Characterization of Ca(OH)2/fly ash sorbents for flue gas desulfurization

Ca(OH)2/fly ash sorbents prepared by varying the Ca(OH)2/fly ash weight ratio (CH/FA) and the slurrying time at 65 °C and a water/solid ratio (L/S) of 10:1 were characterized to study the influence of the preparation variables on their physical properties and chemical compositions. The pozzolanic reaction between Ca(OH)2 and fly ash taking place in slurry resulted in the formation of calcium silicate hydrates (CSHs) (C–S–H(I)), which, having shapes of foil, plate, and sphere, tended to cause particles to cohere and to form a porous structure. Particles formed by foils of calcium silicate hydrates were compressible under high pressures. The reaction was rapid before 1 h and abruptly slowed down thereafter due to the covering of fly ash particles by calcium silicate hydrates; but the structure of calcium silicate hydrates continued to change with slurrying time, hence more pore volume and surface area were generated. Pores in the sorbents were mainly in the mesopore range and slit-shaped. The particle sizes, specific surface areas, and pore volumes of the sorbents, being larger than those of Ca(OH)2 or fly ash alone, changed with Ca(OH)2/fly ash weight ratio. The sorbent with a ratio of 70:30 had the largest specific surface area, mesopore volume, and micropore volume.