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

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.

GRAPHICAL ABSTRACT

KEYWORDS:

• Torrefaction
• slow pyrolysis
• biomass
• kinetic analysis
• PCA
• SEM

Highlights

• The torrefaction and pyrolysis behaviors of softwood and hardwood were explored.

• TG/DTG curves shift to a higher temperature as pyrolysis heating rate increases.

• The principal component analysis reveals the correlation among torrefied samples.

• The estimated activation energies broadly increase with the torrefaction severity.

• SEM images show severe structural damage in sample at raised torrefaction severity.

Acknowledgement

This work is financially supported by the Zhejiang Provincial Natural Science Foundation of China (project number LXR22A020001), the National Natural Science Foundation of China (project numbers 12172328 and 51876191), and the Fundamental Research Funds for the Central Universities (project number 2021FZZX001-11).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability

The data that support the observations of this work would be made available by the corresponding author upon reasonable request.

Additional information

Funding

The work was supported by the Key Projects of National Natural Science Foundation of Zhejiang Province [LXR22A020001]; National Natural Science Foundation of China [51876191, 12172328]; Fundamental Research Funds for the Central Universities [2021FZZX001-11]