Improvements to Absorber for Direct Air CO2 capture

Abstract

An absorber for the capture of carbon dioxide (CO2) from a CO2 containing gas stream. The absorber comprises an absorber volume to contact the CO2 containing gas stream with a sorbent. The absorber comprises a first portion equipped with spray nozzles and a second portion containing packing material arranged to be wetted by sorbent. The first portion and second portion each have a respective duty which together contribute to a required absorber duty defined as a rate of capture of CO2. A method of capture of carbon dioxide is also disclosed. The absorber may provide high-efficiency mass transfer surfaces to overcome worsening kinetics due to progressive depletion of sorbent.

Description

Technical Field The present disclosure concerns an absorber for a direct air CO2 capture process. Background Carbon capture and storage is expected to be a significant way to reduce the effects of global warming from the combustion of fossil fuels. Capture of carbon dioxide (CO2) may involve systems for extracting CO2 from a CO2 containing gas using an absorbent medium. Typically, this involves creating a gas flow over the absorbent medium under conditions where the medium will absorb CO2 from the gas, and then altering the conditions so that the medium releases the absorbed CO2 allowing it to be captured and stored. This process may be used to reduce atmospheric CO2 to mitigate the anthropogenic emissions that are associated with global warming, or climate change. Direct Air Capture (DAC) is the capture of CO2 from atmospheric air which, as the atmosphere contains less than 0.05% CO2, involves processing large volumes of air. Some Direct Air Capture systems use a liquid medium to absorb CO2 in an absorber or liquid-air contactor. There is for example a sorbent containing solution, distributed within an absorber, subject to a high air flow necessary to process large quantities of air. The sorbent may be distributed by spray nozzles, which produces an aerosol of sorbent in solution with a high surface area. Alternatively, the air may be brought into contact with liquid films of sorbent solution propagated across solid surfaces, such as conventional structured packing / film fill. The high energy use and other costs of running a direct air capture system may be improved by improving the design of the absorber. Summary of Invention In an aspect of this disclosure, an absorber for the capture of carbon dioxide (CO2) from a CO2 containing gas stream is provided, the aborber comprising: an absorber volume to contact the CO2 containing gas stream with a sorbent, the absorber volume having a length dimension parallel to the direction of travel of an air stream through the absorber from an inlet to an exit when in use; the absorber comprising at least a first portion nearest to the inlet and a second portion further from the inlet, wherein the first portion is equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream where it can absorb CO2 from the gas stream while droplets of the aerosol are suspended in the gas stream, and the second portion contains packing material arranged to be wetted by sorbent, wherein the first portion and second portion each have a respective duty which together contribute to a required absorber duty defined as a rate of capture of CO2. The duty of the first portion and the duty of the second portion may be selected to minimise the pressure drop across the absorber while providing the required absorber duty. The second portion may configured to provide between 10% and 70% of the absorber duty. The second portion may additionally configured to capture drift from the first portion. There may also be a drift eliminator downstream of the second portion. The absorber may have a third portion between the first portion and the second portion, the third portion containing no packing or packing material arranged differently to the packing in the second portion. The third portion may arranged to be only wetted by sorbent that has been carried from the first portion as an aerosol suspension. The first portion may contain no packing material. That is, the first portion may be a purely spray type contactor. The CO2 containing gas stream may flow perpendicular to the sorbent flow. The CO2 containing gas stream may flow in a horizontal direction or a vertical direction, or in intermediate direction. The CO2 containing gas stream may be ambient air. There is also provided a method of capture of carbon dioxide (CO2) from a CO2 containing gas stream, the method comprising: providing an absorber for contacting sorbent with the CO containing gas stream, supplying sorbent as an aerosol to one portion of the absorber using spray nozzles, and supplying sorbent to a second portion of the absorber onto packing. The duty of the first portion and the duty of the second portion may be selected to minimise the pressure drop across the absorber while providing the required absorber duty. The second portion may be configured to provide between 10% and 50% of the absorber duty. The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and / or combined with any other feature described herein. Brief Description of Drawings Embodiments will now be described by way of example only, with reference to the Figures, in which: Fig. 1 is a schematic of a prior art carbon capture system; Fig. 2 is a schematic of an absorber for a carbon capture system. Fig. 4 is an illustration of performance of a packed absorber for a carbon capture system. Fig. 4 is an illustration of performance of a hybrid absorber for a carbon capture system. Detailed Description With reference to Fig. 1, which shows a system for carbon capture from a CO2 containing gas. Absorber 10 receives a CO2 containing gas stream from the inlet 20 to exit 30. A sorbent stream flows through the absorber, a lean stream 40 enters the absorber, contacts the CO2 containing gas and becomes a rich stream 50. Sorbent may be recirculated within the absorber as a recirculation stream 110, which increases the effective residence time of each portion of the lean stream of sorbent in the absorber. The gas stream may flow horizontally as shown, or may flow vertically from bottom to top of the absorber in counterflow to the sorbent stream, or from top to bottom in parallel with the sorbent flow. The rich stream is typically passed through heat exchanger 100 to recover some heat from the lean stream returning from the desorber 60. Desorber 60 receives the rich stream and heats it up to a temperature where the CO2 will be released form the sorbent, typically using heating means 70 which may also generate steam to form vapour bubbles into which the desorbed CO2 can diffuse, leaving a lean stream of sorbent to return to the absorber to repeat the process. The vapour and desorbed CO2 exit the desorber at 80, where a condenser 90 is usually used to cool the mixture causing the vapour to condense leaving a purer CO2 product stream. The component or components that bring the sorbent solution into contact with the air, are referred to here as the contactors, being one of the sub-components that make up the composite parts of the Absorber sub-system. The common design of these contactors is to provision a solid surface over which a liquid film of sorbent solution can be propagated to provide a large surface area of fluid in contact with the air stream at any one time. This type of component is conventionally referred to as “packing”, of which there are a myriad of generic and proprietary forms utilised extensively for both mass transfer and heat transfer unit operations within industry (e.g. Wet cooling towers, stripper columns). Fig. 2 shows an absorber 10, which may be used in a system like the one in Fig. 1, with a vessel for storing sorbent 210 positioned above the absorber, and an optional distribution mechanism 220 for distributing sorbent across the length and width of the absorber when it is released from the vessel 210. Vessel 210 comprises a release mechanism 240 for controlling the release of sorbent, which may be continuous or released intermittently in pulses either directly into the absorber or via the distribution tray 220. Sorbent supply 260 delivers sorbent into the vessel, which passes through the absorber downwards due to gravity, where it may be collected in a sump 270 after spending a residence time in contact with the airflow between inlet 20 and exit 30. From the sump, the sorbent may be recirculated through the absorber via sorbent supply 260, while a portion of the sorbent leaves the absorber as rich flow to the desorber 60. Lean sorbent from the desorber may be mixed with the recirculated sorbent in sorbent supply 260, or it may enter the absorber via a different stream. The absorber may include packing 250 filling or partially filling the absorber 10. The packing may comprise layers or stacked pieces to distribute the sorbent within the absorber and increase the effective surface area of sorbent exposed to the airflow between inlet 20 and exit 30. The absorber may also include nozzles to distribute the sorbent as a spray or droplets within the absorber. These nozzles may deliver sorbent onto the packing, or create a spray or mist with a high effective surface area directly in the airflow within the absorber. The release mechanism 240 may configured to release a portion of the sorbent into the absorber, optionally via the distribution tray 220 or directly into nozzles, when the level of sorbent in vessel 210 exceeds a threshold depth 230. The release mechanism 240 may form part of vessel 210 or it may be a separate module fitted to vessel 210, either inside or outside the vessel. The release mechanism 240 may be electro-mechanical, such as variable flow valves or may be a passive mechanism that operates without the use of external power and does not rely on external control signals or sensors. The release mechanism 240 may be driven by gravity and the mass of sorbent, using buoyancy, siphons and / or tilting mechanisms. There may optionally or alternatively be a continuous supply of sorbent 260’ into the absorber. For example, there may be a continuous spray of sorbent, for example as a mist or trickle of sorbent which may be into open space or onto packing, and a second intermittent supply of pulses of sorbent delivered by the release mechanism. The release mechanism may be configured to deliver sorbent at different rates to different parts of the absorber, for example supplying spray nozzles in one portion of the absorber to produce mist of sorbent, and supplying other nozzles to wet a packed portion of the absorber. Fig.3 illustrates the CO2 concentration and pressure drop in the gas as it passes through a packed absorber 300. The gas flow is indicated by the arrow from bottom to top on the page from inlet 20 to exit 30, the absorber may in fact be oriented horizontally or vertically. Chart 320 illustrates how the CO2 concentration in the gas reduces as the gas passes though the absorber,reaching a minimum at the exit. The curve is approximately exponential, as the rate of absorption decreases as the concentration decreases. Chart 330 illustrates the pressure drop through the packing 250, which is approximately linear as there will be roughly equal pressure drop per unit length assuming homogenous packing is used throughout the length, at the typical gas flow rates used in a carbon capture absorber. Wetted surface type contactors, can have high mass transfer efficiencies due to having Specific Surface Areas in the 100s of m2 per m3, depleting the air of the solute gas as it transits through the component. Thus, the depth of the component relative to the air stream direction of travel sets the overall mass transfer duty of the sub-system. Being set to meet overall design requirement of the system. This is not constant through the depth of the bed, as the rate at which the solute is captured is partly controlled by the concentration of solute gas at that given point in its transit. Thus, mass transfer can be said to be higher in earlier sections of the component, reducing as the air penetrates further. Therefore, the relationship of contactor depth to mass transfer duty is not linear given a consistent sorbent solution loading, so is subject to optimisation against the pressure drop incurred by the component. As any component placed within air stream will cause blockage and aerodynamic perturbances, to said airflow, causing pressure losses within the flow path. These pressure losses need to be overcome by any method of actively inducing air through the sub-system, which will incur an energy efficiency penalty on the system, which is undesirable and needs to be minimised and traded against the mass transfer. Conventional packing materials are often in excess of 10s of Pa / m, which are significant for a DAC system given the amount of air that needs to be induced through the sub-system to achieve the required mass transfer duties. The energy requirement of the fan to drive air through the contactor is significant. An alternative to a wetted surface to provide contact between the two streams for the purpose of mass transfer, is the usage of droplets. The primary assumption is that they are seeded introduced into the air stream via spray / aerosol methods. However again, multiple methods are possible, to which this invention is not restricted, but could the provision of a spray or an array of multiple spray nozzles to achieve this. The method chosen would be able to achieve high specific surface areas comparable to that of the packings mentioned earlier. And if the components required to seed the air are mounted on the bounding walls that define the primary air flow path, then pressure drop per meter would not be significantly noticeable above the inherent duct pressure losses due to the presence of the bounding walls. If seeded was required to be performed by components protruding into the air flow, this penalty would increase but would be reduced compared to a unit depth of packing. However, droplets have several shortcomings: • The surface area of the droplets is not fixed, being susceptible to inter droplet collision and coalescence, and collision with the bounding walls of the air path. This can be validated experimentally in proposed DAC flow conditions. Furthermore, evaporation of the droplet solvent (H2O) into the air stream will also cause the droplet shape to change. This evaporation is not unique to a spray based system however. • The transit / residence time of the droplet may be less than that of an equivalent volume of fluid in a surface film on a wetted surface. • These are both parameters for optimisation through design and operation. Therefore, the depth of bed for the same duty of solute gas capture for a droplet based system, is likely to be much deeper than an equivalent conventional packed bed type system. However, the pressure drop reduction is attractive, and when combined with benefits in CAPEX (amount of material, transportation and assembly reduced) further promotes this contactor type. This drawback in contacting ability per unit depth, will be further inflated as the air becomes depleted of the solute gas, as mentioned before, So the X meters of spray contactor required to perform the latter fractions of absorption duty will become significantly larger compared to that of a surface type system, with the impacts of extra duct lengths and sorbent solution flow eroding the benefit provided by low pressure drop. Therefore, this invention proposes a hybrid system where a combination of both droplet contacting methods and surface film contact methods are used in combination. Primarily it is envisaged that the air flow through the contactor sequentially, utilising the low pressure drop droplet system first to achieve the majority of the reduced mass transfer duty, then the remaining fraction is achieved with a high efficiency surface film type contactor. There is no restriction in numbers of these contactor “beds” and their configurations, being suitable for optimisation. However, the likely configuration would be a instances of spray contactors, followed by instances of surface contacting beds. A generic example to compare against Fig 3, is shown in Fig 4 to illustrate the functional behaviour through a hybrid contactor type arrangement. Fig.3 illustrates the CO2 concentration and pressure drop in the gas as it passes through a hybrid absorber 400. The gas flow is indicated by the arrow from bottom to top on the page from inlet 20 to exit 30, the absorber may in fact be oriented horizontally or vertically. This hybrid absorber has a first portion 440 where there is little or no packing, and absorption takes places in a droplet based system. Spray nozzles (not shown) deliver sorbent solution into the first portion as an aerosol or mist. Second portion 250 is a packed section where the packing is wetted as described above. There is an optional third portion 450 that may be simply a break between the first and second portions to allow the mist or droplets to settle, or it may be a section of packing without a separate wetting mechanism, that is wetted by the droplets from the first portion that are entrained by the gas flow. Chart 340 illustrates how the CO2 concentration in the gas reduces as the gas passes though the absorber, reaching a minimum at the exit. The curve has three sections. The graph sections are aligned with the respective absorber portions. Each section is approximately exponential, as the rate of absorption decreases as the concentration decreases. However the curve of the first section approaches an asymptotic limit more quickly as the performance of the mist / droplets reduces more quickly with length than the packed portion. Without wishing to be bound by theory, this maybe because the droplets begin to coalesce, reducing the rate of diffusion of gas into the sorbent droplets. Chart 330 illustrates the pressure drop through the packing 250, which again has three sections aligned with the portions of the absorber 400. Each section is approximately linear as there will be roughly equal pressure drop per unit length, however the pressure drop though the spray portion 440 of the absorber is much lower than through the packed portion 250. By selecting a length for the spray portion and the packed portion, an optimum performance for pressure drop can be achieved, each portion of the absorber contributing to a total absorber duty. This overcomes the performance limitations of a purely spray absorber, as the packed section with a thin film of sorbent and longer residence time absorbs more CO2 than would have been achievable with a purely spray absorber of a similar size. The overall pressure drop and hence fan duty of the absorber is reduced compared to a fully packed absorber. It has been found that providing a packed portion that delivers between 10% and 70% of the overall absorber duty achieves the lowest pressure drop compared to a fully packed absorber of the same duty. Furthermore, there is an additional physical minimum suggested for the required depth of wetted surface type contactor if placed as the final contacting component, in a hybrid type system. This is suggested to ensure function for both drift elimination as well as secondary mass transfer of impinged droplets. That being 5-10% of the depth required of wetted surface type packing to achieve the full absorption duty in a non-hybrid system. The fraction of duty to be provided by the packed portion may vary due to the relative costs of sorbent loss, fan power and absorption performance, depending on the environment in which the plant is operated and the type of sorbent used. Thus the minimum duty fraction of the packed portion may be at least 10%, at least 20%, at least 30%, at least 40%, at least 50%. The benefits realised by introducing a spray section of the packing will reduce if the majority of the absorber is filled with packing. Therefore the maximum fraction of duty to obtain a benefit may be less than 90%, less than 80%, less than 70% or less than 60% or less than 50%. An optimal fraction can be obtained by modelling absorber performance for the intended environmental conditions using a chosen sorbent solution. Overall the hybrid absorber will require fewer components than a packed absorber, making it lighter and less resource intensive to build. Because the packed portion captures any drift from the mist portion, it avoids the high sorbent loss normally found in purely spray absorbers. A smaller drift eliminator may be installed at the exit from the absorber to catch andy droplets entrained into the air flow form the packed portion, but this will be smaller than would be required in a purely spray absorber. The invention reduces the amount of surface film contacting sub-components within the Absorber air stream and sub-systems, providing further benefits including: • Reduced Pressure Drop. • Reduced Operational Wetted Weight / Liquid Hold-up. • Simplifies Maintenance Activities, due to improved access and reducing in fouling surfaces. • Reduced Volumes of Surface Film Contactor Components - Further benefits through reducing volume of components to be purchased, transported, and assembled. It is also highlighted that the invention is applicable to any arrangement of Absorber Module, with respect to the flow of the air and the sorbent solution relative to each other. Including, but not limited to, Vertical Counterflow Systems and Horizontal Cross-flow systems. Compared to a fully spray absorber, the invention: provides high-efficiency mass transfer surfaces to over come worsening kinetics due to progressive depletion of sorbent in air stream. Provides a drift elimination function, due to promoting collision and coalescence of droplets in air stream and collision with the contactor surfaces, through the tortuous path the latter provides. Furthermore, this portion of contactor could perform the combined function of mass transfer and drift elimination, either replacing the requirement for a dedicated drift eliminator in its entirety or at least reduces the duty it would need to perform. Thus, being beneficial in terms of pressure loss and the life cycle cost of components. The droplets colliding with the contactor provide a passive wetting of the contactor surfaces, increasing absorption of the solute per pass of sorbent solution through the absorber system and reduces the amount of sorbent solution that needs to be moved around the system to purely wet the surface film-based contactor components. Further benefits of the surface-based contactors in proximity to a droplet-based contactor previously described, include both the occurrence of passive wetting and supplementary mass transfer from the droplets impinging onto the surface. This adds to both the overall mass transfer, reduces the amount of sorbent solution required to be transferred around the system purely for wetting of the contactor surfaces (improved capture per unit pass of sorbent through the module) and drift elimination. The latter is provisioned as a surface-based contactor does provide blockage and a tortuous path for the airflow, whose form is also beneficial for removing drift as well as mass transfer in most instances. Within vertical counter-flow systems, the airflow could be seeded with droplets in a vertical counter flow manner (e.g. a fountain), this would provide increase residence times in the flow if the droplets behaved ballistically (with gravity), either returning to the sump, or wetting surfaces for further mass transfer beneath the injection point. Whilst the smaller droplets, would travel with the air flow, impinging on the contacting surfaces above, removing their potential to be drift, adding to the wetting of said surface, adding additional mass transfer and also providing the ability to coalesce and drip from the surface in large droplets that behave ballistically, returning to the sump or wetted surface contactors below. Droplet production can be produced by a wide range of spray nozzle types, forms, arrays, and arrangements. They may be stationary or could be actively or passively locomoted for more optimal sub-system performance. They could also be intermittently operated for further efficiency and operational benefits. Sorbent properties: Sorbents for carbon capture generally have a changing equilibrium between the carbonate / carbamate forms and being in solution with CO2 that depends on temperature, concentration and other factors. Sorbents are carried in a solvent, for example water, which may contain further additives that can act as catalysts, modify the solution physical properties, reduce degradation or other desirable properties. In general, where the absorber is an air to liquid contactor in a DAC system, the temperature of the sorbent and the temperature of the air within the absorber will be very close, as the sorbent is distributed in a thin film or droplets to achieve a high surface area and contact time, and the air flow is relatively high. In an operating model of operating a CO2 capture system, other factors than absorbing efficiency may need to be taken into account, such as energy efficiency, sorbent degradation rate, loss of sorbent, maintenance costs etc. For any CO2 capture system the threshold for rate of absorbing may be set taking into account all the other factors, to give the best overall operation of the system. Sorbents may include alkaline absorbents such as hydroxides or organic sorbents. Alkaline sorbents may include potassium hydroxide or calcium hydroxide. Organic sorbents may include amines, amino acids. Amines may include Ethanolamine (2-aminoethanol, monoethanolamine, ETA, or MEA). Preferred sorbents include amino acids or alkali salt solutions of amino acids. The amino acids may be derived from the group consisting of alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, ornithine, phenylalanine, proline, sarcosine, selenocysteine, serine, taurine, threonine, tryptophan, tyrosine, or valine. The amino acid may be a compound of an amino acid, such as a methyl amine or diethyl amine. Preferred alkali component of the amino acid salts is potassium or sodium. 5 Examples of amino acid salts include, sodium glycinate, potassium lysinate. Amino acids are preferred because they are understood to have lower heat requirements for desorption, have less degradation than amines, and are less hazardous in use than many of the alternatives. They are often also less volatile 10 so that less sorbent evaporates from the solution. It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, 15 any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and subcombinations of one or more features described herein.

Claims

1. An absorber for the capture of carbon dioxide (CO2) from a CO2 containing gas stream, the absorber comprising:an absorber volume to contact the CO2 containing gas stream with a sorbent, the absorber volume having a length dimension parallel to the direction of travel of an air stream through the absorber from an inlet to an exit when in use;the absorber comprising at least a first portion nearest to the inlet and a second portion further from the inlet, wherein the first portion is equipped with spray nozzles configured to deliver the sorbent as an aerosol into the gas stream where it can absorb CO2 from the gas stream while droplets of the aerosol are suspended in the gas stream, and the second portion contains packing material arranged to be wetted by sorbent, wherein the first portion and second portion each have a respective duty which together contribute to a required absorber duty defined as a rate of capture of CO2.

2. The absorber of claim 1, wherein the duty of the first portion and the duty of the second portion are selected to minimise the pressure drop across the absorber while providing the required absorber duty.

3. The absorber of claim 1or claim 2, wherein the second portion is configured to provide between 10% and 70% of the absorber duty.

4. The absorber of any previous claim, wherein the second portion is configured to capture drift from the first portion.

5. The absorber of any previous claim, further comprising a drift eliminator downstream of the second portion.

6. The absorber of any previous claim, further comprising a third portion between the first portion and the second portion, the third portion containing no packing or packing material arranged differently to the packing in the second portion.

7. The absorber of claim 6, wherein the third portion is arranged to be only wetted by sorbent that has been carried from the first portion as an aerosol suspension.

8. The absorber of any previous claim, wherein the first portion contains no packing material.

9. The absorber of any previous claim wherein the CO2 containing gas stream flows perpendicular to the sorbent flow.

10. The absorber of any previous claim wherein the CO2 containing gas stream flows in a horizontal direction.

11. The absorber of any previous claim wherein the CO2 containing gas stream is ambient air.

12. A method of capture of carbon dioxide (CO2) from a CO2 containing gas stream, the method comprising:providing an absorber for contacting sorbent with the CO containing gas stream, supplying sorbent as an aerosol to one portion of the absorber using spray nozzles, and supplying sorbent to a second portion of the absorber onto packing, wherein the first portion and second portion each have a respective duty which together contribute to a required absorber duty defined as a rate of capture of CO2.

13. The method of claim 12, wherein the duty of the first portion and the duty of the second portion are selected to minimise the pressure drop across the absorber while providing the required absorber duty.

14. The method of claim 12 or 13, wherein the second portion is configured to provide between 10% and 50% of the absorber duty.

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