Lab-scale experiments were carried out to investigate a new approach of preparing biochar-based catalyst for transesterification reaction. Black liquor (BL) from cotton pulping process was used as a precursor for preparing the catalyst via spray drying followed by fast carbonization in a fluidized bed reactor. Response surface methodology (RSM) was employed to statistically analyze and optimize the entire process of catalytic synthesis of biodiesel from a waste frying oil, consisted of the catalyst preparation and transesterification steps. Several major operation parameters of the biodiesel synthesis process, including alkaline loading in biochar, methanol to oil ratio, catalyst loading, and the carbonization temperature of catalyst, were studied for their effects on biodiesel conversion. The optimal condition for achieving the maximum biodiesel conversion (i.e. 91.5 %) was found to be: 27 wt% alkaline loading in biochar, 600 degrees C carbonization temperature, 7.7 methanol to oil ratio, and 6.5 wt% catalyst loading. Overall, the spry drying fluidized bed carbonization demonstrated its viability for significantly reducing duration of high temperature carbonization, opening up the prospects of manufacturing the catalyst in a much less energy consumption manor. (C) 2020 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.