CO₂ fixation using magnesium silicate minerals. Part 2: Process energy efficiency

This paper describes the energy economy of the staged process described in part 1 of this short paper series. The optimal conditions for Mg(OH)₂ production depend to some extend on the mineral that is used: the different temperatures, reactants' mass ratios and (to some extent) residence times will give differing Mg(OH)₂ and by-product amounts which also affects energy input requirements. The heat release from the carbonation step is directly dependent on the degree of carbonation conversion. One benefit of carbonating Mg(OH)₂ is that only this is processed in the pressurised fluidised bed carbonation reactor: other, non-reactive material would add only to pressure drop (= power loss) and reactor size. Also, the product gas will be hot, pressurised steam (mixed with unreacted CO₂). Here, a process energy analysis is made based on the most energy intensive steps, being the heat treatment of the magnesium silicate rock and the carbonation reaction, respectively (a more detailed analysis is given in paper part 3). It is found that the heat requirements at 450-500°C for Mg(OH)₂ production are around 4x higher than the heat generated at 500-550°C by the carbonation reaction, giving heat input requirements (for example from another process) of 4-5 MJ/kg CO₂.