*3.2. FT-IR Analysis*

In addition to the goals of undertaking FT-IR analysis described in Section 2, data from the analysis also helped compare and establish differences in the mechanism of bonding between the pure and blended pellet samples relevant to quality. The FT-IR spectra of pure and blended pellet samples of NSP and PSP are presented in Figure 2. The data were obtained from the experimental analysis described in Section 2.4.2. To permit convenient comparisons, the spectra are presented as a single plot with the legends clearly displayed in the plot and spectral bands numbered for clarity.

**Figure 2.** Fourier-transform infrared (FT-IR) spectra of pure and blended samples of Norway spruce pellet (NSP) and pea starch pellet (PSP).

Prior to interpretation of the FT-IR spectra in Figure 2, it is vital to mention that some spectral bands are more relevant than the others in the differentiation of the content (in terms of functional groups) of the pure and blended pellet samples. To avoid ambiguity, numbers were used to represent the most important bands considered relevant to the type of attraction forces holding particles of the pellets together, and to the quality of the pellets.

It is obvious from Figure 2 that the spectra were quite similar to each other, which does not necessarily mean the same characteristics. Using the pure pellet samples (100% each of NSP and PSP) as reference points, it is also apparent that the composite material (NSP/PSP 50%/50%) showed a synergistic effect. This synergistic behavior was more pronounced around 2893 to 1254 cm<sup>−</sup><sup>1</sup> (bands 2–7), which were bands associated with active bonding groups such as C=O, C–O, C–O–C, and C–H, attributed to amylose and amylopectin, as well as cellulose, hemicellulose, and lignin in all three pellet samples [7]. All bands in this region (bands 2–7) had different intensities in the spectra of the three pellet samples. Some synergy can also be observed in band 2 of the spectra. The wide transmittance band around 3312 cm<sup>−</sup><sup>1</sup> (band 1) for all samples was assigned to the hydroxyl group (–OH), an active bonding group responsible for band broadening due to a combination of intra and intermolecular hydrogen bonding. The presence and concentration of active bonding groups determine the type of attraction forces between individual particles of biomass pellets [7]. According to Popescu et al. [44], the broadening of the –OH band in IR spectra is often caused by the presence of intra and intermolecular hydrogen bonds. However, the –OH group region of the IR spectrum is particularly useful for explaining patterns of hydrogen bonding, because each distinct –OH group offers a single stretching at a frequency that decreases with increasing strength of the hydrogen bonds, which are responsible for various properties of cellulose, hemicellulose, and lignin that are associated with NSP (100%) and NSP/PSP (50%/50%) [44].

Band intensities of samples in FT-IR analyses are true representations of the concentration of components [45]. Therefore, the active bonding groups previously listed absorbed light with greater intensity and indicated the presence of polysaccharides such as starch [46], leading to the more pronounced band intensity of PSP (100%). This suggests that PSP (100%) contained higher proportions of polysaccharides with greater amounts of active bonding groups (C–O and –OH groups) capable of stronger dipole–dipole attraction and hydrogen bonding (which are two types of intermolecular forces of attraction) as compared to pure NSP (100%) and NSP/PSP (50%/50%). Nonetheless, in the spectrum of NSP (100%), the transmittance band around 1275 and 1478 cm<sup>−</sup><sup>1</sup> (bands 4 and 6) was due to distance-dependent interactions associated with van der Waals contact distances as a result of groups such as C=O and C–H, which are known to form bands within the wavelengths specified above. In addition to being active bonding groups, these groups are equally polar in nature because of their ability to form di fferent types of attraction forces [7]. Thus, for the blend (NSP/PSP 50%/50%), it was presumed that its particles were held together by strong hydrogen bonding because of the presence of the C=O group, which is a polar functional group capable of initiating structural changes by introducing more intermolecular forces of attraction.

With respect to the quality of the pellets, defined in this study in terms of strength and burning efficiency, it is fair to allude that, in relation to the strength of the pellets, PSP (100%) seemed to be a higher-quality pellet than NSP (100%) and NSP/PSP (50%/50%). This was because of its more pronounced band intensity, which was construed to mean greater proportions of polar functional groups that were capable of forming a combination of dipole–dipole attraction and hydrogen bonding between particles. A union of these two attraction forces creates stronger bond energies/bond strength, whereas the functional groups associated with NSP (100%) formed more of van der Waals attraction forces than hydrogen bonding between particles of the pellet, which may increase the possibilities of the formation of fines. Particles of the blend (NSP/PSP 50%/50%) were more connected by a combination of multiple forces of adhesion and cohesion, some of which may have reduced bond strengths that were easily broken. These findings are in agreemen<sup>t</sup> with the fact that the quality of biomass pellets, among other factors, depends on the type and strength of attraction forces between particles, as alluded by Kaliyan [41]. In view of this, the order of strength of the pellets was as follows: PSP (100%) > NSP/PSP (50%/50%) > NSP (100%). For the quality of the pellets in terms of burning e fficiency, the thermal analysis data presented in Sections 3.3 and 3.4 has provide the needed information.

It is worth mentioning that spectral interpretation from FT-IR analysis of samples is non-exhaustive. Thus, for clearer spectral interpretation and understanding, the peak absorption range of di fferent functional groups can be found in Reference [47].
