Fly-ash calcium hydroxide mixtures for SO2 removal: Structural properties and maximum yield

The structural properties and sulfation capacity of mixed solids obtained from pressurized hydration of commercial calcium hydroxide and coal combustion fly-ashes have been analyzed. Hydration experiments were planned according to a 2(5) fractional factorial design, studying the influence of temperature, time, pressure, fly-ash/Ca(OH)(2) weight ratio and water/solid weight ratio, on the BET specific surface area (S) and pore volume (V-p) of the final solids, as well as on the dissolved calcium concentration [Ca] in the slurries. Whereas a strong dependence of the ratio fly-ash/Ca(OH)(2) on (S) and [Ca] functions was observed, with values of these parameters in the ranges 2.5-64.3 m(2) g(-1) and 6.0-748.0 mg l(-1), respectively, no dependence was found of the studied variables on the pore volume (determined by Hg intrusion porosimetry), obtaining an average Value of 1.077 cm(3) g(-1). Sulfation runs using the hydrated solids were conducted in a packed bed reactor with a pure humidified SO2 gas phase, leading to a constant sulfation yield of 0.56 +/- 0.07 mol SO2 per mol Ca; this yield was independent of the solids specific surface area, and related only to the sorbents calcium content. (C) 1997 Elsevier Science S.A.