Science

NNadir

(33,511 posts) Mon Dec 30, 2019, 04:07 PM Dec 2019

Direct Air Capture of CO2 with Aqueous Amino Acids and BIG (bis-aminoguanidines).

The paper I'll discuss in this post is this one: Direct Air Capture of CO2 with Aqueous Amino Acids and Solid Bis-iminoguanidines (BIGs) (Radu Custelcean,* Neil J. Williams, Kathleen A. Garrabrant, Pierrick Agullo, Flavien M. Brethomé, Halie J. Martin, and Michelle K. Kidder Ind. Eng. Chem. Res. 2019, 58, 51, 23338-23346).

The authors work at Oak Ridge National Laboratory.

The figure for concentrations of the dangerous fossil fuel waste carbon dioxide's concentration in the planetary atmosphere, as reported yesterday at the Mauna Loa CO2 Observatory are as follows:

Up-to-date weekly average CO2 at Mauna Loa:

Week beginning on December 22, 2019: 412.21 ppm
Weekly value from 1 year ago: 409.24 ppm
Weekly value from 10 years ago: 388.17 ppm
Last updated: December 30, 2019

This concentration is thus 2.97 ppm higher than one year ago, and 24.04 ppm higher than ten years ago. The average rate of the accumulations of the dangerous fossil fuel waste carbon dioxide is thus, as represented by this particular figure, 2.4 ppm per year. On the week beginning on December 26, 1999, the figure comparing the carbon dioxide with that measured 10 years earlier, December 24, 1989, was 15.64 ppm higher, implying an average of 1.5 ppm per year at the end of the 20th century.

Things are getting worse, not better.

I have made pretty clear in my posts here, why I think this is, a refusal to "go nuclear" to address climate change coupled with an unwarranted belief that so called "renewable energy" would save the day. It didn't. It isn't saving the day. It won't save the day.

A smarter generation than the one to which I belong may therefore need to remove our waste from the atmosphere, where we have let it accumulate with severe and unconscionable indifference to reality, a willing to be impervious to facts, both on the political right and on the political left.

As I have approached the end of my life, I have worked to make myself familiar with possible approaches to addressing the very challenging issue of removing carbon dioxide from the atmosphere. Hence this paper which came up in my regular reading caught my eye.

I'll let some excerpts and graphics from the paper speak for themselves. Full access to the paper may be found at a good academic library or by subscription.

From the introduction:

With the projected increase in the world’s population, the global energy demand will continue to grow for decades to come. Given our continuing reliance on fossil fuels as a major source of energy, effective emission reductions through large-scale deployment of carbon capture and storage (CCS) technologies have become critical for mitigating climate change.(1) While CCS technologies have traditionally been implemented at point sources of CO2 emissions, such as coal- or gas-fired power plants, recent integrated assessment models have increasingly emphasized the need for negative emission technologies (NETs), that is, technologies that remove CO2 out of the atmosphere, to limit global warming below 2 °C by 2100.(2?4) NETs have a unique place among the various technological solutions to the climate change problem, as they provide the only means to cut past emissions and restore the atmospheric composition to an optimal level with respect to the CO2 concentration. Furthermore, NETs can capture the CO2 from dispersed emitters involved in transportation, which currently account for about 50% of the annual greenhouse gas emissions. Finally, when coupled with efficient methods to convert the CO2 removed from air into fuels using renewable energy sources, NETs have the potential to close the carbon cycle and generate carbon-neutral fuels.(5)

I oppose giant carbon dioxide waste dumps which appears under the rubric of carbon dioxide capture and storage (CCS), but am enthusiastic about carbon dioxide capture and utilization (CCU). Reference 5 is to a National Academy Press Monograph on the latter subject. Gaseous Carbon Waste Streams Utilization

The introduction continues:

Most existing DAC processes fall into one of the two categories, employing either aqueous solutions of strong alkaline bases (e.g., NaOH, KOH) or solid-supported amines for CO2 capture.(2,6,8?10) The aqueous alkaline DAC systems consist of two cycles.(11) In the first cycle (absorption), the aqueous base is contacted with air, so the hydroxide anions react with CO2 and convert it into sodium or potassium carbonate salts. In the second cycle (regeneration), the carbonate anions are reacted with Ca(OH)2 and precipitated as CaCO3, thereby regenerating the hydroxide base. The CaCO3 solid is then heated to 900 °C in a pure oxygen atmosphere to release the CO2 and regenerate the calcium oxide.(11) This regeneration process is very energy intensive, requiring 6–9 GJ/t CO2 and high-grade heat that needs to be supplied from a low-carbon source to make the overall process net carbon negative.(2,10) On the other hand, the solid-sorbent DAC approach employs primary or secondary amines on porous organic or inorganic supports, which are loaded by contacting with air at ambient conditions, then regenerated by applying heat, vacuum, or a combination of the two.(12?15) These systems typically require less energy than the aqueous bases (5–7 GJ/t CO2) and significantly lower temperatures (80–150 °C) that can be supplied from low-grade waste heat,(2,10) but they tend to have slower CO2 sorption kinetics compared to aqueous sorbents.(16) A remaining challenge with solid-supported amines is their tendency to thermally and chemically degrade with repeated capture/regeneration cycles, though polyimine and polyamine adsorbents with improved resistance to oxidation have been recently reported.(17,18) Water vapor condensation in the pores from the ambient air can also increase the energy demand for sorbent regeneration.(19) Another DAC approach has been developed using ammonium-based anion exchange resins as solid adsorbents, which involve a completely different mechanism of CO2 adsorption and release driven by swings in the ambient humidity rather than temperature changes.(20,21) While lower-energy requirements have been reported for this moisture-swing process, as the sorbent regeneration is done passively using ambient wind conditions, its performance is inherently weather-dependent.(21) Furthermore, as with other solid adsorbents, the kinetics of CO2 adsorption and desorption are relatively slow.(22)

The bold here is mine, and reflects the energy cost of capturing carbon dioxide from the air. Each year at current rates, humanity dumps about 35 billion tons of CO2 into the planetary atmosphere, with another 7 to 10 tons additional being added because of land use changes, such as those involved in the destruction of rain forests to develop palm oil plantations for the production of "renewable" biodiesel. If direct air capture requires an intermediate figure for the two processes, ignoring the carbon cost of constructing plants to accomplish this task, the capture of 35 billion tons of dumped waste, a single year's worth, the energy required would be about 245 exajoules of energy. For comparison purposes, world energy demand for all purposes was, according to the IEA 2019 World Energy Outlook, was 599.34 exajoules.

2019 Edition of the World Energy Outlook Table 1.1 Page 38] (I have converted MTOE in the original table to the SI unit exajoules in this text.)

Thus were we to begin capturing carbon dioxide from the air to reverse climate change, we would need to consume 845 exajoules of energy from carbon free sources. For comparison, all the world's solar, wind, geothermal and tidal energy produced, after more than half a century of increasingly mindless cheering, produced, as of 2018, according to the source immediately above, 12.27 exajoules.

Facts matter.

The authors have proposed a new and interesting scheme. They use amino acid salts, amino acids being the building blocks of proteins, to capture the carbon dioxide - biologically carbon dioxide is captured in plants using the amino acids lysine and/or arginine - and then regenerate the carbonated salts by passing the solution over an insoluble organic compound featuring two of the functional group that is found on the amino acid arginine, guanidyl groups.

The amino acids they use are the simplest proteogenic amino acid, glycine, and its N-methyl derivative known as sarcosine.

The scheme below shows how the system works, showing just glycine and not sarcosine:

The caption:

Scheme 1. DAC of CO2 by Absorption with Aqueous Amino Acids (Potassium Glycinate is Shown as a Representative Example) and (Bi)carbonate Formation, Followed by Crystallization with BIGs

This scheme shows how the "BIGs" were synthesized:

The caption:

Scheme 2. Synthesis of m-BBIG and the Interconversion between the Free Base and the Chloride and Carbonate Salts

Here is how the experiment was run:

Direct Air Capture Cycles
For the multicycle experiments, 2 L of 1 M Sarcosine and 1 M KOH were loaded with CO2 for 24 h using an Envion HumidiHeat humidifier. The solution level was kept at 2 L by pumping in freshwater using a minipump to compensate for the water evaporation. The solution pH was monitored in situ using a pH probe dipped into the reservoir. After 24 h, the loaded amino acid solution was transferred to a 4 L beaker and stirred with a mechanical stirrer set at 200 rpm. Solid m-BBIG (147.8 g, 0.6 mol) was then added to the solution, and the resulting slurry was stirred for 1 h, monitoring the pH throughout the regeneration. The suspension was subsequently vacuum-filtered to separate the solid mixture of m-BBIG and its carbonate salt from the regenerated amino acid solution. The regenerated amino acid solution was then measured with a graduated cylinder to record its volume, then transferred back into the humidifier’s reservoir for the next cycle. The filter cake was transferred to a crystallizer dish and broken up into smaller pieces, then placed in the oven at 120 °C for 2 h. The regenerated m-BBIG was weighed and reused in the next cycle. To measure the CO2 cyclic capacities for each cycle, 50 ?L of samples was drawn with a 1 mL syringe equipped with a 0.22 ?m syringe filter, diluted with 450 ?L of D2O, and left at room temperature for 24 h prior to being analyzed by 1H NMR spectroscopy to measure the carbamate concentrations. The (bi)carbonate concentrations were subsequently measured by IC, taking 20 ?L of the NMR solutions and further diluting them with 980 ?L of H2O before the IC analyses. The DAC cycles were run in triplicate, and average values and standard deviations are reported.

Some additional graphics from the paper tell much of the story:

The caption:

Figure 1. X-ray crystal structure of m-BBIG (50% ellipsoids shown), viewed orthogonal (top) and parallel (bottom) to the benzene ring.

The caption:

Figure 2. X-ray crystal structure (50% ellipsoids shown) of (m-BBIGH22+)(?OOC–CH2–NH–COO–)(H2O)4, obtained by crystallization of m-BBIG from an aqueous potassium glycinate solution loaded with CO2 by DAC. The water molecules included in the crystal are not shown.

The caption:

Scheme 3. CO2 Capture by Absorption with Aqueous Potassium Sarcosinate Followed by Carbonate Crystallization with m-BBIG

The caption:

Figure 3. CO2-loading curves for DAC with 1 M potassium glycinate (red squares) and potassium sarcosinate (blue dots), using an air humidifier (shown on the left) as the air–liquid contactor.

The caption:

Figure 4. Time-dependent regeneration of 1 M glycine (red squares) and sarcosine (blue dots) with m-BBIG. The molar amounts of CO2 removed relative to the amino acid concentrations (mol/mol) were monitored by measuring the concentrations of carbamate and (bi)carbonate left in solution by 1H NMR spectroscopy and IC. The error bars are defined as the standard deviations from three separate experiments.

The caption:

Figure 5. Time-dependent regeneration of 1 M glycine (red squares) and sarcosine (blue dots) by refluxing. The molar amounts of CO2 removed relative to the amino acid concentrations (mol/mol) were monitored by measuring the concentrations of carbamate and (bi)carbonate left in solution by 1H NMR spectroscopy and IC. The error bars are defined as the standard deviations from three separate experiments.

The caption:

Figure 6. Consecutive loading/regeneration DAC cycles with sarcosine/m-BBIG. The error bars are defined as the standard deviations from three separate experiments.

The caption:

Figure 7. Comparison of regeneration energies (kJ/mol) for m-BBIG, PyBIG, and the CaCO3 and aqueous sodium glycinate (30 wt %) benchmarks.

The caption:

Figure 8. Proposed flow diagram for the overall DAC process based on the amino acid/BIG system.

This is a lab scale process, and helpfully, the authors sketch the requirements of scaling the process:

While this study has focused mostly on the fundamental and early-applied aspects of DAC, such as the design, synthesis, and characterization of the BIG/amino acid sorbents, thermodynamic analysis, and the bench-scale process, considerations of future R&D needs for further developing and improving the DAC technology are appropriate here. First, the prototype system needs to be scaled up a few orders of magnitudes, from the current scale of about 100 g of CO2/day, to the pilot scale of 1 ton of CO2/day, and finally to the full scale of 1 Mt of CO2/year. Before that can be achieved, the various components of the DAC process need to be optimized, starting with the air–liquid contactor, continuing with the crystallizer unit, and finishing with the CO2 stripping unit. While the air humidifier used in this early study offered a simple and economical set-up for comparing different sorbents and CO2 loading conditions, better contactors need to be designed to maximize the air–liquid interfacial area and minimize water evaporation. Along this line, different real-world conditions need to be tested, including variable air temperatures and humidity levels. Next, the crystallizer unit needs to be designed so that it can handle large volumes of solids and operate under continuous crystallization conditions. That will require an effective solid–liquid separator, such as a filter-press or a cyclone, and an effective mode of moving the solids between the crystallizer and the stripper units, using for example extruders. The CO2 release from the carbonate solid inside the stripping unit also needs to be optimized, to minimize the time, temperature, and energy required, improve the heat flow, and maximize the pressure of the CO2 output. The use of renewable or waste-heat sources of energy can be explored to increase the sustainability of the process. Finally, all of the components need to be integrated into the overall DAC process, which ideally would be operated in a continuous mode. Figure 8 illustrates a proposed flow diagram for the overall DAC process.

Note that it would require 1000 1MT/year capacity plants just to capture 1/35th of the carbon dioxide waste we dump each year, and having been dumping for decades.

From the author's conclusions:

A bench-scale direct air capture process has been demonstrated, comprising CO2 absorption with aqueous amino acid salts (i.e., potassium glycinate, potassium sarcosinate), followed by room-temperature regeneration of the amino acids by carbonate crystallization with a readily available and inexpensive bis-iminoguanidine (m-BBIG). Finally, CO2 release by mild heating (60–120 °C) of the m-BBIG carbonate crystals regenerated the m-BBIG solid quantitatively, thereby closing the DAC cycle. Three consecutive absorption/regeneration cycles have been run, with observed cyclic capacities of 0.12–0.20 mol/mol and a measured regeneration energy of 360 kJ/mol (8.2 GJ/ton). While the energy requirement is higher than the corresponding energy for the CaCO3 DAC benchmark (278 kJ/mol, 6.3 GJ/ton CO2), a significant advantage of the m-BBIG/amino acid system is that it requires much lower temperatures of regeneration (60–120 °C) compared to CaCO3 (900 °C), which can be easily supplied from low-grade waste heat or carbon-free renewable energy sources (i.e., solar, geothermal). Alternatively, the amino acid can be regenerated by boiling under reflux, with a measured cyclic capacity as high as 0.64 mol/mol and a regeneration energy as low as 253 kJ/mol (5.8 GJ/ton).

While the energy requirement of this system is higher than other approaches, the low temperatures required may make it possible to use heat routinely rejected to the atmosphere, thus lacking much a requirement for new energy, and perhaps moderately improving the thermal efficiency of nuclear reactors that might drive the carbon capture.

I hope you're having a very pleasant holiday season. I know I am, since I am having the joy of studying the equations of state for syn gas hydrogen and carbon dioxide/carbon monoxide mixtures.