Burning dense, viscous combustibles such as heavy fuel-oil as a water-in-oil emulsified combustible enables to decrease the emission of solid carbonaceous residue, in comparison with raw, non-emulsified combustible. This is due to the phenomenon of micro-explosion, meaning the rapid (<0.1 ms) vaporization of the water droplets inside the emulsion, breaking up the initial emulsion droplet into numerous and faster 'daughter-droplets'. The present work is based on a small-scale furnace (300 kW max.) feed with heavy fuel-oil mixed with 10-20% of gasoil, with and without emulsion of water. The emulsification of combustible enables to record a reproducible lowering in emission of carbonaceous residue from the combustion of emulsified fuel, in comparison with raw fuel. This is added to a variation in granulometry of carbonaceous residue, hereby considered as an indicator of second atomization. Copyright (C) 2009 John Wiley & Sons, Ltd.