A great potential exists for increasing the use of bioenergy in thermochemical processes by utilizing agricultural biomass, forest residues, and sewage sludge that have high availability. Many of these biomass assortments have high ash contents with relatively high concentrations of ash-forming elements such as potassium (K), calcium (Ca), silicon (Si), and phosphorus (P). These elements can, during thermal conversion, cause several ash-related problems, such as deposit formation, slagging, and particle emissions. In particular, P has been found to play a vital role in such ash-related problems even at relatively low concentrations. In addition, ashes obtained from these biomass assortments could be an important source of valuable elements such as P and K. Therefore, detailed knowledge about the ash transformation and fate of P during thermal conversion of these opportunity biomass resources is of immense importance to mitigate ash-related problems and to recover valuable nutrient elements from the ash. 

The overall objective of this work was to determine the ash transformation and fate of P during single-pellet and fixed-bed combustion/gasification of different opportunity biomass fuels in the process temperature range of 600-1250°C. Different agricultural biomasses (poplar, wheat straw, grass, and wheat grain residues), forest residues (bark and twigs), and sewage sludge (pure and in mixtures with agricultural residues) were used. These fuels cover a wide range of overall ash compositions and different chemical associations of P in the fuel. The bark and poplar represent fuels rich in K and Ca with minor P content. The wheat straw, grass, and twigs represent typical Si- and K-rich fuels with minor to moderate P contents. The wheat grain residues (WGR) represent typical K- and P-rich fuels with a significant amount of Mg. The produced residual materials, i.e., char, different ash fractions and fine flue gas particles, were morphologically and chemically characterized by scanning electron microscopy-energy dispersive X-ray spectroscopy, X-ray diffraction, inductively coupled plasma, and ion chromatography. The interpretation of the results was supported by thermodynamic equilibrium calculations.  

For all fuels, a major part of the P (> 80%) was found in coarse ash fractions because the studied process conditions favored the formation of stable condensed phosphates. The thermal conversion atmosphere (i.e., gasification/combustion) only caused small effects on the P release and the speciation of the P-compounds formed. Ash transformation pathways generally lead to the formation of orthophosphates (PO₄^3-) such as Ca₅(PO₄)₃(OH), CaKPO₄, and Ca₃(PO₄)₂ with the partial substitution of Ca by some cation forming elements (Fe, Mg, and/or K), as the main P containing crystalline phases. Crystalline pyrophosphate (P₂O₇^4-) compounds were also found in the residual ashes from the seed-based fuel (WGR), where P originates from phytate in the biomass. For the fuels containing a certain (sufficient) amount of Si, orthophosphates interact with silicate phases to form both amorphous and crystalline phosphosilicates. For the sewage sludge mixtures, a surplus of available K was needed to form K-bearing phosphates due to side reactions of K with Si and Al.  

The chemical form of P in the formed ash residues is thus strongly dependent on both the type of P association in the fuel and the relative concentrations of other major ash-forming elements, such as K, Ca, Si, and Al. For the fuels with a high (Ca+Mg)/P molar ratio (AER), i.e., for the typical wood-derived fuels bark and poplar, hydroxyapatite was the main P-containing crystalline phase found in the ash. For the studied fuels/fuel mixtures with moderate AER and a high (K+Na)/(Si+Al) molar ratio (AR), e.g., twigs, grass, wheat straw, and sewage sludge with high mixtures of agricultural residues, there was also a possibility to form alkali-bearing phosphates such CaKPO₄ and K-Mg whitlockite, besides hydroxyapatite. Since these fuels contain a high amount of Si, the P can be found in both amorphous phases, i.e. phosphosilicate, and Si substituted crystalline phases, i.e. Ca₁₀(SiO₄)_x(PO₄)_6-XOH_2-x and Ca₁₅(PO₄)₂(SiO₄)₆. For fuels with moderate AER and low AR, e.g., pure sewage sludge and sewage sludge with low mixtures of agricultural residues, K-bearing phosphates were not formed. Instead, P was found in phases such as whitlockite and phosphosilicates. For the WGR fuel with relatively low AER and high AR, K-bearing phosphates were formed in the ashes, where the P was found in crystalline K-Mg/Ca pyrophosphates and K-Mg orthophosphate, as well as amorphous K-Mg-Ca phosphates. 

The produced knowledge can potentially be used to, e.g., i) suggest efficient measures to mitigate ash-related problems associated with P during thermochemical conversion of opportunity biomass fuels, ii) suggest potential pathways to form plant-available phosphates directly in the thermal conversion process to enable recovery of P and K from the obtained ashes, and iii) find optimal thermal conversion process conditions to obtain bio charcoals that are suitable as alternative fuels and reducing agents in the metallurgical industry. 

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.

In recent years, the thermal conversion of sewage sludge has proven its applicability for managing this inevitably generated waste. The viability arises from the concomitant features of recovering energy or valuable compounds, the breakdown of potentially harmful organic compounds, the separation or immobilization of heavy metals, and the formation of volume-reduced, sanitized residues. The inorganic residue after thermal conversion of municipal sewage sludge, i.e., the ash, is generally rich in phosphorus (P). However, P in sewage sludge ash is mostly present in a chemical association that is poorly plant-available, e.g., apatite and whitlockite. Since sewage sludge ashes represent a P-rich resource, a number of different post-processing methods have been fathomed to extract P or alter its association in the ash. While extraction methods often focus on eluting P with acids, methods to alter the P-association in the ash rely mostly on thermochemical post-processing with additives. A way of enhancing the plant-availability of P in the ash is the thermochemical treatment with alkali additives, e.g., (Na,K)₂SO₄ and (Na,K)₂CO₃, leading to the formation of alkali-bearing phosphates of improved plant-availability. Providing the necessary physiochemical conditions for this phosphate alteration process, there is a potential to achieve the formation of alkali-bearing phosphates already during the thermal conversion of sewage sludge.

This work investigates the potential of forming K-bearing phosphates in fluidized bed co-combustion and co-gasification processes of P-rich sewage sludge and K-rich agricultural residues. The focus was set on the fate and role of P in the interaction of the main ash-forming elements based on thermodynamic equilibrium studies, lab-scale investigations, and bench-scale fluidized bed experiments. Additionally, the benefits, e.g., fuel flexibility and high conversion rate, and ash-related risks due to interaction of ash and bed material when using fluidized bed systems are elaborated with a focus on bed material selection and investigating the operational modes of combustion and gasification.

The applicability of K- and Na-feldspar bed material in a pilot-scale indirect gasification system was investigated to provide a potential substitute for commonly used bed materials such as olivine and quartz. Olivine often contains heavy metals that could contaminate recovered ashes. Quartz may react with fuel-derived K, which could hamper the targeted formation of K-bearing phosphates and lead to bed material fragmentation and bed agglomeration. The bed material analysis of feldspar used in indirect wood gasification showed significant differences in the interaction phenomena between K- and Na-feldspar with the fuel ash. While both feldspar types displayed Ca-reaction and ash deposition layers on the particle surface, the interaction of Na-feldspar with K additionally led to the formation of K-reaction layers, possibly decreasing the bed particle integrity. The results suggest that K-feldspar is the preferred bed material option in terms of process stability and limiting the potential for side reactions of K when aiming for phosphate alteration toward K-bearing phosphates.

Thermodynamic equilibrium calculations (TEC) were conducted with a focus on the fate of P and melting tendencies for a wide range of chemical compositions in biomass ashes and fuel mixtures between sewage sludge and the agricultural residues wheat straw (rich in Si and K) and sunflower husks (rich in K and Ca). The results for the K-Ca-Mg-P-Si-O system were validated with literature references, and an outline of practical implications was given. The results for sewage sludge and mixtures with agricultural residues functioned as a seminal tool for fuel design in experimental investigations. The thermodynamic preference for forming alkali-bearing phosphates in competition with pure Ca-phosphates and incorporating K in silicates could be shown. The analysis of the K-Ca-Mg-P-Si-O system highlighted the influence of elemental ratios between and within the Lewis acid formers (Mg, K, Ca) and the Lewis base formers (Si, P) on the fate of P and the ash melting tendency. The TEC for sewage sludge and mixtures with agricultural residues predicted that these elemental ratios are also the determining factors in the presence of large quantities of Al and Fe.

Experimental research regarding the underlying ash chemistry with a focus on the fate of P was conducted in a single-pellet reactor and bench-scale fluidized bed combustion and gasification processes. The approach used sewage sludge pellets and co-pelletized mixtures of sewage sludge with wheat straw and sunflower husks to determine the P-recovery potentials and ash-related operational risks. The parameters were chosen with relevance to practical applications of fluidized bed technologies. The experimental findings supported the TEC results in several aspects, such as the preference for Ca-phosphate formation in sewage sludge ash and the exclusion of Fe from the bulk ash matrix. However, the results also showed practical limitations for the formation of K-bearing phosphates in fuel mixture ashes. The identified limitations were the reaction of K with Si, the high stability of Ca-rich orthophosphates, and the limited interaction of ash-forming elements in char residues from gasification processes. Furthermore, the results from the fluidized bed experiments highlighted the necessity for amendments in terms of fuel selection and fuel mixing to avoid operational risks such as bed agglomeration. The results of the conducted experimental investigations suggest that using K-feldspar as bed material in sewage sludge co-conversion setups with agricultural residues might benefit the incorporation of K in the P-rich ash fractions.

The results and discussions presented in this work allowed for the assessment of crucial process and fuel parameters for fluidized bed conversion systems using sewage sludge fuels and biomass fuel mixtures focusing on the formation of K-bearing phosphates. The importance of the ash transformation chemistry and its impact on selecting a suitable bed material could be outlined based on experimental and modeling data. The outcome shall assist the design of future large-scale applications in terms of a viable process and fuel design for energy and resource recovery from sewage sludge and agricultural residues.