The combustion of phosphorus (P)-rich biomass has a significant potential to increase the stock of biomass resources available for renewable heat and power production. In addition, the P-rich ashes have the potential as a fertilizer and could reduce the need for non-renewable P resources in agriculture. However, several technical challenges must be resolved to realize this potential.

During combustion, the ash-forming matter in the fuel undergoes numerous chemical transformations, which can result in the formation of ash melts. Excessive melt formation can lead to durable ash deposits in and around the combustion zone and on heat exchangers, which can lower combustion performance and, in severe cases, lead to a complete shutdown of the process. Further, mono-combustion of P-rich residues such as sewage sludge results in the formation of phosphates with poor plant availability, which can significantly limit the value of the ash as a fertilizer. By co-combustion the sludge with K-rich biomass, it may be possible to alter the chemical speciation of P towards more plant-available phases while simultaneously managing the risk of ash-related operational issues. This work investigates the effect of combustion technology, fuel ash composition, and chemical association of P in the fuel on the fate, i.e., distribution and speciation, and ash transformations of P in combustion and co-combustion of biomass and sewage sludge.

The basis of the study was experiments performed in three different combustion technologies, including a fluidized bed (5 kW, 730-800 °C), a fixed bed (20 kW, 950-1250 °C), and a powder burner (150 kW, ~1100°C). The fuels and fuel mixtures included P-rich and P-poor woody biomass, agricultural residues, and sewage sludge, which constitute a wide range of ash compositions in terms of K, Ca, Mg, Fe, Al, Si, and P. The residual ashes from the experiments were collected and chemically characterized with the original fuels and fuel mixtures to determine the ash transformation reactions of P. The experiments were complemented by thermodynamic equilibrium calculations (TECs), which aided the interpretation of experimental data and predicted the risk for operational issues related to the melting of coarse ash fractions.

The major share of fuel P was found in coarse ash fractions such as bed ash particles, bottom ash, slag, cyclone ash, wind side deposits, and coarse fly ash. A low share of fuel P was found in fine ash fractions such as leeside deposits and PM1. This generally matched the predictions by TEC, which indicated that P was stable in condensed phases at the relevant compositions and conditions during the combustion experiments. The powder burner experiments produced the highest share of fuel P in PM1 (4-14 wt.%), followed by fixed bed combustion (<4 wt.%), with fluidized bed combustion having the lowest share (<0.6 wt.%). In addition, the experiments with sewage sludge indicated a significantly lower P share in PM1 for a given combustion technology than the other biomass fuels, ranging from <0.2 wt.% in the fluidized bed and <1.2 wt.% in the fixed bed.

Combustion and co-combustion of woody biomass and agricultural residues resulted in the formation of a wide range of ortho-, pyro-, and metaphosphates associated with K, Ca, and Mg. Combustion of woody biomass generally resulted in a high share of Ca-orthophosphates, whereas agricultural residues had a higher share of K-rich ortho- and pyrophosphates. Irrespective of biomass assortment, the speciation of P in the ash from combustion and co-combustion followed general trends with respect to the fuel ash composition of the biomass mixture. The frequency and share of pyro- and metaphosphates identified in the coarse ash fractions tended to increase with the relative concentration of P to K, Ca, and Mg in the fuel mixture. A similar correlation was found between the share of K-rich phosphates and the relative concentration of K to Ca and Mg.

The crystalline phosphate phases identified in the coarse ash fractions from sewage sludge and K-rich biomass experiments were mainly Fe-rich and Ca-rich orthophosphate. The frequency and share of Fe-rich orthophosphates decreased with the relative P to K, Ca, and Mg concentration in the fuel mixture. However, the sewage sludge mixtures were less prone to form K-rich orthophosphates than the biomass mixtures for a given composition in terms of P to K, Ca, and Mg.

Based on TECs, it was possible to qualitatively predict ash-related issues related to the melting behavior of coarse ash fractions, such as slag formation, for woody biomass and agricultural residues by considering the K, Ca, Mg, Si, and P content in the fuel. The share of network formers (SiO2, PO2.5) to total ash oxides had the largest overall influence on the melting tendency of the ash mixture, followed by the ratio of K2O to total network modifiers (K2O, CaO, MgO), which had a high impact on ash mixtures with high relative shares of SiO2. The slagging tendency of fuel mixtures with a high share of sewage sludge could not be predicted based on the melting behavior of the K-Ca-Mg-Si-P-O system due to the high relative share of Fe and Al. The experimental results indicated that the slagging tendency of the sewage sludge was significantly improved by co-combustion with moderate amounts of wheat straw or sunflower husk.

Based on the combined results, it was possible to establish four fuel ash molar ratios correlated with the speciation of P in the produced coarse ash fractions and the risk of slag formation in fixed-bed combustion. These ratios were used to recommend practical fuel mixing strategies that could enable the production of combustion ashes with high P-plant availability while simultaneously managing the risk of severe slag formation.

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.