The combustion of coal and/or biomass (sludge, wood waste, RDF, etc.) in a circulating fluidized bed has been a commercial topper for over20 years, and references to principles and applications are numerous and widespread although few data are presented concerning the operation oflarge scale CFB-units. The authors studied the CFB-combustion at UPM-Kymmene (Ayr), a major paper mill relying for its steam production uponthe combustion of coal (80-85%), wood bark (5-10%) and wastewater treatment sludge (5-10%). The maximum capacity of the CFB is 58 MWth.A complete diagnostic of the operation was made, and additional tests were performed to assess the operating mode. The plant schematics,relevant dimensions and process data are given. To assess the operation of the UPM-CFB, it is important to review essential design parameters andprinciples of CFB combustors, which will be discussed in detail to include required data, heat balance and flowrates, operating versus transportvelocity, kinetics and conversion (including the possible effect of the Bouduard reaction if carbon is present).Since the residence time in the riser and the cyclone efficiency determine the burnout of circulating fuel-particles, the UPM-CFB was subjectedto a stimulus response technique using nickel oxide as tracer. Results illustrate the efficiency of the cyclone separation and the number of recycleloops for particles of a given size. Results will also be used to assess the cyclone operation and efficiency and to comment upon expected andmeasured carbon conversion. The combustion of coal and / or biomass (sludge, wood waste, RDF, etc.) in a circulating fluidized bed has been a commercial topper for over 20 years, and references to principles and applications are numerous and there those few data are listed in the relevant The authors studied the CFB-combustion at UPM-Kymmene (Ayr), a major paper mill relying for its steam production uponthe combustion of coal (80-85%), wood bark (5-10%) The maximum capacity of the CFB is 58 MWth. A complete diagnostic of the operation was made, and additional tests were performed to assess the operating mode. The plant schematics, relevant dimensions and process data To assess the operation of the UPM-CFB, it is important to review essential design parameters and principals of CFB combustors, which will be discussed in detail to include required data, heat balance and flowrates, operating versus transportvelocity, kinetics and conversion (including the possible effect of the Bouduard reaction if carbon is present) .Since the residence time in the riser and the cyclone efficiency determine the burnout of circulating fuel-particles, the UPM-CFB was subjectedto a stimulus response technique using nickel oxide as tracer. Results illustrate the efficiency of the cyclone separation and the number of recycleloops for particles of a given size. Results will also be used to assess the cyclone operation and efficiency and to comment upon expected andmeasured carbon conversion.