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

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.

Figure 1

Schematic overview of the underfeed pellet burner (20 kW nominal) in the reference boiler, ash sampling points, flue gas measurements, and temperature and pressure measurements. Adapted from Hedayati et al. Copyright 2022 American Chemical Society.

Figure 2

A visual comparison of the combustion zone between the combustion experiments with SH, SHS15, and SHS40.

Figure 3

Distribution of ash (wt %) between hard slag, weak slag, coarse BA, fine BA, PM_1–10, and PM₁ on a CHN-free basis. The collected ash fractions are normalized to the theoretical amount of ash fed with the fuel based on a standard ashing test (550 °C).

Figure 4

The concentration of PM₁ and PM_1–10 in the flue gas with error bars indicating ±1 cumulative standard deviation (WS: n = 2, other fuels: n = 3).

Figure 5

Average elemental composition on an O- and C-free basis (mol %) of PM₁ (<1 μm) based on three SEM/EDS mappings. Error bars indicate ±1 standard deviation.

Figure 6

Average elemental composition (mol %, O- and C-free basis) of hard slag, weak slag, coarse bottom ash, fine bottom ash, and fuel composition based on three to seven SEM/EDS mappings. Error bars indicate ±1 standard deviation.

Figure 7

Elemental distribution of K and P between the collected ash fractions normalized to the theoretical amount of K and P fed with the fuel mixture. The standard deviation is the cumulative uncertainty of the elemental composition based on the standard deviation of each ash fraction (n = 3–7).

Figure 8

SEM-BSE micrographs of ash fractions: (A) SS33 fine BA, (B) SS33 weak slag, (C) SHS40 fine BA, (D) SHS40 weak slag, (E) WSS10 fine BA, and (F) WSS weak slag at 75–85× magnification.

Figure 9

Mass distribution of P between crystalline and amorphous phases in the residual ash based on the mass balance of the bottom ash slag ash fractions, the bulk elemental composition, and the quantitative XRD results.

Figure 10

BSE micrographs and SEM/EDS mappings within the image area of hard slag cross sections from SS33 at ×250 magnification.

Figure 11

BSE micrographs and SEM/EDS mappings within the image area of hard slag cross sections from SHS40 at ×250 magnification.

Figure 12

BSE micrographs and SEM/EDS mappings within the image area of hard slag cross sections from WSS10 at ×250 magnification.