NOVELTY - The reactor comprises a chamber (1) that is delimited into an upstream zone (1A) and a downstream zone (1B) and comprises a unit for mixing and transferring a biomass from one end to another end of the chamber, and a transfer and sealing system (5) allowing the transfer of the dried biomass to the downstream zone with time intervals and to seal the zones during each interval. The upstream zone includes a drying unit ready to carry out drying of the biomass introduced into the reactor. The downstream zone includes a roasting unit ready to carry out roasting of the dried biomass.    USE - The reactor is useful for drying and roasting biomass such as lignocellulosic biomass in a biomass treating installation (all claimed).    ADVANTAGE - The reactor allows easy processing of liquid waste and ensures uniform heat treatment on the biomass.    DETAILED DESCRIPTION - The reactor comprises a chamber (1) that is delimited into an upstream zone (1A) and a downstream zone (1B) and comprises a unit for mixing and transferring a biomass from one end to another end of the chamber, and a transfer and sealing system (5) allowing the transfer of the dried biomass to the downstream zone with time intervals and to seal the zones during each interval. The upstream zone includes a unit for drying the biomass introduced into the reactor. The downstream zone includes a roasting unit ready to carry out roasting of the biomass dried in the upstream zone. The reactor further comprises: a kiln provided with overlying beds defined by fixed plates in which a transfer opening is provided; and a heat pipe element maintained inside a V-shaped throat practiced in the fixed plates. The mixing and transfer unit comprises a set of arms ready to mix and transfer the biomass on each fixed plate according to a trajectory of the transfer opening through which the biomass runs out by gravity. Each of the fixed plate includes two plates superimposed one on the other by a V-shaped spacer whose interior consists of the throat. The transfer and sealing system is a rotary lock including a upstream plate that is mounted in the chamber and placed in a lower end of the upstream zone, and includes a transfer opening, a downstream plate that is mounted on a rotary shaft in the chamber and placed in an upper end of the downstream zone and includes a transfer opening. The transfer opening of the upstream plate is adapted to be in regard with the transfer opening of the downstream plate at time intervals into functions rotational speed of the rotary shaft. A distance between the upstream plate and the downstream plate is 1-5 mm. The upstream and downstream plates have a same number of transfer openings such that all stacks of the upstream plate are arranged in order with stacks of the downstream plate. INDEPENDENT CLAIMS are included for:    (1) an installation for treating biomass; and    (2) a method for drying and roasting biomass.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of an installation for drying and roasting a biomass integrating a reactor and including fluid circuits (Drawing includes non-English language text).    Chamber (1)    Upstream zone (1A)    Downstream zone (1B)    Transfer and sealing system (5)    Cyclonic separator. (23)