This article presents a novel air-blown bubbling fluidized bed device that has the ability to sample gas and bed materials at various axial positions during the gasification experiments. The reactor was operated with olivine, as bed material, and Miscanthus, a biomass rich in potassium and silica, and thus prone to bed agglomeration. The comparison of gas and char axial profiles along the bed allows a better understanding of the biomass gasification: it shows in particular that 02 consumption and CO, production at the bottom of the bed are mainly due to char oxidation, even if few pyrolysis gases may also be produced and oxidized near the grid. Regarding bed defluidization, the agglomerate fraction is followed by taking bed samples during the operation: it is shown that the rate of agglomeration is linear while defluidization signs appear when the agglomerate fraction reaches 6% near the grid. Small agglomerates are observed on the top of the bed, whereas big agglomerates are segregated near the grid. The SEM-EDX analysis shows that the layer that sticks olivine particles together does not strictly correspond to biomass ashes melt: it contains also particles and atoms that come from the erosion of olivine and the stainless steel wall.