WO2010020017A1

WO2010020017A1 - Treatment of co2-depleted flue gases

Info

Publication number: WO2010020017A1
Application number: PCT/AU2009/001084
Authority: WO
Inventors: Louis Wibberley
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2008-08-22
Filing date: 2009-08-24
Publication date: 2010-02-25
Also published as: JP2012500713A; AU2009284712A1; CN102170957A; KR20110063759A; CN102170957B; JP5663479B2

Abstract

A method of recovering carbon dioxide from a stream of CO₂-containing flue gases, includes the steps of contacting the stream at a gas pressure above atmospheric pressure with an aqueous solvent system to effect absorption of CO₂ from the stream whereby the stream becomes a stream of CO₂-leaner flue gases, and separating the solvent containing the absorbed CO₂ from the stream of CO₂-leaner flue gases to form a CO₂ rich solvent stream. The stream of CO₂-leaner flue gases is expanded in a manner whereby the CO₂-leaner flue gases are cooled, and one or more process streams are cooled by heat exchange with the cooled flue gases. In one aspect, the aqueous solvent system contains dissolved ammonia, and ammonium, carbonate and bicarbonate ions, to effect absorption of CO₂ from the stream whereby the stream becomes a stream of CO₂-leaner flue gases. In this case, the CO₂-leaner flue gases are contacted with water that dissolves ammonia therefrom, which dissolved ammonia may be recycled back to the solvent system, and the CO₂-leaner flue gases are thereafter contacted with a sub-stream of CO₂-rich flue gases that contains sufficient sulphur and/or nitrogen oxides to react with a proportion of the ammonia in the CO₂-leaner flue gases. The products of the reaction are recovered from the CO₂-leaner flue gases. Apparatus for carrying out these methods are also disclosed.

Description

TREATMENT OF CO₂-DEPLETED FLUE GASES

Field of the invention

This invention relates generally to the post combustion capture of carbon dioxide from flue gases. In one or more aspects, the invention is concerned with the treatment of Cθ₂-leaner flue gases resulting from the pressure absorption of CO₂ from flue gas streams, for example using solutions of ammonia in equilibrium with ammonium carbonate/bicarbonate. In one or more other aspects, the invention is concerned with reducing the release of ammonia (known as "ammonia slip") during the absorption of CO₂ from flue gas streams using such solutions of ammonia.

The invention has particular, though not of course exclusive, application to the post combustion capture of CO₂ from the flue gases of power stations or from process gases in a wide variety of industrial processes including steel plants, cement kilns, calciners and smelters.

Background of the invention There is rapidly growing pressure for stationary sources of CO₂ emissions such as power stations, to make step reductions in greenhouse gas (GHG) emissions through 1) capturing the CO₂ formed from the process, and 2) storing the CO₂ by various geological means. This involves injection of CO₂ in a supercritical or "liquefied" state into deep aquifers, coal seams, or deep ocean trenches in the ocean floor, or storage of CO₂ as solid compounds.

The process for capturing the CO₂ from power station or combustion device flue gases is termed post combustion capture. In post combustion capture, the CO₂ in flue gas is preferentially separated from nitrogen and residual oxygen using a suitable solvent in an absorber. The CO₂ is then removed from the solvent in a process called stripping (or regeneration), thus allowing the solvent to be reused. The stripped CO₂ is then liquefied by compression and cooling, with appropriate drying steps to prevent hydrate formation. Post combustion capture in this form is applicable to a variety of stationary CO₂ sources as well as power stations, such as steel plants, cement kilns, calciners and smelters.

The use of solutions of ammonia, in equilibrium with ammonia carbonate and ammonium bicarbonate, as the absorbent for CO₂ has recognised advantages relative to systems that employ organic amines (of which monoethanolamine (MEA) is a well known CO₂ absorbent):

• SOx and NOx can be absorbed, with the possibility of advantageously selling the spent solvent solution as a fertiliser (SOx and NOx degrade amine solvents).

• Ammonia is a low cost chemical, in widespread commercial use.

• Oxygen in the flue gas does not degrade the solvent (but it does degrade amines).

The overall energy required for such a process is projected to be around 40% of that required for MEA systems.

For the ammonia process, the solvent solution consists of ammonium, carbonate and bicarbonate ions, in equilibrium with dissolved ammonia (aqueous), and dissolved CO₂ (aqueous). In the absorber, water and ammonia react with CO₂ (aqueous) to form carbonate, bicarbonate or carbamate ions, with the reaction reversed in the stripper by the application of energy. The relevant aqueous phase reactions can be summarized by the following overall equations:

CO₂ + H₂O + NH₃ <→ HCO₃- + NH₄ ⁺ (eqn. 1)

CO₂ + 2NH₃ → NH₂COO- + NH₄ ⁺ (eqn. 2)

HCO₃ ^" + NH₃ «→ CO₃ ^2" + NH₄ ⁺ (eqn. 3)

CO₃ ^2" + H₂O + CO₂ <→ 2HCO₃- (eqn. 4) It should be noted that the formation of carbamate is undesirable, and has a high heat of reaction. However, the reaction is reversible, and does not have a significant impact.

The amount of free ammonia in the gas phase exiting the absorber is proportional to the amount of ammonia (aqueous), which is controlled by the concentration of the other species in the solution, and the temperature: higher temperatures increase the amount of ammonia in the gas phase.

One of the important issues to address with ammonia solution absorbents is the slip of ammonia from the absorber system with the CO₂-leaner flue gases. , , _0>

International patent publication WO 2006/022885 proposes to address the problem of ammonia slip by cooling the flue gas to 0 - 2O⁰C and operating the absorption stage in this temperature range, preferably in the range 0 - 1O⁰C. Regeneration is by elevating the pressure and temperature of the CO₂-rich solution from the absorber. The CO₂ vapour pressure is high, and a pressurized CO₂ stream, with low concentration of NH₃ and water vapour, is generated. The high pressure CO₂ stream is cooled and washed to recover the ammonia and moisture from the gas. This process, known as a chilled ammonia process, is reported to reduce the degree of ammonia slip, but requires considerable energy for chilling, particularly when it is considered that the reaction heat (the carbonate to bicarbonate reaction involved is exothermic) must be removed to maintain the low temperature.

Applicant's international patent publication WO 2009/000025 discloses a process that reduces ammonia slip by effecting the absorption of CO₂ at a gas pressure above atmospheric pressure, and/or by cooling the CO₂-leaner flue gases after absorption by contact with water that dissolves ammonia therefrom for recycling back to the solvent system.

It is an object of this invention, in one or more of its aspects, to address the problem of ammonia slip via the CO₂-leaner flue gases.

In general, the use of pressure in the absorption step of post combustion capture of CO₂ provides two key benefits: the size of the absorber column is reduced substantially, with a direct proportionality between the pressure of the flue gas and the cross sectional area of the absorber column.

• the kinetics of absorption are increased, since the amount of Cθ₂(aqueous) in the solution is directly related to the total flue gas pressure, according to Henry's Law.

However, this approach requires an increase in power to drive the compressor, and the need for aftercooling of the compressed gases before they enter the absorber. More generally, an issue for all CO₂ post combustion capture systems is the overall energy cost of the system. Disclosures are known, for example WO 2000/057990, US 6,655,150 and US2008/0104958, in which CO₂-depleted gas from a CO₂ separator is expanded e.g. in a turbine to provide power for a compressor, generator or other power plant.

In one or more of its aspects, the present invention addresses this issue, and does so in a manner that is especially useful in ammonia-based systems for removing CO₂ from flue gases.

It is not admitted that any of the information in this specification is common general knowledge, or that the person skilled in the art could be reasonably expected to have ascertained, understood, regarded it as relevant or combined it in any way at the priority date.

Summary of Invention

The invention provides, in a first aspect, a method of recovering carbon dioxide from a stream of CO₂-containing flue gases, comprising the steps of:

contacting the stream at a gas pressure above atmospheric pressure with an aqueous solvent system to effect absorption of CO₂ from the stream whereby said stream becomes a stream of CO₂-leaner flue gases; separating the solvent containing the absorbed CO2 from the stream of CCVIeaner flue gases to form a CO₂ rich solvent stream;

expanding said stream of CO₂-leaner flue gases in a manner whereby the CU₂-leaner flue gases are cooled; and

cooling one or more process streams by heat exchange with said cooled flue gases.

The one or more process streams may include the stream of Cθ2-containing flue gases, and a CU₂-lean regenerated solvent stream as it is returned for the aforesaid contacting step.

The expansion may be effected in an expansion turbine, and energy recovered from the expansion.

An embodiment of the invention includes preheating said Cθ₂-leaner flue gases upstream of said expansion, in order to obtain enhanced expansive work during said expansion. This preheating may be effected, for example, by indirect heat exchange with flue gases upstream of said contacting step or, alternatively, by combustion of fuel with residual oxygen in the Cθ₂-leaner flue gases.

Advantageously, the aqueous solvent system is an aqueous solvent system containing dissolved ammonia, and ammonium, carbonate and bicarbonate ions, in which case the CO₂-leaner flue gases may be contacted, prior to said expansion, with water that dissolves ammonia therefrom, preferably for recycling the dissolved ammonia back to said solvent system, and said expansion results in further condensation of residual ammonia which is also preferably recycled to said solvent system.

Absorption of the CO₂ may typically be according to equations (1) to (4) above.

Advantageously, the steps of contacting the stream of flue gases with the aqueous solvent system and contacting the CO₂-leaner flue gases with water are carried out in a common vessel, e.g. a tower vessel. The pressure in the tower vessel is preferably in the range 100 to 3000 kPa (1 to 30 bar), most preferably in the range 500-1500 kPa (5 to 15 bar).

Typically, the method includes the further steps of desorbing CO₂ from the COa-rich solvent stream by application of heat to the solvent stream to desorb the CO₂. The now Cθ2-lean solvent stream may be conveniently recycled to said solvent system. Typically, CO₂ desorbed from the CO2-rich solvent stream is compressed, cooled and liquefied for storage.

In its first aspect, the invention further provides apparatus for recovering carbon dioxide from a stream of Cθ₂-containing flue gases, comprising:

an absorber stage for contacting the stream at a gas pressure above atmospheric pressure with an aqueous solvent system to effect absorption of CO₂ from said stream whereby said stream becomes a stream of Cθ₂-leaner flue gases, and for separating the solvent containing the absorbed CO₂ from the stream of CO₂-leaner flue gases to form a CO₂-rich solvent stream;

gas expansion means for expanding said stream of CO₂-leaner flue gases in a manner whereby the CO₂-leaner flue gases are cooled; and

means for cooling one or more process streams by heat exchange with said cooled flue gases.

There is preferably a cooler for cooling the stream of CO2-containing flue gases before it enters the absorber stage. The aforesaid means for cooling one or more process streams may include this cooler, and/or may include heat exchange coupling for cooling a CO₂-lean regenerated solvent stream as it is returned to the absorber stage.

The gas expansion means may include an expansion turbine.

Advantageously, an embodiment of the invention may include means for preheating said CO₂-leaner flue gases upstream of said expansion, in order to obtain enhanced expansive work during said expansion. Such preheating means may be or include, for example, a heat exchanger for indirect heat exchange with the flue gases upstream of said absorber stage, or, alternatively, means to combust fuel with residual oxygen in the CO₂-depleted stream.

Advantageously the aqueous solvent system is an aqueous solvent system containing dissolved ammonia, and ammonium, carbonate and bicarbonate ions, in which case means may be provided by which the CO₂-leaner flue gases are contacted with water that dissolves ammonia therefrom, preferably for recycling the dissolved ammonia back to said solvent system, and said expansion results in further condensation of residual ammonia, means being provided to recycle the residual ammonia to said solvent system.

Preferably, the means for cooling one or more process streams includes a heat exchange coupling for cooling said water for contacting the CO₂-leaner flue gases to dissolve ammonia therefrom.

In a second aspect of the invention a portion of the original flue gas (from prior to the absorber), containing significant quantities of sulphur and/or nitrogen oxides, bypasses the CO₂ absorber stage and is mixed with the CO₂-leaner flue gas after the absorber and the first water wash, and allowed to react. This may be followed by a further water wash before the CO₂-leaner gases are released to the atmosphere.

The invention provides, in its second aspect a method of recovering carbon dioxide from a stream of CO₂-containing flue gases, including: contacting the stream with an aqueous solvent system, containing dissolved ammonia, and ammonium, carbonate and bicarbonate ions, to effect absorption of CO₂ from the stream whereby said stream becomes a stream of CO₂-leaner flue gases; separating the solvent containing the absorbed CO₂ (as carbonate, bicarbonate and CO_2(aq)) from the stream of CO₂-leaner flue gases to form a CO₂ and/or bicarbonate-rich solvent stream; contacting said CO₂- leaner flue gases with water that dissolves ammonia therefrom, preferably for recycling the dissolved ammonia back to said solvent system; thereafter contacting the CO₂- leaner flue gases with a sub-stream of CO₂-rich flue gases, which sub-stream contains sufficient sulphur and/or nitrogen oxides to react with a proportion of the ammonia in the CO₂-leaner flue gases, and recovering the products of said reaction from the CO₂- leaner flue gases.

The conditions of said contact with the sub-stream may be such that the products of said reaction include one or more of ammonium sulphite, ammonium sulphate, ammonium nitrite and ammonium nitrate.

In its second aspect, the invention also provides apparatus for recovering carbon dioxide from a stream of CO₂-containing flue gases, comprising: an absorber stage for contacting the stream with an aqueous solvent system containing dissolved ammonia, and ammonium, carbonate and bicarbonate ions, to effect absorption of CO₂ from said stream whereby said stream becomes a stream of CO₂-leaner flue gases, and for separating the solvent containing the absorbed CO₂ from the stream of CO₂-leaner flue gases to form a CO₂ and/or bicarbonate-rich solvent stream whereby said stream becomes a stream of CO₂-leaner flue gases; first contacting means for contacting said CO₂-leaner flue gases with water that dissolves ammonia therefrom, preferably for recycling the ammonia back to said solvent system; second contacting means for contacting the CO₂-leaner flue gases with a sub-stream of CO₂-rich flue gases, which sub-stream contains sufficient sulphur and/or nitrogen oxides to react with a proportion of the ammonia in said CO₂-leaner flue gases; and means for recovering the products of said reaction from the CO₂-leaner flue gases. The second contacting means may include a contact chamber downstream of the first contacting means, and a bypass duct for conveying the sub-stream from upstream of the absorber stage to the contact chamber.

The temperature of the aqueous solvent system is preferably greater than 15°C, more preferably greater than 20⁰C, and most preferably in the range 20-50⁰C. A temperature in the range 25°C to 45⁰C is suitable.

If required, the stream of flue gases is cooled before being contacted with the solvent system, for example to about 4O⁰C.

Advantageously, said absorption of CO₂ is catalysed by the presence of selected enzymes to promote the rate of absorption of CO₂ to form bicarbonate in solution. A suitable such enzyme is carbonic anhydrase.

An alternative to using enzymes to promote the rate of CO₂ conversion to bicarbonate in solution is the use of inorganic Lewis bases, such as arsenate (AsO₄ ^3") or phosphate (PO₄ ^3"). The enzyme or Lewis base (promoters) can be circulated at low concentration in the liquid solvent or supported on solid structures over which the solvent solution and CO₂ containing gases flow. In the latter case, the surface of the support material has been chemically modified, so that the enzymes or Lewis base attach securely, and is configured to maximise gas-liquid transfer of CO₂.

With the solid support option, the type and configuration of the enzyme or Lewis base, and its support, can be varied to accommodate variations in the composition of the CO₂ containing gas, the local loading of the solvent, and local temperature and pressure conditions.

The invention also extends to method and apparatus incorporating both aspects of the invention.

Brief description of the drawings

The invention will now be further described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a diagram of CO₂ post-combustion capture (PCC) plant in accordance with a preferred embodiment of the first aspect of the invention, utilising an ammonia-based solvent system;

Figures 2 and 3 are variations of the PCC plant depicted in Figure 1 ; and Figure 4 is a further variation of the PCC plant incorporating an embodiment of the second aspect of the invention.

Description of embodiments of the invention

CO₂-lean solvent solution is pumped and sprayed in at the top 13 of an absorber stage 11 in the form of a packing column 14 in the lower part of a tower vessel 15. This solution flows around and downwardly through the packing material of the column 14, while the CO₂-rich stream of flue gases 8 is compressed by compression plant 6, thereafter cooled if necessary at 9 (to, for example, about 40⁰C), and then introduced at 16 to the bottom of the absorber. The compressed and cooled flue gases pass up through the packing material and thereby contact the solvent system comprising the solvent solution flowing down through the packing material. CO₂ is transferred to the solvent solution, a process that is preferably enhanced by the interaction with appropriate added enzymes or a Lewis base.

Compressor plant 6 may comprise a gas turbine compressor which is suitable for compressing relatively high volumes of gas up to 30 bar. In this case, it is thought that a gas pressure of about 10 bar in column 14 will achieve satisfactory results.

The presence of a base such as ammonia/ammonium ions maintains a basic absorber solution pH to keep the dissolved CO₂ as HCO₃VCO₃ ^2' ions. Ammonia can also directly react with dissolved CO₂ to form carbamates. At sufficiently high concentrations, the bicarbonate/carbonate ions can also precipitate out of solution as the ammonium salts, resulting in a slurry, which allows more CO₂ to be transferred by the loaded solvent system.

At the top 17 of the absorber column 14, the CO₂-leaner gases leave the process, while the CO₂-rich solution (containing carbamate, carbonate and bicarbonate) is extracted via line 35 from the bottom of vessel 15, at 20, for further processing. Ammonia slip is ameliorated by subjecting the CO₂-leaner exit gases, before they are passed to a flue stack 27, to a series of further treatments. A first treatment is a water wash from overhead sprays 39 in a scrubber 22 in the upper part of the tower vessel 15. A further small column 26 of suitable packing material facilitates contact. The water, e.g. at 0- 1O⁰C, dissolves ammonia from the CO₂-leaner flue gases, and is collected in a tray system 28 for recirculation by a pump 29 via a cooling device 31. A proportion of the recirculating ammonia-loaded wash water 23 is recycled via conduit 23a to the solvent system in the absorber stage at 19.

The pressurised CO₂-leaner flue gases leaving the absorber scrubber 22 at substantially above atmospheric pressure are expanded, in an expansion turbine 40 (or similar device able to extract work energy) in a controlled manner whereby the gases are further cooled and further residual ammonia condenses from the gases.

The expanded flue gas from the turbine 40 is then water washed in a packed scrubber 50 to dissolve the condensed ammonia before being discharged to the atmosphere via stack 27, preferably at a temperature above the dew point. The water in this final gas cleaning step is recycled, as indicated at 52, by pump 53 and via cooler 54, until the ammonia reaches a significant concentration for recycling to the absorber system. Alternatively, the spent solution might be advantageously mixed with spent solution from the absorber and used as a fertiliser component.

While most of the free ammonia in the flue gases after CO₂ absorption will be typically removed in the primary scrubber (cold water wash) 22 incorporated into the top of the vessel 15, the addition of the expander 40 and associated scrubber 50 enables additional flexibility:

the temperature of the cold water can be higher, thereby substantially reducing the cooling energy required and the size of the water wash sections.

the amount of ammonia slip can be controlled by optimising the overall post absorber flue gas treatment.

^• lower ammonia concentrations in the CO_2-lean gases emitted to the atmosphere will be possible with the two stages of water wash than when using the chilled water wash alone. Two features of the post-absorber flue gas configuration enhance the overall energy utilisation of the illustrated PCC plant. Expander turbine 40 is coupled to a generator 42 whereby work energy is recovered for electricity generation from the gas expansion at the turbine, while a heat exchanger 70 downstream of scrubber 50 allows cooling of another process stream by heat exchange with the cool Cθ2-leaner flue gases. Two such other process streams are the incoming Cθ2-rich flue gas stream 8 and recirculating ammonia-loaded wash water in scrubber 22: these transfers of "coolth" are indicated by broken lines 71 ,72 to coolers 9,31 respectively in Figure 1.

Recovered energy at turbine 40 may be utilised in a process step upstream or downstream of absorber 14, either more directly via heat exchange or less directly via electrical power produced by generator 42. A particularly advantageous application of this concept is the utilisation of the electrical power produced by generator 42 to operate motor 44 of compressor 6, as indicated by broken line 45 in Figure 1.

The bicarbonate-rich solvent solution extracted at the bottom 20 of vessel 15 is delivered via line 35 to be heated in a stripper or absorbent regeneration stage, in this case a packing column 30, to release the CO₂ for storage or other chemical applications. The recovered CO₂-lean solvent solution 34 is re-circulated via reboiler 33 and conduit 32 back to the top 13 of the absorber column 14: it is cooled en route as necessary by heat exchange at 36 with the CO₂-rich solvent stream in line 35, and by a second cooler 37 (which may be in heat exchange communication with heat exchanger

70 - indicated by line 73). The recovered CO₂ stream 38 is typically treated at 60 by being compressed, cooled and liquefied for storage.

It will of course be appreciated that columns 14, 26, 30 may each comprise more than one absorber or stripper. Moreover, within an individual column 14, 26 or 30, there may well be multiple stages. It will also be appreciated that while the PCC plant of Figure 1 has been described as employing an ammonia based solvent system, other solvents, e.g. amine or MEA in particular, may be employed. In such cases, where the vapour pressure of the solvent it relatively low, the additional water wash 50, and perhaps the water washing stage 22, would not be required. Figure 2, in which like components relative to Figure 1 are indicated by like reference numerals preceded by a "2", depicts a modification of the PCC plant of Figure 1 in which the pressurised gases from absorber scrubber 222 are preheated (80) before being expanded in the expansion turbine 240 (ie an indirect heated expander). The preheater 80 could advantageously use low grade heat, such as from flue gases before they enter the absorber. By preheating the pressurised gases, it is possible to obtain increased expansive work at turbine 240. By optimising the operating pressure of the absorber 214 and the amount of preheat, and how the capture process is heat integrated with the host process (for example a pulverised coal power plant), it will be possible to minimise overall energy consumption for the capture process and to maximise the power output from the power plant.

Figure 3, in which like components relative to Figure 1 are indicated by like reference numerals preceded by a "3", depicts an alternative to the PCC plant of Figure 2 in which advantage is taken of the residual oxygen in the CO₂-leaner flue gases to allow direct heating of the gases, by fuel combustion in a combustor 90, before being expanded in expansion turbine 340. Combustor 90 and turbine 340 may be integrated as a fired expander. Again, by preheating of the pressurised gases, it is possible to obtain increased expansive work. By optimising the operating pressure of the absorber and the amount of preheat, and how the capture process is heat integrated with the host process (for example a pulverised coal power plant), it will be possible to minimise overall energy consumption for the capture process and to maximize the power output from the power plant. This embodiment uses the residual oxygen in the flue gases (around 3% by volume for coal boilers and 7% for gas turbines), which, after the bulk of the Cθ₂ Ϊs removed in absorber 314, will increase significantly, depending on the source of the flue gas and the amount of CO₂ removed.

Depending on the amount of fuel used in the fired expander, it will be possible to recover heat, or provide a source of cooling, through the use of a heat exchanger 370 after the turbine, prior to discharge to the atmosphere at stack 327. Figure 4, in which like components relative to Figure 1 are indicated by like reference numerals preceded by a "4", depicts a PCC plant incorporating an embodiment of the second aspect of the invention.

In this case, turbine 40 and generator 42 are omitted (though it is emphasised they may be maintained in a further variation) and the CU₂-leaner flue gases leaving the absorber scrubber 422 are reacted, in a chamber 502 of a slip control reactor 500, with a substream of the original CO₂-rich flue gas stream 408 extracted at 505 upstream of absorber 411 and delivered to reactor 500 via a bypass duct 508. This substream typically contains nitrogen oxides and/or sulphur oxides that substantially reduce the amount of free ammonia, by reacting with ammonia to form the ammonium compounds noted above. To ensure formation of the ammonium compounds, an excess of the original flue gas is added to the slip control reactor, in the range of 1-10% of the original flue gas (the exact amount depends on the particular concentrations of SOx and NOx in the flue gas). While this will increase slightly the amount of CO₂ in the gases emitted to the atmosphere, this can be compensated for by increased CO₂ absorption in the absorber as required to meet CO₂ emissions targets.

The cleaned flue gas from the slip control reactor 500 is then water washed in a packed scrubber 450 to remove the ammonium salts before being discharged to the atmosphere via stack 427. The water in this final gas cleaning step is recycled, as indicated at 452, by pump 453 and via cooler 454, until the ammonium salts reach a significant concentration, with the spent solution advantageously mixed with spent solution from the absorber and used as a fertiliser component.

While most of the free ammonia in the flue gases after CO₂ absorption will be typically removed in the primary scrubber (cold water wash) 422 incorporated into the top of the vessel 415, the addition of the slip control reactor 500 and associated scrubber 450 enables additional flexibility:

the temperature of the cold water can be higher, thereby substantially reducing the cooling energy required and the size of the water wash sections. the amount of ammonia slip can be controlled by optimising the overall post absorber flue gas treatment.

lower ammonia concentrations in the CO2 lean gases emitted to the atmosphere will be possible than using the conventional chilled water wash alone.

It will be understood that any one or more of turbine 40 and generator 42, preheater 80, and combustor 90 may be incorporated into the plant of Figure 4, preferably downstream of reactor 500.

Claims

CLAIMS:

1. A method of recovering carbon dioxide from a stream of Cθ2-containing flue gases, including the steps of:

contacting the stream at a gas pressure above atmospheric pressure with an aqueous solvent system to effect absorption of CO₂ from the stream whereby said stream becomes a stream of Cθ2-leaner flue gases;

separating the solvent containing the absorbed CO₂ from the stream of CO₂- leaner flue gases to form a CO₂ rich solvent stream;

expanding said stream of CO₂-leaner flue gases in a manner whereby the CO₂- leaner flue gases are cooled; and

2. A process according to claim 1 wherein said one or more process streams includes said stream of CO₂-containing flue gases.

3. A process according to claim 1 or 2 wherein said one or more process streams includes a CO₂-lean regenerated solvent stream as it is returned for said contacting step.

4. A method according to claim 1 , 2 or 3 wherein said expansion is effected in an expansion turbine and energy is recovered from the expansion.

5. A method according to any one of claims 1 to 4 including preheating said CO₂- leaner flue gases upstream of said expansion, in order to obtain enhanced expansive work during said expansion.

6. A method according to any one of claims 1 to 5 wherein the aqueous solvent system is an aqueous solvent system containing dissolved ammonia, and ammonium, carbonate and bicarbonate ions.

7. A method according to claim 6 wherein the Cθ₂-leaner flue gases are contacted, prior to said expansion, with water that dissolves ammonia therefrom.

8. A process according to claim 7 wherein said one or more process streams includes said water for contacting the CO₂-leaner flue gases to dissolve ammonia therefrom.

9. A method according to claim 7 or 8 wherein the steps of contacting the stream of flue gases with the aqueous solvent system and contacting the Cθ2-leaner flue gases with water are carried out in a common vessel.

10. A method according to claim 7, 8 or 9 including, after contacting said Cθ₂-leaner flue gases with water that dissolves ammonia therefrom, then contacting the Cθ₂-leaner flue gases with a sub-stream of CO₂-rich flue gases, which sub-stream contains sufficient sulphur and/or nitrogen oxides to react with a proportion of the ammonia in said CO₂-leaner flue gases, and recovering the products of said reaction from the CO₂- leaner flue gases.

11. A method according to claim 10 wherein the conditions of said contact with said sub-stream are such that the products of said reaction include one or more of ammonium sulphite, ammonium sulphate, ammonium nitrite and ammonium nitrate.

12. A method according to any one of claims 1 to 11 wherein the temperature of the aqueous solvent system is greater than 15°C.

13. A method according to any one of claims 1 to 12 including cooling the stream of CO₂-containing flue gases before being contacted with the solvent system.

14. A method according to any one of claims 1 to 13 further including desorbing CO₂ from the Cθ₂-rich solvent stream by application of heat to the solvent stream to desorb the CO₂.

15. A method according to any one of claims 1 to 14 further including recovering energy from the expansion of the stream of COa-leaner flue gases, and utilising said energy for electricity generation or in a process step upstream or downstream of said steps.

16. A method of recovering carbon dioxide from a stream of CO₂-containing flue gases, including:

contacting the stream with an aqueous solvent system, containing dissolved ammonia, and ammonium, carbonate and bicarbonate ions, to effect absorption of CO₂ from the stream whereby said stream becomes a stream of CO₂-leaner flue gases;

separating the solvent containing the absorbed CO₂ (as carbonate, bicarbonate and CO₂(aq)) from the stream of CO₂-leaner flue gases to form a CO₂ and/or bicarbonate-rich solvent stream;

contacting said CO₂-leaner flue gases with water that dissolves ammonia therefrom, which dissolved ammonia may be recycled back to said solvent system;

thereafter contacting the CO₂-leaner flue gases with a sub-stream of CO₂-rich flue gases, which sub-stream contains sufficient sulphur and/or nitrogen oxides to react with a proportion of the ammonia in said CO₂-leaner flue gases, and recovering the products of said reaction from the CO₂-leaner flue gases.

17. A method according to claim 16 wherein the conditions of said contact with said sub-stream are such that the products of said reaction include one or more of ammonium sulphite, ammonium sulphate, ammonium nitrite and ammonium nitrate.

18. A method according to claim 16 or 17 further including desorbing CO₂ from the CO₂-rich solvent stream by application of heat to the solvent stream to desorb the CO₂.

19. A method according to claims 16, 17 or 18 wherein the temperature of the aqueous solvent system is greater than 15°C.

20. A method according to any one of claims 16 to 19 including cooling the stream of CO₂-containing flue gases before being contacted with the solvent system.

21. A method according to any one of claims 16 to 20 wherein the steps of contacting the stream of flue gases with the aqueous solvent system and contacting the CO₂-leaner flue gases with water are carried out in a common vessel.

22. Apparatus for recovering carbon dioxide from a stream of CO₂-containing flue gases, including:

an absorber stage for contacting the stream at a gas pressure above atmospheric pressure with an aqueous solvent system to effect absorption of CO₂ from said stream whereby said stream becomes a stream of CO₂-leaner flue gases, and for separating the solvent containing the absorbed CO₂ from the stream of CO₂-leaner flue gases to form a CO₂-rich solvent stream;

23. Apparatus according to claim 22 including a cooler for cooling the stream of CO₂- containing flue gases before it enters said absorber stage.

24. Apparatus according to claim 23 wherein said means for cooling one or more process streams includes said cooler.

25. Apparatus according to claim 22, 23 or 24 wherein said means for cooling one or more process streams includes heat exchange coupling for cooling a CO₂-lean regenerated solvent stream as it is returned to said absorber stage.

26. Apparatus according to any one of claims 22 to 25 wherein said gas expansion means includes an expansion turbine.

27. Apparatus according to any one of claims 22 to 26 including means for preheating said CU₂-leaner flue gases upstream of said expansion, in order to obtain enhanced expansive work during said expansion.

28. Apparatus according to any one of claims 22 to 27 wherein the aqueous solvent system is an aqueous solvent system containing dissolved ammonia, and ammonium, carbonate and bicarbonate ions, and first contacting means is provided by which the CO₂-leaner flue gases are contacted with water that dissolves ammonia therefrom, and said expansion results in further condensation of residual ammonia, means being provided to recycle the residual ammonia.

29. Apparatus according to claim 28 wherein said means for cooling one or more process streams includes a heat exchange coupling for cooling said water for contacting the CO₂-leaner flue gases to dissolve ammonia therefrom.

30. Apparatus according to claim 28 or 29 wherein said absorber stage and said first contacting means by which the CO₂-leaner flue gases are contacted with water, are provided by a common vessel.

31. Apparatus according to claim 27, 28 or 29, further including second contacting means for contacting the CO2-leaner flue gases with a sub-stream of CO₂ rich flue gases, which sub-stream contains sufficient sulphur and/or nitrogen oxides to react with a proportion of the ammonia in said CO₂-leaner flue gases and means for recovering the products from said reaction from the CO₂-leaner flue gases.

32. Apparatus according to claim 31 wherein said second contacting means includes a contact chamber downstream of said first contacting means, and a bypass duct for conveying said sub-stream from upstream of said absorber stage to said contact chamber.

33. Apparatus according to any one of claims 22 to 32 wherein said expansion means is configured whereby energy is recovered from the expansion, and wherein means is provided for utilising said recovered energy for electricity generation or in a process step upstream or downstream of said absorber stage.

34. Apparatus for recovering carbon dioxide from a stream of Cθ₂-containing flue gases, comprising: an absorber stage for contacting the stream with an aqueous solvent system containing dissolved ammonia, and ammonium, carbonate and bicarbonate ions, to effect absorption of CO₂ from said stream whereby said stream becomes a stream of CO₂-leaner flue gases, and for separating the solvent containing the absorbed CO2 from the stream of CU₂-leaner flue gases to form a CO₂ and/or bicarbonate-rich solvent stream whereby said stream becomes a stream of Cθ₂-leaner flue gases; first contacting means for contacting said CU₂-leaner flue gases with water that dissolves ammonia therefrom; second contacting means for contacting the Cθ₂-leaner flue gases with a sub- stream of CO₂ rich flue gases, which sub-stream contains sufficient sulphur and/or nitrogen oxides to react with a proportion of the ammonia in said CO₂-leaner flue gases; and means for recovering the products of said reaction from the CO₂-leaner flue gases.

35. Apparatus according to claim 34 wherein said second contacting means includes a contact chamber downstream of said first contacting means, and a bypass duct for conveying said sub-stream from upstream of said absorber stage to said contact chamber.

36. Apparatus according to claim 34 or 25 further including means for recycling the dissolved ammonia back to the solvent system.