Main physicochemical factors affecting the aqueous carbonation of oil shale ash

Pulverised firing oil shale ash (PF OSA) is characterised by a theoretical CO2 capture capacity of 350–500 kg (CO2)/t, 30–50% of which is based on free lime. A low porosity inhibits the availability of free lime, especially in the case of continuous flow processes where the recirculating process water is saturated with different species leached from the OSA (Ca2+, K+, Na+, OH−, SO 4 2 - ) and absorbed from flue gases ( CO 3 2 - , HCO 3 - , HSO 3 - ). Referred ions in the slaking water are known to inhibit the diffusion of Ca(OH)2 from the particle’s surface, which, in turn, reduces the slaking rate and causes reactions to take place on the particle’s surface. The aim of the current study was to elaborate the effect of the listed parameters of liquid and solid phases on the availability of lime for hydration and following carbonation reactions. The experiments showed that the excess amount (⩾0.05 M) of CO 3 2 - and HCO 3 - ions in the slaking water had the severest effect on the availability of lime from PF OSA and its utilisation (<16%) in correlation with the solution concentration. Scanning electron microscope (SEM) analysis showed the coherent product layer on the particles’ surface. X-ray photoelectron spectroscopy (XPS) analysis indicated that the shell contained mainly CaCO3, as, according to energy dispersive X-ray (EDX) analysis, free CaO inside the particles remained unreacted. The utilisation of lime could be enhanced substantially by diluting the process water or using ageing pre-treatment to increase the porosity of PF OSA. Comparative experiments with circulating fluidised bed combustion (CFBC) OSA showed that the porous structure of its particles supported the fast and full hydration of lime as well as the diffusion of Ca2+-ions into solution, which resulted in complete carbonation even under inhibiting conditions. The amount of CO2 bound in the process reached 130 kg (CO2)/t (CFBC OSA) and 155 kg (CO2)/t (PF OSA) under favourable conditions.