Bed agglomeration is a problem related to fluidized bed combustion. It is a result of bed particles and/or fuel ash gluing together, forming agglomerates, i.e., large clusters. These agglomerates can, over time, increase in the bed to the point where the fluidization of the bed is affected. In the case of defluidization or collapse of the bed, a boiler would have to be shut down for maintenance. It is mainly a problem related to burning biomass, especially those exhibiting large amounts of alkali. Researchers have made significant efforts to find preventive measures to avoid a costly shutdown of boilers. There are two main mechanisms causing agglomeration. Coating-induced agglomeration is a result of reactions between the fuel ash and the bed material. This results in a coating on the bed material. In comparison, melt-induced agglomeration stems directly from the ash-forming matter, and a melt is formed from compounds within the fuel ash. In both cases, the melt then acts as a glue between bed particles resulting in interparticle forces. When this interparticle force exceeds the forces pulling them apart, agglomerates can form. Additives, such as kaolinite, have been shown to prevent melt-formation. This is attributed to the capture of alkali in the case of kaolinite, thus preventing it from forming molten alkali-silicates. The main research objectives for this thesis were the following: • Determining the inhibiting effects of limestone and kaolinite • Expand upon the knowledge of agglomerations mechanism and the influence of additives • Determine the amount of additive to prevent agglomeration Initially, a literature review was conducted to review the current state of research in the area. This included understanding the bed agglomeration phenomena itself as well as current methods for preventing it. In addition, an experimental campaign was conducted with a laboratory-scale fluidized bed reactor. The campaign aimed to determine the influence of temperature and additives on agglomeration. Sunflower seed shells and wheat straw were used as fuels, while kaolinite and limestone were used as additives. Samples from the spent bed were analyzed with scanning electron microscopy and x-ray spectroscopy. The results show that kaolinite is a proficient countermeasure for agglomeration. The effect is dependent on the agglomeration mechanism. It works better in the case of coating-induced agglomeration. With melt-induced agglomeration, the initiation was delayed but not completely avoided. Limestone did not yield any significant improvement compared to tests without additives.