After harvesting, the biomass is commonly stored and air-dried before it is trans­ported and further processed. The rate of drying depends strongly on environmental factors, such as temperature and humidity, the particle size and the stacking method. Whole logs lose their moisture slowly and reach a MC of about 30 % after drying anything between 4 weeks and 6 months. This time can be decreased, if the logs are sheltered and cut into smaller pieces. Chips of a few cm size can reach a MC of about 12 % after a few weeks in sunny, arid conditions (Sturos et al. 1983).

Good ventilation is vital to good drying results. This becomes especially a problem, when the biomass is first comminuted and then dried. If the chips are piled on top of each other, ventilation is inhibited and fungi and bacteria can attack the particles. Biological and chemical degradation caused by bacteria and other organisms lead to an increase of temperature, which can in some cases lead to self­ignition of the entire pile.

Air drying should ideally happen on a flat area without contact to moisture (e. g. on a concrete slab under shelter) and the pile should be turned over on a regular basis.

All these problems can be avoided by oven or kiln drying the biomass, which on the other hand increases the processing costs drastically and sets free carbon dioxide, which affects the energy balance negatively (see Chap. 10).