Transformation of calcareous oil-shale circulating fluidized-bed combustion boiler ashes under wet conditions

Composition, diagenetic transformation and alkalinity potential of oil shale ash sediments

“…The composition and transformation of oil shale ash with hydration has been studied since the 1950s [6][7][8][9][10][11][12]. However, the mineral and geochemical transformation of the ash plateau sediments has not yet been addressed.…”

Open-Air Deposition of Estonian Oil Shale Ash: Formation, State of Art, Problems and Prospects for the Abatement of Environmental Impact

“…Similarly, the same mechanism is in place during the carbonation of hydrated particles in the CFBC ash, when the outer shell of anhydrite and/or gypsum retards the CO 2 diffusion and slows the carbonation of CFBC ash, as noted by Kaljuvee et al [51]. Additionally, CO 2 transport is in this case probably further inhibited by CaCO 3 formation in the outer perimeter of the hydrated portlandite particles, and the experimental carbonation in the CFBC ash took place during a much longer period, 58 days [49]. In contrast, in the PF ash portlandite was already carbonated during 28 days of the reaction [48].…”

“…In the experimental hydration of CFBC ash the reaction was completed in 72 hours, together with the dissolution of anhydrite and partial dissolution of periclase [49]. The slower hydration of this ash, as already noted by Anthony et al [50], is most probably related to that CFBC ash solids are typically composed of an unreacted CaO core covered by a Ca-sulphate shell [43,44].…”

“…In contrast, in the PF ash portlandite was already carbonated during 28 days of the reaction [48]. Also, Liira et al [49] report that in the CFBC ash, ettringite formation is accompanied by the precipitation of excess gypsum (CaSO 4 ·2H 2 O), which implies that aluminium, not sulphate as in the PF ash, is a limiting component in this type of ash, which is rich in anhydrite due to the effective SO 2 immobilisation in the INTREX unit of CFBC boilers.…”

“…Laboratory-scale hydration experiments [48,49] are indicative of a rapid carbonation of the metastable portlandite by binding atmospheric CO 2 , which is completed under ambient conditions (allowing the free transport of CO 2 ) in a few weeks or months. However, on a large scale, in waste deposits, where the contact with atmospheric CO 2 is hindered by the added layers of ash, a significant, yet not complete, carbonation of portlandite is only achieved in the uppermost 0.5-1 m layer of the deposit, and the content of calcite decreases over about a 5-m depth interval, whereas portlandite is well preserved in the deeper parts of the ash deposit [40].…”