Torrefaction

Torrefaction of biomass, e.g., wood or grain, is a mild form of pyrolysis at temperatures typically between 200 and 320 °C. Torrefaction changes biomass properties to provide a much better fuel quality for combustion and gasification applications. Torrefaction leads to a dry product with no biological activity like rotting. Torrefaction combined with densification leads to a very energy-dense fuel carrier of 20 to 21 GJ/ton lower heating value (LHV). Torrefaction makes the material undergo Maillard reactions.

Biomass can be an important energy source. However, nature provides a large diversity of biomass with varying characteristics. To create highly efficient biomass-to-energy chains, torrefaction of biomass in combination with densification (pelletisation or briquetting) is a promising step to overcome logistic economics in large-scale sustainable energy solutions, i.e. make it easier to transport and store it. Pellets or briquets are lighter, drier and stable in storage as opposed to the biomass they are made of.

Torrefaction is a thermochemical treatment of biomass at 200 to 320 °C (392 to 608ºF). It is carried out under atmospheric pressure and in the absence of oxygen, i.e. with no air. During the torrefaction process, the water contained in the biomass as well as superfluous volatiles are released, and the biopolymers (cellulose, hemicellulose and lignin) partly decompose, giving off various types of volatiles. The final product is the remaining solid, dry, blackened material that is referred to as torrefied biomass or bio-coal.

During the process, the biomass typically loses 20% of its mass (bone dry basis) and 10% of its heating value, with no appreciable change in volume. This energy (the volatiles) can be used as a heating fuel for the torrefaction process. After the biomass is torrefied it can be densified, usually into briquettes or pellets using conventional densification equipment, to increase its mass and energy density and to improve its hydrophobic properties. The final product may repel water and thus can be stored in moist air or rain without appreciable change in moisture content or heating value, unlike the original biomass from which it is made.

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Wikipedia