as claimed in any one of Claims 1-5, the arrangement being such that the jets (22) of spent impingement cooling air entering the post-primary combustion zone mix with the combustion gases therein to produce a substantially uniform radial temperature distribution in said post-primary combustion zone (17). A method of operating a gas- or liquid-fuelled turbine wherein compressed air is supplied to a combustor (1) for combustion and cooling, a first proportion of the air supplied to the combustor (1) is mixed with fuel in a mixing zone (15) of the combustor, a second proportion of the air supplied to the combustor acts to cool a wall (2) of a primary combustion zone (16) of the combustor (1) by impingement cooling, the spent impingement cooling air thereafter being directed into a post-primary combustion zone (17) of the combustor (1) downstream of the primary combustion zone (16), the method being characterised by the fact that all the spent impingement cooling air enters the post-primary combustion zone as jets (22) directed transverse to the flow of combustion gases, the first proportion constituting at least 50% of the air supplied to the combustor (1). A method as claimed in Claim 7, wherein the arrangement is such that air entering the post-primary combustion zone (17) is at a temperature of at least 700 °C. A method as claimed in Claim 8, wherein the temperature is at least 800 °C. A method as claimed in any one of Claims 7-9, the arrangement being such that the jets (22) of spent impingement cooling air entering the post-primary combustion zone mix with the combustion gases therein to produce a substantially uniform radial temperature distribution in said post-primary combustion zone.

Inventors: ALKABIE HISHAM SALMAN, ALKABIE, HISHAM SALMAN
Applicants: EUROP GAS TURBINES LTD, EUROPEAN GAS TURBINES LIMITED
Application year: 1996
Main classification: F23R3
IPC codes: F23R3, F02C7, F23M5