By B. Jackson, R. Schroeder, and S. Ashton
Biomass feedstock moisture content is dependent on post-harvest handling and storage conditions. Moisture may be lost by transpiration drying, through foliage, or from open wood surfaces. The rate of drying depends on many factors including ambient temperature, relative humidity, wind speed, season, rainfall pattern, tree species, and tree size. The best season for drying usually is dry summer in the South.
Transpirational drying, also known as “leaf seasoning”, “biological drying”, and “delayed bucking” occurs when felled trees are left for several weeks with the tops, branches, and leaves intact (Stokes and Kelly 1993). Such drying can be successfully used with most species. Moisture contents as low as 30 percent may be reached under favorable conditions.
Research studies have been conducted on transpirational drying in the US, and a review of several studies is reported by McMinn (1986) and Stokes and Kelly (1993). They report several earlier findings related to transpirational drying, including the following.
Stokes (1987) provides a basis for predicting transpirational drying of species groups in the southern US based on these variables. It was noted that pine stem weights began to stabilize after about 50 days, soft hardwoods after about 30 days, and hard hardwoods after 40 days. Moisture contents after that period were about 37 percent for pine, 33 percent for soft or low-density hardwoods, and 32 percent for hard or high-density hardwoods (wet weight basis). This pretreatment is important from the standpoint of transport costs as well as combustion efficiency. Each transport load of drier wood will carry less water. Assuming total payload capacities stay the same, there will be more wood (and less water) delivered per load. This will reduce the transport cost per unit of wood, and reduce the amount of water being handled, transported, and evaporated through combustion.
Additional research was performed on another approach to field drying. Sirois and others (1991) reviewed tests performed on a prototype roller crusher. This machine was designed to crush round, smaller diameter stems and facilitate drying by opening the wood to transpirational drying effect. The study found that crushing facilitated drying during periods when rain was absent, and any drying benefit was attained during the first five weeks of drying. The study concluded that there is no guaranteed benefit from crushing trees to increase the rate of moisture loss over long drying periods or periods of heavy rainfall.
Excessive precipitation or low temperatures may hinder the efficiency of transpiration drying (Lehtikangas and Jirjis 1993a). If on-site storage is extended to late fall or even until winter in temperate countries, the advantage gained by transpiration drying will be lost due to absorption of moisture directly from the air and from precipitation (Lehtikangas 1991). One benefit of on-site storage in small heaps is that leaves may be left in the forest, resulting in reduced nutrient loss from the site and better fuel quality.