Technology behind torrefaction
In the past, a great deal of research has been carried out (by ECN, among others) into torrefaction of clean wood. The advantages of these processes have been determined and are well established. In recent years, others have investigated the torrefaction of more difficult biomass materials and residues, which demonstrated similar and even additional advantages.
The raw material is introduced into a reactor, excluding oxygen. The raw material is then heated in the reactor to a temperature of 280 to 310 °C. During this process, a flammable gas is emitted, which is redirected to deliver the heat required for regenerating the own process. An external energy source is used for starting up the plant. Once the plant has reached the required operation temperature, the process becomes self generating or sufficient. After a given period in the reactor, a semi-finished product of 70 percent of the original mass, but containing 90 percent of original energy, is arrived at. Torrefaction results in a homogeneous fuel, with a much improved grinding quality, a higher energy density and containing fewer impurities. The fuel is also hydrophobic, preventing it from absorbing water during storage and making it resistant to rot and fermentation. Further, the fuel has a considerably higher bulk weight to the original biomass, realising substantial savings on transport.
The semi-finished product is mechanically treated once the thermal process is complete. The semi-finished product is then physically reduced in size and sifted, to remove any foreign bodies or impurities liable to cause problems. Due to the diminishing of size, the surface area of the semi-finished product becomes larger, which is important in view of the following step.
The substance is dried to a humidity content of < 5%, using the available heat from the torrefaction process.
The production and substance are tested against quality standards, then stored in silos, which facilitates easy handling and transport by barge or truck.