Abstract Characteristics of biomass used as fuel and distinct ash flows in a thermal power plant with BFBC (bubbling fluidised bed combustion) technology were studied. An integrated approach involving chemical composition, microscopy analysis, mineralogy, and thermal behaviour were used to characterize the ash. The ashes have a low unburned content, typically below 3 wt.% (dry basis), with higher values in ash from the electrostatic precipitator. The chemical element present in higher concentration in the several ash flows is Si (>20 wt.%, dry basis). The fly ash from the electrostatic precipitator are enriched in heavy metals when compared to other ash, and Zn is the heavy metal found in higher concentration (but