Energy-efficient carbon capture process for flue gas

Abstract

A process for separating CO2 from a flue gas (1) comprising at least N2, CO2 and water with a relative humidity above 70%, the process comprising the steps of: a.) heating the flue gas stream (1) with a first heat exchanger (10) to obtain a first feed gas stream (F1) with a desired relative humidity of less than 70%, preferably less than 60%, most preferably less than 50%, at predefined pressure; b.) feeding the first feed gas stream (F1) to a first membrane unit (11, 11a-11d) comprising a first CO2-selective membrane and separating the first feed gas stream (F1) to obtain a first CO2-depleted retentate gas stream (R1) and a first permeate gas stream (P1) comprising CO2 and water; c.) feeding the first permeate gas stream (P1) as a second feed gas stream (F2) to a second membrane unit (21, 21a- 21d) comprising a second CO2-selective membrane to separate the second feed stream (F2) into a second CO2-depleted retentate gas stream (R2) and a second permeate gas stream (P2) comprising CO2 and water; d.) further processing the second permeate gas stream (P2).

Description

[0001] Energy-efficient Carbon Capture Process for Flue Gas

[0002] Technical Field

[0003] The present invention relates to carbon dioxide (CO2) separation processes, specifically designed for flue gas emitted from industrial processes, power plants, waste to energy plants or other combustion sources. More specifically, the invention pertains to novel methods and systems for efficient and cost-effective removal of CO2 from flue gas streams, contributing to the mitigation of greenhouse gas emissions.

[0004] Technical Background

[0005] The increasing concern over climate change and the environmental impact of CO2 emissions has led to extensive research and development in the field of carbon capture technologies. The combustion of fossil fuels, such as coal, oil, and natural gas, releases substantial amounts of CO2 into the atmosphere, contributing to the global rise in greenhouse gas concentrations. Various methods for capturing and sequestering CO2 have been proposed and implemented, aiming to address the environmental challenges associated with these emissions.

[0006] Post-combustion capture is the most widely applied method and involves separating CO2 from the flue gas after the combustion process. Flue gas, which contains CO2 along with other gases, is produced through combustion. The flue gas is directed through a separation unit, typically an absorption column or membrane system.

[0007] Using membrane systems, the flue gas is fed to one or more separation stages with membrane units comprising CO2-selective membranes. The stream which penetrates the membrane and is thus separated from the feed material is called the permeate stream. The stream which does not penetrate the membrane and leaves the separation unit, is called the retentate stream.

[0008] Flue gas typically contains CO2 (generally 12-15% v / v), nitrogen (from the combustion air), water vapor and other gases. Depending on the source of the flue gas, it is often cleaned by passing through a filter to remove particulate matter and / or a flue gas desulfurization unit to mitigate the SOx, before feeding it to the membrane units.

[0009] The CO2 separated during the process is eventually subjected to a confining operation, where it is used, stored, or otherwise disposed of in a confined manner. US11358093 describes a standard two-stage process for separating CO2 from flue gas using membrane separation. High pressure ratios across the membranes can be achieved by compressing the feed gas to a high pressure, by using vacuum pumps to create a lowered pressure on the permeate side of the membrane, by using a sweep stream, or a combination thereof. After the two stages, a condenser is used to remove water.

[0010] US11117092 describes a process for separating CO2 from flue gas. The process comprises two separation stages wherein in a second separation stage is fed with the permeate from the first stage. In one example of the process the retentate of the second stage is fed back into the first stage. After each stage, water is recovered using a condenser to form a noncondensable CO2-rich stream.

[0011] LIS20210354078 describes a membrane process for separating CO2 from flue gas. The process involves passing a fluid stream including the flue gas across a membrane permeable to CO2 and H2O, obtaining retentate that has less CO2 and permeate comprising CO2 and H2O. The permeate is then cooled to remove at least some of the H2O. The process is based on the co-permeation of water vapor to enhance the CO2 separation. Therefore, the flue gas is pre-treated to bring the flue gas to have greater than 70% of its saturation water concentration, preferably by sending the gas through a direct contact water spray tower in which water at an appropriate temperature is sprayed into the gas. A disadvantage of the method is that the added water must be removed again from the permeate after the separation process leading to increased energy consumption. Additionally, there is a risk of condensation of water within the membrane unit leading to "clogged" membranes and reduced separation efficiency.

[0012] US9433887 describes a process involving membrane-based gas separation for separating and recovering carbon dioxide emissions from combustion processes in partially concentrated form. The process uses a sweep stream of air, oxygen-enriched air or oxygen on the permeate side of the membrane. It is not possible to obtain a permeate with high CO2 contents.

[0013] US8246718 discloses a gas separation process for treating flue gases from combustion processes. The process involves routing a first portion of the flue gas stream to be treated in an absorption-based carbon dioxide capture step, while simultaneously flowing a second portion of the flue gas across the feed side of a membrane, flowing a sweep gas steam, usually air, across the permeate side, then passing the permeate / sweep gas to the combustor. Highly concentrated CO2 is thus captured by an absorption process and not by a membrane separation process. Existing CO2 capture technologies, while effective, often face challenges such as high energy consumption, large footprint, and high capital costs. Additionally, the deployment of these technologies at scale is hindered by economic viability and technical limitations. There is, therefore, a need for innovative and improved CO2 capture processes that overcome these challenges, providing enhanced efficiency, reduced costs, and increased feasibility for widespread adoption.

[0014] Summary of the Invention

[0015] It is an objective of the invention to provide energy efficient process for separating CO2 from a flue gas. It is a further objective to reduce the cost of CO2 separation.

[0016] At least one of the objectives of the present invention is achieved by a process according to claim 1 and a system according to claim 14.

[0017] The process for separating CO2 from a flue gas comprising at least N2, CO2 and H2O with a relative humidity above 70%, preferably above 60%, most preferably above 50% and temperature below 150°C, preferably below 100°C, comprises the steps of: (a.) heating the flue gas stream with a first heat exchanger to obtain a first feed gas stream with a desired relative humidity of less than 70%, preferably less than 60%, most preferably less than 50%, at predefined pressure; (b.) feeding the first feed gas stream to a first membrane unit comprising a first CO2-selective membrane and separating the first feed gas stream to obtain a first CO2-depleted retentate gas stream and a first permeate gas stream comprising CO2 and H2O; (c.) feeding the first permeate as a second feed gas stream to a second membrane unit comprising a second CO2-selective membrane to separate the second feed gas stream into a second CO2-depleted retentate gas stream and a second permeate gas stream comprising CO2 and H2O; (d.) further processing the second permeate gas stream.

[0018] In many situations, flue gas from a combustion process (with or without cleaning steps) is emitted with a temperature in the range of 30 - 100°C and has a high relative humidity of about 50 - 100%. The inventors have realized that flue gas with a high relative humidity may lead to condensation in the membrane units resulting in considerable loss of separation efficiency. Known separation processes typically remove the water already from the flue gas using a condenser before the flue gas is fed to the membrane unit. The present process however does not aim to remove the water from the flue gas but rather reduces the relative humidity by heating the flue gas before it is fed to the membrane unit. No condensers are introduced directly before any of the separation steps in the membrane units. Thus, the process can be run at a temperature for which water does not condensate and is particularly suitable when using membranes performing at high temperature. In rather rare situations, where the emitted flue gas is very hot, i.e. above 100°C and with a high relative humidity near 100%, the heating only may not be energy-efficient any more as temperature far above 100° would be necessary to reduce the relative humidity to the required range. In these case, the flue gas may be treated via a condenser to partially remove water and obtain a flue gas with a reduced temperature. However, instead of removing most of the water from the flue gas it may be more energy-efficient to partially remove water and then adjust the relative humidity according to the above described process by heating.

[0019] The process thus avoids a more energy intensive condensation step for removing water from the first feed stream prior to the separation steps, but rather controls the relative humidity of the feed gas by a heating step. If needed, water may be removed from the second permeate. This way, only a part of the first feed gas stream permeating through the membrane units has to be cooled down, which considerably reduces energy consumption. In addition, process heat of the compression or other processes nearby may be used for the heating. By controlling the relative humidity, any condensation of water within the membrane units is avoided.

[0020] As H2O is typically enriched in the permeate the relative humidity of the feed gas stream is set to less than 70%, preferably less than 60%, most preferably less than 50%, to avoid condensation of H2O over the entire membrane unit including permeate gas streams.

[0021] The second permeate with a high CO2 concentration may eventually be subject to a confining operation, where it is used, stored, or otherwise disposed of in a confined manner. It could be pressurised to pipeline to storage, liquified to storage, pressured to be used as raw material.

[0022] Further embodiments of the invention are set forth in the dependent claims.

[0023] In some embodiments a first and second vacuum pump may be arranged at the permeate side of the first and the second membrane unit to create a driving force for separation.

[0024] In some embodiments the flue gas may be provided to step a.) using a blower. Flue gas typically arrives at 0.97 to 1.05 bara and reaches a pressure of approx. 1.1 to 1.3 bara after the blower.

[0025] In some embodiments water may be removed from the second permeate stream using a condenser to obtain dry second permeate for further processing. In some embodiments the second retentate may be release into the environment or at least partially fed back into the first feed gas stream. The first retentate may be released into the environment.

[0026] In some embodiments the relative humidity of the flue gas stream may be continuously monitored or monitored at predefined intervals (e.g. hours, days, weeks) before and / or after the heating step with the first heat exchanger and the heating is continuously adjusted to obtain the desired relative humidity.

[0027] In some embodiments the first permeate may be heated with a second heat exchanger to obtain a second feed gas stream with a second desired relative humidity of less than 70%, preferably less than 60%, more preferably less than 50%.

[0028] In some embodiments the relative humidity of the first permeate may be continuously monitored before and / or after the heating step with a second heat exchanger and the heating is continuously adjusted to obtain the second desired relative humidity.

[0029] In some embodiments the predefined pressure of the first feed gas stream may be 0.9 to 2.0 bara, preferably approx. 1.1 bara. The predefined pressure of the second feed gas stream may be may be 0.9 to 2.0 bara, preferably approx. 1.15 bara. If there is a feedback of the second retentate into the first feed gas stream the pressure of the second retentate stream is slightly higher, approx. 0.05 - 0.1 bar higher than the pressure of the first feed gas stream.

[0030] In some embodiments the first CO2-selective membrane and / or the second CO2-selective membrane may be a hybrid inorganic / polymeric membrane, preferably having CO2 / N2- selectivity above 10 (at 100°). Such hybrid inorganic / polymeric membranes have the advantage that they are operable at higher temperatures up to 150°C than pure polymeric membranes, without losing selectivity. Whereas the selectivity of polymeric membranes considerably drops at higher temperatures. Using hybrid inorganic / polymeric membranes allows to reduce the relative humidity in the flue gas by heating instead of removing water by condensation.

[0031] In some embodiments the second stage membrane has a CO2 / N2 selectivity above 30 (at 100°C).

[0032] In some embodiments, the first feed gas stream in step b.) may be fed to several valve- controlled first membrane units comprising a first CO2-selective membrane and the second feed gas stream in step c.) may be fed to several valve-controlled second membrane units, wherein each of several first and second membrane units has an on-stream or closed state. As any CO2 separation process remains energy intensive, the process using several valve- controlled membrane units can be easily scaled up or down depending on the availability and the price of the electricity used, the feed flux, the CO2 credit price, CO2 purity or combinations thereof. Thus, the number of on-stream first and second membrane units may be controlled (automatically by a controlling unit) by one of availability of energy, feed flux, CO2 credit price and CO2 purity, or combinations thereof.

[0033] In some embodiments, process heat from the first and / or second vacuum pump may be used for heating the first and / or second feed gas stream via the first and / or second heat exchanger. The heat may be harvested from the vacuum pumps by heat exchangers.

[0034] In some embodiments, the flue gas stream may be a cleaned flue gas stream. Prior to feeding the flue gas stream to the membranes, the gas stream coming out from a combustor may be cleaned by passing through a filter to remove particulate matter and / or a flue gas desulfurization unit to mitigate the SOx.

[0035] The invention further relates to a system for separating CO2 form a flue gas according to the above-described process. The system comprises a first membrane unit with a downstream first vacuum pump on the permeate side of the membrane unit and an upstream first heat exchanger, and a second membrane unit with a downstream second vacuum pump on the permeate side of the membrane unit and an upstream second heat exchanger.

[0036] In some embodiments, the system may further comprise a blower upstream of the first heat exchanger and / or a condenser downstream of the second vacuum pump.

[0037] In some embodiments, the system may comprise several valve-controlled first membrane units and several valve-controlled second membrane units, wherein each of several first and second membrane units has an open or closed state.

[0038] The described process may be used for carbon removal from fuel based power plants, waste incineration plants or any manufacturing plant generating flue gas from fuel combustion. It may also be used to remove carbon from (wet-type) biogas plants or in gas sweetening processes.

[0039] Brief Explanation of the Figures

[0040] The invention is described in greater detail below with reference to embodiments that are illustrated in the figures. The figures show:

[0041] Fig. 1 a block diagram of a two-stage separation process; Fig. 2 a block diagram of a two-stage separation process with several membrane units in each stage.

[0042] Embodiments of the Invention

[0043] Figs. 1 and 2 show each a block diagram of a two-stage separation process and system for separating CO2 from a flue gas 1. The flue gas may be a cleaned flue gas stream which has been passed through a filter to remove particulate matter and / or a flue gas desulfurization unit to mitigate the SOx, before feeding it to the membrane units.

[0044] The flue gas stream is fed to a first membrane unit 11 by a pump or blower 30. Before the flue gas stream enters the membrane unit 11 it is heated by a first heat exchanger 10 to obtain a relative humidity of a first feed gas stream F1 of less than 70%, preferably less than 60%, and more preferably less than 50%. The resulting first feed gas stream F1 typically has a temperature of approx. 20-150 °C. No condensation (cooling energy) is required before the membrane separation step. Only at the very end of the process, where water will be enriched, it may be removed much simpler with less energy consumption.

[0045] Example: The change in relative humidity in a membrane unit has been simulated for a flue gas at 71°C and 1.15 bara having a relative humitidy of 69%. Without heating the relative humidity in the membrane unit reaches 100% in the retentate and the permeate leading to condensation of water. Heating the flue gas by 10°C results in a feed gas stream with relative humidity of 46%. In the membrane unit the retentate reaches 68% and the permeate reaches 97%. In many cases the flue gas stream 1 has a constant composition of N2, CO2 (3-25 mol%) and H2O and the amount of heating may be adjusted once to obtain the desired relative humidity of the feed gas stream F1. At defined intervals, the relative humidity of the flue gas stream 1 and the first feed gas stream F1 may be determined to adjust the right amount of heating. Alternatively, and when the composition of the flue gas stream varies within time, the relative humidity of the flue gas stream and / or the first feed gas stream may be monitored continuously or at short intervals to adjust the amount of heating.

[0046] T o create a driving force across the membrane of the first membrane unit 11 , a first vacuum pump 12 is arranged downstream the permeate side of the first membrane unit 11. The first membrane unit 11 comprises a CO2-specific membrane, which separates CO2 (and H20) from the first feed gas stream F1 producing a first permeate gas stream P1 of CO2 (30-50 mol%) and H2O, which is further used as second feed gas stream for the second membrane unit 21. The first retentate gas stream R1 is not further processed and may be released to the environment. The first permeate gas stream P1 is then heated by a second heat exchanger 20 to obtain a second feed gas stream F2 with relative humidity of less than 70%, preferably less than 60%, and more preferably less than 50%, fed to the second membrane unit 21. The second feed gas stream F2 typically has a temperature of approx. 20-80 °C.

[0047] The second membrane unit 21 also comprises a CO2-specific membrane, which separates CO2 (and H20) from the second feed gas stream F2 producing a second permeate gas stream P2 of CO2 (90-99 mol%) and H2O and a second retentate gas stream R2. To create a driving force across the membrane of the second membrane unit 21 , a second vacuum pump 22 is arranged downstream the permeate side of the second membrane unit 21.

[0048] The second retentate gas stream R2 may also be released to the environment or may be fed back into the flue gas stream 1 or into the first feed gas stream F 1. The second permeate gas stream P2 may be directly subjected to a confining operation or its water content may be removed using a condenser 40. The resulting "dry" second permeate gas stream P2 may then be subjected to a confining operation.

[0049] The CO2-specific membrane of the first and second membrane unit 11 , 21 may be a hybrid inorganic / polymeric membrane with CO2 / N2-selectivity above 10. These membranes are particularly well suited to perform at elevated temperatures.

[0050] The process or system as shown in Fig. 2 differs from the process in Fig. 1 in that the first stage comprises several first membrane units 11a-11d and the second stage comprises several second membrane units 21a-21d. Each first and second membrane unit is independently controlled by a valve and may be in an open (or operating) state or a close (or non-operating) state. In other words, the process or system can be quickly scaled up or down depending on the numbers of operating membrane units. For example, when there is an abundance of electric energy and the energy price is low, the process is scaled up and CO2 can be separated from the flue gas at lower costs. When energy prices rise, the process is scaled down.

[0051] Reference Signs

[0052] I flue gas

[0053] 10 first heat exchanger

[0054] I I first membrane unit

[0055] 11a-11d valve-controlled first membrane unit

[0056] 12 first vacuum pump 20 second heat exchanger

[0057] 21 second membrane unit

[0058] 21a-21d valve-controlled first membrane unit

[0059] 22 second vacuum pump 30 blower

[0060] 40 condenser

[0061] F1 first feed gas stream

[0062] F2 second feed gas stream

[0063] P1 first permeate gas stream P2, P2' second permeate gas stream

[0064] R1 first retentate gas stream

[0065] R2 second retentate gas stream

Claims

Claims1. A process for separating CO2 from a flue gas (1) comprising at least N2, CO2 and water with a relative humidity above 70%, preferably above 60%, most preferably above 50% and temperature below 150°C, preferably below 100°C, the process comprising the steps of: a. heating the flue gas stream (1) with a first heat exchanger (10) to obtain a first feed gas stream (F1) with a desired relative humidity of less than 70%, preferably less than 60%, most preferably less than 50%, at predefined pressure; b. feeding the first feed gas stream (F1) to a first membrane unit (11 , 11 a-11d) comprising a first CO2-selective membrane and separating the first feed gas stream (F1) to obtain a first CO2-depleted retentate gas stream (R1) and a first permeate gas stream (P1) comprising CO2 and water; c. feeding the first permeate gas stream (P1) as a second feed gas stream (F2) to a second membrane unit (21 , 21a-21d) comprising a second CO2- selective membrane to separate the second feed stream (F2) into a second CO2-depleted retentate gas stream (R2) and a second permeate gas stream (P2) comprising CO2 and water; d. further processing the second permeate gas stream (P2).

2. Process according to claim 1 , wherein a first and second vacuum pump (12, 22) is arranged at the permeate side of the first and second membrane unit (11 , 21) to create a driving force for separation.

3. Process according to one of the preceding claims, wherein the flue gas is provided to step a.) using a blower (30).

4. Process according to one of the preceding claims, wherein water is removed from the second permeate stream (P2) using a condenser (40) to obtain dry second permeate (P21) for further processing.

5. Process according to one of the preceding claims, wherein the second retentate (R2) is release into the environment or at least partially fed back into the first feed gas stream.

6. Process according to one of the preceding claims, wherein the relative humidity of the flue gas stream (1) is continuously monitored or monitored at predefined intervals before and / or after the heating step with the first heat exchanger (10) and the heating is continuously adjusted to obtain the desired relative humidity.

7. Process according to one of the preceding claims, wherein the first permeate (P1) is heated with a second heat exchanger (20) to obtain a second feed gas stream (F2) with a second desired relative humidity of less than 70%, preferably less than 60%, more preferably less than 50%.

8. Process according to claim 6, wherein the relative humidity of the first permeate (P1) is continuously monitored before and / or after the heating step with a second heat exchanger (20) and the heating is continuously adjusted to obtain the second desired relative humidity.

9. Process according to one of the preceding claims, wherein the predefined pressure of the first feed gas stream is 0.9 to 2.0 bara, preferably approx. 1.1 bara and / or the predefined pressure of the second feed gas stream is 0.9 to 2.0 bara, preferably approx. 1.15 bara.

10. Process according to one of the preceding claims, wherein the first CO2-selective membrane and / or the second CO2-selective membrane is a hybrid inorganic / polymeric membrane preferably having a CO2 / N2-selectivity above 10 at 100°C, and wherein the first and second CO2-selective membrane are operable at temperatures in the range of up to 150°C.

11. Process according to one of the preceding claims, wherein in step b.) the first feed gas stream (F1) is fed to several valve-controlled first membrane units (11a-11d) comprising a first CO2-selective membrane and in step c.) the second feed gas stream (F2) is fed to several valve-controlled second membrane units (21a-21d), wherein each of several first and second membrane units (11 a-11d, 21a-21d) has an on-stream or closed state.

12. Process according to claim 12, wherein the number of on-stream first and second membrane units (11 a-11d, 21a-21d) is controlled by one of availability of energy, feed flux, CO2 credit price and CO2 purity, or combinations thereof.

13. Process according to one of claims 2 to 12, wherein process heat from the first and / or second vacuum pump (12, 22) is used for heating the first and / or second feed gas stream (F1, F2) via the first and / or second heat exchanger (10, 20).

14. A system for separating CO2 from a flue gas according to a process of one of the preceding claims, the system comprising: a first membrane unit (11) with a downstream first vacuum pump (12) on the permeate side of the membrane unit (11) and an upstream first heat exchanger (10), and a second membrane unit (21) with a downstream second vacuum pump (22) on the permeate side of the membrane unit and an upstream second heat exchanger (21).

15. The system of claim 13, further comprising a blower (30) upstream of the first heat exchanger (10) and / or a condenser (40) downstream of the second vacuum pump (22).

16. The system of one of claims 12 to 13, comprising several valve-controlled first membrane units (11a-11d) and several valve-controlled second membrane units (21a-21d), wherein each of several first and second membrane units (11a-11d, 21a-21d) has an open or closed state.

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