## 3 Production of Biogenic Silica
### Combustion of Rice Husk and Rice Straw  
During thermochemical processes, the temperature of the RH and RS is increased under either oxygen or air atmosphere (combustion, \(\lambda\geq 1\)), inert atmosphere such as nitrogen or argon (pyrolysis, \(\lambda=0\)), or with stoichiometric amounts of air, oxygen, or CO\({}_{2}\) (gasification, \(0<\lambda<1\)) in order to decompose organic components. In general, combustion is typically applied to produce biogenic silica [20]. Therefore, the focus of the present work is on combustion of RH and RS. For silica purity, temperature and residence time are two main influential factors. The purity of resulting silica increases by increasing the conversion temperature (CT) and residence time (RT) [20; 69; 187; 188]. However, higher CT and RT can increase the risk of crystallization of the obtained biogenic silica [64; 189]. According to Table 3, during the combustion process, the carbon content of RHA dramatically diminishes and the purity regarding residual carbon content sharply raises by increasing the CT from 300 to 600 \({}^{\circ}\)C [20]. By increasing the CT, carbon content and silica purity remain around the same until SiO\({}_{2}\) crystallization takes place [19; 20; 69]. On the other hand, higher CT and RT reduce specific surface area (SSA) and total pore volume of RHA [19; 71]. Often, this is attributed to the reduction in the carbon content and ash agglomeration, resulting in diminished porosity [189]. According to Table 3, close to 600 \({}^{\circ}\)C, SSA and pore volume strongly depend on the CT, and they drastically decrease by increasing CT from 600 to 700 \({}^{\circ}\)C or above [19; 71]. Therefore, to produce high quality silica, it seems that the optimum CT for combustion of untreated RH is around 600 \({}^{\circ}\)C. The quality of the resulting biogenic silica also depends on the combustion technology [179; 190]. Sufficient air flow rate is required for complete combustion in order to produce pure biogenic silica; otherwise, the resulting ash will contain unburned carbon [67]. The heating rate (HR) during combustion also plays an important role in the quality of the remaining ash [54; 188; 191; 192]. The purity of the silica and consequently the brightness of the resulting ash deteriorate when HR increases, although pore volume and SSA of the remaining ash increase at the same time [54; 192]. Usually, a multi-step decomposition of RH and RS has been applied in lab-scale studies to obtain high purity biogenic silica with high SSA and low carbon [72; 98; 193]. For the decomposition of the individual macromolecules cellulose, hemicellulose, and lignin, as described in Section 2, different temperatures are used [72; 98; 100; 193; 194]. This strategy guarantees amorphous structure of the resulting silica since the maximum temperature never exceeds crystallization temperature [72]. The range of crystallization temperature is defined in Section 3.2. However, upscaling of such time-consuming sequential combustion processes at industrial and bench-scales is not feasible. A sequential combustion process can also be designed by pyrolysis of RH followed by an oxidization process (combustion). This strategy was shown to produce amorphous silica at low CTs with silica purity higher than 99.9 wt % with a very low carbon content [195]. Table 3 summarizes the result of different studies in the field of biogenic silica production using combustion of RH. In conclusion, quality of biogenic silica can be adjusted by combustion parameters such as CT, RT and heating rate. To achieve purities exceeding approx. 97.2 wt %, further purification strategies have to be employed. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Input Material, Source, Silica Content in Fast Ash** & **Combustion Process** & **Exp.