Polish energy sector comprises nearly exclusively the conventional power plants, based on the domestic raw material, i.e. hard coal and brown coal. Coal is the basic source of energy, as well as the main factor providing the positive balance in the country's energy balance. The assumptions of Poland's Energy Policy for years to come anticipate a gradual reduction of the coal share in the electricity production. Hiwever, coal will continue to play the leading role in the sector of electricity production due to lower costs comparing to other energy carriers and due to the energy safety of the country. Run of mine coal is characterised by various qualitative parameters. The quality of the coal is not always in conformity with the standard conditions designed for the boiler. The combustion of the coal of parametrs considerably different than those designed causes the deterioration of the boiler operation quality, thereby, lowering the efficiency. In extreme cases, the boiler may even become damaged. In order for the recipient of the coal to receive the product with parameters exactly reflecting those ordered the coal must be subject to preparation processes. The process that is most often used is coal preparation in dense media, in jigs, or in centrifugal separators. The deeper the preparation is, the purer coal is obtained, of the better quality parameters. However, too deep preparation causes large amount of grains (organic substance) to transfer into the waste, thereby increasing the losses of chemical energy. Also the quality of processing machines contributes to the amount of losses. The quality of preparation can be described by performance curves. These curves are prepared on the basis of densimetric analysis of tested coal. The boiler is another place where the losses of chemical energy occur. The losses in the boiler are indicated through the boiler instrumentation systems, which allows drawing up the corresponding diagrams. The relation between yield of coal in the preparation process and the losses in the losses, in the boiler can be determined by a two step analysis. In the first step the relation between the densimetric fractions yield and the calorific value of prepared coal is found. In the second step the relation between the losses in the boiler and the calorific value of burnt coal is determined. The superposition of the two functions gives the relation between yield of coal and losses in boiler. The relation in question allows setting such preparation conditions at the preparation plant that the losses in the boiler during combustion are minimised.