Torrefaction of biomass

Fuel wood is often difficult to use because of its poor combustion characteristics such as low heating value, variable moisture content which is often high, hydroscopic nature, smoking during combustion, etc. For a number of other applications, wood is often upgraded to charcoal. The charcoal-making process is inefficient with the product containing only about 55% of the energy of the original raw material in well-managed, commercial operations and as little as 20% in traditional processes.
Torrefaction
Torrefaction is a thermo chemical treatment of biomass at 200 to 320 °C. It is carried out under atmospheric conditions and in the absence of oxygen. During the process, the water contained in the biomass as well as superfluous volatiles are removed, and the biopolymers like cellulose, hemicellulose and lignin partly decompose giving off various types of volatiles. The final product is the remaining solid, dry, blackened material which is referred to as “torrefied biomass” or “bio-coal”. After the biomass is torrefied it can be densified, usually into briquettes or pellets using conventional densification equipment, to further increase the density of the material and to improve its hydrophobic properties.
Benefits
Torrefaction appears to be an attractive option for upgrading wood to a product which retains about 90% of its energy and can be substituted for charcoal in a variety of applications. Biomass which is typically thermally unstable usually leads to formation of those condensable tars in gasifiers, making problems in down-stream equipment such as choking and blockage of piping. Torrefaction eliminates this problem.
The important advantages of torrefied wood include high energy yield and hydrophobicity so that it does not regain moisture during storage. Torrefaction achieves a stable low moisture content of 3%, reduction of mass by 30%, retention of 90% of original energy content and removal of smoke producing agents. All biological activity is eliminated reducing the risk of fire and stopping biological decomposition. Torrefied wood has a heating value of approximately 22,500 kJ/kg and highly friable and can be easily crumbled or pulverized. Torrefied biomass has excellent combustion properties; the fuel can be readily co-fired with coal, further gasified or fed to pyrolysis units.