Studies of ash transformation processes in thermochemical co-conversion of phosphorus-rich manure and sludge with biomass residues

• Efficient use of resources and sustainable recovery of various materials are important to minimize the anthropogenic impact on the climate and environment. One such resource is the phosphorus (P) present in manure and sewage sludge. Various technologies are currently being developed to recover the element for application as fertilizer in agricultural applications. Thermochemical conversion presents the opportunity to recover energy from these materials. In a single process, elements can be recovered in ash fractions, potentially harmful organic substances can be destroyed and heavy metals fractionated from the P. Mono-combustion of sewage sludge mainly produce apatite, a phosphate mineral with low plant availability and therefore less useful for fertilization. Co-combustion/-gasification with other fuels enables modification of the ash transformation reaction pathways and remedies potential problems, such as bed agglomeration, associated with forestry and agricultural residues when used as fuels.The overall objective of this work was to increase the current knowledge in ash transformation of P-rich materials in co-conversion with forestry and agricultural residues in order to facilitate the P-recovery by formation of suitable phosphates in the ash. The work focuses on i) the influence of co-conversion on ash transformation of P with a focus on altering speciation of P towards the potentially more plant-available K-bearing phosphates ii) the influence of fuel ash composition and chemical association of P in the fuel, temperature and particle interaction on the fate, i.e. speciation and distribution, of P and iii) practical implications of co-conversion in fluidized bed and pulverized fuel systems for P-recovery, specifically interaction of P-rich ash with bed material in fluidized beds and strategies for extracting P-rich ashes.Experiments were carried out in a bench-scale bubbling fluidized bed reactor (BFB), macro-thermogravimetric analysis (TGA) conversion reactor, a dual fluidized bed (DFB) gasification reactor, and an entrained flow reactor (EFR) for pulverized fuel combustion. The fuels studied were mixtures of chicken litter together with wheat straw and bark, and mixtures of digested sewage sludge combined with wheat straw and sunflower husk. The process temperature ranges studied were 800-950 °C for BFB and single-pellet macro-TGA studies, whereas 1000 °Cand 1400 °C were investigated in pulverized fuel combustion studies using the EFR. Ash fractions and bed materials were collected and analyzed using scanning electron microscopy with energy-dispersive Xray spectroscopy (SEM-EDS), powder X-ray diffraction (XRD), inductively coupled plasma with atomic emission spectroscopy (ICPAES) and ion chromatography (IC). The results were interpreted with the support of thermodynamic equilibrium calculations (TECs) using FactSage software with the GTOX & SGPS databases.For all investigated conditions and fuel mixtures, the major part of P (> 90 %) was found in coarse ash fractions, suggesting that the recovery potential is highest in these fractions. This also means that P and volatile heavy metals can be separated in different ash fractions. Crystalline P was to a higher degree observed in the form of K-bearing whitlockite structures and CaKPO4 in mixtures containing low amounts of sewage sludge and high amounts of agricultural residues rich in K. K-bearing whitlockites were also found in ash of chicken litter and its mixture with wheat straw, as well as in ash deposits formed in pulverized combustion with a sewage sludge and wheat straw mixture combusted at 1000 °C. In mixtures with higher shares of sewage sludge, crystalline P was mainly found as Fe- and Mg-substituted whitlockites and hydroxyapatite. The reaction pathway of P appears to mainly occur through substitution and addition reactions in the condensed phase. The findings show that it is possible to modify the ash transformation of P towards K-bearing phosphates by co-conversion and that the difference between combustion and gasification is small.For the mixture of chicken litter and K- and Si-rich wheat straw combusted in BFB, P and Si together with K and Ca formed homogeneous ash particles with large amounts of potentially amorphous content. A similar behavior was observed in sewage sludge and wheat straw mixtures, where P and Si were likely present in a melt that was amorphous after extraction. In addition to these particles, P was also observed in crystalline orthophosphate compounds such as hydroxyapatite, aluminium phosphate, whitlockites and CaKPO4. In the mixture of chicken litter with Ca-rich bark, crystalline P was found in the form of hydroxyapatite. In fuel mixtures with higher amounts of Al with Si, the capture of K in aluminosilicates was higher, making it unavailable to form K-bearing phosphates. Small differences in the fate of P, between organically and inorganically associated P found in the fuels were seen in this work. Lower temperatures (800 °C compared to 950 °C) favored the formation of crystalline K-bearing phosphates in single-pellet combustion of sewage sludge and agricultural residues. In pulverized fuel combustion experiments, more crystalline K-bearing phosphates were found at 1000°C compared to 1400 °C. Fuel ash interaction mainly occurred in condensed phases in ash deposits compared to interactions between particles entrained in the flow.In fluidized bed experiments, P captured Ca and K in relatively high temperature melting phosphates in the fuel ash, decreasing the interactions of these elements with the bed material and thus decreased the risk for bed agglomeration. Possible extraction strategies involve the separation of coarse ash particles from bed material particles or in heated cyclones, avoiding fine ash fractions known to be rich in volatile heavy metals. Mixtures of coarse ash and bed material can potentially also be used for P-recovery. Co-conversion increases the possibility of utilizing existing boilers for recovery of P and increasing their flexibility to different fuels. The results indicate that a powder combustor operating in slagging mode could be a feasible strategy for P recovery because the interaction potential between the formed individual coarse ash particles increases at the hot wall. Plant growth studies have to be performed to further validate the agricultural value of the produced ashes for direct soil application.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Energiteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Energy Engineering (hsv//eng)

Nyckelord

sewage sludge

agricultural residues

forest residues

ash transformation

potassium

phosphorus

combustion

gasification

phosphorus recovery

Energy Engineering

Energiteknik

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