Investigation of the regeneration of a CO2-loaded ammonia solution with solid acid catalysts: A promising alternative for reducing regeneration energy

Abstract

Ammonia-based CO2 capture is a promising option for suppressing CO2 emissions from thermal power plants. However, the regeneration of a CO2-loaded ammonia solution requires a large heat duty, hindering its industrial application. Herein, we investigated the regeneration of a CO2-loaded ammonia solution with the aid of solid acid catalysts, i.e., protonated Zeolite Socony Mobil-5 (HZSM-5), γ-Al2O3, and TiO2. The results demonstrate that all the catalysts can effectively promote the regeneration and the catalytic performance follows the trend: HZSM-5 > TiO2 > γ-Al2O3. Especially, the presence of HZSM-5 can reduce the energy consumption by 23.9% compared to the non-catalytic regeneration. The catalysts were also characterized to reveal their various acid and textural properties. The characterization shows HZSM-5 possesses the most Brønsted acid sites amounting to 3143.8 μmol/g; while γ-Al2O3 has the most Lewis acid sites amounting to 3554.1 μmol/g. Furthermore, the relationships between the catalytic performance and catalyst properties were analyzed. Unlike amine-based regeneration, the CO2 desorption rate increases linearly with the BET surface area × Brønsted acid sites. This is attributed to two factors: (1) smaller molecular volume of NH2COO−, and (2) a large proportion of HCO3− in the CO2-loaded solution. Finally, a plausible catalytic mechanism was proposed. It suggested that Brønsted acid sites can provide accessible free protons to promote CO2 released from HCO3− and CO32−. However, the Brønsted acid sites and Lewis acid sites played a synergistic effect on the breakdown of NH2COO−.