Torrefaction is a promising thermal pretreatment method for biomass in which many material properties are enhanced. The biomass components (hemicellulose, cellulose and lignin) degrade in this thermal process to different extents depending on type of process, treatment temperature, residence time and biomass type. Torrefaction severity is usually defined by biomass weight loss or mass yield, but other approaches to determine degree of torrefaction have also been suggested. For continuous and large scale facilities, mass yield can be challenging to determine and another approach to determine torrefaction severity is therefore desired. In this study, one existing and two new approaches for determining degree of torrefaction are presented, compared and evaluated including uncertainty analysis. The three approaches were based on analysis of; volatile matter, thermochemical properties (enthalpy of formation), and higher heating value.
All three methods were highly correlated to mass yield and independent of torrefaction process. The degree of torrefaction based on higher heating value predicted mass yield most accurate, had lowest measurement uncertainty and the results were independent of biomass type. In the evaluation of the method based on formation enthalpy it was revealed that the increase in heating values for torrefied biomasses could be explained by the combination of increase in enthalpy of formation and decrease in oxygen content.