Agglomeration mechanisms of a fluidized bed burning agricultural residues – Laboratory defluidization experiments coupled with thermal analyses

The use of agricultural residue biomass fuels (agrofuels) in power and heat production is increasing steadily as they are readily available and affordable. However, the high content of alkali, phosphorus, and silicon can lead to agglomeration challenges. To effectively mitigate agglomeration, it is essential to identify the underlying mechanisms and assess the influence of various elements present in the ash. In this study, four agrofuels − wheat straw, sunflower seed shells, wheat bran, and rape seed residue − were studied in a laboratory-scale fluidized bed reactor at 850 and 900 °C using a quartz bed. These fuels had high potassium contents, and varying silicon, phosphorus, calcium, magnesium, and sulfur contents, which drastically affected the agglomeration mechanisms and tendencies. Detailed analyses including SEM/EDX, XRD, DSC/TGA and thermodynamic calculations were conducted to understand the agglomeration mechanisms, ash species formation, and melting behavior. The results showed that if the fuel had a high potassium content relative to both silicon and phosphorus, a reactive K-silicate forming agglomeration mechanism was observed. Using an Si-free bed material (ilmenite) proved an effective countermeasure against this mechanism. In contrast, if the fuel had a K:Si or K:P molar ratio equal to 1 or below, agglomeration was mainly caused by molten ash. Increasing the Ca content for a fuel with a high K and P content, decreased agglomeration due to formation of K-Ca-phosphates. Additionally, a high calcium or sulfur content reduced the agglomeration tendency of the fuel due to formation of high-temperature melting K-Ca-phosphates, Ca-phosphates and K2SO4.