1. Introduction - Firenze University Press

Fig. 5 Schematic diagram of the PFB setup at ÅA [26,27].
The appropriate fluid velocity is regulated/maintained by two flow controllers that allow for the
mixing of two gas streams. In addition a HPLC-pump is used for adding water to the system prior to
the preheater when steam is required in the reaction gas.
In contrary to previous studies, where a simple method of reacting the carbonated product with
hydrochloric acid could be used for determining the carbonate content [51], the now both
carbonated and sulphated Mg(OH)2 samples were analysed for elemental carbon and sulphur using
an ELTRA elemental analyser.
7.3 – Thermodynamics
We have recently concluded that the reaction between Mg(OH)2 and CO2 is likely taking place in
accordance to the following overall equations [27]:
2 2 2
Mg OH MgO H O* MgO + H O(g)
(5)
MgO H2O* + CO 2(g) MgCO 3 + H2O(g) (6)
The equations have been simplified to emphasize the importance of H2O, but it is clear that the
elementary reactions taking place on the surface of Mg(OH)2 particles are more complex than what
the simple direct carbonation of Mg(OH)2 would suggest. To further highlight the role of H2O, the
assumed intermediate product of Mg(OH)2 dehydroxylation is MgO H 2O*
, which corresponds to
magnesium capable of forming carbonate. In the absence of water the reaction between MgO and
CO2 is slow and likewise if the temperature for Mg(OH)2 dehydroxylation is not exceeded very
little carbonation will take place [52]. Adding sulphur dioxide to the reaction gas will compete with
the formation of magnesium carbonate, but to what extent is highly dependent on the SO2
concentration. In any case, a SO2 concentration at < 0.5%-vol is much lower than that of CO2 in a
typical flue gas. Similar conclusions have also been established for calcium-based species [53].
In a study by Hartman and Svoboda [44] a number of alternative reaction mechanisms between
magnesium species and SO2 were suggested. However, in the reaction conditions investigated here,
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