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

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 CaKPO 4 . 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.

Early layer formation on K-feldspar during fluidized bed combustion with phosphorus-rich fuel

“…This could hinder the deposition of ash particles and therefore be the reason for a comparably thinner average layer formation [15]. At the interface of the deposited ash particles and the bed particle, Ca 5 P 2 SiO 12 was calculated to form, which agrees with the previously suggested formation of silicophosphates occurring when different P-rich fuels were thermally converted [9,[49][50].…”

Ash Transformation during Fixed-Bed Combustion of Agricultural Biomass with a Focus on Potassium and Phosphorus

“…However, the high concentrations of these ash-forming elements also present several opportunities since ashes obtained from these biomasses could be a great source of nutrients and valuable elements. The recovery of these elements has recently received more attention. − Therefore, detailed knowledge about ash transformation reactions during the combustion of agricultural biomasses plays an important role in mitigating ash-related problems as well as in recovering valuable nutrient elements such as K and P from ash.…”

Role of Phosphorus and Iron in Particle Adhesiveness at High Temperatures Using Synthetic Ashes

“…Since the reserves of the minerals that contain phosphate are limited, sources of P are required for future technology needs. − Furthermore, the release of P into the environment from human activity can cause eutrophication of surface water that can adversely impact ecological systems. Therefore, recovery technology of P from sewage is important because it can protect water resources and recover P for other uses. − Recovery systems that use chemical and biological techniques have been developed and applied to sewage treatment systems and can produce solid waste containing a high concentration of P. ,− The sewage sludge generated during the water treatment process usually is incinerated to decrease the volume of waste. The sludge contains a burnable portion derived mainly from organic substances and an unburnable portion derived from minerals. , The unburnable portion forms ash particles during incineration.…”