Oxygen-enriched air assisting system for improving the efficiency of cogeneration system

Systems and methods for exhaust gas recirculation in which at least a desired effective oxygen concentration is maintained for stable combustion at increased recirculation rates. Oxygen-enriched gas is injected into the recirculated exhaust gas to achieve the desired effective oxygen concentration.

BACKGROUND

The power generation research and development community faces an important challenge in the years to come: to produce increased amounts of energy under the more and more stringent constraints of increased efficiency and reduced pollution. The increasing costs associated with fuel in recent years further emphasize this mandate.

Gas turbines offer significant advantages for power generation because they are compact, lightweight, reliable, and efficient. They are capable of rapid startup, follow transient loading well, and can be operated remotely or left unattended. Gas turbines have a long service life, long service intervals, and low maintenance costs. Cooling fluids are not usually required. These advantages result in the widespread selection of gas turbine engines for power generation. A basic gas turbine assembly includes a compressor to draw in and compress a working gas (usually air), a combustor where a fuel (i.e., methane, propane, or natural gas) is mixed with the compressed air and then the mixture is combusted to add energy thereto, and a turbine to extract mechanical power from the combustion products. The turbine is coupled to a generator for converting the mechanical power generated by the turbine to electricity.

A characteristic of gas-turbine engines is the incentive to operate at as high a turbine inlet temperature as prevailing technology will allow. This incentive comes from the direct benefit to both specific output power and cycle efficiency. Associated with the high inlet temperature is a high exhaust temperature which, if not utilized, represents waste heat dissipated to the atmosphere. Systems to capture this high-temperature waste heat are prevalent in industrial applications of the gas turbine.

Examples of such systems are cogeneration systems and combined cycle systems. In both systems, one or more heat exchangers are placed in the exhaust duct of the turbine to transfer heat to feed-water circulating through the exchangers to transform the feed-water into steam. In the combined cycle system, the steam is used to produce additional power using a steam turbine. In the cogeneration system, the steam is transported and used as a source of energy for other applications (usually referred to as process steam).

A prior art cogeneration system typically includes a gas turbine engine, a generator, and a heat recovery steam generator. As discussed earlier, the gas turbine engine includes a compressor, a combustor (with a fuel supply), and a turbine. A compressor operates by transferring momentum to air via a high speed rotor. The pressure of the air is increased by the change in magnitude and radius of the velocity components of the air as it passes through the rotor. Thermodynamically speaking, the compressor transfers mechanical power supplied by rotating a shaft coupled to the rotor to the air by increasing the pressure and temperature of the air. A combustor operates by mixing fuel with the compressed air, igniting the fuel/air mixture to add primarily heat energy thereto. A turbine operates in an essentially opposite manner relative to the compressor. The turbine expands the hot and pressurized combustion products through a bladed rotor coupled to a shaft, thereby extracting mechanical energy from the combustion products. The combusted products are exhausted into a duct. Feed-water is pumped through the steam generator located in the duct where it is evaporated into steam. It is through this process that useful energy is harvested from the turbine exhaust gas. The turbine exhaust gas is expelled into the atmosphere at a stack.

Due to deregulation of the energy market and volatility in energy prices, many cogeneration operators prefer to have the option of shutting down the turbine assembly while retaining the steam generation capability of the cogeneration system (known as fresh air mode operation). To enable operation of this fresh air mode, a furnace is disposed in the exhaust duct. The furnace provides an alternate source of hot gas for steam generation. To increase the efficiency of the fresh air mode, a portion of the exhaust gas may be recirculated back to the furnace. Generally, the efficiency of the fresh air mode increases with an increase in recirculation rate of the exhaust gas. Heat energy lost through the stack also decreases with an increase in recirculation rate of the exhaust gas. However, with the increase of the recirculation rate of exhaust gas, the oxygen concentration at the inlet of the furnace decreases, which, eventually adversely affects combustion stability (of the mixture in the furnace) and generates pollutants. Thus, maintaining stable combustion at the high recirculation rates of exhaust gas is problematic.

SUMMARY

Embodiments of the present invention generally relate to an exhaust gas recirculation system which maintains a desired oxygen concentration for stable combustion at increased recirculation rates. In one embodiment, a method for generating heat energy is provided. The method includes the acts of mixing a first stream of exhaust gas with a stream of fresh air, thereby forming a mixture; injecting the mixture, a stream of fuel, and a stream of oxygen-enriched gas into a burner; combusting and mixing the mixture with the stream of fuel and the stream of oxygen-enriched gas, thereby forming a second stream of the exhaust gas; and dividing the second stream of the exhaust gas into at least the first stream of exhaust gas and a third stream of the exhaust gas.

In another embodiment, a steam generator is provided. The steam generator includes a main duct; a furnace in fluid communication with the main duct. The furnace includes a combustion chamber having a first axial end and a second axial end; and a burner located proximate to the first axial end. The steam generator further includes a heat exchanger having a first chamber physically separate from and in thermal communication with a second chamber, the first chamber either in fluid communication with the main duct or being part of the main duct, the first chamber in fluid communication with the second axial end of the combustion chamber. The steam generator further includes a recirculation system. The recirculation system includes a first diverter damper in fluid communication with the first chamber of the heat exchanger and a recycle duct; the recycle duct in fluid communication with the diverter damper and a mixing damper; and the mixing damper in fluid communication with the main duct and fresh air. The steam generator further includes an oxygen-enrichment system. The oxygen-enrichment system includes a source of oxygen-enriched gas in fluid communication with the burner via an oxygen line.

In another embodiment, a control system for use with a cogeneration system is provided. The control system includes a memory unit containing a set of instructions; a control valve configured to meter a flow rate of oxygen-enriched gas; an oxygen sensor configured to measure an oxygen concentration of a mixture of exhaust gas and fresh air, the oxygen sensor in electrical communication with a processor; and a processor configured to control operation of the control valve and perform an operation, when executing the set of instructions. The operation includes acts of comparing the measured oxygen concentration of the mixture with a predetermined oxygen concentration; and if the measured oxygen concentration is not substantially equal to the predetermined oxygen concentration, then calculating a flow rate of oxygen-enriched gas in order to maintain a predetermined oxygen concentration only in a volume proximate to an ignited flame of the fuel; and adjusting the control valve to provide a flow rate substantially equal to the calculated flow rate.

In another embodiment, a method for generating heat energy using a cogeneration system including a gas turbine engine and a steam generation system, where the method includes acts of operating the cogeneration system in a first mode in which the gas turbine engine is operated to produce energy; and operating the cogeneration system in a second mode in which the gas turbine engine is disabled and the steam generation system operates to generate energy, The operation in the second mode includes acts of flowing a combustible mixture into an ignition unit in order to combust the combustible mixture and produce exhaust gas; introducing a recirculated portion of the exhaust gas at a location of the steam generation system upstream of the ignition unit; and introducing an oxygen-enriched gas at a location of the steam generation system proximate to the ignition unit.

In another embodiment, a method for generating heat energy is provided. The method includes the acts of mixing a first stream of exhaust gas with a stream of fresh air, thereby forming a mixture; injecting the mixture, a stream of fuel, and a stream of oxygen-enriched gas into a burner of a cogeneration system comprising a gas turbine engine and a steam generation system; combusting and mixing the mixture with the stream of fuel and the stream of oxygen-enriched gas, thereby forming a second stream of the exhaust gas; controlling a flow rate of the stream of oxygen-enriched gas being injected into the burner; dividing the second stream of the exhaust gas into at least a third stream of exhaust gas and a fourth stream of the exhaust gas; and recirculating the third stream of exhaust gas to form the first stream of exhaust gas.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1is a process flow diagram of a cogeneration system100, according to one embodiment of the present invention. The cogeneration system100includes a gas turbine engine5, a furnace50, at least one heat exchanger20, and a main stack70. The furnace50and the heat exchanger20are typically referred to as a heat recovery steam generator. The cogeneration system100is operable in either cogeneration mode or fresh air mode. In cogeneration mode, the gas turbine engine5is operating, whereas, in fresh air mode, the gas turbine engine5is shut-down and the heat recovery steam generator is operated using an alternative fuel source. The furnace50includes a combustion chamber50band a duct burner50aconnected to a fuel supply F. The furnace50provides an alternate source of hot gas for steam generation in fresh air mode.

In one embodiment of operation, a first stream25aof exhaust gas is mixed with a stream of fresh air A, thereby forming a mixture25b. The first mixture25bis injected, along with a stream of fuel F and a stream of oxygen-enriched gas O2, into the duct burner50a. Combustion and mixing of the first mixture with the fuel stream F and the oxygen-enriched stream of gas O2 substantially occur in the combustion chamber50b(some mixing and/or combustion may occur in the duct burner50a). A second stream25dof the exhaust gas results from the mixture and combustion of the composite stream25c. Heat energy is extracted from the second stream25dof the exhaust gas in the heat exchanger20to produce steam. The second stream25dof the exhaust gas is divided into at least the first stream25aof the exhaust gas and a third stream25eof the exhaust gas. The third stream25eof the exhaust gas may be released into the atmosphere at the main stack70.

FIG. 2is a schematic diagram of the cogeneration system100, according to one embodiment of the present invention. The gas turbine engine5includes a compressor205a, a combustor205b(with a fuel supply F), and a turbine205c. The gas turbine engine5is coupled to a generator215. The combusted products from the gas turbine engine5are exhausted into a main exhaust duct210. Disposed in the exhaust duct210are one or more heat exchangers20. In the illustrative embodiment, the one or more heat exchangers20include a super-heater220a, an evaporator220b, and an economizer220c. Since the super-heater220ais disposed closest to the turbine205c, it is exposed to the highest temperature combustion products, followed by the evaporator220band the economizer220c.

Feed-water W is pumped through these exchangers220a, b, cfrom feed-water tank240wby feed-water circulation pump235. The feed-water W first passes through the economizer220c. At this point, the exhaust gas is usually below the saturation temperature of the feed-water W. The term saturation temperature designates the temperature at which a phase change occurs at a given pressure. The exhaust gas is cooled by the economizer220cto lower temperature levels for greater heat recovery and thus efficiency. The heated feed-water W then passes through the evaporator220bwhere it achieves saturation temperature and is at least substantially transformed into steam S. The steam S then proceeds through the super-heater220awhere further heat energy is acquired by the steam to raise its temperature above saturation, thereby increasing the availability of useful energy therein. The superheated steam S is then transported for utilization in other processes, for example, refining crude oil, manufacturing chemicals, or generating electricity using a steam turbine. It is through this process that useful energy is harvested from the turbine exhaust gas. The turbine exhaust gas is expelled into the atmosphere at the main stack70.

To enable operation of the fresh air mode, the furnace50is disposed in the exhaust duct210. A by-pass stack270band by-pass damper272are used for transition between cogeneration mode and fresh air mode. The by-pass damper272also prevents air leakage into the gas turbine engine5during fresh air mode. To increase the efficiency of the fresh air mode, a diverter damper245is disposed in the main stack70so that a stream25aof the exhaust gas may be recirculated back to the furnace50. Alternatively, the diverter damper245could be located in the exhaust duct210at a location downstream of the economizer220c. The recycled exhaust gas25astream is transported from the diverter damper245by a recirculation duct210r. The recirculation duct210rcarries the stream25aof exhaust gas to a mixing duct260where the stream25aof exhaust gas is mixed with a stream A of fresh air. A damper265is provided to shut in the recirculation duct210rduring cogeneration mode.

A fan255provides the necessary power for recirculation of the stream exhaust gas and mixing thereof with the fresh air A. The fresh air/exhaust gas mixture25bis usually injected into the exhaust duct210at a distance upstream of the furnace250to allow complete mixing of the exhaust gas with the fresh air. The mixture25bthen travels through the exhaust duct210to the duct burner50awhere the fuel stream F and the oxygen-enriched gas stream O2are injected and the fuel stream F is ignited into a fuel flame410f(seeFIG. 4). Combustion and mixing of the fresh air/exhaust gas mixture25bwith the fuel stream F and the oxygen-enriched stream of gas O2substantially occur in the combustion chamber50b(some mixing and/or combustion may occur in the duct burner50a).

The oxygen-enriched gas may be stored in liquid form in a tank240o. Alternatively, an oxygen generator (not shown) may be located on-site. In one embodiment, the oxygen-enriched gas O2may be any gas having an oxygen concentration greater than about 21%. In particular embodiments, the oxygen-enriched gas O2may be any gas having an oxygen concentration greater than about 25%, or greater than about 50%, or greater than about 90%. It is also contemplated that the oxygen-enriched gas O2is commercially-pure oxygen. The oxygen-enriched gas O2is carried from the oxygen tank240o, via pipe or tubing210o, through a control valve275to a header pipe210odisposed in the duct burner50a. The oxygen-enriched gas O2is injected into the duct burner50athrough nozzles310o(seeFIG. 3). An oxygen sensor285is disposed in the recirculation duct210rand is in electrical communication with a controller275cin the control valve275. The controller275cmay also be in electrical communication with other sensors, for example, a carbon monoxide sensor (not shown) and/or a second oxygen sensor (not shown) disposed in the combustion chamber50b. Alternatively, the oxygen sensor285may be located in the combustion chamber50b. Alternatively, the oxygen-enriched gas O2may be mixed with fresh air prior to injection in the duct burner50a. In this scenario, the controller may also control a control valve to meter a ratio of the oxygen-enriched gas to the fresh air. Alternatively, a fan may be disposed in the oxygen pipe210o. The controller275cis a device configured by use of a keypad or wireless interface with machine executable instructions to execute desired functions. The controller275cincludes a microprocessor for executing instructions stored in a memory unit.

Preferably, the controller275cadjusts a flow rate of the oxygen-enriched gas O2so that a predetermined oxygen concentration (POC) is maintained only in a volume410o(seeFIG. 4) proximate to the ignited flame410fof fuel F. Maintaining the POC only in a localized blanket410osurrounding the flame410fminimizes the amount of precious oxygen used. Preferably, the POC for stable combustion is between about 18% and about 18.5%, less preferably, about 17.5% and, least preferably, at about 17%, according to one embodiment of the present invention (depending on specific burner and combustion chamber configuration). Alternatively, the controller275cmay adjust the flow rate so that the POC for the entire stream of the fresh air/exhaust gas mixture25bis maintained or any portion of the fresh air/exhaust gas mixture25b.

The fuel F may be stored in a fuel tank (not shown) and carried to a header pipe210fin the duct burner50aby a fuel pipe (not shown). The fuel F is injected into the duct burner through nozzles310f(seeFIG. 3). The fuel may be delivered to the fuel nozzles310fby a fuel pump (not shown) disposed along and in fluid communication with the fuel pipe.

FIG. 3is a simplified end view of the duct burner50a, according to one embodiment of the present invention. The end of the duct burner50ashown is the end that faces the combustion chamber50b. The duct burner50aincludes a flange305having holes for receiving fasteners to couple the end to the combustion chamber50b. One or more sub-ducts315are formed in the duct burner50a. The sub-ducts315are in fluid communication with the exhaust duct210. The duct burner also includes one or more burners310. Each burner310includes the fuel nozzles310fin fluid communication with the header pipe210fand the oxygen nozzles310oin fluid communication with the header pipe210o. As shown, an oxygen nozzle310ois disposed proximately above and below each fuel nozzle310fand, optionally, between each fuel nozzle310f. An oxygen nozzle310ois also optionally disposed at each horizontal end of the fuel nozzles310f. Alternatively, an oxygen nozzle may be disposed concentrically around each fuel nozzle310f.

FIG. 4is a schematic of a duct burner nozzle in operation being blanketed by oxygen-enriched gas O2, according to one embodiment of the present invention. The ignited stream of fuel F forms a flame410fthrough an opening in a flame shield405. Streams of oxygen-enriched gas O2are injected at other openings in the flame shield405to form blankets510around each of upper and lower portions of the flame410f. If the optional side nozzles or the alternative concentric nozzle is used, then the blanket(s) will substantially surround a periphery of the flame410f. As shown, the blankets410oeach longitudinally extend along a periphery of the flame410fa distance Lowhich is a substantial portion of the flame length Lf. Past Lo, the blanket may dissipate so that the POC is no longer maintained. The ratio Lo/Lfof the blanket length Loto the flame length Lfmay range from three-tenths to one, depending on the specific duct burner50aconfiguration and cogeneration system100. In one embodiment, the ratio Lo/Lfis ranges from five-tenths to one. Alternatively, the ratio Lo/Lfmay be greater than one. As shown, each blanket also has a maximum thickness X measured from the periphery of the flame410fradially outward to the periphery of a respective blanket410o. The maximum thickness X may range from five to twenty centimeters, depending on the duct burner50aconfiguration and cogeneration system100. Alternatively, the maximum thickness X may be less than or equal to ten centimeters. Some variables that may effect these ranges are the orientation of the oxygen nozzles310o, the velocity of the oxygen-enriched gas O2exiting the oxygen nozzles310o, the size of the nozzles310o, the shape of the nozzles310o, and the configuration of the nozzles310o.

EXAMPLES

Table 1 exhibits effects of varying recirculation rates on combustion and efficiency of a conventional cogeneration system operating in free air mode. The entries marked by an “X” indicate cases where the oxygen concentration in a fresh air/exhaust gas mixture injected into the duct burner are insufficient for stable combustion. This oxygen-deficient condition results for increased rates (greater than or equal to about 30%) of recycled exhaust gas.

TABLE 1Effect of Various Recirculation Rates on Combustionand Efficiency of a Cogeneration System Operating inFree Air ModeUnstableRecirculationGlobalO2ToO2InComb.RateEfficiencyBurnerExhaust Gas0%83%20.7%13.5%20%85.8%18.9%11.9%30%87.2%17.45%10.6%X35%88.0%16.73%9.95%X40%88.8%16%9.3%X45%89.6%14.6%7.98%

In operation, especially during increased recirculation rates, the fresh air and recycled gas mixture25bflows through the sub-ducts315and begins combustion when it reaches the burners310. If the oxygen sensor285detects an oxygen deficient condition, the controller275copens the control valve275to compensate the oxygen deficient mixture25bby injecting the oxygen-enriched gas O2through the oxygen nozzles310oin the duct burner50a. The oxygen-enriched gas O2increases the oxygen-concentration of the mixture25bin the localized volume410osurrounding the ignited flame410f, thereby allowing stable ignition of the fuel F. The stable ignition of the flame410fprovided by the blanket410oof oxygen-enriched gas O2facilitates stable combustion of the oxygen-deficient mixture25bwith the fuel F in the combustion chamber50b. Stable combustion allows for realization of higher global efficiencies (see Table 1) associated with increased recirculation rates without the unwanted side effects, i.e. increased pollution, that would otherwise accompany unstable combustion at the increased recirculation rates. In one embodiment, the oxygen-enriched cogeneration system100may maintain an effective oxygen concentration in the burner50aat a level that is acceptable for stable combustion up to about a 45% recirculation rate. In another embodiment, the oxygen-enriched cogeneration system100may maintain an effective oxygen concentration in the burner50aat a level that is acceptable for stable combustion up to about a 60% recirculation rate.

Thus, in one embodiment, oxygen-enriched cogeneration system100operates to vary the oxygen concentration in a fresh air/exhaust gas mixture injected into the duct burner50aaccording to different recirculation rates. In this way, the oxygen-enriched cogeneration system100is capable of maintaining an effective substantially constant oxygen concentration in the duct burner50aat different recirculation rates of the exhaust gas. Different recirculation rates give a cogeneration system greater flexibility for design while relatively effective constant oxygen content to the burner50afacilitates better control of combustion in the system100.

Alternatively, the oxygen-enrichment may also be used in cogeneration mode and in other steam generation systems, such as combined cycle systems and any system using a heat recovery steam generator or integrated boiler system.

Preferred processes and apparatus for practicing the present invention have been described. It will be understood and readily apparent to the skilled artisan that many changes and modifications may be made to the above-described embodiments without departing from the spirit and the scope of the present invention. The foregoing is illustrative only and that other embodiments of the integrated processes and apparatus may be employed without departing from the true scope of the invention defined in the following claims.