Currently a new method of producing pellets of high quality – torrefaction (roasting, drying) is gaining popularity in bioenergy. Torrefaction is a "soft" pyrolysis – the heating of biomass without oxygen at temperatures 200 – 300°C and atmospheric pressure.
Moisture and volatile substances, formed during the partial decomposition of cellulose and lignin, are removed from biomass during pyrolysis process. This reduces the weight of the material by 20-30% and simultaneously leads to an increase in the specific calorific value.
The dry residue obtained in the form of dust is called “bio coal”. Dust subsequently is pelletized by analogy with wood fuel pellets.
One of the biggest advantages of this process is the processing of various wood raw materials (with different moisture content and sizes) in almost monotonous raw material for the manufacture of pellets. Pellets made from such raw materials are distinguished by almost black color, high energy intensity and, as a consequence, reduced transportation costs.
In addition, burning of these pellets takes place practically without producing smoke.
Thus, the main advantages of torrefied biofuels include:
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