PETROLOGY, MINERALOGY, AND GEOCHEMISTRY OF LIGNITES FROM CRETE, GREECE

Coal from the deposit of Plakia, Island of Crete, Greece, was examined petrologically for the determination of rank, maceral composition, an d trace element geochemistry. The coal is of lignite to subbituminous B rank (%R(o,ran) = 0.36-0.44). Some samples are rich in resinite. Thi s was subdivided into two groups based on its morphological and fluore scence properties. Mineral matter in the coals consists predominantly of silicate and sulphur-bearing minerals. The low temperature ash (LTA ) is dominated by quartz, illite and gypsum, whereas the mineralogy of the high temperature ash (HTA) is comprised of silicates, Fe-bearing minerals and anhydrite. The coals are enriched in the elements As, Cr, Co, Cu, Mn, Mo, Rb, Se, U, V, Ce and Nd, when compared to world coals . Thorium, Zn and most of the REEs are present in concentrations withi n the mean range for most coals Bromine, Cl and Se are volatilized at 1000 degrees C, while U, Th, V, Cr, Co, Cu, Mo and Zn are enriched in the coal ash by a factor of 2-8. Also, there are positive correlations among As, Mo, U and S in the coal, as well as between the amount of s ulphur-bearing minerals in coal ash and total concentration of As, Mo and U in the coal. Sulphur content in the coals ranges from 3.5% to 5. 7%; approximately 4.8-33.6% of sulphur is volatilized at 120 degrees C and a large percentage of total sulphur is volatilized at 1000 degree s C. This has an important implication for the use of the Plakia coals for power generation, because of the emission of SO, compounds to the atmosphere. Fluidized bed combustion and flue gas desulphurization pr ocesses are recommended to achieve reduction in trace element and sulp hur emissions. The HTA is relatively acidic and the high content of Si O2 in the ash may create erosion problems in the boilers. Finally, pot ential problems of slagging and fouling are anticipated, based on the indices (e.g., base-acid ratio) of the lignitic ash.