Carbon storage by mineralisation (CSM): serpentinite rock carbonation via Mg(OH)₂ reaction intermediate without CO₂ pre-separation

CO₂ mineral sequestration, a.k.a. mineral carbonation offers an alternative to "conventional" CCS that involves underground storage of pressurised CO₂. It is being developed for locations that lack access to underground storage capacity for CO₂ and/or have access to suitable mineral resources, or for users that aim at marketable (hydro-) carbonate or otherwise useful solid product. The "Abo Akademi route" of producing Mg(OH)₂ from serpentinite rock followed by gas-solid carbonation in a pressurised fluidised bed (PFB) has been further developed and optimized towards industrial demonstration. Recoverable ammonium sulphate salt is used as the fluxing agent for Mg extraction from rock. While Mg(OH)₂ production and its scale-up and subsequent carbonation arc vet to be demonstrated beyond 78 and 65% efficiency, respectively, the carbonation reaction reaches an equilibrium already after 10-15 minutes. Process energy requirements are similar to 3 GJ (heat)/t CO₂ (similar to the capture stage of "conventional" CCS), while using similar to 3 t rock/ t CO,, with separate streams of unreacted rock, FeOOH and MgCO₃ as main products. The scale-up activities involve defining reactor types and conditions for the Mg(OH)₂ production and the carbonation, respectively, using flue gas at similar to 500 degrees C, 20 bar CO₂, partial pressure. This implies compression of a complete flue gas stream. It was shown that carbonation at a given (wet) CO₂ pressure gives results similar to when operating with diluted gas streams at higher pressures, at the same CO₂ partial pressure. Also simultaneous carbonation and sulphation of Mg(OH)₂ was found to be realizable. The beneficial role of increased yet reasonable levels of water vapour pressure is another research topic. While serpentinite-derived Mg(OH)₂ shows good reactivity the production of particle sizes suitable for bubbling PFB reactors is a complicating challenge.