In biomass pyrolysis, final product selectivity is governed not only by major reaction conditions like temperature and heating rate but also by complex vapor–solid interactions and secondary reactions. Yet, the influence of internal flow configuration on pyrolysis vapor remains poorly understood in continuous pyrolysis systems. This study aims to evaluate how controlled vapor–solid interactions via changes in the vapor outlet port location affect the distribution and transformation of pyrolysis products. Experiments were performed in a continuous laboratory-scale auger reactor, processing pine bark at highest treatment temperatures (HTT) of 600, 700, and 800 °C. The reactor featured five independently heated zones and six selectable vapor outlet ports, enabling three vapor flow modes: parallel flow (PF, conventional cocurrent flow operation) and two counterflow (CF) configurations to systematically manipulate vapor–solid contact. Results showed that one of the CF configurations, where vapors passed through the coldest (the incoming) biomass zone before exiting, enhanced vapor condensation on incoming biomass and promoted secondary reactions, leading to up to a 15.5% relative increase in biochar yield compared to PF. The increase in biochar yield was accompanied by an increase in fixed carbon yield, and H2 and CH4 yields, indicating intensified thermal cracking and polymerization of pyrolysis vapors. In contrast, the CF configuration involving vapor recirculation without interaction with the coldest zone favored external condensation and achieved the highest bio-oil recovery. The PF configuration exhibited the lowest char yield and the highest unaccounted carbon fraction due to poor vapor condensation at elevated outlet temperatures. These findings demonstrate that the manipulation of vapor–solid interactions serves as a critical parameter for steering pyrolysis pathways toward targeted product enhancement, offering a scalable approach for optimizing biochar, gas, and bio-oil yields through in situ vapor recirculation.

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.

Oxygen-enriched air combustion of pulverized biomass fuel is an effective method to improve char combustion and improve flame stability. Moreover, understanding the impact of O2 addition is an important step toward oxyfuel combustion, one of the most promising technologies for bioenergy with carbon capture and storage (BECCS). Our previous studies focused on flow manipulation methods, e.g., swirling co-flow and acoustic forcing, to enhance particle dispersion during biomass combustion and gasification. This work aims to extend the understanding of the effect of different manipulation methods on oxygen-enriched combustion at different levels in a lab-scale entrained flow reactor. This methodology combines the analysis of visible flame characteristics, CO and NO gas emissions, and coarse particle emissions characterization with thermogravimetric analysis and particle size distribution by dynamic imaging. The results indicated that oxygen-enriched combustion leads to lower liftoff distance and higher flame brightness. Moreover, oxygen-enriched combustion presented coarse particle emissions with finer particle size distribution and lower carbon content. The acoustic forcing further decreased the flame liftoff and decreased CO emissions, increasing combustion efficiency under conditions with similar equivalence ratios and lower momentum flux at the secondary air.

One of the crucial issues in the chemical looping technology lies in its bed material: the oxygen carrier. Particle size analysis of an oxygen carrier is important since in a fluidized bed the material can only work well within a specific size range. While the favorable size ranges for oxygen carrier materials have already been reported, none of the published studies has analyzed the particle size and shape of oxygen carriers in detail. Furthermore, the effect of oxygen carriers' oxidation degree on such properties has not been considered either. This study aimed to report the particle size and shape analysis of five iron-based oxygen carriers, one natural ore, one synthetic material, and three residue products, at different oxidation degrees using dynamic image analysis (DIA). The oxygen carriers were prepared at different mass conversion degrees in a fluidized bed batch reactor. The size distribution, sphericity, and aspect ratio of the oxygen carrier particles were examined experimentally using a Camsizer instrument. Our results show that the DIA method was successfully able to analyze the particle size and shape of our oxygen carriers with satisfying accuracy for comparison. The oxidation state of the investigated materials seems to only affect the particle size and shape of oxygen carriers to a minor extent. However, exposures to redox cycles in a fluidized bed reactor may alter the particle size and shape of most oxygen carriers.

The use of by-products from forestry and agricultural sectors can increase the bioenergy share for heat/power production and industrial processes. Moreover, the integration with carbon capture technologies has a significant potential for CO₂ reduction with BECCS (bioenergy with carbon capture and storage) technologies. Entrained flow reactors (EFRs) are commonly applied in the direct combustion and gasification of pulverized fuels. In both technologies, particle-laden flow characteristics can significantly influence the reactor operation, with an impact on performance and emissions. This thesis investigates a broad range of particle flow parameters in EFRs, with an experimental analysis combining high-speed imaging methods with sampling techniques. A comprehensive analysis was carried out using different biomass feedstocks (sawdust, pine bark, and rice husk), operating conditions (non-reacting, air and oxygen-enriched combustion, and gasification), and flow manipulation techniques (swirling flow and acoustic forcing).

The latter technique, acoustic forcing, resulted in a high potential for soot reduction in previous experiments when applied to biomass injection in small lab-scale reactors under laminar conditions. Soot emissions represent important environmental concerns and a major technical problem due to the required downstream cleaning processes. For this reason, acoustic forcing was further studied in this work using a larger pulverized swirl burner. Post-processed shadowgraph images from cold-flow experiments provided insights into the near-field particle distribution and quantified particle dispersion in a broad range of operating conditions. Particle dispersion increased near-linearly with the pressure amplitude of the acoustic forcing, which presented the strongest effect followed by the swirl intensity of the secondary air. Both techniques applied simultaneously had a synergetic effect, especially for small particle size (e.g. dispersion angle increased from 0.9 to 9.1° for particles in the size range of 63-112 μm).

High particle dispersion significantly reduced the flame liftoff distance (ignition characteristic) during combustion, which was identified by the high-speed imaging technique. The reduction in liftoff distance, caused by the acoustic forcing in combustion conditions, varied from 6 to 28%. Higher reduction was identified for high oxygen level enrichment and small particles. Acoustic forcing applied at conditions with low secondary air momentum flux resulted in lower CO emissions and higher combustion efficiency, with higher NO emissions. Under gasification, the ignition occurred at earlier stages than in combustion as demonstrated by the changes in liftoff distance, which was strongly affected by the producer gas recirculation (containing CO and H2). The acoustic forcing presented a sharper effect on liftoff in such conditions, decreasing by 42% at low equivalence ratios (λ of 0.4). Moreover, acoustic forcing increased cold-gas efficiency by 12%, by increasing the yields of CO and H₂.

Particle emissions were characterized by particulate matter (PM) isokinetic sampling and coarse particle collection with further thermogravimetric, elemental, and particle size distribution analysis. The coarse particles presented a small reduction of carbon content for combustion conditions under acoustic forcing. In gasification conditions, acoustically forced cases presented up to 25% lower PM emissions, while coarse particle emissions increased substantially. Ultimate and thermogravimetric analysis suggests that soot was an important component of the PM emissions. Coarse particles during gasification mainly consisted of fragmented char, which yield increased with acoustic forcing, apparently due to high velocities imposed on the particles around the flow centerline, which gave them a shorter residence time at high temperatures.

Experiments in a larger scale reactor, with 100 kW thermal capacity, were used for studies focused on the particle emissions and deposition from high-temperature oxygenenriched combustion of rice husks. A completely different ash morphology was identified in such experiments, which mainly presented coarse ash fraction deposit build-ups with high Si content and minor ash-forming elements. These characteristics can be beneficial both for bioenergy applications and ash valorization processes. The current work brings new experimental results of EFRs under different particle-laden flow characteristics. The implications in particle dispersion, flame morphology, and emissions could be addressed in further investigations, from fundamental aspects to optimization of burners of EFRs.

Inter-particle distance and particle dispersion during gasification of biomass have been found to significantly affect soot emission. Consequently, enhanced particle dispersion decreases energy losses and the risk for blockages of downstream equipment, increasing the efficiency and reliability of entrained flow reactors (EFRs). In this work, we investigated the interactions between imposed acoustic oscillations and particle dispersion under non-reacting conditions in a co-axial burner for a lab-scale EFR. A flow of air, laden with pulverized stem wood particles (Norwegian Spruce) of three different sizes (63–112 μm, 200–250 μm, and 500–600 μm), was forced axially through the burner center tube at Reynolds numbers ranged from 800 to 1700, and loading ratio of 0.7–4.2. The influences on particle dispersion from variations of the Strouhal number (0.12–0.6), the pressure amplitude at synthetic jet cavity (0.5–4.0 kPa_p-p), the swirl number (0–2.3), and the center jet velocity (1.9–3.9 m s⁻¹) were investigated. Post-processed shadowgraph images revealed the influence of acoustic perturbations, which generate large structures with high particle concentration for both swirling and non-swirling conditions. Time-averaged contour maps showed a significantly higher particle dispersion, quantified as dispersion angle, for higher values of forcing amplitude and swirl numbers, with a stronger influence from the forcing amplitude, especially at lower Stokes number.

Forced intermittent combustion with periodical variations of pressure, velocity, and air-fuel ratios is a promising method to increase efficiency and reduce emissions from combustion and gasification applications. In this work, flame characteristics and emissions from a pulverized biomass burner are investigated under oscillations induced by an acoustically-driven synthetic jet. Instantaneous images of incandescent light emitted from flame were captured using high-speed cameras. The images were analyzed to identify the liftoff distance, flame length, and shape. The flame liftoff distance decreased under excited conditions, notably at high forcing amplitude applied to small particle size distribution (63-112 μm). In such conditions, acoustic forcing increases particle dispersion as presented in the previous work, providing conditions for earlier ignition due to enhanced fuel-air mixing besides reducing CO emissions. Flue gas emissions were influenced mainly by the particle size distribution, from which the 63-112 μm particle size presented the lowest values of CO and highest levels of NO emissions. The results presented stable flame edge positions for the particle size of 63-112 μm, while wide range particle distributions (0–600, 0-400 μm) had strong fluctuations, indicating high flame instability. The experimental work adds new insights regarding acoustic excitation in swirl burners, which could be used to optimize pulverized fuel combustion.