Abstract:
A device for capturing carbon dioxide includes a supply source for supplying a compressed flue gas; a multi-stream heat exchanger for pre-cooling the compressed flue gas and a gas expansion device located downstream of the multi-stream heat exchanger. The multi-stream heat exchanger is configured to separate the compressed flue gas into a first compressed stream and a second compressed stream. The gas expansion device is configured to expand the compressed flue gas into a first sub-stream of carbon dioxide depleted gas and a second sub-stream of carbon dioxide. The device includes a first recirculation channel that recirculates a portion of the first sub-stream into the multi-stream heat exchanger and a second recirculation channel that recirculates at least a portion of the second sub-stream into the multi-stream heat exchanger, wherein the multi-stream heat exchanger is configured to pre-cool the compressed flue gas using the first sub-stream and the second sub-stream.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &amp; DEVELOPMENT 
       [0001]    This invention was made with United States Government support under contract DE-AR0000101, awarded by the Department of Energy (DoE). The United States Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The field of the present disclosure relates generally to low temperature capture of carbon dioxide (CO 2 ) from a carbon dioxide containing gas. More particularly, the present disclosure relates to systems and methods for separating carbon dioxide from a gas stream and utilizing the carbon dioxide to pre-cool flue gas. 
         [0003]    Combustion of fuels for energy production generates large quantities of exhaust gas, for example, exhaust gas produced at fossil fuel burning power plants. The exhaust gas is commonly referred to as flue gas because the exhaust gas exits the combustion chamber via a flue and is typically exhausted to the atmosphere. The composition of the flue gas is dependent upon the fuel being combusted. Typical flue gas comprises nitrogen, carbon dioxide, water vapor, oxygen, carbon monoxide, oxides and particulate matter. 
         [0004]    Carbon dioxide gas has been found to be a greenhouse gas, which may contribute to global warming. Carbon dioxide gas is also an ingredient used in the food and beverage industry, and contributes to the growth of plants through photosynthesis. Typically, carbon dioxide may be removed from flue gas using amines Alternatively, low temperature capture of carbon dioxide, wherein flue gas is cooled to low temperature temperatures until solid CO 2  is formed, is an alternative method to currently existing technologies that utilize amine-based solvents. However, the direct heat exchange between the cold streams and the flue gas results in large temperature differences between the two streams and is not energy efficient. Further, solid CO 2  forms on the surfaces of tubes containing the cooling stream, thus reducing efficiency of heat transfer between the cold tubes and the flue gas. In addition, removal of solid CO 2  from the surfaces of the tubes presents technical challenges. 
         [0005]    The present disclosure describes systems and methods that enable effective heat transfer between a cold stream of a heat exchanger and a warm stream of flue gas in a low temperature carbon dioxide removal processes. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In one aspect, a device for capturing carbon dioxide comprises a supply source for supplying a compressed flue gas; a multi-stream heat exchanger for pre-cooling the compressed flue gas; a gas expansion device located downstream of the multi-stream heat exchanger, the gas expansion device expanding the compressed flue gas into a first sub-stream of carbon dioxide depleted gas and a second sub-stream of carbon dioxide, a first recirculation channel that recirculates at least a portion of the first sub-stream into the multi-stream heat exchanger, and a second recirculation channel that recirculates at least a portion of the second sub-stream into the multi-stream heat exchanger. The multi-stream heat exchanger is configured to pre-cool the compressed flue gas using the first sub-stream and the second sub-stream. 
         [0007]    In another aspect, a method of capturing carbon dioxide comprises providing a compressed gas containing carbon dioxide; pre-cooling the compressed gas in a multi-stream heat exchanger; expanding the compressed gas in a gas expansion device to provide a first sub-stream of carbon dioxide depleted gas and a second sub-stream of carbon dioxide, and supplying the first sub-stream and the second sub-stream to the multi-stream heat exchanger to facilitate the pre-cooling of the compressed gas. 
         [0008]    In yet another aspect, a carbon capturing system comprises a supply for supplying a compressed flue gas; a water pre-cooler that cools the compressed flue gas; a multi-stream heat exchanger, located downstream of the water-pre-cooler, for further pre-cooling the compressed flue gas; a gas expansion device located downstream of the multi-stream heat exchanger, the gas expansion device expanding the compressed flue gas into a first sub-stream of carbon dioxide depleted gas and a second sub-stream of carbon dioxide, a first recirculation channel that recirculates at least a portion of the first sub-stream into the multi-stream heat exchanger, and a second recirculation channel that recirculates at least a portion of the second sub-stream into the multi-stream heat exchanger. The multi-stream heat exchanger is configured to pre-cool the compressed flue gas using the first sub-stream and the second sub-stream. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram of an exemplary low temperature carbon capturing system according to the present disclosure. 
           [0010]      FIG. 2  is a cross section of an exemplary heat exchanger according to the present disclosure. 
           [0011]      FIG. 3  is a chart showing an exemplary plot of flue gas temperature and cold stream temperature in a heat exchanger according to the present disclosure. 
           [0012]      FIG. 4  is a chart showing an exemplary plot of energy and heat exchanger sections according to the present disclosure. 
           [0013]      FIG. 5  is a chart plotting net efficiency points and exhaust gas recirculation values. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The present disclosure describes systems and methods that provide the technical effect of facilitating effective heat transfer between a cold stream of a heat exchanger and a warm stream of flue gas in a low temperature carbon dioxide removal process. 
         [0015]    Shown generally in  FIG. 1  is an exemplary embodiment of a cryogenic carbon capturing system according to the present disclosure. In one embodiment, the carbon capturing system includes a compressed stream of carbon dioxide containing gas  100  (e.g., a flue gas), a multi-stream heat exchanger  102  comprising heat exchangers  104 ,  106  and  108 , a manifold  110 , a secondary heat exchanger  112 , an expansion device  114 , a refrigeration device  116 , a pair of solid to liquid phase change devices  118 ,  120 , storage chambers  122 ,  124  and a water pre-cooler  126 . 
         [0016]    In one embodiment, compressed stream of carbon dioxide gas  100  is a flue gas extracted from a flue of a fossil fuel fired power plant, such as an electrical power plant. The pressure and temperature of compressed stream of carbon dioxide gas  100  are dependent upon the contents of the gas, and the compressor used for compression. In one embodiment, the compressor is controlled by an operator to provide a temperature and pressure selected by the user. Alternatively, the compressor may automatically adjust and provide the compressed stream of carbon dioxide gas  100  at a predetermined pressure and temperature. For example, in one embodiment, compressed stream of carbon dioxide gas  100  is provided at a temperature of approximately 25° C. and a pressure of 4.8 bar. In another embodiment, the contents of compressed stream of carbon dioxide gas  100  are, by mole fraction, 0.668 nitrogen, 0.167 water vapor, 0.133 carbon dioxide, 0.024 oxygen and 0.008 argon, with the flow rate of the compressed stream of carbon dioxide gas  100  being approximately 5,811,370 lbm/hr. 
         [0017]    In one embodiment, compressed stream of carbon dioxide containing gas  100  enters multi-stream heat exchanger  102  via an input  128 . In one embodiment, multi-stream heat exchanger  102  comprises a gas to liquid heat exchanger  104 , a gas to gas heat exchanger  106  and a gas to solid heat exchanger  108 . In other embodiments, heat exchangers  104 ,  106  and  108  are any suitable heat exchanger that allows the system to function as described herein. 
         [0018]    In one embodiment, compressed stream of carbon dioxide gas  100  is separated into two streams  130 ,  132  before entering multi-stream heat exchanger  102 . Alternatively, multi-stream heat exchanger  102  is configured to separate compressed stream of carbon dioxide gas  100  into two streams after entering multi-stream heat exchanger  102 . In one embodiment, streams  130  and  132  are not equal in flow rate, for example, stream  132  is approximately 60% to 90% and stream  130  is approximately 10% to 40% of the total flow of compressed stream of carbon dioxide gas  100 . In another embodiment, stream  132  is approximately 75% to 90% and stream  130  is approximately 10% to 25% of the total flow of compressed stream of carbon dioxide gas  100 . In yet another embodiment, stream  132  is approximately 80% and stream  130  is approximately 20% of the total flow of compressed stream of carbon dioxide gas  100 . However, the percentage flow rate of streams  130  and  132  may be any percentages that allow the system to function as described herein. 
         [0019]    Multi-stream heat exchanger  102  is configured to pre-cool compressed stream of carbon dioxide gas  100 . In one embodiment, in order to pre-cool compressed stream of carbon dioxide gas  100 , multi-stream heat exchanger  102  utilizes a stream of carbon dioxide depleted material  134  and a stream of carbon dioxide  136 . Streams  134  and  136  may be in solid, liquid or gas form. In one embodiment, stream  134  is a stream of carbon dioxide depleted gas and stream  136  is a stream of solid carbon dioxide. 
         [0020]    Stream of carbon dioxide depleted material  134  and stream of carbon dioxide  136  are provided from an expansion device  114  located downstream of multi-stream heat exchanger  102 . Compressed stream of carbon dioxide containing gas  100  flows into expansion device  114 . Expansion device  114  expands compressed stream of carbon dioxide containing gas  100 , which cools compressed stream of carbon dioxide containing gas  100 . Expansion device  114  cools, by expansion, carbon dioxide containing gas  100  to an extent that separates carbon dioxide from other components of carbon dioxide containing gas  100 . In one embodiment, expansion device  114  outputs, after expansion, carbon dioxide stream  136  at −119° C. and carbon dioxide depleted stream  134  at −119° C. 
         [0021]    In one embodiment, carbon dioxide stream  136  is fed to heat exchanger  104  via a solid to liquid phase change device  118 . The solid to liquid phase change device  118  warms the incoming stream of solid carbon dioxide  136  until stream  136  becomes a liquid stream of carbon dioxide  138 . In another embodiment, the temperature of liquid carbon dioxide stream  138  is approximately −56° C. Liquid carbon dioxide stream  138  is fed into heat exchanger  104 , and cools stream  130 . In one embodiment, the temperature of stream  130  at the output of heat exchanger  104  is −51° C. and the temperature of the liquid carbon dioxide stream  138  output from gas to liquid heat exchanger  104  is −22° C. 
         [0022]    As shown in  FIG. 1 , heat exchanger  104  outputs carbon dioxide stream  138  to a warming device  120 . Alternatively, warming device  120  warms carbon dioxide stream  138  from approximately −22° C. to 20° C. As a further embodiment, warming device  120  outputs carbon dioxide stream  138  to a storage chamber  122  for storage or sequestration. 
         [0023]    In one embodiment, a secondary heat exchanger  112  is disposed between multi-stream heat exchanger  102  and expansion device  114 . In another embodiment, secondary heat exchanger  112  is supplied a refrigerant  140  from refrigerator device  116 . Secondary heat exchanger  112  is configured to further pre-cool compressed carbon dioxide containing gas stream  100  before stream  100  enters expansion device  114 . In one embodiment, secondary heat exchanger  112  outputs a stream of carbon dioxide  142  that is combined with carbon dioxide stream  136 . In another embodiment, carbon dioxide stream  142  is solid carbon dioxide at −97° C. Additionally, secondary heat exchanger  112  outputs a pre-cooled stream of carbon dioxide containing gas  100  to expansion device  114 . In one embodiment, pre-cooled stream of compressed carbon dioxide containing gas  100  supplied to expansion device  114  from secondary heat exchanger  112  is at the same temperature as stream  140 . Alternatively, pre-cooled stream of compressed carbon dioxide containing gas  100  is at a different temperature than stream  140 . In another embodiment, when stream  142  is combined with stream  136 , the resulting temperature of the combined carbon dioxide streams is −102° C. 
         [0024]    Secondary heat exchanger  112  comprises a ice-phobic coating  144  to prevent, or substantially prevent, solid carbon dioxide  146  from sticking to the coated surface. In one embodiment, secondary heat exchanger  112  comprises a collection portion  148  for collecting solid carbon dioxide particles  146 . In another embodiment, solid carbon dioxide particles  148  are collected and stored. In yet another embodiment, solid carbon dioxide particles  148  are output as carbon dioxide stream  142 . In yet another embodiment, secondary heat exchanger  112  comprises a vibrating device  150  that vibrates heat exchanger  112  to prevent or substantially prevent solid carbon dioxide  146  from sticking to coated surface  144 . 
         [0025]    In one embodiment, carbon dioxide depleted gas stream  134  is supplied back to heat exchanger  108 . In another embodiment, carbon dioxide depleted stream  134  is supplied to heat exchanger  108  at a temperature of −119° C. and cools gas stream  132  from an input temperature of approximately −83° C. to approximately −87° C., and carbon dioxide depleted stream  152  exits heat exchanger  108  at approximately −88° C. In yet another embodiment, carbon dioxide depleted stream  152  is supplied to heat exchanger  106  to cool compressed gas stream  132 . In still another embodiment, heat exchanger  106  cools stream  132  from a temperature of approximately 25° C. to approximately −83° C. and exhausts the carbon dioxide depleted gas to storage chamber  124 . 
         [0026]    In one embodiment, a temperature difference between the cooling medium and the compressed gas stream in one or more heat exchangers  104 ,  106 ,  108  and  112  is 5° C. or less. In one embodiment, the 5° C. temperature differential is facilitated by one or more of heat exchangers  104 ,  106 ,  108  and  112  being counter-flow heat exchangers. For example, as shown in  FIG. 3 , the system has been segmented into  11  exemplary segments  1 - 11 . Each segment represents a different point along a path of the system. In this manner, when carbon dioxide containing gas stream  100  interfaces with a cold stream in each of heat exchangers  104 ,  106 ,  108  and  112 , the counterflow arrangement of heat exchangers  104 ,  106 ,  108  and  112  provides a temperature differential (i.e., a pinch point) of the cold stream (e.g., cold streams  134 ,  136 ,  138 ,  140 ) and the warm stream (e.g., carbon dioxide containing gas stream  100 ) within the heat exchangers to be approximately 5° C. The 5° C. temperature differential facilitates a controlled and efficient manner of low temperature capture of carbon dioxide from a carbon dioxide containing gas. 
         [0027]    Shown in  FIG. 4  is an exemplary plot of the energy balance in each of segments  1 - 11  of  FIG. 3 . Each of the bars in  FIG. 4  represents an amount of energy that is required to be added  154  or removed  156  from a stream to maintain the 5° C. temperature difference between the respective cooling stream and the warm stream within a heat exchanger. Negative energy values correspond to energy that has to be removed from a specific stream. For example, since the compressed carbon dioxide containing gas stream  100  is cooled in each heat exchanger, the energy balance for stream  100  is always negative. In one embodiment, in zones  1 - 4 , energy has to be added  158  to the cooling stream in an amount larger than the amount to be removed from stream  100 . In zones  5 - 10  more energy has to be removed from stream  100  than needs to be added to the cooling stream. In one embodiment, zones  5 - 10  represent a path through heat exchanger  112 , wherein refrigeration system  116  is employed to provide refrigerant  140  to secondary heat exchanger  112  to remove energy from stream  100  in zones  5 - 10 . In another embodiment, refrigeration system  116  is used to remove heat from stream  100  in zone  11 . In another embodiment, heat removed from refrigerant  140  in refrigeration system  116  in zones  5 - 11  is transferred to warming device  120  and liquid to gas phase change device  118  in zones  1 - 4 . Zones  1 - 11  are exemplary, and may be distributed along the system in a manner that allows the system to function as described herein. 
         [0028]    Shown in  FIG. 5  is an exemplary plot of net efficiency points and exhaust gas recirculation levels of different combined cycle systems, which may include a carbon capture system. The exhaust gas recirculation (EGR) level is an operator controlled parameter of a combined cycle system, such as a natural gas combined cycle system. Typically, a combined cycle system runs at approximately 50% efficiency (i.e., 50 net efficiency points). However, when a carbon capture system is added to a combined cycle system, a reduction in efficiency occurs, which decreases the net efficiency points of a system. Line  160  plots the net efficiency points of a natural gas combined cycle system without a carbon capture system, and represents a baseline combined cycle system, such as a power plant. Line  162  plots the net efficiency points of a natural gas combined cycle system including an amine-based carbon capture system. As shown, a loss of approximately  7  efficiency points (i.e., an efficiency penalty) is incurred at all EGR levels when utilizing an amine-based carbon capture system. Line  164  plots the net efficiency points of a natural gas combined cycle system including a known low temperature carbon capture system not including a multi-stream heat exchanger according to the present disclosure. As shown in  FIG. 5 , an efficiency penalty ranging between −9 to −7 points is incurred with a traditional low temperature carbon capture system. Line  166  plots the net efficiency points of a natural gas combined cycle system including a low temperature carbon capture system according to the present disclosure. As shown, an efficiency penalty of approximately −8 to −6 points is incurred. Line  168  plots the net efficiency points of a natural gas combined cycle system including a low temperature carbon capture system according to the present disclosure and an amine-based carbon capture system. Thus, as shown in  FIG. 5 , the low temperature carbon capture system according to the present disclosure allows for the possibility of gaining 1-2 net efficiency points (a reduced penalty) for natural gas combined cycle systems in comparison to known carbon capture systems (i.e., lines  162  and  164 ). 
         [0029]    In some embodiments, the above described systems and methods are electronically or computer controlled. The embodiments described herein are not limited to any particular system controller or processor for performing the processing and tasks described herein. The term controller or processor, as used herein, is intended to denote any machine capable of performing the calculations, or computations, necessary to perform the tasks described herein. The terms controller and processor also are intended to denote any machine that is capable of accepting a structured input and of processing the input in accordance with prescribed rules to produce an output. It should also be noted that the phrase “configured to” as used herein means that the controller/processor is equipped with a combination of hardware and software for performing the tasks of embodiments of the invention, as will be understood by those skilled in the art. The term controller/processor, as used herein, refers to central processing units, microprocessors, microcontrollers, reduced instruction set circuits (RISC), application specific integrated circuits (ASIC), logic circuits, and any other circuit or processor capable of executing the functions described herein. 
         [0030]    The embodiments described herein embrace one or more computer readable media, including non-transitory computer readable storage media, wherein each medium may be configured to include or includes thereon data or computer executable instructions for manipulating data. The computer executable instructions include data structures, objects, programs, routines, or other program modules that may be accessed by a processing system, such as one associated with a general-purpose computer capable of performing various different functions or one associated with a special-purpose computer capable of performing a limited number of functions. Aspects of the disclosure transform a general-purpose computer into a special-purpose computing device when configured to execute the instructions described herein. Computer executable instructions cause the processing system to perform a particular function or group of functions and are examples of program code means for implementing steps for methods disclosed herein. Furthermore, a particular sequence of the executable instructions provides an example of corresponding acts that may be used to implement such steps. Examples of computer readable media include random-access memory (“RAM”), read-only memory (“ROM”), programmable read-only memory (“PROM”), erasable programmable read-only memory (“EPROM”), electrically erasable programmable read-only memory (“EEPROM”), compact disk read-only memory (“CD-ROM”), or any other device or component that is capable of providing data or executable instructions that may be accessed by a processing system. 
         [0031]    A computer or computing device such as described herein has one or more processors or processing units, system memory, and some form of computer readable media. By way of example and not limitation, computer readable media comprise computer storage media and communication media. Computer storage media include volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Communication media typically embody computer readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism and include any information delivery media. Combinations of any of the above are also included within the scope of computer readable media. 
         [0032]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention 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.