NOVELTY - Torrefaction (m1) of optionally predried biomass in torrefaction reactor (2) to obtain torrefied biomass and gases (18) involves supplying oxygen-based gas (9, 10, 11, 12) to reactor at a position in reactor to react oxygen with gases under heat formation, and removing gases at second position, where first position is located downstream of second position in relation to biomass transport direction in the reactor such that gases moves through the reactor countercurrent to biomass transport and optionally predried biomass has temperature of 30-230 degrees C when entering the reactor.    USE - For torrefaction of optionally predried biomass (i.e. lignocellulosic biomass, such as an agriculture residue or a forestry residue such as a wood material, such as wood chips) in torrefaction reactor to obtain torrefied biomass and gases (claimed).    ADVANTAGE - The present method solves the problems of known torrefaction processes, and thus increases the heating value at a fixed energy yield, increases the energy yield in the torrefaction process at a fixed heating value, decreases the energy needed for grinding of a torrefied biomass in to a fine powder at a fixed energy yield, increases the energy yield, mass yield and heating value, and decreases the energy needed for grinding to a fine powder at a fixed torrefaction degree. Also the method is more energy efficient and decreases the heating and torrefaction time. The present method utilizes torrefaction reactor having smaller size thus the reactor is cheaper if the heating and torrefaction process is fast.    DETAILED DESCRIPTION - Torrefaction (m1) of an optionally predried biomass in a torrefaction reactor (2) to obtain torrefied biomass and torrefaction gases (18) involves supplying an oxygen-containing gas to the torrefaction reactor at a first position in the reactor such that oxygen reacts with components of the torrefaction gases under the formation of heat, removing torrefaction gases from the torrefaction reactor at a second position of the torrefaction reactor, where the first position is located downstream of the second position in relation to a biomass transport direction in the torrefaction reactor such that the torrefaction gases moves through the torrefaction reactor countercurrent to the biomass transport and the optionally predried biomass has a temperature of 30-230 (preferably 50-200, especially 60-180) degrees C when entering the torrefaction rector. INDEPENDENT CLAIMS are included for the following:    (1) producing (m2) a pulverized torrefied biomass involving: performing torrefaction of optionally predried biomass in torrefaction reactor according to method (m1); and milling the obtained torrefied biomass;    (2) use of a pulverized torrefied biomass or a slurry or an extrudable paste produced according to the method (m2) for production of heat and/or electricity in a combustion process or for production of product gas or synthesis gas in a gasification process; and    (3) a system (1) for torrefaction of a biomass, comprising a torrefaction reactor for torrefying the biomass having a biomass inlet (3) and a biomass outlet (4) where biomass can be transported from the inlet to the outlet in a biomass transport direction during a torrefaction process, and further comprising at least one gas inlet for injection of an oxygen containing gas to the reactor (where the gas inlet is connected to oxygen supply unit for a controlled supply of an oxygen-containing gas to the torrefaction reactor), and an outlet for torrefaction gases (where the outlet for torrefaction gases is located upstream of the gas inlet in relation to a biomass transport direction within the reactor); and further comprising a source of the oxygen-containing gas, which source is connected to the oxygen supply unit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a system for torrefaction of a biomass comprising a torrefaction reactor having a biomass inlet and a biomass outlet.    System (1)    Torrefaction reactor (2)    Biomass inlet (3)    Biomass outlet (4)    Rotary vane feeders (5)    Transport screw (7)    Oxygen containing gas (9, 10, 11, 12)    Torrefaction gases (18)