Composition of Wood Pellets

Green Wood

• Green wood is waste material from wood that is not dried or seasoned, such as raw sawdust and bark that results from sawmills and logging. When green wood is used for pellets, it is dried in a drum dryer. Obtaining green wood to make pellets broadens the manufacturer's options and offers a wider range of materials and sources. To further enhance the efficiency of making wood pellets, waste wood is also used to heat the drums where green wood is dried. No electricity or gas is necessary for this process, according to the Northeast Regional Biomass Program.

Dry Feedstock

• Dry feedstock wood has the ideal moisture content for manufacturing pellets, according to the Northeast Regional Biomass Program. Feedstock is a term used in the biomass fuel production industry that refers to the raw materials needed to create fuel. Dry feedstock wood is obtained in the form of wood chips, bark and shavings, and less often as sawdust. These larger pieces of waste wood are pre-seasoned or dried because they are obtained from wood workers and other manufacturers that don't work with green wood. Although dry feedstock is most often larger pieces that are unsuitable for creating pellets, it can be ground into particles, which allows it to be fed into a compressing machine.

Over Dried Materials

• Some wood waste is too dry to compress. The lignin, or natural adhesive in all wood species, binds the pellets together when they are compressed under heat. However, without an adequate amount of moisture, the lignin will not bind. Water must be added to the raw materials prior to grinding the wood into particles, and before compressing into pellets.

Wood Pellet Grades

• Wood pellets are categorized in three industry-wide grades: Premium, standard and industrial. Each grade represents the ash content, or volume of ash produced when the pellets are burned. It does not refer to ash material in unburned pellets. Premium pellets produce no more than 1 percent ash and are the cleanest choice for home heating. Standard pellets produce no more than 2 percent ash and are also suitable for use in the home. Industrial wood pellets produce 3 percent ash or more. As the name implies, industrial pellets are best left for commercial burning applications where ash content is less of a nuisance, according to Biomass-Events.com. This grading system does not indicate which species of wood were used in production; all pellets contain a variety of woods unless the manufacturer indicates otherwise.