NaNO3 and other alkali
nitrate salts, which are present
in the molten state during use, have been described as facilitators
or catalysts for CO2 absorption by both MgO and MgO-containing
double salts. Although MgO exhibits a high capacity (exceeding 70
wt %), its regenerability in multicycle tests shows a significant
loss of capacity with cycle number prior to lining out. On the other
hand, the MgO–Na2CO3 double salt shows
a lower (∼16 wt %) but stable capacity over multiple cycles
under pressure swing operation. The purpose of this paper is to elaborate
on the concept of molten salts as catalysts for CO2 absorption
by MgO, and extend these observations to the MgO-containing double
salt oxides. We will show that the phenomena involved with CO2 absorption by MgO and MgO-based double salts are similar
and general, but with some important differences. This paper focuses
on the following key concepts: (i) identification of conditions that
favor or disfavor participation of isolated MgO during double salt
absorption, and investigation of methods to increase the absorption
capacity of double salt systems by including MgO participation; (ii)
examination of the relationship between CO2 uptake and
melting point of the promoter salt, leading to the recognition of
the role of premelting (surface melting) in these systems; and (iii)
extension of the reaction pathway model developed for the MgO–NaNO3 system to the double salt systems. This information advances
our understanding of MgO-based CO2 absorption systems for
application with precombustion gas streams.
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