3.3.4. Thermodynamics Analysis

Table 9 shows the thermodynamic parameters for the thermal decomposition of GG at 20 K/min in N2 and air atmospheres. These values were calculated from the apparent activation energy *E<sup>θ</sup>* based on DFM and a first-order differential model. The choice of DFM was informed by the fact that the temperature function was not modeled as an integral approximation, and the result may thus be expected to be relatively more accurate. Though the global reaction mechanism was assumed here, it is noteworthy that the underlying principle for iso-conversional methods supposes that the description of the process kinetics is based on multiple single-step kinetic equations, in which each equation is associated with a given conversion extent over a narrow temperature range, thus validating the use of the DFM model. The model was used for the computation of the pre-exponential frequency factor (A), which gives insight into the frequency of collisions between reactants [60]. It has been shown that for pre-exponential factor values with orders of magnitude ≤ <sup>10</sup><sup>9</sup> min<sup>−</sup>1, the surface reaction pathway is predominantly manifested [61]—this pathway was clearly noticeable for *θ* = 0.6 − 0.7, in the air atmosphere. The Δ*H* was negatively correlated with the conversion for air (r = −0.27; *p* < 0.05), while it was positively correlated for N2 (r = 0.84; *p* < 0.05). The average value of Δ*H* for decomposition in the air (156 kJ/mol) is higher than for N2 (142 kJ/mol)—an indication of a relatively higher reactivity in an oxidative reaction, as earlier noted. Zou et al. [50] observed that the Δ*H* is a reflection of the exchange of heat between complex activated species and reactants so that the higher the value, the higher the reactivity and the faster the rate of reaction. It has been noted that a positive value of Δ*H* indicates an endothermic reaction [8]. By implication, the pyrolytic and oxidative processes were endothermic throughout the conversion range. The average values of Δ*H* obtained in this study for both thermal conditions are higher than that for peanut shells, (74.8 kJ/mol; air) and (29.3 kJ/mol; N2) [1]. The values of Δ*G* for both atmospheres were positively correlated with conversion. The Δ*G* is an indication of

the total energy increase for the reaction in a thermal degradation process and portrays the difficulty and direction of the reactions. Chen et al. [8] stated that large values of Δ*G* are suggestive of the low possibility of reactions, and positive values indicate non-spontaneity in reactions. The mean values of Δ*G* for thermal degradation in N2 and air are 143 and 135 kJ/mol, respectively. This conforms with the findings in the literature [1]. This suggests a less favorable reaction of GG in N2 relative to air. In addition, it may be inferred that both the oxidative and pyrolytic processes of GG proceeded in a non-spontaneous manner. Ahmad et al. [60] opined that Δ*G* is a measure of the amount of energy that is available from the thermal degradation of a given biomass. In comparison to peanut shells and red pepper waste, the thermal decomposition of GG will generate more energy [1,62], whereas it will liberate relatively less energy than Camel grass and Napier grass [38,63]. Generally, the Δ*S* provides information on the degree of disorder for the reactions taking place in the thermal decomposition process. Unlike in air (r = −0.33; *p* < 0.05), the values of Δ*S* for N2 (r = 0.84; *p* < 0.05) were positively correlated with conversion. The trend for Δ*S* was quite similar to that reported by Cai et al. [57] for tea leaves in a N2/O2 atmosphere and that by Ahmad et al. [64] for Para grass, particularly with respect to the appearances of positive and negative values, whereas in air, at *θ* ≤ 0.55, which corresponds mainly to hemicellulose and cellulose decomposition, Δ*S* had positive values and Δ*S* in N2, at *θ* ≤ 0.40, had negative values. This may suggest that the earlier stages of the pyrolytic process had a relatively lower degree of disorder for the product formation, while the converse was true for the oxidative process. Significantly, the dominancy of Δ*S* positive values, which attests to a high degree of disorder for the thermal processes, has been observed in the literature [57]. The average value of Δ*S* in N2 (0.043 kJ/(mol\*K) is lower than in air (0.1132 kJ/(mol\*K).

**Table 9.** Thermodynamic parameters for GG thermal decomposition at 20 K/min based on DFM.


## **4. Conclusions**

Guinea grass (*Megathyrsus maximus*) was subjected to thermal degradation in a nonisothermal TGA (N2 inert and dry air atmosphere) at multiple heating rates of 5, 10, and 20 K/min. The Coats–Redfern, Flynn–Wall–Ozawa, and Starink techniques were utilized to evaluate the kinetic parameters, and these were subsequently used in the combustion characteristics and thermodynamics analyses. A couple of integral models, representative of various decomposition mechanisms, was tested in the model-fitting technique. This was with the primary objective of evaluating the bioenergy potential of GG.

The thermal profile of the GG proceeded distinctly under the different heating scenarios—the average residual mass after the GG decomposition was 29.1% and 13.2%, respectively, for the N2 and air environments. The model-fitting technique suggested that the chemical reaction and diffusional models play critical roles in the thermal decomposition processes of GG, both in the N2 and the air atmospheres. The kinetics model also revealed three distinctive stages of decomposition, which correspond to moisture

evaporation, devolatilization, and solid residue burnout/char formation. On average, the activation energy for the decomposition in the air is relatively higher than in the N2 atmosphere. The decomposition process in air showed relatively higher reactivity, with an average value enthalpy change being 156 kJ/mol, as opposed to 141 kJ/mol for the N2 environment. According to the change in Gibbs free energy, it was also shown that both processes proceeded in a non-spontaneous manner. It may be concluded from the foregoing that the thermal decomposition process of GG, either in an N2-pyrolytic or oxidative environment, follows a complex pathway that involves parallel and successive reactions. It has been demonstrated that GG would be a suitable feedstock for biofuel production and bioenergy purposes. The information derived from the combustion characteristics would be useful in the development and application of combustion technology for the GG.

**Author Contributions:** A.O.B., A.A.A. and P.P.I. conceived the research idea; A.O.B., A.A.A., P.P.I. and S.O.A. provided the materials for the research; A.O.B., A.A.A. and P.P.I. did the preliminary sample preparation in the laboratory; A.M.A. and A.G.M. carried out the further laboratory experiments on the prepared samples to obtain the required data; A.O.B. did the data analyses and wrote the first draft of the manuscript; A.A.A., P.P.I., S.O.A., A.M.A. and A.G.M. contributed to the scientific discussion of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.
