DACCS

Direct Atmospheric Carbon Capture and Storage

This uses a lot of energy, but will be feasible if we can produce enough basic energy in a CO₂-free manner.

Man-made CO₂ emissions are currently running at about 40Gt/year

Pre industrial CO₂ concentration in the atmosphere was 280 ppm. By 2000 it was about 370ppm, it is now nearly 412ppm and still seems to be increasing by 2ppm per year. This is looking out of control, despite the efforts to reduce emissions. We need to pull a very large amount of CO₂ back out of the atmosphere.

There are several different proposed methods for extracting CO₂ from the atmosphere, which fall broadly into two categories. The most established in terms of chemistry knowledge is allowing air to come into contact with sodium or potassium hydroxide – either by bubbling the air through the liquid or by spraying the liquid through the air, making sodium or potassium carbonate in the solution. Mixing the resulting solution with calcium hydroxide produces solid calcium carbonate and restores the potassium or sodium hydroxide. The CO₂ is then extracted from the calcium carbonate – the same chemistry which is needed to make cement, producing pure CO₂ and calcium oxide, which is then slaked (water added) to make calcium hydroxide.

The problems with this process are – it needs a very high temperature (over 800^oC) to split the calcium carbonate into calcium oxide and CO₂, and slaking the resulting calcium oxide to produce calcium hydroxide uses up a lot of water. So it is very energy intensive. However, it is a known process – it should be reliably doable – the process will just work.

The other class of processes use various solid materials which will adsorb CO₂ onto their surfaces when air is blown past them. To extract the CO₂ and restore the surface to being able to catch the CO₂ again takes only a fairly low temperature – around 100^oC. This process is compatible with using up waste heat from various other chemical and power generating processes or using energy efficient heat-pumps to provide the thermal energy. The test plants for these processes currently run in cooler areas of the world where the adsorption happens at a low temperature. It is not known how well they will work in warmer areas – they may need active cooling for the collection phase of CO₂ to happen, but less warming for the extraction of the CO₂ and restoration of the surface.

However, the amount of DACCS needed to stop us increasing the amount of CO₂ in the atmosphere and after that, hopefully to start to decrease it again, will need dedicated energy input – it will not be enough to just do an odd bit on the side as an afterthought. It needs actively progressing and developing as a large scale process. Ultimately this needs to become the default carbon supply for future liquid fuels and carbon required in the chemical industry, so it needs to be developed quickly and in a determined fashion, both as a carbon feed-stock and for carbon storage. If the cost of extracting CO₂ from the atmosphere and producing liquid fuel from it drops below the cost of pulling fossil fuels out of the ground, then there will no longer be any economical advantage for companies to extract the fossil fuels. This has to be the target.

We also need to make sure that every significant industrial point source of CO₂, has that CO₂ captured – so that most of the CO₂ in flues from the current fleet of coal and gas fired power-stations is captured, as well as CO₂ produced by chemical processes, like the production of cement. This is not yet done widely enough – CO₂ scraping has been added to a few plants, but there are thousands still belching CO₂ into the atmosphere, to say nothing of coal burning power stations – which China is still building. It should be mandatory for all power production plants, otherwise they should be summarily shut down.

All this is futile until burning coal is completely banned globally just as CFCs were banned to “fix the ozone hole” problem.

Even the CO₂ scraping from point sources is insufficient – it reduces the CO₂ produced, but it does not eliminate it. Gathering the CO₂ from flue-gases, where it is much more concentrated is often done by the high-temperature sodium or potassium hydroxide method. For very high concentration CO₂ emissions of (over 95% CO₂) from, for example, steam reformation of methane, ammonia production and a few other processes, carbon capture can be done by cooling and compressing the gas. It needs no complicated chemistry.

Current state of technology

There have been many papers written about the process of Atmospheric Carbon Capture.

There are several companies developing DACCS systems, most have working demonstration plants. There are proposals to install bigger systems.

Carbon Engineering has Megaton-scale carbon-capture technology and claims carbon capture and synthetic fuel production. It is aiming to license its technology to local plant developers across the world to help with wide-spread, rapid take up of their process, rather than their company physically building all the plants. They are using the liquid process using hydroxides, but have spent nearly ten years improving the efficiency. They have had a 1Mt CO₂/year pilot plant which they have been running since 2016. That plant actually uses methane burnt in pure oxygen for the high temperature heat source, to convert the calcium carbonate to CO₂ and calcium oxide, but they also capture the CO₂ produced from burning the methane – given the methane is burnt in pure oxygen the flue gases contain just CO₂ and water, so can be cooled and compressed to produce pure CO₂. Their figures are 0.98 Mt atmospheric CO₂ captured, but 1.48 Mt CO₂ produced from their test plant per year. It is sited on a 0.5 hectare industrial site for the whole plant.

They slake the calcium oxide with steam, which produces even more heat, and that heat is used to pre-heat the calcium carbonate, reducing the amount of energy needing to be put in to split the CaCO₃ to CaO and CO₂.

Their system needs 4.7 tons of water per ton of CO₂ produced. Also, 1% of calcium is lost in each cycle – costing around $0.22 per ton of CO₂ produced. Their figures from a smaller prototype plant were that it took in air at 1.4m/s processing 180 tons of air per hour. The maximum CO₂ production was 45kg CO₂ per hour at a 42% capture fraction.

They are working on a plant to capture CO₂ and produce synthetic fuel where electrolysis is used to produce the H₂ needed for the synthetic fuel production, and the O₂ needed to run the DAC heating part of the system. They are also working on a system where the required heat is purely electrical.

They have used commercially available industrial components throughout, having partnered with three other companies when they required custom modifications to an otherwise relatively standard part. Carbon Engineering have modified the temperatures and concentrations of chemical used, speed of fans etc, so they claim, quite plausibly, that this is technology which is ready to deploy with no unknown costs.

Climeworks in conjunction with Carbfix have set up a plant in Iceland, using geothermal energy, to extract CO₂ from the atmosphere and sequester it as carbonates, in the Basalt rock below.
According to their site, their CO₂ collectors selectively capture carbon dioxide in a two-step process. First, air is drawn into the collector with a fan. Carbon dioxide is captured on the surface of a highly selective filter material that sits inside the collectors. Second, after the filter material is full and cannot absorb any more carbon dioxide, the collector is closed. The temperature is increased to between 80^oC and 100^oC – this releases the carbon dioxide. Finally, the high-purity, high-concentration carbon dioxide is collected.

In the CarbFix process, the CO₂ is mixed with water (which will produce Carbonic acid) and is pumped underground where it will react with Basalt – the “new” rock found under Iceland. This process would normally happen over time when the basalt was exposed to the air, rain etc, but by pumping high concentration of H₂CO₃ down into the new rock, it happens faster. There are references to this as “enhanced weathering” – it is a good way of storing CO₂ in a non-flammable way, with a very low risk of the CO₂ leaking back out into the atmosphere again. The process does change the physical structure of the basalt by depositing chemicals like iron carbonate, and magnesium carbonate etc. within the rock. The earth has a very large quantity of basalt in its crust where this process could be used to store the CO₂. The long term goal has to be to get the CO₂ levels in the atmosphere down to a sensible level, stop using fossil fuels completely and then use DAC systems to recycle the CO₂ from the atmosphere to provide liquid fuels and carbon feed-stocks for the chemical industry. Hopefully at that point, the systems will be balanced, and very little additional CO₂ storage will be needed. We have a long way to go before that point is reached though.

Global Thermostat (quoting from their website) uses custom equipment and proprietary (dry) amine-based chemical “sorbents” that are bonded to porous, honeycomb ceramic “monoliths” which act together as carbon sponges. These carbon sponges efficiently adsorb CO₂ directly from the atmosphere, smokestacks, or a combination of both. The captured CO₂ is then stripped off and collected using low-temperature steam (85^o-100^o C), ideally sourced from residual/process heat at little or no-cost. The output results in 98% pure CO₂ at standard temperature and pressure. During the process only steam and electricity are consumed, without the creation of emissions or other effluents. This entire process is mild, safe, and carbon negative.

Global Thermostat produce modules which can absorb 40,000 tonnes of CO₂ per year. Their website gives few details of the process, but their pitch appears to be that you can capture the CO₂ from any fossil fuel-based power plant, with almost the implication that you therefore do not need to worry about using fossil fuels. Though this may be a good place to start capturing CO₂, that should not mean that you can otherwise leave things unchanged. They claim that installing their modules on the flues of the large emitters, and also adding some more modules to capture the CO₂ from the air in the vicinity will make even fossil fuel power stations potentially carbon negative! If the CO₂ is stripped off their devices at only a relatively low temperature of 85^o-100^oC, presumably any flue gases have to be cooled a lot before their devices will capture any CO₂ from them, unless they are only being put on the flues for cosmetic purposes.

Their website invites people who want CO₂ to buy it from them. CO₂ is used in the food industry in fizzy drinks and in food preparation and in medicine, but any sold to those markets will end up in the atmosphere again. CO₂ is used in the oil industry to help push out the remaining oil from a well, but using captured CO₂ with the intention of extracting more fossil fuels, just seems wrong.

Creating a sales market for CO₂ as the only allowable source of Carbon in the chemical industry is a plausible long-term goal, but at the moment we need to store far more than we use.

There is no discussion about compressing the resulting CO₂, but they can capture it and supply it to anyone who needs it. This is a good, half solution, but if there do end up being financial penalties for producing CO2, and financial rewards for capturing it, then adding loads of capturing technology to a fossil fuel plant in order to claim to be reducing carbon emissions, feels – dubious.

Hydrocell Have a Direct Air Capture unit which combines their two primary technologies – a high performance heat exchanger and a regenerative CO₂ scrubber. They claim their novel solution can be used in the sustainable production of all types of fossil fuel-free hydrocarbons and fuels, provided the required primary energy and hydrogen are produced from non-fossil fuel sources such as solar and wind energy. It is not clear whether this is a theoretical or actual claim. Their website has very few technical details.

SkyTree provide domestic air-cleaning products scrubbing CO₂, SO₂ etc from local environments eg inside cars, not earth scale systems. They started as part of ESA to provide space station air scrubbers

Infinitree provides relatively small scale CO₂ moving solutions – decreasing the level of CO₂ outside a greenhouse and increasing it inside – making a better environment in which plants can grow. They do not appear to have an industrial earth-scale CO₂ removal process.