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Climate Change

Getting carbon dioxide out of the atmosphere

Some say, it's about time for geo-engineering if we want to meet the Paris Agreement goals. But which technologies are ready for use, which options are advisable, which risks are manageable?

CO2 is in the air. People demonstrate against climate change.
Carbon Dioxide Removal (CDR) is hoped to be an alternative method against global warming.

As the climate crisis accelerates, one thing is becoming increasingly clear: humanity is running out of time. There are many indications that it might be too late already for "natural" methods of solving the global warming problem, in particular the afforestation of large areas to filter carbon dioxide out of the air using photosynthesis. (Watch out for our report on afforestation efforts in our FairPlanet-dossier on climate change.) So humanity could ultimately be forced to use artificial interventions into nature: geo-engineering.

There are basically two types of geo-engineering that are considered by scientists and engineers. However, while Solar Radioation Management (SRM) seems to incorporate too many unknown factors - and risks - to be used on a large scale, for example spraying sulfate aerosols into the stratosphere for artificial cloud formation, thereby reflecting more sun rays back into space before they can hit the earth's surface, Carbon Dioxide Removal (CDR) appears to be an alternative that can be used shortly. Let‘s take a closer look at the state of the art.

1. Carbon Capture & Storage (CCS)

The best known of these methods is the so-called Carbon Capture & Storage (CCS). This process has not made headlines in recent years, as there have been fears that pressing carbon dioxide in former mine tunnels or boreholes could pose risks and that long-term storage would not be possible because CO2 could leak. But latest innovations shine a new, better light on CCS technology. In 2012, a team of international engineers and scientists started project CarbFix by injecting carbon dioxide into porous basalt rock at an underground test site in Iceland, which was formed there from cooling lava. Two years laster, about 95 percent of the carbon dioxide had morphed into carbonate minerals. It has been captured and dissolved in water, and was then pumped into the injection site, where it was converted into minerals by chemical reaction. Trapped in porous rock, the carbon dioxide cannot leak out of the ground and into the atmosphere.

Basalt rock formation on icelandic coast.Suitable basalt formations can be found globally. They cover most of the oceanic floors and around ten percent of the continents.

The results, published in 2016 in the Science magazine led to a scale up of CarbFix. In fact, suitable basalt formations can be found all around the world, since basalt is the most commen rock type on Earth. It covers most of the oceanic floors and around ten percent of the continents. „Whereever there‘s basalt and water, this model would work“, as Iceland‘s CarbFix-geologist Sandra Osk Snaebjornsdottir told BBC. CCS technologies, like CarbFix, today seem so promising that UN‘s Intergovernmental Panel on Climate Change (IPCC) recomments them together with other negative emissions technologies (NET) for further adoption.

2. Artificial leaf

Converting CO2 into mineral rock would be a “natural” solution. Engineering professor Yimin Wu and his team from the University of Waterloo‘s Institute of Nanotechnology in Canada also take nature as a role model by inventing an artificial leaf. “We call it an artificial leaf because it mimics real leaves and the process of photosynthesis,” Wu said in a press statement. “A leaf produces glucose and oxygen. We produce methanol and oxygen.

In turn, methanol could be used as an alternative fuel, for example by heating it and letting water evaporate, which could then drive turbines to generate electricity. The process, which was presented in Nature Energy in November 2019, even surpasses the natural role model in terms of efficiency by a factor of ten. It comes as a chemical reaction that involves four substances – glucose, copper acetat, sodium hydroxide and sodium dodecyl sulfate – all added to water. The water then has to be heated to a certain temperature and carbon dioxide is blown through it while a beam of light shines on it. It might take a few years for the process to be commercialized though, but once the scientists partner with potent industry companies they might be able to scale it up quickly.

3. electro-swing reactive adsorption

The most promising way to get carbon dioxide out of the atmosphere again, however, might turn out to be a device developed by T. Alan Hatton, the Ralph Landau Professor and the Director of the David H. Koch School of Chemical Engineering Practice at Massachusetts Institute of Technology, and MIT-postdoc Sahag Voskian, who developed the idea during his Ph.D. It is a type of battery that can absorb carbon dioxide as it flows along the electrodes of the device. What makes it so special: It can work at virtually any concentration level, even at around 400 parts per million (ppm) which is the current concentration of CO2 in the atmosphere. In other words: there are no intermediate steps required, such as chemical processing, input of energy, or differences in pressure.

The whole system operates at room temperature and normal air pressure. It could theoretically be installed somewhere outside and immediately start sucking carbon dioxide out of the air. And it comes with yet another advantage: its binary nature. The electrode material has either a high affinity or no affinity whatsoever, depending on the battery's state of charging or discharging. "This binary affinity allows capture of carbon dioxide from any concentration, and allows its release into any carrier stream, including 100 percent CO2," Voskian tells Techxplore. If the desired end-product is, say, pure carbon dioxide to be used in the carbonation of beverages, then a stream of the pure gas can be blown through the plates. The captured gas is then released from the plates and joins the stream. Alternatively, it could be compressed and injected underground for ong-term disposal, or even made into fuel through a series of chemical and electrochemical processes.

In an industry plant, two sets of such stacks of electrochemical cells could be set up side by side to operate in parallel, with flue gas being directed first at one set for carbon capture, then diverted to the second set while the first set goes into its discharge cycle. By alternating back and forth, the system could always be both capturing and discharging the gas. „The process is revolutionary“, Voskian claims.

In the lab, the team has proven the system can withstand at least 7,000 charging-discharging cycles, with a 30 percent loss in efficiency over that time. The researchers think that they might be able to improve it to 20,000 to 50,000 cycles. With about one gigajoule of energy per ton of carbon dioxide is quite energy-efficient, compared to other existing methods which comsume up to ten gigajoule per ton.

The electrodes themselves can be manufactured by straight forward chemical processing methods. They are coated with polyanthraquinone, a composition of carbon nanotubes. Ultimately, they could be produced in large quantities through a roll-to-roll manufacturing process similar to a newspaper printing press, with costs of just a few dollars per square meter of electrode. So, even the scale up seems straightforward. For more capacity you just need to make some more electrodes, as Sahag Voskian put it.

Ultimately, global warming can only be mitigated if we not only get the excess carbon dioxide out of the atmosphere again, but also stop pumping new CO2 into it. There is simply no alternative to leaving fossil fuels in the ground.