## 3 Production of Biogenic Silica
### Combination of Fuel Pre-Treatment with Combustion  
Since RH and RS are composed of organic matter such as cellulose, hemicellulose, and lignin as well as inorganic matter [50; 53; 91], washing or leaching the fuel samples with specified chemical solutions can alter the organic and inorganic composition of RH and RS as well as the composition of resulting RHA and RSA. Ash-forming elements can be divided into water soluble, acid soluble and insoluble parts [196; 71; 197]. In general, alkali metals (potassium, sodium) belong to the water soluble portion of the ash while alkaline earth metals (calcium, magnesium) and manganese are acid soluble. Silicon, aluminum and iron species remain insoluble. Pre-treatment of RS can change the concentration of ash-forming elements such as potassium. Consequently, the location of RS in Figure 2b can be adjusted to a position associated with higher DT. Figure 3 shows the melting behavior of the RS and tap-water washed RS during the combustion process. Accordingly, washing with water can prevent melting and slag formation during combustion of RS [198]. As illustrated by Figure 4, the effect of pre-treatment is more prominent in RS than in RH. According to Figure 4, simple water washing can change the position of RS from close to low melting temperature region to a high melting temperature part. This shift is far less pronounced for RH, which shows that washing does not have a drastic effect on the melting tendency of RH. In Figure 4, data points for original and treated RH and RS are outside of the defined areas and it shows gaps in knowledge, which should be closed in future studies. Table 4 provides an overview of pre-treatment strategies which were used in Figure 4. \begin{table}
\begin{tabular}{c c} \hline \hline
**Symbol** & **Fuel Pre-Treatment Strategy** \\ \hline \hline WRH 1 & RH was soaked in distilled water. No details have been reported for water washing. \\ \hline \hline WRS 1 & RS was collected after having received a total of 451 mm/m\({}^{2}\) of rain. \\ \hline \hline WRS 2 & Laboratory washed, 100 \(g\) whole straw, hand sprayed for 1 min with tap water. \\ \hline WRS 3 & Laboratory washed, 100 g whole straw, submerged in 7 liters of distilled water for 24 h. \\ \hline WRS 4 & RS was collected after having received a total of 65 mm of rain (after first rain). \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of fuel washing strategies for RH and RS used in Figure 4 [102; 105; 109]. Figure 3: Rice straw: (**a**) before combustion; (**b**) untreated RS combusted at 700–800 \({}^{\circ}\)C; (**c**) untreated RS combusted at 900–1000 \({}^{\circ}\)C; (**d**) washed RS combusted at 900–1000 \({}^{\circ}\)C [198]. Reprinted from Waste Management, 33, [PERSON], [PERSON], [PERSON], [PERSON], Effect of water washing on the thermal behavior of rice straw, 2250–2256, Copyright (2013), with permission from Elsevier. Another advantage of fuel pre-treatment is that it can reduce the risk of crystallization in the resulting biogenic silica [67; 71; 187; 189; 199]. Pre-treated RHs remained amorphous up to 1000 \({}^{\circ}\)C [200]. In contrast, untreated RH crystallized at temperatures between 600 and 900 \({}^{\circ}\)C depending on source of the RH and combustion conditions [19; 20; 66; 189]. Pre-treatment was also shown to improve physisorption and pore structure characteristics of biogenic silica [53; 71; 73; 100; 200; 201]. Accordingly, both SSA and pore volume are improved by pre-treatment of the fuels (Table 5), thus being a prerequisite for catalytic applications of biogenic silica. Table 5 summarizes the results of combustion experiments performed with pre-treated RH and RS. According to Table 3, the maximum silica purity, SSA and pore volume are about 97.2 wt %, 220 m\({}^{2}\)/g and 0.26 cm\({}^{3}\)/g, respectively, among the RHA obtained from combustion of untreated RH. According to Table 5, however, fuel pre-treatment improves purity of up to 99.77 wt %, increases SSA of 353 m\({}^{2}\)/g and doubling of the pore volume (0.52 cm\({}^{3}\)/g).