Apparatus and process for capturing and recovering CO2 from process flue gas

Abstract

The present invention relates to an apparatus (10) and a process for capturing and recovering CO2 from the flue gas of a reheating furnace. Carbon dioxide is captured in a CO2 capture and separation unit (C) by membrane separation, adsorptive separation (such as PSA technique) or chemical absorption and gas regeneration and is divided into two CO2 streams (F1, F2). Wherein a CO2 stream (F1) is fed to a washing device (18), in which CO2 forms M2CO3 by means of a solution of an alkali metal hydroxide MOH, in particular NaOH or KOH, and M2CO3 reacts with a second CO2 stream (F2) in a downstream reactor (24) to form alkali metal bicarbonate (MHCO3). The heat recovered from the flue gas may be used in various parts / stages of the apparatus / process of the invention.

Description

Technical Field

[0001] The present invention relates to a process and production equipment for capturing, separating and using a carbon dioxide stream for the production of bicarbonates (such as sodium bicarbonate) by a CCU (carbon capture and utilization) method. In particular, the CO2 contained in the process flue gas of a capture and use furnace (especially a reheating furnace) will be captured and used. Background Art

[0002] During the melting and reheating stages, the CO2 emissions of steel mills are quite large because hydrocarbons and carbonaceous materials are burned in different process stages.

[0003] In particular, the melting furnace has a particularly high production efficiency, which generates a discrete amount of CO2 in the flue gas, but the flue gas contains a large amount of dust, while the methane reheating furnace, on the contrary, generates a combustion flue gas stream with a lower dust content.

[0004] The second type of flue gas contains about 10% CO2, and the annual CO2 emissions of one furnace can reach 200,000 metric tons.

[0005] In addition, these flue gases are transported at high temperatures, so in order to be processed, they must first be cooled and then introduced into the atmosphere.

[0006] Therefore, due to the emissions of reheating furnaces that can change the climate, these reheating furnaces have a considerable impact and are a heavy burden on the factory in terms of paying emission taxes (called carbon taxes), and it is expected that this impact will increase in the next few years. Other CO2 production furnaces in the metal or steel industry are, for example, furnaces for heat treatment of metal products. Summary of the Invention

[0007] The object of the present invention is to overcome the above-mentioned drawbacks and proposes a device and related process for capturing CO2 generated in a furnace, such as a reheating furnace, a heat treatment furnace, a melting furnace, etc. Another object of the present invention is to find a solution for recycling and reusing CO2 to reduce the impact of steel production on the climate, as well as the impact of products to be replaced by products produced by new technology processes.

[0008] Through the following description, other objects and advantages of the present invention will become obvious.

[0009] In a first aspect of the present invention, this object is achieved by a device for capturing and recovering CO2 from process flue gas, the device comprising:

[0010] (a) A furnace that generates heat and flue gas containing CO2 during use;

[0011] (b) A CO2 capture and separation unit, located downstream of the reheating furnace, having a first outlet and an associated first pipeline, and a second outlet and an associated second pipeline, wherein, during use, a gas stream containing a portion of the captured and separated CO2 is supplied to each outlet and the associated pipeline, and the CO2 concentration of the gas stream is different from that of the gas stream supplied to the CO2 capture and separation unit;

[0012] (c) A first gas scrubbing device, preferably an absorption tower, adapted to scrub CO2 with aqueous MOH, where M is an alkali metal, preferably sodium (Na) or potassium (K), and connected to the first pipeline to be supplied with CO2;

[0013] (d) A reactor, adapted to carry out the reaction between an alkali metal carbonate (preferably Na or K) and CO2 (preferably in aqueous solution), connected to the second pipeline and the first gas scrubbing device to be supplied with CO2 and an alkali metal carbonate, preferably Na or K respectively, and provided with an outlet for extracting the alkali metal bicarbonate formed by the reaction between CO2 and the alkali metal carbonate during the use of the equipment.

[0014] The equipment helps to recover CO2 in the form of alkali metal bicarbonate, which has various uses, including food applications, especially NaHCO3 (such as leavening agents), medical (such as hemodialysis), and hygiene (cleaning with bicarbonate). The proposed production of alkali metal bicarbonate can be carried out on an industrial scale. The bicarbonate of alkali metal M is here formed in aqueous solution by the reaction between the carbonate of alkali metal M and CO2 according to the following reaction formula (I):

[0015] (I) M2CO3 + H2O + CO2 → 2MHCO3,

[0016] The required alkali metal carbonate is obtained according to the following reaction formula (II):

[0017] (II) 2MOH + CO2 → M2CO3 + H2O,

[0018] wherein the alkali metal hydroxide in the aqueous solution reacts with CO2 in appropriate contact to form water and alkali metal carbonate. Both of these reactions capture and recover CO2 in the form of salts.

[0019] The preferred alkali metals are sodium and potassium, especially preferably sodium.

[0020] The furnace can have various properties, for example, melting furnace, reheating furnace, heat treatment furnace, etc. The particularly preferred furnace types are those that produce flue gas with a relatively low dust content (for example, the indicative dust content is on the order of about 50 mg / Nmc), such as reheating furnace or heat treatment furnace.

[0021] Reheating furnaces are commonly used in metal and steel processes. They are usually powered by natural gas and generate CO2 during high-temperature combustion, so the heat of the flue gas can also be utilized.

[0022] The reheating furnace allows for the generation of flue gas with little dust and limits the necessary pre-treatment so that its flue gas can be treated together with alkali metal hydroxides and their derivatives to sustainably produce alkali metal bicarbonates. If necessary, i.e., for flue gas containing a large amount of dust, a dust removal system is preferably inserted between the furnace and the CO2 capture unit, and its market scope is well-known to those skilled in the art.

[0023] Given the characteristics of the flue gas generated in the furnace and the chemical reactions required for the production of alkali metal bicarbonates, the present invention describes an apparatus and process for producing alkali metal carbonates, which, in addition to using the heat of the flue gas, can also use the CO2 generated during the reheating process (or at least a part thereof) to produce alkali metal bicarbonates.

[0024] This may reduce the environmental impact of these furnaces, expand the production fleet of the steel plant, and in addition to steel products, the steel plant can also sell chemical products, but it can also compensate for the emissions in other areas of the plant with the non-emissions of some plant themes of the present invention.

[0025] The CO2 capture unit allows for the capture of the gas and its separation from the flue gas (i.e., any other included gas and powder), and then its concentration for recovery and reuse in the form of alkali metal bicarbonates or their derivatives.

[0026] The gas stream or flue gas supplied to the capture unit has a certain CO2 concentration (e.g., expressed in vol%), and this concentration changes during the process of passing through the capture unit, generating two different gas streams, whose concentrations can vary between 40 vol% and 95 vol%. Therefore, a concentration difference expressed in a given concentration quantity (e.g., m%, vol%, mol%, etc.) means a difference in any other concentration quantity, and these concentration quantities can be converted between them.

[0027] A gas stream is understood to be a material stream that is gaseous at room temperature, but in this example, it can also be liquefied and is not necessarily gaseous, especially in the case of a fluid that has passed through a compression system.

[0028] As described above, the thermal energy of the flue gas generated by the reheating furnace can also be recovered and used in other parts of the plant or related processes.

[0029] In this regard, in a preferred embodiment, the CO2 capture and recovery device according to the present invention includes a first heat exchanger downstream of the furnace in order to recover heat from the flue gas within the device. The recovered heat may be useful, as will be explained later, for example, to heat the CO2 regeneration device or the solution processed therein, or to evaporate water in the concentrator, which is part of the extraction system for extracting alkali metal bicarbonate from the reactor and is adapted to treat the remaining solution (after salt crystallization) to recover it.

[0030] In order to be able to separate the alkali metal bicarbonate, the CO2 capture and recovery device according to the present invention advantageously further includes a crystallizer, connected to the reactor discharge; a filter or centrifuge for separating the alkali metal bicarbonate generated during the use in the reactor; and a concentrator downstream of the filter or centrifuge, which is connected to the reactor to supply it with the concentrate generated during the use of the device. Any steam generated within the concentrator can be used, after condensation, to supply a tank mixer, which serves as a source of alkali metal hydroxide for the first gas scrubbing device. Preferably, the reactor has an exhaust port connected to a first pipe in order to recycle the CO2 that has not been converted to alkali metal bicarbonate or formed by the possible decomposition of carbonates and bicarbonates, and to handle any pressure peaks.

[0031] The reactor is understood as a reaction unit, that is, as a container, device, instrument, apparatus in which a chemical reaction takes place. A particularly preferred reactor is a three-phase reactor because there is a three-phase reaction between CO2 (gas), water (liquid), and sodium carbonate (at least partially solid and not in solution).

[0032] In its first embodiment, the capture unit uses a chemical reaction or absorption phenomenon to separate CO2 from the other components of the flue gas, and then performs a first purification of the CO2; the CO2 that has not been captured by the chemical reaction or absorption (e.g., by potassium carbonate or amine) is captured during the formation of a carbonate (M2CO3) that binds CO2 to MOH, such that both CO2 fractions reach the reactor for producing alkali metal bicarbonate, the first by reaction with potassium carbonate or amine or other separation systems, and the second by reaction with MOH. Amine forms carbamate with CO2 (Reaction III); and potassium carbonate forms potassium bicarbonate in the reaction with CO2 (Reaction IV). During the CO2 regeneration process, the reaction occurs in the opposite direction and the gas is released, recovering the corresponding capture agent:

[0033] (III)

[0034] (IV)

[0035] In the case of producing potassium bicarbonate by CO2 capture and recovery, amine system capture is preferred.

[0036] The absorption of CO2 with aqueous amine solutions is one of the most widely used methods for removing CO2 from gas mixtures. The reaction of amine solutions with CO2 has two different mechanisms: primary and secondary amines produce carbamates (Reaction III), the mechanism of which is first the formation of zwitterions, followed by deprotonation by a second amine molecule. The carbamate ions can in turn undergo partial hydrolysis to form bicarbonates. Therefore, in addition to the above reactions, we can also consider the overall reaction V:

[0037] (V)

[0038] Tertiary amines do not have free protons and do not form carbamates, but in any case undergo a basic reaction to form bicarbonate ions (Reaction VI):

[0039] (VI)

[0040] In order to reuse the amine solution used for absorption, it is necessary to include a regeneration step, which can be carried out by steam stripping at a high temperature (e.g., about 120 - 130 °C). This makes the process energy expensive, but it can be managed with heat recovered from the same furnace, i.e., heat recovered from other available heat sources. Solutions of monoethanolamine, diethanolamine, and dimethylethanolamine supplied with components for activation are usually used.

[0041] In its first embodiment, for the chemical capture or absorption of CO2, the CO2 capture and separation unit includes a second gas scrubbing device, preferably an absorption column, adapted to chemically bind or absorb CO2 with an aqueous solution preferably containing an amine or potassium carbonate; and a regeneration device, downstream of the second gas scrubbing device, adapted to release the CO2 absorbed or bound in the gas scrubbing device; wherein the second gas scrubbing device includes a first outlet and is connected to the first gas scrubbing device through a first pipeline; and wherein the regeneration device includes a second outlet and is connected to the reactor through a second pipeline.

[0042] A person skilled in the art knows various types of absorption columns, among which he / she selects the most suitable one for his / her purpose with his / her common sense. Chemical absorption can also be achieved with other chemical systems known to a person skilled in the art. Physical type of capture can also be assumed, for example, various forms of adsorption known in the art.

[0043] Advantageously, heat exchange can be included within the capture and separation unit to heat / cool the material stream. One embodiment of the present invention contemplates that the second gas scrubbing device and the regeneration device are connected by a second heat exchanger to heat the solution stream containing the captured CO2 that leaves the second gas scrubbing device and travels to the regeneration device, and correspondingly cool the regenerated solution that leaves the regeneration device and is heated by the heat source included in the equipment for supply to the second gas scrubbing device. Advantageously, the equipment according to the present invention includes a compressor for compressing CO2 downstream of the regeneration device. This system makes it possible to cross-prepare the material streams in the heat exchanger during the corresponding CO2 capture and its treatment steps.

[0044] In its second embodiment, the CO2 capture and separation unit includes at least one membrane separator, preferably a polymer membrane, adapted to separate CO2 from a gas stream, producing a first gas stream rich in CO2 and a second gas stream with a reduced CO2 content, wherein the at least one membrane separator includes a first outlet and a second outlet, and supplies the gas stream with a reduced CO2 content to the first gas scrubbing device through a first pipe connected to the first outlet, and supplies the gas stream rich in CO2 to the reactor through a second pipe connected to the second outlet.

[0045] In this second embodiment of the capture unit, the ability of a membrane (especially a polymer membrane) to separate gases from a gas stream is utilized. The separation effect is based on the differential diffusion mechanism in the membrane. In this case, the flue gas generated by the furnace passes through the membrane separation unit, where the separation occurs in two streams with different gas concentrations. The gas permeating through the membrane is enriched in the permeate and depleted in the retentate. Separation occurs due to the different diffusion rates of the individual components in the membrane material. The driving force for mass transfer through the membrane is the partial pressure difference of the permeating components between the feed side and the permeate side. In terms of process technology, this difference is usually caused by a lower pressure on the permeate side.

[0046] Generally, the separation efficiency of such a membrane is very good, so a single-stage device is sufficient, but a multi-stage device is also feasible. In this case, a variant of the membrane separation embodiment contemplates inserting at least one additional membrane separator between the at least one membrane separator and the first gas scrubbing device, which additional membrane separator is supplied with the low CO2 content fraction leaving the first membrane separator, and supplies a CO2-rich fraction to the second pipe and a fraction with a reduced CO2 content to the first gas scrubbing device. Additional membrane separators can be inserted. All these separators can be connected in series, where the retentate discharged from one separator is supplied to the next separator, and where the permeate of each separator can be guided countercurrently through the separator.

[0047] Tangential feed (cross-flow) is usually used. In tangential flow, the feed flows tangentially with respect to the membrane and is forced through the membrane by the pressure gradient acting on both sides of the membrane itself. This results in a permeate flow orthogonal to the membrane and a retentate flowing tangentially thereto. For example, this type of flow is used to treat fluids with a high content of suspended solids.

[0048] There are various membrane variants. Spiral-wound membranes consist of a series of flat membrane pairs that are glued together on three sides and whose fourth side is connected to a central permeate collection channel. The membrane is then wound around the channel. Two membrane sheets are separated by a spacer mesh for permeate discharge. Hollow fiber membranes consist of a plurality of small tubes of gas-selective material inserted into a tube, and flat module or tubular module membranes are then known. A person skilled in the art selects the membrane that is most suitable for their purpose according to their needs, for example, by evaluating parameters such as flow rate or velocity, selectivity, fouling, and membrane cleaning.

[0049] To improve the separation effect, membrane separation can use a purge gas, which is a gas present on the permeate side of the membrane separator, to reduce the partial pressure of the permeating species and increase the driving force. This gas is different from the gas to be separated. The driving force can be increased by operating the partial pressure by increasing the feed pressure or by reducing the pressure of a specific gas permeate. In particular, the partial pressure of the permeating species can be reduced in two ways: by reducing the total pressure on the permeate side, for example, by applying a vacuum, and / or by using a purge gas on the permeate side.

[0050] This makes membrane separation more advantageous when a pressure difference can be effectively generated and a limited separation efficiency is required. This is the case here, where the flue gas needs to be separated into two CO2 streams for use in washing with MOH and reacting with alkali metal carbonates.

[0051] Polymer membranes have high permeability to CO2 and good selectivity for other gases, depending on the polymer and the gas to be separated. A multi-stage solution is required to achieve high separation and purity. Membrane separation is particularly competitive for treating pressurized gases (>5 bar). The degree of separation can be controlled by selecting a polymer from the various polymers known to a person skilled in the art.

[0052] The third embodiment of the present invention contemplates the physical capture of CO2 as described above, i.e., by adsorption. Pressure swing adsorption (PSA) separation of substances is particularly suitable.

[0053] PSA utilizes the different adsorption behaviors of gas molecules: under pressure, the adsorbent binds CO2 better than other components of the gas, and these components can penetrate into the adsorption material, such as hypercrosslinked functionalized polymers, as described, for example, by Alex M. James et al., in A Pressure Swing Approach to Selective CO2 Sequestration Using Functionalized Hypercrosslined Polymers (Materials 2021, 14, 1605), or used in combination with zeolites, carbon molecular sieves, etc. If the adsorption material is eventually saturated with the predominantly adsorbed CO2, the process can be reversed, the pressure released and the adsorbent regenerated, or the adsorption material can also be washed with a portion of the CO2 stream that is first separated in the opposite flow direction. The CO2 passing through the adsorbent can be split into two different streams and leave the capture unit through two relatively different outlets and pipes for supply to the first gas scrubber and the reactor.

[0054] It is also conceivable to have a capture unit that combines different types of CO2 capture, selected from membrane separators, adsorption separators (such as PSA), and chemical absorption systems.

[0055] The second aspect of the present invention relates to a method for capturing and recovering CO2 from flue gas, comprising the following steps:

[0056] (i) generating heat and flue gas containing CO2 with a furnace;

[0057] (ii) capturing and separating the CO2 in a capture unit;

[0058] (iii) splitting the captured and separated CO2 into a first CO2 stream and a second CO2 stream;

[0059] (iv) washing the first stream with aqueous MOH, thereby generating an alkali metal carbonate M2CO3, wherein M is preferably Na or K;

[0060] (v) reacting the alkali metal carbonate with the second CO2 stream (F2) to form an alkali metal bicarbonate MHCO3; and

[0061] (vi) separating the alkali metal bicarbonate.

[0062] Preferably, the process uses at least in part the heat from the furnace, in particular the heat contained in the flue gas. The possible uses of the heat have been described above with reference to the apparatus of the present invention.

[0063] Features described in connection with one aspect of the invention can be transferred, with necessary modifications in detail, to other aspects of the invention. The transfer is implicit because certain elements of the device (e.g., the membrane separator) correspond to the respective steps of the process (e.g., membrane separation), and vice versa.

[0064] In an advantageous embodiment of the process for capturing and recovering CO2 according to the invention, the capture and separation of CO2 in step (ii) takes place.

[0065] (ii-a) washing the CO2 with a solution that absorbs or chemically binds CO2 (the solution preferably contains amines or potassium carbonate); and

[0066] (ii-b) subsequently releasing or regenerating the CO2 absorbed or bound in step (ii-a),

[0067] wherein the unabsorbed or unbound portion of CO2 in step (ii-a) forms a first CO2 stream, and the CO2 released or regenerated in step (ii-b) forms a second CO2 stream. This capture and separation of CO2 perfectly reflects the corresponding parts of the above-mentioned device, and vice versa.

[0068] In its preferred variant, according to the process of the invention, using the principle of chemical absorption, it is envisaged to heat the solution free of CO2 and then reuse it in step (ii-a). Before reusing it in step (ii-a), the heated solution is used to heat the solution containing the CO2 absorbed or bound between step (ii-a) and step (ii-b) in a heat exchanger. These additional phases allow two material streams at different temperatures to be used in a "cross" form in the heat exchange to heat / cool them according to the needs of the process, using the heat sources inherent in the process instead of obtaining an energy burden from the outside.

[0069] In an alternative embodiment for capturing and separating CO2, the process of the invention advantageously envisages the use of membrane gas separation technology to be able to produce two separate CO2 streams: in this case, advantageously, the capture and separation of CO2 in step (ii) is carried out by membrane separation, and the membrane separation produces an enriched CO2 stream, which forms the second CO2 stream, and in the stream with reduced CO2 content, this forms the first CO2 stream.

[0070] In another alternative embodiment for capturing and separating CO2, the process according to the invention envisages the use of adsorption separation technology, especially PSA technology: in this case, advantageously, the capture and separation of CO2 in step (ii) is achieved by adsorption separation and the production of two CO2 streams.

[0071] The device and process according to the invention are applicable to the production of bicarbonates of different alkali metals, especially sodium and potassium. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 Depicts a basic schematic diagram of the principle of CO2 capture and recovery from a reheating furnace in a metallurgical plant.

[0073] Figure 2 Shows in detail Figure 1 The figure of the part related to the formation of bicarbonate from carbonate in

[0074] Figure 3 Shows in detail the Figure 2 Schematic diagram related to the capture unit by showing the first capture variant (in this case, capture by chemical absorption).

[0075] Figure 4 Shows in detail the Figure 2 Schematic diagram related to the capture unit by showing the second capture variant (in this case, capture by single-stage membrane separation).

[0076] Figure 5 Shows in detail the Figure 2 Schematic diagram related to the capture unit by showing the third capture variant (in this case, capture by two-stage membrane separation).

[0077] Figure 6 Depicts the gas separation membrane in a sectional view.

[0078] Figure 7 Depicts the hollow fiber separation membrane in a perspective view.

[0079] Figure 8 Depicts the spiral separation membrane in a sectional view.

[0080] Figure 9 Depicts the configuration of the permeation membrane (left side) and the trend of the partial pressure of the gas to be separated relative to the surface traversed.

[0081] Figure 10 Depicts the configuration of the permeation membrane combined with the purge gas and the trend of the partial pressure of the gas to be separated relative to the surface traversed. DETAILED DESCRIPTION OF THE INVENTION

[0082] In the following, the embodiments implemented are directed to the production of sodium bicarbonate, but by replacing sodium with another alkali metal such as potassium or lithium where appropriate, they can be transferred to the production of other alkali metal bicarbonates, such as potassium bicarbonate or lithium bicarbonate.

[0083] Figure 1Depicts a basic schematic diagram constituting the present invention. The reheating furnace generates combustion flue gas containing CO2, which is captured by the capture unit. Then, the captured CO2 is divided into two streams. The first stream is washed with NaOH solution to produce sodium carbonate (Na2CO3). The second stream reacts with the carbonate generated from the first stream in a reactor to form sodium bicarbonate (NaHCO3), which is then separated. The heat recovered from the reheating furnace can be used in the bicarbonate production process.

[0084] For the CO2 capture step, the present invention includes various embodiments, while the captured CO2 recovery and utilization part (at least most of it) is the same or similar for all variants.

[0085] Therefore, Figure 2Details the common parts of various devices according to the present invention, and represents the part of the device related to the initial capture and separation of gas with box C. 10 is instead used to indicate a CO2 capture and recovery device for CO2 discharged from a reheating furnace (not shown) and entering (arrow 12) the device 10. Hot (i.e., high-temperature) gas with a concentration of about 10% CO2 from the reheating furnace (RHF) is cooled in the heat exchanger 14 and pumped into the CO2 capture system C through the compression system 16. The heat (Δ) generated by the reheating furnace is preferably at least partially used for the device 10, as described below. However, the heat generated can also be used for other purposes. After capture, the gas is divided into two streams F1 and F2. The first stream F1 is directed to the absorption tower 18, where the first stream F1 encounters a countercurrent of caustic soda injected from the tank 20. The tank 20 receives NaOH in a concentrated solution and dilutes it in the mixer 20 before introducing it into the device 10. For example, the concentration of sodium hydroxide (NaOH) in the solution corresponds to 10 - 25 m%. In the absorption tower, for a capture unit that can sequester about 50% of the CO2, further sequestration of CO2 (e.g., for about 45% of the part) already corresponds to its first recovery step. Thus, the gas purified from about 95% of the CO2 then passes through the condenser 22 or other similar devices and is treated through the chimney according to known procedures. Sodium carbonate (Na2CO3) leaves the absorption tower 18 in the form of an aqueous solution, which is generated by the reaction of NaOH with CO2 and is fed into the reactor 24. The reactor 24 is simultaneously supplied by the above-mentioned second CO2 stream F2. In the reactor 24, CO2 is recovered in the form of sodium bicarbonate (NaHCO3), which is generated by the reaction of Na2CO3 and CO2. The vent 26 provided on the reactor 24 optionally supplies "unconsumed" CO2 to the pipeline of the first stream F1 and allows any pressure peaks to be handled. The bicarbonate in the aqueous solution reaches the crystallizer 28 from the reactor 24 for salt precipitation. Downstream of the crystallizer 28 is a separation system 30, such as a filter or centrifuge, which separates the solid salt (NaHCO3) for different uses (arrow 32), while sending the separated water containing bicarbonate residues (arrow 34) to the concentrator 36, which in turn sends a part of the water containing bicarbonate residues (arrow 38) back to the reactor 24 after heating to evaporate the excess water by using various recovered heats (e.g., the heat from the reheating furnace itself). Then, the steam passes through the condenser 40, which cools the steam by condensing it into water, so that it can be extracted (arrow 42) for other uses or reused in the tank 20 to dissolve solid NaOH or dilute the caustic soda already present in any aqueous solution.

[0086] Figure 3Shows an embodiment of the capture unit C of the previous figure. In order to recover CO2 as carbonate / bicarbonate, the part of the device for further capturing CO2 is the same as Figure 2 which will not be elaborated here. The capture unit C operates based on the principle of chemical absorption. The flue gas cooled by the exchanger 14 enters the first stage in the absorption tower 44 after passing through the compression system 16. The cold flue gas is introduced from the bottom of the tower 44 and rises upward. At the same time, a stream of absorbent liquid (such as potassium carbonate or amine in aqueous solution) flows countercurrently through the cold flue gas, and the absorbent liquid combines with the CO2 contained in the flue gas to form potassium bicarbonate or potassium carbamate, thereby reducing about 50% or more of the CO2. High pressure (e.g., >3 bar) and low temperature (preferably <70 °C) are beneficial to the CO2 reduction process. Therefore, the flue gas purified from the first part of CO2 in the tower 44 is not captured by K2CO3 or amine, forming the above-mentioned first CO2 stream F1, which is supplied to the absorption tower 18. The liquid potassium carbonate or potassium carbamate solution rich in captured CO2 is sent to the regeneration tower 48 through the heat exchanger 46. The solution rich in captured CO2 is heated when passing through the exchanger 46 so that it can have a sufficient temperature to allow the release of CO2. In fact, in the regeneration tower 48, the liquid solution with high temperature and low pressure (atmospheric pressure) is inserted into the regeneration tower 48 from above and countercurrently passed through by the steam stream 56 generated by the reboiler 54 with a heat source. This steam stream further raises the temperature of the liquid solution (above 100 °C) and removes the CO2 content therein, and the CO2 then mixes with the water vapor. The water vapor is generated by the heat source 54 from the regenerated amine or potassium carbonate solution and is cooled in the regenerator 48. The mixture of water vapor and CO2 passes through the condenser 50, and the condensed water (arrow 52) is discharged from this condenser. The condensed water is supplied to the regeneration tower 48 and the purified CO2, and the purified CO2 forms the above-mentioned second compressed CO2 stream F2. Once the CO2 is released (captured by the steam), the solution with the "absorbent" element accumulates at the bottom of the tower 48, is cooled and mixed with the water in the reintroduced condensate (arrow 52). Then, as described above, all is sent to the reboiler with the heat source 54 to evaporate part of the water that can return to the cycle in the form of steam (arrow 56) to drag new CO2 into the regeneration tower 48, and the heated "absorbent" liquid returns (arrow 58) through the heat exchanger 46, where it is cooled and leaves its heat to the stream entering the regeneration tower 48 in the absorption tower 44. The heat source 54 can utilize the recovered heat from, for example, the heat exchanger 14 or from other systems.

[0087] Figure 4Shows an amine capture system or an alternative via K2CO3. The capture unit C includes a membrane separation system. The flue gas cooled by the heat exchanger 14 preferably enters a blower or compressor 16 and enters a cross-flow membrane separator 60, where the membrane M separates the gas into a fraction with a reduced CO2 content (-CO2) and a fraction with an increased CO2 content (+CO2). The reduced CO2 fraction is fed as stream F1 into the absorption tower 18, while the increased CO2 fraction is passed through a compressor 62 or a vacuum pump and then fed as stream F2 into the reactor 24.

[0088] In contrast to Figure 4 Instead, Figure 5 It includes a two-stage membrane separator system (obviously, multiple separation stages can also improve the recovery efficiency). Another membrane separator 64 is inserted between the membrane separator 60 and the absorption tower 18, and the absorption tower is supplied with the CO2-lean fraction leaving the first membrane separator 60. This another membrane separator 64 in turn separates the gas into a fraction with a reduced CO2 content (-CO2) and a fraction with an increased CO2 content (+CO2). The fraction with a reduced CO2 content (-CO2) is fed as stream F1 to the absorption tower 18; while the fraction with an increased CO2 content (+CO2) is fed to (via 68) stream F2.

[0089] In Figures 3 to 5 it, solid lines represent room temperature streams, dashed lines represent cold streams, dash-dotted lines represent hot streams, and mixed dashed / dash-dotted lines represent compressed gas streams. Other CO2 capture systems known to those skilled in the art can also be envisaged for the capture unit.

[0090] Figure 6 Shows a cross-section of the porous membrane M of the membrane separator. Note the porous support 84 with an exemplary thickness of 50 - 100 μm, then the channels 86, and subsequently the selective layer 88 representing the actual separation element (with a typical thickness of ). All of these are protected by the coating 90. Thus, such a composite structure membrane has a very thin selective layer that is bonded to a microporous support layer that provides mechanical strength to support the pressure difference between the feed side and the permeate side.

[0091] In another exemplary embodiment (not shown), capture unit C may include a system for capturing and separating CO2 by adsorption, such as a system operating according to the PSA principle. In this regard, two-reactor systems are known in the prior art, where each reactor includes an adsorbent material permeable to CO2 while other gases are retained (adsorbed) in the adsorbent material. First, a gas mixture is supplied to the first reactor at high pressure (5 - 10 bar), and then the feed to the second reactor is closed. The CO2 leaving the reactor can be used in the second step of the process according to the present invention. As the load of the adsorbent material from the adsorbed gas increases, the pressure in the first reactor drops, and the feed to the first reactor is closed to open the feed to the second reactor, thus starting the CO2 separation. At the same time, under reduced pressure, the gas adsorbed in the first reactor is separated from the adsorbent material and discharged from the system. When the pressure in the second reactor drops and reaches a certain value, its feed is interrupted and it is directed back to the first reactor. Regeneration starts at low pressure in the second reactor. Thus, due to the pressure changes within the associated reactors, there is a continuous exchange between the adsorption separation between the first and second reactors and the regeneration of the adsorbent material.

[0092] Figure 7 A membrane separator is shown, which includes a plurality of hollow fibers M1 within a tube 61, through which a feed 92 passes during use. A portion of the gas passes through the fiber walls as permeate 93 (i.e., in this example, the CO2-rich fraction), while the remaining stream exits as retentate 91 (i.e., the CO2-lean fraction).

[0093] Figure 8 An alternative membrane configuration is shown, where a plurality of membrane sheets are spirally wound together, creating spaces for the feed FS and permeate PS between the individual sheets. Numeral 98 represents an outer covering, while opening 100 represents the discharge of the permeate. A purge gas can be introduced at position 96. The feed stream and the retentate stream are perpendicular to the cross-section, while the permeate stream is perpendicular to the other streams within the spiral pattern. In these membrane separators, a single module has a cross-flow arrangement, but by appropriately connecting them in series and always passing the permeate through the previous spiral, a counter-current configuration can be obtained with respect to the feed and retentate streams.

[0094] Figure 9 A membrane separator 64 is shown on the left, which is schematically divided into two sectors by a membrane M. The feed 92 enters the separator 64. The retentate stream RF exits from one sector (in the same direction as the feed 92), while the permeate stream PF exits from the other sector perpendicular to the other streams. The adjacent graph (right) shows that the partial pressure of the permeate is constant with respect to the surface passed by the membrane, while the pressure on the feed side is decreasing.

[0095] Figure 10The membrane separator 65 on the left is shown, which is schematically divided into two sectors by the membrane M. The feed 92 enters the separator 65. The retentate stream RF exits from one sector (in the same direction as the feed), while the permeate stream PF exits from the other sector in the opposite direction, relative to Figure 9 the stream PF in []. Its direction has changed because in this case, a purge gas SG is introduced in countercurrent with the feed stream and the retentate stream RF. The adjacent graph (right figure) shows that the partial pressure of the permeate increases at the start of the surface traversed by the membrane and decreases on the feed side.

Claims

1. An apparatus (10) for capturing and recovering CO2 from process flue gas, comprising: (a) A furnace that generates heat and flue gas containing CO2 during use; (b) A CO2 capture and separation unit (C) located downstream of the reheating furnace, having a first outlet and an associated first pipeline, and a second outlet and an associated second pipeline, wherein, during use, a gas stream (F1, F2) containing a portion of the captured and separated CO2 is supplied to each outlet and the associated pipeline, and the CO2 concentration of the gas stream (F1, F2) is different from the CO2 concentration (12) supplied to the CO2 capture and separation unit (C); (c) A first gas scrubbing device (18), preferably an absorption tower, adapted to scrub the CO2 with aqueous MOH, wherein M is an alkali metal, preferably sodium (Na) or potassium (K), and is connected to the first pipeline (F1) to be supplied with CO2; (d) A reactor (24) adapted to carry out a reaction between an alkali metal carbonate, preferably Na or K, and CO2, connected to the second pipeline (F2) and the first gas scrubbing device (18) to be supplied with CO2 and an alkali metal carbonate, preferably Na or K, respectively, and provided with a discharge port for extracting the alkali metal bicarbonate produced by the reaction of CO2 with the alkali metal carbonate during use.

2. The CO2 capture and recovery device (10) according to claim 1, characterized in that, The apparatus downstream of the furnace includes: (e) A first heat exchanger (14) for recovering the heat of the flue gas within the apparatus (10).

3. The CO2 capture and recovery device (10) according to claim 1 or 2, characterized in that, The apparatus further includes: (f) A crystallizer (28) connected to the discharge port; (g) A filter or centrifuge (30) for separating the alkali metal bicarbonate produced during the use of the reactor (24); (h) A concentrator (36) downstream of the filter or centrifuge (30), connected to the reactor (24) and the first gas scrubbing device (18), for supplying the concentrate produced during use to the reactor (24) after condensation in a tank mixer used as a source of alkali metal hydroxide, and for supplying any steam generated within the concentrator (36) to the first gas scrubbing device (18); and wherein, preferably, the reactor (24) has an exhaust port (26) connected to the first pipeline (F1).

4. The CO2 capture and recovery device (10) according to any one of the preceding claims, characterized in that, The CO2 capture and separation unit (C) includes: (b-1) A second gas scrubbing device (44), preferably an absorption tower, adapted to chemically combine or absorb CO2 with an aqueous solution preferably containing an amine or potassium carbonate; (b-2) A regeneration device (48) downstream of the second gas scrubbing device (44), adapted to release the CO2 absorbed or combined in the second gas scrubbing device (44); wherein the second gas scrubbing device (44) includes the first outlet and is connected to the first gas scrubbing device (18) through the first pipeline (F1); and wherein the regeneration device (48) includes the second outlet and is connected to the reactor (24) through the second pipeline (F2).

5. The CO2 capture and recovery device (10) according to claim 4, characterized in that, The second gas scrubbing device (44) and the regeneration device (48) are connected by a second heat exchanger (46) to heat a solution stream containing captured CO2 that leaves the second gas scrubbing device (44) and is intended for the regeneration device (48), while cooling a regenerated solution discharged from the regeneration device (48) and heated by a heat source (54) contained in the apparatus (10) for supply to the second gas scrubbing device (44), and is characterized in that the apparatus (10), preferably, includes a compressor (50) downstream of the regeneration device (48) to compress the CO2.

6. The CO2 capture and recovery device (10) according to any one of claims 1 to 3, characterized in that, The CO2 capture and separation unit (C) includes a first membrane separator (60), preferably having a polymer membrane, adapted to separate CO2 from a gas stream, thereby producing a first gas stream rich in CO2 and a second gas stream with a reduced CO2 content, wherein the first membrane separator (60) includes the first outlet and the second outlet, and supplies the gas stream with a reduced CO2 content to the first gas scrubbing device (18) through the first pipe (F1) connected to the first outlet, and supplies the gas stream rich in CO2 to the reactor (24) through the second pipe (F2) connected to the second outlet.

7. The CO2 capture and recovery device (10) according to claim 6, characterized in that, At least one additional membrane separator (64) is inserted between the first membrane separator (60) and the first gas scrubbing device (18), the additional membrane separator (64) is supplied with a low CO2 content fraction discharged from the first membrane separator (60), and supplies a fraction rich in CO2 to the second pipe (F2), and supplies a fraction with a reduced CO2 content to the second gas scrubbing device (18).

8. The CO2 capture and recovery device (10) according to any one of claims 1 to 3, characterized in that, The CO2 capture and separation unit (C) includes an adsorption capture and separation system, in particular a PSA system, adapted to separate CO2 from the flue gas.

9. The CO2 capture and recovery device (10) according to any one of the preceding claims, characterized in that, The reactor (24) is a three-phase reactor.

10. The CO2 capture and recovery device (10) according to any one of the preceding claims, characterized in that, The furnace is a reheating furnace.

11. A process for capturing and recovering CO2 from process flue gas, comprising the following steps: (i) generating heat and flue gas containing CO2 using a furnace; (ii) capturing and separating the CO2 in a capture unit (C); (iii) dividing the captured and separated CO2 into a first CO2 stream (F1) and a second CO2 stream (F2); (iv) washing the first stream with aqueous MOH, thereby generating an alkali metal carbonate M2CO3, wherein M is preferably Na or K; (v) reacting the alkali metal carbonate with the second CO2 stream (F2) to form an alkali metal bicarbonate MHCO3; and (vi) separating the alkali metal bicarbonate; wherein, preferably, the process at least partially uses heat from the furnace.

12. The process for capturing and recovering CO2 according to claim 11, wherein, The capturing and separating of CO2 in step (ii) is carried out by at least one of the following options: (A) using (ii-a) washing the CO2 with an aqueous solution that absorbs or chemically binds the CO2, the aqueous solution preferably containing an amine or potassium carbonate; and (ii-b) Subsequently release the CO2 absorbed or bound in step (ii-a), wherein a portion of the CO2 that is not absorbed or bound in step (ii-a) forms the first CO2 stream, and the CO2 released in step (ii-b) forms the second CO2 stream, and wherein, preferably, the solution free of CO2 is heated and then reused in step (ii-a), and before being reused in step (ii-a), the solution containing the CO2 absorbed or bound between step (ii-a) and step (ii-b) is heated using the heated solution in a heat exchanger (46); (B) Use membrane separation to produce a CO2-rich stream, which forms the second CO2 stream (F2), and produce a stream with reduced CO2 content, which forms the first CO2 stream (F1); (C) Use, preferably, adsorption separation carried out according to PSA technology.

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