Agglomeration during reactive extrusion of particle-based biomass pellets

Creating mechanically sound aggregates from powder or grain-like feedstocks is a fundamental challenge across industries. One example is the production of pellets from organic materials, like wood residues and cereal grains, for biofuel and animal feed applications. However, the handling and processing of these organic powders is complex, and the binding mechanisms between particles remain poorly understood at physicochemical level. In this work, we address this knowledge gap by analyzing the energy consumption of physicochemical binding mechanisms during reactive extrusion of biomass pellets, by conducting experiments in an industrially relevant pilot plant. Understanding the mechanisms that drive this agglomeration process under various conditions and within various application domains are therefore important subjects of research into granular material dynamics. We focus on the heat flow into the organic ingredients during the agglomeration process induced by reactive extrusion and find that not all the heat increases the product temperature. Instead, some heat is absorbed by internal changes such as phase transitions or chemical reactions. We identify that the energy absorption mechanism changes significantly once the material reaches the so-called stickiness temperature (T∗). Below T∗, the ingredient temperature increases linearly with the energy input and pellets do not bind well; above T∗, the material absorbs energy to undergo transformations that greatly improve the agglomerate strength. Our findings indicate how further work can probe agglomeration kinetics in more detail and show that industrial processes can fine-tune product quality by controlling heat flow during steam conditioning and extrusion. We tested this framework with various ingredient mixtures and confirmed its robustness despite chemical differences. Our results highlight the importance of ingredient-specific physicochemical properties in reactive extrusion and open new directions for optimizing particle agglomeration processes.