## 3 Results and Discussion
### Ash Analysis
Physical Morphology of Ashes from Untreated and Acid-Treated Biomass Fuels  
Figure 5 shows the effects of the pre-treatment step on the physical appearance of the ashes of the untreated and the acid-leached biomass fuels. The results indicated that white biogenic silica could be generated by pre-treating the biomass in a hot citric acid solution. According to [PERSON] et al. [99] and [PERSON] et al. [36], the colouration of the ashes is an indication of completeness of a combustion process and low concentration of remaining impurities. The metallic oxides in the untreated biomass fuels result in the formation of black particles in the ashes [106]. Furthermore, the impurities increase the risk of slag formation and their role during the combustion of silica-rich biomasses have been discussed in the literature [50; 107]. From Figure 5, the ashes of the untreated biomass fuels indicated the presence of black particles (fixed carbon). A plausible explanation for the formation of black particles, as explained by [PERSON] et al. [106], is that, during the combustion of biomass fuels  
Figure 5: Visual appearances of thermally produced ashes of the raw and the acid-leached biomass fuels at 600 \({}^{\circ}\)C. **First row from the top**: untreated biomass fuels; **second row**: ashes of untreated biomass fuels; **third row**: ashes of \(1\,w/\
u\)% biomass fuels, and; **last row**: ashes of \(5\,w/\
u\)% biomass fuels. From left to right, biomass fuels and ashes of cassava peelings (CasP), yam peelings (YamP), coconut husk (CocH), corncobs (CorC) and cornhusk (CorH) generated at 600 \({}^{\circ}\)C for 2 h.  
from room temperature to high temperature, there is a simultaneous decomposition and oxidation of the organic matter into carbon. Prior to the removal of all the carbon, if the temperature rise is higher than the melting temperature of potassium oxide (350 \({}^{\circ}\)C), surface melting or sintering of the ash occurs, and the K\({}^{+}\) ions trap the carbon inside the organic matrix. This phenomenon inhibits carbon removal and results in the formation of black particles at high temperatures [106; 107; 108]. This explains the basis for the higher LOI values (indirect carbon content) measured in the untreated samples compared to the acid-treated biomass samples. Since the potassium oxide concentration is higher in the untreated biomass fuels than the acid-treated biomass, there was a high propensity for black particles formation in the ashes [106]. Furthermore, chemical pre-treatment hydrolyses the organic (i.e., cellulose, hemicellulose and lignin) compounds and break their structures down [109; 110; 111]. Therefore, the evolution of volatile matter in the pre-treated samples could easily occur, decreasing the amount of residual carbon in ashes [112]. Comparatively, the ashes at 1 \(w/v\%\) CA had few remnants of black particles than the ashes of 5 \(w/v\%\) CA for the same reasons explained above. Analysis of the metallic impurities shown in Figure 4 confirms a higher concentration of K\({}^{+}\) ions in the 1 \(w/v\%\) CA than the ashes of 5 \(w/v\%\) CA. Hence, the tendency to form black particles was more pronounced than the ashes from 5 \(w/v\%\) CA treated samples. [PERSON] et al. [113] obtained ash completely white in colour after leaching rice husk in dilute 1 N HCl, thus proving the effectiveness of acid pre-treatment in mitigating the formation of black particles. The complete white colour of CorC-A-5% and CorH-A-5% indicates total amorphous structured silica [114], as it is proven by the XRD measurements.