Carbon Storage by Mineralisation (CSM): Serpentinite Rock Carbonation via Mg(OH)2 Reaction Intermediate Without CO2 Pre-separation

Abstract

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 “Åbo Academi 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 are yet to be demonstrated beyond 78 and 65% efficiency, respectively, the carbonation reaction reaches an equilibrium already after 10-15minutes. Process energy requirements are ∼ 3 GJ (heat)/t CO2 (similar to the capture stage of “conventional” CCS), while using ∼ 3 t rock/t CO2, 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 ∼500°C, 20bar 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.