A process model is developed to simulate the performance of a biomass torrefaction unit, which consists of a dryer, a torrefaction reactor, a combustor, and two heat exchangers. The model is capable of predicting the composition of volatiles and torrefied biomass, mass and energy yields, thermal efficiency, process heat requirement, and CO2 emissions. Useful correlations are presented for the heating value, molecular weight, and specific heat of the volatiles. A comparison of the model prediction with the experimental data reported in the literature shows a very good agreement. The performance of a torrefaction unit with production capacity of 150 MWth is predicted with an acceptable accuracy using the presented process model. The effect of moisture content, torrefaction temperature, residence time, and adiabatic flame temperature on key process parameters are examined. The thermal efficiency of willow torrefaction unit is found to be 88% at a moisture content of 50% (dry basis), which increases to 94% as the moisture content drops to 20%. The results show that the carbon dioxide produced in the process is notably affected by the torrefaction temperature and the moisture content. A higher moisture content or torrefaction temperature may lead to a higher CO2 emission. Furthermore, the conditions at autothermal operation are identified and discussed.