Sustainable carbon adsorbents have been produced from biomass residues by single-step activation with CO 2 . The activation conditions were optimised to develop narrow micropores in order to maximise the CO 2 adsorption capacity of the carbons under post-combustion conditions. The equilibrium of adsorption of pure CO 2 and N 2 was measured between 0 and 50 ˚C up to 120 kPa for the outstanding carbons. The CO 2 adsorption capacity measured at low pressures is among the highest ever reported for carbon materials (0.6-1.1 mmol g -1 at 15 kPa and 25-50 ˚C), and the average isosteric heat of adsorption is typical of a physisorption process: 27 kJ mol -1 . Dynamic experiments carried out in a fixed-bed adsorption unit showed fast adsorption and desorption kinetics and a high CO 2 -over-N 2 selectivity. These adsorbents are able to separate a mixture with 14 % CO 2 (balance N 2 ) at 50 ˚C, conditions that can be considered as representative of post-combustion conditions, and they can be easily regenerated.

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1 1 2 3 4 5 AbstractThere is an urgent need to develop materials and processes that reduce the energy penalty 6 associated to the CO 2 capture step. Biochars are appealing adsorbents for post-combustion 7 CO 2 capture applications due to their low cost, stability in moisture conditions and 8 microporous nature. Series of carbon adsorbents were prepared from almond shells and olive stones by single-step activation with air at 400-500 ˚C, and with lower O 2 concentration in the activating gas, 3-5%, at higher temperatures (500-650 ˚C). This process entails energy savings compared to conventional activation with carbon dioxide or steam. It has been found that the pore size distribution can be tailored by adequately selecting the activating conditions.Carbons obtained under lower oxygen partial pressures and higher temperatures present narrow microporosity, which is essential for the adsorption of CO 2 at low partial pressures.These appealing low-cost adsorbents have competitive CO 2 working capacities and high CO 2 /N 2 equilibrium selectivity in conditions that can be considered representative for post-combustion CO 2 capture, thus showing potential for this application.

Adsorption processes can be used for removing the CO 2 present in flue gas streams, contributing to reduce greenhouse gas emissions. To evaluate adsorbents for this application, many studies focus on the adsorption capacity of pure CO 2 or, in the best cases, on binary mixtures of N 2 and CO 2 . However, the role of water vapor, which is one of the main components of flue gas and is strongly adsorbed by many adsorbents, is less explored. Carbon materials are selective toward CO 2 and have an intrinsic hydrophobic nature, being appealing candidates for postcombustion CO 2 capture. In this work, the influence of water vapor on CO 2 adsorption is evaluated under postcombustion conditions using a biomass-based microporous carbon produced from olive stones by single-step activation with carbon dioxide. The adsorption isotherms of water vapor at 25, 50, and 70 °C present a type V topology that will facilitate the desorption of H 2 O compared to other adsorbents with type I or II isotherms, reducing the energy consumption of the adsorption process. Multicomponent adsorption experiments carried out under dynamic conditions with the main flue gas components, N 2 , CO 2 , O 2 , and H 2 O, showed that CO 2 and H 2 O are preferentially adsorbed over N 2 and O 2 . However, although water vapor is coadsorbed with CO 2 , no significant decrease in the adsorbent CO 2 capture capacity was observed. Moreover, the adsorbent can be easily regenerated by temperature swing adsorption (TSA) or vacuum and temperature swing adsorption (VTSA), recovering its full adsorption capacity. These characteristics, added to their low cost and environmentally friendly character, make these adsorbents appealing adsorbents for postcombustion applications.

Coal fly ashes (CFA) are generated in large amounts worldwide. Current combustion technologies allow the burning of fuels with high sulfur content such as petroleum coke, generating non-CFA, such as petroleum coke fly ash (PCFA), mainly from fluidized bed combustion processes. The disposal of CFA and PCFA fly ashes can have severe impacts in the environment such as a potential groundwater contamination by the leaching of heavy metals and/or particulate matter emissions; making it necessary to treat or reuse them. At present CFA are utilized in several applications fields such as cement and concrete production, agriculture and soil stabilization. However, their reuse is restricted by the quality parameters of the end-product or requirements defined by the production process. Therefore, secondary material markets can use a limited amount of CFA, which implies the necessity of new markets for the unused CFA. Some potential future utilization options reviewed herein are zeolite synthesis and valuable metals extraction. In comparison to CFA, PCFA are characterized by a high Ca content, suggesting a possible use as neutralizers of acid wastewaters from mining operations, opening a new potential application area for PCFA that could solve contamination problems in emergent and mining countries such as Chile. However, this potential application may be limited by PCFA heavy metals leaching, mainly V and Ni, which are present in PCFA in high concentrations.

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BACKGROUND: Carbon materials are appealing adsorbents for postcombustion CO 2 capture