Search Results (135)

Fly ash (FA) from Zhundong coal combustion features high alkali/calcium content and a low Si/Al ratio, limiting its potential for conventional utilization. To enable its high-value application, six size-fractionated samples (FA1–FA6) were characterized via laser particle sizing, SEM-EDS, XRF, XRD, FT-IR, and TGA, to elucidate particle-size-dependent physicochemical and thermal properties. The results show that the size distribution centered at 48–150 μm (~71%). With decreasing size, the morphology shifted from irregular aggregates to smooth vitreous spheres. The chemical composition exhibits significant elemental segregation; the SiO₂ content decreases with decreasing particle size, while active components such as CaO, MgO, and Fe₂O₃ are significantly enriched in fine particles. The thermal conversion behavior is regulated by particle size: The combustion reaction under an air atmosphere conforms to the second-order kinetic model, with the activation energy decreasing from 192.73 kJ·mol⁻¹ for coarse particles (>150 μm) to 63.53 kJ·mol⁻¹ for fine particles (<43 μm); under a nitrogen atmosphere, the weight loss originates from the removal of structural water and the decomposition of carbonates, and fine particles exhibit a higher pyrolysis activation energy (504.15 kJ·mol⁻¹) in the high-temperature stage (850–940 °C) due to being rich in high-crystallinity carbonates. The results of this study elucidate the structure–activity relationship of “particle size-composition-activity” for Zhundong coal fly ash and propose a graded utilization scheme where coarse fractions are suitable for low-grade building fillers, while fine fractions can be used as feedstocks for coal pyrolysis catalysts and functional adsorbents, providing a theoretical basis for its targeted resource utilization based on particle size fractionation. Full article

The progressive phase-out of coal-fired power plants in Portugal has significantly reduced the availability of fly ash (FA) as a supplementary cementitious material (SCM), reinforcing the need for sustainable alternatives. Waste glass powder (WGP), characterized by its high amorphous silica content, has emerged as a promising candidate; however, most studies focus on ultrafine particles or isolated performance indicators, lacking an integrated technical, environmental, and economic assessment. This study evaluates cement pastes incorporating 25% WGP (by volume) with different particle size distributions, including fineness levels comparable to cement and FA. Mechanical performance, grinding energy demand, carbon footprint, and cost were systematically analyzed. The results indicate that WGP is technically viable as an SCM, with a median particle size (D₅₀) of approximately 48 µm providing the most balanced performance. Although finer particles enhance pozzolanic reactivity, the associated increase in grinding energy and economic cost offsets these gains. The findings demonstrate that optimizing particle size, rather than maximizing fineness, enables a technically robust and industrially realistic use of WGP. This approach supports circular economic strategies and contributes to the decarbonization of the construction sector by identifying an efficient replacement pathway for FA under resource-scarcity conditions. Full article

In thermal power plants, fly ash produced from coal combustion is a solid waste that requires large storage areas and poses environmental risks. In addition, coal ash can contain significant amounts of critical elements, including rare earth elements and yttrium (REY). Despite high supply risks, demand for REY is increasing in parallel with technological developments. Therefore, the recovery of REY from coal ash is becoming increasingly important for both solid waste disposal and as a raw material source. This study presents an integrated geochemical assessment of REY in fly ashes from coal-fired thermal power plants in Türkiye, based on systematically compiled and harmonised datasets. The REY concentration of fly ash varies between 134.00 and 429.48, with an average of 230.06 ppm. Light REY are predominant in all samples. The proportion of critical REY averages 34.75, with the highest value calculated at 42% in fly ash from the Yatağan thermal power plant. While most fly ashes show L-type enrichment, there are also samples showing M-type and H-type enrichment. According to initial national-scale estimates, coal fly ashes in Türkiye may contain approximately 3.7–5 kt of rare earth oxides per year. Despite their low REY content, Turkish fly ashes can be considered a potential source for REY recovery when considering the large waste volume, in conjunction with an integrated evaluation strategy. This study establishes a geochemical basis for future process-oriented and recovery-focused investigations. Full article

Corona quenching is a major obstacle to the stable and efficient operation of electrostatic precipitators (ESPs) in coal-fired power plants, particularly under high-ash coal combustion. This study evaluates a novel double-V labyrinth pre-collection device as an active strategy to mitigate corona quenching. Field measurements from a 660 MW ultra-supercritical coal-fired unit, combined with computational fluid dynamics (CFD) simulations, demonstrate that the retrofit significantly improved inlet flow uniformity and reduced fly ash concentration before the ESP. Consequently, corona discharge stability was enhanced, overall collection efficiency increased from 99.42% to 99.92%, and outlet fly ash concentration decreased from 81 mg/m³ to 20.5 mg/m³. Although the pressure drop rose modestly (128 Pa to 187.5 Pa), the overall ESP energy demand was reduced due to more stable operation at lower voltages. These results confirm the technical feasibility and engineering applicability of pre-collection technology, providing a cost-effective solution to overcome corona quenching and ensure ultra-low emission compliance in large coal-fired units. Full article

Growing renewable penetration increases deep peak-shaving demands, making stable wide-load operation of coal-fired boilers essential. A full-process CFD model of a 660 MW ultra-supercritical boiler was established, covering the furnace, heat-transfer surfaces, rear-pass duct, and selective catalytic reduction (SCR) system. Simulations at 25–100% boiler maximum continuous rating (BMCR) quantified load effects on combustion and emissions. Predicted furnace outlet temperature and major flue-gas species matched field data with deviations within ±6%. Lowering the load from 100% to 25% BMCR contracted the high-temperature core in the furnace and reduced mean temperature and mixing. Furnace nitrogen oxides (NO_x) formation decreased as the load decreased. However, NO_x at 25% BMCR increased because separated over-fire air (SOFA) was not applied. Reduced combustion intensity increased the level of unburned carbon in fly ash, which rose by approximately 3.5% at 25% BMCR, relative to the rated condition. Pronounced flow maldistribution also appeared at 25% BMCR. The SCR-inlet flow analysis indicated that the original guide vane design was not suitable for wide-load operation and that inlet-velocity uniformity deteriorated, especially at low loads. An optimized guide vane scheme is proposed, improving SCR-inlet uniformity over the full load range while mitigating ash deposition and erosion risks. Full article

The coal-fired Plomin Thermal Power Plant (Plomin TPP) in Croatia is located in the center of the east coast of the Istrian peninsula (northern Adriatic) and is considered the main source of historical air pollution in the region. Between 1970 and 2000, sulfur-rich coal from the local Raša coal mine was primarily used. In this study, a screening of content and fate of TPP-derived sulfur in soil around the power plant was made two decades after the S-rich coal was banned from use. Soil samples were collected at varying distances from the TPP in the prevailing wind direction (NE), along with a control sample taken more than 10 km away. The samples were analyzed for total sulfur, sulfate, organic sulfur (humic and fulvic), and the stable isotope composition of total sulfur (δ³⁴S). Additionally, coal and coal ash were analyzed for total sulfur, sulfate and δ³⁴S. Soil sampling along the prevailing wind direction from the Plomin TPP revealed markedly elevated sulfur content, with levels at 100 m downwind reaching up to 4 wt.%, which is over 100 times higher than the 0.04 wt.% measured at the control site located upwind. Sulfur content decreases sharply with increasing distance from the TPP, reflecting the deposition gradient along the prevailing wind path. Speciation analysis showed that over 95% of the sulfur in the soil is now present in organic form, mainly bound to humic acids. The δ³⁴S_VCDT values of the bulk coal used in the TPP ranged from −10.0 to −5.0‰. In most soil samples, the bulk δ³⁴S values were positive (+7.0 to +20.0‰). The values of sulfate in soil range from +1.0 to +5.5‰, while those in organic sulfur range from −3.5 to +6.0‰. This indicates that atmospheric deposition of ³⁴S-depleted fly ash and sulfate from coal are the most important sulfur sources, while some of the sulfur in the soil is also of marine origin. Finally, we showed that natural attenuation was a significant and efficient process within the sustainable management of the site historically contaminated by anthropogenic atmospheric sulfur deposition. Full article

As the energy structure evolves, low-load operation of coal-fired boilers is becoming common, posing challenges to combustion stability. This study explored the co-combustion of brown gas (HHO) with bituminous coal and anthracite in a one-dimensional furnace. Results indicate that introducing HHO significantly elevated combustion temperatures, with maximum increases of 158 °C and 207 °C, respectively. In the premixed mode, the flame front shifted upstream, indicating advanced ignition timing. Moreover, HHO co-combustion notably enhanced the combustion stability of anthracite, as reflected in stabilized furnace temperatures. With increasing HHO flow rate, CO concentrations from both bituminous coal and anthracite were reduced by over 80%. The combustion efficiency of bituminous coal reached 98%, while the combustion efficiency of anthracite increased by 19% (premixed) and 13% (staged), confirming the premixed mode’s superiority in promoting complete combustion. HHO co-combustion increased SO₂ emissions but had a complex effect on NO_X emissions due to the competition between NO_X reduction caused by HHO and NO_X formation caused by the increased combustion temperature. HHO co-combustion changed the melting point of fly ash, increased the content of Al₂O₃, and reduced the content of Na₂O, K₂O, and MgO, influencing the slagging behavior of the boiler and the subsequent management of fly ash. Full article

Growing efforts to reduce air pollution have accelerated the development of advanced flue gas treatment technologies for coal-fired power plants. This study presents the design, development, and industrial-scale implementation of a microwave-assisted non-thermal plasma reactor, powered by a 75 kW, 915 MHz magnetron, for simultaneous sulfur dioxide (SO₂) removal and fly ash agglomeration. The reactor was installed on the outlet line of the selective catalytic reduction (SCR) system of a 22 MW_e pulverized-coal-fired boiler and evaluated under real flue gas conditions. The flue gas stream, extracted by an induced-draft fan, was supplied to the reactor through two configurations—radial and axial injection—to investigate the influence of gas flow rate and microwave power on SO₂ abatement performance. Under radial injection, the system achieved a maximum SO₂ removal efficiency of 99.0% at 5194 Nm³/h and 75 kW, corresponding to a specific energy consumption of 14.4 Wh/Nm³. Axial injection resulted in a removal efficiency of 97.5% at 4100 Nm³/h. Beyond SO₂ mitigation, exposure of flue gas to the microwave-assisted plasma environment significantly enhanced particle agglomeration, as confirmed by means of SEM imaging and particle size distribution analyses. Notably, the proportion of fine particles smaller than 2.5 µm (PM_2.5) decreased from 70.25% to 18.63% after plasma treatment, indicating improved capture potential in the downstream electrostatic precipitator (ESP). Overall, microwave-assisted plasma provides efficient SO₂ removal and enhanced particulate capture, offering a compact and potentially waste-free alternative to conventional systems. Full article

Mercury adsorption/oxidation plays a crucial role in mercury transformation during coal combustion. To gain an intuitive understanding of the adsorption/oxidation mechanisms between mercury and fly ash, changes in mercury speciation of fly ash before and after Hg adsorption were investigated using temperature-programmed decomposition–atomic fluorescence spectroscopy (TPD-AFS). The results directly reveal that the primary adsorption/oxidation mechanism between mercury and fly ash is the heterogeneous oxidation reaction of Hg⁰ to HgCl₂. The mercury adsorption capacity exhibits a strong positive correlation with both the unburned carbon (UBC) content and the specific surface area (SSA) of the fly ash, whereas the presence of metal oxides has a negligible effect on mercury adsorption. Higher inlet concentrations of Hg⁰ enhance mercury adsorption, while flue gas components such as N₂, O₂, and CO₂ have minimal influence on mercury adsorption. Full article

The ordinary Portland cement (OPC) manufacturing process is highly resource-intensive and contributes to over 5% of global CO₂ emissions, thereby contributing to global warming. In this context, researchers are increasingly adopting geopolymers concrete due to their environmentally friendly production process. For decades, industrial byproducts such as fly ash, ground-granulated blast-furnace slag, and silica fume have been used as the primary binders for geopolymer concrete (GPC). However, due to uneven distribution and the decline of coal-fired power stations to meet carbon-neutrality targets, these binders may not be able to meet future demand. The UK intends to shut down coal power stations by 2025, while the EU projects an 83% drop in coal-generated electricity by 2030, resulting in a significant decrease in fly ash supply. Like fly ash, slag, and silica fume, natural clays are also abundant sources of silica, alumina, and other essential chemicals for geopolymer binders. Hence, natural clays possess good potential to replace these industrial byproducts. Recent research indicates that locally available clay has strong potential as a pozzolanic material when treated appropriately. This review article represents a comprehensive overview of the various treatment methods for different types of clays, their impacts on the fresh and hardened properties of geopolymer concrete by analysing the experimental datasets, including 1:1 clays, such as Kaolin and Halloysite, and 2:1 clays, such as Illite, Bentonite, Palygorskite, and Sepiolite. Furthermore, this review article summarises the most recent geopolymer-based prediction models for strength properties and their accuracy in overcoming the expense and time required for laboratory-based tests. This review article shows that the inclusion of clay reduces concrete workability because it increases water demand. However, workability can be maintained by incorporating a superplasticiser. Calcination and mechanical grinding of clay significantly enhance its pozzolanic reactivity, thereby improving its mechanical performance. Current research indicates that replacing 20% of calcined Kaolin with fly ash increases compressive strength by up to 18%. Additionally, up to 20% replacement of calcined or mechanically activated clay improved the durability and microstructural performance. The prediction-based models, such as Artificial Neural Network (ANN), Multi Expression Programming (MEP), Extreme Gradient Boosting (XGB), and Bagging Regressor (BR), showed good accuracy in predicting the compressive strength, tensile strength and elastic modulus. The incorporation of clay in geopolymer concrete reduces reliance on industrial byproducts and fosters more sustainable production practices, thereby contributing to the development of a more sustainable built environment. Full article

The global reliance on coal-fired power generation continues to produce vast quantities of fly ash, exceeding 500 million tons annually, with limited recycling rates. Given its high silica (SiO₂) and alumina (Al₂O₃) contents, fly ash represents a promising alternative raw material for sustainable refractory production. In this study, four aluminosilicate refractory castables were formulated using bauxite, calcined flint clay, kyanite, calcium aluminate cement, and microsilica, in which the fine fraction of flint clay was partially replaced by 0, 5, 10, and 15 wt.% fly ash. The specimens were dried at 120 °C and sintered at 850, 1050, and 1400 °C for 4 h. Their physical and mechanical properties were systematically evaluated, while phase evolution and microstructural development were analyzed through X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results revealed that the incorporation of 10 wt.% fly ash (10FAC) provided the optimal balance between densification and strength, achieving compressive strengths of 45.0 MPa and 65.3 MPa after sintering at 1050 °C and 1400 °C, respectively. This improvement is attributed to the formation of a SiO₂-rich liquid phase derived from fly ash impurities, which promoted the in-situ crystallization of acicular secondary mullite and enhanced interparticle bonding among corundum grains. The 10FAC castable also exhibited only a slight increase in apparent porosity (26.39%) compared with the reference (25.74%), indicating effective sintering without excessive vitrification. Overall, the study demonstrates the technical viability of using fly ash as a sustainable substitute for flint clay in refractory castables. The findings contribute to advancing circular economy principles by promoting industrial waste valorization and resource conservation, offering a low-carbon pathway for the development of high-performance refractory materials for structural and thermal applications in energy-intensive industries. Full article

Fly ash from coal-fired power plants is a promising precursor for zeolite synthesis due to its aluminosilicate-rich composition. However, its direct utilization is often limited by impurities and a low silicon-to-aluminum ratio (SAR). This study demonstrates the conversion of Class C fly ash from the Soma thermal power plant (Turkey) into FAU- and CHA-type zeolites through optimized acid leaching and hydrothermal synthesis. Acid treatment increased the SAR from 1.33 to 2.85 and effectively reduced calcium-, sulfur-, and iron-bearing impurities. The SAR enhancement by acid leaching was found to be reproducible among Class C fly ashes, whereas Class F materials exhibited a limited response due to their acid-resistant framework. Subsequent optimization of alkaline fusion-assisted synthesis enabled selective crystallization of FAU and CHA, while GIS and MER appeared under prolonged crystallization or higher alkalinity. SEM revealed distinct morphologies, with MER forming rod-shaped clusters, and CHA exhibiting disc-like aggregates. Water sorption analysis showed superior uptake for metastable FAU (~23 wt%) and CHA (~18 wt%) compared to stable GIS and MER (~12–13 wt%). Overall, this study establishes a scalable and sustainable route for producing high-performance zeolites from industrial fly ash waste, offering significant potential for adsorption-based applications in dehumidification, heat pumps, and gas separation. Full article

The release of heavy metals during coal combustion may pose potential hazards to the surrounding environment and human health. In this study, we investigated the migration characteristics and ecological risks of heavy metals during the combustion of two distinct raw coal samples (C1 and C2) sourced from Shanxi Province. The analytical results demonstrate significant differences in volatilization behavior between the samples, with total heavy metal release rate ranging from 30.25% to 98.92% for C1 and from 17.77 to 98.16% for C2. Four elements—Cd, As, Pb, and Hg—exhibited preferential migration to fly ash fractions A1 and A2, displaying higher transfer coefficients compared to other monitored heavy metals. Chemical speciation analysis revealed that elemental release behavior was predominantly governed by residual phases (2.2–81.4%), Fe-Mn oxide-bound forms (3.7–45.6%), and sulfate-associated fractions (1.3–56.8%). Combustion temperature showed nonlinear positive correlations with the volatilization rates of Cd, As, Pb, and Hg. Hg volatilization decreases at a combustion temperature below 600 °C, whereas for Cd, As, and Pb, this temperature is below 800 °C. Ecological risk indices (RI) indicate substantial contamination potential in fly ash matrices: A1 (RI = 285.32) is dominated by Hg (I_geo = 1.9, $E_r^i$ = 224) with a notable contribution from Cd ($E_r^i$ = 51), whereas A2 (RI = 246.67) showed a predominance of Cd (I_geo = 1.6, $E_r^i$ = 138) over Hg ($E_r^i$ = 94.4). These findings underscore the need for optimized combustion parameters and enhanced particulate filtration systems to mitigate environmental impacts associated with coal-fired power generation. Full article

In response to escalating environmental concerns, the construction industry is under growing pressure to adopt sustainable practices. As a major consumer of natural resources and a significant emitter of greenhouse gases, it paradoxically holds the potential to become a leader in green transformation. This study investigates the development of innovative, fire-resistant, and alkali-activated hybrid binder foams incorporating recycled materials: fly ash, coal slag, and ground brick waste, as sustainable alternatives to traditional building materials. The fire resistance performance at a technical scale and the thermal behavior of fiber-reinforced, alkali-activated hybrid binder foams synthesized from recycled aluminosilicate precursors were determined. The properties of unreinforced composite were compared with the composites reinforced with merino wool, basalt fibers, polypropylene fibers, and coconut fiber. Small-scale fire-resistance tests revealed that merino wool-reinforced composites exhibited the best thermal insulation performance, maintaining structural integrity, that is, retaining shape and continuity without delamination or collapse for 83 min under fire exposure. Analyses combining chemical characterization (X-ray fluorescence) with microstructural methods (computed tomography and colorimetry) confirmed that fire performance is strongly influenced not only by fiber type but also by pore distribution, phase composition, and oxide migration under thermal loading. These findings demonstrate the potential of fiber-reinforced foamed, alkali-activated hybrid binder as eco-efficient, printable materials for fire-safe and thermally demanding construction applications. Full article

Supplementary cementitious materials (SCMs) provide a practical solution for reducing greenhouse gas emissions associated with Portland cement production while enhancing the economy, performance, and service life of concrete and mortar. Currently, there is a significant disparity in the availability, supply, and utilisation levels of SCMs worldwide, particularly in South Africa. This paper presents an in-depth analysis of the characteristics and performance of various SCMs, including local availability, factors driving demand, production, and utilisation. The findings indicate that fly ash and limestone calcined clay are the most widely available SCM resources in South Africa, with deposits exceeding 1 billion tonnes each. Fly ash stockpiles continuously increase due to the reliance on coal-fired power plants for 85% of generated electricity and a low fly ash utilisation rate of 7%, significantly below international utilisation levels of 10–98%. Conversely, slag resources are depleting due to the steady decline of local steel production caused by energy and input costs, alongside the growing importation of steel products. Combined, the estimated production of slag and silica fume is about 1.4 million tonnes per annum, leading to their limited availability and utilisation in niche applications such as high-performance concrete and marine environments. Furthermore, 216,450 tonnes of SCM could potentially be processed annually from agricultural waste. In addition to quality, logistics, costs, and other challenges, this quantity can only replace 1.5% of clinker in South Africa, raising concerns about the viability of SCMs from agricultural waste. Based on its findings, this study recommends future research areas to enhance the performance, future availability, and sustainability of SCMs. Full article