Abstract Detailed measurements have been performed for heavy fuel oil (HFO) sprays, generated by a twin-fluid atomiser, in a large-scale laboratory cylindrical furnace. Data are reported for mean gas temperature, major species concentrations (02, C02, CO and unburnt hydrocarbons), incident wall radiation flux, flue-gas NOx and solid particulates, Together they allow for an assessment of the effect of the secondary air swirl and excess air levels on the combustion characteristics. The main conclusions drawn are: (i) an increase in secondary air swirl (0.9 to 1.3) leads to improvements in mixing, droplet vaporisation and chemical reaction in the near burner region (NBR). The implications of the changes are manifest in the pollutant emissions wherein the higher swirled flame presents lower exit NOx, level due to less “fuel NOx, ” formation but higher solids emissions due to lower cenospheres residence times in the NBR; (ii) an increase in combustion excess air (15% to 25%) leads to more combustion in the NBR...