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Sat Sep 26, 2020, 11:26 PM

Screening Study of Different Amine-Based Solutions as Sorbents for Direct CO2 Capture from Air

The paper I'll discuss in this post is this one: Screening Study of Different Amine-Based Solutions as Sorbents for Direct CO2 Capture from Air (Francesco Barzagli, Claudia Giorgi, Fabrizio Mani, and Maurizio Peruzzini ACS Sustainable Chemistry & Engineering 2020 8 (37), 14013-14021).

Let me start this commentary by repeating myself: We will be damned for all time in history for leaving future generations the task of picking through our garbage dumps to survive. We will not be forgiven and we should not be forgiven.

Of course, we already have people picking through landfills to survive, but in my view, the most egregious dump of them all is precisely the one which almost no higher living thing can escape, our atmosphere.

Some years back, there was a moderately prominent energy website on the internet - it apparently operated from 2005 to 2013 -The Oil Drum which was built around the idea advanced by James Kunstler a journalist, once at Rolling Stone, in his book, The Long Emergency, that the world was experiencing "Peak Oil" and that we were all going to die when oil ran out.

(I could offer my standard joke that one cannot get a degree in journalism if one has passed a college level science course, but it appears that Kunstler does not have a degree in journalism; and certainly doesn't have one in a scientific discipline either.)

Personally, although I was certainly known to ridicule Kunstler despite that he was inexplicably popular among many of us on the left - the same people who opposed the two Iraq wars which were about claims of the essential nature of petroleum, also embraced Kunstler's fetishizing that pernicious substance - but I wish he'd been partly right, that oil was running out, if not about everyone dying without it. Regrettably it hasn't run out, even though the destruction we wrought to get at it is increasingly odious.

As of 2018, according to the 2019 Edition of the World Energy Outlook, dangerous petroleum was the largest single source of primary energy on this planet, producing 184.34 exajoules of energy out of 599.34 exajoules. It was the third fastest growing source of energy in the 21st century, after dangerous coal and dangerous natural gas; together they made up 81% of the world energy supply in 2018, as compared to 80% in the year 2000.

Things are getting worse, not better, but thank you Germany for pretending to care, even if pretending to care has been expressed by an embrace of stupidity. You're excused Germany, inasmuch as we live in the age of stupidity, and the stupidity of the German Energy Policy is simply an embrace of our times.

Eventually though, irrespective of the fate of Kunstler's mentality over the short term, the world will run out of oil, at least if we don't drown in its waste. I personally hope it is sooner rather than later.

This said, if we are ever to have any hope of reaching human development goals, which were first succinctly codified in Article 25, section 1 of the largely ignored 1948 Universal Declaration of Human Rights, an industrial society will require sources of carbon for essential chemicals and materials. Even though we live in the pyritic age of stupidity, we also live in the Golden Age of Chemistry, and an obvious source for carbon, the source in fact utilized by living things, is the otherwise dangerous fossil fuel waste carbon dioxide.

This paper is about the much discussed concept of "Direct Air Capture," often abbreviated in the scientific literature as "DAC," of carbon dioxide. This is an energetically expensive proposition, because in a purely thermodynamic sense, one must overcome the Entropy of Mixing, said entropy having contributed to the dubious embrace of dangerous fossil fuels by providing an efficiency kick. Sophisticated arguments have been advanced about why it might work; other sophisticated arguments have been advanced stating why it won't work. I come in on the side of saying it is feasible, not easy, but feasible, but only if no carbon dependent energy source (with the possible exception to a limited extent of bioenergy) is utilized to address overcoming the entropy that we, and all generations before us beginning in the 19th century, have dumped on future generations. From my perspective it is obviously feasible, since plants and algae do it all the time, albeit from the agency of providing a huge surface area via the self replicating function of life.

I personally think that a better industrial choice for capturing carbon dioxide from the air is indirect air capture, utilizing seawater, but that's another topic entirely.

Even I concede however that under limited circumstances, there are circumstances under which direct air capture might be viable, as a side product.

This involves my view of the wisest approach to what I'll call - since it involves a massive electrical circuit, the grid - capacitance, although I'm not a fan of the sometimes discussed idea of massive "super capacitors," designed to store electricity on a grand scale in the same way as it is stored, for example, in cell phones, or TV's in a short term fashion.

Capacitance is a refined word for energy storage. Energy storage is widely discussed as a scheme to make so called "renewable energy" a practical source of energy, by throwing good money after bad: So called "renewable energy" is an expensive failure, and attempts to store it to make its availability fit better into energy demand are misguided because they will certainly fail, just as so called "renewable energy" has failed to address climate change, particularly because what would be required would be the storage of electrical energy for a very long time in many circumstances. The mass requirements of doing so, and the toxicological and carbon associated with accumulating that mass, would surely be incredibly destructive and expensive.

Nevertheless, on an electrical grid, short term capacitance is a necessary feature. Here is the CAISO graphic for electricity demand in California during the recent extreme heat wave, accessed on September 6, 2020 at 3:05 pm Pacific Coast Daylight time:



Note that the distance between the forecasted peak power on that date, 45,168 MW, and the minimum at around 6:45 am on the same date, looks to be, from the graph, about 26,000 MW is roughly 20,000 MW. There are two ways to address this discrepancy, one being to build redundant power plants to cover these exigencies. This is extremely wasteful and therefore environmentally and economically unattractive, and it represents the reason that the highest electricity prices in the OECD are found in Germany and in Denmark. The other is capacitance, but this need not - in my opinion should not - involve the storage of electricity itself either in batteries or in massive super capacitors since this approach will clearly be environmentally odious. A better option would be to store the energy as heat, as in a phase change material, or as compressed air, or perhaps both.

For the purposes of this discussion, I will only discuss compressed air. Compressing air generates heat according to - on the simplest level - Charles Law, although vastly more sophisticated gas laws are obviously well known and widely used. It follows that gases cool when they expand adiabatically, that is, without heat being added. However, if one adds heat, in particular waste heat, one can under the right circumstances increase the exergy derived from the heat, where exergy is the usable energy extracted from the system.

If the air is compressed over a solution containing a carbon capture agent, similar to the amines discussed here, or - more to my personal liking - metal hydroxides, one can remove carbon dioxide from the air as a side product of the effort.

Another possibility is to use air as the working fluid in a Brayton cycle, during which the air is continuously cycled over carbon capture agents. This is certainly possible; all jet engines are Brayton cycle heat engines, and all use air as the working fluid.

If the air is superheated after compression, say to temperatures approaching 1000° or even higher, this will have the effect of combusting the greenhouse gas methane as well as carbon particulate matter, the latter a serious health risk, the former a potent greenhouse gas. If the heat transfer medium is highly radioactive it will have the effect of destroying the ozone depleting greenhouse gas nitrous oxide, residual CFC's, HFC's, sulfur hexaflouride, carbon tetrafluoride.

Although unlike the hyped up energy charlatan Amory Lovins, I am aware of Jevon's Paradox, I still think that high efficiency is desirable, particularly if we consider human development goals of justice and opportunity and health for all of humanity, not just those of us who live in wealthy countries. A very high temperature Brayton cycle, or a series of them, coupled to a Rankine cycle and perhaps even a Stirling cycle offers a number of opportunities, including the opportunity of providing sensible heat for chemical processing and, in fact, carbon dioxide recovery and reduction into useful products.

From the paper's introduction:

The recent climate conference COP21 (Paris, 2015) underlined the need to take actions by most of the world’s countries to mitigate climate change and keep the global temperature rise well below 2 °C above preindustrial levels.(1) In addition to the reduction of the combustion of fossil fuels and the improvement of the CO2 capture from large-point sources, the so-called carbon capture and sequestration (CCS) technology,(2,3) a strategy that is emerging as crucial for achieving the ambitious Paris’ target, is the development of negative emission technologies (NETs).(4) NETs relate to CO2 removal from the atmosphere through techniques such as the chemical CO2 capture from ambient air, called direct air capture (DAC).(5) DAC is a developing technology with the potential to contrast the dispersed emissions coming from transport and residential heating, which cannot be captured at their sparse sources and represent approximately half of the annual anthropogenic CO2 emissions.(6,7) In the DAC process, large air-absorbent contactors equipped with many fans blow the air to the absorber, where the ultradiluted CO2 (approximately 410 ppm) is selectively removed and the “clean” air is returned to the atmosphere. Afterward, the sorbent is regenerated and the captured CO2 is released for disposal or, more interestingly, for direct utilization, as, for example, in the catalytic methanation.(8) Moreover, DAC systems benefit from their inherent flexibility of placement, and careful location planning can favor the use of renewable energy and can reduce the cost of CO2 transportation from the capture site to the storage or utilization sites.(9) An ideal DAC process should combine a quick and efficient CO2 capture with low-energy inputs for air handling, sorbent regeneration, and CO2 release. Although DAC processes were considered prohibitively expensive until a few years ago, with costs in the range 200–1000 $/ton of CO2 (10 times higher than conventional capture from flue gas), the most recent economic analyses suggest that with the latest improvements (mainly engineering) the DAC technology is approaching commercial viability, with capturing costs that can be reduced to less than 100 $/ton of CO2.(10−12) In particular, several studies demonstrated that an air–liquid cross-flow scheme, which reduces the pressure drop, can dramatically lower the capture cost...(9)


The "approximately 410 ppm" remark is bitterly amusing to anyone who pays attention to carbon dioxide concentrations in the air. I am certainly such a person, as I monitor these levels closely on a weekly basis. I note that it was only a few years ago that scientific papers were talking about "approximately 390 ppm."

Depending on this year's carbon dioxide minimum, which will probably occur this week, measured at Mauna Loa we may never see a level as low as 410 ppm again, so dramatic is our failure to address climate change. In the last 52 weeks, going back to the week beginning of the week of September 29, there have been six where the concentration at Mauna Loa was lower than 410 ppm. That week, represented the annual minimum. We have not, as of this year, seen a value as low as 410 ppm: The last data point, the week beginning September 20, 2020, reported a concentration of 411.27 ppm. If values fall this year to 410 ppm - I doubt they will - it will be the last time in the lifetime of anyone now living that it will do so. (This year's maximum was 417.43 ppm, measured in the week beginning May 24, 2020, during worldwide Covid shutdowns.)

So there's that.

Later the introduction continues:

So far, the main potential technologies involve chemisorbent materials; (6) in particular, many researchers have focused on the development of solid-based sorbent systems, especially immobilized amine/silica sorbents or hollow fiber sorbents.(13−18) Alkaline liquid sorbents have also been taken into consideration for their fast and efficient CO2 capture in continuous (not batch) processes; (6) however, their development has so far been limited due to the high costs of regeneration. Aqueous solutions of sodium and potassium hydroxide have been extensively studied as sorbents for DAC processes for their strong alkalinity and their high reaction rate even with ultradiluted CO2.(19,20) Despite a good capture efficiency, the process is energy intensive: the sorbent regeneration is based on the formation of CaCO3 by adding Ca(OH)2, and the subsequent calcination of CaCO3 to release pure CO2 requires very high temperatures (900 °C), which entail high energy costs, up to 180 kJ/mol CO2.(9,10,19)

With the aim of developing new liquid sorbents for the efficient capture of ultradiluted aerial CO2 with a lower regeneration energy compared to KOH and NaOH solutions, we decided to investigate the performance of several amine-based sorbents in DAC systems. Aqueous amines are well-known (and widely investigated) sorbents for the efficient CO2 capture from large-scale emission points (CO2 12–15% v/v), which can be regenerated at T = 100–120 °C, a temperature well below that required for the CaCO3 calcination.(21,22) Currently, many researchers are working to develop innovative amine-based absorbents able to combine the most efficient CO2 capture with the lowest heat of CO2 desorption,(23−26) an important parameter for assessing the regeneration energy (the opposite of the heat of CO2 absorption, usually lower than 90 kJ/mol CO2 for all of the most studied aqueous amines).(22,27)


For various reasons, I'm not quite sanguine about giving up on alkali metal hydroxides, although - despite it's limited availability - for various reasons I won't discuss presently, I favor cesium or at least rubidium hydroxides. I note that alkali hydroxides can be made into continuous systems by the expedient of drizzling in saturated solutions of group 2 hydroxides, those of calcium, strontium or barium. The authors are nonetheless focused on reducing the regeneration heat and energy required, as they state above, and study various amines.

The amines tested and their structures are shown in a table in the text:




Here is a photograph of their equipment, accompanied by a schematic:



The caption:

Figure 1. Apparatus for the determination of the percentage of CO2 absorbed and its schematic flow diagram. Blue lines refer to air and black lines to the liquid sorbent.


Carbon dioxide captured by amines, including the commercial carbon capture amine, monoethanolamine, is generally in the form of carbamates, structures in which a carbon dioxide is loosely bound to a amine nitrogen.

Some tables of results:




The formation of carbamates tracked by NMR:



The caption:

Figure 2. 13C NMR spectra of aqueous MEA, 2A1B, AMP, and AMPD at the end of the absorption experiment. The numbers indicate the carbon atoms referred to both free and protonated amine fast exchanging in the NMR scale. Asterisks denote the chemical shifts of carbon backbones of amine carbamate. C indicates the carbonyl atoms of amine carbamate, while b/c refers to the signal of fast exchanging bicarbonate/carbonate ions. The intensity of the signals at 163–167 ppm is not in scale.


The authors explore the use of non-aqueous solvents. This table gives results.




In some cases the carbamates react with the alcoholic functions in the nonaqueous solvents having them to produce alkylcarbonates by the proposed mechanism:



The caption:

Figure 4. Scheme of the proposed two-step reaction mechanism for the formation of alkyl carbonate in nonaqueous EMEA solutions, including (A) the initial formation of the carbamate of the amine and (B) its subsequent reaction with an alcohol.


Excerpts from the conclusion:

With the aim of identifying the most crucial chemical peculiarities for the development of new liquid absorbents for DAC processes, we carried out a screening study on the performance of different aqueous alkanolamine solutions, under the same operating conditions: their ability to absorb CO2 from an air stream was correlated with their chemical structure and with the species formed by the absorption reaction, and useful information on the reaction mechanism has been obtained. As a general finding, aqueous unhindered primary amines are the most suitable sorbents for DAC processes, as they are as efficient as aqueous alkali hydroxides but with a potential energy saving due to the lower temperatures required for sorbent regeneration. The formation of a high yield of amine carbamate seems to be the decisive factor for an efficient CO2 capture, but the formation of an appreciable amount of carbonate/bicarbonate because of the strong basicity of some amines (EMEA, BUMEA) can contribute to attain a high percentage of CO2 absorbed. The amines that are unable to form carbamate have provided poor absorption values...

...These findings highlighted the differences of DAC processes compared to conventional CCS processes and, consequently, the best CCS absorbents cannot be the best choice for the DAC process. The obtained results also showed that aqueous amines are more efficient than the same amines in organic diluents. MEA and DGA in EG/PrOH display slightly lower abs% compared to the aqueous solution by virtue of the high percentage of carbamate formed...


This is a fine paper; I like it, although I'm not sure I agree with the idea of amine carbon capture reagents, in particular because in the case of a commercial example MEA, monoethylamine, the stability of the amine proves to be a long term problem. Another is the recognition that the air is hardly clean, and besides the formation of sulfates, there is a considerable amount of nitrogen oxides in the air. A recently discovered problem in the pharmaceutical and, albeit to a lesser extent, the food industries is the formation of highly carcinogenic and genotoxic nitrosoamines. At the scale of air capture - we're talking billions of tons here - this may be problematic for these amines, even if they are designed to be used in closed systems.

I wish you a pleasant and safe Sunday.

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