Motivations for carbonating magnesium silicates using a gas-solid process route

In Finland, work on carbonation of minerals for long-term storage has been ongoing for several years, motivated by greenhouse gas emission reduction commitments under the 1997 Kyoto Protocol, the absence of locations suitable for geological storage and the presence of vast resources of magnesium silicate minerals. A major focus on high-temperature gas-solid chemistry has been a specific feature of the work, aiming at taking maximum benefit of the fact that the overall chemistry of the carbonation process is exothermic. Thus, proper optimization and integration of the process would in principle allow for operation at zero or negative net energy input. While work on this route in the U.S. ended already a while ago to make way for the more promising routes that make use of aqueous solutions, the Finnish work on gas-solid carbonation has continued and produced data on thermodynamic feasibility and chemical kinetics, and identified a three-staged process for serpentine carbonation. In this paper the pros and contras of using a gas-solid process route for serpentine carbonation for long-term CO2 storage are addressed. The results and current state-of-the-art are summarized and compared with what has been achieved with wet processes for serpentine and olivine carbonation. Also the stability of the carbonate product and the issue of what to do with large amounts of it are addressed. Finally, current activities and near-future plans for the research in Finland are reported, which focuses on the use of fluidized bed reactors for the carbonation of magnesium silicates via magnesium oxide and magnesium hydroxide intermediates. It is shown that while gas-solid carbonation of magnesium hydroxide at acceptable rates requires temperatures above 500°C and pressures above 30-40 atm, the production of magnesium hydroxide from serpentine is not straightforward either.