Combustor and method of combusting fuel

Abstract: A gas turbine combustor is provided which reduces nitric oxide emissions through premixing (10,11) of the fuel gas (12) and air (18) and feeding the mixture through a venturi (24), providing an air cooled passage (36) around the venturi, and extending (44) the passage downstream toward the combustion zone to optimize the stability of the combustion and reduce NO x and CO emissions.

Description: The present invention relates to combustor and method of combusting fuel. The combustor may be used in for example a gas turbine. In recent years, gas turbine manufacturers have become increasingly concerned with pollutant emissions. Of particular concern has been the emissions of nitrogen oxides (NO x ) because such oxides are a precursor to air pollution. It is known that NO x formation increases with increasing flame temperature and with increasing residence time. It is therefore theoretically possible to reduce NO x emissions by reducing flame temperature and/or the time at which the reacting gases remain at the peak temperatures. In practice, however, this is difficult to achieve because of the turbulent diffusion flame characteristics of present day gas turbine combustors. In such combustors, the combustion takes place in a thin layer surrounding either the evaporating liquid fuel droplets or the dispensing gaseous fuel jets at a fuel/air equivalence ratio near unity regardless of the overall reaction zone equivalence ratio. Since this is the condition which results in the highest flame temperature, relatively large amounts of NO x are produced. It is also known that the injection of significant amounts of water or steam can reduce NO x production so that the conventional combustors can meet the low NO x emission requirements. However, such injection also has many disadvantages including an increase in system complexity, an increase in operating costs due to the necessity for water treatment, and the degrading of other performance parameters. The problem of realizing low NO x emissions becomes even further complicated when it is necessary to meet other combustion design criteria. Among such criteria are those of good ignition qualities, good crossfiring capability, stability over the entire load range, low traverse number or flat exhaust temperature profile, long life and the ability to operate safely. Some of the factors which result in the formation of nitrogen oxides from fuel nitrogen and air nitrogen are known and efforts have been made to adapt various combustor operations in light of these factors. See, for example, United States Patent Numbers 3,958,413; 3,958,416; 3,946,553; and 4,420,929. The processes used heretofore, however, have either not been adaptable for use in a combustor for a stationary gas turbine or adequate for the reasons set forth below. US-A-3,905,192 discloses a gas turbine engine having an annular burner with a plurality of staged premixing tubes extending from the forward end thereof. Each tube directs flow to the burner through two concentric flow passages. A movable tube section is arranged to direct all the air through both flow passages or just through one passage. Fuel is directed into the stage premixing tube for mixing with air flowing therethrough. Cooling is provided around the primary zone of the burner so as to provide a minimum of cooling flow into the primary zone. A venturi configuration can be used to stabilize the combustion flame. In such arrangements, lowered NO x emissions are achieved by lowering peak flame temperatures through the burning of a lean, uniform mixture of fuel and air. Uniformity is achieved by premixing fuel and air in the combustor upstream of the venturi and then firing the mixture downstream of the venturi sharp-edged throat. The venturi configuration, by virtue of accelerating the flow preceding the throat, is intended to keep the flame from flashing back into the premixing region. Further, the nature of the flow adjacent the downstream wall of the venturi is a zone of separated flow and is believed to serve as a flame holding region. This flame holding region is required for continuous, stable, premixed fuel burning. Because the venturi walls bound a combustion flame, they must be cooled. This is accomplished with back side impingement air which then dumps into the combustion zone at the downstream end of the venturi. However, such arrangements have not been entirely satisfactory. United States Patent number 4,292,801 of Wilkes and Hilt, and which is hereby incorporated by reference, describes a gas turbine combustor which has an upstream combustion chamber and a downstream combustion chamber separated by a venturi throat or constriction region. US-A-4,413,477 of Deanard White discloses a development of US-A-4,292,801 in which a plurality of convolutions are formed in the downstream periphery of an insert forming the venturi throat. The convolutions form air-flow passages which aid the cooling of the downstream periphery of the insert. The passages extend only to the periphery of the insert. Other patent applications directed at reducing the NO x emissions include European Patent Applications EP-A-273126 (15DV-2910) and EP-A-269824 (51DV-2903), which are hereby incorporated by reference. EP-A-269824 (51DV-2903) is directed at premixed fuel and air combustor arrangements including a venturi. Premixed fuel combustion by its nature is very unstable. The unstable condition can lead to a situation in which the flame cannot be maintained, which is referred to as "blow-out". This is especially true as the fuel-air stoichiometry is decreased to just above the lean flammability limit, a condition that is required to achieve low levels of NO x emissions. The problem to be solved with the premixed dry low NO x combustor is to lean out the fuel-air mixture to reduce NO x while maintaining a stable flame at the desire operating temperature. Further, it is desirable to have stable premixed burnilng over a wide range in combustion temperature to allow for greater flexibility in operation of the gas turbine, and to increase the product life of turbine combustion systems. According to one aspect of the present invention, there is provided a combustor comprising: a premixing chamber for mixing fuel gas and air; a combustion chamber positioned downstream of said premixing chamber for the combustion of the premixed fuel gas and air and including a separated zone and a combustion zone downstream from said separated zone in use; a venturi positioned between said premixing chamber and said combustion chamber through which said premixed fuel gas and air pass to said combustion chamber; and a passageway for cooling gas flow extending axially along at least a portion of the downstream surface of said venturi in the region of said combustion chamber; said passageway positioned on the side of said venturi opposite that which said premixed fuel gas and air passes to said combustion chamber; and said passageway extending downstream in said combustion chamber beyond the mid-region of said separated zone; whereby said combustor may be effectively fired over a larger temperature range to reduce NO x emissions of said combustor. According to another aspect of the invention, there is provided a method of providing fuel to a gas turbine combustor including a separated zone and a combustion zone with low nitric oxide and carbon monoxide emissions comprising: mixing fuel gas and air in a premixer; passing the mixture of fuel gas and air after mixing through a venturi constriction within said gas turbine combustor to accelerate its flow; cooling at least the wall of said venturi in region of said combustion zone with a cooling gas; passing said cooling gas through a passageway which is adjacent the wall of said venturi and extends beyond the mid region of said separated zone; and igniting said mixture to burn within the combustion zone of said combustor. For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings, in which: FIG. 1 is a simplified representation of a cross section of an illustrative gas turbine combustion system incorporating the present invention; FIG. 2 is a plot of the improved operating characteristics realized through use of the present combustion system; FIG. 3 is a partial cross section, shown in reduced size, of a portion.of FIG 1 incorporating an alternative embodiment of the present invention. Referring first to FIG. 1, 10 and 11 are sections of an annular premixing chamber or individual chambers in which fuel gas and air are premixed. The fuel gas 12, which may, for example, be natural gas or other hydrocarbon vapor, is provided through fuel flow controller 14 to one or more fuel nozzles such as 16 and 17 in premixing chambers 10 and 11, respectively. In accordance with the above referenced United States Patents and European patent applications, there may be a plurality of premixing chambers arranged circumferentially around the upstream end of combustor. While 2 chambers 10 and 11 are shown in FIG. 1, there can be any suitable number of combustion chambers. A single axisymmetric fuel nozzle such as 16 and 17 may be used for each premix chamber. Air is introduced through one or more entry ports such as 18. The air is provided to ports 18 from the gas turbine compressor (not shown) under an elevated pressure of five to fifteen atmospheres. The premixed fuel and air is provided to the interior of the combustion chamber 22 through venturi 24 formed by angular walls 32 meeting at the constriction or constricted throat 30. The combustion chamber 22 is generally cylindrical in shape about combustor centerline 26 and enclosed by outer walls 28 and 29. The venturi 24 causes the fuel-air mixture moving downstream in the direction of arrows 31 and 33 to accelerate as it flows through the constricted throat 30 to the combustion chamber 22. Because the venturi wall, 32 is adjacent the combustion chamber 22, it is necessary to cool the wall with back side impingement air-flowing along and through passageway or channel 36 bounded by the venturi walls 32 and generally parallel walls 33. The cooling air 23 may be provided from the turbine compressor (not shown) through the wall 33, at inlet 25, or alternatively through louvers in the wall as described in the aforesaid United States Patent Number 4,292,801. The cooling medium may also be, or include, steam or water mixed with the air. Arrangements which have dumped the cooling air from the passageway 36 of the venturi 24 have not proven to be as stable in operation over a wide a temperature range as desired, and/or have not provided the optimum low NO x emissions desired. In studying this during the development of an improved low NO x combustor 20, we have observed with flow visualization techniques on a full scale plexiglass model of the combustor, that the venturi cooling air dumping into the combustion zone 22, proceeds to "reverse" flow into the separated region or zone adjacent the venturi wall in the downstream area 37. The separated zone is characterized by a detachment of the bulk flow from walls 32 with a small amount of air, and burned and unburned fuel recirculating in the area bounded by the bulk flow and walls 32. The bulk flow detachment is caused by the rapid increase in geometric area downstream of the venturi throat 30. The path of the venturi cooling dump flow in a combustor in which the downstream exit 36 is directly connected to the interior of the combustion chamber 22 was found to be the reverse flow shown by dotted flow lines and arrows 42. Subsequent actual "fired" testing of that dry low NO x system has shown that reducing the amount of venturi cooling air entering the separated zone improved the stability of the premixed fuel burning operation. Thus, we have proven that the reverse flow cooling air adversely affects the stability of such venturi combustion systems. Through further experimentation it was determined that the performance of the combustor could be improved greatly and unexpectedly by providing a controlled cooling air flow dump downstream from the venturi wall 32 toward the combustion zone in the interior of the combustor, and furthermore that this could be accomplished with relatively simple hardware. Referring again to FIG. 1, the exit channel 36 is connected through the passageway 44 extending downstream from the exit channel and formed by a cylindrical wall 46 which is concentric with and within combustor wall 28 to form the passageway therebetween. The wall 46, since it is also adjacent to the combustion chamber 22, is provided with some cooling such as back side impingement air, film air, or fins such as 48, to transfer heat away from the wall. The wall 46 may be the combustor shroud wall which is adjacent to the combustion process. The length 49 of the passageway 44 is optimized for each combustor design although it is in general some 8 to 10 times the radial width of the venturi exit channel 36. One embodiment of the invention was on a combustor 20 having an internal diameter of 254mm (10 inches), a distance 47 of 76.2mm (3 inches) axially from the constricted throat 30 of venturi 24 to the downstream exit 49 of the exit channel 36 of the venturi, a throat diameter 30 of 178mm (7 inches), and a 50.8mm (2 inch) axial length 49 of the passageway 44 formed by cylindrical wall 46 and wall 28. On other embodiments, the internal diameter of the combustor 20 was varied from 254mm-356mm (10-14 inches), the distance 47 was varied from 76.2-127mm (3-5 inches), the diameter of the throat 30 was varied from 178-229mm (7-9 inches), and the length of the passageway 44 was varied from 50.8 - 178 mm (2 - 7 inches). With this arrangement, the dump cooling air 52 from the venturi 24 was found to be mostly in the downstream flow in the combustion chamber as shown by the arrows 52 with only a small reverse flow 55. We have found that this provides significant benefits as described in more detail below. However, prior to the actual combustor testing and flow visualization testing on a full scale plexiglass model, it was thought that the venturi cooling air flow through passageway 44 exited to the combustion zone 58 along the wall 28 and did not in its entirety, or substantial entirety, flow upstream against the flow of fuel gas and air into the separated zone 54 as shown by the arrows 42. Contrary to that existing belief, we now believe that the low pressure zone in the separated region or separated zone 54 adjacent to the venturi downstream wall 32 (due to high velocity combustion gases created by the vena contracta of the venturi throat 30) induces the venturi cooling air, which was dumped at the downstream edge of the passageway 36, to flow backwards upstream into the separated zone 54. The present combustor provides a passageway of significant and sufficient length to carry the venturi cooling gas flow further downstream. It is believed that the cooling gas dump should be at least beyond the mid region of the separated zone 54. Subsequent testing on full pressure, fired combustion equipment with varying length passageways led to the discovery that controlling the amount of cooling fluid entering the separated zone 54 significantly improved the stability of a premixed fuel-air combustor. The improved results included a significant increase in the temperature range over which premixed operation is possible. and, in addition, the ability to operate the combustor 20 with lower combustion system dynamic pressures. It is inferred from temperature measurements of a full pressure, fired combustion system without the passageway 44 that the venturi cooling air significantly cools and dilutes the combustion gases recirculating in the separated zone 54 resulting in reducing the flame holding stability of this region. FIG. 2 shows the effects of varying the length 49 of the passageway 44. Referring to FIG. 2, the combustor exhaust temperatures in °C (°F) are plotted on the Y axis and the ratio of the passageway 44 length/width are plotted on the X axis. The stable flame region is above the resultant plot or curve 57 while the cycling or unstable flame region is below the plot. It is to be noted that increasing the length/width ratio lowers the range of temperatures at which the combustor 20 provides a stable flame. FIG. 2 shows how the combustor exhaust temperature varies with changing the length of the venturi air dump 46, made dimensionless using the venturi diameter 30. Below the curve, the combustor begins to operate in a cyclic mode where the premixed combustion is unstable. Below 871°C (1600°F) the premixed fuel gas and air blows out. As an example, if the dimensionless venturi air dump length is 0.25, the dry low No x combustor 20 can be operated stably at an exhaust temperature above 1037 o C (1900°F). Further, if the full load operating temperature is 1149°C (2100 °F), then the combustor can be operated in the premixed firing mode at partial load conditions corresponding to the range in exhaust temperature from 1037°C to 1149°C (1900 to 2100°F). It is to be noted that the stable flame temperature may be lowered from in excess of 1149°C (2100°F) to less than 927°C (1700°F). This ability to maintain stable combustion over a wide range, including lower temperatures, has achieved a desired reduction in the NO x and carbon monoxide (CO) emissions. The benefits of the present combustor due to the improvement in the premixed operating mode of the dry low NO x combustor 20 are: (1) greater flexibility in operating the gas turbine because of a larger temperature range, including lower temperatures, over which 5 the combustor is stable and can be fired in the premixed mode, (2) lowered resultant NO x emissions, (3) lowered CO emissions, (4) increased combustor lifetime and time between inspections due to lower system dynamic pressures, and (5) provision of a means of adjusting the combustor operation such that the emissions can be optimized for a given combustor nominal operating temperature. FIG. 3 shows an alternate embodiment of the present invention. Referring to FIG. 3, the length of the passageway 44 is made adjustable to enable adjustable optimization of the present invention under variable operating conditions. A cylindrical sleeve 60 is slidably mounted closely within the passage to enable adjustment of the effective length of passageway 44. Because of the high temperatures and harsh environment of the interior of combustor 20 most installations may include a non-adjustable wall 46 which is designed for optimum operating characteristics. The adjustment mechanism shown schematically as controls 62 may be of any suitable type for the combustor 20 environment such as a rack and pinion mechanism or simply movement of the sleeve 60 by the control 62 moving within an axial slot 64 in wall 28, with control 62 being threaded fasteners to secure the sleeve in the- desired location by screwing the fasteners tightly into the threaded bores 66 in the sleeve. While the present invention has been described with respect to certain preferred embodiments thereof, it is to be understood that numerous variations in the details of construction, the arrangement and combination of parts, and the type of materials used may be made without departing from the scope of the invention.

Claims: A dry low nitric oxides (NO x) emission combustor comprising: a premixing chamber (10,11) for mixing fuel gas and air; a combustion chamber (22) positioned downstream of said premixing chamber (10,11) for the combustion of the premixed fuel gas and air and including a separated zone and a combustion zone downstream from said separated zone in use: a venturi (24) positioned between said premixing chamber (10,11) and said combustion chamber (22) through which said premixed fuel gas and air pass to said combustion chamber (22); a passageway (36) for cooling gas flow extending axially along at least a portion of the downstream surface (32) of said venturi (24) in the region of said combustion chamber (22); said passageway (36) positioned on the side of said venturi (24) opposite that which said premixed fuel gas and air passes to said combustion chamber (22); and said passageway (36) extending downstream in said combustion chamber (22) beyond the mid region of said separated zone (54); whereby said combustor may be effectively fired over a larger temperature range to reduce the NO x emissions of said combustor. The combustor of claim 1 wherein said venturi (24) includes a constriction to the flow of said fuel gas and air, and said passageway (36) includes an exit downstream from said constriction adjacent the periphery of said combustion chamber (22). The combustor of claim 2 wherein said downstream exit of said passageway (36) if provided by a second passageway (44) along the periphery of said combustion chamber extending said venturi exit further downstream to avoid significant backflow of the cooling fluid into said separated zone. A method of providing fuel to a gas turbine combustor including a separated zone and a combustion zone with low nitric oxide and carbon monoxide emissions comprising: mixing fuel gas and air in a premixer; passing the mixture of fuel gas and air after mixing through a venturi constriction within said gas turbine combustor to accelerate its flow; cooling at least the wall of said venturi in region of said combustion zone with a cooling gas; passing said cooling gas through a passageway which is adjacent the wall of said venturi and extends beyond the mid region of said separated zone; and igniting said mixture to burn within the combustion zone of said combustor. The method of providing fuel to a gas turbine combustor of claim 4 comprising the additional step of adjusting the axial length of said passageway to stabilize the combustion of said mixture at a lowered temperature to minimize the emission of nitrous oxides. The method of providing fuel to a gas turbine combustor of claim 5 wherein air is provided for said cooling gas. The method of providing fuel to the gas turbine combustor of claim 4 wherein the outer wall of said combustor is substantially cylindrical, and said passageway is formed by providing a wall within said combustor and substantially concentric with said outer wall. The method of providing fuel to the gas turbine combustor of claim 7 wherein the length of said passageway is adjusted to be substantially greater than the distance between said walls. The method of providing fuel to a gas turbine combustor of claim 8 wherein the length of said passageway is adjusted to place the downstream exit of said passageway at least to the mid region of the separated zone.

Inventors: BECHTEL WILLIAM THEODORE II, KUWATA MASAYOSHI, WASHAM ROY MARSHALL, BECHTEL, WILLIAM THEODORE, II, KUWATA, MASAYOSHI, WASHAM, ROY MARSHALL
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1991
Main classification: F23R3
IPC codes: F23R3