WO2007012143A1

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Publication number: WO2007012143A1
Application number: PCT/AU2006/001078
Authority: WO
Inventors: Louis Wibberley
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2005-07-29
Filing date: 2006-07-28
Publication date: 2007-02-01

Abstract

A method of recovering carbon dioxide from a stream of flue gases includes cooling the stream of flue gases to a temperature suitable for efficient absorption of CO2 from the stream by a predetermined sorbent system. The stream is contacted with the predetermined sorbent system to effect absorption of CO2 from the stream, and the sorbent and absorbed CO2 are separated from the stream of flue gases to form a CO2-rich absorbent stream. CO2 is absorbed from the CO2-rich absorbent stream by application of heat to the absorbent stream to desorb the CO2. Energy released in the cooling step is utilised to provide the heat in the CO2 desorption step. The invention also provides apparatus for recovering carbon dioxide from a stream of flue gases, and a plant energy management system.

Description

RECOVERY OF CARBON DIOXIDE FROM FLUE GASES

Field of the invention

This invention relates generally to the recovery of carbon dioxide from flue gases generated by industrial processes, for example coal-fired power plants, steel plants, smelters, cement kilns and calciners.

Background of the invention

There is rapidly growing pressure for coal-fired power plants and other CO₂-producing industrial processes to make step reductions in greenhouse gas emissions by capturing the CO₂ formed from the process, and then storing the CO₂. Most storage schemes involve injecting CO₂ in a supercritical state into deep aquifers, coal seams and adjacent strata, or at depth in the ocean, or converting the CO₂ into a solid mineral. This invention is concerned, in one application, with the capture of CO₂ from flue gases, typically for processing to supercritical conditions.

There are at present three main approaches to CO₂ separation from new power plants:

(i) Post-combustion capture, in which the CO₂ in the flue gases is separated from nitrogen and residual oxygen using a range of liquid and solid sorbents. The captured CO₂ is then liquefied by compression and cooling. The main disadvantage of this process is that the CO₂ partial pressure is relatively low (compared to the two alternative approaches below), which necessitates the use of CO₂ selective sorbents. The regeneration of these sorbents releases an essentially pure CO₂ stream, but this step is energy intensive. Overall, around 20% of the power of the base power plant is consumed in providing heat (around 2/3 of the loss) and power to operate the CO₂ capture process and CO₂ liquefaction plant. This would reduce the thermal efficiency of the plant by around 9 percentage points. (N) Integrated combined cycle with gasification and pre-combustion capture, where coal is gasified with oxygen at high pressure, the resulting syngas is treated by catalytically converting the CO to CO₂ and H₂ in a reaction called shift, and then the C0₂ is removed. In this process the high concentration and high pressure of the CO₂ greatly facilitates CO₂ capture, which may involve a number of chemical or physical sorbents. The captured CO₂ is then liquefied by¹ additional compression, with cooling. The disadvantage is that the overall power plant costs are high, which offsets the lower energy consumption of the process. Overall, around 16% of the power of the base power plant is consumed in providing power to operate the air separation unit and CO₂ liquefaction plant. This would reduce the thermal efficiency of the plant by around 6 percentage points

(iii) Oxygen combustion, in which the coal is combusted in a "synthetic air" comprised of oxygen produced from an air separation unit and recycled flue gases taken from after the combustion chamber. In this process, the flue gases contain predominantly CO₂ and water vapour, and after condensing the water, the CO₂ can be liquefied by compression and cooling, without requiring separation. The disadvantage of this approach is the need to separate oxygen from air, a costly and energy intensive process. Overall, around 20% of the power of the base power plant is consumed in providing power to operate the air separation unit and CO₂ liquefaction plant. This would reduce the thermal efficiency of the plant by around 9 percentage points.

In practice, the three routes offer similar overall costs for new plants, however, for retrofit situations, post-combustion capture is also applicable to other stationary CO₂ sources, such as steel plants, cement kilns, calciners and smelters.

Post-combustion capture is already practised for producing CO₂ from combustion gases for a range of high value uses (eg. food additives or for enhanced oil recovery). In greater detail, the process involves:

(a) cooling and pretreating the flue gases to remove particulates and other contaminants in general; (b) contacting the flue gases with a suitable sorbent system, eg a solution containing amines or similar compounds, that selectively absorbs the CO2;

(c) regenerating the sorbent, by pumping the CO₂ enriched solution in a separate desorption process in which the solution is heated to cause it to desorb the CO₂; and

(d) compressing, cooling and liquefying the captured CO₂.

The heat energy required for desorption and sorbent regeneration, also sometimes termed the stripping process, is around 3-4 GJ/t CO₂ captured. This energy is required at a temperature of 110-125⁰C, and is generally provided by low pressure steam extracted from the low pressure stages of the steam turbine that drives the generator of the power plant. This steam flow reduces the output of the turbine by around 15-20%, and the CO₂ compressor and various pumps consume electrical power that increases the overall loss in output to around 20-25%.

Optimising the integration between the steam plant and the desorber has been highlighted in several studies as a key opportunity for reducing overall capture costs for CO₂ abatement in power generation.

It is accordingly an object of this invention to improve the energy efficiency of post- combustion capture of CO₂ from flue gases so as to reduce the detriment of the process to the thermal efficiency of the primary plant.

Summary of the invention

The present invention entails the realisation that opportunities for enhanced energy efficiency of post-combustion capture of CO₂ lie within the process steps themselves and within the overall energy management of the plant.

The invention accordingly provides, in a first aspect, a method of recovering carbon dioxide from a stream of flue gases, comprising:- cooling the stream of flue gases to a temperature suitable for efficient absorption of CO₂ from the stream by a predetermined sorbent system;

contacting the stream with the predetermined sorbent system to effect said absorption of CO₂ from the stream, and separating the sorbent and absorbed CO₂ from the stream of flue gases to form a CO₂-rich absorbent stream; and

desorbing CO₂ from the CO₂-rich absorbent stream by application of heat to the absorbent stream to desorb the CO₂;

wherein energy released in said cooling step is utilised to provide said heat in the CO₂ desorption step.

Preferably, the energy released in said cooling step is recovered in a first heat exchange zone and carried by an intermediate fluid from said first heat exchange zone.

Advantageously, sorbent produced in said desorption step is recycled to the Co₂ absorption step. The recycled CO₂-lean sorbent may be subjected to heat exchange with the CO₂-rich sorbent for cooling the sorbent to the temperature of the cooled flue gases in the CO₂ absorption step.

Optionally, CO₂ desorbed from the CO₂-rich absorbent stream is compressed, cooled and liquefied for storage. Optionally, energy released by this step is recycled to the aforesaid CO₂ desorption step.

The invention also provides in the first aspect an apparatus for recovering carbon dioxide from a stream of flue gases, comprising:

means for cooling the stream of flue gases to a temperature suitable for efficient absorption of CO₂ from the stream by a predetermined sorbent system; means for contacting the stream with said sorbent system to effect said absorption of the CO₂ from the stream, and for separating the sorbent and absorbed CO₂ from the stream to form a CO2-rich absorbent stream;

sorbent regeneration means for desorbing CO₂ from the CO₂-rich absorbent stream by application of heat to the absorbent stream to desorb the CO₂; and

means for recovering energy released by said cooling means and transferring said energy to provide said heat in said sorbent regeneration means.

Preferably the apparatus further includes a first heat exchange zone for recovering energy released by said cooling means and from which the recovered energy is carried by an intermediate fluid.

Advantageously, means are included for recycling CO₂-lean sorbent produced by said sorbent regeneration means. Heat exchange means may be provided for subjecting the recycled CO₂-lean sorbent to heat exchange with the CO₂-rich sorbent for cooling the sorbent to the temperature of the cooled flue gases in the CO₂ absorption.

Optionally, means are provided for compressing, cooling, and liquefying CO₂ desorbed from the CO₂-rich absorbent stream for storage. Optionally, the apparatus includes means for recycling energy released by this step to the aforesaid CO₂ desorbing step.

Advantageously, the plant utilises a heat pump system including one or more heat exchange stages for effecting said energy recovery in an integrated fashion.

The invention further provides a plant energy management system that takes advantage of the improved energy efficiency of post-combustion capture of CO₂ enabled by the method and apparatus described above.

In a second aspect, the invention accordingly provides a plant energy management system including: selectively recovering carbon dioxide from a stream of flue gases in accordance with the first aspect of the invention;

monitoring carbon dioxide emissions from the plant;

managing said selective recovery of carbon dioxide such that carbon dioxide emissions are within a predetermined limit of carbon dioxide emissions; and

wherein said selective recovery includes relatively reduced recovery of carbon dioxide during periods of high energy demand to maximise power generation by the plant, and relatively increased recovery of dioxide at other times.

Advantageously, in this second aspect, the predetermined limit of carbon dioxide emissions is a total or averaged limit over a prescribed time period, such that selected said periods of relatively reduced recovery of carbon dioxide do not cause the relevant limit to be exceeded.

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 much simplified schematic diagram of a coal-fired electricity generation power plant having post combustion capture of CO₂ configured to be operated in accordance with an embodiment of the invention;

Figure 2 is a diagram of energy transfers in accordance with an embodiment of the invention; and

Figure 3 is a diagram of percentage capture of CO₂ from a flue gas in the CO₂ absorption stage of post combustion capture, as a function of the temperature to which the flue gases are cooled before contacting the absorbent system. Detailed description of embodiments of the invention

Figure 1 depicts the essentials of a coal-fired electricity generation power plant 10. Coal and air are delivered to a large scale boiler system 12 which heats large volumes of water to generate steam 14 for driving a steam turbine 16. Turbine 16 in turn powers a generator 18 that produces electricity as its output. The steam recovered from turbine 16 passes through a condenser 22 for recycling to the boiler and release into the atmosphere via a cooling tower 24.

Flue gases 26 from boiler 12 are treated to remove most particulates (27) and then passed via conduit 29 to a four-stage plant 30 for post-combustion capture of carbon dioxide. In stage 1 , indicated at 32, the cleaned flue gases are cooled to a temperature suitable for efficient absorption of CO₂ from the gases by a suitable sorbent system. In stage 2, indicated at 34, the cleaned and cooled flue gases are scrubbed by contact with such a sorbent system, e.g. a system containing amines or similar compounds. The amines selectively absorb CO₂ in a weekly bonded form. The CO₂-rich absorbent stream is then passed to the third stage comprising a desorber 36, where the amine is regenerated by heating it to desorb the CO₂ .

The CO₂-lean flue gas from absorber stage 34 is passed to a flue stack 39 for release into the atmosphere while the desorbed CO₂ from desorption regeneration stage 36 is compressed, cooled and liquefied, in the fourth stage 38, for subsequent transport and storage.

It will be appreciated that, in particular plants, there may be more than one CO₂- absorber stage 34 and/or more than one CO₂-desorber stage 36. Moreover, within an individual desorber, there may well be multiple stages within the column.

In conventional practice of post-combustion capture of CO₂, the heat energy for regeneration/desorption stage 36 is obtained from steam turbine 16, with the effects previously discussed. In accordance with a preferred practice of the invention, cooling stage 32 is in two parts 40, 42. The clean gas is initially cooled in a first heat exchanger 40 by indirect heat exchange with one or more fluids in a fluid circuit 41. Further cooling of the gas in second stage 42, forming part of an overall heat pump system 50, to a temperature in the order of 4O⁰C causes partial condensation of the contained moisture and other condensable species, with cooling provided by heat pump evaporation stage or stages 52.

The energy recovered in these ways during the cooling steps is supplied directly via fluid circuit 41 and via heat path 51 of heat pump system 50 to provide the heat required for the regeneration/desorption stage 36. It is believed that for warm flue gas containing of the order of 20% or more moisture, a substantial component, in some cases all, of the heat required for regeneration of the absorbent can be supplied from the cooling stage, resulting in a smaller parasitic power loss to the power plant than arises using low pressure steam from turbine 16 in accordance with conventional practice. This available low grade heat can be readily supplemented by heat released in the CO₂ compression and liquefaction stage 38, as indicated by energy path 56 of heat pump system 50: this path may of course be a carrier fluid from a heat exchange zone at stage 38.

The regenerated CO₂-lean sorbent from regeneration/desorption stage 36 is recycled to absorption stage 34 via conduit 60. Beneficially, this recycled stream may pass through a heat exchanger 62 in which the recycling sorbent stream recovers heat from the CO₂- rich absorbent stream, for heating the recycled sorbent stream to a temperature of the order of the cooled flue gases.

Figure 2 schematically illustrates the energy transfers, both specific and optional, referred to above, save that in this case all heat recovery and use is managed by heat pump system 50. This diagram also shows the options of additional waste heat sources 70 for further reducing the parasitic effect on the power plant of the post-combustion CO₂ capture plant, and of a heat path 72 for some controlled recovery of energy from the turbine of the steam power plant itself.

A number of heat pump systems could be employed, with a number of sequential stages or multiple effect stages being used to provide heat energy at the desorber reboiler temperature. The condenser pressure of the heat pump stages could also be varied according the required reboiler temperature and duty. It will also be appreciated that the illustrated plant might further include sorbent systems, either integrated in stage 34 or separately provided, for removing other gases such as NO_x and SO_x.

The amount of regeneration, and therefore the amount of capture, that can be obtained will depend on several factors including the moisture content of the flue gases, the energy and temperature required for the desorber, and the efficiency of the heat pump system. These factors will also directly affect the parasitic power requirements for the heat pump.

It will be appreciated that, for the purposes of plant energy management, the flue gases in conduit 29 may be selectively diverted direct to flue stack 39 via a bypass conduit 39a and suitable valving (not shown), i.e. the CO₂-capture process is switched off. This action might be taken, for example, at time of peak effective power demand or cost, so as to remove the parasitic effect of the CO₂ capture plant (which, as discussed, is a significant improvement on the conventional 9 percentage points using low pressure steam from the power station turbine but is nevertheless still present with the inventive capture process).

Alternatively, for energy management purposes, instead of diverting the flue gases via bypass conduit 39a, the desorber and CO₂ liquefaction stages 36, 38 might be disabled, or operated at lower throughput, with similar effect. In this case, the main flue gas flow path from boiler 12 to stack 29 is maintained.

More generally, this energy management system will typically involve relatively reduced recovery of carbon dioxide during periods of high energy demand to maximise power generation by the plant, and relatively increased recovery of carbon dioxide at other times. Such a system can be utilised to optimise the revenue of the electricity generation plant from varying spot electricity prices, and is feasible because limits on CO₂ emissions, or minimums for CO₂ reduction, are usually a total or averaged limit over a prescribed time period, such that selected periods of relatively reduced recovery of carbon dioxide do not cause the relevant limit to be exceeded. The outlined energy management should thus include monitoring carbon dioxide emissions from the plant and managing the selective recovery of carbon dioxide such that carbon dioxide emissions are within a predetermined limit of carbon dioxide emissions.

The inventive process facilitates easy switching off and on of CO2 capture, because the process is "work-based", and the powering and depowering of the conventional turbine source for the required heat is less easily managed and is relatively inefficient.

Example

The following example relates to application of the invention for CO₂ capture from the flue gas from a Victorian lignite (brown coal) power plant without coal drying. The application relates either to a retro-fit installation in an existing power station, or to a new installation. Victorian lignites typically contain 60-65% water in a number of combinations with the coal. This water, together with water produced from combustion of the hydrogen in the coal, results in a flue gas with the composition shown in Table 1 , and with a temperature of around 17O⁰C. The flue gas is cooled to around 12O⁰C in a heat exchanger, with the heat energy being transferred directly to a reboiler of the desorber (i.e. path 41 in Figure 1 ).

Table 1 Typical flue gas composition from Victorian power station

The cooled flue gas was then further cooled in a series of heat exchangers which were also the evaporators of a number of heat pump stages. These heat exchangers cooled the flue gases to around 4O⁰C. For heat exchanger temperatures below around 65⁰C the water vapour in the flue gases will progressively condense, releasing the latent heat of condensation of the water vapour to the heat pump system.

Figure 3 gives an indication of the proportion of CO₂ that can be captured assuming a heat pump coefficient of performance of 3 and a desorber energy requirement of 3.5GJ/t CO₂. It is noted that the lineal portion of the curve shows the amount that can be captured using the sensible heat of the flue gases only.

For 80% capture, the overall reduction in thermal efficiency is found to be below 7 percentage points, a significant improvement on the conventional 9 percentage points using low pressure steam from the power station turbine.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims

1. A method of recovering carbon dioxide from a stream of flue gases, comprising:-

cooling the stream of flue gases to a temperature suitable for efficient absorption of CO₂ from the stream by a predetermined sorbent system;

desorbing CO₂ from the CO₂-HCh absorbent stream by application of heat to the absorbent stream to desorb the CO₂;

wherein energy released in said cooling step is utilised to provide said heat in the

CO₂ stripping step.

2. A method according to claim 1 , wherein energy released in said cooling step is recovered in a first heat exchange zone and carried by an intermediate fluid from said first heat exchange zone.

3. A method according to claim 1 or 2, wherein CO₂-lean sorbent produced by said desorption step is recycled to the CO₂ absorption step.

4. A method according to claim 3, further including subjecting the recycled C0₂-lean sorbent to heat exchange with the CO₂-rich sorbent for cooling the sorbent to the temperature of the cooled flue gases in the CO₂ absorption step.

5. A method according to any preceding claim, wherein CO₂ desorbed from the CO₂-rich absorbent stream is compressed, cooled and liquefied for storage.

6. A method according to claim 5, wherein energy released by compressing, cooling and liquefying the desorbed CO₂ is recycled to the CO₂ desorption step.

7. An apparatus for recovering carbon dioxide from a stream of flue gases, comprising:

means for cooling the stream of flue gases to a temperature suitable for efficient absorption of CO₂ from the stream by a predetermined sorbent system;

means for contacting the stream with said sorbent system to effect said absorption of CO₂ from said stream, and for separating the sorbent and absorbed CO₂ from the stream to form a Cθ₂-rich absorbent stream;

8. Apparatus according to claim 7, further including a first heat exchange zone for recovering energy released by said cooling means and from which the recovered energy is carried by an intermediate fluid.

9. Apparatus according to claim 7 or 8, further including means for recycling CO₂- lean sorbent produced by said sorbent regeneration means.

10. Apparatus according to claim 9, further including heat exchange means for subjecting the recycled CO2-lean sorbent to heat exchange with the CO₂-rich sorbent for cooling the sorbent to the temperature of the cooled flue gases in the CO₂ absorption.

11. Apparatus according to any one of claims 7 to 10, further including means for compressing, cooling, and liquefying CO₂ desorbed from the CO₂-rich absorbent stream for storage.

12. Apparatus according to claim 11 , further including means for recycling to the sorbent regeneration means energy released by said compressing, cooling, and liquefying.

13. Apparatus according to any one of claims 7 to 12, wherein said means for recovering energy includes a heat pump system including one or more heat exchange stages for effecting said energy recovery in an integrated fashion.

14. A plant energy management system including:

selectively recovering carbon dioxide from a stream of flue gases in accordance with any one of claims 1 to 6;

monitoring carbon dioxide emissions from the plant;

wherein said selective recovery includes relatively reduced recovery of carbon dioxide during periods of high energy demand to maximise power generation by the plant, and relatively increased recovery of carbon dioxide at other times.

15. A plant energy management system according to claim 14, wherein the predetermined limit of carbon dioxide emissions is a total limit or averaged limit over a prescribed time period, such that selected said periods of relatively reduced recovery of carbon dioxide do not cause the relevant limit to be exceeded.