System and method for capturing CO2 from combustion flue gas

Abstract

The invention discloses a system and a method for capturing CO2 from combustion flue gas. The method comprises the following steps: performing multi-stage compression and multi-stage cooling on flue gas to enable the flue gas to reach the pressure and temperature range of CO2 condensation, and separating and recovering liquid CO2; the separated non-condensable gas is heated through compression heat generated in the multi-stage compression process; and the heated high-pressure non-condensable gas is introduced into a multi-stage expansion machine to do work through expansion, so that the pressure energy of the non-condensable gas is recycled, and the work recycled by the multi-stage expansion machine is used for compensating the power consumption of the multi-stage compressor. According to the system, waste heat in the compression process is used for heating inlet air of the expansion machine, work recycled by the expansion machine is used for compensating power consumption of the compressor, gradient utilization and efficient coupling recycling of internal energy of the system are achieved, energy consumption and operation cost of CO2 liquefaction trapping are remarkably reduced, and economic benefits are high.

Description

Technical Field

[0001] This invention relates to the field of greenhouse gas control technology, and more specifically, to a system and method for capturing and liquefying CO2 from industrial combustion flue gas, particularly a cryogenic capture process that utilizes internal energy coupling to achieve high thermal efficiency and low energy consumption. Background Technology

[0002] Since the Industrial Revolution, the large-scale consumption of fossil fuels has led to a sharp rise in atmospheric CO2 concentration, triggering a series of severe problems such as global climate change, glacial melting, and extreme weather events. Therefore, carbon capture, utilization, and storage (CCUS) technology, which captures CO2 from the combustion flue gas of large stationary emission sources (such as thermal power plants, cement plants, and steel plants), is considered a key technological path to address the climate crisis.

[0003] Currently, existing CO2 capture technologies mainly include chemical absorption, physical absorption, membrane separation, low-temperature separation, and adsorption.

[0004] Among these methods, chemical absorption, particularly absorption based on amine solvents (such as monoethanolamine MEA), is currently the most mature and widely used capture technology. However, this technology has fatal flaws. First, it has extremely high energy consumption. The energy consumption of this process is mainly concentrated in the solvent regeneration process, i.e., heating the rich solution at high temperatures (about 120°C) to desorb CO2. This process requires a large amount of high-quality steam, resulting in a parasitic load as high as 25-40%, significantly reducing the net power generation efficiency of the power plant. Second, it presents numerous operational problems. Amine solvents are prone to oxidation and thermal degradation under high temperature and aerobic conditions, leading to continuous consumption and replenishment of the solvent; at the same time, amine solutions and their degradation products (such as thermally stabilized salts) are highly corrosive to pipelines and equipment. In addition, the escape of degradation products and amine vapors not only causes secondary environmental pollution but also generates toxic waste that requires expensive treatment, greatly increasing operating costs (OPEX) and environmental footprint.

[0005] Cryogenic separation is a physical separation method based on the difference in phase transition temperatures of components. It achieves separation by cooling flue gas to the phase transition point of CO2 (liquefaction or sublimation). A major challenge of traditional cryogenic processes (such as cryogenic distillation or anti-sublimation techniques) is the high moisture content (H2O) in the flue gas and the high sublimation point (dry ice point) of CO2. During deep cooling (e.g., below -100°C), moisture and CO2 readily form ice or dry ice (CO2 frost) on the heat exchanger surface. These solid substances rapidly clog the heat exchanger surface and pipes, hindering heat transfer, leading to a sharp decrease in system efficiency, and forcing the system to periodically shut down for defrosting and regeneration, which complicates operation and increases energy consumption. Furthermore, to achieve such low temperatures (far below the triple point of CO2 at -56.6°C), the system must rely on energy-intensive external refrigeration cycles (such as cascaded refrigeration systems or Stirling refrigerators). If the system's cold energy cannot be efficiently recovered, its total energy consumption may even exceed that of chemical absorption methods.

[0006] Membrane separation utilizes the difference in permeation rates between CO2 molecules and other gases (such as N2) in a specific membrane material to separate CO2. The main bottleneck of this technology lies in an inherent "selectivity-permeability" trade-off: membranes with high selectivity (i.e., high CO2 purity) typically have low permeability (i.e., processing flux), and vice versa. The CO2 concentration and partial pressure in combustion flue gas are usually very low (approximately 10-15%). To obtain sufficient transmembrane driving force, high-pressure compression of the upstream flue gas or application of a vacuum on the downstream permeate side is usually required, both of which consume significant amounts of electrical energy. To obtain high-purity CO2 products, multi-stage membrane separation systems are often necessary, further increasing compressor power consumption and equipment investment. Simultaneously, the performance and lifespan of membrane materials are susceptible to contamination and chemical degradation by impurities such as SOx and NOx in the flue gas.

[0007] Some existing technologies have recognized the potential of combining compression and expansion processes, such as utilizing the cold energy of an expander to liquefy CO2, or broadly mentioning the recovery of thermal and pressure energy. However, in-depth analysis of related technologies reveals that they fail to disclose or inspire the core of this invention: using the waste heat (a low-grade energy) generated during multi-stage compression to preheat the high-pressure, non-condensable gas (exhaust gas) rich in N2 after CO2 separation, and then feeding this high-temperature, high-pressure exhaust gas into the expander to maximize its expansion work potential (a high-grade energy). Thermodynamically, increasing the temperature of the gas at the expander inlet (i.e., increasing its enthalpy) is one of the most effective means of increasing its output work. This invention, through this specific thermodynamic cycle design, converts the compression heat that would otherwise be wasted into high-value mechanical work to compensate for the compressor's own power consumption. This specific closed-loop path of internal energy coupling has not been achieved in existing technologies.

[0008] In summary, existing technologies either suffer from poor economic efficiency due to high energy consumption, low stability due to complex operation (such as frost clogging), or inherent performance bottlenecks. There is an urgent need in this field for a novel CO2 capture method that is low in energy consumption, stable in operation, and highly economical. Table 1 below summarizes the performance comparison between this invention and existing mainstream CO2 capture technologies.

[0009] Table 1: Performance Comparison of Existing CO2 Capture Technologies

[0010] Summary of the Invention

[0011] The purpose of this invention is to overcome the shortcomings of existing CO2 capture technologies (especially chemical absorption and traditional cryogenic methods) in the background art, such as high energy consumption, high operating costs, equipment corrosion, solvent degradation, and frosting blockage, and to provide an energy coupling system and method for CO2 liquefaction capture of combustion flue gas that is energy-efficient, economical, and stable in operation.

[0012] The technical solution of the present invention:

[0013] To achieve the above objectives, the present invention provides a method for capturing CO2 from combustion flue gas. This method is characterized by comprising an internally energy-coupled cycle. First, the combustion flue gas is subjected to multi-stage compression, generating heat of compression during the process; simultaneously, the compressed gas is cooled to achieve CO2 liquefaction conditions. Second, the condensed liquid CO2 is separated from a high-pressure non-condensable gas stream (primarily N2). Next, the separated high-pressure non-condensable gas stream is heated using the heat of compression generated during the preceding multi-stage compression. Then, the heated, high-temperature, high-pressure non-condensable gas stream is introduced into a multi-stage expander to expand and perform work, generating mechanical work. Finally, the mechanical work generated by the expander is recovered and used to at least partially drive the multi-stage compressor.

[0014] Accordingly, the present invention also provides a system for implementing the above method. The system is characterized by comprising: multiple multi-stage compressors for compressing combustion flue gas and equipped with coolers to remove the heat of compression; a phase separator for separating liquid CO2 and the main stream of non-condensable gases; a heat exchange network whose core function is to transfer the heat of compression removed by the compressors and coolers to the high-pressure main stream of non-condensable gases discharged from the phase separator; multiple multi-stage expanders for expanding the heated main stream of non-condensable gases to generate mechanical work; and a power transmission device (such as a drive shaft or a motor-generator set) for transmitting the mechanical work generated by the expanders to the multi-stage compressors to compensate for their power consumption.

[0015] Compared with the prior art, the present invention has the following significant advantages:

[0016] 1. Significant energy-saving effect: The greatest advantage of this invention lies in its highly efficient internal energy coupling cycle.

[0017] (a) Work recovery: Non-condensable gases such as N2, which dominate the combustion flue gas, can be recovered as high-value mechanical work (or electrical energy) after being compressed to high pressure through a multi-stage expander.

[0018] (b) Utilization of Waste Heat: The core innovation of this invention lies in the creative use of the low-to-medium grade compression heat generated during multi-stage compression, which would otherwise be wasted, to heat the inlet gas of the expander. According to thermodynamic principles, increasing the enthalpy (i.e., temperature) of the expander inlet gas is key to maximizing its work capacity. Through this design, this invention upgrades low-grade "waste heat" into high-grade "mechanical work," greatly increasing the expander's output work and thus significantly reducing the net electrical energy consumption required to drive the compressor.

[0019] 2. High operational stability, fundamentally avoiding blockage: Traditional cryogenic methods suffer from frosting blockage caused by dry ice (CO2 solid phase). This invention overcomes this problem through a non-obvious design: pressurizing the flue gas to a high-pressure state (e.g., 200-700 bar) far above the CO2 triple point pressure (5.18 bar), and cooling it under this high pressure. According to the CO2 phase diagram, at this pressure, under the partial pressure of CO2, CO2 will directly condense from the gas phase to the liquid phase, rather than sublimating into the solid phase. This invention, by separating in the liquid phase region, fundamentally avoids the formation of any dry ice, thereby eliminating the risk of blockage and ensuring the long-term stable operation of the system.

[0020] 3. Environmentally friendly and free of secondary pollution: This invention is a purely physical separation process that does not rely on any chemical solvents (such as amines). Therefore, it completely avoids a series of problems inherent in chemical absorption methods, such as solvent degradation, equipment corrosion, and the treatment of toxic and harmful waste liquids, making the process cleaner and more environmentally friendly.

[0021] 4. High purity and easy transport of CO2 product: This method directly obtains high-pressure liquid CO2 product through high-pressure phase separation. This product is of high purity and is already in a high-pressure liquid state, allowing for direct pipeline transport or geological storage (EOR / CCS) without the need for additional large-scale compression. Attached Figure Description

[0022] Figure 1 A detailed schematic diagram of the core energy coupling unit according to an embodiment of the present invention illustrates the coupling relationship between the multi-stage compressor, the multi-stage expander, and the heat exchange network.

[0023] In the diagram: 1 Compressor A, 4 Compressor B, 7 Compressor C, 2 Cooler A, 5 Cooler B, 8 Cooler C, 3 Liquid CO2 Collector A, 6 Liquid CO2 Collector B, 9 Liquid CO2 Collector C, 10 Heat Exchange Medium Pump A, 14 Heat Exchange Medium Pump B, 18 Heat Exchange Medium Pump C, 11 Heater A, 15 Heater B, 19 Heater C, 12 Expander A, 16 Expander B, 20 Expander C, 13 Power Transmission Device A, 17 Power Transmission Device B, 21 Power Transmission Device C, 100 Combustion Flue Gas, 200 Non-condensable Component Flue Gas. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0025] Example 1: Overall flow chart of an energy-coupled CO2 capture system (see...) Figure 1 )

[0026] Figure 1 A flowchart illustrating the overall process flow of a CO2 capture system according to a basic embodiment of the present invention is shown. The system mainly includes a 3-stage compressor (1, 4, 7), 3 sets of coolers (2, 5, 8), 3 sets of liquid CO2 collectors (3, 6, 9), 3 sets of heat exchange medium pumps (10, 14, 18), 3 sets of heaters (11, 15, 19), a 3-stage expander (12, 16, 20), and 3 sets of power transmission devices (13, 17, 21).

[0027] Step 1: Deep cooling of inlet flue gas and heating and reheating of outlet flue gas

[0028] The deeply purified combustion flue gas 100 (mainly composed of N2 and CO2) and the final expanded non-condensable component flue gas 200 enter the heat exchanger 22 for heat exchange.

[0029] (a) Flue gas pre-cooling: The combustion flue gas 100 (mainly composed of N2 and CO2) after deep purification and dehydration enters the heat exchanger 22 for heat exchange and cooling, and the temperature is reduced to room temperature or lower (such as 25°C or -20°C).

[0030] (b) Heating and warming of the non-condensable component flue gas after final expansion: The low-temperature non-condensable component flue gas 200 (mainly composed of N2) after final expansion enters the heat exchanger 22 for heating, so that its temperature rises back to room temperature or a slightly higher temperature (such as 25°C or 35°C). This ensures that no white smoke is visible when it is discharged from the chimney.

[0031] Step 2: Multi-stage compression and CO2 liquefaction

[0032] The preheated and purified combustion flue gas (mainly composed of N2 and CO2) enters the multi-stage compressor unit (1, 4, 7).

[0033] (a) Compression: Given the large flue gas flow rate and high compression ratio (from 1 bar to 200-700 bar), the compressor units (1, 4, 7) are preferably high-efficiency integral gear centrifugal compressors or large axial-centrifugal hybrid compressors. For example... Figure 1 As shown, the compressor unit may include multiple compression stages (e.g., 1, 4, 7) and multiple interstage coolers (2, 5, 8).

[0034] (b) Cooling and Liquefaction: After being compressed by compressor A1, the high-temperature gas enters interstage cooler A2 for cooling. The cooled gas then enters compressor B4 for further compression, and the high-temperature gas after compression enters interstage cooler B5 for cooling. This process continues sequentially through compressor C7 for compression and interstage cooler C8 for cooling. In each cooler (2, 5, 8), the high-temperature, high-pressure gas (e.g., 90 bar) from the corresponding compressor is sufficiently cooled to ambient temperature or lower (e.g., 25°C or -5°C).

[0035] (c) Phase transition: According to the phase diagram of CO2, at 90 bar and 25°C, the pressure is much higher than its triple point pressure (5.18 bar) and the temperature is lower than its critical temperature (31.1°C). Therefore, most of the CO2 components in the flue gas will undergo a phase transition to a liquid state under these conditions. The main components of the flue gas, N2, and small amounts of O2, have extremely low critical points and remain in a high-pressure gas state under these conditions.

[0036] (d) Compression heat recovery: In the cooler group (2, 5, 8), the heat released by the high-temperature compressed gas (i.e., the heat of compression Q_comp) is carried away by the cooling medium (e.g., a closed-loop hot water or hot oil circuit). This heat is not wasted, but is transferred to the heater group (11, 15, 19) in step four.

[0037] Step 3: Phase separation (Units 3, 6, 9)

[0038] Liquid CO2 from cooler groups (2, 5, 8) settles into liquid CO2 collector groups (3, 6, 9) under gravity.

[0039] (a) Liquid CO2 product: The denser liquid CO2 settles to the bottom of the cooler (2, 5, 8) under the action of gravity, and is drawn out as a high-purity, high-pressure liquid CO2 product to the liquid CO2 collector group (3, 6, 9), which can be directly used for subsequent transportation or storage.

[0040] (b) High-pressure non-condensable gas: The main stream of non-condensable gas (NCG) with a lower density (mainly composed of N2, still at, for example, 90 bar and 25°C) is discharged from the top of the coolers (2, 5, 8).

[0041] Step 3: Energy Coupling and Recovery

[0042] This step is the core of the invention, achieving efficient internal coupling of thermal energy and mechanical work.

[0043] (a) Exhaust gas heating: The high-pressure non-condensable gas from step three does not expand directly, but is first introduced into heaters (19, 15, 11) to be heated and become high-temperature and high-pressure gas (e.g., temperature exceeding 210°C).

[0044] (b) Waste heat supply: The heat of compression (Q_comp) from step two is used as a heat source and is introduced into the heater heat (19, 15, 11) by heat exchange medium pumps (18, 14, 10).

[0045] (c) Heat exchange: In the heaters (11, 15, 19), the heat of compression (Q_comp) heats the high-pressure non-condensable gas to a high temperature (e.g., close to the compressor outlet temperature, such as 210°C), forming a high-temperature, high-pressure gas.

[0046] (d) Expansion and work: The high-temperature, high-pressure gas is then fed into a turbine expander (12, 16, 20) (e.g., a multi-stage centripetal expander). Due to its high inlet temperature (enthalpy), the gas has a very strong capacity to expand and perform work in the expander.

[0047] (e) Work Compensation: The expanders (12, 16, 20) output a large amount of mechanical work (W_exp). This mechanical work is recovered through a power transmission device (13, 17, 21) (e.g., a common drive shaft connected to the compressors (1, 4, 7), or a "generator-motor" system) and used directly to drive the multi-stage compressor unit (1, 4, 7), thereby significantly compensating for its total power consumption (Wcomp).

[0048] (f) Emissions and Cold Energy Recovery: After the non-condensable gas 200 expands and does work in the last expander C20, its pressure drops to near atmospheric pressure (e.g., 1.1 bar), and its temperature also drops sharply to a low temperature (e.g., -100°C). This low-temperature exhaust gas (mainly N2) is sent (as described in claim 10) to the heat exchanger 22 in step one to pre-cool the flue gas entering the compressor unit, further recovering cold energy and reducing the total energy consumption of the system.

[0049] Example 2: Quantitative Simulation Working Case

[0050] To quantitatively demonstrate the beneficial effects of the present invention, the inventors used Aspen HYSYS process simulation software to conduct a thermodynamic simulation of a typical 600MW coal-fired power plant flue gas treatment case.

[0051] Input conditions: Total flue gas flow rate is 1,000,000 Nm³ / h, CO2 volume fraction in flue gas is 14%, pressure is 1 bar, and temperature is 40℃. The target for CO2 capture is 90%.

[0052] Simulation benchmark (Prior Art-MEA): According to literature data, the specific energy consumption (SEC) (mainly regeneration heat consumption) of MEA chemical absorption method to achieve the same capture target is about 3.2 GJ / tCO2.

[0053] Simulation results (Example 1 of the present invention):

[0054] (1) After deep purification and pretreatment, the flue gas enters a 5-stage integral gear compressor and is compressed to 90 bar. The total shaft power of the compressor unit (Wcomp,gross) = 150MW.

[0055] (2) The interstage cooler and aftercooler (IC / AC) together generate recoverable heat of compression (Q_comp) = 135MWth.

[0056] (3) After separation in the separator, the flow rate of captured liquid CO2 (T) is 200t / h.

[0057] (4) The high-pressure N2 exhaust gas is heated in the heat exchange network by the 135MWth heat of compression (Q_comp).

[0058] (5) The heated high-temperature and high-pressure exhaust gas enters the 3-stage turboexpander, and the recovered mechanical work (W_exp) = 105MW.

[0059] (6) The net power consumption of the system (W_net) = W_comp_gross - W_exp = 150MW - 105MW = 45MW.

[0060] (7) The specific energy consumption (SEC) of the present invention is W_net / T = (45MW*3600s / h) / 200t / h = 810,000MJ / h / 200t / h = 810MJ / tCO2, that is, 0.81GJ / tCO2.

[0061] Conclusion: Simulation results show that the specific energy consumption (SEC) of this invention is only 0.81 GJ / tCO2. This value is not only far lower than that of the traditional MEA chemical absorption method (3.2 GJ / tCO2), but also superior to other reported advanced hybrid low-temperature membrane processes (approximately 850 MJ / tCO2 or 0.85 GJ / tCO2). The energy consumption of this invention is only approximately 25.3% of that of the MEA method (0.81 / 3.2).

[0062] This result strongly demonstrates that the internal coupling cycle of "compression heat-expansion work" proposed in this invention has a revolutionary energy-saving effect in achieving CO2 capture. At the same time, it fundamentally solves the problems of frosting and clogging in traditional low-temperature methods and solvent degradation and corrosion in chemical methods, and has extremely high industrial application value and economic benefits.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of their technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for capturing CO2 from a combustion flue gas, characterized in that, The process comprises the steps of: (a) multi-stage compression of the deeply cleaned combustion flue gas, said multi-stage compression comprising the use of at least one compressor and one or more inter-stage coolers and / or post-coolers and generating compression heat in said coolers; (b) cooling the compressed combustion flue gas to at least partially condense CO2 in said combustion flue gas into liquid CO2; (c) separating said liquid CO2 from a non-condensable gas main stream; (d) using said compression heat generated in step (a) to heat said non-condensable gas main stream separated in step (c) by heat exchange; (e) introducing said heated non-condensable gas main stream in step (d) into a multi-stage expander for work expansion to generate mechanical work; (f) recovering said mechanical work generated in step (e) for at least partially driving said multi-stage compression in step (a).

2. A system for capturing CO2 from a combustion flue gas, characterized in that, The process comprises: (a) a multi-stage compressor configured for compressing the combustion flue gas, said multi-stage compressor having at least one compressor and one or more coolers for removing compression heat; (b) a phase separator fluidly connected to the end of a cooler of said multi-stage compressor for separating liquid CO2 from a non-condensable gas main stream; (c) a heat exchange network having a first input fluidly connected to said cooler for receiving said compression heat and a second input fluidly connected to the non-condensable gas main stream outlet of said phase separator; said heat exchange network being configured for heating said non-condensable gas main stream using said compression heat; (d) a multi-stage expander having an inlet fluidly connected to the heated non-condensable gas main stream outlet of said heat exchange network for expanding said non-condensable gas main stream to generate mechanical work; (e) a power transmission device mechanically or electrically connecting said multi-stage expander to said multi-stage compressor for transferring said mechanical work to said multi-stage compressor. The process further comprises, prior to step (a), a pre-treatment of the combustion flue gas, said pre-treatment comprising a dehydration step and a step for removing at least one impurity selected from the group consisting of SOx, NOx and particulate matter.

3. The method of claim 1, wherein, The dehydration step uses a temperature swing adsorption (TSA) or pressure swing adsorption (PSA) device and a molecular sieve adsorbent.

4. The method of claim 3, wherein, The heating in step (d) and the expansion in step (e) are staggered, comprising: heating the non-condensable gas main stream using first stage compression heat; expanding the heated non-condensable gas main stream in a first stage of said multi-stage expander; re-heating the first stage expanded non-condensable gas main stream using second stage compression heat; expanding the re-heated non-condensable gas main stream in a second stage of said multi-stage expander.

5. The method of claim 1, wherein, The compressors in said multi-stage compressor are centrifugal compressors or integrally geared compressors.

6. The system of claim 2, wherein, The heat exchange network and / or the coolers in said multi-stage compressor comprise plate-fin heat exchangers (PFHE) or printed circuit board heat exchangers (PCHE).

7. The system of claim 2, wherein, The multi-stage expander is a turbo-expander and the power transmission device is a common drive shaft connecting said expander and said compressor.

8. The system of claim 2, wherein, ​ 9. The method of claim 1, wherein, In the step (b), the combustion flue gas is compressed to a pressure of 70 bar to 100 bar and cooled to a temperature range of -10°C to 30°C.

10. The method of claim 1, wherein, The cold energy of the non-condensable gas main stream discharged after expansion in the step (e) is also used for the cooling of the combustion flue gas in the step (b).

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