Torrefaction is considered as a kind of mild pyrolysis that is carried out in inert atmosphere. During this process, the moisture of the initial fuel and part of volatiles are removed leading to advanced properties like high energy density, hydrophobicity and durability. The application of torrefaction process to biomass feedstock has beneficial effect on reducing storage, transportation and handling costs. The development of a comprehensive process model, which takes into account the effect of basic parameters, such as fuel quality, reaction temperature and residence time, is considered as necessary step for the design of a new pilot scale torrefaction reactor. The developed model will be used for construction of the torrefaction reactor of straw pellets, at Russia (Rostov region) and will have a capacity of 1 ton/h. The commercial simulation tool “ASPEN Plus” is used as a main modelling tool. The required data for straw is obtained from proximate and ultimate analysis carried out at CERTH/ ISFTA’s laboratory. The calculation of product yield composition is based on post processed results obtained from the available literature after their correlation with relevant temperatures and residence times. Detailed kinetic data obtained from TGA analysis carried out at CERTH/ ISFTA, is also used for the accurate estimation of the reduction rate of torrefied biomass and the composition of solid residue. Through the process energy balance, the required heat flux of the heating medium to keep constant process conditions is calculated. Finally, after the initial model set up, sensitivity analysis including process optimisation is carried out, so that the main design parameters and the proposed mass and energy balance will be determined. The present paper is based on the results from a study prepared by Clean Energy Ltd., spin off company of CERTH/ ISFTA.