Over five years ago I discussed the issue that addressing global warming would involve both the reduction of new human based releases of carbon dioxide (CO2) into the atmosphere (carbon mitigation) and developing a method of increasing the rate of removal of already existing CO2 in the atmosphere either spurred through natural and/or technological means (carbon remediation). This dual requirement is born from the inability of nature to currently manage existing and future CO2 levels to ensure the maintenance of a viable environment to accommodate both the existing global human population and any increases that are seen in the near-future.
For both carbon mitigation and remediation two elements take precedence: effectiveness and speed. Effectiveness is rather self-explanatory; if the applied strategies are unable to reduce the release of new CO2 concentrations and remove more CO2 from the air versus what is added over the life-cycle of the remediation processes then such strategies are not worth exploring. Speed is necessary because there is already a dangerous amount of CO2 in the atmosphere and the rate of carbon mitigation is not proceeding nearly fast enough relative to the capacity of natural sinks to remove CO2. Basically with each passing year the total concentration of CO2 in the atmosphere is increasing not decreasing and based on current mitigation patterns this reality is not going to change in the near future. Note while both mitigation and remediation are important, the remainder of this discussion will focus on remediation.
With the idea of speed in mind, while there are more cost-effective (i.e. more economically attractive) remediation strategies available, largely those involving planting trees or synthesizing bio-char, these methods are significantly slower than various technological methods. In addition to the issue of speed, the efficiency of natural methods like planting trees could be called into question for there is the potential for natural sinks to decline in overall CO2 capacity between less CO2 absorption from trees, a more acidic ocean beginning to out-gas due to changes in the concentration gradient or even decreased levels of material weathering.
Even if there was no threat of lost absorption capacity from natural sinks, it is difficult to conclude that natural sinks will be able to remove enough CO2 from the atmosphere, even in a scenario of rapid emission reduction due to the already existing concentration, before the occurrence of serious negative environmental outcomes. Therefore, while it may not be a popular notion for some environmentalists and some economists, the simple reality is that technology will have to be at the forefront of removing existing CO2 from the atmosphere leaving nature to play more of an auxiliary role.
Of the two major strategies for large-scale carbon remediation, direct air capture and ocean fertilization, initial tests with ocean fertilization have not been positive. While the initial theory is solid, in practice the increased phytoplankton concentrations have been unable to demonstrate any real gains in CO2 removal, largely due to increased predation from zooplankton.1 These complications have soured the chief advantage of ocean fertilization, simplicity, leaving direct air capture as the theoretical best strategy for carbon remediation.
To ensure clarity, the term “Direct Air Capture” is being interpreted as: the technological removal of atmospheric CO2 from a non-point source (versus a point source which would be a power plant or automobile) by reacting atmospheric CO2 with a sorbent (usually an alkaline NaOH solution). This reaction with the sorbent typically forms sodium carbonate and water. The carbonate then reacts with calcium hydroxide (Ca(OH)2)) resulting in the generation of calcite (CaCO3) and reformation of the sodium hydroxide. The process of causticization transfers a vast majority of the carbonate ions (»94-95%) from the sodium to the calcium cation and the calcium carbonate precipitate is thermally decomposed to regenerate the previously absorbed gaseous CO2. The final step involves thermal decomposition of the calcite in the presence of oxygen along with the hydration of lime (CaO) to recycle the calcium hydroxide.2,3 Obviously some of the details can differ depending on the type of sorbent utilized and other side elements of the process, but the above description entails the general chemical operation of direct air capture.
Obviously direct air capture is not without its own challenges mostly due to the incredibly small concentration of CO2 in the atmosphere for while 400+ parts per million (ppm) is very significant from an environmental standpoint, it is clearly not a large amount from a chemical reaction standpoint. This CO2 “deficiency” is largely responsible for the significant costs associated with CO2 removal via direct air capture, which have been estimated at a cost floor of $300 per ton of CO2 (which is optimistic in isolation) to a $1200+ per ton of CO2 ceiling (which is rather pessimistic).4 However, regardless of these potential costs it does appear that whatever the actual costs, it is one that humanity will have to foot the bill for if it wants to maximize its probability of surviving from a societal standpoint into the near future.
There are three major issues surrounding the proper functionality of the process of direct air capture not involving the specifics of the direct process of capturing the CO2: power use, water use, and end destination of the absorbed CO2. Not surprisingly each of these issues must be addressed to optimize the overall process of CO2 removal from the atmosphere and maximize its overall economics.
The consideration of the power source is important relative to speed and efficiency regarding the total net CO2 captured and removed from the atmosphere. For example, if a trace emission source is utilized (nuclear, geothermal, wind or solar) then the process can be reasonably estimated as 90-99% efficient (10-100 tons of CO2 will be captured and removed for every 1 ton of CO2 used to power the process). With this estimate the net cost per ton will be 1.01-1.1 times more than the gross cost relative to the power use component. However, if a fossil fuel source is utilized then, largely dependent on the exact fuel mix, the process will be 50-70% efficient and the net cost will be about 1.3-1.5 times larger than the gross estimated cost for the power use component.
Obviously due to this significant efficiency disparity the utilization of a trace emission source for the process is imperative, but which process is most appropriate? Speed is the most important element in the removal process because of the existing and future damage to the environment, something that money really cannot replace so the process must operate as close to 24 hours a day 7 days a week as possible. This requirement heavily limits the viability of using wind or solar as the energy medium, thus leaving two principal contenders: geothermal and nuclear.
Now while one could attempt to argue that wind or solar could work with the appropriate level of storage as backup, such an argument does not sit on solid ground with the existing lack of storage options and the empirical track record of such a design. While small pilot plans exist and have received flashy headlines and hype, the output of these plants is basically irrelevant to any expected energy requirements for air capture. Also recall that energy can only stored if it is in excess, which will not be true most of the time, for the solar and/or wind elements are already providing energy to various elements associated with the capture process. Pumped hydro shares the same problem, as well as limiting the location for the process because of its required topography.
In the past geothermal was thought to be the better choice over nuclear largely due to any potential nuclear waste issues associated with nuclear power, with enhanced geothermal systems (EGS) being the preferred geothermal methodology. Note that the issue of safety regarding nuclear power has long been a foolish reason to oppose it for safety issues only arise when the operator (be it government or corporation) is allowed to cut corners and/or does not adhere to proper and standard safety operating procedures.
Unfortunately, there has been few rigorous studies concerning EGS especially relative to any expansion of seismic activity pertaining to its application. In short the EGS process can produce an environment that increases seismic activity of low Richter scale earthquakes (the occurrence of 2 to 3 scale quakes appear to increase in probability). However, unlike fracking, which increase both earthquake probability and severity, little is known regarding whether EGS will increase earthquake severity (from 2 or 3 to 4+). This uncertainty, which could have been and should have been studied in earnest years ago, makes it difficult to support going forward with EGS. Thus, nuclear becomes the better choice with at least a generation 2 design as the standard in order to limit or outright eliminate resultant waste or one could utilize a small modular unit design.
Water utilization is also an important issue for regardless of the system, the chemical reaction involved in the absorption of CO2 from the atmosphere requires water, commonly as a catalyst. However, despite the general nature of a catalyst (lack of consumption at the conclusion of the reaction) the open-air nature of the reaction system results in a significant percentage of the utilized water being lost to the atmosphere as water vapor making inherent water recovery within the process itself more difficult. Therefore, there are two important questions involving water use in the process: 1) How will the initial amount of water for beginning the process be procured? 2) How will atmospheric water losses be minimized?
The best solution for obtaining the required starting water is from desalination, which is suitable because direct air capture units can be built almost anywhere due to the natural mixing of the atmosphere maintaining relatively constant global CO2 concentrations over the long-term. Regarding the question of how atmospheric water losses will be minimized there are two potential strategies. First, the use of properly placed atmospheric condensers could recover a significant portion of the lost water and recycle it back into the beginning of the process. Second, depending on the economic and environmentally efficiency of the desalination process, there may be no need for any type of recycling, instead drawing all required water from desalination including that which is lost.
However, this method is inherently risky because of the potential detriments associated with desalination and any potential issues involving the hydrological cycle due to the new levels of water evaporation from the direct air capture process. Overall the better option appears to initially provide water via desalination and allow further desalination to fill-in any gaps in recycling missed by the water condensers. Fortunately, either option seems valid from an energy standpoint with the nearby nuclear reactor powering the direct air capture devices.
The infrastructure to transport water needs to be considered both from an efficiency and economic standpoint. The two most viable methods for the initial water application would be constructing a piping infrastructure to transport the desalinated water to the direct air capture units or simply using transport vehicles, like large trucks, to move the water to the direct air capture units. An important element to determining which method is best involves the rate of recycling from any water atmospheric collectors near the direct air capture units. The more water recycled the more attractive a less permanent infrastructure appears (trucks) due to the lower overall capital and even maintenance costs. However, while theory is fine, the overall scale requirements of the operation may require a more permanent source of water due to the sheer amount of water required regardless of recycling.
Also an important consideration is what to do with desalination byproducts, mostly the removed salt, some of the chemicals in the desalination process and the possibility of certain contaminants from pipe and process breakdown (copper, iron, zinc, etc.). At the moment many desalination plants dispose the brine in the ocean or a closed watercourse through a direct disposal strategy sometimes involving salinity concentration reduction by discharging the brine with wastewater or a cooling stream from a power plant.
Obviously there is concern about releasing a stream of heavily concentrated brine into the ocean for it can produce both eutrophication and significant pH changes creating problems for the local flora and fauna.5 Other common management strategies include minimization or direct reuse.5 Minimization commonly involves membrane or thermal methods whereas reuse involves recovering salts from the waste brine via crystallization or evaporative cooling and utilizing that salt for other processes or goods.5
While some are high on the idea of selling salt to offset the operation of a desalination plant such an idea seems optimistic due to the overall expected scale of the operation. Some have proposed ammoniating the brine and using it to increase the volume of CO2 capture.5 The concern with that strategy is providing the necessary ammonium to react with the brine to create a consistent and worthwhile process. Another option that has been floated is incorporating the brine into a set of molten salts that would be used in either nuclear power reactors or batteries. However, the viability of such an idea is still questionable.
Desalination is not the only aspect of the process that produces a byproduct. The more important environmental byproduct is obviously the CO2 that is extracted from the atmosphere. The most important aspect of this absorption is what process will be utilized to ensure that the newly capture CO2 is not reintroduced into the environment? Some of the more desired solutions involve dreaming of using the captured CO2 as an economic product within enhanced oil recovery processes, as a means to producing a methane or hydrocarbon based fuel for vehicles or a marketed product in a commercial industry (soda, etc.).
Unfortunately, those first two options return the captured CO2 back to the atmosphere at some percentage, which limits the overall efficiency of the CO2 absorption, increasing overall costs and decreasing the speed of net removal. Also the commercial option will not provide sufficient funds to the operation of the process. While this reality eliminates the idea that commercial product distribution can carry the finances of the process, tapping into commercial process should still be worthwhile as a means to eliminate a very minor portion of the captured CO2.
Another method to remove atmospheric carbon gaining in popularity is the use of bio-char. In essence bio-char is black carbon synthesized through pyrolysis of biomass. Bio-char is effective because it is believed to be a very stable means of retaining carbon, sequestering it for hundreds to thousands of years. Depositing the captured CO2 into one-sided greenhouses could be another method to disposing of some of the captured CO2 then turning the grown flora into bio-char would remove the CO2. While a possibility, again the scale of absorbed CO2 limits the total value of this process.
A new method for potentially removing CO2 is utilizing it in an electrolytic conversion to create molten carbonates and later converting those carbonates into Carbon Nanofibers and potentially later even Carbon Nanotubes.6 While this process has yet to be scaled to what would be classified as commercial levels, it does demonstrate some level of promise. The versatility and usefulness of carbon nanotubes or fibers have more commercial value than pure CO2 as a commercial product. However, similar to the other potential options listed above, it is difficult to presume that most of the captured CO2 will be eliminated via this process.
Mineral sequestration via olivine, serpentine or wollastonite has drawn attention as a possible avenue for CO2 “storage”. However, this strategy does not appear economically or rationally viable for natural weathering is too slow and technologically induced weathering, by grinding down these materials to dramatically increase available surface area, is emission inefficient and costly. So despite some of these more flashy or “economic” choices, overall it is reasonable to suggest that a majority of the captured CO2 will be stored long-term in underground rock formations.
With all of these additional considerations to take into account it does not appear wise to simply build these air capture units at random. These units clearly need to be constructed in an orderly and cohesive manner, perhaps even in a localized autonomous network. This network needs to contain a water source, a power source and a means of utilizing the captured CO2 in addition to having recycling pathways for all necessary materials used in the selected air capture reactions.
Overall it is also important to understand that one should not attempt to portray this type of above complex or even direct air capture in general as some new budding industry that will produce a profit. While certain elements will provide some form of revenue, envisioning a new profitable industry does not appear appropriate at this time. So if profitability is not viable, what is the economic argument for direct air capture? The response is adjusting how one looks at the economic issue. The economics of direct air capture and any resulting complex is not profitability, but prevention and to some extent, survivability.
For example, Person A does not eat broccoli on a regular basis because he is paid a sum of money by Person B to do so, but instead consumes broccoli because it is a healthy food and there is reason to believe that the consistent consumption of broccoli will result in a reduced probability of various diseases and ailments in the future relative to a person who does not consume broccoli (all other elements being accounted for). Therefore, the economic benefit for consuming broccoli is derived from lower future costs associated with healthcare and perhaps a reduction in lost wages due to less work missed versus immediate short-term incentive/reward.
No reasonable person disputes the fact that global warming will increase the probability and severity of future extreme weather events in addition to producing detrimental changes in general climate and weather patterns. These changes will produce significant levels of environmental and economic damage and will eventually threaten the very viability of human society. Therefore, a reasonable person would come to the conclusion that it is important to lessen the detrimental impacts of global warming as much as possible. Such a reduction would also result in the savings of billions of dollars in the short-term (10-20 years from now) and trillions of dollars in the long-term (20-50 years from now). Therefore, similar to the broccoli example, the prevention model is how people should look at direct air capture versus attempting to inappropriately sell it as some form of short-term “money-making” venture. The “profitability” comes from the money saved in the future by reducing the probability of detrimental outcomes associated with global warming.
With this mindset, how would such projects be funded? It is difficult to see venture capitalists getting involved because most only have a nose for eventual profits and as discussed above, this project will not produce profits in that manner. Ironically the only venture capitalists that might get involved are those who are very young and/or have large stock holdings in insurance companies. In a just world every major corporation in the world would have to pay into some form of “carbon remediation and mitigation” fund as a form of restitution for championing a carbon heavy global economy. Money from this fund would then be used to fund direct air capture in addition to other direct CO2 mitigation projects. One could argue that the funds procured from a carbon tax would also serve this purpose.
Unfortunately, the likelihood of such a program where corporations foot a lot of the bill is unlikely for it is difficult to envision most multi-national corporations agreeing to fund such a program; most companies typically do not do something unless profit is available, which here it is not, or if government is footing the bill. Therefore, it appears that various world governments will have to foot the bill. With that said what governments should go first so to speak: Well the United States is definitely a candidate as it is responsible for the most cumulative CO2 out of any other country. China is in a very close second being responsible for the most CO2 in the last few decades in addition to choosing coal and oil to grow their economy without taking into consideration the environmental realities of that choice when nuclear, wind, solar and/or geothermal were also valid, albeit slower, choices. However, in the end such funding would have to be worked out by international treaty, which does not lend much confidence when considering the success of past international environmental based treaties.
In the end, it is understandable that if the economic cost of developing an air capture complex of sorts was quantitatively calculated that it would be high; however, the nature of the complex is that all of these elements will be required in the future based on the current environmental-use path humans have embarked upon with regards to expelling CO2 into the atmosphere, thus the cost is not based on luxury, but necessity. The idea behind such a complex for direct air capture is to lower overall net costs by tying many of the air capture units into the same required operational elements, thus making the direct air capture strategy more economical on an overall scale; saving money for investment in other environmentally necessary avenues like emission reduction. Overall while the manifestation of such a complex may not be exactly as described in this blog post, the reality is that as it current stands such a complex will be needed in one form or another.
1. "Lohafex project provides new insights on plankton ecology: Only small amounts of atmospheric carbon dioxide fixed." International Polar Year. March 23, 2009.
2. Zeman, Frank. “Energy and Material Balance of CO2 Capture from Ambient Air.” Environ. Sci. Technol. 2007. 41(21): 7558-7563.
3. Perez, E, et Al. “Direct Capture of CO2 from Ambient Air.” Chem. Rev. 2016. 116:11840-11876
4. American Physical Society. Direct Air Capture of CO2 with Chemicals: A Technology Assesment for the APS Panel on Public A?airs; APS: 2011.
5. Giwa, A, et Al. “Brine Management Methods: Recent Innovations and Current Status.” Desalination. 2017. 407:1-23.
6. Ren, J, et Al. “One-Pot Synthesis of Carbon Nanofibers from CO2.” Nano Lett. 2015. 15:6142-6148.