This study examined the operational characteristics of an air-oil furnace adapted for oxy-oil combustion. A 300 kW(th) multi-fuel combustion test furnace at National Cheng-Kung University in Tainan, Taiwan was chosen as the experimental facility. The effect of oxygen enrichment (from 21% to 100%) on the operational characteristics of heavy oil combustion in the retrofitted furnace with flue gas recirculation was examined along with the influences of operating pressure. The experiment results include temperature variation, pressure distribution and flue gas composition (O-2, CO2, CO, NO, SO2). Due to air leakage, CO2 concentration in the flue gas was much lower than the expected value. CO2 concentration was increased by oxygen enrichment from 13% to 34.4% and 14.7% to 61.1% in sub-atmospheric and over-atmospheric operation respectively, with air leakage estimated at 30% and 10% of total flue gas (vol.%, wet basis). The transition from air-oil to oxy-oil combustion did not bring negative impacts to the flame stability or unexpected changes to the operational characteristics. (c) 2012 Elsevier Ltd. All rights reserved.