Generalized heating value estimation of torrefied woody biomass based on pyrolysis kinetics of primary constituent polymers

In 2021, more than 40 countries have agreed with de-coal statement at the COP26. The torrefied solid biofuels have come to attract attention as promising alternative fuels to coal in coal-fired power plants and industrial boilers. However, the energy property such as higher heating value (HHV) and energy yield of torrefied biomass, which is one of the most important fuel properties, has been evaluated by means of experiments for various biomass species so far. That is, the torrefaction conditions to produce torrefied solid biofuel with predetermined HHV have been provided only empirically. In this study, a generalized HHV estimation method applicable to any torrefied woody biomass is investigated based on pyrolysis kinetics of primary constituent polymers of woody biomass, cellulose, lignin and xylan. Analyses of pyrolysis kinetics of constituent polymers are conducted by assuming that the process in non-isothermal pyrolysis experiments is modeled as the single reaction. The HHVs of torrefied constituent polymers are obtained by isothermal torrefaction experiments, and the experimental correlations of HHV of torrefied constituent polymers are proposed as a function of solid mass yield. For a given woody biomass whose mass fractions of three constituent polymers are known, HHV of torrefied woody biomass produced with any torrefaction condition can be estimated by applying pyrolysis kinetic models and experimental correlations of HHV for three constituent polymers to the torrefaction process of woody biomass. From the comparison between estimated HHV of torrefied woody biomass and experimental data, it is concluded that the generalized HHV estimation method can be useful for evaluating HHV of torrefied woody biomass with about 10% accuracy, although the HHV is somewhat underestimated.

Automated summary

This study investigates a generalized higher heating value (HHV) estimation method for torrefied woody biomass, using the pyrolysis kinetics of cellulose, lignin, and xylan as primary constituents. Experimental correlations of HHV for torrefied constituent polymers are proposed, enabling the estimation of HHV for torrefied woody biomass based on the mass fractions of the constituent polymers. The method shows approximately 10% accuracy in evaluating HHV, although there is a slight underestimation.