Page 315 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

292 10 Post-combustion Air Emission Control

To increase the conversion rate of the spent sorbent, steam reactivation tech-
nology was first proposed by Shearer et al. [26]. Steam reactivation appeared more
economical and promising as compared with the other technologies that use
additive or catalyst.
There are two main mechanisms of steam reactivation of spent Ca-based sorbent.
One mechanism is the water penetration theory, according to which during a steam
reactivation process, water penetrates the sulfation product layer and reacts with the
unconverted CaO trapped inside the sorbent and produced Ca(OH) 2 . The mole
volume of Ca(OH) 2 is greater than CaO, the fresh Ca(OH) 2 produced inside
expands and increases the specific surface and specific volume of the spent sorbent.
When reactivated sorbent is used for the second sulfation at higher temperatures, Ca
(OH) 2 decomposes and results in fresh pores. These fresh pores make more SO 2
accessible to the internal CaO. This theory indicates that the existence of uncon-
verted CaO inside the spent sorbent is required for a successful steam reactivation.
However, Couturier et al. [10] studied steam reactivation for spent sorbent with
particle sizes smaller than 1 mm in diameter. Without trapped CaO, the conversion
rate of the spent sorbent was still increased by reactivation from 45 to 80 % or
higher. They proposed that there were unconverted CaO between the produced
sulfate crystals, and the CaO was converted into Ca(OH) 2 through the reactivation
process, resulted in volume expansion. This volume expansion produced not only
fresh pores but also the fresh surfaces due to the breakdown of the sorbent particles.
Wang et al. [35] reported the mechanism of steam reactivation of spent Ca-based
sorbent at 200–800 °C using multiple techniques, including mercury porosimeter,
X-ray diffraction (XRD), scanning electron microscopy (SEM), and weight change
analysis. Compared to the conversion rates of 10 and 12 % after first sulfation and
direct second sulfation reaction, the conversion rate with sorbent regeneration
reached a high range of 20–45 %. Regeneration temperature played a more
important role than the retention time for regeneration. For example, the conversion
rate reached the highest value of 45 % at 200 °C, and then decreased to about 30 %
at 300 °C. There was no big difference between 400–800 °C with a conversion rate
between 20 and 25 %.
Being reactivated at 200 °C, the sorbent particles broke down obviously and the
reactivated sorbent was almost entirely Ca(OH) 2 . While at 300–500 °C, reactivated
particles did not change much in size, and both CaO and Ca(OH) 2 exist in the
reactivated sorbent. At 600–800 °C, negligible Ca(OH) 2 was observed in the
reactivated sorbent, but mainly in the form of CaO.
The overall mechanisms of the steam reactivation are summarized in Fig. 10.6.
Because the molar volume of Ca(OH) 2 is greater than that of CaO, the produced Ca
(OH) 2 also resulted in the volume expansion of the sorbent particles. Depending on
the quantity of produced Ca(OH) 2 , the volume expansion results in:
(1) break-down of the particles or pore size increase
(2) penetration of part of the Ca(OH) 2 through the surface of the particles, or
(3) the lumps on the surfaces of the particles.