Ash Transformation during Fixed-Bed Co-combustion of Sewage Sludge and Agricultural Residues with a Focus on Phosphorus

Joel Falk; Thomas Karl Hannl; Marcus Öhman; Ali Hedayati; Nils Skoglund

doi:10.1021/acsomega.3c00415

Ash Transformation during Fixed-Bed Co-combustion of Sewage Sludge and Agricultural Residues with a Focus on Phosphorus

ACS Omega. 2023 Mar 31;8(14):13162-13176. doi: 10.1021/acsomega.3c00415. eCollection 2023 Apr 11.

Authors

Joel Falk¹, Thomas Karl Hannl¹, Marcus Öhman¹, Ali Hedayati¹, Nils Skoglund^{2

3

4}

Affiliations

¹ Energy Engineering, Department of Engineering Sciences and Mathematics, Luleå University of Technology, SE-97187 Luleå, Sweden.
² Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, SE-90187 Umeå, Sweden.
³ BEST-Bioenergy and Sustainable Technologies GmbH, Inffeldgasse 21b, AT-8010 Graz, Austria.
⁴ Institute of Chemical, Environmental & Bioscience Engineering, TU Vienna, AT-1060 Vienna, Austria.

Abstract

This work investigates the ash transformation during fixed-bed co-combustion of sewage sludge mixtures with the agricultural residues wheat straw and sunflower husks, focusing on the fate of phosphorus (P) in the resulting ash fractions. The study aims to determine suitable process parameters for fixed-bed combustion of fuels previously investigated in single-pellet experiments. The pure fuels and fuel mixtures were combusted in a 20 kW_th residential pellet burner while monitoring the flue gas composition, temperature, and particulate matter formation. Subsequently, the different ash fractions were collected and characterized by CHN, SEM/EDS, and XRD analysis. The results showed that co-combustion of sewage sludge and agricultural residues reduced the formation of particulate matter as well as the formation of slag. Co-combustion of sewage sludge with either agricultural residue resulted in a change in phosphate speciation, displaying higher shares of Ca and lower shares of Fe and Al in the formed orthophosphates as well as amorphous phases containing higher shares of K. The formation of K-bearing phosphates was hindered by the spatial association of P with Ca and Fe in the sewage sludge, the incorporation of available K in K-Al silicates, and the depletion of K in the P-rich melt phase. Compared to mono-combustion, co-combustion experiments showed the potential for improving the combustion performance and reducing the risk of slag formation. The outcome suggests that co-combustion is a feasible path to integrate waste streams in fixed-bed energy conversion with simultaneous formation of phosphates enabling P recovery.