A reaction engineering analysis of charcoal formation in batch kilns

Charcoal formation in batch kilns is a process that has traditionally relied on operator experience and trial-and-error rather than a fundamental understanding of the phenomena of wood pyrolysis and combustion. Recent regulations to control the harmful emissions from these kilns will presumably drive the industry towards better operating practices; a thorough scientific analysis of the physicochemical phenomena governing wood pyrolysis and combustion is critical to support a rigorous pollution prevention assessment and develop guidelines for improved operation of the kilns. This research addresses the gap in scientific understanding of the process by adopting a chemical reaction engineering approach through refined kinetic, modeling, comprehensive particle scale modeling and bench scale experiments.; Currently available kinetic models for wood pyrolysis are not amenable to a pollution impact analysis of the process; hence, a refined kinetic scheme was proposed which differentiates the gaseous fraction into inorganic and organic gases. Proof-of-concept of the kinetic model was demonstrated by estimating kinetic parameters from experimental data published in the literature. The estimated parameters were reasonably successful in predicting product yields of independently conducted experimental studies; the significant contribution of this work was the development of a parameter estimation approach that does not necessitate a priori assumptions about the ultimate yields of any product.; The proposed kinetic scheme was then incorporated into the broader framework of a particle scale model for wood pyrolysis and combustion which provides a transient description of the major phenomena involved in charcoal formation—kinetics of pyrolysis and combustion, transport of gaseous species through the porous matrix, heat transfer due to an externally imposed heat flux, and variation of the effective physical properties with temperature and extent of reaction. Simulations conducted for a wide range of cases illustrated the transient nature of the process and provided insights into the governing physicochemical phenomena. Cursory experiments were conducted to investigate some of the model-predicted trends. A parametric analysis of the sensitivity to operating conditions demonstrated the variation of system performance indices—such as product yields at 99% wood conversion, and the time required to achieve those yields—with heat flux, particle size and oxygen concentration. The analysis indicated the potential for controlled variation in operating conditions on the kiln scale to shift the distribution of volatile products towards presumably less-harmful emissions.; The particle scale model provided insights into the important phenomena that merit attention and identified key parameters that require experimental measurement. The model provides the crucial framework for effective reactor scale modeling to achieve a complete description of charcoal formation and explore pollution prevention options which do not compromise overall product quality and yield.