Multi-scale analysis of drying thermally thick biomass for bioenergy applications

Highlights: • Existing drying models are often feedstock-dependent and cannot be generalized. • A new generalizable drying model for thermally thick biomass particles was proposed. • The kinetics model for drying was integrated into a single-particle model. • Spatial inhomogeneity was quantified against experimental data for drying time. • The model was used to size and optimize a commercial-scale dryer as a case study. -- Abstract: Drying is a crucial process in many thermochemical processes for bioenergy. However, most existing drying models often have the following shortcomings: they are feedstock-dependent, or they are unable to describe the spatial inhomogeneity that often develops within thermally thick biomass particles under higher temperature gradients. In this paper, a multi-scale analysis was undertaken on the dynamics of drying thermally thick biomass under high temperatures, based on a new physical drying kinetics approach that is independent on feedstock-specific empirical parameters. A single-particle approach was then layered onto this kinetics model to understand how the spatially inhomogeneous moisture and temperature profiles inside a biomass particle evolve over time during drying. This process generates predicted temperature and drying time profiles that were successfully validated against experimental data. Subsequently, the impact on drying by various factors—particle size, geometry, initial moisture content, and reactor temperature—was studied. These observations were used to consider the design choices of a commercial-scale dryer, highlighting the key trade-offs, as well as optimized combinations of temperature and particle size that minimizes the operating cost. The approach described in this paper can be readily integrated into other mathematical descriptions of bioenergy conversion processes such as gasification and combustion.