Check wikipedia etc. for Torrefaction - the usual disclaimers about taking wikipedia as gospel apply. Here is a portion:
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.[3] The final product is the remaining solid, dry, blackened material[4] 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.
The "absence of oxygen" part makes it not so simple. Here is a part that may be relevant to this audience:
Miscellaneous uses:
C. F. Martin & Co. uses torrefaction to obtain more dimensionally stable product than traditional kiln-drying or air-drying provides, resulting in guitar parts that they claim are similar to older pieces of wood. They state that, "We believe this allows us to approximate the tone of a vintage guitar." [7]