Comparison and characterization of torrefaction performance and pyrolysis behaviour of softwood and hardwood

Abstract

ABSTRACT In this study, the torrefaction and pyrolysis behaviors of softwood (pinewood) and hardwood (teakwood) were experimentally explored. Three different torrefaction temperatures (i.e. 210, 240, and 270°C) and two different residence times (20 and 40 min) were adopted. Various property indicators, for example, solid and energy yields, enhancement factor of higher heating value (HHV), and torrefaction severity index, were used to characterize the torrefaction performance. The principal component analysis (PCA) demonstrated the correlation among different torrefied samples with the help of chemical composition, solid yield, and HHV. Moreover, slow pyrolysis experiments were conducted at three different heating rates (i.e. 15, 30, and 40°C/min) to investigate the thermal degradation behavior of the torrefied biomass samples. Both thermogravimetric and differential thermogravimetric curves shifted to a higher temperature as the pyrolysis heating rate increased. The activation energies E a were estimated by using three isoconversional methods, that is, Kissinger-Akahira-Sunose (KAS), Starink and Flynn-Wall-Ozawa (FWO) methods. The E a broadly increased with torrefaction severity. When using the FWO method, the average E a values ranged from 97.51 to 168.68 kJ/mol and 120.73 to 180.25 kJ/mol for torrefied pinewood and teakwood samples, respectively. Scanning electron microscopy (SEM) images revealed that at higher torrefaction conditions, the samples experienced severe structural damages, leading to crack formation and augmentation. All these observations not only reveal that torrefaction can turn raw biomass into fuel with improved properties but also help to clarify the differences between soft and hardwood for energy utilization purpose.