Abstract
Recycling of phosphorus in combination with increased utilization of bioenergy can mitigate material and global warming challenges. In addition, co-combustion of different fuels can alleviate ash-related problems in thermal conversion of biomass. The aim of this study is to investigate the ash transformation reactions of mainly P in co-combustion of P-rich sewage sludge (SS) with K-rich sunflower husks (SH) and K- and Si-rich wheat straw (WS). Single pellets of 4 mixtures (10 and 30 wt % SS in WS and 15 and 40 wt % SS in SH) and pure SS were combusted in an electrically heated furnace at process temperatures relevant for fluidized bed combustion (800 and 950 °C). Collected ash fractions were analyzed by inductively coupled plasma techniques, ion chromatography, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and X-ray diffraction. Thermodynamic equilibrium calculations were performed to interpret the results. Over 90% of K and P was found to be captured within the residual ash with 30–70% P in crystalline K-bearing phosphates for mixtures with low amounts of SS (WSS10 and SHS15). The significant share of K and P in the amorphous material could be important for P recovery. For the lower percentage mixtures of SS (WSS10 and SHS15), P in crystalline phases was mainly found in K-whitlockite and CaKPO4. For the higher percentage SS mixtures, most of P was found in whitlockites associated with Fe and Mg, and no crystalline phosphates containing K were detected. For P recovery, co-combustion of the lower SS mixtures is favorable, and they are suggested to be further studied concerning the suitability for plant growth.
Highlights
There is a potential growth in bioenergy supply from widely unused agricultural residues.[1]
The majority of K and P was retained within the residual ash
K present in the amorphous phase could be bonded in silicates, Science, Luleå University of Technology, SE-971 87 Luleå, Sweden
Summary
There is a potential growth in bioenergy supply from widely unused agricultural residues.[1]. Utilization of biomass can help decrease the dependence on fossil fuels for heat, power, and biofuel production. This aids in the mitigation of CO2 addition to the atmosphere and combats global warming, a contemporary topic for environmental research. Another topic is that of resources and efficient utilization of existing natural reserves, including recycling of materials essential for the human ecosystem. One element vital for global food production is phosphorus.[13] This essential element is currently mined and extracted from ore deposits for further processing into mineral fertilizers. In the European context, some of the P in SS is utilized for fertilization of agricultural land, but large proportions are still deposited as materials with no utilization of the inherent P content.[16]
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