CO2 sequestration with magnesium silicates—Exergetic performance assessment

• Oxyfuel flue gas mineralisation needs less energy than conventional fired flue gas.
• Ammonium bisulphate (ABS) lowers the Mg extraction's energy requirements by 40%.
• Regeneration of ABS with H2SO4 allows for a net exothermic heat effect.
• Ammonium sulphate thermal decomposition for ABS recovery adds 60–75% energy penalty.
• It is possible to recover up to 90% of the process’ total electricity input.

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.