Thermodynamic Modeling of an Indirectly Heated Rotary Kiln for Biomass Slow Pyrolysis: A Methodological Framework and Illustrative Pilot-Scale Application

Abstract

Biomass slow pyrolysis in indirectly heated rotary kilns involves coupled heat transfer among the external flue gas, the kiln wall, the moving solid bed, and the internally generated gas. This study presents a transparent, steady-state, one-dimensional thermodynamic modeling framework that couples these four domains and accommodates counter-current external heating. The external heating side is represented explicitly through flue-gas convection and radiation and radial conduction across the cylindrical wall, while the internal formulation accounts for wall–solid, wall–gas, and solid–gas heat exchange. Moisture removal and biomass devolatilization are described through a phase-based relation derived from thermogravimetric analysis (TGA). Because the TGA relation is imposed as a constitutive input, its reproduction by the model is treated as an implementation consistency check rather than as an independent prediction. The framework is illustrated using the geometry, feedstock data, operating conditions, and limited temperature records available from a pilot-scale rotary kiln. For the baseline conditions, the calculation generates axial temperature and mass-flow profiles and a conditional breakdown of the heat-transfer pathways. The available pilot measurements are used mainly as inputs or boundary conditions and do not constitute a complete independent experimental validation, particularly for axial solid and wall temperatures, local conversion, or heat flux. The present contribution is therefore positioned as a reproducible engineering calculation framework for screening and hypothesis generation. Predictive use for reactor design, optimization, or scale-up requires subsequent validation against independent measurements collected over multiple operating conditions.