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An efficient chemical absorption method capable of cyclic fixed-bed operations under moist conditions for the recovery of carbon dioxide from flue gases has been proposed employing K2CO3-on-carbon. Carbon dioxide was chemically absorbed by the reaction K2CO3 + CO2 + H2O ⇄ 2KHCO3 to form potassium hydrogencarbonate. Moisture, usually contained as high as 8−17% in flue gases, badly affects the capacity of conventional adsorbents such as zeolites, but the present technology has no concern with moisture; water is rather necessary in principle as shown in the equation above. Deliquescent potassium carbonate should be supported on an appropriate porous material to adapt for fixed-bed operations. After breakthrough of carbon dioxide, the entrapped carbon dioxide was released by the decomposition of hydrogencarbonate to shift the reaction in Eq.1 in reverse on flushing with steam, which could be condensed by cooling to afford carbon dioxide in high purity. Among various preparations of alkaline-earth carbonates (X2CO3, X = Li, Na, K) on porous materials, K2CO3-on-activated carbon revealed excellent properties for the present purpose. Preparation and characterization of K2CO3-on-carbon and illustrative fixed-bed operations under flue gas conditions in laboratory columns and a bench-scale plant are described.

A Cl'-intercalated hydrotalcite-like compound (HTAL) was prepared by neutralizing magnesium and aluminum chlorides with sodium hydroxide. The HTALs were characterized by chemical analyses, X-ray diffraction (XRD), differential thermal (DTA)/thermogravimetry (TG) analyses, and scanning electron microscopic (SEM) observation. The HTAL crystal with a single unit cell, aa = b0 = 0.31 and c0 = 2.33 nm as the lattice constant, was formed by aging at 353 K. The crystal growth proceeded for all three axes a, b, and c up to 5 h of aging, while the stacking along the c axis was preferentially observed by aging beyond 5 h. Phosphate ion-exchange properties of HTAL were investigated by a batch method. The HTAL obtained by longer aging time showed a slightly larger phosphate uptake, owing to better crystallization of the hydrotalcite structure. The pH dependence of the phosphate ion exchange showed that phosphate uptake has a maximum around pH 7. Chemical analyses of HTALs before and after the phosphate loading showed that phosphate ions are mostly ion-exchanged with interlayer Cl' ions, and the composition of phosphate ions in the solid phase is similar to that in the solution phase. The isotherm for phosphate uptake followed the Langmuir equation at pH 7.53 and 310 K; it gave an ion-exchange capacity of 2.37 mmol of P/g for phosphate.

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A modified chemical-absorption method capable of cyclic fixed-bed operations under moist conditions for the recovery of carbon dioxide from flue gases has been proposed employing K2CO3-on-carbon. Deliquescent potassium carbonate was favorably supported on activated carbon as well as on bound carbon dioxide in the presence of water vapor by the following reaction: After breakthrough, the entrapped carbon dioxide was released by the decomposition of hydrogencarbonate at a higher temperature above 150 °C to shift the reaction in reverse by steam, which could be condensed by cooling to afford high-purity carbon dioxide. An effective use of the pore space as microtanks for aqueous potassium carbonate was verified with XRD evidence under a very humid condition of 7.3% H2O, as anticipated. The amount of CO2 uptake was very small under a dry condition, but increased abruptly in the presence of moisture. Potassium carbonate on carbon was crystalline 2K2CO3·3H2O, even when in contact with moisture of 1.7% H2O; this revealed that it is not necessary for K2CO3 to be in the aqueous phase. Illustrative cyclic fixed-bed operations for the recovery of carbon dioxide under a condition close to the actual flue gas of 13.8% CO2 with 10% H2O at 100 °C under atmospheric pressure were demonstrated up to 10 cycles.

Potassium carbonate supported on an activated carbon has been proposed as an efficient sorbent to recover CO 2 from moist flue gases. As a characteristic of the present CO 2 sorption process, which can be described as K 2 CO 3 ‚1.5H 2 O + CO 2 ) 2KHCO 3 + 0.5H 2 O, moisture in the feed gases had no influence on the CO 2 sorption. By the temperature-swing operation of a fixed-bed, the CO 2 recovery was achieved as follows: carbon dioxide in moist flue gases at around 363 K was sorbed by the K 2 CO 3 sorbent, followed by steam flushing at 433 K to release the CO 2 , and then cooling the sorbent for the next CO 2 sorption. In the present study employing a bench-scale apparatus, the material (CO 2 and H 2 O) balances, together with those of heat during each step, were measured to elucidate the CO 2 sorption/release and the cooling behaviors. To evaluate the practical adaptability of this process, the heat consumption for the CO 2 recovery on a commercial-scale was estimated. When compared with that for other processes such as the conventional amine process, it provided a remarkable energy-conservative effect. The cost for the CO 2 recovery by K 2 CO 3 -on-activated carbon is also discussed.

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