Assessing the potential of the invasive grass Cenchrus echinatus for bioenergy production: A study of its physicochemical properties, pyrolysis kinetics and thermodynamics

Abstract

The current study aims to investigate the physicochemical properties and pyrolysis characteristics (kinetic triplet and thermodynamic parameters) of the invasive grass Cenchrus echinatus to assess its bioenergy potential. A thermogravimetric analyzer was employed to obtain the pyrolysis behavior of the grass under slow non-isothermal conditions. First, a multi-component deconvolution analysis of differential thermogravimetry (DTG) curves using the Fraser–Suzuki function was performed, aiming to quantify the individual devolatilization reactions of hemicellulose (P–HC), cellulose (P–CL), and lignin (P–LG). The survey of the activation energy employing four isoconversional methods (Friedman, Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose, and Starink) indicated the following devolatilization order: P–LG (329.9 − 376.7 kJ mol−1) > P–CL (178.4 − 188.2 kJ mol−1) > P–HC (157.8 − 161.2 kJ mol−1). With values of pre-exponential factors from 1.1 × 1015 to 1.4 × 1031 min−1, as estimated from the kinetic compensation effect, it was deduced that the chemical reactions with simpler nature are predominant. With the method of integral master plot, a geometrical contraction mechanism matched the devolatilization of P–CL, and an nth-order-based reaction model matched the devolatilizations of P–HC and P–LG. Besides, the thermodynamic study suggested that the conversion process is endothermic (ΔH≠ = 153.4 to 245.2 kJ mol−1) and nonspontaneous (ΔG≠ = 146.8 to 174.4 kJ mol−1). With the three kinetic triplets, one overall rate expression for the pyrolysis of invasive grass C. echinatus was established. The simulation results were then compared to experimental kinetic curves, and the agreement was deemed satisfactory. The outcomes from this research recommend the C. echinatus as a promising feedstock for bioenergy production and are decisive for scheming large-scale pyrolysis reactors for this invasive grass.