Patent Publication Number: US-2017348638-A1

Title: System and method of reducing oxygen concentration in an exhaust gas stream

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part and claims priority to U.S. patent application Ser. No. 15/171,775, filed Jun. 2, 2016 for “SYSTEM AND METHOD OF RECOVERING CARBON DIOXIDE FROM AN EXHAUST GAS STREAM”, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates generally to reducing emissions from power plant exhaust and, more specifically, to systems and methods of reducing emissions by scavenging oxygen from an exhaust gas stream. 
     Power generating processes that are based on combustion of carbon-containing fuel produce carbon dioxide as a byproduct. Typically, the carbon dioxide is one component of a mixture of gases that results from, or passes unchanged through, the combustion process. It may be desirable to capture or otherwise remove the carbon dioxide and other components of the gas mixture to prevent the release of the carbon dioxide and other components into the environment or to use the carbon dioxide for industrial purposes. 
     To achieve complete combustion of fuel some amount of air or oxygen in excess of stoichiometric is charged to the combustion chamber. The excess oxygen is contained in the exhaust gas. The oxygen concentration in the mixture of gases resulting from the combustion process is typically controlled, or reduced, when carbon dioxide is intended for use in industrial applications. One known method of scavenging oxygen in an exhaust gas stream is in cryogenic distillation separation process. However, the equipment used to facilitate cryogenic distillation typically has a large physical footprint and may require a significant capital investment to implement. 
     BRIEF DESCRIPTION 
     In one aspect, an oxygen scavenging system is provided. The system includes a first catalytic converter unit configured to receive an exhaust stream from a power production unit. The exhaust stream includes oxygen. The system also includes a hydrocarbon injection unit configured to channel a hydrocarbon stream for injection into the exhaust stream upstream from the first catalytic converter unit such that hydrocarbons from the hydrocarbon stream react with the oxygen from the exhaust stream within the first catalytic converter unit. 
     In another aspect, a method of reducing oxygen concentration in an exhaust stream is provided. The method includes channeling an exhaust stream towards a first catalytic converter unit. The exhaust stream includes oxygen. The method further includes injecting a hydrocarbon stream into the exhaust stream upstream from the first catalytic converter unit such that a mixed exhaust stream is formed, and channeling the mixed exhaust stream into the first catalytic converter unit such that hydrocarbons from the hydrocarbon stream react with the oxygen from the exhaust stream. 
     In yet another aspect, an oxygen scavenging system is provided. The system includes a first catalytic converter unit configured to receive an exhaust stream from a power production unit, wherein the exhaust stream includes oxygen. A second catalytic converter unit is positioned downstream from the first catalytic converter unit, wherein the second catalytic converter unit is configured to receive a treated exhaust stream discharged from the first catalytic converter unit. A hydrocarbon injection unit is configured to channel a hydrocarbon stream for injection into the treated exhaust stream upstream from the second catalytic converter unit such that hydrocarbons from the hydrocarbon stream react with the oxygen from the treated exhaust stream within the second catalytic converter unit. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a schematic diagram of an exemplary system for use in recovering carbon dioxide from an exhaust gas stream; 
         FIG. 2  is a schematic diagram of an alternative system for use in recovering carbon dioxide from the exhaust gas stream; 
         FIG. 3  is a schematic diagram of another alternative system for use in recovering carbon dioxide from the exhaust gas stream; 
         FIG. 4  is a perspective view of a transport apparatus; 
         FIG. 5  is a schematic diagram of an exemplary scavenging system for use in scavenging oxygen from the exhaust gas stream shown in  FIG. 1 ; and 
         FIG. 6  is a schematic diagram of an alternative scavenging system for use in scavenging oxygen from the exhaust gas stream shown in  FIG. 1 . 
     
    
    
     Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of the disclosure. These features are believed to be applicable in a wide variety of systems comprising one or more embodiments of the disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein. 
     DETAILED DESCRIPTION 
     In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings. 
     The singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged. Such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 
     Embodiments of the present disclosure relate to systems and methods of reducing emissions by recovering carbon dioxide from an exhaust gas stream. In the exemplary embodiment, a turboexpander compresses the exhaust gas stream and a carbon dioxide membrane selectively removes carbon dioxide from the compressed exhaust gas stream. More specifically, the exhaust gas stream is produced by a power generation unit and is received by a first heat exchanger configured to exchange heat between the exhaust gas stream and a lean carbon dioxide stream. The cooled exhaust gas stream is compressed by a compressor which is driven by a turbine as part of a turboexpander. The compressed exhaust gas stream is channeled to the carbon dioxide membrane which selectively removes carbon dioxide from the compressed exhaust gas stream to produce the lean carbon dioxide stream and a rich carbon dioxide stream. The rich carbon dioxide stream is channeled to a cryogenic separation unit which further refines the rich carbon dioxide stream into a carbon dioxide product stream. The lean carbon dioxide stream is channeled to the first heat exchanger to recover energy from the exhaust gas stream. The lean carbon dioxide stream is channeled to the turbine where it is expanded and drives the compressor. The energy recovered from the exhaust gas stream by the lean carbon dioxide stream is used to drive the compressor in the turboexpander. Using the recovered energy to drive the compression needed to separate carbon dioxide from the exhaust gas stream reduces the energy consumption (kilowatt-hour (kWh) (British Thermal Unit (BTU))) per unit mass (kilogram (kg) (pound (lb))) of carbon dioxide recovered of the process. As such, the systems and methods described herein embody the process changes and equipment for use in recovering carbon dioxide from a carbon dioxide-rich gas stream using a carbon dioxide membrane and a turboexpander to reduce the energy consumption per unit of carbon dioxide recovered of the process. The system and methods described herein reduces energy consumption per unit mass of carbon dioxide recovered by 0.33 kWh/kg (510.75 BTU/lb). The system and methods described herein also reduces the capital cost of the system by 15 percent to 30 percent because an engine or motor is no longer needed to drive the exhaust gas compressor. 
       FIG. 1  is a schematic diagram of an exemplary recovery system  100  for use in recovering carbon dioxide from an exhaust gas stream. In the exemplary embodiment, a power production unit  102  is coupled in flow communication with recovery system  100 . Non-limiting examples of power production unit  102  include internal combustion engines, gas turbine engines, gasifiers, landfills which produce energy through combustion, furnaces (e.g., blast furnaces or chemical reduction furnaces), steam generators, rich burn reciprocating engines, simple cycle combustion turbines with exhaust gas recycle, boilers, combinations including at least two of the foregoing examples, or any other unit which produces energy by combustion. In one embodiment, power production unit  102  includes a reciprocating engine at a gas pipeline booster station. In another embodiment, power production unit  102  includes a portable power production generator. 
     Power production unit  102  receives fuel from a fuel stream  104 . Fuel stream  104  delivers a carbon rich fuel to power production unit  102 . Non-limiting examples of a carbon rich fuel delivered by fuel stream  104  include natural gas, liquefied natural gas, gasoline, jet fuel, coal, or any other carbon rich fuel that enables power production unit  102  to function as described herein. Power production unit  102  receives air from an air stream  106 . Power production unit  102  oxidizes fuel from fuel stream  104  with oxygen from air stream  106  to produce electricity and an exhaust gas stream  108 . Oxidation of carbon rich fuels produces, among many other byproducts, water and carbon dioxide. Exhaust gas stream  108  generally includes about 12 percent by volume carbon dioxide. However, exhaust gas stream  108  may include a range of concentrations of carbon dioxide ranging from about 7 percent by volume to about 15 percent by volume. Additionally, the temperature of exhaust gas stream  108  is generally 500 degrees Celsius (° C.) (932 degrees Fahrenheit (° F.)) or higher. However, the temperature of exhaust gas stream  108  may include any temperature which enables recovery system  100  to operate as described herein. The high concentration of carbon dioxide in exhaust gas stream  108  enables membrane separation of the carbon dioxide from the rest of exhaust gas stream  108 . Additionally, the high temperature of exhaust gas stream  108  provides thermal energy to drive a turboexpander. Carbon dioxide is useful for other industrial applications such as, but not limited to, enhanced oil recovery, tight oil and gas fracturing, hydrogen production, ammonia production and fermentation. Recovery system  100  captures exhaust gas carbon dioxide for use in other industrial applications. 
     Recovery system  100  includes a first heat exchanger  110 , a turboexpander  112 , a second heat exchanger  113 , and a carbon dioxide membrane unit  114 . Turboexpander  112  includes a compressor  116  drivingly coupled to a turbine  118  by a shaft  120 . Compressor  116  is a centrifugal compressor driven by turbine  118  through shaft  120 . First heat exchanger  110  is coupled in flow communication with power production unit  102 , carbon dioxide membrane unit  114 , compressor  116 , and turbine  118 . Second heat exchanger  113  is coupled in flow communication with carbon dioxide membrane unit  114 , compressor  116 , and a cooling water system (not shown). First and second heat exchangers  110  and  113  are configured to exchange heat between two streams. Non-limiting examples of first and second heat exchangers  110  and  113  include shell and tube heat exchangers, plate and frame heat exchangers, or any other heat exchanger which enables first and second heat exchangers  110  and  113  to function as described herein. Turbine  118  and carbon dioxide membrane unit  114  both produce product streams. 
     During operation, first heat exchanger  110  receives exhaust gas stream  108  from power production unit  102  and a lean carbon dioxide stream  122  from carbon dioxide membrane unit  114 . First heat exchanger  110  exchanges heat between exhaust gas stream  108  and lean carbon dioxide stream  122 . Exhaust gas stream  108  is reduced in temperature to produce a cooled exhaust gas stream  124  and lean carbon dioxide stream  122  is increased in temperature to produce a heated lean carbon dioxide stream  126 . Compressor  116  and carbon dioxide membrane unit  114  require the temperature of exhaust gas stream  108  to be reduced to operate safely. As such, first heat exchanger  110  recovers energy from exhaust gas stream  108  and protects compressor  116  and carbon dioxide membrane unit  114 . During cooling, some water entrained in exhaust gas stream  108  may separate from exhaust gas stream  108  by condensation. In the exemplary embodiment, the concentration of carbon dioxide in cooled exhaust gas stream  124  after water has condensed out of the stream is about  14  percent by volume. 
     Compressor  116  receives cooled exhaust gas stream  124  from first heat exchanger  110 . The pressure of cooled exhaust gas stream  124  is atmospheric pressure or approximately 101 kilopascals absolute (kPa) (14.7 pounds per square inch absolute (psia)). Carbon dioxide membrane unit  114  requires an increased pressure to selectively remove carbon dioxide. In the exemplary embodiment, carbon dioxide membrane unit  114  requires the pressure of cooled exhaust gas stream  124  to be increase to approximately 483 kPa (70 psia). Compressor  116  compresses cooled exhaust gas stream  124  to approximately 483 kPa (70 psia) to produce a compressed exhaust gas stream  128 . 
     Turbine  118  receives heated lean carbon dioxide stream  126  from first heat exchanger  110 . Turbine  118  expands heated lean carbon dioxide stream  126  and rotates shaft  120 . Shaft  120 , in turn, rotates compressor  116  and compresses cooled exhaust gas stream  124 . As such, turbine  118  recovers the energy recovered from exhaust gas stream  108  and uses the recovered energy to power compressor  116 . Using recovered energy to power compressor  116  saves energy and reduces the energy consumption per unit of carbon dioxide recovered by recovery system  100 . Turbine  118  produces an expanded lean carbon dioxide stream  130  which is discharged to the atmosphere. 
     Second heat exchanger  113  receives compressed exhaust gas stream  128  from compressor  116 . Second heat exchanger  113  exchanges heat between compressed exhaust gas stream  128  and a cooling fluid  129 . In the exemplary embodiment, cooling fluid  129  includes cooling water from a cooling water system (not shown). Cooling fluid  129  may be any fluid which enables recovery system  100  to function as described herein. Compressed exhaust gas stream  128  is reduced in temperature to produce a cooled compressed exhaust gas stream  131 . During compression, the heat of compression from compressor  116  increases the temperature of compressed exhaust gas stream  128 . Carbon dioxide membrane unit  114  requires the temperature of compressed exhaust gas stream  128  to be reduced to operate safely. As such, second heat exchanger  113  cools compressed exhaust gas stream  128  to protect carbon dioxide membrane unit  114 . 
     Carbon dioxide membrane unit  114  receives cooled compressed exhaust gas stream  131  from second heat exchanger  113 . Carbon dioxide membrane unit  114  selectively removes carbon dioxide from cooled compressed exhaust gas stream  131  to produce a rich carbon dioxide stream  132  and lean carbon dioxide stream  122 . Rich carbon dioxide stream  132  includes more carbon dioxide than lean carbon dioxide stream  122 . In the exemplary embodiment, cooled compressed exhaust gas stream  131  enters carbon dioxide membrane unit  114  with about  20  percent by volume carbon dioxide. Rich carbon dioxide stream  132  leaves carbon dioxide membrane unit  114  with about  70  percent by volume carbon dioxide and lean carbon dioxide stream  122  leaves carbon dioxide membrane unit  114  with about  5  percent by volume carbon dioxide. Rich carbon dioxide gas  132  may be the final product or may be further refined as shown in  FIG. 2 . 
     Carbon dioxide membrane unit  114  includes a plurality of carbon dioxide selective membranes (not shown). Carbon dioxide passes through walls of the carbon dioxide selective membranes to an enclosed area (not shown) on the other side of the carbon dioxide selective membranes, while cooled compressed exhaust gas stream  131  continues through carbon dioxide membrane unit  114 . The membrane(s) are carbon dioxide selective and thus continuously remove the carbon dioxide produced, including carbon dioxide which is optionally produced from carbon monoxide in catalyst portion(s), which can be added to carbon dioxide membrane unit  114  if required. The carbon dioxide selective membranes include any membrane material that is stable at the operating conditions and has the required carbon dioxide permeability and selectivity at the operating conditions. Possible membrane materials that are selective for carbon dioxide include certain inorganic and polymer materials, as well as combinations including at least one of these materials. Inorganic materials include microporous carbon, microporous silica., microporous titanosilicate, microporous mixed oxide, and zeolite materials, as well as material combinations including at least one of these materials. 
       FIG. 2  is a schematic diagram of an exemplary recovery system  200  for use in recovering carbon dioxide from exhaust gas stream  108 . Recovery system  200  includes the equipment included in recovery system  100  with the addition of a third heat exchanger  202  and a cryogenic separation unit  204 . Third heat exchanger  202  receives a first cooled exhaust gas stream  206  from first heat exchanger  110 . Third heat exchanger  202  exchanges heat between cooled exhaust gas stream  206  and a cooling fluid  208 . In the exemplary embodiment, cooling fluid  208  includes cooling water from a cooling water system (not shown). Cooling fluid  208  may be any fluid which enables recovery system  200  to function as described herein. First cooled exhaust gas stream  206  is reduced in temperature to produce a second cooled exhaust gas stream  210 . Compressor  116  and carbon dioxide membrane unit  114  require the temperature of exhaust gas stream to be reduced to operate safely. As such, first heat exchanger  110  recovers energy from exhaust gas stream  108  and protects compressor  116  and carbon dioxide membrane unit  114 . However, first heat exchanger  110  may not cool exhaust gas stream  108  to a safe operating temperature. To ensure that exhaust gas stream  108  is reduced to a safe operating temperature, third heat exchanger  202  further cools first cooled exhaust gas stream  206 . 
     Cryogenic separation unit  204  separates rich carbon dioxide stream  132  into a liquid carbon dioxide product stream  212  and a recycle stream  214 . Cryogenic separation unit  204  generally includes a cryogenic distillation column (not shown), a refrigeration unit (not shown), a plurality of heat exchangers (not shown), and a dehydration unit (not shown). The dehydration unit removes water from rich carbon dioxide stream  132 . The refrigeration unit cools rich carbon dioxide stream  132  with the plurality of heat exchangers. The cryogenic distillation column separates the constituents of rich carbon dioxide stream  132  by boiling point. Liquid carbon dioxide product stream  212  may include a range of concentrations of carbon dioxide ranging from about 99 percent by volume to about 99.99 percent by volume. However, a substantial amount of carbon dioxide is not captured in liquid carbon dioxide product stream  212 . Recycle stream  214  contains a substantial amount of carbon dioxide. Recycle stream  214  may include a range of concentrations of carbon dioxide ranging from about 50 percent by volume to about 90 percent by volume. In order to capture the carbon dioxide lost to recycle stream  214 , recycle stream  214  is channeled to carbon dioxide membrane unit  114  for further separation. 
       FIG. 3  is a schematic diagram of an exemplary recovery system  300  for use in recovering carbon dioxide from exhaust gas stream  108 . Recovery system  300  includes the equipment included in recovery system  200  with the addition of a second turboexpander  302  and a fourth heat exchanger  303 . Second turboexpander  302  includes a second compressor  304  drivingly coupled to a second turbine  306  by a second shaft  308 . Fourth heat exchanger  303  receives a first compressed exhaust gas stream  310  from compressor  116 . Fourth heat exchanger  303  exchanges heat between first compressed exhaust gas stream  310  and a cooling fluid  309 . In the exemplary embodiment, cooling fluid  309  includes cooling water from a cooling water system (not shown). Cooling fluid  309  may be any fluid which enables recovery system  300  to function as described herein. First compressed exhaust gas stream  310  is reduced in temperature to produce a second compressed exhaust gas stream  311 . During compression, the heat of compression from compressor  116  increases the temperature of second cooled exhaust gas stream  210 . Second compressor  304  requires the temperature of first compressed exhaust gas stream  310  to be reduced to operate safely. As such, fourth heat exchanger  303  cools first compressed exhaust gas stream  310  to protect second compressor  304 . Second compressor  304  receives second compressed exhaust gas stream  311  from fourth heat exchanger  303 . Second compressor  304  further compresses second compressed exhaust gas stream  311  to produce a third compressed exhaust gas stream  312 . 
     Second turbine  306  receives a first expanded lean carbon dioxide stream  314  from turbine  118 . Second turbine  306  expands first expanded lean carbon dioxide stream  314  and rotates second shaft  308 . Second shaft  308 , in turn, rotates second compressor  304  and compresses second compressed exhaust gas stream  311 . As such, second turbine  306  recovers more energy recovered from exhaust gas stream  108  and uses the recovered energy to power second compressor  304 . Using recovered energy to power second compressor  304  saves energy and reduces the energy consumption per unit of carbon dioxide recovered by recovery system  300 . Second turbine  306  produces a second expanded lean carbon dioxide stream  316  which is discharged to the atmosphere. Recovery system  300  is not limited to two turboexpanders. Recovery system  300  may include any number of turboexpanders that enable recovery system  300  to function as described herein. 
       FIG. 5  is a schematic diagram of an exemplary scavenging system  500  for use in scavenging oxygen from exhaust gas stream  108 . System  500  includes exemplary recovery system  100 , the components and operation of which are described above in at least the description of  FIG. 1 . In the exemplary embodiment, scavenging system  500  includes a first catalytic converter unit  502  that receives exhaust gas stream  108  from power production unit  102 . As noted above, exhaust gas stream  108  generally includes about 12 percent by volume carbon dioxide. In addition, exhaust gas stream  108  also includes oxygen of less than about  1  percent by volume. Scavenging system  500  is operable to reduce a concentration of oxygen in exhaust gas stream  108 , and thus in rich carbon dioxide stream  132 . 
     In one embodiment, first catalytic converter unit  502  is a three-way catalytic converter that reduces a concentration of carbon monoxide, nitrous oxides, and volatile organic compounds in exhaust gas stream  108 . More specifically, first catalytic converter unit  502  contains a catalyst that induces combustion of methane and oxygen to produce carbon dioxide when exhaust gas stream  108  is channeled through first catalytic converter unit  502 , for example. As such, the concentration of oxygen in exhaust gas stream  108  is reduced. 
     In some embodiments, it is desirable to reduce the concentration of elemental oxygen in exhaust gas stream  108  to less than a predetermined threshold, such as when rich carbon dioxide stream  132  is intended for implementation in industrial applications. For example, the presence of oxygen in exhaust gas stream  108  increases the corrosiveness of carbon dioxide and water mixtures, and can facilitate growth of biological systems in underground reservoirs, for example, which may cause operational issues with enhanced oil recovery. 
     In one embodiment, the predetermined threshold is about 100 parts per million (ppm). In another embodiment, the predetermined threshold is less than about 50 ppm. Moreover, the hydrocarbon content of exhaust gas stream  108  may be insufficient to reduce the concentration of oxygen in exhaust gas stream  108  to less than the predetermined threshold. In the exemplary embodiment, scavenging system  500  further includes a hydrocarbon injection unit  504  that channels a hydrocarbon stream  506  for injection into exhaust gas stream  108  upstream from first catalytic converter unit  502 . As such, a mixed exhaust stream  508  formed from exhaust gas stream  108  and hydrocarbon stream  506  is channeled into first catalytic converter unit  502 . Hydrocarbons from hydrocarbon stream  506  react with oxygen from exhaust gas stream  108  within first catalytic converter unit  502  to produce carbon dioxide. As such, the concentration of oxygen in exhaust gas stream  108  is reduced. 
     In the exemplary embodiment, hydrocarbon injection unit  504  includes a source  510  of hydrocarbons and a nozzle  512  in flow communication with source  510  of hydrocarbons. In one embodiment, source  510  of hydrocarbons contains methane, such that hydrocarbon injection unit  504  channels hydrocarbon stream  506  that includes methane for injection into exhaust gas stream  108 . Moreover, nozzle  512  is operable to distribute the hydrocarbons in exhaust gas stream  108  substantially uniformly. As such, the hydrocarbons are positioned for reacting with the oxygen in exhaust gas stream  108  when channeled across first catalytic converter unit  502 . In operation, hydrocarbon injection unit  504  injects hydrocarbon stream  506  into exhaust gas stream  108  in an amount such that a hydrocarbon-oxygen ratio in mixed exhaust stream  508  is at least stoichiometric to facilitate reducing the concentration of oxygen to less than the predetermined threshold. 
     Scavenging system  500  also includes a lambda sensor  518  and a controller  520  in communication with lambda sensor  518 . Lambda sensor  518  monitors the air-fuel ratio within power production unit  102 , and controller  520  controls the air-fuel ratio within power production unit  102  such that exhaust gas stream  108  contains a predetermined concentration of oxygen. In addition, a catalyst performance map is integrated into the control scheme implemented by controller  520  to account for the formulation of the catalyst in first catalytic converter unit  502  and the fuel composition of that used on power production unit  102 . 
       FIG. 6  is a schematic diagram of an alternative scavenging system  500  for use in scavenging oxygen from exhaust gas stream  108 . In the exemplary embodiment, scavenging system  500  further includes a second catalytic converter unit  514  positioned downstream from first catalytic converter unit  502 . Second catalytic converter unit  514  receives a treated exhaust gas stream  516  discharged from first catalytic converter unit  502 , and is operable to further reduce a concentration of oxygen in treated exhaust gas stream  516 . Second catalytic converter unit  514  contains a catalyst designed to mitigate the oxygen concentration in treated exhaust gas stream  516 . For example, second catalytic converter unit  514  contains a catalyst that induces combustion of methane and oxygen to produce carbon dioxide when treated exhaust stream  516  is channeled through second catalytic converter unit  514 . 
     In addition, hydrocarbon injection unit  504  channels hydrocarbon stream  506  for injection into treated exhaust gas stream  516  downstream from first catalytic converter unit  502  and upstream from second catalytic converter unit  514 . As such, a mixed exhaust stream  517  formed from treated exhaust gas stream  516  and hydrocarbon stream  506  is channeled into second catalytic converter unit  514 . Hydrocarbons from hydrocarbon stream  506  react with oxygen from treated exhaust gas stream  516  within second catalytic converter unit  514  to produce carbon dioxide. As such, the concentration of oxygen in treated exhaust stream  516  is reduced. 
     Recovery systems  100 ,  200 , and  300 , and scavenging system  500  may be permanently installed as a unit at a power production facility. In an alternative embodiment, recovery systems  100 ,  200 , and  300 , and scavenging system  500  are mobile recovery systems disposed on a transport apparatus  400 .  FIG. 4  is a perspective view of transport apparatus  400 . In the exemplary embodiment, transport apparatus  400  is a trailer. Transport apparatus  400  includes a flatbed  402  and a plurality of wheels  404  configured to transport flatbed  402  and recovery systems  100 ,  200 , or  300 , or scavenging system  500 . In an alternative embodiment, transport apparatus  400  includes an enclosed trailer or any other transport apparatus that enables recovery systems  100 ,  200 , or  300 , or scavenging system  500  to operate as described herein. Mobile recovery systems  100 ,  200 , and  300 , and mobile scavenging system  500  are transported to sites with mobile power production units such as, but not limited to, oil wells and constructions sites. Mobile recovery systems  100 ,  200 , and  300 , and mobile scavenging system  500  produce rich carbon dioxide stream  132  as described herein for use on the oil wells and construction sites. 
     The above-described carbon dioxide recovery system provides an efficient method for removing carbon dioxide from an exhaust gas stream. Specifically, the turboexpander compresses the exhaust gas stream and the lean carbon dioxide stream drives the turboexpander. Additionally, the carbon dioxide membrane unit selectively removes carbon dioxide from the compressed exhaust gas stream. Finally, the first heat exchanger transfers energy from the exhaust gas stream to the lean carbon dioxide stream. Using the energy recovered from the exhaust gas stream by the lean carbon dioxide stream to drive the compression needed to separate carbon dioxide from the exhaust gas stream reduces the energy consumption per kg (lb) of carbon dioxide recovered of the process. As such, the systems and methods described herein embody the process changes and equipment for use in recovering carbon dioxide from a carbon dioxide-rich gas stream using a carbon dioxide membrane and a turboexpander to reduce the energy consumption per unit of carbon dioxide recovered of the process. 
     An exemplary technical effect of the system and methods described herein includes at least one of: (a) recovering carbon dioxide from an exhaust gas stream; (b) recovering heat from an exhaust gas stream; (c) powering a compressor with a turbine; and (d) decreasing the energy consumption per kg (lb) of carbon dioxide recovered. 
     Exemplary embodiments of carbon dioxide recovery system and related components are described above in detail. The system is not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the configuration of components described herein may also be used in combination with other processes, and is not limited to practice with only power generation plants and related methods as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many applications where recovering carbon dioxide from a gas stream is desired. 
     Although specific features of various embodiments of the present disclosure may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of embodiments of the present disclosure, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the embodiments of the present disclosure, including the best mode, and also to enable any person skilled in the art to practice embodiments of the present disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the embodiments described herein is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.