The combustion of heavy fuel oils such as Bunker C and vacuum residual oil (VRO) are widely used for industrial applications such as furnaces, power generation and for large marine engines. There is also the possible use of bio-oils derived from biomass. Combustion of these oils generates carbonaceous particulate emissions and polynuclear aromatic hydrocarbons (PAH) that are both health hazards and have an adverse effect on the climate. This paper explores the mechanism of the formation of fine particulate soot and cenospheres. The chemical structure of petroleum asphaltene have been investigated via pyrolysis techniques. The results are consistent with a structure made up of linked small aromatic and naphthenic clusters with substituent alkyl groups, some in the long chains, with the building blocks held together by bridging groups. Other functional groups also play a role. The corresponding bioasphaltene is made up of similar aromatic and oxygenated species and behave in an analogous way. (C) 2016 Elsevier Ltd. All rights reserved.