Method and system for recovering of carbon dioxide

Abstract

The invention relates to a method for the recovery of carbon dioxide. The method comprises providing gas containing carbon dioxide to an absorption step, the absorption step comprises contacting the gas containing carbon dioxide in an absorption unit (1) with an absorbing composition comprising solid particles of bicarbonate and an aqueous solution comprising the bicarbonate and one or more carbonate compounds, thereby reacting at least a part of the carbon dioxide to form bicarbonate; and obtaining a part of the solid bicarbonate particles from the absorption step. The invention also relates to a system for recovering carbon dioxide.

Description

[0001] METHOD AND SYSTEM FOR RECOVERING OF CARBON DIOXIDE FIELD OF THE INVENTION The present invention relates to a method for recovering carbon dioxide from a gas containing it and more particularly to a method for recovering carbon dioxide from a gas containing it according to preamble of claim 1. The present invention relates to a system for recovering carbon dioxide from a gas containing it and more particularly to a system for recovering carbon dioxide from a gas containing it according to preamble of claim 14. The present invention relates to use of solid bicarbonate particles for providing an autocatalytic effect according to preamble of claim 19. BACKGROUND OF THE INVENTION The recovery of carbon dioxide has traditionally involved the use of a so-called packed column, operating in a counter-current mode. It comprises a tank filled with loose blocks. From a top end of the tank is poured liquid absorbent solution so as to soak the loose blocks, and from a bottom end is supplied a flue gas or the like carbon dioxide containing gas. The absorption liquid can be water or mixture of water and different amines. The surface of the loose blocks constitutes a large area, thereby enhancing the absorption or regeneration taking place at an interface between gas and absorption liquid. Gas and absorption liquid are supplied from the opposite ends for the purpose of generating a so-called countercurrent process, in which the concentration gradient remains high across the entire column. In boilers, the same countercurrent principle is referred to as superheating. A drawback with the packed column is its remarkably large size and thereby also the manufacturing costs. The height of packed column can be several tens of meters, and in some cases even a hundred meters. Such columns are inefficient to build. Typically, a cryogenic process is needed to store liquid carbon dioxide. The cryogenic process consisting of dropping the temperature of the gas stream to for instance -20 °C and increasing the pressure to 20 bar. That increases investment and operating costs. BRIEF DESCRIPTION OF THE INVENTION An object of the invention to provide a method and a system for recovering carbon dioxide from a gas containing it so as to solve or at least alleviate the prior art disadvantages. The objects of the invention are achieved by a method for recovering carbon dioxide from a gas containing it which is characterized by what is disclosed in the independent claim 1. The objects of the invention are further achieved by a system for recovering carbon dioxide from a gas containing it which is characterized by what is disclosed in the independent claim 14. The objects of the invention are further achieved by use of solid bicarbonate particles for providing an autocatalytic effect which is characterized by what is disclosed in the independent claim 19. Preferred embodiments are disclosed in the dependent claims. The invention is based on the idea providing method for recovering carbon dioxide from a gas containing it. The method comprising:- providing gas containing carbon dioxide to an absorption step, the absorptionstep comprises contacting the gas containing carbon dioxide in an absorption unit with an absorbing composition comprising solid particles of bicarbonate and an aqueous solution comprising the bicarbonate and one or more carbonate compounds, thereby reacting at least a part of the carbon dioxide to form bicarbonate; and- obtaining a part of the solid bicarbonate particles from the absorption step.In some embodiments, the absorption step is carried out at a temperature in the range from 20 degrees C – 85 degrees C. In some embodiments, the absorption step is carried out at a temperature in the range from 40 degrees C – 85 degrees C. That enhances the absorption reaction. It should be noted that typically flue gas may further comprise other substances than carbon dioxide. In some embodiments, the method comprising: -providing gas containing carbon dioxide to an absorption step, the absorption step comprises contacting the gas containing carbon dioxide in an absorption unit with an absorbing composition comprising solid particles of bicarbonate salt or bicarbonate and an aqueous solution comprising the bicarbonate and one or more carbonate compounds as solutes, thereby reacting at least a part of the carbon dioxide to form bicarbonate; and -obtaining a part of the solid bicarbonate particles from the absorption step. In some embodiments, solid particles of bicarbonate means solid particles of bicarbonate salt. In some embodiments, recovering carbon dioxide means recovering carbon dioxide in bicarbonate. In some embodiments, the method comprising: -providing gas containing carbon dioxide to an absorption step, the absorption step comprises contacting the gas containing carbon dioxide in an absorption unit with an absorbing composition comprising solid particles of bicarbonate salt and an aqueous solution comprising the bicarbonate and one or more carbonate compounds as solutes, thereby reacting at least a part of the carbon dioxide to form bicarbonate; and -obtaining a part of the solid bicarbonate particles from the absorption step. In some embodiments, the one or more carbonate compounds of the absorbing composition comprises potassium carbonate (K₂CO₃). In some embodiments, the one or more carbonate compounds of the absorbing composition consists of potassium carbonate (K₂CO₃). In some embodiments, the one or more carbonate compounds of the absorbing composition being potassium carbonate (K₂CO₃). Potassium carbonate and carbon dioxide efficiently react and provide potassium bicarbonate (KHCO₃). It should be noted that the absorbing composition may contain other hydroxide or carbonate solutes, including but not limited to sodium, calcium or magnesium cations and thus the one or more carbonate compounds may further comprise the respective carbonate of the said cations. In some embodiments, the absorbing composition comprises bicarbonate in the range from 4,05 wt-% to 33,9 wt-% based on the total weight of the absorbing composition. In some embodiments, the absorbing composition comprises bicarbonate in the range from 22,98 wt-% to 33,57 wt-% based on the total weight of the absorbing composition. In some embodiments, the absorbing composition comprises bicarbonate in the range from 22,98 wt-% to 30,86 wt-% based on the total weight of the absorbing composition. In some embodiments, the absorbing composition comprises bicarbonate in the range from 22,98 wt-% to 25,19 wt-% based on the total weight of the absorbing composition. Potassium carbonate and carbon dioxide efficiently react and provide bicarbonate in above mentioned range so that there the amount of solids is not too high for the system. In some embodiments, the absorbing composition comprises bicarbonate in the range from 10,83 wt-% to 25,19 wt-% based on the total weight of the absorbing composition. In some embodiments, the absorbing composition comprises bicarbonate in the range from 6,79 wt-% to 31,32 wt-% based on the total weight of the absorbing composition. In some embodiments, the absorbing composition comprises solid bicarbonate in the range from 4,05 wt-% to 33,9 wt-% based on the total weight of the absorbing composition. The solid bicarbonate particles in the above-mentioned range provide an autocatalytic effect so that carbonate and carbon dioxide react efficiently. In some embodiments, the absorbing composition comprises solid bicarbonate in the range from 22,98 wt-% to 33,57 wt-% based on the total weight of the absorbing composition. The solid bicarbonate particles in above mentioned range provide an efficient autocatalytic effect. In some embodiments, the absorbing composition comprises solid bicarbonate in the range from 22,98 wt-% to 30,86 wt-% based on the total weight of the absorbing composition. The solid bicarbonate particles in above mentioned range provide an efficient auto-catalyst effect so that carbonate and carbon dioxide react still even more efficiently so slurry is easy to obtain from the reactor. In some embodiments, the absorbing composition comprises solid bicarbonate in the range from 22,98 wt-% to 25,19 wt-% based on the total weight of the absorbing composition. The solid bicarbonate particles in above mentioned range provide an efficient autocatalytic effect, and slurry is easier to obtain from the reactor. In some embodiments, the absorbing composition comprises solid bicarbonate in the range from 10,83 wt-% to 25,19 wt-% based on the total weight of the absorbing composition. The solid bicarbonate particles in above mentioned range provide an efficient autocatalytic effect, so that slurry is still easier to obtain from the reactor. It should be noted that the concentration of solid particles may be vary within the absorption unit. It should be noted that too high concentration of solid particles may block pipelines. A chemical reaction is autocatalytic if one of the reaction products is also a catalyst for the same reaction. Tables 1,2 and 3 show how amount of bicarbonate affect to efficiency of reaction of carbon dioxide and potassium carbonate. The tables are shown below in the detailed description of the invention. Furthermore, tables 1,2 and 3 show more details how solid bicarbonate provides an autocatalytic effect and improve efficiency of reaction of carbon dioxide and potassium carbonate. In some embodiments, pH of the absorbing composition being in the range from 9,0 to 11,0. In some embodiments, pH of the absorbing composition being in the range from 9,5 to 10,5. Potassium carbonate and carbon dioxide very efficiently react and provide bicarbonate in above mentioned range. In some embodiments, the absorbing composition comprises the one or more carbonate in the range from 0,66 wt-% to 31,14 wt-% based on the total weight of the composition. In some embodiments, the absorbing composition comprises the one or more carbonate in the range from 1,12 wt-% to 13,15 wt-% based on the total weight of the composition. In some embodiments, the absorbing composition comprises the one or more carbonate in the range from 10,80 wt-% to 13,15 wt-% based on the total weight of the composition. In some embodiments, the absorbing composition comprises the one or more carbonate in the range from 4,42 wt-% to 13,15 wt-% based on the total weight of the composition. In some embodiments, the absorbing composition comprises the one or more carbonate in the range from 10,80 wt-% to 25,0 wt-% based on the total weight of the composition. Tables 1,2 and 3 shows how amount of carbonate affect to efficiency of reaction of carbon dioxide and potassium carbonate. The tables are shown below in the detailed description of the invention. In some embodiments, absorbing composition comprising solid particles of bicarbonate and an aqueous solution comprising bicarbonate and one or more carbonate compounds, the one or more carbonate compounds consists of potassium carbonate, bicarbonate being potassium bicarbonate, and the potassium carbonate being dissolved and a part of the potassium bicarbonate is dissolved. In some embodiments, the solid bicarbonate particles obtained from the absorption step being in a form of slurry, and the method comprises a step of removing liquid from the slurry. The slurry with lower liquid content is more efficient to recover. In some embodiments, the solid bicarbonate particles obtained from the absorption step being in a form of slurry, the method comprises a step of drying the slurry, and the drying being carried out with filter. Filtering is an efficient way to remove liquid from the slurry. In some embodiments, the solid bicarbonate particles obtained from the absorption step being in a form of slurry, the method comprises a step of drying the slurry, and providing heat from the absorption step to the step of removing liquid from the slurry. In some embodiments, the gaseous flow from the absorption step being used for drying. In some embodiments, the gaseous flow from the absorption step being used for drying the slurry obtained from the absorption step. In some embodiments, the solid bicarbonate particles obtained from the absorption step being in a form of slurry, and the method comprises a step of removing liquid from the slurry, and the method comprising a step of storing dried bicarbonate slurry after the step of removing liquid from the slurry. In some embodiments, the solid bicarbonate particles obtained from the absorption step being in a form of slurry, the method comprises a step of drying the slurry, and the drying being carried out with filter, and the method comprising a step of storing dried bicarbonate slurry after the step of removing liquid from the slurry. In some embodiments, the solid bicarbonate particles obtained from the absorption step being in a form of slurry, the method comprises a step of drying the slurry, and providing heat from the absorption step to the step of removing liquid from the slurry, and the method comprising a step of storing dried bicarbonate slurry after the step of removing liquid from the slurry. In some embodiments, the method comprises a regeneration step and the method comprises feeding the solid bicarbonate particles to the regeneration step, and the regeneration step comprises producing recovered carbon dioxide gas from the solid bicarbonate particles. In some embodiments, the method comprises a regeneration step and the method comprises feeding the solid bicarbonate particles obtained from the absorption step to the regeneration step, the regeneration step comprises producing recovered carbon dioxide gas from the solid bicarbonate particles and the regeneration step comprises heating. In some embodiments, the method comprises a regeneration step and the method comprises feeding the solid bicarbonate particles obtained from the absorption step to the regeneration step, the regeneration step comprises producing recovered carbon dioxide gas from the solid bicarbonate particles and the regeneration step comprises heating at a temperature selected to be in a range from 150°C to 250 °C; or at a temperature selected to be in a range from 180°C to 230 °C. Carbon dioxide is obtained from the regeneration step. The bicarbonate decomposes to carbonate and carbon dioxide when heated. In some embodiments, the method comprising a step of storing solid bicarbonate particles obtained from the absorption step; or In some embodiments, the method comprising a step of storing solid bicarbonate particles obtained from the absorption step and feeding the stored solid bicarbonate particles to the regeneration step after the step of storing. Bicarbonate provides an efficient way to store and transport carbon dioxide. In some embodiments, the absorbing composition comprises water in the range from 48,7 wt-% to 57,4 wt-% based on the total weight of the composition. In some embodiments, the absorbing composition comprises water in the range from 52,26 wt-% to 51,60 wt-% based on the total weight of the composition. In some embodiments, the absorbing composition comprises water in the range from 35 wt-% to 70 wt-% based on the total weight of the composition. The wider range in the water amount enables further possibilities for using the process. In some embodiments, the absorbing composition comprises water in the range from 40 wt-% to 60 wt-% based on the total weight of the composition. This range in the water amount enables further possibilities for using the process so that process is still efficient. Tables 1, 2 and 3 show how amount of water affect to efficiency of reaction of carbon dioxide and potassium carbonate. The tables are shown below in the detailed description of the invention. In some embodiments, the method further comprises adjusting the bicarbonate concentration in the absorbing composition with the ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step. In some embodiments, the method further comprises adjusting the bicarbonate concentration in the absorbing composition with the ratio of concentration of carbonate in the absorbing composition and a volume of carbon dioxide gas feed to the absorption step. In some embodiments, the method further comprises providing the amount of bicarbonate of the absorbing composition to be in the range from 4,05 wt-% to 33,9 wt-% based on the total weight of the absorbing composition by a ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step. In some embodiments, the method further comprises measuring amount of bicarbonate in the slurry obtained from absorption step and adjusting amount of bicarbonate in the absorbing composition by a ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step based on the measured amount of bicarbonate in the slurry. In some embodiments, the method further comprises measuring pH of the absorbing composition and adjusting amount of bicarbonate in the absorbing composition by a ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step based on the measured pH of the absorbing composition. In some embodiments, the method further comprises adjusting the solid bicarbonate concentration in the absorbing composition with the ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step. In some embodiments, the method further comprises adjusting the solid bicarbonate concentration in the absorbing composition with the ratio of concentration of carbonate in the absorbing composition and a volume of carbon dioxide gas feed to the absorption step. In some embodiments, the method further comprises providing the amount of solid bicarbonate of the absorbing composition to be in the range from 4,05 wt-% to 33,9 wt-% based on the total weight of the absorbing composition by a ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step. In some embodiments, the method further comprises providing the amount of solid bicarbonate of the absorbing composition to be in the range from 22,98 wt- % to 33,57 wt-% based on the total weight of the absorbing composition, or in the range from 10,83 wt-% to 25,19 wt-% based on the total weight of the absorbing composition. In some embodiments, the method further comprises measuring amount of bicarbonate in the slurry obtained from absorption step and adjusting amount of solid bicarbonate in the absorbing composition by a ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step based on the measured amount of bicarbonate in the slurry. In some embodiments, the method further comprises measuring pH of the absorbing composition and adjusting amount of solid bicarbonate in the absorbing composition by a ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step based on the measured pH of the absorbing composition. In some embodiments, the method further comprises recovering carbonate in the regeneration step and obtaining the recovered carbonate. In some embodiments, the method further comprises recovering carbonate in the regeneration step and feeding the recovered carbonate to the absorption step. It should be noted that the recovery may be interpreted as desorption or regeneration. In some embodiments, the absorption unit 1 having an absorption volume, and 50 % or more of the absorption volume being filled with the absorbing composition; or 70 % or more of the absorption volume being filled with the absorbing composition; or 90 % or more of the absorption volume being filled with the absorbing composition. It should be noted that even some reactor types, for instance, a bubble column comprises an arrangement to homogenize composition inside the reactor, the content may have different concentrations. Furthermore, some reactor types may have gaseous components in a top the reactor. In some embodiments, method may be carried out with any following embodiment of the system for recovering carbon dioxide. The invention further relates to a system for recovering carbon dioxide from a gas containing it, the system comprising an absorption unit for absorbing the carbon dioxide contained in the gas, the absorption unit comprises a gas supplying arrangement arranged to supply the gas into the absorption unit, the absorption unit comprises an absorbing composition comprising solid particles of bicarbonate and an aqueous solution comprising bicarbonate and one or more carbonate compounds, and the absorption unit comprises an discharging arrangement arranged to discharge bicarbonate particles in a form of slurry from the absorption unit. In some embodiments, the solid particles of bicarbonate are arranged to provide an autocatalytic effect. In some embodiments, the solid particles of bicarbonate are arranged to provide an autocatalytic effect, and the absorbing comprises solid particles of bicarbonate in any of the following range: in the range from 4,05 wt-% to 33,9 wt- % based on the total weight of the absorbing composition, in the range from 22,98 wt-% to 33,57 wt-% based on the total weight of the absorbing composition, and in the range from 10,83 wt-% to 25,19 wt-% based on the total weight of the absorbing composition. In some embodiments, the system comprises a regeneration unit for regenerating the carbon dioxide absorbed in the bicarbonate particles. In some embodiments, the system comprises a filtering unit arranged to remove liquid from bicarbonate particles discharged from the absorption unit. In some embodiments, the system comprises a regeneration unit for regenerating the carbon dioxide absorbed in the bicarbonate particles, and the system comprises a filtering unit arranged to remove liquid from bicarbonate particles discharged from the absorption unit. In some embodiments, the slurry comprises liquid 5 – 9 wt-%, or 10 wt-% or less, or 7 – 8 wt-% after a step of drying. In the context of this application wt-% means weight percentage. Removing liquid from the slurry improves efficiency of recovery. In some embodiments, the system further comprises a recycling arrangement arranged to recycle water from the regeneration unit to the absorption unit. In some embodiments, the system further comprises a recycling arrangement arranged to feed water from the regeneration unit and carbonate from the regeneration unit to the absorption unit. In some embodiments, the system further comprises a recycling arrangement arranged to recycle liquid from the regeneration unit to the absorption unit. In some embodiments, the system further comprises a recycling arrangement arranged to feed liquid from the regeneration unit and carbonate from the regeneration unit to the absorption unit. In some embodiments, the system or the units of the system having features of any above disclosed embodiment of the method. The present invention further relates to use of solid bicarbonate particles for providing an autocatalytic effect in an absorbing composition of carbon dioxide absorption in an aqueous solution comprising the bicarbonate and one or more carbonate compounds. In some embodiments, the absorbing composition comprises solid bicarbonate in any of the following range: in the range from 4,05 wt-% to 33,9 wt-% based on the total weight of the absorbing composition, in the range from 22,98 wt-% to 33,57 wt-% based on the total weight of the absorbing composition, and in the range from 10,83 wt-% to 25,19 wt-% based on the total weight of the absorbing composition, In some embodiments, use having any features of any above disclosed embodiment of the method. The invention provides the following advantages: -The process is simple and efficient to build up as the absorption process isefficient. -Furthermore, bicarbonate provides an easy and energy efficient way to storeand transport carbon dioxide. -Furthermore, the regeneration unit may be compact when most of theexcessive liquid being removed from the slurry before the regeneration step. the solid bicarbonate in the defined ranges provides an autocatalytic effect and thus, improves efficiency of carbon dioxide capturing. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in detail by way of example with reference to the accompanying drawings, in which: Figure 1 shows schematically a system according to one embodiment of the present invention; Figure 2 shows schematically a system according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION In prior art solutions yield of bicarbonate from a reaction of carbon dioxide and carbonate has been increased by increasing size of absorption unit. The inventors surprisingly found out that certain amounts of bicarbonate in the absorbing composition increase absorption efficiency when reacting at least a part of carbon dioxide with an aqueous solution of carbonate to form bicarbonate. Thus, the absorbing composition comprises aqueous solution of carbonate and certain amount of bicarbonate. The increased absorption efficiency and thus, enables using smaller absorption unit which it is more efficient to build and operate. The present invention will now be described more fully hereinafter. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and for fully convey the scope of the invention to a skilled person. Although individual features may be included in differ-ent embodiments, these may possibly be combined in other ways, and the inclusion in different embodiments does not imply that a combination of features is not feasible. In addition, singular references do not exclude a plurality. In the context of the present invention, the terms "a", "an" does not preclude a plurality. As used herein, the term "comprising" includes the broader meanings of "including", "containing", and "comprehending", as well as the narrower expressions "consisting of" and "consisting only of. As used herein, the term "containing" includes the broader meanings of "including", "comprising", and "comprehending", as well as the narrower expressions "consisting of" and "consisting only of. In a continuous process carbonate and carbon dioxide are continuously fed into the absorption unit 1 wherein carbonate and carbon dioxide form bicarbonate. Amount of bicarbonate in the absorbing composition may be adjusted by selecting the ratio of carbonate and carbon dioxide fed into the absorption unit. Furthermore, temperature and pressure in the absorption unit 1 affect to the amount of solid bicarbonate. The absorption unit 1 having an absorption volume. In the context of this application the absorption volume means space in the absorption unit 1 which is intended to receive aqueous solution of carbonate and gaseous carbon dioxide. It should be noted that upper part of the absorption unit 1 may comprise gaseous components and the lower part of absorption unit 1 may comprise concentrated suspension of bicarbonate, and the absorbing composition being between the gaseous components and concentrated suspension of the bicarbonate so that 50 % or more of the absorption volume being filled with the absorbing composition; or 70 % or more of the absorption volume being filled with the absorbing composition; or 90 % or more of the absorption volume being filled with the absorbing composition. In the system for recovery of carbon dioxide, control parameters, heat pumps, liquid flow, etc. may be controlled in each situation in an economically optimized way. According to an embodiment, the absolute pressure used in the absorption unit is in the range from 1 to 10 bar, preferably from 2 to 7 bar, more preferably from 2 to 5 bar. According to an embodiment, the absorption unit 1 has a usable temperature of in the range from 10 to 50 °C; or 40 °C or less; or in the range from 15 to 30 °C. It should be noted that the method comprises a starting step wherein the amount of bicarbonate raising to the intended amount. It should be noted that the method may comprise a shutdown step wherein the amount of bicarbonate is adjusted to 4,05 wt-% or less based on the total weight of the absorbing composition. That prevents problems when the process will be started again. Inventors further found that drying the potassium bicarbonate slurry with excessive gas from the absorption step further increases efficiency of the process. Inventors further found out that the heat from regeneration step may be used for drying the potassium bicarbonate slurry obtained from the absorption step. Furthermore, the excessive heat from the regeneration step and / or from absorption step may be used for district heating. There are many possibilities for recovery and utilization of carbon dioxide, but they can be divided into the following categories:1. Usage as a displacement gas for oxygen.An example of this is the packaging of meat so that carbon dioxide acts as a protective gas to prevent oxygen from entering the food. Also, as a fire extinguishing gas, carbon dioxide prevents the entry of oxygen and extinguishes the fire seat.2. Usage as a raw material for another substance.Carbon dioxide can be used, for example, in the manufacture of plastics or fuels, in which case carbon dioxide forms a chemical bond with other elements, in the so- called synthesis process.3. In a greenhouse carbon dioxide as a fertilizer.Plant photosynthesis requires carbon dioxide. In nature, this is done by plants using carbon dioxide and light, with the leaf green acting as a catalyst. In this way, the plants grow to form fibers in the structure and fruits of the plant. 4. Usage as a bubbling gas in beverages. Carbon dioxide is formed or added to soft drinks, beer and, for example, sparkling wine to effect bubbling. It is always profitable to recover carbon dioxide if it can be used close to the recovery point. The recovery as solid bicarbonate salt and the subsequent regeneration allows for purification of the absorbed CO2 by using conventional process technology and without costly liquefaction. It is costly to store and / or transport liquefied carbon dioxide. The carbon dioxide may be stored and transported as potassium bicarbonate and recovered from the potassium bicarbonate. Potassium bicarbonate (KHCO3) is an inorganic chemical, that when stored under right conditions, does not change nor chemically degrade with time. It is recommended that the Potassium bicarbonate should be stored at room temperature in a sealed container. Under these conditions, potassium bicarbonate would have a very long shelf life. If dry potassium bicarbonate contacts with air or moisture, the total alkalinity will decrease because of the increased water content. When the temperature reaches 88°C, anhydrous potassium bicarbonate will yield about 1% per hour potassium carbonate. Significant decomposition occurs at 177°C. Heating above 100°C provides dangerous levels of carbon dioxide gas. When solutions of potassium bicarbonate are at 98°C or higher, decomposition is 0.29% per hour. In real life, containers of potassium bicarbonate are not a perfect seal but if potassium bicarbonate is kept dry and well below 100°C, it is considered stable. In some embodiments, the regenerated gaseous composition obtained from the regeneration step comprises water (H20) and carbon dioxide (CO2). In some embodiments, the regenerated gaseous composition comprises water and carbon dioxide so that the ratio of water and carbon dioxide in the composition being in the range of 1:1 – 3:1 based on moles of water and carbon dioxide. It should be noted that higher moisture in the bicarbonate slurry increases amount of water in the regenerated gaseous composition and thus, the ratio of moles may be 3:1. In some embodiments, the regeneration step being carried out in pressure more than 1 bar. This decreases a risk of contamination caused by air. In some embodiments, the regeneration step being carried out in pressure less than 1 bar. This increases speed of regeneration process. In some embodiments, water is removed from the regenerated gaseous composition with cooling. For instance, the regenerated gaseous composition cooled to a temperature of 25 °C comprises 97 mol-% carbon dioxide and 3 mol-% water. It should be noted that any typical drying method may be used for drying. In some embodiments, pressure of gaseous carbon dioxide is increased after the regeneration step. In some embodiments, pressure of gaseous carbon dioxide is increased after the regeneration step with a compressor. In some embodiments, the pressure of gaseous carbon dioxide is in the range of 40 – 100 bar or in the range of 40 – 60 bar. In some embodiments, the gaseous carbon dioxide is provided to methane or methanol synthesis process. In some embodiments, the method for recovering carbon dioxide from a gas containing it according to the invention comprises:- pressurizing gas containing carbon dioxide,- supplying pressurized gas to an absorption step, wherein carbon dioxidecontained in the pressurized gas is reacted with carbonate in an absorption unit 1,- absorption step wherein at least a part of carbon dioxide in pressurized gas isreacting with an aqueous solution of carbonate to bicarbonate,- obtaining bicarbonate slurry form the absorption step,- filtering step carried out in a filtering unit 4, in the filtering step at least a partof the water of the bicarbonate slurry being removed, and- recovering carbon dioxide desorbed from water. Figure 1 shows schematically a system according to one embodiment of the present invention. The system for recovering carbon dioxide from a gas containing it, comprising: - an absorption unit 1 for absorbing the carbon dioxide contained in the gas, the absorption unit 1 comprises a gas supplying arrangement 11 arranged to supply the gas into the absorption unit 1, the absorption unit 1 comprises an absorbing composition 13 comprising solid particles of bicarbonate and an aqueous solution comprising bicarbonate and one or more carbonate compounds, and the absorption unit 1 comprises an discharging arrangement 12 arranged to discharge bicarbonate particles from the absorption unit 1, and a regeneration unit 2 for regenerating the carbon dioxide absorbed in the bicarbonate particles. The absorption unit 1 having an absorption volume. In the context of this application the absorption volume means space in the absorption unit 1 which is intended to receive aqueous solution of carbonate and gaseous carbon dioxide. It should be noted that upper part of the absorption unit 1 may be filled with gaseous component and the lower part of absorption unit 1 may be filled with bicarbonate slurry, and the absorbing composition being between the gaseous component and the bicarbonate slurry so that 50 % or more of the absorption volume being filled with the absorbing composition; or 70 % or more of the absorption volume being filled with the absorbing composition; or 90 % or more of the absorption volume being filled with the absorbing composition. In some embodiments, the system for recovery of carbon dioxide having control parameters, heat pumps, liquid flow, etc. which may be controlled in each situation in an economically optimized way. According to an embodiment, the absolute pressure used in the absorption unit 1 is in the range from 1 to 10 bar, preferably from 2 to 7 bar, more preferably from 2 to 5 bar. According to an embodiment, the absorption unit 1 has a usable temperature in the range from 10 to 50 °C; or 40 °C or less; in the range from 15 to 30 °C. In some embodiments, the system comprises a filtering unit 4 arranged to remove water from bicarbonate particles discharged from the absorption unit 1. In some embodiments, the system further comprises a recycling arrangement 5 arranged to recycle liquid from the filtering unit 4 to the absorption unit 1. In some embodiments, the system further comprises a recycling arrangement 5 arranged to feed liquid from the filtering unit 4 and carbonate from the regeneration unit 2 to the absorption unit 1. In some embodiments, the filtering unit 4 comprises a pressure filter or low- pressure filter or vacuum filter. In some embodiments, the absorption unit 1 comprises a bubble column or mixer reactor. Preferably, the absorption unit 1 comprises a bubble column. In some embodiments, the regeneration unit 2 comprises a screw dryer or rotary kiln or drum dryer. Preferably, the regeneration unit 2 comprises a screw dryer. In some embodiments, the regeneration unit 2 being indirectly heated. In some embodiments, the regeneration unit 2 comprises heating medium. In some embodiments, the screw dryer comprises a screw arranged to move material to be dried along the dryer, a heating tube arranged to receive heating medium, and a material tube arranged to surround the material to be dried. In some embodiments, the heating medium flows inside the heating tube, and the material tube is arranged to surround the heating tube. In some embodiments, the heating medium is heated with electricity, or gas, or steam, oil, or water, or liquid. In some embodiments, the system comprises a first storage unit 7 arranged to store bicarbonate prior to regeneration. In some embodiments, the system comprises a second storage unit 6 arranged to store regenerated carbonate to be recycled to the absorption unit 2 via the recycling arrangement 5. In some embodiments, the recycling arrangement 5 comprises a container comprising a pump and a pipeline between the absorption unit 1 and the recycling unit 5 and a pipeline between the regeneration unit 2 and the recycling unit 5. In some embodiments, the system comprises a product outlet 21 arranged to transfer recovered carbon dioxide from the system. Figure 2 shows schematically a system according to one embodiment of the present invention. The system corresponds the system shown in figure 1 except the regeneration unit 2 shown in figure 2 comprises two screw dryers 2a, 2b. The regeneration unit 2 shown in figures 1 and 2 may provide heat energy H1 about 550 kW. This may be used for drying the bicarbonate slurry or district heating. The absorption unit 1 shown in figures 1 and 2 may provide heat energy H2. This may be used for drying the bicarbonate slurry or district heating. Example 1 (comparative) First inventors tried to increase yield of potassium bicarbonate when reacting a definite amount of carbon dioxide and aqueous solution of potassium carbonate so that the solution contained a high amount of unreacted potassium carbonate. Inventors found out that 51,4 % - 62,4 % of carbon dioxide was reacted with the potassium carbonate during 50 sec when the reactor comprised an aqueous solution of potassium carbonate so that the solution comprised 0 wt-% - 11,7 wt-% potassium bicarbonate as can be seen on table 1. In other words, the concentration of solid bicarbonate is in the range 0 - 3,52 wt%. Each value of experiment in table 1 was analyzed when carbon dioxide has been together with aqueous potassium carbonite or with aqueous potassium carbonite and bicarbonate during 50 sec. The example 1 was carried out so that pressure was 3 bar and temperature was room temperature which was about 20 °C. Table 1 shows amounts and concentration of carbon dioxide, potassium carbonate, water, potassium bicarbonate, pH and percentage of reacted carbon dioxide (%) of tests 1 - 20. Table 1. Example 2 (according to the invention) The inventors continued their efforts to invent how to increase an efficiency of potassium bicarbonate production when reacting the said definite amount of carbon dioxide and aqueous solution of potassium carbonate. Inventors repeated previous experiment so that the solution comprised 12,3 wt-% to 50,0 wt-% of potassium bicarbonate. Inventors found out 65,3 % and 91,8 % of carbon dioxide was reacted with the potassium carbonate during 50 sec when the reactor comprised an aqueous solution of potassium carbonate so that the solution comprised above mentioned amount of potassium bicarbonate as can be seen on table 2. Each value of experiment in table 2 was analyzed when carbon dioxide has been together with aqueous potassium carbonate or with aqueous potassium carbonite and bicarbonate during 50 sec. The example 2 was carried out so that pressure was 3 bar and temperature was room temperature which was about 20 °C. It is surprising that more reacted potassium carbonate provides more efficient reaction of carbon dioxide and potassium carbonate compared to less reacted potassium carbonate. In other words, solid bicarbonate particles provide an autocatalysis effect when the concentration of solid bicarbonate is in the range according to table 2. In other words, the concentration of solid bicarbonate is in the range 4,05 – 33,95 wt%. The concentration of solid bicarbonate is in the range 22,98 wt-% to 33,57 wt-% based on the total weight of the absorbing composition provides an efficient autocatalysis effect. The concentration of solid bicarbonate is in the range from 10,83 wt-% to 25,19 wt-% based on the total weight of the absorbing composition provides an autocatalysis effect so that the bicarbonate is easy to obtain from the reactor. Table 2 shows amounts and concentration of carbon dioxide, potassium carbonate, water, potassium bicarbonate, pH and percentage of reacted carbon dioxide (%) of tests 21- 79.

[0002] Wt-% in slurry:Test solution water K2CO3 KHCO3 Slurry tot. Water K2CO3 KHCO3(in solution) Solution+KHCO3 pH reacted CO2gram / s gram / s gram / s gram / s gram / s wt-% wt-% wt-% %21 1890,23 1130,48 613,47 79,75 1969,98 57,39 % 31,14 % 4,05 % 10,59 63,322 1882,27 1129,36 604,90 90,44 1972,70 57,25 % 30,66 % 4,58 % 10,57 64,423 1874,19 1128,22 596,18 101,29 1975,48 57,11 % 30,18 % 5,13 % 10,55 65,424 1865,99 1127,07 587,30 112,31 1978,31 56,97 % 29,69 % 5,68 % 10,53 66,925 1857,70 1125,89 578,28 123,48 1981,18 56,83 % 29,19 % 6,23 % 10,51 68,326 1849,30 1124,70 569,11 134,80 1984,10 56,69 % 28,68 % 6,79 % 10,49 70,127 1840,81 1123,48 559,80 146,24 1987,06 56,54 % 28,17 % 7,36 % 10,47 71,828 1832,24 1122,25 550,35 157,82 1990,06 56,39 % 27,66 % 7,93 % 10,45 72,129 1823,60 1121,01 540,79 169,51 1993,11 56,24 % 27,13 % 8,50 % 10,42 72,530 1814,89 1119,75 531,10 181,30 1996,19 56,09 % 26,61 % 9,08 % 10,40 73,731 1806,12 1118,47 521,30 193,19 1999,31 55,94 % 26,07 % 9,66 % 10,38 74,932 1797,29 1117,18 511,39 205,17 2002,46 55,79 % 25,54 % 10,25 % 10,35 75,733 1788,43 1115,87 501,37 217,22 2005,65 55,64 % 25,00 % 10,83 % 10,33 76,634 1779,52 1114,56 491,27 229,35 2008,87 55,48 % 24,45 % 11,42 % 10,30 77,535 1770,59 1113,23 481,07 241,53 2012,11 55,33 % 23,91 % 12,00 % 10,28 78,536 1761,63 1111,89 470,78 253,76 2015,39 55,17 % 23,36 % 12,59 % 10,25 79,337 1752,65 1110,54 460,41 266,04 2018,69 55,01 % 22,81 % 13,18 % 10,23 80,238 1743,66 1109,18 449,97 278,35 2022,01 54,86 % 22,25 % 13,77 % 10,20 80,739 1734,66 1107,80 439,44 290,70 2025,36 54,70 % 21,70 % 14,35 % 10,18 81,340 1725,66 1106,42 428,85 303,08 2028,73 54,54 % 21,14 % 14,94 % 10,15 81,941 1716,65 1105,03 418,18 315,47 2032,13 54,38 % 20,58 % 15,52 % 10,12 82,642 1707,65 1103,64 407,45 327,89 2035,54 54,22 % 20,02 % 16,11 % 10,10 83,843 1698,66 1102,23 396,64 340,32 2038,98 54,06 % 19,45 % 16,69 % 10,07 8544 1689,67 1100,81 385,77 352,77 2042,44 53,90 % 18,89 % 17,27 % 10,04 85,445 1680,70 1099,39 374,83 365,22 2045,92 53,74 % 18,32 % 17,85 % 10,01 85,746 1671,74 1097,95 363,83 377,68 2049,42 53,57 % 17,75 % 18,43 % 9,98 86,847 1662,80 1096,51 352,75 390,15 2052,95 53,41 % 17,18 % 19,00 % 9,95 87,848 1653,87 1095,06 341,61 402,63 2056,50 53,25 % 16,61 % 19,58 % 9,92 8849 1644,96 1093,60 330,39 415,11 2060,07 53,09 % 16,04 % 20,15 % 9,89 88,250 1636,07 1092,13 319,10 427,59 2063,66 52,92 % 15,46 % 20,72 % 9,86 88,651 1627,20 1090,65 307,74 440,08 2067,28 52,76 % 14,89 % 21,29 % 9,83 89,152 1618,35 1089,16 296,31 452,57 2070,91 52,59 % 14,31 % 21,85 % 9,80 89,453 1609,53 1087,66 284,80 465,05 2074,58 52,43 % 13,73 % 22,42 % 9,76 89,754 1600,73 1086,15 273,22 477,53 2078,26 52,26 % 13,15 % 22,98 % 9,73 90,255 1591,97 1084,63 261,56 490,00 2081,97 52,10 % 12,56 % 23,54 % 9,69 90,856 1583,25 1083,10 249,83 502,46 2085,71 51,93 % 11,98 % 24,09 % 9,66 91,357 1574,56 1081,57 238,03 514,91 2089,46 51,76 % 11,39 % 24,64 % 9,62 91,858 1565,91 1080,02 226,17 527,32 2093,24 51,60 % 10,80 % 25,19 % 9,59 91,659 1557,32 1078,47 214,25 539,71 2097,04 51,43 % 10,22 % 25,74 % 9,55 91,560 1548,79 1076,91 202,28 552,06 2100,85 51,26 % 9,63 % 26,28 % 9,51 91,661 1540,32 1075,34 190,26 564,36 2104,68 51,09 % 9,04 % 26,81 % 9,47 91,862 1531,93 1073,77 178,20 576,59 2108,51 50,93 % 8,45 % 27,35 % 9,42 91,863 1523,62 1072,20 166,13 588,74 2112,36 50,76 % 7,86 % 27,87 % 9,38 91,764 1515,41 1070,62 154,05 600,79 2116,21 50,59 % 7,28 % 28,39 % 9,33 91,765 1507,31 1069,05 141,99 612,73 2120,05 50,43 % 6,70 % 28,90 % 9,28 91,766 1499,34 1067,49 129,97 624,54 2123,88 50,26 % 6,12 % 29,41 % 9,23 91,367 1491,51 1065,93 118,01 636,18 2127,69 50,10 % 5,55 % 29,90 % 9,18 9168 1483,85 1064,38 106,15 647,63 2131,48 49,94 % 4,98 % 30,38 % 9,12 90,769 1476,36 1062,86 94,42 658,86 2135,22 49,78 % 4,42 % 30,86 % 9,06 90,570 1469,08 1061,35 82,86 669,84 2138,92 49,62 % 3,87 % 31,32 % 9,00 89,571 1462,02 1059,87 71,52 680,53 2142,55 49,47 % 3,34 % 31,76 % 8,92 88,672 1455,22 1058,43 60,45 690,87 2146,10 49,32 % 2,82 % 32,19 % 8,84 87,973 1448,74 1057,04 49,77 700,79 2149,53 49,18 % 2,32 % 32,60 % 8,75 87,274 1442,82 1055,75 39,88 709,91 2152,74 49,04 % 1,85 % 32,98 % 8,65 85,375 1437,67 1054,61 31,16 717,92 2155,59 48,92 % 1,45 % 33,31 % 8,54 83,376 1433,54 1053,69 24,10 724,40 2157,94 48,83 % 1,12 % 33,57 % 8,42 80,577 1430,64 1053,03 19,06 729,03 2159,66 48,76 % 0,88 % 33,76 % 8,32 77,678 1428,88 1052,63 15,98 731,88 2160,76 48,72 % 0,74 % 33,87 % 8,24 71,879 1427,94 1052,41 14,31 733,43 2161,37 48,69 % 0,66 % 33,93 % 8,19 65,9Table 2. Example 3 (comparative) The inventors further continued their efforts to find out how to increase an efficiency of potassium bicarbonate production when reacting carbon dioxide and aqueous solution of potassium carbonate. Inventors dried to further increase efficiency by further increasing the amount of potassium bicarbonate. Table 3 shows amounts and concentration of carbon dioxide, potassium carbonate, water, potassium bicarbonate, pH and percentage of reacted carbon dioxide (%) of tests 80-83. The example 3 was carried out so that pressure was 3 bar and temperature was room temperature which was about 20 °C. Wt-% in slurry:Test solution water K2CO3 KHCO3 Slurry tot. Water K2CO3 KHCO3(in solution) Solution+KHCO3 pH reacted CO2gram / s gram / s gram / s gram / s gram / s wt-% wt-% wt-% %80 1427,47 1052,30 13,48 734,21 2161,68 48,68 % 0,62 % 33,96 % 8,17 58,281 1427,25 1052,25 13,08 734,59 2161,84 48,67 % 0,61 % 33,98 % 8,15 50,482 1427,15 1052,22 12,89 734,76 2161,91 48,67 % 0,60 % 33,99 % 8,15 41,283 1427,10 1052,21 12,81 734,84 2161,94 48,67 % 0,59 % 33,99 % 8,14 31,9Table 3. Inventors surprisingly found out that 33,96 wt-% or more potassium bicarbonate provides less efficient reaction of carbon dioxide and potassium carbonate compared to 33,93 wt-% or less potassium bicarbonate in the slurry. Example 4 (according to the invention) The inventors made an experiment of regeneration of the solid potassium bicarbonate particles obtained from the absorption step to recover carbon dioxide gas from the solid potassium bicarbonate particles by heating the solid potassium bicarbonate particles. The inventors found out that the temperature may be selected to be in a range from 150°C to 250 °C. Preferably, the temperature may be selected to be in a range from 180°C to 230 °C. The inventors found out the yield of regenerated carbon dioxide was in the range of from 80 % to 90 % based on the carbon dioxide fed in the absorption step. Example 5 (according to the invention) The inventors found out that the hot potassium carbonate from regeneration step may generate also about 552 kW heat energy. The heat energy may be used in district heating systems. Example 6 (according to the invention) The inventors found out that the gases of the regeneration step and drying step may generate also about 1520 kW heat energy. The heat energy may be used in district heating systems. The heat energy may be used for drying and heating bicarbonate slurry. For instance, the heat may be provided to the bicarbonate slurry with an indirect counter current drying. The inventor found out that required heating power in regeneration step increases from 3143 kW to 3248 kW when liquid content of the bicarbonate slurry increases from 7 wt-% to 15 wt-% with the indirect counter current drying.

Claims

Claims1. A method for recovering carbon dioxide from a gas containing it, characterizedin that, the method comprising: -providing gas containing carbon dioxide to an absorption step, the absorptionstep comprises contacting the gas containing carbon dioxide in an absorption unit (1) with an absorbing composition comprising solid particles of bicarbonate and an aqueous solution comprising the bicarbonate and one or more carbonate compounds, thereby reacting at least a part of the carbon dioxide to form bicarbonate; and -obtaining a part of the solid bicarbonate particles from the absorption step.

2. A method for recovering carbon dioxide according to claim 1, characterized inthat: - the one or more carbonate compounds of the absorbing composition comprises potassium carbonate (K₂CO₃); or - the one or more carbonate compounds of the absorbing composition consists of potassium carbonate (K₂CO₃); or - the one or more carbonate compounds of the absorbing composition being potassium carbonate (K₂CO₃).

3. A method for recovering carbon dioxide according to claim 1 or 2, characterizedin that: -the absorbing composition comprises bicarbonate in the range from 4,05 wt-% to 33,9 wt-% based on the total weight of the absorbing composition; or - the absorbing composition comprises solid bicarbonate in the range from 22,98 wt-% to 33,57 wt-% based on the total weight of the absorbing composition; or - the absorbing composition comprises solid bicarbonate in the range from 22,98 wt-% to 30,86 wt-% based on the total weight of the absorbing composition; or - the absorbing composition comprises solid bicarbonate in the range from 22,98 wt-% to 25,19 wt-% based on the total weight of the absorbing composition; or - the absorbing composition comprises solid bicarbonate in the range from 10,83 wt-% to 25,19 wt-% based on the total weight of the absorbing composition.

4. A method for recovering carbon dioxide according to any one of claims 1 to 3, characterized in that: - pH of the absorbing composition being in the range from 9,0 to 11,0; or - pH of the absorbing composition being in the range from 9,5 to 10,5.

5. A method for recovering carbon dioxide according to any one of the precedingclaims, characterized in that:- the absorbing composition comprises the one or more carbonate in the range from 0,66 wt-% to 31,14 wt-% based on the total weight of the composition; or - the absorbing composition comprises the one or more carbonate in the range from 1,12 wt-% to 13,15 wt-% based on the total weight of the composition; or - the absorbing composition comprises the one or more carbonate in the range from 10,80 wt-% to 13,15 wt-% based on the total weight of the composition; or - the absorbing composition comprises the one or more carbonate in the range from 4,42 wt-% to 13,15 wt-% based on the total weight of the composition; or - the absorbing composition comprises the one or more carbonate in the range from 10,80 wt-% to 25,0 wt-% based on the total weight of the composition.

6. A method for recovering carbon dioxide according to any one of the precedingclaims, characterized in that:- the solid bicarbonate particles obtained from the absorption step being in aform of slurry, and the method comprises a step of removing liquid from the slurry; or - the solid bicarbonate particles obtained from the absorption step being in a form of slurry, the method comprises a step of drying the slurry, and the drying being carried out with filter; or - the solid bicarbonate particles obtained from the absorption step being in a form of slurry, the method comprises a step of drying the slurry, and providing heat from the absorption step to the step of removing liquid from the slurry; or - the solid bicarbonate particles obtained from the absorption step being in a form of slurry, and the method comprises a step of removing liquid from the slurry, and the method comprising a step of storing dried bicarbonate slurry after the step of removing liquid from the slurry; or - the solid bicarbonate particles obtained from the absorption step being in a form of slurry, the method comprises a step of drying the slurry, and the drying being carried out with filter, and the method comprising a step of storing dried bicarbonate slurry after the step of removing liquid from the slurry; or- the solid bicarbonate particles obtained from the absorption step being in a form of slurry, the method comprises a step of drying the slurry, and providing heat from the absorption step to the step of removing liquid from the slurry, and the method comprising a step of storing dried bicarbonate slurry after the step of removing liquid from the slurry.

7. A method for recovering carbon dioxide according to any one of the precedingclaims, characterized in that:- the method comprises a regeneration step and the method comprises feeding the solid bicarbonate particles to the regeneration step, and the regeneration step comprises producing recovered carbon dioxide gas from the solid bicarbonate particles; or - the method comprises a regeneration step and the method comprises feeding the solid bicarbonate particles obtained from the absorption step to the regeneration step, the regeneration step comprises producing recovered carbon dioxide gas from the solid bicarbonate particles and the regeneration step comprises heating; or - the method comprises a regeneration step and the method comprises feeding the solid bicarbonate particles obtained from the absorption step to the regeneration step, the regeneration step comprises producing recovered carbon dioxide gas from the solid bicarbonate particles and the regeneration step comprises heating at a temperature selected to be in a range from 150°C to 250 °C; or at a temperature selected to be in a range from 180°C to 230 °C 8. A method for recovering carbon dioxide according to any one of the precedingclaims, characterized in that:- the method comprising a step of storing solid bicarbonate particles obtained from the absorption step; or - the method comprising a step of storing solid bicarbonate particles obtained from the absorption step and feeding the stored solid bicarbonate particles to the regeneration step after the step of storing.

9. A method for recovering carbon dioxide according to one of the preceding claims, characterized in that - the absorbing composition comprises water in the range from 48,7 wt-% to 57,4 wt-% based on the total weight of the composition; or - the absorbing composition comprises water in the range from 52,26 wt-% to 51,60 wt-% based on the total weight of the composition.

10. A method for recovering carbon dioxide according to any one of the precedingclaims, characterized in that:- the method further comprises adjusting the bicarbonate concentration in the absorbing composition with a ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step; or - the method further comprises providing the amount of solid bicarbonate of the absorbing composition to be in the range from 4,05 wt-% to 33,9 wt-% based on the total weight of the absorbing composition by a ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step; or - the method further comprises measuring amount of bicarbonate in the slurry obtained from absorption step, and adjusting amount of bicarbonate in the absorbing composition by a ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step based on the measured amount of bicarbonate in the slurry; or - the method further comprises measuring pH of the absorbing composition and adjusting amount of bicarbonate in the absorbing composition by a ratio of concentration of carbonate in the absorbing composition and carbon dioxide gas feed to the absorption step based on the measured pH of the absorbing composition.

11. A method for recovering carbon dioxide according to any one of the precedingclaims, characterized in that.- the method further comprises recovering carbonate in the regeneration step, and obtaining the recovered carbonate; or - the method further comprises recovering carbonate in the regeneration step and feeding the recovered carbonate to the absorption step.

12. A method for recovering carbon dioxide according to any one of the precedingclaims, characterized in that the absorption unit (1) having an absorption volume,and 50 % or more of the absorption volume being filled with the absorbing composition; or 70 % or more of the absorption volume being filled with the absorbing composition; or 90 % or more of the absorption volume being filled with the absorbing composition.

13. A method for recovering carbon dioxide according to any one of the precedingclaims, characterized in that:- the method comprising providing an autocatalytic effect in the absorption step with the solid bicarbonate particles; or- the method comprising providing an autocatalytic effect in the absorption step with the solid bicarbonate particles, the absorbing composition (13) comprises solid particles of bicarbonate in any of the following range: in the range from 4,05 wt-% to 33,9 wt-% based on the total weight of the absorbing composition, in the range from 22,98 wt-% to 33,57 wt-% based on the total weight of the absorbing composition, and in the range from 10,83 wt-% to 25,19 wt-% based on the total weight of the absorbing composition.

14. A system for recovering carbon dioxide from a gas containing it, characterizedin that the system comprises, -an absorption unit (1) for absorbing the carbon dioxide contained in the gas,the absorption unit (1) comprises a gas supplying arrangement (11) arranged to supply the gas into the absorption unit (1), the absorption unit (1) comprises an absorbing composition (13) comprising solid particles of bicarbonate and an aqueous solution comprising bicarbonate and one or more carbonate compounds, and the absorption unit (1) comprises an discharging arrangement (12) arranged to discharge bicarbonate particles in a form of slurry from the absorption unit (1).

15. A system for recovering carbon dioxide according to claim 14, characterized inthat: - the system comprises a regeneration unit (2) for regenerating the carbon dioxide absorbed in the bicarbonate particles; or - the system comprises a filtering unit (4) arranged to remove liquid from bicarbonate particles discharged from the absorption unit (1); or - the system comprises a regeneration unit (2) for regenerating the carbon dioxide absorbed in the bicarbonate particles, and the system comprises a filtering unit (4) arranged to remove liquid from bicarbonate particles discharged from the absorption unit (1).

16. A system for recovering carbon dioxide according to claim 14 or 15, characterized in that: -the system further comprises a recycling arrangement (5) arranged torecycle water from the regeneration unit (2) to the absorption unit (1); or - the system further comprises a recycling arrangement (5) arranged to feed water from the regeneration unit (2) and carbonate from the regeneration unit (2) to the absorption unit (1).

17. A system for recovering carbon dioxide from a gas containing it, characterizedin that: - the solid particles of bicarbonate are arranged to provide an autocatalytic effect; or- the solid particles of bicarbonate are arranged to provide an autocatalytic effect,and the absorbing composition (13) comprises solid particles of bicarbonate in any of the following range: in the range from 4,05 wt-% to 33,9 wt-% based on the total weight of the absorbing composition, in the range from 22,98 wt-% to 33,57 wt-% based on the total weight of the absorbing composition, and in the range from 10,83 wt-% to 25,19 wt-% based on the total weight of the absorbing composition.

18. Use of solid bicarbonate particles for providing an autocatalytic effect in an absorbing composition of carbon dioxide absorption in an aqueous solution comprising the bicarbonate and one or more carbonate compounds.

19. Use according to claim 18, characterized in that the absorbing composition comprises solid bicarbonate in any of the following range: in the range from 4,05 wt- % to 33,9 wt-% based on the total weight of the absorbing composition, in the range from 22,98 wt-% to 33,57 wt-% based on the total weight of the absorbing composition, and in the range from 10,83 wt-% to 25,19 wt-% based on the total weight of the absorbing composition.

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