Prediction of pyrolysis kinetics for torrefied biomass based on raw biomass properties and torrefaction severity

Abstract

Torrefaction is a thermal pretreatment of biomass at mild temperatures to upgrade the fuel properties. The torrefied biomass consists of residual chemical structures after partial decomposition; therefore, its pyrolysis kinetics largely inherit those of raw biomass. This study proposed a novel kinetic model for pyrolysis of torrefied biomass, established from three-parallel reaction model (TPRM) of raw biomass. Three structural changes caused by torrefaction were incorporated into the model. First, the residual lignocellulosic composition was estimated using the TPRM. Second, the partial decomposition of weaker chemical bonds was considered using the cumulative Rayleigh distribution with a scale parameter determined from the residual lignocellulosic components. Third, the increase in the amount of fixed carbon due to char-forming reactions was treated by a correlation acquired from the experimental data. When applied to hinoki cypress, kenaf, and wood pellets at various torrefaction severities, the proposed model accurately reproduced the pyrolysis kinetics of torrefied biomass. The prediction quality (R2) for typical torrefaction severities was greater than 0.9967 and 0.9833, respectively, depending on the correlation of fixed carbon used for individual biomass and the average trend. In the latter, the model required only a single thermogravimetric analysis to derive all model parameters at any torrefaction severity.