Torrefied biomass, which has better fuel properties than the crude biomass is used as intermediate product for co-firing and gasification with coal. The stability of torrefied biomass, especially moisture content, deteriorates during direct cooling stage of the production process in case of sub-optimal operating conditions and condensation of liquids onto the solid. In this study, three operating parameters of direct cooling stage were optimised statistically with operating cost to produce a stable torrefied biomass. Their effect on the moisture content and final temperature of product was thoroughly investigated in this study. Furthermore, the temperature distribution in the cooling chamber was examined by using a thermal imaging camera. Even though the coolant flow rate played a more prominent role only for product properties and operating cost, the residence time was more effective parameter for cooling. The optimisation results indicated that the stability of torrefied biomass having similar properties with lignite could be improved by using the speed of 2.87 rpm and a residence time of about 20 min at nitrogen flow rate of 4 l/min. The minimum operating cost was calculated when there was a deviation of 20.7% from target product quality. (C) 2019 Elsevier Ltd. All rights reserved.