The majority of the thin intercalated sterile layers, which are coexcavated with lignite in the Southern Field mine of Ptolemais, Northern Greece, are petrologically characterized as marly limestones. The), consist of 90-95% calcite, 5-7% micas + clays, and < 4% feldspars. The coexcavation of lignite with marly limestones and/or carbonaceous marls increases a) the content of the carbonate minerals (already contained in lignite), b) the ash content, and c) the total and free CaO content of the lignite-ash and decreases the calorific value of the mined lignite. The increase of total CaO deteriorates the power plant operation due to folding effects, while the increase of the CaO(free) content of the lignite ash results in an increase of the solid particle emissions from the electrostatic precipitators of Agios Dimitrios Power Plant. The temperature increase, which follows the hydration of CaO(free), results in the evaporation of the ash water and the long-term damage of the ash transportation equipment as well. The concentration of CaO(free) in the lignite ash is estimated through a coefficient, which is obtained from the ratio of the CO2 content (db) and the ash content (db) of the mined lignite. The improvement of lignite quality, reducing the CaCO3 content, results in a better power plant operation in terms of economy. and environmental protection.