Topping combustor for an indirect fired gas turbine

Abstract: A gas turbine power plant comprising a gas turbine (1) having a compressor section (2) for compressing air and a turbine section (3) for expanding a hot gas, a solid fuel furnace (5) including a heat exchanger (13) connected so as to receive at least a first portion of said compressed air from said compressor section (2), and a topping combustor (6) connected so as to receive said hot compressed air and adapted to burn liquid or gaseous fuel, wherein said topping combustor has an inner conduit (29) containing a reaction zone with a burner for burning said liquid or gaseous fuel in said hot compressed air which is then supplied to said turbine section (3) and an outer conduit (30) defining an annular passage (31) in flow communication with said compressor section (2) and with said heat exchanger (13), such that said first portion of said compressed air flows through said passage (31), the burner including a plate assembly (37) which has a plurality of fuel nozzles (36) mounted thereon and which is in flow communication with said compressor section (2), such that a second portion of said compressed air produced by said compressor section (2) is conducted through said plate assembly (37) and into said fuel nozzles (36) for low NO x -forming combustion.

Description: The current invention concerns a gas turbine combustor. More specifically, the current invention concerns a topping combustor for a gas turbine fired primarily by indirect heating in a furnace burning a solid fuel. The high efficiency, low capital cost and short lead time of gas turbine based systems make them particularly attractive to electric utilities as a means for producing electrical power. However, traditionally, gas turbine operation has been limited to expensive, sometimes scare, fuels -- chiefly distillate oil and natural gas. As a result of the ready availability and low cost of coal, considerable effort has been expended toward developing a gas turbine system for generating electrical power which can utilize coal as its primary fuel. Unfortunately, burning coal directly in the hot gas which is to flow through the turbine section of the gas turbine presents a variety of problems, such as erosion of the turbine blades due to particulate matter entrained in the gas and corrosion due to various contaminants in the coal. Consequently, interest has focused on indirect heating of the gas to be expanded in the turbine section by flowing the compressed air discharged from the compression section of the gas turbine through a heat exchanger disposed in a coal fired furnace, such as a fluidized bed combustor. However, such heat exchangers are incapable of heating the air entering the turbine to the temperatures required for high efficiency in modern turbines. Consequently, it has been proposed to employ a topping combustor burning a conventional gas turbine fuel (i.e., natural gas or no. 2 distillate oil) to further heat the gas from the heat exchanger. Such a scheme is shown in U.S. Patent No. 4,369,624. Unfortunately, conventional gas turbine combustors are not suitable for use as topping combustors for several reasons. First, they rely on combustion air for cooling. In conventional gas turbines this presents no problem since the combustion air is the compressed air discharged by the compressor section. The temperatures of such air is typically only 370°C (700°F) and, hence, is suitable for cooling purposes. However, topping combustors use air from the coal furnace heat exchanger as combustion air and the temperature of such air must be relatively high to obtain high efficiency, typically 980°C (1800°F). As a result, a conventional gas turbine combustor would be insufficiently cooled to achieve adequate durability when used as a topping combustor. Second, the high temperature of the combustion air supplied to the topping combustor promotes the formation of nitrogen oxides (NO x ), a harmful air pollutant. Hence, the amount of NO x produced by a conventional gas turbine combustor supplied with such high temperature combustion air would be objectionable. Consequently, it is the principal object of the present invention to provide a topping combustor for an indirect fired gas turbine which employs hot compressed air from a heat exchanger as combustion air yet which achieves adequate cooling for good durability and which minimizes the formation of NO x . With this object in view of the present invention resides in a gas turbine power plant, comprising: a gas turbine having a compressor section for compressing air and a turbine section for expanding a hot gas; a solid fuel furnace including a heat exchanger connected to receive at least a first portion of said compressed air from said compressor section thereby producing hot compressed air; and a topping combustor connected so as to receive said hot compressed air and adapted to burn liquid or gaseous fuel, characterized in that said topping combustor has an inner conduit containing a reaction zone for burning said liquid or gaseous fuel for raising the temperature of said hot compressed air before it is supplied to said turbine section and an outer conduit defining an annular passage in flow communication with said compressor section and with said heat exchanger, such that said first portion of said compressed air flows through said passage; and that a burner is disposed in said inner conduit and includes a plate assembly which has a plurality of fuel nozzles mounted thereon, and which is in flow communication with said compressor section, such that a second portion of said compressed air produced by said compressor section is conducted through said plate assembly and into said fuel nozzles. The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, wherein: Figure 1 is a schematic diagram of an indirect fired gas turbine combined cycle power plant employing a topping combustor according to the prior art. Figure 2 is a longitudinal cross-section through the upper half of the a gas turbine showing the topping combustor according to the current invention integrated into a conventional gas turbine. Figure 3 is a detailed view of the portion of the topping combustor shown in Figure 2 enclosed by the circle designated III. Figure 4 is a cross-section through line IV-IV shown in Figure 3. Figure 5 is an isometric view of the fuel nozzle shown in Figure 3. There is shown in Figure 1 an indirect fired gas turbine combined cycle power plant according to the prior art. Ambient air 11 enters the compressor section 2 of a gas turbine 1. The compressed air 16 produced by the compressor section 2 enters a heat exchanger 13 disposed in a furnace 5 burning a solid fuel 10, such as coal. The furnace 5 may be of the atmospheric fluidised bed type. As previously discussed, as a practical matter, the heat exchanger 13 is only capable of heating the air to about 980°C (1800°F). Therefore, the hot compressed air 22 from the heat exchanger 13 is directed to a topping combustor 6 supplied with a conventional fuel 12, such as natural gas or no. 2 distillate oil. The topping combustor 6 raises the temperature of the compressed air to a level consistent with high efficiency in the turbine section 3, typically approximately 1290°C (2350°F). The hot gas 17 produced by the topping combustor 6 is then directed to the turbine section 3 of the gas turbine 1, where it is expanded, thereby producing power to drive an electric generator 4. The gas 18 exhausting from the turbine section 3 is directed to a heat recovery steam generator 7 (HRSG) where much of the latent heat remaining in the exhaust gas 18 is used to convert feedwater 20 to steam 21 via a heat exchanger 14. The steam 21 produced by the HRSG 7 is directed to a steam turbine 8 which expands the steam, thereby producing power to drive a second electric generator 4. The expanded steam 59 is then exhausted to a condenser 9. The gas 58 exhausting from the HRSG 7 is directed to the furnace 5 where it provides oxygen for combustion of the solid fuel 10. The gas 19 exhausting from the furnace 5 is discharged to atmosphere via a stack 15. As shown in Figure 1, a portion of the gas flowing through the HRSG 7 may be exhausted directly to the stack 15, thereby bypassing the furnace. Figure 2 shows the integration of the topping combustor 6 of the current invention into the gas turbine 1. According to the current invention, an extraction pipe 35 formed in the outer shell of the gas turbine 1 directs the air 16 discharging from the compressor to the topping combustor 6. Although only one topping combustor 6 is shown in Figure 2, it should be understood that a plurality of extraction pipes 35 could be located around the outer shell, each directing compressed air to one of several topping combustors 6. As shown in Figure 2, the topping combustor 6 is comprised of inner and outer cylindrical shells 29 and 30 and a burner assembly. The burner assembly is comprised of a plurality of fuel modules 43 mounted in a fuel distributor plate assembly 37 extending transversely into the inner shell. The inner shell 29 serves as a conduit to direct the flow of the hot compressed air 22 from the furnace 5 to the burner assembly, as discussed further below. The inner shell 29 also serves as a conduit to direct the flow of hot gas 17 from the burner assembly to a toroidal conduit 34 which distributes the hot gas around the inlet of the turbine section 3. Accordingly, the inner shell 29 has necked down portions 56 and 57 at its upper and lower ends which form a combustion air inlet and a hot gas outlet, respectively. The inner shell 29 also serves to enclose the reaction zone 36 downstream of the burner assembly in which combustion occurs. According to an important aspect of the current invention, the outer shell 30 encloses the inner shell 29, thereby forming an annular passage 31 between the two shells. Extraction pipe 35 directs the compressed air 16 discharging from the compressor section 2 to the inlet 54 of the annular passage 31. The compressed air 16 flows though the annular passage 31 and thence on to the heat exchanger 13. As a result, although the hot compressed air 22 from the furnace 5 forms the combustion air for the topping combustor 6, the relatively cool (i.e., typically less than 370°C (700°F)) compressed air 16 from the compressor serves to cool the inner shell 29 of the topping combustor. Moreover, as shown in Figure 2, the flow of relatively cool air 16 is counter-current with respect to the flow of the hot air 22 and combustion gas 17, so that the coolest air flows over the hottest portions of the inner shell 29. Although Figure 2 shows the compressed air 16 as only flowing over the outer surface of the inner shell 29, in some cases, it may be desirable to transport a portion of the air through the inner shell to provide it with film or transpiration cooling. As shown in Figure 2, a portion 25 of the compressed air 16 is withdrawn from the annular passage 31 and directed by pipe 64 to a boost compressor 24 and air-to-air coolers 60. The air-to-air coolers are disposed upstream and downstream of the boost compressor 24 and transfer heat from the compressed air 25 to ambient air 23. As a result, in the preferred embodiment, the compressed air 25 is cooled to approximately 150°C (300°F) and its pressure is increased by approximately 138 kPa (20 psi). The cooled further compressed air 25 is then split into two portions 26 and 27. Portion 26 is directed to the fuel distributor plate assembly 37 and portion 27 is directed to a fuel supply pipe jacket 33. As shown in Figure 2, a plurality of fuel injection modules 43 are formed in the plate assembly 37. As shown in Figure 3, the plate assembly 37 is comprised of upper and lower substantially planar circular plates 39 and 40, respectively. The diameter of the plates 39 and 40 is only slightly less than the inner diameter of the inner shell 29. Thus, substantially all of the hot air 22 entering the combustor 6 flows through the plate assembly 37. Moreover, the plates 39 and 40 are parallel to each other and spaced apart to form a cooling air passage 41 between them which extends transversely through the inner shell 29. Portion 26 of the further compressed cooled air flows through the cooling air passage 41, thereby cooling the plate assembly 37. As shown in Figures 3 and 4, each fuel module 43 is comprised of a fuel nozzle 50 and seven combustion air passages 53. As shown in Figures 3, 4 and 5, each fuel nozzle 50 is comprised of a fuel supply pipe stub 61, which receives fuel 12 from the fuel supply pipe 32, and a nozzle body 52. The nozzle body 52 surrounds the distal end of the pipe stub 61 and forms an annular passage 48 therebetween. As shown in Figure 3, in the preferred embodiment, seven radially oriented air inlet ports 49 are formed in each fuel nozzle body 52. The air inlet ports 49 serve to direct the cooled air 26 flowing through the plate assembly 37 into the annular passage 48 in each fuel nozzle 50. The cooling air 26 entering the annular passage 48 is divided into two portions 46 and 47. Portion 46 is used to cool the plate 39, as explained further below, while portion 47 is mixed directly with the fuel 12. As shown in Figures 3 and 4, seven radially oriented orifices 42 are formed in the distal end of each pipe stub 61. The orifices 42 serve to mix the fuel 12 flowing into the pipe stub 61 into the portion 47 of the cooled air which entered the annular passage 48. As a result, a rich fuel/air mixture 44 is formed which exits the annular passage 48 through seven radially oriented discharge ports 51 distributed around the fuel nozzle body 52 and aligned with the orifices 42. As shown best in Figure 3, the combustion air passages 53 are formed in the plate assembly 37 by sleeves 45 mounted therein. As shown in Figure 4, the seven air passages 53 are uniformly spaced around a circle concentric with the fuel nozzle 50. Moreover, the air passages 53 are radially aligned with the discharge ports 51 in the fuel nozzle body 52. The air passages 53 serve to distribute the combustion air 22 around each of the fuel nozzles 50 and to form axial jets of hot air 22 which interact with the radial jets of fuel/air mixture 44 formed by the discharge ports 51. As a result, there is vigorous mixing of the combustion air 22 with the rich fuel/air mixture 44 in the reaction zone 36 downstream of the plate assembly 37. This mixing results in an ultra-lean fuel/air ratio in the reaction zone 36. As is well known in the art, ultra-lean combustion minimizes the formation of NO x . In the current invention, combustion occurs at an overall equivalence ratio of approximately 0.15. (The equivalence ratio is a relative value used to characterize the rich/lean nature of a combustion process. Stoichiometric combustion is defined as an equivalence ratio of 1.0, while conventional gas turbine combustors typically operate at an overall equivalence ratio of approximately 0.37.) Such ultra-lean overall combustion is achieved by the dispersal of a large number of fuel modules 43 around the plate assembly 37 with the local fuel air ratio in the vicinity of each fuel module just high enough to sustain combustion. According to the current invention, flame stability at such lean fuel/air ratios is made possible by two factors. The first and most important factor is the high temperature of the combustion air 22 flowing through the plate assembly 37. In the preferred embodiment, the heat exchanger 13 is designed to heat the combustion air 22 to approximately 980°C (1800°F). The second factor allowing good flame stability is the low velocity of the combustion gas 17 in the reaction zone 36 downstream of the plate assembly 37. The diameter of the inner shell 29 is sized so that its flow area -- that is, its area in a plane perpendicular to the direction of the flow through the inner shell -- in the reaction zone 36 results in the average reference velocity (the average reference velocity is defined as the velocity through the combustor calculated based on continuity considerations, using the temperature, pressure, and density of the fuel/air mixture at burner inlet conditions) of the gas 17 flowing therein being relatively high (i.e., approximately 30 m/s (100 fps)). However, the plate assembly 37 blocks a substantial portion of this flow area by confining the flow of the combustion air 22 in the vicinity of the plate assembly to the air passages 53. In the preferred embodiment, the size and quantity of the air passages 53 are such that the portion of flow area blocked by the plate assembly 37 is in the range of approximately 65 to 70%. This flow blockage results in a non-uniform velocity distribution in the reaction zone 36 and the formation of eddy currents downstream of the plate assembly 37. These eddy currents form low velocity zones (i.e., approximately 3 m/s (10 fps)) -- sometimes collectively referred to as a "sheltered zone" -- which prevent the flame in the reaction zone 36 from being blown out. As shown in Figure 3, the portion 46 of the cooled air 26 which enters the fuel nozzle through the air inlet port 49, but which does not mix with the fuel 12, flows out of the top of the annular passage 48 and is directed by a baffle 38, attached to the fuel pipe stub 61, to flow over the surface of the upper plate 39 of the plate assembly, thereby providing film cooling for the plate assembly. As shown in Figure 2, a second portion 27 of the cooled further compressed air 25 is directed to an annular passage formed by a jacket 33 which encloses the portion of the fuel supply pipe 32 which extends through the inner shell 29, thereby cooling the pipe. As shown in Figure 3, the fuel pipe jacket 33 forms an outlet 62 just upstream of the plate assembly 37 at each fuel module 43 location. After cooling the fuel pipe 32, the portion 27 of the cooling air discharges from the outlet 62 and flows over the upper plate 39 of the plate assembly 37, thereby providing additional cooling.

Claims: A gas turbine power plant, comprising: a gas turbine (1) having a compressor section (2) for compressing air and a turbine section (3) for expanding a hot gas; a solid fuel furnace (5) including a heat exchanger (13) connected to receive at least a first portion of said compressed air from said compressor section (2) thereby producing hot compressed air; and a topping combustor (6) connected so as to receive said hot compressed air and adapted to burn liquid or gaseous fuel, characterized in that said topping combustor has an inner conduit (29) containing a reaction zone for burning said liquid or gaseous fuel for raising the temperature of said hot compressed air before it is supplied to said turbine section (3) and an outer conduit (30) defining an annular passage (31) in flow communication with said compressor section (2) and with said heat exchanger (13), such that said first portion of said compressed air flows through said passage (31); and that a burner is disposed in said inner conduit and includes a plate assembly (37) which has a plurality of fuel nozzles (36) mounted thereon, and which is in flow communication with said compressor section (2), such that a second portion of said compressed air produced by said compressor section (2) is conducted through said plate assembly (37) and into said fuel nozzles (36). A gas turbine power plant according to claim 1, characterized in that said furnace (5) is adapted to heat at least a portion of said compressed air (16) from said compressor section (2) by burning a second solid fuel (10), thereby producing hot compressed air (22), said furnace (5) being in flow communication with said inner conduit (29) so that said hot compressed air (22) flows through said inner conduit and said annular passage (31) being in flow communication with said heat exchanger (3) in such a manner that the compressed air (16) produced by said compressor section (2) flows through said annular passage (31) in a direction counter-current to the direction of flow of said hot air (22) through said inner conduit (29). A gas turbine power plant according to claim 1 or 2, characterized by a boost compressor (24) for further pressurizing said second portion (25) of said compressed air (16); and a conduit (64) for directing said second portion (25) of said compressed air (16) from said annular passage (31) through said boost compressor (24) to said plate assembly (37). A gas turbine power plant according to claim 3, characterized by a cooler 60 for cooling said second portion (25) of said compressed air (16). A gas turbine power plant according to any of claims 1 to 4, characterized in that said plate assembly (37) extends across said inner conduit (29) and has cooling air passages formed therein, said cooling air passages being in communication with said fuel nozzles (36) for supplying said cooling air to said fuel nozzles (36).

Inventors: PILLSBURY PAUL WALTER, PILLSBURY, PAUL WALTER
Applicants: WESTINGHOUSE ELECTRIC CORP, WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1992
Main classification: F02C3
IPC codes: F02C6, F23D23, F02C3, F02C7, F02C9, F23R3