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Solid biomass oxy-fuel combustion with external recirculation was demonstrated in a 100-kW atmospheric, down-fired combustor operated close to stoichiometry. The external recirculation setup comprised a particulate filter, a condenser, a fan and O₂ addition before the burner inlet. A theoretical description of the recirculation process is presented and validated. Two fuels, softwood (SW) and forest residues (FR), with similar residence times, were compared. Gaseous species, including potassium (K) compounds (atomic K, KOH, and KCl), and gas temperature were quantified in real-time by tunable diode laser absorption spectroscopy (TDLAS) and photofragmentation TDLAS at two locations in the reactor core. Major species (CO₂, H₂O, O₂, and N₂) were also measured at the exhaust. Flue gas particles collected with a low-pressure impactor at the exhaust were analyzed by X-ray powder diffraction and scanning electron microscopy. The average CO₂ purity (dry) was 90 % for SW and 86 % for FR. The NO concentration was higher for FR due to the larger nitrogen content in the fuel. The gaseous K species concentrations were higher for FR than for SW (factor 2–3), but not as high as expected from the difference in fuel K content (factor 7), likely due to the high content of Si and Al in FR. Gas-phase K was significantly lower than predicted by thermodynamic equilibrium calculations (TEC), probably due to K adsorption by soot particles. The fine and coarse particle concentrations were significantly higher for FR than for SW due to the higher ash content of FR. The FR fine mode particles consisted mainly of K₂SO₄ and KCl, in good quantitative agreement with TEC of gas phase condensation. Apatite, Ca₅(PO₄)₃OH, likely formed from vaporized Ca and P, was found in the fine mode in all recirculation cases.

Biochar performance in applications like soil amendment depends on properties including porosity, surface area, and pore structure, which are influenced by both process conditions and feedstock characteristics such as wood anatomy. However, their effects across micro- and nanometre length scales remain poorly understood. This study therefore investigates how feedstock and process conditions affect properties of pine wood biochar produced in industrially relevant pilot-scale continuous reactor for torrefaction (291–315°C, 6–12 min) and pyrolysis (350–400°C, 25 min). High-resolution X-ray microtomography combined with AI-assisted image analysis was used to quantify earlywood–latewood distribution, porosity, pore size, cell wall thickness, and micrometre-scale surface area, while N₂ and CO₂ sorption probed nanometre-scale structure. The wood microstructure was largely preserved across treatments, resulting in highly anisotropic pore networks. Porosity remained high (33–77%), primarily influenced by earlywood–latewood variability rather than temperature. From torrefaction to pyrolysis, cell wall thickness and pore diameter decreased, while microporosity (<∼0.7 nm) and accessible surface area increased significantly above 350°C. Discrepancies between N₂ and CO₂ measurements indicate differences in pore accessibility, likely associated with ultramicroporosity, pore constrictions and/or partially inaccessible pores. Water holding capacity ranged from 1.7 to 5.3 times dry weight, with highest average at 350°C, although differences were not significant. Combined use of microtomography and gas sorption provides a multiscale framework for linking biochar structure across micro- and nanometre scales to performance-relevant properties. These findings demonstrate that wood anatomy dominates over process conditions under mild thermal treatment, highlighting new possibilities for tailoring process design through consideration of wood anatomy.

Rotary kilns are widely used for various high temperature industrial applications, e.g., quicklime and cement production. Most of these operations rely on fossil fuels like coal; however, there is growing interest in increasing the share of biomass fuels. During the high-temperature combustion process, ash transport onto, and reactions with, refractory bricks can significantly influence refractory wear. Ash infiltration may impair refractory performance through chemical reactions and exacerbated cracking caused by structural changes in combination with thermal shock. Analysis of spent refractory materials provides valuable insights for understanding and predicting corrosion mechanisms. In the present study, three MgO-based spent refractory bricks were collected from different locations within the burn zone of a rotary lime kiln that was cofired with a mixture of coal, olive pomace (a potassium-rich biomass), and oil. Infiltrated ash and reaction products within the refractory bricks were sampled and characterized using SEM-EDX for elemental mapping and morphology analysis. EBSD analysis was employed to measure the grain size distributions. Micrographic images revealed that all three spent refractory bricks were more sintered and cracked on their hot sides compared to their middle sections and cold sides. Si-rich and K-rich ashes from the coal and biomass fuels, respectively, all infiltrated into the refractories as well as Ca-rich constituents from the limestone/quicklime. XRD analyses revealed the formation of phases such as Mg2SiO4, Ca12Al14O33, and Ca2SiO4. No potassium from the fuel ash was found on the hot side of the refractory bricks, but some were detected deeper within the middle section and cold side of the bricks. The combined use of analytical techniques enabled detailed mapping of ash-forming elements and identification of newly formed phases from the reactions between refractory bricks, ash, and quicklime. These findings provide critical insights into fuel-specific interactions and highlight potential risks for refractory degradation.

Refractory liner bricks in the hot zone of rotary lime kilns can sustain wear and corrosion during contact with fuel ashes and quicklime (QL), a product composed mainly of CaO. The effects on a MgO-based refractory after exposure at 1400 °C for 96 h to olive pomace ash (OPA) and coal ash (CA), with and without QL, were investigated. Exposure of the refractory to only OPA caused slag intrusion with no ash deposits remaining on top, while CaMgSiO4 (monticellite) was also identified as a new phase. When exposed to only CA, the refractory exhibited dissolution into the molten slag and 0.5–2 mm cracks were found on the surface interfacing the ash. Mg2SiO4 (forsterite) and CaMgSiO4 were identified as new formed phases. Exposure of the refractory to OPA + QL and CA + QL caused less slag intrusion and substantial amounts of ash/QL deposit remained afterwards. No new phases were identified. The differences in interactions between the exposure materials and refractory were supported by thermochemical equilibrium analysis. Apparent Ca-Si–rich or Ca-rich melts were found in all the exposed samples, but potassium (K) was found to be depleted in all samples, including those involving OPA, which was rich in K. Furthermore, with the exception of exposure to only CA, the other exposures caused the cold crushing strength (CCS) of the refractory to increase compared to its original value. This was attributed to the sintering of the refractory microstructure. The CCS of the refractory decreased after exposure to only CA. The findings of this study enhance understanding of how CA and OPA impact MgO refractories in lime kilns, supporting initiatives aiming at reducing fossil fuel use. The results are encouraging and motivate further investigation.

Oxy-fuel biomass combustion can facilitate carbon capture in heat and power plants and enable negative carbon dioxide (CO₂) emissions. We demonstrate oxy-fuel combustion (OFC) of softwood powder in a 100-kW atmospheric down-fired pulverized combustor run at a global oxidizer-fuel equivalence ratio of around 1.25. The simulated oxidizer was varied between oxygen (O₂)/CO₂ mixtures of 23/77, 30/70, 40/60 and 54/46, and artificial air. The concentrations of the main gaseous potassium (K) species: atomic K, potassium hydroxide (KOH) and potassium chloride (KCl), were measured at two positions in the reactor core using photofragmentation tunable diode laser absorption spectroscopy (PF-TDLAS). Major species were quantified by TDLAS in the reactor core and with Fourier transform infrared spectroscopy and mass spectrometry at the exhaust. Flue gas particles were collected at the exhaust employing a low-pressure impactor and analyzed by X-ray powder diffraction and scanning electron microscopy. The measured individual K species concentrations in the reactor core agreed with predictions by thermodynamic equilibrium calculations (TEC) within one order of magnitude and the sum of K in the gas phase agreed within a factor of three for all cases. Atomic K was underpredicted, while the dominating KOH and KCl were slightly overpredicted. The ratios of measured to predicted total K were similar in artificial air and OFC, but the distributions of the individual species differed at the upper reactor position. The gaseous K species and fine particle concentrations in the flue gas were directly proportional to the O₂ content in the oxidizer. The crystalline phase compositions of the coarse mode particles were rich in K- and calcium-containing species. The fine mode particles, which contained most of the K, consisted mainly of K₂SO₄ (94%) and K₃Na(SO₄)₂, which is in excellent agreement with TECs of gas phase condensation. As supported by the solid phase analysis, complete sulfation of K species was achieved for all studied cases. A CO₂ purity (dry) of up to 94% was achieved for OFC.

In the future, cement clinker formation is likely to take place in high temperatures and high carbon dioxide atmospheres in carbon-neutral production processes as part of, for example, electrified processes. The aim of this study was thus to compare the volatilisation of minor and trace elements during cement clinker formation in a high carbon dioxide atmosphere and a conventional combustion atmosphere. Raw meal samples were exposed, at high temperature, to the two different atmospheres, with elemental analysis performed before and after. For both atmospheres, the minor elements potassium and sulfur, and the trace elements rubidium, lead, thallium, caesium, cadmium and mercury were highly volatile. For most of the analysed elements, no difference was observed between the two atmospheres. However, volatilisation of potassium, sodium and sulfur was lower in the high carbon dioxide atmosphere. It is suggested that this should be further studied in relation to the molar ratio of sulfur to alkalis in the clinker and the effect on clinker quality.

Quicklime, rich in CaO(s), is generated by calcining limestone at high temperatures. Parallel-flow regenerative lime kilns are the most energy-effective industrial method available today. To prevent major disruptions in such kilns, a high raw material quality is necessary. Under some conditions, impurity-enriched material may adhere to limestone pebbles and enter the kiln. In this study, limestone and corresponding quicklime were analyzed to evaluate the extent and composition of surface impurities and assess the effect on quicklime product quality, here defined as free CaO. This was performed by sampling and analyzing limestone, quarry clay, laboratory-produced quicklime, and industrially produced quicklime with XRF, SEM/EDX, and XRD; interpretations were supported by thermodynamic equilibrium calculations. In the laboratory-produced quicklime, the surface impurities reacted with calcium forming Larnite, Gehlenite, Åkermanite and Merwinite, reducing the quicklime quality. The results showed that the limestone surface layer comprised 1.2 wt.-% of the total mass but possessed 4 wt.-% of the total impurities. The effect on industrially produced quicklime quality was lower; this indicated that the limestone surface impurities were removed while the material moved through the kiln. Multicomponent chemical equilibrium calculations showed that the quarry clay was expected to be fully melted at 1170 °C, possibly leading to operational problems.

Quicklime is produced through the thermal processing of limestone in industrial kilns. During quarry operations, fine particulate quarry dust adheres to limestone lump surfaces, increasing the bulk concentration of impurities in limestone products. During thermal processing in a kiln, impurities such as Si, Mg, Al, Fe, and Mn react with Ca, reducing quicklime product quality. Which reactant phases are formed, and the extent to which these result in a reduction in quality, has not been extensively investigated. The present study investigated as-received and manually washed limestone product samples from two operational quarries using elemental compositions and a developed predictive multi-component chemical equilibrium model to obtain global phase diagrams for 1000–1500 °C, corresponding to the high-temperature zone of a lime kiln, identifying phases expected to be formed in quicklime during thermal processing. The results suggest that impurities found on the surface of the lime kiln limestone feed reduce the main quality parameter of the quicklime products, i.e., calcium oxide, CaO (s), content by 0.8–1.5 wt.% for the investigated materials. The results also show that, in addition to the effect of impurities, the quantity of CaO (s) varies greatly with temperature. More impurities result in more variation and a greater need for accurate temperature control of the kiln, where keeping the temperature below approximately 1300 °C, that of Hatrurite formation, is necessary for a product with higher CaO (s).