Power generation facilities are under constant pressure to comply with increasingly stringent environmental regulationsnamely, coal utilities. One possibility to aid in compliance is through the use of torrefied biomass as a replacement or supplement for traditional fossil coal. To obtain a continuous understanding of the changes to biomass as it is torrefied, a three-stage chemical reaction model with intrinsic rate coefficients was used. This gas/vapor phase model predicts the evolution of primary torrefaction products and was further used to predict changes in elemental composition and energy content of torrefied biomass. From the model, over the typical torrefaction region (up to similar to 25-30% mass loss), the relative amount of carbon increased by approximately 9%, the oxygen decreased approximately 10%, and the hydrogen content was relatively unchanged. The material retained 90% of the original available energy, and the higher heating value increased by 22.5% from 19.1 to 23.4 MJ/kg. These properties were modeled as a function of mass loss and correspond well to values measured by others for woody feedstock (root mean squared error of 2.0% for carbon, 0.43% for hydrogen, and 2.8% for oxygen).