## 1 Introduction  
According to the report of International Energy Agency (IEA) in 2017, the worldwide share of renewable energy reached 23.9% in the electricity sector, 10.3% for power production and 3.4% for transportation. A further increase to 29.4%, 11.8% and 3.8%, respectively, is expected until 2023 [1]. Besides solar, hydrothermal and wind energy, bioenergy, in particular the exploitation and valorization of agricultural side products and biogenic residues, will play an important role to enable this sustainable development. Biomass needs to fulfill the following criteria as a sustainable resource of energy: (1) it should be readily available; (2) arable land for food resources should not be affected by biomass for energy generation; and (3) it should produce zero waste and have no negative impact on the environment [2]. In this regard, rice husk (RH) and rice straw (RS) as by-products in rice production and milling processes can fulfill these criteria. Rice represents the second largest share of any crop in the world based on the report of Food and Agriculture Organization of the United Nations (FAO) [3], and the amount is steadily increasing, Figure 1. The world capacity in production of paddy rice in 2015 and 2016 was around 739 and 755 million tons, respectively [4, 5]. According to the FAO rice market monitor [4], there are more than 50 countries with paddy rice production, with the largest cumulated production originating from Asia (681.8 million tons) followed by America (36.3 million tons) and Africa (32.6 million tons), while Europe and Oceania provide only marginal contributions of 4.1 million tons and 0.3 million tons, respectively. Depending on the crop and harvesting method, approximately 20-25 and 40-60 wt % db (dry basis) of paddy rice are RH and RS, respectively [17; 18]. RH and RS do not compete with food resources for land usage, and because of the abrasive structure and low nutritional value, they are not suitable for food and fodder, and usually are disposed [19; 20; 21]. Consisting predominantly of organic matter, i.e., cellulose, hemicellulose, and lignin, RH and RS are applicable for the use as sustainable fuel for energy generation [22; 23; 24]. After combustion of RH and RS, approx. 10-20 wt % of the initial fuel remains as ash rich in silica (i.e., \"biogenic silica\"). It can be an economically valuable material for various applications including the cement and concrete industry [25; 26; 27], an adsorbent to remove heavy metal ions such as lead (II), mercury (II), zinc (II) and nickel (II) ions from wastewater streams, as catalyst [28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38], for synthesis of zeolites and mesoporous silica [39; 40; 41; 42; 43; 44; 45; 46] or for drug delivery systems [47; 48]. Depending on the anticipated application, biogenic silica is required with different purities. For the utilization as pozzolan in concrete, a silica purity of at least 97 wt % db is sufficient [49]. In contrast, advanced applications such as electronics [50] and solar applications [51] require silica purity of up to 99.9 wt % db. Similarly, different characteristics are desirable depending on the desired application, e.g., negligible slagging tendencies [52], low carbon content and high silica purity for synthesizing advanced materials [53], whiteness and proper particle size for filler applications [54], amorphous structure and an optimized pore system (high specific surface area) [53; 55]. Commonly, porous silica is produced on an industrial scale by precipitation from alkaline silicates [50]. Alkaline silicates (water glass) are typically obtained from carbonate powders reacting with silica sand, which is a very energy intensive process and requires very high temperatures (approx. 1400 \({}^{\circ}\)C) [50; 56]. Water glass can also be produced by hydrothermal treatment of sand with lyes, which is also an energy consuming procedure [57].