*3.1. Characterization of Guinea Grass*

Table 3 shows the results of the proximate, elemental, compositional, and calorific value analyses of the GG. It is shown that the GG contained 40% C, 1.3% N, and 5.1% ash. The ash content was well within the range reported for other grass species, such as Elephant grass [37], Camel grass (6.31%) [38], and *Echinochloa stagnina* (6.31%) [23], as well as other biomass wastes, such as jackfruit peel (5.56%) and seeds (6.64%) [39]. In comparison to low-rank coals, the lower ash content of biomass makes it more suitable for combustion processes [39]. This is due to technical problems, such as slagging and fouling, which impede heat and mass transfer. The estimated protein content of 8.1% was in the range (5.3–8.8%) for fresh GG, as documented by Aganga and Tshwenyane [40]. An FC value

of 16.1% was in the range (8.5–16.9%) for Napier grass [41]. The calorific value obtained for the GG was 15.5 MJ kg<sup>−</sup>1, and is comparable to Napier grass (16.2–18.1 MJ kg<sup>−</sup>1) [41], tamarind residues (17.5 MJ kg<sup>−</sup>1) [42], smoked cigarette butts (18.5 MJ kg<sup>−</sup>1) [43], and jackfruit wastes (16.3–17.2 MJ kg−1) [39]. An extractives content of 1.4% agreed with the literature [40,44]. A lignin content of 21.5%, which was nearly twice that reported by Ratsamee et al. [45] for GG, using the acetyl bromide method, was recorded, while Mohammed et al. [41] obtained a lignin content for Napier grass of 24%. The higher lignin value is attributable to protein interference in the Klason lignin determination of grasses [46]. Detailed carbohydrate analysis showed mainly glucan (34%) and xylan (18%), together with galactan (1.2%) and arabinan (6.4%). The total carbohydrate value is lower than that reported by Ratsamee et al. (hemicellulose (27.1%) + cellulose (41.7%)) for GG [45], but higher than for the other grasses listed in Table 1 [15]. The variations observed in some parameters may be due to differences in genetics and/or environmental conditions.


**Table 3.** Proximate, Elemental, and Compositional data for GG sample.

Fatty acids are an important source of unsaturated acids in grasses for foraging animals [47]. The fatty acid profile of GG extractives was determined as FAME derivatives and given in Table 4. The fatty acids were from C12 (lauric acid) to C24 (lignoceric acid), with the most abundant being palmitic acid (74 mg/g extract), linoleic (39 mg/g extract), and oleic (24 mg/g extract) acids. Lauric (C12) to stearic (C18) acids, saturated and unsaturated, have been observed in several types of forage grass [47].

**Table 4.** Fatty acid profile of GG extract.



Cellulose crystallinity is a key factor in the biological or thermal degradability of biomass. The XRD analysis of the GG (Figure 1a) showed a typical diffractogram of cellulose I, with 2θ peaks at 15◦ and 22◦, which were assigned to the cellulose planes of (101) and (002), respectively [22]. The CCI was determined after peak fitting at 0.431, and it was found to be higher than that of Napier grass, having a CCI of 0.327 [46].

**Figure 1.** (**a**) X-ray diffractogram and (**b**) FTIR spectrum of guinea grass.

The GG was analyzed for chemical properties by FTIR spectroscopy (Figure 1b). The vibrational band assignments in GG by Balogun et al. [22] were used. An O-H stretching band around 3300 cm−<sup>1</sup> was assigned to the polysaccharides and lignin. The C–H stretching band at 2920 cm−<sup>1</sup> was assigned to the aliphatic structures, while the carbonyl band around 1735 cm−<sup>1</sup> was assigned to the acetyl and uronic acid groups in xylan. The presence of lignin was confirmed by the distinct bands at 1514 and 1604 cm<sup>−</sup>1, assigned to the aromatic skeletal vibrations. The large band centered at 1037 cm−<sup>1</sup> was assigned to the C-O stretching in the cellulose, hemicellulose, and lignin polymers. As mentioned earlier, cellulose degradation is associated with its crystallinity. Cellulose crystalline information was determined by its TCI (crystallinity) and LOI (cellulose I), and the values obtained were both 1.1 for the GG. The values of TCI and LOI for Napier grass were 1.25 and 0.53, respectively [48], while sorghum glume has a lower value of LOI (0.75) [49]. The glass transition temperature (softening point) and reactivity of lignin are influenced by its S/G ratio [34]. The lignin S/G ratio of GG was calculated at 1.2, and it was higher than straw soda lignin (1.05) [49]. Sun et al. [34] used Raman spectroscopy to determine S/G ratios for switchgrass (0.92) and maize (1.1).
