Pellet fuels (or pellets) are biofuels made from compressed organic matter or biomass.[1] Pellets can be made from any one of five general categories of biomass: industrial waste and co-products, food waste, agricultural residues, energy crops, and virgin lumber.[2]Wood pellets are the most common type of pellet fuel and are generally made from compacted sawdust[3] and related industrial wastes from the milling of lumber, manufacture of wood products and furniture, and construction.[citation needed] Other industrial waste sources include empty fruit bunches, palm kernel shells, coconut shells, and tree tops and branches discarded during logging operations.[4][5] So-called "black pellets" are made of biomass, refined to resemble hard coal and were developed to be used in existing coal-fired power plants.[6] Pellets are categorized by their heating value, moisture and ash content, and dimensions. They can be used as fuels for power generation, commercial or residential heating, and cooking.[7] Pellets are extremely dense and can be produced with a low moisture content (below 10%) that allows them to be burned with a very high combustion efficiency.[8]