The lignite thermal plant operation is affected by the fuel quality. In the lignite mines of Northern Greece, both inorganic impurities and thin intercalations, especially when they consist of carbonate minerals, co-excavated with lignite, reduce the run-of-mine lignite quality. Before any lignite exploitation or combustion process study, both the lignite and the lignite ash quality have to be thoroughly investigated in order for the recoverable blocks of lignite to be determined. Marly layers, characterized as marly limestones, containing up to 95% CaCO3, are thermally decomposed producing a high percent of CaO and CO2. The high CaO(total) and CaO(free) contents of the lignite ash cause fouling problems in the combustion systems of the power plants and increase the solid particle emissions to the atmosphere. To control the CaO(free) content of the ash below a critical limit of 11%, an index (Sigma) = [CO2 in lignite/ash((d.b).())] x 100 may be used. For values of (Sigma) lower than 17, the CaO(free) of the lignite ash is controlled approximately lower than 11%. The use of the index (Sigma) in determining the CaO(free) content of the ash and controlling the blending process of lignite layers has improved the operation of the Agios Dimitrios Power Plant, leading to economic and environmental advantages, such as reduction of lignite specific consumption, increase of the calorific value, and reduction and stabilization of the solid particle emissions.