Background or part 1 for this post can be viewed here:
The current infeasibility of available oceanic remediation mechanisms is troubling because as previously discussed it does not appear that global CO2 neutrality will be achieved at any point in the near future. This lack of neutrality will lead to further ocean acidity raising the probability of catastrophic loss of ocean biodiversity. Therefore, new strategies need to be proposed in effort to alleviate the problem of ocean acidity. Note that this proposal is theoretical and has not been tested in any way, shape or form.
As the situation currently stands it appears that the most viable economic route to CO2 removal would be to design a piece of technology that could somehow remove the unassociated CO2 from the ocean by facilitating a chemical reaction to bind it and then dissociate from the CO2 at a later time, making the material reusable. This strategy eliminates various problems with the catalytic option used in iron fertilization by anchoring any catalyzing agent to a device and even if needed sequestering it away from any detrimental elements. It also redirects the limiting factor of CO2 turnover to the material that is absorbing the CO2, which is more controllable and can be manipulated more easily than biological organisms or limestone deposits. In addition if the material can be manufactured at reasonable cost, the material being the limiting factor in CO2 absorbed would only be a minor inconvenience. Although such a strategy seems daunting, there is reason to be optimistic. Below is a description of the type of device that may accomplish the desired reduction in acidity.
Considering the solubility factors and the role of the natural carbon cycle, it appears that withdrawing CO2 closer to the surface is preferable. Therefore, it would be useful for the device to behave in similar fashion to a buoy in that the absorption portion of the device would be submerged below the surface, but most of the device remains above the surface. The main reason for this strategy is the fact that permanently submerging the entire unit may be counter-productive as the non-submerged portion could be used to support a solar panel system to power any autonomous actions for the device or some other non-aquatic advantage. Also salvaging the system after reaching maximum CO2 storage would be made more complicated if it were fully submerged.
Due to the sheer size of the ocean, reducing total average acidity without significant removal of atmospheric CO2 is rather farfetched. The principle idea behind the device presented here is not to reduce the acidity of the entire ocean, but instead focus on small critical portions to delay or even prevent the erosion of oceanic biodiversity and food chains. For example it would be difficult to argue that some portion of the Pacific Ocean in the middle of nowhere is of equal importance to oceanic biodiversity to that of the Great Barrier Reef. Granted that it is highly likely that due to the mixing differential of the ocean a point location reduction of acidity would not be straightforward, but by applying continuous acidic reduction pressure at a particular point, there is a high probability that acidity levels at that point will fall faster than will be recouped by mixing. In fact there is a small probability that if significant points of action are established total average ocean acidity throughout the system will be reduced. However, such reduction would not be anything of significance beyond the point locations.
There are a number of different materials that are known to interact with and bind CO2 either as a catalyst or in a chemical reaction (NaOH, different resins, amines, aqueous ammonia, ionic liquids, membranes, etc). Most of these elements have been explored or are currently being used in the design of carbon capture mechanisms for coal power plants. Unfortunately very few of these options, for obvious reasons due to the focus on source capture in power plants, have been tested in aquatic conditions. Another problem is that most of these processes are scaled-up to function over a much larger area than that which would be economically feasible for an ocean CO2 absorbing device. However, metal organic frameworks (MOF), a hybrid material constructed from metal oxide clusters with organic linkers1,2 appear to be a possibility. The reason MOFs are an attractive option is they do not appear to require as much supporting infrastructure as other CO2 absorption materials. Also MOFs have a fairly unique selectivity for CO2, which may increase the efficiency of ‘filtering’ CO2 from other molecules in the ocean while also reducing the probability of contamination and fouling.1
The selectivity of MOF for CO2 is derived from its ability to interact with the large quadrupole moment possessed by CO2.1,3 At certain times CO2 oscillates into a state where its electrons are not evenly distributed (the quadrupole moment). At this point in time based on the molecular arrangement of the particular species of MOF the CO2 binds to the MOF. Technically the quadrupole moment for CO2 is thought to be –4.1 to –4.4 x 10^26 e.s.u. cm^-24,5 Another useful attribute for MOF is the fact that although selective, the bond with CO2 is still rather weak; therefore, less heat and pressure is required to remove the CO2 from the MOF vs. other processes (most notable amine CO2 binding).3 However, it must be noted that similar to the methods listed above, MOF has yet to be tested in an aqueous environment, so there could definitely be some future concerns.
While there are a wide variety of MOFs to choose from, the best option appears to be MOF-177 because so far in empirical studies it has the greatest surface area of all MOF and MOF-similar compounds and has the highest CO2 capacity between all of these compounds.6 MOF-177 has a BET surface area of 4,508-4,750 m^2/g, a bulk density of 0.43 g/cm^3 and absorbs CO2 at a capacity of 1,470 mg/g.6 Note that if covalent organic frameworks (COF) 102 and 103 are much cheaper to produce, they may become viable alternatives to MOF-177.6
Due to the presence of the target CO2 in ocean water, water would need to make contact with the material (probably MOF) doing the binding with CO2, for any attempt to collect out-gassed CO2 would be a rather inefficient means of reducing ocean acidity. There are two primary ways to accomplish this interaction, passive or active. Passive interaction would rely on the natural movement of the water to initiate contact with the material. Active interaction would work to create some form of pressure difference that would draw the water over the material, thus the material would be in contact with the water at certain periods of time instead of random periods of time. For the sole purpose of driving the reaction between the CO2 and the material there does not appear to be a significant difference between passive and active interaction, with the exception that active interaction would require additional energy and/or complexity to power the pump or other drawing mechanism.
Similar to CO2 absorption through technological means via either point source capture or air capture, ocean CO2 absorption has the important lingering question of where to transport the CO2 after absorption. It makes little economic sense to keep the CO2 bound to the material in question; therefore, the CO2 needs to be relocated to an environment where it will not easily re-enter either the atmosphere or the ocean. This question has always been somewhat problematic because there are few options for the collected CO2. As previously discussed in the air capture/sequestration post there are some that would like to utilize capture CO2 in industrial applications like making carbon-neutral fuel, enhancing oil retrieval or augmenting greenhouse-based crop growth; however, none of these options are viable long-term to utilize the amount of CO2 that would be collected and it is difficult to view anything, but enhancing oil retrieval as viable in the short-term. Due to the lack of a viable long-term industrial application and the sheer amount of CO2 that needs to be sequestered, most view storage in natural sinks as the best option.
Based on the specific location of the acidity reduction, storage in sinks could be useful for implementation of such a device. However, if the device is floating on the surface transport to an appropriate storage site could require either a long transfer line or increasing the depth of operation. A short transfer line would not be a significant problem, but when considering that the device will be at a depth of 5-20 ft when on the surface and the typical storage region will have a depth ranging from 5,000-10,000+ ft one could understand how such a long transfer line/pipe would be cumbersome. Therefore, it seems reasonable that the device would have to change depth.
Unfortunately storage in this manner from the device itself is highly unlikely because oceanic sequestration requires that the CO2 be in liquid form, which involves the application of a significant amount of heat and pressure, to be applied within the device, which would probably be largely isolated to a specific compartment. This phase change would provide increased complexity in design because not only would there need to be a separate storage area for the CO2 in gaseous form, but a storage area would be required for CO2 in liquid form as well as the means to generate the necessary levels of heat and pressure. These additional pieces will increase the total weight of the device reducing the maximum capacity of CO2 acquisition and things that could go wrong with the device in general. However, as will be seen, the idea involving a depth changing cycle is still viable.
If direct from the device oceanic sequestration is not rational, then the CO2 collected from the device will need to be manually retrieved and taken to a processing plant to be prepared for sequestration. If this is the case then it is important that the device have as high a maximum capacity for CO2 absorption as possible. It is unlikely that such a capacity can be achieved if passive interaction is used because too little of the material would be in contact with water at a given time. Therefore, it would be wise to create isolated compartments where a large percentage of the environment could contain the material and react with CO2 from water that is moved using active interaction through these areas. However, in order to make the attempt to maximize CO2 capacity mass and density shifts will be expected in the device creating depth changes.
So how will the change in depth be achieved in a device that has the primary function of floating on the surface of the water while trying to maximize CO2 capacity? To best illustrate the process first begin with the question of how a ship floats on water. Basically a ship floats on water because the bulk density of the ship is less than the bulk density of the liquid supporting it (i.e. the water). For reference recall that density is defined as the mass of an object divided by its volume. A ship will no longer float when its density becomes greater than the density of water; most notably this change occurs when the ship’s hull is breached and water begins to flow into the ship increasing its mass. In normal function a ship will sink to an overall depth relative to its density vs. the density of the water (the closer its density is to water the more it will sink). Note that overall object buoyancy is more complex than a simple relationship between densities, (weight related liquid displacement relative to exotically shaped objects and their buoyancy) but for general practice, restricting the discussion to density is fine for non-exotically shaped objects.
Clearly it would be a mistake to breach any portion of the device to induce sinking; however, there is something to be learned from adding water to change the density of the device to initiate sinking. A controlled rate of water acquisition would require compartmentalization and a form of active transport, which is exactly what was proposed to increase CO2 absorption capacity and efficiency. The water would be driven into an alternative compartment(s) in the device via a pump. This alternative compartment would also contain the absorption material. As water continues to flow into these compartments, the device should begin to sink. Once the compartments fill and enough time is allotted for binding reactions, the device can then eject the stored water from the compartments both lowering its density and creating a concentrated jet propulsion stream to hasten its ascent to the surface.
Upon returning to the surface the material should have absorbed a significant amount of CO2 from the water. Regardless of the material, to release the CO2 a considerable amount of heat (temperature increase) will need to be applied. This temperature increase can be achieved through activation of heating units placed on the wall opposite the material. Once released the CO2 will be drawn into a gaseous CO2 storage compartment, which is normally restricted via a valve or some other obstruction. Then the process begins anew with the device changing depth and sinking again.
For this device to function in such a capacity it needs to have a significant level of autonomy. Various sensors and valves (for restricting access) in addition to a centralized computer system would be required. Although difficult to accomplish, autonomous action can also facilitate a form of repeating action or multiple passing, which will increase the probability of reaching the maximum level of CO2 extraction before collecting the CO2 for sequestration. Fortunately the autonomous action elements of the device are not developed for use in a blind environment. Information can be acquired regarding the maximum depth, submergence time, device surface area and volume, etc. which can generate versatile designs for a given region reducing the work required to attain autonomy. For example it is reasonable to know deployment depth, thus timing mechanisms can be utilized to start and end certain processes such as pump action, heating and valve opening and closing.
In some respects think of this device as a significantly more complicated APEX type float. The pump would have to be of greater horsepower and the communication systems more advanced, but the general descent and ascent properties would operate in a similar capacity. The biggest difference is instead of using the pump to transfer fluid to and from a hydraulic bladder, the pump transfers ocean water to and from the MOF absorption regions.
With all that has been said, an example description of how such a device would operate is given below:
The device consists of four units, one main unit and three wing units. The main unit is a sealed rectangle constructed out of titanium or some other non-corrosive metal, which is airtight and houses all of the electronics that issue the commands to facilitate autonomy. The electronics in the main unit are powered by either a lithium-ion battery or a series of solar cells that are positioned on the top of the main unit.
The wing units are attached to the main unit in a way that forms a tripod base structure to aid stability and uniformity of shape when on the surface and sinking and are connected to the main unit through ascending sealed pipes/tubes. The volume of each wing unit is approximately 30%-90% the size of the main unit, the size is dependent on how much space in the main unit is required for the necessary electronics, with a spherical bottom and rectangular top and a centralized ascending pipe sealed by mechanical valves ascending from the spherical bottom. Spherical bottoms are used because the conical shape further aids stability and buoyancy. Behind the valves are grated sieves covering the pipes, which allow for the influx of water, but not elements of significant size like various forms of marine life.
A wing unit has a secondary compartment containing the absorption material that can be sealed off from the main portion of the wing unit. The material, which is MOF-177 in this example, is lined on all of the sidewalls of the rectangular portion of the wing unit. As the water fills the wing unit it will come into contact with the MOF-177. Test results demonstrate that MOF-177 interacts better with CO2 as pressure increases to about 30-40 bars.1,6 However, if such a pressure increase proves to be too complicated or too detrimental within the device (there is a sufficient probability that it may) MOF-177 can still recover CO2 at atmospheric pressure although at a very significant efficiency loss. The external pressure during submersion could aid in the reaction process, but the extent of that aid is unclear if even significant.
Fortunately, the repetitive action of the device should compensate for this efficiency loss. Heating units are sandwiched between the inner wall of the wing unit and an outer wall, which shields the units from the outer environment. These heating units increase the temperature of the inner wall up to at least 70 C to facilitate separation of the CO2 from the MOF. The heating units will automatically shutoff after a preset time determined through empirical study.
The newly freed gaseous CO2 will then be moved to a storage unit attached to the top of each wing unit. These storage units will have a sensor reporting to a base station when the unit is full and will also be detachable so that recovery crews can remove the collected CO2 and transfer it to a storage unit on the recovery ship. The storage unit will then be reattached to the device and the device can be reinitialized. If such a design proves too cumbersome, there is the possibility of storing the CO2 in the main unit with the electrical equipment, but a minor concern of long-term corrosive damage would need to be addressed.
A summary of the lifecycle of the device:
- The device is placed in the water at a point of interest for ocean acidity reduction where it floats/bobs like a buoy on the surface
- after an initial acclimation time the valves in at the bottom of the wing units open and the corresponding pumps activate increasing the uptake of water and the mass of the device causing it to sink - during the uptake of water and the descent of the device the pressure of the water within each wing unit increases to increase the efficiency of the interaction rate between the material and the CO2
- once the carrying capacity of the wing units is reached (identified by a sensor), the pumps reverse action and push the water out of the wing units resulting in the device ascending back to the surface
- once on the surface heating units opposite the material activate separating the CO2 from the material
- after a pre-determined time the heating units turn off, triggering activation of vacuums transferring the free gaseous CO2 from the wing units to the storage units
- the storage units are sealed and the process begins anew.
1. Walton, Krista, et, Al. “Understanding Inflections and Steps in Carbon Dioxide Adsorption Isotherms in Metal-Organic Frameworks.” Journal of American Chemical Society. 2008. 130: 406-407.
2. Long, Jeffrey, and Yaghi, Omar. “The pervasive chemistry of metal–organic frameworks.” Chemical Society Reviews. 2009. 38: 1213-1214.
3. Voosen, Paul. “New Material Could Vastly Improve Carbon Capture.” Scientific American Online. June 30, 2009. http://www.scientificamerican.com/article.cfm?id=metal-organic-frameworks-carbon-capture
4. Buckingham, A, and Disch, R. “The Quadrupole Moment of the Carbon Dioxide Molecule.” Proceedings of the Royal Society of London. Mathematical and Physical Sciences. 273(1353): 275-289.
5. Xu, Ruren, Chen, Jiesheng, Gao, Zi, and Yan, Wenfu. From Zeolites to Porous MOF Materials. The 40th Anniversary of International Zeolite Conference. Vol. 170. 2009.
6. Furukawa, Hiroyasu, and Yaghi, Omar. “Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications.” Journal of American Chemical Society. 2009. 131: 8875-8883.