Ultimate analysis: The determination of the elemental composition of the organic portion of carbonaceous materials, as well as the total ash and moisture. See elemental analysis.
Uneven-aged Management: A regeneration and management technique that removes some trees in all size classes either singly, in small groups, or strips in order to maintain a multi-aged stand.
Under-fire air: Combustion air added under the grates. Serves the function of drying the fuel, cooling the grates, and supplying oxygen to the pyrolysis reactions.
Understory: Refers to vegetation that grows on the forest floor. This can include shrubs, bushes, grasses, flowers, and even very small trees.
Unmerchantable wood: Material which is unsuitable for conversion to wood products due to poor size, form, or quality.
Urban Residues: Wood and yard waste;construction and demolition debris from an urban source.
Urban wood waste: Woody biomass generated from tree and yard trimmings, the commercial tree care industry, utility line thinning to reduce wildfire risk or to improve forrest health, and greenspace maintenance.
Uronic acid: A simple sugar whose terminal -CH2OH group has been oxidized to an acid, COOH group. The uronic acids occur as branching groups bonded to hemicelluloses such as xylan.
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