1. Introduction - Firenze University Press

the formation of MgSO3 is found thermodynamically infeasible [44] (see also Fig. ), but in the
presence of oxygen the following reaction has been suggested [44,47]:
1
MgO + SO (g) + O (g) MgSO
2 2 4
(7)
2
Increasing the concentration of oxygen further, increases the amount of sulphur trioxide in the gas
and thus the reaction between MgO and SO3 also needs to be considered [44]:
MgO + SO 3(g) MgSO4
(8)
In addition to the reactions above, the possibility of MgCO3 reacting with SO2 (or SO3) to form
MgSO4 cannot be ignored. The conversion of MgCO3 to MgSO4 is given by the equation below:
1
MgCO + SO (g) + O (g) MgSO + CO (g)
3 2 2 4 2
(9)
2
From thermodynamic equilibrium calculations (HSC Chemistry 5.11 software) it can be concluded
that both Reactions (8) and (9) are thermodynamically favoured under the experimental conditions
investigated here. It appears that MgSO4 is stable up to 640 °C for SO2 and SO3 concentrations
above 0.1 ppmv. Furthermore, as long as the CO2/SO2 ratio is below 10 10 , sulphation of MgCO3 is
also feasible as can be seen from Fig. 6. In other words, even if the concentration of SO2 is only
0.1 ppbv (parts per billion, volumetric) in CO2, sulphate is stable. In order to perform the
equilibrium calculations, the amount of oxygen in the gas was arbitrarily chosen to be 3.5%-vol.
Fig. 6. Equilibrium concentrations of SO2 (Reaction 3) and SO3 (Reaction 4), together with the
equilibrium ratio, CO2/SO2 (Reaction 5) as a function of temperature based on Gibbs energy
minimisation calculations (HSC Chemistry 5.11). O2 (arbitrarily chosen) in the gas phase:
3.5%-vol.
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