Torrefaction is a mild pyrolysis, which is used for biomass pretreatment aimed to increase the heating value and hydrophobicity. Nowadays there are a lot of various torrefaction techniques. The heating of the wood pellets or briquettes inside the layer of mineral particles under atmospheric conditions can represent one of the prospective torrefaction techniques. Significant diffusion resistance of the mineral layer can favor substantial reduction in the oxygen flux to the biomass from the environment. Due to this one can carry out torrefaction without the use of inert gas. The paper analyzes experimentally the torrefaction of pine pellets inside the layer of bentonite clay or microcalcite. It is established experimentally that the height of mineral layer does not affect significantly on the mass yield at 230 degrees C. However, under the conditions of severe torrefaction (290 degrees C) the increase in mass yield with height of mineral layer is observed. The calorific properties of torrefied biomass are estimated on the basis of elemental analysis. It is shown that the value of energy yield increases together with the mass yield.