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Aenert. Research Laboratory news
In view of the looming climate crisis we are currently facing, finding a feasible solution to mitigating the harmful influence of CO₂ emissions is of paramount importance. In this context, capturing carbon dioxide and pumping it deep underground by means of geological carbon capture and storage (CCS) appears to be an essential solution for achieving the objectives of carbon neutrality. The classic approach to geological storage of CO₂ is in supercritical form, where the CO₂ is densely compressed and enables storing of several million tonnes of CO₂ per year. However, this is a complex process in terms of technical and economic feasibility (choice of reservoir, durability of storage, leak-tightness, transport of the CO₂ to the injection site) which is not necessarily suitable for facilities that emit only a small volume of the gas. Also, ensuring the carbon dioxide remains well separated from the atmosphere is no mean feat.

Now (2023), scientists at Sandia National Laboratories have designed and tested a device under lab conditions which uses the temperature difference caused by periodically pumping carbon dioxide down a borehole to charge batteries. The heat flowing from the hot earth through the device to the cooler carbon dioxide creates a voltage that can be used to charge a battery and eventually power sensors. The Sandia-developed device functions in similar fashion to the radioisotope thermoelectric generators used to power NASA space probes and Mars rovers. It forms a multilayered tube consisting of an array of 1-by-1-inch square thermoelectric generators which can convert the heat flowing through them into a voltage and then power. The inner tube is resilient and can withstand the temperatures and pressures from carbon dioxide being pumped through it, while the outer tube is able to withstand the temperatures and pressures existing deep underground. In the area between the outer and inner tube, the electronics are located to capture and convert the voltage from the thermoelectric generators to charge a battery.

Also, a small circuit board was designed whose purpose was to convert and regulate the energy from the generators so that a battery could be charged without damaging surges. The challenge was to find batteries that could work above 71 degrees Celsius, the typical temperature downhole at the depths used for carbon sequestration.

Power generation was tested in the lab using the initial, foot-long prototype. Also, thermal imaging and computer modeling helped get an idea of how the temperature changed around the device when hot or cold fluid flowed through it. The modeling and tests played an important role in refining the prototype for an in-the-field test.

In the second prototype, the team installed several improvements to ensure the thermoelectric generators had good contact with the inner and outer shells, and that the heat could not take a shortcut around the generators through the rest of the device. For the field-test prototype, thermal insulators around the device were added and the heat-highway metal screws holding the thermoelectric generators together with spring-based clamps were replaced.

For the first field test, the prototype was inserted into a shallow borehole to a depth of about 19 metres. Then 170-degree water was pumped through the interior tube of the device to test the thermoelectric generators and the system. During the test the device received a leak, damaging the power conditioning board and battery. The second test, however, a repeat of the first, was a success. Finally, the ability of the field prototype to survive high-pressure environments was also tested. The pressures 400 times atmospheric pressure acted on the inner shell of the device and pressures 34 times atmospheric pressure on the outer shell of the device. The device inside the pressure chamber was heated up and the current from the thermoelectric generators measured, ensuring they worked under pressure.

Scientists have long tried to mitigate harmful CO₂ emissions during hydrocarbon production. In 2020, they designed a green carbon capture and conversion technology which offered scalability and economic viability for mitigating CO₂ emissions. Suspensions of gallium liquid metal were used to reduce CO₂ into carbonaceous solid products and O₂ at near room temperature. The nonpolar nature of the liquid gallium interface was responsible for the solid products to instantaneously exfoliate and keeping active sites accessible. The solid co-contributor of silver-gallium rods enabled a cyclic sustainable process. Mechanical energy was used as the input, which drove nano dimensional triboelectrochemical reactions. It was also found that through altering the secondary solvent and changing the reactor height, the dissolution and conversion efficiency could be modified. The optimum reactor height was 27 cm, if a gallium/silver fluoride mix at 7:1 mass ratio was used as the reaction material. At CO₂ input of ~8 sccm, 92% efficiency was achieved using a record low input energy of 228.5 kW∙h for the capture and conversion of a tonne of CO₂.

Image: Schematics and Raman spectra of solid carbon produced from CO₂ using liquid metal. a-d, Schematic illustrations for the preparation of a suspension of catalyst (a,b) and the CO₂ reduction process using different mechanical energy inputs (c,d). e, Schematic illustration of the formation and detachment of carbon flakes on the surface of Ga droplets in the presence of the solid rods. f-k, Raman spectra of the samples obtained from the reaction mixes of Ga with different silver salts as precursors in DMF: AgF (f, versus time), AgCl (g), AgBr (h), AgI (i), AgOTf (j) and AgNO3 (k). The D and G bands at 1350 and 1600 cm^-1, respectively, emerged after the reactions occur. l,m, Raman spectra (versus time) from the surface of mixtures from the 10-times diluted reaction system (Ga and AgF mix) by employing DMF (l) and DMF+ETA (m) as the reaction solutions. The blue and red curves in f-m are Raman spectra for the samples before and after reaction, respectively



Source: Junma Tang, Jianbo Tang, Mohannad Mayyas, Mohammad Bagher Ghasemian/ Mechanical energy-induced CO₂ conversion using liquid metals/ University of New South Wales, November 2020/ DOI:10.21203/rs.3.rs-112257/v1/ Open Source This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

In 2023, scientists constructed an electrochemical conversion system consisting of proton-bicarbonate-CO₂ mass transport management coupled with an in-situ copper (Cu) activation strategy to achieve high CH₄ selectivity at high currents. The scientists discovered that open matrix Cu electrodes retained sufficient local CO₂ concentration by combining both dissolved CO₂ and in-situ generated CO₂ from the bicarbonate. In-situ Cu activation was achieved through an alternating current operation and made the catalyst highly selective towards CH₄. Combining these strategies enabled CH₄ Faradaic efficiencies of over 70% in a wide current density range (100 – 750 mA cm⁻²) which was stable for at least 12 h at a current density of 500 mA cm⁻². The system also delivered a CH⁴ concentration of 23.5% in the gas product stream.

Image: Modeling reaction environment and CO₂ availability a Schematic illustration of the aqueous solution-fed ECR system using porous Cu cathode, Ni foam anode and bipolar membrane (BPM). b Schematic of modeled domain with key physics annotated. c CO₂ flux components at three current densities, 250, 500, and 750 mA cm⁻² for dense matrix and open matrix catalysts with N₂ sparging or CO₂ sparging. 0.3 M KHCO₃ was used as an electrolyte for all modeling. d Modeled CH₄ FE as a function of current density for a dense matrix catalyst (solid lines) and open matrix catalyst (dashed lines) with CO2 sparging and N₂ sparging



Source: Cornelius A. Obasanjo, Guorui Gao, Jackson Crane, Viktoria Golovanova/ High-rate and selective conversion of CO₂ from aqueous solutions to hydrocarbons/ Nature Communications 14(1), June 2023/ DOI:10.1038/s41467-023-38963-y/ Open Source This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

There are several potential advantages to be gained from the device: In NASA’s radioisotope thermoelectric generators the temperature difference from hot plutonium pellets and the cold of space is used to produce power. Sandia’s thermoelectric generator device makes use of the temperature difference from the hot Earth underground and the carbon dioxide being pumped down. Even though the technology is not as efficient at producing electricity as the internal combustion engine in cars, its greatest advantage is that it has no moving parts that could jam, which makes it suitable for hard-to-reach places such as space and deep boreholes. Eventually, the scientists successfully generated sufficient current to power downhole sensors with limited current draw.

Future plans include testing the device for longer times. For this purpose, more memory would be needed as well as the power conditioning board would have to be rebuilt so that it could operate with higher temperature differences. Also, a diode would need to be added so that the board can charge the battery regardless of whether hot or cold fluids flow through the device. Also, downhole sensor researchers would need to ensure that the power conditioning board can provide the right power for the sensors. If successful, a useful device for mitigating the harmful effects of CO₂ released during production processes could contribute to saving the climate.

By the Editorial Board