High resolution 3D mapping of grain kinematics during high temperature sequestration of SO₂ from flue gas by carbonate aggregates

Sulphur dioxide (SO₂) is removed from flue gases prior to discharge into the atmosphere by high temperature sulphation reactions with the mineral calcite (CaCO₃) in the form of calcite aggregates such as limestone. The efficiency of this industrial-scale process is constrained by the self-inhibiting growth of anhydrite (CaSO₄) along calcite grain boundaries. Using very high resolution X-ray μCT and Scanning Electron Microscopy we show, for the first time, how the sulphation reaction is initiated by the anisotropic thermal expansion of calcite grains to produce high inter-grain permeability. In turn fast gas-solid reaction occurs to produce a network of porous anhydrite layers between grains. Individual calcite grains are then free to rotate and translate with respect to each other as the sulphation reaction proceeds. Grain translations of up to 24 μm and rotations of up to 0.64 degrees have been tracked in samples of a highly compacted calcite aggregate (Carrara Marble) across up to 600,000 grains through heating and cooling cycles during exposure to SO₂ gas flow at temperatures from 600 to 750 °C at one atmosphere. Such grain kinematics help to maintain gas phase permeability in the solid reactant and mitigate the inhibitory growth of porous anhydrite on grain boundaries.