A reactor for grinding and roasting biomass, including: a chamber interiorly delimited with internal walls; a grinder laid out inside the chamber, including a central rotary shaft rotatably mounted in the chamber and grinding elements present on the central rotary shaft for grinding against internal walls and of biomass, or lingo-cellulosic biomass, present inside the chamber; a heater for heating and maintaining by thermal conduction via the grinder the biomass present inside the chamber, at a predetermined called roasting temperature between 200° C. and 350° C., to simultaneously achieve grinding and roasting of the biomass in the chamber.