# Patent

## Title
Systems and methods for carbon dioxide capture in low emission combined turbine systems

## Abstract

Systems, methods, and apparatus are provided for generating power in combined low emission turbine systems and capturing and recovering carbon dioxide from the exhaust. In one or more embodiments, the exhaust from multiple turbine systems is combined, cooled, compressed, and separated to yield a carbon dioxide-containing effluent stream and a nitrogen-containing product stream. Portions of the recycled exhaust streams and the product streams may be used as diluents to regulate combustion in each combustor of the turbine systems.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is the National Stage entry under 35 U.S.C. 371 of PCT/US2012/027780, that published as WO 2012/128928 and was filed on 5 Mar. 2012 which claims the benefit of U.S. Provisional Application Nos. 61/542,039, filed on 30 Sep. 2011; U.S. Provisional Application 61/466,384 filed Mar. 22, 2011; U.S. Provisional Application 61/542,030 filed Sep. 30, 2011; U.S. Provisional Application 61/466,385 filed Mar. 22, 2011; U.S. Provisional Application 61/542,031 filed Sep. 30, 2011; U.S. Provisional Application 61/466,381 filed Mar. 22, 2011; and U.S. Provisional Application 61/542,035 filed Sep. 30, 2011, each of which is incorporated by reference, in its entirety, for all purposes.
This application contains subject matter related to U.S. Provisional Application 61/542,037 filed Sep. 30, 2011 (PCT/US2012/027776, that published as WO 2012/128927 and was filed on 5 Mar. 2012); U.S. Provisional Application 61/542,041 filed Sep. 30, 2011 (PCT/US2012/027781, that published as WO 2012/128929 and was filed on 5 Mar. 2012); and U.S. Provisional Application 61/542,036 filed Sep. 30, 2011 (PCT/US2012/027774, that published as WO 2012/128926 and was filed on 5 Mar. 2012).


FIELD OF THE DISCLOSURE
Embodiments of the disclosure relate to low emission power generation. More particularly, embodiments of the disclosure relate to methods and apparatus for carbon dioxide capture for increased efficiency and reduced cost in low emission combined gas turbine systems.
BACKGROUND OF THE DISCLOSURE
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.
Many oil producing countries are experiencing strong domestic growth in power demand and have an interest in enhanced oil recovery (EOR) to improve oil recovery from their reservoirs. Two common EOR techniques include nitrogen (N2) injection for reservoir pressure maintenance and carbon dioxide (CO2) injection for miscible flooding for EOR. There is also a global concern regarding green house gas (GHG) emissions. This concern combined with the implementation of cap-and-trade policies in many countries makes reducing CO2 emissions a priority for those countries as well as for the companies that operate hydrocarbon production systems therein.
Some approaches to lower CO2 emissions include fuel de-carbonization or post-combustion capture using solvents, such as amines. However, both of these solutions are expensive and reduce power generation efficiency, resulting in lower power production, increased fuel demand and increased cost of electricity to meet domestic power demand. In particular, the presence of oxygen, SOX, and NOX components makes the use of amine solvent absorption very problematic. Another approach is an oxyfuel gas turbine in a combined cycle (e.g., where exhaust heat from the gas turbine Brayton cycle is captured to make steam and produce additional power in a Rankine cycle). However, there are no commercially available gas turbines that can operate in such a cycle and the power required to produce high purity oxygen significantly reduces the overall efficiency of the process.
Moreover, with the growing concern about global climate change and the impact of carbon dioxide emissions, emphasis has been placed on minimizing carbon dioxide emissions from power plants. Gas turbine power plants are efficient and have a lower cost compared to nuclear or coal power generation technologies. Capturing carbon dioxide from the exhaust of a gas turbine power plant is very expensive, however, because the concentration of carbon dioxide in the exhaust stack is low, a large volume of gas needs to be treated, and the pressure of the exhaust stream is low. These factors, among others, result in a high cost of carbon dioxide capture.
Accordingly, there is still a substantial need for a low emission, high efficiency power generation process with incorporated CO2 capture and recovery at a reduced cost.
SUMMARY OF THE DISCLOSURE
In the low emission power generation systems described herein, exhaust gases from low emission gas turbines, which are vented in a typical natural gas combined cycle (NGCC) plant, are instead separated and recovered. The apparatus, systems, and methods of the invention combine and recycle the exhaust streams of two gas turbines to efficiently generate power while concentrating and recovering carbon dioxide.
In systems and methods of the present invention, two gas turbine systems, each comprising a compressor, a combustion chamber, and an expander, are fluidly linked to operate in tandem and the exhaust gases from each system are combined. The combined exhaust gases are then cooled, compressed, and recycled. A portion of the recycled exhaust stream is supplied to one of the combustion chambers to act as a diluent to control or otherwise moderate the temperature of the combustion and flue gas entering the succeeding expander. The other portion of the recycled exhaust stream is separated to capture CO2 in one stream and generate a separate product stream comprising nitrogen, oxygen, argon, or combinations thereof. A portion of the product stream from the CO2 separator is then similarly supplied to the other combustion chamber to act as a diluent, while the remainder of the product stream may be used to generate additional power or for another purpose either within the system or externally. By combining the exhaust gases in this manner, the combustion products from each gas turbine within the overall system are concentrated and CO2 can be captured and removed more easily, thereby leading to more efficient power generation.



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 combined low emission power generation system comprising two fluidly linked gas turbine systems and incorporating CO2 separation.
FIG. 2 depicts a variation of the combined low emission power generation system of FIG. 1 in which the exhaust streams from each turbine are combined prior to recovering heat from the streams.
FIG. 3 depicts a variation of the combined low emission power generation system of FIG. 1 in which the heat is recovered from the exhaust streams from each turbine separately and the streams are combined after heat recovery.



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) and/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 (CH4) 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 (C2H6), higher molecular weight hydrocarbons (e.g., C3-C20 hydrocarbons), one or more acid gases (e.g., hydrogen sulfide), 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 an equivalence ratio ranging from about 0.9:1 to about 1.1:1, or more preferably from about 0.95:1 to about 1.05:1. Use of the term “stoichiometric” herein is meant to encompass both stoichiometric and substantially stoichiometric conditions unless otherwise indicated.
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 may be used to produce low emission electric power and CO2 for enhanced oil recovery (EOR) or sequestration applications. According to embodiments disclosed herein, a mixture of compressed oxidant (typically air) and fuel is combusted in the combustion chambers of two separate but fluidly linked gas turbines and the exhaust gases from each are expanded to generate power. The exhaust gases are then combined, cooled, compressed, and separated to capture CO2 and generate a product stream comprising nitrogen. In EOR applications, the recovered CO2 is injected into or adjacent to producing oil wells, usually under supercritical conditions. The CO2 acts as both a pressurizing agent and, when dissolved into the underground crude oil, significantly reduces the oil's viscosity enabling the oil to flow more rapidly through the earth to a removal well. The product stream comprising nitrogen (and frequently oxygen and argon as well) may be used to generate additional power, and may also be used for a variety of purposes, including for pressure maintenance applications. In pressure maintenance applications, an inert gas such as nitrogen is compressed and injected into a hydrocarbon reservoir to maintain the original pressure in the reservoir, thus allowing for enhanced recovery of hydrocarbons. The result of the systems disclosed herein is the production of power and the concentration and capture of CO2 in a more economically efficient manner.
In one or more embodiments of the present invention, power generation systems are provided comprising a first compressor configured to receive and compress one or more oxidants to generate a compressed oxidant; a first combustion chamber configured to receive and combust a first portion of the compressed oxidant, at least one first fuel, and a first diluent to generate a first exhaust stream; and a first expander configured to receive the first exhaust stream and generate a first gaseous exhaust stream. The systems further comprise a second compressor configured to receive and compress a cooled recycle stream to generate a compressed recycle stream; a second combustion chamber configured to receive and combust a second portion of the compressed oxidant, at least one second fuel, and a second diluent to generate a second exhaust stream; and a second expander configured to receive the second exhaust stream and generate a second gaseous exhaust stream. The systems additionally comprise one or more heat recovery steam generators configured to receive and cool the first and second gaseous exhaust streams to generate steam and a combined exhaust stream; a recycle cooling unit configured to receive and cool the combined exhaust stream and generate a cooled recycle stream; and a separator configured to receive and separate a portion of the compressed recycle stream into a separator effluent stream and a separator product stream.
The one or more oxidants may comprise any oxygen-containing fluid, such as ambient air, oxygen-enriched air, substantially pure oxygen, or combinations thereof. In one or more embodiments of the invention, all or substantially all of the oxidant supplied to the combined system as a whole is compressed in and supplied by the first compressor. The compressed oxidant exiting the first compressor may then be split into two streams, such that a first portion of the compressed oxidant is directed to the first combustion chamber and a second portion of the compressed oxidant is directed to the second combustion chamber. The split may be equal or unequal, and may also be varied during operation to adapt to changes in the process.
The first and second fuels may be the same or different, and each fuel may comprise natural gas, associated gas, diesel, fuel oil, gasified coal, coke, naphtha, butane, propane, syngas, kerosene, aviation fuel, bio-fuel, oxygenated hydrocarbon feedstock, any other suitable hydrocarbon containing gases or liquids, hydrogen, or combinations thereof. Additionally, the fuels may comprise inert components including but not limited to N2 or CO2. In some embodiments, the first and/or the second fuels may be at least partially supplied by a hydrocarbon reservoir that is benefiting from enhanced oil recovery via injection of CO2 captured via the process described herein. In certain embodiments, the first and the second fuels comprise natural gas.
The combustion conditions in each of the first and second combustion chambers may be lean, stoichiometric or substantially stoichiometric, or rich. In one or more embodiments, the combustion conditions in the first and second combustion chambers are stoichiometric or substantially stoichiometric. The first and second diluents are supplied to the first and second combustion chambers to control or otherwise regulate the temperature of the combustion and flue gas to meet the material requirements of the succeeding expanders. The flow of the first and second diluents, however, may also be adjusted to help maintain stoichiometric conditions in the respective combustion chambers, moderating changes in composition, volumetric flow, or other variations in the oxidant and fuel streams. In one or more embodiments, the first diluent provided to the first combustion chamber comprises at least a portion of the separator product stream. In the same or other embodiments, the second diluent provided to the second combustion chamber comprises at least a portion of the compressed recycle stream.
In some embodiments, high pressure steam may also be employed as a coolant in one or both of the first and second combustion chambers. In such embodiments, the addition of steam would reduce power and size requirements in the system, but would require the addition of a water recycle loop or loops. Additionally, in further embodiments, the compressed oxidant feed to one or both of the combustion chambers may comprise argon. For example, the oxidant may comprise from about 0.1 to about 5.0 vol % argon, or from about 1.0 to about 4.5 vol % argon, or from about 2.0 to about 4.0 vol % argon, or from about 2.5 to about 3.5 vol % argon, or about 3.0 vol % argon.
The first and second compressors may each be a single compressor or may be two or more compressors operating in parallel or in series. Each compressor may comprise a single stage or multiple stages. In multiple stage compressors, interstage cooling may optionally be employed to allow for higher overall compression ratios and higher overall power output. When more than one compressor is used to compress a process stream, the compressors taken together are considered herein to be the “first compressor” or the “second compressor,” as appropriate. Each compressor may be of any type suitable for the process described herein. Such compressors include, but are not limited to, axial, centrifugal, reciprocating, or twin-screw compressors and combinations thereof. In one or more embodiments, the first and second compressors are axial compressors.
Combustion of the oxidant and fuel in each of the first and second combustion chambers generates a first exhaust stream and a second exhaust stream, respectively. The first and second exhaust streams comprise products of combustion, and their individual compositions will vary depending upon the composition of the fuel and the oxidant used in each combustion chamber. In one or more embodiments, the first and second exhaust streams may each comprise vaporized water, CO2, carbon monoxide (CO), oxygen (O2), nitrogen (N2), argon (Ar), nitrogen oxides (NOX), sulfur oxides (SOX), hydrogen sulfide (H2S), or combinations thereof.
The first exhaust stream may be expanded in the first expander to form a first gaseous exhaust stream, and the second exhaust stream may be expanded in the second expander to form a second gaseous exhaust stream. The first and second expanders may each be a single expander or may be two or more expanders operating in parallel or in series. Each expander may comprise a single stage or multiple stages. When more than one expander is used to expand an exhaust stream, the expanders taken together are considered herein to be the “first expander” or the “second expander,” as appropriate. Each expander may be any type of expander suitable for the process described herein, including but not limited to axial or centrifugal expanders or combinations thereof. Expansion of the first and second exhaust streams generates power, which may be used to drive one or more compressors or electric generators. In one or more embodiments of the invention, the first expander is coupled to the first compressor via a common shaft or other mechanical, electrical, or other power coupling, such that the first compressor is at least partially driven by the first expander. In the same or other embodiments, the second expander is coupled to the second compressor via a common shaft or other mechanical, electrical, or other power coupling, such that the second compressor is at least partially driven by the second expander. In other embodiments, the first or the second compressors, or both, may be mechanically coupled to an electric motor with or without a speed increasing or decreasing device such as a gear box. When taken together, the first compressor, first combustion chamber, and first expander may be characterized as a Brayton cycle. Similarly, the second compressor, second combustion chamber, and second expander may also be characterized as a Brayton cycle.
After expansion, the first and second gaseous exhaust streams may in some embodiments be cooled in a heat recovery steam generator (HRSG). Although reference is made herein to a single HRSG, in practice two or more HRSGs may be used. Alternatively, any device designed to cool or recover heat from the first and second gaseous exhaust streams, such as for example one or more heat exchangers, regenerators, cooling units, or the like may be used in place of the HRSG. The HRSG may be configured to receive the first and second exhaust streams and utilize the residual heat in the streams to generate steam. The steam generated by the HRSG may be used for a variety of purposes, such as to drive a steam turbine generator in a Rankine cycle or for water desalination.
In one or more embodiments of the present invention, the first and second gaseous exhaust streams are fed to the HRSG separately and combined within the HRSG to form a combined exhaust stream. In other embodiments, the first and second gaseous exhaust streams may be combined upstream or downstream of the HRSG. When the streams are combined downstream, each of the first and second gaseous exhaust streams may be cooled in a separate HRSG and then combined to form the combined exhaust stream.
In one or more embodiments of the present invention, the combined exhaust stream may be sent to one or more cooling units configured to reduce the temperature of the combined exhaust stream and generate a cooled recycle stream. The cooling unit may be any type of apparatus suitable for lowering the temperature of the exhaust gases, such as for example a direct contact cooler (DCC), trim cooler, mechanical refrigeration unit, or combinations thereof. In some embodiments, the cooling unit is a DCC. The cooling unit can also be configured to remove a portion of condensed water from the cooled recycle stream via a water dropout stream. In some embodiments, the water dropout stream may optionally be routed to a HRSG to provide a water source for the generation of additional steam.
In some embodiments, the cooled recycle stream exiting the cooling unit may be sent to the second compressor and compressed to generate a compressed recycle stream. Cooling and compressing the exhaust gases helps to address issues related to the large volume of gas that must be treated and the low pressure of the exhaust streams that ordinarily lead to a high cost of CO2 capture, thus making CO2 capture and recovery in the present systems more efficient and more cost effective.
Upon exiting the second compressor, the compressed recycle stream may be split, and a first portion of the compressed recycle stream may be provided to the second combustion chamber for use as a diluent, as described previously. In such cases, the second portion of the compressed recycle stream not directed to the second combustion chamber may be fed to one or more separators, in which CO2 and other greenhouse gases are separated from the compressed recycle stream. Although not described in detail herein, it will be recognized by those skilled in the art that intermediate heating, cooling, or other process operations may be required between the second compressor and the separator, so that the recycle stream enters the separator at conditions optimized for the particular separation process employed. In one or more embodiments, for example, a cooling unit may be used to cool the compressed recycle stream before it enters the separator.
The CO2 separation process employed in the one or more separators may be any suitable process designed to separate the compressed recycle gases and result in a separator effluent stream comprising CO2 and a separator product stream comprising nitrogen. In some embodiments, the product stream may also comprise oxygen, argon, or both. Separating the components of the compressed recycle stream allows different components in the exhaust to be handled in different ways. Ideally, the separation process would segregate all of the greenhouse gases in the exhaust, such as CO2, CO, NOX, SOX, etc. in the effluent stream, leaving the remainder of the exhaust components such as nitrogen, oxygen, and argon in the product stream. In practice, however, the separation process may not withdraw all of the greenhouse gases from the product stream, and some non-greenhouse gases may remain in the effluent stream.
Any suitable separation process designed to achieve the desired result may be used. In one or more embodiments, the separation process is an oxygen-insensitive process. Examples of suitable separation processes include, but are not limited to, hot potassium carbonate (“hot pot”) separation processes, amine separation, molecular sieve separation, membrane separation, adsorptive kinetic separation, controlled freeze zone separation, and combinations thereof. In some embodiments, the separator uses a hot pot separation process. In one or more embodiments of the invention, the separation process may be operated at elevated pressure (i.e., higher than ambient) and configured to keep the product stream pressurized. Maintaining pressure on the process in this manner allows for smaller separation equipment, provides for improved separation effectiveness, and allows for increased energy extraction from the product stream. In some embodiments, the CO2 separation process is selected and configured to maximize either the outlet pressure or the outlet temperature, or both, of the product stream.
The separator effluent stream may be used for a variety of applications. For example, the effluent stream may be injected into a hydrocarbon reservoir for enhanced oil recovery (EOR) or may be directed to a reservoir for carbon sequestration or storage. The separator effluent stream may also be sold, vented, or flared.
In one or more embodiments, the separator product stream comprises nitrogen and also optionally comprises oxygen or argon (or both). The separator product stream may be split, and a first portion of the product stream may be provided to the first combustion chamber for use as a diluent, as described previously. In such cases, the second portion of the separator product stream not directed to the first combustion chamber may be directed to a third expander. In one or more embodiments, the third expander may be configured to receive the separator product stream and output the same gases at approximately ambient pressure. As will be appreciated by those skilled in the art, the third expander generates power, and the power generated may be used to drive one or more compressors or electric generators in any configuration, either within the described system or externally.
In one or more embodiments, the product stream may pass through one or more additional heat recovery steam generators (HRSGs) after expansion. The one or more HRSGs may be configured to utilize the residual heat in the stream to generate steam. The steam generated by the one or more HRSGs may be used for a variety of purposes, such as to drive a steam turbine generator in a Rankine cycle or for water desalination. Further, if any residual heat remains in the product stream exiting the one or more HRSGs, the system may further comprise one or more heat exchangers configured to transfer that heat to a non-steam working fluid. In such embodiments, the non-steam working fluid may optionally be used to drive an expander in a Rankine cycle.
The separator product stream may be used, wholly or in part, for a variety of applications. For example, the product stream may be injected into a hydrocarbon reservoir for pressure maintenance. The product stream may also be sold or vented. In one or more embodiments when pressure maintenance is not a viable option (or when only a portion of the product stream is required for pressure maintenance), the product stream may be cooled, by expansion or another method, and used to provide refrigeration in the systems described herein. For example, the cooled product stream may be used to provide refrigeration to reduce the suction temperature of one or more compressors within the system, or to chill water for use in one or more cooling units within the system.
In other embodiments when all or part of the product stream is not used for pressure maintenance, the product stream may instead be heated so that additional power may be generated for use elsewhere in the system or for sale. Some methods of heating the product stream include cross-exchanging the product stream with another process stream in a heat exchanger or using a supplementary combustor to supply additional heat to the product stream. It will be appreciated that the use of an additional combustor will require additional fuel. If a carbon-containing fuel is used in the combustor, additional CO2 will be generated that will be unrecoverable from the product stream. Therefore, in some embodiments, the fuel used in the combustor may be a non-carbon fuel source, such as hydrogen. The oxidant required by the supplementary combustor may be supplied via a separate oxidant stream, or there may be sufficient oxidant in the product stream such that an additional supply of oxidant is unnecessary. Other possible methods for heating the separator product stream include using a heating coil in the HRSG to heat the product stream, using catalysis to combust any CO present in the product stream, or heating the stream as a consequence of using the product stream for cooling (i.e., as the product stream provides cooling to other streams or apparatus, the stream itself is heated).
Referring now to the figures, FIG. 1 illustrates a combined power generation system 100 configured to provide separation and capture of CO2 after combustion. In at least one embodiment, the power generation system 100 can have a first compressor 118 coupled to a first expander 106 through a common shaft 108 or other mechanical, electrical, or other power coupling, thereby allowing a portion of the mechanical energy generated by the first expander 106 to drive the first compressor 118. The first expander 106 may generate power for other uses as well, such as to power another compressor, an electric generator, or the like. The first compressor 118 and first expander 106 may form the compressor and expander ends, respectively, of a standard gas turbine. In other embodiments, however, the first compressor 118 and first expander 106 can be individualized components in a system.
The system 100 can also include a first combustion chamber 110 configured to combust a first fuel stream 112 mixed with a first portion of compressed oxidant 114. In one or more embodiments, the first fuel stream 112 can include any suitable hydrocarbon gas or liquid, such as natural gas, methane, naphtha, butane, propane, syngas, diesel, kerosene, aviation fuel, coal derived fuel, bio-fuel, oxygenated hydrocarbon feedstock, or combinations thereof. The first fuel stream 112 may also comprise hydrogen. The first portion of compressed oxidant 114 supplied to the first combustion chamber 110 can be derived from the first compressor 118 fluidly coupled to the first combustion chamber 110 and adapted to compress a feed oxidant 120. While the discussion herein assumes that the feed oxidant 120 is ambient air, the oxidant may comprise any suitable gas containing oxygen, such as air, oxygen-rich air, substantially pure oxygen, or combinations thereof. In one or more embodiments, the first compressor 118, the first combustion chamber 110, and the first expander 106, taken together, can be characterized as a Brayton cycle. In one embodiment, the system is operated by mixing ambient air with a portion of recycle gas and compressing the mixture in a main air compressor (MAC). The high pressure mixture of air and recycle gas is mixed and combusted with fuel gas and additional high pressure recycle gas in a gas turbine combustor to produce high pressure products of combustion that are delivered to the gas turbine expander. A portion of the high pressure recycle gas may optionally be extracted from recycle gas compressor at various location and pressure levels and used to cool some parts of a gas turbine expander. The gas turbine expander exhaust is measured to determine the content of one or more components, e.g., oxygen, carbon dioxide, UBH, etc. by one or more sensors and the result of this measurement is used to adjust the position of a metering valve to control the mixture of ambient air and the portion of recycle gas at the inlet of the MAC. As a result, the oxygen content of the stream may be actively controlled to affect the desired stoichiometry of the combustion.
A first exhaust stream 116 is generated as a product of combustion of the first fuel stream 112 and the first portion of compressed oxidant 114 and directed to the inlet of the first expander 106. In at least one embodiment, the first fuel stream 112 can be primarily natural gas, thereby generating a first exhaust stream 116 including volumetric portions of vaporized water, CO2, CO, oxygen, nitrogen, argon, nitrogen oxides (NOX), and sulfur oxides (SOX). In some embodiments, a small portion of unburned first fuel 112 or other compounds may also be present in the first exhaust 116 due to combustion equilibrium limitations. As the first exhaust stream 116 expands through the first expander 106, it generates mechanical power to drive the first compressor 118 or other facilities, and also produces a first gaseous exhaust stream 122.
The power generation system 100 may also have a second compressor 180 coupled to a second expander 170 through a common shaft 188 or other mechanical, electrical, or other power coupling, thereby allowing a portion of the mechanical energy generated by the second expander 170 to drive the second compressor 180. The second expander 170 may generate power for other uses as well, such as to power another compressor, an electric generator, or the like. The second compressor 180 and second expander 170 may form the compressor and expander ends, respectively, of a standard gas turbine. In other embodiments, however, the second compressor 180 and second expander 170 can be individualized components in a system.
The system 100 can also include a second combustion chamber 150 configured to combust a second fuel stream 152 mixed with a second portion of compressed oxidant 114. In one or more embodiments, the second fuel stream 152 can include any suitable hydrocarbon gas or liquid, such as natural gas, methane, naphtha, butane, propane, syngas, diesel, kerosene, aviation fuel, coal derived fuel, bio-fuel, oxygenated hydrocarbon feedstock, or combinations thereof. The second fuel stream 152 may also comprise hydrogen. The second portion of compressed oxidant 114 supplied to the second combustion chamber 150 can be derived from the first compressor 118 fluidly coupled to the second combustion chamber 150. In one or more embodiments, the second compressor 180, the second combustion chamber 150, and the second expander 170, taken together, can be characterized as a Brayton cycle.
A second exhaust stream 156 is generated as a product of combustion of the second fuel stream 152 and the second portion of compressed oxidant 114 and directed to the inlet of the second expander 170. In at least one embodiment, the second fuel stream 152 can be primarily natural gas, thereby generating a second exhaust stream 156 including volumetric portions of vaporized water, CO2, CO, oxygen, nitrogen, argon, nitrogen oxides (NOX), and sulfur oxides (SOX). In some embodiments, a small portion of unburned second fuel 152 or other compounds may also be present in the second exhaust 156 due to combustion equilibrium limitations. As the second exhaust stream 156 expands through the second expander 170, it generates mechanical power to drive the second compressor 180 or other facilities, and also produces a second gaseous exhaust stream 172.
From the first expander 106 and the second expander 170, the first and second gaseous exhaust streams 122 and 172, respectively, are directed to a heat recovery steam generator (HRSG) 126 configured to use the residual heat in the gaseous exhaust streams 122 and 172 to generate steam 130 and combined exhaust stream 132. The steam 130 generated by the HRSG 126 may have a variety of uses, such as for example to generate additional power by driving a steam turbine generator in a Rankine cycle or for water desalination.
The combined exhaust 132 can be sent to at least one cooling unit 134 configured to reduce the temperature of the combined exhaust 132 and generate a cooled recycle stream 140. In one or more embodiments, the cooling unit 134 is considered herein to be a direct contact cooler (DCC), but may be any suitable cooling device such as a direct contact cooler, trim cooler, a mechanical refrigeration unit, or combinations thereof. The cooling unit 134 can also be configured to remove a portion of condensed water via a water dropout stream 136.
In one or more embodiments, the cooled recycle stream 140 can be directed to the second compressor 180 fluidly coupled to the cooling unit 134. The second compressor 180 can be configured to increase the pressure of the cooled recycle stream 140, thereby generating a compressed recycle stream 182. Upon exiting the second compressor 180, the compressed recycle stream 182 may be split, such that a portion of the compressed recycle stream 182 is directed to the second combustion chamber 150, where it may serve as a diluent to regulate combustion conditions within the second combustion chamber 150.
In one or more embodiments, the remaining portion of compressed recycle stream 182 not directed to the second combustion chamber is directed to a separator 162. The separator 162 may employ any of a variety of separation processes designed to separate the compressed recycle stream 182 into a separator effluent stream 166 comprising CO2 and a separator product stream 164 generally comprising nitrogen and, in some cases, oxygen and/or argon. For example, the separator 162 may be designed to separate the compressed recycle stream 182 using a chemical separation process, such as hot potassium carbonate (“hot pot”) separation, amine separation, or separation using a molecular sieve. Other separation processes include physical separation using membranes, or processes such as adsorptive kinetic separation or controlled freeze zone separation. In some embodiments, combinations of the foregoing separation methods may be used. In one or more embodiments, the CO2 separation process may be configured to maximize the temperature or the pressure of the product stream 164. The separator effluent stream 166 may be used for a variety of downstream applications, such as injection into a hydrocarbon reservoir for enhanced oil recovery (EOR), carbon sequestration, storage, or sale. The effluent stream 166 may also be vented or flared.
In one or more embodiments, a portion of the product stream 164 exiting the separator 162 may be diverted and directed to the first combustion chamber 110 in recycle product stream 184. In this manner, recycle product stream 184 may serve as a diluent to regulate combustion conditions within the first combustion chamber 110. The remainder of product stream 164 may then be used to generate additional power. For example, product stream 164 may be directed to a third expander 190. The power generated by the third expander 190 may be used for a variety of purposes, such as to at least partially drive one or more additional compressors (not shown) or to drive an electric generator. In some embodiments, when the product stream is injected into a reservoir for pressure maintenance, the third expander 190 may be used to drive a pipeline or injection compressor.
In one or more embodiments, the expanded product stream 192 exiting the third expander 190 may be directed to a heat recovery steam generator (not shown) for additional power generation. The product stream 192, like the effluent stream 166, may also be used for a variety of other applications, including pressure maintenance, storage, or venting.
Referring now to FIG. 2, depicted is an alternative configuration of the power generation system 100 of FIG. 1, embodied and described as system 200. As such, FIG. 2 may be best understood with reference to FIG. 1. In system 200 of FIG. 2, first gaseous exhaust stream 122 and second gaseous exhaust stream 172 are combined upstream of the HRSG 126 to form combined exhaust stream 132. The combined exhaust stream 132 is then directed to the HRSG 126 to cool the combined exhaust stream 132 and generate steam 130, and the remainder of the system operates as previously described with respect to FIG. 1.
Referring now to FIG. 3, depicted is an alternative configuration of the power generation system 100 of FIG. 1, embodied and described as system 300. In system 300 of FIG. 3, each of the gaseous exhaust streams is cooled separately before being combined. Specifically, the first gaseous exhaust stream 122 is cooled in the HRSG 126, while the second gaseous exhaust stream 172 is cooled in an additional HRSG 326. First and second gaseous exhaust streams 122 and 172 exit HRSGs 126 and 326, respectively, and are then combined downstream of the HRSGs to form combined exhaust stream 132. The remainder of the system operates as previously described with respect to FIG. 1.
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. Any features or configurations of any embodiment described herein may be combined with any other embodiment or with multiple other embodiments (to the extent feasible) and all such combinations are intended to be within the scope of the present invention. Additionally, it should 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.

## Claims

1. A method for generating power comprising:
compressing one or more oxidants in a first compressor to generate a compressed oxidant;
supplying a first portion of the compressed oxidant, at least one first fuel, and a first diluent to a first combustion chamber;
substantially stoichiometrically combusting the first portion of the compressed oxidant and the at least one first fuel in the first combustion chamber to generate a first exhaust stream;
expanding the first exhaust stream in a first expander to generate a first gaseous exhaust stream;
using one or more sensors to measure one or more components of the first gaseous exhaust stream;
controlling a flow of the first portion of the compressed oxidant into the first combustion chamber by adjusting a metering valve based on one or more measurements from the one or more sensors in order to maintain substantially stoichiometric combustion conditions in the first combustion chamber, wherein substantially stoichiometric combustion is a combustion reaction having an equivalence ratio in a range of 0.9:1 to 1.1:1;
compressing a cooled recycle stream in a second compressor to generate a compressed recycle stream;
supplying a second portion of the compressed oxidant, at least one second fuel, and a second diluent to a second combustion chamber;
combusting the second portion of the compressed oxidant and the at least one second fuel in the second combustion chamber to generate a second exhaust stream;
expanding the second exhaust stream in a second expander to generate a second gaseous exhaust stream;
recovering heat from the first and second gaseous exhaust streams;
combining the first and second gaseous exhaust streams to form a combined exhaust stream;
cooling the combined exhaust stream to generate the cooled recycle stream;
splitting the compressed recycle stream into a first portion and a second portion of the compressed recycle stream; and
separating the first portion of the compressed recycle stream to generate an effluent stream primarily comprising carbon dioxide and a product stream primarily comprising at least one of nitrogen, oxygen, and argon;
wherein the first diluent comprises a first portion of the product stream and the second diluent comprises at least a portion of the second portion of the compressed recycle stream.

2. The method of claim 1, wherein heat is recovered from the first gaseous exhaust stream, the second gaseous exhaust stream, or both in one or more heat recovery steam generators.

3. The method of claim 1, wherein the first and second gaseous exhaust streams are combined before recovering heat from the first and second gaseous exhaust streams.

4. The method of claim 1, wherein the first and second gaseous exhaust streams are combined after recovering heat from the first and second gaseous exhaust streams.

5. The method of claim 1, wherein the combined exhaust stream is cooled in a direct contact cooler.

6. The method of claim 1, wherein the first portion of the compressed recycle stream is separated using a process selected from a hot potassium carbonate separation process, an amine separation process, a molecular sieve separation process, a membrane separation process, an adsorptive kinetic separation process, a controlled freeze zone separation process, or combinations thereof.

7. The method of claim 6, wherein the first portion of the compressed recycle stream is separated using the hot potassium carbonate separation process.

8. The method of claim 1, further comprising compressing at least a portion of the effluent stream to generate a compressed effluent stream and injecting the compressed effluent stream into a hydrocarbon reservoir for enhanced oil recovery.

9. The method of claim 1, further comprising injecting a second portion of the product stream into a hydrocarbon reservoir for pressure maintenance.

10. The method of claim 1, further comprising expanding a second portion of the product stream to generate power.

11. The method of claim 10, further comprising heating the second portion of the product stream prior to expansion.

12. The method of claim 1, further comprising cooling the first portion of the compressed recycle stream prior to separation.

13. A power generation system comprising:
a first compressor configured to receive and compress one or more oxidants to generate a compressed oxidant;
a first combustion chamber configured to receive and substantially stoichiometrically combust a first portion of the compressed oxidant, at least one first fuel, and a first diluent to generate a first exhaust stream;
a first expander configured to receive the first exhaust stream from the first combustion chamber and generate a first gaseous exhaust stream;
one or more sensors used to measure one or more components of the first gaseous exhaust stream;
a metering valve configured to be adjusted to control a flow of the first portion of the compressed oxidant into the first combustion chamber based on one or more measurements from the one or more sensors in order to maintain substantially stoichiometric combustion conditions in the first combustion chamber, wherein substantially stoichiometric combustion is a combustion reaction having an equivalence ratio in a range of 0.9:1 to 1.1:1;
a second compressor configured to receive and compress a cooled recycle stream to generate a compressed recycle stream;
a second combustion chamber configured to receive and combust a second portion of the compressed oxidant, at least one second fuel, and a second diluent to generate a second exhaust stream;
a second expander configured to receive the second exhaust stream from the second combustion chamber and generate a second gaseous exhaust stream;
a heat recovery steam generator configured to receive and cool the first and second gaseous exhaust streams to generate a combined exhaust stream and steam;
a cooling unit configured to receive and cool the combined exhaust stream and generate the cooled recycle stream; and
a separator configured to receive and separate a first portion of the compressed recycle stream into a separator effluent stream primarily comprising carbon dioxide and a separator product stream primarily comprising at least one of nitrogen, oxygen, and argon;
wherein the compressed recycle stream is split into the first portion and a second portion upstream of the separator, the first diluent comprises a first portion of the separator product stream, and the second diluent comprises at least a portion of the second portion of the compressed recycle stream.

14. The power generation system of claim 13, wherein the cooling unit is a direct contact cooler.

15. The power generation system of claim 13, wherein the separator uses a separation process selected from a hot potassium carbonate separation process, an amine separation process, a molecular sieve separation process, a membrane separation process, an adsorptive kinetic separation process, a controlled freeze zone separation process, or combinations thereof.

16. The power generation system of claim 15, wherein the separator uses the hot potassium carbonate separation process.

17. The power generation system of claim 13, wherein the separator effluent stream is used for enhanced oil recovery in a hydrocarbon reservoir.

18. The power generation system of claim 13, wherein the separator product stream is used for pressure maintenance in a hydrocarbon reservoir.

19. The power generation system of claim 13, further comprising a third expander configured to a second at least a portion of the separator product stream.

20. The power generation system of claim 19, further comprising a product heating unit configured to receive and heat the second portion of the separator product stream directed to the third expander.

21. The power generation system of claim 13, further comprising a separator cooling unit configured to receive and cool first the portion of the compressed recycle stream directed to the separator.