## 4 Summary and Conclusions  
In this comprehensive review, combinations of fuel pre- and ash post-treatment strategies with combustion of rice husk (RH) and rice straw (RS) have been discussed. With respect to the review objective, we conclude that:  
1. By studying the ash melting tendency of original and treated RH and RS, it was shown that there is a gap in knowledge, and further investigation is required in this field. 2. Fuel pre-treatment has a significant impact on controlling ash melting issue in RH and RS, and it seems that at least water washing prior to combustion is essential in RS to avoid slag formation. 3. Increasing combustion temperature (CT) and residence time (RT) improves the purity of the biogenic silica with respect to the carbon content. At the same time, specific surface area (SSA) and pore volume of the biogenic silica decrease while the risk of crystallization is increased. The alternative strategy to improve all quality characteristics of rice husk ash (RHA) and rice straw ash (RSA) is using fuel pre-treatment prior to the combustion. Crystallization tendency of RHA possibly correlates with the alkali metal content, which facilitates formation of cristobalite. Crystallization temperature is around 600-900 \({}^{\circ}\)C in untreated RH and RS depending on the elemental composition, whereas pre-treated RH and RS remain amorphous up to higher temperatures of about 1000 \({}^{\circ}\)C.
4. In RHA from untreated RH, the maximum silica purity, SSA and pore volume are around 97.2 wt %, 220 m\({}^{2}\)/g and 0.26 cm\({}^{3}\)/g, respectively. However, these values can be improved to around 99.8 wt %, 353 m\({}^{2}\)/g and 0.52 cm\({}^{3}\)/g using fuel pre-treatment strategies prior to the combustion. Further improvement of silica purity is possible by a combination of fuel pre-treatment, combustion and ash post-treatment strategies. 5. Although high quality biogenic silica can be produced from RH and RS, most studies were performed in lab-scale muffle furnaces and investigations at a bench scale are scarce. 6. Economically feasible production of biogenic silica from RH was demonstrated at a bench scale though with silica purity limited to 98 wt %. For higher silica purities, a harsh pre- and post-treatment environment may be required, which could increase the technical efforts and thus the overall process costs. 7. Finally, it was shown that a preliminary practical study has been done to scale the production procedure up to a bench scale with the aim of developing an environmentally friendly and economically feasible process with coupled energetic and material utilization of RH. However, further investigations are required to understand the behavior of RH and RS in combustion. Moreover, there is no report in literature to predict the quality of biogenic silica as well as the ash melting tendency and gaseous emissions using thermodynamic calculations. By using thermodynamic calculations, the effect of chemical composition of input material can also be evaluated in order to transfer to other biomasses since there are limited experimental data available in literature. No information is available in literature on computational fluid dynamic (CFD) simulation of the combustion process of biomass to simulate their conversion mechanism to biogenic silica inside the boiler. If a CFD simulation is used, thermal efficiency of the combustion unit can also be evaluated and optimized. In addition, such a simulation will be valuable to calculate the exact CT and RT of each singular biomass particle during the real operational condition of the combustion unit. In the literature, fundamental lab-scale studies are usually carried out in static airflow. However, the effect of dynamic airflow and air to fuel ratio on the quality of resulting ash have not being studied in the literature.