Description

Combustion pellets are produced by first turning wood material through a hammer mill into a homogeneous dough-like mass, which is then compressed. These substances are fed into a compressor, where they are extruded into pellets through a die with through-holes of the desired size (usually 6 mm in diameter, sometimes 8 mm or more). The high pressure of the compressor greatly increases the temperature of the wood, and the lignin slightly plasticizes to form a natural “glue” that holds the pellets together as it cools.

Pellets can be made from grass biomass and other non-woody forms that do not contain lignin: Dried distillers grains (a by-product of the brewing industry) can be added to provide the necessary durability. A 2005 news report from Cornell University News indicated that grass pellet production is more advanced in Europe than in North America. The benefits it recommends as a raw grass include its short growth time (70 days) and ease of growing and processing. The story quotes Jerry Charney, a professor of agriculture at the school, stating that grass produces 96 percent of the heat of wood, and that “any mix of grass can be used, cut in mid to late summer, left in the ground to leach minerals, then packaged and pelletized. The hay from which pellets are made does not need to be dried, making processing less expensive than wood pellets.”