(104d) Useful Correlations for Describing and Analyzing Torrefaction Behavior of Biomass

Torrefaction of biomass is a useful technology for improving its properties as solid fuel. There are a few important variables for describing behavior of torrefaction, such as the solid mass yield (Y_m), calorific value (CV), and solid energy yield (Y_e) [1]. A factor S was proposed for integrating the effects of reaction time (t) and temperature (T) in the field of hydrothermal treatment: S=t∗exp((T-100)/14.75) [2]. This idea was transferred to the field of torrefaction as the torrefaction severity factor: SF=log(t∗exp((T-T_R)/14.75)), where T_R is a constant parameter, called the reference temperature [3,4]. Recently, this factor SF was correlated with the mass loss (ML or 1-Y_m): ML=A∗SF+B, where A and B are system constants [5]. Thus, ML is correlated with t and T by combining ML=A∗SF+B and SF=log(t∗exp((T-T_R)/14.75)). A fact that just still a small number of torrefaction studies have used the torrefaction severity parameter and/or the equation ML=A∗SF+B for discussion implies that there may be still something to be improved for these from a practical point of view. The most serious drawback of this correlation, combination of SF and ML=A∗SF+B, is that it can’t reproduce the inflexion point which is observed in ML vs. T relationship at higher torrefaction temperatures [6,7]. This presentation proposes a new one-step correlation of ML=ƒ(t, T), which can reproduce the inflexion point. Furthermore, this presentation proposes a correlation of CV/CV_raw=k∗ML, where k is a constant parameter. By combining these two correlations, CV/CV_raw and Y_e can be predicted from T and t. CV_raw is the calorific value of the raw biomass.

References

[1] T. G. Bridgeman, et al., Torrefaction of reed canary grass, wheat straw and willow to enhance solid fuel qualities and combustion properties, Fuel, 87, 844-856 (2008).

[2] R.P. Overend and E. Chornet, Fractionation of Lignocellulosics by Steam-Aqueous Pretreatments, Phil. Trans. R. Soc. Lond. A, 321, 523-536 (1987).

[3] J.-W. Lee, et al., Optimizing the Torrefaction of Mixed Softwood by Response Surface Methodology for Biomass Upgrading to High Energy Density, Bioresource Technology, 116, 471-476 (2012).

[4] B.-I. Na, et al., Torrefaction of oil palm mesocarp fiber and their effect on pelletizing, Biomass and Bioenergy, 52, 159-165 (2013).

[5] W.-H. Chen, et al., Torrefaction performance prediction approved by torrefaction severity factor, Fuel, 251, 126-135 (2019).

[6] T. Sawai, et al., Estimation of energy properties of torrefied Japanese cedar with calorimetric values, Mechanical Engineering Journal, 4(1), 1-14 (2017).

[7] J. Huang, et al, Valorization of Food Waste via Torrefaction: Effect of Food Waste Type on the Characteristics of Torrefaction Products, Energy & Fuels, 34, 6041-6051 (2020).