A method for choosing the optimum fineness of coal in pulverised-fuel boilers is presented. This method is based on economic criteria and takes into account variations of the power unit load as well as the durability and the output of pulverisers. Calculations are performed for short periods of time (e.g. 1 h), provided that it is possible to change the fineness of the coal by controlling the angle of the classifier guide vanes. Therefore, the optimum value, adapted to the current parameters, may be easily set. A program was created in MS Excel to perform the optimization calculations. It enables making decisions concerning the modernisation of grinding systems and their proper operation when changing the fuel characteristics as well as the load and costs of power unit operation. The work is based on tests of pulverisers of a 650 t/h pulverized fuel boiler (OP 650) in a Polish power plant. On the basis of these studies, relationships were derived between the fineness of the produced pulverized fuel and operating parameters, the efficiency of the boiler and parameters of the milling system, e.g., energy consumption of the mill and its fan. The influence of variation of excess air in the boiler on the maximum grain size that can burn at an acceptable unburned combustibles level is also shown. The analysis of the impact of the composition of the mineral part of the Polish hard coal to the abrasion of mills was made. Based on the measurements carried out and the actual periods between overhauls of mills, which were determined during the operation, a correlation was found to determine the abrasive wear rate of the elements of ring-ball mills with nominal output of 33 t/h (type MKM 33). Exemplary calculations of optimum value of ROM were also performed. (C) 2017 Elsevier Ltd. All rights reserved.