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Cement production is an energy- and resource-intensive industry accounting for approximately 7% of global carbon dioxide emissions. Therefore, a key decarbonisation option for the cement industry is to substitute the clinker with so-called supplementary cementitious materials (SCMs). Due to its properties and availability, the bottom ash from the biomass combustion process could be suitable as an SCM. Several agricultural residues were collected and analysed. The materials were applied for ashing experiments in a lab-scale muffle furnace, which was operated at different temperatures. The chemical, physical, and mineralogical characterisation of the ashes produced was carried out. In addition, the reactivity of the cementitious paste made from the ashes was measured through lab-scale experiments. The influence of the different ashing temperatures and the additive mixing on the properties of the ashes and cementitious paste was analysed. The results show that the spelt husk ash is the most promising biomass ash, with its high silica content and high pozzolanic reactivity. The bound water of the cementitious paste made from spelt husk ash reaches 7.3 g/100 g paste at 700 °C but decreases to 2.5 g/100 g paste at 900 °C due to the formation of a crystalline structure. Nevertheless, the addition of kaolin to the spelt husk can maintain the reactivity of the spelt husk ash produced at high ashing temperatures by stabilising the amorphous structure in the ash.

In this study, a low-temperature drying plant based on renewable energies to dry food processing wastes is investigated. The demand-oriented heat supply is realized by a solar wall in combination with a biomass boiler. Due to the operational complexity of such a system with different sub-units and process parameters, steady-state simulations were performed in Aspen Plus to provide an insight into the process. Moreover, a time-resolved energetic evaluation was conducted to analyze the influence of varying capacity of the heat sources and operational strategy in addition to economic calculations. The simulations showed that an overall control strategy needs to consider the air properties as well as the flow rate of wet input material. In the reference case, the boiler must be operated at full load through the year to supply as much heat as possible. The revenue from the dried material was the most crucial parameter on the drying economics. Although the current plant configuration operating at 12 h per day and five days per week enable feasible results, the drying process can be more profitable by doubling the boiler capacity and increasing operational hours to 24 h per day and five days per week. The proposed plant can provide an environmentally friendly and cost-effective solution for the re-valorization of food-processing wastes into added-value compounds.

Utilization of biomass either as a renewable energy source or for the generation of biogenic materials has received considerable interest during the past years. In the case of rice husk (RH) and rice straw (RS) with high silica contents in the fuel ash, these approaches can be combined to produce high-grade biogenic silica with purities >98 wt % from combustion residues. The overall process can be considered nearly neutral in terms of CO2 emission and global warming, but it can also address disposal challenges of rice husk and rice straw. For the resulting biogenic silica, several advanced application opportunities exist, e.g., as adsorbents, catalysts, drug delivery systems, etc. This article provides a comprehensive literature review on rice husk and rice straw combustion as well as applied strategies for raw material pre-treatment and/or post-treatment of resulting ashes to obtain high quality biogenic silica. Purity of up to 97.2 wt % SiO2 can be reached by combustion of untreated material. With appropriate fuel pre-treatment and ash post-treatment, biogenic silica with purity up to 99.7 wt % can be achieved. Studies were performed almost exclusively at a laboratory scale.