Hydrothermal reaction of lime with fly ash to produce calcium silicates for dry flue gas desulfurization

Experimental work was performed to determine the chemical interactions involved in the formation of lime/fly ash solids for flue gas desulfurization. Three types of experiments were used: fly ash dissolution, lime/fly ash reaction, and reagent chemical. Work was also performed to determine the effects of recycle (i.e. sulfite and sulfate) on the reaction of hydrated lime with fly ash.;The fly ash dissolution experiments showed that the high-calcium fly ashes were more reactive than the low- and medium-calcium fly ashes. The aluminum present in the high-calcium fly ashes was more reactive than the aluminum present in the low- and medium-calcium fly ashes.;The dissolution rate of silica from the low-calcium fly ash was first-order in the hydroxide concentration of the solution. The activation energy for silica dissolution was 20.5 kcal/mol. The dissolution rate of silica from the high calcium fly ash was more sensitive to the hydroxide concentration in the solution.;The lime/fly ash experiments showed a clear effect of fly ash type on the reaction of hydrated lime with fly ash. The solids produced with the high-calcium fly ash had a lower surface area and reactivity towards SO;The reagent chemical experiments showed that the low reactivity of the solids from the high-calcium fly ash is due to the reactive aluminum content of the fly ash. The reagent chemical experiments showed that the calcium-silicates produced by the reaction of hydrated lime with silica fume were amorphous and had a high surface area. These solids were reactive towards SO;The experiments investigating the effect of recycle materials showed that the addition of gypsum to the slurry inhibited the formation of the unreactive calcium-aluminate materials, but had no effect on the formation of the calcium-silicate solids. The addition of calcium sulfite hemihydrate to the slurry changed the calcium-aluminate product, but the new product also had a low surface area and was unreactive towards SO...