The chemistry and mineralogy of waste from retorting and combustion of oil shale

Oil shales are comprised of clastic, organic, carbonate and minor sulphide fractions. The relative proportions of these fractions influence the composition and potential environmental impact of wastes produced by mining, combustion, and retorting of these shales. Mining produces spoils and gangues, which may or may not produce acidic leachates, depending on relative proportions of sulphide and carbonate minerals. Combustion of oil shale for power production produces slag and ash, which, when deposited as huge heaps and plateaux, can generate highly alkaline leachates. Power plants also emit acid-producing and greenhouse gases (SO2, NOx, and CO2) and particulate matter. Emitted particulate matter is dominated by basic oxides (e.g., CaO), the fallout of which tends to neutralize any tendency to environmental acidification. Estonian oil shales are highly unfavourable in terms of greenhouse gas emissions: CO2 is generated both by decomposition of carbonates and oxidation of organic carbon, and 0.029 t/GJ carbon (C) is emitted for an energy yield of only 9 GJ/t shale. Finally, retorting of oil shales produces organically contaminated condensate water and semi-coke solid residue, leachates from which can contain several hundred mg/L phenols.