Four industrial coal-fired atmospheric fluidized bed systems used to produce process steam and hot oil were investigated. Chemical and particle size analysis of the coal, bed ash, and fly ash samples taken from the investigated fluidized bed systems were performed. Minimum fluidization velocity was calculated for each combustion system and compared to its operation velocity. The combustible parts of the fly ash samples were determined and related to the operation conditions. The reasons for inefficient combustion were indicated and some suggestions were made to increase combustion efficiency.