A comparison of CO2 mineral sequestration processes involving a dry or wet carbonation step

CO₂ mineral sequestration is one method of the CCUS (carbon capture, utilisation and storage) portfolio, and work on stepwise carbonation of serpentinites (serpentine-rich rock, 3MgO·2SiO₂·2H₂O) in Finland has resulted in what is known as “the ÅA (Åbo Akademi) route”. This involves extraction of magnesium from rock using ammonium sulphate salt, precipitation of magnesium hydroxide and finally carbonation in a high temperature pressurised fluidised bed. Besides magnesium carbonate (MgCO₃) significant amounts of iron (hydr)oxides are produced. Disadvantages are the complexity and exergy consumption associated with alternating (hot/cold/hot) treatment conditions. Therefore, an alternative ÅA route has been developed that, like the conventional route, can operate directly on flue gas. Here, the final carbonation step is accomplished in an aqueous solution. Products are magnesium (hydrocarbonates), hydromagnesite (4MgCO₃·Mg(OH)₂), besides iron (hydr)oxides. Early results obtained with this route method are reported, along with a comparison (using process simulation) of the both routes, operating on flue gas from: 1) a lime kiln and 2) a natural gas fired power, addressing the external heat and power input requirements. It was found that conversion levels and rates are similar for the two routes, although excess NH₃ may be needed to establish the working pH for hydromagnesite precipitation.