Experimental data on ultimate analysis of different biomasses torrefied at various conditions are collected from past studies to establish a new semi-empirical model for predicting the C-H-O compositions of a torrefied wood and volatiles at a given temperature and residence time. The model is used to conduct a comparative analysis of torrefaction of two popular woody biomasses: willow and beech. Key parameters such as the energy yield and the lower heating value of the torrefied wood can be predicted using this model. In particular, the heat of reaction is calculated at temperatures in the range 523-573K and residence time between 10-40 min for the two woods considered in this study. The reaction heat of willow torrefaction was found to be between 140 kJ/kg and 300 kJ/kg, whereas for beech torrefaction it was observed to vary between 170 kJ/kg and -130 kJ/kg. The results suggest that at a low temperature (e.g. 523 K), the reaction is less endothermic at a longer residence time for both willow and beech. Willow torrefaction is found to be more endothermic at 573 K than 523 K. On the other hand, for beech, the process shifts from endothermic to exothermic when the temperature increases from 523 K to 573 K.