WO2011112069A1

WO2011112069A1 - Method for capturing and fixing carbon dioxide and apparatus for carrying out said method

Info

Publication number: WO2011112069A1
Application number: PCT/MY2010/000214
Authority: WO
Inventors: Mohd Roslee Othman; Martunus; W. James Noel Fernando
Original assignee: Universiti Sans Malaysia (U.S.M)
Priority date: 2010-03-09
Filing date: 2010-10-21
Publication date: 2011-09-15
Also published as: CH703745B1; MY170412A

Abstract

The present invention relates to a method for capturing and fixing carbon dioxide, particularly from (but not limited to) flue gas from power plants, said method comprising the steps of capturing carbon dioxide from flue gases using a pressure swing adsorption (PSA) unit, containing an adsorbent; reacting the captured carbon dioxide with sodium chloride and ammonium hydroxide, yielding ammonium chloride and precipitating sodium hydrogen carbonate; calcinating said sodium hydrogen carbonate, yielding sodium carbonate, water and excess carbon dioxide; reacting said ammonium chloride with sodium hydroxide yielding sodium chloride, water and ammonia; reacting said ammonia with water yielding ammonium hydroxide. The invention further relates to an apparatus for carrying out said method.

Description

TITLE

Method for capturing and fixing carbon dioxide and apparatus for carrying out said method

FIELD OF INVENTION

The present invention relates to a method for capturing and fixing C0₂ from flue gas and to an apparatus for carrying out this method.

BACKGROUND OF INVENTION There are 7 coal fired plants operated in Malaysia with 8120 MW capacity. Total C0₂ emissions from these coal fired power plants are growing at 4.1 % per year, and projected to reach 98 million tons in 2020. The emission will increase in the future with the construction of new coal fired power plants and the increase on the capacity of the existing coal fired power plants to accommodate the increasing demand for electricity.

Carbon dioxide is a greenhouse gas that is claimed to contribute to global warming as a result of the increase of the gas content in the atmosphere. It is usually released into the air together with the other exhaust gases from the combustion of fossil fuels in coal fired power plants and other plants. The emission of C0₂, especially from coal fired power plants, is reportedly high and the treatment or capture of this gas using traditional methods is considered very costly and complicated. Even if C0₂ is successfully captured, the gas needs to be injected underground for permanent storage. This solution is clearly cost ineffective and not sustainable. U.S. Pat. No. US 6,245, 127 discloses the use of an adsorption process and apparatus for carbon dioxide adsorption. That is to say, for the separation of C0₂ from gas mixtures containing 12 % (molar %) C0₂ and 88 % (molar %) N₂. The adsorbent system consists of zeolite NaY supported on alumina. The feed gas is fed to adsorption column at a pressure of 1.05 bar and at a temperature of 27 °C (low temperature). This process incurs: pressurization, adsorption, depressurization from 1.05 bar to 0.06 bar, evacuation and purging. The process produces 80 % product purity at a recovery of 66 %. Recovery is defined herein as fraction of C0₂ in the feed that is recovered as product. The disadvantages of the prior art are: (a) It cannot be applied at high temperature such as flue gas from coal fired power plant, (b) It exhibits low adsorbent capacity and C0₂ product purity, (c) It is not economical, and (d) It does not convert carbon dioxide into commercially usable products. WO 2009/139813 dicloses the use of an absorption column system for C0₂ capture that is accomplished by reacting carbon dioxide in flue gas with an alkali metal carbonate, or a metal oxide, particularly containing an alkaline earth metal or iron, to form a carbonate salt. A preferred carbonate for C0₂ capture is a dilute aqueous solution of additive-free Na₂C0₃. Other carbonates include ₂C0₃ or other metal ions that can produce both a carbonate and a bicarbonate salt. Examples of suitable metal oxides include several alkaline earths including CaO and MgO. The captured C0₂ is preferrably sequestered using any available mineral or industrial waste that contains calcium magnesium or iron in non-carbonate forms, or iron in the Fe⁺² oxidation state. The disadvantages of the prior art are: (a) It is not effective/economical and not flexible because the prior art requires complicated and costly equipment for C0₂ permanent storage, (b) It consumes high energy because the process is not adsorption but absorption, (c) It introduces and makes use of corrosive, degradable and volatile chemicals, (d) It requires a new backpressure steam generator, C0₂ scrubber and C0₂ compressor/dryer. In order to operate the C0₂ scrubber and compressor, additional energy of approximately 30% of the power plant output is needed, and (e) The noticeable weakness of the prior art is that, the preparation of initial CaO and/or MgO liberates C0₂, thus defeating the purpose of C0₂ abatement.

U.S. Pat. Appl. No. US 2005/304,356 dicloses the use of an apparatus for reducing the amount of carbon dioxide released into the atmosphere through fixation by dumping carbon dioxide in seawater or fresh water. At certain temperature and pressure conditions carbon dioxide hydrate or its clathrate is formed. Carbon dioxide is pumped into the pipeline and is discharged by the compressor into the reacting housing from the ejector nozzles under pressure to form carbon dioxide hydrate or its clathrate; the thus produced hydrate or its clathrate is dispersed by the propeller out of the reaction housing to the deep bottom of the sea. The disadvantages of the prior art are: (a) It is not effective and not flexible because the prior art requires C0₂ injection into subsurface/seawater reservoir for permanent storage, (b) The system is complicated, costly, and it may not be environmentally favourable, (c) It cannot convert carbon dioxide into commercially usable products.

The disadvantages of the technical solutions known in the prior art are overcome by the process and apparatus of the current invention.

SUMMARY OF THE INVENTION

A preferred embodiment of the present invention relates to a method for capturing and fixing carbon dioxide, particularly from flue gas from power plants, the method comprising capturing carbon dioxide using a pressure swing adsorption (PSA) unit, containing an adsorbent; reacting the captured carbon dioxide with sodium chloride and ammonium hydroxide, yielding ammonium chloride and precipitating sodium hydrogen carbonate; calcinating said sodium hydrogen carbonate, yielding sodium carbonate, water and excess carbon dioxide; reacting said ammonium chloride with sodium hydroxide yielding sodium chloride, water and ammonia; reacting said ammonia with water yielding ammonium hydroxide. The adsorbent in the pressure swing adsorption (PSA) unit contains hydrotalcite potassium-sodium or modified intercalated adsorbents containing or resembling hydrotalcites or their structures or combination of hydrotalcite with other adsorbents by means of doping, impregnation,coating, juxtapositioning or other means.

Also, the presenst invention discloses an apparatus for carrying out the above methods , characterized in that it comprises a continuous stirred tank reactor (1 ), a separation means (2) connected with the reactor (1 ), a calciner (3) connected with the separation means (2), a stripper column (4) connected with the separation means (2) and a scrubber column (5) connected with the stripper column (4) and in that the outlets of the stripper column (4) and the scrubber column (5) are preferabl connected with a stirred tank (6) which supplies reagents to the reactor. BRIEF DECRYPTION OF THE FIGURES

Fig. l represents a schematic complete view of C0₂-capture and fixation unit placement within a power plant.

Fig. 2 shows schematically the pressure swing adsorption (PSA) unit for capturing C0₂.

Fig. 3 shows schematically the C0₂ fixation unit.

Fig. 4 shows desorption percentage as a function of purge gas amount.

Fig. 5 shows the effect of purge to feed ratio on C0₂ product purity .

Fig. 6 shows C0₂ reacted as a function of agitation speed.

Fig. 7 shows the performance of stripper column as a function of its length.

Fig. 8. shows the schematic flow of total material balance in the fixation unit.

Fig. 9 shows eflluent mole fraction profiles of the adsorber at adsorption step.

o

Fig. 10 shows molar flow rate of gases at the outlet of the column.

Fig. 1 1 shows C0₂ concentration profiles at the end of each step.

Brief description of Tables

Table 1 Total material balance in Fig. 8.

Table 2 Mass balance for equipment in Figure 3.

DETAILED DESCRIPTION OF THE INVENTION

The present invention objectives are to capture C0₂ from flue gas of coal fired power plant and send it to a converter (fixation) unit to convert it into soda ash by reacting the C0₂ with the ammoniated brine in a continuous stirred tank reactor (CSTR). The temperature of flue gas is in the range of 400 to 600 °C (high temperature) and it contains about 15 % (molar %) C0₂, 10 % (molar %) water vapor and 75 % (molar %) N₂. The heat from the flue gas is recuperated during fixation. All the by products can be re-used into the system, making the system highly economical and greener. No prior art suggests a technical solution having all these features. While the method in the present invention is specially tailored for high temperature flue gas, it is also applicable for low temperature operation by conveniently replacing the adsorbent. The present invention provides a method for capturing and fixing carbon dioxide, particularly from flue gas from power plants, said method comprising the steps of a) capturing carbon dioxide using a pressure swing adsorption (PSA) unit, containing an adsorbent;

b) reacting the captured carbon dioxide with sodium chloride and ammonium hydroxide, yielding ammonium chloride and precipitating sodium hydrogen carbonate;

c) calcinating said sodium hydrogen carbonate, yielding sodium carbonate, water and excess carbon dioxide, which can be re-used in step b);

d) reacting said ammonium chloride with sodium hydroxide yielding sodium chloride, water and ammonia, wherein sodium chloride and water can be re-used to prepare the reaction mixture of step b);

e) reacting said ammonia with water yielding ammonium hydroxide, which can be reused to prepare the reaction mixture of step b).

In the preferred embodiment, the process can make use of steam for purging and regenerating the adsorbents, so that the process is more economic than prior arts. The process also interconnects to the fixation unit for ease of C0₂ conversion into soda ash. The detail structure of the invention is provided in the following figures.

The adsorbent in the pressure swing adsorption (PSA) unit contains (but not limited to) hydrotalcite potassium-sodium or modified intercalated adsorbents containing or resembling hydrotalcites or their structures or combination of hydrotalcite with other adsorbents known to those of skill in the art such zeolites by way of doping, impregnation, coating, juxtapositioning or other means.

The present invention further provides an apparatus for carrying out the method of fixing carbon dioxide, which comprises a continuous stirred tank reactor (CSTR) in which the reaction of carbon dioxide, sodium chloride and ammonium hydroxide takes place, preferably at temperature of about 30 °C to produce sodium bicarbonate (sodium hydrogen carbonate), ammonium chloride and water. The solid sodium bicarbonate can be separated from the mother liquor using any separation means known to those of skill in the art, such as rotary filter. The solution is sent to rotary filter for sodium bicarbonate separation. The rotary filter preferably operates at a temperature of about 30 °C and vacuum pressure. The solid sodium bicarbonate is fed to calciner. The remaining solution is sent to stripper column for ammonia recovery. The calciner is preferably operated at a temperature of about 180° C and at a pressure of about 1 atm. In a preferred embodiment, the heater for calciner is supplied from the heat of the flue gas. The sodium bicarbonate forms sodium carbonate, carbon dioxide and water vapor. The solution from the separation means, preferably rotary filter, is fed to stripper column on the top column, stripping the ammonium hydroxide to form ammonia, hydrogen ion, hydroxide ion and chloride ion and, then, reacting the sodium hydroxide (inlet from the top column) with chloride ion to form sodium chloride. Sodium chloride and water formed are sent back to CSTR. The stripper preferably operates at a temperature of 100°C and at a pressure 1 atm. The ammonia from stripper column is fed to scrubber column and reacts with water to form ammonium hydroxide, preferably at a temperature of about 30 °C and at a pressure of about 1 atm. The ammonium hydroxide is re-used as feed in CSTR.

The concept of PSA for C0₂ is based on the preferential or selective adsorption of the gas on a microporous-mesoporous solid adsorbent (such as, but not limited to, hydrotalcite potassium -sodium or modified intercalated adsorbents having or resembling those of hydrotalcites or their structures) at a relatively high pressure by contacting the gas with the solid in a packed column of the adsorbent in order to produce a gas stream enriched in the less strongly adsorbed components of the feed gas. The adsorbed components are then desorbed from the solid by lowering their superincumbent gas-phase partial pressures inside the column so that the adsorbent can be re-used. The desorbed gases are enriched in the more strongly adsorbed components of the feed gas. No external heat is generally used for desorption. A PSA process carries out the adsorption step at a superambient pressure, and the desorption is achieved at a near-ambient pressure level. A PSA unit is based on the fact that adsorbents can be easily regenerated by a pressure change. The more adsorbable components (C0₂) are removed from a gas mixture by adsorption at high pressures, PH. Subsequently, the adsorbed species are desorbed using low pressures, Pi. Two Packed-bed adsorbers are used. Each bed undergoes a cyclic operation involving four stages: (1 ) The first stage is pressurization (with feed) to a pressure of P_H, fed in at a constant flow rate, Fpress, and for a time, t_press. (2) The second stage is adsorption of the more adsorbable components (C0₂), from a mixture (flue gas of power generation as a feed) fed in at a constant pressure of PH, at a constant flow rate, Ff_ee(j, and for a time, t_ads- A raffinate stream richer in the less adsorbable components is produced and, then, goes to gas stack . (3) The third stage is depressurization (reducing the pressure from P_H to PL) using purge with a steam stream at a lower inlet pressure, flowing in at a constant rate, Fpu_rge, for a time, t_purge. The gas coming out of the purge stage goes as waste (gas stack). (4) The fourth stage is blowdown with no inlet stream, but with a constant outlet flow rate, F_Wow, of the extract (C0₂ product) for a time, tbiow

The present invention proposes a method for capturing and fixing carbon dioxide in its entirety. The only end product will be commercially attractive that can generate economy. There are 2 main steps to this method:

1 ) Capturing carbon dioxide using pressure swing adsorption (PSA) unit: In this unit, carbon dioxide is captured and separated from flue gas using adsorption technology.

2) Converting carbon dioxide to commercial products: In this step, carbon dioxide is reacted with other benign, eco-friendly chemicals to become useful product such as sodium carbonate (soda ash). The process achieves almost a complete loop, making it sustainable.

Soda ash is required in many industrial processes, such as glass industry, wherein it is used in the manufacturing of flat and container glass. Acting as a network modifier or fluxing agent, it allows lowering the melting temperature of sand and therefore reduces the energy consumption. Further, soda ash may be used in detergent industry, in a large number of prepared domestic products: soaps, scouring powders, soaking and washing powders containing varying proportions of sodium carbonate, where the soda ash acts primarily as a builder or water softener. Soda ash is also used in steel industry, where it is used as a flux, a desulfurizer, dephosphorizer and denitrider. In non-ferrous metallurgy industry, soda ash may be used, e.g., for treatment of uranium ores, oxidizing calcination of chrome ore, lead recycling from discarded batteries, recycling of zinc, aluminium. And finally, soda ash is used in a large number of industrial chemical reactions to produce organic or inorganic compounds used in very different applications. Soda ash also finds use in electrolysis, water softeners, anti-mold agents, wetting and bonding agents, food additives and stabilizers, etc. Sodium chloride finds many uses as food additive and is important in the manufacture of medicaments. Ammonia finds its uses as fertilizer, cleaner and fuels. The process of the present invention is effective and flexible, it can effectively replace the need for C0₂ injection into subsurface/seawater reservoir for permanent storage that requires complicated and costly equipment. Thanks to the fact that the process of the present invention converts carbon dioxide into commercially usable products such as sodium carbonate (soda ash), it is economically advantageous. The process according to the present invention is also sustainable, because the recovery of ammonia does not use calcium chloride in stripper column. So that there are no cost for the environmentally safe disposal of this calcium chloride. Moreover, the regulation of storage and disposal of calcium chloride could be neglected. The process achieves almost a complete loop, making it sustainable.

Examples of carrying out the Invention The invention is herein further described by way of examples, which should not be construed as further limiting.

Fig. 1 shows a diagram showing the entire structure, including the location at which all the important units are placed. First, flue gas from a coal fired power plant is fed to pressure swing adsorption (PSA) unit. The C0₂ separated from the flue gas in the PSA unit is then fed into the converter unit, in which it is converted to sodium carbonate. Other components of the flue gas go to stack.

Example 1 The PSA unit for capturing C0₂ as shown in Fig. 2 contains a steam tank _Π with a gas pump Γ2, a gas pump 13. for flue gas inlet, two adsorbent beds 14, 15, a cooler 16, cooled by water from a water tank 17 pumped by a water pump J_8.

The process described herein (Figure 2) is from a bench scale unit using adsorption column packed type with 3 mm of hydrotalcite potasium-sodium modified adsorbent pellet (particle) diameter and 814 kg/m³ of bulk density of the adsorbent bed. The C0₂ adsorption capacity of adsorbent is 0.84 mol/kg at the temperature of 300 °C. The adsorption column diameter is 2.2 cm and its length is 25 cm. The adsorption column is initially pressurized with feed at the flow rate of 0.568 L/min, at the pressure of 4 bar and at the temperature of 300 °C for 25 minutes. A feed gas containing 15% (v/v) C0₂, 10% (v/v) water vapor and 75% (v/v) N₂ at the pressure of 4 bar and at the temperature of 302 °C is introduced into the column for 40 minutes. The column is then co-currently depressurized from 4 bar to 1.1 bar purged with steam at a purge to feed ratio of 1.6 and at a pressure of 1.1 bar utilizing a gas pump for 40 minutes. The adsorption column is then evacuated (blowdown) co-currently for 25 minutes, thus yielding a C0₂ product in excess of 99.9% (v/v) purity with 0.1 %. (v/v) water vapor as impurity.

Fig. 4 shows desorption percentage as function of purge gas amount at desorption step; 300 °C, 1.1 bar. Fig. 5 shows the effect of purge to feed ratio on C0₂ product purity. The purge step in PSA is actually the desorption step that a fraction of the product stream is withdrawn to purge the bed and expended to a low pressure (1.1 bar and 300 °C). As the purge volume increases, purging becomes more effective (optimum at the purge to feed ratio of 1.6). Example 2

The details of the C0₂ fixation unit are shown schematically in Fig. 3. The present invention provides a carbonation process using a continuous stirred tank reactor (CSTR) 1 as shown in Fig. 3. In this reactor JL, carbon dioxide reacts with ammonium hydroxide and sodium chloride to form sodium bicarbonate, ammonium chloride and water. The reaction is carried out at the temperature of 30 °C, the pressure of 1 atm and agitation speed of 150 rpm. Carbon dioxide is fed into the reactor at the pressure of 1.2 atm. The reaction is as follows:

C0₂ + H₂0 + NH₄OH + NaCI ► NaHC0₃ + NH₄CI + Ij O

(1 )

In the preferred embodiment, the reactor gives conversion of 85%, higher than the carbonator of the prior art at 70%.

The solid sodium bicarbonate can be separated using any method known to those of skill in the art such as rotary filter 2. The solution is sent to rotary filter 2 for sodium bicarbonate separation. The rotary filter 2 operates at a temperature of 30° C and at a vacuum pressure. The solid sodium bicarbonate is fed to calciner 3. The remaining solution is sent to stripper column 4 for ammonia recovery.

The calciner 3 operates at 180 °C and 1 atm. The sodium bicarbonate forms sodium carbonate, carbon dioxide and water vapor as shown in reaction below:

NaHC0₃ ► 0.5 Na₂C0₃ + 0.5 C0₂ +0.5 H₂0 (2) In the stripper 4, ammonium hydroxide/chloride is stripped into ammonia, hydrogen ion, hydroxide ion and chloride ion and, then, reacting the sodium hydroxide (inlet from the top column) with chloride ion to form sodium chloride at 100 °C and 1 atm,

NH OH ^si

4 !__!_ NH, +

3 (3) strip

₄CI ► N H₃ + CI

(4)

NaOH + CI + H ► NaCI+ H₂0 (5)

Sodium chloride and water formed from the reaction are sent back to CSTR \ .

The ammonia from stripper column 4 is fed to scrubber column 5 and reacted with water to form ammonium hydroxide at 30 °C and 1 atm. The ammonium hydroxide is used as feed to the CSTR 1. The reaction in scrubber column 5 is given by,

NH₃ +H₂0 ► NH₄0H

(6) The ammoniated brine for feeding into CSTR 1 is prepared in ammoniated brine stirred tank 6. Further, in the system are coolers 7, compressor 8 , KO drum 9 and heater 10.

The total material balance for the system as described in Fig. 3 for 99.9 % (v/v) purity C0₂ of 0.004294 kmol/hr with 0.1 %. (v/v) water vapor as impurity is shown in Table 1.

The material balance for equipments such as CSTR, calciner, stripper and srubber as described in Fig. 3 for for 99.9% (v/v) purity C0₂ of 0.004294 kmol/hr with 0.1%. (v/v) water vapor as impurity is shown in Table 2.

Fig. 6 shows the conversion of C0₂ as function of agitation speed of 85% to form sodium bicarbonate at 30° C and 1 atm. Fig. 7 shows the performance of stripper column at 100 °C and 1 atm. Reaction of ammonium chloride with sodium chloride and stripping of ammonium hydroxide are obtained at 100% conversion.

Fig. 8. shows the schematic flow of total material balance for system as described in Fig. 3. The C0₂ from PSA unit with 99.9% (v/v) purity C0₂ of 0.004294 kmol/hr with 0.1%. (v/v) water vapor is fed to this system.

Fig. 9 shows eflluent mole fraction profiles of the adsorber: adsorption step; 303 K, 2 bar, feed flow rate 3.34 L/min (STP), feed composition = 20% C0₂, 10% N₂ (balance CH₄) for Example 3.

Fig. 10 shows molar flow rate of gases at the outlet of the column, 303 K, 2 bar, feed flow rate 3.34 L/min (STP), feed composition = 20% C0₂, 10% N₂ (balance CH₄) for Example 3.

Fig. 1 1 shows carbon dioxide concentration profiles inside the column at the end of each step in cyclic steady state: si. pressurization; s2. feed; s3. counter-current blowdown; s4. purge for Example 4 . Example 3

This example relates to low temperature application of the invention for separation of carbon dioxide from natural gas (methane). The low quality of natural gas is due to some contaminants like nitrogen and corrosive carbon dioxide. In order to meet the standard quality of natural gas the low-quality gas has to be upgraded according to grade specifications: 4% for nitrogen and 2% for carbon dioxide. The composition of natural gas as a feed composed of 70% methane, 20% carbon dioxide and 10% nitrogen. In the example that follows, the PSA utilizes adsorption column (diameter of 2.2 cm and length of 25 cm). The adsorption column is packed with hydrotalcite doped on zeolite adsorbent pellet (3 mm diameter particle) having 792 kg/m³ of bulk density of the adsorbent bed. The C0₂ adsorption capacity of the adsorbent is 3.63 mol/kg at 30° C. The adsorption column is initially pressurized with feed flow rate of 3.34 L/min, at 1.2 bar and 30° C for 10 minutes. A feed gas containing 20% (v/v) C0₂, 70% (v/v) methane and 10% (v/v) N₂ at 2 bar and 30° C is introduced into the column for 15 minutes during adsorption. The column is then counter currently evacuated (blowdown) from 2 bars to 0.2 bar for 10 minutes utilizing a gas pump connected to yiel C0₂ product in excess of 91.3% (v/v) purity by nitrogen contamination. The adsorption column is then purged co-currently for 15 minutes using pure methane at a purge to feed ratio of 0.75, at 30° C and 0.2 bar. Results of from this operation are shown in Figure 9 and 10. The C0₂ containing N₂ may need to be treated using PSA again until it is free from N₂ before C0₂ can be sent to fixation unit for Na₂C0₃ conversion.

Example 4

The following example demonstrates the applicability of the system to remove C0₂ from natural gas at low temperature operation using commercial zeolite 13X adsorbent packed in 2.2 cm diameter, 25 cm length column. The bulk density of the adsorbent adsorption is 758 kg/m³ and the C0₂ adsorption capacity of the adsorbent is 4.5 mol/kg at the temperature of 30° C. The adsorption column is initially pressurized with feed flow rate of 3.47 L/min, at 1.2 bars and 30° C for 5 minutes. A feed gas containing 20% (v/v) C0₂, 70% (v/v) methane and 10% (v/v) N at 1.2 bar and 30° C is introduced into the column for 8 minutes during adsorption. The column is then counter currently evacuated (blowdown) from 1.2 bars to 0.1 bar for 5 minutes utilizing a gas pump connected to ejector to yiel C0₂ product in excess of 92.5% (v/v) purity by nitrogen contamination. The adsorption column is then purged co-currently using pure methane for 8 minutes at a purge to feed ratio of 0.4, temperature of 30° C and pressure of 0.1 bar. The carbon dioxide concentration profiles inside the column at the end of each step in cyclic steady state; si . pressurization; s2. feed; s3. counter-current blowdown; s4. purge; is shown in Figure 1 1.

Claims

1. A method for capturing and fixing carbon dioxide, particularly from flue gas from power plants, said method comprising the steps of

a) capturing carbon dioxide using a pressure swing adsorption (PSA) unit, containing an adsorbent;

c) calcinating said sodium hydrogen carbonate, yielding sodium carbonate, water and excess carbon dioxide;

d) reacting said ammonium chloride with sodium hydroxide yielding sodium chloride, water and ammonia;

e) reacting said ammonia with water yielding ammonium hydroxide.

2. The method according to claim 1, wherein the sodium carbonate, water and excess carbon dioxide produced in step c) are re-used in step b).

3. The method according to claim 1, wherein the sodium chloride and water produced in step d) are re-used to prepare the reaction mixture of step b).

4. The method according to claim 1 , wherein the ammonium hydroxide produced in step e) is re-used to prepare the reaction mixture of step b).

5. The method according to claim 1, wherein steam is used for purging and regenerating the adsorbents.

6. The method according to claim 1, wherein the adsorbent in the pressure swing adsorption (PSA) unit contains hydrotalcite potassium-sodium or modified intercalated adsorbents containing or resembling hydrotalcites or their structures or combination of hydrotalcite with other adsorbents known to those of skill in the art such zeolites by means of doping, impregnation, coating, juxtapositioning or other means.

7. An apparatus for carrying out the method of claim 1 , characterized in that it comprises a continuous stirred tank reactor (1), a separation means (2) connected with the reactor (1 ), a calciner (3) connected with the separation means (2), a stripper column (4) connected with the separation means (2) and a scrubber column (5) connected with the stripper column (4) and in that the outlets of the stripper column (4) and the scrubber column (5) are preferably connected with a stirred tank (6) which supplies reagents to the reactor.