A temperature dependent framework to predict and control physical pellet quality in biomass extrusion

Pellet manufacturing of biomass (food, feed, bioenergy) presses powders or particles into dense pellets with improved nutritional, calorific, and handling properties. This process upgrades industrial co-products from agriculture, forestry, and bioenergy into higher-value products. However, processing particulate streams raises the scientific question: Under which conditions do loose particles bind to form rigid, durable pellets? This work answers this question for biomass extrusion. Systematic experiments reveal how steam conditioning temperature, production rate, and die geometry interact to determine pellet quality. We propose an overarching framework introducing the stickiness temperature ($T^*$), marking the onset of enthalpic reactions required for particle agglomeration. $T^*$ serves as the boundary for inter-particle bond formation and is reached through a combination of steam conditioning and friction, both controllable via process parameters. Results highlight the combined role of pellet temperature and die residence time in optimizing pellet durability while lowering specific energy use (J/kg). Validation with experiments and literature confirms that this framework offers practical guidance to enhance efficiency and sustainability of pelleting. By providing operational parameters to control bonding and energy input, this work supports a more circular economy through efficient conversion of diverse biomass streams into valuable products while reducing energy consumption and greenhouse gas emissions.