The comminution of coarse forest biomass into fine products suitable to feed pulverized fuel burners could be an efficient way to produce usable renewable energy in the short term. However, size reduction of lignocellulosic forest materials is an intensive operation that requires energy. Pulverized fuel burners suitable for biomass usually require that the biomass fuel meet certain particle size specifications. In the present work, the specifications for the comminuted product are set as 95% by weight passing a 1000 gin mesh and 12% weight passing a 125 put mesh. Three different forest biomasses, namely poplar chips, pine chips and pine bark, have been selected for size reduction study. Four different types of open-circuit processes have been designed and assayed utilizing a pilot plant comprising two hammermills, one screener and one dynamic air separator, in order to obtain the desired fineness for the product. The biomass raw materials and the products obtained have been characterized and the electrical energy requirements recorded. The process consisting of a primary grinding with 4 mm screen size, followed by selection with a dynamic air separator and secondary grinding of the separator reject, has been identified as the process most suitable for obtaining the desired product. The extrapolation to a full-scale plant for processing 10 o.d. t h(-1) gives estimated energy requirements of 120, 150 and 35 kW It o.d. t(-1) for poplar chips, pine chips and pine bark, respectively, with initial moisture contents between 10% and 15% (w.b.). (c) 2006 Elsevier B.V All rights reserved.