Field and laboratory measurements of the air permeability of chipped forest fuel materials

Abstract

The air permeability in different forest materials has been measured in the field and at different degrees of compaction in the laboratory. A new method was developed to make rough measurements of the air permeability in situ and was used to determine air permeability in field test piles. A sonde was put into the pile, then air was blown into the pile through a perforated part of the sonde. A rough value for the permeability may then be calculated from the air flow rate and the difference between the static pressure in the air at the perforated part of the sonde and the atmospheric pressure. The method should be suitable for other types of materials stored in piles, such as coal. According to the measurements, the permeability for air in the compacted logging residues is less than 2 × 10 −9 m 2 after 7 months storage. In the piles with compacted oak chips and uncompacted logging residues the permeability is a factor of 10 higher than the compacted logging residues. The permeability is then too high to make reliable measurements of the difference in pressure in the laminar region. The method developed for in situ measurement of permeability works well for permeabilities less than 10 −8m 2. For materials with higher permeabilities, measurements in a laboratory column are recommended. In the laboratory measurements, the permeabilities of torn bark, fresh logging residues and chips from the sawmill are found to be of the magnitude of 10 −7m 2. The permeability decreases moderately due to compression of the material. The permeability may decrease considerably during the storage period due to compaction of the pile, redistribution of moisture within the pile and vigorous mycelial growth in some parts of the pile.