Agglomeration is one of the main challenges for combustion and gasification of biomass in fluidized beds. Bed agglomeration is related to K species present in biomass. Understanding the role of different types of K species on formation of agglomerates at different conditions can reveal the mechanisms of biomass ash induced agglomeration. Extensive experiments are conducted in a laboratory scale fluidized bed reactor, using mixtures of quartz sand and K species, including KCl, K2SO4 and K2CO3, to study the agglomeration mechanisms. The effects of gas composition, including air, H-2(-) and H2O- containing gas are investigated. The morphology and elemental analyses of the agglomerate samples are examined by SEM/EDS analysis. Thermodynamic equilibrium calculations are performed for verifying the proposed mechanisms. The results showi that the role of various forms of potassium salts on agglomeration in fluidized beds is different. Gas composition also has strong impact on the agglomeration tendency. In the air and H-2-containing gas, defluidization of KCl in a sand bed is caused by the melt of KCl. However, KCI reacts with SiO2 to form K-silicates in the H2O-containing gas, which results in a lower defluidization temperature. No defluidization is observed for K2SO4 in the presence of oxygen or water. However, K2SO4 decomposes and reacts with SiO2 to form K-silicates, causing defluidization at 850 degrees C in the H-2(-) containing gas. In the air, H-2(-) or H2O-containing gas, K2CO3 would react with SiO2 to form potassium silicates and KOH may be additionally formed in the H-2(-) and H2O-containing gas. The possible mechanisms of agglomeration of various potassium salts at different conditions are discussed.