bio-FLASHCHAIN® theory for rapid devolatilization of biomass 1. Lignin devolatilization

摘要

This study validates a modeling framework to accurately predict the total and tar yields from any lignin at any devolatilization conditions. The original reaction mechanism and depolymerization kinetics in bio-FLASHCHAIN (R) are slightly modified for pure lignin, and used to accurately interpret how the molecular weight distribution (MWD) of a lignin governs its primary devolatilization behavior. The only sample-specific input requirements are an ultimate analysis and a weight-average molecular weight (M-W), although a measured M-W-value is not needed for lignin in whole biomass. Predicted total and tar yields are validated with measured values from 20 diverse lignins for heating rates from 10 degrees C/min to 5000 degrees C/s to temperatures from 430 to 950 degrees C with contact times of several seconds. Predicted tar yields increase for progressively lighter lignin MWDs, and accurately depict how variations in lignin MWD affect total volatiles yields. The strong impact of lignin MWD on its primary devolatilization behavior is accurately interpreted by the flash distillation analogy without any adjustments to the reaction kinetics for a diverse assortment of lignins. The entire reaction mechanism in bio-FLASHCHAIN (R) is required with non-pyrolytic lignins, whose predicted total and tar yields increase in inverse proportion to reductions in M-W. For pyrolytic lignins, bridge scission is superfluous because the whole lignin falls within the size range for tar precursors initially and throughout devolatilization. Consequently, neither total nor tar yields are affected by M-W variations because the tar vaporization rates are maximized throughout this M-W range. Devolatilization shifts toward hotter temperatures for progressively faster heating rates, in good agreement with data, whereas predicted total and tar yields are insensitive to heating rate variations. Total yields increase for progressively hotter temperatures due to the transition from tar production to char decomposition in the reaction mechanism. Tar yields saturate to an asymptotic ultimate value at approximately 550 degrees C for fast heating rates.