Ash transformation processes during the entrained flow combustion of brewer's spent grains (BSG), a P-rich grain- and seed-based agricultural biomass, were investigated in a 150 kW horizontal powder burner operating at ∼1400 °C. Coarse ash particles (> 1 μm) were obtained from several locations within the furnace and heat exchanger zone. The collected ash samples were characterized using energy-dispersive X-ray fluorescence (ED-XRF), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Complementary thermodynamic equilibrium calculations were conducted to interpret experimental findings. The results showed that the majority of P was retained in the residual coarse ash fractions (> 1 μm) as Ca-Mg-rich phosphosilicate melt. Coarse ash samples collected near the flame zone exhibited intensified interaction between partially molten Ca-Mg-phosphates and Si-rich particles, promoting an increased melt fraction in the condensed phases. These insights deepen the understanding of ash transformation during entrained flow combustion of P-rich grain- and seed-based agricultural biomasses. They also carry practical implications for slag mitigation, system design, and phosphorus recovery from residual ash fractions.

Efficient biomass production through management like thinning is crucial for increasing the supply of renewable and carbon neutral feedstock. However, change in growth rates may alter feedstock properties and affect subsequent bioenergy conversion, material and chemical production. This study evaluated the effects of thinning treatments and stem diameter on the fuel, elemental, and structural composition of stemwood and bark from second-rotation poplar plantation (original stand: 1100 stumps ha−1). Two different thinning methods were applied: row thinning (removing all stems for every other row of plantation and reducing stump density to 550 stumps ha−1) and stem thinning (retaining only the single largest stem per stump). The results showed that thinning method and stem diameter affect fuel and lignocellulosic composition. Single-stem trees at high stump density had the best fuel traits, with low ash and high volatile matter to fixed carbon (VM/FC) ratios, reflecting reduced growth competition. Smaller stems contained more ash and VM/FC in bark. Carbon, hydrogen, and nitrogen contents were not affected by treatments. Single-stem trees had higher hemicelluloses and lower lignin, indicating more complete cell wall development, while crowded, multi-stem conditions increased lignin. Highest extractives were found in bark from low-density single-stem trees. Both total biomass and structural components yields were highest for single-stem trees without row thinning. It highlights the benefits of stem thinning. This study suggests that both quality and quantity of biomass from second-rotation poplar plantation can be influenced by thinning treatments and stem diameter, with potential implications for bioenergy and bio-based chemicals or fuels.

Understanding the characteristics and formation process of bed particle layers resulting from interactions between ash-forming matter and bed material during fast pyrolysis is crucial for optimizing fast pyrolysis bio-oil (FPBO) production. However, research on this topic remains limited. In this study, the evolution of the bed particle layers formed on quartz bed particles during fast pyrolysis of wood was investigated across bench-, pilot-, and industrial-scale units. Bed material samples with different exposure times were characterized using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM/EDS), and focused ion beam-SEM (FIB-SEM) to assess the morphology, elemental composition, and thickness of the bed particle layers. Overall, the time-dependent formation and characteristics of the quartz bed particle layers were similar to those reported for the fluidized-bed combustion of woody biomass. The key difference, however, was that the layers formed during fast pyrolysis were significantly thinner and contained less Ca. The bed particle layer formation began early in the process through the deposition of Ca-rich ash particles, primarily on convex surfaces, likely followed by solid–solid diffusion of Ca²⁺ into the quartz core, forming a Ca-silicate-rich inner layer. The inner layer developed later and more sparsely in concave regions, resulting in thinner layers. After approximately 1 day, an outer layer developed on the convex surfaces due to continued ash particle deposition, while a K–Si-rich inner-inner layer (likely composed of K-rich silicates and associated with gaseous alkali diffusion) formed primarily in concave regions. Over time, the bed particle layer thickness approached a limiting value of approximately 4 μm, likely due to reduced growth of the inner layer, which may be attributed to diminished inward transfer of Ca²⁺ as the diffusion distance increased.

The detailed ash transformation process during the combustion of agricultural biomass containing moderate to high amounts of P was studied in entrained flow conditions. The selected fuels were grass and brewer’s spent grain (BSG) containing a moderate and high amount of P in the fuel, respectively. The experiments were conducted in a lab-scale drop tube furnace at 1200 and 1450 °C. The residual chars, ashes, and particulate matter (PM) were collected and analyzed by scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and ion chromatography (IC), and CHN-analysis. Additionally, the obtained results were interpreted through thermodynamic equilibrium calculations (TECs). For both fuels, P was primarily identified in the residual coarse ash (>1 μm) fractions. In contrast, a minor to moderate amount of fuel inherent P was detected in the fine particulate (<1 μm) fraction at 1200 and 1450 °C, respectively. For grass, the retained P in the residual coarse ash fractions was mainly identified as amorphous K–Ca–Mg-rich phosphosilicate melt. These phosphosilicates were most likely formed through the initial formation of molten K-rich silicates, with subsequent incorporation of Ca, P, and Mg. For BSG, a P–Si-rich fuel with moderate to minor amounts of Ca, Mg, and K, most P was retained in a Ca–Mg-rich phosphosilicate melt, likely originating from phytate-derived Ca–Mg phosphates interacting with fuel-inherent Si-rich particles. The results obtained from this study could be used to address the ash-related challenges and potential P-recovery routes during pulverized fuel combustion of P-containing biomass.

Rice husks were combusted in a 150 kW pilot-scale powder burner connected to a horizontal ceramic-lined furnace to investigate the ash transformation processes, including deposit formation at high surface temperatures. Residual coarse ash samples (>1 μm) were collected from different positions along the furnace and heat exchanger path. Fine fly ash samples (<1 μm) were collected from the furnace outside the flame, and high-temperature deposits were collected on deposition probes having different surface temperatures. The collected samples were analyzed via scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. Additionally, thermodynamic equilibrium calculations were employed to interpret experimental results. The results showed different ash transformation processes occurring at the outer surface and inner part of the rice husks. A high share of minor ash-forming elements (i.e., K, P, Ca, and Mg) together with Si was retained in the residual coarse ash particles. The retained minor ash-forming elements were mainly incorporated in the spherical Si-rich particles with moderate amounts of K, Ca, Mg, and P that were partially molten and originated from the inner part of the rice husks. The outer surface of the rice husks primarily formed skeleton-like coarse ash particles dominated by Si. The high surface temperature deposits only contained skeleton-like coarse ash particles that were partially molten.

Despite extensive research on the bed particle layer formation and characteristics of ilmenite bed particles in fluidized bed combustion and gasification of woody biomass (characterized by high Ca and K content), a significant knowledge gap exists when P-rich fuels are used. Chicken litter, as a P-rich biomass, presents a promising alternative biomass for energy conversion processes. Therefore, this study aims to investigate the bed particle layer formation and characteristics of ilmenite bed particles during fluidized bed combustion of chicken litter. The bed particle layer formation process and characteristics for ilmenite bed particles utilized in a 5 kWth bubbling fluidized bed combustion of chicken litter were studied in this work. Bed samples were taken after 4, 8, 12, and 16 h from the start-up and were analyzed via SEM/EDS. The findings highlighted that the initial stage of bed particle layer formation is similar to what was observed in fluidized bed combustion of woody biomass and is driven by the reaction of fuel-derived Ca-rich particles with the bed particle, leading to the formation of the inner layer. As time progresses, the deposition of fuel-derived bed ash, containing mainly Ca, P, Mg, and K, forms an outer layer that uniformly covers the entire bed particle surface, irrespective of the bed particle’s surface morphologies. Additionally, the outward migration of Fe from ilmenite bed particles is significantly constrained when utilizing chicken litter, a Ca- and P-rich fuel, due to the formation of a bed particle layer that uniformly covers the bed particle surface.

Biocarbon is a potential alternative to fossil coal use in the industrial sector. Fluidized-bed technology, known for its exceptional thermal mixing and reactor integration capabilities, holds promise for large-scale biocarbon production. However, the successful implementation of this technology requires overcoming technical challenges such as high concentrations of potassium (K) and phosphorus (P) in forest-based biocarbon, which can limit its applicability in certain industrial processes.

The objective of this study was to identify the potential effects of biomass-bed material interactions that can affect the presence of these ash-forming elements in the resulting biocarbon. Laboratory-scale fluidized bed experiments were conducted in a weakly oxidizing atmosphere (86.2 % vol N2, 10 % vol CO2, and 3.8 % O2) at various temperatures and residence times. Pine bark, which is a low-cost Ca-K-rich biomass with a minor amount of P, was used as raw biomass. The experimental results were analyzed using scanning electron microscopy-energy-dispersive scanning electron microscopy (SEM-EDS) and thermodynamic equilibrium calculations (TECs), providing insights into the ash transformation process. Resulting biocarbon had a high carbon content (75–90 wt% d.b.), with mass yields ranging from 13 to 30 wt%. The K retention in the biocarbon after 400 s of conversion was between 67 % and 44 % at a bed temperature of 550–900 °C, whereas the P retention was between 58 % and 43 %. The results suggested that additional inorganic removal mechanisms, different from K and P volatilization, are present in fluidized bed reactors compared to other commercial pyrolysis technologies. This highlights fluidized bed reactors (FBRs) as a promising alternative for producing biocarbon with lower K and P levels. The findings of this study contribute to the development of the design and operational criteria for fluidized beds used in biocarbon production. In addition, the results strongly indicate that the interaction between the ash-forming elements and bed materials begins before the raw material fractions are completely converted, and further investigation is recommended.

Agricultural biomass is today largely underutilized in combustion and gasification processes because of the abundant supply of other easier-to-process biomass fuels. These biomass types generally have a moderate to high ash content comprising valuable elements, such as Si, K, and P, which can lead to ash-related operational problems. The high share of Si, K, and P in agricultural biomass assortments also has a significant economic value. These elements are usually retained in the coarse or fly ash fractions. Extracting valuable Si- and K–P-containing compounds with high purity from these ash fractions often requires further postprocessing steps, which increases operational costs. Therefore, a potential novel design concept could be to control the combustion/gasification processes so that Si, K, and P can be extracted by condensation from the flue/hot gases at a quality that implies added value instead of extra costs. This work aims to identify the possibilities of extracting valuable Si and K–P compounds from the hot gases generated during entrained flow conditions via stepwise controlled condensation in the close-flame regions or heat exchanger zone. Thermodynamic equilibrium calculations were performed by employing the databases (GTOX and SGPS) in FactSage 8.0 software. The calculations were performed under varying conditions ,i.e., temperatures, atmospheres, and fuel compositions. The selected fuels were rice husks (Si-rich), brewer’s spent grains (P–Si-rich with moderate to minor amounts of Ca, Mg, and K), and grass (K–Si-rich with moderate amounts of Ca, Mg, and P). The results indicate that the high-temperature formation of the valuable Si compounds, such as SiC (s) and Si2N2O (s), would require an inert atmosphere during both the release and cooling stages. Moreover, a high Si/P molar ratio is needed to form valuable Si-containing compounds. The predicted K-bearing phosphates during the gas cooling near the burner zone were formed in the same temperature range as Ca-, Mg-, and Si-containing compounds with all of the fuels. The results obtained by this study can guide experimental research on the practical extraction of Si and K–P compounds from different types of agricultural biomass during thermochemical conversion in entrained flow conditions.

This study investigates the detailed ash transformation process during the combustion of rice husks in entrained flow conditions. The experiments were conducted in a lab-scale drop tube furnace at 1200 and 1450 °C in pyrolysis/devolatilization (using N2) and combustion (using air) conditions. The detailed ash transformation process during the different fuel conversion stages in combustion (i.e., devolatilization and char combustion) was investigated by comparing the results obtained in the pyrolysis/devolatilization experiments with the combustion experiments. The resulting residual chars, ashes, and particulate matter (PM) were collected and characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM–EDS), X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), ion chromatography (IC), and CHN analyses. Furthermore, the obtained results were interpreted via thermodynamic equilibrium calculations (TECs). For all investigated conditions, Si, Ca, and Mg were retained entirely in the coarse ash and char fractions (>1 μm). Meanwhile, K and P were found in coarse ash/char fractions and fine particulate fractions (<1 μm). A moderate, at 1200 °C, to high share, at 1450 °C, of the detected K and P was found in the fine particle fractions after combustion. The majority (>95%) of the detected S and Cl were volatilized during the experiments. The study showed an accumulation of minor ash-forming elements (i.e., K, Ca, Mg, P) on the inner part of rice husk chars, initiating melt formation during the char combustion stage. The identified melt at 1200 °C after combustion was rich in Si with minor amounts of K, Ca, Mg, and P. The share of molten ashes was increased at 1450 °C compared to that at 1200 °C. Overall, the results presented in this work reveal detailed insights into the ash transformation processes taking place in different parts of the fuel during the combustion of rice husks in entrained flow conditions.

In this work, the fate of the ash-forming elements during bubbling fluidized bed combustion and gasification of P-rich sewage sludge (SS) and mixtures with either Si-K-rich wheat straw (WS) or K-Ca-rich sunflower husks (SH) were investigated. The focus of the study was assessing the feasibility of using fuel blends in fluidized bed systems and potential P recovery from the resulting ashes. The used fuels were pure SS and mixtures including 90 wt.% WS (WSS) and 85 wt.% SH (SHS). The analyzed operating conditions were combustion (930–960 °C, λ: 1.2–1.5) and gasification (780–810 °C, λ: 0.4–0.7) in a 5 kW bench-scale reactor. Residual ash and char fractions were collected from different parts of the 5 kW bubbling fluidized bed (bed, cyclone, filter) and analyzed by CHN, SEM/EDS, XRD, and ICP-AES.

The conversion of the fuel mixtures achieved a steady state under the used process conditions except for the combustion of WSS, which led to the formation of large bed agglomerates with the bed material. The morphology of ash samples after combustion showed that SS fuel pellets mostly maintained their integrity during the experiment. In contrast, the ash and char particles from fuel mixtures were fragmented, and larger quantities were found in the cyclone, the filter, or on interior reactor surfaces. The fate of P was dominated by crystalline Ca-dominated whitlockites in all ash fractions, partially including K for the fuel mixtures SHS and WSS. 76–81 % of ingoing P was found in the bed residue after combustion and gasification of the SS-fuel. After conversion of the fuel mixtures SHS and WSS, the share was lower at 22–48 %, with larger shares of P in the entrained fractions (25–34 %). The quantity of identified crystalline compounds was lower after gasification than combustion, likely due to the limited interaction of ash-forming elements in the residual CHN matrix. Altogether, the results show that fuel mixtures of sewage sludge with agricultural residues could expand the fuel feedstock and enable P recovery. This may be used in the fuel and process design of upscaled fluidized bed processes or systems employing both combustion and gasification.