NOVELTY - The method involves autothermal gasification of biomass in presence of air in a reactor under reduced pressure and at temperature greater than 800`C using an autothermal gasification reactor (1) for obtaining syngas. The syngas is cooled at a temperature which is not less than 300`C by a cooling device (2). The syngas is washed by a washing device (3) for removing tars and/or ammonia. The washed gas is injected in a gas engine (4) coupled to an electrical generator for producing electrical energy.    USE - Used for upgrading a biomass e.g. wood under form of billet, pellet or plate, wooden vegetal sub product e.g. leaf, rod, pod, spindle and herbs, produced by forest operation e.g. crown, bark and sawdust, by sawmill e.g. saw mill and plate, by a wood transformation industry, by a carpenter, furniture fabricant, panel fabricant and pallet fabricant, an industrial sub product e.g. slime from pulp e.g. black liquid and waste of agro-food industry e.g. grape-cake, coffee grounds, pulp, grape seed, endive root, linen waste and bagasse, products from traditional agriculture e.g. cereals and oleaginous, residue e.g. straw, rice hull, and bagasse, new plantation e.g. willow and miscanthus, organic waste such as urban waste having biosolid, household waste, waste from agriculture such as agricultural effluent, waste from breeding like manure, litter and liquid manure, for producing electrical energy from the biomass (claimed).    ADVANTAGE - The washed syngas is injected in the gas engine coupled to the electrical generator for producing electrical energy in an efficient manner at less cost, while permitting to process and upgrade all sorts of biofuel such as vegetal waste, without necessitating prior extraction step of plants and isolation of specific parts of the plants. The method avoids production of atmospheric pollutants during gasification.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for an installation for producing electrical energy from a biomass.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of an electrical energy producing installation.    Autothermal gasification reactor (1)    Cooling device (2)    Washing device (3)    Gas engine (4)    Vapor turbine (6)