In the ever-increasing demand for alternative fuels, one promising solution is the partial substitution of conventional fossil fuel with biofuel rather than completely replacing it. In this experimental study, a 300 kWth (thermal power) oil-fired furnace is fueled with blends of crude castor oil and diesel to examine the feasibility of using these blends as a substitute for pure diesel in industrial applications. Castor oil is derived from castor seeds, which possesses low heating value, high oxygen content and high viscosity. A furnace test was conducted for pure diesel and castor oil/diesel blends with various mixing ratios. Pure diesel and blends with 5%, 10%, 20% and 30% castor oil were comparatively investigated by focusing on wall temperatures in the radiative section of the furnace, gas temperatures in the convective section and emission products from combustion. All experiments were performed under the operating condition of optimum (minimum) excess O-2 concentration in the flue gas. The air supply rates for the minimum excess oxygen requirement were 245, 241, 240, 237 and 236 Nm3/h, respectively, for pure diesel and 5%, 10%, 20% and 30% blended castor oil at the fixed liquid-fuel supply rate of 20 L/h. Excellent stable combustion was observed during the experiments with the castor oil/diesel blended fuels. Both the wall temperature and gas temperature dropped slightly with increases in castor oil content in the blends; nevertheless, they were very close compared with pure diesel. Additionally, with increasing castor oil content in the blends, NO and CO emissions only slightly decreased and increased, respectively. Accordingly, it is verified that the use of 5-30% castor oil in the blends produces similar furnace temperature distributions and comparable emission levels of CO, NO and SO2 when compared to pristine diesel. (C) 2017 Elsevier Ltd. All rights reserved.