Point source carbon capture by porous inorganic carbonates
Abstract: Mesoporous inorganic carbonates (MIC) was synthesized and tested as adsorbents for CO2, using vacuum and temperature swing adsorption. Mesoporous magnesium carbonate (MMC), mesoporous calcium carbonate (MCC) and mesoporous calcium magnesium carbonate (MCMC), all included in MIC, are exceedingly porous with an amorphous structure. MMC was first reported in 2013, where it was synthesized in a methanol and MgO mixture under CO2 pressure. In this work, the synthesis of MCC and MCMC was developed from the synthesis of MMC. Further effects on the CO2 adsorption characteristics of the MIC materials with several additives (Al(NO3)3, Al2O3, K2CO3 and KNO3) introduced into the porous structures were also investigated. The MCC materials CO2 adsorption capacity (14.96 mmol g-1) was drastically lowered (7.29 mmol g-1) by severe sintering after continuous cycles when heat was used for sorbent regeneration. The combined structure of MCMC improved the stability, mitigating the sintering for high temperature adsorption/desorption (650 °C, 850 °C). The addition of Al(NO3)3 improved the stability further, with an optimum additive amount of 35 wt.%, giving a high initial CO2 uptake (12.23 mmol g-1) and maintaining a high CO2 uptake after 23 cycles (10.96 mmol g-1). The pure gas CO2 uptake of MMC was around 1.52 mmol g-1 at 101 kPa (0 °C) using vacuum swing adsorption. The N2 uptake under the same conditions was less than 0.10 mmol g-1. All of the additives tested increased the CO2 uptake of MIC under these conditions, with the most promising additives being low weight percentages of potassium carbonate (5-10 wt.%) added to MMC for low temperature adsorption (0 °C). The incorporation of 5 wt.% K2CO3 increased the CO2 uptake of MMC up to 3.24 mmol g-1, suggesting that the required energy for adsorption on this sample, due to the sorbent surpassing 3 mmol g-1 CO2 capacity, could be less than for conventional chemical absorbents. Vacuum swing cyclic CO2 adsorption/desorption showed a decrease in CO2 uptake on MMC with 5 wt.% K2CO3 after each cycle. Heat regeneration (150 °C, for 30 minutes) could recover most of the lost CO2 capacity each cycle. Heat indicatively improved the cyclic performance of this adsorbent without damaging the nanoporous structure. MMC with 5 wt.% K2CO3 was the best performing adsorbent when vacuum was used for sorbent regeneration and can potentially be further developed into a good CO2 adsorbent for temperature swing adsorption (TSA) processes.
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