Brown Coal Fly Ash Research Articles

Thermal Studies of Fractionated Lignite and Brown Coal Fly Ashes.

Coal fly ash (CFA), a by-product of coal combustion, is a valuable raw material for various applications. However, the heterogeneous nature of the composition and properties of CFA provides challenges to its effective usage and utilisation. This study investigates the thermal behaviour of the fly ashes of lignite (FA1) and brown coal (FA2) and their fractions obtained by dry aerodynamic separation. Thermal analysis techniques, including thermogravimetry (TG), differential scanning calorimetry (DSC), and evolved gas analysis (EGA), were used to characterise the behaviour of the fly ash fractions while heating up to 1250 °C. The results reveal distinct differences in the thermal behaviour between ash types and among their different size fractions. For the FA1 ashes, the concentration of calcium-rich compounds and the level of recrystallisation at 950 °C increased with the decrease in particle size. The most abundant detected newly formed minerals were anhydrite, gehlenite, and anorthite, while coarser fractions were rich in quartz and mullite. For the FA2 ashes, the temperature of the onset of melting and agglomeration decreased with decreasing particle size and was already observed at 995 °C. Coarser fractions mostly remain unchanged, with a slight increase in quartz, mullite, and hematite content. Recrystallisation takes place in less extension compared to the FA1 ashes. The findings demonstrate that the aerodynamic separation of fly ashes into different size fractions can produce materials with varied thermal properties and reactivity, which can be used for specific applications. This study highlights the importance of thermal analysis in characterising fly ash properties and understanding their potential for utilisation in various applications involving thermal treatment or exposure to high-temperature conditions. Further research on advanced separation techniques and the in-depth characterisation of fly ash fractions is necessary to obtain materials with desired thermal properties and identify their most beneficial applications.

Evaluation of brown coal fly ash for stabilising expansive clay subgrade: A sustainable solution for pavement construction

Brown coal fly ash (BCFA) is a lower-quality type of fly ash produced through brown coal combustion, and its disposal in landfills raises environmental concerns. This study, for the first time, evaluates the feasibility of using BCFA for stabilising expansive clay subgrade in pavement construction. Various engineering tests were conducted, including unconfined compressive strength (UCS), repeated load triaxial (RLT), soaked California bearing ratio (CBR), direct shear, and shrink-swell tests. The results indicated that adding BCFA increased UCS, resilient modulus, soaked CBR, and cohesion while reducing shrinkage-swell index and shrinkage/swell strain. The highest resilient modulus (82.52 MPa) and soaked CBR (3.41 %) were achieved with 3 % BCFA content. These findings recommend using 3 % BCFA as the optimum content for stabilising expansive clay subgrade. This research fills a gap by assessing the application of BCFA for stabilising expansive clay soil, providing insights for sustainable utilisation of this abundant waste material in construction.

Life Cycle Assessment for Geopolymer Concrete Bricks Using Brown Coal Fly Ash

Traditionally, the construction industry has predominantly used Portland cement (PC) to manufacture bricks, as it is one of the most-commonly available building materials. However, the employment of waste industrial material for brick production can lead to a significant improvement in terms of sustainability within the construction sector. Geopolymer bricks made from brown coal fly ash, a promising industrial waste by-product, serve as a potential alternative. Conducting a life cycle assessment (LCA), this study thoroughly evaluated the entire manufacturing process’s environmental impact, from source material acquisition and transportation to brick manufacturing, distribution, usage, and end-of-life, for brown coal bricks as compared to PC bricks. The LCA of the brown coal bricks revealed that their primary environmental impacts stemmed from the raw material manufacturing and usage, while exhibiting substantial reductions in ozone depletion, water depletion, and metal depletion. These findings highlighted the environmental advantages of the brown coal bricks and their potential to revolutionize sustainable construction practices.

Synthesis of (111) facet-engineered MgO nanosheet from coal fly ash and its superior catalytic performance for high-temperature water gas shift reaction

Synthesis of high-value products from coal ash, an otherwise valueless waste is crucial to both sustainable waste treatment and low-cost material fabrication. However, research in this area is still sparse, with majority of the researches being conducted on the synthesis of alumina/silica-based nanomaterials such as zeolite from fly ash. Herein, we report a facile synthesis of high-purity, magnesium hydroxide (Mg(OH)2) nanosheet from a brown coal fly ash (BCFA) waste via leaching, precipitation and ethanol-mediated hydrothermal processing in sequence. The resultant nanosheet from fly ash bears a specific surface area of around 100 m2/g, and an averaged aspect ratio of 11.8, as opposed to only 1.5 for the commercial Mg(OH)2 and the one synthesed without the mediation of ethanol during hydrothermal processing. The fly ash derived Mg(OH)2 nanosheet was subsequently loaded with 30 wt% Fe3+ (as Fe2O3) via incipient wetness impregnation and calcined into a (111) facet-rich MgO, which was proved for a superior catalysis performance for high-temperature water gas shift reaction (HT-WGSR). At the reaction temperatures of 400−450 °C, the CO conversion was increased by a factor of two compared to a reference catalyst consisting of identical Fe loading on the Mg(OH)2 support synthesized without ethanol, and by a factor of seven compared to a nano-sized hematite catalyst synthesized from same hydrothermal process without ethanol addition. This is mainly due to the abundant oxygen vacancy on the (111) facet of the MgO nanosheet support, which improved the reducibility of Fe3+ as well as facilitated the formation and decomposition of intermediate formates that in turn promoted the forward reaction. This study is expected to open a new direction for the valorisation of fly ash and any Mg-bearing solid wastes into high-value materials such as catalyst or catalyst support for a broad range of catalytic applications.

Synthesis of in-situ Al3+-defected iron oxide nanoflakes from coal ash: A detailed study on the structure, evolution mechanism and application to water remediation

The recovery of value-added materials from coal ash waste is of highly economic value and sustainable significance. However, researches on the synthesis of defect-engineering nanomaterials from coal ash are still blank. Herein, iron oxide (Fe1.72Al0.28O3, simplified as FAO) nanoflakes were successfully synthesized from a brown coal fly ash (BCFA) waste. The obtained FAO nanoflakes possess a round-shape morphology with a diameter of around 300 nm and 50 nm in thickness. With the progress of hydrothermal treatment, the impure Al3+ gradually replaced part of the Fe3+ in the α-Fe2O3 crystal. Specifically, Al3+ was preferentially adsorbed on the (001) facet, hindering the growth of Fe3+ on the [001] direction and thus causing the flattening of the resultant FAO. The introduced Al3+ also serves as the disordered defects on the hematite surface, leading to decreased crystal parameters for hematite, the formation of a compact first shell and a reduced periodical symmetry for the central cation Fe3+. The defects were also found to significantly improve the adsorption capacity of the resultant FAO for Cr(VI), As(V), As(III) and Congo red in waste water, with the maximum adsorption capacity of 68.3, 80.6, 61.1 and 213.8 mg g−1, respectively. Cyclic tests also confirmed a relatively strong stability for the as-synthesised adsorbents.

Compressive strength and microstructure evolution of low calcium brown coal fly ash-based geopolymer

A comprehensive experimental study has been conducted to investigate the geopolymerisation and compressive strength development of mortar made from brown coal fly ash from two separate locations in the storage ponds of an Australian power plant. The specimens gave similar compressive strengths but had significantly different material and performance characteristics despite being from the same storage location. The Loy Yang‒A (LYA) geopolymer mortar demonstrated an approx. 30% strength increase while Loy Yang‒B (LYB) gave an approx. 18% strength drop over the period from 7 and 90 d, though both geopolymer mortars initially achieved a similar 28-d strength of approx. 23 MPa. The LYA ash had almost double the alumina content compared to LYB and a higher proportion of AlVI compared to the LYB. The lower alumina content coupled with the low quantity of AlVI in the ash and its lower conversion to AlVI during geopolymerisation is identified as the primary reason for the reduction in strength observed in the LYB geopolymer. The increase of Q4(3Al) during geopolymerisation and some conversion to Q4(4Al) coordination over time resulted in the increase in the compressive strength observed in the LYA mortar. This strength increase of LYA mortar is further correlated with an increase in Quartz phases coupled with a reduction in the Moganite phase. Formation of sodium carbonate due to atmospheric carbonation of unreacted sodium hydroxide in Loy Yang geopolymer additionally contributed to the strength development of LYA geopolymer.

Effect of Curing Conditions on Microstructure and Pore-Structure of Brown Coal Fly Ash Geopolymers

This study reports the effect of heat curing at 120 °C on the geopolymeric reaction and strength evolution in brown coal fly ash based geopolymer mortar and concrete. Moreover, an examination of this temperature profile of large size geopolymer concrete specimens is also reported. The specimen temperature and size were observed to influence the conversion from the glassy (amorphous) phases to the crystalline phases and the microstructure development of the geopolymer. The temperature profile could be divided into three principal stages which correlated well with the proposed reaction mechanism for class F fly ash geopolymers. The geopolymerisation progressed more rapidly for the mortar specimens than the concrete specimens with 12 to 14 h providing an optimum curing time for the 50 mm mortar cubes and 24 h being the optimum time for the 100 mm concrete cubes. The 50 mm and 100 mm concrete specimens’ compressive strengths in excess of 30 MPa could be obtained at 7 days. The structural integrity was not achieved at the center of 200 mm and 300 mm concrete specimens following 24 h curing at 120 °C. Hence, the optimal curing time required to achieve the best compressive strength for brown coal geopolymer was identified as being dependent on the specimen size.

Complex utilisation of ekibastuz brown coal fly ash: Iron & carbon separation and aluminum extraction

Fly ash landfills that accumulate a by-product of coal combustion and gasification represent a permanent threat to the surrounding environment due to many factors (air and water pollution, soil contamination, wildlife poisoning, etc). Moreover, disposed coal fly ash may contain significant amounts of valuable elements that are not extracted and potentially wasted. To improve the above situation, a combined ash treatment process was developed for utilisation of the coal fly ash waste from coal-fired power stations. The ash treatment includes three stages: 1) magnetic separation of an iron-containing fraction, 2) carbon separation by floatation, and 3) extraction of aluminum by the autoclave hydrochloric acid leaching. The lab-scale results of the ash treatment applied to the Ekibastuz brown coal fly ash from the Omsk power stations (Russia) were presented and discussed. The XRD analysis showed that the fly ash consists primarily of quartz, mullite and magnetite. It was found that the magnetic fraction separated at the first stage is enriched in magnetite (over 20 wt %), the carbon content in the concentrate after flotation increases to 27 wt %, and 90–95% of aluminum can be extracted during the autoclave acid leaching. The SEM analysis showed that the magnetite phase is grown on the surface of alumosilicate spheres as ∼1 μm cubic crystals. The effect of the autoclave temperature and exposure time on the Al extraction efficiency was also investigated and analysed in the present paper. The optimal autoclave temperature and exposure time were found to achieve the maximum Al extraction efficiency. It was also found by the SEM microanalysis that further extraction of aluminum is not economically feasible since the remaining Al is evenly surrounded by SiO2 in the fly ash particles.

Evaluation of Co and Zn competitive sorption by zeolitic material synthesized from fly ash using 60Co and 65Zn as radioindicators

Zeolitic material prepared by hydrothermal treatment of brown coal fly ash was used for sorption separation of Co2+ and Zn2+ ions from single and binary solutions. Adsorption in single systems was rapid, pH dependent and follow the Langmuir isotherm. 3D sorption surfaces revealed competitive effects and mutual interactions between Co2+ and Zn2+ and significant preference for Zn2+ in binary system was confirmed. Surface retention and ion-exchange were most probably the principal mechanisms of Co2+ and Zn2+ removal. Zeolitic material with markedly high sorption capacity for Co2+ and Zn2+ can be a promising sorbent for removal of toxic metals and radionuclides.

Coal Waste to Two-Dimensional Materials: Fabrication of α-Fe2O3 Nanosheets and MgO Nanosheets from Brown Coal Fly Ash

High-purity 2-D α-Fe2O3 nanosheets and MgO nanosheets have been successfully fabricated with a high extraction yield of 45 and 25 wt %, respectively, from a brown coal fly ash, an otherwise valuele...

Study of mineral additives for cement materials for 3D-printing in construction

The present work provides the results of a study of the effect of thermally activated brown coal fly ash from the coal basin in Podmoskovye, microsilica, Crimean shell rock flour and quartz flour on the rheological and physical and mechanical properties of the model system (of the cement mixture) in order to select the most effective mineral components for cement materials for 3D-printing. Optimization of the structure in order to form a dense and tight structure of high-quality concrete on cement binders can be carried out by introducing fine mineral additives of different material composition. In the presence of polycarboxylate plasticizers, such additives allow to regulate the mobility of cement systems at constant W/C ratio. At the same time, the increase in strength characteristics can be achieved both by introducing reaction active additives (microsilica, fly ash, shell rock flour) and components that perform the structural and topological function (shell rock flour, quartz flour). The effectiveness of regional mineral additives for use in cement systems is presented.

Insights into Carbonation Kinetics of Fly Ash from Victorian Lignite for CO2 Sequestration

Mineral carbonation of fly ash can both capture and store CO2 permanently in a single process without long-term monitoring. Previous studies indicate that fly ash with high calcium and magnesium contents exhibit promising CO2 fixation capability. However, the reaction mechanisms and kinetics involved in the carbonation reaction of fly ash is still not fully understood. In this study, a typical Victorian brown coal fly ash from Hazelwood power plant was selected to sequestrate CO2 in a direct aqueous carbonation process. Experiments were conducted in a vessel reactor at various temperatures (40, 50, 60, and 70 °C), stirring rates (900, 1050, 1200, and 1350 rpm), and CO2 pressures (3, 4, 5, 6, and 7 bar) to investigate the reaction kinetics and identify the rate-limiting steps of carbonation. The results show that both the carbonation rate and the maximum carbonation efficiency could be improved by optimizing parameters and by the introduction of NaHCO3. Also, the complex effects of the operating parameters...

Effect of brown coal fly ash on dispersive clayey soils

Dispersive soils exhibit erosive behaviour in contrast to normal clayey soils. Several chemical stabilisers have been identified as being capable of stabilising these soils. However, the effect of brown coal fly ash (BFA) on the stabilisation of dispersive soil has not yet been investigated adequately. BFA is known to be a weak contributor to strength development. In this work, the stabilisation mechanism of BFA for strength improvement and erosion resistance over time was investigated using two different dispersive soils. The experimental results revealed that the selected BFA is an effective stabiliser in enhancing the erosion resistance of the tested dispersive soils in the short-term. On the other hand, it was found that the addition of hydrated lime further enhanced the strength development of dispersive clayey soils and facilitated early strength gain. Furthermore, soil samples treated with an effective combination of BFA and hydrated lime were investigated at the micro-level by means of energy dispersive x-ray spectroscopy and scanning electron microscopy. The micro-level investigations revealed that the cation exchange rate was higher in soils with a low calcium/sodium ratio.

Suitability of Brown Coal Fly Ash for Geopolymer Production

AbstractThe use of low-calcium Class F fly ash both as a replacement material for cement and to produce geopolymers has become established practice. However, much of the fly ash produced globally i...

Physical, mechanical and chemical properties of biosolids and raw brown coal fly ash, and their combination for road structural fill applications

Worldwide, 100 to 125 million tonnes of biosolids are produced annually, and this number is continuously increasing with millions of dollars expended on the treatment and management processes. Biosolids are nutrient-rich materials comprising a mix of inorganic, organic and water materials that are a by-product of the sewage treatment process. This paper investigates the combination of Raw Brown Coal Fly Ash (BCFA) with biosolids samples from the Eastern Treatment Plant (ETP) and Western Treatment Plant (WTP) in Melbourne, to form a material that can be used as a substitute for engineering applications. Biosolids samples were prepared along with samples of biosolids mixed with 10%, 25% and 50% BCFA by dry weight. The mechanical, chemical and physical properties of ETP22, WTP10 and BCFA were investigated using X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD), Standard Compaction and California Bearing Ratio (CBR) to assess their suitability for use in engineering applications. A Leachate Analysis was conducted using the Toxicity Characteristic Leaching Procedure (TCLP) and Australian Bottle Leachate Procedure (ABLP) to classify the wastes and determine the possibility of ground or surface water contamination through the possible leaching of heavy metals. The CBR testing aimed to analyse the relationship between mixes by preparing samples at OMC cured for 24 h, OMC cured for 7 days and OMC soaked for 48 h for investigating the effect of moisture conditions and time on the strength of the materials. The results showed that the load bearing capacity of ETP biosolids samples, with an organic content of 7.1%, significantly improved with the addition of BCFA, producing strong CBR values of 10%, 13% and 19% after 7 days curing with the addition of 10%, 25% and 50% BCFA. The WTP biosolids samples with an organic content of 22.1%, stabilized with the same percentages of BCFA, produced CBR values of 5.5%, 6% and 4% when cured for 7 days, compared with a CBR value of 3% for the unstabilized WTP biosolids sample. The results revealed that the combination of biosolids samples with BCFA that has no organic content has the potential for use in road embankments or as a subgrade material as it produced a good indication of general strength and load bearing capacity. The addition of BCFA significantly improved the CBR values of biosolids samples used in this study and reduced the organic content of the mixtures.

Dry separation of brown coal fly ash, determination of its physical, morphological, chemical, and mechanical properties of its cementitious mixes

The dry separation of brown coal fly ash of density 2.21 g/cm3, specific surface area 5112 cm2/g, having d50 and d97 of 60 and 231 µm was carried out in this research using ultrafine air classifier. Classifiers wheel speed was increased from 2000 to 10000 rpm to obtain fine and coarse products. Median diameter of 5.62 µm was obtained for fine products at a speed of 10000 rpm with nearly 90% decrease in median particle size as compared raw fly ash. Particle morphology was observed on optical, electron microscopes which showed that at 10000 rpm classifiers wheel speed, average fine particles morphology changed from angular and rounded slaggy particles to spherical particles. Cement – 60% fine products samples showed an increase of 26% of compressive strength at 90 days as compared to raw fly ash and linear relationship was developed between median diameter of fines and compressive strength.

Contribution to Sustainable Environment through Examination of Durability of Materials in an Aggressive Environment

Abstract The paper presents the results of leaching tests of concrete samples with 5 wt. % cement's replacement by fly ash and applies the statistical approach to an interpretation the results. Concrete samples with of two types of fly ashes, originating from anthracite and lignite burning, respectively were exposed to solution of Al2(SO4)3 to sulphate corrosion testing. Deterioration process was manifested by leaching the elementary components of concrete (Ca, Si, Fe, Al) and particular pH changes of leachates in 5 cycles. The leached-out concentrations of elements from concrete matrix measured by X-ray fluorescence method (XRF) and measured pH values were used for the subsequent statistical analysis. A dependency between pH of leachates and leaching trends of Ca and Al, respectively, was confirmed. The correlation coefficients ranged from 0.79 to 0.85. No significant differences between durability of concrete materials based on black or brown-coal fly ashes regarding to leachability were noticed.

High-temperature tube corrosion upon the interaction with Victorian brown coal fly ash under the oxy-fuel combustion condition

This study has examined the corrosion of tubes exposed to Victorian brown coal fly ash under the oxy-fuel combustion mode. The temperature of 650°C, flue gas composition (steam 30vol%, O2 5%, SO2 3000ppm, HCl 1000ppm, N2 5% and CO2 balanced), and an exposure time of 50h were employed. Through the joint use of advanced instruments such as synchrotron X-ray adsorption spectroscopy and thermodynamic equilibrium calculation, it has been confirmed that, from both mass loss and penetration rate perspectives, the corrosion of low-Cr tubes under the oxy-firing mode is severe compared to the respective air-firing mode. Both flue gas-related and ash-related corrosions are responsible for tube corrosion. In contrast, the corrosion rate of high-Cr tubes (∼20% for SUS304 and SUS347) is comparable between the two modes, due to the protective effect of Cr-oxide. The existence of fly ash on tube surface suppressed the chlorination reaction. Oxygen is the principal element permeating into tube under the air-firing mode, whereas both oxygen and sulphur penetrated into the tube under the oxy-firing mode, accelerating sulphation/sulphidation reactions for a rapid corrosion. Although Cr2O3 forms a protective layer under the air-firing mode, it broke down due to the accelerated corrosion attack caused by sulphates/sulphide under the oxy-firing mode. The interaction between ash and tube surface promoted the formation of pure oxides and oxide complexes with the involvement of Na and Ca as well. The presence of alkali sulphates in ash promoted the sulphidation reaction, the extent of which is however tube-specific and correlates little with Cr content. The use of advanced material SUS347 was found to be the most suitable candidate for heat-exchange tubes to be used in the oxy-firing boiler burning low-rank coal such as Victorian brown coal which is rich in alkali and alkaline earth metals.

Effect of accelerated carbonation on the chemical properties and leaching behaviour of Australian coal fly ash, to improve its use as a compost amendment

Direct agricultural use of most Australian coal fly ashes is limited by their (1) extreme alkalinity, (2) extreme salinity and (3) trace metal leachability. Therefore, in order to use fly ash as a fertiliser source, the bio-availability of its nutrients needs to be improved by reducing its toxic elements and salinity. Application of fly ash as an amendment to carbonaceous compost has the potential to dilute the toxic effects of fly ash while enhancing the release of more nutrients to the soil through the chemical reaction between compost and fly ash. However, the elevation of pH and salinity levels in compost by the addition of fly ash may reduce the availability of some indigenous micro-organisms in the compost, soil respiration, and nitrogen release and may cause toxic heavy metal intrusion into crops, creating health issues. Therefore, the present study examined the applicability of coal fly ash as an amendment to compost for agricultural use, after neutralization by mineral carbonation, taking into consideration the changes which occur in the geo-chemical properties of fly ash, after the mineral carbonation reactions for CO2 sequestration. The changes in the alkalinity (pH), salinity/electrical conductivity and trace metal concentration of the leachates from column leaching tests were measured and compared before and after the carbonation of three types of Victorian brown coal fly ashes. The results showed a significant drop of pH of the coal fly ash samples from 11–12 to 7.5–8.5 after mineral carbonation, and the salinity reduced from 6–7 to 3.2–3.8 dS/m. Along with the pH reduction, the leachable concentrations of a number of heavy metals, including B, Cd, Cu, Pb, Hg, Zn, As and Se, reduced in the carbonated ash. These favourable changes indicate the enhanced capacity of Victorian brown coal fly ashes as a compost amendment after mineral carbonation reactions.

Mineral carbonation of Victorian brown coal fly ash using regenerative ammonium chloride – Process simulation and techno-economic analysis

This report examined the technical and economic feasibility of four process scenarios for the mineral carbonation of Victorian brown coal fly ash. The first two design scenarios aimed to compare the performance of two leaching agents, namely, ammonium chloride (NH4Cl) and a mixture of ammonium chloride and hydrochloric acid (NH4Cl+HCl), on product yields and cost, whereas the other two scenarios were designed to recycle the leaching residue via single or multi-stage leaching steps to co-produce a carbonate precipitate and cement additive-grade by-product. Detailed designs were developed in Aspen Plus to determine the technical and economic potential of the selected process configurations and identify the concept with the lowest overall costs relative to the product yields. As has been confirmed, the overall production costs and carbon dioxide (CO2) capture cost of the evaluated process scenarios range from ∼$61 to 333 per tonne of product and from $135 to 1091 per tonne of CO2, respectively. The process scenario that used NH4Cl+HCl as the leaching reagent had a significantly larger cost and a higher carbonation conversion compared to the other scenarios. The process configuration that recycled the leaching residue resulted in the lowest cost per tonne of fly ash and the lowest CO2 capture cost among the four proposed scenarios. The largest net present value (NPV) and the internal rate of return (IRR) as well as the shortest payback period for this scenario further confirmed its highest profitability. The NPV, IRR and payback period of $49million, 53.4% and 2.3years, respectively, could be achieved using Victorian brown coal fly ash in this scenario. A sensitivity analysis suggests that the change in the ammonium chloride price exerts the largest effect on the production cost. A 50% increase in the ammonium chloride cost could result in the production cost increasing by 29.5%. Additionally, the selling price of the carbon precipitate product and the production cost strongly affect the financial indices. However, the production of the cement-additive by-product exerts a marginal role on the process profit. The extra income created from the cement-additive by-product is counteracted by the larger cost related to the purchase and consumption of hydrochloric acid used in the final leaching stage.