WO2014096251A1

WO2014096251A1 - Improvements in layout of absorber for co2 capture

Info

Publication number: WO2014096251A1
Application number: PCT/EP2013/077480
Authority: WO
Inventors: Karin BØRTER; Kjell TØRRES
Original assignee: Aker Engineering & Technology As
Priority date: 2012-12-20
Filing date: 2013-12-19
Publication date: 2014-06-26
Also published as: NO335542B1; NO20121540A1

Abstract

An absorber column (3) for a CO2 capture plant, the absorber column (3) comprising a lean absorbent line (4) for introduction of lean absorbent onto the top of one or more packing(s) (3') arranged across the cross section f the absorber column (3), one or more exhaust gas inlet(s) (23) arranged in the side wall of the absorber column and below the packing(s) (3') for introduction of incoming exhaust gas into the absorber column so that the exhaust gas is caused to flow countercurrent to the absorbent in the packing(s), an exhaust gas outlet (6) arranged above the packing(s), an absorber column bottom part for collecting rich absorbent having absorbed CO₂, and a rich absorbent line (5) for withdrawal of rich amine from the bottom part of the absorber (3), wherein an elevated bottom part (22) is provided below the surface of the absorbent collected in the bottom part of the absorber, where the elevated bottom part is arranged to displace a at least 50% of the rich absorbent collected in the bottom part, is described.

Description

Improvements in layout of absorber for C02 capture

Technical Field

[0001] The present application relates to an absorber for a plant for removal of an acid gas from a gas stream, such as a plant for capturing CO2 from a CO2 containing gas stream, such as an exhaust gas from combustion of carbonaceous fuels.

Background Art

[0002] Methods and plants for capturing acid gases, such as CO2, from gaseous mixtures, such as exhaust gas, have been known decades. The last years the activity for developing improved methods and plants for CO2 capture has increased due to the high increase in atmospheric CO2 mostly due to combustion of carbonaceous fuels.

[0003] The commonly suggested plants and methods for CO2 capture from an exhaust gas are methods based on aqueous CO2 absorbents that are caused to flow counter current to the exhaust gas in a packed section in an absorber to give a CO2 exhausted exhaust gas, that is released into the surroundings, and a CO2 rich absorbent, that is withdrawn and

regenerated to give a CO2 stream that is compressed and treated further for export from the plant, and regenerated absorbent that is recycled into the absorber.

[0004] A typical plant of this kind is illustrated in the attached figure 1 , and is

described i.e. in WO201002877. Exhaust gas is introduced into the plant through an exhaust pipe 1 and is led into a cooler 2, here illustrated with a direct contact cooler 2. Exhaust gas is introduced close to the bottom of the direct contact cooler and is caused to flow countercurrent to water introduced by spraying water into the cooler via a cooling water

recirculation line 2' and a recirculation pump 2".

[0005] Cooled and humidified exhaust gas is withdrawn from the direct contact cooler and introduced into an absorber 3, where the exhaust gas is caused to flow countercurrent to an aqueous CO2 absorbent, such as an aqueous amine solution. CO2 lean exhaust gas is withdrawn through a cleaned exhaust pipe 6 and released into the surroundings, whereas rich amine, having absorbed CO2, is collected at the bottom of the absorber and withdrawn through a rich amine line 5. A pump 6 is arranged in line 5 to give the pressure required to pump the rich amine in line 5 via a heat exchanger into a regenerator 8 to be regenerated by stripping of the amine in countercurrent flow to steam that is introduced into the regenerator through a steam line 12. Regenerated, or lean, amine is collected in the bottom of the regenerator 8, and is recycled through a lean exhaust line 4 via the heat exchanger 9 and reintroduced into the absorber 3. Steam for stripping in the regenerator 8 is generated by boiling a part of the lean absorbent in line 4 in a reboiler 1 1 using steam introduced in line 13 as a heating medium. The lean absorbent being reintroduced into the absorber 3 is introduced at the top or a packing 3' provided to optimize the contact between the exhaust gas and the absorbent.

[0006] Due to the large volumes of exhaust gas exiting from a thermal power plant, and the required contact time between the absorbent and gas to obtain sufficient contact between the exhaust gas and the absorbents, the absorber for a 400 MW gas fired power plant has a cross section area of typically about 320 m², and a height of more than 60 meters. The exhaust gas is entering the absorber from the cooler 2 through one or more inlet(s) typically having a total cross section of 54 m², at a speed of about 17 m/s.

[0007] The exhaust inlets into the absorber is normally arranged 2 to 3 meters, such as about 2.5 meter above the surface of the absorbent collected in the bottom part of the absorber to reduce the "wind" impact on the liquid surface. A wind of incoming exhaust gas at a wind speed of near gale to fresh gale combined with a heavy rain of absorbent precipitating from the absorber packing will, however, result in that gas bubbles are mixed into the absorbent collected in the bottom of the absorber.

[0008] The level of liquid collected in the bottom of the absorber may vary during the operation of the plant as the absorber is often used as a buffer for absorbent. The minimum liquid level is, however, 1 ,5 or more preferred 2 meters, both to give the necessary liquid pressure to avoid cavitation in pumps for withdrawal of the liquid from the absorber, and to substantially reduce, or preferably avoid, that gas bubbles are present in the absorbent withdrawn from the absorber. The required suction height, i.e. liquid depth in the sump, or bottom of the absorber, results in a large absorbent volume of absorbent in the absorber sump.

[0009] To avoid or reduce the amount of bubbles in the absorbent withdrawn from the absorber, the outlet(s) for the absorbent from the absorber is (are) preferably arranged close to the side of the absorber where the exhaust gas is introduced as the wind impact close to the exhaust gas inlet is relatively low as the incoming exhaust gas due to the geometry and direction of flow will meet the surface of the liquid at a distance from the exhaust gas inlet, typically at about the same distance as the height of the exhaust gas inlet above liquid surface. Due to geometrical restrictions caused by the cooler being close to the inlet to the absorber, and the requirement for a barrier to collect any leakage from the absorber, the pipelines has to enter into the absorber from the opposite side of the exhaust gas inlet, and have to be securely fastened to the bottom of the bottom of the absorber in a length of about 15 meters, an arrangement that may be a weak point of the construction. Alternatively, the outlet may enter the absorber through the bottom of the absorber, but this

arrangement will require extra space below bottom of the absorber to allow access to the pipelines and pumps arranged below the absorber, and would increase the building costs of the total plant.

Summary of invention

[0010] According the present invention, an absorber column for a CO2 capture plant is provided, the absorber column comprising a lean absorbent line for introduction of lean absorbent onto the top of one or more packing(s) arranged across the cross section of the absorber column, one or more exhaust gas inlet(s) arranged in the side wall of the absorber column and below the packing(s) for introduction of incoming exhaust gas into the absorber column so that the exhaust gas is caused to flow countercurrent to the absorbent in the packing(s), an exhaust gas outlet arranged above the packing(s), an absorber column bottom part for collecting rich absorbent having absorbed CO2, and a rich absorbent line for withdrawal of rich amine from the bottom part of the absorber, wherein an elevated bottom part is provided below the surface of the absorbent collected in the bottom part of the absorber, where the elevated bottom part is arranged to displace a substantial part of the rich absorbent collected in the bottom part.

[001 1] As mentioned in the introductory part, a substantial volume of absorbent is normally collected in the bottom of an absorber column, due to the requirements for suction height for pumps, and the cross section of the absorption column. By arranging an elevated bottom part displacing a substantial part of the absorbent volume, i.e. filling up a substantially part of the volume that could be filled with absorbent, the volume of the amine that at any time is present in the absorbent column to give a required suction height, is substantially reduced as full depth of the liquid absorbent is only present at a minor part of the cross section of the absorption column. Reduction of the volume collected at the bottom of the absorber means a reduction in the total absorbent and may thus reduce the cost both for buying new absorbent and depositing used amine. Additionally, it is assumed that a substantial part of the degradation of the absorbent occurs during its residence time in the absorber. By reducing the volume in the absorber sump while the circulation rate is maintained, the residence time in the absorber is reduced. Reduction of the total volume of the absorbent will also improve the response time of the absorber to

operational changes.

[0012] According to one embodiment, the elevated bottom part is arranged to displace 50% or more of the volume of the rich absorbent collected in the bottom part. To have a sufficient beneficial effect, it is assumed that the volume of the rich absorbent being present at the bottom of the absorber has to be reduced by 50 % or more, without reducing the suction height for the pumps. It is preferred that the volume is reduced more, such as more than 55 %, or more than 60%.

[0013] The elevated bottom part preferably fills a volume starting at the opposite side of the absorber relative to the side of the exhaust inlets(s) continuing towards the side of the exhaust inlet(s) in the total width of the column to leave a well part extending to the bottom of the absorber allowing for the suction height required by rich absorbent pump, between the elevated part and the absorber wall below the exhaust gas inlet(s). By building up the elevated bottom part from the part of the absorber that is furthest away from the exhaust gas inlet(s) to form a well part of the absorber directly below the exhaust gas inlets into the absorber, the deepest part of the absorber and the part from where rich absorbent is withdrawn are placed where the influence of the incoming exhaust due to its incoming speed is lowest, as the exhaust gas blows into the absorber higher up. Accordingly, the liquid absorbent in the well part has less gas bubbles due to waves and heavy winds in the absorber than other parts of the absorber.

Additionally, the streams in the absorbent is substantially lower in this part of the absorber than the parts were the speed incoming exhaust gas causes formation of waves and wirling motion of the liquid.

[0014] The absorber column according to any of the preceding claims, wherein one or more rich amine line(s) are arranged through the elevated bottom part opening into the bottom of the absorber for withdrawal of rich amine. By allowing the rich amine lines to go through the elevated bottom part, there is no need to fasten pipelines at the bottom of the absorber, fastenings that may loosen and require reparations.

Brief description of drawings

[0015]

Fig. 1 is a principle sketch of an amine based carbon capture plant according to the state of the art,

Fig. 2 is a cross section through an absorber in a carbon capture plant according to the present invention, and

Fig. 3 is a length section through a bottom part of the absorber of fig. 2. Detailed description of the invention

[0016] Figure 2 is a cross section through an absorber 3 produced in concrete.

Concrete is an attractive building material for an absorber. The rectangular cross section, so that the absorber is a right rectangular prism, is a practical choice of geometry both from a practical and economical point of view. The inner walls of the absorber are preferably lined by a lining of a polymer material to reduce or avoid erosion of the walls during operation of the plant.

[0017] The lower part of the absorber is surrounded by a tray 20, to collect any leakage of absorbent from the absorber 3 to avoid spillage into the surroundings. The tray 20 is in many countries a requirement for allowing the operation of such plants, as spillage of absorbent into the surroundings is not allowed, and the tray is constructed to be able to collect the maximum volume of absorbent that may escape from the absorber in a major accident.

[0018] An elevated bottom part 22 of the absorber is arranged in the absorber 3, to reduce to liquid depth of a substantial part of the bottom of the absorber, and thus reduce the total volume of the absorbent collected in the bottom part of the absorber. The elevated bottom part is elevated from the lowest part of the absorber except in an area preferably starting from the wall through which the incoming exhaust gas is introduced, in which area a well part 7 is formed. The presently preferred elevated bottom part is a solid block of concrete molded during the preparation of the

foundation for slip forming of the concrete absorption column. The skilled person will, however, understand that the elevated bottom part 22 may be any kind of construction that is capable of displacing a substantial part of the volume of the absorbent in the bottom of the absorber, and may be a hollow structure formed by concrete walls and top.

[0019] Pumps 6 for withdrawal of absorbent from the bottom of the absorber are arranged outside of the tray 20. Due to geometric restrictions caused by the direct contact cooler 2 being arranged close to the absorber and connected to the absorber by a cooled exhaust duct 24 opening into the exhaust entrance 23, the pumps 6 are preferably arranged at side of the absorber being opposite to the exhaust gas entrance 23, as illustrated in figure 3. Pipelines 5 are therefore arranged from the pumps 6, through the wall of the tray 20, through the wall of the absorber, and are cemented into the elevated bottom part 22, ending into the well part 7 at the bottom of the absorber to act as absorbent withdrawal pipes. Open ends 21 of the pipelines 5 are arranged close both the bottonn of the well part 7 and which are arranged close to the side of the absorber at which the exhaust gas is entering the absorber through one or more exhaust entrance(s) 21.

[0020] As mentioned above, the typically required depth of the absorbent

collected at the bottom of the absorber to give the required suction height for the pumps 6, is at least 1.5 meters, and more preferably about 2 meters. To be at the safe side, the typical average depth during operation is about 2.8 meters. The depth may, however, vary during operation, between about 3 meters and 2.3 meters. Accordingly, the elevated bottom part will normally be from about 1.5 to 2.5 meters, such as about 2 meters higher than the bottom of the absorber. The minimum amine depth should be sufficient to cover the top of the elevated bottom section to avoid the elevated bottom formed by the elevated bottom part to the physical stress of exposure to the liquid absorbent raining down from the above packing, and the heavy wind from the incoming exhaust gas. Preferably, the elevated bottom part is covered by at least 0.5 m of absorbent, more preferably about 0.8 m during stable operation of the capture plant.

[0021] By reducing the volume of absorbent in bottom part of the absorber, the total volume of absorbent in the carbon capture plant may be reduced substantially. A reduced volume of absorbent is an advantage, with regard to the investment cost in purchasing the chemicals.

[0022] The elevated bottom part is preferably arranged starting opposite of the exhaust gas entrance(s) into the absorber, continuing towards the exhaust gas entrance(s), ending at a distance from absorber side of the exhaust gas entrance(s) to leave the well part 7 having the full depth of the absorber. By this kind of arrangement, the well part 7 will be in the "wind shadow" of the incoming exhaust gas, the well part will be arranged in the part of the bottom section least influenced wind and undesirable streams in the liquid.

[0023] Additionally, an important part of the thermal and oxidative degradation of the absorbent is assumed to be closely connected to the residence time in the bottom part of the absorber, where the absorbent is exposed to high temperature in combination with oxygen from the exhaust gas. A reduction of the residence time in the bottom part of the absorber is assumed to be advantageous with regard to degradation of the absorbent.

[0024] A reduced total volume of absorbent is, due to the reduced degradation rate, also advantageous with regard to the operating cost associated with purchase of new amine with disposal of used amine, as the amine absorbents are environmentally unacceptable and have to be handled as special category waste. The cost for handling of used amine may be sufficiently higher than the cost for new amine.

[0025] A reduction of the total volume of absorbent is also advantageous from an absorbent regeneration point of view. Degradation, i.e. thermal and/or oxidative degradation, is inevitable. A bleed stream of lean absorbent is therefore withdrawn from the lean absorbent line 4 an introduced into a not illustrated reclaimer for reclaiming not degraded amine as a mixture of amine vapour and steam that is introduced into the regeneration column as additional stripping steam, and where insoluble material is removed and disposed. This is a process requiring substantial amount of energy, and a reduction in the volume introduced into the reclaimer will therefore reduce the total energy requirement of the plant.

[Example

[0026] An exemplary absorber for capturing CO2 from the exhaust gas from a 400 MW gas fired power plant, receives about 700 m³/sec exhaust gas at a temperature of about 30 °C from the direct contact cooler 2. Having a cross section of the exhaust gas inlet 23 of about 80 m², the incoming exhaust gas has a velocity of about 13 m/sec into the absorber. The exhaust gas inlet(s) is (are) provided in one of the walls of the absorber having a rectangular cross section, and is (are) situated below the packing 3', but above the surface of the absorbent collected in the bottom part of the absorber.

[0027] To allow sufficient contact time between the absorbent and exhaust gas in the packing to obtain a sufficient level of CO2 absorption, the gas velocity in the absorber is reduced to 2 - 3 m/sec by increasing the cross section of the absorber relative to the cross section of the exhaust gas inlet(s), to give a cross section of > 300 m². The exemplified absorber has a rectangular cross section, having side lengths of 16 m and 20 m, respectively, i.e. an area of about 320 m². The distance between the exhaust gas inlet and the opposite wall is 16 m.

[0028] The absorbent collected at the bottonn of the absorber has a depth in the well part 7 of about 2.75 m. The elevated bottonn part has a height of about 2 meters, which leaves about 0.75 meters absorbent liquid depth above the elevated bottom in steady state operation. The elevated bottom part fills the bottom part of the absorber, except for the well part starting from the wall in which the exhaust inlet(s) is (are) arranged and extending about 2 meters from said wall. In this example, the volume of the absorbent present in the bottom of the absorber is reduced from 880 m³ to 320 m³, or a reduction in the volume of about 560 m³, or a reductionof about 64 % compared to an absorber without an elevated bottom section.

[0029] The incoming exhaust gas will cause waves to be formed at the surface of the absorbent collected in the bottom part. Waves and amine raining down from the packing will cause formation of gas bubbles that are mixed into the absorbent. The bigger the waves are the more gas bubbles to be mixed into the absorbent. Gas bubbles in the absorbent are unwanted as the bubbles may cause additional contact between the gas containing oxygen, and the absorbent, which may cause additional oxidative degradation. Additionally, the gas bubbles may cause cavitation problems for the pumps.

[0030] To reduce the waves, the exhaust gas inlet(s) is (are) arranged at a certain height above the surface of the amine basin, such as at least 1 meter above the surface, more preferably, at least 2 meters above the surface. If the height between the exhaust gas inlet and the surface of the absorbent basin exceeds a certain height no further advantages are obtained, but construction cost is added due to the increased total height of the absorber. It is currently assumed that the distance should be less than 4 meters, and more preferably less than 3 meters. A currently preferred distance is between 2 and 2.5 meters between the lower part of the exhaust gas inlet(s) and the surface of the absorbent basin during normal, steady state operation of the plant. In other words, the extension of the well part from the wall including the exhaust inlet(s) in the direction of the exhaust gas flow is preferably shorter than the height between the surface of the absorbent basin and the lower part of the exhaust gas inlet(s), such as more than 5 % shorter or more preferably more than 10% shorter.

Claims

1. An absorber column (3 ) for a CO2 capture plant, the absorber column (3) comprising a lean absorbent line (4) for introduction of lean absorbent onto the top of one or more packing(s) (3') arranged across the cross section of the absorber column (3) , one or more exhaust gas inlet(s) (23) arranged in the side wall of the absorber column and below the packing(s) (3') for introduction of incoming exhaust gas into the absorber column so that the exhaust gas is caused to flow countercurrent to the absorbent in the packing(s), an exhaust gas outlet (6) arranged above the packing(s), an absorber column bottom part for collecting rich absorbent having absorbed CO2, and one or more rich absorbent line(s) (5) for withdrawal of rich amine from the bottom part of the absorber (3), wherein an elevated bottom part (22) is provided below the surface of the absorbent collected in the bottom part of the absorber, where the elevated bottom part is arranged to displace 50% or more of the volume of the rich absorbent collected in the bottom part.

2. The absorber according to claim 1 , wherein the exhaust gas inlet(s) (23) is (are) arranged from one side of the absorber column.

3. The absorber column (3) according to claim 1 , wherein elevated bottom part (22) fills a volume starting at the opposite side of the absorber relative to the side of the exhaust inlets(s) continuing towards the side of the exhaust inlet(s) in the width of the column to leave a well part extending to the bottom of the absorber allowing for the suction height of 1.5 meters or more, between the elevated part and the absorber wall below the exhaust gas inlet(s).

4. The absorber column according to any of the preceding claims, wherein the one or more rich amine line(s) (5) are arranged through the elevated bottom part (22) opening into the bottom of the absorber for withdrawal of rich amine.

5. The absorber according to any of the preceding claims, wherein the elevated bottom part is constituted by a solid concrete block.

6. The absorber according to any of the claims 1 to 6, wherein the elevated

bottom part is formed by concrete walls and top..