Thermochemical equilibrium study of ash transformation during combustion and gasification of sewage sludge mixtures with agricultural residues with focus on the phosphorus speciation

Abstract

The necessity of recycling anthropogenically used phosphorus due to depleting quantities of available phosphate ores encouraged recent research to identify potential alternative resource pools. One of these resource pools is the ash derived from the thermochemical conversion of sewage sludge. This ash is rich in phosphorus, although most of it is chemically associated in a way where it is not plant available. The purpose of this work was to identify the P-recovery potential of ashes from sewage sludge co-conversion processes with two types of agricultural residues, namely wheat straw (rich in K and Si) and sunflower husks (rich in K), employing thermodynamic equilibrium calculations. The results indicate that both the melting behavior and the formation of plant available phosphates can be enhanced by using these feedstock mixtures in comparison to pure sewage sludge feedstock. This enhanced bioavailability of phosphates was mostly due to the predicted formation of K-bearing phosphates in the mixtures instead of Ca/Fe/Al-phosphates in the pure sewage sludge ash. According to the calculations, gasification conditions could have a negative effect on the melting behavior of the mixtures dominated by the sewage sludge ash. Furthermore, the possibility of precipitating phosphates from ash melts could be shown. It is essential to emphasize that the results of this theoretical study ought to be used with caution since non-equilibrium influences were not investigated. Nevertheless, the applicability of thermodynamic calculations in the prediction of molten and solid phases may motivate experimental research in the future.