A central composite design of experiments was performed to optimize a free-fall reactor for the production of bio-oil from red oak biomass. The effects of four experimental variables including heater set-point temperature, biomass particle size, sweep gas flow rate and biomass feed rate were studied. Heater set-point temperature ranged from 450 to 650 degrees C, average biomass particle size from 200 to 600 mu m, sweep gas flow rate from 1 to 5 sL/min and biomass feed rate from 1 to 2 kg/h. Optimal operating conditions yielding over 70 wt.% bio-oil were identified at a heater set-point temperature of 575 degrees C, while feeding red oak biomass sized less than 300 mu m at 2 kg/h into the 0.021 m diameter. 1.8 m tall reactor. Sweep gas flow rate did not have significant effect on bio-oil yield over the range tested. (C) 2011 Elsevier Ltd. All rights reserved.