Production
Pellets are produced by compressing the wood material which has first passed through a hammer mill to provide a uniform dough-like mass. This mass is fed to a press where it is squeezed through a die having holes of the size required (normally 6 mm diameter, sometimes 8 mm or larger). The high pressure of the press causes the temperature of the wood to increase greatly, and the lignin plastifies slightly forming a natural 'glue' that holds the pellet together as it cools.
Pellets conforming to the norms commonly used in Europe (DIN 51731 or Ö-Norm M-7135) have less than 10% water content, are uniform in density (density in excess of 1 ton / cubic meter, so they do not float if placed in water), have good structural strength, and low dust and ash content. Because the wood fibres are broken down by the hammer mill, there is virtually no difference in the finished pellets between different wood types. Pellets can be made from nearly any wood variety, provided the pellet press is equipped with good instrumentation, the differences in feed material can be compensated for in the press regulation.
Pellets conforming to the European standards norms cannot contain any recycled wood or outside contaminants. Recycled materials such particle board, treated or painted wood, melamine resin-coated panels and the like are particularly unsuitable for use in pellets, since they may produce noxious emissions and / or uncontrolled variations in the burning characteristics of the pellets.
New pellet mills are being opened in the United Kingdom and other nations of Europe on a rapid basis, decreasing the price of a tonne of pellets to new lows. The scarcity and unreliable supply that used to come with the dependence on wood pellets has vanished.
Standards used in the United States are different, developed by the Pellet Fuel Institute, are not mandatory, and are generally less strict than those of Europe; for example, it is accepted that pellets exposed to large volumes of water in the US may significantly degrade (turning into "mush"). Still, many manufacturers comply, as warranties of US-manufactured or imported combustion equipment may not cover damage by pellets non-conformant with regulations. Prices for US pellets surged during the fossil fuel price inflation of 2007 - 2008, but subsequently have decreased signficantly in late 2008 and early 2009, and are generally lower on a per-BTU basis than most fossil fuels, excluding coal, which is not an option highly favored for heating by many residential and commercial consumers, due to frequent maintenance/tending requirements for end users, high carbon emissions, air pollution (often leading to nuisance complaints from neighbors and/or investigation by boards of health and environmental agencies), the mess generated by coal dust and/or improper storage, as well as potentially dangerous fly ash.
