This study deals to improve the understanding of biomass thermo-chemical conversion in a dense and fast internally circulating fluidized bed. The experimental rig used in this study ran either as a dense fluidized bed or as a fast internally circulating fluidized bed. The effect of operating parameters, such as bed temperature (ranging from 750 to 850 degrees C), steam gas velocity (between 3 and 9.5 times the minimum fluidization velocity of bed material), steam to biomass mass ratio (ranging from 0.7 to 5.6 kg.kg(-1)) and biomass shape (sawdust and pellets) is investigated in both dense and fast internally circulating fluidized bed with olivine as bed material. In addition, the effect of bed material nature (olivine and sand) is also studied. Results indicated that syngas composition is strongly dependent on steam to biomass mass ratio. Moreover, it was found that syngas yield is favored by rising bed temperature and steam to biomass mass ratio while it is disadvantaged when fluidizing gas velocity increases. In addition, a greater syngas yield was obtained with olivine particles, compared to sand particles. The effect of bed material circulation flow rate was also investigated in fast internally circulating fluidized bed. Results showed that an increase in this parameter leads to a lower syngas yield. Finally, the experimental results presented in this paper were compared to ones reported in the literature and a good agreement was found.