Fluidized bed gasification is an attractive conversion technology for biomass. However, processing biomass in fluidized beds suffers from ash related problems. In this work, bed agglomeration of wood pellets and grass pellets was compared, focusing on de-fluidization temperatures and variation in microstructure and composition of agglomerates. The results showed that both biomasses form agglomerates at temperatures between 700 °C and 900 °C. After each test run, residual sand and agglomerates were discharged and the ash/bed material ratio was calculated. Larger amount of accumulated ash and agglomerates were collected from the gasification of grass pellets than that from wood pellets, indicating that different mechanisms are present in the agglomerate formation. The ash/bed material varied from 3 to 10 wt% for the grass pellets and from 0.8 to 1.5 wt% for the wood pellets. A scanning electron microscope with energy dispersive X-ray spectroscopy was used for characterizing morphology and elemental composition of the agglomerates. The findings show that the surface of agglomerates from grass were dominated by Si, Ca and K, while the surface of agglomerates from wood mainly were dominated by Si and K. The cross-sectional analysis concluded that Si, Ca, and K are the ash forming elements that play the most significant role in agglomeration processes.