Ash transformation during single-pellet gasification of sewage sludge and mixtures with agricultural residues with a focus on phosphorus

The recovery of phosphorus (P) from sewage sludge ashes has been the focus of recent research due to the initiatives for the use of biogenic resources and resource recovery. This study investigates the ash transformation chemistry of P in sewage sludge ash during the co-gasification with the K-Si- and K-rich agricultural residues wheat straw and sunflower husks, respectively, at temperatures relevant for fluidized bed technology, namely 800 °C and 950 °C. The residual ash was analyzed by ICP­AES, SEM/EDS, and XRD, and the results were compared to results of thermochemical equilibrium calculations. More than 90% of P and K in the fuels were retained in the residual ash fraction, and significant interaction phenomena occurred between the P-rich sewage sludge and the K-rich ash fractions. Around 45–65% of P was incorporated in crystalline K-bearing phosphates, i.e., K-whitlockite and CaKPO4, in the residual ashes with 85–90 wt% agricultural residue in the fuel mixture. In residual ashes of sewage sludge and mixtures with 60–70 wt% agricultural residue, P was mainly found in Ca(Mg,Fe)-whitlockites and AlPO4. Up to about 40% of P was in amorphous or unidentified phases. The results show that gasification provides a potential for the formation of K-bearing phosphates similar to combustion processes