Carbon Dioxide from the Atmosphere

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To deal with small and dispersed CO2 emitters and to avoid the need to develop and construct a huge CO2-collecting infrastructure, CO2 could be captured from the atmosphere - an approach that has already been proposed by some in the past [224-227]. The atmosphere could thus serve as an effective means of transporting CO2 emissions to the site of capture. This would make the CO2 collection independent of CO2 sources, and CO2 could be captured from any source - small or large, static or mobile. With the concentration of CO2 in air being at equilibrium all around the world, CO2 extraction facilities could be located anywhere, but to allow for any subsequent methanol synthesis they should ideally be placed close to the hydrogen production sites. As mixing and equilibration of air is relatively rapid, local depletion in CO2 is not likely to pose any problem. If this were not the case, emissions from power plants would cause much higher local concentration of CO2 near the plants, which is not the case.

Despite the low concentration of CO2 of only 0.037% in the atmosphere, Nature routinely recycles CO2 by photosynthesis in plants, trees, and algaes to produce carbohydrates, cellulose and lipids, and eventually new plant life, while simultaneously releasing oxygen and thereby sustaining life on Earth. Following Nature's example, mankind will be able to capture excess CO2 from air and to recycle it to generate hydrocarbons and their products. CO2 can be captured from the atmosphere using basic absorbents such as calcium hydroxide (Ca(OH)2) or potassium hydroxide (KOH) which react with CO2 to form calcium carbonate (CaCO3) and potassium carbonate (K2CO3), respectively [228]. Due to its low CO2 content, large volumes of air should be contacted with the sorbent material, and this could be achieved with minimum energy input, preferably using natural air convection. After capture, CO2 would be recovered from the sorbent by desorption, through heating, vacuum or electrochemically. Calcium carbonate, for example, is thermally calcinated to release CO2. CO2 absorption is an exothermic reaction, which liberates heat, and is readily achieved by simply contacting CO2 with an adequate base. The energy-demanding step is the endothermic desorption, requiring energy to regenerate the base and to recover CO2. Calcium carbonate or sodium carbonate, requiring high energy input for recovery are therefore probably not the ideal candidates for CO2 capture from air, and other bases might be more appropriate for this application. Research into this area, though still in its relatively early phases of development, should determine the best absorbents and technologies to remove CO2 from air, with the lowest possible energy input. For example, when using KOH as an absorbent, it has been shown that the electrolysis of K2CO3 in water could efficiently produce not only CO2 but also H2 with relatively modest energy input [229]. With further developments and improvements, CO2 capture from the atmosphere, which has already been described as technically feasible, will also become economically viable [228].

Among the various advantages of CO2 extraction from air is the fact that CO2 capture, in being independent of CO2 sources, allows more CO2 to be captured than is actually emitted from man- made activities. This means that this technology could allow mankind not only to stabilize CO2 levels, but also eventually to lower them.

The removal of a significant share of CO2 from industrial emissions and capture of CO2 from the atmosphere would make huge amounts of CO2 available. As proposed presently, the captured CO2 could be stored/sequestered in depleted gas- and oil- fields, deep aquifers, underground cavities, or at the bottom of the seas. This approach, however, does not provide a permanent solution, nor does it assist in mankind's future needs for fuels, hydrocarbons, and their products. The recycling of CO2 via its chemical reduction with hydrogen to produce methanol (i.e., the "Methanol Economy") is, therefore, an attractive alternative. As fossil fuels become more scarce, the capture and recycling of atmospheric CO2 would become and remain feasible for the production of methanol, together with synthetic hydrocarbons and associated products. The hydrogen required would be obtained by the electrolysis of seawater (an unlimited resource), while also releasing oxygen. The electrical power required would be provided by atomic energy, and/or by any suitable alternate energy source. Upon their combustion, methanol and the synthetic hydrocarbons produced would be transformed back to CO2 and water, thereby closing the methanol cycle. This would constitute mankind's artificial version of Nature's CO2 recycling via photosynthesis. By using this approach, there would be no need for any drastic change in the nature of our energy storage and transportation systems, as would be required by switching to the hydrogen economy, while also providing synthetic hydrocarbons (Fig. 12.13). Furthermore, as CO2 is available to everybody on Earth, it would liberate us from the reliance on diminishing and non-renewable fossil fuels and all the geopolitical instability associated with them.

Figure Co2 Recycling For Methanol
Figure 12.13 CO2 recycling for methanol and synthetic hydrocarbons production.

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