CO2 sequestration with magnesium silicates - Exergetic performance assessment

A staged process for CO₂ sequestration by mineralisation, using magnesium silicates, studied at Åbo Akademi (ÅA) involves the production of magnesium hydroxide from suitable rock (requiring heat at ∼350–450 °C) using recoverable ammonium salts and its subsequent carbonation (generating heat at ∼500 °C). In addition, the process gives substantial amounts of solid by-products making the integration of mineral carbonation with other industries an opportunity to both reduce CO₂ emissions and substitute raw material inputs.

Aspen Plus^® v7.2 software is used to optimize the ÅA process towards minimal energy use and to study the impact of the flue gases' CO₂ concentration (conventional coal firing (CCF) vs. oxyfuel combustion (OXY) flue gases) in the carbonation step instead of pre-separated and compressed CO₂ (providing an additional benefit compared to “conventional” CO₂ capture and storage (CCS) options). Also the influence of using either ammonium sulphate (AS) or bisulphate (ABS) as the fluxing salts on the process's exergetics is evaluated. It was concluded that ABS lowers the energy requirements in the Mg extraction by 40% but its regeneration (fundamental feature for the route's success) by: (i) thermal decomposition of AS appears to be unviable, (ii) addition of H₂SO₄ saturates the process with sulphur. The simulation results showed that the extraction with ABS and carbonation with OXY flue gases requires less energy input.