1. Introduction - Firenze University Press

(see Fig 5). Carbon dioxide uptake of the former sorbent was ~94% higher that the latter at the
second cycle. In subsequent cycles the uptake was at least ~60% higher than that obtained with dry
precursor.
Fig. 5. Comparison between material obtained with hydrated precursor and dry precursor when
subjected to moderately severe regeneration.
Such an improvement could be likely be explained with the formation of calcium hydroxide in the
sorbent precursor due to the presence of water and the subsequently calcination (for the binder
formation see Sec 2.1) leaving more pore volume. In fact the molar volume of Ca(OH)2 is greater
than that of CaO. Thus when the hydrated precursor would undergo calcination process water vapor
would be formed and it would leave the particle producing extra pore volume. Moreover the
migration of water vapor towards the outer part could likely create cracks throughout the sorbent
particle exposing more specific surface area to the carbon dioxide. Consequently the specific
surface area is higher and more CaO surface is brought in contact with the CO2. Moreover as Fig 6
suggests the chemical treatment of the sorbent precursor leads to a major change in the pore size
distribution. The formation of larger and smaller pores was indeed found as reported in Fig. 6 where
the BJH curves for the two kinds of sorbent are presented
dV/dlog(D) Pore Volume (cm³/
g·nm)
0,0018
0,0016
0,0014
0,0012
0,001
0,0008
0,0006
0,0004
0,0002
0
Dry
precursor
Hydrated
precursor
1 10 100
Pore size [nm]
Fig. 6. Pore size distribution of material obtained with hydrated precursor and dry precursor.
As you can see the sorbent synthesized with dry precursor shows an uni-modal pore size
distribution (average size 30 nm) whereas for the material obtained from hydrated precursor a wider
180