CO 2 capture from the atmosphere

The capture of carbon dioxide directly from the atmosphere would provide several benefits. It would enable the capturing of CO2 emitted from small, distributed sources such as building heating and transportation. The capture and subsequent collection of CO2 directly from these sources would be impractical and uneconomical [26]. By direct air capture, the CO2 emitted from any source could be removed from the atmosphere, with the capture facilities potentially

located anywhere in the world. Performed at large scale, this would allow reduction of the overall CO2 concentration in the atmosphere, instead of being limited to the reduction of further emissions. Simultaneously, the captured CO2 could provide an essentially limitless carbon source for the synthesis of fuels and chemicals [7].

The main obstacle to energy efficient and economical capture of CO2 from the atmosphere is the low concentration of CO2 in air. Additional difficulties arise from the presence of moisture and the need to perform the separation of CO2 at close to ambient temperature and pressure;

heating, cooling and compression of the massive volumes of air to be treated would very likely be uneconomical. These limitations eliminate many of the established and researched CO2

separation technologies from the consideration for direct air capture. Physical adsorbents such as zeolites and activated carbon are ruled out due to the low adsorption capacity at ambient pressure and in the presence of moisture, physical solvents are similarly not applicable at the low pressure and the amine based solvents suffer from degradation in the presence of air [26].

The theoretical energy requirement for the separation of CO2 from atmospheric concentration (approx. 20 kJ/mol of CO2, based on enthalpy of mixing) is only 1.8 to 3 times the energy required for the separation from concentrated sources [25]. To minimize energy consumption, the capture processes could be optimized for efficiency rather than for complete separation of CO2. While high (>90%) degree of CO2 separation is desired in capture from point sources, separation of only 25% of the CO2 might be satisfactory for an air capture unit [25]. Clearly, the energy consumption of actual processes would not be comparable to the theoretical minimum.

Indeed, energy intensity is the main issue with many of the proposed processes for direct air capture. In addition to energy, significant land area would be required for large scale operation.

The plants for direct air capture would be much larger compared to capture units for point sources due to the much larger volumes of gas to be treated [26].

The proposed technology options for direct air capture include chemisorption by inorganic, mainly basic, materials and the use of hybrid adsorbents consisting of organic amines supported on inorganic solids [26]. The use of basic materials such as sodium hydroxide can be considered the conventional route [52], while the development of solid hybrid sorbents has seen rise in more recent years [53].

2.1.3.1 Chemisorption by aqueous bases

Strong bases such as calcium, potassium and sodium hydroxides absorb CO2 by chemical reaction, forming the respective carbonates [54]. Especially the absorption of CO2 by aqueous sodium hydroxide has been considered [26]. Various methods for contacting the basic solution with air have been developed. In packed absorption columns, efficient separation of CO2 can be reached but the pressure drop associated with blowing large volumes of air through the packed bed is problematic. As a result, a column geometry with a large cross-section combined with a low height has been proposed [55]. Another issue caused by the large volume of air through the column is the significant evaporation of water. A water loss of 90 g per gram of CO2

captured has been noted [56]. Avoiding significant pressure drops by employing open absorption towers with no packing has also been considered [57].

The most energy intensive phase in the sodium hydroxide based scrubbing process is the regeneration of NaOH from the sodium carbonate formed in the reaction with CO2. In the causticization process, sodium carbonate is reacted with calcium hydroxide, forming NaOH and precipitating CaCO3. The calcium carbonate is then calcined at temperatures above >700^oC, releasing CO2 and resulting in calcium oxide. By hydration with water, calcium hydroxide is again obtained, closing the cycle. The causticization process is widely employed at Kraft pulp mills for the recovery of sodium hydroxide.

Usually the significant amount of heat required for the calcination reaction is provided by firing fossil fuels in air, meaning that a secondary CO2 capture unit would be required to remove CO2

from the outlet gas [26]. By oxygen firing, a stream of concentrated CO2 would be obtained instead, simplifying the separation of CO2 [54]. The overall energy requirement of the sodium hydroxide capture process has been estimated at 12 to 17 GJ per ton of CO2 captured [25, 55].

For comparison, the combustion of coal provides 9 GJ of energy per ton of CO2 emitted [55]. It can be concluded, that the economic and environmental feasibility of this energy intensive process is questionable [26].

Alternative causticization cycles have been developed with the goal of reducing the energy intensity. Cycles utilizing borates [58] and titanates [59] have been proposed. However, high temperatures are still required in both processes for the release of CO2. Alternatively, the use of calcium hydroxide as the absorbent material instead of sodium hydroxide has been researched. In this simplified process, calcium carbonate is precipitated by reaction of calcium

hydroxide with CO2. Calcium carbonate is then calcined as previously described, again leading to high energy consumption. Additional problems are caused by the low solubility of calcium hydroxide in water and mass transfer limitations [26]. As a summary, the high energy requirement, water losses by evaporation and also corrosion issues associated with the aqueous base scrubbing of CO2 make these types of processes seem impractical [60].

2.1.3.2 Supported amine adsorbents

Solid, amine based adsorbents were already introduced in the context of CO2 capture from point sources (Section 2.7.1). However, these types of materials are particularly interesting when capture from air is considered, as they seem to offer properties particularly well suited for this purpose. Through the chemical interaction of the amine functional groups with CO2, the binding of CO2 is weaker compared to strong bases, leading to more energy effective regeneration. The interaction is however stronger compared to physical adsorbents such as zeolites, generally leading to improved adsorption capacities at ambient conditions [61].

Opposed to aqueous amine solutions, evaporation is not an issue with the solid amine based adsorbents.

The amine based adsorbents can be classified based on the mechanism used to embed the active amine component onto the inorganic support [62]. In the first group, the support material is physically impregnated with monomeric or polymeric amines. These materials generally suffer from degradation due to the weak interaction between the amine and the support [26].

Alternatively, amines can be covalently bonded with the support, leading to increased stability.

This can be performed by binding amines to silica through silane bonds or by creating polymeric supports with amine side chains. The final option is the in situ polymerization of polyamines with an inorganic support material.

Silica and mesoporous silica (such as MCM-41 and SBA-15) are commonly used as support materials, alternatives including alumina and carbon fibers [26]. A wide range of amines have been studied. Polyethylenimines (PEIs) physically loaded onto supports have been found promising, combining simple and inexpensive preparation with good CO2 adsorption capacity and regenerability [26]. As an example of in situ polymerized adsorbents, hyperbranched aminosilicas (HAS) have been found promising [63]. Varied desorption methods have been used for the regeneration of amine based solid adsorbents [26]. Pressure, vacuum and

temperature swing adsorption processes have all been studied. Moisture swing adsorption based on desorption in contact with moisture or water has also been demonstrated with an anionic ion exchange resin used as the adsorbent [64]. In addition to regeneration ability, the adsorbents should have good stability under process conditions to allow practical use.

Adsorption capacity can be irreversibly reduced by degradation of either the amine or the support [26], with degradation by acidic gases (NOx, SOx) and oxygen the main concern.

Despite the low concentration of CO2 in air, direct air capture has been considered technically feasible [26]. However, the concept is often considered uneconomical due to excessive costs [65]. Cost estimations of CO2 capture from the air range from under 20€ all the way to over 800€ per ton of CO2, with the low estimates considered overly optimistic by some [26]. In comparison, the estimated cost of capture from concentrated sources is reported from under 30€ to 90€ per ton of CO2. Pilot and demonstration projects of direct air capture have already been launched by multiple companies [66, 67, 68]. These operations should provide more information about the technological and economic feasibility of capturing CO2 from the atmosphere.