## 3 Results and Discussion
### Ash Analysis
#### 3.5.3 Phase Analysis of the Biomass Ash Samples  
[PERSON] et al. [122] reported that the presence of alkali ions such as K, Mg, and Na in the ashes results in low melting temperature alkali-rich compounds, which drives the transition to different crystalline phases of silica. In addition, [PERSON] et al. [123] reported that these metallic species accelerate the crystallization of amorphous silica above 1000 \({}^{\circ}\)C. Thus, eutectic mixtures or silica phases are induced, decreasing the phase transition temperature from amorphous to crystalline silica. These minerals, introduced as fuel contaminants, result in a less active amorphous silica with low textural properties [115] due to the strong correlation between the crystallinity fraction and porosity changes of the silica-rich ashes [50]. This phenomenon is even more pronounced for untreated biomass fuels, as mentioned in the study of [PERSON] et al. [35; 47] during their investigations with rice husk. Figure 7 shows the XRD patterns and crystalline phases identified in the ash samples of the biomass fuels pre-treated with 1 \(w\)/\(v\)% citric acid. In all the ash samples, the following dominant phases were detected: quartz (SiO\({}_{2}\)), calcite (CaCO\({}_{3}\)) and anhydrite [Ca(SO\({}_{4}\))]. The presence of these dominant phases has also been reported during the investigations of wheat straw, miscanthus and cereal remnant pellet, all leached in hot solution of sulfuric acid [20]. Other minor crystalline phases, such as feldspar of the plagioclase series such as albite [Na(A\(\mathrm{AlSi}_{3}\mathrm{O}_{8}\))], were found in cassava and yam peelings as well as coconut husk in addition to an amorphous fraction. Potassium feldspar, such as sanidine (KAlSi\({}_{3}\mathrm{O}_{8}\)) was clearly detected in CoCH-A-1%. The formation of sanidine (KAlSi\({}_{3}\mathrm{O}_{8}\)), as reported in the study of [PERSON] et al. [20] could be as a result of a thermodynamic effect (i.e., \"back-formation\" of feldspar or arcanite) that reduces part of the amorphous silica particles [10; 100]. The major differences in the XRD patterns of the samples were that of the CorC-A-1%, CorH-A-1%, which were characterized by a clear and dominant amorphous component with traces of arcanite (K\({}_{2}\)SO\({}_{4}\)) and anhydrite [Ca(SO\({}_{4}\))], respectively. These last two phases are because of the remaining low impurities of K\({}_{2}\)O and CaO present in the ashes, as confirmed by the EDX spectrum of the CorH-A-1% sample in Figure 8.  
According to the literature, the phase formations in the ashes can be attributed to the high presence of critical ash forming elements (such as Ca, K and Na) and their interactions with the silica particles [50; 124; 125; 126]. Comparing the elemental compositions of the ashes of the biomass fuels leached at 1 \(w/v\%\) citric acid, all ash samples except for CorC-A-1% and CorH-A-1% had a higher content of potassium and sodium oxide (see \"Elemental Composition of ashes in Figure 4). Thus, the tendencies for the formation of these minerals (fuel contaminants), i.e., anhydrite Ca(SO\({}_{4}\)), quartz (SiO\({}_{2}\)), arc  
Figure 8: EDX spectrum (30 kV) of the ash sample (wt.% db = 100) resulting from the combustion of CorH-A-1%. Figure 7: X-ray diffraction pattern of the ash samples resulting from the combustion of 1 \(w/v\%\) acid-treated biomass fuels at 600 \({}^{\circ}\)C.  
sylvite (KCl), calcite and albite [Na(AlSi\({}_{3}\)O\({}_{8}\))] were more evident in those samples as confirmed by the XRD measurements and summarized in Table 5. The broad peak in the range of 2 Theta = 20-30\({}^{\circ}\) found in the comrhusk ash samples was predominantly due to the amorphous silica in the analyzed samples in Figure 7. The thermally unstable phase of calcite (CaCO\({}_{3}\)) was also observed in some of the samples, especially cassava and yam peels, and this was due to the high concentration of Ca\({}^{2+}\) ions in the water used for the leaching.