Patent Publication Number: US-2023134621-A1

Title: Carbon Capture System and Method with Exhaust Gas Recirculation

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of U.S. Provisional Application No. 63/274,652, filed Nov. 2, 2021, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to systems and methods for purifying gases and, more particularly, to a system and method for carbon capture that uses exhaust gas recirculation to increase carbon dioxide concentration of a cryogenic carbon capture system feed stream. 
     BACKGROUND 
     Gas purification of carrier or feed gases has been an important process in industry for many years. An example is the processing of combustion flue gases. Combustion flue gas consists of the exhaust gas from a fireplace, oven, furnace, boiler, steam generator, or other combustor. The combustion fuel sources include coal, natural gas, liquid hydrocarbons, black liquor and biomass. Combustion flue gas varies greatly in composition depending on the method of combustion and the source of fuel. Combustion using air leads to most of the flue gas consisting of nitrogen. The non-nitrogen flue gas consists of mostly carbon dioxide (or CO2), water, and unconsumed oxygen. Small amounts of carbon monoxide, nitrogen oxides, sulfur dioxide, and trace amounts of hundreds of other chemicals are present, depending on the source. Entrained dust and soot will also be present in most combustion flue gas streams. 
     The separation of carbon dioxide from other light gases such as nitrogen is called carbon capture and is important for reducing CO2 emissions and their associated environmental impacts. It is commonly believed that this CO2 represents a significant factor in increasing the greenhouse effect and global warming. Therefore, there is a clear need for efficient methods of capturing CO2 from flue gases to produce a concentrated stream of CO2 that can readily be transported to a safe storage site or to a further application. 
     The minimum work required to separate a unit mass of CO2 from the remaining flue gas depends on the purity of the CO2 product, the fraction of CO2 separated (captured), the initial amount of CO2 in the gas. The initial CO2 content affects this minimum energy much more than the other two variables, with specific energy demand (energy per unit mass of captured CO2) increasing as the initial CO2 content decreases. 
     The initial CO2 content in an exhaust or flue gas increases in many combustion systems when a portion of the exhaust gas recirculates to the combustor inlet: a process known as exhaust gas recirculation (EGR). EGR applies primarily to systems that process substantially more air than is required for combustion, the primary examples of which are turbines (both simple- and combined-cycle) and reciprocating engines. EGR in reciprocating engines is a common technique used primarily for pollution control, but EGR in turbines is rare. Both turbines and reciprocating engines should often cool the recirculated exhaust gas to manage peak combustion temperatures or to improve process efficiency. For example, in a combined cycle where a turbine drives a generator and a compressor, an exhaust stream is cooled in a heat recovery steam generator with the resulting stream directed to a condenser. 
     As carbon capture becomes more prevalent, new systems and methods are needed to improve efficiency and/or provide other operational advantages for other carbon capture technologies. 
     SUMMARY OF THE DISCLOSURE 
     There are several aspects of the present subject matter which may be embodied separately or together in the methods, devices and systems described and claimed below. These aspects may be employed alone or in combination with other aspects of the subject matter described herein, and the description of these aspects together is not intended to preclude the use of these aspects separately or the claiming of such aspects separately or in different combinations as set forth in the claims appended hereto. 
     In one aspect, a system for separating carbon dioxide from an exhaust gas of a heat engine includes a heat engine configured to receive and consume a fuel stream so that a heat engine exhaust stream is produced and a cryogenic carbon capture system. The cryogenic carbon capture system includes a flue gas cooling device in fluid communication with the heat engine and having a flue gas cooling device outlet. The flue gas cooling device is configured to receive a fluid stream that is downstream from the heat engine and a cooled liquid coolant stream so that the fluid stream is cooled by the cooled liquid coolant stream and a cooled flue gas stream is formed. The cooled flue gas stream exits the flue gas cooling device through the flue gas cooling device outlet. A cryogenic carbon capture unit is in fluid communication with the flue gas cooling device outlet and is configured to receive at least a portion of the cooled flue gas stream and to separate carbon dioxide from the first portion of the cooled flue gas stream so that a clean flue gas stream and a carbon dioxide stream are formed. A liquid coolant cooling device is configured to receive the clean flue gas stream from the cryogenic carbon capture unit and a liquid coolant stream and to cool the liquid coolant stream using the clean flue gas stream so that the cooled liquid coolant stream is formed and provided to the flue gas cooling device. The heat engine is in fluid communication with the cryogenic carbon capture system and is configured to receive: (i) a portion of a split stream that is downstream from the flue gas cooling device as an exhaust gas recirculation stream; and (ii) an air stream. 
     In another aspect, a method for separating carbon dioxide from an exhaust gas of a heat engine includes the steps of cooling a fluid stream downstream from the heat engine using a cooled liquid coolant stream so that a cooled flue gas stream is formed; separating carbon dioxide from the cooled flue gas stream in a cryogenic carbon capture process so that a clean flue gas stream and a carbon dioxide stream are formed; cooling a liquid coolant stream using the clean flue gas stream so that the cooled liquid coolant stream is formed; and directing a portion of a split stream that is downstream from the initial cooling stop to the heat engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a process flow diagram and schematic illustrating a first embodiment the system and method of the disclosure. 
         FIG.  2    is a process flow diagram and schematic illustrating a second embodiment of system and method of the disclosure. 
         FIG.  3    is a process flow diagram and schematic illustrating a third embodiment the system and method of the disclosure. 
         FIG.  4    is a process flow diagram and schematic illustrating a fourth embodiment the system and method of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     It should be noted herein that the lines, conduits, piping, passages and similar structures and the corresponding streams are sometimes both referred to by the same element number set out in the figures. 
     Also, as used herein, and as known in the art, a heat exchanger is that device or an area in the device wherein indirect heat exchange occurs between two or more streams at different temperatures, or between a stream and the environment. In addition, all heat exchangers referenced herein may be incorporated into one or more heat exchanger devices or may each be individual heat exchanger devices. As used herein, the terms “communication”, “communicating”, and the like generally refer to fluid communication unless otherwise specified. And although two fluids in communication may exchange heat upon mixing, such an exchange would not be considered to be the same as heat exchange in a heat exchanger, although such an exchange can take place in a heat exchanger. 
     As used herein, the terms, “high”, “middle”, “warm”, “cold” and the like are relative to comparable streams, as is customary in the art. 
     Reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures for shared elements or components without additional description in the specification in order to provide context for other features. 
     In the claims, letters are used to identify claimed steps (e.g. a., b. and c.). These letters are used to aid in referring to the method steps and are not intended to indicate the order in which the claimed steps are performed, unless and only to the extent that such order is specifically recited in the claims. 
     Systems and methods of the disclosure use exhaust gas recirculation (EGR) to increase the CO2 concentration in an exhaust gas by separating a partial stream of the exhaust gas downstream of a heat engine, preferably cooling the gas, and mixing the separated stream with the incoming air. Systems and methods of the disclosure provide innovations with regard to the locations and extent of cooling involved in the recirculating stream and the potential use of the heat from the cooling in the carbon capture process. 
     In the case of simple- and combined-cycle turbines, the recirculated exhaust stream mixes with the inlet air upstream of the compressor. The compressed mixed stream exiting the compressor is directed to the turbine to aid in combustion and cooling. The illustrations in this disclosure show a combined-cycle system as the heat engine. However, the same procedures apply to other heat engines such as simple-cycle turbines, internal combustion engines and any system in which exhaust gas can substitute for a portion of the air feed stream. 
     A process flow diagram and schematic illustrating a first embodiment of the system and method of the disclosure is provided in  FIG.  1   . A fuel stream  10  is received and consumed by a heat engine  8 . In one embodiment, the heat engine may include a turbine  12  of a combined-cycle that receives the fuel stream. As a result, the turbine  12  powers a compressor  14  via shaft  16 . The turbine may also power a generator (not shown) or other system components. As an example only, the fuel stream  10  may include natural gas. 
     In alternative embodiments, the heat engine  8  may include a simple-cycle turbine, an internal combustion engine or any system in which exhaust gas can substitute for a portion of the air feed stream. 
     An exhaust gas stream  18  exits the heat engine  8  (which in the illustrated embodiment is turbine  12 ) and is directed in part or in total to an optional heat recovery heat exchanger that, in the illustrated embodiments, is integrated into a heat recovery steam generator (HRSG)  22 . As is known in the art, the HRSG includes one or more heat exchangers that receive the flue gas stream  18  and a liquid stream  24  (typically water). The liquid stream is warmed by the exhaust gas stream in the HRSG  22  so that a stream of steam  26  is produced. The steam is directed to and turns a steam turbine  32  which may be used to power generators or other components. The steam turbine exhaust stream  34  exits the steam turbine while a cooled turbine exhaust stream exits the HRSG as flue gas stream  36 . The flue gas stream  36  is directed in part or in total to a flue gas cooling device, such as cooling tower  38  of a cryogenic carbon capture system  40 . Alternative cooling devices known in the art may be used in place of cooling tower  38 , including, but not limited, to a horizontal duct with a liquid spray. 
     A liquid coolant cooling device, such as cooling tower  42  of cryogenic carbon capture system  40 , receives a liquid coolant feed stream  44  and, as explained below, is cooled. Alternative cooling devices known in the art may be used in place of cooling tower  42 , including, but not limited, to a horizontal duct with a liquid spray. As an example only, the liquid coolant may be water. The cooled liquid coolant stream  46  flows to the flue gas cooling tower  38 . Due to passage of the flue gas stream  36  through the flue gas cooling tower  38 , and contact with the liquid coolant stream  46  within the flue gas cooling tower  38 , a cooled flue gas stream  48  exits the flue gas cooling tower  52 . A warmed liquid coolant stream  54  exits the bottom of the flue gas cooling tower  12 . 
     A portion of the cooled flue gas stream  48  branches off as exhaust gas recirculation stream  56 . This stream is directed back to the heat engine  8 , which in the illustrated embodiment includes a compressor  14 , which also receives cooling air stream  58 . Steams  56  and  58  are combined and compressed in compressor  14  of the heat engine so that a compressed mixed stream  62  is formed. This stream is directed to the turbine  12  to aid in cooling and combustion within the heat engine  8 . 
     As an example only, approximately 30% of the cooled flue gas stream  48  may be diverted to form the exhaust gas recirculation stream  56 . As examples only, a diverter, deflector plate or blowers positioned in line  48  or  56  may be used to divert a portion of stream  48  to form stream  56 . 
     In alternative embodiments of the system of the disclosure, the exhaust gas recirculation steam  56  may branch off of any alternative location after the flue gas cooling tower  38  of the cryogenic carbon capture system  40 . For example, the exhaust gas recirculation stream  56  may branch off of a stream of a cryogenic capture unit  64  or a clean flue gas stream  72 , both of which are described below. 
     The cooled flue gas stream portion remaining after the exhaust gas recirculation stream  56  is diverted from stream  48  is directed to a cryogenic carbon capture unit  64  of the cryogenic carbon capture system  40  as carbon capture feed stream  66 . Due to the exhaust gas recirculation (EGR) as described above, the carbon capture feed stream  66  has a higher carbon dioxide concentration and a lower mass flow rate (when compared to a system without EGR). As an example only, embodiments of the system of the disclosure may increase the carbon dioxide concentration of the carbon capture feed stream  66  from a level of 4% concentration to 10% concentration. A higher portion of the cooled flue gas stream  48 , or other streams referenced above, may be diverted to stream  56  to further increase the carbon dioxide concentration of the carbon capture feed stream  66 . 
     The carbon capture feed stream  66  entering the cryogenic carbon capture unit  64  is processed so that carbon dioxide is separated and directed out of the cryogenic carbon capture unit  64  as stream  68 . A resulting clean flue gas stream  72  also exits the cryogenic carbon capture unit  64  and, in some embodiments, may be at a temperature at or below ambient temperature, and is directed to the coolant liquid cooling tower  42  of the cryogenic carbon capture system so that the coolant liquid stream  44  also entering the cooling tower  42  is cooled. The resulting warmed clean flue gas stream exits the coolant liquid cooling tower  42  as vent gas stream  74 . 
     Any carbon capture technology known in the art may be used in the carbon capture system  40  or cryogenic carbon capture unit  64 . As examples only, the carbon capture system  40  or the cryogenic carbon capture unit  64  may use the technology disclosed in U.S. Pat. Nos. 9,250,012; 9,410,736; 9,766,011; 10,537,823; 10,724,793; 10,213,731; 10,739,067; 10,969,169 and 10,995,984, all to Sustainable Energy Solutions, Inc., the contents of each of which are hereby incorporated by reference, as well as U.S. Patent Application Publication Nos. US 2020/0318900; US 2021/0299591; US 2018/0031315 and 2019/0192999, all owned by Sustainable Energy Solutions, Inc., the contents of each of which are also hereby incorporated by reference. 
     In alternative embodiments of the system, the flue gas cooling tower  38  and/or the liquid coolant cooling tower  42  may be integrated into or a part of the cryogenic carbon capture unit  64 . 
     In the embodiment of  FIG.  2   , a blower or booster fan  82 , or other flow boosting device, and a pair of dryers, such as drying columns  84  and  86 , have been added to the cryogenic carbon capture system  40  between the flue gas cooling tower  38  and the cryogenic carbon capture unit  64  of  FIG.  1   . While two drying columns are illustrated in  FIG.  2   , the system could instead feature a single drying column or more than two drying columns. Furthermore, alternative types of dryers known in the art could be used in place of drying columns  84  and  86 . In addition, the blower or booster fan  82  could be omitted in alternative embodiments of the system. One or more blower or booster fans could also instead be positioned within any or all of lines  94 ,  96  and/or  98 . 
     As illustrated in  FIG.  2   , the cooled flue gas stream  48  passes through the blower or booster fan  82  and is then directed to a first drying column  84 . 
     The top portion of drying column  84  receives desiccant liquid stream  92 . Cooled flue gas stream  48 , after leaving the flow boosting device  82 , enters the drying column  84  wherein it contacts the desiccant stream. As a result, the desiccant stream captures the water vapor from the cooled flue gas stream, and other potentially other components such as carbon dioxide, and exits as stream  92 . 
     As examples only, the desiccant liquid streams may consist of a mixture of water and a compound from either of the following two groups: i) ionic compounds including potassium carbonate, potassium formate, potassium acetate, calcium magnesium acetate, magnesium chloride, sodium chloride, lithium chloride, and calcium chloride; and, ii) soluble organic compounds including glycerol, ammonia, propylene glycol, ethylene glycol, ethanol, and methanol. 
     A partially dried flue gas stream  94 , which contains a reduced amount of water, exits the top of drying column  84 . Similar to the embodiment of  FIG.  1   , a portion of the partially dried flue gas stream  94  branches off as exhaust gas recirculation stream  96 . This stream is directed back to compressor  14  of the heat engine  8 , which also receives cooling air stream  58 . Steams  96  and  58  are combined and compressed in compressor  14  so that a compressed mixed stream  62  is formed. This stream is directed to the turbine  12  of the heat engine  8  to aid in cooling and combustion. 
     A remaining portion of the partially dried flue gas stream  94  flows as stream  98  to the second drying column  86  of the cryogenic carbon capture system  40 . As for first drying column  84 , the top portion of the second drying column  86  receives desiccant liquid stream  102 . Stream  98  enters the second drying column  86  wherein it contacts the desiccant stream. As a result, the desiccant stream captures water vapor from the partially dried flue gas stream  98 , and other potentially other components such as carbon dioxide, and exits as stream  104 . 
     A further dried flue gas stream  106  exits the second drying column  86  and is directed to the cryogenic carbon capture unit  64 , where further processing occurs as described above for  FIG.  1   . 
     In some embodiments, there can be removal of between 1% and 100% of the water vapor from the cooled flue gas stream  48  in the first and second drying columns  84  and  86 . 
     Alternative types of dryers known in the art may be substituted for the desiccant dryers  84  and  86  of  FIG.  2   . 
     The embodiment of  FIG.  2   , by reducing the water content in exhaust gas recirculation stream  96 , permits the temperature of stream  96  to be lower. More specifically, by reducing the water content of stream  96 , freezing within the compressor  14  of the heat engine  8  does not occur until much lower temperatures. A lower temperature for stream  96  increases the mass flow rate through the compressor  14 , which results in an increase in power for the turbine  12  of the heat engine  8 . In some embodiments of the system of the disclosure, the temperature of the partially dry stream  94 , and thus the temperature of exhaust gas recirculation stream  96 , may be as low as 0° C. 
     In alternative embodiments of the system, the flue gas cooling tower  38 , the liquid coolant cooling tower  42 , the flow boosting device  82  and/or one or both of the first and second drying columns  84  and  86  may be integrated into or a part of the cryogenic carbon capture unit  64 . Furthermore, in an alternative embodiment, a carbon capture unit dryer may be provided in the carbon capture unit  64  in place of dryers  84  and  86  along with a cryogenic carbon capture heat exchanger, where the cryogenic carbon capture unit dryer receives and dries the cooled flue gas stream so that a dried flue gas stream is formed and the cryogenic carbon capture unit heat exchanger receives and further cools the dried flue gas stream with a portion of the resulting stream being directed to the heat engine as the exhaust gas recirculation stream. 
     In the system of  FIG.  3   , a supplemental heat exchanger  112  has been added between the flue gas and liquid coolant cooling towers  38  and  42  of the cryogenic carbon capture system  40  and the cryogenic carbon capture unit  64 . The supplemental heat exchanger  112  receives a refrigerant stream  114  and therefore provides additional cooling for the cooled flue gas stream  48 . This provides a lower temperature for the exhaust gas recirculation stream  56  so as to increase the mass flow rate through the compressor  14  and thus increase power in turbine  12 . The supplemental heat exchanger  112  also provides further cooling of the clean flue gas stream  72  to enable lower cooling temperatures in the liquid coolant cooling tower  42 . 
     A warmed refrigerant stream  116  exits the supplemental heat exchanger  112  and may be compressed and cooled in a closed loop refrigeration cycle to provide refrigerant stream  114 . Alternatively, an open loop refrigeration system may be used to provide refrigerant stream  114 . 
     As examples only, refrigerant stream  114  may include propane, a mixed refrigerant, R134A or any other refrigerant known in the art. In addition, alternative cooling devices known in the art may be used in place of the supplemental heat exchanger  112  of  FIG.  3   . 
     While illustrated in  FIG.  3    as being part of the cryogenic carbon capture system  40 , in alternative embodiments, the supplemental heat exchanger  112  may not be part of the cryogenic carbon capture system  40 . 
     In the system of  FIG.  4   , a supplemental heat recovery heat exchanger  120  has been added between the HRSG  22  and the flue gas cooling tower  38  of the cryogenic carbon capture system  40  of  FIG.  2   . The supplemental heat recovery heat exchanger  120  receives a cooling stream  124  from the cryogenic carbon capture unit  64  and therefore provides additional cooling for the flue gas stream  36  prior to the flue gas cooling tower  38 . As an example only, the cooling stream  124  and the warmed return stream  126  may be a reboiler service from a distillation column of the cryogenic carbon capture unit  64 . 
     While illustrated in  FIG.  4    as being part of the cryogenic carbon capture system  40 , in alternative embodiments, the supplemental heat recovery heat exchanger  120  may not be part of the cryogenic carbon capture system  40   
     While the preferred embodiments of the disclosure have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the disclosure, the scope of which is defined by the following claims.