Fuel-Wetting Processes

Dead vegetation retains its original structure of cells, intercellular spaces, and capillaries. It can soak up liquid water like a blotter, only more slowly, until all these spaces are filled. Dead vegetation may hold two or more times its own dry weight in water. Fine materials may absorb that much in a matter of minutes, while large logs may require a season or more of heavy precipitation. In some climatic regimes, the centers of large materials may never become completely saturated. One reason is that the rate of penetration slows down with increasing distance from the surface.

A second and equally important consideration in understanding fuel-wetting processes is that the materials making up the dead cell walls are hygroscopic. Hygroscopic materials have an affinity for moisture that makes it possible for them to absorb water vapor from the air. This process is one of chemical bonding.

Molecules of water are attracted to, penetrate, and are held to the cell, fiber, or walls by the hygroscopic character of the cell material. The water molecules that penetrate and the few molecular layers that adhere to the cell walls are called bound water. The hygroscopic bond between the cell walls and the water molecules is strong enough to effectively reduce the vapor pressure of the bound water. The layer of water molecules immediately in contact with a cell wall has the strongest hygroscopic bond and lowest vapor pressure. Successive molecular layers have progressively weaker bonds until the cell walls become saturated. At that point, the vapor pressure in the outer layer of water on the cell wall is equal to that of free water, or saturation pressure. The amount of bound water at the fiber-saturation point varies with different materials. For most plant fuel it is in the range of 30 to 35 percent of the fuel dry weight.

The result of the bonding phenomenon is that free water cannot persist in a cell until the cell walls become saturated. Then free water can pass through the cell walls by osmosis. Below the saturation level, moisture is evaporated from cell walls of higher moisture content and taken up by cell walls of lower moisture content until the moisture in each cell attains the same vapor pressure. In this manner, much of the moisture transfer within fuels is in the vapor phase and always in the direction of equalizing the moisture throughout a particular piece of fuel.

Dead fuels will extract water vapor from the atmosphere whenever the vapor pressure of the outer surface of the bound water is lower than the surrounding vapor pressure. In a saturated atmosphere, this may continue up to the fiber-saturation point. Full fiber saturation rarely persists long enough in the absence of liquid water to permit the necessary internal vapor transfer.

See also: Fuel-drying Processes.