Abstract Heavy fuel oil (HFO) is an economical fuel alternative for power generation as its low production cost and high energy density. However, its incomplete combustion induced by the presence of long-chain petroleum molecules in the fuel results in high levels of emissions. Here, we investigate the influence of the swirl flow on the combustion and emissions of a spray HFO swirling flame. To this end, HFO is sprayed into a hot swirling air, using an air-blast nozzle. The flame blowout limits are tested under different swirl flows. An investigation of the in-flame temperature fields, gaseous emissions including CO, CO2, O2, NOX, SOX, UHC (Unburned Hydrocarbon) and solid particles in the form of cenospheres are used to quantify the performance of the HFO combustion. The influence of the HFO swirling flame is tested under different conditions of global equivalence ratio, swirling number, and tangential and axial airflow rates. A comparison of two different flame regimes that fuel-jet dominate flame and air-driven vortex flows are investigated and compared in various swirling flow conditions. The results show that the tangent air is the primary factor for preheating and evaporating the fuel, thus defining the flame operating regimes.