The present invention relates to a combustion system for gas turbine engines and, more particularly, to an improved combustor design and method for using direct air injection and a substantially inert gas feed such as an exhaust gas recycle stream (“EGR”) as the primary working fluid. (The terms “inert gas stream” and “substantially inert gas stream” are used interchangeably herein to refer to an EGR type gas stream having small, residual amounts of oxygen). The method and apparatus according to the invention reduce the formation of undesirable gas emissions from the gas turbine engine while increasing the overall efficiency of the combustor.
Gas turbine engines typically include a compressor section, a combustor section and a turbine that rotates to generate electrical power. The compressor discharges higher pressure gas directly into the combustor where hydrocarbon fuel is injected, mixed and burned to produce thermal and kinetic energy. The combustion gases are then channeled into and through one or more stages of the turbine which extracts rotational energy from the exhaust gases with the rotational energy then being used to generate electrical power. Most existing gas turbine engines include multiple combustors positioned circumferentially around a rotational axis. In order to achieve maximum operating efficiency, the combustors operate over a wide range of different fuel compositions, pressures, temperatures and fuel/air ratios, preferably with the ability to use either liquid or gas fuels. Most gas turbine engines also tend to operate in a slightly “rich” mode, i.e., with less than the precise stoichiometric amount of oxygen in the feed necessary to burn the entire hydrocarbon fuel mixture, resulting in at least some incomplete combustion, and hence the production of carbon monoxide and other by-products.
In most systems, the combustion exhaust invariably contains small amounts of excess CO and oxygen, with some CO2 disassociating into CO and O2, depending on the specific combustor operating conditions. Unless removed, the same components, including the residual amount of oxygen, remain in the system as they pass into and through components downstream of the combustor, e.g., the turbine and heat recovery steam generator. As a result, the final exhaust stream from conventional gas turbine engines includes N2 and CO2 with small amounts of CO, O2 and oxides of nitrogen, all of which eventually must be removed or reacted in downstream operations (such as by catalyst treatment) in order to maintain desired gas turbine efficiencies.
The need for combustors with improved emissions is a direct outgrowth of air pollution concerns resulting from more stringent domestic and international pollution control standards such as those promulgated by the Environmental Protection Agency regulating the release of oxides of nitrogen, unburned hydrocarbons and carbon monoxide. The unwanted emissions generally fall into two broad categories—those resulting from high flame temperatures (e.g., NOx) and those formed at lower flame temperatures, such as CO, that do not allow the basic fuel-air reaction to proceed to completion. Operating at low combustion temperatures in order to lower the NOx emissions can result in incomplete or partially incomplete combustion, which in turn leads to the production of excessive amounts of unburned hydrocarbons and carbon monoxide, as well as lower power output and lower thermal efficiency of the engine. Higher combustion temperatures, on the other hand, tend to improve thermal efficiency and lower the amount of unburned hydrocarbons and CO, but can still result in a higher amount of NOx if the combustion mixture and operating conditions are not properly monitored and controlled.
Two additional practical concerns influence the need to control the amount of unwanted emissions formed in the turbine exhaust. First, the fuel to air ratio is subject to finite control since only a limited ability exists to control the exact amount of fuel being fed to each combustor in a multi-combustor design. Second, individual combustors tend to operate with a slightly different fuel to air ratio, resulting in variations in the emissions on a “can-to-can” basis. As noted, in the past those design issues have contributed to the need to operate gas turbine engines with a slight amount of excess fuel with all (or virtually all) of the oxygen present in the exhaust eventually being consumed and/or eliminated using catalyst treatment. However, it has been difficult to control the exact amount of CO produced during the initial combustion, particularly on an individual combustor basis.
While numerous combustor designs have been proposed over the years to enhance the mixing of fuel and air prior to combustion, the need remains for improvements in combustor architecture in order to reduce the level of undesirable components such as NOx formed when the flame temperatures occasionally become too high or when the combustion is incomplete leaving residual oxygen and CO in the exhaust. Thus, the industry continues to search for pre-combustion systems that provide improved fuel/air mixing upstream of the combustor and better thermal efficiency, but with reduced NOx and unburned hydrocarbon emissions during and after combustion takes place.
The present invention addresses the above concerns by providing a new method for reducing and controlling the CO and oxygen emissions in a gas turbine combustor using a hydrocarbon fuel that is burned with an oxidizer such as air or “oxyfuel.” Significantly, the invention includes the use of a significant volume of a low to near zero oxygen content gas, such as the exhaust gas recycle (“EGR”) from the gas turbine engine as a major part of the working fluid. The use of an inert combustor feed offers particular advantages for applications in which the gas turbine system is designed to achieve stoichiometric exhaust gas recirculation in order to reach target emissions goals. As detailed below, it has been found that the combustion dynamics can be significantly improved by changing the nature and composition of the primary working fluid and the manner in which it is fed to the combustor head end prior to ignition, resulting in improved emissions (less oxygen and CO formed during combustion) without sacrificing engine efficiency.
The invention described below includes a new method for reducing the amount of carbon monoxide and oxygen emissions in a hydrocarbon combustor in a gas turbine engine using the following basic steps: feeding hydrocarbon fuel and an oxidizer component (such as air) directly into the head end of the combustor while injecting defined amounts of a substantially inert gas (nominally an exhaust gas recycle or “EGR”) directly into the combustor head with the fuel and oxidizer; igniting the fuel mixture to form a combustor exhaust stream that includes EGR; cooling the combustor exhaust using a portion of the same EGR circulated around the combustor shell and then exhausted from the system; detecting the amount of carbon monoxide and oxygen present in the combustor exhaust; and adjusting the amount of fuel, oxidizer and inert gas feeds based on the detected amount of carbon monoxide and oxygen.
The invention can be incorporated into a gas turbine engine comprising a gas compressor, one or more combustors as described above, a turbine driven by the expanded exhaust from the combustors, a primary fuel circuit for the combustors that feeds hydrocarbon fuel and air to each combustor, means for feeding the substantially inert gas stream into each combustor, carbon monoxide and oxygen sensors disposed in the exhaust stream, and a control for determining the amount of fuel, air and substantially inert gas stream to be fed to each combustor based on the detected amount of carbon monoxide and oxygen.
The invention thus includes both a method and combustor architecture that uses direct air injection into the head end of the combustor in combination with an inert feed, namely a very low oxygen-content gas recycle stream as the working fluid. In an exemplary embodiment, both the external air and a portion of the gas turbine exhaust (having little or no oxygen present and reduced in temperature) are combined to form the primary working fluid after the combustor reaches a desired level of operation following startup. The lower temperature inert gas stream (comprising mostly nitrogen and CO2) serves to improve the stoichiometry of the combustion while acting as a cooling medium for the combustor itself.
The method as described below in connection with
As noted above, the new combustor design and related method are capable of supporting stoichiometric combustion with minimum resulting O2 and CO emissions. The amount of NOx and CO emissions can also be adjusted by an additional diluent injection (such as nitrogen) directly into the combustor head end. In addition, the combustor nozzle designs described below for use with the head end result in improved mixing of the fuel, air and EGR before the mixture is injected into the combustor. The invention also includes an integrated fuel and air combustion system for a gas turbine engine comprising one or more combustors, a modified fuel supply for providing hydrocarbon fuel to the combustors, a compressed external air supply for the combustor (such as that generated by a main air compressor), means for providing the inert exhaust gas recycle stream to the combustor and a modified fuel and air nozzle positioned in the head end of the combustor for providing the desired mixing of components prior to ignition.
The present invention includes means for injecting air directly into the combustor head end along with a significant volume of an inert gas into the combustor at particular specified locations along the exhaust flow path in order to reduce unwanted emissions, particularly CO and oxygen, while improving overall combustor performance. An exemplary system uses a specified amount of exhaust gas recycle (which is substantially inert and at a lower temperature after passing through the turbine) in an amount up to about 50% of the total combustor feed, with the remaining 50% comprising about 5% hydrocarbon fuel and 45% air. The use of an inert gas as part of the working fluid also serves to cool the combustor and reduce the combustor flame temperature while improving the stoichiometry of the gas combustion. That is, it has been found that the inert gas feed helps to establish an equivalence ratio approaching 1.0 for the fuel and oxygen fed to the combustor. Minimizing the final oxygen content in the combustor exhaust also reduces potential metallurgical damage resulting from the presence of residual oxygen in the system.
In the embodiments of
In operation, an inert gas stream containing very little oxygen (approximately 5% of the total feed) is fed to the combustor during start-up, with the amount and volume of EGR increasing as the combustor reaches steady state operating conditions. The amount of inert gas fed to the head end of the combustor thus gradually increases over time (up to about 50%) based on the detected amount of CO and oxygen in the exhaust stream (hopefully approaching zero as the combustor continues to operate). The invention thereby achieves three benefits over conventional combustor designs: (1) lowers the residual oxygen content in the combustor exhaust to a near zero stoichiometric level; (2) helps control the composition and temperature of the combustor emissions; and (3) assists in lowering and controlling the combustor flame temperature. The low oxygen content of the final exhaust gas also helps to prevent uneven combustion (such as combustion occurring at or near the combustor wall). Further, since the cooling gases being recycled into the head end of the combustor are substantially inert, the risk of forming unwanted CO from CO2 is much lower because less oxygen is present in the system.
Each combustor also normally includes multiple fuel nozzles, each of which has a dedicated premixing section so that fuel is premixed with air prior to ignition in the combustion chamber. In this way, multiple dedicated premixing sections allow for the thorough premixing of fuel and air prior to being burned, a feature that assists in achieving low NOx, CO and O2 levels. Typically, the plurality of diffusion/premix fuel nozzles are arranged in a circular array about the longitudinal axis of a cylindrical combustor casing, with the nozzles mounted within the combustor end cover assembly at the head end. The fuel nozzles extend into a combustor shroud with a central fuel injection port that premixes the combustion air with injected fuel as described below. See
The figures also illustrate the use of a cylindrical flow sleeve integral with and concentrically aligned with the cylindrical combustor shell. The flow sleeve is formed from an interior combustion liner coupled to the end cap assembly. In the exemplary embodiment of
Referring now to the specific figures,
Controlled air feed 1 passes into a main air compressor 2 and exists at elevated pressure from the last stage of the compressor as shown at compressor exit stream into combustor 6. The main air compressor and gas turbine compressor 4 are operatively connected via rotating shaft 3 as shown. Gas turbine compressor 4 operates to drive gas turbine 8 via rotating shaft 9. The higher pressure compressor exit stream 5 leaving main air compressor 2 feeds directly into combustor 6 with the flow rate controlled using control valve 11. A controlled amount of hydrocarbon fuel 29 (such as natural gas) also feeds directly into combustor 6 through control valve 26 where it ignites and produces thermal and kinetic energy.
The spent exhaust gasses 7 exiting from the combustor drive gas turbine 8 and then pass at a lower pressure as shown at gas turbine exhaust line 14 into heat recovery steam generator (“HRSG”) 15.
The exhaust gas recirculation (“EGR”) loop 16, which comprises an inert mixture of nitrogen, CO, CO2 and only residual amounts of oxygen, is recycled as shown back to gas turbine compressor 4 and thereafter fed at higher pressure directly into combustor 6 as shown by gas turbine feed 10. Exhaust stream 20 from HRSG 15 is used to drive steam turbine 19 to generate electrical power via generator 22 in a conventional manner. Typically, a portion of the spent exhaust from HRSG 15 will be removed from the HRSG through HRSG valve 12 and vented to the atmosphere as shown through vent line 18. In order to increase the operating efficiency of the steam turbine, the spent exhaust gasses leaving the last stage of steam turbine 19 are recycled back to the HRSG 15 via HRSG recycle line 21.
In performing the method according to the invention, substantially inert gas stream 16 (typically EGR), which is now reduced in temperature after leaving HRSG 15 and has water removed as shown at water discharge line 17, becomes part of the main feed into combustor 6 (see
Once the combustor approaches a steady state condition after the initial air and fuel feeds, the inert exhaust gas recycle stream can gradually be increased by injection into the combustor core using cylindrical flow sleeve 37 which is concentrically aligned with the combustor casing. Flow sleeve 37 includes an interior combustion liner 38 coupled to the end cap assembly which, together with the combustor exterior shell, form an EGR flow passage. The interior wall of flow sleeve 37 includes a plurality of flow apertures 36 which enable the compressed EGR gas from the gas turbine compressor (item 4 in
Thus, at one point during combustion, the substantially inert gas (EGR) performs three functions in the embodiment of
A separate diluent feed line, typically cylindrical, may also be included which extends around the perimeter of fuel nozzle 50 with additional flow passages sized to permit a secondary diluent flow to be included in the combined feed to the nozzle. A shroud guides the additional diluent flow toward the plurality of holes thus ensuring that a portion flow passes through the air swirler holes and into the mixing chamber where it mixes with the exhaust gas, air and fuel.
The cross-sectional view of
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.