Thermochemical equilibrium study of ash transformation during combustion and gasification of sewage sludge mixtures with agricultural residues with focus on the phosphorus speciation

Abstract

The necessity of recycling anthropogenically used phosphorus to prevent aquatic eutrophication and decrease the economic dependency on mined phosphate ores encouraged recent research to identify potential alternative resource pools. One of these resource pools is the ash derived from the thermochemical conversion of sewage sludge. This ash is rich in phosphorus, although most of it is chemically associated in a way where it is not plant available. The aim of this work was to identify the P recovery potential of ashes from sewage sludge co-conversion processes with two types of agricultural residues, namely wheat straw (rich in K and Si) and sunflower husks (rich in K), employing thermodynamic equilibrium calculations. The results indicate that both the melting behavior and the formation of plant available phosphates can be enhanced by using these fuel blends in comparison with pure sewage sludge. This enhanced bioavailability of phosphates was mostly due to the predicted formation of K-bearing phosphates in the mixtures instead of Ca/Fe/Al phosphates in the pure sewage sludge ash. According to the calculations, gasification conditions could increase the degree of slag formation and enhance the volatilization of K in comparison with combustion conditions. Furthermore, the possibility of precipitating phosphates from ash melts could be shown. It is emphasized that the results of this theoretical study represent an idealized system since in practice, non-equilibrium influences such as kinetic limitations and formation of amorphous structures may be significant. However, applicability of thermodynamic calculations in the prediction of molten and solid phases may still guide experimental research to investigate the actual phosphate formation in the future.

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

“…The GTOx database was used as the primary database for the modeling of the stoichiometric compounds, the solution phases, and the gas phase as it has been demonstrated to be suitable for P-rich inorganic systems. 32 The oxide melt model contained in the GTOx database was used for the prediction of molten phases. The SGPS database was used as the secondary database source for complementary compounds in the gas phase and stoichiometric solids including H. As calculations including S and Cl determined predominantly volatilization of those elements in the calculations, the implementation of S, Cl, and a salt melt model was neglected.…”

“…Comparison of the results with previous research experiments about the formation of K-bearing phosphates in SS ashes by addition of a K source suggests that the effect of kinetic restrictions and side reactions of K is higher than what the calculations predicted. 32 3.7. General Behavior of P and Practical Implications.…”

“…29,31 However, thermodynamic equilibrium calculations (TECs) for the co-combustion of SS and K-rich agricultural fuels indicate that the formation of K-bearing phosphates is thermodynamically favored at lower temperatures relevant for fluidized bed combustion. 32 There are also new TEC databases 33 with enhanced applicability in P-rich systems that need to be assessed and compared to experimental results. Since thermodynamic equilibrium calculations suggest a possibility to modify the speciation of P into K-phosphates, experimental studies are needed to confirm and investigate these findings.…”

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

“…Ash particles can adhere inside of the incineration machinery and form large aggregates, decreasing efficiency of heat recovery, blocking ducts, and stopping fluidization in fluidized incineration beds. − Since these issues inhibit stable and efficient plant operation, evaluation and control of ash adhesiveness is important. The P in ashes can induce ash adhesion. − To realize recovery of P with high efficiency and stability, understanding the adhesiveness of P in ashes at high temperatures is an important research subject.…”

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

“…However, 18% of the P contained in the fertilizer remains in an unavailable form [112]. Obtaining P in a solid phase constitutes the basis for forming new phosphate compounds with high plant availability [2,113] 2) process relies on P recovery from thermally treated SSA at 850 • C. The process leads to receiving phosphoric acid from SSA. The process relies on the P and HMs dissolving.…”