Abstract
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.
Highlights
SO2 released into the atmosphere by many large-scale industrial processes has the potential to produce acid rain and destroy ozone
Desulfurization is mostly conducted by wet scrubbing using slurries of calcite aggregate materials, such as limestone or marble[2], but a proportion is undertaken by the simpler, but less efficient, dry process of reacting high temperature flue gas with a spray of these same raw materials. For this direct-sulphation reaction involving SO2 as a gaseous component of the flue gas mixture, the reaction may be written as a disproportionation, CaCO3 + 1.5SO2 → CaSO4 + CO2 + 0.25S2 (1a)
At the sub-micron scale, these coupled, rate-limiting chemical and physical processes using a combination of high temperature gas-solid reaction experiments, high resolution micro-computed X-ray tomography (X-ray μCT), Field Emission-Scanning Electron Microscopy (FE-SEM) and X-ray Diffraction (XRD)
Summary
SO2 released into the atmosphere by many large-scale industrial processes has the potential to produce acid rain and destroy ozone. Desulfurization is mostly conducted by wet scrubbing using slurries of calcite aggregate materials, such as limestone or marble[2], but a proportion is undertaken by the simpler, but less efficient, dry process of reacting high temperature flue gas with a spray of these same raw materials For this direct-sulphation reaction involving SO2 as a gaseous component of the flue gas mixture, the reaction may be written as a disproportionation, CaCO3 + 1.5SO2 → CaSO4 + CO2 + 0.25S2. At the sub-micron scale, these coupled, rate-limiting chemical and physical processes using a combination of high temperature gas-solid reaction experiments, high resolution micro-computed X-ray tomography (X-ray μCT), Field Emission-Scanning Electron Microscopy (FE-SEM) and X-ray Diffraction (XRD) This approach has enabled us to document, for the first time, the dynamics of individual grain movements (rotation and translation) in a reacting material as a contributing process in maintaining both inter-grain cohesion due to grain boundary interlocking and material permeability. XRD patterns were obtained for each disc and analysed for proportions of anhydrite, remaining calcite and intermediate products
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