Fate of Phosphorus in Fixed Bed Combustion of Biomass and Sewage Sludge

Abstract

The recovery of phosphorus (P) from societal waste streams, such as sewage sludge, could make a significant contribution to alleviating the global dependency upon non-renewable phosphate sources, such as phosphate rock. This study aims to determine the effect of fuel ash composition, chemical association, and combustion technology on the fate of P in ashes from the combustion of sewage sludge and biomass blends to enable more efficient P recovery from combustion ashes. Experiments were performed in a fixed bed pellet burner (20 kW), combusting two sewage sludge blends and three biomass blends of similar fuel ash composition but with different P source (sewage sludge, dried distiller's grain with solubles, or phosphoric acid). Slag, bottom ash, and particulate matter samples were collected and analyzed by scanning electron microscopy−energy-dispersive X-ray spectroscopy and X-ray diffraction for morphology and elemental and crystalline phase composition and compared to results from experiments in fluidized bed combustion using the same fuel blends reported separately. The distribution and elemental composition of ash fractions indicated that sub-micrometer particles contained a minor share of fuel P, with the significant share of fuel P found in the slag and bottom ash fractions. No apparent difference in phosphate speciation could be observed between the slag and bottom ash from sewage sludge blends and biomass blends, with a range of crystalline Ca, Mg, and K phosphates detected in the ash. By comparison, only Ca-rich phosphates were detected in the ashes from the combustion of the sewage sludge blends in the bench-scale fluidized bed. The difference in P speciation between the technologies was attributed to a difference in the process temperature between the two technologies. In comparison to fluidized bed combustion, fixed bed combustion favored the formation of (Ca, Mg)−K phosphates rather than Ca phosphates for similar fuel blends.

Single Pellet Combustion of Sewage Sludge and Agricultural Residues with a Focus on Phosphorus

“…The observation of such insignificant P release agrees with previous results reported for the combustion of SS-derived biosolids, where the majority of P was captured in the coarse ash fraction. 27,29,41 The low release rates obtained for all of the major ash-forming elements except S and Cl indicate that the complex elemental composition enhances the formation of non-volatile compounds. Release rates of Cl above 90% were obtained regardless of the temperature and the fuel mixture.…”

“…This has previously also been reported for SS and WS co-combustion. 12,29 In the identified whitlockite, the general β-Ca 3 (PO 4 ) 2 structure partially contains Fe, Mg, or K as substitute elements for Ca in cationic positions. For WSS10 at 800 °C, the quantification of whitlockite is very low compared to the others, which might be an error, and the amount of amorphous material is overestimated.…”

“…Potential reaction pathways have been discussed, but detailed studies lack information regarding the reactions between P, Si, Al, Fe, and K during the fuel particle conversion. Previous studies have further shown that by adding K to the SS fuel mixture, plant-available phosphates in the form of K-whitlockites can be formed .…”

Phosphorus Recovery from Sewage Sludge Ash Based on Cradle-to-Cradle Approach—Mini-Review

“…Thermochemical methods are included in the later process of P production in an electric furnace and ash calcination. The plant availability of P in SSA depended on the thermal conversion process, causing a significant change in the molecular environment of phosphates compared to the original sludge [111]. During these processes, P is transferred into a mineral form available for plants [96].…”

Extraction and Purification of Phosphorus from the Ashes of Incinerated Biological Sewage Sludge

“…The main drawbacks of these technologies are represented by (i) the need for pre-dewatering treatment; (ii) the need for disposal of final residues in combustion, such as bottom ash (BA) and fly ash (FA); and (iii) limited experience of real scale plant management for pyrolysis and gasification [5,[19][20][21]. Recent studies [22,23] evaluated the presence of P within these residues, which could therefore be a potentially exploitable source.…”