NOVELTY - The method comprises supplying biomass, which contains an amount of moisture to a first treatment device (3) of a torrefaction system, heating the biomass containing moisture in the first treatment device of the torrefaction system to a torrefaction temperature of 260-290 degrees C within a first time period, in which the biomass containing moisture is completely dried by evaporating the moisture and the dried biomass is partially torrefied to form torrefied biomass, and removing the partially torrefied biomass from the first treatment device after the first time period.    USE - The method is useful for treating a biomass such as mowed grass and prunings, using a torrefaction system, where the biomass is useful as a fuel for cofiring in coal-fired power plants.    ADVANTAGE - The method is capable of economically and rapidly treating the biomass with less energy consumption.    DETAILED DESCRIPTION - The method comprises supplying biomass, which contains an amount of moisture, to a first treatment device (3) of a torrefaction system, heating the biomass containing moisture in the first treatment device of the torrefaction system to a torrefaction temperature of 260-290 degrees C within a first time period, in which the biomass containing moisture is completely dried by evaporating the moisture and the dried biomass is partially torrefied to form torrefied biomass, removing the partially torrefied biomass from the first treatment device after the first time period, supplying the biomass removed from the first treatment device to a plug flow reactor type second treatment device (3) of the torrefaction system, and keeping the biomass supplied to the second treatment device at the temperature of 230-260 degrees C for a second time period, which is longer than the first time period. The first time period is at most 3 minutes. The second time period is 5 minutes, 10 minutes or 20 minutes. The biomass in the second treatment device is kept at a temperature, which is lower than the highest temperature, when the biomass reaches in the first treatment device. The biomass is cooled at the temperature of 240 degrees C during passage from the first treatment device to the second treatment device. The cooling step is carried out by bringing the biomass into direct contact with a coolant gas. The hot gas is supplied to the first treatment device for fluidizing biomass particles of the biomass in the first treatment device. The first treatment device includes continuous ideally stirred-tank reactor (CISTR) type first treatment device, and is provided with an annular treatment chamber, which defines a vertical axis. The treatment chamber comprises a supply for biomass particles, a discharge for biomass particles and a bottom. The bottom of the treatment chamber is provided with supply openings, which debouch obliquely with respect to the vertical axis upwards into the treatment chamber, in which a hot gas is supplied to the supply openings for forming a fluidized bed of biomass particles on the bottom, which is displaced in the peripheral direction of the annular treatment chamber. The hot gas is supplied to the first treatment device, and contains at most 2% of oxygen on a volume basis. The biomass in the second treatment device is kept in the temperature range during the second time period by supplying a hot gas to the second treatment device, which is brought into direct contact with the biomass in the second treatment device. The biomass in the second treatment device is kept in the temperature range during the second time period by a heating agent. The heating agent is separated from the biomass in the second treatment device by a unit of a partition wall, and is in heat-exchanging contact with the biomass in the second treatment device via the partition wall. The first treatment device further comprises a drying device and a torrefaction device. The biomass containing moisture is dried completely in the drying device. The dried biomass is transferred from the drying device to the torrefaction device, and is partially torrefied in the torrefaction device. The biomass is heated in the drying device to the temperature of 80-180 degrees C. The dried biomass is heated in the torrefaction device of the first treatment device to the torrefaction temperature. The method further comprises supplying the biomass, which contains a first amount of moisture to a pre-drier, and heating the biomass containing the first amount of moisture in the pre-drier in order to evaporate moisture from the biomass until it contains a second amount of moisture which is smaller than the first amount of moisture. The biomass containing the second amount of moisture is supplied to the first treatment device of the torrefaction system. An INDEPENDENT CLAIM is included for a torrefaction system for treating biomass.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a system for treating a biomass.    First treatment device (3)    Inlet (6)    First cooling device (10)    Second treatment device (18)    Second cooling device. (24)