Potential for Compression Drying of Green Wood Chip Fuel

Equipment currently designed for and in use by the forest products industry for the dewatering of wood bark and chips by mechanical pressure can only reduce the moisture content (MC) to 45 or 50 percent (wet basis). Thus, this equipment is suited to remove water added in processing but not free water normally present in green wood. Exploratory research described in this paper has shown on a laboratory scale that the proper application of high pressure to green wood chips can remove the water from cell lumens, thus reducing the MC to 35 percent or less. Further, this water removal may be accomplished in an energy efficient manner. The ratio of energy increase of the fuel (increase in recoverable heat) to the mechanical energy required for water removal was found to vary from 67:1 to 240:1. This ratio depends upon wood specific gravity (SG) green MC, and mode of compression. A theoretical relationship between volumetric compression, MC, SG, and water extracted was developed and tested on several species of softwoods and hardwoods. The cost savings for fuel and boiler investment in a hypothetical situation are presented to illustrate the economic potential of the process if production scale equipment can be developed.

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