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An experimental study of thermally thick biomass (beech wood spheres) pyrolysis under high radiative heat flux was performed. The influence of sample diameter (5-20 mm), incident heat flux (60-180 kW/m 2 ) and initial moisture content (1-50 wt%) was studied. Char yields and temperature histories were monitored. Initial moisture content impact was highlighted. Indeed, steam coming from the sample core drying can gasify the external char layer, reducing therefore the char yield and increasing syngas production. This study was supported by a 2D unsteady numerical model of biomass degradation (mass, momentum and heat conservation coupled with Broido-Shafizadeh reaction scheme). This model gave more insight about phenomena occurring inside the degrading sample. It revealed that a pyrolysis front follows up a drying one. Therefore, steam is forced out of the sample through a high temperature char layer, making char steam gasification chemically possible.