Physico-chemical characterization, thermal decomposition and kinetic modeling of Digitaria sanguinalis under nitrogen and air environments

Abstract

The study undertook the thermal degradation of a tropical grass species, Digitaria sanguinalis, in nitrogen (pyrolysis) and air (combustion) atmospheres through thermogravimetric analysis as well as comparative kinetic investigation. The differential (Friedman) and integral (Flynn-Wall-Ozawa and Straink) isoconversional methods in conjunction with the Coats-Redfern method were utilized. This was to obtain the kinetic parameters and also predict the probable reaction mechanisms involved in the decomposition process. Before the thermal and kinetic investigations, the grass was analyzed for its physical, chemical, and structural properties utilizing diverse wet-chemistry and spectroscopic techniques. This research attempt is part of a larger project designed to investigate a couple of local grass species, which are invasive by nature, as potential energy crops for pyrolytic and combustion applications. The grass had a fixed carbon content of 17.85% and a calorific value of 13.7 MJ kg−1. The fatty acids detected were from C12 (lauric acid) to C24 (lignoceric acid), with the three most abundant being palmitic (94 mg/g extract), linoleic (27 mg/g extract), and oleic (19 mg/g extract) acids. The average residual weight in air (25.3%) was relatively less than in nitrogen (38.7%), affirming the higher rate of reaction in an oxidative process (combustion). The activation energy profiles in both atmospheres were markedly different, as shown by the Flynn-Wall-Ozawa technique for a conversion ratio of 0.1–0.2 (nitrogen, 149 kJ/mol; air, 177 kJ/mol) and 0.65–0.8 (nitrogen, 366 kJ/mol; air, 170 kJ/mol). Of all the models tested, the model-fitting technique indicates that the chemical reaction and diffusional models play predominant roles in the thermal decomposition of the grass under investigation. The thermal degradation of Digitaria sanguinalis proceeded mainly as complex multi-step reaction mechanisms. Aside from the potential suitability of the grass species for bioenergy applications and biofuels production, it also demonstrated huge capability for biochemical extraction. Future work will incorporate the kinetic data for the associated thermochemical processes development, and the design and optimization of reactors/combustors.