Forest chip drying in self-heating piles during storage as affected by temperature and relative humidity conditions

Storing is an important part in the value chain of the energy use of forest chips as severe losses of dry mass and energy content and increases in greenhouse gas emissions may occur. During the storage the temperature in the biomass increases affecting the dry mass loss and the material drying. In a field-scale experiment, we monitored temperature inside two uncovered wood chip piles for 175 days and measured the chip gravimetric moisture content at the beginning and end of the experiment. In the laboratory the drying rate of the wood chips under different temperature and relative humidity conditions was determined. We constructed a new two-dimensional simulation model where water vapor diffusion within the pile, and chip moisture change was solved using a finite volume method. The determined chip drying rate was used as a source term in the simulation. The simulation model results were in close agreement with the experimental data in the middle sections of the pile, whereas the model was not able to describe the chip moisture redistribution in the pile tails. Modeling revealed that the pile size and porosity were important factors in determining the chip drying, which indicates that chip drying can be controlled by management, such as planning of pile dimensions and compaction.