Torrefaction and steam explosion of flax straw: Combustion characteristics, kinetics, and thermodynamics

Agri-residues can be utilized as coal-like biofuels for power generation. Treatment methods can upgrade their fuel properties and combustion behavior. This study tested the effects of torrefaction and steam explosion on the combustion characteristics, kinetics and thermodynamics of flax straw. Flax straw was torrefied at a temperature range of 250–300 ° C and a residence time of 1 h. Steam explosion was conducted at 300 psi and a reaction time of 1 min. Combustion of raw and treated flax straw was executed through thermogravimetric analysis (TGA) at four heating rates, 5, 7.5, 10, and 15 ° C/min. The findings revealed that the effect of torrefaction on the ignition and burnout temperatures of flax straw was more pronounced than that of steam explosion, highlighting the thermal stability of torrefied biofuels. Combustion kinetics were analyzed following the Friedman, Kissinger-Akahira-Sunose (KAS), and Ozawa-Flynn-Wall (OFW) methods. Torrefied biofuels showcased lower activation energy values relative to raw and steam exploded biomasses, where the mean activation energy at a 0.1–0.8 conversion range was 42.9 kJ/mol for flax straw torrefied at 250 ° C, as determined by the OFW method. Furthermore, the activation energy was used to obtain the change in activation enthalpy, Gibbs free energy, and activation entropy of combustion using the Eyring equations. Thermodynamic properties were enhanced after torrefaction, with lower mean activation enthalpy and entropy values and higher Gibbs free energy. The results provide an insight into the potential of torrefied biomass as a promising biofuel for power generation. • Torrefaction had a greater impact on combustion behavior than steam explosion. • Torrefaction at 250 °C increased the ignition temperature from 259.1 to 280.9 °C. • Activation energy decreased post torrefaction at 250 °C from 105.0 to 42.9 kJ/mol. • Activation enthalpy and entropy decreased after torrefaction. • Gibbs free energy increased from 117.9 to 157.8 kJ/mol post torrefaction at 250 °C.