The vaporizalion and pyrolysis of a heavy fuel droplet moving through a high temperature inert gas is analyzed, with special emphasis on fuel chemical composition and how it changes during the droplet lifetime. A mathematical model is proposed based on a number of simplifying assumptions, including distillation-like vaporization. With this model the relation between droplet size, asphaltene fraction and coke residue formation has been examined. Calculation results show that finer atomization is a good way to reduce coke formation. These results further indicate that experiments with large droplets or with thermogravimetric apparatus may lead to wrong conclusions as to the coke residue farming tendency of heavy fuel sprays in atmospheric combustion devices.