The purpose of this work is to prove the technical feasibility of the bubbling fluidized bed co-combustion, using biomass and low-grade coal mixtures and applying the exergy method. The pilot plant modelled is an atmospheric bubbling fluidized bed combustion chamber with a nominal capacity of 1 MWth. We have applied the mass balance, the energy balance and the exergy balance to the plant in nine experiments, which have been performed at different operation conditions. The exergy analysis includes the calculation of the exergy destruction and the exergetic efficiency of the plant for these experiments. An estimation of the irreversibility cost is also evaluated. (c) 2005 Elsevier Ltd. All rights reserved.