## 3 Results and Discussion
### Ash Analysis  
Impact of the Pre-Treatment Process on the Chemical Composition of the Inorganic Fraction of Biomass Fuels  
The changes in elemental composition upon treatments (1 and 5 \(w\)/\(v\)% citric acid (CA)) of the biomass fuels are shown in Figure 4. The results indicated that in untreated ash samples, the share of oxides of silica, potassium, sodium and calcium are in higher proportion compared to other ash forming elements (such as Al\({}_{2}\)O\({}_{3}\), CaO, K\({}_{2}\)O, MnO, etc.). Acidification of the biomass fuels proved to be effective in removing the above metals to substantially lower levels, as the main component of the samples was SiO\({}_{2}\). The reduction in the metal impurities after the pre-treatment process could be attributed to the dissolution of weakly and strongly bonded metals within the organic matrix of the biomass fuels [89], and the volatilization of metals that occurred during the thermal decomposition process [90]. Nevertheless, the ashes of some of the samples contained a considerable amount of potassium and oxides of Al constituents relative to silica even after the acidic pre-treatment. These observed effects align with similar observations made by [PERSON] et al. [89] during their investigations on Egyptian diatomic. According to the analyses, the removal efficiency of K\({}_{2}\)O by the citric acid in yam peel was the lowest (79 wt.% db) in comparison to 92, 97, 98 and 97 wt.% db calculated for corncob, cassava peel, coconut husk and cornhusk, respectively. [PERSON] et al. [91] have reported a K\({}_{2}\)O removal in the proximity of 98 wt.% db after acetic acid pre-treatment of rice husk. According to [PERSON] [92], hydrolysis of cellulose and hemicellulose to produce monosaccharides occurs during the leaching of biomass fuels. Consequently, the monosaccharides were assumed to block access of the leaching agent to the inorganic impurities [92]. Thus, the volatilization of K\({}^{+}\) ions is hindered due to their attachment to the organic matrix at temperatures below 400 \({}^{\circ}\)C [93]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Wavenumber** & **Functional Groups** & **Compounds** & **Reported Values** & **References** \\
**(cm\({}^{-1}\))** & **Functional Groups** & **Compounds** & **(cm\({}^{-1}\))** & **References** \\ \hline
3600–3000 & O-H stretching & Acid, methanol & 3600–3000 & [PERSON] et al. [60] \\
2900–3000 & C-H stretching & Alkyl, aliphatic, aromatic & 2860–2970 & [PERSON] et al. [60] \\  & & & 1440–1400, & [PERSON] et al. [77] \\
1402–1415 & OH bending, CO\({}_{3}\)\({}^{2-}\), & Acid and CaCO\({}_{3}\) & 1400–1460, & [PERSON] et al. [78] \\  & CH bending & & 1417–1425 & [PERSON] et al. [65] \\
1159–1108; 1027–1051 & C–O–C stretching vibration & Pyranose ring skeletal & 1170, 1082 & [PERSON] et al. [60] \\  & & & & [PERSON] et al. [79] \\
1035–1125 & C–O stretching, C–O deformation, OH association and Si–O–Si & C–OH (ethanol), SO\({}_{4}\)\({}^{2-}\) and SiO\({}_{2}\) & 1035–1065, 1108 & [PERSON] et al. [80] \\  & & & and SiO\({}_{2}\) & [PERSON] et al. [78] \\  & & & & [PERSON] et al. [60] \\
956–972 & Si–O–Si & SiO\({}_{2}\) & 956–972 & [PERSON] et al. [81] \\  & & & & [PERSON] et al. [83] \\  & & & & [PERSON] et al. [84] \\  & C–H and stretching & Aromatic hydrogen and vibration of CO\({}_{3}\)\({}^{2-}\) & CaCO\({}_{3}\) & 700–900 & [PERSON] et al. [26] \\
778–799 & Si–O–Si & SiO\({}_{2}\) & 796, 798 & [PERSON] et al. [85] \\
711 & Stretching vibration of C–O & CaCO\({}_{3}\) & 713, 709 & [PERSON] et al. [86] \\
400–600 & C–C stretching & Aromatic hydrogen & 700–400 & [PERSON] et al. [87] \\ \hline \end{tabular}
\end{table} 
Table 3: The main functional groups of the three main components of the biomass fuels and the ashes produced at 600 \({}^{\circ}\)C during combustion characterized by FTIR. In this study, further removal of metals from the biomass fuels occurred in diminutive amounts with increasing concentration of the citric acid (5 \(w\)/\(v\)%).