Pyrolysis characteristics and kinetics of the thermal degradation of elephant grass (Pennisetum purpureum): a comparative analysis using the Flynn–Wall–Ozawa and the Kissinger–Akahira–Sunose methods

Pyrolysis is a very fast and effective thermochemical conversion scheme, hence suitable for the processing of elephant grass, a fast-growing and invasive grass, into bioenergy. This research investigates the pyrolysis characteristics and the thermal degradation of elephant grass using thermogravimetric analysis (TGA). Different characterization techniques were carried out to study the biomass and its bio-oil in order to ascertain their fuel properties. The two most widely used iso-conversional methods, Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS), were used to analyze the TGA data obtained over a range of temperatures at heating rates of 10, 15, and 25 °C/min in order to calculate the activation energy and other thermodynamic parameters. From the pyrolysis study, the highest bio-oil yield of 39 wt% was achieved at 500 °C. The obtained bio-oil has excellent fuel properties because of its higher heating value (HHV) of 20.9 MJ/kg, low ash and sulphur contents. The average activation energies (Ea) were 172.48 kJ/mol and 164.11 kJ/mol, respectively, for the FWO and KAS methods, while the average enthalpy changes (∆H) were 168.31 kJ/mol and 159.32 kJ/mol. These values of Ea and ∆H revealed that the overall reaction is favourable since their difference is less than 5 kJ/mol. FWO produces higher and more accurate results since it can capture the decomposition behaviour of the different biomass components for any conversion fraction, while KAS is based on the overall reaction.