Combustion of heavy fuels is one of the main sources of greenhouse gases,particulate emissions,ashes,NOx and SOx.Gasification is an advanced and environmentally friendly process that generates combustible and clean gas products such as hydrogen.Some entrained flow gasifiers operate with Heavy Fuel Oil(HFO)feedstock.In this application,HFO atomization is very important in determining the performance and efficiency of the gasifiers.The atomization characteristics of HFO(Mazut)discharging from a pressure-swirl atomizer(PSA)are studied for different pressures difference(Δp)and temperatures in the atmospheric ambient.The investigated parame-ters include atomizer mass flow rate((m)),discharge coefficient(CD),spray cone angle(θ),breakup length(Lb),the unstable wavelength of undulations on the liquid sheet(λs),global and local SMD(sauter mean diameter)and size distribution of droplets.The characteristics of Mazut sheet breakup are deduced from the shadowgraph tech-nique.The experiments on Mazut film breakup were compared with the predictions obtained from the liquid film breakup model.Validity of the theory for predicting maximum unstable wavelength was investigated for HFO(as a highly viscous liquid).A modification on the formulation of maximum unstable wavelength was pre-sented for HFO.SMD decreases by getting far from the atomizer.The measurement for SMD and θ were compared with the available correlations.The comparisons of the available correlations with the measurements of SMD and θ show a good agreement for Ballester and Varde correlations,respectively.The results show that the experimen-tal sizing data could be presented by Rosin-Rammler distributions very well at different pressure difference and temperatures.