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.
The pellets contain a low humidity content that allows them to be burned with very high combustion efficiency. Their regular geometry and size allows automatic feeding with fine calibration. In addition their high density permits compact storage which facilitates transport over long distances.
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, cannot contain any recycled wood or outside contaminants, 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.
Wood pellets produced from bamboo have an 11,000 Btu/lb value, which is almost equivalent to that of coal. Only minor changes are required to the boiler and feed-in systems in order to use in co-fired coal power plants. The specification of bamboo wood pellets conforms to the current EU and US standards. The demand for wood pellets continues to increase due to mandatory emissions reduction targets being enforced by many Western Governments on coal fired power plants.
This picture shows a palletizing factory which can produce wood pellets from the bamboo Woodchips.