Coal-fired power stations will continue to form a major portion of the power generation mix in the immediate future. Much research has been done on reducing CO2 emissions from these sources by CO2 capture, but very little on emission from smaller sources. An approach which captures CO2 directly from the atmosphere, remotely from the generating site, offers a useful solution.
To date most approaches have been based on carbon capture at the source and sequestration in geologically suitable formations. However the long term effects of carbon sequestration are uncertain, and CO2 recycling or CO2 neutral usage methods are being considered. Most attempts at CO2 capture have focused on large emitters, such as power plants, but ignored the large number of small emitters such as motor vehicles, gas driven appliances, industrial boilers, aircraft, etc., which are responsible for 40 to 50% of all CO2 emissions. The cost of applying carbon capture to these small emitters individually would be horrendous, and for these cases, direct capture of CO2 from the air is the most viable option.
Direct air capture (DAC) refers to a range of technologies which capture and concentrate carbon dioxide (CO2) from ambient air, and produce a high purity product which can be re-used in various processes. The current techniques use large fans which move ambient air through a capture structure, which uses chemical scrubbing processes to capture CO2 through absorption or adsorption separation processes. Fig. 1 shows an artist’s impression of a possible DAC plant .
CO2 is increasingly being regarded as a valuable commodity, as a feedstock to many chemical processes, as a greenhouse fertilizer, and a basic component in the formation of hydrocarbon liquids and gases, using surplus renewable energy, or using solar energy directly.
Carbon dioxide has no direct immediate impact in the vicinity of its point of release, and other than preventing its release in to the atmosphere, there is no necessity to capture CO2 at its source. In addition there are the costs of storage at transport to the site of ultimate use or sequestration. Cost of capture methods are high in terms of energy, and plant, and cause problems with the removal of other substances from flue gas.
Recent developments have led to a variety of plants which capture CO2 directly from the atmosphere at an energy cost claimed to be of the same order as bulk capture systems, or only marginally more expensive. This method has the advantage that the operation could be performed at any site, for instance at a plant that uses CO2, such as a greenhouse complex, or at a site with surplus renewable energy to generate syngas, etc.
Systems are being developed by a number of organisations – research and commercial – and a pilot plant is being operated in Switzerland by the company Climeworks, where the captured CO2 is being fed to a greenhouse installation close to the facility, where it is used to increase the production of greenhouse crops. An increase in yield of the order of 20 to 30% is claimed for CO2 fertilization. Another plant, which has the ultimate aim of producing synthetic fuel, is under development by Carbon Engineering, an organisation based in Canada.
The ultimate aim of large scale plant is not only greenhouse fertilization, but the production of synthetic liquid and gaseous hydrocarbons (SHC), by combining the CO2 with hydrogen produced from electrolysis using “surplus” renewable energy (RE). RE sources such as wind and solar are highly variable, and a high penetration of RE often results in production exceeding demand. Under these conditions, the price of RE is very low, in fact sometimes going negative when curtailment of production is not an option. The question of how to handle surplus production has produced some ingenious solutions. Surplus RE is used in Germany to drive electrolysis to produce hydrogen which is fed into the gas mains, for instance.
A popular solution is to store the surplus energy, and one of the options is to produce SHCs. While it may seem paradoxical to focus on producing SHC from atmospheric CO2, we have to consider that this is exactly what the biofuels industry is doing: using crops as the means to capture CO2. One of motivations behind the direct capture SHC method is that the process uses much less water and arable land than biofuel production, and, as it is not subject to the vagaries of weather, is much more controllable. The argument against biofuels, that the process uses arable land that could be used for food crops, is not applicable to this system.
The process of direct capture is not new, and has been under investigation since the 1990s. One of the most important questions applied to any process is cost. A study done by a review panel of the American Physical Society found that DAC would likely cost about $600/t of captured CO2 which would make the process uneconomic . But as with all new processes, developers have found ways of reducing costs and are now estimating the cost at $100/t, with projected further reduction to $ 92/t by 2030 . Current estimated costs of carbon capture from flue gas at site range from $69 to $103, but this is for bulk capture.
Most units under development consist of vertical banks of collector modules, giving a low footprint. There are several systems being developed or at trial stage, but all use the same process of chemical adsorption and desorption to capture CO2. Both liquid and solid sorbents are used. In the solvent-based approach, a strong alkaline solution is used to capture CO2 from the air in a simple acid-base reaction that results in the formation of a stable carbonate, which then has to be heated in a kiln to release high-purity CO2 and lime to be recycled throughout the process. In a solid-sorbent-based DAC process, CO2 from the air binds to a sorbent using a gas-solid contactor.
An approach based on both heat and vacuum is used to desorb the CO2 from the sorbent, producing a concentrated stream of CO2. This system is subsequently cooled to begin the cycle again. In the liquid sorbent based system, air is passed through a spray of sorbent, or through a matrix of surfaces over which the sorbent flows. Liquid sorbents allow a continuous process, whereas solid sorbents may require a batch type process requiring multiple units to ensure continuous production.
A number of different sorbents have been used or proposed. The operations cycle for a system based on two stage adsorption and resorption using potassium hydroxide as the collector and calcium hydroxide as the extractor is shown in Fig. 2 .
The plant uses fans to push air through towers containing potassium hydroxide (KOH) solution, which reacts with CO2 to form potassium carbonate (K2CO3). The remaining air, now containing less CO2, is released. The K2CO3 is fed to the reactor stage where it reacts with calcium hydroxide Ca(OH)2 to form calcium carbonate CaCO3 and KOH. Heating of the CaCO3 releases the captured CO2, and regenerates the capture solution for reuse.
Direct air capture has the advantage that plant can be located close to the site of re-use, and can in fact be collocated with the end user. A proposal collocates the DAC unit with a greenhouse installation, and similar applications could apply to other industries .
 D Keith, et al: “A Process for Capturing CO2 from the Atmosphere”, Joule 2, August 2018.
 R Socolow, et al: “Direct Air Capture of CO2 with Chemicals”, APS 2011.
 T Wang: “CO2 fertilization system integrated with a low-cost direct air capture technology”, Energy Procedia 63 (2014).
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