The kinetics of the reaction of Ca(OH)2 with CO2 in humid N2 has been studied at 60−90 °C by using a differential reactor. Ca(OH)2 reacted with CO2 to form CaCO3 only when the relative humidity exceeded 0.08. The reaction ceased before 1 h, and Ca(OH)2 was incompletely converted. The reaction rate and final conversion of Ca(OH)2 were significantly affected by relative humidity, were slightly dependent on temperature, and were zeroth order with respect to gas phase CO2 concentration. The carbonation of Ca(OH)2 was well described by the asymptotic equation which is obtained by assuming chemical reaction control and considering the surface coverage by product. The rate-controlling step might be the dissolution of Ca(OH)2 at the water-adsorbed surface. The reaction behavior of Ca(OH)2 with CO2 was similar to that with SO2. The model derived for the reaction with CO2 may be applied to that with SO2 with proper modifications. The results of this study are useful to the design and efficient operation of the processes using hydrated lime to remove SO2 from the flue gas, in which both SO2 and CO2 are present.

Hydrated lime [Ca(OH) 2 ] is frequently used as a sorbent in the spray-drying flue gas desulfurization (FGD) and injection/humidification FGD processes to remove SO 2 from the flue gas. In the present study, the kinetics of the reaction of Ca(OH) 2 with SO 2 has been studied by using a differential reactor and a gas mixture of N 2 , SO 2 , and H 2 O under the conditions of 0-80% relative humidity, 60-90 °C, and 1000-5000 ppm SO 2 . The reaction ceased before 1 h, and Ca(OH) 2 was incompletely converted. The relative humidity of the gas significantly affected the initial reaction rate and the maximum conversion of Ca(OH) 2 . The temperature and SO 2 concentration had mild effects on the reaction. The sulfation of Ca(OH) 2 was well described by the asymptotic equation, which is obtained by assuming that the rate was controlled by the chemical reaction occurring on the water-adsorbed sorbent surface and by considering the surface coverage by the product. The results of this study are useful to the design and efficient operation of the processes using hydrated lime to remove SO 2 from the flue gas.

The influence of CO2 and O2 in the flue gas on the reaction of hydrated lime sorbent with SO2 was studied using a fixed-bed differential reactor under conditions prevailing in the spray-drying flue gas desulfurization process. With the presence of CO2, the sulfation and carbonation reactions of Ca(OH)2 took place simultaneously until Ca(OH)2 ceased to react. The CaCO3 produced reacted further to form CaSO3·1/2H2O. The apparent sulfation rate, total reaction rate, and final total conversion of Ca(OH)2 were greater than those for the case without CO2. The final total conversion was about 1.45 times that for the latter case at the conditions of 1000 ppm SO2, 60 °C, and 70% relative humidity. The same effect was observed whether CO2 was present with O2 or not. Kinetic expressions obtained by assuming chemical reaction control and considering the surface coverage by product crystals best represented the kinetic data.

The characteristics and the SO2 capture capacities of sorbents prepared from products of spray‐drying flue gas desulfurization (FGD) have been studied. Sorbents were prepared by first slurrying Ca(OH)2 and CaSO3 and/or CaSO4 with and without the addition of fly ash and then drying. Compared to the use of pure Ca(OH)2, the SO2 capture and Ca(OH)2 utilization decreased for sorbents prepared without fly ash and increased for sorbents with fly ash. Flakelike ill‐crystallized tobermorites were observed for all the sorbents containing fly ash. In addition, significant amounts of needle‐shape Ca4Al2(OH)12SO4 . 6H2O and Ca6Al2 (OH)12(SO4)3 . 26H2O (ettringite) were also observed for the sorbents containing CaSO3 and/or CaSO4. These newly formed compounds dissociated into CaSO3 . 0.5 H2O and inert precursors upon sulfation, and were responsible for the high SO2 capture capacities and Ca(OH)2 utilizations of the sorbents prepared with fly ash.