Thermodynamic Equilibrium Study of Ash Transformation during Entrained Flow Conversion of Agricultural Biomass Focusing on the Potential Extraction of Valuable Si and K–P Compounds via Condensation from the Gas Phase

Agricultural biomass is today largely underutilized in combustion and gasification processes because of the abundant supply of other easier-to-process biomass fuels. These biomass types generally have a moderate to high ash content comprising valuable elements, such as Si, K, and P, which can lead to ash-related operational problems. The high share of Si, K, and P in agricultural biomass assortments also has a significant economic value. These elements are usually retained in the coarse or fly ash fractions. Extracting valuable Si- and K–P-containing compounds with high purity from these ash fractions often requires further postprocessing steps, which increases operational costs. Therefore, a potential novel design concept could be to control the combustion/gasification processes so that Si, K, and P can be extracted by condensation from the flue/hot gases at a quality that implies added value instead of extra costs. This work aims to identify the possibilities of extracting valuable Si and K–P compounds from the hot gases generated during entrained flow conditions via stepwise controlled condensation in the close-flame regions or heat exchanger zone. Thermodynamic equilibrium calculations were performed by employing the databases (GTOX and SGPS) in FactSage 8.0 software. The calculations were performed under varying conditions ,i.e., temperatures, atmospheres, and fuel compositions. The selected fuels were rice husks (Si-rich), brewer’s spent grains (P–Si-rich with moderate to minor amounts of Ca, Mg, and K), and grass (K–Si-rich with moderate amounts of Ca, Mg, and P). The results indicate that the high-temperature formation of the valuable Si compounds, such as SiC (s) and Si₂N₂O (s), would require an inert atmosphere during both the release and cooling stages. Moreover, a high Si/P molar ratio is needed to form valuable Si-containing compounds. The predicted K-bearing phosphates during the gas cooling near the burner zone were formed in the same temperature range as Ca-, Mg-, and Si-containing compounds with all of the fuels. The results obtained by this study can guide experimental research on the practical extraction of Si and K–P compounds from different types of agricultural biomass during thermochemical conversion in entrained flow conditions.