The typical moisture content of freshly cut wood ranges from 30 to 60%, and for some biomass it can exceed 90% (see Table 2.9). Every kilogram of mois­ture in the biomass takes away a minimum of 2260 kJ of extra energy from the gasifier to vaporize water, and that energy is not recoverable. For a high level of moisture this loss is a concern, especially for energy applications. While we cannot do much about the inherent moisture residing within the cell structure, efforts may be made to drive away the external or surface moisture. A certain amount of predrying is thus necessary to remove as much moisture from the biomass as possible before it is fed into the gasifier. For the production of a fuel gas with a reasonably high heating value, most gasification systems use dry biomass with a moisture content of 10 to 20%.

The final drying takes place after the feed enters the gasifier, where it receives heat from the hot zone downstream. This heat dries the feed, which releases water. Above 100 °C, the loosely bound water that is in the biomass is irreversibly removed. As the temperature rises, the low-molecular-weight extractives start volatilizing. This process continues until a temperature of approximately 200 °C is reached.