Combustion is a great source of carbonaceous and inorganic particle emissions, even though the combustion technologies and their efficiency are improving. Flameless oxy-combustion technology provides frontier emission performances, with the most advanced Rankine+Joule Bryton high yield thermodynamic cycle. The process ISOTHERM PWR® is an innovative high temperature, pressurized oxy-combustion process recently developed. A 5 MWth demonstration unit, settled at Gioia del Colle (BA) scores more than 5000 hr of tests on different fuels, i.e. solid, non vaporizable liquid, vaporizable liquid, gas, and wastes. In this paper, we focused our attention on the size distribution function of the particles formed during the combustion process and emitted when burning different fuels (heavy oils, wastes and coal). Different particle-size measuring techniques are used including atomic force microscopy and spectroscopic characterization of the material collected in water samples and on-line inertial classifier and SEM analysis.