Patent Publication Number: US-9903316-B2

Title: Stoichiometric combustion of enriched air with exhaust gas recirculation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage entry under 35 U.S.C. 371 of PCT/US2011/039829, that published as WO 2012/003079 and was filed on 9 Jun. 2011 which claims the benefit of U.S. Provisional Application No. 61/361,178, filed on 2 Jul. 2010, each of which is incorporated by reference, in its entirety, for all purposes. 
     This application contains subject matter related to PCT/US2011/042870, that published as WO 2012/003489 and was filed on 1 Jul. 2011; PCT/US2011/039824, that published as WO 2012/003076 and was filed on 9 Jun. 2011; PCT/US2011/039826, that published as WO 2012/003077 and was filed on 9 Jun. 2011; PCT/US2011/039828, that published as WO 2012/003078 and was filed on 9 Jun. 2011; and PCT/US2011/039830, that published as WO 2012/003080 and was filed on 9 Jun. 2011. 
     This application contains subject matter related to U.S. Patent Application No. 61/361,169, filed Jul. 2, 2010 entitled “Systems and Methods for Controlling Combustion of a Fuel”; U.S. Patent Application No. 61/361,170, filed Jul. 2, 2010 entitled “Low Emission Triple-Cycle Power Generation Systems and Methods”; U.S. Patent Application No. 61/361,173, filed Jul. 2, 2010, entitled “Low Emission Triple-Cycle Power Generation Systems and Methods”; U.S. Patent Application No. 61/361,176, filed Jul. 2, 2010, entitled “Stoichiometric Combustion With Exhaust Gas Recirculation and Direct Contact Cooler”; U.S. Patent Application No. 61/361,180 filed Jul. 2, 2010, entitled “Low Emission Power Generation Systems and Methods”. 
     FIELD 
     Embodiments of the disclosure relate to low emission power generation in combined-cycle power systems. More particularly, embodiments of the disclosure relate to methods and apparatus for combusting a fuel for enhanced carbon dioxide (CO 2 ) manufacture and capture. 
     BACKGROUND 
     This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art. 
     With the growing concern on global climate change and the impact of CO 2  emissions, emphasis has been placed on CO 2  capture from power plants. This concern combined with the implementation of cap-and-trade policies in many countries make reducing CO 2  emissions a priority for these and other countries as well as the companies that operate hydrocarbon production systems therein. 
     Gas turbine combined-cycle power plants are rather efficient and can be operated at relatively low cost when compared to other technologies, such as coal and nuclear. Capturing CO 2  from the exhaust of gas turbine combined-cycle plants, however, can be difficult for several reasons. For instance, there is typically a low concentration of CO 2  in the exhaust compared to the large volume of gas that must be treated. Also, additional cooling is often required before introducing the exhaust to a CO 2  capture system and the exhaust can become saturated with water after cooling, thereby increasing the reboiler duty in the CO 2  capture system. Other common factors can include the low pressure and large quantities of oxygen frequently contained in the exhaust. All of these factors result in a high cost of CO 2  capture from gas turbine combined-cycle power plants. 
     At least one approach to lowering CO 2  emissions in combined-cycle systems includes stoichiometric combustion and exhaust gas recirculation. In a conventional exhaust gas recirculation system, such as a natural gas combined cycle (NGCC), a recycled component of the exhaust gas is mixed with ambient air and introduced into the compressor section of a gas turbine. Typical CO 2  concentrations in the exhaust of a NGCC are around 3%-4%, but can increase above 4% with exhaust recirculation. In operation, conventional NGCC systems require only about 40% of the air intake volume to provide adequate stoichiometric combustion of the fuel, while the remaining 60% of the air volume serves as a diluent to moderate the temperature and cool the exhaust to a temperature suitable for introduction into the succeeding expander. Recirculating a portion of the exhaust gas increases the CO 2  concentration in the exhaust, which can subsequently be used as the diluent in the combustion system. 
     However, due to the molecular weight, specific heat, Mach number effects, etc. of CO 2 , without significant modifications to either the compressor or the expander sections, standard gas turbines are limited as to the concentration of CO 2  that can be tolerated in the compression section of the gas turbine from the exhaust. For example, the limit on CO 2  content in the exhaust recirculated to the compression section of a standard gas turbine is about 20 wt % CO 2 . 
     Moreover, the typical NGCC system produces low pressure exhaust which requires a fraction of the power produced via expansion of the exhaust in order to extract the CO 2  for sequestration or enhanced oil recovery (EOR), thereby reducing the thermal efficiency of the NGCC. Further, the equipment for the CO 2  extraction is large and expensive, and several stages of compression are required to take the ambient pressure gas to the pressure required for EOR or sequestration. Such limitations are typical of post-combustion carbon capture from low pressure exhaust associated with the combustion of other fossil fuels, such as coal. 
     The foregoing discussion of need in the art is intended to be representative rather than exhaustive. A technology addressing one or more such needs, or some other related shortcoming in the field, would benefit power generation in combined-cycle power systems. 
     SUMMARY 
     The present disclosure is directed to integrated systems and methods for improving power generation systems. In some implementations, the present disclosure provides a gas turbine system, comprising a first compressor, a second compressor, a combustion chamber, and an expander. The first compressor may be configured to receive and compress a recycled exhaust gas into a compressed recycle exhaust gas. The second compressor may be configured to receive and compress enriched air to generate a compressed oxidant. The combustion chamber may be configured to receive the compressed recycle exhaust gas and the compressed oxidant and to stoichiometrically combust a fuel stream. The compressed recycle exhaust gas serves as a diluent to moderate combustion temperatures. The expander may be configured to receive a discharge from the combustion chamber to generate the recycled exhaust gas. The expander further may be coupled to the first compressor to drive, at least partially, the first compressor. 
     Additionally or alternatively, the present disclosure provides methods of generating power. Exemplary methods include: a) compressing a recycled exhaust gas in a main compressor to generate a compressed recycle exhaust gas; b) compressing enriched air in an inlet compressor to generate a compressed oxidant; c) stoichiometrically combusting the compressed oxidant and a fuel in a combustion chamber and in the presence of the compressed recycle exhaust gas, thereby generating a discharge stream, wherein the compressed recycle exhaust gas acts as a diluent configured to moderate the temperature of the discharge stream; and d) expanding the discharge stream in an expander to at least partially drive the main compressor and generate the recycled exhaust gas and at least partially drive the main compressor. 
     Still additionally or alternatively, the present disclosure provides integrated power generation systems. Exemplary integrated power generation systems include both a gas turbine system and an exhaust gas recirculation system. The gas turbine system may comprise a first compressor, a second compressor, a combustion chamber, and an expander. The first compressor may be configured to receive and compress a recycled exhaust gas into a compressed recycle exhaust gas. The second compressor may be configured to receive and compress enriched air to generate a compressed oxidant, the enriched air having an oxygen concentration between about 30 wt % and about 50 wt %. The combustion chamber may be configured to receive the compressed recycle exhaust gas and the compressed oxidant and to stoichiometrically combust a fuel stream, wherein the compressed recycle exhaust gas serves as a diluent to moderate combustion temperatures. The expander may be configured to receive a discharge from the combustion chamber to generate the recycled exhaust stream. The expander further may be coupled to the first compressor and adapted to drive, at least partially, the first compressor. The exhaust gas recirculation system may include a heat recovery steam generator, one or more cooling units, and a boost compressor. The heat recovery steam generator may be communicably coupled to a steam gas turbine. The heat recovery steam generator may be being configured to receive the exhaust gas from the expander to create steam that generates electrical power in the steam generator. The one or more cooling units may be configured to cool the recycled exhaust gas received from the heat recovery steam generator and to remove condensed water from the recycled exhaust gas. The boost compressor may be configured to increase the pressure of the recycled exhaust gas before injection into the first compressor to provide the compressed recycle exhaust gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other advantages of the present disclosure may become apparent upon reviewing the following detailed description and drawings of non-limiting examples of embodiments in which: 
         FIG. 1  depicts a schematic of an integrated system for low emission power generation and enhanced CO 2  recovery, according to one or more embodiments described. 
         FIG. 2  depicts another schematic of an integrated system for low emission power generation and enhanced CO 2  recovery, according to one or more embodiments described. 
         FIG. 3  depicts another schematic of an integrated system for low emission power generation and enhanced CO 2  recovery, according to one or more embodiments described. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description section, the specific embodiments of the present disclosure are described in connection with preferred embodiments. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present disclosure, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the disclosure is not limited to the specific embodiments described below, but rather, it includes all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims. 
     Various terms as used herein are defined below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. 
     As used herein, the term “natural gas” refers to a multi-component gas obtained from a crude oil well (associated gas) or from a subterranean gas-bearing formation (non-associated gas). The composition and pressure of natural gas can vary significantly. A typical natural gas stream contains methane (CH 4 ) as a major component, i.e. greater than 50 mol % of the natural gas stream is methane. The natural gas stream can also contain ethane (C 2 H 6 ), higher molecular weight hydrocarbons (e.g., C 3 -C 20  hydrocarbons), one or more acid gases (e.g., hydrogen sulfide, carbon dioxide), or any combination thereof. The natural gas can also contain minor amounts of contaminants such as water, nitrogen, iron sulfide, wax, crude oil, or any combination thereof. 
     As used herein, the term “stoichiometric combustion” refers to a combustion reaction having a volume of reactants comprising a fuel and an oxidizer and a volume of products formed by combusting the reactants where the entire volume of the reactants is used to form the products. As used herein, the term “substantially stoichiometric combustion” refers to a combustion reaction having a molar ratio of combustion fuel to oxygen ranging from about plus or minus 10% of the oxygen required for a stoichiometric ratio or more preferably from about plus or minus 5% of the oxygen required for the stoichiometric ratio. For example, the stoichiometric ratio of fuel to oxygen for methane is 1:2 (CH 4 +2O 2 &gt;CO 2 +2H 2 O). Propane will have a stoichiometric ratio of fuel to oxygen of 1:5. Another way of measuring substantially stoichiometric combustion is as a ratio of oxygen supplied to oxygen required for stoichiometric combustion, such as from about 0.9:1 to about 1.1:1, or more preferably from about 0.95:1 to about 1.05:1. 
     As used herein, the term “stream” refers to a volume of fluids, although use of the term stream typically means a moving volume of fluids (e.g., having a velocity or mass flow rate). The term “stream,” however, does not require a velocity, mass flow rate, or a particular type of conduit for enclosing the stream. 
     Embodiments of the presently disclosed systems and processes can be used to produce ultra low emission electric power and CO 2  for enhanced oil recovery (EOR) and/or sequestration applications. In one or more embodiments, a mixture of enriched air and fuel can be stoichiometrically or substantially stoichiometrically combusted and simultaneously mixed with a stream of recycled exhaust gas. The stream of recycled exhaust gas, generally including products of combustion such as CO 2 , can be used as a diluent to control, adjust, or otherwise moderate the temperature of combustion and the exhaust that enters the succeeding expander. As a result of using enriched air, the recycled exhaust gas can have an increased CO 2  content, thereby allowing the expander to operate at even higher expansion ratios for the same inlet and discharge temperatures, thereby producing significantly increased power. 
     Combustion in commercial gas turbines at stoichiometric conditions or substantially stoichiometric conditions (e.g., “slightly rich” combustion) can prove advantageous in order to eliminate the cost of excess oxygen removal. By cooling the exhaust and condensing the water out of the cooled exhaust stream, a relatively high content CO 2  exhaust stream can be produced. While a portion of the recycled exhaust gas can be utilized for temperature moderation in the closed Brayton cycle, a remaining purge stream can be used for EOR applications and/or electric power can be produced with little or no sulfur oxides (SO X ), nitrogen oxides (NO X ), and/or CO 2  being emitted to the atmosphere. The result of this process is the production of power in three separate cycles and the manufacturing of additional CO 2 . 
     Referring now to the figures,  FIG. 1  depicts a schematic of an illustrative integrated system  100  for power generation and CO 2  recovery using a combined-cycle arrangement, according to one or more embodiments. In at least one embodiment, the power generation system  100  can include a gas turbine system  102  characterized as a power-producing, closed Brayton cycle. The gas turbine system  102  can have a first or main compressor  104  coupled to an expander  106  via a shaft  108 . The shaft  108  can be any mechanical, electrical, or other power coupling, thereby allowing a portion of the mechanical energy generated by the expander  106  to drive the main compressor  104 . In at least one embodiment, the gas turbine system  102  can be a standard gas turbine, where the main compressor  104  and expander  106  form the compressor and expander ends, respectively. In other embodiments, however, the main compressor  104  and expander  106  can be individualized components in the system  102 . 
     The gas turbine system  102  can also include a combustion chamber  110  configured to combust a fuel introduced via line  112  and mixed with an oxidant introduced via line  114 . In one or more embodiments, the fuel in line  112  can include any suitable hydrocarbon gas or liquid, such as natural gas, methane, ethane, naphtha, butane, propane, syngas, diesel, kerosene, aviation fuel, coal derived fuel, bio-fuel, oxygenated hydrocarbon feedstock, or any combinations thereof. The oxidant via line  114  can be derived from a second or inlet compressor  118  fluidly coupled to the combustion chamber  110  and adapted to compress a feed oxidant introduced via line  120 . In one or more embodiments, the feed oxidant in line  120  can include atmospheric air, enriched air, or combinations thereof. When the oxidant in line  114  includes a mixture of atmospheric air and enriched air, the enriched air can be compressed by the inlet compressor  118  either before or after being mixed with the atmospheric air. The enriched air can have an overall oxygen concentration of about 30 wt %, about 35 wt %, about 40 wt %, about 45 wt %, or about 50 wt %. 
     The enriched air can be derived from several sources, including implementing various technologies upstream of the inlet compressor  118  to produce the enriched air. For example, the enriched air can be derived from such separation technologies as membrane separation, pressure swing adsorption, temperature swing adsorption, nitrogen plant-by-product streams, and/or combinations thereof. The enriched air can also be derived from an air separation unit (ASU), such as a cryogenic ASU, for producing nitrogen for pressure maintenance or other purposes. The reject stream from the ASU can be rich in oxygen, having an overall oxygen content of about 50 wt % to about 70 wt %. This reject stream can be used as at least a portion of the enriched air and subsequently diluted, if needed, with unprocessed atmospheric air to obtain the desired oxygen concentration for the application. 
     As will be described in more detail below, the combustion chamber  110  can also receive a compressed recycle exhaust gas in line  144 , including an exhaust gas recirculation primarily having CO 2  and nitrogen components. The compressed recycle exhaust gas in line  144  can be derived from the main compressor  104  and adapted to help facilitate a stoichiometric or substantially stoichiometric combustion of the compressed oxidant in line  114  and fuel in line  112  by moderating the temperature of the combustion products. As can be appreciated, recirculating the exhaust gas can serve to increase the CO 2  concentration in the exhaust gas. 
     An exhaust gas in line  116  directed to the inlet of the expander  106  can be generated as a product of combustion of the fuel in line  112  and the compressed oxidant in line  114 , in the presence of the compressed recycle exhaust gas in line  144 . In at least one embodiment, the fuel in line  112  can be primarily natural gas, thereby generating a discharge or exhaust gas via line  116  that can include volumetric portions of vaporized water, CO 2 , nitrogen, nitrogen oxides (NO X ), and sulfur oxides (SO X ). In some embodiments, a small portion of unburned fuel in line  112  or other compounds can also be present in the exhaust gas in line  116  due to combustion equilibrium limitations. As the exhaust gas in line  116  expands through the expander  106  it generates mechanical power to drive the main compressor  104 , an electrical generator, or other facilities, and also produces a gaseous exhaust in line  122  having a heightened CO 2  content resulting from the influx of the compressed recycle exhaust gas in line  144 . In some implementations the expander  106  may be adapted to produce additional mechanical power that may be used for other purposes. 
     The power generation system  100  can also include an exhaust gas recirculation (EGR) system  124 . In one or more embodiments, the EGR system  124  can include a heat recovery steam generator (HRSG)  126 , or similar device, fluidly coupled to a steam gas turbine  128 . In at least one embodiment, the combination of the HRSG  126  and the steam gas turbine  128  can be characterized as a power-producing closed Rankine cycle. In combination with the gas turbine system  102 , the HRSG  126  and the steam gas turbine  128  can form part of a combined-cycle power generating plant, such as a natural gas combined-cycle (NGCC) plant. The gaseous exhaust in line  122  can be introduced to the HRSG  126  in order to generate steam via line  130  and a cooled exhaust gas in line  132 . In one embodiment, the steam in line  130  can be sent to the steam gas turbine  128  to generate additional electrical power. 
     The cooled exhaust gas in line  132  can be sent to a first cooling unit  134  adapted to cool the cooled exhaust gas in line  132  and generate a cooled recycle gas stream  140 . The first cooling unit  134  can include, for example, one or more contact coolers, trim coolers, evaporative cooling unit, or any combination thereof. The first cooling unit  134  can also be adapted to remove a portion of any condensed water from the cooled exhaust gas in line  132  via a water dropout stream  138 . In at least one embodiment, the water dropout stream  138  may be routed to the HRSG  126  via line  141  to provide a water source for the generation of additional steam in line  130  therein. In other embodiments, the water recovered via the water dropout stream  138  can be used for other downstream applications, such as supplementary heat exchanging processes. 
     In one or more embodiments, the cooled recycle gas stream  140  can be directed to a boost compressor  142 . Cooling the cooled exhaust gas in line  132  in the first cooling unit  134  can reduce the power required to compress the cooled recycle gas stream  140  in the boost compressor  142 . As opposed to a conventional fan or blower system, the boost compressor  142  can be configured to compress and increase the overall density of the cooled recycle gas stream  140 , thereby directing a pressurized recycle gas in line  145  downstream, where the pressurized recycle gas in line  145  has an increased mass flow rate for the same volumetric flow. This can prove advantageous since the main compressor  104  can be volume-flow limited, and directing more mass flow through the main compressor  104  can result in higher discharge pressures, thereby translating into higher pressure ratios across the expander  106 . Higher pressure ratios generated across the expander  106  can allow for higher inlet temperatures and, therefore, an increase in expander  106  power and efficiency. As can be appreciated, this may prove advantageous since the CO 2 -rich exhaust gas in line  116  generally maintains a higher specific heat capacity. 
     Since the suction pressure of the main compressor  104  is a function of its suction temperature, a cooler suction temperature will demand less power to operate the main compressor  104  for the same mass flow. Consequently, the pressurized recycle gas in line  145  can optionally be directed to a second cooling unit  136 . The second cooling unit  136  can include, for example, one or more direct contact coolers, trim coolers, evaporative cooling units, or any combination thereof. In at least one embodiment, the second cooling unit  136  can serve as an aftercooler adapted to remove at least a portion of the heat of compression generated by the boost compressor  142  on the pressurized recycle gas in line  145 . The second cooling unit  136  can also extract additional condensed water via a water dropout stream  143 . In one or more embodiments, the water dropout streams  138 ,  143  can converge into stream  141  and may or may not be routed to the HRSG  126  to generate additional steam via line  130  therein. 
     While only first and second cooling units  134 ,  136  are depicted herein, it will be appreciated that any number of cooling units can be employed to suit a variety of applications, without departing from the scope of the disclosure. In fact, contemplated herein are embodiments where the cooled exhaust gas in line  132  is further directed to an evaporative cooling unit associated with the exhaust gas recirculation loop, such as generally described in the concurrently filed U.S. Patent Application entitled “Stoichiometric Combustion with Exhaust Gas Recirculation and Direct Contact Cooler,” the contents of which are hereby incorporated by reference to the extent not inconsistent with the present disclosure. As described therein, the exhaust gas recirculation system may include any variety of equipment adapted to provide the exhaust gas to the main compressor for injection into the combustion chamber. 
     The main compressor  104  can be configured to receive and compress the pressurized recycle gas in line  145  to a pressure nominally at or above the pressure of the combustion chamber  110 , thereby generating the compressed recycle exhaust gas in line  144 . As can be appreciated, cooling the pressurized recycle gas in line  145  in the second cooling unit  136  after compression in the boost compressor  142  can allow for an increased volumetric mass flow of exhaust gas into the main compressor  104 . Consequently, this can reduce the amount of power required to compress the pressurized recycle gas in line  145  to a predetermined pressure. 
     In at least one embodiment, a purge stream  146  can be recovered from the compressed recycle exhaust gas in line  144  and subsequently treated in a CO 2  separator  148  to capture CO 2  at an elevated pressure via line  150 . The separated CO 2  in line  150  can be used for sales, used in another processes requiring CO 2 , and/or further compressed and injected into a terrestrial reservoir for enhanced oil recovery (EOR), sequestration, or another purpose. Because of the stoichiometric or substantially stoichiometric combustion of the fuel in line  112  combined with a boosted pressure from the boost compressor  142 , the CO 2  partial pressure in the purge stream  146  can be much higher than in conventional gas turbine exhausts. As a result, carbon capture in the CO 2  separator  148  can be undertaken using low-energy separation processes, such as employing less energy-intensive solvents. At least one suitable solvent is potassium carbonate (K 2 CO 3 ) which absorbs SO X  and/or NO X , and converts them to useful compounds, such as potassium sulfite (K 2 SO 3 ), potassium nitrate (KNO 3 ), and other simple fertilizers. Exemplary systems and methods of using potassium carbonate for CO 2  capture can be found in the concurrently filed U.S. Patent Application entitled “Low Emission Triple-Cycle Power Generation Systems and Methods,” the contents of which are hereby incorporated by reference to the extent not inconsistent with the present disclosure. 
     A residual stream  151 , essentially depleted of CO 2  and consisting primarily of nitrogen, can also be derived from the CO 2  separator  148 . In one or more embodiments, the residual stream  151  can be introduced to a gas expander  152  to provide power and an expanded depressurized gas, or exhaust gas, via line  156 . The expander  152  can be, for example, a power-producing nitrogen expander. As depicted, the gas expander  152  can be optionally coupled to the inlet compressor  118  through a common shaft  154  or other mechanical, electrical, or other power coupling, thereby allowing a portion of the power generated by the gas expander  152  to drive the inlet compressor  118 . However, during start-up of the system  100  and/or during normal operation when the gas expander  152  is unable to supply all the required power to operate the inlet compressor  118 , at least one motor  158 , such as an electric motor, can be used synergistically with the gas expander  152 . For instance, the motor  158  can be sensibly sized such that during normal operation of the system  100 , the motor  158  can be configured to supply the power short-fall from the gas expander  152 . In other embodiments, however, the gas expander  152  can be used to provide power to other applications, and not directly coupled to the inlet compressor  118 . For example, there may be a substantial mismatch between the power generated by the expander  152  and the requirements of the compressor  118 . In such cases, the expander  152  could be adapted to drive a smaller (or larger) compressor (not shown) that demands less (or more) power. 
     An expanded depressurized gas in line  156 , primarily consisting of dry nitrogen gas, can be discharged from the gas expander  152 . In at least one embodiment, the combination of the gas expander  152 , inlet compressor  118 , and CO 2  separator  148  can be characterized as an open Brayton cycle, or a third power-producing component of the power generation system  100 . Illustrative systems and methods of expanding the nitrogen gas in the residual stream  151 , and variations thereof, can be found in the concurrently filed U.S. Patent Application entitled “Low Emission Triple-Cycle Power Generation Systems and Methods,” the contents of which are hereby incorporated by reference to the extent not inconsistent with the present disclosure. 
     Referring now to  FIG. 2 , depicted is another schematic of an illustrative integrated system  200  for power generation and CO 2  recovery using a combined-cycle arrangement, according to one or more embodiments. The system  200  of  FIG. 2  is substantially similar to the system  100  of  FIG. 1  and therefore will not be discussed in detail where like elements correspond to like numerals. The system  200  of  FIG. 2 , however, can replace the gas expander  152  of system  100  with a downstream compressor  158  configured to compress the residual stream  151  and generate a compressed exhaust gas via line  160 . In one or more embodiments, the compressed exhaust gas in line  160  can be suitable for injection into a reservoir for pressure maintenance applications. In applications where methane gas is typically reinjected into hydrocarbon wells to maintain well pressures, compressing the residual stream  151  may prove advantageous. For example, the pressurized nitrogen gas in line  160  can instead be injected into the hydrocarbon wells and any residual methane gas can be sold or otherwise used as a fuel in related applications, such as providing fuel in line  112 . 
     Referring to  FIG. 3 , depicted is another schematic of an illustrative integrated system  300  for power generation and CO 2  recovery using a combined-cycle arrangement, according to one or more embodiments. The system  300  of  FIG. 3  is substantially similar to the systems  100  and  200  of  FIGS. 1 and 2 , respectively, and therefore will not be discussed in detail where like elements correspond to like numerals. As depicted, the system  300  can be a characterized as a hybrid arrangement of the power-producing nitrogen gas expander  152  as discussed with reference to  FIG. 1 , and the pressure maintenance downstream compressor  158  as discussed with reference to  FIG. 2 . In one or more embodiments, the residual stream  151  can be split, thereby directing a first portion of the residual stream  151  to the gas expander  152 , and at the same time directing a second portion of the residual stream  151  to the downstream compressor  158  via line  162 . In at least one embodiment, the respective volumetric mass flow of the first and second portions can be manipulated so as to provide predetermined and/or desired amounts of the residual stream  151  to either location to maximize production. 
     By using enriched air as the compressed oxidant in line  114  and pressurizing the exhaust gas in the boost compressor  142 , the power generation system  100  can achieve higher concentrations of CO 2  in the exhaust gas, thereby allowing for more effective CO 2  separation and capture. Embodiments disclosed herein can effectively increase the concentration of CO 2  in the exhaust gas in line  116  to CO 2  concentrations ranging from about 10 wt % to about 20 wt %. To achieve such CO 2  concentrations, the combustion chamber  110  can be adapted to stoichiometrically or substantially stoichiometrically combust an incoming mixture of fuel in line  112  and compressed oxidant in line  114 , where the compressed oxidant in line  114  includes enriched air having an overall oxygen concentration of about 30 wt %, about 35 wt %, about 40 wt %, about 45 wt %, or about 50 wt %. 
     In order to moderate the temperature of the stoichiometric combustion and meet expander  106  inlet temperature and component cooling requirements, a portion of the exhaust gas with increased CO 2  content derived from the compressed recycle exhaust gas in line  144  can be injected into the combustion chamber  110  as a diluent. Thus, embodiments of the disclosure can essentially eliminate excess oxygen from the exhaust gas in line  116  while also increasing its CO 2  concentration to about 20 wt %. As such, the gaseous exhaust in line  122  can have less than about 3.0 wt % oxygen, or less than about 1.0 wt % oxygen, or less than about 0.1 wt % oxygen, or even less than about 0.001 wt % oxygen. 
     At least one benefit of having an increased CO 2  concentration is that the expander  106  can be operated at an even higher expansion ratio for the same inlet and discharge temperatures, and thereby produce increased power. This is due to the higher heat capacity of CO 2  relative to nitrogen found in ambient air. In one or more embodiments, the expansion ratio of the expander  106  can be increased from about 17.0 to about 20.0 corresponding to about 10 wt % and about 20 wt % CO 2  recycle streams, respectively. Embodiments using enriched air having about 35 wt % oxygen can be used in order to achieve the about 20 wt % in the CO 2  recycle stream. 
     Other benefits of having an increased CO 2  concentration in the recycle gas include an increased concentration of CO 2  in the extracted purge stream  146  used for CO 2  separation. Because of its increased CO 2  concentration, the purge stream  146  need not be as large in order to extract the required amounts of CO 2 . For example, the equipment handling extraction for CO 2  separation can be smaller, including its piping, heat exchangers, valves, absorber towers, etc. Moreover, increased concentrations of CO 2  can improve the performance of CO 2  removal technology, including using low-energy separation processes, such as employing less energy-intensive solvents that would otherwise be untenable. Consequently, capital expenditures for capturing CO 2  can be dramatically lowered. 
     The specifics of exemplary operation of the system  100  will now be discussed. As will be appreciated, specific temperatures and pressures achieved or experienced in the various components of any of the embodiments disclosed herein can change depending on, among other factors, the purity of the oxidant used and/or the specific makes and/or models of expanders, compressors, coolers, etc. Accordingly, it will be appreciated that the particular data described herein is for illustrative purposes only and should not be construed as the only interpretation thereof. In an embodiment, the inlet compressor  118  can provide compressed oxidant in line  114  at pressures ranging between about 280 psia and about 300 psia. Also contemplated herein, however, is aeroderivative gas turbine technology, which can produce and consume pressures of up to about 750 psia and more. 
     The main compressor  104  can be configured to recycle and compress recycled exhaust gas into the compressed recycle exhaust gas in line  144  at a pressure nominally above or at the combustion chamber  110  pressure, and use a portion of that recycled exhaust gas as a diluent in the combustion chamber  110 . Because amounts of diluent needed in the combustion chamber  110  can depend on the purity of the oxidant used for stoichiometric combustion or the particular model or design of expander  106 , a ring of thermocouples and/or oxygen sensors  131   a ,  131   b  can be disposed on the outlet of the expander  106 . In operation, the thermocouples and sensors  131   a ,  131   b  can be adapted to regulate and determine the volume of exhaust gas required as diluent needed to cool the products of combustion to the required expander inlet temperature, and also regulate the amount of oxidant being injected into the combustion chamber  110 . Thus, in response to the heat requirements detected by the thermocouples  131   a  and the oxygen levels detected by the oxygen sensors  131   b , the volumetric mass flow of compressed recycle exhaust gas in line  144  and compressed oxidant in line  114  can be manipulated or fluctuate to match the demand. Illustrative embodiments and more detailed descriptions of systems and methods for controlling the composition of an exhaust gas produced by combusting a fuel can be found in the concurrently filed U.S. patent application Ser. No. 13/808,073 entitled “Systems and Methods for Controlling Combustion of a Fuel,” the contents of which are hereby incorporated by reference to the extent not inconsistent with the present disclosure. 
     In at least one embodiment, a pressure drop of about 12-13 psia can be experienced across the combustion chamber  110  during stoichiometric or substantially stoichiometric combustion. Combustion of the fuel in line  112  and the compressed oxidant in line  114  can generate temperatures between about 2000° F. and about 3000° F. and pressures ranging from 250 psia to about 300 psia. As described above, because of the increased mass flow and higher specific heat capacity of the CO 2 -rich exhaust gas derived from the compressed recycle exhaust gas in line  144 , higher pressure ratios can be achieved across the expander  106 , thereby allowing for higher inlet temperatures and increased expander  106  power. 
     The gaseous exhaust in line  122  exiting the expander  106  can exhibit pressures at or near ambient. In at least one embodiment, the gaseous exhaust in line  122  can have a pressure of about 13-17 psia. The temperature of the gaseous exhaust in line  122  can be about 1225° F. to about 1275° F. before passing through the HRSG  126  to generate steam in line  130  and a cooled exhaust gas in line  132 . In one or more embodiments, the cooling unit  134  can reduce the temperature of the cooled exhaust gas in line  132  thereby generating the cooled recycle gas stream  140  having a temperature between about 32° F. and about 120° F. As can be appreciated, such temperatures can fluctuate depending primarily on wet bulb temperatures during specific seasons in specific locations around the globe. 
     According to one or more embodiments, the boost compressor  142  can be configured to elevate the pressure of the cooled recycle gas stream  140  to a pressure ranging from about 17 psia to about 21 psia. As a result, the main compressor  104  eventually receives and compresses a recycled exhaust with a higher density and increased mass flow, thereby allowing for a substantially higher discharge pressure while maintaining the same or similar pressure ratio. In order to further increase the density and mass flow of the recycle exhaust gas, the pressurized recycle gas in line  145  discharged from the boost compressor  142  can then be further cooled in the second cooling unit  136 . In one or more embodiments, the second cooling unit  136  can be configured to reduce the temperature of the pressurized recycle gas in line  145  to about 105° F. before being directed to the main compressor  104 . 
     In at least one embodiment, the temperature of the compressed recycle exhaust gas in line  144  discharged from the main compressor  104 , and consequently the temperature of the purge stream  146 , can be about 800° F., with a pressure of around 280 psia. The addition of the boost compressor  142  and the stoichiometric combustion of enriched air can increase the CO 2  purge pressure in the purge stream  146 , which can lead to improved solvent treating performance in the CO 2  separator  148  due to the higher CO 2  partial pressure. 
     Embodiments of the present disclosure can be further described with the following simulated example. Although the simulated example is directed to a specific embodiment, it is not to be viewed as limiting the disclosure in any specific respect. 
     To illustrate the superior performance of using enriched air as the compressed oxidant in line  114 , the system  100  was simulated using standard air and then using enriched air having an oxygen concentration of about 35 wt % for the same gas turbine system  102  under the same ambient conditions. The following table provides these testing results and performance estimations. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Triple - Cycle Performance Comparison 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Cycle with 
               
               
                   
                 Power (MW) 
                 Cycle with Air 
                 Enriched Air 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Gas Turbine Expander Power 
                 1234 
                 1386 
               
               
                   
                 Main Compressor 
                 511 
                 561 
               
               
                   
                 Fan or Boost Compressor 
                 18 
                 20 
               
               
                   
                 Inlet Compressor 
                 251 
                 176 
               
               
                   
                 Total Compression Power 
                 780 
                 757 
               
               
                   
                 Net Gas Turbine Power 
                 444 
                 616 
               
               
                   
                 Steam Turbine Net Power 
                 280 
                 316 
               
               
                   
                 Standard Machinery Net Power 
                 724 
                 931 
               
               
                   
                 Aux. Losses 
                 16 
                 18 
               
               
                   
                 Nitrogen Expander Power 
                 181 
                 109 
               
               
                   
                 Combined Cycle Power 
                 889 
                 1022 
               
               
                   
                 Combined Cycle Eff. (% lhv) 
                 55.6 
                 56.4 
               
               
                   
                   
               
            
           
         
       
     
     As should be apparent from Table 1, embodiments including enriched air as the compressed oxidant in line  114  can result in an increase in expander  106  power, due to an increased expansion pressure ratio and an increased mass flow through the expander  106 . Moreover, while the main compressor  104  may experience a slight increase in power demand, partially arising from the removal of a portion of the N 2  component from the air, the increase is more than offset by the reduced air compression power in the inlet compressor  118 , thereby resulting in an overall decrease in the total compression power required. As can be appreciated, because of the reduced airflow for the same amount of oxygen, the inlet compressor  118  can exhibit a considerable decrease in required compressor power. Importantly, Table 1 indicates a large increase in the combined-cycle power output which reflects about 1.0% uplift in combined-cycle efficiency. 
     While the present disclosure may be susceptible to various modifications and alternative forms, the exemplary embodiments discussed above have been shown only by way of example. However, it should again be understood that the disclosure is not intended to be limited to the particular embodiments disclosed herein. Indeed, the present disclosure includes all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.