CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present invention is a non-provisional of and claims priority to U.S. Provisional Patent Application Serial No. 62/825,947, filed March 29, 2019, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] Embodiments of the invention are directed to C02 separation & liquefaction, and more particularly, to C02 capture technology that uses exhaust gas as a refrigerant to liquify and separate C02 from the exhaust gas.

[0003] Global climate change, or global warming, is generally considered by the scientific community to be a result of C02 emissions originating from human activity. A primary source of C02 emissions is the combustion of fossil fuels for energy production. To slow the increase of global temperatures, society must decrease the amount of C02 emitted to the atmosphere as our demand for energy increases. One method of reducing C02 emissions is to capture C02 generated as a byproduct of combustion at fossil fuel burning power plants. Carbon capture and sequestration (CCS) technologies remove C02 from power plant emissions, called flue gas, often as liquid C02 to be pumped and stored underground to prevent increase in C02 emissions to the atmosphere. Technologies to improve carbon capture are heavily researched, including technologies that retrofit existing power plants with CCS systems. For instance, the FLExible Carbon Capture and Storage (FLECCS) program is a government initiative that funds development of carbon capture and storage (CCS) technologies that enable power generators to improve response to grid conditions in a high variable renewable energy (VRE) penetration environment. Lower capital cost carbon capture technology is of significant interest for FLECCS.

[0004] Unfortunately, existing carbon capture and sequestration (CCS) systems typically have high capital costs. Typical CCS systems include conventional amine carbon capture technology and cryogenic carbon capture with external cooling or refrigeration systems. Reversible reactions between amines and C02 make amines suitable for separating C02 from post combustion exhaust, including flue gas. Conventional amine carbon capture technologies tend to have high capital costs, at the rate of 4 or more times that of some cryogenic systems. Further, amine CCS technologies may not meet the FLECCS objectives without additional retrofits, like thermal energy storage (TES), to the power plant.

[0005] Other CCS systems also include cryogenic carbon capture systems that use external cooling or refrigeration cycles. Externally cooled cryogenic carbon capture uses refrigeration systems to cool pressurized exhaust or flue gas. Using external refrigeration systems allows for the design of smaller temperature differences within the heat exchange processes, which can lead to higher efficiency. Unfortunately, the higher efficiency comes at the price of higher capital cost, primarily from bigger heat exchangers. Additionally, external cooling systems require specialized refrigerants and multiple heat pump subsystems, which increases system complexity, operation challenges, and drives up costs. Further, externally cooled cryogenic carbon capture usually requires use of a refrigerant with high global warming potential. For example, a typical refrigerant for such processes is R-14 (tetrafluoromethane), which has a 100-year global warming potential (GWP) estimated at over 6,500. This means that very small leaks in the R-14 refrigerant loop could significantly reduce the impact of the C02 that is captured by the system.

[0006] Therefore, it would be desirable to having a CCS system that minimizes capital costs by using modular construction to retrofit existing fossil fuel burning power plants. It would further be desirable to have a CCS system that uses low-cost heat exchangers that operate with relatively large temperature differences as compared to prior cryogenic systems, and which can also accommodate solid ice and solid C02 formation if operated at low temperatures. A carbon capture process that uses no chemicals or refrigerants other than the exhaust gas itself would reduce environmental footprint, operation, and maintenance costs. BRIEF DESCRIPTION OF THE INVENTION

[0007] In accordance with one aspect of the invention, a C02 separation and liquefaction system includes a first cooling stage to cool flue gas with liquid C02, a compression stage coupled to the first cooling stage to compress the cooled flue gas, a second cooling stage coupled to the compression stage and the first cooling stage to cool the compressed flue gas with a C02 melt and provide the liquid C02 to the first cooling stage, and an expansion stage coupled to the second cooling stage to extract solid C02 from the flue gas that melts in the second cooling stage to provide the liquid C02.

[0008] In accordance with another aspect of the invention, a CCS system includes a first cooling stage to cool a flue gas with C02 to be sequestered, a compression stage coupled to the first cooling stage to compress the cooled flue gas, a second cooling stage coupled to the compression stage to cool the compressed flue gas with solid C02, and an expander coupled to the second cooling stage to extract solid C02 from the flue gas, the expander coupled to the first cooling stage and the second cooling stage to provide the solid C02 and the C02 to be sequestered.

[0009] In accordance with yet another aspect of the invention, a method of operating a C02 liquefaction system includes extracting flue gas from a flue gas producer, compressing the flue gas in a first compressor, cooling the flue gas in a heat exchanger cooled by the ambient environment, cooling a first stream of the flue gas from the heat exchanger using liquid C02, compressing the flue gas in a second compressor, cooling the flue gas by melting solid C02 and producing the liquid C02, and expanding the flue gas to extract the solid C02.

[0010] In accordance with still another aspect of the invention, a CCS system for a power plant includes a first compressor to compress air and a first heat exchanger coupled to the first compressor to cool the compressed air, the first heat exchanger cooled by an ambient environment. The CCS system also includes a first cooling stage coupled to the power plant to receive flue gas and coupled to the first heat exchanger to receive the air, the first cooling stage cooling the flue gas and the air with liquid C02. The CCS system further includes a second compressor coupled to the first cooling stage to receive and compress the air, a second cooling stage coupled to the second compressor to receive and cool the air, the second cooling stage coupled to the first cooling stage to cool and extract C02 from the flue gas and provide the liquid C02, and an expansion stage coupled to the second cooling stage to expand the air and provide the expanded air to the second cooling stage to cool the flue gas.

[0011] Various other features and advantages will be made apparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The drawings illustrate preferred embodiments presently contemplated for carrying out the invention.

[0013] In the drawings:

[0014] FIG. 1 is process diagram of a C02 separation and liquefaction system with interwarming expansion coupled to a combined cycle power plant and a thermal energy storage system, according to an embodiment of the invention.

[0015] FIG. 2 is a process diagram of a C02 separation and liquefaction system with frosting expansion coupled to a combined cycle power plant and a thermal energy storage system, according to an embodiment of the invention.

[0016] FIG. 3 is a process diagram of a C02 separation and liquefaction system with interwarming expansion, according to an embodiment of the invention.

[0017] FIG. 4 is a process diagram for a C02 separation and liquefaction system with air Brayton cooling, according to an embodiment of the invention.

[0018] FIG. 5 is a process diagram of a C02 separation and liquefaction system with frosting expansion, according to an embodiment of the invention.

[0019] FIG. 6 is a graph of C02 building up on the walls of heat exchangers used in a C02 separation and liquefaction system, according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] The operating environment of the invention is described with respect to a C02 separation and liquefaction system for use with exhaust gases from fossil fuel combustion. However, those skilled in the art will appreciate that the invention is equally applicable for separating and liquifying C02 from other carbon gas streams. While embodiments of the invention will be described with respect to a carbon capture and sequestration system for a fossil fuel burning power plant, embodiments of the invention are equally applicable for use with C02 separation and liquefaction for other industrial processes.

[0021] Referring to FIG. 1, a C02 separation and liquefaction system 20 with interwarming expansion is shown, in accordance with an embodiment of the invention. The C02 separation and liquefaction system 20 is shown coupled to a combined-cycle power plant 22 which includes a natural gas power plant 24, coal steam plant 26, and a thermal energy storage (TES) system 28. The natural gas power plant 24 burns fossils fuels (e.g. natural gas) to generate electrical power and can reject heat through its exhaust, or flue gas, to the TES system 28. The TES system 28 couples to the coal steam plant 26 to preheat water used in the steam plant. The coal steam plant 26 burns fossil fuels (e.g. coal) to generate electrical power and can reject heat through its flue gas to the TES system 28.

[0022] Thermal energy storage systems, such as TES system 28, may consist of a wide range of storage technologies that allow excess thermal energy from industrial processes to be stored for later use. Thermal energy storage may be used for balancing energy demands between daytime and nighttime or other periods of high variable energy demand. For example, U.S. Patent Application Ser. No. 14/919,535 filed October 21, 2015 and issued as U.S. Patent No. 10,054,373 on August 21, 2018, the disclosure of which is incorporated herein by reference in its entirety, discloses a thermal heat capture, storage, and exchange arrangement. The arrangement includes at least one thermal exchange and storage (TXES) array that receives a fluid flow of a heat source fluid, and at least one heat engine operable with the TXES array that extracts heat from the TXES array and converts it to mechanical energy. [0023] In a preferred embodiment, the C02 separation and liquefaction system 20 may receive flue gas from the natural gas power plant 24 and/or the coal steam plant 26, either directly or indirectly via the TES system 28. The C02 separation and liquefaction system 20 can utilize the flue gas as an auto-refrigerant by compressing, cooling, and then expanding the flue gas to further reduce its temperature so that C02 from the flue gas will frost (e.g., change phase from a gas to a solid). With the C02 in solid phase, it can then be separated from the rest of the flue gas, converted to a liquid or superheated state, and directed underground for sequestration. Thus, the C02 separation and liquefaction system 20 may provide a cryogenic carbon capture and sequestration (CCS) system 30 for the combined-cycle power plant 22.

[0024] A key benefit of the CCS system 30 is that it is relatively simple, and therefore very low capital cost, while remaining competitively efficient with other carbon sequestration systems. Compared to other cryogenic carbon capture and sequestration systems, which invoke highly complex refrigeration systems, the CCS system 30 can use the flue gas as the only refrigerant. The simplicity of this approach provides the potential for a very low capital cost CCS system while achieving acceptable efficiency.

[0025] FIG. 1 shows the CCS system 30 first cooling the flue gas received from the TES system 28 in a precooler 32 to condense out water and acid, and to minimize future compression work. At this point, if the natural gas power plant 24 is in use, some flue gas may be recycled back to the natural gas power plant 24. Flue gas that is not recycled may then be directed to one or more compressors 34, generally referred to as hot compressor(s) if compressing hot flue gas. At this stage, the flue gas can behave as an air-like gas and thus undergoes vapor compression in the hot compressor(s). The flue gas is further cooled in one or more post-compression heat exchangers 36 following compression in each of the one or more compressors 34. Each of the post-compression heat exchange^ s) 36 is preferably cooled by the ambient environment, and more water/acid is removed. In one embodiment, the post-compression heat exchanger(s) 36 may be cooled by water from the ambient environment. FIG. 1 shows the flue gas being compressed and cooled in three stages of hot compressors 34 and post-compression heat exchangers 36. [0026] Next, the flue gas preferably enters a flue split 38 which separates the flue gas into a first stream 40 and a second stream 42 and directs each stream to a first cooling stage 43 comprising two parallel recuperative cooling processes. The first stream 40 of flue gas is cooled using liquid and/or supercritical C02 by a first plurality of series connected heat exchangers 44. The second stream 42 of flue gas is cooled using N2 by a second plurality of series connected heat exchangers 46. As will be described in more detail below, the first stream 40 of flue gas may be cooled using C02 extracted from the flue gas before the C02 is sequestered. The second stream 42 of the flue gas may be cooled using N2 from the flue gas after the C02 is extracted. That is, the second stream 42 of flue gas may be cooled using ambient (or near ambient) pressure, nitrogen-rich flue gas before it exits to the atmosphere. Thus, the first stream 40 recuperates cooling potential from the extracted C02, and the second stream 42 recuperates cooling potential from the nitrogen-rich flue gas.

[0027] The first plurality of series connected heat exchangers 44 and the second plurality of series connected heat exchangers 46 (i.e., the C02 and N2 coolers) each preferably include three identical sets of heat exchangers in series, though each could have 2, 4, or any suitable number of heat exchangers. The first heat exchanger 48(a), 48(b) in each series preferably cools the flue gas to a temperature just above the freezing point of water, which allows further water removal from the flue gas. The second heat exchanger 50(a), 50(b) in each series preferably cools the flue gas to temperatures where water begins freezing on the walls of the heat exchangers. The third heat exchanger 52(a), 52(b) in each series preferably further cools the flue gas to remove additional water and ice from the flue gas (e.g., to near 220K in some embodiments). Additional ice should build up on each of the third heat exchangers 52(a), 52(b), though at a slower rate than in the second heat exchangers 50(a), 50(b) since the flue gas contains less water.

[0028] It is recognized that the ice build-up in the first plurality of series connected heat exchangers 44 and the second plurality of series connected heat exchangers 46 should be removed periodically to prevent clogging and poor heat transfer performance therein. To remove the ice-buildup, the first plurality of series connected heat exchangers 44 and the second plurality of series connected heat exchangers 46 may be swapped within each respective series on a regular basis such that the heat exchangers with ice build-up can exchange heat with the C02 or nitrogen rich flue streams above the freezing point of water, and thus force the ice to melt off. Swapping the first plurality of series connected heat exchangers 44 and the second plurality of series connected heat exchangers 46 allows the heat exchangers previously operating below the freezing point of water to be regenerated, with water, SOx, and other condensates drained therefrom. There may still be water in the flue gas after the heat exchangers in 44 and 46, so this equipment may require an infrequent regeneration step of bringing those units up to a temperature above the triple point of water to melt off ice build-up. Another method to handle the water ice build-up would be to install a dehydration system before the flue gas temperature gets below the triple point of water, which could alleviate the need for these heat exchanger regeneration processes.

[0029] Swapping the first plurality of series connected heat exchangers 44 and the second plurality of series connected heat exchangers 46 may include reordering/rearranging the heat exchangers in their respective series by moving one or more heat exchangers operating along a colder portion of the flue gas (e.g. C02 stream and nitrogen rich flue stream) to a warmer portion, and vice versa, rather than by replacing an iced-up heat exchanger so that it can thaw once removed from the series. By contrast, traditional methods of deicing heat exchangers can require turning them off, swapping one out of a cooling operation to thaw or adding heat in some form to melt the ice buildup. Embodiments of the present invention provide energy benefits over other systems by reducing costs associated with heat addition or removing heat exchangers from a cooling operation. Further, the current invention may include control valving (not shown) that includes controllers timed to swap series heat exchangers when efficiencies start to fall, which can optimize the performance of the series.

[0030] As shown in FIG. 1, the first stream 40 and the second stream 42 of flue gas enter a flue mixer 54 which combines the streams for entry into a compression stage 56 and a second cooling stage 58. The compression stage 56 may include a first compressor 60 and a second compressor 62 in series to compress the flue gas from the first cooling stage 43. The second cooling stage 58 may include a first heat exchanger 64 in series between the first compressor 60 and second compressor 62, and a second heat exchanger 66 downstream from the second compressor 62. The first compressor 60 and the second compressor 62 may be referred to as cold compressors since they are located downstream from the first cooling stage 43. The first compressor 60 and the second compressor 62 may perform vapor compression of the flue gas existing in an air-like state.

[0031] The first stream 40 and the second stream 42 of flue gas may leave the third heat exchangers 52(a), 52(b), at a temperature below the freezing point of water (e.g., at around 220K in some embodiments) and be mixed together before being compressed by the first compressor 60 (e.g., to approximately 8.4 bar with the flue gas containing about 15% C02 by volume in some embodiments). The flue gas may then enter the first heat exchanger 64, where it is preferably cooled by high mass flow liquid C02 near the melting point of C02. The flue gas may then be compressed by the second compressor 62 (e.g. to approximately 12.2 bar in some embodiments). The flue gas may then enter the second heat exchanger 66 where it is again cooled by high mass flow liquid C02 near the melting point of C02. In some embodiments, C02 melts/freezes at a temperature within a range of approximately 216-217 K at the pressures of the flue gas following compression by the first compressor 60 and the second compressor 62, therefore first heat exchanger 64 and/or the second heat exchanger 66 preferably cools the flue gas to a temperature just above the freezing point of C02 or within a range of approximately 218-235K.

[0032] The cooling potential for the first heat exchanger 64 and the second heat exchanger 66 comes from the melting energy of the solid C02 extracted from the flue gas (discussed below). The first compressor 60 and the second compressor 62 of the second cooling stage 58 provide significant efficiency improvements over prior cryogenic CCS systems. The first compressor 60 and the second compressor 62 allow the C02 separation and liquefaction system 20 to recuperate melting energy of the solid C02 near the melting point (e.g., near 220K in some embodiments) while using flue gas as the only refrigerant. In contrast, other cryogenic concepts require an external refrigeration system which adds significant complexity and cost. An external refrigeration cycle also requires additional energy to run the cycle, the costs of which can be avoided using embodiments of the current invention. [0033] After leaving the second cooling stage 58, the flue gas can enter a pre-frost heat exchanger 68 which cools the flue gas to a temperature just above where C02 begins to frost (desublimate). Next, the flue gas can enter an expansion stage 69 which may include a plurality of heat exchangers 70 followed by a plurality of expanders 72, though embodiments of the invention may include other arrangements including a single heat exchanger and/or expander. The plurality of heat exchangers 70 includes a first frost heat exchanger 74 to cool the flue gas preferably to a temperature where flue gas frosts on the walls of the heat exchanger (e.g., to approximately 200K in some embodiments). FIG. 1 shows the plurality of heat exchangers 70 also including a second frost heat exchanger 76, a third frost heat exchanger 78, and a fourth frost heat exchanger 80 to extract additional C02 from the flue gas. The pre-frost heat exchanger 68 and each of the plurality of heat exchangers 70 may use ambient pressure, nitrogen-rich flue gas as a coolant. That is, the flue gas is cooled in the plurality of heat exchangers 70 using recuperated nitrogen-rich flue gas from the plurality of expanders 72 to the point where the C02 begins frosting on the heat exchanger tube walls.

[0034] Next, the flue gas is preferably expanded in a plurality of expanders 72 reducing temperature to provide a coolant to each of the plurality of heat exchangers 70. FIG. 1 shows the plurality of expanders 72 as including a first expander 82, second expander 84, third expander 86, and a fourth expander 88. The first expander 82 may reduce the temperature of the flue gas received from the fourth frost heat exchanger 80 to provide a coolant back to the fourth frost heat exchanger 80. The second expander 84 may reduce the temperature of the flue gas received from the first expander 82 after it is warmed in the fourth frost heat exchanger 80 to provide a coolant to the third frost heat exchanger 78, etc. Thus, the expansion stage 69 may include a plurality of heat exchangers 70 coupled in series to extract the solid C02 from the flue gas, and a plurality of expanders 72 coupled in series and downstream from the plurality of heat exchangers, with each of the heat exchangers coupled to a separate one of the expanders to receive the flue gas as a refrigerant. The process of expansion and reheating occurs for a total of four expansion steps in the expansion stage 69, though any number of suitable expansion steps may occur. After the last expansion step, the nitrogen rich flue stream is sent to cool the second plurality of series connected heat exchangers 46 (e.g., N2 coolers), which preferably warms the nitrogen rich flue stream to near ambient temperatures before its released to the environment.

[0035] With regard to operation of the heat exchangers 70 and expansion stage 69, it is noted that - after the flue gas leaves the plurality of heat exchangers 70 - enough of the C02 has been removed such that the C02 is so diluted in the flue gas that it will have negligible impact on the expansion process. That is, the formation/build-up of solid C02 during the expansion process performed by expansion stage 69 is avoided by taking nearly all of the C02 out first in the heat exchangers 70. For example, approximately 97% of the C02 could be removed from the flue gas by operation of the heat exchangers 70 prior to the flue gas passing to the expansion stage 69.

[0036] Formation of solid C02 on the frost heat exchangers 74, 76, 78, 80, may be removed to use in cooling heat exchangers in the first cooling stage 43 and the second cooling stage 58 prior to sequestration or use in other industrial applications. In order to remove the solid C02 from the frost heat exchangers 74, 76, 78, 80, one or more of the frost heat exchangers can run a recuperation mode 90 while the others remain online. The recuperation process could involve circulating liquid C02 (e.g., at approximately 12 bar in some embodiments) through the respective frost heat exchanger to melt solid C02 from walls of the heat exchanger (described further below).

[0037] The liquid C02 used in the recuperation mode 90 may be stored in a liquid C02 reservoir 92. The liquid C02 from the liquid C02 reservoir 92 is circulated through the respective frost heat exchanger 74, 76, 78, 80, to melt off solid C02 formed on the walls of the heat exchanger, which is added to the liquid C02 reservoir 92. The plurality of heat exchangers 70 preferably includes extra capacity sufficient to allow one or more heat exchangers to run the recuperation mode 90 while the others remain online.

[0038] The liquid in the liquid C02 reservoir 92 is also used as a refrigerant in the first cooling stage 43 and the second cooling stage 58. A pump 94 may pump the liquid C02 from the liquid C02 reservoir 92 to a first valve 96 that directs some of the liquid C02 to the heat exchanger in the recuperation mode 90 and directs the rest of the liquid C02 to the second cooling stage 58. Next, a second valve 98 may be used to direct some of the liquid C02 to the first heat exchanger 64 and the rest of the liquid C02 to the second heat exchanger 66. The liquid C02 from the first heat exchanger 64 and the second heat exchanger 66 are combined at a third valve 100, also referred to as a mixing valve, which directs some of the liquid C02 back to the liquid C02 reservoir 92 and the rest to a second pump 102. The second pump 102 pumps liquid C02 to the first plurality of series connected heat exchangers 44, which can subsequently enter to a C02 pipeline 104 preferably as a liquid or supercritical fluid for sequestration or other industrial use.

[0039] The liquid C02 that returns to the liquid C02 reservoir 92 from the second cooling stage 58 can be cooled by the melting energy of the solid C02 during the recuperation mode 90. Thus, the second cooling stage 58 may include a cooling loop 105 comprising one or more heat exchangers 64, 66 to cool compressed flue gas with liquid C02, and a liquid C02 reservoir 92 to cool liquid C02 expelled from the one or more heat exchangers using solid C02 (e.g. a C02 melt).

[0040] As stated above, solid C02 builds upon on the walls of the frost heat exchangers 74, 76, 78, 80, which can reduce flow distribution and heat transfer. To promote even distribution of C02 buildup, the frost heat exchangers 74, 76, 78, 80 may use a single-pass cross-flow configuration, with short path length tubes of the heat exchanger which contain the desublimating fluid. Further, as C02 builds up in certain areas, it creates a flow blockage slowing additional buildup of C02 in those areas, thus promoting even distribution of solid C02 across the heat exchanger. The shorter path lengths also promote a redistribution of flow and C02 buildup in the direction of the desublimating fluid since temperature gradients along the tube lengths is limited. Also, large flow area within the heat exchanger tubes allows for longer periods of C02 buildup before the recuperation mode 90 (C02 melting) must be performed.

[0041] As referred to above, the C02 separation and liquefaction system 20 removes solid C02 from the frost heat exchangers 74, 76, 78, 80 periodically to ensure consistent and adequate performance of the heat exchanger and to remove C02 from the flue stream. As such, the recuperation mode 90 periodically melts the solid C02 off the walls of the heat exchanger using liquid C02 from the liquid C02 reservoir 92. The frost heat exchangers 74, 76, 78, 80 may be grouped into sets of one or more heat exchangers that allow at least one set to be online while the remaining sets are in the recuperation mode 90 (e.g. melting process), and then allow periodic swapping of the different sets in/out of the melting process for continuous operation of the C02 separation and liquefaction system 20.

[0042] A melting process for the frost heat exchangers 74, 76, 78, 80 using the recuperation mode 90 includes a first step of determining whether a predetermined amount of C02 has formed on the walls of a first set of heat exchangers. The first step continues by operating valve(s) (not shown) to turn on the flow of flue gas to a second set of frost heat exchangers that were previously in the recuperation mode 90, and then operating valve(s) to turn off flow of flue gas to the first set of frost heat exchangers. During the first step, solid C02 will be formed on the walls of the first set of heat exchangers, which will also contain pressurized flue gas (e.g., at approximately 12 bar in some embodiments).

[0043] Step two involves operating valve(s) to turn on liquid C02 to the first set of heat exchangers, with the liquid C02 being provided from the first heat exchanger 64, second heat exchanger 66, or the liquid C02 reservoir 92. The liquid C02 will push the flue gas out of the first set of frost heat exchangers and into the liquid C02 reservoir 92 where it can be periodically bled off through a pressure relief valve. The liquid C02 also strips (i.e., melts) off the solid C02 from the first set of frost-heat exchanges, absorbing its cooling potential (i.e., cold/melting energy) and transferring it to the liquid C02 reservoir 92. The mass flow of the liquid C02 is high enough to prevent additional freezing within the first set of frost heat exchangers.

[0044] After the solid C02 is removed from the first set of heat exchangers in the recuperation mode 90, the process continues at step three where flue gas is pulled into the first set of heat exchangers to displace the liquid C02. The first set of frost heat exchanger is now cleaned of solid C02 and is appropriately pressurized with flue gas. The aforementioned valves can be operated to swap out another set of frosting heat exchangers when needed.

[0045] Referring to FIG. 2, a process diagram of a C02 separation and liquefaction system with frosting expansion is shown, according to an embodiment of the invention. The C02 separation and liquefaction system 20 is shown coupled to a combined-cycle power plant 22 that includes a natural gas power plant 24, a coal steam plant 26, and a TES system 28 thus providing a CCS system 30 for the combined-cycle power plant 22.

[0046] FIG. 2 shows the CCS system 30 including a first cooling stage 43 to cool a flue gas with C02 to be sequestered, and a compression stage 56 coupled to the first cooling stage to compress the cooled flue gas. A second cooling stage 58 may be coupled to the compression stage 56 to cool the compressed flue gas with solid C02, and an expander 106, also referred to as a frost expander, may be coupled to the second cooling stage to extract solid C02 from the flue gas. The expander 106 is preferably coupled to the first cooling stage 43 and the second cooling stage 58 to provide the solid C02 and the C02 to be sequestered. The embodiment of FIG. 2 shares many elements with the embodiment of FIG. 1, and discussion of these elements are found with respect to FIG. 1.

[0047] In the embodiment of FIG. 2, after the flue gas is cooled in the pre-frost heat exchanger 68, the flue stream is further cooled in the first frost heat exchanger 74 to the point where the C02 begins frosting on the walls of the first frost heat exchanger 74. The flue gas then enters the expander 106 and is preferably expanded to near atmospheric pressure. During the expansion process, more C02 will solidify (i.e., frost), which can be filtered from the flue gas using a solid C02 filter 108, also referred to as a bag house. The solid C02 may then be pressurized and pressed into the liquid C02 reservoir 92 using an air lock 110, which may include a solid compressor. The solid C02 pressed into the liquid C02 reservoir 92 cools liquid C02 received from the second cooling stage 58, thus recover its melting energy.

[0048] Upon removal of C02 in the frost expander 106, the flue gas may reach the point where only 3% of the original C02, in this embodiment, remains in vapor form.

The remaining nitrogen-rich flue stream may then be used as a refrigerant in the first frost heat exchanger 74, the pre-frost heat exchanger 68, and then the second plurality of series connected heat exchangers 46, before being discharged to the atmosphere. The first frost heat exchanger 74 may be periodically swapped with a replacement so that it can enter the recuperation mode 90, as discussed above with respect to FIG. 1. Extra capacity, in the form of additional frost heat exchangers, allows for the first frost heat exchanger 74 to enter the recuperation mode 90 without turning offline the C02 separation and liquefaction system 20.

[0049] The frost expander 106 may comprise an expansion turbine to produce work used to drive a compressor or generator, and can reduce heat transfer area over that of heat exchangers, thereby reducing capital costs. It is recognized that drawbacks of inclusion of the expander 106 may include solid C02 buildup, erosion from solid C02 particles, high subcooling requirements in order to nucleate the C02, and maintaining high efficiency, but that such drawbacks can be mitigated.

[0050] Solid C02 buildup on the walls of the expander 106 can clog the expander or lead to balance issues with turbine blades. C02 buildup can occur through two main mechanisms: heterogeneous nucleation as the C02 cools during expansion, and sticking to the expander upon impingement as C02 particles flow through the expander. Both of these issues may be overcome by heating the turbine wall or using microchannels to provide an inert gas along the walls for protection. Additionally, special coatings may be used to increase the activation energy of heterogeneous nucleation on the walls and/or prevent sticking of impinged solid C02.

[0051] Additionally, impacts from solid C02 particles against the expander 106 can cause erosion. Turbine blade erosion can be minimized by employing three different strategies. First, erosion can be reduced by coating the rotor blades with a high Vickers hardness coating including WC-10Co-4Cr Tungsten Carbide Cobalt Chrome. Such coatings have high resistance to erosion and are commonly used to protect slurry pumps and turbine blades from erosion. Second, erosion can be reduced by minimizing impact pressure below the impacted material yield strength, accomplished by reducing impact velocity. Third, erosion can be minimized by reducing particle impact angle, since the highest impact force generally occurs when the impact angle is 90° to an impact surface.

[0052] C02 nucleation from the flue gas in the expander 106 may require increasing subcooling of the C02. However, increased subcooling requirements can reduce efficiency of the C02 separation and liquefaction system 20 by requiring large over pressurization of the flue gas to drive the nucleation. A default strategy includes seeding the flue gas at the inlet of the expander 106 with either liquid C02 spray, a non-C02 liquid contact fluid spray, solid C02 dust, or non-C02 dust particles.

[0053] With regard to the CCS system 30, a CCS system 30 has been simulated in ChemCAD software using flue gas from an NETL Case B31 A natural gas combined cycle power plant. Capital cost estimations were performed using a ground-up design process to account for all materials and fabrication steps. Table 1 below shows the key performance parameters associated with the CCS system 30. With over 97% capture efficiency, the cost to capture C02 is only $29/tonne. This cost accounts for the installed capital cost, electricity consumed, operation and maintenance costs, and lost capacity of a power plant. Table 2 gives the key assumptions used to get the CCS system 30 performance parameters. A relatively low electricity price of $29/MWh is possible since the CCS system 30 can be installed at legacy power plants to leverage their low costs of energy. A capacity factor of 90% was assumed since the CCS system 30 will be producing a commodity of nearly pure C02, which can be sold for revenue. The commodity factor coupled with the performance and maintenance cost advantages of running the power plant at baseload lead to such a high capacity factor assumption.

[0054] Table 1 : CCS System key performance parameters for a natural gas combined cycle power plant.

[0055] Table 2: CCS System key assumptions.

[0056] The cost energy penalty at a natural gas combined cycle power plant for the CCS system 30 may be about 0.517 MWh/tonne (or for example less than .525 or .550 MWh/tonne), which at a state of the art new natural gas combined cycle power plant leads to 22% lost energy production.

[0057] Benefits of the CCS system 30 include a simple, low-risk system using vapor compression, expansion, and heat exchange, with the ultimate benefit being a low capital cost CCS system. For such a low capital cost, the energy penalty is within an acceptable range. The cold compressor technique (e.g., using first compressor 60 and second compressor 62) is more efficient than if the CCS system were to use only hot compression, thereby reducing energy input required to operate the CCS system 30.

[0058] Referring to FIG. 3, a simplified process diagram for C02 separation and liquefaction with interwarming expansion is shown, such as would be carried out by the C02 separation and liquefaction system of FIG. 1, according to an embodiment of the invention. The C02 separation and liquefaction process can avoid any substantial desublimation of C02 during expansion (e.g., to prevent erosion and clogging of the expander). The C02 separation and liquefaction process prevents frosting in the expander

111 by forcing the frosting to occur in one or more recuperating cold heat exchangers 112 and performing an inter-stage warming process during expansion. The inter-stage process involves expanding some flue gas and sending the expanded cooled flue gas back to a recuperating cold heat exchanger 112 (which warms the flue gas), then expanding the gas further, returning it to the heat exchanger(s) for further warming and pre-cooling of incoming flue gas before expansion.

[0059] Referring to FIG. 4, a simplified process diagram for C02 separation and liquefaction using an air Brayton cooling cycle is shown, in accordance with an embodiment of the invention. The air Brayton cooling cycle preferably uses interwarming expansion with air as the working fluid, in contrast to flue gas as a working fluid like the interwarming expansion process of FIG. 3. Referring back to FIG. 4, the air Brayton cooling cycle decouples the flue gas from an expansion cooling step.

[0060] The C02 separation and liquefaction system 20 may receive and cool flue gas at a first heat exchanger 150, preferably cooled by the ambient environment. The system may also receive and compress air from the ambient environment at a first compressor 152. Air from the first compressor 152 may be cooled in a second heat exchanger 154, preferably cooled by the ambient environment. The flue gas from the first heat exchanger 150 and the air from the second heat exchanger 154 are cooled in a first cooling stage 156 using liquid C02, cold nitrogen-rich flue gas, and cold air. Air from the first cooling stage 156 is compressed in a second compressor 158 and then cooled in a second cooling stage 160. The second cooling stage 160 operates with an expander 162 to perform an inter-stage warming process during expansion, as discussed in more detail previously with respect to FIG. 3 (and FIG. 1).

[0061] Referring back to FIG. 4, flue gas from the first cooling stage 156 is further cooled in the second cooling stage 160 to extract solid or liquid C02, with the flue gas in the second cooling stage 160 cooled by the expanded air. The cooling potential of the extracted solid or liquid C02 is recovered in a cooling loop that provides liquid C02 as a refrigerant to the first cooling stage 156, but could recover cooling potential within the second cooling stage 160. After the C02 extraction, the nitrogen-rich flue stream and the air from the second cooling stage 160 is also used as a refrigerant to the first cooling stage 156. After the first cooling stage 156, the air and the nitrogen-rich flue stream can be vented to the atmosphere, and the extracted C02 can be used in industrial processes or sequestered.

[0062] Accordingly, the flue gas can be cooled using the cooling potential created by expanded air, rather than by expanding the flue gas and using the flue gas as the only refrigerant source. Further, either of the first or second cooling stages 156, 160 may comprise one or more heat exchangers. Since the flue gas is decoupled from the expansion step, either pressurized or unpressurized flue gas can be cooled in the C02 separation and liquefaction system 20. A system with pressurized flue gas could include compressors or expanders for the flue gas in addition to the compressors and expanders shown in FIG. 4.

[0063] Further, the second cooling stage 160 may comprise a thermal energy storage device that stores cooling potential from the expanded air. If the second cooling stage 160 comprises a thermal energy storage device, the air Brayton cooling cycle could be operated on an independent schedule from that of the flue gas, with the cooling potential of the expanded air stored for later use when the flue gas is available. Thus, the flue gas could be chilled to extract solid or liquid C02 when the air Brayton cycle is not in operation. For example, the thermal energy storage device could store cooling potential from the air Brayton cycle when electricity prices are low and cycle operation is less expensive, and then use the stored cooling capacity when needed during periods of high electricity prices.

[0064] Referring to FIG. 5 a simplified process diagram for C02 separation and liquefaction with frosting expansion is shown, such as would be carried out by the C02 separation and liquefaction system of FIG. 2, according to an embodiment of the invention. The C02 separation and liquefaction process may include extracting flue gas from a flue gas producer 114 and cooling the flue gas from the flue gas producer in a first heat exchanger 116 cooled by the ambient environment. The flue gas may then be compressed by a first compressor 118 and cooled by a second heat exchanger 120 cooled by the ambient environment. The C02 separation and liquefaction process may include cooling a first stream of the flue gas from the second heat exchanger 120 using liquid C02 in a third heat exchanger 122 and compressing the flue gas in a second compressor 124. The process may also include cooling the flue gas in a fourth heat exchanger 126 by melting solid C02 and producing the liquid C02, and expanding the flue gas in one or more expanders 128 to extract the solid C02. Either of the first heat exchanger 116 and the second heat exchanger 120 could be a water-cooled heat exchanger cooled by water from the ambient environment.

[0065] The C02 separation and liquefaction process may further include cooling the flue gas in a pre-frost heat exchanger 68 (FIG. 1) and a first frost heat exchanger 74 (FIG. 1) after cooling the flue gas in the fourth heat exchanger 126 by melting solid C02 and producing the liquid C02. The process may also include swapping the first frost heat exchanger 74 (FIG. 1) with a second frost heat exchanger to continue cooling the flue gas, and running liquid C02 through the first frost heat exchanger to melt the solid C02.

[0066] Referring to FIG. 6, a graph of simulated C02 buildup up on the walls of a heat exchanger (e.g., any of frost heat exchangers 74, 76, 78, 80) used in a C02 separation and liquefaction system is shown, in accordance with an embodiment of the invention. That is, FIG. 6 is a graph of solid C02 thickness build up in inches on heat exchanger tube walls, each having a 2 inch diameter, along the length of different rows 180, 182, 184, 186, 188 of tubes at 4.2362 hours into the simulation. As can be seen, the heat exchanger design was able to achieve a relatively even distribution of solid C02 buildup.

[0067] Beneficially, embodiments of the invention thus provide a C02 separation and liquefaction system that may be used to improve carbon capture and sequestration. In addition, new and novel processes and techniques for implementing a carbon capture and sequestration (CCS) system are disclosed. The CCS system can use flue gas as the only refrigerant, with compression split up into a warm component and a cold component. After the warm compression, flue gas may be cooled by the ambient environment and then cooled using recirculated flue gas and liquid C02. After cold compression, the flue gas may be cooled by liquid C02 that has been cooled by recovering melting energy of solid C02. The CCS system also exhibits substantial cost advantages as a result of lower capital costs and improved efficiency.

[0068] Therefore, according to one embodiment of the invention, a C02 separation and liquefaction system includes a first cooling stage to cool flue gas with liquid C02, a compression stage coupled to the first cooling stage to compress the cooled flue gas, a second cooling stage coupled to the compression stage and the first cooling stage to cool the compressed flue gas with a C02 melt and provide the liquid C02 to the first cooling stage, and an expansion stage coupled to the second cooling stage to extract solid C02 from the flue gas that melts in the second cooling stage to provide the liquid C02.

[0069] According to another embodiment of the invention, a carbon capture and sequestration (CCS) system includes a first cooling stage to cool a flue gas with C02 to be sequestered, a compression stage coupled to the first cooling stage to compress the cooled flue gas, a second cooling stage coupled to the compression stage to cool the compressed flue gas with solid C02, and an expander coupled to the second cooling stage to extract solid C02 from the flue gas, the expander coupled to the first cooling stage and the second cooling stage to provide the solid C02 and the C02 to be sequestered.

[0070] According to yet another embodiment of the invention, a method of operating a C02 liquefaction system includes extracting flue gas from a flue gas producer, compressing the flue gas in a first compressor, cooling the flue gas in a heat exchanger cooled by the ambient environment, cooling a first stream of the flue gas from the heat exchanger using liquid C02, compressing the flue gas in a second compressor, cooling the flue gas by melting solid C02 and producing the liquid C02, and expanding the flue gas to extract the solid C02.

[0071] According to still another embodiment of the invention, a CCS system for a power plant includes a first compressor to compress air and a first heat exchanger coupled to the first compressor to cool the compressed air, the first heat exchanger cooled by an ambient environment. The CCS system also includes a first cooling stage coupled to the power plant to receive flue gas and coupled to the first heat exchanger to receive the air, the first cooling stage cooling the flue gas and the air with liquid C02. The CCS system further includes a second compressor coupled to the first cooling stage to receive and compress the air, a second cooling stage coupled to the second compressor to receive and cool the air, the second cooling stage coupled to the first cooling stage to cool and extract C02 from the flue gas and provide the liquid C02, and an expansion stage coupled to the second cooling stage to expand the air and provide the expanded air to the second cooling stage to cool the flue gas.

[0072] 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.