Distinguishing between natural and recarbonated calcite in oil shale ashes

Oil shale ashes from the PAMA demonstration power plant in the Negev region of Israel are produced by fluidized bed combustion (700-850°C) under short residence time. The FED is organic-rich calcareous raw material rich in carbonate rather than clays. Thus it is important to ascertain whether the calcite in the ashes is original natural calcite from the raw material or the product of recarbonation of lime. Three groups of ashes from the power plant, Ash Cooler (AC), Fly Ash (FAS) and Boiler Bank (BB) were examined using XRD, FT-IR, SEM and isotope analysis methods. The recarbonated calcite is distinguished from the natural original by smaller crystal size, lower degree of crystallinity and the presence of impurities. High negative δ¹³C values in oil shale ashes are explained by assuming recarbonation of lime with CO₂ originating from the combustion of the organic matter of the raw oil shale. Fly Ash, FAS, and BB, produced from organically-rich FED, contain more recarbonated calcite than bottom ash, AC. This observation can be explained by the larger grains of the AC, which do not reach the highest temperature area, and thus most of the original calcite does not decompose.