Bio-FLASHCHAIN® theory for rapid devolatilization of biomass 2. Predicting total yields for torrefied woods

Abstract

Apparently, no detailed reaction mechanisms for any torrefied biomass have yet been put forward. This study expands bio-FLASHCHAIN® to accurately predict the total volatiles yields from any torrefied wood. Whereas the original reaction mechanism and depolymerization kinetics are retained, the analysis introduces a fuel constitution submodel that uses the carbon aromaticity added during torrefaction to estimate the changes in torrefied woods that are responsible for the impacts on primary devolatilization behavior. Torrefaction replaces bridges with char links within wood macromolecules of both cellulose and lignin and these added char links are entirely responsible for the reductions in volatiles yields. The added char links are estimated without any adjustable parameters, given only an ultimate analysis for the torrefied wood. The primary devolatilization of torrefied woods abides by the same reaction kinetics as the parent raw woods, except that the energy distribution for bridge scission narrows for progressively more severe torrefaction conditions to coarsely represent the elimination of the most reactive components during torrefaction. The predictions are validated with volatiles yields from 13 woods and 22 torrefied woods for heating rates from several hundred to more than 104 °C/s to temperatures from 500 to 1000 °C with contact times from tens of milliseconds through several minutes. According to this interpretation, the most important chemistry to unravel in raw and torrefied woods is the accumulation of furans and benzenoid and condensed aromatics in cellulose throughout all stages of both torrefaction and primary devolatilization.